Effect of austenitising conditions on the impact properties of an alloyed austempered ductile iron of initially pearlitic matrix structure

This study sets out to investigate the effect of austenitising conditions on the microstructure and impact properties of an austempered ductile iron containing Copper and Nickel and having an initially fully pearlitic structure. Un-notched Izod impact test specimens were solution treated in the range 850–1000 °C for durations between 15 and 360 min. and then austempered at 360 °C for 180 min.

It was shown that increasing the austenitising temperature increases the amount of carbon taken in solution by the original austenite. This reduces the driving force controlling the transformation of the austenite to the ausferrite product, ferrite and austenite. As a result, the retained austenite volume in the final microstructure increases but simultaneously its stability falls. This places an upper limit on the austenitising temperature and the amount of retained austenite permissible. On the other hand, for optimum properties, the austenitising temperature and time must be high and long enough respectively to ensure complete austenitisation.

It was also shown that generally, in irons with an initially pearlitic structure, the impact properties increase steadily to a maximum value as the austenitising time increases to about 180 min. and remain constant as the soaking period extends further. Optimum properties are obtained following austenitising between 850 and 900 °C for durations of 120 to 180 min. and correspond to heat treatment cycles which saturate the initial austenite with carbon.