Laser cladding coatings were successfully prepared on the surface of high manganese steel using FeCrVSi and Ni+WC coating powders, and the coating microstructure, microhardness and wear resistance were studied. The results show that both coatings can improve the wear resistance and microhardness of the high manganese steel substrate, and the Fe-CrVSi coating has a better improvement on the substrate performance. The surface wear of the material with FeCrVSi and Ni+WC coatings is reduced by 9.5% and 6.3%, respectively, and the hardness is 470-550 HV and 500-630 HV, respectively, which is higher than the 250 HV of the substrate. This is mainly due to the solid solution strengthening effect of the alloy elements and the chilling effect of the laser cladding process. During the high stress load impact process, the coating provides the first layer of protection for high manganese steel, resisting abrasive damage with high hardness particles; at the same time, the surface substrate undergoes plastic deformation and strengthening, producing deformation-induced martensite and luan crystal hardening, providing a high hardening effect, and providing higher strength and hardness for high manganese steel under the effect of synergistic strengthening, improving its wear resistance.
High manganese steel has excellent wear resistance and low cost, and is widely used in mining, machinery, metallurgy, railway, power and other industries, such as crusher lining, moving cone, fixed cone, hammer head, etc. For the hardening mechanism of high manganese steel, there are deformation-induced martensitic phase transformation, twinning hardening, dislocation hardening and dynamic strain aging hardening. These theories all show that high manganese steel has low hardness and strength when subjected to lower impact loads. Only under large impacts will deformation-induced martensite and a large number of dislocations be produced to produce a hardening effect and play a good wear resistance. The hardness and wear resistance of high manganese steel are low at low impact wear, and greater material wear will occur during abrasive wear. At the same time, the yield limit of high manganese steel after impact hardening is not very high, and it is easy to break when the external force exceeds the yield limit. How to improve the wear resistance of high manganese steel has always been an important topic. Domestic and foreign scholars have conducted surface coating research on high manganese steel and alloy steel. Pi et al. prepared Fe-WC composite cladding layers with different WC contents on the surface of high manganese steel and studied the effects of different WC contents on the structure and performance of the cladding layer. Ye et al. studied the effects of cladding coating Ni/WC on the morphology, structure and wear resistance of high manganese steel under different laser powers, and investigated the effects of laser power on hardness and wear resistance. Li et al. designed a new type of multi-element low-alloy wear-resistant steel crushing liner and developed a new casting and heat treatment process to make the material hardness reach more than 50 HRC, improve the mechanical properties and wear resistance of the material, and extend the service life of the equipment.
The research on high manganese wear-resistant materials and coatings involves multiple cross-disciplinary subjects such as laser, optics, metallurgy, metal powder, welding, and materials. The difficulties lie in the design of wear-resistant steel substrates, the selection of coating materials, coating preparation, wear resistance evaluation, and mechanism analysis. Laser cladding technology is a new type of surface treatment technology. Under the action of a high-energy laser beam, the substrate and coating powder are heated and melted at the same time to obtain a metallurgical bonded cladding layer, which significantly improves the performance of the material. Laser cladding technology is widely used in aerospace, molds, automobiles, biomedicine and other fields, and has good scientific research and application prospects. In this paper, Ni+WC and FeCrVSi metal powders (coaxial powder feeding) are used to modify the surface of high manganese steel to obtain a high manganese steel cladding layer, and its effect on the microstructure, hardness and wear resistance of high manganese steel is studied. The purpose is to explore the theory of improving the wear resistance of high manganese steel and new application methods.
1 Experimental materials and methods
1. 1 Experimental materials
Nickel-based NJ-23+60%WC and FeCrVSi powders are selected as coatings, and their compositions are listed in Table 1. The substrate is cast high manganese steel ZGMn13Cr2, the substrate size is 150 mm×100 mm×10 mm, and its composition is listed in Table 2. Before the test, the substrate was polished to expose the metal substrate to improve the cladding bonding strength and avoid non-metallic defects on the surface. Finally, the surface was cleaned with ethanol.
1. 2 Preparation of cladding layer
The cladding test was carried out using a multi-mode fiber laser from IPG, Germany. The powder was fed using a coaxial powder feeding process. During the cladding process, inert gas protection and an overlap rate of 50% were used to perform multiple double-layer laser cladding. The laser cladding process parameters are listed in Table 3. Among them, the laser power has a greater impact on the coating quality. Low power input will lead to low cladding efficiency and insufficient coating bonding strength, while high power energy input will easily cause warping and cracking of the plate. Based on a large number of previous experiments, the laser power of 1 600-2 200 W was selected for the experiment.
1. 3 Characterization method
After laser cladding, the substrate was machined into a standard sample of 25 mm×75 mm×10 mm for wear resistance testing. The laser cladding substrate was processed into a 10 mm × 10 mm × 10 mm sample for microstructure and hardness testing. The required test samples were prepared using a metallographic mounting machine, and then polished with 100#, 200#, 400#, 1000# and 2000# sandpapers, and then polished with diamond abrasives with a particle size of W2.5 to prepare a mirror-like metal sample, which was then corroded in nitric acid alcohol etching solution for 10-30 s. The microstructure of the coating interface was observed using a Zeiss metallographic microscope; the Vickers hardness was tested within 2 mm from the interface between the coating and the substrate according to the GB/T 4340.1-2009 test standard, and the hardness distribution was investigated at the same time; the coating element composition line scan was conducted at the interface between the coating and the substrate based on the GB/T 17359-2012 test standard to investigate the distribution and changes of elements near the coating; the steel wear resistance test was based on the ASTM G65-16 test standard, with a load of 130 N applied to the sample, the rubber rotated at a speed of 200 r∙min‘−1, and the gravel contacted the interface at a speed of 325 g∙min‘−1. The mass loss of the sample was tested after 718 m of linear wear (rubber 1 000 r∙min‘−1), and the test method is shown in Figure 1.
2 Results and analysis
2. 1 Coating powder morphology
Figure 2 shows the morphology of atomized FeCrVSi powder and nickel-based WC powder particles. As shown in Figure 2, FeCrVSi powder particles are spherical with a particle size of about 100 μm; nickel-based WC powder particles are spherical and irregular in shape, among which nickel-based powder is spherical with a particle size of 80-100 μm and uniform in size, while WC particles are irregular in shape after mechanical crushing. Nickel-based powder and WC powder are evenly arranged by mechanical mixing.
2. 2 Microstructure of coating
The high manganese steel laser cladding coating was polished and then subjected to XRD analysis. Its composition is shown in Figure 3. As shown in Figure 3, the FeCrVSi coating prepared by laser cladding is mainly composed of Cr and V carbides, and there are also Fe oxides on the surface; while the Ni/WC coating is mainly composed of hard phases SiC, CrB and WC, and there are also cementite structures such as Fe3C. The formation of these strong carbides is conducive to the formation and dispersion of secondary particles.
Figure 4 shows the microstructure morphology of the substrate and coating after laser cladding of FeCrVSi on high manganese steel. As shown in Figure 4, the cast high manganese steel matrix is austenite with coarse grains and an average grain size of 600 μm; the coating is an iron-based high-carbon high-chromium material with high hardenability. The growth of fine regular carbides can be seen at high magnification, and the carbides diffuse into the matrix to form a diffusion zone of about 100 μm. The carbides in the diffusion zone are distributed in a dotted manner. The finely dispersed hard carbides in the coating can improve the local hardness and uniform wear resistance. At the same time, the high content of Cr can improve the hardenability and coating strength and toughness to resist dynamic abrasive wear and impact. Due to the quenching during the laser cladding process, the coating is cooled and solidified first, while the cooling rate of the heat-affected zone and the substrate is relatively slow, which promotes the growth of the coating structure into the substrate and the diffusion of components. The diffusion of elements in the coating (such as strengthening elements such as Cr, V, Si, etc.) forms a secondary precipitation phase, which greatly improves the strength and wear resistance of the substrate.
Figure 5 shows the microscopic morphology of the substrate and coating after laser cladding of Ni/WC powder on high manganese steel. As can be seen from Figure 5, the coating morphology is divided into three parts from the outside to the inside. The outer layer is a nickel-based material layer, which forms fine grains and dispersed carbides after quenching in a protective atmosphere. WC particles can improve the overall wear resistance and hardness of the coating; the middle layer has a diffusion zone of about 100 μm, which is formed by the growth and diffusion of dendrites into the substrate after rapid cooling of the coating. At the same time, the rapid diffusion of elements such as Ni and Cr also forms elemental solid solution and carbides in the diffusion zone. The good compatibility of nickel-based materials and iron substrates ensures the bonding force of the transition zone; the inner layer is a high manganese steel substrate with coarse grains, which can ensure the toughness of the core area. At the same time, when the surface coating is damaged by wear, impact plastic deformation and deformation-induced phase transformation hardening are produced in advance. There are a large number of undissolved WC particles in the coating. This is because the energy of the laser beam is mainly concentrated on the surface of the substrate during the laser cladding process. Under the action of the vortex and temperature gradient of the molten pool, many high-melting-point WC particles do not have time to melt by heat, resulting in a large number of undissolved carbides in the coating that remain in their original state. These carbides have good compatibility with nickel-based coatings and play a direct role in improving abrasive and abrasive wear. At the interface between the coating and the substrate, due to the high energy density of the laser beam, a large number of WC particles are dissolved and diffused into the substrate, and then point-like dispersion precipitation occurs during rapid cooling. These solid-solution carbides play a role in solid-solution strengthening, and the precipitated carbides can play a role in dispersion strengthening on the surface of the substrate, and the substrate is strengthened twice, which can improve the overall performance.
2.3 Microhardness of coating and substrate
Figure 6 shows the hardness distribution of FeCrVSi and Ni/WC laser cladding coatings. As shown in Figure 6 (a), the microhardness of the FeCrVSi coating is 470-550 HV, which is much higher than the 250 HV of the substrate. This is mainly due to the solid solution strengthening of alloy elements and the quenching during the laser cladding process. Because Cr and Si elements are strengthening elements in the iron matrix, they can not only improve the strength and hardness of the coating, but also improve the hardenability of the coating. The coating forms martensite during the cooling process, which improves the microhardness of the coating. The precipitation of carbides such as VC can not only disperse and improve the microhardness of the coating, but also inhibit the growth of grains, refine the grains, and improve the performance of the coating. In the heat affected zone of the FeCrVSi coating, the high temperature promotes the diffusion of alloy elements and the growth of carbides. The microhardness of this area is about 450 HV. This area can play a good role in connecting the coating and the substrate. The diffusion of elements and the growth of tissues also improve the bonding strength of the interface. As shown in Figure 6 (b), the microhardness of the Ni/WC coating is 500-630 HV, which is also higher than the microhardness of the substrate. This is mainly due to the strengthening phase WC and the microstructure of the substrate. Because there is a large amount of undissolved WC in the coating, there are also some dissolved and reprecipitated WC particles near the heat affected zone of the coating. These can improve the hardness of the coating, but not as good as the strengthening effect of the original WC. At the same time, the γ phase matrix of the nickel-based alloy also has high strength and hardness, especially the addition of strengthening elements Cr and Si, which can also form dispersed carbides during the cooling process to improve the strength and bonding of the coating; the strengthening effect in the heat affected zone is not obvious, mainly because the main elements such as W diffuse slowly in this area, and no large amount of solid solution and carbide strengthening phases are formed.
2. 4 Composition and element distribution of the coating
Figure 7 shows the element line scan and EDS analysis results of the FeCrVSi coating. As shown in Figure 7, the FeCrVSi coating has uniform element distribution and good compatibility with the substrate. The Mn content increases at the substrate 1.5 mm away from the surface. At the same time, carbides (such as VC) are found to precipitate in the coating, causing fluctuations in the element peaks. Compared with other alloy systems, the iron-based alloy has better metallurgical compatibility due to its closer composition system to the substrate. There is no obvious interface between the coating and the substrate without the addition of corrosive agents.
The coating dissolves, solidifies and cools under the action of the laser beam, forming a cladding layer, a molten layer, a heat-affected zone and a substrate area near the coating and the substrate. The small-scale fluctuations in the element composition are mainly due to uneven diffusion, carbide precipitation, and a small amount of segregation between dendrites. The strengthening of FeCrVSi surface coating mainly comes from three aspects: (1) the solid solution strengthening effect of alloy elements. Strengthening elements such as Cr and Si can greatly improve the strength and hardenability of the alloy; (2) the surface quenching after laser cladding is equivalent to a heating quenching, which makes the coating metal structure quickly cool into martensite. These high-carbon, high-alloy martensite structures can improve the hardness and wear resistance of the material surface; (3) the addition of V promotes the fine dispersion of VC. The microhardness of VC reaches 2900HV. The fine dispersed VC in the coating can continuously resist the impact wear of abrasive particles. The iron-based alloy system coating also has good wettability and strong bonding with the high-manganese steel substrate, which can effectively solve the spalling problem during laser cladding. The coating structure is a dendrite rich in C, Si, etc. and a Fe-Cr martensite structure. Among them, Si and C can improve the hardness and wear resistance of the cladding layer, and Cr can also improve the surface corrosion resistance.
Figure 8 shows the element line scan and point scan EDS analysis results of Ni-based/WC coating. It can be seen from Figure 8 that irregular WC particles are evenly distributed in the coating. When the insoluble particles of WC are scanned, the peak value of the W element becomes very high. At the same time, some WC exists in the matrix in the form of element solid solution, so that the peak of the W element is always maintained at a certain level. The element composition varies greatly from the coating surface to the core of the substrate, mainly due to the insolubility of carbides on the coating surface and the partial dissolution and precipitation of carbides inside the coating. On the one hand, the decomposition, solid solution and re-precipitation of WC particles themselves in nickel-based alloys make the element distribution of W, C, Ni and other elements different; on the other hand, the hard phases and alloys formed between the solidified dendrites by the W, C, Cr, Si and other elements dissolved and diffused in the Ni-based and Fe-based alloys also bring about changes in the element composition of the cladding layer. The nickel-based coating mainly contains Fe, Cr, and W elements for austenite strengthening. The addition of C element obtains higher hardness and dispersion-strengthened carbides. Si and B elements can be used as deoxidizers and self-fluxing agents. The microhardness of WC can reach 2 000 HV, and W2C can even reach 3 000 HV. They can be well wetted by nickel-based alloys. WC and W2C can be dissolved in nickel-based alloys at high temperatures and can be reprecipitated after cooling. The composite addition of nickel-based alloys and WC can give full play to the advantages of both parties and improve the overall performance of the coating.
2.5 Wear resistance of coating
Figure 9 shows the wear resistance of high manganese steel and coating. As can be seen from Figure 9, the wear loss of high manganese steel is 158 g. The wear loss of FeCrVSi and Ni/WC coating materials is reduced by 9.5% and 6.3% respectively compared with high manganese steel. It shows that both coatings have good wear resistance.
Figure 10 is a wear metallographic diagram of high manganese steel and coating. As can be seen from Figure 10, the wear marks of FeCrVSi coating and high manganese steel substrate are relatively flat, and the wear marks of Ni/WC coating have a large number of adhesion and plowing grooves caused by abrasive wear. The mesh skeleton structure formed after the addition of WC particles is conducive to the improvement of wear resistance; and the addition of FeCrVSi forms finer carbides, which improves the wear resistance and also improves the bonding force between carbides and substrates, so that carbides do not peel off when resisting abrasive erosion, so the wear surface is flat.
The wear resistance of surface-modified high manganese steel depends on the protection of the coating and the strengthening effect at high strain rate, that is, the pre-wear of the wear-resistant coating and the twin hardening characteristics of high manganese steel. When the coating of the high manganese steel substrate is not subjected to impact wear, plastic strain will be generated, and the plastic deformation is evenly distributed on each grain on the surface and a certain plastic deformation strengthening will be generated. At this time, the relationship between the yield strength of the material surface and the strain rate (see formula (1) in the figure), where σ SY is the static yield strength of the material, a1 and α1 are the characteristic constants of the material. When the external load increases the surface strain rate, the dynamic yield strength of the metal material increases exponentially; when the strain rate is less than 10 s’−1, the material deformation speed is slow, and the dynamic yield strength of the material will not increase too much; when the strain rate is greater than 10 s−1, the external load produces a large material strain, and the dynamic yield strength will increase sharply, resulting in a large surface hardening of the material. In addition, twin hardening and deformation-induced phase transformation hardening will also greatly affect the wear resistance of high manganese steel. High manganese steel has a low stacking fault energy, among which dislocation slip and deformation twinning determine the mechanical properties of high manganese steel. Stress of metal materials (see formula (2) in the figure) where ρ d is the dislocation density, l is the intergranular spacing, and D is the grain size. The density of prisms and dislocations are the main factors that increase the hardening of materials. When high manganese steel is subjected to impact load, as the stress propagates from the surface coating and the surface matrix to the inside, it undergoes plastic deformation from the surface to the inside, continuously generating deformed prisms and induced martensite strengthening organizations. The deformed prisms will divide the grains into very small areas, promote the continuous accumulation of dislocations at grain boundaries and prisms, and increase the surface hardening and strengthening of high manganese steel. Figure 11 is a schematic diagram of the strengthening mechanism of the material under impact load. In summary, during high stress load, the laser cladding coating provides the first layer of protection for high manganese steel, using high-hardness carbide material points to resist the damage of abrasive particles; at the same time, the surface substrate forms strong plastic deformation and strengthening, which improves the dynamic yield strength of the material; in addition, deformation-induced martensite and prism hardening provide a high hardening effect. Under the synergistic strengthening effect, it provides high manganese steel with higher strength and hardness, and better improves its wear resistance.
3 Conclusions
(1) FeCrVSi and Ni/WC coatings were successfully prepared on ZGMn13Cr2 high manganese steel substrate by laser cladding process, and the cladding layer had uniform microstructure.
(2) The microhardness of FeCrVSi coating was much higher than that of substrate, mainly due to the solid solution strengthening effect of alloy elements and the quenching effect of laser cladding process. In the heat affected zone of FeCrVSi coating, the high temperature promoted the diffusion of alloy elements and the growth of carbides. The microhardness of this area was about 450 HV. This area could well connect the coating and substrate. The diffusion of elements and the growth of microstructure also improved the interfacial bonding strength.
(3) The microhardness of Ni/WC coating was also higher than that of substrate, mainly due to the properties of strengthening phase WC and substrate microstructure. There was a large amount of undissolved WC in the coating that was not melted in time during the cladding process and remained in its original state, showing irregular shape. In the coating near the heat affected zone, some WC particles were dissolved and reprecipitated, which could also improve the hardness of the coating. At the same time, the γ phase matrix of the nickel-based alloy also has high strength and hardness, especially the addition of strengthening phases Cr, Mn, and Si, which can also form dispersed carbides during the cooling process, thereby improving the strength and bonding strength of the coating.
(4) The wear resistance of high manganese steel, FeCrVSi coating, and Ni/WC coating was analyzed. The wear loss of FeCrVSi and Ni/WC coating materials was reduced by 9.5% and 6.3% respectively compared with high manganese steel. The wear marks of FeCrVSi coating and high manganese steel substrate are relatively smooth, and the wear marks of Ni/WC coating have a large number of adhesion and plowing grooves caused by abrasive wear. FeCrVSi coating can form finer carbides, which improves the wear resistance and the bonding strength between carbides and substrates. Carbides do not peel off when resisting abrasive erosion, and the wear surface is smooth.
