Classi cation and quanti cation of microstructures in steels

Transcription

1 aterials perspective Classi cation and quanti cation of microstructures in steels G. Thewlis The International Institute of Welding (IIW) microstructure classi cation scheme for ferrous weld metals has been investigated as a basis for the quanti cation of complex microstructures in steels. The aim has been to cover the full range of microstructures observed in plain carbon and low alloy steel products, as well as ferritic weld metals and parent plate heat affected zones. The mechanisms of formation of the principal structures and the characteristic ferrite morphologies produced in the reconstructive and displacive transformation regimes of ferrous materials have been brie y reviewed. The classi cation and terminology used for intragranular as well as austenite grain boundary microstructural constituents have been considered, and also the way in which transformation products are orientated in space. Problems encountered in relating microstructural constituents to principal structures have been discussed in detail and solutions proposed. The microstructure classi cation and terminology used in the IIW scheme have been built upon and new terminology incorporated into a table providing descriptions of the principal structures and sub-category components. A new classi cation scheme has been de ned in the form of ow charts with guidelines for identifying the principal structures. Evaluation exercises have been carried out with the new scheme. These have shown that a reasonable degree of consistency may be obtained between operators in identifying primary ferrite, pearlite, martensite and the transformation products constituting ferrite sideplate and acicular ferrite structures, notably Widmanstätten ferrite and bainite. A means is thus provided of obtaining database information for developing microstructure property relationships, or generating data for calibrating physical models, which have the principal structures as their output. ST/5675 Keywords: Steel microstructures, Low alloy steels, Ferrite, Bainite, artensite, etallography, icrostructure classi cation, Phase transformation products The author is with Corus Research, Development and Technology, Swinden Technology Centre, oorgate, Rotherham, S6 3AR, UK (graham.thewlis@corusgroup.com). anuscript received 17 October 22; accepted 22 September 23. # 24 Io Communications Ltd. Published by aney for the Institute of aterials, inerals and ining. Introduction With the advances in computer power in recent years, there has been increasing interest in the development of models to predict the microstructure of steel products, particularly those processed by thermomechanical treatments 1 4 or fabricated by welding The driving force for much of the modelling work has been the need for computer based systems to control and optimise microstructure and mechanical properties. Linear regression analysis on large databases of information has proved a useful tool in generating composition structure property relationships.ore recently, neural network techniques 9 have enabled prediction of situations too complex for simple analytical models or multiple regression techniques. Regression and neural network models often nd use as online models for process control. However, they are restricted to well de ned data over limited ranges of composition and process parameters. Such constraints can to a large extent be circumvented by the development of physical models based on fundamental metallurgical principles. The major advantage of physical models is their general applicability. They can be used as design tools for a wide variety of new materials and products. Classical nucleation and growth theory may, for example, be used to predict the microstructure of steels processed to a given austenite grain size and cooled at different rates through the austenite to ferrite transformation temperature range. 4,6 8 The transformation sequence during cooling and the phase proportions of allotriomorphic ferrite, pearlite, Widmanstätten ferrite, bainite and martensite may be the outputs. The latter are the principal structures in the reconstructive (diffusion controlled with slow rates of reaction) and displacive (shear dominated with rapid rates of reaction) transformation regimes of continuous cooling transformation (CCT) diagrams. 1 2,1 3 While the development of regression and neural network models requires good quality database information, the development of sophisticated physical models for microstructure prediction in steels has led to a need for accurate calibration data. However, the microstructures observed in steel products are complex. A variety of reaction products may form at austenite grain boundary sites in thermomechanically processed or heat treated steels. In the fusion zone of welds, the simultaneous and competitive formation of a variety of phases from both austenite grain boundary and intragranular sites may occur while, in the parent plate heat affected zone (HAZ), steep thermal gradients may give rise to a wide range of transformationproducts. A scheme is thus required for classifying and quantifying complex steel microstructures. Classifying and quantifying the microstructures of steels has long been a contentious issue Depending on the plane of observation, constituents that are part of the same principal structure may appear morphologically different giving rise to sub-category components. Furthermore some structures may have similar morphological or generic features but be mechanistically different. A scheme for identifying the various ferrite morphologies in isothermally transformed steels was rst used by Dubé et al. 1 7 and later extended by Aaronson. 1 8 However, the effect of continuous cooling was to render the distinguishing morphological features much less distinct. Allotriomorphic ferrite morphologies were readily identi ed and also various sideplate DOI / aterials Science and Technology February 24 Vol

2 144 Thewlis Classi cation and quanti cation of microstructures in steels morphologies (often classed as bainite). Widmanstätten ferrite was dif cult to place but was regarded as a generically similar structure to bainite. Intragranular component phases such as acicular ferrite posed a much greater degree of dif culty. uch effort was made by the welding fraternity in the 198s to develop an overall microstructurequanti cation scheme for weld metals incorporating both prior austenite grain boundary and intragranular nucleated constituents, and addressing stereological effects, i.e. the way constituents are orientated in space. 1 9,2 A scheme was devised which became recognised as the International Institute of Welding (IIW) classi cation. 1 9 ost of the constituents de ned in the IIW scheme were relatively easily identi ed. Furthermore, the scheme could just as readily be applied to steels where austenite grain boundary transformations dominate, as to weld metals where intragranulartransformationsare the rule. However, identi cation of the actual transformation products constituting component structures such as ferrite sideplate and acicular ferrite has proved dif cult. Anelli and Di Nunzio 2 1 recently devised a scheme providing guidance on identifying transformation products associated with sideplate structures which has had some success, but stereological effects and intragranular constituents were not treated in depth. The objective in the current work has therefore been to investigate the IIW microstructure classi cation scheme as a basis for quanti cation of complex microstructures in steels. The overall aim has been to develop a scheme that, although requiring a basic knowledge as to the mechanism of formation of the principal structures, will be relatively easy to use given optical microscopy, standard specimen polishing and etching techniques and appropriate guidance. The approach has been to review microstructural constituents in the IIW scheme in the context of the development of principal structures found in the reconstructive and displacive transformation regimes of steels. Detailed intragranular as well as austenite grain boundary transformation products have been considered and also stereological effects. Problems relating microstructural constituents to principal structures in the IIW scheme have been investigated together with possible solutions so that a new quanti cation scheme may be developed with a much broader application range. The intention has been to cover microstructures observed in carbon (up to abount. 8%) and low alloy (up to approximately 5%) steels, as well as weld metals (up to. 1%C and 5% alloy) and weld HAZs. Classi cation of microstructures and terminology In this section, the mechanisms of formation of the principal structures and the characteristic ferrite morphologies produced in the reconstructive and displacive transformation regimes of ferrous materials are brie y reviewed. The classi cation and terminology used in the IIW scheme are described, together with that of Dubé et al. 1 7 to provide a link with the early work on classi cation of prior austenite grain boundary ferrite morphologies. Terminology used in recent work by the present author and co-workers 2 2 is also included to provide a contemporary view of complex intragranular transformations, including those generating the microstructure commonly known as acicular ferrite. RECONSTRUCTIVE TRANSFORATION REGIE In the high temperature, reconstructive transformation regime, a change from the austenite to ferrite crystal structure occurs by a reconstruction process involving movement of atoms across the c/a transformationinterface. The principal 1 Allotriomorphic and idiomorphic primary ferrite phases are ferrite and pearlite. Reactions tend to be diffusion controlled with slow rates. Ferrite In low hardenability materials, the rst phase usually forming on prior austenite grain boundaries during cooling below the A e3 temperature is classically referred to as allotriomorphic ferrite, as shown schematically in Fig. 1. The ferrite nuclei have a Kurdjumov Sachs (K S) orientation relationship with one austenite grain and grow into the adjacent austenite grain with which they should normally have a random orientation relationship. 2 3 At some lower temperature, ferrite may begin to nucleate on inclusions inside the austenite grains 2 2,2 4 and this is termed idiomorphic ferrite (see Fig. 1). The indications are that ferrite idiomorphs do not have a xed orientation relationship with the matrix grains into which they grow. 2 5 Growth at reconstructive transformation temperatures tends to be controlled by substitutional element diffusion away from the c/a interface at low undercooling and carbon diffusion at high undercooling. Various growth modes are recognised, in order of decreasing transformation temperature: 2 6 (i) local equilibrium with bulk partition of substitution alloying elements (PLE) (ii) local equilibrium with negligible partition of substitutional alloying elements (NPLE) (iii) paraequilibrium, where only the interstitial carbon atoms diffuse. The diffusion rate of carbon in austenite may be many orders of magnitude greater than that of substitutional atoms at reconstructive transformation temperatures. True equilibrium segregation during phase transformations at migrating interfaces is therefore unlikely to be achieved with regard to all components. Growth under diffusion control with local equilibrium at the interface is then envisaged. Two phases may differ either signi cantly (PLE) or negligibly (NPLE) in terms of substitutional alloy content. Element concentration or depletion spikes are invoked to satisfy the thermodynamic constraints. In many cases, as the transformation temperature is decreased, the relative rates at which elements are able to diffuse negate the assumption of local equilibrium, since the interface composition spike would be only several atomic layers thick. In such cases, the concept of paraequilibrium is applied, i.e. there is no redistribution of iron or substitution atoms at the interface between the phases and only the interstitial carbon atoms diffuse The different growth modes described above may result in signi cant changes in ferrite growth morphology from equiax grains towards a plate shape (see below). Dubé et al. 1 7 refer to prior austenite grain boundary allotriomorphic ferrite as GBF. The IIW classi cation scheme refers to the rst phase forming at reconstructive transformation temperatures as primary ferrite, termed PF. Prior austenite grain boundary primary ferrite allotriomorphs are termed PF(G) in the IIW classi cation scheme aterials Science and Technology February 24 Vol. 2

3 Thewlis Classi cation and quanti cation of microstructures in steels 145 1: intragranular ferrite idiomorphs; 2: grain boundary ferrite allotriomorphs 2 orphologies of ferrite at prior austenite grain boundary and intragranular sites in. 6%C, 1. 46%n submerged arc weld metal, continuously cooled, iced brine quenched from 67 C 22 and are usually observed in the form of polygonal grains or veins, as shown schematically in Fig. 1. Reference is made in the IIW scheme to polygonal ferrite grains in the intragranular regions (see Fig. 1) of a size approximately three times greater than those of the surrounding ferrite laths or grains. These ferrite grains in reality may be cross-sections of ferrite allotriomorphs that have grown from prior austenite grain boundaries beneath the plane of observation and have a wide range of sizes. They are termed PF(I) in the IIW scheme. The present author and co-workers 2 2 have referred to the different forms of prior austenite grain boundary primary ferrite as GB(PF), so that a distinction may be made with idiomorphic primary ferrite as described below. In weld metals, stable particle dispersed steels and some microalloyed steels, ferrite may nucleate not only at the austenite grain boundaries but also on particles inside the austenite grains 2 2,2 7 (see Fig. 2). The author and coworkers 2 2 have termed these intragranular ferrite idiomorphs I(PF). Depending on the temperature in the reconstructive regime, the intragranular ferrite morphologies 2 2 may take the form of blocks, loops, ellipses, rose petals or wedges. The IIW classi cation scheme does not have a terminology for these primary ferrite idiomorphs. Pearlite Classically, pearlite transformation may occur at austenite grain boundaries or an inhomogeneitysuch as an inclusion. 2 3 Ferrite or cementite nucleation may initiate the pearlite transformation depending on whether the steel is hypo- or hyper-eutectoid in composition. Growth of a pearlite nodule into an austenite grain proceeds with the formation of alternate ferrite and cementite plates or lamellae. Both the cementite and ferrite possess unique crystallographic orientations within the pearlite nodule. 2 3 Edgewise growth of the plates may occur and also branching of the cementite lamellae. The rate controlling process in the growth of pearlite is the diffusion of carbon. As the transformation temperature is lowered, the driving force for the reaction is increased but the diffusivity of carbon is decreased so that the pearlite interlamellar spacing is decreased. At high transformationtemperatures, pearlite is generally observed as nodules of alternate ferrite and cementite lamellae that may be quite coarse and degenerate. When viewed in cross-section, the lamellae may appear as a ferrite carbide aggregate. As the transformation temperature is lowered, the lamellae become increasingly ne until the structure becomes irresolvable under the light 1: alternate ferrite/cementite lamellae; 2: ne ferrite ± carbide aggregate; 3: irresolvable pearlite 3 Resolvable and irresolvable pearlite in. 83%C,. 5%n, as rolled rod microscope (see Fig. 3). The pearlite may then have a light etching response. Alternatively, the lamellae may become subjected to distortion and bending, appearing as a dark etching, ferrite carbide aggregate or barely resolvable, somewhat non-lamellar pearlite, often described in older nomenclature as primary troostite. 2 8,2 9 In the IIW scheme, FC(P) is used to describe lamellar pearlite, degenerate or coarse pearlite, and ne colony or irresolvable pearlite. The term FC is used to describe ferrite carbide aggregate. At reconstructive transformation temperatures, large islands of pearlite or ferrite carbide aggregate may be interspersed with prior austenite grain boundary primary ferrite PF(G). A similar situation may occur with idiomorphic primary ferrite I(PF) (see Fig. 4). 2 7 In some cases pearlite may be present as microphase (see below). DISPLACIVE TRANSFORATION REGIE In the low temperature, displacive transformation regime, a change from the austenite to ferrite crystal lattice occurs by an invariant plane strain shape change with a large shear component. Diffusion of interstitial carbon atoms may accompany the shear transformation. For a purely displacive transformation there is no movement of atoms across the c/a interface. Reactions in the displacive transformation regime tend to be rapid. The principal phases are Widmanstätten ferrite, bainite and martensite. 1: idiomorphic ferrite; 2: ferrite ± carbide aggregate; 3: irresolvable pearlite 4 Intragranular primary ferrite and pearlite in as cast,. 13%C, 2. %n, cerium sulphide particle dispersed steel 27 aterials Science and Technology February 24 Vol. 2

4 146 Thewlis Classi cation and quanti cation of microstructures in steels 5 Primary and secondary Widmanstätten ferrite WidmanstaÈ tten ferrite A classic feature of Widmanstätten ferrite formation is that it may occur at relatively low undercooling. 2 3 The growth mechanism is thought to involve the simultaneous formation of pairs of mutually accommodating plates so that less driving force is required for transformation than with bainite or martensite. 3 The ferrite plates grow rapidly with a high aspect ratio (~1 : 1), resulting in parallel arrays. Widmanstätten ferrite is not the result of a purely displacive transformation but forms by a paraequilibrium mechanism, 3,3 1 involving the rapid diffusion of interstitial carbon atoms across the advancing interface into the remaining austenite during the shear transformation. At the relatively low undercooling required for Widmanstätten ferrite formation, microphases of retained austenite, martensite or ferrite/carbide aggregate (pearlite) may be formed between the growing ferrite plates. Widmanstätten ferrite can easily be confused with bainite. Dubé et al. 1 7 describe both prior austenite grain boundary Widmanstätten ferrite and bainite as ferrite sideplate FS but reference is also made to intragranular plates IP. The IIW classi cation scheme refers to all forms of Widmanstätten ferrite and bainite as ferrite with second phase FS, although a distinction may be made in the terminology when Widmanstätten ferrite can be positively identi ed, e.g. FS(SP). Characteristically, primary Widmanstätten ferrite plates grow directly from a prior austenite grain boundary, whereas secondary Widmanstätten ferrite plates grow from allotrimorphic ferrite at the grain boundaries, as shown schematically in Fig. 5. Primary Widmanstätten ferrite plates may also grow from inclusions, while secondary Widmanstätten ferrite plates grow from intragranular idiomorphic ferrite. 2 2,3 2 Widmanstätten ferrite that grows from prior austenite grain boundary sites is usually seen as colonies of coarse sideplates with aligned microphase (see Fig. 6), which are termed FS(A) in the IIW scheme. The individual plates within an array are separated by low angle boundaries that are dif cult to resolve under the light microscope, although careful specimen polishing and etching may reveal them. Depending on the plane of observation, the microphases may appear non-aligned. When viewing a cross-section of ferrite laths that have grown from prior austenite grain boundaries beneath the plane of observation, all that may be seen are islands of microphase in a matrix of ferrite within the prior austenite grains (see Fig. 6). The Widmanstätten ferrite is then classi ed as FS(NA). The present author and co-workers 2 2 have referred to the different forms of prior austenite grain boundary Widmanstätten ferrite as GB(WF) so that a distinction may be made with intragranular Widmanstätten ferrite as described below. In the intragranular regions of weld metals and in 2 2,2 7,3 2 some steels, multiple large plates (aspect ratio> 4 : 1) of Widmanstätten ferrite with aligned microphase 1: idiomorphic ferrite; 2: prior austenite grain boundary WidmanstaÈ tten ferrite with aligned microphase; 3: prior austenite grain boundary WidmanstaÈ tten ferrite with non-aligned microphase 6 Interlocking colonies of Widmanstätten ferrite in. 5%C, 1. 35%n, HSLA steel, submerged arc weld HAZ may be observed that grow from inclusions (primary Widmanstätten ferrite) or from idiomorphic ferrite (secondary Widmanstätten ferrite) as shown in Fig. 7. The IIW classi cation scheme does not have a terminology for these plates. However, they have been designated intragranular ferrite sideplates FS(I) in recent work by the present author. 3 2 In many cases, individual plates may be observed that have grown relatively unimpeded from intragranular inclusions (see Fig. 8). These plates do not have aligned microphase and may be interspersed with bainite or martensite. 2 2,2 7,3 2 The inclusions from which the plates grow may not be viewed since they may be under the plane of observation. These plates have been designated IFP by the present author, 3 2 who summed FS(I) and IFP to give a total quantity of intragranular Widmanstätten ferrite, referred to as I(WF). Where there is a high density of inclusions, multiple hard impingements of individual Widmanstätten ferrite plates growing from inclusions 2 2,3 2 may produce a ne interlocking structure (see schematic diagram, Fig. 5). The IIW classi cation scheme refers generally to this type of structure as acicular ferrite AF (see below). Bainite Bainite is generally recognised as forming at temperatures where diffusion controlled transformations are sluggish and has features in common with low temperature martensitic 7 Intragranular Widmanstätten ferrite sideplates in as deposited,. 8%C, 2. 87%n,. 35%o,. 27%B,. 19%Ti, submerged arc weld metal: 32 arrow indicates multiple plates of Widmanstätten ferrite with aligned microphase nucleated on large intragranular inclusions aterials Science and Technology February 24 Vol. 2

5 Thewlis Classi cation and quanti cation of microstructures in steels Bainite sheaves and sub-units 1: idiomorphic ferrite; 2: individual plate of WidmanstaÈ tten ferrite nucleated on large intragranular inclusions 8 Growth of intragranular Widmanstätten ferrite plates in. 6%C, 1. 37%n,. 17%o,. 28%B,. 27%Ti submerged arc weld metal, continuously cooled, helium quenched from 62 C 22 transformations. 2 6 It grows as individual plates or sub-units to form parallel arrays or sheaves. The growth of each subunit is accompanied by an invariant plane strain shape change with a large shear component. There is no redistribution of iron or substitutional solute atoms at the transformation interface. Classically, bainite has been categorised into two component structures, notably upper and lower bainite, depending on the transformation temperature. Carbon partitions into the residual austenite in upper bainite, and precipitates as cementite between the bainitic ferrite plates. In lower bainite, the ferrite becomes supersaturated with carbon and some carbide precipitation occurs within the ferrite sub-units as well as between them. The exact growth mechanism of bainite is still the subject of much debate A paraequilibrium mechanism in upper bainite involving a shear transformation accompanied by the rapid diffusion of interstitial carbon atoms across the c/a interface would mean that bainitic growth was in part similar to Widmanstätten ferrite. However, a purely displacive transformation would require no redistribution of atoms across the c/a interface. A temperature curve T o may be identi ed on the Fe C phase diagram de ning thermodynamically where austenite and ferrite of the same composition have identical free energy. 2 6,3 3 At the T o temperature there is no driving force for transformation. The T o curve has a negative slope with carbon concentration, lying between the A e 1 and A e 3 lines of the Fe C phase diagram. In a steel with a carbon concentration lower than that de ned by the T o curve, bainitic ferrite plates may begin to grow without diffusion at an appropriate hold temperature, then partition excess carbon into the residual austenite. Further diffusionless growth of plates may take place from the carbon enriched austenite, and the process continues until such transformation becomes thermodynamically impossible at the T o curve. This is termed the incomplete reaction phenomenon. Continuous undercooling of the steel below T o will cause the bainite reaction to be maintained. Carbide precipitation occurs when the transformation conditions are kinetically favourable. For a purely displacive transformation, therefore, a rapid redistribution of carbon atoms is envisaged after the diffusionless growth of bainitic ferrite sub-units. 2 6 Bainite can easily be confused with Widmanstätten ferrite, as noted above. Both structures are referred to as ferrite with second phase, FS in the IIW classi cation scheme, although a distinction may be made in the terminology where bainite can be clearly identi ed, e.g. FS(B). A further distinction may be made where upper and lower bainite can be positively identi ed, e.g. FS(UB) and FS(LB) respectively. Characteristically, bainite may grow directly from a prior austenite grain boundary 2 6 or an intragranular inclusion, 3 6 as shown schematically in Fig. 9. Sympathetic nucleation of bainite plates from existing sheaves is a common feature. Bainite that grows from prior austenite grain boundaries is commonly observed in the form of interlocking sheaves of very ne plates with aligned cementite particles (see Fig. 1), which are designated FS(A) in the IIW scheme. In upper bainite, FS(UB), carbide particles are observed between the plates, while in lower bainite, FS(LB), the carbides are within as well as between the plates and the structure tends to have a darker etching response. The individual plates within a sheaf are separated by low angle boundaries that are virtually irresolvable under the light microscope. The sheaves are shown in the process of growth in Fig. 11. Extensive sympathetic nucleation is evident. Depending on the plane of observation, cementite particles may appear non-aligned. When viewing a cross-section of ferrite laths that have grown from prior austenite grain boundaries beneath the plane of observation, all that may be seen are carbide particles in a matrix of ferrite within the prior austenite grains (see Fig. 1). The bainite is then classi ed as FS(NA). The present author and co-workers 2 2 have referred to the different forms of prior austenite grain boundary bainitic ferrite as GB(B) so that a distinction may be made with intragranular bainite as described below. In some steels and weld metals, 2 6,3 2,3 6 bainite sheaves may be seen to grow from intragranular inclusions (see Fig. 12). Individual ne plates of bainitic ferrite may also be observed that grow relatively unimpeded from intragranular inclusions (see Fig. 13). The latter plates do not have aligned carbide particles and may be dif cult to distinguish from Widmanstätten ferrite plates IFP (see above). The inclusions from which the plates grow may not be observed 1: lower bainite with carbide particles between as well as within subunits; 2: upper bainite with aligned carbide; 3: bainitic ferrite with non-aligned carbide 1 Interlocking sheaves of upper and lower bainite in. 17%C, 1. %n steel, laser weld HAZ aterials Science and Technology February 24 Vol. 2

6 148 Thewlis Classi cation and quanti cation of microstructures in steels 11 Growth of bainite sheaves and (arrowed) sympathetic nucleation of laths in. 38%C, 1. 39%n,. 39%S,. 9%V steel, isothermally transformed, 45 s at 4 C 14 Lath martensite in. 13%C laser weld metal: arrow indicates martensite laths with highly dislocated substructure 12 Growth of bainite sheaves from intragranular inclusions in. 38%C, 1. 39%n,. 39%S,. 9%V,. 13%N steel, isothermally transformed, 38 s at 45 C: arrow indicates multiple laths of bainite with carbide particles between as well as within subunits 15 Plate or twin martensite in. 27%C laser weld metal: arrow indicates lenticular martensite with twinned substructure result in a very ne interlocking structure 2 6,3 2 (see schematic diagram, Fig. 9). The IIW classi cation scheme refers generally to this type of structure as acicular ferrite AF (see below). 13 Growth of intragranular bainite plates in. 38%C, 1. 39%n,. 39%S,. 9%V,. 13%N steel, isothermally transformed, 38 s at 5 C: arrows indicate individual plates of bainitic ferrite nucleated on small intragranular inclusions since they are under the plane of observation. The IIW classi cation scheme does not have a terminology for the different forms of intragranular bainite, but the author and co-workers 2 2 have termed them I(B). Where there is a high density of inclusions, multiple hard impingements of individual bainitic plates growing from the inclusions may artensite artensite is classically an extremely rapid, diffusionless transformation where carbon is retained in solution. 3 7 As the austenite lattice changes from fcc to the required martensite bcc or bct lattice, strain energy considerations dominate and the martensite is constrained to be in the form of thin plates. In low carbon steels (less than ~. 2%C) lath martensite with a bcc crystal structure is the commonly occurring form 3 7 and is designated or (L) in the IIW scheme. The martensite units are formed in the shape of laths that are grouped into larger sheaves or packets (see Fig. 14). The sub-structure consists of a high density of dislocations arranged in cells; each martensite lath is composed of many dislocation cells. As the steel carbon content increases signi- cantly above about. 2%C, plate martensite tends to form with either a bct or bcc crystal structure. 3 7 The martensite units form as individual lenticular plates (see Fig. 15) with a substructure consisting of very ne twins. This form of martensite is termed twinned martensite in the IIW scheme and is designated or (T). artensite, whether in plates or lath form, is generally irresolvable under the light microscope and tends to have a slow etching response. aterials Science and Technology February 24 Vol. 2

7 Thewlis Classi cation and quanti cation of microstructures in steels Nature of acicular ferrite Acicular ferrite Conventionally, 2 6 acicular ferrite is recognised as an intragranular nucleated morphology of ferrite in which there are multiple impingements between grains. The acicular ferrite nucleates on inclusions inside the prior austenite grains during the cda transformation. Provided there is a high density of inclusions, a ne interlocking structure (generally <5 mm) can be produced. In the IIW scheme, acicular ferrite is designated AF. For a long time acicular ferrite was thought to be a single transformation product. Early work 3 8 suggested that it was intragranularly nucleated Widmanstätten ferrite. Later research 2 6 provided evidence for intragranularly nucleated bainite. However, recent research by the author and coworkers 2 2 has demonstrated that the nature of acicular ferrite may be as shown schematically in Fig. 16. Different reaction products may nucleate on intragranular inclusions at reconstructive and displacive transformation temperatures during continuous cooling depending on the nature, size and amount of inclusions (see Figs. 2 and 17). Acicular ferrite results from multiple hard impingements of the different transformation products. The sequence of transformations is consistent with the theoretical activation energy barrier to nucleation of the different sites. Acicular ferrite development may thus be de ned in terms of conventional steel transformation products and CCT diagrams incorporating both intragranular and grain boundary transformations. Under continuous cooling transformation conditions AF~I(PF)zI(WF)zI(B) This leads to acicular ferrite that may have a variety of forms depending on steel composition, cooling rate and inclusion characteristics. Acicular ferrite may consist of mixtures of different intragranular transformation products (see Fig. 18). 2 2,3 2 Alternatively, Widmanstätten acicular ferrite or bainitic acicular ferrite may form per se. 2 6,3 8 However, if reactions are completed at purely reconstructive transformation temperatures, it may be preferable to use the term idiomorphic primary ferrite instead of acicular ferrite to describe the microstructure, since intragranular primary ferrite is likely to be coarse and non-acicular in morphology (see Fig. 4). Acicular ferrite is usually observed as a ne interlocking ferrite structure interspersed with microphases (see Fig. 18). The shape of the ferrite plates may not appear to be needlelike as the use of the term acicular would imply. This is because the different ferrite morphologies cannot grow very far before mutual hard impingement. It is evident from Fig. 18 that the degree of re nement of the acicular ferrite is dependent on the nature of the transformation products inherent in its formation. a a b a idiomorphic ferrite (arrowed) nucleated on large inclusions; b WidmanstaÈ tten ferrite plates (arrowed) nucleated on small inclusions 17 Acicular ferrite development in. 6%C, 1. 37%n,. 17%o,. 28%B,. 27%Ti submerged arc weld metal, continuously cooled, iced brine quenched from 615 C 22 b a intragranular primary ferrite± WidmanstaÈ tten ferrite in C ± n weld metal; 22 b intragranular WidmanstaÈ tten ferrite ± bainite in Ti ± o ± B alloyed weld metal Forms of acicular ferrite aterials Science and Technology February 24 Vol. 2

8 15 Thewlis Classi cation and quanti cation of microstructures in steels icrophases The different ferrite growth modes of the principal structures described above result in carbon enrichment of the remaining austenite, leading to associated second phases of retained austenite, martensite, bainite or ferrite carbide aggregate (pearlite), depending on the degree of carbon enrichment of the austenite and the prevailing cooling conditions. The second phases associated with Widmanstätten ferrite and acicular ferrite are generally quite small (2 5 mm), and are termed microphases. IIW classi cation scheme problem areas and solutions The objective in the present work was to investigate the IIW microstructure classi cation scheme for weld metals as a basis for quantifying the full range of microstructures found in plain carbon and low alloy steels, as well as ferritic weld metals and parent plate heat affected zones. A means may thus be provided of obtaining database information for developing microstructure property relationships, or generating data for calibrating physical models that have the principal structures primary ferrite, pearlite, Widmanstätten ferrite, bainite and martensite as output. It is clear from the above review that while the IIW scheme provides a sound structure for quantifying complex microstructures in steels, the classi cation of constituents such as ferrite sideplate and acicular ferrite is incompatible with the principal structures found in the reconstructive and displacive transformation regimes of ferrous materials. Knowledge of the actual transformation products constituting ferrite sideplate and acicular ferrite structures is required. Classi cation is also needed of idiomorphic ferrite and ferrite sideplate structures growing relatively unimpeded from intragranular inclusions. Problems that may be encountered in relating subcategory microstructural components to principal structures at prior austenite grain boundary and intragranular sites are discussed below together with possible solutions. The ways in which transformation products associated with ferrite sideplate and acicular ferrite structures may be identi ed will be addressed. The use of optical microscopy with specimens polished to a. 25 mm nish and etched in 2% nital is assumed as standard. However, instances will be given where different instruments and techniques may be needed to solve problems. Where possible, the effects of steel composition and heat treatment will be highlighted, but detailed examples are outside the scope of the present paper. PRIARY FERRITE In low alloy weld metals, care has to be taken in identifying primary ferrite due to stereological effects. Ferrite allotriomorphs growing from prior austenite grain boundaries beneath the plane of observation may appear as polygonal ferrite grains in the intragranular regions (see Fig. 1). If these ferrite allotriomorphs are of a size approximately three times greater than those of surrounding acicular ferrite laths or grains, it is likely that they are the constituent PF(I) described in the IIW scheme. It is unlikely that such large grains are idiomorphic ferrite, I(PF), nucleated on inclusions as referenced in the literature, 2 2 since the latter tend to nucleate at lower temperatures with relatively little time for growth (see Fig. 2). PEARLITE Problems may arise in classifying pearlite when it is present along with displacive transformation products. Lamellar pearlite, FC(P) in the IIW classi cation scheme, may be confused with martensite if the ferrite/ cementite plates are irresolvable under the light microscope. A distinguishing feature is the generally rapid etching response and lower hardness of the pearlite. The dark etching, non-lamellar pearlite known as ferrite carbide aggregate, FC in the IIW classi cation scheme, may sometimes be confused with bainite. The nodular appearance of pearlite as opposed to the sheaf appearance of bainite may provide a distinguishing feature. The carbon content of the steel may also give an indication as to how much pearlite may be expected; high volume fractions should not be present in low carbon steels. Ultimately, however, knowledge of the thermal history and transformation conditions of the steel may be needed to provide a check on classi cation (see below). The reconstructive pearlite transformation should take place slowly at high temperatures and over a wide temperature range. A displacive transformation to bainite should take place rapidly at lower temperatures and over a relatively small temperature range. It is notable that in bainitic steels, prolonged holding at a given temperature may result in the incomplete reaction phenomenon (see above). Continued isothermal treatment can result in pearlite formation from the remaining carbon enriched austenite. 2 6 Dif culties in identi cation of pearlite may be compoundedby a eutectoid transformationthat has been noted in continuously cooled plain carbon steel (. 11%C,. 5%n). This involves ferrite growing in conjunction with repeated nucleation of alloy carbides on the moving c/a interphase boundary. 3 9 The reaction has been termed interphase precipitation of cementite. Dark etching equiaxed ferrite grains containing a ne dispersion of carbides are observed under the light microscope while, under the transmission electron microscope, the cementite is seen in sheets. FERRITE SIDEPLATE Bainite and Widmanstätten ferrite may be present in signi cant amounts in heat treated steels and the coarse grained HAZ of welds but they are dif cult to classify individually so that both structures have been generally referred to as ferrite sideplate. WidmanstaÈ tten ferrite Classi cation of Widmanstätten ferrite can prove dif cult because of its similarity to upper bainite, but certain guidelines may be followed to avoid confusion. The free energy requirement or driving force would be expected to be lower for Widmanstätten ferrite formation than for the upper bainite transformation, since the former is thought to grow by the mutual accommodation of plates and the latter by sub-units (see above). All else being equal, therefore, Widmanstätten ferrite may be expected to occur at higher temperatures than upper bainite and exhibit a generally coarser structure with a lower dislocation density. Furthermore,the microphasesbetween Widmanstätten ferrite laths may be expected to be a mixture of pearlite, bainite, martensite or retained austenite, whereas the nature of bainite formation (see above) means that cementite particles may generally be observed between the bainitic ferrite plates. 2 6 icrophases may be revealed by the use of different chemical etchants (see below). The identi cation of secondary Widmanstätten ferrite with aligned microphase, FS(A) in the IIW scheme, is relatively easy since it grows from existing allotriomorphic ferrite, but care has to be taken in distinguishing the boundary between the two structures. Identi cation of primary Widmanstätten ferrite is signi cantly more dif - cult; it grows directly from prior austenite grain boundaries and may be more easily confused with upper bainite. The use of colour etching methods 4,4 1 in conjunction with aterials Science and Technology February 24 Vol. 2

9 Thewlis Classi cation and quanti cation of microstructures in steels 151 optical microscopy may prove helpful in distinguishing Widmanstätten ferrite from bainite. These techniquesinvolve complex electrochemical reactions and require careful experimentation, but can provide a means of distinguishing various phases by their colouring response. Nanohardness measurements may also prove useful; these are obtained using a modi ed scanning force microscope (SF). 4 2 The nanoindentation technique allows very small regions of grains to be investigated and different phases to be distinguished. All else being equal, Widmanstätten ferrite should exhibit a lower hardness than bainite. Although Widmanstätten ferrite may be distinguished from upper bainite using the above guidelines, care has to be taken with stereological effects. Widmanstätten ferrite plates within a colony tend to grow in a common crystallographic orientation. They are therefore generally separated by low angle boundaries. When prior austenite grain boundary Widmanstätten ferrite is seen end-on with nonaligned microphase, FS(NA) in the IIW scheme, the plates can give the appearance of ferrite grains interspersed with microphase, thereby creating confusion with regions of intragranular acicular ferrite, AF. In the case of acicular ferrite, hard impingements of the different ferrite morphologies growing from inclusions results in high angle boundaries, which are signi cantly more distinct than the low angle boundaries of Widmanstätten ferrite. Careful specimen polishing and etching may be required to distinguish the two structures. In the intragranular regions of welds, it may be relatively straightforward to identify multiple plates of Widmanstätten ferrite with aligned microphase growing unimpeded from large inclusions, described as FS(I) in the literature. 3 2 Recognising single plates of Widmanstätten ferrite without aligned microphase, designated IFP, may be more dif cult, but these plates are likely to be quite coarse and grow from large inclusions. Formation of the latter may appear contradictory from a mechanistic viewpoint. It is possible that the second plate is beneath the plane of observation (see Fig. 8). Alternatively, the absence of aligned microphase may be because, during plate growth, carbon is rejected into the remaining austenite, which then undergoes a secondary transformation at lower temperatures to bainite, martensite or ne acicular ferrite nucleated on small inclusions. Bainite The effects of steel composition may compound many of the problems associated with distinguishing Widmanstätten ferrite from upper bainite described above. Low carbon content in bainitic steels can increase the transformation temperature and result in a coarse lath size so that bainitic ferrite with aligned second phase, FS(A) in the IIW scheme, appears similar to Widmanstätten ferrite. High silicon content in bainitic steels (generally >1%) can retard the precipitation of carbide from austenite 2 6 and result in martensite or retained austenite microphases between the bainitic ferrite laths, thereby creating confusion with Widmanstätten ferrite. Granular bainite, which tends to form in continuously cooled, low carbon bainitic steels, poses a similar problem. 2 6 This structure appears as a relatively coarse aggregate of bainitic ferrite and retained austenite or martensite islands; the bainitic sub-units have very thin regions of austenite between them, which cannot be resolved under the light microscope. 2 6 Ultimately, high resolution SE, TE or electron back-scattering diffraction (EBSD) techniques 4 3,4 4 may be needed to distinguish these forms of bainite from Widmanstätten ferrite by revealing the crystallographic sub-structure and thereby the mechanism of formation, but some electron metallographic techniques are time consuming and often dif cult. When trying to distinguish upper, FS(UB), and lower, FS(LB), bainite in the IIW scheme, stereological effects may cause confusion. Cross-sections of upper and lower bainite sheaves may appear similar. In general, however, the carbides are likely to be ner and the etching response darker in the lower bainite. In weld metals, individual plates of bainitic ferrite, I(B), growing unimpeded from intragranular inclusions may be dif cult to separate from Widmanstätten ferrite plates, IFP. However, the former are likely to be signi cantly ner than the latter and the nucleating inclusions may be smaller. Colour etching methods 4,4 1 may be helpful for identi cation but, ultimately, electron metallographic techniques may be required to determine the nature of the plates. ARTENSITE artensite is often present together with bainite in the HAZ of laser welds and to some extent electron beam welds; these phases also occur in high strength weld metals. 3 2 ost low carbon steels have martensite start temperatures above room temperature so that, at slower cooling rates, carbon atoms can redistribute and precipitate, i.e. autotempering can take place. It is then dif cult to distinguish between autotempered martensite,, and lower bainite, FS(LB), in the IIW scheme. The carbides precipitated inside the laths in lower bainite are, however, likely to be coarser and some interlath carbide should be evident (see above). Colour etchingmethods 4,4 1 maybe investigatedas a means of distinguishing between bainite and martensite. Comparatively simple nanohardness measurements 4 2 may also prove useful in separating martensite from other principal structures,and in distinguishingthe different forms of martensite. Since carbon content generally governs the martensitic hardness, twinned martensite, (T), may be expected to exhibit a much higher hardness than lath martensite, (L). ACICULAR FERRITE Distinguishingthe intragranulartransformationproducts that compose acicular ferrite, AF in the IIW scheme, is likely to be very dif cult comparedwith identifyingthe structureitself.it is recommended, therefore, that for the purposes of calibrating models, a pragmatic solution be adopted. Thus measured volume fractions of acicular ferrite should be compared with the sum of the intragranularconstituents I(PF)zI(WF)zI(B) predicted by modelling. However, care should be taken to distinguish between acicular ferrite, AF, where multiple impingementoccurs between the different intragranularferrite morphologies, and the intragranular transformationproducts I(PF), I(WF) and I(B), which may grow relatively unimpeded and may be identi ed in their own right. ICROPHASES icrophases are normally revealed using a standard etch polish technique with a 2% nital etch. However, problems may arise in distinguishing martensite and retained austenite, which often occur together as A phase. TE techniques may be employed to separate the phases but are time consuming and dif cult. The proportion of austenite in the A phase may be determined using X-ray diffraction techniques. In some cases, etching in picral can reveal the nature of the microphases. Thus cementite may appear black; a light brown coloration indicates lath martensite; a yellow-brown colour is likely to be twin martensite while a grey-white colour is indicative of retained austenite. New classi cation scheme In the previous section, problems in the IIW microstructure classi cation scheme were discussed and guidelines proposed for identifying the principal structures associated aterials Science and Technology February 24 Vol. 2

10 152 Thewlis Classi cation and quanti cation of microstructures in steels Table 1 Classi cation scheme for microstructural constituents Category terminology Principal structure classi cation Overall ain Sub Component structure description Comments Reconstructive transformations (diffusion controlled, with slow rates of reaction) Ferrite PF* PF(GB) PF(G)* Grain boundary primary ferrite Allotriomorphic ferrite Polygonal ferrite Ferrite veins PF(NA) Polygonal primary ferrite nonaligned Ferrite veins or polygonal grains aligned with prior austenite grain boundaries Polygonal ferrite grains within the prior austenite grains, of a size approximately three times greater than the surrounding ferrite laths or grains; cross-sections of ferrite allotriomorphs that have grown from prior austenite grain boundaries below the plane of observation PF(I) PF(I) Idiomorphic ferrite Ferrite idiomorphs associated with intragranular nucleation sites (large oxide/sulphide inclusions) in weld metals and particle dispersed steels Nodules of alternate ferrite/cementite lamellae, which are often dif cult to resolve under the optical microscope. The structure has a rapid etching response in 2% nital and a generally low hardness. Pearlite may be present as a microphase FC* Ferrite ± carbide aggregate Pearlite lamellae viewed in cross-section. Distorted pearlite lamellae may appear as a dark etching Pearlite P* P* FC(P)* Lamellar pearlite Degenerate pearlite Fine colony pearlite Displacive transformations (shear dominated, with rapid rates of reaction) WidmanstaÈ tten ferrite WF WF(GB) FS(A)* WidmanstaÈ tten ferrite with aligned microphase WidmanstaÈ tten ferrite sideplates virtually irresolvable ferrite/carbide aggregate known as primary troostite. Dif cult to distinguish ferrite/carbide aggregate from bainite Colonies of parallel ferrite laths (or sideplates) with microphases aligned between the laths ranging from pearlite to martensite. Lath boundaries are dif cult to resolve. Primary WidmanstaÈ tten ferrite grows from the prior austenite grain boundaries, whereas secondary WidmanstaÈ tten ferrite grows from allotriomorphic ferrite at the boundary FS(NA)* WidmanstaÈ tten ferrite with Aggregate of microphase islands and WidmanstaÈ tten ferrite within the prior austenite grains; non-aligned microphase cross-sections of WidmanstaÈ tten ferrite sideplates that grow from prior austenite grain boundaries below the plane of observation ultiple coarse WidmanstaÈ tten ferrite plates (aspect ratio greater than 4 : 1) with aligned microphases, which grow from intragranular inclusions. Primary intragranular ferrite sideplates grow from inclusions, whereas secondary sideplates grow from ferrite idiomorphs associated with inclusions FP(I) Intragranular WidmanstaÈ tten Individual coarse plates of WidmanstaÈ tten ferrite that grow relatively unimpeded from intragranular ferrite plates inclusions AF* WidmanstaÈ tten acicular ferrite Fine interlocking structure formed by multiple impingements of individual WidmanstaÈ tten ferrite plates growing from intragranular inclusions Sheaves of parallel ferrite laths (or sub-units) with cementite particles aligned between the laths. Lath boundaries are generally irresolvable under the light microscope. Sheaves grow from prior austenite grain boundaries; sympathetic nucleation of laths from existing sheaves is a common feature FS(NA)* Bainitic ferrite with non-aligned Aggregate of coarse carbides and bainitic ferrite within the prior austenite grains; cross-sections of carbide bainite sheaves that grow from prior austenite grain boundaries (or existing sheaves) below the plane WF(I) FS(I) Intragranular WidmanstaÈ tten ferrite sideplates Bainite B B(GB) FS(A)* Bainitic ferrite with aligned carbide Bainite sheaves of observation FS(UB)* Upper Bainite Carbide particles are precipitated between the bainite sub-units. Upper bainite has a higher dislocation density than primary WidmanstaÈ tten ferrite. Bainite may appear as a microphase between WidmanstaÈ tten ferrite sideplates FS(LB)* Lower bainite Fine cementite particles precipitated within as well as between bainitic ferrite plates. Lower bainite has a generally darker etching response than upper bainite. Dif cult to distinguish lower bainite from autotempered martensite aterials Science and Technology February 24 Vol. 2

11 Thewlis Classi cation and quanti cation of microstructures in steels 153 Table 1 (Continued) Category terminology Principal structure classi cation Overall ain Sub Component structure description Comments B(I) FS(I) Intragranular bainite sheaves Sheaves of ne bainitic ferrite plates with aligned carbide, which grow from intragranular inclusions FP(I) Intragranular bainite plates Individual ne plates of bainitic ferrite that grow relatively unimpeded from intragranular inclusions. AF* Bainitic acicular ferrite Very ne interlocking structure formed by multiple impingements of individual bainitic ferrite plates growing from intragranular inclusions artensite * * (L)* Lath martensite Low carbon martensite with a lath structure and heavily dislocated sub-structure. Lath martensite has a slow etching response in 2% nital and a generally high hardness. Colonies of martensite may form within the prior austenite grains. Smaller colonies may be treated as microphases. icrophases may consist of martensite with retained austenite (A) (T)* Twin martensite High carbon martensite with a plate structure and twinned sub-structure *Retained IIW terminology. with prior austenite grain boundary and intragranular sites, taking into account stereological effects. In this section, the information gained has been used to develop a new classi- cation scheme. The application and accuracy of the new scheme have been addressed, and consideration given to its evolution. DEFINITION Using the information gained above, the traditional IIW classi cation scheme has been modi ed and new terminology de ned as in Table 1. The main and sub-categories of microstructural constituents of the table re ect the mechanisms of formation of the principal structures and the characteristic ferrite morphologies produced in the reconstructive and displacive transformation regimes of steels. Traditionally, the IIW classi cation scheme terminology places the transformation product rst and the location second, whereas the reverse is often the case in the wider 1 7,2 2,3 2 published literature. For consistency, therefore, the terminology described in Table 1 follows the traditional IIW notation. Thus, the constituents GB(PF), I(PF), GB(WF), I(WF), GB(B), I(B) described in the literature 2 2 are replaced by PF(GB), PF(I), WF(GB), WF(I), B(GB), B(I) as main category terms in Table 1. Likewise, the constituent IFP in the literature 3 2 is replaced by the subcategory constituent FP(I) in Table 1. To avoid con ict in Table 1 between the terminology adopted for idiomorphic primary ferrite PF(I) and that for cross-sections of ferrite allotriomorphs growing from prior austenite grain boundaries below the plane of observation, the latter terminology has been changed from PF(I) to PF(NA), i.e. primary ferrite not aligned with prior austenite grain boundaries. PF(NA) may be added together with PF(G) to give an overall quantity of reconstructive prior austenite grain boundary nucleated ferrite PF(GB). It should be noted in Table 1 that the new sub-category component terminology automatically de nes its location either at prior austenite grain boundaries or in intragranular regions. In practice, therefore, an identi cation system may be employed which directly links a sub-category component to the principal structure, e.g. B-FS(A) and WF-FS(A). Flow charts that incorporate the classi cation and terminology of Table 1 but provide detailed guidance on identifying principal structures are shown in Fig. 19. The key to the ow charts is given in Fig. 2. Separate charts are provided for austenite grain boundary and intragranular microstructural components. Progression through the charts from sub-category component structures to the principal structures is dependent on answering a number of boxed questions on a yes/no basis. The questions are derived from the considerations made in this paper. If the answer to a question is yes, progression is made to the right of the chart towards the principal structure. If the answer is no a move vertically downwards is needed to obtain more information before, eventually, progress is made to the right again. The ow charts thus potentially provide a means of quantifying complex steel microstructures in terms of the principal structures, thereby enabling the generation of either database information or data for calibration of theoretical models. APPLICATION To assess the accuracy of the new classi cation scheme and identify discrepancies between operators, exercises were carried out to quantify widely different microstructures. The microstructures were obtained by thermally cycling steels of compositions %C, %n in a dilatometer to peak temperatures of 9 13 C and cooling at rates between 2 and 2 K s 2 1. Full details of the quanti cation exercises, including a complete statistical aterials Science and Technology February 24 Vol. 2

12 154 Thewlis Classi cation and quanti cation of microstructures in steels a prior austenite grain boundary constituents; b intragranular constituents 19 Guidelines and terminology for identi cation of principal structures aterials Science and Technology February 24 Vol. 2

13 Thewlis Classi cation and quanti cation of microstructures in steels Key to ow charts analysis, are outside the scope of the present paper. However, the results for selected steels are summarised below. Six dilatometer sample microstructures covering a wide transformation temperature range were photographed using an appropriate magni cation. The resulting microstructural elds are shown in Fig. 21. A mesh grid inscribed on transparent acetate paper was overlaid in a xed position on the photographs so that those microstructural constituents under or just touching the grid cross-lines could be quanti ed. Each cross-line was identi ed from the grid scale, e.g. A1, A2, A3,, B1, B2, B3, A total of 5 points was counted of each eld. Because the grid points were xed, results from different operators could be compared and the constituents that were most dif cult to quantify could be relatively easily identi ed. Initially, a single operator was employed to point count the volume percentages of microstructural constituents in the six microstructural elds using the traditional IIW and the new classi cation schemes. The results (Table 2) demonstrate the advantages of the new scheme in being able to rationalise the principal structures associated with ferrite sideplate. Ultimately the microstructural output is reduced to the ve principal constituents. Following the above exercise, different operators were employed to determine the volume percentages of the principal structures in the six microstructural elds using the new scheme per se. The results are shown in the form of histograms in Fig. 22. ost operators chose to identify the major transformation products directly, although some operators chose to classify subcategories and thereby the major components. In all cases, microphases associated with primary ferrite and Widmansta tten ferrite were treated separately, while bainitic ferrite was quanti ed together with the carbide. Because of the xed position of the point counting grid, the variations in phase proportions in Fig. 22 are due to differences in microstructural interpretation by the individual operators, rather than point counting errors that would emerge between operators from random repositioning of the grid in the dilatometer sample microstructure. When quantifying the volume fraction of secondary Widmansta tten ferrite, some discrepancy occurred between operators owing to the need to distinguish the boundary between allotriomorphic ferrite and Widmansta tten ferrite (see Fig. 22a). Further differences occurred because of the need to distinguish between ferrite carbide aggregate (pearlite) and bainite (see Fig. 22b and c), and to some extent lower bainite and autotempered martensite (see Figs. 22d and f). These dif culties were compounded by the low resolution of the photographic images. A signi cant improvement in the consistency between operators was achieved, after appropriate training, when quantifying phase proportions randomly over a relatively large area in actual steel samples. In this case different magni cations could be used to reveal dif cult features. A light microscope with a Swift point counting stage was employed to count 5 points of various dilatometer sample microstructures, again covering a wide transformation temperature range. The statistical errors in point counting were determined using the formula according to Gladman and Woodhead4 7 svf =Vf ~ (1{Vf )=Pa Š1=2 where sv f is one standard deviation, Pa the fraction of counts in the a phase and Vf the volume fraction of a phase. The phase proportions obtained by two operators on six steels are shown in Fig. 23. The 95% con dence limits (2sv f ) are superimposed. The results show that the phase proportions obtained by the individual operators were in many cases within the statistical error de ned in the point counting exercise. However, to obtain a sensible statistical analysis aterials Science and Technology February 24 Vol. 2

14 156 Thewlis Classi cation and quanti cation of microstructures in steels a b c d e f a. 51%C,. 51%n, 12 C, 1 K s2 1; b.17%c,.52%n, 13 C, 1 K s2 1; c.13%c, 1.2%n, 13 C, 2 K s2 1; d.13%c, 1.2%n, 12 C, 1 K s2 1; e. 13%C, 1.2%n, 13 C, 5 K s2 1; f.13%c, 1.2%n, 13 C, 2 K s icrostructural elds of steels thermally cycled in dilatometer to temperatures of 12 or 13 C and cooled at rates between 2 and 2 K s2 1 (8 5 C) of operator bias, a larger number of operators is needed. Further work is required in the form of round robin exercises to determine the statistical uncertainty between operators when quantifying different types of microstructure, and to provide appropriate training measures for widespread dissemination of the scheme. The above studies were carried out without prior knowledge of the thermal history of the specimens examined. However, transformation behaviour knowledge can provide a useful check on results. The six microstructural elds in Fig. 21 were largely representative of the parent dilatometer sample microstructures. The corresponding dilation curves, aterials Science and Technology February 24 Vol. 2 percentage transformed versus temperature graphs and peak rate transformation curves are shown in Fig. 24. The dilatometer data in Fig. 24a show that for this particular steel, transformation began at 793 C and took place over a wide temperature range, nishing at 628 C. As the transformation proceeded, the rate of transformation increased slowly to a peak at 715 C and then decreased slowly, indicative of transformation controlled by diffusion. This supports the operator classi cation for the steel of about 7% primary ferrite and 5% pearlite, i.e. predominantly reconstructive transformation (see Fig. 22a). By contrast, the dilatometer data in Fig. 24c show that for this steel