A study of sulphur effect in high silicon ductile irons

Abstract

Late sulphur additions to post-inoculated high silicon ductile irons were studied by means of microstructural characterisation and thermal analysis, complemented with statistical tools, image analysis and thermodynamic modeling. Using a 4.31% carbon equivalent iron melt, two low sulphur levels were investigated in an equivalent post-inoculated iron: 0.005 wt.% and 0.012 wt.% S, where the higher sulphur level was produced by the addition of iron sulfide after treatment and inoculation. A thorough image analysis was used for graphite particle characterisation and fraction ferrite, revealing that, under these experimental conditions, sulphur does not significantly affect the iron nodularity at the investigated levels. Two Weibull distributions fitted to the frequency distributions of the graphite shape factor in both iron sample sets indicated that there were no significant differences between shape factor populations. In addition, nodule counts increased with addition of sulphur to the post-inoculated iron. A larger amount of ferrite was detected in the higher sulphur ductile iron, and was attributed to the higher nodule count. The use of the inverse cooling rate method made it possible to verify this correlation by inspecting thermal events in the eutectoid range. A more energetic reaction in the higher sulphur iron upon cooling was attributed to the formation of more ferrite. Differential thermal analysis was used to evaluate those results and thermodynamic modeling of phase formation in an equivalent Fe-C-Si was used to validate the overall findings. Finally, the inverse cooling rate method was demonstrated to be a reliable and robust tool when minor differences in solid state transformations are investigated.