SUS347 Stainless Steel: Niobium-Stabilized Alloy for High-Temperature Service

What Is SUS347

SUS347, equivalent to ASTM 347 and UNS S34700, is a niobium-stabilized austenitic stainless steel developed to solve the intergranular corrosion problem of unstabilized 18-8 grades. During welding or service in the sensitization temperature range, chromium carbides can precipitate at grain boundaries and deplete the adjacent chromium, leaving the material vulnerable to intergranular attack. Niobium ties up the carbon as stable niobium carbides, keeping chromium in solution and preserving corrosion resistance.

Chemical Composition

Limits are carbon 0.08 percent maximum, chromium 17.0 to 19.0 percent, nickel 9.0 to 13.0 percent, niobium plus tantalum at least 10 times the carbon content up to 1.0 percent maximum, manganese 2.0 percent, silicon 1.0 percent, phosphorus 0.045 percent, and sulfur 0.030 percent. The EN equivalent 1.4550 requires niobium at least 8 times the carbon content.

Mechanical Properties

Minimum values are tensile strength 515 MPa, yield strength 205 MPa, and elongation 40 percent, with maximum hardness 201 HB. Physical properties include density 7.93 g per cubic centimeter, elastic modulus 193 GPa, and thermal conductivity 16.3 W per meter per Kelvin at 20 degrees C.

Stabilization and Elevated-Temperature Behavior

Niobium stabilization is generally less affected by welding heat input than titanium stabilization, since titanium can be lost during welding, making 347 a common choice for heavy-section welded construction where post-weld heat treatment is impractical. The grade also provides excellent resistance to oxidation and scaling at elevated temperatures, comparable to 321 and superior to 304 under cyclic heating and cooling. For continuous service above about 600 degrees C where creep strength matters, the high-carbon variant 347H, with carbon 0.04 to 0.10 percent, is recommended.

Applications

SUS347 is used for boiler superheater and reheater tubes and headers, catalytic reforming reactors and furnace tubes in petroleum and petrochemical plants, shell-and-tube heat exchangers with many welded joints, high-temperature reactor internals, and industrial exhaust and aerospace-related components.

Frequently Asked Questions

Q: What is the difference between SUS347 and SUS321?

A: 347 uses niobium stabilization, 321 uses titanium; niobium resists burn-off during welding.

Q: What is 347H?

A: The high-carbon version with 0.04 to 0.10 percent carbon for better creep strength.

Q: Does SUS347 require post-weld heat treatment?

A: Generally no, thanks to niobium stabilization.

Q: What is the tensile strength of SUS347?

A: 515 MPa minimum.

Q: What applications use SUS347?

A: Boiler superheaters, petrochemical furnace tubes, and high-temperature heat exchangers.

Q: What are the equivalents of SUS347?

A: ASTM 347, UNS S34700, EN 1.4550, and GB 06Cr18Ni11Nb.

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