Almost all fasteners are made of carbon steel, alloy steel, general fasteners hope to prevent corrosion. In addition, the coating of the surface treatment must adhere firmly.
For surface treatment, people generally pay attention to aesthetics and anti-corrosion, but the main function of fasteners is to fasten the connection, and surface treatment also has a great influence on the fastening performance of fasteners. Therefore, when choosing the surface treatment, the factor of fastening performance should also be considered, that is, the consistency of installation torque-pretightening force.
A high-level fastener designer should not only consider the design, but also pay attention to the manufacturability of assembly and even environmental protection requirements. The following is a brief introduction to some commonly used coatings for fasteners based on the above factors for reference by fastener practitioners.
1 、Plating
The electroplating of fasteners refers to immersing the parts of the fasteners that need to be electroplated in a specific aqueous solution. The aqueous solution will contain some deposited metal compounds, so that after the current passes through the aqueous solution, the metal substances in the solution are precipitated and attached to the surface. Immersed portion of a fastener. Electroplating of fasteners generally includes galvanized, copper, nickel, chromium, copper-nickel alloy, etc.
2 、phosphating
Phosphating is cheaper than galvanizing, and its corrosion resistance is worse than galvanizing. There are two commonly used fastener phosphating, zinc phosphating and manganese phosphating. Zinc-based phosphating has better lubricity than manganese-based phosphating, and manganese-based phosphating has better corrosion resistance and wear resistance than galvanized. Phosphating products such as engine connecting rod bolts, nuts, cylinder heads, main bearings, flywheel bolts, wheel bolts and nuts, etc.
3 、oxidation (blackening)
Blackened+oiled is a popular plating for industrial fasteners because it's the cheapest and looks good until the oil runs out. Since blackening has almost no anti-rust ability, it will rust quickly without oil. Even in the state of oil, the neutral salt spray test can only reach 3~5 hours.
4 、hot dip galvanizing
Hot-dip galvanizing is a heat-diffusion coating in which zinc is heated into a liquid. The thickness of the coating is 15~100μm, and it is not easy to control, but it has good corrosion resistance and is mostly used in engineering. Due to the temperature of hot-dip galvanizing (340-500C), it cannot be used for fasteners above grade 10.9. The hot-dip galvanizing treatment of fasteners is more expensive than electroplating.
5、 sherardizing
Zinc sheathing is a solid metallurgical thermal diffusion coating of zinc powder. Its uniformity is good, and a uniform layer can be obtained in threads and blind holes. The coating thickness is 10~110μm, and the error can be controlled within 10%. Its bonding strength with the substrate and anti-corrosion properties are the best among zinc coatings (electrogalvanized, hot-dip galvanized, Dacromet). Its processing process is pollution-free and the most environmentally friendly. If the price of chromium and environmental protection issues are not considered, it is actually most suitable for high-strength fasteners with high anti-corrosion requirements.
The main purpose of the surface treatment of fasteners is to make the fastener itself obtain corrosion resistance, so as to increase the reliability and adaptability of the fastener.
