Method for longitudinally seam-welding pipe-bland for welded steel pipe from inside along groove

In a welding method for longitudinally seam-welding a pipe-blank from the inside thereof using the metal arc welding process comprising supplying direct electric current as the welding current to a plurality of consumable electrodes in tandem through at least one cable, feeding said respective consumble electrodes through a respective plurality of welding torches in tandem, while moving a boom carrying said welding torches through said pipe-blank relative to said pipe-blank in the same direction as that of welding, said boom having a free end which carries said welding torches and having a length at least equal to that of said pipe-blank,

• the improvement comprising:
• providing at least two consumable electrodes, said electrodes being provided in an even number;
• connecting half of said at least two consumable electrodes to a source of power so as to be positive in polarity and arranging said half of said electrodes as the leading electrodes in the direction of welding;
• connecting the remaining half of said at least two consumable electrodes to a source of power so as to be negative in polarity, and arranging said remaining half of said electrodes as the trailing electrodes in the direction of welding;
• applying the GMA welding process on the side of said leading positive polarity consumable electrodes;
• and
• applying the submerged-arc welding process on the side of said trailing negative polarity consumable 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 current for welding flowing through said at least one cable introduced into said pipe-blank to cancel each other, and also preventing spattering of molten droplets from said negative polarity consumable electrodes.