The most widely used welding system, the electric-resistance welding (ERW) line, starts with a descaled hot-rolled strip that is first slit into coils of a specific width to fit a desired tube diameter. In the entry section is an uncoiler, a welder that joins the ends of coils for continuous operation, and a looping pit, which permits constant welding rates of, typically, three meters per minute. Several consecutive forming rolls then shape the strip into a tube with a longitudinal seam on top.

Two squeeze rolls press the seam together, while two electrode rolls or sliding contacts feed the electric power to the seam for resistance heating and welding. A cutting tool removes the flash created during welding, and, after a preliminary inspection, the tube is cut into cooling-bed length by a saw that moves with the tube.

Tubes up to 500 millimeters in diameter with walls 10 millimeters thick are produced on ERW lines. Larger-diameter pipes are often produced by forming the strip into an endless spiral. Forming is followed by continuous welding of the seam, often by automatic arc welding. Pipes up to 1.5 meters in diameter and with a 12-millimetre wall thickness are sometimes produced by this spiral welding process. Still larger pipes are produced from plates by a U-ing and O-ing process, which applies heavy presses to form plates into a U and then an O. The longitudinal seam (or seams) are then welded by automatic arc-welding equipment.