PURPOSE: To prevent the discoloration of the tube due to the carbonization of lubricating oil during annealing process by the method in which the reducing gas confined in tank is made to gush by rupturing a rupture disk at a predetermined temperature and then the reducing gas is replaced by the decomposed vapor of lubrication oil inside metal tube.
CONSTITUTION: The rupture disk 3A of the rupture disk device 3 connected to the one end 1A of the long metal tube 1 is set so as to rupture at 475°C, for example. Then, the metal tube 1 and the tank 2 in which reducing gas from the bomb 9 is confined are charged into an annealing furnace and gradually heated to 475°C at which the rupture disk 3A is broken to spout the reducing gas inside the tank 2 into the metal tube 1. At this time, lubricating oil adhered to the inside of the metal tube 1 is decomposed to vapor and the decomposed vapor is sweeped away by the reducing gas. Thus, the inside of the tube is completely cleansed. Afterwards, the metal tube 1 is subjected to annealing in the furnace heated up to 600°C. In this way, it is possible to obtain a metal tube which is free of discoloration due to the carbonization of lubricating oil which is responsible for the corrosion of the inner wall of tube.
MATSUMOTO JIYOUJI
TODA KAZUO
