Grade B Welded Pipe and the Submerged Arc Process
API 5L Grade B welded pipe refers to L245 line pipe in which the seam is produced by submerged arc welding, abbreviated SAW. In this process an electric arc is struck under a blanket of granular flux, so the arc and the molten pool are shielded from the atmosphere and the weld metal solidifies with a low oxygen and nitrogen content. The result is a tough fusion welded seam that can be produced at high deposition rates on thick walls, which is why the SAW family dominates large diameter line pipe. The grade designation is Grade B or L245, with a specified minimum yield strength of 245 MPa and a specified minimum tensile strength of 415 MPa.
LSAW and SSAW Compared
| Item | LSAW (longitudinal SAW) | SSAW (helical or spiral SAW) |
|---|---|---|
| Starting material | Plate, formed by JCOE, UOE or roll bending | Coil, formed at a helix angle |
| Seam orientation | One longitudinal seam, welded inside and outside | Helical seam over the full length |
| Diameter flexibility | Each diameter needs its own tooling | One strip width covers a range of diameters |
| Typical strength of seam | Weld strength and toughness match the pipe body closely | Weld quality depends closely on forming and seam tracking control |
| Common use | Cross country transmission mains and high pressure lines | Water transmission, piling, low pressure lines |
Mechanical and Chemical Requirements
Grade B pipe is supplied to PSL1 or PSL2. Both levels carry the same minimum yield and tensile strength of 245 MPa and 415 MPa; the difference lies in the chemical limits, the mandatory impact testing and the carbon equivalent control of PSL2, plus tighter tolerances and additional inspection. Elongation acceptance is derived from the API 5L tables, which relate the required value to the test piece area and tensile strength rather than using a single fixed figure. Weld tensile tests, transverse or longitudinal as applicable, confirm that the weld and heat affected zone perform consistently with the pipe body.
Testing the SAW Seam
Visual inspection of the internal and external weld bead for profile, undercut and overlaps.
Ultrasonic examination along the full seam length to detect lack of fusion, slag inclusions and porosity.
Radiographic examination of the weld ends, where the start and stop of the weld are most critical.
Hydrostatic testing of every length where specified, followed by dimensional checks on diameter, ovality, wall thickness and end squareness.
Flattening or bend tests on the weld to confirm ductility of the joint.
Where Grade B SAW Pipe Fits
Because the grade has a modest yield strength but excellent weldability and toughness, SAW pipe in L245 is chosen for raw water and potable water mains, cooling water and firewater systems, slurry and tailings pipelines, low pressure gas and air lines, marine and piling applications, and as structural tube. In refinery and plant piping it is used for utility and cooling circuits. Where the transported medium is sour, the material is ordered to PSL2 and evaluated against ISO 15156 hardness and chemistry limits before it is accepted for hydrocarbon service.
Welding, Fabrication and Handling
Field girth welds in Grade B pipe are made by manual metal arc, gas metal arc or flux cored arc processes, and the procedures are qualified under API 1104 for pipeline work or under ASME Boiler and Pressure Vessel Code Section IX for plant piping. Preheat is normally limited for this low carbon steel, but it is increased for thick walls or low ambient temperatures to control hydrogen assisted cracking. Coating is usually applied after the weld and dimensional inspection, and pipe ends should be protected with caps during storage so that the bevel and the weld ends remain in the condition in which they were tested.
FAQ
Q: What does SAW mean in API 5L welded pipe?
SAW stands for submerged arc welding, in which the arc is shielded by a layer of granular flux. It is used for the longitudinal and helical seams of large diameter line pipe.
Q: Is Grade B welded pipe the same as Class B pipe?
No. The correct API 5L designation is Grade B, which defines the specified minimum yield and tensile strength; descriptions of pipe as Class B are a translation error for the same grade.
Q: Which is better, LSAW or SSAW pipe?
LSAW pipe generally offers a seam that matches the pipe body more closely and is preferred for high pressure transmission; SSAW pipe is economical for water transmission, piling and lower pressure service.
Q: What weld testing is applied to SAW pipe?
Visual inspection, ultrasonic examination of the full seam, radiographic examination of the weld ends, hydrostatic testing and weld mechanical tests, with the extent agreed in the purchase order.
Q: Can Grade B SAW pipe be used for sour service?
Yes, when ordered to PSL2 with hardness and chemistry controls that meet ISO 15156, with the NACE MR0175 reference retained for older specifications.
Q: How are the pipe ends prepared?
Ends are cut square and bevelled for field welding, with the bevel angle and face dimensions agreed in the order so that they match the qualified field welding procedure.
