Effect of Antimony on Graphite Growth in Ductile Iron

Lutz Dekker,Babette Tonn,Gerhard Lilienkamp

doi:10.1007/s40962-020-00434-1

Lutz Dekker, Babette Tonn + Show 1 more

Open Access

https://doi.org/10.1007/s40962-020-00434-1

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Abstract

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.

Highlights

The current study presents thermodynamic calculations showing that at the beginning of the eutectic solidification, Mg3Sb2 is formed
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 in graphite spheroids
During solidification of mostly thick-walled ductile iron castings, the graphite has a tendency to degenerate during eutectic growth, which results in exploded or even chunky graphite

Summary

Introduction

During solidification of mostly thick-walled ductile iron castings, the graphite has a tendency to degenerate during eutectic growth, which results in exploded or even chunky graphite. Possible reasons are long solidification times, high amounts of Si and the presence of Ce in the melt.[1]. This paper is an invited submission to IJMC selected from presentations at the 2nd Carl Loper 2019 Cast Iron Symposium held September 30 to October 1, 2019, in Bilbao, Spain. According to References 2–4, small amounts of Pb, Bi or Sb in a defined stoichiometric ratio with Ce can neutralize the detrimental effects of Ce on graphite growth by forming intermetallic phases in the melt. That is why mainly in large ductile iron castings very small amounts of Sb between 20 and 40 ppm are added to the cast iron melt to avoid the formation of chunky graphite. Even in ductile cast irons without any Ce in the melt, the addition of Sb to the melt is a proven instrument to suppress these undesirable graphite morphologies

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