Effects of quenching and tempering conditions of a low-alloy pressure vessel steel (A508 Gr.3) on its brittle fracture initiation and toughness

The fracture toughness and the brittle fracture initiation of a quenched/tempered A508 Gr.3 low-alloy steel were studied to reveal the effects of heat treatment conditions. The results show better fracture toughness properties (i.e. lower fracture toughness reference temperature T 0 (according to ASTM E1921 standard)) at higher quenching rate and lower tempering parameter, as expected. Three types of brittle fracture initiation mechanisms have been observed at the microscopic scale: at grain boundary carbides; on inclusions; at grain boundaries with no detectable particle. The proportion of occurrences of crack initiations cases observed at inclusions was higher when the reference temperature T 0 is lower, either with a smaller tempering parameter or a higher quenching rate. Overall, the mechanism of crack initiation on a carbide as observed in this study was predominant.