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for silica with 10% zircon is similar to baseline silica sand. However, from 20% zircon onwards, a reduc- tion in the alpha-beta phase transi- tion peak expansion can be seen with silica with 40% zircon having the lowest peak of 0.005 in./in., which is lower than baseline silica by 56%. Another trend that can be observed from the thermal expan- sion of the blends is that the cristo- balite phase transition induced at a lower temperature when compared to the baseline silica sand sample. At the 2,372-2,462F (1,300-1,350C) core surface temperature, the silica and zircon blend samples are already going through a secondary expan- sion. Tough a steep contraction can be seen after the alpha-beta phase transition at 1,063F (573C), the large secondary expansion at a lower temperature negates the strain on the surface of the core at higher temperatures as both the surface and sub-surface of the core are expand- ing, thereby reducing the network of cracks on the surface of the core. Silica with 7.5% chromite has an


expansion profile similar to the zircon blends, with slightly higher tempera- ture of cristobalite transition. Te secondary expansion seen after the cristobalite transition is also lower. Silica with 10% chromite shows sudden steep contraction at 2,012F (1,100C) while silica with 30% chro- mite shows a small secondary expan- sion at 1,652F (900C) before a steep contraction at the same temperature. Tis steep contraction for the 10% and 30% chromite samples and the relatively smaller secondary expan- sion after the cristobalite transition may lead to the sand grains fusing to each other in steel castings.


Fig.4. Here’s the temperature profile of a step-cone core at 5% solid fraction.


Fig.5. Depicted is the silica with zircon blends thermal expansion results.


Table 2. The Sinter Temperature, Peak Viscosity and Specific Heat Capacity Results Sample ID


Baseline Silica


Silica With 10% Zircon Silica With 20% Zircon Silica With 30% Zircon Silica With 40% Zircon Silica With 7.5%Chromite Silica With 10% Chromite Silica With 30% Chromite


Sinter Temp. (C) 1437.4 1252.7 1238.6 1234.3 1231.1 1255.7 1165.1 1153.1


Peak Viscosity (Pa.s) 5.030x108 9.282x108 8.819x108 1.122x109 1.724x109 1.084x109 8.857x108 5.637x108


Specific Heat Capacity at 1,200C(J/g.C) 1.2 1.1


1.09 1.07 1.07 1.04 1.04 0.98


February 2017 MODERN CASTING | 33


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