Abstract
Sulfur, an element that belongs to group 16 (chalcogens) of the periodic table, is an excellent promoter of nucleation substrates for graphite in cast iron. In ductile iron, sulfur favors a higher nodule count, which inhibits the risk of carbides and of microporosity. It is reasonable to expect that other elements from group 16, such as selenium or tellurium, play similar roles in the nucleation of graphite. The objective of this paper was to investigate the effect of selenium on the process of graphite formation. Thermal analysis cups were poured to evaluate the nodule count and size distribution. Some of the cups were not inoculated, while others were inoculated with a Ce-bearing inoculant, or with the Ce inoculant and additions of Se. Cross-shaped castings were also poured to quantify the microporosity regions by tomography. It appears that selenium additions modify the number and size of graphite particles, as well as the volume of microshrinkage. Direct correlations between these three parameters were found. Advanced Extensive Field Emission Gun Scanning Electron Microscope (FEG-SEM) techniques were used to identify the nature of the main nucleation compounds. Selenides, combined with Mg and rare earths, were observed to serve as nuclei for graphite. Their presence was justified by thermodynamics calculations.
Highlights
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It is well known that sulfur, an element that belongs to group 16, plays a major role in the nucleation process of spheroidal graphite (SG) cast iron, usually in combination with other active minor elements, such as aluminum, calcium, cerium, or barium, introduced through the inoculation and/or spheroidization processes [1,2,3,4]
Similar observations were made in gray cast irons [11,12]
Summary
It is well known that sulfur, an element that belongs to group 16 (chalcogens), plays a major role in the nucleation process of spheroidal graphite (SG) cast iron, usually in combination with other active minor elements, such as aluminum, calcium, cerium, or barium, introduced through the inoculation and/or spheroidization processes [1,2,3,4]. Many theories support the assumption of the heterogeneous nucleation on oxy-sulfides, Mg-Ca sulfides, or complex Mg-Ca-RE sulfides, as the main nuclei for spheroidal and compacted graphite [5,6,7,8,9,10]. These sulfides are among the most stable nonmetallic compounds. Similar observations were made in gray cast irons [11,12]
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