Basic Knowledge of Stainless Steel

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Stainless Steel refers to steel that is resistant to corrosion by weak corrosive agents such as air, steam, and water, as well as by chemically aggressive substances such as acids, alkalis, and salts; it is also known as rust-resistant and acid-resistant steel. Steel grades that are resistant to weak corrosive agents such as air, steam, and water, or that possess rust resistance, are called stainless steel ； Steel grades that can resist chemical corrosive agents such as acids, alkalis, and salts are called acid-resistant steel. Due to the differences in their chemical composition, the former is not necessarily resistant to corrosion by chemical agents, whereas the latter generally possesses rust resistance. The corrosion resistance of stainless steel depends on the alloying elements contained in the steel. Generally, based on their microstructure, ordinary stainless steels are classified into three categories: austenitic stainless steels, ferritic stainless steels, and martensitic stainless steels. Based on these three basic microstructural types, dual-phase steels, precipitation-hardening stainless steels, and high-alloy steels with an iron content of less than 50% have been developed to meet specific requirements and purposes. Classified by microstructure: 01 Austenitic stainless steel. A stainless steel in which the matrix is primarily composed of austenite structure (CY phase) with a face-centered cubic crystal structure, is non-magnetic; it is strengthened mainly through cold working (which may also result in some magnetism). The American Iron and Steel Institute uses numbers from the 200 and 300 series to identify them, such as 304. 02 Ferritic stainless steel. It is a stainless steel in which the matrix is primarily composed of ferrite structure (phase a) with a body-centered cubic crystal structure; it is magnetic and generally cannot be hardened through heat treatment, but can be slightly strengthened by cold working. The American Iron and Steel Institute uses 430 and 446 as designations. 03 Maraging stainless steel. Stainless steel with a martensitic structure (body-centered cubic or cubic), which is magnetic and whose mechanical properties can be adjusted through heat treatment. The American Iron and Steel Institute uses the numbers 410, 420, and 440 to designate them. Martensite has an austenitic structure at high temperatures; when cooled to room temperature at an appropriate rate, the austenitic structure can transform into martensite (i.e., hardening). 04 Austenite-ferrite (duplex) stainless steels. The matrix consists of both austenite and ferrite phases; in materials with a smaller proportion of one of these phases, the content of the matrix is generally greater than 15%. It is a stainless steel that is magnetic and can be strengthened through cold working, and 329 is a typical example of a duplex stainless steel. Compared to austenitic stainless steels, duplex steels have higher strength, as well as significantly improved resistance to intergranular corrosion, chloride stress corrosion, and pitting corrosion. 05 Precipitation-hardening stainless steel. Stainless steel with an austenitic or martensitic structure that can be hardened through precipitation hardening. The American Iron and Steel Institute uses numbers from the 600 series to identify them, such as 630, which corresponds to 17-4PH. Generally speaking, aside from alloys, austenitic stainless steels exhibit relatively excellent corrosion resistance. In environments with low corrosivity, ferritic stainless steels can be used; in mildly corrosive environments, if high strength or hardness is required from the material, martensitic stainless steels and precipitation-hardening stainless steels can be employed.