US3647558A

US3647558A - Protected thermocouple and protection tube

Info

Publication number: US3647558A
Application number: US648821A
Authority: US
Inventors: Carl H Mcmurtry
Original assignee: Carborundum Co
Current assignee: Stemcor Corp
Priority date: 1967-06-26
Filing date: 1967-06-26
Publication date: 1972-03-07
Anticipated expiration: 1989-03-07
Also published as: GB1184306A; DE1773710C3; FR1569518A; SE352737B; DE1773710A1; BE717107A; DE1773710B2

Abstract

A COMPOSITE PROTECTION TUBE FOR PROTECTING A TEMPERATURE MEASURING MEANS IN A MOLTEN METAL BATH COMPRISING AN INNER REFRACTORY TUBE, A CONSUMBLE OUTER METALLIC CASING, AND AN INTERMEDIATE LAYER OF A CARBON IMPREGNATED REFRACTORY GRAIN TO MINIMIZE THERMAL SHOCK AND RESIST THE CORROSIVE ATTACK OF MOLTEN METALS AND THEIR ASSOCIATED SLAG.

Description

PROTECTED THERMOCOUPLE AND PROTECTION TUBE Filed June 26, 1967 CARL H. MCMURTRY United States Patent t fine 3,647,558 PROTECTED THERMOCOUPLE AND PROTECTION TUBE Carl H. McMurtry, Lewiston, N.Y., assignor to The Carborundum Company, Niagara Falls, N .Y. Filed June 26, 1967, Ser. No. 648,821 Int. Cl. H01r 1/04 U.S. Cl. 136-234 11 Claims ABSTRACT F THE DISCLOSURE A composite protection tube for protecting a temperature measuring means in a molten metal bath comprising an inner refractory tube, a consumable outer metallic casing, and an intermediate layer of a carbon impregnated refractory grain to minimize thermal shock and resist the corrosive attack of molten metals and their associated slag.

This invention relates to a composite protection tube and more particularly to a composite protection tube for a temperature measuring means for enabling utility of the same at elevated temperatures under adverse conditions.

Although the present invention may be applicable in various metal producing environments, it will be convenient to refer specifically to its use in association with the steel producing art. In the steelmakin-g art, an oxygen blowing process, of relative contemporary origin, is becoming increasingly popular and is rapidly replacing the open hearth steelmaking process. In the rapid oxygen steelmaking process, oxygen of high purity is blown onto the surface of a molten steel bath to produce a product comparable to, or better than, that obtained in an open hearth furnace. Moreover, the time required to rene the melt may be as little as thirty minutes as contrasted to ten hours with the open hearth process.

One of the critical problems encountered in the rapid steel-making process is in the measurement of the bath temperature in order to ascertain when the proper tapping temperature, which is in the neighborhood of 2900-3000 F., is reached. Because of the relatively short time involved as contrasted to the open hearth process, the techniques employed in the open hearth process for measuring the temperature of the molten bath are unsatisfactory for the oxygen process. One temperature measuring device currently in use comprises a thermocouple encased in a sheathing which is lowered into the molten bath. The temperature is quickly measured before the device is consumed within the molten bath. Since the life of these temperature measuring devices is approximately tive seconds, any temperature indication below or above the tapping temperature requires that the molten bath be retreated accordingly and repeated temperature measurements must be taken during the heat. It is readily apparent that eiliciency and production are seriously impaired resulting in excessive costs.

The general purpose of the present invention, as hereinafter described, provides a solution to the above problem by utilizing a composite protection tube for a temperature measuring device which may be immersed in a molten metal bath for relatively long periods of time to enable the temperature measuring device to continuously measure the temperature of said bath.

Accordingly, it is an object of the present invention to provide a new and improved composite protection tube.

It is another object of the present invention to provide a new and improved composite protection tube for encasing a temperature measuring device.

3,647,558 Patented Mar. 7, 1972 It is a further object of the present invention to provide a new and improved means for protecting thermocouples for relatively long periods of time at very high temperatures and in many different environments.

It is still another object of the present invention to provide a new and improved composite protection tube 'which is durable, rugged, compact and simple in construction, requires a minimum amount of parts, and is reliable in operation.

It is still a further object of the present invention to provide a new and improved composite protection tube having an outer consumable tube or casing, an inner tube, and an intermediate layer of a carbon impregnated refractory material for resisting thermal shock and the corrosive attack of molten metals at high temperatures.

These and other objects of the present invention will become more apparent upon consideration of the following detailed description thereof when taken in conjunction with the following drawings, in which:

FIG. 1 is a longitudinal sectional 'View of the composite protection tube constructed in accordance with the principles of this invention illustrating a temperature measuring means mounted and encased therein;

FIG. 2 is an enlarged fragmentary longitudinal sectional view of the composite protection tube of FIG. 1 showing the tube before use; and

FIG. 3 is an enlarged fragmentary longitudinal sectional vieW of the composite protection tube of FIG. 1 illustrating the tube after it has been immersed in a molten bath of metal.

Referring now to the drawings in detail, it will be seen that a composite protection tube, generally designated 10, constructed in accordance with the principles of this invention comprises an elongated, impervious, refractory oxide inner tube 12, preferably made of alumina and having a closed end as at 13, encased lWithin an outer elongated metallic tube or casing 14, preferably composed of steel or cast iron. The annulus 16 defined by the exterior wall surface of the inner tube 12 and the interior wall surface or the casing 14 is lled with a slag resisting material 18, such as carbon impregnated magnesia grain, for example, which is known to have excellent resistance tcl) the corrosive attack of most metals and to high-lime s ags.

With reference to FIG. l, it will be seen that a temperature measuring device, such as a thermocouple, generally designated 20, is encased within protection tube 10. The thermocouple comprises a pair of

thermocouple lead wires

22 and 24 having a suitable insulated sheathing 26 disposed thereabout, said lead wires joined together at one end as by means of welding for example, to form a hot junction 28.

The protection tube 10 is closed at one end as indicated at 30 and is provided with a threaded portion 32 at the other end releasably secured to a coupling 34 having a threaded bore 36 at one end and a threaded counterbore 38 at the other end thereof for releasably securing one end of an externally threaded hollow member 40 thereto. Releasably secured to the other end of member 40 is a terminal assembly, generally designated 42, having a housing comprised of a metallic shroud 44 and a dielectric collar 46 suitably secured in axial abutting relationship to shroud 44 by means of screws 48. Extending axially from the face 50 of collar 46 are a pair of

terminals

52 and 54 each having a pair of screws S6 and 58, respectively. The

thermocouple lead wires

22 and 24 are connected to the screws 56 of

terminals

52 and 54, respectively. Screws 58 of

terminals

52 and 54 are adapted to accommodate electrical leads (not shown) connected to an indicating meter (not shown) in a conventional manner.

A composite protection tube constructed according to a preferred embodiment of the invention is made in the following manner:

An inner tube having a closed end is inserted in a vertically extending iron pipe having a threaded portion at one end. The open end of the inner tube protrudes beyond the threaded portion of the pipe, said inner tube being disposed coaxially in the pipe and maintained in position by means of a pig or the like.

A suitable granular refractory material, such as dry MgO grain, is poured into the annulus dened by the exterior wall surface of inner tube 12 and the interior wall surface of casing 14. The granular refractory material is vibrated as it is poured in order to obtain optimal density. Also, the provision of various size graded particles increases the density of the finished product. A suitable carbonizable binder, such as pitch for example, is heated to a liquid state and poured into the casing to infiltrate or impregnate the granular refractory grain. The resulting mixture is heated slowly to a temperature of 800 C. in

order to carboriize the binder and drive off the volatiles therefrom. To further increase the density of the mixture and to more completely fill the pores in and around the refractory particles, the above process is repeated with the exception that instead of pitch, the mixture is reimpregnated with a suitable resin, such as liquid furfuryl alcohol polymer for example. The process is repeated a third time to further increase density, completely lill the voids in the mixture, and increase the carbon yield. Not only does the carbon serve as an efficient bonding agent for the refractory particles, but also aids the refractory body in resisting the chemical attack of the slag present in the vessel during the metalmaking process. Although not necessary, it is desirable to subject the assembly to a vacuum treatment before each impregnation or reimpregnation step to facilitate the absorption of the impregnant by the refractory particles. If desired, the bonding agent can be mixed with the refractory grain prior to pouring.

In order to obtain the optimal bulk density of refractory mix, various mixtures of different sized MgO grain were prepared and poured under vibration into an iron pipe. In test mixes, it was established that MgO mixtures having various percentages of graded sizes ranging from -196 mesh to -6 mesh (U.S. Standard series) are acceptable in forming a refractory body having a satisfactory density. A typical grain size distribution used in this invention is set forth below:

Percent -l +16 mesh 60 -80 +100 mesh l0 -200 mesh The following specific example details suggested mixes and steps for optimum results and sets forth the preferred embodiment of the invention.

A composite protection turbe was constructed by inserting an alumina inner tube into a vertically extending iron pipe having a threaded portion at one end. The alumina tube was coaxially located relative to the pipe and the closed end of said tube was disposed inwardly from the end of the pipe remote from the threaded portion. A jig or the like may be employed to maintain the alumina tube in the desired position relative to the pipe. A size graded MgO mix, of about the following screen analysis, -lO +16 mesh, 10% -80 -l-lOO mesh and 30% -200 mesh, Was poured into the annulus between the alumina tube and pipe. The pipe and mixture were vibrated as the mix was poured in order to obtain the optimal bulk density of said mix. After the pipe had been filled to the top adjacent the threaded end portion thereof, the jig was removed, the pipe inverted and the remainder of the annulus lilled with the refractory grain. The assembly was placed in an autoclave and subjected to a vacuum in order to facilitate the absorption of an impregnant by the granular material. A carbonizable binder, such as a heated liquied bonding pitch, comprising approximately 11.5 percent by weight of the refractory material, was pressure poured into the annulus of the assembly to impregnate or infiltrate the MgO mix. The assembly was then heated slowly up to a temperature of 800 C. to carbonize and drive off the volatiles. At the completion of the carbonizing cycle the assembly was cooled and the impregnating and carbonizing cycles were repeated two more times with the exception that a liquid furfuryl alcohol polymer catalyzed with 5 percent maleic anhydride was utilized as the impregnant rather than pitch. The reimpregnating and carbonizing cycles strengthen the carbon bond between the refractory particles by completely filling the pores in and around the refractory particles.

Although it has been found that increased carbon results upon the reimpregnating and reheating cycles with the use of any carbonizable bonding agent in accordance with the present invention, it is preferred that such liquidified bonding agent comprise about l-20 percent by weight of the refractory material used. Although three impregnations are preferable, it should be realized that the principles of this invention envisage more or less than three impregnations, as desired. It should be appreciated by those skilled in the art that either of the two carbonizable bonding agents mentioned above may be substituted for the other in any of the three impregnating steps. Also, other impregnants, such as the phenol-formaldehyde types for example, may be substituted for those heretofore mentioned. Although the closed end of the inner tube is preferably disposed inwardly of the bottom end of the outer casing, the principles of this invention also contemplate mounting the closed end of the inner tube flush with the bottom end of the casing or protruding therebeyond, as desired. Also, the length and diameter of the protection tube may be varied dependent on the specific environment in which it is to be used and the economics of design.

Tests were conducted on composite protection tubes made in accordance with the above example to determine its resistance to thermal shock, corrosion and resistance to chemical attack from molten metal and its associated slag.

Under laboratory conditions, a composite protection tube was immersed into a molten bath of steel contained in an induction heated furnace. The portion of the steel pipe extending into the metal melted rapidly exposing the carbon impregnated MgO grain to provide protection at the slag steel surface. The tube remained in the bath continuously for a period of l hour during which time the temperature of the steel ranged from 3067-3l50 F. The tube withstood the erosion and thermal shock conditions of this test quite well. There was no indication of any fracture or cracking or breakage caused by thermal shock. A small eroded band 15 (see FIG. 3) was observed at the slag line of the protection tube but subsequent sectioning disclosed that no slag or steel had penetrated the inner alumina tube indicating that the useful life of the tube was not expended and could survive immersion for a longer period of time. The encased thermocouple, made of a platinum and platinum rhodium alloy, held up satisfactorily and could be used again.

Another test was conducted under actual operating conditions at a steel manufactures facility. In this test the protection tube was mounted in a trough which conveys molten iron from the tap hole of a blast furnace to a ladle car. The temperature of the molten iron flowing through the trough varied from 2720-2790 F. The iron pipe exposed to the molten metal rapidly melted eX- posing the carbon impregnated MgO grain to the flowing melt. The velocity of the iron in the trough was estimated to be l-Z feet per second. The protection tube and encased thermocouple remained in the molten iiow for a period of 52 minutes. After removal there was no evidence of erosion or corrosion on the tube exposed to the iron and slag and there were no fractures or cracks due to thermal shock. The pipe casing had melted away to a height of approximately 2 inches above the level of the flow and some carbon had been oxidized on the downstream side of the tube above the slag line resulting in the loss of some MgO grain. The thermocouple survived the immersion test and could be used again and the protection tube was serviceable for further use.

A third test was conducted in the research laboratory of a large steel manufacturer in connection with an oxygen blowing process for making steel. The protection tube was immersed 2 inches below the slag line in a bath of molten steel for the entire duration of a 30-minute blow, and temperatures were recorded continuously ranging from 2650 F. at the beginning to 3080 F. at the end. After removal, little erosion or corrosion was detected and no fissures or cracks were observed. The thermocouple was unaffected by this immersion test and could be used again. In this test the slag composition was basic with a CaO/SiOz ratio of approximately 2/1 and was similar to the slag which prevails under normal operating conditions. The rate of surface removal of the MgO layer of the tube at the slag line was 0.12 inch per hour and there was no surface removal of the MgO` layer on that portion of the tube exposed to the melt.

It should be realized that although magnesia grain is preferable as the protective layer, any suitable refractory oxide grain may be used such as zirconia, alumina, mullite, and dolomite by Way of example. In addition to oxides, other carbides, borides and nitrides may be used, such as zirconium carbide, zirconium diboride, zirconium carbonitride and aluminum nitride, each suitably impregnated with carbon in a manner similar to that described in connection with the refractory oxides, within the purview of the present invention. In describing the composite protection tube, reference has been repeatedly made to the use of an inner tube made of alumina. However, this invention contemplates the use of inner tubes that are made, for example, of zirconia, thoria, beryllia, mullite, an alumina-zirconia mixture, and other refractory materials resistant to the molten metal environment in which it is to be used. Likewise, the outer tube or casing need not be restricted to a cast iron composition but may be made of the same metal in which it is to be immersed, such as nickel, stainless steel, copper and aluminum by way of example.

The composite protection tube of this invention has utility in applications other than for shielding temperature measuring devices. For example, the tube may be hollow throughout its length and serve as a pressure pouring tube or a lance for directing oxygen therethrough in those metal producing operations that require the lance to be inserted through the slag line into the melt. Still other uses Will be readily apparent to those skilled in the art.

As a result of the present invention, a new and improved composite protection tube is provided for housing a temperature measuring device in an improved and more efficient manner. An expendable outer metallic tube or casing is employed for providing the mechanical strength required to pierce the semi-molten material at the slag line and to distribute heat evenly during insertion to minimize thermal shock. An inner refractory tube, resistant to the chemical attack of the molten metal is provided for encasing a temperature measuring device and an intermediate layer of a carbon impregnated refractory grain surrounds the inner refractory tube for resisting the corrosive attack of slag upon melting of the expendable outer casing. It is believed that the dominant factor contributing to the thermal shock resistance of this material is that there are no grain to grain bonds as would occur if the intermediate layer was sintered. Thus, a fracture occurring in one or more particles would terminate at the surface of the particle and would not be propagated throughout the remainder of the particles.

Preferred embodiments of the principles of this invention having been hereinabove described and illustrated, it is to be realized that modifications thereof can be made without departing from the broad spirit and scope of this invention as defined by the appended claims.

I claim:

1. A composite protection tube adapted to be immersed in a molten metal bath comprising: an elongated inner tube having an exterior wall surface; an elongated outer tube having an interior wall surface spaced radially from said exterior wall surface and defining an annulus therebetween; and a solid body filling said annulus, said solid body consisting of granular refractory material bonded together by carbon.

2. A composite protection tube as defined in claim 1 wherein said refractory solid body comprises granular refractory material selected from the group consisting of oxides, borides, carbides and nitrides bonded with carbon.

3. A composite protection tube as defined in claim 2 wherein said granular refractory material is selected from the group consisting of magnesia, zirconia, dolomite and mullite.

4. A composite protection tube as defined in claim 2 wherein said granular refractory material is magnesia.

5. A composite protection tube as defined in claim 3 wherein said inner tube is composed of a heat resistant refractory oxide.

6. A composite protection tube as defined in claim 5 wherein said refractory oxide is selected from the group consisting of alumina, zirconia, thoria and beryllia.

7. A composite protection tube as defined in claim 5 wherein said inner tube is composed of alumina.

8. A composite protection tube as defined in claim 6 wherein said inner tube is closed at one end and a thermocouple junction is located adjacent said closed end.

9. A composite protection tube as defined in claim 7 wherein said outer tube has one end adapted for immersion in molten metal and said closed end of said inner tube is located inwardly from said one end of said outer tube.

10. A composite protection tube as defined in claim Z in which said refractory material is magnesia and said inner tube is composed of alumina.

11. A composite protection tube as defined in claim 10 in which said outer tube has one end adapted for immersion in molten metal and said inner tube is closed at one end; said closed end of said inner tube being located inwardly from said one end of said outer tube; and a thermocouple junction located adjacent said closed end of said inner tube.

References Cited UNITED STATES PATENTS 1,773,825 8/1930 Simms 136-242 1,773,826 8/1930 Simms 13G-242 2,405,075 7/ 1946 Vollrath 136-242 2,676,195 4/1954 Hart 136-242 2,697,734 12/ 1954 Zvanut 136-242 LELAND A. SEBASTIAN, Primary Examiner S. I. LECHERT, l R., Assistant Examiner U.S. Cl. X.R.