the bainitic phase transformation in aluminium containing ductile irons after short time of austempering

the influence of aluminium on the microstructure of austempered ductile cast iron (adi) has been studied. anumber of irons of different compositions have been made by green sand casting and gravity die casting ofappropriate design to provide the experimental materials. austenitising was carried out at different temperaturesand holding times for a variety of experimental irons to achieve sufficient homogeneity for further isothermalheat treatment. the transformation to a bainitic microstructure during austempering under different conditionswas then examined for the most successful of the experimental casts. austenitising temperature of 920°c, andaustempering temperature of 350 °c for 1 minute have been used. microstructures have been examined by thelight and scanning electron microscopy (sem). the microstructure of austempered ductile cast iron (adi)consists of nodular graphite which is randomly dispersed in a bainitic matrix consisting of bainitic ferrite andcarbon enriched retained austenite. it has been shown that the former austenite grain boundaries are the preferredsites for bainite nucleation and the best sites for nucleation are grain corners.