Impact fatigue tests were carried out using various kinds of carburized steels. It was shown that internal oxidation was remarkably reduced with decreasing Si and Mn contents, and resulted in higher impact fatigue strength. The effects of residual stress, retained austenite, hardness near the surface, and fatigue fracture toughness were investigated. The difference of the fatigue strength in various carburized steels could be explained in terms of those effects.
The effect of shot peening application to carburized steel was investigated using JIS standard type steels. As a result, high cycle impact fatigue strength increased 40 to 60% gain by shot peening. This improvement was caused by increase of hardness near surface and compressive residual stress. On the other hand, low cycle fatigue strength showed no gain. For this phenomenon, it was suggested that decrease of the amount of retained austenite by shot peening showed negative effect for fatigue strength. Further, the difference of the fatigue strength in shot peened steels was very slightly. For this reason, it was considered that the life of shot peened steel was influenced by releasing its compressive residual stress rather than the effect of alloying elements.