Welding unit for longitudinal pipes

A welding unit for longitudinal pipes comprises a gantry having a welding bridge movably mounted thereon, the welding bridge carrying welding equipment with a first welding head for welding the outerside of a pipe billet. A pipe billet rotation system and assembly jigs, each of which contains a clamping mechanism for clamping the longitudinal edges of a pipe billet, are mounted in the gaps between the supporting posts of the gantry. The welding unit is equipped with a cantilevered cross bar mounted in a supporting assembly and having a second welding head for welding the inner side. The clamping mechanism is in the form of hydraulic stops. The pipe billet rotation system is in the form of a welding carriage movably mounted on a guide and having rotating rollers for positioning a pipe billet in a welding position, and supporting and rotating rollers situated close to the assembly jigs and mounted such as to be capable of moving apart in a transverse direction relative to the guide to allow the welding carriage