Influences of Hardness, Defect Size and Retained Austenite on Fatigue Strength at N = 10⁷ for Carburized Steels

Abstract

The fatigue strength at N = 10⁷ for carburized steels, which indicates the fatigue strength for specimens whose fatigue origins are not at inner defects but at as-carburized surfaces in this study, is affected by a softened structure, oxidized intergranular, which is considered to be mechanically equivalent to a defect, retained austenite and residual stress. Therefore, the influence of each factor should be separately considered for evaluating the fatigue strength at N = 10⁷ quantitatively. From this viewpoint, this study aims to clarify the influences of hardness, which exceeds the range to which the √area parameter model is applicable, defect size and retained austenite on fatigue strength at N = 10⁷. To clarify these influences, carburized SCM420s and quenched-tempered SUJ2s were applied, and rotating bending fatigue tests were conducted using specimens with an artificial defect. Tension-compression fatigue tests were also performed by specimens with a single edge notch in order to observe crack closure behavior. The fatigue strength at N = 10⁷ was influenced by not only the hardness and defect size, but also the retained austenite ; the fatigue strength at N = 10⁷ of the specimens, for which the volume of the retained austenite contained in the microstructure was about 20%, was approximately 1.5 times higher than that estimated by the √area parameter model. It is considered that the improvement of the fatigue strength is due to developed crack closure in the threshold stress intensity factor range and the development is mainly caused by the strain-induced transformation of the retained austenite generated during a fatigue test. Based on these results, a new prediction equation as a substitute for the conventional one was proposed by considering the influence of the retained austenite.