EN 10253-2 Type B buttweld pipe fittings in 1.4550 — 347 niobium-stabilised austenitic stainless steel (X6CrNiNb18-10, S34700). Niobium is added at minimum 10×C to stabilise carbon as NbC instead of Cr₂₃C₆ — preventing sensitisation during service in the 400–800°C range. Because niobium is not lost in the welding arc the way titanium is, 347 is the preferred stabilised grade for welded fittings and fabricated assemblies.
Titanium (Ti ≥5×C) ties up carbon as TiC to prevent sensitisation at 400–800°C. Excellent for wrought and machined components, but titanium oxidises and is partly lost across the welding arc — so weld metal must be stabilised separately with an Nb-bearing filler (ER347).
Niobium (Nb ≥10×C) forms stable NbC carbides and, unlike titanium, transfers cleanly across the weld arc. This makes 347 the more robust choice for welded fittings and fabrications, and it offers slightly better high-temperature creep strength than 321.
| Element | Min | Max | Note |
|---|---|---|---|
| C (Carbon) | — | 0.080% | Tied up by Nb as NbC |
| Cr (Chromium) | 17.0% | 19.0% | Passive film |
| Ni (Nickel) | 9.0% | 12.0% | Austenite stabiliser |
| Nb (Niobium) | 10×C min | 1.00% | Stabilises C — prevents sensitisation |
| Mo | — | — | None — no Mo in 347 |
| W.Nr. | 1.4550 |
| EN Name | X6CrNiNb18-10 |
| ASTM UNS | S34700 |
| ASTM Grade | A403 WP347 |
| Common Name | 347 |
Filler: ER347 (Nb-stabilised, AWS A5.9) — a matching-composition filler. No PWHT required. Niobium stabilisation is retained across the weld, which is the principal reason 347 is favoured over 321 for welded fittings.
