X1CrNiMoCuN20-18-7 · UNS S31254 · PRE ≥43 · Fully Austenitic
Super austenitic 254 SMO — the highest PRE (≥43) stainless steel available without crossing into nickel alloy territory. 6% Mo and copper addition deliver exceptional chloride pitting and SCC resistance. Fully austenitic microstructure means no sigma phase temperature ceiling — unlike duplex grades, 1.4547 can be used above 250°C where duplex fails. The bridge between standard duplex and nickel alloys.
Duplex and super duplex stainless steels are limited to 250°C by sigma phase embrittlement. EN 1.4547 (254 SMO) is fully austenitic — there is no sigma phase transformation. It retains full toughness and corrosion resistance from cryogenic temperatures to approximately 350°C, making it the correct choice for hot seawater or chloride-bearing process above 250°C where duplex is code-restricted.
| System | Designation |
|---|---|
| EN Werkstoffnummer | 1.4547 |
| EN Grade Name | X1CrNiMoCuN20-18-7 |
| UNS | S31254 |
| Trade name | 254 SMO (Outokumpu/Avesta trademark) |
| ASTM Fittings | A403 (to S31254 chemistry) |
| PRE | ≥43 |
| AWS Filler (GTAW) | ERNiCrMo-3 (Inconel 625 filler) — no SS filler matches 6Mo heat |
| Element | 1.4547 (254 SMO) | 1.4410 (2507) | Purpose in 1.4547 |
|---|---|---|---|
| Cr | 19.5–20.5% | 24.0–26.0% | Baseline passive film |
| Ni | 17.5–18.5% | 6.0–8.0% | High Ni → fully austenitic, no sigma phase |
| Mo | 6.0–7.0% | 3.5–4.5% | Primary pitting resistance driver — PRE formula: 3.3×Mo |
| Cu | 0.50–1.00% | None | Additional pitting resistance in reducing acid environments |
| N | 0.18–0.22% | 0.24–0.32% | Stabilises austenite, contributes to PRE: 16×N |
| PRE (calc.) | ≥43 | ≥40 | — |
| Grade | W.Nr. | PRE | Temp Limit | Step Up When… |
|---|---|---|---|---|
| 316L | 1.4404 | ~24 | 450°C | Chlorides >200 ppm at ambient |
| Duplex 2205 | 1.4462 | ≥35 | 250°C | Warm seawater, hypochlorite dosing |
| Super Duplex 2507 | 1.4410 | ≥40 | 250°C | Hot seawater, sour brine >2205 PRE |
| 254 SMO | 1.4547 | ≥43 | ~350°C | Chloride service above 250°C; duplex code-restricted |
| Inconel 625 | 2.4856 | ~51 | 980°C | Extreme chloride, crevice seawater, high-temp |
| Hastelloy C-276 | 2.4819 | ~73 | 1040°C | Most aggressive chloride + reducing acid environments |
DN15–DN600 and larger. Outside diameters follow the EN 10220 / EN ISO 1127 pipe series; wall thickness is matched to your connecting pipe.
| Size | OD (mm) |
|---|---|
| DN 15 | 21.3 |
| DN 20 | 26.9 |
| DN 25 | 33.7 |
| DN 32 | 42.4 |
| DN 40 | 48.3 |
| DN 50 | 60.3 |
| DN 65 | 76.1 |
| DN 80 | 88.9 |
| DN 100 | 114.3 |
| DN 125 | 139.7 |
| DN 150 | 168.3 |
| DN 200 | 219.1 |
| DN 250 | 273.0 |
| DN 300 | 323.9 |
| DN 350 | 355.6 |
| DN 400 | 406.4 |
| DN 450 | 457.0 |
| DN 500 | 508.0 |
| DN 600 | 610.0 |
EN 1.4547 (254 SMO) is fully austenitic — no ferrite phase — so it has no sigma phase embrittlement temperature limit. Super Duplex 2507 (1.4410) is limited to 250°C by the sigma phase ceiling. 254 SMO maintains its PRE ≥43 and full toughness at temperatures up to 350°C. This makes 254 SMO the correct choice when service temperature exceeds 250°C AND high chloride resistance is required — for example, hot seawater piping above deck, process condensers above 250°C, or chloride-bearing process at intermediate temperatures above duplex limits. The tradeoff is cost: 254 SMO is more expensive than Super Duplex 2507.
With 6% molybdenum and copper it has a PRE of 43 or more, the highest of the stainless steels before nickel alloys.
X1CrNiMoCuN20-18-7 (UNS S31254).
