AL-6XN® (UNS N08367) is a low-carbon, nitrogen-bearing super-austenitic stainless steel alloy containing 23.5–25.5% nickel, 20–22% chromium, and 6–7% molybdenum. It achieves a Pitting Resistance Equivalent Number (PREN) of approximately 46–48, suitable for seawater, chloride-bearing, and acidic environments where standard stainless steels such as 316L are insufficient. It is available in forged forms including flanges, fittings, and valves per ASTM B462 / ASME SB462.
AL-6XN® is a registered trademark of ATI Properties, Inc. (d/b/a Allegheny Ludlum Corporation). Jiangsu Liangyi Co., Limited is an independent manufacturer and supplier of forged components in UNS N08367 alloy and is not affiliated with, endorsed by, or a licensed distributor of ATI Properties, Inc. All references to "AL-6XN" on this page are used solely for descriptive and informational identification of the alloy by its widely recognized trade name.
Quick Summary: UNS N08367 (commonly referred to as AL-6XN®) is a super-austenitic stainless steel engineered for environments where 316L and duplex grades fail. Its high nickel (24%), chromium (22%), and molybdenum (6.3%) content delivers exceptional chloride corrosion resistance, 95 ksi tensile strength, and full weldability — available as forged flanges, fittings, valves, and billets to ASTM B462 / ASME SB462 from Jiangsu Liangyi Co., Limited.
When engineers specify materials for seawater heat exchangers, offshore chemical injection systems, or flue gas desulfurization scrubbers, the question is never simply "which stainless steel?" — it is "which stainless steel survives long enough to be economical?" That question is where UNS N08367 consistently earns its place on the specification sheet.
This guide covers the alloy's full chemistry, mechanical and corrosion properties, available UNS N08367 forged parts from Jiangsu Liangyi Co., Limited, governing standards, and the industries that depend on it daily.
Origin and Development of UNS N08367
The alloy now standardized as UNS N08367 was originally developed by Allegheny Ludlum Corporation under the trade name AL-6XN® and has been commercially available since the late 1970s. It was purpose-built as a seawater-resistant material after engineers recognized that standard austenitic stainless steels — including the widely used 316L — were inadequate in chloride-rich environments. Failures caused by pitting and crevice corrosion in desalination plants and offshore structures drove demand for a grade with significantly higher molybdenum and nickel content.
The term "super-austenitic" distinguishes UNS N08367 from conventional austenitic grades by its elevated alloying content — molybdenum above 6% and nickel above 20%. These additions push the PREN above 40 (the accepted seawater suitability threshold) while retaining full austenitic ductility and weldability.
Today UNS N08367 is produced by multiple qualified mills worldwide and covered by multiple ASTM and ASME specifications. It has become a benchmark alloy for demanding industrial service, with forged components serving as critical connectors in process piping systems across chemical, offshore, desalination, and power generation industries.
UNS N08367 Chemical Composition
The chemical composition is defined per ASTM B462 / ASME SB462 — the primary specification governing forged flanges, fittings, and valves. The three dominant elements — nickel, chromium, and molybdenum — directly drive corrosion performance.
| Element | Min % | Max % | Role in Performance |
|---|---|---|---|
| Nickel (Ni) | 23.5 | 25.5 | Stabilizes austenitic structure; primary defense against chloride SCC |
| Chromium (Cr) | 20.0 | 22.0 | Passivation agent; resistance in oxidizing environments |
| Molybdenum (Mo) | 6.0 | 7.0 | Pitting and crevice corrosion resistance; reducing acid environments |
| Nitrogen (N) | 0.18 | 0.25 | Elevates tensile strength; enhances PREN |
| Carbon (C) | — | 0.03 max | Low carbon prevents sensitization and intergranular corrosion |
| Manganese (Mn) | — | 2.00 max | Deoxidizer; minor austenite stabilizer |
| Silicon (Si) | — | 1.00 max | Deoxidizer; higher Si can impair hot workability |
| Copper (Cu) | — | 0.75 max | Mild benefit in reducing acid environments |
| Phosphorus (P) | — | 0.04 max | Low P preserves grain boundary integrity |
| Sulfur (S) | — | 0.03 max | Low S ensures weldability and toughness |
| Iron (Fe) | Balance ~44% | Matrix element | |
| Source: ASTM B462 / ASME SB462. Highlighted rows are the primary performance elements. AL-6XN® is a registered trademark of ATI Properties, Inc. | |||
The combination of high chromium + high molybdenum + nitrogen elevates the PREN to approximately 46–48 — far exceeding 316L's ~25 and clearing the seawater service threshold of ≥ 40. The strict 0.03% carbon maximum prevents intergranular corrosion at weld heat-affected zones, critical for forged components welded into active piping systems.
PREN = %Cr + 3.3 × %Mo + 16 × %N
Using nominal values: 21% Cr + 3.3 × 6.3% Mo + 16 × 0.22% N = 21 + 20.8 + 3.5 = ~45.3 to 47.5. The accepted seawater service minimum is PREN ≥ 40. UNS N08367 comfortably exceeds this.
Mechanical Properties at Ambient Temperature
Nitrogen acts as both a corrosion enhancer and solid-solution strengthener in UNS N08367. The result is tensile and yield strengths substantially above conventional austenitic grades, without sacrificing ductility or impact toughness.
PREN Comparison: UNS N08367 vs Common Alloys
PREN = %Cr + 3.3×%Mo + 16×%N. Seawater suitability threshold: PREN ≥ 40. Higher is better.
*Hastelloy® is a registered trademark of Haynes International. UNS N08367 occupies the optimal cost-performance position for most chloride and seawater applications.
Corrosion Resistance of UNS N08367
The corrosion resistance of UNS N08367 results from four alloying elements working in concert, each protecting against a specific corrosion mechanism.
Chloride Pitting and Crevice Corrosion
With a PREN of ~46–48, UNS N08367 resists pitting initiation at chloride concentrations and temperatures where 316L and duplex 2205 fail. In ASTM G48 testing, the critical pitting temperature (CPT) exceeds 72°C in 6% FeCl₃ — a benchmark standard austenitics cannot approach. This makes it the preferred material for seawater heat exchangers, offshore piping, and coastal process equipment.
Chloride Stress Corrosion Cracking (SCC) Resistance
The 24% nickel content is the primary defense against chloride-induced SCC — a catastrophic failure mode in conventional 304 and 316 grades in hot, chloride-laden service. At nickel levels above 20%, SCC susceptibility drops dramatically. Combined with molybdenum, UNS N08367 is rated for hot seawater service that would rapidly crack lower-nickel grades.
Acid and Reducing Environment Resistance
Chromium and molybdenum together extend corrosion resistance into acidic environments. UNS N08367 performs reliably in dilute sulfuric acid, phosphoric acid, and moderate-concentration hydrochloric acid — valuable in chemical process tanks, pipelines, and tall oil distillation columns.
Intergranular Corrosion (Sensitization) Resistance
With carbon at 0.03% maximum, UNS N08367 is a low-carbon grade. This suppresses chromium carbide precipitation during weld thermal cycles, preserving corrosion resistance at weld heat-affected zones — critical when forged flanges and fittings are welded into active piping.
UNS N08367 Forged Parts: Product Forms & Standards
Jiangsu Liangyi Co., Limited is an ISO-certified manufacturer of forged industrial components. Our UNS N08367 forged parts are manufactured to the following specifications:
Third-party inspection (TPI) by customer-nominated inspector is accepted upon request. We do not claim certification from SGS, Bureau Veritas, TÜV, or Lloyd's Register unless specifically arranged for your order.
| Product Type | Common Configurations | Governing Standard |
|---|---|---|
| Forged Flanges | Weld Neck, Slip-On, Blind, Socket Weld, Lap Joint, Threaded, Long Weld Neck | ASTM B462 · ASME B16.5 / B16.47 |
| Forged Fittings | Elbows 45°/90°/180°, Equal/Reducing Tees, Reducers, Caps, Stub Ends, Crosses | ASTM B366 · ASME SB366 |
| SW & Threaded Fittings | Elbows, Tees, Couplings, Unions, Bushings, Plugs, Nipples | ASME B16.11 |
| Forged Valve Bodies | Gate, Globe, Check | ASTM B462 · ASME B16.34 |
| Forged Bar / Billet | Round bar, hex bar for machined components | ASTM B691 |
| Pressure classes 150# through 2500# per ASME B16.5. Sizes ½″ NPS through 24″ NPS standard; larger diameters per ASME B16.47. | ||
Full heat-number traceability and Certified Material Test Reports (CMTR/MTR) are provided with every order. Contact us for custom dimensions and inspection requirements.
How UNS N08367 Forged Parts Are Manufactured
Forging UNS N08367 requires tighter process controls than standard austenitic grades due to its higher strength and work-hardening rate. The five key manufacturing stages are:
Raw Material Selection & Certification
Ingot or billet conforming to ASTM B691 / ASME SB691. Full heat-number traceability and CMTR maintained. Carbon verified below 0.03% maximum.
Hot Forging with Temperature Control
Forged in the range of approximately 1,900–2,250°F (1,040–1,230°C). Pyrometric control is mandatory to prevent matrix damage or excessive grain growth.
Solution Annealing & Water Quench
Post-forging solution anneal at minimum 2,100°F (1,149°C), followed by rapid water quench. This achieves the fully corrosion-resistant condition required by ASTM B462.
CNC Machining & Dimensional Inspection
Machined to ASME B16.5 / B16.11 / B16.47 tolerances. RF, RTJ, and FF facing options available. Full dimensional verification performed.
PMI, Hardness Testing & Final Release
PMI (Positive Material Identification), hardness verification (max 217 HB per ASTM B462), and visual/dimensional inspection. Customer-nominated TPI accepted.
Where UNS N08367 Forged Parts Are Specified
The combination of seawater corrosion resistance, acid tolerance, chloride SCC immunity, and high mechanical strength makes UNS N08367 the first-choice material across the following industries:
- Reverse osmosis pressure vessels
- Seawater intake piping flanges
- High-pressure pump components
- Brine disposal system headers
- Process tanks and reactor vessels
- Acid transfer piping flanges
- Tall oil distillation columns
- FGD scrubber internals
- Subsea chemical injection headers
- Offshore production manifolds
- Sour service fittings (H₂S)
- Topside seawater cooling systems
- FGD scrubber ducting flanges
- Cooling water heat exchangers
- Nuclear auxiliary piping systems
- Flue gas condensate drain lines
- Bleach plant piping systems
- Digester vessel flanges
- Washer vat connections
- Chlorine dioxide handling lines
- Sterile process connections
- CIP/SIP system flanges
- High-purity water fittings
- Aggressive CIP chemical service
UNS N08367 vs Alternative Alloys
| Alloy | PREN | SCC Resistance | Relative Cost | Choose When… |
|---|---|---|---|---|
| UNS N08367 (AL-6XN®) | ~47 | Excellent | $$$ mid-high | Chloride-bearing, seawater, moderate acids; lifecycle value is critical |
| 316L Stainless | ~25 | Poor–Moderate | $ low | Mildly corrosive, low chloride, budget-constrained projects |
| Duplex 2205 | ~35 | Moderate | $$ moderate | High strength + moderate chloride; avoid elevated temperature service |
| Hastelloy® C-276 | ~71 | Excellent | $$$$$ very high | Extreme acid concentrations; highest corrosion severity |
| Alloy 625 (N06625) | ~52 | Excellent | $$$$ high | High-temperature combined with corrosion; critical fatigue service |
| AL-6XN® is a registered trademark of ATI Properties, Inc. Hastelloy® is a registered trademark of Haynes International. PREN values are approximate and may vary by exact heat composition. | ||||
Frequently Asked Questions About UNS N08367
- ASTM B462 — Standard Specification for Forgings, UNS N08367 Alloy Pipe Flanges, Forged Fittings, and Valves (American Society for Testing and Materials)
- ASME SB462 — Equivalent ASME Boiler and Pressure Vessel Code specification
- ASTM B366 — Standard Specification for Factory-Made Wrought Nickel and Nickel Alloy Fittings
- ASME B16.5 — Pipe Flanges and Flanged Fittings: NPS ½ Through NPS 24 Metric/Inch Standard
- ASME B16.11 — Forged Fittings, Socket-Welding and Threaded
- ASTM G48 — Standard Test Methods for Pitting and Crevice Corrosion Resistance of Stainless Steels and Related Alloys
- AWS A5.14 — Specification for Nickel and Nickel-Alloy Bare Welding Electrodes and Rods
- AL-6XN® is a registered trademark of ATI Properties, Inc. (d/b/a Allegheny Ludlum Corporation). Hastelloy® is a registered trademark of Haynes International.