Abstract: In a vessel, two tubes are mutually spaced apart by a given distance, are to be welded together at ends of the tubes, are accessible with difficulty or not at all, and one of them is introduced into the vessel through an opening in a wall of the vessel. A method and apparatus for forming a partial protective-gas atmosphere for the two tubes includes a sheath brought to a welding location through one of the tubes or through the opening for the tube in the wall of the vessel. Both of the tube ends are overlapped with the sheath or one of the tube ends is overlapped with the sheath and the sheath is placed above the opening in the wall of the vessel for the other tube. This forms a chamber about the welding location. The two tube ends are subsequently pushed toward each other until only a slight intermediate space remains therebetween. A protecive gas is subsequently introduced into the chamber through the intermediate space. The tubes are subsequently centered in a desired position for welding.