Microstructural characterisation and kinetics modelling of vermicular cast irons

Abstract

Several experimental techniques are used for phase identification and microstructure characterisation of austempered vermicular cast irons (XRD, SEM, TEM and Mössbauer 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→bainite transformation assuming a plate-like nucleation shape for bainite gave an average characteristic length close to that observed from statistical length distributions. The kinetics of the transformation was modelled in the Avrami framework; both the diffusion controlled and the diffusionless growth hypotheses were considered to elucidate the mechanism underlying the austempering phase transformation. Results indicated that diffusion of C is responsible for the nucleation process of the bainite sheaves, which appear as a consequence of a localised displacive transformation when the C concentration is adequate, but further growth of the bainite plates is almost suppressed.