Welded Steel Tube from Coil (ERW)

Most everyday steel tube is electric-resistance welded (ERW). Steel strip slit from a coil is bent cold through a row of shaped rolls until its edges meet as a round tube; a current passed across the edges heats them, and rolls squeeze them together into a weld with no filler metal. The bead squeezed out is trimmed off, sizing rolls bring the tube to its final diameter, and a flying saw cuts it to length. Low-frequency current was used from the 1920s until 1970, when high-frequency welding replaced it because it gives a better seam.