Niobium-Stabilized 18/10 Austenitic Stainless for Welded High-Temp Service
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UNS NUMBER
S34700
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ALLOY FAMILY
Austenitic (Nb-stabilized)
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STRUCTURE
100% Austenite
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UTS (MIN)
≥515 MPa
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MAX TEMP
~870°C
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DENSITY
7.93 g/cm³
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Stainless Steel 347 (UNS S34700) is 304 stabilized with niobium (minimum 10× the carbon content). Niobium forms NbC, tying up carbon so the steel resists intergranular sensitization after welding and keeps high-temperature strength to about 870°C — the niobium-stabilized alternative to 321, preferred for the highest creep duty.
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Cr
17–19%
Oxidation resistance
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Ni
9–13%
Austenite stabilizer
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Mo
— (none)
Not added
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Nb
10×C min
Stabilizer, forms NbC
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UTS
≥515 MPa
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YS 0.2%
≥205 MPa
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Elong.
≥40%
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Service
up to ~870°C
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What is Stainless Steel 347?
347 is the niobium-stabilized counterpart of 304. Adding niobium to at least 10× the carbon content makes it preferentially form NbC, leaving chromium in solution and preventing sensitization during and after welding. Because NbC is more stable than TiC above ~1000°C, 347 also holds creep strength better than titanium-stabilized 321 at the top of its range.
347 is chosen for welded high-temperature systems — headers, thermowells, and refinery transfer lines — where 304L would lack high-temperature strength. The trade-off is that niobium-stabilized welds need controlled heat input and the grade costs a bit more than 321. For the hottest creep duty, 347H (higher carbon) is specified. For moderate, non-stabilized service, 304L is cheaper.
Chemical Composition (Weight %)
| Element |
Min (%) |
Max (%) |
Role in Alloy |
| Chromium (Cr) |
17.0 |
19.0 |
Oxidation resistance |
| Nickel (Ni) |
9.0 |
13.0 |
Austenite stabilizer |
| Niobium (Nb) |
10×C min |
1.0 max |
Stabilizer; forms NbC |
| Carbon (C) |
— |
0.08 |
At 304 level |
| Manganese (Mn) |
— |
2.00 |
Deoxidizer |
| Silicon (Si) |
— |
1.00 |
Deoxidizer |
| Phosphorus (P) |
— |
0.045 |
Impurity |
| Sulfur (S) |
— |
0.030 |
Impurity |
| Nitrogen (N) |
— |
0.10 |
Minor |
| Iron (Fe) |
Balance |
— |
Matrix former |
★ Niobium ≥ 10×C (max 1.0%) forms NbC that prevents chromium-carbide sensitization after welding. NbC is more stable than TiC above ~1000°C, giving 347 better very-high-temperature creep stability than 321.
Mechanical Properties
Values are for the solution-annealed condition per ASTM A240, A182 (F347), and A312 (TP347).
| Condition / Form |
UTS (MPa) |
YS 0.2% (MPa) |
Elong. % (min) |
Hardness (max) |
| Minimum (ASTM A240 / A182 F347) |
≥515 |
≥205 |
40 |
201 HBW (92 HRB) |
| Typical mill-annealed |
515–700 |
205–350 |
45–55 |
150–195 HBW |
Modulus of Elasticity 193 GPa. Niobium stabilization avoids sensitization and gives better high-temperature creep stability than 321. Match filler ER347.
Physical Properties
| Property |
Value |
Note |
| Density |
7.93 g/cm³ |
Same as 304 |
| Melting Range |
1400–1420°C |
Solidus ~1400°C |
| Thermal Conductivity (100°C) |
16.1 W/m·K |
Comparable to austenitics |
| Thermal Expansion (20–100°C) |
17.0 ×10⁻⁶ /K |
Higher than carbon steel |
| Specific Heat |
500 J/kg·K |
At 21°C |
| Electrical Resistivity |
0.73 µΩ·m |
At 21°C |
| Magnetic Property |
Non-magnetic* |
*Slight after cold work |
| PREN (Pitting Resistance) |
~18 |
No Mo |
Corrosion Resistance
Weld Sensitization Immunity
Niobium forms NbC, so 347 does not sensitize in the 425–815°C range after welding. It is chosen where post-weld annealing is impractical and high-temperature strength is needed.
High-Temperature Service
347 carries useful strength and oxidation resistance to about 870°C. Because NbC is more stable than TiC above ~1000°C, it has better creep stability than 321 at the top of the range.
General & Atmospheric Corrosion
Same 18/8 resistance as 304 in fresh water, food, and mild chemicals (PREN ~18, no molybdenum). Not for chloride-heavy duty.
Pitting in Chlorides
No molybdenum means PREN ~18; 347 pits in chloride environments like 304. For chloride + heat, use 316H or 317L.
Stainless Steel 347 vs Similar Grades — Selection Guide
| If you need... |
Choose... |
Why |
| Welded high-temp, best creep |
347 / 347H |
Nb stabilization, stable NbC |
| Welded high-temp, lower cost |
321 |
Ti stabilization |
| Hottest creep >800°C |
347H |
Higher carbon |
| Chloride + heat |
316H / 317L |
Need molybdenum |
| General, non-welded |
304 / 304L |
Lower cost |
Applicable Standards
| Standard |
Product Form |
| ASTM A240 |
Plate, sheet, strip |
| ASTM A182 F347 |
Forged flanges & fittings |
| ASTM A312 TP347 |
Seamless & welded pipe |
| ASTM A403 WP347 |
Butt-weld fittings |
| ASTM A213 / A269 |
Tube |
| ASTM A276 / A479 |
Bar, rod, forging stock |
HT PIPE Supply — Stainless Steel 347 Product Forms
| Product Form |
Specifications |
HT PIPE Supply |
| Seamless Pipe |
ASTM A312 TP347 |
✅ Available |
| Forged Flanges |
ASTM A182 F347, ASME B16.5 |
✅ In stock |
| Butt-Weld Fittings |
ASTM A403 WP347 |
✅ Available |
| Plate / Sheet |
ASTM A240 |
✅ Available |
| Bar / Rod |
ASTM A276 / A479 |
✅ Available |
| Welded Pipe |
ASTM A312 TP347 |
✅ Available |
HT PIPE supplies 347 in pipe, flanges, fittings, plate, and bar for welded high-temperature service, with MTRs and matching ER347 welding consumables.
Project Experience — Stainless Steel 347
HT PIPE supplies 347 for welded high-temperature process, header, and thermowell systems.
| Project |
Product |
Specification |
| Refinery Transfer Line (Middle East) |
347 Pipe + Flanges |
A312, A182 F347 |
| Thermowell Assemblies (Asia) |
347 Bar + Forgings |
A276, A182 |
| High-Temp Header (China) |
347 Plate + Fittings |
A240, A403 |
We advise 347 vs 321/347H and supply matching ER347 filler for stabilized welds.
| FREQUENTLY ASKED QUESTIONS |
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FAQ — Stainless Steel 347
| What is the difference between 347 and 321? |
| Both are stabilized 304 grades avoiding weld sensitization, but with different stabilizers: 347 uses niobium (NbC, ≥10×C); 321 uses titanium (TiC, ≥5×(C+N)). NbC is more stable than TiC above ~1000°C, so 347 has better very-high-temperature creep stability. |
| What temperature can 347 withstand? |
| Oxidation resistance and useful strength extend to about 870°C. For the highest creep duty, the high-carbon 347H is specified. |
| Is 347 good in seawater? |
| No. Like 304 it has no molybdenum (PREN ~18) and pits in chlorides. For chloride service use 316L or higher. |
| What filler is used for 347? |
| ER347 / E347 to preserve niobium stabilization. Plain 308/316 fillers lack the stabilization concept. |
| When would I choose 347H over 347? |
| For the highest creep duty: 347H raises carbon to 0.04–0.10% (with grain-size control) to carry ASME allowable stresses from about 525°C to ~870°C, where plain 347 has no code-listed allowable stresses. |
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Need Stainless Steel 347 for Welded High-Temperature Service?
HT PIPE supplies 347 (UNS S34700) pipe, flanges, fittings, plate, and bar with full MTR documentation and matching welding consumables. Contact our technical team for advice.
Request a Quote |
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