Fatigue design of heavy section ductile irons: Influence of chunky graphite

The influence of chunky graphite on mechanical properties, especially on fatigue resistance, of an EN-GJS-400 ferritic ductile iron, was investigated by tensile and rotating-bending tests. The samples for the experiments were machined from two heavy section castings. The first one was cast with the addition of a Ce-containing inoculant to promote chunky formation and the second one by using a standard Ce-free inoculant. The presence of chunky graphite in the samples decreased tensile strength and elongation at break, while no significant effect on the hardness and yield strength was measured. Also fatigue strength was negatively affected, with a decrease of fatigue limit of about 12%. This value was calculated also taking into account the different amount of microporosity in the two sets of samples and its effect on the fatigue strength by using the Murakami approach. Fracture analysis showed that chunky graphite has a limited influence on fatigue crack nucleation if compared to micro-shrinkages, while it behaves as a preferential path for crack propagation. A simple way to consider the presence of chunky graphite in the fatigue design of heavy section ductile irons is proposed on the basis of the present results and taking into consideration the published data too.