## 1 Introduction  
Cavitation erosion is a major cause of damage to fluid components, including turbine blades, propellers, pumps, and valves [1; 2; 3; 4; 5]. During the operation of these components, low-pressure zones form on their surfaces, resulting in the creation of cavitation bubbles [6]. The collapse of these bubbles due to pressure fluctuations generates shock waves/microjets that damage the surface of the material [7; 8], significantly reducing the equipment's service life and increasing maintenance and operation costs. Strengthening the surface of the material is a feasible way to improve their cavitation erosion resistance, as surface damage is the primary mode of material damage. Researchers have conducted many studies on this topic [9; 10; 11; 12]. They found that refining the surface grain size of materials to increase surface hardness effectively dissipates the energy generated by cavitation and greatly enhances cavitation erosion resistance [13].  
[PERSON] et al. studied the cavitation erosion behavior of nickel-aluminum bronze after friction stir processing [14]. The initial grain size of the sample was around 190 \(\upmu\)m, which was reduced to about 20 \(\upmu\)m after treatment: a nearly 10-fold decrease. After cavitation erosion testing in a 3.5 wt % NaCl solution, the cavitation erosion resistance was greatly improved. [PERSON] et al. constructed a microstructured layer with an average grain size of 600 nm on the surface of austenitic stainless steel using friction stir welding [15]. After 20 h of cavitation erosion testing, the cumulative volume loss was only 16.7% of that of the original samples. [PERSON] et al. used friction stir processing to treat the surface of 532205 duplex stainless steel, reducing the grain size of the austenite and martensite phases to 1 \(\upmu\)m and 3 \(\upmu\)m, respectively, resulting in a significant improvement in mechanical properties and a 70% reduction in cavitation erosion resistance compared to the original sample [16]. Although excellent cavitation erosion resistance was achieved through the microstructure refinement of the material surface and reduction in grain size, the grain size of the material was still relatively large and did not reach the nanometer scale (less than 100 nm). As is known, materials at the nanometer scale usually exhibit excellent mechanical properties [17]. Therefore, it is necessary to investigate whether refining the surface grain size of materials to the nanometer scale further enhances their cavitation erosion resistance.  
Mechanical grinding technology has been proven to be used to prepare nanostructured layers on the surface of materials. [PERSON] et al. used surface mechanical grinding technology to prepare a layer of nanostructured layer on the surface of AISI 304 stainless steel [18]. The results showed that the nanocrystalline surface layer was mainly composed of martensitic phase, and the grain size was distributed between 8 and 60 nm. Thus, this study proposes using cutting-induced mechanical grinding to transform the austenitic phase in 304 stainless steel into martensitic phase with the aim of refining the surface microstructure to the nanometer level. In addition, the strain-hardening ability of martensite phase is low during cavitation erosion, which may be detrimental to cavitation erosion performance [19; 20]. Therefore, in this study, it was also expected to further enhance the cavitation erosion resistance of the nanostructured layer by adjusting the content of austenitic and martensitic phases in the nanostructured layer through heat treatment.