1. Introduction
G20Mn5QT steel has excellent strength and toughness [
1,
2,
3], so it is widely used in key components of rail vehicles [
4]. When the rail vehicle is running at high speed, due to the high-frequency impact between the wheel and rail for a long time, the key components are worn out, which poses a huge hidden danger to the operation of high-speed trains. So far, the replacement of high-speed train parts requires huge costs. In order to save costs, laser cladding (LC) repair technology is used to repair them. Previously, many surface modification techniques [
5,
6,
7,
8] have also been used to repair other materials and thus improve their service life. Examples include surface remanufacturing [
9], thermal spraying [
10], electroplating [
11], and vapor deposition [
12]. These techniques enhance the surface properties of the substrate without changing the overall properties of the substrate material. Among these surface modification techniques, laser cladding technology stands out. Laser cladding repair technology [
8,
13] has the characteristics of high energy density, low dilution rate, good metallurgical bonding with the substrate, high automation, enhanced mechanical properties, and fine quality uniformity, which is suitable for surface repair and modification of hole parts, shaft parts, and flat parts. In this investigation [
14], the mechanical properties, degree of anisotropy, and microstructure of the LC sheets produced by using commercial 316L stainless steel powder were studied. The test results revealed the elastic isotropy for the elastic modulus and Poisson’s ratio, and the plastic anisotropy for the proof stresses, ultimate stress, and elongation, but the degree of anisotropy for most of the plastic mechanical properties was less than 20%. Some scholars [
15] have used laser cladding technology to study the friction characteristics of copper-based powder metallurgy materials. The results show that the friction layer prepared by laser cladding technology has a compact structure, without obvious crack defects, and has good wear resistance. Some researchers [
16] synthesized that TiB
2 and TiC particles reinforced dr40-based composite coatings in situ on shaft parts via laser cladding. This composite coating was found to have a clean wear surface with shallower and more uniform scratches. In the past, many authors [
17,
18,
19,
20,
21] have repaired different materials using LC technology and studied their wear resistance. It was found that the wear resistance of the coating repaired by LC technology has been improved. The sensitivity of the LC process parameters to the effect of cladding quality has been investigated [
22]. It was found that the laser power was more sensitive to the effect of the melting temperature, while the powder feeding rate was more sensitive to the effect of the melt pool flow rate.
In the past [
4,
23,
24], scholars have mainly focused on the study of butt welds of G20Mn5QT cast steel and other materials. However, little research has been reported on the laser cladding repair of G20Mn5QT steel. According to the performance requirements of G20Mn5QT steel, the properties after LC require better toughness, plasticity, wear resistance, and fatigue properties than the substrate. This paper requires that the mechanical properties of the selected cladding material are similar to and better than those of the substrate, but the hardness should not be so high as to cause wear to the shaft. After the study of various powder materials, it was found that the 316L stainless steel powder is more suitable for the repair of the substrate selected in this paper. Due to its low cost [
25], good corrosion resistance, high toughness and plasticity, good formability [
26], and good weldability [
25,
27], 316L stainless steel is an austenitic stainless steel commonly used for various industries [
28,
29,
30,
31,
32] and structures in corrosive environments [
33,
34,
35]. Some scholars [
36] have studied the effect of heat treatment on the wear behavior of additively manufactured 316L stainless steel and found that the heat treatment process leads to a decrease in hardness and wear resistance. Some researchers [
37] have investigated the wear anisotropy of SLM-formed 316L stainless steel by scratch tests. The crystal structure and grain orientation were found to be critical for wear resistance. The effect of cryogenic treatment on the wear behavior of additively manufactured 316L stainless steel has also been studied [
38], and the treatment was found to have a significant effect on reducing the friction coefficient. This work takes the volumetric energy density as an energy input variable to explore the synergy mechanism of the high energy efficiency and excellent mechanical properties of 316L parts [
35]. The results showed that a lower energy density helped to improve 316L parts while ensuring fewer defects. This study provides a direction to improve the performance of the parts by optimizing the energy input parameters. Some scholars [
39] also used laser cladding repair technology to repair 304 stainless steel by powdering 316L stainless steel. The results showed that the wear performance was significantly higher than that of the substrate. An experimental study of 316L stainless steel by Zhan [
40], who chose the laser cladding repair technique, showed that thermal cycling had a significant effect on the microstructure and hardness of the cladding layer. Several researchers [
41] investigated the effect of powder composition on the organization and mechanical properties of the repaired 316L layer by the laser cladding repair technique. The results showed that there was a good metallurgical bond between the repair layer and the substrate, the organization consisted of ferrite and austenite, and the overall properties were good after repair. In past studies [
38,
39,
42], scholars mainly focused on the study of laser cladding repair of 316L stainless steel powder on materials such as 304 stainless steel, but the repair of indentation and rust of G20Mn5QT steel was rarely studied.
In this context, the repair capability of 316L stainless steel powder on G20Mn5QT cast steel was investigated in this paper using laser cladding repair technology. The microstructure and properties of G20Mn5QT cast steel and 316L coating were studied, and the sensitivity of the laser cladding process parameters to the effect of the properties was analyzed. Finally, the laser cladding repair process parameters were optimized by extreme difference analysis. The mechanical properties of the substrate and 316L coating were compared, and it was found that 316L coating could improve the service life of key vehicle components, thus improving the use of the parts and reducing the maintenance cost repair cost. Therefore, laser cladding repair technology is of great significance to the service performance and service life of repaired G20Mn5QT cast steel.
4. Conclusions
The 316L cladding was successfully prepared on G20Mn5QT steel substrate by different process parameters. In this paper, the microstructure, microhardness, impact properties, tensile properties, and frictional wear properties of the deposited specimens were investigated and the following conclusions were drawn.
The microstructure of G20Mn5QT steel is mainly composed of ferrite and pearlite. The microstructure of the 316L melt layer is austenite, which is mainly composed of planar crystal, cellular crystal, columnar crystal, and equiaxed crystal. Through the range analysis of the mechanical properties of the cladding samples, it is found that the microhardness, wear resistance, and micro-shear strength of the cladding samples increase with the increase in laser power, while the tensile strength and yield strength first increase and then decrease with the increase in laser power. When the laser power is low, due to insufficient energy, the metallurgical bond between the 316L stainless steel cladding layer and G20Mn5QT steel is weak, resulting in a lower shear strength. When the laser power is higher, although the shear strength is higher, the dilution rate does not meet the repair requirements. The optimal process parameters obtained by range analysis are as follows: laser power of 2300 W, scanning speed of 500 mm/min, and powder feeding rate of 14 g/min. Under the optimal process parameters, there are few defects such as pores and cracks in the cladding sample structure. Compared with G20Mn5QT cast steel, the mechanical properties of the cladding sample under the optimal parameters are better than those of the substrate. Among them, the micro-shear strength, tensile strength, and yield strength of the cladding specimens were increased by 17.8%, 10.91%, and 32.5%, respectively, and the elongation after tensile fracture and low-temperature impact properties reached 1.3 and 2 times that of G20Mn5QT cast steel, respectively. The comprehensive study shows that, under the optimal process parameters, the cladding specimens have high strength, plasticity, and toughness, which can meet the repair requirements of G20Mn5QT steel.
At present, there are few studies on the laser cladding of 316L on G20Mn5QT cast steel, especially about the relationship between microstructure and properties. In the future, a large number of studies can be conducted using laser cladding technology on G20Mn5QT cast steel, which is of great significance for the future maintenance cost and operation cost of high-speed trains.