Development of high strength and reliable spheroidal graphite cast iron

球墨铸铁具有高强度和高韧性的静态性能。然而，众所周知，疲劳强度不是那么大。另一方面，表面处理，硬化表面和产生的残余压应力是一种有效的方法来中继裂纹萌生.在这项研究中，自由基渗氮，上级表面处理方法之一，应用于铁素体球墨铸铁和机械性能的渗氮效果（硬度，耐磨性和疲劳强度）进行了比较，与铸态。渗氮提高了疲劳强度。断裂是由显微收缩或异常石墨引起的，与铸态铸铁的断裂方式相同。其原因是硬化层和残余压应力抑制了表面裂纹的萌生。

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.