Patent ID: 7638005
Filing Date: 2009-12-29
Classification: B62D,C21D,Y02P

Abstract:
1. A method of producing a track bushing wherein, in a series of quenching processes in which by use of a quenching system capable of carrying out induction heating from the outer circumferential surface of a track bushing and capable of independently carrying out inner circumferential surface cooling and outer circumferential surface cooling while pressing a partition jig against each end face of the track bushing such that a first cooling medium for the inner circumferential surface of the track bushing and a second cooling medium for the outer circumferential surface are isolated from one another, after setting the track bushing, which has been entirely heated to a quenching temperature from the outer circumferential surface, in the quenching system capable of additional induction heating from the outer circumferential surface, preceding cooling of the track bushing from the inner circumferential surface is carried out, or cooling from the inner circumferential surface is effected while carrying out induction heating from the outer circumferential surface and, after an elapse of a specified time, cooling from the outer circumferential surface is carried out, or alternatively, the induction heating from the outer circumferential surface is stopped to effect cooling, by adding a step in which, during the induction heating from the outer circumferential surface or during the time from when the induction heating is stopped until when the cooling from the outer circumferential surface is finished, the cooling from the inner circumferential surface is stopped and the inner circumferential surface is tempered within a short time by heat diffusion from the outer circumferential surface to the inner circumferential surface, quench-hardened cases extending from the outer and inner circumferential surfaces, respectively, to the wall center of the track bushing are formed, with an unhardened quenched layer disposed between the quench-hardened cases on an inner circumferential surface side and an outer circumferential surface side, such that the quench-hardened case on the outer circumferential surface side extends deeper than the quench-hardened case on the inner circumferential surface side, such that the unhardened quenched layer has a structure containing one or more selected from ferrite, pearlite and bainite which precipitate in the course of cooling from the quenching temperature, and such that the inner circumferential surface quench-hardened case has a tempered martensitic structure, wherein the jigs for dividing the inner circumferential surface cooling medium from the outer circumferential surface cooling medium have, at their inner circumferential surfaces, thin cylindrical sections for preventing cooling of the inner circumferential surface of the track bushing and are respectively pressed against flat sealing portions and/or chamfered inner circumferential surface portions at both end faces of the track bushing so that leakage of the cooling mediums at their contact portions is minimized.