Method for longitudinally seam-welding pipe-bland for welded steel pipe

Abstract

In longitudinally seam-welding a pipe-blank for welded steel pipe from the inside along a groove by the metal arc welding process which comprises using direct electric current supplied to a plurality of consumable electrodes in tandem through at least one current supply cable as the welding current, an even number of consumable electrodes are used; half of said even-numbered consumable electrodes are connected in the reverse-polarity manner, i.e., to be positive in polarity, and are used as leading electrodes, and the remaining half are connected in the straight-polarity manner, i.e., to be negative in polarity, and are used as trailing electrodes; the GMA welding process is applied on the side of said reverse-polarity consumable electrodes which are the leading electrodes; and the submerged-arc welding process is applied on the side of said straight-polarity consumable electrodes which are the trailing electrodes; thereby preventing magnetization of said pipe-blank and the resulting magnetic arc blow of the welding arc by causing lines of magnetic force produced by said direct electric currents for welding flowing through said current supply cables introduced into said pipe-blank to cancel each other, and also preventing spattering of molten droplets from said straight-polarity consumable electrodes.