Quantitative analysis of austempering kinetics of ductile iron with Cu addition

The kinetics of austempering of two ductile iron alloys, Fe-3.5C-2.5Si and Fe-3.6C-2.7Si-0.7Cu, was investigated. The alloys were austempered for various times at 623 K (350 degrees C) and 673 K (400 degrees C), followed by water quenching. The first ferrite needles nucleate mainly at the graphite/austenite interface. Copper addition decreases the ferrite needles' dimensions, increases the autocatalysis factor and increases the austempering rate. Decreasing the austempering temperature from 673 K (400 degrees C) to 623 K (350 degrees C) results in a decrease in the ferrite needles' dimensions, an increase in the autocatalysis factor and a decrease in the austempering rate. A modified quantitative kinetic model for austempering of ductile iron, which takes into account the graphite nodules as ferrite nucleation sites, was developed. The change in the ferrite needles' dimensions is explained by the interplay of the gamma ->alpha transformation driving force and the dislocation density that formed near the ferrite needles during their growth.