缺口对渗氮CrNiW钢超高周疲劳特性的影响

In order to study the influence of notch sensitivity on fatigue properties of nitrided CrNiW steel, a smooth specimen and two notched specimens with concentration factor K_t=1.20 and K_t=1.55 were designed, respectively. The very high cycle fatigue tests were carried out by using nitrided CrNiW steel. The microstructure of the fatigue specimen was observed by SEM. The relationship of notch fatigue factor K_f, notch sensitivity index q and number of cycles to failure N_f were analyzed. The results show that, the case-nitriding has little effect on the improvement of fatigue strength of steel. For the smooth specimen and the notch specimen with K_t=1.20, the failure modes can be classified as surface induced failure and the interior induced failure where the defects are limited in the non-nitriding area. For the notch specimen with K_t=1.55, interior failure induced by the inhomogeneous matrix area within the nitrided layer is observed, which is a new failure characteristic. The duplex S-N property of smooth specimen is not obvious. However, there is a duplex S-N property with surface induced failure and interior induced failure as K_t=1.20, and there is a duplex S-N property only with surface induced failure which occurs in the short life regime and the VHCF regime as K_t=1.55. With the increase of K_t, the phenomenon of multiple interior crack sites is replaced by the single interior crack site, and the interior defect position gradually moves towards the surface of specimen, and the shape of fisheye first approximately exhibits the oblate with major axis at x direction, followed by the circular shape, and then the elliptical shape with major axis at y direction. The K_t has a large effect on fatigue life with surface failure, but has a little effect on fatigue life with interior failure.