Effect of tellurium on the nucleation process of spheroidal graphite in cast iron

The crystallization of graphite nodules in cast iron is initiated by compounds acting as heterogeneous nuclei. Sulfur promotes precipitation of such non-metallic substrates, increasing the nodule count. It is reasonable to assume that homologous elements in the periodic table, such as selenium or tellurium, could play a similar role in graphite nucleation. Indeed, selenides were found to act as nuclei. To investigate the effect of tellurium on graphite nucleation three different levels of tellurium were added. Cooling curves were recorded to study the variation of the main parameters of solidification. Thermal analysis cups were poured to evaluate the nodule count and size distribution. A Ca–Zr-bearing inoculant was added in some of these cups. Cross-shaped castings were also poured to quantify the microporosity regions by tomography. It was found that tellurium additions alter significantly the solidification process, modifying the nodule count, the size of the graphite, as well as the volume of microshrinkage. Their effect is more evident as the content of tellurium increases. Early precipitation and growth of graphite seems to be favored. Advanced Extensive Field Emission Gun Scanning Electron Microscope (FEG-SEM) techniques were used to identify the nature of the main nucleation sites. Tellurium compounds combined with oxygen and Rare Earths were observed acting as nuclei for graphite. Their presence was supported by theoretical thermodynamics calculations. Tellurium effect on graphite growth was discussed.