Influences of crystallography and delamination on anisotropy of Charpy impact toughness in API X100 pipeline steel

The anisotropic behavior of Charpy toughness has been investigated in API X100 pipeline steel, and the combination of crystallographic textures and delamination is considered to be the reason for the observed anisotropy in Charpy toughness. The crystallographic textures can contribute to anisotropy by increasing the probability of {001} cleavage planes parallel to the fracture surfaces of the Charpy impact specimens, and biasing available slip systems for ductile fracture. Thus, the Charpy impact toughness is worst at 45° to the rolling direction, because the grains with {001} cleavage planes parallel to the Charpy specimen fracture surfaces is much more at that orientation. The existence of much more {001} cleavage planes at the 45° orientation can lead to a greater probability of cleavage fracture compared with the other orientations at the same temperature. Delamination related to elongated grain structures and {001} grain planes parallel to the rolling plane also lead to anisotropy in Charpy properties. The deep groove shape cleavage crack induced by delamination in the upper shelf region is responsible for a decrease in upper shelf energy. Delamination as a part of the fracture energy absorption process in the ductile-brittle transition region also entails additional plasticity, resulting in an increase in impact energy. Therefore, delamination can also play a role in the anisotropy of Charpy toughness.