High temperature decomposition of austempered microstructures in spheroidal graphite cast iron

Abstract

Spheroidal graphite (SG) cast iron is often plasma nitrided for corrosion resistance, and plasma nitriding has been proposed as a surface engineering treatment to improve wear resistance. However, the microstructure of austempered SG iron comprises constituents that may be unstable at nitriding temperatures. Therefore, the thermal stability of austempered SG cast iron has been studied at high temperature. Differential scanning calorimetry shows that microstructures obtained by austempering at low (300°C) and intermediate (380°C) temperatures, and which contained retained austenite, underwent a large exothermic transition during heating to typical nitriding temperatures. The transition began at approximately 470°C and peaked at 510–520°C, and was due to the decomposition of retained austenite to ferrite and cementite. A microstructure obtained by austempering at a higher temperature (440°C), and which consisted entirely offirst and second stage bainite, was stable up to nitriding temperatures. After tempering for 2 h at 570°C all austempered microstructures consisted offerrite and cementite, but cementite was most finely distributed in the material that had been austempered at 300°C, and coarsest in that austempered at 440°C. It is concluded that if SG cast iron is to be nitrided conventionally at temperatures >500°C, then prior austempering to obtain controlled microstructures is of limited value.

MST/3106