Microstructural characterization and kinetics modelling of vermicular cast irons

Abstract

Several experimental techniques are used for phase identification and microstructure characterization of austempered vermicular cast irons (XRD, SEM, TEM and Mossbauer spectroscopy). Acicular structures were found to be composed by ferrite and austenite with average sizes compatible with those reported for bainitic ferrite in steels and Austempered Ductile Iron. An assessment of the free energy change involved in the austenite to bainite transformation assuming a plate-like nucleation shape for bainite gave an average characteristic length close to the observed from statistical length distributions. The kinetics of the transformation was modelled in the Avrami framework; both the diffusion controlled and the diffusionless growth hypothesis were considered in order to elucidate the mechanism underlying the austempering phase transformation. Results indicated that diffusion of C is the responsible of the nucleation process of the bainite sheaves, that appear as a consequence of a localized displacive transformation when the C concentration is adequate, but further growth of the bainite plates is almost suppressed.