JPH03146434A - Method for heat-treating high purity quartz-porous glass preform and heating furnace therefor - Google Patents

Abstract

PURPOSE:To prevent the contamination of a porous glass preform with impurities and to obtain the preform excellent in transparency, etc., by inserting the preform into a furnace core tube piercing furnace casing from the vertical lower part of the tube, heat-treating the preform and then discharging the preform from the lower part. CONSTITUTION:The furnace core tube 3 is allowed to pierce the hollow furnace casing 1 having a heating element 2, and an opening 10 directed vertically downward is provided at the part of the tube projecting downward from the casing 1. An inert gas is supplied into the tube 3 from an inlet 5 provided at the upper part of the tube 3, and a porous glass preform 7 is inserted into the tube 3 from the opening 10 and heat-treated. The heat-treated preform 7 is discharged from the tube 3 vertically downward. Consequently, the mixing of the ambient atmosphere into the heating atmosphere is prevented, the contamination with the dust in the atmosphere contg. Al2O3, Fe2O3, etc., is obviated, and the preform 7 is not devitrified.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a heating furnace for a high-purity quartz glass base material such as a matrix for optical fibers, and more specifically, for heating silica-based porous glass @particles. The present invention relates to a heating furnace that processes (for example, dehydration, dopant addition, sintering, etc.) to produce a transparent high-purity quartz glass base material used in the manufacture of optical fibers and the like.

[Prior Art] Conventionally, for example, Japanese Patent Publication No. 5B=42136 and No. 58-58299 and Japanese Patent Application Laid-Open No. 60-8604 have been used in heating furnaces used for manufacturing glass preforms for optical fibers.
As shown in Japanese Patent No. 9, it has been proposed to use a quartz glass tube as a furnace core tube.

However, a serious problem with quartz glass tubes is that they are easily deformed at high temperatures. In fact, when using a heating furnace at temperatures above 1500°C, the quartz glass tube will deform if the supporting method of the furnace core tube and the differential pressure inside and outside the furnace core tube are not strictly controlled.
It may become unusable. Further, when used for a long time at 1150° C. or higher, devitrification (crystallization) occurs in the quartz glass tube. Since the glass layer and the devitrification layer have different coefficients of thermal expansion, there is a problem that the furnace core tube may be destroyed by the resulting strain.

The present inventors have already discovered that a carbon tube is effective as a furnace core tube in order to solve this problem (for example, Japanese Patent Application Laid-Open No. 64-76927 and International Application Publication No. WO 38106145 (PCT/ JP8B1001
51)).

Carbon tubes are not only stable at temperatures above 2,000°C and have excellent heat resistance, but also have ash content of 20 ppm or less, are easy to purify, and are useful for heat treatment of glass base materials for optical fibers. sexual gases (e.g. cQ, , C
It has the advantage of not reacting with CQ, SiF+, S F m, CCl2tFt, etc.). Since carbon tubes have good processing accuracy, they can be assembled to reduce costs, and the outer surface can be coated with SiC or carbon to improve airtightness, making it a higher quality glass base for optical fibers. material can be obtained.

In the conventional heat treatment, for example, as shown in FIG. 7, the heat treatment is performed in a furnace equipped with a zone heater. A carbon heater 2 is provided inside a hollow furnace body 1, and a furnace core tube 3 passes through the furnace body 1. This heating furnace has a nitrogen gas population 4 for purging the furnace body, an atmospheric gas population 5 in the furnace core tube, and a glass base material support jig 6, and a porous glass body 7 is inserted into the heating furnace. The furnace core tube 3 has an upper part 31, a middle part 32, and a lower part 33.
At least the central portion 32 is made of carbon, and the surface of the carbon may be coated with SiC or carbon.

In the conventional heat treatment, the heating furnace is constructed as shown in FIG. 7, so when the glass body is taken in and out, the surrounding atmosphere (atmosphere of the working chamber) enters the furnace core tube.

FIG. 8 is a schematic diagram of an apparatus for measuring the amount of ambient air mixed into the furnace core tube. This device consists of a furnace core tube 1011, a purge gas population 102, a gas sampling tube 103, and an oxygen concentration measuring device 10.
4, has a pump 105 and a zone furnace 106. The inner diameter of the furnace core tube 101 is 150 mm, and the tip of the gas sampling tube is fixed at a point 1 m from the opening of the furnace core tube.

Using the apparatus shown in FIG. 8, the extent to which ambient air entered the heating furnace was measured. The results are shown in the graph of FIG. 9 as a relationship between the oxygen concentration in the heating furnace and the amount of nitrogen to be purged. As is clear from the graph, a considerable amount of ambient air is mixed into the furnace core tube, and it is understood that even if the flow rate of the purge nitrogen gas is increased, it is impossible to prevent ambient air from being mixed in.

When air enters the heating furnace in this way, the following problems occur.

First, the inside of the furnace core tube is contaminated by dust in the atmosphere.

The dust is composed of 5ide, AQvOs, Fe1O3, etc., among which Al2tOa causes base material devitrification, and Pet's causes increased loss.

Second, oxidation of the inner surface of the carbon furnace tube occurs.

It is known that when carbon fired bodies are oxidized, tar and pitch used as binders are first oxidized. Therefore, the remaining graphite particles fall off or scatter and fly around in the furnace. Since these particles adhere to the surface of the sintered glass base material, a fiber manufactured from a glass base material heat-treated in such a state will have many low-strength portions. Furthermore, as a matter of course, the life of the carbon furnace tube becomes extremely short.

The first method to prevent oxidation of the furnace core tube is as follows:
Carbon does not oxidize the temperature at which the glass body is taken in and out 400
It is possible to reduce the temperature to below ℃.

However, with this method, the operating rate of the furnace decreases significantly, and furthermore, since the carbon furnace core tube is porous, once exposed to the surrounding atmosphere, a considerable amount of moisture in the surrounding atmosphere will be adsorbed to the furnace core tube. It is impossible to completely prevent oxidative consumption of carbon.

The second method is to provide a front chamber in the upper part of the furnace core tube, store the porous glass base material in the front chamber once, replace it with inert gas, and then move the porous glass base material into the furnace core tube. is disclosed in Japanese Patent Application Laid-Open No. 64-76927. The furnace described in this patent application is shown in FIG. 1O.

The heating furnace shown in Fig. 1O has a carbon heater 2 inside the furnace body l.
and a carbon furnace core tube 3 are provided. This heating furnace has a nitrogen gas population 4 for purging the furnace body, an atmosphere gas population 5 in the furnace core tube, a glass base material support jig 6, and a front chamber 11. It has a front chamber gas outlet 14, a front chamber purge gas port 15, and a partition 16, and a porous glass body 7 is inserted into the heating furnace.

In order to insert the porous glass body into the heating furnace shown in FIG. 10, the following steps are performed.

! A porous glass body 7 is attached via a support rod 6 to a chuck that can move up and down once.

2. Open the top lid of the front chamber 11 and insert the porous glass body 7 into the front chamber 1.
Lower it to within 1.

3. Close the top lid and fill the front chamber with inert gas (N or He).
etc.).

4. Open the partition 16 that separates the front chamber 11 from the heating atmosphere, and introduce the porous glass body 7 into the heating atmosphere maintained at the heat treatment temperature in advance.

5. Close the partition 16.

Further, in order to take out the base material from this heating furnace, the following procedure is performed.

Open the l1 partition 16.

2. After the heat treatment, the base material 7 is removed from the heating atmosphere into the front chamber 1.
Raise it to 1. At that time, the temperature of the heating atmosphere does not necessarily need to be lowered.

3. Close partition ■6.

4. Open the upper lid of the front chamber 11 and take out the base material 7.

[Problems to be Solved by the Invention] Although the heating furnace shown in Fig. 1O described above has a good effect in terms of the function of preventing oxidative wear and tear of the carbon furnace core tube due to entrainment of the surrounding air, it has the following problems. A new problem arises.

That is, (1) since it is necessary to provide a front chamber, the total height of the device becomes too long. Figure 2 shows the total length 800 ++
This is an example of the heating furnace shown in FIG. 1O when heat-treating a porous glass base material with a seed rod of 200 mm. In this case, a distance of 6760+ m is required from the tip of the furnace core tube to the lower end of the chuck, and the total height of the heating furnace is nearly 8000 mm, taking into account the space required for work.

Furthermore, the zero partition I6 has a complicated structure.

The partition 16 needs to accommodate both cases when the porous glass base material support is penetrated and when it is not.

A method of operating the partition 16 will be specifically described below with reference to FIGS. 12, 13, and 14.

Fig. 12 shows a state in which the porous glass body is held in the front chamber 11, and Fig. 13 shows the state in which the porous glass body is held in the front chamber 11, and Fig. 13 shows the state in which the inside of the front chamber 2 is replaced with inert gas, the partition 16 is opened, and the porous glass body is heated in the atmosphere. FIG. 14 shows the state in which the device is inserted into the interior and is subjected to heat treatment.

When performing these three operations, it is necessary to operate three members, namely, the penetration seal M72 and the two half lids 71 using the two slide rods 73.

Therefore, for example, the following problems arise: (i) Since the partition 16 is composed of a total of three members, it takes time to open and close the partition; (ii) The structure of the partition 16 is complicated; In particular, it is difficult to confirm that the half-lid is completely closed, and there is a high possibility that the front chamber will be opened to the atmosphere in an insufficiently sealed state: and (iii) the structure of the partition 16. Due to the complexity of the system, the volume of the partition increases and it takes time to replace the gas.

[Means for Solving the Problems] As a means for solving the above-mentioned problems, the present invention provides high-purity quartz porous glass using a hollow furnace body having a heating element and a furnace core tube penetrating the furnace body. In this method, a porous glass base material is inserted into the furnace core tube from vertically below the furnace core tube, and the glass base material is inserted vertically downward into the furnace core tube. Provided is a heat treatment method characterized by taking out the heat treatment.

Furthermore, as a more preferable embodiment of the heat treatment method, the present invention prevents ambient air from entering the furnace core tube by supplying an inert gas above the furnace core tube at least when the furnace core tube is opened. The heat treatment method includes preferentially allowing an inert gas to flow into the furnace core tube instead of the surrounding atmosphere.

Further, the present invention has a hollow furnace body having a heating element and a furnace core tube passing through the furnace body, and heat-treats a high-purity quartz porous glass base material in the furnace core tube to produce a high-purity quartz base material. A heating furnace for manufacturing, the furnace core tube portion protruding downward from the furnace body opens vertically downward and has an opening that can be closed with a cap member, To provide a heating furnace in which a glass preform to be heat treated can be inserted into a furnace core tube through an opening and taken out from the furnace core tube. The heating furnace of the present invention can most conveniently be used in the heat treatment method of the present invention described above.

As used herein, the term "heat treatment" is used to include all operations that heat the furnace core tube, including dehydration, dopant addition, sintering, and the like.

[Function] The heat treatment method and heating furnace of the present invention will be explained in detail with reference to FIGS. 1 to 6.

1 and 2 schematically show a method of heat treatment using the heating furnace of the present invention. FIGS. 3 to 6 are schematic diagrams for consideration as the basis of the present invention.

First, an experiment was conducted to visualize the gas flow in the atmosphere-opening part of the furnace core tube in order to examine the mixing state of the surrounding atmosphere with respect to the conventional furnace core tube.

The results are schematically shown in FIG. 3(a). As shown by the arrows, it was found that a strong downward flow occurred at the center of the furnace core tube, and an upward flow occurred near the inner wall surface of the furnace core tube. Also,
At the opening (radius of the cross section passing through the center of the furnace core tube (r)
The velocity (v) distribution in this direction is schematically shown in FIG. 3(b). In FIG. 3(b), the shaded area is the area where the surrounding atmosphere enters the furnace core tube. The heater temperature during the experiment was 80°C, the temperature of the surrounding atmosphere was 25°C, and the outer surface temperature of the upper part of the furnace core tube was 200°C.

This phenomenon is due to the effect of natural convection caused by the temperature difference between the low temperature surrounding atmosphere and the high temperature furnace core tube.

Next, let's consider the natural convection phenomenon using a simple example. As shown in FIG. 4, when the red part of the bottom of a beaker containing water is heated, the water near the center of the bottom of the beaker is warmed, becomes lighter, and rises due to buoyancy. After being cooled on the water surface, it descends along the vicinity of the beaker wall and reaches the bottom, as shown by the arrowed line in the figure. On the other hand, when heating the lower side of the beaker, the opposite phenomenon occurs as shown in FIG. 5.

The natural convection phenomenon can be said to be a phenomenon in which a density difference occurs in a fluid due to a temperature difference, and some kind of flow phenomenon occurs.

Based on the above considerations, the phenomenon of burning in the core tube of a heating furnace can be considered as follows.

The gas in the furnace core tube is heated at the high temperature portion, that is, the inner wall surface of the furnace core tube, and a high temperature, low density upward flow is generated along the wall surface.

Due to this upward flow, the gas inside the furnace core tube is released outside the furnace core tube, so in order to compensate for the released gas, it is relatively lower temperature and denser than the upward flow, and exists above the gas inside the furnace core tube. The surrounding atmosphere descends through the center of the furnace tube. This downward flow is the cause of mixing with the surrounding air, entraining the dust contained in the outside air into the furnace core tube, and if the furnace core tube is made of carbon material, the oxygen contained in the surrounding atmosphere causes carbon will be oxidized and consumed.

Based on the above considerations, in order to prevent the air from being mixed in due to the natural convection mentioned above, the high temperature, low density gas heated by the inner wall of the furnace core tube must always exist above the surrounding low temperature, high density atmosphere. It is thought that a furnace core tube with a structure like this would be sufficient.

That is, in a heating furnace having a conventional structure as shown in FIG. 3, the opening of the furnace core tube is open upward, allowing the high-density surrounding atmosphere to enter from the outside. That is, the surrounding atmosphere, which has a high density, tends to descend more easily due to the action of gravity than the gas in the reactor core tube, which has a lower density. Therefore,
As long as the opening of the furnace core tube is open upward, ambient air tends to enter the furnace core tube.

However, as shown in FIG. 6(a), when the opening of the furnace core tube is provided in the lower part of the furnace core tube and the opening of the furnace core tube opens downward, the inside of the furnace core tube Gases with low density are difficult to escape to the outside, and surrounding air is difficult to enter the furnace core tube. Therefore, the gas heated on the inner wall of the furnace core tube rises, merges at the upper part of the furnace tube, and then descends, and this downward flow replenishes the gas that has risen near the inner wall surface of the furnace core tube.

Furthermore, near the opening of the furnace core tube, the high-temperature, low-density gas heated in the furnace core tube exists above the low-temperature, high-density surrounding atmosphere, so there is almost no exchange of gas due to density differences. Therefore, the intrusion of air from outside can be minimized. Therefore, the velocity (in the radial (') direction of the cross section passing through the center of the furnace core tube) at the opening in the case of Fig. 6 (a)
v) The distribution can be schematically shown as in FIG. 6(b).

The heat treatment method and heating furnace of the present invention are based on the above considerations.

That is, in the present invention, as shown in FIGS. 1 and 2,
The glass preform 7 is taken in and out of the furnace core tube 3 through a vertically downward opening 10 of the furnace core tube portion that protrudes from the furnace body 1. The aspect of the furnace core tube shown in FIGS. 1 and 2 is
The embodiment shown in the figure has been modified into a practical embodiment.

Therefore, when the furnace core tube is opened and the glass base material is put in and taken out, even if convection occurs in the furnace core tube due to the high temperature of the inner wall of the furnace core tube, the surrounding atmosphere will flow inside the furnace core tube due to the above-mentioned reason. Contamination of the pipes is minimized.

In a more preferred embodiment, when the furnace core tube is opened, an inert gas, such as nitrogen, is supplied into the furnace core tube from above, thereby further suppressing the intrusion of the surrounding atmosphere into the furnace core tube.

FIG. 1 is a schematic sectional view of the heating furnace of the present invention before or after heat treatment, in which the cap member 9 is removed from the furnace core tube 3 and the opening 10 of the furnace core tube is open downward. .

FIG. 2 is a schematic sectional view of the heating furnace of the present invention during heat treatment, and the cap member 9 is attached to the furnace core tube 3.

Next, the following experiment was conducted to evaluate the performance of the heating furnace according to the present invention. In the experiment, the heating furnace shown in FIG. 1 was used, and the performance was evaluated by measuring the oxygen concentration inside the furnace core tube 3 with the cap member 9 open.

The surface temperature of the carbon heater 2 is 850"C1. The flow rate of the furnace atmosphere gas (introduced from 5) is IOR/min (N.

When using gas), the oxygen concentration in the furnace core tube 3 is 1100
It was less than pp. This value of 1100pp is 0.05g per hour when the material of the furnace core tube 3 is carbon.
This is a level that converts carbon into COt, which is a practically acceptable value.

When inserting a porous glass base material into a heating furnace, the following steps are performed, for example.

The description will be made starting from the state in which the porous glass base material 7 is not placed in FIG. 2.

(2) Set the temperature of the carbon heater 2 to 850° C. or lower, and supply a purge atmosphere gas (for example, N gas) from the furnace core tube atmosphere gas port 5.

■Open the carap member 9. During this time, the purge gas is
The flow rate is 1091 minutes.

(2) Pull down the cap member 9 as shown in FIG. 1 to fix the porous glass base material 7 to the support rod 6 (which can rotate and move up and down). Purge gas is supplied in the same way as in (■).

(2) The porous glass base material 7 is raised together with the cap member 9 using, for example, the cap member lifting device 8 and inserted into the furnace core tube 3. Purge gas is supplied in the same way as in (■).

■ After closing the cab 1 part vI9, exhaust the inert gas from the inert gas exhaust boat 91. (State a in Fig. 2) ② Start processing of the porous glass base material 7.

The above-mentioned procedure for taking out the base material from the furnace core tube of the heating furnace may be reversed. (In step (3), the base material is removed from the support rod.) [Example] Examples of the present invention are shown below.

Using a conventional heating furnace having a front chamber above the furnace core tube shown in Fig. 11, a porous glass base material (total length 800a++o,
When we designed a heating furnace for heat-treating seed rods (200 am in length), the total height of the equipment, including the space required for design and work, was approximately 8000xx.

On the other hand, when designing the heating furnace of the present invention shown in Fig. 1 for processing the same porous glass base material, the height of the device is 4560 mm from the lower end of the chuck to the lower end of the cap member lifting device.
It was s-. The total height of the actually designed equipment including the space required for design and work is 5800+
nn, and the 11th to process the base material of the same size as mentioned above.
It has been found that a heating furnace approximately 2.2 m lower than the apparatus shown in the figure is sufficient.

Example 2 A heating furnace with a diameter of 200 u in Fig. 1 (the furnace core tube part has a thickness of 8
A porous glass base material was heat-treated using a high-purity carbon material whose surface was coated with 30 μm of pyrolytic carbon. The processing conditions are shown below: Temperature during dehydration
1100℃ Clearing temperature 1650℃ In this example, the conditions when inserting the porous glass base material into the furnace core tube are shown below: Carbon heater temperature 850℃ Carbon heater soaking section length 500xx Carbon heater soaking section length Temperature distribution at 850±1O℃ Purge gas type N.

Purge gas flow rate 10. ! l/l/tube core tube inner diameter 200xm Furthermore, under the above conditions, O when the cap member 9 is opened at the same time! Concentration was also measured. The time the cap is open is 3 minutes to set the porous gas base material, and the maximum O1 concentration at this time is 12Bp.
It was p-.

Further, when the furnace core tube portion exposed outside the heater was also heated and the length of the soaking section was set to 11,000 i, the O2 concentration was measured in the same manner, and the maximum value was 60 ppm.

After heat treatment of the base material, this glass base material is used as a core material and drawn at a drawing speed of 150x/min to obtain an outer diameter of 125μ.
When an optical fiber with dimensions and shape of Rin was made, the transmission loss was 0.18 dB/km at an optical wavelength of 1.55 μ-.
The loss was low.

X Shikigoi i The porous glass body was sintered in the same manner as in Example 2.
Repeated 0 times. The weight loss of the carbon furnace core tube during this period was 14
9 (corresponding to oxidative wear of 35 μm in the heating section). This amount of consumption corresponds to the amount that allows the carbon furnace core tube to be used for about two years.

Comparative Example 1 A heating furnace shown in Fig. 1O (the furnace core tube part is made of high-purity carbon whose surface is coated with 30 μl of pyrolytic carbon with a thickness of 8R1) was used. The porous glass body was placed in the front chamber, the upper lid of the front chamber was closed, and nitrogen gas was flowed into the front chamber at 10 Q/min for 10 minutes to replace the inside of the front chamber with nitrogen gas. Thereafter, the partition was opened, the porous glass preform was moved from the front chamber into the furnace core tube, and after the partition was closed, heat treatment was performed to produce a transparent glass preform for optical fiber.

The heat treatment conditions were the same as in Example 2.

When taking out the base material, the partition was first opened, the glass base material was moved to the front chamber, the partition was closed, and then the top cover was opened and the glass base material was taken out.

When an optical fiber was made using the obtained glass base material as a core material in the same manner as in Example 2, the transmission port was as low as 0.18 dB/kn at an optical wavelength of 1.55 μl.

Comparative Example 2 In the same manner as in Comparative Example 1, the heat treatment of the porous glass body was repeated 40 times. During this period, the weight loss of the carbon furnace tube was 2.
0g (corresponding to oxidative consumption of 50μ debris from the surface).

For this amount of consumption, use a carbon furnace core tube! , equivalent to the amount that can be used for about 5 years.

[Effects of the Invention] Contamination of the surrounding atmosphere (atmosphere of the working room) into the heating atmosphere is eliminated, and contamination by impurities in the furnace core tube is eliminated. Therefore, devitrification of the base material can be prevented, and the transparency of the manufactured base material is improved.

If the furnace core tube is made of carbon, the oxidation consumption of carbon is suppressed and the life of the furnace is extended. Furthermore, when compared with a heating furnace with a front chamber for the same purpose, the heating furnace of the present invention has the same or better effect, and has the additional advantage that the structure of the heating furnace of the present invention is simpler and the overall height of the apparatus is lower.

Furthermore, since there is no need to lower the temperature of the furnace body when loading and unloading the glass body, the operating rate of the furnace is increased.

[Brief explanation of the drawing]

Figures 1 and 2 are schematic cross-sectional views of the heating furnace of the present invention, Figures 3 (a) and (b) are diagrams showing the state of gas flow in the upper part of the furnace core tube, and Figures 4 and 5. is a schematic diagram of natural convection,
FIGS. 6(a) and (b) are diagrams showing the principle of the present invention,
Figure 7 is a schematic cross-sectional view of a conventional sintering furnace for producing high-purity quartz base material, Figure 8 is a schematic configuration diagram of a device for measuring the oxygen concentration inside the furnace core tube, and Figure 9 is a schematic cross-sectional view of a conventional sintering furnace for producing high-purity quartz base material. A graph showing the relationship between oxygen concentration and purge gas flow rate, Figure 1O is a schematic diagram of another conventional heating furnace for producing high-purity quartz base material, and Figure 11 explains the height of the heating furnace in Figure 10. 12, 13, and 14 are explanatory diagrams showing a method of operating the partition of the heating furnace shown in FIG. 1O. I...Furnace body, 2...Carbon heater, 3...Furnace core tube, 4...Furnace body purge gas inlet, 5...Atmospheric gas population in the furnace core tube, 6...Glass motherboard Material support jig, 7... Porous glass base material, Cap member lifting device, 9... Cap member,
lO...opening, 11...front chamber, 14...front chamber gas outlet, 15...front chamber purge gas inlet, 16...partition,
1.32.33... Furnace core tube component, ■... Half lid, 72... Penetration sealing lid, 3... Slide rod, 1... Inert gas discharge boat, 02 ...Gas population for purging the furnace body, 03...Gas sampling pipe, 04...Acid '1g111 degree measuring device, 06...Furnace body. 101... Furnace core tube, 105... Pump,

Claims (1)

[Scope of Claims] 1. A method for heat-treating a high-purity quartz porous glass base material in a furnace core tube using a hollow furnace body having a heating element and a furnace core tube passing through the furnace body, the method comprising: A heat treatment method comprising inserting a porous glass preform into a furnace core tube from vertically below the core tube, and removing the glass base material from the furnace core tube vertically below the furnace core tube. 2. The heat treatment method according to claim 1, further comprising supplying an inert gas above the furnace core tube. 3. A hollow furnace body having a heating element and a furnace core tube passing through the furnace body, for producing a high purity quartz base material by heat-treating a high purity quartz porous glass base material in the furnace core tube. A heating furnace, the furnace core tube part of which protrudes downward from the furnace body, opens vertically downward and has an opening that can be closed with a cap member, and the glass to be heat treated. A heating furnace in which a base material can be inserted into a furnace core tube through an opening and taken out from the furnace core tube. 4. The heat treatment method according to claim 1 or 2, using the heating furnace according to claim 3.