HIGH STRENGTH MACHINE PARTS AND SHAFT EXCELLENT IN FATIGUE CHARACTERISTICS, AND METHOD FOR IMPROVING FATIGUE CHARACTERISTICS THEREOF

摘要

High strength machine parts excellent in fatigue strength which contain 0.1 to 1.2 mass % of C, and have a notch having a radius of curvature of 25 mm or less, wherein the surface thereof has an HV hardness of 250 or more and a surface layer of the notch has a compression residual stress of -100 to -1500 MPa; and a method for improving the fatigue strength thereof. A shaft excellent in fatigue resistance characteristics which comprises a steel product having a chemical composition, in mass %, that C: 0.1 to 1.2 %, Si: 0.05 to 2.5 %, Mn: 0.2 to 3 %, Al: 0.005 to 0.1 %, N: 0.001 to 0.02 % and the blance: Fe and inevitable impurities, and exhibiting a tensile strength of 800 MPa or more, characterized in that the shaft has an oil hole, and the surface layer of the oil hole has a compression residual stress of 50 to 90 % of the tensile strength of the above steel product; and a method for improving the fatigue strength thereof.