CONSTANT VELOCITY UNIVERSAL JOINT HAVING EXCELLENT ROLLING FATIGUE PROPERTY, POWER TRANSMISSION SHAFT HAVING EXCELLENT TORSIONAL FATIGUE PROPERTY, AND PROCESSES FOR PRODUCING THEM

摘要

This invention provides a constant velocity universal joint having an excellent rolling fatigue property, in which a carbon steel for machine structures having machinability favorably comparable with conventional Pb-added free-cutting steels and, at the same time, having excellent properties after nitriding is used in at least a part of the constant velocity universal joint, and a power transmission shaft having an excellent torsional fatigue property. In the constant velocity universal joint, an outer member and an inner member comprise a steel having a graphite phase of which the carbon content in the molding of the members is not less than 5% by mass. In the surface part after induction hardening/tempering, residual graphite particles and graphite-derived pores have an equivalent circular diameter of not more than 7 μm. The power transmission shaft comprises a constant velocity universal joint provided at one end of a stub shaft. The stub shaft comprises a steel having a graphite phase of which the carbon content in the molding is not less than 5% by mass. In the surface part after induction hardening/tempering, residual graphite particles and graphite-derived pores have an equivalent circular diameter of not more than 7 μm.