## 5 The Influence of Surface Residual Stress on Hydrogen Permeation Resistance
### The Analysis of Hydrogen Hardening
However, the variation of material hardness becomes less significant as the depth reaches 50 \(\upmu\)m.  
Based on the phenomenon mentioned above, it can be seen that after 5 h of hydrogen charging, the depth of the hydrogen-induced hardening layer in the specimens with surface residual compress stresses of 293 MPa and 336 MPa is only 50 \(\upmu\)m. The reasons for the phenomenon are that compared with the unprocessed specimen with a surface residual compress stress of 120 MPa, the surface and subsurface material microstructures of the  
Figure 12: The effect of surface residual stress on the hydrogen penetration hardness gradient. (**a**) Hardness test after hydrogen charging; (**b**) increase in hardness at different depths. surface residual compressive stress of 215 MPa, 293 MPa, and 336 MPa are significantly refined due to the shot peening strengthening effect. The densification of the microstructure also improves, leading the hardness of the surface and subsurface of the specimens to be higher than that of the specimen without the shot peening process. The brittle hybrids would form when the specimen metal material contacts with the hydrogen medium, which improves the specimen material's hardness on the original basis. Therefore, the surface and subsurface hardness of the specimens with surface residual compress stress processed by shot peening is generally higher than that of unprocessed specimens. During the pretreatment, the evident plastic deformation and microstructure refinement of the reinforced layer would be generated on the specimen surface and subsurface by shot peening. As the depth from the specimen surface increases, the impact-strengthening effect of shot peening gradually weakens, diminishing microstructure refinement. Meanwhile, the hydrogen traps on the surface and subsurface of the specimen intercept hydrogen atoms within the material, significantly reducing the amount of hydrogen atoms permeating into deeper layers of the material, which reduces the improvement in specimen material hardness caused by hydrogen-induced hardening. Therefore, the hardness of the specimen decreases with the increasing depth from the surface under the combined influence of two effects. In the case of the specimen with a surface residual compress stress of 120 MPa, which is not processed by shot peening, the microstructure of the specimen surface and subsurface is relatively coarse. Consequently, the proportion of the hydrogen traps within the material's surface is relatively small, and the intercept effect of hydrogen atoms is relatively weak. Therefore, the depth of the hydrogen-induced hardening effect is more significant, and the increase in hardness growth after 5 h of hydrogen charging is the largest among the four groups of specimens. By comparing the microstructure of the specimen surface and subsurface after shot peening in Figure 6, it is evident that due to the larger pressure of the shot peening pretreatment, the specimens form a surface residual compress stress of 293 MPa and 336 MPa. The refinement of the surface and subsurface could be clearly observed, which creates a strong intercept effect on the permeation of hydrogen atoms in a pro-hydrogen environment. Therefore, the increase in material hardness near the surface of specimens is relatively small. Meanwhile, the refinement grains increase the grain boundary area per unit volume of the specimen material. The density of grain boundary hydrogen traps within the specimen material increases, the hydrogen permeation is inhibited, and the hydrogen concentration of the per unit grain boundary area is reduced [38], which interferes with the further permeation of penetrated hydrogen atoms into the deeper depth of the specimen. Therefore, the material hardness of the two regions of the specimens only increases significantly within the depth range of 0-50 \(\upmu\)m.