Surface treatment of self – tapping screws: most self – tapping screws are coated with zinc or zinc phosphate.Cadmium plating is rarely used because of its high cost and toxicity. If appearance is very important, nickel or chromium plating can be selected.

One is for

Surface treatment will not only increase the size (such as hole reduction, etc.), but also different surface treatment will affect the relationship between torque, tightening degree and strength. Therefore, different rules should be followed for zinc plating, cadmium plating or phosphate treatment when conducting the penetration experiment.When testing the suitable tapping hole diameter, the screws used must be of the same appearance.

The second is

High hardness, small scale self – tapping screws often fail due to hydrogen embrittlement.In order to break into the iron plate, the self-tapping screw shall not be carburized.High hardness and high carbon content will occur after carburizing treatment, and hydrogen embrittlement will occur when electroplating or pickling treatment in this situation.The recommended methods for eliminating hydrogen embrittlement are :a). Baking within 1 hour after electroplating or pickling. B).Temperature 375 ~ 425 oF(190 ~ 220 oC). Time at least 4 hours.

Hydrogen embrittlement test

Tapping screw hydrogen embrittlement test is mainly to test whether residual hydrogen in the screw arrangement and whether will make the embrittlement, self tapping screw attack (generally not soon attack, brittle fracture and should be giving must stress will emerge) plating or lie to the rules of the screw should be loaded as table 3 of the test plate and steel smooth screw, outstanding head type screw head bearing surface using the standard flat washers, countersunk type (Oval) screws using a combination of the chamfer distance sheet steel.The thickness of the washers or distance plate installed on the underside of the head shall match the maximum invalid tooth length of the screw.