## 5 The Influence of Surface Residual Stress on Hydrogen Permeation Resistance  
### The Analysis of Hydrogen Hardening  
Figure 12 illustrates the test results of the hydrogen penetration hardness gradient with different surface residual stress specimens after hydrogen charging. It can be seen from Figure 12a that after 5 h of hydrogen charging, the hardness of both the surface and subsurface material shows a decreasing trend as the test depth increases. Additionally, the tested hardness increases as the specimens' surface residual stress increases. In particular, the hardness of the specimen with a surface residual compress stress of 120 MPa slightly  
Figure 11: The performance curve of samples with different surface roughness after being charged with hydrogen for 5 h. (**a**) Curves of tensile stress and elongation; (**b**) hydrogen embrittlement sensitivity index curve. decreases within the depth range of 0-25 \(\upmu\)m. Within the depth range of 25-75 \(\upmu\)m, the hardness significantly decreases, and the varying trend of hardness and the increasing amount trend of hardness eventually tend to be stable at a depth of 75 \(\upmu\)m to the surface. The specimens with a surface residual compress stress of 336 MPa show the most apparent decrease in the decay rate of the hardness value, which slows down at a depth of 100 \(\upmu\)m. The specimens with surface residual compress stresses of 215 MPa and 293 MPa show a stable trend of hardness decline; the hardness at a depth of 100 \(\upmu\)m is close to that of the specimen with a surface residual compress stress of 336 MPa. By comparing the measured hardness at different depths of specimens before hydrogen charging in Figure 7b with the measured results after hydrogen charging in Figure 12a, the variable quantity of hardness at different depths is shown in Figure 12b. The goal is to minimize the interference of hardening caused by shot peening on the hydrogen analysis. Based on the observations in Figure 12b, it is evident that the surface and subsurface hardness of the specimens with the surface residual compress stress values of 120 MPa and 215 MPa after hydrogen charging increase significantly compared to before hydrogen charging. Specifically, the hardness of the surface residual compress stress 120 MPa hydrogen-charged specimen increases by 17.82 HV compared to that before hydrogen charging, which increases by 8.14%; the increase in hardness is the largest among four specimens with different surface residual compress stresses. Meanwhile, the hardness of the two groups of specimens changes significantly before and after hydrogen charging within a depth range of 0-75 \(\upmu\)m from the surface, which indicates that the depth of the hydrogen-hardening layer in the specimens after hydrogen charging is approximately 75 \(\upmu\)m. The surface and subsurface hardness of specimens with surface residual compress stresses of 293 MPa and 336 MPa change slightly before and after hydrogen charging. The variation of two groups of specimens tends to be stable at a depth of 50 \(\upmu\)m from the surface. Among all the different residual compress stress specimens, the surface residual compress stress of 336 MPa only increased the specimen's surface hardness by 0.75 HV after hydrogen charging. The increase in hardness is the smallest among the four specimens with different surface residual compress stresses. The material hardness of the specimen with a surface residual stress of 293 MPa shows a noticeable increase within the depth range of 0-25 \(\upmu\)m after 5 h of hydrogen charging.