Abstract:
A cementitious formulation which is self-setting when mixed with water, and which can be cast into monolithic refractory components capable of resisting molten metal and repeated thermal shocks, has three main components: high purity magnesia (at least 94% by weight being MgO); high purity alumina (at least 98% by weight Al 2  O 3 ) and high alumina cement (preferably 75% Al 2  O 3 ) by weight, or greater). The magnesia may be 60 to 95% by weight of the three components and the alumina amounts to at least 1% by weight of the three components. Castings produced from the formulation can survive repeated flame testing even after exposure to firing at elevated temperature: known, conventionally pressed and similarly fired magnesia refractories may not survive a single flame test.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to basic refractory cementitious material and components thereof. 
     The invention moreover relates to a method of making such cementitious components as are exposed to chemical attack, wear and erosion by molten metals such as steel. 
     Refractory components of valves, and refractory nozzles for various purposes in the metal pouring art, having conventionally been produced by pressing and firing at high temperaturs. Costly, high purity materials such as zirconia and 85 to 95% Al 2  O 3  -based refractories have been considered necessary in view of the extremely harsh service conditions to which the components are subjected. Energy expended in producing components by pressing and firing is substantial, since firing temperatures normally exceeding 1500° C. must be created and maintained throughout the firing process. The energy expenditure contributes significantly to the unit costs of components made from such fired refractory materials. 
     Despite the use of strong fired refractories in the metal pouring art, items such as valve plates commonly need frequent replacement at great cost. 
     Early prejudices against the use of refractory concretes--revolving around the belief that they could not possibly withstand harsh working conditions--are being overcome and concrete systems are being considered for the manufacture of metal pouring refractory components. 
     Recently, chemically-bonded concretes have been proposed, for instance for sliding gate valve plates. Like fired refractory plates, chemically bonded concrete plates are unlikely to withstand repeated thermal shocks. Thus, their use in valves for ingot teeming is expected to be beset by inconvenient stoppages for their replacement. 
     We have found certain hydraulically-bonded basic cementitious materials surprisingly possess the ability to withstand thermal shock extremely well, and the production of components from these materials is particularly straightforward. Our proposed materials are composed of magnesia and an hydraulic aluminous cement, and in addition free alumina, the latter being thought to contribute significantly to stabilizing and strengthening components made from our materials. 
     SUMMARY OF THE INVENTION 
     According to the present invention, there is provided an hydraulic refractory cementitious formulation for making cast refractory parts resistant to molten metals, the formulation comprising an admixture including three components, namely fused or sintered magnesia, alumina and hydraulic aluminous cement, the magnesia component being present in an amount of 60 to 95%, the alumina component being present in an amount of at least 1% and the cement component being present in an amount of 4 to 15%, the percentages being by weight of the total weight of the three components, and the formulation after wetting being capable of hydraulically setting at room temperature. 
     The invention also provides cast refractory shapes moulded from the above formulation and caused to self set. 
     Accordingly, the invention for instance provides a cast refractory shape produced from the formulation defined hereinabove, which has a cold crushing strength after subjection to temperatures of 110° , 1000° C. and 1500° C. of 492, 517 and 868 kp/cm 2  respectively and a bulk density after subjection to these temperatures of 2.83. 2.78 and 2.85 g/cc. 
     The invention further provides a method of making a refractory part which is exposed to wear or erosion by molten metal, which comprises vibration casting the part in a mould from hydraulic concrete prepared by adding water to the formulation as defined hereinabove, curing or setting the casting and allowing it to dry. 
     Manufacture of refractory shapes from the present formulations could involve firing to say 1500° C. before they are placed in service. Such pre-firing will usually be unnecessary, however, as firing will occur in service. 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Formulations according to the invention essentially comprise three components, namely magnesia, alumina per se and aluminous hydraulic hydraulic cement. Optionally, minor amounts of other components can be added for specific purposes, e.g., plasticizing compounds, wetting agents and C-containing materials such as tar or pitch. The latter is commonly used in valve plates and nozzles to prevent slags adhering to such components. 
     In formulations according to the invention, it appears that the first two components should be of high purity for best results. 
     Thus, the magnesia component should have an MgO content of at least 94% by weight of the magnesia component, and the alumina component should have an Al 2  O 3  content of at least 98% by weight of the alumina component. The alumina can be sintered, fused or preferably calcined. 
     The cement component could in principle be any high-alumina cement (Al 2  O 3  content greater than 45% by weight of the cement). 
     Preferably, however, the aluminous cement component has an Al 2  O 3  content of not less than 75% by weight of the cement component. 
     The magnesia component is a major part of the formulation by weight, and the alumina component must be present in an amount at least 1% by weight of the total weight of the three components. 
     The magnesia component may be present in an amount in the range 60 to 95% by weight of the total weight of the three components. On the same percentage basis, the alumina component is in the range 1 to 36% and the cement component is in the range 4 to 15%. 
     The said percentage basis will be used for all percentage figures hereinafter unless stated to the contrary. 
     Preferred formulations according to the invention have the three components within the following ranges: magnesia 70 to 86%, alumina 5 to 15% and cement 9 to 15%. 
     As is known in the art, the formulation should be prepared from graded particulate materials. The cement component, for instance, should have a particle size of 75 microns or less. It is tolerable for some cement particles to be larger, but preferably at least 90% of the cement (by weight of the total cement) has a particle size of 75 microns or less. 
     Regarding the magnesia component, 20 to 40% e.g., 20 to 30% (of the total weight of the three components) is desirably magnesia having particle sizes within the range -5 mm to +1 mm, preferably -3 mm to +1 mm. Similarly, 15 to 35% e.g., 20 to 30% is magnesia having particle sizes within the range -1 mm to +0.3 mm and 25 to 40% e.g., 30 to 40% is magnesia having particle sizes less than or equal to 0.3 mm. 
     Regarding the alumina component, 1 to 20% e.g., 5 to 10% of the total weight of the three components is preferably alumina having a particle size equal to or less than 45 microns. Up to 20% e.g., 0 to 5% of the total weight of the three components can be alumina having a particle size of 0.3 mm or less. 
     The magnesia component can be Steetley Britmag 31 PHD, the alumina component can be Alcoa calcined alumina A17 and the cement component can be Alcoa CA-25. 
     Formulations according to the invention can be selected from the data given in the following tabulation: 
     
         __________________________________________________________________________     SIZE RANGE        PERCENTAGE RANGEMATERIAL  Overall  Preferred                       Overall                             Preferred__________________________________________________________________________Magnesia (Sintered     -5 mm + 1 mm              -3 mm + 1 mm                       20-40 20-30or Fused)Magnesia (Sintered     -1 mm + 0.3 mm              -1 mm + 0.3 mm                       15-35 20-30or Fused)Magnesia (Sintered     &lt;0.3 mm  &lt;0.3 mm  25-40 30-40or Fused)Alumina (sintered,     &lt;0.3 mm  &lt;0.3 mm   0-20 0-5fused or calcined)Alumina (calcined,     &lt;45 microns              &lt;45 microns                        1-20  5-10fused or sinteredbut preferablycalcined)Hydraulic cement     Min 90%  Min 90%   4-15  9-12with Al.sub.2 O.sub.3 content     &lt;75 microns              &lt;75 micronsgreater than 75%__________________________________________________________________________ 
    
     The present formulations are mixed with water in an amount adequate to yield a workable mix. Such a mix may, for instance, contain 7% water by weight of the mix. 
     Formulations according to the invention are self-setting at room temperature. Application of heat is unnecessary, although moderate heating to accelerate curing of cast shapes may be permissible. 
     Without heating, however, curing to a state allowing de-moulding can be achieved within one hour or so. Thus high productivity can be achieved. 
     The present hydraulically-bonding formulations possess significant advantages over chemically-bonded systems. A problem ever present with chemically-bonded systems is that when they are in the process of heat setting and drying, the binder tends to migrate to exposed surfaces. Bond migration and resulting non-uniformity of the integrity of castings do not arise with the present formulations. Moreover, a rigid set occurs with the present formulations so that handling of castings, e.g., when de-moulding, it free from the risk of introducing internal stress flaws. It is not impossible with chemically-bound castings to adversely affect them during handling. 
     The present formulations have surprisingly excellent resistance to thermal shock. They are therefore expected to find use in parts of sliding gate valves and associated pouring nozzles used in the intermittent teeming of molten metals, e.g., steel, during ingot casting, for instance. 
     A commonly used test for thermal shock resistance is the torch test developed by U.S. Steel Corporation&#39;s Research Laboratories. In this test, an oxy-propane torch flame is slowly traversed over a refractory being tested at 1.7 mm per second, the torch being held 6.4 mm from the refractory surface. 
     Conventional pressed and fired magnesia valve plates cannot ordinarily withstand just one pass of the oxy-propane flame without significant surface and internal damage. Known chemically-bonded magnesia valve plates are better able to resist the flame, but our tests have revealed moderate degradation following one pass. 
     By contrast, valve plates made from our present formulations have been found capable of withstanding repeated passes, numbering twelve, without significant surface degradation. This implies their ability to cope with the temperature variations encountered during repeated valve throttling and open/shut valve operations will show a marked improvement over fired or chemically bonded plates. 
     As indicated above, the present formulations can be used for casting valve plates for sliding gate valves as well as nozzles such as collectors and extended pouring tubes associated therewith. Ladle wells and dispensing nozzles can also be produced from the formulations and other applications will be apparent to the addressee. 
     Articles cast from the present formulations will ordinarily be supplied to the metallurgical industry in the hydraulically set state. Nevertheless, it may sometimes be desired to supply the cast articles in a pre-fired condition, rather than allow them to fire in service. Pre-firing may be applicable for example to articles such as replaceable wear and erosion resistant sleeves or liners for discharge nozzles. 
    
    
     A specific example of the invention is now given: 
     EXAMPLE 
     This formulation comprised a dry admixture of the Steetley and Alcoa materials noted above, in the following proportions. The percentages given are again by weight of the total weight of the magnesia, alumina and cement components. 
     
         ______________________________________Magnesia, size range -3 to +1 mm                       26%Magnesia, size range -1 to +0.3 mm                       25%Magnesia, size less than or equal to 0.3 mm                       34%Calcined alumina, size less than or equalto 75 microns               6%High alumina cement         9%______________________________________ 
    
     The cement had an Al 2  O 3  content greater than 75% by weight of the cement, and at least 90% by weight of the cement had a particle size less than 75 microns. The magnesia and alumina respectively had MgO and Al 2  O 3  contents of 94 and 98% by weight of these components. 
     The formulation yielded a workable and adequately fluent concrete for casting when mixed with water amounting to 7% of its weight of these components. 
     The formulation yielded a workable and adequately fluent concrete for casting when mixed with water amounting to 7% of its weight. Mould filling can be asisted by vibration, an exemplary vibration frequency being 3000 Hz. 
     Vibration cast concrete samples prepared as above, after curing and drying, possessed the following properties at the stated temperatures: 
     
         ______________________________________      FIRING TEMPERATURE °C.PROPERTY     110      1000     1500    1700______________________________________Bulk Density g/cc        2.83     2.78     2.85Apparent Porosity %        16.0     19.3     17.0Permanent Linearchange Dryto Fired %            +0.01    -1.24   -3.44Cold CrushingStrength p.s.i.        7000     7350     12350MNM.sup.-2   48.3     50.8     85.2Kp.cm.sup.-2 492      517      868Flame Test 1 Cycle        Pass                      PassFlame Test 12 Cycles        Pass                      Pass______________________________________ 
    
     The foregoing properties are considered entirely suitable for making monolithic (i.e., cast) sliding gate valve components which, if desired, can be supplied in a subsequently fired condition. 
     The effect of the alumina component in the present formulations is not fully understood. Without wishing to be bound by theory, we suspect the alumina component may play a dual role. Firstly, it may react with the magnesia component to form magnesium aluminate spinel. At any rate, the alumina appears to minimise shrinking upon firing. By contrast, magnesia-calcium aluminate cements suffer significant shrinkage, unless alumina is specifically added. Secondly, high melting point inter-phases of magnesia, alumina and lime may be formed and contribute to the advantageous refractory properties attained by the present formulations. 
     In summary, the present formulations possess extremely favourable refractory properties coupled with excellent thermal shock resistance. These properties commend the formulations to applications such as the flow-controlling refractories used in interrupted metal teeming operations. Manufacture of such refractories is by the advantageously simple routine of casting monolithically, followed by air setting. The use of hydraulic presses is unnecessary and firing the set castings to high temperature is only optional.