Microstructure and mechanical properties of 4% cobalt and nickel alloyed ductile irons

In the study, 4% cobalt and nickel elements were added to ductile iron (DI) to investigate the effects of the alloying elements on both its microstructure and its mechanical properties. The results show that 4% cobalt alloyed ductile iron (4% Co-DI) not only increases the nodule count and ferrite content in the microstructure, but also improves the mechanical properties such as yield strength, tensile strength. impact toughness and fracture toughness (K-1C) as compared to unalloyed DI. In contrast, with 4% nickel addition, there is a decreased nodule count and increased pearlite content in the matrix. However, this material (4% Ni-DI) has the highest hardness and strength but the lowest ductility and toughness among all the ductile irons. In the case of microstructure, all kinds of iron have a sufficient nodularity up to 90% and the sequence of the nodule count is 4% Co-DI (150nodules/mm(2)) > Dl (106nodules/mm(2)) > 4% Ni-DI (85nodules/mm(2)). Moreover, it appears that cobalt plays an important role in stabilizing the ferrite phase, whereas 4% nickel accelerates the formation of pearlite in ductile iron. (c) 2006 Elsevier B.V. All rights reserved.