Influence of austenitising temperature on austempering kinetics of high manganese alloyed ductile cast iron

Abstract

X-ray diffraction, optical microscopy, and hardness measurements were used to determine the austenitising kinetics of an alloyed ductile iron containing 0·67%Mn, 0·25%Mo, and 0·25%Cu, during austempering at 285 and 375°C after austenitising at 870, 900, and 920°C. The austenitising kinetics show that 120 min is sufficient time to produce afully austenitic matrix. The stage I reaction during austempering occurs in two distinct steps: first in the eutectic cell and then in the intercellular areas. Decreasing the austenitising temperature is shown to increase the driving force for the stage I reaction but to have only a small effect on the stage II kinetics. Decreasing the austenitising temperature results in a more uniform austempered microstructure and reduces the amount of martensite in the structure. These changes shift the heat treatment processing window for high Mn irons to shorter timesfor austempering at 285°C and come close to, but do not open the processing window at 375°C.MST/3117