Types, grades, performance characteristics and technical parameters of duplex stainless steel wire mesh products

Duplex Stainless Steel (DSS) refers to stainless steel in which ferrite and austenite each account for about 50%, and the content of the lesser phase generally needs to reach at least 30%. In the case of low C content, the Cr content is 18%~28%, and the Ni content is 3%~10%. Some steels also contain alloying elements such as Mo, Cu, Nb, Ti, and N.
This type of steel has the characteristics of both austenitic and ferritic stainless steels. Compared with ferrite, it has higher plasticity and toughness, no room temperature brittleness, and significantly improved intergranular corrosion resistance and welding performance. At the same time, it also maintains the 475℃ brittleness and high thermal conductivity of ferritic stainless steel, and has the characteristics of superplasticity. Compared with austenitic stainless steel, it has high strength and significantly improved resistance to intergranular corrosion and chloride stress corrosion. Duplex stainless steel has excellent pitting resistance and is also a nickel-saving stainless steel.
The main feature is that the yield strength can reach 400-550MPa, which is twice that of ordinary stainless steel, so it can save materials and reduce equipment manufacturing costs. In terms of corrosion resistance, especially in harsh media environments (such as seawater with high chloride ion content), the resistance to pitting, crevice corrosion, stress corrosion and corrosion fatigue of duplex stainless steel is significantly better than that of ordinary austenitic stainless steel, and can be comparable to high-alloy austenitic stainless steel.

Classification of duplex stainless steel:
The first category is low alloy type, the representative grade is UNS S32304 (23Cr-4Ni-0.1N), the steel does not contain molybdenum, the PREN value is 24-25, and it can replace AISI304 or 316 in terms of stress corrosion resistance.
The second category is medium alloy type, the representative grade is UNS S31803 (22Cr-5Ni-3Mo-0.15N), the PREN value is 32-33, and its corrosion resistance is between AISI 316L and 6%Mo+N austenitic stainless steel.
The third category is high alloy type, generally containing 25%Cr, molybdenum and nitrogen, and some also contain copper and tungsten. The standard grade is UNSS32550 (25Cr-6Ni-3Mo-2Cu-0.2N), the PREN value is 38-39, and the corrosion resistance of this type of steel is higher than that of 22%Cr duplex stainless steel.
The fourth category is super duplex stainless steel, 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 media conditions, has good corrosion resistance and mechanical comprehensive properties, comparable to super austenitic stainless steel.

Duplex stainless steel grades
Many duplex stainless steel grades have been added to China's new standard GB/T 20878-2007 "Stainless steel and heat-resistant steel grades and chemical composition". For example: 14Cr18Ni11Si4AlTi, 022Cr19Ni5Mo3Si2N, 12Cr21Ni5Ti. For more grades, see the standard.
In addition: the famous 2205 duplex steel is equivalent to China's 022Cr23Ni5Mo3N.

chemical composition

Mechanical behavior

Duplex stainless steel performance characteristics
Due to the characteristics of the two-phase structure, through the correct control of chemical composition and heat treatment process, duplex 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 higher strength and resistance to
chloride stress corrosion of ferritic stainless steel. It is these superior properties that have made duplex stainless steel develop rapidly as a weldable structural material. Since the 1980s, it has become a steel type on par with martensitic, austenitic and ferritic stainless steel. Duplex stainless steel has the following performance characteristics:
(1) Molybdenum-containing duplex stainless steel has good chloride stress corrosion resistance under low stress. Generally, 18-8 type austenitic stainless steel is prone to stress corrosion cracking in neutral chloride solutions above 60°C. Heat exchangers, evaporators and other equipment made of this type of stainless steel in trace chloride and hydrogen sulfide industrial media have a tendency to produce stress corrosion cracking, while duplex stainless steel has good resistance.
(2) Molybdenum-containing duplex stainless steel has good pitting corrosion resistance. When having the same pitting resistance equivalent value (PRE=Cr%+3.3Mo%+16N%), the critical pitting potential of duplex stainless steel and austenitic stainless steel is similar. The pitting resistance of duplex stainless steel and austenitic stainless steel is equivalent to that of AISI 316L. The pitting and crevice corrosion resistance of high chromium duplex stainless steel containing 25% Cr, especially nitrogen, exceeds that of AISI 316L.
(3) It has good corrosion fatigue and wear corrosion resistance. Under certain corrosive media conditions, it is suitable for making power equipment such as pumps and valves.
(4) It has good comprehensive mechanical properties. It has high strength and fatigue strength, and its yield strength is twice that of 18-8 austenitic stainless steel. The elongation in the solid solution state reaches 25%, and the toughness value AK (V-notch) is above 100J. (5) It has good
weldability and low tendency to thermal cracking. Generally, no preheating is required before welding, and no heat treatment is required after welding. It can be welded with dissimilar materials such as 18-8 austenitic stainless steel or carbon steel.
Duplex stainless steel (6) Duplex stainless steel containing low chromium (18% Cr) has a wider hot working temperature range than 18-8 austenitic stainless steel, has lower resistance, and can be directly rolled into billets to produce steel plates without forging. Duplex stainless steel containing high chromium (25% Cr) is slightly more difficult to hot work than austenitic stainless steel, and can produce products such as plates, tubes and wires.
(7) During cold working, the work hardening effect is greater than that of 18-8 austenitic stainless steel. In the early stage of deformation of tubes and plates, a large stress needs to be applied to deform.
(8) Compared with austenitic stainless steel, it has a large thermal conductivity and a small linear expansion coefficient, and is suitable for use as equipment lining and production of composite plates. It is also suitable for making the core of heat exchangers, and its heat transfer efficiency is higher than that of austenitic stainless steel.
(9) It still has various brittle tendencies of high chromium ferritic stainless steel and is not suitable for use under working conditions above 300°C. The lower the chromium content in duplex stainless steel, the less harmful the brittle phases such as σ are.
Application: Used
in heat exchangers and cooling devices and components for oil refining, fertilizer, papermaking, petroleum, chemical industry, etc. that are resistant to seawater, high temperature and concentrated nitric acid.