Thermal effects due to tempering of austenite and martensite in austempered ductile irons

Typical compositions of austempered ductile cast irons in different metallurgical states were studied with the aim of making a detailed assignment of thermal effects seen using differential scanning calorimetry (DSC) to transformations occurring in the alloy phase constitution and microstructure. In austempered specimens the transformation of retained austenite is revealed by an exothermic reaction at temperatures above 450°C which depend on the content of alloying elements. Martensite transformations are seen in a wide temperature range starting at ∼100°C and giving exothermic and endothermic overlapped DSC signals due to relaxation, carbon diffusion, and tempering. It is concluded that DSC is a powerful tool for detecting tiny traces of martensite in austempered materials.