DESC:
Riser tube
Field of the Invention
The present invention relates to riser tubes and more specifically metal- composite riser tubes for use in low pressure die casting.
Background of the Invention
In the low-pressure die casting (LPDC), the riser tube or pipe is used as a passage for the molten metal (typically aluminium) to enter the mould filling form from the holding furnace on the low-pressure casting machine. The traditional riser tube is made of high-temperature resistant steel. In use, the riser pipe is exposed to high-temperature molten metal. The phenomenon of iron infiltration occurs, causing the iron component from the riser tube to enter the molten metal which can greatly affect the quality of the cast metal parts, such as aluminium parts.
To avoid the abovementioned problem, the riser tube may be made of a ceramic material, such as fused silica, aluminium titanate, silicon nitride and sialon. However ceramic materials generally have poor impact strength with the resultant riser tubes being easily worn or broken under the pressure of the mould.
CN201201047 addressed some the above limitations through disclosing a silicon carbide or a refractory cement tube either on the outside or inside of a stainless steel tube with the exposed stainless steel surface preferably coated with a silicon carbide coating. The composite tube is able to prevent iron contamination of the molten aluminium, whilst the refractory components improved the temperature resistance of the riser tube.
However, there still need for a riser tube with an improved service life which is able to withstand prolonged periods of temperature fluctuations whilst maintaining excellent operational performance.
Summary of the present invention
In a first aspect of the present invention there is provided a metal- composite riser tube comprising:
A. a metallic tube comprising an inner surface and an outer surface; and
B. an inner composite tube fixedly disposed against the inner surface of the metallic tube,
wherein the inner composite tube includes a ceramic matrix which comprises and is bound together with a binder, said ceramic matrix comprising a source of alumina and a carbon compound, such as graphite.
The riser tube may further comprise an outer composite tube fixedly disposed against the outer surface of the metallic tube. The additional of the outer composite tube further protects the metallic tube from corrosion through contact with the molten metal, thereby further extending the effective life of the riser tube.
While graphite provides excellent molten metal non-wetting properties to the riser tube, upon oxidation the mechanical properties of graphite significantly diminish and compromise the mechanical integrity of the riser tube.
While there are a variety of oxidation inhibitors available, such as glassy coatings, their use often comes with a compromise in mechanical properties of the composite. The composite of the present disclosure is able to provide the ideal combination of molten metal non-wetting properties with mechanical properties required to maintain a long lasting riser tube.
Preferably the mixture further comprises an oxidation inhibitor for said carbon compound. The oxidation inhibitor preferably forms at least part of the binder phase.
The oxidation inhibitor preferably comprises fibrous silicon carbide (e.g. beta silicon carbide) and/or a glass ceramic phase. The glass ceramic phase preferably comprises mullite. For the fibrous silicon carbide and/or glass ceramic phase to function as an oxidation inhibitor in relation to the carbon compound, the fibrous silicon carbide and/or glass ceramic phase should be located in close proximity to the graphite material such to form part or all of a barrier layer to prevent the ingress of oxygen. Fibrous silicon carbide is typically formed in situ during sintering from the use of an organic binder in combination with silicon metal powder, whilst the glass ceramic phase is preferably formed in situ during sintering from a clay based inorganic binder. The in situ formation of these compounds enables these material to form nano or microstructural components which inhibit the ingress of oxygen whilst maintaining or enhancing the mechanical properties of the composite.
The binder phase, in combination with the other components of the ceramic matrix and the metallic tube, provides both the required oxidation resistance and the mechanical properties, including erosion resistance and refractory properties to contribute to the long service life of the riser tubes of the present disclosure. The riser tuber of the present invention provides long lasting service due to a combination of:
• mechanical strength;
• high temperature resistance;
• hardness;
• thermal shock resistance;
• wear resistance; and
• non-stick surface properties.
• Better insulating property

The metallic tube may be made of any suitable metallic material and preferably iron or iron alloy based materials, such as cast iron or stainless steel. The metallic tube provides excellent impact resistance to complement the refractory properties of the inner composite tube. In some embodiments, the metallic tube may extend the complete length of the riser tube. In some embodiments, the metallic tube extends part of the length of the riser tube (preferably at least 30% of the length of the riser tube, with the composite tube forming the outer surface of the riser tube for the portion of the riser tube not comprising the metallic tube. Preferably, in use, the metallic tube extends above the molten metal level, thereby protecting the riser tube from contamination. In addition, as the metallic tube has substantially no porosity, pressure drops are avoided during the casting operation thereby reducing defects due to incomplete filling of the mould.

In embodiments comprising an outer composite tube, the outer composite tube may extend at least above the molten metal level, when in use. The outer composite tube may extend from the bottom of the riser tube at least 30% of the length of the riser tube. In some embodiment, the outer composite tube extends the total length of the riser tube and terminates adjacent the flange. In some embodiments, the outer composite tube extends to cover at least 50% or at least 80% of the outer surface of the metallic tube. The outer and inner composite tubes may extend beyond the distal end of the metallic tube. In such embodiments, the void space between the ends of the inner and outer composite tubes, may be filled with refractory mortar.
Preferably, the composite tube comprises at least 5 wt% or 10 wt% or 20 wt% or 30 wt% or 40 wt% or 50 wt% alumina, preferably crystalline alumina. The alumina provides chemical resistance, thermal insulative properties and high temperature strength.
To provide thermal shock resistance, the composite tube preferably comprises a source of crystalline silica, such as fused silica and/or cordierite. Alumina and/or silica may also be present in an inorganic refractory binder, such as clay. The clay may at least partially form mullite upon firing of the composite tube during the production process.
The graphite content is preferably between 5 and 20 wt%. Higher amounts of graphite may result in a riser tube which is more susceptible to oxidation and erosion. Lower graphite levels may not have sufficient molten metal non-wetting properties. Preferably, the graphite content is less than 15 wt% or less than 12 wt% or less than 9.0 wt%.
To impart thermal shock resistance properties and anti-sticking surface properties, the composite tube preferably comprises a source of carbon, such as graphite (e.g. flaked graphite). To prolong the service life of the source of carbon, the composite tube preferably further comprises a carbon oxidation inhibitor, such as a refractory glass phase, mullite, silicon carbide (beta form), or precursors thereof (e.g. silicon metal powder), aluminium powder or boron carbide. The oxidation inhibitors form a barrier to the ingress of oxygen into the composite to react with the graphite particles.
It has been found the oxidation inhibitors derived through the sintering of clay or carbonised resin and silicon metal provide the required mechanical strength and oxygen barrier properties.
In one embodiment, the ceramic matrix comprises:
A. 10 to 60 parts by weight alumina based filler
B. 5 to 45 parts by weight of a binder phase.
C. 0 to 30 parts by weight fused silica and/or cordierite
D. 5 to 20 parts by weight graphite
E. 0 to 25 parts by weight silicon carbide (preferably comprising alpha form); and
F. 0 to 10 parts by weight additives
The parts by weight of the inner composite tube preferably total 100 parts by weight.
The riser tube may further comprise an outer composite tube fixedly disposed against an outer surface of the metallic tube, said outer composite tube includes a ceramic matrix comprising:
A. 10 to 60 parts by weight alumina based filler;
B. 5 to 45 parts by weight binder phase;
C. 0 to 30 parts by weight fused silica and/or cordierite;
D. 5 to 20 parts by weight graphite;
E. 0 to 25 parts by weight silicon carbide;
F. 0 to 10 parts by weight additives and
the sum of A+B+C+D+E+F = 100 parts by weight.
The composition of the inner composite tube and the other composite tube may be the same or different.
The alumina filler preferably comprises a source of alumina selected from the group consisting of fused alumina, calcined alumina, tabular alumina, corundum, or combinations thereof.
The binder phase preferably functions to bind components A, C, D and E together. As such, the binder phase preferably interfaces and/or coats the surfaces of these components providing both corrosive and erosion resistance as well as being a gas barrier to prevent the oxidation of the graphite component of the ceramic matrix.
The binder phase preferably comprises a glass ceramic phase and/or fibrous silicon carbide. These components are preferably formed in situ during sintering from the green binder (e.g. clay and/or carbonaceous resin). In embodiments in which the binder is derived from a carbonaceous resin, the ceramic matrix preferably comprises at least 5 parts by weight or at least 10 parts by weight or at least 12 parts by weight fused silica and/or cordierite.
The binder phase may comprise a glass ceramic component and/or a fibrous silicon carbide phase. The glass ceramic phase may comprise mullite crystals and other alumina and/or silica crystals disperse within a glass phase. The crystal size distribution has a d50 of typically less than 3 µm. The glass ceramic component preferably comprises at least 70 wt% or at least 75 wt% or at least 80 wt% of at least 85 wt% or at least 90 wt% aluminium and silicon in an oxide form. The glass phase is preferably a refractory glass phase which has a high melting point (e.g. greater than 900oC or greater than 1000oC), such that it maintains its mechanical strength at the operating temperature of the riser tube (e.g. 700-800oC). Typically the glass phase has less than 20 wt% or less than 15 wt% or less than 10 wt% or less than 5 wt% alkali or alkaline earth metal oxides. The glass phase preferably comprises at least 70 wt% or at least 75 wt% or at least 80 wt% of at least 85 wt% or at least 90 wt% aluminium and silicon in an oxide form. The high alumina/silica content combined with the low alkali and alkaline earth metal oxide content of the glass phase, combined with the dispersion of hard ceramic particles therein, results in a corrosive and erosion resistant refractory binder capable of providing long lasting mechanical and oxidation inhibiting properties.
The carbon compound is preferably graphite. The graphite may be in the form of flaked graphite (e.g. mean particle size distribution between 100 to 500 µm. The flake thickness is typically between 1 to 150 µm). The flaked graphite provides a relatively high surface area compared to granular graphite. This has the advantage of providing effective molten metal non-wetting properties to the exposed surface of the riser tube, whilst minimising the adverse effects (e.g. loss in mechanical properties) if the graphite flakes become oxidised.
The additive may further comprise alternative refractory materials; refractory sealant; and carbon oxidation inhibitors or precursors thereof. Additives may include silicon metal, FeSi, aluminium, boron alumina-silicate, borax and/or boric acid. Additives preferably make up between 1 to 8 parts by weight or between 2 and 5 parts by weight of the inner composite tube.
The alumina based filler preferably comprises at least 50 wt% or at least 60 wt% or at least 70 wt% or at least 80 wt% crystalline material. Alumina based crystalline materials are typically hard, refractory and have good insulative properties. Preferably, at least part (e.g. at least 50 wt% or at least 60 wt% or at least 70 wt%) of the alumina based filler is in the form of fused alumina (e.g. brown or white), calcined alumina, tabular alumina, corundum, mullite or combinations thereof.
Preferably, the binder phase is derived from an alumina-silicate clay. The binder phases preferably comprises mullite. The mullite is preferably formed in situ during a firing/sintering step which partially converts the clay, or other mullite precursor material, to mullite. In one embodiment the ceramic matrix comprises 5 to 35 wt% (preferably 10 to 25 wt%) clay (including mullite).
Preferably, the weight ratio of the binder phase to the alumina based filler (preferably crystalline alumina based filler) is in the range of 1:1 to 1:20 and more preferably in the range 1:3 to 1:8. The optimal ratio may depend upon the particle size of the alumina based filler and the desired properties of the inner composite tube.
Due to the wide temperature range which the riser tube is exposed to, the composite tube preferably has a porosity of less than 20% v/v or less than 15% v/v or less than 14% v/v or less than 13% v/v or less than 12% v/v. The lower the porosity the less prone the composite tube is to cracking due to the internal stresses created by gases expanding with increasing temperature.
In a preferred embodiment, the inner composite and/or outer composite tube further comprises a refractory sealant. The impregnation of the composite tube with a refractory sealant fills the open pores on the surface of the composite tube, thereby further decreasing the porosity of the composite tube. The inner and/or outer surface of the composite tube may be vacuum impregnated with the refractory sealant. The outer/inner surface of the composite tube may also be coated with the refractory sealant prior to being sealed to the metallic tube. In contrast to some conventional tubes, the refractory sealant does not function as a binder and, as such, the mechanical strength of the refractory sealant (typically a glassy material) does not detrimentally affect the mechanical properties of the composite tube(s) as a whole.
The sealed composite tube may comprise 0.5 to 10 parts by weight refractory sealant (or approximately 0.5 to 10.0 wt%). The exact amount will depend upon the open porosity of the outer surface of the inner composite tube. The porosity of the composite tube comprising the refractory sealant may be less than 10% v/v or less than 8% v/v or less than 6% v/v or less than 4% v/v. Prior to impregnation, the porosity may be between 5% v/v and 15% v/v.
In a particularly preferred embodiment, the composite tube comprises a refractory sealant extending from the outer surface of the composite tube towards the centre of the composite tube. The refractory sealant is preferably a refractory glass which is able to penetrate the open pores of the outer surface of the composite tube as a molten glass or a precursor thereof. The refractory sealant may comprise a borate glass or a silicate glass. The refractory glass may comprise silica, boron, phosphate, magnesium, aluminium or combination thereof. Refractory sealant precursors include boron alumina-silicate, borax, boric acid, aluminium phosphate, calcium sulphate, magnesium sulphate or combination thereof.
To further enhance the service life of the riser tube, the inner surface of the composite tube may further comprise an outer coating comprising a refractory non-stick material. The non-stick material may be a non-oxide material or an oxide material. The non-oxide refractory coating may be selected from the group consisting of carbide, nitride and boride coatings or combinations thereof. The oxide refractory coating may comprise alumina.
The non-stick coating works in co-operation with the non-stick components of the core of the composite tube (e.g. graphite), with the non-stick component of the composite tube able to maintain the non-stick properties of the internal surface of the riser tube after erosion or other wear of the non-stick coating results in intermittent exposure of the core composite tube to molten metal.
The inner tube may be fixedly attached to the outer tube by an adhesive layer. The adhesive layer may comprise alumina and/or silica. The adhesive layer may be a refractory cement or clay as known by those skilled in the art; or the vacuum impregnation solution (e.g. borate or silica glass solution).
In a second aspect of the present invention, there is provided a method of manufacturing a riser tube comprising the steps of:
a. providing a metallic tube (the metallic tube preferably covers the outer surface of the inner composite tube completely or partially);
b. providing an inner composite tube mixture comprising a binder; a source of alumina, a source of silica; a graphite and a oxidation inhibitor for said graphite;
c. filling a mould with the mixture;
d. isostatically pressing the filled mould to form a green tube;
e. curing the green tube to form a cured tube;
f. machining the cured tube to form a predetermined external diameter of the inner composite tube;
g. firing the cured and machined tube to form an inner composite tube;
h. vacuum impregnating the inner composite tube with a refractory sealant;
i. applying a layer of refractory adhesive to the inner surface of the metallic tube and/or the outer surface of the inner composite tube;
j. fitting the metallic tube onto the inner composite tube;
k. filling the gap between the inner composite tube and the metallic tube with a refractory cement or mortar;
l. optionally applying at least one non-sticking coating to the inside surface of the inner composite tube; and
m. optionally applying at least one refractory paint coating to the outer surface of the metallic tube.
The above process may be appropriately modified to further fixedly dispose an outer composite tube onto the outer surface of the metallic tube.
The refractory adhesive may be an alumina-clay-sodium silicate based refractory adhesive.
The composite tube mixture forms a ceramic matrix in which the components are preferably uniformly distributed. The mixture is preferably bound together with about 5 to 20 wt% binder. The isostatic pressing further contributes to improved isotropy of both the microstructure and mechanical properties of the ceramic matrix. Isotropic properties are achieved as the pressure is applied from all sides, shrinkage is uniform in all directions; and directionality originating from the pressing operation is prevented.
The mixture preferably has a maximum particle size of no more than 1.0 mm. The fused alumina preferably comprises the largest mean particle size distribution to maximise the mechanical and refractory properties of these components. Similarly, the binder components (e.g. clay) preferably have a relative small particle size (e.g. maximum of less than 0.1 mm) to enable the binder to penetrate in between the larger components and reduce void space in the structure.
The components may have different particle size distribution resulting in a mixture with multi-modal particle size. The combination of relative small and multi-modal particle size enables the porosity of the resultant composite tube to be reduced.
Alternatively, or in addition to, the binder may be an organic binder (preferably 5-13 wt%), such as a resin, tar or sugar binder or an inorganic binder, such as clay. The binder provides sufficient strength of the composite tube for shaping and handling prior the firing step. The firing/sintering step may take place at greater than 1000oC and preferably between 1200oC and 1500oC (or higher) depending upon the materials used. Furthermore, during the firing step the organic resin may provide a source of carbon, which may react with silicon metal powder to form the beta form of silicon carbon which functions as an oxidation inhibitor for the carbon/graphite within the ceramic matrix. The refractory sealant may also function as an oxidation inhibitor.
The vacuum impregnation step preferably comprises vacuum impregnating the outer and inner surfaces of the inner composite tube with an impregnation solution (e.g. a borax-boric acid; aluminium phosphate; and/or calcium/magnesium sulphate solution) and allowing the dry. The impregnated surfaces are then preferably cleaned. The outer surface is then preferably coated with a layer of refractory sealant solution prior to fitting inner composite tube into the metallic tube. Mortar is applied to seal the gap between the metal and ceramic tubes, preferably at both ends.
In a third aspect of the present invention, there is provided a riser tube (100) comprising:
a. a metallic tube (110) comprising an outer surface (120) and an inner surface (130)
b. a refractory mortar layer (140) fixedly disposed against the inner surface of the metallic tube (130);
c. an inner composite tube (150) attached to the inner surface of the metallic tube (130) by the refractory mortar layer (140); and
d. a non-stick coating (160) disposed on an inner surface of the inner composite tube (170).
The riser tube may also comprise an outer composite tube (155) attached to the outer surface of the metallic tube (120), with a refractory mortar layer (135). A non-stick coating (125) may be disposed on the outer surface of the outer composite tube (155).
The inner composite tube (200) may comprise a glassy pathway (210) extending from a peripheral surface of the ceramic tube (220) towards the centre of the inner composite tube (230).
The inner composite tube (150) preferably further adheres to the inner surface of the metallic tube (130) by a refractory seal, the same or similar to that used in impregnation of the composite tube (e.g. borate or silica glass).
The inner surface (240) of the inner composite tube (200) and, when present outer composite tube, comprises particles of a graphite. The graphite is preferably dispersed within a ceramic matrix. The graphite preferably provides a non-stick surface to the inner surface of the composite tube, thereby minimising the amount of molten metal which sticks to the tube’s inner surface.
The inner and, when present, outer composite tube preferable comprises a ceramic matrix comprises crystalline ceramic and graphite particles (310, 320, 330, 340) bound together by the binder phase (300). The binder phase preferably comprises fibres of silicon carbide and/or a glass ceramic phase. The glass ceramic phase preferably comprises ceramic crystalline particles (preferably including mullite) with a d50 of less than 3 µm embedded with the glassy phase.
The riser tube comprises alumina particles with an average particle size in the range of 0.1 to 1.0 mm. The alumina particles are preferably crystalline particles and form part of a ceramic matrix.
The metallic tube (100) typically has a thickness of between 2 mm and 40 mm.
The inner composite tube (150) preferably has a thickness of between 5 mm and 100 mm or between 10 mm and 30 mm.
The outer composite tube (155) preferably has a thickness of between 5 mm and 100 mm or between 10 mm and 30 mm.
The refractory mortar layer(s) (140) preferably has a thickness of between 0.1 mm and 10 mm.
The non-stick coating(s) (160) has a thickness of between 0.001 mm and 10 mm.
The riser tube preferably comprises a length in the range of 0.2 metres to 3 metres.
The riser tube may comprise a flanged end (180). The flanged end preferably provides an attachment mechanism, or part thereof, to enable the riser tube to attach to a part of a die/mould.
Riser tube flange (180) preferably has a thickness (beyond the metallic tube thickness) of between 5 mm and 200 mm.
The internal diameter of the riser tube may be in the range of 20 mm to 400 mm.
The porosity (preferably the open porosity) of the composite tube may be in the range of 3 to 30% v/v.
The glassy pathways (110) may comprise no more than 20 wt% of the total weight of the inner composite tube (150). The glassy pathway may comprise at least 0.2 wt% or at least 1.0 wt% or at least 2.0 wt% of the total weight of the inner composite tube (150).
Unless otherwise indicated, reference to silicon carbide is reference to the alpha form thereof. silicon carbide referenced as a binder/oxidation inhibitor is in the beta form.
Reference to the composite tube is taken to be reference to the inner and/or outer composite tube, when present.
For the purposes of the present invention, an alumina based filler excludes sintered clay components of the binder, including mullite.
Brief Description of the Figures
Figure 1 is a schematic diagram of low pressure die casting equipment.
Figure 2a is a schematic diagram of a cross-sectional view of a riser tube of the present invention comprising an inner composite tube.
Figure 2b is a schematic diagram of a cross-sectional view of a riser tube of the present invention comprising an inner and an outer composite tube.
Figure 3 is a schematic diagram of a cross-sectional view of a portion of an inner composite tube of Figure 2a.
Figure 4 is a process flow diagram illustrating the process steps required to produce a riser tube of the present invention.
Figure 5 is a photograph of a metal/composite riser tube of the present invention
Figure 6 is a photography of the metal/composite riser tube of Figure 5 after 504 hours in use.
Figure 7 is a SEM image of SiC fibres in a composite sample of the present invention sintered at 1350oC.
Figure 8 is a SEM of a glass ceramic binder phase in a composite sample of the present invention sintered at 1350oC.
Detailed description of a preferred embodiment
With reference to Figure 1, the Low Pressure Die Casting (LPDC) process is mostly used to cast aluminium components. Traditionally LPDC was used to form symmetrical parts such as alloy wheels, although now it is being used for an increasing variety of aluminium cast components.
The die components 10 are closed together with the lowering of a moving platen 20. Air 30 with a controlled pressure is applied to the furnace 40. Metal is fed into the crucible 50 via a feed port 60 and the temperature of the furnace raised to form molten metal. The air pressure is increased to displace the molten metal in the crucible up the riser tube 70 (also known as a “fill stalk” or “stalk tube”) and into the die 10.
The air pressure is maintained to enable the casting (not shown) to solidify in the die 10. The air pressure is then released to enable the molten metal 80 to drop away from the die. Final solidification of the casting takes place prior to opening the die by raising the platen 20 and ejecting the casting. The process may then be repeated.
The process exposes the riser tube to both high temperatures of the molten metal and the pressurised gases. The riser tube must also withstand the mechanical stresses of the molten metal being forced through the internal conduit.
The desired properties of a riser tube used in this application include the following:
• near impermeability to air at application temperature so that the applied pressure acts on the molten metal and does not take the "path of least resistance" through the tube;
• non-reactivity with the molten metal being cast, to yield high purity metal castings and to enhance life of the riser tube;
• controlled thermal conduction and insulation so that as the metal is cast into the mould and allowed to solidify, the tube allows the metal to remain in a molten state which allows back-flow and drainage of the tube; and
• controlled mechanical properties so that as pressure is applied to the tube/mould cavity interface to ensure a tight enough seal (to prevent molten metal leakage), the tube is not damaged and can therefore be used again.
As illustrated in Figure 2a, the riser tube 100 of the present invention comprises a metallic tube 110. The metallic tube may be any suitable metal able to withstand the operating temperature of the furnace. The metallic tube may be made of cast iron or stainless steel or any other suitable iron or nickel based alloy. The metallic tube preferably extends the entire length of the riser tube.
The inner composite tube 150 is positioned concentrically within the metallic tube 110. The inner surface of the metallic tube 130 adheres to the inner composite tube through a refractory mortar layer 140. The thickness of the mortar layer 140 is typically greater than 100 µm and less than 20mm.
Figure 2b illustrates a riser tube similar to the riser tube of Figure 2a, but with an outer composite tube 155 in addition to the inner composite tube 150. The outer composite tube 155 adheres to the outer surface of the metallic tube 110 by a mortar layer 135, which may be of similar thickness to mortar layer 140 adhering the inner composite tube 150 to the metallic tube 110. The inner and/or outer composite tubes 150, 155 may extend past the metallic tube 110. In such embodiments, mortar 147 may be applied to fill the gap(s) between the ends of the layers (150, 155). The mortar 147 seals the end of the riser tube, preventing ingress of gases contacting the metallic tube 110, thereby maintaining its structural integrity for longer. The outer surface of the outer composite tube may comprise a non-stick coating.
Figure 3 illustrates the inner composite tube 200 which has been impregnated with a glassy refractory sealant 210, with the glassy phase extending from an outer surface 220 of the composite tube towards the centre of the tube 230. The reduction in the porosity of the tube reduces the detrimental effects of repetitive gas expansion on internal ceramic microstructure, thereby increasing the service life of the tube. The refractory sealant also functions as an oxidation inhibitor to prevent the degradation of graphite in the ceramic matrix. The inner surface of the composite tube 240 (and optionally the outer surface of the outer composite tube 155 ) is also preferably impregnated with a glassy refractory sealant (not shown) The glass refractory sealant cooperates with the graphite and non-stick coating to provide a resilient surface which is able to withstand molten metal flow over long periods without losing its non-stick properties.
The riser pipe is made of an insulating refractory material having the general characteristics of low thermal expansion, high thermal shock resistance, good high temperature strength, good oxidation resistance and no wettability by non-ferrous metals. The inner composite tube comprises a mixture of components which enable it to possess the required impact resistance and non-stick properties.
As indicted in Table 1, the metal/composite riser tube of the present disclosure provides a unique combination of properties which outperform conventional riser tubes in terms of the balance between service life, productivity and ease of use.
Conventional ceramic riser tubes require additional set up, for example the use of a separate metal adaptor with gasket to fit the tube for handling and support. This assembly requires skill for proper fitting otherwise the ceramic tube will get damage. Further, with a metal adaptor on the ceramic tube there is a greater propensity of ceramic tube leakages resulting in incomplete filling of casting, resulting in increased deflects and/or shorter riser tube service life.
Table 1 Relative performance of Riser Tubes
Properties Cast iron Ceramic Metal/Cement Metal/Composite
Mechanical/impact strength Excellent Low Good High
Service Life Low Excellent Moderate Excellent
Productivity Low Excellent Good Excellent
Iron pick up by molten metal High N/A Moderate N/A
Resistance to metal non-sticking Low Excellent Low Excellent
Care/skill in handing Not required High Low Not required
Additional set up required Not required Required Required Not required

The process of producing the riser tube is illustrated in the flow diagram in Figure 4. In a preferred embodiment, a ceramic mixture is prepared comprising a weight ratio of clay (maximum particle size of (0.1mm), brown fused alumina (max. 1mm), fused silica (max. 0.5mm), graphite (max. 0.5mm), silicon carbide (alpha phase) (max. 1.5mm), silicon metal (max. 0.1mm), boron carbide (max. 0.1mm) and liquid resin (e.g. NovolacTM with about 80% solids) of 12:32:20:10:12:2:2:10.
The clay’s comprised alumina 28-35%, silica 50-58%, iron oxide 2-3%, titanium oxide 1-3% and a balance of alkali and alkaline metals 2-3%.
The iso-pressing (isostatic pressing) may be performed over a range of pressures (e.g. 10 to 400 MPa or 150 MPa to 350 MPa). Isostatic pressing densifies the green ceramics, whilst reducing internal stresses which result in subsequent cracking during firing and in use.
Upon firing at 1350oC for sufficient time (e.g. at least 60 min), the clay is preferably at least partially converted to mullite, with the carbonised resin reacting with silicon metal to form further silicon carbide which acts as an oxidation inhibitor.
The vacuum impregnation, coating and sealing steps are all readily performed within the scope of those skilled in the art. It would be understood that variations and/or modifications to the process steps or material used are envisaged and encompassed by the present inventive disclosure.
The composition of the inner composite tube reflects the raw materials used, with the proportion of clay equivalent to the glass refractory binder phase (e.g. about 12 wt%) and about 15wt% of a SiC phase derived predominately from the liquid resin and silicon metal, resulting in a binder content of about 27 wt%. The other components would be substantially in the same quantities as provided in the raw material mix.
The finished product is illustrated in Figure 5, having the following dimensions:
• Internal diameter: 78 mm
• External diameter: 126 mm
• Thickness of the metallic outer tube: 4 mm
• Thickness of inner composite tube: 22.5 mm
• Thickness of flange (inclusive of thickness of the metallic tube thickness): 18 mm
• Length: 865 mm
• Porosity before impregnation 13-15 %
• Porosity after impregnation and dried 11-12%
• Weight of refractory sealant – 0.5 wt% of total weight of composite tube
• Thickness of anti-sticking coating- 0.5 to 1mm
• Thickness of refractory mortar 0.5 to 1 mm
Porosity was determined using the water immersion method.
Figure 6 illustrates the condition of the riser tube after operating in a LPDC process for 63 eight hour shifts with no additional maintenance, after which the riser tubes were still in good operable condition. This was a substantial improvement (133%) over the service life of conventional cast iron tubes, which had a service life of 27 eight hour shift, with the need to reapply a non-stick coat every 9 shifts. The tubes achieved a combined manufacturing and customer reject rate about 11% less than the reject rate for cast iron riser tubes. In contrast to the cast iron riser tubes, the riser tubes of the present disclosure exhibited no iron pick-up by the molten metal.
Effect of sintering temperature
The above formulation was sintered at 950oC and 1350oC to assess the effect of the sintering temperature. As indicated in Table 2, the composite sintered at 1350oC has improved mechanical strength and oxidation resistance, measured by a reduction in weight loss of the composite.
Density and Porosity measurements were conducted in accordance with ASTMC20. The tensile bending strength measurement was conducted in accordance with ASTMC1161-13. Crystalline phase analysis was conducted using XRD analysis and elemental analysis was conducted using SEM/EDS.
Table 2
Sintered at 950oC Sintered at 1350oC
Density g/cc 2.13 ± 0.02 2.14 ± 0.02
Porosity % 14 ± 1 13 ± 1
Transverse Bending Strength MPa 16 ± 1 22 ± 1
Weight loss % (750oC for 1 hr) 26 12

A key aspect in the decreased porosity, increased strength and lower graphite oxidation is the in situ formation of the binder phase. The improved properties and performance is attributable to the formation of SiC fibres (280) as illustrated in Figure 7 due to the reaction between silicon metal and carbonised resin at the higher sintering temperature. A refractory glass phase, embedded with mullite, was also formed which co-operated with the SiC fibres to provide both mechanical strength and a graphite oxidation inhibitor.
The chemical composition of the fibrous silicon carbide phase is indicated in Table 3, with the dominant components being fibrous silicon carbide and amorphous carbon derived from the liquid resin. Other components included residual silicon metal which did not react with the amorphous carbon to form the fibrous silicon carbon.

Table 3: Chemical Composition of fibrous SiC binder
Chemical composition % wt
Al2O3 6
SiO2 11
SiC 46
Fe2O3 1
Si 3
TiO2 0.2
Free carbon 33

A variation of the ceramic matrix (having a similar raw material particle size distribution) was produced using only the clay as the binder (Table 4). Similar properties and performance was achieved to that in Table 2, with the transformation of the clay to a refractory glass ceramic phase being pivotal to the high strength and graphite oxidation resistance. The proportion of alumina and graphite components would be expected to be substantially the same as in the raw material mix.
Table 4: Clay bonded mix composition

Raw material wt%
Calcined Alumina 30
Fused white alumina 15
Clay 40
Graphite 15

The composition (crystalline portion) of the clay used in the raw material is provided in Table 5, with the sintered clay products provided in Table 6.

Table 5: Crystalline component in clay
Clay Quartz Kaolinite Illite Montmorillonite Anatase Rutile
Formula SiO2 Al2O3.2SiO2.2H2O K2Al4Si8O24 CaAl4Si8O24 TiO2 TiO2
wt% 10.6 58.9 22.7 3.1 3.7 1.0

As illustrated in Tables 5 & 6 after sintering at 1350oC the sintered clay is transformed with the formation of mullite and cristobalite. Additionally, the alkali and alkaline earth metals illite and montmorillonite facilitate the formation of a refractory glass phase with a composition as indicated in table 7.
Table 6: Crystalline components in sintered clay
Clay Mullite Quartz Cristobalite
Formula Al4.5Si1.5O9.74 SiO2 SiO2
wt% 57 4.7 38.3

Table 7: glass composition
Glass SiO2 Al2O3 CaO K2O Fe2O3 TiO2 MgO
%wt 50-60 25-30 1-2 2-5 2-5 1-3 1-2

The high temperature sintering enables the refractory glassy phase to soften and thereby densify the ceramic matrix above 1200oC. The resultant binder comprises a highly refractory glassy phase embedded with small (e.g. d50 < than 3µm) crystalline silica or mullite phases 350, which exhibit high strength and excellent resistant to erosion as well as inhibiting the oxidation of the graphite phase. Figure 8 illustrate the presence of the glass ceramic binder phase 300 (lighter phase) binding particles 310, 320, 330, 340 of the ceramic matrix together.
It will be appreciated that the skilled artisan may readily vary the ceramic matrix or other components of the present disclosure and still achieve advantageous results.
,CLAIMS:We claim:

1. A metal-composite riser tube comprising:
• a metallic tube comprising an inner surface and an outer surface; and
• an inner composite tube fixedly disposed against the inner surface of the metallic tube,
wherein the inner composite tube includes a ceramic matrix comprising:
A. 10 to 60 parts by weight alumina based filler;
B. 5 to 45 parts by weight binder phase;
C. 0 to 30 parts by weight fused silica and/or cordierite;
D. 5 to 20 parts by weight graphite;
E. 0 to 25 parts by weight silicon carbide;
F. 0 to 10 parts by weight additives and
the sum of A+B+C+D+E+F = 100 parts by weight.
2. The riser tuber according to claim 1, further comprising an outer composite tube fixedly disposed against an outer surface of the metallic tube, said outer composite tube includes a ceramic matrix comprising:
A. 10 to 60 parts by weight alumina based filler;
B. 5 to 45 parts by weight binder phase;
C. 0 to 30 parts by weight fused silica and/or cordierite;
D. 5 to 20 parts by weight graphite;
E. 0 to 25 parts by weight silicon carbide;
F. 0 to 10 parts by weight additives and
the sum of A+B+C+D+E+F = 100 parts by weight.
3. The riser tube according to claim 1 or 2, wherein the binder phase comprises a glass ceramic phase and/or fibrous silicon carbide.

4. The riser tube according to claim 3, wherein the binder phase comprises fibrous silicon carbide and the ceramic matrix comprises between 12 and 30 parts by weight fused silica and/or cordierite.

5. The riser tube according to claims 1, 2 or 3, wherein the glass ceramic phase comprises mullite.

6. The riser tube according to any one of claims 3 to 5, wherein oxides of aluminium and silicon account of at least 70 wt% of the glass ceramic phase.

7. The riser tube according to any one of the preceding claims, wherein the binder phase comprises a glass phase component which comprises less than 20 wt% of alkali and alkaline earth metal oxides.

8. The riser tube according to any one of the preceding claims, wherein the graphite is flaked graphite.

9. The riser tube according to any one of the preceding claims wherein the graphite has a mean particle size of between 100 and 500 µm.

10. The riser tube according to any one of the preceding claims, wherein the alumina based filler comprises crystalline alumina.

11. The riser tube according to any one of the preceding claims, wherein the weight ratio of the binder phase to alumina based filler is in the range of 1:1 to 1:20.

12. The riser tube according to any one of the preceding claims, wherein at least 50 wt% of the alumina based filler is selected from the group consisting of fused alumina, calcined alumina, tabular alumina, corundum or combinations thereof.

13. The riser tube according to any one of the preceding claims, wherein the porosity of the inner and/or outer composite tube in the range 5 to 25 %v/v.

14. The riser tube according to any one of the preceding claims, further comprising a refractory sealant impregnated into the inner and outer surfaces of the inner composite tube to form a sealed inner composite tube.

15. The riser tube according to any one of the preceding claims, wherein the inner and/or composite tube comprises 0.1 to 10 parts by weight refractory sealant.

16. The riser tube according to any one of the preceding claims, wherein the additives are selected from the group consisting of silicon metal, ferrosilicon, aluminium, boron carbide, boron alumina-silicate, borax, boric acid, aluminium phosphate, calcium sulphate, magnesium sulphate or combination thereof.

17. The riser tube according to any one of the preceding claims, wherein the inner surface of the inner composite tube further and/or an outer surface of the outer composite tube comprises a non-oxide refractory coating or an oxide refractory coating.

18. The riser tube according to claim 17, therein the non-oxide refractory coating is selected from the group consisting of carbide, nitride and boride coatings or combinations thereof.

19. The riser tube according to claim 17, therein the oxide refractory coating comprises alumina.

20. The riser tube according to any of the preceding claim, wherein the inner composite tube and/or the outer composite tube is fixedly attached to the metallic tube by an adhesive layer.

21. The riser tube according to claim 20, wherein the adhesive layer comprises alumina and/or silica based mortar or cement.

22. A process for producing a metal-composite riser tube according to any one of the preceding claims comprising the steps of:
a. providing a metallic tube;
b. providing an inner composite tube mixture comprising a binder; a source of alumina, a source of silica; silicon carbide; and graphite;
c. filling a mould with the mixture;
d. Isostatically pressing the filled mould to form a green tube;
e. curing the green tube to form a cured tube;
f. machining the cured tube to form a predetermined external diameter of the inner composite tube;
g. firing the cured and machined tube to form an inner composite tube;
h. vacuum impregnating the inner composite tube with a refractory sealant;
i. applying a layer of refractory adhesive to the inner surface of the metallic tube and/or the outer surface of the inner composite tube;
j. fitting the metallic tube onto the inner composite tube;
k. filling the gap between the inner composite tube and the metallic tube with a refractory cement or mortar;
l. optionally applying at least one non-sticking coating to the inside surface of the inner composite tube; and
m. optionally applying at least one refractory paint coating to the outer surface of the metallic tube.
23. The process according to claim 22, further comprising the step of fixedly disposing an outer composite tube to the outer surface of the metallic tube.

24. The process according to claim 22 or 23, wherein the firing step is performed at a temperature of at least 1000oC.

25. The process according to any one of claims 22 to 24, wherein the firing step is performed at sufficient temperature for sufficient time to result in the formation of fibrous silicon carbide and/or the formation of a glass ceramic phase within the binder phase.

Dated this 4th day of June 2021.

(ASHISH KUMAR SHARMA)
IN/PA-858
Of SUBRAMANIAM & ASSOCIATES
ATTORNEYS FOR THE APPLICANTS