904L Stainless Steel Flanges Manufacturer — UNS N08904 / A182 F904L / EN 1.4539 High-Alloy Austenitic

904L is a high-alloy austenitic stainless steel, designated UNS N08904, ASTM A182 Grade F904L and EN 1.4539. It contains roughly 20% chromium, 25% nickel, 4.5% molybdenum and a 1.5% copper addition. The high nickel, molybdenum and copper give it far better resistance to acids — especially sulphuric and phosphoric — and to chloride pitting and stress-corrosion cracking than standard 316L. Its low carbon content (the “L”) keeps it resistant to intergranular corrosion after welding.

904L is UNS N08904. For forged flanges the ASTM grade is A182 F904L (ASME SA182 F904L); plate and sheet are ASTM A240 N08904, pipe is ASTM A312 / A790 N08904, fittings are ASTM A403 WP904L, and bar/wire is ASTM B649. The European designation is EN 1.4539, material name X1NiCrMoCu25-20-5. A flange marked N08904, F904L or 1.4539 is the same 904L material.

Typical N08904 / F904L composition: carbon 0.020% max, chromium 19.0–23.0%, nickel 23.0–28.0%, molybdenum 4.0–5.0%, copper 1.0–2.0%, manganese 2.0% max, silicon 1.0% max, phosphorus 0.045% max, sulphur 0.035% max, balance iron. The high nickel and the deliberate copper addition (about 1.5%) are what give 904L its hallmark resistance to sulphuric acid.

In the solution-annealed condition, A182 F904L / N08904 has tensile strength about 490 MPa (71 ksi) minimum, 0.2% yield about 220 MPa (32 ksi) minimum, elongation about 35% minimum, hardness up to ~90 HRB (~240 HBW), and density about 8.0 g/cm³. The steel is fully austenitic, tough and essentially non-magnetic.

904L has a PREN (= %Cr + 3.3×%Mo + 16×%N) of about 35. PREN indexes resistance to chloride pitting and crevice corrosion. At ~35, 904L sits well above 316L (~24) and on par with duplex 2205, but below the 6-molybdenum super austenitics such as 254 SMO (~43). So 904L is a strong all-rounder for acids and moderate chloride service, while 254 SMO is preferred for the most aggressive seawater duty.

904L is far more highly alloyed: about 25% nickel, 4.5% molybdenum and 1.5% copper, against roughly 10% nickel, 2% molybdenum and no copper in 316L. That gives 904L a PREN of ~35 versus ~24, dramatically better resistance to sulphuric and phosphoric acid, and much greater resistance to chloride stress-corrosion cracking. Where 316L corrodes in warm acids or chloride streams, 904L keeps working.

Both are high-alloy austenitic stainless steels, but they target different enemies. 254 SMO has about 6% molybdenum and a nitrogen addition, giving a PREN of ~43 — the better choice for warm seawater, desalination and the most aggressive chloride service. 904L has about 4.5% molybdenum and a copper addition, giving it the edge in sulphuric and phosphoric acid. For pure chloride pitting resistance choose 254 SMO; for reducing acids such as sulphuric, choose 904L.

904L is austenitic while 2205 is duplex (austenitic-ferritic). Their PREN values are similar (~35), but 2205 has roughly double the yield strength and is more economical, whereas 904L offers better toughness over a wider temperature range, non-magnetic behaviour, easier fabrication and superior performance in reducing acids such as sulphuric. The choice depends on whether strength or acid resistance and fabricability matter most.

The copper addition (about 1.5%) is the key. Copper, together with the high nickel and molybdenum, suppresses corrosion in reducing acids — particularly sulphuric acid across a wide range of concentrations and temperatures, and phosphoric acid. This is why 904L was originally developed and why it remains a standard material for sulphuric acid plants, pickling lines and acid handling where ordinary stainless steels fail.

All standard types — weld neck (WNRF), slip-on (SORF), blind, socket-weld, threaded, lap-joint (stub-end), spectacle blind, RTJ and orifice — to ASME B16.5 (½″–24″), ASME B16.47 / MSS SP-44 for larger diameters, and EN 1092-1 (1.4539). Classes 150 to 2500 with RF, FF and RTJ facings. Custom sizes are made to drawing.

Wherever acids and chlorides defeat ordinary stainless: sulphuric acid production and handling, pickling plants, phosphoric acid and fertilizer plants, pulp and paper bleaching, flue-gas desulphurization (FGD), seawater and brackish water systems, organic acids such as acetic and formic, oil refining, and pharmaceutical processing. It is often a cost-effective alternative to nickel alloys in acid service.

By GTAW, GMAW, SMAW and SAW. It can be welded with a matching 904L filler, but for the most demanding corrosion service an over-alloyed nickel-base filler such as ERNiCrMo-3 (Alloy 625 type) is often preferred to keep weld corrosion resistance equal to the base metal. No preheat is required; interpass temperature is kept low and heat input controlled, and no post-weld heat treatment is normally needed because of the low carbon content.

Yes. Every 904L flange ships with EN 10204 Type 3.1 (or 3.2 third-party witnessed) mill test certificates confirming chemistry, mechanical properties, solution-annealed condition and heat traceability, with PMI / XRF confirming the molybdenum and copper content, plus hydrostatic testing and third-party inspection on request. Tesco Steel & Engineering is ISO 9001:2015 certified, based in Mumbai, India.

No. 904L is fully austenitic, so it is essentially non-magnetic in the solution-annealed condition (a slight magnetic permeability can develop after heavy cold work). This distinguishes it from duplex grades such as 2205, which are magnetic because of their ferrite content.