Effect of electrochemical hydrogen charging on an API X70 pipeline steel with focus on characterization of inclusions

Environmental conditions of service affect the resistance of pipeline steels to failure, as a result, the applied stress which would be otherwise considered safe can be responsible for fracture due to hydrogen induced cracking (HIC). The presence of hydrogen sulfide in natural gas is one of the main sources of hydrogen in steel. HIC testing showed that cracks initiated from inclusions and propagated through the steel matrix along the crack path that was occupied by inclusions. The number of inclusions and the distance between them as well as the grain size between them was considered to be important for predicting failure. Shorter distance between inclusions and smaller grains were found to facilitate HIC propagation. The hydrided specimens were subjected to tensile stress until failure to generate stress-strain curves, ductility and toughness data. It was revealed that even though most of the cracks initiate from rectangular and spine] inclusions, the globular inclusions have also contributed to reducing the tensile strength, ductility and toughness. Synchrotron radiation 3D imaging of steel structure was used to explain the features of the HIC cracks formation during tensile tests.