The present invention discloses a welding system, particular a welding system using an inert gas in order to provide an oxide free fusion zone, where the object to be welded has a first side and a second side, said first and second sides being opposite sides of the object to be welded, where the system comprises: —a first wagon, in use suitable to be arranged on a first side of the object to be welded, where said wagon comprises means for remaining on the surface of the object to be welded, and being able to travel on the surface of the object to be welded, where a first welding device is arranged in connection to or on said wagon; —a slave wagon, in use suitable to be arranged on a second side of the object to be welded, where said slave wagon comprises a tub, said tub having an upper rim adapted to the contour of the object to be welded, and where further a gas inlet is provided such that a suitable gas may be introduced into the tub, and where the slave wagon comprises means such that in use the rim of the tub on the slave wagon may remain proximate the second side of the object to be welded, and where means are provided for moving the slave wagon such that the tub is superposed the fusion zone on the object to be welded.