Modified DHTT Equipment for Crystallization Studies of Mold Slags

Abstract

The double hot thermocouple technique (DHTT) enables simulations of the temperature gradient at near-service conditions during continuous casting of steel. With the equipment applied so far, a rectangular slag film of even thickness often cannot be achieved. Further, the minimum temperature frequently lies within the slag film. Modified equipment can avoid these disadvantages via the following design features. The entire furnace chamber is heated to the selected temperature of the cold wire, and the minimum temperature is not located within the slag film. Furthermore, the shape of the heating wire is improved, which enables mounting of a thin, rectangular slag film between four platinum wires. This modification allows for investigations on transparent and translucent slags. So far, the results from DHTT investigations were represented via snapshots of the samples at certain experimental times. Therefore, appropriate methods for the graphical representation of the results were suggested: the maximum crystallinity, the time related to certain crystallinities with a dependence on the position within the slag film, and the crystal growth rate. The CaO-MgO-Al 2 O 3 -SiO 2 slag investigated with this equipment was mineralogically examined additionally, and based on thermodynamic calculations, the allocation of temperatures to certain positions within the crystallized slag film was possible.

High-temperature investigation of mould slag crystallization by single and double hot thermocouple techniques

“…The new device consists of a furnace chamber (5) heated to the cold wire temperature. The correct location of the minimum temperature at the cold wire was verified by thermochemical calculations and compared with mineralogical investigations after the experiment [32]. Due to the control of the heating of U-shaped platinum wire (9) in close contact with the cold end of the slag film (see below), this approach contributes to a continuous temperature decrease within the slag film.…”

“…Whereas one total half-period is used for heating, only 1/3 of the subsequent period can be used for temperature measurement to reduce sample noise [28]. An additional heater (3) positioned near the sample reduces heat loss from the sample furnace to the surroundings [31,32]. To achieve higher cooling rates of the sample for isothermal experiments, the equipment may also include an air cooling device [38,47].…”

“…Thus, the crystallization of slags containing small amounts of colouring oxides can also be observed. Additionally, heating of U-shaped platinum wire (9) is controlled by the temperature detection of the cold wire, which shifts the position of the minimum temperature to the cold wire when the furnace temperature is identical to the cold wire temperature [32].…”

“…Three possible approaches can be used to investigate the crystallization behaviour of mould slags [52]: isothermal or continuous cooling experiments using SHTT set-up and experiments with different thermal gradients using DHTT equipment. For the isothermal investigations, the slag is liquefied either within one SHTT [21,45,52] or between both DHTT thermocouples [28,32,52] and quenched to the desired temperatures below the melting point; it is held until no further crystallization can be observed. During the test, pictures of the sample in defined time intervals are taken and then evaluated to create time-temperature-transformation (TTT) diagrams for relevant crystallinities (e.g.…”

“…Even though all of these methods reveal important information regarding the crystallization behaviour of fluxes, an experimental set-up enabling the simulation of the temperature gradient between the mould and strand as well as simultaneous in situ observation of the slag film was necessary. Therefore, the single/double hot thermocouple technique (SHTT/DHTT) has been established by Kashiwaya et al [28,29] and further modified [30][31][32]. Thus far, a critical assessment of the set-ups used is given in the present paper.…”