Interaction of Phase Transformation and Diffusion in Steels

Abstract

During continuous casting low alloy steels undergo a proeutectoid γ (austenite)/α (ferrite) transformation at temperatures of about 1000 K and above. The continuous rearrangement of the iron-lattice (fcc to bcc) is accompanied by diffusion of dissolved components (C, Mn). The diffusion coefficient of carbon exceeds that of a substitutionally dissolved element by a factor of 104 to 105 at these temperatures. Therefore, Mn-diffusion does not influence the γ/α phase transformation provided that the mole fraction of Mn and the transformation temperature are low enough. In this case the γ/α transformation in the Fe-C-Mn system finally reaches paraequilibrium rather than orthoequilibrium. The time-dependent driving force of the phase transformation has been evaluated as a function of temperature and composition. Furthermore, the growth kinetics of the ferrite phase was calculated by a numerical routine.