Hot dip galvanizing is a surface treatment process that involves immersing pre treated parts into a zinc bath for high-temperature metallurgical reactions to form a zinc coating The three steps of hot dip galvanizing are as follows:
① The surface of the product is dissolved by zinc liquid, and the iron based surface is dissolved by zinc liquid to form a zinc iron alloy phase.
② The zinc ions in the alloy layer further diffuse towards the matrix to form a zinc iron mutual solution layer; Iron forms a zinc iron alloy during the dissolution of zinc solution and continues to diffuse towards the surrounding area The surface of the zinc iron alloy layer is wrapped with a zinc layer, which cools and crystallizes at room temperature to form a coating. At present, the hot dip galvanizing process for bolts has become increasingly perfect and stable, and the coating thickness and corrosion resistance can fully meet the anti-corrosion requirements of various mechanical equipment. However, there are still the following problems in actual production and installation of machine facilities:
1. There is a small amount of zinc residue on the bolt thread, which affects installation,
2. The influence on the connection strength is generally achieved by enlarging the machining allowance of the nut and tapping back after plating to ensure the fit between the hot-dip galvanized nut and the bolt. Although this ensures the fit of the fastener, mechanical performance testing often occurs during the tensile process, which affects the connection strength after installation.
3. The impact on the mechanical properties of high-strength bolts: Improper hot-dip galvanizing process can affect the impact toughness of bolts, and acid washing during the galvanizing process can increase the hydrogen content in the matrix of 10.9 grade high-strength bolts, increasing the potential for hydrogen embrittlement. Research has shown that the mechanical properties of the threaded parts of high-strength bolts (grade 8.8 and above) after hot-dip galvanizing have a certain degree of damage.
Mechanical galvanizing is a process that utilizes physical, chemical adsorption deposition, and mechanical collision to form a coating of metal powder on the surface of a workpiece at room temperature and pressure. By using this method, metal coatings such as Zn, Al, Cu, Zn-Al, Zn-Ti, and Zn-Sn can be formed on steel parts, providing good protection for the steel iron substrate. The mechanical galvanizing process itself determines that the coating thickness of threads and grooves is thinner than that of flat surfaces. After plating, nuts do not require back tapping, and bolts above M12 do not even need to reserve tolerances. After plating, it does not affect the fit and mechanical properties. However, the particle size of zinc powder used in the process, the feeding intensity during the plating process, and the feeding interval directly affect the density, flatness, and appearance of the coating, thereby affecting the quality of the coating.
Post time: Dec-12-2023
