An analysis of the fatigue performance of 316L stainless steel bar drilled and tapped is performed. The effect of flow drilling and flow tapping on the material microstructure, microhardness and fatigue life is compared to the characteristics of conventional cutting processes.
The hardness recorded at the surface of flow formed holes is 62% higher than that of the raw material. In addition, grains are refined and plastically deformed by the flow processes.
Four-points bending fatigue tests were performed at 3 stress amplitudes and with a stress ratio of 0.1. The results revealed no significant differences in fatigue life for tests performed bending moment is equal to 75% and 60% of the yield bending moment. Nevertheless, when the maximum bending moment applied was limited to 50% of the yield bending moment, the specimens containing holes manufactured by the cutting endure more cycles. Fractographic observations revealed, for both specimens, that the failure initiated from the thread beneath the surface of maximum tensile stress. On the fracture surfaces of flow processed specimens, cracks initiated from the discontinuities observed at the peak of threads. In addition, secondary cracks are observed at the thread roots where to material is hard and the grains are refined.