Improved Hydrogen Embrittlement Resistance of Steel by Shot Peening and Subsequent Low-temperature Annealing

The effect of shot peening and subsequent low-temperature annealing (SP treatment) on hydrogen embrittlement in tempered martensitic steel was investigated comparing the typical hydrogen charging methods, constant current controlled cathodic charging test and combined cyclic corrosion test (CCT). The hydrogen entry behavior was evaluated by hydrogen permeation technique and hydrogen visualization using secondary ion mass spectrometry. The SP treatment improved hydrogen embrittlement resistance in both hydrogen charging methods. On the other hand, the effect of SP treatment on the hydrogen entry was different depending on hydrogen charging method. The hydrogen entry was suppressed by SP treatment in the cathodic charging test because the compressive residual stress reduced hydrogen concentration in the surface layer and the potential increased by the increase of surface roughness and the formation of a surface film. In CCT, although the surface hydrogen concentration decreased due to compressive residual stress, the total hydrogen content did not decrease by SP treatment since the surface film disappeared by corrosion and an increase of surface roughness led to an increase in hydrogen entry sites. The improved hydrogen embrittlement resistance by the SP treatment in the cathodic charging test is a result of hydrogen entry suppression in addition to the effect of compressive residual stress. In CCT, the hydrogen embrittlement resistance was improved by SP treatment due to effect of compressive residual stress, i.e., the suppression of stress concentration as well as the stress-induced hydrogen diffusion in stress concentration area and the reduction of hydrogen concentration in the surface layer.