Inoculation, Solidification Studied.

The 13 presentations of the Cast Iron Div. focused on numerous technical subjects, ranging from age strengthening of gray iron to compacted graphite iron to thin-wall ductile iron production concepts. Below is a sampling of some of the papers presented.

Inoculation was an area of high interest. T. Skaland, Elkem ASA, Kristiansand, Norway, presented a new approach to ductile iron inoculation that has improved casting performance and properties (01-078).

The new ferrosilicon (FeSi) alloy contains levels of calcium (Ca) and cerium (Ce) that are adjusted to minimize chill formation and neutralize subversive trace elements in the iron. In addition, the inoculant contains small and controlled amounts of nonmetallic sulfur (S) and oxygen (O) powders that simultaneously react with the Ca and Ce during introduction into the iron. This special composition is designed to give highly powerful graphite nucleation conditions in ductile irons along with very effective chill and shrinkage reductions, said Skaland.

In four field trials, the new inoculant was found to be more powerful than conventional FeSi-based inoculants and showed the effective reduction in shrinkage tendencies. Results also showed improvements in tensile properties as well as machinability for ferritic ductile irons, he said.

Another paper by Skaland, as well as I. Riposan, M. Chisamera and S. Stan, Politehnica Univ. of Bucharest, Romania, and M.I. Onsoien, SINTEF, Norway, studied the inclusions that may act as nucleation sites for graphite in over-inoculated gray irons (01-094). They examined low-Si hypoeutectic gray irons that were overinoculated by the addition of 2.0 wt. % high-purity Fe-Si, Ca-FeSi or Sr-FeSi inoculants.

Using electron microscopy and electron probe microanalysis, the researchers found that while Ca was distributed evenly in the inclusion volume; strontium (Sr) was found mainly in the core of the inclusions. They also reported that the particles associated with graphite had a lower manganese to sulfur (Mn/S) ratio than the matrix-embedded particles, and that Ca-FeSi and Sr-FeSi inoculated irons had a lower Mn/S than high-purity Fe-Si treated irons. "The presence of aluminum (Al) and O in the inclusions core in all cases is a peculiar conclusion," they said. The polyogonal or rounded (MnX)S particles (where X = Fe, Al, O, Ca, Si, Sr, Ti, etc.) nucleated on [Al.sub.2][O.sub.3]-based sites, and sometimes covered with a thin layer of silicate, are the major nucleation sites for graphite flakes, they concluded.

Solidification was also examined by several authors. R. Ruxanda, L. Beltran-Sanchez, J. Massone and D.M. Stefanescu, Univ. of Alabama-Tuscaloosa, described the results of their experimental and mathematical study (01-066) on the eutectic solidification of ductile iron. They found that the eutectic structure of ductile iron includes primary austenite dendrites as well as eutectic dendrites containing several nodules. They proposed a computational solidification model that describes microstructure evolution of the multi-nodule eutectic grains in ductile iron. According to the authors, such a model calculates microstructures that correctly depict the experimental microstructure obtained through SEM and color-etching metallography.

Another paper by F. Mampaey, WTCM Foundry Center, Zwijnaarde, Belgium, examined the solidification morphology of white iron (01-068). Using cooling curve measurements, quenching experiments and metallographic examination on hypoeutectic white iron poured into 1.5-in, diameter cylinder castings, the authors gained insight into how the solid phase is distributed over the casting during freezing. They also reported comparisons to gray and ductile irons.

They found that the white eutectic freezes with a slightly rough skin and that no solid eutectic phase precipitates within the interior liquid. The austenite dendrites previously formed do not interfere with the eutectic. In the compositions examined, the volume fraction of the dendrites was equal to the equilibrium value as predicted by the lever rule. This, they reported, is in contrast to gray iron, where about 19% of dendrites are present in a eutectic composition. The different behavior of the dendrite formation in gray and white iron can be explained by the coupled-zone concept.

The experiment also showed that a carbide-free rim is formed early during solidification and its size at the end of solidification is nearly equal to the final thickness after normal cooling. The white eutectic, which occurs at the edges of thin-wall ductile iron plate castings, is the plate-like eutectic, which probably forms at much higher undercooling than ledeburite (the usual structure available in white iron). However, at larger distances from the edge where only carbide traces remain within the ductile iron structure, the white eutectic was again found to be the ledeburite type.