A Mechanistic Study of Carbon Steel Cracking in 360°C Air and Hydrogen Environments

Thirty percent cold-worked (CW) carbon steel tensile specimens were exposed to 360°C air and hydrogen environments (2 MPa H₂ and 20 MPa H₂) under an applied load to produce intergranular creep cracking. In this study, cutting-edge microscopy techniques were applied to characterize cracking on multiple length scales and in three dimensions. The objective was to develop a better mechanistic understanding of creep cracking in carbon steel, and the known deleterious effect of hydrogen (attack) at the micro-to-nanoscale. Amorphous carbon along the fracture path was observed in all experiments, with evidence for nanoscale cavities/methane bubbles in hydrogen exposures, particularly at cementite-ferrite boundaries. Results suggested that creep or residual stress led to breakdown of cementite to amorphous carbon, cavitation, and/or formation of methane (depending on H₂ content); it is suggested that the combination of deleterious mechanisms leads to initiation and/or acceleration of creep cracking in CW carbon steel. Comparisons are made between the morphology of creep cracking in these laboratory experiments and recent results from characterization of creep cracking in ex-service carbon steel piping from a CANDU nuclear power plant. Although more subtle, similar morphology and chemistry at crack tips in laboratory and ex-service CW carbon steel suggests that the mechanism(s) of creep cracking is similar.