Investigation of the effect of electrolytic hydrogen charging of X70 steel: I. The effect of microstructure on hydrogen-induced cold cracking and blistering

By using electrolytic hydrogen charging, differences in hydrogen pick-up, trapping, hydrogen-induced cold cracking (HICC) and blistering were investigated for an X70 steel for a range of processing and microstructural conditions: as-rolled strip (banded ferrite-pearlite, BFP); transfer bar (ferrite-granular bainite, FGB); normalised and annealed transfer bar (equiaxed ferrite-pearlite, EFP); and a simulated grain coarsened heat affected zone (GCHAZ) (bainitic ferrite, BF). The microstructure was found to have a profound effect on the response to electrolytic hydrogen charging, with the BFP structure being the most susceptible to HICC and the development of surface blisters. In contrast, the simulated GCHAZ structure did not show any blistering for the maximum charging time of 24 h. These trends are consistent with the ratios of residual to total hydrogen content obtained for the same charging conditions (charging time; electrolyte, current density and sample geometry). The ratio decreased in the order BFP (46%), EFP (34%), FGB (33%), and BF (14%), reflecting the relative capacities of the different microstructures for strong trapping of hydrogen and the related susceptibility to HICC.