HARDENING METHOD OF STEEL PIPE

PURPOSE:To prevent the back flow of a refrigerant from an inside cooler surface to heater side even if a steel pipe has a small bore in a hardening method wherein the refrigerant is ejected to the inside and outside surface of the steel pipe conveyed in axial direction by providing an adequate space on the side of the cooler in the direction in which the steel pipe advances. CONSTITUTION:A cooler 6 for inside surface which is inserted into a steel pipe 10 and ejects a refrigerant from the inside of the steel pipe is inserted into an induction heater 1 from the front thereof, and a refrigerant is supplied to the nozzle part 7 in front thereof. The pipe 10 is conveyed to the front surface of the heater 1 by conveyance rolls 4. Devices 8, 8' for supplying the refrigerant to the rolls 4 and the cooler 6 are retreatable in this case. The steel pipe is conveyed between the heaters at a specified speed by pinch rolls 3 and during this time, the piping part of the cooler 6 is maintained in the central part of the steel pipe by means of a spacer 11. The steel pipe heated to a prescribed temp. is successively conveyed to the position of the nozzle part 7 and the cooler 2 where it is cooled. The refrigerant ejected from the nozzle 7 is thus discharged smoothly.