How to treat the surface of the self-locking tuning screw after processing?

As a precision industrial fastener, the surface treatment of self-locking tuning screws should take into account the preservation of corrosion resistance, lubricity, wear resistance, insulation (in some scenes) and self-locking performance. The specific process needs to be selected in combination with the use scenes (such as aerospace, communication equipment, automotive electronics, etc.) and materials (carbon steel, stainless steel, titanium alloy, etc.). The following are the mainstream surface treatment schemes and process flows:

Anti-corrosion treatment of foundation

This kind of treatment is mainly used to improve the anti-rust ability of screws, and it is suitable for ordinary industrial environment or scenes with basic requirements for anti-corrosion.

Galvanizing passivation

This is the most commonly used basic treatment process for self-locking tuning screws made of carbon steel. Firstly, a uniform zinc coating is formed on the surface of the screw by electroplating zinc, and the thickness of the coating is generally controlled at 6-12 μ m; After that, passivation treatment (color passivation or black passivation) is carried out to form a passivation film on the surface of the zinc layer, which greatly improves the corrosion resistance, and the neutral salt spray test can reach 72-120 hours. The cost of this process is low, and the thread accuracy of the screw can be maintained without affecting the cooperation of the self-locking structure, but the thickness uniformity of the coating should be paid attention to to avoid the failure of thread occlusion.

Dacromet treatment

Also known as zinc-chromium coating, it is suitable for scenes with high anti-corrosion requirements and no hydrogen embrittlement (such as high-strength carbon steel screws). The process is that the screws are immersed in dacromet coating solution, and then baked and solidified to form an inorganic coating containing zinc, aluminum and chromium, and the thickness is usually 4-8 μ m. Its corrosion resistance is far better than that of ordinary zinc plating (neutral salt spray can reach more than 500 hours), and it is resistant to high temperature and has no risk of hydrogen embrittlement. However, the hardness of the coating is low, which is not suitable for high wear-resistant scenes. At the same time, the thickness of the coating on the thread self-locking surface should be controlled to prevent the self-locking torque from being affected.

Lubrication and anti-loosening strengthening treatment

The core function of self-locking tuning screw is anti-loosening, so some surface treatments will be combined with lubrication or anti-loosening coating, which not only improves performance but also ensures self-locking effect.

Nickel plating+locking glue coating

Firstly, the screw is electroplated with nickel. The thickness of the nickel coating is 8-15μm, which has good corrosion resistance and conductivity and is suitable for the field of electronic communication. After that, pre-coated locking glue (such as nylon glue and anaerobic glue) is applied to the self-locking section of the thread, and the thickness of the glue layer needs to be accurately controlled (generally 0.05-0.15mm). After curing, an elastic locking structure is formed, which can not only enhance the self-locking and locking ability, but also reduce the friction coefficient when the thread is matched, so as to avoid slipping teeth in the locking process. It should be noted that the adhesive layer should cover the effective contact area of the self-locking thread and cannot block the through hole or positioning structure of the screw.

Sulphurizing treatment

Mainly used for screws made of stainless steel or alloy steel, a layer of FeS lubrication layer with a thickness of 0.5-2μm is formed on the thread surface through low-temperature sulphurization. The coating can significantly reduce the friction coefficient between threads (the friction coefficient can be reduced to 0.08-0.12), improve the torque stability during locking, and maintain the corrosion resistance and hardness of the metal matrix, so it is suitable for high-precision and high-torque locking scenes (such as aerospace equipment). After sulphurizing, hydrogen should be removed to prevent hydrogen embrittlement and cracking.

Special function class processing

For scenes with special requirements such as insulation, high temperature resistance and high wear resistance, special surface treatment technology is needed.

Anodizing+hole sealing

Self-locking tuning screw suitable for aluminum alloy material. Firstly, anodic oxidation is carried out to form a porous alumina film (thickness 5-20μm) on the surface, and then the pores of the film are filled by hole sealing treatment (hot water sealing or nickel salt sealing) to improve corrosion resistance and insulation. The insulation resistance can reach more than 10 8 Ω, which is suitable for electronic insulation scenes. If higher wear resistance is needed, hard anodic oxidation can be carried out after oxidation, and the hardness of the film can reach more than HV300, but the influence of the thickness of the oxide film on the thread matching accuracy should be paid attention to.

Silver plating treatment

Used in high conductivity and high temperature resistant scenes (such as tuning screws in RF communication equipment). The thickness of silver coating is generally 5-10μm, which has excellent electrical conductivity, thermal conductivity and lubricity, and can maintain stable performance in high temperature environment above 200℃. After silver plating, anti-tarnishing treatment (such as passivation or coating anti-tarnishing agent) should be carried out to prevent the silver layer from oxidation and blackening in wet environment, which will affect the conductivity and appearance.

General process precautions

Before surface treatment, it is necessary to thoroughly clean the screws to remove the oil stain, iron filings and scale left after processing, so as to avoid affecting the adhesion of the coating or coating;

For high-strength carbon steel screws, dehydrogenating treatment (baking at 200-230℃ for 2-4 hours) is required after electroplating to prevent the screw from breaking due to hydrogen embrittlement;

All treatment processes need to control the coating/coating thickness of the threaded part to ensure that the fit clearance of the self-locking structure meets the design requirements. After treatment, self-locking torque test and salt spray test are needed to verify whether the performance meets the standard.