Influence of austenitising temperature on austempering of an Mn-Mo-Cu alloyed ductile iron .1. Austempering kinetics and the processing window

X-ray diffraction, optical microscopy, and hardness measurements were used to determine the austempering kinetics of an alloyed ductile iron of composition (wt-%) Fe-3.49C-2.33Si-0.42Mn-0.25Cu-0.23Mo-0.035 Mg at austempering temperatures of 300, 350, 375, and 400 degrees C and austenitising temperatures of 870 and 920 degrees C. The stage I reaction during austempering occurs in two steps, the first in the eutectic cell and the second in the intercellular area. Decreasing the austenitising temperature is shown to increase the driving force for the stage I reaction but to have a lesser effect on the stage II reaction. Decreasing the austenitising temperature produces a more uniform austempered microstructure and reduces the amount of martensite in this structure. These changes move the processing window to shorter austempering times and increase the temperature at which the processing window closes.