JPH07256462A - Structure of welding part in gas shield arc welding equipment - Google Patents

Abstract

PURPOSE:To reduce sticking of spatter and to easily remove its spatter. CONSTITUTION:This equipment consists of a torch 11, a tip 7 to be attached on the top end of this torch 11, and a nozzle 1 which is attached on the torch 11, cylinder aligns this tip 7 into the axial line and receives it in a hollow part. The hollow part of the nozzle part 1 has a taper part 18 which gradually make the dimension of its inner diameter large from an almost central part in the longitudinal direction toward the top end part, and a ceramics thin film 10 is formed on the internal wall of the taper part 18 in the almost central part in the longitudinal direction of this taper part 18.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is used in welding equipment used for manufacturing, maintaining, repairing structures and the like, and particularly used for inert gas (hereinafter referred to as "inert gas") arc welding, carbon dioxide arc welding, etc. The present invention relates to the structure of a welded part of a gas arc welding device equipped with a nozzle for injecting gas into a welded part.

[0002]

2. Description of the Related Art Conventionally, as welding methods, there are welding methods such as covered arc welding and gas welding. Inert gas arc welding using an inert gas such as argon or helium or carbon dioxide gas is used instead of the covering material. Carbon dioxide arc welding was known.

In gas arc welding, an arc is generated between an electrode rod and a portion to be welded in a gas atmosphere, and the electrode rod or the portion to be welded is melted by the high heat generated by this arc to perform welding. . For this gas arc welding,
It is classified into a TIG welding method and a MIG welding method depending on the difference in the electrodes.

The TIG welding method is a method in which a welding rod is used as an electrode and the welding is performed while melting the welding rod with an arc. A bare tungsten electrode rod is used as the welding rod.
Since this tungsten electrode rod hardly wears down,
It is also called non-electrode electrode gas arc welding. In the MIG welding method, instead of using a tungsten electrode rod as an electrode, an electrode wire made of a filler material is continuously fed, and an arc is generated between this electrode wire and a welded part to form the electrode wire. It is a method of melting and welding. Therefore, this MIG welding method
It is also called hot-electrode gas arc welding because the electrodes are consumed.

The welding electrode type gas arc welding apparatus is a welding power source for supplying a voltage for generating an arc, a welding torch facing the welded portion, and an electrode wire for feeding an electrode wire as a welding rod to the tip of the welding torch. A feeder, an inert gas,
It is composed of a gas cylinder for supplying a gas such as carbon dioxide. A cable to which current, electrode wire and gas are supplied is attached to the welding torch.By opening and closing the switch equipped with this welding torch as needed, an arc is generated to melt the electrode wire and weld it. I do.

There are various welding torches depending on the current used, the torch shape, and the cooling method. There are curved torches and pistol torches in the torch shape, and generally curved torches are used. Further, there are air cooling type and water cooling type cooling systems, and in general, there are many air cooling types. However, when a large current is used, a water cooling type is used because of high temperature. Commonly used welding torches are air-cooled curved torches and water-cooled pistol torches.

The tip of the conventional welding device will be described with reference to FIG. 6 showing an air-cooled curved torch. The welding portion 40 of the welding device using this air-cooled curved torch is
Torch 41, tip 44, insulating joint 42, baffle 4
3 and the nozzle 46. An electrode wire supply hole corresponding to the diameter of the electrode wire to be supplied is axially penetrated in the chip 44, and a voltage is applied from the chip 44 to the electrode wire (not shown) inserted in the electrode wire supply hole. It The insulating joint 42 serves to insulate the torch 41 and the nozzle 46 from each other. The baffle 43 is generally made of ceramics for the purpose of rectifying the flow of the delivered gas and preventing the torch 41 and the nozzle 46 from being electrically short-circuited.

Then, the gas is supplied to the torch 41 from a gas cylinder (not shown), ejected from a predetermined number of gas supply small holes 45 formed at the tip of the torch 41, rectified by the baffle 43, and the tip 44. And flows out through the gap between the nozzle 46 and the inner wall of the nozzle 46, and is ejected from the tip of the nozzle 46.

[0009]

By the way, during welding, a part of the molten metal is not fused and solidified on the welded portion but is scattered to form molten particles 66 (hereinafter referred to as spatter). Further, part of the scattered spatter 66 collides with and adheres to the inner wall of the nozzle 46, etc., as shown in FIG.
Particularly, since the spatter 66 adheres to the tip of the nozzle 46, the nozzle 46 has a problem that the gap between the tip 44, which is a gas passage, and the inner wall of the nozzle 46 is closed by the adhered spatter 66. .

Then, the gas is spattered by the sputter 66 in the nozzle 4
Even if the gap between the tip 44 and the inner wall of the nozzle 46 is not completely closed by adhering to the inner wall of the nozzle 6,
If this gap becomes narrow, it will not be sufficiently ejected from the tip portion of the nozzle 46. Therefore, unless the gas is sufficiently injected into the welded part, the welded part is insufficiently shielded from the outside air (so-called gas shield), and is oxidized and nitrided by the air, so There was a problem that the physical properties were made brittle.

Therefore, it is necessary for the nozzle 46 to remove the attached spatter 66 from the inner wall or the like by using a cleaning tool such as a bamboo spatula, and the cleaning work is performed so as not to damage the inner wall of the nozzle 46. ing. However, this cleaning operation has a problem that it is difficult and difficult to put a cleaning tool in this gap because the gap between the inner wall of the nozzle 46 and the tip 44 is narrow.

The nozzle 46 is provided with an inner wall of the nozzle 46 and the tip 44 in order to facilitate the cleaning operation of the nozzle 46.
It is necessary to increase the distance between the nozzles and the inner diameter of the nozzle 46. However, the tip portion of the electrode wire, which is the welded portion, is hidden by the nozzle 46 when the outer diameter dimension of the nozzle 46 increases as the inner diameter dimension of the nozzle 46 increases.
Therefore, there is a problem in that it becomes difficult to visually recognize the welded portion during the welding work, which causes a hindrance to the welding work.

Therefore, the welded portion of the gas arc welding apparatus is periodically disassembled and the nozzle 46, the baffle 43,
Maintenance work and cleaning work are performed to remove the spatter 66 adhering to the tip 44 and the torch 41.

Therefore, the present invention has been proposed for the purpose of providing a structure of a welded portion of a gas arc welding apparatus in which spatter adhered to the inner wall of a nozzle is reduced and the adhered spatter can be easily removed. It is a thing.

[0015]

In order to achieve the above-mentioned object, the welding structure of the gas arc welding apparatus according to the present invention is attached to the outer peripheral surface on the base end side and the inner peripheral surface on the tip end side, respectively. A cylindrical torch provided with a means, a cylindrical tip member provided with a mounting means attached to the mounting means on the tip end side of the torch at the base end, and a base end portion of the torch mounted on the base end. It is provided with a mounting means to be mounted on the mounting means, and is composed of a cylindrical nozzle member for accommodating the tip member in the hollow portion with its axis aligned. The hollow portion of the nozzle member includes a taper portion whose inner diameter gradually increases from the substantially central portion in the longitudinal direction toward the tip portion. In addition, at a substantially central portion in the longitudinal direction which is a taper portion, it is detachably divided into two parts, and a ceramic coating is further formed on the inner wall.

[0016]

According to the structure of the welded portion of the gas arc welding apparatus according to the present invention having the above-described structure, the inner diameter is gradually increased from the substantially central portion in the longitudinal direction toward the tip portion. As a result, the area of the end surface of the tip end portion of the nozzle member, which is a portion where spatter is likely to adhere, is reduced, and the surface area of the nozzle member is also reduced. Then, the gap between the inner surface wall of the nozzle member and the tip member also becomes large, so that spatter hardly adheres to the inner surface wall of the nozzle member.

Further, the ceramic coating formed on the inner wall of the nozzle member reduces the surface roughness of the inner wall of the nozzle member. Therefore, the spatter adhered on the ceramic coating is easily peeled off because of its low adhesion. further,
The ceramic coating protects the inner wall of the nozzle member from heat received by the collision and attachment of spatter and heat received by the arc. In addition, during cleaning work to remove spatter adhering to the inner wall of the nozzle,
By being divided, the spatter removal work can be easily performed.

[0018]

EXAMPLES The structure of the welded portion of the gas arc welding apparatus according to this example will be described in detail below with reference to the examples shown in FIGS. Welding part 20 of this gas arc welding device
Has substantially the same basic configuration as the welding portion 40 of the conventional welding apparatus, and includes a torch 11, a tip 7 attached to the tip of the torch 11, an insulating joint 4 attached to the torch 11, and the insulating joint 4. 4, a baffle 13 attached to 4, and a nozzle 1 attached to the insulating joint 4,
It is composed of an electrode wire 5 sent by an electrode wire feeder not shown.

The torch 11 is provided at the tip of the curved torch 11 with a cylindrical tip having a diameter smaller than that of the torch 11. A predetermined number of gas supply small holes 6 are formed on the outer peripheral surface of the cylindrical tip portion of the torch 11, and gas is delivered from the gas supply small holes 6. Also, torch 1
1, a threaded portion 12 to which the insulating joint 4 is attached is provided on the outer peripheral surface on the base end side of the cylindrical tip portion, and a tip 7 is formed on the inner peripheral surface on the tip end side of the cylindrical tip portion as shown in FIG. Is provided with a screw portion 32.

The tip 7 has a torch 11 on the inner peripheral surface of the base end.
A screw portion 33 is provided which is screwed into the screw portion 32 provided in, and the diameter is gradually reduced from the base end portion to the tip end portion. Also, chip 7
Is an electrode wire supply hole 64 corresponding to the diameter of the electrode wire 5 to be sent.
Is provided so as to penetrate in the axial direction, and when the electrode wire 5 is inserted into the electrode wire supply hole 64, it serves to apply the voltage supplied from the welding power source to the electrode wire 5.

The insulating joint 4 is provided with a screw portion 15 on the inner peripheral surface of the cylindrical base end portion, which is screwed into the screw portion 12 provided on the torch 11. Further, the insulating joint 4 is provided with a threaded portion 9 to which a threaded portion 8 provided on the nozzle 1 described later is screwed, on the outer peripheral surface of the cylinder distal end portion having a diameter smaller than the diameter of the cylinder proximal end portion. Further, in the insulating joint 4, a screw portion 34 to which the baffle 4 described later is attached is provided at the tip portion of the screw portion 9 provided on the outer peripheral surface of the cylindrical tip portion. The insulating joint 4 is disposed between the torch 11 and the nozzle 1 and serves to insulate the torch 11 and the nozzle 1 from each other.

The baffle 13 is provided on the inner peripheral surface of the base end of the cylinder.
A screw portion 35 that is screwed to the screw portion 34 provided on the insulating joint 4 is provided. The baffle 13 is provided with a predetermined number of gas rectification small holes 14 on the outer peripheral surface of the cylinder tip end portion whose diameter is smaller than that of the cylinder base end portion, and gas flows out from these gas rectification small holes 14. That is, the baffle 13 is disposed between the inner wall of the nozzle 1 and the tip 7,
The torch 11 and the nozzle 1 are electrically short-circuited by the spatter 66 adhering to the inner wall of the nozzle 1 while rectifying the flow of the gas delivered from the gas supply small hole 6 formed in the cylindrical tip portion of the torch 11. There is also a function to prevent doing.

The electrode wire 5 is inserted into an electrode wire supply hole 64 provided in the chip 7 by an electrode wire feeding device (not shown) and projected from the tip of the chip 7. Also, the electrode wire 5
Is applied with a voltage when it is inserted into the electrode wire supply hole 64, so that an arc is generated between the welded part and the welded part.

Welded portion 20 of the gas arc welding apparatus of the embodiment
Nozzle 1 used for is, on the inner peripheral surface of the cylindrical base end portion 60,
A screw portion 8 that is screwed with a screw portion 9 provided on the insulating joint 4 is provided. The nozzle 1 has a taper shape in vertical cross section whose inner diameter gradually increases from the substantially central portion in the longitudinal direction toward the tip portion 61. That is, the nozzle 1 is provided with a straight portion 17 that is parallel to the axis of the nozzle 1 and a taper portion 18 that is inclined with respect to the axis of the nozzle 1.

Therefore, the welded portion 2 of the gas arc welding apparatus is
In the nozzle 1 used for 0, the area of the end surface of the tip portion 61 of the nozzle 1 which is a plane orthogonal to the axis of the nozzle 1 becomes small, so that the spatter 6 scattered from the welded portion
Adhesion of 6 to the tip 61 of the nozzle 1 is reduced. Therefore, it is possible to reduce the possibility that the gap between the nozzle 1 and the chip 7 is closed by the spatter 66 attached to the inner peripheral corner of the tip portion 61 of the nozzle 1. Further, since the surface area of the nozzle 1 is reduced by providing the nozzle 1 with a tapered portion, the area where the sputter 66 is attached is also reduced.
Further, since the gap between the inner wall of the nozzle 1 and the tip 7 is widened, the spatter 66 separated from the inner wall of the nozzle 1 is prevented from adhering to the nozzle 1 again.

The nozzle 1 has a tapered portion 18 on the inner wall.
A ceramic thin film 10 having a predetermined thickness is formed by spraying the ceramics on the inner surface of the thin film 10. The surface roughness of the inner wall is reduced to reduce the adhesive force of the sputter 66 and prevent the sputter 66 from adhering. . Therefore, the attached spatter 66 is easily peeled off, so that the sputter 6
The removal work of 6 is facilitated. Further, the ceramic thin film 10 covering the tapered portion 18 of the inner wall of the nozzle 1 also serves to protect the inner wall from being damaged by the heat received by the collision and adhesion of the spatter 66 or the heat received by the arc.

An outline of the principle of gas arc welding performed by the welded portion 20 of the gas arc welding apparatus configured as described above will be described. The outline of this welding principle is that an electrode wire feeder (not shown) applies voltages of different polarities to the electrode wire 5 which is the filler material sent from the tip of the tip 7 and the welded part. Then, when these are brought into contact with each other and then separated slightly, an arc is generated in the gap between the electrode wire 5 and the welded portion. The electrode wire 5 is melted by the high temperature generated by this arc and becomes a droplet of molten metal and is welded to the welded portion.

In the gas arc welding, the gas supplied from a gas cylinder (not shown) has a predetermined number of gas supply small holes 6 formed in the outer peripheral surface of the cylindrical base end portion of the torch 11.
From the nozzle 61, is rectified by the baffle 13 and flows out along the inner wall of the nozzle 1,
Is jetted from. The injected gas wraps the electrode wire 5, the welded portion and the arc delivered from the tip portion of the tip 7 so as to shield the gas from the outside air and the electrode wire 5 and the welded portion. .

Since the gas shields the electrode wire 5 and the welded portion from the outside air, the droplets of the electrode wire 5 are protected from the air, and the welded portion is prevented from being oxidized or nitrided. That is,
During welding, the gas prevents the welded part from being adversely affected by air, and the mechanical properties of the welded part are prevented from becoming brittle.

The structure of the welded portion of the second embodiment gas arc welding apparatus will be described below with reference to FIGS. 4 and 5. The welding portion 70 of this gas arc welding apparatus has the same basic configuration as the welding portion 40 of the conventional welding apparatus, and includes a torch 52, a tip 27 attached to the tip of the torch 52, and an insulation attached to the torch 52. Joint 2
4 and a baffle 48 attached to the insulating joint 24
And a nozzle 2 attached to the insulating joint 24, and an electrode wire 25 delivered by an electrode wire feeder not shown.

The torch 52 is provided at the tip of the curved torch 52 with a cylindrical tip having a diameter smaller than that of the torch 52. A predetermined number of gas supply small holes 26 are formed on the circumferential surface of the cylindrical tip end portion of the torch 52, and gas is delivered from the gas supply small holes 26. Further, the torch 52 is provided with an insulating joint 2 on the outer peripheral surface on the base end side of the cylindrical tip end portion.
4 is provided with a screw portion 56, and the tip 2 is attached to the inner peripheral surface on the tip side of the cylindrical tip portion as shown in FIG.
A screw portion 57 to which 7 is attached is provided.

The tip 27 is provided at its base end with a threaded portion 58 which is screwed into a threaded portion 57 provided on the torch 52, and is formed so that the diameter gradually decreases from the base end portion to the tip end portion. There is. Further, the tip 27 is provided with an electrode wire supply hole 65 axially penetrating according to the diameter of the electrode wire 25 to be sent out, and when the electrode wire 25 is inserted into the electrode wire supply hole 65, a welding power source is provided. It serves to apply the voltage supplied from the electrode wire 25 to the electrode wire 25.

The insulating joint 24 is provided on the inner peripheral surface of the base end of the cylinder.
A threaded portion 59 that is screwed to the threaded portion 56 provided on the torch 52 is provided. Also, the insulation joint 24 is
A screw part 29 is provided on the outer peripheral surface of the tip end of the cylinder that is smaller than the diameter of the base end part of the cylinder to which a screw part 28 provided in the nozzle 2 described later is screwed. Further, the insulating joint 24 is provided with the screw portion 2 provided on the outer peripheral surface of the cylindrical tip portion.
A screw portion 53, to which a baffle 48 described later is attached, is provided at the tip portion of 9. And the insulating joint 24
Is disposed between the torch 52 and the nozzle 2 and serves to insulate the torch 52 and the nozzle 2 from each other.

The baffle 48 is provided on the inner peripheral surface of the base end of the cylinder.
A threaded portion 54 that is screwed into the threaded portion 53 provided on the insulating joint 24 is provided. The baffle 48 has a predetermined number of gas rectification small holes 49 formed in the outer peripheral surface of the cylinder tip end portion having a diameter smaller than that of the cylinder base end portion, and gas flows out from these gas rectification small holes 49. That is, the baffle 48 is arranged between the inner wall of the nozzle 2 and the tip 27, and the small hole 26 formed in the cylindrical tip portion of the torch 53 main body.
Rectifies the flow of gas discharged from the nozzle 2
It also has a function of preventing the torch 52 and the nozzle 2 from being electrically short-circuited by the spatter 66 attached to the inner wall of the.

The electrode wire 25 is inserted into an electrode wire supply hole 65 provided in the chip 27 by an electrode wire feeding device (not shown) and projected from the tip of the chip 27. Further, when the electrode wire 25 is inserted into the tip 27, a voltage is applied to generate an arc between the electrode wire 25 and the welded portion.

In the nozzle 2 used in the welding portion of the gas arc welding apparatus of the embodiment, the inner peripheral surface of the cylindrical base end portion 62 is provided with the screw portion 28 which is screwed into the screw portion 29 provided in the insulating joint 24. ing. And this nozzle 2
It has a vertical cross-sectional taper shape whose inner diameter gradually increases from the substantially central portion in the longitudinal direction toward the tip portion 63. That is, the straight portion 50 parallel to the axis of the nozzle 2
And a tapered portion 51 that is inclined with respect to the axis of the nozzle 2.

Therefore, the area of the end face of the tip portion 63 of the nozzle 2, which is a plane orthogonal to the axis of the nozzle 2, becomes smaller, and the spatter 66 scattered from the welded portion adheres to the tip portion 63 of the nozzle 2. Is reduced. Therefore, it is possible to reduce the possibility that the gap between the nozzle 2 and the chip 27 is closed by the spatter 66 attached to the inner peripheral corner of the tip portion 63 of the nozzle 2. The surface area of the nozzle 2 is
Since it is reduced by providing the tapered portion in the nozzle 2, the area where the spatter 66 adheres is also reduced. Further, since the gap between the inner wall of the nozzle 2 and the tip 27 becomes wider, the sputter 6 separated from the inner wall of the nozzle 2
The redeposition of 6 on the nozzle 2 is reduced.

The nozzle 2 has a tapered portion 51 on the inner wall.
A ceramic thin film 55 having a predetermined thickness is formed by spraying ceramics onto the inner surface of the inner wall, and the surface roughness of the inner wall is reduced to reduce the adhesive force of the spatter 66 and prevent the spatter 66 from adhering. . Therefore, the attached spatter 66 is easily peeled off, so that the sputter 6
The removal work of 6 is facilitated. Further, the ceramic thin film 55 covering the tapered portion 51 of the inner surface wall serves to protect the inner surface wall from being damaged by heat received by collision and adhesion of the spatter 66 or heat received by the arc.

Further, the nozzle 2 has a tip portion 6 of the nozzle 2 at a substantially central portion in the longitudinal direction where the nozzle 2 has a tapered cross section.
The first nozzle main body 21 on the third side and the second nozzle main body 22 on the base end 62 side of the nozzle 2 are divided and configured. The first nozzle body 21 and the second nozzle body 22 are provided with threaded portions 30 and 31 at their respective ends to which the first nozzle body 21 and the second nozzle body are attached. 22 is assembled. By dividing the nozzle 2, the nozzle 2 can be divided and cleaned when the spatter 66 adhering to the inner wall of the nozzle 2 is removed, and the cleaning tool can be installed in every corner of the inner wall. As it arrives, cleaning work becomes easier.

An outline of the principle of gas arc welding performed by the welding portion 70 of the gas arc welding apparatus configured as described above is described in the principle of gas arc welding performed by the welding portion 20 of the gas arc welding apparatus of the first embodiment described above. The description is omitted because it is the same as the outline.

According to the structure of the welded portion of the gas arc welding apparatus according to the present embodiment described above, the number of nozzle cleanings can be reduced by improving the effect of preventing spatter adhesion.
It is possible to reduce interruption of welding work for cleaning and improve welding efficiency. Therefore, it is suitable for use in a welding process using an automatic welding machine.

The structure of the welded portion of the gas arc welding apparatus of the embodiment can also be used in, for example, an inert gas arc welding apparatus or a carbon dioxide gas arc welding apparatus. Also,
Although the nozzle body is divided into two, it may be divided into multiple stages as necessary. Further, although the screw type is adopted as the attaching means for detachably attaching the respective divided nozzle bodies, for example, other attaching means such as a coil built-in ring may be used.

[0043]

As described above, according to the structure of the welded portion of the gas arc welding apparatus according to the present invention, the tapered portion is provided so that the inner diameter gradually increases from the substantially central portion in the longitudinal direction of the nozzle toward the tip portion. As a result, it is possible to reduce the adhesion of spatter generated during welding. Therefore, cleaning work for removing spatter can be easily performed. Further, by disposing the ceramic thin film on the inner wall of the nozzle, it is possible to prevent the adhesion of spatter, facilitate the removal of the adhered spatter, and protect the inner wall of the nozzle from the heat received. Further, by detachably dividing the nozzle into two parts, it is possible to facilitate the cleaning work for removing spatter adhering to the inner wall of the nozzle.

[Brief description of drawings]

FIG. 1 is a vertical cross-sectional view showing a nozzle which is a main part of the structure of a welded portion of a gas arc welding apparatus according to an embodiment of the present invention.

FIG. 2 is a vertical cross-sectional view showing a structure of a welded portion of the gas arc welding apparatus.

FIG. 3 is an exploded perspective view showing a structure of a welded portion of the gas arc welding apparatus.

FIG. 4 is an exploded vertical cross-sectional view showing a nozzle that is a main part of the structure of the welded portion of the gas arc welding apparatus.

FIG. 5 is a vertical cross-sectional view showing a structure of a welded portion of the gas arc welding apparatus.

FIG. 6 is an exploded perspective view showing a structure of a welded portion of a conventional gas arc welding apparatus.

FIG. 7 is a vertical cross-sectional view showing a nozzle used in a welding portion of a conventional gas arc welding apparatus.

FIG. 8 is a perspective view showing an example of a state in which spatter adheres to a welded portion of a conventional gas arc welding apparatus.

[Explanation of symbols]

 1 Nozzle 2 Nozzle 7 Chip 8 Threaded Part 10 Ceramics Thin Film 11 Torch 17 Straight Part 18 Tapered Part 21 First Nozzle Main Body 22 Second Nozzle Main Body 27 Tip 28 Screw Part 30 Screw Part 31 Screw Part 50 Straight Part 51 Tapered Part 52 Torch 55 Ceramic thin film

Claims (3)

[Claims] 1. A cylindrical torch having mounting means provided on the outer peripheral surface on the base end side and an inner peripheral surface on the tip end side, and a mounting that is mounted on the base end part by mounting means on the tip end side of the torch. A cylindrical tip member provided with a means and a mounting means for mounting the mounting means on the base end side of the torch at the base end, and a cylindrical shape for accommodating the tip member in the hollow portion with the axes aligned. Nozzle member, wherein the hollow portion of the nozzle member is provided with a taper portion whose inner diameter gradually increases from a substantially central portion in the longitudinal direction toward the tip portion. Structure. 2. The structure of the welded part of the gas arc welding apparatus according to claim 1, wherein the taper part is detachably divided into two parts at a substantially central part in the longitudinal direction. 3. The structure of the welded portion of the gas arc welding apparatus according to claim 1, wherein a ceramic coating is formed on the inner wall.