WO2018111210A1

WO2018111210A1 - A holding furnace for low pressure casting benches

Info

Publication number: WO2018111210A1
Application number: PCT/TR2017/050450
Authority: WO
Inventors: Şükrü IRMAK
Original assignee: Irmak Suekrue
Priority date: 2016-12-15
Filing date: 2017-09-26
Publication date: 2018-06-21

Abstract

The present invention is a holding furnace (10) developed for use in low pressure casting benches (1) and having a furnace body (20) positioned under a casting machine plate (60) whereon a casting mold (50) is provided and which seats onto a furnace vehicle (40), and having a chamber (21) positioned in said furnace body (20) and wherein a liquid metal (22) is poured, characterized in that at least the base section of said chamber (21) is made of a refractor material with thermal transmission characteristic, and a combustion gas path (26) is provided which is embodied in a manner at least partially contacting the outer surface of the chamber (21) and wherein flame and at least one type of combustion gas advance and where the heat, formed therein, is transferred to the crucible (21), and thanks to this, the heating of the furnace is provided by means of gas or liquid fuel from outside of the pressured region (28).

Description

A HOLDING FURNACE FOR LOW PRESSURE CASTING BENCHES TECHNICAL FIELD

The present invention relates to an air-tight holding furnace embodiment developed for use in low pressure casting benches. PRIOR ART

The low pressure casting method consists of filling liquid metal, held at desired temperature inside an air-tight holding furnace, into the mold by rising through a feeding tube whose one end is opened to the liquid metal and whose other end is opened to the casting mold thanks to the pressured air applied into the furnace; keeping said liquid metal at a desired pressure (in general, 0.7-1 .2 bars) inside the mold; and solidifying liquid metal by operating the cooling circuits in the mold meanwhile. Since pressured air is applied into the furnace, the furnace shall be air-tight. When solidification is completed, the pressure inside furnace is discharged and the liquid inside feeding tube returns to the holding furnace. The cast piece, solidified inside the mold and thus which took the shape of the mold, is taken outwardly after opening the mold. This process is repeated for each piece.

The sealed holding furnaces used in low pressure casting machines are generally produced in two types. The properties of permanent refractory furnaces used frequently are as follows:

• Electrical heaters are provided above the liquid metal, at the furnace ceiling. Heating is provided substantially by radiation.

• The furnace refractory is formed by a number of layers from the inner section of the steel body towards the furnace center; in general, it is a pourable refractor with isolation characteristic (in general, including high level of Al₂0₃) and resistant to liquid metal contact and isolation bricks and ceramic fiber. The lifetime of such refractor is approximately 5 years or more.

• There is a big volume which has to be filled with pressured air, even in case the metal level is highest, between the furnace ceiling and liquid metal level since the electrical heaters are placed onto the liquid metal. The second type which is used less frequently is the crucible sealed holding furnace. Inside these furnaces, there is graphite and/or SiC based crucible except the insulated refractor and in a separate manner from said insulated refractor. The properties of such furnaces are as follows:

· The electrical heaters are assembled on the walls of the insulation refractor and they are placed in a manner encircling the crucible.

• The liquid metal is provided in a crucible whose lifetime is in general changing between 4-8 months. Since the crucible lifetime is substantially low when compared with furnaces having permanent refractor, they are not preferred except some exceptional cases.

• The volume, which is to be pressured, is again big due to the shape of the crucible and the resistances provided on the side walls.

o The heating method is radiation again. In both types of furnaces, since the volume which shall be pressured and where the liquid metal is provided for providing sealing and the heaters which shall meet the thermal losses of the liquid metal shall be provided in the same volume, electrical heating in these furnaces becomes necessary. The low pressure casting method used frequently in production of particularly aluminum cast pieces, in the technology level of today, it accommodates some problems caused by the sealed furnace structure.

1. The heat losses of the liquid metal provided in the holding furnace are met by electrical heaters placed into the furnace. However, electrical power is one of the most expensive power types.

2. Heating method is radiation. Since the surface of liquid aluminum is very brilliant, most of the arriving radiation is reflected back, moreover, Al₂0₃ (aluminum oxide) layer formed on the liquid aluminum surface in a duration much less than one second becomes thicker in every air application into the furnace and more strongly prevents permeation of the radiation arriving from above. This means excessive energy usage for meeting thermal losses of the liquid metal. The thermal losses in the ceiling of the holding furnace are more than the thermal losses in the other regions except the loading-unloading doors.

3. Since heaters are positioned inside the furnace, the volume which shall be pressured is substantially great. In every cast of pieces, said whole volume is increased to casting pressure, and at the end of casting, the pressured air inside the furnace is discharged. The pressured air consumption per piece is substantially high. This increases the process cost due to the electrical power used in pressured air production. 4. High amount of pressured air requirement per cast piece increases the first investment consumptions and maintenance costs for the compressors.

5. Since great amount of air is applied into the holding furnace in every casting process, the metal gas level is rapidly increased and the quality of the cast piece is compromised. A section of cast piece is scraped due to the porosity problems caused by the high gas level in liquid aluminum. This affects production efficiency and thus production costs in an unfavorable manner. Another effect of application of high amount of air into the sealed furnace is that aluminum oxide is formed as a result of reaction of liquid aluminum with oxygen and thus, the metal losses increase. As a solution to these problems, in general, noble gases like dried air and/or nitrogen, argon are applied into the furnace. This solution solves the quality problem and substantially increases the production costs.

6. The amount of air applied to the holding furnace receives the heat existing inside the furnace into its body and it is heated and at the end of casting, this heat is lost when discharged. In pluralities of holding furnaces, thermal exchangers are used for minimizing this loss. The heat of the discharged air is transferred to the heat holding media, and in the next casting process, the air applied into the furnace is passed through this media and it is heated. This method decreases thermal loss; however it cannot provide a complete solution; since the main problem is the requirement of applying great amount of air into the furnace. Secondly, the temperature difference between the heating air and the air to be heated used in the thermal exchanger is relatively lower. This leads to insufficient heating of the air applied into the furnace. Due to all of these reasons, the energy efficiency of the sealed holding furnaces used today decreases, and the electrical power consumption per cast piece weight is higher and thus, it leads to high process cost.

As a result, because of all of the abovementioned problems, an improvement is required in the related technical field.

BRIEF DESCRIPTION OF THE INVENTION

The present invention relates to air-tight holding furnaces, for eliminating the above mentioned disadvantages and for bringing new advantages to the related technical field. The main object of the present invention is to provide an air-tight liquid metal holding furnace which can be heated by means of gas fuel.

Another object of the present invention is to provide a liquid metal holding furnace where first investment and operational costs are reduced in the low pressure casting process.

In order to realize all of the abovementioned objects and the objects which are to be deducted from the detailed description below, the present invention is a holding furnace developed for use in low pressure casting benches and having a furnace body positioned under a casting machine plate whereon a casting mold is provided and which seats onto a furnace vehicle, and having a chamber positioned in said furnace body and wherein a liquid metal is poured. Accordingly, said invention is characterized in that at least the base section of said chamber is made of a refractor material with thermal transmission characteristic, and a combustion gas path is provided which is embodied in a manner at least partially contacting the outer surface of the chamber and wherein flame and at least one type of combustion gas advance and where the heat, formed therein, is transferred to the crucible.

In another preferred embodiment of the invention, said combustion gas path extends at least along the base of the chamber.

In another preferred embodiment of the invention, all surfaces of said chamber are made of a refractory material with thermal transmission characteristic.

In another preferred embodiment of the invention, said combustion gas path is embodied in a manner encircling the chamber for at least 3/4 tours in a spiral manner.

In another preferred embodiment of the invention, said combustion gas path is embodied in a manner encircling the chamber for at least two tours in a spiral manner. In another preferred embodiment of the invention, a combustion gas outlet is provided where the combustion gases leave the combustion gas path.

In another preferred embodiment of the invention, an exchanger, connected to the combustion gas outlet, is provided. In another preferred embodiment of the invention, said exchanger comprises a pressured air inlet where the heat received from the combustion gas outlet is transferred in order to be applied to the pressured section. In another preferred embodiment of the invention, a protective coating is provided in at least one section of the combustion gas path.

In another preferred embodiment of the invention, said protective coating is provided in a manner encircling all surfaces of the combustion gas path.

In another preferred embodiment of the invention, said protective coating is provided in a manner covering all outer surfaces of the chamber.

In another preferred embodiment of the invention, said protective coating is selected from at least one of a conductive ceramic material and a metal whose melting temperature is higher than the melting temperature of the combustion gas.

In another preferred embodiment of the invention, a thermal insulation region is provided between said combustion gas path and the outer surface of the furnace body.

In another preferred embodiment of the invention, said thermal insulation region comprises an insulation refractor which fills the gap between the chamber and the furnace body.

In another preferred embodiment of the invention, said thermal insulation region comprises a ceramic plate provided between said insulation refractor and the furnace body.

In another preferred embodiment of the invention, at least one sealing element is positioned at the joining locations of all items provided on the furnace body. BRIEF DESCRIPTION OF THE FIGURES

In Figure 1 , a representative general view of the low pressure casting bench is given. In Figure 2a, a representative detailed view of the thermal insulation region is given.

In Figure 2b, a representative detailed view of the alternative thermal insulation region is given. In Figure 3, a representative alternative view of the low pressure casting bench is given.

REFERENCE NUMBERS 1 Low pressure casting bench

10 Holding furnace

20 Furnace body

21 Chamber

22 Liquid metal

23 Feeding inlet

24 Sealed cover

25 Combustion gas inlet

26 Combustion gas path

261 Protective coating

27 Combustion gas outlet

28 Pressured section

29 Thermocouple

30 Thermal insulation region

301 Insulation refractor

302 Ceramic plate

32 Exchanger

33 Pressured air inlet

34 Sealing elements

35 Mold feeding tube

36 Intermediate tube

37 Pressure sensor

40 Furnace vehicle

50 Casting mold

60 Casting machine plate

DETAILED DESCRIPTION OF THE INVENTION

In this detailed description, the subject matter is a holding furnace (10) developed for use in a low pressure casting bench (1 ) and is explained with references to examples without forming any restrictive effect only in order to make the subject more understandable. With reference to Figure 1 , said holding furnace (10) essentially comprises a furnace body (20), and a combustion gas path (26) which at least partially contacts the base and/or side surfaces of a chamber (21 ) provided on said furnace body (20) and wherein a liquid metal (22) is positioned and wherein flame and at least one type of combustion product gas advance. In the preferred application, the combustion gas path (26) encircles at least 3/4 tours around the chamber (21 ). In alternative embodiments, the combustion gas path (26) encircles pluralities of tours around the chamber (21 ). In order to provide feeding of liquid metal (22) into the chamber (21 ), a feeding inlet (23) is embodied which is connected to the chamber (21 ). A sealed cover (24) is positioned at the section of the feeding inlet (23) opened outwardly. A pressured section (28) is defined at the upper section of the liquid metal (22) inside the chamber (21 ). The chamber (21 ) is made of a refractor which has strong thermal transmission characteristic.

The furnace body (20) has a box-like structure. There is at least one thermocouple (29) extending from the upper section of the furnace body (20) into the liquid metal (22) in order to measure the temperature of the liquid metal (22). At one end of said combustion gas path (26), there is a burner which will burn the gas fuel and there is a combustion gas inlet (25) where the combustion flame and combustion product gases formed as a result of combustion are applied to the combustion gas path (26). In a similar manner, there is a combustion gas outlet (27) provided at the other end of the combustion gas path (26) and where the combustion gas exits the system. At least one section of the combustion gas path (26) is preferably coated with a protective coating (261 ). In alternative embodiments, whole of the combustion has path (26) is coated with said protective coating (261 ). In another alternative embodiment, at least some section or whole of the outer surfaces of the chamber (21 ) is coated with protective coating (261 ). The protective coating (26) is made of a conductive ceramic or a metal of which the melting temperature is higher than the melting temperature of combustion gas.

There is an exchanger (32) provided in a connected manner to the combustion gas outlet (27) and there is a pressured air inlet (33) connected to said exchanger (32). There is a pressure sensor (37) positioned in the vicinity of said pressured air inlet (33) and which can control the pressure of air in the pressured section (28). A thermal insulation region (30) is provided between the furnace body (20) outer section and the chamber (21 ). The thermal insulation region (30) comprises an insulation refractor (301 ) in a manner filling the gap between the chamber (21 ) and the furnace body (20) and a ceramic plate (302) provided between said insulation refractor (301 ) and the furnace body (20). At least one sealing element (34) is positioned at the joining locations of all items provided on the furnace body (20).

The furnace body (20) seats onto a furnace vehicle (40). There is a casting machine plate (60) positioned in an independent manner and in a manner not contacting the furnace body (20) at the upper section of the furnace body (20). In alternative embodiments, the furnace body (20) does not have an upper section, and it is directly pressed onto the casting machine plate (60), and the sealing in between is provided by means of a sealing element cooled by means of a media like water, air or another media. There is a casting mold (50) positioned on said casting machine plate (60). There is an intermediate tube (36) which passes through the casting machine plate (60) and which reaches the casting mold (50) and there is a mold feeding tube (35) provided at the continuation of said intermediate tube (36) and which extends in a manner immersed into the liquid metal (22). The chamber (21 ) is positioned such that the combustion gas path (26) at least partially contacts the chamber (21 ). In the preferred application, the combustion gas path (26) is positioned so as to encircle the chamber (21 ) in a spiral manner. The flame and combustion gases applied through the combustion gas inlet (25) advance along the combustion gas path (26) and they reach the combustion gas outlet (27). Here, by means of using exchanger (32), the temperature of the exiting gas is used for heating the air entering into the pressured air inlet (33) and for heating the air to be used in burning the fuel in the burner.

During application of the low pressure casting method, the total inner pressure of the chamber (21 ) is going to be increased by means of the air to be applied to the pressured section (28) provided inside the chamber (21 ) through the pressured air inlet (33) and the liquid metal (22) passes through the mold feeding tube (35) and through the intermediate tube (36) positioned at the continuation, and said air advances into the casting mold (50). By means of applying suitable and sufficient pressure, the liquid metal (22) which completely fills the casting mold (50) takes the shape of the casting mold (50) and said liquid metal (22) is solidified and it is discharged by means of opening the casting mold (50). By means of removing the product from the casting mold (50), this process can be repeated at the desired number and at the desired frequency.

In the present invention, sealing elements (34) which are resistant to high temperature and refractors which are in tile form and/or which can be cast and which are SiC and/or SiC+ Graphite based whose thermal transmission coefficients are high are used, and the heater section and the pressured section provided inside the chamber (21 ) are separated from each other. Thanks to this, the usage of gas burners is provided. Since the sealing elements (34) can lose their functions within the duration where the holding furnace (10) is used; as a second sealing security, the combustion gas path (26) and/or the outer surface of the chamber (21 ) is/are taken into metal or ceramic protective coating (261 ) which is sealed and of which the thermal transmittance is high and which is resistant to high temperature. At least one of steel, cast iron, stainless steel, Inconel, sialon, aluminum nitrate, fused silica, etc. or the mixtures thereof, which are at a specific proportion, can be used as the protective coating (261 ). Refractor of which the thermal transmittance is high has been used in the structure of the chamber (21 ) provided in the holding furnace (10), and while the liquid metal (22) is held inside the chamber (21 ), the liquid metal (22) is heated by means of the gas burner burnt around the chamber (21 ). The chamber (21 ), made of thermal transmission refractor where the liquid metal (22) is provided, is isolated from the section where the combustion gases are provided by means of the sealing elements (34) which are resistant to high temperature, in other words, said chamber (21 ) is isolated from the combustion gas path (26), and by means of this, the chamber (21 ) can be pressured.

Thanks to the subject matter embodiment, since the heaters, in other words, the combustion gas path (26) do/does not have to stay inside the chamber (21 ), the volume of the section (28) to be pressured is reduced. By means of this, the pressured air consumption and depending on this, the electrical consumption of the process and the costs are reduced.

Thanks to said holding furnace (10) embodiment, by using the chamber (21 ), made of casting refractors of which the thermal transmittance is high, and the sealing elements (34) which are resistant to high temperature and/or metal and/or ceramic protective coatings (261 ) which are resistant to high temperature and which encircle the combustion gas path (26) from the inner side, the liquid metal (22) is heated from outside and the liquid metal (22) is pressured inside the chamber (21 ). The thermal insulation refractor (301 ), provided externally, is embodied such that the flame of the natural gas burner and the hot gases formed as a result of combustion encircle the chamber (21 ). The thermal insulation refractor (301 ) determines the path of the hot combustion gases around the chamber (21 ) and at the same time, the thermal insulation refractor (301 ) realizes the main insulation function which prevents outward transmission of heat. Moreover, by using the waste heat in flue gases, the air, which is to be applied into the holding furnace (10), is heated and by means of this, the thermal losses of the liquid metal (22) are minimized. Additionally, again by using the waste heat existing in the flue gases, the air used for burning the fuel in the burner is also heated and thus, power saving and thus, cost advantage is provided.

The combustion product gases follow the combustion gas path (26) and they transfer their heat to the liquid metal (22) through the chamber (21 ) made of conductive refractor, and they reach the combustion gas outlet (27) and from there, they are guided to the exchanger (32) in order to heat the pressured air to be applied to the chamber (21 ) and the air to be applied to the gas burner. The present invention does not use the waste heat of the air discharged at the end of casting through the pressured section (28) and it uses the waste heat of the combustion products formed by the natural gas burner, in the pre-heating of the air to be applied into the chamber (21 ), in a different manner from the prior technology. Because the heat of the combustion products formed by the burner is higher than the heat of the pressured air discharged through the holding furnace (10), and thanks to the present invention, the amount of air to be applied into the holding furnace (10) and to be discharged after casting is substantially reduced.

By means of the subject matter product and application method, in the sealed holding furnace (10) of the low pressure casting benches, the pressured section (28) and the heater section are separated during casting, heating can be realized by means of power types except electrical power and by means of lower cost power types, for instance by means of natural gas, and the pressured volume is reduced. By means of this, the following advantages will be provided:

- Since the sealed holding furnace (10) can be heated by means of lower cost power types like natural gas, LPG or LNG, the power cost consumed in the process will be reduced.

- Since the heating method is realized by means of thermal transmission, the heating efficiency will be higher when compared with holding furnaces (10) of the present technology level heated by means of radiation.

- Since the volume which shall be pressured during casting will be reduced, the amount of pressured air consumed per cast piece will be reduced. Since this will reduce the electrical power used in pressured air production, this will affect the process cost in a favorable manner.

- Since less amount of pressured air is needed per cast piece, the compressor initial investment and maintenance costs will be reduced.

- Since less air is applied into the sealed holding furnace (10), the metal gases will be substantially reduced, reduction in porosity errors will occur. Even in the casting of pieces which need high quality, there will remain no need to use noble gas, and the usage of physically dried air (-5^SC) may be sufficient.

- Since the reduction of the air applied into the sealed holding furnace (10) will reduce the amount of oxygen with which the liquid metal (22) may enter into reaction, there will also be reduction in liquid metal (22) loss proportions.

- Since less pressured air will be applied into the sealed holding furnace (10) for every casting, the amount of heat lost as a result of the discharge of this air and thus, power consumption will reduce. Moreover, thanks to recycle of the waste heat existing in the flue gases of the holding furnace (10) heated by means of natural gas and thanks to heating of the air, applied into the holding furnace (10), by means of the waste heat existing in these combustion gases, the thermal losses of the liquid metal (22) will be minimized.

- Thanks to the control of the gas burner, which will provide heating, by means of feedback to be taken from the measurement of the metal temperature and by means of PI D control unit and the proportional valve, the natural gas consumption may be minimized as much as possible. In other words, since in the technology of today, the thermal control by means of electricity can also be provided in gas burning systems with an accuracy which is close to the accuracy provided in electrical heating, the fluctuations which may affect the process in the liquid metal (22) temperature are prevented.

The protection scope of the present invention is set forth in the annexed Claims and cannot be restricted to the illustrative disclosures given above, under the detailed description. It is because a person skilled in the relevant art can obviously produce similar embodiments under the light of the foregoing disclosures, without departing from the main principles of the present invention.

Claims

A holding furnace (10) developed for use in low pressure casting benches (1 ) and having a furnace body (20) positioned under a casting machine plate (60) whereon a casting mold (50) is provided and which seats onto a furnace vehicle (40), and having a chamber (21 ) positioned in said furnace body (20) and wherein a liquid metal (22) is poured, characterized in that at least the base section of said chamber (21 ) is made of a refractor material with thermal transmission characteristic, and a combustion gas path (26) is provided which is embodied in a manner at least partially contacting the outer surface of the chamber (21 ) and wherein flame and at least one type of combustion gas advance and where the heat, formed therein, is transferred to the crucible (21 ).

A holding furnace (10) according to claim 1 , wherein said combustion gas path (26) extends at least along the base of the chamber (21 ).

A holding furnace (10) according to claim 1 , wherein all surfaces of said chamber (21 ) are made of a refractory material with thermal transmission characteristic.

A holding furnace (10) according to claim 1 , wherein said combustion gas path (26) is embodied in a manner encircling the chamber (21 ) for at least 3/4 tours in a spiral manner.

A holding furnace (10) according to claim 1 , wherein said combustion gas path (26) is embodied in a manner encircling the chamber (21 ) for at least two tours in a spiral manner.

A holding furnace (10) according to claim 1 , wherein a combustion gas outlet (27) is provided where the combustion gases leave the combustion gas path (26).

A holding furnace (10) according to claim 6, wherein an exchanger (32), connected to the combustion gas outlet (27), is provided.

A holding furnace (10) according to claim 7, wherein said exchanger (32) comprises a pressured air inlet (33) where the heat received from the combustion gas outlet (27) is transferred in order to be applied to the pressured section (28).

9. A holding furnace (10) according to claim 1 , wherein a protective coating (261 ) is provided in at least one section of the combustion gas path (26).

10. A holding furnace (10) according to any one of the preceding claims, wherein said protective coating (261 ) is provided in a manner encircling all surfaces of the combustion gas path (26).

11. A holding furnace (10) according to any one of the preceding claims, wherein said protective coating (261 ) is provided in a manner at least partially covering the outer surfaces of the chamber (28).

12. A holding furnace (10) according to claim 9, wherein said protective coating (261 ) is selected from at least one of a conductive ceramic material and a metal whose melting temperature is higher than the melting temperature of the combustion gas.

13. A holding furnace (10) according to claim 1 , wherein a thermal insulation region (30) is provided between said combustion gas path (26) and the outer surface of the furnace body (20).

14. A holding furnace (10) according to claim 12, wherein said thermal insulation region (30) comprises an insulation refractor (301 ) which fills the gap between the chamber (21 ) and the furnace body (20).

15. A holding furnace (10) according to claim 13, wherein said thermal insulation region (30) comprises a ceramic plate (302) provided between said insulation refractor (301 ) and the furnace body (20).

16. A holding furnace (10) according to claim 1 , wherein at least one sealing element (34) is positioned at the joining locations of all items provided on the furnace body (20).