AL-6XN® (AL6XN / UNS N08367) Super-Austenitic Stainless Steel
- 6.3% molybdenum
- Over 25 years of proven seawater service
- Practical immunity to chloride stress cracking
- 50% stronger than stainless
- ASME coverage up to 800°F
- Easily welded
- Available in ALL product forms
AL-6XN® alloy (UNS N08367) is a low carbon, high purity, nitrogen-bearing “super-austenitic” stainless alloy. The AL-6XN alloy was designed to be a seawater resistant material and has since been demonstrated to be resistant to a broad range of very corrosive environments. The high strength and corrosion resistance of the AL6XN alloy make it a better choice than the conventional duplex stainless steels and a cost effective alternative to more expensive nickel-base alloys where excellent formability, weldability, strength and corrosion resistance are essential.
Performance Profile
The levels of chromium, molybdenum and nitrogen in AL-6XN all serve to provide resistance to acidic oxidizing chloride solutions previously achieved only by the nickel base alloys. High nickel (24%) and molybdenum (6.3%) contents make AL-6XN a solution to chloride ion stress corrosion cracking. Because of its nitrogen content, AL-6XN has greater tensile strength than common austenitic stainlesses, while retaining high ductility and impact strength. The ASME allowable stresses for AL-6XN are up to 75% higher than for 316L stainless, and more than twice those for the copper-nickel alloys.
Chemistry
| Cr | Ni | Mo | C | N | Mn | Si | P | S | Cu | Fe | |
|---|---|---|---|---|---|---|---|---|---|---|---|
| MIN | 20.00 | 23.50 | 6.00 | — | 0.18 | — | — | — | — | — | |
| MAX | 22.00 | 25.50 | 7.00 | 0.03 | 0.25 | 2.00 | 1.00 | 0.04 | 0.03 | 0.75 | Balance |
Specifications
ASME B-31.1 Case 155-1, ASME SA 182, ASME SA 240, ASME SA 249, ASME SA 312, ASME SA 479, ASME SB 276, ASME SB 366, ASME SB 462, ASME SB 564
ASME SB 675, ASME SB 688
ASME SB 691, ASME SB-366 Code Case N-438-3
ASTM A 182, ASTM A 240, ASTM A 249, ASTM A 312, ASTM A 479, ASTM B 366
ASTM B 462, ASTM B 472
ASTM B 564, ASTM B 675, ASTM B 676, ASTM B 688
ASTM B 691
ASTM B 804
UNS N08367
Inventory
AL-6XN® Coil, AL-6XN® Concentric Reducers, AL-6XN® Fittings, AL-6XN® Forged Billet, AL-6XN® Pipe, AL-6XN® Plate, AL-6XN® Round Bar, AL-6XN® Sheet, AL-6XN® Stub Ends, AL-6XN® Tees, AL-6XN® Weld Wire
Common Trade Names
Features
- Excellent resistance to pitting and crevice corrosion in chloride solutions
- Practical immunity to stress corrosion cracking in NaCl environments
- High strength and toughness
Applications
- FGD Scrubbers
- Reverse Osmosis
- Distillation Columns
- Offshore Oil and Gas
- Heat Exchangers
Physical Properties
Density: 0.291 lb/in 3
Melting Point: 2410-2540°F
Electrical Resistivity: 535 Ohm-circ mil/ft
| Temperature, °F | 70 | 200 | 300 | 400 | 600 | 700 | 800 |
| Coefficient* of Thermal Expansion, in/in°F x 10 -6 | – | 7.9 | 8.3 | 8.4 | 8.6 | 8.7 | 8.8 |
| Thermal Conductivity Btu • ft/ft2 • hr • °F |
6.7 | 7.5 | 8.1 | 8.7 | 10.0 | 10.6 | 11.2 |
| Modulus of Elasticity, Dynamic psi x 10 6 | 28.3 | 27.4 | – | 26.1 | 24.8 | – | 23.4 |
* 70°F to indicated temperature.
Mechanical Properties
Minimum Specified Properties, ASME SB-688 Plate
| Ultimate Tensile Strength, ksi | 95 |
| 0.2% Yield Strength, ksi | 45 |
| Elongation | 30 |
| Hardness MAX, HRC | 30.5 |
Typical Tensile Properties, Plate
| Temperature, °F | -450 | -320 | 70 | 200 | 400 | 600 | 800 | 1000 |
| Ultimate Tensile Strength, ksi | 218.0 | 196.0 | 108.0 | 99.9 | 90.3 | 86.0 | 87.0 | 83.6 |
| 0.2% Yield Strength, ksi | 142.0 | 107.0 | 53.0 | 49.4 | 40.4 | 36.3 | 36.0 | 34.0 |
| Elongation, % | 36 | 49 | 47 | 47 | 46 | 47 | 48 | 50 |
| Charpy Impact V-notch, ft-lbs | 322* | 85 | 140 | — | — | — | — | — |
*KIC Fracture Toughness
ASME Maximum Allowable Stresses, Section VIII, Division 1, ksi
| 200°F | 400°F | 600°F | 800°F | |
| AL-6XN Alloy | 27.1 | 24.6 | 23.3 | 22.6 |
| 316L | 20.0 | 19.3 | 17.0 | 15.9 |
| Alloy 400 | 18.7 | 18.7 | 18.7 | 15.0 |
Fabrication
The low carbon and high nitrogen contents in AL-6XN alloy (UNS N08367) minimize the precipitation of carbides and secondary phases that can occur during welding. Therefore as-welded assemblies can be placed in service, provided a suitable over-matched filler metal, such as alloy 625 is used. AL6XN alloy can be welded in the field using procedures similar to those used with other austenitic stainless steels.
Corrosion Resistance
AL-6XN has excellent corrosion resistance in high chloride environments as well as many acids. Below is a quick summary of the critical pitting and crevice corrosion resistance compared to many other alloys. AL-6XN has improved properties over stainless steels and all duplex grades. The practical immunity to chloride stress corrosion cracking has pushed AL-6XN (UNS N08367) into many seawater applications including seawater reverse osmosis. The resistance to acid and chloride environments is proven by over 20 years of service in the flue gas desulfurization process at coal fired electric power plants.
| Alloy | % Mo | CCCT, °F | CPT, °F | PREN |
|---|---|---|---|---|
| 316L | 2.1 | < 28 | 68 | 24 |
| 2205 | 3.1 | 68 | 120 | 35 |
| 904L | 4.4 | 75 | 130 | 36 |
| ZERON 100 | 3.5 | 108 | 180 | 41 |
| AL-6XN | 6.3 | 110 | 172 | 44 |
| 625 | 9.0 | 113 | – | 51 |
* CCCT – 10% FeCl3 6H20, per ASTM G 48 Practice B, CPT – 1 M NaCl, per ASTM G 150, PREN = Cr + 3.3 Mo + 16N
