Effect of Cobalt Additions on the Microstructure and Mechanical Properties of As-Cast Thin-Wall Ductile Iron

The production of thin-wall ductile iron can be challenging due to the formation of carbides during the solidification process. One way to reduce carbides in thin sections is by the addition of silicon, but this is detrimental for the impact strength and fracture toughness of ductile iron. It is known that cobalt as an alloying element in ductile iron increases the nodule count resulting in a larger fraction of ferrite. Therefore, the ability of cobalt to inhibit carbide formation was studied. The goal was to inhibit carbide formation without increasing silicon content. Five heats were produced: Co-free ductile iron, 1 wt%, 2 wt%, 3 wt%, and 4 wt% Co. These were cast into plates with thicknesses of 2, 2.5, 3, 3.5, 4, and 6 mm. Metallography was performed to evaluate the percent nodularity, nodule count (N/mm(2)), and ferrite/pearlite/carbide percentages. Tensile testing was performed using flat sub-size dogbone samples. Also, the fracture surfaces of the 2 and 6 mm sections were analyzed using SEM. Finally, Brinell hardness was performed on each section thickness. There was no increase in the percent nodularity with the addition of cobalt, but with 4 wt% Co an increase in nodule count was observed. With the addition of 3 wt% and 4 wt% Co, there was a reduction in carbide percentage due to an increase in ferrite content. With the addition of 4 wt% Co, there is a reduction in yield strength and an increase in elongation due to a higher ferrite percentage. It was discovered that with the addition of 4 wt% Co the production of carbide-free sections is possible without increasing the silicon percentage above 2.41 wt% Si.