Kinetic transitions during non-partitioned ferrite growth in Fe-C-Mn alloys

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.