A device for thermally welding workpieces including a heating element and a shielding gas supply is provided. The device includes a heating plate consisting of a thermal insulating material in which a groove is formed, said groove includes a heating element, relative to which a surface of a workpiece to be welded is positioned in close proximity to the heating element. A feed channel for a shielding gas is formed in the heating plate and communicates with the groove via at least one through-channel, and the feed channel for supplying shielding gas can be connected to a shielding gas source via a feed line and a valve. The device may further include an assembly for welding a first and a second workpiece, said assembly includes a first and second movement device for moving the workpieces towards one another.