Late sulfur inoculation of spheroidal graphite cast irons

Abstract

The addition of small amounts of sulfur in the form of pyrite granules to post-inoculated ductile irons was investigated. Two types of nodular iron were used in this study: high silicon ductile iron and high carbon equivalent (CE) ductile iron. Due to the low levels of sulfur employed careful statistical analysis of the results was conducted. In high silicon ductile irons, the late addition of sulfur resulted in a larger fraction ferrite as revealed by image analysis. Differential thermal analysis and thermodynamic modeling of comparable ternary Fe-C-Si alloys further verified these results. Study of shape factors of graphite nodules indicated no significant effect of the low levels of sulfur added to the post-inoculated irons. For the high carbon equivalent irons, higher temperature primary graphite arrests were observed, suggesting that sulfur promoted the formation of potent substrates for the formation of graphite nodules at higher temperatures. Concurrently, through microstructural characterization it was corroborated that higher ferrite volume fractions were also attained. Under similar experimental conditions, the sulfur effect differs with different carbon equivalents. In high silicon (2.9 wt.% Si) eutectic irons, low sulfur additions did not noticeably alter the cast iron solidification. Conversely, in high CE (hypereutectic) irons there exists a stronger influence on the primary graphite arrest temperature. In both ductile iron types, image analysis of the resulting microstructures indicated that the late additions of sulfur to the post-inoculated melts produced larger nodule counts. These results reaffirmed the conclusion that sulfur plays a direct role in the formation of graphite, for instance by reacting and forming complex Mg oxysulfide particles that develop into potent nucleation sites for graphite.