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The life of the high temperature bolts for thermal power used in the fasteners
1) Changes in high temperature bolts during operation
1. Structural features
The stress concentration area at the root of the bolt thread, especially the root of the first buckle of the bolt, is particularly serious. The time required to maintain air tightness after the bolts are tightened is generally a period of overhaul, that is, (2~3)*10000 hours. As long as the bolts can continue to be used, the bolts need to be re-tightened for each overhaul.
2. Changes in high temperature bolts during operation
The creep and relaxation bolts operate within the creep temperature range, stress relaxation will occur during the operation and cause creep damage.
Fatigue The bolts used in certain parts of the thermal power unit (such as speed control valve bolts) will cause fatigue damage, which is a mechanical fatigue damage.
Changes in microstructure properties Some low-alloy CrMoV steel bolts will have obvious microstructure properties changes during high-temperature operation, mainly hot brittleness. The development of thermal brittleness increases the brittle transition temperature of bolt materials and significantly reduces the impact toughness at room temperature.
(2) Factors affecting the life of high-temperature bolts
1. Choose bolts
Bolt structure has a significant impact on bolt life. The life of general bolts is generally shorter than that of thin waist bolts of the same material. This is due to the large stress concentration factor of the bolts and the local stress concentration is not easy to transfer.
2. Manufacturing and installation
The surface quality of the bolt is processed poorly, which will cause the bolt to seize and shorten the life of the bolt.
Incorrect installation and tightening process will bring additional bending stress to the bolt and shorten the life of the bolt; overtightening the bolt will also shorten the life of the bolt.
3. Material
The low impact toughness of the material will shorten the life of the bolt. Therefore, when there are coarse crystals and heat treatment defects in the raw material, the development of thermal brittleness of the material during operation will affect the life of the bolt.