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AL6XN stainless steel is also known as UNS N08367 and is known in China as 00Cr20Ni24Mo6N. Another designation is AL-6XN plus, known in China as 00Cr21Ni25Mo6CuN. Both grades are super-austenitic stainless steels developed by Allegheny Ludlum Co. in the United States.
AL6XN plus is based on AL-6XN, but the chromium, nickel, molybdenum, and nitrogen content in the steel is controlled to the upper limit of 6X-6XN. This results in a PRE value of approximately 50 without compromising the thermal stability of the structure. In actual production, if the chromium, nickel, molybdenum, and nitrogen content in AL-6XN is controlled to the upper limit, the chemical composition of AL-6XN plus will be the same. Therefore, AL6XN plus is not considered a new grade.
AL6XN super austenitic stainless steel has a single austenite in the solid solution state without intermetallic phases. However, if the temperature is ≥540℃, intermetallic phases such as carbon, nitride and σ phase will precipitate. In this regard, those with chromium and molybdenum contents at the upper limit of the chemical composition will be more sensitive.
Grade | C≤ | Mn≤ | P≤ | S≤ | Si≤ | Cr | Ni | Mo | Cu≤ | N |
AL6XN | 0.03 | 2.0 | 0.04 | 0.03 | 1.0 | 20.0- 22.0 | 23.5- 25.5 | 6.0- 7.0 | 0.75 | 0.18- 0.25 |
| Parameter | Value |
|---|---|
| Density | 8.06 g/cm³ (0.291 lb/in³) |
| Tensile Strength | 795–830 MPa (115–120 ksi) |
| Yield Strength (0.2% Offset) | 390 MPa min (57 ksi min) |
| Elongation (50mm) | 40% min |
| Hardness (Rockwell) | HRB 80–95 |
| Charpy V-Notch Impact (RT) | >150 J (-150 ft·lb avg) |
| Young's Modulus | 200 GPa (29 × 10⁶ psi) |
| Shear Modulus | 73 GPa (10.6 × 10⁶ psi) |
Thermal Properties
| Parameter | Value |
|---|---|
| Melting Range | 1350–1390°C (2462–2534°F) |
| Thermal Expansion (20–100°C) | 14.7 μm/m·°C |
| Mean CTE (20–400°C) | 15.5 μm/m·°C |
| Thermal Conductivity | 11.5 W/m·K (80 Btu·in/ft²·h·°F) |
| Specific Heat Capacity | 500 J/kg·K (0.12 Btu/lb·°F) |
Electrical and Magnetic Properties
| Property | Value |
|---|---|
| Electrical Resistivity | 87 μΩ·cm @ 20°C |
| Relative Magnetic Permeability | ≤1.02 |
Pitting and Crevice Corrosion Resistance
| Environment | Test Standard | Temperature Limit (°C/°F) | Performance Rating |
|---|---|---|---|
| Neutral Chlorides (Seawater) | ASTM G48 Method A&D | 85°C / 185°F (CPT) 40°C / 104°F (CCT) | Excellent |
| Bleach (NaOCl, 15% pH=12) | ASTM G61 Potentiodynamic | 50°C / 122°F | Excellent |
| FeCl₃ (10%) | ASTM G48 Method A | 35°C / 95°F | Good* |
| H₂S-Saturated Brine (pH=3.5) | NACE TM0177 Method A | 80°C / 176°F | Excellent |
Acid Resistance
| Acid Solution | Concentration | Max. Temp (°C) | Max. Temp (°F) | vs. 316L Gain |
|---|---|---|---|---|
| Sulfuric Acid (H₂SO₄) | 10% | 90 | 194 | +40°C |
| 50% | 50 | 122 | +30°C | |
| Phosphoric Acid (H₃PO₄) | 30% | 120 | 248 | +35°C |
| 85% | 75 | 167 | +25°C | |
| Acetic Acid (CH₃COOH) | 100% Glacial | Boiling | 118 | +Full boiling |
| Nitric Acid (HNO₃) | 20% | 60 | 140 | –15°C▼ |
Stress Corrosion Cracking Resistance
| Environment | Standard/Test | Threshold | Result |
|---|---|---|---|
| MgCl₂ Boiling Test | ASTM G36 | 45% MgCl₂ @ 155°C | Immune (vs. 316L fails@110°C) |
| Sour Service (H₂S/Cl⁻) | NACE MR0175/ISO 15156-3 | 0.1 psi H₂S + 50,000 ppm Cl⁻ | NACE Level VII Qualified to 163°C |
| Caustic Cracking | ASTM C692 | 50% NaOH @ 140°C | Resistant |
| Concern | Test Method | AL-6XN Performance |
|---|---|---|
| Galvanic Action | ASTM G82 | Electrode Potential: +0.3V SCE Low risk when coupled with CrMo steel |
| Intergranular Attack | ASTM A262 Practice E | Sensitization-Immune @ 650°C No attack after HNO₃-HF test |
Environment-Specific Field Performance
| Industry Application | Key Corrodents | Observed Performance |
|---|---|---|
| Seawater Desalination (RO) | Cl⁻ >30,000 ppm | 15+ years zero pitting (Red Sea plants) |
| Flue Gas Desulfurization | H₂SO₄ mist + Fly ash + Cl⁻ | 7× life vs. 317LMN (FGD spray headers) |
| Offshore Oil Brine Transfer | H₂S 50 ppm + CO₂ + SRB | No SCC in 1000 psi subsea manifolds |
| Pharmaceutical CIP Systems | HCl/citric acid blends @ 60°C | Passes FDA-ESI/USP <661> leachables test |
STANDARD | UNS |
AL6XN | N08367 |
| Products | Standards | Show |
| AL6XN Pipe | ASTM A312 |  |
| AL6XN Tubing SMLS | ASTM A213/ ASTM A269 |  |
| AL6XN Welded Tube | ASTM A269 |  |
| AL6XN Sheet/Plate | ASTM A240 |  |
| AL6XN Bar | ASTM A479/ ASTM A276 |  |
| AL6XN Forgings | ASTM A182 |  |
| AL6XN Fittings | ASTM A403 |  |
FGD (Flue gas desulfurization) scrubbers.
Reverse osmosis desalination.
Distillation columns.
Heat exchangers.
Biopharm tanks and fermenters.
Pressure vessels.
Agitators and mixers.
Seawater coolers in nuclear plants.
Alloy Family: Both are high-molybdenum, high-nitrogen, low-carbon "super austenitic" stainless steels.
Corrosion Resistance:
AL-6XN PREN ≈ 43-45 (Cr% + 3.3Mo% + 16N%)
254 SMO PREN ≈ 43-45 (Cr% + 3.3Mo% + 16N%)
Very High PREN: Both have excellent resistance to pitting and crevice corrosion.
Chloride Resistance: Outstanding resistance to seawater, brine, brackish water, and chloride-containing process streams.
Acid Resistance: Excellent resistance to a wide range of acids, including sulfuric, phosphoric, acetic, and organic acids, particularly at moderate temperatures and concentrations.
Stress Corrosion Cracking (SCC): Highly resistant to chloride-induced SCC.
Base Mechanical Properties: Similar tensile and yield strength at room temperature. Good toughness and ductility.
Fabrication: Generally weldable using similar techniques (TIG, MIG, SAW) but require careful control to avoid harmful precipitates (sigma, chi, Laves phases). Both benefit from post-weld annealing for maximum corrosion resistance in the HAZ. Good formability.
Applications: Overlap significantly in demanding environments like:
Seawater handling (pumps, valves, piping, heat exchangers)
Desalination plants (MSF, MED, RO)
Chemical processing (FGD, pulp & paper, petrochemical)
Pharmaceutical and food processing
Pollution control equipment
| Feature | 254 SMO (S31254) | AL-6XN (N08367) | Winner |
|---|---|---|---|
| Composition (Typical) | 20% Cr, 18% Ni, 6.1% Mo, 0.20% N, 0.7% Cu | 20.5% Cr, 24% Ni, 6.3% Mo, 0.22% N, 0.3% Cu | AL-6XN: Higher Ni, Mo, N. 254 SMO: Higher Cu. |
| Corrosion Resistance Nuances | Slightly better in oxidizing acids (e.g., HNO₃, CrO₃, FeCl₃, CuCl₂) due to higher Cr/Cu. | Slightly better in reducing acids (e.g., H₂SO₄, H₃PO₄) and environments containing sulfides (H₂S) due to higher Ni/Mo/N. | Oxidizing: 254 SMO (Marginally). Reducing/Sulfides: AL-6XN (Marginally). Test for critical apps! |
| Mechanical Properties | Higher Strength: Slightly higher yield (YS) and tensile (UTS) strength at RT. Lower ductility (Elongation %). | Slightly lower YS/UTS at RT. Higher ductility. Better impact toughness at cryogenic temperatures. | Strength (RT): 254 SMO. Ductility/Cryo Toughness: AL-6XN. |
| High Temp Strength | Significantly Better: Maintains strength better above ~300°C (570°F). | Loses strength more rapidly above ~300°C. | >300°C: 254 SMO (Clear Advantage). |
| Cost | Generally Higher than AL-6XN. | Generally Lower than 254 SMO. | Cost: AL-6XN (Advantage). |
| Availability & Forms | Widely available, but sometimes slightly longer lead times or less stock for niche forms than AL-6XN. | Extremely widespread availability in plate, sheet, tube, pipe, bar, fittings. Often considered the "workhorse" super austenitic. | Availability: AL-6XN (Slight Advantage). |
| Weldability | Similar challenges (need low heat input, fast cooling). Slightly higher risk of hot cracking due to higher Cu & slightly lower Ni/Cr ratio. | Similar challenges. Marginally easier to weld without hot cracking due to higher Ni/Cr ratio. | Weldability: AL-6XN (Slight Advantage). Both require expertise! |
Highest Priority on Oxidizing Acid Resistance: If the primary corrosive media includes strong oxidizing acids (like concentrated nitric acid) or highly oxidizing salts (like ferric or cupric chloride).
Higher Temperature Strength: For applications operating consistently above 300°C (570°F) where maintaining mechanical strength is critical.
Slightly Higher Room Temperature Strength: Where maximizing YS/UTS at ambient conditions is a key design factor (though AL-6XN is still strong).
Cost Sensitivity: When the budget is tighter, and AL-6XN's slightly lower cost provides significant savings, especially on large projects.
Sulfide or Reducing Acid Environments: Where resistance to H₂S or reducing acids like sulfuric acid is paramount.
Cryogenic Applications: Where excellent impact toughness at very low temperatures is required.
Maximizing Formability: For complex forming operations where higher elongation is beneficial.
Lead Time: When immediate, wide-ranging stock availability is crucial.
Marginally Easier Welding: In situations where weldability is a major concern and any slight edge matters.
