SS 904L – Chemical Composition, Equivalents & Products
SS 904L is a high-alloy, low-carbon austenitic stainless steel – often called super-austenitic – with about 20% chromium, 25% nickel, 4.5% molybdenum and 1.5% copper. The high nickel and molybdenum give much better resistance than 316L to pitting, crevice corrosion and chloride stress corrosion cracking, and the copper gives good resistance to sulphuric acid. The very low carbon content keeps welds resistant to intergranular corrosion.
Giriraj Impex supplies SS 904L in every product form – we manufacture flanges, fittings, bars and fasteners, and stock and supply pipes, tubes, plates and sheets.
Chemical composition
| Grade | UNS | C | Mn | Si | P | S | Cr | Ni | Mo | Other elements |
|---|---|---|---|---|---|---|---|---|---|---|
| 310S | S31008 | 0.08 | 2.00 | 1.50 | 0.045 | 0.030 | 24.0–26.0 | 19.0–22.0 | 0.75 | – |
| 321 | S32100 | 0.08 | 2.00 | 0.75 | 0.045 | 0.030 | 17.0–19.0 | 9.0–12.0 | – | N 0.10; Ti 5 × (C + N) min, 0.70 max |
| 904L | N08904 | 0.020 | 2.00 | 1.00 | 0.045 | 0.035 | 19.0–23.0 | 23.0–28.0 | 4.0–5.0 | Cu 1.0–2.0; N 0.10 |
Limits for plate, sheet and strip to ASTM A240. Pipe (A312), bar (A276/A479) and forging (A182) limits are similar; check the standard for the product form.
904L is assigned a UNS number in the N series (N08904) because of its high nickel content, although it is classed as a stainless steel. The mill test certificate shows the standard and actual analysis.
Mechanical properties
| Grade | Tensile strength, MPa (ksi) | Yield strength 0.2%, MPa (ksi) | Elongation, % | Hardness max |
|---|---|---|---|---|
| 310S | 515 (75) | 205 (30) | 40 | 217 HB |
| 321 | 515 (75) | 205 (30) | 40 | 217 HB |
| 904L | 490 (71) | 220 (31) | 35 | 90 HRB |
Values for plate, sheet and strip to ASTM A240.
904L has slightly lower minimum tensile strength than 316L but a slightly higher yield strength, with good ductility and toughness, including at low temperatures.
Equivalent grades
| Grade | UNS | EN number | EN name | JIS | BS |
|---|---|---|---|---|---|
| 310S | S31008 | 1.4845 | X8CrNi25-21 | SUS310S | 310S16 |
| 321 | S32100 | 1.4541 | X6CrNiTi18-10 | SUS321 | 321S31 |
| 904L | N08904 | 1.4539 | X1NiCrMoCu25-20-5 | SUS890L | 904S13 |
Equivalent grades are similar but not identical; composition and property limits differ between standards. Order to the standard you need.
See the full grade equivalent chart for other grades and systems.
Physical properties
| Property | 310S | 321 | 904L |
|---|---|---|---|
| Density, g/cm³ | 7.9 | 7.9 | 8.0 |
| Melting range, °C | approx. 1400–1450 | approx. 1400–1425 | approx. 1300–1390 |
| Thermal conductivity at 100 °C, W/m·K | approx. 14 | approx. 16 | approx. 13 |
| Modulus of elasticity, GPa | approx. 200 | approx. 193 | approx. 195 |
| Magnetic (annealed) | No | No | No |
Typical values from recognised industry references, for guidance only; they are not specification limits.
Corrosion resistance
Sulphuric acid: the combination of nickel, molybdenum and copper makes 904L one of the few stainless steels that resist sulphuric acid across a useful range of concentrations and temperatures, which is its original purpose.
Chlorides: with a pitting resistance equivalent number (PREN) of about 34–36, 904L resists pitting and crevice corrosion much better than 316L (about 24–26), and its high nickel gives strong resistance to chloride stress corrosion cracking. In warm seawater, however, 6Mo super-austenitic grades or super duplex steels are preferred.
Other media: 904L performs well in phosphoric acid, many organic acids, and process streams containing halides that would attack 316L.
SS 904L products we supply
- Seamless pipes and welded pipes – seamless to ASTM B677, welded to ASTM B673.
- Tubes – ASTM B677 seamless and B674 welded, for heat exchangers.
- Flanges and forged fittings – ASTM A182 F904L and ASTM B649 bar.
- Butt-weld fittings – ASTM A403 WP904L.
- Sheets and plates – ASTM A240 and B625 N08904.
- Round bars – ASTM B649 and A479 N08904.
All stainless steel grades and forms are on the stainless steel page.
Comparable grades
SS 316L is the standard step down, adequate for many chloride and acid duties at lower cost. Duplex 2205 has a similar PREN to 904L and about twice the yield strength, and is often chosen where strength and chloride resistance matter more than resistance to sulphuric acid. 6Mo super-austenitic grades such as UNS S31254 and super duplex steels are used for seawater and more severe chloride conditions, and nickel alloys such as Incoloy 825 and Hastelloy C276 where 904L is not enough.
Typical applications of SS 904L
904L is used for sulphuric acid plants and coolers, phosphoric acid and fertiliser plants, pulp and paper bleach equipment, flue-gas desulphurisation and scrubbers, seawater-cooled heat exchangers and condensers in moderate conditions, pharmaceutical and fine-chemical process equipment, oil and gas process components, and pickling equipment. Its corrosion resistance and ability to take a fine polish also make it a choice for some high-end consumer products.
Fabrication
904L forms well cold, though it needs more force than 316L, and is welded by all common processes. Matching 904L filler or over-alloyed nickel-based fillers such as ERNiCrMo-3 are used, with low heat input and clean joints, because fully austenitic welds can be sensitive to hot cracking. After welding, pickling and passivation restore the corrosion resistance of the weld zone. Post-weld heat treatment is not normally needed.
Frequently Asked Questions
SS 904L is a super-austenitic stainless steel with about 20% chromium, 25% nickel, 4.5% molybdenum and 1.5% copper, designed for sulphuric acid and chloride service.
904L has much more nickel and molybdenum, plus copper, giving far better resistance to pitting, chloride stress corrosion cracking and sulphuric acid than 316L.
SS 904L is UNS N08904. The EN equivalent is 1.4539 (X1NiCrMoCu25-20-5).
The pitting resistance equivalent number of 904L is typically about 34–36, compared with about 24–26 for 316L.
904L is used in moderate seawater conditions, but for warm seawater, 6Mo super-austenitic or super duplex steels are preferred.
No. 904L is fully austenitic and non-magnetic in the annealed condition.
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