焼なました工具鋼SK５の疲労限度の評価について

概要

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Fatigue limit is related to the crack initiation and growth process. The length of a surface crack at fatigue limit is related to the size of a single crystal in the case of a plain specimen of low carbon steel, copper, etc. However, the initial crack length is independent of the crystal size in the case of pure aluminum, a lamellar pearlitic steel, etc. So, the relationship between the initial crack length and fatigue limit was investigated in this study by using tool steel SK5. The original microstructure of this material includes spherical microstructure. However, after annealing under some temperatures, that microstructure changed to a lamellar microstructure. It was found that the fatigue limit of a plain specimen of this material could not be evaluated by average hardness. However, its fatigue limit could be evaluated by a relation in which the parameters are the hardness and size of the crack initiation area, independent of the heat treatment conditions. This result was discussed based on the observations of crack growth behavior and fracture surface.