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. A number of irons of different compositions have been made by green sand casting and gravity die casting of appropriate design to provide the experimental materials. Austenitising was carried out at different temperatures and holding times for a variety of experimental irons to achieve sufficient homogeneity for further isothermal heat treatment. The transformation to a bainitic microstructure during austempering under different conditions was then examined for the most successful of the experimental casts. Austenitising temperature of 920°C, and austempering temperature of 350 °C for 1 minute have been used. Microstructures have been examined by the light 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 and carbon enriched retained austenite. It has been shown that the former austenite grain boundaries are the preferred sites for bainite nucleation and the best sites for nucleation are grain corners.