Comparison of 347 and 347H Stainless Steel: Standard vs High-Carbon Niobium-Stabilized Grade

347 and 347H are niobium-stabilized austenitic stainless steels, with the core difference being carbon content. 347H improves high-temperature creep strength through high carbon content, while 347 has better toughness and corrosion resistance. Both use niobium for stabilization, with better welding performance than titanium-stabilized grades, suitable for high-temperature welding-intensive components.

Core Parameter Comparison

Parameter	347 Stainless Steel	347H Stainless Steel
Chemical Composition (wt%)	C≤0.08, Cr=17.00-19.00, Ni=9.00-13.00, Nb=0.80-1.20, Fe=Balance	C=0.04-0.10, Cr=17.00-19.00, Ni=9.00-13.00, Nb=0.80-1.20, Fe=Balance
Mechanical Properties (Annealed)	Tensile Strength ≥520MPa, Yield Strength ≥205MPa, Elongation ≥40%, Hardness ≤201HB	Tensile Strength ≥520MPa, Yield Strength ≥205MPa, Elongation ≥40%, Hardness ≤201HB
Service Temperature	400℃ to 870℃ (continuous service)	500℃ to 870℃ (continuous service)
Equivalent Grades	SUS347 (JIS), EN 1.4550, UNS S34700	SUS347H (JIS), EN 1.4551, UNS S34709

Key Performance Differences: 1. High-temperature strength: 347H has higher creep strength than 347 at 500-870℃, creep rupture life 2 times that of 347 at 700℃. 2. Welding performance: Both have excellent performance, niobium not easy to burnout; 347 has better weld toughness than 347H. 3. Corrosion resistance: 347 has slightly better intergranular corrosion resistance than 347H due to lower carbon content.

Applicable Scenario Distinction: 347 is suitable for high-temperature welding-intensive components requiring good toughness, such as high-temperature furnace tubes, expansion joints, and aircraft engine exhaust pipes. 347H is suitable for high-temperature stress-bearing components in nuclear power and chemical industries, such as nuclear power plant reactor internals, high-temperature chemical reactor tubes, and thermal power plant superheater tubes.

Practical Q&A

Q1: What is the difference between niobium stabilization and titanium stabilization (321 series)? A1: Niobium is not easy to burnout during welding, weld performance more stable; niobium-stabilized grades have higher high-temperature strength than titanium-stabilized grades above 600℃; 347 series cost higher than 321 series.

Q2: Can 347H be used in nuclear power plants? A2: Yes. It has excellent radiation swelling resistance, meets nuclear grade standards, and is used as reactor internals and steam generator components; its performance is stable after irradiation.

Q3: What heat treatment precautions are there for 347 and 347H? A3: Hot working temperature 1150-1260℃, avoid long stay at 700-900℃ to prevent sigma phase embrittlement; cold working needs intermediate annealing.

Q4: How to distinguish 347 and 347H? A4: Chemical composition analysis (carbon content 0.04-0.10% for 347H, ≤0.08% for 347); 347H has higher hardness after high-temperature aging, 347 has better toughness.

Q5: What is the application limit of 347/347H? A5: Not suitable for environments with high chloride ions (e.g., seawater), chloride ion corrosion resistance worse than 316 series; for coastal high-temperature components, 316H is recommended.