Effect of Casting Skin Condition on Fatigue Strength of Gray Cast Iron

Abstract

The microstructure around the casting skin of cast iron changes according to conditions in the mold, wash material and cooling speed. When a self-hardened mold was used, a matrix microstructure containing all pearlite was obtained in the casting skin. When a greensand mold was used, the matrix microstructure was a mixture of pearlite and ferrite. In this study, tests were conducted to determine the influence of a casting skin on fatigue strength in gray cast iron with one sample containing a fully pearlitic matrix compared with another having a ferrite–pearlite mixture. For comparison, fatigue tests were also conducted on the sample with a pearlitic matrix taken from inside the sample and machined. The surface roughness of the test piece with casting skin was rougher than the test piece with the machined surface. The fatigue strength of the test piece with coarser surface roughness was lower. In a smaller eutectic cell, fatigue strength became higher. The fracture path was observed to be along the interface of graphite. It was considered that the fracture propagation was suppressed by smaller eutectic cells. On the other hand, a little ferrite generated in a casting skin made the fatigue strength lower.