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.