INFLUENCE OF AUSTENITISING TEMPERATURE ON AUSTEMPERING KINETICS OF HIGH MANGANESE ALLOYED DUCTILE CAST-IRON

X-ray diffraction, optical microscopy, and hardness measurements were used to determine the austenitising kinetics of an alloyed ductile iro n containing 0.67%Mn, 0.25%Mo, and 0.25%Cu, during austempering at 285 and 375 degrees C after austenitising at 870, 900, and 920 degrees C. The austenitising kinetics show that 120 min is sufficient time to pr oduce a fully austenitic matrix. The stage I reaction during austemper ing occurs in two distinct steps: first in the eutectic cell and then in the intercellular areas. Decreasing the austenitising temperature i s 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 aust enitising temperature results in a more uniform austempered microstruc ture and reduces the amount of martensite in the structure. These chan ges shift the heat treatment processing window for high Mn irons to sh orter times for austempering at 285 degrees C and come close to, but d o not open the processing window at 375 degrees C. (C) 1995 The Instit ute of Materials.