The cast steel (No. 25 steel) for traction pins was used as the substrate, and the austenitic stainless steel coating was clad by laser cladding technology. The organization and structure of the cladding layer were analyzed and characterized by optical microscope, scanning and transmission electron microscope, and X-ray diffractometer. The hardness and wear performance were tested by microhardness tester and wear tester. The results show that the austenitic stainless steel cladding layer is composed of equiaxed crystals, dendrites and planar crystals, and the heat affected zone is composed of martensite and troostite. The dendrites of the cladding layer are phases, and the interdendritic regions are composed of phases and carbides Cr, C². The microhardness decreases from cladding layer-heat affected zone-substrate to 310HVo.1-280HVo.1-170HVo.1 in sequence. The No. 25 steel substrate is adhesive wear and abrasive wear, and the cladding layer is abrasive wear; after 1h of wear, the wear resistance of the cladding layer is twice that of the No. 25 steel substrate.
As an important part of the traction device of electric locomotives, the traction pin bears the friction and impact of the traction rod body during the operation of the locomotive. The traction pin of the Harmony Electric 3C electric locomotive (HXD3C) produced by a certain company is made of ZG230-450 (No. 25 steel). During the casting process, sporadic defects such as sand holes and pores are often produced near the surface, which can easily cause the traction pin to fail before reaching its service life. Therefore, it is necessary to repair the defects near the surface locally.
Laser cladding layer has excellent wear resistance, corrosion resistance, high temperature oxidation resistance, fatigue resistance and other advantages, which has led to the rapid development of laser cladding technology. Compared with traditional cladding techniques (such as arc welding and thermal spraying), laser cladding can produce better coatings, such as dense microstructure, high wear resistance, low dilution rate and metallurgical bonding with the substrate. This technology has become an important method for surface repair and modification of parts.
In this paper, 25 steel was used as the substrate, and austenitic stainless steel coating was prepared by laser cladding technology. The coating structure, microhardness and wear performance were analyzed and tested.
1 Experimental method
The experimental substrate was 25 steel, and the surface was pre-grinded with sandpaper for rust removal. Austenitic stainless steel powder was used as the laser cladding material, and its particle size range was 50~100μm. The micromorphology of austenitic stainless steel powder is shown in Figure 1. Its chemical composition (mass fraction) is as follows: C is 0.02%, Si is 0.82%, Mn is 1.68%, Mo is 0.08%, Cr is 19.34%, Ni is 9.97%, and Fe is 68.09% .
Before the laser cladding experiment, the austenitic stainless steel powder was fully dried in a drying oven at 120℃ for 1h. In order to simulate the defect characteristics of sand holes near the surface of the traction pin to the maximum extent, the surface of 25 steel was mechanically dug before laser cladding, with a hole diameter of 4 mm and a hole depth of 0.5 mm.
In this paper, YLS-6000 fiber laser was used and coaxial powder feeding method was used for laser cladding. The optimized process parameters were: negative defocus of 30mm, spot diameter of 3mm, laser power of 2600W, scanning rate of 6mm/s, and powder feeding rate of 18.9g/min in single-pass laser cladding experiment; laser power of 2500W, scanning rate of 6mm/s, overlap rate of 40%, and powder feeding rate of 18.9g/min in multi-pass single-layer laser cladding experiment.
Using Leica DMi8 A optical microscope, PANalytical Empyrean X-ray diffractometer, Zeiss Supra The organization and structure of the coating were analyzed and characterized by a 55-type field emission scanning electron microscope and a JEM2100F transmission electron microscope. The hardness of the substrate and the cladding layer was tested by a microhardness tester model HV-1000B, with a loading time of 15s and an experimental load of 100g. The test was carried out from the coating surface to the No. 25 steel substrate along the longitudinal cladding layer thickness direction, and the hardness of 3 points was tested transversely at the same thickness in the longitudinal direction, and the average value was taken.
The wear test used a UMTTriboLab friction and wear tester, the grinding ball was CCr15 material with a diameter of 5mm, the grinding ball hardness was 62HRC, the wear load was 100N, the grinding ball reciprocating distance was 6mm, and the reciprocating speed was 4 mm/s. The mass of the substrate and coating before and after wear was weighed with an electronic balance.
2 Experimental results and discussion
2.1 Microstructure analysis
Figure 2 is a scanning electron microscope image of the cladding layer (CL) structure. During the laser cladding process, the laser beam is a Gaussian heat source with uneven energy distribution. During the heating and cooling process, different heating and cooling rates will lead to different organizational morphologies in each area. According to the solidification theory, the solid-liquid interface stability (G/R) factor determines the growth morphology of the cladding layer structure, where G is the temperature gradient and R is the solidification rate. From the microstructure of the upper and bottom of the cladding layer, it can be seen that the upper part of the cladding layer is mainly composed of equiaxed crystals and dendrites, and the bottom of the cladding layer is mainly composed of a single coarse columnar crystal, and its growth direction is basically perpendicular to the fusion line. This is because the heat dissipation in the direction perpendicular to the fusion line is faster during the cladding process, and the organization of the fusion line area is composed of planar crystals.
Figure 3 shows the microstructure of the heat affected zone of a single-pass cladding sample. Since the highest temperatures experienced by each position during the cladding process are different, the organizational structure is also different, and can be divided into a quenching zone and a normalizing zone. The quenching zone is composed of lamellar martensite. During the cladding process, the substrate close to the molten pool is rapidly heated to above Acl for austenitization, and then rapidly cooled to form lamellar martensite. The organization of the normalizing zone is basically the same as that of the substrate, except that the organization of the normalizing zone is finer troostite relative to the matrix organization.
2.2 X-ray diffraction analysis
The XRD diffraction spectrum of the cladding layer is shown in Figure 4. It can be seen from Figure 4 that the austenitic stainless steel cladding layer is composed of a single austenite (γ) phase, and no other diffraction peaks appear.
2.3 TEM analysis of the cladding layer
Figure 5 shows the microstructure and selected diffraction spots of the austenitic stainless steel cladding layer. Figure 5 (a) is a scanning electron microscope photo of the microstructure of the cladding layer; Figures 5 (b) and 5 (c) are transmission electron microscope bright field images of regions b and c, respectively; Figures 5 (d) and 5 (e) show the selected electron diffraction spots of regions d (inside the dendrite) and e (between the dendrites). The structure inside the dendrite in Figure 5 (a) appears as a white matrix under a transmission electron microscope, and the corresponding diffraction spot in Figure 5 (d) is a phase with a crystal band axis of the FCC structure, which is consistent with the XRD diffraction result in Figure 4. The combined spots in Figure 5 (e) were calibrated to determine that its strip-like structure is Cr, Cz of the orthorhombic system (crystal band axis is), and the surrounding white matrix is the γ phase of the face-centered cubic system (crystal band axis is). This shows that when laser cladding austenitic stainless steel powder, the γ phase is first precipitated from the liquid phase, and then a eutectic reaction occurs, forming a eutectic structure with γ phase and chromium carbide lamellar layers. Since the above X-ray analysis is of the surface layer of the cladding layer, it is very likely that the carbide content at the surface is less than 5%, so no Cr3C2 is shown.
2.4 Microhardness distribution
Figure 6 is the microhardness curve from the cladding layer to the No. 25 steel substrate. It can be seen from Figure 6 that the hardness of the self-melting cladding layer, heat-affected zone, and substrate shows a gradually decreasing trend. It can also be seen from the figure that the temperature of the top layer is at the critical value of the powder melting point, resulting in the powder adhering to the surface of the molten pool and not being completely melted through, and the microhardness fluctuates slightly; with the increase of distance, the quality of the cladding layer tends to be stable, the maximum microhardness value is 314HV0.1, and the microhardness is stable at about 310 HV0.1.
During the laser cladding process, the maximum temperature experienced by each position of the heat-affected zone is different, and the degree of austenitization is also different. The quenching zone of the heat-affected zone close to the fusion line has a temperature higher than the liquidus during the cladding process. After rapid cooling, a high-strength martensitic structure is obtained, so that its microhardness is maintained at about 280HV0.1. With the increase of the surface distance, the proportion of martensitic structure gradually decreases. When it reaches the normalizing zone of the heat affected zone, its structure is mainly fine ferrite and troostite. As the depth increases, the cooling rate after cladding gradually slows down, and its microhardness gradually and steadily decreases to the hardness of the 25 steel substrate, that is, 170 HV0.1.
2.5 Friction and wear experimental analysis
Figure 7 shows the change curve of mass wear of the substrate and austenitic stainless steel laser cladding layer under dry friction conditions over time. Under the same wear time, the wear weight loss of the cladding layer is always less than that of the substrate, and as the wear time increases, the gap between the two is getting bigger and bigger. After 30 minutes of wear, the wear weight loss of the substrate is about 1.5 times that of the cladding layer; after 1 hour of wear, the wear weight loss of the substrate is about 2 times that of the cladding layer. This shows that after laser cladding of austenitic stainless steel coating, the wear resistance is significantly improved.
According to the wear mechanism, wear can be divided into fatigue wear, erosion wear, adhesive wear, abrasive wear, etc. Generally speaking, wear behavior is often dominated by a certain mechanism, and multiple mechanisms are carried out simultaneously. Figure 8 shows the wear morphology of the substrate and the austenitic stainless steel cladding layer after 1 hour of wear. As can be seen from Figure 8, the furrows of the substrate material after wear are deep and have a certain degree of plastic deformation. The adhesion phenomenon at the edge is more obvious, which is a typical feature of adhesive wear and abrasive wear. This is because the substrate hardness is low and peeling pits appear during the wear process. The wear marks of the austenitic stainless steel cladding layer are shallow, with a small amount of peeling marks, mainly due to abrasive wear. This is because the rapid cooling of the laser cladding makes the austenitic stainless steel cladding layer finer, which plays a role in supporting and connecting the wear-resistant reinforcement phase during the wear process. The tough phase and hard phase Cr, Cz in the cladding layer work together to improve its wear resistance.
3 Conclusions
(1) The austenitic stainless steel cladding layer is composed of equiaxed crystals, dendrites and planar crystals, and the heat-affected zone is composed of martensite and troostite. The dendrites of the cladding layer are composed of phases, and the interdendritic phases are composed of phases and carbides Cr; C2.
(2) The microhardness of the self-melting coating-heat affected zone-substrate gradually decreases to 310 HVo.1-280 HVo.1-170 HVo.10
(3) The No. 25 steel substrate is subjected to adhesive wear and abrasive wear, and the cladding layer is subjected to abrasive wear. After 1 hour of wear, the wear weight loss of the substrate is about twice that of the cladding layer, that is, the wear resistance of the austenitic stainless steel cladding layer is twice that of the No. 25 steel substrate.
