a second method step preferably by means of an electric resistance welding so as to provide a butt weld seam not illustrated alternatively to this the two ring ends 29 30 may also be joined together by means of other suitable thermal joining methods for example by laser welding or the like before thermal joining it is usually necessary to subject the region of the ring ends 29 30 to mechanical retreatment for example by detaching a short piece in the region of the two ring ends 29 30 and or by lathe turning the ring ends 29 30 before the actual welding operation at least two continuous cylindrical venting bores 32 33 are introduced into the ring 28 by drilling or punching preferably in the region of the ring ends 29 30 on the one hand these venting bores 32 33 prevent excess pressure from occurring in the ring 28 as a result of the welding gases arising due to the thermal joining process on the other hand the venting bores 32 33 during the subsequent process of shaping into the flanged disk 16 in which a change in volume of an inner space of the ring 28 generally also occurs prevent the situation where excess pressure within the ring 28 arises before the shaping process an outside diameter 34 of the tube portion lies in a range of between 15 mm and 30 mm but may.