Stainless steel knowledge, thoroughly explain the 300 series

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Stainless Steel is the abbreviation of stainless acid-resistant steel. Steel types that are resistant to weak corrosive media such as air, steam, and water or are stainless are called stainless steel; and steel that is resistant to chemical corrosive media (acid, alkali, salt, etc.) The type of steel that corrodes is called acid-resistant steel.

Stainless steel refers to steel that is resistant to corrosion by weakly corrosive media such as air, steam, and water and chemically corrosive media such as acids, alkali, and salts. It is also called stainless acid-resistant steel. In practical applications, steel that is resistant to corrosion by weakly corrosive media is often called stainless steel, while steel that is resistant to corrosion by chemical media is called acid-resistant steel. Due to the difference in chemical composition between the two, the former is not necessarily resistant to corrosion by chemical media, while the latter is generally stainless. The corrosion resistance of stainless steel depends on the alloying elements contained in the steel. Usually, ordinary stainless steel is divided into three categories according to its metallographic structure: austenitic stainless steel, ferritic stainless steel, and martensitic stainless steel. On the basis of these three basic metallographic structures, duplex steels, precipitation hardening stainless steels and high alloy steels with an iron content of less than 50% are derived for specific needs and purposes.

According to the metallographic structure, it is divided into:

Austenitic stainless steel

The matrix is mainly austenite structure (CY phase) with a face-centered cubic crystal structure. It is non-magnetic and is mainly strengthened by cold working (and may lead to certain magnetism). The American Iron and Steel Institute uses numbers in the 200 and 300 series, such as 304.

ferritic stainless steel

The matrix is mainly a ferrite structure ((a phase) with a body-centered cubic crystal structure. It is magnetic and generally cannot be hardened by heat treatment, but cold working can make it slightly strengthened. The American Iron and Steel Institute is labeled 430 and 446.

martensitic stainless steel

The matrix is a martensite structure (body-centered cubic or cubic), which is magnetic and its mechanical properties can be adjusted through heat treatment. The American Iron and Steel Institute uses numbers 410, 420 and 440. Martensite has an austenite structure at high temperatures. When cooled to room temperature at an appropriate rate, the austenite structure can transform into martensite (that is, hardened).

Austenitic-ferritic (duplex) stainless steel

The matrix has both austenite and ferrite two-phase structures, of which the content of the smaller phase matrix is generally greater than 15%. It is a stainless steel that is magnetic and can be strengthened by cold working. 329 is a typical duplex stainless steel. Compared with austenitic stainless steel, duplex steel has high strength, and its resistance to intergranular corrosion, chloride stress corrosion and pitting corrosion is significantly improved.

Precipitation hardened stainless steel

Stainless steel whose matrix is austenite or martensite and can be hardened through precipitation hardening. The American Iron and Steel Institute uses 600 series numbers, such as 630, which is 17-4PH.

Generally speaking, in addition to alloys, austenitic stainless steel has excellent corrosion resistance. In less corrosive environments, ferritic stainless steel can be used. In mildly corrosive environments, if the material is required to have high For strength or high hardness, martensitic stainless steel and precipitation hardened stainless steel can be used.