Abstract: In a process and an apparatus for the longitudinal-side welding of tubes (1) to flat steel bars (2), in which the welding in each case is carried out on the top side and on the underside of the flat steel bars (2), the welding torches (4) are arranged in a fixed position, the parts (1, 2) to be joined are pressed against one another before the welding and are moved in the longitudinal direction by means of a conventional transport device (10) during the welding operation, and the welding is carried out on the top side in a known manner by means of a SAW process, combined pulse oscillation GMAW is carried out on the underside at the same time as the SAW carried out on the top side, during which pulse oscillation GMAW the torches (11) are set in very rapid lateral motion by means of an oscillation device (15), the oscillating speed for guiding the welding torches (11) being coordinated with the feed rate of the parts (1, 2) to be welded.