Influence of Cu and Ni Alloying on the Microstructure and Mechanical Properties of Austempered Ductile Iron Castings

Abstract

Austempered ductile cast iron (ADI) offers a good combination of high tensile and fatigue strength, good ductility, toughness, wear resistance and damping characteristics, lower density in an economical way. This excellent combination of properties is due to the specific microstructure of ADI; which is composed of spheroidal graphite particles on an ausferritic matrix. The ausferrite consists of acicular ferrite and high carbon retained austenite; which is produced via austempering heat treatment after casting. The alloying additions of Cu or Cu + Ni increases austemperability, which means completely ausferritic structures can be produced on larger cross-sections. In the present study the effect of the alloying additions of Cu and Cu + Ni on mechanical properties and microstructure of ADI was studied. For that purpose, Y-block specimens having a lean composition, 0.8% Cu and 0.8% Cu + 0.4% Ni alloying additions were cast. After austempering treatment, mechanical tests, fractographic and metallographic examinations were performed. The results show that the Cu + Ni alloyed specimen has higher strength and elongation. The lean alloy on the other hand, has the highest nodularity and matrix hardness but the lowest strength and ductility. Those differences in mechanical properties were attributed to the fraction and morphology of the retained austenite regions of the matrix.