What ASTM A671 CD70 Class 50 Pipe Is
ASTM A671 is the standard specification for electric-fusion-welded (EFW) steel pipe produced from pressure-vessel-quality plate with filler metal added, and written for atmospheric and lower-temperature service. A CD70 Class 50 pipe is therefore a plate-based welded product, not seamless pipe and not resistance-welded pipe: the cylinder is rolled from plate, the longitudinal seam is welded with filler metal, and the finished length is heat treated and examined according to the class stated in the order.
The designation A671 CD70 Class 50 reads in three blocks. The letter pair identifies the pressure-vessel plate specification from which the cylinder is rolled, and the number that follows the letters keeps the grade numbering of that plate specification. The two-digit class number states the heat-treatment condition of the finished pipe together with the non-destructive examination and testing regime that applies to it. The class number must always be quoted together with the plate letters and the grade, because pipes of identical chemistry can be supplied in different heat-treatment conditions and with quite different levels of radiographic examination of the longitudinal seam.
The result is a pipe that combines the through-thickness quality of pressure-vessel plate with a weld that can be examined, repaired under qualified procedures and heat treated together with the pipe body. Where a project design temperature sits below the practical range of ordinary line pipe, this construction is usually the reason the material is selected.
Manufacturing Route and Quality Control
Plate is purchased against the referenced pressure-vessel plate specification, with ladle analysis and mechanical test certificates traced to each heat.
Plate edges are machined or planed, the shell is formed and the longitudinal edges are aligned and tacked for welding.
The longitudinal seam is welded with filler metal, normally by the submerged arc process using carbon steel electrode and flux combinations classified under AWS A5.17 / SFA-5.17. Both sides are welded where wall thickness or the order requires it.
The weld is heat treated as required by the class designation; the pipe is then straightened, end-finished and cleaned.
The longitudinal seam is examined by radiography to the coverage called for by the class, and weld ends and repaired areas are normally examined by magnetic particle or liquid penetrant methods.
Each length is hydrostatically tested, or examined by the method permitted in place of the test, and is then marked and released with mill test reports.
Specification and Dimensional Data
| Item | Reference or range |
|---|---|
| Product standard | ASTM A671, electric-fusion-welded pipe for atmospheric and lower-temperature service |
| Base material | Pressure-vessel-quality steel plate, identified by the letter pair in the designation |
| Outside diameter | NPS 8 to NPS 48 as standard, outside diameters per ASME B36.10M |
| Wall thickness | Selected against the order, up to 76 mm for heavy-wall service |
| Lengths | 6 m to 12 m, or specific cut lengths |
| Seam type | One or two longitudinal seams, welded with filler metal |
| Filler metal | Carbon steel electrode and flux combinations to AWS A5.17 / SFA-5.17 |
| Heat treatment | As stated by the class number in the designation |
| Examination | Radiography of the longitudinal seam and hydrostatic test as required by the class |
| End finish | Plain, bevelled or as ordered |
Service Applications
A671 CD70 Class 50 pipe is specified where low design temperature and heavy wall meet large diameter: LNG receiving terminals and cryogenic transfer lines, air separation and industrial gas plants, refrigeration and ethylene units, low-temperature sections of petrochemical and gas processing trains, ammonia and propylene storage piping, cold boxes and compressor suction lines. It is also used for buried and offshore low-temperature headers where plate pipe offers a wall thickness and diameter combination that seamless mill capacity cannot reach.
Ordering, Fabrication and Inspection Points
A purchase order should state the product standard, the plate letter pair and grade, the class number, nominal size and wall thickness, length and end finish, hydrostatic test pressure or the examination applied in place of the test, radiographic coverage of the seam, impact test temperature and minimum absorbed energy, heat-treatment condition, surface condition and coating, marking, and the contents of the mill test report.
Fabrication follows a welding procedure specification qualified to ASME BPVC Section IX. Low-hydrogen consumables, controlled preheat and interpass temperature and, where the class requires it, post-weld heat treatment of field joints are normally specified. Because the pipe serves at low temperature, the impact energy of the weld metal and heat-affected zone rather than tensile strength alone governs acceptance, so procedure qualification records should carry low-temperature impact results for the production joint, including the heat-affected zone.
FAQ
Q: What does the CD70 block of the designation mean in ASTM A671?
The letter pair identifies the pressure-vessel plate specification used for the cylinder of the pipe, and 70 follows the grade numbering of that plate specification. The same plate grade can be made into different classes, so the letters and grade must always be read together with the class number.
Q: Is ASTM A671 CD70 Class 50 pipe seamless?
No. ASTM A671 covers electric-fusion-welded pipe made from plate with filler metal added. The longitudinal seam is welded, and the class number governs the heat treatment and the examination applied to that weld and to the pipe body.
Q: What examination does the class number govern?
The extent of radiographic examination of the longitudinal seam, the examination of weld ends and repairs, the heat-treatment condition and the pressure test are all tied to the class number and to the requirements of the order. Each length is marked with the designation and released with a mill test report covering chemical analysis, tensile results and any impact tests performed.
Q: Why is filler metal used instead of resistance welding?
Filler-metal welding allows thick plate to be formed into large-diameter cylinders with single or double longitudinal seams, and it allows the weld metal to be treated and heat treated together with the pipe. Resistance-welded pipe is limited in wall thickness and diameter by comparison.
Q: Can this pipe be used above ambient temperature?
It may be used within the design limits of the applicable pressure design code, but where high-temperature service governs the design, plate-based pipe to ASTM A672 is generally specified instead, because A672 is written specifically for high-temperature service at moderate temperatures.
Q: What should be verified on the mill test report?
The report should identify the plate specification and grade, the class number, the heat and pipe numbers, the chemical analysis, the tensile properties, the heat-treatment record, the results and coverage of radiographic examination and the hydrostatic test record, so that the finished pipe can be traced back to the plate.
