SIMULATION OF LEDGEWISE GROWTH KINETICS OF PROEUTECTIOD FERRITE UNDER INTERFACIAL REACTION-DIFFUSION MIXED CONTROL MODEL

A mixed control model considering both carbon diffusion and interfacial reaction is established for ledgewise growth kinetics of proeutectiod ferrite during gamma -> alpha isothermal transformation in low carbon steels. In this model. the growth rate of ledgewise ferrite and the carbon concentration at the ledge riser are determined by both carbon diffusion at ledge riser in austenite and interfacial reaction rate of gamma -> alpha transformation, which is different from the traditional local equilibrium(LE) model. Simulation is done by utlizing tins model to analyze the ledgewise growth kinetics of proeutectiod ferrite. And based on analysis of the characteristics of growth kinetics of proeutectiod ferrite obtained by the simulation, a factor S. which includes the effects of carbon diffusion, interface migration, temperature and composition, is proposed to characterize the control mode of growth kinetics. By judging tins factor S. we can predict that if the carbon diffusion or interfacial reaction dominates the growth kinetics of proeutectiod ferrite. In the case that the value S is relatively small, the growth kinetics is mainly controlled by interfacial reaction. In the case that the value of S is quite large, the growth kinetics is controlled by carbon diffusion, which returns to the traditional local equilibrium model. The mixed control simulation results for ledgewise growth of proeutectoid ferrite at 720 degrees C in Fe-1%C (atomic fraction) alloy show a good agreement with the experimental results previously reported.