Duplex stainless steel material introduction
Duplex steel / super duplex steel stainless steel pipe has the structure of austenite and ferrite, so its performance features are austenitic stainless steel and ferritic stainless steel, which is a kind of high strength and high corrosion resistance stainless steel. Generally, Cr content is 18% - 28% and Ni content is 3% - 10% when the carbon C content is low. Some steels also contain Mo, Cu, Nb, Ti, N and other alloy elements. Compared with ferrite, the plasticity and toughness are higher, and the room temperature brittleness is not available. The intergranular corrosion resistance and welding performance are significantly improved. Meanwhile, the 475 ℃ brittleness and high thermal conductivity of ferritic stainless steel are maintained, and the properties of superplasticity are also maintained. Compared with austenitic stainless steel, the yield strength is significantly improved, and the performance of intergranular corrosion, stress corrosion, corrosion fatigue and abrasion resistance is improved significantly.Duplex stainless steel pipe fittings can be generally divided into four categories
(1) Low alloy pipe fittings - the representative brand is UNS s32304 (23cr-4ni-0.1n), and the pitting corrosion resistance index prEN value (prEN = CR% + 3.3 × Mo% + 16 × n%) is 24-25. The low alloy UNS s32304 does not contain molybdenum, and can replace AISI304 or 316 in the aspect of stress corrosion resistance.
(2) The corrosion resistance of medium alloy type UNS S31803 is between AISI 316L and 6% Mo + n austenitic stainless steel.
(3) High alloy type pipe fittings -- the standard grade is UNS s32550 (25cr-6ni-3mo-2cu-0.2n), and the pitting corrosion resistance index prEN is 38-39. The high alloy type pipe fittings generally contain 25% Cr, molybdenum and nitrogen, and some also contain copper and tungsten. The corrosion resistance of this kind of steel is higher than 22% CR.
(4) Super duplex stainless steel pipe fittings - the standard grade is UNS S32750 (25cr-7ni-3.7mo-0.3n), and the pitting corrosion resistance index prEN value is more than 40. The super duplex stainless steel pipe fittings contain high molybdenum and nitrogen, and some also contain tungsten and copper. It can be applied to harsh medium conditions, and has good corrosion resistance and comprehensive mechanical properties, which can be compared with super austenitic stainless steel pipe fittings.