The aim of this study was to determine the influence of alumina increase on the functioning of casting powder during continuous casting. Two aspects of the powder's performance were experimentally measured, namely the influence of alumina on the viscosity and the crystallisation behaviour. These two aspects were then related to the in-mould functioning of the casting powder with the aid of literature references. Casting slag must provide lubrication between the mould surface and the thin steel shell. Alumina increase will cause viscosity increase in casting slags and will hinder lubrication when the inflow of casting slag into the mould/strand gap deteriorates. Heat transfer across the gap is determined by the amount of solid casting slag and the form (glass or crystalline) thereof. Heat transfer across a crystalline material may be up to seven times lower than that across a glass phase. The increased alumina will serve to increase the ratio of glassy phase to crystalline phase in the gap, so increasing the heat transfer. To determine the true extent of alumina increase with modern clean steel practice, samples were taken from the moulds of both the VI and V2 continuous casters at ISCOR Vanderbijlpark. The influence of this alumina increase on the viscosity and the crystallisation behaviour of the casting slag were experimentally measured. Viscosity measurements were done with a rotating bob viscometer (in a vertical tube furnace) on two commercial casting powders (with increasing AI2O3 content). Viscosity prediction models were evaluated with the data from the measured viscosity values. Crystallisation measurements were done by quench experiments with the aid of the hot thermocouple technique. Crystallisation behaviour after a specified heat cycle was presented as the percentage opaque material (crystalline) to vitreous material (glassy) measured with an optical microscope. For the seven sequence casts during which samples were collected from the mould, it was found that the alumina content of the casting slag reached a steady-state value within the first ladle (first 40 minutes) with an increase of 3 to 4 mass %. The influence of a 4 mass % alumina increase on the measured viscosity is small enough that proper lubrication function of the slag will not be negatively affected. Viscosity prediction models vary in their accuracy and are limited with respect to the composition range of the casting powder and the temperature range for which they are valid. The increased alumina content was found to have a strong influence on the percentage crystalline material present. After the sample was heated to 1300°C, kept there for 10s, and then quenched, the crystalline material will decrease from 60 % to 30 % for 5 mass % alumina added. This decreased crystalline material present may notably increase heat transfer. Full crystallisation does not occur at a single temperature, and crystallisation occurs over a temperature interval (of up to 200°C). This means that crystallisation temperature values quoted by casting powder manufacturers depend on the heat cycle and the technique used during crystallisation measurements. Several crystalline phases are usually present in solidified casting slag and these crystalline phases are strongly influenced by the alumina content: the stable crystalline phases may change as alumina content increases. For the current alumina increase in casting powders the effect on viscosity is small, while crystallisation behaviour may be influenced to a greater extent. / Dissertation (M Eng (Metallurgical Engineering))--University of Pretoria, 2006. / Materials Science and Metallurgical Engineering / unrestricted

Made available in DSpace on 2014-12-17T15:01:49Z (GMT). No. of bitstreams: 1 ErikaASG_TESE_partes_autorizadas.pdf: 3587859 bytes, checksum: 8731a4ebad1b8f6f11b36c879b4a1a76 (MD5) Previous issue date: 2009-12-30 / The WAT is the temperature at the beginning of the appearance of wax crystals. At this temperature the first wax crystals are formed by the cooling systems paraffin / solvents. Paraffins are composed of a mixture of saturated hydrocarbons of high molecular weight. The removal of petroleum from wells and the production lines means a surcharge on produced oil, thus solubilize these deposits formed due to modifications of thermodynamics has been a constant challenge for companies of oil exploration. This study combines the paraffin solubilization by microemulsion systems, the determination of WAT systems paraffin / solvent and performance of surfactant in reducing the crystallization. We used the methods: rheological and the photoelectric signal, validating the latter which was developed to optimize the data obtained due to sensitivity of the equipment used. Methods developed for description of wax precipitation are often in poor agreement with the experimental data, they tend to underestimate the amount of wax at temperatures below the turbidity point. The Won method and the Ideal solution method were applied to the WAT data obtained in solvent systems, best represented by the second interaction of Won method using the solvents naphtha, hexane and LCO. It was observed that the results obtained by WAT photoelectric signal when compared with the viscosity occur in advance, demonstrating the greatest sensitivity of the method developed. The ionic surfactant reduced the viscosity of the solvent systems as it acted modifying the crystalline structure and, consequently, the pour point. The curves show that the WAT experimental data is, in general, closer to the modeling performed by the method of Won than to the one performed by the ideal solution method, because this method underestimates the curve predicting the onset of paraffin hydrocarbons crystallization temperature. This occurs because the actual temperature measured was the crystallization temperature and the method proposes the fusion temperature measurement. / A TIAC ? a temperatura de in?cio do aparecimento dos cristais de parafina. Nesta temperatura os primeiros cristais de parafina s?o formados, pelo resfriamento de sistemas parafina/solventes. As parafinas s?o compostas de uma mistura de hidrocarbonetos saturados de alto peso molecular. A remo??o de parafinas dos po?os e das linhas de produ??o significa um custo adicional ao petr?leo produzido, portanto solubilizar estes dep?sitos formados decorrentes das modifica??es termodin?micas tem sido um desafio constante das empresas exploradoras de petr?leo. Este estudo alia a solubiliza??o paraf?nica por sistemas microemulsionados, ? determina??o da TIAC dos sistemas parafina/solvente e a atua??o do tensoativo na redu??o da cristaliza??o. Utilizaram-se os m?todos: reol?gico e o do sinal fotoel?trico, validando este ?ltimo o qual foi desenvolvido visando aperfei?oar os dados obtidos devido a sensibilidade do equipamento utilizado. M?todos desenvolvidos para descri??o da precipita??o da cera est?o freq?entemente em pobre concord?ncia com os dados experimentais; eles tendem a subestimar a quantidade de cera ?s temperaturas abaixo do ponto de turbidez. Foram aplicados os m?todos de Won e o da solu??o ideal aos dados de TIAC da parafina obtidos em sistemas com solventes, sendo melhor representado pela segunda intera??o do m?todo de Won com os solventes: nafta, hexano e LCO. Foi observado que os resultados de TIAC obtidos pelo sinal fotoel?trico quando comparados com a viscosidade, ocorrem antecipadamente, demonstrando a maior sensibilidade do m?todo desenvolvido. O tensoativo i?nico reduziu a viscosidade dos sistemas paraf?nicos, pois ele atuou modificando a estrutura cristalina, conseq?entemente o ponto de fluidez. As curvas que representam os dados experimentais da TIAC est?o, de maneira geral, mais pr?ximas da modelagem realizada pelo m?todo de Won do que da solu??o ideal, pois este m?todo subestima a curva de predi??o do in?cio da temperatura de cristaliza??o da parafina com os hidrocarbonetos. Isto ocorre porque a temperatura real medida foi a de cristaliza??o, e a proposta pelos m?todos ? a de fus?o