Duplex stainless steel

Duplex stainless steel

Duplex Stainless Steel (Duplex Stainless Steel, referred to as DSS), refers to ferrite and austenite each accounted for about 50%, generally less phase content also needs to reach at least 30% of stainless steel. In the case of low C, the Cr content is 18% to 28%, and the Ni content is 3% to 10%. Some steels also contain Mo, Cu, Nb, Ti, N and other alloying elements.

This kind of steel has the characteristics of austenitic and ferritic stainless steel, compared with ferritic, higher plasticity, toughness, no room temperature brittleness, intergranular corrosion resistance and welding performance are significantly improved, while also maintaining the 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 pitting resistance and is also a nickel-free stainless steel.

Performance characteristics:

Due to the characteristics of the two-phase structure, through the correct control of the chemical composition and heat treatment process, the duplex stainless steel has the advantages of ferritic stainless steel and austenitic stainless steel, which combines the excellent toughness and weldability of austenitic stainless steel with the high strength and resistance of ferritic stainless steel

Combined with chloride stress corrosion properties, it is these superior properties that make duplex stainless steel develop rapidly as a weldable structural material, and since the 1980s has become a steel class alongside martensitic, Austenitic and ferritic stainless steel. Duplex stainless steel has the following performance characteristics:

(1) Molybdenum containing duplex stainless steel has good resistance to chloride stress corrosion under low stress. Generally, 18-8 austenitic stainless steel is prone to stress corrosion fracture in neutral chloride solution above 60°C, and heat exchangers, evaporators and other equipment made of this kind of stainless steel in trace chloride and hydrogen sulfide industrial media have a tendency to produce stress corrosion fracture, while duplex stainless steel has good resistance.

(2) Molybdenum containing duplex stainless steel has good pitting resistance. With the same pitting resistance value (PRE=Cr%+3.3Mo%+16N%), the critical pitting potential of duplex stainless steel is similar to that of austenitic stainless steel. The pitting resistance of duplex stainless steel and austenitic stainless steel is equivalent to AISI 316L. The resistance to pitting and crevice corrosion of 25%Cr, especially high chromium duplex stainless steels containing nitrogen, exceeds that of AISI 316L.

(3) It has good corrosion resistance to fatigue and wear corrosion. Under the conditions of some corrosive media, it is suitable for making power equipment such as pumps and valves.

(4) Good comprehensive mechanical properties. It has high strength and fatigue strength, and the yield strength is twice that of 18-8 type austenitic stainless steel. The elongation of the solid solution state reaches 25%, and the toughness value AK(V-notch) is more than 100J.

(5) Good weldability, small hot crack tendency, generally do not need to preheat before welding, no heat treatment after welding, can be welded with 18-8 type austenitic stainless steel or carbon steel and other dissimilar.

(6) Dual-phase stainless steel with low chromium (18%Cr) hot working temperature range is wider than 18-8 austenitic stainless steel, resistance is small, without forging, direct rolling production of steel plates. Duplex stainless steel with high chromium (25%Cr) is slightly more difficult to process than austenitic stainless steel, and can produce products such as plates, tubes and wires.

(7) cold processing than 18-8 type austenitic stainless steel work hardening effect is large, in the initial deformation of the tube and plate, the need to apply greater stress to deformation.

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

(9) There are still various brittleness tendencies of high chromium ferritic stainless steel, which should not be used in working conditions higher than 300°C. The lower the chromium content of duplex stainless steel, the less harmful the brittle phase such as .

Structure and type:

Duplex stainless steel has the characteristics of austenitic stainless steel and ferritic stainless steel because of its austenitic + ferritic biphase structure, and the content of the two phase structures is basically equivalent. The yield strength can reach 400Mpa ~ 550MPa, which is twice that of ordinary austenitic stainless steel. Compared with ferritic stainless steel, duplex stainless steel has higher toughness, lower brittleness transition temperature, higher intergranular corrosion resistance and higher weldability. At the same time, it retains some characteristics of ferritic stainless steel, such as 475℃ brittleness, high thermal conductivity, small linear expansion coefficient, superplasticity and magnetism. Compared with austenitic stainless steel, the strength of duplex stainless steel is high, especially the yield strength is significantly improved, and the performance of pitting corrosion resistance, stress corrosion resistance, corrosion fatigue resistance is also significantly improved.

Duplex stainless steel according to its chemical composition classification, can be divided into Cr18 type, Cr23(without Mo) type, Cr22 type and Cr25 type four categories. For Cr25 type duplex stainless steel can be divided into ordinary type and super duplex stainless steel, of which Cr22 type and Cr25 type are more used. The biphase stainless steel used in China is mostly produced in Sweden, and the specific grades are :3RE60(Cr18), SAF2304 (Cr23), SAF2205 (Cr22), SAF2507(Cr25).