Basic knowledge of fastener products

Update:10 Feb 2020
Summary:

The standards for screws are: product name (standard), […]

The standards for screws are: product name (standard), material, strength level, specifications and surface treatment
1. In terms of product type, the torque of the outer hexagonal screw is relatively large, the torque of the inner hexagonal screw is small, and the torque of the cross groove is smaller. In the product supporting use, we generally recommend that the selection of screws is one level higher than that of nuts, which is the most economical.
2. Product material grade. Here we mainly talk about the carbon steel commonly used by Hongyi Screws. According to the carbon content, we are divided into: c1008 (corresponding to 4.8), c1035 (corresponding to 8.8), c1045 (corresponding to 10.9), scm435 (corresponding to 12.9 and 45h), the higher the carbon content, the harder the material. Screws of grade 8.8 and above are high-strength screws.
3. For specifications, for example, m4x8, the outside diameter of the 4-finger teeth is 4mm, and the effective length of the 8-finger embedded object is 8mm. Generally, the countersunk head screws are loaded with the total length, and half countersunk head screws must be added with a half head. length. The pan head screw cross does not include the head size.
4. When the same material is heat-treated, the higher the hardness, the worse the toughness. Heat treatment is required for plating of grade 8.8 to above. We currently have two types of heat treatment: high-strength screws need to be quenched and tempered, that is, the hardness of the screws is uniform from the inside to the outside; self-tapping screws need carburizing heat treatment, that is, the surface of the screws is carburized with carbon, which is very hard , But the inside is very soft, if the carburizing to the inside, the screw will be burnt.
5. In general, the hardness is> 32hrc, and the plating has the risk of hydrogen embrittlement. Hydrogen embrittlement means that when the product is pickled (oil stained), h + enters the inside of the metal to form bubbles, and the screw is continuously used on the spot, and it only breaks after 24 hours. Therefore, this product with a risk of hydrogen embrittlement needs to be dehydrogenated after plating.