Smelting steel in industrial furnaces has several technical, economic, health and hygiene, and ecological advantages over other steel-smelting methods. The widespread use of induction smelting of steel, particularly of alloyed steel, is hindered by the low resistance of the main lining, which is the result of ignoring the special features of the manufacture of the lining, its bringing on line, and operation. The consumption of refractories in induction smelting sometimes reaches 40 kg per ton of liquid steel.An important feature of induction furnaces is the presence of a large (up to 20-30=C/mm) temperature gradient between the working surface and the water-cooled inductor, which leads to the emergence of significant thermal and mechanical stresses in the lining as well as to intermittent working.The high working temperature of smelting and the intense electromagnetic perturbation of melts cause significant interaction to occur with the lining.The material of the lining should not be wetted by the molten metal and slag if one is to avoid migration of the melts into the depth of the lining. Corundum, fused magnesite, and composites of these in various compositions are most frequently used as the lining materials for induction furnaces, particularly vacuum furnaces; mineralizing and building components are added to the main refractory material in the batch for the manufacture of the lining.All the operations of preparing the crucible for the first standard smelting are directed to obtaining a crucible with the optimum material compositions and the optimum porosity of the structure. In order to reduce the wetting it is necessary to strive for a reduction in .•130
A comparison between the adhesion and electrophysical characteristics of oxides may be useful in predicting the interaction between the latter and a metal.The experimental and calculated data for these characteristices taken both from our own studies and from the published literature [i] are given in Table i. The values of the limiting wetting angle and the electrical conductivity of oxides are given for temperatures of 1700-1800~ at which the region of intrinsic conductivity is reached.The published data which is lacking for the electrical conductivity of oxides at this temperature were obtained by extrapolation using the activation energy [I]. Table 1 also gives the calculated values of log electrical conductivity; the experimental values [I] and the values we calculated using the method in [2] involving the band gap and assuming that the oxides have a semiconducting conduction mechanism.The limiting wetting angle between specimens of several oxides and a liquid metal was measured at 1823~ by the resting drop method developed at the M. I. Kalinin Polytechnic Institute.From the results of the measurements we calculated the adhesion energy.As the liquid metal we used OZZhR-grade iron whose chemical compositions in the initial state and after fining in an ISB-001-PI vacuum furnace are given in Table 2. A comparison of the parameters indicates the absence of any correlation between the adhesion energy and the band gap but the existence of a correlation between the former and the log electrical conductivity. Figure 1 shows the dependence of the adhesion energy on the log conductivity of the oxide.Disregarding the data for oxides of rare-earth elements, we can see that there is a linear relationship with an acceptable spread.The dependence of the electrical conductivity on the reciprocal temperature has the same form; there is an exponential dependence in the activation model for the formation of current carriers, i.e., a ~ aoe~Ea/kT or c = enH, where o 0 is the electrical conductivity extrapolated to I/T = 0; e, charge on an electron; k, Boltzmann's constant; T, absolute temperature; n, concentration of current carriers; n--Noe-Ea/kT; No, concentration of electrically active centers; and H, their mobility. T(O~ / ZOO0
Impurities intensify the structural changes in the articles being tested and change theirproper~ies. A concentration of SiO 2 and Fe203 in amounts of 0.5-1.0% increases the extent of the chemical reaction betnveen the refractory and the alkalis and lowers the electrical resistivity by an order of magnitude.The erosion of the periclase articles made from sintered and electrosmelf~d material is approximately the same at high temperatures and in a medium with a higher concentration of oxygen (23%)o The conditions under which the bushing is used in plasma-chemical apparatus with an are-column temperature of 4500--6000~ are somewhat complex and therefore it is necessary to develop an optimum design of the plasmatron unit and better methods of protecting the articles in use. LITERATURECITED
Refractory materials based on oxides of the MgO--AI203 system have become very widely used in recent years in vacuum metallurgy. We have studied the electron conductivity of these materials as a function of temperature and composition.Experimental specimens were prepared using the normal method of sintering a mechanical mixture of MgO and A1203 powders with a grain size of ~3 ~m at 2000OK. The results of the study of the electron conductivity of the specimens of the MgO--AI203 system (one of a series of measurements) are shown in Fig. i. In this particular system there is p-type impurity conductivity as is indicated by the thermo-emf data.It should be noted that there is a sharp increase in the electron conductivity and a marked rise in the activation energy of the impurity conductivity even with low concentrations of MgO in A1203 ~Fig. 2). This suggests that the refractory materials in the study are an essentially nonuniform system consisting of two phases with different conductivities,The conductivity in such a system, even with small concentrations of MgO, must be effected primarily in relation to the phase whose resistivity is significantly less than that of corundum.The MgO--AI=03 system is two-phase as is also indicated by the method of preparing the specimens. The sintering temperature was clearly low in relation to the melting of either component of the refractory but at 20006K mutual diffusion between MgO and A1203 is possible. These features of the structure and properties of the MgO--AI203 system allow us to make use of the methods of occurrence theory [I, 2] to explain the data in Figs. 1 and 2. The essence of this theory is that in a nonuniform system there is some critical parameter (in this partic ular case , the composition of the MgO--AI203 refractory) which can only be realized by a sharp change in the properties of the system, To do this, the temperature dependences log ~-(l/T) (Fig, I) were extrapolated to T + ~. The values of ~o obtained in this way depend only on the composition of the MgO--AI203 system in accordance with the expression M, I, Kalinin Leningrad Technological Institute.
Almost all the fibers have a thin edging of a very finely dispersed, dark, opaque material which is clearly the product of the strong surface interaction between the fibers and Y203.The Y203 in the specimens was sintered to a high density: intercrystal and intracrystal porosity is absent.
The lattice in an oxide is disrupted by ions and electrons diffusing from a hot zone to a cold one, which favors metal-component mass transport in linings, ioe., from the contact layer into the lining~ The density of the thermal diffusion current i is [1] i~_vj.n2 ( e$ dT dq~ ee dT d~ T dx +q-'d~x )+v~n~ (--~.-+e--~'-x),in which v i and v e are the ion and electron mobilities, with n i and n, the concentrations of the ionic and electronic defects, while ~i and se are the corresponding activation energies, with q and e the charges on the positive ions and electrons and dT/dx and d~o/dx are the temperature gradient and the potential gradient. Refractory oxides are considered as virtually stoichiometric compounds, but this is so only for moderate temperatures; they become nonstoichiometric at high temperatures~ For example, zirconium dioxide at 1200~ has a component ratio corresponding to ZrO1.86, not ZrO 2 [2]. Therefore, one can estimate the currents flowing between phases in the metal-lining system by using both parts of (1), which characterize the contributions from the ionic and electron defects to the thermal-diffusion current density.The ionic-defect and free-electron concentrations are represented by the same type of equation [1]:3.If the lining or stack in an oven is not in contact with the liquid metal but is subject to thermal stress, the thermal diffusion current is gradually attenuated because of the concentration-dependent diffusion flux. When the lining contacts the liquid metal, such balance is not attained because ions from readily ionized atoms continuously enter the contact layer.(1) indirectly implies that the transfer rate under otherwise equal conditions is governed by the oxide disorder, i.e., the defect formation energies, so we consider the kinetic relationships for the metal-lining transfer from the viewpoint of disordering, which can be estimated from the resistivity.The experiments were performed with steel containing 2% titanium at 1600-18500C in a vacuum induction furnace having a capacity of 1 kg and crucibles made from fine-grain powder (fraction around 100 #m) of aluminum, beryllium, zirconium, and magnesitim oxides. The final sintering for the crucibles was for 2 h at 1700~ in an oven with graphite heaters. The crucible wall thickness was about 10 mm (top) or 15 mm (bottom)~ The initial porosity was determined from the theoretical density together with the mass and volume, and the values were 38, 39, 34, and 36% correspondingly for the aluminum, beryllium, zirconium, and magnesium oxide crucibles. These high porosities occur because the refractory materials for making the crucibles were powders of about the same fraction size. The firing temperature and time were insufficient for the crucibles to sinter further. The impregnated-zone thickness was dependent on the material and on the temperature and time in the experiments; it was 2-10 mm. We found that the impregnation and blackening occurred throughout the wall and bottom of the crucible at 17500C and above.The liquids were kept...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.