Flange Bolt Classification and Common Materials

Flange bolts are a type of fastener consisting of a hexagonal head and a flange plate (the gasket under the hexagonal is fixed with the hexagonal), and a screw (a cylinder with an external thread), which needs to be matched with the nut., used to connect two parts with through holes securely. In this article, the editor of Xianji.com mainly introduces its classification and common materials. Let's look!

Classification of flange bolts:

50. Hexagonal Flange Bolt Types: Hexagonal heads come in flat and recessed varieties.
50. Surface Color Classification: To suit various requirements, the surface has options including white, military green, vibrant yellow, and Dacromet for long-lasting rust resistance.
50. Flange Variation: Flange bolts are categorized based on their specific application areas, with varying disk size requirements. These categories include flat-bottomed and serrated flanges, with serrated ones providing enhanced anti-slip functionality.
50. Based on the connection force, flange bolts can be classified into standard and swivel types. For reamed holes, flange bolts must match the hole size and are ideal for lateral force applications. Additionally, to prevent loosening due to vibration post-installation, some bolts feature holes in the rod section for locking purposes. Thin rod flange bolts, characterized by unthreaded polished rods, are slender and suitable for variable force connections. Special high-strength bolts are designed for steel structures and are distinguished by larger heads and varying sizes.

Comments: GB5789 and GB5787 are widely recognized national standards in the market.

Common materials for flange bolts:

50. High-carbon steel with a carbon content greater than 0.45%: Hexagonal flange bolt materials of this composition are not presently utilized in the market.
50. Medium carbon steel with 0.25% carbon content: Hexagon head bolts made from medium carbon steel are commonly referred to as No. 35 and No. 45 steel in China. Internationally, they are typically labeled as 1035, CH38F, 1039, 40ACR, etc. These materials are primarily employed for grade 8 nuts, grade 8.8 bolts, and grade 8.8 hexagon socket products.
50. Low-carbon steel with carbon content ≤ 0.25% is commonly known as A3 steel in China. Internationally, it is typically called 1008, 1015, 1018, 1022, etc. These materials are primarily utilized for grade 4.8 bolts, grade 4 nuts, small screws, and other products that do not require high hardness. (Note: Drill tail screws are primarily manufactured from 1022 material.)
50. Alloy Steel: Alloying elements are incorporated into standard carbon steel to enhance its specific properties. Examples include 35, 40 chromium molybdenum, SCM435, and 10B38. Hexagon flange bolts primarily employ SCM435 chromium molybdenum alloy steel, comprising key components such as C, Si, Mn, P, S, Cr, and Mo.

Why should flanged joints not be bolted in materials such as 304 and 316?

Flange connections experience separation between the two sealing surfaces due to internal pressure, resulting in decreased stress on the gasket and ultimately leading to leakage and failure of the flange joint.

During practical operation, bolt force loosening is unavoidable, and the initially applied tightening force gradually diminishes over time. This effect is particularly pronounced in flange joints subjected to high temperatures and severe cyclic conditions. After 10,000 hours of operation, bolt load loss often surpasses 50%, diminishing further with prolonged exposure to time and temperature fluctuations.

Conclusion

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