What the A671 CC65 Class 70 Designation Means
ASTM A671/A671M covers electric-fusion-welded (EFW) steel pipe made from pressure-vessel-quality plate with filler metal added, intended for high-pressure service at atmospheric and lower temperatures. Pipe of this type is built from plate that has been rolled to a cylinder, longitudinally seam welded by the submerged arc process, and finished to the dimensional tolerances of the pipe specification. Because plate is the starting product, the wall thickness is uniform and the pipe can be produced in the large diameters and heavy walls needed for cryogenic and refrigeration systems.
The designation carries three pieces of information. CC identifies the grade family, produced from ASTM A516 pressure-vessel plate, and 65 identifies the plate grade within that family, so the base metal chemistry and tensile requirements follow A516 Grade 65. Class 70 identifies the heat-treatment condition of the finished pipe and the corresponding test schedule required by the purchaser. Grade and class must always be ordered together, because two pipes of identical grade but different classes are not interchangeable at low temperature.
Plate Chemistry and Mechanical Requirements
| Property | Requirement |
|---|---|
| Carbon, max | 0.28 % |
| Manganese | 0.85 - 1.20 % |
| Silicon | 0.15 - 0.40 % |
| Phosphorus, max | 0.035 % |
| Sulfur, max | 0.035 % |
| Tensile strength | 485 - 620 MPa (70 - 90 ksi) |
| Yield strength, min | 255 MPa (37 ksi) |
| Elongation, min | 20 % |
These values are those of the A516 Grade 65 plate specified for A671 pipe. The chemistry is deliberately lean: low carbon and controlled manganese keep the ferrite tough at sub-zero temperature, and the low phosphorus and sulfur limits reduce the risk of brittle fracture at the weld. Plate for low-temperature pipe is normally ordered in the normalized condition, and the finished pipe is tested by transverse tension tests on specimens taken across the weld, in accordance with the mechanical test requirements of the pipe specification.
Class, Heat Treatment and Testing
The class number tells the mill how to heat treat the finished pipe and which supplementary tests to run. Heat-treatment options range from the as-welded condition through stress relieving to normalizing, or normalizing followed by tempering, and each option produces a different combination of yield strength, hardness and notch toughness. For sub-zero design temperatures the selection of class is driven by three inputs: the minimum design metal temperature, the wall thickness, and the toughness rules of the construction code that governs the plant.
Charpy V-notch impact testing is the decisive acceptance test for this grade. Specimens are taken from the pipe body, from the weld and from the heat-affected zone, and are tested at the temperature stated on the purchase order, which is normally tied to the minimum design metal temperature. Absorbed energy results are recorded for each location. In addition, the pipe receives a hydrostatic test, weld reinforcement and alignment are checked, and radiographic or ultrasonic examination of the longitudinal seam is performed when specified. A mill test report covering heat number, heat treatment record, chemical analysis, tension and impact results accompanies every lot.
Where the Pipe Is Used
Liquefied gas and refrigerant piping in storage, loading and regasification terminals
Sub-zero process lines in gas processing and petrochemical plants
Low-temperature sections of ammonia and refrigeration circuits
Power plant cooling and auxiliary systems in cold climates
Compressor and pump station piping subject to low ambient temperatures
Fabrication and Inspection Points
Field fabrication must preserve the toughness that the mill established. Cutting and beveling should not overheat the plate edges, and welding procedures must be qualified with impact testing of the weld and heat-affected zone at the design temperature. Heat input, interpass temperature and preheat should be controlled and recorded, and low-hydrogen consumables used throughout. After welding, tie-in joints should be inspected against the acceptance criteria of the governing code, and hardness limits applied where the service is wet or sour.
Storage and handling also matter. Pipe ends should remain protected, and plates and pipe should be separated from carbon steel that has been in a hydrogen or wet-sour environment. Impact test specimens should be taken from the production pipe rather than from plate remnants, because the weld thermal cycle changes the toughness of the heat-affected zone.
FAQ
Q: What does the number 65 in CC65 refer to?
It identifies the grade of the ASTM A516 pressure-vessel plate used to make the pipe. The designation is built from the plate specification family and the plate grade, not from a yield strength value in ksi.
Q: Why must the class also be specified?
The class fixes the post-weld heat treatment and the test schedule. Two pipes with the same grade but different classes can differ in toughness and hardness, so they cannot be substituted for one another in low-temperature service.
Q: What is the purpose of the Charpy impact test?
Ferritic steel can fracture in a brittle manner when it is cold. Charpy V-notch testing at the minimum design metal temperature demonstrates that the pipe body, the weld and the heat-affected zone retain adequate absorbed energy.
Q: Is the pipe seamless?
No. A671 pipe is electric-fusion-welded from plate with filler metal added. The longitudinal seam is made by submerged arc welding, and the weld is examined radiographically or ultrasonically when required.
Q: Can A671 CC65 pipe be supplied in heavy walls?
Yes. Plate-based production allows heavy walls and large diameters, which is one reason this specification is chosen for cryogenic and low-temperature headers where thick sections are required.
Q: What should be included in the purchase order?
Grade, class, outside diameter, wall thickness, length, weld seam examination method, impact test temperature and energy requirement, hydrostatic test requirement and the need for a mill test report with heat treatment records.
