Bainite in steels: theory and practice

616 Pages·2015·176.505 MB·English

by  Bhadeshia, Harshad Kumar Dharamshi Hansraj

BAINITE IN STEELS

BAINITE IN STEELS
Theory and Practice
Third edition
H. K. D. H. Bhadeshia
University of Cambridge and POSTECH
Boca Raton London New York
The Institute of Materials,
CRC Press is an imprint of the
Minerals and Mining
Taylor & Francis Group, an informa business
First published 2015 by Maney Publishing on behalf of the Institute of Materials, Minerals & Minning
First edition published 1992; second edition published 2001
Published 2018 by CRC Press
Taylor & Francis Group
6000 Broken Sound Parkway NW, Suite 300
Boca Raton, FL 33487-2742
© 2015 by H.K. D. H. Bhadeshia
CRC Press is an imprint of Taylor & Francis Group, an Infonna business
No claim to original U.S. Government works
ISBN 13: 978-1-909662-74-2 (hbk)
This book contains infonnation obtained from authentic and highly regarded sources. Reasonable efforts
have been made to publish reliable data and information, but the author and publisher carmot assume
responsibility for the validity of all materials or the consequences of their use. The authors and publishers
have attempted to trace the copyright holders of all material reproduced in this publication and apologize
to copyright holders if pennission to publish in this fonn has not been obtained. If any copyright material
has not been acknowledged please write and let us know so we may rectify in any future reprint.
Except as pennitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced,
transmitted, or utilized in any fonn by any electronic, mechanical, or other means, now known or hereafter
invented, including photocopying, microfilming, and recording, or in any infonnation storage or retrieval
system, without written pennission from the publishers.
Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used
only for identification and explanation without intent to infringe.
Visit the Taylor & Francis Web site at
http://www.taylorandfrancis.com
and the CRC Press Web site at
http://www.crcpress.com
The author has asserted his moral right.
CONTENTS
Preface xv
Acknowledgments xvii
Acronyms xix
Nomenclature xx
1 Introduction 1
1.1 The Discovery of Bainite 2
1.2 The Early Research 4
1.2.1 Crystallography 5
1.2.2 The Incomplete Reaction Phenomenon 5
1.2.3 Carbon Redistribution 6
1.2.4 Thermodynamics 9
1.2.5 Paraequilibrium IO
1.2.6 Kinetics 12
1.3 Bainitic Steels: Industrial Practice 14
1.4 Summary of the Early Research 15
2 Bainitic Ferrite 17
2.1 Sheaves of Bainite 17
2.1.1 Morphology 17
V
vi CONTENTS
2.1.2 Thickness of bainite plates 21
2.1.3 Stereology 23
2.2 Dislocation Density 24
2.2.1 Quantitative Estimation of Dislocation Density 26
2.3 Chemical Composition 27
2.3.1 Substitutional Alloying Elements 27
2.3.2 Interstitial Alloying Elements 31
2.4 Crystallography 33
2.4.1 Crystallography: Block Size, Austenite Grain Boundary 39
2.4.2 Autocatalytic Nucleation 40
2.5 Crystallographic Theory 42
2.5.1 Application to Bainite 43
2.5.2 High-Resolution Studies of the Shape Change 46
2.5.3 The Shape Change: Further Considerations 48
2.5.4 Shape Change and the Superledge Mechanism 52
2.5.5 The Structure of the Interface 52
2.5.6 The Crystallography of a Lath of Bainite 54
2.6 Unit Cell Symmetry 55
2.6.1 Symmetry of Interstices 55
2.6.2 Tetragonality of Bainitic Ferrite 56
2.7 Microstructure of Bainite: The Midrib 58
2.8 Summary 59
3 Carbide Precipitation 61
3.1 Upper Bainite 61
3.2 Lower Bainite 64
3.2.1 Precipitation within Lower Bainitic Ferrite 64
3.2.2 Precipitation between Lower Bainitic Ferrite Platelets 67
3.3 Kinetics of Carbide Precipitation 68
3.3.1 Partitioning and Distribution of Carbon 68
3.3.2 Kinetics of Precipitation from Residual Austenite 70
3.3.3 Kinetics of Precipitation within Bainitic Ferrite 72
3.4 Crystallography of Carbide Precipitation in Bainite 75
3.4.1 Cementite: Orientation Relationships 75
3.4.2 The Habit Plane of Cementite 76
3.4.3 Three-Phase Crystallography 76
3.4.4 Interphase Precipitation 77
3.4.5 Relief of Strain Energy 79
3.4.6 Epsilon-Carbide 79
3.4.7 Eta- and Chi-Carbides 81
3.5 Chemical Composition of Bainitic Carbides 83
3.6 Summary 85
CONTENTS vii
4 Tempering of Bainite 87
4.1 Introduction 87
4.2 Tempering Kinetics 90
4.3 Tempering of Steels Containing Austenite 90
4.3.1 Redistribution of Substitutional Solutes 92
4.3.2 Decomposition of Austenite 93
4.3.3 Tempering of Nanostructured Bainite 95
4.4 Coarsening of Cementite 98
4.5 Secondary Hardening and the Precipitation of Alloy Carbides 99
4.6 Changes in the Composition of Cementite 100
4.6.1 Remanent Life Prediction 102
4.6.2 Theory for Carbide Enrichment 103
4.6.3 Effect of Carbon on Carbide Enrichment 105
4.7 Low-Temperature Tempering of Mixed Microstructures 106
4.8 Sequence of Alloy Carbide Precipitation 106
4.8.1 Effect of Starting Microstructure on Tempering Reactions 111
4.9 Changes in the Composition of Alloy Carbides 111
4.10 Precipitate-free Zones 112
4.11 Precipitation Hardening with Copper 113
4.12 Summary 115
5 Thermodynamics 117
5.1 Deviations from Equilibrium 117
5.2 Chemical Potential 118
5.3 Stored Energy due to Transformation 120
5.4 Thermodynamics of Growth 122
5.4.1 Substitutional Solutes during Growth 122
5.4.2 Interstitial Solutes during Growth 122
5.4.3 Approach to Equilibrium 127
5.5 Quench and Partitioning 128
5.6 Summary 129
6 Kinetics 131
6.1 Thermodynamics of Nucleation 131
6.1.1 Transformation-Start Temperature 132
6.1.2 Evolution of the Nucleus 135
6.2 Possible Mechanisms of Nucleation 138
6.3 Bainite Nucleation 141
6.4 Empirical Methods for the Bainite-Start Temperature 142
6.5 The Nucleation Rate 143
6.5.1 Grain Boundary Nucleation 144
viii CONTENTS
6.6 Growth Rate 144
6.6.1 Theory for the Lengthening of Plates 144
6.6.2 Growth Rate of Sheaves of Bainite 148
6.6.3 Growth Rate of Sub-Units of Bainite 148
6.6.4 Solute Drag 150
6.7 Partitioning of Carbon from Supersaturated Bainitic Ferrite 153
6.8 Growth with Partial Supersaturation 154
6.8.1 Stability 155
6.8.2 The Interface Response Functions 157
6.8.3 Data on Transformation with Partial Supersaturation 160
6.8.4 Summary 161
6.9 Cooperative Growth of Ferrite and Cementite 161
6.10 Overall Transformation Kinetics 162
6.10.1 Isothermal Transformation 162
6.10.2 Incorrect formulation of incubation time 164
6.10.3 Mechanistic Formulation of the Avarmi Equation 165
6.10.4 Austenite Grain Size Effects 167
6.10.5 Anisothermal Transformation Kinetics 167
6.11 Simultaneous Transformations 169
6.11.1 Special Cases 169
6.11.2 Precipitation in Secondary Hardening Steels 170
6.11.3 Time-Temperature-Transformation Diagrams 171
6.11.4 Continuous Cooling Transformation Diagrams 173
6.11.5 Boron, Sulphur and the Rare Earth Elements 176
6.11.6 Niobium and hardenability 179
6.12 Superhardenability 181
6.13 Effect of Chemical Segregation 183
6.14 Martensitic Transformation in Partially Bainitic Steels 185
6.14.1 Autocatalysis 187
6.15 Phase Field Models 190
6.15.1 Introduction to Phase Fields 190
6.15.2 Phase Field Simulation of Bainite 192
6.16 Summary 194
7 Upper and Lower Bainite 195
7.1 Matas and Hehemann Model 195
7.2 Quantitative Model 197
7.2.1 Time to Decarburise Supersaturated Ferrite 197
7.2.2 Kinetics of Cementite Precipitation 197
7.2.3 Quantitative Estimation of the Transition Temperature 199
7.2.4 Comparison of Theory and Experimental Data 200
7.3 Mixed Microstructures by Isothermal Transformation 203
CONTENTS IX
7.4 Other Consequences of the Transition 204
7.5 Comparison with Tempering of Martensite 204
7.6 Summary 205
8 Stress and Strain Effects 207
8.1 Mechanical Driving Force 208
8.2 The Temperature 211
8.3 General Observations 212
8.3.1 Externally Applied Stress 212
8.3.2 Internally Generated Stress 213
8.4 Effect on Microstructure 214
8.4.1 Extent of Variant Selection 215
8.5 Effect of Hydrostatic Pressure 217
8.6 Mechanical Stability of Retained Austenite 218
8.7 Transformation under Constraint: Residual Stresses 220
8.8 Anisotropic Strain due to Transformation Plasticity 223
8.8.1 Interactions between Variants 225
8.9 Influence of Plastic Strain 225
8.10 Stress-Affected Carbide Precipitation 226
8.11 Plastic Deformation and Mechanical Stabilisation 228
8.11.1 Theoretical Basis 228
8.11.2 Experimental Evidence 230
8.11.3 Transformation from Recovered Austenite 235
8.11.4 Technological Implications of Mechanical Stabilisation 235
8.12 Summary 236
9 From Bainite to Austenite 239
9.1 Heating a Mixture of Austenite and Upper Bainitic Ferrite 240
9.1.1 One-Dimensional Growth from a Mixture of Austenite and
Bainitic Ferrite 244
9.1.2 Estimation of the Parabolic Thickening Rate Constant 245
9.2 Anisothermal Transformation 247
9.3 Heating a Mixture of Cementite and Bainitic Ferrite 247
9.4 Irradiation-Induced Rapid Heating 249
9.5 Summary 249
10 Acicular Ferrite 251
10.1 General Characteristics and Morphology 251
10.2 Mechanism of Growth 254
10.3 Mechanism of Nucleation 257
10.4 Nucleation and The Role of Inclusions 258
10.4.1 Aluminium and Titanium Oxides 262