Properties: refers to steel mainly composed of ferrite phase and martensite phase, which can be obtained by critical zone treatment or controlled rolling of low carbon steel or low alloy steel. This kind of steel has a good combination of high strength and high ductility, and has become a new type of stamping steel with high strength and good formability, which has been successfully applied to the automobile industry. Due to the characteristics of dual-phase structure, dual-phase stainless steel has the advantages of ferritic stainless steel and austenitic stainless steel by correctly controlling chemical composition and heat treatment process. It combines the excellent toughness and weldability of austenitic stainless steel with the high strength and chloride stress corrosion resistance of ferritic stainless steel. It is these excellent properties that make duplex stainless steel develop rapidly as a weldable structural material, and it has become a steel grade alongside martensite, austenite and ferrite stainless steel since 1980s. Duplex stainless steel has the following performance characteristics:
(1), molybdenum-containing duplex stainless steel has good chloride stress corrosion resistance under low stress. General 18-8 austenitic stainless steel is prone to stress corrosion cracking in neutral chloride solution above 60℃. In industrial medium with trace chloride and hydrogen sulfide, heat exchangers, evaporators and other equipment made of this stainless steel are prone to stress corrosion cracking, while duplex stainless steel has good resistance. (2) Mo-containing duplex stainless steel has good pitting corrosion resistance. When duplex stainless steel and austenitic stainless steel have the same pitting corrosion resistance equivalent value (PRE=Cr%+3.3Mo%+ 16N%), their critical point corrosion potentials are similar. The pitting corrosion resistance of duplex stainless steel and austenitic stainless steel is equivalent to AISI 3 16L. The pitting and crevice corrosion resistance of high chromium duplex stainless steel containing 25%Cr, especially nitrogen, exceeds AISI3 16L. (3) It has good corrosion resistance, fatigue resistance and wear and corrosion resistance. Under some corrosive medium conditions, 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 austenitic stainless steel. The solid solution elongation reached 25%, and the toughness value AK(V-notch) was above 100J. (5) Good weldability and small hot cracking tendency. Generally, preheating before welding or heat treatment after welding is not needed, and it can be welded with 18-8 austenitic stainless steel or carbon steel. (6) The hot working temperature range of low chromium (18%Cr) duplex stainless steel is wider than that of 18-8 austenitic stainless steel, and the resistance is small, so the steel plate can be directly rolled without forging. The hot working difficulty of duplex stainless steel containing high chromium (25%Cr) is slightly greater than that of austenitic stainless steel, and it can produce plates, pipes, wires and other products. (7) Austenitic stainless steel with work hardening effect greater than 18-8 during cold working. In the early stage of tube sheet deformation, it is necessary to apply greater stress to deform. (8) Compared with austenitic stainless steel, it has higher thermal conductivity and lower linear expansion coefficient, and is suitable for lining equipment and producing composite plates. It is also suitable for making the tube core of heat exchanger, and the heat exchange efficiency is higher than that of austenitic stainless steel. (9) High chromium ferritic stainless steel still has various brittle tendencies, and should not be used under working conditions higher than 300℃ ... The lower the chromium content in duplex stainless steel, the less the harm of brittle phases such as σ. Duplex stainless steel has developed to the third generation since it was born in the United States in the 1940s. Its 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 the manufacturing cost of equipment. In terms of corrosion resistance, especially in the harsh medium environment (such as seawater with high chloride ion content), the pitting corrosion, crevice corrosion, stress corrosion and corrosion fatigue resistance of duplex stainless steel are obviously superior to ordinary austenitic stainless steel, and can be comparable to high-alloy austenitic stainless steel.
Duplex stainless steel has good weldability. Compared with ferritic stainless steel and austenitic stainless steel, it is neither like ferritic stainless steel whose plastic toughness is greatly reduced due to severe grain coarsening, nor like austenitic stainless steel which is sensitive to welding hot cracks.
Duplex stainless steel is widely used in petrochemical equipment, seawater and wastewater treatment equipment, oil and gas pipelines, paper machinery and other industrial fields because of its special advantages. In recent years, it has also been studied in the field of bridge load-bearing structure, which has a good development prospect. Classify duplex stainless steel according to PRE value;
PRE (that is, the value of Cr%+3.3× (Mo+1/2w)%+16× n% in steel).
1. Low alloy dual-phase steel: representative brand: UNS S32304 (23Cr-4Ni-0. 1N). The steel contains no molybdenum and has considerable pitting corrosion resistance, and the PRE value is 24-25. It can replace AISI304(0Cr 18Ni9) or 366 in stress corrosion resistance.
2. Medium alloy dual-phase steel: rePREsentative brand: UNSS 31803 (22cr-5ni-3mo-0.15n), pre value of 32-33, and corrosion resistance of AISI 316L (00cr17Ni).
3. High-alloy dual-phase steel: This dual-phase stainless steel contains as much as 25% chromium, which first appeared in dual-phase stainless steel series, such as 329 in the United States, and then developed in major industrial countries, with many brands and wide applications. In the early days, there were low-carbon and molybdenum-free Cr25Ni5 dual-phase steels, such as IN-744(0Cr26Ni6Ti) and 2323(0Cr26Ni5). Later, many countries developed a batch of 1% ~ 3% Mo high chromium duplex stainless steel, which improved the corrosion resistance of steel, expanded the application scope, and was gradually included in the standard, such as 329J655. Din1.4582 (0cr25ni7mo1.5), etc. However, after welding, the number of ferrite near the weld zone increases, and it may even become a single-phase structure, which deteriorates the properties of steel and limits its use. In order to solve this problem, ultra-low carbon nitrogen-containing duplex stainless steel was developed after 1970s. In addition to molybdenum, some brands have added elements such as copper and tungsten to further improve corrosion resistance. Such as ntk r-4 (00cr25ni5mo2n), dp3 (00cr25n7mo3cun) and so on.
4. Super dual-phase steel: refers to PREN & gt；; 40, containing 25%Cr, high molybdenum (>: 3.5%) and high nitrogen (0.22% ~ 0.30%). The main brands are UNS S32550(UR52N+), S32750(SAF 2507) and S32760(Zeron 100. The composition of these three steels is similar, but the difference lies in the different contents of tungsten and copper. Many grades of duplex stainless steel have been added to the new standard GB/T 20878-2007 "Grade and Chemical Composition of Stainless Steel and Heat Resistant Steel". Such as:14cr18ni1si4alti, 022Cr 19Ni5Mo3Si2N, 12Cr2 1Ni5Ti, etc. See the standard for more brands.
In addition, the famous 2205 dual-phase steel is equivalent to China's 022Cr23Ni5Mo3N. Chemical composition of duplex stainless steel and austenitic stainless steel in common use
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Chemical composition of USNP brand alloy brand, Wt%
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PREN*
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Duplex stainless steel S32304 SAF 2304 0.03 23 4.5-0.1.25
s 3 1803 SAF 2205 0.03 22 5.0 3.2 0. 18 35
S32750 SAF2507 0.03 25 7.0 4.0 0.3 43
Austenitic stainless steel s3040304l0.03188.0-0.120.
s 3 1603 3 16L 0.03 16 1 1.0 2.0 0. 1.24
s 3 1703 3 17L 0.03 18 1 1.0 3.0 0. 1.30
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Finally, it is wrong for some web pages and even papers to write duplex stainless steel as bidirectional stainless steel. Duplex means that there are two kinds of metallographic structures, not the "direction" of direction. Taigang duplex stainless steel won the bid for Hong Kong-Zhuhai-Macao Bridge.
On the morning of 201365438+1October 16, a new type of duplex stainless steel reinforcement independently developed by TISCO was successfully awarded the bid for the Hong Kong-Zhuhai-Macao Bridge project. In the bidding of many internationally renowned stainless steel enterprises, TISCO stood out with its strong technical research and development strength, perfect quality assurance system and high-quality supporting services, and became the only enterprise that won the bid for the dual-phase stainless steel reinforcement of the Hong Kong-Zhuhai-Macao Bridge project.
TISCO won the bid for 3,462 tons of duplex stainless steel reinforcement, which will be used in many parts of the bridge, such as pile caps, tower foundations and pier shafts. This product has the unique advantages of good chloride ion corrosion resistance, high strength, light weight, maintenance-free and long service life. This is the first time that stainless steel bars have been used in bridge construction in Chinese mainland.
On October 20 12, 165438 10, TISCO duplex stainless steel bar passed CARES (professional certification body for reinforced concrete products) certification, becoming the only enterprise in China with the qualification to produce duplex stainless steel bar according to British standard. (Source People's Network-Shanxi Channel)