The cooling rate of the melt and the crystallization conditions are the main factors in the technology of producing fused refractories and determine the structure of the material. To study the effect of these factors on the structure of fused forsterite material, batches based on serpentinites from the Dzhetygarinsk Deposits with additions of metallurgical magnesite were melted, followed by cooling of the melt at a rate of 100~ and quenching in water or oil.The chemical composition of the serpentinite and of the metallurgical magnesite we used is given in Table i.The batch components in fractions less than 0.3 mm were milled and fired at 13730K after careful stirring; the compositions obtained (Table 2) were fused in graphite molds at a voltage of 100-120 V and a current of 200-220 A.Lumps of material, diameter 3-4 mm, were taken from the blocks obtained and small I0 • I0 • 60 mm blocks were cut out. The small blocks were melted in an electric arc between two horizontal graphite electrodes with a voltage of 50 V and a current of 35-45 A. Drops of the melt falling freely from a height of 200 mm were cooled in water or oil. The temperature of the falling drops was determined using an MRS-I multiple-spectra recorder in the Physics of Arc-Discharge Laboratory at the Institute of Thermophysics, Siberian Branch of the Academy of Sciences of the USSR.The measured temperature of the drops did not depend on their chemical composition and was 2350-2450~ probably because the velocity of the melt decreases simultaneously with an increase in the melting temperature of the MgO material and less superheating is required to break off the drop.The rate of cooling of the melt during the crystallization of the drops was measured in the MRS-I instrument and was 7.2.108 ~ The drops are brittle, hollow granules with a variable wall thickness of I to 3 mm with a volume of 350-900 mm3; the average mass of the drops was 0.96 g. No dependence of the volume and mass of the drops on the concentration of magnesite was observed. There were no differences in the phase composition or structure of the granules quenched in water or in oil in spite of the different thermophysical characteristics of these two media.The interference lines on the x-ray patterns of the specimens obtained are quite distinct indicating the excellent crystallization of the material and the absence of any significant concentrated nonuniformities caused by intracrystalline liquid-phase separation which normally increases with the cooling rate of the melt.In all the specimens there are lines of forsterite, magnesite, and spinel and in specimen No. i, very weak lines of clinoenstatite.No halo, characteristic of the existence of a glass phase is present. The absence of glass is also confirmed by the data from the petrographic analysis.* There was no diffferences in the phase composition of specimens cooled at a rate of 100~ and those at 7.2.108 ~ The forsterite of the specimens is represented by prismatic crystals slightly extended in a single direction and randomly oriented ...
which are individual for each impurity being determined. These coefficients are intended to eliminate the breakdown of the equalities of the intensities of the lines in the samples and the graduation specimens with the same concentrations in them of the elements being determined caused by structural noncorrespondences.
Overheating of the melt above the melting point is inexpedient, because it does not lead to substantial improvement of the structure although the energy consumption is increased. Holding of hardened materials at high temperatures increases the porosity substantially without changing the volume, which means that the crystals formed have multiple defects.The previous works of this series [1,2] were devoted to the effect of the cooling rate of a melt of forsterite composition in the period of crystallization and its chemical composition on the structure of the materials obtained.As a rule, when a melt is heated above the melting point and poured and cooled under the same conditions, the curve describing the temperature dependence of the grain size has a nonlinear nature [3] and a minimum at the overheating point characteristic for each particular substance. The heterogeneous crystallization is changed to a homogeneous one and control of the pouring temperature becomes very important for controlling the structure of the material.In order to determine the dependence between the grain size and the overheating temperature we prepared specimens by the following method. The material of blocks of electrofused forsterite was melted in an electric arc with hardening of the molten drops in water, which ensured a homogeneous fine-crystalline structure for the material and a uniform chemical composition. Pieces of drops with the same mass (0.30-0.35 g) were heated in a high-temperature hardening microfumace [4] to the melting point, the melt was overheated, held at the needed temperature for 30 see, and then (1) cooled by hardening in the water-cooled pipe of the furnace (2) cooled to a temperature 50°C higher than the temperature of the end of melting, followed by 30-sec holding and cooling in the water-cooled pipe of the furnace (Fig. I). This regime provides similar conditions for the cooling of specimens and is a traditional technique for constructing such dependences [3].When hardening the material from different temperatures the drops have different internal energies, which leads to substantially different temperature gradients in cooling and I Alma-Atinskii Power Institute, Alma-Ata, Kazakhstan. hence to substantially different crystallization conditions at the surface of the drop and at its center for different specimens. Holding of the melt at the same overheating temperature creates similar conditions of crystallization of the material although the thermal history of the specimens can be different. The thermal history affects the degree of ordering of the melt, which becomes worse after melting the crystals at a higher temperature or with lengthier holding times at the same temperature. As the overheating temperature is increased, the number of undissolved crystals of the high-temperature phase of the material and impurities capable of becoming centers of heterogeneous crystallization decreases.The experiments were carried out with specimens containing asbestos waste and periclase of near-eutectic and post-eutectic...
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