Stainless Steel 347: Niobium-Stabilized Austenitic Grade for High-Temperature and Nuclear Service

What Is Stainless Steel 347?

Stainless steel 347 is an austenitic grade stabilized with niobium and tantalum, designated UNS S34700 in ASTM A240 and EN 1.4550 (X6CrNiNb18-10) in EN 10088, with the Japanese equivalent SUS347 per JIS G4303 and G4304. Stabilization means that the carbon is bound into stable niobium carbides (NbC) instead of being free to react with chromium. This prevents chromium carbide precipitation at grain boundaries during welding or sustained high-temperature exposure, so the steel resists intergranular corrosion and can be used in the as-welded condition without post-weld annealing. Grade 347 is the preferred choice when components face frequent heating and cooling cycles, because niobium carbides do not dissolve and re-precipitate during thermal cycling as readily as titanium carbides.

Chemical Composition

The composition limits below follow ASTM A240 for plate, sheet and strip; the same chemistry applies to pipe and tube under ASTM A312 and A269.

Element	Composition (wt%)
Carbon (C)	0.08 max
Manganese (Mn)	2.00 max
Phosphorus (P)	0.045 max
Sulfur (S)	0.030 max
Silicon (Si)	0.75 max
Chromium (Cr)	17.0-19.0
Nickel (Ni)	9.0-13.0
Niobium + Tantalum (Nb+Ta)	10 x C min, 1.00 max

The Nb+Ta requirement of at least 10 times the carbon content, with a practical minimum of 0.70%, guarantees that enough niobium is present to combine with all the carbon in the steel.

Mechanical Properties

In the annealed condition, 347 provides the following minimum properties per ASTM A240.

Property (Annealed)	Value
Tensile Strength min (MPa)	515
Yield Strength min (MPa)	205
Elongation min (%)	40
Hardness max (HB)	217

At elevated temperatures, 347 retains useful strength and, unlike non-stabilized grades, keeps its corrosion resistance even after long periods in the sensitizing temperature range of 425-815°C.

Niobium Stabilization vs Titanium Stabilization

Grade 321 uses titanium for the same stabilizing purpose, but the two elements behave differently at high temperature. Titanium carbides (TiC) remain stable up to about 1000°C, while niobium carbides (NbC) remain stable up to about 1200°C. In service above 1000°C, or under repeated thermal cycling, titanium carbides can dissolve and re-precipitate at grain boundaries, which locally depletes chromium and reduces corrosion resistance. Niobium carbides are much less affected by thermal cycling, which is why 347 is specified for aircraft exhaust components, superheater parts and nuclear components that experience frequent heating and cooling. Grade 321 remains the more economical choice for static high-temperature service below 1000°C.

Welding and Typical Applications

Grade 347 is readily weldable by TIG, MIG and MMA processes using matching 347 filler metal that carries the same niobium content. Low heat input minimizes grain growth in the heat-affected zone, and argon shielding of 99.99% purity prevents porosity on the weld face. Preheating is not required. Because the steel is stabilized, welded joints keep their intergranular corrosion resistance without post-weld annealing, which is a major advantage for large vessels and complex fabrications. Typical applications include aircraft engine components and exhaust systems, nuclear pressure vessels and reactor internals, chemical and petrochemical reactors, superheater and reheater tubing, and furnace parts operating above 800°C.

FAQ

How does niobium stabilization differ from titanium stabilization in 321?

Niobium forms NbC carbides that stay stable up to about 1200°C, while titanium forms TiC stable to about 1000°C. Under repeated thermal cycling, niobium carbides resist dissolution and re-precipitation far better, so 347 keeps its intergranular corrosion resistance in fluctuating high-temperature service. Grade 347 also requires 10 times the carbon content in niobium, versus 5 times for titanium in 321, which gives a more complete carbon lock-up.

Why is 347 used in nuclear pressure vessels?

Welded joints in nuclear vessels must not lose corrosion resistance, because any intergranular attack could become a leak path for radioactive media. The niobium stabilization of 347 keeps welded joints corrosion-resistant without post-weld annealing, the austenitic structure resists radiation embrittlement better than ferritic grades, and the fine distribution of NbC helps trap radiation-induced defects while preserving ductility.

How does 347 compare with 304L?

304L relies on a very low carbon content (0.03% max) to avoid carbide precipitation, but the protection is lost in welded or high-temperature service above 425°C. Grade 347 resists intergranular corrosion through stabilization and retains strength above 600°C. 304L is cheaper for low-stress applications such as food equipment; 347 is the correct choice for welded high-temperature components such as boiler tubing.

What is the best way to weld 347?

Use TIG or MIG with matching 347 filler metal so the weld metal has the same niobium stabilization. Keep heat input low to limit grain growth in the heat-affected zone, shield with high-purity argon to prevent porosity, and avoid wide weaving. No preheating is needed, and post-weld grinding or pickling removes oxide scale from the weld area.

Can 347 be used at cryogenic temperatures?

Yes. The austenitic structure of 347 remains stable down to -196°C, so it can serve in non-critical cryogenic applications such as small liquid nitrogen vessels. For highly stressed cryogenic components, 321 is usually preferred because its finer titanium carbides give better impact toughness; the larger niobium carbides in 347 act as minor stress concentrators.