JP2008309412A - Heating unit protective tube - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a heating unit protective tube, restraining radiation of heat from the surface of the protective tube into the air and to a fixed member provided on a furnace for fixing the protective tube on the upper part of the protective tube not dipped in molten metal, and achieving excellent heat efficiency of the heating unit to attain high output of heating energy on the lower part of the protective tube dipped in the molten metal. <P>SOLUTION: The protective tube for the heating unit dipped in the molten metal in the furnace to heat the molten metal is formed by joining a lower tube body having the bottom part dipped in the molten metal and closed and a hollow tubular upper tube body disposed on the upside of the lower tube body to each other through sintering. The lower tube body and the upper tube body are formed of ceramics, and the thermal conductivity of the lower tube body is larger than that of the upper tube body. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a protective tube for a heating body that is immersed in a heating furnace or a molten metal such as aluminum in a melting furnace to heat the molten metal.

  In a heating furnace or melting furnace for a non-ferrous metal such as aluminum, a heating body protective tube provided with a heating body such as an electric heater or a gas burner is used inside a hollow tube body in order to melt the material. Among them, a heating element protective tube made of a ceramic sintered body such as silicon nitride, sialon, silicon carbide, boron nitride and the like, which has excellent resistance to melting damage, is known.

  For example, in Patent Document 1, a heat source is arranged inside a bottomed hollow tubular body made of a ceramic refractory material, and the upper end of the tubular body is fixed to a heating furnace top wall and held in the heating furnace. In a submerged tube heater that is immersed vertically in a non-ferrous alloy melt and is held vertically, the portion including the range in contact with the molten metal surface that fluctuates during operation in the heating furnace at the intermediate portion in the axial direction of the tube body It is made of small, dense ceramics and is made of ceramics with a low content of substances that can be reduced to oxides that are reduced by high-temperature molten metal above the set temperature of the molten metal, and other parts are made of ceramics with high porosity. A dip tube heater in which these are integrally connected is described.

  Patent Document 2 discloses a molten metal heating element protective tube made of a silicon nitride sintered body containing silicon nitride as a main component, wherein the silicon nitride sintered body has a thermal conductivity of 70 W / (m · K) or more, and a heated body protective tube is described in which the silicon nitride sintered body has a 4-point bending strength at room temperature of 600 MPa or more.

JP 7-43075 A Japanese Patent Laid-Open No. 2002-249381

  Of the conventional heating element protective tubes, those made of a silicon nitride sintered body manufactured by the reactive sintering method have the advantage that there is almost no shrinkage during the reactive sintering and that high dimensional accuracy can be obtained. However, even a product obtained by completely nitriding silicon is a relatively low-density sintered body having a porosity of about 20%, so that the bending strength is only about 300 MPa, and mechanical stress and impact during use Is not enough to withstand. In addition, since the density is low, the surface of the sintered body is rough and the molten metal is liable to adhere.

  Patent Document 1 can improve the service life by making only the middle part of the tube in contact with the molten metal surface in the furnace with a material that does not change or break with the molten metal at a set temperature or higher. In this method, an intermediate portion formed of a dense ceramic and the original pipe body of the conventional material above and below the intermediate portion are bonded with a fireproof adhesive. However, since the intermediate portion is at a position where the molten metal swings most, a load is always applied, and bonding strength with a refractory adhesive is insufficient. In addition, when the protective tube is used for a long period of time, the bonded portion of the intermediate portion is damaged, and the molten metal enters the protective tube, and the tube body is easily damaged.

  In Patent Document 2, since the thermal conductivity of the silicon nitride sintered body mainly composed of silicon nitride forming the heating body protective tube is high, the heat generated by the heating body provided inside the protective tube can be quickly and efficiently generated. It can be transmitted to the molten metal through the surface of the protective tube, and heating energy can be increased in output. However, since the thermal conductivity of the entire protective tube is high, heat easily escapes from the upper surface of the protective tube not immersed in the molten metal into the air. In addition, heat is easily transmitted from the upper part of the protective tube via a fixed member for fixing the protective tube provided in the furnace, so that there is a problem that the heat efficiency of the heating body is lowered.

  Accordingly, an object of the present invention is to provide a heating element protective tube made of a ceramic sintered body, in the upper part of the protective tube not immersed in the molten metal, to dissipate heat from the surface of the protective tube to the air, or to a protective tube provided in the furnace. It is possible to suppress the heat radiation to the member to be fixed for fixing. Moreover, it is providing the heating body protective tube which is excellent in the thermal efficiency of the heating body which can output high heating energy in the lower part of the protective tube immersed in molten metal.

  The heating element protective tube of the present invention is a heating element protective tube that is immersed in the molten metal in the furnace to heat the molten metal. A hollow tubular upper tube is joined by sintering. The lower tube and the upper tube are made of ceramics, and the thermal conductivity of the lower tube is larger than that of the upper tube. To do.

  The second heated body protective tube of the present invention includes a lower tubular body having a bottom portion in which a heated tubular protective tube that is immersed in molten metal in a furnace to heat the molten metal is closed by being immersed in the molten metal, and an upper portion of the lower tubular body A hollow tubular upper tube disposed between the lower tube and the upper tube, and an intermediate layer formed of ceramics having a thermal expansion coefficient between the lower tube and the upper tube. The tubular body and the upper tubular body are made of ceramics, and the thermal conductivity of the lower tubular body is larger than that of the upper tubular body.

  In the present invention, the lower pipe preferably has a thermal conductivity of 50 W / (m · K) or higher, and the upper pipe has a thermal conductivity of 20 W / (m · K) or lower.

  Preferably, the lower tube is formed from silicon nitride and the upper tube is formed from sialon.

  It is preferable that the thermal conductivity of the upper tubular body is smaller than the thermal conductivity of a member to be fixed for fixing the upper tubular body provided in the furnace.

It is desirable that the difference in thermal expansion coefficient between the lower tube body and the upper tube body is 2.0 × 10 ⁻⁶ / K or less.

  It is desirable that the uppermost end position of the lower tubular body is below the molten metal surface to be heated.

  The heating body protective tube of the present invention has a two-layer structure in which the lower tube body and the upper tube body are integrated by sintering. Ceramics having a high thermal conductivity are arranged as the lower tube body, and the lower tube body is used as the upper tube body. Place ceramics with low thermal conductivity. With this configuration, the lower pipe immersed in the molten metal can conduct heat generated from the heating body to the molten metal through the surface of the lower tubular body with high output, and maintains the molten metal at a predetermined temperature.

  In addition to the action of the lower pipe, an upper pipe having a low thermal conductivity is arranged on the upper part of the lower pipe, so that heat generated from the heating body in the lower pipe is transferred from the lower pipe to the upper pipe. It is possible to suppress the transmission and output the heating energy in the lower pipe immersed in the molten metal stably and greatly with little loss.

  Usually, a flange part is provided in the upper end part of a heating body protective tube. Moreover, the furnace filled with the molten metal is provided with a member to be fixed such as a mold for attaching the heating element protection tube. The heating element protection tube is suspended and fixed in the vertical direction by fixing its flange portion to a fixed member with a bolt or the like. Since the flange portion and the fixed member are in contact with each other, the heat generated from the heating body easily moves from the flange portion at the upper end of the protective tube and its peripheral portion to the fixed member.

  In the case of the present invention, since the upper pipe body has a low thermal conductivity, heat loss from the heating body is reduced because heat dissipation from the flange part at the upper end of the upper pipe body and its peripheral part to the fixed member on the furnace side is reduced. Can be reduced. In addition, the amount of heat that escapes from the surface of the portion of the upper tube that is not immersed in the molten metal to the air in the furnace can be suppressed, and the loss of heat generated from the heater can be reduced. The thermal efficiency of the heating body can be improved by superimposing the effects of the lower tubular body and the upper tubular body as described above.

  Further, since the lower tube body and the upper tube body are integrally joined by sintering, a high bonding strength can be ensured. Since it is a strong joint by sintering, the molten metal does not enter the protective tube from the joint, and the breakage of the protective tube can be prevented.

  The second heated body protective tube of the present invention has a three-layer structure in which a lower tube, an intermediate layer, and an upper tube are integrated by sintering. The intermediate layer is formed of ceramics interposed between the lower tube body and the upper tube body and having an intermediate thermal expansion coefficient. The intermediate layer is a so-called functionally graded material, and is used to ensure sufficient bonding strength when the difference in thermal expansion coefficient between the lower tube and the upper tube is relatively large. The intermediate layer is preferably a mixture in which two or more kinds of ceramics of different materials are easily mixed and the thermal expansion coefficient can be easily adjusted. Further, two or more intermediate layers may be interposed in order to gradually incline the thermal expansion coefficient.

  In order to sufficiently obtain the effects of the present invention, it is desirable that the lower tube body and the upper tube body have an appropriate difference in thermal conductivity. The desirable thermal conductivity combination of the present invention is that the lower tube has a thermal conductivity of 50 W / (m · K) or more at room temperature, and the upper tube has a thermal conductivity of 20 W / (m · K) or less at room temperature. It is.

  In the lower tube and the upper tube, combinations of various ceramics are effective if the condition that the thermal conductivity of the lower tube is higher than that of the upper tube is satisfied. In particular, the lower tube is preferably formed of silicon nitride having excellent resistance to melting and thermal shock, and the upper tube is preferably formed of sialon having excellent resistance to melting and heat.

  When the thermal conductivity of the upper tube is smaller than the thermal conductivity of the member to be fixed for fixing the upper tube provided in the furnace, the heat retention is improved, and the flange portion at the upper end of the upper tube and its surroundings This is preferable because the heat radiation from the section to the fixed member on the furnace side is reduced and the loss of heat generated from the heating body can be reduced.

In order to obtain a high joint strength between the lower tube body and the upper tube body, it is desirable that the difference in thermal expansion coefficient between them is 2.0 × 10 ⁻⁶ / K or less. More desirably, it is 1.0 × 10 ⁻⁶ / K or less.

  The lower tubular body with a high thermal conductivity is designed so that the heating energy can be supplied to the molten metal as much as possible, and in order to reduce the release of the heating energy into the air in the furnace and to the fixed member, It is desirable to position it below the surface of the molten metal to be heated. In other words, it is desirable that the uppermost end position of the lower tubular body is below the molten metal surface to be heated.

  FIG. 1 is a schematic cross-sectional view of a heating element protective tube according to an embodiment of the present invention. In FIG. 1, the heating body protective tube is divided in the vertical axis direction, and the lower tube 1 and the upper tube 2 having substantially the same outer diameter and inner diameter are sintered and integrated.

  The lower tubular body 1 has a hollow portion 11 in which a heating body 3 such as a heater is inserted and accommodated therein, and a lower end thereof has a closed bottom portion 12. The uppermost end position 13 of the lower tube 1 is joined to the lower end surface of the upper tube 2.

The lower tubular body 1 was formed of a sintered body mainly composed of silicon nitride. The sintered body mainly composed of silicon nitride has a thermal expansion coefficient of 3.0 × 10 ⁻⁶ / K at room temperature, a relative density of 99.6%, and a thermal conductivity of 51 W / (m · K) at room temperature. The 4-point bending strength at room temperature is 900 MPa.

  The upper tubular body 2 has a hollow portion 21 into which a heating body 3 such as a heater is inserted, and a flange portion 22 is provided at the upper end portion. The flange portion 22 of the upper tubular body 2 is attached and fixed to a fixed member 4 provided in the furnace with a bolt or the like.

The upper tube body 2 was formed of a sialon sintered body. The sialon sintered body has a thermal expansion coefficient of 3.0 × 10 ⁻⁶ / K at room temperature, a relative density of 99.6%, a thermal conductivity of 17 W / (m · K) at room temperature, and a four-point bending strength at room temperature. Is 880 MPa.

  A heating body 3 such as an electric heater is placed in the hollow portions 11 and 21 of the heating body protection tube in which the lower tube body 1 and the upper tube body 2 are joined by sintering. And the heating body protection tube was immersed in molten metal 5 such as aluminum. It was immersed so that the uppermost end position 13 of the lower tubular body 1 was below the molten metal surface 51 of the molten metal 5 to be heated. And by making the heating body 3 generate | occur | produce heat, the heat is transmitted to the molten metal 5 through the surface of a heating body protection tube, and a molten metal is maintained at predetermined | prescribed temperature.

  The arrow Qa is the heating energy supplied to the molten metal, the arrow Qb is the thermal energy emitted from the surface of the upper tube 2 not immersed in the molten metal into the air in the furnace, and the arrow Qc is the flange portion 22 of the upper tube 2. Further, the thermal energy and the magnitude of the arrow appearing from the peripheral portion to the fixed member 22 provided in the furnace represent the magnitude of the output. By the structure of this invention, it showed that Qa was large and the loss of Qb and Qc was small.

  Next, the manufacturing method of a heating body protection tube is demonstrated. A core metal having an outer diameter of 40 mm is disposed in a rubber mold having an inner diameter of 60 mm, and the raw material powder of the silicon nitride serving as a lower tubular body is filled in a lower layer portion of a gap formed between the rubber mold and the core metal, Next, the upper layer portion was filled with the sialon raw material powder serving as the upper tube, and cold isostatic pressing (CIP) was performed by hydrostatic pressure to produce a tubular molded body serving as a heating body protective tube. This molded body was fired in a nitrogen gas atmosphere at 1800 ° C. and 9 atm for 10 hours to obtain a heated body protective tube in which the lower tube body and the upper tube body of the present invention were joined by sintering. The boundary strength between the lower tube and the upper tube was 600 MPa.

  FIG. 2 is a schematic cross-sectional view of a heating element protective tube according to a second embodiment of the present invention. FIG. 2 shows a configuration similar to that of the embodiment of FIG. 1, except that an intermediate layer 6 formed of ceramics having a thermal expansion coefficient intermediate between the lower tubular body 1 and the upper tubular body 2 is sintered. It is characterized by being joined. The intermediate layer 6 is a mixture obtained by mixing different types of ceramics with silicon nitride.

  When the heating element protective tube of the present invention was immersed in molten aluminum at 680 ° C. and subjected to molten metal heating, almost no erosion by the molten aluminum was observed, and no damage or cracking occurred due to impact load or thermal shock during use. . Also, compared to the conventional silicon nitride protective tube made of silicon nitride, heat is released from the surface of the protective tube to the air and the protective tube provided in the furnace is fixed at the top of the protective tube that is not immersed in the molten metal. Therefore, it is possible to suppress heat dissipation to the fixed member. In addition, it was confirmed that heating energy can be output at a lower portion of the protective tube immersed in the molten metal, and the thermal efficiency of the heating body is excellent.

  According to the heating body protective tube of the present invention, it is possible to contribute to energy saving by suppressing the loss of heating energy with excellent thermal efficiency of the heating body.

It is a schematic sectional drawing of the heating body protective tube of this invention Example. It is a schematic sectional drawing of the heating body protection tube of 2nd Example of this invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 Lower pipe body, 2 Upper pipe body, 3 Heating body, 4 Fixed member, 5 Molten metal, 6 Middle layer

Claims (7)

A protective tube of a heating body that is immersed in the molten metal in the furnace and heats the molten metal is fired into a lower tubular body that has a closed bottom that is immersed in the molten metal, and a hollow tubular upper tubular body that is disposed above the lower tubular body. A heating body protective tube, wherein the lower tube body and the upper tube body are formed of ceramics, and the thermal conductivity of the lower tube body is greater than the thermal conductivity of the upper tube body. A protective tube of a heating body that is immersed in the molten metal in the furnace and heats the molten metal, a lower tubular body having a closed bottom portion immersed in the molten metal, a hollow tubular upper tubular body disposed on the upper portion of the lower tubular body, and a lower portion An intermediate layer formed by ceramics interposed between the tube and the upper tube and having an intermediate thermal expansion coefficient between them is joined by sintering, and the lower tube and the upper tube are formed of ceramics, A heating body protective tube, characterized in that the thermal conductivity of the lower tubular body is larger than that of the upper tubular body. The thermal conductivity of the lower tubular body at room temperature is 50 W / (m · K) or more, and the thermal conductivity of the upper tubular body at room temperature is 20 W / (m · K) or less. Or the heating body protective tube of 2. The heating body protective tube according to any one of claims 1 to 3, wherein the lower tube body is formed of silicon nitride, and the upper tube body is formed of sialon. The heating body according to any one of claims 1 to 4, wherein the thermal conductivity of the upper tubular body is smaller than the thermal conductivity of a member to be fixed for fixing the upper tubular body provided in the furnace. Protective tube. The heating body protective tube according to any one of claims 1 to 5, wherein a difference in thermal expansion coefficient between the lower tubular body and the upper tubular body is 2.0 × 10 ^-6 / K or less. The heating body protective tube according to claim 1 or 2, wherein an uppermost end position of the lower tubular body is below a molten metal surface to be heated.