Effect of shot peening coverage on hydrogen embrittlement of a ferrite-pearlite steel

Effect of a wide range of shot peening (SP) coverage on hydrogen diffusion and hydrogen embrittlement (HE) of a steel consisting of ferrite and pearlite was investigated. Conventional SP (CSP) reduces diffusivity and HE because of increase of dislocation trapping sites. Although severe SP (SSP), which uses higher coverages than CSP, increases the area of ferrite grain boundaries as a result of grain refinement, it cannot suppress further the diffusion and HE, as it cannot induce a further evident increase in strong hydrogen trapping sites such as dislocations. However, when coverage is increased to so high that causes the breaking and refinement of pearlite phase, diffusion and HE is suppressed further, because the dispersed distribution of ultrafine pearlite particles within ferrite matrix provides more strong interface trapping sites. SP-induced change of trapping sites plays a more critical role in HE and diffusion, in comparison with induced residual compressive stresses.