Fastening knowledge-Screw Factory
Surface treatment types:
Surface treatment is the process of forming a covering layer on the surface of a workpiece by certain methods, the purpose of which is to endow the surface of the product with an aesthetic and anti-corrosive effect. The surface treatment methods carried out all boil down to the following methods:
1.Plating: the part to be plated is immersed in an acid solution containing the metal compound being deposited, and the plating metal is precipitated and deposited on the part by passing an electric current through the plating solution. General plating includes galvanising, copper, nickel, chromium, copper-nickel alloys, etc. Sometimes blackening (bluing) and phosphating are also included.
2.Hot dip galvanising: This is done by immersing carbon steel parts in a bath of dissolved zinc at a temperature of approximately 510°C. The result is a surface coating of zinc on the steel part. The result is a gradual transformation of the iron-zinc alloy on the surface of the steel part into passivated zinc on the external surface of the product. Hot dip aluminium plating is a similar process.
3.Mechanical plating: the surface of the product is impacted by particles of plated metal and the coating is cold welded to the surface of the product.
Generally screws are mostly electroplated, but hexagonal wood screws used in electricity, highways and other outdoor areas are hot dipped in zinc; the cost of electroplating is generally 0.6 - 0.8 RMB per kg, while hot dipped zinc is generally 1.5 - 2 RMB per kg, which is more costly.
Plating effectiveness: The quality of plating is measured mainly by its corrosion resistance, followed by its appearance. Corrosion resistance is the ability to imitate the product's working environment and set it up as a test condition for corrosion testing. The quality of the plated product is controlled in the following ways:
1.Appearance:
The surface of the product is not allowed to have local no plating, burnt, rough, grey, peeling, crusty condition and obvious streaks, no pinhole pockmark, black plating slag, loose passivation film, cracking, peeling and serious passivation traces.
2.Plating thickness:
The operating life of fasteners in a corrosive atmosphere is directly proportional to the thickness of its plating. The generally recommended economic plating thickness is 0.00015in to 0.0005in (4 to 12um).
Hot dip galvanizing: the standard average thickness is 54um (43um for appellation diameter ≤ 3/8) and the minimum thickness is 43um (37um for appellation diameter ≤ 3/8).
3.Plating distribution:
Using different deposition methods, the way the plating is gathered on the fastener surface is different. Plating metal is not uniformly deposited on the peripheral edges when plating, and thicker plating is obtained at the corners. In the threaded part of the fastener, the thickest layer is located at the top of the threaded tooth and gradually thins out along the side of the thread, with the thinnest deposition at the bottom of the tooth, while hot dip galvanising is the opposite, with thicker layers deposited at the inner corners and at the bottom of the thread.
4.Hydrogen embrittlement:
During the processing and handling of the fastener, especially during the pickling and alkaline washing before plating and the subsequent plating process, the surface absorbs hydrogen atoms and the deposited metal coating then captures the hydrogen. When the fastener is tightened, the hydrogen is transferred towards the part of the fastener where the stress is most concentrated, causing the pressure to increase beyond the strength of the base metal and producing a small surface rupture. The hydrogen is particularly active and quickly penetrates into the newly formed fracture. This pressure-rupture-infiltration cycle continues until the fastener fractures. This usually occurs within a few hours of the first stress application.
To eliminate the threat of hydrogen embrittlement, fasteners are heated and baked as quickly as possible after plating to allow the hydrogen to leach out of the coating, with baking typically taking place at 375-4000F (176-190°C) for 3-24 hours.
As mechanical galvanising is non-electrolytic, this virtually eliminates the threat of hydrogen embrittlement.