Bolts made of high-strength steel, or bolts that must be applied with a relatively large pre-tightening force, can be called high-strength bolts. High-strength bolts are mostly used for the connection of highway bridges, railroad tracks, ultra-high pressure and high-pressure facilities.
The fracture of such bolts is mostly brittle fracture. For the high-strength bolts used in high-pressure equipment, in order to ensure the tightness of the vessel, a large prestressed steel bar must be applied. Some definitions of high-strength bolts 1. According to the regulations, bolts with a characteristic level above 8.8 are called high-strength bolts. The current national industry standard only lists M39. For large-size specifications, especially high-strength bolts with a length of more than 10 to 15 times, China's manufacturing is still a short-term stock. The difference between high-strength bolts and ordinary bolts: High-strength bolts can bear a larger load than ordinary bolts of the same size. The raw materials of general bolts are produced by Q235 (namely A3). The raw material of high-strength bolts is 35# steel or other high-quality raw materials, and after they are made, heat treatment is carried out to improve the strength. The difference between the two lies in the strength of the raw materials.
High-strength bolts from the perspective of raw and auxiliary materials: High-strength bolts are manufactured with high-strength raw materials. The extruder screw, screw cap and sealing ring of high-strength bolts are all made of high-strength stainless steel plates, such as No. 45 steel, 40 boron steel, 20 manganese titanium boron steel, 35CrMoA and so on. General bolts are usually made of Q235 (equivalent to the former A3) steel. From the perspective of strength level: high-strength bolts are increasingly used. There are two intensity levels of 8.8s and 10.9s, among which 10.9 is the majority. Generally, the bolt strength level is lower, generally 4.4, 4.8, 5.6 and 8.8. From the perspective of bearing characteristics: high-strength bolts increase pre-tension and transmit external force by sliding friction. Generally, the bolt connection relies on the bending of the bolt rod and the pressure of the hole wall to transmit the shear stress.
The pretension force generated when the nut is tightened is not large, and its impact can be ignored. In addition to the high material strength of the high-strength bolt, the return bolt increases a large pretension force, which causes extrusion pressure between the connected prefabricated components, and then makes the direction of the vertical bisecting extruder screw larger. The sliding friction force, and the pretension force, anti-slip index and stainless steel plate type are all directly related to the bearing capacity of the high-strength bolt.
According to the characteristics of bearing force, it can be divided into pressure bearing type and friction type. The calculation methods of the two are different. The minimum specification model of high-strength bolts is M12, and M16~M30 is common, and the characteristics of super-large bolts are unstable, so they should be used with caution in design.