International audienceA self-consistent model for non-partitioning planar ferrite growth from alloyed austenite is presented. The model captures the evolution with time of interfacial contact conditions for substitutional and interstitial solutes. Substitutional element solute drag is evaluated in terms of the dissipation of free energy within the interface, and an estimate is provided for the rate of buildup of the alloying element "spike" in austenite. The transport of the alloying elements within the interface region is modeled using a discrete-jump model, while the bulk diffusion of C is treated using a standard continuum treatment. The model is validated against ferrite precipitation and decarburization kinetics in the Fe-Ni-C, Fe-Mn-C, and Fe-Mo-C systems

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International audienceThe kinetics of ferrite growth in the Fe-C-Co and Fe-C-Si systems has been quantified using controlled decarburization experiments. The Fe-C-Co system is a particularly interesting system since a large range of Co contents can be considered providing a suitable data set for examination of the composition dependence of the solute drag effect. Six Fe-C-Co alloys containing Co from 0.5 to 20 pct have been considered. Three Fe-C-Si alloys have also been considered and each has been transformed at three temperatures proving a suitable data set for examining the temperature dependence of the solute drag effect. This data set, along with ferrite growth data from decarburization experiments on an Fe-C-2Cr alloy has been used to test the ferrite growth model proposed in the companion article by Zurob et al. It is shown that this model for ferrite growth, that includes diffusional dissipation due to interaction between the solute and the migrating boundary, quantitatively captures both the temperature and composition dependence of the deviation of experimental ferrite growth kinetics from the PE and/or LENP models

International audienceThe segregation of solutes to austenite/ferrite transformation interfaces during decarburization/denitriding of Fe-Mn-C, Fe-Mn-N, and Fe-Si-C ternary alloys was studied by using atom probe tomography. Manganese was found to segregate noticeably to the transformation interface in the presence of carbon, while no segregation could be detected in the presence of nitrogen. This result might indicate that manganese interacts little with the interface itself and that its interaction with the interstitial controls its segregation behavior. In the case of Fe-Si-C, the experiments were complicated by interface motion during quenching. Preliminary results suggest that silicon was depleted at the interface in contrast to the commonly observed segregation behavior of silicon at grain boundaries of ferrite and austenite. This observation could be explained by taking into account the repulsive interaction between silicon and carbon along with the intense segregation of carbon to the interface. This would lead to a net repulsive interaction of silicon with the interface even when considering the intrinsic tendency of silicon to segregate to the boundary in the absence of carbon. The results presented here emphasize the need to account for the interaction of all solutes present at the interface in ferrite growth models

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Recent years have seen an increasing emphasis on the identification of transitions in growth modes during the diffusional decomposition of austenite to ferrite in Fe-C-X alloys. Of particular interest are transitions between the extremes of non-partitioned growth represented by the ParaEquilibrium (PE) and local equilibrium negligible partition limits. Identification of such transitions requires high-quality measurements of ferrite growth kinetics, and recent uses of the decarburization approach have allowed access to very high-precision growth kinetic measurements. However, one of the limitations of the decarburization approach is that the lower limit of its applicability is the eutectoid temperature and this has so far compromised its usefulness to probe the temperature dependence of kinetic transitions. In this contribution, analogous denitriding studies have been performed that allow access to much lower temperatures than are possible using decarburization. It is shown that the kinetics of ferrite growth in the Fe-N-Mn system become closer to the PE limit as the temperature is lowered. The growth kinetic data are interpreted quantitatively in the framework of the Zurob et al. model that includes diffusional dissipation due to Mn diffusion across the migrating interface. Furthermore, comparisons are made between decarburization and denitriding in Fe-Mn alloys of the same Mn content at the same temperature. The interface velocities are much faster under denitriding conditions, and this allows inferences to be made about the effect of the interface velocity on dissipation and contact conditions. It is also suggested that the binding energy of Mn to the migrating interface may be slightly higher in the Fe-C-Mn system than in the Fe-N-Mn system, and it is speculated that this is due to the strong segregation of C to the interface and the associated co-segregation of Mn.

Ferrite growth behavior in Fe-C-Mn alloys has been studied using controlled decarburization experiments. Two types of kinetic transition are considered. A first transition is proposed which involves a change from ParaEquibrium (PE) contact conditions at short times to Local-Equilibrium with Negligible Partitioning at longer times (LENP). This transition is attributed to the gradual build up of an alloying element spike due to the diffusion of Mn across the interface. The cross-interface mobility of Mn is estimated based on the experimental results. In some alloys, we observe a transition to extended PE states at high temperatures. A simple model which quantitatively describes the experimental observations over a range of composition and temperature is proposed. A key feature of this model is the introduction of an alloying element capacity of the moving ferrite/austenite interface, X*. The introduction of this quantity is purely guided by the experimental data and, at present, there is no physically based method for calculating it.

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