Microstructural Effects on the Toughness of a High Co-Ni Steel

Purpose of this work is to investigate the effects of reverted and retained austenite on the fracture toughness of high Co–Ni steels. Different cryogenic and tempering treatments are applied to a high Co–Ni steel in order to alter the austenite content, its chemical composition and hence its stability against transformation to martensite. Austenite stability is analyzed by applying thermal cycles in the dilatometerand by X-ray diffraction. For each heat treatment, the fracture toughness is characterized by J-integral and fractographic crack growth resistance measurements. The results indicate that cryogenic and two-step tempering treatments lead to a higher austenite stability and impede austenite transformation during cooling after tempering. Consequently, the toughness is considerably improved.