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It was hypothesized that the dramatic inverse peak in the molecular weight measurements at 585C (1085F) was due to the effects of hydrogen dissolution into the liquid metal. Figure 5 (b) clearly shows that hydrogen dissolution does not cause any peak in the measured molecular weight. In addition, the composition data shown in Table 1 indicate that hydrogen evolution is insignificant at temperatures around 750C (1382F). This further supports the conclu- sion that the peak in the measured binder gas molecular weight is not due to hydrogen dissolution. Regardless of these facts, the possible issue of hydrogen dissolution and its potential effects on the measured binder gas molecular weight were investigated further. The rate of hydrogen dissolution into the liquid metal was determined from the pure hydrogen molecular weight measurements. As a worst case approximation, it was assumed that binder gas hydrogen dissolves into the liquid metal at the same rate as pure hydrogen. The volume of hydrogen corresponding to the mass of binder gas hydrogen that “hypothetically” dissolved during heating was added back into the binder gas molecular weight calculations. On a worst case basis, hydrogen dissolution changes the binder gas molecular weight measurements above 560C (1040F) by a maxi- mum of only 4 g/mol. Since hydrogen is not expected to evolve within the temperature range of the present mea- surements, the above described hydrogen dissolution cor- rection of the measured binder gas molecular weight was discarded. The details of correction for hydrogen dissolu- tion are described elsewhere.27

It was also hypothesized that the inverse peak in the binder gas molecular weight measurements at 585C (1085F) was related to a volume change in the silica sand, caused by the increase in thermal expansion accompanying the sand’s α–β phase transformation at 573C (1063F).33

periment was performed to evaluate this hypothesis. The ex- pansion of 1.018 g of pure silica sand, 3.8 cm3

An additional ex- (0.232 in3

) of

argon, and the liquid metal in the GED was measured during heating at a rate of 2°C/min (3.6°F/min). The results show that no peak in the molecular weight measurements occurs when argon and sand are heated in the GED. Additional de- tails are given elsewhere.27

It is also possible that the inverse peak in the measured binder gas molecular weight at 585C (1085F) is caused by a reaction between the binder gas and the sand. However, it is more likely that the peak simply reflects the actual binder gas evolution behavior. It can be seen from Figure 3 that the fraction of original binder mass remaining does not experi- ence any unusual behavior at 585C (1085F). This implies that the inverse peak in the measured binder gas molecu- lar weight may simply be due to a rapid formation of low molecular weight gaseous compounds followed by the for- mation of higher molecular weight compounds. Recall that the bonded sand samples in the GED were immersed in the evolved binder gas and that the binder gas was not purged during the tests. Therefore, it is possible that the simultane-

International Journal of Metalcasting/Spring 2012

ous presence of binder gas and bonded sand influences the manner in which additional gas is evolved from the bonded sand or the manner in which the previously evolved binder gas components react with one another during heating at temperatures near 585C (1085F).

The measured binder gas behavior during all tests, including both the heating and cooling portions, can also be compared in terms of the moles of binder gas per original binder mass, nb

. This quantity was calculated from Eqn. 13

The second equality in Equation (13) indicates that nb not depend on the mass of the binder gas, mb

does , but can be

directly calculated from the measured gas volumes, pres- sures, and temperatures. The moles of binder gas per origi- nal binder mass for all binder gas evolution tests are plotted as a function of temperature in Figure 8. Again, the testing conditions and curve coloring correspond to those of Fig- ure 6, and only gas measurements meeting the gas cut-off criterion ( g

seen that, as expected, all heating curves nicely coincide. A significant decrease in the evolved binder gas moles per original binder mass during cooling indicates condensa- tion of the binder gas. As shown in Figure 8, the binder

∆h2 ≥ 0.8 cm [0.315 in]) are included. It can be

Figure 8. Measured binder gas moles generated per original binder mass as a function of temperature during heating and cooling of different PUNB bonded sand sample masses. The color scheme of the curves follows that of Figure 6. The heating rate was 2°C/min (3.6°F/ min), and a significant decrease in nb

indicates condensation of the binder gas. 35

during cooling

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