Production of corundum-graphite complex articles by isostatic pressing

Abstract

Following the introduction of high-output continuous ingot-casting machines (MNLZ) it became necessary to create new types of large refractory articles of complicated shape, and possessing a high metal and slag resistance under the dynamic action of liquid metal, and also a good thermal-shock resistance. These included batching devices for intermediate ladles for continuous ingot casting (MNLZ) (immersion nozzles and stopper-blocks) of tubular design with a large length to diameter ratio.These articles are best shaped by isostatic pressing* [i, p. 4; 2, p. 3], a method that is widely used in powder metallurgy and in recent years being rapidly developed in the manufacture of ceramics and refractories [3][4][5]. After being isostatically pressed, the green articles have a greater density uniformity and are in fact denser than, e.g., articles that have been dry pressed hydraulically. A useful method is vibration and vacuum densification of the powder before pressing [6; 7, p. 2] which improves the product's quality.The Bogdanovich Refractories Factory has put into action a section for making immersion nozzles and stopper-blocks which includes equipment for preparing corundum--graphite bodies, a plant for vibration-vacuumdensification, isostatic pressing machinery, and a division for glazing the articles and preparing elastic shells. The vibration-vacuum machine (Fig. i) has a double vibrator operating at a frequency of 50 cycles/sec and an amplitude controllable up to 4 mm; its vacuum chamber is connected to a vacuum pump. The hydrostatic unit, developed and built by VNIImetmash (Research Institute for Metallurgical Machinery) is designed towork at pressures of up to 2000 kgf/cm 2, and is fitted with two alternatively operating containers 400 mm in diameter and 2000 mm in length. The hydrostatic unit's feature is the use of the "free press-molds," i.e., molds extractable from the containers after each pressing cycle.In addition to radial pressure created during the forming of the elastic shell, easing the lateral surface of the article, axial densification is provided for the compound by means of the hydraulic counterweights, which are activated also by the elastic shell, and which are built into the cover of the press-molds. The presence of the hydraulic counterweight during pressing of stopper-blocks facilitates the formation of their conical--spherical heads without additional machining. Figure 2 shows the overall flow sheet for making corundum--graphite articles. These are made from batches containing crystalline silicon, silicon carbide, electrocorundum, foliated graphite, fireclay, and other components. Part of the clay is added as a slip with a density of 1.2 g/cm s in a water solution of sulfite lye. Uniform distribution of the components is obtained first by blending the silicon, carbide, graphite and part of the clay slip, and then the corundum and other slip constituents are added; the clay is put in last. The total blending time is 60 min. The water content of the prepared compound is 2.8-3.2%.The isosta...

Pressability of refractory materials on hydrodynamic machines

“…The general character of the rise in relative density with pressure increases from hydrodynamic pressing is the same as the law for hydrostatic densification of refractory bodies [5][6][7][8]. But with the same pressing force (Fig.…”