Results are provided for a study of refractory concrete before and after service in a cementation furnace with the aim of revealing reasons for rapid wear of a concrete block after four months of operation at 850 -930°C in an endogas atmosphere. It is established that the reason for the rapid loss of concrete strength is destruction of the concrete matrix as a result of the chemical action of endogas. Destruction of the concrete matrix is connected with partial reduction of SiO 2 to volatile SiO in microsilica and in calcium aluminosilicates.
Recently, refining of steel outside the furnace, which permits a considerable reduction in the content of gases and nonmetallic inclusions in the metal and improvement of its quality, has attained wide use. Existing procedures for refining metal outside the furnace --vacuum treatment, treatment with synthetic slags, and purging with inert gases --differ from one another in complexity, cost of the equipment, and the technical efficiency.The advantages of purging metal with gases are the minimal capital costs and the possibility of using simple inexpensive equipment with the existing stock of steel-teeming ladles without significant reconstruction of the casting bays. Treatment of the metal with inert gases in the steel-teeming ladle also increases the output of the smelting units, reduces the consumption of electricity (by finishing the metal outside the furnace) and alloying additions (by transferring alloying to the steel-teeming ladle), and improves the quality of the metal by reducing its gas saturation and averaging the temperature and composition during its mixing --this is particularly important when the capacity of the units is increased.Purging of metal with inert gases in a steel-teeming ladle by means of devices with relatively large orifices (tubes, nozzles, etc. ) does not ensure preparation of a sufficiently developed gas/metal contact surface and reduces the refining efficiency; therefore, widespread use has been attained by the method of metal purging through gas-permeable refractories, laid in the bottom of the ladle [1][2][3][4][5][6][7][8][9][10][11][12]. Both outside [13][14][15][16] and in the USSR [17] the gas is fed downward to the steel-teeming ladle through a special refractory tuyere fixed into the ladle bottom (Fig. 1). The porous tuyere may be in the form of a truncated cone or pyramid with an internal porous sector, or two truncated cones with a common base.This article summarizes the experience gained in the development of refractory materials, used for metal purging, based on corundum, high-alumina chamotte, and periclase.The purging efficiency depends on the number and size of the gas bubbles and their distribution in the body of the metal, the purging time, the gas pressure, etc. Experience gained shows that with attainment of the required efficiency the gas consumption per ton of metal treated is 0.5-1 m a, corresponding, e.g., to a gas consumption of 50-100 m 3 at an excess pressure of 1 atm for 10 rain in a 100-ton ladle. An increase in the pressure of the purging gas (in comparison with the accepted excess pressure of 3-6 atm) may lead to jet effusion of the gas --this is undesirable [18]; the increase in the purging time is limited by the cooling of the metal [1].
The production of high-alumina refractories on the basis of technical alumina and clay is being extensively developed. The basic versions of the production technology are being perfected as a result of research being done at the D. I. Mendeleev Moscow Institute of Chemical Technology [i], the Ukrainian Scientific-Research Institute of Refractories [2, 3], the Podol'sk refractories factory* [4,5], and in other organizations. However, this technology for high-alumina refractories, being traditional, is complicated by the need to make two firings (briquet and finished goods), and double milling (alumina and chamotte).The Eastern Institute of Refractories has developed a simplified method for making synthetic, high-alumina refractories by direct synthesis from finely milled alumina and granular quartz [6,7].The working proprties of aluminous refractories, including their load resistance at high temperatures, largely depend on the ratio of the crystalline phases making up the brick. Suitable distribution of crystalline phases and a small amount of glassy phase are the distinctive features of high-alumina materials obtained by the direct synthesis of mullite from alumina and silica of different grain sizes, using a method developed by the Eastern Refractories Institute.Synthetic mullite refractories made from coarse-crystalline quartzite and fine corundum, after firing at 1600~ contain (parts by volume) 40-75% mullite, 20-45% corundum, 5-12% metacristobalite, and 5-7% glass. The peculiarity of the microstructure of these materials is that the finely dispersed bond, i.e., the matrix, has a mullite-corundum basis, and the granular framework consists of mullite and cristobalite. Therefore, the technical properties of the synthetic mullite refractories mainly determine the high-temperature subsystem 3AI2Oa'2SiO=--AI2Oa in which the liquid phase appears at 1850~[8].This article gives the results for the deformation properties of synthetic mullite, made from granular quartzite (fractions 0.I-i mm) and fine corundum (particles sizes 7-40 ~m), in relation to the applied loads, test temperature, and soaking time. The behavior of these materials at high temperatures was compared with that for high-alumina brick made with the conventional methods. In addition, in order to predict the behavior of the new materials in actual conditions, we studied their deformation after prolonged heat treatment (aging) at 1600 and 1700~ for 50 and I00 h.For comparison, we chose high-alumina articles MKP-72 and MKS-18 made at the Semiluki Refractory Plant (SOZ) and also mullite and mullite--corundum articles whose production technology was developed by the Ukrainian Institute of Refractories [9]. High-Alumina (mullite) chamotte used in them was obtained from kaolin and technical alumina. Calcined finely milled alumina was also added to the mulllte--corundum batches, in addition to mullite chamotte.In terms of the weight proportions of alumina, the selected high-alumina refractories, compared with the new articles, were the same, or had a higher con...
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