What is duplex stainless steel
What is duplex stainless steel?
Duplex stainless steel (DSS for short) refers to stainless steel with ferrite and austenite accounting for about 50% respectively, and the content of less phase is generally at least 30%. In the case of low C content, Cr content is 18% ~ 28%, Ni content is 3% ~ 10%. Some steels also contain Mo, Cu, Nb, Ti, N and other alloy elements.
This kind of steel has the characteristics of austenite and ferritic stainless steel. Compared with ferrite, it has higher plasticity and toughness, no room temperature brittleness, significantly improved intergranular corrosion resistance and welding performance, and maintained the 475 ℃ brittleness, high thermal conductivity and Superplasticity of ferritic stainless steel. Compared with austenitic stainless steel, it has higher strength, intergranular corrosion resistance and chloride stress corrosion resistance. Duplex stainless steel has excellent pitting corrosion resistance and is also a nickel saving stainless steel.
Due to the characteristics of two-phase structure of duplex stainless steel, the duplex stainless steel has the advantages of both ferritic stainless steel and austenitic stainless steel through proper control of chemical composition and heat treatment process. Due to the presence of austenite, duplex stainless steel reduces the brittleness and the tendency of crystal growth of high Cr ferrite steel, and maintains its excellent toughness and weldability. The presence of ferrite improves the yield strength, intergranular corrosion resistance and chloride stress corrosion resistance of austenite.
Development of duplex stainless steel
Duplex stainless steel has a history of more than 80 years. The early grades were alloys of chromium, nickel and molybdenum. In 1930, the first batch of forged duplex stainless steel was produced in Sweden and used in sulfite papermaking industry. These grades were developed to reduce intergranular corrosion of early high carbon austenitic stainless steel. In 1930 Finland produced duplex stainless steel castings. The predecessor of Uranus 50 was patented in France in 1936. After World War II, AISI 329 stainless steel has become a mature brand and is widely used in heat exchanger pipes of nitric acid plant. 3re60 is one of the first two-phase stainless steel grades specially developed to improve the chloride stress corrosion cracking (SCC) resistance. Later, forged and cast duplex stainless steel grades were used in a variety of processing industry applications, including vessels, heat exchangers, and pumps.
The first generation of duplex stainless steel has good performance, but it has limitations in the welding state. The HAZ of weld has low toughness because of too much ferrite, and its corrosion resistance is obviously lower than that of base metal. These limitations limited the application of the first generation duplex stainless steel to some specific applications in the non welded state. In 1968, the invention of stainless steel refining and argon oxygen decarburization (AOD) technology made a series of new stainless steel grades possible. One of the many advances AOD has brought is the deliberate addition of nitrogen as an alloying element. The toughness and corrosion resistance of HAZ in welding state are close to that of base metal when nitrogen is added to duplex stainless steel. With the improvement of Austenite Stability, nitrogen also reduces the formation rate of harmful intermetallic phase.
The nitrogen containing duplex stainless steel is called the second generation duplex stainless steel. This new commercialization progress started in the late 1970s, which coincides with the development of offshore oil and gas fields in the North Sea and the market demand for stainless steel with excellent chloride corrosion resistance, good processing performance and high strength. 2205 has become the main brand of the second generation duplex stainless steel and is widely used in gas gathering pipelines and treatment facilities of offshore oil platforms. Because of its high strength, the wall thickness can be reduced, thus reducing the weight of the platform, so the application of this stainless steel has great attraction.
Classification of duplex stainless steel
Up to now, the development of duplex stainless steel has gone through three generations. At present, all countries in the world have their brands. Typical duplex stainless steels can be divided into four categories:
- The first type belongs to low alloy type, the representative brand is UNS s32304 (23cr-4ni-0.1n), the steel does not contain molybdenum, the equivalent pre value of pitting corrosion resistance is 24-25, which can replace AISI304 (0Cr18Ni9) or 316 (ocr17ni12mo2) in stress corrosion resistance.
- The second type belongs to medium alloy type, the representative brand is UNS S31803 (22cr-5ni-3mo-0.15n), pre value is 32-33, its corrosion resistance is between AISI316L (oocr17ni14mo2) and 6% Mo + n austenitic stainless steel.
- The third type is high alloy type, generally containing 25% Cr, molybdenum and nitrogen, and some copper and tungsten. The standard brand is UNS s32550 (25cr-6ni-3mo-2cu-0.2n), and the pre value is 38-39. The corrosion resistance of this kind of steel is higher than that of duplex stainless steel with 22% CR.
- The fourth type is super duplex stainless steel with high molybdenum and nitrogen content. The standard brand is UNS S32750 (25cr-7ni-3.7mo-0.3n). Some of them also contain tungsten and copper. The pre value is more than 40, which is suitable for harsh medium conditions. It has good corrosion resistance and comprehensive mechanical properties, comparable to super austenitic stainless steel.
The local corrosion resistance of stainless steel has a strong correlation with the content of alloy elements. Cr, Mo, and N. W are the main elements that can increase the pitting resistance. Although W is not commonly used, its effective contribution is about half of Mo (in weight percentage). The relationship between the relative pitting resistance of stainless steel in chloride solution and the composition of stainless steel can be described by an empirical formula, which is called pre. The prEN calculation formula for austenitic stainless steel and duplex stainless steel is as follows:
- PrEN (pitting equivalent) =% Cr + 3.3 (% Mo + 0.5% w) + 16% n
UNS S31803
UNS S31803 is a typical duplex stainless steel very similar to UNS S32205 except that it may have a bit lower content requirement of chromium, molybdenum and nitrogen. Generally, both UNS 31803 and S32205 can be called as “Duplex 2205” in practice.
ASTM A789 UNS S31803 seamless pipes, 33mm x 1mm.
Chemical Composition of UNS S31803
| The Chemical Composition of S31803, % | |
|---|---|
| Carbon | 0-0.03 |
| Chromium | 21.00-23.00 |
| Manganese | ≤2.00 |
| Silicon | ≤1.00 |
| Phosphorous | 0-0.03 |
| Sulphur | 0-0.02 |
| Nickel | 4.50-6.50 |
| Nitrogen | 0.08-0.20 |
| Molybdenum | 2.50-3.50 |
| Iron | Balance |
Mechanical Properties of UNS S31803
| Tensile Strength,min. | Yield Strength,min. | Elongation,min. | Hardness Brinell, max |
|---|---|---|---|
| Mpa | Mpa | % | HB |
| 620 | 450 | 25 | 290 |
Physical Properties of UNS S31803
A floating ball valve with ASTM A182 F51 body.
| Density | Thermal Expansion | Modulus of Elasticity | Thermal Conductivity | Electrical Resistivity |
|---|---|---|---|---|
| g/cm3 | K | Gpa | W/m.k | Ω.m |
| 7.805 | 13.7*10-6 | 200 | 19.0 | 0.085*10-6 |
Product Forms and Standards
Duplex stainless steel UNS S31803 can be supplied in the following forms:
- Plates, sheets & strips: ASTM A240;
- Bars and shapes: ASTM A276, A479;
- Seamless and weld pipe/ tubes: ASTM A789, A790, A928;
- Wrought duplex pipe fittings: ASTM A815;
- Forgings (flanges, fittings, valve parts): ASTM A182, ASTM A988.
2205
Designated as UNS S32205 or DIN W.Nr. 1.4462, duplex 2205 is an austenitic & ferritic two-phase Cr-Mo-Ni alloyed stainless steel which has a roughly mixed 1:1 ferrite and austenite structure. It is the most widely used duplex stainless steel grade and is characterized by high-yield strength, double that of standard austenitic stainless steel grades. Piping components made from duplex 2205 have good fatigue strength, as well as outstanding resistance to stress-corrosion cracking, crevice, pitting, and general corrosion in severe environments. They are widely used in chemical processing, oil & gas piping system, marine & high chloride environments, pulp and paper industry, and food industry, etc. Duplex 2205 materials can be furnished in the form of plates, bars, pipes, tubes, fittings, flanges, and valves in accordance with relative standards such as ASTM A240, ASTM A479, ASTM A484, ASTM A276, ASTM A789, ASTM A790, ASTM A182, etc.
A valve body made of duplex 2205 stainless steel.
Chemical Composition of Duplex 2205
| The Chemical Composition of Duplex Stainless Steel 2205, % | |
|---|---|
| Carbon | ≤0.03 |
| Manganese | ≤2.00 |
| Silicon | ≤1.00 |
| Phosphate | ≤0.03 |
| Sulfur | ≤0.02 |
| Chromium | 21.0-23.0 |
| Nickel | 4.5-6.5 |
| Molybdenum | 2.5-3.5 |
| Nitrogen | 0.08-0.2 |
Mechanical Properties of Duplex 2205
| Tensile Strength | Yield Strength | Elongation | Rockwell | Charpy V-Notch Impact | |
|---|---|---|---|---|---|
| ksi.(Mpa) | ksi. (Mpa) | % | Hardness | mm-N/mm2 | mm-N/mm2 |
| 125(862) | 90(621) | 28 | 25C | 1310 | 844 |
*All values are minimum unless otherwise specified.
Manufacture, Heat Treatment & Weldability
Duplex stainless steel 2205 can be manufactured from billets, bars, pipes, or tubes through forging, bending, extruding, or pressing process. It has better formability than that of the ferritic stainless steel grades, but not quite as good as the austenitic stainless steels. Duplex 2205 shall be solution-annealed at 1020-1100°C and then be quenched in air or water quickly. 2205 duplex stainless steel can be readily welded with GTAW, GMAW, plasma and SMAW welding methods. The alloy shall be welded without preheating, followed by cooling to at least 300°F[150°C] between each step. A welding procedure should be developed so that a ferrite content of 30-55% is present in the weld fusion and heat affected zones of the final welding.
Properties of duplex stainless steel
Advantages of duplex stainless steel
Compared with austenitic stainless steel, the advantages of duplex stainless steel are as follows:
- (1) The yield strength of austenitic stainless steel is more than twice that of ordinary stainless steel, and it has enough plastic toughness for forming. The wall thickness of tank or pressure vessel made of duplex stainless steel is 30? 50% less than that of austenite, which is helpful to reduce the cost.
- (2) Even duplex stainless steel with the lowest alloy content has a higher ability of stress corrosion cracking resistance than austenitic stainless steel, especially in chloride containing environment. Stress corrosion is an outstanding problem that is difficult to solve for ordinary austenitic stainless steel.
- (3) The corrosion resistance of 2205 duplex stainless steel, which is widely used in many media, is better than 316L austenitic stainless steel, while super duplex stainless steel has very high corrosion resistance. In some media, such as acetic acid, formic acid, etc., it can even replace high alloy austenitic stainless steel and even corrosion resistant alloy.
- (4) Compared with austenitic stainless steel with the same alloy content, it has better wear resistance and fatigue corrosion resistance than austenitic stainless steel.
- (5) Compared with austenitic stainless steel, the coefficient of linear expansion is lower, close to carbon steel, suitable for connection with carbon steel, which has important engineering significance, such as production of composite plates or linings.
- (6) No matter in the condition of dynamic load or static load, duplex stainless steel has higher energy absorption capacity than austenitic stainless steel, which has obvious advantages in dealing with sudden accidents such as collision, explosion and so on. Duplex stainless steel has practical application value.
Compared with ferritic stainless steel, the advantages of duplex stainless steel are as follows:
- (1) The comprehensive mechanical properties of ferritic stainless steel are better than that of ferritic stainless steel, especially the plastic toughness, which is not as sensitive to brittleness as ferritic stainless steel.
- (2) In addition to stress corrosion resistance, other local corrosion resistance are better than ferritic stainless steel.
- (3) The cold processing and cold forming properties of duplex stainless steel are much better than that of ferritic stainless steel.
- (4) The welding performance is also far better than that of ferritic stainless steel. Generally, preheating is not required before welding and heat treatment is not required after welding.
- (5) The application range is wider than that of ferritic stainless steel.
Disadvantages of duplex stainless steel
Compared with austenitic stainless steel, the disadvantages of duplex stainless steel are as follows:
- (1) The universality and versatility of the application are not as good as austenitic stainless steel, and the heat resistance is lower. The service temperature must be controlled below 300 ℃.
- (2) Its plastic toughness is lower than that of austenitic stainless steel, and the work hardening effect of cold working is greater than that of austenitic stainless steel. Duplex stainless steel containing 25% Cr is more difficult to hot work than austenitic stainless steel.
- (3) In order to avoid the appearance of harmful phase and damage the performance, the process system of heat treatment and welding should be strictly controlled.
Compared with ferritic stainless steel, the disadvantages of duplex stainless steel are as follows:
The content of alloy elements is high, the price is relatively high, and generally ferritic stainless steel does not contain nickel.
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 first processing industry using duplex stainless steel. Using duplex stainless steel instead of 304 and 316 austenitic stainless steel to make sulfuric acid and sulfite cooking pot can improve the corrosion resistance and strength of the cooking pot, reduce the wall thickness and the total weight of the cooking pot.
Chemical processing industry: the chemical processing industry involves a wide range and complex working conditions. The storage tank for industrial wet process phosphoric acid is usually made of 316L or 317L austenitic stainless steel, but its wall is very thick. Adopting duplex stainless steel 2205, the strength is almost twice as high as 316L and 317L, and the wall thickness can be reduced, thus reducing the importance and cost of storage tank. At the same time, in the industrial sulfuric acid environment, especially in the presence of chloride ions, 2205 has better local corrosion resistance than austenitic stainless steel.
Food and beverage industry: the production of food and beverage industry is generally processed in the hot solution containing chloride ion. Duplex stainless steel has unique advantages in stress corrosion cracking resistance. Therefore, different duplex stainless steels can be selected according to different environments, but the total cost is also superior to austenitic stainless steel.
Transportation industry: due to the high strength and excellent corrosion resistance of duplex stainless steel, it has a great advantage in making chemical ships. It reduces the importance of the ship and the fuel consumption. In this respect, 22% chromium duplex stainless steel is the most widely used. At the same time, it can meet the corrosion resistance of the materials needed for the goods.
Construction industry: in the coastal areas, the air is full of the marine atmosphere evaporated from the sea water, in which the concentration of chloride ion is much higher than that of the air in the inland. Therefore, seawater corrosion-resistant materials are needed. The corrosion resistance of duplex stainless steel has this advantage, at the same time, its high strength can also reduce the weight. At present, many offshore oil platforms use duplex stainless steel to make explosion-proof walls.
Petrochemical Industry: the corrosion environment of petrochemical industry and chemical industry is characterized by high reaction temperature and medium concentration of chloride, which is easy to induce stress corrosion cracking of stainless steel. Therefore, this field not only needs duplex stainless steel, but also uses super duplex stainless steel.
The oil refining industry is one of the fields where duplex stainless steel is widely used. It is mainly used in atmospheric and vacuum distillation, catalytic cracking, hydrodesulfurization and other units, both at home and abroad. In addition, the use of duplex stainless steel in the production of acid gas and oil is gradually increasing, which is mainly used for production pipe lining, onshore and offshore pipeline systems and heat exchangers.
In addition, duplex stainless steel has also been successfully used in waste water treatment heat exchanger, hot water pipe and water heater.
Comparison of advantages and disadvantages between duplex stainless steel and other stainless steel
The so-called two-phase stainless steel is in its solid solution structure, the ferrite phase and austenite phase account for about half each, and the content of the minimum phase generally needs to reach 30%. Therefore, it has the performance characteristics of both ferritic stainless steel and austenitic stainless steel. Compared with austenitic stainless steel, the advantages of duplex stainless steel are as follows:
- ① The yield strength of austenitic stainless steel is more than twice that of ordinary stainless steel, and it has enough plastic toughness for forming. The wall thickness of tank or pressure vessel made of duplex stainless steel is 30% ~ 50% less than that of austenitic stainless steel, which is beneficial to reduce the cost.
- ② It has excellent resistance to stress corrosion cracking, especially in the environment containing chloride ions, even the duplex stainless steel with the lowest alloy content has higher resistance to stress corrosion cracking than austenitic stainless steel. Stress corrosion is a prominent problem that ordinary austenitic stainless steel is difficult to solve.
- ③ The corrosion resistance of 2205 duplex stainless steel, which is widely used in many media, is better than that of 316L austenitic stainless steel, while super duplex stainless steel has very high corrosion resistance. In some media, such as acetic acid, formic acid, etc., it can even replace high alloy austenitic stainless steel and even corrosion resistant alloy.
- ④ Compared with austenitic stainless steel with the same alloy content, duplex stainless steel is superior to austenitic stainless steel in terms of its high strength and good corrosion resistance.
- ⑤ Compared with austenitic stainless steel, the coefficient of linear expansion is lower, close to carbon steel, suitable for connection with carbon steel, which has important engineering significance, such as the production of composite plates or linings.
- ⑥ It has higher energy absorption capacity than austenitic stainless steel under dynamic or static load. It has obvious advantages and practical application value for structural parts to deal with sudden accidents such as collision, explosion and so on.
Compared with austenitic stainless steel, the disadvantages of duplex stainless steel are as follows:
- ① The universality and versatility of application are not as good as austenitic stainless steel. For example, the service temperature of long-term application must be controlled below 250 ℃.
- ② Its plastic toughness is lower than that of austenitic stainless steel, and its cold and hot working process and forming properties are not as good as austenitic stainless steel.
- ③ In order to avoid the appearance of harmful phase and damage the performance, the process system of heat treatment and welding should be strictly controlled.
The disadvantage of duplex stainless steel compared with austenitic stainless steel is due to the large amount of ferrite structure in duplex stainless steel.
Compared with ferritic stainless steel, the advantages of duplex stainless steel are as follows:
- ① The comprehensive mechanical properties are higher than that of ferritic stainless steel, especially the plastic toughness. It is not as sensitive to brittleness as ferritic stainless steel.
- ② In addition to stress corrosion resistance, other local corrosion resistance are better than ferritic stainless steel.
- ③ The properties of cold working process and cold forming are much better than that of ferritic stainless steel.
- ④ The welding performance is also far better than that of ferritic stainless steel. Generally, preheating is not required before welding and heat treatment is not required after welding.
- ⑤ The application range is wider than that of ferritic stainless steel.
The advantage of duplex stainless steel compared with ferritic stainless steel is due to the large amount of austenite structure in duplex stainless steel.
Compared with ferritic stainless steel, the other disadvantage of duplex stainless steel is the high content of alloying elements and relatively high price, and generally ferritic stainless steel does not contain nickel.
In conclusion, it can be concluded that duplex stainless steel has a complete picture of service performance and process performance. It has won the favor of users with its superior mechanical and corrosion-resistant comprehensive performance, and has become an excellent corrosion-resistant engineering material that saves both weight and investment.
Source: China Duplex Stainless Steel Flanges Manufacturer – Yaang Pipe Industry (www.epowermetals.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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