Simple Solutions That Work! Issue 7

It was further demonstrated by Riposan (1998) that a small sulfur addition (less than 0.010%), when added concurrently with calcium silicon-based inoculants increased graphite nucleation potential in ductile iron, but without affecting graphite nodularity. Chisamera and Riposan (1998) also showed that the strong sulfide forming tendencies of calcium and rare earth metals, when used in conjunction with controlled sulfur additions, strongly promoted the formation of sulfide compounds assisting their effectiveness as nodular graphite nuclei. The amount of sulfur addition in magnesium-treated iron needed to obtain a critical nodular graphite / compacted graphite (NG/CG) ratio depends on the residual magnesium content after magnesium treatment as well as holding time prior to pouring. Other important factors are casting wall thickness, mold type and thermal gradient effects. Kelley also showed that it was also possible to produce both compacted graphite irons and ductile iron from the same base iron melt (suitable for ductile iron production) using cored wire containing a high magnesium containing ferrosilicon. Kelley, et al. also showed that it was possible to use such a magnesium treated ductile iron, with low residual Mg levels (0.025 to 0.04%) but with an addition of “fresh” sulfur which was in the form of a rapid dissolving iron sulfide briquette since loose, granulated iron pyrites produced inconsistent results. Less than 0.02 weight percent sulfur was needed to “denodulize” the iron. Thus, it was possible to have the same furnace melt chemistry and have a controlled transition from ductile iron to compacted graphite iron in the same campaign. The key to his success was having complete control over the magnesium reaction. Sulfur can be both detrimental and beneficial element in ductile iron and compacted graphite iron. Sulfur’s harmful and beneficial effects are related to the amount present before magnesium treatment (nodularizing process) as well as its concentration during graphite nucleation. A high base iron sulfur content is generally considered harmful because it will lower the magnesium efficiency and result in increased dross formation in both ductile and compacted graphite irons. However, in ductile iron, a minimum sulfur level of at least 0.005 to 0.008% is necessary after magnesium treatment to insure proper post-inoculation and reduce the risk of carbides. Thus, after magnesium treatment, the presence of critical sulfur levels is considered beneficial for the promotion of graphite nuclei. Further, the reaction of sulfur with sulfide forming elements such as rare earths and calcium enhance nucleation of graphite in ductile irons. In compacted graphite irons and after magnesium treatment, control of sulfur levels is critical for controlling graphite nodularity and promoting compacted formation. In Kelley’s, two sources of sulfur were used experimentally to resulfurize the magnesium treated iron. These included 1) FeS2 or iron pyrites, nominally containing 49ercent sulfur having a particle size of 70 mesh by down and 2) briquetted iron pyrites (Resulf 30) FeS2 briquettes. Excellent and consistent control of the sulfur recovery has been found to be an essential feature of this technology and it has been demonstrated in foundry conditions, for both ductile iron and compacted graphite iron. In the early stages of the investigation, granular FeS2 additions after Mg-treatment were used to re-introduce sulfur (resulfurize) to an iron melt. Since iron pyrites are normally available only in very fine mesh sizes, difficulties are often encountered during the addition to ladles, resulting in inconsistent recoveries. The fine sized FeS2 particles, when added to molten irons, tend to become airborne due to convection currents of super-heated air, leading to the generation of obnoxious fumes and odors. For all these reasons, it was necessary to improve control over the sulfur addition. It was found that briquetted FeS2 (Resulf 30) can circumvent the inconsistencies of adding powdery iron pyrites. The “iron pyrite briquettes” are formulated to go into solution rapidly without odor. A second and important benefit of these briquettes is that they supply a “fresh sulfur” source to the iron, which affects the surface activity and speculatively changes the graphite growth mechanism promoting the “compacted” growth mode. The sulfur additions were calculated from the charts developed by Riposan (1998). Continued on page 30 29