BAINITE IN STEELS[13:43 3/9/01 C:/3B2 Templates/keith/3750 BAINITE.605/3750-prelims.3d] Ref: 0000 Auth: Title: Chapter 00 Page: 1 1-24[13:43 3/9/01 C:/3B2 Templates/keith/3750 BAINITE.605/3750-prelims.3d] Ref: 0000 Auth: Title: Chapter 00 Page: 2 1-24BAINITEIN STEELSTransformations, Microstructureand PropertiesSECOND EDITIONH. K. D. H. BHADESHIAProfessor of Physical MetallurgyUniversity of CambridgeFellow of Darwin College, Cambridge[13:43 3/9/01 C:/3B2 Templates/keith/3750 BAINITE.605/3750-prelims.3d] Ref: 0000 Auth: Title: Chapter 00 Page: 3 1-24Book 0735Second edition rst published in 2001 byIOM Communications Ltd1 Carlton House TerraceLondon SW1Y 3DB# 2001 IOM Communications LtdAll rights reservedISBN 1-86125-112-2IOM Communicataions Ltdis a wholly-owned subsidiary ofThe Institute of MaterialsFirst edition published in 1992 byThe Institute of MaterialsTypeset in the UK byKeyset Composition, ColchesterPrinted and bound in the UK atThe University Press, Cambridge[13:43 3/9/01 C:/3B2 Templates/keith/3750 BAINITE.605/3750-prelims.3d] Ref: 0000 Auth: Title: Chapter 00 Page: 4 1-24PrefaceComputational metallurgy has grown rapidly over the last twenty years andthe subject has been embraced by industry with remarkable enthusiasm, result-ing in close collaborations and long term partnerships between industry andacademic research laboratories. No longer are alloys designed from experiencealone but calculations are used to reduce the task and to introduce creativity.There are now numerous examples of protable commercial products resultingfrom the application of this type of research.The fact that bainitic steels have featured prominently in this kind of metal-lurgy is a testimony to the depth of understanding that has been achieved. Thehighest ever combinations of strength and toughness (1600 MPa, 130 MPa m12)have been obtained in bainitic steels invented using theory alone. Opticallyvisible bainite has been obtained under conditions where the diffusion distanceof an iron atom is just 1017m. Automobiles have become safer because of theincorporation of bainitecontaining strong steels to protect against sidewayscollisions. Gigantic magnetic elds have been used to stimulate bainite. Newtungstencontaining creepresistant bainitic steels, which can be used withoutpostweld heat treatment have now been in service for more than four years.Experimental techniques invented to characterise the nucleation of bainite onceramic particles have been emulated in other elds of metallurgy.Atomic resolution has shown that like ordinary bainite, substitutionalsolutes simply do not diffuse during the growth of acicular ferrite. Themechanism of carbide precipitation in bainite is better understood; butwouldn't it be nice if the displacements due to precipitation could becharacterised?The focus has shifted from stress to strainaffected transformation. Indeed, ithas been proposed that `there is no mechanism by which plastic strain canretard reconstructive transformation. Likewise, only displacive transform-ations can be mechanically stabilised.' This provides a simple way of estab-lishing the atomic mechanism of transformation. The proposal has not yet beencontradicted.Bainite is thriving as a material. Most of the new products based on bainiteare manufactured by large steel industries. There are in addition, universityspinoffs. In one case, a large company has been created to manufacture andmarket only bainitic steels; the company concerned is possibly unique in[13:43 3/9/01 C:/3B2 Templates/keith/3750 BAINITE.605/3750-prelims.3d] Ref: 0000 Auth: Title: Chapter 00 Page: 5 1-24vhaving the word `bainite' in its title. In another case a edgling `dot-com' hasbeen created to market the software useful in modelling the microstructure andproperties of bainitic and other steels. A short monograph on bainite is nowavailable in seven different languages on the world wide web.Much has changed since the rst edition of this book. There is a new clarityin the concepts associated with solidstate transformations. There is eventransparency in the denition of problems which are not yet understood. Tosummarise, I sense real progress. It was useful therefore to write a secondedition rather than just reprint the rst. As with the rst edition, this book ismeant for all who are interested in transformations in steels or who are curiousabout phase changes in general.Preface[13:43 3/9/01 C:/3B2 Templates/keith/3750 BAINITE.605/3750-prelims.3d] Ref: 0000 Auth: Title: Chapter 00 Page: 6 1-24viAcknowledgementsThis book has developed out of a long standing interest in the subject of bainiteand I am grateful to many friends for their help and advice. In particular, Ihave beneted enormously from the support of Professor J. W. Christian,Professor M. Cohen, Dr. S. A. David, Professor D. V. Edmonds, Dr. H.Harada, Professor Sir Robert Honeycombe, Professor D. Hull, Professor C. J.Humphreys, Professor J. F. Knott, Professor G. B. Olson and Professor C. M.Wayman.I have over the years enjoyed the privilege of working with many colleagueswho have contributed to my understanding of bainite; J. R. Yang, M.Strangwood, A. Sugden, A. Ali, Shahid A. Khan, S. Mujahid, M. Takahashi,G. Rees and S. Babu, J. M. Gregg, S. V. Parker, N. Chester, S. B. Singh, S. J.Jones, M. Lord, E. Swallow, P. Shipway, P. Jacques and F. G. Caballero, T.Sourmail, H. S. Lalam and M. A. YescasGonzalez, deserve a special mentionin this respect.I should also like to express my gratitude to John Garnham for being sogenerous with his knowledge on bainitic rail steels, to David Gooch for dis-cussions on creep resistant bainitic steels, to LarsErik Svensson for introdu-cing me to the acicular ferrite, and to Greg Olson for so many inspiringdiscussions on bainite. In addition, I would like to thank H.O. Andren, S. S.Babu, G. Barritte, P. Clayton, D. V. Edmonds, M. Farooque, G. Fourlaris, I.Gutierrez, P. Jacques, B. Josefsson, T. Maki, Y. Ohmori, H. Ohtsuka, M. Oka,J. Race, G. Rees, J. M. RodriguezIbabe, M. Takahashi, H. Tamehiro, R.Thomson, B. J. P. Sandvik, M. Umemoto and the late Javier J. Urcola for pro-viding micrographs, as acknowledged in the text. Fig. 1.1 is reprinted withpermission from E. C. Bain, The Alloying Elements in Steel, American Societyfor Metals, 1939.I would like to express my gratitude to Peter Danckwerts of the Institute ofMaterials for the care with which he has produced this book and for hispatience throughout the venture.I dedicate this book to Anika, Maya, Narmada and Dharamshi.[13:43 3/9/01 C:/3B2 Templates/keith/3750 BAINITE.605/3750-prelims.3d] Ref: 0000 Auth: Title: Chapter 00 Page: 7 1-24vii[13:43 3/9/01 C:/3B2 Templates/keith/3750 BAINITE.605/3750-prelims.3d] Ref: 0000 Auth: Title: Chapter 00 Page: 8 1-24ContentsPREFACE vACKNOWLEDGEMENTS viiNOMENCLATURE xvii1. INTRODUCTION 1The Discovery of Bainite 2The Early Research 4Crystallography 5The Incomplete Reaction Phenomenon 6Carbon Redistribution 8Thermodynamics 8Paraequilibrium 10Kinetics 12Bainitic Steels: Industrial Practice 15Summary of the Early Research 162. BAINITIC FERRITE 19Sheaves of Bainite 19Morphology 19Thickness of Bainite Plates 23Dislocation Density 26Quantitative Estimation of the Dislocation Density 28Chemical Composition 29Substitutional Alloying Elements 29Interstitial Alloying Elements 34Crystallography 35Autocatalytic Nucleation 42Crystallographic Theory 44Application to Bainite 47High-Resolution Studies of the Shape Change 50The Shape Change: Further Considerations 51The Shape Change and The Superledge Mechanism 56The Structure of the Interface 57The Crystallography of a Lath of Bainite 58[13:43 3/9/01 C:/3B2 Templates/keith/3750 BAINITE.605/3750-prelims.3d] Ref: 0000 Auth: Title: Chapter 00 Page: 9 1-24ixMicrostructure of Bainite: The Midrib 59Summary 603. CARBIDE PRECIPITATION 63Upper Bainite 63Lower Bainite 66Precipitation within Lower Bainitic Ferrite 68Precipitation between Lower Bainitic Ferrite Platelets 70Kinetics of Carbide Precipitation 71Partitioning and Distribution of Carbon 71Kinetics of Precipitation from Residual Austenite 73Kinetics of Precipitation within Bainitic Ferrite 74Crystallography of Carbide Precipitation in Bainite 76Cementite: Orientation Relationships 76The Habit Plane of Cementite 77Three-Phase Crystallography 77Interphase Precipitation 79Relief of Strain Energy 81Epsilon-Carbide 81Eta-Carbide 82Chi-Carbide 83Chemical Composition of Bainitic Carbides 85Summary 884. TEMPERING OF BAINITE 91Introduction 91Tempering Kinetics 94Tempering of Steels Containing Austenite 94Redistribution of Substitutional Solutes 95Decomposition of Austenite 96Coarsening of Cementite 98Secondary Hardening and The Precipitation of Alloy Carbides 100Changes in the Composition of Cementite 101Remanent Life Prediction 103Theory for Carbide Enrichment 106Effect of Carbon on Carbide Enrichment 107Sequence of Alloy Carbide Precipitation 108Effect of Starting Microstructure on Tempering Reactions 112Changes in the Composition of Alloy Carbides 113Precipitation Hardening with Copper 113Summary 115Contents[13:43 3/9/01 C:/3B2 Templates/keith/3750 BAINITE.605/3750-prelims.3d] Ref: 0000 Auth: Title: Chapter 00 Page: 10 1-24x5. THERMODYNAMICS 117Deviations from Equilibrium 117Chemical Potential 118Stored Energy due to Transformation 120Thermodynamics of Growth 122Substitutional Solutes during Growth 122Interstitial Solutes during Growth 122Approach to Equilibrium 126Summary 1286. KINETICS 129Thermodynamics of Nucleation 130Transformation-Start Temperature 131Evolution of the Nucleus 132Possible Mechanisms of Nucleation 135Bainite Nucleation 139Empirical Equation for the Bainite-Start Temperature 140The Nucleation Rate 141Growth Rate 142Theory for the Lengthening of Plates 143Growth Rate of Sheaves of Bainite 146Growth Rate of Sub-Units of Bainite 146Solute-Drag 147Partitioning of Carbon from Supersaturated Bainitic Ferrite 150Growth with Partial Supersaturation 152Stability 153The Interface Response Functions 155Calculated Data on Transformation with Partial Supersaturation 159Summary 161Cooperative Growth of Ferrite and Cementite 161Overall Transformation Kinetics 163Isothermal Transformation Kinetics 163Mechanistic Formulation of the Avrami Equation 164Austenite Grain Size Effects 166Anisothermal Transformation Kinetics 168Simultaneous Transformations 169Special Cases 169Precipitation in Secondary Hardening Steels 170Time-Temperature-Transformation (TTT) Diagrams 171Continuous Cooling Transformation Diagrams 174Boron, Sulphur and the Rare Earth Elements 177Superhardenability 180Contents[13:43 3/9/01 C:/3B2 Templates/keith/3750 BAINITE.605/3750-prelims.3d] Ref: 0000 Auth: Title: Chapter 00 Page: 11 1-24xiThe Effect of Chemical Segregation 182Martensitic Transformation in Partially Bainitic Steels 185Autocatalysis 185Summary 1877. UPPER & LOWER BAINITE 189The Matas and Hehemann Model 189Quantitative Model 191Time to Decarburise Supersaturated Ferrite 191Kinetics of Cementite Precipitation 191Quantitative Estimation of the Transition Temperature 194Comparison of Theory and Experimental Data 196Mixed Microstructures Obtained By Isothermal Transformation 196Other Consequences of the Transition 199Comparison with the Tempering of Martensite 199Summary 2008. STRESS AND STRAIN EFFECTS 201The Mechanical Driving Force 202The Bd Temperature 204General Observations 206Externally Applied Stress 206Internally Generated Stress 206Plastic Deformation and Mechanical Stabilisation 207Technological Implications of Mechanical Stabilisation 214The Effect on Microstructure 214The Effect of Hydrostatic Pressure 216Mechanical Stability of Retained Austenite 217Transformation under Constraint: Residual Stresses 218Anisotropic Strain Due to Transformation Plasticity 219Stress-Affected Carbide Precipitation 220Summary 2219. FROM BAINITE TO AUSTENITE 225Heating a Mixture of Austenite and Upper Bainitic Ferrite 226OneDimensional Growth From a Mixture of Austenite andBainitic Ferrite 230Estimation of the Parabolic Thickening Rate Constant 232Anisothermal Transformation 234Heating a Mixture of Cementite and Bainitic Ferrite 234Effects Associated with Rapid Heating 235Summary 235Contents[13:43 3/9/01 C:/3B2 Templates/keith/3750 BAINITE.605/3750-prelims.3d] Ref: 0000 Auth: Title: Chapter 00 Page: 12 1-24xii10. ACICULAR FERRITE 237General Characteristics and Morphology 237Mechanism of Growth 240Mechanism of Nucleation 243Nucleation and the Role of Inclusions 245Aluminium and Titanium Oxides 248Sulphur 250Phosphorus 252Nitrogen, Titanium and Boron 254Boron and Hydrogen 259Stereological Effects 259Effect of Inclusions on the Austenite Grain Size in Welds 260Inuence of Other Transformation Products 260Some Specic Effects of Allotriomorphic Ferrite 262Lower Acicular Ferrite 265Stress-Affected Acicular Ferrite 269Effect of Strain on the Acicular Ferrite Transformation 269Inoculated Acicular Ferrite Steels 269Structural Steel 271Steelmaking Technology for the Inoculated Alloys 274Summary 27511. OTHER MORPHOLOGIES OF BAINITE 277Granular Bainite 277Inverse Bainite 279Columnar Bainite 279`Pearlitic' Bainite 281Grain Boundary Lower Bainite 282Summary 28312. MECHANICAL PROPERTIES 285General Introduction 285The Strength of Bainite 286Hardness 286Tensile Strength 289Effect of Austenite Grain Size 289Effect of Tempering on Strength 291The Strength Differential Effect 291Temperature Dependence of Strength 293Ratio of Proof Stress to Ultimate Tensile Strength 293Ductility 296Ductility: The Role of Retained Austenite 297Impact Toughness 298Contents[13:43 3/9/01 C:/3B2 Templates/keith/3750 BAINITE.605/3750-prelims.3d] Ref: 0000 Auth: Title: Chapter 00 Page: 13 1-24xiiiFully Bainitic Structures 300Fracture Mechanics Approach to Toughness 301Microstructural Interpretation of KIC 302Cleavage Crack Path 307Temper Embrittlement 3076508C Reversible Temper Embrittlement 3073003508C Temper Embrittlement 3093003508C Tempered-Martensite Embrittlement 309The Fatigue Resistance of Bainitic Steels 310Fatigue of Smooth Specimens 311Fatigue Crack Growth Rates 314Fatigue in Laser-Hardened Samples 318Fatigue and Retained Austenite 319Corrosion Fatigue 319Stress Corrosion Resistance 321The Creep Resistance of Bainitic Steels 323Heat Treatment 326214Cr1Mo Type Steels 3271CrMoV Type Steels 32714CrMoV Type Steels 329Enhanced CrMo Bainitic Steels 329Tungsten-Strengthened Steels 331Regenerative Heat Treatments 332Transition Metal Joints 334Reduced-Activation Steels 336Steels with Mixed Microstructures 339Summary 34013. MODERN BAINITIC ALLOYS 343Alternatives to the FerritePearlite Microstructure 343Strength 345Bainitic Steels 347Controlled-Rolling of Bainitic Steels 348Crystallographic Texture 350Rapidly Cooled Control-Rolled Steels 353Pipeline and Plate Steels 353Process Parameters 355Chemical Segregation 358Steels with a High Formability 358TRIP-Assisted Steels 362Transformations During Intercritical Annealing 365Dieless Drawn Bainitic Steels 366Contents[13:43 3/9/01 C:/3B2 Templates/keith/3750 BAINITE.605/3750-prelims.3d] Ref: 0000 Auth: Title: Chapter 00 Page: 14 1-24xivUltra-Low Carbon Bainitic Steels 368Bainitic Forging Steels 370High Strength Bainitic Steels without Carbides 373Thermomechanically Processed High-Strength Steels 377Ausformed Bainitic Steels 378Strain-Tempered Bainitic Steels 380Creep Tempering of Bainite 380Bainite in Rail Steels 382Track Materials 382Silicon-rich Carbide-free Bainitic Rail Steels 385Wheels 387Bearing Alloys 387Bainitic Cast Irons 388Austempered Ductile Cast Irons 389Wear of Bainitic Cast Irons 39514. OTHER ASPECTS 397Bainite in Iron and its Substitutional Alloys 397The Weldability of Bainitic Steels 397Electrical Resistance 399Internal Friction 401Internal Stress 401Bainite in IronNitrogen Alloys 402Effect of Hydrogen on Bainite Formation 40315. THE TRANSFORMATIONS IN STEEL 405Key Characteristics of Transformations Steels 408Notes Related to Table 15.1 40816. REFERENCES 41117. AUTHOR INDEX 44118. SUBJECT INDEX 449Contents[13:43 3/9/01 C:/3B2 Templates/keith/3750 BAINITE.605/3750-prelims.3d] Ref: 0000 Auth: Title: Chapter 00 Page: 15 1-24xv[13:43 3/9/01 C:/3B2 Templates/keith/3750 BAINITE.605/3750-prelims.3d] Ref: 0000 Auth: Title: Chapter 00 Page: 16 1-24Nomenclaturea Length of an edge crack^am Minimum detectable increase in austenite layer thicknessA Mean areal intercept in stereologyAc3 Temperature at which a sample becomes fully austenitic duringheatingAe3 Temperature separating the c and phase elds for a specicalloyAr3 Temperature at which an austenitic sample begins to transform toferrite during coolingAf Temperature at which the transformation to austenite is completeAi Atomic weight of element iAs Temperature at which the transformation to austenite beginsAs Mean free slip area in statistical theory for plasticity (Kocks, 1966)B Matrix representing the Bain deformationBd Highest temperature at which bainite forms under the inuence ofanexternally applied stressBS Bainite-start temperatureBo A temperature below which bainitic transformation is consideredto be stress-assisted and above which it is considered to be strain-induced, during transformation under the inuence of an exter-nally applied stressc Length of an edge crack, or length of a microcrack nucleuscd Diameter of a pennyshaped crack in a spheroidal particlecc0i Concentration of element i in phase c which is in equilibriumwith phase 0co Carbide thicknessCi Constants, with i = 1. 2. 3 . . .d Interatomic spacing along a specic crystallographic directiond Vector describing the shear component of an IPSDc Diffusivity of carbon in ferriteD or D Diffusivity of carbon in austeniteDci Diffusivity of element i phase cDeff Effective diffusion coefcientD Weighted average diffusivity of carbon in austenite[13:43 3/9/01 C:/3B2 Templates/keith/3750 BAINITE.605/3750-prelims.3d] Ref: 0000 Auth: Title: Chapter 00 Page: 17 1-24xviiE Young's Modulusf1 Normalised supersaturationfC Activity coefcient for carbon in austenitef+ Attempt frequency for atomic jumps across an interfaceG Growth rate^Gm Molar Gibbs free energyGN Function specifying the free energy change needed in order toobtain a detectable rate of nucleation for Widmanstatten andbainiteGcN Function specifying the critical value of ^Gcat the MS tempera-tureG+ Activation free energy for nucleation, or for interfacial motionG+O Activation free energy to overcome the resistance to dislocationmotion without the aid of a chemical driving forceG+1 Activation free energy for the growth of an embryo into a nucleusG+2 Activation free energy for the transfer of atoms across thenucleus/matrix interfaceGdd Free energy dissipated in the process of solute diffusion ahead ofan interfaceGF Free energy per unit area of fault planeG0i Molar Gibbs free energy of pure iGid Free energy dissipated in the transfer of atoms across an interfaceG/id Free energy term describing the maximum glide resistance of dis-locationsGs Strain energy per moleGSB Stored energy ofGSW Stored energy of bainite^G General term representing driving force^GCHEM Chemical driving force^Gm Molar Gibbs free energy change on transformation; alternatively,the maximum molar Gibbs free energy change accompanyingnucleation^GMECH Mechanical driving force^GSTRAIN Coherency strain energy during nucleation^G c Free energy change for transformation without compositionchangehc Ledge height at the interface between c and the parent phaseH Hardness of martensiteHF Hardness of tempered martensite when all excess carbon hasprecipitatedH0 Hardness of virgin martensiteH1 A function in the theory of diffusioncontrolled growthNomenclature[13:43 3/9/01 C:/3B2 Templates/keith/3750 BAINITE.605/3750-prelims.3d] Ref: 0000 Auth: Title: Chapter 00 Page: 18 1-24xviii^HcEnthalpy change during the c transformationvI Nucleation rate per unit volumeJ Diffusion uxk Boltzmann constantkA Constant in the Avrami equationke Equilibrium solute partitioning coefcientkg Constant relating lath size to strengthki Partitioning coefcient for alloying element ikp Coefcient representing the strengthening effect of cementite par-ticles; alternatively, a solute partitioning coefcientkc Coefcient in an equation for the strength of tempered martensiteKI Stress intensication factor in fracture mechanicsKIC Critical value of KI, a measure of the toughness of a materialKISCC Threshold value of the stress intensity below which stresscorrosion cracks do not grow at a perceptible rate^K Stress intensity range during fatigue testing^KO Threshold value of the stress intensity range during fatigue crackgrowth studies^lm Maximum relative length contraction due to isothermal reausteni-tisationL Mean intercept length in stereology, grain sizeLS Lower bainite start temperaturem Paris constant in fracture mechanicsmi Mass fraction of element iM Mobility of an interfaceMd Highest temperature at which martensite forms under theinuence of an externally applied stressMS Martensite start temperaturen Time exponent in the Avrami equationnA Number of atoms in an embryo involved in nucleationnFe Number of iron atoms per unit volume of cnp Number of closepacked planes involved in the faulting processduring displacive nucleationN Number of cycles in fatigue loadingNv Number of particles per unit volumep Peclet number (a dimensionless velocity) or autocatalytic factorP PressureP Matrix representing a homogeneous invariantplane straindeformationq Half the increase in the thickness of austenite during one-dimensional growthQ Activation energyNomenclature[13:43 3/9/01 C:/3B2 Templates/keith/3750 BAINITE.605/3750-prelims.3d] Ref: 0000 Auth: Title: Chapter 00 Page: 19 1-24xixQ Matrix representing an inhomogeneous latticeinvariant deform-ationr Radius of a disc; alternatively, the distance ahead of a crack tip;alternatively the tip radius of a growing plater1 Proof stress to ultimate tensile stress ratior2 Ratio of oa to osrC Critical distance in fracture mechanics, related to KIC; alternatively,critical tip radius at which the growth of a plate ceasesre Value of r2 at the endurance limit in fatiguer Mean particle radius at time tro Mean particle radius at time zeroR Universal gas constant; alternatively, the semiaxis of an oblateellipsoidRd Rate at which growing austenite dilutess Shear component of the IPS shape deformationS Deformation matrix in the crystallographic theory of martensiteS1, S2 Functions in the Trivedi model for the growth of paraboliccylindersSV Interfacial area per unit volumet Time; alternatively, the thickness of a disct1 Time for isothermal transformation to bainite during austemper-ing of cast iront2 Time to the beginning of carbide precipitation from austeniteduring austemperingta Time required to reach a given fraction of isothermal trans-formationtc Time required for a subunit to reach a limiting sizetd Time required to decarburise a plate of bainiteti Time interval for step i in a series of isothermal heat treatmentst0 Time for the precipitation of cementite from ferrite^t Time interval between the nucleation of successive subunitduring sheaf lengtheningT TemperatureTc Critical Zener ordering temperature for carbon atoms in ferrite;alternatively, the temperature below which cementite can inprinciple precipitate in association with upper bainitic ferriteTh The temperature below which the nucleation of displacive trans-formations rst becomes possible at a detectable rateTi Isothermal transformation temperatureTF Temperature at which accelerated cooling is stoppedT0 Temperature at which and c of the same composition have thesame free energyNomenclature[13:43 3/9/01 C:/3B2 Templates/keith/3750 BAINITE.605/3750-prelims.3d] Ref: 0000 Auth: Title: Chapter 00 Page: 20 1-24xxTom As T0, but forcing the Zener ordering of carbon atoms in the ferriteT/0 As T0, but accounting for the stored energy of ferriteTM Melting temperatureTr Temperature below which a midrib is found in lower bainiteplatesTR Temperature at which rolling deformation is stoppedTt Transition temperature for impact toughnessT Isothermal reaustenitisation temperatureTcAustenite to ferrite transformation temperaturev+ Activation volumeV Volume of a sampleVcVolume of phase cVce Extended volume of phase cVd Diffusion eld velocityVi Velocity of an interface calculated on the basis of its mobilityVk Velocity of an interface calculated using a solute trapping functionVI Volume fraction of inclusionsVl Plate lengthening rateVmax Maximum volume fractionVSmax Maximum volume of a sheaf^Vm Change in molar volume on transformationVS Sheaf lengthening rate^Vv Minimum detectable change in volume fractionVsc Velocity of steps in the c/parent phase interfaceV0m Molar volume of phase 0Vt Volume per particlew Thickness of a bainite subunitwi Weight percent of element iwsoli Weight percent of element i, in solutionW Width of a fracture toughness specimen for a KIC testx Average mole fraction of carbon in an alloyxm Maximum carbon supersaturation permitted in ferrite, on thermo-dynamic groundsxc Carbon in cat interfacex Carbon concentration in austenitexI Carbon concentration in austenite before the start of austenitegrowthxcMole fraction of carbon in ferrite which is in equilibrium orparaequilibrium with austenitexcMole fraction of carbon in austenite which is in equilibrium orparaequilibrium with ferritexT/0Carbon concentration given by the T/0 curveNomenclature[13:43 3/9/01 C:/3B2 Templates/keith/3750 BAINITE.605/3750-prelims.3d] Ref: 0000 Auth: Title: Chapter 00 Page: 21 1-24xxixAe3 Carbon concentration given by the Ae3 curvex0Thickness of cementite particlexX Concentration of X in cementitexX Average concentration of X in cementitexc0X Concentration of X in ferrite which is in equilibrium with cemen-titey Semiaxis of an oblate ellipsoidY Compliance function in fracture mechanics; alternatively, a con-stant in the theory of thermally activated dislocation motionz Coordinate normal to the interface plane; alternatively, a constantin the theory of thermally activated dislocation motionzd Effective diffusion distanceZ Position of the interface along coordinate z.c Allotriomorphic or idiomorphic ferrite which forms by reconstruc-tive transformationc1 Onedimensional parabolic thickening rate constantu Constant in weld metal inclusion formation theory; alternatively,an autocatalytic factor AusteniteI Capillarity constantcb Boundary thickness^ Uniform dilatation accompanying transformation; alternatively,the average distance between neighbouring particles in temperedmartensite0 Cementitec1 Average transverse thickness of dislocation cell structure in mar-tensitei Mean % planar mist between inclusion and ferrite` Interledge spacing; alternatively an intersite jump distance duringdiffusionj Shear modulusji Chemical potential of element ii Poisson's ratioj DensityjA Spacing of closepacked planesjd Dislocation densityt Incubation time before the growth of an individual particle beginsduring isothermal transformation, or before a detectable degree ofoverall transformation. Alternatively, the shear stress resolvedalong the shear directionto Resistance to dislocation motiontj Athermal resistance to dislocation motionNomenclature[13:43 3/9/01 C:/3B2 Templates/keith/3750 BAINITE.605/3750-prelims.3d] Ref: 0000 Auth: Title: Chapter 00 Page: 22 1-24xxiii Percent planar matching during epitaxial nucleation Constant in weld metal inclusion formation theoryo Applied stressoa Cyclic stress amplitude in a fatigue testoC Critical stress in fracture mechanics, related to KIC; alternatively,solid solution strengthening due to carbonoF Stress necessary for the propagation of cleavage fractureoFe Strength of pure annealed ironog Strengthening due to grain boundariesoN Normal stress on the habit planeop Work of fracture, per unit area of crack surfaceor Stress as a function of the distance r ahead of the crack tipos Saturation value of oiy in a fatigue testoSS Solid solution strengthening due to substitutional solutesoiy Instantaneous ow stress at any particular stage of a testoy Yield stress or proof stress in monotonic loading testso0c 0c interface free energy per unit areao0 Intrinsic strength of martensite, not including microstructuralstrengthening! Volume per atom!Fe Volume of an atom of Fe in c!c Volume of a molecule of Fe3C less 3!Fe Volume fraction, or volume fraction divided by the equilibrium orsome other limiting volume fractiona A specic value of . Uniaxial dilatation normal to the habit planeASM American Society for MetalsASTM American Society for Testing MaterialsBCC Body-centred cubicBCT Body-centred tetragonalCE Carbon equivalentFATT Fracture assessed ductile-brittle transition temperatureFCC Face-centred cubicHAZ Heat-affected zone of welded jointsHREM High-resolution transmission electron microscopyHSLA High-strength low-alloy (steels)HV Vickers HardnessIIW International Institute for WeldingIPS Invariant-Plane Strain shape changeKS Kurdjumov-SachsLEFM Linear-Elastic-Fracture-MechanicsNW Nishiyama-WasermannNomenclature[13:43 3/9/01 C:/3B2 Templates/keith/3750 BAINITE.605/3750-prelims.3d] Ref: 0000 Auth: Title: Chapter 00 Page: 23 1-24xxiiip.p.m. Parts per million by weightSCR Stress corrosion cracking resistanceSSAW Self-Shielded Arc WeldTRIP TransformationInduced PlasticityTTT Time-Temperature-Transformation diagramULCB Ultralow carbon bainitic steelUTS Ultimate tensile strengthNote: The term residual austenite refers to the austenite that exists at the reac-tion temperature during transformation to bainite, whereas the term retainedaustenite refers to the austenite which remains untransformed after cooling thespecimen to ambient temperature.Nomenclature[13:43 3/9/01 C:/3B2 Templates/keith/3750 BAINITE.605/3750-prelims.3d] Ref: 0000 Auth: Title: Chapter 00 Page: 24 1-24xxiv1 IntroductionWe begin with a historical survey of the exciting early days of metallurgicalresearch during which bainite was discovered, covering the period up to about1960, with occasional excursions into more modern literature. The earlyresearch was usually well conceived and was carried out with enthusiasm.Many of the original concepts survive to this day and others have been con-rmed using the advanced experimental techniques now available. The thirtyyears or so prior to the discovery of bainite were in many respects formative asfar as the whole subject of metallurgy is concerned. The details of that periodare documented in the several textbooks and articles covering the history ofmetallurgy,| but a few facts deserve special mention, if only as an indication ofthe state-of-the-art for the period between 19201930.The idea that martensite was an intermediate stage in the formation of pear-lite was no longer accepted, although it continued to be taught until well after1920. The u-iron controversy, in which the property changes caused by theparamagnetic to ferromagnetic transition in ferrite were attributed to the exis-tence of another allotropic modication (u) of iron, was also in its dying days.The rst evidence that a solid solution is an intimate mixture of solvent andsolute atoms in a single phase was beginning to emerge (Bain, 1921) and it soonbecame clear that martensite consists of carbon dispersed atomically as aninterstitial solid solution in a tetragonal ferrite crystal. Austenite was estab-lished to have a face-centred cubic crystal structure, which could sometimes beretained to ambient temperature by quenching. Bain had already proposed thehomogeneous deformation which could relate the face-centred cubic andbody-centred cubic or body-centred tetragonal lattices during martensitictransformation. It had been established using X-ray crystallography that thetempering of martensite led to the precipitation of cementite, or to alloy car-bides if the tempering temperature was high enough. Although the surfacerelief associated with martensitic transformation had been observed, its impor-[12:07 3/9/01 C:/3B2 Templates/keith/3750 BAINITE.605/3750-001.3d] Ref: 0000 Auth: Title: Chapter 00 Page: 1 1-181|Notable historical works include: The Sorby Centennial Symposium on the History of Metallurgy,published by the A.I.M.E. in 1965 (includes an article by Bain himself), the commentary by H. W.Paxton, Metallurgical Transactions 1 (1970) 34793500, and by H. W. Paxton and J. B. Austin,Metallurgical Transactions 3 (1972) 10351042. Paxton's 1970 article is published along with areproduction of the classic 1930 paper on the discovery of bainite by Davenport and Bain,and is based on rst hand historical knowledge obtained directly from Davenport and Bain.tance to the mechanism of transformation was not fully appreciated.Widmanstatten ferrite had been identied and was believed to precipitate onthe octahedral planes of the parent austenite; some notions of the orientationrelationship between the ferrite and austenite were also being discussed.It was an era of major discoveries and great enterprise in the metallurgy ofsteels. The time was therefore ripe for the discovery of bainite. The term`discovery' implies something new. In fact, microstructures containing bainitemust have been encountered prior to the now acknowledged discovery date,but the phase was never clearly identied because of the confused microstruc-tures that followed from the continuous cooling heat treatment procedurescommon in those days. A number of coincidental circumstances inspiredBain and others to attempt isothermal transformation experiments. That aus-tenite could be retained to ambient temperature was clear from studies ofHadeld's steel which had been used by Bain to show that austenite has aface-centred cubic structure. It was accepted that increasing the cooling ratecould lead to a greater amount of austenite being retained. Indeed, it had beendemonstrated using magnetic techniques that austenite in low-alloy steelscould exist at low temperatures for minutes prior to completing transforma-tion. The concept of isothermal transformation was already exploited in indus-try for the manufacture of patented steel wire, and Bain was aware of thisthrough his contacts at the American Steel and Wire Company. He began towonder `whether exceedingly small heated specimens rendered whollyaustenitic might successfully be brought unchanged to any intermediatetemperature at which, then their transformation could be followed' and he`enticed' E. C. Davenport to join him in putting this idea into action.1.1 The Discovery of BainiteDuring the late 1920s, in the course of these pioneering studies on the isothermaltransformation of austenite at temperatures above that at which martensite rstforms, but below that at which ne pearlite is found, Davenport and Bain(1930) discovered a new microstructure consisting of an `acicular, dark etchingaggregate' which was quite unlike the pearlite or martensite observed in thesame steel (Fig. 1.1). They originally called this microstructure martensitetroostite' since they believed that it `forms much in the manner of martensitebut is subsequently more and less tempered and succeeds in precipitatingcarbon'.The structure was found to etch more rapidly than martensite but less sothan troostite (ne pearlite). The appearance of `low-range' martensitetroostite (formed at temperatures just above the martensite-start temperatureMS) was found to be somewhat different from the `high-range' martensitetroostite formed at higher temperatures. The microstructure exhibited unusualIntroduction[12:07 3/9/01 C:/3B2 Templates/keith/3750 BAINITE.605/3750-001.3d] Ref: 0000 Auth: Title: Chapter 00 Page: 2 1-182and promising properties; it was found to be `tougher for the same hardnessthan tempered martensite' (Bain, 1939), and was the cause of much excitementat the newly established United States Steel Corporation Laboratory in NewJersey. It is relevant to note here the contributions of Lewis (1929) andRobertson (1929), who were the rst to publish the results of isothermalBainite in Steels[12:07 3/9/01 C:/3B2 Templates/keith/3750 BAINITE.605/3750-001.3d] Ref: 0000 Auth: Title: Chapter 00 Page: 3 1-183Fig. 1.1 Microstructures in a eutectoid steel: (a) Pearlite formed at 720 8C; (b)bainite obtained by isothermal transformation at 290 8C; (c) bainite obtained byisothermal transformation at 180 8C; (d) martensite. The micrographs were takenby Vilella and were published in the book The Alloying Elements in Steel (Bain,1939). Notice how the bainite etches much darker than martensite, because itsmicrostructure contains many ne carbides.transformation experiments on eutectoid steel wires, probably because of theirrelevance to patented steel. But the Davenport and Bain experiments wereunique in showing the progressive nature of the isothermal transformationof austenite, using both metallography and dilatometry. Their experimentswere successful because they utilised very thin samples. Their method ofrepresenting the kinetic data in the form of time-temperature-transformationcurves turned out to be so simple and elegant, that it would be inconceivable tond any contemporary materials scientist who has not been trained in the useor construction of `TTT' diagrams.In 1934, the research staff of the laboratory named the microstructure`Bainite' in honour of their colleague E. C. Bain who had inspired the studies,and presented him with the rst ever photomicrograph of bainite, taken at amagnication of 1000 (Smith, 1960; Bain, 1963).The name `bainite' did not immediately catch on. It was used rathermodestly even by Bain and his co-workers. In a paper on the nomenclatureof transformation products in steels, Vilella, Guellich and Bain (1936) men-tioned an `unnamed, dark etching, acicular aggregate somewhat similar tomartensite' when referring to bainite. Hoyt, in his discussion to this paperappealed to the authors to name the structure, since it had rst been producedand observed in their laboratory. Davenport (1939) ambiguously referred to thestructure, sometimes calling it `a rapid etching acicular structure', at othertimes calling it bainite. In 1940, Greninger and Troiano used the term`Austempering Structures' instead of bainite. The 1942 edition of the bookThe Structure of Steel (and its reprinted version of 1947) by Gregory andSimmons contains no mention of bainite.The high-range and low-range variants of bainite were later called `upperbainite' and `lower bainite' respectively (Mehl, 1939) and this terminologyremains useful to this day. Smith and Mehl (1942) coined the term `featherybainite' for upper bainite which forms largely, if not exclusively, at theaustenite grain boundaries in the form of bundles of plates, and only at highreaction temperatures, but this description has not found frequent use. Bothupper and lower bainite were found to consist of aggregates of parallel plates,aggregates which were later designated sheaves of bainite (Aaronson andWells, 1956).1.2 The Early ResearchEarly work into the nature of bainite continued to emphasise its similarity withmartensite. Bainite was believed to form with a supersaturation of carbon(Wever, 1932; Wever and Jellinghaus, 1932; Portevin and Jolivet, 1937,1938;Portevin and Chevenard, 1937). It had been postulated that the transformationinvolves the abrupt formation of at plates of supersaturated ferrite alongIntroduction[12:07 3/9/01 C:/3B2 Templates/keith/3750 BAINITE.605/3750-001.3d] Ref: 0000 Auth: Title: Chapter 00 Page: 4 1-184certain crystallographic planes of the austenite grain (Vilella et al., 1936). Theferrite was then supposed to decarburise by rejecting carbon at a rate depend-ing on temperature, leading to the formation of carbide particles which werequite unlike the lamellar cementite phase associated with pearlite. The trans-formation was believed to be in essence martensitic, `even though the tempera-ture be such as to limit the actual life of the quasi-martensite to millionths of asecond'. Bain (1939) reiterated this view in his book The Alloying Elements inSteel. Isothermal transformation studies were by then becoming very popularand led to a steady accumulation of data on the bainite reaction, still variouslyreferred to as the intermediate transformation', `dark etching acicular consti-tuent', `acicular ferrite', etc.In many respects, isothermal transformation experiments led to the clarica-tion of microstructures, since individual phases could be studied in isolation.There was, however, room for difculties even after the technique became wellestablished. For alloys of appropriate composition, the upper ranges of bainiteformation were found to overlap with those of pearlite, preceded in some casesby the growth of proeutectoid ferrite. The nomenclature thus became confusedsince the ferrite which formed rst was variously described as massive ferrite,grain boundary ferrite, acicular ferrite, Widmanstatten ferrite, etc. On a laterview, some of these microconstituents are formed by a `displacive' or `military'transfer of the iron and substitutional solute atoms from austenite to ferrite,and are thus similar to carbon-free bainitic ferrite, whereas others form by a`reconstructive' or `civilian' transformation which is a quite different kineticprocess (Buerger, 1951; Christian, 1965a).1.2.1 CrystallographyBy measuring the crystallographic orientation of austenite using twin vestigesand light microscopy, Greninger and Troiano (1940) were able to show that thehabit plane of martensite in steels is irrational. These results were consistentwith earlier work on non-ferrous martensites and put paid to the contempor-ary view that martensite in steels forms on the octahedral planes of austenite.They also found that with one exception, the habit plane of bainite is irrational,and different from that of martensite in the same steel (Fig. 1.2). The habitplane indices varied with the transformation temperature and the averagecarbon concentration of the steel. The results implied a fundamental differencebetween bainite and martensite. Because the habit plane of bainite approachedthat of Widmanstatten ferrite at high temperatures, but the proeutectoidcementite habit at low temperatures, and because it always differed fromthat of martensite, Greninger and Troiano proposed that bainite from thevery beginning grows as an aggregate of ferrite and cementite. A competitionbetween the ferrite and cementite was supposed to cause the changes in theBainite in Steels[12:07 3/9/01 C:/3B2 Templates/keith/3750 BAINITE.605/3750-001.3d] Ref: 0000 Auth: Title: Chapter 00 Page: 5 1-185bainite habit, the ferrite controlling at high temperatures and the cementite atlow temperatures. The competition between the ferrite and cementite was thusproposed to explain the observed variation of bainite habit plane. The crystal-lographic results were later conrmed using an indirect and less accuratemethod (Smith and Mehl, 1942). These authors also showed that the orientationrelationship between bainitic ferrite and austenite does not change veryrapidly with transformation temperature and carbon content and is within afew degrees of the orientations found for martensite and Widmanstatten fer-rite, but differs considerably from that of pearlitic ferrite/austenite. Since theorientation relationship of bainite with austenite was not found to change,Smith and Mehl considered Greninger and Troianos' explanation for habitplane variation to be inadequate, implying that the habit plane cannot varyindependently of the orientation relationship.1.2.2 The Incomplete Reaction PhenomenonIt was known as long ago as 1939 that in certain alloy steels in which thepearlite change is very slow', the extent of transformation to bainite decreases,ultimately to zero, as the transformation temperature is increased (Allen et al.,1939). For example, the bainite transformation in a Fe2.98Cr0.2Mn0.38Cwt%alloy was found to begin rapidly but cease shortly afterwards, with the max-imum volume fraction of bainite obtained increasing with decreasing transfor-mation temperature (Klier and Lyman, 1944). At no temperature investigateddid the complete transformation of austenite occur solely by decomposition toIntroduction[12:07 3/9/01 C:/3B2 Templates/keith/3750 BAINITE.605/3750-001.3d] Ref: 0000 Auth: Title: Chapter 00 Page: 6 1-186Fig. 1.2 An example of the results obtained by Greninger and Troiano (1940),showing the irrational habit of bainite, which changed as a function of the trans-formation temperature. Notice also that the habit plane of bainite is different fromthat of martensite in the same steel.bainite. The residual austenite remaining untransformed after the cessation ofthe bainite reaction, reacted by another mechanism(pearlite) only after a furtherlong delay. Cottrell (1945) in his experiments on a low-alloy steel, found that theamount of bainite that formed at 525 8C (