Experience with the use of mullite-corundum refractories in the roofs of electric-steel furnaces

Abstract

Starting in the 1960s as a result of continuous improvements in electric arc furnaces and the melting technology for electric steels, there have been increases in outputs, and the grades of steels have been altered, all of which demanded basic changes in the electrical equipment, the energy and technological melting schedules and management procedures in the electric steel-melting shops.In these conditions, periclase--chromite refractories, having found extensive use in the roofs of steel-melting furnaces, are now proving to have inadequate roof resistance. Thus, the life of roofs in the arch-suspension design of powerful 100-ton furnaces with a roof rise of from 0.13 to 0.14 of its diameter (to provide adequate constructionstrength) is on average 80-90 heats [i]; and in this case the chief causes of destruction of the refractories in the peripheral zones are cracking and spalling under the action of stresses developing during regeneration and volume changes in the refractory, saturated with slag, and also the influence of thermal impacts, gravity forces, etc; and in the central part --spalling and melting as a result of the formation of relatively low-melting compounds due to the saturation of the working zone of the refractory with iron oxides, silica and other elements from the furnace dust and the action of high temperatures.Investigations show that the life of the roofs made from periclase--chromite refractories can be increased by 15-20% as a result of design changes in the structure (for example, annular construction instead of sector-arch, etc.) [2,3]. In foreign plants, for electric furnace roofs extensive use is being made of high-alumina refractories and materials containing from 65 to 97% A1203. However, working experience shows that only refractories containing 85% or more A1203 will provide adequate roof life. It is noted that the successful use in these roofs of refractories made of alumina materials should possess, in addition to good slag resistance, an enhanced mechanical strength at elevated temperatures, and also significant spalling resistance [4].In this article we shall present results from the use of mullite corundum refractories in the roofs of electric furnaces of average specific capacity (250 kVA/ton), melting stainless and ball-bearing steels.The Institute together with the Semiluksk refractories factory has developed grade MK-80 refractories of mullite corundum composition with structural reinforcement using nonisometric grains of fused mullite [5]; the production of these refractories was mastered at the Semiluksk plant. Testing of the mullite corundum articles MK-80 possessing high spalling resistance (Table i) was done in stages in the roofs of 50-ton (actual capacity 65 tons) electric furnaces at the Dneprospetsstal' factory; in the first stage for melting stainless, and in the second ballbearing steels.The roofs had a sector-arch structure with a rise of from 0.08 to 0.09 of their diameters. In the first stage the roof was laid dry; one roof was almost completely made up of exp...

A test of synthetic mullite-corundum parts in the roof of an arc furnace

“…In addi~ tion to high temperatures (up to 1800~ and higher) and thermal shocks, the roof refractories experience local thermal stresses and mechanical loads and are also subject to the action of melting dust having variations in chemical and mineral compositions [I, 2].Investigations conducted earlier showed that the use of mullite-corundum refractories in electric furnace roofs [3,4] is most effective with the use of them in combination with PKhS periclase-chromite parts (COST 10888-76) using specially developed methods of lining [5,6].This article presents the results of investigations of two lots of synthetic mullitecorundum refractories produced by direct synthesis with a single firing in the Experimental Plant of Eastern Refractory Institute [7] after service in the lO0-ton arc steel melting furnaces (DSP-I00) of Chelyabinsk Metallurgical Combine. In the first lot of parts the weight % of AI203 varied from 58 to 72%.…”