JPS624354B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a thermocouple protection tube, and particularly to a thermocouple protection tube that has excellent corrosion resistance against molten metals such as aluminum, mainly composed of silicon nitride, and has excellent thermal shock resistance.

It is extremely important to detect and control the temperature of the molten material in the casting and smelting of various metals.
Thermocouples such as platinum rhodium and chromel-alumel are widely used.

When measuring the temperature of molten metal using such a thermocouple, it is necessary to use a thermocouple protection tube (hereinafter referred to as protection tube) to prevent damage due to the reaction between the thermocouple and the molten metal. , currently heat-resistant steel,
Metals such as cast iron, refractories such as carborundum, and heat-resistant porcelain such as alumina and mullite are used as materials for protective tubes. The protective tube is made of a dense material that can withstand the thermal shock that occurs when it is thrown into molten metal, has corrosion resistance against molten metal, does not allow molten metal to seep in, and is heat conductive so that there is no time delay in temperature measurement. It is necessary that the ratio is high.

No conventional protection tube has been able to satisfy all of these characteristics, and there has been a long-awaited need for a protection tube that has both corrosion resistance and thermal shock resistance.

In other words, metal protection tubes were severely attacked by molten metal, and in the case of aluminum in particular, the reaction was severe, and not only did the aluminum become contaminated, but the lifespan of the protection tube was extremely short, around 100 hours. However, in the case of carborundum protection tubes, there still existed problems such as reaction with aluminum melt and aluminum contamination. Furthermore, since the carborundum protection tube has many pores, molten metal sometimes penetrates and reacts with the thermocouple inside, making temperature measurement impossible. Furthermore, alumina and mullite-based heat-resistant porcelain protective tubes have excellent corrosion resistance against molten metal, but on the other hand, they have a drawback in terms of thermal shock resistance, and they can be used only two or three times. It will be damaged during temperature measurement. On the other hand, silicon nitride is a molten metal,
In particular, it is a material that is completely uncorroded by nonferrous molten metals such as aluminum, and has extremely excellent heat resistance, oxidation resistance, and thermal shock resistance, and attempts have been made to utilize it as a protective tube. However, since silicon nitride is a material with poor sinterability, silicon nitride molded bodies can only be produced at low cost by using the reaction sintering method or by adding a large amount of sintering accelerator and sintering at normal pressure. I couldn't get it.

That is, the reaction sintering method is a method in which metal silicon powder is preformed and reacted with nitrogen at 1000 to 1400°C to form a silicon nitride molded body.
It has 30% porosity, so molten metal can seep into it, there is a time delay during temperature measurement due to poor thermal conductivity, and moisture can be adsorbed inside, causing cracks when placed in molten metal. It often occurs,
It was not satisfactory as a protection tube. In addition, in the case of sintering with the addition of a sintering accelerator,
When trying to obtain a compact molded body with low porosity and good thermal conductivity using conventional sintering accelerators such as alumina, it is necessary to add a large amount of sintering accelerator, The inherent excellent corrosion resistance of silicon nitride has been significantly impaired by the sintering accelerator.

As a result of various studies on sintering accelerators for silicon nitride, the present inventor found that if magnesium oxide and lanthanum group oxides are added and sintered, a dense molded body can be obtained, and silicon nitride originally has The present invention was completed based on the knowledge that excellent corrosion resistance and thermal shock resistance are not impaired at all.

The purpose of the present invention is to provide a protective tube that does not have the drawbacks of the conventional ones, and uses a powder mixture containing silicon nitride as a main component and magnesium oxide and lanthanum group oxides as sintering accelerators. By preforming and pressureless sintering using a method, it has a small apparent porosity of 1% or less, is dense, has good heat conduction, and allows for quick temperature measurement, and does not absorb moisture or seep in molten metal. It is a protective tube with no scratches, excellent corrosion resistance, and thermal shock resistance.

The protective tube of the present invention contains 70 to 99% by weight of silicon nitride, and the balance is magnesium oxide and lanthanum group oxide.
Powder mixtures each containing 0.5% by weight or more are preformed by a slurry casting method, an isostatic press method, an extrusion method, etc.
Obtained by normal pressure sintering at 1800℃.

The reasons for these limitations will be explained below.

70% silicon nitride by weight in powder mixture formulation
If the amount is less, the sintering accelerator will be too large, which will significantly impair the corrosion resistance and thermal shock resistance of silicon nitride against molten substances, resulting in a shortened life span.
If it exceeds % by weight, the sintering accelerator is too small and a dense molded body with an apparent porosity of 1% or less cannot be obtained, resulting in poor thermal conductivity. If either the sintering accelerator, magnesium oxide or lanthanum group oxide, is less than 0.5% by weight, the thermal shock resistance will deteriorate rapidly. Note that the sintering accelerator does not necessarily have to be added as an oxide from the time of blending the raw materials, and carbonates, nitrates, etc. of these which change into oxides during the sintering process may be used. Even if more than one species is selected and used, the effect of promoting sintering remains the same.

The sintering atmosphere should be a non-oxidizing atmosphere such as nitrogen or argon to prevent oxidation of silicon nitride, and the sintering temperature should be 1600℃ or higher to ensure rapid sintering progress. to prevent decomposition
Must be below 1800℃.

Hereinafter, the present invention will be explained in detail according to Examples and Comparative Examples.

Example 1 A powder mixture consisting of 15% by weight of magnesium oxide, 15% by weight of cerium oxide, and 70% by weight of silicon nitride
After adding 3% by weight of CMC and an appropriate amount of water and granulating it, it was filled into a rubber mold of the desired shape and preformed using an isostatic press method. The preform was heated and sintered at 1600°C in a nitrogen atmosphere for 2 hours to give an outer diameter of 15 mm.
A protective tube with an inner diameter of 10 mm and a length of 80 cm was obtained. This sintered body is dense with an apparent porosity of 0.1%, making it difficult to crack or
was not recognized at all.

This protection tube contains platinum-platinum rhodium (PR-13).
A thermocouple was inserted to measure the temperature of molten iron at 1400℃. The protective tube, which was initially kept at room temperature, was placed in molten iron at 1400°C, immersed for 5 minutes to measure the temperature, and then allowed to cool.The cycle was repeated, and it was found that the conventional alumina protective tube In contrast, the protective tube of the present invention showed no abnormalities even after 20 repetitions, demonstrating that it has excellent thermal shock resistance. In addition, the time delay from immersion in molten iron to reading the true temperature was only 20 seconds, fully demonstrating the effectiveness of its dense structure and good thermal conductivity.

Example 2 A slurry was made by adding 1.5% by weight of polyvinyl alcohol and an appropriate amount of water to a powder mixture consisting of 5% by weight of magnesium oxide, 0.5% by weight of praseodymium oxide, and 94.5% by weight of silicon nitride, and the slurry was molded into a plaster mold of the desired shape. After casting and drying, it was heated and sintered at 1800℃ for 30 minutes in an argon atmosphere to form a mold with an outer diameter of 15 mm and an inner diameter.
A protective tube with a length of 10 mm and a length of 1 m was obtained. The apparent porosity of this sintered body was 0.4%, which was dense, and no cracks or pores were observed at all.

Example 3 Magnesium oxide 0.5% by weight, cerium oxide 0.5%
3% by weight of methylcellulose and an appropriate amount of water were added to a powder mixture consisting of 99% by weight of silicon nitride, thoroughly kneaded together, and then preformed into the desired shape using an extrusion molding machine. This was heated and sintered at 1,800℃ for 2 hours in a mixed atmosphere of nitrogen and hydrogen to obtain an outer diameter of 15 mm.
A protective tube with an inner diameter of 10 mm and a length of 1 m was obtained. This sintered body was dense with an apparent porosity of 1.0%, and no cracks or pores were observed.

Comparative Example 1 A powder mixture containing 20% by weight of magnesium oxide, 20% by weight of cerium oxide, and 60% by weight of silicon nitride was preformed in the same manner as in Example 1 using a powder mixture having a blending amount outside the scope of the present invention. 30 at 1600℃ in atmosphere
After heating and sintering for minutes, the outer diameter is 15 mm, the inner diameter is 10 mm, and the length is 1 m.
A protective tube was obtained. This sintered body has an apparent porosity of 0.2%
It was hot.

Comparative example 2 Magnesium oxide 0.2% by weight, cerium oxide 0.3
A powder mixture having a blending amount of 99.5% by weight of silicon nitride, which is outside the range of the present invention, was preformed in the same manner as in Example 2, heated and sintered at 1800°C for 2 hours in an argon atmosphere, and then extruded. A protective tube with a diameter of 15 mm, an inner diameter of 10 mm, and a length of 1 m was obtained. This sintered body has an apparent porosity of
It was a large 23%.

A platinum-platinum-rhodium (PR-13) thermocouple was inserted into a cast iron protection tube of the same shape as the protection tubes obtained in Examples 2 and 3 and Comparative Examples 1 and 2, and continuous temperature measurement of molten aluminum at 800°C was carried out. I went there. While the cast iron protection tube melted away in 96 hours, making temperature measurement impossible, the protection tubes of Examples 2 and 3 and Comparative Example 1 showed no signs of corrosion by aluminum.
However, after 200 hours, the surface of the protective tube of Comparative Example 1 began to be corroded, and it became impossible to measure the temperature after 300 hours. In addition, the protection tube of Comparative Example 2 had a large porosity of 23%, so aluminum seeped into it, and after 50 hours, the soaked aluminum reacted with the thermocouple, making temperature measurement impossible. However, the protection tubes of Examples 2 and 3 have a resistance of 3000
Even after hours, there is no surface erosion or aluminum seepage.
Furthermore, the thermocouple protection tube of the present invention could be used for more than 1000 hours, demonstrating that it has extremely excellent corrosion resistance.

Claims (1)

[Claims] 1. Preform a powder mixture containing 70 to 90% by weight of silicon nitride and the balance of magnesium oxide and lanthanum group oxides each of 0.5% by weight or more, and heat it at a temperature of 1600 to 1800°C in a non-oxidizing atmosphere. A silicon nitride thermocouple protection tube with an apparent porosity of 1% or less, which is made by pressure sintering.