Effect of surface residual stress and inclusion size on fatigue failure mode of matrix HSS in very high cycle regime

Fatigue properties in high cycle and very high cycle fatigue regime of matrix high-speed tool steel (1.3C-6W-5Mo-3V-8Co) were investigated through the experimental results obtained from a cantilever-type rotary bending fatigue test in air at room temperature using hourglass shaped specimens. Two kinds of specimen were processed by grinder and cutting tool, which led to different surface compressive residual stress. Both specimens showed clear duplex S-N curve, composed of three or two types of failure mode depending on the stress amplitude, such as, a surface inclusion-induced failure mode (S-mode), a subsurface inclusion-induced failure mode without (I-mode) and with granular bright facet (GBF) area in the vicinity of inclusion (IGmode). The transition of the failure mode from S-mode to I-mode and from I-mode to IG-mode was occurred at high stress amplitude level by the effect of large compressive residual stress on the surface layer. A map of the appearance of failure modes relating the stress amplitude with the residual stress was proposed and showed a good agreement with the experimental results. Fatigue life in very high-cycle regime was almost same between the both specimens because of the existence of almost same size inclusion at crack origin. S-N curve depended on the inclusion size was provided with the estimation of crack growth rate from the S-N data.