Thermal measuring and testing – Temperature measurement – Of molten metal
Reexamination Certificate
2000-12-28
2001-08-28
Gutierrez, Diego (Department: 2859)
Thermal measuring and testing
Temperature measurement
Of molten metal
C374S179000, C136S234000
Reexamination Certificate
active
06280083
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
[0002] The present invention relates to a thermocouple lance with a protective sheath suitable for measuring relatively high temperatures in molten metal baths of steel or the like.
2. Description of the Prior Art
[0003] Conventionally used for measuring a high temperature of about 1700° C. encountered in a molten steel bath are thermocouples made from wires of Pt—Rh alloy relatively high in melting point and stable in the atmosphere, the wires being fitted in a tubular sheath of alumino silicate fibers. The prior thermocouples are currently expensive as well as low in life expectancy of repetitive use because they are apt to become not capable of accurate temperature measurement and thus have to be disposed after only one or two measurements in the molten steel bath.
[0004] A type of the prior sheathed thermocouples has W—Re alloy wires that are enveloped in the metallic protective tubular sheath made of, for example, stainless steel so as to be adapted for use at high temperatures. Any such type of thermocouples with stainless steel protective sheaths has been designed to be used in atmospheres of above 1000° C. This protective sheath is usually produced from a special heat resisting alloy of, for example, Inconel which is a nickel-base alloy including chrome and iron. Another type of the prior thermocouples is well known in which the Pt—Rh wires are encapsulated in a protective tube of cermet.
[0005] Disclosed in Japanese Patent Laid-Open No. 160200/1994 is a sheathed thermocouple assembly hermetically sealed with a sealing plug, which may be free from errors in its measured temperature resulting from a temperature gradient occurring in the sealing plug owing to transient variations in temperature. The sheathed thermocouple assembly includes therein an alumel-chromel thermocouple consisting of two wires of different metals, or alumel and chromel. The two wires are contained in the stainless steel sheath together with inorganic insulating material electrically insulated from each other and further hermetically sealed at the open end of the sheath by means of the plug. The plug is provided in a ceramic end plate thereof with two tubes, which are made of Kovar, or iron-nickel-cobalt alloy, and coated on the inner surfaces thereof with electrically insulating sleeves. The two wires extend outside the sheath through tubes with no direct contact with their associated tubes.
[0006] On the other hand, the thermal shock resistance of the cermet-made protective sheath is 1.5 times of that of a Si
3
N
4
-made protective sheath. Moreover, the Si
3
N
4
-made protective sheath, when directly immersed into the molten steel bath above 1700° C., may crack in a relatively short interval after immersion resulting in breaking of the sheath.
[0007] It is to be noted that the Pt—Rh thermocouple may not be used in an inert-gas atmosphere and its permissible temperature limit in an environmental atmosphere is at 1500° C.
[0008] In contrast, the W—Re thermocouple may be used in any of inert-gas atmosphere and environmental atmosphere and its permissible temperature limit is at 400° C. in environmental atmosphere while at 2300° C. in inert-gas atmosphere.
[0009] The Pt—Rh thermocouple of platinum-rhodium alloy wires usually has a thermo-electromotive force that is {fraction (1/15)} times of that of the alumel chromel thermocouple while {fraction (1/7)} times of that of the W—Re thermocouple. Hence, the Pt—Rh thermocouple has a shortcoming in that it is inferior in accuracy as well as rapid response of the temperature measurement when compared with the other types of thermocouples.
[0010] In order to measure the temperature of the molten bath at the site of the furnaces by means of the prior thermocouples described above, the operator has consequently been compelled to stay near the furnaces for about eight seconds until the measured temperature becomes stable.
[0011] The prior thermocouples have a further disadvantage in which they are apt to be adhered to by molten metals so as to become deteriorated in rapid response of temperature measurement. This causes a collateral problem in which a troublesome process is inevitably required to remove iron or steel adhering to the Pt—Rh alloy wires and protective sheath of the thermocouple assembly. Another problem faced in using such thermocouples is the need to frequently replace the thermocouples because they are apt to become not capable of taking accurate temperature measurements after one or two uses in the molten steel bath. It is to be understood that the W—Re alloy wires for the thermocouples tend to be subjected to oxidation in environmental atmosphere so that they are not permitted to be used for measuring high temperatures in the molten iron for casting. The outer protective sheath is also disadvantageously apt to be adhered to by molten metals.
SUMMARY OF THE INVENTION
[0012] A primary object of the present invention is to overcome the shortcomings in the prior art as having been described above, and to improve the rapid response of the temperature measurement and the durability of the thermo-couple. The present invention has for its object to provide a thermocouple lance for measuring high temperatures in molten metal baths, which comprises a thermocouple consisting of tungsten-rhenium alloy wires, and a protective sheath of a layered structure consisting of cermet layers, having a matrix of molybdenum which does not easily react with molten metal, and other layers high in thermal shock resistance. This makes it possible to provide the sufficient thermal shock resistance for the protective sheath and also to improve the life expectancy of repetitive measuring services.
[0013] Another object of the present invention is to provide a thermocouple lance for measuring high temperatures in molten metal baths, which has a protective sheath of layered structure, each layer of which includes therein filaments of heat resisting property, the filaments being arranged in the circumferential direction of the protective sheath, or in a direction along which may occur a thermal stress and a tensile stress, to thereby provide the sufficient thermal shock resistance for the protective sheath and also to provide the life expectancy of repetitive measuring services.
[0014] A further object of the present invention is to provide a thermocouple lance for measuring temperatures, in molten metals, comprising a protective sheath with one closed end and one opened end wherein a filler and a pair of temperature-measuring alloy wires are contained, there being only one pair of wires, the pair being composed of two wires of different alloys joined with each other at their confronting ends to form a junction where the temperature is to be measured, and the protective sheath being formed in a multi-layered structure of layers different in composition which are laid concentrically one on top of another in alternate layers.
[0015] As the temperature-measuring alloy wires are made of tungsten rhenium (W—Re) wires having a melting point of above 2300° C., the thermocouple lance having such wires kept hermetically from the atmosphere is endurable in heat resistance to the temperature in the molten steel bath for the steel making.
[0016] Another object of the present invention is to provide a thermocouple lance for measuring temperatures in molten metals, wherein the layered structure of the protective sheath is made from the layers of a molybdenum matrix and other layers of at least one material selected from the class consisting of C, MgO, CaO, Al
2
O
3
, and ZrO
2
, the filler is of porous heat-resisting ceramics, and the protective sheath is hermetically closed at its open end by means of a sealing plug composed of dense heat-resisting substance and glass.
[0017] According to the present invention, the protective sheath has the multi-layered structure made of carbon, ma
Kita Hideki
Suzuki Takayuki
Browdy & Neimark
Gutierrez Diego
Isuzu Ceramics Research Institute Co. Ltd.
Pruchnic Jr. Stanley J.
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