Effect of double shot peening using super-hard fine particles on fatigue strength of 18% Ni maraging steel

Rotating bending fatigue tests up to 10⁸ cycles were carried out in order to investigate the effects of double shot peeing using super-hard fine particles on the fatigue strength and the fracture mechanism in an 18%Ni maraging steel. The effects of surface roughness, residual stress and work hardened layer on the fatigue strength were examined in connection with the size of shot particles. The S-N curves of double shot peened specimens showed the same two steps as those seen in the S -N curves of single shot peened ones. The fatigue strength in the short life region is greatly increased by the double shot peeing irrespective of the size of shot particles, especially in the specimens with rough surface caused by the first stage of double shot peening. The high fatigue strength is due to very small surface roughness and high compressive residual stress generated just under the surface due to the second stage of double shot peening using super-hard fine particles. On the other hand, the fracture mechanism in the double shot peened specimens is just the same as that in the single shot peened specimens, i. e., in the short life region, surface fracture occurs, while in the long life region, fish-eye fracture, a typical interior fracture happens. As a matter of course, the fracture mechanisms can transit from surface fracture to interior fracture depending on the stress applied. However, at the stress levels between surface fracture and interior one, a unique interior fracture with the initiation site in the weak structure like a grain boundary was observed. This fracture surface showed a radial and rough one instead of the typical fish-eye surface which showed a flat surface.