1. What is ASTM A671 CD70 Class 31 Steel Pipe?
Answer:
ASTM A671 CD70 Class 31 is an electric-fusion-welded (EFW) carbon steel pipe designed for low-temperature service (down to -70°F/-57°C). Key specifications:
CD70: Material grade requiring Charpy V-notch impact testing at -70°F to validate cryogenic toughness.
Class 31: Minimum yield strength of 31 ksi (214 MPa), engineered for moderate-to-high-pressure cryogenic systems (e.g., LNG storage tanks, high-pressure gas pipelines).
2. What are the manufacturing and certification requirements?
Answer:
Welding: EFW process with filler metal, 100% radiography (RT) or ultrasonic testing (UT) for weld integrity.
Heat Treatment: Stress-relieved or normalized to enhance low-temperature ductility.
Certification: Mill test reports (MTRs) include:
Chemical composition (e.g., C ≤ 0.30%, Mn ≤ 1.40%).
Mechanical properties (yield/tensile strength, elongation).
Impact test results at -70°F.
3. How does Class 31 compare to other ASTM A671 classes?
Answer:
| Parameter | CD70 Class 31 | CD70 Class 20 | CD70 Class 60 |
|---|---|---|---|
| Yield Strength | 31 ksi (214 MPa) | 20 ksi (138 MPa) | 60 ksi (414 MPa) |
| Application | Moderate-high pressure | Moderate-pressure | High-pressure cryogenic |
| Cost | Mid-to-high range | Mid-range | Premium |
4. What are standard dimensions and tolerances?
Answer:
Diameter: 16"–120" OD (standard for large-scale cryogenic pipelines).
Wall Thickness: Tolerance ±10% nominal thickness (per ASTM A671).
Length: Typically 40ft (12.2m), with custom lengths up to 60ft.
5. How to ensure corrosion protection and longevity?
Answer:
External:
3LPE coating (3-layer polyethylene) for buried/submerged pipelines.
Cathodic protection in corrosive environments (e.g., coastal/industrial areas).
Internal:
Epoxy/FBE linings for chemical resistance (e.g., acidic LNG contaminants).
Maintenance: Annual inspections of welds and coatings in cyclic conditions.
