The so-called duplex stainless steel is in its solid quenching organisation in the ferrite phase and austenite phase each half, generally the least phase content of perhaps to reach 30%. Duplex stainless steel is based on 18-8 austenitic stainless steel, increase the Cr content or add other ferrite elements formed, so that the steel has austenitic plus ferrite bidirectional organisation, but also save the Ni alloy.
Due to the characteristics of two-phase organisation of two-way stainless steel, through the correct control of the chemical composition and heat treatment process, so that the duplex stainless steel has the advantages of both ferrite stainless steel and austenitic stainless steel. Duplex stainless steel due to the presence of austenite reduces the brittleness and crystal growth tendency of high Cr physique steel, but also to maintain its excellent toughness and welding performance. The presence of ferrite improves the austenitic yield strength, resistance to intergranular corrosion and chloride stress corrosion resistance.
First, the performance of duplex stainless steel
(A) the advantages of duplex stainless steel
1, compared with austenitic stainless steel, the advantages of duplex stainless steel are as follows:
(1) yield strength than ordinary austenitic stainless steel is more than twice as high, and has sufficient plastic toughness needed for moulding. The use of duplex stainless steel manufacturing tanks or pressure vessels wall thickness than the commonly used austenitic reduction of 30?50%, is conducive to reducing costs.
(2) has excellent resistance to stress corrosion rupture ability, even the lowest alloy content of duplex stainless steel than austenitic stainless steel has a higher resistance to stress corrosion rupture ability, especially in the environment containing chloride ions. Stress corrosion is common austenitic stainless steel is difficult to solve the outstanding problems.
(3) in many media, the most commonly used 2205 duplex stainless steel corrosion resistance is better than ordinary 316L austenitic stainless steel, and super duplex stainless steel has a very high corrosion resistance, in some media, such as acetic acid, formic acid, etc., and can even replace the high-alloyed austenitic stainless steel, and even corrosion-resistant alloys.
(4) has good resistance to local corrosion, and the alloy content of austenitic stainless steel, compared with its resistance to wear corrosion and fatigue corrosion are better than austenitic stainless steel.
(5) than the austenitic stainless steel has a low coefficient of linear expansion, and carbon steel is close to the carbon steel, suitable for connection with carbon steel, has important engineering significance, such as the production of composite panels or linings.
(6) Whether in dynamic or static load conditions, duplex stainless steel than austenitic stainless steel has a higher energy absorption capacity, which is the structural components to cope with sudden accidents such as impact, explosion, duplex stainless steel advantage is obvious, there are practical applications.
2, compared with ferritic stainless steel, the advantages of duplex stainless steel are as follows:
(1) comprehensive mechanical properties than ferritic stainless steel is good, especially plastic toughness, not like ferritic stainless steel as sensitive to brittleness.
(2) In addition to stress corrosion resistance, other local corrosion resistance is better than ferritic stainless steel.
(3) Duplex stainless steel cold working process performance and cold forming properties are far better than ferritic stainless steel.
(4) Welding performance is also far better than ferritic stainless steel, generally do not need preheating before welding, no heat treatment after welding.
(5) The application range is wider than ferritic stainless steel.
(B) the disadvantages of duplex stainless steel
1, compared with austenitic stainless steel, duplex stainless steel disadvantages are:
(1) the universality of application and versatility is not as good as austenitic stainless steel, heat resistance is low, the use of its temperature shall be controlled below 300 ℃.
(2) its plastic toughness is lower than austenitic stainless steel, cold working than the austenitic stainless steel work hardening effect is large, in the pipe, plate subjected to deformation at the beginning of the deformation, the need to apply a larger force in order to deform, so that the pipe, the plate produced by the internal stress is large. Duplex stainless steel containing 25% Cr than austenitic stainless steel hot working difficulties.
(3) The existence of medium temperature brittle zone, need to strictly control the heat treatment and welding process system, in order to avoid the emergence of harmful phases, damage performance.
2, compared with ferritic stainless steel, duplex stainless steel disadvantages are as follows: high content of alloying elements, the price is relatively high, generally ferritic does not contain nickel.
Second, the classification and grade of duplex stainless steel
So far, the development of duplex stainless steel has gone through three generations, and currently all countries in the world have their grades. Typical duplex stainless steel can be divided into four categories:
The first category is low alloy type, the representative grade is UNS S32304 (23Cr-4Ni-0.1N), the steel does not contain molybdenum, pitting corrosion resistance equivalent PRE value of 24 ~ 25, in the stress corrosion resistance instead of AISI304 (0Cr18Ni9) or 316 (OCr17Ni12Mo2) use.
The second type is medium alloy type, the representative grade is UNS S31803 (22Cr-5Ni-3Mo-0.15N), PRE value of 32 ~ 33, its corrosion resistance between AISI316L (OOCr17Ni14Mo2) and 6% Mo + N austenitic stainless steel.
The third category is a high-alloy type, generally containing 25% Cr, also contains molybdenum and nitrogen, some also contain copper and tungsten, the standard grade UNS S32550 (25Cr-6Ni-3Mo-2Cu-0.2N), PRE value of 38-39, the corrosion resistance of this type of steel is higher than that of the duplex stainless steel 22% Cr.
The fourth category is a super duplex stainless steel type, containing high molybdenum and nitrogen, standard grade UNS S32750 (25Cr-7Ni-3.7Mo-0.3N), some also contain tungsten and copper, PRE value is greater than 40, can be applied to harsh media conditions, has good corrosion resistance and mechanical properties, can be comparable with super austenitic stainless steel.
Third, the application of duplex stainless steel
Duplex stainless steel is widely used in various fields with its unique advantages:
Pulp and paper industry: pulp and paper industry is the earliest use of duplex stainless steel processing industry. The use of duplex stainless steel instead of 304 and 316 austenitic stainless steel made of sulphuric and sulphurous acid cooking pots to improve the pot's corrosion resistance and strength, while the wall thickness is reduced by a lot, reducing the total weight of the pot.
Chemical processing industry: chemical processing industry involves a very wide range of working conditions are also more complex. Preservation of industrial wet process phosphoric acid storage tanks, usually with 316L or 317L type austenitic stainless steel to manufacture, but the wall is very thick. The use of duplex stainless steel 2205, almost double the strength of 316L and 317L, and can reduce the wall thickness, thus reducing the importance and cost of storage tanks. At the same time, in industrial sulphuric acid environment, especially in the presence of chloride ions, 2205 has better resistance to localised corrosion than austenitic stainless steel.
Food and beverage industry: food and beverage industry production is generally processed in hot solutions containing chloride ions. Duplex stainless steel has a unique advantage in resistance to stress corrosion rupture. Therefore, depending on the use of different environments, different duplex stainless steel can be used, but also in the total cost than the austenitic stainless steel advantage.
Transport industry: due to the high strength and excellent corrosion resistance of duplex stainless steel, so that it is used in the production of chemical ships, there is a great advantage. Reduces the importance of the ship, reduces fuel consumption, in this regard, the most used is 22% chromium type duplex stainless steel. At the same time can meet the cargo carried need material corrosion resistance.
Construction: In coastal areas, the air is filled with marine atmosphere evaporated from seawater, in which the concentration of chloride ions is much higher than the air inland, therefore, the need for seawater corrosion-resistant materials. The corrosion resistance of duplex stainless steel is having this advantage, at the same time, its high strength can also reduce weight. At present, there are many offshore oil platforms are using duplex stainless steel to manufacture explosion-proof wall.
Petrochemical industry: petrochemical and chemical industry corrosive environment is characterised by high reaction temperatures, the medium often contains high or medium concentration of chloride, easily induced stress corrosion fracture of stainless steel. Therefore, this field not only need duplex stainless steel, more to use super duplex stainless steel.
The oil refining industry is one of the fields that use duplex stainless steel more, mainly used in atmospheric distillation, catalytic cracking hydrodesulfurisation and other devices, both at home and abroad are used. In addition, duplex stainless steel in the production of sour gas and oil use gradually increased, mainly used as a production pipe lining, onshore and offshore piping systems and heat exchangers.
In addition, duplex stainless steel is also successfully used in wastewater treatment heat exchangers, hot water pipes, water heaters.
Fourth, conclusion
Duplex stainless steel after three generations of development and improvement, from the point of view of production, processing and manufacturing and application, replaced austenitic stainless steel in some applications. It has won the favour of users with its superior mechanical and corrosion resistance performance, and has become an excellent corrosion-resistant engineering material that saves both weight and investment. In recent years, with the super duplex stainless steel into the market, to expand the selection of duplex stainless steel and the development of new steel to further create the conditions.
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