At present in the USSR submersible nozzles of quartz glass are widely used in machines for continuous casting of steel. A high heat resistance is an advantage of quartz material.The wise use of continuous casting of steel in the whole World and the ever-increasing use of series casting, especially of mold steels, had led to a significant increase in the use of amorphous silica parts [i, 2].In the USSR submersible quartz nozzles to TU 14-8-231-77 are produced by the Podoisk Refractory Part Plant.The parts are made by slip casting of a suspension with a density of up to 1.80 g/cm 3 in gypsum molds.The formed green part is dried and fired in tunnel kilns at I150~The possibility of crystallization of the material in high-temperature firing is a significant disadvantage of this method.The presence of cristobalite reduces the heat resistance of the parts as the result of a sharp change in the linear dimensions in the polymorphic $ § ~-transformation of cristobalite.In connection with the above there is significant interest in the method of unfired quartz parts [3] in which the material is hardened by hydrothermal treatment.Such a treatment eliminates the formation of cristobalite and, in addition, provides a reduction in the consumption of thermal energy in production of the parts.The purpose of this work was the development and comparison of different variations of the method for submersible quartz nozzles for continuous casting machines.The work was done in collaboration with Podolsk Refractory Part Plants.In the production of unfired quartz refractories the properties of the original highly concentrated suspension have a special influence.During wet grinding of the paticles of fused quartz complex processes causing not only obtaining of the specified fineness but also mechanicochemical activation of the dispersed phase occur. The main problem consists of creation of a cast system revealing binding properties.The latter are determined by a combination of characteristics (volume share of the solid phase CV, degree of dispersion, pH, viscosity, etc.) formed directly in grinding [4].The rate of grinding, evaluated, for example, by the content in the system of fine (<5 pm) or coarse (>50 or >63 pm) fractions, is determined by many factors.The most general of them, which are the result of the type of grinder and the composition of the charge, are the effective density of the grinding bodies (APg.b is the difference in the density of the grinding bodies and the suspension), the active zone of grinding (~ is the ratio of the volume of voids between the grinding bodies to the volume of suspension being ground), and the size (diameter) of the grinder.It is known that in the highly concentrated suspension method there are two methods of wet grinding, with single-step and with portion (step-by-step) charging of the material. The first method is used in grinding comparatively coarse porosity-free materials such as quartz glass or fused quartz.In wet grinding of such a material in a ball mill the processes of crushing and fine gr...

666.762.2.017:620~!79oi T. I. Litovskaya, and S. P. Shmitt-FogelevichThe formation of cristobalite during the preparation of articles made from quartz ceramic leads to the emergence of cracks in the articles and to a loss of strength [i] as a result of the ~ + B cristobalite transition on cooling.As a result, the properties of the articles are degraded in use.It is therefore necessary to eliminate as far as possible the supply to the customer of products in which a critical amount of cristobalite has been formed.Sometimes the formation of cristobalite leads to cracks which emerge on the surface of the article.In that case defective articles can be revealed by visual inspection.Since the reasons for the formation of cristobalite may vary and be associated with very slight and unrecorded deviations in the production process, the output of articles in which the concentration of cristobalite goes beyond permissible limits is random in character.At the present time there are no inspection methods which make it possible to reveal~ reliably and immediately, those articles which contain cristobalite.Normally under laboratory conditions the concentration of cristobalite is determined using an x-ray method [2] but for several reasons this cannot be used for production control.According to [3,4] as well as from some theoretical considerations, the emergence of cristobalite must produce a change in the dielectric properties of a quartz ceramic. This fact may be used as the basis of a method which makes it possible to organize on-line production control of the concentration of cristobalite in the articles.The aim of the studies described in the present paper was to investigate the dependence of the dielectric properties, porosity, and density, on the phase composition of the ceramic.The studies were carried out on cylindrical specimens made by a slip-casting method. As the starting materials we used vitreous silica with a mass fraction of SiO2 > 99%. The specimens were fired in a furnace with silicon carbide heaters at a heating rate of 200~ with a 3-h dwell.It was possible to produce specimens with different concentrations of cristobalite by changing the firing temperature between I150~and 1300~ The open porosity (Pop) and apparent density (Papp) of the specimens were determined in accordance with GOST 2409-80; the concentration of cristobalite was determined using an x-ray method.Samples were prepared by quartation down to 2-5 g and milling in a chalcedony mortar to particles <0.063 nun. The measurement error in the concentration of crystalline phase depends on the amount and extent of defectiveness in the cristobalite [2].The dielectric properties of the specimens were studied in the UHF range at wavelengths of % = 3.935 and 0.884 cm.The measurements at % = 3.935 cm were carried out in an INF-2 instrument [5] using horn-type antennae with an aperture of 2.4 x 2.4 cm; the measurements at X = 0.884 cm were carried out in equipment assembled from standard units in accordm~ce with the diagram in Fig. i; horn antennae with an ...

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

In a recent study [I], we established the main regularities of the centrifugal casting process and some of the technological parameters required for casting the highly concentrated binder suspensions (HCBS) of fused quartz.This paper deals with a specific study of the effect of the technological and the theological properties of HCBS of fused quartz on the porosity Pcsg and the strength of the castings based on them. Some aspects of the strengthening process of-the castings used for obtaining unfired ceramics [2] are also examined.During centrifugal casting, besides the concentration of HCBS and the coefficient of excess solid phase Kex s [i, 2], the circumferential velocity ~ has a significant effect on the parameter Pcsg (Fig. I, region i). In this region, the range of variation of Pcsg at different values of ~ is determined by various technological factors (for example, Kexs). However, a regular increase of the porosity of the castings is observed with increasing of the casting process. On increasing ~ from 3 up to 20 m/sec, Pcsg increases from 11-14 up to 19-22%. The corresponding values of the apparent density of the castings are found to decrease from 1.86-1.89 up to 1.71-1.78 g/cm s. The aforementioned regularity owes to the rheologicai properties of the HGBS (in particular, their dilatancy) and to the excessive layering of the castings (that, in turn, hinders dense packing of particles) at high values of ~ of the process.It is known [3,4] that the increased porosity of the slip-cast quartz ceramics in the limits indicated in Fig. 1 (see region i) is accompanied by a significant loss of their strength.During centrifugal casting, abn d of the castings in the as-dried condition (region 2) as well as in the strengthened condition (region 3) remain unaltered. The aforementioned anomaly possibly owes to the structure (in particular, the specific features of the grain size distribution in the castings) and requires a special investigation.Just as in the case of slip casting, the pH value of the original HCBS (that, in turn, determines the rheolQgical properties [5,6]) is another significant factor. Figure 2 shows that thixotropic properties (in the acidic region) and distinct dilatant characteristics (in the alkaline region) become apparent depending on the pH value of the suspension. Figure 3 shows the results of a comparative study of the porosity of the castings obtained by slip casting and centrifugal casting from a given HCBS over a wide range of pH values. In contrast to the slip casting process during which a significant increase of Pcsg is observed in the region of low pH values (during thixotropic flow of the original HCBS), even a decrease of Pcsg can occur during the centrifugal casting process. On the other hand, in the pH range corresponding to dilatant flow, an increase of Pcsg is observed during centrifugal casting whereas in the case of slip casting, minimum values of Pcsg are observed in the same range.The aforementioned difference can be attributed to the 'deformation' characteristics of the pr...

Until the present ime, slip casting in gypsum molds has been the preferred method for shaping quartz ceramics [I, 2].When using highly concentrated binder suspensions (HCBS) that are based on quartz glass or fused quartz, one can attain low porosity levels in the castings [3,4]. This is extremely important from the standpoint of technology (in particular, during the production of unfired materials [4]). However, this method has significant shortcomings such as slow rate of shaping and rapid wear of the gypsum molds.In view of this, the method of centrifugal forming of castings from the same HCBS that are often used for slip casting is of considerable practical interest. This method makes it possible to eliminate the aforementioned shortcomings of slip casting.It is mainly used for shaping solids of revolution such as the nozzles of the steel casting ladles (downtubes), heat insulating pipes, and roller casings of the "Tandem" type furnaces. However, when the process is perfected, products having inserts (lateral parts) can also be molded.As concerns the shaping of ceramics and refractories, the process of centrifugal casting has been used rarely up to the present time and, therefore, virtually no studies have been carried out on these aspects. In view of this, we examined the general regularities of the process of centrifugal casting and the effect of the main technological factors on it.The highly concentrated (C V = 0.70-0.75) fused quartz suspensions that are used foe obtaining unfired quartz ceramics [4,5] formed the basic experimental materials. Experimental specimens were obtained in a metallic (porefree) mold using the previously described installation [5]. The experiments conducted using HCBS of the mullite, chamotte, and other systems showed that the main regularities established during the study of the HCBS of fused quartz are generally valid for the centrifugal tasting process of all the investigated systems.Just as in the process of centrifugal forming of ferro-concretes [6,7], when a mold containing a uniformly distributed suspension is rotated around a fixed axis at a predetermined speed, a centrifugal pressure Pc is developed in the molding system, and under the action of this compressive pressure the dispersing medium is squeezed out from the suspension along with the finest particles suspended in it. During this process, the larger particles of the solid phase are simultaneously drawn closer to each other.The dispersing medium of HCBS receives the centrifugal pressure directly. In view of this, the excess hydrostatic pressure increases and leads to filtration of the liquid and simultaneous densification of the solid phase of HCBS. The nmnber of revolutions of the mold per unit time n (or the circumferential or the linear velocity ~ derived from it) forms the most important parameter of the process.The compressive (compacting) pressure is distributed nonuniformly across the wall thickness of the product (just as the hydrostatic pressure). Its maxim~n and the minimum values are obtained at the inte...