Key Feature

The Columbium-Stabilized High-Temperature Austenitic Stainless Steel Pipe. Niobium addition prevents sensitization in welded structures and provides improved high-temperature strength compared to 304/304L.

The preferred choice for welded piping systems exposed to corrosive environments in the 425-900°C range.

Common Designations

UNS S34700
AISI 347
ASTM A312 TP347 (Seamless/Welded)
ASTM A358 TP347 (EFW)
EN 1.4550, X6CrNiNb18-10
JIS SUS347
BS 347S17

Columbium (Cb) is also known as niobium (Nb); both designations refer to the same element.

Chemical Composition (Typical wt.%)

C: ≤ 0.08
Cr: 17.0 - 19.0
Ni: 9.0 - 12.0
Cb (Nb): ≥ 10×C (min 0.80%)Mn: ≤ 2.00
Si: ≤ 0.75
P: ≤ 0.045
S: ≤ 0.030

Columbium content is critical: minimum 10 times carbon content, typically 0.8-1.1%.

Mechanical Properties

Tensile Strength: ≥ 515 MPa
Yield Strength: ≥ 205 MPa
Elongation: ≥ 40%
Hardness: ≤ 201 HB

Similar to 304 in annealed condition, but with better high-temperature properties.

Elevated Temperature Properties

Continuous Service: Up to 900°C
Intermittent Service: Up to 870°C
Creep Strength: Better than 304
Stabilized up to: 900°C

Superior to 304/304L for long-term exposure in the 425-900°C range.

Physical Properties

Density: 7.90 g/cm³
Melting Range: 1399 - 1454 °C
Thermal Expansion: 17.3 × 10⁻⁶/K (20-100°C)
Thermal Conductivity: 16.3 W/m·K (100°C)
Non-magnetic when annealed

Essentially identical to 304 in physical properties.

Corrosion Resistance

Excellent general corrosion resistance similar to 304. Superior resistance to intergranular corrosion after exposure to sensitizing temperatures (425-900°C).

Specifically designed to prevent weld decay in corrosive high-temperature environments.

Welding Characteristics

Excellent weldability
Columbium stabilization prevents sensitization
No post-weld annealing required for most applications
Matching filler metals recommended

Superior weld performance compared to non-stabilized grades like 304.

Pressure-Temperature Ratings

At 600°C: ~60% of room temp strength
At 700°C: ~45% of room temp strength
At 800°C: ~30% of room temp strength
ASME B31.3 compliant

Good high-temperature strength retention for pressure applications.

Key Standards

ASTM A312 TP347: Seamless & Welded Pipe
ASTM A358 TP347: EFW Pipe
ASTM A213 TP347: Boiler Tubes
EN 10216-5: Seamless Pressure Tubes
ASME SA312: Code Construction
ISO 1127: Stainless Steel Tubes

Widely specified in pressure vessel and high-temperature codes.

Common Applications

• High-Temperature Process Piping: Chemical reactors, reformer piping, pyrolysis systems
• Power Generation: Superheater tubes, boiler components, high-pressure steam lines
• Aerospace: Jet engine exhaust systems, afterburner components
• Chemical Processing: Welded tanks, heat exchangers, corrosive service piping
• Oil & Gas: Refinery piping, heat exchangers, high-temperature lines
• Food Processing: High-temperature processing lines, sterilization systems
• Industrial Heating: Furnace piping, radiant tubes, heat treatment fixtures

Specifically selected where welding and high-temperature exposure combine with corrosive conditions.