1. Definition & Standard Compliance
Q: What is ASTM A671 CC70 Class 20?
A:
Type: Electric-fusion-welded carbon steel pipe designed for low-temperature and moderate-pressure service (rated for -50°F/-45°C).
Grade CC70: Higher carbon-manganese content (C ≤0.35%, Mn ≤1.20%) than CC65, offering improved strength.
Class 20: Specifies minimum yield strength of 20 ksi (138 MPa) and standardized wall thickness/pressure ratings.
Key Standard: ASTM A671, with supplementary compliance to ASME B31.3 for pressure piping.
Note: Class 20 provides a balance between cost and performance for moderate低温applications.
2. Typical Applications
Q: Where is Class 20 typically used?
A:
Refrigeration Systems: Ammonia/Freon pipelines in industrial cooling.
Oil & Gas: Low-pressure cryogenic transfer lines (non-critical).
Chemical Plants: Transport of mildly corrosive fluids at reduced temperatures.
Power Generation: Auxiliary cooling systems in moderate低温environments.
Exclusion: Not suitable for LNG or extreme cryogenic services (requires CC65 grades).
3. Material Properties & Testing Requirements
Q: What are the key material properties and tests?
A:
Mechanical Properties:
Yield Strength: ≥20 ksi (138 MPa).
Tensile Strength: 60–85 ksi (414–586 MPa).
Impact Testing: Optional (project-specific; Charpy V-notch at -50°F/-45°C if required).
Non-Destructive Testing (NDT): Spot radiography (RT) or visual inspection (VT) unless stricter codes apply.
4. Comparison with Other Grades
Q: How does Class 20 compare to CC65 grades (e.g., Class 70/71)?
A:
| Parameter | CC70 Class 20 | CC65 Class 70/71 |
|---|---|---|
| Temperature Range | -50°F/-45°C | -65°F/-54°C |
| Yield Strength | 20 ksi (138 MPa) | 70+ ksi (483+ MPa) |
| Cost Efficiency | Higher (lower alloy content) | Lower (specialized) |
Trade-off: Class 20 is more economical for moderate低温applications but lacks the ultra-low-temperature performance of CC65 grades.
5. Fabrication & Welding Guidelines
Q: What are the critical fabrication practices?
A:
Welding:
Preheat to 150–250°F (66–121°C) for thickness >1 inch (25 mm).
Use AWS E6010/E7018 electrodes (low-hydrogen optional for critical welds).
Post-Weld Heat Treatment (PWHT): Optional unless specified (e.g., for thick sections or high restraint).
Corrosion Protection: Epoxy paint or hot-dip galvanizing for mild environments.
Compliance Tip: Always verify project-specific requirements (e.g., NDT extent, PWHT) against ASTM A671-XX25 and ASME B31.3.
