Hydrogen role in strain aging of low alloy steels under operation

Strain aging of structural steels is generally considered as an important factor in their embrittlement. In the laboratory, it is initiated by preliminary plastic deformation of a metal for the generation of dislocations, followed by short-term heating. In the study, a new hypothesis about the possibility of strain ageing occurring without any initial plastic deformation at the micro-scale under the presence of hydrogen in the process that induces internal stresses in steel was proposed. It was presumed that areas with local plastic deformation induced by hydrogen would be the preferred locations for steel to undergo strain aging during subsequent heating. This assumption was substantiated through experimental tests using the low-alloy pipe steel in different states (as-delivered state, after low-temperature tempering and after preliminary electrochemical hydrogen charging followed by low-temperature tempering). The mechanical properties of the steel, including strength, plasticity, impact strength, fracture toughness, and resistance to stress corrosion cracking, were assessed. Low-temperature tempering did not affect the mechanical behaviour of the steel. However, the steel subjected to the procedure of combining preliminary hydrogen charging with subsequent low-temperature tempering was characterized by a significant decrease in fracture toughness and resistance to stress corrosion cracking. Hydrogen had an impact on the embrittlement of the steel through the strain aging at local sites being preferable for hydrogen diffusion.