Definition and classification of stainless steel

One definition
Stainless steel refers to the general term for steel with certain chemical stability in the atmosphere, water, acid, alkali and salt solutions, or other corrosive media. Generally speaking, steel that is resistant to corrosion by weak media such as atmosphere, steam and water is called stainless steel, and steel that is resistant to corrosive media such as acid, alkali and salt is called corrosion-resistant steel, or acid-resistant steel. Stainless steel has rust resistance, but not necessarily corrosion resistance, while corrosion-resistant steel generally has good rust resistance.

The good corrosion resistance of stainless steel is due to the addition of chromium to the iron-carbon alloy. Although other elements, such as copper, aluminum, and silicon, nickel, and molybdenum, can also improve the corrosion resistance of steel, the effect of these elements is limited without the presence of chromium. Therefore, chromium is the most important element in stainless steel. The minimum chromium content required for stainless steel with good corrosion resistance depends on the corrosive medium.

The American Iron and Steel Institute? (AISI) uses 4% chromium as the boundary between stainless steel and other steels. According to the Japanese Industrial Standard JIS G 0203, the so-called stainless steel is an alloy steel containing chromium or chromium-nickel for the purpose of improving corrosion resistance. Generally, the chromium content is greater than 11%. German DIN standard and European standard EN10020 stipulate that the chromium content of stainless steel is not less than 10.5%, and the carbon content is not more than 1.2%. my country generally sets the chromium content of stainless steel to be no less than 12%.

The corrosion resistance of stainless steel is generally considered to be the result of a “passivation film” formed on its surface under the action of corrosive media, and the corrosion resistance depends on the stability of the “passivation film”. This is not only related to the chemical composition of stainless steel, but also related to the type, concentration, temperature, pressure, flow rate, and other factors of the corrosive medium.

Two categories

There are many types of stainless steel. According to China’s national standard GB/T13304-1991 “Steel Classification” and the international general classification method is divided into five categories according to the metallographic structure of steel, namely, austenitic stainless steel, austenite- Ferritic stainless steel, ferritic stainless steel, martensitic stainless steel, and precipitation hardening stainless steel.

In addition, according to the alloying elements in steel, it can be divided into chromium series stainless steel, chromium nickel series stainless steel, chromium nickel molybdenum series stainless steel, chromium manganese nickel (nitrogen) series stainless steel, and low carbon stainless steel, ultra low carbon stainless steel and high purity stainless steel. .

Stainless steel has a wide range of uses, and can be divided into nitric acid resistant stainless steel, sulfuric acid resistant stainless steel, pitting corrosion resistant stainless steel, stress corrosion resistant stainless steel, low temperature stainless steel, non-magnetic stainless steel, high strength stainless steel, easy to cut Stainless steel, and some special stainless steel, etc. Certain stainless steels can have multiple characteristics mentioned above.