Effect of Heating Rate on the Elastic Modulus Variation
In order to investigate the effect of heating rate, additional tests were performed at heating rates of 5C/min (9F/min), 2C/min (3.6F/min), and 0.8C/min (1.44F/min). In view of Figure 1b, such low heating rates occur in steel casting molds only far from the mold-metal interface. The results of these elastic modulus measurements are shown in Fig- ure 12 and compared to the fit of the elastic modulus data from Figure 9 for a heating rate of 8C/min (14.4F/min). It can be seen that the heating rate has a strong effect on the elastic modulus variation with temperature. Within the present range, the elastic modulus at a given temperature can vary by more than a factor of two depending on the heating rate. With decreasing heating rate, the minimum in the elastic modulus at 125C (257F) shifts to higher val- ues. For the lower three heating rates, and above 175C to 200C (347F to 392F), the elastic modulus increases almost linearly to some maximum value. The maximum value is reached at a temperature between 375C to 425C (707F to 797F) and is equal to about 2,800 MPa. Beyond that temperature, the elastic modulus decreases steeply and the specimens lose their strength.
Although DSC data is not available for the lower heating rates, the large differences in the elastic modulus varia- tions observed in Figure 12 are clearly linked to the de- pendence of the chemical changes of the binder on the heating rate. In particular, the urethane bond breakage ap- pears to be a very time dependent process. For the lower heating rates, there is enough time for the urethane bond breakage to complete at lower temperatures. Once the urethane bonds are broken, the breakdown to polymer ar- omatics can commence, which in turn coincides with the linear increase in the elastic modulus with temperature. The breakdown to polymer aromatics does not appear to be a time dependent process, since the slopes of the linear increase in the elastic modulus with temperature are ap- proximately the same for all heating rates. However, the temperature at which all bonds are broken shifts to lower values with decreasing heating rate.
Also shown in Figure 12 is a curve labeled “steady-state”. This curve will be discussed in greater detail in the next sub-section, but it can be thought of as corresponding to an “infinitely” slow heating rate. It can be observed in Figure 12 that with decreasing heating rate, the elastic modulus curves approach this steady-state curve.
Elastic Modulus During Holding at Elevated Temperatures
The strong dependence of the elastic modulus on the heating rate observed in Figure 12 raises the question what the elas- tic modulus variation is for an infinitely slow heating rate. For example, does the elastic modulus at about 150C (302F)
International Journal of Metalcasting/Fall 10
Figure 12. Comparison of the elastic modulus variation with temperature for specimens heated at 0.8, 2, 5, and 8°C/min with a fit line along with the steady state data.
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continue to increase with decreasing heating rate? In order to investigate this issue, a special set of experiments was conducted where a specimen was heated (at approximately 8C/min (14.4F/min)) to a pre-selected “hold” temperature and then held at this temperature for a time sufficient for the elastic modulus to attain a constant “steady-state” value. Representative results are shown in Figure 13 for four dif- ferent hold temperatures: (a) 50C (122F), (b) 200C (392F), (c) 300C (572F), and (d) 370C (698F). In each of the graphs, the measured elastic modulus and temperature are plotted as a function of time. It can be seen that during the heating peri- od, the elastic modulus varies in a similar fashion as already discussed in connection with Figure 8. However, during holding of the specimen at a given temperature, the elastic modulus can be seen to continue to change with time, except for the experiment with the lowest holding temperature (50C (122F), Figure 13a). For the higher three hold temperatures, the elastic modulus always increases during the holding pe- riod, until a constant steady-state value is achieved.
The constant steady-state values for the elastic modulus mea- sured in the present set of experiments are plotted as a func- tion of the hold temperature in Figure 14. It can be seen that the steady-state values for the elastic modulus for hold tem- peratures below 100C (212F) are close to the instantaneous values measured in the continuous heating experiments. This indicates that below 100C (212F), the heating rate does not have an influence on the elastic modulus (see Figure 12). For hold temperatures between 100C (212F) and 200C (392F), the steady-state elastic modulus is equal to about 2,000 MPa. Then, the elastic modulus increases to about 3,000 MPa at 270C (518F) and drops to 2,400 MPa at 300C (572F). Be-
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