>The Grade Behind the Process
GR.B is the API 5L designation for the lowest standard line pipe grade, known as L245 in ISO 3183. It has a specified minimum yield strength of 245 MPa and a specified minimum tensile strength of 415 MPa, and it is the grade most often selected for low-pressure water lines, utility piping, piling and general structural service where the highest strength is not required but a defined, certified quality level is.
Because the strength requirement is moderate, GR.B can be produced by several routes. The manufacturing process is therefore chosen on diameter, wall thickness, quantity and the inspection level demanded by the purchase order, not on the grade alone.
>Chemistry and Raw Material
| Element | PSL1 requirement | PSL2 requirement |
|---|---|---|
| Carbon | 0.28 % max seamless, 0.26 % max welded | 0.24 % max seamless, 0.22 % max welded |
| Manganese | 1.20 % max | 1.20 % max |
| Phosphorus | 0.030 % max | 0.025 % max |
| Sulphur | 0.030 % max | 0.015 % max |
| Minimum yield strength | 245 MPa | 245 MPa |
| Minimum tensile strength | 415 MPa | 415 MPa |
The feedstock is continuously cast slab, bloom or round billet produced by basic oxygen or electric arc steelmaking, followed by secondary metallurgy for desulphurisation and inclusion control. The cast product is reheated and rolled, and for welded pipe it is further rolled into strip or plate on a hot strip mill or a plate mill. Cleanness of the steel, expressed through sulphur content and inclusion rating, is the single largest factor behind impact toughness and resistance to hydrogen-induced cracking.
>Seamless Production Route
Round billets are cut to length, centred and heated in a rotary hearth furnace to a rolling temperature.
A piercer punches a hole through the rotating billet between two inclined rolls and a plug, producing a thick-walled hollow shell.
A mandrel mill, plug mill or Pilger mill elongates the shell and reduces the wall to the ordered thickness.
A sizing mill or stretch reducing mill brings the outside diameter and wall thickness into tolerance.
The pipe is cooled, straightened, cut to length, and where required normalised or otherwise heat treated.
Seamless GR.B pipe has no weld seam, which makes it the preferred choice for high-pressure, cyclic or critical service and for small diameters where welding is impractical.
>Welded Production Routes
Welded pipe is made from coil or plate and accounts for most of the large-diameter GR.B supply.
Electric resistance welded pipe is formed from coil by progressive roll forming and joined by high-frequency induction or contact welding. The weld is normalised in line by induction heating, and the internal and external flash is removed. Electric resistance welded pipe covers the small and medium diameter range economically and is the standard choice for water and utility lines.
Longitudinal submerged arc welded pipe is formed from plate by pressing or roll bending into a cylinder, then joined by a double-sided submerged arc weld. It is used for medium and large diameters and for thicker walls where the electric resistance welding process reaches its limit.
Helical submerged arc welded pipe is formed from coil that is fed at an angle so that the seam spirals along the pipe. A single coil width can therefore produce a wide range of diameters, which makes this route flexible for large-diameter low-pressure lines.
>Finishing, Testing and Inspection
Heat treatment: normalising, stress relieving or thermomechanical processing, depending on the product specification level and the ordered delivery condition.
Dimensional control: outside diameter, wall thickness, ovality, straightness, length and end squareness.
Weld quality: ultrasonic or radiographic examination of the seam, plus a flattening test or guided bend test on welded pipe.
Mechanical testing: tensile test on a specimen cut from the pipe body to confirm yield strength, tensile strength and elongation, with Charpy V-notch impact testing when PSL2 or a low-temperature requirement applies.
Pressure testing: a hydrostatic test at the pressure stated in the purchase order, or an approved non-destructive substitute.
Finishing: bevelling, end protection, marking with standard, grade, size, heat number and manufacturer mark, and application of the specified external coating and internal lining.
>FAQ
Q: Which production route gives the better GR.B pipe?
There is no universally better route. Seamless pipe suits small diameters and high-pressure or cyclic service, while electric resistance welded and submerged arc welded pipe offer large diameters and lower cost per metre for lower-pressure duties.
Q: What is the yield strength of GR.B steel pipe?
The specified minimum yield strength is 245 MPa, equivalent to 35,500 psi, and the minimum tensile strength is 415 MPa for both product specification levels.
Q: Is GR.B the same as L245?
Yes. GR.B is the API 5L designation and L245 is the ISO 3183 designation for the same grade, and pipe for export is normally dual marked.
Q: Why is the electric resistance weld normalised after welding?
Normalising the weld zone removes the hard, brittle structure produced by rapid heating and cooling during welding, restoring toughness and making the seam suitable for bending and further fabrication.
Q: Is the hydrostatic test always required?
The standard permits an approved non-destructive examination instead of hydrostatic testing for certain sizes and specifications, but a hydrostatic test at the ordered pressure remains the normal requirement for line pipe.
Q: What information should the marking carry?
The manufacturing standard, the grade and product specification level, the outside diameter and wall thickness, the heat number, the length and the manufacturer mark, together with any third-party inspection stamp required by the project.
