浸炭および浸炭窒化材のピッティング疲労に関する研究

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This paper attempts to clarify the mechanism of pitting fatigue, one of the contact rolling fatigue failures at tooth faces in a gear. Carburized and carbonitrided JIS-SCr420 steels with as-heat treated surface were tested by geared roller tester with a contact stress of 3.68 GPa and a slip ratio of - 40%. At the surface of the carburized specimens, nonmartensitic areas derived from low hardenability due to internal oxidation were worn and detached with increase in the contact rolling cycles. It roughened the contact track and accelerated main crack propagation. On the other hand, carbonitrided specimens with little nonmartensitic area on their surfaces showed higher pitting resistance than carburized ones. It was further clarified that the surface layer of carbonitrided specimen was strengthened not only by stress induced martensitic transformation but nitrides precipitation during roller contact loading.