Stainless Steel (Duplex) (Fe+Cr+Ni+Mo)
Duplex stainless steels combine austenitic and ferritic microstructures in approximately equal proportions, delivering roughly twice the yield strength of austenitic grades while maintaining excellent corrosion resistance. They are the material of choice for aggressive environments in oil & gas, chemical processing, and marine applications.
Properties
- Density
- 7,800 kg/m³
- Melting Point
- 1,350–1,400 °C
- Boiling Point
- 2,750 °C
- Thermal Conductivity
- 19 W/(m·K)
- Electrical Resistivity
- 8.0 × 10⁻⁷ Ω·m
- Tensile Strength
- 620–900 MPa
- Yield Strength
- 450–550 MPa
- Hardness
- 250–320 HB
- Elongation
- 20–30%
- Crystal Structure
- Mixed BCC + FCC
Common Grades
- 2205 (1.4462): The standard duplex grade — 22% Cr, 5% Ni, 3% Mo. Covers 80% of duplex applications.
- 2507 (1.4410): Super duplex — 25% Cr, 7% Ni, 4% Mo. For the most aggressive chloride environments.
- LDX 2101 (1.4162): Lean duplex — lower nickel content, cost-effective alternative to 304/316 with higher strength.
- 2304 (1.4362): Lean duplex without molybdenum. Good alternative to 316L with double the strength.
- Ferralium 255 (1.4507): High-performance duplex — 25% Cr, 6% Ni, 3% Mo, 2% Cu. Pumps, valves, and fasteners in severe service.
Applications
- Oil & Gas: Subsea pipelines, process piping, separators, heat exchangers, manifolds
- Chemical Processing: Pressure vessels, reactors, scrubbers, pulp & paper digesters
- Marine: Propeller shafts, seawater systems, desalination, offshore structures
- Construction: Reinforcement bars in bridges, swimming pools, coastal structures
- Water Treatment: Reverse osmosis systems, water storage tanks, pump components
FAQ
What is the advantage of duplex over austenitic stainless steel?
Duplex stainless steel offers approximately double the yield strength of 304/316, superior resistance to stress corrosion cracking, and better pitting resistance. This allows thinner wall sections, reducing material costs by 20–40% while improving performance in aggressive environments.
What does PREN mean for duplex stainless steel?
PREN (Pitting Resistance Equivalent Number) is calculated as %Cr + 3.3×%Mo + 16×%N. A higher PREN indicates better pitting corrosion resistance. Standard duplex 2205 has PREN ≥35, while super duplex 2507 achieves PREN ≥42.
Can duplex stainless steel be used at high temperatures?
Duplex stainless steels should not be used above 300°C for extended periods, as sigma phase and other intermetallic compounds can form, causing embrittlement. For high-temperature service, austenitic grades like 321 or 310 are preferred.
What is sigma phase embrittlement in duplex steel?
Sigma phase is a hard, brittle intermetallic compound (Fe-Cr-Mo) that forms at 600–1,000°C. It severely reduces toughness and corrosion resistance. Avoiding prolonged exposure above 300°C, controlling welding heat input, and proper solution annealing (1,020–1,100°C with rapid quench) prevent sigma formation.