Today's advanced manufacturing represented by large aircraft, large-scale power generation equipment, automobiles, high-speed trains, large ships, and large-scale complete sets of equipment has entered an important development direction. As a result, the fasteners will enter an important stage of development. High-strength bolts are used for important mechanical connections, and repeated disassembly and assembly or various mounting torque methods are extremely demanding for high-strength bolts. Therefore, the surface condition and thread accuracy will directly affect the service life and safety of the mainframe. In order to improve the friction coefficient and avoid rust, seizure or seizure during use, the technical requirements stipulate that the surface should be nickel-phosphorus plated. The thickness of the coating is guaranteed to be in the range of 0.02 to 0.03 mm, and the plating layer is uniform, dense, and free of pinholes.
The bolt material is: 18Cr2Ni4W, 25Cr2MoV steel; bolt specifications: M27 ~ M48. Since the steel is easy to form a passivation film on the surface, and the passivation film will make the bolt unable to obtain a chemical nickel-phosphorus layer with good adhesion, special pre-treatment measures must be taken to remove the film first, and measures should be taken. Preventing it from regenerating can ensure a good bond between the plated coating and the substrate. At the same time, due to the large geometrical size of the bolt, it is difficult to detect the quality of the nickel-phosphorus plating process and the process.
Second, the process flow and quality control points
High-strength bolt nickel-phosphorus plating process Nickel-phosphorus plating consists of three parts. The first part is the pre-treatment process, including the precision and appearance inspection of high-strength bolt plating, manual degreasing, soaking and degreasing, pickling, electroactivation and Flash nickel plating and other processes; the second part of the electroless nickel plating process; the third part is the post-treatment process, including hydrogen flooding heat treatment, polishing and finished product inspection. As follows: bolt chemical composition inspection â†’ bolt plating accuracy, appearance inspection â†’ manual degreasing â†’ appearance inspection â†’ soaking and degreasing â†’ hot water washing â†’ cold water washing â†’ pickling â†’ cold water washing â†’ electric activation â†’ cold water washing â†’ flash nickel plating â†’ cold water washing â†’ Deionized water washing â†’ electroless nickel plating â†’ deionized water washing â†’ cold water washing â†’ hydrogen driving â†’ polishing â†’ finished product inspection.
2.2 pre-treatment process
The pretreatment process is a key process to determine the quality of high-strength bolt nickel-phosphorus plating. The purpose of this process is to remove the passivation layer on the surface of the bolt and prevent the regeneration of the passivation film. The execution status of this process directly determines the degree of bonding between the substrate and the coating. Most of the quality accidents that occurred during production were caused by poor pre-treatment of Lu. Before the plating, the oil, rust and scale attached to the surface of the bolt must be carefully removed; the difference from the plating should be checked more carefully, and the bolts that are not cleaned are absolutely not allowed to be plated.
1 bolt inspection; visual inspection of the bolt surface quality, requires that any machining burrs must be removed, sharp edges and edges must be rounded. 2 Manual degreasing; ensure that the surface of the substrate is free of oil. 3 Soak and remove oil; put the bolt into alkaline water to remove surface oil. 4 pickling; in order to prevent the alkaline degreasing solution from contaminating the flash nickel plating bath, the acid washing solution is used for electroactivation treatment before flash nickel plating. 5 electroactivation; electroactivation treatment with an acid solution. 6 flash nickel plating; flash alloy nickel should be used for low alloy steel to increase the bonding strength between the coating and the substrate.
2.3 Electroless nickel plating
The electroless nickel plating solution requires a service life of two months, a use temperature of 75 to 90 Â° C, and a pH of 7 to 9. 1 temperature; the plating temperature has a great influence on the deposition rate and the phosphorus content. With the increase of temperature, the deposition rate of the plating solution increases exponentially and the phosphorus content of the coating decreases slightly. The effect of 2PH value; as the pH value increases, the deposition rate increases, and the phosphorus content in the coating decreases. Conversely, when the deposition rate decreases, the phosphorus content increases. The amount of phosphorus in the coating determines the physical properties and corrosion resistance of the coating. The phosphorus content is high, and the corrosion resistance of the plating layer is better.
2.4 post-treatment process
The nickel-phosphorus plating treatment includes two main processes of hydrogen driving and polishing.
1 flooding; according to the relevant standards, the hydrogen drive temperature after plating is 200 Â± 10 Â° C, the treatment time is 2 h. 200 Â°C is beneficial to eliminate hydrogen embrittlement, relax internal stress, improve the bonding force between the coating and the substrate, and improve the corrosion resistance of the coating. 2 polishing; polished bolts look bright, but in order to better improve the quality of the coating, flatten the tiny traces, get a shiny mirror-like surface, polishing with a polishing machine.
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