Graphite Nucleation in Compacted Graphite Cast Iron

During the last several decades, a multitude of theories have attempted to explain the process of graphite nucleation in lamellar (LG) and spheroidal (SG) graphite iron castings. Nevertheless, the complex 3D morphology of compacted graphite (CG) has hindered significant advances in similar theories for this type of graphite. To bring clarity to this issue, interrupted solidification experiments were conducted on compacted graphite irons at two different levels of titanium content in the melt (0.008% and 0.037%), with and without addition of commercial inoculants (Ce, MnZr). Nucleation sites were characterized through detectors, spectrums, mapping, and line scans utilizing FEG-SEM equipment. It was found that the nature of the inclusions acting as nucleation sites is directly related to the titanium content in the base metal. Nucleation in samples with low level of Ti occurs on Mg–Ca sulfides or Mg–Si–Al nitrides, which usually appear alone and seem to affect the growth of graphite. In the case of high percentage of Ti, double inclusions formed by Ti carbonitrides growing on Mg–Ca sulfides and restricting their growth seem to be the best combination for the nucleation of graphite. This is in line with our earlier findings for spheroidal graphite.