Quantitative analysis of austempering kinetics of ductile iron with Cu addition

Abstract

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°C) and 673 K (400°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°C) to 623 K (350°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 γ→α transformation driving force and the dislocation density that formed near the ferrite needles during their growth.

Keywords:

• Austempered ductile iron
• Ausferrite transformation kinetics
• Austenite stabiliser
• Quantitative model

This joint research project was financially supported by the state of Lower-Saxony (Niedersachsen), Hannover, Germany, within a joint Niedersachsen–Israel research project. The authors are also grateful to Ingo Schmidt, ACO Guss GmbH, Kaiserslautern, Germany, for the casting and to Casting Laboratory, Israel Institute of Metals, Technion, Israel, for using their facility for heat treatment.