This invention relates to a thermocouple protecting tube which can withstand the temperature measuring operation within molten metal for many hours, thereby providing for the continuous measuring of the temperature of molten metal with high precision.
With the increase of continuous casting facilities in steel making plants, the continuous measuring of the temperature of molten metal has become a matter of vital importance.
The purposes for continuous temperature measurement of molten steel may vary depending on the particular steel making plant; however, in general, the purposes can be summarized as involving quality control and the lowering of production costs.
For quality control, continuous measuring is effective in the production of metal having a uniform quality and for the prevention of segregation within the metal. Furthermore, continuous measuring facilitates the refining operation since the temperature of molten metal can be continuously measured.
With a view toward lowering of production costs, continuous measuring enables the complete computarization of the control of the refining operation and also improves the drawing speed of the continuous casting operation.
Conventionally, for measuring the temperature of molten steel, ceramic-made thermocouple protecting tubes, especially alumina protecting tubes, have been predominantly used.
In general, ceramics (sintered ceramics) used for the above purpose are easily harmed by thermal; in fact, a sharp rise or fall of temperature causes the rupture thereof.
However, the above ceramics have a rather high heat resistance, and they can withstand a temperature of up to 2000.degree. C.
This heat resistance employed hereunder implies erosion resistance against molten metal, molten slag, molten glass or other chemicals of high temperature, loading resistance and fluidity resistance at a high temperature.
The basic concept of this invention involves the use of such ceramics, which have high heat resistance and low thermal shock resistance for producing a thermocouple protecting tube, which can be used for the continuous measurement of the temperature of a molten body for a considerable length of time under conditions of immersion.
The applicant of this invention has previously filed several patent applications such as U.S. Pat. Application Ser. No. 715,023, now U.S. Pat. No. 4,060,095, related to thermocouple protecting tubes, wherein the following provisions for protecting the ceramic thermocouple protecting tube from rupture caused by thermal shock have been disclosed, namely:
(1) coating a refractory powder onto the outer surface of the ceramic tube, PA1 (2) providing a silica glass tube concentrically over the ceramic tube. PA1 (a) an inner ceramic tube having one end closed, and containing a thermocouple element therein, PA1 (b) an outer silica glass tube disposed concentrically over the inner ceramic tube and over the one closed end of the inner tube forming an annular space therebetween, and PA1 (c) an intermediate heat-insulating layer within the annular space.
Although, the above ceramic tubes are effective when their diameter is small, it has been found that they tend to rupture when their diameter becomes large.
For example, when an alumina tube containing Cr.sub.2 O.sub.3 in an amount of 5% by weight and having an outer diameter of 5 mm, an inner diameter of 5 mm and a wall thickness of 2.5 mm (the tube of this composition has been specifically developed for the purpose of this invention and accordingly forms a part of this invention) was provided with either conventional provision 1 or 2 and subsequently immersed in molten steel at about 1500.degree. C., the mean lifetime of this thermocouple protecting tube was about 150 to 180 minutes at maximum, due to the erosion caused by molten slag floating on the molten steel.
To prolong the lifetime of the thermocouple and thus facilitate the more complete operation thereof, the tube must be more resistant to erosion by molten slag, and this can be achieved only by making the wall of the tube thicker.
However, the tube must accommodate the plutinum thermocouple element therein, and therefore the inner diameter of the tube must be at least more than 5 to 6 mm.
Accordingly, the tube must have an increased outer diameter to provide for a thicker tube wall, e.g. from the conventional 10 mm to 12 or 14 mm.
However, as described above, it has been found that in proportion to the increase of the outer diameter, the thermal shock resistance of the tube decreases by the square value of the outer diameter, and thus provisions 1 and 2 cannot protect tubes of increased outer diameter from rupturing.
To be more specific, the maximum outer diameters that are available for provisions 1 and 2 are 10 mm and 12 mm respectively. When the outer diameter of the tubes exceeds the above values, the tubes rupture, even if they are preheated before they are immersed in a molten body.
In the case of provision 1, when a tube having an outer diameter greater than the allowable diameter was immersed in molten steel at above 1500.degree. C., the coating layer of the tube peeled off or the temperature of the tube sharply rose due to heat transfer through the coating layer.
In the case of provision 2, when a tube of excessive diameter was immersed in molten steel, the silica glass tube which is disposed over the ceramic tube was softened by the heat of the molten steel and subsequently adhered to the ceramic tube due to the buoyancy of the molten steel, whereby the temperature of the ceramic thermocouple protecting tube sharply increased. That is, the air layer (low heat conducting layer) disposed between the ceramic tube and the outer silica glass tube, which reduced the rate of heat transfer within a limited range, was not sufficiently effective, thereby the tube ruptured.
Furthermore, since the silica glass was transparent or semi-transparent, the glass tended to transfer the radiation heat readily.
It is believed that the temperature of the ceramic thermocouple protecting tube rose sharply when it was provided with either of two provisions due to the above two unfavorable phenomena discussed above and accordingly, a tube of excessive diameter could not withstand or absorb thermal shock and ruptured.