The orientation dependence of Charpy toughness has been investigated in API-X80 linepipe steel. The occurrence of delamination and preferential alignment of {100} 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 {100} 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.

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There has been a huge expansion in the laying of pipelines for the transmission of fossil fuels over large distances and in dire environments. Large diameter pipes can be manufactured by welding spirals of hotrolled linepipe steels. This process has a cost advantage relative to one in which the steel is seam welded after bending into a tubular shape. However, one particular problem associated with the steels used to fabricate the pipes is that of the anisotropy of mechanical properties, especially the toughness. Even though properties such as the Charpy toughness and strength meet minimum specifications, the existence of orientation dependence can compromise, for example, the stability of the pipe to buckling. There is, therefore, a large international activity on understanding the anisotropy of pipeline steels. This review represents an attempt to critically assess the steels and the orientation dependence of their mechanical properties, with the aim of establishing a basis for further progress.