With the development of methods of processing steel outside the furnace there has been a change in the function of the ladle -from an intermediate container for transporting the metal it has been changed into a most important unit in which the steel is sequentially subjected to refining, blowing with inert gas, and vacuum treatment. Thus, the conditions of service for the refractory lining have been made much rigorous, since the dwell time of the metal in the ladle has increased from 20-30 min to 2-3 h, which has led to a reinforcement of the corrosive action of the steel and slag on the lining. Furthermore, there has been an increase in the abrasive action of the liquid steel on account of the more intense mixing during blowing and vacuum treatment [1][2][3].The use of chamotte linings in such conditions is undesirable, not only because of the sharp fall in the resistance of the ladle, but also as a result of its unfavorable influence on the purity of the metal [i, 2].For lining ladles used in processing steel outside the furnace more resistant high-alumina refractories are bing used [1][2][3][4], including those with additions of chromite [5].In Soviet and foreign practise there is experience with the use of unfired refractories of various compositions for lining ordinary steelcasting ladles [6][7][8][9].The unfired refractories compared with fired products possess a higher resistance to scaling under the action of liquid steel and slag, and low porosity, as a result of which they are less prone to penetration by metal and slag [8, 9].The present article presents the results of investigations into the development of a technology for unfired, mullite-corundum, and fired, mullite-siliceous refractories with additions of chromium ore, and also the results of comparative tests on experimental batches of the newly develope products and industrial mullite-corundum articles in the linings of steel ladles used in treating steel outside the furnace.The following material were used in the investigations: high-alumina chamotte containing 68% AI203 by weight, electrocorundum, Latnensk clay, and chrome ore. The chemical compositions of the starting materials are shown in Table i. Orthophosphoric acid was added to the bodies for the unfired products.The laboratory investigations established the optimum content of orthophosphoric acid in the bodies, e,~uring the production of high-quality green bricks and products, and these results also established the heat process schedule for the unfired and fired refractories.On the basis of the studies at the Semiluksk refractories factory we prepared trial batches of unfired mullite-corundum articles in amounts of about 220 tonnes and fired mullitesiliceous articles with additions of chrome ore in amounts of 50 tonnes.The articles were pressed on SM-I085 toggle presses with a moisture content in the body of 4-4.5%; the apparent density of the green brick for the unfired mullite-corundum articles was on average 2.73, and for fired brick containing chrome ore 2.90 g/cm 3.

A number of modern production operations in metallurgical, chemical, electronic, and other branches of industry are done in hydrogen-containing protective atmospheres at high temperatures. Hydrogen, even in low concentrations, is an active reducing agent and, therefore, for heating equipment with hydrogen-containing atmospheres refractories based on aluminum, zirconium, and magnesium oxides, which are resistant to reduction, are used [I].However, as experience shows, the factor of chemical resistance to the atmosphere predetermines the length of service only of the stationary lining of equipment while for the refractories of moving parts their heat resistance is the decisive factor. Therefore under conditions of thermal cycling it is desirable to use more heat resistant ones, particularly mullite-corundum refractories [2]. Mullite, as a silica-containing component, decomposes in a reducing atmosphere [3] but there is information [4] that as the result of a small quantity of glassy phase in fused mullite its decomposition starts somewhat later than that of sintered.In this work the changes in structure and the changes related to them in the properties of grains of fused mullite of the finer than 3 mm fraction and of specimens of heat resistant corundum parts based on fused mullite, corundum, and GK alumina [5] under conditions of thermal cycling were investigated.The investigation was made on powders of type PMAP-3 fused mullite to TU 14-8-450-83 produced by Kazakh Refractory Plant. The parts were pressed from a charge containing 35% electrocorundum, 30% fused mullite, and 35% GK alumina and fired at 1500~ after which cubes with an edge of 30 mm were cut from them.The tests of powders and specimens of parts were made in production of continuous hydrogen tunnel electric kilns. The test conditions were atmosphere of thorougly dried hydrogen, dew point -40~hydrogen consumption 3-5 m3/h, and length of cycle i0 h, including I h at 1500~ and cooling rate from 1500 to 700~ 12-15~ After i, 2, 5, 8, i0, and 12 cycles a portion of the powders was taken and the water absorption, open porosity, apparent density of grains of the 2-3 mm fraction, and the chemical analysis determined. Then they were investigated by fraction in immersion under a microscope.Specimens of parts based on fused mullite were similarly investigated after i, 6, i0, and 17 thermal cycles. The losses of weight, the open porosity, the apparent density, and the compressive strength were determined and the structure was investigated on polished specimens under the microscope.After the first thermal cycle (passage through the kiln) there is a sharp reduction in the open porosity of the grains of fused mullite from 4.2 to 1.5% while with further thermal cycling up to 12 passages the open porosity of the grains increases to 5.8% (Fig. i).The change in chemical analysis of the 3-0.5 mm fraction grains was insignificant while in the grains of the finer than 0.5 mm fraction a decrease in weight share of SiO 2 related to decomposition of silica and an increase in we...

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