1.4550 Stainless Steel (X6CrNiNb18-10 / UNS S34700)

What Is 1.4550 Stainless Steel?

1.4550 is the EN material number for X6CrNiNb18-10, an austenitic stainless steel stabilized with niobium (columbium). In American practice the same material is Grade 347 with UNS S34700, covered by ASTM A240 for plate, sheet, and strip, A312/A213 for pipe and tube, and A276 for bar. The stabilizing element niobium combines preferentially with carbon, so that during heating in the sensitization range chromium remains in solid solution in the matrix instead of precipitating as chromium carbides at grain boundaries. This makes 1.4550 highly resistant to intergranular corrosion after welding or prolonged high-temperature exposure, and it also gives the grade better high-temperature strength than unstabilized 304.

International Equivalents

Chemical Composition

The table combines the limits of EN 10088-2 for X6CrNiNb18-10 and ASTM A240 for UNS S34700; both are consistent for the main elements.

Mechanical Properties (Annealed, ASTM A240)

Stabilization and Sensitization Resistance

When unstabilized austenitic steel is held between about 450 C and 850 C, carbon migrates to grain boundaries and forms chromium carbides, leaving a chromium-depleted zone that corrodes preferentially - a phenomenon called sensitization. In 1.4550, niobium reacts with carbon first, forming stable niobium carbides, so the matrix chromium stays available for the passive film. The effect is that welded joints and components serving in this temperature range do not lose corrosion resistance, and the grade is exempt from post-weld heat treatment in most design codes.

High-Temperature Performance and Applications

1.4550 maintains good creep and stress-rupture strength at elevated temperature, with continuous service typically possible up to about 870 C where oxidation resistance is the limiting factor. Its combination of stabilization and high-temperature strength suits it to heavy welded sections that other stabilized grades cannot serve.

Aerospace: jet engine components, afterburner parts, high-temperature fasteners, exhaust systems

Power generation: superheater and reheater tubes, boiler internals, turbine components

Chemical processing: welded vessels, heat exchangers, piping at elevated temperature

Heat treatment equipment: retorts, muffles, radiant tubes, furnace fixtures

Frequently Asked Questions

Why is niobium added to 1.4550?

Niobium is a strong carbide former. By tying up carbon as stable niobium carbides, it prevents chromium carbide precipitation at grain boundaries during welding or high-temperature exposure, preserving corrosion resistance and preventing intergranular attack.

What is the difference between 1.4550 and 1.4541 (321)?

The stabilizing element differs: 1.4550 uses niobium, 1.4541 uses titanium. 347 generally offers better high-temperature creep strength and is preferred for heavy welded sections; 321 may be easier to form in some cold-working operations.

Does 1.4550 require post-weld heat treatment?

No. Because the material is stabilized, welded joints keep their corrosion resistance in the as-welded condition, and most pressure-vessel codes do not require post-weld heat treatment for this grade.

What is the maximum service temperature of 1.4550?

For continuous service the practical limit is about 870 C, governed by oxidation and scaling. Creep strength is a design consideration at temperatures above approximately 550-600 C.

Is 1.4550 magnetic?

Like other austenitic grades, 1.4550 is essentially non-magnetic in the annealed condition. Slight magnetism can develop after cold work but does not affect its performance.

In which product forms is 347 / 1.4550 available?

Plate, sheet, and strip per ASTM A240; seamless and welded pipe and tube per ASTM A312 and A213; bar and shapes per ASTM A276; and forgings per ASTM A182 (F347 and F347H).