Process introduction of hot galvanizing bolt fasteners

Most hot-dip galvanized fasteners adopt solvent method. Before the purified steel parts are immersed in the plating pot, a flux layer is formed on the surface to prevent oxidation and further remove the surface residues. This flux layer is heated and volatilized during dipping, which makes the fresh steel surface directly contact with the molten metal, resulting in infiltration, dissolution, chemical reaction and diffusion, which are formed on the steel surface in sequence γ Phase δ 1 phase sum ζ A layer of molten zinc is attached to the alloyed gold layer and formed after cooling η Phase zinc coating. The content of zinc in the alloys increased gradually from 77% to 99.97%. In modern hot-dip galvanizing production, a small amount of aluminum is usually added to molten zinc to improve the adhesion of zinc coating. At this time, the main phase in the alloy layer is fe2al5, and δ 1 phase sum ζ The number of phases is very small. Non brittle γ Phase.

Hot dip galvanized coating is a kind of coating with low cost and good corrosion resistance. Due to the electrochemical characteristics of zinc, it has a "sacrificial" protective effect on the steel matrix; The corrosion product (basic zinc carbonate) of the zinc layer in the atmosphere is relatively dense, well attached and insoluble in water, which has a certain protective effect on the zinc layer. Therefore, hot-dip galvanizing is widely used to protect steel and its products from atmospheric corrosion, especially suitable for mass production of standard parts and fasteners.

It is generally considered that during hot-dip galvanizing, the formation process of zinc coating is as follows: molten zinc penetrates into the matrix iron - iron reacts with zinc to form iron zinc alloy - a pure zinc layer is formed on the iron zinc alloy layer. The basic principle of the formation of zinc coating is high-temperature metallurgical reaction. The hot-dip galvanizing of hardware and fasteners usually adopts the plating aid method (also known as the solvent method). The hot-dip galvanizing process of the plating aid method can be divided into wet method and dry method. The coating obtained by wet process has poor adhesion and is rarely used at present. The dry process allows a wide selection range of plating AIDS and good coating quality. Therefore, the hot-dip galvanizing of structural parts, fasteners and hardware generally adopts the dry process.

Hot dip galvanized fasteners have the following characteristics: ⑴ thick coating can be obtained, with good corrosion resistance. (2) the coating has good adhesion. After plating, proper forming, welding, coating and other processing can be carried out.

Due to the small size of fasteners, irregular shape, and some require thread matching, hot-dip galvanizing also has some obvious defects.

(1) it is difficult to screw in the threads after plating. After hot-dip galvanizing, the residual zinc is stuck in the thread, which is not easy to remove, and the thickness of the zinc layer is uneven, which affects the coordination of threaded parts. It is specified in gb/t13912-1992 technical requirements for hot dip galvanized coatings of iron and steel products with metal coatings and gb/t2314-1997 general technical requirements for power fittings; The external thread of fasteners shall be processed or rolled according to GB196 before hot-dip galvanizing, while the internal thread can be processed before or after hot-dip galvanizing. However, in practical application, customers often require both internal and external threads to have zinc coating, so people adopt a variety of measures to solve the problem of hot-dip galvanizing of thread fittings. Such as back tapping of threaded parts after plating; Large fit clearance shall be reserved; Centrifugation and other methods. Back tapping can easily damage the coating of the thread part, or even expose the steel matrix, causing corrosion of fasteners. It is very easy to reduce the fitting strength by deliberately enlarging the nut hole diameter or reserving the fitting clearance beyond the range, which is not allowed for high-strength fitting. At present, it is common in China to throw residual zinc by mechanical centrifugation. In addition, considering the hot-dip galvanizing process itself, adjusting the hot-dip temperature and time to improve the coating quality has not fundamentally solved the problem.

⑵ high operating temperature of hot-dip galvanizing will reduce the mechanical strength of high-strength fasteners. The strength of some threads of grade 8.8 bolts after hot-dip galvanizing is lower than the standard requirements; The strength of bolts above grade 9.8 after hot-dip galvanizing basically cannot meet the requirements.

(3) poor working environment and serious pollution. The hot-dip galvanizing process of fasteners is carried out at high temperature. When the solvent is dried and the workpiece to be plated is dipped into the bath, strong irritant hydrogen chloride gas will be released; When the zinc bath is under high temperature for a long time, zinc vapor is generated on the surface of the zinc bath, and the atmosphere of the whole working environment is bad. At present, many private enterprises still use coal-fired reflection heating furnace for hot-dip galvanizing production, which emits a large amount of smoke containing SO2, CO2, CO and dust into the atmosphere, causing serious air pollution.

Although there are many defects in the hot-dip galvanizing of fasteners, it has the advantages of thick coating, good bonding strength and good long-term corrosion effect. It has always been respected in power, communication and transportation. For example, jb/t8177-1999 general technical conditions for hot dip galvanizing layer of flange metal accessories has put forward specific requirements for hot dip galvanizing of flange terminals, and dl/t764-2002 cold heading hot dip galvanizing bolts and nuts for fastening transmission line towers and power fittings has made specific provisions for hot dip galvanizing layer on thread parts.

With the great development of electric power and transportation in China, the development of hot-dip galvanizing of fasteners will be promoted; Therefore, it is very important to develop automatic centrifugal casting equipment, improve the hot-dip galvanizing process of fasteners and improve the coating quality of hot-dip galvanized fasteners.