Ferritic stainless steel (chromium steel, such as 400 series)
Stainless steel, which is mainly ferritic in service, has a body-centered cubic crystal structure with a CR mass fraction of 11% – 30%. Compared with austenitic stainless steel, this kind of steel generally does not contain Ni, and sometimes contains a small amount of Mo, Ti, Nb, and other elements. The application of Argon Oxygen Decarburization and Vacuum Oxygen Decarburization can greatly reduce the elements such as C and N. It has the advantages of Ni saving, low price, good stress corrosion resistance, and good oxidation resistance. It is mostly used in environments resistant to atmospheric, steam, water vapor, and oxidizing acid corrosion, It has certain corrosion resistance in dilute nitric acid and weak organic acid at room temperature, so this kind of steel is widely used. However, it is not resistant to reducing acid and other media corrosion. The general ferritic stainless steel has some disadvantages, such as poor impact toughness, poor plasticity and corrosion resistance after welding, sensitivity to intergranular corrosion, poor pitting corrosion resistance, and more, which limits its use.
Austenitic stainless steel (chromium-nickel steel, such as 300 series and 200 series)
Austenitic stainless steel is widely used in various pressure components of pressure vessels because of its non-magnetic properties, high strength, toughness, and ductility in a wide temperature range, easy cold processing such as rolling and pressing, and resistance to oxidation medium corrosion. Because austenitic stainless steel has a face-centered cubic structure, it does not undergo phase transformation and is easy to weld. Austenitic stainless steel generally has no cold brittle transition temperature, so it is often used as low-temperature steel.
18-8 series solid solution stainless steel has good corrosion resistance in oxidizing acid, atmosphere, water, steam, and other media, and the intergranular corrosion resistance of low carbon or stabilized element Ti or Nb is better, and the pitting corrosion resistance of Mo is better. Those containing Mo, Cu, and other elements can also resist the corrosion of nonoxidizing acids such as dilute sulfuric acid and phosphoric acid and organic acids such as formic acid and acetic acid. However, the pitting corrosion resistance is poor, and stress corrosion cracking is easy to occur in halide solution. Creep and sensitization will occur after long-term use at temperatures above 500 ℃ -600 ℃.