Stainless Steel

Here, you will learn about:

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

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.

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. As the development of new grades added, the grades of duplex stainless steels  be categorised into two main types by their end application:

  • 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×%Mo+16×%N

Duplex stainless steel sub-groups in the table below:

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

Duplex stainless steel with high resistance to chloride attack. UNS S32750 (2507) is a super duplex stainless steel designed for applications which require exceptional strength and corrosion resistance.

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.

Future 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.

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