Phase Relations in Ce–Al–Fe–S Based Grain Refiners for Steels

Abstract

In the present investigation the phase relations within the Ce–Al–Fe–S 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 FeS₂ are melted and superheated to about 2000°C within small tantalum crucibles inside a dedicated laboratory furnace filled with cleaned argon. The phases detected in the as-solidified samples were CeS, Ce₃Al, Fe₂Ce and γ-Ce, along with Ce₂O₂S, which is an undesirable microconstituent in CeS-based grain refiners. It is concluded that FeS₂ can be used as a sulphur source for addition up to about 4 wt% sulphur. At higher levels the Fe–Ta interaction becomes so vigorous that tantalum no longer acts as an inert refractory metal and wetting becomes a major problem. In contrast, aluminium is an essential alloying element in the sense that it prevents the grain refiners from disintegrating in contact with air due to internal oxidation of free cerium by promoting the formation of Ce₃Al.