Effect of titanium in the nucleation process of spheroidal and compacted graphite cast iron

Nucleation and growth phenomena that control the solidification directly affect the soundness and the mechanical properties of the castings. Understanding the kinetics of these processes is thus a key objective in managing the soundness of cast iron parts. The drive to understand this problem has been responsible for the development of a multitude of theories that during the last decades have tried to clarify these mechanisms for the three main types of graphite (lamellar, compacted and spheroidal). 

Silicates, oxides, nitrides and sulfides have been reported to act as nuclei for graphite precipitation in most cases. Previous studies carried out by the authors of this paper have demonstrated that the addition of small levels of Ti in the base metal can modify the preferred nucleation sites. In order to analyze the influence of titanium on the nucleation of spheroidal and compacted graphite cast iron by studying their similarities and differences, a series of quenching experiments with different titanium levels (0.007-0.037%) were produced and poured into standard thermal analysis cups, with and without inoculation. The nucleation sites were characterized through a Field Emission Gun Scanning Electron Microscope (FEG-SEM).

Mg-Ca sulfides, and depending on the Ti content, complex Mg-Si-Al nitrides or/and Ti(CN) were found acting as the main nuclei for both types of graphite. Possible sequences of events during solidification are discussed. Disregistry calculations were performed to support the experimental findings. The effect of the inoculation process is studied. An extensive discussion on the growth of graphite and the type of inclusions is provided. No differences in the nature of the nuclei were observed between the samples quenched and the ones obtained at room temperature, which confirms the reliability of the research methodology.