In various construction projects, bolt selection requires systematic consideration of multiple factors such as structural characteristics, load characteristics and environmental conditions. First, the type of bolt needs to be determined according to the functional requirements of the connection part: hexagonal head bolts are universal fasteners and are suitable for most conventional connection scenarios; flange bolts can effectively disperse the contact surface pressure with their integrated gasket design and are often used for anti-loosening requirements under vibration conditions; anchor bolts are specifically used for anchoring equipment bases and concrete foundations, and their embedding depth and hook structure need to be accurately calculated according to the equipment load. For connection nodes that bear shear force, the use of hinged hole bolts can significantly improve the shear resistance, and high-strength bolts for steel structures transfer loads through the friction force generated by the preload force, which has become the core connection method of modern steel structure buildings.
The selection of bolt performance grade is directly related to structural safety. In the common 4.8 grade, 8.8 grade and other identifications, the value before the decimal point represents 1/100 of the nominal tensile strength, and the value after the decimal point is the yield strength ratio. Ordinary mechanical connections mostly use 8.8 grade carbon steel bolts, whose tensile strength of about 800MPa can meet most static load requirements. For working conditions that are subject to alternating loads or impact loads, it is necessary to upgrade to 10.9 or even 12.9 high-strength bolts. It is worth noting that the strength grade system of stainless steel bolts is different from that of carbon steel. For example, the 70 in the A2-70 mark represents the minimum tensile strength of 700MPa. When selecting, special attention should be paid to the differences in the standard system.
Material selection requires comprehensive consideration of mechanical properties and environmental adaptability. Q235 carbon steel bolts are the most economical, but have poor corrosion resistance and are suitable for dry indoor environments; 35CrMo alloy steel can obtain excellent strength and toughness matching through heat treatment and is often used for heavy-duty equipment connections; stainless steel A4-80 series has become the first choice for chemical equipment due to its excellent acid and alkali resistance. In high-temperature environments, nickel-based alloy bolts can maintain stable mechanical properties, while austenitic stainless steel with good low-temperature toughness must be selected for low-temperature conditions. For dissimilar metal contact scenarios with the risk of electrochemical corrosion, the corrosion circuit must be blocked by material matching or insulating gaskets.
The surface treatment process directly affects the durability and adaptability of bolts to working conditions. Hot-dip galvanizing can provide a protective layer of about 85μm, which is suitable for ordinary outdoor environments; Dacromet coating has both corrosion resistance and high temperature resistance, and is often used in complex working conditions such as automobile chassis; the microporous structure formed by phosphating treatment is conducive to subsequent coating adhesion, and is often used in conjunction with anti-rust oil inside mechanical equipment. In marine corrosive environments, zinc-nickel alloy coatings show better protection than traditional galvanizing, while the zinc-diffusion process can better maintain thread accuracy. It should be noted that the risk of hydrogen embrittlement exists in the electroplating process of high-strength bolts, and dehydrogenation treatment is usually required within 24 hours after plating.
Bolt selection requires a comprehensive understanding of the characteristics of the entire connection system. Engineers must first analyze the specific stress mode of the structure, the conditions of the actual use environment, and the possibility of later maintenance. For the stress conditions of key parts, computer simulation can be used for auxiliary verification. Now with the advancement of technology, special sensors can be installed to monitor the tightness and corrosion of bolts in real time, which is particularly useful for the maintenance of important connection parts. This management method throughout the entire life cycle of the bolt is shifting the bolt selection from past experience judgment to scientific decision-making supported by data.
Post time: Mar-21-2025