Abstract
Ion-nitriding treatment has been brought into greater use as a surface hardening technique, for it offers such potential advantages as saving energy, clean procedure and so forth. Though the fundamental mechanical properties of ion-nitrided specimens have been investigated from various points of view, there exist few papers on their fatigue behavior under repeated impact load. In the present study, a series of low cycle impact fatigue tests were carried out on the ion-nitrided SM50 steel specimens by using the Matsumura type impact fatigue testing machine. The results obtained at four impact energy levels of 0.2, 0.3, 0.4 and 0.5 kgf·m indicated that the surface hardened layer of the ion-nitrided specimen behaved in a brittle manner, that is, surface cracks initiated at the first impact regardless of the impact energy level. It was concluded that such brittle cracks shortened the fatigue lives of the ion-nitrided specimens, when compared with the results for the annealed SM50 specimens. Furthermore, the above mentioned fatigue properties were discussed in relation to the fracture mode in monotonic tensile tests.
