Metallographer's Guide: Practices and Procedures for Irons and Steels

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© 2002 ASM International. All Rights Reserved. Metallographer’s Guide: Irons and Steels (#06040G)

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Metallographer’s Guide Practices and Procedures for Irons and Steels

Bruce L. Bramfitt Homer Research Laboratories Bethlehem Steel Corporation Arlan O. Benscoter Department of Materials Science and Engineering Lehigh University

Materials Park, OH 44073-0002 www.asminternational.org

© 2002 ASM International. All Rights Reserved. Metallographer’s Guide: Irons and Steels (#06040G)

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Copyright © 2002 by ASM International® All rights reserved No part of this book may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the written permission of the copyright owner. First printing, March 2002

Great care is taken in the compilation and production of this book, but it should be made clear that NO WARRANTIES, EXPRESS OR IMPLIED, INCLUDING, WITHOUT LIMITATION, WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE, ARE GIVEN IN CONNECTION WITH THIS PUBLICATION. Although this information is believed to be accurate by ASM, ASM cannot guarantee that favorable results will be obtained from the use of this publication alone. This publication is intended for use by persons having technical skill, at their sole discretion and risk. Since the conditions of product or material use are outside of ASM’s control, ASM assumes no liability or obligation in connection with any use of this information. No claim of any kind, whether as to products or information in this publication, and whether or not based on negligence, shall be greater in amount than the purchase price of this product or publication in respect of which damages are claimed. THE REMEDY HEREBY PROVIDED SHALL BE THE EXCLUSIVE AND SOLE REMEDY OF BUYER, AND IN NO EVENT SHALL EITHER PARTY BE LIABLE FOR SPECIAL, INDIRECT OR CONSEQUENTIAL DAMAGES WHETHER OR NOT CAUSED BY OR RESULTING FROM THE NEGLIGENCE OF SUCH PARTY. As with any material, evaluation of the material under end-use conditions prior to specification is essential. Therefore, specific testing under actual conditions is recommended. Nothing contained in this book shall be construed as a grant of any right of manufacture, sale, use, or reproduction, in connection with any method, process, apparatus, product, composition, or system, whether or not covered by letters patent, copyright, or trademark, and nothing contained in this book shall be construed as a defense against any alleged infringement of letters patent, copyright, or trademark, or as a defense against liability for such infringement. Comments, criticisms, and suggestions are invited, and should be forwarded to ASM International. Prepared under the direction of the ASM International Technical Book Committee (2001-2002), Charles A. Parker, Chair ASM International staff who worked on this project included Veronica Flint, Manager of Book Acquisitions; Bonnie Sanders, Manager of Production; Nancy Hrivnak, Production Project Manager; and Scott Henry, Assistant Director of Reference Publications. Library of Congress Cataloging-in-Publication Data Bramfitt, B. L. Metallographer’s guide: practices and procedures for irons and steels / Bruce L. Bramfitt, Arlan O. Benscoter. p. cm. Includes bibliographical references and index. 1. Metallography. I. Benscoter, Arlan O. II. Title. TN690 .B78 2001 669’.951—dc21 2001045910 ISBN: 0-87170-748-9 SAN: 204-7586 ASM International® Materials Park, OH 44073-0002 www.asminternational.org Printed in the United States of America Micrographs on cover: Top, left: Annealed AISI/SAE 1005 steel showing equiaxed ferrite grains photographed using dark field illumination. Marshall’s etch. 200⫻ Top, right: Annealing twins in AISI 347H austenitic stainless steel photographed using differential interference contrast (Nomarski). Electrolytically etched in 60% nitric acid in water. 5 volts, stainless steel cathode. 500⫻ Bottom, left: Partially spheroidized AISI/SAE 1060 steel photographed using bright field illumination. 4% picral etch. 500⫻ Bottom, right: Lath martensite in austenitized and quenched AISI/SAE 1040 steel photographed using bright field illumination. Etched in 2% nital. 500⫻

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To Joan and Sandy for their enduring patience and encouragement.

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© 2002 ASM International. All Rights Reserved. Metallographer’s Guide: Irons and Steels (#06040G)

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Contents Optical Defects in Objectives .............................................. 118 Types of Objectives.............................................................. 120 Types of Eyepieces .............................................................. 123 The Illumination System...................................................... 126 The Components of the Illumination System ..................... 126 Types of Illumination........................................................... 131 Accessories for the Microscope........................................... 137 The Metallograph ................................................................. 139 Special Procedures for the Metallurgical Microscope ........ 140

Preface ........................................................................................ vi Acknowledgments..................................................................... vii Atlas of Microstructures ........................................................ viii Chapter 1: Introduction to Steels and Cast Irons ................. 1 Steels......................................................................................... 1 Carbon and Low-Alloy Steels ................................................. 2 High-Alloy Steels..................................................................... 9 Cast Irons ............................................................................... 16

Chapter 6: The Expanded Metallographic Laboratory .... 149 The Image Analyzer............................................................. 149 The Electron Microscope..................................................... 152 The X-Ray Diffractometer ................................................... 162 The Hot Stage Microscope .................................................. 164 The Microhardness Tester.................................................... 165 The Hot Microhardness Tester ............................................ 166 Other Specialized Techniques.............................................. 167

Chapter 2: Origin of Microstructure..................................... 23 Microstructural Development Resulting from Heat Treatment............................................................................ 24 The Iron-Carbon Phase Diagram........................................... 24 Kinetics of Phase Transformations........................................ 28 The Microstructural Constituents in Steel............................. 31 Microstructural Development Resulting from Solidification....................................................................... 41 Phase Transformations in Cast Irons..................................... 42 Transformations in a 3% C Cast Iron ................................... 43 General Description of Microstructures in Cast Irons.................................................................................... 44 Commercial Cast Irons .......................................................... 46

Chapter 7: Metallographic Specimen Preparation ............ 169 Information Gathering.......................................................... 169 Sectioning ............................................................................. 170 Mounting .............................................................................. 183 Grinding................................................................................ 198 Polishing ............................................................................... 202 Specimen Storage ................................................................. 211

Chapter 3: Alteration of Microstructure............................... 49 The Intentional Alteration of Microstructure in Steels and Cast Irons .................................................................... 49 The Unintentional Alteration of Microstructure in Steels and Cast Irons .................................................................... 66

Chapter 8: The Art of Revealing Microstructure .............. 215 Etching Response ................................................................. 215 Revealing Microstructure in an As-Polished Specimen ..... 217 Revealing Microstructure by Etching.................................. 219 The Basic Etchants for Carbon and Low-Alloy Steels and Cast Irons .................................................................. 221 The Attack Etchants ............................................................. 222 Picral..................................................................................... 227 Variations of Picral............................................................... 228 4% Picral and 2% Nital ....................................................... 233 Basic Tint Etchants for Carbon and Low-Alloy Steels and Cast Irons .................................................................. 233 General Procedure in Using Tint Etchants.......................... 234 The Common Tint Etchants................................................. 234 The Basic Etchants for Stainless Steels .............................. 236 Attack Etchants for Stainless Steels.................................... 237 Electrolytic Etchants for Stainless Steels............................ 239

Chapter 4: The Metallographer and the Metallographic Laboratory............................................. 87 The Metallographer................................................................ 87 The Metallographer versus the Chemist ............................... 89 The Metallographer’s Workday ............................................. 92 The Metallographic Laboratory ........................................... 103 Safety in the Metallographic Laboratory ............................ 106 Chapter 5: The Metallurgical Microscope.......................... 109 The Microscope.................................................................... 109 The Objective ....................................................................... 112 The Nosepiece ...................................................................... 116 iv

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Alloy steel compositions applicable to billets, blooms, slabs, and hot-rolled and cold-finished bars.................... 308 Chemical compositions for typical low-alloy steels ........... 309 ASTM specifications for chromium-molybdenum steel product forms ................................................................... 310 Nominal chemical compositions for heat-resistant chromium-molybdenum steels ......................................... 310 Compositions of standard stainless steels ........................... 311 Compositions of nonstandard stainless steels ..................... 312 Nominal compositions of wrought iron-base heat-resistant alloys.......................................................... 314 Composition limits of principal types of tool steels ................................................................................. 315 Standard composition ranges for austenitic manganese steel castings..................................................................... 316 Typical compositions for malleable iron............................. 316 Nominal compositions of commercial maraging steels...... 316 Typical base compositions of SAE J431 automotive gray cast irons for heavy-duty service ............................ 316 Chemical compositions and mechanical properties of austenitic manganese steels for nonmagnetic and cryogenic applications...................................................... 317 Composition of selected cast irons...................................... 317 Temperature Conversions..................................................... 318

The Basic Etchants for Coated Steels ................................. 241 Special Etching Procedures ................................................. 241 The Use of the Microscope to Enhance Microstructural Features............................................................................. 242 Chapter 9: Glossary............................................................... 245 Appendix: Tables Helpful to the Metallographer .............. 297 List of ASTM standards that pertain to ferrous metallography ................................................................... 297 List of vendors for metallographic supplies ....................... 298 List of light optical microscope manufacturers .................. 298 Microscope reticle manufacturer ......................................... 298 Scientific imaging products ................................................. 299 Used and/or reconditioned equipment................................. 299 Conversion of average grain intercept length (microns) to ASTM number ............................................................. 300 Chemical polishing solutions............................................... 300 Electroless and electrolytic coatings for edge protection ... 301 Etchants for revealing macrostructures in iron and steel ... 302 Etchants for carbon and alloy steels ................................... 303 Carbon steel compositions ................................................... 307 Free-machining (resulfurized) carbon steel compositions .. 307 Free-machining (rephosphorized and resulfurized) carbon steel compositions................................................ 307 High-manganese carbon steel compositions ....................... 307 High-manganese carbon steel compositions ....................... 307

Index ........................................................................................ ................................................................................ 321

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Preface This guide was prepared not only for the beginning metallographer but also for the experienced metallographer who may be looking for alternatives and new approaches to metallographic practice. For the beginning metallographer, little or no knowledge of steels and cast irons is necessary since the first three chapters provide the basic information needed to understand the various types of steels and cast irons available in the commercial world. These chapters also provide examples of the multitude of microstructures that the metallographer will encounter, how these microstructures are created, and how they can be altered by heat treatment and other means. Some metallographers may be working in a small laboratory where no metallurgical support is available. The authors feel very strongly that to be effective, the metallographer must understand as much as possible about the metallurgy of the material he or she is preparing. Without this knowledge, the metallographer can offer little interpretation of the microstructure he or she develops even after applying the best metallographic practices. Also, without a proper background in recognizing metallographic constituents, he or she may produce an artifact through improper specimen preparation that will lead to a totally inappropriate result. Thus, it is important that the metallographer read the first three chapters to obtain a basic understanding of steel microstructures before proceeding to the metallographic techniques chapters. As part of this guide, the authors felt that a metallographer should know some of the history of metallography. In this new century, we have come a long way from the early days of Sorby and Widmanstätten who pioneered what we now know as metallography of steels and cast irons a century and a half ago. Chapter 4 gives a brief history of these early metallographers and defines the identity of a metallographer by comparing the vast amount of information gained from a metallographic analysis to that produced from a chemical analysis. The chapter also describes the types of things that a metallographer will encounter in a typical workday in a large metallographic laboratory in the research department of a large steel company and a small metallography laboratory associated with an iron foundry. Actual metallographic tasks in both situations are described in detail. Chapter 6 discusses some of the tools that are available beyond the typical metallographic laboratory. In today’s world, there has been an explosion in technology to aid the metallographer. Not only can one reveal the microstructural constituents in a steel or cast iron, but also one can determine the chemical analysis of each constituent even on a nanometer scale. To be effective, the metallographer must be familiar with the capabilities of these modern-day instruments. Since this guide concentrates on light (optical) metallography, Chapter 5 has been added to describe in detail how a metallurgical microscope works. This is the instrument located in all metallographic laboratories. The metallographer must have an intimate knowledge of the microscope to use it properly. An understanding of the different types of oculars (eyepieces) and objectives is important so that the microstructure can be revealed in its truest form. Knowledge of the various types of illumination (bright field, dark field, interference contrast, etc.) is important to enhance the image of the microstructural features. The metallographer also must know how to maintain and clean the microscope to keep it in the best condition possible. Specimen preparation procedures were saved for Chapters 7 and 8. The procedures presented in this guide have proven to work effectively to prepare the specimen. However, the authors recognize that other procedures also can work as effectively. This book guides the metallographer through the specimen preparation procedures in a step-by-step manner. Various options are offered, and preferred methods are described in detail. The authors provide a basic understanding of how and why the methods work. As the metallographer becomes more experienced, he or she may develop his or her own adaptations of the procedures presented here. This guide will get the metallographer started with a sound procedure that works. A unique feature of this guide is a separate and complete index of the various steels and cast irons used as examples throughout the book. The index makes the hundreds of micrographs essentially an Atlas of Microstructures, and it precedes Chapter 1. Although this book is for the novice metallographer, an experienced metallographer may find it useful in that dozens of special metallographic tips are scattered through the chapters on specimen preparation and the art of revealing microstructure. This guide could be used as a university or technical school text to accompany the teaching of a laboratory course in metallography. Bruce L. Bramfitt Arlan O. Benscoter

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Acknowledgments The authors wish to acknowledge the reviewers of the various chapters who provided valuable insight for improvement. The list includes L.E. Samuels, Keith A. Taylor, Albert Brandemarte, Samuel Lawrence, Colin McCrindle, and Robert O’Reilly. Special thanks go to Samuel Lawrence, Metallographer at Bethlehem Steel’s Homer Research Laboratories, for his advice and assistance in the chapters on specimen preparation. A very special acknowledgment goes to metallographers Ernie Kozak (deceased) and George Ruyak (deceased) who nurtured and guided one of the authors (AOB) in his early years of metallography at Bethlehem Steel’s Homer Research Laboratories. The various students at Lehigh University who provided specimens and micrographs for this book are also acknowledged. Particular thanks go to the editors at ASM International: Timothy Gall who started this venture and Sunniva Collins, our first editor, who provided us with lots of help and support and who married, had children, and received her Doctorate degree while we were in the process of writing this book. Also, thanks go to Veronica Flint, who through diplomatic prodding and helpful encouragement brought us to the point of completing the book in a somewhat timely manner, and to Nancy Hrivnak for her help and guidance in preparing the book for publication.

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Atlas of Microstructures Steels/irons

Carbon steels AISI/SAE 1005 AISI/SAE 1008 AISI/SAE 1010

AISI/SAE 1015 AISI/SAE 1018

AISI/SAE 1020

AISI/SAE 1025 AISI/SAE 1035 AISI 10B36 AISI/SAE 1040

AISI/SAE 1045 AISI/SAE 1060

AISI/SAE 1070 AISI/SAE 1080

Fig. No.

Cover 7.1 1.2(a) 3.10 4.20 4.21 7.39 3.41 8.31 8.32 8.52 8.53 2.42 3.5 3.42 3.51 3.52 5.35 6.2 6.3 7.4 7.7 7.45 8.9 8.33 2.14 7.14 3.39 3.40 8.37 Cover 1.2(b) 2.6 2.23 3.1 3.3 3.4 3.15 5.64 5.65 3.29 3.30 6.18 Cover 2.15 2.16 2.22 2.41 4.8 4.9 2.1 2.19 2.20 2.21 3.28 3.47 3.55 3.56 3.58 3.59 3.60 4.5 4.6 5.24

Page No.

Steels/irons

Fig. No.

Page No.

6.9 6.11 7.3 7.8 7.41 8.2 8.21 8.38 1.2(c) 2.26 3.48 3.31 3.32 7.6 8.5 1.3 8.34 2.43 7.22 4.14

153 153 171 174 206 218 228 236 4 36 75 67 68, 69 173 219 5 234 42 188 97

0.75%C-3.25% Cr Cr-Mo Ni-Cu-Cr-Mo

6.27 7.15 3.21 4.15 5.41 4.11 3.6 3.7 3.8 3.17 3.19 3.20 2.33 2.34 8.35 8.7 4.11 8.17 3.33 3.34 3.35 3.36 3.37 3.38 6.37 3.45 2.30 2.31 2.32 2.39 3.57 8.26 8.28 7.46 7.47 5.32 8.30 2.39

161 182 63 97 132 96 53 53 54 60 61 62 39 39 235 221 96 226 69 69 70 70 70 73 167 75 37 38 38 41 82 231 232 211 212 126 232 41

Tool steels AISI/SAE A2 AISI/SAE A8

8.12 5.67

224 147

Carbon steels (continued) AISI/SAE 1080

ii 170 4 55 100 100 205 73 233 233 243 243 42 53 74 77 78 128, 129 150 150 172 174 210 222 233 33 180, 181 73 73 235 ii 4 28 35 50 51 52, 53 58 146 147 66 67 156 ii 33 33 35 41 94 95 23, 24 34 34 35 66 75 80 81 83 83 84 91 92 122

AISI/SAE 1095 AISI/SAE 1117 AISI/SAE 1144 AISI/SAE 11L44 AISI/SAE 1213 AISI/SAE 1524 Steel, screw Steel, shot (S-230) Low-alloy steels AISI/SAE 1335 AISI/SAE 4327 AISI/SAE 4340 AISI 52100 AISI/SAE 8630

AISI/SAE 8720 AISI/SAE 8860 AISI/SAE 9425 AISI/SAE 52100 Low-alloy 0.7%C-3%Cr

E660 3%Cr 0.2%C-1%Mn-5%Ni Ni-Cr-Mo

0.23%C-3.4%Ni-1.7%Cr-0.5%Mo 0.5%Mo-B 2.5%Ni-0.4 Nb

viii

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Atlas of Microstructures Steels/irons

Tool steels (continued) AISI/SAE D2 AISI/SAE M2 AISI/SAE W1 Stainless steels AISI/SAE 301 AISI/SAE 303 AISI/SAE 304 AISI/SAE 309 AISI/SAE 316

AISI/SAE 316L AISI/SAE 347H AISI/SAE 410 AISI/SAE 430F Custom 630 Duplex 7Mo Plus Duplex 2205 Very-low carbon steels Interstitial-free Enameling Motor lamination Very-low carbon Low-carbon

Low-carbon (0.02%) Low-carbon (0.04%) Low-carbon cold-rolled

Iron alloys Pure iron Iron-0.2%C Iron-0.4%C Iron-0.6%C

Fig. No.

Page No.

5.46 8.27 1.19 8.22

135 231 15 229

8.45 4.17 4.18 4.19 3.12 8.6 1.11 2.36 3.43 3.44 3.53 3.54 5.47 5.60 8.44 8.46 8.47 7.33 Cover 1.15 2.28 1.13 1.17 8.42 8.43 1.16 8.40 8.48

240 98 99 99 57 220 11 40 74 74 79 79 136 141 239 240 241 197 ii 14 37 12 14 238 239 14 237 241

3.16 8.20 8.55 8.56 1.23 8.39 5.44 7.5 7.18 7.21 8.8 8.13 8.14 8.23 8.24 2.13 2.17 3.11 8.15 8.16

59 227 244 244 16 236 134 173 184 186 222 224 225 229 229 32 33 56 225 226

8.19 2.24 8.10 8.1 8.10 8.10

226 36 223 216 223 223

Steels/irons

Iron alloys (continued) Iron-1.0%C Iron-1.2%C Iron-1.4%C Iron-1.75%C Iron-1.86%C Wrought iron Lancashire iron Meteorite High-strength steels X65 linepipe HSLA

Dual-phase

1/2Mo-B Comp F (MIL-S-23194) Alloy steels Maraging Invar A286 superalloy 1%C-14%Cr 0.93%C-1.45%Mn 0.93%C-14.5%Ni 25%Cr-12%Ni 19%Cr-9%Ni Grinding ball Coated steels Galvalume

Aluminized Type 1 Aluminized Type 2 Nickel plate Enamel ASTM steels ASTM A1 rail

ASTM A 36 structural

ix

Fig. No.

Page No.

7.17 3.49 3.50 2.7 2.18 3.46 5.42 2.27 8.4 8.3 4.4

184 76 77 28 34 75 133 137 218 218 88

1.7 3.13 6.10 6.16 6.28 7.42 1.8 2.38 3.9 7.12 8.18 8.36 8.28 8.29

8 57 153 156 161 207 9 40 55 179 226 235 232 232

1.21 8.41 1.22 1.18 7.26 2.37 8.11 5.36 5.63 7.10

15 238 15 14 192 40 224 129, 130 145 176, 177

6.5 6.6 8.49 8.50 8.51 4.7 4.7 7.1 7.18 7.29 8.55 8.56

151 151 242 242 242 93 93 170 184 195 244 244

1.4 3.55 3.56 4.5 4.6 4.16 1.5 7.11 7.40

6 80 81 91 92 98 6 178 206

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Atlas of Microstructures Steels/irons

ASTM steels (continued) ASTM A 128 Hadfield ASTM A 247 ASTM A 470 rotor ASTM A 514 ASTM A 516 ASTM A 710 ASME steels ASME SA 210 tubing ASME SA 213 T22 tubing Other steels Ancient steel Blister steel Cast irons Gray iron

White iron

Malleable iron

Ductile (nodular) iron

Mottled iron Austempered ductile iron

Fig. No.

Page No.

1.20 1.25 4.22 1.9 2.25 3.61 6.10

15 17 101, 102 9 36 84 153

3.62 1.6 8.25

85 7 230

7.9 4.2

175 88

1.24 1.25 2.48 2.56 3.63 5.13 5.14 1.26 2.45 2.46 2.49 2.54 2.55 3.22 5.17 1.28 2.53 3.23 3.24 3.25 1.30 2.51 2.52 3.26 3.27 4.23 4.24 1.27 2.50 1.31

16 17 44 47 85 115 115 18 43 44 45 47 47 64 117, 118 18 46 64 65 65 20 45 46 65 65 103 103 18 45 20

Microstructures

Microstructures Acicular ferrite Alloy carbides

Alloy layer Annealing twins

Austenite

Austenite/ferrrite Bainite

Bainite/ferrite Bainite/graphite Bainite (granular) Bainite (lower) Bainite (upper) Bainite transformation Banding

Bull’s-eye structure Carburized surface Cementite platelets Central bursts in wire Cold work

x

Fig. No.

2.16 1.19 5.46 7.10 7.26 8.12 8.22 8.27 4.7 1.11 1.18 1.22 2.36 8.44 1.11 1.18 1.20 1.22 2.36 3.12 3.43 3.44 3.53 3.54 5.47 8.44 8.46 5.60 8.47 2.1(b) 2.31 2.32 6.15 7.46 7.47 8.7 3.21(a) 7.15(d) 1.31 2.35 2.34 1.9 2.33 6.33 1.3 2.43 3.1 3.13 3.15 3.19 3.61 6.27 7.4 8.34 8.40 3.26 4.23(a) 3.31 3.32 2.7 2.18 5.17 4.8 3.11

Page No.

33 15 135 176, 177 192 224 229 231 93 11 14 15 40 239 11 14 15 15 40 57 74 74 79 79 136 239 240 141 241 23 38 38 155 211 212 221 63 182 20 39 39 9 39 165 5 42 50 57 58 61 84 161 172 234 237 65 103 67 68, 69 28 34 117, 118 94 56

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Atlas of Microstructures Microstructures

Microstructures (continued) Cold work

Copper precipitates Copper penetration Creep voids Decarburized surface Deformation bands Delta ferrite

Dendritic structure

Dimpled fracture surface Enamel coating Epitaxial ferrite

Equiaxed ferrite

Eutectic carbides Exaggerated grain growth Fayalite Ferrite

Ferrite/austenite

Fig. No.

3.16 5.47 6.36 6.37 7.5 7.11(a) 8.15 8.16 8.32 8.45 8.52 6.10 3.40 3.57 3.28 3.29 3.30 3.12 3.53 5.47 4.19 5.60 8.47 8.48 2.42 7.10 8.50 8.51 8.6 6.19 8.55 8.56 3.10 3.32(e) 3.32(f) 5.44 8.18 8.36 1.2(a) 1.23 2.13 8.8 8.13 8.20 8.27 3.4 6.18 1.13 1.23 2.13 3.11 3.16 5.44 7.5 7.21 8.8 8.15 8.16 8.18 8.20 8.39 8.55 1.16 4.19

Page No.

Microstructures

Microstructures (continued) Ferrite/austenite Ferrite/bainite Ferrite/carbides

59 136 167 167 173 178 225 226 233 240 243 153 73 82 66 66 67 57 79 136 99 141 241 241 42 176 242 242 220 156 244 244 55 69 69 134 226 235 4 16 32 222 224 227 231 52, 53 156 12 16 32 56 59 134 173 186 222 225 226 226 227 236 244 14 99

Ferrite/graphite

Ferrite/pearlite

Ferrite/martensite

xi

Fig. No.

8.40 3.21(a) 2.17 3.17 3.19 8.1 8.13 8.14 8.23 1.28 1.30 3.23 3.48 3.49 4.23 4.24 1.2(a) 1.2(b) 1.3 1.5 1.7 2.6 2.14 2.23 2.42 2.43 3.1 3.3 3.4 3.5 3.6 3.7 3.13 3.15 3.19 3.52(c) 3.61 3.62 4.20 4.21 6.2 6.27 7.3 7.4 7.9 7.11 7.14(d) 7.29 7.40 8.31 8.32 8.34 8.52 8.53 1.6 1.8 2.16 2.38 3.9 3.10 3.32(e) 3.32(f) 3.52(b) 7.12

Page No.

237 63 33 60 61 216, 217 224 225 229 18 20 64 75 76 103 103 4 4 5 6 8 28 33 35 42 42 50 51 52, 53 53 53 53 57 58 61 78 84 85 100 100 150 161 171 172 175 178 181 195 206 233 233 234 243 243 7 9 33 40 55 55 69 69 78 179

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Atlas of Microstructures Microstructures

Microstructures (continued) Ferrite/martensite Ferrite/martensite/bainite/ pearlite Flow lines Fractured weld Grain boundary carbides

Grain boundary segregation Graphite flakes

Graphite nodules

Graphitic corrosion Graphitization Heat-affected zone Hot cracking (weld) Hot shortness Hydrogen flakes

Inclusions Inclusions Inclusions Inclusions

(CaS) (FeS) (lead) (MnS)

Inclusions (oxide)

Inclusions (slag) Inclusions (TiN) Internal oxidation Lath martensite

Fig No.

Page No.

8.36 8.37

235 235

4.15 4.10 7.45 8.6 2.17 5.36 8.8 8.13 8.14 8.25 3.36 3.38 1.24 2.48 3.63 5.13 5.14 2.52 3.26 3.27 4.22 4.23 4.24 3.63 3.48 3.49 3.50 4.10 8.6 3.39 3.40 3.58 3.59 3.60 3.61 3.62 6.25 3.41 8.5 1.3 1.13 3.33 3.34 3.35 3.42 6.19 7.6 7.39 8.2 8.34 6.25 6.26 7.7 8.3 8.34 8.4 8.54 5.67 7.8 3.30 2.24

97 95 210 220 33 129, 130 222 224 225 230 70 73 16 44 85 115 115 46 65 65 101, 102 103 103 85 75 76 77 95 220 73 73 83 83 84 84 85 160 73 219 5 12 69 69 70 74 156 173 205 218 234 160 160 174 218 234 218 243 147 174 67 36

Microstructures

Microstructures (continued) Lath martensite

Ledeburite

Martensite

Martensite/austenite

Martensite/carbides Martensite/pearlite Meteorite Microcracks Oxide layer Oxide penetration Oxidized surface Pearlite

xii

Fig. No.

2.25 2.28 3.8 3.20 3.32(c) 3.32(d) 3.52(a) 5.35 8.9 8.10 8.17 8.33 8.41 8.42 8.43 1.26 1.27 2.45 2.46 2.49 3.22 1.15 1.17 1.21 2.1(c) 3.17(a) 4.9 7.14(c) 2.37 2.39 2.54 2.55 3.32(a) 3.32(b) 3.46 8.11 7.10 7.26 8.27 2.22 3.33 5.32 4.4 3.46 3.47 7.17 7.22 8.26 6.18 8.26 1.4 2.1(a) 2.19 2.20 2.21 3.56(d) 3.58 4.5 4.6 5.24 6.9 6.11 7.3 7.8

Page No.

36 37 54 62 68 68 78 128, 129 222 223 226 233 238 238 239 18 18 43 44 45 64 14 14 15 24 60 95 181 40 41 47 47 68 68 75 224 176, 177 192 231 35 69 126 88 75 75 184 188 231 156 231 6 23 34 34 35 81 83 91 92 122 153 153 171 174

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Atlas of Microstructures Microstructures

Microstructures (continued) Pearlite Pearlite/bainite Pearlite/bainite/martensite Pearlite/cementite

Pearlite colonies Pearlite/ferrite Pearlite/graphite

Pearlite lamella

Pearlite/martensite Pearlite nodules Phosphorus segregation Plate martensite

Prior austenite grain boundaries

Fig No.

8.21 8.38 3.21(b) 2.1(d) 1.26 1.27 2.7 2.45 2.46 2.49 3.22 5.17 8.22 2.21 8.21 2.15 2.41 7.3 1.24 2.48 2.50 2.51 2.53 2.56 3.24 3.25 3.26 3.63 4.23 4.24 5.13 5.14 2.19 2.20 4.5 4.6 5.13 5.14 5.24 6.9 6.11 5.32 2.22 3.36 2.26 2.27 3.32(a) 3.32(b) 2.37 3.46 3.47 3.56(b),(c) 5.42 7.17 8.10 8.11 3.33 3.36 3.38 3.57 5.41 8.17 8.18 8.25

Page No.

Microstructures

Microstructures (continued) Prior austenite grain boundaries

228 236 63 24 18 18 28 43 44 45 64 117 229 35 228 33 41 171 16 44 45 45 46 47 65 65 65 85 103 103 115 115 34 34 91 92 115 115 122 153 153 126 35 70 36 37 68 68 40 75 75 81 133 184 223 224 69 70 73 82 132 226 226 230

Proeutectoid cementite Proeutectoid ferrite Recrystallized grains Retained austenite

Rusted surface Sensitized stainless steel

Silicon carbide particles Spangle dendrites Spheriodized carbides

Steadite Stress-corrosion cracking Sulfur segregation Surface carbides Surface damage (abrasive wheel) Surface damage (EDM) Surface damage (polishing) Surface damage (shearing) Surface damage (torch-cut) Surface damage (wheel burn on rail) Temper carbon

Temper carbides Temper embrittlement Tempered martensite

xiii

Fig. No.

Page No.

8.28 8.29 8.30 1.2(c) 2.7 2.18 2.15 2.41 7.41 3.16 8.15 2.37 2.38 2.39 3.10 3.32(b) 7.12 8.11 7.46 7.47 3.43 3.44 3.54 8.25 8.44 7.1 8.50 8.51 3.17(b) 3.19(b) 3.48 8.1 2.56 5.14 4.18 4.19 4.16 8.33 7.9 7.10 7.15 7.41 7.42 7.11 7.12 3.51 3.52 7.14 3.55

232 232 232 4 28 34 33 41 206 59 225 40 40 41 55 68 179 224 211 212 74 74 79 230 239 170 242 242 60 61 75 216, 217 47 115 99 99 98 233 175 176 182 206 207 178 179 77 78 180 80

1.28 2.53 3.23 3.24 3.25 8.33 3.45 1.19 2.30 3.20 3.21(c) 3.56(b) 4.11 8.25

18 46 64 65 65 233 75 15 37 62 63 81 96 230

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Atlas of Microstructures Microstructures

Microstructures (continued) Tempered martensite Tempered martensite/alloy carbides Titanium-molybdenum carbides Veining Vermicular graphite

Fig No.

Page No.

8.33 8.35 1.19 7.10 6.16 6.28 8.19 4.24

233 235 15 176 156 161 226 103

Microstructures

Microstructures (continued) Voids White surface layer Widmanstatten ferrite Widmanstatten structure Wustite

xiv

Fig. No.

Page No.

3.36 3.37 3.57 3.55 3.56 2.14 4.4 6.18

70 70 82 80 81 33 88 156

Metallographer's Guide: Practices and Procedures for Irons and Steels Bruce L. Bramfitt, Arlan O. Benscoter, p1-21 DOI:10.1361/mgpp2002p001

Copyright © 2002 ASM International® All rights reserved. www.asminternational.org

CHAPTER 1

Introduction to Steels and Cast Irons STEELS AND CAST IRONS are basically alloys of iron and various other elements in the periodic table. The vast majority of steels and all cast irons contain carbon as a principal alloying element. As a general definition, a steel is an alloy of iron, carbon (under 2% C), and other alloying elements that is capable of being hot and/or cold deformed into various shapes. A cast iron, on the other hand, is an alloy of iron, carbon (over 2% C), and other elements and is not normally capable of being hot and/or cold deformed. A cast iron is used in its cast form. Steels and cast irons are the most widely used and least expensive metallic materials on earth. This Chapter introduces the metallographer to the various types of steels and cast irons and explains how they are classified and defined. The classification includes the plain carbon and alloy steels as well as the gray, white, ductile, and malleable cast irons, beginning with the steels.

Steels There are thousands of different steel compositions currently available around the world. To the beginning metallographer, the variety and terminology may at first be overwhelming. In fact, the way that steels are classified may be quite confusing even to the seasoned metallographer and metallurgist. However, in many cases the steels fall into a limited number of well-defined classes. An attempt is made in this chapter to summarize these classes. More detailed information can be found in the ASM Handbook (Volume 1), the selected references listed at the end of the Chapter, and in the Appendix.

Classification of Steels Generally, the carbon and low-alloy steels come under a classification system based on composition. The higher-alloy steels (the stainless, heat-resistant, wear-resistant steels, etc.) can be classified according to many different systems, including composition, microstructure, application, or specification. The flow diagram in Fig. 1.1 shows very generally how steels are classified. On the left side, they are classified by commercial name or application, and on the right side, by microstructure. The flow diagram may look complicated at first, but this Chapter attempts to explain it. Mostly, the classifications on the left side of the diagram are examined.

The easiest way to classify steels is by their chemical composition. Various alloying elements are added to iron for the purpose of attaining certain specific properties and characteristics. These elements include, but are not limited to, carbon, manganese, silicon, nickel, chromium, molybdenum, vanadium, columbium (niobium), copper, aluminum, titanium, tungsten, and cobalt. The functions of each of these elements and others are listed in Table 1.1. Most steels contain several of these elements, particularly, carbon, manganese, and silicon.

Formal Classification Systems Many nations have their own classification system for steels and cast irons. Because of the complexity of these different classification systems, only those used in the United States are described in this Chapter. The American Iron and Steel Institute (AISI) and Society of Automotive Engineers (SAE) System. For many decades, plain carbon, low-alloy steels have been classified by composition using a system devised by SAE and eventually AISI. In this chapter, the steels thus classified have “AISI/SAE” before the steel code number, for example, AISI/SAE 1040 steel. The system is based solely on composition. In the four- or five-digit code designation, the last two or three digits represent the carbon content (three digits for steels with a carbon content of 1.00% and above), and the first two digits represent the compositional class. Thus, in the example of AISI/SAE 1040 mentioned previously, the “10” represents the class of plain carbon steels, and the “40” represents the carbon content of 0.40% C. The AISI/SAE designations and compositions are listed in the Appendix. The American Society for Testing and Materials (ASTM) System. Another system was devised by ASTM. This system is not based on composition but on the steel product and application, for example, railroad rails, boiler tubes, plate, and bolts. ASTM has devised a system of specifications that contain composition, mechanical properties, and other required characteristics of steels and cast irons. The ASTM system reaches far beyond ferrous materials and includes other materials, such as rubber, cement, wood, fabric, copper, and so on. The American Society of Mechanical Engineers (ASME) devised a similar system, but it is generally limited to boiler and heat exchanger steels and other materials that are covered by the boiler code specifications.

2 / Metallographer’s Guide The Unified Numbering System (UNS). Because of the confusion of different systems, a number of technical societies and U.S. governmental agencies devised what is known as the Unified Numbering System. There is a UNS designation for each steel composition, and it consists of a letter followed by five digits. The system fully incorporates the AISI/SAE system. For example, the UNS designation for AISI/SAE 1040 is G10400. The letter “G” represents the AISI/SAE plain carbon and alloy steels. Other ferrous alloys have different letters, such as “F” for cast irons and cast steels (cast steels can also have the letter “J”), “D” for steels with specific mechanical properties, “S” for heat- and corrosionresistant steels, “T” for tool steels, and “H” for steels with enhanced hardenability. In this book, the AISI/SAE designations are favored only because they are, at the present time, more

Fig. 1.1 Classification chart for steels

widely used than the UNS designations. However, in the reference tables in the Appendix, both designations are listed. In this Chapter, all systems are used where appropriate. For some steels, it may be easier to use the AISI/SAE system, for others, the ASTM system. We first examine the way the steels are classified simply by composition, using the AISI/SAE system. This system has been established for many years and is widely used in industry.

Carbon and Low-Alloy Steels The general category of carbon and low-alloy steels encompasses plain carbon steels, alloy steels, high-strength low-alloy

Introduction to Steels and Cast Irons / 3 Table 1.1 Element

Carbon (C)

Manganese (Mn)

Phosphorus (P)

Sulfur (S) Silicon (Si)

Nickel (Ni)

Chromium (Cr)

Molybdenum (Mo)

Copper (Cu)

Cobalt (Co)

Tungsten (W)

Essential and incidental elements in steel and cast iron Function

An essential alloying element in most steels. Added to increase solid-solution strength and hardness as well as to increase hardenability. Dissolves in iron to form ferrite and austenite. Combines with iron to form a carbide (cementite-Fe3C). The carbide is a component of pearlite. An essential alloying element in most steels. Added to increase solid-solution strength and hardness as well as to increase hardenability. A weak carbide former (greater than iron). Counteracts brittleness caused by sulfur (iron sulfide) through the formation of a manganese sulfide (MnS). High levels of manganese produce an austenitic steel with improved wear and abrasion resistance. Usually considered an impurity in most steels. Can be added to low-carbon steels to increase strength and hardness. Improves machinability of free-machining steels. Promotes temper embrittlement. Forms an undesirable iron phosphide (Fe3P) at high phosphorus levels (especially in cast irons) Usually considered an impurity in steel. Added to special steels for improved machinability An essential alloying element in most steels. Added to increase solid-solution strength and hardness as well as to increase hardenability. Is added to molten steel to remove oxygen (deoxidize). As a result of deoxidation, can form silicate stringers (silicon dioxide inclusions). Does not form a carbide in steels. Improves oxidation resistance. Added to special steels to improve electrical and magnetic properties as well as hardenability. Increases susceptibility to decarburization. Promotes graphitization in cast irons An essential alloying element in some steels. Added to increase solid-solution strength and hardness as well as to increase hardenability. Toughens steels, especially at low temperatures. Does not form a carbide in steel. Renders high-chromium stainless steels austenitic An essential alloying element in some low-alloy steels and all stainless steels. Added to slightly increase solid-solution strength and hardness as well as to increase hardenability. Increases resistance to corrosion and high-temperature oxidation. A carbide former (greater than manganese); its carbides improve wear and abrasion resistance and provide high-temperature strength. An essential alloying element in some low-alloy steels and tool steels. Added to increase solid-solution strength and hardness as well as to increase hardenability. A strong carbide former (stronger than chromium). Improves high-temperature properties, including creep strength. Counteracts temper embrittlement. Enhances corrosion resistance in stainless steels Usually considered an impurity or tramp element in most steels, because it promotes hot shortness. Added to some steels for improved corrosion resistance. Added in special steels for increased strength and hardness through heat treating (aging). Very insoluble in iron at room temperature and does not form a carbide An essential alloying element in some steels. Added to increase strength and hardness. Improves hot hardness. Weak carbide former. An important element in some tool steels and heat-resistant steels. Decreases hardenability An essential alloying element in some steels. Added to increase solid-solution strength and hardness as well as to increase hardenability. Strong carbide former; the carbides form hard, abrasion-resistant particles in tool steels.

Element

Vanadium (V)

Columbium (Cb) Niobium (Nb) Aluminum (Al)

Titanium (Ti)

Boron (B)

Nitrogen (N) Lead (Pb) Bismuth (Bi) Tin (Sn) Antimony (Sb) Arsenic (As) Oxygen (O)

Hydrogen (H)

Calcium (Ca)

Zirconium (Zr) Cerium (Ce) Magnesium (Mg)

Function

An important element in microalloyed steels. Added to increase strength and hardness of steel by grain-size control (grain refinement) as well as to increase hardenability. Strong nitride former; also forms a carbide. Minimizes loss in strength during tempering An important element in microalloyed steels. Added to increase strength and hardness of steel by grain-size control (grain refinement) as well as to increase hardenability. Strong carbide former; also forms a nitride An important alloying element in nitrided steels and deep-drawing sheet steels. Added to increase strength and hardness of steel by grain-size control (grain refinement). A common deoxidizer. Forms undesirable alumina inclusions (aluminum oxides). A strong nitride former. Does not form a carbide in steel An important element in microalloyed steels. Added to increase strength and hardness of steel by grain-size control (grain refinement). Very strong carbide and nitride former. Important element to “getter” or tie up nitrogen in steels (protects boron from nitrogen in boron-treated steels). Also a strong deoxidizer. Can combine with sulfur to form titanium sulfides Added to steel to increase hardenability. Enhances the hardenability characteristics of other alloying elements. Added to steel for nuclear reactor applications because of its high cross section for neutrons Added to some microalloyed steels to increase the amount of nitrides required for strengthening or grain-size control (e.g., in a vanadium steel) Insoluble in steel. Added to special leaded steels for improved machinability. Environmentally sensitive Similar to lead. Added to special steels for improved machinability An impurity or tramp element in steel. Promotes temper embrittlement An impurity or tramp element in steel. Promotes temper embrittlement An impurity or tramp element in steel. Promotes temper embrittlement Undesirable in steel. Combines with other elements (manganese, silicon, aluminum, titanium, etc.) to form oxide inclusions that can degrade toughness and fatigue resistance. Usually minimized in steel by deoxidation with aluminum and/or silicon and vacuum degassing Undesirable in steel. If entrapped, can cause crack formation (hydrogen flakes, microcracks, etc.). Usually minimized in liquid steel by vacuum degassing or slow cooling after the austenite-to-ferrite transformation Added to steel for sulfide shape control (combines with sulfur to form rounded, undeformable inclusions). Strong deoxidizer. Forms calcium oxide and calcium aluminate inclusions Added to steel for sulfide shape control (forms rounded, undeformable zirconium sulfides). Strong deoxidizer. Forms zirconium oxide and is a strong nitride former Added to steel for sulfide shape control (forms rounded, undeformable cerium sulfide inclusions). Strong deoxidizer Added to liquid cast iron to nucleate graphite nodules in ductile (nodular) iron

4 / Metallographer’s Guide (HSLA) steels, and a variety of other low-alloy steels. Each of these subcategories is described in the following sections.

Plain Carbon Steels The more commonly used steels are classified according to composition. These steels include the plain carbon steels, with the following general subclasses: Subclass

The microstructures of typical low-carbon, medium-carbon, and high-carbon steels are shown in Fig.1.2(a), (b), and (c), respectively. The low-carbon steel is represented by an AISI/SAE 1010 steel, the medium-carbon steel by an AISI/SAE 1040 steel, and the high-carbon steel by an AISI/SAE 1095 steel. As carbon content increases, the amount of pearlite (the dark etching constituent) increases. Actually, the amount of pearlite increases up to a maximum of 100% at a carbon content near 0.8%. Below

Carbon content (a), %

Low-carbon steels Medium-carbon steels High-carbon steels

Under 0.2 0.2–0.5 Above 0.5

(a) All percentages in this Chapter are weight percent, unless otherwise noted.

AISI/SAE Classification System for Plain Carbon Steels. The plain carbon steels can be further classified by specific composition according to the AISI and SAE designations. As a specific example, the designation AISI/SAE 1040 signifies a mediumcarbon steel with a nominal carbon content of 0.40% and with the following range of composition: Element

Content, %

Carbon Manganese Phosphorus Sulfur

0.37–0.44 0.60–0.90 0.040 max 0.050 max

The AISI/SAE designations for the plain carbon steels are listed in the Appendix.

Fig. 1.2(a) Micrograph of low-carbon AISI/SAE 1010 steel showing a matrix of ferrite grains (white etching constituent) and pearlite (dark etching constituent). Etched in Marshall’s reagent followed by 2% nital. 200⫻

Fig. 1.2(b) Micrograph of medium-carbon AISI/SAE 1040 steel showing ferrite grains (white etching constituent) and pearlite (dark etching constituent). Etched in 4% picral followed by 2% nital. 300⫻

Fig. 1.2(c) Micrograph of high-carbon AISI/SAE 1095 steel showing a

matrix of pearlite and some grain-boundary cementite. Etched in 4% picral. 500⫻

Introduction to Steels and Cast Irons / 5 0.8% C, the other constituent in the microstructure is ferrite, as seen in Fig. 1.2(a) and (b). Above 0.8%, the other constituent is cementite, as seen in Fig. 1.2(c). More details about these constituents are found in the next Chapter. Within the AISI/SAE plain carbon steel designations there are five subclasses, namely 10xx, 11xx, 12xx, 13xx, and 15xx. These are broadly based on the following categories of steel composition: AISI/SAE designation

10xx 15xx 13xx (a) 11xx 12xx

Type of steel

Plain Plain Plain Plain Plain

carbon: carbon: carbon: carbon: carbon:

Mn 1.00% max Mn 1.00–1.60% Mn 1.60–1.90% resulfurized resulfurized and rephosphorized

(a) Actually, the 13xx series of steels is classified as low-alloy steels because of the high manganese level. (Generally a steel with an alloying element content above 1.5% is considered a low-alloy steel; above 8% it is considered a high-alloy steel.) However, in the case of the 13xx series, one is basically dealing with a simple extension of the 10xx and 15xx plain carbon steels.

The AISI/SAE 15xx and 13xx series represent high-manganese, plain carbon steels. The higher manganese levels impart higher hardness and strength to the steels. The complete series of AISI/SAE 15xx and 13xx steels are listed in the Appendix. The 11xx series of plain carbon, resulfurized steels contains intentionally added sulfur. The sulfur does not actually alloy with the iron but combines with manganese to form manganese sulfide (MnS) inclusions. The sulfur level is much higher in the 11xx series than the 10xx series of plain carbon steels where sulfur is generally considered as an impurity. The higher sulfur level in the resulfurized steels imparts improved machinability to the steel because of the chip-breaking effect of the manganese sulfides. An example of a resulfurized steel is AISI/SAE 1140 steel, with the following composition:

Element

Content, %

Carbon Manganese Phosphorus Sulfur

0.37–0.44 0.70–1.00 0.040 max 0.08–0.13

For a given carbon content, the manganese levels are slightly higher in the 11xx series than in the previously discussed 10xx series. The higher manganese levels compensate for the higher sulfur levels, because manganese is added to tie up all the sulfur to form manganese sulfides. The AISI/SAE 11xx series of resulfurized steels is listed in the Appendix. The AISI/SAE 12xx series represents resulfurized and rephosphorized, plain carbon steels that are also free-machining steels, with both sulfur and phosphorus as alloy additions. The phosphorus addition increases the strength of the steel and promotes chip breaking during machining operations. In order to limit the strength of the steel, the carbon content is restricted to a level under 0.15%. For example, an AISI/SAE 1213 steel is represented by the following composition:

Element

Content, %

Carbon Manganese Phosphorus Sulfur

0.13 max 0.70–1.00 0.07–0.12 0.24–0.33

Fig. 1.3 shows the microstructure of a typical resulfurized, rephosphorized steel containing manganese sulfides (the oblong, gray particles). The AISI/SAE 12xx series of steels is listed in the Appendix. As has been shown, the AISI/SAE system for classifying the plain carbon steels is quite simple and is based solely on chemical composition. However, many plain carbon and alloy steels are classified according to a much more complex system based on the product application, the chemical composition, and the mechanical properties. This system has been devised by ASTM. The ASTM system consists of a set of detailed specifications for each steel, depending upon how it is used. Thus, there are specifications for plate, strip, sheet, rod, railroad rails, pipe, bolts, wire, nuts, structural shapes, and so on. The system is much different than the AISI/SAE system, and it is only touched on in this Chapter. The ASTM Specification System for Plain Carbon Steels. ASTM has very elaborate specifications for steels that include the type of product (sheet, plate, bar, wire, rail, etc.), the composition limits, and the mechanical properties. The ASTM specifications for iron and steel products comprise six 25 mm (1 in.) thick books weighing over 5.5 kg (12 lb). (The specifications can also be obtained in compact diskette form.) The specification code consists of the letter “A” followed by a number. A partial list of the plain carbon steels according to the ASTM specification system is given subsequently:

Fig. 1.3 Micrograph of a resulfurized, rephosphorized AISI/SAE 1213 steel

showing manganese sulfide inclusions (the gray, oblong particles marked by arrows). The remaining microstructure is ferrite (white etching constituent) and pearlite (dark etching constituent). Etched in 4% picral followed by 2% nital. 200⫻

6 / Metallographer’s Guide ASTM designation

A1 A 36 A 131 A 228 A 307 A 510 A 529 A 570 A 709

Type of steel

Carbon steel, tee rails Structural steel Structural steel for ships Steel wire, music spring quality Carbon steel, bolts and studs, 420 MPa (60 ksi) tensile strength Carbon steel wire rods Structural steel with 290 MPa (42 ksi) minimum yield point Steel, sheet and strip, carbon, hot rolled, structural quality Structural steel for bridges

As examples, two of these ASTM specifications are described in more detail: ASTM A 1 for railway rails and ASTM A 36 for structural steels (structural beams, plate, etc.). ASTM A 1 requires that railroad rails have certain composition limits and a minimum hardness. For example, for a common rail size of 60 kg/m (132 lb/yd) the requirements are:

Carbon Yield point Tensile strength Total elongation (in 50 mm, or 2 in.)

The microstructure of a typical ASTM A 36 structural steel is shown in Fig. 1.5. The microstructure is a mixture of pearlite and ferrite, with some manganese sulfide stringers. The ASTM specifications illustrated previously are rather simple. As the product becomes more critical and the composition more complex, the requirements expand considerably.

Alloy Steels The alloy steels are generally divided into two classes: the low-alloy steels and the high-alloy steels. They are divided according to composition as follows: Type

Carbon Manganese Phosphorus Sulfur Silicon Hardness

0.72–0.82% 0.80–1.10% 0.035% max 0.040% max 0.10–0.20% 269 HB min

The microstructure of a typical ASTM A 1 rail steel is shown in Fig. 1.4. The microstructure is 100% pearlite. ASTM A 36 for structural steels is very different from ASTM A 1 for rail steel in that it specifies only a minimum carbon content and certain tensile properties. ASTM A 36 has the following requirements:

Fig. 1.4 Micrograph of ASTM A 1 rail steel showing the fully pearlitic microstructure. Etched in 4% picral. 500⫻

0.26% max 248 MPa (36 ksi) min 400–552 MPa (58–80 ksi) 21% min

Low-alloy steels High-alloy steels

Alloying elements, %

8

The AISI/SAE Classification System for Low-Alloy Steels. As with the plain carbon steels, there is an established classification system of AISI/SAE designations for the low-alloy steels. The classification is based on the principal alloying element(s) in the steel. These principal elements include carbon, manganese, silicon, nickel, chromium, molybdenum, and vanadium. Each element, either singly or in combination with other elements, imparts certain properties and characteristics to the steel. The role of each element was described in Table 1.1. The subsequent list gives the breakdown of the AISI/SAE classification for the low-alloy steels:

Fig. 1.5 Micrograph of ASTM A 36 structural steel showing a microstructure consisting of ferrite (light etching constituent) and pearlite (dark etching constituent). Etched in 4% picral followed by 2% nital. 200⫻

Introduction to Steels and Cast Irons / 7 AISI/SAE designation

13xx 40xx 41xx 43xx 44xx 46xx 47xx 48xx 50xx 51xx 50xxx 51xxx 61xx 81xx 86xx 87xx 88xx 92xx 93xx 94xx xxBxx xxLxx

Type of steel

1.75% Mn steels 0.25% Mo steels 0.50 and 0.95% Cr-0.12 and 0.25% Mo steels 1.80% Ni-0.50 and 0.80% Cr-0.25 and 0.40% Mo steels 0.40% Mo steels 0.85 and 1.80% Ni-0.20 and 0.25% Mo steels 1.05% Ni-0.45% Cr-0.20 and 0.35% Mo steels 3.5% Ni-0.25% Mo steels 0.28 and 0.50% Cr steels 0.80, 0.88, 0.95 and 1.00% Cr steels 1.05–1.45% Cr steels 1.03% Cr steels 0.60 and 0.95% Cr-0.13 and 0.15% (min) V steels 0.30% Ni-0.40% Cr-0.12% Mo steels 0.55% Ni-0.50% Cr-0.20% Mo steels 0.55% Ni-0.50% Cr-0.25% Mo steels 0.55% Ni-0.50% Cr-0.35% Mo steels 1.40 and 2.00% Si-0.00 and 0.7% Cr steels 3.25% Ni-1.20% Cr-0.12% Mo steels 0.50% Ni-0.40% Cr-0.98% Mo steels Boron steels (“B” denotes boron) Leaded steels (“L” denotes lead)

The composition ranges for the previously mentioned AISI/ SAE low-alloy steels (except for the boron and leaded steels) are listed in the Appendix. There are many low-alloy steels that are not classified under the previously mentioned AISI/SAE system (some of these steels are also listed in the Appendix). Thus, the situation with low-alloy steels becomes much more complicated. For example, HY-80, a steel widely used for high-strength plate and forging applications, is a Ni-Cr-Mo steel but does not have an AISI/SAE designation. This particular steel is covered by a specification designation, ASTM A 543. ASTM has dozens of specifications for low-alloy steels. This system is discussed subsequently.

The ASTM Specification System for Low-Alloy Steels. As with the plain carbon steels, ASTM specifications also cover many of the low-alloy steels. However, as mentioned previously, the ASTM system is driven by the application for the particular steel. The system for low-alloy steels is quite large and is only touched on in this chapter; for example, a fairly common low-alloy steel is 21⁄4Cr-1Mo steel. In the ASTM system there are 13 separate specifications covering this steel, depending on the product form that is manufactured, as shown subsequently: Product form

Forgings Tubes Pipe Castings Plate

ASTM designations

A 182, A 336, and A 541 A 199, A 220, and A 213 A 335, A 369, and A 462 A 217 and A 356 A 387 and A 542

As an example, ASTM A 213 has the title “Seamless Ferritic and Austenitic Alloy Steel for Boiler, Superheater, and Heat Exchanger Tubes.” The standard actually covers 14 different grades of ferritic steels and 14 different grades of austenitic steels. The 21⁄4Cr-1Mo steel is grade T22. Because the grade is used in tubing for boilers and heat exchangers, it is also part of the specification system of ASME. The ASME adopts the ASTM code and places an “S” before it as, for example, ASME SA213 type T22. The ASTM and ASME grade (type) T22 has the following composition: Element

Content, %

Carbon Manganese Silicon Chromium Molybdenum

0.15 max 0.30–0.60 0.50 max 1.90–2.60 0.87–1.13

The microstructure of a typical ASTM A 213 grade T22 steel (ASME SA213 type T22) is shown in Fig. 1.6. It is interesting to note that if the same steel was used for a forging or plate, it may have a different microstructure because of the different specified heat treatment. Even for tubes (ASTM A 213), it can be furnished in the full-annealed, isothermal annealed, or normalized and tempered condition. Each condition would have a different microstructure.

High-Strength, Low-Alloy Steels

Fig. 1.6 Micrograph of ASME SA213-T22 boiler tube steel showing a

microstructure consisting of ferrite (light etching constituent) and a small amount of pearlite (dark etching constituent). Light tan areas are martensite. Etched in 4% picral. 200⫻

Although many of the previously mentioned AISI/SAE lowalloy steels also have high strength and, in some cases, ultrahigh strength (a yield strength above 1380 MPa, or 200 ksi), there is a rather loose class of steels called HSLA steels that do not fit the previously mentioned AISI/SAE classification. Although attempts have been made by AISI, SAE, and ASTM to classify the HSLA steels, the metallographer can easily become confused about these classifications. An attempt is made here to explain the basic classification systems. These HSLA steels are a group of low- and medium-carbon steels that generally use small amounts of alloying elements to attain yield strengths usually above about 345 MPa (50 ksi) in the hot-rolled, cold-rolled, annealed, stress-relieved, accelerated-

8 / Metallographer’s Guide cooled, direct-quenched, or normalized condition. In some cases they are called microalloyed steels because of the small amounts of vanadium, columbium (niobium), and/or titanium that are added for grain refinement and precipitation strengthening. The microstructure of a typical microalloyed steel is shown in Fig. 1.7. The ASTM Specification System. ASTM specifies most of the HSLA steels according to composition, mechanical property requirements, and application. A partial list of ASTM specifications for various HSLA steels is given subsequently (a complete list can be found in the Appendix): ASTM designation

A 242 A 572 A 588 A 656 A 714 A 715 A 808 A 871

Type of steel

HSLA structural steel HSLA columbium (niobium)-vanadium structural steel HSLA structural steel with 345 MPa (50 ksi) minimum yield point HSLA hot-rolled structural V-Al-N and titanium-aluminum steels HSLA welded and seamless steel pipe HSLA, hot-rolled sheet and strip, and sheet steel, coldrolled, high-strength, low-alloy, with improved formability HSLA with improved notch toughness HSLA steel with atmospheric corrosion resistance

Within each ASTM specification, one can find the mechanical property requirements as well as the range of chemical composition allowed. There are numerous other ASTM specifications involving low-alloy steels, depending on the particular application. The SAE Classification System for High-Strength, LowAlloy Steels. The Society of Automotive Engineers has developed

a classification for HSLA steels used in automotive applications. The steels are classified according to minimum yield strength level. The latest SAE classification system for HSLA steels consists of a three-digit code representing the minimum yield strength in ksi. Thus, a code of 080 would represent a 552 MPa (80 ksi) minimum yield strength. In the SAE system, there are usually one or more letters following the three-digit number to describe the chemical composition, carbon level, or deoxidation practice. The composition could be structural quality (S), lowalloy (L), or weathering (W). The carbon content could be low (L) or high (H). The deoxidation practice could be killed (K), killed plus inclusion control (F), or nonkilled (O). For example, SAE grade 080XLK would represent a low-alloy (X), low-carbon (L), killed, inclusion-controlled (K) steel with a minimum yield strength of 552 MPa (80 ksi). (The older SAE J410.c system would have a grade code of 980XK.)

Content, % SAE designation

050XLK 060XLK 070XLK 080XLK

Carbon

0.23 0.26 0.26 0.26

max max max max

Manganese

1.35 1.45 1.65 1.65

max max max max

Other

Cb, V Cb, V, N Cb, V, N Cb, V, N

The AISI Classification System for High-Strength, LowAlloy Steels. The AISI classification for high-strength, low-alloy steels is somewhat similiar to the SAE classification system, except that it does not have a carbon level but includes more strength levels and the dual-phase steels. The dual-phase steels, which contain about 10 to 20 vol% martensite in a matrix of ferrite, have a “D” as part of the designation. Also, dual-phase steels are different from other HSLA steels in that they are not generally classified by minimum yield strength but by minimum tensile strength in ksi. For example, an AISI code of DF090T would be a dual-phase (D) killed steel with inclusion shape control (F) and has a minimum tensile strength (T) of 620 MPa (90 ksi). A typical microstructure of a dual-phase steel is shown in Fig. 1.8. Some HSLA steels have commercial trade names. Recent designations HSLA 80 and HSLA 100 are being used for steels of a very specific steel composition with a minimum yield strength level of 552 MPa (80 ksi) and 690 MPa (100 ksi). In reality, there can be many HSLA 80 and HSLA 100 steels, depending upon composition, thermomechanical treatment, and heat treatment. Thus, there is not a standard classification system that encompasses all high-strength, low-alloy steels.

Other Low-Alloy Steels

Fig. 1.7 Micrograph of a microalloyed 450 MPa (65 ksi) yield strength linepipe steel showing a microstructure consisting of ferrite (light etching constituent), a small amount of pearlite (dark etching constituent), and martensite (gray etching constituent). Etched in 4% picral followed by 2% nital. 500⫻

There are many low-alloy steels that are not designed for just their room-temperature strength properties. These steels have additional properties that are important, such as corrosion or heat resistance and formability. Low-Alloy Steels for High-Temperature Properties. An example of a low-alloy steel that is used for its high-temperature

Introduction to Steels and Cast Irons / 9 properties is ASTM A 470 turbine rotor steel. These steels are used in steam turbines for electric power generation and usually contain combinations of nickel, chromium, molybdenum, and/or vanadium. An example of the microstructure of ASTM A 470 rotor steel is shown in Fig. 1.9. Low-Alloy Steels for Improved Corrosion Resistance. There are a number of low-alloy steels that have improved corrosion resistance. These steels usually have additions of copper, nickel,

Fig. 1.8 Micrograph of AISI DF090T dual-phase steel showing a micro-

structure consisting of ferrite (light etching constituent) and a small amount of martensite (dark etching constituent). Etched in 4% picral. 500⫻

or chromium and are called weathering steels. The ASTM specifications cover several of these steels. Low-Alloy Steels with Formability. There are some steels that are designed for optimal formability in sheet-forming applications. One common steel is specified as drawing quality, special killed. This cold-rolled, low-carbon sheet steel has a specified aluminum content. The aluminum combines with nitrogen in the steel to form aluminum nitride precipitates during the annealing process. These aluminum nitride precipitates are instrumental in the development of a specific crystallographic texture in the sheet that favors deep drawing. Another type of steel used for applications requiring optimal formability is interstitial-free steel. In this very-low-carbon sheet steel, the interstitial elements, carbon and nitrogen, are combined with carbide- and nitride-forming elements, such as titanium and columbium (niobium). The steel is rendered “free” from these interstitial elements that degrade formability. Bake-Hardenable, Low-Alloy Steels. Specific sheet steels have been designed to increase strength during the paint-baking cycle of automobile production. These bake-hardenable steels contain elements that develop compounds that precipitate at the paint-baking temperatures. These precipitates harden the steel. Dual-Phase, Low-Alloy Steels. A special class of steels known as dual-phase steels are used in applications where the yield strength of the sheet is increased during the forming process itself. These steels are designed to have a microstructure consisting of about 10 to 20% martensite in a matrix of ferrite. The steels have relatively low yield strength before forming a particular component (e.g., a wheel rim) and develop strength by a process called continuous yielding. The dispersed martensite regions are required for this process. Dual-phase steels were discussed in an earlier section, and a typical microstructure is seen in Fig. 1.8. As mentioned previously, many of the low-alloy steels are classified according to composition, properties, or application. The same is true for the high-alloy steels. Some of the high-alloy steels fall under a classification system described subsequently.

High-Alloy Steels High-alloy steels generally contain more than 8% total alloying elements. These steels include the corrosion-resistant (stainless) steels, the heat-resistant steels, and the wear-resistant steels (tool steels). The stainless steels and the tool steels fall under an established classification system. First the corrosion-resistant steels are examined.

Corrosion-Resistant (Stainless) Steels

Fig. 1.9 Micrograph of ASTM A 470 rotor steel showing a microstructure consisting of tempered upper bainite. Etched in 4% picral. 500⫻

For the corrosion-resistant steels, the system established by the AISI is not based on composition, but on microstructure. Thus, the stainless steels are classified as austenitic, ferritic, austeniticferritic, martensitic, duplex, and precipitation-hardening types, as shown in the flow diagram in Fig. 1.1. Most of the steels are classified by a three-digit designation. The system is not as clearly organized as the AISI/SAE system for plain carbon steels, because the number designations overlap. For example, within the 4xx

10 / Metallographer’s Guide series, 405 and 409 designate ferritic stainless steels, while 403 and 410 designate martensitic stainless steels; within the 3xx series, 321 and 330 designate austenitic stainless steels, and 329 designates a duplex stainless steel. Therefore, the metallographer must be aware that the system for stainless steels is somewhat inconsistent. The basic classification system is discussed subsequently.

Fig. 1.10 Family relationships for standard austenitic stainless steels

Austenitic Stainless Steels. These stainless steels have a microstructure of austenite at room temperature. Thus, they are nonmagnetic. Austenitic stainless steel (such as the popular type 304) has been called 18/8 stainless steel, because it contains nominally 18% Cr and 8% Ni. There are 30 compositional variations in the standard austenitic stainless steels, and a summary of the family relationships is shown in Fig. 1.10. All the

Introduction to Steels and Cast Irons / 11 austenitic stainless steels are essentially chromium-nickel alloys. The chromium varies between 15 and 24% and the nickel between 3 and 22%. The family is derived from two basic, general-purpose

Fig. 1.11 Micrograph of AISI 316 austenitic stainless steel showing a

microstructure consisting of 100% austenite. The straight-edged areas (marked by arrows) within the grains are annealing twins. Electrolytically etched in 60 parts nitric acid in 40 parts water, stainless steel cathode, at 6 V direct current. 200⫻

Fig. 1.12 Family relationships for standard ferritic stainless steels

alloys, types 302 and 202. The type 302 grade expands into 26 other types with specific compositional variations to impart particular properties, for example, better weldability, increased strength, increased heat resistance, better corrosion resistance, and improved machinability. The type 202 series is limited to only three grades and was designed to replace nickel, a rather expensive alloying element, with nitrogen and manganese. The corrosion resistance of the austenitic stainless steels is superior to other types of stainless steel. The specific composition ranges for each of the austenitic stainless steels are listed in the Appendix. The microstructure of a typical austenitic stainless steel is shown in Fig. 1.11. Ferritic Stainless Steels. The number of standard grades of ferritic stainless steel is much smaller than the austenitic grades. Fig. 1.12 shows the family relationships for the standard ferritic stainless steels. All the grades are variations on the basic, general-purpose type 430. The ferritic stainless steels are basically chromium steels with chromium ranging between 10.5 and 27%. These alloys deliberately lack high nickel contents, because nickel renders the steels austenitic (as previously mentioned). The ferritic stainless steels are the lower-cost stainless steels, because they contain less alloy, and they do not contain nickel (nickel is more expensive than chromium). The composition ranges of the ferritic stainless steels are listed in the Appendix. The microstructure of a free-machining ferritic stainless steel is shown in Fig. 1.13. Martensitic Stainless Steels. The family relationships of the martensitic stainless steels are shown in Fig. 1.14. The compositional variations all lead from the general-purpose type 410. The

12 / Metallographer’s Guide specific compositional ranges are listed in the Appendix. The martensitic stainless steels are essentially chromium steels with higher carbon content than either the ferritic or austenitic stainless steels. The chromium and carbon contents are balanced to ensure a martensitic microstructure after hardening by heat treatment. These steels are harder than the austenitic or ferritic series of stainless steel and are used for applications such as knife blades. The microstructure of a typical martensitic stainless steel is shown in Fig. 1.15. Duplex Stainless Steels. These steels have a mixed microstructure of ferrite and austenite. There is only one standard type of duplex stainless steel, grade 329, which contains 23 to 28% Cr, 2.5 to 5.0% Ni, and 1.0 to 2.0% Mo. The specific composition is listed in the Appendix. The duplex stainless steels have corrosion resistance similiar to an austenitic stainless steel but possess higher tensile and yield strengths and improved resistance to stress-corrosion cracking. The microstructure of a duplex stainless steel is shown in Fig. 1.16. Precipitation-Hardening Stainless Steels. These steels are mainly chromium-nickel steels with precipitation-hardening elements such as copper, aluminum, and titanium. They can possess either a ferritic or martensitic microstructure. In the classification, the letters “PH”, for precipitation hardening, may appear in the type code. The specific compositional variations are listed in the Appendix. The microstructure of a typical precipitation-hardening stainless steel is shown in Fig. 1.17.

Heat-Resistant Steels These alloy steels are used for elevated-temperature applications. Unfortunately, there is not a basic classification system for

these steels. This is because they include the low-alloy steels as well as the high-alloy stainless steels, tool steels, and iron-base superalloys. In other words, it depends on the elevated-temperature range and the application. At the lower temperatures, the low-alloy steels are adequate, whereas at higher temperatures, the higher-alloy steels are more appropriate. There are four AISI designated steels for elevated-temperature service. They are classified by a three-digit number beginning with “6”. These steels are 601, 602, 603, and 610 and are all medium-carbon, low-alloy steels containing chromium, molybdenum, and vanadium. Many of the heat-resistant steels are considered pressure vessel steels and are covered under the ASME Boiler and Pressure Vessel Code. These steels usually contain chromium and molybdenum (sometimes vanadium). The popular 2 1⁄4 Cr-1 Mo steel comes under this family of steels (discussed earlier in this Chapter). Under ASME specifications, the grade codes begin with the letters “SA”, and thus, a 21⁄4Cr-1Mo steel could have a code number SA-387 Gr22. The ASME grade code incorporates the ASTM grade code A 387 grade 22, which is the ASTM specification for 21⁄4Cr-1Mo steel for pressure vessels. Thus, in most cases the ASME grade codes are linked to the ASTM grade codes. A list of ASME compositions can be found in the Appendix. Many of the ferritic, austenitic, martensitic, and precipitation-hardened stainless steels are also used at elevated temperatures. In addition, many variants exist with proprietary names, such as Nitronic 60, Carpenter 18-18 Plus, Lapelloy, Greek Ascoloy, and so on. A list of compositions of the high-alloy, heat-resistant steels can be found in the Appendix. There are also several iron-base superalloys, including Discaloy, Haynes 556, Incoloy 800, and Pyromet CTX-1. A list of the compositions of the iron-base superalloys is also found in the Appendix. The microstructure of a typical iron-base superalloy (heat-resistant steel) is shown in Fig. 1.18.

Wear-Resistant Steels Tool Steels. By far the largest class of wear-resistant steels is the tool steels. There is an AISI classification system for the tool steels that has been in existence for many years. The AISI twoand three-digit codes begin with a letter representing the class of tool steel. The general classes are as follows: Letter

W A O S M T H D L P

Fig. 1.13 Micrograph of AISI 430F free-machining ferritic stainless steel

showing a microstructure consisting of ferrite and globular manganese sulfides (gray constituent). Etched in three parts glycerol, two parts HCl, and one part HNO3. 750⫻

Type of steel

Water-hardening tool steels Air-hardening, medium-alloy, cold-work steels Oil-hardening, cold-work steels Shock-resisting steels Molybdenum high-speed steels Tungsten high-speed steels Chromium high-speed steels High-carbon, high-chromium, cold-work steels Low-alloy, special-purpose tool steels Low-carbon mold steels

Most all of the tool steels contain molybdenum and/or tungsten. Other alloying elements, such as vanadium, cobalt, nickel, and chromium, are also added for certain characteristics. The microstructure of a typical tool steel is shown in Fig. 1.19. A complete list of the composition ranges of the tool steels can be found in the Appendix.

Introduction to Steels and Cast Irons / 13 Austenitic Manganese Steels . Another type of wear-resistant steel is the series of austenitic manganese steels. These steels are basically varients of Hadfield manganese steel invented in 1882 by the Englishman Sir Robert Hadfield. These steels contain between 11 and 14% Mn and between 0.7 and 1.45% C. The alloys are austenitic at room temperature and possess a unique combination of toughness and ductility coupled with high workhardening capacity and good wear resistance. Hadfield manganese steels are widely used in applications requiring abrasion resistance, for example, ore crushers. They are classified by ASTM as grade A 128. A list of the compositional variations can be found in the Appendix. The microstructure of a typical Hadfield steel is shown in Fig. 1.20.

Special-Purpose Alloy Steels There are a number of special-purpose steels that do not fit in any of the previous categories. These steels include the magnetic

Fig. 1.14 Family relationships for standard martensitic stainless steels

steels, the ultrahigh-strength steels, the electrical steels, and so on. In this Chapter, some of these special-purpose steels are discussed briefly. Ultrahigh-Strength (Maraging) Steels. The maraging steels (mar-aging represents martensitic aging) can be classified as ultrahigh-strength steels. These steels develop their high strength (above 1380 MPa, or 200 ksi, yield strength) through a precipitation reaction that occurs by an aging-type heat treatment. The steels have a martensitic microstructure. The more common maraging steels contain 18% Ni, 8.5 to 10% Co, 3.3 to 5% Mo, 0.2 to 1.6% Ti, and 0.1 to 0.3% Al. Because of the high cost of cobalt, there are a number of low-cobalt and cobalt-free alloys. The composition of a number of maraging steels can be found in the Appendix. The microstructure of a typical maraging steel is shown in Fig. 1.21. Magnetic Steels. A popular permanent (hard) magnetic steel is Alnico V, which contains 20% Co, 14% Ni, 8% Al, and 3% Cu. There are a number of high-permeability steels, such as Permal-

14 / Metallographer’s Guide loy, which contains 45% Ni, and Permendur, which contains 50% Co. All these steels have very high alloy contents. Nonmagnetic Steels. The austenitic stainless steels and Hadfield manganese steels described previously are nonmagnetic. However, there are specific steels designed to have very low magnetic permeability. These steels usually contain 14 to 30% Mn. These steels can also be used for cryogenic (low-temperature)

applications. The compositions of these steels are listed in the Appendix. Low-Thermal-Expansion Steels. Some steels are designed to have a very low coefficient of thermal expansion. One such alloy is Invar, which contains 36% Ni. Super Invar contains 31% Ni and 4 to 6% Co and has a coefficient of thermal expansion near zero. Another low-expansion alloy, Kovar, with 29% Ni and 17% Co,

Fig. 1.15 Micrograph of AISI 410 martensitic stainless steel showing a

Fig. 1.16 Micrograph of a duplex stainless steel (7Mo Plus) showing a

microstructure consisting of 100% martensite. Etched in Kalling’s No. 1 reagent. 500⫻

Fig. 1.17 Micrograph of a precipitation-hardening stainless steel (Custom

630) showing a microstructure consisting of martensite. Etched in Fry’s reagent. 320⫻

microstructure consisting of ferrite and austenite. The ferrite is the continuous matrix constituent. Electrolytically etched in 10% oxalic acid at 5 V. 200⫻

Fig. 1.18 Micrograph of an iron-base superalloy A286 showing a micro-

structure consisting of austenite. The straight-edged regions are annealing twins. Etched in a solution of 100 ml water, 20 ml HCl, 1 g NH4F HF, and 0.5 g K2S2O5. 200⫻

Introduction to Steels and Cast Irons / 15 matches the coefficient of thermal expansion of glass and is used for glass-to-metal seals. The microstructure of a typical lowexpansion steel is shown in Fig. 1.22. High-Thermal-Expansion Steels. The maximum possible coefficient of thermal expansion for a steel is obtained in a steel called Hi-Span-Hi, which contains 29% Ni, 8.5% Cr, 2.4% Ti, 0.4% Mn, 0.4% Si, and 0.4% Al. High-expansion alloys are used

in bimetallic strips for thermally activated switches and thermostats. Electrical Steels. There are very-low-carbon steels that are used for transformer cores and electric motor laminations. All these steels contain silicon (sometimes they contain aluminum). The transformer steels contain 3.25 to 4% Si (some contain 4.5 to 5% Si), and the motor lamination steels contain 0.5 to 1.0% Si

Fig. 1.19 Micrograph of AISI M-2 high-speed tool steel showing a micro-

Fig. 1.20 Micrograph of ASTM A 128 austenitic manganese steel (Had-

structure consisting of tempered martensite and carbides (white etching constituent). Etched in 12% sodium metabisulfite solution. 500⫻

Fig. 1.21 Micrograph of a maraging steel showing a microstructure consisting of martensite. Etched in Kalling No. 1. 200⫻

field steel) casting showing a microstructure consisting of austenite. Etched in 2% nital (5 s) followed by removal of the stain in aqueous 3% EDTA and etching in 20% sodium metabisulfite solution. 64⫻

Fig. 1.22 Micrograph of a low-expansion steel (Invar) showing a micro-

structure consisting of austenite. The straight-edged regions (arrows) are annealing twins. Etched in 4% picral. 200⫻

16 / Metallographer’s Guide (sometimes they also contain aluminum). Carbon is not necessary in these steels and, in fact, is considered detrimental. The microstructure of a typical motor lamination steel is shown in Fig. 1.23. Electrical-Resistance Steels. Some steels with high electrical resistance are used for heating elements in household stoves and industrial ovens and furnaces. One such steel is Kanthal, which contains 25% Cr, 5% Al, and 3% Co. Ohmax is another steel with high resistance and is used for electrical resistors. It contains 20% Cr, 5 to 8.8% Al, and 0 to 5% Co.

refers to the flake graphite that is present. The microstructure of a typical gray iron is shown in Fig. 1.24. Common unalloyed gray cast iron has the following broad range of chemical composition:

Cast Irons The cast irons generally contain more than 2% C and a variety of other alloying elements. They are classified by a rather simple, somewhat archaic system. They are classified by the appearance of their fracture surface, their microstructure, or their properties. Historically, there were two different cast irons: the cast iron that has a gray fracture appearance and the cast iron that has a white fracture appearance. Thus, the names gray iron and white iron evolved. Those irons that have a mixed gray and white appearance are called mottled iron. These names still apply today. Other cast irons appeared over the years and have names associated with a mechanical property, such as malleable iron and ductile iron. More recently, compacted graphite iron and austempered ductile iron have been introduced. This section examines each of these cast irons.

The gray irons have a high silicon content, because silicon (a graphitizer) promotes the formation of graphite during solidification. Gray iron has almost no ductility because of the presence of the graphite flakes. However, it is inexpensive and can be cast into complex shapes. The microstructure of gray iron results from a rather slow cooling rate in the casting process. ASTM has classified the type, distribution, and size of graphite in gray iron according to specification A 247. The flake graphite patterns have been subdivided into five types, as shown in Fig. 1.25. Each type imparts a different characteristic to the properties of the gray iron. For example, type A is preferred for most applications, because it is superior in certain wear applications. An important application is in cylinders of internal combustion engines. ASTM has also classified gray iron by a number of specifications that require certain compositional ranges and mechanical properties. A partial list follows:

Gray Iron

ASTM designation

By far the most common of all cast irons is gray iron. This iron has a gray fracture appearance, because it contains a high volume fraction of graphite flakes (the graphite flakes have a gray appearance). Gray iron is sometimes identified as “FG,” which

Fig. 1.23 Micrograph of a motor lamination steel showing a microstructure consisting of ferrite. Etched in 2% nital. 200⫻

A 48 A 126 A 159 A 278

Element

Content, %

Carbon Silicon Manganese Phosphorus Sulfur

2.5–4.0 1.0–3.0 0.2–1.0 0.002–1.0 0.02–0.25

Type of iron

Gray iron castings Gray iron castings for valves, flanges, and pipe fittings Automotive gray iron castings Gray iron castings for pressure-containing parts for temperatures up to 650 °F

Fig. 1.24 Micrograph of a gray cast iron showing a microstructure con-

sisting of pearlite (gray etching constituent), ferrite (light etching constituent), and graphite flakes (dark constituent). Etched in 4% picral. 100⫻

Introduction to Steels and Cast Irons / 17 ASTM A 48 is the general specification for gray cast iron and classifies grades of gray iron according to minimum tensile strength in ksi, as follows: ASTM A 48 class

Minimum tensile strength MPa ksi

Class Class Class Class Class Class Class Class Class

138 172 207 241 276 310 345 379 414

20 25 30 35 40 45 50 55 60

20 25 30 35 40 45 50 55 60

There are no compositional requirements specified in ASTM A 48. The SAE has also classified gray iron for automotive applications by specification SAE J431. There are five grades of gray cast iron included in SAE J431, as follows: SAE J431 grade

Minimum tensile strength MPa ksi

Grade Grade Grade Grade Grade

125 172 207 241 276

G1800 G2500 G3000 G3500 G4000

18 25 30 35 40

As one can see, the grade number is linked to the minimum tensile strength in ksi, as it is in ASTM A 48, but each SAE grade has other requirements. For example, the composition and property requirements for SAE grade G1800 are as follows: Element

Content, %

Carbon (TC) (a) Manganese Silicon Phosphorus Sulfur

3.40–3.70 0.50–0.80 2.80–2.30 0.15 max 0.15 max

(a) Carbon is expressed as total carbon (TC).

Property

Hardness, HB Bend Properties Minimum transverse load, kg (lb) Minimum deflection, mm (in.) Minimum tensile strength, MPa (ksi)

Required value

187 max 780 (1720) 3.6 (0.14) 124 (18)

According to SAE J431, grade G1800 is used for soft iron castings (as-cast or annealed) in which strength is not a primary factor. The other higher-strength grades are used in specific automotive applications. The compositional ranges for the previously mentioned grades can be found in the Appendix.

White Iron If a gray iron is solidified rapidly, white iron results. Graphite flakes are not present in white iron. Instead of graphite flakes, an iron carbide network forms that gives the white appearance on the fracture surface. The microstructure of a typical white iron is shown in Fig. 1.26. The composition range of elements in unalloyed white iron are: Element

Carbon Silicon Manganese Phosphorus Sulfur

Content, %

1.8–3.6 0.5–1.9 0.25–0.8 0.06–0.2 0.06–0.2

The silicon content is lower in white iron to minimize its graphitizing effect. White irons are extremely hard and abrasion resistant. To enhance their abrasion resistance, they are usually alloyed with nickel, chromium, and/or molybdenum. ASTM specification A 532 covers the abrasion-resistant white cast irons. There are ten different white cast irons, according to ASTM A532. These are listed subsequently:

Designation

Class I Type A Type B Type C Type D Class II Type A Type B Type C Type D Type E Class III, type A

Type of white cast iron

Ni-Cr high-carbon Ni-Cr low-carbon Ni-Cr GB (grinding balls) Nickel high-chromium 12% Cr 15% Cr-Mo low-carbon 15% Cr-Mo high-carbon 20% Cr-Mo low-carbon 20% Cr-Mo high-carbon 25% Cr

Fig. 1.25 Types of graphite flakes in gray iron (American Foundryman’s Society-ASTM). In the recommended practice (ASTM A 247), these charts are shown at a magnification of 100⫻. They have been reduced to one-third size for reproduction here.

18 / Metallographer’s Guide Mottled Iron. This type of cast iron is not intentionally produced. It results from a transition between gray and white iron in a casting and is not necessarily a desirable material. The microstructure of a mottled iron is shown in Fig. 1.27.

Malleable Iron Malleable iron is produced by heat treating white iron to break down or decompose the iron carbide into a temper carbon (a form of graphite). Malleable iron is sometimes referred to as TG iron because of the temper graphite present. Usually, white iron is heated to 800 to 970 °C (1470 to 1780 °F) for long periods of time, on the order of 20 h. During this time, irregularly shaped nodules of graphite form. Because of the absence of the hard and brittle carbide constituent, the iron becomes malleable. The microstructure of a typical malleable iron is shown in Fig. 1.28. The typical compositional limits of malleable iron are (wt%): Element

Content, %

Carbon (TC) (a) Manganese Silicon Sulfur Phosphorus

2.2–2.9 0.2–0.6 0.9–1.9 0.02–0.2 0.02–0.2

(a) Carbon is expressed as total carbon, TC.

ASTM has a number of specifications for malleable iron, depending on the microstructure and application. A partial list is given subsequently: ASTM designation

Fig. 1.26 Micrograph of a white cast iron showing a microstructure

consisting of pearlite (gray etching constituent), cementite (light etching constituent), and ledeburite (regions of rounded clusters). Etched in 4% picral. 250⫻

Fig. 1.27 Micrograph of a mottled cast iron showing a microstructure

consisting of pearlite (dark gray etching constituent), cementite (light etching constituent), ledeburite (clusters of small, rounded pearlite particles), and graphite flakes (dark constituent). Etched in 4% picral. 250⫻

A 47 A 197 A 220 A 338 A 602

Type of malleable iron

Ferritic malleable iron castings Cupola malleable iron Pearlitic malleable iron castings Malleable iron fittings and valve parts for railroad, marine, and other heavy-duty service at temperatures up to 345 °C (650 °F) Automotive malleable iron castings

Fig. 1.28 Micrograph of a malleable cast iron showing a microstructure

consisting of ferrite (light etching constituent) and temper carbon (dark gray irregular-shaped constituent). Etched in 2% nital. 200⫻

Introduction to Steels and Cast Irons / 19 As an example, ASTM A 220 for pearlitic malleable iron castings has eight grades of malleable iron, depending on minimum yield strength and elongation. The class or grade code is quite unusual and incorporates the minimum yield strength and elongation levels. For example, the grades are listed subsequently:

Grade

40010 45008 45006 50005 60004 70002 80002

Yield strength (min), MPa (ksi)

276 310 310 345 414 383 552

(40) (45) (45) (50) (60) (70) (80)

Tensile strength (min), MPa (ksi)

Hardness, Brinell

Elongation (min), %

414 (60) 448 (65) 448 (65) 483 (70) 552 (80) 586 (85) 724 (105)

149–197 156–197 156–207 179–229 197–241 217–269 269–321

10 8 6 5 4 3 1

More details of the compositional ranges of the malleable irons can be found in the Appendix.

Ductile Iron Ductile iron, also known as nodular iron and spheroidal graphite cast iron, is a cast iron where the graphite is in the form of spheres or nodules. The nodules are not as irregularly shaped as in malleable iron and are formed during solidification, not by heat treatment. The code SG is sometimes used to refer to the spheroidal graphite that is present in ductile iron. ASTM has classified the types of graphite shapes found in ductile cast irons and other irons. Sketches of the ASTM A 247 classification are shown in Fig. 1.29. The ductile irons contain type I graphite (type II being imperfectly formed nodular graphite). Type III would

Fig. 1.29 Typical graphite shapes after ASTM A 247. I, spheroidal graphite; II, imperfect spheroidal graphite; III, temper graphite; IV, compacted graphite: V, crab graphite; VI, exploded graphite; VII, flake graphite

20 / Metallographer’s Guide typify a malleable iron, type IV a compacted graphite iron (described at the end of this chapter), and type VII would typify the flake graphite of a gray iron. The microstructure of a typical ferritic ductile iron is shown in Fig. 1.30. To produce ductile iron, a fairly high-purity cast iron is inoculated with 0.03 to 0.06% Mg or 0.005 to 0.20% Ce. These elements provide nuclei on which the graphite spheroids grow. Because of the shape of the graphite, this type of cast iron exhibits far greater ductility than gray iron. The chemical composition limits of ductile iron are given below (wt%): Element

Content, %

Carbon Silicon Manganese Phosphorus Sulfur

3.0–4.0 1.8–2.8 0.1–1.0 0.01–0.1 0.01–0.03

Ductile irons are classified by both ASTM and SAE according to the application and properties desired. A partial ASTM list is shown subsequently: ASTM designation

A 377 A 395 A 439 A 476 A 536 A 716

Type of ductile iron

Ductile iron pressure pipe Ferritic ductile iron pressure-retaining castings for use at elevated temperatures Austenitic ductile iron castings Ductile iron castings for paper mill dryer rolls Ductile iron castings Ductile iron culvert pipe

Fig. 1.30 Micrograph of a ferritic ductile (nodular) cast iron showing a

microstructure consisting of ferrite (light etching constituent) and graphite nodules (dark gray constituent). Etched in 2% nital. 100⫻

A look at ASTM A 536 shows that five grades are defined according to minimum tensile strength, minimum yield strength, and elongation. In fact, the grade code number incorporates all three properties, as follows:

Grade

60-40-18 60-42-10 65-45-12 70-50-05 80-55-06 80-60-03 100-70-03 120-90-02

Tensile strength (min), MPa (ksi)

415 (60) 415 (60) 448 (65) 485 (70) 555 (80) 555 (80) 689 (100) 827 (120)

Yield strength (min), MPa (ksi)

276 290 310 345 379 415 485 621

(40) (42) (45) (50) (55) (60) (70) (90)

Elongation (min), %

18 10 12 5 6 3 3 2

To attain these various properties, the matrix microstructure varies from ferritic for the lower strengths, to pearlitic for the intermediate strengths, to martensitic for the higher strengths. Further details on the composition of ductile irons can be found in the Appendix. Austempered Ductile Iron. This is essentially a subclass of ductile iron. Austempered ductile irons have the same spherical or nodular graphite as a normal ductile iron, but the matrix is a combination of bainite (acicular ferrite and carbides) and stabilized austenite. The microstructure of a typical austempered ductile iron is shown in Fig. 1.31. The microstructure is obtained by a special heat treatment called austempering. The heat treatment involves austenitizing, followed by a quench and an isothermal hold at a specific temperature (usually obtained by quenching

Fig. 1.31 Micrograph of an austempered ductile cast iron showing a

microstructure consisting of bainite (acicular constituent) and graphite nodules (dark gray constituent). Etched in 2% nital. 500⫻

Introduction to Steels and Cast Irons / 21 into a molten salt bath). ASTM has recently established a specification, A 895, for austempered ductile iron. The specification consists of five grades that incorporate the minimum tensile strength, minimum yield strength, and elongation (similar to the system discussed previously for ductile iron). ASTM A 895 has the following property requirements:

Grade

125-80-10 150-100-7 175-125-4 200-155-1 230-185

Tensile strength (min), MPa (ksi)

Yield strength (min), MPa (ksi)

Elongation (min), %

Hardness, HB

Impact energy, J (ft · lbf)

862 (125) 1034 (150) 1207 (175) 1379 (200) 1586 (230)

552 (80) 690 (100) 862 (125) 1069 (155) 1276 (185)

10 10 4 1 ...

269–321 302–363 341–444 388–477 444–555

102 (75) 81 (60) 61 (45) 34 (25) ...

Note the much higher strength levels attained in austempered ductile iron compared with all the other irons described previously. Applications for austempered ductile iron include gears, automotive crankshafts and universal joints, and other parts requiring wear resistance, and high fatigue and impact strength.

Compacted Graphite Iron

Compacted graphite appears as short, thick flakes similar to type IV shown in Figure 1.29. The iron is also referred to as CG iron because of the presence of compacted graphite. The shape of the graphite is controlled by the addition of minor alloying elements, such as magnesium, titanium, calcium, cerium, and/or aluminum. The basic compositional range of standard elements is as follows (wt%): Content, %

Carbon Silicon Manganese Phosphorus Sulfur

2.4–4.0 1.0–3.0 0.2–1.0 0.01–0.10 0.01–0.03

The basic range of composition is similiar to ductile iron, described previously. Currently, there are no ASTM or SAE specifications for compacted graphite cast irons. One of the largest applications for compacted graphite cast iron is in the manufacture of ingot molds, where the mold life can be extended by 20 to 70% over the life of molds made from normal gray cast iron. SELECTED REFERENCES •

Compacted graphite cast iron has a graphite shape somewhere between the flake graphite found in gray cast iron and the nodular graphite found in ductile iron. Thus, the properties of compacted graphite cast irons are in-between those of gray and ductile iron.

Element

•

J.R. Davis, Ed., ASM Specialty Handbook: Cast Irons, ASM International, 1996 ASM Handbook, Properties and Selection: Irons, Steels, and High-Performance Alloys, Vol 1, ASM Handbook, ASM International, 1990

Metallographer's Guide: Practices and Procedures for Irons and Steels Bruce L. Bramfitt, Arlan O. Benscoter, p23-48 DOI:10.1361/mgpp2002p023

Copyright © 2002 ASM International® All rights reserved. www.asminternational.org

CHAPTER 2

Origin of Microstructure EVERY METALLOGRAPHER should know and understand how the various microstructures in steels and cast irons originate. This is because a key part of a metallographer’s job is to interpret what he or she sees under the microscope and to make sound judgments and recommendations based on these observations. A metallograhper is not just an “iron polisher,” as perceived by many people outside the profession. An effective metallographer needs to have a basic understanding of the fundamentals of ferrous physical metallurgy. The basic tenet of ferrous physical metallurgy is that the properties of steels and cast irons are controlled by both microstructure and chemical composition. It is the metallographer’s job to determine and characterize the microstructure, and it is the chemist’s job to determine the composition. This chapter discusses, in an introductory way, some of the basic ferrous physical metallurgy principles that are needed by the metallographer. The discussion focuses on the numerous microstructures that are generated as a result of the phase transformations that occur during both heat treatment (as in steels) and solidification (as in cast irons). The next chapter shows how numerous factors can influence microstructural development and how one can alter microstructure to attain desired properties.

Fig. 2.1(a) Various microstructures formed in an AISI/SAE 1080 steel. A fully pearlitic microstructure. 4% picral etch. 500⫻

As an example of microstructural diversity, Fig. 2.1 shows four different microstructures, all derived from the same steel by increasing the cooling rate, in an American Iron and Steel Institute/Society of Automotive Engineers (AISI/SAE) 1080 steel (a plain carbon steel containing 0.79% C, 0.90% Mn, and 0.25% Si). The microstructure shown in Fig. 2.1(a) is pearlite (a constituent consisting of the two phases, ferrite and cementite. A constituent is a phase or a combination of phases that occurs in a characteristic configuration in a microstructure. A phase is a homogenous and distinct portion of a material system.) The microstructure shown in Fig. 2.1(b) is bainite (a constituent also consisting of the two phases, ferrite and cementite, but with a totally different morphology). The microstructure shown in Fig. 2.1(c) is martensite (a constituent consisting of ferrite supersaturated with carbon). The microstructure shown in Fig. 2.1(d) is a mixture of pearlite, bainite, and martensite. Detailed discussion on each of these constituents can be found in later sections in this chapter. In the next section, it becomes clear as to how the microstructures in Fig. 2.1 originated.

Fig. 2.1(b) A fully bainitic microstructure. 4% picral etch. 1000⫻

24 / Metallographer’s Guide

Fig. 2.1(c) A fully martensitic microstructure. Sodium metabisulfite etch.

phase diagram and the time-temperature-transformation (TTT) diagram, more commonly called the isothermal transformation (IT) diagram. Early 20th century metallurgists, such as Floris Osmond, John Edward Stead, Albert Sauveur, and Henry Marion Howe, established the basic principles of transformation behavior in iron and steel. Early 20th century classic works, such as Metallography and Heat Treatment of Iron and Steel by Sauveur in 1912 (Ref 1), The Microscopic Analysis of Metals by Osmond and Stead in 1913 (Ref 2), and The Metallography of Steel and Cast Iron by Howe in 1916 (Ref 3), were replete with micrographs of microstructures of various irons and steels processed under a wide variety of heat treatment conditions. Also, an understanding of the iron-carbon phase diagram was fairly well developed, as evidenced by the diagrams published in each of the previously mentioned books. In the early 1930s, Edgar Bain and his colleagues developed the concept of the IT diagram that extended our understanding into the realm of the kinetics of phase transformations (Ref 4). The iron-carbon phase diagram and the IT diagram brought the metallography of iron and steel into perspective. From these simple diagrams, one was able to more fully understand the origin of a microstructure observed in the microscope.

1000⫻

The Iron-Carbon Phase Diagram

Fig. 2.1(d) A microstructure consisting of pearlite (dark), bainite (gray), and martensite (white). 4% picral and 2% nitral etch. 500⫻

Microstructural Development Resulting from Heat Treatment Considerable progress has been made in the past several decades to understand ferrous phase transformations. The basis for this understanding came with the evolution of the field of metallography, which led to the development of the iron-carbon

The iron-carbon phase diagram, coupled with the understanding of the kinetics of phase transformations through the IT diagram, were the breakthroughs necessary for the advancement of ferrous physical metallurgy. The phase diagram of today is much the same as that published early in this century. In taking a close look at the iron-carbon phase diagram shown in Fig. 2.2, it can be seen that the diagram is essentially a map of the phases that are stable (or metastable) at a given temperature and carbon content. The diagram represents equilibrium or quasi-equilibrium conditions, which are rarely completely achieved in industrial processes. Before discussing the diagram, we must first understand what a microstructure consists of in steel. Examples of microstructures are given in Fig. 2.1. The term “microstructure” relates to the overall structure, as usually observed in a microscope, which generally consists of one or more discrete phases. These phases are the building blocks of the various constituents that make up the microstructures, and each phase, singly or in combination with other phases, imparts certain properties to steel, for example, hardness, strength, toughness, and ductility, or certain characteristics, such as machinability and formability. This chapter examines the origin of those phases that are commonly observed in the microscope. There are basically only three phases normally present in steels: ferrite, austenite, and cementite (graphite is also a phase found in cast irons and is discussed in the last section of this chapter). A phase, in this case, can only exist according to the iron-carbon phase diagram. Several important features of the iron-carbon phase diagram are discussed here. First, there are two sets of

Origin of Microstructure / 25 phase boundaries superimposed in Fig. 2.2. One set represents the iron-cementite (Fe3C) phase diagram (solid lines), and the other set represents the iron-graphite phase diagram (dashed lines). The reason that there are two sets of phase boundaries is that the stable (equilibrium) phase for carbon in the iron-carbon system is graphite, whereas cementite is a metastable phase. A metastable phase can transform to a stable phase, given enough time. For example, cementite (Fe3C) can decompose with time (generally, very long periods of time) into iron and graphite (carbon). However, for all practical purposes, cementite remains a “stable” phase in steels. The iron carbide (cementite) phase diagram ranges from 0 to 6.7% C, the carbon content of iron-cementite. The iron-graphite phase diagram ranges from 0 to100% C (the diagram in Fig. 2.2 does not extend up to 100% C, because there are no important phases, other than graphite, to consider beyond 6.7% C). When discussing steels, the iron-cementite phase diagram is applicable, and for cast irons both diagrams are applicable. As a very general

rule, steels contain less than about 2% C, and cast irons contain more than 2% C. Although both steels and cast irons at some stage of their history undergo the process of solidification, a discussion of the influence of solidification on microstructure is reserved until the end of the chapter. Most steel products are subsequently hot worked and/or heat treated to alter the cast microstructure and to improve the properties of the steel. On the other hand, cast irons are most often used commercially in the as-cast condition. The various regions or fields in which the phases are stable are shown in Fig. 2.2. For example, the fields for the three basic phases in steel, austenite, cementite, and ferrite, can be located on the iron-cementite phase diagram. Austenite exists over a large phase field extending to just above 2% C and within the temperature range above 727 °C (1340 °F) and below 1495 °C (2713 °F). Cementite exists as a single composition at 6.7% C. Cementite exists up to 1227 °C (2240 °F), which is its melting temperature. The term “ferrite” appears twice on the ironcementite diagram: as delta ferrite at high temperatures of 1394 to

Fig. 2.2 The iron-carbon phase diagram. Solid lines indicate Fe-Fe3C diagram; dashed lines indicate iron-graphite diagram. Source: Ref 5

26 / Metallographer’s Guide 1538 °C (2340 to 2800 °F) and as alpha ferrite at temperatures below 912 °C (1675 °F). In this Chapter, when we mention ferrite in a steel microstructure, we are referring only to the lower-temperature form (if we need to discuss the higher-temperature form, we will call it delta ferrite). As shown in Fig. 2.2, ferrite appears as essentially pure iron. However, as seen in the expanded version of the iron-carbon phase diagram in Fig. 2.3, ferrite can contain up to about 0.02% C. Thus, ferrite can contain only a minor amount of carbon in contrast to austenite, which can contain up to 2.1% C (i.e., up to 100 times more carbon than ferrite). Historically, some of the phase boundaries shown in Fig. 2.3 are labeled as A1, A3, and Acm (the “A” from the French word arret, meaning arrest). If one is cooling a steel, the boundaries become Ar1, Ar3, and Arcm (the “r” from the French word refroidissement, meaning cooling), and if one is heating, the boundaries become Ac1, Ac3, and Accm (the “c” from the French word chauffage, meaning heating). Often, an “e” is used to represent equilibrium, as Ae1, Ae3, and Aecm. Also, in Fig. 2.3 there is a very important point on the diagram at 0.77% C and 727 °C (1340 °F). This point is called the eutectoid point. (The eutectoid point is the composition of a solid phase that undergoes transformation into two or more other solid phases upon cooling.) There is another point at 4.3% C and 1148 °C (2100 °F) on the iron-carbon diagram (see

Fig. 2.3 Portion of the iron-carbon diagram

Fig. 2.2). This latter point represents a eutectic point. (The eutectic point is the composition of a liquid phase that undergoes transformation into two or more solid phases upon cooling.) These transformations are important in steels and cast irons and are described in detail later in this chapter. In Fig. 2.3, steels with carbon contents less than 0.77% are called hypoeutectoid steels, and above 0.77% C, hypereutectoid steels (the prefix hypo means under, and the prefix hyper means over). It must be kept in mind that the iron-carbon phase diagram in Fig. 2.3 represents only iron-carbon binary alloys and does not apply to iron-carbon alloys containing other elements, such as manganese, silicon, nickel, chromium, molybdenum, vanadium, and so on. Any of these elements, if added to an iron-carbon alloy, would expand or contract the phase fields shown in Fig. 2.3. Thus, a ternary or quaternary phase diagram is needed for alloy systems with a total of three or four alloying elements, respectively. Nevertheless, for all practical purposes, the iron-cementite diagram can be used as a basic guide to the origin of microstructure in steels. However, a time scale is not associated with the phase diagram, because the data used to construct the diagram was established at near-equilibrium conditions (extremely slow heating/cooling rates). Departure from near-equilibrium conditions, for example, heating a piece of steel to 1000 °C (1832 °F) and quenching in water, requires an understanding of transformation

Origin of Microstructure / 27 kinetics, which is discussed in a later section on describing the concept of the IT diagram. But first, we must understand the basic phase diagram as it applies to the heat treatment of steel. For pure iron, one must follow the very left side of the diagram in Fig. 2.3. At a temperature above 912 °C (1675 °F), the stable phase of iron is gamma iron, which is a face-centered cubic (FCC) form of iron. Below 912 °C (1675 °F), alpha iron is the stable phase and has a body-centered cubic (BCC) crystal structure. In transforming from gamma to alpha iron or vice versa, the iron has passed through an allotropic change, that is, a change in crystal structure. It is important to understand this allotropic change, because it is an important part of the foundation for phase transformations in steel. The characteristics or properties of gamma iron and alpha iron are very different. The face-centered crystal structure imparts different properties than a body-centered crystal structure. A simplified sketch of the basic FCC unit cell is shown in Fig. 2.4. One can think of it as a cube with an iron atom at each corner and one at the center of each face of the cube. The body-centered unit cell is shown in Fig. 2.5. Here, iron atoms are also at the cube corners, as in the FCC structure, but in the body-centered structure, an iron atom occupies a position at the center of the cube. Of course, a crystal of iron consists of many millions of cells aligned in an orderly array in three dimensions. These crystals are the grains that one observes in the microscope. An important aspect of the allotropic change in iron is the accompanying change in volume. For example, in transforming from gamma iron to alpha iron, upon cooling there is an expansion, and upon transforming back (heating), there is a contraction. This volume change can be detected by sensitive

instruments. In fact, the dilatometer, an instrument found in most steel-related research laboratories, is based on measuring the volume changes that take place during a phase transformation. If we now look at an iron-carbon alloy, we find that the situation is different from pure iron. In an iron-carbon alloy, the FCC phase is called austenite, and the BCC phase is called ferrite (alpha ferrite). The term “austenite” is named after Sir William Chandler Roberts-Austen, an early English metallurgist, and the term “ferrite” is derived from the Latin word ferrum, for iron. The spaces between the iron atoms in the BCC and FCC lattices are called interstitial sites. Carbon atoms have a much smaller atomic diameter than iron and can fit into some of these sites. Thus, carbon is called an interstitial element (another important interstitial element is nitrogen). When an element occupies sites in the iron lattice, the result is called a solid solution. With interstitial elements, the alloy is an interstitial solid solution. When an atom substitutes for an iron atom in the FCC or BCC lattice, a substitutional solid solution forms. Substitutional elements, such as nickel, chromium, and manganese, have atomic diameters similar to iron. Interstitial and substitutional solid solutions form the basis for the design of steels with particular properties and characteristics. To illustrate the phase transformations in an iron-carbon alloy, the cooling paths shown in Fig. 2.3 can be followed. As an example, an AISI/SAE 1040 steel is used. An AISI/SAE 1040 steel contains nominally 0.4% C and some manganese and silicon, but for the moment we will treat this steel as a binary iron-carbon alloy. As one cools this steel very slowly, that is, at equilibrium, from the austenite phase field, for example, starting at 1000 °C (1832 °F), it remains single-phase austenite until 785 °C (1445 °F), the Ar3, at which point the austenite begins to transform to ferrite. Between 785 and 727 °C (1445 and 1340 °F), the Ar1, austenite continues to transform to ferrite. At 727 °C (1340 °F), the remaining austenite transforms through the eutectoid reaction. The ferrite that forms above 727 °C (1340 °F) is called proeutectoid ferrite (the prefix pro meaning before the eutectoid transformation). The eutectoid transformation in this case results in austenite transforming into two separate phases: ferrite and cementite. Thus: Austenite ↔ Ferrite ⫹ Cementite

Fig. 2.4 Face-centered cubic crystal structure (unit cell)

Fig. 2.5 Body-centered cubic crystal structure (unit cell)

Pearlite is the name given to the two-phase constituent of ferrite and cementite that results during slow cooling from this reaction. In fact, many steels and cast irons contain pearlite in their microstructure. An iron-carbon alloy containing 0.77% C is a pure eutectoid alloy; that is, it transforms to 100% pearlite. An example of a microstructure consisting of 100% pearlite is shown in Fig. 2.1(a). This microstructure was produced by very slow cooling of a eutectoid steel. Under very slow heating/cooling conditions, the iron-carbon phase diagram can be used to determine the resulting microstructure. In commercial steels, a fully pearlitic structure is sometimes desirable, as in rail steel and piano wire, because of the hardness, wear resistance (rail), and high strength (piano wire) imparted. Steels with a carbon content under 0.77%, processed to approach equilibrium conditions (slow cooling, etc.), would have ferrite and pearlite microstructures.

28 / Metallographer’s Guide In the 0.4% C steel (an AISI/SAE 1040 steel) described previously, only the austenite remaining at 727 °C (1340 °F) transforms to pearlite. The proeutectoid ferrite that already formed above 727 °C (1340 °F) remains essentially unchanged through this eutectoid transformation. As mentioned earlier, steels to the left of the eutectoid point (carbon content less than 0.77%) are called hypoeutectoid steels, and those above 0.77% C are called hypereutectoid steels. The hypoeutectoid steel described previously would have a microstructure similar to that shown in Fig. 2.6. This steel has a microstructure consisting of about 50% ferrite (white-appearing constituent) and 50% pearlite (dark-appearing constituent). Next, the transformation of a hypereutectoid steel cooling from the austenite phase field at 1000 °C (1832 °F) to room temperature is discussed. As an example, a representative hypereutectoid binary iron-carbon alloy would be iron Fe-1.4% C. From Fig. 2.3, upon cooling from 1000 to 960 °C (1832 to 1760 °F), the microstructure remains 100% austenite. At 960 °C (1760 °F), the Arcm, proeutectoid cementite begins to form. At the eutectoid temperature of 727 °C (1340 °F), the remaining austenite transforms to pearlite through the eutectoid reaction. The microstructure for this hypereutectoid alloy is shown in Fig. 2.7. Here, one can see a pearlite matrix (dark etching constituent) with proeutectoid cementite (the white etching network at the prior austenite grain boundaries). There are also some plates of cementite that grew within the austenite grains during transformation. This is an undesirable structure, because the hard cementite phase at the grain boundaries imparts brittleness to the steel. There are, however, some very useful hypereutectoid low-alloy steels. As is shown in the next chapter, one can attain very desirable properties in hypereutectoid steels when a spheroidized structure is developed by heat treatment. Spheroidization means that the form of

the carbide phase has been changed to small spherical particles uniformly distributed in a continuous matrix of soft ferrite. The next chapter illustrates that microstructural manipulation of this nature is vital to developing commercially useful steels and cast irons. Metallographers must not only understand the origin of microstructures, but must also understand how the microstructure can be altered to a more desirable form.

Fig. 2.6 A hypoeutectoid AISI/SAE 1040 steel showing a ferrite (white

Fig. 2.7 A hypereutectoid Fe-1.4% C binary alloy showing proeutectoid

etching constituent) and pearlite (dark etching constituent) microstructure. 4% picral and 2% nital etch. 800⫻

Kinetics of Phase Transformations It has been shown that the iron-carbon phase diagram is a plot of temperature and composition (carbon content). The transformations that were discussed previously took place at equilibrium conditions. However, in commercial practice, equilibrium is rarely approached. For example, some steels are heated to the austenite phase field and quenched in water to attain certain desired properties. Under these conditions, cooling takes place over a short period of time, and one can no longer use the iron-carbon phase diagram to predict transformation temperatures or the constituents that may form. In fact, as is shown later, in the previously mentioned AISI/SAE 1040 steel, a constituent (martensite) can form that does not contain cementite, as would be predicted by the phase diagram. To predict the effects of nearequilibrium cooling, we need to know something about the kinetics of the transformations concerned, that is, the rate at which they occur at different temperatures. Bain and his colleagues at the research laboratory of the United States Steel Corporation conducted pioneering work on this topic in the 1930s and showed that the transformation kinetics could most easily be summarized in a TTT diagram, also called an IT diagram.

cementite (and cementite needles) at the prior austenite grain boundaries in a matrix of pearlite. 4% picral etch. 500⫻

Origin of Microstructure / 29 An IT diagram for AISI/SAE 1080 steel is shown in Fig. 2.8. The IT diagram has a C-curve shape that is characteristic of the transformation behavior of most ferrous constituents. (Actually the C-curve is an envelope of C-curves, one for each constituent. See Chapter 2 in Ref 8 for further discussion.) The region marked “A” represents austenite, whereas “F” represents ferrite, and “C” represents cementite. The equilibrium eutectoid temperature of 727 °C (1340 °F) is shown as Ae1 on the diagram. Austenite is stable above this temperature. The austenite that exists below the Ae1 is subcritical austenite and is unstable. The left-hand curve represents the time required for the transformation to start at a particular temperature, and the right-hand curve represents the time required for it to finish the transformation. For example, if the steel is held at 400 °C (752 °F), the unstable austenite will start to transform to ferrite and cementite in about three seconds and complete the transformation in about 100 seconds. The tip of the C-curve at the left of the diagram is called the “nose” and represents the temperature at which transformation of austenite to a ferrite and cementite mixture is most rapid. To construct an IT diagram such as that in Fig. 2.8, one heats very thin steel samples to the austenite phase field above the Ae1 and then quickly transfers the specimens into a salt (or lead) pot held at a constant temperature below the Ae1. Thin samples must be used in order to minimize surface-to-center temperature gradients. When quenched in water to room temperature without holding in a salt pot, the specimen transforms to martensite (named after the early German metallographer Adolph Martens). For example, Fig. 2.9 shows a schematic of the cooling path required to form martensite.

Martensite forms in the steel as soon as the specimen is cooled below 210 °C (410 °F), which is shown on the IT diagram in Fig. 2.8 as Ms (martensite start temperature). One-hundred percent martensite has formed when the specimen has been cooled below 100 °C (212 °F), the Mf (martensite finish temperature). Figure 2.1(c) (representing an AISI/SAE 1080 steel) shows the fully martensitic microstructure resulting from this treatment. To determine the rest of the diagram, specimens are held at constant temperatures for a measured amount of time to allow the phase transformation to progress, and then the specimens are quickly quenched in cold water to arrest the progress of the transformation. This process is repeated for several hold times. As specimens are held for several consecutive times, one can follow the progression of transformation and determine the start and finish times at each temperature. An example is shown in Fig. 2.10, where six thin specimens are heated in a furnace at 900 °C (1650 °F) for reaustenitization and then rapidly transferred within a half-second into a salt pot and held at an isothermal temperature of 650 °C (1200 °F). The hold times are 2, 5, 10, 20, 50, and 200 seconds (see arrows). At a hold time of two seconds and water quench, the resulting microstructure would be martensite, because the pearlite transformation does not begin until about five seconds have lapsed. The fully pearlitic microstructure shown in Fig. 2.1(a) would represent the samples transformed at 50 and 200 seconds. Specimens held for 10 and 20 seconds would be pearlite and martensite mixtures, as seen in Fig. 2.1(d). The same process is repeated for other temperatures to complete the IT diagram, for example, six specimens held at 300 °C (572 °F) and then individually removed from the salt pot and quenched

Fig. 2.8 An isothermal transformation diagram for AISI/SAE 1080 steel. A, austenite; F, ferrite; C, cementite; Ms, martensite start temperature

30 / Metallographer’s Guide

Fig. 2.9 An isothermal transformation diagram for AISI/SAE 1080 steel showing cooling path to obtain a fully martensitic microstructure

Fig. 2.10 An isothermal transformation diagram for an AISI/SAE 1080 steel showing cooling path and isothermal treatment at 650 °C (1200 °F)

Origin of Microstructure / 31 into cold water after 10, 100, 200, 500, 1000, and 5000 seconds. Figure 2.11 shows a schematic of the isothermal treatment. Upon metallographic examination of each specimen, we find that after ten seconds, no transformation has taken place at 300 °C (572 °F), but after 100 seconds, a small amount of transformation has begun (the transformation actually starts in about 60 seconds). Metallographic examination of the specimen held for 100 seconds reveals that the transformed component of the microstructure has a different form than pearlite, described previously. It is called bainite (named after Edgar C. Bain). After 500 seconds, the transformation is over 50% complete, and after 1000 seconds, all the austenite has transformed to bainite. A microstructure of fully transformed bainite is shown in Fig. 2.1(b), which represents this AISI/SAE 1080 steel. Thus, from these simple experiments, Bain and his colleagues were able to show that departures from equilibrium can result in transformation products that do not appear on the iron-carbon phase diagram. The IT diagram is actually an oversimplification of the transformation behavior and kinetics of each of the different constituents illustrated previously. (For more detailed information on the link between transformation kinetics and IT diagrams, consult Ref 8 and 9.) Although Bain established the basic understanding of kinetics of ferrous phase transformations by transformations at constant temperature, practical heat treatments are carried out by heating and cooling at various rates. The same concepts developed by Bain can be used to develop continuous cooling transformation (CT) diagrams. These diagrams are constructed by cooling specimens at different rates from the austenite phase field. These rates may simulate air cooling, oil quenching, water quenching, and so on. A CT dia-

gram for an AISI/SAE 1080 steel is shown in Fig. 2.12. It looks somewhat similar to the IT diagram shown in Fig. 2.8 and can, in fact, be related approximately to it. The CT diagram is constructed by cooling specimens at different cooling rates and, by some means, measuring the start and finish of transformation. Metallographic analysis is used to determine the type and amount of each microstructural constituent that has formed. The C-shaped curves connecting the data points in Fig. 2.12 indicate the start and finish temperatures for each constituent, for example, Ps and Pf represent the pearlite start and finish temperatures. For example, in Fig. 2.12, a cooling rate of 750 °C/min (1350 °F/min) resulted in the microstructure of pearlite, bainite, and martensite, shown in Fig. 2.1(d). The cooling curve for this specimen is highlighted in bold on the CT diagram. Upon cooling at this rate, the first constituent that forms is pearlite (the dark etching constituent in Fig. 2.1d), which begins at the pearlite start temperature (Ps). Upon further cooling, bainite (the gray etching constituent) begins to form at the Bs temperature. The bainite in this case (Fig. 2.1d) has nucleated upon the pearlite nodules. The bainite transformation is completed at the Bf temperature. Upon further cooling, the martensite transformation (the light etching constituent) begins at the Ms temperature.

The Microstructural Constituents in Steel In the previous section, the discussion was centered on how a constituent can form, but did not describe the morphological features of the constituents. Examples of some of these features are shown subsequently.

Fig. 2.11 An isothermal transformation diagram for an AISI/SAE 1080 steel showing cooling path and isothermal treatment at 300 °C (572 °F)

32 / Metallographer’s Guide

Fig. 2.12 A continuous cooling transformation diagram of an AISI/SAE 1080 steel. Eight cooling rates are shown, with data points representing transformation temperatures.

Ferrite. One of the most common phases in steel is ferrite. The term was derived from the Latin word ferrum, meaning iron. A micrograph of low-carbon steel with a fully ferritic microstructure is shown in Fig. 2.13. This structure in the micrograph consists of hundreds of individual ferrite grains separated by grain boundaries, shown as the dark borders of each grain. Each of these grains is a single crystal of ferrite. A grain or crystal of ferrite has a BCC crystal structure. A grain boundary separates a ferrite grain of one crystal lattice orientation from a ferrite grain of another

Fig. 2.13 Microstructure of ferrite in a 0.02% C steel. Marshall’s etch. 500⫻

orientation. The microstructure shown in Fig. 2.13 is typical of a very-low-carbon steel. The microstructure or morphology is equiaxed, meaning that the grain dimensions are approximately equal in all directions (equal axes). It is also called polygonal ferrite, which implies that ferrite forms as an array of polygons. Ferrite can have many other morphological forms. Ferrite exists as a phase in the constituents pearlite and bainite, which is discussed later. A form of ferrite other than its polygonal form is acicular ferrite, sometimes referred to as Widmanstätten ferrite. The term “Widmanstätten” was named after Aloys Joseph Franz Xavier Beck von Widmanstätten, a 19th century Austrian museum curator, who observed and studied microstructures found in polished and etched meteorites. The morphology of Widmanstätten ferrite is shown as the light etching constituent in Fig. 2.14. It appears needlelike in the two-dimensional plane of polish, but in three dimensions, the morphology is lath or platelike. The steel in Fig. 2.14 is AISI/SAE 1025 (the dark etching constituent is pearlite). In steels, ferrite usually forms at prior austenite grain boundaries during cooling from the austenite phase field. The grainboundary form of ferrite is called proeutectoid ferrite and occurs in hypoeutectoid steels. Figure 2.15 shows proeutectoid ferrite (the white etching network phase) in a plain carbon AISI/SAE 1060 steel. The dark etching matrix is pearlite. There are different morphologies of grain-boundary ferrite, with the two most common being a blocky, somewhat equiaxed form and an acicular, or needlelike, form somewhat similar to Widmanstätten ferrite. The blocky form is shown previously in Fig. 2.15, and the acicular grain-boundary form is shown in Fig. 2.16. In this microstructure, the needle-like white etching phase is ferrite, and the gray etching matrix is martensite. The acicular ferrite (shown with arrow) has grown from the prior austenite grain boundaries.

Origin of Microstructure / 33 Cementite is iron carbide, that is, a compound of iron and carbon, Fe3C. The name cementite comes from cementation steel, which was an early process used to produce high-carbon steel through carburization, by which carbon is absorbed into the steel by packing it in charcoal and heating to a high temperature. In steels, cementite is never present as 100% of a microstructure. It can be present as a precipitate in ferrite; as a grain-boundary constituent at the junctures of ferrite grains in low-carbon steels; as a continuous grain-boundary phase (proeutectoid cementite) in high-carbon, hypereutectoid steels; as spherical particles in a

Fig. 2.14 A form of ferrite called Widmanstätten ferrite in a coarse-grained AISI/SAE 1025 steel. 4% picral etch. 100⫻

Fig. 2.16 Acicular form of ferrite nucleated at prior austenite grain

boundaries in an AISI/SAE 1060 steel. Matrix is martensite. 2% nital etch. 500⫻

spheroidized steel; or associated with ferrite as a constituent of pearlite, bainite, and tempered martensite. Figure 2.17 shows a micrograph of cementite as a grain-boundary phase in a lowcarbon steel. The cementite generally forms at the junctures of ferrite grains. The matrix is ferrite. In the previous example (using picral as an etchant), the cementite etches as a light gray phase. Figure 2.18 shows proeutectoid cementite or grain-boundary cementite (light grayappearing grain-boundary phase) in a hypereutectoid steel. The matrix is pearlite.

Fig. 2.15 Proeutectoid ferrite (white etching network) at the prior austen-

ite grain boundaries in an oil-quenched AISI/SAE 1060 steel. Dark etching constituent is pearlite. 4% picral etch. 500⫻

Fig. 2.17 Cementite (arrows) at ferrite grain boundaries in a batchannealed 0.04% C sheet steel. Marshall’s etch. 500⫻

34 / Metallographer’s Guide Cementite is a hard and brittle compound. Some steels and cast irons depend on the presence of cementite to impart wear resistance, for example, in steels for railroad rails and ball bearings and in alloy cast irons for rolls. A number of steels are designed to contain a minimal amount of cementite, for example, very-low-carbon automotive sheet steels. These latter steels need ductility and formability, and the presence of cementite tends to degrade these properties. Pearlite is a constituent that contains both ferrite and cementite. According to the iron-carbon equilibrium diagram, the carbon content of pearlite is 0.77%. Pearlite forms through the eutectoid reaction, as described earlier. Figure 2.19 shows a micrograph of pearlite. After etching with 4% picral, the cementite appears as a light gray phase, and the ferrite is white. This is because of the surface relief developed by the chemical etching process. As is explained in Chapter 5, surface relief influences the amount of light that is reflected away from the objective of a metallurgical microscope. The more light that is reflected away, the darker the image. Pearlite generally has a distinctive lamellar or alternating platelike morphology. The alternating plates of cementite and ferrite are shown more clearly in the higher-magnification view in Fig. 2.20 (this photo was taken on a scanning electron microscope). By volume, pearlite consists of about 13% cementite and 87% ferrite. The lamellar structure of pearlite arises from the side-by-side growth (or coupled growth) of the two phases during transformation from austenite. The pearlite transformation depends on diffusion of carbon in the austenite (or movement of carbon in the iron lattice) during the eutectoid reaction. Thus, pearlite forms through a diffusional process. A region of pearlite having a single growth orientation (i.e., with all the lamella aligned in the same general direction) is called

a pearlite colony. Many pearlite colonies can be seen in Fig. 2.19, which illustrates a microstructure consisting of coarse pearlite. Many more colonies appear in Fig. 2.21, which illustrates a microstructure consisting of very fine pearlite. “Fine” and “coarse” usually refer to the spacing between the lamella. The spacing is called the interlamellar spacing. During transformation, many colonies nucleate and grow within a single prior austenite grain, and because each colony has a different crystallographic orientation, they reflect light at different angles in the microscope, giving rise to the variation in appear-

Fig. 2.19 Coarse pearlite in an AISI/SAE 1080 eutectoid steel. 4% picral etch. 500⫻

Fig. 2.18 Proeutectoid cementite (white etching phase) at the prior aus-

tenite grain boundaries in an Fe-1.4% C binary alloy. Matrix is pearlite. 4% picral etch. 500⫻

Fig. 2.20 Scanning electron microscope (SEM) micrograph of pearlite. 5000⫻

Origin of Microstructure / 35 ance shown in Fig. 2.21. Usually, pearlite nucleates and grows as a nodule (spherical shape) originating at a prior austenite grain boundary or the juncture of prior austenite grain boundaries. A pearlite nodule contains many pearlite colonies. Looking closely at Fig. 2.21, many pearlite nodules can be seen. The nodular pattern can be more easily seen in Fig. 2.22, where the nodules are still isolated from one another. This micrograph was obtained from an AISI/SAE 1060 steel that was interrupted in its cooling process while transforming to pearlite by quenching quickly to

Fig. 2.21 Fine pearlite colonies in an AISI/SAE 1080 steel. 4% picral etch.

room temperature. This means that initially, at a fairly slow cooling rate, austenite was transforming to pearlite, as evidenced by the pearlite nodules. Upon quenching, the remaining austenite transformed to martensite (light etching matrix), leaving the nodules at their point of interrupted growth. In a steel having a fully pearlitic microstructure, as in Fig. 2.21, it is difficult to distinguish the individual nodules. Pearlite is most often seen in plain carbon steels as a constituent associated with ferrite, as seen in the hypoeutectoid AISI/SAE 1040 steel in Fig. 2.23, which shows a typical microstructure of ferrite (light etching regions) and pearlite (dark etching regions). Steels with fully pearlitic microstructures, as shown in Fig. 2.21, are used for applications requiring wear resistance. A fully pearlitic AISI/SAE 1080 steel (eutectoid steel) takes advantage of the strength and hardness of pearlite. However, pearlitic steels lack toughness and formability. Music wire is also made from a fully pearlitic steel. Pearlite, because of its lamellar structure consisting of soft ferrite and hard cementite, when drawn into fine wire, develops a very high tensile strength. Music wire is the strongest metal commonly available. Although pearlitic structures can be useful in a limited number of applications, many modern steels are designed to minimize the amount of pearlite in the microstructure, because it tends to degrade toughness. Martensite is a constituent that forms during rapid cooling (quenching) of steel. It is different from the pearlitic transformation in that the martensitic transformation does not depend on diffusion of carbon in the austenite and is thus a diffusionless transformation. The interstitial carbon atoms in solid solution in the parent austenite before transformation are “trapped” in the martensite lattice. Carbon steels with a fully martensitic structure are generally hard and have very little ductility in the as-quenched condition.

500⫻

Fig. 2.22 Pearlite nodules in a partially transformed water-quenched AISI/SAE 1060 steel. Matrix is martensite. 4% picral etch. 250⫻

Fig. 2.23 Pearlite (dark) associated with ferrite (light) in an air-cooled AISI/SAE 1040 steel. 4% picral and 2% nital etch. 800⫻

36 / Metallographer’s Guide In the light microscope it is usually difficult to resolve the fine microstructural details of martensite. Because of the fine microstructure, martensite is studied using an electron microscope. In carbon steels, there are two types of martensite: lath martensite and plate martensite. An example of lath martensite in an Fe-0.2% C alloy is shown in Fig. 2.24. Note that in the micrograph of lath martensite the laths have grown in different orientations. Each orientation has a different etching response and reflects light at different angles in the microscope, thus giving the different shades of gray. All the laths with a single orientation comprise what is called a “packet.” In the micrograph in Fig. 2.24, the packet size is very coarse (a packet is outlined by arrows). A lath would be one of the subunits within the packet. An example of lath martensite in an as-quenched commercial alloy steel is shown in Fig. 2.25. This microstructure represents a more typical fine packet size. An example of plate martensite is shown in Fig. 2.26. This microstructure represents a commercial water-quenched AISI/ SAE 1095 steel and has a small martensitic plate size. Lath martensite forms in lower-carbon steels (under 0.6% C), and plate martensite forms in higher-carbon steels (above 1.0% C). Mixed lath and plate martensite can form between 0.6% and 1.0% C. Other than their lath and plate morphologies, plate martensite differs from lath martensite by its unique midrib within each plate, as seen in Fig. 2.27. Not all martensitic steels contain carbon. In fact, the SAE 410 martensitic stainless steel shown in Fig. 2.28 is almost carbonfree. Many of the carbon-free martensites have some degree of ductility when compared with carbon-bearing martensites. In iron-carbon alloys, the crystal structure of martensite is body-centered tetragonal (BCT), with a unit cell configured as shown in Fig. 2.29. Basically, the unit cell is a distorted BCC cell. In steels containing carbon, the lattice distortion is due to carbon

atoms that occupy certain positions (in the interstices between the larger iron atoms, as shown in Fig. 2.29). Because of their location, they stretch the cell in one direction. In martensitic iron-carbon alloys, the carbon is trapped during transformation, leaving the lattice supersaturated with carbon. This is different from a steel that is slow-cooled, such as in the case where ferrite

Fig. 2.25 Lath martensite in an as-quenched commercial ASTM A 514

steel (0.13% C, 0.62% Mn, 0.29% Si, 0.5% Mo, 0.054% Al, 0.036% Ti, and 0.0019 %B). 2% nital etch. 500⫻

Fig. 2.26 Plate martensite in an AISI/SAE 1095 steel (0.97% C, 1.05% Mn,

Fig. 2.24 Lath martensite in an Fe-0.2% C binary alloy. A packet of laths is outlined with arrows. 2% nital etch. 500⫻

0.25% Si, and 0.20% Cr). Unetched areas are retained austenite. Sodium metabisulfite etch. 1000⫻. Courtesy of S. Lawrence, Bethlehem Steel Corporation

Origin of Microstructure / 37 forms. Ferrite, having the normal BCC lattice, cannot contain much carbon in solid solution (less than 0.002% C at room temperature). This means that during slow cooling, the carbon diffuses into the remaining austenite, eventually forming iron carbide (cementite) along with the ferrite. If martensite is heated or tempered, the trapped carbon will precipitate and eventually form iron carbide. The microstructure of a steel with a fully tempered martensitic structure is shown in Fig. 2.30. In the tempered martensite shown previously, the acicular appearance of the martensite is still evident. The small black particles in the microstructure are cementite. Essentially, the

lattice has changed from the BCT crystal structure to the BCC crystal structure during tempering, because the carbon atoms are removed from the lattice, thus relieving its distortion. The tempering process restores ductility to a martensitic steel, but with a sacrifice in strength. The effect of tempering is discussed in detail in the next chapter. Because as-quenched martensite has

Fig. 2.27 Plate martensite showing the characteristic midrib morphology

Fig. 2.29 Body-centered tetragonal crystal structure of martensite in iron-

Fig. 2.28 Martensite in a SAE 410 stainless steel. Vilella’s etch. 500⫻.

Fig. 2.30 A fully tempered martensite in a 0.2% C, 5% Ni, and 1% Mn

(arrow) in an Fe-1.86% C binary alloy. 2% nital etch. 1000⫻

Courtesy of K. Luer, Lehigh University

carbon alloys

steel. 4% picral etch. 500⫻

38 / Metallographer’s Guide very little ductility, it is almost exclusively found commercially in the tempered condition. One application of an untempered martensitic steel is in razor blades. In this case, the martensite imparts high strength and hardness, which allows the blade to maintain a sharp cutting edge. Bainite. When some steels are cooled at intermediate cooling rates, for example, between water quenching and air cooling, a constituent known as bainite can form. During etching in 4% picral, bainite appears in the light microscope as a dark etching, acicular constituent, as seen in Fig. 2.31. It is dark etching because it contains a dispersion of small carbide particles that provide surface relief from preferential attack by the etchant when observed in the microscope. As is seen later in this chapter, bainite appears so similar to tempered martensite that they can only be distinguished by observation in the electron microscope. There is also some untempered martensite in the micrograph in Fig. 2.31. In this sample, the untempered martensite etched much lighter than the bainite, because it does not contain carbides and hence, has a different etching response. The formation of bainite, like pearlite, involves diffusion of carbon in the austenite during transformation. Another bainitic microstructure representing a different steel than the steel in the previous micrograph is shown in Fig. 2.32. Note that the microstructure is somewhat similar to martensite (see Fig. 2.24) in that they both have an acicular morphology. However, if one could resolve the structure in the light microscope (usually an electron microscope is required), substantial differences would be found. This is because the slower cooling rates during the bainite transformation allow carbide precipitaton, as opposed to no diffusion during the martensitic transformation where the carbon atoms remain dissolved in the lattice. Thus, bainite has the normal BCC lattice of ferrite. Bainite

also differs from pearlite in that it does not form the lamellar morphology characteristic of pearlite. One form of bainite (called lower bainite) does, however, have a microstructure very similar to tempered martensite. In fact, in the light microscope, lower bainite is indistinguishable from tempered martensite. However, at higher magnifications in the transmission electron microscope (TEM), distinctions can generally be made, because the carbides in tempered martensite usually precipitate in a multivariant Widmanstätten-like pattern. In bainite, the carbides do not generally have this pattern and form at discrete locations at a single variant or specific angle (between 55° and 65°) to the major axis of the acicular ferrite. The discrete morphology of the carbides also distinguishes between what is called upper and lower bainite. The terms upper and lower bainite were first applied to the microstructures that develop during IT. The terms are also applied to bainite that transforms during normal continuous cooling. Upper bainite is the term attributed to bainite formed between 400 and 550 °C (752 and 1022 °F) and that contains carbides aligned along the ferrite lath boundaries. Figure 2.33 represents upper bainite produced by IT of a commercial low-alloy AISI/SAE 8720 steel at 425 °C (797 °F). Lower bainite, on the other hand, forms between 250 and 400 °C (482 and 752 °F) and contains carbides within the acicular ferrite laths (or plates) and not at the boundaries. Figure 2.34 represents lower bainite in an AISI/SAE 8720 steel that has transformed isothermally at 325 °C (617 °F). In most real-world processes, steel does not usually transform isothermally but transforms over a range of temperature (i.e., oil quenching, air cooling, etc.). However, the basic morphologies described previously are still found as a result of continuous cooling. Unfortunately, the structures that develop during continuous cooling are not as easily classified as those that form during

Fig. 2.31 Bainitic microstructure (dark etching constituent) in a 0.3% C,

Fig. 2.32 Bainitic microstructure in a low-carbon alloy steel (0.3% C,

0.8% Mn, 0.26% Si, 1.05% Cr, 1.07% Mo, and 0.25% V steel. The lighter etching constituent is martensite. 4% picral etch. 500⫻

0.62% Mn, 0.2% Si, 2.73% Ni, 0.23% Cr, and 0.5% Mo). 4% picral ⫹ HCl etch. 500⫻

Origin of Microstructure / 39 IT. The microstructures that develop during continuous cooling often consist of mixed upper and lower bainite, and many times, retained austenite is found instead of carbides at the acicular ferrite boundaries (Ref 10). Another form of bainite is so-called “granular” bainite, which consists of acicular ferrite and discrete islands or regions of retained austenite and/or martensite (sometimes called the M-A constituent). The term originated from a description of its granular appearance in the light microscope. An example of this form of

Fig. 2.33 Upper bainite in an AISI/SAE 8720 steel isothermally trans-

bainite is shown in Fig. 2.35. Generally, this form of bainite is found in alloy steels (e.g., Ni-Cr-Mo steels) that have been cooled at slow-to-intermediate cooling rates. Because of the higher alloy content in these steels, the pools of austenite can be stable at room temperature or partially stable with some portion of the austenite transformed to martensite. Because of the mixed and rather complicated bainitic microstructures that develop during continuous cooling, Bain’s experiments focused only on constant temperature transformation in order to develop uncomplicated microstructures. Work is currently underway to sort out the microstructures that have been termed bainite in continuously cooled steels (Ref 10). Bainitic steels are important because of their high strength and good toughness balance. Commercially, bainitic steels are used for nuclear reactor components, pressure vessels, large steam turbine rotors, and numerous other applications. Austenite. Although not commonly seen in plain carbon steels at room temperature (as evidenced in the iron-carbon phase diagram), austenite is the parent phase of ferrite, cementite, pearlite, bainite, and martensite. Austenite has a FCC crystal structure, as represented in Fig. 2.4. In most steels, austenite is a high-temperature phase and generally exists only above about 700 °C (about 1300 °F). However, some steels, for example, the austenitic stainless steels (e.g., AISI 304 and AISI 316 stainless steels), are 100% austenite at room temperature. Figure 2.36 shows a micrograph of austenite found in an AISI 316 austenitic stainless steel. In some plain carbon steels and many alloy steels, some austenite can be retained at room temperature, if the steel is quenched (usually in water or iced brine) from the austenite phase field. In some alloy steels and cast irons, if there is a sufficient carbon and alloy content in the austenite, it is stable at room

Fig. 2.34 Lower bainite in an AISI/SAE 8720 steel isothermally trans-

Fig. 2.35 Granular bainite in a 0.2% C, 0.3% Mn, 3% Ni, 1.5% Cr, and

formed at 425 ºC (797 ºF) for 90 s. 4% picral etch. 500⫻

formed at 325 ºC (617 ºF) for 100 s. 4% picral etch. 800⫻

0.4% Mo steel. 4% picral etch. 800⫻

40 / Metallographer’s Guide temperature. When examined in the microscope, retained austenite usually appears as a white etching constituent, because it has a different etching response than most other constituents. In many cases, retained austenite is not desirable in a steel. This means that the steel must be further heat treated (tempered) or cooled to very low temperatures to transform the retained austenite. In some steels, retained austenite is desirable, because it may impart an added degree of ductility to the steel. The following micrographs show retained austenite in a variety of steels. In Fig. 2.37, retained austenite (white) is seen between plate martensite (dark, needlelike constituent) in a high-carbon, low-alloy steel. Islands of retained austenite as well as islands of martensite can exist in a matrix of ferrite in a dual-phase steel (a steel used in some limited automotive applications). Generally, a dual-phase steel is produced by cooling from the two-phase, austenite plus ferrite region. Some of the austenite transforms to martensite upon cooling, while some of the austenite is retained. The term “dual-phase steel” was coined to describe a type of steel that contains both ferrite and martensite (however, in reality it is a tri-phase steel, with both martensite and austenite, the M-A constituent, existing as regions within the ferrite matrix). In this type of steel, the retained austenite is desirable, because it promotes enhanced ductility. Figure 2.38 shows an example of a dual-phase steel. The small, white, rounded regions (arrows) are retained austenite, and the dark regions are martensite. The matrix is ferrite. Figure 2.39 shows about 15% retained austenite (small, white, rounded areas indicated by an arrow) in a matrix of martensite. In all these cases, the austenite phase, because of its higher carbon content and/or higher alloy content, is stable at room temperature. However, it is sometimes possible to transform the

Fig. 2.36 Austenite grains in an AISI/SAE 316 austenitic stainless steel. Straight-edged regions are annealing twins. 4% picral and HCl

etch. 500⫻

retained austenite to martensite by cooling the steel well below room temperature, for example, in liquid nitrogen. This subcooling process is actually carried out in industry, for example, large-diameter, quench-hardened steel rolls are cooled in liquid nitrogen to transform retained austenite to martensite. However, many other applications do not require such extreme hardness in a steel, and tempering can be performed. To transform the retained

Fig. 2.37 Retained austenite (white etching phase) in a water-quenched

0.93% C-1.45% Mn steel. Gray etching phase is plate martensite. 2% nital etch. 1000⫻

Fig. 2.38 A dual-phase sheet steel (0.11% C, 1.4% Mn, 0.58% Si, 0.12%

Cr, and 0.08% Mo) showing small regions of retained austenite (see arrows indicating white areas) and martensite (dark etching constituent) in a matrix of ferrite. 12% aqueous sodium metabisulfite etch. 1000⫻

Origin of Microstructure / 41

Fig. 2.39 Retained austenite (arrow indicating small, rounded, white

areas) in a 0.16% C, 1.8% Mn, 0.7% Ni, 0.75% Cu, 2% Cr, and 0.3% Mo steel. Dark matrix is martensite. Etched in a solution of 10 g sodium thiosulfate and 3g potassium metabisulfite in 100 cm3 of water (Beraha’s etch). 1000⫻

Fig. 2.40 A drawing of a dendrite

austenite, for example, the steel is heated to 600 °C (1112 °F) for several hours and cooled to room temperature.

Microstructural Development Resulting from Solidification In the previous sections of this chapter, microstructural development focused on steels experiencing a cooling process, that is, the steel was reaustenitized and cooled to room temperature. Most steels used commercially are heat treated to obtain a desired microstructure for specified mechanical properties. However, all commercial steels undergo a solidification process (the transformation from the liquid state to the solid state) during the initial stage of their manufacture. The solidified, or as-cast, microstructure is generally destroyed through mechanical deformation (hot deformation) and subsequent heat treatment. In all the microstructures shown in the previous sections of this chapter, the as-cast microstructure is no longer evident. The main reason for the elimination of the as-cast microstructure is that it imparts three undesirable attributes: (a) directionality of properties because of the alignment of the dendrites (crystals with a “treelike” branching pattern); (b) coarse microstructures; and (c) micro- and macrosegregation of the elements contained in the steel (carbon, manganese, sulfur, phosphorus, etc.). First, we must look at the dendritic structure of steel. A dendrite is shown schematically in Fig. 2.40. The main “trunk” of the dendrite is called the primary arm, and the other branches or arms are secondary and tertiary arms.

Fig. 2.41 As-cast microstructure of an AISI/SAE 1060 steel. Pearlitic matrix

with ferrite in the prior austenite grain boundaries. 4% picral

etch. 32⫻

Figure 2.41 shows a representative as-cast microstructure of AISI/SAE 1060 steel. The microstructure consists mainly of pearlite (dark etching constituent) and some ferrite (light etching constituent). Note that the overall microstructure has a directionality and that ferrite has formed a network around the pearlite. The ferrite has actually nucleated and grown on prior austenite grain boundaries that formed at the last stage of solidification as dendritic boundaries (the boundaries formed by the impingement

42 / Metallographer’s Guide of two or more dendrites). It is interesting to note that a dendritic pattern is not obvious in this microstructure. This is partly because the microstructure is a result of transformations that took place upon cooling in the solid state after solidification. If one takes the same piece of as-cast steel and reaustenitizes and cools it slowly to room temperature, the microstructure in Fig. 2.42 results. Here, we can see that the carbon has been redistributed and has concentrated into the regions between the secondary dendrite branches or arms to form pearlite. This condition is an example of microsegregation and is generally undesired in steels. A consequence of microsegregation is banding, which is shown in an AISI/SAE 1524 steel in Fig. 2.43. Banding occurs upon hot rolling of carbon steels that usually contain high manganese levels. Banding can be minimized by homogenizing heat treatments, which allow enough time to redistribute the manganese by diffusion. Although most steels are not used in their as-cast condition, cast irons are almost exclusively used in their solidified and undeformed state (possibly with some subsequent heat treatment to optimize the microstructure). The following section describes the microstructural development of these commercially important materials. Various forms of cast iron have been used for over 2000 years for household utensils, stoves, water pipe, cannons, farm implements, and so on. As a class of metals, cast irons are generally inexpensive and are fairly common, because they have lower melting temperatures than most steels and are thus easy to melt and cast. There are a number of different types of cast iron that are widely used today for numerous applications. They are generally classified according to their microstructure, for example, gray iron, for its gray fracture appearance due to the presence of graphite flakes in the microstructure; white iron, for its white

Fig. 2.42 Annealed condition showing dendritic structure of the as-cast AISI/SAE 1020 steel shown in Fig. 2.41. 4% picral etch. 32⫻

fracture appearance due to the presence of cementite (iron carbide and no graphite); and ductile iron or nodular iron, for the presence of graphite in the form of nodules instead of flakes. Each of these types of cast iron is discussed later in the chapter. In order to understand the origin of the numerous, fairly complex microstructures that form in cast irons, we need to first understand the phase transformations that take place during both solidification and subsequent cooling to room temperature.

Phase Transformations in Cast Irons To understand the phase transformations in the various cast irons, we must examine the iron-carbon phase diagram. Actually, because most cast irons contain either graphite (pure carbon) or iron carbide (cementite), we must understand the iron-carbon phase diagram with respect to carbon in the form of either graphite or cementite. The phase diagram shown earlier in Fig. 2.2 can be roughly divided into two parts: cast irons for carbon levels above 2% and steels for carbon levels below 2%. Thus, for all the cast irons, the eutectic portion of the phase diagram must be considered. The Iron-Graphite and Iron-Cementite Phase Diagrams. As shown previously, Fig. 2.2 represents the iron-carbon phase diagram, with phase boundaries for both iron-graphite and ironcementite. The iron-cementite phase boundaries are indicated as solid lines, and the iron-graphite phase boundaries are indicated as dashed lines. For steels, the iron-cementite phase boundaries are more applicable, but for cast irons, both sets of phase boundaries are applicable. For example, to describe the transformations that take place in gray and nodular cast iron, the iron-graphite phase diagram is appropriate, and for transformations that take place in white iron, the iron-cementite phase diagram is appropriate. It

Fig. 2.43 An AISI/SAE 1524 steel showing the condition of banding. Dark

bands are pearlite, and light bands are ferrite. 4% picral etch.

200⫻

Origin of Microstructure / 43

At this carbon content, we are dealing with a hypoeutectic cast iron. We follow a cooling path from solidification to room temperature, using the appropriate phase diagram. The iron-cementite phase diagram is shown in Fig. 2.44. Tracing the solidification path in Fig. 2.44 (or Fig. 2.2) for a binary Fe-3% C alloy, one can see that solidification begins with the formation of primary austenite at 1300 °C (2372 °F). This is the point where the liquidus temperature is crossed upon cooling. The liquidus is the temperature where the solid phase begins to form upon cooling. Cooling from 1300 to 1148 °C (2372 to 2100 °F), the austenite continues to grow generally in the form of dendrites. At 1148 °C (2100 °F), the eutectic temperature, the remaining liquid transforms through the eutectic reaction to a mixture of austenite and cementite as:

pearlitic areas are portions of the austenite dendrites (that transformed to pearlite upon cooling to room temperature). One can also estimate that at a point just below 1148 °C (2100 °F), the total microstructure will consist of about 20% cementite and 80% austenite. However, in continued cooling from 1184 to 727 °C (2100 to 1340 °F), the proportions of austenite and cementite in the total microstructure will change to 38% cementite and 62% austenite. The additional 18% cementite grew at the expense of both the primary austenite dendrites and the austenite in the ledeburite and appears as a continuous rim of cementite surrounding each austenite region. The rim around the primary austenite dendrites can be seen (see arrows) in Fig. 2.46 (a rim can also be seen in Fig. 2.45). Upon cooling to room temperature, the primary austenite and the austenite contained in the eutectic constituent must transform. Depending on the cooling rate and alloy content, the austenite could transform to either pearlite, bainite, or martensite. The resulting cast iron would be white iron free of graphite. Figure 2.45 represents a white cast iron where the austenite has transformed to pearlite. The iron-graphite phase diagram is shown in Fig. 2.47. For an Fe-3% C alloy, the solidification begins the same as previously mentioned, with the formation of primary austenite crystals between 1300 and 1154 °C (2372 and 2110 °F). However, according to Fig. 2.47 (or Fig. 2.2), at the eutectic temperature of 1154 °C (2110 °F), the eutectic reaction would be:

Liquid ↔ Austenite ⫹ Cementite

Liquid ↔ Graphite ⫹ Austenite

This eutectic constituent, called ledeburite (named after Karl Heinrich Ledebur, a noted 19th century German professor of metallurgy), should contain about 48% cementite and 52% austenite by volume. The morphology of ledeburite can be seen in the central region of Fig. 2.45 as the cluster of small, rounded particles. The white etching continuous phase in the ledeburite shown in Fig. 2.45 is cementite, and the darker etching phase is pearlite (transformed from the eutectic austenite phase). The large

The eutectic of graphite and austenite does not have a name, as in the case of ledeburite. In continued cooling from 1154 to 738 °C (2110 to 1360 °F), the graphite would grow at the expense of the surrounding austenite.

must be kept in mind that while using these binary diagrams, commercial cast irons usually contain over 1% Mn and/or Si, and these and other elements will shift the phase boundaries. In this section, we take a closer look at these phase diagrams, especially the hypoeutectic side (carbon content less than eutectic composition) of each diagram, because the vast majority of cast irons contain less than 4.3% C.

Transformations in a 3% C Cast Iron

Fig. 2.45 The eutectic constituent ledeburite (center of photo with small,

Fig. 2.44 The iron-cementite phase diagram

elongated and rounded particles) in a pearlitic white cast iron (3.21% C, 0.32% Mn, and 0.47% Si). White-appearing area is cementite. 4% picral etch. 500⫻.

44 / Metallographer’s Guide Just below the eutectic temperature of 1154 °C (2110 °F), about 2% graphite and 98% austenite would be present in the microstructure. However, at room temperature, up to 4% graphite and 96% transformed austenite would be present. Figure 2.48 represents the solidified microstructure resulting from the previous eutectic reaction. In Fig. 2.48, the austenite transformed to pearlite, while the graphite remained stable in the form of flakes. This is called a pearlitic gray iron and is the most common form of gray cast iron. If the cooling rate was extremely slow, the carbon in the austenite,

instead of forming cementite in the pearlite, would diffuse to and precipitate around the graphite flakes and leave a matrix of ferrite. This structure would represent ferritic gray iron. Because of the ferritic matrix, this type of iron would have a lower strength but higher ductility than pearlitic gray iron.

General Description of Microstructures in Cast Irons Gray Cast Iron. The distinguishing microstructural characteristic of gray cast iron is the presence of graphite in the form of flakes, as shown in Fig. 2.48. In this micrograph, the flakes (dark gray) are surrounded by a matrix of pearlite. The matrix of an as-cast gray iron can also consist of 100% ferrite, 100% pearlite, or various mixtures of pearlite and ferrite. In addition, the matrix of heat treated gray iron can consist of other transformation products, such as martensite and bainite. In the pearlitic gray iron shown in Fig. 2.48, the graphite flakes are rather long. Gray cast irons can have flakes of various lengths and of different morphologies. These morphologies are discussed in more detail in Chapter 1. Gray cast iron usually contains between 1 and 3% Si, because silicon promotes the formation of graphite, that is, silicon is a graphitizer. In Fig. 2.48, the graphite flakes appear separated, but if observed in three dimensions, the flakes would be interconnected. The three-dimensional array would encompass what is called a cell. The flakes impart a degree of brittleness to this form of cast iron. Therefore, gray cast iron is generally not used in applications where critical parts are subjected to impact or bending stresses. However, the flakes also impart a degree of damping capacity (the

Fig. 2.46 Pearlitic white cast iron (3.21% C, 0.32% Mn, and 0.47% Si)

showing the rim of cementite (note arrows) that formed upon cooling around the primary dendrites of austenite (now pearlite). 4% picral etch. 500⫻

Fig. 2.48 Pearlitic gray cast iron (3.22% C, 0.77% Mn, and 1.98% Si). Elongated, dark particles are graphite flakes. 4% picral etch.

Fig. 2.47 The iron-graphite phase diagram

500⫻

Origin of Microstructure / 45 ability of a material to absorb vibrations). Gray cast iron is very useful as frames or bases in all kinds of heavy machinery. Gray iron, because of the soft graphite flakes, is easily machinable. White Cast Iron. As seen in Fig. 2.49, the microstructure of white cast iron is entirely different from gray cast iron. In this microstructure, there are no graphite flakes present. The larger, dark gray, rounded areas in Fig. 2.49 consist of pearlite. The aligned array of these rounded areas is a result of the original dendritic structure that formed during solidification. Each large, rounded patch of pearlite formed between a branch or arm of a dendrite. The white continuous phase in Fig. 2.49 is cementite, and the regions containing the small, rounded pools are the eutectic constituent called ledeburite. Another view of ledeburite is shown in Fig. 2.45 and 2.46. The eutectic constituent consists of cementite (white) and pearlite (dark). The eutectic formed during solidification as cementite and austenite (discussed previously). These rounded regions within the ledeburite can also be martensitic, depending on the alloy content of the cast iron and the cooling rate during transformation. Because of the large amount of the continuous cementite phase, white cast irons are usually brittle, very hard, and difficult to machine. In fact, the high hardness makes white cast iron very useful for applications requiring wear and abrasion resistance, for example, rolls and ore/rock crushing machinery. Mottled Cast Iron. When a cast iron consists of a mixture of gray and white iron, the cast iron is called mottled cast iron, because of its mottled or speckled appearance in a fractured surface. Figure 2.50 shows an example of the microstructure of mottled iron. Note the presence of graphite flakes in a matrix of pearlite, that is, the gray iron microstructure, adjacent to a continuous network of cementite, pools of ledeburite, and rounded regions of pearlite, that is, the white iron microstructure. Gener-

Fig. 2.49 Pearlitic white cast iron (3.21% C, 0.32% Mn, and 0.47% Si). 4% picral etch. 250⫻

ally, a mottled microstructure is found as a transition microstructure between gray iron and white iron in a casting. Mottled iron is not a desirable form of cast iron, particularly from the viewpoint of machinability, where, for example, in a gray iron, hard spots are introduced by the presence of patches of white iron. Ductile (Nodular) Cast Iron. The nodular or spherical form of graphite in ductile cast iron is shown in Fig. 2.51, which illustrates a pearlitic ductile iron. In producing ductile cast iron, the liquid is inoculated with magnesium or cerium. These elements form compounds in the liquid cast iron that act as nuclei for the

Fig. 2.50 Mottled pearlitic cast iron. Gray iron at upper left and white iron at lower right of photo. 4% picral etch. 250⫻

Fig. 2.51 Pearlitic ductile (nodular) cast iron (2.95% C, 0.36% Mn, 2.75%

Si, and 0.09% Mg). Dark, rounded particles are graphite. 4% picral etch. 500⫻

46 / Metallographer’s Guide graphite. Without these nuclei, the graphite would form as flakes. Because of the nuclei, the graphite grows in a radial pattern to form spheres, an example of which is shown in Fig. 2.52. As in gray cast iron, the matrix of ductile iron can consist of a variety of different constituents. The nodular form of the graphite imparts a high degree of ductility to this type of cast iron. Ductile iron is used in applications requiring ductility and toughness, where gray or white cast iron cannot be used.

Malleable Cast Iron. Malleable iron cannot be produced as-cast and is produced only by heat treating white cast iron at temperatures and times that are sufficient to decompose the cementite into carbon (graphite). These heat treatment cycles are generally about 870 to 925 °C (1600 to 1700 °F) for 15 to 24 hours. The characteristic microstructure of malleable iron is shown in Fig. 2.53. In this example, the irregular-shaped dark areas are called “temper carbon,” a form of graphite, and the matrix is pearlite. Represented in Fig. 2.53 is pearlitic malleable cast iron. The temper carbon is formed by the decomposition of the cementite in the original white iron by the following reaction: Fe3C ↔ 3Fe ⫹ C

Graphite is the stable form of carbon in cast irons and steels, whereas cementite is metastable, which means that with sufficient time and temperature, cementite will eventually revert to graphite. Because of the rounded form of the temper carbon, the properties of malleable iron are somewhat similar to ductile cast iron. There is more discussion of malleable iron in the next chapter.

Commercial Cast Irons Because all commercial cast irons contain fairly large amounts of manganese, silicon, and other alloying elements, the phase diagrams discussed previously can only be used as a rough guide to transformation behavior. It has been shown that silicon and phosphorus have a strong effect on shifting the eutectic composition. The following equation can be used to approximate the eutectic composition in terms of a carbon equivalent value: Fig. 2.52 A graphite nodule in a ductile (nodular) cast iron. Unetched. 500⫻

Fig. 2.53 Pearlitic malleable cast iron (3.21% C, 0.32% Mn, and 0.47% Si). Dark areas are “temper carbon.” 4% picral. 500⫻

Carbon equivalent (CE) ⫽ % Total carbon ⫹ (% Si ⫹ % P)/3

Thus, for a cast iron with 2.6% C, 1.0% Si, and 0.2% P, the CE value would be 3.0%, which would be a hypoeutectic cast iron (hypoeutectic if below the equilibrium value of 4.3% C in Fig. 2.2). The additional elements not only shift the eutectic composition and eutectic temperature, but they also have other profound effects on microstructural development. Manganese, chromium, molybdenum, and vanadium are carbide stabilizers, which means that they promote the formation of stable, iron-rich carbide, that is, cementite containing these elements in solid solution. Silicon, aluminum, nickel, and copper, on the other hand, are graphitizers, which means that they promote the formation of graphite. Thus, having manganese, chromium, molybdenum, or vanadium would favor the iron-cementite phase diagram, and silicon, aluminum, nickel, and copper would favor the iron-graphite phase diagram. Thus, proper control of these elements is essential if one desires a gray cast iron or a white cast iron. Just as important as composition is the cooling rate during solidification and subsequent cooling during the solid-state transformations. As in the case of steels, the proper combination of composition and cooling rate can determine a particular microstructure in cast iron. Thus, gray irons usually contain substantial amounts of silicon and are solidified rather slowly. For white irons, the silicon content is

Origin of Microstructure / 47 usually lower than that in gray irons, and one or more of the carbide stabilizers are present. Also, white cast irons are solidified more rapidly and are usually cast in thin sections or are cast in molds containing chills. Foundry molding practices can be tailored to give fast or slow cooling rates during solidification and solid-state transformation. Unfortunately, there are only a few continuous CT diagrams for the cast irons, but the same principles of continuous cooling in

Fig. 2.54 Plate martensite transformation in the primary austenite dendrites of an alloy (Ni-Hard) white cast iron (2.95% C, 0.63% Mn, 0.73% Si, 3.08% Ni, and 2.17% Cr). 4% picral etch. 500⫻

Fig. 2.55 Additional formation of plate martensite in the same sample of Ni-Hard cast iron shown in Fig. 2.54 after cooling in liquid nitrogen. 4% picral etch. 500⫻

steels still apply to cast irons, especially in controlling the matrix microstructure. The graphite and the cementite phases are controlled mostly by solidification (except for malleable iron, where the graphite is controlled by heat treatment). In cooling below the eutectic temperature, the austenite can decompose into pearlite, ferrite, martensite, or bainite, depending mainly on cooling rate and on the composition of the austenite. Gray cast irons, in general, have a pearlitic matrix at slow-to-moderate cooling rates and a ferritic matrix at very slow cooling rates. In white cast irons, the austenite decomposes by either a pearlitic, bainitic, or martensitic transformation. An example of the structure observed after a martensitic transformation in a commercial alloyed white cast iron (called Ni-Hard) is shown in Fig. 2.54. The martensite is plate martensite, due to its high carbon content. In this micrograph, the martensite was tempered at about 100 °C (212 °F), which caused it to appear darker after etching. The gray regions between the plates of martensite are retained austenite. This retained austenite would transform if the cast iron was cooled to a very low temperature (this alloy has a martensite finish temperature below room temperature). The same field of view shown in Fig. 2.54 of the Ni-Hard cast iron is shown again in Fig. 2.55 after being subcooled in liquid nitrogen. Note that much of the retained austenite has transformed to more plate martensite, as evidenced by the white needles of freshly formed (untempered) plate martensite. There is a fairly large amount of retained austenite still existing in this microstructure. One way to change the microstructure of cast iron is through heat treatment by reaustenitizing and cooling at a specified rate. The next chapter deals with various ways to alter the microstructure of cast iron. Also, in commercial cast irons, several impurity elements may be present, often at levels much higher than in steels. The major impurities are sulfur and phosphorus. As in steels, manganese is added to tie up the sulfur in the form of manganese sulfide

Fig. 2.56 The eutectic constituent steadite (see arrow) in a pearlitic gray cast iron. 4% picral etch. 500⫻

48 / Metallographer’s Guide inclusions. Phosphorus, on the other hand, is not tied up by manganese and forms a eutectic constituent called steadite, named after John Edward Stead, a noted 19th century English metallurgist. The eutectic consists of iron and iron phosphide, Fe3P, and is formed through the following reaction: Liquid ↔ Fe ⫹ Fe3P

An example of the constituent, steadite, is seen in Fig. 2.56 as a region containing a cluster of small iron phosphide particles. The microstructure shown in Fig. 2.56 represents a pearlitic gray cast iron containing high phosphorus. REFERENCES 1. A. Sauveur, The Metallography and Heat Treatment of Iron and Steel, Sauveur and Boylston, Cambridge, Mass., 1912 2. F. Osmond and J.E. Stead, The Microscopic Analysis of Metals, Charles Griffin and Co., London, 1913

3. H.M. Howe, The Metallography of Steel and Cast Iron, McGraw-Hill, New York, 1916 4. E.S. Davenport and E.C. Bain, Trans. AIME, Vol 90, 1930, p. 117–144. 5. Properties and Selection: Irons, Steels, and High-Performance Alloys, Vol 1, ASM Handbook, ASM International, 1990 6. G.V. Raynor and V.G. Rivlin, Phase Equilibria in Iron Ternary Alloys, The Institute of Metals, London, 1988 7. V. Raghavan, Phase Diagrams of Ternary Iron Alloys, Part 1, ASM International, 1987 8. G. Krauss, Steels: Heat Treatment and Processing Principles, ASM International, 1989 9. L.E. Samuels, Optical Microscopy of Carbon Steels, American Society for Metals, 1980, p. 41–58 10. B.L. Bramfitt and J.G. Speer, Metall. Trans. A, Vol 21, 1990, p. 817–829

Metallographer's Guide: Practices and Procedures for Irons and Steels Bruce L. Bramfitt, Arlan O. Benscoter, p49-86 DOI:10.1361/mgpp2002p049

Copyright © 2002 ASM International® All rights reserved. www.asminternational.org

CHAPTER 3

Alteration of Microstructure IN THE PREVIOUS CHAPTER, it was shown how a great variety of microstructures can be formed in steels and cast irons. It was learned that cooling rate and composition play important roles in determining the type of microstructure that is produced. Accordingly, the same steel can have different microstructures, depending on the way the steel is processed. It is important to understand the principles in the previous chapter in order to get more meaning from the information and examples given in this chapter. A particular microstructure can be altered, either intentionally or unintentionally. It is the metallographer’s task to prepare a specimen, examine it in the light microscope, and in many cases, interpret the microstructure. Many times, the metallographer (and metallurgist) must diagnose what may have happened to the steel or cast iron, based on its microstructural details, and, in turn, explain why the steel or cast iron is exhibiting abnormal behavior. The first section provides examples of how microstructure can be intentionally altered during heat treatment, solidification, and deformation (hot and cold working). These treatments are considered intentional alterations, because they are meant to achieve a specific goal, for example, to soften the steel, to strengthen the steel, to increase surface hardness, and to produce a more ductile cast iron. In many cases, a steel or cast iron must be heat treated for a second time to return it to its original microstructure and properties. Remember, here we use terms “microstructure” and “properties” synonymously. That is, processing and composition determine the microstructure that in turn dictates the properties of the material. From the metallographer’s viewpoint, the golden rule is that microstructure influences properties. Knowing this link can be very useful in making recommendations about intentionally retreating a steel or cast iron component to restore properties. In the second section, some specific examples are shown to illustrate what can go wrong through unintentional changes in microstructure, for example, the loss of carbon from the surface of the steel by the process known as decarburization or the buildup of brittle carbides on the grain boundaries of an austenitic stainless steel by the process known as sensitization. Unintentional changes can usually relate to a process that has gone out of control. The metallographer (and metallurgist) must be aware of these unintentional microstructural alterations and be able to recognize them when they have taken place. Also, the metallographer and metallurgist should know what needs to be done to change the microstructure back to a normal condition. In some cases, the unintentional changes are not reversible, for example, a steel in the burned condition or with hydrogen damage cannot be

salvaged. However, many times, a simple heat treatment can restore the microstructure and properties. This chapter provides a basis to assist the metallographer and metallurgist in making rational decisions that can save time and money in the workplace.

The Intentional Alteration of Microstructure in Steels and Cast Irons As shown in the previous chapter, one of the easiest ways to change a microstructure is through heat treatment. Many steel and some cast iron products are heat treated to achieve specific properties, such as a particular hardness level or tensile strength. Marked changes in microstructure can even take place by simply heating a steel to various temperatures and cooling to room temperature. The cooling rate from these temperatures can have a profound effect on the resulting microstructure and properties. Some of these simple heat treatments are discussed subsequently. This portion of the chapter is structured so that each heat treatment is used to achieve a specific purpose or goal. Sometimes microstructures can be intentionally altered by processes other than heat treatment, as in the case of cold working of steel and inoculation of liquid iron. These processes are also discussed subsequently.

Goal: Produce a Uniform Microstructure Normalizing. A normalizing heat treatment is designed to produce a uniform microstructure. Normalizing also produces a finer grain size and improves machinability. To illustrate the effect of normalizing, a hot-rolled, 19 mm (0.75 in.) diameter bar of AISI/SAE 1040 steel is used. The microstructure of the original hot-rolled bar is seen in Fig. 3.1. In the micrograph (longitudinal view), it can be seen that the microstructure is obviously nonuniform and consists of alternating bands of pearlite and ferrite. In a plain carbon steel, the bands of pearlite contain higher carbon and manganese contents than the ferrite bands. This compositional “banding” is a result of the chemical segregation that occurred during solidification. (The solidification took place when the molten steel was cast into a mold, such as a water-cooled copper mold of a continuous casting machine found in a high-productivity steel plant. The cooling rate during solidification and the composition largely dictate the amount of segregation that develops.)

50 / Metallographers’ Guide

Fig. 3.1 Microstructure of a typical as-rolled AISI/SAE 1040 steel bar consisting of bands of ferrite (light) and pearlite (dark). 2% nital and 4% picral etch. 200⫻

In the previously mentioned AISI/SAE 1040 steel, the hotworking process from the as-cast state to the final rolled bar was insufficient to destroy this segregation. The hot-rolling process merely stretched out the compositional segregation into elongated bands. Sometimes, this type of microstructure is undesirable, because banding produces anisotropic (directional) properties. For example, the strength and notch toughness measured parallel to the bands may differ significantly from the strength and notch toughness measured perpendicular to the bands. However, for many applications, these differences are so small that banding is not considered a problem. The simplest way to eliminate pearlite banding is to reheat the steel to an appropriate normalizing temperature and air cool to room temperature. Most normalizing temperatures are specified as being about 55 to 80 °C (100 to 150 °F) above the upper critical temperature (Ac3). This is represented as the shaded band shown in the iron-carbon equilibrium diagram in Fig. 3.2. Actually, an equilibrium diagram is not the correct way to indicate normalizing temperatures, because we are not dealing with equilibrium conditions. However, this representation is used because most metallographers/metallurgists are familiar with the iron-carbon equilibrium diagram. The actual recommended heat treating temperatures for a specific steel can be found in the references

Fig. 3.2 Iron-carbon equilibrium diagram showing region (shaded) of typical normalizing temperatures

Alteration of Microstructure / 51 listed at the end of this chapter. For AISI/SAE 1040, the specified normalizing temperature range is 860 to 900 °C (1575 to 1650 °F). The microstructure of the normalized AISI/SAE 1040 steel bar is shown in Fig. 3.3. This microstructure was produced by heating the steel to 900 °C (1650 °F) for one hour, followed by air cooling. Note that the microstructure is very uniform and consists of a fine ferrite grain size and regions of pearlite. The banding has disappeared. During the heat treatment, the carbon was redistributed in the austenite, that is, the carbon diffused from the banded pearlite regions, which transformed to austenite along with the ferrite and distributed itself uniformly in the austenite. Remember from the previous chapter, for a 0.4% C steel at a temperature of 900 °C (1650 °F), carbon is completely soluble in austenite. The air cooling provided a rapid enough cooling rate to prevent the carbon from returning to the banded regions where manganese and other segregated alloying elements still reside. If the steel was cooled slowly, the carbon would have had time to diffuse back to the compositional bands. The only way to remove the compositional bands is to subject the steel to a high-temperature, long-time homogenization treatment described in the following section. This treatment allows the manganese, which is a substitutional alloying element in the iron lattice, to diffuse. (A substitutional element is one that can replace an iron atom in the face-centered cubic or body-centered cubic lattices [see Chapter 2].) Carbon, being an interstitial alloying element, can diffuse far more easily than manganese, because it can move in the interstices between the iron atoms in the crystal lattice. It is important to follow the recommended temperatures of each type of heat treatment found in the Handbooks. There are metallurgical reasons why these temperature ranges are specified. For example, let’s look at what happens to the microstructure of the same hot-rolled bar of AISI/SAE 1040 steel when heated to

Fig. 3.3 Microstructure of a typical normalized AISI/SAE 1040 steel bar showing uniformity of ferrite (light) and pearlite (dark). 2% nital and 4% picral etch. 200⫻

five different austenitizing temperatures ranging from 840 to 980 °C (1550 to 1800 °F) and subsequently air cooled to room temperature. Figure 3.4 shows the microstructure produced by air cooling from these five temperatures. In heating to temperatures between 840 and 940 °C (1550 and 1725 °F), the resulting microstructure is uniform and consists of small pearlite colonies in a matrix of ferrite. This range covers the specified range for normalizing an AISI/SAE 1040 steel of 860 to 900 °C (1575 to 1650 °F). The result of heating at temperatures above 980 °C (1800 °F) is that the microstructure is drastically different and consists of some extremely large grains containing pearlite with a grain boundary network of proeutectoid ferrite. The question is, why the sudden change in microstructure? The answer is that there is a sudden growth of the austenite grains that takes place between 955 and 980 °C (1750 and 1800 °F). It actually begins to take place just below 955 °C (1750 °F), as shown in Fig. 3.4. In this micrograph, one can see the beginning of large austenite grains (now pearlite) that have absorbed the smaller grains. At 970 °C (1775 °F), the grain-coarsening process is more advanced. The exaggerated growth of the austenite grains is due to the dissolution of small aluminum nitride precipitates that were pinning the grain boundaries (the particles are much too small to be seen in the light microscope and must be examined in the transmission electron microscope). Once the particles dissolved upon heating, the grain boundaries were free to move. The AISI/SAE 1040 steel, shown in Fig. 3.4, was aluminum killed, and during the cooling of the original hot-rolled bar, the aluminum nitride particles precipitated on the austenite grain boundaries before the austenite transformed into ferrite and pearlite. In a steel that was not treated with aluminum or other nitride former, for example, titanium, the exaggerated grain growth would take place at lower temperatures because of the lack of the pinning action of the aluminum nitride precipitates. Because of the drastic microstructural changes that can take place upon heating, heat treatment guidelines should always be followed for a particular steel. As in the case of normalizing, the 860 to 900 °C (1575 to 1650 °F) range specified for a AISI/SAE 1040 steel allows some safety to avoid the problem of excessive grain growth. Homogenizing. A heat treatment called homogenization is carried out to eliminate or minimize the compositional segregation discussed previously, that is, banding in a steel. This treatment is seldom used, because it requires high-temperature treatments that take place for an extended period of time. Compositional segregation in a steel product can be traced back to the dendritic solidification process. During dendritic solidification, alloying elements, for example, manganese and chromium, are rejected from the growing dendrites. These alloying elements end up in liquid pools between the dendrite arms and between adjacent dendrites (a portion of the manganese and chromium will also be in the dendrites themselves). The resulting microstructure will reveal these regions, called microsegregation and macrosegregation. Microsegregation is represented by the segregation of alloying elements between the dendrite branches or arms. Macrosegregation is considered bulk segregation that usually takes place at the front of the growing dendrites and is therefore located toward the center of the cast shape. Figure 3.5 shows a dendritic

52 / Metallographers’ Guide microstructure in a continuous-cast AISI/SAE 1020 steel slab. One can see that the interdendritic areas and the areas between the dendrite arms consist of pearlite. The subsequent hot-working processes simply distort these and other segregated regions into elongated bands oriented along the rolling or forging direction (see Fig. 3.1). During the hot-working process, the steel is not at a high temperature long enough to diffuse substitutional alloying elements, such as manganese and chromium. The manganese and chromium atoms are of similar size to the iron atom and can only move about the lattice at high temperatures. To move a substantial distance, the steel must remain at this high temperature for an

extended time. An example of a homogenization treatment is given for an AISI/SAE 8630 steel that contains substitutional elements, manganese and chromium. The starting material is an as-rolled, 25 mm (1 in.) diameter bar that has the banded microstructure shown in Fig. 3.6. After heat treating the bar at a temperature of 1090 °C (2000 °F) for 120 hours, the microstructure shown in Fig. 3.7 was developed. The grain size is much coarser in the homogenized sample, because no effort was made to normalize the sample after the homogenization treatment. It is important to notice that in the homogenized condition, the compositional banding is eliminated. The proof is in the fact that

Fig. 3.4 Microstructure of a series of AISI/SAE 1040 steel bars heat treated at different temperatures. (a) At 840 °C (1550 °F). (b) At 940 °C (1725 °F). (c) At 955 °C (1750 °F). (d) At 970 °C (1775 °F). (e) At 980 °C (1800 °F). 2% nital and 4% picral etch. 100⫻

Alteration of Microstructure / 53 this bar was furnace cooled from 1090 °C (2000 °F), allowing ample time for the carbon to diffuse back to manganese- and chromium-rich bands, if they were still present. This means that the manganese and chromium also diffused and were thus redistributed in the iron matrix. Obviously, a five-day (120 hour) heat treatment at 1090 °C (2000 °F) is an expensive route to achieve homogenization. Most industrial processes do not use such treatments. A more economical way is to minimize segrega-

Fig. 3.4 (continued) (e) At 980 °C (1800 °F). 2% nital and 4% picral etch. 100⫻

Fig. 3.6 Microstructure of an as-rolled AISI/SAE 8630 steel bar. Longitudinal plane of polish showing banding of pearlite (dark etching constituent). 2% nital and 4% picral etch

tion during the solidification process. This can be done by decreasing the cross section of the casting (decreasing the solidification distance), increasing the cooling rate during solidification, or using a grain refiner to promote the nucleation of many independent crystals during solidification. These methods will produce finer dendrite arm spacings and shorter diffusion distances. The shorter diffusion distances mean less time is needed for homogenization.

Fig. 3.5 A dendritic microstructure in a continuous-cast AISI/SAE 1020 steel showing pearlite (dark etching constituent) in the interdendritic regions and between the dendrite arms. 4% picral etch. 50⫻

Fig. 3.7 Microstructure of the AISI/SAE 8630 steel bar after a homogenization heat treatment. 2% nital and 4% picral etch. 100⫻

54 / Metallographers’ Guide

Goal: Produce a Higher Hardness Quenching. Centuries ago, craftsmen knew that quenching a “red hot” sword in water or into the fleshy parts of a “Nubian slave” would produce high hardness and a better cutting edge. The process was finally understood at the end of the 19th century when metallurgists began looking at steel samples under the light microscope. A steel that has been quenched in water contains a constituent known as martensite. As-quenched martensite, in medium- and high-carbon steels, has a very high hardness, that is, 60 to 65 HRC. An example of an as-quenched fully martensitic AISI/SAE 8630 steel is shown in Fig. 3.8(a) and (b). In this example, lath martensite has formed. (As discussed in Chapter 2, there are two common forms of martensite: lath martensite and plate martensite. Lath martensite forms in low- to medium-carbon steels, and plate martensite forms in high-carbon steels. For example, as carbon content increases in a plain carbon steel, plate martensite begins to form at about 0.6% C, and above about 1% C, the as-quenched microstructure will be all plate martensite (between 0.6 and 1% C, there will be a mixture of lath and plate martensite). These carbon percentages can change with alloying.) Martensite is certainly one way to achieve high hardness in steel. Razor blades are an example of a product having an as-quenched martensitic microstructure. However, in carbon steels, as-quenched martensite lacks ductility and therefore has limited commercial usefulness. Therefore, martensitic steels are used in the tempered condition, because tempered martensites have some ductility. Tempered martensitic steels are used in many products. Tempering is discussed in an upcoming section, because it is a process to increase ductility and toughness. Intercritical Annealing. Generally, heat treatment is not carried out in the intercritical region, that is, between the Ar1 and Ar3

temperatures, because the austenite transformation from ferrite and pearlite upon heating is not completed, that is, only a portion of the microstructure at the intercritical temperature will be austenite and the remainder untransformed ferrite. However, one form of heat treatment takes advantage of this partial transformation to austenite. The process is called intercritical annealing to produce what is known as a dual-phase steel. Dual-phase steels, developed in the 1970s, became noteworthy for special automotive applications requiring high strength and good formability. One such application is in producing automotive wheel rims and wheel discs. An example of a typical dual-phase microstructure is shown in Fig. 3.9. The microstructure consists of regions or islands of martensite in a matrix of ferrite, hence the term “dual-phase steel.” The martensite formed as a result of quenching a steel sheet from the intercritical (two-phase) region where the microstructure at the annealing temperature was approximately 15% austenite and 85% untransformed (original) ferrite. The austenite is rich in carbon, and during quenching, the austenite transformed to martensite, and the ferrite remained essentially unchanged. Usually, the presence of martensite in an automotive sheet is undesirable (except in very special high-strength applications). However, in the case of a dual-phase steel, the martensite is embedded in a soft ferrite matrix, and the martensite provides a continuous source of dislocations during forming of the part. A source of dislocations is needed to promote continuous yielding behavior, that is, lack of a yield point, during forming in the stamping operations. This means that the deformed areas of the sheet actually strengthen due to a process known as work hardening. Normal high-strength automotive sheet will deform by a process of discontinuous yielding and will have limited workhardening capability. A steel with discontinuous yielding will produce a yield point during the tensile test, and a steel with

Fig. 3.8 Microstructure of a quenched AISI/SAE 8630 steel bar consisting of lath martensite. 2% nital etch. (a) 500⫻ and (b) 1500⫻

Alteration of Microstructure / 55 continuous yielding will produce a smooth or “round house” stress-strain curve. Discontinuous yielding can create Lüders lines in a sheet, which, for exposed parts, is objectionable. The amount of martensite in the microstructure of a dual-phase steel is controlled by the annealing temperature and the carbon content of the steel. The greater the percentage of martensite, the higher the tensile strength of the sheet. However, more martensite also means lower ductility. The metallographer should be able to recognize a dual-phase microstructure and determine if it received the proper heat treatment. Many times, dual-phase steels will contain retained austenite in addition to the martensite. Retained austenite can provide an added degree of ductility during a deformation process as it transforms to additional martensite. Figure 3.10(a) shows an intercritically annealed AISI/SAE 1010 steel that contains martensite (dark etching constituent) and retained austenite (light gray etching constituent) in a matrix of ferrite. During forming, the austenite would transform to martensite by a process called strain-induced transformation. If examined with differential interference contrast (this technique, called Nomarski, is described in Chapter 5), as in Fig. 3.10(b), the same microstructure reveals a region surrounding each area of martensite and retained austenite. This region is “new” ferrite that formed during cooling, that is, the new ferrite grew into the austenite regions that were present at the intercritical temperature. The new ferrite is thought to be a simple extension of the ferrite already present and is thus referred to as epitaxial ferrite, because it formed epitaxially (growing on a material of identical crystal structure and similar interatomic spacing) on the existing ferrite lattice. Cold Working. If a steel is rolled at room temperature, or at a temperature below the recrystallization temperature of ferrite, the steel will increase in hardness. The deformation introduces a high density of dislocations. Dislocations are defects in the body-

centered cubic lattice, for example, a dislocation could be created by a missing plane of atoms. Figure 3.11 shows the microstructure of a cold-rolled, low-carbon steel sheet that has been cold rolled 30, 50, 70, and 90% (measured by the percent change in final thickness from the original thickness). As the sheet lengthened, the degree of elongation of the ferrite grains increased in

Fig. 3.10 Microstructure of an intercritically annealed AISI/SAE 1010 steel

Fig. 3.9 Microstructure of a typical dual-phase steel bar consisting of

martensite (dark) in a matrix of ferrite (light). 2% nital etch. 1000⫻

bar consisting of (a) martensite (dark etching constituent) and retained austenite (gray etching constituent) in a matrix of ferrite in bright-field illumination and (b) the same field of view in differential interference contrast (Nomarski). Note the formation of “epitaxial” ferrite (see arrows) surrounding the martensitic areas. 4% picral etch. 500⫻

56 / Metallographers’ Guide proportion to the amount of cold reduction. The ferritic microstructure was thus altered by the deformation process. Note that the elongated ferrite grains contain dark etching regions. These dark regions represent deformation bands that contain a high density of dislocations. The dislocations cannot be resolved in the light microscope and thus can only be observed in the transmission electron microscope. The deformation-induced dislocations increase the hardness of the steel. However, steels are rarely used in the cold-worked condition, because they lack ductility and formability. Steels in the cold-rolled “full-hard” condition are used in limited applications, such as steel strapping for banding

cargo and freight shipments. Another form of cold-rolled sheet is duo-reduced tinplate. Usually, steels in the cold-rolled condition are annealed (see the subsequent section on recrystallization annealing) to enhance ductility and formability. Cold-rolled steels can be stress relieved at temperatures of about 315 to 425 °C (600 to 800 °F) before use. There are commercial sheet steels called stress-relief annealed steels that are essentially steels in the cold-worked condition that have been stress relieved just enough to provide some ductility, but not enough to drastically lower strength. The process is also called recovery annealing, where the metallurgical process of

Fig. 3.11 Microstructure of a cold-rolled, low-carbon steel sheet. Cold-worked (a) 30%, (b) 50%, (c) 70%, and (d) 90%. Marshall’s etch. 500⫻

Alteration of Microstructure / 57 recovery takes place. Recovery is an intermediate stage between cold work and recrystallization. During the recovery process, most of the dislocations that were induced by deformation can move, interact, and cancel each other. The remaining dislocations can regroup to form subgrain boundaries within the ferrite grains themselves. The dislocations in subgrain boundaries can only be seen in the transmission electron microscope.

Fig. 3.12 Microstructure of a cold-worked AISI 304 stainless steel rod at

(a) a near-surface region and (b) at the center. Differential interference contrast (Nomarski). Etched in 60 parts nitric acid in 40 parts water, stainless steel cathode, 5 V for 10 s. 800⫻

Another example of the use of cold working to harden and strengthen a steel is in the case of an austenitic stainless steel. Because austenitic stainless steels are fully austenitic from room temperature up to their melting point, they cannot be hardened by quenching, as in the case of a plain carbon or low-alloy steel. Therefore, austenitic stainless steels are generally cold worked to achieve a higher strength. Cold-worked austenitic steels can be found in numerous applications. An example of a cold-worked steel can be seen in the 9.5 mm (0.375 in.) diameter AISI 304 austenitic stainless steel rod shown in Fig. 3.12. Note that the cold work is delineated by numerous deformation bands within the austenite grains. The degree of cold work is much greater near the surface region (Fig. 3.12a) than at the center of the rod (Fig. 3.12b). Also note that the deformation bands can occur at three specific orientations within each grain. These orientations represent the {111} planes of the austenite crystal lattice, where deformation (or slip) along these planes can take place more easily. The center of the rod has fewer locations of deformation, because there is a deformation gradient from the surface inward. This leads to uneven deformation and surface-to-center variation and is one of the disadvantages of using cold working as a strengthening mechanism in thicker sections. The metallographer should be able to recogonize a cold-worked condition in both plain carbon and austenitic stainless steels. In many cases, a cold-worked condition is not desired, and proper metallographic procedures will reveal the extent of cold working. In some cases, the metallographic preparation process itself can introduce cold working into a sample. This produces an “artifact,” or false microstructure, that will mislead the metallographer in analyzing the microstructure. More about artifacts can be found in Chapter 7.

Fig. 3.13 Microstructure of a hot-rolled, high-strength microalloyed steel

plate with elongated pearlite bands (dark constituent) in a ferrite matrix. 4% picral followed by 2% nital. 500⫻

58 / Metallographers’ Guide

Fig. 3.14 Iron-carbon equilibrium diagram showing region (shaded) of annealing temperatures

Fig. 3.15 Microstructure of an annealed AISI/SAE 1040 steel bar consist-

ing of bands of ferrite (light) and pearlite (dark). Longitudinal section. 2% nital and 4% picral etch. 500⫻

Precipitation Hardening. A very popular way to increase hardness and strengthen a steel is through a mechanism known as precipitation hardening or precipitation strengthening. The steel is processed in such a way as to allow precipitates to form during cooling, during hot deformation, or during a subsequent aging treatment. Once the precipitate nucleates and grows, the surrounding matrix is locally hardened, because the precipitates take up more volume in the iron matrix. Precipitates can also form during tempering of low-alloy steels and give rise to what is termed “secondary hardening.” These precipitates are usually alloy carbides of chromium, molybdenum, and vanadium. The tempering heat treatment usually involves a temperature below the lower critical temperature (A1). The precipitation process also is an important step in the strengthening of maraging steels, where, after quenching, a precipitate (an intermetallic compound) forms in the martensite during an aging heat treatment (usually at 425 °C (800 °F) for two hours). A similar process is also involved in precipitationstrengthened stainless steels. Another example of precipitation strengthening is a process involving carbide- and nitride-forming elements (columbium (niobium), vanadium, titanium, aluminum, etc.). These carbonitride particles precipitate in austenite during hot rolling or forging.

Alteration of Microstructure / 59 The small precipitates restrict austenite grain-boundary movement and thus provide grain refinement. This process is described subsequently. Thus, the metallographer should be aware of the role of precipitates in altering the microstructure of steel. The light microscope can be used to observe clues about the precipitation process, but an electron microscope is often necessary to observe the precipitate itself. An analytical electron microscope, such as a scanning transmission electron microscope (STEM), is vital to

identify the chemical composition of individual precipitate particles. Grain Refinement. Precipitates also inhibit dislocation and grain-boundary movement, thus producing a fine grain size that promotes higher hardness and strength of the steel. Precipitates can also prevent recrystallization of austenite during hot rolling, a process that allows the austenite grains to elongate along the rolling direction. When the flattened austenite grains transform to ferrite, the flattened grains restrict the growth distance and thus

Fig. 3.16 Microstructure of cold-rolled, interstitial-free steel sheet that has been annealed for 1 min at (a) 649 °C (1200 °F), (b) 676 °C (1250 °F), (c) 704 °C (1300 °F), and (d) 732 °C (1350 °F). Marshall’s etch. 400⫻

60 / Metallographers’ Guide

Fig. 3.17 Microstructure of (a) an as-quenched AISI/SAE 8630 steel bar and (b) a fully spheroidized structure consisting of spheroids of cementite in a matrix of ferrite. 4% picral etch. 1000⫻

the size of the new ferrite grains. The fine ferrite grains provide higher hardness and strength. An example of the results of this process is shown in Fig. 3.13, which represents a hot-rolled plate steel that was “microalloyed” with the element columbium (niobium). The columbium (niobium) combined with carbon and nitrogen to form columbium carbonitride, Cb(C,N), precipitates. During hot rolling, austenite recrystallization was retarded, and elongated grains developed. Some evidence of the elongated austenite grains can be seen in the microstructure shown in Fig. 3.13. The final ferrite grain size is controlled largely by the thickness of the prior austenite grains. Strain Aging. One such strain-aging treatment would be the paint-bake cycle for automobile outerbody panels. In this process, called bake hardening, not only is the paint cured, but the steel itself increases in hardness and strength.

Goal: Produce a Lower Hardness

Fig. 3.18 Iron-carbon equilibrium diagram showing region (shaded) of spheroidizing temperatures

Annealing. This heat treatment is generally used to soften a steel or cast iron. The austenitizing temperatures used are not as specific as in the case of normalizing, but are limited to temperatures that avoid the exaggerated grain growth seen in Fig. 3.4. Annealing temperatures can range from 840 to 900 °C (1550 to 1650 °F). Annealing temperatures for plain carbon steels are

Alteration of Microstructure / 61 shown as a shaded band on the iron-carbon equilibrium diagram in Fig. 3.14. Recommended annealing temperatures for various steels can be found in the references at the end of the chapter. For an AISI/SAE 1040 steel, the annealed microstructure is seen in Fig. 3.15. Annealing has softened the steel, but did not produce a uniform microstructure, as in the case of normalizing (Fig. 3.3). The differences between annealing and normalizing are due to the cooling rate of the steel after the austenitizing treatment; in annealing, the steel is slow cooled, usually furnace cooled, and in normalizing, the steel is cooled at a faster rate, generally, by forced air cooling or static air cooling. The slower cooling process allows for carbon diffusion. Many times, an annealed microstructure will be banded if the starting microstructure was banded. Annealed microstructures are not as uniform or as fine-grained as a normalized microstructure. Recrystallization Annealing. Annealing can also be used to soften a cold-worked steel and is a very important step in the production of cold-rolled steel sheet. For example, automotive sheet steels for outerbody panels are produced by cold rolling (rolling at room temperature) to obtain the desired sheet thickness, followed by either batch or continuous annealing. As described in the section on cold working, a cold-rolled steel has limited ductility and cannot be easily formed into contoured parts. The annealing process for steel sheet is called recrystallization annealing. New ferrite grains (recrystallized grains) are formed from the elongated cold-worked ferrite grains. Figure 3.16(a) shows the microstructure of a cold-rolled, interstitial-free (IF) sheet steel after annealing at 650 °C (1200 °F) for one minute. (Interstitial elements, such as carbon and nitrogen, have a much smaller atom size than iron, and the atoms can reside in special positions within the iron lattice (in the interstices). “Interstitial-free”steel is a term used to describe a steel in which carbon and nitrogen are almost completely removed from the iron lattice in the form of carbides or nitrides. Commercially, titanium or columbium (niobium) are added for this purpose.) This treatment illustrates the very early stage of recrystallization. The arrows show the small recrystallized grains that have nucleated. After one minute at 675 °C (1250 °F) (Fig. 3.16b), more recrystallized grains are shown in the microstructure, and after annealing one minute at 705 °C (1300 °F), the cold-rolled IF steel shows that recrystallization is almost complete, with some evidence of the original cold-worked grains remaining (Fig. 3.16c). Figure 3.16(d) shows a fully recrystallized microstructure that occurred after annealing for one minute at 730 °C (1350 °F). In the fully recrystallized condition, this IF steel will have excellent formability, and strain aging will be absent, because the carbon and nitrogen interstitial elements are tied up as carbides, nitrides, or carbonitrides. The metallographer must be able to recognize cold-worked and recrystallized microstructures, especially in monitoring sheet steel annealing processes. Spheroidizing. To develop the softest possible condition, a steel is usually given a spheroidizing treatment. This kind of treatment is used to enhance machinability. The term “spheroidizing” means that the cementite constituent is converted to spheroids, or rounded particles, in a matrix of ferrite. An example of a fully spheroidized microstructure of AISI/SAE 8630 steel is

shown in Fig. 3.17(b). Note the small, rounded particles of cementite in the micrograph. The hardness of this steel is 82 HRB, which is well below the hardness of 97 HRB of the same steel in the normalized condition. To alter the microstructure to a spheroidized condition, the steel is heated for a fairly long time (10 to 15 hours) at a temperature just above and then just below the lower critical (A1) temperature. This is indicated by the shaded band on the iron-carbon equilibrium diagram, shown in Fig. 3.18. The fairly long treatment time allows dissolution or breakup of the

Fig. 3.19 Microstructure (b) of a spheroidized AISI/SAE 8630 steel bar

produced from the original banded microstructure shown in (a). 2% nital and 4% picral etch. 1000⫻

62 / Metallographers’ Guide cementite lamella in pearlite. The carbon is then redistributed by the diffusion process, and rounded particles are formed. For a uniform distribution of cementite particles, it is recommended that the steel be in the quenched condition before spheroidizing, that is, the steel should have a martensitic microstructure (see Fig. 3.17a). The martensitic microstructure has a uniform distribution of carbon dissolved in the iron lattice, and the microstructure is usually very fine and thus contains numerous nucleation sites for

the new carbides. With martensite present, the process does not depend on the dissolution of the cementite lamella in pearlite, and thus spheroidization can take place in a much shorter period of time. An example of a spheroidized microstructure produced from a bar of AISI/SAE 8630 steel with an as-rolled ferrite/pearlite microstructure (Fig. 3.19a) is shown in Fig. 3.19(b). Note the uniform distribution of carbides in the microstructure from the martensitic bar and the nonuniform distribution of carbides resulting from the ferrite/pearlite bar, even though both steels were heat treated at 700 °C (1290 °F) for 20 hours and air cooled. In the latter case, one can see where the pearlite bands existed. Tempering. Generally, a steel that has been quenched is subsequently tempered to provide some softening and ductility as well as stress relief. Tempering is a subcritical treatment, where the part is heated to temperatures generally ranging from 260 to 650 °C (500 to 1200 °F) and air cooled to room temperature. During the tempering process, carbides begin to precipitate from the carbide-free martensitic microstructure. An example of a quenched and tempered AISI/SAE 8630 steel is shown in Fig. 3.20(a) and (b). The original steel in the as-quenched condition was shown in Fig. 3.8(a) and (b). The tempering temperature was 650 °C (1200 °F), and the tempering time was 2.5 hours. If the tempered microstructure is compared with the quenched microstructure, it can be seen that the tempered microstructure does not reveal the sharpness of the lath boundaries of the quenched steel, and the microstructure contains many small, dark particles barely resolvable at 500⫻. The picral etchant that was used darkened the carbides in the tempered steel. During tempering, the carbon that was in solution in the martensite precipitated as carbides. The hardness of the steel shown in Fig. 3.20 is 35 HRC, which is much lower than the 63 HRC hardness of the as-quenched steel.

Goal: Produce a Hardened Surface

Fig. 3.20 Microstructure of tempered lath martensite in an AISI/SAE 8630 steel bar. 2% nital etch. (a) 500⫻ and (b) 1500⫻

Carburizing. In some applications, hardness is only needed at the surface of a part, for example, a gear where only the wear surfaces of the teeth are hardened, or a motor shaft where the surface requires wear resistance and the core requires toughness. An inexpensive way of surface hardening is carburizing, where carbon is allowed to diffuse into the surface of a component that is heated to temperatures between 840 to 955 °C (1550 to 1750 °F). Pack carburizing is a process where the carbon can be provided by “packing” or burying the part in a carbon-rich substance, for example, charcoal. Gas carburizing provides the carbon from a carbon-rich gas, for example, methane (CH4), and liquid carburizing is a process where the part is submerged in a molten salt bath containing a carbon-rich liquid. An example of the microstructure of a carburized surface is shown in Fig. 3.21, which represents the surface of a large AISI/SAE 4340 cast steel gear in the as-received, carburized, and heat treated conditions, respectively. This example is somewhat unique, because the steel is in the as-cast dendritic state, and the diffused carbon produces different microstructures within and outside the dendrites. The starting microstructure (Fig. 3.21a) consists of alternating regions of upper bainite and mixed upper bainite, pearlite, and ferrite. The lower bainite regions represent the dendrites, and the upper bainite/pearlite/ferrite regions repre-

Alteration of Microstructure / 63 sent the interdendritic areas. This is not the normal microstructure for carburizing, due to the segregation in the steel. The carburized microstructure (Fig. 3.21b) consists of alternating regions of pearlite in the dendrites and upper bainite in the interdendritic regions. The gear received a final heat treatment of austenitizing, quenching, and tempering, and the resulting microstructure is martensitic and is shown in Fig. 3.21(c). Even after heat treatment, there is some evidence of the original dendritic structure.

However, the heat treatment has produced a fairly uniform martensitic microstructure that will provide the hardness required at the surface of the gear.

Goal: Produce a More Ductile Cast Iron Malleabilizing. One of the most striking ways of intentionally altering microstructure is the process of producing malleable iron

Fig. 3.21 Microstructure of an AISI/SAE 4340 cast steel gear in the (a) as-cast condition consisting of dendrites of bainite (gray etching constituent) and

interdendritic regions of ferrite (light etching constituent) and pearlite (dark etching constituent), (b) carburized condition with pearlite (dark) and bainite (gray), and (c) quenched and tempered condition with martensite. 2% nital and 4% picral etch. 40⫻

64 / Metallographers’ Guide from white cast iron. As discussed in Chapter 2, white cast iron forms through the eutectic reaction of: Liquid → Cementite ⫹ Austenite

Also, because many cast irons are hypoeutectic, some of the liquid transforms to primary austenite. Both the eutectic austenite and the primary austenite transform at lower temperatures to ferrite plus cementite, or essentially pearlite (adding more cementite to the microstructure). Of course, with rapid cooling, the austenite could transform to martensite and/or bainite. An example of the microstructure of white iron is shown in Fig. 3.22. In this microstructure, the eutectic, called ledeburite, is shown as the region of small globules of pearlite (once austenite) in a matrix of cementite. The primary austenite can be seen as the larger pearlitic regions and associated cementite. The large amount of cementite renders white cast iron very brittle. Although white iron has many uses, early investigators found that through an extended heat treatment, the cementite can decompose into graphite and ferrite. The result is malleable iron. The microstructure of malleable iron can be seen in Fig. 3.23, where most of the carbon that was present in the cementite has formed irregular-shaped graphite particles called “temper carbon.” In this microstructure, the matrix is ferrite, which means that the remaining carbon diffused from the austenite into the temper carbon particles. If carbon diffusion is not fully complete, as in Fig. 3.24, a pearlitic malleable iron is produced. Thus, Fig. 3.24 shows a pearlitic malleable iron with rims of ferrite around the temper carbon in a matrix of pearlite. Because of its appearance, this type of microstructure is called “bull’s-eye” malleable iron. The heat treatment must be closely monitored to avoid incomplete malleabilization where large regions of undis-

Fig. 3.22 Microstructure of a white cast iron. Light etching constituent is

cementite, and the darker etching constituent is pearlite. The small pearlite globules surrounded by cementite in the center of the micrograph are a eutectic called ledeburite. 4% picral etch. 250⫻

sociated cementite still remain. Such a case is shown in Fig. 3.25, where gross cementite pockets (the rounded regions unattacked by the etch) still exist. The heat treatment process involves three important steps: (1) the nucleation of temper carbon during heating to the annealing temperature of 900 to 970 °C (1650 to 1780 °F); (2) the holding at the annealing temperature for an extended time (up to 20 hours, depending upon thickness and composition) to enhance the growth of the temper carbon by the conversion of the carbides to graphite; and (3) cooling as rapidly as practical to 740 to 760 °C (1360 to 1400 °F), a temperature regime below the critical temperature range. At this latter stage, called second-stage malleabilization, the remaining carbon in the austenite diffuses to the temper carbon particles. Manipulation of this second stage can produce a fully ferritic matrix, a fully pearlitic matrix, a ferritepearlite, that is, the bull’s-eye microstructure, or a martensiticpearlitic matrix (produced by quenching after the first-stage anneal). The metallographer must be able to disinguish malleable iron by the appearance of the graphite. The graphite is easily distinguished from the graphite flakes formed in gray cast iron. The irregular-shaped graphite in malleable iron is also distinguished from the rounded graphite nodules of ductile (nodular) iron discussed in the next section. Malleable iron is an extremely important member of the cast iron family because of its desirable mechanical properties. Since the early 1700s, malleable iron has provided the foundryman with a way to produce a nonbrittle form of cast iron that has been used in thousands of applications. Malleable iron is used in many automotive parts, such as transmission gears, connecting rods, steering gear housings, and universal joint yokes, and in those

Fig. 3.23 Microstructure of a malleable iron consisting of “temper carbon,” a form of graphite, in a matrix of ferrite. 2% nital etch.

200⫻

Alteration of Microstructure / 65 parts that require machinability and cold forming. Today, malleable iron is being replaced by nodular or ductile iron (discussed subsequently), because the extended heat treatment is no longer necessary. Inoculation. Another interesting example of intentionally altering the microstructure of a cast iron is the process of inoculation of the liquid iron. Inoculation is a process where a substance

is injected into a liquid bath. As we learned in the previous chapter, cast irons are usually associated with graphite flakes, as in gray cast iron, or cementite, as in white cast iron. By simply inoculating the liquid with magnesium (or cerium), the classic forms of gray and white cast iron are altered during solidification to produce a cast iron with nodules of graphite, as shown in Fig. 3.26. This type of cast iron is known as ductile cast iron or nodular

Fig. 3.24 Microstructure of a malleable iron consisting of “temper car-

Fig. 3.25 Microstructure of a malleable iron with an incomplete mallea-

bon,” a form of graphite, surrounded by ferrite in a matrix of pearlite. 2% nital etch. 200⫻

blization heat treatment. Large, rounded, light etching areas (arrows) are undissolved cementite. 2% nital etch. 500⫻

Fig. 3.26 Microstructure of a ductile iron showing graphite nodules (gray)

Fig. 3.27 Microstructure of a graphite nodule in ductile iron showing the

with rims of ferrite (white) in a matrix of pearlite. 4% picral etch.

100⫻

internal structure of the nodule radiating from the central nucleus. Polarized light. Unetched. 500⫻

66 / Metallographers’ Guide cast iron (the term “ductile iron” is preferred internationally). In the example shown in Fig. 3.26, the nodules are surrounded by a rim of ferrite in a matrix of pearlite. Some people refer to this microstructure as bull’s-eye ductile iron because of its appearance (as described previously for malleable iron). Other important ductile iron microstructures can have a fully ferritic matrix, a fully

pearlitic matrix, a ferritic-pearlitic matrix, a martensitic matrix, or a bainitic matrix. The latter two microstructures are developed through heat treatment. The ferritic and pearlitic microstructures can be formed in the as-cast condition or through heat treatment. At the very center of each nodule is a small particle of a complex magnesium (cerium) compound that provided the nucleation site for the growth of the graphite in the liquid. Figure 3.27 shows the graphite radiating from the central nucleating point. This micrograph was taken with polarized light to show the internal structure of the graphite nodule. The shape of the graphite cannot be altered by heat treatment once it has formed (other than being dissolved at high temperature or enlarged by carbon diffusion from the matrix). However, as mentioned previously, the matrix can be intentionally altered by heat treatment. The mechanical properties of the ductile iron would greatly depend on the type of matrix present. The metallographer must be able to intrepret the microstructure of ductile irons and understand the origin of the microstructure. Ductile iron is very useful for such parts as automotive crankshafts and universal joints, as well as industrial gears and valve and pump bodies.

The Unintentional Alteration of Microstructure in Steels and Cast Irons Quite often, a metallographer is confronted with a situation where the microstructure of a steel or cast iron component has been altered by a process that has gone out of control or by a heat treatment that was incorrectly carried out. Recognition of an unintentionally altered microstructure is an important first step in

Fig. 3.28 Microstructure of an AISI/SAE 1080 steel bloom showing exces-

sive decarburization along (a) the bloom surface and a shallow crack and (b) a magnified view at the crack tip. Note that ferrite has nucleated on the prior austenite grain boundaries. 4% picral etch. (a) 40⫻ and (b) 100⫻

Fig. 3.29 Microstructure of an AISI/SAE 1045 steel billet showing a

decarburized layer of uniform thickness (a zone of complete decarburization) consisting of columnar grains of ferrite. The microstructure below decarburized layer is ferrite and pearlite. 2% nital and 4% picral etch. 32⫻

Alteration of Microstructure / 67 finding a solution to a problem. This section of the chapter illustrates some examples of unintentionally altered microstructures that are fairly common. What caused the altered microstructure is discussed in order to give the metallographer some guidance in problem solving. The types of altered microstructures are broadly classified as being altered by improper heat treatment or as being altered by handling or in service.

Microstructures Altered by Improper Heat Treatment Decarburization is the loss of carbon from the surface of a steel or cast iron component. Many times during heat treatment, the surface of a part loses carbon by a reaction with oxygen

Fig. 3.30 Microstructure of an AISI/SAE 1045 steel billet showing (a) a

large oxidized crack in the decarburized zone and (b) internal oxidation along a small crack (see arrow in a). 2% nital and 4% picral etch. (a) 32⫻ and (b) 500⫻

contained in the heat treating furnace atmosphere (oxygen is present in air, carbon dioxide, and water vapor). Hydrogen can also decarburize steel or cast iron. Just heating a steel or cast iron in air will cause some decarburization. However, detrimental decarburization usually requires an extended time in an oxygenrich atmosphere. For example, excessive decarburization in an AISI/SAE 1080 steel bloom is shown in Fig. 3.28(a). This steel bloom was heated to 1275 °C (2330 °F) in a gas-fired furnace for six hours. Note the high volume fraction of ferrite at the bloom surface and along the surface of the small crack. Also note that the ferrite penetrates deeper into the steel, as shown in the magnified view in Fig. 3.28(b). The ferrite occurs along the prior austenite grain boundaries. Below the grain-boundary ferrite, the microstructure is 100% pearlite. Grain boundaries provide an ideal path for diffusion of oxygen into the steel. In this example, carbon was removed from the bloom surface and crack surface during the six hours at temperature. Upon transformation of the austenite during cooling, ferrite formed in those areas of carbon depletion. This condition of decarburization is not desired, because the surface hardness is much lower than the expected hardness for an AISI/SAE 1080 steel. Most heat treating shops prevent decarburization by heating steels and cast irons in a endothermic gas atmosphere where most of the decarburizing agents, that is, oxygen, have “reacted” before the gas enters the furnace. Another form of decarburization is shown in Fig. 3.29. In this case, hot-rolled AISI/SAE 1045 steel billets were slow cooled

Fig. 3.31 Microstructure of an improperly carbonitrided AISI/SAE 1117

steel shaft showing various layers below the treated surface. The layers “A” through “E” are shown at higher magnification in Fig. 3.32. 2% nital and 4% picral etch. 40⫻

68 / Metallographers’ Guide from the austenite phase region over a period of several days to prevent surface cracking and to remove hydrogen by diffusion (the hydrogen originated in the steelmaking process). As a result of the very slow cooling process, the carbon diffused from the surface in a uniform layer. This condition was due to the slow cooling rate through the austenite-to-ferrite transformation region where accelerated diffusion of carbon occurs. At these temperatures, the diffusion of carbon in ferrite is more rapid than diffusion of carbon in austenite. Also, the incentive for carbon diffusion is greatly increased, because the solubility of carbon in ferrite is very low compared to that in austenite. Thus, there is a high incentive (called

driving force) for carbon to leave. In a sense, the transformation to ferrite controlled the carbon diffusion process, and one can see that columnar ferrite grains formed perpendicular to the surface during this transformation process. At the interface between the equiaxed ferrite/pearlite region and the columnar ferrite surface region, the billet was cooled from a temperature too low for effective diffusion, and the process stopped. As in the case shown in the first example, this condition is not desirable, because the surface hardness will be well below that of an AISI/SAE 1045 steel. The metallographer should have little problem recognizing decarburization in a medium- or high-carbon steel. Many times,

Fig. 3.32 Magnified view of the various layers in the carbonitrided AISI/SAE 1117 steel shaft in Fig. 3.31. (a) Outer layer of retained austenite and plate

martensite (needlelike constituent), (b) a layer of plate martensite and some retained austenite, (c) a layer of fine lath martensite, (d) a layer of coarse lath martensite, and (e) the core of ferrite and pearlite in bright field and (f) in Nomarski (arrows point to epitaxial ferrite). (a), (b), and (c) sodium metabisulfite etch and (d), (e), and (f) 2% nital and 4% picral etch. 800⫻

Alteration of Microstructure / 69 the metallographer is required to measure the depth of decarburization in a steel part. The ASTM standard E 1077 describes the proper procedures. In the case shown previously in Fig. 3.29, the measurement would be fairly easy and would involve measuring the depth of the zone of complete decarburization. However, in the case shown in Fig. 3.28, the measurement is a little more difficult, because it includes the measurement of the partial

decarburization zone into the depth where the proeutectoid ferrite eventually disappears. Internal Oxidation. During heat treatment, exposure of steel or cast iron to an oxygen-rich atmosphere for a long time and at a high temperature can cause oxygen to penetrate austenite grain boundaries and react with carbon and other elements, such as silicon, manganese, and aluminum, that are contained in the steel

Fig. 3.32 (continued) (e) the core of ferrite and pearlite in bright field and (f) in Nomarski (arrows point to epitaxial ferrite). (e) and (f) 2% nital and 4% picral etch. 800⫻

Fig. 3.33 A manganese sulfide stringer (dark gray) appearing as a “pearl

necklace”-type morphology (see long arrow) at a prior austenite grain boundary in a 0.7% C-3% Cr steel forging exposed to 1170 °C (2500 °F) for 1.5 h. Pearlite (the dark etching constituent) has nucleated on the sulfide particles. The matrix is plate martensite (note the microcracks—see stubby arrows). 4% picral and HCl etch. 500⫻

Fig. 3.34 A SEM micrograph of a fracture surface of the 0.7% C-3% Cr steel forging in Fig. 3.33 showing a manganese sulfide dendrite.

2000⫻

70 / Metallographers’ Guide or cast iron. Figure 3.30(a) shows a condition of internal oxidation that occurred when a small surface crack formed on the surface of an AISI/SAE 1045 steel billet that was exposed to an oxygen-rich atmosphere for an extended time at 1260 °C (2300 °F). The enlarged view in Fig. 3.30(b) shows a region of small, spherical particles around an oxidized grain boundary extending from the surface crack. Electron probe microanalysis results reveal that these spherical particles are Si-Mn-Al oxides that formed by oxygen reacting with the silicon, manganese, and aluminum in

the steel. These elements are less noble than iron, that is, they oxidize in preference to iron, because they have a higher affinity for oxygen than iron. (The term “less noble” means that the elements have more negative potential than iron on the electromotive series of elements. These elements will oxidize in the presence of iron. Elements more noble than iron are copper, gold, and platinum.) This condition is called internal oxidation (selective oxidation), because the oxidation process took place

Fig. 3.36 Microstructure of the same steel in Fig. 3.35 showing voids

(rounded, black regions—see arrows) that formed at the junctures of the prior austenite grain boundaries. White-appearing areas at the grain boundaries indicate phosphorus segregation. 4% picral and HCl etch. 100⫻

Fig. 3.35 (a) A manganese sulfide dendritic array in the 0.7% C-3% Cr

forging steel exposed to 1170 °C (2500 °F) for 1.5 h and (b) type II manganese sulfide inclusions in an as-cast ASTM A36 steel slab. Unetched. 1000⫻

Fig. 3.37 A SEM micrograph showing a void in the fracture surface of the steel shown in Fig. 3.36. Note the rounded features.

Alteration of Microstructure / 71 internally in the steel as opposed to surface oxidation where an oxide scale forms. Internal oxidation is commonly found when steel is exposed for extended times to an oxygen-containing atmosphere at temperatures above 1200 °C (2200 °F). Grain boundaries and surface cracks enhance the flow of oxygen from the steel surface to the interior regions. The metallographer should be able to determine if a steel has internal oxidation. In some cases, a composition check by the microprobe may be needed to confirm that the particles contain manganese, silicon, or other elements less noble than iron. If internal oxidation is found, it usually means that a heat treatment process may be out of control, that is, the steel was exposed to an excessive temperature for an extended period of time. Improper Carbonitriding. An example of an improperly carbonitrided surface is shown in Fig. 3.31 which represents a quenched AISI/SAE 1117 steel shaft (this is a resulfurized steel that contains a high density of manganese sulfide inclusions for improved machinability). This steel shaft has been carburized at 955 °C (1750 °F) for 5.5 hours to a carbon potential of 0.80, that is, with a case of 0.80% C, followed by nitriding in ammonia (NH3) for 45 minutes at 845 °C (1550 °F). In this example, the diaphragm on the gas cylinder of the ammonia tank ruptured, causing a much higher concentration of ammonia gas in the furnace for the entire treatment. Because of the excess concentration of ammonia, the steel shaft developed a very high level of nitrogen (along with carbon) at the surface, and retained austenite was produced. The presence of retained austenite means that the nitrogen and carbon content decreased the martensite start (Ms) temperature to a point well below room temperature. In Fig. 3.31, one can see many distinct microstructural zones from the carburized surface inward. The actual outer surface zone consists of retained austenite, appearing as the white nonetching constituent in Fig. 3.32(a). The retained austenite has a hardness of 298 HK (Knoop). There are also many voids that developed on the austenite grain boundaries. These voids are the result of excess molecular nitrogen (N2) that formed as gas pockets at the austenite grain boundaries. The next layer consists of retained austenite and plate martensite, with a hardness of 640 HK (also shown in Fig. 3.32b). The plate martensite appears as the dark, needlelike constituent in this figure. The next inner zone consists of fine lath martensite (Fig. 3.32c), with a hardness of 742 HK. The next layer consists of coarser lath martensite (Fig. 3.32d), with a hardness of 511 HK. The core consists of ferrite and pearlite (Fig. 3.32e), with a hardness of 192 HK. An interesting feature of the core microstructure, shown in Fig. 3.32(e) is the evidence of epitaxial ferrite (discussed previously), as revealed by differential interference contrast (Nomarski) in Fig. 3.32(f) (see arrows). This indicates that the shaft was exposed for a period of time to temperatures within the intercritical region. From the previously mentioned hardness values, it is obvious that the surface is far too soft, due to the presence of retained austenite. Although stresses in service may transform the surface austenite to martensite, this is evidence of a very poor carbonitriding practice. This process is not aimed at producing retained austenite at the surface. If the amount of austenite is in excess of 50%, not only will the hardness be lower, but the bending fatigue

resistance will be significantly lowered. The skilled metallographer should be able to identify the retained austenite in a carburized part and make recommendations about the carburizing heat treatment. Many times, a simple microhardness measurement is all that is needed, as shown previously. However, if further confirmation is required, x-ray diffraction can easily detect the presence of retained austenite. Exposure to Excessively High Temperatures. On occasion, steels are heated to temperatures that are too high, and the result can be very deleterious to mechanical properties. The terms that are used to describe the condition of a steel that has been exposed to excessive temperatures are “overheating” and “burning.” These are rather subjective terms, but they are widely used in industry. In this section, we consider overheating and burning as simply different degrees of exposure to high temperature; overheating is essentially the first stage of microstructural alteration, and burning is the second, more severe, irreversible stage. In normal heat treating operations where standard practices and procedures are followed, the steel will not become damaged. However, if there is an accidental temperature excursion in the heat treating furnace, the steel could be affected. Also, in forging operations where alloy steels are heated to rather high temperatures, that is, above 1315 °C (2400 °F), there is a danger of overheating/burning taking place. If overheating takes place, the steel may be salvaged by an additional low-temperature heat treatment. However, if burning takes place, it is not reversible, and the steel must be scrapped. When a steel has experienced overheating/burning, there are important microstructural changes that have occurred, which, in a way, leave classic internal markers or identifiers for the metallographer to recognize. The example that we use, a 0.7% C-3% Cr steel forging heated to 1315 and 1370 °C (2400 and 2500 °F) for 1.5 hours, will have these identifiers. These include manganese sulfide dissolution, segregation at austenite grain boundaries, incipient melting of lower-melting constituents, and voids that form at grain-boundary junctions (triple points). Instead of attempting to distinguish between overheating and burning, we examine the previously mentioned alterations of microstructure. First of all, it is important to understand what happens when a steel is exposed to excessively high temperatures. In nearly all cases, the material at and near the austenite grain boundaries is affected; for example, manganese sulfides change morphology, and phosphorus segregates to the grain boundaries. For example, the exposure to high temperature causes manganese sulfide stringers to dissolve in the austenite. During the initial stage of exposure, the manganese sulfide stringers appear to break up into a “pearl neckace”-type morphology, seen in Fig. 3.33. This breakup is usually associated with the manganese sulfide stringer dissolving in the surrounding austenite. However, is this really what happened? When viewed in three-dimensions, this may not be the case. For example, when the sample is fractured and examined in the scanning electron microscope (SEM), one can see dendrites of manganese sulfides, an example of which is shown in Fig. 3.34. In this SEM micrograph, there is a primary dendrite arm (the main stem aligned left-to-right in the micrograph), with a number of secondary arms aligned perpendicular to the primary arm. If the metallographer happened to section the specimen through the secondary arms, the resulting microstructure would

72 / Metallographers’ Guide reveal a necklace-type morphology, shown in Fig. 3.33. Thus, the metallographer can sometimes be deceived by examining only one plane of a given sample. The dendrites can occur when regions at and around the austenite boundaries melt as a result of the excessive temperature. The melting takes place because these regions contain a higher level of alloy concentration and lowermelting constituents, such as manganese sulfide. This incipient melting process is called liquation. These regions were the locations of the last liquid to solidify during the initial solidification process, for example, during continuous casting. Once melting has taken place, the manganese sulfides can solidify upon cooling, as the dendritic morphology shown in Fig. 3.34. Also in Fig. 3.33, the manganese sulfide particles have nucleated pearlite (dark etching constituent), and the plate martensite matrix (gray etching constituent) contains microcracks (see arrows). Microcracks, which form when two or more martensite plates strike each other during transformation, are discussed in an upcoming section. These latter microstructural changes are not necessarily the result of the excessive temperature, because these changes can also occur during normal heat treatment. Another view of a possible manganese sulfide dendritic array can be seen in Fig. 3.35(a) for the same 0.7% C-3% Cr steel. There is similarity between these manganese sulfide dendrites formed during liquation and subsequent solidification, and eutectic manganese sulfide colonies (called type II sulfide inclusions) that form during solidification of a steel casting. For example, the manganese sulfide inclusions in Fig. 3.35(b) represent type II sulfides that formed in a continuous-cast ASTM A36 steel slab. In this case, the last liquid to freeze contained a higher concentration of manganese and sulfur, and the liquid solidified as a eutectic of manganese sulfide and iron. In a continuous-cast slab, this morphology will be altered into stringer-type inclusions during the hot-rolling process. However, in the case of the steel forging described previously, the manganese sulfide colonies are in their final form and will not be further altered by hot deformation. Thus, this morphology, which is deleterious to mechanical properties (particularly notch toughness and fatigue), will render the steel forging useless, and it must be scrapped. This condition is usually associated with the term “burned steel.” Another deleterious burned condition created by excessively high temperatures is shown in Fig. 3.36, where voids have formed at grain-boundary junctions (triple points) in the previously mentioned forging steel heated to 1370 °C (2500 °F). In this micrograph, there are four voids that can be identified by their rounded features (see arrows indicating two of the larger voids). When examined in the SEM, the rounded features become more evident, as shown in Fig. 3.37. This irreversible condition was created by localized melting that resulted in shrinkage porosity or entrapped hydrogen gas. This condition is unacceptable, and the part must be scrapped. A more subtle microstructural alteration that occurs during excessive heating is caused by sulfur and phosphorus segregation at the austenite grain boundaries. The white-appearing grain boundaries in Fig. 3.36 would indicate segregation. An example of grain-boundary segregation is shown at higher magnification in Fig. 3.38. Sometimes this condition is difficult to detect in the light microscope, and special etching procedures may have to be

used to reveal the segregation. However, these etching techniques are, at times, unreliable. Here, the SEM can be useful. The fracture surface of a steel with phosphorus or sulfur segregation will reveal intergranular austenite grain surfaces similar to those found in temper embrittlement (described in an upcoming section). A simple heat treatment can eliminate the segregation. Hot shortness is a term used to describe grain-boundary separation that can create a rough surface condition in hot-rolled and forged steel. In many cases, hot shortness results in surface cracks that do not heal during the hot-rolling process. Internal grain-boundary separations can also be created from low-meltingpoint inclusions. Hot shortness is caused when impurities or residual elements in the steel are liquid at hot-rolling temperatures. Two common examples of hot shortness are caused by copper, which is molten above 910 °C (1670 °F), and iron sulfide, FeS, which is molten above 1190 °C (2170 °F). Copper enters the steelmaking process through scrap and thus is a potential problem in electric furnace steelmaking operations. The copper from the scrap cannot be removed in the steelmaking process and thus is carried through into the final product. Many ASTM specifications limit residual copper to 0.40%. Even at this level, problems of hot shortness can exist. The mechanism for hot shortness caused by copper starts in the formation of oxide scale on the steel surface. Scale forms during continuous casting and during reheating in soaking pits and reheat furnaces. Because copper is more noble than iron on the electromotive series, it cannot be oxidized in the presence of iron. Therefore, as the scale grows on the steel surface, the copper remains behind and accumulates at the scale/steel interface. Many times, the copper will alloy with other noble elements, such as nickel, tin, arsenic, and antimony. All these elements are impurities in the steel (although in some steels, nickel may be added as an alloying element). The liquid alloy has a tendency to penetrate grain boundaries that are exposed at the scale/steel interface. Because the alloy is molten at rolling temperatures, the grain boundaries easily separate to form surface cracks, allowing more scale to form in the crack. In steels that contain copper as an alloying element, for example, ASTM A710, nickel is added in quantities at least 50% of the copper content to prevent hot shortness. The nickel increases the melting point of the copper, and the copper-nickel alloy that forms becomes part of the scale during scale growth. Thus, when the steel is hot rolled, the copper-nickel alloy is removed along with the scale. Figure 3.39 shows surface cracks on an AISI/SAE 1035 steel bloom that were caused by an excessive copper level in the steel. This form of hot shortness can be controlled by lowering the copper content of the steel, that is, using a scrap charge that is lower in copper. In the light microscope, the copper can be seen in its elemental form as reddish “fingers” extending along grain boundaries, as shown in Fig. 3.40. In this micrograph, one can see also a few of the copper droplets at the scale/steel interface. Many times, to identify the copper at the scale/steel interface, the electron probe microanalyzer can be used to provide x-ray maps of the copper concentration. The second source of hot shortness is due to the presence of iron sulfides. Iron sulfides can develop in a steel if the manganese level is insufficient to combine with all the sulfur present in the steel. Historically, one of the main reasons for adding manganese to

Alteration of Microstructure / 73 steel was to combine with sulfur in the form of manganese sulfide, MnS, to prevent hot shortness. Without sufficient manganese, the sulfur would combine with the iron and create iron sulfides. Iron sulfide has a much lower melting point than manganese sulfide and thus remains liquid at rolling temperatures. Also, iron sulfides are more brittle than manganese sulfides and degrade the mechanical properties of the steel. Manganese sulfides, on the other hand, are easily deformed at rolling temperatures and are not as deleterious to mechanical properties.

Internal hot shortness can be created by the presence of iron sulfides. Iron sulfides have a lighter gray appearance than manganese sulfides, as shown in Fig. 3.41. In this micrograph, taken from a normal AISI/SAE 1015 steel casting, the iron sulfide constituent surrounds a manganese sulfide particle. The dove gray color of a normal manganese sulfide inclusion can be seen in Fig. 3.42 for comparison. In Fig. 3.41, small cracks can be seen in the brittle iron sulfide rim, and the rim has separated from the steel matrix. The iron sulfide has formed around (nucleated on) the manganese sulfide, because it was liquid when the manganese sulfide was solid during the solidification process. Upon heating to

Fig. 3.38 Microstructure of segregation along a prior austenite grain

Fig. 3.39 Macrograph of an AISI/SAE 1035 steel showing surface cracking

boundary in the 0.7% C-3% Cr steel shown in Fig. 3.36. 1000⫻

due to a hot-shortness condition caused by copper. 2⫻

Fig. 3.40 Microstructure of the steel shown in Fig. 3.39 with elemental

copper along grain boundaries (see arrow). This is a condition known as hot shortness. Note: the micrograph is out of focus, because the ferrite matrix was chemically attacked by the etch, thus leaving the focused copper in relief. 2% nital etch. 500⫻

Fig. 3.41 Microstructure of an AISI/SAE 1015 casting showing iron sulfide surrounding a manganese sulfide inclusion. Unetched. 2000⫻

74 / Metallographers’ Guide hot-rolling temperatures, the iron sulfide would remelt and create an internal flaw. This condition can lead to internal cracking. Because they are undesirable in steel, a metallographer must be able to identify iron sulfides. However, with modern steelmaking technology, it is quite rare to see iron sulfides in steel. Sensitization. Stainless steels rely on their chromium content to prevent corrosion. Generally, about 12% Cr is needed for this task. One example of unintentional microstructural alteration that

Fig. 3.42 Microstructure of an AISI/SAE 1020 cast bloom with a normal

manganese sulfide inclusion. Note the color difference of iron sulfide in Fig. 3.41. Unetched. 1000⫻

Fig. 3.43 Microstructure of an AISI/SAE 316 stainless steel showing

sensitization. Note the chromium carbides at the austenite grain boundaries. The steel was exposed to 675 °C (1250 °F) for 12 days. HCl/HNO3/H2O etch. 1000⫻

has serious consequences for a stainless steel is the result of exposure to temperatures in the range of 425 to 870 °C (800 to 1600 °F). During exposure, chromium carbides form at the grain boundaries and deplete the regions near the boundaries of chromium. The longer the exposure, the greater the depletion of chromium, until eventually the level drops below 12% locally, and corrosion along grain boundaries can result. The process of chromium loss is called sensitization. An example of sensitization in an AISI 316 austenitic stainless steel sheet is shown in Fig. 3.43. In this micrograph, the austenite grain boundaries are “decorated” with chromium-rich carbides (Cr,Fe)23C6. This stainless steel sheet was exposed to 675 °C (1250 °F) for 12 days. A more severe example of sensitization is shown in Fig. 3.44, where the same AISI 316 stainless steel sheet was exposed to 730 °C (1350 °F) for two months. Here, the chromium carbides have precipitated on annealing twins in addition to the austenite grain boundaries. Because of the long-term exposure, large chromium carbides have also formed. The metallographer must be aware of the possibility of a sensitized condition in a stainless steel that has been exposed to an elevated temperature. In the early stages, it is very difficult to detect the carbides at the grain boundaries, and careful observation at high magnification is required. It is important to know that the condition of sensitization can be eliminated by another heat treatment at temperatures above 870 °C (1600 °F), where all the carbides dissolve in the austenite. A typical treatment is heating a sensitized stainless steel to 980 °C (1800 °F) for four hours, followed by rapid cooling. Temper Embrittlement. If an alloy steel is slow cooled from above 595 °C (1100 °F), it may become embrittled. The specific embrittlement temperature range is about 375 to 575 °C (710 to 1070 °F). Indications of embrittlement are evident in the Charpy

Fig. 3.44 Microstructure of an AISI 316 stainless steel showing severe

sensitization. Exposed to 730 °C (1350 °F) for two months. HCl/HNO3/H2O etch. 1000⫻

Alteration of Microstructure / 75 V-notch impact test, where very low values of toughness are recorded. The embrittlement takes place at the austenite grain boundaries. Phosphorus segregation to the austenite grain boundaries is usually attributed as the cause of the problem. An easy way to detect temper embrittlement is to observe the fracture surface of a broken Charpy bar or other fractured surface in the SEM. An example of such a fracture surface can be seen in Fig.

Fig. 3.45 A SEM micrograph of the fracture surface of a 3% Cr steel

showing intergranular fracture, indicating a condition of temper embrittlement. 500⫻

Fig. 3.47 Microstructure of an AISI/SAE 1080 steel showing a microcrack

at the center of the micrograph (see arrow) in a martensite plate. 12% sodium metabisulfite tint etch. 1000⫻

3.45. This example is a 3% Cr steel. Many of the austenitic grain boundaries can be seen as the flat, faceted surfaces in the SEM micrograph. This type of fracture is called intergranular fracture. In this case, phosphorus has accumulated on the austenite grain boundaries and created a weakness in the bond between the austenite grains. In most chromium-bearing steels, an addition of 0.5 to 1.0% Mo is sufficient to eliminate the problem. The

Fig. 3.46 Microstructure of a 1.4% C steel showing numerous micro-

cracks (dark lines) in the martensite plates. The white-appearing constituent is retained austenite. 12% sodium metabisulfite tint etch. 500⫻

Fig. 3.48 Microstructure of a cold-drawn and spheroidized AISI/SAE 1095

steel bar showing regions of graphite (elongated, dark bands). 4% picral etch. 1000⫻. Courtesy of S. Lawrence, Bethlehem Steel’s Homer Research Center

76 / Metallographers’ Guide molybdenum changes the activity of phosphorus in the steel (austenite) and minimizes its ability to segregate to the grain boundaries. Microcrack Formation. When higher-carbon steels (above about 0.6% C) are rapidly cooled from the austenitic state, plate martensite can form. Plate martensite differs from the lath martensite that forms in lower-carbon steels. The differences have been described in Chapter 2. One problem with plate martensite is that it is very brittle. When two or more martensite plates strike

each other during transformation, the plates can crack at the point of impingement. The result is a microcrack. An example of numerous microcracks that have formed in a 1.4% C steel is shown in Fig. 3.46. In this micrograph, one can see the small cracks in the martensite plates. These cracks can grow to larger cracks when a component with microcracks is used in service, especially under a cyclic loading environment. Cyclic loading will eventually lead to a fatigue failure. A microcrack can also be seen in Fig. 3.47, representing both lath and plate martensite in a

Fig. 3.49 Microstructure of a 1.2% C steel that has formed graphite (dark etching constituent), or “graphitized,” after exposure to 700 °C (1290 °F) for (a) 190, (b) 375, and (c) 565 h. 4% picral etch. 500⫻. Courtesy of B. Lindsay and A.R. Marder, Lehigh University

Alteration of Microstructure / 77 quenched AISI/SAE 1080 steel rod containing centerline segregation (a region of higher hardenability enhancing the formation of martensite). In this figure, the few martensite plates can be seen (arrows). Whenever a metallographer observes plate martensite in a microstructure, he or she should carefully look for microcracks, because their presence will be an important indication of a potential problem. If microcracks form, the part may have to be scrapped. It is possible that a component with microcracks can be salvaged by annealing for an extended period of time to allow healing of the cracks. This treatment, however, does not always work. Graphitization of Steel. It is obvious that cast irons contain free graphite. However, it is also possible for a steel to contain graphite. An example of graphite formation in steel is shown in Fig. 3.48, where the graphite is shown as dark, elongated particles. This steel is AISI/SAE 1095 that has been annealed, hot rolled, and spheroidized. However, the spheroidization treatment was extended too long, and the cementite (Fe3C) phase decomposed to pure carbon (graphite) and iron. This graphitization process occurs when a steel is held for long times at temperatures just below the lower critical temperature (A1). The optimal temperature range for graphitization is between 600 and 700 °C (1110 and 1290 °F). Graphitization can also occur in steam power plant components that are exposed to elevated temperatures for extended periods of time. Graphitization is not desirable, because the regions of graphite are very soft and provide locations of weakness within the steel. The development of graphite in a 1.2% C steel is shown in Fig. 3.49. This steel was given a spheroidizing heat treatment at 700 °C (1290 °F) for 190, 375, and 565 hours. At 190 hours (Fig. 3.49a), very small regions of graphite begin to form on cementite

Fig. 3.50 Microstructure of a 1.2% C steel that was graphitized after

exposure to 700 °C (1290 °F) for 190 h, then heat treated at 980 °C (1800 °F) to dissolve the graphite. Note void (dark region) in center of micrograph. 4% picral etch. 500⫻

particles. At 375 hours (Fig. 3.49b), more graphite forms, replacing some of the cementite particles, and at 565 hours (Fig. 3.49c), all the cementite particles have decomposed into graphite. This latter treatment is unusually long, but illustrates the graphitization process. The only way to remove graphitization is to retreat the steel in the austenite regime to dissolve the graphite and to redistribute the carbon uniformly in the steel. From the iron-carbon equilibrium diagram shown in Fig. 3.2, one can see that carbon is very soluble in austenite. Once the graphite dissolves in the austenite, the steel can be cooled to room temperature. Figure 3.50 shows the result of reversing the graphitization process by reheating the 1.2% C steel at 980 °C (1800 °F) for two hours. The dark, rounded region in the center is a void. One of the results of dissolving the graphite into austenite is the development of voids, because of differences in specific volume between graphite and austenite.

Microstructures Altered during Cutting/Machining Many times, the microstructure of a steel or cast iron is unintentionally altered during a cutting operation using an oxygen-acetylene torch. The microstructural alteration could be detrimental. This section shows examples of how a microstructure can be altered simply by cutting the steel. Improper Cutting. Many steel products are cut to a specific size or length by an oxygen-acetylene or plasma-arc cutting torch. However, this practice may not always be the best procedure. Let’s look at an example of a plasma-arc-cut steel plate. Figure 3.51 shows the microstructure at the cut surface of an AISI/SAE 1020 steel plate. The dark surface region is 100% lath martensite, as shown in Fig. 3.52(a). Adjacent to the lath martensite surface

Fig. 3.51 Microstructure of a plasma-arc-cut surface of an as-rolled

AISI/SAE 1020 steel plate showing surface damage (top). Regions “A”, “B”, “C” are shown at higher magnification in Fig. 3.52. 2% nital and 4% picral etch. 100⫻

78 / Metallographers’ Guide region is a region of lath martensite and ferrite, as shown in Fig. 3.52(b). The base microstructure is shown in Fig. 3.52(c). The lath martensite layer has a hardness of 349 HK (35 HRC), whereas the base steel has a hardness of 165 HK (80 HRB). This hard layer at the surface may cause problems in machining the material. Residual stresses will also develop, because of the formation of a martensitic layer in a ferritic-pearlitic component. These stresses and the higher hardness level can be reduced by a stress-relief heat treatment. In a higher-carbon steel, the martensite would be in the form of plate martensite, which would have a tendency to form

microcracks during transformation (as shown in a previous section on microcracks). Microcracks form when martensite plates impinge on one another. This condition would be unacceptable, because the microcracks would create larger cracks during service. Improper Machining. Machining can induce cold work into the surface layers of a part. An example is shown in Fig. 3.53(a), which represents the surface layer of a 13 mm (0.5 in.) diameter bar of AISI 316 stainless steel. The surface layer consists of a high density of mechanical twins that were induced during machining.

Fig. 3.52 Microstructure of the affected layers in the plasma-torch-cut AISI/SAE 1020 steel plate in Fig. 3.51 showing (a) lath martensite at the surface, (b) lath martensite and ferrite just below the surface, and (c) ferrite and pearlite of the base steel. 4% picral etch. 500⫻

Alteration of Microstructure / 79 The microstructure at the center of the bar is shown in Fig. 3.53(b). Here, the microstructure contains only a few twins. In an effort to remove the mechanical twins at the surface layers, the bar was heat treated at 800 °C (1470 °F) for 0.5 hours. Unfortunately, the mechanical twins were not removed, and the bar was inadvertently sensitized, as shown in the high-magnification micrograph in Fig. 3.54. Sensitization was discussed in the section “Microstructures Altered by Improper Heat Treatment.” In this micrograph, one can see carbides that formed at the grain boundaries and twins. A very high magnification in the light

microscope was necessary to reveal this sensitized condition. This is a caution to the metallographer who could miss the subtle features if only a cursory examination is used. This is a unique sample in that not only is the surface cold worked, but the remaining bar is sensitized. The only way to salvage this material would be to anneal the steel to remove both the cold work and the sensitized condition.

Microstructures Altered under Service Conditions In many cases, the original microstructure is altered in service. When steels and cast irons are exposed to high temperatures, oxidation and corrosion processes produce microstructural changes that can adversely affect the properties of the material. Also, deformation and friction can create high temperatures in steel. These temperatures can alter microstructure. The metallographer must be aware of the effects of these service conditions on microstructure. A few examples are given in this section to illustrate some of these changes. Friction Effects. A metallographer examining a specimen taken from a railway track should expect to find a fully pearlitic microstructure. However, sometimes the pearlitic microstructure is unintentionally altered in an unusual manner in service. For example, the running surface of a rail (the surface in contact with the railway wheel) can be heated into the austenitic range by the friction of a seized or slipping locomotive wheel rubbing along the rail surface. This can take place during slippage, where the locomotive wheels are turning, but the train does not move. Also, severe friction could occur if the locomotive wheel is stationary during emergency breaking, with the wheels sliding along the track. The surface damage to the rail is termed a “wheel burn.”

Fig. 3.53 Microstructure of a machined AISI 316 stainless steel bar

showing (a) deformation bands at the surface and (b) annealing twins at the center. Electrolytic etch of 10% oxalic acid in water, stainless steel cathode, 6V, 10 s. 200⫻

Fig. 3.54 A sensitized condition found in the central region of the AISI

316 stainless steel bar in Fig. 3.53. Note the carbides on the grain boundaries and annealing twins. Electrolytic etch of 10% oxalic acid in water, stainless steel cathode, 6V, 10 s. 1500⫻

80 / Metallographers’ Guide Upon cooling from the austenitic field, the microstructure that forms is not pearlite, because the cooling rate is very high due to the self-quenching effect of the underlying steel. Martensite and bainite layers form from the austenite. There is also retained austenite contained within the martensite layer. An example of a wheel burn is shown in Fig. 3.55. At this low magnification, the microstructural constituents are not resolved, but one can see a shallow, nonetching white layer at the surface, several surface cracks due to the thermal stresses, and a dark layer followed by a light etching layer and a gray layer. The thin white layer and a portion of the dark etching layer are shown in Fig. 3.56(a). The white layer has been the focus of many metallurgical investigations, and its identity is controversial. It is usually considered as martensite. In some cases, it may be highly deformed ferrite. In the example shown in Fig. 3.56, the white layer is martensite. The dark etching layer in Fig. 3.56(a) and the region just below is tempered martensite, as shown in Fig. 3.56(b). The light etching regions within the martensite are retained austenite. The light etching region below the tempered martensite is as-quenched martensite and mixtures of as-quenched martensite and pearlite, as seen in Fig. 3.56(c). The dark etching constituent is the base pearlitic microstructure (see Fig. 3.56d). This example shows the

Fig. 3.55 Microstructure of a “wheel burn” condition on a railroad rail

(eutectoid steel) showing the surface “white layer” and other layers. These layers are shown at higher magnification in Fig. 3.56. 2% nital etch. 32⫻

metallographer the complexity of microstructures that can develop within one sample. This is why it is important to understand the origin of microstructures in steels and cast irons. This basic understanding is vital to metallographic interpretation. The previous example is a case where the light microscope has reached the limit of resolving power, and an electron microscope is necessary to add to the interpretation through higher magnification and resolving power. Void Formation during Creep. When steel and cast iron components are exposed to a stress or load for a long period of time at an elevated temperature, a process known as “creep” can take place. Generally, the stress is well below the normal yield strength of the material. Metal flow takes place by mechanisms related to grain-boundary sliding and void formation at grain boundaries. An example of the beginning of the creep process is shown in Fig. 3.57. The alloy is a 0.28% C, 2.5% Ni, 1 % Cr, 1% Mo steel that was held at 565 °C (1050 °F) under a stress of 210 MPa (30 ksi). Voids can be seen nucleating and growing at grain boundaries in Fig. 3.57(a). In Fig. 3.57(b), the voids begin to link up, and in Fig. 3.57(c), they form separations along grain boundaries. Once this grain-boundary separation occurs, the material will begin to fail at an accelerated rate. Under these conditions, it took about 5000 hours to develop the stage of extensive grain-boundary separation. Creep occurs in parts such as rotors in steam-powered generators and in the blades of aircraft gas turbine engines. Once in the final stages of creep, the parts are prone to fail in a catastrophic manner. Special techniques are used to monitor the life of parts that are exposed to environments that promote creep. One of these techniques is field metallography, where a small area of the part is polished and etched while the part is still in service. Field metallography is used when the part or a sample cannot be brought to the metallographic laboratory. Hydrogen Damage. Damage due to hydrogen gas can occur under many circumstances. Hydrogen can be picked up in steels through the steelmaking process where the liquid steel is exposed to moisture from the furnace/ladle/tundish refractories, from exposure to humidity in the air, and from moisture in alloying additions. Also, in cast steels and cast irons, sand binders and cores containing hydrocarbons can break down to form hydrogen gas upon exposure to molten steel and cast iron. Hydrogen can also be absorbed into steel by exposure to acids and electrolytic plating processes. Hydrogen can also enter a weld from moistureladen electrodes. Thus, hydrogen can come from many sources, and if hydrogen damage is found in a steel, the metallographer may have to find clues to determine the source of hydrogen. Hydrogen, being the smallest element, is easily absorbed in the iron lattice as elemental hydrogen (H⫹). To eliminate this form of hydrogen, it can be easily diffused from the steel under controlled conditions, for example, vacuum degassing of liquid steel and slow cooling from an elevated temperature, for example, 540 °C (1000 °F), immediately after rolling or forging (not allowing the component to cool to room temperature first). However, when the hydrogen atoms combine into the molecular form (H2), diffusion is almost impossible, and hydrogen is thus trapped inside the steel. Once inside the steel, hydrogen prefers to reside at a surface such as an inclusion interface, for example, a manganese sulfide. At

Alteration of Microstructure / 81

Fig. 3.56 Microstructure of the various layers of the rail steel in Fig. 3.55. (a) The white layer at the surface (unattacked by the etchant), (b) tempered plate martensite, (c) as-quenched plate martensite and pearlite (dark), and (d) pearlite base microstructure. 4% picral etch. 1000⫻

82 / Metallographers’ Guide these sites, sufficient hydrogen gas pressure can create a separation at the steel/inclusion interface. As the component is stressed, the separation can extend into a crack. Hydrogen “flakes” or cracks form in this manner. An example of hydrogen flakes in an AISI/SAE 1080 steel bar is shown in Fig. 3.58. From these micrographs, it is evident that hydrogen cracks can more easily be detected in a sample in the unetched condition (Fig. 3.58a) than in the etched condition (Fig. 3.58b). Many times, the etched micro-

structure can obscure the cracks, because there are so many other features present. Also, differential interference contrast (Nomarski) can be very helpful in locating cracks. Note the vast improvement in the same AISI/SAE 1080 steel shown in both bright-field (Fig. 3.59a) and differential interference contrast (Fig. 3.59b). One way to detect hydrogen damage is in the fracture appearance of through-thickness tensile specimens. If hydrogen flakes are present, “fisheyes” appear on the fracture

Fig. 3.57 Microstructure of a Ni-Cr-Mo steel held at 565 °C (1050 °F) under a load of 210 MPa (30 ksi), showing (a) initial void formation at the austenite grain boundaries, (b) void linkup, and (c) separation of an austenite grain boundary. 4% picral and HCl etch. 500⫻

Alteration of Microstructure / 83 surface. Fisheyes are shiny, rounded regions that have developed by internal cracks caused by hydrogen damage. These regions were there before the tensile fracture took place. An unusual example of a decarburized region surrounding a hydrogen flake in an AISI/SAE 1080 steel can be seen in Fig. 3.60. In this case, the component was heat treated after the hydrogen crack formed. Here, exposure to 870 °C (1600 °F) for five hours allowed the hydrogen gas to react with the carbon in the steel to form methane gas (CH4). The result was carbon depletion

(decarburization) along the crack surface, seen as the white nonetching constituent in Fig. 3.60. An example of severe hydrogen damage is shown in Fig. 3.61(a), representing an ASTM A516 steel plate. In this micrograph, one can see cracks that developed by grain-boundary separation along the banded regions. Usually these cracks are not noticed until the part is in service, where it is exposed to some kind of stress, that is, thermal or mechanical stress. Under continued stress, the part usually fails as the crack or cracks grow

Fig. 3.58 Hydrogen flakes (cracks—see arrows) found in an AISI/SAE

Fig. 3.59 Another example of a hydrogen flake (crack) in an AISI/SAE

1080 steel bar in the (a) unetched and (b) etched condition. 4% picral etch. 1000⫻

1080 bar showing (a) the crack in bright-field illumination and (b) in differential interference contrast (Nomarski). Unetched. 1000⫻

84 / Metallographers’ Guide in length. Also, as atomic hydrogen diffuses into the steel, it forms molecular hydrogen that cannot leave, because in its molecular state, the diffusivity is extremely low. As hydrogen diffusion continues, the molecular hydrogen begins to form gas pockets that are under very high pressure. In the previous example, one can see in the the enlarged view of Fig. 3.61(b) that the gas pressure actually expanded the cracks and plastically deformed the regions between cracks (note the bending of the ferrite/pearlite bands between the two expanded cracks). Once a hydrogen-related crack forms, the process is irreversible, and the part must be taken out of service before catastrophic failure occurs. Another form of hydrogen damage can occur in service from electrochemical corrosion reactions. Such a reaction can occur in steel boiler tubes exposed to high temperature and high-pressure steam. Water can react with iron at high temperature to create atomic hydrogen (as opposed to molecular hydrogen, H2) and iron oxide. The atomic hydrogen can easily diffuse into the steel boiler tube and cause damage. One form of damage occurs when hydrogen can combine with the carbides (cementite) in the steel to form methane gas, CH4, which can cause internal grain-boundary separation similar to the hydrogen flakes described previously. An example of this type of damage is illustrated in a SA 210 boiler tube exposed for hundreds of hours to superheated steam at 360 °C (675 °F) under a pressure of 18 MPa (2.6 ksi). Figure 3.62(a) shows the microstructure of the boiler tube in its original condition, and Fig. 3.62(b) shows the result of the methane damage. The damage is in the form of grain-boundary separation created by the increased pressure developed by the reaction of hydrogen with cementite. The methane gas is in molecular form, and the molecules are too big to easily diffuse in the steel. Note

the dissolution of the pearlite colonies adjacent to the cracks caused by the reaction of the hydrogen with the iron carbide. Once grain-boundary separation occurs, the strength of the tube will decrease, and the tube will eventually fail. Corrosion Effects. Corrosion is generally thought to affect only the outer surface layer of a part. This is not always the case. For example, as in the case of a gray cast iron, corrosion can penetrate into the interior of a component, for example, an underground water pipe, due to the connectivity of the graphite flakes. An

Fig. 3.60 An internal hydrogen flake (crack) in an AISI/SAE 1080 steel bar

that was exposed to a temperature of 870 °C (1600 °F) for 5 h. The crack surface was decarburized (white area surrounding crack) by the reaction of the hydrogen with the carbon in the steel. Pearlite matrix. 4% picral etch. 1500⫻

Fig. 3.61 Microstructure of an as-rolled ASTM A516 steel plate showing

hydrogen flakes along the pearlite bands. 2% nital and 4% picral etch. (a) 50⫻ and (b) 400⫻

Alteration of Microstructure / 85 example is shown in Fig. 3.63(a). Here, corrosion products can be found surrounding the flake graphite and graphitic cells (a cell is a connected network of graphite flakes). Figure 3.63(b) shows the corrosion product and graphite flakes at higher magnification. The penetration of corrosion, in a way, proves that the graphite flakes are interconnected.

Corrosion engineers often call this type of corrosion “graphitization,” which, to a metallurgist, is obviously incorrect, because the graphite was present before the corrosion took place. What has happened in this case is selective leaching of the iron matrix around the graphite flakes. In very severe cases, the entire outer skin of the corroded pipe or component is graphite, with all the

Fig. 3.62 Microstructure of an ASME SA 210 steel tube consisting of (a) ferrite (light etching constituent) and pearlite (dark etching constituent) and (b) a hydrogen-damaged region showing cracks (arrows) at the pearlite/ferrite interfaces. 4% picral etch. 1000⫻

Fig. 3.63 Microstructure of a gray cast iron water pipe with corrosion penetrating below the surface along graphite flake networks (cells) (see arrows). (a) unetched, 50⫻ and (b) 4% picral etch, 500⫻

86 / Metallographers’ Guide iron leached from the surface. From a corrosion viewpoint, the graphite and iron act as a galvanic cell, where the graphite is the cathode and the iron is the anode. Because of the galvanic action, corrosion is enhanced next to the graphite flakes. In a previous section in this chapter, the term “graphitization” has been shown to be the formation of graphite during a long-term exposure of a carbon steel to temperatures between 315 and 370 °C (600 and 700 °F). SELECTED REFERENCES •

Atlas of Microstructures of Industrial Alloys, Vol 7, Metals Handbook, 8th ed., American Society for Metals, 1972

• • • •

• •

The Heat Treaters Guide—Standard Practices and Procedures for Steel, American Society for Metals, 1982 Heat Treating, Vol 4, ASM Handbook, ASM International, 1991 G. Krauss, Principles of Heat Treatment, 2nd ed., ASM International, 1993 Properties and Selection: Irons, Steels, and High-Performance Alloys, Vol 1, ASM Handbook, ASM International, 1990 L.E. Samuels, Optical Microscopy of Carbon Steels, American Society for Metals, 1980 H. Thielsch, Defects and Failures in Pressure Vessels and Piping, Reinhold Publishing, 1965

Metallographer's Guide: Practices and Procedures for Irons and Steels Bruce L. Bramfitt, Arlan O. Benscoter, p87-107 DOI:10.1361/mgpp2002p087

Copyright © 2002 ASM International® All rights reserved. www.asminternational.org

CHAPTER 4

The Metallographer and the Metallographic Laboratory MATERIALS PLAY A MAJOR ROLE in the world economy and in the development of nations. This is particularly true of metals, with steels and cast irons being the most widely used. Metals not only enhance our life-style, but have become a necessity of modern life. Science and engineering, particularly in the areas involving metal, ceramic, polymeric, electronic, and superconducting materials, have advanced rapidly and will continue to outpace technology as a whole. In this swirl of activity, a metallographer is vital to our basic understanding of the link between microstructure and the properties of these materials. By understanding microstructure and its origin, one can begin to develop a basis on how to achieve specific properties tailored for a particular engineering application. For example, an application for an earth-moving shovel in mining requires a steel with very high tensile strength and hardness, combined with a high degree of wear resistance and some toughness. What optimal microstructure would provide these properties? This book provides a beginning to the basic understanding of the development of microstructure and gives instructions on how to employ proper techniques to reveal microstructure, that is, the techniques of metallography. In this book we, of course, restrict our attention to the metallography of iron and steel.

ing wrought iron, wrought iron armor plate, and blister steel. He found that the samples had definite microstructural features. A copy of Sorby’s 1864 macrograph of blister steel can be seen in Fig. 4.2. This macrograph, taken at 9⫻, shows distinct grain boundaries. This discovery was significant, because it eventually led to the realization that microstructural features imparted certain properties to steel. The development of ferrous physical metallurgy, and physical metallurgy in general, depended on this important link.

The Metallographer What is a metallographer? In general terms, a metallographer is a person who has the skill to properly prepare a specimen of a metal or alloy in order to allow examination and interpretation of its microstructure. In this technological age, the term “metallographer” is becoming somewhat of a misnomer, because today it covers not only metals but ceramics and the other materials mentioned previously. The term at some time in the future may be changed to “materiallographer,” but for the moment, the term “metallographer” is still in place. The field of metallography, which involves the study of the microstructure of metals, is almost a century and a half old. It all began with Henry Clifton Sorby on July 28, 1863. Sorby, whose photograph can be seen in Fig. 4.1, was an English geologist, petrographer, and mineralogist who was the first person to examine polished and chemically etched metal samples under the microscope. His samples included Swedish wrought iron, Bowl-

Fig. 4.1 Henry Clifton Sorby, the father of metallography

88 / Metallographer’s Guide In Sorby’s day, the examination of structure was limited by the fairly low magnification of the light (optical) microscope. (In this book, we prefer the term “light” microscope to “optical” microscope, because light is the source of radiation or illumination of the specimen. Other microscopes, such as an electron microscope, depend on electrons as the source of radiation, and these microscopes still use an “optical” system of magnetic lenses to obtain an image.) We can assume that in the 1860s, microscopes allowed a magnification of only a few hundred times. Sorby’s microscope, purchased in 1861 from Messrs. Beck, had a maximum magnification of about 400⫻. A sketch of the microscope is shown in Fig. 4.3. Most microscopes of the day were for biological studies, and Sorby needed a microscope using reflected light. Sorby modified Beck’s microscope with an additional small mirror inclined at a

45° angle, as shown by the letter “g” in the lower right of the sketch. This mirror allowed light rays to reflect from the sample in normal reflected illumination as opposed to the oblique illumination in Beck’s parabolic mirror, “f” in the lower left of the sketch. Sorby’s mirror not only increased the amount of light reflected back into the objective “a,” it also improved the resolving power of the microscope. Resolving power and light-gathering ability are discussed in much greater detail in Chapter 5. Although etching was used at least four centuries before Sorby etched his specimens, it was only used for macroetching to bring up the damask patterns of swords and various pieces of armor, as well as the structure of polished meteorites. In 1808, Alos von Widmanstätten, a geologist and museum curator in Vienna, and his co-worker Carl von Schreibers etched various meteorites to show the outstanding crystalline patterns (these patterns are now called Widmanstätten structure). An excellent example of their work is shown in Fig. 4.4 which shows the structure of the etched Elbogen iron meteorite that fell in 1751. The meteorite was polished and deeply etched with nitric acid. After rinsing in water and drying, printer’s ink was rolled on the etched surface, and the sample was pressed onto a piece of paper. The direct transfer is seen in Fig. 4.4. Sorby cut and polished his specimens to remove all “traces of roughness.” After polishing, he used extremely dilute nitric acid to etch his specimens. He actually followed the progress of etching in order not to overetch the specimen. Widmanstätten and

Fig. 4.2 The first macrograph of the microstructure of steel, Sorby’s 1864 macrograph of blister steel. Etched in very dilute nitric acid. 9⫻

Fig. 4.4 A macrograph of the Elbogen iron meteorite prepared in 1808 by Fig. 4.3 A sketch of Beck’s microscope with Sorby’s flat mirror (g)

Widmanstätten and Schreibers using heavy etching in nitric acid to outline the structure in relief. The macrograph was made by rolling printer’s ink over the sample and then transferring the inked specimen onto paper.

The Metallographer and the Metallographic Laboratory / 89 Schreibers etched specimens that could be viewed with the naked eye, but Sorby was the first to etch specimens and observe the microstructure with a microscope. This was the beginning of metallography. It is sad that the constituent Sorbite, named to honor Sorby, was simply very fine pearlite unresolved in the light microscopes at the end of the 19th century. Because it was not a new constituent, the term “Sorbite” didn’t survive. The term “pearlite” survived to this day and is actually connected to Sorby, because he described the “pearly constituent” as having the appearance of mother of pearl. Today, with electron microscopes we examine the structure of materials at over 1,000,000⫻ magnification. As newer instruments are developed, the metallographer is able to probe deeper and deeper into the fine structure of metals. However, even with the sophisticated instruments available, the first and most important stage of a metallographer’s investigation is still in the use of the light microscope to examine the general microstructure. In bypassing the light microscope and going directly to the electron microscope, many metallographers have been misled in their intrepretation of the structures they observe. Thus, the first rule in metallography is to examine the microstructure in a light microscope (usually 50 to 1000⫻) and then, if required, move up to higher magnifications with the scanning electron microscope, transmission electron microscope, or scanning transmission electron microscope. The metallographer must be able to prepare a specimen of iron or steel with the proper metallographic procedures to reveal the true microstructural features in the specimen. It is important that the metallographer understand the metallurgical aspects of the iron or steel sample that is being prepared. Often, an unskilled person will employ improper techniques and unknowingly create a false “structure” or artifact in the specimen that will lead to a misinterpretation of the microstructure. (We use the term “artifact” as a generic term for a false microstructure, that is, the appearance or creation of a “microstructure” that is not the actual microstructure of the steel or cast iron. An example is an unintentional water stain on the specimen surface.) False information can be very costly if wrong decisions are made based on poor sample preparation. An example is if a critical step in a manufacturing process was varied in the wrong direction because of the misinterpretation of an artifact induced in a metallographic specimen. Chapter 7 discusses a number of examples of artifacts that have been introduced into metallographic samples of iron and steel.

The Metallographer versus the Chemist An example to show the importance of a metallographer is illustrated using a simple situation. A small manufacturing plant orders 12 mm (1⁄2 in.) diameter American Iron and Steel Institute/ Society of Automotive Engineers (AISI/SAE) 1040 barstock in truckload quantities to fabricate brackets. One particular week, the plant was experiencing difficulty in machining, bending, and welding the brackets. In one case, the bars were much too hard and cracked when bent into the 90° bend required, and these same bars were very difficult to machine, and were wearing the cutting

tool at an unacceptable rate. At the other extreme, some of the bars were difficult to machine, because they were too soft and clogged and broke several cutting tools. The foreman decided to trace the bars back to the truck shipments and found that three truckloads were delivered the same week, but each load was from a different supply warehouse. To determine what was possibly different in the three shipments, the foreman gave a sample of bar from each of the different shipments to a chemist for a complete chemical analysis, and an identical set of samples to a metallographer. After drilling chips from each bar and conducting a “wet” chemical analysis, the chemist determined the chemical composition of the steel and found that the three samples consist of the following composition: Composition, %

Carbon Manganese Phosphorus Sulfur Silicon Nickel Chromium Molybdenum Copper Aluminum

Sample 1

Sample 2

Sample 3

0.39 0.72 0.013 0.018 0.21 0.015 0.026 0.010 0.23 0.021

0.41 0.70 0.015 0.014 0.23 0.027 0.028 0.014 0.18 0.018

0.40 0.68 0.010 0.016 0.22 0.002 0.005 0.002 0.05 0.023

The chemical analysis shows that three samples have similar composition. From these analyses, the steel is AISI/SAE 1040, thus showing that the shipments were of the correct grade of steel. The analysis reveals a lot of information about how the steel was manufactured. For example, the steel from all three shipments is silicon-aluminum killed. Samples 1 and 2 contain a fairly high level of residual (tramp) elements, such as copper. The level of residual elements indicates that the steel was produced by the electric furnace steelmaking process, which uses steel scrap as the melting source. Steel scrap can consist of crushed automobiles, which contain copper wires, zinc alloy die castings, and so on. Sample 3 was produced from a basic oxygen furnace (BOF), where the levels of copper, nickel, chromium, and molybdenum are much lower, because BOF-based steel is produced from the liquid iron of a blast furnace. The blast furnace uses iron ore, coke, and limestone as the raw materials to make iron. Generally, these residual elements are not present in these raw materials. However, some residuals can be picked up if some scrap is melted in the BOF. From all this information, it is impossible to tell anything about the specific properties of the steel bars, for example, hardness, tensile strength, and so on. However, in the reference Engineering Properties of Steel, one finds that AISI/SAE 1040 steel can have the following range of properties: Hardness Yield strength Tensile strength Total elongation (in 2 in.) Reduction in area

149 to 529 BHN 50 to 85 ksi (50,000 to 85,000 psi) 70 to 130 ksi (70,000 to 130,000 psi) 17 to 35% 45 to 70%

From these wide ranges of properties found in the handbook, we do not know if the steel bars are hard, soft, or ductile. These

90 / Metallographer’s Guide specific properties depend on the processing history and condition of the steel. For example, were the steel bars received in the as-hot-rolled condition or heat treated, for example, austenitized and water quenched? Thus, the chemist can provide very important information about chemical composition that will give clues about the steel, but the information is very limited in its usefulness in the prediction of properties. In fact, as seen subsequently, the chemical information can sometimes lead to the wrong conclusions about the steel. The metallographer, on the other hand, after sectioning, polishing, etching, and examining samples of the same three steels under the microscope, reports that each steel bar has a different microstructure and thus, different properties and characteristics. Following is what the metallographic analysis revealed. Sample 1. Details about these constituents are described in Chapter 2. A skilled metallographer would recognize that this bar is in the hot-rolled condition, because of the banded microstructure where the pearlite constituent appears in bands running parallel to the rolling direction of the bar. Banding is common in hot-rolled steels and is caused mainly by manganese segregation during the solidification process (the carbon diffuses to the manganese bands to form pearlite upon cooling from the hotrolling temperature). The segregation pattern elongates during the hot-rolling process. The metallographer determines by standard quantitative microscopy procedures outlined in ASTM E 562 that the bar is 50% ferrite and 50% pearlite. The ferrite constituent is equiaxed (not elongated, but equal dimensions in all directions), and from standard grain size measurements according to the three-circle method in ASTM E 112, has an average grain size of 21 microns (ASTM number 6). From these proportions of 50% ferrite and 50% pearlite, the metallographer can estimate that the steel has a carbon content close to 0.4% and that the steel is hot-rolled AISI/SAE 1040 (the chemical analysis is important for this information). This carbon content can be calculated by using the lever rule and the iron-carbon equilibrium diagram described in Chapter 2. The lever extends along the eutectoid horizontal line of the iron-carbon diagram from 0.02 to 0.77% C. With the fulcrum of the lever at the unknown carbon content of “X,” the right side of the lever (the amount of ferrite) is 0.77 ⫺ X, divided by the entire length of the lever, or 0.77 ⫺ 0.02, as shown: 0.77% C ⫺ X% C ⫻ 100% ⫽ 50% (ferrite) 0.77% C ⫺ 0.02% C

By solving this equation, X ⫽ 0.4% C. It must be pointed out that this is only an estimate, because the percentages of ferrite and pearlite can change, depending on processing parameters such as cooling rate from austenite to ferrite and pearlite. The austenite grain size before transformation can also vary the amount of ferrite and pearlite in a microstructure, because a large grain size has fewer sites for nucleation of ferrite (and vice versa for a fine grain size), which can alter the percentage transformed. However, in this example where the bar was hot rolled followed by air cooling, the estimate of 0.4% C is reasonable. From the previously mentioned handbook, an AISI/SAE 1040 steel in the as-rolled condition would have the following approximate properties:

Hardness Yield strength Tensile strength Total elongation (in 2 in.) Reduction in area

150 BHN 50 ksi (50,000 psi) 75 ksi (75,000 psi) 30% 60%

Sample 2. The microstructure of this sample consists of martensite. This microstructure indicates that the bar was austenitized and rapidly cooled (probably water quenched) to room temperature. From close examination of the microstructure, the steel does not appear to be tempered (softened by heat treatment between 425 and 650 °C, or 800 and 1200 °F) and is thus in the as-quenched condition. This steel will be much harder and stronger than the steel represented by sample 1. The metallographer, by knowing the aim chemical analysis, can estimate the properties from the previously mentioned handbook as: Hardness Yield strength Tensile strength Total elongation (in 2 in.) Reduction in area

500 BHN 95 ksi (95,000 psi) 130 ksi (130,000 psi) 17% 45%

Sample 3. The metallographer reveals that this bar has an abnormal microstructure. The bar is heavily decarburized, which means that during processing at high temperature, the carbon in the steel was allowed to diffuse out of the surface layers of the bar into the surrounding atmosphere. Decarburization can be caused by improper furnace atmosphere practice during heat treatment. Because of the severe decarburization, approximately the outer 1⁄4 of the cross section of the bar is ferrite, and the microstructure at the central region of the bar is 100% pearlite. The center of the bar represents the original microstructure, because it was unaffected by the decarburization phenomenon. This means that the original bar was actually AISI/SAE 1080 (a fully pearlitic steel with 0.75 to 0.88% C), not AISI/SAE 1040 (0.37 to 0.44% C), as indicated by the chemist. The loss in carbon by the severe decarburization lowered the overall carbon content (the bulk carbon content) of the bar to about 0.40%. Thus, the chemical analysis information alone would have been misleading in this case. However, coupled with a metallographic analysis, a great deal of relevant information can be obtained about the steel. Based on the previously mentioned chemical and metallographic analyses, logical decisions can be made concerning the properties and processing history of the steel bars. The plant foreman knew that each sample came from a different truckload of bars from three different steel suppliers. All the bars were ordered to a specification mandating a standard silicon-aluminum killed, AISI/SAE 1040, as-rolled bar stock. The analysis by the chemist indicates that the three shipments would meet the AISI/SAE 1040 chemical analysis specification. However, the foreman knew that the bars had different properties. Only by conducting a metallographic analysis was it determined why the bars are different. The information from the metallographic analysis explained why the bars were cracking and breaking cutting tools. The bracket manufacturing process employs steps that run the material through normal, closely controlled forming/machining/welding

The Metallographer and the Metallographic Laboratory / 91 operations designed and optimized around a hot-rolled AISI/SAE 1040 steel. In this example, the 12 mm (0.5 in.) diameter bar stock is cold sheared into 254 mm (10 in.) lengths, threaded at one end, bent 90° at a position 100 mm (4 in.) from the threaded end, and welded at the other end to a large steel tank. First, the bars from each supplier behaved very differently during cold shearing. The martensitic bar (steel 2) would damage the shear blade and be unable to be sheared. The bending operation would also eliminate the martensitic steel, because the bars would crack and break during bending. Machining the threads on steel 2 would also be difficult using tooling setup for a softer steel. Also, the heavily decarburized bars (steel 3), having a very soft surface, would machine much differently than an AISI/SAE 1040 steel. Because of the softer surface, the machined threads would also be very weak in service. The welding characteristics of each of these three steels would also be very different. However, if a metallographer examined the samples from each shipment before processing, the foreman would realize that the steels were different and that two out of the three shipments did not meet the specifications of a hot-rolled AISI/SAE 1040 steel. This example is not meant to downgrade the role of the chemist, because in most cases, knowing the chemical composition is very important. But the composition of the steel in combination with the microstructure provides much more information for an accurate decision to be made about the properties and characteristics of the steel. The main point in the previous example, in contrasting chemical and metallographic information, is that both are important, but the metallographic information about the microstructure is of utmost importance in understanding the properties and characteristics of the steel. This means that the metallographer must be able to prepare a sample of the steel to reveal its microstructure. A skilled and educated metallographer can interpret the microstructure to assist others in the proper course of action. The interpretation of microstructure requires a sound knowledge of ferrous physical metallurgy. Many metallographers leave this interpretation to metallurgists/metallurgical engineers. However, the authors feel that to be more effective, a metallographer should have some knowledge of structure-property relationships. The field of metallography has rapidly progressed since the 1950s to the point where all major steel manufacturing and fabricating companies use some kind of metallographic analysis in quality-control procedures. Metallographic data is sometimes indispensable. Many product specifications are written with metallographic standards, such as minimum grain size, maximum inclusion size/distribution (microcleanliness), maximum depth of decarburization, and so on. The modern-day metallographer must not only know how to properly prepare a specimen and interpret its microstructure, but also how to quantitatively measure the metallographic features found in each specimen. Most times, these measurements can be made with the aid of a simple metallographic microscope with a scale of measurement installed in an eyepiece, or by projecting the image on a ground glass screen. In larger operations, where thousands of repetitive measurements are made, a computer-assisted image analysis system is used. In this case, the specimen is automatically scanned using a

special mechanized stage on the microscope. Hundreds of measurements are made and stored in the microprocessor. The system is programmed to give data on the number, volume fraction, size and distribution of the particles, and phases in the microstructure. Many times, a metallographer is needed to assist in programming such systems, because of his or her knowledge of various microstructural characteristics that the image analysis system will measure. The modern-day metallographer must also have some proficiency in metallography beyond the light microscope. The light microscope, in using reflected light to illuminate the specimen, is limited in resolving power, so that the useful magnification is generally under 1000⫻. In fact, special procedures and highquality lenses are needed to achieve 1500⫻. This magnification limit is very restricting for some fine microstructures. For example, as described in Chapter 2, pearlite is a constituent in steel consisting of alternating plates or lamella of soft ferrite (nearly pure iron) and hard cementite (iron carbide). This microstructure is mostly unresolved at these magnifications. For example, the microstructural details of fine pearlite in a sample of rail steel (AISI/SAE 1080), taken on a light microscope, are shown in Fig. 4.5. One can see evidence of discrete pearlite colonies, due to the differences in the reflectivity of light from the different orientations of the colonies (the chemical etching process attacks each colony, exposing a different reflection plane to the incident light of the microscope). However, at this magnification, one cannot discern the fine details of the individual cementite and ferrite lamella. Figure 4.6 shows the same sample at a magnification of 10,000⫻, taken on a scanning electron microscope (SEM). This magnification can be easily obtained on a SEM that employs reflected electrons instead of reflected light. Scanning electron microscopes are now common in many industrial and university metallographic laboratories. A skilled metallographer must prop-

Fig. 4.5 Microstructure of a fully pearlitic steel rail. 4% picral etch. 1000⫻

92 / Metallographer’s Guide steels in the form of rod, wire, plate, forgings, structural shapes, hot- and cold-rolled sheet, rail, bar, and castings. Some of the steel samples are from experimental studies, but a large portion of samples are from failed parts and from the companies’ existing commercial products. Samples are submitted with as much documentation as possible, for example, chemical composition and details of prior heat treatment and processing. Although metallographers may tend to specialize their expertise in particular types of samples, such as coated steels, in this laboratory they prepare samples in rotation as they are submitted. To be specific, the metallographic submissions on one particular working day included the following nine examples. The actual metallographic techniques used with each of these examples are described in order to give the reader an idea of the work required. Chapters 7 and 8 provide much more detail on metallographic technique than can be described here.

Fig. 4.6 A SEM micrograph of a fully pearlitic microstructure of a steel rail. 4% picral etch. 10,000⫻

erly prepare a specimen for SEM analysis. In addition to the very useful SEM, many other instruments are available to analyze the microstructure. In fact, these instruments, including the SEM, can provide data on the chemical composition of the individual phase or particle in a microstructure. The electron probe microanalyzer (EPMA, or simply “microprobe”) is the ultimate instrument to provide chemical composition. These instruments and others are discussed in detail in Chapter 6.

The Metallographer’s Workday In order to have some idea of what a typical metallographer does in a particular workday, the following examples are presented of a metallographer working as part of a team of metallographers in a research laboratory of a large steel company, and a metallographer working alone at a small iron foundry.

The Metallographer’s Workday at a Research Laboratory of a Large Steel Company In the United States, the major steel corporations have research/ technology departments with up to 100 or so employees. To support a staff of this size, a typical metallographic laboratory employs one to three full-time metallographers, not including supporting personnel who operate the specialized instruments, such as a SEM, EMPA, or x-ray diffractometer. In this example, a metallographic laboratory with three metallographers was selected. These metallographers can each process well over 1000 samples a year. All three metallographers work closely with physical and process metallurgists as well as corrosion and welding engineers. Although this is a steel research laboratory, the metallographers must also have the skills to prepare samples of nonferrous metals, for example, copper used in blast furnace tuyeres to zinc coatings on steel. However, ever, about 98% of the samples submitted to the laboratory are

Submission 1 is two lots of steel sheet, coded “1” and “2,” with an aluminized coating. Work Required. Determine if the aluminized coating is type 1 or type 2 and measure the overall coating thickness and the thickness of the alloy layer at the interface between the aluminum coating and the steel. A type 1 coating contains about 9% Si, whereas a type 2 coating is essentially pure aluminum. Specimen preparation is as follows. • •

Sectioning: Hand shear three samples 8 ⫻ 25 mm (0.30 ⫻ 1.0 in.) from each lot. Mounting: The samples will be mounted in epoxy. First, make a sandwich of the six samples, using thin strips of doublebacked tape placed at each end of each strip (this procedure is described in Chapter 7). The tape will keep each specimen apart and allow epoxy to fill the gap. It is important to make a sketch of the samples showing how they were stacked. Place the sandwich with the required face down onto the bottom of a cup-shaped mold. Place a 30 mm (1.17 in.) diameter, 25 mm (1 in.) high plastic cylinder (mold) around the sandwich (center the sandwich in the mold). Place two steel nuts in the mold, at both sides of the 25 mm (1 in.) dimension of the sandwich. These nuts will help keep the polished surface of the sandwich flat during grinding and polishing. Place a “V” marker insert on one side of the sandwich to orient how the specimens were stacked according to the sketch. The marker can be a bent piece of sheet or other material. Thoroughly mix the epoxy according to the manufacturer’s instructions. This should be carried out under an exhaust hood. In this particular case, heating the liquid epoxy container in a beaker of warm water will reduce the viscosity and allow better filling of the gaps between the specimens. After filling the mold with the epoxy mixture to just cover the sandwich, place the mount in a vacuum chamber to remove excess air bubbles. The bubbles appear as a foam forming on the top surface of the mount. Allow air to enter the chamber, and repeat the evacuation several times. Remove mount from the chamber and fill the cylinder with the remaining epoxy mixture. Allow the mount to cure according to the manufacturer’s instructions.

The Metallographer and the Metallographic Laboratory / 93 •

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Grinding: Measure the height of the sample. Silicon carbide grinding paper will be used, starting with 120-grit paper. The wheel rotation will be 300 rpm. During the wet grinding operation, remove approximately 1 mm (0.040 in.) from the surface of the mount. This will remove any deformation induced from shearing the specimen. Continue wet grinding with successively finer-grit papers, 240, 320, 400, 600, 12 μm, and 8 μm, rinsing the specimen in running water after each paper. Rotate the specimen 90° between each paper, so that it can be seen if the scratches from the previous grind are removed. After the first grind on the 120-grit paper, only 15 to 20 seconds is usually required per paper. In this case, it is important to end the last grinding step on 8 μm paper, with the grinding scratches parallel with the 25 mm (1 in.) length of the specimens. This will minimize rounding of those edges of the specimen with the coated layer. The mount should then be cleaned by swabbing with cotton under running water, followed by flushing with alcohol and drying. Check the surface of the specimen in the microscope to ensure that there is no rounding of the edges of the coating. If rounded, the mound should be reground, starting at 600 grit until flat. Polishing: The specimens will be polished using an aqueous 1 μm alumina (Al2O3) solution on a napless cloth. First, use a stationary wheel or flat surface while rotating the mount until all the parallel scratches from the last grinding step are removed. If staining occurs by a galvanic reaction using the aqueous alumina solution, change the pH of the water to 7 or replace the water with ethylene glycol. Clean by swabbing with cotton under running water, and flush the specimen with alcohol and dry. Second, polish the specimen using a stationary wheel or flat surface with an aqueous 0.3 μm alumina solution on a low-nap cloth (e.g., rayon ) for 90 seconds. Rinse with cotton swabbing, and flush with alcohol and dry. Third,

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polish on a stationary low-nap cloth with aqueous colloidal solution of 0.04 μm silica (SiO2) for two minutes. Polishing on a low-nap or napless cloth will minimize rounding of the edges of the specimens. Etching: Etch the specimen in 2% nital for 15 to 20 seconds and rinse in running water. Flush the specimen with alcohol and blow dry. Observation: The etched specimens were examined in a light microscope equipped with a measuring reticle. From this scale, the coating thickness can be measured by rotating the scale so that it is perpendicular to the coating thickness. Results: After examining the etched specimens in the microscope, it was revealed that the aluminized coating on lot 1 was type 1, with an average coating thickness of 15 μm. Figure 4.7(a) shows a representative field of the coating on the steel surface. The microstructure reveals particles of silicon (dark gray needles) in an aluminum matrix and an alloy layer (light gray region between steel and coating) about 5 μm thick. Cracks can be seen in the brittle alloy layer in Fig. 4.7(a). For lot 2, the coating thickness averaged 25 μm, and the alloy layer averaged 10 μm. A representative microstructure is shown in Fig. 4.7(b). The coating of the sample in lot 2 is type 2.

Submission 2 is a defective 1 mm (0.040 in.) diameter AISI/SAE 1070 steel wire. Work Required. Determine the cause of wire failure during service. Specimen preparation is as follows. •

Sectioning: Cut four pieces of wire to a length of 12 mm (0.50 in.) and four pieces of wire to a length of 18 mm (0.70 in.). Include an 18 mm (0.70 in.) length of wire with the fracture surface.

Fig. 4.7 Micrograph of a steel sheet with (a) type 1 and (b) type 2 aluminized coating. 2% nital etch. 1000⫻

94 / Metallographer’s Guide •

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Mounting: Place the four longest wires in a parallel array on a flat magnetic surface covered with aluminum foil or on a flat surface of double-backed tape. Leave a 3 mm (0.12 in.) spacing between each wire. Center a 30 mm (1.17 in.) diameter by 25 mm (1 in.) high plastic cylinder (mold) around the wires. If using aluminum foil, use quick-drying cement to secure the mold in place to prevent the mold from moving. Prepare an epoxy mixture and pour the mixture into the cylinder until a depth of 20 mm (0.80 in.) is reached. Allow the epoxy to cure in accordance with instructions. Drill four holes slightly larger than the wire diameter into the cured mount. The holes should be about 2 mm (0.080 in.) from the end of each wire. Place the four short wires in the holes, with the cut surface of each wire extending out of the holes (this ensures that the cutting damage will be removed during grinding). Fill the remaining cavity of the mold with epoxy. To lower the viscosity of the epoxy mixture, it is recommended that the epoxy mixing container be heated by placing it in warm water (about 50 °C, or 120 °F). Place the mount into a vacuum chamber to remove any air bubbles surrounding the wires in the holes. The epoxy will fill any gaps remaining when brought back to atmospheric pressure. Allow the mount to cure. Grinding: Wet grind the surface of the mount on 400-grit silicon carbide paper until all the epoxy from the second pouring has been removed. Use wheel speed of 300 rpm. Then grind the mount on 600-grit silicon carbide paper until the center of the four longer wires has been revealed. Use the ends of the short wires to judge the diameter. Polishing: On a rotating wheel, polish (150 rpm) the specimens with 6 μm diamond paste or spray on a very-low-nap or napless cloth. Use heavy pressure until all the 600-grit

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Submission 3 is a failed weld in a 12 mm (0.50 in.) steel plate. Work Required. Determine the cause of the failed weld. Specimen preparation is as follows. •

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Fig. 4.8 Microstructure taken at the center of a fractured AISI/SAE 1070

steel wire. Fracture is associated with a central bursting condition. Unetched. 50⫻

scratches are removed. Clean the specimen by swabbing with cotton in flowing water, followed by an alcohol rinse and blow dry. Continue the polishing operation on a rotating wheel with an aqueous 0.3 μm alumina solution on a low-nap cloth for 60 to 120 seconds. Use firm pressure. Clean the specimen by swabbing with cotton under flowing water, and flush the specimen with alcohol and blow dry. As the last polishing step, use a 0.04 μm colloidal silicon dioxide slurry on a mediumnap cloth for 15 seconds (stationary surface). Use firm pressure. Clean the specimen by cotton swabbing under flowing water, and flush with alcohol and blow dry the specimen. Etching: First observe the polished specimens without etchng. If defects are found in the wires, photograph the areas of importance. Etch the specimens in 4% picral. Rinse the specimen in running water, followed by an alcohol rinse and blow drying. Observation: As mentioned previously, examine the specimens in both the unetched and etched conditions. Sometimes etching will obscure a defect. Results: Upon examination in the unetched condition, the wire with the fracture surface had a condition called central bursting, as seen in Fig. 4.8. This condition can develop in the cold-drawing process used to produce the wire. However, once the wire specimen was etched, it was determined that the central bursts were created by regions of hard martensite along the centerline of the wire. These martensite regions can be seen in Fig. 4.9(a) and (b). The martensite regions, because of their hardness, did not flow through the drawing die as easily as the matrix, thus causing a rupture.

Sectioning: Remove a 15 mm long by 10 mm (0.60 by 0.40 in.) section from each side of the fractured weld, using a water-cooled abrasive cut-off machine. Use a medium-hardness aluminum oxide wheel. Mounting: Not necessary Grinding: Wet grind each specimen, starting with 320-grit silicon carbide paper. Then continue wet grinding on 400- and 600-grit papers, rotating the specimen 90° after each paper. The specimen should be rinsed in running water after each paper. Polishing: Polish each specimen on a rotating wheel (150 rpm) with 6 μm diamond paste or spray on a very-low-nap cloth until all scratches from the last grind are removed. Clean the surface with a cotton swab under running water, followed by an alcohol rinse and drying. Polish each specimen on a rotating wheel with aqueous 0.3 μm alumina solution on a low-nap cloth for two minutes. Clean the specimens by cotton swabbing under running water, followed by rinsing in alcohol and blow drying. Polish the specimens in an aqueous 0.04 μm silicon dioxide slurry for 45 to 60 seconds on a stationary flat surface using a low-nap cloth. Clean by swabbing with cotton

The Metallographer and the Metallographic Laboratory / 95

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under running water, followed by an alcohol rinse and blow drying. Etching: Etch the two specimens in 2% nital. Rinse the specimen in running water, rinse in alcohol and blow dry. Observation: Place two specimens together at the fracture, using a leveling device and clay. Examine the weld in the microscope.

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Submission 4 is a heat treated AISI/SAE 52100 steel bearing race. Work Required. Determine why material did not meet the hardness specification of 700 Knoop required by the customer. Specimen preparation is as follows. •

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Fig. 4.9 Micrograph taken at the fracture of an AISI/SAE 1070 steel wire

showing hard regions of martensite that caused the rupture of the wire. Picral etch. (a) at 100⫻ and (b) at 200⫻

Results: It was observed that the failure occurred along the heat-affected zone (HAZ) of the weld and not in the weld fusion zone, as seen in Fig. 4.10.

Sectioning: Cut two specimens 12 mm (0.50 in.) in length on a water-cooled abrasive cut-off machine, using a soft alumina blade. Mounting: Mount one specimen in epoxy. Grinding: Wet grind the specimen, starting with 240-grit silicon carbide paper. Then continue wet grinding on 320-, 400-, and 600-grit papers, rotating the specimen 90° after each paper. The specimen should be rinsed in running water after each paper. Grinding time for each paper is about 15 to 20 seconds. Polishing: Polish the specimen on a rotating wheel (150 rpm) with 6 μm diamond paste or spray on a very-low-nap cloth until all scratches from the last grind are removed. Use heavy pressure while polishing. Clean the surface with a cotton swab under running water, followed by an alcohol rinse and blow drying. Polish the specimen on a rotating wheel (150 rpm) with a 1 μm diamond paste or spray on a low-nap cloth for 90 seconds, using heavy pressure. Clean the specimen by cotton swabbing under running water, followed by rinsing in alcohol and blow drying. Polish the specimen in an aqueous 0.04 μm silicon dioxide slurry for 45 to 60 seconds on a stationary flat surface, using a medium-nap cloth for 45 seconds, using firm pressure. Clean by swabbing with cotton under flowing water, followed by a rinse in alcohol and blow drying.

Fig. 4.10 Macrograph of a fractured weld showing through the heataffected zone (HAZ). 3% nital etch. 5⫻

96 / Metallographer’s Guide • •

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Etching: Etch in 4% picral. Rinse the specimen in flowing water, followed by an alcohol rinse and blow drying. Hardness measurement: Make hardness measurements (Knoop) of the specimen. Use 500 gram load with a 15 second hold time. Results: The specimen cut from the original lot of steel had an average Knoop hardness of 730. This converts to a Rockwell hardness of 60 Rc. The microstructure is martensite, as seen in

Fig. 4.11(a). To reconstruct what may have happened to achieve a much lower hardness, another piece of the bearing race was sectioned from the same sample, but was removed using a cut-off wheel without water cooling. That specimen was prepared in the same manner described previously. The specimen had a similar microstructure to the first specimen, as seen in Fig. 4.11(b). However, the second specimen had an average Knoop hardness of only 326 (Rc 32). Thus, it was concluded that the original parts met the hardness specification and that the low reading achieved by the customer was created by improperly cutting the specimen from the bearing race. Submission 5 is a grade S-230 chilled cast iron shot. Work Required. Determine the particle shape of the shot. Sample preparation is as follows. • •

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Sectioning: Not required Mounting: Mix a small amount of shot with an epoxy mixture. Place a solid aluminum cylinder inside a 30 mm (1.17 in.) hollow plastic cylinder (mold), leaving enough depth to pour the liquid into the mold for a layer of about 2 mm (0.080 in.) thick (see Fig. 4.12). Place mold into a vacuum chamber and evacuate to remove air bubbles. Allow the epoxy to cure according to the instructions from the manufacturer. Grinding: Remove the aluminum cylinder and attached cured epoxy/shot mixture from the mold (see Fig. 4.13). Wet grind the epoxy/shot mixture, starting on 400-grit silicon dioxide paper. Rinse the specimen in running water. Rotate the specimen 90° and wet grind on 600-grit silicon carbide paper. Again, rinse the specimen in running water. Polishing: Polish the specimen on a rotating wheel (150 rpm) with 6 μm diamond paste or spray on a napless cloth, for example, nylon. Clean the specimen by swabbing with cotton in flowing water, followed by an alcohol rinse and drying. Polish the specimen on a rotating wheel, using an aqueous solution of 0.3 μm alumina on a low-nap cloth. Clean the specimen by swabbing with cotton under flowing water, followed by rinsing in alcohol and blow drying. Polish in a

Fig. 4.11 (a) Martensitic microstructure with a Knoop hardness impression

of AISI/SAE 52100 bearing race at a Knoop hardness of 730 and (b) the same material improperly cut using an abrasive wheel without water cooling at a Knoop hardness of only 326. 2% nital etch. 1000⫻

Fig. 4.12 Photograph of mold and aluminum cylinder insert used for preparation of a shot specimen. 1⫻

The Metallographer and the Metallographic Laboratory / 97

• • •

vibratory polishing machine for one hour, using an aqueous 0.04 μm silica solution, using a napless or low-nap cloth (woven wool). Clean the specimen with cotton under flowing water, followed by an alcohol rinse and blow drying. Etching: Not required Observation: Examine the specimen in a microscope and take several photographs of the shot morphology. Results: A typical example is shown in Fig. 4.14. The shot is in the form of fairly uniformly rounded particles.

• • • • •

Sectioning: Cut a 12 mm (0.50 in.) thick slice from the rail. Mounting: Not required Grinding: Surface grind one side of the cross-sectional surface of the rail. Use the finest grinding wheel available. Polishing: Not required Etching: A normal metallographic etch will not be applied. A technique similar to etching, called sulfur printing, will be used to produce a pattern of the segregation that occurs during

Submission 6 is a small section AISI/SAE 4340 forging. Work Required. Examine the flow lines to estimate the effectiveness of the forging operation. Specimen preparation is as follows. •

• •

• •

• •

Sectioning: Remove a 12 mm (0.50 in.) cross-sectional slice from the forging, using a water-cooled abrasive cut-off machine with a bonded alumina wheel. Mounting: Not required Grinding: Wet grind on 320-grit silicon dioxide paper, followed by wet grinding on 400- and 600-grit silicon dioxide papers. Rinse the specimen in running water, and rotate the specimen 90° after each grind. Polishing: Not necessary Etching: Etch the specimen in a solution of three parts water, two parts sulfuric acid, and one part hydrochloric acid. Heat the solution to 76 °C (170 °F) before etching. After etching, rinse the specimen in flowing water, followed by an alcohol rinse and blow drying. Observation: The flow lines in the specimen can be observed without a microscope. Results: Figure 4.15 shows the flow lines in the cross section of the forged specimen.

Fig. 4.14 Micrograph of grade S-230 chilled cast iron shot. Unetched. 160⫻

Submission 7 is a railroad rail. Work Required. Determine the chemical segregation pattern in the rail cross section. Specimen preparation is as follows.

Fig. 4.13 Photo shows a thin layer of epoxy with powder metal bonded to

an aluminum slug. This technique has many advantages over an all-epoxy mount.

Fig. 4.15 Macrograph of a steel forging showing flow lines. Etched in

three parts water, two parts sulfuric acid, and one part hydrochloric acid. 1.25⫻

98 / Metallographer’s Guide solidification. Soak a bromide-type photographic paper (for example, Kodak AZO F3 fiber-based photographic paper, Eastman Kodak Corporation) in a 2% solution of sulfuric acid for two to five minutes. Remove the paper from the solution and allow the excess solution to drain away. Place the emulsion side of the paper on the ground surface of the rail. Quickly remove any air bubbles from underneath the paper with a roller or sponge. Hold the paper flat with one hand while rolling or sponging to prevent movement. Allow the image to develop over a time period of between three to ten

• •

minutes (generally five minutes, depending on the sulfur content of the steel). Carefully peel the paper from the rail. Rinse the paper in water, and place the paper in a solution of photographic fixer (for example Kodak Rapid Fixer, Eastman Kodak Corporation) for about ten minutes. Rinse the paper in water for about a half-hour to remove the fixer, and allow the paper to dry. Observation: A microscope is not required to observe the image. Results: The sulfur print will reveal a pattern of sulfur distribution in the rail. Figure 4.16 shows the result. The sulfuric acid attacks the manganese sulfides in the steel and produces hydrogen sulfide gas. The hydrogen sulfide gas reacts with the silver bromide in the paper to form silver sulfide. The brownish image is a result of the silver sulfide reaction.

Submission 8 is a broken 6 mm (0.24 in.) diameter AISI/SAE 303 stainless steel bolt (see Fig. 4.17). Work Required. Determine the cause of failure. Specimen preparation is as follows. •

• •

Fig. 4.16 Sulfur print of a steel rail showing regions of sulfur segregation. 1⫻

•

•

•

• Fig. 4.17 Photograph of a broken AISI/SAE 303 stainless steel bolt. 2.5⫻

Sectioning: Cut a 12 mm (0.50 in.) thick slice from the fractured end of the bolt, using a low-speed, gravity-feed saw with a diamond blade. Mounting: Mount the bolt section in epoxy, using normal procedures described previously. Grinding: Using a belt sander with 80-grit silicon carbide paper, grind under water until about 1/3 of the cross section is removed. Continue grinding, using 120-grit silicon carbide paper. Rotate the specimen 90° between each grind, and rinse in flowing water. Stop just prior to the midpoint of the cross section. Continue wet grinding on 240-, 320-, 400-, and 600-grit papers until the midpoint is achieved. Rinse with cotton under running water, rinse in alcohol, and blow dry. Polishing: Polish the specimen on a low-nap cloth on a rotating wheel (150 rpm) with 6 μm diamond paste or spray. Polish until the scratches from the 600-grit grind are removed. Clean with cotton under flowing water, rinse in alcohol and blow dry. Polish on a low-nap cloth on a rotating wheel (150 rpm) with 1 μm diamond paste or spray for two minutes. Clean with cotton under flowing water, rinse in alcohol and blow dry. On a stationary wheel or flat surface, polish the specimen on a napless cloth for 20 seconds, using an aqueous colloidal solution of 0.04 μm silica. Clean with cotton under flowing water, rinse in alcohol and blow dry. Observation: Before etching, observe the polished specimen in the microscope. Figure 4.18 shows an unetched region at the root of one of the threads of the bolt. The corrosion seen at the root of the thread is called stress corrosion. Etching: Electrolytically etch the specimen in a solution of 10% oxalic acid and 90% water at 5 volts for 10 to 20 seconds, using a stainless steel cathode. Rinse the specimen in flowing water, followed by an alcohol rinse and blow drying. Results: In the etched condition, the stress-corrosion condition and cracking is seen following regions of delta phase, which is

The Metallographer and the Metallographic Laboratory / 99 aligned longitudinally along the axis of the bolt. Figure 4.19 shows the cracks along the delta phase stringers. Submission 9 is a 2.5 mm (0.10 in.) thick hot-rolled AISI/SAE 1010 steel sheet for hot water heater tanks. Work Required. Measure the ferrite grain size and the amount of pearlite at the center of the sheet. Specimen preparation is as follows. •

•

•

•

Fig. 4.18 Stress-corrosion cracking found at the root of a thread on the fractured bolt shown in Fig. 4.17. Unetched. 32⫻

•

•

•

Fig. 4.19 The stress-corrosion cracks in Fig. 4.18 are shown following

delta ferrite stringer in the austenite matrix. Electrolytically etched in 10% oxalic acid solution at 5 V. 200⫻

•

Sectioning: Using an abrasive cut-off machine with an alumina-bonded blade, cut a 12 mm ⫻ 18 mm (0.50 ⫻ 0.70 in.) sample from the plate. The 18 mm (0.70 in.) length should be parallel to the rolling direction of the plate (this is the longitudinal plane). Mounting: Mount the specimen longitudinal face down in a Bakelite (Union Carbide Corporation) mount, using standard procedures. Grinding: Wet grind the specimen using 320-, 400-, and 600-grit silicon carbide papers. Between each paper, rotate the specimen 90°, and rinse the specimen in flowing water, followed by an alcohol rinse and blow drying. Polishing: Polish the specimen on a rotating wheel (150 rpm), using a low-nap cloth with 6 μm diamond paste or spray. Polish until all the 600-grit scratches are removed. Clean the specimen with cotton in flowing water, rinse with alcohol and blow dry. Polish for two minutes on a rotating wheel (150 rpm), using a low-nap cloth with 0.3 μm alumina. Clean with cotton in flowing water, rinse with alcohol and blow dry. Continue polishing on a stationary wheel or flat surface for 30 to 45 seconds, using a low-nap cloth with aqueous colloidal solution of 0.04 μm silica. Clean the specimen with cotton in flowing water, rinse with alcohol and blow dry. Etching: Etch the specimen in 4% picral plus a few drops of 17% zepherin chloride (a wetting agent). Flush with running water, rinse with alcohol and blow dry. Observation and measurement: The microstructure is shown in Fig. 4.20. The picral etchant only attacks the pearlite colonies or carbides, thus allowing the measurement of the volume fraction of these constituents. Using the procedure described in ASTM E 562, measure the percentage of pearlite in the specimen. Use a 100-point reticle placed in one of the eyepieces (oculars) of the microscope. The ocular must have the ability to adjust the image of the reticle to coincide with the image of the microstructure. (Reticles can be custom-made and purchased from suppliers such as Klarmann Ruling, Inc., Manchester, New Hampshire.) Count the number of points of the grid that fall on top of a pearlite patch (count only a half if the point falls on an edge of the patch). Divide the total counts by 100, and the volume fraction is obtained. In this example, the volume fraction of pearlite is 0.068, or 6.8%. Etching: In order to measure the ferrite grain size, the same etched specimen must be etched in 2% nital (do not repolish to remove the picral etch). Figure 4.21 shows the final microstructure. Observation and measurement: Using the three-circle method described in ASTM E 112, measure the ferrite grain size. This

100 / Metallographer’s Guide procedure can be carried out using a three-circle reticle located in an adjustable-focus ocular. The total number of grainboundary intersections with the three circles must be counted. The total length of the three circles is 500 mm (20 in.). Because pearlite is present in the microstructure, each area must be considered as only a single grain boundary if it is intersected by one of the circles. To calculate the ferrite grain size, the following formula is used:

Grain size (microns) ⫽ (500 millimeters) minus (the percentage of pearlite times 5) divided by (the average number of grain boundaries intersected by three circles per field of view) divided by (the total magnification) multiplied by (1000 microns per millimeter). In this example, the ferrite grain size is 10.8 μm. This converts to an ASTM grain size number of 9.8. From the previous nine submissions, one should have an idea how the metallographer must plan each job at the beginning. The cutting and the mounting procedures must be carefully chosen, so as not to disturb or alter the microstructure. The selection of the specimen orientation with respect to the rolling or forging direction must be considered up front, so that the proper plane is examined. The metallographer must have a basic knowledge of grinding, polishing, and etching procedures, so that the sample is prepared properly the first time and the correct information is obtained. Without this basic knowledge, a metallographer may introduce an artifact into the sample. An artifact is not an actual part of the original microstructure and misleads the metallographer into obtaining false information.

The Metallographer’s Workday at a Small Iron Foundry

Fig. 4.20 Microstructure of AISI/SAE 1010 steel showing ferrite-pearlite

microstructure. Etched in 4% picral to enhance pearlitic areas for percent pearlite measurement. 500⫻

Fig. 4.21 Microstructure of AISI/SAE 1010 steel showing ferrite-pearlite

microstructure. Etched in a solution of equal parts 2% nital and 4% picral to reveal both the patches of pearlite and the ferrite grain boundaries for grain size measurement, using ASTM E 112 standard. 500⫻

An example of a small metallography laboratory is in an iron foundry that manufactures pearlitic ductile cast iron (nodular cast iron) crankshafts with a limited number of sizes and shapes. Ductile cast iron (discussed in Chapters 1 and 2) is produced by inoculating molten iron with magnesium. The magnesium provides nuclei on which graphite nucleates and grows into spheroids. Because of the spheroidal morphology, ductile iron has superior strength and ductility, compared to conventional gray cast iron where the graphite is in the form of flakes. In this foundry, one metallographer handles all the sample preparation for routine quality control. For example, two test blocks (also called keel blocks) are selected, representing the beginning and end of the pour of each heat produced. The test blocks are produced according to ASTM specification A 395 for microstructure and SAE specification J434 for hardness. Therefore, the test blocks are sent to the metallographic lab for examination of the nodular graphite and matrix microstructure and hardness testing. There are approximately 260 to 380 crankshafts cast for each 1000 kg (2200 lb) heat of iron. Eight heats are cast during each eight-hour shift. A single metallographer working the day shift can prepare 48 test block samples sent to the laboratory, representing the production of each 24-hour day. The laboratory is small but fully automated with automatic grinding and polishing equipment. For example, six samples can be ground and polished simultaneously in a matter of minutes. After polishing, the metallographer must examine each specimen in the microscope to rate the distribution and the size of the nodular graphite particles. Rating is done using ASTM A 247 comparison charts, which consist of six standard photographs at 100⫻, showing ranges of graphite shape and size. For example, the ASTM A 247 plate III used by this foundry is shown in Fig. 4.22. Note that the graphite nodules are sized from size 3 (large nodules) to size 8 (small nodules).

The Metallographer and the Metallographic Laboratory / 101

Fig. 4.22 ASTM A 247 Plate III method for evaluating the microstructure of nodular graphite in iron castings. Graphite size: magnification 100 diameters. Courtesy of ASTM

102 / Metallographer’s Guide

Fig. 4.22 (continued) ASTM A 247 Plate III method for evaluating the

microstructure of nodular graphite in iron castings. Graphite size: magnification 100 diameters. Courtesy of ASTM

In addition to a metallographic analysis, six Brinell hardness readings are taken on each test block. A designated area on each block is surface ground to remove any decarburization and surface roughness. The average values must meet a minimum specified hardness, acccording to SAE specification J434 for automotive ductile (nodular) iron castings. This SAE specification is used because the crankshafts are intended for automotive applications, and ASTM specification A 536 for ductile iron castings does not have a hardness specification. If any of the six hardness measurements are below the minimum level of hardness or outside the acceptable range for nodular graphite size and distribution, the metallographer must notify the quality-assurance manager, who in turn will make a decision about the disposition of the crankshafts from that particular heat. In some cases, more crankshafts may be sampled and tested from the same heat. As examples, the results of two test block samples are given. The two test blocks represent two different ferritic-pearlitic ductile iron heats. Microstructure is as follows. Test Block 1. Figure 4.23 shows the microstructure of the ferritic-pearlitic ductile iron test block in the etched condition. The microstructure indicates that the matrix is pearlite with halos of ferrite arround the graphite nodules. This halo formation, called a bull’s-eye microstructure, is typical of an as-cast ferriticpearlitic ductile iron. The nodules are spherical and fairly randomly distributed. The graphite would be rated as type 1, size 4, and falls within the specified acceptable limits. Test Block 2. This test block is also an as-cast ferritic-pearlitic ductile iron and should be similar to the microstructure of test block 1. However, as seen in Fig. 4.24, the matrix is pearlite, but the graphite has a different morphology than test block 1. Much of the graphite is nonspherical and has a degenerate form called “vermicular” graphite. The graphite shape and size would fall outside the acceptable limits. The nonspheroidal graphite is the result of “fading” during the pouring of the heat. During fading, the magnesium level in the liquid iron, between the time it was added to the time the iron solidified, decreased to the point where the inoculation was ineffective. The elapsed time between adding the magnesium and pouring the heat must be closely controlled. Obviously, something happened during this process in casting this heat. Because of the fading, the crankshafts from this heat will be rejected. Vermicular graphite differs from nodular graphite and appears to grow by a different mechanism. Hardness is as follows. Test Block 1. This casting is a ferritic-pearlitic ductile iron and must meet the SAE J434 specifications of grade D5506, which has a hardness range of 187 to 255 HB. This grade would typically have a yield strength (if measured) of 379 MPa (55 ksi) and total elongation of 6%. This casting has an average hardness of 235 HB and is within specification. Test Block 2. This casting is also a ferritic-pearlitic iron, with an average hardness of 303 HB. This level of hardness means that the hardness is higher than the top of the range for grade D5506 and therefore, does not meet the specification. The metallographer in this particular job is not faced with the variety of materials submitted in a large research laboratory or

The Metallographer and the Metallographic Laboratory / 103 failure analysis laboratory. However, the metallographer must know the basic skills of sample preparation, microstructural interpretation, and hardness testing. The previous examples illustrate the basic skills that are necessary for a metallographer. The next section examines the metallographic laboratories in which these procedures can be carried out.

The Metallographic Laboratory

Fig. 4.23 Representative micrographs of ferrite-pearlite ductile cast iron

Fig. 4.24 Representative micrographs of the ferritic-pearlitic cast iron test

test block 1. (a) Note the ferrite halo around the graphite nodules. The matrix is pearlite. 4% picral etch. 100⫻. (b) Same specimen unetched to compare with ASTM A 247, plate III, in Fig. 4.22. 100⫻

To be effective, a metallographer must have the proper tools and equipment. In the Appendix, there is a list of companies that sell metallographic equipment and supplies. To describe the setup of a metallographic laboratory, two examples are used: a basic laboratory associated with a small manufacturing plant, a foundry, or a heat treating shop employing a single metallographer, similar to

block 2. (a) Note the degenerate form of graphite called “vermicular” graphite. 4% picral etch. 100⫻. (b) Same specimen unetched to compare with ASTM A 247, plate III, in Fig. 4.22. 100⫻

104 / Metallographer’s Guide the small foundry laboratory described previously; and a full-scale laboratory associated with a research department of a major steel company, with a team of two to four metallographers (also described previously). These are probably the extreme examples of a metallographic laboratory. Most laboratories fall somewhere in between. It is recognized that there are obviously physical size constraints and financial restraints that impact the setup of a particular laboratory. However, the following examples may be useful as a guideline.

A Basic Metallography Laboratory All metallographic laboratories, no matter what their size, are broken down into three basic areas: the specimen preparation area, the polishing/etching area, and the observation/photographic area. Ideally, the areas should be separated by walls or partitions, because of potential contamination problems. For example, it is mandatory that the dust and debris generated in the sample preparation area, where specimen cutting and grinding take place, be kept away from the polishing and observation areas. In a basic laboratory, it is recommended that the preparation area be in a separate, well-ventilated room. Also, the observation microscope/ metallographs and other delicate optical instruments in the observation area must be protected from the fumes from etching solutions (usually acids) in the etching area. Therefore, the etching area must be well ventilated, usually with a fume hood, and should be separated from the microscopes by a wall and door. Thus, before any equipment is purchased, make sure that the allocated laboratory space is adequate to partition the three areas. The work flow in the laboratory is very important for optimal efficiency. The details of each area are described subsequently. The Specimen Preparation Area. Samples submitted to the metallographic laboratory should be of a manageable size. As was described with the nine submissions in a previous section in this chapter, the metallographer must know the history of the sample in order to be able to perform an effective job. For example, if the steel has been quenched, the metallographer must avoid any heating of the sample during cutting or mounting. Heating will cause the sample to temper and thus alter the original microstructure. In the preparation area, heating can be caused by the cutting operation or by mounting the specimen in a thermosetting plastic in the mounting press. In the previous example, the specimen must be cut under water or flooded with water and mounted in a cold-setting plastic. These procedures are described in detail in Chapter 7. The sample preparation area must have the equipment to cut or shear a metallographic specimen from the submitted piece. Usually, the specimen should be small enough to fit into a 25.4 or 31.8 mm (1 or 1.25 in.) diameter mount. The equipment in this area includes a band saw and/or a hack saw. Also, an abrasive cut-off machine is useful to obtain a damage-free flat surface. For sheet specimens, a small shear is necessary. After the piece is cut to specimen size, a small belt sander is useful to remove rough edges and saw marks. A sink should be nearby to rinse the specimen of cutting debris.

As discussed in Chapter 7, mounting the specimens can take place in many ways. Usually, the metallographic laboratory has a mounting press to mount the specimen in a thermosetting or thermoplastic mounting compound. The mounting press can be either manual or automatic. In laboratories that deal exclusively with sheet products, the specimens can be mounted in a steel clamp, eliminating the need of the mounting press. Also, a mounting press may not be necessary if all the mounting is done with a cold mounting compound. Some cold mounting compounds, such as epoxy, require a fume hood, because many of these compounds are toxic. Once the specimen (or specimens) is mounted, it must be identified, usually by scribing an identification code on the reverse side of the mount. A small, hand-held electric scriber is useful for this purpose. If a transparent mount is used, the specimen identification can be embedded within the mount. The next step in preparing the mounted specimen is grinding, to provide a flat surface. The grinding operation could take place in the preparation area or in its own area, because of the grinding debris and dust that is generated. Grinding can be done by a hand grinding device, by rotating wheels, or by an automatic grinding/ polishing machine. Either way, the specimen must be ground flat through several consecutive grinding papers, usually starting at 120 grit and ending at 600 grit. Grinding is usually done wet. In most laboratories using hand grinding procedures, three or more consecutive and adjacent rotary wheels are in place, each wheel with a different interchangeable grinding paper, for example, 120-, 240-, 320-, 400-, and 600-grit papers. The Polishing/Etching Area. This area must be kept free of dust and is thus separated from the preparation area by a wall. All specimens entering the polishing area should be cleaned to remove any dust and particles. Manual or automatic polishing can be used. Manual polishing requires one or more rotating wheels, each with a different polishing cloth and polishing compound. For example, one wheel would be fitted with a low-nap cloth with a 6 μm diamond paste, and one wheel would be fitted with a low-nap cloth with a 1μm diamond paste. Another one would use a medium-nap cloth with a 0.3 μm alpha aluminum oxide (alumina) slurry. A fourth wheel would use a medium nap-cloth with a 0.04 to 0.06 μm silicon dioxide slurry. Details of polishing are found in Chapter 7. If automatic polishing is used, the same configuration of rotating wheels are used. In automatic polishing, three or more mounted specimens are polished simultaneously in a special holder. Special care must be taken if both grinding and polishing are carried out on the same machine. A sink should be nearby to rinse each specimen or specimen holder after each polishing step. Near the sink, there should be a ventilated etching area. A fume hood is usually required in the etching area. The freshly polished specimens are etched, rinsed, and dried with a heated-air blow dryer. The Observation/Micrography Area. In this area, one needs a metallurgical microscope. Microscopes are described in detail in the next chapter. For a basic metallographic laboratory, the microscope can also have a camera attachment for photographing the microstructure through the lenses of the microscope. The basic laboratory can use instant (e.g., Polaroid, Polaroid Corporation)

The Metallographer and the Metallographic Laboratory / 105 film or a digital camera to avoid the necessity of a dark room for developing and printing cut film. Thus, for a small laboratory, this microscope with camera attachment would suffice for routine micrography. If necessary, a macrocamera, also using instant film, may be required to photograph larger specimens and parts. Near the observation area, the metallographer should maintain a desk and files. Samples can be submitted in the office area so that they can be cataloged. It is important to have a submission form that has the date, name of submitter, number of samples, type of material, composition, specimen history, and what needs to be done. The submission form is the document that is filed for future reference. The office area should have a location for the storage of specimens and records.

A Full-Scale Metallographic Laboratory This type of laboratory is set up to provide full metallographic services, including publication-quality micrographs, extensive quantitative measurements, and so on. This laboratory is equipped for routine and nonroutine metallographic analysis and is usually adjacent to an image analysis system, a SEM, an EPMA, an x-ray diffractometer, and a transmission electron microscope (TEM). For analytical electron microscopy, a scanning transmission electron microscope (STEM) may be required. Although some of these specialized instruments have their own specimen preparation facilities, most depend on support from the metallographic laboratory. The use of these instruments is described in Chapter 6. The Specimen Preparation Area. In the preparation area, a band saw, an automatic hack saw, a shear, a drill press, a grinding wheel, a vice, and an abrasive cut-off machine are available. A full-scale laboratory will have a belt grinder to dress specimens. A diamond saw is useful in sectioning samples without inducing deformation. Usually, more than one automated mounting press is needed, because different materials/jobs are being prepared simultaneously by different metallographers. Also, different mounting materials are required, depending on the sample being mounted. A hand-operated mounting press allows the metallographer some flexibility in adjusting and controlling mounting temperature and pressure. In addition to thermosetting and thermoplastic mounting materials, some specimens require epoxy-type mounting. These materials require a mixing area and a fume hood to exhaust the toxic fumes generated from these organic compounds. A small oven should be inside the hood to cure the materials, if required. Also, a vacuum bell jar is needed to remove bubbles and provide good bonding between the mount and specimen. In the preparation area, there should be a sink to rinse specimens and to cool mounting press pistons and cylinders between cycles. Grinding is accomplished by an automatic grinding/rough polishing machine. This type of machine can maintain a high level of productivity in the laboratory and prevent any bottleneck in hand grinding and rough polishing. Although automatic preparation is available, a set of rotating grinding wheels should also be in the grinding area. The Polishing/Etching Area. In the polishing area, only a limited number of polishing wheels are necessary. These may be set up for the finest alumina and silica polishing compounds and

the finest diamond paste. The selection depends on what is being polished and what will be done with the specimen. For example, alumina (aluminum oxide) or silica (silicon dioxide) would never be used if the specimen was being prepared for the SEM or EPMA, where a qualitative or semiquantitative chemical analysis of inclusions in the specimen is required. This is because many inclusions from the steelmaking process contain complex oxides of aluminum and silicon, and contamination from the polishing compound would make their identification difficult. An ultrasonic cleaner should be available to remove polishing debris from the specimen after polishing. The polishing area should also have equipment for electrolytic polishing/etching located within a hood, because some materials, such as Hadfield steel and certain stainless steels, require electrolytic procedures. A hot-air blow dryer is nearby to dry the specimens just after etching and rinsing in alcohol. Acid storage and flammable-material storage cabinets are installed near the hood and sink area. These cabinets should be ventilated to the outside of the laboratory. Premixed etchants can be stored in these cabinets. Some premixed nonflammable and nonexplosive etchants can be temporarily stored on a shelf inside the fume hood. The Observation/Micrography Area. In a full-scale laboratory, the observation room is separated from the micrography room (with digital cameras, a micrography room is not necessary). This is because of the various people using both areas simultaneously; one room requires darkness, the other full lighting. The observation room contains two or more metallurgical microscopes for general observation of the polished and etched specimens. The microscopes can be used to carry out manual grain size or other simple quantitative measurements by having the eyepieces/oculars retrofitted with graduated scales. These microscopes can also be used to quickly examine the prepared specimen before proceeding to the micrography area. One of the microscopes should be equipped with an instant film (e.g., Polaroid) camera attachment or digital camera, if a quick micrograph is needed. Also, a digital camera with a television monitor should be attached to one of the microscopes to allow more than one person to observe the microstructure of the prepared specimen. Wet film micrography can be done on a metallograph in a separate room. However, with the recent advances in digital photography, a metallograph may not be necessary. A macrocamera setup is needed to take photographs at 1⫻ to 20⫻. The macrocamera is ideal for photographing fractures and in documenting the regions where specimens were cut from a larger piece. The metallograph and the macrocamera can use either instant film, 35 mm film, 4⫻5 in. sheet film, or a digital camera. The 35mm and 4⫻5 in. sheet film require separate adjacent darkroom facilities. One darkroom should be used for film loading and film development and a second darkroom for printing. Separate rooms are necessary, because one metallographer may be printing while another may be developing negatives. Interconnecting light-sealed doors are used between the darkrooms. The developing and printing rooms require sinks and good ventilation. The print-developing room should be set up with both manual and automatic development, washing, and drying equipment. A refrigerator should be nearby to store film and darkroom chemicals.

106 / Metallographer’s Guide Many metallographic laboratories have replaced their film development facilities by using digital cameras mounted on a microscope. Near the observation area, an image analysis system is located. Depending on the size of the system, a separate room may be required. The image analysis system, used for automatic quantitative measurements, consists of a microscope, with special automated stage, a video camera mounted on the microscope, and a video monitor to observe the image being measured. The associated computer is usually located in an inconspicuous area. Another portion of the metallographic laboratory should be set up as an office area for the metallographers and for storage of records, specimens, prints, and negatives. Each metallographer should have his or her own dessicator to store polished and/or etched specimens overnight while they are being prepared. A specimen submission table should be in the office area, and a completed submission form should accompany each specimen or group of specimens. Other equipment that can be part of a full-scale metallographic laboratory includes a microhardness tester for measuring hardness traces across regions of a metallographic specimen or measuring the hardness of individual constituents in the microstructure, a Rockwell or Vickers hardness tester for conducting bulk hardness, and possibly a hot hardness tester for measuring hardness at high temperatures. This hardness testing equipment may be in a separate room near the metallographic laboratory. Also, a hot stage accessory for a microscope is useful in studying changes in microstructure on heating and cooling. The hot stage setup should also include a video camera to record microstructural changes that take place upon heating and cooling.

Safety in the Metallographic Laboratory As with any laboratory containing toxic, flammable, corrosive, and explosive chemicals, as well as moving machinery, certain precautions are necessary to protect both the metallographer and the equipment. Some of the important safety issues are briefly mentioned subsequently. In the specimen preparation area, safety glasses must be worn for eye protection. Safety guards must be placed on the band saw and hack saw for protection against accidental injuries. The cut-off machine must be totally enclosed with a safety shield to protect the metallographer from flying shards of the cut-off wheel, in the event the wheel bursts while rotating at high speed. Usually, these machines cannot be started unless the shield is in position. When mounting specimens, especially using cold mounting compounds such as epoxy, proper ventilation is mandatory to protect the metallographer from exposure to the toxic fumes. A hooded area is needed for this purpose. Also, the organic compounds should not come into contact with skin, because the chemicals can be absorbed into the body through the skin. Latex gloves must be worn when handling these chemicals. Moderate precautions must be taken with the thermosetting plastic mounting materials, particularly in avoiding the breathing of dust generated during the

handling of the various powders. Good ventilation would be prudent in the entire mounting area. Precautions should be taken around the mounting presses, because they operate under fairly high pressure and at temperatures of about 150 °C (300 °F). Protective gloves must be worn to prevent burns during handling the hot components of the presses and hot mounted specimens. The belt grinder must have the proper safety shields in place, and extreme care must be taken when grinding a mounted sample by hand. A shield must also be in place to prevent a sample/mount from flying across the preparation room, if a sample is accidentally released while in contact with the moving belt. In the specimen grinding area, the metallographer must take precautions to prevent accidental injury incurred by contact with the moving parts of the automatic grinding/polishing machine or with rotating manual grinding wheels. During manual grinding, a mount or specimen can accidentally fly free of a spinning wheel and cause injury. Shielding can be used for protection. It is important to remove rings from one’s finger while working with rotating equipment. In the polishing/etching area, the same precautions as previously mentioned apply when operating automatic or manual polishing equipment. The most dangerous operations in this area involve the mixing of chemicals, the use of electrolytic polishing/ etching equipment, and the etching of metallographic specimens. In etching, the metallographer stores, mixes, and handles various chemicals, including hydrochloric acid, nitric acid, picric acid, phosphoric acid, potassium ferrocyanide, and so on. References at the end of the chapter offer further details about the safety procedures used for handling these and other chemicals commonly found in the metallographic laboratory. Safety glasses or a face shield must be worn when using and mixing most chemicals. The metallographer must understand the toxic, flammable, corrosive, and explosive nature of each chemical used and must be aware of the proper mixing precautions of the various chemicals employed in etching. This information is located at the “right-toknow” stations located at the entrance to the metallographic laboratory. Here you will find a book containing a materials safety data sheet (MSDS) for each of the chemicals and materials used in the laboratory. Always consult the MSDS if you are not sure about the chemical or substance you are using. You can obtain an MSDS directly from the manufacturer of the chemical or material being used. In addition to proper handling of acids and flammable chemicals, it should be pointed out that two common chemicals used for etching steels are potentially highly dangerous (most chemicals are potentially dangerous). Picric acid, in the form of its characteristic yellow crystals, if allowed to dry out during storage or in a beaker, becomes a powerful explosive. Picric acid is always stored wet. Just the friction and static electricity involved in simply unscrewing the lid or cap of a bottle containing picric acid, which has become dry from long-term storage, can cause an explosion that could seriously harm and possibly kill the metallographer. Many people do not realize that picric acid was used as an explosive before the invention of dynamite. However, picric acid has been successfully and safely used for over 90 years as an etchant for iron and steel specimens. With proper precautions

The Metallographer and the Metallographic Laboratory / 107 taken, picric acid is a safe and vital part of the metallographic laboratory. Picric acid is always stored in a special metal cabinet, and frequent examination of the moisture content must be monitored. If picric acid becomes dry, special precautions must be used to remove the material from the laboratory. The other dangerous chemical is methyl alcohol (also called wood alcohol or methanol), which, up until recent times, was commonly used for the basic ingredient in the most widely used etchants for steel—picral and nital. Methyl alcohol was also used in rinsing excess water from an etched specimen after rinsing in water. It is well known that methyl alcohol is highly toxic to humans, especially if taken internally, for example, by drinking. However, the alcohol is quickly and easily absorbed through the skin and has the same toxicity effect as if drinking the liquid. It is important to note that methyl alcohol can, for the most part, be replaced in the metallographic laboratory by ethyl alcohol. Ethyl alcohol has very little toxicity by absorption through the skin or by simply breathing the fumes. Many modern laboratories have eliminated methyl alcohol and have substituted ethyl alcohol in its place. It must be kept in mind that denatured ethyl alcohol has some concentration of methyl alcohol. Thus, many metallographic laboratories use pure ethyl alcohol (200 proof). Other chemicals are also dangerous; for example, mixing perchloric acid with other acids can cause an explosion from the heat generated during mixing. Perchloric acid mixtures must be kept cold during use and used only in special hoods equipped with wash-down capabilities. It should be remembered that it is no longer permissible to dispose of chemical waste “down the drain.” A modern laboratory has provisions for the proper storage and disposal of hazardous and toxic waste. Another point to remember is to buy only the quantities of chemicals that will be needed in the short term. Procuring large amounts of chemicals, because it is more economical, is no longer a rule of thumb. The shelf life of the chemicals must be considered, as well as the potential dangers involved in storing bulk chemicals. In general, the shelf life of most chemicals should be no more than five years. Many times, it costs far more to dispose of chemicals through a waste disposal company than it does to purchase them. Of course, smoking and/or eating should never be allowed in the metallography laboratory. This is not only because of the presence of flammable chemicals in the polishing/etching area, but the tars from the smoke coat the delicate lenses of the microscopes and delicate metallographic equipment. “No Smoking” signs should be visible throughout the metallography laboratory. Also, flammable chemicals should be stored in a specially designed and well-ventilated safety cabinet.

Other than smoking, there are only a few precautions to observe when in the observation/photographic area. One serious danger with metallographic equipment is in the handling and use of high-pressure zenon lamp bulbs. Special handling is required in replacing this type of bulb. Gloves must be worn at all times, not only to protect the hand, but to prevent fingerprints on the quartz bulb, which can cause premature failure during use. A face shield must also be worn when handling these bulbs. In the photographic areas, the dark room chemicals should be handled properly and spills cleaned up immediately. These chemicals, although toxic, are not usually life threatening under normal use. A metallographic laboratory should also be equipped with an eye rinse facility, in the event that chemicals are splashed into the metallographer’s eyes. A water shower should be nearby in case of a serious chemical spill on clothing and body. Type A, B, and C fire extinguishers must be placed in or near the preparation area and the polishing/etching area, in case of a fire from flammable liquids or other sources. A fire alarm station should be in or nearby the metallography laboratory. A safety cabinet with a fireproof blanket to smother a clothing fire, as well as fully stocked first-aid supplies and special absorbent materials for chemical spills must be in or near the metallographic laboratory. Some industrial laboratories have a chemical spill team that is trained to deal with any kind of chemical spill or gas leak. If a chemical spill team is not viaible, at least someone in the metallographic laboratory should be trained in first aid and chemical spill procedures. A good rule to remember is—if you are not familiar with a chemical or substance, always check for the toxicity and explosive and flammability level before proceeding. This information must be available to the metallographer. Today’s metallography laboratories usually have this information in the form of MSDSs. SELECTED REFERENCES • • • • • •

Engineering Properties of Steel, American Society for Metals, 1982 Prudent Practices for Handling Hazardous Chemicals in Laboratories, National Academy Press, Washington, D.C. N.I. Sax, Dangerous Properties of Industrial Materials, Reinhold Book Corporation, New York C.S. Smith, A History of Metallography, University of Chicago Press, 1960 C.S. Smith, A Search for Structure—Selected Essays on Science Art, and History, MIT Press, 1981 C.S. Smith, Ed., The Sorby Centennial Symposium on the History of Metallurgy, Metallurgical Society Conferences, Gordon & Breach Science Publishers, 1965

Metallographer's Guide: Practices and Procedures for Irons and Steels Bruce L. Bramfitt, Arlan O. Benscoter, p109-148 DOI:10.1361/mgpp2002p109

Copyright © 2002 ASM International® All rights reserved. www.asminternational.org

CHAPTER 5

The Metallurgical Microscope THE METALLOGRAPHER’S most important tool is the metallurgical microscope. Every metallographic laboratory has at least one metallurgical microscope for observing microstructures. This microscope is different from the conventional microscope, which uses transmitted light for transparent material, for example, stained biological specimens. Metallographic specimens are opaque to light, and therefore, a metallurgical microscope needs a source of reflected light. This source of light is discussed in the following sections. Both kinds of microscopes are commonly called optical microscopes. This term is not used in this book, because it is more appropriate to use the name of the source of the incident “beam” being used to illuminate the specimen. For example, a controlled beam of light is used in both the metallurgical and biological microscopes. Thus, they are called light microscopes or light optical microscopes. If a beam of electrons is employed, the microscope would be called an electron microscope, and if a beam of ions is employed, it is called an ion microscope. Electron microscopes are valuable tools for the metallographer and are discussed in the next chapter. When discussing microscopes, one is entering a field of physics called optics, and many terms, concepts, and mathematical expressions are used that are generally unfamiliar to the metallographer. This chapter describes only those items that are necessary for the metallographer to develop a basic understanding of the microscope. Some of the basic terms described in this chapter include resolving power, the virtual image, bright- and dark-field illumination, numerical aperture, focal length, image contrast, depth of field, and spherical and chromatic aberration. For more detailed technical descriptions, there are several references listed at the end of the chapter. The metallographer must obviously know the basics of the microscope in order to use it properly. These are sophisticated scientific instruments, and the metallographer must have the required working knowledge in order to obtain the optimal benefit from the microscope. The modern-day metallographer is very involved with metallographic interpretation and must fine-tune the microscope to obtain the ultimate image for accurate microstructural interpretation. Also, a metallographer may be in the position to recommend or purchase a microscope or metallograph (a dedicated microscope with built-in camera for taking micrographs). A full understanding of the various features of a metallurgical microscope is necessary in order to intelligently procure this type of instrument. These features include such things as apochromatic objectives, hyperplane oculars, vertical illuminators, counting reticles, widefield

oculars, polarization filters, field diaphragms, interferometers, and tungsten-halogen lamps. This chapter discusses all these features. In addition to developing a basic understanding of the metallurgical microscope, the metallographer must also develop a basic understanding of methods to record the microstructural image. The latter part of this chapter is devoted to the metallograph, a metallurgical microscope that is dedicated to micrography, that is, recording the microstructure. First, the metallographer must understand the microscope.

The Microscope The term “microscope” is derived from the Greek words mikros (small) and skopein (to see). The words were combined and given a Latin form by Giovanni Faber, a Roman scientist, in 1625. The microscope is thus an instrument that can see small things. The Dutch eyeglass maker, Zacharias Janssen, has been credited with developing the principles of the compound microscope in 1590. In the mid-1600s, Anthony Van Leewenhoek, a Dutch amateur scientist, was the first to observe microscopic life in pond water and has been called the father of the microscope. He constructed a simple microscope (not a compound microscope) with a power of 270⫻, which, at the time, was the most powerful microscope ever built. An early example of a compound microscope is seen in Fig. 5.1. This type of microscope was used by Robert Hooke, an English microscopist, in 1665. Although the microscope has been used for over 300 years, it was only in the latter part of the 19th century that the microscope was first used for observing metals. As mentioned in Chapter 4, Sir Henry Clifton Sorby, the father of metallography, used the microscope to observe the microstructure of a polished and etched steel specimen.

The Basic Metallurgical Microscope In this book, only the metallurgical microscope is discussed. Examples of this type of microscope are seen in Fig. 5.2 and 5.3. There are two types of metallurgical microscopes: the upright and inverted microscope. In the upright microscope (Fig. 5.2), the specimen is positioned below the objective, and in the inverted microscope (Figure 5.3); the specimen is upside down above the objective. Each type of microscope has advantages and disadvantages, some of which are listed subsequently:

110 / Metallographer’s Guide Upright microscope

The beam of light can be seen on the specimen surface. The specimen must be leveled in order to maintain focus while moving specimen. x-y movement not limited by stage (entire specimen surface can be observed) Polished specimen surface does not contact stage surface Specimen thickness generally limited by distance between objective and stage Limited stage weight capacity Specimen surface can be scribed with a circle while on the stage for special identification (special scribing device needed)

Inverted microscope

Difficult to see the beam of light on the specimen surface Leveling of the specimen is not required. x-y movement not limited by stage opening (diameter of hole limits the area of specimen to be seen) Polished surface may contact the stage surface (potential for scratches) Ideal for large specimens Usually built with ample weight capacity Scribing not possible

address the differences between manufacturers or describe any particular manufacturer’s design. However, the basic features common to all metallurgical microscopes are described subsequently.

The Mechanical System The mechanical system includes those components that are required for moving the specimen beneath or above the light beam and for focusing the image of the microstructure. The stage is a movable flat platform that supports the specimen. A top view of a typical stage on an upright microscope can be seen in Fig. 5.4. On the stage, one can see the specimen directly below the objective of the microscope. All stages have mechanical movement in two horizontally perpendicular directions. This allows the metallographer to move the specimen from left to right

The basic upright metallurgical microscope shown in Fig. 5.2 consists of many important components (the same components are on the inverted metallurgical microscope). These components can be roughly divided into having three main functions: the mechanical system, the optical system, and the illumination (light) system. All these systems are supported and aligned by the stand, the structural frame or body of the microscope, as shown in Fig. 5.2. This microscope has a far different appearance than microscopes of the past. The older metallurgical microscopes were simply adaptations of the standard biological microscope. Up until a few decades ago, most microscope manufacturers produced the standard upright tube-type microscope. Because the metallographer requires a microscope with reflected light, special attachments were made to fit onto the standard microscope. Today, microscope manufacturers make dedicated metallurgical microscopes that meet the special needs of the metallographer. Each manufacturer has its own unique design and special features. This book does not

Fig. 5.2 An upright metallurgical microscope

Fig. 5.1 A sketch of an early 17th century microscope used by Robert Hooke in 1665

Fig. 5.3 An inverted metallurgical microscope

The Metallurgical Microscope / 111 (x-movement) and front to back (y-movement). Coaxial knurled knobs, that is, one knob within the other, are used to make these horizontal adjustments. These knobs can be seen in the center of Fig. 5.5. The amount of x- and y-movement of the stage can be measured by graduated scales along the edge of the stage. These scales are shown along the edges of the stage in Fig. 5.6. Some stages also allow rotation of the specimen. This is generally the case for an inverted microscope. Also, as described subsequently, all metallurgical microscope stages have a third, up-and-down z-motion that is used for coarse and fine focus of the specimen.

Some metallographers use the scales on the stage to obtain a rough measure of the thickness of certain features in the specimen, such as the thickness of a coating or the length of a crack. The scale is usually calibrated in 0.1 mm divisions. For more accurate measurements, an eyepiece reticle is used. The reticle is a graduated scale within the eyepiece that can be focused along with the image of the microstructure. Reticles are discussed later in this chapter. The Coarse- and Fine-Focus Knobs. On older tube-type microscopes, the coarse- and fine-focus knobs adjusted the barrel of the tube up and down, and the stage was at a fixed vertical position. On modern-day microscopes, the upward and downward movements of the stage are controlled by coaxial knurled knobs easily

Fig. 5.5 Coaxial knurled knobs used to move stage in x- and y-directions Fig. 5.4 The stage of an upright metallurgical microscope with a mounted specimen directly beneath the incident beam of light

Fig. 5.7 Coaxial knurled knobs for fine (small, inside knob) and coarse Fig. 5.6 Scales on the stage that indicate the amount of x- and y-motion

(large knob) focus. Note the graduated scale on the fine-focus knob, which indicates the amount of vertical (z-axis) movement.

112 / Metallographer’s Guide accesssible to the metallographer. In metallurgical microscopes, these are the coarse- and fine-focus knobs similar to those shown in Fig. 5.7. For coarse focus, the large-diameter knob is used, and for fine focus, the small-diameter knob is used. Most microscopes have sets of knobs on both sides of the microscope stand for both right- and left-handed operators. In Fig. 5.7, the fine-focus knob has a graduated scale from 0 to 180, with each increment representing a movement of 0.001 mm. The metallographer can use the graduations on the fine-focus knob to measure depth of a feature within a specimen. For example, the depth of a pore can be measured by magnifying the pore so that both its edge and bottom are in the field of view. With the field aperture of the light source fully open, the metallographer first focuses on the edge of the pore and records the graduated location on the barrel of the fine-focus knob. The knob is then moved to focus on the bottom of the pore. By subtracting the location at the bottom of the pore from that at the top of the pore, the depth is determined. Generally, the graduations on the fine-focus barrel are 0.001 to 0.005 mm, depending on the microscope manufacturer. This procedure is described in more detail in the section “Special Procedures for the Metallurgical Microscope” at the end of this chapter. Also, the proper way to focus the microscope on a specimen is described in that section.

The Optical System

Magnification. The main function of the objective is to create a magnified image. This magnified “real” image is created within the tube length of the optical system at a point called the image plane of the eyepiece. Figure 5.8 shows a simplified ray diagram of a metallurgical microscope. In this diagram, the reflected light from the specimen passes through the objective in the form of a magnified image of the specimen. This magnified image is projected onto the image plane of the eyepiece (called the real image). At this point, the image is again magnified by the eyepiece and observed through the lens of the human eye. What the eye actually sees is an image called the virtual image. The virtual image appears about 250 mm (10 in.) from the eye (see Fig. 5.9). The image is inverted, because each time the image passes through a lens system, that is, through the objective, through the eyepiece, and through the lens of the eye to the retina, the image is inverted. Many metallurgical microscopes are purchased with a set of 5, 10, 20, 50, and 100⫻ objectives. A 40 or 60⫻ objective can be used in place of the 50⫻ objective. Some microscopes can be purchased with objectives as low as 1⫻ and as high as 200⫻. The symbol “⫻” is a shortened way to represent the number of times in diameter the image is magnified, that is, 50⫻ means 50 times the diameter, or 50 times magnification. Most modern-day objectives have the magnification boldly engraved on the barrel, as shown in Fig. 5.10. This is a 100⫻ bright-field objective with a

The optical system consists mainly of an objective (the lens assembly close to the object or specimen) and an eyepiece or ocular (the lens assembly close to the eye). They are called lens assemblies, because each objective and eyepiece contains more than one lens element, that is, compound lenses. Some objectives contain up to 14 different lenses. Older microscopes are constructed with a fixed distance between the objective and the eyepiece, called the mechanical tube length. The tube length is measured from the top of the tube to the last thread of the objective, that is, at the point where the unthreaded portion of the objective meets the microscope or nosepiece. Each microscope manufacturer has a fixed tube length that generally varies between 160 and 250 mm. For this reason, most objectives could not be interchanged between microscopes of different manufacturers. However, more and more manufacturers are developing infinity-corrected objective lenses that depend less on tube length. These lenses are discussed in a later section.

The Objective The objective is the most important component of the optical system, because it defines the quality of the magnified image. For this reason, the metallographer, when purchasing a microscope and particularly a metallograph, should always obtain the best objectives possible, even if some other feature of the microscope needs to be sacrificed because of cost. The objective lens controls six important properties of the optical system of a microscope. These are the magnification, numerical aperture, resolving power, depth of field, working distance, and light-gathering capability.

Fig. 5.8 Sketch of the ray diagram for the typical upright metallurgical

microscope showing the location of the human eye, the eyepiece, the real image, the objective, and the polished specimen

The Metallurgical Microscope / 113 numerical aperture (discussed subsequently) of 0.90. Other markings indicate that the objective is to be used “dry.” This means that it is used with air between the tip of the objective and the specimen (the normal situation). Some objectives cannot be used in air, as in the case of an oil-immersion objective, where a special oil is placed between the tip of the objective and the specimen. The oil-immersion objectives provide more resolving power than a dry objective. The oil-immersion objective is discussed later in this chapter. Also in Fig. 5.10, the number “210” indicates that this objective must be used in a microscope with a tube length of 210 mm, and the “0” indicates that the objective is not corrected for a cover slip or cover glass (a thin, glass sheet that is placed on top of the specimen in a biological microscope). If a cover slip is necessary, the objective will indicate its required thickness in millimeters instead of a “0.” Cover slip correction is discussed later. The final magnification in the microscope is usually the magnifying power of the objective times the magnifying power of the eyepiece. For example, a 100⫻ objective with a 10⫻ eyepiece yields a total magnification of 1000⫻. This assumes that the tube factor is 1⫻. Some older microscopes can have tube factors greater or less than 1⫻ (a range from 0.8 to 1.25⫻). The tube factor is usually engraved on the microscope tube. A tube factor is necessary when there is an intermediate lens in the light path between the objective and the eyepiece. Newer microscopes that use infinity-corrected objectives have a special built-in tube lens (discussed later), and a tube factor is not necessary.

Magnification can be achieved by any combination of objectives and eyepieces that produce the required magnification. For example, for a magnification of 250⫻, the metallographer may choose any of the following combinations: Objective

10⫻ 25⫻ 50⫻

Eyepiece

25⫻ 10⫻ 5⫻

Even though the magnification of each combination is identical, the quality of the image produced is different. As discussed previously, the objective defines the quality of the image, and in the three examples listed previously, for the best quality image, the metallographer should usually choose the objective with the highest magnification (numerical aperture), in this example, the 50⫻ objective with the 5⫻ eyepiece. With this combination, most of the total magnification is obtained with the 50⫻ objective, and the higher-power objective will also have the greatest resolving power (discussed subsequently). Numerical Aperture. As can be seen in Fig. 5.10, there are other words and numbers engraved on the barrel of the objective. These include the type of lens and the numerical aperture (NA). The objective shown in Fig. 5.10 has a NA of 0.90. The NA is a measure of the light-gathering ability of the objective lens

Fig. 5.10 Photograph of a 100⫻ bright-field objective with a numerical

Fig. 5.9 Sketch of the same ray diagram in Fig. 5.8, but showing the location of the virtual image

aperture of 0.90. The “dry” indicates that the objective is used without oil between the objective lens and the specimen; the “210” indicates the required 210 mm tube length of the microscope; and the “0” indicates that the objective is used without a cover slip (cover glass).

114 / Metallographer’s Guide assembly. The higher the number, the greater the amount of light collected. Mathematically it is represented by the formula: NA ⫽ ␮ sine ␣

where ␣ is the aperture half-angle of the most tilted (oblique) light rays entering the objective lens, and μ is the refractive index of the medium (i.e., air where μ ⫽ 1.0) in front of the lens. In Fig. 5.11, which is a schematic representation of light rays being reflected from the specimen to the objective lens, the aperture half-angle ␣ is shown as 18°. From the previous equation, this objective would have a NA of 0.30. Figure 5.12 has been included to further illustrate the importance of the light-gathering ability of an objective. In these figures, ray diagrams represent objectives with a NA of 0.70 (Fig. 5.12a) and a NA of 0.25 (Fig. 5.12b). In both cases, the objectives are assumed to be correctly focused on the specimen. In Fig. 5.12(a), the aperture half-angle is 45°, and all seven reflected rays pass through the objective lens. In Fig. 5.12(b), with a aperture half-angle of 15°, only three of the seven rays pass through the objective lens. In both figures, the angular relationship of the seven rays are identical. Therefore, the objective in Fig. 5.12(a) has the greatest light-gathering ability and thus, the highest NA. With air having a refractive index of 1.0, the maximum practical NA is about 0.95. However, when a special oil is placed between the lens and the specimen, the NA can be increased to 1.40. Most oils that are supplied by microscope manufacturers have a refractive index of 1.515, which matches the refractive index of the glass in the lens. Special objectives, called oil-immersion objectives, are used with oil. With this almost-perfect matching, the specimen is actually coupled to the lens, and the lens can gather more reflected light. This is discussed subsequently in the subsection “Oil-Immersion Objective.” Resolving Power. The higher the numerical aperture, the greater the resolving power or resolution of the microscope.

Resolving power is the ability of the objective to distinguish features that are closely spaced. The resolution can be determined by the following formula: d ⫽ (0.61 ␭)/NA

where d equals the distance between two points or lines being resolved and ␭ is the wavelength of the light from the illumination source (i.e., tungsten-filament lamp, etc.). Thus, the resolving power is larger when the NA is large and the wavelength of the light is short. With a good oil-immersion lens (high NA), the limit of resolution is about 0.2 μm. To show the effect of NA on the quality of a microstructural image, Fig. 5.13 shows micrographs of the same field of pearlite in a gray cast iron taken with 40⫻ 0.55 NA and 0.65 NA objectives. Both micrographs were taken at the same magnification of 600⫻. Note the sharper image in Fig. 5.13(b), the micrograph taken with the 0.65 NA objective, especially the resolution of cementite lamella in the pearlite (see arrows). Filters can also be used to enhance the resolving power of an objective. For light, the shorter wavelengths are at the violet-bluegreen end of the spectrum, and the higher wavelengths are at the orange-red end of the spectrum. The wavelength of the light beam can be controlled by filters and the actual light source itself. Figure 5.14 shows coarse pearlite in a pearlitic gray cast iron. The

Fig. 5.11 Sketch of an objective and a specimen showing the cone of light

that can enter the objective lens from reflection from the specimen; ␣ is the half-angle and is used to calculate numerical aperture. The working distance is indicated as the distance between the bottom of the lens and the top of the specimen.

Fig. 5.12 Sketches showing two different objectives with half-angles of (a)

45° and (b) 15° and numerical apertures of 0.70 and 0.25,

respectively

The Metallurgical Microscope / 115 micrographs in Fig. 5.14 were taken without a filter and with a green filter. Note the improved resolution of the pearlite in Fig. 5.14(b), the micrograph taken with the green filter (see the arrow). Useful magnification is a term that applies to resolving power. In the microscope, magnifying an image beyond the resolving power of the lens system produces a magnified image but no further detail in the image. The magnification range up to the point of optimal detail is useful magnification (also called significant or meaningful magnification). Any magnification above this point is called empty magnification. From theory, empty magnification is pro-

duced above a magnification of 1000 times NA. Therefore, using this rule, an objective with a NA of 0.95 would produce a useful magnification limit of 950⫻, and from experience, most metallographic work is conducted at or below a magnification of 1000⫻.

Fig. 5.13 Micrographs of pearlitic gray cast iron taken with 40⫻ objec-

Fig. 5.14 Micrographs of pearlitic gray iron taken (a) with and (b) without

tives of different numerical apertures. Micrograph (a) taken with an objective with NA ⫽ 0.55 and micrograph (b) taken with NA ⫽ 0.65. Note the better resolution in (b) (see arrows). The white constituent is ferrite, the lamellar constituent is pearlite, and the dark gray flakes are graphite. 4% picral etch. 600⫻. Courtesy of J. Wright, Lehigh University

a green filter. Note the improved resolution in the micrograph taken with the green filter (see arrows). The light gray speckled constituent is steadite (iron phosphide eutectic), the lamellar constituent is pearlite, and the dark gray flakes are graphite. 4% picral etch. 600⫻. Micrographs by J. Wright, Lehigh University

116 / Metallographer’s Guide Depth of Focus. Another important property of an objective lens is depth of focus (also called depth of field), which is the maximum amount of vertical movement of the stage or objective without any noticeable change in focus on the specimen. The depth of field is inversely proportional to the square of the NA as seen in the following equation: μ␭ Depth of field ⫽ ⫾ 2NA2

where μ is the index of refraction of the medium between objective and specimen, for example, μ ⫽ 1 for air; and ␭ is the wavelength of the light (mm), for example, green ⫽ 548 nm. This means that an objective with a lower NA will allow a greater depth of focus. Thus, for rough or heavily etched surfaces, a low NA lens must be used. Working Distance. The working distance is the distance between the tip of the objective lens and the specimen, as illustrated in Fig. 5.11. The working distance decreases as the power of the objective increases and generally, as the NA increases. For example, the working distance of a 10⫻ objective may be about 10 mm, whereas the working distance of a 100⫻ objective may be less than 0.5 mm. With short working distance objectives, the metallographer must take extreme care in order to avoid damaging the tip of the objective by chipping, cracking, or scratching the glass lens by contact with the specimen. Some objectives with short working distances are spring loaded, which allows the bottom portion of the objective to retract into the objective barrel if the tip is inadvertently pressed against the specimen. This is a safety feature to minimize damage to the exposed objective lens. However, there is no reason to ever touch the specimen surface with the objective lens. As is seen later in this chapter, there are specially designed objectives with long working distances. For example, a long working distance objective is required when observing a specimen through a glass or quartz window of a hot stage (a special attachment that heats the specimen under vacuum or inert gas). Basic Characteristics of the Objective. The following basic rules apply to all objective lenses (within their class). As magnifying power increases: • • • •

5.15 shows a nosepiece with five objectives. The objectives are mounted in progressive rotational order. As the metallographer rotates the nosepiece, there are obvious mechanical stops built into the nosepiece that allow for proper optical alignment with the beam of light. This alignment with the optical axis means that the objective is parcentric. During rotation of a nosepiece with five objectives, the first objective in rotation might be the 5⫻ objective, followed by the 10⫻ objective, the 20⫻ objective, the 50⫻ objective, and finally, the 100⫻ objective. The nosepiece mounted on a microscope shown in Fig. 5.16 is rotated clockwise, looking at the nosepiece from the specimen, to achieve the

Fig. 5.15 A typical nosepiece from a metallurgical microscope. This nosepiece accomodates five objectives.

Working distance decreases Lens diameter decreases Field of view decreases Depth of field decreases As NA increases:

• • •

Resolving power increases Useful magnification increases Light-gathering increases

The Nosepiece For convenience, most metallurgical microscopes have a rotating nosepiece where three or more objectives are mounted. Figure

Fig. 5.16 Nosepiece mounted on a typical upright metallurgical microscope

The Metallurgical Microscope / 117 previously mentioned order of magnification. When the nosepiece is rotated from objective to objective, the field remains in approximate focus. This is known as being parfocal. The only adjustment may be to slightly adjust the fine-focus knob. However, the central features of the microstructure remain centered. Of course, the field is much larger at the lower magnifications. This is shown in Fig. 5.17 where the same field is photographed after

rotation of the nosepiece to 5, 10, 20, 50, and 100⫻ (using a 10⫻ eyepiece). In this example, a white cast iron is shown with a dominant feature of massive cementite platelets (white constituent). The rotating nosepiece is an important advantage in metallography, because the metallographer can quickly find the area of interest at low magnification and then rotate the nosepiece to a higher magnification for better detail of the features in the field. In

Fig. 5.17 Micrographs taken of the same field of a white cast iron by rotating a five-objective nosepiece through (a) 5⫻, (b) 10⫻, (c) 20⫻, (d) 50⫻, and (e) 100⫻. The white constituent is cementite, and the dark constituent is pearlite. 4% picral etch. Final magnification (a) 50⫻, (b) 100⫻, (c) 200⫻, (d) 500⫻, and (e) 1000⫻

118 / Metallographer’s Guide fact, this should be normal procedure when examining a specimen for the first time. Always scan the specimen at low magnification, that is, using the 5⫻ objective, to find the field of interest and then rotate the nosepiece to progressively higher magnifications. It is not advisable to rotate directly from a 5⫻ objective to a 100⫻

objective, because changing the magnification from 50⫻ to 1000⫻ is too large a change. Also, there may be a chance of hitting the specimen with the lens of the highest magnification objective (the longest objective with the shortest working distance), especially if the objective is not fully screwed into the nosepiece. This loosening of objectives can actually happen when the nosepiece is rotated by using the objectives as a handle instead of properly using the knurled ring on the nosepiece for rotation. If the metallographic laboratory has more than one microscope, always place the objectives in the same rotational order in the nosepiece. This simple procedure will prevent damage to the expensive higher-power objectives with the very short working distances.

Optical Defects in Objectives Microscope manufacturers strive to produce the perfect objective. This is almost impossible, because even if the glass lens is ground to perfection, there are errors or aberrations that are present due to the inherent nature of refraction when light passes through air and into a glass lens, and due to the different wavelengths within the visible light spectrum. Therefore, all lenses need to be corrected for these aberrations so that the metallographer will observe a near defect-free image of the microstructure, that is, an image that is as close as possible to the actual microstructure. In order to understand how these aberrations arise, the metallographer must understand how light interacts with a lens. In most cases, we are dealing with convex lenses where the lens surfaces are curved outward, as seen in Fig. 5.18(a), as opposed to concave lenses where the lens surfaces curve inward, as seen in Fig. 5.18(b). The convex lens is a magnifying lens that is necessary to produce the highly magnified image observed in the microscope (most compound objectives contain both convex and concave lenses). When a beam of light travels through air and then through the glass lens, the speed of the beam of light decreases. The speed change creates a phenomenon called refraction. A simple example of refraction is observed when a pencil is placed into a container of water, as seen in Fig. 19. The portion of the pencil in the water appears to bend at an angle to the portion of the pencil in the air above the water. In a lens, this bending of light, or refraction, is

Fig. 5.17 Micrographs taken of the same field of a white cast iron by

rotating a five-objective nosepiece through (a) 5⫻, (b) 10⫻, (c) 20⫻, (d) 50⫻, and (e) 100⫻. The white constituent is cementite, and the dark constituent is pearlite. 4% picral etch. Final magnification (a) 50⫻, (b) 100⫻, (c) 200⫻, (d) 500⫻, and (e) 1000⫻

Fig. 5.18 Sketches of a (a) convex and a (b) concave lens

The Metallurgical Microscope / 119 the feature that makes the lens work. However, refraction can create aberrations. Because the beam of light (called white light) is composed of various primary colors (red, orange, yellow, green, blue, and violet), the colors observed in a rainbow, and each color has a different wavelength, the speed of each color is slowed at a different rate as it passes through the glass. For example, red has a wavelength of 656 nm, green a wavelength of 548 nm, blue a wavelength of 486 nm, and violet a wavelength of 405 nm. These differences in wavelength can be seen when a beam of light passes through a prism and the beam splits into its primary colors. This effect creates two kinds of lens aberrations: chromatic aberration and spherical aberration. Chromatic aberration is created by the different refraction angles (different wavelengths) of the various colors of the light beam passing through the lens. Axial (longitudinal) chromatic aberration is illustrated in Fig. 5.20 where light rays passing through a convex lens separate into blue, yellow-green and red colors along the optic axis. This aberration must be corrected, because all the colors should have one common focal point and not a separate focal point for each color. The lens designer must

Fig. 5.19 A photograph of a pencil immersed in a dish of water showing

correct for this disparity in focus. There is also a lateral (transverse) chromatic aberration, where the light of one color has a greater magnification than another color. Without correction, the image as seen in the microscope will have color fringes. Spherical aberration is created by the different bending angles of the light rays as they pass through a convex lens. This is illustrated in Fig. 5.21. The light rays entering the outer diameter of the lens (the thinnest part of the lens) are called marginal rays. These rays have a different focal point than the axial rays that enter and exit the central portion of the lens (the thickest part of the lens). The lens must be corrected in order to create one common focal point. Coma is another lens defect that produces comet tails on images of points in the specimen. It is more pronounced in those light rays that pass through the lens further away from the lens axis. Coma can be corrected by grinding the proper surface curvature of the lens. Astigmatism is a lens defect that creates lines in the image from points in the specimen. The word astigmat means “not a point,” and, as with coma, it can be corrected by grinding the proper curvature. Curvature of field is a serious defect in images produced by convex lenses. In the metallurgical microscope, the image must be as flat as possible so that the center of the image is in as sharp a focus as the outer edges. Curvature of field creates blurred outer edges. It is very difficult to design an objective without having some degree of curvature of field, and thus, these specially designed objectives are very expensive. In some microscopes, the curvature of field created in the image of the objective can be corrected by the eyepiece. Distortion is another defect that cannot be tolerated in the metallurgical microscope. The image created by the objective can bow in or out at the outer edges. In other words, if examining a square grid of perpendicular lines, the grid at the center (axis) would represent the proper spacing, but near the edge of the image, the lines become curved and no longer represent the true grid spacing. This can be seen in Fig. 5.22 where it is greatly exaggerated. Distortion is produced by a lens having a different magnification at the central axis than at the outer edges of the lens. Most modern metallurgical microscopes have been corrected for

the effect of refraction of light, that is, the apparent bending of the pencil in water due to a difference in refractive index between water and air

Fig. 5.20 Sketch of a ray diagram of light passing through a convex lens

with chromatic aberration. The rays passing through the outer portion of the lens split into different focal lengths of blue, yellow-green, and red light because of the different wavelengths.

Fig. 5.21 Sketch of a ray diagram of light passing through a convex lens

with spherical aberration. The rays passing through the center of the lens (axial rays) have a different focal point than the rays passing through the edge of the lens (marginal rays).

120 / Metallographer’s Guide

Fig. 5.22 Sketch of a distorted image. (a) Barrel-type image and (b) pincushion-type image

parallax, which eliminates distortion. Therefore, it is very rare to see distortion in modern microscopes.

Types of Objectives The selection of an objective can be quite confusing for the beginning metallographer. This is because there are many different types of objectives. Some have corrections for various lens aberrations. These corrections, discussed previously, include spherical aberration, coma, astigmatism, distortion, and chromatic aberration. Some of these same aberrations can be found in the lenses of the human eye, and eyeglasses are used to correct for these errors. Some objectives have glass lenses; some have lenses made from calcium fluoride (fluorite). Lens designers even use different types of glass, for example, flint glass or crown glass. Some objectives are produced for exceptional flatness of field, some are produced with a longer focal length, and others are produced for dark-field applications. The metallographer should be familar with the various types of objectives available for the microscopes in the metallographic laboratory. Some of these objectives are briefly described subsequently. Achromatic Objective. This is the most commonly used and least-expensive objective for the metallurgical microscope. The objective is partially corrected for chromatic aberration (discussed previously) by correcting for two colors, usually green and red. This means that the red and green colors of the spectrum of light are brought into the same focal point. An achromatic objective, or simply an “achromat,” is also partially corrected for spherical aberration for one color, usually the green color or the yellowgreen color. Therefore, when using an achromat, it is best to use a green or yellow-green filter to take into account this later correction. Because they are only partially corrected, they are not

suitable for color micrography but are well suited for black and white micrography. An advantage of the achromatic objective is that it has a larger working distance than the more highly corrected objectives, for example, the apochromatic objective. This is because there are fewer lens elements in the objective. A larger working distance provides for more clearance between the tip of the objective and the specimen surface. With a larger clearance, there are fewer chances of chipping, scratching, and cracking the exposed lens. Some objectives are spring loaded to minimize such damage. An achromatic objective is generally not identified with a code or symbol engraved on the barrel of the objective, whereas most all the other objectives have special identification. Semiapochromatic (Fluorite) Objective. These objectives, called fluorites, employ fluorite instead of glass for the lens elements. Fluorite is the mineral fluorspar (calcium fluoride) and can be ground into lenses from either natural or synthetic crystals. These fluorite objectives are more expensive than the achromats. They are corrected for chromatic aberration for two colors, the red and blue or the red and green colors, and are corrected for spherical aberration in two colors. This means that these objectives can be used for color micrography as well as black and white micrography. Usually these objectives are identified by a “Fl” or “Fluor” symbol that is engraved on the barrel. Apochromatic Objective. These objectives are more expensive than the achromats and fluorites, because they are more highly corrected. Therefore, these are the finest objectives available and are particularly suited for higher-magnification metallography. They are chromatically corrected for three colors, the primary colors red, blue, and green, and spherically corrected for at least two colors, generally green and blue. This means that an apochromatic objective will perform best with a green or blue filter. These objectives are used for color micrography. They are also very suitable for black and white micrography. Apochromatic objectives usually have higher NAs than the achromatic and fluorite objectives and thus have the potential for higher resolving power. Generally, these objectives are only used for magnifications of 500⫻ and above. Even though these objectives are chromatically corrected for three colors and spherically corrected for two colors, they are still not optimally corrected. Further corrections of the apochromatic objective can be made by the proper selection of a compensating eyepiece. Therefore, when using apochromatic objectives for optimal correction, it is important to couple the objective with the compensating eyepiece of the same microscope manufacturer. Generally an apochromatic objective is identified with an “Apo” engraved on the barrel. One manufacturer uses “CF,” or chromatic-free, on the barrel of their apochromatic objective. All the aberrations have been corrected for in the objective, and they do not depend on the eyepiece for additional corrections. This type of objective is ideal for chargecoupled device (CCD) digital cameras that do not require an eyepiece. Plano Objective. The plano objectives are corrected for flatness of field and therefore are sometimes called flat-field objectives. The previous three types of objectives are generally not corrected for flatness of field unless they are engraved plan-achromatic, planfluorite, or plan-apochromatic. If the objec-

The Metallurgical Microscope / 121 tive is engraved with just “plano,” “plan,” or “pl,” it is a plan-achromatic objective. A plan-apochromatic objective may be engraved as “Pl apo.” The plan-apochromatic objective is very expensive and is the most highly corrected objective available for the microscope. These expensive objectives may not be needed for a microscope that is only used for observation, because the metallographer can accept some curvature of field and simply focus and refocus on the field of interest. However, if the instrument is used for micrography, a plano objective will produce the flatness of field necessary for the entire micrograph. Phase Contrast Objective. When the optical system is functioning under phase contrast conditions (transmitted light), special phase contrast objectives are used with a special vertical illuminator (condenser). A phase contrast objective may have “Phaco,” “Ph,” “DL,” or “DM” engraved on the barrel of the objective. Phase contrast illumination is not used for specimens of steel or cast iron but is widely used in transmitted light for biological specimens. Strain-Free Objective. For ferrous specimens, differential interference contrast, or the Nomarski technique, is more commonly used than phase contrast (previously mentioned). In this case, a strain-free objective is used, and this objective may be engraved with an “N” for Nomarski. Differential interference contrast is discussed later in this chapter. Also, strain-free objectives are used for interference illumination, described later in this chapter. These objectives can be engraved with an “IK” for interference contrast, “NIC” for Nomarski interference contrast, “DIC” for differential interference contrast, “POL” for polarized light, or “Pol. Interf.” for polarization interference contrast. A

Fig. 5.23 Photograph of a 40⫻ dark-field objective (left) and a 40⫻

bright-field objective (right). Both objectives have a numerical aperture of 0.65, a required tube length of 210 mm, and are used dry. In the dark-field objective, note the annular opening around the central objective lens for the incident light path.

strain-free lens may also be engraved with the letters “SF” for strain-free. Dark-Field Objective. Most objectives are produced for bright-field illumination. Bright field is the normal operating illumination of the metallurgical microscope, where the incident light passing through the objective is perpendicular to the specimen surface. However, there are certain applications where dark-field illumination is required. In these cases, the incident light radiates from the objective at oblique angles to the specimen surface. The dark-field objective illuminates the specimen with a 360° circle of light around the periphery of the objective barrel, thus allowing only the reflected light to pass through the glass lenses of the objective. Dark-field and bright-field objectives are shown in Fig. 5.23. Note the annular opening surrounding the central lens of the dark-field objective (left). This circular opening allows the illumination light to pass around the outer portion of the objective (just inside the objective barrel) while the reflected light passes through the lens of the objective. More details on dark-field illumination can be found later in this chapter. A dark-field objective usually has a “D,” “DF,” or “BD” engraved on its barrel and is larger in diameter than a bright-field objective, as seen in Fig. 5.23. Long-Working-Distance Objective (Quartz Corrected). Special objectives with a long working distance are produced for hot-stage microscopy. The long working distance is necessary, because the objective must be kept at a safe distance from the heated specimen. Generally, the working distances range from 6 to 16 mm. Also, these objectives may be corrected for quartz. This is because there is usually a clear quartz window between the specimen and the objective. As described previously, these objectives are corrected for the quartz window. An objective used for the hot stage may display an “H” (hot) or “L” (long working distance) engraved on the barrel. Sometimes the barrel of the objective will be engraved with the word “Quartz.” Oil-Immersion Objective. Special oil-immersion objectives are designed to give the highest resolution obtainable with a light microscope. These objectives are used with a drop of special oil that is placed between the tip of the objective and the specimen. The oil has the same refractive index as the glass used in the objective lens (μ ⫽ 1.515). Because the NA is determined by the refractive index and aperture half-angle of light entering the tip of the objective (explained previously in the section “Numerical Aperture”), an oil-immersion objective can have a much higher NA than a dry objective. In all the other objectives described previously, the light must pass through air with a refractive index of unity (μ ⫽ 1). In passing from the glass lens with μ ⫽ 1.515 into air with μ ⫽ 1.0, the rays of light are bent by the phenomenon of refraction (see Fig. 5.19). This also occurs when the same light is reflected off the specimen back through air into the glass lens of the objective. This bending of light causes a portion of the light to be lost. If the bending of light is eliminated, very little light is lost, and therefore, most of the light passes to the specimen and back through the lens. Earlier in this chapter it was mentioned that the more light gathered by the objective, the higher the NA. Oilimmersion objectives can have a NA up to about 1.40. The upper limit for dry objectives is NA ⫽ 0.95. Oil-immersion objectives at

122 / Metallographer’s Guide 160⫻ are available for high-resolution requirements. Figure 5.24 shows a comparison of the image produced with an oil-immersion objective (NA ⫽ 1.3) and a dry objective at the upper limit of NA (NA ⫽ 0.95). Both micrographs were taken at the same magnification of 1300⫻. In this fully pearlitic microstructure, one can see the improved resolution using the oil-immersion objective.

Fig. 5.24 Micrographs of an AISI/SAE 1080 steel showing the same field of

pearlite taken with (a) a dry objective of NA ⫽ 0.90 and (b) and oil-immersion objective of NA ⫽ 1.30. Note the improved resolution in the micrograph taken with the oil-immersion objective. 4% picral etch. 1300⫻. Courtesy of S. Lawrence, Bethlehem Steel Corporation

Generally, oil-immersion objectives are only rarely used, because they are not required for the vast majority of metallographic specimens. Oil-immersion objectives require special care and must be cleaned after each use. The special procedure used for an oil-immersion objective can be found in the procedures section at the end of this chapter. An oil-immersion objective is usually engraved with “Oil,” “Oel,” or “HI” (homogeneous immersion). Also, any objective with a NA greater than 1.0 is usually an oil-immersion objective. Infinity-Corrected Objectives. Newer metallurgical microscopes have objectives that are “infinity” corrected. In a conventional microscope, the rays of reflected light emerging from the back lens of the objective lens are not parallel. With an infinitycorrected objective, the reflected light rays emerging from the back lens of the objective are parallel (projected toward infinity). To use these objectives, a tube lens is mounted in the path between the objective and the eyepiece (above the vertical illuminator). This lens brings the parallel rays from the objective into convergence (focus) onto the focal plane of the eyepiece diaphragm (described in the section “Types of Eyepieces” later in this chapter). The advantage of an infinity-corrected lens is that the light rays are parallel when passing through the vertical illuminator (bright-field reflected mirror) region of the microscope tube. This means that optical components, that is, polarizers, can be inserted into the column of parallel light rays without introducing optical errors (aberrations) and at an exact magnification of 1⫻. Some infinity-corrected objectives have the symbol “⬁” engraved on the barrel. These objectives are less restricted to a fixed tube length of the microscope and in some special microscopes, allow movement of the objective rather than the stage for focus. Retractable Objectives. Some objectives with a short working distance are spring loaded to prevent damage to the exposed lens of the objective when inadvertently pressed against the specimen. (Note that there is no circumstance where any objective should physically touch the specimen.) A spring-loaded assembly allows the bottom part of the objective to retract into the objective barrel. This type of objective may be useful for students who are not familiar with a metallurgical microscope. Experienced metallographers have less need for this feature. Cover-Glass-Corrected Objectives. It must be remembered not to mix objectives and eyepieces of different microscope manufacturers and even from different microscopes of the same manufacturer. Generally, when a metallurgical microscope is purchased, the set of objectives and eyepieces will only be used for that model of microscope. This is because of different tube lengths, tube factors, and other basic characteristics of the microscope. It should also be pointed out that the objectives for a biological microscope should not be used on a metallurgical microscope. This is because the objectives for the biological microscope are corrected for a cover glass (also called cover slip). The cover glass is a thin glass sheet that is placed over the biological specimen (blood stain, biopsy tissue slice, etc.). The standard cover glass thickness is 0.17 mm (other thicknesses are available). One way to distinguish a cover-slip-corrected objective from an uncorrected objective is by “0.17” engraved on the barrel of the objective. For example, in Fig. 5.25 a cover-glass-corrected

The Metallurgical Microscope / 123 eyepieces, as seen in Fig. 5.27. In a metallographic laboratory with many people using the microscope, the metallographer should remember his or her unique interpupillary distance so that the binocular attachment can be reset each time the microscope is used. Some microscopes have another eyepiece adjustment called the diopter scale, as seen in Fig. 5.28. This adjustment compensates for differences in focus of the left and right eyes. In Fig. 5.28, the diopter is set at the zero position. The metallographer should know his or her unique diopter settings.

Fig. 5.25 Photograph of two objectives. The objective on the left, marked

“170/-,” is for a tube length of 170 mm to be used without a cover slip (cover glass). The objective on the right, marked “170/0.17,” is for a tube length of 170 mm but must be used with a 0.17 mm thick cover slip.

objective is shown on the right, where one can see the cover glass thickness alongside the tube length as “170/0.17.” The 10⫻ uncorrected objective at the left in Fig. 5.25 has a dash, “-”, next to the tube length. The dash means with or without cover glass. The 100⫻ cover-glass-corrected objective in this photograph is an oil-immersion lens (“Oel”) with a NA of 1.30. The uncorrected objective is a dry objective with a NA of 0.25. Some microscope manufacturers engrave a “NCG,” “NC,” or “0” on the objective to indicate no cover glass.

Types of Eyepieces The eyepieces or oculars of the metallurgical microscope are not as complex as the objectives. The function of the eyepiece is to magnify the image produced by the objective. Therefore, the image of the microstructure is magnified twice: first by the objective and then again by the eyepiece. Thus, a 20⫻ objective would magnify the image 20 times, and a 10⫻ eyepiece increases the magnification to 200 times (assuming no other lenses are between the eyepiece and the objective). This final image in the eye appears as a “virtual image” at a distance of about 250 mm (10 in.) from the eye (as shown in Fig. 5.9). Eyepieces usually are manufactured with magnifications of 5, 6.3, 8, 10, 12.5, 20, 25, and 30⫻. The magnification is engraved on the top ring of the eyepiece. For most metallographic work, the 10⫻ eyepiece is appropriate. The eyepiece fits into the eyepiece tube of the microscope. Many metallurgical microscopes have two eyepieces that fit into a binocular attachment to the main tube of the microscope, as seen in Fig. 5.26. The image from the objective is split into the two eyepieces by a prism. With two eyepieces, there is less eye strain for the operator. The eyepiece tubes are adjusted for the correct interpupillary distance, that is, the distance between the pupils of one’s eyes. For most people, this is between 60 and 70 mm. Usually, an interpupillary scale is located between the two

Fig. 5.26 A binocular attachment for a metallurgical microscope

Fig. 5.27 A binocular attachment showing the interpupillary scale (between the eyepieces)

124 / Metallographer’s Guide In addition to a binocular, many metallurgical microscopes have a trinocular tube arrangement, where the third tube is used for mounting a ground-glass screen, video camera, or photographic camera attachment, as shown in Fig. 5.29. In purchasing a microscope, always select the types of objectives first, then select the eyepieces that match these objectives. One cannot mix objectives and eyepieces and expect optimal performance from the microscope. In addition to the magnification being engraved on the top ring of the eyepiece, some eyepieces have a field number engraved on the ring. The field number specifies the specimen area that will appear in the field of view. For example, an eyepiece may have the number 18 engraved near the magnification number. This is the field number. If the field number is divided by the objective magnification, the result will be the diameter of the field of view on the specimen. For example, at 100⫻, if the field number is 18, then the field of view on the specimen will be 0.18 mm in diameter. Wide-field eyepieces have field numbers of 25 to 32. Simple eyepieces are constructed, generally, with two glass lenses. The top lens near the eye is called the eye lens and the inner lens is called the field lens. The lenses are planoconvex, which means that one surface is flat and the other convex or curved outward. There are three basic types of eyepieces: the Huygenian eyepiece, the Ramsden eyepiece, and the compensating eyepiece. These are described subsequently. The Huygenian Eyepiece. This is the least expensive and most commonly used eyepiece. The Huygenian eyepiece consists of a planoconvex eye lens and a planoconvex field lens with the convex sides of both lenses facing downward. The lens was designed by Christian Huygens, a 17th century Dutch scientist. Between the two lenses there is a fixed circular opening or diaphragm where the magnified image from the objective is

projected. A schematic representation of the cross section of a Huygenian eyepiece is shown in Fig. 5.30. The diameter of the diaphragm determines the circular size of the field that is observed

Fig. 5.29 A trinocular arrangement for an upright metallurgical micro-

scope. In addition to the binocular eyepieces, there is a close-coupled device attached to the camera port of the microscope.

Fig. 5.28 A binocular attachment showing the diopter scale at the base of one of the eyepieces

Fig. 5.30 Sketch of the cross section of a Huygenian eyepiece

The Metallurgical Microscope / 125 in the eyepiece. Also, special measuring micrometer discs or grids (called reticles or graticules) can be placed on the diaphragm. These scales and grids thus appear as if they are superimposed on the microstructural image. Simple Huygenian eyepieces are somewhat self-corrected for lens aberrations, because many of the aberrations in the eye and field lenses cancel each other. Some manufacturers produce more highly corrected Huygenian eyepieces that contain two-part (doublet) or three-part (triplet) eye lenses. These are multiple lenses that are bonded together as one lens. In general, Huygenian eyepieces are used with the lesser-corrected, lower-power achromatic objectives of 5⫻ to 40⫻ magnification. This type of eyepiece should not be used with the higher-power achromatic objectives or any of the fluorite, apochromatic, or plano objectives. These objectives require eyepieces of special correction. The Huygenian eyepiece usually can be recognized by the lack of any identification on the top eyepiece ring. The corrected eyepieces have identification letters or symbols. The Ramsden Eyepiece. The Ramsden (or orthoscopic-type) eyepiece differs from the Huygenian eyepiece in that the convex portion of the field lens faces upward and the circular diaphragm is positioned below the field lens. A schematic representation of a cross section of a Ramsden eyepiece is shown in Fig. 5.31. With the diaphragm below both lenses, the quality of the image of any measuring scale or grid is better than with the Huygenian objective. Otherwise, the Ramsden eyepiece suffers with the same limitations as the Huygenian eyepiece. Many Ramsden eyepieces contain four lens elements: two in the eye lens and two in the field lens. These eyepieces can be used for medium-power observation. The Hyperplane (Compensated) Eyepiece. Compensated eyepieces may be of either the Huygenian or Ramsden type. They have doublet or triplet eye lenses and are more highly corrected for chromatic aberration and curvature of field. This type of eyepiece can be used with the more powerful, higher-magnification achromatic objectives and the more highly corrected fluorite, apochromatic, and plano objectives. These special compensated eyepieces should be matched with the corrected objectives when

Fig. 5.31 Sketch of the cross section of a Ramsden eyepiece

the microscope is purchased. They are usually engraved with “comp,” “K,” or “C” on the top ring of the eyepiece. The eyepieces used with flat-field objectives may be engraved with “plano-comp” or “plan-comp.” The Wide-Field or Wide-Angle Eyepiece. Some eyepieces are manufactured to produce a wider field of view (larger diameter of view on the specimen) and are engraved with a “WF” or “S.W..” They consist of many lens elements. They are engraved with a field number that, when divided by the objective magnification, specifies the diameter of the field of view on the specimen (in mm). At 100⫻, the diameter of the field of view of a standard eyepiece is about 0.18 mm. Special wide-field eyepieces can increase the field of view to 0.32 mm by using a larger diameter eyepiece with a higher field number (in this case, a field number of 32). The High-Point Eyepiece. Most modern metallurgical microscopes have high-point eyepieces. High-point, or high-eyepoint oculars, are produced for metallographers who must wear eyeglasses. Although the metallurgical microscope will allow the metallographer who normally wears eyeglasses to view the microstructure without wearing the eyeglasses, some eye aberrations, such as astigmatism, cannot be corrected by the optical system of the microscope. People who are farsighted or nearsighted can use a metallurgical microscope with or without eyeglasses. In the high-point eyepiece, the eye lens is elevated to allow a larger distance between the eye lens and the eye. This extra space of about 25 mm (1 in.) allows ample room for the eyeglasses. Some high-point eyepieces are inscribed with an “H.” The Adjustable Eyepiece. When using a binocular attachment to the metallurgical microscope, at least one eyepiece should be adjustable. This means that one eyepiece should have a focusing eye lens. The focusing eyepiece can be adjusted to compensate for people who do not have perfect 20/20 vision. The use of the adjustable eyepiece is explained in the section “Procedure to Personalize the Microscope for Viewing.” Measuring Eyepieces. Some eyepieces contain special measuring scales called reticles (or graticules). When viewed through the eyepiece, these scales are superimposed on the image of the specimen. Generally, the barrel of the eyepiece will rotate in order to bring the scale into the same focus as the field of view. An eyepiece with a graduated linear scale is normally used. A reticle scale is important when measuring the length or diameter of a feature in the microstructure. These eyepiece scales must be calibrated with a stage micrometer (see the section “Special Procedures for the Metallurgical Microscope” at the end of this chapter). One such eyepiece scale is called an eyepiece micrometer. Some elaborate eyepieces have a movable vernier scale for making measurements. These are called filar measuring eyepieces. Special reticles with circular or square grids can be used to determine the volume fraction of a particular constituent or a grain-size measurement. The Photoeyepiece. For micrography, the normal observation eyepiece is not used. In this case, a photoeyepiece or projector lens is placed in the trinocular port of the microscope. The photoeyepiece projects the image from the objective to the film plane of the camera. These lenses are generally of low magnifi-

126 / Metallographer’s Guide cation (1.5 to 8⫻), because the image projected onto the film is subsequently enlarged during the photographic process (as in the case of a 35 mm camera). The film plane can also be at a fixed distance, for example, 250 mm, from the photoeyepiece.

The Illumination System Another very important part of the metallurgical microscope is the system that provides light to the specimen. Although it is the part of the microscope that the metallographer can do the most to control microscope performance, it is also the most overlooked part of the microscope. This is probably due to a lack of understanding of how to control and adjust the illumination system for peak performance. If the illumination system is not optimized by the metallographer, the properties of the optical system, particularly the properties of the objectives, will not be properly used. For example, the metallographer may purchase the most expensive plan-apochromatic objectives available for the microscope. These objectives may have a NA of 0.95. However, in order to use the NA of the objective, an intense beam of optically centered light must be available to fill the backside of the lens. If the beam is off-center or not properly focused, the objective will not function the way it is designed to function and the metallographer will not obtain the full benefit of the high-resolution objective. Figure 5.32 shows an example of how an improperly adjusted illumination system can affect microstructural image quality. The micrograph in Fig. 5.32(a) was taken with no filter, a misaligned light source, and an aperture diaphragm that was opened too wide. Sharpness and quality improvement can been seen in the micrograph in Fig. 5.32(b), which shows the same field taken with a green filter, correctly aligned light source, and a properly adjusted aperture diaphragm. The next section describes the various parts of the illumination system on most metallurgical microscopes. A later section describes the procedures used to control the illumination system for optimal results.

The Components of the Illumination System

Fig. 5.32 Micrographs of a 0.75% C-3.25% Cr steel with an improperly

and properly aligned illumination system. Micrograph (a) was taken with no filter, a wide-open aperture diaphragm, and misaligned light source. Micrograph (b) was taken with a green filter, correctly adjusted aperture diaphragm, and properly aligned light source. Note the improved clarity in micrograph (b). Pearlite nucleating on a prior austenite grain boundary in a matrix of martensite. Vilella’s etch. 1000⫻

The standard upright biological microscope has a substage condenser as part of the illumination system (some older microscopes use a detached lamp and mirror system). The light is thus transmitted through the specimen from below. However, because the metallurgical microscope uses opaque specimens, this type of system cannot be used. The light must pass vertically through the objective to the specimen. The light is then reflected from the specimen back through the objective to the eyepiece or ocular. Figure 5.33 shows a very simplified schematic representation of a typical illumination system. The system consists of a light source, a condenser lens, an aperture diaphragm, a field diaphragm, a second condenser lens (field lens), and the half-mirror or illuminator. This optical system is termed “Kohler illumination” and produces high-intensity and uniform illumination on the specimen. August Kohler, a German scientist, developed the system in the early 1890s. The part of the system that includes the two diaphragms and the mirror is called the vertical illuminator.

The Light Source For the metallurgical microscope, the source of light should be bright, steady, and collimated (controlled as an intense single

The Metallurgical Microscope / 127 beam of light). In older microscopes and metallographs, carbon arc lamps were used, because they produce light of high intensity. The arc was generated as the two carbon rods touched and then developed a small space between them. A major problem was that the space between the rods could not be controlled sufficiently to produce constant illumination. There was flickering of light, because the arc wandered about the end of the rod, and because the rods had a fairly rapid consumption rate. These lamps are no longer in use, because they provided an unstable source of illumination, especially for micrography. Many technological advances have been made in the past two decades to develop high-intensity, stable light sources. The metallographer has many options to choose from for the metallurgical microscope. Tungsten-Filament Lamps. Low-cost, tungsten-filament incandescent lamps (bulbs) were common years ago as a light source for the metallurgical microscope. That is particularly true for the simple metallurgical microscope that is used mainly for observation. These lamps use a low voltage (6 or 12V) and can be purchased at various wattages. The level of intensity is controlled by a transformer with a variable rheostat. The intensity of light is suitable for most ordinary black-and-white micrography. However, for high-magnification and polarized-light metallography, very long exposure times are required. For such work, a brighter source of light is required (see subsequent paragraphs). Tungsten-filament lamps operate at color temperatures between 2700 and 3200 K. “Color temperature” is a term used in photography, for example, a color temperature of 3200 K is the same as the light color produced by a camera flashbulb. One problem with tungsten-filament bulbs is the buildup of tungsten on the inside surface of the glass bulb with extended use. This buildup will decrease the intensity of the light produced and is particularly a problem in micrography, because the color temperature gradually decreases. If the bulb surface is darkened, it should be replaced. Tungsten-Halogen Lamps. The tungsten-halogen lamp has essentially replaced the simple tungsten-filament lamp described previously. The tungsten-halogen lamp employs a tungsten filament in a halogen gas. The halogen gas is usually iodine, but other halogens, such as chlorine or bromine, can be used. The main

Fig. 5.33 Schematic of a ray diagram of a basic illumination system for a metallurgical microscope

advantage of the tungsten-halogen bulb is that tungsten from the filament is not deposited on the inside of the glass bulb. The tungsten atoms that evaporate from the filament collide and combine chemically with the halogen gas atoms. The gas then deposits the tungsten back onto the filament. This means longer life of the filament, and the lamp is self-cleaning (never darkens). Lamp life is about 50 hours. These lamps can also operate at higher voltages and can produce maximum light output. For these reasons, this type of lamp has replaced the lower-cost tungstenfilament lamp in many metallurgical microscopes. Some older microscopes can be upgraded with a new lamp housing and transformer. The tungsten-halogen lamp is also called a quartz-halogen or quartz-iodine lamp, because quartz is used instead of glass for the bulb. Some bulbs are made from a high-silica glass called Vycor (Corning Glass Works). Quartz and Vycor have high melting temperatures compared with glass and can thus be used at higher temperatures. Also, the wall of the bulb can be closer to the filament, and thus, tungsten-halogen bulbs can be made smaller than a conventional tungsten-filament bulb. The metallographer is cautioned never to touch the quartz (or Vycor) surface, because any fingerprint will result in a short lifespan of the bulb. When skin oils contact a quartz surface, the quartz will undergo a crystallization process, which will change the affected region from transparent to translucent. This crystallization (devitrification) will weaken the quartz, and the bulb will soon fail at this region. Tungsten-halogen lamps generally operate at 6 V and can be purchased at 10, 20, 50, 80, 100, 150, and 250 W capacity. Some lamps can operate at higher voltages; check the manufacturer’s recommendations. Most tungsten-halogen lamps operate at a color temperature of 3200 K. Xenon Arc Lamps. Very stable, high-intensity illumination can be provided with a xenon arc lamp. However, the xenon arc lamp is an expensive source of illumination. The light is of daylight quality (color temperature of 6400 K), which is an advantage when using daylight-type color film for micrography without filters. The quartz bulb is designed so that an arc is produced between two tungsten electrodes. The lamp requires a special direct current power source, and the atmosphere within the bulb is xenon gas (an inert gas) under high pressure. Xenon arc lamps usually operate at high wattages (75, 150, and 450 W). The color temperature of a typical zenon arc lamp is 6400 K, and these lamps are ideal for color daylight film. The metallographer must be very careful in handling the pressurized bulb, and gloves and safety glasses or face shield should be worn in case of an explosion. Care must also be taken not to touch the glass bulb, because any fingerprint on the quartz surface will result in a short lifespan of the bulb (as described in the subsection “Tungsten-Halogen Lamps”). Replacement bulbs are very expensive. These lamps also emit some ultraviolet and infrared light, and the metallographer must make certain not to be exposed to direct light. Mercury Arc Lamps. The mercury arc or mercury vapor lamp is not commonly used for metallographic purposes but can provide what is called a high-intensity monochromatic source of light (sodium vapor lamps can also do this). This means that the

128 / Metallographer’s Guide source of light is at a particular wavelength. For example, the mercury arc lamp emits very bright light at wavelengths of 546, 435, and 365 nm. Because of their precise wavelengths, these lamps are useful for interferometry (discussed later in this chapter). However, because of the discontinuous light spectrum, a mercury arc lamp is a problem in color micrography. The lamps require their own direct current power source, are expensive, and have a tendency to flicker. The mercury arc lamp takes about ten minutes to reach full intensity and cannot be turned on again until the bulb has cooled to room temperature. These lamps emit some ultraviolet light, and care must be taken to shield the metallographer’s eyes from direct exposure. The mercury arc bulbs are under high pressure and should be handled with care (gloves and safety glasses or face shield must be worn). Zirconium Arc Lamps. As with the xenon arc lamp, the zirconium arc lamp provides a stable, high-intensity source of light for the metallurgical microscope. The intensity is not as bright as a tungsten-halogen or xenon lamp but is sufficient for most micrography (color temperature of 3200 K). The zirconium arc lamp also requires its own direct current power source, making the system quite expensive. The same precautions as the xenon arc lamp in handling and exposure to ultraviolet light must be maintained. Zirconium arc lamps are not commonly used for metallography. The Lamp Housing. The lamps mentioned previously are surrounded by a lamp housing, which shields the surroundings from extraneous light. The housing allows for cooling by louvers or radiation fins. An example of a lamp housing can be seen in Fig. 5.34. In some cases, there is a reflective device (partialhemispherical mirror) located in back of the lamp. This reflector directs the back light of the lamp toward the optical system. In most metallurgical microscope lamp housings, there are adjustment knobs to allow the metallographer to vertically and laterally center the light source (the procedure for centering the light source is discussed in the section “Special Procedures for the

Metallurgical Microscope”). These knobs can be seen in Fig. 5.34 (arrow).

The Collector Lens Just in front of the lamp housing is the collector lens or lamp condenser lens (sometimes called field condenser or lamp collector). There actually may be more than one lens element in the collector lens. The main purpose of the collector lens is to collimate a light beam from the light source to the aperture diaphragm. The collector lens also provides even illumination of the specimen.

The Aperture Diaphragm With the collector lens forming a collimated light beam on the aperture diaphragm (also called the aperture iris diaphragm or aperture stop), the aperture, which the metallographer can open and close, controls the resolution, contrast, and the amount of light to the specimen. Actually, for optimal conditions, the aperture diaphragm should maintain a beam of light that fills about 75% of the back lens of the objective. This way, the full potential of the objective NA is used. Too small an aperture opening will reduce resolution, and too large an opening will cause haziness of the entire field. This is shown in Fig. 5.35 where the same field is viewed with the aperture diaphragm at too large and at too small an opening compared with the correct opening. Note that with a large opening, the field is fuzzy and thus has poor contrast. With the small opening, the field is sharpened, but certain fine features

Fig. 5.35 Micrographs of a water-quenched AISI/SAE 1020 steel with a

Fig. 5.34 A simple lamp housing for a metallurgical microscope. Note the adjustment knob (arrow).

fully martensitic microstructure. (a) Taken with fully open aperture diaphragm, (b) taken with a closed-down aperture diaphragm, and (c) taken with a properly set aperture diaphragm. Note the improved clarity of the structure with the properly set aperture diaphragm. 2% nital etch. 500⫻

The Metallurgical Microscope / 129 are missing. The correct aperture setting shows a much clearer martensite microstructure. It is important for the metallographer to understand how to properly adjust the aperture diaphragm. Proper adjustment is discussed in the section “Special Procedures for the Metallurgical Microscope.”

The Field Diaphragm The field diaphragm (also called the field iris diaphragm or field stop) controls the field of view of the specimen. The diaphragm, which opens and closes, prevents glare caused by stray light in the

Fig. 5.36 Micrographs of a cast 25% Cr-12% Ni heat-resistant HH steel

Fig. 5.35 (continued) (b) taken with a closed-down aperture diaphragm, and (c) taken with a properly set aperture diaphragm. Note the improved clarity of the structure with the properly set aperture diaphragm. 2% nital etch. 500⫻

with grain-boundary carbides showing (a) an unfocused field diaphragm, (b) a focused field diaphragm, and (c) the focused field of view. When the field diaphragm is in focus, the field of view on the specimen is in focus. Electrolytic etch (60 ml nitric acid in 40 ml water, stainless steel cathode). 400⫻

130 / Metallographer’s Guide system. The image of the field diaphragm can be seen on the specimen when the specimen is in focus. It is important to remember that when the field diaphragm is in focus, the specimen is in focus. This can be seen in Fig. 5.36. In this series of micrographs, the field diaphragm is out of focus when the specimen is out of focus, and the field diaphragm is in focus when the specimen is in focus. The microstructure consists of primary carbides in a cast 25% Cr-12% Ni heat-resistant grade HH steel. Generally, for best results, the field diaphragm is opened just beyond the field of view. Proper adjustment of this diaphragm is discussed in the section “Special Procedures for the Metallurgical Microscope.”

The Condenser Lens There is another lens, also called the field lens, located just beyond the field diaphragm. It is the purpose of this condenser lens and the objective itself to image the field diaphragm onto the surface of the specimen. This feature should not be used as a brightness control. Instead, judge the image quality and adjust the intensity by the rheostat on the light source, or use filters.

the illumination system of a metallurgical microscope. This critical component is the main feature that distinguishes the metallurgical microscope, using reflected light, from the biological microscope, using transmitted light. There are various types of illuminators. The Glass Reflector. One form of illuminator is a very thin glass plate or disc that is mounted at a 45° angle to the incident light beam. The front of the plate generally has a lightly mirrored surface. If it has a fully mirrored surface, it will not allow the reflected light from the specimen to pass up the tube of the microscope to the eyepiece, and thus, the metallographer will not see the image. If the plate is not mirrored, a large portion of light from the collimated beam will travel straight through the glass, and very little light is reflected down onto the specimen. Even in a properly designed illuminator, it is estimated that only about 10 to 25% of the original beam at the aperture diaphragm passes to the eyepieces. Thus, when using a plane glass illuminator, the light source must be of high intensity. The Prism Reflector. In some microscopes, a right-angle prism reflector is used to direct the light beam 90° down to the specimen. Although almost 100% of the light reaches the specimen, this type

The Illuminator The purpose of the illuminator is to direct the light beam 90° down the microscope tube to the back lens of the objective. A schematic representation of the illuminator is shown in Fig. 5.37, where the light from the light source is reflected toward the objective. The light is represented by the two parallel rays at the right side of the sketch. The illuminator must also allow the beam reflected from the specimen to pass through with minimal absorption and distortion. Thus, the illuminator is a critical part of

Fig. 5.36 (continued) Micrographs of a cast 25% Cr-12% Ni heat-

resistant HH steel with grain-boundary carbides showing (a) an unfocused field diaphragm, (b) a focused field diaphragm, and (c) the focused field of view. When the field diaphragm is in focus, the field of view on the specimen is in focus. Electrolytic etch (60 ml nitric acid in 40 ml water, stainless steel cathode). 400⫻

Fig. 5.37 Sketch of a ray diagram showing reflective mirror mounted in the microscope tube above the objective

The Metallurgical Microscope / 131 of illuminator has a serious drawback, in that the light reflected from the specimen back through the objective cannot pass through the prism on its way to the eyepiece. To solve this problem, the prism is offset about halfway into the incident light beam so as to deflect some of the light beam to the specimen while allowing ample free space to allow the reflected beam to pass the prism. This means that only half of the objective lens is being used for the incident light and one-half for the reflected light. With this arrangement, there is some loss in resolution. Also, because the prism is off-center, some of the light entering the objective is at an angle. This produces oblique illumination of the specimen, which may not be desirable for all circumstances. Because of the drawback of the prism illuminator, it should only be used with low-magnification objectives. The Bright-Field/Dark-Field (BD) Reflector. Many modern metallurgical microscopes use a thin glass plate reflector that has a mirrored outer ring and clear center. The outer ring reflects light from the light source down to the outer rim of the back objective lens. The clear center of the reflector allows almost full transmission of the reflected light from the specimen to the eyepieces. Because the reflector is at a 45° angle to the light beam, the clear region is elliptically-shaped on the glass surface. However, the ellipse appears circular to the reflected light from the specimen. This reflector is used both for bright-field and dark-field illumination.

would appear dark in bright-field illumination appear bright in dark-field illumination. This is accomplished by producing incident light at an angle to the specimen, as seen in the ray diagram in Fig. 5.39. In this schematic representation of dark-field illumination, the scratch on the specimen surface reflects light into the objective, and the remaining light is reflected at an angle and never passes through the objective. In order to produce incident light at an angle to the specimen, a special dark-field objective is required. In this objective, light rays are directed in a circular area only around the inside periphery of the barrel, and the incident light is excluded from the normal central area of the objective, that is, as in the normal bright-field objective. The annular opening can be seen in the dark-field objective in Fig. 5.23. To produce the ring of light, a dark-field stop in the illuminating system eliminates the light in the central portion of the beam, as shown in the sketch in Fig. 5.40. The illuminator (reflecting mirror) also has a clear elliptical center, so that any light in the center of the beam passes through the clear portion of the glass and is not reflected downward toward the objective. The outer portions of the elliptical mirror reflect the light beam to the annular path of the dark-field objective. Dark-field illumination is ideal for observing grain boundaries, scratches, and pores on the specimen surface. For example, Fig. 5.41 compares an etched specimen of quenched American Iron

Types of Illumination The metallurgical microscope is a remarkably versatile instrument, and the more the metallographer knows about the microscope illumination system, the more tools that become available to achieve the perfect image. Not only can the metallographer magnify the image of the microstructure to 1000⫻ and beyond, but the image can be enhanced by special optical techniques that produce different types of illumination on the specimen. The normal type of illumination in the microscope is called brightfield illumination, which provides an image of reflected light from the specimen. This bright-field image is quite appropriate for representing microstructural details and has been used for most of the micrographs in this book. However, on occasion, certain features in the microstructure need to be enhanced beyond what can be done with bright-field illumination. Some of the more common types of illumination are described subsequently. Bright-Field Illumination. In bright-field illumination, most of the light that passes through the objective to the specimen is reflected back through the objective to the eyepiece of the microscope, as seen in the ray diagram in Fig. 5.38. Those specimens with features that are at an angle to the incident beam of light (as shown in Fig. 5.38) appear dark, because the light reflected from these features does not pass back through the objective. Therefore, in etched specimens, grain boundaries appear dark, because the light is reflected away from the objective lens. This is why etching is so important in revealing microstructural features in a specimen. Dark-Field Illumination. Dark-field illumination is essentially the opposite of bright-field illumination, that is, those features that

Fig. 5.38 Sketch of a ray diagram showing bright-field illumination. Note

that light rays impinging on a scratch on the specimen surface are reflected away from the objective lens, while the other rays are reflected back through the lens. Thus, the scratch appears dark, while the remaining surface is bright.

132 / Metallographer’s Guide and Steel Institute/Society of Automotive Engineers (AISI/SAE) 4340 steel in both bright-field and dark-field illumination. The microstructure is lath martensite. The prior austenite grain boundaries are more clearly seen in dark-field illumination than in bright-field illumination. The dark-field technique is very useful

for metallographic interpretation, and most metallurgical microscopes are equipped with the necessary dark-field attachments. Because most of the incident light to the specimen is reflected away from the objective lens during dark-field illumination, the

Fig. 5.39 Sketch of a ray diagram showing dark-field illumination. Note

that light rays impinging on a scratch on the specimen surface are reflected through the operative lens, while the other rays are reflected away from the lens. Thus, the scratch appears bright, while the remaining surface is dark.

Fig. 5.41 Micrographs of a water-quenched AISI/SAE 4340 steel with a

Fig. 5.40 Sketch of a ray diagram for typical dark-field illumination. Note

the special dark-field stop that restricts the light path to an annular ring of light, and the special reflecting mirror that reflects that annular ring of light to the outer ring of the objective.

fully martensitic microstructure. Micrograph (a) was taken in bright-field illumination, and micrograph (b) was taken with dark-field illumination. Note the clarity of the prior austenite grain boundaries in the dark-field micrograph. 1% picric acid in water with a few drops of sodium tridecylbenxene (wetting agent). 500⫻

The Metallurgical Microscope / 133 aperture and field diaphragms should be fully open and the lamp intensity increased. Oblique Illumination. Somewhat similar to dark-field illumination is oblique illumination, where the rays of incident light are decentered, or off-center, from the bright-field position. This means that the incident light strikes the specimen at an angle in one direction only, as opposed to dark-field illumination, which strikes the specimen at an angle as a 360° circle. Oblique illumination can be produced by deliberately decentering the aperture diaphragm. This technique is used to show relief on the

Fig. 5.42 Micrograph of a water-quenched Fe-1.75% C binary alloy

showing plate martensite in a matrix of austenite. Micrograph taken with oblique illumination. The martensite plates, because of their surface relief, are shadowed by the oblique light source. 2% nital etch. 500⫻

Fig. 5.43 Sketch of a ray diagram for an illumination system for polarized

light. The polarizer filter is placed between the second condenser lens and the illuminator, and the analyzer filter is placed above the illuminator.

specimen surface. An example of the use of oblique illumination is shown in Fig. 5.42. Polarized Light. Although a technique not widely used for steels and cast irons, polarized light can reveal crystallographic orientation differences in the specimen. To produce polarized light, a polarizer must be placed in the light path between the light source and the objective, and an analyzer must be placed in the light path between the mirror and the eyepiece. There are no differences between the polarizer and analyzer, they are just given different names to differentiate their roles. A schematic representation is shown in Fig. 5.43. The polarizer and analyzer are filters that allow only certain directions of light waves to pass through the filter. These filters present parallel optical slits to the light beam. When the light passes through the filter, only one orientation of light waves can pass through. As an example, in Fig. 5.43, the polarizer allows only that component of light wave oriented in a direction perpendicular to the plane of the specimen to pass through, and the analyzer allows only that component of light wave that is parallel to the plane of the specimen to pass through. The polarizer and analyzer filters can be rotated, and when they are crossed, that is, when the slits are perpendicular (90°) to each other, the field of view becomes dark. At this point, the specimen is rotated perpendicular to the incident beam. If no change in reflection occurs, the material is considered isotropic, and if changes occur, the material is considered anisotropic. Isotropic means that the material has equal characteristics (mechanical properties, refractive index, etc.) in all directions (x-, y-, and z-axes). The body-centered cubic and face-centered cubic metals, such as iron and copper, are isotropic and do not respond well to polarized light. On the other hand, the hexagonal metals titanium, zirconium, magnesium, and zinc are anisotropic and respond well to polarized light. Many minerals and oriented polymers, as well as common sugar, are anisotropic. Therefore, polarized light is much more useful for these anisotropic materials that are not cubic by nature. However, polarized light can be used on steels that have been tint etched. A tint etch deposits a chemical residue on the surface of each grain in the specimen. The residue itself can have an orientation that is anisotropic. For example, Fig. 5.44 illustrates the use of polarized light on a very-low-carbon electrical steel containing silicon that was tint etched with one of Baraha’s tint etchants. The bright-field image is seen in Fig. 5.44(a), and the same field under polarized light is shown in Fig. 5.44(b). Note the clarity of the grains in the polarized image. More about the use of tint etches is discussed in Chapter 8. Differential Interference Contrast (Nomarski). This technique is used in most modern bright-field microscopes. In this technique, a polarizer is placed between the light source and the vertical illuminator (mirror), and an analyzer is placed between the vertical illuminator and the eyepiece, as with polarized illumination described previously. For differential interference contrast (also called differential phase contrast), the polarizers must be crossed (at 90° to each other). A special prism, called a Wollaston prism, is placed between the vertical illuminator and the objective, as shown in Fig. 5.45. This prism divides the polarized light beam into two beams of incident light. The split beams have a slight lateral separation when they illuminate the

134 / Metallographer’s Guide specimen surface. After being reflected back through the objective, the two beams are recombined by the prism and pass through the analyzer to the eye. The slight displacement of the beams at the specimen surface produces the differential contrast. Differential interference contrast works only when the surface of the

specimen has uneven surface features or if there is a difference in index of refraction in coated surfaces. To show the effect of differential interference contrast, a series of micrographs have been taken at crossed polarization and two positions of moving the Wollaston prism. Figure 5.46(a) shows the flat, as-polished surface of the D2 tool steel specimen under normal bright-field illumination. The only features that are noticeable are five small inclusions. The same field of view under differential interference contrast can be seen in Fig. 5.46(b). In this micrograph, large carbide arrays can be seen along with fine carbides dispersed within the grains. Because this specimen was not etched, the relief shown in Fig. 5.46(b) is caused by polishing. The carbides, being harder than the matrix, did not abrade away as much as the matrix, and thus the carbides are in higher relief than the matrix. There is a danger of confusion in using this technique. For example, by moving the Wollaston prism too far out of alignment, the image can be reversed, as seen in Fig. 5.46(c). Here, the carbides appear as depressions in low relief with respect to the matrix, thus creating a false image. In using differential interference contrast, the metallographer must be fully aware of the characteristics of the material that is being examined. If there is uncertainty, the metallographer can verify if the relief is high or low by using an alternate technique, such as oblique illumination.

Fig. 5.44 Micrographs of a very-low-carbon electrical steel with a fully

Fig. 5.45 Sketch of a ray diagram of differential interference contrast

ferritic microstructure taken under (a) bright-field illumination and (b) polarized light. Beraha’s etch (3 g potassium metabisulfite and 10 g sodium thiosulfate in 100 ml water). 200⫻

illumination (Nomarski) in a metallurgical microscope. Note the polarizer and analyzer for polarized light and the Wollaston prism to split the light beam.

The Metallurgical Microscope / 135 Another example of the use of differential interference contrast can be seen in Fig. 5.47. This is an example of cold-worked AISI 316 stainless steel that has deformation twins. Note the excellent contrast of the deformation twins in the micrograph using differential interference contrast (Fig. 5.47b) when compared with the micrograph taken in bright-field illumination (Fig. 5.47a).

This technique is also called Nomarski, named after G. Nomarski who developed the technique in the 1940s. More details about this technique and other phase contrast and interference techniques can be found in the selected references at the end of this chapter. Interference Illumination. Some advanced metallurgical microscopes have special optical accessories called interferometers.

Fig. 5.46 Micrographs of an as-polished D2 tool steel taken in (a) bright-field and (b) differential phase contrast. Note the carbides in (b) in high relief

compared with the matrix. In (c), the same field taken after moving the Wollaston prism slightly out of alignment. Note in (c) that the carbides are in low relief (as pits), giving a false image. Unetched. 500⫻

136 / Metallographer’s Guide These interferometers (e.g., polarization interferometers), either double-beam or multiple-beam, produce interference fringes on the specimen surface. For example, Fig. 5.48 shows a series of parallel interference fringes on a flat, polished surface of steel. With everything properly aligned, the spacing between the fringes

is one-half the wavelength of the light being used. Thus, in this case where a green filter is used, the wavelength is 548 nm, and the spacing between fringes is 274 nm. Any change in surface flatness, for example, a grain boundary or a scratch, produces a change in the fringe spacing and direction. A simple way to visualize this effect is by noting the shadows of electric power lines or telephone wires produced by the sun. These shadows change direction and spacing as they cross from a flat surface to a hill or depression. For example, on a totally flat surface, such as the outside wall of the building, the shadows would appear as straight, parallel dark lines. As the shadows cross a recessed window in that wall, the shadows would change direction and spacing at the recessed surface and change direction and spacing again along the flat window surface itself, and so on. Another way of visualizing interference fringes is by observing the constant elevation contour lines on a topographical map. At a flat surface, the elevation contour lines are spaced far apart. At a cliff along a stream bed, the elevation lines are closely spaced. Therefore, the observer can visualize the hills and valleys on the surface of the earth shown in the map. The spacing between the lines can be used to determine the steepness of the hill or valley with respect to a reference elevation, for example, sea level. Thus, interference illumination is usually used to examine very small height changes on a specimen surface. Some interferometers can measure height differences as small as 30 nm. To illustrate the effectiveness of interference illumination, the same polished specimen shown in Fig. 5.48 contained a scratch that was examined with and without interference illumination, as seen in Fig. 5.49. The micrograph in Fig. 5.49(a) was taken in bright-field illumination to show the scratch. The scratch was produced by a small diamond-pointed tool. The micrograph in

Fig. 5.47 Micrographs of a cold-worked AISI 316 stainless steel sheet

taken with (a) bright-field illumination and (b) differential interference contrast illumination. Note the excellent clarity of surface relief in the differential interference contrast illumination. Electrolytic etch (10% oxalic acid in water, stainless steel cathode). 400⫻

Fig. 5.48 Micrograph showing interference fringes on a flat, polished specimen surface. Unetched. Taken with a green filter. 500⫻

The Metallurgical Microscope / 137 Fig. 5.49(b) was taken in interference illumination (the same illumination in Fig. 5.48). Note in Fig. 5.49(b), the interference fringes changed direction and spacing along the sidewall of the scratch. From these changes, the metallographer can then calculate the depth of the scratch with respect to the polished surface by measuring the the number of fringes on the scratch sidewall with

respect to the number of fringes of known spacing on the flat surface. More details about the use of this important technique can be found in the references.

Accessories for the Microscope Bertrand Lens. Some advanced metallurgical microscopes are equipped with an auxiliary lens, called a Bertrand lens, that focuses the aperture diaphragm within the tube of the microscope. Without a Bertrand lens, the image of the aperture diaghragm is small and difficult to accurately adjust, because the eyepiece is removed from the tube of the microscope. However, if the metallographer uses a Bertrand lens, the eyepiece remains in place, and the image of the aperture is enlarged and in focus, making adjustments to the aperture diaphragm much easier. An example of the image produced by a Bertrand lens is shown in Fig. 5.50. In this micrograph, the larger circular area is the light passing through the back lens of the objective. The aperture diaphragm is shown as the eight-sided opening. Note that in this example, only about 75% of the light from the objective passes through the aperture diaphragm. This is the recommended aperture opening to allow for the full potential of the NA of the objective to be obtained. See the section “Special Procedures for the Metallurgical Microscope” at the end of this chapter for instructions on the use of the Bertrand lens to adjust the aperture diaphragm. Scribe. On occasion, the metallographer will need to permanently mark an area on the specimen surface for further evaluation by another instrument, that is, the scanning electron microscope or electron probe microanalyzer. The scribe device screws into one of the objective locations in the nosepiece of the microscope. Figure 5.51 shows an example of a scribe before mounting on the

Fig. 5.49 Micrographs of a scratch on the same polished specimen in Fig.

5.48 in (a) bright field and (b) the same scratch under interference illumination. Taken with a green filter. Unetched. 500⫻

Fig. 5.50 Micrograph of the image of the eight-sided aperture diaphragm and back of the 5⫻ objective (the light gray circular image) through a Bertrand lens mounted in the microscope tube. Note the aperture diaphragm is closed down about 25%.

138 / Metallographer’s Guide nosepiece of a metallurgical microscope. At the tip of the scribe is a carbide or diamond point that is offset from the optical axis. The tip is lightly touched to the specimen surface and is rotated to encircle the area of interest. The diameter of the circle can be adjusted by selecting the proper number on the barrel. The proper procedure for using the scribe can be found in the section “Special Procedures for the Metallurgical Microscope” at the end of this chapter. Stage Micrometer. A stage micrometer is a glass or metal slide that has been accurately engraved with a calibrated scale. An example of the image of a stage micrometer is shown in Fig. 5.52. The slide is placed on the stage of the microscope and is used to calibrate the magnification of the microscope through the objectives and eyepieces of the microscope. The procedures used for calibration are described in detail in the section “Special Procedures for the Metallurgical Microscope” at the end of this chapter. Reticles (Graticules). If the metallographer needs to make quantitative measurements of the features in the microstructure during observation on the microscope, a reticle eyepiece can be used. Reticles are small glass disks with graduated scales and grids that are located in one of the eyepieces of the microscope. The scales can be used to measure grain size, coating thickness, particle diameter, and so on. Eyepieces can be purchased with specific grids and scales, or the metallographer may order special

or custom-made reticles to be inserted into the eyepiece. A micrometer eyepiece (described previously in the subsection “Types of Eyepieces” and in the section “Special Procedures for the Metallurgical Microscope” at the end of this chapter) is an eyepiece with a graduated linear scale. An example of a reticle scale is shown in Fig. 5.53. Filar Eyepiece. A filar eyepiece, or filar micrometer, is used for accurate linear measurements of microstructural features (about 1 μm accuracy). These eyepieces have a screw-micrometer attachment (similar to a machinist’s micrometer) that is rotated for the measurements. Within the eyepiece, there is a movable cross hair that is connected to the micrometer movement. There are also digital filar eyepieces that are automated for direct digital readout or printout of the measurements. Leveling Device. With an upright metallurgical microscope, a device to level the specimen for observation on the stage is

Fig. 5.52 Micrograph of a stage micrometer. 40⫻

Fig. 5.51 Photograph of a scribe device for the upright metallurgical

microscope. The device fits into an objective port on the nosepiece. Note the offset diamond tip that is rotated to produce a scribed circle around the feature on the specimen surface.

Fig. 5.53 An example of eyepiece reticle scale

The Metallurgical Microscope / 139 required. Most microscope manufacturers supply a leveling device with their upright microscopes. The device is described in detail later in this chapter in “Procedure to Properly Level the Specimen on an Upright Microscope.” Lens Tissue. The metallographer should always keep an ample supply of lens tissue in order to clean dust from the eyepiece and objective lenses. Also, a sheet of lens tissue can be used to protect the polished specimen surface in a leveling device. Lens tissue is a lint-free, thin paper specially made for this purpose. Before using lens tissue, it is recommended that the lens be dusted with a burst of air from a rubber bulb or from a pressurized can of air or carbon dioxide (never use Freon gas [E.I. Du Pont de Nemours, Inc.]) Video Monitor. A video monitor, attached to a video camera on the trinocular port of the microscope, can be used to show the image of the microstructure to one or more observers. Video monitors also come with video printers that can print the image seen on the monitor. Camera Attachment. Metallurgical microscopes with a trinocular port are designed for a camera attachment (see Fig. 5.29). What the metallographer observes through the binocular eyepieces can be photographed on a 35 mm camera or instant film camera by a prism that splits the image. There are also attachments for cut film photography. Some microscopes may use a mirror that the metallographer uses to rotate the image from the binocular to the trinocular port. In addition to conventional photographic cameras, there are a number of electronic devices that can be attached to the trinocular port to capture an image. These include CCD (charge-coupled device) cameras that can provide a digital image to a video monitor or a computer.

metallograph is seen in Fig. 5.54. Most metallographs are inverted microscopes with built-in photographic capability. The metallograph shown in Fig. 5.54 has a 35 mm camera mounted in a pocket on the side and a rectangular ground glass viewing screen at the lower front facing the metallographer, as shown in Fig. 5.55. The viewing screen can be removed to allow a film holder to be positioned in its place. Figure 5.56 shows the electronic shutter speed control system for photography. Because all of the principles described in this chapter for the metallurgical microscope apply to the metallograph, no further description is needed here. Many modern metallurgical microscopes are equipped with excellent camera accessories, and a metallograph may not be needed for many metallographic laboratories. The camera accessories include not only 35 mm and instant film cameras, but also digital cameras that connect to a

The Metallograph The micrographs in this book were produced on a special microscope called a metallograph. This instrument is a metallurgical microscope that is dedicated to taking micrographs. A typical

Fig. 5.54 Photograph of a metallograph

Fig. 5.55 A close-up view of the 35 mm camera and ground glass screen on the metallograph in Fig. 5.54

Fig. 5.56 A view of a electronic shutter speed control for the metallograph in Fig. 5.54

140 / Metallographer’s Guide video monitor or printing device. With the swirl of technology development in digital processing, common film micrography is being replaced by digital micrography. However, many metallographers still prefer the unexcelled resolution of a photographic print to a digitally produced image.

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Special Procedures for the Metallurgical Microscope Procedure to Properly Level the Specimen on an Upright Microscope. One of the drawbacks of the upright microscope is that the specimen must be level (perpendicular to the light path) for observation and micrography. Most manufacturers of upright microscopes provide a leveling device when the microscope is purchased. An example of a typical leveling device is shown in Fig. 5.57. This device has two flat, parallel circular platens. The bottom platen is stationary, and the top platen is spring mounted and moves vertically. The procedure to properly level a specimen is described in the steps that follow: •

Step 1: Place a small mound of stiff clay (Plasticine) on the center of a small, rectangular metal plate or glass slide. Place the mounted or unmounted specimen on top of the clay mound and center the assembly on the bottom platen of the mounting device, as shown in Fig. 5.57.

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Procedure to Level a Large Specimen with Uneven Surface. On occasion, the metallographer will have to place a large specimen or a specimen with an uneven surface on the stage of an upright microscope. An example can be seen in Fig. 5.58, which shows a specimen of an AISI 316 stainless steel sheet with an uneven surface (the sheet has a test weld). For this kind of specimen, the normal leveling procedure described previously cannot be used. The following multistep procedure is recommended: • •

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Fig. 5.57 A device used to level metallographic specimens on clay for the upright metallurgical microscope

Step 2: Place a folded, lint-free piece of microscope lens tissue on top of the specimen. Alternatively, if the specimen is mounted, an oval-shaped metal ring (usually aluminum) can be carefully placed on the mount itself without touching the specimen. This oval ring can be easily made by partially compressing a circular aluminum ring, making sure that the shorter opening dimension is always greater than the typical specimen size. For a 31 mm (1.25 in.) mount, a 31 mm (1.25 in.) diameter ring should be compressed to about 25 mm (1 in.). Step 3: With the lens tissue or oval ring in place, slowly push the top platen downward toward the specimen until contact is made. Then press hard on the top platen to deform the clay and level the specimen. Step 4: Slowly release the top platen and remove the lens tissue or oval ring. The specimen is now firmly attached to the clay and slide. Remove the assembly from the leveling device and place it on the microscope stage. When placing the assembly on the stage, always make sure that the stage has been lowered sufficiently to easily allow the specimen enough clearance to not contact the objective. It is advisable to always have the lowest magnification objective (the shortest objective) in position in the nosepiece.

Step 1: Place the specimen on an ample amount of clay on a small metal plate and place on the microscope stage. Step 2: Remove one of the objectives from the nosepiece of the microscope. Align the nosepiece opening over the specimen. Step 3: Closedown both the aperture and the field diaphragms. Step 4: Without looking into the microscope, approximately level the area of the specimen to be observed, using the the alignment of the beam of light from the nosepiece opening, as shown in Fig. 5.58. Step 5: Center the light in the field of view as accurately as possible by slightly tilting and rotating the specimen on the clay mount. When the field diaphragm is seen and uniformly illuminated as best as possible, shown in Fig. 5.59, the plane of the specimen should now be perpendicular to the light beam. (Note that it is difficult to obtain perfect, uniform illumination, because of the uneven specimen surface.) Step 6: Rotate the nosepiece so that one of the low-power objectives is over the specimen. Focus the specimen. The metallographer can then rotate the nosepiece to the highermagnification objectives. Figure 5.60 shows the microstructure of the weld in the specimen. This technique shows that a large and uneven specimen can be observed and photographed even up to a magnification of 1000⫻.

The Metallurgical Microscope / 141 Procedure to Use an Oil-Immersion Objective. As mentioned earlier in this chapter, the oil-immersion objective is used to obtain a high-magnification image with excellent resolution. Oil-immersion objectives have NAs above 1.0, generally between 1.2 and 1.4 NA. Because oil is required between the nose of the objective and the specimen surface, a special procedure is required as follows: •

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Step 1: Place the leveled specimen on the stage of the microscope. Center and focus the field of interest first with a lower-magnification objective (usually the 10⫻ objective), and then focus with the oil-immersion objective (usually 100⫻). Once the field is in focus, turn the nosepiece back to the low-magnification objective. Step 2: Place a drop of oil (special oil supplied from the manufacturer, which has a refractive index to match the refractive index of the objective glass lens) in the center of the lighted region on the specimen. Step 3: Turn the nosepiece from the low-magnification objective directly to the oil-immersion objective. This prevents the other intermediate magnification objectives from touching the oil drop. Step 4: Focus the field with only the fine-focus knob. The oil will form a bridge between the objective and the specimen.

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Step 5: When finished with the observation, lower the stage, remove the specimen, and turn the oil-immersion objective toward you. Step 6: Clean the oil from the tip of the objective first with a dry, lint-free lens tissue. Then place a few drops of ethanol (ethyl alcohol) on a clean, lint-free lens tissue and clean the tip of the objective with a gentle circular motion. A commercial lens cleaner can also be used. Dry the tip of the objective with a clean, lint-free piece of lens tissue. Step 7: Clean the specimen as in step 6.

Fig. 5.59 Micrograph of the field diaphragm in view. Large specimen in

Fig. 5.58 is moved by hand in order that the field diaphragm is in focus (objective is not in nosepiece, as shown in Fig. 5.58).

Fig. 5.58 A large, unmounted sheet of AISI 316 stainless steel with a test

weld placed on the stage of an upright metallurgical microscope. The specimen rests on a large ball of clay. Note that the objective is missing from the nosepiece, exposing the specimen surface to the light beam.

Fig. 5.60 Micrograph of the delta ferrite (dark) in the welded area of the

AISI 316 stainless steel specimen in Fig. 5.58. Electrolytic etch (10% oxalic acid in water, stainless steel cathode). 1000⫻

142 / Metallographer’s Guide Procedure to Clean the Microscope Lenses. The metallographer must, on occasion, clean exposed surfaces of the objective and eyepiece lenses to remove dust and airborne films. (Note that if these lenses need to be cleaned internally, the microscope manufacturer should be notified.) These glass surfaces should be treated with utmost care. The following procedure is recommended: •

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Step 1: Clean dust from the lens surface by gently brushing the surface with a camel-hair brush or blowing the dust from the surface with a rubber bulb (ear syringe). Step 2: Clean films from the glass surfaces with a clean, lint-free lens tissue dampened with a few drops of ethanol (ethyl alcohol). Use a gentle circular motion.

microstructural feature using an eyepiece reticle with a graduated linear scale. Before making such a measurement, the scale must be accurately calibrated. This is done by using a calibrated stage micrometer in combination with the reticle eyepiece. The procedure is as follows: •

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Precautions: Never touch the glass lens surface with the fingers. Never smoke around a microscope, because the airborne tars create a dark film on the glass surfaces. Always keep microscopes in a dust-free, smoke-free, and acid-free environment. • Procedure to Maintain a Clean, Dust-Free Microscope Environment. The microscope should always be protected from dust by placing a dust cover over the microscope when not in use. The microscope manufacturer will usually supply a dust cover. If not, a dust cover can be a simple plastic bag that completely covers the microscope down to the table. Always keep the microscope in a smoke-free room, because tars from the smoke will form on the lens surfaces. Also, make certain that the microscope is in a room away from the acids used in etching. Acids can damage the delicate parts of the microscope. Procedure to Calibrate an Eyepiece Reticle Scale. Many times, the metallographer will need to make a measurement of a

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Step 1: Place a stage micrometer on the stage of the microscope or metallograph. A stage micrometer consists of a closely spaced graduated scale inscribed on a slide. Figure 5.52 shows an example of a stage micrometer scale magnified to 50⫻. Remember, to ensure accuracy, the stage micrometer must be traceable to the National Institute of Standards and Technology (NIST) or a similar organization. Step 2: Place the eyepiece with the reticle scale in one of the ports of the binocular eyepiece attachment or in the tube of the microscope. The metallographer can also use a special reticle scale called an eyepiece micrometer. The eyepiece micrometer has a ruled reticle with a graduated linear scale similar to the scale on the stage micrometer shown in Fig. 5.52. Step 3: Focus on the stage micrometer using the lowest-power objective. Rotate to a higher-power objective if required. Step 4: Bring the image of the stage micrometer into coincidence with the reticle scale in the eyepiece. Fig. 5.61 shows an example of the image of a stage micrometer (the scale at the right) and the reticle scale (the scale at the middle of the field of view). (Note that this is not the same stage micrometer shown in Fig. 5.52). Line up the two scales until they are parallel, as shown in Fig. 5.61 and align the “0” of the two micrometer scales, as shown. In this stage micrometer, one small division equals 0.01 mm (100 divisions ⫽ 1 mm). Step 5: As shown in Fig. 5.61, count the number of divisions on the stage micrometer scale that match the full 100 divisions on the eyepiece reticle scale. In this example, 100 divisions on the eyepiece scale equal 122 divisions on the stage micrometer. Therefore, 122 divisions equal a length of 1.22 mm. From the following equation, calculate the length of each division on the eyepiece scale:

Eyepiece scale division ⫽

Fig. 5.61 Micrograph of a stage micrometer scale (right) and an eyepiece

micrometer scale (center) as seen through an eyepiece. Note that the “0” position of the stage micrometer scale coincides with the “0” position of the eyepiece micrometer scale. Courtesy of E. Leitz GmbH

Length of stage micrometer divisions No. of divisions on eyepiece reticle scale

Thus, each eyepiece scale division ⫽ 1.22 mm/100 divisions ⫽ 0.0122 mm per division, or 12.2 μm per division (1 mm ⫽ 1000 μm). This means that everything that is magnified by this objective-eyepiece combination on this microscope is 1.22⫻ greater than actual. Thus, a 5⫻ objective with a 10⫻ eyepiece would have a magnification of 5 ⫻ 10 ⫻ 1.22 ⫽ 61⫻. • Step 6: Repeat steps 3 and 4 for each objective. • Step 7: Prepare a table of eyepiece scale (or eyepiece micrometer) calibrations and magnification corrections for each objective-eyepiece combination for each microscope in the metallographic laboratory. A similar procedure can be used to calibrate a filar measuring eyepiece. A filar eyepiece has a moveable cross hair on the linear

The Metallurgical Microscope / 143 eyepiece scale. The moveable cross hair is adjusted by rotating a micrometer barrel attached to a graduated vernier scale (similar to a machinist’s micrometer) mounted outside the eyepiece. The cross hair is placed at the extreme edge or “0” division of the stage micrometer scale image, as seen in the eyepiece of the microscope. This “0” position should coincide with the “0” position on the graduated micrometer barrel scale. The cross hair is then moved to the next division on the stage micrometer scale, and the graduated position on the micrometer barrel scale is noted, and so on. Similar procedures can be used to calibrate digital filar eyepieces and other electronic devices (e.g., cross-hair movement of lines on a monitor screen). Procedure to Determine the Magnification of a Micrograph. When micrographs are produced through the optical system of a microscope or metallograph, it is not always guaranteed that the assumed magnification will be achieved on the micrograph. The actual magnification at the film plane in the camera attachment or film holder should be physically measured to ensure the magnification of the micrograph. The following procedure is recommended: •

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Step 1: Place a stage micrometer on the stage of the microscope or metallograph. A stage micrometer consists of a closely spaced graduated scale inscribed on a slide. For ensured accuracy, the stage micrometer must be traceable to NIST or a similar organization. An example of a magnified image of a stage micrometer photographed at 50⫻ is seen in Fig. 5.52. There are two scales on this stage micrometer. In the large main scale at the left, each division is 0.1 mm apart, and with 20 divisions, the total length of the larger scale is 2 mm. In the small scale at the right, the smallest divisions are 0.01 mm apart (the total length of the smaller scale is 0.2 mm). Step 2: After carefully focusing the image of the micrometer, adjust the microscope and produce a micrograph. Step 3: Using the larger scale on the micrograph in Fig. 5.52, measure the length between the 20 divisions. The stage micrometer length is 2 mm. Use an accurate engraved ruler with a millimeter scale (do not use a cheap plastic ruler found in some desk drawer). In this case, the distance measured on the micrograph between the 20 divisions in the larger scale is 100 mm. Step 4: To determine the magnification, divide the measured length from the micrograph by the length of the divisions in the actual stage micrometer: Magnification ⫽

task, but an inexperienced person can easily damage the objective lens as well as gouge the specimen with the tip of the objective. Never allow the objective to touch the specimen during focus, and never allow the specimen to touch the tip of the objective when placing the specimen under the objective. The experienced metallographer should not let an inexperienced person use the microscope without first instructing that person on the proper procedure to focus the microscope. The recommended procedure is as follows. For an inverted microscope: •

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Measured length on micrograph Measured length on micrometer

In this case, the magnification equals 100 mm/2 mm, or 50⫻. The same procedure can be performed on the ground glass screen of the camera attachment of the microscope or metallograph and also on the screen of a video monitor. Procedure to Properly Focus the Microscope on the Specimen. Obviously, focusing the microscope appears to be a simple

Step 1: Without the specimen on the stage, rotate the nosepiece so that the objective with the lowest magnification (shortest objective) will be under the specimen. This is usually the 5⫻ objective. Step 2: Place the specimen on the stage. When doing this, make sure that the tip of the objective is well below the stage opening, so that the objective lens will not be damaged. Step 3: While looking at the objective tip, move the stage downward (or upward if the stage is fixed and the nosepiece moves up or down) with the coarse-focusing knob until the objective tip is close to but not touching the specimen. (Note: never lower the specimen toward the objective, or vice versa, while looking into the eyepieces.) Step 4: While looking into the eyepieces, raise the stage until the specimen is in focus. Close down the field diaphragm until it can be seen through the eyepieces, and focus on the edge of the field diaphragm. When the field diaphragm is in focus, the specimen is in focus. Step 5: Once in this focused position, move the stage in the xor y-direction, and if the specimen image moves, everything is proper. However, if nothing moves, the focus may be on dirt or dust on the eyepiece or objective lenses, particularly for a polished, featureless specimen. Many times, the inexperienced person is deceived by focusing on the dirt instead of the specimen itself. For ease of focus on a highly polished specimen, align the specimen so that the light beam is about half-on and half-off the specimen (look at the specimen and light beam without looking into the microscope). By looking into the eyepieces, very slowly adjust the coarse-focus knob to bring the edge of the specimen into reasonable focus. Use the fine-focus knob to obtain proper focus of the specimen edge. Step 6: Once proper focus is achieved, the nosepiece can be rotated to the objective with the next highest magnification (do not go directly from 5 to 100⫻ without intermediate focus at the other objectives). As a rule, keep in mind that the coarseor fine-focus knobs should be used only for the 5 and 10⫻ objectives. For 20⫻ and above, to prevent damage to the objective lens and damage to the specimen, only use the fine-focus knob because of the short working distance of the higher-power objectives.

For an upright microscope: •

Step 1: Without the specimen on the stage, rotate the nosepiece so that the objective with the lowest magnification (shortest

144 / Metallographer’s Guide

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objective) will be over the specimen. This is usually the 5⫻ objective. Step 2: Place the leveled specimen on the stage. When doing this, make sure that there is ample room between the specimen and the tip of the objective. To ensure this, fully lower the stage. Step 3: While looking at the objective tip, move the stage upward with the coarse-focusing knob until the objective tip is close to but not touching the specimen. (Note: never raise the stage toward the objective while looking into the eyepieces, because the specimen can impact the objective and cause damage to the lens). Step 4: While looking into the eyepieces, lower the stage until the specimen is in focus. Close down the field diaphragm until it can be seen through the eyepieces, and focus on the edge of the field diaphragm. When the field diaphragm is in focus, the specimen is in focus. Step 5: Once in this focused position, move the stage in the xor y-direction, and if the specimen image moves, everything is proper. However, if nothing moves, the focus may be on dirt or dust on the eyepiece or objective lenses. Many times, the inexperienced person is deceived by focusing on the dirt instead of the specimen itself, particularly for a polished, featureless specimen. For ease of focus on a highly polished specimen, align the specimen so that the light beam is about half-on and half-off the specimen (align the specimen and light beam without looking into the eyepieces). Then, by looking into the eyepieces, very slowly adjust the coarse-focus knob to bring the edge of the specimen into reasonable focus. Use the fine-focus knob to obtain proper focus of the specimen edge. Step 6: Once proper focus is achieved, the nosepiece can be rotated to the objective with the next highest magnification (do not go directly from 5 to 100⫻ without intermediate focus at the other objectives). As a rule, keep in mind that the coarseor fine-focus knobs should be used only for the 5 and 10⫻ objectives. For objectives at 20⫻ and above, to prevent damage to the objective lens and the specimen, only use the fine-focus knob, because of the short working distance of the higher power objectives.

Procedure to Personalize the Microscope for Viewing. Every person does not have perfect 20/20 vision. Some people wear eyeglasses to correct for vision errors such as astigmatism or nearsightedness. Also, the spacing between the eyes can vary from person to person, depending on facial bone structure. Therefore, a microscope must be adjusted or “personalized” to obtain the optimal magnified image for each person. The following step-bystep instructions should be taken each time the metallographer uses a microscope. If the metallographer is the only one using the microscope, this procedure needs to be done only once. However, in a laboratory where more than one person is using the same microscope, personalized adjustments need to be conducted by each person. In the following procedure, an upright microscope is assumed: •

Step 1: Turn on the microscope light source and place a leveled specimen on the stage.

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•

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Step 2: Rotate the nosepiece, allowing for ample clearance of the specimen, so that the lowest power objective is over the specimen. Step 3: Without looking into the eyepiece, rotate the coarseadjustment knob so that the tip of the objective is fairly close to the specimen (about 3 mm, or 0.125 in., clearance). Step 4: While looking into the eyepiece, lower the stage away from the objective. (Note: never raise the stage toward the objective while looking into the eyepieces.) If observing an as-polished specimen, it is easier to focus by having the light beam half-on the specimen and half-on the mount, or if the specimen is unmounted, half-on the specimen and half-off the edge. While looking at the specimen from the side (not through the eyepieces), the beam of light can be seen illuminating the specimen. Continue to move the stage away from the specimen until the image is in focus. If the specimen is highly polished, an alternate procedure is to close down the field diaphragm until it can be seen. Move the stage downward until the edge of the field diaphragm is in focus. If the diaphragm is in focus, the specimen is in focus. Step 5: In a binocular microscope, adjust the eyepieces to take into account the distance between the pupils of your eyes, called the interpupillary distance. By slowly moving your head back and forth and adjusting the interocular distance, a single circular field should appear, as if the circular fields of both eyepieces merged into one field. Once adjusted, remember the interpupillary distance for future reference. Step 6: Adjust each eyepiece for each eye at a magnification of 500⫻. Some microscopes allow adjustment of one eyepiece; some microscopes allow both to be adjusted. With the former, close the eye over the adjustable eyepiece and rotate the fine focus for the eye over the fixed eyepiece. Then, close that eye and adjust the other eyepiece for focus. Do not adjust the fine focus for that eye, but adjust the eyepiece itself. Now the images in both eyepieces are in focus. During this process, changes can be made to the aperture diaphragm for optimal imaging, remembering the rule that for high magnification, the field-stop iris is closed down, and at low magnifications, the iris is opened up. Some eyepieces have a diopter scale at the bottom of each eyepiece, as seen in Fig. 5.62. Before starting step 6, set the diopter mark at zero for each eyepiece. After completing this step, note the diopter number for future reference. Step 7: Adjust the field diaphragm so that it is just out of sight in the field of view. This can be done by closing the diaphragm until one can see it at the outer edge of the field. Then, slowly open the diaphragm until it just disappears from the field of view. Now the field diaphragm does not have to be further adjusted; the microscope can now be used for observation.

Procedure to Center and Focus the Light Source. When purchasing a new microscope or when changing the light source (bulb), the metallographer must center and focus the light source before using the microscope. If the lamp is not centered, the light will not be evenly distributed on the specimen. This is particularly important for micrography. This can be seen in Fig. 5.63 which

The Metallurgical Microscope / 145 represents pearlite on austenite grain boundaries in a cast 19% Cr-9% Ni heat-resistant grade HF steel. The micrograph shown in Fig. 5.63(a) was taken with an off-center halogen light source. The light source was centered for the micrograph shown in Fig. 5.63(b). The differences can be seen in the sharpness of the carbide lamella in some of the lamellar colonies (see arrows). To center and focus the lamp, follow the instructions supplied from the microscope manufacturer. Each microscope is different, and therefore, the manufacturer’s instructions are important to

follow. Some microscopes allow for centering and focusing on a portable screen or on the wall of the laboratory. However, in most microscopes, the procedure is done inside the microscope tube. In all cases, the lamp must be centered and focused through the condenser lens, aperture diaphragm, and field diaphragm system, as follows: • •

•

•

•

• Fig. 5.62 Photograph of a binocular attachment showing diopter scale at the base of one of the eyepieces

Step 1: Fully open the aperture diaphragm. Step 2: For this procedure, a highly reflective specimen is needed. This can usually be achieved by placing a polished but unetched metallographic specimen on the stage. Step 3: Focus on the specimen. For easier focusing, close the field diaphragm and obtain a clear, sharp image of the diaphragm. Once the diaphragm is in focus, the specimen is in focus. Step 4: Remove the eyepiece from the observation tube. If the microscope has a binocular, remove one of the eyepieces. Look into the empty tube to see the bright, circular image at the back side of the objective. This circle is created by the light from the specimen coming back through the lenses of the objective. This step can also be performed through the trinocular tube of a microscope. If possible, the diffusion filter must be removed. Step 5: Adjust the screws on the lamp housing in order to move the lamp forward and backward in the optical axis until an image of the filament of the lamp can be clearly seen in the circle of light on the back lens of the objective. At this point, tighten the screws in this position. Step 6: Adjust the screws that move the lamp side to side and up and down until the image of the filament is centered on the back lens of the objective. The light should now be focused and centered.

Fig. 5.63 Micrographs of lamellar carbide colonies on an austenite grain boundary in cast 19% Cr-9% Ni heat-resistant HF steel. In (a) the light source is

not centered, and in (b) the light source is centered. Note the improved sharpness of the carbide lamella in (b) (see arrows). Electrolytic etch (60 ml nitric acid in 40 ml water, stainless steel cathode). 500⫻

146 / Metallographer’s Guide •

Step 7: Replace the eyepiece in the eyepiece tube.

Procedure to Adjust the Aperture Diaphragm. It is very important to adjust the aperture diaphragm for optimal performance of the microscope. The resolution, contrast, and depth of focus are all affected by the aperture diaphragm. One should always follow the procedures outlined in the instruction manual that was included with the microscope when it was purchased. In general, the following steps are usually performed: • •

•

Step 1: Place a metallographic specimen on the stage in order to obtain a focused, highly reflective surface. Step 2: Remove the eyepiece from the observation tube or one of the eyepieces from the binocular tube. By looking into the tube, one will see a small circle of light at the back lens of the objective (created by the light returning back through the objective from the reflectivity of the specimen). (An alternate method to step 2 is described subsequently, using a Bertrand lens. This lens magnifies the circle of light for more accurate adjustment.) Step 3: Close down the aperture diaphragm until one can see the outer edges of the diaphragm at the periphery of the circle of light. In other words, the image of the aperture diaphragm is now visible on the circle of light. Now adjust the image of the aperture until it is about 70 to 80% of the original circle of light (70 to 80% of the diameter). With this amount of area, the aperture diaphragm is properly adjusted.

• • •

Procedure to Measure the Depth of a Depression in the Specimen. There may be instances when the metallographer will be required to measure the depth of a pore, crack, or other depression on the surface of a metallographic specimen. The following procedure can be followed (the example is a hemispherical depression): •

• • •

Important note: Remember to adjust the aperture diaphragm every time a different objective and different sample are used. The light intensity will need adjusting as the aperture is adjusted. Procedure to Adjust the Aperture Diaphragm Using a Bertrand Lens. Some advanced metallurgical microscopes have a special lens, called a Bertrand lens, mounted in the tube of the microscope (actually, the lens is moved in and out of the optical axis, as needed). With the Bertrand lens in the light path, the eyepiece of the microscope does not have to be removed from the tube (as in step 2 of the previous procedure), and by simply focusing, the metallographer can observe the edge of the aperture diaphragm very clearly with an enlarged view. Figure 5.50 shows an example of the image created by a Bertrand lens. Note the light gray outer circle; this is the outer image of the cone of light from the back of the objective. The eight-sided image is the aperture diaphragm. In this example, the aperture diaphragm is closed about 25% (see step 3 of the previous procedure). Procedure to Adjust the Field Diaphragm. The field diaphragm determines the size of the illuminated area on the specimen surface with relation to the field of view that the metallographer observes through the eyepieces. Proper adjustment will decrease light flare in the image and improve contrast. For micrography, the proper adjustment of the field diaphragm is vital. Adjustment of the field diaphragm is quite easy, as indicated in the following steps: •

Step 1: Place a metallographic specimen on the stage of the microscope.

Step 2: Bring the microstructure into focus. Step 3: Close down the field diaphragm until it is seen at the outer edge of the field of view. Step 4: Open the field diaphragm until it just disappears from view. At this point, the field diaphragm is properly adjusted. During micrography, if the image is projected onto a ground glass screen, close down the field aperature until it is just outside the film format markings on the screen.

Step 1: Place the leveled specimen on the stage of the microscope. Upon observation, make sure that the top edge and bottom of the feature are in the same field of view, as seen in Fig. 5.64. This is important so that the stage does not have to be moved in the horizontal x- and y-directions. (Note in this micrograph that the out-of-focus disk in the black region is the reflected light from the bottom of the depression.) Step 2: Choose the highest-magnification objective possible in order to provide the smallest depth of field. Step 3: Open the aperture diaphragm to its largest diameter, in order to reduce the depth of field. Step 4: Focus on the edge of the feature, as seen in Fig. 5.64, and record the reading on the fine-focus knob scale. An example of the scale on the fine-focus knob can be seen in Fig.

Fig. 5.64 Micrograph of the top edge of a hemispherical depression in an

AISI/SAE 1040 steel. The microstructure consists of ferrite (white constituent) and pearlite (dark constituent). Note the small circle of reflection from the bottom of the indentation. 2% nital etch. 200⫻

The Metallurgical Microscope / 147 5.7. Check the manufacture’s instructions for the microscope to determine the dimensions of the scale increments. These increments are usually 0.001 mm (1μm). • Step 5: Focus on the bottom of the feature, as seen in Fig. 5.65,

Fig. 5.65 Micrograph of the bottom surface of a hemispherical depression

in an AISI/SAE 1040 steel. The microscope is focused on the small, circular area in this micrograph, which contains a manganese sulfide inclusion. 2% nital etch. 200⫻

•

and record the reading on the fine-focus knob. In this micrograph, one can see a manganese sulfide inclusion at the bottom of the depression. Step 6: Subtract the two measurements to calculate the depth

Fig. 5.66 Scribing device mounted in one of the ports of the nosepiece.

Before scribing, the specimen should be firmly mounted in clay on a glass or metal slide on the microscope stage.

Fig. 5.67 Micrographs of a small titanium nitride inclusion in A8 tool steel. (a) Surrounded by three circular scribe marks and (b) a closer view of the inner circle showing the inclusion (see arrow). Vilella’s etch. (a) 50⫻ and (b) 300⫻

148 / Metallographer’s Guide of the feature. The depression in this example has a depth of 116 μm. The same procedure can be used to measure the height of a surface feature. Procedure to Scribe a Selected Region on the Specimen Surface. It may become necessary for the metallographer to mark a feature in the microstructure for further analysis by another instrument or to revisit the feature at some other time. One procedure is to use a scribe, which physically “plows” a permanent circle around the area of interest. Figure 5.51 shows a scribing device that replaces an objective in the nosepiece of a metallurgical microscope. The procedure to scribe a circle around the area of interest is as follows: •

•

• •

•

•

•

Step 1: Remove one of the objectives from the nosepiece of the microscope. Screw the scribing device into that location, as shown in Fig. 5.66. It is best that the scribing device is next to the objective being used to select the field of interest. Step 2: Place the specimen on the microscope stage. Make sure that the specimen will not move on the microscope stage. One way to ensure this is to push the specimen (mount) firmly into Plasticine (clay), using a leveling device. The Plasticine should, in turn, be firmly attached to a glass or metal slide. Step 3: Select the feature or area of interest, and center it in the field of view. Step 4: Rotate the nosepiece so that the scribe is centered over the specimen. At this point, the metallographer cannot see the field of view, because the scribe replaced the objective, but the scribe should be parcentric with the observation objective. Step 5: Select the diameter of the circle by selecting the appropriate number on the scale on the barrel of the scribe (see manufacturer’s instructions about the scale dimensions). The scribe shown in Fig. 5.51 can produce circles from 250 to 4000 μm in diameter. Step 6: While watching, carefully raise the stage until the specimen makes direct contact with the diamond tip of the scribe. The tip is usually spring loaded, and as soon as there is a slight movement of the tip of the scribe, there is contact with the specimen surface. Step 7: Gently rotate the knurled ring on the scribe in order to rotate the diamond tip (the tip is off-center from the optical axis). The tip will “plow” a circle or semicircle around the

feature. Typical scribe marks are shown in Fig. 5.67. The micrograph in Fig. 5.67(a) shows three concentric scribed circles, and the micrograph in Fig. 5.67(b) shows the inner circle around an area that includes a small titanium nitride inclusion (see arrow) in an A8 tool steel. The diameter of the large circle is 1498 μm, the diameter of the middle circle is 764 μm, and the diameter of the small circle is 262 μm. Many times, the first circle is too large, and the metallographer must make one or more smaller inner circles to “zero in” on the feature. • Step 8: After scribing, carefully lower the stage away from the tip of the scribe. • Step 9: Rotate the nosepiece to the lowest-magnification objective, focus, and view the selected area and the circle. If the circle is too large, repeat steps 3 to 8 by selecting a smaller radius on the scribe (step 5). If the circle is too small, select a larger radius on the scribe. This method can be used to scribe many features on a specimen surface. Different diameter circles or different semicircular shapes can be used for identification of each feature. SELECTED REFERENCES General references • • • •

J.G. Delly, Photography through the Microscope, Eastman Kodak Company, 1988 R.C. Gifkins, Optical Microscopy of Metals, American Elsevier Publishing Company, Inc., 1970 Metallography and Microstructures, Vol 9, ASM Handbook, American Society for Metals, 1985 Metallography, Structures and Phase Diagrams, Vol 8, Metals Handbook, 8th ed., American Society for Metals, 1973

Microscope manufacturer booklets • • •

M. Abramowitz, “Microscope Basics and Beyond,” Vol 1, Olympus Corp., 1985 M. Abramowitz, “Optics: a Primer,” Olympus America Inc., 1994 F. Restivo, “A Simplified Approach to the Use of Reflected Light Microscopes,” Leco Corp., 1991

Metallographer's Guide: Practices and Procedures for Irons and Steels Bruce L. Bramfitt, Arlan O. Benscoter, p149-168 DOI:10.1361/mgpp2002p149

Copyright © 2002 ASM International® All rights reserved. www.asminternational.org

CHAPTER 6

The Expanded Metallographic Laboratory MANY METALLOGRAPHIC LABORATORIES of today are rather sophistcated, with tools that go well beyond the simple light microscope. For example, a scanning electron microscope (SEM) is quite useful in conducting failure analysis. An image analysis system can be justified when large numbers of grain size and volume fraction measurements are required. Chapter 4 introduced the basic equipment required for typical large and small metallographic laboratories. These laboratories are designed to include facilities for sample preparation and microscopes for observing and photographing microstructures. Chapter 5 introduced and described the use of the metallurgical microscope. The microscope is known as a “light” microscope, because it employs light as the source of illumination of the specimen. These microscopes are commonly called “optical” microscopes, but in this book the authors identify the microscope by the type of illumination used. The first stage in any metallographic analysis involves the observation of the microstructure in a light microscope or a light stereomicroscope before going to other instruments. This is extremely important, because there are numerous cases where key information was missed because the light microscope was bypassed. Once the specimen is examined in the light microscope, in many cases it is desirable to go beyond the light microscope to gather further information about the chemical composition of the microstructural constituents, the amount (volume fraction) of each constituent in the microstructure, finer details of the morphology of the various constituents present in the sample, and the hardness of the constituents. There are a number of specialized instruments available for the metallographer to use as tools to gather additional key information on characteristics of the microstructure being analyzed. Some of these instruments are microscopes that use electrons as a source of illumination instead of light; some are instruments that use x-rays. This chapter describes these instruments in terms of how they can be used to gather important information about a microstructure. Also, the limitations of each instrument are described. If additional information is needed on these instruments, several references have been included at the end of the chapter for further reading.

The Image Analyzer Many metallographic laboratories have an image analyzer as part of the observation/photographic facilities. As the name

implies, the specimen image is “analyzed” according to its geometric features. This technique cannot give you a chemical analysis of the constituents in the microstructure, but it can measure the volume fraction, size, shape, and distribution of each constituent in the microstructure. The instrument is simply a light microscope equipped with a moving specimen stage and a video scanner that records the image of a particular location or field in the specimen. Information obtained from the scanned image is digitized, and many analytical functions are performed by a computer. A typical image analyzer setup is shown in Fig. 6.1. The specimen is moved according to a programmed sequence of steps so that the camera can record the features of the microstructure at each position. Hundreds of individual fields are digitally recorded for each specimen and stored in a computer. The components in the microstructure are distinguished from each other by their different levels of gray, with white and black levels being the extremes. For example, in a typical American Iron and Steel Institute/Society of Automotive Engineers (AISI/SAE) 1040 steel, the pearlite constituent would have a darker gray level than the ferrite constituent. Before the specimen is analyzed, a gray

Fig. 6.1 A typical image analyzer setup showing light microscope with

camera attachment at the left, video monitor in the center, and microprocessor with command monitor at the right. The microstructure displayed on the center monitor is the AISI/SAE 1020 steel shown in Fig. 6.2.

150 / Metallographer’s Guide threshold level is established for each constituent. The instrument can then distinguish each constituent and make accurate measurements of its shape, size, and distribution. Many fields are automatically analyzed in the same manner to obtain a meaningful statistical population that accurately characterized that constituent. As an example, the constituents in a simple microstructure are measured. An annealed specimen of AISI/SAE 1020 is chosen for the example, its microstructure being shown in Fig. 6.2. For the analysis, the specimen was moved in a preset pattern on the microscope stage so that 100 different fields of view were analyzed. The digitized image of a typical field displayed on the monitor is shown in Fig. 6.3. After completing the collection of data and allowing the computer time to analyze the data, the following information was obtained about the constituents in this AISI/SAE 1020 specimen: Volume percent pearlite Volume percent ferrite Average ferrite grain diameter ASTM grain size

⫽ ⫽ ⫽ ⫽

20% 80% 19.4 μm 8.1

One can also obtain the statistical error in the measurements and have the computer print a plot of the data as a histogram, pie chart, and so on. The obvious advantages of the image analysis system are the speed and accuracy of the measurements. The previous analysis of 100 different fields took about five minutes, whereas in the same period of time, only about five fields could be measured by manual techniques. Thus, 20 times more data can be collected

and analyzed by automated image analysis (this is continuing to improve as computers become faster). Another advantage is that there is less fatigue of the metallographer. One disadvantage is the cost of an image analysis system. The additional cost can be justified if large amounts of quantitative information are needed on a routine basis. An example would be in evaluating cleanliness of steels for quality control in a production environment. The analysis would yield quantitative information on the number, type, size, and distribution of nonmetallic inclusions in production samples as well as give a microcleanliness rating for the specimen analyzed. Some image analysis systems are programmed to give the steel an ASTM E 45 ranking for sulfides, silicates, aluminum oxides, and globular oxides. Many production metallographic laboratories have image analysis capability for routine quality control. The Digitizing Pad. Another instrument that can be used for image analysis is the digitizing pad. This is a low-cost option for the laboratory that cannot afford a full-scale computerized image analysis system. A digitizing pad is a semiautomatic electronic device that allows the metallographer to interact with a specimen through a light microscope or with a micrograph of the specimen to collect data on microstructural details, such as areas, lengths, point counts, maximum diameters, distances between constituents, and so on. The information can be collected and analyzed in a personal computer. A photograph of a digitizing pad attached to a light microscope can be seen in Fig. 6.4. The device can be used in two different ways. One is to lay a micrograph to be analyzed on the pad and move the electronically sensitive cursor by hand about the metallographic features to be measured. This procedure

Fig. 6.2 Micrograph of AISI/SAE 1020 steel shown on the video monitor in

Fig. 6.1. Pearlite is the gray-appearing constituent, and ferrite is the white-appearing constituent. Marshall’s reagent was specifically used to delineate the ferrite grain boundaries for image analysis. The pearlite is etched brown by Marshall’s reagent as opposed to a picral or nital etch, which produces a darker pearlite. 400⫻

Fig. 6.3 A closer view of the digitized image of the microstructure of the AISI/SAE 1020 steel that was shown on the video monitor in the center of Fig. 6.1.

The Expanded Metallographic Laboratory / 151 is shown in Fig. 6.5. The other way is to view a metallographic specimen through a light microscope. The cursor is still moved by hand on the “empty” pad. However, the cursor and microstructural image are superimposed in the view seen in the microscope. The merger of the microscope image and the cursor, shown in Fig. 6.6, takes place by means of a “drawing” tube. In Fig. 6.4, the drawing tube is attached to the right side of the microscope.

Fig. 6.4 Digitizing pad setup with a light microscope attached to the pad

The advantage of the digitizing pad, other than its lower cost, is that the metallographer can collect data on microstructural features that have very little gray-level difference. Also, less training is needed to operate a digitizing pad, and a “perfectly” prepared sample is not absolutely necessary, because the human eye can overlook irregularities that are not part of the microstructure. As is seen later in Chapter 7, the specimen preparation for an automatic image analyzer must be closely controlled in order to prevent inaccurate measurements. Sample Requirements. It must be pointed out that the metallographic specimens for an image analyzer must be more carefully prepared than for manual measurements. In manual measurements (including the digitizing pad device described previously), the human eye can compensate and adjust for minor irregularities in microstructure, but removal of scratches and other artifacts is absolutely necessary in a specimen prepared for image analysis. Relief (constituents of differing height) caused by overetching of constituents also should be minimized. Also, because the image analyzer depends on the distinction between differing gray levels of the constituents being measured, special etching techniques may be needed to distinguish the various constituents so that they intentionally have differing gray levels. For grain-size measurements, the specimen must be etched to reveal all the grain boundaries; otherwise, the measurement will be in error. The specimen of AISI/SAE 1020 shown in Fig. 6.2 was etched in Marshall’s reagent to highlight the ferrite grain boundaries without creating unacceptable relief in the specimen. Marshall’s reagent and other etchants used for image analysis are described in Chapter 8. More detailed information on image analysis and specimen preparation for image analysis can be found in the references at the end of the chapter.

(or tablet) at right. The drawing tube (horizontal tube) is attached to the right side of the microscope. A printer is shown at the left.

Fig. 6.6 The image of the cursor superimposed on the field of the Fig. 6.5 Digitizing pad with a micrograph of a Galvalume (BHP, Ltd.) coating on a low-carbon steel. The movable cursor is shown.

microstructure. This image was taken through the light microscope to illustrate what the metallographer would observe if a specimen on the microscope stage was being measured.

152 / Metallographer’s Guide

The Electron Microscope There are several different types of electron microscopes used for metallographic analysis. The more common electron microscopes include the transmission electron microscope (TEM), the SEM, and the electron probe microanalyzer (EPMA). Another electron microscope, used mainly in a research environment, is the scanning transmission electron microscope (STEM). A description of these four electron microscopes is given subsequently. First, we must understand why electron microscopes are important tools for metallographic analysis. This is because the properties or characteristics of electrons are different than those of light used in the light microscope. Metals are opaque to light but are transparent to electrons, at least to a limited extent. Thus, electrons can penetrate a steel sample, and, if it is thin enough, it can become transparent. Therefore, with electrons as the source of illumination on a steel sample, a metallographer can see the internal microstructure of that sample. Moreover, electrons have a much shorter wavelength (about 0.0037 nm) than light (about 1000 nm), and, as a consequence, the theoretical resolving power in the electron microscope is thousands of times greater than the light microscope. This means that magnifications in the hundreds of thousands of times are quite common, and very small features can be observed in the sample that cannot be seen in a light microscope (e.g., precipitates as small as 2 nm in diameter). Electrons also interact with the atoms in the steel sample in a number of ways, and, as a result, additional useful information can be generated. In fact, the different electron microscopes mentioned previously are designed to take advantage of these electron/ metal interactions. The various interactions are shown schematically in Fig. 6.7. For example, the primary electrons (the electrons in the electron beam) interact to generate x-rays that have energies and wavelengths that are uniquely characteristic of the elements contained within the sample. These x-rays can be collected within the microscope to provide information about the chemical com-

Fig. 6.7 A sketch showing some of the different types of electrons and x-rays generated by the interaction of the primary incident electron beam with the specimen in the electron microscope

position of the constituents in the sample. The primary electrons also interact to generate other electrons, called secondary electrons, which can provide topographical information about the surface features of the sample. Other interactions include backscattered electrons, Auger electrons, and, photons. The results of some of these interactions as used in the various electron microscopes are discussed subsequently. First, the basic electron microscope, the TEM, is described.

The Transmission Electron Microscope When people think of an electron microscope, they usually think of the TEM. This instrument has been used since the middle of the 20th century to examine the “fine” microstructural details of steels and cast irons. Because the electrons can penetrate thin foils of specially prepared samples, discoveries were made by transmission electron microscopy in the late 1940s and early 1950s that led to major advances in our understanding of the fundamentals of physical metallurgy. Confirmation of the presence of dislocations in the late 1940s was a key breakthrough needed to understand how metals plastically deform and work harden. The electron microscope was also used to make major advances in our understanding of precipitation and dislocation strengthening of metals and alloys. The TEM employs the same optical principles of the light microscope. Instead of a bright light source, it uses a beam of electrons that is focused by means of magnetic fields that act as “lenses,” similiar in principle to the glass lenses of the light microscope. Figure 6.8 shows a typical TEM. The top of the microscope contains the filament, or source of electrons. A high voltage, usually over 100,000 volts, accelerates the electrons emitted by the filament down through the series of magnetic condenser and objective lenses toward the specimen (located about two-thirds of the way down the column). The image of the electrons as they pass through the specimen can be seen on a phosphorescent screen located below the specimen. The windows located at the bottom of the column are used to observe the image on the screen. A camera is located beneath the screen to photograph the image. It must be remembered that the TEM is only useful for examining very thin specimens. The higher the accelerating voltage of the microscope, the thicker the specimen that can be examined. Generally, two types of specimens are employed: a thin foil or a surface replica. A thin foil is prepared from a bulk sample and prepared to develop a wafer-thin specimen. A replica is prepared from the surface of an etched specimen or fracture surface. The techniques used to prepare these thin foils and replicas are discussed later in this section. In the TEM, the image that is observed is the result of transmitted electrons. Thus, the internal microstructure of a thin section of a metal can be seen, consisting of dislocations, precipitates, grain boundaries, and subgrain boundaries. Figure 6.9 shows a transmission electron micrograph taken of a thin foil of a pearlitic steel. The cementite lamella in the pearlite can be seen, as well as dislocations (the dark lines) in the ferrite. If the micrograph is examined closely, the top and bottom surfaces of

The Expanded Metallographic Laboratory / 153 the cementite lamella can be seen. The through thickness of the lamella is shown as the lighter midsection. The lines running

almost perpendicular to the lamella are subgrain boundaries. Figure 6.10 is a transmission electron micrograph taken from a thin foil of a high-strength, low-alloy (HSLA) steel showing round copper precipates in a matrix of ferrite. Also seen are arrays of dislocations and grain boundaries. Figure 6.11 shows a transmission electron micrograph of a replica of the surface of the same pearlitic steel shown in Fig. 6.9. In this micrograph, taken at

Fig. 6.10 Transmission electron micrograph of a thin foil showing numer-

ous copper precipitates in a hot-rolled, high-strength, low-alloy (HSLA) steel (0.1 C, 0.5 Mn, 1.4 Cu, and 1.0 Ni). Dislocations aligned in a low-angle grain boundary can also be seen. 61,500⫻. Courtesy of K.A. Taylor, Bethlehem Steel Corporation

Fig. 6.8 A transmission electron microscope

Fig. 6.9 Transmission electron micrograph of a thin foil of AISI/SAE 1080

pearlitic steel. The top and bottom surfaces of the cementite lamella can be seen. 15,000⫻

Fig. 6.11 A transmission electron micrograph of a two-stage, shadowed

surface replica of AISA/SAE 1080 pearlitic steel showing a boundary between two pearlite colonies. Specimen etched for 10 s in 2% nital followed by 20 s in 4% picral. 4,000⫻

154 / Metallographer’s Guide a prior austenite grain boundary, one can see the cementite lamella in relief. One disadvantage of the standard TEM is its lack of capability to give the metallographer direct compositional information of the constituents in a microstructure. The SEM, EPMA, and STEM described subsequently have this important capability. However, in the TEM, one can obtain key information about the constituents based on their crystal structure. For example, iron at room temperature has a crystal structure that is called body-centered cubic (BCC). This means that the atoms of iron are in an array of cubes, with atoms at the cube corners and one at the center of each cube. Figure 2.2 in Chapter 2 shows a schematic representation of the BCC unit cell. A grain of ferrite (or iron) consists of millions of BCC unit cells all stacked in an orderly array or lattice. When a primary electron beam in the TEM penetrates this crystal lattice of BCC cells, the beam is diffracted. The angle the electron beam is diffracted is determined by the spacing of the atomic planes in the lattice. The spacing of the atomic planes of iron is unique, because it is based on the distance between the iron atoms in the unit cell. The interatomic spacings of the planes, called “dspacings,” are thus characteristic of each element. These spacings can be measured by the TEM because of the diffraction process. Because the lattice is unique to the particular atomic element in the specimen, electron beams that are diffracted from the specimen lattice can be collected on a photographic film. The resulting spots exposed on the film can be used to identify the particular element present in the sample. The collection of spots is called an electron diffraction pattern. For example, Fig. 6.12 is an electron diffraction pattern for BCC iron (ferrite). Each bright spot in the

photograph indicates the junction where a beam of diffracted electrons, each from a different atomic plane, passed through the photographic emulsion. By measuring the spacing of all the spots on the diffraction pattern with respect to the primary beam (large center spot) and knowing the geometric details (i.e., the camera constant) within the microscope chamber, a microscopist can establish that this pattern is characteristic of BCC iron. If the primary beam is focused on a cementite (Fe3C) particle, it would yield a different electron diffraction pattern unique to cementite, which has an orthorhombic unit cell and thus, a different set of d-spacings. Thus, in a somewhat tedious and indirect way, compositional information can be obtained about the constituents in a particular microstructure. However, the TEM is not normally thought of as an analytical instrument, and other electron microscopes, such as the SEM, EPMA, and STEM, are available for such purposes. Specimen Requirements. Specimens are usually in one of two forms: a thin foil produced from a wafer that is machined from the bulk sample, or a “replica” of the sample surface. To prepare a thin foil, one must obtain a thin section, generally less than 0.5 mm thick, from the sample of interest. This thin section can be cut with a precision wafering machine or by cutting a thicker section and surface grinding the section to the desired thickness (wafer). The thin foil is made by electrolytic thinning of the wafer. The resulting thin foil is on the order of 50 to 200 nm thick. Care is taken to remove all mechanical damage (cold work) created during the cutting operation. Using a special device, the foil is further thinned until a tiny hole appears in the foil. It is the knife-edge thin area around the hole that is actually used for observation in the microscope. One such device, a jet thinning unit, is shown in Fig. 6.13. In this device, a hole is produced in the

Fig. 6.12 An electron diffraction pattern of body-centered cubic iron

taken from a region in a thin foil representing a single grain. Large white spot is the primary beam, and the surrounding spots are the diffracted beams, each representing a different crystallographic plane in the iron lattice. This particular pattern indicates that the grain is orientated with a cube face perpendicular to the primary beam. Courtesy of K.A. Taylor, Bethlehem Steel Corporation

Fig. 6.13 A jet thinning device for preparing thin foils. Source: South Bay Technology Corporation

The Expanded Metallographic Laboratory / 155 foil by the impingement of “jets” of electrolyte on the opposing surfaces at the center of the thin foil. The applied voltage and jet flow is stopped when the first indication of a hole is detected with a photocell device. Another thinning technique is called ion milling, or ion bombardment thinning. In this device, a beam of energetic, positively charged ions, such as argon (Ar⫹), is used to thin the specimen. The technique is extremely slow but produces a smooth surface without grain-boundary etching or etching around particles and precipitates. However, some “damage” can occur in the specimen because of the ion bombardment. Replicas are prepared by different techniques. One type of replica, called a surface or carbon replica, is produced from a specimen as prepared for light microscopy. The specimen surface is coated with a cellulose acetate film (e.g., collodion) to produce a negative impression of the surface features. Then the plastic film is stripped from the specimen surface and coated with a layer of carbon (about 10 to 20 nm thick). The carbon-coating process takes place in a vacuum evaporator unit. A photograph of a typical vacuum evaporator is seen in Fig. 6.14. The plastic is then removed (dissolved) from the carbon replica by a solvent. Before the replica is observed in the microscope, it is usually shadowed with carbon or a heavy element to enhance the topographical features of the surface. The shadowing takes place in a vacuum chamber where atoms of the heavy element are evaporated or

Fig. 6.14 A vacuum evaporator used for depositing carbon on the surface

of a metallographic specimen as the first step in preparing surface replicas. Source: Denton Vacuum, Inc.

“sputtered”onto the surface at an oblique angle to the plane of the replica. Thus, when observing in the microscope, you are not looking at the actual sample but a replica of the surface of the sample. Surface replicas are useful for observing details of surface features in the specimen. Examples of the features in a surface replica are seen in Fig. 6.11 and 6.15. Another type of replica is called an “extraction” replica. The metallographic specimen is etched beforehand to put particles and carbides in relief. In an extraction replica, a carbon film is deposited on the surface of the etched specimen. The carbon film itself is not physically stripped from the specimen surface, but etched or “floated” away from the surface so that those particles physically attached to the deposited carbon film will be extracted from the specimen. An example of the features in an extraction replica can be seen in Fig. 6.16. The dark particles represent titanium-molybdenum carbides and are images of precipitates in a microalloyed steel. The “halos” around the particles are the result of the etching and film deposition technique used. Further details on these and other specimen preparation techniques can be found in references at the end of the chapter.

The Scanning Electron Microscope The most versatile electron microscope is the SEM. A typical SEM is shown in Fig. 6.17. A major difference between the SEM and TEM is that in the SEM, the stationary electron beam is not transmitted through a thin specimen, but the electron beam scans an opaque specimen surface. Thus, the SEM is used to observe and characterize surface features over large (or small) areas of the specimen. The primary electron beam interacts with the specimen surface to produce secondary electrons, backscattered electrons,

Fig. 6.15 Example of a carbon surface replica of a bainitic steel shadowed with carbon. 5000⫻

156 / Metallographer’s Guide and x-rays. With detectors mounted just above the specimen, these electrons and x-rays can be collected and analyzed to provide important information about the specimen surface. Subsequently, we look at each interaction. The backscattered electrons are those electrons that are scattered from the specimen surface and can be collected as the primary beam scans the specimen surface. The collected backscattered electron image of the specimen surface is displayed on a cathode ray tube (CRT) and can be photographed. There is some electron energy loss during the backscattering process. The higher the atomic number of the elements in the sample, the greater the degree of backscattering (less energy loss). This means that

elements with higher atomic numbers, such as iron with an atomic number of 26, will appear brighter on the backscattered electron image on the CRT than elements with lower atomic numbers, such as oxygen with an atomic number of only 8. An example of a backscattered electron image of oxide scale penetration into the surface of an AISI/SAE 1045 steel is shown in Fig. 6.18. The steel appears as the light gray constituent, and the oxide appears as two darker gray constituents. Note that, because of its higher atomic number (i.e., the higher density), the steel appears brighter than

Fig. 6.18 A SEM backscattered electron micrograph of oxide scale pen-

etration on the surface of an AISI/SAE 1045 steel. The dark-grayappearing constituent is silicon-rich iron oxide (fayalite-Fe2SiO4), the medium gray constituent is iron oxide (wustite-FeO), and the light gray constituent is steel. The black constituent is a calcium-aluminum oxide (Ca aluminate). 540⫻

Fig. 6.16 An extraction replica showing titanium-molybdenum carbides in a high-strength, low-alloy steel. 130,000⫻

Fig. 6.17 A scanning electron microscope

Fig. 6.19 A SEM secondary electron micrograph of a fractured steel bar. Inclusions can be seen in many of the voids. 1000⫻

The Expanded Metallographic Laboratory / 157 the oxide. Thus, backscattered electrons are useful in giving what is called “atomic number contrast” in a microstructure. Secondary electrons, on the other hand, are the result of the interaction of primary (beam) electrons with those electrons contained within the atoms in the sample. The primary electrons can actually knock the loosely held orbital electrons from atoms. These displaced electrons are called secondary electrons. The secondary electrons have much lower energy than the backscattered electrons described previously. This means that secondary electrons are only detected from the surface and near-surface regions of the specimen, because those from deeper regions are easily absorbed by the sample. Thus, secondary electrons yield a secondary electron image on the cathode ray tube that reveals surface topography and produces an image with enhanced depth of field. It is this depth of field that makes the SEM one of the

most useful of all electron microscopes. Figure 6.19 shows a secondary electron image of the surface of a fractured steel bar. In this micrograph, inclusions can be seen in many of the voids on the fracture surface. Note the tremendous depth of field that is obtained. This capability is what makes the SEM a popular instrument for studying surface features. The remarkable advantage is that the images can be obtained at magnifications varying from 10 to 30,000⫻. As an example, a small copper support grid for viewing surface replicas and thin foils is examined in the SEM. The grid viewed in the SEM shows a secondary electron image at 1000⫻ in Fig. 6.20(a) and a backscattered electron image in Fig. 6.20(b). The grid openings are produced by a punching process. Another SEM secondary electron image of a different type of copper grid made by an electrolytic process is shown at the bottom of Fig. 6.21 (also at 1000⫻). The rougher surface details of the image of this grid show that it was manufactured by a different process. X-rays are also emitted from the sample, because of the interaction of the primary and backscattered electrons with the inner shell electrons of atoms in the sample. The primary/ backscattered electrons have sufficient energy to knock inner shell electrons (the shell closest to the atom nucleus) out of orbit. Where an electron is knocked out of a particular inner electron shell, an x-ray is emitted when an electron (from an electron shell further out from the nucleus) moves into its place. These x-rays have a characteristic energy (and wavelength) for the particular atomic species present. Because of this, they are called characteristic x-rays. This means that every x-ray that is collected has an energy and wavelength that is unique to the particular element present in the sample. The importance of collecting these x-rays is that compositional information can be thus obtained. Generally, in a SEM, only the x-ray energy is analyzed. The technique is called energy dispersive spectroscopy, or EDS. An example of an EDS analysis is shown in Fig. 6.22 where a nonmetallic inclusion in a fractured specimen, shown in Fig. 6.19, is analyzed to determine

Fig. 6.20 A copper grid made by the punching process, as seen in the

Fig. 6.21 A copper grid made by an electrolytic process, as seen in the

scanning electron microscope. (a) Secondary electron image (760⫻) and (b) backscattered electron image

scanning electron microscope. Secondary electron image.

750⫻

158 / Metallographer’s Guide the chemical elements present. By placing the primary electron beam on the nonmetallic inclusion itself and collecting the x-rays emitted, the elements contained within the inclusion can be determined. The energy spectrum of the x-rays collected from the inclusion is shown in Fig. 6.22. The intensity, or number of x-rays, is plotted on the vertical axis, and the x-ray energy is plotted on the horizontal axis. In this particular case, the peaks in the energy spectrum indicate that calcium, aluminum, and sulfur are present (there are traces of manganese, silicon, and potassium; the iron is from the specimen itself). This analysis indicates the elements present but does not reveal the distribution of elements within the inclusion. As is seen in the next section on the EPMA, this inclusion is a complex inclusion that has an aluminum oxide core and a calcium-manganese rim similiar to the inclusion shown later in Fig. 6.25. Some SEMs can provide an EDS x-ray map of the elements contained on the surface of the sample. However, x-ray mapping is not discussed here but is described as one of the primary features of the EPMA (discussed subsequently). Specimen Requirements. Special techniques are not usually necessary in preparing specimens for the SEM. Fracture specimens need to be cut to a size, without contaminating the surface, that will fit within the specimen chamber of the microscope. The specimen can be glued to a metallic stub that fits into the specimen chamber. Usually, more than one specimen can be placed in the chamber. Mounted metallographic specimens are also used in the SEM. Specially prepared conductive paint is used to “bridge” the metallic specimen to the metallic specimen holder in order to electrically ground the specimen. If not electrically grounded, the electrical charges will build up on the specimen surface and cause imaging problems. In fact, on certain specimens, gold, because of its excellent electrical conductivity, is deposited on the surface by sputtering in order to drain away the excess electrons and to enhance the secondary electron image. Gold sputtering is essential

for nonconducting surfaces (e.g., oxide layers) as well as surfaces containing large cavities. The sputtered layer is applied uniformly using a special device shown in Fig. 6.23. If EDS analysis is required to qualitatively analyze the composition of nonmetallic inclusions, it is important not to polish the specimen with alumina or silica polishing compounds, because they can contaminate the specimen, with small particles being lodged in porous areas around the inclusions. A diamond polishing compound is usually more appropriate, unless one is analyzing for carbides. Because many nonmetallic inclusions in ferrous alloys contain various amounts of aluminum and silicon oxides, the EDS analysis would yield incorrect results, especially if the inclusions do not contain these oxides. More details on specimen preparation for the SEM can be found in the references at the end of the chapter.

The Electron Probe Microanalyzer Another important instrument in the metallographer’s arsenal is the EPMA, an instrument often called simply, the “microprobe.” The microprobe is essentially a SEM equipped with extra x-ray detectors to maximize the compositional information generated from the x-rays created during the interaction of the primary electron beam with the sample. The microprobe is an analytical instrument capable of obtaining both qualitative and quantitative chemical analysis of constituents in a metallographic sample. The SEM, on the other hand, offers better imaging versatility but usually only qualitative chemical analysis data. However, with the tremendous advances recently achieved in electron microscope

Fig. 6.22 An EDS spectrum of the elements contained in the nonmetallic

inclusion in Fig. 6.19. The inclusion contains calcium, aluminum, and sulfur, with traces of silicon, potassium, and manganese (the iron peak is from the surrounding steel). The vertical axis (intensity) has a full scale (VFS) of 8192 x-ray counts, and the horizontal axis (energy) has a full scale of 10.240 electron volts (eV). This EDS spectrum was photographed from the cathode ray tube of the EDS unit.

Fig. 6.23 A vacuum sputtering device used to deposit a thin coating of

gold, or some other element, onto the surface of a SEM or EMPA specimen. Source: Polaron Instruments, Inc.

The Expanded Metallographic Laboratory / 159 instrumentation, quantitative analytical capability can be added to a SEM so that the SEM and EPMA can have similar features. A typical setup of an EPMA can be seen in Fig. 6.24. The important feature is the addition of the x-ray detectors mounted on the side of the microscope column (the rectangular boxlike attachments shown on the microscope column in Fig. 6.24). These detectors analyze the wavelength of the x-rays generated from the interaction of the primary and backscattered electrons and the specimen. The technique is called wavelength dispersive spectroscopy (WDS), as opposed to EDS generally used on the SEM. With WDS, the microprobe is capable of detecting x-rays from all elements in the periodic table above boron (atomic number of 5) and can yield much more accurate quantitative data than the EDS technique. Most microprobes also have EDS detectors similar to those found on a SEM. Wavelength dispersive spectroscopy is much more time-consuming than EDS and is thus used for a more accurate and detailed analysis. The microprobe can also generate images of the backscattered and secondary electrons, similar to the SEM. From the backscattered electron image (BEI), one can obtain atomic number contrast and some topographic information from the specimen. From the secondary electron image, one can obtain detailed topographic information. An example of the output of a typical microprobe is now described. In the example, the metallographically polished specimen to be analyzed contains a complex inclusion, as shown in the BEI in the upper left-hand corner of Fig. 6.25. In the backscattered image, one can see that the inclusion has a light gray rim and a dark gray core. The difference in gray level is due to the atomic number contrast of the elements contained in both constituents of the inclusion. The lighter gray color of the rim means that the rim contains elements with higher atomic numbers than in the core. To determine which elements are present in both the rim and core, the microprobe can generate a WDS x-ray map of each element

present. These maps are shown in Fig. 6.25, all at the same magnification as the BEI. Each map represents a different element. For example, the rim contains mostly calcium, manganese, and sulfur. Thus, the rim is a calcium-manganese sulfide. The core, on the other hand, contains mainly aluminum, magnesium, and oxygen. Thus, the core is a magnesium aluminate. There are some fine features in the inclusion that indicate that there is also a calcium aluminate constituent in the core and a minor amount of a calcium aluminate constituent in the rim. In addition to the time-consuming WDS analysis shown previously, the microprobe can use its EDS capability to give a “rapid” semiquantitative analysis of the constituents present in the inclusion. From the x-ray energy data, a “standardless” EDS analysis (details of the technique can be found in the references at the end of the chapter) can be used to determine the actual composition of each constituent. For example, the analysis of the magnesium aluminate is as follows: Element

Aluminum Magnesium Iron Manganese Oxygen(a)

Atomic %

Weight %

33.7 7.2 0.4 0.3 58.4 100.0

44.3 8.6 1.1 0.6 45.4 100.0

(a) Oxygen is not analyzed, but determined by difference.

Because inclusions are usually composed of inorganic chemical compounds, one can determine the stoichiometric proportions from the previous data as follows: Al2O3 MgO FeO MnO

83.6% 14.2% 1.4% 0.8%

Note: These were assumed compounds from which oxygen content was derived.

The same EDS analysis can be applied to the other constituents. The calcium aluminate portion of the core has the following composition: Element

Aluminum Calcium Iron Oxygen

Atomic %

Weight %

33.3 8.0 0.5 58.2 100.0

41.2 14.7 1.3 42.8 100.0

or in compound form: Al2O3 CaO FeO

77.9% 20.5% 1.6%

and the calcium-manganese sulfide rim consists of the following: Element

Calcium Manganese Iron Sulfur

Fig. 6.24 An electron probe microanalyzer

Atomic %

Weight %

33.3 12.3 1.7 53.7 100.0

34.2 17.8 2.6 45.4 100.0

160 / Metallographer’s Guide Another important feature of the microprobe is that it can perform quantitative analysis. This is achieved with the use of “pure” element standards representing each element being determined. The primary electron beam can focus on a particular spot of the constituent being analyzed. The quantity of x-rays that have a characteristic wavelength of the particular element are collected and compared to the the quantity of x-rays from the pure standard. All other conditions remain constant, and a quantitative value can be determined for that element. This technique is quite useful if one wants to determine the chemical composition of various regions in a metallographic specimen, for example, the composition of areas of segregation, such as pearlite banding. Specimen Requirements. Generally, polished metallographic specimens are necessary for the microprobe. If mounted, the specimen itself is grounded to the metallic specimen holder with a “bridge” of electrically conductive paint. Also, the surface may be coated with carbon in a vacuum evaporator (Fig. 6.14) or with

gold, using a sputtering device (Fig. 6.23) for better conduction. If gold is used, special attention is needed in the analysis, because the gold layer will “filter” certain x-rays, for example, characteristic x-rays of molybdenum and sulfur, thus complicating analysis of the specimen. Specimens with a rough surface cannot be used for precise compositional analysis, because of the accurate focus required by the x-ray wavelength detectors. Even metallographic specimens that have been etched will not yield accurate results, because of the roughened and chemically altered surface. In an etched specimen, the metallographer must find the area of interest, mechanically scribe a circular groove around the area, and then lightly polish the specimen to remove the etch but not the scribed circle itself. The microprobe operator will analyze the area inside the scribed circle. A standard light microscope can be equipped with a scribing device. The special scribing device, described in Chapter 5, is mounted in one of the port locations of the objective turret (nosepiece). It has a knurled ring used to rotate the scribing knob (a diamond that protrudes from the bottom of the device). Figure 6.26 shows an example of a scribed area on a metallographic specimen. Within the scribed area, an oxide stringer can be seen. Another method used to mark an area of interest is placing microhardness impressions at strategic locations on the specimen (more on microhardness testing can be found later in this chapter). Figure 6.27 shows an example of using Vickers microhardness indentations to identify the location of ferrite banding in a specimen prepared from an AISI/SAE 1335 steel bar. After placing the indentations on the specimen, the etch was removed by light polishing (shown at the bottom of Fig. 6.27).

Fig. 6.25 Backscattered electron micrograph (upper left) and WDS x-ray

maps of the elements contained in a nonmetallic inclusion that resulted from calcium treating liquid steel for sulfide shape control. The x-ray maps are for oxygen, calcium, aluminum, manganese, magnesium, sulfur, and iron.

Fig. 6.26 An example of a scribed groove around a oxide stringer-type nonmetallic inclusion. 100⫻

The Expanded Metallographic Laboratory / 161 The same precautions mentioned for preparing specimens for the SEM, that is, avoiding certain polishing compounds that contain alumina or silica, especially apply to preparing specimens for the microprobe. If nonmetallic inclusions are to be analyzed by

EDS or WDS, these polishing compounds may contaminate the specimen and lead to an incorrect analysis. In this case, a diamond polishing compound would be appropriate (unless the inclusions contain carbides). More on specimen preparation for the microprobe can be found in the references listed at the end of the chapter.

The Scanning Transmission Electron Microscope Although mostly confined to a research laboratory environment because of its high cost and complex instrumentation, the STEM is another tool that the metallographer can use for analysis. The STEM is known as an analytical electron microscope. The STEM is essentially a combination of the TEM and SEM. Thus, it can produce high-resolution micrographs of very fine microstructural details, as in the TEM, and can also perform compositional analysis of small regions and particles within the specimen, as in the SEM. The STEM uses a thin foil or extraction replica as a specimen. There are basically two types of STEMs: a TEM that has been configured with EDS analytical components to produce a TEM-STEM, and a dedicated STEM that is constructed from the beginning as a STEM. An example of the analytical capability of a STEM is shown in Fig. 6.28. Here, one can see small precipitates of titaniummolybdenum carbides in a carbon extraction replica of a HSLA steel. The micrograph was taken in dark field, thus showing the precipitates with a white appearance. These precipitates are on the order of 20 nm in diameter. They are far too small to detect in the EPMA described previously. However, as with the SEM and EPMA, one can use an EDS detection system to obtain compo-

Fig. 6.27 Preparing a specimen from an AISI/SAE 1335 steel bar for

compositional analysis on the electron probe microanalyzer. (a) Vickers microhardness indentations used to mark the location of ferrite banding and (b) the same location with the etch removed by light polishing. 4% picral etch (top). 500⫻

Fig. 6.28 A STEM micrograph of titanium-molybdenum carbides in an

extraction replica of a HSLA steel. Micrograph taken in dark field, thus the precipitates appear white in a dark matrix. 230,000⫻. Courtesy of K.A. Taylor, Bethlehem Steel Corporation

162 / Metallographer’s Guide sitional information about the particles. For example, the EDS spectra obtained in the STEM from the particles is shown in Fig. 6.29. The peaks for titanium and molybdenum are quite obvious in the EDS spectrum (the copper peaks are from the copper support grid holding the carbon extraction replica, and the carbon peak is from the replica itself). Because of the carbon replica, it is difficult to determine carbon in the particles. Even if the replica was of some other element, the EDS method would have a difficult time detecting carbon and nitrogen, because of their low atomic numbers of 6 and 7, respectively. Generally, conventional EDS will not detect elements below fluorine, with an atomic number of 9 (some detectors are equipped with “windowless” detectors that can detect elements with lower atomic numbers). In the STEM, a special detector can be mounted on the opposite side of the specimen from the primary beam to measure the inelastically scattered electrons that pass through the specimen. The detector measures the energy loss of the electrons passing through the specimen. The technique is called energy loss spectroscopy, or EELS. From an EELS spectrum, one can detect light elements, such as carbon and nitrogen. Thus, the EELS and EDS data combined can yield the complete composition of precipitates, such as those shown in Fig. 6.28. Similar to the SEM and EMPA, the STEM can also do qualitative EDS x-ray mapping of the elements contained in a particular constituent. Sample Requirements. Specimens used for the STEM are similar to those used in the TEM, mainly thin foils and extraction replicas. Specimens must be “clean” in order to avoid excessive buildup of contamination while collecting x-ray spectra.

Fig. 6.29 An EDS spectrum of the elements contained in the precipitates

in Fig. 6.28. Note that only titanium and molybdenum are shown. The copper peaks are from the support grid. Vertical scale (intensity) is from 0 to 8,000 counts, and horizontal scale (energy) is from ⫺0.2 to 20.3 keV.

The X-Ray Diffractometer Another instrument that can extend the capabilities of a metallographic laboratory is the x-ray diffractometer. This instrument is not a microscope, as in the case of the instruments described previously, but does use a beam of x-rays to strike the specimen. The x-ray beam is produced from a special x-ray tube that emits monochromatic radiation, that is, radiation consisting of x-rays of a single wavelength. This wavelength depends on the metal used for the target of the x-ray tube. For example, a chromium target will emit characteristic x-rays for chromium. An x-ray beam of chromium radiation has a wavelength of 2.92092 angstroms (in x-ray terminology, the angstrom unit, Å, is commonly used; one angstrom ⫽ 0.1 nm). When the monochromatic beam strikes the sample, some of the x-rays are diffracted from certain planes of the crystal lattice of the specimen, and the angles at which they are diffracted are unique to the spacing of the atoms that are present in the crystal. The diffraction process occurs when certain conditions, described in a simple law called Bragg’s law, are satisfied. These conditions may be represented by the following equation (Bragg’s law): ␭ ⫽ n2d sin ␪

where ␭ is the wavelength of the x-rays, d is the spacing between crystallographic planes in the crystal lattice, ␪ is the angle of the x-ray beam with relationship to the sample surface, and n is an integer. Thus, by scanning the specimen at a continuously changing angle (represented by 2␪, the angle between the incident x-rays and diffracted x-rays) and by knowing the wavelength of the x-rays in the incident beam, one can measure the d-spacings of the planes of the crystalline lattice responsible for the diffraction. Often, the diffractometer is used to identify chemical compounds that are associated with a specimen. For example, a corrosion or oxidation product on the surface of a specimen can be analyzed for the compounds present. As an example of the use of a diffractometer, the oxidation product on a heavily scaled steel part was analyzed by x-ray diffraction. The oxide was scraped from the surface of the part, ground into a fine powder, and glued to a glass slide. The slide was placed in the beam of x-rays, and the beam was scanned at 2␪ angles ranging from 20 to 160°. The intensity of the diffracted x-rays was measured by a device called a scintillation counter (or other type of detector) and plotted against the scanning 2␪ angle of the x-ray beam. The results (Fig. 6.30) show a number of peaks of various intensities at different 2␪ angles. Each set of peaks is unique to a particular chemical compound and can be identified from published information. The compounds so identified are labeled next to the peaks. The oxide contains FeO (wustite) and Fe3O4 (magnetite), with traces of Fe2SiO4 (fayalite) and FeCr2O4 (chromite). To identify the peaks, a d-spacing is used that is unique to the particular compound. For convenience, the Joint Committee on Powder Diffraction Standards (JCPDS) are computerized or are in the form of cards and books. The JCPDS card, an example of which is shown in Fig. 6.31, lists the d-spacings (the d-spacing is related to the 2␪ angle by Bragg’s law) for the particular element or compound. The card

The Expanded Metallographic Laboratory / 163 shown in Fig. 6.31 is for iron oxide (wustite, FeO), and the d-spacings are listed along with the relative intensity of the peak associated with that d-spacing. The three peaks with the highest relative intensities are used for identification. In the case of iron oxide, these peaks would be the (200) peak at 100% intensity (d ˚ ), the (111) peak at 80% intensity (d ⫽ 2.49 A ˚ ), and the ⫽ 2.153 A

Fig. 6.30 X-ray diffraction data showing the oxides present in scale on a

steel surface. Each peak represents a particular 2␪ value and corresponding d-spacing for a constituent. Each peak is labeled with the constituent matching that particular 2␪ angle. The vertical axis (intensity) is from 0 to 500 counts/s, and the horizontal axis is from 25 to 155°.

˚ ). No other element or (220) peak at 60% intensity (d ⫽ 1.523 A compound has the same order or combination of identifying d-spacings and relative peak intensities. Thus, in a way, x-ray diffraction can yield a unique “fingerprint” of the elements or compounds present in a particular sample. An x-ray diffractometer can also be used to determine the magnitude of any residual strains, and from this, the residual stresses present in a sample. Residual stresses in cold-worked or quenched samples elastically stretch or compress the crystal, and an x-ray beam set at the proper Bragg angle can measure this distortion. For example, in a quenched steel containing martensite, a certain diffraction line, known as the (211) peak, is measured and compared with the (211) peak of a stress-free sample. The width of the peak is an indicator of the amount of residual stress in the sample. If the sample is austenitic (face-centered cubic, or FCC) the (311) peak is used. For further information about using x-ray diffraction for measuring residual stresses, consult the references at the end of the chapter. Another application of x-ray diffraction is to measure the crystallographic texture or orientation of the crystalline grains in a sample. Inverse, reflection, or transmission pole figures can be constructed to determine the degree to which a particular crystallographic plane is oriented parallel to the specimen surface. (A pole figure is essentially a map of the intersections of diffracted x-ray beams with respect to a particular orientation plane, for example, a (100), (110), or (111) plane.)

Fig. 6.31 The d-spacings for iron oxide, FeO (wustite), are listed in the upper right corner of the table.

164 / Metallographer’s Guide A transmission or reflection pole figure can also be constructed to yield a three-dimensional representation of the crystallographic orientation of the sample. A transmission pole figure requires a thin sample, whereas a reflection pole figure can be obtained with a polished bulk specimen. Specialized specimen holders and goniometers (the moveable stage that holds and rotates the specimen in three axes) are required. An x-ray diffractometer can also be used to determine the amount of retained austenite in a steel specimen. Austenite, being FCC, has a different set of diffraction peaks than ferrite, which is BCC. Specimen Requirements. For most x-ray diffraction investigations, a ground powder is used as the specimen, that is, a powdered corrosion product. The material to be analyzed is usually removed from the bulk specimen and ground to a powder in a mortar and pestle. A powdered sample will give sharp, distinctive diffraction peaks of the constituents present in the sample. A portion of the powder can be glued to a glass slide and placed in the specimen holder of the diffractometer. The glue and glass slide are amorphous (noncrystalline) and do not produce diffracted peaks. When a specimen cannot be removed from the bulk specimen, the entire specimen can be scanned by the x-ray beam. However, the results are then less accurate, and the diffraction pattern can contain other peaks originating from the material below the surface being investigated. For residual-stress analysis, the actual bulk sample must be used. However, for accuracy, a flat surface is desired. Cutting and machining operations carried out on the specimen themselves introduced residual stresses (cold work) in the specimen, and this must be avoided. Thus, care must be taken in specimen preparation. For the determination of inverse pole figures, a flat, polished metallographic specimen can be used. It is important to have a large enough surface area of the specimen to totally cover the full region of x-ray beam impingement. For transmission pole figures, a thin specimen is used. The specimen is mechanically ground flat and parallel by surface grinding. After a thickness under 0.25 mm (0.010 in.) is achieved, the specimen is further thinned by chemical thinning. The final specimen thickness should be around 0.05 mm (0.002 in.). For reflection pole figures, a bulk specimen can be used. The specimen surface should be metallographically polished for better accuracy. References at the end of the chapter describe each of these techniques in detail.

window through which the microscope objective lens can focus. The objective must be corrected for observation through the quartz window and must have a long focal length, because of the extended distance between the specimen and the lens (see more on corrected lenses in Chapter 5). Figure 6.32 shows a view of a hot-stage attachment on a conventional microscope. The rotating quartz window can be seen at the top of the hot stage. Although not seen in the photograph, a small, unmounted, polished specimen (usually in the form of a flat sheet) rests on a flat heating element located just below the window. The temperature of the specimen is measured and controlled by a thermocouple spot welded to the specimen. To record events taking place during observation, a video camera is attached to the microscope. An example of what can be observed in a hot-stage setup is shown in Fig. 6.33. This sequence of micrographs, taken with a high-speed camera, shows the formation of bainite during cooling of a low-alloy steel. In the upper left micrograph, an austenite grain can be seen. In the subsequent micrographs, the bainitic laths grow across the austenite grain along specific crystallographic directions. In the micrograph in the bottom right corner of Fig. 6.33, the austenite grain is completely transformed to bainite. The total lapsed time in the sequence was 10 s. Of course, in the light microscope, only events that take place on the surface of the sample can be observed. However, using a thin foil in the TEM, three-dimensional changes can be observed. Specimen Requirements. Because the specimens used in the hot stage for a light microscope are not mounted, they must be prepared with flat and parallel surfaces. Also, for better heat transfer, the specimen should be in the form of thin sheet, less than 1 mm (0.040 in.) thick. In most cases, specimens are not etched, because new grain boundaries form at the higher temperatures.

The Hot Stage Microscope Most of the time, the metallographer observes the microstructure of metallographic specimens at room temperature. However, in some cases, it may be necessary to observe changes in the microstructure of a particular steel at elevated temperatures and at different heating or cooling rates. For this, a hot-stage attachment mounted on a light microscope (or electron microscope) is used. For the light microscope, the specimen is contained within a vacuum chamber (or a chamber filled with inert gas) to protect the polished surface from oxidation. The chamber has a quartz

Fig. 6.32 A hot-stage attachment mounted on the stage of a light micro-

scope. The view of the specimen is through the rotating quartz window, shown at the tope of the device.

The Expanded Metallographic Laboratory / 165 These grain boundaries are “etched” at the higher temperatures by a phenomenon called “thermal grooving.” More details about hot-stage microscopy can be found in the references at the end of the chapter.

The Microhardness Tester Sometimes, a metallographer needs to know the hardness of the individual constituents in a microstructure. For example, small regions of retained austenite can be identified from the harder martensite matrix in a quenched steel, or the hardness of a thin, carburized or decarburized steel surface can be measured. Another example would be determining the hardness of individual carbides in a tool steel. These measurements cannot be performed by a conventional Brinell, Vickers, or Rockwell hardness tester, because the indentation size far exceeds the dimensions of an individual constituent. In fact, the conventional hardness tester obtains an average or bulk hardness of a particular microstructure. A microhardness tester has to be used for “fine-scale” hardness measurements. A microhardness tester is shown in Fig. 6.34. A microhardness tester uses loads of 1 kg or less, whereas a conventional hardness tester uses higher loads. The indenter is an important feature of the microhardness tester. There are basically two types of indenters: the Vickers indenter, which leaves a diamond-shaped “pyramid” impression, and the Knoop indenter, which leaves an elongated, rhomboid-shaped impression. Ex-

amples of the two types of indenters and the shapes of the indentations are shown in Fig. 6.35. The Vickers (also referred to as diamond pyramid) indentation has diagonals of equal length, whereas the Knoop indentation has one diagonal longer than the other. The depth of the Vickers impression is about 1⁄7 of the diagonal dimension, and the depth of the Knoop impression is about 1⁄30 of the length of the long diagonal. Because the Vickers indentation is deeper than the Knoop indentation, it is less sensitive to surface conditions. Because the Knoop indentation has a longer diagonal than the Vickers indentation, it is less sensitive to measurement errors. The hardness is determined by measuring the length of the diagonals of the impression. With the Knoop indenter, only the long diagonal is measured, but with the Vickers indenter, both diagonals are measured, and the average diagonal length is used. For both indenters, there are tables that relate impression length to hardness for a fixed load, for example, 10 g. It is important not to place indentations too close to one another or too close to the edge of the specimen. The general rule would be to keep indentations at least three diagonal lengths apart. This is because a volume surrounding the indentation is deformed by cold working. The deformation created by Vickers and Knoop microhardness indentations can be seen in Fig. 6.36 and 6.37. In Fig. 36, two Vickers indentations, one from a 10 g load (on the left) and one from a 50 g load (on the right), are present. Note the deformation markings (called slip lines) around each hardness impression. In Fig. 6.37, the deformation markings can be seen around a Knoop indentation

Fig. 6.33 A series of micrographs, taken on the hot-stage microscope, of the bainite transformation in a low-alloy steel. The specimen was cooled at a rate of 17 °C/s. The times in seconds are shown below each frame. Unetched.

166 / Metallographer’s Guide at 50 g. Overlapping deformation markings from two indentations might yield inaccurate readings, because the cold-worked region around the preexisting indentation would affect the hardness value for the new indentation. Specimen Requirements. Generally, a good metallographically prepared specimen is required for microhardness testing. The surfaces of the specimen must be flat and parallel to avoid erroneous readings. For uneven samples, a leveling device can be used, an example of which is shown in Fig. 6.38. This device has

a gimballed platform that adjusts for specimens that do not have parallel top and bottom surfaces. The top surface of the specimen is kept level by the two leaves that support the top and side of the specimen. Before making any microhardness tests, the indenters should be examined for chipped or cracked surfaces. A defective indenter should be replaced.

The Hot Microhardness Tester

Fig. 6.34 A microhardness tester

In some cases, a need arises to determine the hardness of a metallographic sample at temperatures above room temperature. Knowing the elevated-temperature hardness may be important for those steels that are used in high-temperature environments. To measure “hot” hardness, a special hardness tester equipped with a furnace to heat the sample is needed, as well as a furnace to heat the indenter. The temperature of both the specimen and indenter needs to be controlled for accurate results. If the indenter is not heated, it can chill the specimen in the area of the indentation. The maximum temperature allowable for most equipment is usually 1500 °C (2732 °F). The sample is protected from oxidation either by vacuum or inert atmosphere. A hot microindentation hardness tester is shown in Fig. 6.39. Diamond indenters cannot be used at temperatures above 1000 °C (1832 °F), and for higher temperatures, sapphire or cubic boron carbide indenters have to be used. Sapphire can be used to 1200 °C (2192 °F), and cubic boron nitride can be used above 1200 °C (2192 °F). The same indenter configuration used in a microhardness tester, for example, the Vickers-pyramid shaped indenter or the Knoop rhombic-shaped indenter, is used. As in other microhardness testers, the loads used in hot hardness testers are below 1 kg. Depending on the type of machine employed, either the depth of penetration or the size of the indentation are used to measure hardness. It is important that

Fig. 6.35 A comparison between the (a) Knoop and (b) Vickers microhardness indenters

The Expanded Metallographic Laboratory / 167 the diagonal measurements of the hardness impressions are made at the test temperature so as to eliminate errors due to the thermal contraction that would occur if the diagonals were measured at room temperature. Specimen Requirements. The same specimen preparations that are used for microhardness testing will suffice for hot microhardness testing. One difference is that the specimens for hot microhardness testing are not mounted, because the mounting

materials cannot withstand the high temperatures. The specimen surfaces must be flat and parallel in order to avoid inaccurate results.

Other Specialized Techniques If information is required about the chemical analysis of the actual surface layers of a specimen, there are a number of specialized, sophisticated instruments and techniques available to the metallographer. These instruments are usually located in a research laboratory or specialized testing laboratory and include:

Fig. 6.36 A micrograph showing the regions of deformation (see arrows)

around two Vickers microhardness impressions in an as-cast E660 steel. The indentation at the left was produced by a 10 g load, and the indentation on the right was produced by a 50 g load. Nomarski. 200⫻

Fig. 6.38 A special leveling device mounted on the stage of the microhardness tester. The device is used to level an uneven specimen.

Fig. 6.37 A micrograph showing the region of deformation (see arrows)

around a Knoop microhardness impression in an as-cast E660 steel. Nomarski. 200⫻

Fig. 6.39 A hot microindentation hardness tester. (Nikon Inc.)

168 / Metallographer’s Guide • • • • • • •

Auger electron spectroscopy (AES) X-ray photoelectron spectroscopy (XPS) Field ion microscopy (FIM) Atom probe (AP) microanalysis Low-energy ion-scattering spectroscopy (LEISS) Secondary ion mass spectroscopy (SIMS) Rutherford backscattering spectroscopy (RBS)

Because of the highly specialized techniques involved, a discussion of each is beyond the scope of this book. More information can be found in the references at the end of this chapter.

• •

Scanning transmission electron microscopy • • •

SELECTED REFERENCES

•

General reference •

Materials Characterization, Vol 10, ASM Handbook, American Society for Metals, 1986

shin, Ed., Scanning Electron Microscopy and X-Ray Microanalysis, Plenum Press, 1981 C.W. Oatley, The Scanning Electron Microscope, Cambridge Press, 1972 O.C. Wells, Scanning Electron Microscopy, McGraw-Hill, 1974

J.W. Edington, Practical Electron Microscopy in Materials Science, Van Nostrand Reinhold, 1976 D.C. Joy, A.D. Romig, and J.I. Goldstein, Ed., Analytical Electron Microscopy, Plenum Press, 1986 L.E. Murr, Electron Optical Applications in Materials Science, McGraw-Hill, 1970 D.B. Williams, Practical Analytical Electron Microscopy in Materials Science, Philips Electron Instruments, Inc., 1984

Electron probe x-ray microanalysis •

K.F.J. Heinrich, Electron Beam X-Ray Microanalysis, Van Nostrand Reinhold, 1981

Image analysis • • • •

R.T. DeHoff and F.N. Rhines, Quantitative Microscopy, McGraw-Hill, 1968 E.E. Underwood, Quantitative Stereology, Addison-Wesley, 1970 G.F. Vander Voort, Etching Techniques for Image Analysis, Microstructural Science, Vol 9, 1981, p 137–154 G.F. Vander Voort, Metallography: Principles and Practice, McGraw-Hill, 1984

X-ray diffraction • • •

•

L.V. Azaroff and M.J. Buerger, The Powder Method in X-Ray Crystallography, McGraw-Hill, 1958 B.D. Cullity, Elements of X-Ray Diffraction, Addison Wesley, 1978 H.P. Klug and L.E. Alexander, X-Ray Diffraction Procedures for Polycrystalline and Amorphous Materials, John Wiley & Sons, 1974 Taylor, X-Ray Metallography, 1961

Transmission electron microscopy • •

•

M.J. Goringe and G. Thomas, Transmission Electron Microscopy, John Wiley & Sons, 1979 P. Hirsch, A. Howie, R.B. Nicholson, D.W. Pashley and M.J. Whelan, Electron Microscopy of Thin Crystals, Butterworths, 1965 (revised ed., Kreiger, 1977) I.M. Watt, The Principles and Practice of Electron Microscopy, Cambridge University Press, 1985

Hot stage microscopy •

•

B.L. Bramfitt, A.O. Benscoter, J.R. Kilpatrick, and A.R. Marder, “The Use of Hot-Stage Microscopy in the Study of Phase Transformations,” ASTM STP-557, American Society for Testing and Materials, 1974, p 43–70 M.G. Lozinskii, High Temperature Metallography, Pergamon Press, 1961

Scanning electron microscopy

Hardness testing

•

•

J.I. Goldstein, D.E. Newbury, P. Eichlin, C. Fiori, and E.Lif-

H.E. Boyer, Ed., Hardness Testing, ASM International, 1987

Metallographer's Guide: Practices and Procedures for Irons and Steels Bruce L. Bramfitt, Arlan O. Benscoter, p169-213 DOI:10.1361/mgpp2002p169

Copyright © 2002 ASM International® All rights reserved. www.asminternational.org

CHAPTER 7

Metallographic Specimen Preparation THE EARLY CHAPTERS in this book instruct the metallographer about the origin of microstructures and how those microstructures can be altered either intentionally or unintentionally. This information is vital in establishing baseline knowledge for metallographic interpretation. Specimen preparation is the backbone of metallographic practice. Not only should the metallographer know details about microstructures, but the metallographer must know how to prepare a specimen properly. This chapter instructs the metallographer on the basic skills required to prepare a polished metallographic specimen. This book is intended to provide the skills required for a well-rounded metallographer. A skilled metallographer does not simply prepare a polished specimen and then assume the job is done. The metallographer must be fully involved with revealing the microstructure and in the interpretation of the microstructure. In some metallographic laboratories, the metallographer prepares a specimen for someone else to interpret, that is, a metallurgist or materials engineer. In some situations, the metallographer prepares the specimen, interprets the microstructure, and performs any quantitative analyses required. A skilled metallographer can usually add a great deal of knowledge to the interpretation process. The authors feel that the modern-day metallographer must be part of the interpretation process. Metallographic specimen preparation techniques are presented in this chapter. The techniques have been used to prepare many types of steels and cast irons and are proven to be effective. Special “metallographic tips” are added to give the reader special instructions that may be helpful in difficult situations. Further details on technique can be found in the Selected References at the end of the chapter. This chapter is organized in chronological sequence starting with the information-gathering process on the material being investigated and ending with methods used to properly store metallographic specimens. Chapter 8 describes procedures on how to reveal the microstructure of a properly prepared metallographic specimen.

Information Gathering One of the most important steps in metallographic practice is to obtain as much information as possible about the material being evaluated before preparing the sample. In some metallographic

laboratories, a work request form is required. The form details the history and chemical composition of the material. Also, the form lists the special instructions that are to be performed once the metallographic specimen is prepared, such as the need to measure grain size or volume fraction of one or more of the constituents. The history of the material is very important because the metallographer must know how to section the material to obtain a specimen that will represent what is required. For example, is the material in cast or wrought form? If wrought, what is the rolling or forging direction? This is important in order to prepare the proper polishing plane: longitudinal or transverse? Is the material heat-treated? If so, is it in the as-quenched or in the tempered condition? If as-quenched, the metallographer must not mount the specimen in a thermosetting material that requires the specimen and mounting compound to be heated to temperatures around 150 °C (300 °F). This mild heating may alter the quenched (martensitic) microstructure by the process of tempering discussed in Chapter 3. Also, a quenched specimen may require a much different etching procedure than a tempered or annealed specimen. The chemical composition is very important to know since it will influence the way the metallographer will etch the sample. For example, in low-alloy steels containing higher chromium levels, adding a few drops of hydrochloric acid to a 4% picral etchant (discussed in Chapter 8) will enhance the microstructure. Thus, knowing the chemical composition of the steel beforehand will help the metallographer to choose the correct etchant so that important constituents in the microstructure will be properly revealed. However, in some laboratories a work request form is not used. In this situation, the metallographer must ask questions of the person submitting the material in order to find out as much as possible about the material and what is required. Without proper information about the material being evaluated, the metallographer will waste a lot of time through trial and error to develop the proper procedure for an accurate and thorough analysis. Unfortunately, there will be times that information on the material is not available, and the metallographer must proceed with a standard procedure that may or may not be correct. However, all is not lost, since the same specimen can be repolished and etched once the metallographer realizes the type of material being prepared. Quite often, samples submitted to the metallographic laboratory require a detailed analysis that can be used as the basis to solve a problem. An example would be a part or component that fractured

170 / Metallographer’s Guide in a severe service environment. In this situation, the metallographer conducts an investigation not unlike a forensic investigation, where information is gathered to solve a crime. A “fingerprint” or the results of a DNA analysis can be important markers that help identify the person who may have committed the crime. For the metallographer, the microstructure is the “fingerprint” that identifies the unique characteristics of the particular specimen being prepared. Criminal cases are won or lost on the basis of evidence and how that evidence is interpreted and analyzed. For the forensic investigator, it is important to fully document all information on how and where the samples are gathered. The investigator must take extraordinary care to ensure that the samples are not contaminated and that they are stored properly. For the metallographer, the same things are important. The way the specimens are sectioned from a larger sample is very important since the plane of polish can reveal different microstructural features if polished longitudinal, transverse, planar, or in some other orientation. Contamination of the specimen can easily take place during sectioning, grinding, and polishing by the accumulation of debris; for example, silicon carbide, diamond, or aluminum oxide particles can be retained in voids in the specimen or at the specimen surface (at a steel/oxide-scale interface or a steel/mounting material interface, etc.). Figure 7.1 shows an example of silicon carbide particles embedded into the polished surface of a nickel-plated steel specimen. This is an “artifact” that could result in a misleading compositional analysis conducted using a scanning electron microscope (SEM) or an electron probe microanalyzer (EPMA) (microprobe) where silicon and carbon will be detected. Although steels do not contain silicon carbide particles, most steels and cast irons contain inclusions and

compounds containing silicon (silicates) and carbon (cementite). It thus becomes difficult to know if these elements are real or introduced by specimen preparation (contamination). This means that procedures must be carefully documented, and if the metallographer knows that a microprobe analysis will be conducted, these debris-causing materials must be avoided (if possible) and the sample must be carefully cleaned to remove preparation debris. The microprobe operator should also be alerted to the fact that the specimen came into contact with these potential contaminants. If the specimen is being prepared for elemental analysis in the SEM or microprobe, it should be polished only with diamond abrasives. Storage of the specimen is also extremely important since a polished (and etched) surface easily oxidizes (rusts) in a normal laboratory environment by the humidity in the air and by human touch of the polished surface. Surface oxidation leads to normal passive layers that inhibit etching. Fracture surfaces are very vulnerable to oxidation from humidity and human touch. Of course if the fracture occurred outdoors, any exposure to rain and humidity would cause immediate and damaging surface oxide contamination. There are metallographic procedures available to remove oxidation from a specimen surface. These procedures are discussed in the next section. Because of oxidation, metallographic specimens are always stored in a dry atmosphere, for example, a desiccator or a dust cap. (A desiccator is a metal, plastic, or glass container that has a desiccant, that is, a material that absorbs moisture (hygroscopic material). More details can be found in the section “Specimen Storage” in this chapter.) Thus, without proper care, both the forensic investigator and metallographer can create samples with questionable integrity. The following sections explain in detail procedures to properly prepare a metallographic specimen.

Sectioning Once the condition of the material and what is required for the evaluation is determined, the next important step is usually cutting a metallographic specimen (or specimens) from the material that has been submitted. In most cases, the submitted material will be too large for a metallographic investigation, and appropriately sized specimens must be removed. The key is to think before you

Fig. 7.1 Microstructure of a nickel-plated AISI/SAE 1008 steel sheet speci-

men showing embedded silicon carbide particles from the grinding paper. The nickel layer pulled away from the mount during the curing process and produced a gap. The mount appears black, the nickel layer is unattacked by the etchant, and the SiC particles appear gray. 2% nital

Fig. 7.2 The three planes in a rolled product. The rolling direction (RD) is shown by the arrow.

Metallographic Specimen Preparation / 171 cut. Let’s consider an example. Before any sectioning can take place, the metallographer must decide which polishing plane to prepare. Figure 7.2 indicates by schematic representation the longitudinal, transverse, and planar sections in a rolled product. One would normally select the longitudinal section to (a) measure the degree of grain deformation (called aspect ratio, which is the length-to-width ratio of the constituent being measured in a particular field of view) and grain size in a hot-rolled or cold-rolled steel, (b) observe the severity of ferrite-pearlite

banding, (c) determine the inclusion morphology and the degree of inclusion elongation, and (d) observe the effectiveness of a particular heat treatment. The transverse section can be used to (a) determine the surface-to-center microstructural uniformity, (b) measure the distribution of inclusions, (c) measure the depth and uniformity of decarburization, and (d) measure grain size. A planar section would be used to examine a large area at a particular depth below the surface in order to better understand the microstructure.

Fig. 7.3 The microstructure of three parallel planar polished layers from the surface of a AISI SAE 1080 steel plate. (a) Near-surface layer of mostly ferrite and

some pearlite in the decarburized zone. (b) Mostly pearlite and some ferrite at the prior austenite grain boundaries in the second layer near the end of the decarburized zone. (c) Fully pearlitic matrix in the third planar polish below the decarburized layer. 4% picral etch

172 / Metallographer’s Guide When the metallographer prepares a series of consecutive planar sections with depth in a particular specimen, the technique is called serial sectioning. This technique can be used to study how the morphology of a particular constituent changes with depth, that is, to allow the constituent to be viewed in a three-dimensional context. This is illustrated in Fig. 7.3, where the serial-section technique has been applied to the zone of decarburization in a heat treated AISI/SAE 1080 steel plate. In this example, three parallel planar sections have been prepared by grinding and polishing just below the plate surface at a depth of 0.25 mm (0.010 in.), near the end of the decarburized layer at 0.5 mm (0.020 in.), and in the matrix just below the decarburized layer at 1.0 mm (0.040 in.). In the section near the surface, one can see that the matrix is mostly ferrite with pearlite within the ferrite grains. Also, the original austenite grain boundaries contain oxide scale (light gray), which means that they have been exposed to air during the heating and rolling process. At a depth of 0.5 mm (0.020 in.), the microstructure is predominantly pearlite with some ferrite at the original austenite grain boundaries. At 1.0 mm (0.040 in.), the microstructure is fully pearlitic, which is the microstructure of the bulk of the AISI/SAE 1080 steel plate. This series of micrographs shows that the planar section is a poor choice to use to determine the microstructure of a particular steel or cast iron if not enough material is removed from the surface. This is especially true if the surface is decarburized. In some cases, a planar view can be used to advantage, for example, to provide better detail of the pearlitic microstructure in a banded microstructure. Figure 7.4 shows a heavily banded AISI/SAE 1020 hot-rolled steel plate, polished in the transverse and planar planes. The longitudinal section (Fig. 7.4b) shows pearlite bands extending the full length of the micrograph. However, in the planar view (Fig. 7.4a) the definition of the

pearlitic microstructure in the bands is much more evident than in the longitudinal view. In the planar section, one can observe the proeutectoid ferrite surrounding each region of pearlite within an austenite grain. Also, the pearlite bands actually undulate, and therefore a single band cannot be fully revealed in a single planar section. A number of serial sections would reveal more information on the shape of the particular pearlite band. In many flat-rolled products, the longitudinal plane is prepared because it yields the most information. This means that the material must be cut parallel to the rolling direction and perpendicular to the rolled surface, allowing the metallographer to examine the final prepared specimen for the degree of grain elongation, that is, aspect ratio. Figure 7.5 shows the morphology of ferrite grains in a low-carbon steel polished on the three planes shown in Fig. 7.2. Note the differences in ferrite morphology in each of the planes of polish. Also, the elongation of inclusions can be observed. For example, manganese sulfides (MnS), because they have plasticity at rolling temperatures, elongate along the rolling direction in what are called “stringers.” Figure 7.6 shows MnS inclusions on a longitudinal plane of polish and transverse plane of polish in a resulfurized AISI/SAE 1144 steel rod. Note the vast difference in MnS morphology in the two sections of polish. In the longitudinal section the inclusion elongation can be seen, and in the transverse section the cross-sectional shape of the sulfide inclusions can be seen. Oxides, such as aluminum oxide, on the other hand, have no plasticity at rolling temperatures and do not elongate as continuous stringers as do manganese sulfides, but break up into elongated arrays of smaller particles aligned along the rolling direction. Figure 7.7 shows oxide particles in the longitudinal section of polish. Some inclusions, such as titanium nitrides, remain in a cubic form and do not deform or break up during hot rolling, as seen in Fig. 7.8. From these examples it is

Fig. 7.4 Microstructure of an as-rolled AISI/SAE 1020 steel plate. (a) Planar plane. (b) Longitudinal plane. 4% picral etch. 100⫻

Metallographic Specimen Preparation / 173 clear that if the metallographer were to prepare only the transverse plane complete information on grain elongation and inclusion shape would not be available. Sometimes the metallographer will prepare both planes for evaluation. This is particularly important when the rolling or forging direction is unknown. Protecting Specimen Surfaces. In some cases, the metallographer must take special precautions before cutting a specimen from a piece of steel or cast iron that is submitted to the metallographic laboratory. For example, the material may have fractured. In this case, the fracture surface must be protected from moisture (including humidity) to prevent rusting of the surface. If the material is not corroded, it should be stored in a dry atmosphere until examination. A metallographic laboratory should always have one or more desiccators available for short-term specimen storage. In some cases, the specimen may be too large for a desiccator and thus must be protected by other means. One simple way to protect a fracture surface temporarily is to spray the surface with an acrylic lacquer. The lacquer can be removed later by cleaning with a solvent such as acetone. Cleaning Rusted Surfaces. If the fracture surface is rusted, there are procedures that can safely remove most of the rust. All these procedures employ very mild chemical solutions that do not attack the steel surface. These include: •

Fig. 7.5 Microstructure of a very-low-carbon steel plate polished in three views shown in Fig. 7.2. 2% nital. 100⫻

First, try a room-temperature dilute solution of 3 g of disodium ethylenediamine tetraacetate (EDTA) in 100 mL of water. However, sometimes this solution will dissolve inclusions on the surface. Rust removal is accelerated by placing the beaker in an ultrasonic cleaning device. If this is ineffective, heat the solution and then place in the ultrasonic cleaner. An immersion heater placed in the glass beaker with the specimen works

Fig. 7.6 Microstructure of an as-polished resulfurized AISI/SAE 1144 steel bar showing manganese sulfide inclusions. (a) Longitudinal plane of polish. (b) Transverse plane of polish. Unetched. 500⫻

174 / Metallographer’s Guide well. Do not leave the specimen for more than a few minutes since some etching of the surface will take place. After removal from the solution, the surface should be flushed with running water, immediately rinsed in alcohol, and dried with a blast of warm air.

•

•

Fig. 7.7 Microstructure of an as-polished AISI/SAE 1020 steel bar showing

aluminum oxide inclusions. Longitudinal plane of polish. Unetched. Courtesy of Samuel Lawrence, Bethlehem Steel, Homer Research Laboratories. 500⫻

Immerse the specimen in a glass beaker containing 15 g of Alconox (a powder used to clean laboratory glassware) dissolved in 350 mL of water. If the rust is not removed at room temperature, try again with the solution heated to 90 °C (195 °F). The specimen (within the beaker) should be agitated in an ultrasonic cleaner for 30 min. The specimen should be flushed with running water followed by an alcohol rinse and a warm air blast. This solution should be used as a last resort. Apply a solution of equal parts phosphoric acid (H3PO4) and distilled water to the fracture surface. After a few minutes, the specimen is then flushed with water, rinsed in alcohol, and dried with a warm air blast. Use disposable gloves during this procedure.

Usually fracture surfaces are examined first with a stereomicroscope and then in the SEM. Before examination, the metallographer will section the fractured component into an appropriately sized sample for the SEM. This must be done very carefully in order to prevent damage to the fracture surface and contamination. Several examples of sectioning techniques are discussed in the next section. Cleaning Painted Surfaces. Sometimes the metallographer must remove a coating of paint from the component before sectioning into a metallographic specimen. Some oil-based paints can be removed with mineral spirits or commercial paint removers. After removal of the paint, the residue must be completely removed by rinsing the specimen with a quick-drying solvent, that is, acetone. Some paints are more difficult to remove. In some cases, a proprietary solution called Act I, used by the lithography industry, removes paint from metal surfaces without damaging the microstructure.

Sectioning Techniques

Fig. 7.8 Microstructure of a AISI/SAE 1080 steel bar showing titanium nitride inclusions. Arrows indicate light unetched constituent. Longitudinal plane of polish. 4% picral etch. Courtesy of Samuel Lawrence, Bethlehem Steel, Homer Research Laboratories. 1000⫻

There are many ways to section a material to obtain a metallographic specimen. Before cutting, thought must be given to the implications of the procedure selected. This is because the method used may alter the microstructure or damage the specimen. The microstructure is most vulnerable to damage and alteration during the sectioning process. For example, an abrasive wheel cutting without a water coolant (and sometimes with a water coolant) can create enough frictional heat to reaustenitize the steel or cast iron in the region along the cut surface, and this austenite will transform into a totally new microstructure. This obviously is poor metallographic practice. Also, some methods of sectioning create deep regions of plastic deformation (cold work) in the specimen along the cut surface. This situation should also be avoided since an undesirable microstructural change is being introduced, and cold-worked regions in the specimen etch faster than unworked regions. A description of the various sectioning methods that can be used by the metallographer follows. Abrasive Cutoff Saw (Wheel). The abrasive saw is one of the most common methods used to cut a metallographic specimen from a bulk component. The resulting surface of the cut specimen is very flat and smooth and ideal for mounting and grinding. The

Metallographic Specimen Preparation / 175 abrasive wheel is a fairly thin disc with abrasive particles, typically aluminum oxide, embedded in resin, rubber, or a resin/rubber mixture. Pores in the blade enable the resin to more easily fracture, thus allowing the exposed aluminum oxide particles to abrade away from the wheel, exposing fresh new particles. The cutting takes place by the abrasive particles continuously abrading away the specimen. If the abrasive wheel gets smaller in diameter as the cutting process proceeds, the process is working. The bonding material is important since it holds the

abrasive particles in place and allows the wheel to wear away at a controlled rate. The operator generally controls the pressure of the specimen against the wheel. Too much pressure can damage the specimen. For example, the steel shown in Fig. 7.9 was cut with too much pressure applied by the operator. Figure 7.9(a) shows the microstructure of the steel well away from the cut surface. The microstructure consists of patches of coarse pearlite in a ferrite matrix. Figures 7.9(b) and (c) represent areas near the cut surface and show fine pearlite and ferrite (Fig. 7.9b) and

Fig. 7.9 Micrographs of an ancient steel artifact cut with an abrasive wheel without water coolant. (a) Normal bulk microstructure of coarse pearlite regions

in a ferrite matrix away from the cut surface. Fine pearlite (b) and martensitic (c) regions near the cut surface indicating heating due to the absence of water coolant. 4% picral etch. 500⫻

176 / Metallographer’s Guide martensite (Fig. 7.9c). The evidence of fine pearlite and martensite indicates a false microstructure and indicates that the cut surface of the specimen was reheated into the intercritical ferrite ⫹ austenite region (two-phase region) and the original coarse pearlite in the ferrous artifact transformed to austenite and then formed fine pearlite or martensite on cooling. Ample water coolant was used during cutting. For metallographic specimens, a coolant should be used to minimize heating of the specimen. However, even with a coolant, applying too much pressure can

result in damage, as discussed previously. In most metallographic specimens, this damaged region can be removed by grinding. However, it is poor practice. Most coolants contain a rust inhibitor to minimize rusting of the freshly cut surface and the exposed parts of the machine. The coolant solution is usually recirculated through a pump and settling tank (to settle out the cutting debris). Figure 7.10 shows a specimen of a cast high-carbon (2.3%), high-chromium (14.7%) steel ball used in a grinding mill. Figure 7.10(a) shows a surface that was cut using proper water coolant,

Fig. 7.10 Macro- and micrographs of a cast high-carbon, high-chromium grinding ball cut with an abrasive wheel. (a), (c), and (e) With water coolant. (b),

(d), and (f) Without coolant. The specimen cut without the water coolant developed cracks along the chromium carbide networks and had regions of tempered martensite (dark) due to excessive heating. 4% picral ⫹ HCl etch. (a) and (b) at 2⫻. (c) and (d) at 50⫻. (e) and (f) at 500⫻

Metallographic Specimen Preparation / 177 and Fig. 7.10(b) shows a surface that was cut without coolant. Note in the uncooled specimen (Fig. 7.10b), the surface is cracked and has an uneven surface appearance consisting of dark and light bands, whereas the cooled surface is even and uncracked. Figures 7.10(c) and (d) show the same two conditions at higher magnification. The cracks in the improperly cooled surface follow a path along chromium carbide networks as seen in Fig. 7.10(d). The dark bands in the uncooled specimen represent bands of tempered martensite. These dark and light bands are seen more clearly in Fig. 7.10(e) and (f) for the same two conditions. The dark constituent associated with the white etching chromium carbide dendritic constituent is tempered martensite. The lighter region is untempered martensite. The evidence of tempered martensite means that portions of the surface were heated sufficiently to cause tempering. Thus, the metallographer must be careful in applying the proper amount of coolant while using an abrasive saw. Wheel Selection. For steels and cast irons, an abrasive wheel containing aluminum oxide is used. A silicon carbide wheel is generally used for nonferrous materials. For hard steels/cast irons a “soft” wheel is employed, and for soft (low hardness) steels/cast irons a “hard” wheel is used. The “soft” and “hard” terms are used Table 7.1

to describe the ease with which the binder abrades away to expose new particles. A “soft” wheel will abrade much faster than a “hard” wheel. Resin and resin/rubber-bonded wheels wear away faster than rubber-bonded wheels. One can detect a rubber-bonded wheel by the smell of burned rubber during cutting. In the metallographic laboratory, the quality of the cut far outweighs the economy of longer-lasting wheels. As a rule, an abrasive wheel that does not wear is not functioning properly. Sometimes, the edge of the abrasive wheel becomes glazed with specimen debris. This means that the abrasive particles in the wheel are not being effectively exposed to the steel or cast iron specimen. Generally, this happens when cutting an extremely hard material with an aluminum oxide wheel bonded with rubber. The rubber bond tends to hold the particles in place, not allowing them to wear away. This condition can usually be eliminated by “dressing” the wheel by cutting a softer material. (Dressing the wheel means that the glazed cutting surface is cleaned of embedded debris and a fresh new surface of sharp angular particles is exposed.) Table 7.1 lists causes of problems encountered in abrasive cutoff sectioning. Abrasive wheels should be stored flat to prevent warping (never store abrasive wheels on edge). Also, the wheels should be stored at room temperature in a low-humidity environment to minimize

Solutions for problems encountered in abrasive cutoff sectioning

Problem

Burning (bluish discoloration) Rapid wheel wear Frequent wheel breakage Resistance to cutting Cutter stalls

Possible cause

Solution

Overheated specimen Wheel bond breaking down too rapidly Uneven coolant distribution, loose specimen fixturing Slow wheel breakdown Cutter too light for the work

Increase coolant rate; lessen cutting pressure; choose softer wheel. Choose harder wheel; lessen cutting pressure. Distribute coolant uniformly; fix specimen rigidly. Choose softer wheel; reduce coolant flow; use oscillating stroke. Use heavier cutter; limit sample size.

Source: Ref 1

Fig. 7.10 (continued) (e) With water coolant. (b), (d), and (f) Without coolant. The specimen cut without the water coolant developed cracks along the chromium carbide networks and had regions of tempered martensite (dark) due to excessive heating. 4% picral ⫹ HCl etch. (a) and (b) at 2⫻. (c) and (d) at 50⫻. (e) and (f) at 500⫻

178 / Metallographer’s Guide degradation of the bonding material. The wheels have a shelf life of about 1 to 11⁄2 years. Most manufacturers will place a date on the wheel as a set of numbers; for example, 601 means the wheel was made in June 2001. Therefore, when using a supply of wheels always use the oldest wheel first. Because abrasive wheels wear rather easily, there is a buildup of debris in the coolant system. Usually, a tank is provided to allow the abrasive particles and binder to settle to the bottom of the tank. This tank must be cleaned on a periodic basis to maintain a properly functioning machine.

Metallographic Tip: Sometimes, if cutting becomes difficult and glazing takes place, more cutting speed can be achieved if one deliberately notches the rim of the abrasive wheel approximately every 50 mm (2 in.) with a pair of pliers. The notches should be approximately 6.5 to 12.5 mm (1⁄4 to 1⁄2 in.) deep. Also, gently applying pressure to push the abrasive wheel in and out of contact with the workpiece may help accelerate the cutting. However, this procedure presents a safety issue because the blade will be weakened by the notches.

Portable Abrasive Saw. To remove specimens in the field, a handheld abrasive-wheel device is often used. These abrasive saws are commonly used to cut pipe and other metal and masonry materials at construction sites. These saws are generally used without a coolant and thus create damage to the cut surface of the specimen. However, once a saw-cut specimen is obtained in the field, it is further cut to a smaller size using proper metallographic procedures in the laboratory.

Shear. A small shear is a very common way to prepare metallographic specimens from sheet and other thin, flat-rolled products. The shearing process can be automated or can be carried out manually. The shear blades will, however create a shear burr, which is a region of heavy cold work. This cold work alters the microstructure as seen in Fig. 7.11, which represents the microstructure of a shear burr in an ASTM A36 steel plate. This micrograph shows that the deformation created by the shear blades was not removed during grinding. In most cases, the metallographer can prepare a large enough specimen so that the affected area can be ignored during microscopic examination. However, if necessary, proper grinding procedures can be followed to remove the shear burr. Grinding is discussed in the following section of this chapter. Figure 7.12 shows an example of how cold work developed during shearing can affect the microstructure of a dual-phase steel. The micrograph in Fig. 7.12(a) shows the correct microstructure well away from the damage of the shear burr. The constituents in this microstructure are martensite (dark regions) and retained austenite (small white regions) in a ferrite matrix. In Fig. 7.12(b), sectioned in the deformed area of the shear burr, the retained austenite regions no longer exist because they have transformed to martensite by the deformation induced by the shearing process. This is a prime example of how the microstructure can be altered by the specimen preparation process and a false microstructure (artifact) can be produced. To minimize the types of situations described above, the shear blades must be sharpened periodically to minimize the extent of damage introduced by the shear burr. Also, the shear can be adjusted to allow a smaller gap between the blade and the table. Hand Shear (Nibbler). To remove a sheet specimen from a larger steel sheet in the field, a nibbler or hand shear can be used. The sheared specimen can be brought back to the metallographic

Fig. 7.11 Microstructure of an ASTM A36 sheared plate. (a) Cold work at the sheared edge. (b) Unaffected area of ferrite and pearlite. 2% nital. 200⫻

Metallographic Specimen Preparation / 179 laboratory for further cutting. Generally, the nibbler will produce more edge damage than a laboratory shear. Punch. Some metallographic laboratories that prepare thin materials on a routine basis use a punch for round, square, or rectangular specimens. The rectangular specimen is preferred because the long axis can be aligned along the rolling direction of the sheet. This way, the metallographer will always know the orientation of the rolling direction. As in shearing, a punch will result in a shear burr along the outer edges of the specimen. The punched specimens can be mounted in a clamp or mounting compound, and the burr can be removed by grinding. It is important to keep the punch dies as sharp as possible in order to minimize the burr. A band saw uses a continuously rotating saw blade that is welded end to end. An upright or vertical band saw is commonly used for cutting sheet, plate, and bar. With this type of saw, the workpiece is fed into the moving blade. Cutting must be carried out at a speed slow enough to prevent heating of the specimen. Many times the material is hand fed into the blade so that the operator can control the cutting speed. Lubrication is available on some vertical band saws. There are also horizontally mounted band saws where the workpiece is stationary and the horizontal arm containing the continuous blade is weighted to control cutting pressure. This type of saw has a lubricant (coolant) flooding the workpiece to prevent frictional heating and to extend the life of the saw blade. Fairly large pieces can be sectioned by this type of saw. A shallow area along the cut surface is damaged by cold work and must be removed by grinding. However, properly used band saws, because of their controlled speed, pressure, and coolant, do not heat the material sufficiently to create an altered microstructure.

Band-saw blades are available for particular cutting requirements and different grades of steel. Blade life depends on the hardness of the steel or cast iron being cut. A common saw blade will only cut material with a hardness below about 35 HRC. Special blades (bimetal or gulleted blades) are required to cut harder material. Also, the number and depth of the teeth on the cutting edge of the blade determines the cutting capability for different hardness steels and cast irons. Generally, a blade with 16 to 20 teeth/in. is used for hard metals, and a blade with 10 to 16 teeth/in. is used for softer metals. For particularly hard steels, for example, tool steels, special toothless blades are available with tungsten carbide particles bonded to the cutting edge of the blade. Figure 7.13 shows a schematic of this type of blade in the smooth and gulleted (notched) form. The gulleted blade prevents buildup of cutting debris and is used for faster cutting rates of particularly hard steels. Band saws usually operate at a fairly slow speed, and light lubricating oil is recommended for ease of cutting and to prevent heating the workpiece. Handheld portable band saws are also available. These saws are valuable in removing specimens in the field. The same types of blades are available for these saws except that they are of shorter length. A handheld hacksaw is a crude but effective way to remove a metallographic specimen from a larger component. However, the saw-cut surface is very rough, and further preparation by hand grinding is required to remove the roughness and cold work. The hacksaw can be used to remove a portion of material from a larger piece, possibly located in the service environment in the field. This rough-sawed portion of material can be further sectioned in the metallographic laboratory by a more appropriate method to obtain the final metallographic specimen. Thus, the hacksaw is

Fig. 7.12 Microstructure of a dual-phase steel sheet showing the results of deformation by shearing. (a) Correct dual-phase microstructure away from the

shear burr. (b) Shear burr region where pools of retained austenite have transformed to martensite by the plastic deformation. Arrows indicate retained austenite (a) and austenite transformed to martensite (b). 10% sodium metabisulfite etch. 1000⫻

180 / Metallographer’s Guide generally only used as an intermediate step to prepare specimens in the metallographic laboratory. A motorized, oscillating hacksaw is normally used for obtaining a rough specimen from a thicker section than a handheld hacksaw. A lubricant (coolant) is available to flood the workpiece. A flat saw blade with teeth on the cutting side reciprocates on the workpiece. Generally, the horizontal blade and arm of the saw are weighted to control the pressure and the speed of the cut. Because of the coarseness of the teeth, the thickness of the saw blade, and the pressure required, this type of hacksaw can easily damage the steel specimen by introducing cold work. The sectioned specimen is usually sectioned further to final size with an abrasive wheel with proper coolant. Oxyacetylene Torch. Although not used directly for metallographic specimens, an oxyacetylene torch can be used to obtain a bulk sample in the field. These torch-cut pieces can be further

Fig. 7.13 Band-saw blades bonded with tungsten carbide particles along

the cutting edge. The top blade is toothless, whereas the bottom blade is gulleted (notched) to help carry away the cutting debris.

sectioned by a saw or abrasive wheel in the laboratory. The torch-cut surface itself is ruined from a metallographic viewpoint since it is heavily decarburized and contains numerous particles of entrapped oxides. An example of a torch-cut surface is shown in Fig. 7.14. This micrograph is from a AISI/SAE 1025 steel bar and shows a martensitic microstructure at the cut surface. The unaffected region of the bar consists of bands of pearlite and ferrite. Between the unaffected region and the surface martensite is a transition region. Hardness measurements were taken every 0.25 mm (0.01 in.) as indicated by the diamond-shaped Vickers indentations in along the centerline of Fig. 7.14. The hardness near the surface (Fig. 7.14c) is 511 HV (martensite), and the hardness in the unaffected area (Fig. 7.14d) is 274 HV (ferrite and pearlite). In this example, the total affected area is approximately 4 mm (0.2 in.) deep. To be on the safe side, as a rule of thumb, the metallographer should assume that a torch-cut surface has a heat-affected zone (HAZ) of at least 25 mm (1 in.) of material. Thus, a large enough bulk sample must be prepared for further cutting in order to remove the affected region. Generally, torch cutting is only used in the field where other cutting methods are unavailable. Also, torch cutting is only used to obtain a sample of material from a piece that is too large to bring to the metallographic laboratory. A plasma torch is a rather uncommon tool in the metallographic laboratory, but it is used in cutting steel products. A plasma torch can generate enough heat to melt a path through steel or cast iron. Because the workpiece is melted at the cut, the heat alters the microstructure near the cut surface. An example of a plasma torch cut surface is shown in Chapter 3 (Fig. 3.51). Waterjet. Although not commonly used, a waterjet-cutting device is very effective for preparing metallographic specimens because of the complete lack of affected area at the cut surface. The disadvantage of waterjet cutting is that it is relatively expensive when compared with the more popular abrasive-wheel cutoff machine. It takes much longer to cut a thick plate with a waterjet than with an abrasive wheel.

Fig. 7.14 Effect of a oxyacetylene torch cut (longitudinal section) on an AISI/SAE 1025 steel bar. (a) Overall view of affected area with micohardness

impressions. (b) Plot of the hardness change from the cut edge. (c) Martensitic microstructure at the cut edge. (d) Ferrite ⫹ pearlite microstructure in the bulk specimen. 2% nital ⫹ 4% picral etch. (a) at 40⫻. (c) and (d) at 200⫻

Metallographic Specimen Preparation / 181 Electric discharge machining (EDM) is an expensive and slow cutting process that is generally used to provide a stress-free cut. The process uses an electric discharge (sparks) generated in a dielectric fluid such as kerosene or transformer oil to remove material from a specimen. The sparking develops microscopic craters on the specimen surface. Because the tool itself never touches the surface of the specimen, many people have the impression that the surface is microstructurally unaffected. However, because of the localized heating from the electric discharge, a small region at the cut surface is altered. The localized heating is sufficient to actually melt the surface of the specimen. This is shown in Fig. 7.15 where an EDM cut has been made on an AISI/SAE 4327 steel casting. It is obvious that the surface layers shown in Fig. 7.15(a) have been altered by the intense heat from the electrical discharge. Note in particular that melting and solidification has taken place at the surface, as shown in Fig. 7.15(b). This micrograph shows fine dendrites oriented perpendicular to the surface. The outer surface layer is martensitic (white etching constituent), while the base microstructure is bainitic (Fig. 7.15c). There is also an HAZ between the martensitic surface layer and the bainitic base microstructure (Fig. 7.15d). This means that sufficient heat was generated to form austenite, which upon cooling transformed to martensite. Thus, the metallographer should be aware that the EDM process drastically alters the microstructure at the cut surface, and appropriate steps must be taken to remove this damage in the metallographic specimen. Precision Saw (Constant Force). A precision saw consists of a variable-speed motor that drives a small, thin, circular blade— for example, AISI 410 stainless steel—with diamond particles embedded along the outer rim (other types of blades are available). The blades are usually 100 to 125 mm (4 to 5 in.) in diameter. There are different diamond sizes and concentrations.

The motor operates at a fairly low cutting speed, and the specimen is gravity fed into the blade, thereby producing a constant-force cut. Generally, a mineral oil should be used for a lubricant with a diamond saw blade. Weights of differing size are added to the cutting arm that holds the specimen, and the amount of weight determines the “feed” speed of the cut. For many materials, this type of saw usually takes a long time (hours) to complete the cut as opposed to the constant-feed speed saw discussed later in this section. Both types of devices can be used to cut fragile materials or small pieces of hard materials that cannot be secured in the vise of an abrasive-wheel machine. Also, the thin blade removes very little material during cutting; that is, there is low kerf loss. The smooth cut provides an “almost” damage-free surface, which means that there will be much less cumulative damage in subsequent grinding steps, and therefore less grinding is required. It is a device often used for preparing thin wafers that will be further processed into thin foils for the transmission electron microscope. Although a diamond saw blade is effective with tool steels and hardened steels, carbon steels can usually be cut more effectively using a constant-feed speed saw with an aluminum oxide blade. There are aluminum oxide blades available to fit both the constant-force and constant-feed speed saws. These aluminum oxide blades, being very thin, remove a small amount of material (kerf) in the cut area compared with a conventional aluminum oxide abrasive saw. A water-based coolant is used. Precision Saw (Constant Feed Speed). The constant-feed speed saw operates on the principle that the specimen is fed into the rotating blade at a constant speed. The range of blade rotation is between about 100 and 5000 rev/min. The feed speed is determined by the operator and can vary from 0.005 to >3 mm/s (0.002 to >0.12 in./s). As with the constant-force saw described

Fig. 7.14 (continued) (c) Martensitic microstructure at the cut edge. (d) Ferrite ⫹ pearlite microstructure in the bulk specimen. 2% nital ⫹ 4% picral etch. (c) and (d) at 200⫻

182 / Metallographer’s Guide previously, blades with embedded diamond particles and aluminum oxide blades are generally used. Usually with the higher feed speeds, the specimen can be cut at a much faster rate than with the constant-force device (a few minutes versus a few hours). A diamond-impregnated blade would be used to cut stainless steels and steels with a high surface hardness, for example, weld overlays. An aluminum oxide blade is used to cut carbon and low-alloy steels. A wire saw uses a stainless steel wire that is continuously drawn across the cutting surface of the workpiece. Some saws use

a wire that is embedded with diamonds. Figure 7.16 shows micrographs of diamonds bonded to a stainless steel wire. Other saws use a hardened steel or stainless steel wires drawn through a slurry containing abrasive particles. Glycerin, because of its high viscosity, is usually used to carry the abrasive particles on the wire. The abrasive particles can be inexpensive emery, garnet, silicon carbide, silica, alumina, or more expensive synthetic diamond. Diamond, because of its extremely high hardness, is the best overall abrasive for this process. Boron carbide and tungsten carbide, also materials of high hardness, have also been used as

Fig. 7.15 Microstructure of an AISI/SAE 4327 steel casting that was sectioned by electric discharge machining (EDM). (a) Low-magnification view of the cut surface. (b) Melted and solidified region at the surface. (c) Heat-affected zone. (d) Base bainitic microstructure. 4% picral plus HCl etch. (a) 80⫻. (b), (c), and (d) at 500⫻

Metallographic Specimen Preparation / 183 abrasive particles. Wire sawing is a very slow process and is usually used in preparing damage-free specimens for electron microscopy. However, very delicate specimens such as thinwalled tubing, honeycomb structures, and enameled sheet steel (ceramic-coated steel) are sectioned with a wire saw. The kerf (the area cut away) by the wire saw depends on the diameter of the wire. Generally, wires range in diameter between 0.08 and 0.38 mm (0.003 and 0.015 in.). A small high-speed handheld circular saw is used for special applications that require precision in cutting specimens from a larger section. The circular abrasive saw blades are on the order of 25 mm (1in.) in diameter. The cutting rate is slow and rather tedious. However, if special care is required, this may be the tool of choice. Fracturing in Liquid Nitrogen. Because steel and cast iron are brittle at very low temperature, liquid nitrogen at ⫺196 °C (⫺320 °F) can be used to cool the specimen before producing a freshly fractured surface. In some investigations, the metallographer may want to fracture the steel or cast iron component to reveal voids, porosity, internal cracking, and so forth. The fracturing method may be the only acceptable way to prepare such a specimen. The material is usually notched (if possible) near the area to be fractured in order to control the fracture path. The material is immersed in a vacuum-insulated container (dewar) of liquid nitrogen and remains there until the gas evolution stops. After removal from the liquid nitrogen, the material is quickly secured in a vise (the notch being just outside the grip of the vise), and the specimen is broken with a hammer blow. The material will fracture from an initiation point at the notch. The fractured specimens (two halves) are immediately immersed in ethyl or denatured alcohol to allow warming to room temperature. The immersion in alcohol prevents moisture in the air from condensing on the fractured surfaces. This moisture would immediately create a layer of corrosion (rust) on the fracture surface.

is also advisable to remove all rings from the fingers when working around sectioning equipment and machinery.

Mounting Once a metallographic specimen is cut to an appropriate size, it is usually mounted (encapsulated) in a polymeric material or in a metal clamp. Some specimens because of their large size are not mounted. Also, all automatic grinding and polishing machines have holders that do not require mounted samples. However, for best results in hand polishing, the metallographer will mount a specimen to achieve a flatter surface including the edges where flatness can be vital. For example, one may need to observe the oxide-scale formation on the surface of a hot-rolled plate. In this case, edge retention is a priority and the specimen edge must be protected in a mount. Specimen flatness is extremely important since a flat specimen is easier to scan under the microscope without continuously changing the focus of the area of view. It is distracting and frustrating to observe a specimen that is not flat. The metallographer must think before mounting the specimen. The metallographer must think about what is required in the final specimen and think about the procedures that will be used to polish and etch the specimen. It is important to think about the outcome before the specimen is mounted. First, as described in the next section, there are many different mounting materials to choose from. Proper choice is critical in some cases. For example, a metallographer is to examine the prior austenitic grain boundaries in a quenched-and-tempered steel specimen, and the procedure might involve etching the final polished specimen in a solution of boiling alkaline sodium picrate in order to reveal the prior austenite boundaries. Therefore, a phenolic resin should not be used as a mounting medium because the hot etchant will attack and swell the mount to the point where its surface will be above

Safety While Sectioning Accidents can easily occur during sectioning of metallographic specimens. Safety glasses with side shields should always be worn in the specimen preparation room and in the field when using any sectioning equipment. Leather gloves should be worn when handling sheets of steel and steel components with sharp edges. It

Fig. 7.16 Micrographs of the embedded diamond particles on a wire used in a wire saw. (a) 39⫻ and (b) 200⫻

184 / Metallographer’s Guide the polished steel surface. In this case, the metallographer may choose a thermosetting epoxy mounting material that is resistant to most acids and solvents. Another example would be if the metallographer is mounting an as-quenched steel specimen. This example is shown in Fig. 7.17, which represents a 1% C steel bar that was heated to 1095 °C (2000 °F), held for 30 min, and then quenched in water. Figure 7.17(a) shows the resulting untempered

martensitic microstructure. The specimen in Fig. 7.17(a) was mounted in epoxy. The microstructure shown in Fig. 7.17(b) is of the same material mounted in a thermosetting resin. Note that the microstructure is revealed more clearly because tempering has taken place. The tempering was a result of heating the specimen to 150 °C (300 °F) to cure the thermosetting phenolic resin. Even this fairly low temperature altered the microstructure, giving it the

Fig. 7.17 Microstructure of the plate martensite in a 1% C steel mounted in castable epoxy (a) and thermosetting plastic (b). Note the darker tempered microstructure in (b). 2% nital etch. 500⫻

Fig. 7.18 Microstructure of a low-carbon steel sheet that was electroless nickel plated on both sides. (a) Specimen mounted in epoxy. (b) Specimen mounted in thermosetting phenolic resin. Note the damage in (b) due to the thermal-compression mounting process. Unetched. 100⫻

Metallographic Specimen Preparation / 185 appearance of being tempered. This unintentional alteration could lead to the wrong conclusion in a metallographic investigation. Thus, in this situation the metallographer should use a castable mounting material such as epoxy because minimal heat is developed during the curing process. Another example would be if the metallographer were mounting a steel specimen with a fragile coating. In this case, a thermal-compression mounting material should be avoided since the movement of the hot mounting material under high pressure (29 MPa, or 4200 psi) may dislodge the coating from the steel substrate. A castable mounting material would be more appropriate. An example of the dislodgment of a coating from the steel sheet is shown in Fig. 7.18(b). This example shows the cross section of a low-carbon steel sheet that was nickel-plated on both sides (the electroless plating process was used in this case). The specimen mounted in thermosetting phenolic resin caused the

nickel coating to separate from the sheet specimen during the mounting process. Although the bond between the electroless nickel coating is rather weak, this example illustrates how specimens can be damaged by the thermosetting mounting process. In Fig. 7.18(a), the specimen was mounted in an castable epoxy with no damage. Damage can also occur in the actual specimen itself as shown in Fig. 7.19(b), which represents a stack of thin sheet specimens mounted in a thermosetting resin. The specimens were deformed by the movement of the molten resin under the high pressure required in the mounting process. A similar stack of sheet specimens mounted in a castable epoxy was undamaged as seen in Fig. 7.19(a). The metallographer must make the decision whether or not to mount a specimen. In most situations, the metallographer should chose to mount the specimen. A mount provides the following distinct advantages: • • • • •

• •

Maintenance of specimen flatness Protection of fragile specimens Ease of handling the specimen either manually or in automatic grinding and polishing machines Minimal rounding of specimen edges, that is, better edge retention Filling of holes and cracks in the specimen with mounting material to prevent “bleeding” of water, alcohol, and etching solutions Containment of sharp corners within the mount to prevent torn grinding papers and polishing cloths Ease of storage in dessicator cabinets since mounts are of standard size

Some specimens are too large to mount. In one production metallographic laboratory, the entire head portion of railway rails are prepared for microstructural observation on a routine basis. A typical rail head is about 150 mm (3 in.) wide and 100 mm (2 in.) high. The thickness of the metallographic specimen is about 25 mm (1 in.). Under production conditions, where dozens of rail heads are prepared in a single day on an automatic grinding/ polishing machine, it would be time consuming and costly to mount specimens of this size. The rail heads are securely held in a specimen holder for processing. Before mounting, the specimen should be dry and free from cutting debris and cutting oil. Also, any sharp edges should be removed for personal safety and to prevent cracks from forming in the mount. However, certain edges should not be removed, especially any edges that may be crucial to the investigation.

Mounting Materials

Fig. 7.19 Six sheet specimens (with spacers) mounted in castable epoxy

(a) and thermosetting phenolic resin (b). Note the damage caused by the pressure of the thermosetting mounting process. 2⫻

The metallographer has a variety of mounting materials to choose from. They can be categorized as mechanical mounts (clamps), thermally compressible materials, and castable materials. The distinction is important since each mounting process is different and the mounting materials have different characteristics. A good source of information about mounting-material characteristics and safety can be obtained from the vendors of these materials. Before using any material, the Material Safety Data

186 / Metallographer’s Guide Sheet (MSDS) should be examined for details about toxicity, danger from exposure to the lungs and skin, flammability, and so forth. The vendor of the material should supply the MSDS when the material is purchased. Mechanical Mounts. Metallographic laboratories use mechanical mounts because the mounting process is quick, provides excellent edge retention, and does not require any special equipment other than a clamp, vise, and screwdriver. A clamp is particularly useful when mounting a stack of steel sheet specimens of similar composition, for example, cold-rolled and annealed AISI/SAE 1008 steel. Figure 7.20 shows mechanical clamps with a rod specimen (right) and a stack of 20 sheet

Fig. 7.20 Typical mechanical clamps holding 20 sheet specimens (left) and a rod specimen (right). 1⫻

specimens (left). The specimens can be mounted with or without spacers (Teflon, etc.). However, if the specimens are flat, spacers are generally not required. Figure 7.21(a), shows the tight interface that can be achieved between the specimen and the clamp material. Although there may be a small gap between the specimens in the clamp, the edges of each specimen are flat, as seen in Fig. 7.21(b).

Metallographic Tip: For excellent edge retention and to prevent “bleeding,” place the clamp and specimens in a vise. Apply enough pressure to squeeze the clamp and specimens until the specimens are flat and there are no spaces left between the specimens and clamp. Tighten the screws while the clamp is in the vise. Leave approximately 1.5 mm (1⁄16 in.) of the specimen edge to be polished above the clamp surface (this is especially important for a sheared or punched specimen). This extra material will be removed by grinding, thus eliminating the shear burr or other mechanical damage.

The pressure applied to the mechanical mount is important. Not enough pressure will allow the specimens to move and shift location during subsequent operations. Too much pressure may damage the specimens by introducing cold work in very soft steel. The clamp material itself should be from steel so that the mount and specimens have a similar rate of metal removal during the grinding and polishing operations and the same reactivity to the etchant used after polishing. Dissimilar materials will create a galvanic cell and promote corrosion when the clamp and speci-

Fig. 7.21 Micrographs showing low-carbon steel sheet with the tight interface that can be achieved in a mechanical clamp (clamp shown above specimen) (a) and the flat edges of two sheet specimens in a mechanical clamp with a gap between specimens (b). Marshall’s etch. 500⫻

Metallographic Specimen Preparation / 187 mens are immersed in etching solutions. One advantage of a clamp is that the specimens can be easily removed from the mount when the job is completed and the clamp can be reused. Clamps provide very good edge retention because the specimens are directly against the clamp and against each other (see Fig. 7.21a). One disadvantage of mounting specimens in a mechanical mount is that grinding and polishing debris tend to collect in open areas of the mount. This means that when using mechanical mounts, the metallographer must place the mount in a beaker of alcohol in an ultrasonic cleaner after the last grinding step and after each polishing step. Alcohol is used because it dries quickly and does not leave a residue on the specimen. Also, after etching the mounted specimen, all traces of etching solution must be washed from the specimen by rinsing in warm water followed by a rinse in alcohol and blow drying. Any etchant remaining on the specimen may seep from between the clamp and specimen and create a stain. Any acid fumes remaining from the etchant may attack the optical system of the microscope and cause damage. Thermal-compression mounting materials can be categorized into two groups, the thermosetting compounds and the thermoplastic compounds. Both types require a temperature and pressure cycle in a mounting press in order to produce the desired mount. Mounting generally takes place by rapid heating of the specimen and mounting material in a pressurized cylinder. The specimen is placed upside down on the bottom piston of the cylinder. The mounting material is poured over the specimen and the top piston is placed inside the cylinder. Both heat and pressure are applied to the cylinder. It is important to measure the amount of material so that the specimen is completely covered in the final mount. A mounting press should have an automatic temperature controller and an ejection device to remove the specimen. The thermosetting materials—for example, phenolic compounds such as Bakelite—undergo the liquid and hardening stages before reaching the final curing temperature of about 150 °C (300 °F). The final mount can be removed from the mounting press once the curing temperature is achieved. However, to produce a better mount it is advisable to allow these materials to remain under pressure until they have cooled to around 65 °C (150 °F). This produces a better mount. Once cured, a thermosetting material will not remelt, but will char if exposed to high temperature. Thermoplastic materials, for example, acrylic compounds, melt at a temperature of 150 °C (300 °F) and must remain under pressure in the mold of the mounting press until the mount has cooled to about 40 °C (105 °F) in order to allow the material to completely harden. After curing, a thermoplastic material can soften and remelt, distinguishing it from a thermosetting material.

Mold Sizes Generally, thermosetting and thermoplastic mounts are produced to standard diameters of 25 mm (1 in.), 32 mm (11⁄4 in.), 38 mm (11⁄2 in.), and 50 mm (2 in.). A set of standard sizes makes it convenient for equipment manufacturers to design mount holders

for automatic polishing and grinding machines, mounting press cylinders, and storage cabinets. Other instruments that depend on metallographic mounts, such as the EPMA, are designed to use the standard diameter mounts. Other custom-sized mounts are also used in many metallographic laboratories.

Thermosetting Mounting Materials Phenolics. One of the oldest and most common thermosetting mounting material is phenolic resin. It has been in use since the 1920s and has been marketed as Bakelite (after Leo Baekeland the inventor). It is an inexpensive material and readily available. Phenolic powders and granules can be purchased in various colors including red, green, black, and mottled (filled with a wood flour). They also can be obtained as “premolds,” which are cylinders of partially compacted granules ready to form a standard size mount. The premold is manufactured to fit into the cylinder of the mounting press. In mounting fragile specimens, a premold should not be used because of potential damage to the specimen when pressure is applied to the cylinder. As a thermosetting material, a phenolic resin has the advantage of a shorter cycle in the mounting press operation because it can be rejected from the mold after the 29 MPa (4200 psi) pressure treatment at 150 °C (300 °F). Curing time can vary from 5 to 9 min for loose powder and 3 to 7 min for premolds. A shorter mounting time is important in a production metallographic laboratory. However, it is advisable to allow a thermosetting mount to cool under pressure to under 65 °C (150 °F) before removal. It is preferable to cool the specimen under pressure to room temperature. This is because the thermal shock may crack the mount and create a separation between the mount and specimen due to the differences in the thermal expansion of the steel/cast iron and the mounting material. Any separation between the specimen and the mount will make the metallographer’s job more difficult because the gap will allow seepage or (“bleeding”) of etching and rinse solutions and entrapment of grinding and polishing debris. Never remove a hot phenolic mount from the mounting press and cool the hot mount in water. This practice will save time, but will definitely cause the specimen to pull away from the mount to form a gap. A phenolic mount can be attacked by certain etching solutions. For example, boiling alkaline sodium picrate, an etchant used to darken cementite, will attack a phenolic mount. Diallyl phthalate is another thermosetting material. This material is more expensive than a phenolic resin and is marketed with various fillers including mineral and glass particles for added hardness and strength. A filled diallyl phthalate mount will have greater wear resistance than a phenolic mount (improved edge retention). Diallyl phthalate mounts require a pressure of approximately 22 MPa (3200 psi) at 150 °C (300 °F) and a curing time of 7 to 12 min. It is advisable to cool the mount under pressure to room temperature before removing it from the mounting press. Do not remove a hot mount from the press and cool the mount in water since this will cause loss of adhesion to the specimen surface. In contrast to the phenolics, the diallyl phthalates have better

188 / Metallographer’s Guide adhesion to the specimen surface and greater wearability. This is important in minimizing a separation between the mount and specimen. The adhesion is not as good as that of the epoxy materials described later in this section. Electrically Conductive Diallyl Phthalate Mounts. Particles of aluminum, copper, iron, and graphite can be added to a diallyl phthalate mounting material. These composite mounts are electrically conductive and are an advantage in electrolytic polishing and electrolytic etching. Also, electrically conductive mounts are an advantage in the SEM and EPMA, where the specimen must be grounded to conduct away the electrical charge buildup in the metallic specimen by the electron beam. However, the filled diallyl phthalates contaminate grinding and polishing wheels. Many laboratories restrict the use of copper-filled diallyl phthalates, for example, a copper-filled diallyl phthalate mount prepared by automatic grinding/polishing with a recirculating coolant system. In an aqueous environment, the copper can be inadvertently plated onto the specimen surface. In the light microscope, one can actually observe the copper plating by its reddish color. Another outcome is that specimens prepared using coppercontaminated wheels may be used for the EPMA. If metallographic specimens are contaminated with copper-filled diallyl phthalate, the elemental analysis of copper in the microprobe may lead the investigator to false conclusions. For example in Chapter 3 it is shown that elemental copper, a residual element in the steel itself, could create an undesirable condition called hot shortness during the hot rolling of steel. This process begins at the steel/scale interface, where copper accumulates during exposure to oxygen at high temperatures. Thus, a copper-contaminated specimen could produce copper artifacts at the steel/scale interface that would be confused with hot shortness. This is a prime

example of when the metallographer must think ahead before selecting a mounting material. Thermosetting Epoxy. A premixed powder form of an epoxy compound (usually with a filler added for higher hardness) is used for mounting metallographic specimens because of its many advantages. As with the other thermosetting materials, thermosetting epoxy will also require a mounting press, but it has a number of positive attributes. First, thermosetting epoxy resists solvents and acids. It provides excellent edge retention and does not shrink away from the specimen. This is shown in Fig. 7.22, where a steel screw is mounted in a thermosetting epoxy (Fig. 7.22a) and Bakelite (Fig. 7.22b). There is excellent edge retention in the epoxy since the specimen and mount are both in focus. Also, there is a thin oxide-scale layer on the surface of the screw. In the Bakelite mount, there is poor edge retention with the mount out of focus and the oxide-scale layer not evident. There is also a gap between the mount and the specimen. Thermosetting epoxy has the best flow characteristics of the thermosetting materials. This is an important feature in filling cracks and voids in the specimen. Thermosetting epoxy compounds require lower pressures than the phenolic materials, for example, a pressure as low as 8 MPa (1200 psi) versus the 29 MPa (4200 psi) required for the phenolic materials. Also, thermosetting epoxy mounts have the best wearability compared with other common mounting materials. However, the thermosetting epoxy requires a slightly longer cure time than the other thermosetting materials. It is important to follow the vendor recommendations for curing times. Also, the thermosetting epoxy compounds are more expensive than the phenolics and diallyl phthalates. Thus, in general, the thermosetting mounting materials are widely used in the metallographic laboratory. They are resistant to

Fig. 7.22 Micrographs showing the polished edge of a steel screw mounted in thermosetting epoxy (a) and thermosetting phenolic resin (Bakelite) (b). Note

the excellent edge retention of the epoxy mount where a thin oxide layer can be seen on the screw surface. The edge of the screw in the Bakelite mount is rounded, and the oxide layer cannot be seen. 200⫻

Metallographic Specimen Preparation / 189 most solvents and acids. However, some etchants may break down the material and cause swelling. Thermosetting mounts are also resistant to softening during grinding and polishing operations and tend to abrade at a rate similar to most steels and cast irons. This is important for flatness and edge retention. If a mounting material abrades faster than the specimen, rounding will take place and the specimen will not be prepared properly. The filled diallyl phthalates and thermosetting epoxies can match the rate of abrasion of most steels and cast irons. One problem with the thermalcompression mounting process is possible damage to a fragile specimen caused by the movement of the hot mounting material around the specimen at the high pressure used. Another problem would be that the temperatures required for curing the mount may alter the microstructure. From experience, the metallographer will be able to determine the most appropriate mounting material for the steel or cast iron specimen being prepared.

Thermoplastic Mounting Materials In using thermoplastic powders as a mounting material, the mounting practice differs somewhat from that used for thermosetting materials. During the initial heating of the cylinder, a low pressure of around 0.7 MPa (100 psi) is applied. Once the temperature reaches 150 °C (300 °F), the pressure is increased to 29 MPa (4200 psi). The mount is then cooled within the cylinder at this pressure until the temperature drops to below 40 °C (105 °F) or preferably to room temperature. This process, which usually takes about 40 min, can be shortened by applying chill blocks or other cooling device to the outside of the cylinder during the cooling cycle. If removed from the cylinder too early, the mount will not hold its shape. Acrylics. An example of an early acrylic mounting material is methyl methacrylate. It is widely known as Lucite, the common transparent plastic material used for numerous applications. Another tradename for an acrylic is Transoptic. The acrylics are transparent in the solid form, which can be an advantage when the metallographer needs to observe the sides of the specimen embedded inside the mount. Also, an identification label can be placed inside the mount (facing outward), or in some cases the identification code on the specimen itself can be observed through the mount. Figure 7.23 shows an example of a specimen code mounted within a transparent acrylic mount. However, the acrylics suffer from many disadvantages. First, acrylics are not resistant to nitric and acetic acids and many solvents (alcohol, acetone, and ethyl methyl ketone). An improperly prepared (insufficient melting) acrylic will easily dissolve in alcohol and even a properly prepared mount may craze in etchants containing alcohol, for example, nital and picral. Second, an acrylic mount has a tendency to pull away from the specimen and leave a gap that creates the potential for seepage of the etchant and rinse solution. This is because the thermal expansion of a thermoplastic material is much greater than the steel or cast iron specimen. Third, the acrylics require longer curing times than the thermosetting materials because the mount must remain in the mounting press cylinder, under pressure, until it is cooled to room temperature. However, some mounting presses have a rapid

cool-down cycle for thermoplastic materials. Fourth, the metallographer must prevent the mount from heating during grinding operations. A coolant (usually water) must be used since the acrylic material will soften with heat. A thermosetting material will not soften under the same circumstances. Fifth, for many steels and cast irons, an acrylic mount is too soft and will abrade faster than the specimen. This will create rounding and poor edge retention. From these disadvantages, one can see that for steels and cast irons, an acrylic mounting material is not favored by many metallographic laboratories. Polyvinyl Compounds. Other thermoplastic materials are polyvinyl formal and polyvinyl chloride (PVC). Polyvinyl formal is marketed under the tradename of Formvar. It has similar characteristics to the acrylics, but is not widely used as a mounting material. It does have the advantage of being one of the hardest of the organic compounds used as mounting materials. One of its disadvantages is its short shelf life. Apparently, the mount continues to cure with time and will eventually deteriorate and crack during storage. Typical Problems of Compression Mounting Materials. Table 7.2 lists five examples of typical problems of thermosetting mounting materials and two problems with thermoplastic mounting materials. The cause of each problem and a solution to the problem are also listed.

Castable Mounting Materials A castable mounting material means that it is in liquid form at room temperature and can be “cast” around a specimen. Because it does not require heat for curing, it is also called cold mount. The specimen is placed inside the mold, where both mold and specimen are on a flat surface. The mold can be simply an aluminum or phenolic tube cut to a height of about 19 mm (3⁄4 in.). Although castable mounting materials are more expensive than the thermosetting and thermoplastic mounting materials, they are now widely used. This is because they have many advantages over

Fig. 7.23 Specimen code embedded in an acrylic mount

190 / Metallographer’s Guide the thermal-compression materials. Major advantages are that a mounting press is not required, they are relatively easy to mix and use, and many mounts can be made at one time. Epoxy. The metallographic epoxy mounting materials consist of two different liquids that are mixed in certain proportions. One liquid is the resin, and the other liquid is the hardener (also called activator or catalyst). The proper proportion of resin to hardener is very important for optimal curing of the mount. Also, the two liquids must be thoroughly mixed. To ensure a better mix, the metallographer should use a flat paddle instead of a rod to mix the two liquids. After the two liquids are thoroughly mixed, there will be a slight amount of heat generated by the reaction of the hardener and the resin. Because of this heat of reaction, polystyrene (e.g., Styrofoam) and wax-coated mixing cups should be avoided as a mixing container. As a precaution, the handling and mixing of epoxy materials should be conducted in a fume hood. The metallographer should carefully read the MSDS for the materials being used.

Table 7.2

For a castable mount, the specimen is placed in the center of a mold. There are three basic types of molds: • • •

A cylinder open at both ends A cylinder with a removable end A flexible cylinder with a closed end

Some examples of molds are seen in Fig. 7.24. The mold can consist of phenolic resin, rubber, aluminum, and so forth. As a simple setup, the metallographer can use a precut phenolic cylinder usually 25 mm (1 in.), 32 mm (11⁄4 in.), 38 mm (11⁄2 in.), and 50 mm (2 in.) in diameter (standard mount diameters). The cylinder height is usually 25 mm (1 in.) high. Of course, other size molds can be used depending on the size and shape of the specimen. The open-ended mold is the least expensive, but must be glued to a flat surface. During mixing of the two liquids, air bubbles usually become entrapped in the final mixture. These bubbles can create pockets

Typical problems of compression-mounting materials

Problem

Cause

Solution

Thermosetting resins Too large a section in the given mold area; sharp cornered specimens

Increase mold size; reduce specimen size.

Excessive shrinkage of plastic away from sample

Decrease molding temperature; cool mold slightly prior to ejection.

Absorbed moisture; entrapped gases during molding

Preheat powder or premold; momentarily release pressure during fluid state.

Too short a cure period; insufficient pressure

Lengthen cure period; apply sufficient pressure during transition from fluid state to solid state.

Insufficient molding pressure; insufficient time at curve temperature; increased surface area of powdered materials

Use proper molding pressure; increase cure time. With powders, quickly seal mold closure and apply pressure to eliminate localized curing.

Powdered media did not reach maximum temperature; insufficient time at maximum temperature

Increase holding time at maximum temperature.

Inherent stresses relieved upon or after ejection

Allow cooling to a lower temperature prior to ejection; temper mounts in boiling water.

Thermoplastic resins

Metallographic Specimen Preparation / 191 on the polished surface that will be undesirable. The air pockets will fill with grinding and polishing contamination, which will produce scratches and a unacceptable polish.

Metallographic Tip: To prevent the epoxy from bonding with the flat surface in an open-end mold, place a sheet of aluminum foil over the surface. After the mount has cured, the foil can be peeled away from the mount or will easily disappear in the first grinding operation. To prevent the epoxy from seeping under the mold, while pouring the liquid, the mold must be bonded to the foil. Usually this is done with a fast-drying glue. The higher density of the steel or cast iron specimen will prevent it from floating in the epoxy liquid.

Vacuum Processing. To avoid the formation of air pockets and bubbles described previously, the mount while still in the liquid state should be subjected to evacuation in a vacuum chamber. A common transparent glass or plastic vacuum desiccator can be used for this purpose. The vacuum pump can be an inexpensive mechanical “roughing” pump. The three different methods of vacuum treatment are described in the following paragraphs. Placing a Freshly Mixed Epoxy Mount in the Vacuum Chamber. The simplest method is to prepare the epoxy mixture in air and pour the mixture around the specimen in the mold. The mold is then placed in the vacuum chamber, and the chamber is evacuated. The mount should remain under vacuum until foam forms on top of the exposed side of the mount. After evacuation is completed, air is slowly bled into the chamber. For best results, this evacuation-bleeding process should be repeated several times.

Fig. 7.24 Various molds used to produce castable epoxy mounts

This technique assists the epoxy in flowing into cracks and voids in the specimen. Sucking the Epoxy Mixture into Vacuum Chamber. The epoxy is mixed in the air. However, in this method the mold and specimen are placed inside the vacuum chamber without adding the epoxy mixture. The chamber is then evacuated, and the epoxy mixture is sucked into the chamber through a plastic tube to fill the mold as seen in Fig. 7.25. This type of vacuum desiccator requires an excess amount of epoxy mixture in order to prevent air from being sucked into the chamber. The plastic tube must be discarded after use because of the hardened epoxy inside the tube. Pouring Inside the Vacuum Chamber. In this method, the epoxy is mixed and then poured into a cup that is mounted inside the vacuum chamber. After evacuation of the chamber, the cup is tipped and the mixture is poured directly into the mold. As in the method described in the previous paragraph, since the pouring takes place under vacuum the epoxy can fill all the voids and cracks in the specimen. After pouring, air can be bled into the chamber or the chamber can be placed under positive pressure. Curing of Epoxy. Once mixed, the epoxy will cure in 8 to 16 h. To accelerate the solidification process, the mount should be placed in an oven at 65 °C (150 °F) for 1 h followed by curing at room temperature for 2 h. However, the ratio of hardener to resin must be reduced (follow the vendors recommendations). Generally, the metallographer will prepare the metallographic mounts for an overnight cure at room temperature. Some epoxy materials on the market have a catalyst that shortens the curing time, but this material generates much more heat during the curing reaction and this can lead to shrinkage. If, after a long cure time at room temperature, the mount is not completely cured, placing the mount in a oven at 60 to 70 °C (140 to 160 °F) or under a heat lamp for a few hours will produce a more wear-resistant mount. As

Fig. 7.25 System used to cast samples in epoxy under a vacuum

192 / Metallographer’s Guide opposed to a thermal-compression mount (except thermosetting epoxy and diallyl phthalate mounts), an epoxy mount should not shrink away from the specimen. There is usually an excellent bond between the mount and specimen.

Metallographic Tip: To cast a mount larger than 38 mm (11⁄2 in.) diameter, the ratio of hardener to resin must be lowered or the mount will crack during the curing period from the excess heat generated during the reaction. One solution is to place the mount in a refrigerator for about 16 h (overnight). Remove from the refrigerator and let the mount stabilize at room temperature for 8 to 12 h. This technique will prevent the epoxy from producing a high exothermic reaction that would result in a gap between the specimen and a mount.

Acrylics. A castable acrylic usually has a faster curing time (20 to 30 min) than a castable epoxy mounting material. On the other hand, most acrylics have a higher level of heat generated during curing than a castable epoxy material. Depending on the amount of heat generated and the temperature excursion in the specimen itself, this could alter the microstructure by tempering. To minimize heating of the specimen, a conductive mold material such as copper and aluminum can be used to extract the excess heat. Also, acrylics have the greatest shrinkage of any of the castable mounting materials and do not have good edge retention. As with the acrylic thermoplastic mounting material, attack by solvents and strong acids could be a problem. A castable acrylic mount is transparent.

Polyesters. Castable polyester mounting materials are resistant to most solvents and acids. They consist of two liquids that are mixed before pouring into the mold. They cure within 1 to 3 h and have less shrinkage than castable acrylic mounting materials. However, they are more expensive than acrylic resins. Polyester mounts generally have lower hardness than other castable mounts. A finished polyester mount is transparent.

Fillers The metallographer can change the characteristics of a mounting material by adding a filler material. Two examples of fillers added to epoxy mounts are described in this section. Non-charging Epoxy Mounts for Electron Microscopy. Epoxy mounts can be made not to create a “charging effect” when examined in the scanning electron microscope by adding 4.3 g of graphite to 15 g of the epoxy liquid mixture. As described previously, a conductive mount will prevent electrical charge buildup on the specimen surface when used in the SEM or EPMA. However, one must be aware of potential carbon contamination of the specimen, particularly if the element carbon is of importance to the analysis of the specimen. The graphite will also contaminate the grinding papers and polishing cloths. High-Wearability Epoxy Mounts (for Edge Retention). Epoxy mounts can be produced with more abrasion resistance by adding aluminum oxide granules to the liquid epoxy mixture during casting into the mold. An example of a high-wearability mount is shown in Fig. 7.26, which illustrates the excellent edge retention at the sharp tip of a hardened knife blade. An aluminum oxide epoxy mixture was placed around the knife-blade specimen. The hardness of the

Fig. 7.26 Micrographs of a 1% C, 14% Cr stainless steel knife blade mounted in a high-wearability aluminum oxide epoxy mount. Note the excellent edge retention along the surface of the blade and tip. Vilella’s etch. Courtesy of Patrick Kelley, Spiderco Inc. (a) 50⫻ and (b) 1000⫻

Metallographic Specimen Preparation / 193 epoxy mixture was sufficient to prevent rounding of the blade edge, and excellent edge retention was maintained.

of the skin). Under these circumstances, gloves must be worn for protection.

Special Mounting Techniques Metallographic Tip: To increase the surface hardness of an epoxy mount, mix a small amount of epoxy with aluminum oxide (alumina) granules. These are not the same type of alumina particles used for polishing metallographic specimens. The particles are much larger (100 to 200 μm) in size, and they are porous. Pour the epoxy-alumina mixture into the mold with the specimen already in place. A layer of about 3 mm (0.1 in.) in depth is all that is necessary. Allow the layer to partially cure, then fill the remaining volume with the normal epoxy mixture. The final mount will have the aluminum oxide granules on the surface to be ground and polished. This will greatly improve edge retention of the sample.

When epoxy mounts are prepared with aluminum oxide granules, the surface is very hard and special grinding and polishing procedures are necessary to properly prepare the specimen. Generally, special metal discs bonded with diamond particles are used for this purpose. These discs made to fit onto the normal polishing wheel are discussed in the sections on grinding and polishing later in this chapter.

Dyes Colored and fluorescent dyes can be added to castable epoxy mounting material to distinguish the molding material from particular features on the sample. An example would be a layer of thin-oxide scale lining the walls of a crack or a pore in the specimen. In the light microscope it can be difficult, at times, to distinguish the scale or oxide layer from the epoxy mounting material. With a colored or fluorescent dye, the mounting compound should be easily distinguishable from the oxide scale. The effect of the dye can be enhanced by using polarized or darkfield illumination in the microscope. Typical Problems of Castable Mounting Compounds. Table 7.3 lists some of the problems encountered in acrylic, polyester, and epoxy mounts. The cause and solution to the problem are also listed.

Safety During Mounting It is always advisable to use a fume hood to exhaust the organic vapors from mounting compounds. Always read the safety precautions provided by the vendor and the MSDS. Many of the fumes and powders are toxic and should not be inhaled. Some epoxy mounting compounds can irritate the skin. Wear disposable gloves when handling the castable liquids. Safety glasses with side shields should always be worn during the mixing of these materials and around the mounting presses. Continuous exposure to uncured epoxy compounds can cause dermatitis (inflammation

Edge Retention. In many metallographic specimens, the features at the edge of the specimen are of vital importance. These features could include a metallic or organic coating, an oxide scale, or a decarburized layer. Unfortunately, during polishing, the specimen edge tends to abrade faster than the remaining specimen. This rounding of the specimen edge is unacceptable if the features at the edge are to be preserved. There are a number of techniques that are used to ensure a flat edge. One of the easiest techniques is to mount the specimen in a clamp of similar material. This way, the interface between the clamp and specimen is preserved (see Fig. 7.20). In a thermal-compression or castable mount, it is advisable to place a steel sheet next to the edge of interest as shown in Fig. 7.27. A small gap is allowed between the steel sheet and the specimen in order to fill the gap with the mounting material. If the sheet abuts the specimen directly, a space may develop that will allow seepage of etching and rinsing solutions at the final preparation stage. During mounting, there is not enough pressure between the sheet and the specimen to ensure a tight interface. Another way to maintain edge retention is to ensure a flat specimen during grinding and polishing. The flatness can be stabilized by placing steel rods or steel nuts at the outer quadrants of the specimen as shown in Fig. 7.28. In many automatic grinding and polishing machines, the mount surface at the very outer radius of the specimen holder tends to round. By placing one of the steel rods at the outer radius, rounding can be minimized. Edge retention can also be minimized by selecting a mounting material that matches the abrasion resistance of the sample as discussed previously with aluminum oxide fillers. However, other than a steel clamp described previously, the metallographer will find that most mounting materials will abrade somewhat faster than the steel or cast iron specimen. Surface Plating. For ultimate edge retention, some metallographic laboratories plate the surface of interest with nickel (other Metallographic Tip: Before plating can begin, it is important that the specimen surface be clean. First, the specimen should be degreased in a solvent that does not leave a residue, for example, acetone. Second, the specimen should be placed into a solution of dilute sulfuric acid (prepared to pH ⫽ 2) to remove any passive layer. After plating, if the specimen is to be mounted in epoxy, it should be rinsed for a few seconds in a 5% aqueous solution of hydrochloric acid, then rinsed in water, and blown dry. This is because, in the epoxy liquid, bubbles appear on the plated surface that are difficult to remove in the vacuum system, and the dilute acid solution tends to eliminate the surface bubbles.

194 / Metallographer’s Guide plating metals are listed in the Appendix). Electroless nickel plating procedures are preferred over electrolytic methods because it is a simple procedure and the coating has lower internal stresses. A cleaned specimen is immersed into a heated plating solution. The metallographer can obtain premixed solutions from a vendor or prepare a solution in the laboratory (see the Appendix). The time in the solution depends on the thickness of the coating required. Generally, these solutions plate a layer thickness of 10 to 15 μm/h. With a layer of nickel on the surface, subtle features can be retained since the nickel layer will become

Table 7.3

rounded during polishing instead of the edge of the steel or cast iron specimen. An example of a electroless nickel plated surface is shown in Fig. 7.29. In this case, a broken Charpy specimen was plated to maintain edge retention at the fractured surface. Mounting of Sheet Specimens. In most cases, only the cross section of the sheet is of microstructural interest. This means that the sheet must be mounted perpendicular to the prepared surface. To ensure that the sheet specimen remains in the correct alignment during mounting, the sheet can be bent in the shape of an “L” or a metal or plastic binder clip can be secured to one end of the

Typical problems of castable mounting materials

Problem

Cause

Solution

Acrylics Too violent agitation while blending resin and hardener

Blend mixture gently to avoid air entrapment.

Insufficient air cure prior to oven cure; oven cure temperature too high; resinto-hardener ratio incorrect

Increase air cure time; decrease oven cure temperature; correct resin-tohardener ratio.

Resin-to-hardener ratio incorrect; resin has oxidized

Correct resin-to-hardener ratio; keep containers tightly sealed.

Resin-to-hardener ratio incorrect; incomplete blending of resin-hardener mixture

Correct resin-to-hardener ratio; blend mixture completely.

Resin-to-hardener ratio incorrect; incomplete blending of resin-hardener mixture

Correct resin-to-hardener ratio; blend mixture completely.

Insufficient air cure prior to oven cure; oven cure temperature too high; resinto-hardener ratio incorrect

Increase air cure time; decrease oven cure temperature; correct resin-tohardener ratio.

Too violent agitation while blending resin and hardener mixture

Blend mixture gently to avoid air entrapment.

Resin-to-hardener ratio incorrect; oxidized hardener

Correct resin-to-hardener ratio; keep containers tightly sealed.

Resin-to-hardener ratio incorrect; incorrect blending of resin-hardener mixture

Correct resin-to-hardener ratio; blend mixture completely.

Polyesters

Epoxies

Metallographic Specimen Preparation / 195 sheet. Fig 7.30 illustrates the use of binder clips. In some cases, the triangular-shaped spring portion of a paper binder clip can be used. The placement of the clip or short leg of the “L” can indicate specimen orientation. This technique can be used with both thermal-compression or castable mounts.

Many thin sheet specimens can be mounted by stacking the sheets. The specimens can be separated by placing a small strip of double-sided tape (adhesive on both sides) on the outer edges of each sheet specimen as shown in Fig. 7.31. In stacking the taped specimens into a sandwich, the specimens themselves will not touch each other and there will be a small space between layers for epoxy to fill this space. Placing the mount in a vacuum just after casting with epoxy will ensure that the spaces will be free of

Fig. 7.27 Steel sheet placed next to the specimen in an epoxy mount for

Fig. 7.28 Epoxy mount with a steel rod at each quadrant for specimen

edge retention

flatness and edge retention

Fig. 7.29 Micrographs showing the fracture surfaces of the halves of a broken Charpy specimen mounted in epoxy. (a) Fracture surface plated with electroless nickel. (b) Fracture surface unplated. Note the excellent edge retention of the plated fracture surface. 2% nital etch. 500⫻

196 / Metallographer’s Guide air bubbles. Keeping the specimens closely spaced will prevent edge rounding. For those specimens that have a shear burr, the thickness of the mount should be measured beforehand so that an ample amount of mount is removed during grinding. This ensures that all cold work from shearing is removed in the grinding process. Mounting of Tubes and Cylinders. In tubular specimens, the epoxy pulls away from the internal diameter of the tube. One method to prevent this shrinkage is to mix 0.5 g of 0.05 μm

aluminum oxide (used for specimen polishing) with 20 g of epoxy. When the mount is cured, the powder will minimize shrinkage. Mounting of Wires. If the metallographer needs to mount a number of steel wires to observe the cross-sectional area, the wires must be mounted on end, that is, perpendicular to the polished surface. To ensure that the wires will be aligned properly, the following technique can be used. Start with a empty but fully cured thermosetting mount, approximately 38 mm (3⁄4 in.) thick. For specimen orientation, an L-shaped sheet should be placed in the mount as seen in Fig. 7.32(a). In the central region of the mount, rows of equally spaced holes are drilled approximately 6 mm (1⁄4 in.) deep. The hole diameter should be slightly larger than the wire diameter. The wires, approximately 7 mm (1⁄4 in.) long, are placed into the drilled holes as seen in Fig. 7.32(a). A dam of masking tape is placed along the top of the mount, as shown in Fig. 7.32(b). A castable epoxy is poured into the taped region of the mount. For best results, the freshly poured mount should be placed under vacuum, or the epoxy mixture should be poured on top of the specimen while under vacuum. After curing, the epoxy side of the mount is prepared to reveal the wire cross sections. The metallographer should make a sketch of the wire arrangement within the mount for specimen code identification. Mounting of Powders. To prepare a mount of a specimen in powder form, mix a small amount of the steel or cast iron powder with a small amount of well-stirred epoxy mixture. Pour the composite liquid into the mold to form a layer about 5 mm (0.2 in.) thick and place the mold in a vacuum. The vacuum will eliminate any air bubbles that may have formed on or around the metal particles. Remove the mount from the vacuum, and very carefully fill the remaining portion of the mold with a regular epoxy liquid to the desired height. If concerned about disturbing the composite mixture with the additional epoxy, allow the composite layer to cure, then fill the remaining volume with

Fig. 7.30 Epoxy mounts with binder clips to hold the specimen perpen-

Fig. 7.31 Mount with sheet specimens separated by double-sided tape at

dicular to the polished surface

the ends

Metallographic Specimen Preparation / 197 epoxy. Figure 7.33 shows an example of an AISI 316L stainless steel powder specimen prepared using this procedure. Note that all the powder particles are held in place during the grinding/ polishing operations and that all the interstices are filled with epoxy mounting compound. Maintaining Specimen Orientation. For a single specimen in a mount, an L-shaped sheet can be mounted along with the specimen, as shown in Fig. 7.34. The long dimension of the “L” can be aligned along the rolling direction. Often, more than one specimen is placed in the same mount. It is necessary that the

orientation of the specimens be arranged so that they are easily identified after mounting. If a number of sheet or thin plate specimens are in the same mount, a small V-shaped metal sheet placed at one side of the arrangement will suffice as an orientation marker as shown in Fig. 7.35. The metallographer will record the codes of each specimen with regard to the marker. This can be done on the reverse side of the mount or on the work request form. Specimen Identification. The metallographer must identify the specimen or specimens contained within each mount. Generally, a code is permanently placed on the back surface of the

Fig. 7.32 Procedure to mount steel wire specimens. (a) Wires are placed in drilled holes in the mount. (b) A masking tape dam is provided to hold the castable epoxy around the specimens. 1.2⫻

Fig. 7.33 Micrographs of AISI 316L stainless steel powder particles mounted in a castable epoxy mount. (a) 50⫻ and (b) 400⫻

198 / Metallographer’s Guide mount with a handheld vibrating electric scribe. The identification should be placed on the mount as soon as possible after the mount has cooled to room temperature so as not to forget the specimen code. If one uses a transparent mounting material, the specimen code can be mounted within the specimen using a paper tag as seen in Fig. 7.23.

Grinding The next important step in metallographic specimen preparation is the process of grinding. As a general definition, grinding is performed with a fixed abrasive (e.g., the abrasive is bonded physically to a heavy paper or cloth), and polishing is performed with a “rolling” or loose abrasive. Grinding the specimen sets the stage for subsequent preparation processes and therefore must be given a great deal of attention by the metallographer. The main purpose of grinding is to remove the plastic deformation induced into the specimen surface by sectioning. During grinding, it is mandatory that surface flatness is maintained. By using consecutive papers with finer and finer grit sizes, the layer of plastic deformation remaining on the specimen surface from sectioning becomes shallower and shallower until it almost disappears. However, the grinding process itself creates its own plastic deformation. This plastic deformation not only includes the scratches created from the grinding medium, but also a zone in the specimen beneath the scratches. This condition is shown by schematic representation in Fig. 7.36. The valleys represent the scratches induced by the grinding abrasive. The zones beneath the scratches represent the plastic deformation zones from mechanical deformation (cold work). As a rule, the softer the material, the deeper the deformation zones beneath the scratches. The deformation zones are deeper than the scratches themselves. Although the grinding process removes the damage created during sectioning, the damage created from grinding must be removed by subsequent polishing steps described in the next section. For more

Fig. 7.34 Mount with a L-shaped strip to indicate specimen orientation

Fig. 7.35 V-shaped metal strip in a mount to indicate orientation

Fig. 7.36 Depth of scratches from grinding and depth of the deformation area beneath the scratches. Source: Ref 2

Metallographic Specimen Preparation / 199 details on the damage created by sectioning, grinding, and polishing, consult Ref 3.

Grinding Media Paper-Backed Grinding Discs. Silicon carbide, zirconium oxide, and diamond particles are used for grinding steel specimens. Silicon carbide particles are commonly used in the metallographic laboratory with particles bonded to a heavy paper disc. The grinding papers (“C” weight) for the metallographic laboratory are disc shaped to conform to a 200, 250, or 300 mm (8, 10, or 12 in.) diam rotating wheel or in the form of rectangular strips that attach to a grinding table. Some automatic grinding/polishing machines use larger-diameter discs. Diamond is usually not used in manual grinding but is reserved for polishing metallographic specimens. However, diamond particles bonded to metal grinding discs are available for grinding hard steel and cast iron specimens. These diamond discs produce excellent edge retention in tool steel specimens. Some mounts, such as an epoxy mount filled with aluminum oxide granules for edge retention, cannot be effectively ground with silicon carbide paper. In this case, a diamond disc is necessary. These discs are discussed later in this section. Metallographic grinding papers are prepared to standard “grit” sizes. The term grit size means the nominal particle size. It is measured by passing the particles through a series of screens (called sieves), each with known screen openings. The screen opening is called mesh size. For metallographic specimen preparation, the standard grit sizes are shown in Table 7.4. The American National Standards Institute (ANSI) and Coated Abra-

Table 7.4 papers

Standard grit sizes for metallographic grinding

European grit No.

U.S. grit No.

Approximate particle

(FEPA)

(ANSI/CAMI)

size, μm

P-60 P-80 P-120 P-180 P-220

60

250 180 125 75 63 59 46 41 30 26 22 18 15

120 180 240

P-320 320 P-500 400 P-800 P-1000 P-1200

Table 7.5

sives Manufacturers Institute (CAMI) grit numbers are widely used in the United States, whereas the European Federation of Abrasive Particles (FEPA) grit numbers are used in Europe. There are also grinding papers with a finer grit size. Some of these sizes are listed in Table 7.5. Some grinding papers are sold by the actual particle size not grit size, for example, 12, 8, 5, 3, and 1 μm papers. All these fine grinding papers are used for special applications where it may be necessary to extend the grinding process beyond 600 grit to eliminate some of the subsequent polishing steps. By eliminating these steps, the metallographer can minimize the “pulling” of nonmetallic inclusions or graphite particles from the specimen surface. Also, eliminating these steps may prevent “rounding” of the specimen edge. These procedures are described in detail in the section on polishing. If the specimen was sectioned with the proper abrasive wheel, the metallographer can usually start grinding with the 320 grit paper or finer. The initial paper size selection will depend on the type of material and the condition of the cut surface. If the specimen was sectioned by a shear, band saw, or hacksaw, a coarser paper is used, for example, 80 grit. Metal-Backed Grinding Discs. There are also special grinding media in the form of discs bonded with diamond particles. These discs are ideal for maintaining specimen flatness and edge retention. Some discs are marketed as a set for fine grinding, rough polishing, and medium polishing. Other vendors market discs with a particular diamond particle size. Vendors are listed in the Appendix. For convenience, some of these discs are magnetic so that they can be quickly changed between grinding and polishing steps using a magnetic wheel or platen. Some of the metal-backed grinding discs are not magnetic, but have an adhesive backing.

600

Finer grit sizes for grinding papers

European grit No.

U.S. grit No.

(FEPA)

(ANSI/CAMI)

Particle size, μm

P-2400 P-4000

800 1200

8 3

Metallographic Tip: Since many wheels or platens are nonmagnetic (brass) they can be made magnetic by attaching an adhesive-backed magnetic sheet to the wheel surface.

Adhesive Backing versus Plain Backing Grinding papers are manufactured with and without a pressuresensitive adhesive (PSA) backing. For many manual grinding operations, it is important that the side of the paper without the abrasive particles be coated with a PSA backing so that the paper can adhere to the polishing wheel or table. This backing allows the paper to bond and rotate with the wheel. However, papers without an adhesive backing can be held onto a wheel with a retaining ring that attaches over the outer edge of the wheel. Generally, the retaining ring has at least three pressure points that allow the metallographer to secure the ring tightly to the wheel. Some automatic grinding machines use a plain-backed paper with a retaining ring, and some require a paper with PSA backing.

200 / Metallographer’s Guide

Metallographic Tip: When using a plain-backed paper, always wet the wheel before placing the paper on the wheel. Thus, when the paper is pressed to the wheel, a bond is created that keeps the paper tight against the wheel.

ular to the side of the mount and there may be more than one plane of grinding. With experience, the proper technique will evolve. Remember that plenty of water must be used during grinding to avoid damage to the specimen surface. An exception to this rule is in the case of cast-iron specimens containing graphite where the

One factor to consider in selecting the type of backing of a grinding paper is cost. A PSA-backed paper will cost more than a plain-backed paper. Considering that grinding paper is the most costly consumable product in the metallographic laboratory, the metallographer must balance cost with performance.

The Technique of Grinding Before hand grinding, the metallographer should bevel the outer edge of the mount to prevent the edge from catching the grinding paper when it is placed against the rotating wheel. With a sharp edge, the mount could actually be tipped sufficiently to propel the specimen from the hand of the metallographer. This can also occur during grinding on a belt grinder. The outer edge of an unmounted specimen should also be beveled unless the edge is of metallographic interest. In using an automatic grinding machine, the specimen or mount does not have to be beveled because the specimen/mount is held securely in place in the specimen holder. Grinding can be performed manually or by an automated machine. In manual grinding, the specimen is held by the thumb and fingers. For best results, the fingers should be as close as possible to the bottom of the mount, that is, close to the grinding paper. This way, the metallographer has more control of the pressure on the specimen and there are fewer tendencies to tip the specimen. The metallographer must apply equal pressure to each point of contact. However, each person may have a different pressure point. For example, the thumb may apply more pressure than one of the fingers. It is important to understand your pressure point and to develop a way to eliminate it. This is done by trial and error.

Metallographic Tip: When hand grinding and polishing a metallographic specimen, think of the specimen as an extension of the hand. It is important to pay attention to what you are doing and not to daydream. An inattentive metallographer will not produce an acceptable specimen. Thus, one of the rules of manual grinding and polishing is to concentrate on what you are doing.

After grinding on the first paper, the metallographer should examine the pattern of grinding scratches on the specimen. If there is more pressure being applied to one side of the mount than another side, the ground surface will not be perpendic-

Fig. 7.37 Micrographs of scratches on properly ground surfaces. (a) 120

grit. (b) 240 grit. (c) 320 grit. (d) 400 grit. (e) 600 grit papers. (f) 6 μm diamond. 50⫻

Metallographic Specimen Preparation / 201 last two grinding steps should be conducted without water in order to retain the graphite particles. On a wheel rotating counterclockwise, the specimen should be placed in the 3 o’clock position. This way, the rotation of the wheel will produce a grinding direction away from rather than toward the metallographer. The specimen should not be rotated around the wheel, but slowly moved back and forth from near the center to near the edge of the wheel. This way the specimen surface is exposed to as much of the grinding paper surface as possible. Also, by moving back and forth, the platen or wheel will not have a groove worn at a particular location after many years of use. The back-and-forth motion should be slow and deliberate. A rapid motion will create a loss of control resulting in an uneven surface. In the automatic grinding process the specimen may rotate, but the human touch is eliminated. If the specimens are correctly clamped in the specimen holder, flatness should be uniform. If grinding on a fixed, flat grinding table, in order to maintain a flat surface it is usually better to apply pressure while pushing or pulling, but not in both directions.

Metallographic Tip: Upon finishing each step during hand grinding, the surface should be dull with distinct parallel scratches. In the automatic grinding process, the surface should be dull but the scratches, although distinct, will be randomly oriented.

After grinding the specimen on one paper, for example, 240 grit, the specimen should be rotated 90° before grinding on the next paper, for example, 320 grit. In this way, after the ground surface is examined, one can be assured that the surface scratches of the previous grind are properly removed. Figure 7.37 shows properly ground surfaces for 120, 240, 320, 400, and 600 grit papers and 6 μm diamond. Note the 90° orientation of the scratches for each paper. When hand grinding, the specimen should be rinsed in water and dried with a paper towel. If water is allowed to sit on the surface of the specimen, pits can develop on the ground surface.

Metallographic Tip: When grinding a rectangularshaped specimen where one of the long edges needs to be metallographically preserved, for example, a coating, always finish the last grinding step with the long axis of the edge parallel to the direction of rotation of the wheel and facing the center of the wheel. This will minimize smearing of the long edge (coating) onto the remaining specimen and will concentrate any rounding of the specimen on the narrower edges that are of no interest.

Grinding Time Only a short time is spent on each grinding paper, generally 10 to 30 s. One thing to keep in mind is that the harder the specimen the faster the grinding paper will wear. A worn paper can actually damage the surface of the specimen. Worn papers and long grinding times begin to burnish the surface of the specimen; that is, the specimen surface is being buffed instead of being abraded away by the hard particles. If a specimen is burnished, it means that the abrasive particles embedded in the resin of the paper are worn (no longer sharp edged) and clogged with polishing debris. A freshly ground surface should never appear polished, but should be dull in appearance with scratches aligned all in the same direction. If the surface appears polished, burnishing has taken place, and the surface must be reground on a fresh paper.

Metallographic Tip: As a guideline, grind the specimen for a time sufficient to obtain a uniform set of scratches, then continue to grind for a time period twice as long. For example, if it takes 8 s to achieve a uniform set of scratches, then continue to grind for another 16 s in order to remove most of the deformation zone created by the previous, coarser-grit paper.

Incomplete or aggressive grinding can cause changes in microstructure. For instance, a worn grinding paper can have a tendency to transform retained austenite into martensite as shown in Fig. 7.38 for a dual-phase steel. In this plot, one can see that the amount of austenite decreases with a worn paper when

Fig. 7.38 A plot showing the percent of retained austenite in specimens of

a dual-phase steel sheet prepared with new and worn grinding papers (two specimens each). The true amount of retained austenite is 8.5% as seen in the two bars on the right for chemically polished specimens (no grinding deformation). Note the much lower levels resulting from the worn paper and the slight decrease from the new paper.

202 / Metallographer’s Guide compared with a new paper (two sets of specimens are shown because each stage of the experiment was duplicated). The true amount of austenite in the specimen was about 8.5%, as verified by the two bars on the right side of the graph. These specimens were prepared by a chemical polish where any grinding deformation was completely avoided. As seen in the graph, even with a new paper there is some minor decrease in the amount of retained austenite.

Wheel Speed During Grinding As a rule, the speed should be fast enough for metal removal but slow enough to maintain control of the specimen. Generally, wheel speeds ranging from 150 to 300 rev/min are recommended. There are fixed-speed and variable-speed wheels.

Coolant During each grinding step, copious water should be used to cool the specimen surface and to carry away grinding debris. The specimen should never be heated to the point where the microstructure is altered. Some belt grinders are equipped with a coolant in order to (a) minimize the alteration of microstructure and (b) the extend the life of the grinding belt. For some metallographic specimens, water should not be used as a coolant. An example would be a steel containing aluminum precipitates where the metallographer must observe aluminum nitride precipitates in the transmission electron microscope. These aluminum nitride precipitates are water soluble. In this situation, mineral spirits, kerosene, or a lapping oil can be used as a coolant. In grinding a gray, ductile, or malleable cast iron, water should not be used in the last two grinding steps in order to prevent the graphite particles from being pulled from the ground surface.

Storage of Grinding Papers Papers that have a pressure-sensitive adhesive backing have a shelf life, and, with time, the adhesive will lose its adhesive power. Therefore, as with other laboratory consumables, a date should be marked on the package of paper when the material was purchased. The older stock should be used first. Generally, grinding papers such as abrasive-bonded wheels should be stored flat in a dry, room-temperature environment.

Safety During Grinding Obviously, one should keep one’s fingertips away from the rotating grinding paper. However, an additional danger that can occur during grinding is a specimen being propelled at high velocity from the wheel. When hand grinding, a firm grip on the specimen is an absolute necessity. It is always advisable to wear safety glasses with side shields when sectioning, mounting, grinding, and polishing metallographic specimens.

Polishing Polishing is the final mechanical preparation stage in obtaining a flat, scratch-free surface for metallographic examination. In most cases, a few scratches may be permitted as long as the fields of interest in the microscope do not contain scratches. The main purpose of polishing a specimen is to remove all traces of deformation from the last grinding step and each subsequent polishing step. In addition to grinding, the “art” of polishing is one of the most difficult stages in metallographic specimen preparation. Some of the difficulty is in deciding when to go to the next step and in keeping the specimen surface as flat as possible. There are many polishing techniques that can be used, and the metallographer must find a workable technique through practice and experience. This section describes proven techniques for polishing steel and cast iron specimens. Manual polishing techniques are presented, even though automatic polishing machines are widely available in many metallographic laboratories. Automatic methods are designed to eliminate much of the human element in metallographic practice. However, even with automatic methods, the metallographer must first learn the principles involved in polishing metallographic specimens, and the manual techniques presented in this chapter cover these principles. The beginning metallographer should understand that repeated use of the manual technique is essential in order to develop workable metallographic skills. The metallographer must be familiar with the materials in the metallographic laboratory, such as the polishing cloths and polishing abrasives. If the conventional procedures do not work on a particular specimen, try a different approach. Not all laboratories use the same procedure to obtain the exact same result. Even within a laboratory, one metallographer may have a preferred procedure compared with another metallographer’s procedure. Usually, grinding ends with a specimen surface prepared from a 600 grit paper. The abrasive particles on a 600 grit paper are approximately 15 μm in size. These particles create scratches on the specimen surface that must be removed by a sequence of subsequent polishing steps. Also, there is a plastic deformation layer beneath the scratches that add to the total deformation zone depth. For a proper metallographic finish, both the scratches and most of the deformation layer must be completely removed by polishing. This condition is represented in Fig. 7.36. In most cases, the plastic deformation zone extends deeper than the scratches themselves. Most of the deformation zone is removed during the rough-polishing step. Polishing with a series of abrasive particles of decreasing size, for example, 6 μm diamond, followed by 0.3 μm aluminum oxide, followed by 0.05 μm silicon dioxide (silica). Each subsequent step attempts to remove the prior deformation zone. For example, the 6 μm diamond particles remove the deformation created by the 600 grit grinding step. The last deformation zone is removed by etching the specimen surface. The final metallographic specimen should not only be scratch-free but also free of any remaining deformation that will affect the appearance of the microstructure. Many false microstructural features (artifacts) can be generated by improper polishing procedures.

Metallographic Specimen Preparation / 203

Cleaning the Specimen The specimen must be thoroughly cleaned of grinding debris before polishing. If a clamped specimen is being polished, special care must be given in order to remove grinding debris that has accumulated in the open regions of the clamp. The general cleaning procedure is to rinse the specimen in running tap water. While rinsing, the specimen is swabbed with cotton to loosen foreign material, especially in a clamp. After this preliminary step, the specimen should be immersed in a beaker of alcohol and placed in an ultrasonic cleaning device for at least 60 s. Once removed from the beaker, the specimen is rinsed with alcohol and dried.

Metallographic Tip: A hot-air blower is ideal for drying metallographic specimens. One such blower is the type used in public lavatories to dry one’s hands after washing. Forced-air drying minimizes the formation of a film residue on the specimen surface and removes any remaining alcohol from crevices in the mount and specimen, for example, cracks, pores, a separation between the mount and specimen, and so forth. The air is also heated, which accelerates the drying process.

In addition to the specimen, the metallographer’s hands must be washed with soap and warm water to remove polishing debris attached to the skin. It should always be kept in mind that grinding debris (and polishing debris) can create scratches on the surface of the polished specimen. There is nothing more frustrating to the metallographer in attempting to prepare a scratch-free surface than to find that the polishing cloth is contaminated with grinding debris. Even one errant abrasive particle can cause problems.

Polishing Cloths There are numerous types of polishing cloths available in disc form from the various metallographic suppliers listed in the Appendix. Each cloth has a particular polishing characteristic. For example, some cloths are used for a fast rate of metal removal, some cloths are used for inclusion retention, and some cloths are used for improved edge retention. Table 7.6 lists some of the polishing cloths that are available for use in the metallographic laboratory. One of the main characteristics in selecting a polishing cloth is the type of nap. In general, a long nap cloth will provide a faster rate of metal removal than a cloth with a short nap. The longer nap cloth will hold more abrasive and provide sites for larger-sized abrasive particles than a cloth with a shorter nap. Therefore, a long-nap cloth is generally used for rough polishing, and a short-nap cloth is generally used for final polishing. However, a longer-nap cloth tends to round the edges of the

specimen more easily than a shorter-nap cloth. Also, a longer-nap cloth is more prone to “pull” graphite and inclusions on the specimen surface than a shorter-nap cloth. As can be seen in the polishing techniques described later in this chapter, a mediumnap cloth is a good compromise between the long- and short-nap cloths and can be used for rough, intermediate, and final polishing. Most polishing cloths are supplied precut to the same diameter of standard polishing wheels (also called platens). Wheels are generally 200, 250, and 300 mm (8, 10, and 12 in.) in diameter, with the most common being 200 mm (8 in.) for manual polishing. The cloth can be supplied with a PSA backing so that the cloth fits tightly against the platen surface or with a metal disc that attaches to a polishing wheel that is magnetic or to a magnetic film that has been applied to the wheel and not move or rotate during polishing. A wrinkle in a cloth on the polishing wheel cannot be tolerated. The polishing cloth can also be supplied without a PSA backing and is held snug against the wheel surface by a tight-fitting retaining ring. The cloth is larger in diameter than the wheel and thus extends over the outer edge of the wheel. The retaining ring slips over the outer diameter of the wheel and compresses the excess cloth over the outer diameter of the wheel.

Metallographic Tip: To tightly secure a polishing cloth without a PSA backing onto a wheel, soak the cloth in water then pull the cloth tightly over the wheel while tightening the ring. This procedure will ensure that the cloth is tight against the wheel and free of wrinkles.

Table 7.6 Type of Cloth

Selvyt Nylon

Characteristic of polishing cloths Characteristics

Medium-nap cotton cloth for final polishing Napless nylon cloth for rough and intermediate polishing used for maintaining good edge retention Pan (Pan-W) Nonwoven, napless cloth for flatness and edge retention during rough and intermediate polishing Cotton Short- to medium-nap cotton cloth for rough and intermediate polishing Flocked twill Medium-nap, flocked rayon cloth bonded to a cotton twill backing used as a general polishing cloth Billiard Medium-nap, sheared-pile virgin wool cloth for rough polishing Felt Medium-nap, plush (plucked) virgin wool for intermediate polishing Silk Napless woven silk with hard surface for intermediate polishing, has good edge retention for hard materials Velvet Medium- to long-nap soft cloth for final polishing very soft metals Canvas Coarse-woven duck cloth for rough polishing, usually with loose abrasive particles Suede Medium-nap duck cloth on a canvas back used for final polishing soft materials Wool Short-nap wool pile for final polishing Rayon Short-nap rayon flock on a woven cotton backing for final polishing Stainless steel A woven stainless steel cloth for grinding and rough polishing, generally on automatic machines

204 / Metallographer’s Guide

Polishing Discs As previously discussed in the grinding section, a metal disc with diamond particles bonded to the disc surface can replace a polishing cloth. These discs are used to produce an exceptionally flat specimen with excellent edge retention. The discs can be used for rough and medium polishing and therefore require a further step on a cloth to produce the final polish.

Polishing Abrasives Unlike grinding, where the abrasive particles are embedded in the grinding paper, in polishing the abrasive particles are separate and are applied to the cloth. The particles can be applied loose or in an aqueous slurry, an aerosol spray, a paste, or a colloidal suspension. For polishing steels and cast iron specimens, there are three main types of abrasive particles, namely, diamond, aluminum oxide (alumina), and silicon dioxide (silica). The characteristics of each type of abrasive particle are described in this section. Diamond is the hardest material found in nature. It is therefore an excellent material to remove metal during polishing. With the development of the synthetic diamond, the availability of diamond particles as an abrasive is now widespread and is fast becoming the most popular metallographic polishing medium. Although diamond particle sizes are marketed from 0.1 to 45 μm, common particle sizes for preparing ferrous specimens are 1 and 6 μm. Diamond particles are supplied in a wax, water-based paste, or slurry that can be applied directly to the polishing cloth. Diamond particles are also supplied as an aerosol spray where the diamond particles are suspended in a petroleum base liquid or propane gas. Caution is advised in using some aerosol sprays since the carrier can be flammable. Aluminum oxide (also called alumina) is one of the first abrasives to be used in the metallographic laboratory. Although less hard than diamond, alumina is a very hard, inexpensive ceramic material that is ideal as a polishing abrasive. There are two allotropic forms of alumina that are available as metallographic abrasives, namely, ␣ alumina and ␥ alumina. Alpha alumina, the hexagonal form, removes metal at a more rapid rate than the ␥ (cubic) form. Gamma alumina has a slower rate of metal removal and is sometimes used as an abrasive for final polishing. However, ␥ alumina has generally been replaced by 0.04 μm water-base colloidal silicon dioxide (see next section). In those special cases where colloidal silicon dioxide attacks a coated specimen, ␥ alumina has to be used. Gamma alumina can also be mixed with other carriers such as ethylene glycol if the water-based silicon dioxide cannot be applied. Alumina in the dry form can also be used where a slurry is produced by wetting the polishing cloth and mixing the dry powder and water by hand. The most common form is a slurry of alumina in water. The three most common particle sizes for alumina are 0.05, 0.3, and 1 μm for final, intermediate, and rough polishing, respectively. The 0.05 μm particle size is ␥ alumina, and the 0.3 and 1.0 μm particle sizes are ␣ alumina. Silicon dioxide (colloidal silica), which is generally softer than either diamond and alumina, is used as metallographic

abrasive for final polishing. Metallographic supply houses market a premixed colloidal silica with a particle size of 0.05 μm for final polishing. A colloid is a suspension of particles in a liquid. The particles remain in suspension due to a negative surface charge on the particles, causing them to repel each other. In the case of colloidal silica, the negatively charged silica particles are suspended in an aqueous alkaline liquid, for example, sodium hydroxide or aluminum hydroxide. These alkaline liquids can cause skin irritation with prolonged exposure. Rubber surgicaltype gloves can be worn to minimize exposure. Also, the alkaline solution must not come into contact with the eyes. There are two things to remember about colloidal silica liquids: never allow the liquid to freeze, and never allow the liquid to dry on the polishing cloth. If the solution is allowed to freeze, the particles will no longer be in colloidal form and the solution becomes unsuitable for polishing. If allowed to dry on the polishing cloth, a hard glaze forms that renders the cloth useless. This means that when using these liquids, the cloth must be thoroughly rinsed with water fairly soon after use. Some manufacturers of colloidal silica add an inhibitor to the solution to minimize the formation of the glaze. However, the inhibitors may adversely alter the polishing characteristics of the solution. Other Abrasive Particles. There are a number of less common polishing abrasives that are used for special applications. For example, magnesium oxide is used as a final polishing abrasive for aluminum alloys, and rare earth oxides are used to polish other oxides such as silicate glass. Ferric oxide in water can be used as a final polish for steels that contain a high density of inclusions and to polish copper alloys. A few drops of a 10% aqueous solution of chromic oxide are added to the ferric oxide solution. One problem with using ferric oxide is that a passive film forms on the specimen surface, and the film must be removed to allow a proper etching reaction. A couple of seconds on a final polishing wheel will remove the passive film.

Manual (Hand) Polishing As one can see from the previous discussion, there are numerous polishing cloths and polishing abrasives for the metallographer to chose from in preparing a metallographic specimen. However, in polishing metallographic specimens of steel or cast iron, only a few of the choices are necessary. This is shown in the following examples of polishing procedures used for ferrous materials. Keep in mind that the procedures outlined here are for manual (hand) polishing. In automatic polishing machines, the recommendations of the equipment supplier should be followed. Procedure for Medium- to High-Hardness Steels and Cast Irons. This category includes most carbon steels, above about 0.15% C, and most low-alloy steels and cast irons. The softer, lower-carbon steels require a slightly different technique that is described in a later section. All procedures assume that the specimen was finished at the grinding stage with a 600 grit paper. Step 1. After completely cleaning the specimen to remove grinding debris, the first step requires a medium-nap flocked twill

Metallographic Specimen Preparation / 205 rayon cloth mounted on the platen of a rotating polishing wheel. The medium-nap cloth was chosen in order to achieve a moderate rate of metal removal during polishing. As stated previously, one of the principles involving cloth selection is that the higher the nap the higher the metal-removal rate. However, the higher the nap the greater the chance of creating poor edge retention (rounding) and the greater the opportunity to smear inclusions on the polished specimen surface. The polishing abrasive for this initial polishing step is 6 μm diamond applied to the cloth either as a paste, aerosol spray, or slurry. The metallographer should apply the diamond plus lubricant rather sparingly. It is more economical to have a “dry” rather than a “wet” surface because one can lose much of the expensive diamond abrasive by the centrifugal force of the rotating wheel. Also, a wet surface can create a condition similar to hydroplaning where the specimen surface does not come into contact with the polishing cloth and the specimen rides on the top of the liquid layer instead of the cloth. When polishing steel specimens coated with zinc, the pH of the lubricant may have to be changed to prevent a galvanic reaction that may attack-etch the zinc coating. Kerosene can be used as a lubricant. The polishing wheel should rotate between 150 and 300 rev/min, and the specimen should be rotated in a direction counter to the rotation direction of wheel. Thus, on wheels that rotate counterclockwise, when observed from above, the specimen should be rotated clockwise. Never polish a specimen by holding it in one position on a rotating wheel. Figure 7.39(a) shows what can happen to MnS inclusions if the specimen is held stationary while the wheel is rotating. In this specimen of AISI/SAE 1010 steel, the metal flows around the particles of MnS inclusions and in the process creates an unacceptable condition known as comet

tails. Figure 7.39(b) shows the same specimen polished correctly by rotating the specimen in the direction opposite to the wheel rotation. The metallographer should use as much surface of the cloth as practical to ensure that the specimen surface is abraded by freshly exposed diamond particles. A circular or oval pattern is recommended. The clockwise rotation should be at a speed of about 70 rev/min. It has been proven that a faster rotation speed does not yield higher productivity and may in fact prolong the polishing process because it is difficult to apply the proper pressure on the specimen at the faster speed. For this initial polishing step, a heavy pressure should be applied to the specimen for a period of 60 to 90 s.

Metallographic Tip: From experience, a heavy pressure can be subjectively measured by observing the blood flow in the fingers contacting the specimen. If the skin color between the first and second knuckles of the fingers is much lighter than normal flesh color, the proper pressure is being applied.

The purpose of this heavy pressure is to ensure a fast metalremoval rate by the diamond particles. It must be kept in mind that the abrasive particles in the 600 grit grinding paper are about 15μm in diameter. The majority of the deformation layer beneath the scratches produced by these particles must be removed in this initial polishing step. The diamond particles being employed are 6 μm in diameter. With proper pressure, the grinding scratches and deformation layer can be removed in the 60 to 90 s required. As

Fig. 7.39 Microstructure of an as-polished AISI/SAE 1010 steel showing (a) smeared manganese sulfides on the polished surface creating ”comet tails“ by

holding the specimen stationary on a rotating wheel and (b) the MnS inclusions not smeared by rotating the specimen counter to the wheel rotation. Unetched. 100⫻

206 / Metallographer’s Guide a check to see if the 600 grit grinding scratches are removed, the specimen surface should be observed in a microscope at low magnification. Remember, the grinding scratches are all aligned in the same direction because during the last grinding step, the specimen is not rotated. However, the newly formed polishing scratches are randomly oriented because during polishing, the specimen is rotated. If any parallel scratches remain, the polishing step should be repeated. Figure 7.40 shows an example of an

Fig. 7.40 Micrograph of ASTM A 36 steel plate rough polished with 6 μm

diamond paste with insufficient pressure. Note the parallel deformation zones in some of the ferrite grains (see arrow). 2% nital etch. 100⫻

ASTM A36 structural steel that was prepared on 6 μm diamond with “light” pressure. Note that the light pressure did not remove the parallel grinding deformation zones from grinding that are still visible in the regions of ferrite (see Fig. 7.36). Another example is shown in Fig. 7.41 for a fully pearlitic AISI/SAE 1080 steel. Grinding scratches and “disturbed” metal are still evident in Fig. 7.41(a) where light pressure was used with 6 μm diamond. The same specimen polished with heavy pressure is shown in Fig. 7.41(b), and the scratches and disturbed metal are eliminated. If these scratches are not removed by the 6 μm diamond polish, it is a waste of time to go to step 2. Any indication of grinding scratches means that step 1 must be continued until the parallel scratches disappear. An inexpensive, inverted metallurgical microscope can be used to inspect the specimen after each polishing step. Once a routine practice has been developed by the metallographer, every specimen does not have to be examined. The microscope should, however, be used after polishing a new steel or cast iron specimen or after using a nonroutine polishing practice. After polishing the specimen with 6 μm diamond, the specimen must be thoroughly cleaned to remove all traces of polishing debris. It is recommended that the specimen be swabbed with cotton under a flowing stream of water. The specimen should be immediately rinsed in ethyl alcohol and dried with a blast of hot air. Step 2. In this intermediate polishing step, the cleaned specimen is polished on a flocked twill rayon cloth (same type of cloth as in step 1) using 0.3 μm alumina in a water slurry. The slurry should be applied liberally to the center of the rotating wheel and the liquid allowed to spread to the outer edge of the wheel by centrifugal force. The cloth should be maintained as wet as possible without allowing too much of the slurry to spin from the wheel.

Fig. 7.41 Micrographs of a AISI/SAE 1080 steel shown with (a) disturbed metal on the surface using light polishing pressure and (b) the normal microstructure of pearlite with proeutectoid ferrite using heavy polishing pressure. 4% picral etch. 500⫻

Metallographic Specimen Preparation / 207

Metallographic Tip: The slurry can be contained in an Erlenmeyer flask with a single-hole rubber stopper. A short length of glass tubing should be inserted in the hole with both ends of the tube extending from the stopper (approximately 3 mm, or 1⁄8 in., inside and 12 mm, or 1⁄2 in., outside). Before the suspension is added to the polishing cloth, the flask should be shaken to suspend the alumina particles in the water.

The specimen should be rotated counter to the wheel rotation (as in step 1 above) for approximately 2 min. A firm pressure, not the heavy pressure used in step 1, should be applied to the specimen. Figure 7.42 shows the result of polishing with too much pressure on a heavily worn cloth. The specimen, a low-carbon, vanadium-niobium (columbium) microalloyed, hot-rolled plate steel, shows a layer of “disturbed” metal on the surface (see black arrow) where the ferrite is smeared and heavily cold worked. The ferrite grains should appear clean, as indicated by some of the nonetching ferrite grains shown in the micrograph (see white arrow). In this case, the heavy pressure created an unacceptable degree of surface deformation, particularly in a softer matrix such as ferrite. After polishing, the specimen should be thoroughly cleaned by swabbing with cotton under a stream of water, rinsed in alcohol, and dried with a blast of hot air. As in step 1, the surface of the polished specimen should be examined under a low-power mi-

Fig. 7.42 Micrograph showing microstructure of a vanadium microal-

loyed plate steel with too much pressure being applied during polishing with 0.3 μm alumina. The black arrow points to a ferrite grain with a smeared surface. The outlined white arrow shows a normal ferrite grain. 2% nital etch. 500⫻

croscope. In this case, the previous polishing scratches will not be aligned in one direction since those scratches from grinding were removed in step 1. The metallographer should now have a mental note of the random scratches from step 1 and can compare the appearance and depth of those scratches with the new scratches from step 2. If step 1 scratches still remain, the step 2 process should be repeated. Step 3. The final polishing step takes place on a medium-nap flocked twill rayon cloth (same type of cloth used in steps 1 and 2) saturated with a solution of 0.05 μm colloidal silicon dioxide (silica). Many metallographic supply houses market a premixed solution. This solution can be diluted if necessary. The solution should be applied to the center of the polishing cloth to maintain a “wet” condition. The cloth can be attached to a rotating wheel or can be stationary. If a rotating wheel is used, the speed of rotation should be between 50 to 100 rev/min.

Metallographic Tip: A thick glass or plastic plate can be used, since it can be easily cleaned in a sink with a stream of tap water rinsing away the colloidal solution. The PSA backing allows the cleaned cloth and plate assembly to be used over and over. Also, using a plate will free up the polishing wheels for other operations.

After polishing for about 30 to 60 s with a firm pressure, the specimen should be swabbed with cotton under flowing water, rinsed in alcohol, and dried in a hot-air blast. Upon inspection of the polished surface in the microscope, a scratch-free surface should have been produced. If scratches persist, either the final polishing cloth is contaminated or the scratches from step 2 have not been removed. In the first situation where a few deep random scratches persist, the polishing cloth should be thoroughly flushed with a stream of running water and the process repeated. In the second case, where many fine random scratches persist, step 3 should be repeated. Once the colloidal solution is applied, the cloth should not be allowed to dry. If drying takes place, a glaze forms over the surface, and the cloth must be replaced before proceeding. Therefore, after polishing, the cloth must be rinsed and scrubbed with a clean nylon brush to remove the colloidal solution before it dries. Some premixed colloidal solutions contain an inhibitor to prevent the formation of the glaze. However, from personal experience, these mixtures do not perform as well as a solution without the inhibitor. It is also important to keep the colloidal solution from freezing. Also, because some colloidal silica mixtures are alkaline or caustic, prolonged exposure may cause a skin irritation. Rubber (latex) surgical gloves should be worn to minimize exposure to the skin. The solution must be kept away from the eyes. The metallographer should wash exposed areas of the skin. Procedure for Low-Hardness Steels And Cast Irons. For carbon steels under 0.15% C, ferritic cast irons, and ferritic and

208 / Metallographer’s Guide austenitic stainless steels, the procedure differs from that described previously due to the greater extent of plastic deformation that develops during grinding and polishing. As a general rule, the softer the steel or cast iron, the deeper the plastic deformation zone. In addition to the scratches themselves, the zone of plastic deformation must be removed by each polishing step. The objective is to remove all the damage induced by the previous step, leaving just the new damage created by the current step. The very last damage layer from the final polish is removed by etching the specimen surface. In fact, for the lower hardness steels and cast irons, etching is used as a way to remove damage between polishing steps. The following procedure is recommended. Step 1. After grinding on a 600 grit paper and thoroughly cleaning by swabbing the specimen in running water, rinsing in alcohol, and drying in a hot air blast, the specimen is polished on a flocked twill rayon cloth impregnated with 6 μm diamond. The specimen is rotated counter to the wheel rotation in a circular or elliptical pattern for 60 to 72 rev/min. The wheel speed can range from 150 to 300 rev/min. A heavy pressure is used for 60 to 90 s. This is the same step as used for the harder steels and cast irons discussed previously. However for soft materials, after cleaning, the specimen is etched in 2% nital for about 20 s to remove some of the deformed surface layer. After etching, the specimen is rinsed in water followed by a brief rinse in alcohol and immediately dried with a hot-air blast. The surface of the specimen should be examined with a low-power observation microscope to see if all the grinding scratches are removed (the grinding scratches should all be aligned in the same direction). Step 2. After cleaning the specimen, the next polishing step is conducted on a sheared pile, 100% virgin wool billiard cloth using a slurry of 1 μm aluminum oxide (alumina). The specimen is rotated counter to the wheel rotation (150 to 300 rev/min wheel speed). A firm pressure is applied to the specimen for approximately 60 s. After cleaning the specimen, the surface is etched in 2% nital for a period of about 10 s to remove a portion of the deformed layer. The specimen is flushed in water followed by an alcohol rinse then blown dry in hot air. Examination of the cleaned specimen in a low-power observation microscope is recommended to determine the effectiveness of the 1 μm alumina polish. Step 3. This polishing step utilizes a flocked twill rayon cloth and a slurry of 0.3 μm aluminum oxide (alumina). The specimen is rotated counter to the wheel rotation with a firm pressure for approximately 2 min. Etching is not required for this step. The specimen is then cleaned with a cotton swab and water, flushed with alcohol, and dried in a hot-air blast. As in step 1, the specimen should be examined in a low-power observation microscope to observe if the Step 2 scratches were removed. Step 4. The last polishing step involves a flocked twill rayon cloth with a solution of 0.05 μm colloidal silicon dioxide (silica). Although a rotating wheel can be used, it is generally preferable to use a stationary cloth (with PSA backing) attached to a glass or plastic plate. The polishing time is approximately 30 to 60 s. The metallographer must remember not to allow the cloth and colloidal solution to dry after use. It is important to rinse and scrub the cloth under running water to prevent the solution from transform-

ing into a hard glaze on the cloth surface. Swabbing with cotton under flowing water cleans the specimen. After rinsing in alcohol, the specimen is dried in a blast of hot air. At this point, the normal etching procedure is used depending on the type of microstructural investigation involved. Polishing to Maintain Specimen Edge Retention (Soft Edge). In some specimens, it is very important to maintain a flat surface to the very edge of the specimen. An example where this may be difficult to achieve would be a specimen of medium- to highcarbon steel with a decarburized surface. In this case, the surface layer would be very soft when compared with the bulk specimen. With a soft edge, two things can occur: the soft edge can become rounded, and the soft zone will contain more polishing deformation. Also, the mounting material itself may be much softer than the specimen, thus allowing the mount to wear much faster than the specimen leaving a rounded edge. Thus, in order to use the procedures described below, the specimen must be on the same plane as the mount. An example of this is shown in Fig. 7.43. In this micrograph, the scratches on the specimen surface and the mount surface are all in alignment and on the same focal plane of the microscope. In the situation where a soft outer edge is involved, it is important to avoid a polishing cloth with a nap and to avoid using wheel rotation. In addition, there are special procedures (described in the following paragraphs) that the metallographer can use to maximize edge retention. Procedure A. In order to achieve adequate metal removal while polishing on a napless cloth, the metallographer can choose to first prepare the specimen with grinding papers finer than 600 grit, for example, 800, 1000, or 1200 grit. This way, the specimen will not require extensive polishing, thus minimizing rounding of the specimen edges. A 800 grit paper (P-2400) will have an average abrasive particle size of about 10 μm, a 1000 grit paper about 7

Fig. 7.43 Micrograph of a steel specimen mounted in a material that has

high wearability. Note that the grinding scratches in the mount and specimen are in the same focal plane. 100⫻

Metallographic Specimen Preparation / 209 μm, and a 1200 grit paper (P-4000) about 5 μm. One technique would be to continue the grinding operation from 600 grit paper to 1000 grit and then to 1200 grit. After grinding, a first polishing step would be to use 1 μm alumina on a napless cloth such as nylon or pan-w. The cloth should be mounted on a flat stationary surface and the specimen rotated by hand for about 60 s (or until all parallel scratches from last grit are removed) using a firm pressure. The second polishing step would be to polish for 60 s on a stationary medium-nap rayon cloth using 0.3 μm alumina. The final polishing step would be to use colloidal silica on a stationary medium-nap rayon cloth for 60 s. (This polishing procedure works well for the authors, and other polishing procedures may work well in other metallographic laboratories.) Procedure B. The specimen can be polished after a 1200 grit paper using a vibratory polishing machine (described in the section on automatic polishing). Vibratory polishing takes longer than normal manual or automatic polishing procedures, but is noted for excellent edge retention. Procedure C. Stationary wheels can be used with the technique of etch-polish-etch described previously in “Procedure for LowHardness Steels and Cast Irons.” By etching the specimen after each polishing step, the prior deformation zone is removed. Polishing to Maintain Specimen Edge Retention (Fragile Edge). When edge retention is needed for a specimen with a fragile or brittle surface layer—that is, an oxide scale—it may be more important to select the proper mounting material than to be concerned about the polishing procedure. For example, edge retention can be maximized by using a hard mounting material such as thermosetting or castable epoxy with a filler of aluminum oxide granules (described in the section on mounting). Also, the mount can contain steel pellets or a piece of sheet steel next the edge of the specimen to be retained. Figure 7.22 shows edge retention of a thin oxide-scale layer on the surface of an AISI/SAE 1080 steel screw. In this case, the metallographer was interested in the morphology of the scale layer, the steel/scale interface and the steel region just below the interface. In order to observe and to photograph these features, they must all be in focus and edge rounding cannot be tolerated. Another procedure involves plating the surface (the edge to be retained) with electroless nickel. The outer nickel layer will provide hardness similar to the steel specimen and thus provide protection to the scale layer. These procedures are discussed in the section on mounting. Polishing to Maximize Inclusion Retention. As in edge retention, a high-nap (e.g., billiard cloth) on the polishing cloth should be avoided. In using a napless cloth, the metallographer must be aware that the zone of grinding and polishing deformation will be difficult to completely remove especially if the material being polished is soft (low-carbon steel with a matrix of ferrite or an austenitic stainless steel). In preparing a specimen for image analysis, finish the specimen on a low-nap cloth with colloidal silica. Wheel rotation should not be used because the rotation can cause the inclusions to be pulled from the surface. This condition is shown in Fig. 7.39. Polishing to Maximize Graphite Retention. Graphite flakes and nodules are “pulled’ from the surface of a metallographic

specimen during the last grinding steps. Therefore, in order to retain graphite in a polished specimen, the metallographer should use a procedure where the last two grinding steps are performed without water as a coolant. Obviously, special care must be taken to prevent heating of the specimen. It is recommended that the final grinding step be performed on a paper finer than 600 grit. A 3 or 1 μm paper is recommended. The specimen can then be polished on a low-nap cloth with 1 μm diamond for 60 to 120 s followed by a 45 s final polish on a low-nap cloth with 0.05 μm colloidal silica. Polishing Coated Steel. Coatings include metallic coatings such as zinc (galvanized steel) and tin (tinplate), organic coatings (paint), and ceramic coatings (enameled steel). For some metallic coatings, special grinding and polishing procedures are required because the coating and steel specimen can create a galvanic cell in an aqueous environment. The pH of the water is of prime importance in controlling the galvanic reaction. Distilled water should be used in preference to tap water, which may not have a neutral pH. However, the pH of even distilled water should be checked before use. Also, special procedures are required because many of the coatings are fragile and must be protected from damage during grinding and polishing.

Automatic Polishing Vibratory Polishing. Some metallographic laboratories prefer vibratory techniques using specially designed vibratory polishing machines. These devices mildly vibrate the specimen in a slurry of abrasive particles on a standard polishing cloth. The specimens are usually weighted so that a controlled amount of load is applied. A ring usually confines the slurry and keeps the specimen movement within a circular area. The polishing times are generally between about 30 min to 12 h, depending on the initial surface condition. These times are much longer than conventional automatic and manual polishing procedures. However, once the machine is operating, it can run unattended, and one metallographer can oversee many machines, each with many specimens. The entire polishing operation uses a single abrasive material and polishing cloth. It may require some time to determine the proper operating parameters for a particular material. These parameters include the amount of weight applied to each specimen, the type and amount of abrasive slurry, and the type of cloth. One of the advantages of vibratory polishing is that one can achieve better edge retention for coated surfaces and better inclusion and graphite retention than most manual procedures. Also, vibratory polishers are noted for producing scratch-free surfaces particularly in such soft steels as austenitic stainless steel. However, in addition to the long polishing times, there are some disadvantages when compared with conventional techniques. First, one can ”overpolish“ the specimen. This means that the long polishing times can develop relief or localized etching of the microstructure. This is undesirable because the metallographer should be in complete control of the process to reveal the microstructure. That control is usually by a chemical or electrolytic etching procedure. The formation of relief during vibratory polishing is due to the hardness of the various constituents in the microstructure.

210 / Metallographer’s Guide Microprocessor-Controlled Grinding/Polisher Machines. Many metallographic laboratories have automatic grinding and polishing machines. These machines are ideally suited for preparing large numbers of repetitive specimens in a production laboratory. In many cases, these machines are not operated by a metallographer, but by someone skilled in machine operation. However, even though a metallographer is not needed to operate an automatic machine, many laboratories prefer these devices in order to give the metallographer more time in procedures involving metallographic observation, interpretation, and documentation. In automatic grinding/polishing machines, metallographic specimens are placed in specimen holders that contain multiple specimens. Many of the grinding papers and polishing abrasives/ cloths are the same as those used in manual procedures. It is always best to follow the manufacturer’s instructions for these machines. Cleanliness is also important since contamination can easily occur between grinding and polishing steps.

Electrolytic Polishing Only in very special cases are electrolytic polishing (also called electropolishing) procedures used for ferrous materials. These special cases include pure iron and some of the single-phase stainless steels where electropolishing is used quite effectively. However, most stainless steels can be polished using the manual polishing techniques described earlier in this section. Figure 7.44 shows a typical electrolytic polishing cell. It is a rather simple setup that requires a controlled electrical current and voltage (dc). The current passes through the electrolyte, which is an electrically conductive solution, from the anode (⫹) to the cathode (⫺). The specimen is the anode, and the cathode consists of a nondissolving material such as stainless steel or pure iron. Electrolytic polishing is also called anodic polishing because the specimen, which is the anode in the electrolytic cell, undergoes the process of anodic dissolution. This anodic dissolution process must be closely controlled. The following variables need to be controlled:

Fig. 7.44 Basic laboratory setup for electropolishing and electrolytic etching

• • • • • •

Current density (amperes/cm2) Voltage Electrolyte composition, temperature, and flow rate Time Specimen surface condition Cathode composition, size, and shape

The key to proper electropolishing is the development and control of a thin, viscous liquid layer of reaction products at the specimen surface. This layer of liquid, known as the polishing film, controls the polishing process. The layer only forms under certain combinations of current density and voltage. The current density is simply the current (amperes) divided by the surface area of the specimen exposed to the electrolyte.

Polishing for a Two-Dimensional View Sometimes the metallographer may want to observe the microstructure in two planes in order to obtain more information. Figure 7.45 shows a two-dimensional view of a weld in an AISI/SAE 1020 steel. The through-thickness plane is shown on the top side of the micrograph, and the planar view is shown on the front. The through-thickness view shows the orientation of the dendritic morphology in the top and bottom welds, and both views show the extent of the HAZ. One can see that the morphology of the void (the black region at the lower portion of the weld) is revealed in a two-dimensional manner. In order to prepare this two-dimensional view, the specimen was mounted in a mechanical clamp. One face is polished with 6 μm diamond and 0.3 μm alumina. The specimen is carefully removed from the clamp, then rotated with the polished surface against the clamp, while the other surface is

Fig. 7.45 Macrograph of microstructure of a AISI/SAE 1020 welded plate

as a two-dimensional view. 5⫻. 2% nital etch. (Courtesy of Samuel Lawrence, Bethlehem Steel, Homer Research Center)

Metallographic Specimen Preparation / 211 prepared in the same manner. It is important to polish the clamp surface that will be in contact with the first polished specimen surface. The specimen is then carefully removed from the clamp, and each of the polished surfaces are final polished with 0.05 μm colloidal silicon oxide. The specimen is then etched in 2% nital to reveal the microstructure. Care must be taken not to touch either of the polished surfaces with the fingers during handling. The macrograph was taken with a macrocamera at 5⫻. The specimen was tipped so that both etched surfaces are in focus. The tipped specimen is held in place by pressing the specimen into a piece of clay that is secured to a flat surface.

Specimen Storage If the polished (and etched) specimen is not examined and photographed the same day it is prepared, the specimen must be properly stored in a dry environment to prevent atmospheric corrosion (rust formation). Figure 7.46(a) shows a case where a

specimen of 2.5%Ni-0.4%Nb (columbium) steel was not stored in a desiccator, and the surface became corroded by moisture in the atmosphere. The micrographs at the higher magnifcations (Fig. 7.46b and c) show the corroded surface. The corrosion obscures most of the bainitic microstructure. To reveal the true microstructure, the specimen was cleaned in a 3% aqueous solution of EDTA, and the results are shown in Fig. 7.47(a) to (c) at the same magnifications as Fig. 7.46(a) to (c). Before cleaning, a region of the surface was scribed within a circle in order to find the same area after cleaning. In Fig. 7.47(c), the microstructure is clearly bainitic, and all traces of the corrosion have been removed (compare with the same view shown in Fig. 7.46c). The dark streak in Fig. 7.47(b) is a segregation band, not a region of corrosion. The most common storage location in the metallographic laboratory is a desiccator. Desiccators come in various shapes and sizes, and there are several kinds of desiccants. The metallographer should become familiar with what is available for the laboratory.

Fig. 7.46 (a) Photograph of a rusted specimen of 2.5% Ni 0.4% Cb steel not stored in a desiccator. (b) The microstructure of the corroded region at 100⫻ and (c) the same region at 500⫻. 4% picral plus HCl

212 / Metallographer’s Guide

Types of Desiccators There are two general types of desiccators: (1) a glass, plastic, or aluminum cylindrical container with a tight-fitting lid or (2) a painted steel, stainless steel, or clear plastic cabinet. The cylindrical containers are more economical, but have limited storage capacity. The cabinet, on the other hand, has ample storage capacity since it usually has two or more shelves within the cabinet. Some cabinets are designed specifically to hold standard 25 mm (1 in.), 32 mm (11⁄4 in.), and 38 mm (11⁄2 in.) diam metallographic mounts. The specimens are neatly placed in round holes in the drawers in the cabinet. In both types of desiccators, the desiccant is located at the bottom. The specimens should not come into contact with the desiccant. In the cylindrical containers a perforated metal, plastic, or porcelain plate supports the specimens well above the desiccant. Usually the desiccant is located in a tray at the bottom of the cabinet-type desiccator. The lid of the cylindrical-type desiccator is sealed with a stopcock grease, and the door of the cabinet-type desiccator is sealed with a rubber gasket. There is also another form of a desiccator that employs a vacuum. The vacuum desiccator is usually of the glass or plastic

cylinder type and employs a stopcock on the top of the lid. The desiccator is evacuated and the stopcock closed to maintain the vacuum.

Desiccants The most common and inexpensive desiccant is anhydrous calcium sulfate (CaSO4) supplied as a white, granular material. This material absorbs between 10 and 14% of its weight in water. Another form of desiccant is anhydrous calcium sulfate mixed with about 3% blue-colored cobalt chloride (CoCl2). This desiccant is called an indicating desiccant since it changes from blue to pink as it absorbs moisture. Although there is a distinct advantage of the indicating desiccant, its cost is more than twice that of a plain, nonindicating anhydrous calcium sulfate desiccant. One important feature of these desiccants is that they can be dried and used over again. The moisture is removed by drying the material in an oven at 230 °C (450 °F) for 11⁄2 h. For best results, the material should be spread into a single layer of particles. Once dry, the indicating desiccant returns to the blue color (the normal calcium sulfite desiccant remains white). Another type of desiccant is silica gel. It is usually supplied in a perforated can or in a cloth or permeable polyester-felt bag. This material can absorb about 30% of its weight in water. The indicating silica gel is coated with cobalt chloride so that it changes from blue to pink as it absorbs moisture. It can also be dried and reused by heating in an oven at 150 °C (300 °F) for 1 h.

Other Means of Specimen Storage Plastic Caps. An inexpensive and effective way to store a specimen in a standard mount is to slip a plastic cap over the mount (polished surface inside). The inscribed specimen code on

Fig. 7.47 The same specimen as shown in Fig. 7.46 but cleaned with EDTA. 4% picral etch HCl. (a) Actual size. (b) 100⫻. (c) 500⫻

Metallographic Specimen Preparation / 213 the bottom of the mount remains visible for later identification. These plastic caps are available as end caps that are used to protect the threaded ends of a metal pipe. They are manufactured for 25 mm (1 in.), 32 mm (11⁄4 in.), and 38 mm (11⁄2 in.) diam pipes. Thus, they fit tightly around the standard-sized mounts. Protection from moisture in the atmosphere as well as mechanical and handling damage are guaranteed for many years. Coatings. Fracture surfaces, particularly in specimens too large for a desiccator, can be stored for extended periods of time by coating the surface with a protective material. A clear acrylic lacquer sprayed onto the fracture is a common method of preservation. REFERENCES 1. D.V. Miley and A.E. Calabra, A Review of Specimen Mounting

Methods for Metallography, Metallographic Specimen Preparation, J.L. McCall and W.M. Mueller, Ed., Plenum Press, 1974, p 1–40 2. G. Petzow, Metallography, Metallographic Etching, 2nd ed., ASM International, 1999, p 21 3. L.E. Samuels, Metallographic Polishing by Mechanical Methods, 3rd ed., American Society for Metals, 1982

SELECTED REFERENCES • Metallography and Microstructures, Vol 9, ASM Handbook, American Society for Metals, 1985 • L.E. Samuels, Optical Microscopy of Carbon Steels, American Society for Metals, 1980 • G.F. Vander Voort, Metallography, Principles and Practice, American Society for Metals, 1984

Metallographer's Guide: Practices and Procedures for Irons and Steels Bruce L. Bramfitt, Arlan O. Benscoter, p215-244 DOI:10.1361/mgpp2002p215

Copyright © 2002 ASM International® All rights reserved. www.asminternational.org

CHAPTER 8

The Art of Revealing Microstructure ONCE THE POLISHED metallographic specimen is properly prepared, the next stage is to reveal the microstructure. When examining the as-polished metallographic specimen in the light microscope, microstructural constituents such as pearlite, ferrite, bainite, and martensite cannot be observed, because the specimen surface is highly polished and these constituents require a difference in the reflectivity or absorption of the light beam as it is seen by the human eye. As described in Chapter 5, the metallurgical microscope operates with a beam of light that is reflected from the specimen surface (the specimen is opaque to light), as opposed to the biological microscope where the beam of light passes through the specimen (the specimen is transparent to light). In the biological specimen, differences in density and transparency of light reveal the microstructure being examined. In fact, many biological specimens are stained to provide density differences to enhance contrast. With an opaque metallographic specimen, only differences in light reflectivity from the specimen surface can reveal the microstructure, that is, the light that is 100% reflected back to the viewer’s eye, as in a mirror, will appear differently than the light that is only partially reflected. The full reflection will appear much brighter than the partial reflection. One way to create differences in reflectivity at the surface of the as-polished metallographic specimen is to subject the surface to chemical attack. The constituents in the microstructure are then selectively etched by the chemical attack, that is, selective corrosion takes place. For example, in a specimen with a ferritic microstructure, each neighboring ferrite grain has a different crystal lattice orientation, as discussed in Chapter 2, where each ferrite grain is a single crystal of body-centered cubic iron. Each ferrite grain etches at a different rate, depending on its crystallographic orientation with respect to the polished surface. The different crystallographic planes that are exposed by the chemical attack can reflect light at different angles from the incident beam in the microscope. Also, between the individual ferrite grains there is a grain boundary. These boundaries selectively corrode when exposed to the acid contained in most metallographic etching solutions. The light beam from the microscope is thus reflected from these attacked grains and boundaries at an angle to the incident light beam, thus creating the contrast observed in the eyepiece of the microscope. The art of revealing microstructure is the most important tool of the metallographer. An experienced metallographer knows how to use this tool, much like an artist knows how to use and mix the

different color paints on his palette to express the mood of his painting. The selection on the metallographer’s palette is quite extensive and is described in this Chapter.

Etching Response An example of the metallographer’s “palette” is displayed in Fig. 8.1. In this example, a fully spheroidized Fe-0.4% C alloy is etched or stained to obtain a specific chemical response and different microstructural appearance. The metallographer must know and understand how to use etching response to effectively reveal a microstructure. The spheroidized specimen in Fig. 8.1 consists of rounded particles of cementite (spheroidized cementite particles in three dimensions) in a matrix of ferrite. Etching Response—to Reveal Carbides. Figure 8.1(a) shows the spheroidized carbides randomly dispersed in the microstructure. The etchant used is 4% picral (details about all the etchants/ stains used in Fig. 8.1 are described in detail later in this chapter). Picral is an etchant that is used to highlight carbides in a ferritic, bainitic, or martensitic microstructure. The etchant attacks the carbide boundaries but does not effectively attack the ferrite grain boundaries of the matrix. Etching Response—to Darken Carbides. Some etchants even color or darken the cementite particles, as seen in Fig. 8.1(b). The etchant used to darken carbides is boiling alkaline sodium picrate and is valuable in distinguishing carbides from other constituents, such as small regions of retained austenite that could resemble the carbides seen in this specimen. By darkening the carbides, there can be no confusion about which constituents are present. This means that this etch is ideal for image analysis. As in the 4% picral etch, the ferrite grain boundaries on the matrix are unattacked. Etching Response—to Reveal Ferrite Grain Boundaries. Figure 8.1(c) shows an etchant that is used to attack or reveal ferrite grain boundaries. That etchant is 2% nital. Note that in this specimen, the carbide morphology seen in Fig. 8.1(a) and (b) is obscured by the overpowering detail of the ferrite grain boundaries and subgrain boundaries. Nital does not evenly etch carbides. Note the striking difference between Fig. 8.1(a) and (c). Etching Response—to Reveal Carbides and Ferrite Grain Boundaries. The etchant used in Fig. 8.1(d) offers a compromise. The etching response of Marshall’s reagent used in this example reveals the carbides and ferrite grain boundaries. In fact, the

216 / Metallographer’s Guide carbides are slightly colored gray, which aids in distinguishing them from small ferrite grains. Marshall’s reagent attacks the ferrite grain boundaries and is preferred over 2% nital for image analysis. Etching Response—to Color or Tint the Ferrite Grains and Leave the Carbides Unaffected. Figure 8.1(e) shows the response of a tint etchant. In Fig. 8.1(e), a tint etch called Beraha’s reagent stains or tints some of the ferrite grains. The carbides are not affected.

Etching Response—to Darken the Ferrite Grains and Leave the Carbides Unaffected. In Fig. 8.1(f), the more concentrated solution of sodium metabisulfite (15% solution) darkens the ferrite grains to different degrees, depending on grain orientation. The carbides are not affected by this tint etchant. From all of the micrographs in Fig. 8.1, one can see that the metallographer has an important tool available to reveal and distinguish microstructural constituents in a variety of ways. The process of using etchants for different etching response is called

Fig. 8.1 A series of micrographs of a fully spheroidized Fe-0.4% C alloy showing different etching response. (a) 4% picral, (b) boiling alkaline sodium picrate, (c) 2% nital etch, (d) Marshall’s reagent, (e) Beraha’s reagent, and (f) 15% sodium metabisulfite tint etch. 500⫻

The Art of Revealing Microstructure / 217 selective etching and is covered in more detail later in this chapter. An example of its use is to distinguish small regions of retained austenite from carbides. In this case, the alkaline sodium picrate etch in Fig. 8.1(b) would darken the carbides but not the retained austenite. Many times, it becomes necessary to use selective etching to distinguish and identify particular constituents in the microstructure. As seen previously, the most common way to reveal microstructure is by chemical etching or attack-etching, for example, using 2% nitric acid in alcohol (nital), as seen in Fig. 8.1(c). Also important are those etchants that chemically stain, tint, and color the constituents in the microstructure, for example, the Beraha’s tint etch in Fig. 8.1(e). These etchants work by depositing chemical films on the surface of certain constituents. Many more examples of attack and chemical deposit (tint) etchants are discussed later in this chapter. Those etchants that are listed and discussed are proven to work and are recommended by the authors. However, the authors realize that other etchants may work just as well as those recommended. The metallographer should keep an open mind and try different etchants and procedures in order to obtain the correct etching response. The metallurgical microscope itself can be used to reveal and enhance microstructural details. Modern metallurgical microscopes are equipped to operate with bright- and dark-field illumination, differential interference contrast (Nomarski), and polarized light. These procedures were discussed in Chapter 5 and are applied as methods to reveal various microstructural features later in this chapter.

Revealing Microstructure in an As-Polished Specimen The metallographer should always stop and think before applying an attack or tint etch to the specimen. If etched before examination, some important features on the specimen surface may be destroyed or altered. In many cases, these features will be masked by the etched microstructure. Thus, the first step should be to examine the specimen in the as-polished condition. For example, many times it is important to determine the microcleanliness of a steel or cast iron specimen. In Fig. 8.2(a), a number of manganese sulfide inclusions can be seen in the unetched condition of an oil-quenched American Iron and Steel Institute/Society of Automotive Engineers (AISI/SAE) 1080 steel bar polished in the longitudinal plane. However, as seen in Fig. 8.2(b), once the specimen is etched (same field), these inclusions are very difficult to see, and the metallographer cannot easily determine the degree of microcleanliness of the steel bar. Also, small patches of as-quenched martensite (white appearance) can add to the difficulty of distinguishing the manganese sulfide inclusions. The metallographer must be aware that before a specimen is attack-etched or tinted, there may be important information revealed on the as-polished surface. Once chemically altered, this important information is lost. Therefore, it is always prudent to examine the polished surface in the metallurgical microscope before proceeding with any etching techniques. Examining the As-Polished Specimen. Sometimes the metallographer may examine a specimen in the microscope in the

Fig. 8.1 (continued) (e) Beraha’s reagent, and (f) 15% sodium metabisulfite tint etch. 500⫻

218 / Metallographer’s Guide as-polished or unetched condition in order to observe certain features, such as delicate oxide layers, inclusion shape and distribution, graphite morphology, and crack and pore geometry. Some of these features, because they have a different hardness than the steel matrix, develop their own surface relief from polishing. Some features have a different color, that is, manganese sulfides have a dove gray color, titanium nitrides are yellow, zirconium nitrides are orange, and so on. Therefore, in certain instances, some specimens are deliberately not etched in order to observe the features of the inclusions themselves. An example is

seen in Fig. 8.3, which shows a complex oxide inclusion found in an historic specimen of Lancashire iron (from Karlholms Bruk Dennemora in Sweden). A microprobe analysis of the chemical elements in the inclusion would yield important information about the iron-making process, for example, the source of the iron ore, and so on. Etching this specimen may preferentially remove some important constituents of the inclusion. Another example of an unetched specimen is seen in Fig. 8.4, which represents the microstructure of wrought iron. The dark constituent is “slag,” a glassy material that is intentionally added

Fig. 8.2 (a) As-polished specimen of AISI/SAE 1080 steel showing manganese sulfide inclusions and (b) same area etched in 4% picral. In the etched condition, martensite (arrows) and the inclusions are unattacked, adding to possible confusion. 200⫻

Fig. 8.3 As-polished specimen of oxide inclusions in a specimen of Lancashire iron. 100⫻

Fig. 8.4 As-polished specimen of wrought iron showing slag inclusions. 50⫻

The Art of Revealing Microstructure / 219 or worked into the iron during manufacture. The matrix is essentially pure iron. Etching this sample would mask many of the subtle features of the slag particles. Figure 8.5 shows metallic lead attached to a manganese sulfide inclusion in a lead-treated free-machining AISI/SAE 11L44 steel. As in the previous example, etching is not necessary and may disturb the microstructural features. As-polished specimens are used when the metallographer is performing image analysis to measure volume fraction, distribution, and sizing of inclusions and graphite flakes/nodules. Etching the specimen will simply complicate the microstructure and introduce potential errors in the analysis. Specimens for the electron probe microanalyzer (microprobe) are usually analyzed in the as-polished condition. This is because etching will remove some of the constituents, alter the microstructural features, and leave a chemical residue on the surface of the specimen. Many times, a specimen to be analyzed in the electron probe microanalyzer will first be etched to allow the metallographer to find the area and feature of interest. Then that specific area will be marked by a circular scribe or microhardness impressions (discussed in Chapters 4 and 5). Once marked, the specimen is lightly polished to remove the etched surface damage and residue. The microprobe operator will then be able to find and analyze the desired features within the scribed area on the specimen surface.

Revealing Microstructure by Etching Most specimens are etched to reveal the underlying microstructural constituents, and over the decades, many chemical mixtures (etchants) have been developed by trial and error. An extensive table of etchants for steels and cast irons can be found in the

Fig. 8.5 As-polished specimen of free-machining AISI/SAE 11L44 steel showing manganese sulfide inclusions with metallic lead attached. 500⫻

Appendix. However, for most steels and cast irons, the metallographer needs to become familiar with only a few etchants.

The Need to Etch a Specimen Many times, a problem can be solved when a proper etch is applied to a specimen. For example, a hot-cracking problem occurred in a type 309 stainless steel weld. A specimen was prepared of the affected weld area and etched. Figure 8.6 shows that the cracks followed along interdendritic boundaries. Obviously, without etching, this important piece of information would not have been revealed. Therefore, if inclusions are not of concern in an investigation, etching must be performed to reveal the microstructural constituents.

Attack-Etching The chemical attack-etching technique is the oldest and most common way to reveal microstructural details. In a sense, attacketching is a process of controlled corrosion. Every microstructural constituent within a metal or alloy has a different surface potential and thus, different corrosion behavior. In scientific terms, each constituent has a specific electrochemical potential, and those constituents that are more electropositive are attacked more easily than those constituents that are more electronegative. In a sense, the electropositive constituent is the anode and the electronegative constituent is the cathode in an electrolytic cell. For example, a pearlitic microstructure consists of layers (lamella) of ferrite and cementite. When exposed to an acid solution, the ferrite becomes the anode and the cementite becomes the cathode. In this electrolytic cell, the ferrite is attacked faster than the cementite, and thus, when pearlite is viewed in the light microscope, there is surface relief, with the ferrite lamella etched deeper than the cementite lamella. This surface relief results in the ferrite appearing darker than the cementite, because more light is scattered and reflected away from the microscope objective lens. Surface Preparation for Etching. If etching is to be used, it should be performed soon after final polishing. With time, a freshly polished specimen will develop what is called a passive layer. If the polished specimen was exposed to air for a few hours (up to 24 h) before etching, the surface should be lightly repolished, using only the last polishing step for 10 to 15 s. Sometimes, rinsing the specimen in hot water will remove the passive layer. However, this practice is not recommended, because the resultant surface attack may not be uniform. If many steel samples are in the same mount, it may be possible that some specimens etch while others will not etch. The polishing procedure will remove any corrosion film that formed by exposure to air (especially moist air). If not removed, the etching time will be different than that normally used, and the etching reaction may become localized in certain regions of the surface. If the specimen is exposed for long periods, the specimen should be completely repolished, because the corrosion will have penetrated deeper into the surface. The Mixing of Etching Solutions. Most of the etchants described in this Chapter are simple mixtures of an acid in alcohol

220 / Metallographer’s Guide or water, or solid crystals of a particular chemical dissolved in alcohol or water. It is recommended that the metallographer always mix these solutions the same way each time they are prepared. This will provide consistency in etching response and timing. It is generally advised to always add the acid or solid material to the alcohol or water while stirring. This is performed by placing a beaker of the solvent (alcohol or water) on a magnetic stirring device. A plastic-coated magnet is placed inside the beaker to create the stirring action. These magnetic stirring devices are rather inexpensive and very effective. In some cases, for example, adding sodium hydroxide pellets to water, such as in the alkaline sodium picrate etchant, produces an exothermic reaction (heat generated). The heat is more uniformly distributed if the solution is stirred. In many cases, a mildly heated solution hastens the dissolution of solid crystals that may be added as part of the formula. However, if too much heat is generated, the solution should be cooled to room temperature before adding other ingredients. In some cases, for example, adding sodium thiosulfate to water, such as in Klemm’s reagent, the reaction is endothermic (heat absorbed), and the solution has to be allowed some time to warm to room temperature before use. The reactions experienced during the mixing of the etchants described in this chapter are rather mild. However, it is always recommended that the metallographer wear eye protection, a full-length acid- and fire-resistant laboratory coat, and latex gloves while mixing and handling chemicals. It is also recommended that all mixing take place in a well-ventilated hood with a sink. For safety reasons, a shower and eye flush station should always be nearby any laboratory where chemicals are stored and mixed. The Procedure for Attack-Etching. The steps used for attacketching are quite simple. In most cases, the polished specimen is

Fig. 8.6 Hot cracking in a weld of 309 stainless steel. The crack follows

the branches of the dendrites. Electrolytic etch with 60% nitric acid and 40% water using 5 V. 500⫻

completely immersed, polished faceup, in the etchant. The solutions can be prepared by mixing just prior to etching, or they can be drawn from a premixed stock solution stored in a well-labeled glass or plastic bottle. (Note: Etchants containing hydrofluoric acid cannot be stored in glass containers, because the acid attacks the glass itself.) As is seen later in this Chapter, some etchants must be prepared just prior to etching, because they lose their potency in storage. Marshall’s reagent is an example of this type of solution, because it contains hydrogen peroxide, which easily decomposes with time.

Metallographic Tip: A 250 to 400 mL capacity porcelain casserole dish is ideal to hold the etching solution because of its wide shape, that is, it has a greater width to height ratio than a conventional beaker. The casserole dish has a porcelain handle that makes it easier and safer than a beaker to pick up and place in or near the sink. Some casserole dishes have a ground surface on the handle so that the metallographer can write the etchant name in pencil. The pencil code can simply be erased when the dish is cleaned and put away. If the handle does not have a ground surface, simply make your own surface by hand grinding a flat surface at the top of the handle.

Metallographic Tip: An effective way to rinse the remains of the etchant from the specimen is to immerse the specimen into a 1000 ml beaker filled with continuously flowing water from the sink faucet. The vigorous flow and large volume of water will flush away any remaining etching solution. After rinsing in water, the specimen is rinsed in denatured ethyl alcohol to remove the water and then blown dry.

Keeping the polished surface face-up in the etchant allows the metallographer to observe the chemical reaction. Generally, a “haze” develops on the etched surface, indicating that the chemical reaction is taking place. A gentle agitation should be employed in order to expose the specimen surface to fresh solution and to eliminate etching reaction debris and any gas bubbles that accumulate on the surface. (As is discussed later, specimens immersed in tint etchants should not be agitated.) Generally, an etching time of 15 to 30 s is used. However, the metallographer must determine the proper time based on the composition of the steel and the type of microstructure being revealed. It is advisable to use a short time, because repeated etching is easier than repolishing an overetched specimen. Generally, etchants are used at room temperature. However, some etchants are heated for optimal results, for example, alkaline sodium picrate is used at

The Art of Revealing Microstructure / 221 boiling-water temperature. After immersion, the specimen is rinsed thoroughly in a stream of running water to flush away any remaining etchant. The polished specimen is generally held with stainless steel tongs and immersed into the etching solution with the polished surface facing toward the metallographer.

Metallographic Tip: A plastic squeeze bottle containing denatured ethyl alcohol is very handy for rinsing metallographic specimens at the sink just before drying. The specimen is then dried in a blast of warm air.

If the water rinse causes the specimen to stain, only an alcohol rinse should be employed. A steel containing more than 1% Si is prone to stain in water. In this case, a series of alcohol baths and alcohol rinses are necessary. Etching Time. Proper etching times can only be determined through experience. An experienced metallographer will generally know how long to etch a particular specimen. Before etching, it is important to have some idea of the magnification range the microstructure will be examined in under the microscope. For example, if the microstructure is to be viewed and photographed at high magnification, the etching time should be shorter than if the specimen is to be viewed at low magnification. In other words, a short etching time may be optimal to reveal the subtleties of the constituents at the higher magnification range, that is, 500 to 1000⫻, but when viewed at lower magnifications, the microstructure will appear underetched, with insufficient surface relief. Figure 8.7 shows an example of an AISI/SAE 9425 bar steel

etched to reveal the bainitic constituent at 1000⫻ (Fig. 8.7a). However, in Fig. 8.7(b), the same specimen observed at 100⫻ appears washed-out and underetched. It is best to start at shorter etching times, because the specimen can be re-immersed in the etchant to develop more relief. Many times, an optimal time can be determined for both high and low magnifications.

The Basic Etchants for Carbon and Low-Alloy Steels and Cast Irons For many years, the “workhorse” etchants for steels and cast irons have been nital and picral and their variations. However, other etchants play important roles in developing and enhancing microstructural features in a metallographic specimen. In fact, the authors prefer Marshall’s reagent as a replacement for nital in most situations. This section describes these etchants with regard to the etchant ingredients, the etching procedure, and the major uses of each etchant. Table 8.1 is a quick reference guide for most of these etchants. This guide should be used to select the type of etchant that will reveal those desired microstructural features listed. Details on how to prepare and use these common etchants listed in Table 8.1 are discussed subsequently. The authors have found that these etchants will work in most cases. However, other etchants may work just as well as those recommended by the authors. The metallographer should experiment with different etchants in order to fully understand their etching responses. There is an extensive list of etchants in the Appendix. However,

Fig. 8.7 A specimen of AISI/SAE 9425 steel etched (a) to reveal details of a bainitic microstructure at 1000⫻. Note the same specimen (b) appears to be underetched and washed-out at 100⫻.

222 / Metallographer’s Guide the following etchants should work for most low-carbon and low-alloy steels as well as most cast irons.

The Attack Etchants Nital Ingredients of 2% Nital: 2 mL nitric acid (concentrated) (1 to 5% solutions can also be used) 98 mL ethyl alcohol

In mixing the solution, it is recommended that the nitric acid be poured into a beaker containing the measured amount of alcohol. The beaker should be on a magnetic stirring device so that as the nitric acid is poured into the alcohol, it is quickly mixed into the solution. Major Uses. The main use for nital is to reveal ferrite grains and martensite in most carbon and low-alloy steels and cast irons. It is also useful in etching bainitic steels. The etchant attacks the ferrite grains and grain boundaries to produce “relief” on the specimen surface. Figure 8.8 shows a low-carbon sheet steel etched in 2% nital for 30 s. Note that most of the ferrite grains are

Description. Historically, the most common etchant is a simple solution of concentrated nitric acid in alcohol. The etchant name, nital, is an acronym for nitric acid and alcohol. Ethyl alcohol (also called ethanol) is preferred over methyl alcohol (methanol), because it is less toxic. The authors prefer a mixture of 2 ml of nitric acid in 98 mL of ethyl alcohol (2% nital). However, other concentrations of nitric acid can be used, that is, 1 to 5% nitric acid. A 1% solution of nital reacts slower than 2% nital. The higher the concentration of nitric acid, the faster the etching response. If nital is prepared with a nitric acid concentration of over 4%, methyl alcohol should be used, because ethyl-alcoholbased mixtures at these higher nitric acid concentrations can become unstable. Also, these higher concentrations should not be stored. Ethyl alcohol denatured with a small amount of methyl alcohol is a less expensive substitute for pure ethyl alcohol (200 proof). If denatured ethyl alcohol is employed, the metallographer must avoid denaturing agents such as benzene and aviation fuel. These agents produce a different etching response and can usually be detected by their odor. Fig. 8.8 Low-carbon steel etched in 2% nital. Note grain-boundary carTable 8.1 Quick reference guide to common attack etchants for carbon steels, low-alloy steels, and cast irons

bides (arrows) are difficult to see. 180⫻

Etchant

To reveal or enhance General microstructure (unknown beforehand) Ferrite grains and grain boundaries Grain-boundary carbides Carbide networks Both ferrite grains and carbides Pearlite Bainite Granular bainite (martensite-austenite constituent) Martensite, as-quenched Martensite, tempered Prior austenite grain boundaries

First 4% picral, then 2% nital 2% nital or Marshall’s reagent 4% picral 4% picral 4% picral ⫹ 2% nital or Marshall’s reagent 4% picral 4% picral 4% picral

Nonmetallic inclusions Epitaxial ferrite Cold-worked ferrite

2% nital 4% picral ⫹ 2% nital Winsteard’s (modified) or boiling alkaline sodium picrate or Marshall’s reagent or saturated aqueous picric acid 4% picral or Winsteard’s (modified) reagent 4% picral Marshall’s reagent 2 nital or Marshall’s reagent

To color or darken Carbides, but not ferrite

Boiling alkaline sodium picrate

General segregation

Fig. 8.9 A water-quenched AISI/SAE 1020 steel showing lath martensite. 2% nital etch. 320⫻

The Art of Revealing Microstructure / 223 delineated by the ferrite grain boundaries. In this microstructure, several cementite particles can be seen at the ferrite grain boundaries (see arrows). The cementite is difficult to see, because it blends in with the grain boundaries. In the next section, it is seen that Marshall’s reagent is much more effective in revealing all ferrite grain boundaries and highlighting the cementite particles. Figure 8.9 shows a 2% nital etch applied to a water-quenched

AISI/SAE 1020 steel. The packets of lath martensite in this microstructure are clearly seen. The use of 2% nital is also shown in the series of carbon-iron alloys in Fig. 8.10. Figure 8.10 shows packets of lath martensite in a 0.2% C-iron, and Fig. 8.10(b) (0.4% C-iron) and Fig. 8. 10(c) (0.6% C-iron) show lath martensite with some plate martensite (arrows). Plate martensite can be seen in Fig. 8.11, which represents a 0.93% C, 14.5% Ni steel

Fig. 8.10 A series of water-quenched iron-carbon alloys. (a) 0.2% C-iron alloy showing lath martensite, (b) 0.4% C-iron alloy showing lath martensite and a small quantity of plate martensite (see arrows), and (c) 0.6% C-iron alloy showing lath martensite with plate martensite. 2% nital etch. 500⫻

224 / Metallographer’s Guide etched in 2% nital. The unetched constituent is retained austenite. Nital will not attack austenite as it does ferrite and martensite. Nital will not attack carbides but will enhance the interface between the carbide and the matrix. For example, the carbides in the air-cooled specimen of an AISI A2 tool steel in Fig. 8.12 were not attacked by the 2% nital etch, but the carbide interfaces were enhanced and the martensite darkened.

Marshall’s Reagent Ingredients: Part A 5 mL sulfuric acid (concentrated) 8 g oxalic acid 100 mL water Part B 30% solution hydrogen peroxide

Fig. 8.11 Plate martensite in water-quenched 0.93% C, 14.5% Ni steel.

Description. Marshall’s reagent is a two-part etchant, with part A consisting of 5 ml of sulfuric acid and 8 g of oxalic acid (solid white crystals) in 100 mL of water. Part A is prepared by adding the sulfuric acid to the water during stirring. There is a small amount of heat generated as a result of the reaction. The oxalic acid crystals are then added to this warm solution. Just before using Marshall’s reagent, part A is mixed with equal parts of part B, a 30% solution of hydrogen peroxide. Normally a one to three second immersion is adequate for proper results. Always use a freshly mixed solution, because the etchant has a short life span. When using Marshall’s reagent, always immerse the specimen so that the polished surface is vertical in the solution. This orientation prevents pits from forming on the surface. The etching reaction is very effervescent. If no visible reaction occurs, a three second etch in 2% nital is required before Marshall’s reagent is used. Sometimes, the metallographer may use a 20 second etch in 2% nital after etching in Marshall’s reagent. This postetch will increase the chemical attack and further sharpen the grain boundaries.

The unetched areas between the martensite plates are retained austenite. 2% nital etch. 100⫻

Fig. 8.12 Carbides in an AISI A2 tool steel. Note the carbides are not

attacked, but their boundaries are enhanced. 2% nital etch.

1000⫻

Fig. 8.13 Low-carbon steel etched in Marshall’s reagent. Note the grain-

boundary Fe3C carbides (arrows) are clearly seen. Also note that all the ferrite grain boundaries are delineated. Compare with the same steel etched in 2% nital in Fig. 8.8. 200⫻

The Art of Revealing Microstructure / 225 Major Uses. Although 2% nital has been the etchant of choice for many decades, more recently, Marshall’s reagent has been used for low-carbon steels to sharpen the ferrite grain boundaries. Many times, 2% nital does not produce a complete ferrite grain-boundary network, that is, gaps can be found in many of the grain boundaries in a particular field of view (see Fig. 8.8). These gaps can be a problem in measuring ferrite grain size either manually or by image analysis. Marshall’s reagent, on the other hand, provides a complete network. Figure 8.13 shows the same steel in Fig. 8.8 but etched with Marshall’s reagent. Also, it should be noted that the grain-boundary cementite particles (arrows) are more clearly seen when etched with Marshall’s reagent. A higher-magnification view of this effect is seen in Fig. 8.14. Another use for Marshall’s reagent is in revealing recrystallized grains in a partially annealed, cold worked microstructure. Figure 8.15(a) shows a small percentage of newly formed recrystallized grains that have nucleated at prior austenite grain boundaries of a cold-worked low-carbon steel. Figure 8.15(b) shows the same steel after almost complete recrystallization. Note the sharpness in the ferrite grain boundaries. The elongated unetched grains are unrecrystallized areas (recovery has taken place but not recrystallization). Figure 8.16 shows a fully cold-worked low-carbon steel etched with Marshall’s reagent. In some as-quenched low-carbon steels, the prior austenite grain boundaries can be delineated by Marshall’s reagent. Figure 8.17 shows an example of a waterquenched low-alloy steel etched in Marshall’s reagent. Note the sharpness of the prior austenite grain boundaries. The background structure within the austenite grains represents packets of lath martensite not fully resolved by this etchant. Note the small region of ferrite that nucleated on one of the austenite grain boundaries (center of micrograph).

Marshall’s reagent has also been used to delineate epitaxial ferrite in steels that have been heat treated and cooled from the two-phase ferrite plus austenite region. An example of using Marshall’s reagent for epitaxial ferrite in a low-carbon steel is seen in Fig. 8.18. Note the envelopes or halos around the ferrite grains at the prior austenite grain boundaries (see arrows). This

Fig. 8.15 Partially annealed cold-worked low-carbon steel showing (a) a

Fig. 8.14 A higher-magnification view of the low-carbon steel in Fig. 8.14. Note grain-boundary Fe3C carbides. Marshall’s reagent. 500⫻

few recrystallized grains in a cold-worked matrix and (b) an unrecrystallized area (recovered grains) in a fully recrystallized matrix. Marshall’s reagent. 250⫻

226 / Metallographer’s Guide ferrite is a result of austenite forming on the prior austenite grain boundaries when the steel was heated into the two-phase region just above the lower critical temperature. The ferrite in the center of the grains never transformed to austenite. When the steel cooled to room temperature, the austenite transformed back to ferrite, leaving a ghostlike image shown in Fig. 8.18. Marshall’s reagent can also etch the boundaries of substructure in pure iron,

Fig. 8.16 A fully cold-worked low-carbon steel. Marshall’s reagent. 100⫻

as seen in Fig. 8.19. The substructure, sometimes called veining, is more subtle than the darker ferrite grain boundaries.

Modified Marshall’s Reagent Ingredients: Part A 5 mL sulfuric acid (concentrated)

Fig. 8.17 Water-quenched low-alloy steel showing clearly delineated

prior austenite grain boundaries. Matrix is lath martensite. Marshall’s reagent. 200⫻

Fig. 8.18 A dual-phase steel showing epitaxial ferrite (new ferrite) at prior

austenite grain boundaries. The epitaxial ferrite formed when the steel was heated into the two-phase region. Austenite formed at the grain boundaries, and ferrite transformed epitaxially on the old ferrite upon cooling. Marshall’s reagent. 500⫻

Fig. 8.19 Pure iron showing “veining” (substructure) within the ferrite grains. Marshall’s reagent. 200⫻

The Art of Revealing Microstructure / 227 8 g oxalic acid 100 mL water Part B 30% solution hydrogen peroxide Part C 4 drops hydrofluoric acid Description. Marshall’s reagent can be modified by adding a few drops of hydrofluoric acid. This addition produces a more uniform etching response. The hydrofluoric acid is added last. When handling hydrofluoric acid, extreme care must be taken to protect the skin from contact with the acid. Hydrofluoric acid is extremely corrosive to human skin and can cause serious acid burns. Latex gloves, an acid-resistant laboratory coat, and eye protection must be worn when using hydrofluoric acid. This acid is so corrosive that it even attacks glass. Therefore, hydrofluoric acid is never stored in glass bottles and is never used in glass beakers or porcelain dishes. Always use plastic containers when mixing and using this modified Marshall’s reagent. For best results, etch while the solution is in an ultrasonic device. Major uses. Marshall’s reagent is modified with hydrofluoric acid to enhance ferrite grain boundaries even more clearly than those developed in the standard reagent formulation. This etchant thus produces much sharper delineation of ferrite grain boundaries than 2% nital in very-low-carbon steels such as interstitial-free (IF) steels. For example, a titanium-stabilized, IF steel was etched in 2% nital and modified Marshall’s reagent in Fig. 8.20(a) and

(b), respectively. The 2% nital etch in Fig. 8.20(a) shows the ineffectiveness of this etch for very-low-carbon steels. The IF steel etched in modified Marshall’s reagent in Fig. 8.20(b) shows almost every ferrite grain boundary delineated. This modified form of Marshall’s reagent is useful for preparing specimens for ferrite grain size measurements using image analysis techniques. Modified Marshall’s reagent can also be used to delineate the pearlite colony boundaries of a fully pearlitic steel. An example is seen in Fig. 8.21 where the colony boundaries of fully pearlitic AISI/SAE 1080 steel are outlined using the modified Marshall’s reagent. From the previous examples, it can clearly be seen that Marshall’s reagent and modified Marshall’s reagent are extremely useful in microstructural development. The authors are using these reagents for most applications where 2% nital was once employed.

Picral 4% Picral Ingredients: 4 g picric acid 96 mL ethyl alcohol 5 drops zephiran chloride (wetting agent) Description. Picral is an etchant that contains picric acid (2,4,6-trinitrophenol) dissolved in alcohol. The name picral is an acronym for picric acid and alcohol. As with nital, ethyl alcohol is preferred over methyl alcohol for toxicity reasons. As with nital

Fig. 8.20 Very-low carbon interstitial-free steel (a) etched in 2% nital, where only a few of the ferrite boundaries are attacked, and (b) etched in modified Marshall’s reagent, where most ferrite boundaries are delineated. 400⫻

228 / Metallographer’s Guide discussed previously, if denatured ethyl alcohol is used, it is important to avoid benzene and aviation fuel as denaturing additives. Methyl alcohol is an acceptable additive. This etchant is called 4% picral even though the picric acid is not in liquid form and 4 g of solid picral only approximates 4 mL of liquid. The solution should be mixed while being magnetically stirred. The picric acid crystals are added during the stirring action. It must be remembered that the yellow picric acid crystals are explosive, and as a precaution, they are always kept moist (dry crystals of picric acid should never be touched or handled by the metallographer). Caution should be taken when handling and storing picric acid. Some laboratories do not allow the use of picric acid for safety reasons. This is very unfortunate for the metallographers in those laboratories, because if properly handled, picric acid can be used safely. One feature of picric acid is unusual in that the more it is used to etch specimens, the stronger the etching reaction. Upon use, the solution will become darker in color. For its very first use, the metallographer does not know how long to leave the specimen in the solution, because it is unpredictable. However, with more use, the etching time becomes more predictable.

Metallographic Tip: In order to have a predictable etching time when using 4% picral, place a piece of steel (about 30 g [1 oz]) in the solution before using. The steel will darken the solution by producing iron ions. The release of these ions will heighten the reaction for etching metallographic specimens.

When rinsing the specimen after etching in 4% picral, it is advisable to rinse in water to remove the bulk of the etchant, then

immerse the specimen in a beaker of alcohol, followed by flushing the surface with a stream of alcohol from a squeeze bottle. Because picric acid is almost insoluble in water, the alcohol immersion and flush is necessary to remove all the final traces of the etchant from the specimen. If the specimen contains pores or cracks, it is advisable to immerse the specimen in a beaker of alcohol and agitate the beaker, using an ultrasonic cleaning device. One problem with picral is its ability to stain yellow almost anything it touches, particularly the skin and clothing. To prevent staining, do not allow the solution to splash or come into contact with your fingers (latex rubber gloves should be used) or clothing. For best results, a few drops (approximately five) of a wetting agent called zephiran chloride (a 17% solution) in 100 mL, when added to 4% picral, will enhance the contrast between constituents and will increase etching speed. A 17% solution of zephiran chloride can be found in most drug stores. A wetting agent is a surface-active agent or surfactant that modifies the surface energy between the solid specimen surface and the liquid etchant. Major Uses. Picral is used for steels containing pearlite, bainite, tempered martensite, cementite, and other carbides. It is particularly useful for heat treated carbon and low-alloy steels and tool steels. As opposed to nital that attacks ferrite grains and grain boundaries, picral attacks interfaces between ferrite and carbides and is not sensitive to pearlite colony orientation. An example of the use of picral is shown in Fig. 8.22, which represents a water-hardenable tool steel (grade W1) in the normalized condition. Figure 8.22(a) shows the W1 tool steel etched in 2% nital. As can be seen, many of the pearlite colonies are not delineated and appear as grayish patches. In Fig. 8.22(b), where the same steel is etched in 4% picral, most of the pearlite colonies are etched, and the dispersion of rounded carbides is more evident. Picral is not sensitive to orientation of the pearlite constituents and will etch evenly, whereas nital is very sensitive to orientation and will attack the microstructure unevenly. An example of picral attacking the interfaces of carbide particles is seen in Fig. 8.23 and 8.24. This specimen in Fig. 8.23 shows carbides clearly delineated by the 4% picral etch. Figure 8.24 shows carbides (arrows) formed from a carbon stain on the surface of a batched-annealed lowcarbon steel sheet. The carbides prevented uniform phosphating of the surface.

Variations of Picral There are a number of important etchants that are variations of the standard 4% picric formula. Some of these variations employ water instead of alcohol, and others are used hot instead of at room temperature.

Picral Plus Hydrochloric Acid

Fig. 8.21 Pearlite colonies are outlined in an AISI/SAE 1080 steel using modified Marshall’s reagent. 200⫻

Ingredients: 4 g picric acid 96 mL ethyl alcohol 5 drops hydrochloric acid 5 drops zephiran chloride

The Art of Revealing Microstructure / 229 Description. A few drops of hydrochloric acid added to picral provide more effective etching response. Major Uses. Picral plus hydrochloric acid is used to sharpen features in low-alloy steels containing moderate to high amounts of chromium. The enhanced picral reagent sharpens grains and carbides.

Saturated Aqueous Picric Acid Ingredients: 10 g picric acid 100 mL water 1 g sodium tridecylbenzene sulfanate (wetting agent) Description. Water is used instead of alcohol as the solvent of the picric acid. However, only about 1% picric acid will dissolve

Fig. 8.23 Carbides in a low-carbon steel. 4% picral. 1500⫻

Fig. 8.22 A normalized water-hardenable AISI W1 tool steel (1.03% C)

etched in (a) 2% nital, where some of the pearlite colonies appear as grayish patches, and (b) 4% picral, where the pearlite colonies are etched and the carbides more clearly delineated. 1000⫻

Fig. 8.24 Surface carbides resulting from a carbon stain on the sheet

surface of a batch-annealed low-carbon steel (see arrows). 4% picral plus zephiran chloride. 1000⫻

230 / Metallographer’s Guide in water at room temperature. By mixing 10 g of picric acid crystals in 100 mL of water, only a portion of the crystals will dissolve upon stirring. After stirring, the saturated liquid should be decanted from the undissolved crystals. In order to produce etching uniformity and reproducable speed, 1 g of sodium tridecylbenzene sulfanate should be added as a wetting agent to 100 ml of solution. Note that zephiran chloride, which is used in 4% picral, is not added as a wetting agent. Zephiran chloride, which works well in an alcohol-based solution, does not work well in a water-based solution. Also, sodium tridecylbenzene sulfanate works well in water-based solutions but not in alcoholbased solutions. Major Uses. To reveal prior austenite grain boundaries in a fully martensitic microstructure, saturated aqueous picric acid is normally employed. Often, the metallographer is asked to measure prior austenite grain size, because it is a very important metallographic feature that relates to mechanical properties and in determining proper heat treatment. Etching results and etching time can be improved by etching the specimen with the aid of an ultrasonic cleaning device. For steels containing over 1% Si, a small amount of ethylenediamine tetraacetic acid (EDTA) can be added to prevent staining.

Vilella’s Reagent Ingredients: 1 g picric acid 5 mL hydrochloric acid (concentrated) 100 mL ethyl alcohol Description. It consists of 5 mL of hydrochloric acid mixed with 1 g of picric acid and 100 ml of alcohol (ethyl alcohol

preferred). The picric acid is first added to the alcohol while stirring is taking place. The picric acid crystals are then dissolved in the solution with continuous stirring action. The hydrochloric acid is added last. Major Uses. Another etchant that delineates prior austenite grain boundaries in a martensitic quenched and tempered steel is Vilella’s reagent. The etchant works best when the martensite is tempered between 300 and 500 °C (570 and 930 °F). Figure 8.25(a) shows prior austenite grain boundaries in a welded heat-affected zone of American Society of Mechanical Engineers (ASME) T-23 steel. The martensite was tempered at 700 °C (1300 °F). Figure 8.25(b) shows that the austenite grain boundaries are decorated with carbides from the tempering process. These carbides at the boundaries are the reason the prior austenitic boundaries can be seen. A similar example is shown in Fig. 8.26, which shows prior austenitic grain boundaries in a quenched and tempered 0.23% C, 3.4% Ni, 1.7% Cr, 0.5% Mo steel. The steel was subject to surface oxidation, which penetrated the austenite grain boundaries. Vilella’s reagent is also useful in etching martensitic stainless steels and tool steels. This etchant can also be applied to any ferrite-carbide microstructure. Figure 8.27 shows an AISI D2 tool steel etched in Vilella’s reagent. Note the sharpness of the carbide particles and grain-boundary carbide networks in this microstructure.

Boiling Alkaline Sodium Picrate Ingredients: 2 g picric acid 25 g sodium hydroxide 100 mL water

Fig. 8.25 The tempered martensitic microstructure in the heat-affected zone of a welded ASME SA 213 T-23 steel showing (a) prior austenite grain boundaries and (b) the grain boundaries decorated with carbides. Vilella’s reagent. 400⫻ and 1500⫻, respectively

The Art of Revealing Microstructure / 231 Description. Sodium hydroxide (NaOH) dissolved in 100 mL of distilled water provides an alkaline pH to alter the acid pH of the picric acid. First, the sodium hydroxide is added to the water while being stirred. The ensuing exothermic reaction produces enough heat to dissolve most of the picric acid, which is added next with continuous stirring. The warming of the mixture assists

the dissolution of the picric acid crystals. Before etching, the solution is heated to boiling and held for five to ten minutes. The specimen is immersed into the boiling solution. Major Uses. This etchant is useful in darkening cementite. Figure 8.1(b) shows an example of using the boiling alkaline sodium picrate etchant for darkening the cementite constituent in an AISI/SAE 1040 steel. Figure 8.1(a) is the same steel etched in 4% picral. In the darkening of a carbide phase such as cementite, the metallographer has a tool to identify carbide phases in a mixed microstructure. The previously discussed etchants are all used for chemical attack of the constituents in a microstructure. Because none of these etchants darken a constituent, it is sometimes difficult to distinguish between small regions of retained austenite, ferrite, and cementite. With boiling sodium picrate, the cementite is easily distinguishable. Boiling alkaline sodium picrate can be used to enhance prior austenitic grain boundaries. However, as seen in Fig. 8.28(a) and (b), the technique requires addditional etching in nital and picral. The as-quenched 0.5% Mo-B steel was etched for ten minutes in boiling alkaline sodium picrate, followed by etching for ten seconds in 2% nital and 20 seconds in 4% picral with hydrochloric acid. After etching, the specimen was lightly wiped on a dry polishing cloth (rayon type).

Winsteard’s Reagent

Fig. 8.26 Quenched and tempered 0.23% C, 3.4% Ni, 1.7% Cr, 0.5% Mo

steel showing oxidation penetrating the austenitic grain boundaries. The austenite grain boundaries are also delineated, because they are decorated with carbides. Vilella’s reagent. 500⫻

Ingredients: Part A: 2 g picric acid 10 mL ethyl alcohol Part B: 200 mL water 5 mL sodium tridecylbenzene sulfanate (40% solution) Ingredients: (modification) Part A: 2 g picric acid 10 mL ethyl alcohol Part B: 200 mL water 5 mL sodium tridecylbenzene sulfanate (40% solution) 5 drops hydrochloric acid

Fig. 8.27 An AISI D2 tool steel showing large eutectic carbides and small carbides in a martensitic matrix. Vilella’s reagent. 500⫻

Description. This water-based reagent is mixed in two parts, because picric acid is very difficult to dissolve in water. The picric acid/alcohol solution is prepared first. This solution is then added to the water, and sodium tridecylbenzene sulfanate is added as a wetting agent. For effective etching, this reagent is used hot, between 60 and 70 °C (140 and 160 °F). Modified Winsteard’s reagent contains the addition of five drops of hydrochloric acid. As in the unmodified solution, effective etching is achieved if this reagent is used hot, between 60 and 70 °C (140 and 160 °F ). In

232 / Metallographer’s Guide using Winsteard’s reagent, if the unmodified solution does not work, try etching in a beaker placed in an ultrasonic cleaning device. If it still does not work, try using the modified version with five drops of hydrochloric acid. Major Uses. Winsteard’s reagent is used to delineate prior austenite grain boundaries, as shown in Fig. 8.29 and 8.30. Figure 8.29 represents a quenched and tempered MIL-S-23194 compo-

sition F low-alloy steel etched for 1.5 minutes in modified Winsteard’s reagent. The solution was heated to 65 °C (130 °F). The prior austenite grain boundaries in this quenched and tempered low-alloy plate are clearly seen. A similar example of etching in modified Winsteard’s reagent is shown in Fig. 8.30 for a water-quenched 0.27% C, 1.0% Mn, 1.02% Cr, 0.27% Mo steel. In this case, the solution was heated to 70 °C (160 °F).

Fig. 8.29 Prior austenite grain boundaries in a quenched and tempered MIL-S-23194 composition F-steel forging. Modified Winsteard’s

etch. 500⫻

Fig. 8.28 Prior austenite grain boundaries in a quenched 0.5% Mo-B

steel. (a) 200⫻ and (b) 500⫻. Boiling alkaline sodium picrate etch followed by 10 seconds in 2% nital etch and 20 seconds in 4% picral etch

Fig. 8.30 A water-quenched 0.27% C, 1.0% Mn, 1.02% Cr, 0.27% Mo

steel showing prior austenite grain boundaries. The dark bands are due to segregation in the bar. Modified Winsteard’s etch. 250⫻

The Art of Revealing Microstructure / 233

4% Picral and 2% Nital

Basic Tint Etchants for Carbon and Low-Alloy Steels and Cast Irons

Description. For etching specimens with ferrite plus pearlite microstructures, 4% picral (aged) and 2% nital can be mixed together in equal parts. In some cases, the specimen is used as a two-step etching procedure by first etching in 4% picral followed by 2% nital. After step one, rinse in water and alcohol immediately. After etching, the specimen should be rinsed in water and alcohol and dried in a warm air blast. Major Uses. For ferrite-pearlite steels and ferritic steels containing carbides, a combination etch using mixed 4% picral and 2% nital will delineate the boundaries between the carbides and ferrite and the ferrite grain boundaries. Figure 8.31 shows the results of the combination picral/nital etch used on an annealed and air-cooled bar of AISI/SAE 1018 steel. The ferrite grain boundaries are clearly shown, along with the regions of pearlite. A similar but coarser microstructure can be seen in an AISI/SAE 1018 steel bar in the cold-finished (cold worked at the surface) condition in Fig. 8.32. Note in this case, the ferrite grains are slightly elongated. It is interesting that the steel in Fig. 8.31 had a notch toughness absorbed energy value (Charpy test) of 156 J (115 ft · lbf), whereas the steel in Fig. 8.32 had a notch toughness absorbed energy value of only 9 J (7 ft · lbf). The lower value is due to the cold work in the latter steel bar. An example of using the two-step procedure is shown in Fig. 8.33. In this case, a water-quenched and tempered AISI/SAE 1020 steel was etched to bring up the carbides (4% picral) and the residual martensite lath boundaries (2% nital). A free-machining AISI/SAE 1213 steel bar was etched in 4% picral followed by 2% nital, as seen in Fig. 8.34. In this microstructure, there is ferrite plus pearlite banding, manganese sulfide inclusions, and a ferritic matrix. The two-part etch reveals all constituents.

microstructure. Equal parts 4% picral (aged) mixed with 2% nital etch. 200⫻

Fig. 8.31 Air-cooled AISI/SAE 1018 steel showing a ferrite plus pearlite

Fig. 8.33 Water-quenched and tempered AISI/SAE 1020 steel showing

microstructure. Equal parts 4% picral (aged) mixed with 2% nital etch. 200⫻

Some metallographic reagents react with the surface of the constituents in the microstructure to form what appear as films or tints. These reagents are grouped as tint etchants (Table 8.2). The surface is not physically altered, such as the attack etchants

Fig. 8.32 Cold-finished AISI/SAE 1018 steel showing ferrite plus pearlite

fine carbides and residual laths of martensite. 4% picral followed by 2% nital etch. 500⫻

234 / Metallographer’s Guide extremely important and must be properly prepared in order to ensure a uniform and effective chemical deposit. The chemically deposited film is very fragile and requires a different etching procedure. It is recommended that the following set of simple rules be followed for proper tint etching: • •

•

• Fig. 8.34 Free-machining AISI/SAE 1213 steel bar showing ferrite plus

pearlite banding, manganese sulfide inclusions, and a ferrite matrix. 4% picral followed by 2% nital etch. 320⫻

•

containing acids, but result in a chemically deposited film on the surface. They are very useful in distinguishing martensite in a mixed microstructure. They are also important in color metallography. The common tint etchants are aqueous solutions of sodium or potassium metabisulfite and/or sodium thiosulfate. It is the metabisulfite (S2O52⫺) and the thiosulfate (S2O32⫺) ions that are the active ingredients in the etchant. In aqueous or acidic solutions, the metabisulfite and thiosulfate ions decompose into sulfur dioxide, hydrogen sulfide, sulfur, and hydrogen. The presence of sulfur and the sulfur compounds provides the sulfur (S2⫺) ion that creates a sulfide film on the steel surface.

•

General Procedure in Using Tint Etchants

•

Tint etchants are very different than the attack etchants described previously. The surface condition of the specimen is

•

Table 8.2 Quick reference guide to common tint etchants for carbon steels, low-alloy steels, and cast irons Etchant

To reveal or enhance Bainite Granular bainite (martensite-austenite constituent) Martensite, as-quenched Retained austenite To color or darken Ferrite grains Ferrite, but not carbides Martensite (ferrite and retained austenite not attacked)

15% sodium metabisulfite 15% sodium metabisulfite 15% sodium metabisulfite 15% sodium metabisulfite Beraha’s reagent or Klemm’s reagent Beraha’s reagent 15% sodium metabisulfite

•

•

Rule 1: Always use a freshly mixed solution. Generally, tint etchants are not stored for future use. Rule 2: Always use a freshly polished surface. If no reaction is occurring on the specimen surface or if the reaction is spotty, the specimen may have been in the air too long, and a passive film has formed on the surface. To recover the surface, the specimen should be lightly pre-etched in 2% nital. After the pre-etch, immerse the specimen in water and then into the tint etching solution. Do not dry the specimen surface before immersion into the tint etch solution. Rule 3: When etching, always watch the surface of the specimen. It is important to obtain the proper color from tint etchant. Knowing the proper color comes with experience. Rule 4: Do not agitate the specimen in the tint etching solution. Movement of the specimen can disturb the fragile chemically deposited surface film. Rule 5: Do not swab the surface of the specimen while etching. Swabbing will damage and physically remove the chemically deposited surface film. Rule 6: If the specimen surface is underetched (undertinted), the metallographer must start over. Never put the specimen back into the solution without repolishing the surface. A quick repolish (less than ten seconds) with 0.3 μm aluminum oxide (alumina) is all that is necessary. Remember, this is a thin surface stain and it is not formed by chemical attack, as in the case of most etchants. Rule 7: Never flush the specimen surface with a stream of water from the tap. Rinse the specimen by gentle immersion in a beaker of water. Rule 8: Gently rinse the water from the specimen surface with a minimal amount of alcohol. Too much alcohol can change the characteristics of the stain. Rule 9: Never wipe the specimen dry. Gently blow dry the surface of the specimen. Rule 10: Always use distilled water with the pH adjusted to 2 to 4. Water above a pH level of 7 will not work.

The Common Tint Etchants Sodium Metabisulfite Ingredients: 8 to 20 g sodium metabisulfite Dilute in 100 mL water Description. This is a general-purpose tint etchant. Sodium metabisulfite dissolves easily in water. A range of the amount of sodium metabisulfite is given, depending on the desired staining response. The greater the amount of sodium metabisulfite, the darker the stain on the specimen surface. Before using this

The Art of Revealing Microstructure / 235 etchant, it is advisable to pre-etch the specimen in 2% nital to remove a passive layer that may have formed during exposure of the specimen surface to the atmosphere. If not preetched, the chemical deposit (tint) that forms on the surface during etching in this reagent may not be uniform. Etching time can range from ten seconds to one minute. Major Uses. This simple tint etchant is used to darken martensite and highlight prior austenite grain boundaries in quenched steels. Figure 8.35 shows the use of 15% sodium metabisulfite for detecting untempered martensite in a quenched Jominy bar (6.4 mm, or 0.25 in., from the quenched end) of AISI/SAE 8860 steel. In this micrograph, the untempered martensite appears as gray needles. Sodium metabisulfite is also very useful in differentiating martensite from ferrite and retained austenite. It has been used for dual-phase steels that consist of patches of martensite and epitaxial ferrite in a ferrite matrix. Figure 8.36 shows an example of a dual-phase steel etched with a 15% sodium metabisulfite tint etch. Note that the martensite patches are stained (brown if shown in color) and the ferrite is untouched. This etch is also useful for highlighting the prior austenite grain boundaries in a lath martensite microstructure. Sometimes a 15% sodium metabisulfite solution can be used to tint multiple constituents. Figure 8.37 shows a mixture of ferrite, pearlite (dark constituent), bainite, and martensite in a waterquenched Jominy bar of boron-treated AISI/SAE 10B36 steel.

Description. There are a number of tint etchants developed by Dr. E. Beraha (see Selected References at the end of this chapter). This particular formulation contains both sodium and potassium metabisulfite. The potassium and sodium metabisulfite crystals dissolve easily during stirring of the water. Major Uses. This tint etchant darkens ferrite, as shown in Fig. 8.38. Note that the ferrite in the pearlite colonies is dark, and the cementite is unaffected. Figure 8.39 shows a very-low-carbon

Beraha’s Reagent Ingredients: 3 g potassium metabisulfite 10 g sodium metabisulfite Dilute to 100 mL water

Fig. 8.35 Quenched AISI/SAE 8860 steel bar showing a mixture of

tempered and untempered martensite (gray needles). Sodium metabisulfite tint etch. 1000⫻

Fig. 8.36 Epitaxial ferrite in a dual-phase steel. The epitaxial ferrite is

surrounding regions of martensite (dark-appearing constituent) (see arrows). Matrix is ferrite. Sodium metabisulfite tint etch. 1000⫻

Fig. 8.37 The mixed microstructure from a Jominy specimen of boron-

treated AISI/SAE 10B36 steel showing pearlite (dark), bainite, ferrite, and martensite. 15% sodium metabisulfite etch. 1000⫻

236 / Metallographer’s Guide motor lamination steel sheet tinted with Beraha’s reagent. The structure consists of columnar grains of ferrite growing from each surface, with equiaxed grains of ferrite in the central region of the sheet. Each ferrite grain has a different darkness, depending on grain orientation.

Klemm’s Reagent Ingredients: Part A: Sodium thiosulfate (enough for saturation) 50 mL water Part B: 1 g potassium metabisulfite Description. This etch requires a saturated solution of sodium thiosulfate. This is produced by adding crystals of sodium thiosulfate to 50 mL of water. The crystals are very soluble, and to develop saturation, a fairly large quantity of sodium thiosulfate is necessary. Because of this, the volume of the solution increases, and a large beaker, for example, 250 ml, should be used to contain the increased volume. As opposed to the exothermic reaction described earlier for adding sodium hydroxide to water, adding sodium thiosulfate to water results in an endothermic reaction, and the solution becomes cold to the touch. As the solution gets colder, it takes time to add the large amount of crystals. It is recommended that smaller portions be added during stirring. When these smaller quantities are dissolved, add more until no more can be dissolved (there will be crystals at the bottom of the beaker). One gram of potassium metabisulfite is then added to the saturated solution.

Fig. 8.38 Ferrite is darkened in a pearlitic steel, whereas cementite is unaffected. Beraha’s reagent. 1000⫻

Major Uses. Klemm’s reagent tints ferrite grains with a red and blue color.

The Basic Etchants for Stainless Steels Because stainless steels contain a minimum of 11% Cr for corrosion resistance, they are more difficult to etch than carbon and low-alloy steels. The stainless steel attack etchants contain a higher concentration of acid in order to obtain the proper etching response. Tint etchants can also be used for some stainless steels to darken certain constituents in the microstructure. In addition to the attack and tint etchants, one of the most effective etching procedures for stainless steels is through the use of an electrolytic cell. The electrolytic etching process is quite simple and effective and is described in this section. Stainless steels are generally grouped according to the matrix constituent, that is, ferritic, austenitic, and martensitic stainless steel. Some stainless steels have a duplex matrix of ferrite and austenite and are classed as duplex stainless steels. Another group are the precipitation-hardenable stainless steels where elements such as niobium, titanium, and molybdenum are added to form a precipitate in an austenitic or martensitic matrix. Stainless steels differ from the carbon and low-alloy steels in that they have second phases that are called delta ferrite and sigma phase, and the carbides that form are alloy carbides, such as M3C, M23C6, and M6C, where M (metal) represents iron, chromium, molybdenum, and so on. Table 8.3 is a quick reference guide to etchants for stainless steels.

Fig. 8.39 Very-low-carbon motor lamination steel sheet showing colum-

nar grains growing from the sheet surface and equiaxed ferrite grains in the center. Beraha’s reagent. 75⫻

The Art of Revealing Microstructure / 237

Attack Etchants for Stainless Steels

hydrochloric acid to the solution while stirring (a magnetic stirrer is ideal for stirring). Major Uses. Vilella’s reagent is used to etch the martensite in martensitic and precipitation-hardenable stainless steels. This etchant also outlines and enhances delta ferrite, sigma phase, and carbides.

Vilella’s Reagent Ingredients: 1 g picric acid 5 mL hydrochloric acid (concentrated) 95 mL ethyl alcohol Description. This etchant has been described previously carbon and low-alloy steels. However, it is also very useful etching stainless steels that has been sensitized. First, add picric acid to the alcohol while stirring. Then slowly add

for for the the

Table 8.3 Quick reference guide to common etchants for stainless steels Etchant

To reveal or enhance General microstructure (unknown beforehand) Austenite grains (austenitic stainless steel) Ferrite grains (ferritic stainless steel) Martensite structure Carbides Carbides, sensitized Sigma phase Delta ferrite To color or darken Austenite Carbides and not ferrite Sigma phase Martensite

Vilella’s reagent Glyceregia or aqua regia Kalling’s reagent Vilella’s or Fry’s or Kalling’s reagent Vilella’s reagent or glyceregia 4% picral ⫹ HCl or Vilella’s reagent Vilella’s or glyceregia Vilella’s reagent Lichtenegger/Bloech reagent Murakami’s reagent Murakami’s reagent Kalling’s reagent

Lichtenegger/Bloech Reagent Ingredients: 20 g ammonium bisulfide 0.5 g potassium bisulfite 100 mL water Description. After dissolving the two dry chemicals in 100 mL of water, heat the solution to 60 to 80 °C (140 to 175 °F). Etching time is between one and five minutes. Major Uses. This etchant darkens austenite in duplex stainless steel. An example is shown in Fig. 8.40(a) and (b) for a duplex AISI 2205 stainless steel. In these micrographs, the austenite phase is darkened and the ferrite phase is unaffected.

Aqua Regia Ingredients: 45 mL hydrochloric acid (concentrated) 15 mL nitric acid (concentrated) Description. These two strong acids are carefully mixed together while stirring. The process must take place in a wellventilated hood, and eye protection, acid-resistant lab coat, and

Fig. 8.40 An AISI 2205 duplex stainless steel showing austenite (dark constituent) and ferrite. Lichtenegger/Bloech reagent. (a) 500⫻ and (b) 1000⫻

238 / Metallographer’s Guide latex gloves must be worn. There are various formulations for aqua regia. One is a dilute form that consists of 15 mL hydrochloric acid, 5 mL nitric acid, and 100 mL of water. Major Uses. This powerful etchant is used for austenitic stainless steels to reveal the austenite grain structure and to enhance other phases, such as ferrite and sigma phase.

Glyceregia

40 mL hydrochloric acid (concentrated) 25 mL ethyl alcohol 30 mL water Description. These ingredients are mixed in reverse order of the previously mentioned list. Carefully mix the water, alcohol, and hydrochloric acid during stirring. Add the cupric chloride crystals to the solution while continuously stirring the mixture.

Ingredients: 3 parts glycerol 2 to 5 parts hydrochloric acid (concentrated) 1 part nitric acid (concentrated) Description. This is a general-purpose etchant for stainless steels. Because of the amounts of acid used, this etchant must be mixed inside a well-ventilated hood, and proper safety equipment must be worn by the metallographer. The solution should be mixed in the order given previously. Always use a freshly mixed solution for etching. Once discolored, it should be discarded. Never store glyceregia. Major Uses. Glyceregia attacks sigma phase and enhances carbides in stainless steels.

Kalling’s Reagent Ingredients for Kalling’s Reagent #1: 33 mL hydrochloric acid (concentrated) 33 mL ethyl alcohol 1.5 g cupric chloride 33 mL water Ingredients for Kalling’s Reagent #2: 100 mL hydrochloric acid (concentrated) 5 g cupric chloride 100 mL ethyl alcohol

Fig. 8.41 Lath martensite in a maraging steel (18% Ni). Kalling’s reagent #1. 100⫻

Description. These reagents are mixed in reverse order to the ingredients listed. Carefully mix the water, alcohol, and hydrochloric acid during stirring. Add the cupric chloride crystals to the solution while continuously stirring the mixture. Major Uses. Kalling’s reagents are used for etching martensitic stainless steels. The martensite is colored dark. Figure 8.41 shows an 18% Ni maraging steel etched in Kalling’s reagent #2. The microstructure is lath martensite, and the etchant darkened the packets of the martensite, depending on orientation. An example of a precipitation-hardening stainless steel (Custom 630) etched in Kalling’s #2 reagent is shown in Fig. 8.42. In general, austenite is not attacked by reagent #1 but is slightly attacked by reagent #2, and ferrite is attacked by reagent #2 and slightly colored by reagent #1. Neither reagent will attack carbides.

Fry’s Reagent Ingredients: 5 g cupric chloride

Fig. 8.42 Lath martensite in a precipitation-hardening stainless steel (Custom 630). Kalling’s reagent #2. 200⫻

The Art of Revealing Microstructure / 239 Major Uses. Fry’s reagent is used for etching martensitic and precipitation-hardenable stainless steels. The martensite is colored dark. An example of Fry’s reagent used to etch a precipitation hardening stainless steel (Custom 630) is shown in Fig. 8.43.

Picral Plus Hydrochloric Acid Ingredients: 4 g picric acid 96 mL ethyl alcohol 5 drops hydrochloric acid 5 drops zephiran chloride Description. A few drops of hydrochloric acid added to picral provides more effective etching response. Major Uses. Picral plus hydrochloric acid can be used to outline the grain and twin boundaries of a sensitized stainless steel. An example is shown in Fig. 8.44 representing an AISI/SAE 316 stainless steel. This etch only works because of the chromium carbides precipitated at the grain and twin boundaries. Fig. 8.44 Grain and twin boundaries in a sensitized AISI/SAE 316 auste-

Electrolytic Etchants for Stainless Steels In addition to the many attack and tint etchants listed previously for stainless steels, there are a number of etchants that are used with the electrolytic etching method. Many metallographers, including the authors, find the electrolytic etching process for stainless steels the easiest and most effective way of etching most stainless steel specimens. Electrolytic etching is similar to the electropolishing process described in Chapter 7. The process is rather simple in that the specimen becomes the anode (⫹ electrode) and metal ions are

nitic stainless steel. 4% picral plus hydrochloric acid etch.

250⫻

conducted away from the anode into the electrolyte solution, that is, the etching solution. A simple electrolytic cell is shown in Fig. 7.44 in Chapter 7. A simple battery with a direct current (DC) is used to activate the etching process. The metallographer does not need expensive and elaborate equipment to set up a cell. Table 8.4 is a quick reference guide to the common electrolytic etchants for stainless steels. Many more etchants are listed in the Appendix. The electrolytic etchants are described in detail subsequently. These are generally very simple solutions of a chemical component in water. The voltage is usually given with each etching solution.

Oxalic Acid Ingredients: 10 g oxalic acid 100 mL water Use 6 to 8 volts DC for 25 to 30 seconds—stainless steel cathode

Fig. 8.43 Lath martensite in a precipitation-hardening stainless steel (Custom 630). Fry’s reagent. 250⫻

Description. This is a general-purpose electrolyte solution for etching stainless steels. The oxalic acid is added to the water while stirring. Major Uses. This oxalic acid solution is widely used and reveals austenite, austenite grain boundaries, carbides, and sigma phase. It can also be used in heavily cold-worked stainless steels, as seen in Fig. 8.45. This example is a cold-worked AISI/SAE 301 stainless steel. Differential interference contrast (Nomarski) illumination has been used to highlight the microstructure.

240 / Metallographer’s Guide

Hydrochloric Acid Ingredients: 5 mL hydrochloric acid (concentrated) 95 mL methyl alcohol Use 1.5 to 5 volts DC and stainless steel cathode

Description. Carefully add the hydrochloric acid to the methyl alcohol while stirring. Care must be taken in handling hydrochloric acid, and latex gloves, lab coat, and eye protection must be worn. Major Uses. At low voltages (about 1.5 volts DC), this solution is used to attack sigma phase. At higher voltages (3 to 5 volts DC), it is used for general structure.

Nitric Acid (60/40) Table 8.4 Quick reference guide to common electrolytic etchants for stainless steels Etchant

To reveal or enhance General microstructure (unknown beforehand) Austenite grains (austenitic stainless steel) Ferrite grains (ferritic stainless steel) Prior austenite grain boundaries Martensite structure Carbides Sigma phase

Ingredients: 60 mL nitric acid (concentrated) 40 mL water Use 5 volts DC for 45 seconds with stainless steel or platinum cathode

Oxalic acid solution Oxalic acid or nitric acid (60/40) Nitric acid (60/40) Oxalic acid solution

Delta ferrite Duplex structure

Oxalic acid Oxalic acid, hydrochloric acid or potassium hydroxide solution Nitric acid (60/40) Potassium hydroxide solution

To color or darken Ferrite Sigma phase Delta ferrite

Potassium hydroxide solution Potassium hydroxide solution Potassium hydroxide

Fig. 8.45 Heavily cold-worked AISI/SAE 301 austenitic stainless steel.

Electrolytically etched in 10% oxalic acid solution using a stainless steel cathode at 8 V. Enhanced by differential interference contrast illumination (also called Nomarski illumination). 500⫻

Description. This is a strongly acid electrolyte solution for etching stainless steels. The nitric acid is added to the water while stirring. Latex gloves, lab coat and eye protection must be worn when handling nitric acid. Major Uses. With a stainless steel cathode, nitric acid is used to reveal the general structure of austenitic stainless steels. It is interesting that by using a platinum cathode, austenite grains are revealed without etching twin boundaries, as seen in the austenitic AISI/SAE 316 stainless steel in Fig. 8.46. This is important for grain size measurements using image analysis. However, using a stainless steel cathode, both the austenite grain boundaries and twin boundaries are revealed. Delta ferrite can be revealed with this solution, as seen in Fig. 8.47, which represents an austenitic AISI/SAE 316 stainless steel. In this example, the delta ferrite was

Fig. 8.46 Annealed AISI/SAE 316 austenitic stainless steel showing grain

boundaries but the absence of twins. Ideal for grain size measurements by image analysis. Electrolytically etched with 60% nitric acid and 40% water using a platinum cathode at 5 V. 500⫻

The Art of Revealing Microstructure / 241 revealed with a short etching time of ten seconds. Longer etching times would begin to reveal the austenite grains.

Potassium Hydroxide Ingredients: 56 g potassium hydroxide

100 mL water Use 1.5 to 3 volts DC for three seconds with a stainless steel cathode Description. Potassium hydroxide produces a very caustic solution and must be mixed and handled carefully. Use latex gloves, lab coat, and eye protection while mixing. Mixing and etching should take place inside a well-ventilated hood. Major Uses. This solution colors sigma phase and ferrite. An example is seen in Fig. 8.48, which represents a 2205 duplex stainless steel. The sigma phase is the dark etching constituent.

The Basic Etchants for Coated Steels Amyl Nital When etching coatings such as hot dipped and electrogalvanized steels, amyl alcohol is substituted for ethyl alcohol in the standard nital etch. Substituting amyl alcohol for ethyl alcohol provides a slower or moderating etching response. The same mixing procedure as with nital is recommended. Ingredients: 1 mL concentrated nitric acid 99 mL of amyl alcohol

Fig. 8.47 Delta ferrite stringers in an AISI/SAE 316 austenitic stainless

steel. Electrolytically etched with 60% nitric acid and 40% water using a stainless steel cathode at 10 V. 500⫻

Description. Add the nitric acid to the amyl alcohol while stirring. Major Uses. This solution is used to bring out the microstructure of zinc coatings on steel. An example of a hot dipped Galvalume-coated steel (BHP Corporation) etched in 1% amyl nital is seen in Fig. 8.49. The etchant usually darkens the alloy layer between the steel and coating.

Rowland’s Reagent Ingredients: Part A: 0.75 g picric acid 130 mL ethyl alcohol Part B: 480 mL water Description. This water-based reagent is mixed in two parts, because picric acid is very difficult to dissolve in water. Thus, the first step involves dissolving 0.75 g of picric acid in 130 mL of ethyl alcohol (picric acid is soluble in ethyl alcohol) while stirring. In the second step, the picric acid solution is mixed with the 480 mL of water. Major Uses. This etchant is used for zinc coatings on steel.

Special Etching Procedures Fig. 8.48 Duplex AISI/SAE 2205 stainless steel showing delta phase (dark)

and ferrite. Electrolytically etched with potassium hydroxide solution using a stainless steel cathode at 3 V. 200⫻

In all the previous examples of etchants, the specimen was immersed in the etching solution. However, there is a special

242 / Metallographer’s Guide technique where the specimen is suspended above the solution and the fumes etch the specimen surface. This technique is used when the specimen will be overly attacked by immersion in the etchant. An example is illustrated here for etching the outside surface of a Galvalume-coated wire. The wire is suspended above nitric acid heated to 85 °C (185 °F) for one to five minutes, then rinsed in flowing water, followed by an alcohol rinse. The results can be seen in Fig. 8.50. In this micrograph, the central region of a zinc alloy crystal (called a spangle) is shown in dendritic form. Figure 8.51 shows a similar area, as seen in the scanning electron microscope.

clearly delineated, as seen in Fig. 8.52. The DIC shows the ferrite grains with surface features that are due to deformation bands created by the cold work. The annealed and air-cooled sample examined with DIC is seen in Fig. 8.53. In this case, all signs of cold working are absent, and the DIC reveals the step relief between the ferrite grains. Differential interference contrast can

The Use of the Microscope to Enhance Microstructural Features As discussed in Chapter 5, different methods of illumination of light can be used to highlight certain features in a microstructure. Two of the most important methods are diffferential interference contrast and dark-field illumination. Examples are shown subsequently, and details of the methods can be found in Chapter 5. Differential Interference Contrast (Nomarski). Although not a chemical etching procedure, differential interference contrast (DIC) has been called “optical etching.” It is generally used in conjunction with attack-etching to further emphasize microstructural features. For example, earlier in this Chapter, examples were shown in Fig. 8.31 and 8.32 of an AISI/SAE 1018 steel etched in 4% picral plus 2% nital. One specimen (Fig. 8.31) was in the annealed and air-cooled condition, whereas the other specimen (Fig. 8.32) contained cold work. Although the ferrite grains in the latter specimen indicated cold work, the cold work itself was not obvious. However, if examined under DIC, the cold work can be

Fig. 8.50 The dendritic structure of a zinc-aluminum alloy spangle (Gal-

valume) on a steel wire. Specimen etched by suspending over fuming nitric acid. 200⫻

Fig. 8.51 Scanning electron microscope micrograph of a similar dendritic

Fig. 8.49 Hot dipped Galvalume coating on steel. 1% amyl nital. 1000⫻

spangle shown in Fig. 8.50. Specimen etched by suspending over fuming nitric acid. 200⫻

The Art of Revealing Microstructure / 243 also be used on unetched specimens. For example, Fig. 8.54 shows a slag particle entrapped in a stainless steel weld. The structural features of the slag particle itself are clearly visible along with features of the stainless steel. Dark-Field Illumination. As discussed in Chapter 5, dark-field illumination can be a powerful tool in revealing microstructure.

Figures 8.55(a) and (b) compare bright-field to dark-field illumination in a low-carbon steel with an enamel coating (the enamel coating is shown at higher magnification in Fig. 8.56). The specimen has been etched in 2% nital. The dark-field micrograph (Fig. 8.55b) shows the ferrite grain boundaries much more clearly than the bright-field micrograph (Fig. 8.55a).

Fig. 8.52 Differential interference contrast used to reveal cold work in an AISI/SAE 1018 steel (the same sample shown in Fig. 8.32). 4% picral followed by 2% nital etch. 1000⫻

Fig. 8.53 Differential interference contrast used for an air-cooled AISI/SAE

1018 steel (the same sample shown in Fig. 8.31). 4% picral followed by 2% nital etch. 1000⫻

Fig. 8.54 An entrapped slag particle in a stainless steel weld. Differential

interference contrast used to delineate the structure of the inclusion and surrounding stainless steel matrix. Unetched. 500⫻

244 / Metallographer’s Guide

Fig. 8.55 Enamel coating on a low-carbon steel. (a) Bright-field illumination and (b) dark-field illumination. Note the clear delineation of the ferrite grain boundaries in the dark-field image. 2% nital. 100⫻

SELECTED REFERENCES • • • •

Fig. 8.56 Unique microstructure of the faults in the enamel coating shown in Fig. 8.55. 500⫻

E. Beraha and B. Shpigler, Color Metallography, American Society for Metals, 1977 Metallography and Microstructures, Vol 9, ASM Handbook, American Society for Metals, 1985 G. Petzow, Metallographic Etching, 2nd ed., ASM International, 1999 G.F. Vander Voort, Metallography: Principles and Practice, McGraw-Hill, 1984, reprinted by ASM International, 1999

Metallographer's Guide: Practices and Procedures for Irons and Steels Bruce L. Bramfitt, Arlan O. Benscoter, p245-296 DOI:10.1361/mgpp2002p245

Copyright © 2002 ASM International® All rights reserved. www.asminternational.org

CHAPTER 9

Glossary A Å, A., A.U., or ÅU. See angstrom unit. Acm, A1, A3, A4. Same as Aecm, Ae1, Ae3, and Ae4. aberration. Any error that causes image degradation. Such an error may be chromatic, spherical, astigmatic, or comatic and can result from design, execution, or both. See also astigmatism, chromatic aberration, coma, and spherical aberration. abrasion. The process of grinding or wearing away through the use of abrasives; a roughening or scratching of a surface due to abrasive wear. abrasion rate. The rate at which material is removed from a surface during abrasion. It is usually expressed in terms of the thickness removed per unit of time or distance traversed. abrasive. A substance capable of removing material from another substance in machining, abrasion, or polishing that usually takes the form of several small, irregularly shaped particles of hard material. abrasive belt. A coated abrasive product, in the form of a belt, used in production grinding and polishing. abrasive disk. (1) A grinding wheel that is mounted on a steel plate, with the exposed flat side being used for grinding. (2) A disk-shaped, coated abrasive product. abrasive paper. A coated abrasive product in which a paper is used as the backing sheet. abrasive wear. The removal of material from a surface when hard particles slide or roll across the surface under pressure. The particles may be loose or may be part of another surface in contact with the surface being abraded. Compare with adhesive wear. Accm. In hypereutectoid steel, the temperature at which the solution of cementite in austenite is completed during heating. Ac1. The temperature at which austenite begins to form during heating. Ac3. The temperature at which transformation of ferrite to austenite is completed during heating. Ac4. The temperature at which austenite transforms to delta ferrite during heating. accelerating potential. A relatively high voltage applied between the cathode and anode of an electron gun to accelerate electrons. The source of electrons in the electron microscope. achromatic. Free of color. A lens or objective is achromatic when corrected for longitudinal chromatic aberration for two colors. See also achromatic objective.

achromatic objective. Objectives are achromatic when corrected chromatically for two colors, generally red and green and spherically for light of one color, usually in the yellow-green portion of the spectrum. achromatic objective lens. An objective lens with longitudinal chromatic correction for green and blue, and spherical chromatic correction for green. Note: Lens should be used with a green filter. acicular ferrite. A highly substructured nonequiaxed ferrite formed upon continuous cooling by a mixed diffusion and shear mode of transformation that begins at a temperature slightly higher than the transformation temperature range for upper bainite. It is distinguished from bainite in that it has a limited amount of carbon available. acid. A chemical substance that yields hydrogen ions (H⫹) when dissolved in water. Compare with base. acid embrittlement. A form of hydrogen embrittlement that may be induced in some metals by acid treatment. acid extraction. Removal of phases by dissolution of the matrix metal in an acid or other corrosive solution. activation. The changing of a passive surface of a metal to a chemically active state. Contrast with passivation. activity. A measure of the chemical potential of a substance, where chemical potential is not equal to concentration, that allows mathematical relations equivalent to those for ideal systems to be used to correlate changes in an experimentally measured quantity with changes in chemical potential. adhesion. Force of attraction between the molecules (or atoms) of two different phases. Contrast with cohesion. adhesive bonding. A materials joining process in which an adhesive, placed between faying surfaces, solidifies to bond the surfaces together. adhesive wear. The removal or displacement of material from a surface by the welding together and subsequent shearing of minute areas of two surfaces that slide across each other under pressure. Compare with abrasive wear. Aecm, Ae1, Ae3, Ae4. The temperatures of phase changes at equilibrium. age hardening. Hardening by aging, usually after rapid cooling or cold working. See also aging. age softening. A decrease of strength and hardness that takes place at room temperature in certain strain-hardened alloys. aging. A change in properties that occurs at ambient or moderately elevated temperatures after hot working, heat treating, or

246 / Metallographer’s Guide cold working (strain aging). The change in properties is often due to a phase change (precipitation), but does not alter chemical composition. See also age hardening, artificial aging, interrupted aging, natural aging, overaging, precipitation hardening, precipitation heat treatment, progressive aging, quench aging, step aging, and strain aging. air-hardening steel. A steel containing sufficient carbon and other alloying elements so as to harden fully during cooling in air or other gaseous media from a temperature above its transformation range. This term should be restricted to steels that are capable of being hardened by cooling in air in fairly large sections, about 50 mm (2 in.) or more in diameter. Same as self-hardening steel. Airy disk. The image of a bright point object, as focused by a lens system. With monochromatic light, it consists of a central point of maximum intensity surrounded by alternate circles of light and darkness caused by the reinforcement and interference of diffracted rays. The light areas are called maxima and the dark areas minima. The distribution of light from the center to the outer areas of the figure was investigated mathematically by Sir George Airy. The diffraction disk forms a basis for determining the resolving power of an ideal lens system. The diameter of the disk depends largely on the aperture of the lens. The diffraction of light causing the Airy disk is a factor limiting the resolution of a well-corrected optical system. alignment. A mechanical or electrical adjustment of the components of an optical device so that the path of the radiating beam coincides with the optical axis or other predetermined path in the system. See also magnetic alignment, mechanical alignment, and voltage alignment. alkali metal. A metal in group IA of the periodic system— namely, lithium, sodium, potassium, rubidium, cesium, and francium. They form strongly alkaline hydroxides, hence the name. alkaline cleaner. A material blended from alkali hydroxides and such alkaline salts as borates, carbonates, phosphates, or silicates. The cleaning action may be enhanced by the addition of surface-active agents and special solvents. alkaline earth metal. A metal in group IIA of the periodic system—namely, beryllium, magnesium, calcium, strontium, barium, and radium—so called because the oxides or “earths” of calcium, strontium, and barium were found by the early chemists to be alkaline in reaction. alligatoring. The longitudinal splitting of flat slabs in a plane parallel to the rolled surface. Also called fishmouthing. alligator skin. See orange peel. allotriomorph. A particle of a phase that has no regular external shape. allotriomorphic crystal. A crystal whose lattice structure is normal, but whose outward shape is imperfect, because it is determined to some extent by the surroundings; the grains in a metallic aggregate are allotriomorphic crystals. allotropy. The property by which certain elements may exist in more than one crystal structure. See also polymorphism. Iron exists as body-centered cubic up to 910 °C (1670 °F), facecentered cubic between 910 and 1400 °C (1670 and 2550 °F),

and body-centered cubic above 1400 °C (2550 °F) until melting. alloy. A substance having metallic properties and composed of two or more chemical elements of which at least one is metal. alloying element. An element added to and remaining in a metal that usually changes structure and properties. alloy steel. Steel containing significant quantities of alloying elements (other than carbon and the commonly accepted amounts of manganese, silicon, sulfur and phosphorus) added to effect changes in mechanical or physical properties. Those containing less than 8% total metallic alloying elements tend to be termed low-alloy steels, and those containing more than 8% tend to be termed high-alloy steels. alloy system. A complete series of compositions produced by mixing in all proportions any group of two or more components, at least one of which is a metal. alpha ferrite. See ferrite. alpha iron. Solid phase of pure iron that is stable below 910 °C (1670 °F), possesses the body-centered cubic lattice, and is ferromagnetic below the Curie temperature of 768 °C (1415 °F). alpha stabilizer. An alloying element that dissolves preferentially in the alpha phase and raises the alpha-gamma transformation temperature. In iron alloys, silicon, chromium, aluminum, and titanium are alpha stabilizers. alternate etch and polish technique. A polishing technique in which the polished surface is etched at intervals during polishing, each intervening polishing stage ideally being continued for just long enough to remove the effects of the preceding etch. This technique is commonly used to increase the material removal rate, an objective for which it is not very appropriate. It may also be employed to reduce the relief developed between phases during polishing, by use of an etchant that attacks preferentially the slower-polishing phase. This use is an appropriate one. aluminizing. Forming of an aluminum or aluminum alloy coating on a metal by hot dipping, hot spraying, or diffusion. ambient. Surrounding; usually used in relation to temperature, as “ambient temperature” surrounding a certain part or assembly. amorphous. Not having a crystal structure; noncrystalline. amphoteric. Possessing both acidic and basic properties. amplifier. A negative lens used instead of an eyepiece to project under magnification the image formed by an objective. The amplifier is designed for flatness of field and should be used with an apochromatic objective. analyzer. An optical device capable of producing plane-polarized light. It is used for detecting the effect of the object on plane-polarized light produced by the polarizer. angle of reflection. (1) Reflection: the angle between the reflected surface. See also normal. (2) Diffraction: the angle between the diffracted beam and the diffracting planes. angstrom unit. A unit of linear measure equal to 10-10 m, or 0.1 nm. 1 μm ⫽ 10,000 Å. Although not an accepted SI unit, it is occasionally used for small distances, such as interatomic distances, and some wavelengths. Various abbreviations include Å, A., A.U., Å., or ÅU.

Glossary / 247 angular aperture. In optical microscopy, the angle between the most divergent rays that can pass through a lens to form the image of an object. anion. A negatively charged ion; it flows to the anode in electrolysis. anisotropy. Characterized by having different values of a property in different directions. annealing. A generic term denoting a treatment—heating to and holding at a suitable temperature followed by cooling at a suitable rate—used primarily to soften metallic materials, but also to produce desired changes simultaneously in other properties or in microstructure. When applied only for the relief of stress, the process is called stress-relieving or stress-relief annealing. In ferrous alloys, annealing is carried out above the upper critical temperature, but the time-temperature cycles vary widely in maximum temperature attained and cooling rate used, depending on composition, material condition, and desired results. See also black annealing, blue annealing, box annealing, bright annealing, cycle annealing, flame annealing, graphitizing, isothermal annealing, malleabilizing, process annealing, quench annealing, spheroidizing, and subcritical annealing. In nonferrous alloys, annealing cycles are designed to remove part or all the effects of cold working (recrystallization may or may not be involved); cause complete coalescence of precipitates from the solid solution in relatively coarse form; or both, depending on composition and material condition. See also intermediate annealing, recrystallization annealing, and stress relieving. annealing carbon. See temper carbon. annealing twin. A twin formed in a crystal during recrystallization. Twins are commonly formed in austenitic stainless steels. annealing twin bands. See twin bands. anode. The electrode where electrons leave an operating system such as a battery, an electrolytic cell, an x-ray tube, or a vacuum tube. In the first of these, it is negative; in the other three, positive. In a battery or electrolytic cell, it is the electrode where oxidation occurs. Contrast with cathode. anode aperture. In electron microscopy, the opening in the accelerating voltage anode shield of an electron gun through which the electrons must pass to illuminate or irradiate the specimen. anodic etching. Development of microstructure by selective dissolution of the polished surface under application of a direct current. Variation with layer formation: anodizing. anodic protection. Imposing an external electrical potential to protect a metal from corrosive attack. (Applicable only to metals that show active-passive behavior.) Contrast with cathodic protection. aperture, effective. The diameter of the entrance pupil; it is the apparent diameter of the limiting aperture measured from the front. aperture, electron. See anode aperture, condenser aperture, and physical objective aperture. aperture, light. In light (optical) microscopy, the working diameter of a lens or a mirror. See also angular aperture. aplanatic. Corrected for spherical aberration and coma.

apochromatic objective. Objectives corrected chromatically for three colors and spherically for two colors are called apochromatic. These corrections are superior to those of the achromatic series of lenses. Because apochromats are not well corrected for lateral color, special eyepieces are used to compensate. See also achromatic. apparent density. (1) The weight per unit volume of a metal powder, in contrast to the weight per unit volume of the individual particles. (2) The weight per unit volume of a porous solid, where the unit volume is determined from external dimensions of the mass. Apparent density is always less than the true density of the material itself. Arcm. In hypereutectoid steel, the temperature at which precipitation of cementite starts during cooling. Ar1. The temperature at which transformation of austenite to ferrite or to ferrite plus cementite is completed during cooling. Ar3. The temperature at which austenite begins to transform to ferrite during cooling. Ar4. The temperature at which delta ferrite transforms to austenite during cooling. arc cutting. A group of cutting processes that melts the metals to be cut with the heat of an arc between an electrode and the base metal. See also metal-arc cutting. arrest. That portion of a cooling curve in which temperature is invariant with time. For example, thermal or eutectic arrest. arrest lines (marks). See beach marks. artifact. A feature of artificial character, such as a scratch or a piece of dust on a metallographic specimen, that can be erroneously interpreted as a real feature. See also mounting artifact and polishing artifact. artificial aging. Aging above room temperature. Compare with natural aging. aspect ratio. In metallography, usually the ratio of the length to width, length to thickness, or width to thickness of a grain, or the average ratio of many grains. The grain aspect ratio is measured on the longitudinal, transverse, or normal planes of material that is usually rolled or forged. asperity. In tribology, a protuberance in the small-scale topographical irregularities of a solid surface. astigmatism. A defect in a lens or optical system that causes rays in one plane parallel to the optical axis to focus at a distance different from those in the plane at right angles to it. ASTM grain size number. See grain size. athermal. Not isothermal. Changing rather than constant temperature conditions. athermal transformation. A reaction that proceeds without benefit of thermal fluctuations, that is, thermal activation is not required. Such reactions are diffusionless and can take place with great speed when the driving force is sufficiently high. For example, many martensitic transformations occur athermally upon cooling, even at relatively low temperatures, because of the progressively increasing driving force. In contrast, a reaction that occurs at constant temperature is an isothermal transformation; thermal activation is necessary in this case, and the reaction proceeds as a function of time. atomic number. The number of protons in an atomic nucleus,

248 / Metallographer’s Guide which determines the individuality of the atom as a chemical element. atomic percent. The number of atoms of an element in a total of 100 representative atoms of a substance. atomization. The dispersion of a molten metal into small particles by a rapidly moving stream of gas or liquid. attack-polishing. Simultaneous etching and mechanical polishing by adding a weak etching solution to the polishing compound. attritious wear. Wear of abrasive grains in grinding such that the sharp edges gradually become rounded. A grinding wheel that has undergone such wear usually has a glazed appearance. Auger electron spectroscopy (AES). A technique for chemical analysis of surface layers that identifies the atoms present in a layer by measuring the characteristic energies of their Auger electrons. ausforming. Hot deformation of metastable austenite within controlled ranges of temperature and time that avoids formation of nonmartensitic transformation products. austempering. Cooling (quenching) an austenitized steel at a rate high enough to suppress formation of high-temperature transformation products, then holding the steel at a temperature below that for pearlite formation and above that for martensite formation until transformation to an essentially bainitic structure is complete. austenite. Generally, a solid solution of one or more alloying elements in a face-centered cubic polymorph of iron (gamma iron). Specifically, in carbon steels, the interstitial solid solution of carbon in gamma iron. austenitic grain size. The size attained by the grains in steel when heated to the austenitic region. This may be revealed by appropriate etching of cross sections after cooling to room temperature. austenitic steel. An alloy steel whose structure is normally austenitic at room temperature. austenitizing. Forming austenite by heating a ferrous alloy into the transformation range (partial austenitizing) or above the transformation range (complete austenitizing). autoradiography. An inspection technique in which radiation spontaneously emitted by a material is recorded photographically. The radiation is emitted by radioisotopes that are (a) produced in a metal by bombarding it with neutrons, (b) added to a metal such as by alloying, or (c) contained within a cavity in a metal part. The technique serves to locate the position of the radioactive element or compound. autotempering. Tempering occurring immediately after martensite has formed, either as the martensite cools from the Ms temperature to room temperature or at room temperature. Also known as self-tempering. average grain diameter. The mean diameter of an equiaxed grain section whose size represents all the grain sections in the aggregate being measured. See also grain size. axial. Longitudinal, or parallel to the axis or centerline of a part. Usually refers to axial compression or axial tension. axial ratio. The ratio of the length of one axis to that of another

in a crystal lattice, for example, c/a, or the continued ratio of three axes, such as a:b:c. axis, crystal. The edge of the unit cell of a crystal lattice. Any one axis of any one lattice is defined in length and direction relative to other axes of that lattice. axis, optical. The line formed by the coinciding principal axes of a series of optical elements comprising an optical system. It is the line passing through the centers of curvature of the optical surfaces.

B backing. (1) In grinding, the material (paper, cloth, or fiber) that serves as the base for coated abrasives. (2) In welding, a material placed under or behind a joint to enhance the quality of the weld at the root. It may be a metal backing ring or strip; a pass of weld metal; or a nonmetal such as carbon, granular flux, or a protective gas. backing film. A film used as auxiliary support for the thin replica or specimen-supporting film. back reflection. The diffraction of x-rays at a Bragg angle approaching 90°. bainite—upper, lower, intermediate. Metastable microstructure or microstructures resulting from the transformation of austenite at temperatures between those that produce pearlite and martensite. These structures may be formed on continuous (slow) cooling if the transformation rate of austenite to pearlite is much slower than that of austenite to bainite. Ordinarily, these structures may be formed isothermally at temperatures within the above range by quenching austenite to a desired temperature and holding for a period of time necessary for transformation to occur. If the transformation temperature is just below that at which the finest pearlite is formed, the bainite (upper bainite) has a feathery appearance. If the transformation temperature is just above that at which martensite is produced, the bainite (lower bainite) is acicular, resembling slightly tempered martensite. At the higher resolution of the electron microscope, upper bainite is observed to consist of plates of cementite in a matrix of ferrite. These discontinuous carbide plates tend to have parallel orientation in the direction of the longer dimension of the bainite areas. Lower bainite consists of ferrite needles containing carbide platelets in parallel array cross-striating each needle axis at an angle of about 60°. Intermediate bainite resembles upper bainite; however, the carbides are smaller and more randomly oriented. baking. (1) Heating to a low temperature to remove gases. (2) Curing or hardening surface coatings such as paints by exposure to heat. (3) Heating to drive off moisture, as in the baking of sand cores after molding. banded structure. A segregated structure consisting of alternating, nearly parallel bands of different composition, typically aligned in the direction of primary hot working. banding. Inhomogeneous distribution of alloying elements or phases aligned in filaments or plates parallel to the direction of

Glossary / 249 working. See also ferrite-pearlite banding and segregation banding. bar. (1) An obsolete unit of pressure equal to 100 kPa. (2) An elongated rolled metal product that is relatively thick and narrow; most bars have simple, uniform cross sections such as rectangular, square, round, oval, or hexagonal. Also known as barstock. bark. The decarburized layer just beneath the scale that results from heating steel in an oxidizing atmosphere. basal plane. A plane perpendicular to the principal axis (c-axis) in a tetragonal or hexagonal structure. base. A chemical substance that yields hydroxyl ions (OH⫺) when dissolved in water. base metal. (1) After welding, that part of the metal which was not melted. (2) A metal that readily oxidizes, or that dissolves to form ions. Contrast with noble metal. Bauschinger effect. For both single-crystal and polycrystalline metals, any change in stress-strain characteristics that can be ascribed to changes in the microscopic stress distribution within the metal, as distinguished from changes caused by strain hardening. In the narrow sense, the process whereby plastic deformation in one direction causes a reduction in yield strength when stress is applied in the opposite direction. beach marks. Progression marks on a fatigue fracture surface that indicate successive positions of the advancing crack front. The classic appearance is of irregular elliptical or semielliptical rings, radiating outward from one or more origins. Beach marks (also known as clamshell marks or tide marks) are typically found on service fractures where the part is loaded randomly, intermittently, or with periodic variations in mean stress or alternating stress. Not to be confused with striations, which are microscopic and form differently. Beilby layer. A layer that was once thought to cover mechanically polished surfaces. Smoothing of the surfaces during polishing was thought to occur by the layer being spread over the surface to fill irregularities, and the layer was thought to have an amorphous, or at least an amorphous-like, structure. Now known not to exist. Beilby smearing. Lateral spreading of a surface layer during mechanical polishing by a mechanism implied by the Beilby theory of polishing. See also Beilby layer. bellows length. The distance from the eyepiece to the photosensitive material or viewing screen in a micrographic apparatus. Bf. The temperature at which bainite starts to form. Nomenclature used in labeling a continuous cooling transformation and isothermal transformation diagram. bifilar eyepiece. A filar eyepiece with motion in two mutually perpendicular directions. billet. A solid, semifinished steel round or square product that has been hot worked by forging, rolling, or extrusion; usually smaller than a bloom. binary alloy. Any specific composition in a binary system. binary system. The complete series of compositions produced by mixing a pair of components in all proportions. birefringence. A double-refraction phenomenon in anisotropic materials in which an unpolarized beam of light is divided into

two beams with different directions and relative velocities of propagation. The amount of energy transmitted along an optical path through a crystal that exhibits birefringence becomes a function of crystalline orientation. bivariant equilibrium. A stable state among several phases equal to the number of components in a system and in which any two of the external variables of temperature, pressure, or concentration may be varied without necessarily changing the number of phases. Sometimes termed divariant equilibrium. black annealing. Box annealing or pot annealing ferrous alloy sheet, strip, or wire. See also box annealing. blackheart malleable. See also malleable cast iron. black oxide. A black finish on a metal produced by immersing it in hot oxidizing salts or salt solutions. blank carburizing. Simulating the carburizing operation without introducing carbon. This is usually accomplished by using an inert material in place of the carburizing agent, or by applying a suitable protective coating to the ferrous alloy. blank nitriding. Simulating the nitriding operation without introducing nitrogen. This is usually accomplished by using an inert material in place of the nitriding agent or by applying a suitable protective coating to the ferrous alloy. blemish. A nonspecific quality control term designating an imperfection that mars the appearance of a part but does not detract from its ability to perform its intended function. blister. A raised area, often dome shaped, resulting from (a) loss of adhesion between a coating or deposit and the basis metal or (b) delamination under the pressure of expanding gas trapped in a metal in a near-subsurface zone. Very small blisters may be called pinhead blisters or pepper blisters. bloom. (1) Ancient: Iron produced in a solid condition directly by the reduction of ore in a primitive furnace. The carbon content is variable but usually low. Also known as bloomery iron. The earliest ironmaking process, but still used in underdeveloped countries. (2) Modern: A semifinished hot rolled steel product, rectangular in section, usually produced on a blooming mill but sometimes made by forging. blowholes. A hole produced in a casting or weld by gas trapped during solidification. blue annealing. Heating hot-rolled ferrous sheet in an open furnace to a temperature within the transformation range, then cooling in air to soften the metal. A bluish oxide surface layer forms. blue brittleness. Brittleness exhibited by some steels after being heated to some temperature within the range of about 200 to 370 °C (400 to 700 °F), particularly if the steel is worked at the elevated temperature. Killed steels are virtually free of this kind of brittleness. bluing. Subjecting the scale-free surface of a ferrous alloy to the action of air, steam, or other agents at a suitable temperature, thus forming a thin blue film of oxide and improving the appearance and resistance to corrosion. Note: This term is ordinarily applied to sheet, strip, or finished parts. It is used also to denote the heating of springs after fabrication to improve their properties. body-centered. Having an atom or group of atoms separated by

250 / Metallographer’s Guide a translation of 1⁄2, 1⁄2, 1⁄2 from a similar atom or group of atoms. The number of atoms in a body-centered cell must be a multiple of 2. bond. (1) In grinding wheels and other relatively rigid abrasive products, the material that holds the abrasive grains together. (2) In welding, brazing, or soldering, the junction of joined parts. Where filler metal is used, it is the junction of the fused metal and the heat-affected base metal. (3) In an adhesive-bonded or diffusion-bonded joint, the line along which the faying surfaces are joined together. boundary grain. In the Jeffries’ method for grain size measurement, a grain that is intersected by the boundary of the standard area and is therefore counted only as one-half of a grain. box annealing. Annealing of a metal or alloy in a sealed container under conditions that minimize oxidation. See also black annealing. Bragg angle. The angle between the incident beam and the lattice planes considered. Bragg equation. n␭ ⫽ 2d sin ␪, where n is the order of reflection, ␭ is the wavelength of x-rays, d is the distance between lattice planes, and ␪ is the Bragg angle. See also order (in x-ray reflection). Bragg method. A method of x-ray diffraction in which a single crystal is mounted on a spectrometer with a crystal face parallel to the axis of the instrument. brazing. A group of welding processes in which metals are joined by a filler metal that has a liquidus temperature below the solidus of the parent metal, but above 450 °C (840 °F). The filler metal is distributed by capillary action. breaks. Creases or ridges usually in “untempered” or in aged material where the yield point has been exceeded. Depending on the origin of the break, it may be termed a cross break, a coil break, an edge break, or a sticker break. bright annealing. Annealing in a protective medium to prevent discoloration of the bright surface. bright-field illumination. For reflected light, the form of illumination that causes specularly reflected surfaces normal to the axis of the microscope to appear bright. For transmission electron microscopy, the illumination of an object so that it appears on a bright background. bright plate. An electrodeposit that is lustrous in the as-plated condition. Brinelling. Damage to a solid bearing surface characterized by one or more plastically formed indentations brought about by overload. This term is often applied in the case of rollingelement bearings. See also false Brinelling. brittle. Permitting little or no plastic (permanent) deformation prior to fracture. brittle crack propagation. A very sudden propagation of a crack with the absorption of no energy except that stored elastically in the body. Microscopic examination may reveal some deformation not noticeable to the unaided eye. Contrast with ductile crack propagation. brittle fracture. Separation of a solid accompanied by little or no macroscopic plastic deformation. Typically, brittle fracture

occurs by rapid crack propagation with less expenditure of energy than for ductile fracture. brittleness. The tendency of a material to fracture without first undergoing significant plastic deformation. Contrast with ductility. Bs. The temperature at which bainite completes transforming. Nomenclature used in labeling a continuous cooling transformation and isothermal transformation diagram. buffer. A substance added to aqueous solutions to maintain a constant hydrogen-ion concentration even in the presence of acids or alkalis. bull’s-eye structure. The microstructure of malleable or ductile cast iron when graphite nodules are surrounded by a ferrite layer in a pearlitic matrix. burning. (1) During austenitizing, permanent damage of a metal or alloy by heating to cause incipient melting or intergranular oxidation. See also overheating. (2) During subcritical annealing, particularly in continuous annealing, production of a severely decarburized and grain-coarsened surface layer that results from excessively prolonged heating to an excessively high temperature. (3) In grinding, sufficient heating of the workpiece to cause discoloration or to change the microstructure by tempering or hardening. burnishing. Smoothing surfaces through frictional contact between the workpiece and some hard pieces of material, such as hardened steel balls. burr. (1) A turned-over edge on work resulting from cutting, punching, or grinding. (2) A rotary tool having teeth similar to those on hand files.

C calcite. Calcium carbonate, CaCO3. A mineral of the ditrigonal scalenohedral class. Also called calcspar or Iceland spar. In its transparent form, it has double refracting properties (birefringence) and is used in polarizing prisms. It is uniaxial negative and has indexes of refraction of o ⫽ 1.658 and e ⫽ ⫺1.486. Its Mohs hardness is 3 and its specific gravity is 2.7. caliper diameter (Feret’s diameter). The length of a line normal to two parallel lines, tangent to opposite edges of a phase or object. calorizing. Imparting resistance to oxidation to an iron or steel surface by heating in aluminum powder at 800 to 1000 °C (1470 to 1830 °F). carbide. A compound of carbon with one or more metallic elements. carbide tools. Cutting or forming tools, usually made from tungsten, titanium, tantalum, or niobium carbides, or a combination of them, in a matrix of cobalt, nickel, or other metals. Carbide tools are characterized by high hardnesses and compressive strengths and may be coated to improve wear resistance. carbon edges. Carbonaceous deposits in a wavy pattern along the edges of a sheet or strip; also known as snaky edges.

Glossary / 251 carbon equivalent (CE) (for rating weldability). For rating of weldability: CE ⫽ C ⫹

Mn Cr ⫹ Mo ⫹ V Ni ⫹ Cu ⫹ ⫹ 6 5 15

carbonitride. A precipitate of an element containing both carbon and nitrogen. Examples include columbium (niobium) carbonitride Cb(C,N) and titanium carbonitride Ti(C,N). Commonly found in microalloyed steels where the microalloying element combines with both nitrogen and carbon. carbonitriding. A case-hardening process in which a suitable ferrous material is heated above the lower transformation temperature in a gaseous atmosphere having a composition that results in simultaneous absorption of carbon and nitrogen by the surface and, by diffusion, creates a concentration gradient. The process is completed by cooling at a rate that produces the desired properties in the workpiece. carbon potential. A measure of the ability of an environment containing active carbon to alter or maintain, under prescribed conditions, the carbon concentration in a steel. Note: In any particular environment, the carbon level attained will depend on such factors as temperature, time, and steel composition. carbon restoration. Replacing the carbon lost in the surface layer during previous processing by carburizing this layer to the original carbon level. carbon steel. Steel having no specified minimum quantity for any alloying element (other than the commonly accepted amounts of manganese, silicon, and copper) and containing only an incidental amount of any element other than carbon, silicon, manganese, copper, sulfur, and phosphorus. carburizing. Absorption and diffusion of carbon into solid ferrous alloys by heating, to a temperature usually above Ac3, in contact with a suitable carbonaceous material. A form of case hardening that produces a carbon gradient extending inward from the surface, enabling the surface layer to be hardened either by quenching directly from the carburizing temperature or by cooling to room temperature, then reaustenitizing and quenching. carburizing flame. A gas flame that will introduce carbon into some heated metals, as during a gas welding operation. A carburizing flame is a reducing flame, but a reducing flame is not necessarily a carburizing flame. case. That portion of a ferrous alloy, extending inward from the surface, whose composition has been altered during case hardening. Typically considered to be the portion of an alloy (a) whose composition has been measurably altered from the original composition, (b) that appears light when etched, or (c) that has a higher hardness value than the core. Contrast with core. case hardening. A generic term covering several processes applicable to steel that change the chemical composition of the surface layer by absorption of carbon, nitrogen, or both and, by diffusion, create a concentration gradient. See also carbonitriding, carburizing, cyaniding, nitriding, nitrocarburizing, and quench hardening. cassette. A light-tight film or plate holder.

casting. (1) An object at or near finished shape obtained by solidification of a substance in a mold. (2) Pouring molten metal into a mold to produce an object of desired shape. cast iron. A generic term for a large family of cast ferrous alloys in which the carbon content exceeds the solubility of carbon in austenite at the eutectic temperature. Most cast irons contain at least 2% C, plus silicon and sulfur, and may or may not contain other alloying elements. For the various forms, gray cast iron, white cast iron, malleable cast iron, and ductile cast iron, the word “cast” is often left out, resulting in “gray iron,” “white iron,” “malleable iron,” and “ductile iron,” respectively. cast steel. Steel in the form of castings. cast structure. The metallographic structure of a casting evidenced by shape and orientation of grains as well as by segregation of impurities. See also solidification structure. cathode. The electrode where electrons enter an operating system such as a battery, an electrolytic cell, an x-ray tube, or a vacuum tube. In the first of these, it is positive; in the other three, negative. In a battery or electrolytic cell, it is the electrode where reduction occurs. Contrast with anode. cathodic etching. See ion etching. cathodic protection. Partial or complete protection of a metal from corrosion by making it a cathode, using either a galvanic or an impressed current. Contrast with anodic protection. cation. A positively charged ion; it flows to the cathode in electrolysis. caustic cracking. A form of stress-corrosion cracking most frequently encountered in carbon steels or iron-chromiumnickel alloys that are exposed to concentrated hydroxide solutions at temperatures of 200 to 250 °C (400 to 480 °F). Also known as caustic embrittlement. caustic embrittlement. See caustic cracking. cavitation. The formation and instantaneous collapse of innumerable tiny voids or cavities within a liquid subjected to rapid and intense pressure changes. Cavitation produced by ultrasonic radiation is sometimes used to effect violent localized agitation. Cavitation caused by severe turbulent flow often leads to cavitation damage. cementation. (1) Introduction of one or more elements into the outer layer of a metal object by means of diffusion at high temperature. (2) An obsolete process used to convert wrought iron to blister steel by carburizing. Wrought iron bars were packed in sealed chests with charcoal and heated at 1100 °C (2012 °F) for six to eight days. Cementation was the predominant method of manufacturing steels, particularly high-carbon tool steels, prior to the introduction of the Bessemer and open hearth methods. cemented carbide. A solid and coherent mass made by pressing and sintering a mixture of powders of one or more metallic carbides and a much smaller amount of metal, such as cobalt, to serve as a binder. cementite. A very hard and brittle compound of iron and carbon corresponding to the empirical formula Fe3C. It is commonly known as iron carbide and possesses an orthorhombic lattice. In “plain carbon steels,” some of the iron atoms in the cementite lattice are replaced by manganese, and in “alloy steels,” by

252 / Metallographer’s Guide other elements such as chromium or tungsten. Cementite will often appear as distinct lamellae or as spheroids or globules of varying size in hypoeutectoid steels. Cementite is in metastable equilibrium and has a tendency to decompose into iron and graphite, although the reaction rate is very slow. centerline shrinkage. Shrinkage or porosity occurring along the central plane or axis of a cast metal section. central pencil. A bundle of rays originating in the axis with an angular aperture equal to the effective aperture of the lens. These rays pass through the lens aperture and contribute to the formation of the image. centrifugal casting. A casting made by pouring metal into a mold that is rotated or revolved. cermet. A powder metallurgy product consisting of ceramic particles bonded with a metal. CG iron. Same as compacted graphite cast iron. characteristic radiation. X-radiation of a particular set of wavelengths, produced by and characteristic of a particular element used as a target whenever its excitation potential is exceeded. charging. (1) For a lap, impregnating the surface with fine abrasive. (2) Placing materials into a furnace. chemical deposition. The precipitation or plating-out of a metal from solutions of its salts through the introduction of another metal or reagent to the solution. chemical metallurgy. See process metallurgy. chemical polishing. Improving the surface luster of a metal by chemical treatment. chevron pattern. A fractographic pattern of radial marks (shear ledges) that look like nested letters “V”; sometimes called a herringbone pattern. Chevron patterns are typically found on brittle fracture surfaces in parts whose widths are considerably greater than their thicknesses. The points of the chevrons can be traced back to the fracture origin. chromatic aberration. A defect in a lens or lens system that results in different focal lengths of the lens for radiation of diverse wavelengths. The dispersive power of a simple positive lens focuses light from the blue end of the spectrum at a shorter distance than light from the red end. An image produced by such a lens will exhibit color fringes around the border of the image. The difference in position along the axis for the focal points of light is called longitudinal chromatic aberration. The difference in magnification due to variations in position of the principal points for light of different wavelengths, also a difference in focal length, is known as lateral chromatic aberration. chromizing. A surface treatment at elevated temperature, generally carried out in pack, vapor, or salt bath, in which an alloy is formed by the inward diffusion of chromium into the base metal. clad metal. A composite metal containing two or three layers that have been bonded together. The bonding may have been accomplished by co-rolling, welding, casting, heavy chemical deposition, or heavy electroplating. clamshell marks. Same as beach marks. clear glass focusing screen. A glass screen polished on both sides and mounted for use in a camera, in lieu of photosensitive

material, for the purpose of establishing a plane on which to focus an image prior to recording it. cleavage. Fracture of a crystal by crack propagation across a crystallographic plane of low index. cleavage crack. A crack that extends along a plane of easy cleavage in a crystalline material. cleavage fracture. A fracture, usually of a polycrystalline metal, in which most of the grains have failed by cleavage, resulting in bright reflecting facets. It is one type of crystalline fracture and is associated with low-energy brittle fracture. Contrast with shear fracture. cleavage plane. A characteristic crystallographic plane or set of planes in a crystal on which cleavage fracture occurs easily. close annealing. Same as box annealing. close-packed. A geometric arrangement in which a collection of equally sized spheres (atoms) may be packed together in a minimum total volume. coalescence. (1) The union of particles of a dispersed phase into larger units, usually effected at temperatures below the fusion point. (2) In welding, brazing, or soldering, the union of two or more components into a single body, which usually involves melting of a filler metal or of the base metal. coarse grains. Grains larger than normal for the particular wrought metal or alloy or of a size that produces a surface roughening known as orange peel or alligator skin. coated abrasive. An abrasive product (sandpaper, for example) in which a layer of abrasive particles is firmly attached to a paper, cloth, or fiber backing by means of glue or syntheticresin adhesive. coefficient of elasticity. Same as modulus of elasticity. coefficient of thermal expansion. Change in unit of length (or volume) accompanying a unit change of temperature, at a specified temperature. coercive force. The magnetizing force that must be applied in the direction opposite to that of the previous magnetizing force in order to reduce magnetic flux density to zero; thus, a measure of the magnetic retentivity of magnetic materials. coherency. The continuity of lattice of precipitate and parent phase (solvent) maintained by mutual strain and not separated by a phase boundary. coherent precipitate. A crystalline precipitate that forms from solid solution with an orientation that maintains continuity between the crystal lattice of the precipitate and the lattice of the matrix, usually accompanied by some strain in both lattices. Because the lattices fit at the interface between precipitate and matrix, there is no discernible phase boundary. coherent scattering. A type of x-ray or electron scattering in which the phase of the scattered beam has a definite (not random) relationship to the phase of the incident beam. Also termed unmodified scattering. See also incoherent scattering. cohesion. Force of attraction between the molecules (or atoms) within a single phase. Contrast with adhesion. cohesive strength. (1) The hypothetical stress causing tensile fracture, without plastic deformation. (2) The stress corresponding to the forces between atoms. coil breaks. Creases or ridges in sheet or strip that appear as

Glossary / 253 parallel lines across the direction of rolling and that generally extend the full width of the sheet or strip. cold lap. Wrinkled markings on the surface of an ingot, caused by incipient freezing of the surface while the liquid is still in motion; results from insufficient pouring temperature. cold mill. A mill for cold rolling of sheet or strip. cold rolled sheets. A mill product produced from a hot rolled pickled coil that has been given substantial cold reduction at room temperature. The resulting product usually requires further processing to make it suitable for most common applications. The usual end product is characterized by improved surface, greater uniformity in thickness, and improved mechanical properties compared with hot rolled sheet. cold rolling. The process of rolling a sheet or plate without the aid of heating the workpiece. Usually performed at room temperature. cold shortness. Brittleness that exists in some metals at temperatures below the recrystallization temperature. cold treatment. Exposing to suitable subzero temperatures for the purpose of obtaining desired conditions or properties such as dimensional or microstructural stability. When the treatment involves the transformation of retained austenite, it is usually followed by tempering. cold work. Permanent strain in a metal accompanied by strain hardening. cold-worked structure. A microstructure resulting from plastic deformation of a metal or alloy below its recrystallization temperature. cold working. Deforming metal plastically under conditions of temperature and strain rate that induce strain hardening. Usually, but not necessarily, conducted at room temperature. Contrast with hot working. collimation. The operation of controlling a beam of radiation so that its rays are as nearly parallel as possible. collodian replica. A replica of a surface cast in nitrocellulose. colonies. In pearlite, a colony is a region of a single orientation. color filter. A device that transmits principally a predetermined range of wavelengths. See also contrast filter and filter. columnar structure. A coarse structure of parallel, elongated grains formed by unidirectional growth that is most often observed in castings, but sometimes seen in structures. This results from diffusional growth accompanied by a solid-state transformation. column, electron microscope. The assembly of gun, lenses, specimen, and viewing and plate chambers. coma. A lens aberration occurring in the part of the image field that is some distance from the principal axis of the system. It results from different magnification in the various lens zones. Extraaxial object points appear as short, cometlike images, with the brighter small head toward the center of the field (positive coma) or away from the center (negative coma). combined carbon. That part of the total carbon in steel or cast iron that is present as other than free carbon.

comet tails (on a polished surface). A group of comparatively deep unidirectional scratches that form adjacent to a microstructural discontinuity during mechanical polishing. They have the general shape of a comet tail. Comet tails form only when a unidirectional motion is maintained between the surface being polished and the polishing cloth. compacted graphite cast iron. Cast iron having a graphite shape intermediate between the flake form typical of gray cast iron and the spherical form of fully spherulitic ductile cast iron. Also known as CG iron or vermicular iron, compacted graphite cast iron is produced in a manner similar to that for ductile cast iron, but using a technique that inhibits the formation of fully spherulitic graphite nodules. comparison standard. A standard micrograph or a series of micrographs, usually taken at 75 to 100⫻, used to determine grain size by direct comparison with the image. compensating eyepiece. An eyepiece designed for use with apochromatic objectives. They are also used to advantage with highpower (oil-immersion) achromatic objectives. Because apochromatic objectives are undercorrected chromatically, these eyepieces are overcorrected. See also apochromatic objective. complex silicate inclusions. A general term describing silicate inclusions containing visible constituents in addition to the silicate matrix. An example is corundum or spinel crystals occurring in a silicate matrix in steel. component. One of the elements or compounds used to define a chemical (or alloy) system, including all phases, in terms of the fewest substances possible. composite compact. A powder metallurgy compact consisting of two or more adhering layers of different metals or alloys with each layer retaining its original identity. composite material. A heterogeneous, solid structural material consisting of two or more distinct components that are mechanically or metallurgically bonded together (such as a cermet, or boron wire embedded in a matrix of epoxy resin). composite plate. An electrodeposit consisting of layers of at least two different compositions. composite structure. A structural member (such as a panel, plate, pipe, or other shape) that is built up by bonding together two or more distinct components, each of which may be made of a metal, alloy, nonmetal, or composite material. Examples of composite structures include: honeycomb panels, clad plate, electrical contacts, sleeve bearings, carbide-tipped drills or lathe tools, and weldments constructed of two or more different alloys. compressive strength. The maximum compressive stress that a material is capable of developing, based on original area of cross section. If a material fails in compression by a shattering fracture, the compressive strength has a very definite value. If a material does not fail in compression by a shattering fracture, the value obtained for compressive strength is an arbitrary value depending on the degree of distortion that is regarded as indicating complete failure of the material.

254 / Metallographer’s Guide Compton scattering. X-ray scattering by atoms in which the scattered beam has, relative to the incident beam, a longer wavelength and a random phase relationship. Also called incoherent or modified scattering. condenser. A system of lenses or mirrors designed to collect, control, and concentrate light. condenser, Abbe. Originally a two-lens substage condenser combination designed by Ernst Abbe. It lacks chromatic correction, though designed for a minimum of spherical aberration, and has only a very low-angle aplanatic cone. It may be rated with a numerical aperture as high as 1.3. condenser aperture. In electron microscopy, an opening in the condenser lens controlling the number of electrons entering the lens and the angular aperture of the illuminating beam. condenser, dark-field. A condenser forming a hollow cone of light with its apex (or focal point) in the plane of the specimen. When used with an objective having a numerical aperture lower than the minimum numerical aperture of the hollow cone, only light deviated by the specimen enters the objective. Objects are seen as bright images against a dark background. condenser lens. A device used to focus radiation in or near the plane of the object. condenser, variable-focus. Essentially an Abbe condenser in which the upper lens element is fixed and the lower movable. The lower lens may be used to focus the illumination between the elements so that it emerges from the stationary lens as a large-diameter parallel bundle. The field of low-power objectives may thus be filled without removing the top element. At the opposite extreme, it can be adjusted to have a numerical aperture as high as 1.3. conditioning heat treatment. A preliminary heat treatment used to prepare a material for a desired reaction to a subsequent heat treatment. For the term to be meaningful, the exact heat treatment must be specified. congruent melting. An isothermal or isobaric melting in which both the solid and liquid phases have the same composition throughout the transformation. congruent transformation. An isothermal or isobaric phase change in which both of the phases concerned have the same composition throughout the process. conjugate phases. Those states of matter of unique composition that coexist at equilibrium at a single point in temperature and pressure. For example, the two coexisting phases of a two-phase equilibrium. conjugate planes. Two planes of an optical system such that one is the image of the other. constituent. (1) One of the ingredients that make up a chemical system. (2) A phase or combination of phases that occurs in a characteristic configuration in a microstructure. constitution diagram. A graphical representation of the temperature and composition limits of phase fields in an alloy system as they actually exist under the specific condition of heating or cooling (synonymous with phase diagram). A constitution diagram may be an equilibrium diagram, an approximation to an equilibrium diagram, or a representation of metastable conditions or phases. Compare with equilibrium diagram.

contact fatigue. Cracking and subsequent pitting of a surface subjected to alternating Hertzian stresses such as those produced under rolling contact or combined rolling and sliding. The phenomenon of contact fatigue is encountered most often in rolling-element bearings or in gears, where the surface stresses are high due to the concentrated loads and are repeated many times during normal operation. contact plating. A metal plating process wherein the plating current is provided by galvanic action between the work metal and a second metal, without the use of an external source of current. contact potential. The potential difference at the junction of two dissimilar substances. continuous casting. A casting technique in which a cast shape is continuously withdrawn through the bottom of the mold as it solidifies, so that its length is not determined by mold dimensions. Used chiefly to produce semifinished mill products such as billets, blooms, ingots, slabs, and tubes. continuous cooling transformation diagram (CT or CCT diagram). As opposed to an isothermal transformation (IT or TTT) diagram, a continuous cooling transformation diagram indicates the start and finish temperature of the ferrite, pearlite, bainite, and martensite transformation during continuous cooling. Continuous cooling transformation diagrams are of more commercial importance than isothermal diagrams, because they represent the transformation that takes place during most commercial heat treating and thermomechanical processes. continuous phase. In an alloy or portion of an alloy containing more than one phase, the phase that forms the background or matrix in which the other phase or phases are present as isolated units. continuous precipitation. Precipitation from a supersaturated solid solution in which the precipitate particles grow by long-range diffusion without recrystallization of the matrix. Continuous precipitates grow from nuclei distributed more or less uniformly throughout the matrix. They usually are randomly oriented, but may form a Widmanstätten structure. Also called general precipitation. Compare with discontinuous precipitation and localized precipitation. continuous spectrum (x-rays). The polychromatic radiation emitted by the target of an x-ray tube. It contains all wavelengths above a certain minimum value, known as the sort wavelength limit. continuous yielding. A characteristic of steel having a smooth stress-strain curve, usually found in steels containing mobile dislocations. contrast enhancement (electron optics). An improvement in electron image contrast by the use of an objective aperture diaphragm, shadow casting, or other means. See also shadowing. contrast filter. A color filter, usually with strong absorption, that uses the special absorption bands of the object to control the contrast of the image by exaggerating or diminishing the brightness difference between differently colored areas. contrast perception. The ability to differentiate various components of the object structure by various intensity levels in the image.

Glossary / 255 contrast, photographic. The word “contrast” has been used in many different senses in connection with various photographic characteristics; there are different types of photographic contrast and different methods of measuring it. It is frequently used to designate the magnitude of the density difference resulting from a given exposure difference. controlled cooling. Cooling from an elevated temperature in a predetermined manner to avoid hardening, cracking, or internal damage, or to produce desired microstructure or mechanical properties. controlled etching. Electrolytic etching with selection of suitable etchant and voltage resulting in a balance between current and dissolved metal ions. controlled rolling. A hot rolling process in which the temperature of the steel is closely controlled, particularly during the final rolling passes, to produce a fine-grain microstructure. In microalloyed steels the final rolling passes take place below the recrystallization temperature of austenite. conversion coating. A coating consisting of a compound of the surface metal, produced by chemical or electrochemical treatments of the metal. (Examples are chromate coatings on zinc, cadmium, magnesium, and aluminum, and oxides and phosphate coatings on steel.). coolant. In metal cutting, the preferred term is cutting fluid. cooling curve. A graph showing the relationship between time and temperature during the cooling of a material. It is used to find the temperatures at which phase changes occur. A property or function other than time may occasionally be used—for example, thermal expansion. cooling rate. The average slope of the time-temperature curve taken over a specified time and temperature interval. cooling stresses. Residual stresses resulting from nonuniform distribution of temperature during cooling. coordination number. (1) Number of atoms or radicals coordinated with the central atom in a complex covalent compound. (2) Number of nearest neighboring atoms to a selected atom in crystal structure. copperhead. A reddish spot in a porcelain enamel coating caused by iron pickup during enameling, iron oxide left on poorly cleaned basis metal, or burrs on iron or steel basis metal that protrude through the coating and are oxidized during firing. core. (1) A specially formed material inserted in a mold to shape the interior or other part of a casting that cannot be shaped as easily by the pattern. (2) In a ferrous alloy prepared for case hardening, that portion of the alloy structure not part of the case. Typically considered to be the portion that (a) appears dark on an etched cross section, (b) has an essentially unaltered chemical composition, or (c) has a hardness, after hardening, less than a specified value. coring. (1) A condition of variable composition between the center and surface of a unit of microstructure (such as a dendrite, grain, or carbide particle); results from nonequilibrium solidification, which occurs over a temperature range. (2) A central cavity at the butt end of a rod extrusion, sometimes called extrusion pipe. corrosion. The chemical or electrochemical reaction between a material, usually a metal, and its environment that produces a

deterioration of the material and its properties. See also corrosion fatigue, crevice corrosion, dezincification, erosion-corrosion, exfoliation, fretting corrosion, galvanic corrosion, graphitic corrosion, impingement attack, interdendritic corrosion, intergranular corrosion, internal oxidation, oxidation, pitting, rust, stray-current corrosion, stress-corrosion cracking, and sulfide stress cracking. corrosion embrittlement. The severe loss of ductility of a metal resulting from corrosive attack, usually intergranular and often not visually apparent. corrosion fatigue. Cracking produced by the combined action of repeated or fluctuating stress and a corrosive environment. corrosive wear. Wear in which chemical or electrochemical reaction with the environment is significant. corundum. Natural abrasive of the aluminum oxide type that has higher purity than emery. coupon. A piece of metal from which a test specimen is to be prepared—often an extra piece (as on a casting or forging) or a separate piece made for test purposes (such as a test weldment). covalent bond. A bond between two or more atoms resulting from the completion of shells by the sharing of electrons. covered electrode. A composite filler-metal welding electrode consisting of a bare wire or a metal-cored electrode plus a covering sufficient to provide a layer of slag on the deposited weld metal. The covering often contains materials that provide shielding during welding, deoxidizers for the weld metal, and arc stabilization; it may also contain alloying elements or other additives for the weld metal. covering power. The ability of a solution to give satisfactory plating at very low current densities, a condition that exists in recesses and pits. This term suggests an ability to cover, but not necessarily to build up, a uniform coating, whereas throwing power suggests the ability to obtain a coating of uniform thickness on an irregularly shaped object. crack extension, ⌬a. An increase in crack size. See also crack length and effective crack size. crack length (depth) a. In fatigue and stress-corrosion cracking, the physical crack size used to determine the crack growth rate and the stress-intensity factor. For a compact-type specimen, crack length is measured from the line connecting the bearing points of load applications. For a center-crack tension specimen, crack length is measured from the perpendicular bisector of the central crack. See also crack size. crack plane orientation. An identification of the plane and direction of a fracture in relation to product geometry. This identification is designated by a hyphenated code, the first letter(s) representing the direction normal to the crack plane and the second letter(s) designating the expected direction of crack propagation. crack size, a. A lineal measure of a principal planar dimension of a crack. This measure is commonly used in the calculation of quantities descriptive of the stress and displacement fields. In practice, the value of crack size is obtained from procedures for measurement of physical crack size, original crack size, or effective crack size, as appropriate to the situation under consideration. See also crack length (depth).

256 / Metallographer’s Guide crater. In arc welding, a depression at the termination of a bead or in the weld pool beneath the electrode. crater crack. A crack, often star-shaped, that forms in the crater of a weld bead, usually during cooling after welding. creep. Time-dependent strain occurring under stress. The creep strain occurring at a diminishing rate is called primary or transient creep; that occurring at a minimum and almost constant rate, secondary or steady-rate creep; and that occurring at an accelerating rate, tertiary creep. creep limit. (1) The maximum stress that will cause less than a specified quantity of creep in a given time. (2) The maximum nominal stress under which the creep strain rate decreases continuously with time under constant load and at constant temperature. Sometimes used synonymously with creep strength. creep rate. The slope of the creep-time curve at a given time determined from a Cartesian plot. creep recovery. Time-dependent strain after release of load in a creep test. creep-rupture strength. The stress that will cause fracture in a creep test at a given time in a specified constant environment. Also known as stress-rupture strength. creep strain. The time-dependent total strain (extension plus initial gage length) produced by applied stress during a creep test. creep strength. (1) The constant nominal stress that will cause a specified quantity of creep in a given time at constant temperature. (2) The constant nominal stress that will cause a specified rate of secondary creep at constant temperature. creep stress. The constant load divided by the original crosssectional area of the specimen. crevice corrosion. A type of concentration-cell corrosion; corrosion caused by the concentration or depletion of dissolved salts, metal ions, oxygen or other gases, and such, in crevices or pockets remote from the principal fluid stream, with a resultant building of differential cells that ultimately cause deep pitting. critical cooling rate. The rate of continuous cooling required to prevent undesirable transformations. For steel, unless otherwise specified, it is the minimum rate at which austenite must be continuously cooled to suppress transformations above the martensite start temperature. critical current density. In an electrolytic process, a current density at which an abrupt change occurs in an operating variable or in the nature of an electrodeposit or electrode film. critical curve. In a binary or higher order phase diagram, a line along which the phases of a heterogeneous equilibrium become identical. critical illumination. The formation of an image of the light source in the object field. critical point. (1) The temperature or pressure at which a change in crystal structure, phase, or physical properties occurs. Also termed transformation temperature. (2) In an equilibrium diagram, that specific value of composition, temperature, or pressure, or combination thereof, at which the phases of a heterogeneous system are in equilibrium. critical range. The microstructural changes that occur in steel take place at different temperatures (critical points) dependent

on whether the steel is being heated or cooled. The range between critical points on heating and on cooling is known as the critical range. critical shear stress. The shear stress required to cause slip in a designated slip direction on a given slip plane. It is called the critical resolved shear stress if the shear stress is induced by tensile or compressive forces acting on the crystal. critical strain. The strain just sufficient to cause recrystallization; because the strain is small, usually only a few percent, recrystallization takes place from only a few nuclei, which produces a recrystallized structure consisting of very large grains. critical surface. In a ternary or higher order phase diagram, the area upon which the phases in equilibrium become identical. critical temperature. (1) Synonymous with critical point if pressure is constant. (2) That temperature above which the vapor phase cannot be condensed to liquid by an increase in pressure. critical temperature ranges. Synonymous with transformation ranges, which is the preferred term. crop. (1) An end portion of an ingot that is cut off as scrap. (2) To shear a bar or billet. cross breaks. Same as coil breaks. cross direction. See transverse direction. cross rolling. Rolling of sheet or plate so that the direction of rolling is about 90° from the direction of a previous rolling. crucible steel. High-carbon steel produced by melting blister steel in a covered crucible. Crucible steel was developed by Benjamin Huntsman in about 1750 and remained in use until the late 1940s. crystal. A solid composed of atoms, ions, or molecules arranged in a pattern that is repetitive in three dimensions. crystal analysis. A method for determining crystal structure, for example, the size and shape of the unit cell and the location of all atoms within the unit cell. crystal-figure etching. Discontinuity in etching depending on crystal orientation. Distinctive sectional figures form at polished surfaces. Closely related to dislocation etching. crystalline fracture. A pattern of brightly reflecting crystal facets on the fracture surface of a polycrystalline metal, resulting from cleavage fracture of many individual crystals. Contrast with fibrous fracture and silky fracture. crystallite. A crystalline grain not bounded by habit planes. crystallization. (1) The separation, usually from a liquid phase on cooling, of a solid crystalline phase. (2) Sometimes erroneously used to explain fracturing that actually has occurred by fatigue. crystal orientation. See orientation. crystal system. One of seven groups into which all crystals may be divided: triclinic, monoclinic, orthorhombic, hexagonal, rhombohedral, tetragonal, and cubic. cube texture. A texture found in wrought metals in the cubic system in which nearly all the crystal grains have a plane of the type [100] parallel or nearly parallel to the plane of working and a direction of the type (100) parallel or nearly parallel to the direction of elongation.

Glossary / 257 cubic. Having three mutually perpendicular axes of equal length. cubic plane. A plane perpendicular to any one of the three crystallographic axes of the cubic (isometric) system; the Miller indices are {100}. cup fracture (cup-and-cone fracture). A mixed-mode fracture, often seen in tensile-test specimens of a ductile material, where the central portion undergoes plane-strain fracture and the surrounding region undergoes plane-stress fracture. It is called a cup fracture (or cup-and-cone fracture) because one of the mating fracture surfaces looks like a miniature cup—that is, it has a central depressed flat-face region surrounded by a shear lip; the other fracture surface looks like a miniature truncated cone. Curie point temperature. That temperature of magnetic transformation below which a metal or alloy is ferromagnetic and above which it is paramagnetic. In pure iron, the Curie temperature is 768 °C (1415 °F). cut edge. A mechanically sheared edge obtained by slitting, shearing, or blanking. cutoff wheel. A thin abrasive wheel for severing or slotting any material or part. cutting speed. The linear or peripheral speed of relative motion between the tool and workpiece in the principal direction of cutting. cyaniding. A case-hardening process in which a ferrous material is heated above the lower transformation temperature range in a molten salt containing cyanide to cause simultaneous absorption of carbon and nitrogen at the surface and, by diffusion, create a concentration gradient. Quench hardening completes the process. cycle (N). In fatigue, one complete sequence of values of applied load that is repeated periodically. cycle annealing. An annealing process employing a predetermined and closely controlled time-temperature cycle to produce specific properties or microstructures. cyclic load. (1) Repetitive loading, as with regularly recurring stresses on a part, that sometimes leads to fatigue fracture. (2) Loads that change value by following a regular repeating sequence of change. cyclic stressing. See cyclic load.

D DI or DI. Ideal diameter in hardenability. The diameter of a round steel bar hardened (quenched) to produce a microstructure of at least 50% martensite at the bar center. damping capacity. The ability of a material to absorb vibration (cyclical stresses) by internal friction, converting the mechanical energy into heat. dark-field illumination. The illumination of an object such that it appears bright and the surrounding field dark. This results from illuminating the object with rays of sufficient obliquity so that none can enter the objective directly. In electron microscopy, the image is formed using only electrons scattered by the object.

dead soft. A temper corresponding to the condition of minimum hardness and tensile strength produced by full annealing. Debye ring. A continuous circle, concentric about the undeviated beam, produced by monochromatic x-ray diffraction from a randomly oriented crystalline powder. An analogous effect is obtained using electron diffraction. Debye-Scherrer method. A method of x-ray diffraction using monochromatic radiation and a polycrystalline specimen mounted on the axis of a cylindrical strip of film. See also powder method. decalescence. A phenomenon, associated with the transformation of alpha iron to gamma iron upon the heating (superheating) of iron or steel, revealed by the darkening of the metal surface owing to the sudden decrease in temperature caused by the fast absorption of the latent heat of transformation. decarburization. Loss of carbon from the surface layer of a carbon-containing alloy due to the reaction with one or more chemical substances in a medium that contacts the surface. decoration (of dislocations). Segregation of solute atoms to the line of a dislocation in a crystal. In ferrite, the dislocations may be decorated with carbon or nitrogen atoms. deep drawing. Forming deeply recessed parts by forcing sheet metal to undergo plastic flow between dies, usually without substantial thinning of the sheet. deep etching. Severe macroetching. defect. A departure of any quality characteristic from its intended (usually specified) level that is severe enough to cause the product or service not to fulfill its anticipated function. According to American National Standards Institute standards, defects are classified according to severity: Very serious defects lead directly to severe injury or catastrophic economic loss. Serious defects lead directly to significant injury or significant economic loss. Major defects are related to major problems with respect to anticipated use. Minor defects are related to minor problems with respect to anticipated use. define (x-rays). To limit a beam of x-rays by passage through apertures to obtain a parallel, divergent, or convergent beam. definition. The clarity or sharpness of a microscopic image. deformation. A change in the form of a body due to stress, thermal change, change in moisture, or other causes. Measured in units of length. deformation bands. Bands produced within individual grains during cold working that differ variably in orientation from the matrix. deformation lines. Thin bands or lines produced by cold working in grains of some metals, particularly those of face-centered cubic structure. They are not removed by repolishing and re-etching. deformed layer. A plastically deformed surface layer produced during machining. Constitutes one form of damaged layer. degenerate structure. Usually refers to pearlite that does not

258 / Metallographer’s Guide have an ideally lamellar structure. The degree of degeneracy may vary from slight perturbations in the lamellar arrangement to structures that are not recognizably lamellar. degrees of freedom. The number of independent variables, such as temperature, pressure, or concentration, within the phases present that may be adjusted independently without causing a phase change in an alloy system at equilibrium. delayed yield. A phenomenon involving a delay in time between the application of a stress and the occurrence of the corresponding yield-point strain. delta ferrite. Designation commonly assigned to delta iron that indicates inclusion of elements in solid solution. Small amounts of carbon and large amounts of other alloying elements markedly affect the high- and low-temperature limit of equilibrium. delta iron. Solid phase of pure iron that is stable from 1400 to 1539 °C (2550 to 2800 °F) and possesses the body-centered cubic lattice. dendrite. A crystal with a treelike branching pattern. It is most evident in cast metals slowly cooled through the solidification range. dendritic segregation. Inhomogeneous distribution of alloying elements through the arms of dendrites. deoxidation. (1) Removal of oxygen from molten metals by use of suitable chemical agents. (2) Sometimes refers to removal of undesirable elements other than oxygen by the introduction of elements or compounds that readily react with them. deoxidation products. Those nonmetallic inclusions that form as a result of adding deoxidizing agents to molten metal. deoxidizer. A substance that can be added to molten metal to remove either free or combined oxygen. deoxidizing. (1) The removal of oxygen from molten metals by use of suitable deoxidizers. (2) Sometimes refers to the removal of undesirable elements other than oxygen by the introduction of elements or compounds that readily react with them. (3) In metal finishing, the removal of oxide films from metal surfaces by chemical or electrochemical reaction. depth of field. The depth in the subject over which features can be seen to be acceptably in focus in the final image produced by a microscope. depth of fusion. In welding, the distance that fusion extends into the base metal or into a previous pass. descaling. Removing the thick layer of oxides formed on some metals at elevated temperatures. deviation (x-rays). The angle between the diffracted beam and the transmitted incident beam. It is equal to twice the Bragg angle theta. dezincification. Corrosion in which zinc is selectively leached from zinc-containing alloys. Most commonly found in copperzinc alloys containing less than 85% Cu after extended service in water containing dissolved oxygen. diamond wheel. A grinding wheel in which crushed and sized industrial diamonds are held in a resinoid, metal, or vitrified bond. diaphragm. A fixed or adjustable aperture in an optical system. Diaphragms are used to intercept scattered light, to limit field angles, or to limit image-forming bundles or rays. die casting. (1) A casting made in a die. (2) A casting process

wherein molten metal is forced under high pressure into the cavity of a metal mold. differential coating. A coated product having a specified coating on one surface and a significantly lighter coating on the other surface (such as a hot dip galvanized product or electrolytic tin plate). differential heating. Heating that intentionally produces a temperature gradient within an object such that, after cooling, a desired stress distribution or variation in properties is present within the object. differential interference contrast illumination (DIC). A microscopic technique using a beam-splitting double-quartz prism placed ahead of the objective together with a polarizer and analyzer in the 90° crossed positions. The two light beams are made to coincide at the focal plane of the objective, revealing height differences as variations in color. The prism can be moved, shifting the interference image through the range of Newtonian colors. diffraction. (1) A modification that radiation undergoes, for example, in passing by the edge of opaque bodies or through narrow slits, in which the rays appear to be deflected. (2) Coherent scattering of x-rays by the atoms of a crystal that necessarily results in beams in characteristic directions. Sometimes termed reflection. (3) The scattering of electrons by any crystalline material through discrete angles depending only on the lattice spacings of the material and the velocity of the electrons. diffraction grating. An artificially produced periodic array of scattering centers capable of producing a pattern of diffracted energy, such as accurately ruled lines on a plane surface. diffraction pattern (x-rays). The spatial arrangement and relative intensities of diffracted beams. diffraction ring. The diffraction pattern produced by a given set of planes from randomly oriented crystalline material. See also Debye ring. diffusion. (1) Spreading of a constituent in a gas, liquid, or solid that tends to make the composition of all parts uniform. (2) The spontaneous movement of atoms or molecules to new sites within a material. diffusion aid. A solid filler metal sometimes used in diffusion welding. diffusion coating. Any process whereby a basis metal or alloy is either (1) coated with another metal or alloy and heated to a sufficient temperature in a suitable environment, or (2) exposed to a gaseous or liquid medium containing the other metal or alloy, thus causing diffusion of the coating or of the other metal or alloy into the basis metal with resultant change in the composition and properties of its surface. diffusion coefficient. A factor of proportionality representing the amount of substance diffusing across a unit area through a unit concentration gradient in unit time. diffusion zone. The zone of variable composition at the junction between two different materials, such as in welds or between the surface layer and the core of clad materials or sleeve bearings, in which interdiffusion between the various components has taken place. dilatometer. An instrument for measuring the linear expansion or

Glossary / 259 contraction in a solid metal resulting from changes in such factors as temperature and allotropy. dilatometry. The measurement of length or volume changes of a substance undergoing a change in temperature, pressure, or state. dimple rupture. A fractographic term describing ductile fracture that occurs through the formation and coalescence of microvoids along the fracture path. The fracture surface of such a ductile fracture appears dimpled when observed at high magnification and usually is most clearly resolved when viewed in a scanning electron microscope. directional property. Property whose magnitude varies depending on the relation of the test axis to a specific direction within the metal. The variation results from preferred orientation or from fibering of constituents or inclusions. direct quenching. (1) Quenching carburized parts directly from the carburizing operation. (2) Also used for quenching pearlitic malleable parts directly from the malleabilizing operation. discontinuity. Any interruption in the normal physical structure or configuration of a part, such as cracks, laps, seams, inclusions, or porosity. A discontinuity may or may not affect the usefulness of the part. discontinuous precipitation. Precipitation from a supersaturated solid solution in which the precipitate particles grow by short-range diffusion, accompanied by recrystallization of the matrix in the region of precipitation. Discontinuous precipitates grow into the matrix from nuclei near grain boundaries, forming cells of alternate lamellae of precipitate and depleted (and recrystallized) matrix. Often referred to as cellular or nodular precipitation. Compare with continuous precipitation and localized precipitation. discontinuous yielding. Nonuniform plastic flow of a metal exhibiting a yield point in which plastic deformation is inhomogeneously distributed along the gage length. Under some circumstances, it may occur in metals not exhibiting a distinct yield point, either at the onset of or during plastic flow. dislocation. A linear imperfection in a crystalline array of atoms. Two basic types are recognized: (1) an edge dislocation that corresponds to the row of mismatched atoms along the edge formed by an extra, partial plane of atoms within the body of a crystal; (2) a screw dislocation that corresponds to the axis of a spiral structure in a crystal, characterized by a distortion that joins normally parallel planes together to form a continuous helical ramp (with a pitch of one interplanar distance) winding about the dislocation. Most prevalent is the so-called mixed dislocation, which is the name given to any combination of a screw dislocation and an edge dislocation. dislocation etching. Etching of exit points of dislocations on a surface. Depends on the strain field ranging over a distance of several atoms. Crystal figure (etch pits) are formed at exit points. For example, etch pits for cubic materials are cube faces. disordered structure. The crystal structure of a solid solution in which the atoms of different elements are randomly distributed relative to the available lattice site. Contrast with ordered structure. dispersoid. Finely divided particles of relatively insoluble constituents visible in the microstructure for certain alloys.

dissociation. As applied to heterogeneous equilibria, the transformation of one phase into two or more new phases of different composition. dissociation pressure. At a designated temperature, the pressure at which a phase will transform into two or more new phases of different composition. dissolution etching. Development of microstructure by surface removal. disturbed metal. The cold-worked metal layer formed at a polished surface during the process of mechanical grinding and polishing. divariant equilibrium. See bivariant equilibrium. divorced eutectic. A metallographic appearance in which the two constituents of a eutectic structure appear as massive phases rather than the finely divided mixture characteristic of normal eutectics. Often, one of the constituents of the eutectic is continuous with and indistinguishable from an accompanying proeutectic constituent. domain. A substructure in a ferromagnetic material within which all the elementary magnets (electron spins) are held aligned in one direction by interatomic forces; if isolated, a domain would be a saturated permanent magnet. double aging. Employment of two different aging treatments to control the type of precipitate formed from a supersaturated alloy matrix in order to obtain the desired properties. The first aging treatment, sometimes referred to as intermediate or stabilizing, is usually carried out at a higher temperature than the second. double etching. Use of two etching solutions in sequence. The second etchant emphasizes a particular microstructural feature. double tempering. A treatment in which quench-hardened ferrous metal is subjected to two complete tempering cycles, usually at substantially the same temperature, for the purpose of ensuring completion of the tempering reaction and promoting stability of the resulting microstructure. doublet, in characteristic x-ray spectra. A separation of characteristic radiation into subspecies of slightly different wavelength. drawability. A measure of the workability of a metal subject to a drawing process. A term usually expressed to indicate the ability of a metal to be deep drawn. drawing. (1) Forming recessed parts by forcing the plastic flow of metal in dies. (2) Reducing the cross section of barstock, wire, or tubing by pulling through a die. (3) A misnomer for tempering (see temper). dressing. Cutting, breaking down, or crushing the surface of a grinding wheel to improve its cutting ability and accuracy. drift. In electron optics, motion of the electron beam or image due to current, voltage, specimen instabilities, or to charging of a projection, such as dirt on or near the electron beam. drop. A casting imperfection due to a portion of the sand dropping from the cope or other overhanging section of the mold. drop forging. A shallow forging made in impression dies, usually with a drop hammer. dross. The scum that forms on the surface of molten metal

260 / Metallographer’s Guide largely because of oxidation but sometimes because of the rising of impurities to the surface. dry cyaniding. (obsolete) Same as carbonitriding. dry etching. Development of microstructure under the influence of gases. dry objective. Any microscope objective designed for use without liquid between the cover glass and the objective, or, in the case of metallurgical objectives, in the space between objective and specimen. dual-phase steel. A term applied to a class of high-strength steels that consist of a matrix of ferrite and a second phase (10 to 20%) of martensite (often martensite and retained austenite— called MA constituent). Dual-phase steels exhibit continuous yielding behavior. ductile cast iron. A cast iron that has been treated while molten with an element such as magnesium or cerium to induce the formation of free graphite as nodules or spherulites, which imparts a measurable degree of ductility to the cast metal. Also known as nodular cast iron, spherulitic graphite cast iron, and spheroidal graphite iron. ductile crack propagation. Slow crack propagation that is accompanied by noticeable plastic deformation and that requires energy to be supplied from outside the body. ductile fracture. Fracture characterized by tearing of metal accompanied by appreciable gross plastic deformation and expenditure of considerable energy. Contrast with brittle fracture. ductility. The ability of a material to deform plastically without fracturing; measured by elongation or reduction of area in a tensile test, by height of cupping in an Erichsen test, or by other means. duplex coating. See composite plate. duplex grain size. The simultaneous presence of two grain sizes in substantial amounts, with one grain size appreciably larger than the others. Also termed mixed grain size. duplex microstructure. A two-phase structure.

E ECM. An abbreviation for electrochemical machining. edge dislocation. See dislocation. edge strain. Transverse strain lines or Lüders lines located from 25 to 300 mm (1 to 12 in.) in from the edges of cold rolled steel sheet or strip. edge-trailing technique. A unidirectional motion perpendicular to and toward one edge of the specimen during abrasion or polishing used to improve edge retention. EDM. An abbreviation for electric discharge machining. effective crack size (ae). The physical crack size augmented for the effects of crack-tip plastic deformation. Sometimes the effective crack size is calculated from a measured value of a physical crack size plus a calculated value of a plastic-zone adjustment. A preferred method for calculation of effective crack size compares compliance from the secant of a loaddeflection trace with the elastic compliance from a calibration for the type of specimen.

elastic constants. Facors of proportionality that describe elastic responses of a material to applied forces, including modulus of elasticity (either in tension, compression, or shear). elastic deformation. A change in dimensions directly proportional to and in phase with an increase or decrease in applied force. elastic electron scatter. The scatter of electrons by an object without loss of energy, usually an interaction between electrons and atoms. elastic hysteresis. A misnomer for an anelastic strain that lags a change in applied stress, thereby creating energy loss during cyclic loading. More properly termed mechanical hysteresis. elastic limit. The maximum stress a material is capable of sustaining without any permanent strain (deformation) remaining upon complete release of the stress. elastic modulus. Same as modulus of elasticity. elastic ratio. Yield point divided by tensile strength. elastic recovery. In hardness testing, the shortening of the original dimensions of the indentation upon release of the applied load. elastic strain. Dimensional changes accompanying stress where the original dimensions are restored upon release of the stress. Same as elastic deformation. elastic strain energy. See strain energy. electric discharge machining (EDM). Removal of stock from an electrically conductive material by rapid, repetitive spark discharge through a dielectric fluid flowing between the workpiece and a shaped electrode. Variations of the process include electrical discharge grinding and electrical discharge wire cutting. electrochemical corrosion. Corrosion that is accompanied by a flow of electrons between cathodic and anodic areas on metallic surfaces. electrochemical equivalent. The weight of an element, compound, radical, or ion involved in a specified electrochemical reaction during the passage of a unit quantity of electricity. electrochemical etching. General expression for all developments of microstructure through reduction and oxidation (redox reactions). electrochemical machining (ECM). Removal of stock from an electrically conductive material by anodic dissolution in an electrolyte flowing rapidly through a gap between the workpiece and a shaped electrode. Variations of the process include electrochemical deburring and electrochemical grinding. electrochemical series. Same as electromotive series. electrode. (1) In arc welding, a current-carrying rod that supports the arc between the rod and work, or between two rods as in twin carbon-arc welding. It may or may not furnish filler metal. See also covered electrode. (2) In resistance welding, a part of a resistance welding machine through which current and, in most instances, pressure are applied directly to the work. The electrode may be in the form of a rotating wheel, rotating roll, bar, cylinder, plate, clamp, chuck, or modification thereof. (3) An electrical conductor for leading current into or out of a medium. electrode deposition. The weight of weld-metal deposit obtained from a unit length of electrode.

Glossary / 261 electrodeposition. The deposition of a substance on an electrode by passing electric current through an electrolyte. Electroplating (plating), electroforming, electrorefining, and electrowinning result from electrodeposition. electroforming. Making parts by electrodeposition on a removable form. electrogalvanizing. The electroplating of zinc upon iron or steel. electroless plating. A process in which metal ions in a dilute aqueous solution are plated out on a substrate by means of autocatalytic chemical reduction. electrolysis. Chemical change resulting from the passage of an electric current through an electrolyte. electrolyte. (1) An ionic conductor. (2) A liquid, most often a solution, that will conduct an electric current. electrolytic cell. An assembly, consisting of a vessel, electrodes, and an electrolyte, in which electrolysis can be carried out. electrolytic cleaning. Removing soil from work by electrolysis, the work being one of the electrodes. The electrolyte is usually alkaline. electrolytic deposition. Same as electrodeposition. electrolytic etching. See anodic etching. electrolytic extraction. Removal of phases by using an electrolytic cell containing an electrolyte that preferentially dissolves the metal matrix. See also extraction. electrolytic grinding. A combination of grinding and machining wherein a metal-bonded abrasive wheel, usually diamond, is the cathode in physical contact with the anodic workpiece, the contact being made beneath the surface of a suitable electrolyte. The abrasive particles produce grinding and act as nonconducting spacers permitting simultaneous machining through electrolysis. electrolytic machining. Controlled removal of metal by use of an applied potential and a suitable electrolyte to produce the shapes and dimensions desired. electrolytic pickling. Pickling in which electric current is used, the work being one of the electrodes. electrolytic polishing. An electrochemical polishing process in which the metal to be polished is made the anode in an electrolytic cell where preferential dissolution at high points in the surface topography produces a specularly reflective surface. electrolytic protection. See preferred term cathodic protection. electromagnetic lens. An electromagnet designed to produce a suitably shaped magnetic field for the focusing and deflection of electrons or other charged particles in electron-optical instrumentation. See also focusing device. electromechanical polishing. An attack-polishing method in which the chemical action of the polishing fluid is enhanced or controlled by the application of an electric current between the specimen and the polishing wheel. electromotive force. Electrical potential; voltage. electromotive series. A series of elements arranged according to their standard electrode potentials. In corrosion studies, the analogous but more practical galvanic series of metals is generally used. The relative positions of a given metal are not necessarily the same in the two series. electron. An elementary particle that is the negatively charged

constituent of ordinary matter. The electron is the lightest known particle possessing an electric charge. Its rest mass is me ⬵ 9.1 ⫻ 10⫺28g, approximately 1⁄1836 of the mass of the proton or neutron, which are, respectively, the positively charged and neutral constituents of ordinary matter. electron bands. Energy states for the free electrons in a metal, as described by use of the band theory (zone theory) of electron structure. Also called Brillouin zones. electron beam. A stream of electrons in an electron-optical system. electron beam cutting. A cutting process that uses the heat obtained from a concentrated beam composed primarily of high-velocity electrons, which impinge upon the workpieces to be cut; it may or may not use an externally supplied gas. electron diffraction. The phenomenon, or the technique, of producing diffraction patterns through the incidence of electrons upon matter. electron energy loss spectrometry (EELS). A spectrographic technique in the electron microscope that analyzes the energy distribution of the electrons transmitted through the specimen. The energy loss spectrum is characteristic of the chemical composition of the region being sampled. electron gun. A device for producing and accelerating a beam of electrons. electron image. A representation of an object formed by a beam of electrons focused by an electron-optical system. See also image. electron lens. A device for focusing an electron beam to produce an image of an object. electron micrograph. A reproduction of an image formed by the action of an electron beam on a photographic emulsion. electron microscope. An electron-optical device that produces a magnified image of an object. Detail may be revealed by selective transmission, reflection, or emission of electrons by the object. See also scanning electron microscope and transmission electron microscope. electron microscope column. The assembly of gun, lenses, specimen, and viewing and plate chambers. electron microscopy. The study of materials by means of an electron microscope. electron optical axis. The path of an electron through an electron-optical system, along which it suffers no deflection due to lens fields. This axis does not necessarily coincide with the mechanical axis of the system. electron optical system. A combination of parts capable of producing and controlling a beam of electrons to yield an image of an object. electron optics. The science that deals with propagation of electrons, as light optics deals with that of light and its phenomena. electron probe. A narrow beam of electrons used to scan or illuminate an object or screen. electron probe microanalyzer (EPMA). An instrument for selective analysis of a microscopic component or feature in which an electron beam bombards the point of interest in a vacuum at a given energy level. Scanning of a larger area permits determi-

262 / Metallographer’s Guide nation of the distribution of selected elements. The analysis is made by measuring the wavelengths and intensities of secondary electromagnetic radiation resulting from the bombardment. electron trajectory. The path of an electron. electron velocity. The rate of motion of an electron. electron wavelength. The wavelength necessary to account for the deviation of electron rays in crystals by wave-diffraction theory. It is numerically equal to the quotient of Planck’s constant divided by the electron momentum. electroplating. Electrodepositing a metal or alloy in an adherent form on an object serving as a cathode. electropolishing. (1) A technique commonly used to prepare metallographic specimens, in which a high polish is produced by making the specimen the anode in an electrolytic cell, where preferential dissolution at high points smooths the surface. (2) A variation of chemical machining wherein electrolytic deplating promotes chemical cutting, especially at surface irregularities. electrostatic immersion lens. See immersion objective. electrostatic lens. A lens producing a potential field capable of deflecting electron rays to form an image of an object. electrotinning. Electroplating tin on an object. elongated grain. A grain with one principal axis significantly longer than either of the other two. elongation. In tensile testing, the increase in the gage length, measured after fracture of the specimen within the gage length, usually expressed as a percentage of the original gage length. embedded abrasive. Fragments of abrasive particles forced into the surface of a workpiece during grinding, abrasion, or polishing. embrittlement. Reduction in the normal ductility of a metal due to a physical or chemical change. Examples include blue brittleness, hydrogen embrittlement, and temper brittleness. emery. An impure mineral of the corundum or aluminum oxide type, used extensively as an abrasive before the development of electric-furnace products. emf. An abbreviation for electromotive force. emission microscope. A type of electron microscope in which the specimen is the cathode source of the electrons. Sometimes used synonymously with shadow microscope. emulsion. A dispersion of one liquid phase in another. enameling. A cooling process whereby a glasslike coating is applied to steel. enantiotropy. The relation of crystal forms of the same substance in which one form is stable above a certain temperature and the other form is stable below that temperature. For example, ferrite and austenite are enantiotropic in ferrous alloys. end-quench hardenability test. A laboratory procedure for determining the hardenability of a steel or other ferrous alloy; widely referred to as the Jominy test. Hardenability is determined by heating a standard specimen above the upper critical temperature, placing the hot specimen in a fixture so that a stream of cold water impinges on one end, and, after cooling to room temperature is completed, measuring the hardness near the surface of the specimen at regularly spaced intervals along its length. The data are normally plotted as hardness versus distance from the quenched end.

energy-dispersive spectroscopy (EDS). A method of x-ray analysis that discriminates by energy levels the characteristic x-rays emitted from the sample. Compare with wavelengthdispersive spectroscopy. epitaxy. Growth of an electrodeposit or vapor deposit in which the orientation of the crystals in the deposit are directly related to crystal orientations in the underlying crystalline substrate. epsilon. Designation generally assigned to intermetallic, metalmetalloid, and metal-nonmetallic compounds found in ferrous alloy systems, for example, Fe3Mo2, FeSi, and Fe3P. epsilon carbide. Carbide with hexagonal close-packed lattice that precipitates during the first stage of tempering of primary martensite. Its composition corresponds to the empirical formula Fe2.4C. epsilon structure. Structurally analogous close-packed phases or electron compounds that have ratios of seven valence electrons to four atoms. equiaxed grain structure. A structure in which the grains have approximately the same dimensions in all directions. equilibrium. A dynamic condition of physical, chemical, mechanical, or atomic balance that appears to be a condition of rest rather than one of change. equilibrium diagram. A graphical representation of the temperature, pressure, and composition limits of phase fields in an alloy system as they exist under conditions of complete equilibrium. In metal systems, pressure is usually considered constant. Compare with phase diagram. erosion. Destruction of metals or other materials by the abrasive action of moving fluids, usually accelerated by the presence of solid particles or matter in suspension. When corrosion occurs simultaneously, the term “erosion-corrosion” is often used. erosion-corrosion. See erosion. etchant. A chemical substance or mixture used for etching. etch-attack polishing. A mechanical polishing method in which the polishing fluid is chemically active with respect to the material being polished, but only to the extent that it supplements the mechanical action of the abrasive. etch cracks. Shallow cracks in hardened steel containing high residual surface stresses, produced by etching in an embrittling acid. etch figures. Characteristic markings produced on crystal surfaces by chemical attack, usually having facets parallel to low-index crystallographic planes. etching. Subjecting the surface of a metal to preferential chemical or electrolytic attack to reveal structural details for metallographic examination. eutectic. (1) An isothermal reversible reaction in which a liquid solution is converted into two or more intimately mixed solids upon cooling; the number of solids formed equals the number of components in the system. (2) An alloy having the composition indicated by the eutectic point on an equilibrium diagram. (3) An alloy structure of intermixed solid constituents formed by a eutectic reaction. eutectic arrest. In a cooling or heating curve, an approximately isothermal segment corresponding to the time interval during

Glossary / 263 which the heat of transformation from the liquid phase to two or more solid phases is evolving. eutectic carbide. Carbide formed during freezing as one of the mutually insoluble phases participating in the eutectic reaction of a hypereutectic tool steel. See also hypereutectic alloy. eutectic-cell etching. Development of eutectic cells (grains). eutectic colony, grain. A two-phase region that solidified progressively from a simple center and, therefore, has some uniformity of structural relationship. eutectic melting. Melting of localized microscopic areas whose composition corresponds to that of the eutectic in the system. eutectic point. The composition of a liquid phase in univariant equilibrium with two or more solid phases; the lowest melting alloy of a composition series. eutectoid. (1) An isothermal, reversible transformation in which a solid solution is converted into two or more intimately mixed solids. The number of solids formed equals the number of components in the system. (2) An alloy having the composition indicated by the eutectoid point on an equilibrium diagram. (3) An alloy structure of intermixed solid constituents formed by a eutectoid transformation. eutectoid carbide. Usually associated with the transformation structure in an alloy steel that has a eutectoid structure similar to pearlite, except that the composition of the phases is alloy-rich from ferrite and cementite. eutectoid point. The composition of a solid phase that undergoes univariant transformation into two or more other solid phases upon cooling. evaporation. The vaporization of a material by heating, usually in a vacuum. In electron microscopy, this process is used for shadowing or to produce thin support films by condensation of the vapors of metals or salts. Ewald sphere. A geometric construction, of radius equal to the reciprocal of the wavelength of the incident radiation, with its surface at the origin of the reciprocal lattice. Any crystal plane will reflect if the corresponding reciprocal lattice point lies on the surface of this sphere. excitation potential. The applied potential on an x-ray tube required to produce characteristic radiation from the target. exfoliation. A type of corrosion that progresses approximately parallel to the outer surface of the metal, causing layers of the metal to be elevated by the formation of corrosion product. exogenous inclusions. Nonmetallic inclusions generally large in size and representing accidental contamination from materials, such as fireclay refractories. extinction. A decrease in the intensity of the diffracted beam caused by perfection or near perfection of crystal structure. See also primary extinction and secondary extinction. extinction coefficient. The ratio of the diffracted beam intensity when extinction is present to the diffracted beam intensity when extinction is absent. It applies to primary or secondary extinction. extraction. A general term denoting chemical methods of isolating phases from the metal matrix. extraction replica. A replica removed from an etched surface that contains particles and precipitates from the surface embedded in the replica material.

extractive metallurgy. The branch of process metallurgy dealing with the extraction of metals from their ores. extra hard. A temper of nonferrous alloys and some ferrous alloys characterized by values of tensile strength and hardness about one-third of the way from those of full hard to those of extra spring temper. extra spring. A temper of nonferrous alloys and some ferrous alloys corresponding approximately to a cold worked state above full hard beyond which further cold work will not measurably increase strength or hardness. eye clearance. The distance from the back lens of an eyepiece to the proper location of the viewer’s eye, typically about 8 mm (0.31 in.) (about 20 mm, or 0.78 in., for high eyepoint eyepieces that permit the use of eyeglasses). eye lens. The lens in an eyepiece nearest to the eye. eyepiece. A lense or system of lenses for increasing magnification in a microscopy by magnifying the image formed by the objective. eyepiece micrometer. See ocular micrometer. eyepiece, parfocal. Eyepieces with common focal planes so that they are interchangeable without refocusing. eyepiece, positive. An eyepiece in which the real image of the object is formed below the lower lens elements of the eyepiece.

F face angle. The included dihedral angle between two opposite faces of an indenter. face-centered. Having atoms or groups of atoms separated by translations of 1⁄2, 1⁄2, 0; 1⁄2, 0, 1⁄2; and 0, 1⁄2, 1⁄2 from a similar atom or group of atoms. The number of atoms in a face-centered cell must be a multiple of 4. face (crystal). An idiomorphic plane surface on a crystal. false Brinelling. Evenly spaced depressions in a raceway of a rolling-element bearing caused by fretting that occurs when the bearing is subjected to vibration while it is not rotating. Compare with Brinelling. family of crystal planes. The planes in any one crystal that have common Miller indices, regardless of sign. fatigue. The phenomenon leading to fracture under repeated or fluctuating stresses having a maximum value less than the ultimate tensile strength of the material. See also fatigue failure, high-cycle fatigue, low-cycle fatigue, and ultimate strength. fatigue failure. Failure that occurs when a specimen undergoing fatigue completely fractures into two parts or has softened or been otherwise significantly reduced in stiffness by thermal heating or cracking. Fatigue failure generally occurs at loads that, applied statically, would produce little perceptible effect. Fatigue failures are progressive, beginning as minute cracks that grow under the action of the fluctuating stress. fatigue life. The number of cycles of stress that can be sustained prior to failure under a stated test condition. fatigue limit. The maximum stress that presumably leads to fatigue fracture in a specified number of stress cycles. If the

264 / Metallographer’s Guide stress is not completely reversed, the value of the mean stress, the minimum stress, or the stress ratio also should be stated. fatigue ratio. The fatigue limit under completely reversed flexural stress divided by the tensile strength for the same alloy and condition. fatigue strength. The maximum stress that can be sustained for a specified number of cycles without failure, the stress being completely reversed within each cycle unless otherwise stated. fatigue striation. Parallel lines frequently observed in electron microscope fractography of fatigue fracture surfaces. The lines are transverse to the direction of local crack propagation; the distance between successive lines represents the advance of the crack front during one cycle of stress variation. fatigue wear. Wear of a solid surface caused by fracture arising from material fatigue. Feret’s diameter. See caliper diameter. ferrimagnetic material. A material that macroscopically has properties similar to those of a ferromagnetic material but that microscopically also resembles an antiferromagnetic material in that some of the elementary magnetic moments are aligned and antiparallel. If the moments are of different magnitudes, the material may still have a large resultant magnetization. ferrite. A solid solution of one or more elements in bodycentered cubic iron. Unless otherwise designated (for instance, a chromium ferrite), the solute is generally assumed to be carbon. On some equilibrium diagrams, there are two ferrite regions separated by an austenite area. The lower area is alpha ferrite; the upper, delta ferrite. If there is no designation, alpha ferrite is assumed. ferrite banding. Parallel bands of free ferrite aligned in the direction of working. Sometimes referred to as ferrite streaks. ferrite grain size. The grain size of the ferrite in predominantly ferritic steels. See ASTM E 112, “Standard Test Methods for Determining Average Grain Size.” ferrite number. An arbitrary, standardized value designating the ferrite content of an austenitic stainless steel weld metal. This value directly replaces percent ferrite or volume percent ferrite and is determined by the magnetic test described in AWS A4.2. ferrite-pearlite banding. Inhomogeneous distribution of ferrite and pearlite aligned in filaments or plates parallel to the direction of working. ferrite streaks. Same as ferrite banding. ferrite tail. A region of ferrite surrounding a surface defect (crack) that has decarburized due to exposure to air at high temperature. ferritic grain size. The grain size of the ferritic matrix of a steel. ferritic malleable. See malleable cast iron. ferritizing anneal. A treatment given as-cast gray or ductile (nodular) iron to produce an essentially ferritic matrix. For the term to be meaningful, the final microstructure desired or the time-temperature cycle used must be specified. ferroalloy. An alloy of iron that contains a sufficient amount of one or more other chemical elements to be useful as an agent for introducing these elements into molten metal, especially into steel or cast iron.

ferromagnetic material. A material that in general exhibits the phenomena of hysteresis and saturation, and whose permeability is dependent on the magnetizing force. Microscopically, the elementary magnets are aligned parallel in volumes called domains. The unmagnetized condition of a ferromagnetic material results from the overall neutralization of the magnetization of the domains to produce zero external magnetization. Compare with paramagnetic material and ferrimagnetic material. fiber texture. A texture characterized by having only one preferred crystallographic direction. fibrous fracture. A fracture whose surface is characterized by a dull gray or silky appearance. Contrast with crystalline fracture. fibrous structure. (1) In forgings, a structure revealed as laminations, not necessarily detrimental, on an etched section or as a ropy appearance on a fracture. It is not to be confused with silky or ductile fracture of a clean metal. (2) In wrought iron, a structure consisting of slag fibers embedded in ferrite. (3) In rolled steel plate stock, a uniform, fine-grained structure on a fractured surface, free of laminations or shale-type discontinuities. As contrasted with (1), it is virtually synonymous with silky or ductile fracture. field metallography. Metallographic techniques carried out in the field when the part or component is too large to bring to a metallographic laboratory or a specimen cannot be removed. filament. An electrically heated wire used as a source of radiation, such as electrons, or as a source of heat, such as in the vaporization of a metal. filamentary shrinkage. A fine network of shrinkage cavities, occasionally found in steel castings, that produces a radiographic image resembling lace. filar eyepiece or filar micrometer. An eyepiece equipped with a fiducial line in its focal plane, that is movable by means of a calibrated micrometer screw, in order to make accurate measurements of length. filler metal. A third material that is melted concurrently with the parent metals during fusion or braze welding. It is usually, but not necessarily, of different composition from the parent metals. filter. A device that modifies the light from the light source either chromatically or with regard to intensity. Color—A device that transmits principally a predetermined range of wavelengths. Contrast—A color filter, usually with strong absorption, whose function is to use the spectral absorption bands of the subject to control the contrast of the image by exaggerating or diminishing the brightness difference between areas of different color. Maximum contrast is obtained when the transmission of the filter is entirely within the absorption band of an area but not of its surrounds. Interference—A combination of several thin optical films to form a layered coating for transmitting or reflecting a narrow band of wavelengths by virtue of interference effects. Neutral—(1) A color filter that reduces the intensity of the transmitted illumination without affecting its hue. (2) A color filter having identical transmission at all wavelengths through-

Glossary / 265 out the spectrum. Such an ideal filter does not exist in practice. Orthochromatic—A color filter whose function is to modify the illumination quality reaching the film so that the brightness of colored objects will be relatively the same in the resultant black-and-white positive. Photometric—A color filter whose function is to convert the quality of illumination from that of one source to that of another. Most frequently the term is used for a filter altering the illumination quality from that of one color temperature to that of another. X-ray—A material that preferentially absorbs certain wavelengths. final polishing. A polishing process in which the primary objective is to produce a final surface suitable for microscopic examination. fire crack. Cracking, frequently intergranular in nature, that occurs in some metallic materials when too rapidly heated or when stressed and heated rapidly. Not to be confused with quench crack. fisheye. A discontinuity found on the fracture surface of a weld in steel that consists of a small pore or inclusion surrounded by an approximately round, bright area. flake graphite. An irregularly shaped body, usually appearing as long, curved plates of graphitic carbon, such as that found in gray cast irons. flakes. Short, discontinuous internal fissures in ferrous metals attributed to stresses produced by localized transformation and decreased solubility of hydrogen during cooling after hot working. In a fracture surface, flakes appear as bright, silvery areas; on an etched surface, they appear as short, discontinuous cracks. Also called shatter cracks or snowflakes. flame annealing. Annealing in which the heat is applied directly by a flame. flame hardening. A process for hardening the surfaces of hardenable ferrous alloys in which an intense flame is used to heat the surface layers above the upper transformation temperature, whereupon the workpiece is immediately quenched. flash. (1) In forging, the excess metal forced between the upper and lower dies. (2) In casting, a fin of metal that results from leakage between mating mold surfaces. (3) In resistance butt welding, a fin formed perpendicular to the direction of applied pressure. flash welding. A resistance welding process that joins metals by first heating abutting surfaces by passage of an electric current across the joint, then forcing the surfaces together by the application of pressure. Flashing and upsetting are accompanied by expulsion of metal from the joint. flatness of field. A qualitative term describing how well the image of a planar specimen is reproduced as a plane in the image field. flow. Movement (slipping or sliding) of essentially parallel planes within an element of a material in parallel directions; occurs under the action of shear stress. Continuous action in this manner, at constant volume and without disintegration of the material, is termed yield, creep, or plastic deformation. flow brightening. Melting of an electrodeposit, followed by solidification, especially of tin plate. flow lines. (1) Texture showing the direction of metal flow during

hot or cold working. Flow lines often can be revealed by etching the surface or a section of a metal part. (2) In mechanical metallurgy, paths followed by minute volumes of metal during deformation. flow stress. The uniaxial true stress at the onset of plastic deformation in a metal. fluorescence. The emission of characteristic electromagnetic radiation by a substance as a result of the absorption of electromagnetic or corpuscular radiation having a greater unit energy than that of the fluorescent radiation. It occurs only so long as the stimulus responsible for it is maintained. fluorescent magnetic-particle inspection. Inspection using a fluorescent liquid that will penetrate any surface opening; after the surface has been wiped clean, the location of any surface flaws may be detected by the fluorescence, under ultraviolet light, of back-seepage of the fluid. fluorescent screen. A sheet of material that emits visible light when exposed to invisible radiation. fluorescent x-rays (fluorescent analysis). Characteristic x-rays excited by radiation of wavelength shorter than the corresponding absorption edge. fluorite. Calcium fluoride; CaF2. A mineral of the cubic hexoctahedral class. Also called fluorspar. In transparent form it is used as a lens material in semiapochromatic objectives to correct for chromatic aberration in two colors and spherical aberration in two colors. Its Mohs hardness is 4, and its specific gravity is 3.2. focal length. The distance from the second principal point to the point on the axis at which parallel rays entering the lens will converge or focus. focal spot. That area on the target of an x-ray tube that is bombarded by electrons. focus. A point at which rays originating from a point in the object converge or from which they diverge or appear to diverge under the influence of a lens or diffracting system. focusing camera (x-rays). A diffraction camera in which the x-ray source of a divergent x-ray beam, specimen (polycrystalline), and film all lie on one circle, which results in the diffracted beams all being focused on the film. focusing device (electrons). A device that effectively increases the angular aperture of the electron beam illuminating the object, rendering the focusing more critical. focusing magnifier. A low-power microscope, telescope, or simple lens used to observe the electron image formed on a fluorescent screen. focusing (x-rays). The operation of producing a convergent beam in which all rays meet in a point or line. fog quenching. Quenching in a fine vapor or mist. foil. Metal in sheet form less than 0.15 mm (0.006 in.) thick. forgeability. Term used to describe the relative ability of material to flow under a compressive load without rupture. forged structure. The macrostructure through a suitable section of a forging that reveals direction of working. forging. Plastically deforming metal, usually hot, into desired shapes with compressive force, with or without dies. forging range. Temperature range in which a metal can be forged successfully. form. A set of equivalent planes in a crystal. In general, they will

266 / Metallographer’s Guide have the same spacing but different Miller indices. For example, in the cubic system, the planes (101) (110), (011), and so on are planes of the form (110). In the tetragonal system, however, the planes (101) and (110) belong to different forms. Equivalent directions are also spoken of as directions of a form. formability. The relative ease with which a metal can be shaped through plastic deformation. See drawability. forming. Making a change, with the exception of shearing or blanking, in the shape or contour of a metal part without intentionally altering its thickness. fractography. Descriptive explanation of a fracture process, especially in metals, with specific reference to photographs of the fracture surface. Macrofractography involves low magnification (25⫻). fracture. The irregular surface produced when a piece of metal is broken. See also fibrous fracture, granular fracture, intergranular fracture, silky fracture, and transgranular fracture. fracture grain size. Grain size determined by comparing a fracture of a specimen with a set of standard fractures. For steel, a fully martensitic specimen is generally used, and the depth of hardening and the prior austenitic grain size are determined. fracture mechanics. See linear elastic fracture mechanics. fracture stress. (1) The maximum principal true stress at fracture. Usually refers to unnotched tensile specimens. (2) The (hypothetical) true stress that will cause fracture without further deformation at any given strain. fracture test. Test in which a specimen is broken and its fracture surface is examined with the unaided eye or with a low-power microscope to determine such factors as composition, grain size, case depth, or soundness. fracture toughness. A generic term for measures of resistance to extension of a crack. The term is sometimes restricted to results of fracture mechanics tests, which are directly applicable in fracture control. However, the term commonly includes results from simple tests of notched or precracked specimens not based on fracture mechanics analysis. Results from tests of the latter type are often useful for fracture control, based on either service experience or empirical correlations with fracture mechanics tests. fragmentation. The subdivision of a grain into small, discrete crystallite outlined by a heavily deformed network of intersecting slip bands as a result of cold working. These small crystals or fragments differ in orientation and tend to rotate to a stable orientation determined by the slip systems. freckling. A type of segregation revealed as dark spots on a macroetched specimen of a consumable-electrode vacuum-arcremelted alloy. free carbon. The part of the total carbon content in steel or cast iron present in elemental form as graphite in gray and nodular cast iron or temper carbon in malleable cast iron. Contrast with combined carbon. free ferrite. Ferrite that is formed directly from the decomposition of hypoeutectoid austenite during cooling, without the simultaneous formation of cementite. Also called proeutectoid ferrite. free machining. Pertains to the machining characteristics of an

alloy to which one or more ingredients have been introduced to produce small broken chips, lower power consumption, better surface finish, or longer tool life; among such additions are sulfur or lead to steel, lead to brass, lead and bismuth to aluminum, and sulfur or selenium to stainless steel. freezing point. See melting point. freezing range. The temperature range between liquidus and solidus temperatures in which molten and solid constituents coexist. frequency (x-ray). The number of alternations per second of the electric vector of the x-ray beam. It is equal to the velocity divided by the wavelength. Fresnel fringes. A class of diffraction fringes formed when the source of illumination and the viewing screen are at a finite distance from a diffracting edge. In the electron microscope, these fringes are best seen when the object is slightly out of focus. fretting. A type of wear that occurs between tight-fitting surfaces subjected to cyclic relative motion of extremely small amplitude. Usually, fretting is accompanied by corrosion, especially of the very fine wear debris. Also referred to as fretting corrosion, false Brinelling (in rolling-element bearings), friction oxidation, chafing fatigue, molecular attrition, and wear oxidation. fretting corrosion. The deterioration at the interface between contacting surfaces as the result of corrosion and slight oscillatory slip between the two surfaces. fretting fatigue. Fatigue fracture that initiates at a surface area where fretting has occurred. full annealing. An imprecise term that denotes an annealing cycle designed to produce minimum strength and hardness. For the term to be meaningful, the composition and starting condition of the material and the time-temperature cycle used must be stated. full hard. A temper of nonferrous alloys and some ferrous alloys corresponding approximately to a cold worked state beyond which the material can no longer be formed by bending. In specifications, a full hard temper is commonly defined in terms of minimum hardness or minimum tensile strength (or, alternatively, a range of hardness or strength) corresponding to a specific percentage of cold reduction following full annealing. For aluminum, a full hard temper is equivalent to a reduction of 75% from dead soft; for austenitic stainless steels, a reduction of about 50 to 55%. fusion. A change of state from solid to liquid melting. fusion zone. In a weldment, the area of base metal melted as determined on a cross section through the weld.

G galvanic cell. A cell in which chemical change is the source of electrical energy. It usually consists of two dissimilar conductors in contact with each other and with an electrolyte, or of two similar conductors in contact with each other and with dissimilar electrolytes.

Glossary / 267 galvanic corrosion. Corrosion with the current of a galvanic cell consisting of two dissimilar conductors in an electrolyte or two similar conductors in dissimilar electrolytes. Where the two dissimilar metals are in contact, the resulting reaction is referred to as couple action. galvanic series. A series of metals and alloys arranged according to their relative electrode potentials in a specified environment. Compare with electromotive series. galvanize. A process where zinc is coated upon steel for galvanic corrosion protection. The zinc can be applied electrolytically or by passing the steel through a bath or molten zinc. Galvanized coating now includes many zinc alloys, for example, zincaluminum, zinc-nickel, and zinc-iron. galvanneal. To produce a zinc-iron alloy coating on iron or steel by keeping the coating molten after hot dip galvanizing until the zinc alloys completely with the basis metal. gamma iron. The face-centered cubic form of pure iron, stable from 910 to 1400 °C (1670 to 2550 °F). gamma ray. Short-wavelength electromagnetic radiation, similar to x-rays but of nuclear origin, with a range of wavelengths from about 10⫺14 to 10⫺10 m. gamma stabilizer. In iron alloys, manganese and nickel are gamma stabilizers. gas porosity. Fine holes or pores within a metal that are caused by entrapped gas or by evolution of dissolved gas during solidification. ghost lines. (1) Lines running parallel to the rolling direction that appear in a panel when it is stretched. These lines may not be evident unless the panel has been sanded or painted. (Not to be confused with leveler lines.) (2) Metallographic term used to describe white lines sometimes observed after steel has been worked then partially or fully annealed. glancing angle. The angle (usually small) between an incident x-ray beam and the surface of the specimen. glazing. Dulling the abrasive grains in the cutting face of a wheel during grinding. glide. (1) Same as slip. (2) A noncrystallographic shearing movement, such as of one grain over another. goniometer. An instrument devised for measuring the angle through which a specimen is rotated. graded abrasive. An abrasive powder in which the sizes of the individual particles are confined to certain specified limits. grain. An individual crystal in a polycrystalline metal or alloy; it may or may not contain twinned regions or subgrains. grain boundary. An interface separating two grains at which the orientation of the lattice changes from that of one grain to that of the other. When the orientation change is very small, the boundary is sometimes referred to as a sub-boundary structure. grain-boundary corrosion. Same as intergranular corrosion. See also interdendritic corrosion. grain-boundary etching. Development of intersections of grain faces with the polished surface. Because of severe, localized crystal deformation, grain boundaries have higher dissolution potential than grains themselves. Accumulation of impurities in grain boundaries increases this effect. grain-boundary liquation. An advanced stage of overheating in

which material in the region of austenitic grain boundaries melts. Also termed burning. grain-boundary sulfide precipitation. An intermediate stage of overheating in which sulfide inclusions are redistributed to the austenitic grade boundaries by partial solution at the overheating temperature and reprecipitation during subsequent cooling. grain coarsening. A heat treatment that produces excessively large austenitic grains. grain-contrast etching. Development of grain surfaces lying in the polished surface of the microsection. These become visible through differences in reflectivity caused by reaction products on the surface or by differences in roughness. grain flow. Fiberlike lines appearing on polished and etched sections of forgings, which are caused by orientation of the constituents of the metal in the direction of working during forging. Grain flow produced by proper die design can improve required mechanical properties of forgings. grain growth. An increase in the average size of the grains in polycrystalline metal, usually as a result of heating at an elevated temperature. grain size. (1) For metals, a measure of the areas or volumes of grains in a polycrystalline metal or alloy, usually expressed as an average when the individual sizes are fairly uniform. In metals containing two or more phases, the grain size refers to that of the matrix unless otherwise specified. Grain size is reported in terms of number of grains per unit area or volume, average diameter, or as a grain-size number derived from area measurements. See ASTM E 112. (2) For grinding wheels, see the preferred term grit size. grain size comparison eyepiece. An eyepiece provided with calibrated patterns representing a series of standard sizes of grains. The eyepiece must be used at a total magnification for which the patterns have been calibrated. granular bainite. A special form of bainite that consists of a ferrite matrix and martensite plus austenite constituent (instead of a carbide phase). granular fracture. A type of irregular surface produced when metal is broken, characterized by a rough, grainlike appearance as differentiated from a smooth, silky, or fibrous type. It can be subclassified into transgranular and intergranular forms. This type of fracture is frequently called crystalline fracture, but the inference that the metal broke because it “crystallized” is not justified, because all metals are crystalline in the solid state. Contrast with fibrous fracture and silky fracture. graphite. The polymorph of carbon with a hexagonal crystal structure. See also flake graphite, nodular graphite, rosette graphite, and spheroidal graphite. graphite, flake. An irregularly shaped body, usually appearing as long, curved plates of graphitic carbon such as found in gray cast irons. graphite, nodular. Rounded clusters of temper carbon such as obtained, for example, in malleable cast iron as a result of the thermal decomposition of cementite or in ductile iron as a result of inoculation. graphite, rosette. Arrangement of graphite flakes in which the flakes extend radially from centers of crystallization in gray cast iron.

268 / Metallographer’s Guide graphitic carbon. Free carbon in steel or cast iron. graphitic corrosion. Corrosion of gray iron in which the iron matrix is selectively leached away, leaving a porous mass of graphite behind; it occurs in relatively mild aqueous solutions and on buried pipe and fittings (not to be confused with graphitization). graphitic steel. Alloy steel made so that part of the carbon is present as graphite. graphitization. Formation of graphite in iron or steel. Where graphite is formed during solidification, the phenomenon is called primary graphitization; where formed later by heat treatment, secondary graphitization (not to be confused with graphitic corrosion). graphitizing. Annealing a ferrous alloy in such a way that some or all of the carbon is precipitated as graphite. gray cast iron. A cast iron that gives a gray fracture due to the presence of flake graphite. Often called gray iron. grinding. The removal of material from the surface of a specimen by abrasion through the use of hard-abrasive particles randomly oriented to each other (for example, grind stone) or bonded to a suitable substrate such as paper or cloth, where the abrasive particle size is generally in the range of 60 to 1000 grit (approximately 150 to 5 μm) but may be finer. grinding burn. See burning (2). grinding cracks. Shallow cracks formed in the surfaces of relatively hard materials because of excessive grinding heat or the high sensitivity of the material. See also grinding sensitivity. grinding sensitivity. Susceptibility of a material to surface damage such as grinding cracks; it can be affected by such factors as hardness, microstructure, hydrogen content, and residual stress. grinding stress. Residual stress, generated by grinding, in the surface layer of work. It may be tensile or compressive, or both. grit. Individual particles of abrasive. grit size. Nominal size of abrasive particles in a grinding wheel, corresponding to the number of openings per linear inch in a screen through which the particles can pass. Sometimes, but inadvisedly, called grain size. ground-glass focusing screen. A glass screen, one side of which is ground or made diffusing and mounted for use in a camera, in place of photosensitive material, for the purpose of intercepting, viewing, and focusing a real image formed on it. Guinier-Preston (G-P) zone. A small precipitation domain in a supersaturated metallic solid solution. A G-P zone has no well-defined crystalline structure of its own and contains an abnormally high concentration of solute atoms. The formation of G-P zones constitutes the first stage of precipitation and is usually accompanied by a change in properties of the solid solution in which they occur.

H habit plane. The plane or system of planes of a crystalline phase along which some phenomenon, such as twinning or transformation, occurs. hairline crack. See flakes.

half hard. A temper of nonferrous alloys and some ferrous alloys characterized by tensile strength about midway between those of dead soft and full hard tempers. Hall-Petch relationship. A relationship where the yield strength of steel is directly proportional to the reciprocal square root of grain size. hard chromium. Chromium electrodeposited for engineering purposes (such as to increase the wear resistance of sliding metal surfaces) rather than as a decorative coating. It is usually applied directly to basis metal and is customarily thicker than a decorative deposit, but not necessarily harder. hardenability. The relative ability of a ferrous alloy to form martensite when quenched from a temperature above the upper critical temperature. Hardenability is commonly measured as the distance below a quenched surface at which the metal exhibits a specific hardness—50 HRC, for example—or a specific percentage of martensite in the microstructure. hardening. Increasing hardness by suitable treatment, usually involving heating and cooling. When applicable, the following more specific terms should be used: age hardening, case hardening, flame hardening, induction hardening, precipitation hardening, and quench hardening. hardfacing. Depositing filler metal on a surface by welding, spraying, or braze welding to increase resistance to abrasion, erosion, wear, galling, impact, or cavitation damage. hardness. A measure of the resistance of a material to surface indentation or abrasion; may be thought of as a function of the stress required to produce some specified type of surface deformation. There is no absolute scale for hardness; therefore, to express hardness quantitatively, each type of test has its own scale of arbitrarily defined hardness. hardness (indentation). Resistance of a metal to plastic deformation by indentation. Various hardness tests such as Brinell, Knoop, Rockwell, Scleroscope, and Vickers may be used. In the Vickers test, a diamond pyramid with an included face angle of 136° is used as the indenter. hard surfacing. Same as hardfacing. hard temper. Same as full hard temper. hard (x-rays). Of short wavelength. HAZ. See heat-affected zone. H-band steel. Alloy steel produced to specified limits of hardenability; the chemical composition range may be slightly different from that of the corresponding grade of ordinary alloy steel. heat-affected zone (HAZ). That portion of the base metal that was not melted during brazing, cutting, or welding, but whose microstructure and mechanical properties were altered by the heat. heat check. A pattern of parallel surface cracks that are formed by alternate rapid heating and cooling of the extreme surface metal, sometimes found on forging dies and piercing punches. There may be two sets of parallel cracks, one set perpendicular to the other. heating curve. Graphical representation of the course of temperature rise of a sample or body as a function of time. heat-resisting alloy. An alloy developed for very-high-tempera-

Glossary / 269 ture service where relatively high stresses (tensile, thermal, vibratory, or shock) are encountered and where oxidation resistance is frequently required. heat tinting. Coloration of a metal surface through thermal oxidation by heating to reveal details of structure. heat treating film. A thin coating or film, usually an oxide, formed on the surface of metals during heat treatment. heat treatment. Heating and cooling a solid metal or alloy in such a way as to obtain desired conditions or properties. Heating for the sole purpose of hot working is excluded from the meaning of this definition. hematite. The oxide of iron of highest valence that has a composition close to the stoichiometric composition Fe2O3. herringbone pattern. Same as chevron pattern. heterogeneous equilibrium. In a chemical system, a state of dynamic balance among two or more homogeneous phases capable of stable coexistence in mutual or sequential contact. hexagonal close-packed. (1) A structure containing two atoms per unit cell located at (0, 0, 0) and (1⁄3, 2⁄3, 1⁄2) or (2⁄3, 1⁄2, 1⁄2). (2) One of the two ways in which spherical objects can be most closely packed together so that the close-packed planes are alternately staggered in the order A-B-A-B-A-B. hexagonal (lattices for crystals). Having two equal coplanar axes, a1 and a2, at 120° to each other and a third axis, c, at right angles to the other two; c may or may not equal a1 and a2. Heyn method. An intercept method for determining grain size. See Methods E 112, for Determining Average Grain Size. high-cycle fatigue. Fatigue that occurs at relatively large numbers of cycles. The arbitrary, but commonly accepted, dividing line between high-cycle fatigue and low-cycle fatigue is considered to be about 104 to 105 cycles. In practice, this distinction is made by determining whether the dominant component of the strain imposed during cyclic loading is elastic (high cycle) or plastic (low cycle), which in turn depends on the properties of the metal and on the magnitude of the nominal stress. high-strength low-alloy steel (HSLA steel). A high-strength steel that achieves its strength level by a small addition of alloying elements (see microalloying). homogeneous carburizing. Use of a carburizing process to convert a low-carbon ferrous alloy to one of uniform and higher carbon content throughout the section. homogeneous (radiation, monochromatic). Of the same wavelength. homogenizing. Holding at high temperature to eliminate or decrease chemical segregation by diffusion. homogenizing annealing. An annealing treatment carried out at a high temperature, approaching the solidus temperature, for a sufficiently long time that inhomogeneous distributions of alloying elements are reduced by diffusional processes. Hooke’s law. A material in which stress is linearly proportional to strain is said to obey Hooke’s law. This law is valid only up to the proportional limit, or the end of the straight-line portion of the stress-strain diagram. See also modulus of elasticity. hot-cold working. (1) A high-temperature thermomechanical treatment consisting of deforming a metal above its transformation temperature and cooling fast enough to preserve some or

all of the deformed structure. (2) A general term synonymous with warm working. hot crack. A crack formed in a cast metal because of internal stress developed upon cooling following solidification. A hot crack is less open than a hot tear and usually exhibits less oxidation and decarburization along the fracture surface. hot dip coating. A metallic coating obtained by dipping the basis metal into a molten metal. hot forming. See hot working. hot quenching. An imprecise term used to cover a variety of quenching procedures in which a quenching medium is maintained at a prescribed temperature above 70 °C (160 °F). hot rolling. The process of rolling a sheet or plate where the workpiece is heated, usually above the Ac3. hot shortness. A tendency for some alloys to separate along grain boundaries when stressed or deformed at temperatures near the melting point. Hot shortness is caused by a low-melting constituent, often present only in minute amounts, that is segregated at grain boundaries. hot stage. A microscope stage capable of heating and cooling a specimen during observation. hot tear. A crack or fracture formed before completion of solidification because of hindered contraction. A hot tear is frequently open to the surface of the casting and thus exposed to the atmosphere. This may result in oxidation, decarburization, or other metal-atmosphere reactions at the tear surface. hot-worked structure. The structure of a material worked at a temperature higher than the recrystallization temperature. hot working. Deforming metal plastically at such a temperature and strain rate that recrystallization takes place simultaneously with the deformation, thus avoiding any strain hardening. Hull-Davey charts. Charts for indexing the lines of powder patterns on which a function of the interplanar spacing of the Bragg angle is plotted against the axial ratio for a number of different lattice planes. Hull method (for x-ray crystal analysis). See Debye-Scherrer method. Huygens eyepiece. An achromatic eyepiece invented by Huygens and consisting of a plano-convex eyelens and a planoconvex collective, between which is a field diaphragm. hydrogen blistering. The formation of blisters on or below a metal surface from excessive internal hydrogen pressure. Hydrogen may be formed during cleaning, plating, corrosion, and so on. hydrogen damage. A general term for the embrittlement, cracking, blistering, and hydride formation that can occur when hydrogen is present in some metals. hydrogen embrittlement. A condition of low ductility in metals resulting from the absorption of hydrogen. hydrogen-induced delayed cracking. A term sometimes used to identify a form of hydrogen embrittlement in which a metal appears to fracture spontaneously under a steady stress less than the yield stress. There is usually a delay between the application of stress (or exposure of the stressed metal to hydrogen) and the onset of cracking. Also referred to as static fatigue. hypereutectic alloy. In an alloy system exhibiting a eutectic, any

270 / Metallographer’s Guide alloy whose composition has an excess of alloying element compared with the eutectic composition and whose equilibrium microstructure contains some eutectic structure. hypereutectoid alloy. In an alloy system exhibiting a eutectoid, any alloy whose composition has an excess of alloying element compared with the eutectoid composition and whose equilibrium microstructure contains some eutectoid structure. hypereutectoid structure. The microstructure of a hypereutectoid alloy. For example, microstructural aggregate found in iron-carbon alloys that consist of primary crystals of cementite together with nodules of pearlite. hypoeutectic alloy. In an alloy system exhibiting a eutectic, any alloy whose composition has an excess of base metal compared with the eutectic composition and whose equilibrium microstructure contains some eutectic structure. hypoeutectoid alloy. In an alloy system exhibiting a eutectoid, any alloy whose composition has an excess of base metal compared with the eutectoid composition and whose equilibrium microstructure contains some eutectoid structure. hysteresis, magnetic. The lag of the magnetization of an iron or steel specimen behind any cyclic variation of the applied magnetizing field.

I idiomorph. Crystals that have grown without restraint so that the habit planes are clearly developed. idiomorphic crystal. An individual crystal that has grown without restraint so that the habit planes are clearly developed. Compare with allotriomorphic crystal. illumination. See bright-field illumination, dark-field illumination, differential interference contrast illumination, and polarized light illumination. image. A representation of an object produced by radiation, usually with a lens or mirror system. image rotation. In electron optics, the angular shift of the electron image of an object about the optic axis induced by the tangential component of force exerted on the electrons perpendicular to the direction of motion in the field of a magnetic lens. immersion etching. Method in which a microsection is dipped face up into etching solution and is moved around during etching. This is the most common etching method. immersion lens. See immersion objective. immersion objective. An objective in which a medium of high refractive index is used in the object space to increase the numerical aperture and therefore the resolving power of the lens. impact test. A test to determine the behavior of materials when subjected to high rates of loading, usually in bending, tension, or torsion. The quantity measured is the energy absorbed in breaking the specimen by a single blow, as in Charpy and Izod tests. imperfection. (1) When referring to the physical condition of a part or metal product, any departure of a quality characteristic from its intended level or state. The existence of an imperfec-

tion does not imply nonconformance, nor does it have any implication as to the usability of a product or service. An imperfection must be rated on a scale of severity, in accordance with applicable specifications, to establish whether or not the part or metal product is of acceptable quality. (2) Generally, any departure from an ideal design, state, or condition. (3) In crystallography, any deviation from an ideal space lattice. impingement attack. Corrosion associated with turbulent flow of liquid. May be accelerated by entrained gas bubbles. See also erosion. impregnation. (1) Treatment of porous castings with a sealing medium to stop pressure leaks. (2) The process of filling the pores of a sintered compact, usually with a liquid such as a lubricant. (3) The process of mixing particles of a nonmetallic substance in a matrix of metal powder, as in diamond-impregnated tools. impression replica. See replica. impurities. Elements or compounds whose presence in a material is undesirable. inclusion. A particle of foreign material in a metallic matrix. The particle is usually a compound (such as an oxide, sulfide, or silicate), but may be of any substance that is foreign to (and essentially insoluble in) the matrix. Inclusions are usually considered undesirable, although in some cases—such as in free-machining metals—manganese sulfides, phosphorus, selenium, or tellurium may be deliberately introduced to improve machinability. inclusion count. Determination of the number, kind, size, and distribution of nonmetallic inclusions. incoherent scattering. The deflection of electrons by electrons or atoms that results in a loss of kinetic energy by the incident electron. See also coherent scattering. incongruent transformation. Nonisothermal, or nonisobaric, phase change in which one, or both, of the phases involved undergo composition change during the process. indices. See Miller indices. indigenous inclusions. Nonmetallic inclusion that form by reaction of components within the alloy. For example, manganese sulfide (MnS) in steel. inelastic electron scatter. See incoherent scattering. inflection point. Position on a curved line, such as a phase boundary, at which the direction of curvature is reversed. infrared. Invisible light and heat radiation, adjacent to the red end of the visible spectrum with wavelengths from 700 to about 3000 nm (nanometers). ingot. A casting of simple shape, suitable for hot working or remelting. ingot iron. Commercially pure iron. inhibitor. A substance that retards some specific chemical reaction. Pickling inhibitors retard the dissolution of metal without hindering the removal of scale from steel. inoculation. The addition of a material to molten metal to form nuclei for crystallization, as in adding magnesium to produce nodular graphite in ductile cast iron. integrating camera. A diffraction camera in which the specimen is moved relative to the incident beam in order to cause

Glossary / 271 diffraction to occur from an extended area of the specimen surface. intensifying screen. A sheet of a substance that emits visible light, x-rays, or photoelectrons, or combinations of these, under the action of x-rays, thus enhancing the darkening of a film placed in contact with it. intensity of scattering. The energy per unit time per unit area of the general radiation diffracted by matter. Its value depends on the scattering power of the individual atoms of the material, the scattering angle, and the wavelength of the radiation. intensity (x-rays). The energy per unit of time of a beam per unit area perpendicular to the direction of propagation. intercept method. A quantitative metallographic technique in which the desired quantity, such as grain size or amount of precipitate, is expressed as the number of times per unit length a straight line on a metallographic image crosses particles of the feature being measured. See ASTM E 112. intercrystalline. Between crystals, or between grains of a metal. Same as intergranular. intercrystalline cracks. Cracks or fractures that occur between the grains or crystals in a polycrystalline aggregate. interdendritic. Located within the branches of a dendrite or between the boundaries of two or more dendrites. interdendritic corrosion. Corrosive attack that progresses preferentially along interdendritic paths. This type of attack results from local differences in composition, such as coring commonly encountered in alloy castings. interdendritic porosity. Voids occurring between the dendrites in cast metal. interface. A surface that forms the boundary between phases or systems. interfacial tension. The contractile force of an interface between two phases. interference. The effect of a combination of wave trains of various phases and amplitudes. interference filter. A combination of several thin optical films to form a layered coating for transmitting or reflecting a narrow band of wavelengths by interference effects. intergranular. Between crystals or grains. Also termed intercrystalline. Contrast with transgranular. intergranular corrosion. Corrosion occurring preferentially at grain boundaries, usually with slight or negligible attack on the adjacent grains. See also interdendritic corrosion. intergranular cracking. Cracking or fracturing that occurs between the grains or crystals in a polycrystalline aggregate. Contrast with transgranular cracking. intergranular fracture. Brittle fracture of a metal in which the fracture is between the grains, or crystals, that form the metal. Contrast with transgranular fracture. intergranular stress-corrosion cracking. Stress-corrosion cracking, in which the cracking occurs along grain boundaries. interlamellar spacing. The spacing between adjacent lamella in a pearlitic structure. intermediary plane. Any place in a microscope where a real image of a specimen is formed. Reticles can be inserted at intermediary planes for superposition on the image.

intermediate annealing. Annealing wrought metals at one or more stages during manufacture and before final thermal treatment. intermediate phase. In an alloy or a chemical system, a distinguishable homogeneous phase whose composition range of existence does not extend to any of the pure components of the system. intermetallic compound. An intermediate phase in an alloy system having a narrow range of homogeneity and relatively simple stoichiometric proportions; the nature of the atomic binding can be of various types, ranging from metallic to ionic. intermetallic phases. Compounds, or intermediate solid solutions, containing two or more metals, which usually have compositions, characteristic properties, and crystal structures different from those of the pure components of the system. internal oxidation. (1) The formation of isolated particles of corrosion products beneath the metal surface. This occurs as the result of preferential oxidation of certain alloy constituents by inward diffusion of oxygen, nitrogen, sulfur, and so on. Also called subsurface corrosion. (2) Preferential in situ oxidation of certain components of phases within the bulk of a solid alloy accomplished by diffusion of oxygen into the body. This is commonly used to prepare electrical contact materials. interplanar distance. The perpendicular distance between adjacent parallel lattice planes. interpupillary distance. Spacing between the pupils of the eyes; eyepieces on binocular microscopes should be set at this distance for comfortable and accurate viewing. interrupted aging. Aging at two or more temperatures, by steps, and cooling to room temperature after each step. See also aging, and compare with progressive aging and step aging. interrupted quenching. A quenching procedure in which the workpiece is removed from the first quenching at a temperature substantially higher than that of the quenchant and is then subjected to a second quenching system having a cooling rate different from that of the first. interstitial solid solution. A solid solution in which the solute atoms occupy positions that do not correspond to lattice points of the solvent. Contrast with substitutional solid solution. intracrystalline. Within or across the crystals or grains of a metal; same as transcrystalline and transgranular. intracrystalline cracking. See transcrystalline cracking. inverse chill. A condition in an iron casting in which the interior is comprised of chilled or white iron, while the surfaces are either mottled or contain free graphite. inverse segregation. Segregation in case metal in which an excess of lower-melting constituents occurs in the earlier freezing portions, apparently the result of liquid metal entering cavities developed in the earlier-solidified metal. inverted microscope. A microscope arranged so that the line of sight is directed upward through the objective to the object. investment casting. (1) Casting metal into a mold produced by surrounding (investing) an expendable pattern with a refractory slurry that sets at room temperature, after which the wax,

272 / Metallographer’s Guide plastic, or frozen mercury pattern is removed through the use of heat. Also called precision casting or lost wax process. (2) A part made by the investment casting process. ion. An atom, or group of atoms, that has gained or lost one or more outer electrons and thus carries an electric charge. Positive ions, or cations, are deficient in outer electrons. Negative ions, or anions, have an excess of outer electrons. ion etching. Surface removal by bombarding with accelerated ions in vacuum (1 to 10 kV). ionic bond. A bond between two or more atoms that is the result of electrostatic attractive forces between positively and negatively charged ions. ionic crystal. A crystal in which atomic bonds are ionic bonds. This type of atomic linkage, also known as (hetero) polar bonding, is characteristic of many compounds (sodium chloride, for instance). iron. An element that has an average atomic number of 55.85 and that usually, in engineering practice, contains small but significant amounts of carbon. isobar. Section at constant pressure through a phase diagram. isochor. In a phase diagram, a section, or contour, at constant volume. isomorphous. Having the same crystal structure. This usually refers to intermediate phases that form a continuous series of solid solutions. isomorphous system. A complete series of mixtures in all proportions of two or more components in which unlimited mutual solubility exists in the liquid and solid states. isotherm. Section, at constant temperature, through a phase diagram. isothermal annealing. Austenitizing a ferrous alloy, then cooling to and holding at a temperature at which austenite transforms to a relatively soft ferrite-carbide aggregate. See also austenitizing. isothermal transformation. A change in phase that takes place at a constant temperature. The time required for transformation to be completed, and in some instances the time delay before transformation begins, depends on the amount of supercooling below (or superheating above) the equilibrium temperature for the same transformation. isothermal transformation (IT) diagram. A diagram that shows the isothermal time required for transformation of austenite to begin and to finish as a function of temperature. Same as time-temperature-transformation (TTT) diagram or S-curve. isotropic. Having equal values of properties in all directions. Quasi-isotropic refers to material in which statistical uniformity exists, such as polycrystalline metals. isotropy. The quality of having identical properties in all directions.

J Jeffries’ method. A method for determining grain size based on counting grains in a prescribed area. Jeffries’ multiplier. A factor used in the Jeffries’ method for

grain size determinations. See ASTM E 112, “Standard Test Methods for Determining Average Grain Size”. Jominy test. See end-quench hardenability test.

K K. (abbreviation) x-rays. See also K-radiation. Kellner eyepiece. A positive eyepiece consisting of an achromatic eyelens and a single collective in which the image plane and field diaphragm is external and near the collective. kerf. The space that was occupied by material removed during cutting. Kikuchi lines. Light and dark lines superimposed on the background of a single-crystal electron-diffraction pattern caused by diffraction of diffusely scattered electrons within the crystal; the pattern provides information on the structure of the crystal. killed steel. Steel treated with a strong deoxidizing agent, such as silicon or aluminum, in order to reduce the oxygen content to such a level that no reaction occurs between carbon and oxygen during solidification. kink band (deformation). In polycrystalline materials, a volume of crystal that has rotated physically to accommodate differential deformation between adjoining parts of a grain while the band itself has deformed homogeneously. This occurs by regular bending of the slip lamellae along the boundaries of the band. kish. Free graphite that forms in molten hypereutectic cast iron as it cools. In castings, the kish may segregate toward the cope surface, where it lodges at or immediately beneath the casting surface. Kohler illumination. A specular illumination system. In reflected-light microscopy, used directly for the bright-field mode, and as a preliminary setup for all other modes except dark-field. The image of the field diaphragm is focused on the specimen surface, and the image of an undiffused lamp source is focused in the plane of the aperture diaphragm. K-radiation. Characteristic x-rays produced by an atom when a vacancy in the K shell is filled by one of the outer electrons. K-series. The set of x-ray wavelengths comprising K-radiation. KX. (abbreviation) Kilo X unit ⫽ 1000 X units (X.U.) ⫽ 1.00203 angstrom units (␭). See also angstrom unit.

L lamellae, lamellar. A platelike morphology. See also lamellar structure. lamellar structure. A microstructure consisting of parallel plates of a second phase, generally associated with eutectic and eutectoid transformation. Pearlite is a lamellar structure consisting of alternate plates of ferrite and cementite. lamellar tear. A system of cracks or discontinuities aligned generally parallel to the worked surface of a plate. This is usually associated with a fusion weld in thick plate. lamination. (1) A type of discontinuity with separation of weak-

Glossary / 273 ness generally aligned parallel to the worked surface of a metal. May be the result of pipe, blisters, seams, inclusions, or segregation elongated and made directional by working. Lamination may also occur in powder metallurgy compacts. (2) In electrical products such as motors, a blanked piece of electrical sheet that is stacked up with several other identical pieces to make a stator or rotor. lap. A surface imperfection, with the appearance of a seam, caused by hot metals, fins, or sharp corners being folded over and then being rolled or forged into the surface but without being welded. lap joint. A joint made between two overlapping members. lapping. Finishing a surface by abrasion with an object, usually made of copper, lead, cast iron, or close-grained wood, having very fine abrasive particles rolled into its surface. latent heat. Thermal energy absorbed or released when a substance undergoes a phase change. lath martensite. Martensite formed partly in steels containing less than approximately 1.0% C and solely in steels containing less than approximately 0.6% C as parallel arrays of packets of lath-shape units 0.1 to 0.3 μm thick. lattice. (1) A space lattice is a set of equal and adjoining parallelopipeds formed by dividing space by three sets of parallel planes, the planes in any one set being equally spaced. There are seven ways of so dividing space, corresponding to the seven crystal systems. The unit parallelopiped is usually chosen as the unit cell of the system. See also crystal system. (2) A point lattice is a set of points in space located so that each point has identical surroundings. There are 14 ways of so arranging points in space, corresponding to the 14 Bravais lattices. lattice constant. See lattice parameter. lattice parameter. The length of any side of a unit cell of a given crystal structure; if the lengths are unequal, all unequal lengths must be given. Laue equations. The three simultaneous equations that state the conditions to be met for diffraction from a three-dimensional network of diffraction centers. Laue method (for crystal analysis). A method of x-ray diffraction using a beam of white radiation, a fixed single crystal specimen, and a flat photographic film usually normal to the incident beam. If the film is located on the same side of the specimen as the x-ray source, the method is known as the back reflection Laue method; if on the other side, as the transmission Laue method. leaded. Characteristic of a metallic body containing metallic lead in dispersed form. ledeburite. The eutectic of the iron-carbon system, the constituents of which are austenite and cementite. The austenite decomposes into ferrite and cementite on cooling below Ar1. lens. A transparent optical element, so constructed that it serves to change the degree of convergence or divergence of the transmitted rays (light, electrons, etc.). lens, Bertrand. A small convergent lens placed between objectives and eyepiece. The lens focuses an image of the upper focal plane of the objective on the focal plane of the eyepiece. It is chiefly used with polarized light for inspecting the interference

figure. It is also convenient for quickly verifying centering size and for uniform illumination of an aperture. lens, compound. A lens composed of two or more separate pieces of glass or other optical material. These component pieces of elements may or may not be cemented together. A common form of compound lens is a two-element objective, one element being a converging lens of crown glass and the other a diverging lens of flint glass. The combination of suitable glasses or other optical materials (plastics, minerals) properly ground and polished reduces aberrations normally present in a single lens. lens, negative. A lens that is thicker on the edges that in the center and which causes parallel light rays to diverge. Also called diverging lens. lever principle. In a phase diagram, the relative proportions of the conjugate phases, at a stated value of temperature and pressure, or both, are such that a state of mechanical balance would be obtained if the corresponding weight of each phase were placed upon its composition point upon the tie element (tie line, tie triangle, etc.) and the fulcrum were located at the gross composition point of the mixture. lever rule. A method that can be applied to any two-phase field of a binary phase diagram to determine the amounts of the different phases present at a given temperature in a given alloy. A horizontal line, referred to as a tie line, represents the lever, and the alloy composition its fulcrum. The intersection of the tie line with the boundaries of the two-phase field fixes the compositions of the coexisting phases, and the amounts of the phases are proportional to the segments of the tie line between the alloy and the phase compositions. light-field illumination. See bright-field illumination. light filter. See color filter. light metal. One of the low-density metals, such as aluminum, magnesium, titanium, beryllium, or their alloys. light microscope. A microscope that uses light radiation (photons), as opposed to an electron microscope that uses electrons. Sometimes called an optical microscope or light optical microscope. limited solid solution. A crystalline miscibility series whose composition range does not extend all the way between the components of the system; that is, the system is not isomorphous. limiting current density. The maximum current density that can be used to obtain a desired electrode reaction without undue interference such as from polarization. lineage structure. (1) Deviations from perfect alignment of parallel arms of a columnar dendrite as a result of interdendritic shrinkage during solidification from a liquid. This type of deviation may vary in orientation from one area to another from a few minutes to as much as two degrees of arc. (2) A type of substructure consisting of elongated subgrains. linear elastic fracture mechanics. A method of fracture analysis that can determine the stress (or load) required to induce fracture instability in a structure containing a cracklike flaw of known size and shape. linear magnification. See magnification.

274 / Metallographer’s Guide linear strain. See strain. line indices. The Miller indices of the set of planes producing a diffraction line. line (in x-ray diffraction patterns). An array of small diffraction spots arranged so that they appear to form a continuous line on the film. liquation. Partial melting of an alloy, usually as a result of coring or other compositional heterogeneities. liquation temperature. The lowest temperature at which partial melting can occur in an alloy that exhibits the greatest possible degree of segregation. liquid metal embrittlement. The decrease in ductility of a metal caused by contact with a liquid metal. liquid penetrant inspection. A type of nondestructive inspection that locates discontinuities that are open to the surface of a metal by first allowing a penetrating dye or fluorescent liquid to infiltrate that discontinuity, removing the excess penetrant, and then applying a developing agent that causes the penetrant to seep back out of the discontinuity and register as an indication. Liquid penetrant inspection is suitable for both ferrous and nonferrous materials, but is limited to the detection of open surface discontinuities in nonporous solids. liquidus. In a constitution or equilibrium, diagram, the locus of points representing the temperatures at which the various compositions begin to freeze upon cooling or finish melting upon heating. See also solidus. loading. (1) In cutting, building up of a cutting tool back of the cutting edge by undesired adherence of material removed from the work. (2) In grinding, filling the pores of a grinding wheel with material from the work, usually resulting in a decrease in production and quality of finish. (3) In powder metallurgy, filling of the die cavity with powder. local action. Corrosion due to the action of “local cells”—that is, galvanic cells resulting from inhomogeneities between adjacent areas on a metal surface exposed to an electrolyte. local cell. A galvanic cell resulting from inhomogeneities between areas on a metal surface in an electrolyte. The inhomogeneities may be of physical or chemical nature in either the metal or its environment. local current density. Current density at a point or on a small area. localized precipitation. Precipitation from a supersaturated solid solution similar to continuous precipitation, except that the precipitate particles form at preferred locations, such as along slip planes, grain boundaries, or incoherent twin boundaries. longitudinal direction. The principal direction of flow in a worked metal. See also normal direction and transverse direction. low-alloy steel. A steel that contains alloying elements (such as manganese, silicon, chromium, nickel, or molybdenum) up to a level no greater than about 8 wt%. low-cycle fatigue. Fatigue that occurs at relatively small numbers of cycle (0.9% C). plating. Forming an adherent layer of metal on an object; often used as a shop term for electroplating. plating range. The current-density range over which a satisfactory electroplate can be deposited. plowing. In tribology, the formation of grooves by plastic deformation of the softer of two surfaces in relative motion. plumbago. A special quality of powdered graphite used to coat molds and, in a mixture with clay, to make crucibles. P/M. The acronym for powder metallurgy. point projection x-ray microscopy. A method of producing enlarged images by means of x-rays. The specimen is placed close to a point source of x-rays, and the magnification achieved is the ratio of source-image to source-object distance. Resolution depends primarily on the diameter of the source. polarization. A change in the potential of an electrode during electrolysis, such that the potential of an anode becomes more noble, and that of a cathode more active, than their respective reversible potentials. Often accomplished by formation of a film on the electrode surface. polarized light illumination. A method of illumination in which the incident light is plane polarized before it impinges on the specimen. polarizer. A Nicol prism, polarizing film, or similar device into which normal light passes and from which polarized light emerges. pole. (1) A means of designating the orientation of a crystal plane by stereographically plotting its normal plane. For example, the north pole defines the equatorial plane. (2) Either of the two regions of a permanent magnet or electromagnet where most of the lines of induction enter or leave. pole figure. A sterographic projection representing the statistical average distribution of poles of a specific crystalline plane in a polycrystalline metal, with reference to an external system of axes. In an isotropic metal, that is, in one having a completely random distribution of orientations, the pole density is stereo-

graphically uniform; preferred orientation is shown by an increased density of poles in certain areas. pole figure (crystalline aggregates). A graph of the crystal orientations present in an aggregate. polished surface. A surface that reflects a large proportion of the incident light in a specular manner. polishing. Smoothing metal surfaces, often to a high luster, by rubbing the surface with a fine abrasive, usually contained in a cloth or other soft lap. Results in a microscopic flow of some surface metal together with actual removal of a small amount of surface metal. May be extended to include electropolishing. Contrast with burnishing. polishing artifact. A false structure introduced during a polishing stage of a surface-preparation sequence. polishing rate. The rate at which material is removed from a surface during polishing. It is usually expressed in terms of the thickness removed per unit of time or distance traversed. polycrystalline. Pertaining to a solid composed of many crystals. polygonal, ferrite. Ferrite grains that form an equiaxed morphology, that is, the grains can be described as polygons. polymorphic substance. An element or compound capable of stable existence in different temperatures and pressure ranges in two or more different crystalline states. polymorphism. A general term of the ability of a solid to exist in more than one form. In metals, alloys, and similar substances, this usually means the ability to exist in two or more crystal structures, or in an amorphous state and at least one crystal structure. See also allotropy, enantiotropy, and monotropism. pores. (1) Small voids in the body of a metal. (2) Minute cavities in a powder metallurgy compact, sometimes intentional. (3) Minute perforations in an electroplated coating. porosity. Fine holes or pores within a metal. positive distortion. The distortion in the image that results when the magnification in the center of the field is less than that at the edge of the field. Also termed pincushion distortion. Contrast with negative distortion. positive eyepiece. An eyepiece in which the real image of the object is formed below the lower lens elements of the eyepiece. positive replica. A replica whose contours correspond directly to the surface being replicated. postheating. Heating weldments immediately after welding, for tempering, for stress relieving, or for providing a controlled rate of cooling to prevent formation of a hard or brittle structure. pot annealing. Same as box annealing. potentiometer. An instrument that measures electromotive force by balancing against it an equal and opposite electromotive force across a calibrated resistance carrying a definite current. potentiostat. An instrument that automatically maintains an electrode in an electrolyte at a constant potential or controlled potentials relative to a suitable reference electrode. potentiostatic etching. Anodic development of microstructure at a constant potential. Adjusting the potential makes possible a defined etching of singular phases. powder metallurgy. The art of producing metal powders and of using metal powders for production of massive materials and shaped objects.

Glossary / 283 powder method. Any method of x-ray diffraction involving a polycrystalline and preferably randomly oriented powder specimen and a narrow beam of monochromatic radiation. precipitation. Separation of a new phase from solid or liquid solution, usually with changing conditions of temperature, pressure, or both. precipitation etching. Development of microstructure through formation of reaction products at the surface of the microsection. See also staining. precipitation hardening. Hardening caused by precipitation of a constituent from a supersaturated solid solution. See also age hardening and aging. precipitation heat treatment. Artificial aging in which a constituent precipitates from a supersaturated solid solution. preferred orientation. A condition of a polycrystalline aggregate in which the crystal orientations are not random, but rather exhibit a tendency for alignment in a specific direction in the bulk material that is completely related to the direction of working. Also termed texture. preheating. Heating before some further thermal or mechanical treatment. For tool steel, heating to an intermediate temperature immediately before final austenitizing. For some nonferrous alloys, heating to a high temperature for a long time in order to homogenize the structure before working. In welding and related processes, heating to an intermediate temperature for a short time immediately before welding, brazing, soldering, cutting, or thermal spraying. preshadowed replica. A replica formed by the application of shadowing material to the surface to be replicated. It is formed before the thin replica film is cast or otherwise deposited on the surface. See also shadowing. primary creep. The first, or initial, stage of creep, or timedependent deformation. primary crystal. The first type of crystal that separates from a melt upon cooling. primary extinction. A decrease in intensity of a diffracted x-ray beam caused by perfection of crystal structure extending over such a distance (approximately 1 μm or greater) that interference between multiple reflected beams inside the crystal decreases the intensity of the externally diffracted beam. primary metal. Metal extracted from minerals and free of reclaimed metal scrap. Compare with secondary metal. primary (x-ray). The beam incident on the specimen. principal stress (normal). The maximum or minimum value of the normal stress at a point in a plane considered with respect to all possible orientations of the considered plane. On such principal planes the shear stress is zero. There are three principal stresses on three mutually perpendicular planes. The state of stress at a point may be (1) uniaxial, a state of stress in which two of the three principal stresses are zero, (2) biaxial, a state of stress in which only one of the three principal stresses is zero, or (3) triaxial, a state of stress in which none of the principal stresses is zero. Multiaxial stress refers to either biaxial or triaxial stress.

printing. A method in which a carrier material is saturated with an etchant and pressed against the surface of the specimen. The etchant reacts with one of the phases, and substances form that react with the carrier material, leaving behind a life-size image. Used for exposing particular elements—for example, sulfur (sulfur prints). prior austenitic grain size. The grain size of austenite in a metallographic specimen that has transformed to other phases. It is revealed by special etching techniques. prism. A transparent body with at least two polished plane faces inclined with respect to each other, from which light is reflected or through which light is refracted. When light is refracted by a prism whose refractive index exceeds that of the surrounding medium, it is deviated or bent toward the thicker part of the prism. prismatic plane. In noncubic crystals, any plane that is parallel to the principal axis (c axis). process annealing. An imprecise term denoting various treatments used to improve workability. For the term to be meaningful, the condition of the material and the time-temperature cycle used must be stated. process metallurgy. The science and technology of winning metals from their ores and purifying metals; sometimes referred to as chemical metallurgy. Its two chief branches are extractive metallurgy and refining. proeutectoid carbide. Primary crystals of cementite formed directly from the decomposition of austenite exclusive of that cementite resulting from the eutectoid reaction. proeutectoid ferrite. Primary crystals of ferrite formed directly from the decomposition of austenite exclusive of that ferrite resulting from the eutectoid reaction. proeutectoid (phase). Particles of a phase that precipitate during cooling after austenitizing but before the eutectoid transformation takes place. progressive aging. Aging by increasing the temperature in steps or continuously during the aging cycle. See aging and compare with interrupted aging and step aging. projection distance. Distance from the eyepiece to the image screen. projection lens. The final lens in the electron microscope corresponding to an ocular or projector in a compound optical microscope. This lens forms a real image on the viewing screen or photographic film. Ps. The temperature at which pearlite starts to form. Nomenclature used in labeling a continuous cooling transformation or isothermal transformation diagram. pseudobinary system. (1) A three-component or ternary alloy system in which an intermediate phase acts as a component. (2) A vertical section through a ternary diagram. pseudocarburizing. See blank carburizing. pseudonitriding. See blank nitriding. pyramidal plane. In noncubic crystals, any plane that intersects all three axes. pyrometallurgy. High-temperature winning or refining of metals.

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Q quadrivariant equilibrium. A stable state among several conjugate phases equal to two less than the number of components, that is, having four degrees of freedom. quantitative metallography. Determination of specific characteristics of a microstructure by making quantitative measurements on micrographs or metallographic images. Quantities so measured include volume concentration of phases, grain size, particle size, mean free path between like particles or secondary phases, and surface-area-to-volume ratios of microconstituents, particles, or grains. quarter hard. A temper of nonferrous alloys and some ferrous alloys characterized by tensile strength about midway between those of dead soft and half hard tempers. quasi-binary system. In a ternary or higher-order system, a linear composition series between two substances each of which exhibits congruent melting, wherein all equilibria, at all temperatures or pressures, involve only phases having compositions occurring in the linear series, so that the series may be represented as binary on a phase diagram. quasi-cleavage fracture. A fracture mode that combines the characteristics of cleavage fracture and dimple rupture fracture. An intermediate type of fracture found in certain high-strength metals. quasi-isotropic. See isotropic. quaternary system. The complete series of compositions produced by mixing four components in all proportions. quench-age embrittlement. Embrittlement of low-carbon steel evidenced by a loss of ductility upon aging at room temperature following rapid cooling from a temperature below the lower critical temperature. quench aging. Aging induced from rapid cooling after solution heat treatment. quench annealing. Annealing an austenitic ferrous alloy by solution heat treatment followed by rapid quenching. quench cracking. Fracture of a metal during quenching from elevated temperature. Most frequently observed in hardened carbon steel, alloy steel, or tool steel parts of high hardness and low toughness. Cracks often emanate from fillets, holes, corners, or other stress raisers and result from high stresses due to the volume changes accompanying transformation to martensite. quench hardening. (1) Hardening suitable alpha-beta alloys— most often certain copper or titanium alloys—by solution treating and quenching to develop a martensite-like structure. (2) In ferrous alloys, hardening by austenitizing, then cooling at a rate so that a substantial amount of austenite transforms to martensite. quenching. Rapid cooling. When applicable, the following more specific terms should be used: direct quenching, fog quenching, hot quenching, interrupted quenching, selective quenching, spray quenching, and time quenching. quenching crack. Crack formed as a result of thermal stresses produced by rapid cooling from a high temperature. quenching medium. The liquid or gas that is used to quench a steel to produce a rapid cooling rate. Examples include oil,

water, iced brine, forced air cooling, and polymer-water mixture. quench time. In resistance welding, the time from the finish of the welding operation to the beginning of tempering. Also called chill time.

R radial marks. Lines on a fracture surface that radiate from the fracture origin and are visible to the unaided eye or at low magnification. Radial lines result from the intersection and connection of brittle fractures propagating at different levels. Also called shear ledges. See also chevron pattern. radiograph. A photographic shadow image resulting from uneven absorption of penetrating radiation in a test object. radiography. A method of nondestructive inspection in which a test object is exposed to a beam of x-rays or gamma rays and the resulting shadow image of the object is recorded on photographic film placed behind the object. Internal discontinuities are detected by observing and interpreting variations in the image caused by differences in thickness, density, or absorption within the test object. Variations of radiography include electron radiography, fluoroscopy, and neutron radiography. random orientation. A condition of a polycrystalline aggregate in which the orientations of the constituent crystals are completely random relative to each other. Contrast with preferred orientation. rare earth metal. One of the group of 15 chemically similar metals with atomic numbers 57 through 71, commonly referred to as the lanthanides. ratchet marks. Lines on a fatigue fracture surface that result from the intersection and connection of fatigue fractures propagating from multiple origins. Ratchet marks are parallel to the overall direction of crack propagation and are visible to the unaided eye or at low magnification. RE. Abbreviation for rare earth (elements). recarburize. (1) Increasing the carbon content of molten cast iron or steel by adding carbonaceous material, high-carbon pig iron, or a high-carbon alloy. (2) Carburizing a metal part to return surface carbon lost in processing; also known as carbon restoration. reciprocal lattice. A lattice of points, each representing a set of planes in the crystal lattice, so that a vector from the origin of the reciprocal lattice to any point is normal to the crystal planes represented by that point and has a length that is the reciprocal of the plane spacing. recovery. Reduction or removal of strain-hardening effects without motion of large-angle grain boundaries. recrystallization. (1) Formation of a new, strain-free grain structure from the structure existing in cold-worked metal, usually accomplished by heating. (2) The change from one crystal structure to another, as occurs upon heating or cooling through a critical temperature.

Glossary / 285 recrystallization annealing. Annealing cold-worked metal to produce a new grain structure without a phase change. recrystallization temperature. The approximate minimum temperature at which complete recrystallization of a cold-worked metal occurs within a specified time. recrystallized grain size. (1) The grain size developed by heating cold-worked metal. The time and temperature are selected so that, although recrystallization is complete, essentially no grain growth occurs. (2) In aluminum and magnesium alloys, the grain size after recrystallization, without regard to grain growth or the recrystallization conditions. reflection method. The technique of producing a diffraction pattern by x-rays or electrons that have been reflected from a specimen surface. reflection x-ray microscopy. A method of producing enlarged images by means of x-rays. In this method, the radiation is totally reflected at glancing incidence from polished concave mirrors or from the curved surfaces of single crystals by Bragg reflection. The problem of aberration corrections still limits the resolution obtainable. reflection, x-rays. See diffraction. refraction, angle of. The acute angle between the normal to a refracting surface at the point of incidence and the refracted ray. refractive index, electrons. The ratio of electron wavelength in free space to its wavelength in material medium. refractory. (1) A material of very high melting point with properties that make it suitable for such uses as furnace linings and kiln construction. (2) The quality of resisting heat. refractory alloy. (1) A heat-resistant alloy. (2) An alloy having an extremely high melting point. See refractory metal. (3) An alloy difficult to work at elevated temperatures. refractory metal. A metal having an extremely high melting point; for example, tungsten, molybdenum, tantalum, niobium (columbium), chromium, vanadium, and rhenium. In the broad sense, this term refers to metals having melting points about the range for iron, cobalt, and nickel. regular reflection. See specular reflection. reliability. A quantitative measure of the ability of a product or service to fulfill its intended function for a specified period of time. replica. A reproduction of a surface in a material. It is usually accomplished by depositing a thin film of suitable material, such as plastic, onto the specimen surface. This film is subsequently extracted and examined by transmission electron microscopy. See also collodian replica, impression replica, oxide film replica, plastic replica, positive replica, preshadowed replica, and vapor-deposited replica. replicate. In electron microscopy, to reproduce using a replica. residual elements. Elements present in an alloy in small quantities, but not added intentionally. Also called tramp elements. resolution. The ability to separate closely related items of data or physical features using a given test method; also a quantitative measure of the degree to which they can be discriminated. resolving power. The ability of a given lens system to reveal fine detail in an object. See also resolution. resulfurized steel. Steel to which sulfur has been added in

controlled amounts after refining. The sulfur is added to improve machinability. retained austenite. Austenite that is retained in a steel microstructure at room temperature; usually a result of rapid cooling and/or alloy partitioning during transformation. reticle (graticule). A group of lines or some other figure in the focus of an optical instrument, often used as a measuring reference, as a focusing target, or to define the field of view of a camera. rhombohedral. Having three equal axes, with the included angles equal to each other, but not equal to 90°. rimmed steel. Low-carbon steel containing sufficient iron oxide to produce continuous evolution of carbon monoxide during ingot solidification, resulting in a case or rim of metal virtually free of voids. Sheet and strip products made from rimmed steel ingots have very good surface quality. riser. A reservoir of molten metal connected to a casting to provide additional metal to the casting, required as the result of shrinkage before and during solidification. river pattern. A term used in fractography to describe a characteristic pattern of cleavage steps running parallel to the local direction of crack propagation on the fracture surfaces of grains that have separated by cleavage. riveting. Joining of two or more members of a structure by means of metal rivets, the unheaded end being upset after the rivet is in place. rock candy fracture. A fracture that exhibits separated-grain facets; most often used to describe intergranular fractures in large-grained metals. rosebuds. Concentric rings of distorted coating, giving the effect of an opened rosebud. Noted only on minimized spangle. rosette. (1) Rounded configuration of microconstituents arranged in whorls or radiating from a center. (2) Strain gages arranged to indicate at a single position strains in three different directions. rosette graphite. Arrangement of graphite flakes in which the flakes extend radially from the center of crystallized areas in gray cast iron. rough grinding. Grinding without regard to finish, usually to be followed by a subsequent operation. roughness. Relatively finely spaced surface irregularities, the heights, widths, and directions of which establish the predominant surface pattern. rough-polishing process. A polishing process having the primary objective of removing the layer of significant damage produced during earlier machining and abrasion stages of a preparation sequence. A secondary objective is to produce a finish of such quality that a final polish can be produced easily. rust. A corrosion product consisting of hydrated oxides of iron. Applied only to ferrous alloys.

S sacrificial protection. Reduction of the extent of corrosion of a metal in an electrolyte by coupling it to another metal that is electrochemically more active in the environment.

286 / Metallographer’s Guide saturated gun. A self-biased electron gun in which electron emission is limited by space charge rather than filament temperature. scale. A layer of oxidation products formed on a metal at high temperature. scale pit. (1) A surface depression formed on a forging due to scale remaining in the dies during the forging operation. (2) A pit in the ground in which scale (such as that carried off by cooling water from rolling mills) is allowed to settle out as one step in the treatment of effluent waste water. scaling. (1) Forming a thick layer of oxidation products on metals at high temperature. (2) Depositing water-insoluble constituents on a metal surface, as in cooling tubes and water boilers. scanning electron microscope (SEM). An electron microscope in which the image is formed by a beam operating in synchronism with an electron probe scanning the object. The intensity of the image-forming beam is proportional to the scattering or secondary emission of the specimen where the probe strikes it. scanning transmission electron microscopy (STEM). An analytical technique in which an image is formed on a cathode-ray tube whose raster is synchronized with the raster of a point beam of electrons scanned over an area of the sample. The brightness of the image at any point is proportional to the number of electrons that are transmitted through the sample at the point where it is struck by the beam. scoring. In tribology, a severe form of wear characterized by the formation of extensive grooves and scratches in the direction of sliding. scratch. A groove produced in a surface by an abrasive point. scratching. In tribology, the mechanical removal and/or displacement of material from a surface by the action of abrasive particles or protuberances sliding across the surfaces. See also plowing. screw dislocation. See dislocation. seam. On a metal surface, an unwelded fold or lap that appears as a crack, usually resulting from a discontinuity. secondary creep. See creep. secondary etching. Development of microstructures deviating from primary structure through transformation and heat treatment in the solid state. secondary extinction. A decrease in the intensity of a diffracted x-ray beam caused by parallelism or near-parallelism of mosaic blocks in a mosaic crystal; the lower blocks are partially screened from the incident radiation by the upper blocks, which have reflected some of it. secondary hardening. The hardening phenomenon that occurs during high-temperature tempering of certain steels containing one or more carbide-forming alloying elements. Up to an optimal combination of tempering time and temperature, the reaction results either in the retention of hardness or an actual increase in hardness. secondary metal. Metal recovered from scrap by remelting and refining. secondary x-rays. The x-rays emitted by a specimen irradiated by a primary beam.

segregation. Nonuniform distribution of alloying elements, impurities, or microphases. segregation banding. Inhomogeneous distribution of alloying elements aligned in filaments or plates parallel to the direction of working. segregation (coring) etching. Development of segregation (coring) mainly in macrostructures and microstructures of castings. selective quenching. Quenching only certain portions of an object. self-diffusion. Thermally activated movement of an atom to a new site in a crystal of its own species, as, for example, a copper atom within a crystal of copper. self-hardening steel. See preferred term, air-hardening steel. sensitization. In austenitic stainless steels, the precipitation of chromium carbides, usually at grain boundaries, on exposure to temperatures of about 550 to 850 °C (about 1000 to 1550 °F), leaving the grain boundaries depleted of chromium and therefore susceptible to preferential attack by a corroding (oxidizing) medium. serial sectioning. A technique in which an identified area on a section surface is observed repeatedly after successive layers of known thickness have been removed from the surface. It is used to construct a three-dimensional morphology of structural features. shadow angle. The angle between the line of motion of the evaporated atoms and the surface being shadowed. The angle analogous to the angle of incidence in optics. It may be specified as arc tangent a so that a is in the ratio between the height of the object casting the shadow over the length of the shadow. See also shadowing. shadow cast replica. A replica that has been shadowed. See also shadowing. shadowing. (1) Same as shielding in electroplating. (2) Directional deposition of carbon or a metallic film on a plastic replica so as to highlight features to be analyzed by transmission electron microscopy. shadow microscope. An electron microscope that forms a shadow image of an object using electrons emanating from a point source located close to the object. shales. Abrasive particles of platelike shape. The term is applied particularly to diamond abrasives. shape resolution. An electron image exhibits shape resolution when a polygon can be recognized as such in the image. Roughly, the particle diameter—defined as the diameter of a circle of the same area as the particle—must exceed the resolution by a factor equal to the number of sides on the polygon. shatter cracks. See flakes. shear. (1) That type of force that causes or tends to cause two contiguous parts of the same body to slide relative to each other in a direction parallel to their plane of contact. (2) A type of cutting tool with which a material in the form of wire, sheet, plate, or rod is cut between two opposing blades. (3) The type of cutting action produced by rake so that the direction of chip flow is other than at right angles to the cutting edge. shear angle. The angle that the shear plane, in metal cutting, makes with the work surface.

Glossary / 287 shear bands. Bands in which deformation has been concentrated inhomogeneously in sheets that extend across regional groups of grains. Only one system is usually present in each regional group of grains, different systems being present in adjoining groups. The bands are noncrystallographic and form on planes of maximum shear stress (55° to the compression direction). They carry most of the deformation at large strains. Compare with microbands. shear fracture. A ductile fracture in which a crystal (or a polycrystalline mass) has separated by sliding or tearing under the action of shear stresses. shearing strain. See strain. shear ledges. See radial marks. shear lip. A narrow, slanting ridge along the edge of a fracture surface. The term sometimes also denotes a narrow, often crescent-shaped, fibrous region at the edge of a fracture that is otherwise of the cleavage type, even though this fibrous region is in the same plane as the rest of the fracture surface. shear strain. Same as shearing strain; see also strain. shear strength. The stress required to produce fracture in the plane of cross section, the conditions of loading being such that the directions of force and of resistance are parallel and opposite although their paths are offset a specified minimum amount. The maximum load divided by the original crosssectional area of a section separated by shear. shear stress. See stress. sheet. A flat-rolled metal product of some maximum thickness and minimum width arbitrarily dependent on the type of metal. Sheet is thinner than plate and has a width-to-thickness ratio greater than about 50. shelling. A mechanism of deterioration of coated abrasive products in which entire abrasive grains are removed from the cement coating that held the abrasive to the backing layer of the product. shortness. A form of brittleness in metal. It is designated as “cold,” “hot,” and “red” to indicate the temperature range in which the brittleness occurs. shot. Small, spherical particles of metal. shotblasting. Blasting with metal shot; usually used to remove deposits or mill scale more rapidly or more effectively than can be done by sandblasting. shot peening. Cold working the surface of a metal by metal-shot impingement. shrinkage cavity. A void left in cast metals as a result of solidification shrinkage. Shrinkage cavities occur in the last metal to solidify after casting. shrinkage cracks. Hot tears associated with shrinkage cavities. sieve analysis. Particle-size distribution; usually expressed as the weight percentage retained upon each of a series of standard sieves of decreasing size and the percentage passed by the sieve of finest size. Synonymous with sieve classification. sieve classification. Same as sieve analysis. sieve fraction. The portion of a powder sample that passes through a standard sieve of specified number and is retained by some finer sieve of specified number. sigma. Solid phase found originally in binary iron-chromium

alloys that is in stable equilibrium below 820 °C (1510 °F). It is now used to identify any structure having the same complex body-centered crystal structure. sigma phase. A hard, brittle, nonmagnetic intermediate phase with a tetragonal crystal structure, containing 30 atoms per unit cell, space group P42/mnm, occurring in many binary and ternary alloys of the transition elements. The composition of this phase in the various systems is not the same, and the phase usually exhibits a wide range in homogeneity. Alloying with a third transition element usually enlarges the field of homogeneity and extends it deep into the ternary section. silicate-type inclusions. Inclusions composed essentially of silicate glass, normally plastic at forging and hot-rolling temperatures, that appear in steel in the wrought condition as small elongated inclusions usually dark in color under reflected light as normally observed. siliconizing. Diffusing silicon into solid metal, usually steel, at an elevated temperature. silky fracture. A metal fracture in which the broken metal surface has a fine texture, usually dull in appearance. Characteristic of tough and strong metals. Contrast with crystalline fracture and granular fracture. skid-polishing process. A mechanical polishing process in which the surface to be polished is made to skid across a layer of paste, consisting of the abrasive and the polishing fluid, without contacting the fibers of the polishing cloth. skin. A thin outside metal layer, not formed by bonding as in cladding or electroplating, that differs in composition, structure, or other characteristics from the main mass of metal. skin lamination. In flat-rolled metals, a surface rupture resulting from the exposure of a subsurface lamination by rolling. skin pass. See temper rolling. slack quenching. The incomplete hardening of steel due to quenching from the austenitizing temperature at a rate lower than the critical cooling rate for the particular steel, resulting in the formation of one or more transformation products in addition to martensite. slag. A nonmetallic product resulting from mutual dissolution of flux and nonmetallic impurities in smelting, refining, and certain welding operations. slag inclusion. Slag or dross entrapped in a metal. slip. Plastic deformation by the irreversible shear displacement (translation) of one part of a crystal relative to another in a definite crystallographic direction and usually on a specific crystallographic plane. Sometimes called glide. slip band. A group of parallel slip lines so closely spaced as to appear as a single line when observed under an optical microscope. See also slip line. slip direction. The crystallographic direction in which the translation of slip takes place. slip-interference theory. Theory involving the resistance to deformation offered by a hard phase dispersed in a ductile matrix. slip line. The trace of a slip plane on the viewing surface; the trace is usually observable only if the surface has been polished before deformation. The usual observation on metal crystals

288 / Metallographer’s Guide (under an optical microscope) is of a cluster of slip lines known as a slip band. slip plane. The crystallographic plane in which slip occurs in a crystal. sliver. An imperfection consisting of a very thin elongated piece of metal attached by only one end to the parent metal into whose surface it has been worked. smut. A reaction product sometimes left on the surface of a metal after pickling, electroplating, or etching. snap temper. A precautionary interim stress-relieving treatment applied to high-hardenability steels immediately after quenching to prevent cracking because of delay in tempering them at the prescribed higher temperature. soak cleaning. Immersion cleaning without electrolysis. soaking. Prolonged holding at a selected temperature to effect homogenization of structure or composition. soft temper. Same as dead soft temper. solidification. The change in state from liquid to solid upon cooling through the melting temperature or melting range. solidification range. The temperature range between the liquidus and the solidus. solidification shrinkage crack. A crack that forms, usually at elevated temperature, because of the internal (shrinkage) stresses that develop during solidification of a metal casting. Also termed hot crack. solidification structure, cast structure. An as-cast structure, usually polished and etched to reveal the dendritic pattern. solid solution. A single, solid, homogeneous crystalline phase containing two or more chemical species. solidus. In a constitution or equilibrium diagram, the locus of points representing the temperatures at which various compositions finish freezing upon cooling or begin to melt upon heating. See also liquidus. soluble oil. Specially prepared oil whose water emulsion is used as a cutting or grinding fluid. solute. The component of either a liquid or solid solution that is present to a lesser or minor extent; the component that is dissolved in the solvent. solution. In a chemical system, a phase existing over a range of composition. solution heat treatment. Heating an alloy to a suitable temperature, holding at that temperature long enough to cause one or more components to enter into solid solution, then cooling rapidly enough to hold these constituents in solution. solvent. The component of either a liquid or solid solution that is present to a greater or major extent; the component that dissolves the solute. solvus. In a constitution or equilibrium diagram, the locus of points representing the temperature at which the various compositions of the solid phases coexist with other solid phases— that is, the limits of solid solubility. sorbite. (obsolete) A fine mixture of ferrite and cementite produced either by regulating the rate of cooling of steel or by tempering steel after hardening. The first type is very fine pearlite that is difficult to resolve under the microscope; the second type is tempered martensite. This term is no longer used.

source, x-rays. The area emitting primary x-rays in a diffraction experiment. The actual source is always the focal spot of the x-ray tube, but the virtual source may be slit or pinhole, depending on the conditions of the experiment. space-charge aberration. An aberration resulting from the mutual repulsion of the electrons in a beam. This aberration is most noticeable in low-voltage, high-current beams. This repulsion acts as a negative lens, causing rays, which were originally parallel, to diverge. space lattice. A regular, periodic array of points (lattice points) in space that represents the locations of atoms of the same kind in a perfect crystal. The concept may be extended, where appropriate, to crystalline compounds and other substances, in which case the lattice points often represent locations of groups of atoms of identical composition, arrangement, and orientation. spacing, lattice planes. See interplanar distance. spalling. The cracking and flaking of particles out of a surface. spangle. The characteristic crystalline form in which a hot dipped zinc coating solidifies on steel strip. spatial grain size. The average size of the three-dimensional grains, as opposed to the more conventional grain size determined by a simple average of observations made on a cross section of the material. specimen. A test object, often of standard dimensions and/or configuration, that is used for destructive or nondestructive testing. One or more specimens may be cut from each unit of a sample. specimen chamber, electron optics. The compartment located in the column of the electron microscope in which the specimen is placed for observation. specimen charge, electron optics. The electrical charge resulting from the impingement of electrons on a nonconducting specimen. specimen contamination, electron optics. The contamination of the specimen caused by the condensation upon it of residual vapors in the microscope under the influence of electron bombardment. specimen distortion, electron optics. A physical change in the specimen caused by desiccation or heating by the electron beam. specimen grid. See specimen screen. specimen holder, electron optics. A device that supports the specimen and specimen screen in the correct position in the specimen chamber of the microscope. specimen screen, electron optics. A disk of fine screen, usually 200-mesh stainless steel, copper, or nickel, that supports the replica or specimen support film for observation in the microscope. specimen stage. The part of the microscope that supports the specimen holder and specimen in the microscope and can be moved in a plane perpendicular to the optic axis from outside the column. specimen strain. A distortion of the specimen resulting from stresses occurring during preparation or observation. In electron metallography, strain may be caused by stretching during removal of a replica or during subsequent washing or drying.

Glossary / 289 specular reflection. The condition in which all the incident light is reflected at the same angle as the angle of the incident light relative to the normal at the point of incidence. The reflection surface then appears bright, or mirrorlike, when viewed with the naked eye. Sometimes termed regular reflection. spherical aberration. The zonal aberrations of a lens referred to an axial point. When rays from a point on the axis passing through the outer lens zones are focused closer to the lens than rays passing the central zones, the lens suffers positive spherical aberration. If the condition is reversed, that is, the outer zones have a longer focal length than the inner zones, the lens has negative spherical aberration. In the first instance, the lens is uncorrected or undercorrected; in the second, overcorrected. spherical projection. A projection in which the orientation of a crystal plane is represented by the point at which the plane normal intersects a sphere drawn with the crystal as the center. spheroidal graphite. Graphite of spheroidal shape with a polycrystalline radial structure. This structure can be obtained, for example, by adding cerium or magnesium to liquid ductile iron. spheroidite. An aggregate of iron or alloy carbides of essentially spherical shape dispersed throughout a matrix of ferrite. spheroidized structure. A microstructure consisting of matrix containing spheroidal particles of another constituent. spheroidizing. Heating and cooling to produce a spheroidal or globular form of carbide in steel. Spheroidizing methods frequently used are: 1. Prolonged holding at a temperature just below Ae1 2. Heating and cooling alternately between temperatures that are just above and just below Ae1 3. Heating to a temperature above Ae1 or Ae3 and then cooling very slowly in the furnace or holding at a temperature just below Ae1 4. Cooling at a suitable rate from the minimum temperature at which all carbide is dissolved to prevent the re-formation of a carbide network, and then reheating in accordance with method 1 or 2 above. (Applicable to hypereutectoid steel containing a carbide network.) spheroidizing annealing. A subcritical annealing treatment intended to produce spheroidization of cementite or other carbide phases. spherulitic graphite cast iron. Same as ductile cast iron. spiegeleisen, spiegel. A pig iron containing 15 to 30% Mn and 4.5 to 6.5% C. spinning. Forming a seamless hollow metal part by forcing a rotating blank to conform to a shaped mandrel that rotates concentrically with the blank. In the usual application, a flat-rolled metal blank is forced against the mandrel by a blunt, rounded tool; however, other stock (notably, welded or seamless tubing) can be formed, and sometimes the working end of the tool is a roller. spinodal curve. A graph of the realizable limit of the supersaturation of a solution. spinodal decomposition. Mechanism of a phase separation from a solid solution into two homogeneous phases of different

chemical composition, each having the same crystal structure as the parent metal. spinodal structure. A fine, homogeneous mixture of two phases that form by the growth of composition waves in a solid solution during suitable heat treatment. The phases of a spinodal structure differ in composition from each other and from the parent phase, but have the same crystal structure as the parent phase. sponge iron. Either porous or powdered iron produced directly without fusion, such as by heating high-grade ore with charcoal, or an oxide with a reducing gas. spray quenching. Quenching in a spray of liquid. sputtering. The production of specimens in the form of thin films by deposition from a cathode subjected to positive-ion bombardment. stabilizing treatment. (1) Any treatment intended to stabilize the structure of an alloy or the dimensions of a part. (2) Heating austenitic stainless steels that contain titanium, columbium, or tantalum to a suitable temperature below that of a full anneal in order to inactivate the maximum amount of carbon by precipitation as a carbide of titanium, columbium, or tantalum. (3) Transforming retained austenite parts made from tool steel. (4) Precipitating a constituent from a nonferrous solid solution to improve the workability, to decrease the tendency of certain alloys to age harden at room temperature to obtain dimensional stability. stage. A device for holding a specimen in the desired position in the optical path. staining. Precipitation etching that causes contrast by distinctive staining of microconstituents; different interference colors originate from surface layers of varying thickness. stainless steel. Any of several steels containing 12 to 30% Cr as the principal alloying element; they usually exhibit passivity in aqueous environments. stamping. A general term covering almost all press operations. It includes blanking, shearing, hot or cold forming, drawing, bending, or coining. standard grain-size micrograph. A micrograph taken of a known grain size at a known magnification that is used to determine grain size by direct comparison with another micrograph or with the image of a specimen. state of strain. A complete description of the deformation within a homogeneously deformed volume or at a point. The description requires, in general, the knowledge of the six independent components of strain. state of stress. A complete description of the stresses within a homogeneously stressed volume or at a point. The description requires, in general, the knowledge of the six independent components of stress. static fatigue. A term sometimes used to identify a form of hydrogen embrittlement in which a metal appears to fracture spontaneously under a steady stress less than the yield stress. There almost always is a delay between the application of stress (or exposure of the stressed metal to hydrogen) and the onset of cracking. More properly referred to as hydrogen-induced delayed cracking.

290 / Metallographer’s Guide steadite. A hard structural constituent of cast iron that consists of a binary eutectic of ferrite, containing some phosphorus in solution, and iron phosphide (Fe3P). The eutectic consists of 10.2% P and 89.8% Fe. The melting temperature is 1050 °C (1920 °F). Stead’s brittleness. A condition of brittleness that causes transcrystalline fracture in the course grain structure that results from prolonged annealing of thin sheets of low-carbon steel previously rolled at a temperature below about 705 °C (1300 °F). The fracture usually occurs at about 45° to the direction of rolling. steady-rate creep. See creep. steel. An iron-base alloy, malleable in some temperature ranges, as initially cast, containing manganese, usually carbon, and often other alloying elements. In carbon steel and low-alloy steel, the maximum carbon content is about 2.0%; in high-alloy steel, about 2.5%. The dividing line between low-alloy and high-alloy steels is generally regarded as being at about 8% metallic elements. Steel is to be differentiated from two general classes of “irons”: the cast irons, on the high-carbon side, and the relatively pure irons, such as carbonyl iron and electrolytic iron, on the low-carbon side. In some steels containing extremely low carbon, the manganese content is the principal differentiating factor. step aging. Aging at two or more temperatures, by steps, without cooling to room temperature after each step. See aging and compare with interrupted aging and progressive aging. stereo angle. One half of the angle through which the specimen is tilted when taking a pair of stereoscopic micrographs. The axis of rotation lies in the plane of the specimen. stereoradiography. A technique for producing paired radiographs that may be viewed with a stereoscope to exhibit a shadowgraph in three dimensions with various sections in perspective and spatial relation. stereoscopic micrographs. A pair of micrographs of the same area, but taken from different angles so that the two micrographs when properly mounted and viewed reveal the structures of the objects in their three-dimensional relationships. stereoscopic specimen holder. A specimen holder designed for the purpose of making stereoscopic micrographs. It makes possible the tilting of the specimen through the stereo angle. sticker breaks. Arc-shaped coil breaks, usually located near the center of sheet or strip. stiffness. The ability of a metal or shape to resist elastic deflection. For identical shapes, the stiffness is proportional to the modulus of elasticity. For a given material, the stiffness increases with increasing moment of inertia, which is computed from cross-sectional dimensions. stopping off. (1) Depositing a metal (copper, for example) in localized areas to prevent carburization, decarburization, or nitriding in those areas. (2) Filling in a portion of a mold cavity to keep out molten metal. strain. A measure of the relative change in the size or shape of a body. Linear strain is the change per unit length of a linear

dimension. True (or natural) strain is the natural logarithm of the ratio of the length at the moment of observation to the original gage length. Conventional strain is the linear strain over the original gage length. Shearing strain (or shear strain) is the change in angle (expressed in radians) between two reference lines originally at right angles. When the term “strain” is used alone, it usually refers to linear strain in the direction of the applied stress. See also state of strain. strain-age embrittlement. A loss in ductility accompanied by an increase in hardness and strength that occurs when low-carbon steel (especially rimmed or capped steel) is aged following plastic deformation. The degree of embrittlement is a function of aging time and temperature, occurring in a matter of minutes at about 200 °C (400 °F), but requiring a few hours to a year at room temperature. strain aging. Aging induced by cold working. See aging. strain energy. (1) The work done in deforming a body. (2) The work done in deforming a body within the elastic limit of the material. It is more properly termed elastic strain energy and can be recovered as work rather than heat. strain etching. Etching that provides information on deformed and undeformed areas if present side by side. In strained areas, more compounds are precipitated. strain hardening. An increase in hardness and strength caused by plastic deformation at temperatures below the recrystallization range. strain-hardening exponent. A measure of rate of strain hardening. The constant n in the expression: ␴ ⫽ ␴0␦n

where ␴ is true stress, ␴0 is true stress at unit strain, and ␦ is true strain. strain markings. Manifestations of prior plastic deformation visible after etching of a metallographic section. These markings may be referred to as slip strain markings, twin strain markings, and so on, to indicate the specific deformation mechanism of which they are a manifestation. strain rate. The time rate of straining for the usual tensile test. Strain as measured directly on the specimen gage length is used for determining strain rate. Because strain is dimensionless, the units of strain rate are reciprocal time. strain-rate sensitivity. Qualitatively, the increase in stress (s) needed to cause a certain increase in plastic strain rate (␧) at a given level of plastic strain (␧) and a given temperature (T). Strain-rate sensitivity ⫽ m⫽

⌬ log s ⌬ log ␧˙ ␧, T

strain state. See state of strain. stray-current corrosion. Corrosion caused by electric current from a source external to the intended electrical circuit, for example, extraneous current in the earth. stress. The intensity of the internally distributed forces or components of forces that resist a change in volume or shape of a material that is or has been subjected to external forces. Stress is expressed in force per unit area and is calculated on the basis

Glossary / 291 of the original dimensions of the cross section of the specimen. Stress can be either direct (tension or compression) or shear. Usually expressed in pounds per square inch (psi) or megapascals (MPa). stress concentration (K1). A change in contour or a discontinuity that causes local increases in stress in materials under load. Typical are sharp-cornered grooves or notches, threads, fillets, holes, and so on. Also called stress raiser. stress-corrosion cracking (SCC). A cracking process that requires the simultaneous action of a corrodent and sustained tensile stress. This excludes corrosion-reduced sections that fail by fast fracture. It also excludes intergranular or transgranular corrosion, which can disintegrate an alloy without applied or residual stress. See also corrosion. stress-relief annealing. An annealing process that provides for stress relief (movement of dislocations) by nonrecrystallization. A commercial process whereby cold rolled sheet steel is annealed at a time and temperature just below that required for recrystallization. stress relieving. Heating to a suitable temperature, holding long enough to reduce residual stresses, then cooling slowly enough to minimize the development of new residual stresses. stress-strain diagram. A graph in which corresponding values of stress and strain are plotted against each other. Values of stress are usually plotted vertically (ordinates or y axis) and values of strain horizontally (abscissas or x axis). Also known as deformation curve and stress-strain curve. stretcher strains. Elongated markings that appear on the surfaces of some materials when they are deformed just past the yield point. These markings lie approximately parallel to the direction of maximum shear stress and are the result of localized yielding. See also Lüders lines. striation. A fatigue fracture feature, often observed in electron micrographs, that indicates the position of the crack front after each succeeding cycle of stress. The distance between striations indicates the advance of the crack front across that crystal during one stress cycle, and a line normal to the striation indicates the direction of local crack propagation. Not to be confused with beach marks, which are much larger (macroscopic) and form differently. stringer. In wrought materials, an elongated configuration of microconstituents or foreign material aligned in the direction of working. Commonly, the term is associated with elongated oxide or sulfide inclusions in steel. strip. A flat-rolled metal product of some maximum thickness and width arbitrarily dependent on the type of metal. It is narrower than sheet. stripping. Removing a coating from a metal surface. structure. As applied to a crystal, the shape and size of the unit cell and the location of all atoms within the unit cell. As applied to microstructure, the size, shape, and arrangement of phases. sub-boundary structure, subgrain structure. A network of low-angle boundaries (usually misorientations less than one degree) within the main crystals of a metallographic structure. subcritical annealing. A process anneal performed on ferrous alloys at a temperature below Ac1.

subgrain. A portion of a crystal or grain with an orientation slightly different from the orientation of neighboring portions of the same crystal. Generally, neighboring subgrains are separated by low-angle boundaries such as tilt boundaries and twist boundaries. submicroscopic. Below the resolution of the microscope. subsieve analysis. Size distribution of particles, all of which will pass through a 44-μm (No. 325) standard sieve, as determined by specified methods. subsieve fraction. That portion of a powdered sample that will pass through a 44-μm (No. 325) standard sieve. substitutional element. An alloying element with an atomic size and other features similar to the solvent that can replace or substitute for the solvent atoms in the lattice and form a significant region of solid solution in the phase diagram. substitutional solid solution. A solid solution in which the solute atoms are located at some of the lattice points of the solvent, the distribution being random. Contrast with interstitial solid solution. substrate. Layer of metal underlying a coating, regardless of whether the layer is the basis metal. substructure. See sub-boundary structure, subgrain structure. subsurface corrosion. Formation of isolated particles of corrosion products beneath a metal surface. This results from the preferential reactions of certain alloy constituents to inward diffusion of oxygen, nitrogen, or sulfur. sulfidation. The reaction of a metal or alloy with a sulfurcontaining species to produce a sulfur compound that forms on or beneath the surface of the metal or alloy. sulfide spheroidization. A stage of overheating in which sulfide inclusions are partly or completely spheroidized. sulfide stress cracking (SSC). Brittle failure by cracking under the combined action of tensile stress and corrosion in the presence of water and hydrogen sulfide. sulfide-type inclusions. In steels, nonmetallic inclusions composed essentially of manganese iron sulfide solid solutions (Fe,Mn)S. They are characterized by plasticity at hot-rolling and forging temperatures and, in the hot-worked product, appear as dove-gray elongated inclusions varying from a threadlike to oval outline. Zirconium and titanium sulfides appear orange and yellow. sulfur print. A macrographic method of examining for distribution of sulfide inclusions by placing a sheet of wet acidified photographic paper in contact with the polished steel surface to be examined. superalloy. See heat-resisting alloy. supercooling. Cooling below the temperature at which an equilibrium phase transformation can take place without actually obtaining the transformation. superfines. The portion of a metal powder that is composed of particles smaller than a specified size, usually 10 μm. superheating. (1) Heating above the temperature at which an equilibrium phase transformation should occur without actually obtaining the transformation. (2) Heating molten metal above the normal casting temperature so as to obtain more complete refining or greater fluidity.

292 / Metallographer’s Guide superlattice. A lattice arrangement in which solute and solvent atoms of a solid solution occupy different preferred sites in the array. superplasticity. The ability of certain metals to undergo unusually large amounts of plastic deformation before local necking occurs. surface finish. (1) Condition of a surface as a result of a final treatment. (2) Measured surface profile characteristics, the preferred term being roughness. surface hardening. A generic term covering several processes applicable to a suitable ferrous alloy that produces, by quench hardening only, a surface layer that is harder or more wear resistant than the core. There is no significant alteration of the chemical composition of the surface layer. The processes commonly used are induction hardening, flame hardening, and shell hardening. Use of the applicable specific process name is preferred. swabbing. Wiping of the specimen surface with a cotton ball saturated with etchant to remove reaction products simultaneously. swarf. Intimate mixture of grinding chips and fine particles of abrasive and bond resulting from a grinding operation. syntectic. An isothermal reversible reaction in which a solid phase, upon absorption of heat, is converted to two conjugate liquid phases. syntectic equilibrium. A reversible univariant transformation in which a solid phase that is stable only at lower temperature decomposes into two conjugate liquid phases that remain stable at higher temperature. system (crystal). See crystal system.

T target, x-ray. That part of an x-ray tube which the electrons strike and from which x-rays are emitted. tarnish. Surface discoloration of a metal caused by formation of a thin film of corrosion product. temper. (1) In heat treatment, to reheat hardened steel or hardened cast iron to some temperature below the eutectoid temperature for the purpose of decreasing hardness and increasing toughness. The process is sometimes applied to normalized steel. (2) In tool steels, temper is sometimes, but inadvisedly, used to denote carbon content. (3) In nonferrous alloys and in some ferrous alloys (steels that cannot be hardened by heat treatment), the hardness and strength produced by mechanical or thermal treatment, or both, and characterized by a certain structure, mechanical properties, or reduction in area during cold working. (4) To moisten sand for casting molds with water. temper brittleness. Brittleness that results when certain steels are held within, or are cooled slowly through, a certain range of temperature below the transformation range. This brittleness is manifested by an upward shift in ductile-to-brittle transition temperature, but only rarely produces a low value of reduction in area in a smooth-bar tension test of the embrittled material.

temper carbon. Clusters of finely divided graphite, such as that found in malleable iron, that are formed as a result of decomposition of cementite, for example, by heating white cast iron above the ferrite-austenite transformation temperature and holding at these temperatures for a considerable period of time. Also termed annealing carbon. See also nodular graphite. temper color. A thin, tightly adhering oxide skin (only a few molecules thick) that forms when steel is tempered at a low temperature, or for a short time, in air or a mildly oxidizing atmosphere. The color, which ranges from straw to blue depending on the thickness of the oxide skin, varies with both tempering time and temperature. tempered layer. A surface or subsurface layer in a steel specimen that has been tempered by heating during some stage of the preparation sequence. When observed in a section after etching, the layer appears darker than the base material. tempered martensite. The decomposition products that result from heating martensite below the ferrite-austenite transformation temperature. Under the optical microscope, darkening of the martensite needles is observed in the initial stages of tempering. Prolonged tempering at high temperatures produces spheroidized carbides in a matrix of ferrite. At the higher resolution of the electron microscope, the initial stage of tempering is observed to result in a structure containing a precipitate of fine epsilon iron carbide particles. At approximately 260 °C (500 °F), a transition occurs to a structure of larger and elongated cementite particles in a ferrite matrix. With further tempering at higher temperatures, the cementite particles become spheroidal, decreased in number, and increased in size. tempering. See temper. temper rolling. Light cold rolling of steel sheet. This operation is performed to improve flatness, to minimize the tendency toward formation of stretcher strains and flutes, and to obtain the desired texture and mechanical properties. temper time. In resistance welding, that part of the postweld interval during which the current is suitable for tempering or heat treatment. tensile strength. In tension testing, the ratio of maximum load to the original cross-sectional area. See also ultimate strength. tensile stress. A stress that causes two parts of an elastic body, on either side of a typical stress plane, to pull apart. tensile testing. See tension testing. tension testing. A method of determining the behavior of materials subjected to uniaxial loading, which tends to stretch the metal. A longitudinal specimen of known length and diameter is gripped at both ends and stretched at a slow, controlled rate until rupture occurs. Also known as tensile testing. terminal phase. A solid solution having a restricted range of compositions, one end of the range being a pure component of an alloy system. terminal solid solution. In a multicomponent system, any solid phase of limited composition range that includes the composition of one of the components of the system. ternary system. The complete series of compositions produced by mixing three components in all proportions. terne alloy. An alloy of lead containing 3 to 15% Sn, used as a

Glossary / 293 hot dip coating for steel sheet or plate. Terne coatings, which are smooth and dull in appearance, give the steel better corrosion resistance and enhance its ability to be formed, soldered, or painted. tertiary creep. See creep. tetragonal. Having three mutually perpendicular axes, two equal in length and unequal to the third. texture. In a polycrystalline aggregate, the state of distribution of crystal orientations. In the usual sense, it is synonymous with preferred orientation, in which the distribution is not random. thermal analysis. A method for determining transformations in a metal by noting the temperatures at which thermal arrests occur. These arrests are manifested by changes in slope of the plotted or mechanically traced heating and cooling curves. When such data are secured under nearly equilibrium conditions of heating and cooling, the method is commonly used for determining certain critical temperatures required for the construction of equilibrium diagrams. thermal arrest. The plateau found on a cooling curve due to the evolution of heat of transformation. thermal etching. Annealing of the specimen in vacuum or inert atmosphere. Used primarily in high-temperature microscopy. thermal fatigue. Fracture resulting from the presence of temperature gradients that vary with time in such a manner as to produce cyclic stresses in a structure. thermal shock. The development of a steep temperature gradient and accompanying high stresses within a structure. thermal spraying. A group of welding or allied processes in which finely divided metallic or nonmetallic materials are deposited in a molten or semimolten condition to form a coating. The coating material may be in the form of powder, ceramic rod, wire, or molten materials. thermal stresses. Stresses in metal resulting from nonuniform temperature distribution. thermionic cathode gun. An electron gun that derives its electrons from a heated filament, which may also serve as the cathode. Also termed hot cathode gun. thermionic emission. The ejection of a stream of electrons from a hot cathode, usually under the influence of an electrostatic field. thermomechanical working. A general term covering a variety of processes combining controlled thermal and deformation treatments to obtain synergistic effects, such as improvement in strength without loss of toughness. Same as thermal-mechanical treatment. thin foil. A thin specimen prepared for the transmission and scanning transmission electron microscope. A thin specimen transparent to electrons. three-quarters hard. A temper of nonferrous alloys and some ferrous alloys characterized by values of tensile strength and hardness about midway between those of half hard and full hard tempers. throwing power. The ability of a plating solution to produce a uniform metal distribution on an irregularly shaped cathode. Compare with covering power. tie line. In a binary or higher-order phase diagram, an isothermal,

isobaric straight line connecting the compositions of a pair of conjugate phases. tiger stripes. Continuous bright lines on sheet or strip in the rolling direction. tilt boundary. A subgrain boundary consisting of an array of edge dislocations. time quenching. Interrupted quenching in which the time in the quenching medium is controlled. time-temperature curve. A curve produced by plotting time against temperature. time-temperature-transformation (TTT) diagram. See isothermal transformation (IT) diagram. tin plate. Steel coated with tin for corrosion resistance, as found in beverage and food containers. tint etching. Not actually etching, as in attack etching, but a deposition of chemical compounds on preferred sites in a polished metallographic specimen. tinting. See heat tinting. tool steel. Any of a class of carbon and alloy steels commonly used to make tools. Tool steels are characterized by high hardness and resistance to abrasion, often accompanied by high toughness and resistance to softening at elevated temperature. These attributes are generally attained with high carbon and alloy contents. total carbon. The sum of the free and combined carbon (including carbon in solution) in a ferrous alloy. toughness. Ability of a metal to absorb energy and deform plastically before fracturing. It is usually measured by the energy absorbed in a notch impact test, but the area under the stress-strain curve in tensile testing is also a measure of toughness. tramp elements. Residual alloying elements that are introduced into steel when unidentified alloy steel is present in the scrap charge to a steelmaking furnace. Tramp elements in steel include copper, tin, antimony, and phosphorus. transcrystalline. See intracrystalline. transcrystalline cracking. Cracking or fracturing that occurs through or across a crystal. Also termed intracrystalline cracking. transference. The movement of ions through the electrolyte associated with the passage of the electric current. Also called transport or migration. transference number. The proportion of total electroplating current carried by ions of a given kind. Also called transport number. transformation-induced plasticity (TRIP). A phenomenon, occurring chiefly in certain highly alloyed steels that have been heat treated to produce metastable austenite or metastable austenite plus martensite, whereby, on subsequent deformation, part of the austenite undergoes strain-induced transformation to martensite. Steels capable of transforming in this manner, commonly referred to as TRIP steels, are highly plastic after heat treatment, but exhibit a very high rate of strain hardening and thus have high tensile and yield strengths after plastic deformation at temperatures between about 20 and 500 °C (70 and 930 °F). Cooling to ⫺195 °C (⫺320 °F) may or may not be required to complete the transformation to martensite. Tempering usually is done following transformation.

294 / Metallographer’s Guide transformation ranges. Those ranges of temperature within which austenite forms during heating and transforms during cooling. The two ranges are distinct, sometimes overlapping but never coinciding. The limiting temperatures of the ranges depend on the composition of the alloy and on the rate of change of temperature, particularly during cooling. See also transformation temperature. transformation temperature. The temperature at which a change in phase occurs. The term is sometimes used to denote the limiting temperature of a transformation range. transgranular. Through or across crystals or grains. Also called intracrystalline or transcrystalline. transgranular cracking. Cracking or fracturing that occurs through or across a crystal or grain. Also called transcrystalline cracking. Contrast with intergranular cracking. transgranular fracture. Fracture through or across the crystals or grains of a metal. Also called transcrystalline fracture or intracrystalline fracture. Contrast with intergranular fracture. transition lattice. An unstable crystallographic configuration that forms as an intermediate step in a solid-state reaction such as precipitation from solid solution or eutectoid decomposition. transition metal. A metal in which the available electron energy levels are occupied in such a way that the d-band contains less than its maximum number of ten electrons per atom, for example, iron, cobalt, nickel, and tungsten. The distinctive properties of the transition metals result from the incompletely filled d-levels. transition phase. A nonequilibrium state that appears in a chemical system in the course of transformation between two equilibrium states. transition point. At a stated pressure, the temperature (or at a stated temperature, the pressure) at which two solid phases exist in equilibrium—that is, an allotropic transformation temperature (or pressure). transition structure. In precipitation from solid solution, a metastable precipitate that is coherent with the matrix. transition temperature (ductile-brittle transition temperature). (1) An arbitrarily defined temperature that lies within the temperature range in which metal fracture characteristics (as usually determined by tests of notched specimens) change rapidly, such as from primarily fibrous (shear) to primarily crystalline (cleavage) fracture. Commonly used definitions are “transition temperature for 50% cleavage fracture,” “15 ft · lb transition temperature,” and “transition temperature for halfmaximum energy.” (2) Sometimes used to denote an arbitrarily defined temperature within a range in which the ductility changes rapidly with temperature. transmission electron microscope (TEM). A microscope in which the image-forming rays pass through (are transmitted by) the specimen being observed. transmission method. A method of x-ray or electron diffraction in which the recorded diffracted beams emerge on the same side of the specimen as the transmitted primary beam. transverse. Literally, “across,” usually signifying a direction or plane perpendicular to the direction of working. In rolled plate or sheet, the direction across the width is often called long

transverse, and the direction through the thickness, short transverse. transverse direction. Literally, “across,” usually signifying a direction or plane perpendicular to the direction of working. In rolled plate or sheet, the direction across the width is often called long transverse, and the direction through the thickness, short transverse. See also longitudinal direction and normal direction. tribology. The science and art concerned with the design, friction, lubrication, and wear of contacting surfaces that move relative to each other (as in bearings, cams, or gears, for example). triclinic. Having three axes of any length, none of the included angles being equal to one another or equal to 90°. triple point. (1) The point on a phase diagram where three phases of a substance coexist in equilibrium. (2) The junction of these adjacent grains. tripoli. Friable and dustlike silica used as an abrasive. TRIP steel. A commercial steel product exhibiting transformation-induced plasticity. troostite (obsolete). A previously unresolvable, rapidly etching, fine aggregate of carbide and ferrite produced either by tempering martensite at low temperature or by quenching a steel at a rate lower than the critical cooling rate. Preferred terminology for the first product is martensite; for the latter, fine pearlite. true strain. See strain. true stress. See stress. twin. Two portions of a crystal with a definite orientation relationship; one may be regarded as the parent, the other as the twin. The orientation of the twin is either a mirror image of the orientation of the parent about a “twinning plane” or an orientation that can be derived by rotating the twin portion about a “twinning axis.” See also annealing twin and mechanical twin. twin bands. Bands across a crystal grain, observed on a polished and etched section, where crystallographic orientations have a mirror-image relationship to the orientation of the matrix grain across a composition plane that is usually parallel to the sides of the band. twist boundary. A subgrain boundary consisting of an array of screw dislocations.

U ultimate strength. The maximum conventional stress (tensile, compressive, or shear) that a material can withstand. ultramicroscopic. See submicroscopic. ultrasonic cleaning. Immersion cleaning aided by ultrasonic waves that cause microagitation. underbead crack. A subsurface crack in the base metal near a weld. undercooling. Same as supercooling. undercut. In weldments, a groove melted into the base metal adjacent to the toe of a weld and left unfilled. underfill. A portion of a forging that has insufficient metal to give it the true shape of the impression.

Glossary / 295 understressing. Applying a cyclic stress lower than the endurance limit. This may improve fatigue life if the member is later cyclically stressed at levels above the endurance limit. uniaxial stress. A state of stress in which two of the three principal stresses are zero. uniform strain. The strain occurring prior to the beginning of localization of strain (necking); the strain to maximum load in the tension test. unit cell. In crystallography, the fundamental building block of a space lattice. Space lattices are constructed by stacking identical unit cells—that is, parallelepipeds of identical size, shape, and orientation, each having a lattice point at every corner—face to face in perfect three-dimensional alignment. univariant equilibrium. A stable state among several phases equal to one more than the number of components, that is, having one degree of freedom. upper bainite. Forms during isothermal decomposition of austenite, usually in the temperature range of 550 to 400 °C (1022 to 752 °F). Its structure has a close similarity with Widmanstätten ferrite, with carbides at the ferrite bath boundaries. upper yield stress. See yield point.

V vacancy. A type of lattice imperfection in which an individual atom site is temporarily unoccupied. Diffusion (of other than interstitial solutes) is generally visualized as the shifting of vacancies. vapor-deposited replica. A replica formed of a metal or a salt by the condensation of the vapors of the material onto the surface to be replicated. variability. The number of degrees of freedom of a heterogeneous phase equilibrium. Also termed variance. variance. See variability. veining. A type of sub-boundary structure that can be delineated because of the presence of a greater than average concentration of precipitate or possibly solute atoms. Vergard’s law. The relationship that states that the lattice parameters of substitutional solid solutions vary linearly between the values for the components, with composition expressed in atomic percentage. vermicular iron. Same as compacted graphite cast iron. vertical illumination. Light incident on an object from the objective side so that smooth planes perpendicular to the optical axis of the objective appear bright. vibratory polishing. A mechanical polishing process in which the specimen is made to move around the polishing cloth by imparting a suitable vibratory motion to the polishing system. voltage alignment. A condition of alignment of an electron microscope so that the image expands or contracts symmetrically about the center of the viewing screen when the accelerating voltage is changed. See also alignment. voltage efficiency. The ratio, usually expressed as a percentage, of the equilibrium-reaction potential in a given electrochemical process to the bath voltage.

volume fraction. The total volume of a phase or constituent per unit volume of a specimen. Usually expressed in percent.

W Wallner lines. A distinct pattern of intersecting sets of parallel lines, usually producing a set of V-shaped lines, sometimes observed in viewing brittle fracture surfaces at high magnifications in an electron microscope. Wallner lines are attributed to interaction between a shock wave and a brittle crack front propagating at high velocity. Sometimes Wallner lines are misinterpreted as fatigue striations. warm working. Plastically deforming metal at a temperature above ambient (room) temperature but below the temperature at which the material undergoes recrystallization. wavelength-dispersive spectroscopy (WDS). A method of x-ray analysis that employs a crystal spectrometer to discriminate characteristic x-ray wavelengths. Compare with energy-dispersive spectroscopy. wavelength (x-rays). The minimum distance between points at which the electric vector of an electromagnetic wave has the same value. It is measured along the direction of propagation of the wave, and it is equal to the velocity divided by the frequency. wear. Damage to a solid surface, generally involving progressive loss of material, due to relative motion between that surface and a contracting surface or substance. weight percent. Percentage composition by weight. Contrast with atomic percent. weldability. A specific or relative measure of the ability of a material to be welded under a given set of conditions. Implicit in this definition is the ability of the completed weldment to fulfill all functions for which the part was designed. welding. (1) Joining two or more pieces of material by applying heat or pressure, or both, with or without filler material, to produce a localized union through fusion or recrystallization across the interface. The thickness of the filler material is much greater than the capillary dimensions encountered in brazing. (2) May also be extended to include brazing and soldering. wetting. A condition in which the interfacial tension between a liquid and a solid is such that the contact angle is 0° to 90°. wetting agent. A surface-active agent that produces wetting by decreasing the cohesion within the liquid. whiskers. Metallic filamentary growths, often microscopic, sometimes formed during electrodeposition and sometimes spontaneously during storage or service, after finishing. white cast iron. Cast iron that shows a white fracture because the carbon is in combined form. white-etching layer. A surface layer in a steel that, as viewed in a section after etching, appears whiter than the base metal. The presence of the layer may be due to a number of causes, including plastic deformation induced by machining or surface rubbing, heating during a preparation stage to such an extent that the layer is austenitized and then hardened during cooling, and diffusion of extraneous elements into the surface.

296 / Metallographer’s Guide whiteheart malleable. See malleable cast iron. white metal. (1) A general term covering a group of whitecolored metals of relatively low melting points (lead, antimony, bismuth, tin, cadmium, and zinc) and the alloys based on these metals. (2) A copper matte of about 77% Cu obtained from smelting of sulfide copper ores. white rust. Zinc oxide; the powdery product of corrosion of zinc or zinc-coated surfaces. wide-field eyepiece. A positive achromatic eyepiece, having a large eye lens and a high eye point, intended primarily for use with wide-field binocular microscopes. Widmanstätten structure. A structure characterized by a geometrical pattern resulting from the formation of a new phase along certain crystallographic planes of the parent solid solution. The orientation of the lattice in the new phase is related crystallographically to the orientation of the lattice in the parent phase. The structure was originally observed in meteorites, but is readily produced in many alloys by appropriate heat treatment. wipe etching. See swabbing. woody structure. A macrostructure, found particularly in wrought iron and in extruded rods of aluminum alloys, that shows elongated surfaces of separation when fractured. wootz. A carbon steel, containing 1 to 1.6% C, produced by melting a bloomery iron or an inhomogeneous steel with charcoal in a crucible. The process originated in India as early as the third century A.D. work angle. In arc welding, the angle between the electrode and one member of the joint, taken in a plane normal to the weld axis. work hardening. Same as strain hardening. working distance. The distance between the surface of the specimen being examined and the front surface of the objective lens. wrought iron. An iron produced by direct reduction or ore or by refining molten cast iron under conditions where a pasty mass of solid iron with included slag is produced. The iron has a low carbon content. wustite. The oxide of iron of lowest valence that exists over a

wide range of compositions that do not quite include the stoichiometric composition FeO.

X x-radiation. Electromagnetic radiation of the same nature as visible light, but having a wavelength approximately 1/1000 that of visible light. Commonly referred to as x-rays. x-rays. See x-radiation. x-ray tube. A device for the production of x-rays by the impact of high-speed electrons on a metal target.

Y yield point. The first stress in a material, usually less than the maximum attainable stress; at which an increase in strain occurs without an increase in stress. Only certain materials—those which exhibit a localized, heterogeneous type of transition from elastic to plastic deformation—produce a yield point. If there is a decrease in stress after yielding, a distinction may be made between upper and lower yield points. The load at which a sudden drop in the flow curve occurs is called the upper yield point. The constant load shown on the flow curve is the lower yield point. yield point elongation. In materials that exhibit a yield point, the difference between the elongation at the completion and at the start of discontinuous yield. yield strength. The stress at which a material exhibits a specified deviation from proportionality of stress and strain. An offset of 0.2% is used for many materials, particularly metals. Compare with tensile strength. yield stress. The stress level of highly ductile materials at which large strains take place without further increase in stress. yield value. The stress (either normal or shear) at which a marked increase in deformation occurs without an increase in load. Young’s modulus. A term used synonymously with modulus of elasticity. The ratio of tensile or compressive stresses to the resulting strain. See also modulus of elasticity.

Metallographer's Guide: Practices and Procedures for Irons and Steels Bruce L. Bramfitt, Arlan O. Benscoter, p297-319 DOI:10.1361/mgpp2002p297

Copyright © 2002 ASM International® All rights reserved. www.asminternational.org

APPENDIX

Tables Helpful to the Metallographer List of ASTM standards that pertain to ferrous metallography Standards can be purchased from ASTM at the following address: ASTM (American Society of Testing Materials), 100 Barr Harbor Drive, West Conshohocken, PA, 19428-2959; Tel: 610/832-9555; E-mail: [email protected]; Website: www.astm.org

General E7 E 807 E 44 E 691

E 930 Terminology Relating to Metallography Standard Practice for Metallographic Laboratory Evaluation Heat Treating Terms Conducting an Interlaboratory Study to Determine the Precision of a Test Method

E 1181 E 1382 E 562

Test Methods for Estimating the Largest Grain Observed in a Metallographic Section Test Methods for Characterizing Duplex Grain Sizes Test Methods for Determining the Average Grain Size Using Semiand Automatic Image Analysis Practice for Determining Volume Fraction by Systematic Manual Point Count Test Method for Estimating the Depth of Decarburization of Steel Specimens Test Method for Measurement of Surface Layer Thickness by Radial Sectioning Practice for Assessing the Degree of Banding or Orientation of Microstructures

Standards for Sample Preparation to Reveal Microstructure E3 Preparation of Metallographic Specimens E 768 Practice for Preparing and Evaluating Specimens for Automatic Inclusion Assessment of Steel E 1351 Practice for Production and Evaluation of Field Metallographic Replicas E 407 Practice for Microetching Metals and Alloys E 1558 Guide for Electrolytic Polishing of Metallographic Specimens E 1920 Guide for Metallographic Preparation of Thermal Sprayed Coatings

E 1077

Standards for Sample Preparation to Reveal Macrostructure E 340 Test Method for Macroetching Metals and Alloys E 381 Method of Macroetch Testing Steel Bars, Billets, Blooms, and Forgings E 1180 Practice for Preparing Sulfur Prints for Macrostructural Examination

Photomicroscopy E 883 Guide for Reflected-Light Photomicrography

Quantitative Metallography A 247 Standard Test Method for the Classification of Graphite in the Microstructure of Cast Iron E 45 Practice for Determining the Inclusion Content of Steel E 1122 Practice for Obtaining JK Inclusion Ratings Using Automatic Image Analysis E 1245 Practice for Determining the Inclusion or Second-Phase Content by Automatic Image Analysis E 112 Test Methods for Determining the Average Grain Size

E 1182 E 1268

Hardness Testing E 384 Test Method for Microindentation Hardness Testing E 140 Standard Hardness Conversion Tables for Metals

X-Ray and Electron Metallography E 81 Test Method for Preparing Quantitative Pole Figures of Metals E 82 Test Method for Determining the Orientation of a Metal Crystal E 766 Practice for Calibrating the Magnification for a Scanning Electron Microscope E 975 Practice for X-Ray Determination of Retained Austenite in Steel with Near-Random Crystallography E 986 Practice for Scanning Electron Microscope Performance Characterization E 1508 Guide for Quantitative Analysis by Energy-Dispersive Spectroscopy

298 / Metallographer’s Guide List of vendors for metallographic supplies Allied High-Tech Products Inc., 2376 E. Pacifica Place, Rancho Dominguez, CA, 90220; Tel: 800/675-1118 or 310/635-2466; Fax: 310/762-6808 Allied Signal Corp., Chesterfield PLT Fibers/Plastics, Hopewell, VA, 23860 Alpha Resources Inc., P.O. Box 199, 3090 Johnson Road, Stevensville, MI, 49127-0199; Tel: 616/465/5559; Fax: 616/465/3629 Amplex Corporation, P.O. Box 33516, Britton Drive, Bloomfield, CT, 06002; Tel: 203/243-1775 Beta Diamond Products, Inc., P.O. Box 2069, Yorba Linda, CA, 92885-1269; Tel: 714/777-7144; Fax: 714/693-9351 Buehler Ltd., 41 Waukegan Road, Lake Bluff, IL, 60044; Tel: 800/BUEHLER or 847/295-6500; Fax: 847/295-7979 Chemplex Industries, Inc., 160 Marbledale Road, Tuckahoe, NY, 10707; Tel: 800/4-CHEMPLEX; Fax: 914/337-0160 Crystalite Corp., 8400 Green Meadows Drive, Westerville, OH, 43081; Tel: 800/777-2894; Fax: 614/548-5673 Delta Resources, 651 W. Terra Cotta, Ste. 110, Crystal Lake, IL, 60014; Tel: 800/79DELTA; Fax: 815/477-0257 DRB Technologies, Inc., 2221 Lovi Road, Freedom, PA, 15042; Tel: 412/774-8590; Fax: 412/774-0384 DSI (Diamond Systems Inc.), 160 East Alondra Boulevard, Gardena, CA, 90248; Tel: 800/421/1150; Fax: 310/324/1385 Engis Corporation, 105 West Hintz Rd., Wheeling, IL, 60090; Tel: 800/323-4069 Excel Technologies, Inc., 99 Phoenix Avenue, Enfield, CT, 06082; Tel: 203/741-3435; Fax: 203/745-6956 Falex Corp., 2055 Comprehensive Drive, Aurora, IL, 60505; Tel: 708/851-7660; Fax: 708/898-7851 Gilson Co., Inc., P.O. Box 677, Worthington, OH, 43085; Tel: 614/548-7298; Fax: 614/548-5314 Harold Johns Co., P.O. Box 247, Highland Park, IL, 60035; Tel: 708/433-6230; Fax: 708/433-6235 Hudson Supply Company, 4500 Lee Road, Cleveland, OH, 44128-2959; Tel: 800/486-0480 or 216/518-3000; Fax: 216/518-9665 Lapmaster International, 6400 Oakton Street, Morton Grove, IL, 60053; Tel: 708/967-2975; Fax: 708/967-3903 LECO Corporation, 3000 Lakeview Avenue, St. Joseph, MI, 49085-2396; Tel: 616/983-5531; Fax: 616/983-3850 LGE Specialty Products, 1530 Moon Stone Street, Brea, CA, 92621; Tel: 714/870-9400; Fax: 714/870-0609 Mager Scientific, Inc., P.O. Box 160, 1100 Baker Road, Dexter, MI, 48130-0160; Tel: 800/521-8768; Fax: 313/426-3885

Mark V Laboratory, 18 Kripes Road, East Granby, CT, 06026; Tel: 888-MARKLAB; Fax: 860-653-4087 Metallurgical Supply Company, Inc., 13581 Pond Springs Road, Suite 307, Austin, TX, 78729; Tel: 800/638-7826, 512/331-6685; Fax: 512/331-7417; E-mail: [email protected] Metals Technology, Inc., 19801 Nordhoff, Northridge, CA, 91324; Tel: 818/882-6414; Fax: 818/882-4490 Metcut Research Associates, Inc., 3980 Rosslyn Drive, Cincinnati, OH, 45209; Tel: 513/271-5100; Fax: 513/271-9511 Metlab Corporation, P.O. Box 1075, 4011 Hyde Park Blvd., Niagara Falls, NY, 14302; Tel: 716/282-6950; Fax: 716/282-6971 Moyco Industries Inc., 2001 Commerce Drive, Montgomeryville, PA, 18936; Tel: 800/331-8837 Pace Technologies, P.O. Box 853, Skokie, IL, 60076; Tel: 888/PACE-654; Fax: 847/673-1980; E-mail: [email protected] Precision Surfaces International, 922 Ashland St., Houston, TX, 77008-6734; Tel: 800/843-0950; Fax: 800/414-1644; E-mail: [email protected]; Website: psidragon.com Proto Manufacturing Ltd., 2175 Solar Crescent, Oldcastle, Ontario, Canada, N0R 1L0; Tel: 519/737-6330; Fax: 519/737-1692 South Bay Technology, Inc. (including supplies for electron microscopy), 1120 Via Callejon, San Clemente, CA, 92672; Tel: 800/728-2233 or 949/492-2600; Fax: 949/492-1499; E-mail: [email protected] Structure Probe Ind./SPI Supplies, P.O. Box 656, West Chester, PA, 19381-0656; Tel: 610/436-5400; Toll-free: 800/2424-SPI; Fax: 610/436-5755 Struers, Inc., 810 Sharon Drive, Westlake, OH, 44145; Tel: 440/871-0071, 888/STRUERS (787-8377), or 800/321-5834; Fax: 440/871-8188 TBW Industries, Inc., Forest Grove Road, P.O. Box 336, Furlong, PA, 18925-0336; Tel: 215/794-8070; Fax: 215/794-8889 Tech-Met Canada Ltd., 9999 Highway 99, Markham, Ontario, Canada, L3P 353; Tel: 905/475-4686; Fax: 905/475-4683 3M Superabrasives and Microfinishing Systems, 3M Ctr. Bldg. 251-2A-08, St. Paul, MN, 55144; Tel: 612/737-1783; Fax: 612/737-1790 ULTRA TEC Mfg., Inc., 1025 E. Chestnut Ave., Santa Ana, CA, 92701; USA Office: Tel: 714/542-0608; Fax: 714/542-0627; Direct fax: 714/242-1588; Mobile: 714/423-8726; E-mail: [email protected] Warren Diamond Powder Company, Inc., 1401 E. Lackawanna Street, P.O. Box 177, Olyphant, PA, 18447; Tel: 717/383-3261; Fax: 717/383-3218

List of light optical microscope manufacturers Carl Zeiss, Inc., Microscope Div., One Zeiss Drive, Thornwood, NY, 10594; Tel: 800/982-6493; Fax: 914/681-7432 Leica, Inc. (Leitz and Reichart), 2345 Waukegan Road, Bannockburn, IL, 60015; Tel: 800/248-0123; Fax: 708/405-0030 Nikon, Inc. Instrument Group, 1300 Walt Whitman Road, Melville, NY, 11747; Tel: 516/547-8500; Fax: 516/547-0306

Microscope reticle manufacturer Klarman Ruling, Inc., 480 Charles Bancroft Hwy., Litchfield, NH, 03052; Tel: 800/252-2401; Fax: 603/424-0970; www.reticles.com/calibration.htm

Olympus America, Inc., Precision Instrument Division, 4 Nevada Drive, Lake Success, NY, 11042-1179; Tel: 516/488-3880; Fax: 516/222-7920 Unitron, Inc., P.O. Box 469, 170 Wilbur Place, Bohemia, NY, 11716; Tel: 516/589-6666; Fax: 516/589-6975

Appendix / 299 Scientific imaging products Allied High-Tech Products Inc., 2376 E. Pacifica Place, Rancho Dominguez, CA, 90220; Tel: 800/675-1118 or 310/635-2466; Fax: 310/762-6808 BSA/Marketlink Southeast, 1120 West Butler Road, Suite A, Greenville, SC, 29607, Attn: Katherine Patten; Tel: 888/800-5139 x 126; E-mail: [email protected] Cadmet Corp., P.O. Box 24, Malvern, PA, 19355; Tel: 800/543-7282; Fax: 610/695-0290 Clemex Technologies, Inc., 800 Guimond, Longueuil, Quebec, Canada, J4G 1T5; Tel: 450/651-6573, Fax: 450/651-9304 Definitive Imaging, Ltd., 19000 Lake Road, Suite 917, Rocky River, OH, 44116; Tel: 440/333-6557; Fax: 440/333-4183; E-mail: defi[email protected] Eastman Kodak Company, Scientific Imaging Systems, 343 State Street, Rochester, NY, 14652-4115, Technical Support; Tel: 877/SIS-HELP, Fax: 203/786-5657; E-mail: [email protected] Jenoptik L.O.S., Digital Camera Division, 17821 E. 17th St., Tustin, CA, 92780 Media Cybernetics, L.P., Raymond Smith, Sales Info Systems Mgr., 8484 Georgia Avenue, Silver Spring, MD, 20910; Tel: 301/495-3305 x 249; Fax: 301/495-5964; E-mail: [email protected]

Metallurgical Supply Company, Inc., 13581 Pond Springs Road, Suite 307, Austin, TX, 78729; Tel: 800/638-7826, 512/331-6685; Fax: 512/331-7417; E-mail: [email protected] MIS, Inc., 10740 West Grand Avenue, Franklin Park, IL, 60131; Tel: 847/455-0450; Fax: 847/455-6044 Pixera Corporation, 140 Knowles Drive, Los Gatos, CA, 95030; Tel: 408/341-1800; Fax: 408/341-1818; Sales: 888/4 PIXERA; E-mail: [email protected] Polaroid Corporation, Brooks Corl, Senior Applications Manager, 201 Broadway (2nd Floor), Cambridge, MA, 02139; Voice: 781/386-8563; Fax: 781/386-8588 Quality Micro Systems, Inc., 2505 Old Monroe Rd., Matthews, NC, 28105; Tel: 704/821-5220; Fax: 704/821-5221; E-mail: [email protected] Sci-Eye, Incorporated, 2941 Corvin Drive, Santa Clara, CA, 95051, contact Adam Janecka; Tel: 408/328-9776; Fax: 408/245-5503; E-mail: [email protected] or [email protected] Visus Image Analysis Applications, Foresthill Products, 5714 Maywood Dr., Foresthill, CA, 95631; E-mail: Tom Cavallero ([email protected])

Used and/or reconditioned equipment Conneaut Lake Scientific, Specializing in used equipment, 6921 Main Street, Hartstown, PA, 16131; Tel: 814/382-1604; Fax: 814/382-8349 Mark V Laboratory, 18 Kripes Road, East Granby, CT, 06026; Tel: 888/MARKLAB; Fax: 860/653-4087

Micro-Cal, 1099-C Corporate Circle, Grayslake, IL, 60030; Tel: 847/548-2911; Fax: 847/548-2913 Vermont Optechs, P.O. Box 69, Charlotte, VT, 05445; Tel: 802/425-2040; Fax: 802/425-2074; E-mail: [email protected]

300 / Metallographer’s Guide Conversion of average grain intercept length (microns) to ASTM number ASTM No.

14.0 13.9 13.8 13.7 13.6 13.5 13.4 13.3 13.2 13.1 13.0 12.9 12.8 12.7 12.6 12.5 12.4 12.3 12.2 12.1 12.0 11.9 11.8 11.7 11.6 11.5 11.4 11.3 11.2 11.1 11.0 10.9

Average intercept

ASTM No.

Average intercept

ASTM No.

Average intercept

ASTM No.

Average intercept

2.5 2.6 2.7 2.8 2.9 3.0 3.1 3.2 3.3 3.4 3.5 3.7 3.8 3.9 4.1 4.2 4.3 4.5 4.7 4.8 5.0 5.2 5.4 5.5 5.7 5.9 6.1 6.4 6.6 6.8 7.1 7.3

10.8 10.7 10.6 10.5 10.4 10.3 10.2 10.1 10.0 9.9 9.8 9.7 9.6 9.5 9.4 9.3 9.2 9.1 9.0 8.9 8.8 8.7 8.6 8.5 8.4 8.3 8.2 8.1 8.0 7.9 7.8 7.7

7.6 7.8 8.1 8.4 8.7 9.0 9.3 9.6 10.0 10.3 10.7 11.1 11.5 11.9 12.3 12.7 13.2 13.6 14.1 14.6 15.1 15.6 16.2 16.8 17.3 18.0 18.6 19.3 20.0 20.6 21.4 22.1

7.6 7.5 7.4 7.3 7.2 7.1 7.0 6.9 6.8 6.7 6.6 6.5 6.4 6.3 6.2 6.1 6.0 5.9 5.8 5.7 5.6 5.5 5.4 5.2 5.1 5.0 4.9 4.8 4.7 4.6 4.5 4.4

22.9 23.7 24.4 25.4 26.3 27.2 28.2 29.2 30.3 31.3 32.4 33.6 34.7 35.9 37.2 38.5 39.9 41.3 42.8 44.3 45.8 47.4 49.1 53 55 56 58 60 63 65 67 69

4.3 4.2 4.1 4.0 3.9 3.8 3.7 3.6 3.5 3.4 3.3 3.2 3.1 3.0 2.9 2.8 2.7 2.6 2.5 2.4 2.3 2.2 2.1 2.0 1.8 1.6 1.4 1.2 1.0 0.5 0 00

72 74 77 80 83 86 89 92 95 98 102 105 109 113 117 121 125 130 134 139 144 149 154 160 171 183 196 210 225 268 313 453

Chemical polishing solutions Before using any chemicals, materials safety data sheets (MSDS) should be reviewed. Etching reagent

To polish

Composition

Remarks

Plichta

Carbon steels (pure iron to 0.8% C and up to 3% alloys)

100 mL H2O 100 mL H2O2 (30%) 14 mL HF

Temperature: 15–20 °C (59–68 °F) (very important) Time: 3 min Prior to chemical polishing, the sample should be ground to a 600-grit finish. To prevent staining, rinse the sample in H2O2 (30%), followed by a ethyl alcohol rinse, and dry.

Chia

Pure iron

Anderson

Carbon steels

Temperature: 20 °C (68 °F) Time: 30–45 seconds Temperature: room

Anderson

Carbon steels

Wiesinger

Iron, iron-silicon alloys

de Jong

Austenitic stainless steel

70 mL H2O2 (30%) 5 mL HF 80 mL H2O 28 g oxalic acid 4 mL H2O2 (30%) 100 mL H2O 50 g chromic acids 15 mL H2SO4 94 mL H2O2 (30%) 6 mL HF 50 mL acetic acid 40 mL HNO3 10 mL HCl 10 mL H3PO4

Temperature: room

Temperature: room. Rinse in water, flush with ethyl alcohol, and dry Temperature: 70 °C (158 °F) Etch to desired contrast

Appendix / 301 Electroless and electrolytic coatings for edge protection Depositing a metallic coating on a specimen prior to mounting is an excellent technique to ensure edge preservation for examination up to 1000 ⫻. Steel samples with a zinc, tin, or cadmium plating must first be electroplated with copper or sputter coated with gold palladium. Electroless nickel plating is by far the most effortless technique. The authors recommend using Buehler Corporation’s electroless commercial solution. ELECTROLYTIC Electrolytic Copper Plating Solution 500 mL H2O 85 g CuSO4. 5H2O (copper sulfate) 30 mL H2SO4 Temperature: Voltage: Current density: Agitation: Anode: Anode bag: Rate:

15–50 °C (59–122 °F) 1–4 V 10–20 mA/cm2 Mild stirring Copper Desirable 20:μ/h

Electrolytic Copper Plating Solution 500 mL H2O 10 g CuCN 15 g NaCN 1.5 g NaOH

Electrolytic Nickel Plating Solution 500 mL H2O 150 g NiSO4 7H2O 30 g NiCl2 6H2O 20 g boric acid ph 4 (raise with NaOH, lower with H2SO4) Temperature: Voltage: Current density: Agitation: Anode: Anode bag: Rate:

40–50 °C (104–122 °F) 1–3 V 30 mA/cm2 Vigorous stirring Pure nickel Essential 28:μ/h

ELECTROLESS Electroless Nickel Plating Solution

Temperature: Voltage: Current density: Agitation: Anode: Anode bag:

45–60 °C (113–140 °F) 4–6 V 0.5–1 mA/cm2 Mild stirring Copper Desirable

Temperature:

85 °C (185 °F)

Electroless Nickel Plating Solution

Electrolytic Iron Plating Solution 500 mL H2O 144 g FeCl2 (ferric chloride) 28 g NaCl (sodium chloride) Temperature: Current density: Agitation: Anode:

Buehler Corporation’s Edgemet Kit #20-8192 Mix equal parts of solution A with solution B

70–100 °C (158–212 °F) 0.5–4 A/cm2 Rotate specimen, if possible, at 50 rev/min Pure iron

1000 mL H2O 37.3 g NiSO4(nickelous sulfate) 26.4 g Na2H2PO2(sodium hypophosphite) 15.9 g NaC2H3O2(sodium acetate) 5 drops H2SO4(sulfuric acid) Temperature: Deposit rate:

71–82 °C (160–180 °F) 10 μ/h

302 / Metallographer’s Guide Etchants for revealing macrostructures in iron and steel Before using any chemicals, materials safety data sheets (MSDS) should be reviewed. Etching reagent

To reveal

Composition

Remarks

Hydrochloric acid

Segregation, porosity, cracks, depth of hardened zones in tool steels

50 mL H2O 50 mL HCl

Temperature: 71–82 °C (160–180 °F) Time: 1–60 min, depending on type of steel and type of structure to be developed

Mixed acids

Welds, segregation, cracks, and hardened zones

50 mL H2O 38 mL HCl 12 mL H2SO4

Temperature: 93 °C (200 °F) Time: 15–45 s or Temperature: 22 °C (72 °F) Time: 2–4 hours

Ammonium persulfate (Rawdon)

Welds

90 mL H2O 10 mL (NH4)2S2O8

Surface should be rubbed with absorbent cotton during etching. Time: etched to desired contrast

Nitric acid

Same as hydrochloric acid

75 mL H2O 25 mL HNO3

Use cold on large surfaces that cannot be conveniently heated

Oberhoffer’s reagent

Develops dendritic pattern in steel. Iron enriched areas are darkened.

500 mL H2O 30 g FeCl3 0.5 g SnCl3 1 g CuCl2 500 mL ethyl alcohol 500 mL H20

Use at room temperature. Immerse about 20 s.

Kalling’s reagent

Develops dendritic pattern in steel; attacks ferritic and martensitic stainless steels. Ferrite darkened; martensite darker; austenite light

33 mL H2O 1.5 g CuCl2 33 mL methyl alcohol 33 mL HCl

Etch to desired contrast

Humfrey’s reagent

Develops dendritic segregation

500 mL H2O 25 mL HCl 60 g Cu(NH3)4Cl2

Slight abrasion of surface after etching is recommended.

Bell’s reagent (electrolytic)

Welds in stainless steel

40 mL H2O 60 mL H2NO3

Stainless steel cathode 5V Time: to desired contrast

Watertown arsenal

For carbon and stainless steels, general structure

50 mL H2O 12 mL H2SO4 38 mL HCl

Temperature: 71–82 °C (160–180 °F) Time: 10–60 min Used with cut or ground surfaces

Dilute aqua regia

For high-alloy steels, Fe-Co high-temperature alloys

25 mL H2O 25 ml HNO3 50 mL HCl

Time: 10–15 min at room temperature

Piearcy and co-workers

Maraging steel macrostructure

60 mL lactic acid 20 mL HNO3 10 mL HCl

Use at room temperature

Burg and Weiss

Nitriding steels macrostructure

85 mL H2O 15 mL ammonium persulfate

Temperature: 71 °C (160 °F) Time: 10 min

Marbles reagent

General etch for austenitic stainless steel

50 mL H2O 10 g CuSO4 50 mL HCl

Solution can be heated

Miller and Houston

Welds and general macrostructure of austenitic stainless steel

90 mL H2O 10 g CrO3

Use electrolytically with polished surface. Platinum or stainless steel cathode. Temperature: 16–38 °C (60–100 °F) 6V Time: 2–7 min

Loria (nitrosulfuric acid etch)

As-cast structure and grain size in cast steels

20 mL H2O 20 mL HNO3 10 mL H2SO4

Use at room temperature

Nielsen

Stress-corrosion cracks in austenitic stainless steel

95 mL methyl alcohol 5 mL bromine

Temperature: room Time: approx. 1 h crack pattern vividly revealed

Appendix / 303 Etchants for carbon and alloy steels Popular etchants for revealing the microstructures of common phases and constituents found in ferrous materials Etching reagent

Ferrite grain boundaries: Nital

Marshall’s reagent

Composition

Remarks

1 to 5 mL HNO3 (nitric acid) 99 to 95 mL ethyl

The authors recommend a 2% solution; usually samples are etched 10 to 20 s with a gentle agitation and the polished surface facing upward to reveal the ferrite grain boundaries. In order to reveal all the grain boundaries, the last polishing step should be with silicon dioxide for at least 60 s, and the sample should then be etched shortly after polishing before a passive layer forms thus preventing an even etch. If the sample is to be examined in the as-polished condition, it should be repolished for 10 to 15 s with silicon dioxide prior to etching. (see Fig. 1.23). Note: A nital etch is not recommended for etching pearlite, because it does not etch evenly.

Stock solution (A) 100 mL H2O 8 g oxalic acid 5 mL H2SO4 (sulfuric acid)

Excellent etchant to show both equiaxed (Fig. 8.13) and cold-worked microstructures (Fig. 8.16). Mix stock solution A to equal parts of solution B just prior to etching. Place the polished face of the sample on its side during immersing in the etchant to prevent or minimize pitting. If the sample does not react to the reagent, add 1 mL hydrofluoric acid to 100 mL of solution. If a haze covers the surface, remove by placing the sample in a 3% aqueous disodium ethylenediamine tetraacetate EDTA solution. Place the sample and solution in an ultrasonic cleaner for several minutes. Flush the sample with water, then alcohol, then dry. Pre-etch for 3 s with 2% nital. Etch in Beraha’s reagent with the polished side of the sample face up for 45 s; do not agitate. Rinse with water, flush with alcohol, and dry. If the sample is over- or underetched, remove etchant by polishing with 0.3 aluminum oxide for 15 s, followed by silicon dioxide for 45 s, and re-etch. See Fig. 8.1.

Stock solution B H2O2 (30%) (hydrogen peroxide) Beraha’s

Pearlite structure 4% picral

Marshall’s reagent

100 mL H2O 10 g Na2S2O3 (potassium thiosulfate) 3 g K2S2O5

96 mL ethyl alcohol 4 g picric acid 5 drops 17% zephiran (benzalkonium chloride, Sanofi-Synthelabo Inc.) per 75 mL solution

Picral is one of few etchants that improves with use. A common practice is to age the etchant by placing a ground piece of steel into the solution until the etchant turns a dark green. This technique makes the etching time constant, usually 20 s for pearlite or Fe3C carbides. The etchant can be saved for future use. (see Fig. 2.19). Samples with more then 0.5% Cr, add 5 drops of hydrochloric acid per 100 mL of solution (see Fig. 2.19).

Stock solution A 100 mL H2O 8 g oxalic acid 5 mL H2SO4 (sulfuric acid)

Excellent etchant to outline pearlite colony boundries (see Fig. 8.21).

Stock solution B H2O2 (30%) (hydrogen peroxide) Alkaline sodium picrate 100 mL H2O 25 g NaOH 2 g picric acid

Klemm’s reagent

Fe3C Carbides 4% picral

Marshall’s reagent

Bring the solution to a low boil; do not boil dry. Etching time: 5–10 min The solution will attack Bakelite mounts; thermosetting epoxy mounts are recommended. The solution will color cementite but will not color carbides with high (10%) Cr content. The solution will also attack sulfides and delineate grain boundaries in steels that have been cooled slowly.

50 mL (H2O) Na2S2O3 enough for saturation 1 g K2S2O5(potassium metabisulfite)

Tint etches pearlite. Etching time: 40–120 s. Etch in Klemm’s reagent with the polished side of the sample face up, do not agitate. Ferrite appears black-brown, but carbides, nitrides, and phosphides remain white. Phosphorus distribution can be detected more sensitively than with usual phosphorus reagents based on copper salts.

96 mL ethyl alcohol 4 g picric acid 5 drops 17% zephiran chloride per 75 mL solution

Carbides will be delineated from the ferrite by a black boundary. (see Fig. 3.17).

Stock solution A 100 mL water 8 g oxalic acid 5 mL H2SO4 (sulfuric acid)

Excellent reagent which will color carbides a light tan. The color variation will assist in differentiating small carbides on ferrite grain boundaries (see Fig. 2.17).

Stock solution B H2O2 (30%) (hydrogen peroxide) (continued)

304 / Metallographer’s Guide Etchants for carbon and alloy steels (continued) Etching reagent

Composition

Remarks

Alkaline sodium picrate 100 mL H2O 25 g NaOH 2 g picric acid

Ferrite and pearlite 2% nital/4% picral 4% picral followed by 2% nital

Bring the solution to a low boil; do not boil dry. Etching time 5 to 10 min The solution will attach Bakelite mounts. Thermosetting epoxy mounts are recommended. The solution will color cementite but will not color carbides with high (10%) chromium content. The solution will also attack sulfides and delineate grain boundaries in steels that have been cooled slowly.

Mix equal parts 2% nital and aged 4% picral 96 mL ethyl alcohol 4 g of picric acid 5 drops of 17% zephiran chloride per 75 mL solution

Etchant will reveal both ferrite and pearlite boundaries. Etch the sample for 20 s in 4% picral. Rinse in an alcohol bath, flush with alcohol, and dry. Next, etch the sample for 10 to 15 s in 2% natal. (see Fig. 1.3).

2 mL HNO3 ⫹ 96 mL ethyl alcohol Bainite 4% picral

96 mL ethyl alcohol 4 g picric acid 5 drops 17% zephiran chloride per 75 mL solution

Etch to desired contrast (see Fig. 2.31 or 2.33). Sometimes additional etching in 2% nital will reveal substructure. This etchant is excellent for upper and lower banite.

Sodium metabisufite

100 mL H2O 12 g Na2S2O5

Pre-etch for 3 s in 2% nital. Rinse in water and immediately immerse in the sodium metabisulfite; do not agitate. Time may vary from 10 to 30 s. Rinse in water, flush with alcohol, and dry.

4% picral ⫹ HCl

96 mL ethyl alcohol 4 g of picric acid 5 drops 17% zephiran chloride per 75 mL solution plus a few drops to several millimeters of hydrochloric acid

See Fig. 2.32.

Ferrite, pearlite, bainite, and martensite Sodium metabisulfite 100 mL H2O 12 g Na2S2O5 4% picral followed by 2% nital

Pre-etch the sample for 3 s in 2% nital. Rinse the sample in water, then immediately immerse the sample in the sodium metabisulfite. Do not agitate. (see Fig. 8.38). Etchant time is approximately 20 s.

96 mL ethyl alcohol 4 g picric acid 5 drops 17% zephiran chloride per 75 mL solution 2 mL H2NO3 ⫹ 96 mL ethyl alcohol

Martensite 2% nital

2mL HNO3 (nitric acid) 98 mL ethyl or methyl alcohol

Use this etchant for lath and plate martensite. Retained austenite in plate martensite structure appears white between the plates. For this etchant to be effective, it is necessary to slightly temper the martensite. A mounting temperature of 149 °C, or 300 °F is sufficient. For plate martensite, see Fig. 2.37. For martensite, see Fig. 2.24, 2.25.

Sodium metabisulfite

100 mL H2O 12 g Na2S2O5

Pre-etch for 3 s in 2% nital. Rinse in water and immediately immerse in the sodium metabisulfite. Do not agitate. Time may vary from 10 to 30 s. Rinse in water, flush with alcohol, and dry.

4% picral

96 mL ethyl alcohol 4 g picric acid 5 drops 17% zephiran chloride per 75 mL solution

This etchant works best on highly tempered martensite. To reveal residual laths in lath martensite, use 2% nital over the 4% picral etch.

Marshall’s reagent

Stock solution A 100 mL water 8 g oxalic acid 5 mL H2SO4 (sulfuric acid)

Works great on lath martensite, especially to reveal the prior austenite grain boundaries.

Stock solution B H2O2 (30%) (hydrogen peroxide) 4% picral followed by 2% nital

96 mL ethyl alcohol 4 g picric acid 5 drops 17% zephiran chloride per 75 mL solution

Works well on martensite that has been tempered at high temperatures or for long period of time (see Fig. 8.33)

2 mL H2NO3 ⫹ 96 mL ethanol alcohol (continued)

Appendix / 305 Etchants for carbon and alloy steels (continued) Etching reagent

Composition

Remarks

Prior austenite grain boundaries Vilella’s 100 mL ethyl alcohol 1 g picric acid 5 mL hydrochloric acid Alkaline sodium picrate 100 mL H2O 25 g NaOH 2 g picric acid

Bring the solution to a low boil. Do not boil dry. Etching time: 10 min followed by 10 s in 2% nital, then 20 s in 4% picral (see Fig. 8.28). The solution will attack Bakelite mounts. Thermosetting epoxy mounts are recommended.

Modified Winsteard’s

Part A: 10 mL ethyl alcohol 2 g picric acid Part B: 200 mL H2O 5 mL sodium tridecylbenzene sulfunate 5 drops hydrochloric acid

Mix part A with part B. Solution can be stored and reused many times. Sodium tridecylbenzene sulfunate can be substituted with All (Lever Brothers Co.) detergent or Calsoft 90 (Pilot Chemical Co.). Volume of hydrochloric acid can be increased after the following procedures are tried: a) At room temperature up to 5 min, observe using first bright-field. If no results are obtained, try dark-field illumination. (b) If no results, heat the solution 15 to 21 °C (60 to 70 °F) and repeat the room temperature procedure. (c) If no results, place the sample and solution in a ultrasonic cleaner, repeating the room temperature procedure (see Fig. 8.29, 8.30).

Saturated aqueous picric acid

10 g picric acid 100 mL water 1 g sodium tridecylbenzene sulfanate (wetting agent)

Add picric acid crystals to water while stirring. Decant liquid from undissolved crystals. Add wetting agent to liquid. Immerse sample, rinse with water followed by an alcohol rinse and blow dry.

96 mL ethyl alcohol 4 g picric acid 5 drops 17% zephiran chloride per 75 mL solution

Etch no less than 20 s.

Internal oxidation 4% picral

Stainless steel microstructures, austenite 60/40 40 mL H2O 60 mL HNO3 nitric acid

Electrolytic etchant. Stainless steel cathode at 6 V, 10 to 20 s or until desired contrast; will outline austenite grain boundaries and annealing twins (see Fig. 1.11). A platinum cathode will produce austenite grain boundaries without revealing annealing twins (see Fig. 8.46).

HCl / methanol

95 mL methyl alcohol 5 mL HCl

Electrolytic etchant. Stainless steel cathode, 10 V. Suggested use for Invar (see Fig. 1.22).

Oxalic acid

100 mL H2O 10 g oxalic acid

Electrolytic etchant. Stainless steel cathode, 5 to 6 V, 10 to 20 s (see Fig. 3.53).

Aqua regia

45 mL HCl (concentrated) 15 mL HNO3 (concentrated)

Use at room temperature. For austenite grain structure. Outlines carbides

Chromic oxide

10 g CrO3 100 mL water

Electrolytic etchant. Stainless steel cathode, 6V, 10–20 s. For welds, outlines carbides

Stainless steel microstructures, ferrite Glyceregia 30 mL glycerol 20 mL HCl 10 mL HNO3 Murakami’s reagent

10 g K3Fe(CN)6 10 g KOH 100 mL water

Stainless steel microstructures, martensite Kalling’s No. 1 33 mL H2O 1.5 g CuCl2 33 mL ethyl alcohol 33 mL HCl Fry’s

25 mL H2O 5 g CuCl2 25 mL ethyl alcohol 40 mL HCl

Immerse until desired contrast is obtained (see Fig. 1.13).

Use hot 75 °C (⫺165 °F) to darken sigma phase. Use at room temperature to darken carbides.

Immerse until desired contrast is obtained (see Fig. 1.15). For maraging steel, see Fig. 8.41.

Immerse until desired contrast is obtained. (see Fig. 1.17). For Custom 630 (precipitation hardening grade), see Fig. 8.43.

(continued)

306 / Metallographer’s Guide Etchants for carbon and alloy steels (continued) Etching reagent

Vilella’s

Composition

100 mL ethyl alcohol 1 g picric acid 5 mL HCl

Stainless steel microstructures, delta ferrite 60/40 40 mL H2O 60 mL HNO3 nitric acid Sodium hydroxide

20 g NaOH 100 mL water

Stainless steel microstructures, duplex Oxalic acid 100 mL H2O 10 g oxalic acid

Remarks

Recommend for revealing martensitic structure in AISI 410 stainless steel (see Fig. 2.28).

Electrolytic etchant. Stainless steel cathode, 10 V for several seconds (see Fig. 8.47). Electrolytic etchant. Stainless steel cathode, 3V for 5–10 s. Colors delta ferrite tan to orange Electrolytic etchant Stainless cathode, 5 V for 5 to 10 s (see Fig. 1.16).

Kalling’s reagent No. 2

100 mL ethyl alcohol 5 g CuCl2 100 mL HCl

Ferric attacked most readily. Austenite slightly attacked, and carbides not attacked. Etch by immersion.

Potassium hydroxide

100 mL H2O 10 g potassium hydroxide

Electrolytic etchant. Stainless steel cathode. 3 V 5 to 20 s (see Fig. 8.48).

Lichtenegger / Bloech reagent

100 mL H2O 20 g ammonium bifluoride 0.5 g potassium bisulfide

Etchant can be stored in plastic bottle. Etching time 1 to 5 min at 25 to 30 °C (see Fig. 8.40).

Stainless steel microstructures, sensitized 4% picral ⫹ HCl 96 mL ethyl alcohol 4 g picric acid 5 mL HCl Vilella’s reagent

100 mL ethyl alcohol 1 g picric acid 5 mL hydrochloric acid

50/50/50

Equal parts H2O, nitric acid, and hydrochloric acid

Coated steels, zinc-based coatings Amyl nital 1 mL HNO3 (concentrated) 99 mL amyl alcohol Amyl picral/nital

Rowland’s reagent

See Fig. 8.44.

To 50 mL of cold water, add 50 mL nitric acid. Cool to below room temperature, and slowly add 50 mL hydrochloric acid (solution should be clear). See Fig. 3.43, 3.44. For Galvalume coatings (see Fig. 8.49).

Part A 1 g picric acid 99 mL amyl alcohol

Mix Parts A and B together in a beaker. Separate the solution into two beakers. Add 3 – 4 drops HF to first solution. Immerse specimen in first solution for 20 s.

Part B 1 mL HNO3 (concentrated) 99 mL amyl alcohol

Rinse in ethyl alcohol. Immerse specimen in second solution for 10 s. Flush with ethyl alcohol and blow dry.

Part A 0.75 g picric acid

Dissolve picric acid crystals in ethyl alcohol. Add Part A to Part B.

Part B 480 mL water Coated steels, aluminum-based coatings Nital 2 mL HNO3 (concentrated) 98 mL ethyl alcohol

For aluminized coatings (see Fig. 4.7).

Appendix / 307 Carbon steel compositions

Free-machining (resulfurized) carbon steel compositions

(a) When lead ranges or limits are required, or when silicon ranges or limits are required for bars or semifinished products, the values in the table “Carbon steel compositions” apply. For rods, the following ranges and limits for silicon are commonly used; up to SAE 1110 inclusive, 0.10% max; SAE 1117 and over, 0.10% max, 0.10⫺0.20%, or 0.15⫺0.35%.

Free-machining (rephosphorized and resulfurized) carbon steel compositions

(a) When lead ranges or limits are required, the values in the table “Carbon steel compositions” apply. It is not common practice to produce the 12xx series of steels to specified limits for silicon because of its adverse effect on machinability.

High-manganese carbon steel compositions (a) When silicon ranges or limits are required for bar and semifinished products, the following ranges are commonly used; 0.10% max; 0.10 to 0.20%; 0.15 to 0.35%; 0.20 to 0.40%; or 0.30⫺0.60%. For rods, the following ranges are commonly used; 0.10 max; 0.07⫺0.15%; 0.10⫺0.20%; 0.15⫺0.35%; 0.20⫺0.40%; and 0.30⫺0.60%. Steels listed in this table can be produced with additions of lead or boron. Leaded steels typically contain 0.15⫺0.35% Pb and are identified by inserting the letter L in the designation (10L45); boron steels can be expected to contain 0.0005⫺0.003% B and are identified by inserting the letter B in the designation (10B46).

High-manganese carbon steel compositions

(a) When silicon, lead, and boron ranges or limits are required, the values in the table “Carbon steel compositions” apply.

(a) When silicon ranges or limits are required, the following ranges and limits are commonly used: up to SAE 1025 inclusive, 0.10% max, 0.10⫺0.25%, or 0.15⫺0.35%. Over SAE 1025, 0.10⫺0.25% or 0.15⫺0.35%.

308 / Metallographer’s Guide Alloy steel compositions applicable to billets, blooms, slabs, and hot-rolled and cold-finished bars

(a) Small quantities of certain elements that are not specified or required may be found in alloy steels. These elements are to be considered as incidental and are acceptable to the following maximum amount: copper to 0.35%, nickel to 0.25%, chromium to 0.20%, and molybdenum to 0.06%. (b) Electric furnace steel. (c) Boron content is 0.0005⫺0.003%.

Appendix / 309 Alloy steel compositions applicable to billets, blooms, slabs, and hot-rolled and cold-finished bars (continued)

(a) Small quantities of certain elements that are not specified or required may be found in alloy steels. These elements are to be considered as incidental and are acceptable to the following maximum amount: copper to 0.35%, nickel to 0.25%, chromium to 0.20%, and molybdenum to 0.06%. (b) Electric furnace steel. (c) Boron content is 0.0005⫺0.003%.

Chemical compositions for typical low-alloy steels See also the table “Nominal chemical compositions for heat-resistant chromium-molybdenum steels” in this Appendix.

(a) Single values represent the maximum allowable. (b) Zirconium may be replaced by cerium. When cerium is added, the cerium/sulfur ratio should be approximately 1.5/1, based on heat analysis.

310 / Metallographer’s Guide ASTM specifications for chromium-molybdenum steel product forms

Nominal chemical compositions for heat-resistant chromium-molybdenum steels

(a) Single values are maximums. (b) Also contains 0.02⫺0.030% V, 0.001⫺0.003% B, and 0.015⫺0.035% Ti. (c) Also contains 0.40% Ni, 0.18⫺0.25% V, 0.06⫺0.10% Nb, 0.03⫺0.07% N, and 0.04% Al

Appendix / 311 Compositions of standard stainless steels

(a) Single values are maximum values unless otherwise indicated. (b) Optional

312 / Metallographer’s Guide Compositions of nonstandard stainless steels

(a) XM designations in this column are ASTM designations for the listed alloy. (b) Single values are maximum values unless otherwise indicated. (c) Nominal compositions. (d) UNS designation has not been specified. This designation appears in ASTM A 887 and merely indicates the form to be used.

Appendix / 313 Compositions of nonstandard stainless steels (continued)

(a) XM designations in this column are ASTM designations for the listed alloy. (b) Single values are maximum values unless otherwise indicated. (c) Nominal compositions. (d) UNS designation has not been specified. This designations appears in ASTM A 887 and merely indicates the form to be used.

314 / Metallographer’s Guide Nominal compositions of wrought iron-base heat-resistant alloys

(a) Ti ⫹ Nb ⫽ (0.20 ⫹ 4C ⫹ 4N) min. (b) Optional. (c) Minimum for Nb ⫹ Ta

Appendix / 315 Composition limits of principal types of tool steels

(a) All steels except group W contain 0.25 max Cu, 0.03 max P, and 0.03 max S; group W steels contain 0.20 max Cu, 0.025 max P, and 0.025 max S. Where specified, sulfur may be increased to 0.06 to 0.15% to improve machinability of group A, D, H, M, and T steels. (b) Available in several carbon ranges. (c) Contains free graphite in the microstructure. (d) Optional. (e) Specified carbon ranges are designated by suffix numbers.

316 / Metallographer’s Guide Composition limits of principal types of tool steels (continued)

(a) All steels except group W contain 0.25 max Cu, 0.03 max P, and 0.03 max S; group W steels contain 0.20 max Cu, 0.025 max P, and 0.025 max S. Where specified, sulfur may be increased to 0.06 to 0.15% to improve machinability of group A, D, H, M, and T steels. (b) Available in several carbon ranges. (c) Contains free graphite in the microstructure. (d) Optional. (e) Specified carbon ranges are designated by suffix numbers.

Standard composition ranges for austenitic manganese steel castings

Nominal compositions of commercial maraging steels

Typical compositions for malleable iron

Typical base compositions of SAE J431 automotive gray cast irons for heavy-duty service

(a) If either carbon or silicon is on the high side of the range, the other should be on the low side. Alloying elements not listed in this table may be required. (b) Microstructure: size 2 to 4 type A graphite in a matrix of lamellar pearlite containing not more than 15% free ferrite. (c) Microstructure: size 3 to 5 type A graphite in a matrix of lamellar pearlite containing not more than 5% free ferrite or free carbide. (d) Alloy gray iron containing 0.85 to 1.25% Cr, 0.40 to 0.60% Mo, and 0.20 to 0.45% Ni or as agreed. Microstructure: primary carbides and size 4 to 7 type A or E graphite in a matrix of fine pearlite, as determined in a zone at least 3.2 mm (1⁄8 in.) deep at a specified location on a cam surface

(a) All grades contain no more than 0.03% C. (b) Some producers use a combination of 4.8% Mo and 1.4% Ti, nominal. (c) Contains 5% Cr

Appendix / 317 Chemical compositions and mechanical properties of austenitic manganese steels for nonmagnetic and cryogenic applications

(a) At ⫺196 °C (⫺321 °F)

Composition of selected cast irons

(a) TC, total carbon. (b) Optional

318 / Metallographer’s Guide

Temperature Conversions The general arrangement of this conversion table was devised by Sauveur and Boylston. The middle columns of numbers (in boldface type) contain the temperature readings (°F or °C) to be converted. When converting from degrees Fahrenheit to degrees Celsius, read the Celsius equivalent in the column headed “°C.” When converting from Celsius to Fahrenheit, read the Fahrenheit equivalent in the column headed “°F.” °F

°C

°F

°C

°F

°C

°F

°C

°F

°C

Appendix / 319 °F

°C

°F

°C

°F

°C

°F

°C

°F

°C

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Index A A, definition ..........................................................................................245 A., definition .........................................................................................245 Acm definition............................................................................................245 in iron-carbon phase diagram.............................................25(F), 26(F) A1 ......................................................................................................58, 61 definition............................................................................................245 graphitization possibility.....................................................................77 in iron-carbon phase diagram.............................................25(F), 26(F) A3 definition............................................................................................245 in iron-carbon phase diagram.............................................25(F), 26(F) A4, definition.........................................................................................245 Aberration. See also Astigmatism, Chromatic aberration, Coma, and Spherical aberration. definition............................................................................................245 Abrasion, definition .............................................................................245 Abrasion rate, definition .....................................................................245 Abrasive, definition..............................................................................245 Abrasive belt, definition ......................................................................245 Abrasive cut-off machine ........................94-97, 99, 104, 105, 106, 180 Abrasive cut-off sectioning, solutions for problems encountered ..............................................................................177(T) Abrasive cut-off wheel.........................................................174-178(F,T) shelf-life, ...........................................................................................178 Abrasive disk, definition .....................................................................245 Abrasive paper, definition...................................................................245 Abrasive wear, definition ....................................................................245 Accm, definition.....................................................................................245 Ac1 ......................................................................................................32(F) definition............................................................................................245 Ac3 ...............................................................................................32(F), 50 definition............................................................................................245 Ac4, definition .......................................................................................245 Accelerating potential, definition .......................................................245 Acetone.........................................................................................189, 193 as cleaning solvent ...................................................................173, 174 Achromat..............................................................................................120 Achromatic. See also Achromatic objective. definition............................................................................................245 Achromatic objective..................................................................120, 125 definition............................................................................................245 Achromatic objective lens, definition ................................................245 Acicular ferrite ..............................................................32, 33(F), 38, 39 definition............................................................................................245 Acid, definition .....................................................................................245 Acid embrittlement, definition ...........................................................245 Acid extraction, definition...................................................................245 Acrylic castable ..............................................................................................192 castable, cause and solution to mounting problems...........193, 194(T) Acrylic compounds, as mounting materials .......................................187 Acrylic lacquer spray coating............................................................173 Acrylic mounting material ............................................................189(F) Activation, definition ...........................................................................245 Activator...............................................................................................190 Activity, definition................................................................................245

Act I ......................................................................................................174 Adhesion, definition .............................................................................245 Adhesive bonding, definition ..............................................................245 Adhesive wear, definition....................................................................245 Adjustable eyepiece.............................................................................125 Aecm ........................................................................................................26 definition............................................................................................245 Ae1 (equilibrium eutectoid temperature) .........26, 29(F), 30(F), 31(F) definition............................................................................................245 Ae3...........................................................................................................26 definition............................................................................................245 Ae4, definition .......................................................................................245 AES. See Auger electron spectroscopy. Age hardening. See also Aging. definition............................................................................................245 Age softening, definition......................................................................245 Aging. See also Age hardening, Artificial aging, Interrupted aging, Natural aging, Overaging, Precipitation hardening, Precipitation heat treatment, Progressive aging, Quench aging, Step aging, and Strain aging. ..............................................58 definition ....................................................................................245-246 Air hardening steels AISI code classification ......................................................................12 definition............................................................................................246 Airy, Sir George ..................................................................................246 Airy disk, definition .............................................................................246 Alcohol..................................................................................................189 Alconox.................................................................................................174 Alignment. See also Magnetic alignment, Mechanical alignment, and Voltage alignment. definition............................................................................................246 Alkali metal, definition........................................................................246 Alkaline cleaner, definition .................................................................246 Alkaline earth metal, definition .........................................................246 Alkaline sodium picrate, as etchant for carbon steels and alloy steels ...........................................................................301(T), 302(T) Alligatoring, definition.........................................................................246 Alligator skin. See Orange peel. Allotriomorph, definition ....................................................................246 Allotriomorphic crystal, definition.....................................................246 Allotropic change, in iron .....................................................................27 Allotropy. See also Polymorphism. definition............................................................................................246 Alloy, definition ....................................................................................246 Alloy carbides .............................................................236, 237, 238, 239 Alloying element, definition ................................................................246 Alloy steels .........................................................................2, 3(T), 6-7(F) austenite in microstructure ............................................................39-40 compositions applicable to billets, blooms, slabs, and hot-rolled and cold-finished bars ...........................306(T), 307(T) definition............................................................................................246 etchants for revealing microstructures.................................301-303(T) granular bainite in microstructure .................................................39(F) overheating ..............................................................69(F), 70(F), 71-72 special-purpose..........................................................................13-16(F) UNS designations and corresponding SAE and AISI numbers...................................................................306(T), 307(T) Alloy system, definition .......................................................................246 Alnico V..................................................................................................13

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322 / Metallographer’s Guide Alpha alumina .....................................................................................204 Alpha ferrite. See also Ferrite....................................................... 25, 27 Alpha iron.........................................................................................26-27 definition............................................................................................246 Alpha stabilizer, definition..................................................................246 Alternate etch and polish technique, definition ...............................246 Alumina. See also Aluminum oxide.................................................... 97 blade, for abrasive cut-off machine .............................................95, 99 polishing compound .................................................................158, 161 solution for polishing,......................................................93, 94, 96, 99 Aluminized coating, metallographic analysis.............................92-93(F) Aluminizing, definition ........................................................................246 Aluminum function as element in steel and cast iron ......................................3(T) as graphitizer.......................................................................................46 WDS x-ray map in nonmetallic inclusion ..................................159(F) Aluminum carbides...............................................................................58 Aluminum coated steels, etchants for ...........................................304(T) Aluminum nitrides .................................................................51, 58, 202 precipitates in low-alloy steels with formability .................................9 Aluminum oxide. See also Alumina. in abrasive cut-off wheel............................................................94, 177 inclusions .............................................................................172, 174(F) particles, as polishing abrasives .......................................................204 Ambient, definition ..............................................................................246 American Iron and Steel Institute (AISI) and Society of Automotive Engineers (SAE) classification system composition ranges for various designations.......................................7 for high-strength, low-alloy steels ..............................................8, 9(F) for low-alloy steels ............................................................................6-7 for plain carbon steels ..................................................................4-5(F) for steels and cast irons ........................................................................1 American Society for Testing and Materials (ASTM) classification system for steels and cast irons..............................1 A designations....................................................................................5-6 for high-strength, low-alloy steels........................................................8 for low-alloy steels ...............................................................................7 for plain carbon steels ..................................................................5-6(F) American Society for Testing and Materials (ASTM) designations, specific types A 1, type of steel product.....................................................................6 A 1, microstructure ..........................................................................6(F) A 36, manganese sulfide dendrites in overheated slab forging ..............................................................................70(F), 72 A 36, microstructure ........................................................................6(F) A 36, rough polishing of plate ....................................................206(F) A 36, sheared plate microstructure..............................................178(F) A 36, type of steel product...................................................................6 A 47, malleable iron product types....................................................18 A 48, gray iron grade classifications by minimum tensile strength ........................................................................................17 A 48, gray iron product types ............................................................16 A 126, gray iron product types ..........................................................16 A 128, austenitic manganese steels with good abrasion resistance .....................................................................................13 A 128, composition ranges for austenitic manganese steel castings.................................................................................314(T) A 128, microstructure ....................................................................15(F) A 131, type of steel product.................................................................6 A 159, gray iron product types ..........................................................16 A 182, low-alloy steel product types ...................................................7 A 197, malleable iron product types..................................................18 A 199, low-alloy steel product types ...................................................7 A 210, hydrogen damage in steel tube ...................................84, 85(F) A 213, composition of grade (type) T22 .............................................7 A 213, heat treatment conditions available..........................................7 A 213, low-alloy steel product types ...................................................7 A 213, steel grades covered by ............................................................7 A 213-T22, composition .......................................................................7 A 213-T22, microstructure...............................................................7(F) A 217, low-alloy steel product types ...................................................7 A 220, low-alloy steel product types ...................................................7 A 220, malleable iron grades of castings...........................................19 A 220, malleable iron product types..................................................18

A 220, mechanical properties .............................................................19 A 228, steel product types....................................................................6 A 242, high-strength, low-alloy steel product types............................8 A 247, comparison charts for nodular graphite particles............................................................100-102(F), 103(F) A 247, ductile iron graphite shapes...............................................19(F) A 247, graphite type, distribution and size in gray iron........16, 17(F) A 278, gray iron product types ..........................................................16 A 307, steel product types....................................................................6 A 335, low-alloy steel product types ...................................................7 A 336, low-alloy steel product types ...................................................7 A 338, malleable iron product types..................................................18 A 356, low-alloy steel product types ...................................................7 A 369, low-alloy steel product types ...................................................7 A 377, ductile iron product types.......................................................20 A 387, low-alloy steel product types ...................................................7 A 387 grade 22, applications..............................................................12 A 395, cast iron test block microstructure.......................................100 A 395, ductile iron product types.......................................................20 A 439, ductile iron product types.......................................................20 A 462, low-alloy steel product types ...................................................7 A 470, applications............................................................................8-9 A 470, composition...............................................................................9 A 470, microstructure ......................................................................9(F) A 476, ductile iron product types.......................................................20 A 510, steel product types....................................................................6 A 514, lath martensite in microstructure.......................................36(F) A 516, hydrogen damage ........................................................83, 84(F) A 529, steel product types....................................................................6 A 532, abrasion-resistant white cast irons .........................................17 A 536, ductile iron castings..............................................................102 A 536, ductile iron grades defined .....................................................20 A 536, ductile iron product types.......................................................20 A 536, mechanical properties .............................................................20 A 541, low-alloy steel product types ...................................................7 A 542, low-alloy steel product types ...................................................7 A 543, high-strength plate and forging applications ...........................7 A 570, steel product types....................................................................6 A 572, high-strength, low-alloy steel product types............................8 A 588, high-strength, low-alloy steel product types............................8 A 602, malleable iron product types..................................................18 A 656, high-strength, low-alloy product types ....................................8 A 709, steel product types....................................................................6 A 710, hot shortness ...........................................................................72 A 714, high-strength, low-alloy steel product types............................8 A 715, high-strength, low-alloy steel product types............................8 A 716, ductile iron product types.......................................................20 A 808, high-strength, low-alloy steel product types............................8 A 871, high-strength, low-alloy steel product types............................8 A 895, austempered ductile cast iron grades .....................................21 A 895, mechanical properties .............................................................21 American Society for Testing and Materials (ASTM) grain size number..................................................................................100 American Society for Testing and Materials (ASTM) standards, pertaining to ferrous metallography.....................295(T) American Society for Testing and Materials (ASTM) standards, specific types E 45, microcleanliness ranking for sulfides, silicates, aluminum oxides, and globular oxides ....................................150 E 112, three-circle method for measuring ferrite grain size...................................................................90, 99-100(F) E 562, measurement of volume fraction of microstructural constituents ...........................................................................90, 99 E 660, microhardness testing.......................................................167(F) E 1077, decarburization procedure ...............................................68-69 American Society of Mechanical Engineers (ASME), classification system for steels and cast irons .................................1 Ammonium persulfate (Rawdon) etchant composition .....................................................................300(T) as etchant for revealing macrostructures in iron and steel ........300(T) Amorphous, definition .........................................................................246 Amorphous alloy. See Metallic glass. Amphoteric, definition.........................................................................246 Amplifier, definition.............................................................................246

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Index / 323 Amyl nital etchants for coated steels, zinc-based coatings...........................304(T) for etching coated steels......................................................241, 242(F) Amyl picral/nital, etchants for coated steels, zinc-based coatings ....................................................................................304(T) Analyzer .............................................................................133(F), 134(F) definition............................................................................................246 Anderson etching reagent, composition, metals polished, temperature, and etching remarks ...........................................298(T) Angle of reflection. See also Normal. definition............................................................................................246 Angstrom..............................................................................................162 Angstrom unit, definition ....................................................................246 Angular aperture, definition...............................................................247 Anhydrous calcium sulfate ................................................................212 Anion, definition...................................................................................247 Anisotropic material ...........................................................................133 Anisotropy, definition ..........................................................................247 Annealing. See also Black annealing, Blue annealing, Box annealing, Bright annealing, Cycle annealing, Flame annealing, Graphitizing, Intermediate annealing, Isothermal annealing, Malleabilizing, Process annealing, Quench annealing, Recrystallization annealing, Spheroidizing, Stress relieving, and Subcritical annealing. ............................ 58(F), 60-61 definition............................................................................................247 dendrites shown in as-cast microstructure ....................................42(F) to salvage component with microcracks ............................................77 to salvage sensitized steel...................................................................79 Annealing border. See Oxidized surface (on steel). Annealing carbon. See Temper carbon. Annealing twin..................................................................................40(F) and chromium carbides with sensitization ....................................74(F) definition............................................................................................247 in iron-base superalloy...................................................................14(F) machining of stainless steels ...................................................78, 79(F) Annealing twin bands. See Twin bands. Anode ..............................................................................210(F), 219, 239 definition............................................................................................247 Anode aperture, definition ..................................................................247 Anodic dissolution...............................................................................210 Anodic etching, definition ...................................................................247 Anodic polishing. See Electrolytic polishing. Anodic protection, definition ..............................................................247 Antimony, function as element in steel and cast iron.......................3(T) Aperture. See also Angular aperture. effective, definition ...........................................................................247 electron. See Anode aperture, Condenser aperture, and Physical objective aperture. light, definition..................................................................................247 Aperture diaphragm ................126, 128-129(F), 130, 133, 137(F), 140 adjustment procedure ........................................................................146 adjustment procedure using a Bertrand lens....................................146 Aperture half-angle ........................................................................114(F) Aperture iris diaphragm........................................................128-129(F) Aperture stop ..........................................................................128-129(F) Aplanatic, definition.............................................................................247 Apochromatic objective. See also Achromatic................ 109, 120, 125 definition............................................................................................247 Apparent density, definition ...............................................................247 Aqua regia dilute, etchant composition..........................................................300(T) dilute, etchant for revealing macrostructures in iron and steel..300(T) as etchant for carbon steels and alloy steels ..............................303(T) for etching stainless steels.........................................................237-238 Arcm (cooling temperature) ...........................................................26, 28 definition............................................................................................247 Ar1 (cooling temperature) .............................................................26, 54 definition............................................................................................247 Ar3 (cooling temperature) .......................................................26, 27, 54 definition............................................................................................247 Ar4, definition .......................................................................................247 Arc cutting. See also Metal-arc cutting. definition............................................................................................247 Arrest, definition ..................................................................................247

Arrest lines (marks). See Beach marks. Arret .......................................................................................................26 Arsenic, function as element in steel and cast iron...........................3(T) Artifact. See also Mounting artifact and Polishing artifact....... 57, 100, 170(F), 175(F), 176, 178, 179(F) from copper contamination...............................................................188 definition .....................................................................................89, 247 produced by improper polishing procedures....................................202 Artificial aging, definition ...................................................................247 As-cast microstructure.....................................................................41(F) annealed condition .........................................................................42(F) Aspect ratio .................................................................................171, 172 definition............................................................................................247 Asperity, definition...............................................................................247 Astigmatism.........................................................................119, 120, 125 definition............................................................................................247 ASTM grain size number. See also Grain size. .............................. 100 Athermal transformation, definition .................................................247 Atomic number............................................................156-157, 159, 162 and backscattered electron image.....................................................156 definition ....................................................................................247-248 Atomic number contrast ............................................................157, 159 Atomic percent, definition...................................................................248 Atomization, definition ........................................................................248 Atom probe (AP) microanalysis........................................................168 Attack-etching ................................................215, 216, 217, 219-221(F) etchants used for stainless steels ..............................236, 237-239(F,T) etchants used for steels and cast irons..............................222-233(F,T) Attack-polishing, definition.................................................................248 Attritious wear, definition...................................................................248 A.U. See Angstrom unit. ÅU. See Angstrom unit. Auger electrons...............................................................................152(F) Auger electron spectroscopy (AES) ..................................................168 definition............................................................................................248 Ausforming, definition.........................................................................248 Austempered ductile cast iron ............................................16, 20-21(F) microstructure.................................................................................20(F) Austempering.........................................................................................20 definition............................................................................................248 Austenite. See also Retained austenite. ..................................... 39-41(F) in austenitic manganese steels.......................................................15(F) definition............................................................................................248 in duplex stainless steels................................................................14(F) etchants for stainless steels ............................................237(T), 303(T) FCC phase in iron-carbon alloy .........................................................27 graphite dissolution and void development ..................................77(F) in iron-base superalloy...................................................................14(F) in iron-cementite phase diagram .............................................25, 26(F) in isothermal transformation diagram for steel (1080).................29(F) in low-thermal-expansion steel......................................................15(F) in normalized plain carbon steel ........................................................51 primary ................................................................................................64 in stainless steel ........................................................................39-40(F) in white iron........................................................................43(F), 44(F) Austenite grains electrolytic etchants for stainless steels ......................................240(T) etchants for stainless steels..........................................................237(T) Austenite grain size...............................................................................90 Austenite phase field.............................................................................31 Austenitic grain size, definition..........................................................248 Austenitic manganese steels......................................................13, 15(F) applications..........................................................................................13 castings, ASTM grade A 128, composition ranges ....................314(T) compositions ..........................................................................13, 315(T) nonmagnetic and cryogenic applications ....................................315(T) properties ...............................................................................13, 315(T) Austenitic stainless steels ...........................................................10-11(F) chemical polishing, etching reagent............................................298(T) cold working .......................................................................................57 compositional variations, family relationships..............................10(F) compositions ..........................................10-11, 309(T), 310(T), 312(T) electrolytic etchants for stainless steels ......................................240(T) etchants for.............................................................237(T), 240, 303(T)

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324 / Metallographer’s Guide Austenitic stainless steels (Continued) microstructure ..........................................................................10, 11(F) nonstandard, compositions...........................................................310(T) polishing.....................................................................................207-208 Austenitic steel, definition ...................................................................248 Austenitizing ..........................................................................................20 definition............................................................................................248 Automated image analysis .....................................................149-151(F) Automatic grinding/polishing machines..................105, 106, 199, 210 backing of grinding paper .........................................................199-200 grinding discs ....................................................................................199 holders not requiring mounted samples...................................183, 187 mounting techniques ...........................................183, 185, 193, 195(F) Automatic polishing.....................................................................209-210 Automotive components, use of malleable iron.............................64-65 Automotive crankshafts .........................................................100-103(F) Autoradiography, definition ...............................................................248 Autotempering, definition ...................................................................248 Average grain diameter. See also Grain size. definition............................................................................................248 Aviation fuel ................................................................................222, 228 Axial, definition ....................................................................................248 Axial ratio, definition...........................................................................248 Axial rays .............................................................................................119 Axis crystal, definition...............................................................................248 optical, definition ..............................................................................248

B Bf. See also Bainite finish temperature. definition............................................................................................249 Bs. See also Bainite start temperature. definition............................................................................................250 Backing, definition ...............................................................................248 Backing film, definition .......................................................................248 Back reflection, definition ...................................................................248 Backscattered electron image (BEI) ............................................159(F) Backscattered electrons ....................................152(F), 155-157(F), 159 Backscattering process ..................................................................156(F) Baekeland, Leo ....................................................................................187 Bain, Edgar ...............................................................................24, 28, 31 Bainite. See also Granular bainite, Lower bainite, and Upper bainite. ........................................................................... 31, 38-39(F) in austempered ductile cast iron....................................................20(F) in carbon surface replica of steel ................................................155(F) in continuous cooling transformation diagram .......................31, 32(F) etchants for carbon steels, low-alloy steels, and cast irons .......222(T) in gray iron..........................................................................................44 intermediate, definition .....................................................................248 in low-alloy steel .................................................................164, 165(F) in low-alloy steels for high-temperature properties........................9(F) in low-carbon alloy steel microstructure.......................................38(F) lower, definition ................................................................................248 microstructure, etchants for .........................................................302(T) in steel bar microstructure ...........................................................221(F) in steel microstructure ........................................................23(F), 24(F) tint etchants for carbon steels, low-alloy steels, and cast irons......................................................................................234(T) transformation in commercial cast irons............................................47 upper, definition ................................................................................248 Bainite finish temperature (Bf) ...................................................31, 249 Bainite start temperature (Bs).....................................................31, 250 Bainitic steels ....................................................................................39(F) applications..........................................................................................39 Bake hardening .....................................................................................60 low-alloy steel.......................................................................................9 Bakelite....................................................................................187, 188(F) Bakelite mount ......................................................................................99 Baking, definition .................................................................................248 Banded structure, definition ...............................................................248 Banding. See also Ferrite-pearlite banding and Segregation banding................................................................. 42(F), 61, 90, 160

definition ....................................................................................248-249 elimination by homogenization treatment...............................52, 53(F) in free-machining steel bar, 4% picral and 2% nital etched ...........................................................................233, 234(F) in hot-rolled steel plate, planar view ..................................171, 172(F) and normalizing heat treatment ........................................49, 50(F), 51 Band saws..............................................104, 105, 106, 179, 180(F), 199 blades ...................................................................................179, 180(F) handheld portable..............................................................................179 Bar, definition.......................................................................................249 Bark, definition.....................................................................................249 Barrel distortion. See Negative distortion. Basal plane, definition .........................................................................249 Base, definition .....................................................................................249 Base metal, definition ..........................................................................249 Basic oxygen furnace (BOF)................................................................89 based steel ...........................................................................................89 Bauschinger effect, definition..............................................................249 BCC. See Body-centered cubic crystal structure. BCT. See Body-centered tetragonal crystal structure. BD. See Bright-field/Dark-field reflector. Beach marks, definition.......................................................................249 Bearing race, metallographic analysis........................................95-96(F) Bearing steels, compositions ..........................................................307(T) Beck’s microscope ............................................................................88(F) BEI. See Backscattered electron image. Beilby layer, definition ........................................................................249 Beilby smearing. See Beilby layer. Bellows length, definition ....................................................................249 Bell’s reagent (electrolytic) etchant composition .....................................................................300(T) as etchant for revealing macrostructures in iron and steel ........300(T) Belt grinder .................................................................................105, 106 Belt sander ............................................................................98, 104, 202 Bending...................................................................................................91 Benzene ........................................................................................222, 228 Beraha, E. ............................................................................................235 Beraha’s reagent...............................................216, 217(F), 235, 236(F) as etchant for carbon steels and alloy steels ..............................301(T) Bertrand lens..........................................................................137(F), 146 definition............................................................................................273 Bifilar eyepiece, definition...................................................................249 Billet, definition ....................................................................................249 Binary alloy, definition........................................................................249 Binary phase diagram ..........................................................................26 Binary system, definition.....................................................................249 Binder clips.............................................................................195, 196(F) Binocular attachment .........................123(F), 124(F), 125, 144, 145(F) Binocular microscope, personalization procedure .............................144 Birefringence, definition ......................................................................249 Bismuth, function as element in steel and cast iron .........................3(T) Bivariant equilibrium, definition .......................................................249 Black and white micrography...................................................120, 127 Black annealing. See also Box annealing. definition............................................................................................249 Blackheart malleable. See Malleable iron. Black oxide, definition .........................................................................249 Blank carburizing, definition..............................................................249 Blank nitriding, definition...................................................................249 Blast furnace..........................................................................................89 Bleeding........................................................................................187, 191 Blemish, definition ...............................................................................249 Blister, definition..................................................................................249 Blister steel, macrograph of .......................................................87, 88(F) Blocky, equiaxed ferrite ............................................................32, 33(F) Bloom, definition ..................................................................................249 Blowholes, definition............................................................................249 Blue annealing, definition ...................................................................249 Blue brittleness, definition ..................................................................249 Bluing, definition..................................................................................249 Body-centered, definition .............................................................249-250 Body-centered cubic (BCC) crystal structure .........................26-27(F) in ferrite...............................................................................................32 of iron................................................................................................154

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Index / 325 Body-centered tetragonal (BCT) crystal structure, in iron-carbon alloys ................................................................36, 37(F) BOF. See Basic oxygen furnace. Bolt, metallographic analysis of fracture.....................................98-99(F) Bond, definition ....................................................................................250 Bonded alumina wheel .........................................................................97 Boron, function as element in steel and cast iron .............................3(T) Boron carbide, as abrasive particles for wire saw......................182-183 Boundary grain, definition..................................................................250 Box annealing. See also Black annealing. definition............................................................................................250 Bragg angle ..........................................................................................163 definition............................................................................................250 Bragg equation. See also Order (in x-ray reflection). definition............................................................................................250 Bragg method, definition.....................................................................250 Bragg’s law ..........................................................................................162 Brazing, definition................................................................................250 Breaks, definition .................................................................................250 Bright annealing, definition ................................................................250 Bright-field/dark-field (BD) reflector ...........................................131(F) Bright-field illumination ..........109, 121, 131-137(F), 217, 243, 244(F) of carbonitrided steel microstructure ......................................69(F), 71 definition............................................................................................250 of hydrogen damage ................................................................82, 83(F) intercritically annealed bar ............................................................55(F) Bright-field objective .............................................................121(F), 131 Bright plate, definition ........................................................................250 Brillouin zones. See Electron bands. Brinell hardness testing......................................................................102 Brinelling. See also False Brinelling. definition............................................................................................250 Brittle, definition ..................................................................................250 Brittle crack propagation, definition .................................................250 Brittle fracture, definition...................................................................250 Brittleness, definition ...........................................................................250 Bromide-type photographic paper......................................................98 Buffer, definition ..................................................................................250 Bull’s eye ductile iron ....................................................................65, 66 Bull’s-eye malleable iron......................................................................64 Bull’s-eye structure................................................................102, 103(F) definition............................................................................................250 Burg and Weiss reactive etchant etchant composition .....................................................................300(T) as etchant for revealing macrostructures in iron and steel ........300(T) Burned steel ...........................................................................................72 Burning. See also Overheating................................. 69(F), 70(F), 71-72 definition............................................................................................250 Burnished surface ..........................................................................201(F) Burnishing, definition ..........................................................................250 Burn-off rate. See Melting rate. Burr, definition.....................................................................................250

C Calcite, definition .................................................................................250 Calcium function as element in steel and cast iron ......................................3(T) WDS x-ray map in nonmetallic inclusion ..................................159(F) Calcium aluminate......................................................................159, 160 Calcium fluoride (fluorite) lenses ......................................................120 Calcium-manganese sulfide .......................................................159, 160 Calcium sulfite desiccant....................................................................212 Caliper diameter (Feret’s diameter), definition ...............................250 Calorizing, definition ...........................................................................250 Camera attachment ............................................................................139 microscope as ............................................................................104-105 Carbide networks, etchants for carbon steels, low-alloy steels, and cast irons ...........................................................................222(T) Carbide precipitation............................................................................38 Carbides definition............................................................................................250 electrolytic etchants for stainless steels ......................................240(T)

etchants for stainless steels..........................................................237(T) etching response....................................................................215-217(F) not ferrite, etchants for carbon steels, low-alloy steels, and cast irons ..............................................................................222(T) not ferrite, etchants for coloring or darkening ...........................222(T) not ferrite, etchants for stainless steels .......................................237(T) and sensitization from improper machining..................................79(F) sensitized, etchants for stainless steels .......................................237(T) in tool steel microstructures ................................................134, 135(F) Carbide stabilizers ................................................................................47 of cast irons.........................................................................................46 Carbide tools, definition......................................................................250 Carbon, function as element in steel and cast iron...........................3(T) Carbon and low-alloy steels .......................................................2-9(F,T) alloy steels.......................................................................2, 3(T), 6-7(F) bake-hardenable, low-alloy steels.........................................................9 classification ..........................................................................................1 dual-phase, low-alloy steels.............................................................9(F) high-strength, low-alloy steels.....................................2-4, 7-8(F), 9(F) low-alloy steels for high-temperature properties.........................8-9(F) low-alloy steels for improved corrosion resistance .............................9 low-alloy steels with formability..........................................................9 plain carbon steels....................................................................2, 4-6(F) Carbon arc lamps ........................................................................126-127 Carbon-chromium forging steel grain-boundary segregation ..........................................70(F), 72, 73(F) manganese sulfide inclusions ..................................................70(F), 72 Carbon depletion...................................................................................67 Carbon diffusion........................................................................61, 67-68 Carbon edges, definition .....................................................................250 Carbon equivalent (CE) .......................................................................46 for rating weldability, definition.......................................................251 Carbon extraction replica......................................................161-162(F) Carbon-iron alloys, lath martensite and plate martensite with nital etch...................................................................................223(F) Carbonitride, definition.......................................................................251 Carbonitriding definition............................................................................................251 improperly done ................................................67(F), 68(F), 69(F), 71 Carbon potential, definition................................................................251 Carbon replica................................................................................155(F) Carbon restoration, definition ............................................................251 Carbon steels chemical polishing, etching reagent............................................298(T) composition ranges and limits.....................................................305(T) definition............................................................................................251 etchants for revealing microstructures.................................301-303(T) free-machining, composition ranges and limits..........................305(T) free-machining, UNS designations..............................................305(T) graphitization ...........................................................................76(F), 77 graphitization process reversal ......................................................77(F) high-manganese, composition ranges and limits ........................305(T) high-manganese, UNS designations............................................305(T) hot rolling and banding ......................................................................42 manual (hand) polishing ...................................................................204 microcrack formation .........................................................75(F), 76-77 plate martensite ............................................................................184(F) polishing.....................................................................................207-208 rephosphorized and resulfurized, composition ranges and limits.....................................................................................305(T) UNS designations ........................................................................305(T) Carburization ........................................................................................33 Carburized surface ...............................................................................71 Carburizing..................................................................................62-63(F) definition............................................................................................251 Carburizing bearing steels, compositions ....................................307(T) Carburizing flame, definition .............................................................251 Carpenter 18-18Plus .............................................................................12 Case, definition .....................................................................................251 Case hardening. See also Carbonitriding, Carburizing, Cyaniding, Nitriding, Nitrocarburizing, and Quench hardening. definition............................................................................................251 Cassette, definition ...............................................................................251

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326 / Metallographer’s Guide Casting, definition ................................................................................251 Castings, ASTM specifications for chromium-molybdenum steel product forms.......................................................................7, 308(T) Cast irons .....................................................................................16-21(F) applications in as-cast condition ..................................................25, 42 austempered ductile iron ....................................................16, 20-21(F) austenite in microstructure ............................................................39-40 carbon content range...........................................................................25 commercial................................................................................46-48(F) commercial, composition....................................................................43 compacted graphite iron ...............................................................16, 21 composition ranges ......................................................................315(T) decarburization of ...............................................................................67 definition .......................................................................................1, 251 ductile iron..........................................................................16, 19-20(F) gray iron....................................................................................16-17(F) hydrogen damage ................................................................................80 as hypoeutectic....................................................................................64 impurities.............................................................................................47 malleable iron .....................................................................16, 18-19(F) manual (hand) polishing ...................................................................204 matrix microstructure, metallographic analysis ...................100-103(F) metallographic analysis, grade S-230 shot ..............................96-97(F) mottled iron..............................................................................16, 18(F) nodular graphite, metallographic analysis ...........................100-103(F) phase transformation.................................................................42-44(F) 3% carbon, phase transformations ...........................................43-44(F) UNS designations..................................................................................2 white iron...........................................................................16, 17, 18(F) Cast iron shot, metallographic analysis......................................96-97(F) Cast steel definition............................................................................................251 hydrogen damage ................................................................................80 UNS designation ...................................................................................2 Cast structure. See also Solidification structure. definition............................................................................................251 Catalyst ........................................................................................190, 191 Cathode..............................................................210(F), 219, 239, 240(F) definition............................................................................................251 Cathode etching See Ion etching. Cathode ray tube (CRT).......................................................156, 158(F) Cathodic protection, definition...........................................................251 Cation, definition..................................................................................251 Caustic cracking, definition ................................................................251 Caustic embrittlement. See Caustic cracking. Cavitation, definition ...........................................................................251 CCD. See Charge-coupled device digital cameras. CCT. See Continuous cooling transformation diagram. C-curves ................................................................................29, 31, 32(F) CE. See Carbon equivalent. Cell ..........................................................................................................44 Cementation, definition .......................................................................251 Cemented carbide, definition..............................................................251 Cementite...............................................................................25, 33-34(F) from austenite transformation.............................................................27 definition ....................................................................................251-252 in hypereutectoid steel..................................................28(F), 33, 34(F) in iron-cementite phase diagram ........................................................25 in isothermal transformation diagram for 1080 steel ...................29(F) in low-carbon sheet steel ...............................................................33(F) in mottled cast iron.............................................................18(F), 45(F) in pearlitic steel............................................................................153(F) in plain carbon steels...................................................................4(F), 5 properties imparted by presence of ....................................................34 in steel microstructure (1080)........................................................23(F) in white cast iron ..........................18(F), 42-45(F), 65, 117(F), 118(F) Centerline segregation .........................................................75(F), 76-77 Centerline shrinkage, definition .........................................................252 Central bursting....................................................................94(F), 95(F) Central pencil, definition.....................................................................252 Centrifugal casting, definition ............................................................252 Ceramic coatings.................................................................................209 Cerium in compound in ductile iron ...............................................................66

function as element in steel and cast iron ......................................3(T) inoculation of liquid ductile iron........................................................45 Cermet, definition ................................................................................252 CG. See Compacted graphite. CG iron. See Compacted graphite cast iron. Chafing fatigue. See Fretting. Characteristic radiation, definition....................................................252 Characteristic x-rays ..........................................................................157 Charge-coupled device (CCD) digital cameras.......................120, 139 Charging, definition .............................................................................252 Charging effect ....................................................................................192 Charpy V-notch impact test, embrittlement indications shown ....................................................................................74-75(F) Chauffage ...............................................................................................26 Chemical analysis ......................................................................89-90, 91 Chemical attack...................................................................................215 Chemical deposition, definition ..........................................................252 Chemical etching...........................................................................34, 209 Chemical metallurgy. See Process metallurgy. Chemical polishing definition............................................................................................252 solutions .......................................................................................298(T) Chemical segregation..................................................................97-98(F) Chemical spills.....................................................................................107 Chemical thinning...............................................................................164 Chemist, job description ........................................................................23 Chevron pattern, definition ................................................................252 Chia etching reagent, composition, metals polished, temperature, and etching remarks ...........................................298(T) Chills.......................................................................................................47 Chill time. See Quench time. Chromatic aberration ...........................................109, 119(F), 120, 125 axial (longitudinal) .......................................................................119(F) definition............................................................................................252 lateral (transverse).............................................................................119 Chromic oxide, as etchant for carbon steels and alloy steels.......303(T) Chromite ..............................................................................................162 Chromium as carbide stabilizer ............................................................................46 function as element in steel and cast iron ......................................3(T) movement during dendritic solidification process ................51, 52, 53 Chromium carbide networks ................................................176-177(F) Chromium carbides ..............................................................................58 sensitization of stainless steels ......................................................74(F) Chromium high-speed steels, AISI code classification.......................12 Chromium-molybdenum steels heat-resistant, ASTM specifications for product forms..............308(T) heat-resistant, compositions and UNS designations ...................308(T) Chromium steel, temper embrittlement ...........................................75(F) Chromizing, definition.........................................................................252 Circular saw, small high-speed handheld...........................................183 Clad metal, definition ..........................................................................252 Clamshell marks. See Beach marks. Classification systems ..........................................................................1-2 Cleaning of microscope lenses.........................................................................142 rusted surfaces ...........................................................................173-174 of specimens......................................................................................203 Clear glass focusing screen, definition ..............................................252 Cleavage, definition..............................................................................252 Cleavage crack, definition...................................................................252 Cleavage fracture, definition ..............................................................252 Cleavage plane, definition ...................................................................252 Close annealing. See Box annealing. Close-packed, definition ......................................................................252 Coalescence, definition.........................................................................252 Coarse-focus knob ..................................................111-112(F), 143, 144 Coarse grains, definition .....................................................................252 Coated abrasive, definition .................................................................252 Coated steels aluminum-based coatings, etchants for .......................................304(T) etchants for...........................................................................241, 242(F) zinc-based coatings, etchants for.................................................304(T) Coatings. See also Electroless plating process.

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Index / 327 acrylic lacquer...................................................................................213 aluminized, metallographic analysis ........................................92-93(F) aluminum-based, etchants for steels ...........................................304(T) etchants for steels ................................................................241, 242(F) and polishing of steel........................................................................209 storage of...........................................................................................213 tin-based ............................................................................................209 zinc-based, etchants for steels .....................................................304(T) Coaxial knurled knobs ...................................................110-111(F), 112 Cobalt, function as element in steel and cast iron ............................3(T) Coefficient of elasticity. See Modulus of elasticity. Coefficient of thermal expansion, definition .....................................252 Coercive force, definition ....................................................................252 Coherency, definition...........................................................................252 Coherent precipitate, definition..........................................................252 Coherent scattering. See also Incoherent scattering. definition............................................................................................252 Cohesion, definition .............................................................................252 Cohesive strength, definition ..............................................................252 Coil breaks, definition ..................................................................252-253 Cold drawing, central bursting .............................................94(F), 95(F) Cold lap, definition ..............................................................................253 Cold mill, definition .............................................................................253 Cold mount..................................................................................186, 189 Cold rolled sheets, definition ..............................................................253 Cold rolling, definition ........................................................................253 Cold shearing ................................................................................91, 196 Cold shortness, definition....................................................................253 Cold treatment, definition...................................................................253 Cold work.......................................................................179, 242, 243(F) created by sectioning ........................................................................174 definition............................................................................................253 low-carbon steel..............................................................225(F), 226(F) and mechanical mounting.................................................................186 plastic deformation zones from grinding .........................................198 from shearing ....................................................................................196 Cold-worked ferrite, etchants for carbon steels, low-alloy steels, and cast irons................................................................222(T) Cold-worked structure, definition......................................................253 Cold working...............................................................................55-58(F) definition............................................................................................253 of steel.................................................................................................49 Collector lens .......................................................................................128 Collimated light source ......................................................................126 Collimation, definition .........................................................................253 Collodian replica, definition ...............................................................253 Colloid ..................................................................................................204 Colloidal silica. See also Silica. inhibitor added to..............................................................................204 Colonies, definition...............................................................................253 Color filter. See also Contrast filter and Filter. definition............................................................................................253 neutral, definition.......................................................................264-265 orthochromatic, definition .........................................................264-265 photometric, definition......................................................................265 Color fringes ........................................................................................119 Color micrography .....................................................................120, 128 Color temperature ......................................................................127, 128 Columbium. See also Niobium. function as element in steel and cast iron ......................................3(T) Columbium carbides.............................................................................58 Columbium carbonitride...........................................................57(F), 60 Columbium nitrides ..............................................................................58 Columnar structure, definition...........................................................253 Column, electron microscope, definition...........................................253 Coma ............................................................................................119, 120 definition............................................................................................253 Combined carbon, definition ..............................................................253 Comet tails ..............................................................................119, 205(F) on a polished surface, definition ......................................................253 Commercial low-alloy steel, bainite in microstructure.............38, 39(F) Compacted graphite (CG) cast iron.............................................16, 21 alloying elements ................................................................................21 applications..........................................................................................21 compositional range ............................................................................21

definition............................................................................................253 Comparison standard, definition .......................................................253 Compensating eyepiece. See also Apochromatic objective..... 124, 125 definition............................................................................................253 Complex silicate inclusions, definition...............................................253 Component, definition .........................................................................253 Composite compact, definition ...........................................................253 Composite material, definition ...........................................................253 Composite plate, definition .................................................................253 Composite structure, definition ..........................................................253 Compound lenses.................................................................................112 Compound microscope ..........................................................109, 110(F) Compressive-mounting materials, problems and solutions ...........................................................................189, 190(T) Compressive strength, definition........................................................253 Compton scattering, definition ...........................................................254 Computer-assisted image analysis system..........................................91 Concave lenses ................................................................................118(F) Condenser Abbe, definition.................................................................................254 dark-field, definition..........................................................................254 definition............................................................................................254 variable-focus, definition ..................................................................254 Condenser aperture, definition...........................................................254 Condenser lens ...............................................................126, 127(F), 130 definition............................................................................................254 Conditioning heat treatment, definition ............................................254 Congruent melting, definition.............................................................254 Congruent transformation, definition ...............................................254 Conjugate phases, definition...............................................................254 Conjugate planes, definition ...............................................................254 Constant-feed speed saw.............................................................181-182 Constant force saw..............................................................................181 Constituent, definition .........................................................................254 Constitution diagram, definition ........................................................254 Contact fatigue, definition...................................................................254 Contact plating, definition ..................................................................254 Contact potential, definition ...............................................................254 Contamination .....................................................................................170 Continuous casting, definition ............................................................254 Continuous cooling................................................................................39 bainite presence...................................................................................39 and bainite transformation.............................................................38-39 of cast irons.........................................................................................47 Continuous cooling transformation (CT or CCT) diagram ...............................................................................31, 32(F) definition............................................................................................254 Continuous phase, definition ..............................................................254 Continuous precipitation, definition ..................................................254 Continuous spectrum (x-rays), definition .........................................254 Continuous yielding ................................................................................9 definition............................................................................................254 Contraction ............................................................................................27 Contrast, photographic, definition.......................................................255 Contrast color filter, definition...........................................................254 Contrast enhancement (electron optics). See also Shadowing. definition............................................................................................254 Contrast filter, definition.....................................................................254 Contrast perception, definition...........................................................254 Controlled cooling, definition .............................................................255 Controlled etching, definition .............................................................255 Controlled rolling, definition ..............................................................255 Conversion coating, definition............................................................255 Convex lenses.....................................................................118(F), 119(F) Coolant ....................................................................174, 175-177(F), 178 for acrylic mounting material...........................................................189 for band saw......................................................................................179 definition............................................................................................255 for grinding .......................................................................................202 for hacksaw .......................................................................................180 for precision saw...............................................................................181 Cooling curve, definition .....................................................................255 Cooling rate altering microstructure ........................................................................49 of cast irons ...................................................................................46-47

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328 / Metallographer’s Guide Cooling rate (Continued) definition............................................................................................255 Cooling stresses, definition..................................................................255 Coordination number, definition .......................................................255 Copper as cause of hot shortness.........................................................72, 73(F) function as element in steel and cast iron ......................................3(T) as graphitizer.......................................................................................46 as isotropic material..........................................................................133 Copperhead, definition ........................................................................255 Core, definition.....................................................................................255 Coring, definition .................................................................................255 Corrosion. See also Corrosion fatigue, Crevice corrosion, Dezincification, Erosion, Exfoliation, Fretting corrosion, Galvanic corrosion, Graphitic corrosion, Impingement attack, Interdendritic corrosion, Intergranular corrosion, Internal oxidation, Oxidation, Pitting, Rust, Stray-current corrosion, Stress-corrosion cracking, and Sulfide stress cracking. attack-etching................................................................215, 219-221(F) definition............................................................................................255 effects ........................................................................................84-86(F) gray iron....................................................................................84-86(F) Corrosion embrittlement, definition ..................................................255 Corrosion fatigue, definition...............................................................255 Corrosion-resistant (stainless) steels ....................9-12(F), 13(F), 14(F) UNS designations..................................................................................2 Corrosive wear, definition...................................................................255 Corundum, definition ..........................................................................255 Counting reticles .................................................................................109 Coupon, definition................................................................................255 Covalent bond, definition....................................................................255 Covered electrode, definition ..............................................................255 Cover glass..............................................................................113(F), 122 Cover-glass-corrected objectives ...........................................122-123(F) Covering power, definition .................................................................255 Cover slip ................................................................................113(F), 122 Crack extension (⌬a). See also Crack length and Effective crack size. definition............................................................................................255 Crack length (depth)(a). See also Crack size. definition............................................................................................255 Crack plane orientation, definition....................................................255 Crack size (a). See also Crack length (depth). definition............................................................................................255 Crater, definition..................................................................................256 Crater crack, definition.......................................................................256 Creep definition............................................................................................256 formation of voids ...................................................................80, 82(F) Creep limit, definition .........................................................................256 Creep rate, definition...........................................................................256 Creep recovery, definition...................................................................256 Creep-rupture strength, definition.....................................................256 Creep strain, definition .......................................................................256 Creep strength, definition ...................................................................256 Creep stress, definition ........................................................................256 Crevice corrosion, definition...............................................................256 Critical cooling rate, definition ..........................................................256 Critical current density, definition ....................................................256 Critical curve, definition .....................................................................256 Critical illumination, definition..........................................................256 Critical point. See also Transformation temperature. definition............................................................................................256 Critical range, definition .....................................................................256 Critical shear stress, definition...........................................................256 Critical strain, definition.....................................................................256 Critical surface, definition ..................................................................256 Critical temperature, definition .........................................................256 Critical temperature ranges. See Transformation ranges. Crop, definition ....................................................................................256 Cross breaks. See Coil breaks. Cross direction. See Transverse direction.

Crossed polarization ...........................................................................134 Cross rolling, definition.......................................................................256 Crown glass..........................................................................................120 CRT. See Cathode ray tube. Crucible steel, definition .....................................................................256 Crystal, definition.................................................................................256 Crystal analysis, definition..................................................................256 Crystal-figure etching. See also Dislocation etching. definition............................................................................................256 Crystalline fracture, definition ...........................................................256 Crystallite, definition ...........................................................................256 Crystallization, definition....................................................................256 Crystal orientation. See Orientation. Crystal system, definition....................................................................256 CT. See Continuous cooling transformation. Cube texture, definition.......................................................................256 Cubic, definition ...................................................................................257 Cubic boron carbide indenters..........................................................166 Cubic boron nitride indenters ...........................................................166 Cubic plane, definition ........................................................................257 Cup fracture (cup-and-cone fracture), definition ............................257 Curie point temperature, definition...................................................257 Curing of epoxy ..........................................................................190, 191 Cursor image ..................................................................................151(F) Curvature of field .......................................................................119, 125 Cut edge, definition..............................................................................257 Cutoff wheel, definition .......................................................................257 Cutting..........................................................................................77-79(F) Cutting speed, definition .....................................................................257 Cyaniding, definition ...........................................................................257 Cycle (N), definition.............................................................................257 Cycle annealing, definition..................................................................257 Cyclic load, definition..........................................................................257 Cyclic loading, and microcrack formation ...........................75(F), 76-77 Cyclic stressing. See Cyclic load. Cylinders, mounting of........................................................................196

D DI, definition .........................................................................................257 Damping capacity definition............................................................................................257 from graphite flakes.......................................................................44-45 Dark-field illumination.................................109, 120, 121, 131-133(F), 217, 242, 243, 244(F) definition............................................................................................257 to enhance mounting dye effect .......................................................193 Dark-field objective ...............................................................121(F), 131 Dark-field stop........................................................................131, 132(F) Darkroom facilities .............................................................................105 Dead soft, definition.............................................................................257 Debye ring, definition ..........................................................................257 Debye-Scherrer method. See also Powder method. definition............................................................................................257 Decalescence, definition .......................................................................257 Decarburization.......................49, 66(F), 67-69(F), 90, 91, 171(F), 172 in cast iron test blocks......................................................................102 definition............................................................................................257 and polishing to maintain edge retention.........................................208 Decoration (of dislocations), definition..............................................257 Deep drawing, definition .....................................................................257 Deep etching, definition.......................................................................257 Defect classification ......................................................................................257 definition............................................................................................257 Define (x-rays), definition....................................................................257 Definition, definition ............................................................................257 Deformation definition............................................................................................257 hot........................................................................................................41 mechanical...........................................................................................41 Deformation bands...........................................................................57(F)

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Index / 329 definition............................................................................................257 machining of stainless steels ...................................................78, 79(F) Deformation curve. See Stress-strain diagram. Deformation gradient ...........................................................................57 Deformation layer.......................................................................202, 205 Deformation lines, definition ..............................................................257 Deformation twins, in stainless steel ....................................135, 136(F) Deformed layer, definition ..................................................................257 Degenerate structure, definition .................................................257-258 Degrees of freedom, definition............................................................258 de Jong etching reagent, composition, metals polished, temperature, and etching remarks ...........................................298(T) Delayed yield, definition......................................................................258 Delta ferrite. See also Ferrite................... 25-26, 236, 237, 240, 241(F) definition............................................................................................258 electrolytic etchants for stainless steels ......................................240(T) etchants for stainless steels ............................................237(T), 303(T) in stainless steel ...........................................................................141(F) Delta ferrite stringers.................................................................98-99(F) Delta iron, definition............................................................................258 Delta phase ..................................................................................98-99(F) Dendrite arms ...........................................................41(F), 51, 52, 53(F) Dendrites.......................................41(F), 42(F), 51, 52, 53(F), 62, 63(F) austenite, in cast irons .............................................................43, 44(F) definition............................................................................................258 manganese sulfide ...................................................69(F), 70(F), 71-72 primary arm....................................................................................41(F) secondary arm ................................................................................41(F) tertiary arm.....................................................................................41(F) in white iron...................................................................................47(F) Dendritic segregation, definition ........................................................258 Dendritic solidification process .................................................51-53(F) Deoxidation, definition.........................................................................258 Deoxidation products, definition ........................................................258 Deoxidizer, definition...........................................................................258 Deoxidizing, definition .........................................................................258 Depression depth, measurement procedure............................146-148(F) Depth measurement, in specimen ..........................................146-148(F) Depth of field (depth of focus) ..................................................109, 116 definition............................................................................................258 Depth of focus......................................................................................116 Depth of fusion, definition ..................................................................258 Descaling, definition.............................................................................258 Desiccants.............................................................................................212 Desiccator cabinets..............................................................................185 Desiccators..............................................................106, 170, 173, 211(F) desiccants ..........................................................................................212 types of..............................................................................................212 Designation systems.............................................................................1-2 Deviation (x-rays), definition ..............................................................258 Devitrification ......................................................................................127 Dezincification, definition....................................................................258 DI, definition.........................................................................................257 Diallyl phthalate...........................................................................187-188 electrically conductive, as mounting material .................................188 mounts ..............................................................................187, 188, 192 mounts, copper-filled ........................................................................188 Diameter of the field of view.....................................................124, 125 Diamond as abrasive particles for wire saw.......................................182, 183(F) abrasives ............................................................................................170 blade for gravity-feed saw..................................................................98 grinding discs ....................................................................................199 indenters ............................................................................................166 particles .............................................................................................205 particles, for grinding discs ..............................................................199 particles, for polishing abrasives......................................................204 particles, synthetic.............................................................................204 paste..................................................................................94, 95, 96, 98 polishing compound .................................................................158, 161 saw.....................................................................................................105 Diamond wheel, definition ..................................................................258 Diaphragm......................................................................124, 125, 126(F) definition............................................................................................258

field iris .................................................................................129-130(F) DIC. See Differential interference contrast. Die casting, definition ..........................................................................258 Dielectric fluid .....................................................................................181 Differential coating, definition ............................................................258 Differential heating, definition............................................................258 Differential interference contrast (DIC) illumination ..................................121, 133-134(F), 135(F), 136(F), 217, 239, 240(F), 242, 243(F) definition............................................................................................258 of hydrogen damage ................................................................82, 83(F) intercritically annealed bar ............................................................55(F) to locate hydrogen damage.................................................................82 to view stainless steel microstructure............................................57(F) Differential phase contrast ..........................133-134(F), 135(F), 136(F) Diffraction ............................................................................................154 definition............................................................................................258 Diffraction grating, definition.............................................................258 Diffraction pattern (x-rays), definition..............................................258 Diffraction ring. See also Debye ring. definition............................................................................................258 Diffusion ...........................................................................................34, 62 of carbon in ferrite to remove hydrogen ......................................67-68 definition............................................................................................258 to redistribute manganese ...................................................................42 Diffusion aid, definition .......................................................................258 Diffusion coating, definition................................................................258 Diffusion coefficient, definition ...........................................................258 Diffusion zone, definition.....................................................................258 Digital cameras.............................................................104-106, 139-140 with television monitor.....................................................................105 Digitizing pad ..........................................................................150-151(F) Dilatometer ............................................................................................27 definition ....................................................................................258-259 Dilatometry, definition.........................................................................259 Dimple rupture, definition ..................................................................259 Diopter.....................................................................................123, 124(F) Diopter scale .....................................................123, 124(F), 144, 145(F) Directional property, definition..........................................................259 Direct quenching, definition ...............................................................259 Discaloy ..................................................................................................12 Discontinuity, definition ......................................................................259 Discontinuous precipitation, definition..............................................259 Discontinuous yielding ....................................................................54-55 definition............................................................................................259 Dislocation.............................................................55-56, 57, 152, 153(F) definition............................................................................................259 Dislocation etching, definition ............................................................259 Disordered structure, definition .........................................................259 Dispersoid, definition ...........................................................................259 Dissociation, definition.........................................................................259 Dissociation pressure, definition.........................................................259 Dissolution etching, definition ............................................................259 Distortion..............................................................................................120 of image in metallurgical microscope..................................119-120(F) Disturbed metal......................................................................206(F), 207 definition............................................................................................259 Divariant equilibrium. See Bivariant equilibrium. Diverging lens. See Lens, negative. Divorced eutectic, definition ...............................................................259 DNA analysis........................................................................................170 Domain, definition................................................................................259 Double aging, definition ......................................................................259 Double etching, definition ...................................................................259 Doublet, in characteristic x-ray spectra, definition .............................259 Double tempering, definition ..............................................................259 Drawability, definition.........................................................................259 Drawing. See also Temper. definition............................................................................................259 Drawing quality, special killed steels.....................................................9 Drawing tube ..................................................................................151(F) Dressing, definition ..............................................................................259 Drift, definition.....................................................................................259 Drill press.............................................................................................105

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330 / Metallographer’s Guide Driving force, for decarburization.........................................................68 Drop, definition ....................................................................................259 Drop forging, definition.......................................................................259 Dross, definition ............................................................................259-260 Dry cyaniding. See Carbonitriding. Dry etching, definition.........................................................................260 Dry objective, definition......................................................................260 d-spacings........................................................................154, 162, 163(F) Dual-phase steel................................................................................40(F) AISI designations..................................................................................8 austenite in microstructure.............................................................40(F) definition............................................................................................260 deformation by shearing......................................................178, 179(F) epitaxial ferrite .............................................................................235(F) low-alloy steels .....................................................................................9 microstructure.............................................................8, 9(F), 54, 55(F) production by intercritical annealing..................................................54 retained austenite transformed into martensite by worn grinding paper ......................................................................201(F) tensile strength designation ..................................................................8 Ductile-brittle transition temperature. See Transition temperature. Ductile cast iron (or ductile iron).......16, 19-20(F), 65-66, 100-103(F) applications ...................................................................................46, 66 composition limits and ranges ..............................................20, 315(T) definition............................................................................................260 as form of cast iron ..........................................................................251 graphite nodules ..................................................................................42 grinding .............................................................................................202 microstructure.....................................................20(F), 45-46(F), 65(F) phase transformations ....................................................................42-43 production..........................................................................................100 Ductile crack propagation, definition ................................................260 Ductile fracture, definition..................................................................260 Ductility, definition ..............................................................................260 Duo-reduced tinplate ............................................................................56 Duplex coating. See Composite plate. Duplex grain size, definition ...............................................................260 Duplex microstructure, definition ......................................................260 Duplex stainless steels ...............................................................12, 14(F) compositions ................................................................................309(T) electrolytic etchants for stainless steels ......................................240(T) electrolytic etching.......................................................................241(F) etchants for...................................................................................303(T) microstructure ..........................................................................12, 14(F) microstructure etched with Lichtenegger/Bloech reagent ..........237(F) nonstandard, compositions...........................................................311(T) Dust cap................................................................................................170 Dust cover, for microscope .................................................................142 Dyes, mounting .....................................................................................193

E ECM. See Electrochemical machining. Edge dislocation. See Dislocation. Edge protection, electroless and electrolytic coatings for ............299(T) Edge retention ....................................................192, 193, 203, 208, 209 of acrylic mounting material ............................................................189 of castable acrylics............................................................................192 clamp use for mounting....................................................................186 diallyl phthalate mount vs. phenolic mount.....................................187 diamond grinding disks in tool steels ..............................................199 epoxy mounted plated sheet specimen ...............................194, 195(F) high-wearability epoxy mounts for ......................................192-193(F) mounting techniques with steel rod ....................................193, 195(F) polishing cloths used to improve .....................................................203 polishing disc use..............................................................................203 polishing for maintenance ....................................................208-209(F) polishing for maintenance (fragile edge) .........................................209 steel sheet placed next to epoxy mount specimen .............193, 195(F) of thermosetting epoxy mount ............................................188(F), 189 vibratory polishing ............................................................................209 Edge strain, definition .........................................................................260

Edge-trailing technique, definition.....................................................260 EDM. See Electric discharge machining. EDS. See Energy-dispersive spectroscopy. EDTA. See Ethylenediamine tetraacetate. EELS. See Electron energy loss spectrometry. Effective crack size (ae), definition.....................................................260 Elastic constants, definition ................................................................260 Elastic deformation, definition ...........................................................260 Elastic electron scatter, definition......................................................260 Elastic hysteresis, definition................................................................260 Elastic limit, definition ........................................................................260 Elastic modulus. See Modulus of elasticity. Elastic ratio, definition ........................................................................260 Elastic recovery, definition .................................................................260 Elastic strain. See also Elastic deformation. definition............................................................................................260 Elastic strain energy. See Strain energy. Elbogen iron meteorite ....................................................................88(F) Electrical-resistance steels....................................................................16 Electrical steels............................................................................15-16(F) applications..........................................................................................15 composition....................................................................................15-16 microstructure.................................................................................16(F) Electric discharge machining (EDM)..................................181, 182(F) definition............................................................................................260 Electric furnace steelmaking process .................................................89 Electric scriber ....................................................................................104 Electrochemical corrosion....................................................................84 definition............................................................................................260 Electrochemical equivalent, definition...............................................260 Electrochemical etching, definition ....................................................260 Electrochemical machining (ECM), definition .................................260 Electrochemical potential ...................................................................219 Electrochemical series. See Electromotive series. Electrode, definition.............................................................................260 Electrode deposition, definition ..........................................................260 Electrodeposition, definition ...............................................................261 Electroforming, definition ...................................................................261 Electrogalvanized steels, etching of......................................241, 242(F) Electrogalvanizing, definition .............................................................261 Electroless nickel plating and edge retention.............................................................................209 rate................................................................................................299(T) solution compositions ..................................................................299(T) temperature range ........................................................................299(T) Electroless plating .....................................184(F), 185, 193-194, 195(F) definition............................................................................................261 Electrolysis, definition .........................................................................261 Electrolyte ...............................................................................210(F), 239 definition............................................................................................261 Electrolytic cell ....................................................................................239 definition............................................................................................261 Electrolytic cleaning, definition ..........................................................261 Electrolytic copper plating agitation........................................................................................299(T) anode bag requirement ................................................................299(T) anode type ....................................................................................299(T) current density..............................................................................299(T) rate................................................................................................299(T) solution compositions ..................................................................299(T) temperature range ........................................................................299(T) voltage range................................................................................299(T) Electrolytic deposition. See Electrodeposition. Electrolytic etching.................................................. 188, 210(F), 220(F) See also Anodic etching. stainless steel bolt sample ..................................................................98 of stainless steels.......................................................236, 239-241(F,T) Electrolytic extraction. See also Extraction. definition............................................................................................261 Electrolytic grinding, definition .........................................................261 Electrolytic iron plating agitation........................................................................................299(T) anode type ....................................................................................299(T) current density..............................................................................299(T) solution composition....................................................................299(T)

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Index / 331 temperature range ........................................................................299(T) Electrolytic machining, definition ......................................................261 Electrolytic nickel plating agitation........................................................................................299(T) anode bag requirement ................................................................299(T) anode type ....................................................................................299(T) current density..............................................................................299(T) rate................................................................................................299(T) solution compositions ..................................................................299(T) temperature range ........................................................................299(T) voltage range................................................................................299(T) Electrolytic pickling, definition ..........................................................261 Electrolytic polishing.....................................................188, 209, 210(F) definition............................................................................................261 Electrolytic polishing/etching ....................................................105, 106 Electrolytic protection. See Cathodic protection. Electrolytic thinning ...........................................................................154 Electromagnetic lens. See also Focusing device. definition............................................................................................261 Electromechanical polishing, definition.............................................261 Electromotive force, definition ...........................................................261 Electromotive series, definition ..........................................................261 Electron, definition...............................................................................261 Electron bands, definition ...................................................................261 Electron beam, definition ....................................................................261 Electron beam cutting, definition.......................................................261 Electron diffraction, definition ...........................................................261 Electron diffraction pattern, of body-centered cubic iron ..........154(F) Electron energy loss spectrometry (EELS)......................................162 definition............................................................................................261 Electron gun, definition.......................................................................261 Electron image. See also Image. definition............................................................................................261 Electron lens, definition.......................................................................261 Electron micrograph, definition .........................................................261 Electron microscope. See also Scanning electron microscope and Transmission electron microscope................... 109, 152-162(F) definition............................................................................................261 distinguishing of bainite from tempered martensite ..........................38 interpreting of friction effects.............................................................80 magnification range.............................................................................89 radiation source and lenses used ........................................................88 Electron microscope column, definition ............................................261 Electron microscopy, definition..........................................................261 Electron optical axis, definition..........................................................261 Electron optical system, definition.....................................................261 Electron optics, definition ...................................................................261 Electron probe, definition ...................................................................261 Electron probe microanalysis, to study internally oxidized microstructure ...........................................................................70, 71 Electron probe microanalyzer (EPMA) ...................92, 137, 152, 154, 158-162(F) carbon contamination problem .........................................................192 for compositional analysis ................................................................170 definition ....................................................................................261-262 electrically conductive mounts .........................................................188 in full-scale metallographic laboratory ............................................105 to identify copper at scale/steel interface ...............................72, 73(F) mounting of specimens.....................................................................187 operated by metallographer at research laboratory............................92 as-polished specimens used ..............................................................219 Electron trajectory, definition ............................................................262 Electron velocity, definition ................................................................262 Electron wavelength, definition ..........................................................262 Electroplating. See also Plating. definition............................................................................................262 Electropolishing. See also Electrolytic polishing. ............................ 239 definition............................................................................................262 Electrostatic immersion lens. See Immersion objective. Electrostatic lens, definition................................................................262 Electrotinning, definition.....................................................................262 Elevated temperatures, effect on microstructure.....69(F), 70(F), 71-72 Elongated grain, definition .................................................................262 Elongation, definition...........................................................................262

Embedded abrasive, definition ...........................................................262 Embrittlement, definition....................................................................262 Emery, definition..................................................................................262 emf. See Electromotive force. Emission microscope, definition .........................................................262 Empty magnification...........................................................................115 Emulsion, definition .............................................................................262 Enamel coating.......................................................................243, 244(F) Enameling, definition...........................................................................262 Enantiotropy, definition ......................................................................262 Endothermic gas atmosphere, heat treating to prevent decarburization ................................................................................67 Endothermic reaction.................................................................220, 236 End-quench hardenability test, definition.........................................262 Energy dispersive spectroscopy (EDS) .................157-158(F), 159-162 definition............................................................................................262 Engineering Properties of Steel...........................................................89 Envelopes ................................................................................225, 226(F) Epitaxial ferrite ..................................................55, 225, 226(F), 235(F) in carbonitrided steel microstructure.......................................69(F), 71 etchants for carbon steels, low-alloy steels, and cast irons .......222(T) Epitaxy, definition................................................................................262 EPMA. See Electron probe microanalyzer. Epoxy castable.................................................................................196, 197(F) castable, cause and solution to mounting problems...........193, 194(T) castable, mounting material..............................................................209 Epoxy mounting material......................................92, 94-98(F), 184(F), 185(F), 188-191(F), 193 cylinder and tube specimens ............................................................196 non-charging, for electron microscopy ............................................192 sheet specimens ....................................................................194-196(F) Epsilon, definition ................................................................................262 Epsilon carbide, definition ..................................................................262 Epsilon structure, definition ...............................................................262 Equiaxed grain structure ..........................................................32, 33(F) definition............................................................................................262 Equilibrium............................................................................................24 definition............................................................................................262 Equilibrium diagram, definition ........................................................262 Equipment, used and/or reconditioned ...............................................297 Erosion, definition ................................................................................262 Erosion-corrosion. See Erosion. Etchants...................................................................................150(F), 151 and acrylic mounting material..........................................................189 for carbon and low-alloy steels and cast irons.................221-236(F,T) for carbon steels and alloy steels.........................................301-303(T) for coated steels, aluminum-based coatings ...............................304(T) for coated steels, zinc-based coatings .........................................304(T) consistency in mixing solutions .......................................................220 definition............................................................................................262 residual effect on mounted materials ........................................186-187 for revealing macrostructures in iron and steel ..........................300(T) for stainless steels..............................................................236-241(F,T) and thermosetting epoxy mounts......................................................189 Etch-attack polishing, definition ........................................................262 Etch cracks, definition.........................................................................262 Etch figures, definition ........................................................................262 Etching ...........................................................................................88, 106 acid damage to microscope lenses ...................................................142 of aluminized coating sample.............................................................93 of bearing race sample........................................................................96 with bright-field illumination............................................................131 definition............................................................................................262 equipment for metallography laboratory..........................................104 of forged steel sample ........................................................................97 for image analysis of microstructure constituents ...........................151 passive layers inhibiting ...................................................................170 of railroad rail sample ..............................................................97-98(F) removal of damage between polishing steps ...................................208 response, need for .............................................................................219 stainless steel bolt sample ..................................................................98 of steel sheet sample .............................................................99, 100(F) of steel wire sample............................................................................94

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332 / Metallographer’s Guide Etching (Continued) time duration of response .................................................................221 weld failure in steel plate sample ......................................................95 Etching response .....................................................................215-217(F) Etch-polish-etch technique.................................................................209 Ethyl alcohol ........................................................................................107 denatured ...........................................................................................107 Ethylenediamine tetraacetate (EDTA).................................211, 212(F) to dissolve inclusions on the surface ........................................173-174 Ethylenediamine tetraacetate acid (EDTA)21 added to aqueous picric acid to prevent staining ............................230 Ethyl methyl ketone............................................................................189 Eutectic, definition ...............................................................................262 Eutectic arrest, definition ............................................................262-263 Eutectic carbides. See also Hypereutectic alloy.................. 230, 231(F) definition............................................................................................263 Eutectic-cell etching, definition ..........................................................263 Eutectic colony, grain, definition........................................................263 Eutectic composition, alloying elements effect....................................46 Eutectic constituents..................................................................47(F), 48 Eutectic manganese sulfide colonies .................................................. 72 Eutectic melting, definition .................................................................263 Eutectic point .............................................................................25(F), 26 definition............................................................................................263 Eutectic temperature ......................................................................43, 47 alloying elements effect ......................................................................46 Eutectoid, definition.............................................................................263 Eutectoid carbide, definition...............................................................263 Eutectoid point ...........................................................................26(F), 28 definition............................................................................................263 Eutectoid reaction ....................................................................27, 28, 34 Eutectoid steel........................................................................................27 microstructure .....................................................................34(F), 35(F) Eutectoid transformation .....................................................................28 Evaporation, definition........................................................................263 Ewald sphere, definition......................................................................263 Excitation potential, definition ...........................................................263 Exfoliation, definition ..........................................................................263 Exhaust hood. See Fume hood. Exogenous inclusions, definition.........................................................263 Exothermal reaction...........................................................220, 231, 236 Extinction. See also Primary extinction and Secondary extinction. definition............................................................................................263 Extinction coefficient, definition .........................................................263 Extraction, definition ...........................................................................263 Extraction metallurgy. See also Process metallurgy. definition............................................................................................263 Extraction replica.............................................155, 156(F), 161(F), 162 definition............................................................................................263 Extra hard, definition..........................................................................263 Extra spring, definition .......................................................................263 Eye clearance, definition .....................................................................263 Eye lens ........................................................................................124, 125 definition............................................................................................263 Eyepiece......................................................................112(F), 113(F), 138 correcting curvature of field .............................................................119 definition............................................................................................263 negative, definition............................................................................278 parfocal, definition ............................................................................263 photo ..........................................................................................125-126 pinhole, definition .............................................................................281 positive, definition ....................................................................263, 282 types of..................................................................................123-126(F) Eyepiece micrometer. See Ocular micrometer. Eyepiece scale division, formula for...................................................142 Eye rinse facility..................................................................................107

F Faber, Giovanni...................................................................................109 Face angle, definition...........................................................................263 Face-centered, definition .....................................................................263

Face-centered cubic crystal structure .....................................26, 27(F) of austenite ..........................................................................................39 Face (crystal), definition......................................................................263 Fading, during pouring of the heat .....................................................102 Failure analysis....................................................................................149 False Brinelling, definition..................................................................263 False information ..................................................................................89 Family of crystal planes, definition ...................................................263 Fatigue. See also Fatigue failure, High-cycle fatigue, Low-cycle fatigue, and Ultimate strength. definition............................................................................................263 Fatigue failure, definition....................................................................263 Fatigue life, definition..........................................................................263 Fatigue limit, definition................................................................263-264 Fatigue ratio, definition.......................................................................264 Fatigue strength, definition.................................................................264 Fatigue striations. See also Wallner lines. definition............................................................................................264 Fatigue wear, definition.......................................................................264 Fayalite .................................................................................................162 Feret’s diameter. See Caliper diameter. Ferric oxide, as polishing abrasive .....................................................204 Ferrimagnetic material, definition .....................................................264 Ferrite.....................................................................................32(F), 33(F) acicular ........................................................................32, 33(F), 38, 39 alpha..............................................................................................25, 27 in annealed microstructure.............................................................58(F) in as-cast microstructure ................................................................41(F) from austenite transformation.............................................................27 banding after microsegregation .....................................................42(F) BCC phase in iron-carbon alloy.........................................................27 blocky, equiaxed ......................................................................32, 33(F) in carbonitrided steel microstructure ...........................68(F), 69(F), 71 in carbon steel ..............................................................................146(F) in carburized microstructure....................................................62, 63(F) in decarburized steel microstructure .......................................66(F), 67 in decarburized steel plate...................................................171(F), 172 definition............................................................................................264 digitally displayed on video monitor .............................149(F), 150(F) in dual-phase steel............................................................................9(F) in ductile cast iron .........................................................................20(F) in duplex stainless steels................................................................14(F) electrolytic etchants for stainless steels ......................................240(T) enhanced by aqua regia ....................................................................238 epitaxial ............................................................55, 225, 226(F), 235(F) in equiaxed microstructure ......................................................32, 33(F) etchants used for stainless steels.................................................303(T) in ferritic stainless steels................................................................12(F) in gray cast iron..........................................................16(F), 44, 115(F) in hypoeutectoid steel .........................................................28(F), 35(F) in iron-cementite phase diagram .............................................25, 26(F) in isothermal transformation diagram for steel (1080).................29(F) in low-carbon steel ..............................................................172, 173(F) in malleable iron ............................................................................18(F) in microalloyed steels ......................................................................8(F) in motor lamination (electrical) steel ............................................16(F) in normalized plain carbon steel ...................................................51(F) in plain carbon steels .................................4(F), 5(F), 49, 50(F), 55(F) in plasma-arc-cut steel surface.................................................77-78(F) polygonal.............................................................................................32 proeutectoid..............................................................................32, 33(F) in spheroidized microstructure ................................................61(F), 62 in steel microstructure (1080) ............................................23(F), 24(F) in structural steel microstructure .....................................................6(F) transformation in commercial cast irons............................................47 in very-low-carbon steel microstructure........................................32(F) Widmanstäbtten........................................................................32, 33(F) Ferrite, pearlite, bainite and martensite microstructure, etchants for...............................................................................302(T) Ferrite banding, definition..................................................................264 Ferrite (but not carbides), tint etchant for carbon steels, low-alloy steels, and cast irons ...............................................234(T) Ferrite grain boundaries etchants for...................................................................................301(T)

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Index / 333 in low-carbon steel ..............................................................243, 244(F) Ferrite grains electrolytic etchants for stainless steels ......................................240(T) etchants for carbon steels, low-alloy steels, and cast irons .......222(T) etchants for stainless steels..........................................................237(T) etching response....................................................................215-217(F) tint etchants for carbon steels, low-alloy steels, and cast irons ..............................................................................234(T) Ferrite grains and carbides, etchants for carbon steels, low-alloy steels, and cast irons ...............................................222(T) Ferrite grain size definition............................................................................................264 formula used for calculation.............................................................100 measurement in steel sheet.....................................................99-100(F) Ferrite number, definition ..................................................................264 Ferrite-pearlite banding, definition ...................................................264 Ferrite-pearlite microstructure, etchants for ...............................302(T) Ferrite streaks. See Ferrite banding. Ferrite tail, definition ..........................................................................264 Ferritic cast irons, polishing .......................................................207-208 Ferritic grain size, definition ..............................................................264 Ferritic malleable. See Malleable cast iron. Ferritic stainless steels..........................................................11(F), 12(F) composition .........................................................11(F), 309(T), 312(T) compositional variations, family relationships..............................11(F) electrolytic etchants for stainless steels ......................................240(T) etchants for .....................................................................237(T), 303(T) free-machining .........................................................................11, 12(F) nonstandard, compositions .............................................310(T), 311(T) polishing.....................................................................................207-208 Ferritizing anneal, definition ..............................................................264 Ferroalloy, definition ...........................................................................264 Ferromagnetic material, definition ....................................................264 Ferrum .............................................................................................27, 32 FG. See Flake graphite. Fiber-based photographic paper .........................................................98 Fiber texture, definition ......................................................................264 Fibrous fracture, definition.................................................................264 Fibrous structure, definition...............................................................264 Field condenser....................................................................................128 Field diaphragms.................109, 126, 127(F), 129-130(F), 140, 141(F) adjustment procedure ........................................................................146 Field ion microscopy (FIM) ...............................................................168 Field iris diaphragm...............................................................129-130(F) Field lens ........................................................124, 125, 126, 127(F), 130 Field metallography ..............................................................................80 definition............................................................................................264 Field number ...............................................................................124, 125 Field stop..................................................................................129-130(F) 50/50/50, as etchant for carbon steels and alloy steels..................303(T) Filament, definition..............................................................................264 Filamenting shrinkage, definition ......................................................264 Filar eyepiece .......................................................................................138 definition............................................................................................264 Filar measuring eyepieces ..................................................................125 calibration of..............................................................................142-143 Filar micrometer .................................................................................138 definition............................................................................................264 Filler metal, definition .........................................................................264 Filters...............................114, 115(F), 126(F), 130, 133, 136(F), 137(F) color, definition .................................................................................264 contrast color, definition ...................................................................254 definition ....................................................................................264-265 neutral color, definition .............................................................264-265 orthochromatic, definition.................................................................279 orthochromatic color, definition ................................................264-265 photometric color, definition ............................................................265 types of ......................................................................................264-265 FIM. See Field ion microscopy. Final polishing, definition ...................................................................265 Fine-focus knob ......................111-112(F), 117, 141, 143, 144, 146-147 Fingerprint...........................................................................................170 Fire alarm station ...............................................................................107 Fire crack, definition ...........................................................................265

Fire extinguishers................................................................................107 First aid procedures............................................................................107 Fisheyes .............................................................................................82-83 definition............................................................................................265 Fishmouthing. See Alligatoring. Fixer........................................................................................................98 Flake graphite (FG). See also Graphite flakes. definition ...................................................................................265, 267 identification in gray iron ...................................................................16 Flakes, definition ..................................................................................265 Flame annealing, definition.................................................................265 Flame hardening, definition................................................................265 Flash, definition....................................................................................265 Flash welding, definition .....................................................................265 Flat-field objectives .............................................................120-121, 125 Flatness of field, definition..................................................................265 Flint glass .............................................................................................120 Flow, definition.....................................................................................265 Flow brightening, definition ...............................................................265 Flow lines definition............................................................................................265 of steel forging sample ..................................................................97(F) Flow stress, definition ..........................................................................265 Fluorescence, definition .......................................................................265 Fluorescent magnetic-particle inspection, definition .......................265 Fluorescent screen, definition .............................................................265 Fluorescent x-rays (fluorescent analysis), definition........................265 Fluorite .................................................................................................120 definition............................................................................................265 Fluorite objectives...............................................................................125 definition............................................................................................279 Fly’s reagent ...................................................................................237(T) Focal length ....................................................................109, 119(F), 120 definition............................................................................................265 Focal point ...........................................................................................119 Focal spot, definition ...........................................................................265 Focus, definition ...................................................................................265 Focusing camera (x-rays), definition .................................................265 Focusing device (electrons), definition...............................................265 Focusing magnifier, definition ............................................................265 Focusing procedure, inverted microscope..........................................143 Focusing (x-rays), definition ...............................................................265 Fog quenching, definition....................................................................265 Foil, definition.......................................................................................265 Forensic investigator...........................................................................170 Forgeability, definition ........................................................................265 Forged structure, definition ................................................................265 Forging ...................................................................................................58 definition............................................................................................265 metallographic analysis..................................................................97(F) Forging range, definition.....................................................................265 Forgings, ASTM specifications for chromium-molybdenum steel product forms..............................................................7, 308(T) Form, definition ............................................................................265-266 Formability. See also Drawability. definition............................................................................................266 Forming, definition ..............................................................................266 Formvar................................................................................................189 4% picral. See also Picral. as etchant for carbon steels and alloy steels....301(T), 302(T), 303(T) 4% picral followed by 2% nital, as etchant for carbon steels and alloy steels.....................................233(F), 302(T) 4% picral + hydrochloric acid, as etchant for carbon steels and alloy steels.....................................302(T), 303(T) Fractography, definition......................................................................266 Fracture. See also Fibrous fracture, Granular fracture, Intergranular fracture, Silky fracture, and Transgranular fracture. definition............................................................................................266 Fracture grain size, definition ............................................................266 Fracture mechanics. See Linear elastic fracture mechanics. Fracture stress, definition ...................................................................266 Fracture test, definition.......................................................................266 Fracture toughness, definition ............................................................266

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334 / Metallographer’s Guide Fracturing in liquid nitrogen ............................................................183 Fragmentation, definition....................................................................266 Freckling, definition.............................................................................266 Free carbon, definition ........................................................................266 Free ferrite. See also Proeutectoid ferrite. definition............................................................................................266 Free machining, definition ..................................................................266 Free-machining steels .............................................................................5 banding and manganese sulfide inclusions .........................233, 234(F) composition ranges and limits.....................................................305(T) microstructure ..........................................................................11, 12(F) UNS designations ........................................................................305(T) Freezing point. See Melting point. Freezing range, definition ...................................................................266 Frequency (x-ray), definition ..............................................................266 Fresnel fringes, definition ...................................................................266 Fretting, definition ...............................................................................266 Fretting corrosion, definition..............................................................266 Fretting fatigue, definition ..................................................................266 Friction effects, microstructure altered by.......................................79-80 Friction oxidation. See Fretting. Fry’s reagent as etchant for carbon steels and alloy steels ..............................303(T) for etching stainless steels....................................................238-239(F) Full annealing, definition ....................................................................266 Full hard, definition.............................................................................266 Full-hard steels, for strapping...............................................................56 Fume hood ....................................................................92, 104, 105, 106 for handling and mixing epoxy materials ........................................190 for mixing aqua regia .......................................................................237 for mixing etchants ...........................................................................220 for mixing glyceregia........................................................................238 for mounting compound vapors .......................................................193 Fuming nitric acid..........................................................................242(F) Furnaces blast .....................................................................................................89 BOF-based...........................................................................................89 Fusion, definition..................................................................................266 Fusion zone, definition.........................................................................266

G Galvalume coating ............................................................151(F), 242(F) etching by amyl nital...........................................................241, 242(F) Galvanic action, in cast iron .................................................................85 Galvanic cell, definition.......................................................................266 Galvanic corrosion ..............................................................................209 definition............................................................................................267 dissimilar materials in mounting process.........................................186 Galvanic series, definition ...................................................................267 Galvanize, definition ............................................................................267 Galvanneal, definition..........................................................................267 Gamma alumina..................................................................................204 Gamma iron ....................................................................................26, 27 definition............................................................................................267 Gamma ray, definition ........................................................................267 Gamma stabilizer, definition ..............................................................267 Gas carburizing.....................................................................................62 Gas leak................................................................................................107 Gas porosity, definition .......................................................................267 General segregation, etchants for carbon steels, low-alloy steels, and cast iron.................................................222(T) Ghost lines, definition..........................................................................267 Glancing angle, definition ...................................................................267 Glass reflector......................................................................................130 Glass window of a hot stage ..............................................................116 Glazing, definition................................................................................267 Glide. See also Slip. definition............................................................................................267 Glyceregia .......................................................................................237(T) as etchant for carbon steels and alloy steels ..............................303(T) for etching stainless steels ................................................................238 Glycerin ................................................................................................182

to carry abrasive particles on wire of wire saw ..............................182 Gold sputtering ...........................................................................158, 160 Goniometers.........................................................................................164 definition............................................................................................267 G-P zone. See Guinier-Preston zone. Graded abrasive, definition ................................................................267 Grain, definition ...................................................................................267 Grain boundaries............................................................152, 215-217(F) definition............................................................................................267 etchants for carbon steels, low-alloy steels, and cast irons .......222(T) Grain-boundary carbides, etchants for carbon steels, low-alloy steels, and cast irons................................................................222(T) Grain-boundary corrosion, definition ...............................................267 Grain-boundary etching, definition ...................................................267 Grain-boundary liquation. See also Burning. definition............................................................................................267 Grain-boundary network ...................................................................225 Grain-boundary segregation .........................................70(F), 72, 73(F) Grain-boundary separation.....................................80, 82(F), 84, 85(F) Grain-boundary sulfide precipitation, definition .............................267 Grain coarsening, definition ...............................................................267 Grain-contrast etching, definition......................................................267 Grain flow, definition...........................................................................267 Grain growth, definition .....................................................................267 Grain intercept length, conversion to ASTM number.................298(T) Grain refinement ..................................................................57(F), 59-60 Grain refiners ........................................................................................53 Grain size. See also Grit size............................................................. 171 definition............................................................................................267 Grain size comparison eyepiece, definition.......................................267 Grain size measurements.................................................99-100(F), 138 Grain size number ..............................................................................100 Granular bainite...............................................................................39(F) definition............................................................................................267 Granular bainite (martensite-austenite constituent) etchants for carbon steels, low-alloy steels, and cast irons .......222(T) tint etchants for carbon steels, low-alloy steels, and cast irons......................................................................................234(T) Granular fracture. See also Crystalline fracture. definition............................................................................................267 Graphite. See also Flake graphite, Nodular graphite, Rosette graphite, and Spheroidal graphite...................................................25 in austempered ductile cast iron....................................................20(F) definition............................................................................................267 dissolution and austenite volume differences................................77(F) distinguishing malleable iron..............................................................64 in ductile cast iron .........................................................................20(F) graphitization of steel ..............................................75(F), 76(F), 77(F) nodular, definition.....................................................................267, 279 rosette, definition ......................................................................267, 285 vermicular ............................................................................102, 103(F) in white iron...................................................................................44(F) Graphite flakes ............................................................42, 44(F), 65, 209 definition ...................................................................................265, 267 in gray cast iron.................................16(F), 64, 84-86(F), 100, 115(F) in mottled cast iron.............................................................18(F), 45(F) Graphite flakes/nodules, as-polished specimens used.......................219 Graphite nodules. .............................................................65, 100-103(F) See also Nodular graphite. in ductile iron.................................................................................46(F) microstructure ..........................................................................65(F), 66 Graphite retention ..............................................................................209 Graphitic carbon, definition ...............................................................268 Graphitic corrosion, definition ...........................................................268 Graphitic steel, definition....................................................................268 Graphitization .............................................................................84-86(F) definition............................................................................................268 reversal of process .........................................................................77(F) of steels ....................................................................75(F), 76(F), 77(F) temperature range................................................................................77 Graphitizer ......................................................................................16, 44 of cast irons.........................................................................................46 Graphitizing, definition .......................................................................268 Graticule. See also Reticle. ......................................... 124, 125, 138(F) Gravity-feed saw ...................................................................................98

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Index / 335 Gray cast iron (or gray iron) ....................................................16-17(F) applications ....................................................................................44-45 for automotive heavy-duty service (SAE J431), compositions ........................................................................314(T) cast iron form....................................................................................251 composition range .................................................................16, 315(T) corrosion....................................................................................84-86(F) definition............................................................................................268 and ductile iron compared ................................................................100 flake graphite (FG) identification .......................................................16 graphite/graphite flakes in ............................................................64, 65 grinding .............................................................................................202 micrographs of pearlite ................................................................115(F) microstructure ........................................16(F), 42, 44-45(F), 47(F), 48 pearlitic...........................................................................................44(F) phase transformations ....................................................................42-43 Greek Ascoloy........................................................................................12 Greenough, H. .....................................................................................277 Greenough microscope, definition......................................................277 Grinding ...............................................................................................164 of aluminized coating sample.............................................................93 of bearing race sample........................................................................95 cast irons ...........................................................................................202 of cast iron shot sample ..........................................................96, 97(F) damage created by ................................................................198-199(F) definition............................................................................................268 equipment for metallography laboratory..........................................104 of forged steel sample ........................................................................97 grit sizes for grinding papers ......................................................199(T) media ............................................................................................199(T) plastic deformation zones from ...................................................198(F) of railroad rail sample ........................................................................97 to remove rust damage ........................................................174, 176(F) to remove shear burr ................................................................178, 179 stainless steel bolt sample ..................................................................98 steel sheet sample ...............................................................................99 of steel wire sample............................................................................94 time....................................................................................................201 weld failure in steel plate sample ......................................................94 Grinding burn. See Burning. Grinding cracks, definition .................................................................268 Grinding discs, metal-backed..............................................................199 Grinding papers .....................................................................199, 201(F) storage of...........................................................................................202 Grinding sensitivity, definition ...........................................................268 Grinding stress, definition...................................................................268 Grinding technique.................................................................200-201(F) Grinding wheel............................................................................105, 106 speed ..........................................................................................201-202 Grit, definition......................................................................................268 Grit numbers .......................................................................................199 Grit size. See also Grain size........................................................ 199(T) definition ...................................................................................199, 268 Ground-glass focusing screen, definition ..........................................268 Guinier-Preston (G-P) zone, definition..............................................268 Gulleted saw blade.................................................................179, 180(F)

H Habit plane, definition.........................................................................268 Hacksaw...............................................................................104, 106, 199 automatic ...........................................................................................105 handheld.....................................................................................179-180 motorized, oscillating........................................................................180 Hadfield, Sir Robert .............................................................................13 Hadfield manganese steel.....................................................13-14, 15(F) Hairline crack. See Flakes. Half hard, definition ............................................................................268 Hall-Petch relationship, definition .....................................................268 Halo formation .......................................................................102, 103(F) of as-cast ferritic-pearlitic ductile iron ...............................102, 103(F) Halos........................................................................................225, 226(F) Hand shear ...................................................................................178-179 Hard chromium, definition .................................................................268

Hardenability, definition .....................................................................268 Hardener ..............................................................................................190 Hardening. See also Age hardening, Case hardening, Flame hardening, Precipitation hardening, and Quench hardening. definition............................................................................................268 Hardfacing, definition..........................................................................268 Hardness bearing race sample measurements ....................................................96 definition............................................................................................268 Hardness (indentation), definition .....................................................268 Hardness testing......................................................................100-103(F) Hard surfacing. See Hardfacing. Hard (x-rays), definition .....................................................................268 Haynes 556.............................................................................................12 HAZ. See Heat-affected zone. Hazardous waste disposal ..................................................................107 Haze ......................................................................................................220 H-band steel, definition .......................................................................268 Heat-affected zone (HAZ) .................................95(F), 180, 210, 230(F) definition............................................................................................268 with electric discharge machining.......................................181, 182(F) of weld failure in steel plate..........................................................95(F) Heat check, definition..........................................................................268 Heating curve, definition.....................................................................268 Heat-resistant chromium-molybdenum steels ASTM specifications for product forms .....................................308(T) compositions ................................................................................308(T) product forms...............................................................................308(T) UNS designations ........................................................................308(T) Heat-resistant steels ...................................................................12, 14(F) AISI designations................................................................................12 ASME designations ............................................................................12 classification ..........................................................................................1 UNS designations..................................................................................2 Heat-resisting alloy, definition ....................................................268-269 Heat tinting, definition ........................................................................269 Heat treating film, definition ..............................................................269 Heat treatment to alter a microstructure......................................................................49 carburizing ................................................................................62-63(F) definition............................................................................................269 to eliminate sensitization ....................................................................74 homogenizing............................................................................51-53(F) intercritical annealing ...............................................................54-55(F) malleabilizing............................................................................63-65(F) for microstructural development ........................................................24 normalizing ....................................................................49-51(F), 52(F) precipitation hardening .......................................................................58 quenching .......................................................................................54(F) recrystallization annealing .......................................................59(F), 61 spheroidizing..................................................................60(F), 61-62(F) strain aging..........................................................................................60 tempering........................................................................................62(F) Hematite, definition .............................................................................269 Herringbone pattern. See Chevron pattern. Heterogeneous equilibrium, definition ..............................................269 Hexagonal close-packed, definition....................................................269 Hexagonal (lattices for crystals), definition ......................................269 Heyn method, definition......................................................................269 High-carbon, high-chromium, cold-work steels, AISI code classification ....................................................................................12 High-carbon, high-chromium steel, sectioning............................176(F) High-carbon, low-alloy steel, austenite in microstructure..............40(F) High-carbon steels carbon content (%)................................................................................4 decarburization ....................................................................................68 microstructure...................................................................................4(F) plate martensite.............................................................................36, 54 High-cycle fatigue, definition..............................................................269 High-expansion alloys...........................................................................15 High-eyepoint oculars.........................................................................125 High-magnification metallography....................................................127 High-manganese carbon steels composition ranges and limits.....................................................305(T) UNS designations ........................................................................305(T)

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336 / Metallographer’s Guide High-permeability steels..................................................................13-14 High-point eyepiece.............................................................................125 Hi-Span-Hi steel, composition ..............................................................15 High-strength, low-alloy steel (HSLA steel). See also Microalloying.......................................................... 2-4, 7-8(F), 9(F) AISI designations.........................................................................8, 9(F) ASTM designations ..............................................................................8 definition............................................................................................269 SAE classification system.....................................................................8 thin foil showing copper precipitates ..........................................153(F) titanium-molybdenum carbides shown by extraction replica..............................................................155, 156(F), 161(F) yield strengths related to heat treatments used ................................7-8 High-thermal-expansion steels.............................................................15 applications..........................................................................................15 Homogeneous carburizing, definition................................................269 Homogeneous (radiation, monochromatic), definition ....................269 Homogenizing altering of microstructure .........................................................51-53(F) definition............................................................................................269 Homogenizing annealing, definition...................................................269 Homogenizing heat treatments............................................................42 Hooke, Robert .....................................................................................109 Hooke’s law. See also Modulus of elasticity. definition............................................................................................269 Hot-air blow dryer..............................................................................105 Hot cathode gun. See Thermionic cathode gun. Hot-cold working. See also Warm working. definition............................................................................................269 Hot crack. See also Solidification shrinkage crack. definition............................................................................................269 Hot dip coating, definition ..................................................................269 Hot dipped steels, etching of.................................................241, 242(F) Hot forming. See Hot working. Hot hardness tester.............................................................................106 Hot microhardness tester.......................................................166-167(F) Hot quenching, definition....................................................................269 Hot-rolled steels.....................................................................................90 Hot rolling..............................................................................................58 and banding.........................................................................................42 definition............................................................................................269 Hot shortness.......................................................................72-74(F), 188 definition............................................................................................269 Hot stage definition............................................................................................269 quartz window of ..............................................................................116 Hot stage microscope .....................................................106, 164-165(F) Hot tear, definition...............................................................................269 Hot-worked structure, definition .......................................................269 Hot working, definition .......................................................................269 Howe, Henry Marion............................................................................24 HSLA steels. See High-strength, low-alloy steels. Hull-Davey charts, definition..............................................................269 Hull method (for x-ray crystal analysis). See Debye-Scherrer method. Humfrey’s reagent etchant composition .....................................................................300(T) as etchant for revealing macrostructures in iron and steel ........300(T) Huntsman, Benjamin..........................................................................256 Huygens, Christian .............................................................................124 Huygens eyepiece ....................................................................124-125(F) definition............................................................................................269 Hydrochloric acid...............................................97(F), 106, 230, 240(T) etchant composition .....................................................................300(T) as etchant for revealing macrostructures in iron and steel ........300(T) for etching stainless steels ................................................................240 Hydrochloric acid/methanol, as etchant for carbon steels and alloy steels................................................................................303(T) Hydrofluoric acid added to Marshall’s reagent .............................................................227 in etchants, storage containers..........................................................220 Hydrogen decarburization ....................................................................................67

function as element in steel and cast iron ......................................3(T) Hydrogen blistering, definition...........................................................269 Hydrogen cracks.......................................................................82, 83, 84 Hydrogen damage............................................................80-84(F), 85(F) definition............................................................................................269 Hydrogen diffusion .....................................................................83-84(F) Hydrogen embrittlement, definition ..................................................269 Hydrogen flakes ...................................................................82, 83, 84(F) Hydrogen-induced delayed cracking. See also Static fatigue. definition............................................................................................269 Hydrogen peroxide..............................................................................220 Hygroscopic material..........................................................................170 Hypereutectic alloy, definition ....................................................269-270 Hypereutectoid alloy, definition .........................................................270 Hypereutectoid steels ...............................................26, 28(F), 33, 34(F) Hypereutectoid structure, definition..................................................270 Hyperplane (compensated) eyepiece.........................................124, 125 Hyperplane oculars.............................................................................109 Hypoeutectic, cast irons as....................................................................64 Hypoeutectic alloy, definition .............................................................270 Hypoeutectoid alloy, definition...........................................................270 Hypoeutectoid steels ..................................................................26, 28(F) microstructure ...............................................................32, 33(F), 35(F) Hysteresis, magnetic, definition..........................................................270

I Identification code, for identifying test specimens ............................104 Idiomorph, definition...........................................................................270 Idiomorphic crystal, definition ...........................................................270 IF. See Interstitial-free steel. Illumination. See Bright-field illumination, Dark-field illumination, Differential interference contrast illumination, and Polarized light illumination. types of..................................................................................131-137(F) Illumination system........................................................................126(F) components of.......................................................................126-128(F) Illuminator .........................................................126, 127(F), 130-131(F) Image, definition...................................................................................270 Image analysis.....................................................................215, 216, 225 as-polished specimens.......................................................................219 with modified Marshall’s reagent.....................................................227 Image analysis system......................................................91, 149-151(F) in full-scale metallographic laboratory ....................................105, 106 Image analyzer ........................................................................149-151(F) Image contrast.....................................................................................109 Image quality ..................................................................................126(F) Image rotation, definition ...................................................................270 Immersion etching, definition.............................................................270 Immersion lens. See Immersion objective. Immersion objective, definition..........................................................270 Impact test, definition..........................................................................270 Imperfection, definition .......................................................................270 Impingement attack. See also Erosion. definition............................................................................................270 Impregnation, definition......................................................................270 Impression replica. See Replica. Impurities in cast irons.........................................................................................47 definition............................................................................................270 Inclusion ...............................................................................................171 as-polished specimens used ..............................................................219 definition............................................................................................270 manganese sulfide ..............................................73-74(F), 172, 173(F), 205, 217, 218(F), 233-234(F) manganese sulfide in carbonitrided steel ................................67(F), 71 manganese sulfide in carbon steel......................................5(F), 147(F) manganese sulfide in cast iron ......................................................47-48 nonmetallic......................................................................159(F), 222(T) oxide-type, definition ........................................................................280 Inclusion count, definition...................................................................270

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Index / 337 Inclusion retention .................................................................205(F), 209 Incoherent scattering. See also Coherent scattering. definition............................................................................................270 Incoloy 800.............................................................................................12 Incongruent transformation, definition.............................................270 Indicating desiccant ............................................................................212 Indices. See Miller indices. Indigenous inclusions, definition ........................................................270 Induction hardening. See Hardening and Surface hardening. Inelastic electron scatter. See Incoherent scattering. Infinity-corrected objective lenses.....................................................112 Infinity-corrective objectives..............................................................122 Inflection point, definition...................................................................270 Infrared, definition...............................................................................270 Ingot, definition ....................................................................................270 Ingot iron, definition............................................................................270 Inhibitor added to colloidal silica....................................................................204 added to silicon dioxide particles.....................................................207 definition............................................................................................270 Inoculation ...................................................................................65-66(F) definition............................................................................................270 of liquid iron .......................................................................................49 of molten iron with magnesium...............................................100, 102 Instant film camera......................................................104-105, 139-140 Integrating camera, definition.....................................................270-271 Intensifying screen, definition.............................................................271 Intensity of scattering, definition .......................................................271 Intensity (x-rays), definition ...............................................................271 Intercept method. See also ASTM standards, specific types definition............................................................................................271 Intercritical annealing ................................................................54-55(F) Intercrystalline. See also Intergranular. definition............................................................................................271 Intercrystalline cracks, definition ......................................................271 Interdendritic, definition .....................................................................271 Interdendritic boundaries, etching response........................219, 220(F) Interdendritic corrosion, definition ...................................................271 Interdendritic porosity, definition......................................................271 Interface, definition..............................................................................271 Interfacial tension, definition..............................................................271 Interference, definition ........................................................................271 Interference filter, definition ......................................................264, 271 Interference fringes...........................................................136(F), 137(F) Interference illumination ...............................................121, 135-138(F) Interferometers ....................................................................109, 135-136 Interferometry .....................................................................................128 Intergranular, definition .....................................................................271 Intergranular corrosion. See also Interdendritic corrosion. definition............................................................................................271 Intergranular cracking, definition .....................................................271 Intergranular fracture................................................................75-76(F) definition............................................................................................271 Intergranular stress-corrosion cracking, definition .........................271 Interlamellar spacing............................................................................34 definition............................................................................................271 Intermediary plane, definition............................................................271 Intermediate annealing, definition .....................................................271 Intermediate phase, definition............................................................271 Intermetallic compound, definition....................................................271 Intermetallic phases, definition ..........................................................271 Internal oxidation .................................................................67(F), 69-71 definition............................................................................................271 etchants for...................................................................................303(T) Interocular distance ............................................................................144 Interplanar distance, definition ..........................................................271 Interpupillary distance...............................................................123, 144 definition............................................................................................271 Interrupted aging. See also Aging. definition............................................................................................271 Interrupted quenching, definition......................................................271 Interstices....................................................................................36, 37(F) Interstitial alloying element .................................................................51 Interstitial solid solution, definition...................................................271

Interstitial carbon .................................................................................35 Interstitial element ................................................................................27 Interstitial-free (IF) steels.....................................................9, 59(F), 61 etching response...........................................................................227(F) Interstitial sites ......................................................................................27 Interstitial solid solution ......................................................................27 Intracrystalline. See also Transgranular. definition............................................................................................271 Intracrystalline cracking. See Transcrystalline cracking. Intracrystalline fracture. See transgranular fracture. Invar composition .........................................................................................14 microstructure.................................................................................15(F) Inverse chill, definition........................................................................271 Inverse pole figure ......................................................................163, 164 Inverse segregation, definition............................................................271 Inverted microscope...............................................................109, 110(F) advantages and disadvantages ..........................................................110 definition............................................................................................271 to inspect polished steel specimens..................................................206 Investment casting, definition .....................................................271-272 Ion, definition .......................................................................................272 Ion bombardment thinning................................................................155 Ion etching, definition..........................................................................272 Ionic bond, definition...........................................................................272 Ionic crystal, definition........................................................................272 Ion microscope ....................................................................................109 Ion milling............................................................................................155 Iris diaphragm, field ...............................................................129-130(F) Iron chemical polishing, etching reagent............................................298(T) crystal structure.................................................................................154 definition............................................................................................272 electron diffraction pattern...........................................................154(F) etchants for revealing macrostructures........................................300(T) in gray iron ..............................................................................47(F), 48 as isotropic material..........................................................................133 WDS x-ray map in nonmetallic inclusion ..................................159(F) Iron-base heat-resistant alloys, compositions ..............................312(T) Iron-base superalloys annealing twins ..............................................................................14(F) as heat-resistant steels .............................................................12, 14(F) microstructure ..........................................................................12, 14(F) Iron-cementite phase diagram ....................24-25(F), 26, 42, 43(F), 46 Iron carbide phase diagram ................................................................25 Iron-carbon alloys, martensitic microstructure .........................36, 37(F) Iron-carbon (Fe3C) carbides, etchants for ...................................301(T) Iron-carbon equilibrium diagram .....................................50(F), 77, 90 of annealing temperatures..............................................................58(F) spheroidizing temperatures......................................................60(F), 61 Iron-carbon phase diagram .......................................................24-28(F) eutectic portion for cast irons.............................................................42 expanded version............................................................................26(F) Iron foundry, metallographer’s job description, workday.....100-103(F) Iron-graphite phase diagram .......................24-25(F), 42, 43-44(F), 46 Iron oxide, d-spacings.....................................................................163(F) Iron phosphide, in gray iron......................................................47(F), 48 Iron-silicon alloys, chemical polishing, etching reagent...............298(T) Iron sulfide, as cause of hot shortness........................................72-74(F) Isobar, definition ..................................................................................272 Isochor, definition ................................................................................272 Isomorphous, definition.......................................................................272 Isomorphous system, definition..........................................................272 Isotherm, definition..............................................................................272 Isothermal annealing. See also Austenitizing. definition............................................................................................272 Isothermal transformation........................................................38, 39(F) definition............................................................................................272 Isothermal transformation (IT) diagram ....................24, 28-29(F), 31 definition............................................................................................272 Isotropic, definition ..............................................................................272 Isotropic material................................................................................133 Isotropy, definition...............................................................................272 IT. See Isothermal transformation diagram.

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338 / Metallographer’s Guide

J Janssen, Zacharias ..............................................................................109 JCPDS. See Joint Committee on Powder Diffraction Standards. Jeffries’ method, definition .................................................................272 Jeffries’ multiplier, definition .............................................................272 Jet thinning unit......................................................................154-155(F) Joint Committee on Powder Diffraction Standards (JCPDS).......162 Jominy test. See End-quench hardenability test.

K K. See also K-radiation. definition............................................................................................272 Kalling’s reagent ............................................................................237(T) etchant composition .....................................................................300(T) as etchant for revealing macrostructures in iron and steel ........300(T) for etching stainless steels ...........................................................238(F) Kalling’s reagent No. 1, as etchant for carbon steels and alloy steels.........................................................................................303(T) Kalling’s reagent No. 2, as etchant for carbon steels and alloy steels.........................................................................................303(T) Kanthal...................................................................................................16 composition .........................................................................................16 Keel blocks...............................................................................100-103(F) Kellner eyepiece, definition.................................................................272 Kerf...............................................................................................181, 183 definition............................................................................................272 Kerf loss ...............................................................................................181 Kikuchi lines, definition ......................................................................272 Killed steel, definition..........................................................................272 Kink band (deformation), definition .................................................272 Kish, definition .....................................................................................272 Klemm’s reagent.........................................................................220, 236 as etchant for carbon steels and alloy steels ..............................301(T) Knoop hardness ...................................................................71, 95, 96(F) Knoop indenter ..........................................................165-166(F), 167(F) Kohler, August.....................................................................................126 Kohler illumination ...............................................................126, 127(F) definition............................................................................................272 Kovar, composition...........................................................................14-15 K-radiation, definition .........................................................................272 K-series, definition ...............................................................................272 KX. See also Angstrom unit. definition............................................................................................272

L Lamellae. See also Lamellar structure. cementite lamella in pearlitic steel ......................................152-153(F) definition............................................................................................272 Lamellar definition............................................................................................272 pearlite ............................................................................................34(F) Lamellar structure, definition ............................................................272 Lamellar tear, definition .....................................................................272 Lamination, definition..................................................................272-273 Lamp collector.....................................................................................128 Lamp housing.........................................................................128(F), 145 Lamps.......................................................................................126-128(F) Lancashire iron, oxide inclusions..................................................218(F) Lanthanides. See Rare earth metals. Lap, definition ......................................................................................273 Lapelloy ..................................................................................................12 Lap joint, definition .............................................................................273 Lapping, definition...............................................................................273 Latent heat, definition .........................................................................273 Lateral chromatic aberration. See Chromatic aberration. Lath martensite ...............................................36, 54(F), 222(F), 223(F) in carbonitrided steel ....................................................68(F), 69(F), 71 in carbon steels ............................................................................233(F) definition............................................................................................273 in iron-carbon binary alloy ............................................................36(F)

in low-alloy steel .................................................................225, 226(F) in maraging steel..........................................................................238(F) and microcrack formation ..................................................75(F), 76-77 in plasma-arc-cut steel surface.................................................77-78(F) in precipitation-hardening stainless steels......................238(F), 239(F) steels in which it formed ....................................................................54 tempered .........................................................................................62(F) Lattice. See also Crystal system. definition............................................................................................273 Lattice constant. See Lattice parameter. definition............................................................................................273 Laue equations, definition...................................................................273 Laue method (for crystal analysis), definition .................................273 Lead, function as element in steel and cast iron ...............................3(T) Leaded, definition.................................................................................273 Lead inclusions, with manganese sulfides .....................................219(F) Ledebur, Karl Heinrich........................................................................43 Ledeburite...................................................................................43(F), 64 definition............................................................................................273 in mottled cast iron.............................................................18(F), 45(F) in white cast iron..........................................18(F), 43(F), 44(F), 45(F) Leewenhoek, Anthony van .................................................................109 Lens assemblies..........................................................................................112 Bertrand................................................................................137(F), 146 Bertrand, definition ...........................................................................273 cleaning procedure ............................................................................142 compound, definition ........................................................................273 definition............................................................................................273 negative, definition............................................................................273 tissue .................................................................................139, 140, 141 Less noble elements ..............................................................................70 Leveling device.............................................................................138-139 Lever principle, definition...................................................................273 Lever rule...............................................................................................90 definition............................................................................................273 Lichtenegger/Bloech reagent ........................................................237(T) as etchant for carbon steels and alloy steels ..............................303(T) for etching stainless steels ...........................................................237(F) Light-field illumination. See Bright-field illumination. Light filter. See Color filter. Light metal, definition .........................................................................273 Light microscope ..................................................................91, 109, 149 bainite appearance..........................................................................39(F) definition............................................................................................273 with digitizing pad................................................................150-151(F) to examine general microstructure .....................................................89 magnification range.............................................................................89 manufacturers ....................................................................................296 microstructure examination in Sorby’s day .......................................88 to observe aluminized coating sample ...............................................93 to observe bainite ..........................................................................38-39 to observe clues about precipitation process .....................................59 to view sensitization of stainless steel ...............................................79 Light source .............................................................................126-127(F) centering and focusing procedure ........................................144-145(F) Light stereomicroscope.......................................................................149 Limited solid solution, definition .......................................................273 Limiting current density, definition...................................................273 Lineage structure, definition...............................................................273 Linear elastic fracture mechanics, definition ...................................273 Linear magnification. See Magnification. Linear strain. See Strain. Line indices, definition ........................................................................274 Line (in x-ray diffraction patterns), definition.................................274 Linepipe steel ......................................................................................8(F) 450 MPa yield strength, microstructure..........................................8(F) Liquation ................................................................................................72 definition............................................................................................274 Liquation temperature, definition......................................................274 Liquid carburizing ................................................................................62 Liquid metal embrittlement, definition .............................................274 Liquid nitrogen for cooling cast irons .....................................................................47(F)

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Index / 339 for cooling quench-hardened steel rolls.............................................40 Liquid penetrant inspection, definition .............................................274 Liquidus. See also Solidus. definition............................................................................................274 Liquidus temperature ...........................................................................43 Loading, definition ...............................................................................274 Local action, definition........................................................................274 Local cell, definition.............................................................................274 Local current density, definition ........................................................274 Localized precipitation, definition .....................................................274 Longitudinal chromatic aberration. See Chromatic aberration. Longitudinal direction. See also Normal direction and Transverse direction. definition............................................................................................274 Longitudinal section .................................................170(F), 171, 172(F) Long transverse. See Transverse. Long-working-distance objective ......................................................121 Loria (nitrosulfuric acid etch) etchant composition .....................................................................300(T) as etchant for revealing macrostructures in iron and steel ........300(T) Low-alloy, special-purpose tool steels, AISI code classification ....................................................................................12 Low-alloy steels AISI/SAE classification system ........................................................6-7 ASTM specification system..................................................................7 bainite...................................................................................164, 165(F) bake-hardenable ....................................................................................9 compositions ................................................................................307(T) definition............................................................................................274 dual-phase.........................................................................................9(F) with formability ....................................................................................9 for high-temperature properties....................................................8-9(F) for improved corrosion resistance ........................................................9 manual (hand) polishing ...................................................................204 prior austenite grain boundaries when quenched-and-tempered .......................................................232(F) Low-carbon alloy steel, bainite in microstructure ..........................38(F) Low-carbon mold steels, AISI code classification ..............................12 Low-carbon quenched-and-tempered steels, compositions........307(T) Low-carbon steels carbon content (%)................................................................................4 cold rolling................................................................................55-56(F) cold-worked.....................................................................225(F), 226(F) electroless nickel plated and mounted ................................184(F), 185 enamel coating .....................................................................243, 244(F) etched in Marshall’s reagent ..........................................224(F), 225(F) etching response of carbides ...............................................228, 229(F) etching response of carbides to picral variation.................228, 229(F) Galvalume coating micrograph....................................................151(F) lath martensite...............................................................................36, 54 mechanically mounted .................................................................186(F) microstructure .............................................................4(F), 172, 173(F) nital etch revealing grain-boundary carbides ..............................222(F) Low-cycle fatigue, definition...............................................................274 Low-energy ion-scattering spectroscopy (LEISS) ...........................168 Lower bainite.........................................................................................38 in carburized microstructures ..................................................62, 63(F) in commercial low-alloy steel .................................................38, 39(F) definition............................................................................................274 Lower yield stress, definition..............................................................274 Low-nap polishing cloth.................................93, 94, 95, 96, 97, 98, 99 Low-thermal-expansion steels ...................................................14-15(F) microstructure.................................................................................15(F) L-radiation, definition .........................................................................274 L-series, definition................................................................................274 Lucite ....................................................................................................189 Lüders bands, definition .....................................................................274 Lüders lines............................................................................................55 definition............................................................................................274

M Mf. See Martensite finish temperature.

Ms. See Martensite start temperature. Machinability, definition .....................................................................274 Machinability index, definition...........................................................274 Machining ....................................................................................77-79(F) to induce cold work..................................................................78-79(F) Machining stress, definition ................................................................274 M-A (MA) constituent ...............................................................39(F), 40 definition............................................................................................274 Macrocamera .......................................................................................105 Macroetching .........................................................................................88 definition............................................................................................274 Macrograph ...........................................................................95(F), 97(F) definition............................................................................................274 Macroscopic, definition .......................................................................274 Macroscopic stresses, definition .........................................................274 Macrosegregation............................................................................41, 51 Macroshrinkage, definition .................................................................274 Macrostress. See Macroscopic stresses. Macrostructure definition............................................................................................274 etchants for iron and steel ...........................................................300(T) Magnesium as anisotropic material ......................................................................133 in compound in ductile iron ...............................................................66 function as element in steel and cast iron ......................................3(T) inoculation of liquid ductile iron........................................................45 WDS x-ray map in nonmetallic inclusion ..................................159(F) Magnesium aluminate .................................................................159-160 Magnesium oxide, as polishing abrasive............................................204 Magnetic alignment. See also Alignment. definition............................................................................................274 Magnetically hard alloy, definition.............................................274-275 Magnetically soft alloy, definition......................................................275 Magnetic lens, definition .....................................................................275 Magnetic-particle inspection, definition ............................................275 Magnetic pole, definition.....................................................................275 Magnetic shielding, definition.............................................................275 Magnetic steels .................................................................................13-14 Magnetic stirrer ..................................................................................237 Magnetic transformation. See Curie point temperature. Magnetite..............................................................................................162 definition............................................................................................275 Magnetizing force, definition ..............................................................275 Magnetostriction, definition................................................................275 Magnification ...........................................................................112-113(F) calibration of .....................................................................................138 definition............................................................................................275 empty, definition ...............................................................................275 eyepieces manufactured with............................................................123 of micrograph, determination of.......................................................143 Magnification limit................................................................................91 Magnification number ........................................................................124 Major defects, definition .....................................................................257 Malleability, definition.........................................................................275 Malleabilizing ..............................................................................63-65(F) definition............................................................................................275 Malleable cast iron ...................................................16, 18-19(F), 64-65 applications ....................................................................................64-65 compositional limits............................................................................18 composition ranges.........................................................314(T), 315(T) definition............................................................................................275 as form of cast iron ..........................................................................251 graphite in ...........................................................................................64 grinding .............................................................................................202 microstructure ...............................................18(F), 46(F), 64(F), 65(F) Manganese as carbide stabilizer ............................................................................46 content effect in resulfurized steels......................................................5 function as element in steel and cast iron ......................................3(T) movement during dendritic solidification process ................51, 52, 53 WDS x-ray map in nonmetallic inclusion ..................................159(F) Manganese segregation.........................................................................90 Manganese sulfides................................................................................98 color...................................................................................................218

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340 / Metallographer’s Guide Manganese sulfides (Continued) dendrites ...................................................................................70(F), 72 in free-machining ferritic stainless steels......................................12(F) inclusions...........................................................................................172 inclusions and etching response..........................................217, 218(F) inclusions in carbonitrided steel..............................................67(F), 71 inclusions in carbon steel ............................................................147(F) inclusions in cast irons ..................................................................47-48 inclusions in free-machining steel.......................................233, 234(F) inclusions in plain carbon steels......................................................5(F) inclusions in polishing ......................................................................205 inclusions in resulfurized steel bar .....................................172, 173(F) inclusions to prevent hot shortness ....................................73(F), 74(F) stringers and overheating effect on morphology...............69(F), 71-72 Maraging, definition ............................................................................275 Maraging steels ..........................................................................13, 15(F) composition............................................................................13, 314(T) heat treatment......................................................................................13 lath martensite ..............................................................................238(F) microstructure.................................................................................15(F) precipitation hardening .......................................................................58 Marbles reagent etchant composition .....................................................................300(T) as etchant for revealing macrostructures in iron and steel ........300(T) Marginal rays ......................................................................................119 Marquenching. See Martempering. Marshall’s reagent ..............................................................................221 as etchant for carbon steels and alloy steels .................301(T), 302(T) for etching carbon and low-alloy steels and cast irons.......224-226(F) for revealing ferrite grain boundaries ..............................................223 storage ...............................................................................................220 Marshall’s reagent, modified, for etching carbon and low-alloy steels and cast irons..........................................................226-227(F) Martempering, definition ....................................................................275 Martens, Adolph....................................................................................29 Martensite. See also Lath martensite, Plate martensite, Transformation temperature, and Untempered martensite............................................................................. 35-38(F) as-quenched, and microcleanliness determination..............217, 218(F) as-quenched, etchants for carbon steels, low-alloy steels, and cast irons ..............................................................................222(T) as-quenched, tint etchants for carbon steels, low-alloy steels, and cast irons .......................................................................234(T) in bearing race................................................................................96(F) carbon-free ..........................................................................................36 as cause of central bursting in steel wire ..........................94(F), 95(F) definition............................................................................................275 in dual-phase steel............................................................................9(F) etchants for stainless steels..........................................................237(T) as friction effect .......................................................................80, 81(F) in gray iron..........................................................................................44 in isothermal transformation diagram for steel (1080) ...........................................................29, 30(F), 31(F) in martensitic stainless steels.........................................................14(F) in microalloyed steels ......................................................................8(F) in microstructure of steel (1060).............................................32, 33(F) in plain carbon steel.......................................................................55(F) in precipitation-hardening stainless steels...............................12, 14(F) in quenched steel ................................................................................54 in stainless steel .......................................................................36, 37(F) in steel microstructure (1080) .................................................23, 24(F) tempered, etchants for carbon steel, low-alloy steels, and cast irons......................................................................................222(T) transformation in commercial cast irons............................................47 in ultrahigh-strength (maraging) steels ...................................13, 15(F) Martensite (ferrite and retained austenite not attacked), tint etchants for carbon steels, low-alloy steels, and cast irons ...234(T) Martensite finish temperature (Mf)..............................................29, 47 Martensite microstructure, etchants for.......................................302(T) Martensite range, definition ...............................................................275 Martensite start temperature (Ms) ..........................................29(F), 31 of carbonitrided steel ..........................................................................71 Martensite structure electrolytic etchants for stainless steels ......................................240(T)

etchants for stainless steels..........................................................237(T) Martensitic, definition .........................................................................275 Martensitic stainless steels .......................................11-12, 13(F), 14(F) compositional variations, family relationships..............................13(F) compositions ................................................................................309(T) etchants for...................................................................................303(T) etching by Kalling’s reagent ............................................................238 nonstandard, compositions...........................................................311(T) precipitation-hardening, compositions.........................................312(T) quenched and tempered, compositions .......................................312(T) Mass scattering coefficient, definition................................................275 Master alloy, definition .......................................................................275 Materials safety data sheet (MSDS) ........................106, 107, 190, 193 of mounting materials................................................................185-186 Matrix, definition .................................................................................275 M3C..............................................................................236, 237, 238, 239 M6C..............................................................................236, 237, 238, 239 M23C6 ...........................................................................236, 237, 238, 239 McQuaid-Ehn test, definition .............................................................275 Mean free path, definition ..................................................................275 Meaningful magnification...................................................................115 Mean normal stress. See Mean stress. Mean stress, definition.........................................................................275 Measuring eyepieces ...........................................................................125 Mechanical alignment. See also Alignment. definition ....................................................................................275-276 Mechanical (cold) crack, definition ...................................................276 Mechanical equation of state, definition ...........................................276 Mechanical metallurgy, definition .....................................................276 Mechanical mounts.........................................................185, 186-187(F) Mechanical polishing, definition.........................................................276 Mechanical properties, definition.......................................................276 Mechanical stage, definition ...............................................................276 Mechanical testing, definition.............................................................276 Mechanical tube length ......................................................................112 Mechanical twin ....................................................................................79 definition............................................................................................276 Mechanical working, definition..........................................................276 Medium-carbon steels carbon content (%)................................................................................4 decarburization ....................................................................................68 lath martensite formation....................................................................54 microstructure...................................................................................4(F) Medium-carbon ultrahigh-strength steels, compositions ...........307(T) Medium-nap polishing cloth ..........................................94, 95, 204-205 Melting point, definition......................................................................276 Melting pressure, definition ................................................................276 Melting range, definition.....................................................................276 Melting rate, definition........................................................................276 Melting temperature. See Melting point. Melt-off rate. See Melting rate. Mercury arc lamps ......................................................................127-128 Mercury vapor lamps..................................................................127-128 Mesh......................................................................................................199 definition............................................................................................276 Mesh size. See Mesh. Metal, definition ...................................................................................276 Metal-arc cutting, definition ...............................................................276 Metallic bond. See also Covalent bond and Ionic bond. definition............................................................................................276 Metallic glass, definition......................................................................276 Metallograph ...........................................109, 112, 126-127, 139-140(F) electronic shutter speed control system for photography...........139(F) Metallographer definition .....................................................................................87, 276 job description.....................................................................................23 metallographic analysis.......................................................................90 workday at a research laboratory of a large steel company ..........................................................................92-100(F) workday at a small iron foundry..........................................100-103(F) Metallographic analysis .................................................................90, 91 of aluminized coating ...............................................................92-93(F) of bearing race sample .............................................................95-96(F) cast iron nodular graphite and matrix microstructure .........100-103(F)

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Index / 341 cast iron shot.............................................................................96-97(F) documentation for samples.................................................................92 forging of steel ...............................................................................97(F) laboratory, basic equipment and areas......................................104-105 railroad rail................................................................................97-98(F) sample forms.......................................................................................92 stainless steel bolt fracture .......................................................98-99(F) steel sheet for hot water heater tanks ....................................99-100(F) of steel wire, defective ..................................................93-94(F), 95(F) submission form................................................................................105 weld failure in steel plate.........................................................94-95(F) Metallographic specimen preparation ..............................169-213(F,T) Metallographic supplies, list of vendors............................................296 Metallographic tips alumina particles for polishing.........................................................207 cleaning of specimen surface ...........................................................193 concentration during hand grinding and polishing ..........................200 creating bond between plain-backed paper and wheel....................200 denatured ethyl alcohol rinse............................................................221 diallyl phthalate vs. phenolic mounts...............................................188 etching solution mixing in porcelain casserole dish........................220 forced-air drying to minimize film residue formation.....................203 grinding time.....................................................................................202 improving edge retention by increasing surface hardness of epoxy mount..............................................................................193 magnetization of wheels or platens..................................................199 minimum smearing of long edge (coating)......................................201 picral etching time ............................................................................228 polishing cloth with plate assembly.................................................207 to prevent epoxy producing a gap between the specimen and mount.........................................................................................192 preventing epoxy from bonding with the mold flat surface............191 removal of etchant from specimen...................................................220 resin proportion to hardener .............................................................190 tightening polishing cloth against wheel..........................................204 Metallography definition .....................................................................................87, 276 father of...............................................................................87(F), 88(F) first rule of ..........................................................................................89 Metallography laboratory...........................................................103-107 basic equipment and areas ........................................................104-105 expanded ....................................................................................149-168 full-scale.....................................................................................105-106 full-scale, ventilation.........................................................................105 partitioning to separate work areas ..................................................104 safety issues ...............................................................................106-107 ventilation .................................................................................104, 105 Metallurgical microscope ......................................104, 105, 109-148(F) Metallurgists/metallurgical engineers.................................................91 Metallurgy, definition ..........................................................................276 Metal shadowing. See also Shadowing. definition............................................................................................276 Metal spraying. See also Thermal spraying. definition............................................................................................276 Metastable, definition ..........................................................................276 Meteorites..........................................................................................88(F) Methane gas...........................................................................................84 hydrogen formation of ........................................................................84 Methanol ..............................................................................................107 Methyl alcohol .....................................................................................107 as additive to picric acid ..........................................................227, 228 Methyl methacrylate ......................................................................189(F) Microalloyed steels..................................................................................8 hot rolling.................................................................................57(F), 60 microstructure .......................................................................8(F), 57(F) Microalloying, definition .....................................................................276 Microbands, definition .................................................................276-277 Microcleanliness ..................................................................................217 Microcrack. See also Microfissure. definition............................................................................................277 and manganese sulfide stringers..............................................69(F), 72 Microcracking .......................................................................75(F), 76-77 annealing as possible salvage treatment ............................................77 in plasma-arc-cut higher-carbon steel ................................................78

Microetching, definition ......................................................................277 Microfissure. See also Microcrack. definition............................................................................................277 Micrograph definition............................................................................................277 magnification determination .............................................................143 Micrography ........................................................................................109 light source centering and focusing .....................................144-145(F) Microhardness. See also Microhardness test. definition............................................................................................277 Microhardness test..................................................................160-161(F) definition............................................................................................277 Microhardness tester......................................................106, 165-166(F) Microindentation. See Microhardness test. Micromanipulator, definition .............................................................277 Micrometer eyepiece ...........................................................................138 definition............................................................................................277 Microporosity, definition.....................................................................277 Microprobe. See Electron probe microanalyzer. Microprobe analyzer. See Electron probe microanalyzer. Microprocessor-controlled grinding/polisher machines..................210 Microradiography contact, definition..............................................................................277 definition............................................................................................277 Microscope camera attachment .....................................................................104-105 definition............................................................................................277 field emission, definition...................................................................277 Greenough, definition .......................................................................277 hot stage accessory.......................................................106, 164-165(F) metallurgical .........................................................104, 105, 109-148(F) x-ray, definition .................................................................................277 Microscopic, definition ........................................................................277 Microscopic stresses, definition ..........................................................277 Microscopy, definition .........................................................................277 Microsegregation. See also Coring.................................... 41, 42(F), 51 definition............................................................................................277 Microshrinkage, definition ..................................................................277 Microstress. See Microscopic stresses. Microstructure as-cast ..................................................................................................41 definition............................................................................................277 electrolytic etchants for stainless steels ......................................240(T) etchants for carbon steels, low-alloy steels, and cast irons .......222(T) etchants for stainless steels..........................................................237(T) influence on properties........................................................................49 unintentionally altered ..............................................................66-86(F) Microtome, definition ..........................................................................277 Midrib morphology, in plate martensite...................................36, 37(F) Migration. See also Transference. definition............................................................................................277 Mil, definition .......................................................................................277 Mild steel, definition ............................................................................277 Miller and Houstan reagent etchant composition .....................................................................300(T) as etchant for revealing macrostructures in iron and steel ........300(T) Miller-Bravais indices, definition .......................................................277 Miller indices, definition .....................................................................277 Mill scale, definition.............................................................................277 Minimized spangle, definition.............................................................277 Minor defects, definition .....................................................................257 Mirror illuminator, definition .....................................................277-278 Mischmetal, definition .........................................................................278 Miscibility gap, definition ...................................................................278 Mixed acids etchant composition .....................................................................300(T) as etchant for revealing macrostructurres in iron and steel .......300(T) Mixed grain size. See Duplex grain size. Modulus of elasticity (E), definition ..................................................278 Modulus of rigidity. See Modulus of elasticity. Mohs scale, definition ..........................................................................278 Moiré pattern, definition.....................................................................278 Mold materials for a castable mount ...............................................190 Mold sizes.............................................................................................187

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342 / Metallographer’s Guide Molecular attrition. See Fretting. Molybdenum as addition to eliminate temper embrittlement ........................75-76(F) as carbide stabilizer ............................................................................46 function as element in steel and cast iron ......................................3(T) Molybdenum carbides ..........................................................................58 Molybdenum high-speed steels, AISI code classification...................12 Monochromatic (homogeneous), definition.......................................278 Monochromatic objective, definition .................................................278 Monochromator (x-rays), definition ..................................................278 Monoclinic, definition ..........................................................................278 Monotectic, definition ..........................................................................278 Monotropism, definition ......................................................................278 More noble elements.............................................................................70 Mosaic crystal, definition ....................................................................278 Mosaic structure, definition ................................................................278 Motor lamination steel, microstructure...........................................16(F) Mottled cast iron........................................................................16, 18(F) microstructure .....................................................................18(F), 45(F) Mount, definition..................................................................................278 Mounting...............................................................................183-198(F,T) advantages .........................................................................................185 of aluminized coating sample.............................................................92 of bearing race sample........................................................................95 of cast iron shot sample.................................................................96(F) cylinders and tubes ...........................................................................196 materials ................................................................................185-187(F) materials, transparent ........................................................................198 mold sizes..........................................................................................187 of powders ............................................................................196-197(F) sheet specimens ....................................................................194-196(F) stainless steel bolt sample ..................................................................98 steel sheet sample ...............................................................................99 of steel wire sample............................................................................93 thermal-compression...............................................184(F), 185(F), 187 weld failure in steel plate sample ......................................................94 of wires ................................................................................196, 197(F) Mounting artifact, definition ..............................................................278 Mounting dyes .....................................................................................193 Mounting press ...................................................................104, 106, 187 automated ..........................................................................................105 hand-operated ....................................................................................105 Mounting under vacuum....................................................................191 MSDS. See Materials safety data sheet. Mf temperature. See also Transformation temperature. definition............................................................................................276 Ms temperature. See also Transformation temperature. definition............................................................................................278 Multiaxial stresses, definition .............................................................278 Multiple etching, definition.................................................................278 Multiplicity factor, definition .............................................................278 Murakami’s reagent ......................................................................237(T) as etchant for carbon steels and alloy steels ..............................303(T) Music wire..............................................................................................35

N NA. See Numerical aperture. Napless polishing cloth................................................93, 94, 96, 97, 98 Napped cloth, definition ......................................................................278 National Institute of Standards and Technology (NIST), calibration of eyepiece reticle scale......................................142-143 Natural aging. See also Aging. definition............................................................................................278 Natural strain. See Strain. Near-equilibrium cooling .....................................................................28 Necking, definition ...............................................................................278 Negative distortion, definition ............................................................278 Negative eyepiece, definition...............................................................278 Network structure, definition .............................................................278 Neumann band, definition...................................................................278 Neutral color filter, definition .....................................................264-265 Neutral filter, definition.......................................................................278

New ferrite .............................................................................................55 Nibbler. See Hand shear. Nickel function as element in steel and cast iron ......................................3(T) as graphitizer.......................................................................................46 Nickel-chromium-molybdenum steel, void formation during creep .....................................................................................80, 82(F) Nickel plating.............................................184(F), 185, 193-194, 195(F) Nicol prism, definition .........................................................................278 Nielsen reagent etchant composition .....................................................................300(T) as etchant for revealing macrostructures in iron and steel ........300(T) Ni-Hard, microstructure....................................................................47(F) Niobium, function as element in steel and cast iron .........................3(T) Niobium carbides ..................................................................................58 Niobium nitrides....................................................................................58 NIST. See National Institute of Standards and Technology. Nital..................................................................................221, 222-224(F) as etchant for carbon steels and alloy steels ..............................301(T) etchants for coated steels, aluminum-based coatings .................304(T) Nitric acid ............................................................................................106 etchant composition .....................................................................300(T) as etchant for revealing macrostructures in iron and steel ........300(T) Nitric acid (60/40) ..........................................................................240(T) for etching stainless steels....................................................240-241(F) Nitride-carbide. See also Carbonitride. definition............................................................................................278 Nitriding, definition ......................................................................278-279 Nitrocarburizing, definition ................................................................279 Nitrogen, function as element in steel and cast iron.........................3(T) Nitronic 60 .............................................................................................12 Nobility of elements ..............................................................................70 Noble metal, definition ........................................................................279 Nodular cast iron. See Ductile cast iron. Nodular graphite, definition .......................................................267, 279 Nodular iron. See Ductile cast iron. Nodular pearlite, definition.................................................................279 Nomarski, G.........................................................................................135 Nomarski interference contrast.....................55, 121, 133-136(F), 217, 239-240(F), 242, 243(F) of carbonitrided steel microstructure ......................................69(F), 71 of hydrogen damage ................................................................82, 83(F) intercritically annealed bar ............................................................55(F) to locate hydrogen damage.................................................................82 to view stainless steel microstructure............................................57(F) Nominal stress. See Stress. Nondestructive inspection, definition.................................................279 Nonmagnetic steels................................................................................14 Nonmetallic inclusions. See also Inclusions. ............................... 159(F) etchants for carbon steels, low-alloy steels, and cast irons .......222(T) Normal. See also Angle of reflection. definition............................................................................................279 Normal direction. See also Longitudinal direction and Transverse direction. definition............................................................................................279 Normalizing. See also Transformation temperature................. 49-51(F), 52(F) and annealing compared................................................................60-61 definition............................................................................................279 Normalizing temperature...........................................................50-51(F) Normal segregation, definition ...........................................................279 Normal stress. See Stress. Nose.........................................................................................................29 Nosepiece .....................................................112, 116-118(F), 137-138(F) objectives in ......................................................................................140 rotating ..................................................................................116-118(F) Notch sensitivity, definition.................................................................279 Nucleation. See also Nucleus. definition............................................................................................279 Nucleus, definition................................................................................279 Numerical aperture (NA) .........................109, 112, 113-114(F), 121(F) definition............................................................................................279 of oil-immersion objective................................................................121 Nylon napless cloth ...............................................................................96

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Index / 343

O

P

Oberhoffer’s reagent etchant composition .....................................................................300(T) as etchant for revealing macrostructures in iron and steel ........300(T) Objective aperture. See Aperture, electron. Objectives ..............................................................112(F), 113(F), 114(F) characteristics ....................................................................................116 correction for hot stage microscope.................................................164 definition............................................................................................279 dry.................................................................................................113(F) fluorite ...............................................................................................125 fluorite, definition..............................................................................279 in nosepiece.......................................................................................140 oil-immersion ............................................................................113, 114 optical defects .......................................................................118-120(F) parcentric ...........................................................................................116 types of ....................................................................119(F), 120-123(F) Oblique evaporation shadowing. See also Shadowing. definition............................................................................................279 Oblique illumination .....................................................131, 133(F), 134 definition............................................................................................279 Octahedral plane, definition ...............................................................279 Ocular. See Eyepiece. Ocular micrometer .....................................................................125, 142 definition............................................................................................279 Ohmax ....................................................................................................16 composition .........................................................................................16 Oil-hardening, cold-work steels, AISI code classification .................12 Oil-immersion objective ............................................119(F), 121-122(F) procedure for use of..........................................................................141 Optical etching ....................................................................................242 definition............................................................................................279 Optical microscope .....................................................................109, 149 Optics....................................................................................................109 Orange peel, definition ........................................................................279 Order-disorder transformation, definition........................................279 Ordered structure, definition..............................................................279 Ordering, definition .............................................................................279 Order (in x-ray diffraction), definition..............................................279 Orientation (crystal). See also Preferred orientation. definition............................................................................................279 Orthochromatic color filter, definition.......................................264-265 Orthochromatic filter, definition ........................................................279 Orthorhombic, definition ....................................................................279 Orthoscopic-type (Ramsden) eyepiece ........................124, 125(F), 138 Osmond, Floris ......................................................................................24 Oven, inside fume hood .......................................................................105 Overaging. See also Aging. definition............................................................................................280 Overheating................................................................69(F), 70(F), 71-72 definition............................................................................................280 Overpolish ............................................................................................209 Overstressing, definition......................................................................280 Overvoltage, definition ........................................................................280 Oxalic acid .........................................................98, 99(F), 239, 240(F,T) as etchant for carbon steels and alloy steels ..............................303(T) Oxidation..............................................................................................170 definition............................................................................................280 Oxidation grain size, definition ..........................................................280 Oxidative wear, definition...................................................................280 Oxide film replica. See also Replica. definition............................................................................................280 Oxide scale..............................................................................172, 188(F) penetration ....................................................................................156(F) Oxide stringers ...............................................................................160(F) Oxide-type inclusions, definition ........................................................280 Oxidized surface (on steel), definition ...............................................280 Oxyacetylene torch ...........................................................180(F), 181(F) Oxygen function as element in steel and cast iron ......................................3(T) WDS x-ray map in nonmetallic inclusion ..................................159(F) Oxygen-acetylene torch cutting ......................77-79(F), 180(F), 181(F) Oxygen deficiency, definition..............................................................280

Pf. See also Pearlite finish temperature. definition............................................................................................281 Ps. See also Pearlite start temperature. definition............................................................................................283 Pack carburizing ...................................................................................62 Packet ................................................................................................36(F) Packet size..............................................................................................36 Paint coatings ......................................................................................209 Painted surfaces, cleaning of..............................................................174 Pancake grain structure, definition ...................................................280 Panchromatic, definition .....................................................................280 Paracentric ...........................................................................................116 Parallax .........................................................................................119-120 Paramagnetic material, definition......................................................280 Parameter (in crystal). See Lattice parameter. Parfocal.................................................................................................117 Parfocal eyepiece, definition.......................................................263, 280 Partial annealing, definition ...............................................................280 Partial decarburization zone ....................................................66(F), 69 Partial hemispherical mirror.............................................................128 Particle size, definition ........................................................................280 Particle-size distribution, definition...................................................280 Passivation, definition..........................................................................280 Passive film. See Passive layer. Passive layer ................................................................170, 219, 234-235 Passivity, definition ..............................................................................280 Patenting, definition.............................................................................280 Pearlite ...................................................................................34-35(F), 64 in annealed microstructure.............................................................58(F) in as-cast microstructure ................................................................41(F) banding after microsegregation .....................................................42(F) in carbonitrided steel microstructure ...........................68(F), 69(F), 71 in carbon steel ..............................................................................146(F) in carburized microstructure....................................................62, 63(F) and cementite lamella in pearlitic steel ...............................152-153(F) in continuous cooling transformation diagram .......................31, 32(F) in decarburized steel microstructure .......................................66(F), 67 in decarburized steel plate...................................................171(F), 172 definition............................................................................................280 described by Sorby .............................................................................89 digitally displayed on video monitor .............................149(F), 150(F) in ductile iron.................................................................................45(F) in equiaxed microstructure ......................................................32, 33(F) etchants for carbon steels, low-alloy steels, and cast irons .......222(T) in eutectoid steel .................................................................34(F), 35(F) in gray cast iron ........................................16(F), 44, 47(F), 48, 115(F) in homogenized microstructure ...............................................52, 53(F) in hypereutectoid steel ...................................................................28(F) in hypoeutectoid steel .......................................28(F), 32, 33(F), 35(F) in iron-carbon binary alloy......................................................33, 34(F) lamellar structure............................................................................34(F) in malleable iron ............................................................................46(F) in manganese sulfide particles.................................................69(F), 72 in microalloyed steels.....................................................8(F), 57(F), 60 in mottled cast iron.............................................................18(F), 45(F) in normalized plain carbon steel ...................................................51(F) in plain carbon steels ............................................4(F), 5(F), 49, 50(F) in plasma-arc-cut steel surface.................................................77-78(F) properties.............................................................................................27 in rail steel microstructure...............................................................6(F) resulting during slow cooling from eutectoid reaction......................27 in spheroidized microstructure ................................................61(F), 62 in steel microstructure (1080) ............................................23(F), 24(F) in structural steel microstructure .....................................................6(F) transformation in commercial cast irons............................................47 volume fraction measurement ................................................99-100(F) in white cast iron................18(F), 43(F), 44(F), 45(F), 117(F), 118(F) Pearlite colony ..............................................................34(F), 35, 99-100 definition............................................................................................280 ferrite tinted dark by Beraha’s reagent ...............................235, 236(F) Pearlite finish (Pf) temperature .........................................31, 32(F), 281 Pearlite nodules ................................................................................35(F)

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344 / Metallographer’s Guide Pearlite nodules (Continued) definition............................................................................................280 Pearlite spacing. See Interlamellar spacing. Pearlite start temperature (Ps) ........................................31, 32(F), 283 Pearlitic malleable. See Malleable cast iron. Pearlitic structure definition............................................................................................280 etchants for...................................................................................301(T) Pebbles. See Orange peel. Peeling, definition.................................................................................280 Penetration, definition .........................................................................280 Perchloric acid.....................................................................................107 mixtures.............................................................................................107 Peritectic, definition .............................................................................280 Peritectic equilibrium, definition .......................................................280 Peritectoid, definition...........................................................................281 Peritectoid equilibrium, definition .....................................................281 Permalloy ..........................................................................................13-14 Permanent set, definition ....................................................................281 Permendur .............................................................................................14 Personalization procedure for microscope viewing ........................144 Petrographic examination, definition ................................................281 Petrography, definition........................................................................281 pH, definition........................................................................................281 Phase.......................................................................................................24 definition .....................................................................................23, 281 metastable............................................................................................25 stable (equilibrium).............................................................................25 Phase boundaries ............................................................24, 25(F), 26(F) cooling.................................................................................................26 equilibrium ..........................................................................................26 heating .................................................................................................26 Phase contrast illumination, definition..............................................281 Phase contrast objective.....................................................................121 Phase contrast microscopy, definition ...............................................281 Phase diagram...............................................24-25(F), 26, 42, 43(F), 46 definition............................................................................................281 Phase rule, definition ...........................................................................281 Phase transformations ........................................................23, 26(F), 27 kinetics of..................................................................................28-31(F) Phenolic resins ................................................................................188(F) as mounting material ..............................................184(F), 185(F), 187 Phosphating, definition........................................................................281 Phosphatizing. See Phosphating. Phosphoric acid ...................................................................................106 Phosphorus effect on shifting eutectic composition in cast irons.........................46 function as element in steel and cast iron ......................................3(T) as impurity element in cast irons..................................................47-48 Phosphorus segregation .................................................70(F), 72, 73(F) temper embrittlement .....................................................................75(F) Photoeyepiece ...............................................................................125-126 Photographic fixer.................................................................................98 Photographic paper ..............................................................................98 Photomacrograph. See Macrograph. Photometric color filter, definition.....................................................265 Photomicrograph. See Micrograph. Photons .................................................................................................152 Physical etching, definition .................................................................281 Physical metallurgy, definition ...........................................................281 Physical objective aperture, definition ..............................................281 Physical properties, definition ............................................................281 Physical testing, definition ..................................................................281 Pickling, definition ...............................................................................281 Picral.....................................................................................................221 for etching carbon and low-alloy steels, and cast irons......227-231(F) plus hydrochloric acid .......................228-229, 239(F), 302(T), 303(T) Picral (4%), as etchant for carbon steels and alloy steels.....................................................301(T), 302(T), 303(T) Picral 4% + Hydrochloric acid, as etchant for carbon steels and alloy steels ...........................................................302(T), 303(T) Picral (4%)/Nital (2%) solution, for etching carbon and low-alloy steels and cast irons ...................................233(F), 302(T) Picric acid .....................................................................106-107, 224-231

explosiveness.....................................................................................228 saturated aqueous ..............................................................................229 Piearcy and co-workers reagent etchant composition .....................................................................300(T) as etchant for revealing macrostructures in iron and steel ........300(T) Pincushion distortion. See Positive distortion. Pinhole eyepiece, definition.................................................................281 Pinhole porosity, definition .................................................................281 Pinholes, definition...............................................................................281 Pinhole system, definition ...................................................................281 Pipe, ASTM specifications for chromium-molybdenum steel product forms.......................................................................7, 308(T) Pitting, definition..................................................................................281 Plain carbon and alloy steels, UNS designations .................................2 Plain carbon steels......................................................................2, 4-6(F) AISI/SAE classification system....................................................4-5(F) AISI/SAE subclasses ............................................................................5 annealing.............................................................................58(F), 60-61 ASTM specification system..........................................................5-6(F) austenite in microstructure ............................................................39-40 carbon content (%)................................................................................4 free-machining ......................................................................................5 high-manganese.....................................................................................5 martensite .......................................................................................55(F) microstructure .....................................................................23(F), 24(F) normalization ............................................................................50-51(F) rephosphorized .................................................................................5(F) resulfurized.......................................................................................5(F) subclasses ..............................................................................................4 Plan-apochromatic objective .....................................................121, 126 Planar section.............................................................170(F), 171-172(F) Plane crystal, definition...............................................................................281 focal, definition .................................................................................281 Plane glass illuminator .......................................................................130 definition............................................................................................281 Planimetric method. See Jeffries’ method. Planoconvex .........................................................................................124 Plano objectives ...................................................................120-121, 125 Plasma-arc cutting torch....................................................................180 and microstructure alteration....................................................77-79(F) Plastic caps ...................................................................................212-213 Plastic deformation ........................................................................179(F) definition............................................................................................282 Plastic deformation zones from grinding............198(F), 202, 207-208 Plastic flow. See Plastic deformation. Plasticine ..............................................................................................148 Plasticity, definition .............................................................................282 Plastic replica. See also Replica. definition............................................................................................282 Plastic smearing, definition.................................................................282 Plastic strain, definition.......................................................................282 Plate ASTM specifications for chromium-molybdenum steel product forms...................................................................7, 308(T) definition............................................................................................282 weld failure in steel ..................................................................94-95(F) Plate martensite ...................................................................36, 40(F), 54 in carbon-iron alloys ....................................................................223(F) in carbonitrided steel ....................................................68(F), 69(F), 71 in carbon-nickel steel............................................................223-224(F) definition............................................................................................282 in lower-carbon steels ....................................................................36(F) in manganese sulfide particles.................................................69(F), 72 and microcrack formation ..................................................75(F), 76-77 in micrograph taken with oblique illumination ..........................133(F) midrib morphology ..................................................................36, 37(F) in mounted carbon steel...............................................................184(F) in plasma-arc-cut higher-carbon steel ................................................78 in rail steel..........................................................................79-80, 81(F) steels in which it formed ....................................................................54 in white iron...................................................................................47(F) Platen ....................................................................................................203 Plating. See also Electroplating.

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Index / 345 definition............................................................................................282 Plating range, definition......................................................................282 Plichta etching reagent, composition, metals polished, temperature, and etching remarks ...........................................298(T) Plowing, definition ...............................................................................282 Plumbago, definition............................................................................282 P/M. See Powder metallurgy. Point projection x-ray microscopy, definition ..................................282 Polarization, definition ........................................................................282 Polarization filters...............................................................................109 Polarization interference contrast.....................................................121 Polarization interferometers ..............................................................136 Polarized light illumination.............................121, 133(F), 134(F), 217 definition............................................................................................282 to enhance mounting dye effect .......................................................193 Polarized-light metallography ...........................................................127 Polarizer.....................................................................122, 133(F), 134(F) definition............................................................................................282 Pole, definition......................................................................................282 Pole figure .....................................................................................163-164 definition............................................................................................282 Pole figure (crystalline aggregates), definition .................................282 Polished surface, definition .................................................................282 Polishing................................................................................202-211(F,T) abrasives ............................................................................................204 of aluminized coating sample.............................................................93 of bearing race sample........................................................................95 of cast iron shot sample ................................................................96-97 cloths.............................................................93, 94, 96, 97, 98, 203(T) debris .................................................................................................206 definition............................................................................................282 discs............................................................................................203-204 equipment for metallography laboratory..........................................104 final....................................................................................................203 manual (hand) .......................................................................204-209(F) napped cloth, definition ....................................................................278 rough..................................................................................................203 stainless steel bolt sample ..................................................................98 steel sheet sample ...............................................................................99 of steel wire sample............................................................................94 for two-dimensional view ............................................................210(F) weld failure in steel plate sample .................................................94-95 Polishing artifact, definition ...............................................................282 Polishing coated steel..........................................................................209 Polishing film .......................................................................................210 Polishing rate, definition .....................................................................282 Polishing wheels ..........................................................................104, 105 Polycrystalline, definition....................................................................282 Polyesters, castable, as cause and solution to mounting problems ..................................................................192, 193, 194(T) Polygonal ferrite....................................................................................32 definition............................................................................................282 Polymorphic substance, definition .....................................................282 Polymorphism. See also Allotropy, Enantiotropy, and Monotropism. definition............................................................................................282 Polystyrene ...........................................................................................190 Polyvinyl chloride (PVC) ...................................................................189 Polyvinyl formal ..................................................................................189 Polyvinyl mounting material .............................................................189 Pore depth ............................................................................................112 Pores, definition....................................................................................282 Porosity, definition ...............................................................................282 Portable abrasive saw.........................................................................178 Positive distortion, definition..............................................................282 Positive eyepiece, definition ........................................................263, 282 Positive replica, definition...................................................................282 Postheating, definition .........................................................................282 Pot annealing. See Box annealing. Potassium ferrocyanide ......................................................................106 Potassium hydroxide......................................................................240(T) as etchant for carbon steels and alloy steels ..............................303(T) for etching stainless steels ...........................................................241(F) Potassium metabisulfite .................................................234-235(F), 236

Potentiometer, definition .....................................................................282 Potentiostat, definition.........................................................................282 Potentiostatic etching, definition ........................................................282 Powder, mounting of ...............................................................196-197(F) Powder metallurgy, definition ............................................................282 Powder method, definition ..................................................................283 Precious metal. See Noble metal. Precipitates...........................................................................................152 Precipitation, definition .......................................................................283 Precipitation etching. See also Staining. definition............................................................................................283 Precipitation hardening. See also Age hardening and Aging...... 58-59 definition............................................................................................283 Precipitation-hardening stainless steels...................................12, 14(F) composition ..............................................................12, 309(T), 312(T) lath martensite ..............................................................................238(F) lath martensite etched by Fry’s reagent ......................................239(F) microstructure ..........................................................................12, 14(F) nonstandard, compositions...........................................................311(T) Precipitation heat treatment, definition ............................................283 Precipitation strengthening.............................................................58-59 Precision saw .......................................................................................181 constant feed speed....................................................................181-182 Preferred orientation, definition ........................................................283 Preheating, definition...........................................................................283 Premolds...............................................................................................187 Preshadowed replica. See also Shadowing. definition............................................................................................283 Pressure point in grinding .................................................................200 Pressure sensitive adhesive (PSA).............................199-200, 202, 203 Primary creep, definition ....................................................................283 Primary crystal, definition..................................................................283 Primary electrons ..................................................152(F), 157, 159, 160 Primary extinction, definition.............................................................283 Primary metal, definition....................................................................283 Primary (x-ray), definition..................................................................283 Principal stress (normal), definition ..................................................283 Printing, definition ...............................................................................283 Prior austenite grain boundaries...........................183, 230(F), 231(F), 232(F), 235(F) electrolytic etchants for stainless steels ......................................240(T) etchants for ...........................................................................302-303(T) etchants for carbon steels, low-alloy steels, and cast irons .......222(T) Prior austenitic grain size, definition ................................................283 Prism ...........................................................................133-134(F), 135(F) definition............................................................................................283 Prismatic plane, definition ..................................................................283 Prism reflector..............................................................................130-131 Process annealing, definition ..............................................................283 Process metallurgy, definition ............................................................283 Product specifications ...........................................................................91 Proeutectoid carbide, definition .........................................................283 Proeutectoid cementite ....................................................................28(F) in high-carbon, hypereutectoid steels.................................................33 in iron-carbon binary alloy......................................................33, 34(F) Proeutectoid ferrite......................................27, 28, 32, 33(F), 51, 52(F) definition............................................................................................283 Proeutectoid (phase), definition..........................................................283 Progressive aging. See also Aging. definition............................................................................................283 Projection distance, definition ............................................................283 Projection lens, definition ...................................................................283 Projector lens.......................................................................................125 PSA. See Pressure sensitive adhesive. Pseudobinary system, definition.........................................................283 Pseudocarburizing. See Blank carburizing. Pseudonitriding. See Blank nitriding. Punch ....................................................................................................179 Pure iron ................................................................................................26 chemical polishing, etching reagent............................................298(T) microstructure after Marshall’s reagent etch ..............................226(F) PVC. See Polyvinyl chloride. Pyramidal plane, definition.................................................................283 Pyrometallurgy, definition ..................................................................283

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346 / Metallographer’s Guide Pyromet CTX-1 .....................................................................................12

Q Quadrivariant equilibrium, definition...............................................284 Quality assurance................................................................................102 Quality control, use of image analysis system.................................150 Quantitative metallography, definition .............................................284 Quarter hard, definition......................................................................284 Quartz bulbs ........................................................................................107 Quartz corrected objective.................................................................121 Quartz-halogen lamp ..........................................................................127 Quartz-iodine lamp.............................................................................127 Quartz window of a hot stage ...........................................................116 Quasi-binary system, definition..........................................................284 Quasi-cleavage fracture, definition ....................................................284 Quasi-equilibrium .................................................................................24 Quasi-isotropic. See Isotropic. Quaternary phase diagram..................................................................26 Quaternary system, definition ............................................................284 Quench-age embrittlement, definition ...............................................284 Quench aging, definition .....................................................................284 Quench annealing, definition..............................................................284 Quench cracking, definition................................................................284 Quench hardening, definition .............................................................284 Quenching. See also Direct quenching, Fog quenching, Hot quenching, Interrupted quenching, Selective quenching, Spray quenching, and Time quenching.................................... 54(F) definition............................................................................................284 Quenching crack, definition................................................................284 Quenching medium, definition ...........................................................284 Quench time, definition .......................................................................284

R Radial marks. See also Chevron pattern. definition............................................................................................284 Radiograph, definition.........................................................................284 Radiography, definition.......................................................................284 Railroad rail, metallographic analysis........................................97-98(F) Rail steel friction effects................................................................79-80(F), 81(F) metallographic analysis ............................................................97-98(F) microstructure .....................................................................91(F), 92(F) Ramsden eyepiece..........................................................124, 125(F), 138 Random orientation, definition ..........................................................284 Rare earth (RE) metal, definition ......................................................284 Rare earth oxides, as polishing abrasives ..........................................204 Ratchet marks, definition....................................................................284 Razor blades ..........................................................................................54 RE. See Rare earth metal. Real image..........................................................................112(F), 113(F) Reaustenitization ...................................................................................47 Recarburize. See also Carbon restoration. definition............................................................................................284 Reciprocal lattice, definition ...............................................................284 Recovery....................................................................................57, 225(F) definition............................................................................................284 Recovery annealing..........................................................................56-57 Recrystallization .............................................................................225(F) definition............................................................................................284 precipitates preventing during hot rolling, ................................. 59, 60 Recrystallization annealing.......................................................59(F), 61 definition............................................................................................285 Recrystallization temperature and cold working ................................................................................55 definition............................................................................................285 Recrystallized grain size, definition ...................................................285 Refining. See Process metallurgy. Reflection, x-rays. See Diffraction. Reflection method, definition..............................................................285 Reflection pole figure...................................................................163-164

Reflection x-ray microscopy, definition.............................................285 Reflectivity of light..............................................................................215 Refraction ........................................................................118-119(F), 121 angle of, definition............................................................................285 Refractive index .....................................116, 118-119(F), 121, 134, 141 of air ..................................................................................................114 electrons, definition...........................................................................285 glass in the lens.................................................................................114 of medium .........................................................................................114 of oils.................................................................................................114 Refractory, definition...........................................................................285 Refractory alloy. See also Refractory metal. definition............................................................................................285 Refractory metal, definition................................................................285 Refroidissement .....................................................................................26 Regular reflection. See Specular reflection. Reliability, definition ...........................................................................285 Relief.............................................................................................151, 222 Rephosphorized and resulfurized steels composition ranges and limits.....................................................305(T) UNS designations ........................................................................305(T) Rephosphorized steels........................................................................5(F) Replica. See also Collodian replica, Impression replica, Oxide film replica, Plastic replica, Positive replica, Preshadowed replica, and Vapor-deposited replica. definition............................................................................................285 Replicate, definition .............................................................................285 Residual elements. See also Tramp elements. .................................... 89 definition............................................................................................285 Residual stresses, in plasma-arc-cut steel surface...........................78(F) Resolution ........................................................................114-115(F), 131 definition............................................................................................285 formula ..............................................................................................114 Resolving power. See also Resolution. ... 91, 109, 113, 114-115(F), 152 definition............................................................................................285 of microscope......................................................................................88 Resulfurized steel ...............................................................................5(F) carbonitrided improperly .........................................................67(F), 71 composition ranges and limits.....................................................305(T) definition............................................................................................285 microstructure ......................................................................172, 173(F) UNS designations ........................................................................305(T) Retained austenite ..........................................................36(F), 39, 40(F) amount determined by x-ray diffractometer ....................................164 in carbonitrided steel ....................................................68(F), 69(F), 71 in carbon-nickel steel...................................................................224(F) definition............................................................................................285 detected by x-ray diffraction ..............................................................71 in dual-phase steel.........................................................55, 178, 179(F) etching response...................................................................216(F), 217 as friction effect ..................................................................................80 identified by microhardness tester....................................................165 and microcrack formation .......................................................75(F), 76 in plain carbon and alloy steels ....................................................39-40 tint etchants for carbon steels, low-alloy steels, and cast irons......................................................................................234(T) transformation to martensite.....................................................40-41(F) transformed into martensite by a worn grinding paper ..............201(F) in white irons .................................................................................47(F) Reticle. See also Graticule. ....................... 93, 99-100, 124, 125, 138(F) definition ...................................................................................111, 285 suppliers for ........................................................................................99 in upright microscope .......................................................................111 Reticle scale ............................................................................125, 138(F) calibration of............................................................138(F), 142-143(F) Retractable objectives.........................................................................122 Rhombohedral, definition ...................................................................285 ⴖRight-to-knowⴖ stations, in metallographic laboratory ....................106 Rimmed steel, definition......................................................................285 Riser, definition ....................................................................................285 River pattern, definition......................................................................285 Riveting, definition...............................................................................285 Roberts-Austen, Sir William Chandler ..............................................27 Rock candy fracture, definition .........................................................285

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Index / 347 Rockwell hardness ................................................................................96 Rockwell hardness tester....................................................................106 Rolled product........................................................................170(F), 171 Rolling direction (RD)...........................................................170(F), 171 Rosebuds, definition.............................................................................285 Rosette, definition.................................................................................285 Rosette graphite, definition ........................................................267, 285 Rough grinding, definition..................................................................285 Roughness, definition...........................................................................285 Rough-polishing process, definition ...................................................285 Rowland’s reagent etchants for coated steels, zinc-based coatings...........................304(T) for etching coated steels ...................................................................241 Rust, definition .....................................................................................285 Rusting..................................................................................173-174, 176 on fractured surface using liquid nitrogen.......................................183 Rust inhibitor ......................................................................................176 Rutherford backscattering spectroscopy (RBS)..............................168

S Sacrificial protection, definition .........................................................285 SAE. See Society of Automotive Engineers. Safety cabinet ......................................................................................107 Safety issues aqua regia mixing ......................................................................237-238 colloidal silica as grinding particles.................................................207 during grinding..................................................................................202 glyceregia mixing..............................................................................238 for hydrofluoric acid use ..................................................................227 in metallography laboratory ......................................................106-107 for mixing etching reagents..............................................................220 for mixing hydrochloric acid............................................................240 for mixing nitric acid........................................................................240 for mixing potassium hydroxide ......................................................241 of mounting techniques ....................................................................193 picric acid..........................................................................................228 for sectioning procedure ...................................................................183 Sapphire indenters ..............................................................................166 Saturated aqueous picric acid, as etchant for carbon steels and alloy steels................................................................................303(T) Saturated gun, definition.....................................................................286 Sauveur, Albert......................................................................................24 Scale, definition ....................................................................................286 Scale pit, definition ..............................................................................286 Scaling, definition.................................................................................286 Scanning electron microscope (SEM)............................89, 91-92, 137, 149, 152, 154, 155-158(F) carbon contamination problem .........................................................192 for compositional analysis ................................................................170 definition............................................................................................286 electrically conductive mounts .........................................................188 to examine dendrites of manganese sulfides .....................69(F), 71-72 to examine fracture surfaces.............................................................174 in full-scale metallographic laboratory ............................................105 operated by metallographer at research laboratory............................92 Scanning electron microscope analysis .........................................91-92 Scanning transmission electron microscope (STEM)..............89, 152, 154, 161-162(F) in full-scale metallographic laboratory ............................................105 to identify composition of precipitate particles .................................59 Scanning transmission electron microscopy (STEM), definition .......................................................................286 SCC. See Stress-corrosion cracking. Scientific imaging products................................................................297 Scintillation counter............................................................................162 Scoring, definition ................................................................................286 Scratch, definition ................................................................................286 Scratching. See also Plowing. definition............................................................................................286 Screw dislocation. See Dislocation. Scribe...........................................................................137-138(F), 160(F) device...............................................................137-138(F), 147(F), 160

of electron probe microanalyzed specimens ....................................219 handheld vibrating electric ........................................................197-198 Scribe marks...........................................................................147(F), 148 Scribing procedure ...................................................138(F), 147(F), 148 Seam, definition....................................................................................286 Secondary creep. See Creep. Secondary electrons ..........................................155-156(F), 157(F), 159 Secondary etching, definition .............................................................286 Secondary extinction, definition .........................................................286 Secondary hardening ............................................................................58 definition............................................................................................286 Secondary ion mass spectroscopy (SIMS) .......................................168 Secondary metal, definition ................................................................286 Secondary x-rays, definition ...............................................................286 Second-stage malleabilization ..............................................................64 Sectioning..............................................................................170-183(F,T) of aluminized coating sample.............................................................92 of bearing race sample........................................................................95 of forged steel sample ........................................................................97 of railroad rail sample ........................................................................97 stainless steel bolt sample ..................................................................98 steel sheet sample ...............................................................................99 of steel wire sample............................................................................93 techniques ..........................................................................174-183(F,T) weld failure in steel plate sample ......................................................94 Seepage ................................................................................187, 189, 193 Segregation ......................................................................70(F), 72, 73(F) definition............................................................................................286 Segregation banding, definition..........................................................286 Segregation (coring) etching, definition ............................................286 Selective corrosion ..............................................................................215 Selective etching...........................................................................216-217 Selective leaching ..................................................................................85 Selective oxidation ................................................................67(F), 69-71 Selective quenching, definition ...........................................................286 Self-diffusion, definition.......................................................................286 Self-hardening steel. See Air-hardening steel. Self-tempering. See Autotempering. SEM. See Scanning electron microscope. Semiapochromatic (fluorite) objective..............................................120 Semiaustenitic stainless steels, precipitation-hardening, compositions ............................................................................312(T) Sensitization................................................................................49, 74(F) annealing to help salvage steel...........................................................79 definition............................................................................................286 heat treatment to eliminate .................................................................74 machined stainless steel.................................................................79(F) in stainless steels, etchants for ....................................................303(T) Serial sectioning .....................................................................171(F), 172 definition............................................................................................286 Serious defects, definition ...................................................................257 SG. See Spheroidal graphite cast iron. Shadow angle. See also Shadowing. definition............................................................................................286 Shadow cast replica. See also Shadowing. definition............................................................................................286 Shadowing, definition ..........................................................................286 Shadow microscope, definition ...........................................................286 Shales, definition ..................................................................................286 Shape resolution, definition ................................................................286 Shatter cracks. See Flakes. Shear.............................................................................................105, 199 definition............................................................................................286 Shear angle, definition.........................................................................286 Shear bands, definition........................................................................287 Shear burr ......................................................................178, 179(F), 196 Shear fracture, definition ....................................................................287 Shearing strain. See Strain. Shear ledges. See Radial marks. Shear lip, definition .............................................................................287 Shear strain. See also Strain. definition............................................................................................287 Shear strength, definition....................................................................287 Shear stress. See Stress.

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348 / Metallographer’s Guide Sheet definition............................................................................................287 mounting of ......................................................................... 194-196(F) steel, metallographic analysis.................................................99-100(F) Shelf life, of chemicals.........................................................................107 Shell hardening. See Surface hardening. Shelling, definition ...............................................................................287 Shielding, as safety issue..............................................................106-107 Shock-resisting steels, AISI code classification ...................................12 Shortness, definition.............................................................................287 Short transverse. See Transverse. Shot, definition .....................................................................................287 Shotblasting, definition........................................................................287 Shot peening, definition.......................................................................287 Shrinkage cavity, definition ................................................................287 Shrinkage cracks, definition ...............................................................287 Sieve analysis, definition .....................................................................287 Sieve classification. See Sieve analysis. Sieve fraction, definition .....................................................................287 Sieves ....................................................................................................199 Sigma, definition...................................................................................287 Sigma phase ...............................................236, 237, 238, 239, 240, 241 definition............................................................................................287 electrolytic etchants for stainless steels ......................................240(T) etchants for stainless steels..........................................................237(T) Significant magnification ....................................................................115 Silica...........................................................94-97, 99, 107, 204, 207, 208 Silica gel desiccant ..............................................................................212 Silica polishing compound .........................................................158, 161 Silicate-type inclusions, definition......................................................287 Silicon .....................................................................................................93 effect on cast iron solidification .........................................................46 effect on shifting eutectic composition in cast irons.........................46 function as element in steel and cast iron ......................................3(T) as graphitizer.................................................................................16, 46 as graphitizer in gray iron .............................................................44(F) Silicon-aluminum killed steel .........................................................89-90 Silicon carbide in abrasive cutoff wheel....................................................................177 grinding discs ....................................................................................199 grinding paper .....................................................93, 94, 95, 96, 98, 99 particles embedded in nickel-plated sheet ..................................170(F) Silicon dioxide. See also Silica. colloidal slurry...............................................94, 95, 99, 207, 208, 211 paper..............................................................................................96, 97 particles (colloidal silica), as polishing abrasives ...........................204 Siliconizing, definition .........................................................................287 Silicon oxide (colloidal).......................................................................211 Silky fracture, definition .....................................................................287 Silver sulfide reaction ...........................................................................98 Sink, in metallography laboratory ...............................................104, 105 60/40, as etchant for carbon steels and alloy steels .......................303(T) Skid-polishing process, definition.......................................................287 Skin, definition .....................................................................................287 Skin lamination, definition..................................................................287 Skin pass. See Temper rolling. Slack quenching, definition.................................................................287 Slag, definition......................................................................................287 Slag inclusion definition............................................................................................287 in stainless steel weld ..................................................................243(F) in wrought iron .....................................................................218-219(F) Slip, definition.......................................................................................287 Slip band. See also Slip line. definition............................................................................................287 Slip direction, definition ......................................................................287 Slip-interference theory, definition ....................................................287 Slip line.................................................................................................165 definition ....................................................................................287-288 Slippage ..................................................................................................79 Slip plane, definition............................................................................288 Sliver, definition ...................................................................................288 Slow cooling ..............................................................................36, 37, 38 to prevent hydrogen damage ..............................................................80

Smoking .......................................................................................107, 142 Smut, definition ....................................................................................288 Snaky edges. See Carbon edges. Snap temper, definition .......................................................................288 Soak cleaning, definition .....................................................................288 Soaking, definition ...............................................................................288 Society of Automotive Engineers (SAE) carbon level designations......................................................................8 chemical composition designations ......................................................8 deoxidation practice designations.........................................................8 high-strength, low-alloy (HSLA) steel classification system ..............8 Society of Automotive Engineers (SAE) specifications, specific types J431, composition and property requirements ...................................17 J431, gray cast iron grades for automotive applications...................17 J434, ductile cast iron automotive castings hardness..............100, 102 Sodium hydroxide, as etchant for carbon steels and alloy steels.........................................................................................303(T) Sodium metabisulfite.........................................216, 217(F), 234-235(F) as etchant for carbon steels and alloy steels ..............................302(T) Sodium picrate...............................................................216(F), 217, 220 boiling alkaline.....................................................187, 230-231, 232(F) temperature for use....................................................................220-221 Sodium thiosulfate..............................................................220, 234, 236 Sodium tridecylbenzene sulfanate wetting agent ............229-230, 231 Soft edge...............................................................................................208 Soft temper. See Dead soft temper. Soft zone...............................................................................................208 Solidification..........................................................................25, 41-42(F) of cast irons.........................................................................................47 definition............................................................................................288 iron-cementite phase diagram........................................................43(F) of white irons ......................................................................................47 Solidification range, definition............................................................288 Solidification shrinkage crack, definition..........................................288 Solidification structure, cast structure, definition............................288 Solid solution .........................................................................................27 definition............................................................................................288 Solid-state transformation, of cast irons .............................................47 Solidus. See also Liquidus. definition............................................................................................288 Soluble oil, definition ...........................................................................288 Solute, definition...................................................................................288 Solution, definition ...............................................................................288 Solution heat treatment, definition ....................................................288 Solvent, definition.................................................................................288 Solvus, definition ..................................................................................288 Sorbite ....................................................................................................89 definition............................................................................................288 Sorby, Henry Clifton..........................................................87-89(F), 109 Source, x-rays, definition ....................................................................288 Space-charge aberration, definition...................................................288 Space lattice, definition .......................................................................288 Spacing, lattice planes. See Interplanar distance. Spalling, definition ...............................................................................288 Spangle ............................................................................................242(F) definition............................................................................................288 Spatial grain size, definition ...............................................................288 Specialized specimen holders.............................................................164 Specimen, definition.............................................................................288 Specimen chamber ..............................................................................158 electron optics, definition .................................................................288 Specimen charge, electron optics, definition.....................................288 Specimen contamination, electron optics, definition .......................288 Specimen distortion, electron optics, definition ...............................288 Specimen grid. See Specimen screen. Specimen holder, electron optics, definition .....................................288 Specimen identification .......................................................196, 197-198 Specimen leveling device............................................................138, 140 Specimen orientation .............................................................197, 198(F) Specimen preparation .........................................................169-213(F,T) Specimen screen, electron optics, definition .....................................288 Specimen stage, definition...................................................................288 Specimen strain, definition .................................................................288

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Index / 349 Specular reflection, definition.............................................................289 Spherical aberration......................................................109, 119(F), 120 definition............................................................................................289 Spherical projection, definition ..........................................................289 Spheroidal graphite, definition...........................................................289 Spheroidal graphite (SG) cast iron. See also Ductile iron.............. 16, 19-20(F) graphite flakes ................................................................................19(F) Spheroidite, definition .........................................................................289 Spheroidization .........................................................28, 75(F), 76(F), 77 Spheroidized steel..................................................................................33 Spheroidized structure, definition......................................................289 Spheroidizing....................................................................60(F), 61-62(F) definition............................................................................................289 Spheroidizing annealing, definition ...................................................289 Spheroidizing methods, most frequently used...................................289 Spheroids..............................................................................................100 Spherulitic graphite cast iron, definition ..........................................289 Spiegeleisen, spiegel, definition...........................................................289 Spinning, definition..............................................................................289 Spinodal curve, definition ...................................................................289 Spinodal decomposition, definition ....................................................289 Spinodal structure, definition .............................................................289 Sponge iron, definition ........................................................................289 Spray quenching, definition................................................................289 Sputtering ....................................................................................155, 158 definition............................................................................................289 device ................................................................................................160 SSC. See Sulfide stress cracking. Stabilizing treatment, definition.........................................................289 Stage.........................................................................................110, 111(F) definition............................................................................................289 Stage micrometer...........................................................125, 138(F), 143 Staining, definition ...............................................................................289 Stainless steels.........................................................9-12(F), 13(F), 14(F) AISI classification system ...............................................................9-10 austenite in microstructure .......................................................39-40(F) austenitic ...................................................................................10-11(F) bolt fracture, metallographic analysis ......................................98-99(F) classification ..........................................................................................1 cold working ..................................................................................57(F) compositions ...................................................................309(T), 312(T) definition............................................................................................289 delta ferrite ...................................................................................141(F) duplex.......................................................................................12, 14(F) etchants for ...........................................................236-241(F,T), 303(T) etchants for austenite microstructures.........................................303(T) etchants for delta ferrite microstructures ....................................303(T) etchants for ferrite microstructures .............................................303(T) ferritic ..................................................................................11(F), 12(F) high-wearability epoxy mounts for edge retention..............192-193(F) martensitic ...............................................................11-12, 13(F), 14(F) precipitation-hardening......................................................12, 14(F), 58 sensitization............................................................74(F), 79(F), 239(F) Stainless steels, specific types S30100, electrolytic etching ................................................239, 240(F) S30300, bolt fracture, metallographic analysis........................98-99(F) S30900, etching response of hot-cracked weld ..........................220(F) S31600, deformation twins seen using differential interference contrast.....................................................135, 136(F) S31600, delta ferrite stringers shown by electrolytic etching........240, 241(F) S31600, electrolytic etching showing grain boundaries .............240(F) S31600, grain and twin boundaries.............................................239(F) S31600, leveling procedure for large specimen ..................140-141(F) S31600, machining causing deformation bands and annealing twins ................................................................78, 79(F) S31600, sensitization .....................................................................74(F) S31603, powder mounted in castable epoxy mount ...........196-197(F) S41000, martensite in microstructure .....................................36, 37(F) S41000, microstructure ..................................................................14(F) S41000, for precision saw blade ......................................................181 S43020, microstructure............................................................11, 12(F) 2205, delta ferrite and ferrite electrolytically etched .................241(F)

2205, microstructure etched with Lichtenegger/Bloech reagent ..................................................................................237(F) Custom 630, lath martensite...........................................238(F), 239(F) 7MoPlus, microstructure..........................................................12, 14(F) Stamping, definition.............................................................................289 Standard grain-size micrograph, definition......................................289 State of strain, definition.....................................................................289 State of stress, definition .....................................................................289 Static fatigue. See also Hydrogen-induced delayed cracking. definition............................................................................................289 Stead, John Edward .............................................................................24 Steadite...........................................................................47(F), 48, 115(F) definition............................................................................................290 Stead’s brittleness, definition..............................................................290 Steady-rate creep. See Creep. Steel forging, metallographic analysis .............................................97(F) Steel plate, weld failure...............................................................94-95(F) Steels alloying elements.........................................................................1, 3(T) applications ..................................................................................1, 2(F) bake-hardenable ....................................................................................9 carbon content range...........................................................................25 classification..................................................................................1-2(F) commercial names .......................................................................1, 2(F) compositional variations, family relationships...............................................10(F), 11(F), 12, 13(F) decarburization of ...............................................................................67 definition .......................................................................................1, 290 drawing quality, special killed..............................................................9 electrical ....................................................................................15-16(F) electrical-resistance .............................................................................16 enhanced hardenability, UNS designation ...........................................2 etchants for revealing macrostructures........................................300(T) ferrite and pearlite microstructures from carbon content ..................27 graphitization ...........................................................75(F), 76(F), 77(F) heat-resistant ............................................................................12, 14(F) high-alloy ....................................................................................9-16(F) high-permeability...........................................................................13-14 high-thermal-expansion.......................................................................15 hydrogen damage ................................................................................80 interstitial-free .......................................................................................9 low-thermal-expansion..............................................................14-15(F) magnetic.........................................................................................13-14 microalloyed..........................................................................................8 microstructural constituents......................................................31-41(F) microstructure ..............................................................................1, 2(F) nonmagnetic ........................................................................................14 special-purpose alloy ................................................................13-16(F) ultrahigh-strength (maraging)..................................................13, 15(F) wear-resistant ......................................................................12-13, 15(F) weathering .............................................................................................9 Steels, series and classes 1008, mechanical mounting of ....................................................186(F) 1008, nickel-plated sheet with silicon carbide embedded particles ................................................................................170(F) 1010, hot-rolled sheet, metallographic analysis ....................99-100(F) 1010, intercritically annealed.........................................................55(F) 1010, microstructure ........................................................................4(F) 1010, polishing with comet tails created ....................................205(F) 1015, hot shortness involving iron sulfides ............................73(F), 74 1018, differential interference contrast revealing cold work .............................................................................242, 243(F) 1018, ferrite plus pearlite microstructure, 4% picral/2% nital etch...............................................................................233(F) 1020, aluminum oxide inclusions .......................................172, 174(F) 1020, as-cast microstructure in annealed condition......................42(F) 1020, banded hot-rolled plate microstructure .............................172(F) 1020, carbides and lath martensite in etching response .............233(F) 1020, homogenization ........................................................51-52, 53(F) 1020, lath martensite visible after nital etch ......................222(F), 223 1020, manganese sulfide inclusions and hot shortness ..........73, 74(F) 1020, microstructure, by variously set aperture diaphragms......128(F) 1020, microstructure constituent information from image analysis system..........................................................................150

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350 / Metallographer’s Guide Steels, series and classes (Continued) 1020, plasma-arc-cut surface and microstructure alteration ...77-78(F) 1020, polishing for a two-dimensional view ..............................210(F) 1025, microstructure ................................................................32, 33(F) 1025, oxyacetylene torch cutting ...................................180(F), 181(F) 1035, hot shortness ..................................................................72, 73(F) 1040, aluminum killed.............................................................51, 52(F) 1040, annealed microstructure.......................................................58(F) 1040, annealing........................................................................58(F), 61 1040, composition.................................................................................4 1040, designation significance .....................................................1, 2, 4 1040, hot rolling .................................................................................50 1040, machining..................................................................................91 1040, mechanical properties ...................................................89, 90-91 1040, microstructural hemispherical depression measurement ...........................................................146(F), 147(F) 1040, microstructure .....................................4(F), 28, 35(F), 49, 50(F) 1040, microstructure digitally recorded by image analyzer ...................................................................149(F), 150(F) 1040, normalized microstructure ...................................................51(F) 1040, normalizing temperature range.................................................51 1040, phase transformations.........................................................27, 28 1045, decarburized steel microstructure ............................66(F), 67-68 1045, internal oxidation of microstructure ........................67(F), 69-71 1045, oxide scale penetration ......................................................156(F) 1060, as-cast microstructure ..........................................................41(F) 1060, microstructure.....................................................32, 33(F), 35(F) 1070, metallographic analysis of defective steel wire................................................................93-94(F), 95(F) 1080, chemical analysis......................................................................90 1080, continuous cooling transformation diagram .................31, 32(F) 1080, decarburization.............................................66(F), 67, 83, 84(F) 1080, edge retention on polished surface ........................................209 1080, etching response and manganese sulfide inclusions .....................................................................217, 218(F) 1080, hydrogen damage ..........................................................82, 83(F) 1080, hydrogen flakes..............................................................83, 84(F) 1080, isothermal transformation diagram ...............29(F), 30(F), 31(F) 1080, microcrack formation...............................................75(F), 76-77 1080, micrograph taken with oil-immersion objective...............122(F) 1080, microstructure ..............23(F), 24(F), 34(F), 35(F), 91(F), 92(F) 1080, microstructure of planar polished layers of plate.............171(F) 1080, pearlite boundaries in microstructure .......................227, 228(F) 1080, polishing and surface microstructure ................................206(F) 1080, serial sectioning.........................................................171(F), 172 1080, surface replica showing boundary between pearlite colonies.................................................................................153(F) 1080, titanium nitride inclusions.........................................172, 174(F) 1080, transmission electron micrograph of thin foil ..................153(F) 1095, graphitization .................................................................75(F), 77 1095, microstructure ........................................................................4(F) 1095, plate martensite in microstructure.......................................36(F) 1095, spheroidized and cold-drawn ........................................75(F), 77 1117, carbonitrided steel, improperly done, microstructure............................................67(F), 68(F), 69(F), 71 1140, composition.................................................................................5 1144, manganese sulfide inclusions in resulfurized steel bar.................................................................................172, 173(F) 1213, banding and manganese sulfide inclusions...............233, 234(F) 1213, composition.................................................................................5 1213, microstructure ........................................................................5(F) 1335, microhardness .............................................................160-161(F) 1524, banding after microsegregation ...........................................42(F) 4327, electric discharge machining.....................................181, 182(F) 4340, carburized microstructure..............................................62, 63(F) 4340, forging, metallographic analysis .........................................97(F) 4340, micrograph taken in bright-field illumination and dark-field illumination ..................................................131-132(F) 8630, homogenization..............................................................52, 53(F) 8630, quenched and tempered microstructure ..............................62(F) 8630, quenched microstructure......................................................54(F) 8630, spheroidized microstructure .....................................60(F), 61(F) 8720, microstructure ................................................................38, 39(F) 8860, tempered and untempered martensite ...............................235(F)

9425, etching response.................................................................221(F) 52100, bearing race, metallographic analysis..........................95-96(F) 0.75%C-3.25%Cr, microstructure................................................126(F) 0.2%C-iron, lath martensite.........................................................223(F) 0.4%C-iron, lath martensite and plate martensite.......................223(F) 0.6%C-iron, lath martensite and plate martensite.......................223(F) 0.23%C, 3.4%Ni, 1.7%Cr, 0.5%Mo steel, microstructure showing etching response............................................230, 231(F) 0.27%C, 1.0%Mn, 1.02%Cr, 0.27%Mo steel, prior austenite grain boundaries...................................................................232(F) 0.93%C, 14.5%Ni steel, plate martensite ...........................223, 224(F) 21⁄4 Cr-1Mo, ASTM specifications by product form ...........................7 19%Cr-9%Ni heat-resistant HF steel, microstructure showing lamellar carbide colonies, light source centering ...............145(F) 25%Cr-12%Ni heat-resistant HH steel, microstructure with grain-boundary carbides ..............................................129(F), 130 0.5%Mo-B steel, etching response of darkened cementite ......................................................................231, 232(F) AISI DF 090T, microstructure ....................................................8, 9(F) Fe-1.75%C binary alloy, plate martensite in micrograph taken with oblique illumination.....................................................133(F) Fe-0.4%C alloy, etching response, spheroidized ................215, 216(F) HSLA 80, yield strength.......................................................................8 HSLA 100, yield strength.....................................................................8 HY-80, ASTM designation (A 543) .....................................................7 MIL-S-23194, prior austenite grain boundaries in F-steel forging ..................................................................................232(F) SA 213 T-23steel, etching response ............................................230(F) 10B36, boron-treated Jominy specimen microstructure .............235(F) 11L44, manganese sulfide with lead inclusions..........................219(F) Steel scrap ..............................................................................................89 Steel sheet, metallographic analysis..........................................99-100(F) STEM. See Scanning transmission electron microscope (microscopy). Step aging. See also Aging. definition............................................................................................290 Stereo angle, definition ........................................................................290 Stereomicroscope, to examine fracture surfaces ................................174 Stereoradiography, definition .............................................................290 Stereoscopic micrographs, definition.................................................290 Stereoscopic specimen holder, definition ..........................................290 Sticker breaks, definition ....................................................................290 Stiffness, definition ...............................................................................290 Stopping off, definition ........................................................................290 Storage, of specimen .......................................................170, 211-212(F) Strain. See also State of strain. definition............................................................................................290 Strain-age embrittlement, definition..................................................290 Strain aging. See also Aging......................................................... 60, 61 definition............................................................................................290 Strain energy, definition......................................................................290 Strain etching, definition.....................................................................290 Strain-free lens ....................................................................................121 Strain-free objective............................................................................121 Strain hardening ...................................................................................54 definition............................................................................................290 Strain-hardening exponent, definition...............................................290 Strain-induced transformation ............................................................55 Strain marking, definition...................................................................290 Strain rate, definition ..........................................................................290 Strain-rate sensitivity, definition........................................................290 Strain state. See State of strain. Stray-current corrosion, definition ....................................................290 Stress, definition............................................................................290-291 Stress concentration (K1), definition .................................................291 Stress-corrosion cracking (SCC). See also Corrosion. definition............................................................................................291 stainless steel bolt sample .......................................................98, 99(F) Stress raiser. See Stress concentration. Stress-relief annealed steels .................................................................56 Stress-relief annealing definition............................................................................................291 for plasma-arc-cut steel surfaces ........................................................78 Stress relieving.......................................................................................56

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Index / 351 definition .......................................................................................... 291 Stress-rupture strength. See Creep-rupture strength. Stress-strain curve. See Stress-strain diagram. Stress-strain diagram, definition ........................................................291 Stretcher strains. See also Lüders lines. definition............................................................................................291 Striation, definition ..............................................................................291 Stringer.................................................................................................172 definition............................................................................................291 Strip, definition.....................................................................................291 Stripping, definition .............................................................................291 Structure, definition.............................................................................291 Sub-boundary structure. See Grain boundary. Sub-boundary structure, subgrain structure, definition .................291 Subcooling process ................................................................................40 Subcritical annealing, definition.........................................................291 Subgrain, definition .............................................................................291 Subgrain boundaries ..................................................................152, 153 Submicroscopic, definition ..................................................................291 Submission form .........................................................................105, 106 Subsieve analysis, definition ...............................................................291 Subsieve fraction, definition ...............................................................291 Substitutional elements ............................................................27, 51, 52 definition............................................................................................291 Substitutional solid solution.................................................................27 definition............................................................................................291 Substrate, definition.............................................................................291 Substructure. See Sub-boundary structure and Subgrain structure. Subsurface corrosion, definition.........................................................291 Sulfidation, definition...........................................................................291 Sulfide spheroidization, definition......................................................291 Sulfide stress cracking (SCC), definition ..........................................291 Sulfide-type inclusions, definition.......................................................291 Sulfur content effect in plain carbon steels.....................................................5 function as element in steel and cast iron ......................................3(T) as impurity element in cast irons..................................................47-48 WDS x-ray map in nonmetallic inclusion ..................................159(F) Sulfuric acid .......................................................................97(F), 98, 193 Sulfur print ............................................................................................98 definition............................................................................................291 Sulfur printing ............................................................................97-98(F) Sulfur segregation .................................................................................72 Superalloy. See Heat-resisting alloy. Supercooling, definition.......................................................................291 Superfines, definition ...........................................................................291 Superheating, definition ......................................................................291 Super Invar, composition......................................................................14 Superlattice, definition.........................................................................292 Superplasticity, definition ...................................................................292 Surface finish. See also Roughness. definition............................................................................................292 Surface hardening. See also Flame hardening and Shell hardening. definition............................................................................................292 Surface relief .......................................................................219, 221, 222 Surface replicas ........................................152, 153(F), 154, 155(F), 157 Surface rounding ................................................................199, 205, 208 Swabbing, definition ............................................................................292 Swarf, definition ...................................................................................292 Syntectic, definition..............................................................................292 Syntectic equilibrium, definition ........................................................292 System (crystal). See Crystal system.

T Target, x-ray, definition.......................................................................292 Tarnish, definition ................................................................................292 TEM. See Transmission electron microscope. Temper, definition ................................................................................292 Temperature conversions.......................................................316-317(T) Temper brittleness, definition .............................................................292

Temper carbon. See also Nodular graphite.................. 18, 64(F), 65(F) definition............................................................................................292 in malleable iron .................................................................18(F), 46(F) Temper color, definition ......................................................................292 Tempered layer, definition ..................................................................292 Tempered martensite ..........................................37-38(F), 47, 80-81(F), 176-177(F), 230(F), 235(F) definition............................................................................................292 observed by electron microscope .......................................................38 properties.............................................................................................54 Temper embrittlement................................................................74-76(F) Temper graphite (TG) iron..................................................................18 Tempering. See Temper. Tempering ............................................................36-38(F), 40, 54, 62(F) of low-alloy steels...............................................................................58 of mounted material.....................................................................184(F) Temper rolling, definition ...................................................................292 Temper time, definition .......................................................................292 Tensile strength. See also Ultimate strength. definition............................................................................................292 Tensile stress, definition.......................................................................292 Tensile testing. See Tension testing. Tension testing, definition ...................................................................292 Terminal phase, definition ..................................................................292 Terminal solid solution, definition .....................................................292 Ternary phase diagram ........................................................................26 Ternary system, definition ..................................................................292 Terne alloy, definition..........................................................................293 Tertiary creep. See Creep. Test blocks ...............................................................................100-103(F) Tetragonal, definition...........................................................................293 Texture. See Preferred orientation. definition............................................................................................293 Thermal analysis, definition ...............................................................293 Thermal arrest, definition...................................................................293 Thermal-compression mounting material........................................187 Thermal etching, definition.................................................................293 Thermal fatigue, definition .................................................................293 Thermal grooving................................................................................165 Thermal shock.....................................................................................187 definition............................................................................................293 Thermal spraying, definition ..............................................................293 Thermal stresses, definition ................................................................293 Thermionic cathode gun, definition...................................................293 Thermionic emission, definition .........................................................293 Thermomechanical working, definition.............................................293 Thermoplastics, as mounting materials ......................187, 189-198(F,T) Thermosetting epoxy....................................184(F), 185(F), 188-189(F) as mounting material........................................................188, 192, 209 Thermosetting phenolic resins......................................................188(F) Thermosetting resins, as mounting materials .................184(F), 185(F), 187-189(F), 190(T) Thin foil.............................152, 153(F), 154-155(F), 157, 161, 162, 164 definition............................................................................................293 35 mm camera.........................................................................139-140(F) Three-quarters hard, definition .........................................................293 Through-hardened bearing steels, compositions.........................307(T) Throwing power, definition ................................................................293 Tie line, definition ................................................................................293 Tiger stripes, definition .......................................................................293 Tilt boundary, definition .....................................................................293 Time quenching, definition .................................................................293 Time-temperature curve, definition...................................................293 Time-temperature-transformation (TTT) diagram. See Isothermal transformation (IT) diagram. Tin, function as element in steel and cast iron ..................................3(T) Tin plate, definition .............................................................................293 Tint etch ..................................................................................133, 134(F) Tint etchants for carbon and low-alloy steels and cast irons.................233-236(F,T) for stainless steels .............................................................................236 Tint etching ....................................................................215, 216(F), 217 definition............................................................................................293 Tinting. See Heat tinting.

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352 / Metallographer’s Guide Titanium as anisotropic material ......................................................................133 function as element in steel and cast iron ......................................3(T) Titanium carbides .................................................................................58 Titanium-molybdenum carbides extraction replica in high-strength, low-alloy steel ............155, 156(F) in high-strength, low-alloy steel..................................................161(F) Titanium nitrides ..................................................................................58 color...................................................................................................218 inclusion in tool steel microstructure ..........................................147(F) inclusions .............................................................................172, 174(F) Tool steels....................................................................................12, 15(F) AISI classification system ..................................................................12 alloying elements ................................................................................12 carbides in microstructure, nital etched ......................................224(F) classes of steels...................................................................................12 composition limits ..........................................................313(T), 314(T) definition............................................................................................293 etching response of microstructure .....................................228, 229(F) etching response with Vilella’s reagent ..............................230, 231(F) eutectic carbides in microstructure .....................................230, 231(F) micrograph taken in bright-field and differential phase contrast .........................................................................134, 135(F) microstructure ..........................................................................12, 15(F) titanium nitride inclusion in microstructure................................147(F) UNS designation ...................................................................................2 Torch cutting .....................................................................180(F), 181(F) Total carbon, definition .......................................................................293 Toughness, definition ...........................................................................293 Toxic waste disposal............................................................................107 Tramp elements, definition .................................................................293 Transcrystalline. See Intracrystalline and Transgranular. Transcrystalline cracking. See also Transgranular cracking. definition............................................................................................293 Transcrystalline fracture. See Transgranular fracture. Transference, definition.......................................................................293 Transference number, definition ........................................................293 Transformation-induced plasticity, definition ..................................293 Transformation ranges. See also Transformation temperature. definition............................................................................................294 Transformation temperature, definition............................................294 Transgranular, definition ....................................................................294 Transgranular cracking, definition ....................................................294 Transgranular fracture, definition .....................................................294 Transition lattice, definition................................................................294 Transition metal, definition.................................................................294 Transition phase, definition ................................................................294 Transition point, definition .................................................................294 Transition structure, definition ..........................................................294 Transition temperature (ductile-brittle transition temperature), definition...............................................................294 Transmission electron microscope (TEM) .....89, 152-155(F), 163-164 definition............................................................................................294 distinguishing bainite from tempered martensite...............................38 examining austenite grains .................................................................51 in full-scale metallographic laboratory ............................................105 observing aluminum nitride precipitates ..........................................202 observing dislocations.........................................................................56 viewing dislocations in subgrain boundaries .....................................57 Transmission method, definition ........................................................294 Transoptic ............................................................................................189 Transport. See Transference. Transport number. See Transference number. Transverse, definition ..........................................................................294 Transverse direction. See also Longitudinal direction and Normal direction. definition............................................................................................294 Transverse section..................................................................170(F), 171 Tribology, definition.............................................................................294 Triclinic, definition...............................................................................294 Trinocular tube arrangement .......................................................124(F) Tri-phase steel........................................................................................40 Triple point, definition.........................................................................294

Tripoli, definition .................................................................................294 TRIP steel, definition ...........................................................................294 Troostite. See also Fine pearlite and Martensite. definition............................................................................................294 True strain. See Strain. True stress. See Stress. TTT. See Time-temperature-transformation diagram. Tube factor ...........................................................................................113 Tube length ................................................................112, 113(F), 121(F) Tubes ASTM specifications for chromium-molybdenum steel product forms ................................................................................7, 308(T) mounting of.......................................................................................196 Tungsten, function as element in steel and cast iron ........................3(T) Tungsten carbide, as abrasive particles for wire saw ................182-183 Tungsten-filament lamps ....................................................................127 Tungsten-halogen lamps.............................................................109, 127 Tungsten high-speed steels, AISI code classification..........................12 Twin. See also Annealing twin and Mechanical twin. definition............................................................................................294 Twin bands, definition .........................................................................294 Twist boundary, definition ..................................................................294 2% nital, as etchant for carbon steels and alloy steels .................302(T) 2% nital/4%picral, as etchant for carbon steels and alloy steels................................................................................302(T) Type II sulfide inclusions .....................................................................72

U Ultimate strength, definition...............................................................294 Ultrahigh-strength (maraging) steels.......................................13, 15(F) composition .........................................................................................13 heat treatment......................................................................................13 microstructure.................................................................................15(F) Ultramicroscopic. See Submicroscopic. Ultrasonic cleaning definition............................................................................................294 device ...............................................................105, 173, 174, 186, 203 device, agitating picral etch specimen .............................................228 device, etching with Winsteard’s reagent ........................................232 device, for modified Marshall’s reagent ..........................................227 Underbead crack, definition ...............................................................294 Undercooling. See Supercooling. Undercut, definition .............................................................................294 Underfill, definition..............................................................................294 Understressing, definition ...................................................................295 Uniaxial stress, definition ....................................................................295 Unified Numbering System (UNS) classification system for steels and cast irons .....................................2 letter designations .................................................................................2 Uniform strain, definition ...................................................................295 Unit cell, definition...............................................................................295 Univariant equilibrium, definition .....................................................295 UNS. See Unified Numbering System. Untempered martensite .................................38(F), 176-177(F), 235(F) in carbon steel ..............................................................................184(F) in white irons ......................................................................................47 Upper bainite..............................................................................38, 39(F) in carburized microstructure....................................................62, 63(F) in commercial low-alloy steel .................................................38, 39(F) definition............................................................................................295 Upper yield stress. See Yield point. Upright microscope ..............................109, 110(F), 116(F), 138-139(F) advantages and disadvantages ..........................................................110 focusing procedure ....................................................................143-144 personalization procedure .................................................................144 procedure for using leveling device...............................140(F), 141(F) ray diagram of..............................................................................112(F) stage of .........................................................................................111(F) Used and/or reconditioned equipment .............................................297 Useful magnification ...........................................................................115

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Index / 353

V Vacancy, definition...............................................................................295 Vacuum bell jar...................................................................................105 Vacuum chamber......................................................................92, 94, 96 for hot stage microscope ..................................................................164 Vacuum degassing, to prevent hydrogen damage ................................80 Vacuum desiccator ..............................................................................191 Vacuum evaporator ...............................................................155(F), 160 Vacuum sputtering device .............................................................158(F) Vanadium as carbide stabilizer ............................................................................46 function as element in steel and cast iron ......................................3(T) Vanadium carbides ...............................................................................58 Vanadium nitrides.................................................................................58 Vapor-deposited replica, definition ....................................................295 Variability, definition...........................................................................295 Variance. See Variability. Veining .............................................................................................226(F) definition............................................................................................295 Vendors for metallographic supplies ...........................................296(T) Ventilation of metallographic laboratories......................104, 105, 106 of safety cabinet, for flammable chemicals .....................................107 Vergard’s law, definition.....................................................................295 Vermicular graphite...............................................................102, 103(F) Vermicular iron. See Compacted graphite cast iron. Vertical illumination, definition..........................................................295 Vertical illuminators...........................................................109, 126, 133 Very-low-carbon electrical steel, micrograph ......................133, 134(F) Very-low-carbon motor lamination steel, ferrite grains in microstructure ...................................................................235-236(F) Very serious defect, definition ............................................................257 Vibratory polishing .............................................................................209 definition............................................................................................295 machine ..........................................................................................96-97 Vickers hardness tester ......................................................................106 Vickers indenter.........................................................165-166(F), 167(F) Vickers microhardness ..........................................................160, 161(F) Video camera .......................................................................................106 Video monitor......................................................................................139 Video printers......................................................................................139 Vilella’s reagent ........................................................230, 231(F), 237(T) as etchant for carbon steels and alloy steels .................302(T), 303(T) for etching martensitic stainless steels and tool steels ....................230 for etching stainless steels ................................................................237 Virtual image .........................................................109, 112, 113(F), 123 Voids .....................................................................................................157 in carbon-chromium forging steel...........................................70(F), 72 in carbonitrided steel ....................................................68(F), 69(F), 71 formation during creep ............................................................80, 82(F) and graphitization process reversal ...............................................77(F) Voltage alignment. See also Alignment. definition............................................................................................295 Voltage efficiency, definition ...............................................................295 Volume fraction, definition .................................................................295 von Schreibers, Carl........................................................................88-89 Vycor.....................................................................................................127

W Wallner lines. See also Fatigue striations. definition............................................................................................295 Warm working, definition...................................................................295 Water-hardening tool steels, AISI code classification ........................12 Waterjet-cutting device ......................................................................180 Watertown arsenal etchant composition .....................................................................300(T) as etchant for revealing macrostructures in iron and steel ........300(T) Wavelength......................................................................................119(F) Wavelength dispersive spectroscopy (WDS).......................159(F), 161 definition............................................................................................295 Wavelength (x-rays), definition ..........................................................295 WDS. See Wavelength dispersive spectroscopy.

Wear, definition....................................................................................295 Wear oxidation. See Fretting. Wear-resistant steels.............................................................12-13, 15(F) classification ..........................................................................................1 Weathering steels ....................................................................................9 Weight percent, definition...................................................................295 Weldability, definition .........................................................................295 Weld fusion zone, of weld failure in steel plate .............................95(F) Welding...................................................................................................91 definition............................................................................................295 Weldments, failure in steel plate.................................................94-95(F) Wet chemical analysis...........................................................................89 Wet film micrography ........................................................................105 Wetting, definition................................................................................295 Wetting agent...............................................................227, 228, 229-230 definition............................................................................................295 Wheel burn .................................................................................79, 80(F) Wheel dressing ....................................................................................177 Whiskers, definition .............................................................................295 White cast iron (or white iron)..........................................16, 17, 18(F) applications..........................................................................................45 cementite in.........................................................................................65 composition range...............................................................................17 definition............................................................................................295 as form of cast iron ..........................................................................251 malleabilized microstructure..........................................................64(F) microstructure ..........................17, 18(F), 42-45(F), 47(F), 117-118(F) phase transformations ....................................................................42-43 properties.............................................................................................17 solidification ........................................................................................47 White-etching layer, definition ...........................................................295 Whiteheart malleable. See Malleable iron. White layer ............................................................................80(F), 81(F) White light ...........................................................................................119 White metal, definition........................................................................296 White rust, definition...........................................................................296 Wide-angle eyepiece............................................................................125 Wide-field eyepiece .....................................................................109, 125 definition............................................................................................296 von Widmanstätten, Aloys Joseph Franz Xavier Beck ........32, 88-89 Widmanstätten ferrite ...............................................................32, 33(F) Widmanstätten structure .....................................................................88 definition............................................................................................296 Wiesinger etching reagent, composition, metals polished, temperature, and etching remarks ...........................................298(T) Winsteard’s reagent, for etching carbon and low-alloy steels and cast irons ....................................................................231-232(F) Winsteard’s reagent, modified, as etchant for carbon steels and alloy steels................................................................................303(T) Windowless detectors..........................................................................162 Wipe etching. See Swabbing. Wire mounting ..............................................................................196, 197(F) steel, metallographic analysis........................................93-94(F), 95(F) Wire saw ..................................................................................182-183(F) Wollaston prism .........................................................133-134(F), 135(F) Wood alcohol .......................................................................................107 Woody structure, definition ................................................................296 Wootz, definition ..................................................................................296 Work angle, definition .........................................................................296 Work hardening. See Strain hardening. Working distance ...........................................................114(F), 116, 120 definition............................................................................................296 Work request form .............................................................................169 Wrought iron definition............................................................................................296 slag inclusions.......................................................................218-219(F) Wustite ....................................................................................162, 163(F) definition............................................................................................296

X Xenon arc lamps .........................................................................127, 128

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354 / Metallographer’s Guide Xenon arc lamps (Continued) bulbs ..................................................................................................107 XPS. See X-ray photoelectron spectroscopy. X-radiation .........................................152(F), 155-158, 160, 162-164(F) definition............................................................................................296 X-ray beam impingement ..................................................................164 X-ray detectors ....................................................................................159 X-ray diffraction ....................................................................162, 163(F) to detect retained austenite presence..................................................71 X-ray diffractometer...............................................................162-164(F) in full-scale metallographic laboratory ............................................105 operated by metallographer at research laboratory............................92 X-ray filter, definition..........................................................................265 X-ray photoelectron spectroscopy (XPS) .........................................168 X-rays. See X-radiation. X-ray tube, definition ..........................................................................296 X-ray wavelength detectors ...............................................................160

Y Yield point, definition ..........................................................................296

Yield point elongation, definition.......................................................296 Yield strength, definition.....................................................................296 Yield stress, definition .........................................................................296 Yield value, definition..........................................................................296 Young’s modulus. See also Modulus of elasticity. definition............................................................................................296

Z Zephiran chloride wetting agent .................................228, 229(F), 230 Zinc, as anisotropic material................................................................133 Zinc-based coatings.............................................................................209 polishing of .......................................................................................205 Zinc coated steels, etchants for .................................241-242(F), 304(T) Zirconium as anisotropic material ......................................................................133 function as element in steel and cast iron ......................................3(T) Zirconium arc lamps ..........................................................................128 Zirconium nitrides, color....................................................................218 Zirconium oxide grinding discs.........................................................199

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