What materials are used for the large bolts of turbine cylinders?

The materials used for large bolts vary depending on the type of turbine; even within the same turbine, different materials are employed. For example, for N125 and N300 large bolts, some are made of 25Cr2Mo1V, while for N200 turbine cylinder bolts, some are made of 25Cr2Mo1V1 or 20Cr1Mo1V1, and others are made of 25Cr2MoV. These are low-alloy chromium-molybdenum steels; prolonged operation at high temperatures increases their hardness but reduces their toughness. Under certain conditions, such as stress concentration, excessive initial tightening force, or additional bending moments during operation, brittle fracture of these bolts can occur. To prevent such fractures, it is common to improve the heat treatment process in order to enhance toughness while reducing endurance strength and relaxation resistance. New materials are developing quite rapidly these days, and I’m not very familiar with them.

I would like to answer the original poster’s question from the following aspects: 1. The importance of material selection for large bolts in turbines: The connection between turbine cylinders must be tight and durable. The tightness of the cylinder flange connection depends on many factors, among which the key factor is the resistance of the bolts to relaxation at high temperatures. The flange bolts that are tightened experience high elastic stresses. Under the effect of high temperatures over a long period of time, the elastic deformation of these bolts transitions into plastic deformation, which leads to stress relaxation within the bolts and results in a loss of the tightness of the flange connection. 2. Requirements for the mechanical properties of bolt materials: A key requirement is that the material should have good resistance to relaxation. Additionally, it is necessary that the material exhibit low sensitivity to notches and minimal tendency to creep embrittlement, in order to reduce the effects of stress concentration. Of course, good mechanical properties and corrosion resistance are also requirements that bolts should possess. 3. When selecting the material for nuts, it is necessary to take into account the working conditions of the threads; the hardness of the nut material should be lower than that of the bolt, with a hardness difference of approximately HB20-40 between the two. 4. The fasteners (bolts, nuts) for the cylinders of domestic steam turbine units are selected based on the maximum operating temperature; materials such as 35, 45, 35SiMn, 35CrMo, and 25Cr2Mu1V are commonly used. Specific details can be found in the manual. 5. To prevent thread galling, the thread surface should be hardened through processes such as surface sulfurizing, in order to reduce the friction coefficient and minimize galling.