Stainless steel is clearly defined in the national standard. Generally speaking, stainless steel refers to a general term for a type of steel with certain chemical stability in the atmosphere, steam, water or corrosive media such as acid, alkali and salt, which is called stainless steel. The steel that is resistant to weak corrosive media such as atmosphere, water and steam is called stainless steel, and the steel that is resistant to strong corrosive media such as acid, alkali and salt is called corrosion-resistant steel or acid-resistant steel. Stainless steel has stainless steel, but it is not necessarily corrosion-resistant, corrosion-resistant steel it has good rust resistance. The main reason why stainless steel has good stainless steel is that the "chromium" element is added to the iron-carbon alloy, of course, although other alloying elements such as nickel, molybdenum, silicon and other alloying elements also have good stainless steel, but if the chromium element is missing, then the role of these elements will be relatively limited, so chromium is the most important element in stainless steel.
The chromium required for stainless steel with good corrosion resistance depends on the corrosive medium, and the state has some clear regulations on the chromium content between ordinary carbon steel and stainless steel. The American Iron and Steel Association AISI takes 4% chromium as a boundary between ordinary carbon steel and stainless steel, the Japanese industrial standard JIS stipulates that chromium is basically more than 11%, and the German DIN standard defines the chromium content between ordinary carbon steel and stainless steel at 105%, our Chinese national standard defines the chromium content between carbon steel and stainless steel, which is different from ordinary carbon, to 12%.
The corrosion resistance of stainless steel is generally considered to be the result of the chromium "passivation film" formed under the action of chromium in the corrosive medium, and the corrosion resistance depends on the stability of the chromium passivation film. This is not only related to the chemical composition of stainless steel, but also to the type of corrosion, medium, environment, temperature ......There is some relationship.
The main steel grades are as follows:
320hq（0crl8ni9cu3）、sus304（0crl8ni9）、 304m304j3（304hc）、 316（0crl7nil2m02）、 316l（00crl7nil4m02）
302hq：Low carbon, low nitrogen, low sulfur, very low work hardening rate, excellent cold room processability, suitable for applications with complex shapes and high molding difficulty.
The work hardening rate is moderate, suitable for general cold machining and stretching, and the cold working performance is good.
304m：Medium work hardening rate, suitable for general cold room machining and stretching.
304hc：Copper is added instead of nickel to reduce the work hardening rate of steel and maintain low magnetic permeability.
sus316：Molybdenum for better resistance to potting and corrosion.
sus316l：Carbon substitution, better corrosion resistance than 316, better cold workability.
The above materials are ranked in order of magnetism from strong to weak: 304 304M 304HC SUS316
All three grades are 300 series austenitic stainless steels with the following chemical compositions:
Thread:
When assembling, drill and tap the teeth on the assembly first, and the internal teeth tapped, which are consistent with the external teeth of the screws, and use a small torque for assembly.
Self-tapping thread:
When assembling, drill holes in the assembly first, without tapping the internal teeth, and use a large torque for assembly.
Self-drilling thread:
Used directly on the assembly, the screw is drilled and tapped.
As long as the corrosion is more serious or the place where it is not easy to rust is needed, stainless steel screws are used, such as stainless steel bolts are used more in petrochemical enterprises, and the other is the electronics industry, such as screws on computers, screws in household appliances, screws on stainless steel locks, screws on stainless steel products, screws on aluminum alloy bicycles, and stainless steel screws are used in aerospace, nuclear power and other industries.