SS 347 Pipe Composition: Niobium-Stabilised Austenitic Stainless Steel
Jul 07, 2025
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Why the Composition of SS 347 Pipe Matters
SS 347 is a niobium-stabilised austenitic stainless steel developed for welded and high-temperature service. Niobium (also written columbium, Cb) has a stronger affinity for carbon than chromium does, so it ties up carbon as niobium carbides instead of allowing chromium carbides to form at grain boundaries. That single alloying decision is what lets 347 pipe operate in the 425-850 C sensitisation range, and in welded joints, without losing corrosion resistance or requiring a post-weld heat treatment.
For engineers, buyers and fabricators, the exact chemistry determines whether the pipe will pass a 1000-hour sensitisation test, how it behaves in a reformer or superheater, and whether it can be welded on site without a furnace anneal. The composition table below is the reference used for that decision.
Equivalents of SS 347
| Standard system | Grade or designation | Number |
|---|---|---|
| ASTM / AISI (US) | 347 | UNS S34700 |
| EN (Europe) | X6CrNiNb18-10 | 1.4550 |
| JIS (Japan) | SUS347 | - |
| GB (China) | 06Cr18Ni11Nb | - |
The stabilised grades 347 and 321 are often compared directly: 347 uses niobium, while 321 uses titanium. Niobium stabilisation is generally preferred where the metal must survive repeated thermal cycling or where welding is carried out in thick sections.
Chemical Composition of SS 347 Pipe
| Element | Content (%) |
|---|---|
| Cr | 17.0 - 19.0 |
| Ni | 9.0 - 13.0 |
| Nb (Nb + Ta) | 10 x C min, 1.10 max |
| C | 0.08 max |
| Mn | 2.00 max |
| Si | 1.00 max |
| P | 0.045 max |
| S | 0.030 max |
| Fe | Balance |
The niobium addition is the defining element. Because the specification sets it as a multiple of carbon rather than a fixed range, a heat with higher carbon also carries more niobium, which guarantees that enough stabiliser is present to protect the whole heat-affected zone.
Product Specifications for 347 Pipe and Tube
| Standard | Product covered |
|---|---|
| ASTM A312 | Seamless and welded austenitic pipe |
| ASTM A213 | Seamless tube for heat exchangers and boilers |
| ASTM A249 | Welded tube for heat transfer equipment |
| ASTM A240 | Plate, sheet and strip |
| ASTM A182 | Forged fittings and flanges |
| AWS A5.9 | ER347 welding wire |
Type 347H is the high-carbon version of the same grade, with 0.04-0.10% carbon, and is specified when creep strength above 550 C is the governing design condition.
Mechanical and Thermal Behaviour
Room-temperature strength: annealed pipe typically delivers 515 MPa minimum tensile strength, 205 MPa minimum 0.2% yield strength and 40% minimum elongation.
Service temperature: continuous use up to about 870 C, which is why 347 appears in superheaters, reformer tubes and refinery heater coils.
Thermal cycling: niobium carbides are stable well above the sensitisation range, so repeated heating and cooling does not degrade grain-boundary corrosion resistance.
Weldability: no preheat and no post-weld anneal are needed for normal wall thicknesses; ER347 filler keeps the deposit chemistry matched to the parent pipe.
Oxidation: a chromium-rich scale resists scaling and spalling in combustion and process gas atmospheres up to the service limit.
High-Temperature Applications of SS 347 Pipe
Heat exchangers, superheaters and condensers in power generation.
Boiler tubes, refinery heater coils and steam piping.
Reformer tubes and furnace outlet manifolds in petrochemical plants.
High-temperature pressure vessels and welded structures that cannot be heat treated after fabrication.
Exhaust and afterburner components where thermal cycling and oxidation occur together.
Frequently Asked Questions
Q: What is the chemical composition of SS 347 pipe?
Chromium 17.0-19.0%, nickel 9.0-13.0%, niobium at a minimum of 10 times the carbon content and 1.10% maximum, carbon 0.08% max, manganese 2.00% max, silicon 1.00% max, with the balance iron.
Q: What is the equivalent of 347 stainless steel?
347 is UNS S34700 in the ASTM system, 1.4550 (X6CrNiNb18-10) in EN, SUS347 in JIS and 06Cr18Ni11Nb under GB.
Q: Why is niobium added to 347?
Niobium binds carbon as stable niobium carbides, preventing chromium carbide precipitation at grain boundaries and so preventing intergranular corrosion after welding or prolonged high-temperature exposure.
Q: What is the maximum service temperature of 347 pipe?
About 870 C in continuous high-temperature service, with the practical limit set by load, atmosphere and the required creep life rather than by corrosion alone.
Q: Is post-weld heat treatment required for 347?
No. Stabilisation by niobium removes the need for a post-weld solution anneal in normal thicknesses, which is one of the main reasons the grade is chosen for site-welded piping.
Q: What is the difference between 347 and 347H?
Carbon content. 347 is capped at 0.08% carbon, while 347H contains 0.04-0.10% carbon for higher creep strength at elevated temperature.
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