Long solidification time effect on solution strengthened ferritic ductile iron fatigue properties

Nowadays, lightweight design, aimed at reducing the eco-impact of mechanical components, is considered imperative and it asks for ever novel materials. In this scenario, solution strengthened ferritic ductile irons, thanks to their good combination of strength and ductility, are a potential alternative to the more expensive light alloys. Their unique properties come from a fully ferritic matrix solution hardened by high amounts of silicon ranging from 3 wt% to 4.3 wt%. Moreover, the fully ferritic matrix makes the alloy less sensitive to the hardness variation through the thickness and therefore more easily workable, compared to a ferritic-pearlitic cast iron of the same strength. With reference to heavy section castings, wind energy industry could take considerable advantage from this material in reducing the weights of heavy section components, provided that their mechanical properties, compared to those of separately cast samples with relatively thin wall, are well assessed. Keeping this challenge in mind, this work is aimed at characterizing the static and fatigue strength of a heavy section solution strengthened ferritic ductile iron casting. Static mechanical properties were found to decrease slightly with the increase of the solidification time. On the other hand, the fatigue resistance was reduced by 33% when the solidification time increased from 10 min to 10 h. Reasons were attributed to defects induced by long solidification times such as degenerate graphite and shrinkage porosities acting as crack initiation sites. Finally, a ‘master curve’ linking the solidification time with the fatigue strength is proposed as a design tool for designers who decide to exploit the potentialities of the investigated cast iron grade.