To provide a constant-velocity universal joint excellent in rolling fatigue which uses a carbon steel for a machine structure having excellent machinability equal to or above that of the conventional Pb-added free cutting steel and further having excellent characteristics after nitriding.
In the constant-velocity universal joint provided with: an outer member 1 having a plurality of guide grooves in an inner circumference; an inner member 2 having a plurality of guide grooves in an outer circumference; torque transmission balls 3; and a holder 4, the outer member and the inner member are made from a steel which contains a graphite phase in an amount of 5% of the C content or more when the members are formed, and make remaining graphite particles and holes originating from graphite, which exist in a surface layer part after the members have been induction-hardened and tempered, controlled into circle-equivalent diameters of 7 m or smaller. Further, the structure after the induction hardening and tempering is formed of a mixture of a martensite mother phase and either or both of a graphite phase and the holes originating from graphite, and the circumference of the graphite phase or holes originating from graphite is provided with a martensite phase having a C concentration higher than that in the martensite mother phase.
TOYOOKA TAKAAKI
IWAMOTO TAKASHI
KIMURA HIDETO
YOSHIDA KAZUHIKO
NTN TOYO BEARING CO LTD
Shiro Fujitani
Kiyoshi Kuruma