Duplex stainless steel is a family of ferritic-austenitic alloys in which ferrite and austenite are present in roughly equal volume fractions. That two-phase microstructure, rather than any single alloying element, is what separates the family from the austenitic and ferritic grades it sits between.
What Makes Duplex Stainless Steel Different
By controlling chemical composition and heat treatment, producers obtain a microstructure of about 50% ferrite and 50% austenite. The result combines the good toughness and weldability of austenitic stainless steel with the high strength and resistance of ferritic stainless steel, and it also brings chloride stress corrosion resistance that neither family offers on its own.
Those properties made duplex stainless steel one of the fastest-growing weldable structural materials from the 1980s onwards, and it now sits alongside martensitic, austenitic and ferritic grades in pressure, chemical and offshore service. The practical gain is strength: yield strength is broadly twice that of the standard austenitic grades, so wall thickness and component weight can often be reduced for the same design pressure.
Phase Balance and Chemical Composition
The balance is achieved by combining chromium and molybdenum with a controlled nitrogen addition and a comparatively low nickel content. Nitrogen is the key element: it stabilises austenite, raises strength and delays the precipitation of intermetallic phases during welding.
| UNS designation | Common name | Cr (%) | Ni (%) | Mo (%) | N (%) |
|---|---|---|---|---|---|
| UNS S32205 | Standard 22% Cr duplex | 22.0-23.0 | 4.5-6.5 | 3.0-3.5 | 0.14-0.20 |
| UNS S32750 | 25% Cr super duplex | 24.0-26.0 | 6.0-8.0 | 3.0-5.0 | 0.24-0.32 |
The ranges above are the typical plate composition windows of the two most widely used grades; lean duplex grades sit at lower nickel and molybdenum levels, while super duplex grades move to higher chromium, molybdenum and nitrogen. Carbon is normally held at 0.030% maximum to limit carbide precipitation.
Chloride Stress Corrosion Cracking Resistance
Molybdenum-bearing duplex grades resist chloride stress corrosion cracking well even at low applied stress. Standard 18-8 austenitic stainless steel is prone to cracking in neutral chloride solutions above about 60 degrees Celsius, and heat exchangers, evaporators and similar equipment built from it can suffer cracking in media containing traces of chloride and hydrogen sulphide.
Duplex stainless steel behaves far better in exactly those conditions, which is why it became the standard answer for chloride-bearing cooling water, process streams and marine atmospheres where austenitic grades had a history of premature failure.
Pitting and Crevice Corrosion Resistance
Molybdenum-bearing duplex stainless steel also resists pitting attack. Performance is ranked with the pitting resistance equivalent, calculated as PRE = Cr% + 3.3 Mo% + 16 N%. At equal PRE the critical pitting potential of duplex and austenitic stainless steel is broadly comparable, and the pitting resistance of duplex is in the same class as AISI 316L.
The advantage grows with chromium and nitrogen content. High-chromium duplex grades containing about 25% chromium, and especially those with a high nitrogen addition, exceed AISI 316L in both pitting and crevice corrosion resistance.
Corrosion Fatigue, Wear Resistance and Mechanical Behaviour
Duplex stainless steel offers good corrosion fatigue resistance and resistance to wear corrosion, which suits pumps, valves and other power equipment that sees a moving part in a corrosive medium. The higher proof strength and lower thermal expansion than austenitic grades also reduce distortion in service and improve fatigue performance in cyclic loading.
Two limits are worth remembering. Long exposure to temperatures above roughly 300 degrees Celsius can promote sigma-phase formation and loss of toughness, and heavy cold deformation without subsequent annealing disturbs the phase balance. Both are managed by specifying the correct grade and by controlled welding and heat treatment.
Fabrication and Welding Notes
Use filler metals with a higher nickel content than the base metal so the weld metal retains a balanced ferrite-austenite structure.
Control heat input and interpass temperature; excessive heat input coarsens the microstructure and encourages intermetallic precipitation.
Avoid excessive ferrite in the weld heat-affected zone and excessive austenite in the weld metal; both reduce corrosion resistance.
Clean surfaces after fabrication, since embedded iron, scale and chloride contamination are far more damaging than the base metal itself.
Use cold forming within the limits of the grade, and anneal after severe deformation.
Frequently Asked Questions
Q: How does duplex stainless steel compare with AISI 316L?
Duplex grades are roughly twice as strong in yield and much more resistant to chloride stress corrosion cracking, while matching or exceeding 316L in pitting resistance. The trade-off is tighter fabrication control and a temperature limit above which embrittlement becomes a risk.
Q: What does the PRE value tell me?
PRE ranks pitting resistance using PRE = Cr% + 3.3 Mo% + 16 N%. Higher values indicate greater resistance to pitting and crevice attack, which is why super duplex grades with higher chromium, molybdenum and nitrogen sit above standard 22% Cr duplex.
Q: Can duplex stainless steel be welded?
Yes, it is a weldable structural material, but the procedure must protect the phase balance. Consumables with higher nickel, controlled heat input and limits on interpass temperature are essential to retain corrosion resistance in the joint.
Q: Is duplex stainless steel suitable for high-temperature service?
It is normally limited to below about 300 degrees Celsius because prolonged exposure at higher temperature promotes sigma-phase precipitation and reduces toughness and corrosion resistance.
Q: Why is the phase balance so important?
Ferrite supplies strength, pitting and stress corrosion resistance; austenite supplies toughness and weldability. If heat treatment or welding pushes the balance too far in either direction, those properties are lost, even though the chemical analysis still meets the specification.
Q: Where is duplex stainless steel typically used?
Heat exchangers, pressure vessels, storage tanks, desalination and water treatment equipment, chemical process piping, pumps, valves and offshore structural components exposed to chloride-bearing environments.
