Technical breakthrough in the production of China large steel castings

Large steel castings are mostly the main bearing parts and supports in major technical equipment. The castings have huge size, complex shape and great difference in section thickness. This kind of casting is often in high temperature and high pressure working state, and its performance and quality requirements are very strict, especially for high temperature performance. Castings should be large from small, not simple imitation and mechanical amplification; The service conditions and environment have more stringent requirements for its mechanical properties, physical properties and defects, which puts forward higher requirements for its manufacturing. Therefore, the production process of large steel castings is a complex system engineering. The production of high-precision products meeting domestic demand will inevitably involve many new materials, new processes and new technologies, and the key technologies in the manufacturing process will inevitably involve many fields and aspects in the material processing process.

As early as the eighth five year plan and the ninth five year plan, China has organized scientific and technological research on the key technologies of large castings of imported thermal power units and nuclear power plant units. The materials and casting technology of large castings such as upper crown, lower ring and blade for 700000 kW large hydraulic turbine in the Three Gorges of the Yangtze River were studied. These studies have made a series of innovative achievements in the formation mechanism of casting shrinkage cavity and porosity. At present, the problems of shrinkage cavity and porosity have been basically solved by means of solidification simulation and experience accumulation.

In recent years, the research group participated by ZHY casting senior engineer team has carried out a lot of work in the simulation of stress field during solidification of large castings. Zhang Lei and Liu Xiaogang used the finite difference / finite element integrated casting thermal stress analysis system developed by Tsinghua University to carry out numerical simulation of stress and strain and prediction of thermal crack and deformation of large castings such as rolling mill frame, rolling mill beam and turbine blade, which provided valuable scientific guidance for the production of the plant. Figure 1 shows Liu Xiaogang’s simulation calculation of the deformation of a integrally cast lower ring in the secondary production. In the simulation, the contact element method is used to deal with the boundary conditions of the casting / mold, which improves the simulation accuracy and provides a quantitative reference for the determination of the casting shrinkage and other process parameters of the casting. In 2005, the Foundry Branch of Erzhong Group Co., Ltd. cooperated with Tsinghua University on the internal crack problem in thick wall steel castings, and predicted its hot crack tendency and deformation by means of numerical simulation, so as to optimize different process schemes. Preliminary progress has been made. In addition, some research work has also been carried out on the numerical simulation of segregation in castings. Ma Changwen et al. Simulated the macro segregation in the cast steel ingot considering the free equiaxed crystal movement, and simulated the negative segregation and positive segregation of C element in the ingot, as shown in Fig. 2.

At present, domestic foundry enterprises pay more and more attention to the important role of numerical simulation technology in auxiliary process design. Many enterprises, including heavy machinery plants, have preliminarily popularized the calculation of temperature field and the prediction of shrinkage cavity and porosity in solidification simulation technology. Domestic FT-Star, Huazhu CAE, foreign magamasoft, ProCAST and other solidification process numerical simulation software have been widely used in domestic casting enterprises. The problems of shrinkage cavity and porosity have been basically solved by means of solidification simulation and experience accumulation. However, thermal stress analysis and crack prediction, residual stress and deformation prediction based on it are not mature and have not been widely used. The segregation of castings is still in the research stage, which is still a certain distance from practical application.