Transformation Stasis Research Articles

Carbon Enrichment of Austenite during Ferrite – bainite Transformation in Low-alloy-steel

The bainitic transformation kinetics and carbon enrichment of austenite during isothermal holding at 723–923 K were investigated for an Fe-0.1mass%C-0.5mass%Si-2.0mass%Mn alloy. The transformation progressed rapidly until approximately 50 s, after which transformation stasis was observed at 823 K. The carbon concentration of austenite increased as the transformation proceeded, and showed an almost constant value during stasis. It reached approximately 0.45–0.50% at 823 K, which corresponds to the carbon concentration at the T0’ composition with an additional strain energy of 100 J/mol associated with the transformation. After stasis, a slight increase in the ferrite or bainitic ferrite fraction was observed. The carbon concentration of austenite also increased and reached approximately 0.60%, clearly exceeding the carbon concentration at the T0 composition. These results imply that at the first stage, the bainite transformation occurs and shows the incomplete transformation, following which at the second stage, diffusional ferrite transformation proceeds. The additional strain energy associated with the transformation calculated from the carbon concentration at stasis due to the incomplete bainite transformation tends to decrease as the holding temperature increases. This indicates that strain relaxation due to the transformation occurred at higher holding temperatures.

Stasis mechanism of γ → ε martensitic transformation in Fe-17Mn alloy

In the present study, we observed incomplete martensitic transformation in Fe-17Mn alloy that γ austenite → ε martensitic transformation stops at a certain temperature during continuous cooling and the ε phase fraction does not increase with decreasing temperature further. The mechanism of this transformation stasis (TS) was investigated through microstructural observation, thermodynamic calculation and tensile testing using Fe-17Mn specimens with various γ grain sizes. The TS phenomenon was observed in all specimens regardless of prior γ grain size, and the TS temperature was lowered with decreasing prior γ grain size. TS occurred due to the dynamic grain refinement that ε platelets subdivide a γ grain to make several subgrains. The γ/ε interphase boundaries surrounding subgrains revealed the higher suppression effect of γ → ε martensitic transformation than prior γ grain boundaries. Therefore, at the same level of subgrain size, the fine γ-grained specimen revealed more active γ → ε martensitic transformation due to the higher fraction of prior γ grain boundaries than the coarse γ-grained specimen. Considering the difference in boundary characteristic between prior γ grain boundaries and γ/ε interphase boundaries, the driving force for γ → ε martensitic transformation was newly calculated and used to explain the TS phenomenon.

Analysis of transformation stasis by the Gibbs energy balance (GEB) approach modified to consider carbon redistribution into remaining austenite

The Gibbs energy balance (GEB) concept for modeling the transformation stasis during isothermal ferrite formation is revisited. Modification has been made to take into account the carbon redistribution into remaining austenite and the increased carbon solubility in ferrite transformed at low temperature. Both the original and modified models adequately describe the transformation stasis in ferrite formation in alloy systems considered in earlier studies; however, the modifications proposed in the present study are critical for the reasonable description of the effect of Mn content on the transformation stasis in a medium carbon steel where the redistribution of carbon has more significant influence.

Transformation stasis phenomenon and overall transformation kinetics of low-carbon, Nb-bearing ferrous alloy

To illuminate the overall transformation kinetics of low-carbon, Nb-bearing ferrous Alloy (Nb ferrous alloy), a novel method was proposed to investigate the overall transformation behavior on the basis of bainitic transformation stasis theory. The stability and transformation behavior of austenite were controlled by choosing different isothermal temperatures nearby or away from the transformation start temperature (Ts), and the dilatometric variation discipline and microstructural features corresponding to different processes were investigated. Nucleation mechanisms throughout the overall transformation process were further discussed on the basis of the classical J-M-A model. Results indicate the overall transformation kinetics concerning to studied Nb ferrous alloy can be nearly separated into two stages in terms of the dilatometric variation discipline together with microstructural features, they are (a) Acicular ferrite (AF) transformation stage with relative high transformation rate at initial transformation stage, and (b) Conventional bainite (CB) transformation stage with low transformation rate in the late stage. The possible nucleation is the diffusion-controlled mechanism with regard to studied Nb ferrous alloy during isothermal temperature from 580 ℃ to 440 ℃. At the initial stage, AF nucleates on the parent austenite grain boundary or sub-grain boundary and grows in the mode of arbitrary dimensional, and the nucleation rate is decreased gradually. At the second stage, the formed AF impingements each other, and new phase nucleus attached to parent austenite grain boundary, sub-grain boundary or AF boundary begin to grow in the mode of incrassation and transform into CB, and the nucleation rate is almost zero throughout the secondary transformation stage.

Inconsistent effects of austempering time within transformation stasis on monotonic and cyclic deformation behaviors of an ultrahigh silicon carbide-free nanobainite steel

Given the transformation stasis after the incomplete transformation of carbide-free bainite, this paper aimed to study the effects of austempering time within transformation stasis on the bainite microstructure, monotonic deformation behavior, and especially cyclic deformation behavior using an ultrahigh silicon (2.59 wt%) steel. With increasing austempering time, the dislocation density of bainitic ferrite decreased, carbon content of retained austenite slight increased, and carbon distribution in retained austenite blocks gradually homogenized. The best combination of strength and ductility was obtained through a longer austempering time within transformation stasis. But the longer austempering time, indeed, did not result in longer fatigue life. The opposite trend could be explained by the fact that the primary factors affecting these mechanical properties were different. The lower density of mobile dislocation pre-existed in the starting microstructure of the samples austempered for a longer time was primarily responsible for its lower cyclic hardenability. Moreover, one retained austenite block with completely homogeneous carbon distribution was only transformed to one martensite grain, which increased the cyclic softening and degrade the fatigue life. By contrast, the deformation-induced martensite transformation from the retained austenite with higher mechanical stability in the samples austempered for a longer time enhanced strain hardenability at higher monotonic tensile strains and well delayed the necking, thus improving the combination of strength and ductility.

Transformation Stasis Phenomenon of Bainite Formation in Low-Carbon, Multicomponent Alloyed Steel

The transformation stasis phenomenon of bainite formation in low-carbon steel was detected using a high-resolution dilatometer. The phenomenon occurred at different stages for different isothermal temperatures. In combination with microstructural observation, the calculated overall activation energy of transformation and interface migration velocity shed new light on the cause of formation of the stasis phenomenon. The temporary stasis formed at the initial stage of phase transformation for high isothermal temperature was attributed to the drag effect of substitutional atoms, which leads to low-interface migration velocity and large overall activation energy.

Coupled solute drag and transformation stasis during ferrite formation in Fe-C-Mn-Mo

Ferrite growth kinetics in a series of quaternary Fe-C-xMn-0.4Mo (wt. %) alloys (x = 0.5, 0.8, 1.1, 1.3) has been studied using the decarburization technique at temperatures between 755 °C and 806 °C. It is shown for the first time that the ferrite growth kinetics in the quaternary system can be well predicted using solute drag parameters (Eb and Dtrans) tuned from experiments on model ternary Fe-C-Mn and Fe-C-Mo alloys. This should be interpreted as great encouragement for the steel phase transformations community and provides hope for extrapolation of research activities on model ternary Fe-C-X systems to real industrial steels. The important effect of carbon segregation to the migrating interface is discussed in the context of Qiu et al.’s recent unsuccessful attempt to predict the growth behaviour in the Fe-C-Mn-Si quaternary system based on parameters tuned from the Fe-C-Si and Fe-C-Mn systems.Using the successful Fe-C-Mn-Mo growth model, traditional ferrite precipitation at lower temperatures of 550 °C and 650 °C is then described and a new explanation for transformation stasis is proposed (local carbon profile inversion) which is consistent with recent analytical transmission electron microscopy (ATEM) measurements showing negligible interfacial solute segregation at the onset of stasis in an Fe-C-Mn-Mo alloy. We propose that the onset of transformation stasis is controlled by the competition between the carbon flux in the austenite away from the interface and the decrease in the interfacial carbon content due to the interfacial dissipation processes. Critically, it is the rate of change of the interfacial dissipation with velocity that matters, ∂ΔGdiss−total∂v, and not the absolute magnitude of the dissipation, ΔGdiss−total for transformation stasis.

Effect of Mo Addition on the Transformation Stasis Phenomenon During the Isothermal Formation of Bainitic Ferrite

The effect of Mo addition on the transformation stasis (TS) phenomenon during bainitic transformation has been investigated in Fe-0.2C-1.5Mn-1.5Si-(0.2, 1.5, 3)Mo (wt pct) alloys. Both diffusionless and diffusional models failed in predicting the TS that is strongly affected by Mo concentration in experiment. The Gibbs energy balance approach could capture TS in alloys with 0.2 and 1.5 wt pct Mo, but significantly underestimate bainite fraction at stasis for alloy with 3 wt pct Mo, which is deduced to arise from the so-called anti-coupled solute drag effect.

The Effect of Interfacial Element Partitioning on Ferrite and Bainite Formation

The formation of bainitic ferrite and that of grain boundary ferrite in low alloy steels have been two of the most important and interesting research topics in the field of solid state ferrous phase transformation for several decades, and various aspects of these two transformations have been discussed extensively in the literature. Recently, a so-called Gibbs energy balance (GEB) model was proposed by the authors to evaluate alloying element effects on the growth of bainitic ferrite and grain boundary ferrite. The model predicts a growth mode transition from paraequilibrium, negligible partitioning to partitioning during the isothermal formation of bainitic ferrite and grain boundary ferrite. Transformation stasis and bay phenomenon are well explained by the GEB model and both of them are found to be due to alloying element diffusion at the interface. This overview gives a summary of the authors’ recent progress in the understanding of the growth of bainitic ferrite and grain boundary ferrite, with particular focus on the growth mode transition, the transformation stasis phenomenon and the bay phenomenon.

Direct measurement of carbon enrichment in the incomplete bainite transformation in Mo added low carbon steels

The overall kinetics and carbon enrichment in austenite during the incomplete isothermal bainite transformation in Fe–0.1C–1.5Mn–(0, 0.03, 0.3, 0.5, 1)Mo (mass%) alloys were investigated with quantitative metallography and Electron Probe Microanalysis in the transformation temperature range of 773–873K. The incomplete transformation appears at 823–873K when Mo addition exceeds 0.3mass%; at 773K substantial carbide precipitation accompanies bainite transformation and no transformation stasis is observed. Transformed fractions in the stasis stage are hardly affected by prior austenite grain size. Carbon concentrations in austenite in the stasis stage are lower than T0 line and decrease with the increase of Mo addition and temperature. T0′ limit, solute drag and WBs limit theories are used to examine the experimentally measured carbon concentration limits in austenite, and their respective flaws are pointed out.

Characterization of Transformation Stasis in Low-Carbon Steels Microalloyed with B and Mo

In the present study, bainite transformation kinetics was examined in low C-Mn steels with the addition of small amounts of B and Mo. This addition delays the onset of the bainite transformation. Mo addition causes transformation stasis at temperatures between 873 K and 823 K (600 °C and 550 °C) just below the bainite-start (B s) temperature, resulting from an incomplete bainite transformation. Post-stasis transformation after a prolonged hold proceeds by the formation of ferrite with a low dislocation density, and in Mo-containing alloys, often the formation of carbides. The volume fraction at which the transformation stops is higher for lower carbon contents and lower transformation temperatures. By contrast, at 773 K (500 °C), the bainite transformation accompanying cementite precipitation occurs regardless of microalloying and is completed after shorter hold times. EDX measurement performed on the Mo-added 0.15 pct C alloy with aberration-corrected STEM revealed that segregation at the bainite/austenite interphase boundary is small for Mn and negligible for Mo in the early stages of stasis, which does not support the incomplete transformation mechanism based on the solute drag theory for the alloys used.

A general mixed-mode model for the austenite-to-ferrite transformation kinetics in Fe–C–M alloys

Using the Gibbs energy balance (GEB) concept, a concise GEB model assuming a planar interface is proposed to study the austenite-to-ferrite transformation kinetics in Fe–C–M alloys (where M is a substitutional alloying element). Compared with the binary mixed-mode model, the newly developed GEB model is a more general mixed-mode model, and can physically describe the mixed-mode kinetics during the austenite-to-ferrite transformation in binary, ternary and even higher alloy systems. The GEB model can also predict kinetic transitions during the austenite-to-ferrite transformation, and the classical paraequilibrium and Local equilibrium are found to be its two specific cases. Furthermore, the GEB model provides a physical understanding of the effect of solute drag on the effective interface mobility. Experiments in Fe–C–Mn and Fe–C–Ni alloys indicate that the transformation stasis phenomenon can also occur during the austenite-to-ferrite transformation. The degree of incomplete transformation increases with increasing Mn or Ni concentration, while Mn has a stronger effect on the degree of incomplete transformation than Ni does due to its higher partitioning coefficient. The GEB model describes well the austenite-to-ferrite transformation stasis phenomenon.

Predicting the Effect of Mo, Ni, and Si on the Bainitic Stasis

The “transformation stasis” phenomenon during the isothermal bainitic ferrite formation in a series of Fe-C-X (X = Mo, Ni, Si ) alloys has been analyzed. Both the Gibbs energy balance (GEB) approach and the \(T_0\) model have been applied to model the “transformation stasis” phenomenon, and their predictions are compared with experimental results. The \(T_0\) model failed in predicting the transformation stasis (TS) phenomenon for the alloys investigated here, while the GEB predictions are in very good agreement with experimental data. It is found that Mo has a very strong effect on the TS phenomenon, while the effect of Si is found to be negligible. Ni has an intermediate effect.

Analysis of transformation stasis during the isothermal bainitic ferrite formation in Fe–C–Mn and Fe–C–Mn–Si alloys

The “transformation stasis” phenomenon during the isothermal bainitic ferrite formation has been investigated in a series of Fe–C–Mn and Fe–C–Mn–Si alloys. The Gibbs energy balance (GEB) approach is applied to model the transformation stasis phenomenon, and theoretical predictions are compared with experimental observations. The good agreement over several alloy systems demonstrates that the transformation stasis is caused by diffusion of alloying elements into the migrating austenite/bainitic ferrite interfaces. It is found that the occurrence of transformation stasis in the Fe–C–Mn and Fe–C–Mn–Si alloys depends on the concentration of Mn, while the addition of Si has a negligible effect on transformation stasis. The GEB model clearly outperforms the diffusionless T0 model.

Incomplete transformation of upper bainite in Nb bearing low carbon steels

Kinetics and microstructure of bainite transformation in Fe–(0·15 or 0·05)C–0·2Si–1·5Mn (mass%) alloys with Nb addition of 0·03 mass%. Bainite transformation occurs at temperatures below 873 K. At 853 K, transformation rapidly proceeds by formation of bainitic ferrite without carbide precipitation, but transformation stasis appears for a certain period in the Nb added alloys leaving untransformed austenite film between neighbouring bainitic ferrites. On the other band, the Nb free alloys do not show such a stasis until the transformation is completed. By further holding, the transformation in the Nb added alloy restarts by forming the mixture of dislocation free ferrite with cementite precipitation in the austenite films. In contrast, bainite transformation accompanying cementite precipitation occurs in both Nb free and Nb added alloys at 773 K, resulting in no difference in transformation kinetics. It is proposed that the incomplete transformation is caused by suppression of ferrite nucleation at interphase boundaries between pre-existing bainitic ferrite and austenite due to Nb segregation.

Molybdenum accumulation at ferrite: Austenite interfaces during isothermal transformation of an Fe-0.24 pct C-0.93 pct Mo alloy

A scanning transmission electron microscope (STEM) technique was used to measure Mo concentrations at ferrite:austenite (α:γ) interfaces in an Fe-0.24 pct C-0.93 pct Mo alloy partially transformed at 650°C, 630°C, and 610°C. These concentrations were quite small at 650°C, which is just below the bay temperature of the time-temperature-transformation (TTT) curve for the initiation of ferrite formation. There were larger concentrations at 630°C, a temperature at which transformation stasis (incomplete transformation) occurred. Concentrations at 610°C were intermediate between the values observed at 650°C and 630°C. The average accumulation at the latter temperatures increased appreciably as a function of transformation time. After each heat treatment, there was considerable variation in Mo accumulation from one α:γ interface to another and, to a lesser extent, from one region to another along the same interface. These higher Mo concentrations were deduced to have developed largely through volume diffusion of Mo, mainly through ferrite, to interfaces whose ledgewise growth had been interrupted by growth stasis. (Mo2C precipitation at α:γ boundaries occurred only at the end of growth stasis.) It appears that only a very small amount of Mo segregation is needed, probably at specific interfacial sites, in order to produce growth cessation. Growth kinetics anomalies of this kind continue to provide the best evidence available for the operation of a coupled-solute drag effect.

The Bainite Transformation Stage in the Processing of Trip-Aided Sheet Steels

Two fundamental aspects of austenite stabilisation are investigated. Firstly, we examine the ‘transformation stasis’ or ‘incomplete transformation’ of bainite which determines the absolute maximum carbon content of residual austenite. A new criterion is proposed stating that the growth of bainite will stop when the instantaneous driving force for bainite growth vanishes because of the plastic resistance of the matrix. Secondly, we investigate the evolution of carbon enrichment and volume fraction of residual austenite during the growth of bainite and its relation to carbide precipitation. Based on these observations, a kinetic model is proposed for simultaneous transformation into bainite and cementite which contains only two fitting parameters. Considering the low level of complexity of the model, calibration with literature data gives satisfactory results.Nous examinons plusieurs aspects fondamentaux de la stabilisation de l’austénite : 1) arrêt de la transformation bainitique qui détermine le maximum absolu de la teneur en carbone de l’austénite résiduelle. Nous proposons un nouveau critère selon lequel la croissance de la bainite s’arrête lorsque le travail plastique dans l’austénite dissipe toute la force motrice chimique pour faire avancer la transformation ; 2) évolution de la teneur en carbone et de la fraction volumique d’austenite résiduelle en lien avec la transformation bainitique et la précipitation des carbures. Sur la base de ces observations, nous proposons un modèle cinétique rendant compte de la formation simultanée de la bainite et de la cémentite qui contient seulement deux paramètres ajustables. Compte-tenu de sa simplicité, le modèle reproduit les données expérimentales de façon satisfaisante.

Bainite transformation stasis controlled by plastic work in austenite

First, it is demonstrated that the shape change accompanying bainite formation is accommodated by plastic deformation in austenite. Secondly, taking into account the corresponding plastic work, which increases due to carbon enrichment and transformation shear strain, as an entropy production mechanism, a new criterion, depending especially on initial chemical composition and grain size of austenite, is proposed for the stop of isothermal bainite before the expected equilibrium fraction expected from the phase diagram.

Alloying element accumulation at ferrite/austenite boundaries below the time–temperature–transformation diagram bay in an Fe–C–Mo Alloy

A scanning transmission electron microscope (STEM)-energy dispersive X-ray (EDX) analysis was made of the Mo concentration at boundaries between degenerate ferrite and austenite in an Fe–0.28mass%C–3mass%Mo alloy isothermally reacted below the bay temperature of the time–temperature–transformation (TTT)-diagram. The measured Mo concentration scattered widely from one boundary to another and varied considerably along the individual boundaries. The enrichment factor of Mo corrected for the sampling volume (beam diameter) to boundary thickness ratio showed an increasing tendency with isothermal holding time and reached as large as 15 when the alloy entered the transformation stasis. The amount of Mo in the boundary is compared with that calculated under the assumption of quasi-steady state diffusion that is used to analyze the solute drag effects on the motion of interphase boundaries.

Kinetics of ferrite transformation in an Fe-0.28mass%C-3mass%Mo alloy

The morphology and the kinetics of ferrite transformation below the TTT-diagram bay were studied in an Fe-0.28mass%C-3mass%Mo alloy. The three-dimensional reconstruction of the transformed microstructure revealed that the highly irregular morphology of ferrite observed on the polished specimen surface is formed due to extensive impingement and coalescence of thin rod-like crystals. The transformation stasis was observed consistently with previous observations. The fractions of ferrite achieved by fast carbon diffusion-controlled growth (followed by sluggish alloying element-partitioned growth) were calculated under paraequilibrium and local equilibrium. The calculated fractions of ferrite varied with temperature qualitatively in the same manner as those experimentally observed at the stasis. It is likely that above the bay the ferrite growth falls in the low velocity regime of Cahn's solute drag theory, whereas below the bay it may fall initially in the high velocity regime, climb up the peak of the drag and enter the stasis. In alloys of lower C and Mo content the growth stasis does not occur presumably because the carbon diffusion-controlled growth can overcome the drag peak.