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

Fracture toughness tests were carried out on six grades of high-strength ma rtensitic 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 degreesC, (b) plane strain fracture toughness, K-IC, near the onset of crack growth, (c) fracture toughness, J(IC), near the initiat ion of slow crack growth, and (d) fracture toughness, J(iC), and crack tip opening displacement (CTODiC) at the onset of slow crack growth using direc t current potential drop (DCPD) technique. Further, true plane strain fract ure toughness, K-o, at the onset of crack initiation was determined. Fractu re toughness behavior including the measured and determined values of CVN K -IC, & J(o), J(IC), and CTODiC have been interrelated over the entire hardn ess 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 degreesC. Further, the steel exhibits a slight 300 degreesC temper embrittlement phenomenon.