F347
F347 is niobium-containing stabilized austenitic stainless steel, and its core advantage is its strong intergranular corrosion resistance (especially in welding/high-temperature working conditions). The following are analyzed from the aspects of standard, code, composition, performance, heat treatment and application system:
I. Standards and brand system
1. Core Standards
F347 mainly follows ASTM standards:
- ASTM A182: Regulates forged/rolled stainless steel flanges, pipe fittings, and valve components for high temperatures (corresponding to the “F347” grade).
- ASTM A240: Standard for stainless steel plates/strips (covering F347 plates, such as pressure vessel liners).
- ASTM A312: Standard for seamless/welded stainless steel tubes (guiding the production of F347 high – temperature pipelines).
Other related standards: ASME SA – 182 (engineering application), JIS G4303 (Japan), EN 10088 – 3 (Europe), GB/T 20878 (China, corresponding grade 06Cr18Ni11Nb).
2. International Grade Correspondence
| System/Region | Grade | Description |
|---|---|---|
| US UNS | UNS S34700 | Basic type F347 |
| European EN | EN 1.4550 | Equivalent to F347 |
| Japanese JIS | SUS347 | General – purpose grade |
| China (Old) | 0Cr18Ni11Nb | Has been updated to the new national standard |
| China (New) | 06Cr18Ni11Nb | Low – carbon revision (C ≤ 0.08%) |
3. Differences from 321 (containing titanium)
F347 contains niobium (Nb), and 321 contains titanium (Ti):
- NbC is more stable than TiC (harder to dissolve at high temperatures), so F347 has better high – temperature strength and resistance to sensitization (suitable for long – term service above 600°C).
II. Chemical composition (ASTM standard, mass fraction%)
The core design of F347 is “Nb stabilization”: Nb preferentially combines with C to form NbC, preventing Cr₂₃C₆ from precipitating along grain boundaries (to avoid intergranular corrosion).
| Element | Content Range | Function Analysis |
|---|---|---|
| C | ≤0.08 | Allows a slightly higher carbon content (because Nb combines with carbon) |
| Si | ≤1.00 | Assists in deoxidation and improves oxidation resistance |
| Mn | ≤2.00 | Stabilizes austenite and improves workability |
| P | ≤0.045 | Impurity element, strictly controlled |
| S | ≤0.030 | Impurity element, low sulfur ensures corrosion resistance |
| Cr | 17.0~19.0 | Forms a passivation film and resists uniform corrosion |
| Ni | 9.0~13.0 | Stabilizes austenite and improves toughness/corrosion resistance |
| Nb | 8×C ~ 1.0 | Combines with C to form NbC and inhibits Cr depletion (Core! The Nb content needs to be ≥8 times that of C to ensure complete combination with carbon) |
III. Mechanical properties (room temperature after solution treatment)
F347 has both high strength and high plasticity, and excellent high – temperature performance (NbC strengthens the high – temperature structure):
| Performance Indicator | Typical Value (ASTM Requirement) | Remarks |
|---|---|---|
| Tensile Strength | ≥515 MPa | Still maintains ≥300 MPa at 600°C (high – temperature strength is better than 304/316) |
| Yield Strength (σ₀.₂) | ≥205 MPa | Austenitic stainless steel has low yield strength and can be strengthened by cold working |
| Elongation (δ₅) | ≥35% | Gauge length 50mm, excellent plasticity, convenient for forming |
| Hardness | ≤217 HB (or ≤95 HRB) | Non – aged state, hardness increases after cold working |
Advantages of High – Temperature Performance
- NbC remains stable at 600 – 800°C, preventing grain growth. Therefore, F347 can serve for a long time at higher temperatures (such as nuclear power steam pipelines, with a design temperature above 550°C).
IV. Heat Treatment Requirements
The heat treatment of F347 centers on “solution annealing + Nb stabilization”:
- Solution Treatment:
- Temperature: 1010 – 1150°C (1050 – 1100°C is recommended to ensure sufficient dissolution of Nb).
- Cooling: Rapid water cooling (quenching) to dissolve Nb uniformly. During cooling, NbC is preferentially formed to inhibit the precipitation of Cr₂₃C₆.
- Function: Restore a single austenitic structure and maximize the resistance to intergranular corrosion.
- Post – Welding Treatment:
- Due to the presence of Nb, even if the welding heat – affected zone undergoes the 450 – 850°C sensitization range, Nb will still combine with carbon to form NbC. Therefore, no additional heat treatment is required (this is the core advantage of F347 — suitable for complex welded structures).
V. Main application fields (relying on the characteristics of “anti-sensitization+high temperature stability”)
- High – temperature Service Scenarios:
- Nuclear power steam generator pipelines, boiler superheater tubes (resistant to high temperatures of 500 – 600°C, thermal fatigue resistance).
- Components of aerospace engine combustion chambers (resistant to high – temperature oxidation and gas corrosion).
- Complex Welded Structures:
- High – temperature chemical reaction kettles (with many welded joints, resistant to intergranular corrosion without post – heat treatment).
- Food and pharmaceutical equipment (such as sterilization tanks, aseptic pipelines, stable corrosion resistance after welding).
- Strong Corrosion Environments:
- Coastal building curtain walls (welding nodes resistant to salt spray corrosion, long – term maintenance – free).
- Seawater desalination pretreatment systems (resistant to chloride ions and microbial corrosion).
- Special Industries:
- Nuclear waste storage tanks (radiation resistance + intergranular corrosion resistance).
- High – end instruments (such as semiconductor crystal growth furnaces, high – temperature resistance + low pollution).
VI. Key Contrast and Precautions
- vs 304/316:
- Advantages: The resistance to intergranular corrosion is increased by 3 – 5 times (especially in welding/high – temperature working conditions), and the high – temperature strength is higher.
- Disadvantages: The cost is 20 – 30% higher, and it is not necessary to use in ordinary environments (such as atmosphere, weak corrosion).
- vs 321 (containing Ti):
- Advantages: NbC is more stable than TiC (not easy to dissolve at high temperatures), so F347 has better high – temperature performance (suitable for above 600°C).
- Disadvantages: Nb resources are rarer, and the cost is slightly higher than that of 321.
- Processing Tips:
- Can be strengthened by cold working (e.g., the strength of cold – rolled plates is higher), and the weldability is excellent (ER347 welding wire is recommended to match the Nb content).
- Avoid being in a state above 900°C for a long time (NbC will coarsen, reducing strength, and the service temperature needs to be controlled).
In summary, F347 is a “special austenitic stainless steel for welding + high – temperature working conditions”. Through Nb stabilization technology, it breaks through the bottleneck that 304/316 is prone to sensitization after welding, and also has excellent high – temperature performance, becoming a core material in high – end fields such as nuclear power, aerospace, and chemical industry.