Role of Texture, Inclusions and Precipitates in Hydrogen Induced Crack Initiation and Propagation in API X۷۰ Pipeline Steels

In this research, the surface and cross section of an API ۵L X۷۰ pipeline steel was investigated by SEM observation and EDS analysis in order to determine type and morphology of inclusions. Then, the electrochemical hydrogen charging technique using ۰.۲ M sulfuric acid and ۳ g/l ammonium thiocyanate was utilized to create hydrogen induced cracks (HICs) in X۷۰ steel. After hydrogen charging experiments, the cross section of hydrogen charged steels was polished up to ۱ µm then etched with ۲% nital solution to find HIC cracks. The HIC crack initiation and propagation was studied by Energy Dispersive Spectroscopy (EDS) and Electron Backscatter Diffraction (EBSD) techniques. The results showed that there are various types of inclusions including oxide, sulfide and nitride inclusions at the cross section of as-received X۷۰ steel; however, the accumulation of inclusions was higher at the center of cross section than in other regions. But, in hydrogen charged specimens, only some special inclusions, such as sulfide and nitride type inclusions, can initiate HIC cracks. Oxide inclusions and precipices has not any role in HIC crack initiation and propagation, but they reduce the fracture toughness of X۷۰ steel. Moreover, the EBSD results showed that the crack initiates in an area with weak or random texture. Other effective parameters in crack growth, such as orientation of grains involved with HIC crack, was discussed.