The fatigue strength of steel is generally proportional to the hardness up to a certain point. However, the relationship of hardness to fatigue strength is more complicated for the case of high strength material, mainly because the defects in the material sensitively reduce the strength. This paper presents a determination of the defect size in carburized gear teeth and a method of strength evaluation based on this size.
A micro notch is artificially processed at the fillet of carburized gear tooth by means of Focused Ion Beam, and the bending fatigue test is performed. The threshold notch depth αn th, which is the limit not to reduce the fatigue strength, is obtained by the test. The threshold notch depth is used as a measure to determine the size of latent defects in the surface layer. The αn th of the gears of which non-martensitic layer is removed by electro-polishing is shorter than the above one.
The surface layer is observed by using scanning electron microscope and electron probe micro analyzer to clarify the characteristics of latent defects. Many micro cracks besides a long crack in the nonmartensitic layer, Cr oxides and Mn oxides near the surface, and Si segregated in grain boundary are observed. This shows they trigger the initiation of fatigue crack, and therefore, the defect size limits the strength. Consequently, the strength is expressed by an experimental formula of vickers hardness and square root of the projected area of the defect obtained above.