Super Duplex Stainless Steel 2507 (UNS S32750 / 1.4410) represents the leading choice for demanding offshore, marine, and chemical processing applications that require the combined benefits of high strength, excellent corrosion resistance, and good fabricability. Understanding its mechanical properties, fabrication requirements, and application limitations is essential for successful implementation in critical engineering projects.
Chemical Composition
| Element | Content (%) | Role |
|---|---|---|
| Chromium | 24.0-26.0 | Pitting and general corrosion resistance |
| Nickel | 6.0-8.0 | Austenite stabilization, toughness |
| Molybdenum | 3.0-5.0 | Pitting and crevice corrosion resistance |
| Nitrogen | 0.24-0.32 | Strength enhancement, pitting resistance |
| Carbon | ≤0.030 | Minimized for welding |
| Manganese | 0.8-1.5 | Deoxidation, strength |
| Silicon | ≤0.80 | Deoxidation |
| Copper | ≤0.50 | Sulfuric acid resistance |
The balanced austenite-ferrite microstructure (approximately 50/50) gives 2507 its unique combination of properties. The high PREN value of 40+ classifies it as a super duplex grade with exceptional localized corrosion resistance.
Mechanical Properties
Room Temperature Properties
| Property | 2507 (S32750) | 316L (reference) | 2205 (S31803) |
|---|---|---|---|
| Yield Strength (MPa) | ≥550 | ≥170 | ≥450 |
| Tensile Strength (MPa) | ≥795 | ≥485 | ≥620 |
| Elongation (%) | ≥15 | ≥40 | ≥25 |
| Hardness (HRC) | ≤32 | ≤190 HB | ≤31 |
| Impact Energy (J) | ≥60 (Charpy V) | ≥100 | ≥80 |
The yield strength of 2507 is approximately 3 times that of 316L and 20% higher than standard duplex 2205. This allows significant weight reduction in structural applications.
Design Stress Values
Per ASME Section VIII and EN 13445, the allowable design stress for 2507 at various temperatures:
- At 20°C: 245 MPa
- At 100°C: 225 MPa
- At 200°C: 195 MPa
- At 300°C: 175 MPa
Maximum design temperature per most codes is 300°C (572°F) to avoid embrittlement from intermetallic phase precipitation.
Corrosion Resistance
Pitting and Crevice Corrosion
With a PREN of 40-45, 2507 offers outstanding resistance to localized corrosion:
- Critical Pitting Temperature (CPT) in 6% FeCl₃: ≥50°C (per ASTM G48)
- Critical Crevice Corrosion Temperature (CCCT): ≥35°C (per ASTM G48)
- Pitting in seawater: Resistant to pitting in clean, flowing seawater up to approximately 80°C
Stress Corrosion Cracking (SCC)
The duplex microstructure provides excellent resistance to chloride stress corrosion cracking – a failure mode that commonly affects standard austenitic grades like 304 and 316 in chloride environments. 2507 is effectively immune to SCC in environments where 316L would fail rapidly.
Corrosion Fatigue
2507 shows superior corrosion fatigue performance compared to austenitic grades in chloride-containing environments, making it ideal for offshore applications with cyclic loading.
Fabrication Guide
Forming and Bending
Due to its high strength and duplex microstructure, 2507 requires different forming practices than austenitic stainless steels:
- Cold forming: Possible but requires higher forces (approximately 2x compared to 316L). Minimum bend radius is typically 2× thickness for 90° bends
- Hot forming: Recommended for severe forming operations. Temperature range: 1050-1150°C, followed by water quenching
- Springback: Higher than austenitic grades – must be accounted for in tooling design
- Anisotropy: Forming properties vary with rolling direction – plan forming operations accordingly
Machining
2507 is more difficult to machine than austenitic grades due to higher strength and work hardening rate:
- Use rigid setups with maximum tool overhang support
- Carbide tools with positive rake geometry
- Consistent feed rates – avoid dwelling or interrupted cuts
- Generous coolant flow for heat dissipation
- Reduced cutting speeds compared to 316L (approximately 70-80% of 316L speeds)
Welding Procedures
Filler Metal Selection
- GMAW/GTAW: AWS A5.14 ER2594 (25%Cr, 9%Ni, 4%Mo, 0.3%N) – overalloyed to compensate for nitrogen loss
- SMAW: AWS A5.4 E2594 electrodes
- SAW: Matching wire-flux combination per manufacturer recommendations
Welding Parameters
- Heat input: 0.2-2.0 kJ/mm (controlled range is critical)
- Interpass temperature: Maximum 150°C (ideally 100°C)
- Preheat: Not required above 10°C ambient
- Shielding gas (GTAW): Ar + 2% N₂ for root passes; Ar + 1% N₂ for fill/cap
- Shielding gas (GMAW): Ar + 2% CO₂ + 1-2% N₂
- Back purging: Ar + 2% N₂ or Ar + 25% N₂ to maintain ferrite-austenite balance
Weld Quality Considerations
Maintaining the correct ferrite-austenite balance in the weld metal is critical:
- Target ferrite content: 30-70% (ideally 40-60%) in the weld metal
- Too much ferrite: Reduces toughness and corrosion resistance
- Too much austenite: Reduces strength and SCC resistance
- Nitrogen control: Essential – nitrogen loss during welding shifts balance toward ferrite
Heat Treatment
Solution annealing is the standard heat treatment for 2507:
- Temperature: 1020-1100°C (1870-2010°F)
- Holding time: Minimum 30 minutes per 25mm thickness
- Cooling: Water quench or rapid air cooling
- Purpose: Dissolve any intermetallic phases (sigma, chi) formed during welding or hot working
Stress relief annealing is NOT recommended for 2507, as temperatures in the 300-900°C range promote harmful phase precipitation.
Applications
Offshore Oil & Gas
- Subsea flowlines and risers
- Topside process piping
- Heat exchangers in seawater cooling service
- Manifold components
- Christmas tree components
Chemical Processing
- Pressure vessels for chloride-containing acid service
- Heat exchangers in aggressive chemical environments
- Storage tanks for corrosive chemicals
- Agitator shafts and impellers
Marine and Desalination
- Seawater piping systems
- MSF and RO desalination plant components
- Ship propeller shafts
- Fire water systems
Pulp and Paper
- Digester components
- Bleach plant equipment
- Black liquor evaporators
CoreMetal Supply for Super Duplex 2507
CoreMetal Steel supplies Super Duplex 2507 (UNS S32750) in plate, sheet, pipe, tube, fittings, and round bar. Our inventory includes:
- Plate per ASTM A240/A240M (3-50mm thickness)
- Seamless pipe per ASTM A789/A789M
- Seamless tube per ASTM A789/A789M
- Butt-welding fittings per ASTM A815
- Round bar per ASTM A276/A479
All products carry full EN 10204 3.1 mill test certificates with verified PREN, ferrite content, and corrosion test results. Contact CoreMetal Steel for competitive pricing and technical support on Super Duplex 2507 products.