Super-Austenitic Stainless Steel
Super-austenitic stainless steels raise the chromium to 20–25 %, the nickel to 18–25 % and, above all, the molybdenum to 4–7 % (with 0.2–0.5 % nitrogen in the 6 Mo grades), so they stay fully austenitic — non-magnetic, tough to cryogenic temperatures, not hardenable — while resisting chlorides and acids far beyond 316L. 254 SMO and AL-6XN have pitting resistance numbers above 42 and critical pitting temperatures around 70 °C, against about 20 °C for 316L; 904L is the older 4.5 Mo grade for sulfuric and phosphoric acid. Yield strength stays modest, 220–350 MPa.
Popular super-austenitic stainless grades
Compare in the explorer| Grade | Yield, MPa | Tensile, MPa | Density, g/cm³ | Machinability | Weldability | Best for |
|---|---|---|---|---|---|---|
| 904L | 260 | 590 | 8 | Poor | Very good | Sulfuric and phosphoric acid; the first step above 316L |
| 254 SMO | 350 | 700 | 8 | Poor | Very good | Seawater, bleach plant, scrubbers; the 6 Mo standard |
| AL-6XN | 365 | 745 | 8.06 | Poor | Very good | Interchangeable 6 Mo grade; pharma and seawater piping |
| Alloy 926 | 360 | 700 | 8.1 | Poor | Very good | 6.5 Mo upgrade of 904L on EN drawings |
| 654 SMO | 490 | 820 | 8 | Poor | Good | PREN above 55; nearest stainless to C276 in chlorides |
| Alloy 28 | 260 | 620 | 8 | Poor | Very good | Sulfuric acid and sour-gas tubing |
| 27-7Mo | 405 | 800 | 8 | Poor | Good | 7 Mo high-nitrogen grade for the harshest chloride duty |
Choose super-austenitic stainless for
Seawater-cooled heat exchangers and condensers, desalination, flue-gas desulfurization scrubbers, pulp-bleach washers, sulfuric and phosphoric acid plant, chlorinated cooling water, pharmaceutical and food process lines and offshore piping — the step between 316L and a nickel alloy, where duplex is ruled out by temperature, magnetism or fabrication.
Look elsewhere when
Strength matters as much as corrosion — 2205 or 2507 give 450–550 MPa yield against about 300 and cost less for the same chloride duty below 300 °C; the acid is hot, reducing or mixed (Hastelloy C276 or Hastelloy C-22); the medium is hydrofluoric acid (Monel 400); or 316L or 317L already survives — there is no reason to pay for 6 % molybdenum in a fresh-water or mild-chemical part.
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Super-austenitic stainless by application
- Seawater and chlorinated water
- 254 SMO or AL-6XN for tube-and-shell exchangers, condensers, pump columns and piping in flowing seawater up to about 40 °C; 654 SMO where the water is hot or crevices are unavoidable.
- Sulfuric and phosphoric acid
- 904L for dilute and intermediate sulfuric acid and for phosphoric-acid plant; Alloy 28 or Alloy 926 when the acid is hotter or carries chlorides; Alloy 20 for the same duty on North American drawings.
- Scrubbers and bleach plant
- 254 SMO and AL-6XN sheet and plate for FGD absorbers, ducting and pulp-bleach washers; 654 SMO for the outlet and mist-eliminator zones that see the most concentrated chloride.
- Pharma and food process
- AL-6XN and 254 SMO tube for high-purity water, WFI and CIP circuits where rouging and chloride pitting of 316L are a recurring problem, electropolished to below 0.4 µm Ra.
Working with super-austenitic stainless
- Machining
- Like 316L but gummier and slower — expect about 60–70 % of the 316L cutting speed for the 6 Mo grades, with strong work hardening and stringy chips. Sharp positive carbide, rigid setups, heavy constant feed and generous coolant; 904L is the easiest of the group.
- Welding
- Never weld the 6 Mo grades with matching filler: molybdenum segregates as the weld solidifies and the joint pits before the plate does. Use the over-alloyed nickel fillers — ERNiCrMo-3 (625 type) or ERNiCrMo-12 for 254 SMO, AL-6XN and Alloy 926, ER385 or ERNiCrMo-3 for 904L — with low heat input, interpass below 100 °C, no preheat and no post-weld heat treatment. Pickle or electropolish the heat tint.
- Heat treatment
- Solution annealing only: 1,100–1,150 °C for 904L, 1,150–1,200 °C for the 6 Mo grades, followed by a water quench. Slow cooling or long holds between 550 and 1,000 °C form sigma and chi phases that cut toughness and pitting resistance, so long-term service stays below about 400 °C.
- Forming
- Forms and deep-draws like 316L with 35–40 % elongation but noticeably higher forces and springback. Sheet, plate and tube bend cold to normal stainless radii; heavily worked parts are solution-annealed rather than stress-relieved.
Other stainless steel series
Super-Austenitic Stainless Steel — frequently asked
What is super-austenitic stainless?
Super-austenitic stainless steels raise the chromium to 20–25 %, the nickel to 18–25 % and, above all, the molybdenum to 4–7 % (with 0.2–0.5 % nitrogen in the 6 Mo grades), so they stay fully austenitic — non-magnetic, tough to cryogenic temperatures, not hardenable — while resisting chlorides and acids far beyond 316L. 254 SMO and AL-6XN have pitting resistance numbers above 42 and critical pitting temperatures around 70 °C, against about 20 °C for 316L; 904L is the older 4.5 Mo grade for sulfuric and phosphoric acid. Yield strength stays modest, 220–350 MPa.
Which grade of super-austenitic stainless should I use?
904L when the enemy is acid rather than chloride: 20 % Cr, 25 % Ni, 4.5 % Mo and 1.5 % copper handle sulfuric acid across most concentrations up to about 40 °C, phosphoric acid and process liquors where 316L corrodes, and it is the cheapest step up. 254 SMO and AL-6XN are the 6 Mo grades — the standard answer for seawater heat exchangers, pulp-mill bleach plants, flue-gas scrubbers and pharmaceutical process piping, with 300–345 MPa yield; the two are interchangeable in most specifications, AL-6XN carrying slightly more nitrogen. Alloy 926 (1.4529) is 904L pushed to 6.5 % Mo, common on EN drawings. 654 SMO (7.3 % Mo, 0.5 % N, PREN about 56) approaches Hastelloy C276 in chlorides for a fraction of the nickel. Alloy 28 (27 % Cr, 31 % Ni) is the sulfuric-acid and sour-gas tubing grade. Castings are CN3MN for AL-6XN and CK3MCuN for 254 SMO.
Which grade of super-austenitic stainless is the strongest?
654 SMO has the highest yield strength in the series at 490 MPa; 1.4652 follows at 480 MPa. Values are for each grade's default condition.
Which grade of super-austenitic stainless is easiest to machine?
1.4552 (machinability fair, 35/100), then 1.4558 (30/100).
Is super-austenitic stainless heat-treatable?
No. These grades are strengthened by cold work and softened by annealing, not by heat treatment; the conditions are listed on each grade page.
904L or 316L — when is the upgrade worth it?
When there is sulfuric or phosphoric acid, or when 316L is pitting in chloride water. 904L raises the pitting resistance number from about 25 to about 35, lifts the critical pitting temperature from about 20 °C to about 40 °C, and its copper keeps it stable in sulfuric acid where 316L is attacked. It is not stronger — 220 MPa yield against 170 — and it is not the answer for hot seawater, where the 6 Mo grades or 2507 are needed.
254 SMO or 2205?
Same chloride territory, different structure. 2205 is stronger (450 MPa yield against about 300), cheaper and the choice for ambient seawater piping, tanks and structure. 254 SMO is non-magnetic, tough to cryogenic temperatures, has a higher critical pitting temperature (about 70 °C against 35 °C), can be used above the 300 °C ceiling of duplex, and welds without the ferrite-balance worries — so it takes over for hot or chlorinated seawater, scrubbers and pharmaceutical tube.