## 3 Results and Discussion
The results showed a significant improvement in the cavitation erosion resistance of the samples with compressive residual stress. Other studies have also demonstrated that compressive residual stress could improve the cavitation erosion resistance of the materials [13; 42]. [PERSON] et al. investigated the influence of tensile residual stress on the cavitation erosion behavior of alumina-tungsten carbide composite ceramics. The results showed that tensile residual stress significantly reduced the cavitation erosion resistance of the composite ceramics [43]. The above studies indicated that tensile residual stress decreased the resistance of the materials to cavitation erosion, while compressive residual stress enhanced the ability of the materials to resist cavitation erosion damage. In this study, the austenite peak ((200) crystal plane) of the 304-Nano sample showed a significant left shift (compared to the original 304 sample), indicating the presence of tensile residual stress [27]. After heat treatment, the austenite peak ((200) crystal plane) of the samples (N-500 \({}^{\circ}\)C and N-550 \({}^{\circ}\)C) returned to the original position (compared to the original 304 sample), indicating the release of tensile stress, which was beneficial for resisting erosion damage. However, heat treatment increased the austenite phase content, resulting in a significant decrease in the Vickers hardness of the sample. The decrease in hardness was unfavorable for resisting erosion damage; therefore, the cavitation erosion resistance of the heat-treated samples (N-500 \({}^{\circ}\)C and N-550 \({}^{\circ}\)C) was reduced. Figure 6 depicts the 3D surface morphology (\(-1\)) and pit depth distribution statistics (\(-2\)) of the 304, 304-Nano, N-500 \({}^{\circ}\)C and N-550 \({}^{\circ}\)C samples after 15 h of cavitation erosion in deionized water. Sa, the surface roughness after cavitation erosion was analyzed based on the 3D surface morphology in order from largest to smallest: 304, N-550 \({}^{\circ}\)C, N-500 \({}^{\circ}\)C, and  
Figure 5: Cumulative mass loss (**A**) and rate of mass loss (**B**) of the 304, 304-Nano, N-500 \({}^{\circ}\)C and N-550 \({}^{\circ}\)C samples exposed to cavitation erosion in deionized water for 15 h.  
304-Nano. Typically, the more damaged a sample is during the cavitation erosion process, the rougher its surface becomes after cavitation erosion. Additionally, by analyzing the pit depth distribution statistically on the surfaces of the four samples after cavitation erosion, the 304 sample had a pit depth distribution at 0-37.6 \(\upmu\)m, while the 304-Nano sample had a pit depth distribution at 0-18.9 \(\upmu\)m. After heat treatment, the pit depth distributions of the N-500 \({}^{\circ}\)C and N-550 \({}^{\circ}\)C samples were 0-21.7 \(\upmu\)m and 0-22.9 \(\upmu\)m, respectively. The results suggest that cavitation erosion pit growth in the 304-Nano sample was inhibited, and this inhibition effect was weakened significantly after heat treatment. Moreover, the inhibitory effect was more pronounced with increasing heat treatment temperature. This explains why the 304-Nano sample exhibited exceptional cavitation erosion resistance, but its performance deteriorated after heat treatment and gradually weakened with an increasing heat treatment temperature. Figure 6: Three-dimensional (3D) surfaces topography (\(-\)1) and depth distribution on the eroded surface (\(-\)2) of the 304 **(A)**, 304-Nano **(B)**, N-500 \({}^{\circ}\)C **(C)** and N-550 \({}^{\circ}\)C samples **(D)** after cavitation erosion in deionized water for 15 h.  
Figure 7 illustrates the 2D surface morphology (\(-\)1) of the samples after cavitation erosion. The deep blue to black regions represents erosion pits, and the more erosion pits there were, the more severe the surface damage. Figure 7(A-2,B-2,C-2,D-2) corresponds to the linear surface profile marked by the horizontal lines in Figure 7(A-1,B-1,C-1,D-1), with Ra values representing the ups and downs of pits on the lines.