Effect of Ferrite Grain and Graphite Nodule Size on Fatigue Strength of Ferritic Spheroidal Graphite Cast Iron

概要

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To clarify the effect of graphite nodule size and ferritic gain size on fatigue strength of ferritic spheroidal graphite cast iron, tensile tests and fatigue tests were carried out. Six types of test specimens were constructed with different gain sizes from the speroidal graphite cast iron blocks. Two block sizes were considered as 32mm and 80mm in thickness. The ferritic spheroidal graphite cast iron was constructed by three types of ferritization heat treatment of the speroidal graphite cast iron. The ferrite grain refinement improved both tensile and proof strengths. The initiation-site of fatigue fracture was different based on the graphite nodule size. The fatigue cracks of large graphite nodule iron initiated from abnormal graphite, that is, closely adjoining graphite nodules or coarse graphite. On the other hand, the fatigue cracks of small graphite nodule iron initiated from small shrinkage cavity. The fatigue strength with large graphite nodules was improved, while the fatigue strength of small graphite nodule was unchanged by the ferrite gain refinement. It was also noted that the fatigue strength of spheroidal graphite cast iron with different ferritic grain size was determined by the maximum length of the non-propagating micro-crack.