What is the surface treatment of self-tapping screws?

Update:02-08-2018
Summary:

Most of the self-tapping screws are covered with galvan […]

Most of the self-tapping screws are covered with galvanized or zinc phosphate. Cadmium plating is rarely used because of its high cost and toxicity. If the appearance is important, nickel or chromium plating can be selected. There are two questions about the appearance of the treatment: one is that the external treatment will not only increase the scale (such as the reduction of the hole, etc.) and the different appearance treatment will affect the torque, the degree of tightening, the connection between the strengths, so the attack In the experiment, it is necessary to do different rules according to galvanizing, cadmium plating or phosphate treatment. When the test is applied to the tapping hole diameter, the screws used must be the same external screw. The second is that high-hardness, small-scale self-tapping screws are often lost due to hydrogen embrittlement.

In order to break into the iron plate, the self-tapping screws have to be carburized. After the carburizing treatment, high hardness and high carbon content will occur, and when it is electroplated or pickled in this case, hydrogen embrittlement will occur. The method proposed in the specification to eliminate hydrogen embrittlement is: a) baking within 1 hour after plating or pickling treatment. b). Temperature 375 ~ 425 oF (190 ~ 220 oC). At least 4 hours. Hydrogen embrittlement test: The hydrogen embrittlement test is mainly to test whether the self-tapping screw is residual hydrogen in the screw arrangement and whether it will make the self-tapping screw embrittlement. (Generally, the brittle fracture does not occur in a while, and must be given The stress will be revealed.

The screws that are plated or covered should be loaded into the test iron plate and the steel Pinghua Division as shown in Table 3. The flat head of the excellent head screw head is used by the standard Pinghua Division. The Oval screw uses a phase-matched chamfered steel plate. The thickness of the flat or the distance piece attached to the underside of the head should match the maximum dead length of the screw. With regard to the long-sleeve screws, the handle can be supported by a long cold-rolled steel distance plate or a flat washer to ensure that the complete tooth portion can be combined and maintained in the thickness of the test iron plate. The screw should be locked to 80% of the maximum breaking torque. The breaking torque is determined by locking the 5 screws to break the average value. Continue to hold the lock for 24 hours, then loosen the screw and then screw it to the original lock. Tight torque, the screw should have no significant loss.