Investigation of the Effect of Novel Two-Step Austenitization Process on the Microstructure and Mechanical Properties of ADI

A novel two step austenitization heat treatment process was conceived to develop ADI with optimum combination of strength, ductility and fracture toughness. This novel heat treatment process involved austenitizing the ductile iron in the lower intercritical temperature range and then raising the temperature to the fully austenitic temperature range followed by austempering in the bainitic temperature range. The resulting microstructure consisted of very fine scale bainitic ferrite, high-carbon austenite and pro-eutectoid ferrite. The proeutectoid ferrite is nucleated because of prior austenitization in the intercritical range which resulted in ADI with high fracture toughness without significantly compromising the strength and ductility. An analytical theory has been developed based upon the nucleation of proeutectoid ferrite and graphite nodules during intercritical austenitization, to explain this physical outcome.