Effects of successive austempering on the tribological behavior of ductile cast iron

Abstract

A 0.75 wt.% Mn ductile iron with different nodule counts was austempered by conventional and successive austempering processes at 315 and 375°C for different periods. Specimens with optimum mechanical properties were used to study the effect of austempering process on the wear behavior of austempered ductile iron (ADI). Sliding wear tests were performed in a pin-on-ring wear tester with the test materials rubbing under dry atmospheric condition against a surface hardening tool steel ring at speeds of 0.6 and 1.28 m s −1 and normal loads of 100, 200 and 300 N. To study worn surfaces of test specimens, optical and scanning electron microscopy (SEM) were used. It was found that specimens austempered at 315°C (LAT, lower austempering temperature) show the highest wear resistance while specimens austempered at 375°C (HAT, higher austempering temperature) show the lowest. Specimens austempered by successive austempering process (HLAT, high–low austempering temperature) with hardness about equivalent to HAT specimens, in addition to improvement of mechanical properties, show higher wear resistance than that of HAT specimens. Duplex structure, upper and lower bainite, along with higher carbon content of retained austenite and reduction of UAV (untransformed austenite volume) can be the main factors which improve mechanical properties and wear resistance of HLAT specimens. The results showed that specimens with longer solidification time have lower wear rate than specimens with shorter solidification time, supposedly due to effect of longer solidification time on the reduction of nodule count and heavier segregation of alloying elements. SEM study of worn surfaces suggests that delamination could be the probable wear mechanism in this investigation.