AUSTEMPERING OF LOW MANGANESE DUCTILE IRONS .2. INFLUENCE OF AUSTENITISING TEMPERATURE

X-ray diffraction, optical microscopy, and hardness measurements were used to determine the austenitising kinetics and the stage I and stage II austempering kinetics of a low Mn, Cu iron and a low Mn, Ni-Cu iron duping austempering at 300, 370, and 440 degrees C after austenitising at 900, 950, and 1000 degrees C. The study of the austenitising kinetics shows that 60 min is sufficient time to produce a constant C austenite content and that the kinetics are influenced strongly by the volume fraction of pearlite in the structure to be austenitised. 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. This shifts the heat treatment processing window to shorter times and can open a processing window that is closed for higher austenitising temperatures. Decreasing the austenitising temperature results in a more uniform austempered microstructure and reduces the amount of martensite in this structure. Decreasing the austenitising temperature has the disadvantage of reducing the austemperability.