347H stainless steel is a high-carbon austenitic stainless steel stabilized with niobium (Nb), belonging to the 300 series stainless steels. It is designed to withstand high temperatures and maintain excellent corrosion resistance, making it widely used in high-temperature industrial environments. Below is a detailed introduction:
1. Chemical Composition
The key chemical components (wt%) of 347H are strictly regulated to ensure its performance:
Carbon (C): 0.04–0.10% (higher than standard 347 to enhance high-temperature strength)
Chromium (Cr): 17.00–19.00% (provides oxidation and corrosion resistance)
Nickel (Ni): 9.00–13.00% (stabilizes the austenitic structure)
Niobium (Nb) + Tantalum (Ta): ≥10×C (min. 8×C), typically 0.70–1.00% (stabilizes against intergranular corrosion by binding with carbon, preventing chromium carbide precipitation)
Manganese (Mn): ≤2.00%
Silicon (Si): ≤1.00%
Phosphorus (P): ≤0.045%
Sulfur (S): ≤0.030%
2. Physical Properties
Density: ~7.93 g/cm³
Melting Point: 1398–1454°C
Thermal Conductivity: ~16.2 W/(m·K) at 100°C (increases slightly with temperature)
Coefficient of Thermal Expansion: ~17.3 × 10⁻⁶/°C (20–100°C)
Electrical Resistivity: ~72 μΩ·cm at 20°C
Magnetism: Non-magnetic in the annealed state (remains non-magnetic even after cold working).
3. Mechanical Properties (Annealed State)
347H exhibits good strength at both room and high temperatures:
Tensile Strength: ≥515 MPa
Yield Strength (0.2% offset): ≥205 MPa
Elongation: ≥35%
Hardness: ≤95 HRB (Rockwell B) or ≤201 HB (Brinell).
Notably, its high-temperature strength is superior to standard 347 (low-carbon version) due to the higher carbon content, making it suitable for long-term service at 600–800°C.
4. Corrosion Resistance
General Corrosion: Excellent resistance to atmospheric corrosion, water, and mild organic/inorganic acids, similar to 304 stainless steel, thanks to its high chromium content.
Intergranular Corrosion: The addition of niobium stabilizes the steel by forming niobium carbides (NbC) instead of chromium carbides (Cr₂₃C₆). This prevents chromium depletion at grain boundaries during welding or high-temperature exposure, avoiding intergranular corrosion—a critical advantage over unstabilized grades like 304.
High-Temperature Oxidation: Resistant to oxidation in high-temperature air up to ~800°C, making it suitable for continuous use in high-temperature environments.
5. Heat Treatment
Solution Annealing: Typically performed at 1040–1150°C, followed by rapid cooling (water quenching). This process dissolves carbides, homogenizes the structure, and restores corrosion resistance.
Stabilization Heat Treatment: Sometimes applied at 850–900°C to promote niobium carbide formation, further enhancing intergranular corrosion resistance.
6. Weldability
347H has good weldability and can be welded using common methods (TIG, MIG, SMAW). Key points:
No pre-weld or post-weld heat treatment is generally required, but proper cleaning of surfaces (to remove oils, scales) is necessary.
Welding filler metals: Matching 347 or 347H filler metals are recommended to maintain corrosion resistance and high-temperature performance.
7. Applications
Due to its high-temperature strength, oxidation resistance, and resistance to intergranular corrosion, 347H is widely used in:
Power Generation: Boiler tubes, superheater tubes, and heat exchanger components in fossil fuel and nuclear power plants.
Chemical Processing: High-temperature reactors, pipelines, and valves handling corrosive media at elevated temperatures.
Aerospace & Petrochemical: Components in jet engines, gas turbines, and high-temperature petrochemical processing equipment.
Key Differences from 347 Stainless Steel
The main distinction lies in carbon content:
347: Low carbon (C ≤ 0.08%), suitable for general corrosion resistance at moderate temperatures.
347H: Higher carbon (C 0.04–0.10%), optimized for enhanced high-temperature strength, making it ideal for long-term service in high-temperature environments.
In summary, 347H is a reliable material for high-temperature, corrosive applications, balancing strength, weldability, and corrosion resistance effectively.