JP2013049898A - Method for improving insulation resistance of vacuum heating furnace - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for improving insulation resistance of a vacuum heating furnace, which can shorten the time required for a burnout process to improve processing efficiency of vacuum carburization.SOLUTION: The method for improving insulation resistance of a vacuum heating furnace includes: a vacuum heating step of reducing the inner pressure of a heating chamber 3 of the vacuum heating furnace 1 having a graphite heater 5 in the heating chamber 3 to a predetermined vacuum degree and also heating the inside of the heating chamber 3 with the graphite heater 5 to a predetermined temperature; and a burnout step of introducing dry air into the heating chamber 3 after the vacuum heating step.

Description

  The present invention relates to a method for improving insulation resistance of a vacuum heating furnace.

Conventionally, a vacuum carburizing method is known in which a hydrocarbon-based gas is decomposed in a vacuum heating furnace under reduced pressure and high temperature, and the obtained carbon is reacted on a metal surface such as iron or chromium. In such a vacuum carburizing method, carbon that has not reacted with a metal such as iron, that is, carbon generated from a heat insulating material (furnace material), a used gas, a processing material, or the like hatches and adheres to the heating chamber.
Then, for example, by attaching soot (carbon) to the insulating material between the heater and the heat insulating material, the insulating property between the heater and the heat insulating material is lowered, and the insulation resistance is reduced, so that the heat contacting the heat insulating material is reduced. A small amount of electricity leaks into the furnace outer body (furnace body), and the heating efficiency of the heater is reduced.

Therefore, conventionally, in the case of a vacuum heating furnace that does not use graphite, so-called burnout has been performed by introducing air into the heating chamber and burning the soot adhering to the insulating material or the like (for example, Patent Document 1). reference).
On the other hand, in a vacuum heating furnace having a graphite structure in the heating chamber, the graphite structure is consumed by oxidation, so oxygen-containing air cannot be introduced and burned. Therefore, in such a vacuum heating furnace, CO ₂ is introduced, and burnout of the vacuum heating furnace is performed by a reaction of “C + CO ₂ → 2CO” (see, for example, Patent Document 2).

JP 2007-131936 A JP-A-56-98468

However, in the burnout of the vacuum heating furnace performed by introducing CO ₂ , it takes a lot of time to react the adhering carbon to carbon monoxide, and the burnout process takes a long time. This is a cause of lowering the processing efficiency of the vacuum carburizing process.

  The present invention has been made in view of the above circumstances, and the object thereof is to improve the insulation resistance of a vacuum heating furnace that shortens the time required for the burnout process and improves the processing efficiency of the vacuum carburizing process. It is to provide a method.

The method for improving the insulation resistance of a vacuum heating furnace according to the present invention is to reduce the pressure in the heating chamber of the vacuum heating furnace having a graphite heater in the heating chamber to a preset degree of vacuum and to set the heating chamber in advance by the graphite heater. A reduced pressure heating process for heating to
A burnout step of introducing dry air into the heating chamber after the reduced pressure heating step;
It is characterized by providing.

  According to this insulation resistance improving method, the heating chamber is depressurized to a preset vacuum degree, and after the heating chamber is heated to a preset temperature by a graphite heater, a burnout for introducing dry air into the heating chamber is performed. Therefore, while suppressing the exhaustion of the graphite heater due to oxidation, the soot (carbon) adhering to the insulating material between the heater and the heat insulating material is rapidly oxidized and gasified to be removed from the insulating material. Can do. Thereby, the insulation between a graphite heater and a heat insulating material improves, and the heating efficiency of a graphite heater improves.

  In the insulation resistance improving method of the present invention, the time required for the burnout process can be shortened and the processing efficiency of the vacuum carburizing process can be improved.

It is a schematic block diagram of a vacuum heating furnace. It is explanatory drawing of the process pattern of the method of improving an insulation resistance. It is explanatory drawing of the process pattern of the method of improving an insulation resistance.

Hereinafter, embodiments according to the method for improving insulation resistance of the present invention will be described in detail.
FIG. 1 is a diagram for explaining the insulation resistance improving method of the present embodiment, and reference numeral 1 in FIG. 1 denotes a vacuum heating furnace. This vacuum heating furnace 1 includes a heating chamber 3 in a furnace body 2 that serves as an exterior body. The heating chamber 3 is held in the furnace body 2 via a holding member (not shown) and is formed by a heat insulating material 4 such as graphite (graphite or graphite), and has a graphite heater 5 inside. have.

  The graphite heater 5 is pulled out from through holes 4a and 4a formed in the heat insulating material 4 of the heating chamber 3, and is connected to a power source via a control unit (not shown). The controller controls heating by the graphite heater 5 by controlling on / off of energization to the graphite heater 5 and a voltage to be applied.

In the through hole 4a, an insulating material 6 for insulating the graphite heater 5 and the heat insulating material 4 is provided.
Further, a vacuum pump 7 is connected to the furnace body 2 via a pipe so that the inside of the furnace body 2, that is, the inside of the heating chamber 3 can be depressurized to a predetermined degree of vacuum. The vacuum pump 7 of the present embodiment can reduce the pressure in the heating chamber 3 to 160 Pa or less.

  Furthermore, a carburizing gas source 8, an air source 9, and an inert gas source 10 are connected to the heating chamber 3 in the furnace body 2 through piping. The carburizing gas source 8 is a supply source of a carburizing gas made of a hydrocarbon gas such as acetylene. By supplying the carburizing gas into the heating chamber 3, the surface of the object to be processed made of a metal such as iron or chromium is carburized. It is for processing. In addition, the piping connected to the carburizing gas source 8 is provided with a pressure reducing valve and an on-off valve, and further provided with a flow meter. Thereby, the flow rate of the carburizing gas supplied from the carburizing gas source 8 into the heating chamber 3 can be adjusted.

  The air source 9 is composed of, for example, an air cylinder filled with dry air, and is used for performing burnout processing in the heating chamber 3 by supplying air into the heating chamber 3. The piping connected to the air source 9 is also provided with a pressure reducing valve and an on-off valve, and further provided with a flow meter. Thereby, the flow rate of the air supplied from the air source 9 into the heating chamber 3 can also be adjusted. The air source 9 is not limited to an air cylinder, and various air sources can be used as long as they can supply dry air containing no water vapor.

The inert gas source 10 supplies, for example, nitrogen gas into the heating chamber 3 and is mainly for cooling the inside of the heating chamber 3. The piping connected to the inert gas source 10 is also provided with a pressure reducing valve and an on-off valve, and further provided with a flow meter. Thus, the flow rate of nitrogen (inert gas) supplied from the inert gas source 10 into the heating chamber 3 can be adjusted.
The carburizing gas source 8, the air source 9, the inert gas source 10 and the piping connecting the heating chamber 3 are formed from the heating chamber 3 to the outside of the furnace body 2, and then branched and carburized. You may comprise so that it may connect with the gas source 8, the air source 9, and the inert gas source 10, respectively.

The vacuum heating furnace 1 is provided with a thermometer 11 that detects the temperature in the heating chamber 3 and a pressure gauge 12 that detects the pressure in the heating chamber 3. The thermometer 11 has a temperature sensor (not shown) in the heating chamber 3, and the pressure gauge 12 has a pressure sensor (not shown) in the heating chamber 3.
Further, the furnace body 2 and the heating chamber 3 are provided with doors (not shown) for carrying in / out the object to be processed.

  In order to perform the carburizing process by the vacuum heating furnace 1 having such a configuration, first, the doors of the furnace body 2 and the heating chamber 3 are opened, and an object to be processed (not shown) is carried into the heating chamber 3. Close each door. Next, the inside of the furnace body 2 is depressurized (evacuated) by the vacuum pump 7 to make the inside of the heating chamber 3 have a desired degree of vacuum, and the inside of the heating chamber 3 is heated by energizing the graphite heater 5, The treated body is heated to a desired temperature. Then, the operation of the vacuum pump 7 is stopped, and the carburizing gas is supplied from the carburizing gas source 8 at a preset flow rate for a predetermined time. Thereby, a carburizing process is performed with respect to a to-be-processed object. After that, when the energization to the graphite heater 5 is turned off and the inside of the heating chamber 3 is lowered to a predetermined temperature, the inside of the heating chamber 3 and the inside of the furnace body 2 are returned to atmospheric pressure. And the door of the furnace body 2 and the heating chamber 3 is opened, and the to-be-processed object which performed the carburizing process is carried out of a furnace.

When such carburizing treatment is repeated, carbon that has not reacted with the object to be treated, that is, carbon generated from the carburizing gas, hatches and adheres to the heating chamber 3.
Then, the generated soot also adheres to the insulating material 6 between the graphite heater 5 and the heat insulating material 4, thereby reducing the insulation between the graphite heater 5 and the heat insulating material 4 and reducing the insulation resistance. Become.

Therefore, in the present invention, in order to improve the insulation resistance between the graphite heater 5 and the heat insulating material 4 reduced by such carburizing treatment and restore the original large resistance, a process comprising the following steps is performed. The pattern shown in FIG.
First, as the reduced pressure heating process of the present embodiment, the vacuum pump 7 is operated to reduce the pressure inside the furnace body 2 and the heating chamber 3 to a preset degree of vacuum, that is, 160 Pa in the present embodiment. Then, after reducing the pressure to this degree of vacuum, the graphite heater 5 is energized to heat the inside of the heating chamber 3 to a preset temperature, in this embodiment 800 ° C. Here, the heating in the heating chamber 3 by the graphite heater 5 is performed at a rate of temperature rise of about 8 ° C./min and heated to 800 ° C. over 56 minutes.

  In the reduced pressure heating process of the present embodiment, after the pressure in the heating chamber 3 is reduced and heated in this way, as a stabilization process, the power supply to the graphite heater 5 is turned off to stabilize the temperature in the heating chamber 3. And soak. Specifically, after energization of the graphite heater 5 is turned off, the state is maintained, that is, at 160 Pa and 800 ° C. for 10 minutes. When such a stabilization process is performed, the temperature of the graphite heater 5 itself is lowered to a temperature substantially equal to the temperature in the heating chamber 3, and there is almost no temperature difference between them. 4. The heating chamber 3 as a whole including the insulating material 6 has a substantially uniform temperature (800 ° C.).

Next, as a burnout process, the operation of the vacuum pump 7 is stopped, and further, dry air is introduced from the air source 9 into the heating chamber 3 having a substantially uniform temperature, and the inside of the heating chamber 3 is burned out for 60 minutes. . The amount of dry air introduced is set to an appropriate amount set in advance by adjusting the secondary pressure from the air source 9 to about 0.2 to 0.25 MPa by a pressure reducing valve. By performing such a bar-out process, soot generated during the carburizing process and adhering to the insulating material 6 or the like is oxidized and rapidly gasified by a reaction of “C + O ₂ → CO ₂ ”. Thereby, the wrinkles adhering to the insulating material 6 etc. are removed. In addition, since the inside of the heating chamber 3 is maintained at a high degree of vacuum (160 Pa) in advance, it is possible to suppress the graphite heater 5 from being oxidized and consumed by the introduced dry air.
During the burnout process, since the power supply to the graphite heater 5 is turned off, the temperature of the heating chamber 3 gradually decreases.

  Thereafter, nitrogen is introduced into the heating chamber 3 from the inert gas source 10 as a cooling step. The pressure of nitrogen to be introduced is set to 87 kPa, and nitrogen is introduced for 1 minute. Thereby, the inside of the heating chamber 3 is cooled, and the temperature is lowered to 60 ° C., for example.

In this embodiment, the resistance (insulation resistance) between the graphite heater 5 and the furnace body 2 was measured with a tester before and after the treatment for improving the insulation resistance. Note that the graphite heater 5 was measured at two points before and after the heater circuit. The measurement results are shown below.
・ Before the heater circuit Before insulation resistance improvement: 6.9Ω → After insulation resistance improvement: 7.1Ω
・ "After heater circuit" Before insulation resistance improvement treatment: 6.8Ω → After insulation resistance improvement treatment: 7.1Ω
Therefore, it was confirmed that soot (carbon) adhering to the insulating material 6 between the graphite heater 5 and the heat insulating material 4 was removed by the burnout process, and the insulating property of the insulating material 6 was improved.

Further, when the surface resistance of the graphite heater 5 was measured before and after the treatment for improving the insulation resistance, it showed the same tendency as the resistance between the heater 5 and the furnace body 2 as shown below. I understood. The heater 5 was measured at three points: “heater receiver”, “hanger”, and “power lead”.
・ "Heater receiver" Before insulation resistance improvement treatment: 93Ω → After insulation resistance improvement treatment: 92Ω
・ "Hanger" Before insulation resistance improvement treatment: 77Ω → After insulation resistance improvement treatment: 94Ω
・ "Power Lead" Before insulation resistance improvement treatment: 66Ω → After insulation resistance improvement treatment: 76Ω

  Since the graphite heater 5 has a porous surface and many fine holes are formed, a large amount of carbon (soot) generated by the carburizing process adheres to the holes. However, as can be seen from the above results, carbon (soot) adhering in this way is oxidized and removed prior to the graphite heater 5 by the burnout process. Therefore, consumption of the graphite heater 5 due to the burnout process is suppressed.

  Separately from the above embodiment, the heating temperature was set to 700 ° C. in the reduced pressure heating step of the above method, and the other steps were performed in the same manner as in the above method. As a result, although the insulation resistance was improved, the effect was small as compared with the above embodiment in which the heating temperature was 800 ° C.

As described above, in the method for improving the insulation resistance of the vacuum heating furnace of the present embodiment, the inside of the heating chamber 3 is depressurized to a preset degree of vacuum and the inside of the heating chamber 3 is preliminarily formed by the graphite heater 5. After heating to the set temperature, burnout for introducing dry air into the heating chamber 3 is performed, so that the insulation between the heater 5 and the heat insulating material 4 is suppressed while suppressing the consumption of the graphite heater 5 due to oxidation. The soot (carbon) adhering to the material 6 can be quickly oxidized and gasified and removed from the insulating material 6. Thereby, the insulation between the graphite heater 5 and the heat insulating material 4 can be improved, and the heating efficiency of the graphite heater 5 can be improved.
In addition, since air is introduced to directly oxidize soot (carbon) with oxygen, soot (carbon) can be oxidized more quickly than conventional methods that oxidize using carbon dioxide, and therefore burnout is possible. The time required for the process can be shortened, and the processing efficiency of the vacuum carburizing process can be improved.

  In addition, after the pressure in the heating chamber 3 is reduced and heated, a stabilization process is performed in which the temperature in the heating chamber 3 is stabilized by turning off the power to the graphite heater 5. By lowering the temperature to a temperature substantially equal to the temperature in the heating chamber 3, the temperature difference between them can be almost eliminated. Therefore, in the subsequent burnout, it is possible to prevent the graphite heater 5 from being easily reacted and exhausted because the temperature of the graphite heater 5 is higher than the others.

Next, another embodiment according to the insulation resistance improving method of the present invention will be described.
The difference between the present embodiment and the previous embodiment is that the burnout process in the previous embodiment is repeated a plurality of times.
That is, in this embodiment, each process is performed according to the pattern shown in FIG. 3 about the insulation resistance between the graphite heater 5 and the heat insulating material 4 which became small by the carburizing process.

As shown in FIG. 3, in this embodiment, first, the reduced pressure heating process is performed in the same manner as in the previous embodiment. Specifically, after reducing the pressure to a predetermined degree of vacuum (160 Pa) and heating to 800 ° C., the power supply to the graphite heater 5 is turned off to stabilize the temperature in the heating chamber 3.
Next, a burnout process is performed in the same manner as in the previous embodiment. However, in this embodiment, since the burnout process is repeated a plurality of times as described above, the processing time per process is shortened. In the present embodiment, the burnout process is repeated three times as shown in FIG. 3, and therefore the processing time per process is 20 minutes. In addition, about a burnout process, when performing it in multiple times, it is good also as 2 times or 4 times or more.

When the first burnout process is performed in this way, the vacuum pump 7 is operated to temporarily depressurize the inside of the heating chamber 3 to a predetermined degree of vacuum (160 Pa), and further energize the graphite heater 5 to heat the heating chamber 3. The inside is heated to 800 ° C. And after heating to 800 degreeC and stopping the vacuum pump 7, the electricity supply to the graphite heater 5 is turned off, and the temperature in the heating chamber 3 is stabilized.
Next, the second burnout process is performed in the same manner as the first burnout process, and the third process is repeated by repeating the same process.
Thereafter, the cooling step is performed in the same manner as in the previous embodiment.

Even in such a method for improving the insulation resistance of a vacuum furnace, burnout for introducing dry air into the heating chamber 3 is performed after the reduced pressure heating step, so that the graphite heater 5 is prevented from being consumed by oxidation. However, the soot (carbon) adhering to the insulating material 6 can be rapidly oxidized and gasified to be removed. Thereby, the insulation between the graphite heater 5 and the heat insulating material 4 can be improved, and the heating efficiency of the graphite heater 5 can be improved.
In addition, since air is introduced to directly oxidize soot (carbon) with oxygen, soot (carbon) can be oxidized more quickly than in the conventional method, thus shortening the time required for the burnout process, The processing efficiency of the vacuum carburizing process can be improved.
Further, since the bar-out process is repeated a plurality of times and the processing time per time is shortened, the consumption due to oxidation of the graphite heater 5 can be more reliably suppressed.

The present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the gist of the present invention.
For example, in the above-described embodiment, the heating temperature of the heating chamber 3 in the reduced pressure heating step is set to 800 ° C., but the heating temperature is not limited to 800 ° C. and can be set to an arbitrary temperature. In the embodiment shown in FIG. 2, the heating temperature in the reduced pressure heating step is set to 850 ° C., and the other steps are performed in the same manner as in the above method. As a result, the heating temperature is set to 800 ° C. The effect was obtained. Therefore, it turned out that it is preferable to set it as the range of 800 to 850 degreeC as heating temperature of the heating chamber 3 in a pressure reduction heating process.
Moreover, although the pressure (degree of vacuum) of the heating chamber 3 in the reduced pressure heating step is set to 160 Pa, this pressure can also be arbitrarily set within a range where the degree of vacuum is sufficient.

DESCRIPTION OF SYMBOLS 1 ... Vacuum heating furnace, 2 ... Furnace body, 3 ... Heating chamber, 4 ... Heat insulating material, 4a ... Through-hole, 5 ... Graphite heater, 6 ... Insulating material, 7 ... Vacuum pump, 8 ... Carburizing gas source, 9 ... Air source

Claims (3)

A vacuum heating step of reducing the pressure in the heating chamber of a vacuum heating furnace having a graphite heater in the heating chamber to a preset degree of vacuum and heating the heating chamber to a preset temperature by the graphite heater;
A burnout step of introducing dry air into the heating chamber after the reduced pressure heating step;
A method for improving insulation resistance of a vacuum heating furnace, comprising:   The reduced pressure heating step includes a stabilization step of heating the interior of the heating chamber to a preset temperature with the graphite heater and then turning off the power to the graphite heater to stabilize the temperature in the heating chamber. The method for improving insulation resistance of a vacuum heating furnace according to claim 1.   The method for improving insulation resistance of a vacuum heating furnace according to claim 1 or 2, wherein, in the reduced pressure heating step, the heating chamber is depressurized to 160 Pa or less and the heating chamber is heated to 800 ° C to 850 ° C. .

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