Development of a kinetic model for bainitic isothermal transformation in transformation-induced plasticity steels

In this work, we modify existing models to simulate the kinetics of bainitic transformation during the bainitic isothermal transformation (BIT) stage of a typical two-stage heat treatment - BIT is preceded by an intercritical annealing treatment - for TRIP steels. This effort is motivated by experiments performed in a conventional TRIP steel alloy (Fe-0.32C-1.42Mn-1.56Si) that suggest that thermodynamics alone are not sufficient to predict the amount of retained austenite after BIT. The model implemented in this work considers the non-homogeneous distribution of carbon - resulting from finite carbon diffusion rates - within the retained austenite during bainitic transformation. This non-homogeneous distribution is responsible for average austenite carbon enrichments beyond the so-called T0 line, the temperature at which the chemical driving force for the bainitic transformation is exhausted. In order to attain good agreement with experiments, the existence of carbon-rich austenite films adjacent to bainitic ferrite plates is posited. The presence of this austenite film is motivated by earlier experimental work published by other groups in the past decade. The model is compared with experimental results and good qualitative agreement is found.