UNS S32760 represents the pinnacle of super duplex stainless steel technology, engineered to deliver exceptional strength and corrosion resistance in the most demanding environments. Often known by its trade names Sandvik SAF 2507® or Ferrinox 255®, this alloy bridges the gap between standard duplex steels and expensive nickel alloys, offering a compelling combination of performance and value. But what exactly is S32760 material, and why is it specified for critical applications in offshore, chemical processing, and desalination? This comprehensive guide dives into its composition, properties, standards, and uses.
Understanding S32760 Advantage
UNS S32760 belongs to the super duplex stainless steel (SDSS) family. Its "duplex" structure refers to its unique, balanced microstructure – approximately 50% ferrite (α) and 50% austenite (γ). This dual-phase structure is the secret to its outstanding properties:
- Double the Strength: Inheriting high strength from the ferritic phase and toughness from the austenitic phase, S32760 offers roughly twice the yield strength of standard 300-series austenitic stainless steels like 316L. This allows for thinner sections, reducing weight and material costs.
- Superior Corrosion Resistance: The high alloy content provides exceptional resistance to a wide range of corrosive media, particularly chloride-induced attacks like pitting, crevice corrosion, and stress corrosion cracking (SCC). Its Pitting Resistance Equivalent Number (PREN) exceeds 40, placing it among the most corrosion-resistant stainless steels available.
- Excellent Toughness & Ductility: Despite its high strength, S32760 maintains good impact toughness and ductility, essential for fabrication and service integrity.
- Good Weldability: While requiring more care than austenitics, S32760 can be reliably welded using appropriate procedures and consumables (typically over-alloyed with nickel).
S32760 Chemical Composition
The remarkable properties of UNS S32760 stem from its precisely balanced chemical composition:
| Element | Typical Range (%) | Primary Function |
|---|---|---|
| Chromium (Cr) | 24.0 - 26.0 | Forms passive oxide layer, resistance to oxidizing acids, pitting, crevice corrosion |
| Nickel (Ni) | 6.0 - 8.0 | Stabilizes austenite phase, improves toughness, resistance to reducing acids |
| Molybdenum (Mo) | 3.0 - 4.0 | Enhances pitting/crevice resistance, especially in chlorides |
| Nitrogen (N) | 0.20 - 0.30 | Boosts strength, stabilizes austenite, dramatically improves pitting resistance |
| Tungsten (W) | 0.50 - 1.00 | Further enhances pitting/crevice resistance, solid solution strengthening |
| Manganese (Mn) | ≤ 1.00 | Aids hot workability, combines with sulfur |
| Copper (Cu) | 0.50 - 1.00 | Improves resistance to sulfuric acid |
| Silicon (Si) | ≤ 0.80 | Deoxidizer, enhances resistance to strong oxidizing acids |
| Carbon (C) | ≤ 0.030 | Minimized to prevent harmful carbide precipitation |
| Phosphorus (P) | ≤ 0.030 | Controlled impurity |
| Sulfur (S) | ≤ 0.010 | Controlled impurity (improves machinability slightly but can reduce hot ductility) |
| Iron (Fe) | Balance | Matrix |
- Key Synergies: The high Cr, Mo, W, and N content work together to achieve the PREN >40. Copper adds specific acid resistance. Low carbon minimizes sensitization risks during welding or high-temperature exposure.
S32760 Material Properties
S32760 delivers an impressive blend of physical and mechanical properties:
Physical Properties:
- Density: ~7.8 g/cm³
- Melting Range: ~1420-1465°C (2590-2670°F)
- Thermal Expansion: ~13.0 x 10⁻⁶ /K (20-100°C) - Lower than austenitic grades, beneficial for thermal cycling.
- Thermal Conductivity: ~15 W/m·K (20°C) - Lower than carbon steel but higher than austenitic SS.
- Modulus of Elasticity: ~200 GPa (29 x 10⁶ psi) - Higher than austenitics, improving stiffness.
- Magnetic Response: Ferromagnetic due to high ferrite content.
Mechanical Properties (Typical Annealed Condition - Plate/Sheet):
- Yield Strength (Rp0.2): ≥ 550 MPa (≥ 80 ksi)
- Tensile Strength (Rm): 750 - 895 MPa (109 - 130 ksi)
- Elongation (A50mm): ≥ 25%
- Hardness (Brinell): ≤ 290 HB (Rockwell C ~30 HRC)
- Impact Toughness (Charpy V-notch @ -40°C / -40°F): ≥ 45 J (≥ 33 ft-lb) - Excellent low-temperature toughness for a duplex steel.
- Fatigue Strength: High, benefiting from the duplex microstructure.
S32760 Corrosion Resistance
This is the core strength of UNS S32760. It outperforms standard duplex (e.g., 2205/S32205) and austenitic grades (316L, 317L, 904L) in numerous aggressive environments:
- Chloride Environments: Exceptional resistance to pitting corrosion, crevice corrosion, and stress corrosion cracking (SCC) in seawater, brackish water, brine solutions, and chloride-containing process streams. Ideal for offshore platforms, desalination plants (RO membranes, high-pressure tubing), seawater cooling systems, and marine applications.
- Acidic Media: Excellent resistance to sulfuric acid (especially at moderate concentrations and temperatures), phosphoric acid, acetic acid, and many organic acids. Copper content enhances sulfuric acid resistance. Good resistance to nitric acid at moderate concentrations.
- Sour Service: Qualified under NACE MR0175/ISO 15156 for use in oil and gas environments containing H₂S (sour gas), CO₂, and chlorides at elevated temperatures and pressures (within defined limits).
- Oxidizing Conditions: Resists oxidizing salts like ferric chloride (FeCl₃) and cupric chloride (CuCl₂) far better than most stainless steels.
- Erosion-Corrosion: The high strength and inherent corrosion resistance make it suitable for components subject to high flow rates or abrasive particles.
S32760 Material Standards and Specifications
UNS S32760 is governed by strict international standards ensuring quality and consistency:
- ASTM:
- A182/A182M: Forged or Rolled Alloy Pipe Flanges, Forged Fittings, Valves & Parts (F61)
- A276/A276M: Bars & Shapes (F61)
- A479/A479M: Stainless Steel Bars & Shapes for Use in Boilers (F61)
- A790/A790M: Seamless & Welded Ferritic/Austenitic Stainless Steel Pipe (S32760)
- A928/A928M: Ferritic/Austenitic (Duplex) Steel Pipe Electric Fusion Welded with Addition of Filler Metal (S32760)
- EN (European Norm):
- EN 10250-4: Open Die Steel Forgings - Stainless Steels (1.4501)
- EN 10088-3: Technical Delivery Conditions for Semi-finished Products, Bars, Rods, Wire, Sections & Bright Products (1.4501)
- EN 10253-4: Butt-welding Pipe Fittings - Weldable Ferritic/Austenitic (Duplex) Stainless Steels (1.4501)
- EN 10222-5: Steel Forgings for Pressure Purposes - Martensitic, Austenitic & Austenitic-Ferritic Stainless Steels (1.4501)
- EN 10216-5: Seamless Steel Tubes for Pressure Purposes - Stainless Steels (1.4501)
- EN 10217-7: Welded Steel Tubes for Pressure Purposes - Stainless Steels (1.4501)
- Others:
- NACE MR0175/ISO 15156: Materials for use in H₂S-containing environments in oil and gas production.
- PED 2014/68/EU: Pressure Equipment Directive (CE Marking).
UNS S32760 Applications
S32760 is the material of choice when standard stainless steels fail, but nickel alloys are prohibitively expensive. Key sectors include:
- Offshore Oil & Gas: Subsea components (manifolds, X-mas trees, flowlines), umbilicals, pumps, valves, seawater piping, heat exchangers (handling seawater or sour fluids).
- Desalination & Water Treatment: High-pressure RO membranes, piping, pumps, valves, pressure vessels in SWRO plants.
- Chemical & Petrochemical Processing: Reactors, columns, piping, heat exchangers handling aggressive acids, chlorides, and sour streams.
- Marine & Shipbuilding: Seawater piping systems, prop shafts, rudders, offshore platform components.
- Pollution Control (FGD): Absorber towers, ducting, dampers, fans handling corrosive flue gas condensates.
- Pulp & Paper: Equipment in bleaching plants and digesters.
S32760 Material Product Form
UNS S32760/S32760 is readily available from specialized mills and suppliers like us in various forms:
- Plate & Sheet: For pressure vessels, tanks, structural components.
- Seamless & Welded Pipe/Tube: For process piping, heat exchanger tubes, umbilicals.
- Bar Stock: Round, hex, square; for machining shafts, valves, fittings, fasteners.
- Forgings: Flanges (weld neck, slip-on, blind), fittings (tees, elbows, reducers), valve bodies, pump casings (ASTM A182 F61 / EN 10250-4 1.4501).
- Fittings: Butt-weld fittings (ASTM A815 / EN 10253-4).
- Flanges: All standard types to ASME B16.5/B16.47 or EN standards (ASTM A182 F61 / EN 10250-4 1.4501).
Conclusion
UNS S32760 material stands as a premier super duplex stainless steel, offering an unmatched combination of extreme strength, exceptional corrosion resistance (especially against chloride pitting, crevice corrosion, and SCC), and good fabricability. Its PREN >40 makes it suitable for the harshest environments found in offshore oil and gas, desalination, chemical processing, and marine applications. Governed by rigorous standards like ASTM A790, EN 10250-4, and EN 10088-3 (1.4501), S32760 provides a cost-effective, high-performance alternative to nickel-based alloys. When your application demands resilience against aggressive corrosion under high stress, S32760 super duplex steel delivers the reliability and longevity engineers trust. Contact our metallurgical experts today to discuss if S32760 is the optimal solution for your next critical project.
