SS 347: properties, composition, uses and standards
SS 347 is a niobium-stabilised austenitic stainless steel, chosen for welded high-temperature piping in refineries and power plants. UNS S34700, W.Nr 1.4550, PREN ≈ 18, density 7.96 g/cm³.
What is SS 347?
SS 347 is a niobium-stabilised austenitic stainless steel. Engineers choose it for welded high-temperature piping in refineries and power plants. It is also known as AISI 347, UNS S34700, W.Nr 1.4550, Niobium-stabilised stainless.
- UNS
- S34700 · S34709 (347H)
- W.Nr
- 1.4550
- EN
- X6CrNiNb18-10
- JIS
- SUS 347
- BS
- 347S31
- Density
- 7.96 g/cm³
- Melting range
- 1400 – 1425 °C
- Magnetic
- Non-magnetic annealed
- PREN (approx.)
- ≈ 18
Chemical composition and what each element does
| Element | Weight % |
|---|---|
| Chromium | 17.0 – 19.0 |
| Nickel | 9.0 – 13.0 |
| Niobium | 10 × C min, 1.00 max |
| Carbon | 0.08 max |
| Manganese | 2.00 max |
| Silicon | 0.75 max |
| Phosphorus | 0.045 max |
| Sulphur | 0.030 max |
| Iron | Balance |
Per ASTM A240 for UNS S34700.
About 18% chromium forms the passive film that makes the steel stainless.
Nickel keeps the structure austenitic — tough at low temperature and easy to form and weld.
Niobium binds carbon more stably than titanium, so 347 holds up in welded high-temperature service.
347H is the choice for hot hydrogen and high-temperature refinery piping.
Mechanical properties
| Property | Value |
|---|---|
| Tensile strength | 515 MPa · 75 ksi |
| Yield strength 0.2% | 205 MPa · 30 ksi |
| Elongation | 40% |
| Hardness | 201 HB max |
Annealed plate & sheet, minimum values. Values for pipe, bar and forgings differ slightly — see the product standard.
Where SS 347 is used
- Refinery hydroprocessing. Reactor effluent piping and exchangers. Industry →
- Power plants. Superheater and reheater components. Industry →
- Aerospace. Exhaust and heat-resisting parts. Industry →
- Chemical plants. Hot, welded process lines. Industry →
Welding 347
The matching filler is ER347 (AWS A5.9). ERNiCr-3 for dissimilar joints.
Keep heat input moderate and interpass temperature low, clean joints thoroughly, and pickle or passivate after welding where the service is corrosive.
SS 347 compared with similar grades
| Grade | PREN | Choose it for |
|---|---|---|
| SS 347 | ≈ 18 | Welded high-temperature piping in refineries and power plants |
| SS 321 | ≈ 18 | High-temperature service between 425 and 815 °C |
| SS 304 | ≈ 18 | General corrosion resistance, food, water and architectural work |
| SS 310 / 310S | ≈ 25 | Furnaces, burners and heat treatment at high temperature |
SS 347 standards by product
Buying SS 347: what to specify
- The product standard and the UNS or designation — for example ASTM A240 UNS S34700.
- Size, thickness or schedule, and quantity in pieces and kg or metres.
- Heat-treatment condition where the standard offers a choice.
- Certificate: EN 10204 3.1 as standard, 3.2 if a third party must witness.