Surface Hardening of Steels Understanding the Basics Edited by J.R. Davis Davis & Associates ASM International Materials Park, OH 44073-0002
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, November 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 PRO- VIDED SHALL BE THE EXCLUSIVE AND SOLE REMEDY OF BUYER, AND IN NO EVENT SHALL EITHER PARTY BE LIABLE FOR SPECIAL, INDIRECT OR CONSE- QUENTIAL DAMAGES WHETHER OR NOT CAUSED BY OR RESULTING FROM THE NEGLIGENCE OF SUCH PARTY. As with any material, evaluation of the material under enduse 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. ASM International staff who worked on this project included; Bonnie Sanders, Manager of Production; Nancy Hrivnak and Jill Kinson, Production Editors; and Scott Henry, Assistant Director of Reference Publications. Library of Congress Cataloging-in-Publication Data Surface hardening of steels : understanding the basics / edited by J.R. Davis p. c.m. Includes bibliographical references and index. 1. Steel Metallurgy. I. Title: Surface treatment of steels. II. Davis, J.R. (Joseph R.) TN731.S85 2002 672.3 6 dc21 2002034248 ISBN: 0-87170-764-0 SAN: 204-7586 ASM International Materials Park, OH 44073-0002 Printed in the United States of America
Contents Introduction Chapter 1 Process Selection Guide.................................. 1 Diffusion Methods of Surface Hardening.................. 1 Surface Hardening by Applied Energy.................... 5 Surface Hardening by Coating or Surface Modification....... 7 Important Considerations for Process Selection............ 11 Process Comparisons................................. 12 Surface Hardening by Thermochemical Diffusion Treatments Chapter 2 Gas Carburizing....................................... 17 Gas Curburizing Practices............................... 17 Characteristic Features of Carburized Cases............... 17 Carbon Sources..................................... 18 Carburizing Equipment............................... 19 Preparation of Parts for Carburizing..................... 21 Carburizing Process Variables.......................... 21 Gas Carburizing Atmospheres.......................... 25 Carbon Concentration Gradients and Surface Carbon Content......................... 29 Process Planning.................................... 39 Selective Carburizing................................. 42 Dimensional Control................................. 43 Case Depth Measurement............................. 44 Microstructures of Carburized Steels....................... 46 Martensite.......................................... 46 Austenite.......................................... 49 Carbides........................................... 53 Alloying Effects..................................... 55 Intergranular Fracture at Austenite Grain Boundaries........ 57 Microcracking in Carburized Steels...................... 60 Excessive Retained Austenite and Massive Carbides........ 60 Residual Stresses.................................... 62 Surface and Internal Oxidation......................... 64 Properties of Carburized Steels........................... 65 Bending Fatigue Strength.............................. 66 Other Properties of Interest............................ 79 iii
Chapter 3 Vacuum and Plasma Carburizing.......................... 91 Vacuum Carburizing................................. 91 Plasma Carburizing................................. 105 Chapter 4 Pack and Liquid Carburizing............................ 115 Pack Carburizing................................... 115 Liquid Carburizing and Cyaniding..................... 118 Chapter 5 Carbonitriding........................................ 127 Applicable Steels and Applications..................... 127 Composition of Case................................ 128 Depth of Case...................................... 128 Hardenability of Case................................ 131 Hardness Gradients................................. 132 Void Formation.................................... 133 Furnaces and Furnace Atmospheres..................... 133 Control of Retained Austenite......................... 134 Quenching Media and Practices........................ 135 Tempering........................................ 136 Carbonitriding of Powder Metallurgy Parts............... 137 Combined Carburizing/Carbonitriding Treatments......... 138 Chapter 6 Nitriding............................................ 141 Gas Nitriding...................................... 141 Modified Gas Nitriding Processes...................... 165 Pack Nitriding..................................... 167 Liquid Nitriding.................................... 167 Plasma (or Ion) Nitriding............................. 177 Wear Resistance of Nitrided Steels..................... 187 Chapter 7 Nitrocarburizing...................................... 195 The Compound Layer and Diffusion Zone............... 195 Liquid Nitrocarburizing.............................. 195 Gas Ferritic Nitrocarburizing.......................... 204 Plasma Nitrocarburizing............................. 208 Chapter 8 Boriding............................................ 213 Advantages and Disadvantages of Boriding.............. 213 Boride Layer Characteristics.......................... 214 Steel Selection..................................... 215 Effects of Alloying Elements.......................... 215 Boriding Processes.................................. 217 Properties of Borided Steels........................... 221 Applications for Pack Boriding........................ 223 Chapter 9 Thermal Diffusion Process.............................. 227 Process Characteristics............................... 227 Characteristics of TD-Processed Materials............... 232 Applications....................................... 233 iv
Surface Hardening by Non-Diffusion Methods Chapter 10 Surface Hardening by Applied Energy..................... 237 Flame Hardening................................... 237 Induction Surface Hardening.......................... 242 Laser Surface Hardening............................. 264 Electron-Beam Surface Hardening..................... 267 Chapter 11 Surface Hardening by Coating or Surface Modification....... 275 Hard Chromium Plating.............................. 275 Electroless Nickel Plating............................ 282 Hardfacing........................................ 286 Thermal Spraying................................... 293 Chemical and Physical Vapor Deposition Processing...................................... 297 Ion Implantation.................................... 301 Laser Surface Processing............................. 302 Appendix Appendix 1 Iron-Carbon Phase Diagram............................. 311 Appendix 2 Austenitizing Temperatures for Steels..................... 315 Appendix 3 Hardness Conversion Tables............................ 317 Subject Index.................................................. 321 Alloy Index................................................... 349 v
Preface Surface hardening is a generic term covering a number of processes used to improve the wear resistance of ferrous parts without affecting the more soft, tough interior of the part. The combination of a hard surface and softer interior, made possible, for example, by case hardening methods, is of inestimable value in modern engineering practice. By the use of high-quality alloy steels, great strength and toughness in the core can be combined with extreme surface hardness, resulting in a composite structure capable of withstanding certain kinds of stress to a high degree. For less exacting requirements, there are many applications where low or moderate core properties, together with a high degree of surface hardness, can be obtained with cheaply fabricated, low-priced carbon steel. Three of the major types of treatments that are conventionally used to enhance surface wear resistance are classified as thermochemical, thermal, and coating or plating. Examples of thermochemical treatments include carburizing, nitriding, and carbonitriding (case hardening methods). Examples of thermal or applied energy treatments include flame and induction hardening. Coating or plating processes include hard chromium plating, electroless nickel plating, and various hardfacing methods. These long established surface hardening technologies are continually being improved and remain among the most widely used. However, today completely different surface hardening technologies are being applied to and developed for steels. The objective remains the same, that is, enhanced surface performance, but technologies that incorporate highenergy beams, plasmas, magnetic and electrical fields, and vacuums are being applied. Some of these technologies have been used for some time in the electronics industry to fabricate thin film devices and circuits and therefore cannot be considered new technologies. Their application to steel, however, is relatively new, and they elevate the level of sophistication and control of surface hardening. Surface Hardening of Steels: Understanding the Basics is a practical selection guide to help engineers and technicians choose the optimum surface hardening treatment for a given application from an ever increasing number of options. Emphasis is placed on characteristics such as processing temperature, case/coating thickness, bond strength, and hardness level obtained. The advantages and limitations of the various surface modification technologies are compared. Recent developments in the understanding of the relationships between microstructure and fatigue and wear performance are reviewed, as are more recently introduced surface hardening processes such as vacuum/plasmarelated technologies, laser processing, chemical vapor deposition/physical vapor deposition, and ion implantation. Methods for evaluating hardness patterns and depths of hardness for quality control and failure analysis are described. Metallurgical comparisons are made between those processes that offer rapid heating and rapid cooling (selfquenching) characteristics for example, induction hardening and conventional furnace hardening. Metallurgical characteristics and properties obtained by atmosphere and vacuum carburizing are also compared. Wear and corrosion data are also provided to demonstrate the benefits of each process. vii
The successful completion of this book would not have been possible without the generous assistance of the ASM staff. In particular I would like to thank Scott Henry, Assistant Director, Technical Publications; Don Baxter, Managing Editor, Advanced Materials & Processes and Heat Treating Progress; and Eleanor Baldwin from the ASM Library. I called on them often for help, perhaps too often, but they always came through. Thanks again my fellow colleagues and friends. Joseph R. Davis Davis & Associates Chagrin Falls, Ohio viii
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