Grade and Class in ASTM A671 Pipe
ASTM A671/A671M describes electric-fusion-welded (EFW) pipe fabricated from pressure-vessel plate with filler metal added, for high-pressure service at atmospheric and lower temperatures. The designation on a mill certificate contains two independent parts. The grade, written as a two-letter series followed by a number, tells the producer which plate specification and plate grade to use. The class fixes the heat treatment applied to the finished pipe and the tests that follow it.
For CC65 the base metal is ASTM A516 Grade 65 pressure-vessel plate, a fully killed carbon-manganese steel with a minimum yield strength of 255 MPa (37 ksi) and a tensile range of 485 to 620 MPa (70 to 90 ksi). Class 62 then calls for a heat-treated condition rather than the as-welded state, which is what makes the pipe suitable for service well below the atmospheric transition range of a plain carbon steel. Because plate is the starting material, wall thickness is consistent around the circumference and long lengths of large-diameter pipe can be produced with a single longitudinal submerged arc seam.
Chemical Composition of the Base Plate
| Element | Requirement, % |
|---|---|
| Carbon, max | 0.28 |
| Manganese | 0.85 - 1.20 |
| Silicon | 0.15 - 0.40 |
| Phosphorus, max | 0.035 |
| Sulfur, max | 0.035 |
Carbon is held low because carbon is the element most responsible for shifting the ductile-to-brittle transition of ferritic steel upward. Manganese is kept in the 0.85 to 1.20 % window, which lowers the transition temperature while retaining strength. Phosphorus and sulfur are restricted because both segregate at grain boundaries and at the weld fusion line, where they promote brittle fracture. For the most severe cryogenic orders the purchaser may tighten these limits further or add a copper or nickel requirement; the tightening is then written into the purchase order and checked by product analysis.
Mechanical Properties and Impact Testing
| Property | Requirement |
|---|---|
| Tensile strength | 485 - 620 MPa (70 - 90 ksi) |
| Yield strength, min | 255 MPa (37 ksi) |
| Elongation, min | 20 % |
| Impact test | Charpy V-notch, pipe body, weld and heat-affected zone at the ordered test temperature |
| Hydrostatic test | Required, at the pressure stated in the specification |
Impact testing is the controlling acceptance test for cryogenic orders. Specimens are taken in the transverse direction from the pipe body, and additional sets are taken to cover the weld metal and the heat-affected zone. The test temperature is set by the minimum design metal temperature of the system, not by a nominal figure on the certificate, and the absorbed energy requirement is stated explicitly on the purchase order so that there is no ambiguity at acceptance. Where the design temperature is very low, the heat-treated condition becomes essential: heat treatment refines the grain structure of the plate and of the weld heat-affected zone, and it relieves the residual stresses that would otherwise add to the driving force for brittle fracture.
Applications
Cryogenic transfer lines and loading arms at liquefied gas terminals
Low-temperature headers, manifolds and compressor station piping
Refrigerant and ammonia circuits in cold storage and chemical plants
High-pressure process piping where heavy wall and large diameter are both required
Offshore and cold-climate pipelines operating below zero degrees Celsius
Ordering, Welding and Inspection Guidance
A complete purchase order for A671 CC65 Class 62 pipe states the grade and class, outside diameter, wall thickness, length, end finish, weld seam examination method, impact test temperature with the required absorbed energy, hydrostatic test pressure, and any additional plate requirements. It should also state whether the pipe is to be delivered with the longitudinal seam radiographed in full or spot examined, and whether supplementary ultrasonic inspection of the plate is required. This information is normally confirmed at the time of enquiry and repeated on the mill test report.
In fabrication, the same discipline applies. Joint preparation should avoid overheating the pipe ends, welding procedures must be qualified with impact tests at the design temperature, and low-hydrogen consumables with controlled heat input should be used. Post-weld heat treatment, where the construction code requires it, should follow a qualified procedure, and tie-in welds should be inspected to the acceptance criteria in force. Hardness limits apply in wet or sour service, and the finished system should be protected from mechanical damage during storage.
FAQ
Q: What does the number 62 indicate in A671 CC65 Class 62?
It is the class designation. The class defines the heat treatment applied to the finished pipe and the associated test schedule, and it is selected according to design temperature, wall thickness and the governing construction code.
Q: Why is Class 62 pipe chosen for cryogenic service?
Heat treatment refines the grain structure and relieves welding residual stress, so the pipe body, the weld and the heat-affected zone retain toughness at temperatures where an as-welded carbon steel pipe would risk brittle fracture.
Q: How is the impact test temperature decided?
It is tied to the minimum design metal temperature of the system and is written on the purchase order. Specimens are taken from the pipe body, the weld metal and the heat-affected zone.
Q: What is the minimum yield strength of the pipe?
The base plate for grade CC65 is A516 Grade 65 with a minimum yield strength of 255 MPa (37 ksi) and a tensile range of 485 to 620 MPa (70 to 90 ksi); the finished pipe is qualified by transverse tension tests.
Q: Is A671 CC65 Class 62 the same as a seamless pipe?
No. It is an electric-fusion-welded pipe produced from plate with a longitudinal submerged arc seam. That construction suits large diameters and heavy walls, and the seam is examined by radiography or ultrasonic testing.
Q: What documents should accompany delivery?
A mill test report showing heat number, heat treatment record, chemical analysis, tension results, impact results and hydrostatic test data, together with the dimensional and non-destructive examination records specified on the order.
