duplex stainless steel

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Duplex stainless steel refers to stainless steel that possesses both austenite and ferrite microstructural phases. Generally, duplex steel refers to a structure consisting of martensite (bainite) along with a ferritic matrix. Martensite is distributed in islands among the ferrite grains; that is, a structure consisting of 10% to 20% martensite along with ferrite. This steel features low yield strength, high elongation, and a high rate of strain hardening, which makes it suitable for cold drawing; its strength can be increased through cold working, while it also retains good plasticity and toughness. The so-called duplex stainless steel has a microstructure in which ferrite and austenite phases each make up half of it; generally, the minimum content of either phase is around 30%. Due to the characteristics of the two-phase structure, by properly controlling the chemical composition and heat treatment processes, DSS combines the advantages of ferritic stainless steels and austenitic stainless steels. 1. Compared with austenitic stainless steels, the advantages of duplex stainless steels are as follows: (1) Their yield strength is more than twice that of ordinary austenitic stainless steels, and they possess sufficient plasticity and toughness for shaping. Using duplex stainless steel to manufacture the walls of storage tanks or pressure vessels reduces the wall thickness by 30-50% compared to conventional austenitic steels, which helps to lower costs. (2) It possesses excellent resistance to stress corrosion cracking; even the duplex stainless steels with the lowest alloy content have a higher resistance to stress corrosion cracking than austenitic stainless steels, especially in environments containing chloride ions. Stress corrosion is a prominent problem that is difficult to address in conventional austenitic stainless steels. (3) The corrosion resistance of 2205 duplex stainless steel, which is the most widely used in many media, is superior to that of conventional 316L austenitic stainless steel. Super duplex stainless steels possess extremely high corrosion resistance; in some media such as acetic acid and formic acid, they can even replace highly alloyed austenitic stainless steels as well as corrosion-resistant alloys. (4) It possesses good resistance to localized corrosion; compared with austenitic stainless steels with similar alloy contents, it offers superior performance in terms of wear corrosion and fatigue corrosion resistance. (5) It has a lower linear expansion coefficient than austenitic stainless steels and is close to that of carbon steel, making it suitable for bonding with carbon steel; this holds significant engineering importance, such as in the production of composite sheets or linings. (6) Under both dynamic and static loading conditions, austenitic stainless steel possesses a higher energy absorption capacity; this gives biphasic stainless steel a clear advantage in enabling structural components to withstand sudden accidents such as collisions and explosions, making it practically useful. 2. Compared with austenitic stainless steels, the disadvantages of duplex stainless steels are as follows: (1) Their applicability and versatility are lower than those of austenitic stainless steels; for example, their operating temperature must be kept below 250 degrees Celsius. (2) Its plasticity and toughness are lower than those of austenitic stainless steels, and its cold and hot working properties as well as formability are inferior to those of austenitic stainless steels. (3) There is a medium-temperature brittleness zone; therefore, the processing parameters for heat treatment and welding must be strictly controlled to prevent the formation of harmful phases that could degrade the properties. 3. Compared with ferritic stainless steels, the advantages of duplex stainless steels are as follows: (1) Their overall mechanical properties are better than those of ferritic stainless steels, especially in terms of ductility and toughness; they are not as sensitive to brittleness as ferritic stainless steels. (2) Except for resistance to stress corrosion, all other local corrosion resistances are superior to those of ferritic stainless steels. (3) Its cold working process properties and cold forming properties are far superior to those of ferritic stainless steels. (4) Its weldability is also far superior to that of ferritic stainless steels; generally, no preheating is required before welding, and no heat treatment is needed after welding. (5) Its application range is wider than that of ferritic stainless steels. 4. Compared with ferritic stainless steels, the disadvantages of duplex stainless steels are as follows: they contain high levels of alloying elements, resulting in a relatively high price; whereas typical ferritic steels do not contain nickel. In summary, an overview of the performance and process characteristics of DSS can be obtained; it has won the favor of users thanks to its excellent combined mechanical and corrosion resistance properties, and has become an outstanding corrosion-resistant engineering material that saves both weight and costs.