Effect of graphite morphology on the fatigue and fracture resistance of ferritic ductile cast iron

In this paper, four material variants of an EN-GJS-400-type ferritic ductile cast iron (DCI) are produced by varying solidification time and chemical composition. In this way, experimental batches with different size of graphite nodules and spheroidal graphite fraction are obtained. The material variants are characterized through static tensile, rotating bending fatigue, fatigue crack growth and fracture toughness tests. It is shown that by increasing the mean diameter of the spheroidal graphite particles fatigue strength, tensile strength and total elongation decrease, while fatigue crack growth threshold and fracture toughness increase. On the contrary, reducing the spheroidal graphite fraction owing to degeneration into chunky graphite results in lower tensile and fatigue properties as well as fracture toughness, while the near-threshold fatigue crack growth resistance is improved. This effect is discussed by the light of crack closure, fracture surface roughness and fractographic analyses.