Table 3. Penetration and Veining Rankings Sample ID
Surface Viscosity and Specific Heat Capacity Results
Te sintering temperature and the peak viscosity at sintering temperature for each sample are shown in Table 2, along with the associated specific heat capacity at 2,192F (1,200C). Baseline sil- ica has a sinter temperature of 2,619.3F (1,437.4C) with a peak viscosity of 5.030 x 108 Pa.s (5.03 x 1011 cP). Te sinter temperature of both the zircon and chromite blends decreases with increas- ing amounts of the aggregates. However, with the zircon blends, the peak viscosity increases with increasing amounts, while the peak viscosity in the case of the chromite blends decreases with increas- ing amounts.
Casting Quality Analysis Te baseline silica casting obtained
is shown in Fig. 6. Te casting exhibits several veins along the surface, which is typical of silica sand castings. No penetra- tion defects are visible. More veins are formed along the thicker sections of the casting, where the metal takes longer to solidify. Tis would enable the cores to reach higher temperatures while the metal is still in its liquid form. Silica with 10% zircon does not
display any veining defects. Tough the alpha-beta transition peak expansion for silica with 10% zircon is similar to baseline silica, the early inducement of the cristobalite transition, the second- ary expansion and higher viscosity at sintering temperature leads to lower strain on the surface of the core, thereby reducing the veining defect. However, silica with 20%, 30% and
40% zircon display slight veining and penetration defects at the thicker cast- ing sections. Silica with 7.5% chromite (Fig. 7) displays no veining or penetra- tion defects. Silica with 7.5% chromite displays a lower peak expansion at the alpha-beta phase transition temperature when compared to silica sand. Also, the cristobalite phase transition is induced approximately 374F (190C) lower and the peak viscosity is about twice as much as baseline silica. Tis would prevent failure on the surface of the core. However, in the silica with 10%
chromite and silica with 30% chro- mite castings a large extent of fused sand on the castings which increases
34 | MODERN CASTING February 2017 Fig.7. This casting was produced in a mold with a silica blend with 7.5% chromite. Fig.6. The baseline silica casting is shown. Baseline Silica
Silica w 10% Zircon Silica w 20% Zircon Silica w 30% Zircon Silica w 40% Zircon Silica w 7.5% Chromite Silica w 10% Chromite Silica w 30% Chromite
Veining Index 43 0 9 9 5 0 - -
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