Quantitative Evaluation of 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, retained austenite and residual stress. In the early study, it was clarified that the fatigue strength at N=107 of the specimens containing the retained austenite in the structure was higher than that estimated by the √area parameter model.This study aims to quantitatively evaluate these complex influences mentioned above and predict the fatigue strength at N=107 with high accuracy. Therefore, rotating bending and three point bending fatigue tests with several stress ratios were conducted using carburized SCM420s. Since the fatigue crack of all the specimens, whose fatigue origin was at the as-carburized surface, emanated from oxidized intergranular at a non-martensitic structure, the oxidized intergranular was considered to be mechanically equivalent to a surface crack. The dependence of mean stress on the fatigue strength at N=107 was accurately arranged by the modified Goodman diagram. Furthermore, the hardness of a thin softened structure was estimated from the half value breadth of the structure. Based on these results, the prediction of the fatigue strength at N=107 for the carburized SCM420s was performed by applying the √area parameter model, and its fatigue strength was successfully estimated at an unquestionable level for practical use.