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What is stainless steel?

Stainless steel is one of the most common steel materials in instrument work. Understanding the knowledge of stainless steel is helpful to help the instrumenter to better grasp the instrument selection and use.

Stainless steel is the abbreviation of stainless acid-resistant steel. Weak corrosion-resistant medium such as air, steam, water or stainless steel is called stainless steel, while chemical corrosion-resistant medium (acid, alkali, salt, etc.) is called acid-resistant steel.

Stainless steel refers to steel which is resistant to weak corrosive media such as air, steam, water and chemical corrosive media such as acid, alkali and salt, also known as stainless steel. In practical application, steel resistant to weak corrosion medium is often called stainless steel, while steel resistant to chemical corrosion medium is called acid-resistant steel. Because of the difference in chemical composition, the former is not necessarily resistant to chemical medium corrosion, while the latter is generally stainless. The corrosion resistance of stainless steel depends on the alloying elements contained in the steel.

Common classification:

Usually classified according to metallographic structure:

Generally, according to the metallographic structure, ordinary stainless steel is divided into three categories: 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 iron content less than 50% have been derived for specific needs and purposes.

1. Austenitic stainless steel.

The matrix is mainly austenite structure (CY phase) with face-centered cubic crystal structure, which is non-magnetic. Stainless steel is strengthened (and possibly leads to a certain degree of magnetism) mainly by cold working. The American Iron and Steel Association uses 200 and 300 series numbers, such as 304.

2. Ferritic stainless steel.

The matrix is mainly ferrite with body-centered cubic crystal structure ((a phase). It is magnetic and generally can not be hardened by heat treatment, but cold working can make the stainless steel slightly strengthened. The American Iron and Steel Association is marked 430 and 446.

3. Martensitic stainless steel.

Stainless steel whose matrix is martensite (body-centered cubic or cubic) and magnetic can adjust its mechanical properties by heat treatment. The American Iron and Steel Association is marked with 410, 420 and 440 numbers. Martensite has austenite structure at high temperature. When it is cooled to room temperature at an appropriate rate, the austenite structure can be transformed into martensite (i.e. hardening).

4. Austenite-ferrite (duplex) stainless steel.

The matrix has austenite and ferrite two-phase structure, in which the content of less phase matrix is generally more than 15%. It has magnetism and can be strengthened by cold working. 329 is a typical duplex stainless steel. Compared with austenitic stainless steel, duplex steel has higher strength, better resistance to intercrystalline corrosion, chloride stress corrosion and pitting corrosion.

5. Precipitation hardening stainless steel.

Stainless steel whose matrix is austenite or martensite and can be hardened by precipitation hardening treatment. The American Iron and Steel Association is labeled with 600 series numbers, such as 630, or 17-4PH.

Generally speaking, in addition to alloys, austenitic stainless steel has excellent corrosion resistance. Ferritic stainless steel can be used in low corrosive environment. In mild corrosive environment, martensitic stainless steel and precipitation hardening stainless steel can be used if the material is required to have high strength or high hardness.

Post time: Apr-03-2019
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