Stainless steel is an alloy composed of iron, chromium, carbon and many different elements. Iron is the main component element and chromium is the first major alloying element. Generally speaking, the chromium content must be at least 11% to be called stainless steel, because if the chromium content is insufficient, the surface of the stainless steel will not be able to form the dense chromium oxide protective film, and the function of rust prevention (corrosion) will be lost. If the content of chromium is sufficient, it will not rust in the normal temperature atmosphere.
Generally, it can be divided into five categories according to its metallographic structure:
﹝1﹞Ostian stainless steel
The most commonly seen 304 stainless steel is this type of stainless steel. Its standard composition is 18% chromium and 8% nickel, which is generally called 18-8 stainless steel. The characteristics of this type of stainless steel are non-magnetic, its metallographic structure cannot be changed by heat treatment, and its workability is good. Because it contains an alloying element-nickel, its corrosion resistance is better than that of chromium-only (nickel-free). The other two types of stainless steel.
The main uses of 300 series stainless steel are: general civilian use such as kitchen tableware, building materials, medical and health equipment, food processing materials, transportation, chemical equipment and pipe fittings, mechanical equipment and parts. In the minds of ordinary people, the so-called stainless steel refers to 300 series stainless steel (especially 304 stainless steel).
﹝2﹞Matian Sans Stainless Steel
The representative grade of this type of stainless steel is 410, and its chemical composition contains 13% chromium, less than 0.15% carbon and a small amount of other element alloys. The raw material is cheap, magnetic, hardened by heat treatment, and increased in strength. However, because it does not contain nickel, its corrosion resistance is inferior to that of ordinary 304 stainless steel, so it can only be used in mildly corrosive environments. General uses include bearings, medical appliances and cutting tools.
﹝ 3 ﹞ Fertilizer grains are stainless steel
The representative grade of this type of stainless steel is 430, its standard chemical composition is 16~18% chromium, and its carbon content is low. This type of stainless steel is magnetic, and if the stainless steel sheet we see can be attracted by a magnet, it is likely to belong to this type of stainless steel. It is less expensive because it does not contain nickel. Typical uses are kitchen utensils, netting and accessories.
﹝4﹞Precipitation hardening stainless steel
The American Iron and Steel Institute AISI number is 600 series stainless steel. In addition to chromium and nickel, this type of stainless steel also contains precipitation hardening elements such as copper, aluminum and titanium, which harden the material and increase its strength. Typical uses of this type of stainless steel are: valves, motor shafts, petrochemical equipment and golf heads.
﹝5﹞Duplex stainless steel
A typical grade of this type of stainless steel is 2205, which is 22% chromium and 5% nickel. The characteristics of this type of steel are: good strength (much better than 304) and good corrosion resistance, so it is often used in pipelines, pressure vessels and mandrels in the petrochemical industry.