Effect of certain technological factors on the properties of the graphite-containing products obtained using an argillaceous binder

Abstract

In the Soviet Union, graphite-containing refractory crucibles and muffles required for melting and distributing nonferrous metals and alloys are being produced using hydrostatic compaction of the bodies incorporating an argillaceous binder at a pressure of 15 N/mm 2. The service life of these products does not meet the current specifications.Based on the service conditions, the products must possess high density, strength, thermal conductivity, and thermal shock resistance and low porosity and oxidability. The forming (shaping) pressure is one of the most important factors determining the variation of these parameters.This paper deals with a study of the effectiveness of increasing the forming pressure during the production of the graphite-containing products and the technological conditions and parameters required for obtaining the products.In order to obtain refractory products by semidry compaction, the moisture content of the graphite-containing mixtures prepared using an argillaceous binder is maintained at 12%. Figure 1 shows the dependence of the strength of specimens obtained at a pressure of 15 and 60 N/mm 2 on the moisture content of the mixtures. A change in the moisture content of the mixture has virtually no effect on the ultimate bend strength Obn d of the raw specimens (green compacts) and the fired specimens and on the ultimate compressive strength Ocm of the fired specimens compacted at a pressure of 15 N/mm 2. In the case a raw specimens, Ocm was found to increase by 1.8 times with decreasing moisture content. At a compaction pressure of 60 N/mm 2, decreasing the moisture content up to 8% improves Ocm of the raw specimens by almost 3 times and Obn d by 1.5 times and, in both modes of testing, the strength of the fired specimens increases by approximately 1.3 times. In this case, the apparent density of the specimens was found to increase from 1.80-1.85 up to 1.9 g/cm a and their open porosity was found to decrease from 26-28 up to 23-25%.Thus, a significant effect due to an increased compaction pressure is observed only when the moisture content of the mixture is decreased simultaneously. This owes to the high content (38%) of the fairly plastic refractory clay in the mixture.It is not advisable to prepare a mixture having a moisture content of less than 8% since the plasticity of the argillaceous binder is not used adequately for ensuring homogeneity of the mixture and uniform coating of the binder on the particles of graphite and silicon carbide. The mixtures having a moisture content of 14% and more do not pass through the sieve easily, agglutinate, and are separated into layers after removing the compaction load.The moisture content of the mixtures can be decreased by holding (ageing) them before shaping. Figure 2 shows the dependence of the ultimate compressive strength of the as-formed specimens on the compaction pressure at different durations of holding the mixtures r prior to compaction (shaping). On increasing 9 up to 24-48 h, Ocm of the specimens increases by almost 3 times and a furt...