Numerical analysis of hydrogen transport into a steel after shot peening

Surface peening is a potential method of suppressing hydrogen embrittlement (HE). But experimental observations in literatures have shown that it can suppress HE of steels exposed to a mild hydrogen-containing environment, but in a severe hydrogen-containing environment it enhances HE. To explain this from the perspective of hydrogen transport and concentration, this paper gives a numerical analysis of hydrogen transport into a PSB1080 high strength steel after shot peening (SP) focusing on the combined effect of residual compressive stress and SP plastic deformation on diffusion. The diffusion model established by Oriani and Sofronis et al. is used, which considers the effect of the increase of trap sites due to plastic strain and assumes that the populations of hydrogen in trap sites and in normal lattice sites are in equilibrium. The results were related to experimental observations. In mild hydrogen-containing environment, the increase of trap sites due to SP can significantly reduce the apparent hydrogen diffusivity, thus suppress the transport and resulting hydrogen concentration level in normal lattice sites. As a result, hydrogen trapped at grain boundaries is reduced, and HE mechanism changes from HEDE to simultaneous HELP and HEDE. Intergranular fracture is suppressed thus reducing HE. And the suppression of hydrogen transport increases with increasing shot velocity, as a faster shot gives rise to more severe plastic deformation. In severe hydrogen-containing environment, SP deformation has little influence on diffusivity but significantly increases trapped hydrogen, leading to irreversible hydrogen blisters hence enhancing HE.