Austenite Nucleation and Growth Observed on the Level of Individual Grains by Three-Dimensional X-Ray Diffraction Microscopy

Austenite nucleation and growth is studied during continuous heating using three-dimensional X-ray diffraction (3-D XRD) microscopy at the European Synchrotron Radiation Facility (ESRF) (Grenoble, France). Unique in-situ observations of austenite nucleation and growth kinetics were made for two commercial medium-carbon low-alloy steels (0.21 and 0.35 wt pct carbon with an initial microstructure of ferrite and pearlite). The measured austenite volume fraction as a function of temperature shows a two-step behavior for both steel grades: it starts with a rather fast pearlite-to-austenite transformation, which is followed by a more gradual ferrite-to-austenite transformation. The austenite nucleus density exhibits similar behavior, with a sharp increase during the first stage of the transformation and a more gradual increase in the nucleus density in the second stage for the 0.21 wt pct carbon alloy. For the 0.35 wt pct carbon alloy, no new nuclei form during the second stage. Three different types of growth of austenite grains in the ferrite/pearlite matrix were observed. The combination of detailed separate observations of both nucleation and growth provides unique quantitative information on the phase transformation kinetics during heating, i.e., austenite formation from ferrite and pearlite.     

A study of nonisothermal austenite formation and decomposition in Fe-C-Mn alloys

Experiments using a hot-stage confocal scanning laser microscope (CSLM) have been carried out to observe phase transformations in two steels: Si-killed resulfurized Fe-0.38 wt pct C-1.43 wt pct Mn and Al-killed Fe-0.20 wt pct C-0.87 wt pct Mn. Austenite formation during continuous heating was investigated on the surface of samples that were etched to reveal the ferrite and pearlite regions. It was found that the austenite precipitated first at the pearlite colonies and subsequently in the ferrite phase. The measured advance rates of the γ/pearlite front were roughly twice those of the γ/α front and both interfaces were found to be curved. The γ/pearlite migration rate was found to be in qualitative agreement with published rate equations for isokinetic austenite formation where diffusion is the rate-limiting step. Austenite decomposition was studied during cooling. Widmanstätten ferrite laths precipitate as distinct colonies from the existing allotriomorphic ferrite phase but then also at MnS precipitates. The electron backscatter diffraction (EBSD) analysis showed that all of the laths in a particular colony exhibit similar orientation to one another but a slightly different orientation than the parent allotriomorph, supporting a sympathetic nucleation mechanism. The growth rate of the laths was found to vary widely within a range of 1.5 to 11 μm/s. The ferrite formation is finally halted by impingement with other advancing fronts. The results are presented in a phenomological discussion, with some quantitative analysis of the transformation kinetics.     

Modeling of kinetics of austenite-to-allotriomorphic ferrite transformation in 0.37C-1.45Mn-0.11V microalloyed steel

The present article is concerned with the theoretical and experimental study of the growth kinetics of allotriomorphic ferrite in medium carbon vanadium-titanium microalloyed steel. A theoretical model is presented in this work to calculate the evolution of austenite-to-allotriomorphic ferrite transformation with time at a very wide temperature range. At temperatures above eutectoid temperature, where allotriomorphic ferrite is the only austenite transformation product, the soft-impingement effect should be taken into account in the modeling. In that case, the Gilmour et al. analysis reliably predicts the progress of austenite-to-allotriomorphic ferrite transformation in this steel. By contrast, since pearlite acts as a carbon sink, the carbon enrichment of austenite due to the previous ferrite formation is avoided, and carbon concentration in austenite far from the α/γ interface remains the same as the overall carbon content of the steel. Hence, the soft-impingement effect should be neglected, and allotriomorphic ferrite is considered to grow under a parabolic law. Therefore, assumption of a semi-infinite extent austenite with constant boundary conditions is suitable for the kinetics of the isothermal decomposition of austenite. An excellent agreement (higher than 93 pct in R ²) has been obtained between the experimental and predicted values of the volume fraction of ferrite in all of the ranges of temperature studied.     

Formation of Austenite During Intercritical Annealing of Dual-Phase Steels

The formation of austenite during intercritical annealing at temperatures between 740 and 900 °C was studied in a series of 1.5 pct manganese steels containing 0.06 to 0.20 pct carbon and with a ferrite-pearlite starting microstructure, typical of most dual-phase steels. Austenite formation was separated into three stages: (1) very rapid growth of austenite into pearlite until pearlite dissolution is complete; (2) slower growth of austenite into ferrite at a rate that is controlled by carbon diffusion in austenite at high temperatures (~85O °C), and by manganese diffusion in ferrite (or along grain boundaries) at low temperatures (~750 °C); and (3) very slow final equilibration of ferrite and austenite at a rate that is controlled by manganese diffusion in austenite. Diffusion models for the various steps were analyzed and compared with experimental results.     

Solidification of M2 high speed steel

The freezing process in AISI type M2 high speed tool steel (6 pct W, 5 pct Mo, 4 pct Cr, 2 pct V, 0.8 pct C) was studied by metallographic and thermal analysis techniques. Unidirectional solidification of small laboratory melts in a modified crystal growing apparatus was employed to provide metallographic sections of known macroscopic growth direction. Also cooling curves were obtained on 40 g specimens solidified in thimble crucibles. X-ray microradiography, electron probe scanning techniques, and quantitative microanalysis of dendrites and interdendritic carbides were extensively used to supplement conventional metallography. Carbon and vanadium contents of M2 were varied in order to observe the effect of an austenite and ferrite stabilizer on the thermal analysis curves and microstructure. The nonequilibrium freezing process in M2 includes three major liquid-solid reactions: 1) Liquid → Ferrite, 1435°C; 2) Liquid + Ferrite → Austenite, 1330°C; 3) Liquid → Austenite + M₆C + MC, 1240°C. These reactions account for the as-cast structure of the commercial alloy. The addition of carbon depresses the liquidus (1) and solidus temperatures (3) and narrows the gap between the liquidus (1) and peritectic transformation (2). This gap is eliminated at > 1.39 wt pct C, where the initial freezing reaction is the crystallization of austenite. The accompanying microstructural change is the elimination of σ eutectoid dendrite cores. The addition of vanadium promotes ferrite formation by strongly depressing the peritectic reaction and thus widening the gap between the liquidus and the peritectic.     

Modeling of kinetics of austenite-to-allotriomorphic ferrite transformation in 0.37C-1.45Mn-0.11V microalloyed steel

The present article is concerened with the theoretical and experimental study of the growth kinetics of allotriomorphic ferrite in medium carbon vanadium-titanium microalloyed steel. A theoretical model is presented in this work to calculate the evolution of austente-to-allotriomorphic ferrite transformation with time at a very wide temperature range. At temperatures above eutectoid temperature, where allotriomorphic ferrite is the only austenite transormation product, thesoft-impingement effect should be taken into account in the modeling. In that case, the Gilmouret al., analysis reliably predicts the progress of austenite-to-allotriomorphic ferrite transformation in this steel. By contrast, since pearlite acts as a carbon sink, the carbon enrichment of austenite due to the previous ferrite formation is avoided, and carbon concentration in austenite far from the α/λ interface remains the same as the overal carbon content of the steel. Hence, the soft-impingement effect should be neglected, and allotriomorphic ferrite is considered to grow under a parabolic law. Therefore, assumption of a semi-infinite extent austenite with constant boundary conditions is suitable for the kinetics of the isothermal decomposition of austenite. An excellent agreement (higher than 93 pct inR ²) has been obtained between the experimental and predicted values of the volume fraction of ferrite in all of the ranges of temperature studied. C. CAPDEVILA, Research Associate, formerly with the Department of Physical Metallurgy, Centro Nacional de Investigaciones Metalurgicas (CENIM), Consejo Superior de Investigaciones Cientificas (CSIC), 28040 Madrid, Spain     

Analysis of the <Emphasis Type="Italic">γ</Emphasis> → <Emphasis Type="Italic">α</Emphasis> transformation in a C-Mn steel by phase-field modeling

This article deals with the austenite (γ) decomposition to ferrite (α) during cooling of a 0.10 wt pct C-0.49 wt pct Mn steel. A phase-field model is used to simulate this transformation. The model provides qualitative information on the microstructure that develops on cooling and quantitative data on both the ferrite fraction formed and the carbon concentration profile in the remaining austenite. The initial austenitic microstructure and the ferrite nucleation data, derived by metallographic examination and dilatometry, are set as input data of the model. The interface mobility is used as a fitting parameter to optimize the agreement between the simulated and experimental ferrite-fraction curve derived by dilatometry. A good agreement between the simulated α-γ microstructure and the actual α-pearlite microstructure observed after cooling is obtained. The derived carbon distribution in austenite during transformation provides comprehension of the nature of the transformation with respect to the interface-controlled or diffusion-controlled mode. It is found that, at the initial stage, the transformation is predominantly interface-controlled, but, gradually, a shift toward diffusion control takes place to a degree that depends on cooling rate.     

Analysis of lattice parameter changes following deformation of a 0.19C-1.63Si-1.59Mn transformation-induced plasticity sheet steel

Austenite and ferrite lattice parameters were monitored using X-ray diffraction subsequent to deformation in uniaxial and biaxial tension and plane straining of a 0.19C-1.63Si-1.59Mn transformation-induced plasticity (TRIP) sheet steel. Details from peak position results suggest the presence of stacking faults in the austenite phase, especially after deformation in uniaxial tension. The results also indicate residual stress or composition effects (through changes in the average carbon concentration due to selective transformation of lower carbon regions of austenite). Compressive residual stresses in the ferrite matrix were measured, and found to increase with increasing effective strain in specimens tested in biaxial tension and plane strain. Strain partitioning between softer ferrite and harder austenite (and possibly bainite or martensite) may be responsible for these residual compressive stresses in the ferrite, although volume expansion from the γ → α′ transformation and texture gradients through the sheet thickness are also possible contributors.     

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.     

Evaluation of Carbon Partitioning in New Generation of Quench and Partitioning (Q&P) Steels

Quenching and partitioning (Q&P) and a novel combined process of hot straining (HS) and Q&P (HSQ&P) treatments have been applied to a TRIP-assisted steel in a Gleeble®3S50 thermomechanical simulator. The heat treatments involved intercritical annealing at 800 °C and a two-step Q&P heat treatment with a partitioning time of 100 seconds at 400 °C. The “optimum” quench temperature of 318 °C was selected according to the constrained carbon equilibrium (CCE) criterion. The effects of high-temperature deformation (isothermal and non-isothermal) on the carbon enrichment of austenite, carbide formation, and the strain-induced transformation to ferrite (SIT) mechanism were investigated. Carbon partitioning from supersaturated martensite into austenite and carbide precipitation were confirmed by means of atom probe tomography (APT) and scanning transmission electron microscopy (STEM). Austenite carbon enrichment was clearly observed in all specimens, and in the HSQ&P samples, it was significantly greater than in Q&P, suggesting an additional carbon partitioning to austenite from ferrite formed by the deformation-induced austenite-to-ferrite transformation (DIFT) phenomenon. By APT, the carbon accumulation at austenite/martensite interfaces was observed, with higher values for HSQ&P deformed isothermally (≈ 11 at. pct), when compared with non-isothermal HSQ&P (≈ 9.45 at. pct) and Q&P (≈ 7.6 at. pct). Moreover, a local Mn enrichment was observed in a ferrite/austenite interface, indicating ferrite growth under local equilibrium with negligible partitioning (LENP).

     

Ferrite recrystallization and austenite formation in cold-rolled intercritically annealed steel

The recrystallization of ferrite and austenite formation during intercritical annealing were studied in a 0.08C-1.45Mn-0.21Si steel by light and transmission electron microscopy. Normalized specimens were cold rolled 25 and 50 pct and annealed between 650 °C and 760 °C. Recrystallization of the 50 pct deformed ferrite was complete within 30 seconds at 760 °C. Austenite formation initiated concurrently with the ferrite recrystallization and continued beyond complete recrystallization of the ferrite matrix. The recrystallization of the deformed ferrite and the spheroidization of the cementite in the deformed pearlite strongly influence the formation and distribution of austenite produced by intercritical annealing. Austenite forms first at the grain boundaries of unrecrystallized and elongated ferrite grains and the spheroidized cementite colonies associated with ferrite grain boundaries. Spheroidized cementite particles dispersed within recrystallized ferrite grains by deformation and annealing phenomena were the sites for later austenite formation.     

An investigation of the effects of austenite strength and austenite stacking fault energy on the morphology of martensite in Fe-Ni-Cr-0.3C alloys

The relative effects of austenite stacking fault energy and austenite yield strength on martensite morphology have been investigated in a series of three Fe-Ni-Cr-C alloys. Carbon content (0.3 wt pct) andM ₆ temperature (− 15°) were held constant within the series. Austenite yield strength atM _s was measured by extrapolating elevated temperature tensile data. Austenite stacking fault energy was measured by the dislocation node technique. Martensite morphologies were characterized by transmission electron microscopy and electron diffraction techniques. A transition from plate to lath martensite occurred with decreasing austenite stacking fault energy. The austenite yield strength atM _s for the low SFE, lath-forming alloy was found to be higher than previously reported for lath-forming alloys. The relative effects of these variables on martensite morphologies in these alloys is discussed.     

Effects of Initial Structure and Reversion Temperature on Austenite Nucleation Site in Pearlite and Ferrite–Pearlite

Austenite nucleation sites were investigated in near-eutectoid 0.8 mass pct C steel and hypoeutectoid 0.4 mass pct C steel samples with full pearlite and ferrite–pearlite initial structures, respectively. In particular, the prior austenite grain size had been coarsened to compare grain boundaries in the hierarchical pearlite structure, i.e., the low-angle pearlite colony and high-angle block boundaries with ferrite/pearlite interfaces in the austenite nucleation ability. When the full pearlite in 0.8 mass pct C steel underwent reversion at a relatively low temperature, austenite grains preferentially formed at pearlite block boundaries. Consequently, when the full pearlite with the coarse block structure underwent reversion just above the eutectoid temperature, the reversion took a long time due to the low nucleation density. However, austenite grains densely formed at the pearlite colony boundaries as well, as the reversion temperature became sufficiently high. On the other hand, when ferrite–pearlite in the 0.4 mass pct C steel underwent reversion to austenite, the ferrite/pearlite interface acted as a more preferential austenite nucleation site than the pearlite block boundary even in the case of low-temperature reversion. From these results, it can be concluded that the preferential austenite nucleation site in carbon steels is in the following order: ferrite/pearlite interface?>?pearlite block?>?colony boundaries. In addition, orientation analysis results revealed that ferrite restricts the austenite nucleation more strongly than pearlitic ferrite does, which contributes to the preferential nucleation at ferrite/pearlite interfaces. This suggests that austenite grains formed at a ferrite/pearlite interface tend to have an identical orientation even under high-temperature reversion. Therefore, it is thought that the activation of austenite nucleation within pearlite by increasing the reversion temperature may be effective for rapid austenitization and the grain refinement of austenite structure after the completion of reversion in carbon steels.     

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.     

The influence of starting microstructure on the retention and mechanical stability of austenite in an intercritically annealed-low alloy dual-phase steel

Retention of austenite during the intercritical annealing of a low carbon, low-alloy, dual-phase steel and the mechanical stability of retained austenite(γ _R) have been studied as a function of starting microstructure and annealing conditions. A quenched and tempered(QT) starting microstructure has been found to result in higher γ_R volume fractions compared to fully martensitic(Q) and ferrite plus pearlitic(F + P) starting structures for all annealing conditions employed in this work. The austenite formed by annealing up to 792 °C (where the kinetics are dominated by higher nucleation rates) is more prone to retention compared to that formed by annealing beyond 792 °C (where the kinetics are mainly dominated by higher growth rates). A smaller size of γ_R particles has a better mechanical stability against deformation-induced martensite transformation. Formerly Master's Student at the University of Manitoba     

Stabilization mechanisms of retained austenite in transformation-induced plasticity steel

Three stabilization mechanisms—the shortage of nuclei, the partitioning of alloying elements, and the fine grain size—of the remaining metastable austenite in transformation-induced plasticity (TRIP) steels have been studied by choosing a model alloy Fe-0.2C-1.5Mn-1.5Si. An examination of the nucleus density required for an athermal nucleation mechanism indicates that such a mechanism needs a nucleus density as large as 2.5 · 10¹⁷ m⁻³ when the dispersed austenite grain size is down to 1 μm. Whether the random nucleation on various heterogeneities is likely to dominate the reaction kinetics depends on the heterogeneous embryo density. Chemical stabilization due to the enrichment of carbon in the retained austenite is the most important operational mechanism for the austenite retention. Based on the analysis of 57 engineering steels and some systematic experimental results, an exponential equation describing the influence of carbon concentration on the martensite start (M _s) temperature has been determined to be M _s (K)=273+545.8 · e ^−1.362w _c(mass pct). A function describing the M _s temperature and the energy change of the system has been found, which has been used to study the influence of the grain size on the M _s temperature. The decrease in the grain size of the dispersed residual austenite gives rise to a significant decrease in the M _s temperature when the grain size is as small as 0.1 μm. It is concluded that the influence of the grain size of the retained austenite can become an important factor in decreasing the M _s temperature with respect to the TRIP steels.     

Effect of Mn Addition on Microstructural Modification and Cracking Behavior of Ferritic Light-Weight Steels

In the present study, effects of Mn addition on cracking phenomenon occurring during cold rolling of ferritic light-weight steels were clarified in relation to microstructural modification involving κ-carbide, austenite, and martensite. Four steels were fabricated by varying Mn contents of 3 to 12 wt pct, and edge areas of steel sheets containing 6 to 9 wt pct Mn were cracked during the cold rolling. The steels were basically composed of ferrite and austenite in a band shape, but a considerable amount of κ-carbide or martensite existed in the steels containing 3 to 6 wt pct Mn. Microstructural observation of the deformed region of fractured tensile specimens revealed that cracks which were initiated at ferrite/martensite interfacial κ-carbides readily propagated along ferrite/martensite interfaces or into martensite areas in the steel containing 6 wt pct Mn, thereby leading to the center or edge cracking during the cold rolling. In the steel containing 9 wt pct Mn, edge cracks were found in the final stage of cold rolling because of the formation of martensite by the strain-induced austenite to martensite transformation, whereas they were hardly formed in the steel containing 12 wt pct Mn. To prevent or minimize the cracking, it was recommended that the formation of martensite during the cooling from the hot rolling temperature or during the cold rolling should be suppressed, which could be achieved by the enhancement of thermal or mechanical stability of austenite with decreasing austenite grain size or increasing contents of austenite stabilizers.     

The morphology,crystallography, and mechanism of carbide precipitation in an Fe-0.12 Pct C-3.28 Pct Ni alloy

Carbide precipitation during the eutectoid decomposition of austenite has been studied in an Fe-0.12 pct C-3.28 pct Ni alloy by transmission electron microscopy (TEM) supplemented by optical microscopy. Nodular bainite which forms during the latter stages of austenite decomposition at 550 °C exhibits two types of carbide arrangement: (a) banded interphase boundary carbides with particle diameters of about 20 to 90 nm and mean band spacings between 180 and 390 nm and (b) more randomly distributed (“nonbanded”) elongated particles exhibiting a wide range of lengths between 33 and 2500 nm, thicknesses of approximately 11 to 50 nm, and mean intercarbide spacings of approximately 140 to 275 nm. Electron diffraction analysis indicated that in both cases, the carbides are cementite, obeying the Pitsch orientation relationship with respect to the bainitic ferrite. The intercarbide spacings of both morphologies are significantly larger than those previously reported for similar microstructures in steels containing alloy carbides other than cementite (e.g., VC, TiC). Both curved and straight cementite bands were observed; in the latter case, the average plane of the interphase boundary precipitate sheets was near {110}_α//{011}_c consistent with cementite precipitation on low-energy {110}_α//{111}_γ ledge terrace planes (where α,β, andc refer to ferrite, austenite, and cementite, respectively). The results also suggest that the first stage in the formation of the nonbanded form of nodular bainite is often the precipitation of cementite rods, or laths, in austenite at the α:γ interfaces of proeutectoid ferrite secondary sideplates formed earlier. Although these cementite rods frequently resemble the “fibrous” microstructures observed by previous investigators in carbide-forming alloy steels, they are typically much shorter than fibrous alloy carbides. The bainitic microstructures observed here are analyzed in terms of a previously developed model centered about the roles of the relative nucleation and growth rates of the product phases in controlling the evolution of eutectoid microstructures.