A Model of Isothermal and Non Isothermal Transformation Kinetics of Bainite in 0.5% C Steels

The present article proposes a kinetic model for the overall transformation of austenite into bainite including both nucleation and growth of the new phase. Nucleation of bainitic ferrite laths is considered in the general framework of the classical nucleation theory. Analysis of experimental data in a low alloyed 0.5% C steel suggests that austenite grain boundary diffusion of carbon is the limiting step in the formation of ferrite nuclei. Growth of bainitic laths is controlled by volume diffusion of carbon in austenite. Nucleation and growth equations are included in a Kolmogorov-Johnson-Mehl-Avrami analysis to provide a simple relationship for the bainite fraction as a function of time and temperature which contains only two semi-empirical parameters. This model is fitted to isothermal kinetics and extrapolated to continuous cooling conditions without changing the two parameters. Comparison between predictions and experimental kinetics in two different 0.5% C steels gives a very good agreement.