PURPOSE: To obtain long metal tube which is free of discoloration and has a clean surface by the method in which lubricating oil adhered to the inner wall of a long metal tube is decomposed to vapor, the vapor is replaced by a reducing gas for purification and then the metal tube is subjected to annealing.
CONSTITUTION: The air present in a coiled iron tube deprived of phosphorus, for example, cast by using a polybutene lubricating oil is replaced by an inert gas. Then, the long tube is put in a heating furnace to heat at 500°C in a reducing gas atmosphere for decomposing the lubricating oil adhered to the inside of the tube. The tube is then taken out of the heating furnace, the decomposed vapor present in the tube is replaced with an inert gas, and then the inner wall of the tube is replaced with an inert gas, and then the inner wall of the tube is cleanesed. Subsequently, the long tube is placed in an annealing furnace for annealing at 600°C. Thus, with the procedure in which annealing is performed after the lubricating oil adhered to the inside of tube has been completely removed, the adhesion of carbonized substances resulting from lubricating oil can be avoided and thus metal tube having a clean inner wall surface which is free of discoloration can be obtained.
MATSUMOTO JIYOUJI
TODA KAZUO
