## 3 Results and Discussion
In order to find the reasons for the differences in cumulative mass loss among the four samples, a clearer understanding of the mass loss during each stage is needed. Figure 5B shows the average mass loss rate curves of the four samples, from which it can be seen that the incubation period of the 304 sample was 3 h, gradually reaching the steady period through acceleration by 2 h, with a mass loss rate of approximately 2.0 mg/h during the steady period. In contrast, the 304-Nano sample had a prolonged incubation period of 6 h and an acceleration period of 7 h with a significantly lower mass loss rate during the acceleration period of the 304 sample. After entering the steady period, the mass loss rate of the 304-Nano sample was only approximately 0.4 mg/h. However, after heat treatment, the mass loss rates of the two samples treated at different temperatures exhibited different trends. The N-500 \({}^{\circ}\)C and N-550 \({}^{\circ}\)C samples both had an incubation period of 6 h, which was shorter than that of the 304-Nano sample. At this time, the difference in mass loss among the three was not significant. After entering the acceleration period, the acceleration period of the N-500 \({}^{\circ}\)C sample was maintained at 7 h, while that of the N-550 \({}^{\circ}\)C sample was only 5 h. After entering the steady period, the mass loss rates of the two samples were approximately 0.9 mg/h and 1.0 mg/h, respectively, which were much higher than that of the 304-Nano sample. The differences in the duration of different periods led to differences in the mass loss of the four samples. Furthermore, the average mass loss rates of the four samples during the acceleration period were, in order from largest to smallest: 304, N-550 \({}^{\circ}\)C, N-500 \({}^{\circ}\)C, and 304-Nano samples, respectively. This difference during the acceleration period was the main reason for the differences in cumulative mass loss among the four samples. After heat treatment, the austenite phase content in the N-500 \({}^{\circ}\)C and N-550 \({}^{\circ}\)C samples increased significantly to 51.0 \(\pm\) 2.8% and 67.8 \(\pm\) 4.5%, respectively, which was much higher than the austenite content in the 304-Nano sample (35.3 \(\pm\) 0.4%). Generally, the hardness of austenite is lower than that of martensite [35,36], which means that an increase in the austenite phase content will decrease the hardness of the sample. After heat treatment, the hardness values of the two samples were 521 \(\pm\) 15 HV\({}_{0}\)-1 (N-500 \({}^{\circ}\)C) and  
Figure 4: Vickers hardness of the 304, 304-Nano, N-500 \({}^{\circ}\)C and N-550 \({}^{\circ}\)C samples. 433 \(\pm\) 10 HV\({}_{0\text{-}1}\) (N-550 \({}^{\circ}\)C), respectively. The Vickers hardness of the N-500 \({}^{\circ}\)C was 20.3% higher than that of the N-550 \({}^{\circ}\)C, which was consistent with the trend in the change of austenite phase content. During the cavitation erosion test, the hardness of the sample has a significant effect on its resistance to cavitation erosion. Previous studies have shown that increasing the hardness of the material could significantly enhance its ability to resist cavitation damage [38; 39; 40]. In this study, the hardness of the N-500 \({}^{\circ}\)C sample (521 \(\pm\) 15 HV\({}_{0\text{-}1}\)) was much higher than that of the N-550 \({}^{\circ}\)C sample (433 \(\pm\) 10 HV\({}_{0\text{-}1}\)), which might be the reason why the N-500 \({}^{\circ}\)C sample exhibited higher resistance to cavitation erosion than the N-550 \({}^{\circ}\)C sample. Residual stress can be classified into tensile residual stress and compressive residual stress. It was reported that both compressive and tensile residual stresses had a significant effect on the cavitation erosion resistance of the materials. [PERSON] et al. applied alternating magnetic field treatment to EN8 steel and nickel-aluminum bronze alloy, which resulted in the generation of compressive residual stress in the treated samples [41].