Evaluation of Bending Strength of Carburized Gears

The high load capacity of carburized gears mainly originates from the hardened layer and induced residual stress. On the other hand, the decarburization at the surface, which causes a nonmartensitic layer, and the inclusions such as oxides and segregation act as a latent defect, and the defect considerably reduces the fatigue strength. In this connection, the authors have proposed a formula of strength evaluation by separately quantifying the influence of the defect. However, the principal defect that limits the strength of gears that have several kinds of defects remains unclarified. This paper presents a method of inferential identification of the principal defect based on the test results of carburized gears made of SCM420 clean steel, gears with both an artificial notch and nonmartensitic layer at the tooth fillet and so forth. It makes clear the practical use of the presented method, and the strength of carburized gears can be evaluated based on the principal defect size.