Effects of Cementite on Brittle Fracture of Fine-grained Martensitic Steel

Effects of cementite on brittle fracture of fine-grained martensitic steel were examined through fractographic and cross-sectional observation. Despite the grain size of the steel with coarse cementite being smaller than that of the steel without coarse cementite, the absorbed energy of the steel with coarse cementite decreased. In the fine-grained martensitic steels, it is considered that fracture progresses through the generation of micro-cracks, including cleavage and quasi-cleavage cracks, which are halted at grain boundaries in front of the crack tip and subsequently connected. In this fracture mode, several micro-cracks are generated before crack propagation, resulting in the reduction in the connected area of ductile fracture. Thus, the proportion of ductile fracture area was lower in the steel containing coarse cementite than in the steel without coarse cementite. In the fine-grained martensitic steels, particularly in the specimen tested at low temperature, ductile fracture is exhibited at tear ridges, which are the connected regions between micro-cracks. In this shear-type fracture, void formation due to debonding reduces the critical fracture strain, resulting in a decrease in absorbed energy per unit area of ductile fracture. Therefore, coarse cementite lowers toughness of fine-grained martensitic steel by decreasing the ductile fracture area ratio and the absorbed energy per unit area of ductile fracture.