Standards for screws include: name (standard), material, strength grade, specifications, and surface treatment.
1. In terms of product type, the torsional strength of the hex screws is relatively larger, the torque of the hex screws is smaller, and the torque of the cross slot is smaller. In supporting the use of products, it is generally recommended to use a screw level higher than the level of the nut, so that the most economical.
2, the product material grade, here mainly about the screws commonly used carbon steel, according to the level of carbon content, divided into: C1008 (corresponding to 4.8), C1035 (corresponding to 8.8), C1045 (corresponding to 10.9), SCM435 (corresponding to 12.9 Grades and 45H), the higher the carbon content, the harder the material. Screws of class 8.8 to above are all high-strength screws.
3, for specifications, such as M4x8, 4 finger tooth outer diameter is 4mm, 8 refers to the embedded object is the effective length of 8mm, the general countersunk head screw is loaded with the total length, semi-sink screw plus half head length. The Pan Head Screw Cross does not include the head size.
4, the same material for heat treatment, the higher the hardness, the worse the toughness. Heat treatment is required for plating from grade 8.8 to above. At present, there are two kinds of heat treatment: high-strength screws require quenching and tempering heat treatment, that is, the hardness of the screw from the inside to the outside are uniform; self-tapping screws need carburizing heat treatment, that is, the surface of the screw penetrates a layer of carbon, very hard, But the interior is very soft, and if it is carburized to the inside, the screw will be scorched.
5, Under normal circumstances, the hardness> 32HRC, plating hydrogen embrittlement risk. When hydrogen embrittlement is the product's pickling (oil stain), H+ enters the metal to form bubbles, and the screws are used on the spot and break off after 24 hours. Therefore, this kind of product with hydrogen embrittlement risk needs to be dehydrogenated after electroplating.