Effects of nickel and processing variables on the mechanical properties of austempered ductile irons

This study investigates the effects of inoculation techniques, ingate numbers and raw materials on the mechanical properties of ADIs. Five batches of base ductile irons (DI1-DI5) were poured to produce stepped plate castings following designated processing variables. DI1 and DI2 were used to compare the effect of raw materials on the sulphur content of irons and the resulting properties. DI1 and DI4 were produced in order to study the effect of nickel; DI3 and DI5 were poured to evaluate the effect of ingate numbers; DI4 and DI5 were poured to study the effect of inoculation techniques. Effects of these variables on the mechanical properties and microstructures of as-cast and austempered ductile irons (ADIs) were evaluated respectively. Using a given magnesium content (0.033-0.047 wt%), ADIs with a high percentage of sulphur, in the range of 0.004-0.02 wt%, developed a higher percentage elongation. ADIs alloyed with 0.48 wt% Cu, 0.46 wt% Ni and a minimum level of sulphur (0.004 wt%) produced a lower degree of deviation in strength, elongation and impact energy but showed the greatest deviation in the tested hardness of the stepped plate castings. Stream inoculation improved the nodule counts and Modularity of base ductile irons but also introduced dross and/or unreacted inoculant within the stepped plate casting. This was detrimental to the tested elongation of ADIs taken from the stepped plate casting. Two-step austempering treatment could significantly improve the tested elongation of ADIs without significantly affecting the developed strength.