Development of high strength and reliable spheroidal graphite cast iron

球墨铸铁具有高强度、高韧性的静力学性能。然而，众所周知，疲劳强度并不是那么大。另一方面，强化表面并产生压缩残余应力的表面处理是传递裂纹萌生的有效方法。本研究对铁素体球墨铸铁进行了径向渗氮处理，并与铸态进行了对比，研究了渗氮对力学性能(硬度、耐磨性和疲劳强度)的影响。渗氮提高了疲劳强度。断裂是从微缩或异常石墨开始的，与铸铁的断裂方式相同。产生这些结果的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.