Data completeness: partial. Last verified 19 August 2026.
Designations
| Common name | 904L |
| UNS | N08904 |
| EN number | 1.4539 |
| Class | High-alloy austenitic stainless steel |
| Specified by | ASTM A240/A240M, EN 10088-2 |
904L carries a UNS number beginning with N, not S, and that is not a clerical detail — at roughly 24% nickel it is classified with the nickel alloys rather than with the stainless steels, and priced accordingly. The alloy was designed for sulphuric acid, and copper is in it for that reason. Its pitting ranking of 34 is a side effect of the molybdenum, not the design intent, and buying 904L for chlorides alone is usually the expensive way to get there.
Properties, with what each one rests on
| Property | Value | Unit | Condition | Evidence | Source | Retrieved |
|---|---|---|---|---|---|---|
| Proof strength Rp0.2, minimum (EN) | 240 | MPa | cold rolled coil and sheet, 20 °C | E3 | EN 10088-2 via Outokumpu Ultra range datasheet — high-alloy austenitic stainless steels | 2026-08-20 |
| Tensile strength Rm, minimum | 530 | MPa | cold rolled; datasheet gives a 530–730 range | E3 | EN 10088-2 via Outokumpu Ultra range datasheet — high-alloy austenitic stainless steels | 2026-08-20 |
| Elongation A, minimum | 35 | % | cold rolled; A and A80 both 35 | E3 | EN 10088-2 via Outokumpu Ultra range datasheet — high-alloy austenitic stainless steels | 2026-08-20 |
| PREN | 34 | — | from typical composition, not from a heat | E3 | Outokumpu Ultra range datasheet — high-alloy austenitic stainless steels | 2026-08-20 |
| Density | 8.0 | g/cm³ | 20 °C | E3 | Outokumpu Ultra range datasheet — high-alloy austenitic stainless steels | 2026-08-20 |
| Modulus of elasticity | 195 | GPa | 20 °C | E3 | Outokumpu Ultra range datasheet — high-alloy austenitic stainless steels | 2026-08-20 |
| Thermal conductivity | 12 | W/(m·K) | 20 °C | E3 | Outokumpu Ultra range datasheet — high-alloy austenitic stainless steels | 2026-08-20 |
| Corrosion rates in sulphuric acid | not established | — | — | — | isocorrosion data is the reason this alloy exists and it is not in the datasheet consulted | — |
| Composition limits | not established | — | — | — | ASTM A240 is paywalled; the datasheet gives typical values, which are not limits | — |
| Critical pitting and crevice temperatures | not established | — | — | — | not in the datasheet consulted; for a high-alloy grade these decide the selection and the pitting ranking number does not | — |
7 sourced values, 3 gaps named. Publishing the gaps is more useful than filling them from an aggregator that does not say where its numbers came from. The routes to filling them are the standard itself, a mill datasheet, or a mill certificate for your actual heat — which supersedes both at E5.
Where the numbers come from
Proof strength Rp0.2, minimum (EN). Identical to 316L on the same product form. Ten times the alloy cost buys corrosion resistance, not strength — one of the most useful facts on this page, and the reason a 904L section is no thinner than a 316L one.
PREN. 19.8% Cr and 4.3% Mo with no nitrogen give 34.0 in the datasheet’s own formula. Below 2205 despite costing several times as much, because the ranking number does not know about sulphuric acid, which is what the alloy is for.
Thermal conductivity. The lowest of the austenitics in the datasheet consulted, which matters if the part is a heat exchanger and not just a vessel.
Why we do not just copy an aggregator. Because then this page would be a worse copy of MatWeb with no way for you to tell which rows to trust, and the one thing we have that MatWeb does not is the evidence column. Our coverage and its limits.
What 904L is usually chosen for
Sulphuric and phosphoric acid service, and mixed acids with chlorides in them, where austenitic grades below it corrode generally rather than pit. The copper addition is specifically there for reducing acids. Pickling lines, acid plants, flue gas cleaning and some pharmaceutical process equipment are where it earns the price.
And what it is usually rejected for
Price, and the fact that a cheaper grade usually does the job. Where the problem is chlorides rather than acid, 2205 has a higher pitting ranking, twice the proof strength and a fraction of the alloy content; where the problem is severe chlorides, 254 SMO is the better answer. 904L gets specified out of habit more often than out of analysis.
The number to be more careful with than any of the above is the difference between a specified minimum and a typical value — a floor the standard guarantees, versus what a supplier’s production usually does. The full version, and the most expensive routine mistake in material data.
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What this page does not cover
- No corrosion rate data, which is the only reason to choose this alloy over a duplex.
- Critical pitting and crevice temperatures are not established here.
- Elevated temperature strength, fatigue and fracture toughness.
- Composition limits, not established.
- Welding practice for high-alloy austenitics, including filler overmatching.
Generated from data/materials/904l.yaml. Regenerate rather than editing this file by hand — edits here are lost on the next build.