## 3 Results and Discussion
It was observed that the pit ups and downs of the 304-Nano, N-500 \({}^{\circ}\)C, and N-550 \({}^{\circ}\)C samples were relatively smoother than those of the 304 sample. This difference in the form of sample detachment might have caused the difference. After heat treatment, the surfaces became smoother, which could be due to the significant differences in surface morphology after erosion. Nevertheless, further observation of the surface morphology after erosion is required. Figure 8 illustrates the SEM surface morphology of the 304, 304-Nano, N-500 \({}^{\circ}\)C and N-550 \({}^{\circ}\)C samples after 15 h of cavitation erosion in deionized water. As seen in Figure 8(A-1), the surface of the 304 sample was severely damaged and uneven, making the initial surface morphology unobservable. The growth of cracks around the cavitation pit, including small amounts of large cracks, was observed (Figure 8(A-2)). While for the 304-Nano, N-500 \({}^{\circ}\)C and N-550 \({}^{\circ}\)C samples, part of the original surface remained undamaged after cavitation erosion (highlighted by the red arrow in Figure 8(A-1,B-1,C-1,D-1)), and the cracks near the pit were finer but more dispersed (highlighted by the red arrow in Figure 8(A-2,B-2,C-2,D-2)). Compared with the N-500 \({}^{\circ}\)C sample, the surface cracks of the N-550 \({}^{\circ}\)C sample become even larger after cavitation erosion. In general, the more dispersed and finer the cracks, the stronger the ability to inhibit crack propagation, resulting in less mass loss. This indirectly indicates that the anti-cavitation erosion ability of the 304-Nano sample was weakened by heat treatment, and this effect became more pronounced with the increasing heat treatment temperature. From the observation of the surface morphology after cavitation erosion of the four samples, it can be inferred that the anti-cavitation erosion performance of the samples may be related to their grain size and hardness. First of all, the  
Figure 7: Surface profiles (\(-\)1) and linear profiles (\(-\)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.  
deformation-induced martensiticio nano-crystals will not undergo a recrystallization process below 650 \({}^{\circ}\)C during heat treatment [28; 29]. Therefore, it can be assumed that the grain size of the 304-Nano, N-500 \({}^{\circ}\)C and N-550 \({}^{\circ}\)C samples remain basically unchanged and all were nano-crystals. Nano-crystals have an ultra-high density of grain boundaries, and the dislocations formed during cavitation erosion will slip to the grain boundaries, causing stress concentration at the grain boundaries. Stress accumulation was more difficult with the increase in grain boundaries, which slowed down the propagation of cracks [44; 45]. This explains why the anti-cavitation erosion performances of the 304-Nano, N-500 \({}^{\circ}\)C and N-550 \({}^{\circ}\)C samples were significantly better than that of the 304 sample. Secondly, due to the transformation of martensite to austenite after heat treatment, the hardness of the N-500 \({}^{\circ}\)C and N-550 \({}^{\circ}\)C samples decrease relative to the 304-Nano sample, and the decrease was greater with increasing temperature. This led to a decrease in the ability of the N-500 \({}^{\circ}\)C and N-550 \({}^{\circ}\)C samples to resist the energy impact of cavitation erosion, which explains why the anti-cavitation erosion ability of the samples decreases after heat treatment, and this effect became more pronounced with increasing temperature. Figure 8: SEM images of the 304 (**A**), 304-Nano (**B**), N-500 \({}^{\circ}\)C (**C**) and N-550 \({}^{\circ}\)C (**D**) samples after cavitation erosion in deionized water for 15 h.