Abstract: In a method for sheathing a plastic tube with a metal sheathing, a metallic band (20) is laid around a plastic tube (14), the metallic band (20) comprising longitudinal edges (32) bent to a common side thereof, which are, when the metallic band (20) is laid around the plastic tube (14), in abutment and extend substantially parallel to the longitudinal axis of the plastic tube (14) and are welded together. The volume of the bent longitudinal edges (32) and the welding energy are chosen such that the metallic band (20) welded together at its longitudinal edges comprises, in the region of the longitudinal seam (46), an outer surface (50) lying substantially within the outer peripheral surface (48) of the metal sheathing after welding.

Abstract: Workpieces pressed from metallic particles, impregnated with an organic lubricant, are sintered in a muffle furnace after traversing a preheating chamber serving for the removal of the lubricant. A substantially oxygen-free protective gas, consisting at least in part of combustible constituents, is passed in counterflow to the workpieces through the preheating chamber in which it is mixed with a hot oxygen-rich gas, preferably air, admitted into the chamber at locations spaced apart in the transport direction of the workpieces to burn the combustible components of the protective gas along with the evaporating lubricant; the oxygen content decreases progressively from a relatively high value near the entrance end of the preheating chamber, where the workpieces are protected from oxidation by their low initial temperature, toward the exit end. The entering air may be brought to a high temperature by heat exchange with the hot gas mixture leaving the chamber at its entrance end.