EN 10253-4 Type B 347 · X6CrNiNb18-10 Nb-Stabilised · No Mo

1.4550 (347) Pipe Fittings

EN 10253-4 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.

PRE (approx.)

~18

Nb Addition

≥10×C, max 1.00%

ASTM Equivalent

A403 WP347

Why Nb Stabilisation — 347 vs 321

321 (1.4541) — Ti Stabilised

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).

347 (1.4550) — Nb Stabilised

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.

Chemistry (EN 10088-4)

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

Applications

Welded high-temperature headers and fittings operating at 400–800°C where a stabilised grade is required
Refinery and petrochemical piping in continuous elevated-temperature service
Exhaust and flue-gas ducting that cycles through the sensitisation range
Expansion joints, bellows and fabricated assemblies where weld-metal stabilisation matters
Applications specifying 347 by name in preference to 321 for welded construction

Cross-References

W.Nr.	1.4550
EN Name	X6CrNiNb18-10
ASTM UNS	S34700
ASTM Grade	A403 WP347
Common Name	347

Welding

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.

Request 1.4550 Quote

Testing & Inspection

✓ Chemical analysis against EN 10216-5 / EN 10253
✓ Tensile test, with impact test where the standard or your order requires it
✓ Intergranular corrosion test when specified
✓ PMI (positive material identification) on every order
✓ Visual and dimensional inspection to EN 10253
✓ NDT (UT, RT, MT or PT) when your order or code calls for it
✓ EN 10204 3.1 certificate as standard; 3.2 with TÜV, Lloyd's Register, Bureau Veritas, DNV or your own inspector

Size Range

DN15–DN600 and larger. Outside diameters follow the EN 10220 / EN ISO 1127 pipe series; wall thickness is matched to your connecting pipe.

Size	OD (mm)
DN 15	21.3
DN 20	26.9
DN 25	33.7
DN 32	42.4
DN 40	48.3
DN 50	60.3
DN 65	76.1
DN 80	88.9
DN 100	114.3
DN 125	139.7
DN 150	168.3
DN 200	219.1
DN 250	273.0
DN 300	323.9
DN 350	355.6
DN 400	406.4
DN 450	457.0
DN 500	508.0
DN 600	610.0

Export Documents & Packing

✓ Commercial invoice, packing list and certificate of origin
✓ EN 10204 certificates with heat-number traceability
✓ PED 2014/68/EU and AD 2000-Merkblatt W0 documentation for European projects
✓ Seaworthy wooden cases or pallets, bevel ends protected, marked to your instructions
✓ FOB, CFR or CIF delivery terms

Request a 1.4550 (347) Quote

Frequently Asked Questions: 1.4550 (347) Fittings

How is 347 (1.4550) stabilised?

Niobium is added at a minimum of 10 × carbon, so carbon forms niobium carbide instead of chromium carbide, preventing sensitisation in the 400–800°C range.

What is the designation of 1.4550?

X6CrNiNb18-10 (UNS S34700).

Which certificates come with 1.4550 fittings?

EN 10204 3.1 as standard, or 3.2 with third-party inspection.