Mold Slag

Name: Mold Slag
Diagram No.: 1170
Type of diagram: CCT
Chemical composition in weight %: See the table
Group: Slags
Note: CCT diagrams including the DSC and dip test cooling curves used in their construction.
Figure 5 shows the DSC curves for slags S1 and S2 generated at a constant heating rate of 10 C/min and cooling rates of 1 C/min. The sharp endothermic trough of the heating curves at 100oC is due to water evaporation. Figure 5(a) also shows two endothermic troughs occurred at around 450 C and 750 C. These troughs are most likely associated with the decomposition of the two common carbonates found in these mold fluxes: Na2CO3 at 450oC and CaCO3 at 750 C. This is confirmed by the accompanying weight loss included in Figure 5(a) (TG), which is expected because of the volatilization of CO2. The series of endothermic peaks above 1000oC on the heating curve indicates that the powder began to melt incongruently. The end of the last peak encountered on heating presumably represents the liquidus temperature. This is seen to be 1235 C for slag S1.

During the cooling process, the phase transformations are very different from heating, owing to the irreversible chemical reactions that occur during melting. Several exothermic peaks were observed, where again phase transformations are indicated. On cooling, the phase transformation is believed to begin at the inflection point starting each peak. For slag S1 at 1 C/min cooling rate, shown in Figure 5(a), the exothermic peaks begin at 1234 C, 1160 C and 1022 C, which might correspond to the onset of three different crystalline phases forming. In general, a higher cooling rate increases the peak height, whereas a lower rate yields higher resolution. Higher cooling rates delay both the onset and finish of each phase transformation.

The results for Slag S2 show a lower liquidus temperature of 1050 C and different exothermic peaks during cooling, as labeled in Figure 5(b). Similar analysis is conducted for different cooling rates and is presented later to construct continuous cooling transformation (CCT) curves.

The cooling curves recorded during the thermocouple dip tests were smoothed using the Savitzky-Golay polynomial smoothing filter to eliminate noise. The temperature history curves were numerically differentiated with second order accuracy to distinguish points where the thermal gradient changes would be indicative of crystallization. These points were used for constructing CCT curves.

From the time-temperature profiles recorded during the DSC tests and thermocouple dip tests, CCT diagrams were constructed by first taking the time to start at zero when the cooling curves crossed the liquidus temperature (1235 C for slag S1 and 1050 C for slag S2). Figure 6(a) shows the CCT diagram for slag S1. The critical cooling rate to maintain the amorphous structure is estimated to be 50oC/s. At slower cooling rates, the first crystalline phase appears at around 1200 C, which should be cuspidine according to the XRD analysis. Watanabe measured that the liquidus of cuspidine varies between 1114 C (55% CaF2) and 1407 C (20% CaF2) in the CaO-SiO2-CaF2 ternary system. For slag S1, the liquidus temperature should be ~1250 C, which almost matches with the result in this work. The liquidus temperature of slag S2 is found to be lower. This might be due to the effect of many other compounds in the slag, especially Na2O, Al2O3 etc, which decrease the system melting temperature. When the cooling rate is slower than 10 C/s, a second crystalline phase is formed at around 1100 C, which could be silicon oxide (oxy) fluoride (Ca2SiO2F4) phase. The DSC tests with very slow cooling rate (<5 C/min) shows a third peak near 900 C as shown in Figure 5(a). This implies the existence of a third phase which could not be distinguished in the XRD pattern of the thermocouple dip tests. The cooling curves of the dip tests are more ambiguous but do not appear to show the third phase either. This suggests that this phase is suppressed at the faster cooling rate of the dip tests relative to the DSC tests. The XRD results from furnace heating devitrification tests confirm that more crystalline phases form at 900 C to 1100 C. Comparing these results with the phase diagrams, these phases generally show lower –temperature liquidus lines which is due to the eutectic reaction.

Similarly, Figure 6(b) shows the CCT diagram for slag S2 and features a critical cooling rate of only 20 C/sec. This low critical cooling rate confirms the conjecture that slag S2 is glassy. The cuspidine phase starts to form at around 1050 C. Other crystalline phases were observed from isothermal aging tests as described in previous section, though they are difficult to distinguish unambiguously in the XRD pattern and cooling curves of thermocouple dip tests.
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