Characteristics of thermal power generation equipment and requirements for fasteners

The steam turbine of a thermal power plant has different characteristics from other equipment. It is a rotary prime mover that converts thermal energy into mechanical energy with steam as the medium. Compared with other thermal prime movers, it has a single unit with high power, high efficiency and stable operation. , A series of advantages such as low manufacturing cost per unit power and long service life; however, when the steam turbine is running, the metal parts such as the cylinder, the rotor and its fasteners have to withstand high temperature and working stress. After long-term operation, the metal material Creep damage will occur. Under the change of working conditions such as start and stop, components such as cylinders are subjected to cyclical changes in temperature and stress, resulting in low-frequency cyclic damage. The accumulation of such damage will also cause cracks in metal materials, and finally damage components and fasteners. Downtime.

(1) Performance requirements of steel for fasteners

Should have sufficiently high room temperature, high temperature yield strength and good relaxation resistance, and have high enduring strength and creep limit to ensure that the minimum sealing stress is not lower than the minimum sealing stress under low preload. Of resilience. Long-lasting plasticity and low notch sensitivity to reduce the damage of the thread stress concentration.

For fastener materials that work under high temperature and corrosive media conditions, they should have good stability, low thermal brittleness, and good oxidation resistance and corrosion resistance.

(2) Selection of fasteners of different materials

1. In order to avoid the seizure of bolts and nuts and reduce bolt wear, the strength of the nut material should be lower than that of the bolt. Generally speaking, the Brinell hardness value of the nut material should be 20-40 units lower than that of the bolt. At the same time, it should be considered as close as possible to the linear expansion coefficient and thermal conductivity of the bolt material.

2. The working temperature of fasteners is different, and the materials used are also different. When the working temperature is below 370℃, tempered medium carbon steel is generally used; for temperature above 370℃, heat-resistant steel is mostly used; below 480℃, chromium-molybdenum low-alloy pearlitic heat-resistant steel is used; below 540℃, chromium-molybdenum is used Vanadium low-alloy pearlitic heat-resistant steel; below 570℃, high-chromium martensitic heat-resistant steel is used, but the linear expansion coefficient of this kind of steel is small, and the pre-tightening force cannot be too large in the cold state to prevent it from reaching work At temperature, the fastener is overloaded and yielded.