Mohammad Ali Mohtadi Bonab

Hydrogen induced cracking (HIC) in pipeline steels industry causes a huge amount of economic loss to the industry, but despite this, HIC behavior is still not fully understood. We have studied HIC behavior in API 5L X60 and X70 pipeline steels using analysis of hydrogen permeation, hydrogen discharge using microprint technique, HIC standard test and their correlation with microstructure, type and morphology of inclusions. We recognized that mixed oxide inclusions do not seriously influence HIC in either steel. HIC tests in both steels demonstrated that the zone at the thickness center has a high concentration of manganese, silicon and sulfur and most cracks nucleate around this zone. Manganese sulphide inclusion, the most harmful type, was not present in the X60 steel specimen but was found in the center of the cross section of the X70 steel. We recognized a linear correlation between HIC susceptibility and hardness value in both steels. Furthermore, we calculated the number of reversible and irreversible hydrogen trapping sites and amount of discharged hydrogen for both steels through hydrogen permeation test. The obtained results show that the role of reversible hydrogen trapping sites on HIC susceptibility and crack growth is very important. Finally, Hydrogen microprint technique (HMT) test-data demonstrated that there was rather uniform distribution of hydrogen discharge through the grains in both steels; however, the concentration of hydrogen at grain boundaries was higher than in other areas.