Stainless Steel Structural Bolts

Stainless steel structural bolts play an irreplaceable role in bridges, towers and heavy machinery connections. Their materials are usually made of austenitic stainless steel such as A4 or A2 grade. According to the ISO 3506-1 standard, the minimum tensile strength of A4-80 grade bolts reaches 800 MPa, and the non-proportional extension strength is not less than 600 MPa. This strength index ensures that the bolts can maintain the elastic deformation range when subjected to alternating loads and avoid plastic necking. At the same time, its excellent corrosion resistance is due to the chromium-nickel-molybdenum alloy composition. In the periodic salt spray test simulating the marine environment, according to the ASTM B117 specification, this type of bolt can remain free of red rust on the surface after five hundred hours of exposure, thus ensuring the reliability of long-term outdoor service. In actual projects, the diameter specifications of bolts range from M12 to M36. When matched with corresponding nuts and washers, their friction-type connections can effectively transmit shear force and reduce the risk of loosening.

However, the installation torque and pretightening force of stainless steel structural bolts need to be controlled with extreme caution because the thread friction coefficient is usually higher than that of carbon steel bolts. According to the VDI 2230 systematic calculation guideline, when the friction coefficient fluctuates in the range of 0.15 to 0.30, the pretightening force deviation generated by the same tightening torque can reach 40%. Therefore, the torque plus angle method is often used on site for precise tightening. For example, for A4-80 bolts with M20 specifications, the recommended torque value is about 450 Nm, and the pretightening force can reach 125,000 N. This value is based on the standard working conditions where the thread pitch diameter and the friction coefficient of the bearing surface are zero.2. If the lubrication conditions change, the torque needs to be recalibrated, otherwise excessive pretightening force may cause the stainless steel thread to seize, while insufficient pretightening force may cause fatigue fracture. Therefore, torque wrenches and ultrasonic axial force monitors are often used during construction to verify to ensure that the safety redundancy of the connection pair meets the design requirements.

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