## 1 Introduction  
Stainless steel has proved to be a vital material found in modern industries, e.g., energy, chemical industry, and aviation due to its excellent properties, e.g., high strength, weldability, corrosion resistance, easy processing, and surface gloss [1, 2, 3, 4, 5, 6]. There has been high demand for 304 stainless steel with high strength, hardness, precision, wear resistance and corrosion resistance. Currently, the surface modification process mainly includes ultrasonic shot peening, ultrasonic rolling, surface film strengthening and heat treatment strengthening technology (e.g., carburizing and nitriding) [7, 8]. Ultrasonic extrusion is used for surface treatment as a hybrid treating process combining conventional extrusion and ultrasonic vibration, which converts electric power into mechanical vibration at the ultrasonic frequency [9]. The chromizing treatment is a chemical surface heat treatment process in which chromium is infiltrated into the surface of metal parts [10]. In the mid-19 th century, [PERSON] et al. [11] proposed the effect of ultrasonic excitation on the elastic-plastic deformation of the metal. A group of scholars [12, 13, 14, 15] studied the metal wire drawing and extrusion technology by adding ultrasonic vibration, performed complex operations and processing of the metal under different conditions, and analyzed the results by using the finite element method, which shows that the ultrasonic vibration has advantages such as reducing the coefficient of friction and the cutting force. [PERSON] et al. [16; 17] then studied the ultrasonic impact surface strengthening of metal materials, which shows that only a tiny static extrusion pressure was required to produce plastic deformation on the metal surface when the ultrasonic impact was superimposed. [PERSON] et al. [18] focused on the chromizing of 316L stainless steel, which shows that the corrosion resistance of the stainless steel is significantly improved by chromizing. [PERSON] et al. [19] performed chromizing strengthening treatment on the surface of titanium alloy, showing that chromizing treatment significantly improves the fatigue resistance of the specimen. [PERSON] et al. [20] conducted ultrasonic impact research on 65 Mn steel after chromizing treatment, which shows that ultrasonic impact strengthens the chromizing layer and enhances the wear resistance of the stainless steel. [PERSON] [21] studied the chromizing surface modification technology of 316L stainless steel, which indicates that chromizing surface modification significantly improves the corrosion resistance of the stainless steel. Most of the above papers only studied a single surface treatment process. For example, many researchers are studying ultrasonic vibration-assisted machining as a special machining method, with its own machining characteristics and advantages. However, a single process is difficult to meet the actual machining needs at the same time. For example, in this paper, the single ultrasonic vibration-assisted machining can only meet the roughness requirements and cannot meet the hardness requirements. In addition, the traditional chromizing process has some practical problems. The high temperature of traditional chromizing treatment causes obvious deformation of low-temperature alloy workpieces, coarsening of workpiece grain structure and deterioration of working performance. Thus, to solve the problems above, a low-temperature chromizing-ultrasonic extrusion composite surface modification process (composite process) is introduced to make 304 stainless steels achieve better surface properties that combine the advantages of two processes such as high residual compressive stress, hardness and wear resistance while obtaining low surface roughness [22; 23]. The composite process can eliminate the limitations of a single process and reach multiple requirements in practice such as high hardness and low surface roughness.