Study of Graphite Nucleation in High Sulfur Ductile Irons

It is well known that sulfur is an excellent promoter of graphite and plays an important role in its formation. The final sulfur content of ductile iron is usually below 0.015%, in order to avoid the formation of slag and dross, although higher values can be found depending on the content of Ce or Mg. Sulfur can combine with some elements such as rare-earths (lanthanides), magnesium or/and calcium, forming sulfides or oxy-sulfides, which act as nucleation sites for graphite. 

To further understand the influence of sulfur in the nucleation process, a series of interrupted solidification experiments at increasing times were conducted on spheroidal graphite irons poured into standard thermal analysis cups, at two high levels of sulfur in the melt (0.016% and 0.019%). A commercial inoculant rich in Ce was added. Two different Mg-treatment alloy (with and without La and Ce) were used. Nucleation sites were characterized through spectrums, mapping and line scans of a FEG-SEM equipment.

Mg-Si-Al nitrides, Mg-Ca sulfides and Mg oxides to a lesser extent, seem to be the main nuclei, regardless of inoculation. All of them can appear alone or combined between them. Several Mg-Al oxides were observed, mainly when the number of complex nitrides was not too high. Some complex silicates were also detected acting as nuclei for graphite. No Ti-carbonitrides and phosphides were found. The nature of nuclei was not influenced by the quenching time. Not unexpectedly, the number of sulfides was favored by the high percentage of sulfur in the iron. The presence of lanthanides, always as sulfides, was directly related to the Mg-treatment. Some dendritic graphite nucleation on Ca-inclusions was detected.