RU2713049C1 - Method of making ceramic melting crucibles - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to production of melting crucibles and can be used in operation with heat-resistant and chemically active alloys. Refractory charge materials are mixed with a paraffin-containing binder and a crucible is moulded from the obtained mass in a metal meld. According to the disclosed method, the moulding cavity is filled with a paraffin-containing ceramic mixture in a vibrocompression mode under conditions of radial temperature gradient at hardening front of paraffin-containing ceramic mass directed from inner shaping surface to outer shaping surface. Further, partial removal of paraffin-containing thermoplastic binder in backfill with alumina and further impregnation of the crucible from inside with an alcohol solution of salt containing ions of Al, Mg, Zr, Hf or REM or their mixture. Products are annealed at temperature of 1350–1550 °C for 6–8 hours.

EFFECT: technical result of invention is improvement of operating characteristics of crucibles produced by more economical method.

1 cl, 1 ex, 5 dwg

Description

The invention relates to the production of ceramic melting crucibles of increased metal consumption, used in casting blades made of heat-resistant alloys for gas turbine engines on vacuum induction melting plants.

It is known that the ceramic crucible during operation is subjected to physicochemical effects from the molten high-alloy heat-resistant alloys at high temperatures, which can lead to contamination of the alloy with non-metallic inclusions, a change in chemical composition. Along with this, significant alternating thermal stresses arise in the walls of the ceramic crucible, which can lead to the rapid formation of unacceptable through cracks.

In the case of ceramic crucibles with increased metal consumption, the requirements for resistance to erosion, corrosion and thermal resistance increase due to an increase in contact time with molten metal and an increase in metallostatic pressure.

A known method of producing ceramic products, which can be used for the manufacture of crucibles for melting high-purity ferrous, non-ferrous and rare metals (RF Patent No. 2 342 344. Published December 27, 2008).

The claimed method is implemented as follows: the initial magnesium oxide is subjected to grinding to obtain a powder with a particle size of 0.05-0.1 microns; anhydrous magnesium dihydroorthophosphate is added to the magnesium oxide powder as a binder in a ratio of 5-15 wt. % binder and 85-95 wt. % MgO; the mixture is stirred until homogeneous, 2-7 wt. % water in relation to the weight of the binder; the molding mass thus prepared is held at a temperature of 10-30 ° C. for 5-10 hours; the resulting molding material is molded and subjected to heat treatment at a temperature of 150-300 ° C for 3-24 hours. The molding is carried out by pressing with a force of 700-1200 kg / cm ³ .

The disadvantages of the method include the presence of magnesium dihydroorthophosphate in the crucible, which can lead to phosphorus contamination of the melt of high-purity metals, in particular heat-resistant alloys. Pressing of large-sized crucibles causes technological difficulties.

A known method of manufacturing crucibles for melting metals and alloys in furnaces with induction heating (RF Patent No. 2 323 195. Published on April 27, 2007)

The formation of crucibles is carried out by casting a highly concentrated quartz-containing suspension with a filler of quartz glass in a gypsum porous form by a drain method. The time of mass gain, depending on the design of the crucible, is 15-20 minutes. After removal from the mold, the molded product is dried at a temperature of more than 100 ° C until a residual moisture content of not more than 0.3% is reached. After drying, if necessary, eliminate possible surface defects of the casting.

The disadvantage of this method is that quartz is thermally dynamically unstable in contact with the molten heat-resistant alloy and can be restored by contaminating the alloy with silicon.

A known method of manufacturing refractory products, including mixing all the components of the refractory mixture (mullite processed or mullite-corundum products, electrocorundum, alumina, silicon carbide, dispersed aluminum) in a dry form in a paddle mixer. Then in the prepared mixture add water in the amount of 6.5-9 wt. %, in excess of 100% and further mixed. Products are molded on a vibrating stand. After hardening, the masses of the mold are disassembled, and the preforms are dried and fired (RF Patent No. 2191 167. Published on April 16, 2001).

The disadvantage of this method is the high duration of the hardening process of the product, the need for a large number of technological equipment.

A known method of manufacturing ceramic crucibles, including mixing the inventive mixture (mullite processed, mullite-corundum products and alumina) dry in a paddle mixer, then add water and continue mixing the components with water, molding by vibration casting, drying and firing (RF Patent No. 2412133 Published 07/07/2009 g)

The disadvantage of this method is the high duration of the hardening process of the product, the need for a large number of technological equipment.

A known method of manufacturing refractory products from nano-structured corundum ceramics (RF Patent No. 2341493. Published December 20, 2008), comprising preparing a molding mixture containing fractionated electrocorundum, reactive alumina, hydraulically hardening additive and silica fume. Dry mixing of fractionated electrocorundum, reactive alumina with a hydraulically hardening additive is carried out, the resulting mixture is homogenized and moistened with silica. The molding is carried out in the form under the influence of vibration, followed by natural drying, drying in the oven and firing.

A disadvantage of the known is the high duration of natural drying of the product, the need for a large number of technological equipment, as well as the content of expensive additives of the company "Almatis" Alphabond-300 and reactive alumina CL 370.

A known method of manufacturing refractory products from corundum ceramics (RF Patent No. 2 637 264. Published), including dry mixing of electrocorundum fractions, separate mixing of GDT with nano-dispersed technological binder based on alumina, then fractionated electrocorundum is added to the resulting mixture, the resulting mixture is moistened water and homogenized with continuous stirring, molding is carried out in the form of a method of vibrocasting with the application of vibrations along the vertical and horizontal axes of the press rmy. The resulting preform is subjected to air hardening, dried in a drying chamber and fired at temperatures of 1500-1550 ° C.

The disadvantage of this method is its increased fire hazard associated with the release of hydrogen in the preparation of nano-dispersed technological binder, the high duration of air hardening of the product, the need for a large number of technological equipment.

There is a known high-performance method for casting large-sized ceramic products mainly from a slip on a thermoplastic bond used for casting products such as bodies of revolution (AS No. 1752560. Published 07.-08.1992 g), including filling under pressure of the injection mold with a slip from a slip tank, cutting-off of the injection mold from a slip tank, increasing the pressure in the mold and holding it under pressure due to the application of centrifugal pressing pressure.

The disadvantage of this method is the impossibility of using coarse-grained refractory particles with a size of more than 1-5 mm in the composition of the thermoplastic slip, providing service characteristics of ceramic crucibles after their high-temperature firing. Large particles of refractory material, having high hardness, cause increased wear on the surface of the mold cavity and the feeder, and also disrupt the normal operation of the shut-off valve.

The disadvantage of this method is that large particles of refractory material, having high hardness, cause increased wear on the surface of the mold cavity of the injection mold and feeder, and also disrupt the normal operation of the shut-off valve. In addition, in the slip tank there is a rapid stratification of the slip, which negatively affects the uniformity of the crucible walls in density and material composition.

The closest to the claimed object in terms of the essential features and the technical result achieved is a method of manufacturing refractory products (RF Patent No. 2278 090. Published on June 20, 2006).

The method includes preparing powdery, multifractional ceramic mixtures, including electrocorundum and additives of a thermoplastic binder, manufacturing a thermoplastic slip, injection molding into a metal mold, removing the binder and final firing at 1650 ° C for 4-5 hours. 15-20% of a finely dispersed mixture of kaolin and clay is introduced into sphere-like powders of electrocorundum with a ratio of 1: 1 in the finely dispersed mixture of components, and the ratio of specific surfaces of electrocorundum powder and finely dispersed mixture is from 0.05 to 0.07. Thermoplastic binder contains paraffin, wax and microcrystalline paraffin in a ratio of components in wt. %: paraffin - 75-82; microcrystalline paraffin - 15-20; wax - 3-5. The thermoplastic binder is removed on a substrate from a capillary-porous permeable material in three stages with exposure for 1-2 hours at liquidus temperatures of wax, paraffin and microcrystalline paraffin.

The described method has a number of significant technological disadvantages that do not allow it to be implemented in the industrial production of ceramic melting crucibles of increased metal consumption.

Firstly, this method involves the use of a mixture of kaolin and clay in an amount of 15-20% containing undesirable impurities of alkali metal oxides and iron.

Secondly, the described method involves the use of specially prepared expensive sphere-like particles of electrocorundum.

Thirdly, in the preparation of a ceramic slurry, a powdered polyfraction mixture is dosed with constant stirring with simultaneous exposure to ultrasonic vibrations, which complicates this process and excludes its effectiveness in the case of using highly viscous ceramic masses.

Fourth, the described method is practically not feasible in the case of using a slow-moving paraffin-containing ceramic mass, which contains coarse-grained refractory particles with a size of 1-5 mm, which provide service characteristics of ceramic crucibles after their high-temperature firing. Along with this, large particles of refractory material in the composition of the thermoplastic ceramic mass, having high hardness, cause increased wear on the surface of the mold cavity of the injection mold and the feeder both when it is filled and in the isobaric exposure mode at an initial specific pressure of ≈ 50-8MPa

Fifth, this method involves the use of a specially made substrate of capillary-porous permeable material in contact with the free surfaces of the semi-finished product.

The technical result of this invention is the development of a reliable and economical industrial method for producing ceramic melting crucibles having high performance.

The technical result is achieved as follows.

A method of manufacturing a melting ceramic crucible, including mixing refractory charge materials, preparing a paraffin-containing ceramic mass, forming a crucible from a ceramic mass in a metal mold, removing paraffin-containing thermoplastic binder and high-temperature firing, characterized in that the molding cavity of the paraffin-containing ceramic mass is filled under conditions of radial temperature gradient at the front of solidification of paraffin-containing ceramic mass directed from the inner forming surface to the outer forming surface, partial removal of the paraffin-containing thermoplastic binder is carried out in a bed with alumina, and the subsequent impregnation of the crucible from the inside with an alcohol solution of salt containing Al, Mg, Zr, Hf, REM ions or their mixture, then carry out high-temperature firing at a temperature of 1350-1550 ° C for 6-8 hours.

The invention is illustrated by drawings: where in FIG. 1 - heated converter, in FIG. 2 - a vibrating table with a mold, in FIG. 3 - low-temperature processing in an oven; FIG. 4 - impregnation, in FIG. 5 - two stage high-temperature annealing.

The essence of the proposed technical solution consists in a complex of operations and a sequence of their implementation, which allow the technological process to obtain ceramic melting crucibles with a given material structure in products, which are a fundamental factor to achieve the objective of the invention.

When vibration is applied to a slow-moving paraffin-containing ceramic mass, friction and adhesion between the particles of the refractory aggregate are reduced. As a result of this, such a ceramic mass goes into a well-flowing state and fills the forming cavity without applying external pressure. In this case, along with the simplification of the method of filling the forming cavity (free pouring), the abrasive effect of the ceramic mass on the material of the tooling is sharply reduced.

Filling the forming cavity with paraffin-containing ceramic mass in the mode of vibroforming under conditions of a radial temperature gradient at the solidification front of the paraffin-containing ceramic mass directed from the internal forming surface to the external forming surface allows the pores to be displaced to the outer surface of the bottom and the crucible wall.

There are three sources of pore macropores in preforms made of paraffin-containing ceramic mass:

- air entrainment by paraffin-containing ceramic streams during filling of the forming cavity, as well as difficult volumetric displacement of air from the forming cavity;

- the difference in the molecular volumes of the liquid and solid phases during the solidification of the paraffin-containing ceramic mass;

- less solubility of gases in the solid phase than in the liquid.

It should be noted that the porosity of the material of ceramic crucibles, formed by micropores (voids between grains) uniformly throughout the volume, as a rule, is determined by the grain composition of polyfraction ceramic mixtures and its presence is a necessary condition for ensuring the operation of the crucible in the thermal cycling mode.

When using a paraffin-containing ceramic mass, in the composition of which coarse-grained refractory particles with a size of 1-5 mm are present, uniformity in density and material composition of the crucible material is achieved in the case of using inactive ceramic masses.

With directed solidification, conditions are created for the constant supply of molten ceramic mass to the melt-solid interface and thereby exclude the formation of a noticeable amount of shrinkage porosity.

In front of the solidification front, there are air inclusions involved in the flows of paraffin-containing ceramic mass.

Also, near the boundary, the melt - solid phase is released, dissolved in a thermoplastic binder gas. As the solidification front moves toward the melt, an increasing amount of gas accumulates in the thin layer near it.

Vibroforming under conditions of a radial temperature gradient at the solidification front of a paraffin-containing ceramic mass directed from the inner forming surface to the outer forming surface allows the pores to be displaced to the outer surface of the bottom and the crucible wall since accumulated gas inclusions in front of the solidification front are washed off from the moving solidification front and are distributed in the melt, accumulating in it. In this case, excess gas inclusions are captured by the solidification front only near the outer surface of the molded crucible, forming closed and open macropores of various sizes. The structure of the walls of the molded crucible is maintained after high-temperature firing.

The presence of pores on the outer surface of the walls of the ceramic crucible reduces its thermal conductivity, reducing heat loss, a denser inner surface provides a decrease in physicochemical interaction with molten metal.

Particularly stringent requirements are placed on the quality of the surface in contact with the molten metal.

Impregnation with alcohol-soluble salts containing Al, Mg, Zr, Hf, REM ions or their mixture of the inner surface of the crucible increases the thermodynamic stability of the crucible material to melts of high alloys and increases its corrosion resistance.

The crucible is impregnated from the inside after partial removal of the plasticizer at a temperature of 240 ° C to 290 ° C. Partial removal of a paraffin-based thermoplastic binder is carried out backfilled with alumina in a metal container. After partial removal of the plasticizer, a permeable near-surface layer is formed due to the occurrence of open porosity. Impregnation with alcohol-soluble salts containing Al, Mg, Zr, Hf, REM ions or their mixture is carried out to a shallow depth and in this case the consumption of the impregnation material is optimal, the drying time is technologically acceptable. Subsequent high-temperature firing results in the burning of residues of a paraffin-containing thermoplastic binder and the decomposition of salts containing Al, Mg, Zr, Hf, REM ions with the formation of oxides in the active form and the synthesis of complex oxide compounds with a refractory base of the melting ceramic crucible. Complex oxide compounds have increased thermodynamic stability and partially fill the voids between the refractory particles, which together increases the quality of the surface in contact with the molten metal.

An example implementation.

The proposed method was carried out in the manufacture of ceramic melting crucibles of increased metal consumption (up to 100 kg). Open macropores were found on the outer surface, and they are practically absent on the inner surface, which is a consequence of the displacement of pores to the outer surface of the crucible.

At the initial stage, a ceramic mass was prepared based on a mixture of fractionated electrocorundum and mullite with a maximum grain size of 5 mm and a paraffin-containing thermoplastic binder in an electrically heated converter at a temperature of 75-80 ° С.

The obtained inactive paraffin-containing ceramic mass was placed in the forming cavity of an aluminum tool under vibration (frequency of 20 Hz and vibration amplitude of 1.5 mm) until it was completely filled. Vibration was continued until the semi-finished product completely hardened, which was 10-15 minutes. To create conditions for a radial temperature gradient at the solidification front of a paraffin-containing ceramic mass directed from the internal forming surface to the external forming surface, the first forming surface was forcibly cooled from the inside by running water, and a heat-insulating insert made of fluoroplastic or electrical cardboard was installed on the second forming surface.

The molded semi-finished product was removed from the technological equipment, loaded into a metal container, filled with alumina, and the thermoplastic binder based on paraffin was partially removed at a temperature of 270 ° C. Then, the semi-finished product was cleaned from alumina and impregnated on the inside with an alcohol solution of Mg (NO ₃ ) ₂ salt. High-temperature firing was carried out in a gas furnace at a temperature of 1350–1550 ° C for 6–8 hours in the form of MgO⋅Al ₂ O ₃ spin-alumina spinel formed _{on the} inner side from the inner side. The open porosity of the ceramic crucible was 19%, which corresponds to the generally accepted requirements for the material of the crucible.

The manufactured ceramic crucibles were tested on an ALD installation (Germany) when casting blades made of heat-resistant nickel alloy with a positive result.

Claims (1)

A method of manufacturing a melting ceramic crucible, including mixing refractory charge materials, preparing a paraffin-containing ceramic mass, forming a crucible from a ceramic mass in a metal mold, removing paraffin-containing thermoplastic binder and high-temperature firing, characterized in that the molding cavity of the paraffin-containing ceramic mass is filled in the conditions radial temperature gradient at the front of solidification of paraffin-containing ceramic mass directed from the inner forming surface to the outer forming surface, partial removal of the paraffin-containing thermoplastic binder is carried out in a bed with alumina followed by impregnation of the crucible from the inside with an alcohol solution of salt containing Al, Mg, Zr, Hf, rare-earth metals or their mixture, then high-temperature firing is carried out at a temperature of 1350-1550 ° C for 6-8 hours.

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