Duplex stainless steel is the family of corrosion-resistant steels whose microstructure contains two phases at room temperature: ferrite and austenite. The balance is usually close to equal, and the secondary phase must normally be present at 30 % or more for the grade to behave as a true duplex material rather than a conventional austenitic or ferritic steel.
What Defines a Duplex Stainless Steel
Duplex grades combine a low-carbon, high-chromium chemistry with a carefully balanced nickel and nitrogen content. The austenite-forming elements nickel, nitrogen, manganese and copper are balanced against the ferrite-forming elements chromium, molybdenum and silicon, so that when the material is solution annealed and quenched, roughly half of the structure is austenite and half is ferrite.
Typical chemistry is 18 % to 28 % chromium, 3 % to 10 % nickel, with molybdenum, copper, nitrogen and sometimes tungsten added for specific service conditions. Nitrogen is the key addition that allows nickel to be kept low while maintaining austenite and raising pitting resistance at the same time.
The two-phase structure is what gives duplex its characteristic combination of properties. The ferrite phase provides high strength and thermal conductivity, while the austenite phase provides toughness and weldability. Because the material is not a single-phase austenite, it is also far less susceptible to chloride stress corrosion cracking than type 304 or 316.
Grade Families and Typical Chemistry
Duplex grades are usually grouped by their pitting resistance equivalent number, calculated as PREN = Cr + 3.3 Mo + 16 N. Higher PREN means better resistance to pitting and crevice corrosion in chloride-bearing environments.
| Family | Example grade | Typical PREN | Typical use |
|---|---|---|---|
| Lean duplex | 2304 with about 23 % Cr and 4 % Ni | 24-25 | Replacement for 304 and 316L in structural and mild chloride service |
| Standard duplex | 2205 with about 22 % Cr, 5 % Ni, 3 % Mo | 34-35 | Chemical plant, pulp and paper, desalination, offshore piping |
| Super duplex | 2507 with about 25 % Cr, 7 % Ni, 4 % Mo | 40-43 | Seawater systems, topside process piping, heat exchangers |
| Hyper duplex | 27 % Cr grades with 6-7 % Mo and high nitrogen | 48 and above | Severe sour and chloride service, subsea umbilicals |
The same family is designated in several systems: 2205 is UNS S32205 and EN 1.4462, 2507 is UNS S32750 and EN 1.4410, and lean duplex 2304 is UNS S32304 and EN 1.4362. A purchase order should state the standard and the product form rather than the trade name alone.
Mechanical Properties and Behaviour at Temperature
Strength: minimum 0.2 % proof strength of standard duplex is roughly twice that of 304L or 316L, which allows thinner walls and lower weight in pressure equipment.
Toughness: impact energy is good at ambient and low temperature, but is lower than that of austenitic grades and falls off at very low temperature.
Thermal properties: thermal conductivity is higher and thermal expansion lower than austenitic stainless steel, which reduces thermal stress and distortion in heat exchangers.
Temperature limits: long-term service above roughly 250-300 °C should be avoided, because the ferrite phase embrittles through 475 °C embrittlement, and above roughly 600 °C the brittle sigma phase precipitates.
Welding: weldable by all common arc processes, but the filler metal is normally over-alloyed with nickel to restore the ferrite-austenite balance in the weld pool, and heat input and interpass temperature must be controlled.
One frequent misunderstanding is worth clearing up: duplex stainless steel is not a nickel-free material. Nickel content of 3 % to 10 % is essential to the phase balance, and lean duplex grades reduce nickel rather than eliminate it.
Standards and Product Forms
Duplex material is specified through the same route as other stainless steel products, which means the standard must be quoted with each form:
ASTM A240 for plate, sheet and strip.
ASTM A789 and ASTM A790 for seamless and welded tube and pipe.
ASTM A182 and ASTM A479 for forgings and bar.
ASTM A928 for welded pipe with filler metal.
EN 10088-2 and EN 10088-3 for European plate and bar, and EN 10216-5 for seamless tube.
NORSOK M-630 for offshore and subsea piping, where the duplex grades are grouped by PREN and service temperature.
Applications in Industry
Chemical and petrochemical plant: reactors, columns, heat exchanger tubing and piping where chlorides and dilute acids are present.
Desalination and water treatment: seawater intake, brine handling, evaporator shells and tubing.
Oil and gas: offshore flowlines, process piping, manifolds and topside seawater systems.
Pulp and paper: digesters, bleaching stages and liquor handling equipment.
Flue gas desulphurisation: absorber towers, ductwork and spray headers in wet limestone scrubbers.
Structural and architectural: bridges, tunnel linings, water gates and coastal facades that need corrosion resistance without painting.
Frequently Asked Questions
Q: Is duplex stainless steel magnetic?
Yes, it is weakly to moderately magnetic because of the ferrite phase. The magnetic response is much lower than a ferritic or martensitic grade but clearly higher than a fully austenitic grade such as 316L.
Q: What is the maximum service temperature for duplex grades?
Continuous service is normally limited to about 250-300 °C to avoid 475 °C embrittlement of the ferrite phase and sigma phase formation, so duplex is not a substitute for heat-resisting austenitic or nickel alloys.
Q: Can duplex be welded to carbon steel?
Yes. Dissimilar joints are common in offshore and process work, using a nickel-based or over-alloyed duplex filler and a qualified welding procedure that controls heat input and interpass temperature.
Q: What filler metal should be used for duplex welding?
A filler with higher nickel than the base metal is normally specified so that the weld metal solidifies with sufficient austenite. Matching-composition fillers are used where the procedure qualification supports them.
Q: How do I choose between 2205 and 2507?
Choose standard duplex 2205 for chloride levels and temperatures up to roughly 60 °C in seawater service, and move to super duplex 2507 when higher chloride concentration, higher temperature or a more severe pitting margin is required.
Q: Why is nitrogen added to duplex stainless steel?
Nitrogen stabilises austenite, allows nickel to be reduced, raises pitting resistance strongly through the PREN calculation, and improves the strength and phase balance of weld metal.
