Na2O-CaO-Al2O-SiO2 Slag

Name: Na2O-CaO-Al2O-SiO2 Slag
Diagram No.: 1175
Type of diagram: TTT
Chemical composition in weight %: No data
Group: Slags
Note: TTT diagram (a) and related CCT diagram (b) for synthetic Na2O-CaO-Al2O-SiO2 slag (adapted from Ref. [41]), TTT and CCT diagrams for an industrial mould flux (c) (adapted from Ref. [49]) and CHT of conventional CaO–SiO2-based mould flux (d) (adapted from Ref. [54])

For SHTT investigations, the depicted pictures of the samples are evaluated by calculating the crystallinity, which is defined as the crystallized volume percentage, from the crystallized as well as the total slag film area. From these results, the time required to achieve given crystallinities is determined and presented in a TTT diagram (Fig. 3a, c). The same procedure is carried out to create CCT or CHT diagrams (Fig. 3b–d). Here, the cooling rate or the heating rate, respectively, is represented and the start and in some cases the end of crystallization is marked (e.g. Fig. 3b) [41].

SHTT is mainly used to study the crystallization behaviour of mould slags under isothermal conditions to create either TTT or CCT diagram. Examples are given in Fig. 3 for a Na2O–CaO–Al2O3–SiO2 slag with a CaO/SiO2 ratio of 1, 4 wt.% Al2O3 and 6 wt.% Na2O and for an industrial mould flux [41, 49]. For these slags, the start of crystallization (0.5% crystallinity) could be observed for all investigated temperatures. This was not the case for the end of crystallization (95% crystallinity). Here, 95% crystallinity could only be observed for temperatures close to the nose temperature. Away from this temperature, crystalline fractions of only 80% for the slag in Fig. 3a and solely 50% for the one in Fig. 3c were achieved within reasonable experimental time. Thus, these results defined the possible maximum crystalline fractions, which can be plotted in the diagrams for all investigated temperatures.

In quenching experiments, different crystal morphologies were obtained. Thus, a partitioning of the diagram according to the morphologies formed within a certain temperature range was possible. At lower temperatures, for all investigated mould slags, the precipitation of relatively small crystals within the total slag film occurs simultaneously. In contrast, at temperatures close to the liquidus temperature of the flux, columnar or dendritic crystals are formed in contact with the platinum wire and grow towards the centre. In between, several combinations of different crystal shapes may be observed, resulting in further detailing of the diagram (e.g. Fig. 3a). Nevertheless, some samples only show a separation into three areas. Li et al. [60] have reported similar results. For most mould slag compositions, the nose of TTT diagrams is observed within a few seconds and at temperatures between 1173 and 1373 K [29, 39, 61]. In some cases, a second nose appears, indicating the temperature-dependent formation of different mineral phases [39, 49, 53].

Cramb [52] investigated the influence of humidity on the crystallization of CaO–Al2O3 and CaO–Al2O3–MgO slags. Water pressures of 3141, 4256 and 5472 Pa were selected, representing moisture-saturated air at 298, 303 and 308 K, respectively. For CaO–Al2O3 slag in the presence of humidity, the nucleation rate increased and TTT curve is shifted towards shorter elapsed time as well as towards higher temperatures. Additionally, an increase in the crystal growth rate is observed with increasing relative humidity. In contrast, the second slag exhibits different behaviours. In this case, at temperatures lower than 1373 K, an increase in crystallinity is detected. However, for the temperature range between 1373 and 1523 K, the crystallinity is not affected. In addition, for temperatures higher than 1523 K, the crystallization process is impeded. Orrling et al. [55, 56] have reported similar results for a mould slag. Furthermore, they also investigated the influence of dry air on this sample after it crystallized under humidity. Immediately after dry air was introduced into the sample chamber, crystal dissolution and bubble formation occurred. The gas bubbles indicated the evaporation of water. A repeated exposure to humidity did not result in the reformation of crystals. These results reveal that humidity within the continuous casting process strongly affects the crystallization behaviour of mould slags and therefore must be considered for near-service investigations. Further experiments were carried out to study the influence of oxide additions on the crystallization behaviour of mould slags, and TTT diagrams were obtained for different Na2O contents. An increase in Na2O content increased both the crystallization tendency and the growth rate. Therefore, the variation in Na2O content is one possible means of influencing the properties of the flux within the mould [56].

Zhou et al. [54] found that the initial crystallization temperature during the heating of glassy solidified mould fluxes increases with increasing heating rate (Fig. 3d). Depending on the chemical composition of the investigated mould fluxes and the heating rate, it could be detected between 1086 and 1245 K. Kinetic analysis was carried out by calculating Mo exponent, the effective Avrami exponent and the Ozawa exponent. Defined values of the Ozawa exponents are related to certain crystallization mechanisms. Thus, the mechanisms could be identified and were confirmed by SHTT and scanning electron microscopy (SEM) analysis.

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