F55
F55 is a super duplex stainless steel (UNS S32760). Through the “five – element synergy of Cr – Mo – N – W – Cu”, it achieves chloride corrosion resistance 4 times that of austenitic steel and strength twice that of it. The following is a systematic analysis from six dimensions:
Ⅰ. Standard System and Grade Codes
1. Core Executive Standards
- Forgings/Flanges: ASTM A182 (grade “F55”, specifying high – temperature pressure – bearing components such as valves and Christmas trees);
- Bars/Profiles: ASTM A276 (machined parts such as bolts and shafts), ASTM A479 (structural profiles);
- Plates/Pipes: ASTM A240 (pressure vessel liners), EN 10088 – 3 (European standard);
- Chinese National Standard: 022Cr25Ni7Mo4WCuN (low – carbon and highly alloyed to enhance corrosion resistance).
2. Global Grade Correspondences
| System/Region | Grade | Description |
|---|---|---|
| U.S. UNS | UNS S32760 | Core identification (benchmark of super duplex steel) |
| European DIN | 1.4501 | Equivalent to F55 |
| Commercial Name | F55 Duplex Steel | Abbreviated due to Cr≈25% and Ni≈7%, most commonly used |
Ⅱ. Chemical Composition (Mass Fraction %, ASTM Standard)
F55 breaks through performance bottlenecks through “unique W/Cu addition + duplex structure (ferrite 40 – 60% + austenite 60 – 40%)”, and the roles of elements are precise:
| Element | Content Range | Core Role |
|---|---|---|
| C | ≤0.03 | Strictly control carbides, avoid intergranular corrosion (very low sensitization risk) |
| Cr | 24.0 – 26.0 | Ferrite is rich in Cr, forming Cr₂O₃ passive film, resisting pitting/uniform corrosion |
| Ni | 6.0 – 8.0 | Austenite is rich in Ni, stabilizes duplex structure, improves toughness and stress corrosion resistance |
| Mo | 3.0 – 4.0 | Enhance resistance to chloride pitting/crevice corrosion (synergizes with N) |
| N | 0.20 – 0.30 | Stabilize austenite, significantly increase strength (duplex steel strength ≈ twice that of austenite) |
| W ite) | ||
| W | 0.50 – 1.00 | Unique advantage! Improve crevice corrosion resistance (synergizes with Mo, enhances corrosion resistance) |
| Cu | 0.50 – 1.00 | Unique advantage! Improve corrosion resistance to reducing acids such as sulfuric acid |
| Mn/Si | ≤1.00/≤0.80 | Optimize workability, stabilize duplex proportion |
Pitting Resistance Equivalent Number (PREN):
PREN = %Cr + 3.3×%Mo + 16×%N + 1.5×%W ≈ 45 – 50 (far exceeding the 40 – 45 of F53/2507, and the chloride corrosion resistance is increased by more than 15%).
PREN = %Cr + 3.3×%Mo + 16×%N + 1.5×%W ≈ 45 – 50 (far exceeding the 40 – 45 of F53/2507, and the chloride corrosion resistance is increased by more than 15%).
Ⅲ. Mechanical Properties (After Solution Treatment, Room Temperature)
Due to the duplex structure + high N/W strengthening, the performance of F55 is better than that of conventional duplex steels (such as F53/2507):
| Performance Index | Typical Value (ASTM Requirement) | Comparison with F53 (2507) |
|---|---|---|
| Tensile Strength | ≥750 MPa | 5% higher (2507≥720 MPa) |
| Yield Strength | ≥550 MPa | Equal (2507≥550 MPa) |
| Elongation (δ₅) | ≥25% | Slightly higher (2507≥20%) |
| Hardness | 250 – 300 HB | Equal (2507≤310 HB) |
| Physical Properties | Density 7.85 g/cm³, thermal conductivity is better than austenite | Suitable for high – temperature heat exchange |
Ⅳ. Heat Treatment Requirements (Activating Duplex Advantages)
- Solution Treatment (Mandatory):
- Temperature: 1020−1100∘C (hold for 1 – 2 hours to homogenize the duplex structure and dissolve carbides / nitrides / tungstates);
- Cooling: Rapid water quenching (inhibit the precipitation of harmful phases such as σ phase and χ phase, and retain stable ferrite + austenite);
- Role: Maximize corrosion resistance and strength, laying the foundation for processing / service.
- Welding and Post – treatment:
- Welding Materials: Select ER2594 or ER2760 welding wires (match the high W – Cu composition to ensure the duplex proportion of the weld);
- Process: Strictly control heat input (avoid ferrite coarsening). For thick – walled parts, short – term solution treatment at 1050∘C is required after welding (restore corrosion resistance and prevent embrittlement in the weld heat – affected zone).
Ⅴ. Main Application Fields (Extreme Corrosion + High – Pressure Scenarios)
Relying on “chloride corrosion resistance × reducing acid resistance × ultra – high strength”, F55 dominates the following fields (performance exceeds F53, close to nickel – based alloys):
- Marine Engineering:
- Seawater desalination (high – pressure membrane shells, deep – sea water pipes): Resistant to pitting corrosion in 5 – 10% NaCl seawater (service life 4 times that of 316L, 1.2 times that of F53);
- Offshore platforms (drilling equipment, mooring systems): Resistant to seawater + high – pressure impact, reducing cost by 40% compared with titanium alloys.
- Energy and Chemical Industry:
- Acid oil and gas fields (wellhead valves, transmission pipes): Resistant to combined corrosion of H₂S + high Cl⁻ + sulfuric acid (complying with NACE MR0175, temperature resistance up to 300°C);
- Flue gas desulfurization (absorption towers, spray pipes): Resistant to HCl + sulfuric acid dew point corrosion (perforation resistance 20% higher than F53).
- Nuclear Power and Environmental Protection:
- Nuclear power (nuclear waste tanks, cooling pipelines): Resistant to radiation + high – temperature water corrosion, with long – term stability;
- Waste incineration (heat exchangers, flues): Resistant to short – term high temperature of 800°C + chloride corrosion (superior to F53’s upper limit of 600°C).
- High – end Manufacturing:
- Papermaking bleaching (digesters, bleaching towers): Resistant to chlorine – containing chemicals + sulfuric acid corrosion, avoiding pulp pollution;
- Food and pharmaceutical (high – concentration pickling liquid tanks): Resistant to organic acids + hygienic compliance (replacing Hastelloy, reducing cost by 30%).
Key Summary
- Core Advantages:
- Corrosion Resistance: PREN ≈ 50, the ability to resist chloride pitting/crevice corrosion is over 15% higher than that of F53, and W/Cu endows it with resistance to reducing acids;
- Mechanical Properties: Strength is comparable to that of F53, plasticity is better (elongation is 5% higher), and workability is better;
- Cost Ratio: Performance is close to that of Hastelloy C – 276, while the cost is only 1/3 of it.
- Limitations:
- Long – term service temperature ≤ 300°C (σ phase is prone to precipitate at high temperatures, leading to embrittlement);
- High difficulty in cold working (high strength requires customized molds/processes).
F55 (UNS S32760/1.4501) is “the peak performance of super duplex steel”. It overcomes the shortcomings of F53 through W/Cu alloying and becomes the ultimate solution in high – pressure, strong corrosion + reducing acid scenarios, balancing performance and cost, and replacing austenitic stainless steel and some nickel – based alloys.