FERRITE NUCLEATION AND GROWTH DURING CONTINUOUS COOLING

The austenite decomposition has been investigated in two hypoeutectoid plain carbon steels under continuous cooling conditions using a dilat ometer on a Gleeble 1500 thermomechanical simulator. The experimental results were used to verify model calculations based on a fundamental approach for the dilute ternary system, Fe-C-Mn. The austenite-to-ferr ite transformation start temperature can be predicted from a nucleatio n model for slow cooling rates and small austenite grain sizes, where ferrite nucleates at austenite grain corners. The nuclei are assumed t o have an equilibrium composition and a pillbox shape in accordance wi th minimal interfacial energy. For higher cooling rates or larger aust enite grain sizes, early growth has to be taken into account to descri be the transformation start, and nucleation is also encouraged at the remaining sites of the austenite grain boundaries. In contrast to nucl eation, growth of the ferrite is characterized by paraequilibrium; i.e ., only carbon can redistribute, whereas the diffusion of Mn is too sl ow to allow full equilibrium in the ternary system. However, Mn segreg ation to the moving ferrite-austenite interface has to be considered. The latter, in turn, exerts a solute draglike effect on the boundary m ovement. Thus, growth kinetics are controlled by carbon diffusion in a ustenite modified by interfacial segregation of Mn. Employing a phenom enological segregation model, good agreement has been achieved with th e measurements.