On this page 11 sections
What is duplex stainless steel?
Short answer
Duplex stainless steel has a mixed microstructure of roughly half austenite and half ferrite. That gives about twice the yield strength of SS 316L and much better resistance to chloride stress-corrosion cracking. 2205 (PREN about 35) is the workhorse grade, and super duplex 2507 (PREN 40 or more) is used for seawater.
Microstructure and strength
Corrosion resistance
With a PREN of about 35 (2205) and above 40 (2507), duplex grades resist pitting and crevice corrosion far better than SS 316L, and they're highly resistant to chloride stress corrosion cracking. Super duplex is used for seawater.
Limits and fabrication
Long exposure between about 300 and 1,000 °C forms embrittling phases (475 °C embrittlement, sigma), so service is usually limited to about 280–300 °C, and welding must control heat input and interpass temperature. Nickel-enriched fillers (2209, 2594) keep the weld's phase balance. Ferrite measurement and ASTM A923 testing are common acceptance checks.
SS 316L vs 2205 vs 2507
| Property | SS 316L | Duplex 2205 | Super Duplex 2507 |
|---|---|---|---|
| Yield strength (min) | 170 MPa | 450 MPa | 550 MPa |
| PREN | ≈ 24 | ≈ 35 | ≥ 40 |
| Chloride SCC resistance | Low | High | Very high |
| Magnetic | No | Yes | Yes |
| Max service (typical) | Cryogenic–high | ≈ 300 °C | ≈ 280 °C |
Why the phase balance matters
Too much ferrite, often caused by fast cooling or low heat input in welds, reduces toughness and corrosion resistance. Too little ferrite reduces strength and SCC resistance. That is why duplex weld procedures specify heat input and ferrite measurement, and why nitrogen-bearing shielding gas is often used.
Lean duplex grades
Lean duplex grades such as 2101 and 2304 have lower nickel and molybdenum. They replace SS 304 or SS 316 where strength matters more than maximum corrosion resistance, for example in storage tanks and structures.
Materials in this guide
Related guides, comparisons and articles
Frequently asked questions
Can duplex be used at low temperature?
Duplex toughness falls at very low temperatures, so most codes limit it to about −40 to −50 °C. Austenitic grades are used for cryogenic service.
Is duplex harder to machine than SS 316L?
Yes. Its higher strength makes machining slower and tool wear higher.
Is duplex stainless steel more expensive than SS 316L?
Per kilogram it's often similar or only slightly higher, because it contains less nickel. Its higher strength can reduce total weight and cost.
How do I choose between grades?
Start from the service environment (chlorides, acids, temperature, pressure), then check the governing code and cost. Our comparisons and grade pages list typical choices, and our team can help shortlist grades for your conditions.
Does Bright Tubes supply the materials in this guide?
Yes. Duplex 2205 Stainless Steel, Super Duplex 2507 Stainless Steel, SS 316L Stainless Steel and related products are supplied from stock or mill-sourced, with EN 10204 certificates.