Fracture toughness properties of high-strength martensitic steel within a wide hardness range

摘要

Fracture toughness tests were carried out on six grades of high-strength martensitic steel within the hardness range from 270 to 475 HB. Four types of tests were performed: (a) Charpy V-notch (CVN) impact over the temperature range - 120 to 60 ℃, (b) plane strain fracture toughness, K{sub}(IC), near the onset of crack growth, (c) fracture toughness, J{sub}(IC), near the initiation of slow crack growth, and (d) fracture toughness, J{sub}(IC), and crack tip opening displacement (CTOD{sub}(iC)) at the onset of slow crack growth using direct current potential drop (DCPD) technique. Further, true plane strain fracture toughness, K0, at the onset of crack initiation was determined. Fracture toughness behavior including the measured and determined values of CVN, K{sub}(IC), K{sub}0, J{sub}(IC), J{sub}(iC) and CTOD{sub}(iC) have been interrelated over the entire hardness range using the various analytical and empirical correlations reported in the literature. The results indicate that the steel acquires the optimum fracture toughness properties at a hardness of 305 HB, corresponding to a tempering temperature of 630 ℃. Further, the steel exhibits a slight 300 ℃ temper embrittlement phenomenon.