‘Arrowhead’ fractures in controlled-rolled pipeline steels

Arrowhead markings, consisting of a series of through-thickness splits in the shape of an arrowhead, have been noted on drop weight tear test (DWTT) specimens from certain controlled-rolled steels. Two steels, one showing arrowhead markings and the other not showing this phenomenon, have been investigated. In the 'as-received' condition both steels had anisotropic fracture resistance, the transition temperature for fractures running parallel to the plate surface being some 80°C above that for fractures running normal to the plate surface. Normalizing heat treatments almost removed the difference in transition temperature between the two orientations. The microstructural features considered responsible for the anisotropic fracture properties were (a) regions of coarse-grainedferrite lying parallel to the plate surface; (b) a preferred (100) [011]texture parallel to the plate surface, and (c) long thin non-metallic inclusions. It is concluded that the prerequisites for 'arrowhead' fractures are a high transition temperature for fractures parallel to the plate surface, a high yield strength, and the absence of planar weaknesses such as non-metallic inclusions, in order to allow the development of high (constrained) stresses normal to the plate surface.