The kinetics of ferrite nucleation at austenite grain edges in Fe-C and Fe-C-X alloys

Nucleation kinetics of proeutectoid ferrite allotriomorphs at the edges of austenite grains in Fe-C and Fe-C-X alloys, where X is successively Mn, Ni, Co, and Si, have been measured using a modification of the techniques previously developed to study nucleation at grain faces. Analysis of these data with classical heterogeneous nucleation theory has shown that ferrite nuclei formed at grain edges have low energy interphase boundaries. An equivalent conclusion was reached during our previous studies of ferrite nucleation at austenite grain faces. The influence of alloying elements on nucleation rates was also found to follow a pattern similar to that demonstrated for grain face nucleation. Formerly Graduate Student with the Department of Metallurgical Engineering and Materials Science, Carnegie-Mellon University,     

Effects of austenitizing temperature on the kinetics of the proeutectoid ferrite reaction at constant austenite grain size in an Fe-C alloy

Austenitizing an Fe-0.23 pct C alloy at 1300°C and further at 900°C prior to isothermal transformation was found to increase the growth kinetics of grain boundary ferrite allotriomorphs while decreasing their rate of nucleation. A scanning Auger microprobe was used to establish that sulfur segregates to the austenite grain boundaries and does so increasingly with decreasing austenitizing temperature. A binding free energy of sulfur to these boundaries of approximately 13 kcal/mole (54.4 kj/mole) was calculated from theMcLean adsorption isotherm. The kinetic results were explained in terms of preferential reduction of the austenite grain boundary energy decreasig nucleation kinetics, and adsorption of sulfur at α:γ boundaries increasing the carbon concentration gradient in austenite driving growth.     

Partition of Mn during the growth of proeutectoid ferrite allotriomorphs in an Fe-1.6 at. pct C-2.8 at. pct Mn alloy

A STEM analysis is made of the Mn distribution around grain boundary allotriomorphs of proeutectoid ferrite in an Fe-1.6 at. Pct C-2.8 at. Pct Mn alloy. Whereas the Mn enriched region is readily observed to extend along the austenite grain boundary, no substantial build-up or depletion of Mn near the ferrite : austenite interface is detected, consistent with the electron probe microanalysis previously reported. In the temperature range where the partition-local equilibrium (P-LE) mode has been proposed to prevail, measured parabolic growth rate constantsfall 1 to 2 orders of magnitude above that predicted from this model, but also below that calculated from the paraequilibrium (PE) model by roughly the same amount. A modification of the theory of grain boundary diffusion-aided growth of precipitates,i.e., the collector/rejector plate mechanism, on the other hand, accounts fairly well for the observed growth kinetics of ferrite allotriomorphs. However, only a slightly better accounting than the P-LE model is provided by this mechanism for the temperature dependence of Mn partition. Data on Ni partition, obtained in an Fe-0.5 at. Pct C-3.1 at. Pct Ni alloy, are also analyzed with the rejector plate model.     

The kinetics of ferrite nucleation at austenite grain boundaries in Fe-C alloys

The nucleation kinetics of grain boundary allotriomorphs of proeutectoid ferrite at austenite grain faces have been measured in three high purity Fe-C alloys as a function of isothermal reaction time and temperature. Several correction techniques, including discrimination between different nucleation sites and the effect of carbon diffusion fields on further nucleation of ferrite, were incorporated into a stereological procedure utilizing the SchwartzJSaltykov size distribution analysis. This analysis enabled the number of ferrite particles per unit unreacted grain boundary area to be obtained as a function of isothermal reaction time, and thus the time-dependent nucleation kinetics to be obtained as a function of temperature and carbon concentration. These rates were then compared with those predicted by classical heterogeneous nucleation theory using various models for the critical nucleus. It was concluded that viable critical nuclei must have predominately low energy interphase boundaries. Only a very small fraction of the austenite grain face area appears to be capable of supporting nucleation. Formerly Graduate Student, Department of Metallurgical Engineering, Michigan Technological University, Houghton, MI 49931 Formerly Professor at Michigan Technological University     

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.     

Nucleation kinetics of grain boundary allotriomorphs of proeutectoid ferrite in Fe-C-Mn-X2 alloys

The steady-state nucleation rates of ferrite allotriomorphs at the “faces” of austenite grain boundaries were measured in Fe-C-X₁-X₂ alloys, where X₁ was Mn and X₂ was successively Si, Ni, and Co, using techniques previously developed for counterpart studies on Fe-C and Fe-C-X alloys. The results were compared with the predictions of the classical nucleation theory, using the pillbox-shaped critical nucleus model. The volume free energy changes associated with nucleation in Fe-C-X₁-X₂ quaternary systems were evaluated from the central atoms model (CAM) for both para- and orthoequilibrium modes of transformation. The nucleation process was assumed to be controlled by volume and/or grain boundary diffusion of alloying elements. The so-called synergistic effects of alloying elements were considered in terms of the volume free energy change and interfacial energies on the basis of the results of the nucleation rate measurements. Formerly Graduate Student, Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh,PA Formerly R.F. Mehl Professor Emeritus at Carnegie Mellon University, is with GEO-Centers, Inc., Ft. Washington, MD,     

Influence of Crystallography on Ferrite Nucleation at Austenite Grain-Boundary Faces,Edges, and Corners in a Co-15Fe Alloy

The nucleation of ferrite precipitates at austenite grain faces, edges (triple lines), and corners (quadruple points) was studied in a Co-15Fe alloy in which the matrix phase was retained upon cooling to room temperature by serial sectioning coupled with electron backscatter diffraction analysis. Nearly half of the edges and corners were vacant at an undercooling of 60 K from the γ/(α + γ) boundary where the precipitation occurred significantly at grain faces. A significant proportion of precipitates had Kurdjumov–Sachs (K–S) and to a lesser extent Nishiyama–Wassermann (N–W) orientation relationships with more than one grain at all boundary sites. Vacant edges and corners were readily observed, of which the misorientations of matrix grain boundaries would permit a precipitate to have a specific orientation relationship with multiple grains. Small differences in the nucleation activation energy among the grain faces, edges, and corners may lend support to a view proposed from experiments of nucleation in Fe-C base alloys that ferrite nuclei are more or less surrounded by low-energy facets of α/γ phase boundary.     

The transformation to austenite in a fine grained tool steel

The transformation to austenite in a fine grained tool steel has been investigated quantitatively until the disappearance of the ferrite. The initial structure of the steel consisted of ferrite and globular carbides. The nucleation starts at carbides which lie at the ferrite grain boundaries. The kinetics are in good agreement with Cahn's theory of grain boundary nucleated transformation. A constant growth rate was found up to 30 pct transformation. Site saturation occurs early in the reaction; this was confirmed by metallographic examination. The rate law is controlled by growth and is independent of the nucleation rate. The mechanism which is controlling this growth is the advancing ferrite-austenite interface reaction. The alloying elements influence the atomic mobilities, increasing the necessary activation energy to about 110 kcal/mol. Formerly Research Engineer, Metallurgical Department, Delft University of Technology     

Local conditions at moving α/γ boundaries of proeutectoid ferrite transformation in iron alloys

The local conditions at moving α/γ boundaries in iron alloys are examined from the data on growth kinetics, solute partitioning, and critical limit of transformation. In Fe-C alloys, local equilibrium of carbon is likely to be sustained at the majority of α/γ boundaries during the growth of allotriomorphic ferrite except at some boundaries containing immobile low-energy facets. In Fe-C-X alloys, there is experimental evidence that local equilibrium of the substitutional alloying element is established at higher temperatures. However, growth under near paraequilibrium conditions may be prevalent at lower temperatures and at early growth stages. The diffusion of alloying elements in ferrite and along the austenite grain boundary may have a significant influence on the growth of ferrite near the boundary between fast and slow growth. The growth of Widmanstätten and bainitic ferrite is likely controlled by carbon diffusion, that is, without a supersaturation of carbon, while the chemical condition of carbon near the plate edge may not be identical to that of a planar disordered α/γ boundary.     

Evaluation of the effects of segregation on austenite grain boundary energy in Fe-C-X alloys

A scanning Auger microprobe study has been made of the segregation of substitutional alloying elements to austenite grain boundaries in Fe-C-X alloys (where X = Mn, Ni, Si, Co, and Mo). The grain boundary enrichments in X are considerably smaller than those previously estimated from the thermal grooving method but appear consistent with other SAM results in the literature. Mo exhibits the highest enrichment factor, those for Si and Mn are intermediate, and no appreciable grain boundary enrichment of either Co or Ni is observed. In view of the special relevance of this information to nucleation kinetics of austenite decomposition products at austenite grain boundaries, the reductions in grain boundary energy attending the measured enrichments are evaluated using the model of interactive segregation of interstitial and substitutional solutes formulated by Guttmann and McLean. These calculations were performed under two different (limiting) conditions: (i) equilibrium segregation of both solutes is fully achieved at the isothermal reaction temperatures, and (ii) the boundary concentration of X is fully inherited from the austenitizing temperature and only paraequilibrium segregation of carbon is achieved. Various characteristics of interactive segregation are also discussed in terms of the interaction and binding energies of each solute. Formerly a Graduate Student, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA15213 Formerly with Oak Ridge National Laboratory, Oak Ridge, TN 37830     

Phase-field model prediction of nucleation and coarsening during austenite/ferrite transformation in steels

A phase-field simulation is performed to study the kinetics of austenite to ferrite (γ → α) transformation in a low-carbon steel during continuous cooling. Emphasis is placed on the influence of nucleation, along with ferrite grain coarsening behind the transformation front, on microstructural evolution. Results show that grain coarsening is significant even before all nucleation has been completed and occurs via two different coarsening mechanisms, grain boundary migration and ferrite grain crystallographic rotation, both of which can be clearly observed occurring as the simulated microstructure evolves. For some grains, sudden growth jumps are predicted by the model—a phenomenon that has been observed before by synchrotron X-ray diffraction. This study quantitatively demonstrates the phenomenon that increasing cooling rate leads to nucleation off initial austenite grain boundaries, which is also verified by studying the morphology of ferrite grains as predicted using different nucleation mode assumptions. A relationship between nucleation site distribution and the nucleation rate is demonstrated by computer simulation.     

On the critical nucleus composition of ferrite in an Fe-C-Mn alloy

The displacement of the capillarity-corrected nucleus composition of ferrite relative to the two compositions conventionally used to calculate the nucleation rate,i.e., the bulk equilibrium composition and the maximum volume free energy change composition, was evaluated in an Fe-0.5 at. Pct C-3 at. Pct Mn alloy in the regime of the classical nucleation theory. While the nucleus composition is very close to the one at maximum volume free energy change, it is very different from that of bulk equilibrium. The errors arising in the calculation of critical nucleus size and nucleation rate when using inappropriate nucleus compositions are discussed under both para- and orthoequilibrium conditions for the formation of ferrite. M. Enomoto, formerly Graduate Student, Department of Metallurgical Engineering and Materials Science, Carnegie-Mellon University     

The role of solute segregation in promoting the hardenability of steel

The two most potent promoters of hardenability in steel are boron and phosphorus. It appears that these elements function by segregating to austenite grain boundaries and interfering with the nucleation of proeutectoid ferrite. It is suggested that this occurs by the stabilization or alteration of the structure of certain special grain boundary re-gions which serve as favored nucleation sites for ferrite. It is demonstrated how at least part of the effect of alloying elements like manganese and chromium might be ascribed to their enhancement of phosphorus segregation. Under certain conditions phosphorus could be a useful addition to increase the hardenability of low alloy steels.     

On the growth kinetics of grain boundary ferrite allotriomorphs

Previous work has shown that the thickening kinetics of proeutectoid ferrite allotriomorphs in an Fe-0.11 pct C alloy are often more rapid than the kinetics calculated for volume diffusion-control from the Dube-Zener equation for the migration of a planar boundary of infinite extent, assuming the diffusivity of carbon in austenite,D, to be constant at that of the carbon content of the Ae3. Recalculating the thickening kinetics, using a numerical analysis of the infinite planar boundary problem previously developed by Atkinson in which the variation ofD with composition is taken fully into account, was found to increase this discrepancy. Measurements were then made of the lengthening as well as the thickening kinetics of grain boundary allotriomorphs in the same alloy. Application to these data of Atkinson’s numerical analysis of the growth kinetics of an oblate ellipsoid, in which the composition-dependence ofD is similarly considered, produced an acceptable accounting for nearly all of the data. It was concluded that the growth of ferrite allotriomorphs is primarily controlled by the volume diffusion of carbon in austenite; the presence of a small proportion of dislocation facets along one of the broad faces of the allotriomorphs, however, usually results in growth kinetics which are somewhat slower. An alternate treatment of the lengthening and thickening data upon the basis of the theory of interfacial diffusion-aided growth of allotriomorphs indicated that, in the temperature range investigated (735° to 810°C),the diffusivities of carbon along γ:γ and γ:α boundaries required for this mechanism to make a significant contribution to growth are too high to be physically plausible. Formerly with Scientific Research Staff Formerly with Scientific Research Staff, Ford Motor Company     

Phase transformations in an Fe-7.8Al-29.5Mn-1.5Si-1.05C alloy

Phase transformations in an Fe-7.8Al-29.5Mn-l.5Si-1.05C alloy have been investigated by means of optical microscopy and transmission electron microscopy. In the as-quenched condition, a high density of fine (Fe,Mn)₃AlC carbides could be observed within the austenite matrix. When the as-quenched alloy was aged at temperatures ranging from 550 °C to 825 °C, aγ → coarse (Fe,Mn)₃AlC carbide + DO₃ reaction occurred by a cellular precipitation on theγ/γ grain boundaries and twin boundaries. Both of the observations are quite different from those observed by other workers in Fe-Al-Mn-C alloys. In their studies, it was found that the as-quenched microstructure was austenite phase(γ), and (Fe,Mn)₃AlC carbides could only be observed within the austenite matrix in the aged alloys. In addition, aγ →α (ferrite) + coarse (Fe,Mn)₃AlC carbide reaction or aγ →α + coarse (Fe,Mn)₃AlC carbide +β-Mn reaction was found to occur on theγ/γ grain boundary in the aged Fe-Al-Mn-C alloys.     

变形晶界对低碳钢显微组织的影响

对不同温度下变形和变形后再加热到奥氏体区的低碳钢SS400的显微组织进行了研究，结果表明：变形使奥氏体和铁素体晶界呈锯齿状，锯齿状的奥氏体晶界优先成为铁素体的形核位置，锯齿状的铁素体晶界有利于铁素体再结晶核心的形成。     

Growth kinetics of grain boundary allotriomorphs of proeutectoid ferrite in Fe-C-Mn-X2 alloys

The parabolic rate constant for the thickening of grain boundary ferrite allotriomorphs at the faces of austenite grain boundaries was measured as a function of isothermal transformation temperature in three Fe-C-X₁-X₂ alloys where X₁ is Mn and X₂ is successively Si, Ni, and Co. The results were compared with the predictions of the local equilibrium model for multi-component systems and with those derived from the theory of growth under paraequilibrium conditions. The distribution of Mn and Si in ferrite and austenite in the Fe-C-Mn-Si alloy was also measured as a function of reaction temperature with transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDX). The observed temperature below which alloying element partition ceased was in good agreement with the local equilibrium model. Whereas the parabolic rate constant for thickening was considerably larger than the amount predicted by this theory in the alloying element diffusion-controlled regime, the opposite was true in the carbon diffusion-controlled regime. Similarly, the calculated paraequilibrium constant was usually considerably larger than that measured experimentally. Synergistic enhancements of the effects of Mn and X₂ in diminishing thickening kinetics were observed for each X₂. The time-temperature-transformation (TTT) curves for the beginning of transformation were calculated from a modified Cahn analysis for the overall kinetics of grain-boundary-nucleated reactions using values of the nucleation rate and the parabolic growth rate constant computed from various models and compared with experimentally determined TTT curves. Substantial discrepancies between the calculated and measured curves were ascribed to synergistic effects of Mn and X₂ upon nucleation and growth kinetics. Formerly Graduate Student, Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA Formerly Mehl Professor Emeritus at Carnegie Mellon University.     

The incomplete transformation phenomenon in steel

The incomplete transformation (ICT) phenomenon is defined as the temporary cessation of ferrite formation (in the absence of carbide precipitation at α:γ boundaries) before the fraction of austenite transformed to ferrite predicted by the Lever rule is attained. The ICT phenomenon is central to the “overall reaction kinetics” definition of bainite but plays lesser roles in the quite different groups of phenomena comprising the “surface relief” and “generalized microstructural” definitions. Experimental generalizations that can be made about the ICT are briefly noted. Effects of alloying elements, X, upon various aspects of the nucleation and growth of ferrite are listed in order of apparently increasing strength. The ICT is seen to be one of the stronger effects in the latter spectrum. Theories of the ICT are then critically examined. The currently most promising theories involve (1) the cessation of growth induced by the coupled-solute drag effect (C-SDE), accentuated by the overlap of the carbon diffusion fields associated with adjacent ferrite crystals; and (2) the concepts of item (1) plus local alloying element partition between ferrite and austenite (LE-NP), thereby making any further ferrite growth require volume diffusion of X in austenite and thus to take place exceedingly slowly. Distinguishing between these theories will require use of an Fe-C-X system in which the temperature-carbon concentration paths of the paraequilibrium (PE) Ae₃ and of the “no partition” boundary are well separated. Although the Fe-C-Mo system has proved convenient for studying many aspects of the ICT phenomenon, it does not fulfill this specification. Fe-C-Mn alloys do so and should be particularly useful subjects for future investigations of the ICT phenomenon. 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.     

Effect of deformation on the austenite-to-ferrite transformation in a plain carbon and two microalloyed steels

Isothermal compression tests were carried out on three steels: (i) a plain C, (ii) a Mo, and (iii) a Mo-Nb-V microalloyed grade in order to study the effect of deformation on the austenite-to-ferrite transformation. Dynamic TTT (DTTT) curves were determined, which show clearly the extent to which deformation accelerates the decomposition of austenite. The latter effect is diminished by the addition of the alloying elements Mo, Nb, and V and is further reduced as the temperature is increased. The substitutional elements Mo, Nb, and V appear to reduce the nucleation rate through reduction of the austenite grain boundary energy. The growth rate is also reduced by these elements, apparently through the solute drag-like effect. The microstructural results indicate that the ferrite formed under dynamic conditions becomes more homogeneous and finer when the strain rate or the temperature is increased. Under static conditions, increasing the prestrain or the prestraining strain rate accelerates the γ-to-α transformation and reduces the mean grain size of the ferrite, although the highest transformation rate is still associated with the dynamic case.     

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.