Abstract:
Mechanical, corrosion and microstructural response of preconditioned thin wall ductile cast iron at varied ladle addition have been investigated. In a view to accomplishing the sole priority of pre-conditioner in the initial treatment of the molten metal and thereby modifying the insufficient nucleation potential that is associated with ordinary inoculated ductile iron and the carbide promoting tendency of high cooling rates, three different ladle additions were made before the nodularization and the final inoculation process. Magnesium ferro-silicon alloy was used as nodulizer on preconditioned molten cast iron from a rotary furnace held in a 12 kg treatment ladle. Nodularization was carried out in the treatment ladle by sandwich method and post-inoculation was thereafter adopted. Comparative studies were conducted on the basis of effects of cementite on mechanical, microstructural and corrosion properties of both preconditioned and unpreconditioned ductile cast iron and it was revealed that magnesium and silicon present in the molten metal react with the free oxygen and sulphur to provide additional nucleation sites for the subsequent post-inoculation thereby reducing the percentage carbide formed drastically. Investigation also revealed that there was a significant improvement in the response of the preconditioned melts to the subsequent inoculation as compared to the response of the unpreconditioned melt by a drastic reduction in hardness value from 85 HRA to 65 HRA. The optimum effects of Al,Zr-FeSi at 0.1 wt % as ladle addition was observed by increased tensile strength from 385 MPa to 420 MPa, yield strength from 250 MPa to 290 MPa and percentage elongation from 16 % to 20 % and thus resulting in ductile cast iron with reduced under cooling and higher nodule count. However, the results obtained from the use of this preconditioner at a higher percentage above 0.1 wt % (i.e. 0.15 wt %) revealed a slight drop in the material’s properties, most especially, chilling tendency, which was as a result of higher Magnesium consumption that led to exploded nodules. From the results obtained, it was observed that there was a significant reduction in carbide formation after preconditioning just as there was an increased nodule counts and improved nodularity. It could also be inferred that mechanical properties met the acceptable values for the given grade of ductile cast iron with an improved tensile strength and reduced hardness after preconditioning
