Super Duplex S32550
S32550 is a 25.5Cr-5.5Ni-3.5Mo-2Cu super duplex stainless steel with copper as a deliberate alloying element. It has an austenite-ferrite dual-phase microstructure, with tensile strength above 760 MPa and a PREN typically in the 38–42 range.
1. Chemical Composition
The table below lists the controlled elemental ranges for S32550. The copper and nitrogen additions are particularly relevant, as they shape the grade's corrosion behavior in acidic environments.
| Grade | C | Mn | P | S | Si | Cr | Ni | Mo | N | Cu |
| S32550 | ≤ 0.04 | ≤ 1.5 | ≤ 0.04 | ≤ 0.03 | ≤ 1.0 | 24.0–27.0 | 4.5–6.5 | 2.9–3.9 | 0.1–0.25 | 1.5–2.5 |
2. Mechanical Properties
These minimum mechanical requirements apply to S32550 in the annealed condition. The dual-phase microstructure gives it higher strength than single-phase austenitic grades.
| Grade | Condition | Tensile Strength Min, MPa (ksi) | Yield Strength Min, MPa (ksi) | Elongation Min, % | PREN |
| S32550 | Annealed | ≥ 760 (110) | ≥ 550 (80) | ≥ 15% | ≥38 |
3. Equivalent Grade
The table below cross-references S32550 across international standards systems for global procurement and equivalency confirmation in engineering specifications.
| GRADE | UNS | GB | JIS | ISO | DIN/EN | GOST | |
| ISC | NEW | ||||||
| 255 | S32550 | S25554 | 03Cr25Ni6Mo3Cu2N | SUS 329J4L | X2CrNiMoCuN25-6-3 | 1.4507 | - |
4. Key Technical Advantages
- Copper for Reducing Acid Resistance: The 1.5–2.5% Cu content gives this grade better resistance to reducing acids, particularly sulfuric acid at moderate concentrations. Copper-free duplex grades corrode faster in these conditions.
- High Chromium-Molybdenum Synergy: The 24–27% Cr combined with 2.9–3.9% Mo creates a stable passive film in chloride environments. Nitrogen (0.10–0.25%) stabilises the austenite phase and adds to pitting resistance.
- Mechanical Advantage Over Austenitic Grades: A yield strength of >=550 MPa allows engineered replacement of heavier-walled 316L or 317L piping with thinner-wall S32550 tubing. This reduces installed weight without losing pressure containment.
Technical Note: S32550 is commercially offered as Ferralium 255. Welding requires control of heat input to preserve the duplex phase ratio. Insufficient heat input (rapid cooling) produces excess ferrite, while excessive heat input promotes intermetallic precipitation. Both degrade toughness and corrosion resistance.
5. Common Manufacturing Standards
ASTM A790: Standard specification for seamless and welded ferritic/austenitic stainless steel pipe. It covers UNS S32550 in pipe configurations for pressure systems.
ASTM A789: Standard specification for seamless and welded ferritic/austenitic stainless steel tubing in general service, applicable to S32550 tubular products.
DIN EN 10216-5: European standard for seamless steel tubes for pressure purposes. It covers stainless steel designation 1.4507, the EN equivalent of S32550/Ferralium 255.
ASTM A276: Standard specification for stainless steel bars and shapes. S32550 bar stock produced for fittings and valve components is commonly supplied to this standard.
Manufacturing Standards Comparison Table:
| Standard | GB | EN/DIN | JIS | GOST |
| ASTM A790 (Pipe) | - | EN 10216-5 (1.4507) | - | - |
| ASTM A789 (Tube) | - | EN 10216-5 (1.4507) | - | - |
6. Primary Applications
- Flue Gas Desulfurization (FGD): Absorber vessels, spray headers, and ductwork handling hot acidic scrubber liquors containing chlorides and sulfur compounds in power plant wet scrubbing systems.
- Phosphoric Acid Production: Process piping and heat exchangers in wet-process phosphoric acid plants. Copper-bearing alloys outperform standard duplex in reducing acid conditions.
- Seawater Desalination: Pump casings, impellers, and piping in seawater reverse osmosis systems that need both high mechanical strength and seawater corrosion resistance.
- Oil and Gas Downhole Tools: High-strength tubular components and mandrels in completion strings where corrosion resistance and yield strength both drive material selection.
- Pulp Bleaching Equipment: Chlorine dioxide bleaching stage piping and towers exposed to elevated chloride concentrations and oxidizing conditions that challenge standard 2205 duplex.