Influence of Copper Addition and Temperature on the Kinetics of Austempering in Ductile Iron

Austempered ductile iron (ADI) is a material that exhibits excellent mechanical properties because of its special microstructure, combining ferrite and austenite supersaturated with carbon. Two ADI alloys, Fe-3.5 pct C-2.5 pct Si and Fe-3.6 pct C-2.7 pct Si-0.7 pct Cu, austempered for various times at 623 K (350 °C) and 673 K (400 °C) followed by water quenching, were investigated. The first ferrite needles nucleate mainly at the graphite/austenite interface. The austenite and ferrite weight fractions increase with the austempering time until stabilization is reached. The increase in the lattice parameter of the austenite during austempering corresponds to an increase of carbon content in the austenite. The increase in the ferrite weight fraction is associated with a decrease in microhardness. As the austempering temperature increases, the ferrite weight fraction decreases, the high carbon austenite weight fraction increases, but the carbon content in the latter decreases. Copper addition increases the high carbon austenite weight fraction. The results are discussed based on the phases composing the Fe-2Si-C system.