Effect of antimony on graphite growth in ductile iron
Antimony is used in ductile cast iron melts to avoid graphite degenerations like exploded graphite and chunky graphite. However, the effect of Sb in the melt on the crystallization of ductile cast iron is still unknown, therefore the aim of our investigations. The current study presents thermodynamic calculations showing that at the beginning of the eutectic solidification, Mg3Sb2 is formed. Furthermore, the amount of Sb increases in the austenite and decreases in the remaining melt during solidification. An experiment was conducted, in which a higher amount of Sb than usual was added to a ductile iron melt. In the microstructure of the Sb-containing sample, a higher graphite precipitate count was measured than in the reference sample without Sb. The graphite spheroids of the Sb-containing sample show an almost perfect roundness and a very smooth surface. However, lamellar outgrowths from the graphite spheroids were also observed. Using scanning electron microscopy with energy-dispersive X-ray spectroscopy, it could be shown that Sb-containing phases have been formed, which are located in the pearlitic matrix and also in graphite spheroids. As a result of spectroscopic investigations, it can be assumed that Sb-containing phases act as nucleant for graphite nucleation. Further investigations by Auger electron spectroscopy showed that a thin layer of Sb at the interface between graphite and matrix has been formed. It seems that this layer constricts the diffusion of carbon to the spheroidal graphite and acts as a surfactant favoring a layer-by-layer growth of the graphite spheroids.