Conditions for the thermal regime of 25-ton steel ladles lined with periclase-carbon refractories are analyzed. The input data for numerical analysis are the temperatures at the inner surface of the lining measured experimentally. Temperature profiles over the cross-section of the hot layer of the lining are calculated and then used to determine thermal stresses in the refractory material. A conclusion is drawn that sharp temperature gradients during the heating-up should be avoided. The currently employed heating regimes generate thermal stresses that exceed the strength tolerance limits for refractory materials.Heating-up of the refractory lining is a critical operation for any metallurgical plant as it is being put in service. The rapid heat-up of the lining is favorably looked upon by technologists since it allows one to save fuel and reduce the standby time to a minimum. On the other hand, the heat-up rate cannot be too high at the risk of excessive thermal stresses that may generate cracking and spalling of the refractory material and thus to shorten the campaign of the metallurgical plant.To study thermal stresses generated by heating we consider the lining of a 25-ton steel ladle. The lining in question was composed of five layers. The thickness of the operating layer (hot layer) made of a periclase-carbon refractory was 135 mm, and that of the reinforcing layer (chamotte) was 65 mm. Sandwiched between these two layers is a rammed mullite-corundum mixed layer 30 mm thick. The graded lining was insulated from the metal casing by sheets of asbestos cardboard 10 mm thick. In what follows, an analysis of thermal stresses will be given only for the hot layer of the graded lining.The input data for numerical analysis were temperatures at the inner surface of the lining that were measured experimentally. The temperature at the inner surface of the lining varying over time is shown in Fig. 1.For solving the problem of inner heat exchange within the hot layer, some simplifying assumptions were made. We assume that the hot layer is uniform and isotropic, with constant values of the specific heat capacity per unit volume c [J/(m 3 × K)] and heat conductivity l [W/(m × K]. The mathematical formulation of the problem is ¶ ¶ ¶ ¶ T t a T y = 2 2 , 0 < y < H,where a is the thermal diffusivity, m 2 /sec. The temperature of the inner surface of the lining being known, the boundary condition of the 1st kind is written as(2)
An analysis of the methods of increasing the temperature of the lining during its drying and heating is given. Three options for increasing the temperature are considered: the maximum possible initial heating rate and its further decrease, the average heating rate, and the minimum initial heating rate and its further acceleration to the maximum. It is shown that for preserving the resulting thermal stresses below the strength limit of the material, the heating option with the highest initial heating rate followed by its further decrease is the most efficient.
A setup for determining the ultimate strength of refractory materials in compression at high temperatures is examined. The value of the ultimate strength of periclase-carbon materials in compression in the temperature range 20 -500°C are presented. The fracture process and strength are analyzed.High-temperature plants are lined with refractory materials. In addition, the service life of many high-temperature plants is determined by the service life of the lining.Optimization of the factors influencing the stability of the lining makes it possible to increase the working run of a plant several-fold. Even without changing the form of the refractory a significant result can be achieved by simply improving the operating conditions (temperature regimes).Physical factors such as expansion and cracking arise in the case of thermal action on furnace lining. It becomes necessary to operate the plant continually without damaging the integrity of the lining in the working chamber and the technical-economic indices of the process.To prevent the lining from being damaged by the stresses arising during heating it must be operated in a regime where stresses grow at a rate below their relaxation rate. It is important to calculate the values of the thermal stresses when calculating the rate of heating. The stress values calculated using computational relations are compared with the admissible values, and on this basis a conclusion is drawn concerning the rate of heating of the plant. The average heating rate of high-temperature plants is about 60 K/min [1].The ultimate strength of the material is used in the calculations as the admissible value of the stresses arising with a change in the temperature field. Knowing the exact temperature dependence of the ultimate strength of the materials used it is possible to determine the maximum rate of heating of a plant (according to the conditions under which stresses arise).The results of tests performed on carbon-containing refractories produced by 'Kombinat Magnezit' JSC are presented in [2]. The characteristics and the stability of the parts are indicated and it is noted that the refractories have a high resistance to thermal shearing in the presence of temperature fluctuations. It should be noted that the ultimate strength in compression (ranging from 19 to 61 MPa for different types of refractories) is presented only at temperature 20°C. Thus, the plant data do not show the dynamics of the change in this parameter as a function of temperature.In the development of heating regimes for high-temperature plants the values of many parameters are constants, i.e., they are independent of temperature. For example, for calculations the specific heat capacity c, thermal conductivity l and ultimate strength s are often taken as constants. At the same time the value of the ultimate strength of a material in compression depends strongly on the temperature.The temperature dependence of the ultimate strength of ceramic material used as lining is of great importance for the development of heating sc...
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