The effect of alloy solidification path on sulfide formation in Fe-Cr-Ni alloys

Solidification and sulfide precipitation in Fe-Cr-Ni alloys were investigated. Five samples containing 0.3 wt% S and 1.5 wt% Mn were investigated and each sample chemistry was adjusted to solidify as primary austenite, primary ferrite or eutectic by tailoring the Cr and Ni contents. The solidification was observed in-situ in a Confocal Scanning Laser Microscope (CSLM) equipped with a gold-image furnace, and the morphology and distribution of sulfides were characterized through SEM and Optical Microscopy. For high Ni and low Cr alloys which solidified in the primary austenite field, sulfides formed in the enriched melt ahead of the dendrite fronts as a eutectic metal-sulfide structure. On the other hand, for high Cr and low Ni alloys which solidified in the primary ferrite field, liquid sulfide phase precipitated in the inter-dendritic enriched melt first, and it appeared to deform and to concentrate in the dendrite boundaries during cooling. In the near eutectic intermediate Cr and Ni containing alloy, sulfides formed in the solid state. The sulfides formed in all the alloys were Mn and/or Cr rich.