The manufacturing route of a steel pipe decides its dimensional accuracy, weld integrity and cost, and Introduction To ASTM A178 ERW Steel Pipe is the process at the centre of that decision for most line pipe projects. This article explains how the process works, what it produces, and how to choose between the available routes.
Seamless pipe has no weld seam; welded pipe is joined by ERW, HFW or SAW.
ERW/HFW covers diameters up to about 660 mm at high production rates.
LSAW and SSAW serve the large-diameter, thick-wall range.
Hydrostatic and non-destructive testing are mandatory on every route.
Process choice is set by the specification, not by purchasing habit.
The Two Families: Seamless and Welded
Steel pipe is divided into seamless pipe, formed from a solid round billet that is pierced and elongated, and welded pipe, formed from steel strip or plate and joined along a longitudinal or spiral seam. Seamless pipe has no weld seam, which is why it is preferred for high-pressure, high-temperature and sour service; welded pipe wins on cost, size range and wall thickness uniformity for large diameters.
Seamless Manufacturing: Piercing and Rolling
In the seamless process a heated round billet is pierced by a plug or mandrel mill to form a hollow shell, which is then elongated by further rolling to reach the required diameter and wall thickness. The pipe is then sized, straightened, heat treated if required, and finished with non-destructive testing. The absence of a weld makes the whole wall a homogeneous wrought structure, which is the key advantage in cyclic and corrosive service.
ERW and HFW Welding
Electric resistance welding (ERW) forms the pipe from cold strip, and the seam is welded by resistance heating without filler metal; high-frequency welding (HFW) is the modern variant with higher production speeds. After welding, the seam is normalized or post-weld heat treated and the whole pipe is non-destructively tested. ERW/HFW pipe is economical in diameters up to about 660 mm and is widely used for gas gathering, water lines and structural applications.
SAW, LSAW and SSAW: Submerged Arc Welding
Submerged arc welding (SAW) joins thick plate with a continuous wire electrode under a flux blanket. In LSAW (longitudinal SAW) the seam runs along the pipe length, in JCOE or UOE forming for large diameters; in SSAW (spiral SAW) the plate is formed helically so one seam spirals along the pipe. SAW pipe covers the large-diameter, thick-wall range used for trunk lines, water and structural piling, where ERW is not available.
How the Process Affects the Buyer
The process choice sets the practical limits of the order: seamless pipe typically runs to about 660 mm outside diameter, ERW to about 660 mm, while LSAW and SSAW extend to 1500 mm and beyond. Wall thickness, length, tolerance class and testing level all differ between routes, so the project specification, not the habit of the purchaser, should decide the process.
Quality Control Common to All Routes
Every route ends with hydrostatic testing, dimensional inspection and non-destructive examination. Seamless pipe is typically checked by ultrasonic or eddy current testing, ERW/HFW pipe by ultrasonic testing of the weld zone, and SAW pipe by ultrasonic and radiographic testing of the seam. The test certificate lists the results per heat and per pipe where the specification requires it.
Frequently Asked Questions
Seamless pipe is pierced from a solid billet and has no weld seam; welded pipe is formed from strip or plate and joined by ERW, HFW or SAW. Seamless suits high-pressure and sour service, welded pipe is more economical at large diameters.
Electric resistance welding, a process in which the seam is welded by resistance heating without filler metal; HFW is the high-frequency variant.
LSAW pipe has one longitudinal seam along its length; SSAW pipe is formed helically so the seam spirals along the length. Both use submerged arc welding.
Cold-formed ERW and cold-rolled seamless pipe give the tightest tolerances; hot-formed processes require allowance for scale and ovality. Tolerance classes are defined in the product standard.
Yes. ERW, HFW and SAW line pipe are all accepted under API 5L and ISO 3183 for gas service, with the required NDT level depending on the PSL and the project specification.
Start from diameter, wall thickness, service pressure and temperature, then compare availability and cost. Sour or high-temperature service favors seamless; large diameters force SAW; medium diameters are most economical in ERW.
