Characterization of quasi-cleavage fracture of pre-strained X80 pipeline steel subjected to SSRT tests in gas hydrogen

Most hydrogen embrittlement (HE) mechanisms involve interaction between hydrogen and dislocations. The interaction depends on dislocation density and time for hydrogen to diffuse. This study investigated the HE behaviour and determined the cleavage cracking characteristics of an X80 pipeline steel by carrying out slow strain rate tensile (SSRT) tests on specimens with different levels of pre-strain and at different strain rates in gaseous hydrogen and examination of the cracking and fracture and characteristics. Pre-straining history and a slower strain rate increased the tendency of the pipeline steel to HE. The current detailed fractographic examination of the tested specimens revealed that cracking occurred on the {112} and {110} slip planes in this pipeline steel. The terrace morphology was observed on the fracture surfaces and nearly parallel crystallographic cracks on the side surfaces. They were attributed to hydrogen promoted dislocation mobility on localized and preferred slip systems. Coalescence of these discrete cracks led to quasi-cleavage fracture of the specimens.