Role of crystallographic texture, delamination and constraint on anisotropy in fracture toughness of API X70 line pipe steels

Fracture toughness studies of line-pipe steels are extremely crucial as line-pipes are exposed to extreme conditions in application. The effects of crystallographic texture and constraint on fracture toughness of API X70 line pipe steel were investigated in this study. The J-integral method was considered to quantify the fracture toughness across orientations and thicknesses of the investigated steel. The difference in the crack initiation toughness clearly demonstrated the presence of strong anisotropy. Further the counteracting effects of thickness and delamination on constraint were found to govern fracture mechanism as well as the fracture toughness of the specimens. The anisotropy in toughness was investigated with macro-texture measurements of the specimens. The major crystallographic textures and their influences on fracture toughness were analyzed. Amongst the observed texture components, strong intensities of {113}<110>component of α-fiber was identified as the major reason for anisotropy in fracture toughness. The distribution of volume fractions of {112}<110> component also showed a strong correlation with the fracture toughness across orientations.