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Duplex stainless steel (DSS) refers to the stainless steel with ferrite and austenite accounting for 50% and 30% respectively. In the case of low C content, Cr content is 18% ~ 28%, Ni content is 3% ~ 10%. Some steels also contain alloying elements such as Mo, Cu, Nb, Ti and n. This kind of steel has higher plasticity and toughness, no room temperature brittleness, and the intergranular corrosion resistance and welding performance are significantly improved. At the same time, it also maintains the brittleness of ferritic stainless steel at 475 ℃ and high thermal conductivity, and has the characteristics of superplasticity. 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.

Duplex stainless steel can be divided into the following categories: the first type is low alloy type, which represents UNS s32304 (23cr-4ni-0.1n). The steel does not contain molybdenum and the prEN value is 24-25. It can replace AISI304 or 316 in stress corrosion resistance. The second type belongs to medium alloy type, the representative brand is UNS S31803 (22cr-5ni-3mo-0.15n), with prEN value of 32-33. Its corrosion resistance is between AISI 316L and 6% Mo + n austenitic stainless steel. The third type belongs to high alloy type, which generally contains 25% Cr, Mo and N, and some also contains copper and tungsten. The standard grade unss32550 (25cr-6ni-3mo-2cu-0.2n) and prEN value is 38-39. The corrosion resistance of this kind of steel is higher than that of 22% Cr duplex stainless steel. The fourth type is super duplex stainless steel type, containing high molybdenum and nitrogen, standard grade UNS S32750 (25cr-7ni-3.7mo-0.3n), some also contain tungsten and copper, prEN value is greater than 40, can be used in harsh medium conditions, has good corrosion resistance and mechanical properties, comparable to super austenitic stainless steel.