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Introduction

UNS N08367 (AL-6XN) is super-austenitic stainless steel with superior corrosion resistance and strength in some of the most demanding industrial environments. Its exceptional ability to withstand chloride-induced pitting and crevice corrosion makes it widely used in industries such as chemical processing, pulp and paper, marine and offshore, air pollution control, power generation, food processing, and advanced fields like space biospheres and high-efficiency residential furnaces.

In this blog, we’ll explore UNS N08367’s key properties and advantages and provide users with detailed information on its specifications, applications, corrosion resistance, and more.

1. Overview of UNS N08367 (AL-6XN)

UNS N08367 (AL-6XN) is a nitrogen-bearing super-austenitic stainless steel designed for extreme environments. It offers superior resistance to pitting, crevice corrosion, and stress-corrosion cracking (SCC) in chloride-bearing environments, making it a cost-effective alternative to higher-cost nickel-based alloys. This material suits industries that handle aggressive chemicals and seawater or require long-term durability in harsh environments.

2. Available UNS N08367 (AL-6XN) Products and Specifications

• UNS Number: N08367
• Common Name: AL-6XN
• W.Nr.: 1.4529
• ASTM/ASME Standards: A240, A312, A249, A479, B688, B675, among others.

AL-6XN is available in various product forms, including:

• Pipe
• Tube
• Plate
• Sheet
• Strip
• Bar
• Rod
• Wire
• Forging Stock

These product forms make it versatile and suitable for various industrial applications.

3. Applications of UNS N08367 (AL-6XN)

Due to its outstanding corrosion resistance and mechanical strength, UNS N08367 (AL-6XN) is widely used in industries that demand reliable performance in harsh environments:

• Chemical Processing: Ideal for handling corrosive chemicals like sulfuric and hydrochloric acids.
• Pulp and Paper: Resistant to the harsh chemicals used in bleaching.
• Marine and Offshore: Excellent for seawater piping systems, desalination plants, and offshore platforms.
• Air Pollution Control: It is suitable for flue gas desulfurization (FGD) systems and scrubbers and offers resistance to acidic condensates.
• Power Generation: Commonly used in heat exchangers, condenser tubing, and service water systems.
• Space Biosphere: Utilized in advanced environmental systems like Biosphere 2, where long-term exposure to recycled air and water is critical.
• Food Processing: Certified for food contact applications, including brewing and dairy equipment.
• High-Efficiency Residential Furnaces: Used in condensing furnaces to resist acidic condensates.

4. Corrosion Resistance Properties

AL-6XN is engineered to provide top-tier resistance to localized corrosion, including:

• Pitting and Crevice Corrosion: Its high molybdenum content (6.2%) and nitrogen enhance resistance to these forms of corrosion in chloride environments.
• Stress-Corrosion Cracking (SCC): The high nickel content (24%) helps prevent SCC, which is common in chloride-laden environments.
• Resistance to Acidic Environments: AL-6XN can withstand sulfuric, phosphoric, and hydrochloric acid exposure.

5. Physical and Thermal Properties

AL-6XN maintains its physical stability even in extreme conditions:

• Density: 8.03 g/cm³
• Melting Range: 1300°C to 1350°C (2370°F to 2460°F)
• Thermal Conductivity: Lower than standard stainless steels, making it suitable for thermal management in specific applications.
• Thermal Expansion: 8.1 × 10⁻⁶/°F (14.6 × 10⁻⁶/°C) from 70°F to 1000°F.

6. Chemical Composition

The chemical composition of AL-6XN is carefully balanced to optimize its corrosion resistance and mechanical properties:
Nickel (Ni): 24.0%
Chromium (Cr): 20.5%
Molybdenum (Mo): 6.2%
Nitrogen (N): 0.22%
Iron (Fe): Balance
Others: Trace amounts of carbon, silicon, manganese, and copper.

7. Mechanical Properties

AL-6XN offers excellent mechanical strength:
Tensile Strength: 690 MPa (100 ksi)
Yield Strength: 310 MPa (45 ksi)
Elongation: 30% (in 2 inches or 4D)
Its high tensile strength and elongation make it ideal for durability and flexibility applications.

8. Heat Treatment

AL-6XN undergoes a solution annealing heat treatment to achieve optimal mechanical and corrosion resistance properties:
Solution Annealing Temperature: 2050°F (1120°C), followed by rapid cooling.
This heat treatment helps maintain its fully austenitic structure. It ensures that the material remains resistant to detrimental phases such as the sigma phase, which can weaken both corrosion resistance and strength.

9. Forming

AL-6XN demonstrates excellent formability and can be formed using both hot and cold working techniques:
Cold Forming: Its high flexibility makes it easily cold-formed into complex shapes without risk of cracking.
Hot Forming: It should be done between 1700°F and 2150°F (927°C to 1177°C) and followed by rapid quenching to preserve corrosion resistance.
Its formability is critical for complex manufacturing processes in demanding industries.

10. Welding of UNS N08367 (AL-6XN)

The weldability of AL-6XN is one of its strengths, making it highly suitable for complex fabrication:
Welding Processes: TIG (GTAW), MIG (GMAW), SMAW, and SAW are commonly used.
Filler Metals: Over-matching filler materials is recommended to ensure the welded joint maintains corrosion resistance equivalent to the base metal.
The low carbon content reduces carbide precipitation, a common cause of intergranular corrosion in welds.

11. Corrosion of Welds

If not correctly treated, welds can be vulnerable to corrosion. AL-6XN minimizes this risk with low carbon and high nitrogen content, reducing carbide precipitation and leading to intergranular corrosion. Proper heat input control and post-weld cleaning are essential to maintaining corrosion resistance.

12. Descaling, Pickling, and Cleaning

Post-weld cleaning is critical to preserve the corrosion resistance of AL-6XN. Descaling and pickling are commonly performed using nitric-hydrofluoric acid solutions, followed by thorough rinsing with water. These processes remove oxides and surface contaminants that could otherwise initiate localized corrosion.

13. Surface Hardening

Surface hardening is generally not recommended for AL-6XN because it could compromise the material’s corrosion resistance. The alloy’s natural strength and toughness are sufficient for most applications, eliminating the need for additional hardening treatments.

Conclusion

UNS N08367 (AL-6XN) is a versatile, high-performance alloy with unparalleled corrosion resistance, strength, and formability for demanding industrial applications. Its ability to withstand harsh environments and cost-efficiency makes it a reliable choice across chemical processing, marine, pulp and paper industries, and power generation. Whether dealing with corrosive chemicals, seawater, or high temperatures, AL-6XN delivers superior performance, long service life, and reduced maintenance costs.

If you have questions or need more information on how AL-6XN can benefit your application, please get in touch with us at [email protected] for guidance and support.