Broadening of the use in domestic steel plants of submerged graphite-containing nozzles in casting of various types of steel has revealed a known complication in operation of continuous casting machines related to a decrease in the through cross-section of the channel and the holes of the submerged nozzle.This process is an obstacle to long continuous casting of steel.Clogging of submerged nozzles and prevention (slowing) of this process have more than once been the subject of investigations [1][2][3][4].In a number of works [3, 5] the influence of the submerged nozzle material on formation of alumina-containing deposits has been evaluated.The processes occurring at the temperature of casting of the steel in the refractory itself taking into consideration the presence in it of a deoxidizer, carbon (graphite, coke residue of the carbon binder), have been considered.The reaction has the general form:Refractory Refractory Gaseous Products In the gaseous state AI20 and SiO diffuse into the wall of the nozzle in the direction of growth of the temperature gradient toward the surface of the nozzle channel and then into the molten metal.On the interface of the phases and close to it reactions of oxidation of the gaseous suboxides occur :in which the oxidizer is oxygen of the steel liberated as the result of the decrease in the temperature of the metal in the layer near the wall and also oxygen liberated in occurrence of the reactionOne of the most important sources of alumina inclusinos must be assumed to be products of deoxidation of the steel with aluminum and depending upon the method of deoxidation, use of ladle treatment, and casting conditions products of either primary or secondary oxidation of the components of the steel predominate in the deposits.In continuous casting of steel containing more than 0.02% aluminum clogging of the nozzle channel occurs as the result of the aluminate deposit on its walls.Small amounts of Ca impurity form with the alumina type CaO.6AI203 hard refractory inclusions with a melting point above 1800~[6].The process of precipitation of oxides is promoted by the presence of irregularities on the inner surface of the submerged nozzle channel.Obviously a certain contribution to the process is made by the hydraulics of flow of metal through the channel of the submerged nozzle.Two features must be emphasized. In the first in the near-wall layer of the liquid (in the given case molten metal) the rate of flow is significantly less than along the axis of the channel while in the second in certain situations (condition of the tundish nozzle, conditions of stopping and shutting off the steel tapping hole) an eddy effect may occur in the stream of metal, especially in the lower portion of the nozzle in the zone of the holes.

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The physical and technological properties and, in particular, the service characteristics of the products obtained from self-bonding polycrystalline silicon carbide are determined by the microstructural features and the specific properties of the primary (SIC') and the secondary (SIC") silicon carbide.This paper deals with a study of the methods of identifying the secondary silicon carbide on the basis of the structural features and the electrophysical properties, determining its content and properties in the fired polycrystalline silicon carbide materials, and calculating the electrophysical properties of the products on the basis of the obtained data.We studied the bodies containing 70 (M70), 80 (M80), 85 (M85) and 90%* (M90) primary SiC' after carrying out their plastic forming and subsequent direct siliconizing treatment (firing) at a temperature of approximately 2300~Satisfactory sintering of the bodies and evolution of the desired final structure were ensured by introducing silicon and carbon into them; in combination with the carbide-formers present in the siliconizing particulate charge, these elements act as a source for the formation of secondary SIC". The M85 body was subjected to stepwise firing in the 1200-1700~ range maintaining intervals of I00 and 200~ (direct electric heating).Identification of the secondary SIC" was carried out according to a complex method of optical microscopy that involves a study of the electrolytically etched sections and the combined transparent-polished sections. Electrolytic polishing made it possible to determine whether the existing silicon carbide belongs to SiC' or SIC" (the latter constituent gets etched and assumes an irridescent coloration under the reflected light). Based on a study of the second type of (combined) sections, it was possible to distinguish the reaction-sintered SIC" from the recrystallized carbide. We note that in this case, redeposition (condensation) of the substance through a gaseous phase leads to the growth of SIC" on the grains of SiC' according to their optical orientation; in view of this, the narrow borders (rims) of the coarse SiC' grains could be related to recrystallized SIC" if simultaneous extinction occurs under crossed polaroids.Electron microscopic studies were carried out on the fracture surfaces and the polished surface of the specimens using a S-570 electron microscope ('Hitachi', Japan)~ When carrying out fractography, a Pt layer was applied on the fracture surfaces by ion sputtering in order to avoid recharging of the specimen under the influence of the electron probe.The electrical conductivity of SiC' and SIC" was evaluated on the basis of the concentration of the main electroactive donor impurity of nitrogen N D which was determined according to the method of electron paramagnetic resonance (EPR).The temperature corresponding to the beginning of the formation of reaction-sintered (RS) SIC" is less than that corresponding to the beginning of the formation of recrystallized (RC) SIC" (t~Sf = 1340-1400~ and t~Cf. = 1800~ [i])...

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At the present time, the use of high temperatures, low and high pressures, aggressive reagents, and high flow rates of gaseous media is increasing in numerous fields including the metallurgical industry.Approximately 67% refractories [i] fail because of corrosion during their interaction with slags, gases, and molten metals; wear of refractories due to erosion during their service in thermal units is an equally important reason for their failure.One of the methods of improving the service life of refractories under these conditions is to employ protective coatings that are capable of improving their corrosion and erosion resistance.Among the available methods of applying the coatings, the plasma method is prominent owing to its relative advantages.This method is widely used in machine building, energetics, electrical engineering, and other fields.There have been numerous studies [2-4] on various aspects of the application of plasma coatings for refractory protection; however, the service characteristics of the protected materials have not received enough attention in these studies.Our aim was to study the possibility of developing strong protective coatings on the surface of refractories using a plasma and to evaluate their characteristics.The experiments were carried out using a PVK-50 type plasmotron having a power rating up to 50 kW and a constantly regenerated cathode.* The refractory protective coatings were created using pulverized materials (particle size ranging from 40 up to 120 um) supplied through the nozzle-anode section (edge) of the plasmotron where they were fused by the plasma jet (spray).The powders were subjected to predrying at 200-300~ for a period of 3-5 h. The thickness of the sprayed-on layer was in the 0.5-0.3 mm range; in some specimens, it was ranging up to 5.0 mm.The coatings were sprayed on the substrates (measuring 150 • 60 x 30 mm) made from chamotte, periclase, corundum, graphite, and quartz glass. We used electrocorundum, a mixture of corundum and quartz glass, a mixture of corundum and zirconium dioxide, and zirconium dioxide, (fused, calcium oxide-stabilized) as the materials for coatings.The coatings were applied on the preheated (up to 500-I000~ substrates and on the substrates held at the ambient temperature.The plasma spray-coatings duration was varied from 1.5 up to 3 min.When applying plasma spray coating on the periclase and graphite substrates that were not preheated, the coatings peeled off along the substrate boundary during the coating process.Such peeling off was particularly noticed during the application of the coatings on the periclase substrates having low adhesive properties.Preheating up to >500~ makes it possible to activate the surface being coated and, thereby, to avoid peeling off of the coating.Microscopic studies of the specimens obtained after applying zirconium dioxide and corundum coatings on heat treated periclase, chamotte, corundum, and graphite substrates (Table i) showed that there is no chemical interaction between them and that the composition o...