Duplex stainless steel explained

Duplex stainless steels^{[1] [2] [3] [4] [5]} 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 are designed to provide better corrosion resistance, particularly chloride stress corrosion and chloride pitting corrosion, and higher strength than standard austenitic stainless steels such as type A2/304 or A4/316. The main differences in composition, when compared with an austenitic stainless steel is that the 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. They are therefore used extensively in the offshore oil and gas industry for pipework systems, manifolds, risers, etc. and in the petrochemical industry in the form of pipelines and pressure vessels. In addition to the improved corrosion resistance compared with the 300 series duplex stainless steels also have higher strength. For example, a Type 304 stainless steel has a 0.2% proof strength in the region of, a 22%Cr duplex stainless steel a minimum 0.2% proof strength of some and a superduplex grade a minimum of .^[6]

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, .^[7]

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.^[8] 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:^[9]

Composition by weight (%)

ISO Steel designation	EN Number	UNS equiv^[10]	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,S32205	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)^[9] (for product thickness below 160mm):

Mechanical properties at room temperature of solution-annealed austenitic–ferritic stainless steels!ISO desig.!EN num.! 0.2% proof stress, min! Ultimate tensile strength! Elongation, min (%)

X2CrNiN23-4	1.4362			25
X2CrNiMoN22-5-3	1.4462			25
X3CrNiMoN27-5-2	1.4460			20
X2CrNiN22-2	1.4062			30
X2CrCuNiN23-2-2	1.4669			25
X2CrNiMoSi18-5-3	1.4424			25
X2CrMnNiN21-5-1	1.4162			25
X2CrMnNiMoN21-5-3	1.4482			25
X2CrNiMnMoCuN24-4-3-2	1.4662			25
X2CrNiMoCuN25-6-3	1.4507			25
X2CrNiMoN25-7-4	1.4410			25
X2CrNiMoCuWN25-7-4	1.4501			25
X2CrNiMoN29-7-2	1.4477			25
X2CrNiMoCoN28-8-5-1*	1.4658			25

*for thickness ≤

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

EBSD map with austenite grains excluded (white). The scale bar is 500 μm. Colours denote the crystal orientation and are taken from the inverse pole figure at the lower right corner. 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,^{[11] [12] [13]} which makes the ferrite phase a preferential initiation site for micro-cracks.^[14]

Heat treatment

Recommended hot forming and annealing/soaking temperatures! UNS No. Grade! EN No.! Hot forming temperature range! Minimum soaking temperature

S32304	1.4362
S32205	1.4462
S32750	1.4410
S32520	1.4507
S32760	1.4501

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.^[15]

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 ^[16]
  • Louvre Abu Dhabi^[17]
  • La Sagrada Familia^[18]
• Infrastructure:
  • Helix Bridge, Singapore ^[19]
  • Cala Galdana bridge ^[20]
  • Hong Kong–Zhuhai–Macau bridge and undersea tunnel ^[21]
  • sea walls, piers, etc.
  • tunnels
• Oil and gas:
  • a wide range of equipment: flowlines, manifolds, risers, pumps, valves, etc.^[22]
• Pulp and paper:
  • digesters, pressure vessels, liquor tanks, etc.^[23]
• Chemical engineering:
  • pressure vessels, heat exchangers, condensers, distillation columns, agitators, marine chemical tankers, etc.^[24]
• Water:
  • desalination plants, large tanks for water storage, waste water treatment ^[25]
• renewable energy: Biogas tanks
• Mobility: tramcars and bus frames, tank trucks, iron ore wagons
• Engineering: pumps, valves, fittings, springs, etc.

Further reading

• TMR Stainless. Practical Guidelines for the Fabrication of Duplex Stainless Steels. 3rd ed. International Molybdenum Association (IMOA); 2014.

See also

• Age-hardened duplex stainless steel
• Spinodal decomposition

Notes and References

1. Book: Peckner, Donald. Handbook of Stainless Steels. Bernstein. I.M.. McGraw Hill. 1977. 9780070491472. chapter 8.
2. Book: Lacombe, P.. Les Aciers Inoxydables. Baroux. B.. Beranger. G.. Les Editions de Physique. 1990. 2-86883-142-7. chapter 18.
3. Book: International Molybdenum Association (IMOA). Practical Guidelines for the fabrication of Duplex Stainless Steels. 2014. www.imoa.info. 978-1-907470-09-7.
4. Book: Charles, Jacques. Proceedings of the Duplex Stainless Steel Conference, Beaune (2010). EDP Sciences, Paris. 2010. 29–82. 2019-10-27. 2022-05-06. https://web.archive.org/web/20220506081328/https://pubs.kci-webshop.com/Webshop/Product/BOOKS/Duplex-Stainless-Steel---DSS-2010-Conference-Proceedings.html. dead.
5. Web site: Duplex Stainless Steels. International Stainless Steel Forum. 2020.
6. Web site: Dr. James Fritz . A Practical Guide to Using Duplex Stainless Steels . Nickel Institute.
7. Web site: Calculation of Pitting Resistance Equivalent Number (PREN). Bristish Stainless Steel Association. bssa.org.uk.
8. Web site: Knowledge center — Sandvik Materials Technology. www.materials.sandvik. en. 2019-03-25.
9. Web site: The standard is available from BSI Shop.
10. Web site: 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) . 10 March 2023 . International Stainless Steel Forum.
11. Örnek . Cem . Burke . M. G. . Hashimoto . T. . Engelberg . D. L. . April 2017 . 748 K (475 °C) Embrittlement of Duplex Stainless Steel: Effect on Microstructure and Fracture Behavior . Metallurgical and Materials Transactions A . en . 48 . 4 . 1653–1665 . 2017MMTA...48.1653O . 10.1007/s11661-016-3944-2 . 1073-5623 . free . 136321604.
12. Weng . K. L . Chen . H. R . Yang . J. R . 2004-08-15 . The low-temperature aging embrittlement in a 2205 duplex stainless steel . Materials Science and Engineering: A . en . 379 . 1 . 119–132 . 10.1016/j.msea.2003.12.051 . 0921-5093.
13. Beattie . H. J. . Versnyder . F. L. . July 1956 . A New Complex Phase in a High-Temperature Alloy . Nature . en . 178 . 4526 . 208–209 . 1956Natur.178..208B . 10.1038/178208b0 . 1476-4687 . 4217639.
14. Liu . Gang . Li . Shi-Lei . Zhang . Hai-Long . Wang . Xi-Tao . Wang . Yan-Li . August 2018 . Characterization of Impact Deformation Behavior of a Thermally Aged Duplex Stainless Steel by EBSD . Acta Metallurgica Sinica (English Letters) . en . 31 . 8 . 798–806 . 10.1007/s40195-018-0708-6 . 1006-7191 . free . 139395583.
15. Web site: Hot forming and Heat Treatment of Duplex Stainless Steels. International Molybdenum Association (IMOA). www.imoa.info.
16. News: Innovative Facades in Stainless Steel. Euro-Inox. Euro-Inox Publication, Building series. 19. 34. 978-2-87997-372-2.
17. News: Louvre Abu Dhabi: A rain of light. International Molybdenum Association. Moly Review. 2019. 1.
18. News: Basilica de la Sagrada familia. June 2018. Acero Inoxidable. Cedinox. 82.
19. News: Helix Pedestrian Bridge. Steel Construction Institute. 2012.
20. Web site: Cala Galdana Bridge. 2010. Steel Construction Institute.
21. Web site: Hong Kong-Zhuhai-Macau Bridge: the world's longest sea bridge. 2021-04-29. www.roadtraffic-technology.com.
22. Zuili. D. 2010. The use of stainless steels in oil & gas industry. Proceedings of the Duplex Stainless Steel Conference. 575. 2019-10-27. 2022-05-06. https://web.archive.org/web/20220506081328/https://pubs.kci-webshop.com/Webshop/Product/BOOKS/Duplex-Stainless-Steel---DSS-2010-Conference-Proceedings.html. dead.
23. News: The pulp and paper industry turns to duplex. Chater. James. 2007. Stainless steel world.
24. Application of Duplex Stainless Steel in the chemical process industry. Notten. G. 1997. Stainless Steel World. 5th Duplex stainless steel world conference.
25. Book: Duplex stainless steels in storage tanks. Directorate-General for Research and Innovation. Directorate-General for Research and Innovation. EU Publication. 2013. 978-92-79-34576-0. 10.2777/49448.