JPH0421996B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a method of manufacturing so-called metal tube electric heating elements such as sheathed heaters and cartridge heaters, and particularly to improvements in the life characteristics and insulation characteristics thereof. [Prior Art] Examples of metal tubular electric heating elements include so-called cartridge heaters and sheathed heaters. Cartridge heaters are insert heaters that are mainly used by being inserted into a drilled metal part, and there are two types: an inner-wound type in which a heating wire is inserted and held through a holding hole formed in a ceramic core, and an inner-wound type in which a heating wire is wrapped around the outer circumference of a ceramic core. There are two types: an externally wound type, and both types are filled with magnesia, silica,
It is made by densely filling inorganic heat-resistant insulating powder such as alumina. On the other hand, a sheathed heater is a metal tubular electric heating element that has a wide range of uses for various heating purposes such as liquid heating, gas heating, and contact heating on metal surfaces. , 1 is placed in the center of the exterior metal tube 3, and an inorganic heat-resistant insulating powder 4 is placed in the gap.
It has a densely packed structure. Both have similar structures in that heating wires are arranged at predetermined positions in an exterior metal tube, and the voids are densely filled with inorganic heat-resistant insulating powder. By the way, it is well known that stainless steel, Incoloy (trade name), or Inconel (trade name) have traditionally been used as materials when heat resistance is particularly required for exterior metal pipes. However, Incoloy (Ni: 30~
35wt%, Cr: 19-23wt%) and Inconel (Ni:
72 wt%, Cr: 14 to 17 wt%) is extremely expensive and therefore is not used for anything other than limited purposes, and the reality is that stainless steel and the like are generally used. However, among stainless steels, ferritic stainless steel (Fe-Cr-Al) is not suitable for use as armored metal pipes due to its inherent disadvantages such as poor high-temperature strength and brittleness at 475°C. It is rarely used, and austenitic stainless steel (Fe-Cr-Ni) has been widely used. However, even with austenitic stainless steel, spinel-type oxides (Cr ₂ O ₃ ,
Although NiO) is generated to protect the inside of the metal, it has poor spalling resistance, and in particularly humid and high-temperature atmospheres, the metal surface is selectively oxidized, forming an uneven oxide film. There were problems such as the oxide film peeling off easily. On the other hand, NCH (Ni-Cr system) and FCH (Fe
-Cr-Al series) so-called electrothermal alloys have been generally used;
In addition to precipitating chromium carbide and becoming brittle at temperatures between 550 and 750°C, it also forms spinel-type oxides (Cr ₂ O ₃ , NiO) in high-temperature oxidizing atmospheres to protect the interior of the metal, while being resistant to exposure to sulfidic gases. It is also known that under a humid, high-temperature atmosphere, metal surfaces undergo selective oxidation and oxides peel off. In particular, a spinel-type oxide film reacts and decomposes with silicates, boric acid, carbonic acid, and chloride to produce low-melting compounds, resulting in a loss of protective effect. Furthermore, when used in a high-temperature oxidizing atmosphere of 1000℃ or higher, scale adheres to the surface of the heating wire, inhibiting heat radiation from the heating wire, and further inducing local overheating, leading to early disconnection. It is something.
Therefore, NCH must be used under conditions with little oxidation and at a temperature of 1000° C. or less, and therefore its uses are inevitably limited. The latter FCH has excellent oxidation resistance because it forms an Al ₂ O ₃ film on the metal surface, but if it is repeatedly used at temperatures above 1000°C, the crystal grains will become coarser, resulting in poor high-temperature strength. It has some drawbacks. For this reason, even with this type of electric heating alloy, its use has been limited to some parts such as heating elements for electric furnaces. [Problems to be solved by the invention] In this regard, if the Al-containing austenitic heat-resistant steel disclosed in Japanese Patent Publication No. 53-43498 is used for heating wires and outer layer metal tubes, the above-mentioned drawbacks can be almost eliminated. It has been found. Specifically, in order to suppress the growth of crystal grains mainly during high temperature use, the composition was adjusted for the precipitation of the δ-ferrite phase, and Al was contained in excess of 4.5 wt%, and the temperature at 800°C to 1200°C was
It is heat-treated at ℃ to form a stable Al ₂ O ₃ film on the steel surface, resulting in a long-life product with the least amount of oxidation increase even when used in a high-temperature oxidizing atmosphere of 1200℃. . However, from the viewpoint of actual molding, after heat treatment is performed in advance to form an Al ₂ O ₃ film, the surface hardness increases and there is a problem in workability. Moreover, this type of steel is heated to 650℃ before heat treatment.
Embrittlement occurred, and coarsening of crystal grains was observed, especially in areas where internal strain was caused by welding or cold working. Therefore, the object of the present invention is to obtain an exterior tubular electric heating element that has excellent acid resistance and strength even in high-temperature oxidizing atmospheres, and has a long life. It is an object of the present invention to provide a method for manufacturing an exterior tube-wrapped electric heating element which can be very effectively removed and has extremely high workability. [Means for solving the problem] The feature is that the heating wire is arranged at a predetermined position in the exterior metal tube, and the void is densely filled with inorganic heat-resistant insulating powder. In the manufacturing method, mainly austenitic heat-resistant steel containing 4 to 10 wt% Al is used as the material for the heating wire and the exterior metal tube, and the heating wire is first annealed at a temperature of at least 800°C or higher in an oxidizing atmosphere. After that, the heating wire was filled into an exterior metal tube, filled with inorganic heat-resistant insulating powder, and the tube diameter was reduced and contracted, and then annealed again under almost the same conditions as above. There it is. That is, in all electric heating materials such as heating wires and exterior metal tubes, mainly austenitic heat-resistant steel containing 4 to 10 wt% Al is used as the material, and when forming it, the material is compressed, expanded and contracted, Or to eliminate residual stress due to plastic deformation caused by various processing such as shrinkage and diameter reduction, internal strain due to welding to electrical terminals, or embrittlement at 600 to 700℃. After molding, it is annealed at at least 800°C, and at the same time an Al ₂ O ₃ film is formed. As a result, the electric heating element according to the present invention can obtain good values in terms of resistance temperature increase coefficient or specific resistance value, and has extremely low oxidation weight gain at high temperatures, and has excellent oxidation resistance. , which obtained a dramatically superior lifespan value,
It was recognized that it could be used in practical applications as an electric heating element. In addition, the problem of brittleness has been resolved, and since the film itself is made of Al ₂ O ₃ , the insulation performance is even better than before, making it an extremely unique material. It has been recognized that it can have both effects. Furthermore, homogeneous formation of Al ₂ O ₃ film is easily achieved.
At the same time, it has the ability to emit infrared rays with the radiation spectrum of Al ₂ O ₃ , and in addition to obtaining an electric heating element with high heat generation performance at all times, it is also uniquely capable of emitting infrared rays using traditional methods such as melting infrared radiating materials. This eliminates the need to adopt this method and achieves a highly desirable process reduction. [Example] Below, the results of tests conducted on heating wires and armored metal tubes obtained by the method of the present invention will be shown. (a) Resistance temperature increase coefficient and specific resistance value Al-containing austenitic heat-resistant steel with an outer diameter of φ1.0
mm, length 400cm, heat treated at 1200℃ for 10 hours, and then heated at 20℃ using a Wheatstone bridge.
And the resistance value at 1200℃ was measured. The composition of the Al-containing austenitic heat-resistant steel used here is:
In weight%, C: 0.028, Si: 0.59, Mn: 0.32,
P: 0:001, Ni: 23.9, Cr: 16.6, Al: 4.93,
Fe: Consists of the remainder. As a result, the resistance value is 5.29Ω at 20℃, 1200℃
It was 7.47Ω. In addition, when calculating the specific resistance ρ value based on JIS C 2524 “Method for testing conductor resistance and volume resistivity of metal resistance materials”, ρ=R/L・A (μΩm) R: resistance value (Ω), L : length (m), A: cross-sectional area (cm ² ), so ρ ₁₂₀₀ = 146.7 (μΩcm) ρ ₂₀ = 103.9 (μΩcm), which are good values that can be used practically as a heating element. Ta. Note that the ρ value was found to fall within the range of 70 to 200 μΩcm. Also, the resistance temperature increase coefficient is approximately 1.41,
Compared to NCH's coefficient of approximately 1.09, we obtained a nearly equivalent value. (b) Lifespan test Based on the method of JIS C 2524 "Method for lifespan testing of heating wires and bands," lifespan values were determined by a test similar to this. In other words, (a) contains the same composition as that used in the test.
Al austenitic heat-resistant steel was used as a wire with a diameter of 1.0 mm, which was heat treated at 1200°C for 10 hours, and then a 40 gf wire was attached to the lower terminal. As a result, it was found that the product of the present invention had a lifespan of 1027 times, whereas NCH1 had a lifespan of 500 times and NCH2 had a lifespan of about 350 times, which was about 2 to 3 times longer. (c) Brittleness test A non-heat treated heating wire made of the same material as (a) was tested according to JIS
When energization was repeated under the conditions specified in 5.1.1(3) of C.2524, the wire broke near the connection between the terminal and the heating wire after 48 times. According to Thermo Paint, this area was assumed to be in the range of 600 to 700 degrees Celsius. On the other hand, since no breakage was observed in heating wires made of the same material as in (a) that were annealed at temperatures of 800°C or higher, it was found that annealing at temperatures of 800°C or higher is necessary for practical purposes. . (d) Oxidation weight gain test Based on JIS C 2520 “Oxidation weight gain test”, (a)
^A heating wire made of austenitic heat-resistant steel with the same composition as
The test piece was 200 m (=6.28 cm ² ) and annealed at 1200°C for 10 hours. At this time, for comparison, Incoloy 800, FCH2,
This was also done for SUS310S. The results are shown in Table 1.

〔Effect of the invention〕

As stated above, this invention has a 4.5 to 10 wt%
The heating wire is mainly made of austenitic heat-resistant steel containing Al, and in its manufacturing method, the heating wire is first annealed in a high-temperature oxidizing atmosphere of at least 800°C, and then the heating wire is coated with an exterior metal. By adopting a method in which the tube is filled with inorganic heat-resistant insulating powder, the tube diameter is reduced and contracted, and then annealing is performed again under almost the same conditions as above, the high-temperature oxidation resistance and An electric heating element with excellent high-temperature strength and long life without causing brittle fracture was obtained. Furthermore, good characteristics were obtained in terms of resistance temperature increase coefficient and specific resistance value, especially since the homogeneously formed Al ₂ O ₃ film itself has extremely excellent insulation performance. This provides a unique insulation effect that has never existed before. Also,
This homogeneously formed Al ₂ O ₃ film itself has the property of emitting infrared rays when it is self-energized, making it extremely easy to obtain the desired surface temperature, and it can be used in a wide range of applications. It is possible. For example, it serves as an extremely efficient heating source for water, synthetic resins, organic substances, metals, ceramics, etc.

[Brief explanation of drawings]

FIG. 1 is a longitudinal sectional view showing an example of a sheathed heater, FIG. 2 is a characteristic diagram showing the relationship between continuous energization time and insulation resistance of the heater according to the present invention and a conventional heater, and FIG. and the emission spectrum of an infrared lamp.

Claims (1)

[Scope of Claims] 1. A method for producing a metal tube-encased electric heating element, which comprises arranging a heating wire at a predetermined position of an exterior metal tube and densely filling the void with an inorganic heat-resistant insulating powder, wherein the heating wire and the exterior metal Mainly austenitic heat-resistant steel containing 4 to 10 wt% Al is used as the tube material, and the heating wire is first annealed in a high-temperature oxidizing atmosphere of at least 800°C, and then the heating wire is A metal tube-packed electric heating element characterized in that an exterior metal tube is filled with an inorganic heat-resistant insulating powder, the tube diameter is reduced and contracted, and then annealing is performed again under substantially the same conditions as above. Production method. 2 Mainly austenitic heat-resistant steel containing 4 to 10 wt% Al, C: 0.4 wt% or less, Si:
2.0wt% or less, Mn: 10wt% or less, Ni: 12~50wt
%, Cr: 5 to 30wt%, and one or more of Co, Mo, and W as necessary.
Contains 5wt% or less and 2.0wt% or less of one or more of Ti, Zr, Nb, Td, etc., with the balance mainly consisting of Fe, and less than 10wt% in the austenite phase in a high-temperature oxidizing atmosphere. 2. The method for manufacturing a metal tubular electric heating element according to claim 1, wherein the components are adjusted so that a δ-ferrite phase is precipitated.