Role of delamination and crystallography on anisotropy of Charpy toughness in API-X80 steel

Abstract

The orientation dependence of Charpy toughness has been investigated in API-X80 linepipe steel. The occurrence of delamination and preferential alignment of {1 0 0} cleavage planes are found to contribute to the observed anisotropy in Charpy properties. Delamination is also related to the presence of banding in the hot-rolled alloy, and the additional plasticity it entails during the process of fracture leads to an effective increase in toughness. As a consequence, the toughness is worst when the Charpy specimen is machined at 45° to the rolling direction because the extent of delamination at that orientation is minimal. The rolling and transformation textures also lead to a greater propensity of {1 0 0} ferrite planes parallel to the fracture surface for the 45° orientation, leading to a further decrease in toughness. Some revealing results are also reported for unconventional Charpy test orientations in which the notch is prepared parallel to the plate.