Bolts made of high-strength steel or requiring large preloading force can be called high-strength bolts. High-strength bolts are mostly used to connect bridges, rails, high-pressure and ultra-high-pressure equipment. The fracture of this kind of bolt is mostly brittle fracture. In order to ensure the sealing of the container, large prestressing force is needed for the high strength bolts used in the ultra-high pressure equipment.
Processing Technology of High Strength Bolt
Hot rolled wire rod – (cold drawing) – spheroidizing (softening) annealing – mechanical descaling – Pickling – cold drawing – cold forging – thread processing – heat treatment – Inspection
Special High Strength Bolts
Usually, high strength bolts can bear more load than ordinary bolts of the same specification. Common bolts are made of Q235 (A3). High strength bolts are made of 45 # steel, alloy steel (such as 20MnTiB, 35VB) or other high quality materials, which are then heat treated to improve strength.
The mechanical performance requirements of common bolts of the same grade are slightly higher than those of high-strength bolts, but the acceptance requirements of impact work of high-strength bolts are more than that of ordinary bolts.
Identification style comparison:
The mark of common bolt and high strength bolt is the basic method of identifying the same grade bolt on site. Because the values of high-strength bolt torque calculated in Anglo-American standards are different, it is necessary to identify the bolts of the two standards.
The difference between them is the strength of the material.
From the view of raw materials: high strength bolts are made of high strength materials. The bolts, nuts and washers of high strength bolts are made of high strength steel, commonly used 45 steel, 40 boron steel, 20 manganese titanium boron steel. Common bolts are usually made of Q235 steel.
From the strength level, 8.8 and 10.9 strength grades are commonly used, of which 10.9 grade is the most. The strength grade of ordinary bolts is lower, generally 4.4, 4.8 and 5.6. (8.8S and 10.9S are special for steel structures)
Nowadays, advanced manufacturing represented by large aircraft, large power generation equipment, automobiles, high-speed trains, large ships and large complete sets of equipment will enter an important development direction. As a result, fasteners will enter an important stage of development. High strength bolts are used for connection of important machinery. Repeated disassembly or various installation torque methods require high strength bolts. Therefore, the surface condition and thread accuracy will directly affect the service life and safety of the main engine. In order to improve the friction coefficient and avoid rusting, biting or jamming during use, the technical requirements stipulate that the surface should be nickel-phosphorus plated. The thickness of the coating is guaranteed to be in the range of 0.02-0.03 mm. The coating is uniform, compact and without pinholes.
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