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Duplex stainless steel
Stainless steel that has both austenitic and ferritic phases
Stainless steel that has both austenitic and ferritic phases
Duplex stainless steels are a family of stainless steels. These are called duplex (or austenitic-ferritic) grades because their metallurgical structure consists of two phases, austenite (face-centered cubic lattice) and ferrite (body centered cubic lattice) in roughly equal proportions.
They provide better corrosion resistance, particularly chloride stress corrosion and chloride pitting corrosion, and higher strength than standard austenitic stainless steels such as A2/304 or A4/316. The main differences in composition, when compared with austenitic stainless steel is that duplex steels have a higher chromium content, 20–28%; higher molybdenum, up to 5%; lower nickel, up to 9% and 0.05–0.50% nitrogen. Both the low nickel content and the high strength (enabling thinner sections to be used) give significant cost benefits. Duplex steels also have higher strength. For example, a Type 304 stainless steel has a 0.2% proof strength in the region of 280 MPa, a 22%Cr duplex stainless steel a minimum 0.2% proof strength of some 450 MPa and a superduplex grade a minimum of 550 MPa.
Duplex steels are used extensively in the offshore oil and gas industry for pipework systems, manifolds, risers, etc. and in the petrochemical industry for pipelines and pressure vessels.
Grades of duplex stainless steels
Duplex stainless steels are usually divided into three groups based on their pitting corrosion resistance, characterised by the pitting resistance equivalence number, .
; Standard duplex (PREN range 28–38): Typically Grade EN 1.4462 (also called 2205). It is typical of the mid-range of properties and is perhaps the most used today ; Super-duplex (PREN range 38–45): Typically grade EN 1.4410 up to so-called hyper duplex grades (PREN: 45) developed later to meet specific demands of the oil and gas as well as those of the chemical industries. They offer a superior corrosion resistance and strength but are more difficult to process because the higher contents of Cr, Mo, N and even W promote the formation of intermetallic phases, which reduce drastically the impact resistance of the steel. Faulty processing will result in poor performance and users are advised to deal with reputable suppliers/processors. Applications include deepwater offshore oil production. ; Lean duplex grades (PREN range 22–27): Typically grade EN 1.4362, have been developed more recently for less demanding applications, particularly in the building and construction industry. Their corrosion resistance is closer to that of the standard austenitic grade EN 1.4401 (with a plus on resistance to stress corrosion cracking) and their mechanical properties are higher. This can be a great advantage when strength is important. This is the case in bridges, pressure vessels or tie bars.
Chemical compositions
Chemicals composition of grades from EN 10088-1 (2014) Standard are given in the table below:
| ISO Steel designation | EN Number | title=Stainless steel grades listed in the international standard ISO 15510:2010 Comparative designations of grades with similar composition from other important standards. (listed by type of steel structure and by increasing intermediate 3-digits code of the ISO name) | url=https://www.duplexsteelalloys.com/assets/pdf/stainless-steel-grades-listed-in-the-international-standard-iso-155102010.pdf | access-date=10 March 2023 | website=International Stainless Steel Forum}} | C, max. | Si | Mn | P, max. | S, max. | N | Cr | Cu | Mo | Ni | Other |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| X2CrNiN22-2 | 1.4062 | S32202 | 0.03 | ≤1.00 | ≤2.00 | 0.04 | 0.010 | 0.16 to 0.28 | 21.5 to 24.0 | - | ≤0.45 | 1.00 to 2.90 | - | |||
| X2CrCuNiN23-2-2 | 1.4669 | 0.045 | ≤1.00 | 1.00 to 3.00 | 0.04 | 0.030 | 0.12 to 0.20 | 21.5 to 24.0 | 1.60 to 3.00 | ≤0.50 | 1.00 to 3.00 | - | ||||
| X2CrNiMoSi18-5-3 | 1.4424 | S31500 | 0.03 | 1.40 to 2.00 | 1.20 to 2.00 | 0.035 | 0.015 | 0.05 to 0.10 | 18.0 to 19.0 | - | 2.5 to 3.0 | 4.5 to 5.2 | - | |||
| X2CrNiN23-4 | 1.4362 | S32304 | 0.03 | ≤1.00 | ≤2.00 | 0.035 | 0.015 | 0.05 to 0.20 | 22.0 to 24.5 | 0.10 to 0.60 | 0.10 to 0.60 | 3.5 to 5.5 | - | |||
| X2CrMnNiN21-5-1 | 1.4162 | S32101 | 0.04 | ≤1.00 | 4.0 to 6.0 | 0.040 | 0.015 | 0.20 to 0.25 | 21.0 to 22.0 | 0.10 to 0.80 | 0.10 to 0.80 | 1.35 to 1.90 | - | |||
| X2CrMnNiMoN21-5-3 | 1.4482 | 0.03 | ≤1.00 | 4.0 to 6.0 | 0.035 | 0.030 | 0.05 to 0.20 | 19.5 to 21.5 | ≤1.00 | 0.10 to 0.60 | 1.50 to 3.50 | - | ||||
| X2CrNiMoN22-5-3 | 1.4462 | S31803, | 0.03 | ≤1.00 | ≤2.00 | 0.035 | 0.015 | 0.10 to 0.22 | 21.0 to 23.0 | - | 2.50 to 3.50 | 4.5 to 6.5 | - | |||
| X2CrNiMnMoCuN24-4-3-2 | 1.4662 | 0.03 | ≤0.70 | 2.5 to 4.0 | 0.035 | 0.005 | 0.20 to 0.30 | 23.0 to 25.0 | 0.10 to 0.80 | 1.00 to 2.00 | 3.0 to 4.5 | |||||
| X2CrNiMoCuN25-6-3 | 1.4507 | S32520 | 0.03 | ≤0.70 | ≤2.00 | 0.035 | 0.015 | 0.20 to 0.30 | 24.0 to 26.0 | 1.00 to 2.50 | 3.0 to 4.0 | 6.0 to 8.0 | - | |||
| X3CrNiMoN27-5-2 | 1.4460 | S31200 | 0.05 | ≤1.00 | ≤2.00 | 0.035 | 0.015 | 0.05 to 0.20 | 25.0 to 28.0 | - | 1.30 to 2.00 | 4.5 to 6.5 | - | |||
| X2CrNiMoN25-7-4 | 1.4410 | S32750 | 0.03 | ≤1.00 | ≤2.00 | 0.035 | 0.015 | 0.24 to 0.35 | 24.0 to 26.0 | - | 3.0 to 4.5 | 6.0 to 8.0 | - | |||
| X2CrNiMoCuWN25-7-4 | 1.4501 | S32760 | 0.03 | ≤1.00 | ≤1.00 | 0.035 | 0.015 | 0.20 to 0.30 | 24.0 to 26.0 | 0.50 to 1.00 | 3.0 to 4.0 | 6.0 to 8.0 | W 0.50 to 1.00 | |||
| X2CrNiMoN29-7-2 | 1.4477 | S32906 | 0.03 | ≤0.50 | 0.80 to 1.50 | 0.030 | 0.015 | 0.30 to 0.40 | 28.0 to 30.0 | ≤0.80 | 1.50 to 2.60 | 5.8 to 7.5 | - | |||
| X2CrNiMoCoN28-8-5-1 | 1.4658 | S32707 | 0.03 | ≤0.50 | ≤1.50 | 0.035 | 0.010 | 0.30 to 0.50 | 26.0 to 29.0 | ≤1.00 | 4.0 to 5.0 | 5.5 to 9.5 | Co 0.50 to 2.00 | |||
| X2CrNiCuN23-4 | 1.4655 | S32304 | 0.03 | ≤1.00 | ≤2.00 | 0.035 | 0.015 | 0.05 to 0.20 | 22.0 to 24.0 | 1.00 to 3.00 | 0.10 to 0.60 | 3.5 to 5.5 | - |
Mechanical properties
Mechanical properties from European Standard EN 10088-3 (2014) (for product thickness below 160mm):
| ISO desig. | EN num. | 0.2% proof stress, min | Ultimate tensile strength | Elongation, min (%) |
|---|---|---|---|---|
| X2CrNiN23-4 | 1.4362 | 400 MPa | 600 to | 25 |
| X2CrNiMoN22-5-3 | 1.4462 | 450 MPa | 650 to | 25 |
| X3CrNiMoN27-5-2 | 1.4460 | 450 MPa | 620 to | 20 |
| X2CrNiN22-2 | 1.4062 | 380 MPa | 650 to | 30 |
| X2CrCuNiN23-2-2 | 1.4669 | 400 MPa | 650 to | 25 |
| X2CrNiMoSi18-5-3 | 1.4424 | 400 MPa | 680 to | 25 |
| X2CrMnNiN21-5-1 | 1.4162 | 400 MPa | 650 to | 25 |
| X2CrMnNiMoN21-5-3 | 1.4482 | 400 MPa | 650 to | 25 |
| X2CrNiMnMoCuN24-4-3-2 | 1.4662 | 450 MPa | 650 to | 25 |
| X2CrNiMoCuN25-6-3 | 1.4507 | 500 MPa | 700 to | 25 |
| X2CrNiMoN25-7-4 | 1.4410 | 530 MPa | 730 to | 25 |
| X2CrNiMoCuWN25-7-4 | 1.4501 | 530 MPa | 730 to | 25 |
| X2CrNiMoN29-7-2 | 1.4477 | 550 MPa | 750 to | 25 |
| X2CrNiMoCoN28-8-5-1* | 1.4658 | 650 MPa | 800 to | 25 |
*for thickness ≤ 5 mm
The minimum yield stress values are about twice as high as those of austenitic stainless steels.
Duplex grades are therefore attractive when mechanical properties at room temperature are important because they allow thinner sections.
475 °C embrittlement
Duplex stainless is widely used in the industry because it possesses excellent oxidation resistance but can have limited toughness due to its large ferritic grain size, and they have hardened, and embrittlement tendencies at temperatures ranging from 280 to 500 °C, especially at 475 °C, where spinodal decomposition of the supersaturated solid ferrite solution into Fe-rich nanophase (\acute{a}) and Cr-rich nanophase (\acute{a}\acute{}), accompanied by G-phase precipitation, occurs, which makes the ferrite phase a preferential initiation site for micro-cracks.
Heat treatment
| UNS No. Grade | EN No. | Hot forming temperature range | Minimum soaking temperature |
|---|---|---|---|
| S32304 | 1.4362 | 1150 to | 980 C |
| S32205 | 1.4462 | 1230 to | 1040 C |
| S32750 | 1.4410 | 1235 to | 1050 C |
| S32520 | 1.4507 | 1230 to | 1080 C |
| S32760 | 1.4501 | 1230 to | 1100 C |
Duplex stainless steel grades must be cooled as quickly as possible to room temperature after hot forming to avoid the precipitation of intermetallic phases (Sigma phase in particular) which drastically reduce the impact resistance at room temperature as well as the corrosion resistance.
Alloying elements Cr, Mo, W, Si increase the stability and the formation of intermetallic phases. Therefore, super duplex grades have a higher hot working temperature range and require faster cooling rates than the lean duplex grades.
Applications of duplex stainless steels
Duplex stainless steels are usually selected for their high mechanical properties and good to very high corrosion resistance (particularly to stress corrosion cracking).
- Architecture
- Stockholm's waterfront building
- Louvre Abu Dhabi
- La Sagrada Familia
- Infrastructure:
- Helix Bridge, Singapore
- Cala Galdana bridge
- Hong Kong–Zhuhai–Macau bridge and undersea tunnel
- sea walls, piers, etc.
- tunnels
- Oil and gas:
- a wide range of equipment: flowlines, manifolds, risers, pumps, valves, etc.
- Pulp and paper:
- digesters, pressure vessels, liquor tanks, etc.
- Chemical engineering:
- pressure vessels, heat exchangers, condensers, distillation columns, agitators, marine chemical tankers, etc.
- Water:
- desalination plants, large tanks for water storage, waste water treatment
- renewable energy: Biogas tanks
- Mobility: tramcars and bus frames, tank trucks, iron ore wagons
- Engineering: pumps, valves, fittings, springs, etc.
- Duplex and super-duplex stainless steels are also available in a variety of commercial product forms, including bars, pipes, plates, flanges, fittings, and fasteners. Frequently used grades, which are selected based on their mechanical strength and resistance to corrosion. These product forms and grades support a wide range of industrial applications, including chemical processing, marine, and offshore environments.
References
References
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- (1990). "Les Aciers Inoxydables". Les Editions de Physique.
- International Molybdenum Association (IMOA). (2014). "Practical Guidelines for the fabrication of Duplex Stainless Steels".
- Charles, Jacques. (2010). "Proceedings of the Duplex Stainless Steel Conference, Beaune (2010)". EDP Sciences, Paris.
- International Stainless Steel Forum. (2020). "Duplex Stainless Steels".
- "A Practical Guide to Using Duplex Stainless Steels".
- Bristish Stainless Steel Association. "Calculation of Pitting Resistance Equivalent Number (PREN)".
- "Knowledge center — Sandvik Materials Technology".
- "The standard is available from BSI Shop".
- "Stainless steel grades listed in the international standard ISO 15510:2010 Comparative designations of grades with similar composition from other important standards. (listed by type of steel structure and by increasing intermediate 3-digits code of the ISO name)".
- Mohamed Koko, A.. (2022). "In situ full-field characterisation of strain concentrations (deformation twins, slip bands and cracks)". University of Oxford.
- (2022-10-15). "In situ characterisation of the strain fields of intragranular slip bands in ferrite by high-resolution electron backscatter diffraction". Acta Materialia.
- (April 2017). "748 K (475 °C) Embrittlement of Duplex Stainless Steel: Effect on Microstructure and Fracture Behavior". Metallurgical and Materials Transactions A.
- (2004-08-15). "The low-temperature aging embrittlement in a 2205 duplex stainless steel". Materials Science and Engineering: A.
- (July 1956). "A New Complex Phase in a High-Temperature Alloy". Nature.
- (August 2018). "Characterization of Impact Deformation Behavior of a Thermally Aged Duplex Stainless Steel by EBSD". Acta Metallurgica Sinica (English Letters).
- International Molybdenum Association (IMOA). "Hot forming and Heat Treatment of Duplex Stainless Steels".
- Euro-Inox. "Innovative Facades in Stainless Steel". Euro-Inox Publication, Building series.
- International Molybdenum Association. (2019). "Louvre Abu Dhabi: A rain of light". Moly Review.
- (June 2018). "Basilica de la Sagrada familia". Cedinox.
- Steel Construction Institute. (2012). "Helix Pedestrian Bridge".
- (2010). "Cala Galdana Bridge". Steel Construction Institute.
- "Hong Kong-Zhuhai-Macau Bridge: the world's longest sea bridge".
- Zuili, D. (2010). "The use of stainless steels in oil & gas industry". Proceedings of the Duplex Stainless Steel Conference.
- Chater, James. (2007). "The pulp and paper industry turns to duplex". Stainless steel world.
- Notten, G. (1997). "Application of Duplex Stainless Steel in the chemical process industry". Stainless Steel World.
- Directorate-General for Research and Innovation. (2013). "Duplex stainless steels in storage tanks". EU Publication.
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