Effect of the magnesium treatment on the nucleation potential of ductile iron

It is well known that the metallurgical quality and nucleation potential of a cast iron are influenced by a series of factors such as the metallic charge, the chemical composition, the liquid treatment, the inoculation process or the melting and holding temperatures. From all of them, the spheroidizing treatment plays an important role as it is responsible for graphite crystallization as spheroids, and consequently, for obtaining better mechanical properties. Ferrosilicon alloys, which contain from 3 to 9% magnesium and small additions of Al, Ca, La and Ce, are the most widely used composition for the treatment alloys. To further understand the influence of these elements on the nucleation of graphite, melts treated with 3 different Mg treatments (no RE, 1.3%RE and 2.3%RE) were produced and poured into standard thermal analysis cups. An inoculant rich in Ce was also used. Nucleation sites were characterized through spectrums, mapping and line scans of a FEG-SEM equipment.

Polygonal Mg-Si-Al nitrides were the main nuclei for Mg treatments with low level of RE. In several cases, Ti carbonitrides growing in their corners were detected. The presence of Ti(CN) was fully linked to the content of titanium in the base metal. Some Mg oxides or Mg-Al oxides were also observed. The lack of rare earths seems to favor the presence of dendritic graphite nucleating on Ca-inclusions. Mg-Ca sulfides were found as the main nucleation sites for Mg treatments with 2.3%RE. Their dominance is encouraged by the lack of inoculation. Usually, these sulfides are rich in La and Ce, which can appear distributed around all the inclusion, as a hat or as a shell. The origin of these RE-inclusions from the spheroidization process was demonstrated.