High-strength bolts

Basic Information

Bolts made of high-strength steel or those requiring a larger pre-tightening force are referred to as high-strength bolts. High-strength bolts are commonly used in connections for bridges, rails, high-pressure, and ultra-high-pressure equipment. The fracture of such bolts is often brittle in nature. For high-strength bolts used in ultra-high-pressure equipment, a larger pre-stress must be applied to ensure the sealing of the container.

Definition: 

Several concepts about high-strength bolts. 1. Bolts with a performance grade of 8.8 or higher as specified are called high-strength bolts.

Differences Between High-Strength Bolts and Ordinary Bolts:

High-strength bolts can withstand greater loads than ordinary bolts of the same specification.

Ordinary bolts are typically made from Q235 steel (equivalent to A3). High-strength bolts are made from materials like 35# steel or other high-quality steels, which undergo heat treatment after manufacturing to enhance their strength. The fundamental difference lies in the material strength.

From a raw material perspective:

High-strength bolts are manufactured using high-strength materials. The screw, nut, and washer of a high-strength bolt are all made from high-strength steel, commonly 45# steel, 40 boron steel, 20MnTiB steel, 35CrMoA, etc. Ordinary bolts are usually made from Q235 steel.

From a strength grade perspective:

The use of high-strength bolts is becoming increasingly widespread. Common strength grades are 8.8 and 10.9, with grade 10.9 being the most prevalent. Ordinary bolts have lower strength grades, typically 4.4, 4.8, 5.6, and 8.8.

Quality Performance:

Today, advanced manufacturing represented by large aircraft, large-scale power generation equipment, automobiles, high-speed trains, large ships, and large complete sets of equipment has entered an important phase of development. Consequently, fasteners are also entering a crucial stage of advancement. High-strength bolts are used for critical mechanical connections, where repeated disassembly and assembly or various installation torque methods place extremely high demands on them. Therefore, the surface condition and thread precision of these bolts directly impact the service life and safety of the host equipment.

To improve the friction coefficient and prevent issues such as rust, seizing, or jamming during use, technical specifications require that their surfaces undergo nickel-phosphorus plating. The coating thickness should be maintained within the range of 0.02–0.03 mm, with requirements for uniformity, density, and absence of pinholes. The bolt materials are 18Cr2Ni4W and 25Cr2MoV steel, and the bolt specifications range from M27 to M48.

Since these types of steel tend to form a passivation film on the surface, which can prevent the bolt from achieving a chemical nickel-phosphorus plating layer with good adhesion, special pre-treatment measures must be taken to remove this film initially and prevent its reformation. This ensures strong bonding between the plating layer and the substrate after plating. Additionally, due to the large geometric dimensions of these bolts, both the nickel-phosphorus plating process and quality inspection during production present increased challenges.