Wind energy, as a renewable and clean new energy source, has great potential to meet the world's energy demand. Wind turbines typically are installed in harsh environments, such as the sea and the Gobi Desert. Therefore, higher requirements are proposed for the wear resistance and corrosion resistance of wind turbine equipment, especially the surfaces of wind turbine bearings and their raceways. According to relevant regulations, the hardness of the bearing raceway surface should be up to 55-62 HR, and the depth of the hardened layer should exceed 3 mm. 42CrMo, as a common material for wind turbine bearings, has high toughness and excellent fatigue resistance. However, the hardness of 42CrMo is relatively low (350-450 HV), so the surface of the 42CrMo raceway should be strengthened to meet the application requirements of wind turbine equipment. Induction quenching is a common surface treatment technology for wind turbine bearings. However, the limited quenching depth, the existence of quenching soft bands, and quenching cracks limit the development of large-scale wind turbine bearings. In this study, the microstructure, solidification process, and residual stress of boron-doped 4Cr13 stainless steel martensite coating are studied, and the differences in properties such as hardness, frictional behaviors, and salt spray corrosion are analyzed and compared with those of induction-hardened 42CrMo. A high hardness crack-free martensitic stainless steel coating with a thickness of more than 3 mm is successfully prepared on the surface of a wind turbine bearing raceway simulator with a diameter of 1 m under preheated conditions. We hope that our research will help advance laser cladding technology for surface strengthening of wind turbine bearings.
The 42CrMo low alloy high strength steel is selected as the substrate, and the 4Cr13 martensitic stainless steel powder doped with 1%-1.5%(mass fraction) boron element is used as the cladding powder. Before laser cladding, the 42CrMo substrate is polished and cleaned with acetone, and the powder is dried at 120 ℃ for 3 h. Then, the laser cladding is performed using optimized test parameters. Afterward, the microstructure of the coating is observed by the X-ray diffraction, scanning electron microscopy, and transmission electron microscopy, the solidification process of the coating is analyzed by the differential scanning calorimetry and coefficient of thermal expansion measurements, and the residual stress of the coating is measured using the contour method. A microhardness tester is used to test the hardness of the coating, and a vertical universal friction and wear tester is used to test the friction and wear of the sample at room temperature. An electrochemical workstation is used to test the potentiodynamic polarization of the coating. A neutral salt spray test is conducted.
The thickness of the coating exceeds 3 mm, and no defect, such as cracks, pores, and inclusions, is found in the coating. In addition, the interface between the coating and the substrate is smooth, and the dilution rate is less than 5% [Fig. 4(a)]. The microstructure of the coating consists of martensite, residual austenite, and reinforcement phase (Fig. 3). Contrary to the residual tensile stress distributed in the common coating, the residual stress in the boron-doped 4Cr13 stainless steel martensite coating is residual compressive stress (Fig. 6). This phenomenon is primarily related to the transformation stress caused by martensitic transformation during the solidification of the coating. The hardness of the coating is more than 800 HV (Fig. 8), which is 1.2 times that of induction-quenched 42CrMo (650 HV). The high hardness of the coating is caused by the high content of reinforcement phases in the coating and the solid solution strengthening of the martensite matrix by the Cr element. The wear results show that the wear loss of the coating is only 50% of that of induction-quenched 42CrMo under the same wear conditions (Fig. 9). Because the coating's high hardness reinforcement phases effectively protect the matrix from direct grinding by abrasive particles during the wear process, the coating's wear resistance is improved. According to the neutral salt spray test and electrochemical test results, the coating exhibits better corrosion resistance than induction-quenched 42CrMo.The corrosion rate of the coating is 68.8% lower than that of induction-quenched 42CrMo (Fig. 11). Furthermore, the self-corrosion potential of the coating is 0.128 V higher than that of induction-quenched 42CrMo.
In this study, the high-hardness crack-free martensitic stainless steel coating with a thickness of more than 3 mm is successfully prepared on the surface of a wind power turbine bearing raceway simulator by laser cladding technology. The microstructure of the coating consists of martensite, residual austenite, and reinforcement phases (M2B and M23C6). Due to the synergistic effects of thermal stress and phase transformation stress, the residual compressive stress in the cladding layer reduces the risk of cracking. The hardness of the coating is more than 800 HV, which is 2.4 times that of the 42CrMo matrix (335 HV) and 23% higher than that of induction-quenched 42CrMo (650 HV). Under the same wear conditions, the wear loss of the coating is 0.15 g, which is only 50% of that of induction-quenched 42CrMo (0.30 g). According to the neutral salt test results, the average corrosion rate of the coating is 0.352 mg·m-2·h-1, which is significantly lower than that of induction-quenched 42CrMo (1.131 mg·m-2·h-1). According to the polarization curve, the self-corrosion potential of the coating is -0.173 V, which is 0.128 V higher than that of induction-quenched 42CrMo (-0.301 V).
