High-speed steel has high hardness and wear resistance because it contains alloying elements such as high carbon (C) and high tungsten (W). It has important application prospects in the field of laser cladding of surface strengthening layers of high-wear-resistant parts. In this paper, W10CrMoMn high-speed steel alloy powder was prepared by vacuum induction melting gas atomization method (VIGA), and the performance indicators of alloy powder, cladding structure and hardness were studied. The results show that the sphericity of alloy powder reaches 98%, the particle size distribution is 1-180μm, and the structure of powder is mainly α-Fe, γ-Fe and M23 (C, B) 6 type boron carbide. The laser cladding W10CrMoMn alloy layer forms a good metallurgical bond with the substrate. The alloy layer structure is mainly composed of lath martensite (TM), luanite martensite (LM), austenite (A), M7C3 and M23 (C, B) 6 type carbides; the surface hardness of the cladding layer reaches 800-1100 HV. This study provides a useful reference for VIGA preparation of W10CrMoMn alloy powder and laser cladding high hardness strengthening layer on the surface of parts.
Keywords: brake disc; laser cladding; high-speed steel powder; hardness
High-speed rail brake disc is one of the important components to ensure the safety of high-speed rail operation. The traditional brake disc manufacturing process is casting and surface heat treatment technology, which has problems such as long process and need to improve performance. In recent years, laser additive manufacturing of alloy steel brake discs has become a hot topic, and several important progresses have been made in the preparation of 24CrNiMo alloy steel powder, forming microstructure evolution and performance regulation, and parts preparation. The high-speed rail brake surface should have high hardness, while the core needs higher toughness to resist torque and prevent fracture. This requires that the strength and toughness matching should be carried out when designing the brake disc material. The surface hardness reaches 700~900HV; the core hardness reaches 350~450HV, the tensile strength is 1050MPa, and the elongation is greater than 12%. However, the research of domestic and foreign counterparts has been mainly focused on the laser forming of 24CrNiMoY low alloy steel. Due to its hardness of 350~400HV, it cannot meet the requirement of 700~900HV for the surface hardness of the brake disc. Therefore, the research on new alloy powders with high hardness and good laser formability and preparation methods has become a top priority in scientific research.
High-speed steel is a tool steel with high hardness, high wear resistance and high heat resistance. Due to its high C and high W, Cr, Mo and other alloy composition characteristics, it is often used for laser cladding of high hardness strengthening layer materials on the surface of parts. However, due to the high sensitivity of laser forming cracks of high-speed steel alloys, further research is needed to find new high-speed steel alloy powders with fewer crack defects, good formability and high hardness to meet application needs. Therefore, this paper aims to prepare a new type of high-hardness W10CrMoMn high-speed steel alloy powder for laser cladding, and uses vacuum induction melting gas atomization technology (VIGA) to study the preparation of spherical W10CrMoMn alloy powder; the research characterizes the technical indicators such as sphericity, fluidity and bulk density of the alloy powder, and studies the formability, microstructure and hardness changes of the laser cladding alloy sample. It is expected that the new type of high-hardness W10CrMoMn alloy powder and preparation technology for laser cladding can be successfully obtained, which will provide a useful reference for the high-hardness and wear-resistant working layer on the surface of high-speed railway brake disc parts manufactured by laser additive manufacturing.
1 Research Methods
1.1 Preparation Method of Alloy Powder
The powder used in this study is prepared by vacuum induction melting gas atomization (VIGA). First, the designed alloy steel is melted and processed into an alloy ingot with a diameter of 106mm and a height of 230mm to match the size of the crucible used for melting. The polished and cleaned alloy ingot is placed in the melting chamber of the equipment. When the vacuum of the equipment reaches 2.0×10-1Pa or below, argon gas with a purity of ≥99.99% is filled as a protective gas, and the voltage can be increased and melting can be carried out. When the temperature of the molten alloy in the crucible reaches 1600~1650℃, atomization can be carried out, and the atomization argon gas pressure is 10~12MPa. After atomization, the powder in the collection box is cooled and taken out, and W10CrMoMn high-speed steel alloy powder is obtained through an 80-mesh sieve.
1.2 Laser cladding process method
A 100mm×200mm×10mm Q235 steel plate is used as the substrate, and the oxide scale on the surface of the substrate is removed and the oil is removed with acetone, and finally it is cleaned and dried with an alcohol solution. The cladding sample is prepared by a FL-Dlight02-3000W semiconductor laser, and the optimized parameters are: power 2300W, scanning speed 5mm/s, overlap rate 40%, and laser energy density 230J/mm3. The sample preparation process includes: first, 0.5mm thick alloy steel powder is placed on the pretreated substrate and placed on a horizontal X-O-Y surface workbench, and then layer by layer is clad at the selected position to obtain a sample with a cladding layer thickness of 2~3mm.
1.3 Characterization and analysis methods
The bulk density and fluidity of the powder were measured using a HYL-102 Hall flowmeter.
The chemical composition of the powder was analyzed using a TCH-600 nitrogen and oxygen analyzer and an AGILENT-7700 inductively coupled plasma mass spectrometer.
The surface morphology of the powder was analyzed using a field emission scanning electron microscope (FSEM, Shimadzu SSX-550).
The metallographic structure of the powder cross section was observed using a laser confocal microscope (OLYMPUS-OLS31001).
The particle size distribution of the powder was characterized using a LA-920 laser particle size analyzer.
The metallographic structure of the alloy steel sample was observed using a laser confocal microscope (OLYMPUS-OLS31001) and an inverted optical microscope (OLYMPUS-GX71).
The samples were analyzed by a Rigaku X-ray analyzer (SmartLab-9000). Cu target Kα ray was selected in the experiment, the tube voltage was 40kV, the tube current was 200mA, the scanning speed was 3°/min, and the diffraction angle range was 20°~100°.
The WILSON-WOLPER-401MVD micro-Vickers hardness tester was used to test the hardness of the samples. The applied load used in the test was 1.96kN and the load loading time was 10s. The hardness test direction was from the surface of the sample section to the substrate, and the test point spacing was 0.5mm. In order to reduce the test error, each sample was tested 3 times and the average value was calculated.
2 Experimental results and analysis
2.1 Preparation of W10CrMoMn alloy steel powder by VIGA
The chemical composition of the W10CrMoMn alloy powder prepared by VIGA is shown in Table 1. As shown in Table 1, the C content of the alloy steel powder is 1wt%, and the main alloying elements are W, Cr, Mo, Mn, Ni, Si, V, B, Y, Fe, etc. The scanning photos and particle size distribution of W10CrMoMn alloy powder prepared by VIGA are shown in Figure 1. As shown in Figure 1 (a) and (b), most of the powder is in a good spherical shape, with a sphericity of 98%, but there are still a small number of irregular particles and satellite particles. Figure 1 (c) is the particle size distribution of the powder, with a size of 1~180μm and a median particle size D50 of 63μm, which is required by laser cladding technology. In the process of preparing powder by VIGA, the atomized droplets are subjected to the action of surface tension, and most of the powder will present a standard spherical shape with a smooth surface. Satellite powder is a common defect, which is formed by the random adhesion of small-particle powder to the surface of larger powder, as shown in Figure 1 (b). Because the powder moves at high speed with the argon gas flow during the atomization process, its movement trajectory cannot be controlled, so the satellite ball defects cannot be completely eliminated. However, the proportion of satellite ball particles can be reduced by optimizing parameters. The measurement results of the fluidity and bulk density of W10CrMoMn alloy steel powder with a particle size of 1~180μm are shown in Table 2.
2.2 Microstructure and properties of W10CrMoMn alloy steel
The metallographic structure of the laser cladding W10CrMoMn alloy steel sample is shown in Figure 2. As can be seen from Figure 2, there are a small number of micro cracks and pore defects in the sample, but a good metallurgical bond is formed between the molten layers, as shown in Figure 2 (a). Cellular crystals, dendrites and grain boundary precipitation phases (GBP) can be observed from Figure 2 (b). Ultrafine lath martensite (LM) and twin martensite (TM) can be observed in Figure 2 (c). This shows that due to the rapid solidification of laser, ultrafine multiphase structures with different morphology are formed in the cladding alloy structure, and these multiphase structures play an important role in the performance of the alloy. However, there are still a small number of micro cracks and pore defects in the alloy layer, which need to be solved by further optimizing the alloy composition and cladding parameters.
The XRD spectrum and cross-sectional hardness distribution of laser cladding W10CrMoMn alloy steel sample are shown in Figure 3. The structure of the sample is mainly composed of M, A, (Cr, Fe)7C3 and M2 (3 C, B)6. This is because laser cladding has the characteristics of rapid heating and rapid cooling. Elements such as C, Cr, Ni, and W in the alloy will be dissolved in M during the forming process, and a large number of (Cr, Fe)7C3 and M2 (3 C, B)6 carbides will be formed.
As shown in Figure 3 (b), the surface hardness of the sample can reach 800~1100HV. This is because TM, LM, and carbides are all high-hardness phases, and the formation of network GBP will hinder the grain growth of these phases, forming a fine-grain strengthening effect; on the other hand, (Cr, Fe)7C3 and M23 (C, B)6 play a particle strengthening effect. These two strengthening mechanisms make the hardness of the cladding layer reach 800~1100HV, providing a good foundation for laser cladding of high-hardness alloy steel working layer on the surface of brake disc parts.
3 Conclusion
(1) The new W10CrMoMn high-speed steel alloy powder was successfully prepared by VIGA technology. The powder sphericity reached 98%, the hollow ball rate was less than 3%, the particle size distribution was 1~180μm, the fluidity was 18.0s/50g, and the loose density was 4.95g/cm3. The powder indicators met the requirements of laser cladding technology.
(2) Under the optimized parameters of laser energy density of 230J/mm3 and overlap rate of 40%, the cladding layer formed a metallurgical bond with the substrate; the microstructure of the cladding layer was mainly composed of M and a small amount of A, (Cr, Fe)7C3 and M2(3C, B)6; the combined effect of LM and TM strengthening and (Cr, Fe)7C3 and M2(3C, B)6 particle strengthening made the surface hardness of the cladding layer reach 800~1100HV, but there were still a small number of micro cracks and pore defects in the alloy layer, which needed to be further optimized in alloy composition and cladding parameters to solve.
