1. Common Materials of Assembly Screws
Assembly screws are generally pre-assembled with screws, plain washers, spring washers and other accessories, and some types are matched with nuts as complete sets. The materials of assembly screws are mainly divided into carbon steel and stainless steel.
Carbon steel assembly screws are manufactured from various carbon steel wire rods, with mainstream grades including 1010, 1018 and 10B21. Among them, 10B21 boron steel wire is mostly used to produce Class 8.8 high-strength assembly screws. Class 8.8 hexagon socket assembly screws are commonly fabricated with 10B21 wire. After blank forming, overall quenching and tempering heat treatment shall be carried out. Hydrogen baking treatment must be implemented after heat treatment before electroplating, which effectively eliminates hydrogen embrittlement risks and prevents fracture failure of matched spring washers during service.
Commercially available stainless steel assembly screws are dominated by SUS304, while SUS201 stainless steel assembly screws are rarely mass-produced. Most fastener manufacturers do not routinely manufacture SUS201 assembly screws. SUS201 stainless steel wire features poor stability in plasticity; its hardness is hard to control, and cracks tend to occur easily during cold heading and stamping forming. In addition, SUS201 has relatively weak corrosion resistance, leading to insufficient overall applicability.
2. Evaluation Criteria for Electroplating Quality of Assembly Screws
The electroplating quality of assembly screws is mainly evaluated by corrosion resistance, with appearance as a secondary indicator. Corrosion resistance verification adopts corrosion tests under test conditions simulating the actual service environment of products. Electroplated finished products are controlled in the following three aspects:
Appearance Requirements No missing plating, burnt coating, rough surface, dull color, peeling, obvious stripes, pinholes, pits, accumulated plating slag, loose and cracked passivation film, coating shedding or severe passivation marks are allowed on the workpiece surface.
Coating Thickness Specifications The service life of fasteners in corrosive atmospheres is positively correlated with coating thickness. The recommended thickness range for economical electroplating is 4 μm~12 μm (0.00015 in~0.0005 in). For hot-dip galvanizing: fasteners with nominal diameter over 3/8 inch have an average coating thickness of 54 μm and a minimum thickness of 43 μm; fasteners with nominal diameter ≤ 3/8 inch have an average coating thickness of 43 μm and a minimum thickness of 37 μm.
Coating Distribution Characteristics Coating accumulation varies with different deposition processes. Electroplated zinc cannot uniformly cover the outer circle of workpieces, with thicker coatings formed at corners. For threaded sections, the thickest coating appears on thread crests, gradually thins along thread flanks, and reaches the minimum thickness at thread roots. Hot-dip galvanizing follows the opposite rule: thicker coatings deposit on inner corners and thread roots. Mechanical galvanizing has a similar coating deposition tendency to hot-dip galvanizing, yet features a smoother surface and much higher overall coating uniformity.
3. Functions, Plating Types and Salt Spray Performance of Electroplating on Assembly Screws
Most carbon steel assembly screws require electroplating after forming. Electroplating optimizes surface appearance and removes residual processing oil. More importantly, it forms dense protective coatings to pass salt spray tests and meet customers' specified corrosion resistance duration requirements.
Electroplating is classified into conventional electroplating and environmental-friendly electroplating. Names of environmental-friendly plating types are prefixed with "environmental-friendly", and all such coatings comply with environmental protection limits. Common conventional plating finishes: zinc plating (clear), black zinc, blue zinc, iridescent zinc, bright nickel, black oxide, black nickel. Common environmental-friendly plating finishes: environmental-friendly clear zinc, environmental-friendly black zinc, environmental-friendly blue zinc, environmental-friendly iridescent zinc, environmental-friendly bright nickel, environmental-friendly black oxide.
Conventional bright nickel coating generally achieves 8~10 hours of neutral salt spray resistance. If longer rust prevention is required, oil sealing can be applied on the coating surface. Environmental-friendly iridescent zinc and environmental-friendly blue zinc normally reach 48~72 hours of neutral salt spray resistance. There are no mandatory national standards specifying coating types and salt spray duration for assembly screws; all parameters are customized according to customer drawings and purchase agreements.
4. Hot-Dip Galvanizing Process for Assembly Screws
Hot-dip galvanizing of carbon steel parts is completed by immersing workpieces into molten zinc solution at approximately 510°C. Metallurgical reaction takes place between iron on the steel surface and zinc to form iron-zinc alloy layers, with a pure zinc passivation protective layer formed on the outermost surface of workpieces. Hot-dip aluminizing follows basically the same process logic as hot-dip galvanizing.
