Stepped heat treatment for austempering of high manganese alloyed ductile iron

Abstract

A stepped heat treatment is proposed for overcoming the difficulty of obtaining ductility in an austempered alloyed ductile iron. The method is illustratedfor an iron containing 0·67%Mn, 0·25%Mo, and 0·25%Cu, using an austenitising temperature of 920°C, afirst step austempering temperature of 400°C for 120 min, and a second step austempering temperature of 285°C. The change in the microstructure and phase characteristics with time during the second austempering step are described. Related changes in the mechanical properties compared with a single austempering treatment at 400°C are an increase in the ultimate tensile strength from 770 to 970 MN m^?2, an increase in elongation from 2·5 to 7·5%, and an increase in the unnotched Charpy impact energy from 40 to 150 J.

MST/3119