US3715441A

US3715441A - Induction furnace with thermocouple assembly

Info

Publication number: US3715441A
Application number: US00166009A
Authority: US
Inventors: H Collins
Original assignee: Individual
Current assignee: Individual
Priority date: 1971-07-26
Filing date: 1971-07-26
Publication date: 1973-02-06
Anticipated expiration: 1990-02-06

Abstract

An induction furnace for melting metal has a built-in thermocouple assembly for continuously measuring the temperature of the molten metal. A power controller then adjusts the power to the furnace to maintain a predetermined temperature. The thermocouple assembly includes a refractory substrate which forms a portion of the furnace lining partially coated with thin strips of two thermoelectric materials so as to form a thermocouple junction. The refractory substrate and the strips of thermoelectric materials are covered with a protective layer of the same refractory material as the substrate. Electrical leads are attached to the assembly and are in contact with the strips of thermoelectric material. An alternative embodiment utilizes a similarly constructed thermocouple which is immersed in the molten material for intermittent measurements.

Description

United States Patent 1191 1111 3,715,441

Collins Feb. 6, 1973 UCT ON FURNAC WITH Primary Examiner-Roy N. Envall, J r.

THERMOCOUPLE ASSEMBLY Attorney-Richards, Harris & Hubbard [76] Inventor: Henry F. Collins, 917 Alamo, Gar- [57] ABSTRACT land, Tex. 75040 [22] Filed: July 26 1971 An induction furnace for melting metal has a built-in thermocouple assembly for continuously measuring [21] App]. No.: 166,009 the temperature of the molten metal. A power controller then adjusts the power to the furnace to main tain a predetermined temperature. The thermocouple 5 2 1 U 8 Cl 13/35 fg assembly mcludes a refractory substrate which forms a portion of the furnace lining partially coated with thin {g 2 5 2 9 strips of two thermoelectric materials so as to form a thermocouple junction. The refractory substrate and 29/592 the strips of thermoelectric materials are covered with a protective layer of the same refractory material as [56] References Cited the substrate. Electrical leads are attached to the as- UNITED STATES PATENTS sembly and are in contact with the strips of thermoelectric material. An alternatwe embodlment ut1l- 2,519,941 5 Tama ..l3/26 UX izes a similarly constructed thermocouple which is im- 2,655,550 10/1953 Zvanut mersed in the molten material for intermittent mea- 3,006,978 10/1961 McGrath et al. suremems 81 12/1963 Shearman ..l36/233 X 5 Claims, 13 Drawings POWER CONTROLLER Z6 PATEHTEDFEB 6l975 3,715,441 SHEET 10F 4 POWER CONTROLLER INDUCTION CORE FIG!

INVENTORI HENRY E COLLINS ATTORNEYS PATENTED FEB 6 I975 SHEET 30F 4 FIG. 8

PMUHHJFEB 6 III/5 SHEET 0F 4 FIG. l0

FIG. I!

FIG. /3

r a 1 I I 3 .I l 3 .l I 1 I I I 8 9 0 X i 1 6 I 9 f FIG. I2

I I 9 l 4 1 9 x z 1 r I I l I I I 7 /7 HENRY F. COLLINS ATTORNEYS INDUCTION FURNACE WITH THERMOCOUPLE ASSEMBLY BACKGROUND OF THE INVENTION sembly having very thin strips of thermoelectric material deposited on a refractory substrate and covered by a protective layer of refractory material.

It is important in metal pouring to control the temperature of the molten metal as closely as possible to an optimum temperature. Failure to achieve temperature control can result in inferior molded products because of variations in the alloying constituents of the metal being cast. Many attempts have been made to perfect a system for measuring the temperature of molten metal within a furnace with a notable lack of success. The solution to this problem is made very difficult because of the extremely hostile environment within the furnace. Not only are the temperatures very high, but the surface of the molten material is usually covered with oxides of the metal and oxides of impurities, which are very corrosive. In areas of the furnace where stirring occurs, the metal itself is highly abrasive. Only very special refractory materials are suitable for direct contact with the molten material. As a result, various devices have been proposed for measuring these temperatures without directly contacting the molten metal, without notable acceptance by the industry. The thermocouple has also been immersed directly into the molten material by various means.

A typical thermocouple device for the thermoelectric measurement of temperature utilizes two wires of different materials which are welded together at one point. This point, called the hot junction, is placed as near as possible to that which is to be monitored. The other two wire ends, or cold junction, are connected to a measuring instrument. In such a closed circuit a current will flow continuously when the two junctions are maintained at different temperatures.

Prior art thermocouple devices which have been installed in induction furnaces have been unsuccessful in continuously and accurately measuring the temperature of molten metals. A device which is installed in a furnace wall will not give continuously accurate readings unless it is located so close to the metals that the furnace lining is weakened and the protective sheath quickly destroyed. Likewise, commercially available immersion devices give fairly accurate intermittent measurements but are soon destroyed by corrosion and abrasion and have to be discarded. Such rapid deterioration of thermocouple assemblies indicates the importance of devising long-lived and inexpensive thermocouple assemblies.

In thermoelectric devices of the prior art used for measuring the temperatures of molten metals the assembly had to be protected from the heat source. Commonly, the two wires were inserted through a refractory or dielectric insulator, welded or joined together, and inserted into a protection tube ofa suitable material. A terminal assembly was then attached to the loose wire ends. In one commercially available device, a thermocouple is inserted in a U-shaped quartz tube which is immersed in the molten metal. Although the quartz is dissolved in the molten iron, the quartz tube delays contact of the metal with the thermocouple sufficiently long to give a temperature reading. These devices are therefore usually one-shot expendable devices and are therefore not suited for use as the sensor in a continuous control system.

SUMMARY OF THE INVENTION In the present invention a vessel for containing molten metal includes a thermocouple device utilizing thin strips of thermoelectric materials deposited on a refractory substrate to from a thermocouple junction and covered by a layer of protective refractory material.

More particularly, in the preferred embodiment of the present inven tion, there is provided a vessel for containing molten materials in which a refractory member forms a part of the refractory lining of the vessel and has a surface exposed to the molten materials for measurement of the temperature of the melt. Thin strips of two thermoelectric materials coat a portion of the refractory member and contact each other to form a thermocouple junction. A protective layer of refractory material, preferably the same material as that used for the refractory member, covers the thermocouple materials and the refractory member. Electrical leads are attached to the strips of thermo-electric materials and extend outside the protective covering to a measurement device.

The thermoelectric materials may be deposited upon the refractory member in very thin strips on the order of l X l0 inches by a sputtering or evaporative coating process. Likewise, the protective coating, which may be as thin as 4 X 10' inches, may also be deposited by sputtering or by a flame spray. The layers so deposited bond tightly with the refractory member, particularly if the refractory layer is the same material as the substrate.

The thermoelectric assembly is installed in the wall of an induction furnace or a molten metal ladle for the purpose of measuring the temperature of the molten material therein. The measured temperature may be used to automatically control the temperature of the metal. In an alternative embodiment, the thermocouple assembly may be used as a separate immersion device which is dipped into the molten materials after the materials are melted.

BRIEF DESCRIPTION OF THE DRAWINGS For a more complete understanding of the present invention and for further objects and advantages thereof reference may now be had to the following description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a cross-sectional side view of a channel-type induction furnace showing the thermocouple assembly installed;

FIG. 2 is an enlarged cross-sectional partial side view of the furnace in FIG. 1 showing the relationship of the thermocouple assembly to the molten metal during pouring;

FIGS. 3, 4, 5 and 6 are the side, top, front and back cross-sectional views, respectively, of one embodiment of the thermocouple assembly;

FIG. 7 is a cross-sectional side view sketching a drum-type induction furnace with thermocouple assembly installed;

FIG. 8 is a cross-sectional side view of a coreless-type induction furnace showing the installation of the thermocouple assembly;

FIG. 9 is an enlarged cross-sectional partial side view of the furnace in FIG. 8 showing an alternate installation of an easily removable thermocouple assembly P g;

FIG. 10 is a perspective view of the thermocouple assembly plug;

FIG. 11 is a cross-sectional side view sketch of a channel-type induction furnace using an alternative embodiment of the invention in the form of a thermocouple immersion device;

FIGS. 12 and 13 are cross-sectional side and front views, respectively, of the thermocouple immersion device embodiment.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to FIG. 1, apparatus in accordance with the present invention is indicated generally by the reference numeral 8. The apparatus 8 includes a channel-type induction furnace 10. Thermocouple device 12 is shown as one of the bricks in the refractory wall lining 14 of the induction furnace 10. Channel 16 leads to a heating core 17 where heat is supplied by induction means for melting materials in furnace 10. The thermocouple device 12 is shown installed in wall 14 below the minimum level 18 of melt 22 so as to avoid the corrosive conditions that exist at the interface between the surface of the molten materials 22 and the surrounding atmosphere. A thermocouple junction is positioned within thermocouple device 12 near the surface 20 which is exposed to molten materials 22.

Electrical leads 24 are attached to thermocouple device 12 and extend through the outer refractory wall 26 of furnace 10 to a thermohead 28. The leads 24 are then attached to a suitable control circuit 29 which may provide a reading of the temperature and also automatically control the power in the core 17.

In FIG. 2, a part of induction furnace 10 is shown in a tilted position, indicating the position of thermocouple device 12 in relation to melt 22 during pouring. As shown, device 12 is installed in the wall 14 such that the exposed surface 20 of thermocouple device 12 remains covered by the melt 22 during the pouring operation.

Having shown the thermocouple device 12 in one type of installation, description will now be made in FIGS. 3-6 of the construction of the thermocouple assembly. As shown in FIG. 3, a substrate of refractory material 30 has connecting leads 32 which are fixed in grooves in the substrate and have terminal strips 34, best seen in FIG. 4.

Very thin strips of thermoelectric material 36 partially coat one side and end of the substrate 30. A second thermoelectric material 38 partially coats the opposite side of substrate 30 and that portion of the first thermoelectric material 36 on the end of substrate 30, thus forming a thermocouple junction 40. The

thermoelectric materials

36 and 38 and the exposed portion of substrate 30 are covered with a coating of refractory material 42 which is the same or similar to the material of substrate 30.

For clarity, the strips of first and second

thermoelectric materials

36 and 38, as shown in the drawings, are considerably exaggerated in thickness. In practice, the thermoelectric materials are preferably deposited in strips as thin as I X 10' inches on substrate 30 by a sputtering or evaporative coating process. Refractory coating 42, which is preferably on the order of 4 X [0' inches thick, may also be deposited by a conventional sputtering or flame spray process, hereafter described in greater detail.

FIG. 4 is a top view of the thermocouple assembly showing five strips of second thermoelectric material 38 deposited on substrate 30. The strips 38 are commonly connected by a terminal strip 34 to an electrical lead 32. The strips of the first thermoelectric material 36 are likewise commonly connected by a terminal strip 34 to an electrical lead 32 as best shown in FIG. 6.

A number of pairs of

strips

36 and 38 may be deposited on substrate 30. Preferably, a considerable portion of substrate 30 would remain exposed for direct bonding to the refractory coating 42, to minimize the possibility of a heat fracture in the thermocouple assembly 12.

In FIG. 5, the thermocouple junction 40, formed by the overlapping of

strips

36 and 38, is shown deposited in a lattice-type configuration on substrate 30. Such a configuration allows the protective layer of refractory 42 to bond directly to substrate 30 over a considerable portion of the surface area. This design promotes bonding and prevents separation of the

thermoelectric strips

36 and 38 from the refractory layer 42 or the substrate 30 due to differences in coefficients of thermoexpansion. When platinum and platinum (9O percent)-rhodium (10 percent) are used as the thermoelectric materials, the rhodium tends to diffuse into the pure platinum. The lattice-type design extends the usable life of the assembly by creating relatively long diffusion paths for the rhodium. FIG. 6 shows the common connections of the

thermoelectric strips

36 and 38 by terminal strips 34.

In FIG. 7, the thermoelectric assembly 12 is shown installed in a drum-type induction furnace 50. As in the channel induction furnace 10, the thermoelectric device 12 is installed in the refractory lining 52 of furnace 50. Lining 52 is not shown in detail in FIG. 7 but may be composed of other bricks of the same size as thermoelectric assembly 12 or some other similar refractory material. As in the channel furnace 10, it is advantageous that the thermoelectric device 12 be installed in drum furnace 50 in a position to remain below the minimum level (not shown) of melt 54 as much as possible. Electrical leads 56 are connected to device 12 and extend through outer refractory layer 58 to thermohead 59.

FIG. 8 shows a smaller coreless-type induction furnace 60. In furnace 60, the inner lining 62 is composed of a tamped or rammed refractory such as aluminum oxide (M 0 and forms the inner walls and bottom of the furnace 60. An inner refractory brick layer 64 is laid below the rammed lining 62 forming the inner bottom of furnace 60. An outer refractory layer 70, also composed of brick, lies below inner layer 64 and directly above the furnace bottom plate 74.

The coreless-type induction furnace 60 is often emptied of all molten materials 66. The thermocouple assembly 12 is thus located in the base of furnace 60 so that the assembly 12 is covered by the melt 66 as much as possible, to reduce corrosive activity. The thermocouple device 12 is embedded in the refractory lining 64 and projects through the inner lining 62 to expose the thermocouple junction face to melt 66.

82 is secured against furnace bottom plate 74 by bolts 84 thus securing plug 80 in position in the bottom wall of furnace 60. Plug 80 may be easily removed and a new plug installed whenever corrosive activity deteriorates the thermocouple assembly 12 to the point where the thermocouple junction is no longer operative. Electrical leads 88 extend through outer lining 70 to a thermohead 89.

FIG. 10 shows a perspective view of the replaceable thermocouple plug 80. As shown, the thermocouple assembly 12 is mounted on an oversized brick 86 which in turn is mounted on face plate 82. Electrical leads 88 follow the edge of oversized brick 86 and couple to the thermohead 89 shown in FIG. 9.

Another alternate embodiment of the invention is shown in FIG. 11, in which an immersion device 90 is lowered into a melt 92 of a channel-type induction furnace 10 for measurement of the melt temperature. The immersion device 90 is lowered by a rod 94 or some other means through an opening 93 in the top of induction furnace 10 or through the open top of furnace 10. Electrical leads extend inside rod 94 to connect to immersion device 90.

One form of construction of the immersion device 90 is shown in FIGS. 12 and 13. A strip of a first thermoelectric material 94 is deposited on a substrate of refractory material 98. The strip of material 94 par tially covers a part of one side and one end of substrate 98. A strip of a second thermoelectric material 96 partially coats the opposite side of substrate 98 and overlaps the end portion of strip 94 to form a thermocouple junction 99. Electrical leads 97 are attached to strips 94 and 96 and are connected to a suitable measuring device (not shown).

Thus, the construction of immersion device 90 is similar to that shown for the thermocouple assembly 12 in FIGS. 3-6 except that only one pair of thermoelectric strips is used for immersion device 90. A protective layer of refractory material 95 also covers the thermoelectric strips 94 and 96 and the exposed substrate 98. Although only one set of strips is shown, several strips may be used in constructing the immersion device 90 as in the thermocouple assembly 12.

Another alternative embodiment of the replaceable thermocouple plug of FIGS. 9 and 10 utilizes a multiple junction thermocoupled assembly in which thermocouple junctions are sequentially sputtered, one upon another, and separated by a protective coating of refractory material. As the surface of the protective refractory material is exposed to both the melt and the atmosphere, it eventually erodes, and the first thermocouple junction is destroyed. The next junction located behind another protective layer of refractory material is then manually or automatically switched on for monitoring the temperature. This procedure is continued until all junctions are destroyed, at which time the multiple junction plug is replaced.

In the sputtering process, the coatings are preferably applied in a chamber which has been evacuated to a high level of vacuum, usually in the range of 10 Torr. The chamber is then backfilled to a partial pressure of approximately 6 to 20 X 10' Torr with a gas, preferably an inert gas such as Argon. A refractory substrate member, with electrical leads and terminal strips attached, is placed in the vacuum chamber.

As shown in FIG. 3, the thermoelectric strips 36 are sputtered on a side and an end of substrate 30. Strips 38 are then sputtered on the opposite side of substrate 30. Strips 38 are also deposited over the portions of strips 36 coating the end of substrate 30, thus forming a thermocouple junction. Finally, a protective coating 42 of refractory material, similar to or the same as the material of substrate 30, is sputtered over the

thermoelectric strips

36 and 38 and the exposed portion of substrate 30.

The sputtering process is performed using conventional sputtering apparatus and procedures, such as those described in an article entitled, Sputtering: Electronics, Razor Blades, Then What?" Product Finishing, March, 1970. The coatings are applied slowly, molecule by molecule, resulting in a very high bonding strength. For all practical purposes, the sputtered coatings become part of the substrate material.

The thermoelectric materials to be used are often expensive metals such as platinum or rhodium. The sputtering process greatly reduces the cost of production by depositing very thin films of the expensive metals. Furthermore, if the thermocouple junction and the thermoelectric strips are eventually destroyed the remaining cavities are too thin to provide a passage for the flow of molten metal. Although sputtering is preferred for depositing the thin strips of thermoelectric materials, another process which will result in the depositing of strips as thin or thinner than those deposited by sputtering may be used without departing from the broader aspects of the invention.

Sputtering is also preferred for coating the thermocouple assembly with an initial protective refractory layer, because of the excellent bonding resulting from the sputtering process. However, it is sometimes desirable to build up a heavier protective layer than is practical using sputtering. For example, a protective coating of 0.030 inches may be required. Since this would require several days using sputtering apparatus presently available, the protective refractory layer may be thickened using a flame or plasma gun or other suitable device.

Although the thermocouple assembly has been shown installed in induction furnaces, the assembly may be used in other vessels or apparatus for temperature measurement without departing from the scope of the invention. For example, the thermocouple assembly may be installed in a ladle for temperature measurement of the hot or molten materials therein. The thermocouple device may also be employed in other types of heating devices where temperature measurement is desired.

From the foregoing the advantages of the present invention are readily apparent. Using the invention, it is possible to obtain accurate and continuous temperature measurements of molten materials. The thinness of the thermoelectric strips provides for an extremely rapid response to changes in temperature. The protective layer of refractory material is also sufficiently thin so as not to significantly affect response time.

Bonding the strips to a strong substrate by sputtering results in the tensile strength of the conducting strips being essentially that of the substrate. Employing a refractory material similar to the substrate for the protective layer of the device gives strong bonding and resistance to heat fracturing of the thermocouple assembly.

The thinness of the thermoelectric strips significantly reduces the amount of conductive material needed and thus reduces production costs. Furthermore, if the strips are destroyed, the molten materials cannot leak through the furnace walls.

Having described the invention in connection with certain specific embodiments thereof, it is to be understood that further modifications may now suggest themselves to those skilled in the art. It is intended to cover such modifications as fall in the scope of the appended claims.

What is claimed is:

1. The combination comprising:

a system for heating molten metals including a vessel having a refractory lining for containing molten metal,

a substantially solid refractory substrate having one face forming a portion of the interior surface of the refractory lining,

a thin layer of a first thermo-electric material deposited on and bonded to an exterior surface of the substrate,

a thin layer of the second thermoelectric material deposited on and bonded to a substantial portion of the layer of the first thermoelectric material to form a thermocouple,

electrical conductor means extending from each of the thin layers of the first and second thermoelectric materials along a surface of the refractory substrate to a point remote from the face forming a portion of the interior surface of the refractory lining, and

a relatively thin layer of refractorymaterial deposited on and covering the exposed surfaces of the first and second thermoelectric materials and the electrical conductors and at least a portion of the substrate, the refractory material being bonded to at least a portion of the substrate as a result of the deposition process.

2. The combination of claim 1 wherein:

at least a portion of the refractory lining is formed by a plurality of refractory bricks, and wherein the refractory substrate is placed between a plurality of the bricks with one face of the substrate being substantially planar and substantially coplanar with the interior surface of the vessel formed by the refractory bricks.

3. The combination of claim 1 wherein the thermocouple is formed on the face of the substrate that is substantially coplanar with the interior surface of the refractory lining.

4. The combination of claim 1 wherein the thin layer of refractory material is substantially the same material as the refractory substrate.

5. The combination of claim 4 wherein the thermoelectric materials are deposited in a series of thin strips forming a lattice network.

Claims

1. The combination comprising: a system for heating molten metals including a vessel having a refractory lining for containing molten metal, a substantially solid refractory substrate having one face forming a portion of the interior surface of the refractory lining, a thin layer of a first thermo-electric material deposited on and bonded to an exterior surface of the substrate, a thin layer of the second thermoelectric material deposited on and bonded to a substantial portion of the layer of the first thermoelectric material to form a thermocouple, electrical conductor means extending from each of the thin layers of the first and second thermoelectric materials along a surface of the refractory substrate to a point remote from the face forming a portion of the interior surface of the refractory lining, and a relatively thin layer of refractory material deposited on and covering the exposed surfaces of the first and second thermoelectric materials and the electrical conductors and at least a portion of the substrate, the refractory material being bonded to at least a portion of the substrate as a result of the deposition process.

2. The combination of claim 1 wherein: at least a portion of the refractory lining is formed by a plurality of refractory bricks, and wherein the refractory substrate is placed between a plurality of the bricks with one face of the substrate being substantially planar and substantially coplanar with the interior surface of the vessel formed by the refractory bricks.