Duplex Stainless Steel: Definition, Grades, Composition, Properties, and Applications

Duplex stainless steels are a specific group of engineering stainless steel materials consisting of the austenitic and ferritic phase in roughly equal proportions in the microstructure. They are widely popular because of their good corrosion resistance along with high strength and ease of fabrication. Compared to traditional austenitic stainless steel and ferritic stainless steel grades; Duplex Stainless Steels provide a range of benefits like:

  • Improved Strength: Roughly two-times stronger than normal stainless steel grades.
  • High Toughness and Ductility
  • High Corrosion Resistance
  • Cost-Effectiveness: High strength of DSS material required less pipe thickness reducing pipe weight. Also, lower levels of nickel reduce cost.

Applications of Duplex Stainless Steel

Excellent corrosion resistance with increased strength and affordable pricing makes duplex stainless steels (DSS) a popular choice for a variety of industries. Their wide uses are found in:

  • Chemical and liquid processing
  • Offshore (flowlines, risers) and other industrial operations
  • Naval parts and components
  • Pulp and paper production
  • Pollution control equipment
  • Water Treatment/Desalination plants
  • Construction, Infrastructure, Architecture
  • Hot water and brewing tanks
  • Food and Drink Storage
  • Boilers, Heat exchangers, pressure vessels
  • Marine Tanks
  • Renewable Energy
  • Nuclear Industry

Duplex Stainless Steel Grades

Duplex stainless steels have a higher chromium content, 20–28%; higher molybdenum, up to 5%; lower nickel, up to 9% and 0.05–0.50% nitrogen as compared to austenitic stainless steels. For resitance against pitting corrosion, DSS material is an ideal selection. The resistance against pitting corrosion is characterised by the pitting resistance equivalence number, or PREN Number defined as follows:

PREN = %Cr + 3.3 %Mo + 16 %N

Depending on the PREN Number values, Duplex Stainless Steel are categorized into four grades.

  • Lean duplex grades (PREN range: 22–27): No deliberate Molybdenum addition. mainly used in building and construction industry for bridges, pressure vessels or tie bars. Example: S32001, S32101, S32304, S32202.
  • Standard duplex (PREN range: 28–38): The most widely used (More than 80%) duplex stainless steel material with mid-range properties. Example: S32003, S31803, S32205
  • Super duplex (PREN range: 38-45): Higher contents of Cr, Ni, Mo, N and even W.  Specifically designed for highly corrosive oil&gas and chemical industries. Example, S32750, S32760, S32950, S32808.
  • Hyper duplex (PREN >45): These are highly alloyed duplex stainless steels. Example S32707, S33207

Naming Convention for Duplex Stainless Steels

Various naming conventions are followed for duplex stainless steels such as:

  • Composition based Names: For DSS 2205 or 2305; 22 or 23 denotes %Cr and 5 denotes %Ni in that specific DSS material.
  • UNS Designation: Unified Numbering System or UNS designation of DSS materials are the most popular and listed on ASTM specifications.
  • AISI Designation: Only one DSS material type 329 has AISI designation.

Duplex Stainless Steel properties

As informed earlier the duplex name has arrived from the co-existence of both austenitic (FCC Structure) and ferritic (BCC Structure) in approximately equal proportions (Fig. 1). The major alloying elements are Chromium, Silicon, Molybdenum, Carbon, Nickel, Nitrogen, Manganese, Copper, Tungsten.

Duplex Stainless Steel Microstructure
Fig. 1: Duplex Stainless Steel Microstructure

Typical duplex stainless steel exhibits a higher strength value as compared to stainless steel. But, the working temperature range of DSS is normally narrow as at around 300oC  undesirable intermetallic phases (α’ -alpha prime phase) start to precipitate which decreases the mechanical properties and corrosion resistance by embrittlement phenomenon. The following table provides some selected properties of common duplex stainless steel grades.

DSS GradesASTM A789 Grade S32520 Heat-TreatedASTM A790 Grade S31803 Heat-TreatedASTM A790 Grade S32304 Heat-TreatedASTM A815 Grade S32550 Heat-TreatedASTM A815 Grade S32205 Heat-Treated
Elastic Modulus200 GPa 200 GPa 200 GPa 200 GPa 200 GPa 
Elongation25 % 25 % 25 % 15 % 20 % 
Tensile Strength770 MPa 620 MPa 600 MPa 800 MPa 655 MPa 
Brinell hardness310290290302290
Yield Strength550 MPa 450 MPa 400 MPa 550 MPa 450 MPa 
Thermal expansion co-efficient1E-5 1/K 1E-5 1/K 1E-5 1/K 1E-5 1/K 1E-5 1/K 
Specific Heat capacity440 – 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 Conductivity13 – 30 W/(m·K) 13 – 30 W/(m·K) 13 – 30 W/(m·K) 13 – 30 W/(m·K) 13 – 30 W/(m·K) 
Table: Properties of Duplex Stainless Steel

The stress-strain curve of austenitic, ferritic, and duplex stainless steels are plotted in the following curve for reference:

Stress Strain curve comparison for SS and DSS.png
Fig. 2: Stress Strain curve comparison for SS and DSS

Composition of Duplex Stainless Steel

The following table in Fig. 3 provides the chemical compositions in %wt for common DSS materials.

Composition of Lean & Standard Duplex Stainless Steel Materials
Composition of Super & Hyper Duplex Stainless Steel Materials
Fig. 3: Typical Chemical Composition of Duplex Stainless Steel

Corrosion Resistance of Duplex Stainless Steel

Due to the presence of a relatively high % of chromium, molybdenum, and nitrogen, Duplex Stainless Steels exhibit a high level of corrosion resistance capability in a variety of environments. DSS materials are specifically selected for oxidizing, acidic, and hot alkaline environments. To fight against pitting corrosion, DSS is the ideal material choice. The PREN Number defined above describes the resistance of DSS against localized pitting corrosion. An increase in PREN Number increases the resistance against pitting corrosion that is quantified using Critical Pitting Temperature. Materials with higher CPT are more resistant to pitting corrosion.
Duplex stainless steels possess better Stress Corrosion Cracking resistance than austenitic stainless steels.

Fabrication of Duplex Stainless Steel

Duplex Stainless steel is supplied in a pipe, plate, sheet, tube, fittings, or bar form. Depending on the requirement they must be fabricated. Special tools are required for the fabrication of duplex stainless steel materials. They have very good weldability and hot forming capability. However, DSS materials are normally difficult for machining purposes.

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Anup Kumar Dey

I am a Mechanical Engineer turned into a Piping Engineer. Currently, I work in a reputed MNC as a Senior Piping Stress Engineer. I am very much passionate about blogging and always tried to do unique things. This website is my first venture into the world of blogging with the aim of connecting with other piping engineers around the world.

3 thoughts on “Duplex Stainless Steel: Definition, Grades, Composition, Properties, and Applications

  1. Dear Anup,

    When considering manufactured items in either Cast or Forged Duplex Materials it’s worth mentioning the type of quality requirements that are typically required. Vendors should be conformity assessed and qualified by an appropriate Notified body against ISO 17782:2018 or Norsok M650 for the size / weight of the product they are making. MDS such as those contained in IOGP S563 / M630 should be used to ensure the product is manufactured correctly.

  2. If DSS are competitive material then why it is not used in extensively?
    What is the difference between austenitic SS and DSS?
    If we generally use 18-8 that is SS304 material for High Temp. then what is the equivalent DSS to ASTM A304, 310 ?

  3. Hi Anup, very well explained in depth. Really appreciate your efforts for junior / senior level piping engineers.
    In my view Duplex is known as 22% crome and Super Duplex is known as 25% crome. Most of the UK and Norwegian continental shelf uses this material for hydrocarbon due to low temp condition. In gulf region this material is replaced by Inconel 625 for offshore piping.

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