Failure analysis in API X70 pipeline steel in sour environment with an emphasis on fracture surfaces and crack propagation

The aim of this study is to investigate the fracture surfaces and crack propagation in an API X70 pipeline steel in acidic environment. To this purpose, as-received and electrochemically hydrogen-charged tensile specimens are subjected to tensile testing. Moreover, EBSD measurements were carried out on the cross section (RD-ND plane) in as-received and hydrogen-charged specimens. The results showed that hydrogen charging and strain rate of tensile testing were considered as two effective parameters in determining the type of fracture. Moreover, the fracture of the uncharged specimens was quite ductile and the strain rate had little effect on the fracture type. A direct correlation was found between type of fracture and the strain rate in hydrogen-charged specimens. With increasing strain rate, the fracture in hydrogen-charged specimens did not have enough time to initiate from inclusions and precipitates and connect to the main fracture surface. < 001 >||ND oriented grains have the highest hydrogen atoms due to the largest interatomic space and the distortion energy accumulated at grain boundaries due to the lack of sufficient activated slip system. The variation of KAM distribution to higher values of hydrogen-charged specimen could be effectively associated to the formation of dislocation walls and dislocation tangles especially in the vicinity of bcc grain boundaries. The higher volume fraction of the grains with high Taylor factor data formed in the hydrogen-charged specimen were highly susceptible to plastic instability, enhancing the crack propagation.