SMO 254
SMO 254 (UNS S31254) is a superaustenitic stainless steel with high molybdenum (6.0–6.5%), chromium (19.5–20.5%), and nitrogen content. Its PREN is typically 42-45. The alloy was developed for seawater and halide-containing media where conventional and high-alloy austenitic grades fall short.
1. Chemical Composition
The table below details the tightly controlled elemental composition of 254 SMO. Nitrogen and molybdenum are the primary strengthening and pitting-resistance agents.
| Grade | C | Mn | P | S | Si | Cr | Ni | Mo | N | Cu | Other |
| 254SMO | ≤ 0.020 | ≤ 1.00 | ≤ 0.030 | ≤ 0.010 | ≤ 0.80 | 19.50–20.50 | 17.50–18.50 | 6.00–6.50 | 0.18–0.25 | 0.50–1.00 | — |
Note: The narrow compositional window, especially for Mo (6.00–6.50%) and N (0.18–0.22%), is maintained to ensure consistent PREN between 42-45 and stable austenitic phase microstructure.
2. Mechanical Properties
The mechanical property values below are for 254 SMO in the solution-annealed condition as per ASTM and ASME requirements, applicable to seamless tube and pipe products.
| Grade | UNS | Tensile Min, ksi (MPa) | Yield Min, ksi (MPa) | Elong. Min, % |
| S31254 | — | ≥ 94 (650) | ≥ 44 (300) | ≥ 35 % |
Note: 254 SMO has significantly higher yield strength than 904L or 316L because of nitrogen solid-solution strengthening. This allows reduced wall thickness at equivalent pressure ratings.
3. Equivalent Grade
The table maps 254 SMO across major international designation systems for use in engineering specifications, third-party inspection certificates, and global procurement documentation.
| GRADE | UNS | GB | JIS | ISO | DIN/EN | GOST | |
| ISC | NEW | ||||||
| 254SMO | S31254 | S31252 | 015Cr20Ni18Mo6CuN | SUS312L | X1CrNiMCuN20-18-7 | 1.4547 | - |
4. Key Technical Advantages
- High PREN for Chloride Resistance: With a PREN [= %Cr + 3.3x%Mo + 16x%N] typically between 43 and 45, 254 SMO resists pitting initiation in seawater at temperatures up to 50-55°C and in chloride solutions where 316L, 317L, and 904L show active corrosion. It is the benchmark material for seawater service in this category.
- Nitrogen-Enhanced Strength and Stability: The addition of nitrogen (0.18–0.22%) has a dual function. It provides solid-solution strengthening and a yield strength of roughly 50% above 316L. They also stabilize the austenitic phase, suppressing sigma phase formation during fabrication and elevated-temperature service.
- Resistance to Stress Corrosion Cracking: The high nickel content (17.5–18.5%) combined with the austenitic structure provides 254 SMO high resistance to chloride-induced stress corrosion cracking (SCC). This failure affects lean austenitic grades at temperatures above 60°C.
Technical Note: 254 SMO requires welding with matching or overmatched filler metals (e.g., AWS ERNiCrMo-3 or ERNiCrMo-10) to preserve corrosion resistance in the weld metal. Standard 308/316 filler metals are not suitable. Post-weld heat treatment is generally not required due to the ultra-low carbon content. Solution annealing (1150–1200°C / water quench) may be beneficial after significant cold working to restore maximum corrosion resistance.
5. Common Manufacturing Standards
ASTM A312: Standard specification for seamless, welded, and heavily cold-worked austenitic stainless steel pipes. UNS S31254 is listed under the superaustenitic alloy group.
ASTM A213: Standard specification for seamless ferritic and austenitic alloy-steel boiler, superheater, and heat-exchanger tubes. S31254 is applicable for corrosion-sensitive heat exchanger service.
ASTM A269: Standard specification for seamless and welded austenitic stainless steel tubing for general service, applicable to S31254.
DIN EN 10216-5: Seamless steel tubes for pressure purposes, stainless steel (Grade 1.4547 equivalent).
GB/T 14976: Seamless stainless steel tubes for fluid transport, 015Cr20Ni18Mo6CuN designation.
Standards Comparison Table:
| Standard | ASTM | EN/DIN | JIS | GB/T | GOST |
| Seamless Pipe | A312 S31254 | EN 10216-5 (1.4547) | — | GB/T 14976 | — |
| Seamless Tube | A213 S31254 | EN 10216-5 (1.4547) | — | GB/T 13296 | — |
| Welded Tube | A249 S31254 | EN 10217-7 (1.4547) | — | GB/T 12771 | — |
6. Primary Applications
- Seawater Desalination: Multi-stage flash (MSF) and reverse osmosis (RO) plant piping, heat rejection tubes, and brine handling systems where continuous contact with high-salinity water at elevated temperatures demands PREN between 42-45.
- Offshore Platform Process Piping: Topside process piping, sea chest inlet screens, and overboard systems on FPSOs and fixed offshore platforms where untreated seawater is the process medium.
- Flue Gas Desulfurization: Absorber tower internals, spray nozzles, and recirculation pump casings in FGD systems handling hot, chloride-rich, sulfurous condensate at high flow velocities.
- Marine Heat Exchangers: Shell-and-tube heat exchangers using seawater as the cooling medium, where tube pitting failure from warm seawater is the primary degradation concern.
- Pulp Bleaching Circuits: Chlorine dioxide bleaching stages in kraft pulp mills where high-chloride bleach liquors at elevated temperatures are too aggressive for 317L or 904L.