Duplex Stainless Steel
Duplex stainless steel (such as 2205 / UNS S31803 / S32205) has a mixed austenite-ferrite structure giving roughly twice the strength of 316 and much better resistance to chloride stress corrosion cracking and pitting. It is used in oil and gas, chemical, desalination and marine equipment.
What it is
Duplex grades contain about 22% chromium, 5% nickel and 3% molybdenum with nitrogen. The two-phase structure combines austenite's toughness with ferrite's strength and stress-corrosion resistance.
Higher strength allows thinner sections, often offsetting the material cost. Welding and heat treatment need controlled procedures to keep the phase balance.
Typical applications
- Pump shafts and impellers
- Valves and fittings for seawater
- Pressure vessels and heat exchangers
- Oil and gas components
- Desalination plant parts
Materials & grades
- Duplex 2205 (S31803 / S32205)
- Lean duplex (2304, LDX 2101)
- Cast duplex (CD4MCu, 4A)
Points to consider
- Use qualified welding procedures to maintain ferrite/austenite balance.
- Avoid prolonged exposure between about 300–950 °C (embrittlement risks).
- Harder to machine than 316; allow for tooling and time.
- Specify MTCs with ferrite content if required.
Frequently asked questions
Why choose duplex over 316 stainless?
Duplex is about twice as strong and far more resistant to chloride pitting and stress corrosion cracking.
What is PREN?
The pitting resistance equivalent number, estimating resistance to pitting from Cr, Mo and N content; higher is better.
Is duplex stainless steel hard to machine?
It is tougher to machine than 304/316, needing rigid setups and suitable tooling.