AbstractThe effect of heat treatment on the microstructure and mechanical properties of a 0·16C–17Cr–2Ni (wt-%) stainless steel was studied. Ultrahigh temperature (1200°C) austenitising did not improve fracture toughness or impact toughness, owing to the presence of manyδferrite networks and some twinned martensite in the microstructure. Fine massiveδferrite, dislocated martensite laths, and interlath retained austenite were obtained by high temperature (1100°C) austenitising. This microstructure was found to have a good combined strength and toughness after an appropriate tempering process. Temper embrittlement occurred at 550°C, with the appearance of typical brittle intergranular fracture. The mechanism of temper embrittlement is considered to be the segregation of phosphorus, chromium, and nickel to prior austenite grain boundaries. The microstructures, fracture surfaces, and impact and fracture toughness values correlate well.MST/1192