F44
F44 (core code UNS S31254, commonly known as 254SMO) is a super austenitic stainless steel. With the cooperative design of high Cr, Mo, N and Cu, it achieves super corrosion resistance (especially chloride ion/sulfuric acid corrosion resistance), which is systematically analyzed from six dimensions as follows:
I. Standard system (multimodal coverage)
Different product forms of F44 correspond to the core standards of ASTM/ASME and are also compatible with international/Chinese national standards:
| Product Form | Executive Standard | Application Scenario |
|---|---|---|
| Forgings/Flanges | ASTM A182 / ASME SA – 182 | High – temperature and high – pressure pipeline connections (such as desulfurization flanges) |
| Plates/Strip | ASTM A240 / ASME SA – 240 | Lining of pressure vessels, equipment panels |
| Bars/Profiles | ASTM A276 | Machined parts (shafts, bolts) |
| Seamless/Welded Tubes | ASTM A312 / ASME SA – 312 | Chemical fluid, seawater transportation pipelines |
| International Compatibility | EN 10088 – 3 (Europe), JIS G4303 (Japan) | Adaptation for cross – border projects |
| Chinese Reference | GB/T 15180 (cold – rolled plate), GB/T 14975 (structural tube) | Connection with domestic engineering |
II. Name and code (international common name)
F44 is a common engineering name, with the core code UNS S31254. There are also the following identifications internationally:
- Trade names: 254SMO (the most commonly used, emphasizing high Mo corrosion resistance), NAS 185N (Japanese brand), 00Cr20Ni18Mo6CuN (Chinese old brand expression);
- European codes: W-Nr. 1.4547, X1CrNiMoCuN20-18-7 (chemical composition identification);
- Core positioning: UNS S31254 (Unified Numbering System of the United States, globally applicable).
III. Chemical Composition (Precise Corrosion Resistance Design)
F44 achieves ultra – strong corrosion resistance through the “synergy of four elements (Cr, Mo, N, Cu)”. The range and function of key elements are as follows:
| Element | Content Range | Core Function |
|---|---|---|
| C | ≤0.02% | Strictly control carbon to avoid intergranular corrosion (very low risk of carbide precipitation) |
| Cr | 19.5 – 20.5% | Form a Cr₂O₃ passivation film to resist uniform corrosion/high – temperature oxidation |
| Ni | 17.5 – 18.5% | Stabilize austenite and resist stress corrosion cracking (high Ni ensures toughness) |
| Mo | 6.0 – 6.5% | Enhance resistance to pitting/crevice corrosion (core in chloride – containing environments) |
| N | 0.18 – 0.25% | Improve strength and assist in pitting resistance (synergize with Mo, PREN↑) |
| Cu | 0.5 – 1.0% | Improve corrosion resistance in reducing acids such as sulfuric acid |
| Mn/Si | ≤1.00% / ≤0.70% | Stabilize austenite and assist in deoxidation |
| P/S | ≤0.03% / ≤0.01% | Strictly control impurities to reduce the risk of embrittlement/heat cracking |
Pitting Resistance Equivalent Number (PREN):
PREN = 9×%Cr + 3.3×%Mo + 16×%N ≈ 42 – 45 (far higher than 24 – 28 of 316L, and the resistance to chloride ion corrosion is increased by more than 50%).
PREN = 9×%Cr + 3.3×%Mo + 16×%N ≈ 42 – 45 (far higher than 24 – 28 of 316L, and the resistance to chloride ion corrosion is increased by more than 50%).
IV. Mechanical properties (both toughness and toughness)
After solution treatment, F44 has excellent room – temperature mechanical properties, and can be further strengthened by cold working:
| Performance Indicator | Typical Value (ASTM Requirement) | Remarks |
|---|---|---|
| Tensile Strength | ≥650 MPa | Actually can reach 700 – 850 MPa (N element contributes to strength) |
| Yield Strength (σ₀.₂) | ≥310 MPa | Outstanding strength among austenitic alloys, better than 304/316 |
| Elongation (δ₅) | ≥40% | Gauge length 50mm, extremely good plasticity (convenient for pipe bending, stamping) |
| Hardness | 182 – 223 HB | Hardness can rise to above 250 HB after cold working |
| Physical Properties | Density 8.24 g/cm³, melting point 1320 – 1390°C, non – magnetic | Suitable for non – magnetic environments (such as medical equipment) |
V. Heat treatment requirements (solid solution as the core)
F44 needs to maximize corrosion resistance and plasticity through solution annealing. Process parameters:
- Solution Treatment:
- Temperature: 1125 – 1175°C (hold for 1 – 2 hours to ensure uniform dissolution of elements such as N and Mo);
- Cooling: Rapid water cooling (quenching) to obtain a single austenitic structure and inhibit carbide precipitation;
- Necessity: Mandatory for all products before delivery; re – solution treatment is required after welding (to prevent sensitization of thick – walled parts, avoid staying in the 450 – 850°C range).
- Welding Key Points:
- Welding material: ERNiCrMo – 3 welding rod is recommended (high nickel and molybdenum, matching corrosion resistance);
- Process: Small heat input welding (to control heat input) to avoid sensitization in the heat – affected zone.
VI. Main Application Fields (High Corrosion Scenarios)
F44, due to its “resistance to chloride ions/sulfuric acid corrosion + high strength”, dominates the following extreme environments:
- Marine Engineering:
- Seawater desalination (evaporators, membrane modules, pipelines): Resists pitting/crevice corrosion of high – salinity seawater;
- Offshore platforms (oil/sea water pipelines, deep – sea equipment): Resists high pressure + corrosion, replacing titanium alloys to reduce costs.
- Chemical Industry:
- Sulfuric acid/phosphoric acid production (reaction kettles, pickling tanks, pipelines): Resists corrosion by medium – to – high concentration acids (performs better than 904L in chlorine – containing sulfuric acid);
- Flue gas desulfurization (FGD absorption towers, spray pipes): Resists chloride + sulfuric acid dew point corrosion, with a service life 2 – 3 times that of 316L.
- Environmental Protection and Energy:
- Waste incineration (heat exchangers, flues): Resists HCl + high – temperature oxidation (stable at 800°C for a short time);
- Nuclear power condensation systems (pipes): Resists seawater + radiation, replacing Inconel 625 to reduce costs.
- High – end Manufacturing:
- Food and medicine (soy sauce brewing tanks, sterile pipelines): Resists acids and alkalis + meets hygiene regulations;
- Papermaking bleaching (digesters, bleaching towers): Resists chlorine – containing chemical corrosion and avoids pulp pollution.
Key Summary
- Advantages: Corrosion resistance is close to Hastelloy (such as B – 2), with a cost only 1/3 – 1/2 of it; high strength is suitable for high – pressure scenarios.
- Limitations: Nickel – based alloys are still needed in concentrated hydrochloric acid/hydrofluoric acid; high cost (3 – 5 times higher than 316L), limited to strong corrosion scenarios.
F44 (UNS S31254) is a “benchmark for super austenitic stainless steel”. Through extreme alloy design, it becomes a “corrosion – resistant rigid demand” material in marine, chemical, environmental protection and other fields, balances performance and cost, and replaces some nickel – based alloys.