Mohammad Ali Mohtadi Bonab

Mechanism of failure by hydrogen induced cracking (HIC) in pipeline steel has not been extensively investigated in the past. In the present work, an API X70 pipeline steel was electrochemically charged with hydrogen for different durations in order to find crack nucleation and propagation sites. After 3 h charging, suitable regions for crack initiation and propagation were found. These regions were studied by color metallography, EDS and EBSD techniques. The results brought out that HIC cracks nucleated from regions rich of manganese sulphide inclusions, some complex carbonitride precipitates such as (Ti, Nb, V) (C, N) and further propagated through the segregation area of some elements, such as manganese, carbon, silicon and sulfur. It is worth-mentioning that all these potential sites for crack nucleation and propagation appeared at the center of cross section of the specimens. EBSD measurements were carried out at the center of cross section in as-received and hydrogen-charged specimens in order to find a pattern between microstructural parameters (texture, grain boundary nature and Taylor factor) and probability of HIC cracking. The results showed that fine grain colonies (less than 3.5 mm in length) with dominant NDjj<001> orientations were prone to intergranular HIC crack propagation. The grain boundaries identified between two grains with a mismatch in Taylor factor were more susceptible to intergranular fracture while transgranular fracture occurred in fragmented grains with high and similar Taylor factor that were less likely to yield. HIC cracking occurred in a wide range of orientations such as NDjj<123>, NDjj<100>, NDjj<112>, NDjj<110> and even NDjj<111>; however, role of high angle grain boundaries and type of fracture would be of great importance in crack propagation.