347 stainless steel – UNS S34700
Niobium-stabilized Cr-Ni austenitic stainless steel
18-10 stainless stabilized with niobium (Nb ≥ 10×C), with the same function as 321 against sensitization, but with Nb that transfers well across the welding arc. Good creep resistance and intergranular corrosion resistance in service between 425 and 815 °C.
Designations and equivalents
| System | Designation |
|---|---|
| UNS | S34700 (347H: S34709) |
| W.Nr./EN | 1.4550 (X6CrNiNb18-10) |
| DIN | X6CrNiNb18-10 |
| JIS | SUS347 |
| Trade names | AISI 347 |
Standards by product form
| Form | Standard |
|---|---|
| Bar | ASTM A479 / A276 |
| Forging/flange | ASTM A182 F347 |
| Plate | ASTM A240 |
| Tube | ASTM A312 / A213 TP347 |
Tolerances and HS codes
- Forged bars (round and square): machining allowance from 2 mm (Ø 6–16 mm) to 24 mm (Ø 400–500 mm) on the diameter. E.g. Ø 100–125 mm finished: +7 mm, tolerance +6 / −0 (length up to 3,500 mm).
- Forged discs: allowance of 4 to 9 mm on the diameter and 3 to 11 mm on the height, plus-only tolerance.
- Cutting and machining: band saw −0 / +5 to +15 mm on length; waterjet +10 % of thickness on the diameter (min. 2 mm); machined parts −0 / +2 to +5 mm on the diameter. See the full tables (INF 002 and INF 003, the same for every alloy).
| Form | NCM (Brazil, 8 digits) | HS (international, 6 digits) | HTS (USA, 10 digits)* | CN (European Union, 8 digits)* |
|---|---|---|---|---|
| Forged bar | 7222.30.00 | 7222.30 | 7222.30.0024 | 7222 30 51 |
| Hot-rolled bar | 7222.11.00 | 7222.11 | 7222.11.0059 | 7222 11 11 |
| Plate (> 10 mm, width ≥ 600 mm) | 7219.21.00 | 7219.21 | 7219.21.0020 | 7219 21 10 |
| Seamless pipe | 7304.41.90 | 7304.41 | 7304.41.6045 | 7304 41 00 |
| Welded pipe | 7306.40.00 | 7306.40 | 7306.40.5085 | 7306 40 80 |
| Flange and fitting | 7307.21.00 | 7307.21 | 7307.21.5000 | 7307 21 00 |
Tariff class: stainless steel (by chemical composition, under the Chapter 72 Notes and Section XV Note 5).
*: US and EU codes for reference. NCM per current legislation (General Rules of Interpretation, Notes to Chapters 72, 74 and 75 and Section XV Note 5). Confirm the final code with your customs broker according to the item size and finish. HS = first 6 digits of the NCM.
Chemical composition (%)
ASTM A479 / A182 F347
| Element | Range (%) |
|---|---|
| C | ≤ 0.08 |
| Mn | ≤ 2.00 |
| P | ≤ 0.045 |
| S | ≤ 0.030 |
| Si | ≤ 1.00 |
| Cr | 17.0 – 19.0 |
| Ni | 9.0 – 13.0 |
| Nb | 10×C – 1.10 |
| Fe | balance |
Mechanical properties
Solution annealed (ASTM A479)
| Property | Value |
|---|---|
| Yield strength (0.2%) | ≥ 205 MPa |
| Tensile strength | ≥ 515 MPa |
| Elongation | ≥ 30% (A240: ≥ 40%) |
| Reduction of area | ≥ 40% (A182: ≥ 50%) |
| Hardness | ≤ 201 HB (A240) |
Typical physical properties
| Property | Value |
|---|---|
| Density | 7.96 g/cm³ |
| Melting range | 1,398 – 1,446 °C |
| Modulus of elasticity | 193 GPa |
| Thermal conductivity (20–100 °C) | 16.3 W/m·K |
| Thermal expansion (20–100 °C) | 16.6 µm/m·K |
Heat treatment, welding and machining
| Item | Usual practice |
|---|---|
| Service temperature | Up to 816 °C in code applications (ASME); oxidation ≈ 900 °C. Creep: 347H. |
| Heat treatment | Solution annealing at a minimum of 1,040 °C (usual range 1,040 – 1,120 °C) with rapid cooling. Optional stabilizing anneal 845 – 900 °C. |
| Welding | Good. Consumable AWS ER347 / E347. Watch for reheat cracking in thick sections of 347H. |
| Machining | Machinability lower than carbon steel: it work hardens quickly; use sharp tools, positive rake, constant feed without 'rubbing' and abundant coolant. |
Filler metal for welding
The wire composition is usually not identical to the material: each page explains why.
Typical applications
- Refinery piping and vessels (hydrotreating)
- Boiler and superheater tubes
- Exhaust manifolds and turbine parts
- Expansion joints
- Welded tanks for the chemical industry
Alternatives: 321 is equivalent and sometimes cheaper; 304H for high temperature without stabilization; 310S above ≈ 900 °C.
347 stainless steel: complete guide
What 347 stainless steel is
347 belongs to the family of austenitic stainless steels. Like 321, it is a stabilized 304, but with niobium instead of titanium. Niobium forms more stable carbides, is not lost in the welding arc and does not form coarse inclusions, which is why 347 is preferred for welds and for the higher temperatures of the stabilization range.
Chemical composition and role of the elements
Chromium (17 – 19%) and nickel (9 – 13%) form the austenitic base; niobium, from at least ten times the carbon up to 1.00%, ties up the carbon and leaves the chromium free for the passive film. 347 contains no titanium and no nitrogen addition. The 347H version, with carbon controlled between 0.04 and 0.10%, is used for creep design.
Mechanical properties
Solution annealed, it meets a minimum yield strength of 205 MPa, minimum tensile strength of 515 MPa, minimum elongation of 30% in bar and forgings (40% in plate), minimum reduction of area of 50% in forgings (40% in bar) and maximum hardness of 201 HBW.
Service temperature and corrosion
Without molybdenum, aqueous corrosion resistance is that of 304, with a PREN of 17 to 20; the advantage is immunity to intergranular corrosion after exposure to 450 – 800 °C.
Applications
Refinery piping and headers, reformer and cracking furnace components, superheaters and reheaters, expansion joints, heat exchangers and thick welded parts that operate hot.
Supply forms and Dustre services
Round bars and blocks forged in open die, cut to size on a band saw, with rough or finish machining, solution annealing in our own furnaces and hardness, tensile and impact testing. All material is supplied with a 3.1 inspection certificate and heat traceability.
Frequently asked questions
What is the chemical composition of 347 stainless steel?
Per ASTM A479 / A182 F347: C ≤ 0.08; Mn ≤ 2.00; P ≤ 0.045; S ≤ 0.030; Si ≤ 1.00; Cr 17.0 – 19.0.
Which standards cover 347 stainless steel?
Bar: ASTM A479 / A276. Forging/flange: ASTM A182 F347. Plate: ASTM A240. Tube: ASTM A312 / A213 TP347.
Up to what temperature can 347 stainless steel be used?
Up to 816 °C in code applications (ASME); oxidation ≈ 900 °C. Creep: 347H.
Which materials can replace 347 stainless steel?
321 is equivalent and sometimes cheaper; 304H for high temperature without stabilization; 310S above ≈ 900 °C.
