904L stainless steel is a high-alloy austenitic stainless steel known for its exceptional corrosion resistance, particularly in aggressive environments containing acids, chlorides, and other corrosive media. Below is a detailed overview:
Basic Information
Alloy Designations:
UNS: N08904
EN: 1.4539
Common Names: 904L, “super austenitic stainless steel” (due to its high nickel and molybdenum content).
Microstructure: Fully austenitic (no ferrite), which provides good toughness even at low temperatures.
Chemical Composition
The key alloying elements (typical ranges) are:
Chromium (Cr): 19.0–23.0% (enhances general corrosion resistance)
Nickel (Ni): 23.0–28.0% (stabilizes the austenitic structure and improves toughness)
Molybdenum (Mo): 4.0–5.0% (boosts resistance to pitting and crevice corrosion)
Copper (Cu): 1.0–2.0% (improves resistance to sulfuric acid and other reducing acids)
Carbon (C): ≤0.02% (minimizes carbide precipitation, reducing intergranular corrosion risk)
Other elements: Manganese (≤2.0%), Silicon (≤1.0%), Phosphorus (≤0.045%), Sulfur (≤0.035%).
Physical Properties
Density: 8.0 g/cm³
Melting Point: 1300–1390°C
Thermal Conductivity: 11.7 W/(m·K) (at 20°C)
Specific Heat Capacity: 420 J/(kg·K) (at 20°C)
Electrical Resistivity: 0.85 μΩ·m (at 20°C)
Coefficient of Thermal Expansion: 15.9 × 10⁻⁶/°C (20–100°C)
Mechanical Properties
Tensile Strength: ≥520 MPa
Yield Strength (0.2% offset): ≥205 MPa
Elongation: ≥35% (in 50 mm)
Hardness: ≤235 HB (Brinell) or ≤90 HRB (Rockwell)
Toughness: Excellent, even at cryogenic temperatures (no ductile-brittle transition).
Corrosion Resistance
904L is renowned for its superior corrosion resistance, attributed to its high alloy content and low carbon:
Pitting Resistance Equivalent Number (PREN): ~34 (calculated as PREN = %Cr + 3.3×%Mo + 16×%N), indicating strong resistance to chloride-induced pitting.
Acid Resistance: Performs well in sulfuric acid, phosphoric acid, acetic acid, and formic acid (especially in dilute to moderate concentrations).
Chloride Resistance: Resists pitting and crevice corrosion in chloride-rich environments (e.g., seawater, brines) better than 316L or 304.
Intergranular Corrosion: Low carbon content and absence of carbide precipitation (even after welding) minimize intergranular corrosion risk.
Processing and Weldability
Weldability: Generally good, compatible with TIG, MIG, and submerged arc welding. No post-weld heat treatment is required due to low carbon, but clean welding conditions are critical to avoid contamination.
Formability: Can be cold-worked (e.g., bending, rolling) but has higher strength than standard austenitic steels, requiring more force.
Machinability: Moderate; tends to work-harden, so sharp tools and proper cooling are recommended.
Applications
904L is used in industries requiring exceptional corrosion resistance:
Chemical Processing: Reactors, storage tanks, and pipelines handling acids (e.g., sulfuric acid production).
Oil and Gas: Equipment for offshore drilling, sour gas environments, and desalination plants.
Pharmaceuticals: Vessels and piping for high-purity chemical processing.
Food Processing: Equipment for acidic food products (e.g., pickling, citrus processing).
Marine Engineering: Components exposed to seawater (pumps, valves, heat exchangers).
Key Advantages
Superior corrosion resistance in aggressive media compared to standard austenitic stainless steels.
Excellent toughness across a wide temperature range.
No need for post-weld heat treatment, simplifying fabrication.
Its main limitation is higher cost due to high nickel and molybdenum content, making it suitable for critical, high-corrosion applications where performance justifies the expense.