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with increasing amounts of chromite. Looking at the expansion and viscos- ity, we can see that silica with higher amounts of chromite undergoes a sudden contraction at around (2,012F (1,100C) unlike the steady contraction seen in silica with 7.5% chromite. Also, the peak viscosity decreases and the sinter temperatures drop by as much as approximately 212F (100C) for the higher chromite content samples. Table 3 displays the veining rank-

ing for baseline silica and the various blends. Silica with 10% and 30% chro- mite could not be evaluated due to the large extent of fused sand on the cast- ings. A lower content of the specialty aggregates display better performance when compared to the higher content. Baseline silica has a high veining

index, as expected. Silica with 10% zircon and 7.5% chromite both display no indications of veining defects.

Conclusion T e quality of metal castings relates

to the high temperature performance of the refractory aggregates used. T is high temperature performance is determined by the thermal volume stability and resistance to high tem- perature softening. Silica sand with its low cost and abundant availability does have limi- tations not easily overcome. Its high rate of expansion through phase transformations results in casting defects. T e use of molds and cores produced entirely from specialty sands like chromite and zircon are expensive and in many applications do not warrant the increased cost. T e use of sand blends of inexpen- sive silica sand with higher cost specialty sands has been shown to have applications where the quality of the casting can be improved with- out the associated high cost of 100% specialty sand cores. As little as 10% of specialty sands can improve the quality of the fi nal casting by reduc- ing the extent of veining defects,

according to the research results. T e eff ect of blending silica sand and specialty sands highly depends on the thermal input of the metal and the mass of the mold that determines the heating rate of the mold and associated cooling rate of the casting. T e chemical reaction between the base sand and the specialty sand must be accurately determined, as was the case of silica and chromite sand blends. Higher heat inputs in the larger metal sections caused the mixture to fuse, leading to casting defects. Lower percentages of chromite sand

improved casting quality in the test casting. T is illustrates the importance of comparing the high temperature physical properties of the sand blend with the specifi c casting application before practicing the technology.

This article is based on the paper “The Use of Specialty Sand Blends to Reduce Vein- ing Defects in Steel Castings” (16-060) which was presented at CastExpo16.




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