HIGH-TEMPERATURE DECOMPOSITION OF AUSTEMPERED MICROSTRUCTURES IN SPHEROIDAL GRAPHITE CAST-IRON

Spheroidal graphite (SG) cast iron is often plasma nitrided for corros ion resistance, and plasma nitriding has been proposed as a surface en gineering treatment to improve weal resistance. However, the microstru cture of austempered SG iron comprises constituents that may be unstab le at nitriding temperatures. Therefore, the thermal stability of aust empered SG cast iron has been studied at high temperature. Differentia l scanning calorimetry shows that microstructures obtained by austempe ring at low (300 degrees C) and intermediate (380 degrees C) temperatu res, and which contained retained austenite, underwent a large exother mic transition during heating to typical nitriding temperatures. The t ransition began at approximately 470 degrees C and peaked at 510-520 d egrees C, and was due to the decomposition of retained austenite to fe rrite and cementite. A microstructure obtained by austempering at a hi gher temperature (440 degrees C), and which consisted entirely of firs t and second stage bainite, was stable tip to nitriding temperatures. After tempering for 2 h at 570 degrees C all austempered microstructur es consisted of ferrite and cementite, but cementite was most finely d istributed in the material that had been austempered at 300 degrees C, and coarsest in that austempered at 440 degrees C. It is concluded th at if SG cast iron is to be nitrided conventionally at temperatures > 500 degrees C, then prior austempering to obtain controlled microstruc tures is of limited value. (C) 1995 The Institute of Materials.