The bolt which is made of high strength steel or needs to use a larger preload can be referred to as high strength bolts. High strength bolts are widely used in the connection of bridge, rail, high pressure and ultra-high pressure equipment. The fracture of this kind of bolt is brittle fracture, the high strength bolts used in the super high pressure equipment need to be prestressed in order to ensure the sealing of the container.

Several concepts of high strength bolts:
According to the provisions, the performance level of bolts is in 8.8 or more, which is called the high strength bolts. Now the national standard is only listed to M39, for large size specifications, especially the length is 10~15 times more than the high strength bolts.

The high strength bolt is to be able to withstand the load which is bigger than the same specification common bolt. The common bolt material is made of Q235 (namely A3). The material of high strength bolt is made of steel or other high quality material, which is processed by heat treatment to improve the strength. The difference between them is the strength of the material.