Harry Bhadeshia Robert Honeycombe Steels Microstructure and Properties Third Edition20190607 70016 1510lww PDF

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Steels

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Steels Microstructure and Properties Third edition H. K. D. H. Bhadeshia Professor of Physical Metallurgy University of Cambridge and Adjunct Professor of Computational Metallurgy Graduate lnstitute of Ferrous Technology, POSTECH and

Sir Robert Honeycombe Emeritus Goldsmiths’ Professor of Metallurgy University of Cambridge

AMSTERDAM • BOSTON • HEIDELBERG • LONDON • NEW YORK • OXFORD PARIS • SAN DIEGO • SAN FRANCISCO • SINGAPORE • SYDNEY • TOKYO Butterworth-Heinemann is an imprint of Elsevier

Butterworth-Heinemann is an imprint of Elsevier Linacre House, Jordan Hill, Oxford OX2 8DP, UK 30 Corporate Drive, Suite 400, Burlington, MA 01803, USA First edition 1981 Second edition 1995 Reprinted 1976, 2000 Transferred to digital printing 2003 Third edition 2006 Copyright © 2006, R. W. K. Honeycombe and H. K. D. H. Bhadeshia. Published by Elsevier Ltd. All rights reserved No part of this publication 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 prior written permission of the publisher Permissions may be sought directly from Elsevier’s Science & Technology Rights Department in Oxford, UK: phone (+44) (0) 1865 843830; fax (+44) (0) 1865 853333; email: [email protected]. Alternatively you can submit your request online by visiting the Elsevier web site at http://elsevier.com/locate/permissions, and selecting Obtaining permission to use Elsevier material Notice No responsibility is assumed by the publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any 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 verification of diagnoses and drug dosages should be made British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloging in Publication Data A catalog record for this book is available from the Library of Congress ISBN-13: 978-0-750-68084-4 ISBN-10: 0-7506-8084-9 For information on all Butterworth-Heinemann publications visit our website at http://books.elsevier.com

Cover Images, used with permission Inset: δ-TRIP steel, S. Chatterjee Background: magnetic field due to a small particle of iron, enclosed in a carbon tube, T. Kasama, R. Dunin–Borkowski, K. Koziol and A. H. Windle. Typeset by Charon Tec Ltd, Chennai, India www.charontec.com Printed and bound in Great Britain 06 07 08 09 10 10 9 8 7 6 5 4 3 2 1

CONTENTS

Preface to the first edition Preface to the second edition Preface to the third edition

ix x xi

1 Iron and its interstitial solid solutions 1.1 Introduction 1.2 The allotropes of pure iron 1.3 The phase transformation: α- and γ-iron 1.4 Carbon and nitrogen in solution in α- and γ-iron 1.5 Some practical aspects Further reading

1 1 2 4 8 15 16

2 The strengthening of iron and its alloys 2.1 Introduction 2.2 Work hardening 2.3 Solid solution strengthening by interstitials 2.4 Substitutional solid solution strengthening of iron 2.5 Grain size 2.6 Dispersion strengthening 2.7 An overall view 2.8 Some practical aspects 2.9 Limits to strength Further reading

17 17 18 20 27 27 32 33 34 35 38

3 The iron–carbon equilibrium diagram and plain carbon steels 3.1 The iron–carbon equilibrium diagram 3.2 The austenite–ferrite transformation 3.3 The austenite–cementite transformation 3.4 The kinetics of the γ → α transformation 3.5 The austenite–pearlite reaction 3.6 Ferrite–pearlite steels Further reading

39 39 42 44 45 53 67 69

4 The effects of alloying elements on iron–carbon alloys 4.1 The γ- and α-phase fields 4.2 The distribution of alloying elements in steels

71 71 74

v

CONTENTS

vi

4.3

The effect of alloying elements on the kinetics of the γ/α transformation 4.4 Structural changes resulting from alloying additions 4.5 Transformation diagrams for alloy steels Further reading

77 84 91 92

5 Formation of martensite 5.1 Introduction 5.2 General characteristics 5.3 The crystal structure of martensite 5.4 The crystallography of martensitic transformations 5.5 The morphology of ferrous martensites 5.6 Kinetics of transformation to martensite 5.7 The strength of martensite 5.8 Shape memory effect Further reading

95 95 95 100 103 106 112 120 126 127

6 The bainite reaction 6.1 Introduction 6.2 Upper bainite (temperature range 550–400◦ C) 6.3 Lower bainite (temperature range 400–250◦ C) 6.4 The shape change 6.5 Carbon in bainite 6.6 Kinetics 6.7 The transition from upper to lower bainite 6.8 Granular bainite 6.9 Tempering of bainite 6.10 Role of alloying elements 6.11 Use of bainitic steels 6.12 Nanostructured bainite Further reading

129 129 129 132 135 135 139 143 144 145 146 147 152 154

7 Acicular ferrite 7.1 Introduction 7.2 Microstructure 7.3 Mechanism of transformation 7.4 The inclusions as heterogeneous nucleation sites 7.5 Nucleation of acicular ferrite 7.6 Summary Further reading

155 155 155 157 161 162 164 164

8 The heat treatment of steels: hardenability 8.1 Introduction 8.2 Use of TTT and continuous cooling diagrams

167 167 168

CONTENTS

8.3 8.4

Hardenability testing Effect of grain size and chemical composition on hardenability 8.5 Hardenability and heat treatment 8.6 Quenching stresses and quench cracking Further reading

vii

170 176 177 179 181

9 The tempering of martensite 9.1 Introduction 9.2 Tempering of plain carbon steels 9.3 Mechanical properties of tempered plain carbon steels 9.4 Tempering of alloy steels 9.5 Maraging steels Further reading

183 183 184 190 191 207 207

10 Thermomechanical treatment of steels 10.1 Introduction 10.2 Controlled rolling of low-alloy steels 10.3 Dual-phase steels 10.4 TRIP-assisted steels 10.5 TWIP steels 10.6 Industrial steels subjected to thermomechanical treatments Further reading

209 209 210 220 223 229 231 233

11 The embrittlement and fracture of steels 11.1 Introduction 11.2 Cleavage fracture in iron and steel 11.3 Factors influencing the onset of cleavage fracture 11.4 Criterion for the ductile/brittle transition 11.5 Practical aspects of brittle fracture 11.6 Ductile or fibrous fracture 11.7 Intergranular embrittlement Further reading

235 235 235 237 240 243 245 252 258

12

259 259 259 264 267 270 270 273

Stainless steel 12.1 Introduction 12.2 The iron–chromium–nickel system 12.3 Chromium carbide in Cr–Ni austenitic steels 12.4 Precipitation of niobium and titanium carbides 12.5 Nitrides in austenitic steels 12.6 Intermetallic precipitation in austenite 12.7 Austenitic steels in practical applications

CONTENTS

viii

12.8 Duplex and ferritic stainless steels 12.9 Mechanically alloyed stainless steels 12.10 The transformation of metastable austenite Further reading

274 278 281 286

13 Weld microstructures 13.1 Introduction 13.2 The fusion zone 13.3 The HAZ Further reading

287 287 287 298 306

14

307 307 309 315 321 326 329 330 333 334

Modelling of microstructure and properties 14.1 Introduction 14.2 Example 1: alloy design – high-strength bainitic steel 14.3 Example 2: mechanical properties of mixed microstructures 14.4 Methods 14.5 Kinetics 14.6 Finite element method 14.7 Neural networks 14.8 Defining characteristics of models Further reading

Index

335

PREFACE TO THE FIRST EDITION

In this book, I have attempted to outline the principles which determine the microstructures of steels and through these the mechanical properties. At a time when our metallographic techniques are reaching almost to atomic resolution, it is essential to emphasize structure on the finest scale, especially because mechanical properties are sensitive to changes at this level. While this is not a book on the selection of steels for different uses, I have tried to include sufficient information to describe how broad categories of steels fulfil practical requirements. However, the main thrust of the book is to examine analytically how the γ/α phase transformation is utilized, and to explain the many effects that non-metallic and metallic alloying elements have, both on this transformation and on other phenomena. This book is written with the needs of metallurgists, materials scientists and engineers in mind, and should be useful not only in the later years of the first degree and diploma courses but also in postgraduate courses. An elementary knowledge of materials science, metallography, crystallography and physics is assumed. I am indebted to several colleagues for their interest in this book, particularly Dr 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 various sections, and numerous other numbers of my research group who have provided illustrations. I wish also to thank my colleagues in different countries for their kind permission to use diagrams from their work. I am also very grateful to Mr S. D. Charter for his careful preparation of the line diagrams. Finally, my warmest thanks go to Mrs Diana Walker and Miss Rosemary Leach for their careful and dedicated typing of the manuscript. RWKH Cambridge 1980

ix

PREFACE TO THE SECOND EDITION

This new edition retains the basic framework of the original book; however, the opportunity has been taken to introduce several additional chapters dealing with areas which have emerged or increased in significance since the book was first published in 1981. There is now a separate chapter on acicular ferrite which has become a desirable structure in some steels. The control of microstructures during welding is undoubtedly a crucial topic which now requires a chapter, while the modelling of microstructures to achieve optimum properties has emerged as an important approach justifying the inclusion of a further new chapter. The opportunity has also been taken to include a completely revised chapter on bainite transformations. The overall aim of the book remains to introduce students to the principles determining the microstructures of steels, and through these, the mechanical properties and behaviour in service. Steels remain the most important group of metallic alloys, possessing a very wide range of microstructures and mechanical properties, which will ensure their continued extensive use far into the foreseeable future. RWKH HKDHB Cambridge 1995

x

PREFACE TO THE THIRD EDITION

Steel has the ability to adapt to changing requirements. This comes from the myriads of ways in which its structure can be influenced by processing and alloying. This is why it is the standard against which emerging materials are compared. Added to this is the commercial success, with output at record levels and a production efficiency which is uncanny. It is pleasing to see how, all over the world, iron and its alloys contribute to improving the quality of life of so many human beings. The technology is so good that most of these people rightly take it for granted. This new edition captures developments since 1995, e.g., the extremely finegrained alloys, steels with the ability to abnormally elongate and the properties of minute particles of iron. Questions are posed as to the theoretical limit to the finest crystals that can be manufactured on a large scale. In addition, there are major revisions in the explanations of microstructure, strengthening, kinetics and modelling. The original aim of this book, to introduce students and technologists to the principles determining the microstructure and properties of iron and its alloys, has remained the guiding principle in the new edition. HKDHB RWKH Cambridge 2006

xi

Supporting material accompanying this book A full set of accompanying exercises and worked solutions for this book are available for teaching purposes. Please visit http://www.textbooks.elsevier.com and follow the registration instructions to access this material, which is intended for use by lecturers and tutors. The compilation of questions has been designed to stimulate the student to explore the subject within the context of the book. Each question is accompanied by a complete answer, with the exception of the proposed set of topics for essays. Most of the questions and answers have been developed as a consequence of many years of teaching and have been tested on a variety of undergraduates.

1 IRON AND ITS INTERSTITIAL SOLID SOLUTIONS

1.1 INTRODUCTION Steel is frequently the ‘gold-standard’ against which emerging structural materials are compared. What is often not realized is that this is a moving standard, with notoriously regular and exciting discoveries being made in the context of iron and its alloys. This is why steel remains the most successful and cost-effective of all materials, with more than a billion tonnes being consumed annually in improving the quality of life. This book attempts to explain why steels continue to take this pre-eminent position, and examines in detail the phenomena whose exploitation enables the desired properties to be achieved. One reason for the overwhelming dominance of steels is the endless variety of microstructures and properties that can be generated by solid-state transformation and processing. Therefore, in studying steels, it is useful to consider the behaviour of pure iron first, then the iron–carbon alloys, and finally the many complexities that arise when further solutes are added. Pure iron is not an easy material to produce. It has nevertheless been made with a total impurity content less than 60 parts per million (ppm), of which 10 ppm is accounted for by non-metallic impurities such as carbon, oxygen, sulphur and phosphorus, with the remainder representing metallic impurities. Iron of this purity can be extremely weak when reasonably sized samples are tested: the resolved shear stress of a single crystal at room temperature can be as low as 10 MN m−2 , while the yield stress of a polycrystalline sample at the same temperature can be well below 50 MN m−2 . However, the shear strength of small single crystals has been observed to exceed 19,000 MN m−2 when the size of the sample is reduced to about 2 µm. This is because the chances of finding crystal defects such as dislocations become small as the size of the crystal is reduced. The theoretical shear strength of a perfect crystal of iron is estimated to be about 21,000 MN m−2 , equivalent to a tensile strength of about 11,000 MN m−2 . 1

2

CHAPTER 1 IRON AND ITS INTERSTITIAL SOLID SOLUTIONS

For comparison purposes the breaking strength of a very small carbon nanotube has been measured to be about 130,000 MN m−2 ; this number is so astonishing that it has led to exaggerated statements about their potential in structural applications. For example, the tubes are said to be a hundred times stronger than steel; in fact, there is no carbon tube which can match the strength of iron beyond a scale of 2 mm, because of the inevitable defects which arise as the tubes are grown. The lesson from this is that systems which rely on perfection in order to achieve strength necessarily fail on scaling to engineering dimensions. Since perfection is thermodynamically impossible to achieve in large samples, steels must in practice be made stronger by other means which are insensitive to size. The mechanisms by which the strength can be increased will be discussed – suffice it to state here that it is possible to commercially buy steel with a strength of 5500 MN m−2 , with sufficient ductility to ensure safe application. Some of the methods by which such impressive combinations of properties are achieved without compromising safety will be discussed, before the wide range of complex structures which determine the properties is dealt with. 1.2 THE ALLOTROPES OF PURE IRON At least three allotropes of iron occur naturally in bulk form, body-centred cubic (bcc, α, ferrite), face-centred cubic (fcc, γ, austenite) and hexagonal closepacked (hcp, ǫ). The phase β in the alphabetical sequence α, β, γ, δ . . . is missing because the magnetic transition in ferrite was at one time incorrectly thought to be the β allotrope of iron. In fact, there are magnetic transitions in all of the allotropes of iron. The phase diagram for pure iron is illustrated in Fig. 1.1. Each point on any boundary between the phase fields represents an equilibrium state in which two phases can coexist. The triple point where the three boundaries intersect represents an equilibrium between all three phases which coexist. It is seen that in pure iron, the hcp form is stable only at very large pressures, consistent with its high density. The best comparison of the relative densities of the phases is made at the triple point where the allotropes are in equilibrium and where the sum of all the volume changes is zero:  V(bcc → hcp) = −0.34 V(hcp → ccp) = +0.13 cm3 mol−1  V(ccp → bcc) = +0.21

There may exist a fourth natural allotrope in the core of the earth, where the pressure reaches some three million times that at the surface and where the temperature is estimated to be about 6000◦ C. The core of the earth is predominantly iron, and consists of a solid inner core surrounded by a liquid outer core. Knowledge of the core is uncertain, but it has been suggested that the crystal structure of the solid core may be double hcp, although calculations which assume pure iron, indicate that the ǫ-iron remains the most stable under inner-core conditions.

1.2 THE ALLOTROPES OF PURE IRON

3

Fig. 1.1 The phase diagram for pure iron (data from Bundy, 1965).The triple point temperature and pressure are 490◦ C and 110 kbars, respectively. α, γ and ǫ refer to ferrite, austenite and ǫ-iron, respectively. δ is simply the higher temperature designation of α.

1.2.1 Thin films and isolated particles There are two further allotropes which can be created in the form of thin films. Face-centred tetragonal iron has been prepared by coherently depositing iron as a thin film on a {1 0 0} plane of a substrate such as copper with which the iron has a mismatch. The position of atoms in the first deposited layer in this case replicates that of the substrate. A few monolayers can be forced into coherency in the plane of the substrate with a corresponding distortion normal to the substrate. This gives the deposit a face-centred tetragonal structure. Growing iron on a misfitting {1 1 1} surface of a fcc substrate leads to trigonal iron. Very thin films of iron retain their ferromagnetic character, but there are special effects due to the small dimensions. The magnetic moment per atom becomes very large: 3.1 Bohr magnetons compared with 2.2 for bulk α-iron. This is due to the smaller coordination number for atoms in a thin film. The second effect is that magnetic anisotropy greatly increases for thin films because the spins tend to align normal to the surface. The Curie temperature is greatly reduced, again because of the change in coordination. For a monolayer of iron the temperature is just ≃280◦ C. Many classical studies of nucleation theory have been conducted on minute (5–1000 nm) particles of iron where defects responsible for heterogeneous nucleation can be avoided. Such particles have acquired new significance in that they are exploited in the manufacture of carbon nanotubes. The particles are deposited due to the decomposition of ferrocene in chemical mixtures which also contain the ingredients necessary to grow the tubes.

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