Stainless steel refers to steel resistant to weak corrosive medium such as air, steam, water and chemical corrosion medium such as acid, alkali and salt.
Also known as stainless acid resistant steel. In practical application, steel resistant to weak corrosion medium is often called stainless steel, and steel resistant to chemical medium is called acid steel. Because of the difference in chemical composition, the former is not necessarily resistant to chemical corrosion, while the latter is generally stainless. The corrosion resistance of stainless steel depends on the alloy elements contained in the steel.
1. species
Stainless steel is often divided into martensitic steel, ferritic steel, austenitic steel and so on. In addition, it can be divided into chromium stainless steel, chrome nickel stainless steel, chromium manganese nitrogen stainless steel and so on.
1, ferritic stainless steel: chromium containing 12% to 30%. The corrosion resistance, toughness and weldability of the stainless steel increase with the increase of chromium content, and the chloride stress corrosion resistance of the stainless steel is better than that of other stainless steel.
Belonging to this category are Crl7, Cr17Mo2Ti, Cr25, Cr25Mo3Ti, Cr28 and so on. Ferritic stainless steel has good corrosion resistance and oxidation resistance because of its high chromium content, but its mechanical and technological properties are poor. Ferritic stainless steel is mainly used in acid-resistant structure with little stress and as oxidation-resistant steel. This kind of steel can resist the corrosion of atmosphere, nitric acid and brine solution, and has the characteristics of high temperature oxidation resistance, low coefficient of thermal expansion and so on. It is used in nitric acid and food factory equipment, and can also be made parts working at high temperature, such as gas turbine parts.
2. Austenitic stainless steel: containing more than 18% chromium, but also containing about 8% of nickel and a small amount of molybdenum, titanium, nitrogen and other elements. It has good comprehensive properties and can withstand multiple medium corrosion. The commonly used grades of austenitic stainless steel are 1Cr18Ni9 and 0Cr19Ni9. The wC<0.08% and steel number of 0Cr19Ni9 steel is marked as "0". This kind of steel contains a lot of Ni and Cr, which makes the steel austenite at room temperature. This kind of steel has good plasticity, toughness, weldability and corrosion resistance. It has good corrosion resistance in oxidizing and reducing medium. It is used to make acid-resistant equipment, such as corrosion-resistant container and equipment lining, transportation pipeline, nitric acid-resistant equipment parts and so on. Austenitic stainless steel is usually treated by solid solution, i.e. heating the steel to 1050-1150 C and then cooling it to obtain single-phase austenitic structure.
3. Austenitic-ferritic duplex stainless steel: both austenitic and ferritic stainless steel advantages, and has superplasticity.
Austenite and ferrite constitute about half of the stainless steel. In the case of low C content, Cr content is 18%~28% and Ni content is 3%~10%. Some steels also contain Mo, Cu, Si, Nb, Ti, N and other alloy elements. This kind of steel has the characteristics of both austenite and ferritic stainless steel. Compared with ferrite, it has higher plasticity, toughness, no room temperature brittleness, intergranular corrosion resistance and weldability, and maintains the 475 C brittleness and high thermal conductivity of ferritic stainless steel. Compared with austenitic stainless steel, it has higher strength, intergranular corrosion resistance and chloride stress corrosion resistance. Duplex stainless steel has excellent pitting corrosion resistance and is also a nickel saving stainless steel.
4, martensitic stainless steel: high strength, but poor plasticity and weldability.
The common grades of martensitic stainless steel are 1Cr13, 3Cr13 and so on. Because of its high carbon content, it has higher strength, hardness and wear resistance, but its corrosion resistance is slightly worse. It is used in some parts which require higher mechanical properties and corrosion resistance, such as springs, turbine blades, hydraulic valves and so on. This kind of steel is used after quenching and tempering.