347 Stainless Steel: Niobium-Stabilized High-Temperature Weldable Grade

347 stainless steel is a niobium (Nb)-stabilized austenitic stainless steel. It uses niobium (10×C% - 1.0%) to replace titanium in 321 for stabilization. Niobium combines with carbon to form NbC, which also prevents intergranular corrosion. Compared with 321, it has better high-temperature strength and welding performance, and is more suitable for components requiring frequent welding.

Core Parameters

Chemical Composition (wt%): C≤0.08, Si≤1.00, Mn≤2.00, P≤0.045, S≤0.030, 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%, Brinell Hardness ≤201HB

Service Temperature: 400℃ to 870℃ (continuous service), up to 925℃ for short-term service

Equivalent Grades: SUS347 (JIS), EN 1.4550 (EN), UNS S34700 (ASTM)

Performance Advantages: Niobium stabilization has stronger high-temperature stability than titanium, with higher creep strength than 321 above 600℃; niobium is not easy to burn out during welding, and the weld's corrosion resistance and strength are more stable; excellent corrosion resistance to oxidizing acids (nitric acid, acetic acid); no post-weld heat treatment is required, which simplifies the manufacturing process.

Typical Applications: High-temperature furnace tubes and heat exchangers, expansion joints of high-temperature pipelines, nuclear power plant internals (with good radiation swelling resistance), aircraft engine exhaust pipes, and nitric acid storage tanks.

Practical Q&A

Q1: What are the differences between 347 and 321 stainless steel? A1: 347 uses niobium for stabilization, while 321 uses titanium. 347 has better welding performance (niobium is not easy to burn out) and higher high-temperature strength; 321 has lower cost. Choose 347 for components requiring frequent welding and long-term high-temperature service, and 321 for general high-temperature anti-intergranular corrosion scenarios.

Q2: What should be noted in the heat processing of 347? A2: The hot working temperature should be controlled at 1150-1260℃, and prolonged stay in the 700-900℃ range should be avoided to prevent sigma phase precipitation and embrittlement; intermediate annealing (1050-1120℃, rapid cooling) is required during cold working to restore toughness.

Q3: Is 347 suitable for nuclear power plant components? A3: Yes. It has excellent radiation swelling resistance and intergranular corrosion resistance, and can be used as internals of nuclear power plant reactors and steam generators. It meets the nuclear grade material standards (such as ASME SA-213), and its mechanical properties and corrosion resistance are stable after irradiation.

Q4: What welding materials are suitable for 347? A4: ER347 welding wire or E347-16 electrode should be selected, which contains the same niobium content as the base metal, ensuring that the weld has the same stabilization effect and high-temperature performance as the base metal. Avoid using welding materials without niobium, which will lead to intergranular corrosion of the weld.

Q5: How to distinguish 347 and 321 stainless steel through simple methods? A5: The chemical composition analysis is the most accurate-347 contains 0.80-1.20% niobium, while 321 contains 0.10-0.30% titanium. In terms of welding performance, 347 has less weld porosity and better weld forming than 321 under the same welding process, and niobium is not easy to burn out.