Gigacycle fatigue properties of high-strength steels according to inclusion and ODA sizes

Gigacycle fatigue tests were conducted for several versions of JIS-SCM440 low-alloy and JIS-SUJ2 bearing steels using 20-kHz ultrasonic fatigue testing to elucidate the relationship of the inclusion size and type to fish-eye fracture properties. The total number of tested specimens was over 200. Most of the specimens revealed Al2O3 Or (Cr, Fe)(3) inclusion-originating types of fish-eye fractures, while TIN inclusions and the matrix also caused fish-eye fractures in some specimens. Based on these fatigue test results, 10(9)-cycle fatigue limits were estimated according to inclusion size by resorting the obtained data points according to their inclusion sizes at the fish-eye fracture origin. The estimated fatigue limits revealed saturation when the inclusion sizes were smaller than 15 mu m, while those fatigue limits depended on the inclusion sizes to the -1/6th power in the case of inclusions above 15 pin in size. The saturation of the 109-cycle fatigue limits was considered to be caused by the effects of the optically dark areas (ODAs). Moreover, the fatigue limits also depended on inclusion type. In comparing the Al2O3 and (Cr, Fe)(3)C inclusions, the key features causing the difference in the fatigue limits were likely to be bonding between the inclusion and the matrix, i.e., the (Cr, Fe)(3)C inclusions were tightly bonded to the matrix, unlike the Al2O3 inclusions, although both inclusions were of the hard type.