METHOD FOR LONGITUDINALLY SEAM-WELDING PIPE-BLANKS FOR WELDED L PIPE FROM THE INSIDE ALONG A GROOVE

摘要

An improvement is provided in longitudinally seam-welding a pipe-blank for welded steel pipe from the inside along a groove by the G.M.A. welding process. That process comprises supplying D.C. current as the welding current to at least two even number of consumable electrodes in tandem through current supply cables, the current supply cables extending alongside each other longitudinally within the pipe-blank and then longitudinally seam-welding the pipe-blank for welded steel pipe from the inside along a groove by means of the respective consumable electrodes fed through a respective even number of welding torches in tandem, while moving a boom carrying the welding torches through the pipe-blank relative to the pipe-blank in the same direction as that of welding, the boom having a free end which carries the welding torches and having a length at least equal to that of the pipe-blank. The method of this invention comprises the steps of connecting one-half of the even-number of consumable electrodes to the D.C. current so as to be positive in polarity, and connecting the other half of the even number of consumable electrodes to the D.C. current so as to be negative in polarity. In this way, the direction, longitudinally of the pipe-blank, of the D.C. current flow to the one-half of the electrodes is opposite to the direction of the D.C. current flow to the other half of the electrodes so that the lines of magnetic force produced by the direct current for welding flowing through at least one cable introduced into the pipe-blank cancel each other, substantially to prevent magnetization of the pipe-blank. Consequently, this method substantially prevents magnetic arc blow of the welding arc. The weld thus provided is sound and substantially free from welding defects.