Diffusional growth limitation and hardenability

It has long been recognized that the effects of alloying elements on the hardenability of steels is related directly to their effects on the nucleation and growth kinetics of proeutectoid and eutectoid austenite decomposition products. In the present paper, theoretical and experimental studies of proeutectoid-ferrite and pearlite growth are reviewed for systems of the form Fe-C-X, where X is a substitutional alloying element such as Co, Cr, Mn, Mo, Ni, Si, and so forth. Of principle interest is the limitation which an alloying element X imposes on the corresponding diffusional growth kinetics. Although the agreement between theory and experiment is reasonable for most systems, there remain areas of considerable controversy (e.g., the role of interface diffusion, whether or not local equilibrium is maintained at interfaces and solute segregation to interfaces). This paper is based on a presentation made at a symposium on “Hardenability” held at the Cleveland Meeting of The Metallurgical Society of AIME, October 17, 1972, under the sponsorship of the IMD Heat Treatment Committee. Note added in proof: Boswellet al. have initiated a theoretical treatment of the “impurity drag” effect of carbide formers on proeutectoid ferrite growth.     

Growth and overall transformation kinetics above the bay temperature in Fe-C-Mo alloys

The kinetics and morphology of isothermal transformation in the vicinity of the time-temperaturetransformation (TTT) diagram bay have been investigated with optical and transmission electron microscopy (TEM) in 19 Fe-C-Mo alloys at three levels of carbon concentration (approximately 0.15, 0.20, and 0.25 wt pct) and at Mo concentrations from 2.3 to 4.3 wt pct, essentially always at temperatures above or at that of the bay,T _b . Quantitative metallography yielded no evidence for incomplete transformation (stasis) in any of these alloys atT > T _b . Measurements of the thickening kinetics of grain boundary ferrite allotriomorphs (invariably containing either interphase boundary or fibrous Mo₂C) demonstrated four different patterns of behavior. The customary parabolic time law for allotriomorph thickening in Fe-C and in many Fe-C-X systems was obtained only at higher temperatures and in the more dilute Fe-C-Mo alloys studied. With decreasing temperature and increasing solute concentrations, a two-stage and then two successive variants of a three-stage thickening process are found. In the most concentrated alloys and at temperatures nearest the bay, the second stage of the three-stage thickening process corresponds to “growth stasis”—the cessation of allotriomorph thickening. Sufficient prolongation of growth stasis presumably leads to “transformation stasis.” A number of models for growth of the carbide-containing allotriomorphs were investigated during attempts to explain the observed kinetics. It was concluded that their growth is controlled by carbon diffusion in austenite but with a driving force drastically reduced by a very strong solute drag-like effect (SDLE) induced by Mo segregation at disordered-type austenite: ferrite boundaries. Carbide growth in the fibrous structure appears to be fed by diffusion of Mo along austenite: ferrite boundaries, whereas carbides in the interphase boundary structure grow primarily by volume diffusion of Mo through austenite. Formerly Republic Steel Corporation Fellow, Department of Metallurgical Engineering, Michigan Technological University, Houghton, MI, and Visiting Graduate Student, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA. Formerly Professor, Michigan Technological University. This paper is based on a presentation made in the symposium “International Conference on Bainite” presented at the 1988 World Materials Congress in Chicago, IL, on September 26 and 27, 1988, under the auspices of the ASM INTERNATIONAL Phase Transformations Committee and the TMS Ferrous Metallurgy Committee.     

Simulation of ferrite growth in continuously cooled low-carbon iron alloys

The growth of a planar ferrite (α): austenite (γ) boundary in low-carbon iron and Fe-Mn alloys continuously cooled from austenite through the (α+γ) two-phase field and the α single-phase field was simulated by incorporating carbon diffusion in austenite, intrinsic boundary mobility, and the drag of an alloying element. At a very high cooling rate (≥ 10³ °C/s), the width of the carbon diffusion spike in austenite approaches the limit at which spikes are viable, so that the growth of ferrite in which carbon is not partitioned can occur even above the α solvus. In this context, the upper limiting temperature of partitionless growth of ferrite is the T ₀ temperature. In the presence of drag of an alloying element, e.g., Mn, both carbon-partitioned and partitionless growth of ferrite begins to occur at finite undercoolings from the Ae ₃, T ₀, or α-solvus temperature, at which the driving force for transformation exceeds the drag force. The intrinsic mobility of the α:γ boundary may play a significant role at an extremely high cooling rate (≥10⁵ °C/s). This article is based on a presentation made at the symposium entitled “The Mechanisms of the Massive Transformation,” a part of the Fall 2000 TMS Meeting held October 16–19, 2000, in St. Louis, Missouri, under the auspices of the ASM Phase Transformations Committee.     

A mechanism for the formation of lower bainite

A diffusional mechanism for the formation of lower bainite is proposed based primarily on transmission electron microscopy (TEM) observations of isothermally reacted specimens of Fe-C-2 pct Mn alloys. The mechanism involves the initial precipitation of a nearly carbide-free ferrite“spine,” followed by sympathetic nucleation of“secondary (ferrite) plates” which lie at an angle to the initial“spine.” Carbide precipitation subsequently occurs in austenite at ferrite: austenite boundaries located in small gaps between the“secondary plates.” An“annealing” process then occurs in which the gaps are filled in by further growth of ferrite and additional carbide precipitation; the annealing out of ferrite: ferrite boundaries between impinged“secondary plates” completes this process. This annealing stage contributes to the final appearance of lower bainite sheaves as monolithic plates containing embedded carbides. The present mechanism accounts for the single variant of carbides oriented at an angle to the sheaf axis repeatedly reported in lower bainite; it is also consistent with the previous observation of one“rough” side and one“smooth” side of lower bainite“plates.” Formerly Graduate Student, Carnegie Mellon University. Formerly Visiting Professor, Carnegie Mellon University. This paper is based on a presentation made in the symposium“International Conference on Bainite” presented at the 1988 World Materials Congress in Chicago, IL, on September 26 and 27, 1988, under the auspices of the ASM INTERNATIONAL Phase Transformations Committee and the TMS Ferrous Metallurgy Committee.     

Studies of the influence of alloying elements on the growth of ferrite from austenite under decarburization conditions: Fe-C-Nl alloys

We have evaluated controlled decarburization as a method for probing the effect of alloying elements on ferrite growth from austenite. The technique permits the exploration of longer-time ferrite layer growth; it minimizes the effects of interface structure on ferrite growth; and it permits the isolation of the effects of temperature and alloying element concentration on ferrite/austenite interface motion. The study of the decarburization of initially homogeneous Fe-C-Ni alloys was complemented by experiments using specimens with a controlled nickel concentration gradient. Although the decarburization method yields consistent results at longer times, it is found to be less appropriate for the study of initial ferrite growth. Nucleation in the gas/solid interface region, coupled with uncertainties about the precise time of decarburization, leads to large relative errors at the earliest times. For these reasons, the method is considered a valuable complement to studies based on precipitation boundary conditions. This article is based on a presentation given in the symposium “The Effects of Alloying Elements on the Gamma to Alpha Transformation in Steels,” October 6, 2002, at the TMS Fall Meeting in Columbus, Ohio, under the auspices of the McMaster Centre for Steel Research and the TMS-ASM Phase Transformations Committee.     

Kinetic transitions and substititional solute (Mn) fields associated with later stages of ferrite growth in Fe-C-Mn-Si

A low-carbon balloy steel with relatively high Mn and Si concentrations (0.04 wt pct C-3 wt pct Mn-1.9 wt pct Si) has been used to explore the effects of alloy chemistry and austenite grain size on ferrite growth. Even at high levels of supersaturation, the volume fraction of ferrite is found to increase slowly relative to the relaxation time for carbon diffusion. A series of scanning transmission electron microscopy (STEM) analyses for Mn indicates that initial unpartitioned ferrite growth is replaced by partitioned growth, accompanied by a dramatic drop in growth rate, and a persistent level of residual supersaturation in the remaining austenite. The results are interpreted in terms of a transition from an initial paraequilibrium interfacial condition to partitioned ferrite growth. This article is based on a presentation made in the “Hillert Symposium on Thermodynamics & Kinetics of Migrating Interfaces in Steels and Other Complex Alloys,” December 2–3, 2004, organized by The Royal Institute of Technology in Stockholm, Sweden.     

The growth of ferrite in Fe-C-X alloys: The role of thermodynamics, diffusion, and interfacial conditions

The growth of allotriomorphic ferrite from austenite in Fe-C-X alloys is studied. Two systems have been selected: the Fe-C-Ni system, in which the substitutional alloying element is expected to have a weak interaction with both the C and the moving interface, and the Fe-C-Mo system, in which these interactions are expected to be non-negligible. The ferrite growth kinetics was measured using two types of experiments: classical isothermal heat treatments and decarburization experiments. All of the experimental observations can be quantitatively rationalized using a model that describes an evolution in interfacial conditions from paraequilibrium (PE) to local equilibrium with negligible partitioning (LENP) during growth. This article is based on a presentation made in the “Hillert Symposium on Thermodynamics & Kinetics of Migrating Interfaces in Steels and Other Complex Alloys,” December 2–3, 2004, organized by the The Royal Institute of Technology in Stockholm, Sweden.     

Alloying Element Partition and Growth Kinetics of Proeutectoid Ferrite in Hot-Deformed Fe-0.1C-3Mn-1.5Si Austenite

Alloying element partition and growth kinetics of proeutectoid ferrite in deformed austenite were studied in an Fe-0.1C-3Mn-1.5Si alloy. Very small ferrite particles, less than several microns in size, were formed within the austenite matrix, presumably at twin boundaries as well as at austenite grain boundaries. Scanning transmission electron microscopy–energy-dispersive X-ray (STEM-EDX) analysis revealed that Mn was depleted and Si was enriched in the particles formed at temperatures higher than 943 K (670 °C). These were compared with the calculation of local equilibrium in quaternary alloys, in which the difference in diffusivity between two substitutional alloying elements was assumed to be small compared to the difference from the carbon diffusivity in austenite. Although the growth kinetics were considerably faster than calculated under volume diffusion control, a fine dispersion of ferrite particles was readily obtained in the partition regime due to sluggish growth engendered by diffusion of Mn and Si.     

The effect of microstructural banding on failure initiation of HY-100 steel

Microstructural banding of a hot-rolled HY-100 steel plate was accentuated by cooling slowly from the austenite region, which resulted in alternating layers of soft, equiaxed ferrite, and hard “granular ferrite.” The segregation of substitutional alloying elements such as Ni and Cr was identified as the main cause for the microstructural banding. Such banding induces anisotropic flow behavior at large strains, with deformation constrained by “pancake-shaped” bands of the hard granular ferrite. Tensile tests of circumferentially notched HY-100 specimens were performed in order to explore the stress dependence of failure in the slow-cooled as well as the quenched and tempered conditions. The failure behavior of the slow-cooled, microstructurally banded material exhibited a pronounced susceptibility to a void-sheet mode of failure. However, the absence of carbides within the equiaxed ferrite delays void coalescence and material failure to higher strains than in a quenched and tempered microstructure, despite the increased susceptibility to shear localization.     

The effect of microstructural banding on failure initiation of HY-100 steel

Microstructural banding of a hot-rolled HY-100 steel plate was accentuated by cooling slowly from the austenite region, which resulted in alternating layers of soft, equiaxed ferrite, and hard “granular ferrite.” The segregation of substitutional alloying elements such as Ni and Cr was identified as the main cause for the microstructural banding. Such banding induces anisotropic flow behavior at large strains, with deformation constrained by “pancake-shaped” bands of the hard granular ferrite. Tensile tests of circumferentially notched HY-100 specimens were performed in order to explore the stress dependence of failure in the slow-cooled as well as the quenched and tempered conditions. The failure behavior of the slow-cooled, microstructurally banded material exhibited a pronounced susceptibility to a void-sheet mode of failure. However, the absence of carbides within the equiaxed ferrite delays void coalescence and material failure to higher strains than in a quenched and tempered microstructure, despite the increased susceptibility to shear localization.     

An investigation of the generality of incomplete transformation to bainite in Fe-C-X alloys

The overall kinetics of the isothermal transformation of austenite to bainite and to pearlite in high-purity Fe-C-3 at. pct X alloys (X = Mn, Si, Ni, or Cu) containing 0.1 wt pct C and 0.4 wt pct C were investigated with quantitative metallography and transmission electron microscopy (TEM) to ascertain the presence or absence of the incomplete reaction phenomenon. The incomplete transformation of austenite to bainite was not observed in the Fe-C-Si, Fe-C-Ni, Fe-C-Cu, or Fe-0.4C-Mn alloys. It was found, however, in the Fe-0.1C-Mn alloy. Transmission electron microscopy results indicate that sympathetic nucleation of ferrite without carbide precipitation is a necessary but not a sufficient condition for the development of the incomplete reaction phenomenon. Transformation resumes following stasis in the low-carbon Fe-C-Mn alloy with the formation of a nodular bainite. The results support the view that the incomplete transformation of austenite to bainite is a characteristic of specific alloying elements and is not an inherent trait of the bainite reaction. Formerly Graduate Student, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University. Formerly Visiting Professor, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University. Formerly Undergraduate Student, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University. This paper is based on a presentation made in the symposium “International Conference on Bainite” presented at the 1988 World Materials Congress in Chicago, IL, on September 26 and 27, 1988, under the auspices of the ASM INTERNATIONAL Phase Transformations Committee and the TMS Ferrous Metallurgy Committee.     

Influence of cooling rate on the microstructure and retained austenite in an intercritically annealed vanadium containing HSLA steel

The influence of cooling rate from the intercritical αγ region on the microstructure of a vanadium bearing HSLA steel was investigated by transmission electron microscopy. Oil quenching produced an essentially ferrite-martensite dual phase structure with ∼4 vol pct of fine particle and thin film retained austenite. In contrast, the slower air cooling resulted in a larger amount (∼10 vol pct) of retained austenite in addition to the ferrite and martensite phases. A major portion of the retained austenite in the air cooled specimen was of the blocky morphology (1 to 6 μm , with the remainder eing the submicron variety, similar to that of the oil quenched specimen. In conformance with the terminology of earlier studies, “retained” ferrite and “transformed” ferrite were observed in the air cooled steel while oil quenching completely suppressed the transformed ferrite. Retained ferrite, the cleaner of the two in terms of precipitate content, is the high temperature ferrite that coexists with the austenite at the intercritical temperature and which is retained on cooling. The transformed ferrite, on the other hand, forms from the decomposition of the austenite and contains banded carbonitrides (row precipitation) much like the initial microstructure of the HSLA steel. formerly known as B. V. N. Rao formerly with General Motors Research Laboratories, Warren, MI,     

The influence of the alloying elements upon the transformation kinetics and morphologies of ferrite plates in alloy steels

A series of Fe-C-X and Fe-C-X₁-X₂ alloys in which X, X₁ and X₂ either raise or depress the activity of C iny were investigated by autodilatometer, optical microscopy, and transmission electron microscopy (TEM) to reveal the relations among the chemical composition, transformation kinetics, and morphology of ferrite plates. The incubation time of austenite decomposition at the nose temperature in the time-temperature-transformation (TTT) diagrams, the concentration of C in y in contact with theα/gg boundary, and the growth rate of ferrite were evaluated to estimate the magnitude of the solute drag-like effect (SDLE) for the different alloying elements used. All the results are consistent qualitatively with the SDLE hypothesis. This article is based on a presentation made at the Pacific Rim Conference on the “Roles of Shear and Diffusion in the Formation of Plate-Shaped Transformation Products,” held December 18-22, 1992, in Kona, Hawaii, under the auspices of ASM INTERNATIONAL’S Phase Transformations Committee.     

Growth kinetics of grain boundary ferrite allotriomorphs in Fe-C-X alloys

Parabolic rate constants for the thickening (α) and lengthening (β) kinetics of grain boundary allotriomorphs of proeutectoid ferrite have been measured as a function of isothermal transformation temperature in several Fe-C-X’ alloys whereX = Si, Ni, Mn, and Cr. These constants have been corrected approximately for the growth inhibition produced by facets on the allotriomorphs. The corrected α values are compared with those calculated on the basis of three models: equilibrium at α:γ boundaries with partition ofX, local equilibrium with “pile-up” ofX rather than bulk partition, and paraequilibrium. Values calculated from both the paraequilibrium and the “pile-up” models were in order of magnitude or better agreement with the corrected experimental α’s. Similar levels of agreement were obtained for the equilibrium model in the Si and Cr alloys and also in one Ni alloy at lower reaction temperatures. However, an estimate of the maximum possible diffusion distance of alloying element into austenite during growth supported only the paraequilibrium model under nearly all conditions investigated. Even for this model, however, measured rate constants are significantly less than those calculated for Fe-C-Mn and Fe-C-Cr and greater for Fe-C-Si and the higher Ni, Fe-C-Ni alloy. The Mn and Cr discrepancies seem best explained at present by a solute drag-like effect; an accompanying paper indicates that interphase boundary precipitation of carbides is involved in the Si and Ni alloys, though an inverse solute drag-like effect may also be operative. Formerly graduate student, Department of Metallurgical Engineering, Michigan Technological University. Formerly Professor at Michigan Technological University.     

Prediction of alloy hardenability from thermodynamic and kinetic data

A simple algorithm for the calculation of the hardenability of low alloy eutectoid steels is presented. This involves the semiempirical prediction of concentration-dependent CCT start and pearlite velocity curves using fundamental thermodynamic and kinetic data and current theories of nucleation and growth. The combination of analytic cooling curves with these predictions and a pearlite growth model based on site saturation at grain corners leads to a good prediction of the hardenability of steel 4068, extensively examined by Jominy, and a grain-size dependence which is qualitatively correct. It is also demonstrated via Taylor and binomial expansions of the undercooling within the various kinetic expressions that the hardenability at low alloy concentrations is correctly represented by linear or quadratic addition formulas. At the same time, it is unequivocally demonstrated that Grossman type multiplication formulas are theoretically incorrect. The quadratic terms in the addition formula quantify the so-called “synergistic” effects. The most important positive terms involve an interaction between austenite and ferrite stabilizers. Austenite stabilizers, by depressing the effective temperature of nucleation and diffusional growth processes involving partition of ferrite stabilizers in pearlite, make the latter processes more sluggish. In agreement with long-standing experience, it is concluded that the strongest and most economic hardenability effects can be obtained via mixtures which include both austenite and ferrite stabilizers. This paper is based on a presentation made at a symposium on “Hardenability” held at the Cleveland Meeting of The Metallurgical Society of AIME, October 17, 1972, under the sponsorhip of the IMD Heat Treatment Committee.     

Transitions in carbide morphology in a ternary Fe-C-W steel

A ternary steel of composition Fe-0.20C-6.3W (wt pct) was examined with the purpose of characterizing the carbide morphologies that arise during the diffusional decomposition of austenite. Interphase precipitation of M₆C with ferrite in the 700 °C to 800 °C temperature range was observed, as previously reported. At higher temperatures (800 °C to 920 °C), precipitation of M₆C in austenite in a nodular fashion was also observed, never before seen in this alloy. The carbides inside such nodules exhibited a highly degenerate, quasilamellar morphology, and the evidence suggested a discontinuous precipitation mechanism. Comparison with a calculated phase diagram showed that this quasilamellar carbide precipitation reaction dominated both in the stable and metastable ferrite + austenite + carbide three-phase field, since full equilibrium, for a variety of reasons, is not obtained at the “short” reaction times over which this kinetically competitive reaction dominates the microstructural evolution. The formation of carbide-free ferrite preceding the onset of carbide precipitation (found in both cases) is seen to be instrumental in the occurrence of the quasilamellar carbide precipitation reaction, and a simple diffusion model is advanced in support of this view. Finally, the austenite inside these nodules was seen to revert to ferrite at long times, indicating that full equilibrium is not attained at the initial stages of austenite decomposition, in accord with expectation. This article is based on a presentation made in the symposium “Kinetically Determined Particle Shapes and the Dynamics of Solid:Solid Interfaces,” presented at the October 1996 Fall meeting of TMS/ASM in Cincinnati, Ohio, under the auspices of the ASM Phase Transformations Committee.     

Ferrite nucleation and growth during continuous cooling

The austenite decomposition has been investigated in two hypoeutectoid plain carbon steels under continuous cooling conditions using a dilatometer on a Gleeble 1500 thermomechanical simulator. The experimental results were used to verify model calculations based on a fundamental approach for the dilute ternary system, Fe-C-Mn. The austenite-to-ferrite transformation start temperature can be predicted from a nucleation model for slow cooling rates and small austenite grain sizes, where ferrite nucleates at austenite grain corners. The nuclei are assumed to have an equilibrium composition and a pillbox shape in accordance with minimal interfacial energy. For higher cooling rates or larger austenite grain sizes, early growth has to be taken into account to describe the transformation start, and nucleation is also encouraged at the remaining sites of the austenite grain boundaries. In contrast to nucleation, growth of the ferrite is characterized by paraequilibrium;i.e., only carbon can redistribute, whereas the diffusion of Mn is too slow to allow full equilibrium in the ternary system. However, Mn segregation to the moving ferrite-austenite interface has to be considered. The latter, in turn, exerts a solute draglike effect on the boundary movement. Thus, growth kinetics are controlled by carbon diffusion in austenite modified by interfacial segregation of Mn. Employing a phenomenological segregation model, good agreement has been achieved with the measurements. This article is based on a presentation made during TMS/ASM Materials Week in the symposium entitled “Atomistic Mechanisms of Nucleation and Growth in Solids,” organized in honor of H.I. Aaronson’s 70th Anniversary and given October 3–5, 1994, in Rosemont, Illinois.     

Nucleation kinetics of proeutectoid ferrite at austenite grain boundaries in Fe-C-X alloys

The nucleation kinetics of proeutectoid ferrite allotriomorphs at austenite grain boundaries in Fe-0.5 at. Pct C-3 at. Pct X alloys, where X is successively Mn, Ni, Co, and Si and in an Fe-0.8 at. Pct C-2.5 at. Pct Mo alloy have been measured using previously developed experimental techniques. The results were analyzed in terms of the influence of substitutional alloying elements upon the volume free energy change and upon the energies of austenite grain boundaries and nucleus: matrix boundaries. Classical nucleation theory was employed in conjunction with the pillbox model of the critical nucleus applied during the predecessor study of ferrite nucleation kinetics at grain boundaries in Fe-C alloys. The free energy change associated with nucleation was evaluated from both the Hillert-Staffanson and the Central Atoms Models of interstitial-substitutional solid solutions. The grain boundary concentrations of X determined with a Scanning Auger Microprobe were utilized to calculate the reduction in the austenite grain boundary energy produced by the segregation of alloying elements. Analysis of these data in terms of nucleation theory indicates that much of the influence of X upon ferrite nucleation rate derives from effects upon the volume-free energy change,i.e., upon alterations in the path of theγ/(α + γ) phase boundary. Additional effects arise from reductions in austenite grain boundary energy, with austenite-forming alloying elements being more effective in this regard than ferrite-formers. By difference, the remaining influence of the alloy elements studied evidently results from their ability to diminish the energies of the austenite: ferrite boundaries enclosing the critical nucleus. The role of nucleation kinetics in the formation of a bay in the TTT diagram of Fe-C-Mo alloys is also considered. Formerly Graduate Student, Department of Metallurgical Engineering and Materials Science, Carnegie-Mellon University     

Origins of internal structure in massive transformation products

The internal structure in massive phases formed during six massive transformations has been reviewed. A counterpart review has also been made for the proeutectoid ferrite reaction, mainly in alloy steels in which bulk partition of alloying elements between austenite and ferrite has not occurred. Both dislocations and twins comprise this structure unless the stacking fault energy is too high to permit twin formation. Volume and shape changes associated with transformation can explain dislocation loops through stress-induced displacement and multiplication of misfit dislocations into the softer phase by means of either a dissociation reaction followed by Ashby-Johnson prismatic looping or emanation of glide loops from Frank-Reed sources. Following Gleiter et al., the “growth accidents” concept used to explain dislocation and twin formation during grain growth proves equally suitable for explaining formation of the same features during the massive and other diffusional transformations. Climb of interfaces produced by edge-to-edge rather than the usual plane-to-plane matching, introduced by Kelly and Zhang and experimentally supported by Nie and Muddle and by Howe et al. for the α → γ _m transformation in near-TiAl alloys, is proposed as another source of dislocations in the product phase. This paper was prepared following participation of its authors in the symposium “The Mechanisms of the Massive Transformation,” held Oct. 9–11, 2000. during the Fall 2000 TMS/ASM Meeting in St. Louis, MO, under the sponsorship of the ASM INTERNATIONAL Phase Transformations Committee.     

The fine structure and formation mechanism of lower bainite

Isothermal transformation of austenite to lower bainite was studied by optical microscopy and transmission electron microscopy (TEM), in two high-purity Fe-C-4 wt pct Mn-2 wt pct Si alloys containing 0.4 and 0.6 wt pct carbon, in order to elucidate the fine structure and formation mechanism of lower bainite. The present results support a mechanism for lower bainite formation presented previously in which lower bainite sheaves result from the formation of an aggregate of fine ferrite crystals with thin austenite “gaps” between them; carbide precipitation occurs within these austenite gaps. This mechanism accounts for the carbides oriented at an angle to the sheaf axis repeatedly observed in lower bainite but is inconsistent with models based on the precipitation of a high-volume fraction of carbides within highly supersaturated ferrite formed by high-velocity shear. Observations by a number of other researchers are reviewed and shown to include morphological features consistent with the present mechanism. Finally, the orientation relationships typically observed among ferrite, austenite, and carbides in lower bainite are reviewed and shown in most (but not all) cases to be consistent with the view that carbides precipitate in austenite at ferrite-austenite boundaries, also in agreement with the present model. This article is based on a presentation made at the Pacific Rim Conference on the “Roles of Shear and Diffusion in the Formation of Plate-Shaped Transformation Products,” held December 18-22, 1992, in Kona, Hawaii, under the auspices of ASM INTERNATIONAL’S Phase Transformations Committee.