The strength level of high-strength bolts is generally divided into four grades of 8.8 grade, 9.8 grade, 10.9 grade, 12.9 grade. The high strength bolts are usually made of medium carbon steel or medium carbon alloy steel, and non-tempered steel, boron steel, FM duplex steel or low carbon martensitic steel. Glory Steel could supply S20C hex steel bar for bolts production.
Since most of the bolt production requires heading, the bolt steel is mostly cold heading steel. With the increase in the size of cold heading equipment and the speed of production, the demand for cold heading steel is also increasing. The bolt acts role as connection, fastening, positioning, sealing in the mechanism. In addition to the simple positioning of the bolts, the bolts need to be pre-tightened during installation. Therefore, they need bear static tensile load. Some bolts, such as connecting rod bolts, cylinder head bolts are usually subjected to axial pre-tensioning tensile loads, and they are also subjected to additional axial tensile (alternating) loads, transverse shears during operation ( alternating), the effect of the load or the resulting bending load, and the impact load. Under normal circumstances, the alternating load will cause the bolt to loosen, the axial alternating load will cause the fatigue fracture of the bolt, and the axial tensile load under the action of the environmental medium will cause the delayed fracture of the bolt. In summary, high-strength bolt steel and its product bolts should meet the following requirements. S20C hex steel bar is a kind of material for hex bolts production.
1. High strength bolts steel should have good cold heading performance (deformability and deformation resistance), good steel quality (surface quality, non-metallic inclusions, segregation, dimensional accuracy) Wait).
2. High strength bolts steel should have high tensile strength to resist elongation, breakage, buckle and wear.
3. High strength bolts steel should have high plasticity and toughness to reduce sensitivity to deflection, notched stress concentration and surface quality.
4. Bolts operating in humid atmospheres or corrosive atmospheres are required to have sufficiently low delayed fracture sensitivity.
5. Bolts subjected to alternating load and impact load, high fatigue resistance and multiple impact tensile resistance are required to resist fatigue and multi-punch fracture.
6. For bolts working in severe cold areas, bolt steel must also have a low ductile-brittle transition temperature.
High-strength bolts have high requirements on the quality of steel, so it is common to use refining outside the furnace and continuous casting with electromagnetic stirring. The advantage is that the main elements such as C, Si, Mn, Cr, Mo in the steel can be controlled in a relatively narrow range, and the uniformity of the steel and the fluctuation range of the product performance can reduce P, S, O, N to reduce the amount of non-metallic inclusions in the steel and to make it finely dispersed to improve the coldness and improve the surface quality of the steel.
Reducing the P and S content in the steel not only improves the cold heading properties of the steel, but also improves the delayed fracture resistance of the steel. Similarly, reducing the oxygen content in the steel and reducing the content of oxide impurities can also greatly improve the cold workability of the steel and the surface quality of the product. In order to increase the market competitiveness of products, while improving the performance of steel and metallurgical quality, it is also required to minimize production costs, reduce energy consumption, and improve production efficiency. To this end, we will vigorously develop non-tempered steel that omits softening, quenching and other tempering heat treatment processes, and boron alloys that save alloying elements. Usually, the 8.8 bolts are made of SAE10B21 equivalent low carbon manganese-containing boron steel, and the 9.8 grade and 10.9 bolts are made of medium carbon boron steel equivalent to SAE10B35. However, due to the low temper softening resistance of boron steel, the tempering temperature is 80°C to 100 °C lower than that of low alloy bolt steel. Therefore, the 10.9 high strength bolt made of boron steel has high delayed fracture sensitivity. We should develop boron steel for 10.9 grade bolts with delayed fracture resistance.
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