CaO x SiO2 x TiO2 Slag

Name: CaO x SiO2 x TiO2 Slag
Diagram No.: 1137
Type of diagram: TTT
Chemical composition in weight %: See the table
Group: Slag
Note: TTT diagram of the slag CST_2 constructed through SHTT. The TTT diagrams for the slags “commercial” [5] and “CST literature” [26] are also shown.
Figure compares the TTT diagram for the CST_2 slag with two other TTT diagrams for slags from literature. The “CST literature” is a slag of similar composition, also in the primary crystallisation field of CaO.SiO2 in the CaO-SiO2-TiO2 diagram. The TTT diagram of the “CST literature” was constructed through Differential Thermal Analysis (DTA) [26]. The slag “commercial” is an industrial mould powder normally employed in the continuous casting of stainless steels, which contains CaF2 and Na2O, without TiO2, and whose TTT diagram was also constructed through SHTT [5]. For the “CST literature” and “commercial” the reported melting points are 1296 °C and 1262 °C respectively [26]. It is required that the crystals of the CaO-SiO2-TiO2 system precipitate as the phase cuspidine in commercial mould powders, replacing the system CaO-SiO2-CaF2. However, the incubation times for the slag CST_2 are much longer than of the industrial mould slag.
An interesting point is that the TTT diagram of the “CST literature” was obtained through DTA, and the TTT diagram of the CST_2 slag was obtained through SHTT. Even with so different experimental setups the result is similar. According to the researchers who studied the “CST literature” [26] their TTT diagram was obtained in the following way: the continuous-cooling- transformation diagram (CCT) was experimentally determined by using DTA, and then it was transformed into TTT diagram by using the additivity rule of Scheil. The crystallisation temperature was defined considering the temperature observed at the exothermic peak during the continuous cooling process. Besides, the melting point was determined by the heating process of the DTA measurement.
The TTT curve for the CST_2 slag was constructed through SHTT by visual analysis using recorded video files. The starting point for crystallisation was considered as the point where undoubtedly crystallisation is perceived, using appropriate light conditions.
The slags “CST literature” and CST_2 are similar considering the following aspects: chemical composition, basicity (V-ratio), both are in the primary crystallisation field of wollastonite, and the possible phases after solidification are the same according to the phase diagram [46]. Besides, they have similar liquidus temperatures and viscosities. Then, considering these similarities, it is expected a similar crystallisation behaviour, as it is shown in Figure.
However, despite of the similar crystallisation behaviour the incubation times are different below 1020 °C. Regarding the kinetics related to CST_2 and “CST literature”, different conditions for heterogeneous nucleation can exist depending on the raw materials and on the experimental set-up.
It is known that phase boundaries and foreign solid particles may favour nucleation. This effect is due mainly to the diminished thermodynamic barrier as compared to that for homogeneous nucleation, owing to a decrease of the contribution of the effective surface energy to the work of critical cluster formation. Catalyzing surfaces may also be represented by dispersed solid particles that act as nucleation sites. In this case, their curvature and number may strongly affect the nucleation kinetics [38]. The raw materials for production of the slags in the present work are reagent grade, i.e. there are some impurities (for the slag CST_2 for example there are 0.62% impurities according to XRF, e. g. little amounts of Fe2O3, MgO, Al2O3, etc.).
It was suggested that it would be worth to investigate the effect of Na2O addition in CaO-SiO2-TiO2 slags [26], since in this way it would be possible to decrease the incubation times of the crystals, getting a fluorine-free mould powder with crystallisation behaviour more similar to an industrial one.
Additional links: No. 1135, No. 1136, No. 1137, No. 1139, No. 1140
Reference: Not shown in this demo version.

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