Effects of shot peening on fatigue behavior in high speed steel and cast iron with spheroidal vanadium carbides dispersed within martensitic-matrix microstructure

Four-point bending fatigue tests had been performed using high speed steel and cast iron with vanadium carbides (VCs) dispersed within the martensitic-matrix microstructure. Shot peening or shot blast was applied to both the materials and the effect of surface treatments on fatigue behavior was investigated. The fatigue strengths of the high speed steel were improved by both shot peening and shot blast processes due to the high hardness near the specimen surface and residual compressive stress. Although the hardness of cast iron was enhanced by both treatments, the fatigue strengths were not improved by the shot blast because of the existence of large casting defects. Shot peening with higher shot energy could induce the transition of crack initiation mechanism of cast iron, where crack initiated from the cluster of VCs. However the shot peening had small effect on the fatigue strengths of the cast iron because large casting defects were not removed by the shot peening due to the high hardness of the martensitic matrix.