Stainless Steel 347
Stainless Steel 347 (UNS S34700) is a niobium (columbium)-stabilized austenitic grade developed for high-temperature service where sensitization must be prevented. Niobium is added at a minimum of ten times the carbon content, forming stable NbC precipitates that keep chromium available at grain boundaries during prolonged thermal exposure.
1. Chemical Composition
The table below details the elemental composition of 347. The niobium stabilization ratio governs its immunity to intergranular attack in weld-affected zones.
| Grade | C | Mn | P | S | Si | Cr | Ni | Mo | Other |
| TP347 | ≤ 0.080 | ≤ 2.00 | ≤ 0.045 | ≤ 0.030 | ≤ 1.00 | 17.00–19.00 | 9.00–13.00 | — | Nb: 10xC min |
Note: Tantalum is counted together with niobium in the stabilization ratio as both elements form stable carbides.
2. Mechanical Properties
These minimum mechanical property values apply to 347 in the solution-annealed condition per ASTM specifications for pressure-service tubes and pipes.
| Grade | Condition | Tensile Strength Min, MPa (ksi) | Yield Strength Min, MPa (ksi) | Elongation Min, % | Heat Treatment Temp |
| TP347 | Annealed | 515 (75) | 205 (30) | 35 | 1040°C (1900°F) min |
3. Equivalent Grade
This table cross-references 347 designations across major international standards for material traceability, third-party inspection, and specification alignment in multinational projects.
| GRADE | UNS | GB | JIS | ISO | DIN/EN | GOST | |
| ISC | NEW | ||||||
| 347 | S34700 | S34778 | 06Cr18Ni11Nb | SUS347 | X6CrNiNb18-10 | 1.4550 | 08X18H12Б |
4. Key Technical Advantages
- Niobium Stabilization for Weld Zone Protection: Niobium has a stronger carbide-forming tendency than chromium, so carbon is preferentially locked as NbC rather than as chromium carbide at grain boundaries. This fully protects weld heat-affected zones without requiring post-weld annealing.
- High-Temperature Strength Above 321: Solid-solution strengthening from niobium gives 347 marginally better high-temperature tensile and yield strength than titanium-stabilized 321 at temperatures in the 500-700°C range.
- Good Aqueous Corrosion Resistance: Without sensitization, 347 resists a wide range of aqueous corrosive media including dilute organic acids, food-grade chemicals, and mildly reducing environments. Its performance is comparable to unstabilized 304 in the annealed condition.
Technical Note: 347 is not suitable for strong oxidizing halide environments or concentrated sulfuric acid service. For high-pressure, high-temperature applications requiring maximum allowable ASME stresses, specify 347H where the carbon floor and grain size requirements apply.
5. Common Manufacturing Standards
ASTM A312: Standard specification for seamless, welded, and heavily cold-worked austenitic stainless steel pipes — TP347 designation covered.
ASTM A213: Standard specification for seamless ferritic and austenitic alloy-steel boiler, superheater, and heat-exchanger tubes — TP347 grade included.
ASTM A269: Standard specification for seamless and welded austenitic stainless steel tubing for general service — 347 applicable.
ASTM A249: Standard specification for welded austenitic steel boiler, superheater, heat-exchanger, and condenser tubes — TP347 designation.
ASTM A511: Standard specification for seamless stainless steel mechanical tubing — MT347 covered.
DIN EN 10216-5: Seamless steel tubes for pressure purposes — stainless steel (Grade 1.4550 equivalent).
JIS G3459: Stainless steel pipes — applicable to SUS347 designation.
GB/T 14976: Seamless stainless steel tubes for fluid transport — 06Cr18Ni11Nb designation.
GOST 9940 / 9941: Seamless hot- and cold-formed stainless steel tubes — 08X18H12Б equivalent.
Manufacturing Standards Comparison Table:
| Standard | ASTM | EN/DIN | JIS | GB/T | GOST |
| Seamless Pipe | A312 TP347 | EN 10216-5 (1.4550) | G3459 SUS347 | GB/T 14976 | GOST 9940 |
| Seamless Tube | A213 TP347 | EN 10216-5 (1.4550) | G3463 SUS347 | GB/T 13296 | GOST 9941 |
| Welded Tube | A249 TP347 | EN 10217-7 (1.4550) | G3463 SUS347 | GB/T 12771 | — |
6. Primary Applications
- Refinery Furnace Tubes: Hydrocracking and hydrotreating furnace tubes operating in the sensitization temperature range, where stabilization against intergranular attack is a process safety requirement.
- Nuclear Reactor Components: Reactor coolant circuit piping and boiling water reactor (BWR) components where resistance to sensitization under neutron flux and elevated temperatures is mandatory.
- Fossil Fuel Power Plants: Superheater tubes and steam piping in coal-fired and combined-cycle power plants that need stabilized austenitic material at operating temperatures above the sensitization threshold.
- Cryogenic Storage Equipment: Cryogenic tanks and liquefied gas storage systems that benefit from 347's retained toughness and non-sensitized microstructure at sub-zero temperatures.
- Aircraft Engine Components: Combustion chamber liners and afterburner components where thermal cycling through sensitization temperatures demands a stabilized alloy system.