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 by using steam as a medium. Compared with other thermal prime movers, it has large single machine power, high efficiency and stable operation. A series of advantages such as low unit manufacturing cost and long service life; however, when the steam turbine is running, metal parts such as cylinders, rotors and fasteners are subjected to high temperature and working stress. After long-term operation, metal materials Creep damage will occur. Under the condition of starting and stopping, the cylinder and other components are subject to cyclic changes of temperature and stress, which cause low-frequency cyclic damage. The accumulation of such damage will also cause cracks in the metal material, and finally damage the parts and fasteners. Stop.
(1) Performance requirements for steel for fasteners
It should have a sufficiently high temperature, high temperature yield strength and good sag resistance, and has high rupture strength and creep limits to ensure that no less than the minimum sealing stress at a lower preload should have good toughness. Long-lasting plasticity and low notch sensitivity to reduce damage at the stress concentration of the thread.
For fastener materials working under high temperature and corrosive media conditions, it should have good stability, low tendency to hot brittleness and good oxidation resistance and corrosion resistance.
(2) Selection of fasteners for different materials
1. In order to avoid the seizure of bolts and nuts and reduce the wear of the bolts, the strength of the nut material used should be lower than that of the bolts. In general, the Brinell hardness value of the nut material should be 20~40 units lower than the bolt, and should also be considered as close as possible to the linear expansion coefficient and thermal conductivity of the bolt material.
2. The working temperature of the fastener is different, and the materials used are different. The working temperature is below 370 °C, generally use medium-tempered steel after quenching and tempering; more than 370 °C, more heat-resistant steel; below 480 °C, chrome-molybdenum low-alloy pearlitic heat-resistant steel; below 540 °C, chrome-molybdenum Vanadium low-alloy pearlitic heat-resistant steel; high-chromium martensitic heat-resistant steel is selected below 570 °C, but the steel has a small coefficient of linear expansion, and the pre-tightening force in cold state cannot be too large to prevent work. At the temperature, the fastener is overloaded and yielded.
Â
Decorative Tassel,Decorative Tassels for Furniture,Colorful Decorative Tassels for Furniture
Taojin Textile Co., Ltd. , http://www.hzcurtain-accessory.com