both References 4 and 5, the bonded sand specimens were heated in an oven (in an inert atmosphere) to the desired test temperature, and the elastic modulus measurements were performed well after the specimens reached a uni- form and constant temperature. The effect of heating rate was not investigated. In Reference 4, the measurements were performed inside of the oven, whereas in Reference 5 the specimens were removed from the oven before test- ing. In both references, the elastic modulus was measured at no more than ten discrete temperatures. Furthermore, the variation of the elastic modulus during cooling from an elevated temperature was not investigated.
In the present study, elastic modulus measurements are performed on PUNB bonded sand typical of steel casting molds. The experimental setup is similar to the one used in Reference 4 and uses a three-point bend apparatus.6,7
The
specimens are tested inside an oven in an inert atmosphere. Unlike in the previous studies, the present setup allows for elastic modulus measurements to be performed almost instantaneously. Hence, an almost continuous variation of the elastic modulus with temperature can be obtained. The effect of different heating rates is also investigated. In or- der to determine typical mold heating (and cooling) rates, a solidification simulation8
was conducted of a large 7.62
cm (3 in) thick steel casting section with a 7.62 cm (3 in) thick mold. Predicted temperatures as a function of time are shown in Figure 1a, and the variations of the mold heat- ing and cooling rates with distance from the mold-metal interface are plotted in Figure 1b. The heating/cooling rates represent averages over the time periods the temperatures in Figure 1a increase/decrease. It can be seen that within a distance of 2.54 cm (1 in) from the mold-metal inter-
face, the temperatures in the mold reach values higher than 1,000C (1,832F) (Figure 1a) and the heating rates are above 50C/min (90F/min) (Figure 1b). Such high heat- ing rates are difficult to achieve in an oven. In addition, a reasonably sized specimen would not be isothermal when heated at such high rates. However, at distances greater than 2.54 cm (1 in) from the mold-metal interface the heat- ing rates are much lower, while the mold temperatures still reach values above 400C (752F). Between 3.81 cm (1.5 in) to 7.62 cm (3 in) from the mold-metal interface, the mold heating rates vary from about 20C/min (36F/min) to 5C/ min (9F/min). Heating rates of that order of magnitude are utilized in the present measurements. Figure 1 also shows that after the initial heating, the mold cools down again. The cooling rates are much lower in magnitude than the heating rates. They decrease from about 1C/min (1.8F/min) to 0.2C/min (0.36F/min) with increasing distance from the mold-metal interface (Figure 1b). During the time period when the temperatures in the mold decrease, the mold can still have a strong effect on casting distortion.1
Thus, the
behavior of the elastic modulus during cooling, after heat- ing to a certain temperature, is also investigated in the pres- ent study. In particular, it is of interest to understand if the elastic modulus takes on the same values during cooling as during heating and if the original room temperature value is recovered at the completion of cooling. Finally, the ef- fects of the solvent and black iron oxide in the binder on the elastic modulus are investigated.
The specimen preparation and the experimental setup are de- scribed in the next section. The results of the present study are presented in Section 3, and the conclusions are summa- rized in Section 4.
(a) Predicted temperatures in the mold.
(b) Predicted average heating and cooling rates in the mold.
Figure 1. Predicted temperatures and average heating/cooling rates during casting of a three inch thick steel section surrounded by a three inch thick sand mold.
8 International Journal of Metalcasting/Fall 10
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