Super austenitic stainless steel — 904L, 254 SMO, 6Mo, Alloy 20
Super austenitic stainless steels are high-alloy austenitic grades with much more nickel, molybdenum and nitrogen than 316L. That raises the pitting resistance equivalent number (PREN) from about 24 to 35–45, giving resistance to seawater, chlorides and strong acids where 316L would pit or crack.
What super austenitic stainless steel is
The main grades are 904L (N08904, about 4.5 % Mo plus copper, for sulphuric and phosphoric acid), the 6Mo grades 254 SMO (S31254, A182 F44) and Alloy 6XN (N08367, A182 F62) for seawater, and Alloy 20 (N08020) for hot sulphuric acid.
- Nickel 17–38 % and molybdenum 4–7 %
- Nitrogen 0.18–0.25 % in 6Mo grades for strength and pitting resistance
- Copper in 904L, 254 SMO and Alloy 20 for sulphuric acid
- PREN typically 35–45
Key properties
| PREN (Cr + 3.3Mo + 16N) | 904L ≈ 32–35 · 6Mo ≈ 42–45 |
|---|---|
| Minimum yield | 215 MPa (904L) – 310 MPa (6XN) |
| Minimum tensile | 490–655 MPa |
| Critical pitting temperature (6Mo) | Above 60 °C in ferric chloride |
| Chloride stress-corrosion cracking | Much better than 316L |
Super austenitic stainless steel grades compared
Chemistry and minimum room-temperature properties from the governing ASTM / ASME / API specifications. PREN = Cr + 3.3Mo + 16N using specification minimums. Open any grade for its full data sheet and PDF.
| Grade | Form | UNS | Cr% | Ni% | Mo% | N% | PRENmin | YieldMPa min | TensileMPa min |
|---|---|---|---|---|---|---|---|---|---|
| UNS N08904 (904L) | Forgings (A182 F904L) | N08904 | 19.0–23.0 | 23.0–28.0 | 4.0–5.0 | 0.10 max | 32 | 215 | 490 |
| ASTM A182 F44 (254 SMO) | Forged flanges & fittings | S31254 | 19.5–20.5 | 17.5–18.5 | 6.0–6.5 | 0.18–0.25 | 42 | 300 | 650 |
| ASTM A182 F62 (Alloy 6XN) | Forged flanges & fittings | N08367 | 20.0–22.0 | 23.5–25.5 | 6.0–7.0 | 0.18–0.25 | 43 | 310 | 655 |
| ASTM B462 Alloy 20 (UNS N08020) | Forged flanges & fittings | N08020 | 19.0–21.0 | 32.0–38.0 | 2.00–3.00 | — | 26 | 240 | 550 |
Reference information compiled from the published standard for convenience. It does not replace the current edition of the governing standard, which takes precedence. Verify critical dimensions and properties before design, purchase or fabrication. Teshi Engineers & Sales Promoters (P) Ltd accepts no liability for errors or omissions.
Specifications by product form
| Product form | Grades |
|---|---|
| Forgings (A182 F904L) | UNS N08904 (904L) |
| Forged flanges & fittings | ASTM A182 F44 (254 SMO), ASTM A182 F62 (Alloy 6XN), ASTM B462 Alloy 20 (UNS N08020) |
Where super austenitic stainless steel is used
- Seawater cooling and firewater systems
- Desalination (MSF, RO high-pressure)
- Sulphuric and phosphoric acid plants
- Pulp-and-paper bleach plants
- Offshore platforms and FPSOs
Welding and fabrication
Weld 6Mo grades with over-alloyed nickel consumables (ERNiCrMo-3 or -13) to keep weld-metal corrosion resistance, with low heat input and no preheat. Solution annealing after hot forming is essential to dissolve intermetallic phases.
FAQ
Super austenitic stainless steel: common questions
Is 904L better than 316L?
For acids and chlorides, yes. 904L has about 25 % nickel, 4.5 % molybdenum and 1.5 % copper, giving far better resistance to sulphuric acid and pitting. 316L is cheaper and adequate for milder service.
What is 254 SMO?
254 SMO is the trade name for UNS S31254, a 6 % molybdenum super austenitic stainless steel covered by ASTM A182 F44 for forgings. Its PREN of about 43 makes it suitable for seawater.
What is the PREN of 904L?
Using minimum values (Cr 19, Mo 4, N 0) it is about 32; typical values give about 35.
Super austenitic products from Teshi Group
Super Austenitic SS 904L & 254 SMO Flanges & Fittings
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ViewMaterial productsSS 904L Flanges
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ViewMaterial productsSS 904L Pipe Fittings
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ViewMaterial products254 SMO (F44) Flanges
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ViewMaterial productsAlloy 20 (Carpenter 20) Flanges
Grades, sizes and standards we supply
ViewOEM manufacturerIndustrial Flanges
ViewOEM manufacturerButt Weld Fittings
ViewStockist & supplierIndustrial Pipes
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