Changes in the structure of quartz ceramics in service

Abstract

The structure of the pore space in refractories to a large extent determines their important working properties --the strength, wear under the action of metal and slag, the capacity to withstand thermal shocks, etc. At the same time the structure of the articles in service undergoes critical changes.In particular, the operation of quartz nozzles designed for continuous steel casting occurs at temperatures much higher than the firing temperature of the ceramics, and the various zones in the goods are subjected to rapid and uneven heating.In these conditions predicting the properties of nozzles is not possible without detailed study of the structural and phase changes in the ceramics during service.Unfortunately, we cannot be completely clear about the nature of the processes taking place in the fused quartz articles during rapid heating and soaking at high temperatures.A study was made of the change in the properties of various sections of the nozzles under the action of metal and slag [i]. In this case it was found that even after 5 h steel casting when the content of cristobalite reaches 100%, the open porosity of the brick is quite high (8-10%).Similar results were obtained from a study of the sintering of quartz ceramics in air at temperatures above 1300~[2]. It was revealed that with a rise in temperature there is a certain increase in porosity which the authors linked with the development of a network of microcracks as a result of phase changes in the cristobalite.At the same time during a short heating in the range 1300-1500~ in a number of cases practically complete sintering of the slip-cast specimens of SiO2 was noted [3].In our view, in order to understand the nature of the processes taking place in quartz glass articles and to explain means of increasing the resistance of slip-cast refractories based on SiO= it is not enough to investigate only the open porosity and degree of crystallization of the material.Undoubted interest is attached to an all-round study of the changes in the porous structure including electron-microscopic studies, and also differential characteristics (distribution of pores in terms of sizes, the coefficient of linear expansion, etc.) o This paper gives the results of a study of the features of the structure of the pore space in various zones of nozzles used for the continuous casting of steel after service at the Novolipetsk steel factory over different time periods.* The open porosity Pop of the nozzles before service was checked by nondestructive radio-wave methods showing the identical nature of the original stucture of the goods [I]. Pop was 13-18%o

Physical and technological properties and the operational reliability of quartz nozzles (downtubes) used for continuous casting

“…The specific service conditions (performance) of the immersed quartz nozzles used for casting low-alloy and mild steels in the curvilinear type units were previously examined [3][4][5][6][7][8]. The specific service conditions (performance) of the immersed quartz nozzles used for casting low-alloy and mild steels in the curvilinear type units were previously examined [3][4][5][6][7][8].…”

“…The specific service conditions (performance) of the immersed quartz nozzles used for casting low-alloy and mild steels in the curvilinear type units were previously examined [3][4][5][6][7][8].Quartz nozzles are recommended [9] for casting the steels containing less than 1%* Mn. The structure evolution of the periclase-carbon refractories occurs due to sintering of the mineralogical filler under the conditions involving a change in the volumetric proportions of the oxide and the carbonaceous phases because of the oxidation-reduction reactions.…”

“…Kalyada et al [5] suggested that the lower limit of porosity must be standardized maintaining, at the same time, the upper limit above 18%. According to Gavrish et al [ii], the life of the nozzles having a porosity of 11.5-20.5% (measured at a distance of 300 mm from the base) is identical.…”

“…In view of the density gradients (difference) in the products, their life depends differentially on the porosity and, consequently, erroneous conclusions [3,5] are drawn if the porosity gradients are not taken into account when testing the nozzles (in particular, in low-alloy steel melts).…”