S34709 Stainless Steel

S34709 is a high-carbon niobium-stabilized austenitic stainless steel (UNS S34709), corresponding to 347H. With C=0.04-0.10% and 8xC-1.0% Nb, it combines high-temperature creep resistance (carbon) and intergranular corrosion resistance (niobium), suitable for high-heat load-bearing welded components.

Chemical Composition (wt%): C=0.04-0.10, Si ≤1.00, Mn ≤2.00, P ≤0.045, S ≤0.030, Cr=17.00-19.00, Ni=9.00-12.00, Nb+Ta=8xC-1.0, Fe=Balance

Mechanical Properties (Annealed): Tensile Strength ≥515MPa, Yield Strength ≥205MPa, Elongation ≥40%, Hardness (HB) ≤217

Performance Advantages: Superior high-temp creep resistance (500-900°C); niobium stabilization prevents intergranular corrosion; better high-temp strength than S34700; weldable with matching filler.

Equivalent Grades: ASTM A240 347H, EN 1.4551, SUS347H

Applications: High-temperature boiler superheater tubes, gas turbine combustion chambers, nuclear power plant high-pressure parts, chemical reactor headers.

FAQ

1. Q: How does S34709 differ from S34700? A: S34709's higher carbon (0.04-0.10% vs S34700's ≤0.08%) enhances creep resistance. At 650°C and 100MPa, its creep rupture time is 10,000 hours vs 6,000 hours for S34700. This makes it suitable for load-bearing high-temp parts like boiler superheater tubes, where S34700 would deform under long-term stress.

2. Q: Why is S34709 used in gas turbine combustion chambers? A: Combustion chambers operate at 800-900°C with cyclic heat and pressure. S34709's niobium stabilization resists intergranular corrosion, and high carbon ensures creep resistance. Its thermal shock resistance handles rapid heating/cooling cycles, avoiding cracking. It outperforms S32109 here, lasting 15+ years vs 8-10 years in turbine service.

3. Q: What welding filler is required for S34709? A: Use ER347H filler to match carbon and niobium content. ER347L (low C) reduces creep resistance, and ER321H (titanium) lowers high-temp stability. Weld with TIG at 110-150A, controlling heat to avoid niobium segregation. Post-weld anneal at 1050-1100°C for thick plates to relieve stress, critical for pressure vessel headers.

4. Q: Does S34709 have sigma phase risk? A: Low risk below 900°C. High Ni (9-12%) stabilizes the austenitic structure. Above 900°C, hold time should be ≤1 hour to avoid sigma phase (brittle). For long-term service at 800-900°C, it's safer than ferritic steels. Regular metallographic inspection can detect sigma phase if present, allowing timely replacement.

5. Q: How to ensure S34709 meets high-temp specs? A: Verify niobium content (8xC-1.0%) via XRF-too low causes carbide precipitation. Conduct creep rupture testing at 650°C/100MPa (pass if ≥9,000 hours). Use the Huey test (boiling nitric acid) to check intergranular corrosion resistance-weight loss ≤1g/m²·h is acceptable. These tests ensure it's suitable for critical high-temp applications.