Structural and phase changes in fused mullite powder and parts based on it in thermal cycling in a hydrogen atmosphere

Abstract

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...

Properties of materials of the system Al2O3-SiC-C after firing in a reducing atmosphere

“…As is well known, formation of mullite needles, that are distributed over the boundaries of graphite flakes and that strengthen the bond between refractory grains, promotes compaction, strengthens and increases material heat resistance [11,12].…”