SMO 654
SMO 654 (UNS S32654) is the highest-alloyed grade in the SMO superaustenitic family. It contains 24% chromium, 22% nickel, 7.3% molybdenum, and 0.50% nitrogen. Its PREN exceeds 60, which places it at the boundary between superaustenitic steels and nickel-based alloys for resistance to pitting, crevice corrosion, and stress corrosion cracking in aggressive halide media.
1. Chemical Composition
The table below lists the elemental composition of 654 SMO. This grade represents the upper performance limit of commercial austenitic stainless steels in corrosion-resisting alloy content.
| Grade | C | Mn | P | S | Si | Cr | Ni | Mo | N | Other |
| SMO 654 (UNS S32654) | ≤ 0.020 | 2.00-4.00 | ≤ 0.030 | ≤ 0.005 | ≤ 0.50 | 24.0–25.0 | 21.0–23.0 | 7.0–8.0 | 0.40–0.50 | Cu: ≤ 0.3, Fe: Bal. |
Note: The manganese content (2.0–4.0%) is intentionally elevated to support the high nitrogen solubility needed for phase stability and to maintain a fully austenitic structure at this extreme alloy content.
2. Mechanical Properties
These are the minimum mechanical property values for 654 SMO in the solution-annealed condition. The high nitrogen content produces noticeably higher strength compared to lower-alloyed superaustenitics.
| Grade | Condition & Size | Standard | Heat Treating Temp. : min | Tensile Strength Min, MPa |
Yield Strength Min, MPa |
Elongation Min, % |
| 654SMO | - | A312 | 1150°C | ≥ 750 | ≥ 430 | ≥ 35 % |
Note: The yield strength of 654 SMO is roughly 85% above 316L. This allows for meaningful wall-thickness reductions in high-pressure seawater and acid service pressure components.
3. Equivalent Grade
This table maps 654 SMO against international designation systems. Few direct equivalents exist because its extreme alloy content sits above most regional standard grade ranges.
| GRADE | UNS | GB | JIS | ISO | DIN/EN | GOST | |
| ISC | NEW | ||||||
| 654 SMO | S32654 | S32652 | 015Cr20Ni18Mo6CuN | - | X1CrNiMoCuN24-22-6 | 1.4652 | - |
Note: 654 SMO has limited direct equivalents in GB, JIS, and GOST standards because of its proprietary development. EN 1.4652 is the recognized European equivalent.
4. Key Technical Advantages
- PREN Exceeding 60: With PREN [= %Cr + 3.3x%Mo + 16x%N] typically around 63–64, 654 SMO resists pitting initiation in seawater at temperatures well above 50°C. It also withstands chloride brine concentrations that cause rapid attack on 254 SMO and 904L, making it suitable for the most aggressive seawater desalination and brine concentration service.
- Ultra-High Nitrogen for Strength and Phase Stability: The nitrogen range of 0.45–0.55% is among the highest in commercial austenitic grades. It delivers yield strengths approaching nickel-based alloy territory while retaining ductility above 40% elongation. This allows pressure vessels and pipelines to be designed with thinner walls.
- Resistance to Crevice Corrosion in Hot Seawater: The combination of ultra-high molybdenum (7.0–8.0%) and nitrogen makes 654 SMO resistant to crevice corrosion initiation under heat exchanger tube support plates and at gasket interfaces in seawater above 35°C, a condition that initiates attack on 254 SMO in certain geometries.
- Technical Note: 654 SMO is difficult to hot-form due to its high strength at elevated temperatures. Cold-forming and solution annealing (typically 1150–1200°C, water-quenched) is the standard fabrication route. Welding must use high-alloy nickel-based filler metals (e.g., Alloy 625) to maintain corrosion performance in the weld metal. Sigma phase can form in the range 700–900°C; time at temperature during fabrication must be minimized.
5. Common Manufacturing Standards
ASTM A312: Standard specification for seamless, welded, and heavily cold-worked austenitic stainless steel pipes. UNS S32654 is listed within the expanded superaustenitic alloy group.
ASTM A213: Standard specification for seamless ferritic and austenitic alloy-steel boiler, superheater, and heat-exchanger tubes. S32654 is applicable for corrosion-service heat exchanger tubes.
ASTM A269: Standard specification for seamless and welded austenitic stainless steel tubing for general service, applicable to S32654.
DIN EN 10216-5: Seamless steel tubes for pressure purposes, stainless steel (Grade 1.4652 equivalent).
GB/T 14976: Seamless stainless steel tubes for fluid transport, applicable where S32654/1.4652 is accepted by material substitution review.
Manufacturing Standards Comparison Table:
| Standard | ASTM | EN/DIN | JIS | GB/T | GOST |
| Seamless Pipe | A312 S32654 | EN 10216-5 (1.4652) | — | GB/T 14976* | — |
| Seamless Tube | A213 S32654 | EN 10216-5 (1.4652) | — | GB/T 13296* | — |
| Welded Tube | A249 S32654 | EN 10217-7 (1.4652) | — | — | — |
*Subject to material substitution review; direct GB/T designation for S32654 is limited.
6. Primary Applications
- Seawater Desalination Brine Concentration: Brine heater tubes and high-pressure brine recirculation piping in MSF and MED desalination plants operating at the highest salinity and temperature conditions, where 254 SMO is no longer adequate.
- Offshore Produced Water Treatment: Produced water handling systems on offshore platforms where high chloride concentrations, dissolved H2S, and elevated temperatures create a corrosion environment beyond the capability of conventional superaustenitics.
- Chlorine and Chlorine Dioxide Service: Process piping and heat exchanger tubes in chlor-alkali plants and chlorine dioxide generation systems where both oxidizing conditions and high chloride concentrations are present at the same time.
- Deep-Sea Mining and Subsea Equipment: High-pressure subsea manifolds, flowlines, and connector bodies for deep-sea applications where 654 SMO's strength and corrosion resistance reduce the need for cathodic protection.
- Pulp Mill Evaporator Bodies: Black liquor evaporator equipment in kraft pulp mills where aggressive alkaline-chloride conditions and elevated temperatures cause stress corrosion cracking in lower-alloyed materials.