347 Stainless Steel (UNS S34700)
Austenitic (Nb-stabilized) · ASTM A240/A240M · Density 8 g/cm³
Type 347 (UNS S34700) is niobium-stabilized 18/8 stainless steel. Niobium (columbium) prevents chromium-carbide precipitation, so 347 keeps its corrosion resistance and creep strength through welding and long service between 425 and 815 °C — the reason it is standard in refinery piping, boiler tube and aerospace exhaust systems.
Chemical Composition (wt%)
| C | Mn | Si | Cr | Ni | Nb | P | S |
|---|---|---|---|---|---|---|---|
| ≤0.08 | ≤2.00 | ≤0.75 | 17.0–19.0 | 9.0–13.0 | 10×C–1.00 | ≤0.045 | ≤0.030 |
Single values are maximums unless a range is shown. Balance: Fe. Per ASTM A240/A240M.
Mechanical Properties
| Tensile strength | ≥515 MPa (75 ksi) |
|---|---|
| Yield strength (0.2%) | ≥205 MPa (30 ksi) |
| Elongation (50 mm) | ≥40% |
| Hardness | ≤201 HBW |
Room-temperature minimum values are summarized for annealed flat product under ASTM A240/A240M unless a condition is named in the property label; product form and thickness can change the requirement.
Physical Properties & Structure
| Density | 8 g/cm³ |
|---|---|
| Magnetic response | Essentially non-magnetic after solution annealing; cold work can introduce a measurable magnetic response. |
| Microstructure | Austenitic (Nb-stabilized) |
Corrosion Resistance & Limits
- General corrosion resistance comes from the chromium-rich passive film; nickel, molybdenum, nitrogen and copper change performance by grade.
- Check chlorides, temperature, crevices, deposits, process chemistry and cleaning conditions. A grade designation alone is not a service-life guarantee.
Fabrication & Heat Treatment
| Welding | Generally weldable with a qualified procedure and matching or over-alloyed filler where required; remove heat tint when corrosion service demands it. |
|---|---|
| Forming | Good ductility is typical, but rapid work hardening increases forming load and springback; bend radius and intermediate annealing depend on grade and reduction. |
| Machining | Use rigid tooling, positive feeds and controlled heat because the material work-hardens; free-machining grades require separate corrosion and welding review. |
| Heat treatment | Solution annealing and rapid cooling restore the intended condition where applicable. Austenitic grades are not hardened by conventional quench-and-temper treatment. |
Equivalent Grades
The closest international equivalents of 347 Stainless Steel are 1.4550 (X6CrNiNb18-10) in the EN system, SUS347 in the JIS system, 06Cr18Ni11Nb in the GB system.
These are common corresponding designations, not unconditional substitutes. Verify chemistry, mechanical properties, product standard, delivery condition, dimensions and project approval before changing designation.
| System | Designation |
|---|---|
| UNS | S34700 |
| EN | 1.4550 (X6CrNiNb18-10) |
| JIS | SUS347 |
| GB | 06Cr18Ni11Nb |
Reference Product Forms & Dimensional Ranges
Common reference product forms for 347 stainless steel: Sheet / Plate, Pipe / Tube, Bar.
| Finishes | No.1, 2B |
|---|---|
| Sheet / coil thickness | 0.5–6.0 mm |
| Plate thickness | 6–50 mm |
Common surface and delivery options: No.1, 2B, BA or mechanically polished finish when supported by the product route; Annealed and pickled, descaled or polished tube finish as specified; Hot rolled, peeled, bright drawn, ground or polished bar condition according to tolerance.
These published ranges are engineering references, not a live-stock or production-capability promise. Confirm the required product form, dimensions, condition, finish and supply route in the quotation.
Typical Applications
- Refinery and petrochemical piping
- Boiler and superheater tubes
- Aircraft and gas-turbine exhaust
- Expansion joints and bellows
- High-temperature process equipment
Frequently Asked Questions
What is the difference between 347 and 321 stainless steel?
Both are stabilized 18/8 grades: 321 uses titanium, 347 uses niobium. Niobium does not burn off in welding arcs the way titanium can, so 347 weld metal keeps its stabilization — which is why 347 filler is even used to weld 321 base metal. 347 also shows slightly better creep-rupture strength; 321 is usually cheaper.
When is 347 required instead of 304L?
When the part both operates in the 425–815 °C sensitization window and needs aqueous corrosion resistance during shutdowns. Low carbon alone (304L) loses strength at temperature, while stabilized 347 keeps carbon locked up and retains creep strength — typical for refinery streams and boiler circuits.
What is 347H?
The higher-carbon variant (C 0.04–0.10%) of 347 with the same niobium stabilization, specified where design codes need higher allowable stresses at creep temperatures. If your drawing calls 347H, straight 347 with C below 0.04% does not qualify.
347 Compared With Other Grades
Organizational author: Tsingshan (Shandong) Iron & Steel Co., Ltd..Review method, status and limitations.
Other stainless steel grades:
304304L316316L201321310S4104302205202301309S316Ti904L409L4204394414442507303317L254 SMO440C17-4 PH2304
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