Prolonged tests and extensive operating experience for channel furnaces for melting aluminum alloys have shown that their good economic performance is accompanied by major shortcomings, which are due to the poor stability in the refractory lining in the longitudinal and transverse blocks of the tapoff induction unit, particularly on operation at high power. Often, these tapoff units fail because of trapping in the arch coating in the transverse blocks and the flow of liquid into lining junctions in the longitudinal channel and in the transverse block.Cracks arise in the lining for the transverse block because of the large temperature differences over the cross section, particularly in emergency situations, as when a channel blocks or needs to be cleaned or one of the tapoff units needs to be replaced.The lining is made of heat-resistant concrete or refractories; the thermal stresses arising in monolithic refractories relax less than those in sintered ones [i], so heat resistance has to be considered in choosing the refractories.Thermal failure in refractories can be evaluated from a criterion [2] that estimates the resistance to cracking and crack growth. That method has been used to evaluate the stability in unshaped refractories, which were chosen to resist molten aluminum-base alloys. MK-90 mullite-corundum refractory with phosphate bonding agent is resistant to such alloys [3], as is our MKBS mullite-corundummaterial based on a combined bonding agent consisting of lignin sulfonate and borosilicate glass. These materials have been compared with heat-resistant magnesian concrete bonded by waterglass, which is fairly heat-resistant but not very resistant to the metal.The thermal deformation resistance has been evaluated from the R 0 criterion, which is defined by Ots(l -~)/E=, in which Ots is the tensile strength, ~ Poisson's ratio, E elastic modulus, and ~ the linear expansion coefficient. The thermal resistance coefficient R has been determined by the [4] method on the basis of the brittleness measure.The thermal resistance was judged from the capacity to resist crack propagation.Griffiths' theory indi-
'fqne cap in the IAKM-40 mixer consists of a ring with an external diameter of 170 mm, internal 70 mm, and thickness 50 mm.It is made from asbothermosilicate [I] by pressing into a ring mold in the axial direction.Asbothermosilicate consists of 45% asbestos, 30% diatomite, and 25% lime.Using a cutter the laboratories of the Krasnoyar Metallurgical Factory cut three batches of specimens in the form of prisms (base 20 x 20 mm, height 40 mm) in three directions over the ring cap -axial, tangential and radial.At room temperature we obtained the followingcompressive strength in the cut specimens: Obz -axial, 8 N/mm2; Obr radial, 7 N/mm ;and G~_ tangential, 8 N/mm 2. After three heat cycles, the compressive strengths were 8, u~ 6.3, and 5.5 N/mm 2 respectively.Taking into account the method of preparing the ring cap of the mixer it is possible to make the following statement:asbestos thermosilicate possesses orthogonai anisotropy with three mutually perpendicular planes of structural symmetry.The deformation of the orthotropi c body is characterized by nine independent constants of elasticity in the main directions.They should be determined [2] by measuring the deformation in specimens over the three main directions (along the axis of the ring checker, and across it in the tangential and radial directions).During the investigation on each specimen we determined the elasticity modulus along the axis and two coefficients of transverse deformation.After testing three batches of specimens we obtained nine elasticity constants: E a = E3, E t = El, E r = E2 -the moduli under compression, respectively along the axes of the cap of the mixer and transversely in the tangential and radial directions:Vta------'Vl3 ~ Vat=V3h
In modern induction channel furnaces (IAK-25/2.1 and IAK-40 3.5 types) and lining of the channel sections, which is made from refractory concrete containing a high-alumina filler bonded with 18% water glass, experiences temperature variations from 330 to 1050~ The detachable induction units consist of thick-walled cylinders that are heated irregularly over their radius. The temperature drop AT equals 200~The detachable unit has a U-shaped channel section (see Fig. 1) consisting of longitudinal channels 1, 2, and a transverse channel 3, which, upon installation of the unit in the furnace, encompasses a toroidal inductor, including a magnetic circuit 4 and a coil 5. The longitudinal and transverse channels each have two layers of lining: the internal layer 6 made of refractory concrete on which is laid, with a tolerance (before the installation of the unit) in the lon#tudinal channel, a metal cylinder 7 made of nonmagnetic material; and an external layer 8 applied to the cylinder 7 and made of nonsintering, heatinsulating refractory fiber (kaolin wool). Both layers are contained in frames 9 and 10, made of nonmagnetic material. By means of the refractory gasket 11, the unit is fitted to the furnace frame 12, using bolt connections; it operates according to the principles of a channel furnace.When the furnace is working, liquid metal, heated in the channels by the induced currents, creates monolateral heating of the lining of the longitudinal and transverse blocks. This causes, in an annular direction of the lining 6, compression of the internal, and tension of the external, layers. The temperature field is symmetrical relative to the axis of the cylinder, and constant over its length.It is established [1, 2] that cracks develop on the outside of the concrete cylinder, and these are caused mainly by the normal tensile stresses which reach magntidues close to the tensile strength of the refractory concrete used to line the channels of the induction furnace. It is known that the refractory of the lining has a low tensile strength (4-6 MPa). These tensile stresses in the danger zone of the lining can be eliminated by using a band in the form of a cylinder made of nonmagnetic material, placed on the concrete cylinder with a certain clearance.The design-calculation for the detachable unit with the band consists of solving two problems: determining the main stresses in the double-layer composite cylinder that develop during pressing of the layers (normally by means of hot fitting); and determining the main stresses in the double-layer, nonuniform cylinder during its unequal stationary, assymmetrical heating.Working out the first problem with the well-known Lam6 and Gadolin equations [3] does not take into account the axial stress that develops during the coupling of the cylinders with the clearance, and also during tightening of the detachable unit with a screw device. The resulting axial stresses may be of the same order as tangential and radial stresses. Ignoring axial stresses in designing the component tubes leads to...
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