JPS6214505B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to members such as immersion heater protection tubes and temperature measurement protection tubes used in furnaces for melting or holding low melting point metals such as Al and Pb. Conventionally, such members have been made of cast iron coated with a protective film, or fired bodies of silicon carbide, graphite, or the like. For example, a silicon carbide immersion heater protection tube has a heater inserted inside and is heated by immersing it in molten metal, which has much higher thermal efficiency than the external heating method that has been used for a long time. This is good, but on the other hand, because it is in direct contact with the molten metal,
It is easily corroded by the molten metal or the antioxidant floating on its surface, and is not necessarily satisfactory in terms of thermal spalling, so it cannot be expected to have a long life. The present invention provides an immersion heater protection tube that has an extremely long life and is easy to maintain by specifying a silicon carbide base material or applying a protective film to its surface.
It relates to temperature measuring protection tubes, working tools, and other parts used in such furnaces. Due to the manufacturing characteristics of silicon carbide molded bodies, free silicon or silicon oxide tends to remain. Since it is present in the raw material powder and easily generated during the manufacturing process, the amount of SiO ₂ +Si may reach as much as 15% in some cases. The present inventors brought a silicon carbide molded body into contact with molten aluminum, etc., and studied the corrosion process, etc.
It has been found that the presence of these free silicones or silicon oxides makes them susceptible to corrosion, peeling, reactions, etc. Therefore, if the molded product contains less SiO ₂ +Si,
Alternatively, when these components were removed from the molded product through purification treatment, etc., we examined their abundance and degree of corrosion, and found that products containing 5% by weight or less of SiO ₂ +Si have excellent corrosion resistance. was recognized. Silicon carbide is inherently difficult to wet with molten metal and has relatively good corrosion resistance, but when molded into a molded product, it is difficult to prevent other components from getting mixed in, and if this part is corroded, the molding It is thought to have an effect on the lifespan of the body itself. In order to further extend the service life, the present inventors succeeded in protecting the surface of such a silicon carbide molded body by growing silicon carbide in a vapor phase. However, when it is immersed in or extracted from molten metal, severe temperature changes may cause it to clutch and eventually fall off. For this reason, a heat insulating material was further coated to alleviate temperature changes on the surface. This heat insulating material is particularly required not only to have corrosion resistance, but also to take into account the difference in thermal expansion with the silicon carbide substrate, and it has been found that silicon nitride and boron nitride are particularly preferred. Examples of the present invention will be described below. Example 1 Recrystallized silicon carbide molded body with a wall thickness of 7 mm (SiC99.5
%, SiO ₂ +Si0.2%) silicon carbide molded body with silicon nitride bond (SiC64%,
A heater protection tube (Si ₃ N ₄ 27%, SiO ₂ +Si 8%) was immersed in molten aluminum to measure the degree of penetration of Al into the molded body. When immersed continuously at 700°C for 30 days and observing its cross section, it was found that the recrystallized silicon carbide material penetrated 1 mm from the surface, and the silicon nitride bonded silicon carbide material penetrated 7 mm from the surface. Example 2 In order to compare the degree of corrosion depending on the amount of Si+SiO ₂ , tests as shown in the table below were conducted.

[Table] In the above table, the penetration depth was measured under the same conditions as in Example 1. When a silicon carbide film was further formed on the surface of such a member with a small amount of SiO ₂ +Si by vapor phase growth, this permeation state could be further improved. This seems to be because the surface has an apparent porosity of 0, but it is not sufficient, and when the porosity is 2 mm or more, even though the silicon carbide is the same, it seems to be due to differences in the formation conditions, etc., but peeling can be seen. Become. Example 3 As shown in the table below, molten aluminum was protected under the conditions of Example 1 by changing the thickness of the silicon carbide film grown in vapor phase on the surface of only the SiO ₂ +Si0.2% recrystallized silicon carbide body. The number of days it took to reach the inner surface of the tube was compared.

[Table] Furthermore, when these members are used, cracks may occur due to considerable thermal shock, especially when immersed in molten metal. This can be alleviated by coating with a heat insulating material to prevent this, but in consideration of the difference in thermal expansion with the silicon carbide substrate and the corrosion resistance of the heat insulating material, it is most preferable to coat with silicon nitride and/or boron nitride. I found out. To coat this, it is sufficient to add an inorganic binder such as aluminum phosphate to powder of silicon nitride and/or boron nitride, form it into a slip, apply it to the substrate, and sinter it, but there are differences in the coating method. do not have. However, if the coating is thinner than 0.3 mm, the effect will be small, and if it is thicker than 3.5 mm, it will easily peel off, which is not preferable. The member of the present invention having such a structure has an excellent long life and high working efficiency, which is not seen in conventional members.

Claims (1)

[Claims] 1. The content of free silicon and silicon oxide is 3% by weight.
A silicon carbide member for a low melting point metal melting and holding furnace, characterized by the following: 2. The member according to claim 1, wherein at least the surface of the member that comes into direct contact with the molten metal is coated with a silicon carbide film grown in a vapor phase. 3. The member according to claim 1, wherein the vapor-phase grown coating has a thickness of 0.1 to 2 mm. 4. The member according to claims 1 to 3, characterized in that it is provided with a corrosion-resistant heat insulating coating. 5. The member according to claim 4, wherein the corrosion-resistant heat-insulating coating is silicon nitride and/or boron nitride.