JPH0214405B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a reducing atmosphere heat treatment furnace capable of extending the life of furnace internal members made of stainless steel, such as heater tubes.

Generally, in a reducing atmosphere heat treatment furnace, the workpiece is charged into the furnace, and the workpiece is heated and maintained at a predetermined temperature using an electric heater or radiant tube burner in a predetermined atmosphere, or is quenched, tempered, etc. after heating. This is used to adjust the material quality of the object to be processed.

For example, in gas carburizing treatment, a converted gas is supplied into the furnace and heat treatment is performed while maintaining the furnace at a desired carbon potential. Tubes and the like are made of stainless steel, so when the temperature reaches 600°C to 720°C, hydrogen in the atmosphere gas diffuses into the stainless steel, weakening the bond between the chrome, nickel, iron, etc. of the stainless steel. Hydrogen embrittlement, which makes chromium brittle, occurs, and carbon and oxygen in the atmosphere combine with chromium and iron in stainless steel, causing chromium and iron to form at grain boundaries.
Iron oxides and composite carbides precipitate, resulting in a potential difference between metal ions, making stainless steel susceptible to electrolytic corrosion. In addition, the stainless steel is susceptible to corrosion due to so-called carbon attack, in which carbon in the atmospheric gas diffuses onto the surface of the stainless steel, increasing the carbon concentration on the surface and causing structural changes on the surface and inside. There has been a problem in that such corrosion shortens the life of stainless steel members such as heater tubes used in heat treatment furnaces. In addition, when the object to be treated is salt quenched after being heated, the salt adheres to the tray and is brought into the heat treatment furnace, but when the temperature reaches high temperatures, the salt decomposes and generates oxygen, which causes the above-mentioned corrosion. It was something that would have encouraged it. In addition, if you try to extend the life of the heater tube by changing the material of the heater tube to one of the above-mentioned corrosion-resistant materials, you will have to use high-quality stainless steel, which will increase the cost. There was a problem.

The present invention has been made in view of the problems of the conventional products, and the outer surface of the stainless steel member is inserted into the heat insulating layer and is exposed to a temperature of about 600°C to 720°C, which is susceptible to corrosion. The purpose of the present invention is to provide a reducing atmosphere heat treatment furnace that can extend the life of stainless steel members such as heater tubes and stirring fan shafts by forming heat-resistant and corrosion-resistant ceramic coatings on the stainless steel parts. .

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows a first embodiment of the present invention, and in the figure, 1 is a cylindrical heat insulating layer made of a heat insulating material such as heat insulating bricks and long in the furnace length direction (perpendicular to the plane of the paper in FIG. 1). The heat insulating layer 1 is attached to the floor surface 3 by the support member 2.
installed on top. Further, the furnace interior 4 surrounded by the heat insulating layer 1 is filled with a metamorphosed gas consisting of hydrogen, carbon monoxide, nitrogen, etc. to create a reducing atmosphere, and the temperature of the atmosphere is maintained at 910°C.
Further, on the upper surface of the hearth portion 1a of the heat insulating layer 1, two rails long in the furnace length direction are provided to protrude upward.
is installed. Also, the side wall portion 1 of the heat insulating layer 1
A sampling tube 8 made of stainless steel and used to sample the atmospheric gas inside the furnace 4 is attached to the side wall 1 b.
b is installed by inserting it, and the ceiling part 1c
The shaft 9a and blades 9b thereof are made of stainless steel, and a stirring fan 9 for stirring the atmospheric gas in the furnace chamber 4 is fitted into the ceiling 1c.

In addition, near the boundary between the ceiling part 1c and the side wall part 1b of the heat insulating layer 1, there is a wall 4 inside the furnace that penetrates from above.
A large number of heater tubes 10 in two rows on the left and right protruding from each other are arranged along the length direction of the furnace, and electric heaters 11 made of nichrome wire or the like are housed inside the heater tubes 10. Also, a portion 1 of the heater tube 10 that is fitted into the heat insulating layer 1
The temperature of the outer surface near the center of Oa is approximately 600 to 720°C, and stainless steel in this temperature range is susceptible to corrosion due to the hydrogen embrittlement and carbon attack described above. Ceramic fibers 12 are bonded to the outer surface of the fitting portion 10a of the heater tube 10 with an inorganic binder to form a ceramic coating. The ceramic fiber 12 is made by mixing alumina powder and silicon dioxide (silica) powder with chromium oxide powder as an additive, and electrolytizing the mixture to form a fiber, and the binder is made of alumina powder and silica powder. It is a mixture of powder, chromium oxide powder, brick powder, and a hardening agent with a low alkaline content.

In this way, in the reducing atmosphere heat treatment furnace of this embodiment, ceramic fibers are inserted into the heat insulating layer 1 of the heater tube 10 on the outer surface of the portion 10a which is at a temperature where corrosion is likely to occur, that is, approximately 600°C to 720°C. 12 is bonded with a binder to form a ceramic coating, the atmospheric gas in the furnace 4 does not come into direct contact with the fitting portion 10a of the heater tube 10, and corrosion of the heater tube 10 can be prevented.

Although the ceramic fiber 12 was used as the ceramic coating in the first embodiment, the material for the ceramic coating is not limited to this. For example, silicon oxide powder and alumina powder may be mixed and this may be electrolyzed to form fibers. It may also be shaped into a shape,
This is mixed with the binder and the portion 10a is inserted into the heat insulating layer 1 of the heater tube 10.
Corrosion of the heater tube can also be prevented in the same manner as in the first embodiment when the binder is used to adhere the heater tube to the outer surface of the heater tube.

Next, a second embodiment of the present invention will be described.
FIG. 2 shows a portion of a heater tube into which a heat insulating layer is inserted according to a second embodiment of the present invention. In the figure, the same reference numerals as in Figure 1 indicate the same things, and 13 is a ceramic coating formed by aluminum coating treatment, and the aluminum coating treatment is a mixture of aluminum powder, chromium powder, acrylic resin, and solvent. Brush onto the fitting part 10a of the heater tube 10 to form a coating film of about 0.5 mm, and heat this at 700°C.
As a result, a ceramic coating 13 consisting of a layer of aluminum oxide (alumina) and chromium oxide of about 50 μm is formed on the outer surface of the insertion portion 10a of the heater tube 10, and a ceramic coating 13 consisting of a layer of aluminum oxide (alumina) and chromium oxide A diffusion layer of about 5 μm is formed inside the insertion portion 10a.

The effects of this embodiment are the same as those of the first embodiment.

In the second embodiment, the ceramic coating was formed by brushing a mixed solution of aluminum powder or the like and firing it, but the method for forming the ceramic coating is not limited to this. For example, a heater tube may be placed in molten aluminum. A ceramic coating made of alumina can also be formed by dipping it in alumina.

Furthermore, the present invention is not limited to heater tubes, but also applies to stainless steel members installed through the heat insulating layer of the furnace, such as stirring fan shafts, sampling tubes, conveyance devices such as hearth rolls, conveyance position detection rods, etc.
It can also be applied to devices where the surface temperature of the heat insulating layer insertion part is at a temperature where corrosion is likely to occur. Furthermore, it goes without saying that the present invention can also be applied to stainless steel furnace internal materials in annealing furnaces and normalizing furnaces.

As described above, according to the reducing atmosphere heat treatment furnace according to the present invention, the surface temperature of the heater tube inserted into the heat insulating layer of the stainless steel member is 600°C.
Since a heat-resistant and corrosion-resistant ceramic coating is formed on the outer surface of the part where the temperature ranges from ℃ to 720℃, reducing atmospheric gas will not come into direct contact with the part where the heat insulating layer of the stainless steel member such as the heater tube is inserted. This has the effect of preventing corrosion of these parts and extending the life of furnace internal materials made of stainless steel, such as heater tubes.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional front view of a reducing atmosphere heat treatment furnace according to a first embodiment of the present invention, and FIG. 2 is a section where a heat insulating layer is inserted into a heater tube of a reducing atmosphere heat treatment furnace according to a second embodiment of the present invention. FIG. DESCRIPTION OF SYMBOLS 1... Heat insulation layer, 4... Furnace interior, 10... Stainless steel member (heater tube), 10a... Fitted part, 12, 13... Ceramic coating.

Claims (1)

[Scope of Claims] 1. On the surface of a heat insulating layer insertion part where the surface temperature of a stainless steel member that is disposed through a heat insulating layer of a reducing atmosphere heat treatment furnace containing hydrogen gas is 600 to 720°C,
A reducing atmosphere heat treatment furnace characterized by forming a ceramic coating. 2. The reducing atmosphere heat treatment furnace according to claim 1, wherein the stainless steel member is a heater tube.