Kinetic Transition during Ferrite Growth in Fe-C-Mn Medium Carbon Steel

The kinetics of austenite-to-ferrite diffusional transformation in a medium carbon Fe-C-Mn steel was calculated based on classical nucleation and growth theory coupled with CALPHAD multicomponent thermodynamics. The description of the growth rate of proeutectoid ferrite includes a time dependence due to the carbon enrichment in the remaining austenite. The experimentally slower kinetics, especially a stagnating behavior at the later stage, was successfully reproduced when a transition from initial paraequilibrium (PE) to local equilibrium negligible partition (LENP) conditions at the austenite: ferrite (gamma/alpha) interface was assumed. This transition is allowed when the velocity of the moving gamma/alpha interface is slow enough to be compared with Mn diffusivity, which leads to build up of a Mn spike in the interface. This assumption is consistent with a series of scanning transmission electron microscopy (STEM) analyses for Mn and C, which indicates that initial unpartitioned Mn ferrite growth is replaced by partitioned growth.