Austenite-to-ferrite phase transformation in low-alloyed steels

The kinetics of the austenite(γ)-to-ferrite(α) phase transformation in iron-based alloys with low amounts of interstitial and substitutional components has been simulated. The finite mobility and the diffusion of the components determine the γ/α transformation kinetics. Numerical difficulties may occur during simulations of diffusional phase transformations in such systems due to the fact that the diffusion of substitutional and of interstitial components occur on completely different time scales. However, substitutional alloying components (e.g. Mn, Cr, Ni) can be assumed to be immobile, if their amount can be completely dissolved in ferrite and the driving force for the transformation is sufficiently high. For lower, but not too small driving forces the site fractions of the substitutional components remain constant in both phases except a thin concentration spike which occurs at the austenite side of the interface. In a new model the Gibbs energy dissipation due to the diffusional motion of this spike has been considered and the transformation kinetics in the ternary Fe-C-Mn system has been calculated. The simulated transformation kinetics are compared to results obtained by isothermal dilatometer tests on a low-alloyed Fe-C-Mn steel.