High‑strength bolts share basically the same shape and connection configuration as ordinary bolts. The main difference between them is that ordinary bolt joints transfer shear force by means of shank bearing and shear resistance. When tightening the nut, the bolt generates only a small pre‑tension, whose influence is generally neglected. The working principle of high‑strength bolt joints is to intentionally apply a large pre‑tension to the bolt, which creates compressive pressure between the contact surfaces of connected components. Consequently, substantial friction force forms in the direction perpendicular to the bolt shank, and this friction force transfers the joint shear load.
The pre‑tension of high‑strength bolts is achieved by tightening the nuts. For large‑hexagon high‑strength bolts, the torque method and the turn‑of‑nut method are generally adopted to control pre‑tension. For twist‑off‑type high‑strength bolts, pre‑tension is controlled by twisting off the splined tail of the bolt.
High‑strength bolts for steel‑structure connections are manufactured from premium alloy structural steel of grade 10.9S or 8.8S and subjected to quenching‑and‑tempering heat treatment. Holes for high‑strength bolts shall be drilled. For friction‑type high‑strength bolt connections, the hole diameter is 1.5‑2.0 mm larger than the nominal bolt diameter d. For bearing‑type high‑strength bolt connections, the hole diameter is 1.0‑1.5 mm larger than the nominal bolt diameter d.
Under the same specification, high‑strength bolts can bear higher loads than ordinary bolts.
Ordinary bolts are generally made of Q235 (A3) carbon steel. High‑strength bolts are made of 45# steel, alloy steel (such as 20MnTiB, 35VB) or other premium materials. After forming, they undergo quenching‑and‑tempering heat treatment to improve their overall mechanical strength.
Their core distinction lies in material strength after heat treatment.
First, from the perspective of raw materials: High‑strength bolts are manufactured from high‑strength materials. Bolts, nuts and washers of high‑strength bolt assemblies are all made of high‑strength steel, commonly 45# steel, 40‑boron steel and 20MnTiB steel. Ordinary bolts are mostly produced from Q235 carbon steel.
Second, from the perspective of strength grades: High‑strength bolts are seeing ever‑wider application. The commonly used grades are 8.8 and 10.9, among which grade 10.9 is more widely used. 8.8S and 10.9S are special grade designations for steel‑structure high‑strength bolts. Ordinary bolts have lower strength grades, typically 4.4, 4.8 and 5.6.
Wire rods such as 45# steel and C1035 are widely used for high‑strength bolts. Completed parts are hardened by quenching‑and‑tempering treatment. After treatment, grade‑8.8 bolts can reach a hardness of 22‑32 HRC.
For matching assemblies: grade‑8.8 bolts go with grade‑8 nuts, and grade‑10.9 bolts go with grade‑10 nuts. Many users wonder why nuts are softer than mating bolts. During assembly and disassembly, component wear is inevitable. Slightly lower hardness of nuts allows wear to occur preferentially on nuts rather than bolts, thus protecting bolt bodies and extending service life. This matching scheme balances performance and total cost, following the same principle that wrenches are harder than the fasteners they drive.
Nowadays, domestic manufacturing technology for high‑strength bolts has reached a high level. High‑strength bolts are widely used in general industry and the automotive sector, and market requirements for their production processes and quality control keep rising.
