# Duplex Stainless Steel: Properties, Standards, and Applications

Duplex stainless steels have a combination of two phases, ferrite and austenite in approximately equal measure. This allows them to benefit from the advantages of both austenitic and ferritic stainless steels, leading to increased strength, improved weldability, higher toughness and resistance to several types of corrosion. Commercially, they are also cheaper than austenitic stainless steels due to their lower nickel content.

Duplex stainless steel was developed in Sweden in 1930, but commercial production didn’t commence until the 1970s due to the very complicated process of its production. There has been a re-emergence in the steel in modern times after significant advances in steelmaking techniques have made it easier to produce it. However, they are still a relatively under-utilised group of stainless steels, with the world’s usage only between 1 – 3% due to the limitations of their application.

• The properties of duplex stainless steel
• Duplex stainless steel grades and standards
• Duplex stainless steel applications
• Future trends of duplex stainless steel

Figure 1. Duplex stainless steel used in bridge construction.

## The properties of duplex stainless steel

The microstructure of duplex stainless steels features both the crystallographic structures of approximately 50% ferrite (a body-centred cubic structure) and 50% austenite (a face-centred cubic structure). Both of these phases co-exist as a stable mixture, unlike two-phase alloys, where one of the two phases manifests as precipitates. The alloying elements in duplex stainless steels are either ferritisers (e.g. chromium, silicon, molybdenum) or austenitisers (e.g. carbon, nickel, nitrogen), which means they favour the formation of the ferritic and austenitic phases, respectively.

At high temperatures, the relatively unstable ferrite phase in duplex steels gets converted into the undesirable α’ (alpha prime) phase, which causes a decrease in their mechanical properties, such as strength and toughness, and also in their corrosion resistance. This phenomenon is called embrittlement [1]. The worst temperature at which embrittlement occurs is at 475 oC; however,  the production of the alpha prime phase may start to occur at temperatures as low as 300 oC, and this limits the maximum service temperature of duplex steels.

Typical duplex stainless steels show a higher yield strength than typical martensitic, austenitic and ferritic grades. However, they have a narrow range of working temperatures due to the precipitation of intermetallic phases that starts to occur above 300 oC and the onset of brittleness as they approach cryogenic temperatures [2]. The table below presents selected properties of some common grades of duplex stainless steels.

Table 1. Properties of selected heat treated duplex steel grades (all values taken at 20 °C).

 ASTM A789 Grade S32520 Heat-Treated ASTM A790 Grade S31803 Heat-Treated ASTM A790 Grade S32304 Heat-Treated ASTM A815 Grade S32550 Heat-Treated ASTM A815 Grade S32205 Heat-Treated Elastic modulus 200 GPa 200 GPa 200 GPa 200 GPa 200 GPa Elongation 25 % 25 % 25 % 15 % 20 % Tensile strength 770 MPa 620 MPa 600 MPa 800 MPa 655 MPa Hardness, Brinell 310 290 290 302 290 Yield strength 550 MPa 450 MPa 400 MPa 550 MPa 450 MPa Coefficient of thermal expansion 1E-5 1/K 1E-5 1/K 1E-5 1/K 1E-5 1/K 1E-5 1/K Specific heat capacity 440 - 502 J/(kg·K) 440 - 502 J/(kg·K) 440 - 502 J/(kg·K) 440 - 502 J/(kg·K) 440 - 502 J/(kg·K) Thermal conductivity 13 - 30 W/(m·K) 13 - 30 W/(m·K) 13 - 30 W/(m·K) 13 - 30 W/(m·K) 13 - 30 W/(m·K)

## Duplex stainless steel grades and standards

In the early stages of the development of duplex stainless steels, there were only a few grades, the most popular one among them being duplex stainless steel UNS S31803. Thereafter, the development of new grades started, and they were determined by their end application, which can be categorised into two main types:

• Hyper-duplex and super-duplex stainless steels, which were created to operate in very corrosive environments but with less focus on their strength
• Lean and standard duplex stainless steels, which had more focus on increased strength and were to be used in mildly corrosive environments such as in structural applications.

These categories are now identified by the pitting resistance equivalence number (PREN) which is calculated by a formula based on the composition of duplex stainless steels.

PREN = % Cr + 3.3 \times % Mo + 16 \times % N

The table below shows the four sub-groups.

Table 2. Duplex stainless steel sub-groups [3]

 Type Example grades Composition PREN Cr% Ni% Mo% N% Lean S31500, S32304, S32404 20-24 1-5 0.1-0.3 0.1-0.22 24-25 Standard S31803, S32205 21-23 4.5-6 2.5-3.5 0.1-0.22 33-35 Super-duplex S32520, S32550, S32750 24-29 4.5-8 2.7-4.5 0.1-0.35 >40 Hyper-duplex S32707, S33207 27 6.5 5 0.4 49

## Duplex stainless steel applications

Due to the limitations of duplex stainless steels such as poor formability and machinability, in addition to their highly complicated metallurgical process of production compared to ferritic, austenitic and martensitic stainless steels, duplex steels are used mostly for niche applications.

Applications of duplex stainless steels take into account their limitations and advantages, but the major applications require corrosion resistance, such as pitting and crevice corrosion resistance, stress corrosion resistance, fatigue corrosion, abrasion corrosion or resistance to corrosive environments that are acidic or caustic. Some of the common industrial applications are outlined below [4].

### Paper production

Duplex stainless steels are preferred to austenitic and ferritic stainless steels for the fabrication of vessels that contain bleach and other corrosive liquids in paper processing.

### Desalination

Desalination of seawater, a process that presents highly corrosive chlorine in a high-temperature environment, is a sturdy test for a corrosion-resistant material. Duplex stainless steels have become the material of choice for the fabrication of evaporators. They can also be made with thinner cross-sections because of the higher strength and corrosion resistance of duplex stainless steels.

### Oil and Gas

The development of duplex stainless steels with PRE numbers above 40 has made it possible to manufacture components such as pumps, piping and manifolds that require high resistance to pitting and crevice corrosion, which are crucial in the oil and gas industry.

### Construction

Duplex stainless steels are used in the construction of load-bearing members that also require corrosion resistance. Typical applications include the construction of bridges over seawater or structures that are in close proximity to the sea.

### Food and Drink Storage

Lean duplex steels show promise in the commercial storage of food and drinks during processing due to their relatively low cost while retaining excellent corrosion resistance and strength.

Figure 2. Sea bridges at low altitudes are generally made of duplex stainless steel due to their good corrosion resistance.

## Future trends of duplex stainless steel

New grades of duplex stainless steels are still being developed till date. The main driver of this endeavour is to further increase pitting corrosion resistance and this is achieved mainly by increasing key alloying elements such as chromium, molybdenum and nitrogen. The advantages of these increased alloying elements also come with disadvantages, mainly the destabilization of the ferrite phase, leading to unwanted precipitates. The challenge is to strike a balance between desirable properties mainly led by increased pitting corrosion resistance while limiting the production of undesirable intermetallic phases such as Cr2N by very precise heat treatment. A duplex steel grade, SAF 2707 HD, a 27Cr-7Ni-5Mo-0.4N with PRE-number of almost 50 has been developed, which is promising for the future of duplex steels in general [5].

## Sources

[1] Tahchieva, A. B., Llorca-Isern, N., & Cabrera, J. M. (2019). Duplex and superduplex stainless steels: microstructure and property evolution by surface modification processes. Metals, 9(3), 347.

[3] Dr. Sunil D.Kahar. Int. Journal of Engineering Research and Application www.ijera.com. ISSN : 2248-9622, Vol. 7, Issue 4, ( Part -4) April 2017, pp.27-36

[4] International Molybdenum Association. (2009). Practical guidelines for the fabrication of duplex stainless steels. London, UK, 1-64.

[5] Nilsson, J. O., Chai, G., & Kivisäkk, U. (2007). Recent development of duplex stainless steels. CEDINOX-Stainless Steel. Sweden: R&D Centre, Sandvik Materials Technology. Retrieved from http://www.cedinox.es/opencms901/export/sites/cedinox/.galleries/publicaciones-tecnicas/13-DUPLEX-SSSM_proceedings08_EN-5.pdf

Duplex stainless steels are materials that benefit from the properties of both austenitic stainless steels and ferritic stainless steels, as they are made of approximately 50% austenites and 50% ferrites.