JPH0131674B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a protective tube for an immersion heater used in melting and holding furnaces for low-melting metals such as Al and Pb. In recent years, for example, an internal heating method in which a heat source is immersed in heat has been adopted as a method for melting and retaining Al. However, despite the remarkable improvement in thermal efficiency of this method, the protective tube of the heater is corroded by molten metal and antioxidants floating on its surface, or is susceptible to thermal shock caused by significant temperature differences. It often broke. It is thought that the above-mentioned phenomenon occurs because this is generally a cast iron pipe coated with a clay coating. A silicon carbide protective tube may also be used, but although silicon carbide has good thermal conductivity, its spalling resistance is not necessarily satisfactory. The present invention relates to an improvement of a silicon carbide protection tube, and the problem was solved by taking advantage of silicon carbide's characteristics of high strength and high thermal conductivity, and applying a coating that has heat insulation and corrosion resistance to it. be. That is, silicon nitride or boron nitride is used as a coating material, and one or more layers of this are coated,
Furthermore, the thickness is limited to a range of 0.3 to 3.5 mm. Residual thermal strain caused by the manufacturing process of silicon carbide molded bodies such as silicon nitride bonded silicon carbide or self-bonded silicon carbide causes microcracks due to sudden temperature changes when used as a protection tube.
This has grown and led to an unstable usage period. The maximum value that a material can withstand against temperature changes is generally expressed by the following equation. ΔT=σf(1-ν)/Eα ΔT=maximum temperature change, σf=average rupture coefficient,
ν: Poisson's ratio, E: Young's modulus, α: coefficient of thermal expansion. Since this value is almost constant depending on the material,
To prevent thermal shock destruction of materials, it is necessary to suppress the difference in internal temperature to below ΔT. For this purpose, it is effective to delay the rate of increase in surface temperature of the material. In the present invention, it has been found that silicon nitride and boron nitride are preferable as materials that are excellent as heat insulating materials and are not corroded by low melting point metals or their antioxidants. It has been found that the effect is further enhanced when this coating layer has two or more layers. It has become clear that this is because when the molten metal permeates into the coating layer, the permeation is suppressed at the interface between the two layers, and the permeation into the internal coating layer is greatly delayed. The coating method is performed by adding an inorganic binder such as aluminum phosphate to silicon nitride or boron nitride powder to make it into a slip, and applying it by spraying or dipping. Further, the thickness of the coating layer is not necessarily sufficient; if it is 3.5 mm or more, the strength of the coating layer itself becomes insufficient and peeling tends to occur. On the other hand, if the thickness is less than 0.3 mm, no heat insulating effect can be expected, so the preferred thickness range is 0.3 to 3.5 mm. Examples of the present invention will be described below. Example 1 A recrystallized silicon carbide protection tube was coated with the following four types of coating layers, and the tube was repeatedly inserted into a holding furnace at 740° C. for Al molten metal and extracted, and its lifespan was measured. (1) No coating layer is applied. (2) Spray coating of silicon nitride on the outer surface. (3) Spray coating the outer surface with boron nitride. (4) Spray coating Al ₂ O ₃ on the outer surface. In addition, the coating material contains phosphoric acid in each powder.
Al is added as a binding agent, and the film thickness is 1.5 mm in each case. This operation was repeated three times a day with an insertion speed of 30 seconds and a withdrawal speed of 30 seconds into the molten Al, and the service life was investigated and the results were as follows.

[Table] Example 2 The life of a product similar to Example 1 except that the coating layer was two layers was investigated. The first coating layer is silicon nitride with a thickness of 0.7 mm.
The second layer was made of boron nitride with a thickness of 0.7 mm. the result,
The second layer peeled off in 60 to 65 days. When this material was again coated with boron nitride, peeling was observed in the second layer after 50 to 60 days of use. Generally, commercially available silicon nitride powder and boron nitride powder are
Although impurities such as SiO ₂ , Si, and B ₂ O ₃ are present,
Since these are highly reactive, they can be made more durable as coating agents by performing a purification treatment in advance. Comparative tests were also conducted regarding film thickness, and for example, when boron nitride was used, a film of 0.2 mm peeled off in 1 to 2 days, and a film of 3.6 mm
In mm, peeling was observed on the first day. The protection tube according to the present invention takes advantage of the excellent high thermal conductivity of silicon carbide, and can be used as a heater protection tube as a material that is heat resistant and has good thermal responsiveness.It also forms an excellent coating layer. As a result, we were able to create a long-life protection tube.

Claims (1)

[Scope of Claims] 1. The substrate is made of silicon carbide, and a coating layer of 0.3 to 3.5 mm thick made of silicon nitride and/or boron nitride is provided on at least the surface of the part that contacts the low melting point metal. Protection tube for immersion heaters for low melting point metals. 2. A protective tube for an immersion heater for low melting point metals according to claim 1, wherein the coating layer has a structure of two or more layers.