Foreword, as environmental pollution intensifies, traditional energy and resources are increasingly scarce, and mankind's strong demand for clean and renewable energy has prompted unprecedented development challenges for the renewable energy industry. Wind power is also an important energy resource in China. As of the end of 2018 , China Offshore Wind Power has approved more than 17 million kilowatts of capacity , with a capacity under construction of about 6 million kilowatts, and a total installed capacity of about 3.6 million kilowatts. At present, China’s grid-connected offshore wind power installed capacity ranks third in the world, second only to Britain and Germany. Currently, grid-connected offshore wind power projects are mainly located in Jiangsu and Shanghai, but in recent years, Guangdong, Fujian, Zhejiang Ocean wind power development and construction progress has accelerated significantly, but currently most of the wind turbines used in major wind power sites in China rely on imports, and the cost remains high, which restricts the rapid development of China's wind power industry [1-3] .
The complete autonomy of wind power technology is an important issue facing my country's wind power field. The development of large-scale wind power gearboxes with independent intellectual property rights and reliable performance is conducive to reducing the cost of wind power in my country and has broad market application prospects. The gearbox is an important mechanical component in the wind turbine. Its function is to transmit the power generated by the wind wheel under the kinetic energy of the wind to the generator and make it get the corresponding speed. The performance of the shaft directly affects the gearbox. Work efficiency [4-12] . The product developed in this article is a 5MW offshore wind power gearbox shaft.
1. Product introduction
The shaft profile size of the 5MW offshore wind power gearbox is φ1600*2200 , the rough weight is 6.48 tons, the gross weight of the casting is 8 tons, the maximum wall thickness is 248 mm , and the minimum is 80 mm . The casting structure is shown in Figure 1(a) , Figure 1(b) It is a structural section view, with many wall thickness changes, poor feeding gradient, and the structure is prone to thermal cracking. The inherent quality requirements are extremely high, and the products are required to be corrosion-resistant, impact-resistant, and non-destructive testing requirements are strict. Ultrasonic testing meets the requirements of EN 12680-1 specification UV2 . It cannot produce casting defects such as slag inclusion, shrinkage porosity, and cracks. The flatness and concentricity tolerances conform to ISO 8063-3 GCTG level 6 .
The traditional wind power shaft is made of ductile iron  . In order to improve the corrosion resistance and low-temperature impact performance of the offshore wind power shaft, the material is G18NiMoCr3-6 , which belongs to low-alloy high-strength steel and is a European grade material. According to European standards: For EN 10340:2007 "Structural Castings", the chemical composition requirements are shown in Table 1 , and the mechanical properties after quenching and tempering are shown in Table 2 .