Arc welded pipe is made by using the heat of an electric arc to melt the edges of a formed steel strip or plate and to fuse them, with or without added filler metal, into a continuous tube. The arc is the heat source; the pipe is the product. Because the arc can be applied from outside, from inside or from both sides, and because filler chemistry can be chosen independently of the strip chemistry, arc welding covers a wider range of diameters and wall thicknesses than any other pipe-making route.
The Working Principle
Steel strip or plate is progressively formed into a cylinder by rolls. As the two edges come together, an arc is struck between an electrode and the workpiece, melting a narrow band of metal on each edge and, where filler is used, the wire or electrode tip as well. The molten pool solidifies immediately behind the arc and forms the weld seam. A protective medium, either a flux, a shielding gas or a slag system, excludes air from the pool so that nitrogen and oxygen do not embrittle the joint. The result is a pipe whose seam has essentially the same strength as the parent metal when the procedure is correctly qualified.
Arc Welding Processes Used for Pipe
| Process | Heat source and shielding | Typical use in pipe making |
|---|---|---|
| Submerged arc welding (SAW) | arc buried under granular flux | longitudinal and spiral seams, medium to heavy wall |
| Gas metal arc welding (GMAW / MIG) | continuous wire with shielding gas | continuous seams, medium wall, high speed |
| Flux-cored arc welding (FCAW) | tubular wire with flux core | thick wall seams, field and shop work |
| Shielded metal arc welding (SMAW) | covered electrode, slag shielding | short seams, repairs, site joints |
| Gas tungsten arc welding (GTAW / TIG) | non-consumable tungsten electrode with inert gas | root passes, thin wall, stainless and alloy tube |
Submerged arc welding dominates large-diameter pipe production because the flux blanket gives very high deposition rates and a clean, ductile weld. Gas metal arc welding offers a good balance of speed and quality for medium walls, while gas tungsten arc welding is chosen where root-pass cleanliness and penetration control matter, as in alloy and stainless tube. Flux-cored and shielded metal arc processes are the practical choices where welding positions are awkward or where work is done on site.
Arc Welded Pipe Compared with ERW Pipe
| Aspect | Arc welded | Electric resistance welded |
|---|---|---|
| Joining mechanism | melting with or without filler | pressure welding of heated edges, no filler |
| Wall thickness | wide range, including heavy wall | generally thinner wall |
| Diameter range | small to very large | small to medium |
| Filler flexibility | filler chemistry can be matched to service | no filler, seam composition equals strip |
| Typical products | line pipe, structural and pressure pipe | water pipe, conduit, general tube |
The practical consequence for a buyer is that very large diameters, heavy walls and alloy compositions point towards arc welding, while thin-wall, high-volume standard tube is usually more economical in ERW form.
Process Variables That Decide Weld Quality
Heat input: too much coarsens the grain and reduces toughness, too little causes lack of fusion.
Edge preparation and fit-up: a uniform bevel and consistent gap are what allow full penetration.
Shielding and flux condition: moisture in flux or a disturbed gas shield introduces porosity and hydrogen.
Travel speed and arc stability: these control bead shape, penetration and the risk of undercut.
Base metal cleanliness: oil, scale and rust are a direct source of porosity and slag inclusions.
Welding procedure qualification and operator qualification are the formal controls that lock these variables in place before production begins.
Inspection and Non-Destructive Testing
Quality is verified in stages rather than at the end. Visual testing confirms bead profile, undercut and surface defects; radiographic testing reveals internal porosity, slag and lack of fusion; ultrasonic testing detects planar defects such as cracks and incomplete penetration and is often applied to the full length of the seam; magnetic particle testing is used for surface and near-surface flaws in ferromagnetic steel; and dye penetrant testing serves the same purpose where a non-magnetic method is needed. Hydrostatic testing proves the pressure integrity of the finished pipe, and flattening, bending or tensile tests demonstrate the mechanical soundness of the welded joint. Destructive tests taken from the ends of the coil or plate lot confirm that the production weld matches the qualified procedure.
FAQ
Q: What is the main working principle of arc welded pipe?
Arc welded pipe uses the high temperature of an electric arc to melt the metal edges of a formed strip or plate and join them into a pipe, with or without a filler material such as a wire or electrode. A flux or shielding gas protects the molten pool from atmospheric contamination. When the pool solidifies it forms a seam whose properties can match the parent metal if the welding procedure is properly qualified.
Q: Which arc welding processes are used to make steel pipe?
The main processes are submerged arc welding, gas metal arc welding, flux-cored arc welding, shielded metal arc welding and gas tungsten arc welding. Submerged arc welding dominates large-diameter and heavy-wall production, gas metal arc welding suits continuous medium-wall seams, and gas tungsten arc welding is used for root passes and for alloy or stainless tube. Flux-cored and shielded metal arc welding serve repair and site work.
Q: What is the difference between arc welded and ERW pipe?
Arc welding melts the pipe edges, with or without added filler, whereas electric resistance welding joins them by pressure after heating with high-frequency current and uses no filler. Arc welding therefore covers heavier walls, larger diameters and matched filler chemistry, which is why it is used for line pipe and pressure pipe. ERW is generally faster and more economical for thin-wall, standard-dimension tube.
Q: What factors affect the quality of an arc welded seam?
Heat input, edge preparation and fit-up, shielding condition, travel speed and base metal cleanliness are the controlling variables. Excessive heat input coarsens the grain and lowers toughness, while insufficient heat causes lack of fusion. Moist flux or a disturbed gas shield produces porosity, and oil or scale on the edges introduces slag and gas defects. Qualified welding procedures and qualified operators are the formal controls that hold these variables within range.
Q: How is the quality of arc welded pipe inspected?
Inspection combines visual testing of bead profile and surface condition with volumetric methods. Radiographic testing reveals porosity, slag and lack of fusion, and ultrasonic testing detects planar defects such as cracks and incomplete penetration along the seam. Magnetic particle or dye penetrant testing covers surface and near-surface flaws. Hydrostatic testing confirms pressure integrity, and flattening, bending or tensile tests on samples from the ends verify the mechanical soundness of the joint.
