1.4550 Stainless Steel Forgings (X6CrNiNb18-10 / UNS S34700 / AISI 347)

1.4550 key data

EN material number

1.4550

EN steel name

X6CrNiNb18-10

UNS number

S34700

AISI / SAE

347

Steel type

Austenitic, Nb-stabilised

Density

7.90 g/cm³

Yield strength R_p0.2

≥ 205 MPa

Tensile strength R_m

510 to 740 MPa

Elongation A

≥ 40 %

Solution anneal

1020 to 1100 °C, rapid cool

Max. service in air

approx. 870 °C continuous

Magnetic response

Non-magnetic (µ_r < 1.05)

Description of 1.4550

1.4550 is a chromium-nickel austenitic stainless steel with niobium added as a stabilising element. In an unstabilised 18/10 grade such as 1.4301 (304), holding the steel between 425 °C and 815 °C allows chromium to combine with carbon and precipitate as Cr₂₃C₆ along the grain boundaries. The metal next to those boundaries is left short of chromium, loses its passive film, and fails by intergranular attack.

Niobium has a stronger affinity for carbon than chromium has. With niobium specified at ten times the carbon content or more, carbon is fixed as niobium carbide during manufacture and chromium remains in solution. 1.4550 is therefore used for welded assemblies that cannot be re-solution-annealed after welding, and for components that operate inside the sensitisation range.

Against the titanium-stabilised grade 1.4541 (321), niobium does not oxidise in the welding arc and does not form the surface stringers that titanium can produce. Weld metal is cleaner, machined and polished surfaces are better, and creep-rupture strength above 600 °C is higher.

1.4550 equivalent grades and standards

Product standards used for 1.4550 forgings

Dual certification. Jiangyin Jiangnan Metal Co., Ltd. supplies 1.4550 forgings dual-certified to EN 10250-4 or EN 10222-5 and ASTM A182 F347. Because the EN and ASTM limits differ (see Table 3), the heat is melted to the narrower overlap so that one heat satisfies both specifications.

Chemical composition of 1.4550

The niobium requirement is the governing line in Table 3. A heat that meets 17 to 19 % Cr and 9 to 12 % Ni but falls short of Nb at ten times carbon is not a stabilised steel and will sensitise in service. The Nb/C ratio is verified on the ladle analysis before material is released to forging. A product analysis taken from the finished forging is available on request.

Mechanical properties of 1.4550 forgings

Specified minimum values at room temperature

Typical actual values are from a 25 mm (1 in) round bar annealed at 1038 °C (1900 °F). Larger cross-sections cool more slowly and normally return lower strength and hardness. For sections above 160 mm the specified minima of EN 10250-4 for the relevant thickness class apply.

Elevated-temperature proof strength

Design note. Table 5 is indicative, for material selection only. For pressure-equipment design use the allowable stresses of the governing code, EN 13445, ASME BPVC Section II Part D or the applicable national code. Certified test results for the actual heat are supplied with every delivery.

Physical properties of 1.4550

These figures have direct consequences for fabrication. Thermal conductivity is about one third that of carbon steel while thermal expansion is roughly 50 % higher. Austenitic forgings distort more during welding and machining than a ferritic grade of the same size, so fixturing, weld sequence and machining allowances have to be planned for it.

Forging and heat treatment of 1.4550

Cooling rate

Solution annealing dissolves precipitates and restores a fully austenitic structure, provided the forging passes quickly through the 815 to 425 °C band on the way down. A slow furnace cool through that band re-precipitates carbides and cancels the anneal. Heavy 1.4550 rings and discs are water quenched direct from the annealing temperature, and the transfer time from furnace to quench tank is recorded on the heat treatment chart issued with the certificate.

Stabilising anneal for polythionic acid service

Refinery components exposed to polythionic acid, formed when sulphide scale reacts with air and moisture during shutdown, require a stabilising anneal at 870 to 900 °C for at least two hours after solution annealing, in accordance with NACE SP0170. The treatment converts remaining free carbon to niobium carbide and gives maximum immunity to polythionic acid stress corrosion cracking. Specify it on the enquiry if the component operates in hydroprocessing or catalytic reforming service.

Corrosion resistance and high-temperature behaviour

• Intergranular corrosion. Immune in the as-welded and stabilised conditions. Verified by ASTM A262 Practice E, the Strauss copper / copper sulphate / sulphuric acid test, with bend examination, available on request.
• General corrosion. Comparable to 1.4301 (304) in oxidising media. Resists nitric acid, most organic acids, and atmospheric and fresh-water exposure.
• Chloride pitting and crevice corrosion. Limited, PREN approximately 18, essentially the same as 304. 1.4550 contains no molybdenum and is not a substitute for 1.4404 (316L) or 1.4547 (254 SMO) in seawater or high-chloride brines.
• Chloride stress corrosion cracking. Susceptible above about 60 °C in chloride-bearing environments, as with all standard 18/10 austenitics. Duplex grades such as A182 F51 or F53, or nickel alloys, are the alternative.
• Scaling resistance in air. Up to approximately 870 °C in continuous service, about 815 °C under intermittent cycling.
• Creep strength. Higher than 1.4541 (321) above roughly 600 °C, the main reason 1.4550 is specified for superheater and reformer hardware.

Welding and machining 1.4550

Welding

1.4550 is weldable by GTAW/TIG, GMAW/MIG, SMAW and SAW. Use matching stabilised filler, ER347 or E347-15 / E347-16 to AWS A5.9 and A5.4, or niobium-free ER316L where the design permits. No preheat is required. Keep interpass temperature below about 150 °C and heat input moderate to limit distortion. Post-weld solution annealing is not normally necessary with a stabilised grade. Back-purge root passes with argon to avoid oxidation, then remove heat tint by pickling and passivation to restore full corrosion resistance.

Machining

1.4550 work-hardens rapidly. Use rigid setups, sharp positive-rake tooling, generous depth of cut and continuous feed. Dwelling or rubbing glazes the surface and shortens tool life. Cutting speeds are typically 20 to 30 % below those used for free-machining carbon steel, with flood coolant. Forgings can be supplied rough machined to drawing with defined stock allowance, which removes the scaled skin and shortens the customer machining cycle.

1.4550 forged products and size range

All items below are produced to customer drawing or specification by Jiangyin Jiangnan Metal Co., Ltd. There is no fixed catalogue and no stocking list.

Delivery condition options: as-forged with scale, shot blasted, solution annealed, rough machined to drawing, or finish machined with pickled and passivated surface.

Manufacturing route and quality control

Melting and refining

1.4550 is melted in an electric arc furnace (EAF) and refined by vacuum oxygen decarburisation (VOD) to reach the low carbon and low sulphur levels a stabilised grade requires. For critical rotating parts, pressure-retaining components and heavy sections, electroslag remelting (ESR) is added to reduce inclusion content, limit segregation and improve transverse ductility and ultrasonic cleanliness.

Forging

Ingots or continuously cast blooms are upset and drawn on hydraulic open-die presses to a minimum forging ratio of 3:1, higher for critical parts, which closes porosity and develops a grain flow following the part contour. Rings are punched and expanded on a ring-rolling mill to produce a true seamless rolled ring rather than a welded or plate-cut substitute.

Testing scope

Note on NDT standard selection. EN 10228-3 applies to ultrasonic testing of ferritic and martensitic steel forgings. 1.4550 is austenitic, so the correct reference is EN 10228-4, which accounts for the coarse anisotropic grain structure that scatters ultrasound in austenitic material. For the same reason magnetic particle inspection cannot be used on 1.4550 and is replaced by liquid penetrant testing.

Certification

Material test certificates are issued to EN 10204 3.1, or to EN 10204 3.2 with third-party witness and counter-signature by TUV, BV, Lloyd's Register, DNV, ABS or SGS. Traceability from heat number through forging, heat treatment and NDT is maintained on every piece, with the heat number hard stamped or vibro-etched on the part.

Applications for 1.4550 forgings by industry

Grade comparison: 1.4550, 1.4541 and 1.4404

For help selecting between these grades, send the operating temperature, the medium and the chloride level to sales@steelforgepieces.com and our metallurgical staff will return a recommendation with the reasoning.

Manufacturer details

Jiangyin Jiangnan Metal Co., Ltd. is an open-die forging factory in Jiangyin, Jiangsu Province, China. We forge to customer drawings in carbon steel, alloy steel, tool steel, stainless steel and nickel-based alloys. Regular output covers forged rings, seamless rolled rings, flanges, round bars, discs, shafts, sleeves, bushings, tube sheets, valve components and forged pipes.

For stainless grades such as 1.4550 we control the full route under one traceable heat number: melting specification, forging ratio, heat treatment, machining allowance, NDT and certification. We work to EN, ASTM and ASME, JIS and GB standards, and to customer-specific specifications.

This page is maintained by the engineering staff of Jiangyin Jiangnan Metal Co., Ltd. Data is compiled from EN 10088-3, EN 10250-4, EN 10222-5, ASTM A182, ASTM A473 and ASTM A965, and from our own production and test records. Values are typical and given for guidance. The certified test report for the supplied heat governs in all cases. Last reviewed 25 September 2026.

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