Fundamental Classification: UNS S32750 (SAF 2507®, F53, EN 1.4410) is a super duplex stainless steel (SDSS). Its microstructure is engineered for a near-equal balance of ferrite and austenite phases (~50/50).
Primary Alloy Driver: Significantly elevated levels of Chromium (Cr), Molybdenum (Mo), and Nitrogen (N) compared to standard duplex (e.g., S32205/2205) and austenitic (e.g., 316L) grades.
S32750 Chemical Composition
- Chromium (Cr): 24.0% – 26.0% (Primary corrosion resistance)
- Nickel (Ni): 6.0% – 8.0% (Austenite stabilization)
- Molybdenum (Mo): 3.0% – 4.5% (Enhanced pitting/crevice resistance)
- Nitrogen (N): 0.24% – 0.32% (Strengthens, boosts corrosion resistance & austenite formation)
- Manganese (Mn): ≤ 1.20%
- Carbon (C): ≤ 0.030%
- Copper (Cu): ≤ 0.50% (Aids sulfuric acid resistance)
- Iron (Fe): Balance
- S32750 PREN: Typically 40 - 45 (vs. ~34-38 for standard duplex S32205).
- S32750 Corrosion Resistance: Superior Defense Against Aggression
- Pitting and Crevice Corrosion: Industry-leading resistance in chloride-rich environments (seawater, brines, chemical process streams) due to high Cr/Mo/N.
- Stress Corrosion Cracking (SCC): Exceptional resistance to chloride-induced SCC, outperforming austenitic grades like 316L and 317L significantly.
- Acid Resistance: Excellent performance in sulfuric, phosphoric, organic (acetic, formic), and mildly reducing acids, especially benefiting from copper content.
- General and Galvanic Corrosion: High resistance across diverse environments. Use intelligently to minimize galvanic risks.
S32750 Mechanical and Physical Properties
Outstanding Strength: Minimum yield strength exceeds 550 MPa (80 ksi), doubling 316L/317L.
Weight Reduction: High strength enables thinner sections, reducing material use and weight.
- Density: ~7.8 g/cm³
- Thermal Expansion: ~13 µm/m·°C (20-100°C).
- Thermal Conductivity: ~15 W/m·K (20°C).
- Young's Modulus: ~200 GPa.
- Toughness: Excellent down to -50°C.
- Magnetic Response: Magnetic due to ferrite phase (~50%).
S32750 vs. Others
|
Property |
S32750 (2507) |
S32205 (2205) |
316L |
|
PREN |
40-45 |
34-38 |
24-26 |
|
Y.S. Min (MPa) |
550 |
450 |
170 |
|
Thermal Exp. (µm/m·°C) |
13.0 (20-100°C) |
13.0 |
16.0 |
|
Chloride SCC Resist. |
Excellent |
Very Good |
Poor (Mod-High Temp) |
|
Seawater/Pitting Resist. |
Superior |
Very Good (Ambient) |
Limited (Ambient) |
|
Cost Factor |
High |
Medium-High |
Lower |
S32750 Fabrication and Weldability
Welding Best Practices:
- Filler Metals: ER2594 (TIG/MIG), E2594 (SMAW). Over-alloyed Ni (~9%) maintains phase balance.
- Heat Input Control: Strict limits (~0.5 - 1.5 kJ/mm) to avoid detrimental phases.
- Interpass Temperature: Critical: Keep < 100°C (212°F).
- Shielding Gas: High purity Argon (+ 2% N₂ optional).
- Surface Cleanliness: Absolute requirement; prevent carbon/copper contamination.
Solution Annealing: Mandatory post-welding/fabrication: 1020-1120°C (1868-2048°F), followed by rapid water quenching.
Forming: Requires more force than austenitics. Avoid excessive cold work without annealing.
Machining: Requires rigid setups, positive cutting tools and moderate feeds/speeds similar to high-strength alloys.
S32750 Applications
- Offshore Oil and Gas: Subsea systems, downhole tubing, manifolds, pumps (high chloride, sour [H₂S] conditions – NACE MR0175 compliant).
- Seawater Systems: Desalination plants (high-pressure RO tubes, evaporators), heat exchangers, pumps, valves, intake screens.
- Chemical and Petrochemical Processing: Reactors, pressure vessels, piping for acids, chlorides, and corrosive slurries.
- Pollution Control: Flue Gas Desulfurization (FGD) scrubbers, absorber towers, ducting, stack liners.
- Pulp and Paper Industry: Digesters, bleach plants, chip handling near aggressive liquors.
- Marine Components: Shafts, propellers, seawater piping systems, fittings.
Key Specifications & Ordering Guidance
S32750 Product Forms
- Plate/Sheet: ASTM A240 / ASME SA240, EN 10088-2
- Pipe: ASTM A790/A789 (Seamless/Welded), EN 10216-5
- Bar: ASTM A276, ASTM A479
- Fittings: ASTM/ASME B16.9/A815
- Flanges: ASTM/ASME B16.5/B16.47
Conclusion
UNS S32750 stands as a premium solution for applications where standard stainless steels falter. Its super duplex structure, defined by its high PREN, outstanding strength, and robustness against chloride attack and SCC, delivers unparalleled value over the lifecycle of critical assets in energy, chemical, and marine sectors. When failure is not an option in the most aggressive environments, S32750 provides the metallurgical assurance engineers demand.
