Duplex Stainless Steels
Introduction
Duplex stainless steel refers to stainless steel whose microstructure is composed of austenitic phase and ferritic phase. Usually, the two phases each account for about half of the volume fraction at room temperature. Even if one of the phases has a low volume fraction, it will not be less than 30%. The duplex stainless steel has low element content of C, 18-28% Cr, 3-10% Ni, and some contain alloy elements such as Mo, Ti and Nb.
Because of combination of ferritic and austenitic microstructures, the duplex stainless steel can obtain the performance advantages of austenitic stainless steel and ferritic stainless steel at the same time, integrating many advantages in one. These superior properties make the very rapid development of duplex stainless steel and make duplex stainless steel applicable to many industrial fields.
Classification & Property & Application
- Duplex stainless steel can be divided into the following five types according to the pitting resistance number (PREN). (1) Lean duplex stainless steels (LDX): The alloy is characterized by low alloy content without Mo and other precious metals, and its PREN value is 22-26. (2) Standard duplex stainless steel: The alloy belongs to medium alloy type with PREN value of 33-36; (3) Advanced duplex stainless steel: The alloy belongs to high alloy type with increased chromium content of 25%, and contains a certain amount of alloy elements such as Mo, W, N, Cu. Its PREN value is 37-39. (4) Super duplex stainless steel: The alloy contains high molybdenum and high nitrogen, as well as trace Cu, W. Its PREN value is 40-45. (5) Hyper duplex stainless steels (HDSSs): The alloy contains higher alloy content than super duplex stainless steels, and its PREN value exceeds 45.
- Duplex stainless steel has good comprehensive properties, the details are as follows: (1) Good comprehensive mechanical properties; (2) Excellent pitting corrosion resistance and chloride stress corrosion resistance; (3) The large thermal conductivity and small linear expansion coefficient compared with austenitic stainless steel; (4) The cold-working performance and cold-forming performance are much better than that of ferritic stainless steel; (5) High chromium (≥25%) duplex stainless steel has large thermal deformation resistance; (6) Good welding performance and small hot cracking tendency without preheating before welding and without heat treatment after welding. In a word, the duplex stainless steel has the properties of high strength, high toughness, excellent corrosion resistance and excellent welding. However, it has use limitations at working conditions higher than 300 ℃ , and presence of medium-temperature brittle zone.
- At present, the application fields of duplex stainless steel mainly include: (1) Petrochemical and chemical industry: this field is easy to cause stainless steel stress corrosion failure, therefore, the common use is super duplex stainless steel; (2) Neutral chloride environment: duplex stainless steel can be widely used in those structures that are more sensitive to chloride corrosion, such as bridges, ships, etc.; (3) Pulp and paper industry: duplex stainless steel has been used in wood chip presteam, pulp digester, pressure drum machine and recovery equipment in pulp and paper industry, and has achieved good results; (4) Chemical fertilizer industry.
As a leading supplier of metals and alloys, Alfa Chemistry now can provide various duplex stainless steel in the shapes of bar, wire, strip, foil, pipe, tube, sheet, plate, forgings, pipe fittings, flanges, etc. The main products include: