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Introduction

UNS N08904, commonly referred to by its trade name 904L, is a high-performance stainless steel alloy known for its superior resistance to various corrosive environments. Unlike typical stainless steels, 904L contains higher levels of nickel, chromium, and molybdenum, which give it exceptional durability and resistance to corrosion. This makes it suitable for applications in industries such as oil and gas, chemical processing, marine environments, power generation, and pharmaceuticals.

904L is considered an austenitic stainless steel, which means it has a non-magnetic structure and excellent formability. Its high resistance to stress corrosion cracking, pitting, and crevice corrosion under extreme conditions makes it a versatile and reliable material choice for various demanding applications.

UNS N08904 (904L) Product Forms and Specifications

Equivalent Grades

• Common Name: 904L
• UNS No.: N08904
• DIN: 1.4539
• EN: 1.4539
• ISO: 904L
• JIS: No direct equivalent
• W.Nr.: 1.4539

Related Product Standards

• ISO: ISO 3506
• JIS: JIS G 4305
• EN: EN 10088-2
• ASTM: ASTM A240, ASTM B625
• ASME: ASME SA240, ASME SA312

Available Product Forms

UNS N08904 (904L) is available in multiple forms to cater to various industries and manufacturing processes:

• Pipe and Tube: Suitable for high-pressure environments and chemical processing applications.
• Plate and Sheet: Used for structural applications requiring corrosion resistance.
• Bar and Rod: Machined into specific components for mechanical and structural purposes.
• Wire and Strip: Often used for specialized applications such as spring manufacturing or as reinforcing materials.
• Forging Stock: Ideal for custom shapes and complex geometries in demanding applications.

Applications

The superior properties of UNS N08904 (904L) make it an ideal material for several challenging applications:

• Chemical Processing: Used in equipment such as reactors, heat exchangers, and tanks where resistance to aggressive chemicals like sulfuric acid, phosphoric acid, and acetic acid is required.
• Oil and Gas Industry: Employed in pipelines, offshore platforms, and other equipment exposed to high levels of chloride and other corrosive substances.
• Marine and Offshore: Excellent resistance to seawater and chloride stress corrosion makes it a go-to choice for marine piping systems, desalination plants, and offshore platforms.
• Power Generation: Used in heat exchangers, cooling systems, and condensers where both corrosion and temperature resistance are crucial.
• Pharmaceuticals and Food Processing: Because of its high purity and resistance to contamination, UNS N08904 (904L) is also used in pharmaceutical equipment, tanks, and pipelines in the food and beverage industries.

Corrosion Resistance Properties of UNS N08904 (904L)

One of the key advantages of UNS N08904 (904L) is its outstanding corrosion resistance. It offers the following corrosion-related benefits:

• Pitting and Crevice Corrosion Resistance: 904L is highly resistant to pitting and crevice corrosion in chloride environments, including seawater and coastal areas.
• Stress Corrosion Cracking (SCC): Unlike many other stainless steels, 904L excels in environments with chloride ions, preventing stress corrosion cracking in highly corrosive media.
• Oxidation Resistance: Its high content of chromium and molybdenum provides excellent oxidation resistance at elevated temperatures, making it suitable for high-temperature applications where other stainless steels would degrade.

Physical and Thermal Properties of UNS N08904 (904L)

Understanding the physical and thermal properties of UNS N08904 (904L) is essential for determining its performance in various conditions:

• Density: 8.0 g/cm³
• Melting Point: 1375–1400°C
• Thermal Conductivity: 14 W/m·K
• Specific Heat: 500 J/kg·K at 25°C
• Modulus of Elasticity: 200 GPa

These properties show that 904L can perform well under both thermal and mechanical stresses in harsh environments.

UNS N08904 (904L) Chemical Composition

The chemical composition of UNS N08904 (904L) is crucial in giving it its unique corrosion resistance and mechanical properties. Typical composition includes:

• Nickel (Ni): 23–28%
• Chromium (Cr): 19–23%
• Molybdenum (Mo): 4–5%
• Iron (Fe): Balance
• Copper (Cu): 1–2%
• Carbon (C): Max 0.020%
• Manganese (Mn): 2–2.5%
• Silicon (Si): 1.0%
• Sulfur (S): Max 0.005%

This alloy composition provides excellent resistance to various corrosive environments, including sulfuric acid, phosphoric acid, and chlorides.

UNS N08904 (904L) Mechanical Properties

The mechanical properties of UNS N08904 (904L) make it suitable for demanding applications where both strength and corrosion resistance are necessary:

• Tensile Strength: 530 MPa (min)
• Yield Strength: 240 MPa (min)
• Elongation: 40% (min)
• Hardness: 220 HB (max)

These properties provide the necessary balance of strength and ductility, allowing the alloy to be used in both structural and pressure-containing applications.

Heat Treatment

UNS N08904 (904L) is typically used in an annealed condition, which ensures good formability and corrosion resistance. Annealing is performed by heating the material to a temperature range of 1050–1150°C and then cooling it rapidly. This treatment helps relieve internal stresses and improves the alloy’s ductility and corrosion resistance.

Forming

The alloy is easily formed using conventional techniques such as hot rolling, cold rolling, and forging. When hot working, 904L should be heated to temperatures between 1100°C and 1200°C, and rapid cooling should be applied afterward to ensure optimal performance.

Welding

Welding of UNS N08904 (904L) can be carried out using typical methods such as TIG, MIG, and SMAW. It is essential to use a compatible filler material to ensure that the welds retain the same level of corrosion resistance as the base material. Preheating is not required, but post-weld heat treatment may be recommended for thick sections to relieve stresses and enhance weld integrity.

Corrosion of Welds

While 904L exhibits excellent corrosion resistance, welding can sometimes lead to localized corrosion, such as weld decay. To mitigate this risk, proper post-weld treatment is essential. Descaling, pickling, or passivating the weld area can prevent the formation of chromium carbides and restore the material’s resistance to corrosion.

Descaling, Pickling, and Cleaning

To enhance the surface quality and corrosion resistance, UNS N08904 (904L) should undergo descaling or pickling processes. This removes oxide layers formed during manufacturing processes. Pickling typically involves using a mixture of nitric acid and hydrofluoric acid, followed by thorough rinsing and passivation to prevent future corrosion.

Surface Hardening

For specific applications requiring enhanced surface properties, surface hardening techniques such as nitriding or shot peening can be applied. These processes improve wear resistance without compromising the alloy’s corrosion resistance, making it suitable for parts exposed to high mechanical stress.

Conclusion

UNS N08904 (904L) is an ideal material for highly corrosive environments, offering exceptional resistance to pitting, crevice corrosion, and stress corrosion cracking. Its high strength, combined with good formability, makes it suitable for a wide range of applications in industries such as chemical processing, marine, oil and gas, and pharmaceuticals.

By understanding the properties, fabrication processes, and maintenance practices associated with UNS N08904 (904L), engineers and professionals can confidently use this material in demanding environments, ensuring long-term performance and reliability in the most challenging conditions.