904L Stainless Steel ASTM Equivalent: UNS N08904 Specifications and Properties

What 904L Is and Why the ASTM Equivalent Question Is Asked

904L is a super-austenitic stainless steel containing about 25 percent nickel, 20 percent chromium, 4.5 percent molybdenum and 1.5 percent copper, developed for aggressive acid and chloride service where grade 316 is not sufficient. The designation 904L is a widely used trade name rather than the standard reference, which is the reason users search for the ASTM equivalent. In the unified numbering system the alloy is UNS N08904, and in the European system it is EN 1.4539, designated X1NiCrMoCu25-20-5. Other systems list it as JIS SUS 890L and as AFNOR Z2 NCDU 25-20, and all of these describe the same composition range.

The single most important point about the ASTM equivalent is that N08904 is not covered by the austenitic stainless steel pipe specifications. ASTM A312 and ASTM A358 list the chromium-nickel grades in the UNS S3xxxx series, and their tables do not include N08904. Because the alloy carries UNS N-numbering, it is handled by the nickel and nickel alloy specifications in the ASTM B series, and any order that calls for 904L pipe under an A312 reference needs to be corrected before the material is produced.

The ASTM and ASME Specifications That Cover UNS N08904

Product form	Specification	Scope
Plate, sheet and strip	ASTM A240 / ASME SA240	Chromium and chromium-nickel stainless plate for pressure vessels, including N08904
Seamless pipe and tube	ASTM B677	Seamless N08904 pipe and tube for corrosive service
Welded pipe	ASTM B675	Welded N08904 pipe
Welded tube	ASTM B676	Welded N08904 tube
Bar and rod	ASTM A276, ASTM A479	Bars for machined components and for bolting and fasteners
Fittings	ASTM B366	Factory-made wrought nickel alloy fittings, including N08904
Forgings	ASTM B564	Nickel alloy forgings
Welding consumables	AWS A5.9 / ASME SFA-5.9	ER385 bare wire and E385 covered electrode, the matching 20 25 5 Cu L classification

Note that the plate standard A240 does include N08904 explicitly, so a 904L vessel or heat exchanger plate order is straightforward. The equivalent pipe route is B677 for seamless product and B675 for welded product, with B366 covering elbows, tees and reducers. Where a project insists on an austenitic stainless reference, the material must still be ordered to a specification whose table actually lists UNS N08904.

Chemical Composition of 904L

Composition limits for UNS N08904 are given below as percentages by mass.

Element	Limit
Carbon	0.020 max
Manganese	2.00 max
Phosphorus	0.045 max
Sulphur	0.035 max
Silicon	1.00 max
Nickel	23.0 to 28.0
Chromium	19.0 to 23.0
Molybdenum	4.0 to 5.0
Copper	1.0 to 2.0

Three elements control the performance. Molybdenum at 4 to 5 percent and chromium at 19 to 23 percent lift the pitting resistance equivalent number, calculated as chromium plus 3.3 times molybdenum, to roughly 34 for a nominal 20 percent chromium and 4.5 percent molybdenum composition, compared with about 24 to 26 for grade 316 and 18 to 20 for grade 304. Copper improves resistance to sulphuric acid and to reducing acids generally, and nickel at 23 to 28 percent stabilises the fully austenitic structure and raises resistance to stress corrosion cracking in chlorides. The very low carbon limit of 0.020 percent keeps sensitisation risk to a minimum even in the heat affected zone of welds.

Mechanical Properties

Tensile strength: 490 MPa minimum.

0.2 percent proof stress: 220 MPa minimum.

Elongation in 50 mm: 35 percent minimum.

Density: about 8.0 g per cubic centimetre.

Structure: fully austenitic, with the higher strength and lower ductility typical of the super-austenitic family relative to grade 316.

The higher proof stress than grade 316 means thinner walls can often be used for the same design pressure, which partly offsets the higher price per kilogram. A design calculation must still use the allowable stress values listed for N08904 in the governing pressure vessel or piping code, and must allow for the lower thermal conductivity of the alloy, which affects heat transfer and the risk of thermal fatigue in thick sections.

Corrosion Behaviour and Service Limits

904L was developed for sulphuric acid service and performs well across a wide range of concentrations and temperatures, including conditions where grade 316 corrodes rapidly. It also gives good results in phosphoric acid, acetic acid and other reducing acid media, and it resists pitting and crevice attack in chloride-bearing water far better than 316 thanks to the molybdenum content. The high nickel content delays chloride stress corrosion cracking, and the low carbon level means welded joints retain intergranular corrosion resistance without a post-weld heat treatment. It is not, however, a universal answer: strongly oxidising environments and very high chloride concentrations combined with elevated temperature still call for a higher alloy, and the correct material should be confirmed against the specific medium, concentration and temperature before ordering.

Welding and Fabrication of 904L

Use a matching filler: ER385 wire or E385 electrode to AWS A5.9, which gives a deposit close in composition to the parent metal and preserves corrosion resistance.

The weld pool is fully austenitic with no ferrite, so hot cracking is the main risk; low heat input, a slightly convex bead profile and controlled penetration reduce it.

Keep interpass temperature low, in the region of 150 degrees Celsius or below, and avoid excessive weaving.

Cleanliness is critical: stainless-only brushes and tools, degreasing before welding, and internal purge gas on pipe roots.

No post-weld heat treatment is normally required. Pickling and passivation after welding is recommended to remove oxide scale and restore the passive film.

Typical Applications and Ordering Checklist

904L plate, pipe, tube and fittings are used in chemical plant handling sulphuric and phosphoric acid, in phosphate fertiliser production, in seawater cooling and desalination systems, in pulp and paper bleach plants, in pharmaceutical process equipment, in acid gas and flue gas desulphurisation lines, and in heat exchangers where chlorides rule out 316. When ordering, state the UNS number N08904 rather than the trade name alone, name the product form and the correct specification, give size and wall thickness or schedule, specify the delivery condition, and state the inspection, test and documentation requirements. Confirming the UNS number and the specification at enquiry stage avoids the common problem of quoting a nickel alloy product against an austenitic stainless standard.

Frequently Asked Questions

Q: What is the ASTM equivalent of 904L?
The ASTM designation is UNS N08904. There is no single ASTM equivalent standard, because the alloy is covered by product specific specifications such as B677 for seamless pipe, B675 for welded pipe and A240 for plate.

Q: Is 904L the same as 1.4539?
Yes. EN 1.4539, designated X1NiCrMoCu25-20-5, describes the same alloy as UNS N08904; the two are simply the American and European designations of the same material.

Q: Can 904L pipe be supplied to ASTM A312?
No. A312 covers austenitic stainless grades in the UNS S3xxxx series and does not list N08904, so 904L pipe must be ordered to B677 or B675 instead.

Q: What welding consumables are used for 904L?
ER385 bare wire and E385 covered electrodes to AWS A5.9, matching the 20 25 5 Cu L composition, are the standard choice for joining this alloy.

Q: Is 904L better than 316 for seawater?
Yes in most cases. The much higher molybdenum content gives a pitting resistance equivalent number near 34 against 24 to 26 for 316, so 904L tolerates higher chloride levels and temperatures before pitting starts.

Q: Does 904L need post-weld heat treatment?
No. The very low carbon content means welded joints keep their intergranular corrosion resistance as welded, and pickling and passivation are normally all that is required.