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Steels Micro Structure Steels Micro Structure and Properties

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SteelsThis page intentionally left blank SteelsMicrostructure and PropertiesThird editionH. K. D. H. BhadeshiaProfessor of Physical MetallurgyUniversity of CambridgeandAdjunct Professor of Computational MetallurgyGraduate lnstitute of Ferrous Technology, POSTECHandSir Robert HoneycombeEmeritus Goldsmiths Professor of MetallurgyUniversity of CambridgeAMSTERDAM BOSTON HEIDELBERG LONDON NEW YORK OXFORDPARIS SAN DIEGO SAN FRANCISCO SINGAPORE SYDNEY TOKYOButterworth-Heinemann is an imprint of ElsevierButterworth-Heinemann is an imprint of ElsevierLinacre House, Jordan Hill, Oxford OX2 8DP, UK30 Corporate Drive, Suite 400, Burlington, MA 01803, USAFirst edition 1981Second edition 1995Reprinted 1976, 2000Transferred to digital printing 2003Third edition 2006Copyright 2006, R. W. K. Honeycombe and H. K. D. H. Bhadeshia. Published by Elsevier Ltd.All rights reservedNo part of this publication may be reproduced, stored in a retrieval system or transmitted in anyform or by any means electronic, mechanical, photocopying, recording or otherwise withoutthe prior written permission of the publisherPermissions may be sought directly from Elseviers Science &Technology RightsDepartment in Oxford, UK: phone (44) (0) 1865 843830; fax (44) (0) 1865 853333;email: [email protected] Alternatively you can submit your request online byvisiting the Elsevier web site at http://elsevier.com/locate/permissions, and selectingObtaining permission to use Elsevier materialNoticeNo responsibility is assumed by the publisher for any injury and/or damage to persons or property as amatter of products liability, negligence or otherwise, or fromany use or operation of any methods, products,instructions or ideas contained in the material herein. Because of rapid advances in the medical sciences,in particular, independent verication of diagnoses and drug dosages should be madeBritish Library Cataloguing in Publication DataA catalogue record for this book is available from the British LibraryLibrary of Congress Cataloging in Publication DataA catalog record for this book is available from the Library of CongressISBN-13: 978-0-750-68084-4ISBN-10: 0-7506-8084-9For information on all Butterworth-Heinemann publicationsvisit our website at http://books.elsevier.comCover Images, used with permissionInset: -TRIP steel, S. ChatterjeeBackground: magnetic eld due to a small particle of iron, enclosed in a carbon tube,T. Kasama, R. DuninBorkowski, K. Koziol and A. H. Windle.Typeset by Charon Tec Ltd, Chennai, Indiawww.charontec.comPrinted and bound in Great Britain06 07 08 09 10 10 9 8 7 6 5 4 3 2 1CONTENTSPreface to the rst edition ixPreface to the second edition xPreface to the third edition xi1 Iron and its interstitial solid solutions 11.1 Introduction 11.2 The allotropes of pure iron 21.3 The phase transformation: - and -iron 41.4 Carbon and nitrogen in solution in - and -iron 81.5 Some practical aspects 15Further reading 162 The strengthening of iron and its alloys 172.1 Introduction 172.2 Work hardening 182.3 Solid solution strengthening by interstitials 202.4 Substitutional solid solution strengthening of iron 272.5 Grain size 272.6 Dispersion strengthening 322.7 An overall view 332.8 Some practical aspects 342.9 Limits to strength 35Further reading 383 The ironcarbon equilibrium diagram and plain carbon steels 393.1 The ironcarbon equilibrium diagram 393.2 The austeniteferrite transformation 423.3 The austenitecementite transformation 443.4 The kinetics of the transformation 453.5 The austenitepearlite reaction 533.6 Ferritepearlite steels 67Further reading 694 The effects of alloying elements on ironcarbon alloys 714.1 The - and -phase elds 714.2 The distribution of alloying elements in steels 74vvi CONTENTS4.3 The effect of alloying elements on the kinetics of the/ transformation 774.4 Structural changes resulting from alloying additions 844.5 Transformation diagrams for alloy steels 91Further reading 925 Formation of martensite 955.1 Introduction 955.2 General characteristics 955.3 The crystal structure of martensite 1005.4 The crystallography of martensitic transformations 1035.5 The morphology of ferrous martensites 1065.6 Kinetics of transformation to martensite 1125.7 The strength of martensite 1205.8 Shape memory effect 126Further reading 1276 The bainite reaction 1296.1 Introduction 1296.2 Upper bainite (temperature range 550400C) 1296.3 Lower bainite (temperature range 400250C) 1326.4 The shape change 1356.5 Carbon in bainite 1356.6 Kinetics 1396.7 The transition from upper to lower bainite 1436.8 Granular bainite 1446.9 Tempering of bainite 1456.10 Role of alloying elements 1466.11 Use of bainitic steels 1476.12 Nanostructured bainite 152Further reading 1547 Acicular ferrite 1557.1 Introduction 1557.2 Microstructure 1557.3 Mechanism of transformation 1577.4 The inclusions as heterogeneous nucleation sites 1617.5 Nucleation of acicular ferrite 1627.6 Summary 164Further reading 1648 The heat treatment of steels: hardenability 1678.1 Introduction 1678.2 Use of TTT and continuous cooling diagrams 168CONTENTS vii8.3 Hardenability testing 1708.4 Effect of grain size and chemical compositionon hardenability 1768.5 Hardenability and heat treatment 1778.6 Quenching stresses and quench cracking 179Further reading 1819 The tempering of martensite 1839.1 Introduction 1839.2 Tempering of plain carbon steels 1849.3 Mechanical properties of tempered plain carbon steels 1909.4 Tempering of alloy steels 1919.5 Maraging steels 207Further reading 20710 Thermomechanical treatment of steels 20910.1 Introduction 20910.2 Controlled rolling of low-alloy steels 21010.3 Dual-phase steels 22010.4 TRIP-assisted steels 22310.5 TWIP steels 22910.6 Industrial steels subjected to thermomechanical treatments 231Further reading 23311 The embrittlement and fracture of steels 23511.1 Introduction 23511.2 Cleavage fracture in iron and steel 23511.3 Factors inuencing the onset of cleavage fracture 23711.4 Criterion for the ductile/brittle transition 24011.5 Practical aspects of brittle fracture 24311.6 Ductile or brous fracture 24511.7 Intergranular embrittlement 252Further reading 25812 Stainless steel 25912.1 Introduction 25912.2 The ironchromiumnickel system 25912.3 Chromium carbide in CrNi austenitic steels 26412.4 Precipitation of niobium and titanium carbides 26712.5 Nitrides in austenitic steels 27012.6 Intermetallic precipitation in austenite 27012.7 Austenitic steels in practical applications 273viii CONTENTS12.8 Duplex and ferritic stainless steels 27412.9 Mechanically alloyed stainless steels 27812.10 The transformation of metastable austenite 281Further reading 28613 Weld microstructures 28713.1 Introduction 28713.2 The fusion zone 28713.3 The HAZ 298Further reading 30614 Modelling of microstructure and properties 30714.1 Introduction 30714.2 Example 1: alloy design high-strength bainitic steel 30914.3 Example 2: mechanical properties of mixed microstructures 31514.4 Methods 32114.5 Kinetics 32614.6 Finite element method 32914.7 Neural networks 33014.8 Dening characteristics of models 333Further reading 334Index 335PREFACETOTHE FIRST EDITIONIn this book, I have attempted to outline the principles which determine themicrostructures of steels and through these the mechanical properties. At atime when our metallographic techniques are reaching almost to atomic resolu-tion, it is essential to emphasize structure on the nest scale, especially becausemechanical properties are sensitive to changes at this level. While this is not abook on the selection of steels for different uses, I have tried to include sufcientinformation to describe how broad categories of steels full practical require-ments. However, the main thrust of the book is to examine analytically howthe / phase transformation is utilized, and to explain the many effects thatnon-metallic and metallic alloying elements have, both on this transformationand on other phenomena.This book is written with the needs of metallurgists, materials scientists andengineers in mind, and should be useful not only in the later years of the rstdegree and diploma courses but also in postgraduate courses. An elementaryknowledge of materials science, metallography, crystallography and physics isassumed.I amindebted to several colleagues for their interest in this book, particularlyDr D. V. Edmonds, who kindly read the manuscript, Dr P. R. Howell, Dr B.Muddle and Dr H. K. D. H. Bhadeshia, who made helpful comments on varioussections, and numerous other numbers of my research group who have providedillustrations. I wish also to thank my colleagues in different countries for theirkind permission to use diagrams from their work. I am also very grateful toMr S. D. Charter for his careful preparation of the line diagrams. Finally, mywarmest thanks go to Mrs Diana Walker and Miss Rosemary Leach for theircareful and dedicated typing of the manuscript.RWKHCambridge1980ixPREFACETOTHE SECOND EDITIONThis new edition retains the basic framework of the original book; however, theopportunityhas beentakentointroduceseveral additional chapters dealingwithareas which have emerged or increased in signicance since the book was rstpublished in 1981. There is now a separate chapter on acicular ferrite which hasbecome a desirable structure in some steels. The control of microstructures dur-ing welding is undoubtedly a crucial topic which now requires a chapter, whilethe modelling of microstructures to achieve optimum properties has