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UNS S32550 super duplex steel represents a pinnacle of metallurgical engineering, combining the best of austenitic and ferritic stainless steels to deliver unmatched corrosion resistance and mechanical strength. This alloy, often marketed as Ferralium 255, features a balanced duplex microstructure (approximately 50% austenite and 50% ferrite), which yields a unique combination of high tensile strength (≥110 ksi/760 MPa), exceptional ductility, and superior resistance to pitting, crevice corrosion, and stress corrosion cracking (SCC).
The alloy’s composition—24-27% chromium, 4.5-6.5% nickel, 2.9-3.9% molybdenum, and 1.5-2.5% copper—creates a protective oxide layer that excels in highly acidic and chloride-rich environments. The addition of copper enhances resistance to sulfuric and phosphoric acids, while nitrogen strengthens the austenitic phase and boosts overall tensile properties. This results in a Pitting Resistance Equivalent (PREN) of 40-43 (calculated as Cr + 3.3Mo + 16N), making S32550 particularly effective in preventing localized corrosion in seawater, industrial chemicals, and acidic media.
Manufactured via seamless extrusion, these pipes exhibit uniform grain structure and precise dimensional control, eliminating weld lines that could compromise integrity in high-pressure applications. Post-production treatments such as pickling, annealing, and optional polishing further enhance surface quality and corrosion resistance. The alloy’s unique blend of strength and corrosion resistance makes it a cost-effective alternative to nickel-based alloys in demanding industrial environments.
ASTM A789: Seamless and welded ferritic-austenitic stainless steel tubing for general service.
ASTM A790: Seamless, welded, and cold-worked austenitic stainless steel pipes.
EN 10216-5: European standard for seamless steel tubes for pressure purposes.
ISO 1127: Stainless steel pipes and tubes for general purposes.
| Element | Range | Role in Alloy |
|---|---|---|
| C | ≤0.04 | Controls carbide formation to maintain corrosion resistance during welding. |
| Mn | ≤1.50 | Stabilizes austenite and improves weldability. |
| P | ≤0.040 | Minimizes brittleness; controlled for formability. |
| S | ≤0.030 | Reduces hot shortness during fabrication. |
| Si | ≤1.00 | Enhances oxidation resistance at elevated temperatures. |
| Ni | 4.50-6.50 | Strengthens austenitic phase and improves resistance to chloride SCC. |
| Cr | 24.00-27.00 | Forms protective oxide layer, critical for oxidation and corrosion resistance. |
| Mo | 2.90-3.90 | Boosts resistance to pitting and crevice corrosion in chloride environments. |
| N | 0.10-0.25 | Strengthens austenite, improves tensile strength, and stabilizes microstructure. |
| Cu | 1.50-2.50 | Enhances resistance to sulfuric and phosphoric acids. |
Note: PREN (Pitting Resistance Equivalent) ≥40, making it suitable for severe corrosion conditions.
| Property | Value | Significance in Applications |
|---|---|---|
| Tensile Strength | ≥110 ksi (760 MPa) | Enables use in high-pressure systems without excessive wall thickness. |
| Yield Strength | ≥80 ksi (550 MPa) | Ensures structural stability under sustained loading. |
| Elongation at Break | ≥15% | Provides ductility for forming, bending, and impact resistance. |
| Hardness (HBW) | ≤297 | Balances wear resistance with machinability for fabrication. |
| Impact Toughness | ≥40 J/cm² at -40°C | Maintains ductility in sub-zero environments (e.g., LNG processing). |
Outer Diameter (OD): 6 mm to 720 mm (0.24" to 28.3")
Wall Thickness: 1 mm to 65 mm (0.04" to 2.56")
Length: Customizable up to 12 meters (standard: 6 meters)
Process Flow:
Vacuum induction melting (VIM) for chemical purity.
Hot-rolling of ingots into billets.
Cold-drawing/extrusion for precise dimensions.
Solution annealing at 1020-1100°C to refine microstructure.
Pickling to remove scale and ensure surface cleanliness.
Surface Treatments:
Pickled & Annealed (standard): Removes oxidation, ideal for general corrosion resistance.
Polished (Ra ≤0.8μm): Enhances hygienic properties for food/pharmaceutical applications.
Bright Annealed: Maintains surface luster via controlled atmosphere heating.
Testing Regime:
100% Positive Material Identification (PMI)
Hydrostatic pressure testing (1.5x design pressure)
Non-destructive evaluation (UT/ET) for internal defects
Intergranular corrosion (IGC) testing per ASTM A262
Mill Test Certificates (MTC): Compliant with EN 10204 3.1/3.2, including chemical analysis and mechanical properties.
Third-Party Inspection: Available from Lloyd’s, DNV, or customer-specified bodies.
Marking: Laser-engraved with grade, size, heat number, and manufacturer logo.
Acid Production & Handling: Resists sulfuric, phosphoric, and nitric acids in fertilizer plants, chemical synthesis, and pickling lines.
Reactors & Heat Exchangers: Handles aggressive media in petrochemical refining and pharmaceutical manufacturing.
Corrosive Waste Treatment: Ideal for pipes and tanks in industrial wastewater neutralization systems.
Offshore Platforms: Deck hardware, rudders, and shafting in saltwater environments, resisting pitting and erosion from high-velocity fluids.
Seawater Desalination: Heat exchangers and pipelines in reverse osmosis (RO) plants, with Critical Pitting Temperature (CPT) exceeding 50°C.
Marine Fixtures: Pump shafts, mixer components, and seawater intake systems in coastal facilities.
Sour Gas Processing: Resists H₂S and CO₂ corrosion in offshore wellheads, separators, and flowlines.
Downhole Equipment: Tubing and casing in high-pressure, high-temperature (HPHT) wells.
Cargo Handling: Corrosion-resistant pipes for transporting acidic hydrocarbons and chemical additives.
Bleaching & Digestion Systems: Replaces conventional stainless steels in equipment exposed to chloride-based bleaches and recycled process liquids.
Pulp Digesters: Withstands acidic conditions in wood pulping and paper manufacturing.
Flue Gas Desulfurization (FGD): Tubes and components in wet scrubbers, resisting sulfuric acid corrosion.
Geothermal Energy: Heat exchangers and pipelines in high-salinity geothermal fluid systems.
Acid Leaching Systems: Handles sulfuric acid in copper, nickel, and uranium extraction.
Mineral Slurry Transport: Resists abrasion and corrosion in ore processing pipelines.
A: S32550 offers superior corrosion resistance (PREN 40-43 vs. 34-38 for S32205) and higher tensile strength (760 MPa vs. 655 MPa), making it suitable for more aggressive environments. It also excels in sulfuric acid, while S32205 is better suited for moderate chloride conditions.
A: Yes, the alloy maintains ductility and impact toughness down to -40°C, making it suitable for LNG processing, Arctic oil fields, and cryogenic storage systems.
A: Use low-heat-input TIG or MIG welding with ER2553 filler wire. Maintain interpass temperatures ≤250°C to avoid phase imbalance. Post-weld heat treatment is typically unnecessary if parameters are controlled.
A: Service temperatures up to 300°C are recommended for continuous use. Prolonged exposure above 350°C may cause sigma phase precipitation, reducing corrosion resistance. For higher temperatures, consider super duplex S32750 or nickel alloys.
A: Yes, when properly heat-treated, S32550 meets NACE MR0175 requirements for H₂S-containing environments, making it suitable for sour oil and gas applications.
A: Standard sizes are available within 2-4 weeks. Non-standard dimensions or special finishes require 6-8 weeks, including manufacturing, testing, and certification.
