SS 904L: properties, composition, uses and standards
SS 904L is a super-austenitic stainless steel with high nickel, molybdenum and copper, chosen for dilute sulphuric acid, phosphoric acid and chloride-bearing cooling water. UNS N08904, W.Nr 1.4539, PREN ≈ 35, density 7.95 g/cm³.
What is SS 904L?
SS 904L is a super-austenitic stainless steel with high nickel, molybdenum and copper. Engineers choose it for dilute sulphuric acid, phosphoric acid and chloride-bearing cooling water. It is also known as Alloy 904L, UNS N08904, W.Nr 1.4539, 2RK65.
- UNS
- N08904
- W.Nr
- 1.4539
- EN
- X1NiCrMoCu25-20-5
- JIS
- SUS 890L
- Trade names
- 2RK65 · Uranus B6
- Density
- 7.95 g/cm³
- Melting range
- 1300 – 1390 °C
- Magnetic
- Non-magnetic (annealed)
- PREN (approx.)
- ≈ 35
Chemical composition and what each element does
| Element | Weight % |
|---|---|
| Nickel | 23.0 – 28.0 |
| Chromium | 19.0 – 23.0 |
| Molybdenum | 4.0 – 5.0 |
| Copper | 1.0 – 2.0 |
| Carbon | 0.020 max |
| Manganese | 2.00 max |
| Silicon | 1.00 max |
| Nitrogen | 0.10 max |
| Phosphorus | 0.045 max |
| Sulphur | 0.035 max |
| Iron | Balance |
Per ASTM A240 for UNS N08904.
About 25% nickel gives far better resistance to chloride cracking than 316L.
4–5% molybdenum lifts pitting resistance to a PREN of about 35.
Copper is what makes 904L the stainless steel of choice for dilute sulphuric acid.
Very low carbon limits carbide precipitation, so welds keep their corrosion resistance.
SS 904L at a glance
See the SS 904L product page for every form we supply and its standard.
Mechanical properties
| Property | Value |
|---|---|
| Tensile strength | 490 MPa · 71 ksi |
| Yield strength 0.2% | 220 MPa · 31 ksi |
| Elongation | 35% |
| Hardness | 90 HRB max |
Annealed plate & sheet, minimum values unless stated. Values for pipe, bar and forgings differ slightly — see the product standard.
Where SS 904L is used
- Sulphuric acid plants. Coolers, piping and tanks for dilute sulphuric acid. Industry →
- Phosphoric acid & fertiliser. Evaporators, piping and scrubbers. Industry →
- Brackish cooling water. Heat-exchanger tubes and piping for brackish and chloride-bearing cooling water on coastal plants. Industry →
- Pulp & paper. Bleach-plant equipment exposed to chlorides. Industry →
- Pharmaceutical & food. Process vessels where 316L pits. Industry →
- Oil & gas. Heat exchangers and piping in moderately corrosive service. Industry →
Welding 904L
The matching filler is ER385 (AWS A5.9). ERNiCrMo-3 for severe chloride service.
Keep heat input moderate and interpass temperature low, clean joints thoroughly, and pickle or passivate after welding where the service is corrosive.
SS 904L compared with similar grades
| Grade | PREN | Choose it for |
|---|---|---|
| SS 904L | ≈ 35 | Dilute sulphuric acid, phosphoric acid and chloride-bearing cooling water |
| SS 316 / 316L | ≈ 24 | Chloride-bearing process fluids, marine air and pharmaceutical service |
| Duplex 2205 | ≈ 35 | Strength with chloride resistance at a stainless-steel budget |
| Inconel 825 | ≈ 31 | Sulphuric and phosphoric acid service, chloride stress-corrosion cracking and sour gas |
SS 904L standards by product
Buying SS 904L: what to specify
- The product standard and the UNS or designation — for example ASTM A240 UNS N08904.
- Size, thickness or schedule, and quantity in pieces and kg or metres.
- Heat-treatment condition where the standard offers a choice.
- Certificate: EN 10204 3.1 as standard, 3.2 if a third party must witness.