## 3 Results and Discussion
Based on the XRD diffraction result, the phase composition of the samples with different heat treatment temperatures was calculated and presented in Table 1. After strain treatment, some of the austenite was transformed into martensite in the 304-Nano. With increasing temperature, the austenite content began to increase, while the martensite content decreased. It has been reported that nano-grained 304 stainless steel could be prepared by equal-channel angular pressing (ECAP), and its strength and ductility could be balanced through heat treatment [28]. The grain size of 304 stainless steel grains with a size of 50 \(\upmu\)m could be refined to 80-100 nm through strain treatment, and the grain size could still be maintained at 100-150 nm below 650 \({}^{\circ}\)C heat treatment. This was due to the fact that the grain has only undergone recovery and not recrystallization when the heat treatment is below 650 \({}^{\circ}\)C [29]. It has been suggested that the existence of martensite regions at triple junctions of reverted austenite might inhibit the further growth of austenite grains [30], which may result in ultrafine grains in reverse transformation austenite [31,32]. Thus, this study successfully produced a nanocrystalline layer on the surface of 304 stainless steel and achieved a biphasic regulation of the nanocrystalline layer through two different heat treatment temperatures. Figure 2: Grain distribution of the nanocrystalline layer on 304 stainless steel (selected area highlighted by red rectangle in Figure 1). Figure 4 shows the Vickers hardness of the 304, 304-Nano, N-500 \({}^{\circ}\)C and N-550 \({}^{\circ}\)C samples. It is clear that the Vickers hardness of the 304 sample was only 227 \(\pm\) 3 HV\({}_{0\text{-}1}\), while after the cutting process, the Vickers hardness of the 304-Nano sample reached as high as 595 \(\pm\) 18 HV\({}_{0\text{-}1}\), indicating a hardness increase of 106%. However, after the heat treatment at N-500 \({}^{\circ}\)C and N-550 \({}^{\circ}\)C, the hardness decreased to 521 \(\pm\) 15 HV\({}_{0\text{-}1}\) and 433 \(\pm\) 10 HV\({}_{0\text{-}1}\), respectively. Firstly, for the 304-Nano sample, on the one hand, the increase in hardness might be due to grain refinement, where smaller grain size led to higher hardness [33; 34]; on the other hand, during the cutting process, martensitic transformation occurred where the hardness of martensite was greater than that of austenite [35; 36]. Secondly, for the N-500 \({}^{\circ}\)C and N-550 \({}^{\circ}\)C samples, although the heat treatment process eliminated the tensile residual stress in the nanocrystalline layer, which might improve the hardness of nanocrystalline [37], the heat treatment process was accompanied by the transformation of martensite phase to austenite phase. A higher heat treatment temperature resulted in a greater decrease in hardness. The differences in hardness could affect the cavitation erosion resistance of a material. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Samples** & **Austenite (\%)** & **Martensite (\%)** \\ \hline
304 & 100 & 0 \\
304-Nano & 35.3 \(\pm\) 0.4 & 64.7 \(\pm\) 0.4 \\ N-500 \({}^{\circ}\)C & 51.0 \(\pm\) 2.8 & 49.0 \(\pm\) 2.8 \\ N-550 \({}^{\circ}\)C & 67.8 \(\pm\) 4.5 & 32.2 \(\pm\) 4.5 \\ \hline \end{tabular}
\end{table} 
Table 1: The percentage of austenite and martensite content in the 304, 304-Nano, N-500 \({}^{\circ}\)C and N-550 \({}^{\circ}\)C samples. Figure 3: XRD pattern of the 304, 304-Nano, N-500 \({}^{\circ}\)C and N-550 \({}^{\circ}\)C samples. Figure 5A shows that the 304 sample had a cumulative mass loss of approximately 23.1 mg after 15 h of cavitation erosion, while the 304-Nano sample only had a cumulative mass loss of approximately 3.4 mg. After heat treatment, the cumulative mass loss of the N-500 \({}^{\circ}\)C and N-550 \({}^{\circ}\)C samples increased significantly to approximately 5.8 mg and 8.3 mg, respectively.