Steels: Microstructure and Properties (eBook)

Front cover 1
Title page 4
Copyright page 5
Table of contents 6
Preface to the first edition 10
Preface to the second edition 11
Preface to the third edition 12
1 Iron and its interstitial solid solutions 14
1.1 Introduction 14
1.2 The allotropes of pure iron 15
1.2.1 Thin films and isolated particles 16
1.3 The phase transformation: a- and .-iron 17
1.3.1 Mechanisms of transformation 20
1.4 Carbon and nitrogen in solution in a- and .-iron 21
1.4.1 Solubility of carbon and nitrogen in a- and .-iron 21
1.4.2 Diffusion of solutes in iron 24
1.4.3 Precipitation of carbon and nitrogen from .-iron 26
1.5 Some practical aspects 28
Further reading 29
2 The strengthening of iron and its alloys 30
2.1 Introduction 30
2.2 Work hardening 31
2.3 Solid solution strengthening by interstitials 33
2.3.1 The yield point 34
2.3.2 The role of interstitial elements in yield phenomena 36
2.3.3 Strengthening at high interstitial concentrations 39
2.4 Substitutional solid solution strengthening of iron 40
2.5 Grain size 40
2.5.1 Hall–Petch effect 40
2.5.2 Nanostructured steels 43
2.6 Dispersion strengthening 45
2.7 An overall view 46
2.8 Some practical aspects 47
2.9 Limits to strength 48
2.9.1 Theoretical strength 48
2.9.2 Gigatubes 49
2.9.3 Fracture 50
Further reading 51
3 The iron–carbon equilibrium diagram and plain carbon steels 52
3.1 The iron–carbon equilibrium diagram 52
3.2 The austenite–ferrite transformation 55
3.3 The austenite–cementite transformation 57
3.4 The kinetics of the . . a transformation 58
3.4.1 Growth kinetics of ferrite 60
3.4.2 Growth kinetics of Widmanstätten ferrite 65
3.5 The austenite–pearlite reaction 66
3.5.1 The morphology of pearlite 67
3.5.2 The crystallography of pearlite 72
3.5.3 The kinetics of pearlite formation 73
3.5.4 The rate-controlling process 75
3.5.5 The strength of pearlite 78
3.6 Ferrite–pearlite steels 80
Further reading 82
4 The effects of alloying elements on iron–carbon alloys 84
4.1 The .- and a-phase fields 84
4.2 The distribution of alloying elements in steels 87
4.3 The effect of alloying elements on the kinetics of the ./a transformation 90
4.3.1 The effect of alloying elements on the ferrite reaction 91
4.3.2 The effect of alloying elements on the pearlite reaction 93
4.4 Structural changes resulting from alloying additions 97
4.4.1 Ferrite/alloy carbide aggregates 98
4.4.2 Alloy carbide fibres and laths 102
4.4.3 Interphase precipitation 102
4.4.4 Nucleation in supersaturated ferrite 104
4.5 Transformation diagrams for alloy steels 104
Further reading 105
5 Formation of martensite 108
5.1 Introduction 108
5.2 General characteristics 108
5.2.1 The habit plane 109
5.2.2 Orientation relationships 110
5.2.3 Structure of the interface 111
5.2.4 The shape deformation 111
5.3 The crystal structure of martensite 113
5.4 The crystallography of martensitic transformations 116
5.5 The morphology of ferrous martensites 119
5.5.1 Low carbon martensite 123
5.5.2 Medium carbon martensite 123
5.5.3 High carbon martensite 124
5.6 Kinetics of transformation to martensite 125
5.6.1 Nucleation and growth of martensite 125
5.6.2 Effect of alloying elements 129
5.6.3 The effect of deformation 131
5.6.4 Stabilization 132
5.7 The strength of martensite 133
5.8 Shape memory effect 139
Further reading 140
6 The bainite reaction 142
6.1 Introduction 142
6.2 Upper bainite (temperature range 550–400°C) 142
6.3 Lower bainite (temperature range 400–250°C) 145
6.4 The shape change 148
6.5 Carbon in bainite 148
6.6 Kinetics 152
6.7 The transition from upper to lower bainite 156
6.8 Granular bainite 157
6.9 Tempering of bainite 158
6.10 Role of alloying elements 159
Carbon 159
Other alloying elements 160
6.11 Use of bainitic steels 160
6.12 Nanostructured bainite 165
Further reading 167
7 Acicular ferrite 168
7.1 Introduction 168
7.2 Microstructure 168
7.3 Mechanism of transformation 170
7.4 The inclusions as heterogeneous nucleation sites 174
7.5 Nucleation of acicular ferrite 175
7.5.1 Lattice matching theory 175
7.5.2 Other possibilities 176
7.6 Summary 177
Further reading 177
8 The heat treatment of steels: hardenability 180
8.1 Introduction 180
8.2 Use of TTT and continuous cooling diagrams 181
8.3 Hardenability testing 183
8.3.1 The Grossman test 184
8.3.2 The Jominy end quench test 186
8.4 Effect of grain size and chemical composition on hardenability 189
8.5 Hardenability and heat treatment 190
8.6 Quenching stresses and quench cracking 192
Further reading 194
9 The tempering of martensite 196
9.1 Introduction 196
9.2 Tempering of plain carbon steels 197
9.2.1 Tempering: stage 1 198
9.2.2 Tempering: stage 2 199
9.2.3 Tempering: stage 3 199
9.2.4 Tempering: stage 4 201
9.2.5 Role of carbon content 203
9.3 Mechanical properties of tempered plain carbon steels 203
9.4 Tempering of alloy steels 204
9.4.1 The effect of alloying elements on the formation of iron carbides 206
9.4.2 The formation of alloy carbides: secondary hardening 208
9.4.3 Nucleation and growth of alloy carbides 210
9.4.4 Tempering of steels containing vanadium 212
9.4.5 Tempering of steels containing chromium 213
9.4.6 Tempering of steels containing molybdenum and tungsten 214
9.4.7 Complex alloy steels 216
9.4.8 Mechanical properties of tempered alloy steels 217
9.5 Maraging steels 220
Further reading 220
10 Thermomechanical treatment of steels 222
10.1 Introduction 222
10.2 Controlled rolling of low-alloy steels 223
10.2.1 General 223
10.2.2 Grain size control during controlled rolling 224
10.2.3 Minimum achievable grain size 229
10.2.4 Dispersion strengthening during controlled rolling 231
10.2.5 Strength of micro-alloyed steels: an overall view 233
10.3 Dual-phase steels 233
10.4 TRIP-assisted steels 236
10.4.1 Low- or zero-silicon TRIP-assisted steels 239
10.4.2 Galvanizing of TRIP-assisted steels 240
10.5 TWIP steels 242
10.6 Industrial steels subjected to thermomechanical treatments 244
Further reading 246
11 The embrittlement and fracture of steels 248
11.1 Introduction 248
11.2 Cleavage fracture in iron and steel 248
11.3 Factors influencing the onset of cleavage fracture 250
11.4 Criterion for the ductile/brittle transition 253
11.5 Practical aspects of brittle fracture 256
11.6 Ductile or fibrous fracture 258
11.6.1 General 258
11.6.2 Role of inclusions in ductility 260
11.6.3 Role of carbides in ductility 264
11.7 Intergranular embrittlement 265
11.7.1 Temper embrittlement 266
11.7.2 Overheating and burning 269
Further reading 271
12 Stainless steel 272
12.1 Introduction 272
12.2 The iron–chromium–nickel system 272
12.3 Chromium carbide in Cr–Ni austenitic steels 277
12.4 Precipitation of niobium and titanium carbides 280
12.5 Nitrides in austenitic steels 283
12.6 Intermetallic precipitation in austenite 283
12.7 Austenitic steels in practical applications 286
12.8 Duplex and ferritic stainless steels 287
12.9 Mechanically alloyed stainless steels 291
12.10 The transformation of metastable austenite 294
Further reading 299
13 Weld microstructures 300
13.1 Introduction 300
13.2 The fusion zone 300
13.2.1 Weld solidification 300
13.2.2 The as-deposit microstructure 302
13.2.3 Allotriomorphic ferrite 304
13.2.4 Widmanstätten ferrite and acicular ferrite 305
13.2.5 Sensitivity to carbon 307
13.3 The HAZ 311
13.3.1 Heat flow 311
13.3.2 Microstructural zones 312
13.3.3 The coarse-grained austenite 313
13.3.4 Fine-grained austenite zone 318
13.3.5 Partially austenitic regions and local brittle zones 319
Further reading 319
14 Modelling of microstructure and properties 320
14.1 Introduction 320
14.2 Example 1: alloy design – high-strength bainitic steel 322
14.2.1 Calculation of the T'0 curve 324
14.2.2 The improvement in toughness 326
14.2.3 The precision of the model 328
14.3 Example 2: mechanical properties of mixed microstructures 328
14.3.1 Calculation of the strength of individual phases 329
14.3.2 Iron and substitutional solutes 330
14.3.3 Carbon 330
14.3.4 Dislocations 331
14.3.5 Lath size 331
14.3.6 Martensite composition and transformation temperature 331
14.3.7 Strength of mixed microstructures 332
14.4 Methods 334
14.4.1 Electron theory 335
14.4.2 Phase diagram calculations and thermodynamics 336
14.5 Kinetics 339
14.5.1 Finite difference method 341
14.6 Finite element method 342
14.7 Neural networks 343
14.8 Defining characteristics of models 346
Further reading 347
Index 348