Influence of oxide additions on the sintering of corundum refractories

Abstract

Speeding up processes in ferrous metallurgy has placed new demands on refractories. Thus, when metal is being cast from ladles using gate valves, the refractory plate, which is part of the valve design, in addition to the usual properties required of refractories (high density, strength, spalling resistance, metal and slag resistance), should possess properties typical of construction materials (constant dimensions, abrasion resistance, and stability of the strength characteristics during operation).The combination of the above propertiescan be realized in corundum refractories, whose structure, phase composition, and technical factors can be varied in wide limits by introducing various oxide additives.The question of the role of oxides of Mg, Ti, Fe, Si, Cr, and other elements for producing articles from corundum has been widely discussed [1][2][3].It is known, for example, that the addition of magnesia suppresses the growth of corundum crystals and thereby contributes to the formation of a fine-grained structure, ensuring densification and strengthening of the material, and increasing its hardness [4,5].Additions of titanium dioxide on the other hand activate recrystallization of corundum, and considerably reduce the sintering temperature [4,5].Chromium oxide retards the recrystallization of A1203; its presence in the corundum in amounts of up to 3%* in the form of a solid solution leads to a tenfold increase in the hardness of the material [4][5][6].The question of the combined action of oxides on the structure and properties of corundum refractories has been less thoroughly investigated. As shown by experiments, with the addition of magnesia, titania, and chromium oxide in various combinations, titanium dioxide preserves the strong mineralizing action, magnesia retards the growth of the crystals, while Cr203 in the presence of MgO and TiO2 acts as an indifferent oxide [4,5].Existing information relates completely to the finely dispersed compositions of the oxides AI--Mg(Cr, Ti), whose sintering is accompanied by a high linear shrinkage, as a rule, equal to 15-18%. Moreover, modern metallurgical processes require large refractory articles, often of very complex shape. The linear shrinkage of such articles should not exceed 5%. Therefore, in developing high-grade corundum refractories we should be disposed toward preliminary sintering of corundum with additions, followed by milling and the use of various fractions of the resulting material.In this case, it must be considered that the structure of the grains of sintered corundum used as the filler have a marked effect on the structure, technical properties, and operational behavior of the refractory as a whole, since to a large degree it determines the nature of the sintering of the goods, the resistance to thermal stresses, and impregnation by molten metal and slags.The present article deals with a study of the effect of magnesia, titania and chromium oxide and also mixtures of these on the structure of sintered corundum and articles made from it.The structure ...

The influence of the composition and structure of alumina-containing slide plates on their failure and service

“…At the same time in the corundum plate about 65% of coarser than I ~m coarser than I #m pores are observed, including 35% pores with an effective radius of more than 3.5 ~m. According to [5,6] for the structure of a refractory in which the main volume falls within the share of pores with a radius of less than 3.5 ~m the probability of penetration of the components of the teemed metal is very small and they practically do not penetrate into pores with an effective radius of less than 3.5 ~m [7]. For refractories possessing such a fine porous structure thermal cracks become the single factor of risk making possible penetration of the reactants into the depth of the refractory and therefore the role of thermal resistance increases especially.…”

Influence of complex oxide additives on the structure and technical properties of corundum refractories

“…In order to create such refractories with a grained structure, possessing a range of defined technical characteristics, studies are being carried out aimed at discovering the influence of additions of Mg, Ti and Cr oxides, and also mixtures of them, on the sintering and structure of corundum refractories [6]. Determinations have been made of the parameters of the structure, ensuring the maximum densification and strengthening of the material, and the main elements of the technology have been ascertained related to the the formation of such a structure, in particular the substance and quantitative compositions of the complex oxide additives [6].…”

“…Determinations have been made of the parameters of the structure, ensuring the maximum densification and strengthening of the material, and the main elements of the technology have been ascertained related to the the formation of such a structure, in particular the substance and quantitative compositions of the complex oxide additives [6].…”

“…On the fractures of specimens containing the ZrO 2 additions, in contrast to the fractures of specimens without additives, described in detail in [6], there is a certain loosening of the structure of the material; on the boundary of the crystals of corundum pores are formed measuring 5-15 ~m, attached to rounded particles of ZrO2, the dimensions of which equal 2-10 ~m.…”

“…The potential of these methods as applied to the study of the structure of corundum refractories was detailed more thoroughly previously [6]. In that article results are given of a study of the pore space and the characteristics of the sinterability of the corundum refractories with additions of Mg, Ti, Cr oxides and compositions based on them, not containing ZrO 2.…”