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 |
