Investigation of the effect of electrolytic hydrogen charging of X70 steel: II. Microstructural and crystallographic analyses of the formation of hydrogen induced cracks and blisters

Hydrogen-induced cold cracking and blistering in hydrogen-charged X70 steel was found to be highly dependent on microstructure, with the banded ferrite-pearlite microstructure of hot rolled strip showing a higher susceptibility than other microstructures produced by different thermal-mechanical routes. Although crack initiation was particularly sensitive to microstructure, crack growth occurred largely parallel to the rolling plane, at least at a macroscopic level, for all of the microstructures investigated. The crack plane was associated with structural anisotropy arising from processing by rolling and was not found to be related to a preferred grain orientation. At a microstructural level, crack propagation was mostly transgranular and occurred dominantly along slip planes of the ferrite grains. Cracks were initiated at strong traps in the microstructure when the hydrogen and local stress concentrations reached critical levels for hydrogen-induced fracture. The main initiation sites were coarse inclusions, mainly oxides, and ferrite-pearlite interfaces.