Duplex Stainless Steel
General / GeneralDefinition
Duplex stainless steels have a microstructure of roughly 50% austenite and 50% ferrite, combining the best properties of both phases. The result is a material with approximately twice the yield strength of 316SS, superior resistance to chloride pitting and stress corrosion cracking, and good weldability. The two most common grades for valve applications are 2205 (standard duplex) and 2507 (super duplex), with 2205 covering the majority of cooling water and mild chemical services and 2507 handling the most aggressive chloride environments.
Working Principle
The balanced austenite-ferrite microstructure gives duplex its name and its properties. Ferrite provides high strength and resistance to chloride stress corrosion cracking (SCC). Austenite provides ductility, toughness, and weldability. The combination yields a yield strength of 65-80 ksi (vs. 30 ksi for 316SS), allowing thinner wall sections and lighter components at the same pressure rating. The high chromium (22-25%), molybdenum (3-4%), and nitrogen (0.14-0.28%) content gives duplex a Pitting Resistance Equivalent (PRE) of 34-42, far superior to 316SS (PRE ~24). The key manufacturing challenge is maintaining the 50/50 phase balance — too much ferrite reduces toughness, too much austenite reduces SCC resistance.
Types & Variants
| Grade | UNS | PRE | Yield (ksi) | Max Temp | Typical Use |
|---|---|---|---|---|---|
| 2205 (S31803/S32205) | S31803/S32205 | 34-36 | 65 | 600F | Seawater cooling, chemical processing |
| 2507 (S32750) | S32750 | 42-43 | 80 | 600F | Hot seawater, offshore, desalination |
| 2304 (S32304) | S32304 | 24-26 | 53 | 570F | Cost-effective alternative to 316SS |
| 2101 (S32101) | S32101 | 26-28 | 58 | 570F | Lean duplex for general corrosive service |
| Zeron 100 (S32760) | S32760 | 40-42 | 80 | 600F | Super duplex with Cu for H2S service |
Materials
ASTM A890/A995 (cast duplex stainless for valves), ASTM A182 (forged duplex flanges and fittings), ASTM A240 (duplex plate), ASTM A790/A789 (duplex pipe and tube). Grade 2205: 22% Cr, 5.5% Ni, 3% Mo, 0.17% N. Grade 2507: 25% Cr, 7% Ni, 4% Mo, 0.28% N. Casting grades: CD3MN (2205), CE3MN (2507). Forging grades: F51 (2205), F53 (2507), F55 (Zeron 100). All duplex grades require careful heat treatment (solution anneal at 1920-2050F, water quench) to avoid sigma phase and intermetallic precipitation that destroy toughness and corrosion resistance.
Applicable Standards
ASTM A890/A995 define cast duplex stainless grades and mechanical properties. ASME B16.34 assigns pressure-temperature ratings. NACE MR0175/ISO 15156 allows duplex for sour service up to specific H2S partial pressures and chloride concentrations — 2507 is approved for more aggressive conditions than 2205. NORSOK M-630 includes duplex material data sheets for offshore applications. ASME B31.3 provides allowable stress values for duplex piping and components.
Applications
Seawater cooling: 2205 duplex heat exchanger tubes and valve bodies replace 316SS that pits in 2-3 years; 2205 lasts 15-20 years. Offshore oil and gas: 2205 and 2507 for subsea and topside valves handling produced water with chlorides and H2S. Desalination: 2507 for high-temperature brine concentrate stages where 2205's pitting resistance is borderline. Chemical tankers: 2205 cargo tanks and valves for chlorinated and acidic cargoes. Pulp and paper: 2205 bleach plant equipment in hot chloride environments. Piping systems where the strength advantage allows thinner walls and lower weight than 316SS.
Advantages
- Twice the yield strength of 316SS — lighter components, thinner walls
- Superior chloride pitting and crevice corrosion resistance (PRE 34-43)
- Resistant to chloride stress corrosion cracking (which destroys austenitic SS)
- Good weldability with proper procedures and filler metals
- Cost-competitive with 316SS at larger sizes due to thinner wall sections
Disadvantages
- Embrittlement at 700
Related Resources