Patent Application
20060054300
This application claims priority of GB Patent Application No. 0420611.6 filed Sep. 16, 2004.
The present invention relates to an apparatus, for the continuous casting of metals, in which the crucible containing the molten metal is connected to the casting die or mould (hereinafter referred to as “die”) by means of a carrier adaptor which permits replacement of the die without the need for cooling of the crucible furnace.
In the continuous casting of metals, especially non-ferrous metals and alloys, a typical procedure involves the melting and alloying of the metal to be cast in a separate melting furnace. The molten metal or alloy is then poured into a heated holding crucible or tundish from which a semi-finished rod, hollow bar or section is continuously cast through a water-cooled casting die, typically made of graphite, which is connected to a bottom outlet of the crucible. The holding crucible can be heated by various means, for example by a resistance heated furnace. In some applications the initial melting or alloying of the metal to be cast is carried out in the holding crucible. Generally the outlet of the crucible is formed and arranged to deliver the molten metal horizontally to the die which, in turn produces the cast product horizontally.
Graphite is a particularly suitable material for holding and casting molten metals, having several desirable properties including strength, machineability, non-wetting behaviour and it also has a naturally reducing effect. Consequently graphite has been successfully used as the material of construction for crucibles and dies in continuous casting apparatus. However, at elevated temperatures, especially above 400° C., graphite must be used in a non-oxidising atmosphere in order to avoid degradation and erosion caused by atmospheric oxygen.
A similar issue arises when the furnace heating the crucible and casting die assembly utilises electrical resistance heating with graphite elements. These heating elements are also susceptible to degradation by oxygen at elevated temperatures.
Nevertheless, by providing a sealed furnace protected by an inert gas atmosphere, many high-copper alloys, brasses, tin bronzes, phosphor-bronzes, aluminium bronzes and a full range of precious metals, including gold, silver and low-palladium alloys can be successfully cast from graphite crucible and die assemblies, using graphite or silicon carbide electrical resistance heaters.
In a typical graphite crucible based system the furnace becomes a sealed unit while at elevated operating temperature and a positive pressure of inert gas is maintained to exclude oxygen and thus to avoid erosion of the graphite heating elements and the exterior surfaces of the graphite crucible which is located inside the furnace box. (The inner surface of the graphite crucible may also be protected by an inert gas atmosphere but this is not generally required). In such an arrangement, the casting die is thrust against an orifice at the base of the crucible using a special seal (feeder tube) and is held in place by a thrust plate which secures the die in close contact with the furnace wall. The gas-tight seal is completed by the provision of a thermal and gas sealing ring fitted around the die at the junction with the outside furnace wall. Jacket coolers around the casting die or a combination of jacket coolers and probe coolers are used to cool and solidify the metal as the casting process continues. The casting die has a relatively short service life, depending on the alloy and section profile, at the end of which the die must be changed. Alternatively, production requirements may dictate that a die should be changed before the end of its service life for a die for a different product size or profile.
A disadvantage of this arrangement is that in order to effect replacement of the casting die the gas-tight seal between the furnace wall, the casting die and any associated sealing rings must be broken. Ingress of air into the furnace at temperatures above 400° C. would cause rapid erosion of all the graphite parts i.e. the crucible and the heating elements. A particular concern is the sealing face on the orifice at the base of the crucible. If damaged, molten metal leaks are likely during subsequent casting operations. Accordingly replacement of the graphite die necessitates careful cooling of the furnace before the inert gas atmosphere is released. After replacing the die the furnace is then reheated to working temperature under an inert gas atmosphere. Cooling and reheating of the furnace can take many hours, thus reducing the efficiency of the production process. Such regular thermal cycling of the equipment may also introduce undesirable thermal stresses in the furnace construction.
It is an object of the present invention to avoid or minimise one or more of the foregoing disadvantages by the provision of a horizontal continuous casting machine having a carrier adaptor which connects the molten metal outlet at the base of the metal holding crucible with the casting die in a manner which permits safe and efficient exchange and replacement of the casting die, without the furnace having to be cooled or powered down.
The present invention provides a horizontal continuous casting apparatus comprising an inert gas-purged resistance-heated type furnace, a crucible for containing molten metal inside said furnace, in use of the apparatus, and having an outlet for molten metal, coupled, via an aperture in a wall of said furnace, to a casting die, and a first clamping device formed and arranged for securing said casting die in sealing communication with said crucible, wherein is provided a carrier adaptor of an oxidation resistant refractory material and comprising: a feed nozzle formed and arranged for providing fluid communication between said crucible outlet and an inlet of said casting die, a female mounting portion formed and arranged for receiving, in sealing inter-engagement, a male mounting portion of said casting die, and sealing means formed and arranged for sealing inter-engagement with said furnace wall around said aperture, and wherein is provided a second clamping device formed and arranged for releasably securing said carrier adaptor to said crucible and said furnace wall, in sealing inter-engagement therewith, and said first clamping device is formed and arranged for releasably securing said casting die to said carrier adapter.
The inert gas-purged resistance-heated type furnace and crucible are of generally the same forms as found in prior art casting machines. Typically the heating elements of the furnace are of graphite or silicon carbide and the crucible is of graphite.
The carrier adaptor is made of a refractory oxidation resistant material, suitable for use in metal casting operations, that is sufficiently durable to be semi-permanently attached to the crucible and furnace for a substantial number of casting campaigns. A low cost, oxidation resistant clay/graphite mixture has been found to serve the purpose very well, but other suitable oxidation resistant refractory materials can also be contemplated provided they give good sealing with the crucible and the casting die and have reasonable thermal conductivity. Conveniently the carrier adaptor is circular in cross-section and is mounted concentrically with the outlet orifice of the crucible and the corresponding aperture in the furnace wall.
Preferably the carrier adaptor feed nozzle has a frusto-conical form, narrowing slightly to a diameter less than that of the casting die inlet at the casting die end of the nozzle. This has the effect of providing a “lip” at the downstream end of the feed nozzle adjacent to the casting die inlet. This lip has the benefit of resisting residual metal running into the casting die at the end of a casting campaign, when the crucible is substantially emptied of molten metal.
Additionally the carrier adaptor is conveniently provided with a drain passage through the wall of the carrier adaptor, preferably at the upstream end of the female mounting portion adjacent to the feed nozzle. This drain passage is normally occluded by the casting die. However when the casting die is removed for replacement, residual molten metal from the crucible, not held back by the lip of the feed nozzle, drains safely downwards via a suitable return passage back into the body of the furnace, rather than running out of the carrier adaptor where it could endanger the operator changing the die.
The second clamping device, for securing the carrier adaptor conveniently comprises a thrust plate with releasable fasteners, the thrust plate engaging with a downstream end facing clamping face of the carrier adaptor, which is conveniently disposed outside the furnace. The thrust plate urges the carrier adaptor into sealing inter-engagement with the crucible outlet orifice, when the releasable fasteners are fastened. Sealing with the crucible outlet may be assisted by the use of refractory cement, gaskets or other suitable seals.
At the same time the sealing means of the carrier adaptor, which may, for example, simply comprise a portion of the outer wall of the carrier adaptor, is used to form a substantially gas tight seal between the carrier adaptor and the wall of the furnace around the orifice in the furnace wall through which the casting is produced. Sealing contact may be made directly between the sealing portion of the carrier adaptor and the sides of the furnace wall orifice, possibly with the assistance of refractory cement or a gasket. Other means of obtaining the seal can also be envisaged. For example, where the carrier adaptor is formed and arranged to protrude through the furnace wall aperture, a sealing ring mounted on the furnace wall, around the furnace wall aperture, may be used to engage the sealing portion of the carrier adaptor. Such a sealing ring can provide all of the sealing between the furnace wall and the carrier adaptor or can be in addition to direct sealing between the carrier adaptor and the sides of the furnace wall orifice.
Casting dies used with the carrier adaptor of the invention can be of the conventional water-cooled graphite construction, fitted with an inert gas purging system to protect the graphite casting surface during use. Casting dies made of alternative materials, such as cast iron, for example, may also be employed depending on the metal or alloy being cast. The female mounting portion of the carrier adaptor is formed and arranged for receiving the male mounting portion of the casting die. Generally a close sealing fit is made between the carrier adaptor and the die at the upstream end of the female mounting portion proximal the feed nozzle, with a small clearance being provided between the mountings elsewhere. This clearance helps to avoid seizing between the carrier adaptor and the die, which would make replacement of the die more difficult.
The casting die is secured to the carrier adaptor by the first clamping device. This clamping device conveniently comprises a thrust plate, or a thrust plate and thrust collar mounted by releasable fasteners such as, for example, studs and nuts or bolts.
Conveniently the fasteners used to mount the thrust plate of the casting die include fastener elements secured to the carrier adaptor, in which case the carrier adaptor further comprises means for mounting these fasteners, such as threaded boreholes. Alternatively the clamping device for the casting die may also be secured to the wall of the furnace.
In use the casting apparatus operates in the same fashion as prior art casting machines during casting operations. However, when the casting die becomes worn or eroded through use, then it can be changed, when the crucible is substantially empty, without the need to cool the furnace and break the gas tight seal formed between the furnace wall and the carrier adaptor. The casting die is simply released from the carrier adaptor by unfastening the casting die clamping device, without disturbing the carrier adaptor, and then removed and replaced. Replacement of the die can readily be safely carried out by a low skilled operator due to the positive location of the die into the female mounting portion of the carrier adaptor. Precise location can be further assisted by the provision of locating screws (adjusted on first fitting of a die to the carrier adaptor). Any sealing gaskets used in the mounting of the die are conveniently replaced during die replacement.
Thus in a further aspect the present invention provides a method of die replacement in a casting apparatus, comprising the steps of:
Further preferred features and advantages of the present invention will appear from the following detailed description of some preferred embodiments illustrated with reference to the accompanying drawings in which:
In use the molten metal held in the crucible 6 flows through the outlet 8 into the casting die inlet 39 via the feeder tube 12. The cooling applied via the cooling jacket 20 solidifies the molten metal in the casting die 14 at or near to, the point indicated by the line 40. The cast solid is continuously withdrawn by means of the casting rollers 42. The disadvantage of this arrangement is that the furnace 1 must be cooled whenever the casting die 14 is to be exchanged as the gas tight sealing at the furnace wall 34 is made between the casting die 14, the furnace wall 34 and/or the sealing ring 38.
The carrier adaptor 44 projects through an orifice 36 of the furnace wall 34 forming a substantially gas tight seal with the assistance of the sealing ring 38. The carrier adaptor 44 is held securely in place by a clamping device comprising a thrust plate 68 studs 70 and nuts 72. The casting die 14 is secured by its own thrust plate 28, collar 29, studs 32 mounted 73 on the carrier adaptor 44, and nuts 30.
In use, during casting operations, the apparatus operates as the prior art machines such as shown in
It will be appreciated that various modifications may be made to the above described embodiments without departing from the scope of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0420611.6 | Sep 2004 | GB | national |
