Metallurgist, Vol. 57, Nos. 5–6, September, 2013 (Russian Original Nos. 5–6, May–June, 2013)

EFFECTIVE LINING FOR THE BOTTOM OF A PUSHER HEATING FURNACE

A. B. Krasnyi,1 I. N. Palii,1 and A. A. Grigor’ev2

UDC 66.043.1

The Bakor Scientific-Technical Center has developed a new material with a corundum-mullite-zirconium composition to line the bottom of pusher heating furnaces. The material owes its high strength to the use of high-tech raw materials and a modified binder. Positive results have been obtained from the use of preshaped blocks in a continuous furnace at a forging-machine plant (in Ukraine). Keywords: corundum-mullite-zirconium refractory, lining of the bottom of a pusher heating furnace.

Experience with the use of continuous heating furnaces at metallurgical plants shows that to improve profitability and reduce energy costs it is necessary to use new, durable refractories to line the bottom of these units [1]. Results were presented in [2, 3] from research on and trial use of new linings for the bottom of walking-beam heating furnaces. The linings are made of blocks composed of a corundum-mullite-zirconium (CMZ) refractory. It was discovered that a lining made of CMZ refractories has important advantages over linings made from various concretes and fused corundum: 1) the lining material does not react with the scale formed on the semifinished products being heated; 2) the lining is thermally stable and very resistant to erosion, which gives it a long service life – over two years; and 3) the lining is simple to install, and its constituent parts can be made at the refractories plant. These findings were substantiated in tests of the bottoms of walking-beam heating furnaces at different factories. The CMZ refractory used in the lining has a compressive strength of 75–90 MPa, which is adequate to ensure reliable performance from the linings of walking-beam heating furnaces. Refractory materials with higher strength characteristics are needed for pusher furnaces. In light of the many requests from manufacturers for durable linings to line the bottom of pusher heating furnaces, the Bakor Scientific-Technical Center has developed new refractories for furnaces of this type. In deciding on the requirements that refractories used for the bottom of pusher furnaces would have to satisfy, researchers at the center determined that these refractories must have the following properties: 1) they must be highly resistant to mechanical corrosion and, thus, be able to extend the service life of the lining; 2) they must be characterized by a high degree of thermal stability and not lead to reactions between the lining and scale. Since CMZ refractories have been shown to have these properties [4], the decision was made to develop a new highstrength refractory based on the same corundum-mullite-zirconium composition. The resistance of a material to wear is characterized mainly by its cohesive strength, which is determined as its compressive strength. In connection with this, the focus of the investigation was improving the compressive strength of the mate-

1 2

Bakor Scientific-Technical Center, Shcherbinka, Russia; e-mail: [email protected]. Teplopribor Plant, Kramatorsk, Donetsk Oblast, Ukraine.

Translated from Metallurg, No. 6, pp. 73–75, June, 2013. Original article submitted April 18, 2013.

0026-0894/13/0506-0539 ©2013 Springer Science+Business Media New York

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Fig. 1. New lining of the bottom of the furnace.

rial being studied. According to the literature data and the results obtained from analyzing the strength properties of materials that are effective for lining pusher furnaces, the compressive strength of the given material should be at least 150 MPa. Different methods can be used to control the increase in the strength of ceramics with a granular structure during the firing operation. Among these methods are using certain combinations of raw materials, changing the granulometric composition of green refractory, reformulating it to make it denser, and altering the temperature regime during the firing operation [5]. The strength of a granular ceramic is a complex function of the parameters of its microstructure and the strengths of the grains of the filler and the finely dispersed binder. As regards the production of ceramics based on electrofused grains – which do not undergo shrinkage during firing – special attention should be paid to possible shrinkage of the binder. Large amounts of shrinkage of the finely dispersed binder create appreciable stresses that lead to the formation of microcracks in ultrafine binders and/or their separation from the grains of the filler. Such developments lower the mechanical characteristics of the material. We performed petrographic studies of the microstructure and an x-ray phase analysis in order to determine the phase composition of the material. It is known that the firing of finely crushed mixtures of oxides of aluminum and zirconium is accompanied by the decomposition of zircon and the formation of dioxides of zirconium and silicon. The silicon dioxide which is formed as a result of this decomposition may subsequently react with aluminum oxide to form mullite. Whether these processes take place or not depends on the fineness of the components, their distribution, and the presence of impurities and additives in the mixture. The x-ray phase analysis showed that, by analogy with the CMZ-based material, the new material that we developed contained only negligible amounts of zircon after firing. The formation of mullite and zirconium dioxide give both the new material and the CMZ-based material a high resistance to heat. The problem of increasing the strength of materials of the corundum-mullite-zirconium composition was successfully resolved by changing the composition of the binder and adding high-tech raw materials made by leading foreign companies to the charge. The use of such a composition in making the refractory blocks makes it possible to increase the density of the green refractory and obtain a fired product with a porosity no greater than 15%. The new material has an ultimate compressive strength of 150–170 MPa, a heat resistance of at least 30 thermal cycles (1300°C – water), and a density of at least 3.3 g/cm3. The microstructure of the new material was subjected to petrographic studies. The finely dispersed binder is a structure with uniformly distributed pores and microcracks, which accounts for the high heat resistance of the material. The mullite is in the form of fine acicular crystals. The finely dispersed binder – in which the largest grains are 30 μm – is in intimate contact with the grains of the filler. The uniform distribution of the pores and microcracks and the close contact between the fine binder and the filler grains are responsible for the high strength and low porosity of the ceramic. A chemical reaction which results in sintering of the material of the bottom of furnaces and metal semifinished products being heated in them can take place when the semifinished products (or their scale) come into contact with the bottom. Such sintering leads to the formation of crusts that impede the movement of the semifinished products through the furnace. 540

Fig. 2. Condition of the lining after 18 months of service.

These crusts are mechanically broken up as the semifinished products move through the furnace, which disturbs the integrity of certain sections of the bottom and damages the surface of the semifinished products. The lower porosity of the new refractory reduces the penetration of melts of metal oxides into the material and prevents the formation of crusts. The finely dispersed binder of the previously-developed CMZ-based material consists of grains with a size of 20–50 μm, and slit-shaped pores are located at the boundary between the filler grains and the binder. These pores are probably formed as a result of shrinkage of the fine binder and its separation from the grains. The presence of the pores reduces the strength of the CMZ-based material compared to the strength of the ceramic developed in our research. Studies have confirmed the effectiveness of using the new binder to make corundum-mullite-zirconium refractories. A trial batch of blocks composed of the refractory that was recently developed was made at the experimental-test facility of the Bakor center. The first practical use of the new refractory was at the Lazovsky Forging-Machine Plant (Ukraine). The Kramatorsk plant Teplopribor (Donetsk Oblast, Ukraine) designed and built continuous furnaces for the Lazovsky plant to heat steel semifinished products (in the form of squares and cylinders). The first furnace came on line in 2009. The bottom of the furnace was made of chrome-magnesite bricks and lasted only five weeks. As the semifinished products moved along the bottom lined with chrome-magnesite bricks, the surface of the bottom wore and lengthwise channels were formed in it. The extent of the bottom’s wear was determined by the depths of the channels. The chrome-magnesite bricks also rapidly reacted with the scale of the semifinished products, resulting in the formation of crusts. In 2011, the corundum-mullite-zirconium refractory developed specifically for pusher furnaces was used to line the furnace. Ready-to-install blocks of the refractory were made at the Bakor center in accordance with specifications developed by Teplopribor. The blocks were designed so that when wear developed on their surface they could be inverted and continue in service. The bottom of the furnace has now been in use for more than 18 months, and no reactions have taken place between the material of the bottom and the metal of the semifinished products or scale formed on them. A comparative economic analysis of the use of products made of the new refractory instead of chrome-magnesite brick to line the bottoms of pusher heating furnaces showed that the capital investment in the new refractory is fully recovered in less than six months. In light of the positive experience gained from the effective use of the high-strength corundummullite-zirconium refractory in pusher heating furnaces, the management of Teplopribor and the Bakor center have partnered with one another in a program to make further use of the new refractory at factories in Ukraine.

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