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DSS & SDSS

  • Overview

    Duplex Stainless Steel (DSS), refers to ferrite and austenite each about 50%, generally less phase content also needs to reach at least 30% 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, Nb, Ti, N and other alloying elements.

    The steel has the characteristics of both austenitic and ferritic stainless steel. Compared with ferritic, its plasticity and toughness is higher, no room temperature brittleness, intergranular corrosion resistance and welding performance are significantly improved, but also maintain ferritic stainless steel 475℃ brittleness and high thermal conductivity, with superplasticity and other characteristics. Compared with austenitic stainless steel, it has higher strength and higher resistance to intergranular corrosion and chloride stress corrosion. Duplex stainless steel has excellent corrosion resistance, is also a nickel-saving stainless steel.

  • Performance Features

    Due to the characteristics of two-phase structure, through the correct control of chemical composition and heat treatment process, the two-phase stainless steel has the advantages of both ferritic stainless steel and austenitic stainless steel, it combines the excellent toughness and weldability of austenitic stainless steel with the high strength and chloride resistance of ferritic stainless steel. It is these superior properties that make duplex stainless steel developed rapidly as weldable structural materials.

    (1) Molybdenum containing duplex stainless steel has good resistance to chloride stress corrosion under low stress.

    (2) Molybdenum containing duplex stainless steel has good pitting corrosion resistance.

    (3) Good corrosion resistance to fatigue and wear.

    (4) Good comprehensive mechanical properties. High strength and fatigue strength.

    (5) Good weldability, small hot crack tendency, no preheating before welding, no heat treatment after welding, can be welded with 18-8 type austenitic stainless steel or carbon steel and others.

    (6) Dual-phase stainless steel with low chromium (18%Cr) has wider thermal working temperature range than 18-8 austenitic stainless steel, less resistance, and can be rolled directly without forging. Dual-phase stainless steel with high chromium (25%Cr) is slightly more difficult to work than austenitic stainless steel and can be used to produce plates, tubes and wires.

    (7) When cold working, its work hardening effect is larger than 18-8 type austenitic stainless steel, In the production of tube and plate, large stress should be applied at the initial stage of deformation.

    (8) Compared with austenitic stainless steel, high thermal conductivity, low linear expansion coefficient, suitable for equipment lining and production of composite plate. It is also suitable for making the tube core of heat exchanger, and the heat transfer efficiency is higher than that of austenitic stainless steel.

    (9) It still has the brittleness tendency of high chromium ferrite stainless steel and should not be used in operating conditions higher than 300°C. The lower the chromium content in the dual-phase stainless steel, the less harmful the brittle phase such as σ.

  • Classification

    (1) Low alloy DSS

    Typical grade is UNS S32304 (23Cr-4Ni-0.1N), free of molybdenum, with a PREN value of 24-25, can be used in place of AISI304 or 316 in stress corrosion resistance.

    (2) Medium alloy DSS

    Typical grades are UNS S31803 (SAF 2205) and UNS S32205 (22Cr-5Ni-3Mo-0.15N) with a PREN value of 32-33. Its corrosion resistance is between AISI 316L and 6%Mo+N austenitic stainless steel.

    (3) High alloy DSS

    Generally it contains 25%Cr, also molybdenum and nitrogen, some also contain copper and tungsten, the typical grade is UNS S32550 (25Cr-6Ni-3Mo-2Cu-0.2N), PREN value 38-39, this kind of steel corrosion resistance is higher than 22%Cr duplex stainless steel.

    (4) SDSS

    Super duplex stainless steel type, containing high molybdenum and nitrogen, typical grades is UNS S32750 (25Cr-7Ni-3.7Mo-0.3N, SAF 2507), some also containing tungsten and copper (typical grade UNS S32760, Zeron 100), PREN value is greater than 40, can be applied to harsh media conditions, has good corrosion resistance and mechanical properties, comparable to super austenitic stainless steel.

  • Application

    Duplex stainless steels are used in oil refining, chemical fertilizer, paper making, petroleum, chemical industry and other heat exchangers and cold shower and devices resistant to seawater and high temperature concentrated nitric acid.

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