Refractory Article For Molten Metal Handling
The present invention relates to the handling of molten metal, and in particular to articles formed from refractory material for use with molten metal The invention especially relates to refractory articles (e g sleeves or boards) for providing feeders for moulds in which metal is cast and to refractory linings for ladles or other molten metal handling vessels
In the production of metal castings, it is usual to provide so-called ' risers" "feeder heads' or "feeders" (collectively termed "feeders" in the present specification) which contain a reservoir of molten metal to supply metal to the mould cavity in order to compensate for shrinkage of the metal in the mould cavity as it cools and solidifies Feeders are positioned with respect to the mould cavity such that liquid metal contained in them is able to flow into the mould cavity under gravity, to replenish the mould cavity with molten metal during such shrinkage Most feeders are in the form of sleeves formed from refractory material (i e material which is able to withstand the temperature of the molten metal being cast) Other feeders comprise boards which may be used in conjunction with other boards to form a feeder cavity, or they may comprise flexible or articulated boards which may be wrapped or otherwise manipulated to form feeder sleeves, for example
In order to provide effective feeding of molten metal to a mould cavity as the metal in the mould cavity cools and solidifies, it is important for the metal contained in a feeder to remain molten for as long as possible For this reason, conventional feeders are generally formed from heat insulating refractory materials, to maintain the temperature of the metal above its melting point for as long a period as possible Some feeders additionally include materials which react exothermically during use (initiated by the heat of the molten metal) thereby generating their own heat
When molten metal is handled, for example during production, treatment or casting, vessels, e g ladles or the like, which contain the molten metal are commonly lined with refractory material Such refractory linings may comprise preformed shapes such as sleeves or cups, or they may comprise boards etc As
well as needing to withstand the temperature, pressure and chemical nature of the molten metal, and to protect the structure of the vessel itself (normally formed from metal, e g. steel), refractory linings are often required to provide a degree of thermal insulation to retain as much of the heat of the molten metal as possible
According to a first aspect, the present invention provides an article suitable for contact with molten metal comprising at least one wall formed from refractory material, the wall including a region having a thermal conductivity which is lower than that of the refractory material
The article according to the invention is preferably a feeder article (e.g. a sleeve or a board), or a lining for a molten metal handling vessel (e.g. a ladle). However, the article may generally be substantially any refractory article intended to contact molten metal and for which thermal insulation may be advantageous.
The lower thermal conductivity region of the article preferably comprises a material having a lower thermal conductivity than that of the refractory material. Alternatively, the lower thermal conductivity region may comprise a vacuum (or at least a partial vacuum).
In preferred embodiments of the invention, the lower thermal conductivity region is located between at least two spaced apart portions of the refractory material of the wall. Most preferably, the lower thermal conductivity region comprises a cavity within the wall. The cavity may contain a lower thermal conductivity material, or it may contain at least a partial vacuum. The article may contain a single cavity or a plurality of such cavities. The or each cavity may be open to the environment, particularly, for example, for embodiments in which the cavity contains air. Alternatively, the or each cavity may be closed and therefore substantially isolated from the environment. In some embodiments of the invention, the article may include one or more cavities open to the environment and one or more cavities closed to the environment
The cavity (where present) is preferably formed within a wall of refractory material which is formed as a single piece Alternatively the cavity may be formed by the joining together of two or more initially separate wall parts For example, where the article is a sleeve, the sleeve may be formed by the joining together of a first sleeve with a second, larger, sleeve, such that the first sleeve is located within the second sleeve, with the cavity provided by a gap between the sleeves The sleeves may be formed from the same, or different, material For example, the material of one of the sleeves (normally the innermost sleeve) may include exothermic material whereas the outermost sleeve may comprise substantially no exothermic material The article may comprise three or more sleeves joined together Advantageously, the separate wall parts and/or sleeves may include connecting members for mechanically interconnecting the wall parts and/or sleeves together
For embodiments of the invention which include a lower thermal conductivity material, the material is preferably a gas The gas is preferably air, but other gases may be used, for example argon or nitrogen, or another inert gas Mixtures of gases may be used
Additionally or alternatively, the lower thermal conductivity material may be a solid Preferred solid materials include foams, or fibrous materials, for example The lower thermal conductivity region may additionally or alternatively comprise a region of refractory material having a lower density than that of the remainder of the refractory material of the wall Advantageously, the lower thermal conductivity material may be material, preferably refractory material, which has been treated so as artificially to reduce its bulk density This may be achieved, for example, by agglomeration of particles of the material to form low density porous agglomerates, alternatively, for example, the material may be mixed with a foaming agent to produce a setting foam
In general, the greater the difference between the thermal conductivity (or thermal conductivities) of the refractory material and that of the lower thermal conductivity region of the wall of the article, the greater will be the benefit of the
lower thermal conductivity region It is preferred for the thermal conductivity (or thermal conductivities) of the refractory mateπal(s) to be at least twice, more preferably at least three times, even more preferably at least four times, even more preferably at least five times, especially at least ten times, as great as the thermal conductivity of the lower thermal conductivity region In other words, the thermal conductivity of the lower thermal conductivity region of the article is preferably at least an order of magnitude lower than that of the refractory material For example, if the thermal conductivity region of the article comprises air or another gas, it may have a thermal conductivity of approximately 0.03 W/mK, whereas the refractory material may have a thermal conductivity of approximately 0.5 W/mK Following the teaching of the present specification, the skilled person will be able to select the appropriate design and/or materials for the article
As already indicated, the article of the invention preferably comprises a sleeve or a board Where the article is a sleeve, the lower thermal conductivity region of the wall preferably extends around substantially the entire circumference of the sleeve. The lower thermal conductivity region of the wall preferably extends along substantially the entire length of the sleeve If the lower thermal conductivity region is in the form of a cavity, the cavity preferably extends around substantially the entire circumference, and preferably along substantially the entire length, of the sleeve. The cavity may be continuous or discontinuous, i.e. it may comprise a single cavity or it may comprise a plurality of separate cavities.
Where the article is a sleeve, it will normally have a substantially circular cross-section, but other cross-sectional shapes are possible, including oval, or square, or rectangular, or another polygonal shape.
According to a second aspect, the invention provides a method of manufacturing an article according to the first aspect of the invention, comprising forming the lower thermal conductivity region of the wall of refractory material at the same time as the remainder of the wall is formed
Preferably, the wall of refractory material is moulded or pressed or vacuum formed In embodiments in which the lower thermal conductivity region comprises a cavity in the wall, in this aspect of the invention the cavity is preferably formed by means of one or more formers (for example pins or other structures) as the article is moulded or pressed or vacuum formed.
The lower thermal conductivity region may be formed by means of an insert around, or against, which the remainder of the wall of refractory material is formed For example, the insert may comprise a preformed low density insulator. Additionally or alternatively the insert may comprise a hollow casing filled with a gas (e.g. air) or a low density material (e.g. perlite) or a sacrificial material (e.g. a polymeric foam) The insert may comprise a sealed pouch or bag filled with gas, e.g. a polymer bag or "bubble wrap". Additionally or alternatively, the insert may comprise a structure, for example a framework, which provides enhanced structural strength to the sleeve or other article. The structure may have the form of a spiral, "waffle" or lattice shape, for example.
According to a third aspect, the invention provides a method of manufacturing an article according to the first aspect of the invention, comprising forming the lower thermal conductivity region of the wall of refractory material subsequently to the remainder of the wall being formed. For embodiments of the invention in which the lower thermal conductivity region of the wall comprises a cavity, the cavity is preferably formed by the removal of some of the material of the wall subsequent to the wall having been formed. For example, portions of the wall may be removed by drilling, machining, milling, or the like
The lower thermal conductivity region of the wall of refractory material, or at least a part thereof, may, in some embodiments of the invention, be created only once the article is used with molten metal. For example, the lower thermal conductivity material may be provided by means of the incorporation into the refractory material of a sacrificial material which, during use at the elevated temperatures associated with the molten metal which the article contacts, changes its form to create the lower thermal conductivity region, or an even lower thermal
conductivity region For example the sacrificial material may change its form by dissociation vapourisation or volatility creating increased porosity in the refractory material or even creating a cavity within the wall of refractory material
The material from which the article is formed must be sufficiently refractory in order to withstand the temperature of the molten metal to be cast Suitable materials include metals ceramic materials, and cermet materials Bonded particulate refractory materials such as sand, e g silica sand, bonded fibrous and/or microspheπcal refractory materials such as alumina or aluminosiiicate may be used For some applications, the article may also include exothermic materials, i e materials which will react exothermically during use The skilled person will be able to select the appropriate refractory and/or exothermic materials according to each particular application
As mentioned above, the article is preferably moulded or pressed or vacuum formed A particularly preferred manufacturing process comprises partially drying (e g by means of a vacuum) a slurry (preferably an aqueous slurry) of particulate and/or fibrous materials together with a binder, on a former which produces the shape of the article The partially dried and formed slurry is then preferably heated in order to remove the last traces of water or other solvent and to cure or harden the binder
The invention will now be described, by way of example, with reference to the accompanying drawings, of which
Figure 1 is a schematic illustration of a feeder sleeve according to the invention,
Figure 2 is a schematic illustration of the feeder sleeve of Figure 1 , in longitudinal cross-section,
Figure 3 is a schematic illustration of a refractory board according to the invention,
Figure 4 is a schematic illustration of a quarter segment of a second type of sleeve according to the invention
Figure 5 is a schematic illustration of a quarter segment of a third type of sleeve according to the invention
Figure 6 views a and b are schematic illustrations of a fourth type of sleeve according to the invention,
Figure 7 is a schematic illustration of a quarter segment of a fifth type of sleeve according to the invention and
Figure 8 is a schematic illustration of a quarter segment of a sixth type of sleeve according to the invention
Figure 1 shows a feeder sleeve 1 formed from refractory material The sleeve 1 comprises a cylindrical wall 3 As shown in Figure 2, the wall contains a cavity 5 which is itself cylindrical in shape, and extends within the wall around the entire circumference of the sleeve The cavity, which is completely enclosed, is filled with air
Figure 3 shows a board 7 according to the invention The board may be a feeder board or part of a lining for a molten metal handling vessel, such as a ladle The board 7 contains a cavity 9 which is completely enclosed within the refractory wall 1 1 from which the board is formed The cavity 9 contains air
Figure 4 shows a quarter segment of sleeve according to the invention which is similar to that shown in Figure 1 , the only substantial difference being that the sleeve of Figure 4 is closed by an upper wall 13
Figure 5 shows a quarter segment of sleeve according to the invention which is similar to that shown in Figure 4 the only difference being that the cavity 5
in the cylindrical sleeve wall 3 contains an insert in the form of a flexible polymeric bag containing perlite particles 15 (As mentioned previously, perlite is a low density refractory material.)
Figure 6, views a and b, shows a quarter segment, and its corresponding three-quarter segment, of a sleeve according to the invention which has the same outward shape as the sleeves shown in figures 4 and 5 In this case, however, the cavity 5 in the cylindrical wall of the sleeve contains a lattice-like framework of internal ribs 1 7, to strengthen the sleeve
Figure 7 shows a quarter segment of another type of sleeve according to the invention. In this case, the sleeve comprises an outer wall portion 19 formed from insulating refractory material, and an inner wall portion 21 formed from exothermic material. A cavity 5 is defined between the inner and outer wall portions.
Figure 8 shows a quarter segment of a sleeve according to the invention which has a different construction In this case, a main part 23 of the sleeve forms an inner wall portion 25, an upper wall portion 27 and a lower edge portion 29; an outer wall portion 31 is formed from a separate part attached to the exterior of the