Kinetic Modeling of Grain Boundary Cementite Evolution

Prediction of grain boundary cementite growth kinetics in hypereutectoid steels is necessary to control its thickness. It is a question of major industrial importance but has remained unresolved to date. This paper presents and compares two different and new modeling approaches. The first one relies on diffusion-based nucleation and growth of cementite precipitates using a modified SFFK model with short-circuit grain boundary diffusion and accounting for heterogeneous nucleation site energy during isothermal treatments and continuous cooling. It is compared to previously published simulations of diffusion-controlled reaction with moving phase boundaries and a similar simulation using the software Dictra. The second approach implies that cementite thickening is driven by the nucleation of ledges at the stepped austenite/cementite interface, controlled by a structure barrier to ledge formation previously assumed in the literature.([1]) We suggest a semi-empirical formulation of this barrier to ledge nucleation during isothermal transformation. Both approaches lead to an excellent match to experimental data for an almost pure Fe-C system. This implies that modeling the stepped structure of the austenite/cementite interface is not imperative for simulation of thickening kinetics, but also that understanding the governing physics of ledge formation allows for a comprehensive description of secondary cementite formation.