Small Metal Fasteners

The production of small metal fasteners mostly uses a multi-station cold heading forming process. The wire rod is formed in one step through processes such as cutting, pre-heading, final heading and thread rolling. The material utilization rate is as high as over 92%, which is about 30% higher than that of cutting processing. According to the ASTM E18 Rockwell hardness test, the surface hardness of the fastener after cold heading hardening can reach HRC 38 to 42, while the core hardness remains at HRC 28 to 32. This hardness gradient takes into account the thread's wear resistance and impact resistance. The surface roughness of rolled threads can reach Ra 0.4 microns, and the depth of the compressive stress layer generated is about 0.1 millimeter. According to the ASTM E466 axial fatigue test, the fatigue limit of rolled threads is about 40% higher than that of cutting threads. When the stress amplitude is 100 MPa, the fatigue life can reach tens of millions of cycles. This makes small fasteners have excellent anti-fracture reliability in high-speed motor brackets or precision transmission chains.

For humid or chemical media environments, zinc-nickel alloy or Dacromet coating can be used instead of traditional galvanizing for small metal fasteners to extend the maintenance cycle. According to the ASTM B117 neutral salt spray test, when the zinc-nickel alloy coating is eight to twelve microns thick, it takes more than one thousand hours for the first red rust to appear, while the traditional zinc coating can only last for four hundred and eighty hours, more than doubling the corrosion resistance. At the same time, according to the friction coefficient measurement method specified in DIN 946, the friction coefficient of the Dacromet-coated fastener bearing surface is stable in the range of 0.08 to 0.14, which is significantly lower than the 0.15 to 0.22 of ordinary phosphating treatment. This reduces the dispersion of tightening torque and controls the assembly pretightening force deviation within plus or minus eight percent. In addition, these coatings can still maintain chemical stability at high temperatures of 200 degrees Celsius and are suitable for bolted connections around engines or HVAC equipment, which not only meets anti-corrosion requirements but also takes into account assembly controllability.

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