Lengthening Kinetics of Pro-Eutectoid Ferrite Ledge in Fe-C Alloy under Interface-Diffusion Mixed Control Mode

Abstract

A mixed control mode is developed to model the ledge growth of pro-eutectoid ferrite, considering coupled effects of migration of austenite/ferrite interface and carbon diffusion in austenite. Carbon concentration of austenite at the austenite/ferrite interface increases from the bulk carbon concentration to a steady level, which is lower than that in local equilibrium, during the ferrite growth process. Correspondingly, ferrite grows rapidly at the beginning since all the driving force of ferrite transformation is dissipated on the interface migration. In the later stage of isothermal transformation, the growth rate of ferrite decreases towards a steady level since a part of driving force is dissipated on carbon diffusion in austenite. The effect of interface migration on ferrite growth rate by changing the interface mobility is emphatically discussed. In the case of the low interface mobility, the growth rate of ferrite is very small while the growth is dominated by the carbon diffusion ability in the case of large interface mobility. When a medium interface mobility is obtained, the growth rate of ferrite may reach a maximum value, which exceed the limitation of diffusion control and interface control modes. After comparing the modeled growth rate of ferrite with the experimental data of 0.11-0.49 wt% C alloy at 973-1113 K, the pre-expontential factor (M0) of interface mobility is estimated within the range of 0.1-1 mol m J-1 s-1, around the value 0.5 mol m J-1 s-1 theoretically estimated.