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Segment 1


2 3


Ramp Rate


°F/min (°C/min) 9 (5)


36 (20) 36 (20)


Temperature °F (°C)


212 (100)


2550 (1400) 77 (25)


Table 3: Heating profile used during the dilatometer testing.


the creep resistance of various mullites. A listing of these mullites can be found in Table 2. The alumina, silica, iron, and the alkali/alkaline earth oxides contents are shown. Attempts were made to test materials with a wide variety of alumina contents. The dilatometer temperature profile is shown in Table 3. Figure 3 shows the temperature vs


change in length dilatometer curve. The thermal expansion on all samples is very similar up until 2010°F (1100°C). This indicates that all of the sample materials could be used for the same molds due to similar expansion until 2010°F. But after this temperature, performance begins to differentiate among sources. M3 begins to soften and its expansion slows. The next decrease in the thermal expansion occurs in sample M4 at 2280°F (1250°C). It is not surprising that these are the two samples that begin to soften first as they have the lowest alumina contents. The only material that does not show a significant slowing of thermal expansion is the Premium Grade Virginia Mullite™. This can be explained by it having the lowest iron oxide among the samples tested. There is less iron to flux the silica which limits glass formation and extends the maximum usage temperature. Changing the x-axis from


temperature to time allows for an examination in the creep characteristics of the materials. Each mullite was held at 1400°C for one hour with constant pressure applied by the dilatometer. This, in essence, is a quasi-creep test. Looking at the differences in change in length at the start versus the end of the hold can shed light on the creep resistance of each mullite. The dilatometer curve with this data is shown in Figure 4. The Premium Grade


Virginia


Mullite™ showed the least amount of change through the dwell and thus


®


the most creep resistance. The value of creep for Premium Grade Virginia Mullite™ was significantly less than the second most creep resistant material, M1. This is despite M1 having 9% more alumina. Samples M2 and Virginia Mullite™ also contain more alumina than the Premium Grade Virginia Mullite™ sample yet crept more during the dwell. These results suggest that the iron oxide content affects the creep resistance of the refractory material and that more than just alumina content must be considered when choosing the raw materials. Premium Grade Virginia Mullite™ also had the lowest amount of


the alkali/alkaline earth oxides.


These also act as fluxes and can lower creep resistance. These oxides might help explain why M1 and M2 show lower creep resistance than Premium Grade Virginia Mullite™. The Virginia Mullite™ products had similar alkali/ alkaline earth contents so the difference in creep resistance between these two minerals must be due to the iron content. Samples M3 and M4 crept the most during the dwell despite having lower iron contents than M1 or M2. This confirms that while iron content is important to consider, high alumina is still required for creep resistance. A large amount of alkali/alkaline earth oxides also negatively affected sample M4. Creep testing suggests that Premium Grade Virginia Mullite™ should be used over the other aluminosilicates tested in molds where dimensional stability of the casting is crucial.


Conclusion The process of mining kyanite-quartzite rock and beneficiating the ore to make an industrially useful kyanite is a very complex process. Over twenty- five steps are used to make a kyanite


Dwell Time (min) 0


60 100


product with an iron content containing less than 0.2%. This material is then calcined to make Premium Grade Virginia Mullite™. Calcining kyanite to make mullite creates a raw material that exhibits several unique characteristics. The mullite has a low level of impurities that are bound to the crystal surface. This makes the impurities non-homogenous which aids in creep resistance. The high aspect ratio of mullite made from kyanite can be used to increase the strength of the shell, reduce shell splits, and in some cases has led to reduction in the number of backup coats required. Our studies indicated that all the


various mullite sources tested have similar thermal expansion up until 2010°F (1100°C). At this point the various materials began to soften and their expansion rates decreased. The Premium Grade


Virginia Mullite™


was the only raw material to continue expanding at a constant rate until 2550°F (1400°C) and showed the highest creep resistance despite several other samples containing higher alumina contents. This is due to the lower amounts of iron oxide and alkali/alkaline earth oxides present which directly impact creep resistance. This high creep resistance property makes Premium Grade Virginia Mullite™ a great option for shells where high temperature dimensional stability is critical to casting quality and yields.


References 1. T. Vert, Refractory Materials Selection for


Steelmaking. The American Ceramic Society and John Wiley & Sons, 107-113 (2016). 2. S. Aramaki and R. Roy, “Revised Phase


Diagram for the System Al2O3—SiO2,” Journal of the American Ceramic Society, 45 [5] 229-242 (1962). 3. H. Schneider and S. Komarneni, Mullite.


Wiley-VCH, Weinheim, Germany, 2005. 4. B. Joensson and B. Sundman, High Temp.


Sci., 26, 263-273 (1990). 5. R. Brandt, “The Sillimanite Minerals:


Andalusite, Kyanite, and Sillimanite,” Ceramic and Glass Materials, 41-48 (2008). 6. S. Ashlock, “A Property Comparison of


Commercially Available Sillimanite Minerals,” EUROGRESS, 1-10 (2017). 7. F.W. Clarke and H.S. Washington, The Composition of the Earth’s Crust. Government Printing Office, Washington, (1924). 8. Owens and M. Pasek, “Kyanite Quartzites in the Piedmont Province of Virginia: Evidence for a Possible High-Sulfidation System,” Economic Geology, 102 [3] 495–509 (2007).


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