Duplex stainless steel is called “duplex” because it has two different microstructure phases – austenite and ferrite – coexisting in the same material. Duplex stainless steel is a type of steel that combines excellent corrosion resistance, high strength and easy manufacturing and processing. Its physical properties are closer to ferritic stainless steel and carbon steel. 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.
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Typical Compositions of Duplex Stainless Steels
Duplex stainless steel has a different chemical composition than austenitic and ferritic stainless steels. Its main alloying elements include:
Chromium (Cr): The content is relatively high, usually between 18% and 28%, providing excellent corrosion resistance.
Nickel (Ni): The content is relatively low, usually between 3% and 9%, used to stabilize the austenite phase.
Molybdenum (Mo): The content is relatively high, up to 5%, which significantly improves the ability to resist pitting, crevice corrosion and chloride stress corrosion cracking.
Nitrogen (N): An important alloying element that can increase strength, toughness, and further improve corrosion resistance, especially in weld areas.
| Common Name | UNS No | BS EN No | Steel Type | Typical Chemical Composition % | 0.2% proof strength N/mm2 (min) | |||||
| %C | Cr | Ni | Mo | N | Cu | |||||
| 2304 | S32304 | 1.4362 | duplex | 0.015 | 23 | 4 | 0.055 | 0.13 | 400 | |
| 2205 | S31803 | 1.4462 | duplex | 0.015 | 22 | 5.5 | 3 | 0.14 | – | 450 |
| 2205 | S32205 | 1.4462 | duplex | 0.015 | 22.5 | 5.5 | 3.3 | 0.17 | 450 | |
| 255(UR52N) | S32520 | 1.4507 | super duplex | 0.015 | 25 | 7 | 3-5 | 0.28 | 0.13 | 550 |
| 2507 | S32750 | 1.441 | super duplex | 0.015 | 25 | 7 | 4.5 | 0.28 | 0.3 | 550 |
| Zeron 100 | S32760 | 1.4501 | super duplex | 0.015 | 25 | 7 | 3.5 | 0.25 | 0.8 | 550 |
| Sandvik SAF3207 | S33207 | – | hyper duplex | 0.03 | 31 | 7.5 | 4 | 0.5 | 0.75 | 700 |
Classification of Duplex Stainless Steel
The first category is low alloy type, the representative grade is UNS S32304, the steel does not contain molybdenum, the PREN value is 24-25. And it can be used in place of AISI304 or 316 in terms of stress corrosion resistance.
The second category is medium alloy type, the representative grade is UNS S31803, PREN value is 32-33, its corrosion resistance is between AISI 316L and 6% Mo + N austenitic stainless steel.
The third category is a high-alloy type, generally containing 25% Cr, but also contains molybdenum and nitrogen. Some also contain copper and tungsten, standard grade UNSS32550, PREN value of 38-39, the corrosion resistance of this type of steel is higher than that of the duplex stainless steel with a 22% Cr.
The fourth category is super duplex stainless steel type, containing high molybdenum and nitrogen, standard grade UNS S32750. Some also contain tungsten and copper, PREN value is greater than 40, can be applied to harsh media conditions, has a good corrosion resistance and mechanical composite performance, can be comparable with the super austenitic stainless steel.
Duplex Stainless Steel: Advantages and Disadvantages
Advantages：
High strength and corrosion resistance: This is the most significant advantage of duplex stainless steels, making them ideal for use in many harsh environments.
High resistance to stress corrosion cracking: Addresses the susceptibility of austenitic stainless steels to stress corrosion cracking under certain conditions.
Cost Effectiveness: Although the unit price may be higher than normal austenitic stainless steels, the overall cost may be lower due to its high strength allowing for thinner wall thicknesses and reduced weight.
Good weldability and workability.
Disadvantages:
Brittle Phase Precipitation: Prolonged residence in the temperature ranges of 400-500°C and 800-900°C tends to precipitate brittle phases, resulting in a reduction in toughness and corrosion resistance. Therefore, its maximum service temperature is usually limited to about 300°C.
Poor hot working performance: Due to the different deformation behavior of the two phases, uneven stresses are easily generated during hot working, resulting in phase boundary cracks.
Work-hardening tendency: Duplex stainless steel has a stronger tendency to work-hardening, which requires higher requirements for processing equipment and technology.
Corrosion Resistance of Duplex Stainless Steel
1. pitting and crevice corrosion
Duplex stainless steel pitting and crevice corrosion resistance is mainly determined by the elemental content of Cr, Mo and Ni, used to measure the corrosion resistance of this index is the PREN value, high chromium, high molybdenum and nitrogen content to make its resistance to localized corrosion in the chloride environment is very strong. Selection of the appropriate solution treatment temperature so that each of the two phases has a comparable PREN value is the only way to give the steel optimum resistance to pitting corrosion.
2. Stress corrosion
Duplex stainless steel yield strength is higher than ordinary stainless steel, so the critical stress value of stress corrosion cracking (SCC) is also high; in neutral chloride media, stainless steel more pitting as the source of cracking of SCC, and duplex stainless steel composition and organizational characteristics of pitting corrosion is not easy to form, prolonging the gestation period of pitting corrosion. So anti-duplex stainless steel SCC resistance is very strong.
3. Corrosion fatigue
Corrosion fatigue is caused by the combined effect of corrosion and cyclic stress and a corrosion pattern, the stress suffered by most of the cyclic stress. When the cyclic stress is relatively small and the corrosive medium is strong, the source of corrosion fatigue often arises in the surface area of the non-metallic inclusions, which in turn produces localized corrosion, and for the duplex stainless steel, especially high-chromium duplex stainless steel, both with a high resistance to localized corrosion and high strength, so the duplex stainless steel corrosion fatigue resistance is also very strong.
Machinability
Duplex stainless steel also has good processing properties. Compared with pure austenitic stainless steel, it has lower resistance to hot working deformation and is easier to form during hot working processes such as forging and rolling. Although work hardening will occur during cold working, the hardening rate is relatively slow, allowing a certain degree of cold forming operations, such as stamping and bending, to facilitate the manufacture of various complex shapes of parts. In addition, the welding performance of duplex stainless steel is also relatively good. Reasonable selection of welding materials and processes can ensure that the welded joint has similar performance to the parent material.
Weldability
Duplex stainless steel has good weldability. However, its welding process requires fine control to ensure that the weld and heat-affected zone maintain the ideal ferrite-austenite ratio. By selecting suitable welding materials, controlling heat input, and using the correct welding method and shielding gas, high-performance duplex stainless steel welded joints can be successfully obtained, thus giving full play to its advantages of high strength and excellent corrosion resistance.
Main Applications
Oil and gas industry:
Used for fluid pipes, process pipeline systems, separation units, washing units, pumps, umbilical cables, etc., especially in harsh environments containing chlorides and hydrogen sulfide.
Chemical and petrochemical industries:
Manufacturing corrosion-resistant containers, pipelines, heat exchangers, evaporators and other equipment, especially in media with the risk of stress corrosion cracking.
Desalination equipment:
Used for seawater cooling heat exchangers, pumps, valves, etc., effectively resisting seawater corrosion.
Pulp and paper industry:
Used for pulp digesters, bleaching equipment, etc., to resist corrosive chemicals.
Food and beverage industry:
Manufacturing storage tanks, pipelines, equipment, etc., to ensure hygiene and corrosion resistance.
Structural applications:
Due to its high strength, it can be used for bridge load-bearing structures, storage tanks, etc. to achieve lightweight design.