Effect of the cooling rate of the melt on the structure of electrosmelted forsterite refractories

Abstract

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 ...

Effect of overheating of the melt and heat treatment of hardened materials on the structure of forsterite granules

“…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.…”

“…This regularity is not observed in a hardened, and hence defect-possessing, material. The initial temperature of melting corresponds to the melting temperature of the magnesiowiistiteforsterite eutectic, which is equal to 2063 K; 0.98 Teu t = 2020 K. 2 In order to determine the structural changes of hardened materials under the effect of high temperatures, granules obtained by melting a charge with 35% periclase powder were held at 1773-2080 K in a high-temperature hardening furnace for 3 rain and then hardened.…”