904L stainless steel is a high-alloy austenitic stainless steel known for its excellent corrosion resistance, especially in harsh environments such as chemical processing, marine, and offshore applications. The "904L" designation refers to the alloy's composition and its resistance to corrosion, with "L" indicating low carbon content.

There aren't many "types" of 904L, but it can be found in different forms, grades, and finishes, depending on the specific requirements. Here's a breakdown of its variations:

1. Standard 904L Stainless Steel (UNS N08904)

• Composition: Primarily made of iron, nickel (23–28%), chromium (19–23%), and molybdenum (4–5%). It also contains small amounts of copper, manganese, silicon, and nitrogen.

• Key Properties:

  • Excellent resistance to corrosion, especially against pitting, crevice corrosion, and stress corrosion cracking.

  • Good resistance to sulfuric acid and other aggressive chemicals.

  • High resistance to chloride-induced corrosion.

  • Excellent formability and weldability.

• Applications: Chemical processing, pollution control equipment, heat exchangers, offshore oil drilling platforms.

2. 904L Stainless Steel in Different Forms:

904L can be produced and used in various forms, and it can be made to meet specific industrial standards. Some common forms include:

• Sheets/Plates: Often used for applications requiring strength and corrosion resistance.

• Pipes/Tubing: Common in industries like petrochemical, power generation, and desalination.

• Bars/Wire: Used for structural applications and in manufacturing processes.

• Fittings & Flanges: Important in piping systems where high corrosion resistance is required.

3. 904L Stainless Steel in Different Finishes:

The surface finish can affect the material’s appearance, corrosion resistance, and suitability for specific applications. Common finishes include:

• Mill Finish (2B): Standard finish, smooth but with visible lines from rolling.

• Bright Annealed (BA): Polished and shiny finish, often used for aesthetic purposes or for more stringent cleanability requirements.

• Pickled and Passivated: Used for further corrosion protection.

• Cold Rolled: Often used in applications that require a smooth, uniform finish.

4. 904L Stainless Steel in Different Grades:

While 904L itself is quite specialized, there can be slight variations in the composition depending on the specific manufacturer or the intended application. Some grades may vary slightly in carbon content, molybdenum percentage, or other alloying elements to meet specific standards like:

• ASTM A312: Standard for seamless and welded pipe, used in high-temperature, high-pressure applications.

• ASTM A240: Standard for stainless steel sheets and plates for general corrosion-resistant applications.

• ASME SA312: Standard used for pressure piping and industrial applications.

5. 904L Super Duplex Stainless Steel (904L variant)

• A variant of 904L, which has been modified to have improved resistance to pitting and stress corrosion cracking in very harsh environments. It’s a blend of austenitic and ferritic structures.

6. 904L vs. Other Stainless Steels:

• 904L vs. 316L: While both are corrosion-resistant, 904L has a higher content of molybdenum and copper, making it more resistant to aggressive chemicals and environments, such as sulfuric acid.

• 904L vs. 317L: 904L offers superior resistance to chloride-induced corrosion compared to 317L.

The 904L stainless steel has specific chemical and mechanical properties that make it suitable for highly corrosive environments. It is primarily known for its high resistance to corrosion, especially in acidic and chloride-rich environments.

Here’s a detailed breakdown of the specification of 904L stainless steel:

1. Chemical Composition of 904L Stainless Steel (UNS N08904)

The composition of 904L SS (as per ASTM A240 and other standards) is as follows:

• Copper gives 904L excellent resistance to sulfuric acid and other aggressive chemicals.

• The low carbon content (0.02% max) helps prevent carbide precipitation during welding, thereby enhancing its resistance to intergranular corrosion.

2. Mechanical Properties

The mechanical properties of 904L are designed for structural and high-strength applications. These are typical values for 904L stainless steel:

These properties are typical at room temperature but can vary depending on the exact processing or finish.

3. Physical Properties

Here are the physical properties that are important for high-performance applications:

4. Corrosion Resistance

One of the main advantages of 904L is its superior corrosion resistance, especially in:

• Chloride-induced stress corrosion cracking (SCC): The alloy has excellent resistance to cracking in chloride environments (compared to alloys like 304 or 316).

• Sulfuric Acid: 904L has outstanding resistance to sulfuric acid at higher concentrations, making it suitable for chemical processing and industrial environments.

• Pitting and Crevice Corrosion: The high molybdenum and copper content provide strong resistance to pitting and crevice corrosion in seawater and acidic environments.

• Acidic and Alkaline Environments: It offers a good balance of resistance to both acidic (including sulfuric, phosphoric, and acetic acids) and alkaline environments.

5. Heat Treatment

• Solution Annealing: 904L can be solution-annealed at temperatures between 1050°C–1150°C (1922°F–2102°F), followed by rapid cooling (water quenching) to preserve its corrosion resistance and structural properties.

• Post-Weld Heat Treatment (PWHT): Generally, 904L does not require post-weld heat treatment due to its low carbon content, but it may be performed depending on the application to relieve stresses or reduce hardness.

6. Welding Characteristics

• Weldability: 904L is highly weldable using common welding techniques, including TIG, MIG, and stick welding. The low carbon content minimizes carbide precipitation and ensures resistance to intergranular corrosion.

• Filler Material: For welding 904L stainless steel, it’s common to use a matching filler metal like ER904L or welding rods made from 904L.

7. Applications of 904L Stainless Steel

Due to its combination of excellent corrosion resistance, strength, and formability, 904L is used in various critical industries such as:

• Chemical Process Industries: Reactors, heat exchangers, and piping systems exposed to aggressive chemicals.

• Marine & Offshore: Components exposed to seawater and chloride-rich environments (e.g., piping, tanks, pumps).

• Pharmaceutical & Food Processing: Equipment that requires high hygiene standards and resistance to aggressive cleaning agents.

• Petrochemical: Equipment used in petroleum refining and chemical manufacturing.

8. Standards and Specifications

904L stainless steel meets or exceeds various industry standards, including:

• ASTM A240: Standard specification for chromium and chromium-nickel stainless steel plate, sheet, and strip for pressure vessels and for general applications.

• ASTM A312: Standard specification for seamless, welded, and heavily cold-worked austenitic stainless steel pipes.

• ASME SA240: For pressure vessels.

• DIN 1.4539: European standard for the alloy.