  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.
