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.