What are the types of stainless steel classified by chemical composition?
Stainless steel refers to steel that does not rust in weakly corrosive media such as air and fresh water. It is mainly formed by adding a certain amount of alloying elements such as chromium (Cr) to ordinary steel to form a dense oxide film (mainly Cr₂O₃) on the surface of the steel. This film can prevent further corrosion of the inside of the steel by external media. Stainless steel generally has good corrosion resistance, high strength and toughness, and has a bright appearance and is easy to clean.
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According to the chemical composition, there are several types of stainless steel:
(1) Chromium stainless steel
With chromium (Cr) as the main alloying element, the chromium content is generally between 12% and 30%. For example, 430 stainless steel in ferritic stainless steel has a chromium content of about 16-18%. It has good corrosion resistance, especially in oxidizing environments. This is because chromium can form a dense protective film of chromium oxide (Cr₂O₃) on the surface of stainless steel, preventing oxygen and other corrosive media from further corroding the metal matrix. This type of stainless steel is relatively low in cost and is often used in the manufacture of kitchen utensils such as pots and pans.
(2) Chromium-nickel stainless steel
In addition to chromium, it also contains nickel (Ni), and the nickel content is usually around 8%-25%. A typical representative is austenitic stainless steel 304, which contains about 18% chromium and 8-10% nickel. The addition of nickel changes the crystal structure of stainless steel from body-centered cubic (ferrite structure) to face-centered cubic (austenite structure). This structure gives it good toughness, machinability and corrosion resistance, and is widely used in many fields such as architectural decoration (such as stainless steel curtain walls) and chemical equipment (such as reactors).
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(3) Chromium-manganese-nitrogen stainless steel
Manganese (Mn) and nitrogen (N) are used to partially replace nickel to stabilize the austenite structure. For example, 201 stainless steel has a lower cost than chromium-nickel stainless steel, a higher manganese content of about 7.5%, a nitrogen content of about 0.25%, and a chromium content of about 16-18%. This type of stainless steel is used in some occasions where corrosion resistance is not extremely high but cost is more sensitive, such as interior decorative strips.
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