In order to elucidate the mechanism of nodule count increase by Bi, we analyzed spheroidal graphite cast iron with increased nodule count by Bi addition. In the Scanning Electron Microscope - Energy Dispersive X-ray Spectrometry (SEM-EDS) analysis, Bi was identified to be present near the center of the spheroidal graphite. The center of this spherical graphite was pretreated by Focused Ion Beam (FIB) micro sampling method to prepare analytical sampls, which were analyzed by Transmission Electron Microscope - Energy Dispersive X-ray Spectrometry (TEM-EDS). As a result, it was found that Bi exists as Bi-La-Ce-Sb oxide on the outer periphery of the graphite core centered on (Mg Ca) S.
In order to verify whether this Bi-La-Ce-Sb oxide is effective for increasing nodule count, an oxide sample having the same composition as the Bi-La-Ce-Sb oxide analyzed by TEM-EDS was prototyped by mechanical alloying, and the prototype Bi-La-Ce-Sb oxide was added to the molten metal of spheroidal graphite cast iron. As a result, nodule count did not increase with the addition of the prototype Bi-La-Ce-Sb oxide alone, but when the prototype Bi-La-Ce-Sb oxide and Fe-Si inoculant were mixed and added, the nodule count increased more than when only the Fe-Si inoculant was added.
These results indicate that, Bi addition is effective for increasing nodule count because Bi acts on the graphite nucleus as an oxide.
