Stainless Steel: Understanding the Different Types and Their Applications

Stainless steel, an abbreviation for "stainless acid-resistant steel," is a type of steel that is resistant to corrosion by air, steam, water, and other weak corrosive media. It is also known as stainless steel if it is resistant to corrosion by chemical corrosive media (acids, alkalis, salts, and other chemical impregnations). Steel that is only resistant to corrosion by weak corrosive media is often called stainless steel, while steel that is resistant to corrosion by chemical media is called acid-resistant steel. This distinction is made because the two types of steel have different chemical compositions. Stainless steel is not necessarily resistant to chemical media corrosion, while acid-resistant steel is generally stainless. The corrosion resistance of stainless steel depends on the alloying elements contained in the steel.

Common Types of Stainless Steel

Stainless steel is typically divided into three categories based on its metallurgical organization: austenitic stainless steel, ferritic stainless steel, and martensitic stainless steel. These three categories can be further divided into duplex steel, precipitation hardening stainless steel, and high alloy steel containing less than 50% iron, depending on specific needs and purposes.

Austenitic Stainless Steel: Austenitic stainless steel is a type of stainless steel with a face-centered cubic crystal structure (gamma phase) and is non-magnetic. It is primarily strengthened through cold working (and may lead to a certain degree of magnetism). Austenitic stainless steel is designated by the American Iron and Steel Institute using the 200 and 300 series of numerical labels, such as 304.

Ferritic Stainless Steel: Ferritic stainless steel is a type of stainless steel with a body-centered cubic crystal structure (alpha phase) and is magnetic. It cannot be hardened by heat treatment, but it can be slightly strengthened by cold working. Ferritic stainless steel is designated by the American Iron and Steel Institute using the 430 and 446 series of numerical labels.

Martensitic Stainless Steel: Martensitic stainless steel is a type of stainless steel with a martensitic (body-centered cubic or cubic) crystal structure and is magnetic. Its mechanical properties can be adjusted through heat treatment. Martensitic stainless steel is designated by the American Iron and Steel Institute using the 410, 420, and 440 series of numerical labels. Martensitic steel can be transformed into martensite (i.e., hardened) by cooling austenitic steel to room temperature at an appropriate rate after heating it to a high temperature.

Austenitic-Ferritic (Duplex) Stainless Steel: Duplex stainless steel is a type of stainless steel with a matrix that contains both austenite and ferrite phases, with the content of the lesser phase matrix generally being greater than 15%. It is magnetic and can be strengthened by cold working. 329 is a typical example of duplex stainless steel. Compared with austenitic stainless steel, duplex steel has significantly improved strength, resistance to intergranular corrosion, and resistance to chloride stress corrosion and pitting corrosion.

Precipitation Hardening Stainless Steel: Precipitation hardening stainless steel is a type of stainless steel with an austenitic or martensitic matrix that can be hardened by precipitation hardening treatment. Precipitation hardening stainless steel is designated by the American Iron and Steel Institute using the 600 series of numerical labels, such as 630 (17-4PH).