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UNS S31803, also known as F51, is a duplex stainless steel with a dual-phase microstructure consisting of approximately 50% austenite and 50% ferrite. This structure provides the material with an excellent combination of strength and corrosion resistance.
F51/S31803/1.4462 duplex stainless steel offers high strength, excellent impact resistance, and corrosion resistance. In harsh environments, its resistance to pitting, crevice corrosion, stress corrosion, and corrosion fatigue is superior to that of ordinary austenitic stainless steel. It also features a lower coefficient of thermal expansion and higher thermal conductivity. Therefore, it is widely used in oil and gas pipelines, heat exchange equipment, and pressure vessels.
| Composition | Value |
Carbon (C) | 0.0 - 0.03 |
Chromium (Cr) | 21.00 - 23.00 |
Manganese (Mn) | 2.00 max |
Silicon (Si) | 1.00 max |
Phosphorous (P) | 0.0 - 0.03 |
Sulphur (S) | 0.0 - 0.02 |
Nickel (Ni) | 4.50 - 6.50 |
Nitrogen (N) | 0.08 - 0.20 |
Molybdenum (Mo) | 2.50 - 3.50 |
Iron (Fe) | Balance |
Tensile, min, ksi[MPa] | Yield, min, ksi[MPa] | Elongation, %(min) | Hardness, HB（max） |
90 [620] | 65 [450] | 25 | 290 |
STANDARD | WERKSTOFF NR. | UNS | EN |
Duplex S31803 | 1.4462 | S31803 | X2CrNiMoN22-5-3 |
| Products | Standards | Show |
| Duplex S31803/F51 Pipe | ASTM A790 |  |
| Duplex S31803/F51 Tube | ASTM A789 |  |
| Duplex S31803/F51 Sheet | ASTM A240 |  |
| Duplex S31803/F51 Bars | ASTM A276/A479 |  |
| Duplex S31803/F51 Forgings/Flanges | ASTM A182 |  |
| Duplex S31803/F51 BW Fittings | ASTM A815 |  |
Corrosion Resistance
Duplex stainless steels are extremely corrosion resistant. They have high resistance to intergranular corrosion. Even in chloride and sulphide environments, duplex stainless steels exhibit very high resistance to stress corrosion cracking. While the super duplex grades are even more resistant to corrosion.
Heat Resistance
The high chromium content of duplex stainless steels that protects against corrosion, causes embrittlement at temperatures over about 300°C. At low temperatures duplex stainless steels have better ductility than the ferritic and martensitic grades. Duplex grades can readily be used down to at least -50°C.
Heat Treatment
The solution treatment temperature is 1040-1100°C, and it must be quenched after heat treatment. The cooling rate should be as fast as possible, and the time of exposure to the brittle temperature range (475°C brittle temperature range, σ brittle temperature range) must be shortened.
Pickling
Pickling uses a mixture of nitric and hydrofluoric acids. Since the scale of S31803 is slightly more difficult to remove compared to SUS 304, it may be more effective to perform a short alkali immersion before pickling or, if possible, shot peening.
The table below provides a side-by-side comparison across the properties that matter most in aggressive service environments.
| Property / Criterion | Duplex S31803 / F51 | 316L Stainless Steel | Which is Better? |
|---|---|---|---|
| Microstructure | 50% ferrite + 50% austenite (dual-phase) | 100% austenite | Duplex: higher strength, better SCC resistance |
| Yield Strength (min) | 450 MPa (65 ksi) | 170 MPa (25 ksi) | Duplex S31803: 2.6x higher yield |
| Tensile Strength (min) | 620 MPa (90 ksi) | 485 MPa (70 ksi) | Duplex S31803: 28% higher UTS |
| PREN (Pitting Resistance) | ~34-36 | ~24-26 | Duplex S31803: superior pitting resistance |
| Chloride SCC Resistance | Excellent (immune in most oilfield conditions) | Poor (susceptible above 60°C) | Duplex S31803: critical for hot chloride service |
| Critical Pitting Temperature (CPT) | ~25-30°C (ASTM G48) | ~10-15°C (ASTM G48) | Duplex S31803: higher CPT |
| NACE MR0175 / ISO 15156 | Qualified for moderate sour service | Qualified for mild sour service only | Duplex S31803: wider H2S/pH range |
| Service Temperature Range | -50°C to 300°C | -200°C to 450°C+ | 316L: wider high-temp range; duplex limited to 300°C |
| Density | 7.8 g/cm³ | 8.0 g/cm³ | Duplex: ~2.5% lighter (cost saving in large tonnage) |
| Thermal Conductivity | Higher (better heat transfer) | Lower | Duplex: advantage in heat exchangers |
| Weldability | Good (requires heat input control) | Excellent (very forgiving) | 316L: easier for general fabrication |
| Typical Cost (relative) | 100 | 75-85 | 316L: 15-25% lower material cost |
| Primary Applications | Oil & gas flowlines, seawater piping, pressure vessels, chemical tanks, mining slurry lines | Food processing, pharmaceutical, architectural, mild chemical service | See selection guide below |
Key Takeaway: For any application involving chloride-containing environments (seawater, produced water, brine), sour service (H2S), or high mechanical stress, duplex S31803 is the technically superior choice despite its 15-25% higher material cost. The yield strength advantage alone can reduce wall thickness by 30-40%, offsetting much of the cost premium.
Duplex S31803 is widely used in the mining and oil & gas industries where the combination of high strength and corrosion resistance.
| Application | Environment | Why Duplex S31803? | Typical Product Forms |
|---|---|---|---|
| Flowlines & Pipelines | Produced water (1,000-80,000 ppm Cl-), moderate H2S, CO2 | Yield strength 450 MPa reduces wall thickness; SCC immunity in hot chlorides; NACE MR0175 qualified | ASTM A790 seamless pipe, ASTM A815 fittings |
| Subsea Piping (non-critical) | Seawater immersion, 5-35°C | PREN ~35 provides adequate pitting resistance for non-splash-zone subsea service; cost advantage over super duplex 2507 | ASTM A790 pipe, ASTM A182 flanges |
| Topside Process Piping | Chloride-containing process fluids, 20-80°C | SCC resistance in hot chlorides; higher strength than 316L allows thinner walls | ASTM A790/A789 pipe & tube |
| Heat Exchangers | Cooling water (seawater or brackish), process fluid | Higher thermal conductivity than austenitics; resistance to under-deposit pitting | ASTM A789 tube, ASTM A240 plate |
| Pressure Vessels | Sour gas, moderate H2S partial pressure | ASME VIII design benefits from high yield strength; NACE MR0175 qualified | ASTM A240 plate |
| Chemical Injection Lines | Methanol, scale inhibitors, moderate chloride | Good general corrosion resistance; lower cost than Alloy 825 or Inconel 625 | ASTM A789 tube |
| Water Injection Systems | Seawater or produced water reinjection | SCC immunity critical; PREN adequate for non-aerated seawater | ASTM A790 pipe |
Decision Rule: For oil & gas service temperatures below 300°C and chloride concentrations above 1,000 ppm, duplex S31803 should be evaluated as the baseline CRA material. If H2S is present, verify NACE MR0175 / ISO 15156-3 qualification for the specific field conditions.
| Application | Environment | Why Duplex S31803? | Typical Product Forms |
|---|---|---|---|
| Slurry Transport Lines | Abrasive + chloride-containing slurries | High hardness (290 HB max) resists erosion; SCC immunity prevents sudden failures | ASTM A790 pipe, ASTM A815 fittings |
| Heap Leach Piping | Cyanide or acid leach solutions, moderate chloride | Resists SCC in chloride-cyanide environments; lower cost than nickel alloys | ASTM A790 pipe |
| Process Tanks & Vessels | Acidic process solutions, 20-60°C | Good resistance to dilute sulfuric and phosphoric acid; high strength allows thinner plate | ASTM A240 plate |
| Pump & Valve Components | Corrosive mine water, abrasive slurries | High strength + corrosion resistance extends service life | ASTM A276/A479 bar, ASTM A182 forgings |
| Tailings Pipelines | High-chloride tailings water | SCC immunity critical for long-term integrity | ASTM A790 pipe |
| Ventilation Ducting (underground) | Humid, chloride-containing mine air | Resists atmospheric pitting and crevice corrosion | ASTM A240 sheet |
Decision Rule: For mining applications involving chloride-containing process water, slurries, or acid leach solutions, duplex S31803 is often the most cost-effective CRA. For highly acidic conditions (pH <2) or extreme abrasion, consider super duplex 2507 or nickel alloys.
Service temperatures above 300°C: Risk of sigma-phase embrittlement. Use 316L or high-temperature austenitic grades.
Continuous seawater immersion at >50°C: PREN ~35 may be marginal. Consider super duplex 2507 (PREN ~43) or Inconel 625.
Extreme sour service (high H2S partial pressure, elemental sulphur): Verify NACE MR0175 limits. May require Alloy 825 or Inconel 625.
Cryogenic service below -50°C: Impact toughness may decrease. Verify with Charpy testing for specific temperatures.
Seawater systems
Oil field piping
Chemical process vessels, piping, and heat exchangers
FGD scrubber systems
Pulp mill digesters, bleach washers
Food process equipment
Q1: What is the difference between S31803 and S32205?
Both S31803 and S32205 are duplex stainless steels with nearly identical chemical composition ranges. The key difference is in the nitrogen specification: S32205 has a tighter nitrogen range (0.14-0.20%) compared to S31803 (0.08-0.20%), which ensures more consistent corrosion resistance and mechanical properties. In practice, most modern duplex 2205 material is dual-certified to both S31803 and S32205, meeting the stricter S32205 nitrogen requirement while retaining S31803 designation for legacy specifications. JN Alloy supplies dual-certified duplex pipe and plate meeting both standards.
Q2: Can duplex S31803 be used for seawater service?
Yes — duplex S31803 can be used for seawater service under specific conditions. Its PREN of approximately 34-36 provides adequate resistance to pitting in natural seawater at temperatures up to approximately 30-35°C. For continuous seawater immersion at higher temperatures, or in splash zone applications where aeration increases aggressiveness, super duplex 2507 (PREN ~43) or Inconel 625 is preferred. For subsea flowlines in cold seawater (North Sea, deepwater), S31803 is a cost-effective choice when properly qualified per DNV-OS-F101.
Q3: Is duplex S31803 qualified for sour service under NACE MR0175?
Yes — duplex S31803 is qualified for sour service under NACE MR0175 / ISO 15156-3 for moderate sour conditions. The qualification applies to solution-annealed material with hardness ≤28 HRC (typical annealed hardness is 22-25 HRC). The allowable environmental region includes H2S partial pressures up to approximately 0.3-1.0 bar depending on pH and chloride concentration. For specific field conditions, verify against the S31803 region in NACE MR0175 / ISO 15156-3 Table A.23. JN Alloy supplies NACE-compliant S31803 with full CMTR documentation.
Q4: What is the PREN of duplex S31803?
The PREN (Pitting Resistance Equivalent Number) of duplex S31803 is approximately 34-36, calculated as %Cr + 3.3×%Mo + 16×%N using nominal composition (22% Cr, 3% Mo, 0.14% N). This places S31803 above 316L (PREN ~24) and below super duplex 2507 (PREN ~43). For seawater-immersed service, a minimum PREN of 40 is often specified — S31803 meets this threshold for moderate chloride environments but may require upgrading to super duplex 2507 for continuous seawater immersion at elevated temperatures.
Q5: What is the maximum service temperature for duplex S31803?
The maximum recommended service temperature for duplex S31803 is 300°C (572°F) for extended service. Above this temperature, prolonged exposure can cause sigma-phase precipitation in the ferrite phase, leading to embrittlement and reduced corrosion resistance. For applications requiring service above 300°C, austenitic stainless steels such as 316L, 321, or 347 are preferred. For cryogenic service, S31803 retains good toughness down to -50°C; below this, impact testing is required to confirm suitability.
Q6: How does duplex S31803 compare to super duplex 2507 for offshore use?
Duplex S31803 and super duplex 2507 are both used in offshore applications, but they serve different niches:
S31803 (PREN ~35): Suitable for moderate chloride environments, subsea flowlines in cold seawater, topside piping, and heat exchangers. Lower cost (approximately 30-40% less than 2507).
Super duplex 2507 (PREN ~43): Required for continuous seawater immersion at higher temperatures, splash zone, high-chloride formation water, and the most aggressive offshore conditions. Higher strength and CPT (~75°C vs ~25°C for S31803).
Decision rule: If the service conditions require PREN ≥40 or CPT ≥50°C, specify super duplex 2507. Otherwise, S31803 is the more economical choice.
Q7: Can duplex S31803 be welded?
Yes — duplex S31803 has good weldability using standard arc welding processes (GTAW, GMAW, SMAW, SAW). The key considerations are:
Filler metal: Use over-alloyed duplex filler (e.g., ER2209 / E2209) to ensure proper austenite-ferrite balance in the weld metal.
Heat input control: Maintain interpass temperature below 150°C and heat input in the range of 0.5-2.5 kJ/mm to avoid excessive ferrite or sigma-phase precipitation.
No PWHT required: Duplex S31803 does not require post-weld heat treatment in most applications.
Weld procedure qualification: Verify ferrite number (35-60 FN) in weld metal and HAZ per AWS D1.6 or equivalent.
JN Alloy can provide welding procedure guidance and ER2209 filler metal recommendations for S31803 fabrication.
Q8: What product forms are available for duplex S31803?
Duplex S31803 (F51, 1.4462) is available in all major mill product forms:
Pipe & Tube: ASTM A790 (seamless and welded), ASTM A789 (seamless tube)
Plate & Sheet: ASTM A240, ASTM A480
Bar & Rod: ASTM A276, ASTM A479
Forged Flanges: ASTM A182 (F51 grade), ASTM A350
Butt-Weld Fittings: ASTM A815, ASTM A182
Forged Fittings: ASTM A182, MSS SP-43/SP-79
