Conditions for CeS Formation During Manufacturing of Ce–S–Al Based Grain Refiners for Steels

Abstract

In the present investigation the phase relations within the Ce–S–Al system have been clarified, using a combination of optical microscopy and WDS microprobe analyses. As a starting point high-purity charge materials of cerium, aluminium and Ce₂S₃ were melted and superheated to about 2000°C within small tantalum crucibles inside a dedicated laboratory furnace filled with cleaned argon. The main constituent phases detected in the as-solidified samples are CeS, Ce₃Al, CeAl and γ-Ce, where the CeS phase constitutes a discontinuous dendritic network within the grain refiners. The melting experiments show that pure cerium can dissolve about 6 wt% of sulphur at 2000°C, which drops to approximately 1.8 wt% at 1500°C. The measured sulphur solubility is considerable lower than that inferred from the existing binary Ce–S phase diagram, which therefore should be revised to comply with these new measurements. Because alloying with aluminium reduces the sulphur solubility in liquid cerium, the addition of this element should be restricted if a high volume fraction of CeS is desired in the grain refiners. At the same time the use of Ce₂S₃ as a sulphur source in replacement of pyrite (FeS₂) means that aluminium is not actually needed to prevent the grain refiners from disintegrating in contact with air due to internal oxidation.