Development of high strength and reliable spheroidal graphite cast iron

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Spheroidal graphite cast iron has high strength and high toughness in the static property. However, it is well known that the fatigue strength is not so great. On the other hand, surface treatment that hardens surface and generates the compressive residual stress is an effective method to relay the crack initiation.In this study, radical nitriding, one of superior surface treatment methods, was applied to ferritic spheroidal graphite cast iron and the effect of nitriding on the mechanical property (hardness, wear resistance and fatigue strength) were investigated in comparison with as cast one. Fatigue strength was increased by nitriding. Fracture started from microshrinkages or abnormal graphites in the same manner as that of as cast iron. The cause of these results was the hardened layer and the compressive residual stress, which suppressed the initiation of the surface crack.