Stainless Steel 347

Stainless Steel 347 is a niobium-stabilized austenitic alloy (Nb: 10×C–1.00%). It resists intergranular corrosion better than 321 under thermal cycling, ideal for welded or fluctuating high-heat equipment.

Chemical Composition (Key, % ASTM A240)

C: ≤0.08; Cr: 17.0–19.0; Ni: 9.0–13.0

Nb+Ta: 10×C–1.00 (min 0.70)

Mechanical Properties (Annealed)

Tensile Strength: ≥515 MPa; Yield Strength: ≥205 MPa

Elongation: ≥40%; Hardness: ≤217 HB

Performance & Uses

Advantages: 900°C service temp, stable under thermal cycling, low weld cracking risk.

Applications: Aircraft engines, nuclear pressure vessels, welded reactors.

Equivalent Grades

EN 1.4550, JIS SUS347, DIN X6CrNiNb18-10

Q&As

How does Nb stabilize it?Nb (10×C) forms more stable carbides than Ti (dissolves at 1200°C vs. 1000°C). Even after repeated heating/cooling, Nb locks carbon-no Cr carbides, so it resists corrosion better than 321 in cycles.

Compare to 304L?304L is low-C (≤0.03%) but no stabilizer-prone to weld decay above 425°C. 347 resists intergranular corrosion, handles 900°C, and is better for welded/high-heat use (e.g., reactors vs. 304L sinks).

Nuclear applications?Yes. Nb prevents corrosion in 850°C reactor coolants. Austenitic structure resists radiation embrittlement. Welds stay stable-critical for pressure vessels (avoids radioactive leaks).

Thermal cycling vs. 321?321 sensitizes after hundreds of cycles (Ti carbides dissolve). 347 stays stable for thousands (Nb carbides hold). Good for aircraft exhausts (heats/cools daily) where 321 would degrade.

Best welding processes?TIG for thin parts (clean welds, low heat). MIG/SAW for thick parts (fast deposition). Use ER347 filler (matches Nb content). No post-weld treatment-welds resist corrosion.