KR950000306B1 - Heat treat furnace for nitriding steel material - Google Patents

Abstract

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Description

Nitriding method of steel and heat treatment furnace using it

1 is a cross-sectional view of an example of a heat treatment furnace using the present invention.

2 is an explanatory diagram of its use state.

* Explanation of symbols for main parts of the drawings

1: outer cover 2: inner cover

3: steel material 4: exhaust hole

5: base 6: air circulation fan

7: heat exchanger 8: circulation blower for forced cooling

10a, 11a: latching portion 11: the lid body

12: decontamination apparatus 15: frame

18,20: valve 21: vacuum pump

The present invention relates to the formation of a nitride layer on the surface of the steel and to a special pre-treatment to form a nitride layer deep and uniform, and at the same time to improve the throughput per unit time, and a heat treatment furnace using the same.

In order to improve the mechanical properties such as wear resistance, corrosion resistance and fatigue strength, a nitriding method for forming a nitride layer on the surface of steel is adopted.

Among such nitriding methods, a typical method is nitriding (gas nitriding or gas hardening) using ammonia alone gas or a mixture of ammonia and a gas having a carbon source (RX gas).

These methods have a problem in the stability of treatment, such as a tendency for nitriding to occur when processing alloy steel and a material of a complicated shape.

In general, steels are nitrided at temperatures above 500 ° C. However, the adsorption and diffusion of nitrogen in the steel surface layer is required to be free of oxide film as well as contamination by organic and inorganic components. In addition, high activity of the steel surface layer itself is also required.

In practice, however, as described above, it is impossible to nitrate to prevent the formation of an oxide film and to completely activate the steel surface. For example, in the case of an austenitic stainless cold work product, it is difficult to completely remove the passivation film by fluoronitrate washing before removing the above-mentioned stainless steel into the treatment furnace, but it is difficult to completely remove the passivation film. Full activation cannot be realized.

Therefore, it is almost impossible to form a satisfactory nitride layer. In addition, organic and inorganic components on the steel surface are removed by organic cleaning using alkali degreasing or trichloroethane before nitriding. However, the most effective cleaning effect is due to the recent pollution regulations (regulations on destruction of the ozone layer). It is an environment in which organic washing is difficult to perform, and thus this is also a major obstacle to the formation of a favorable disease layer.

Thus, the inventors of the present invention maintain the steel in a fluorine-based gas atmosphere in a fluorine-based gas prior to nitridation, and then nitrate the steel to purify the surface of the steel (removal of organic and inorganic components and remove oxide films) and activation. The patent application has already been found by finding that a good nitride layer can be formed (Japanese Patent Application No. 1-177660).

In this method, first, the steel is placed in a furnace, heated, heated, and in contact with a fluorine-based gas such as NF ₃ to be pretreated. As a result, the activated fluorine atom destroys organic and inorganic contaminants attached to the surface of steel and removes surface contamination, and the passivation film such as an oxide film on the steel surface is changed to a fluoride film and the steel surface is fluorinated film. It will be in the state of covering protection.

Next, nitriding this.

The fluorinated film is destroyed and removed by introducing a mixed gas of a nitrogen gas (for example, NH ₃ gas) having a nitrogen source and H ₂ gas under heating during nitriding treatment.

In more detail, according to the destruction and removal of the fluoride film, the activated steel surface appears, and the N atom in the nitride gas rapidly penetrates and diffuses into the activated steel to form a deep nitride layer uniformly.

The present inventors also advocate a two-room furnace apparatus in accordance with the implementation of the above basic invention (Japanese Patent Application No. 1-333425), but the actual operation experiment was conducted by this apparatus, and as a result, the fluorination treatment time using the fluorine-based gas and Since there is a large difference in the time of nitriding treatment of nitriding steel, a problem arises that a series of operations from the pretreatment of steel to nitriding treatment are not performed efficiently.

SUMMARY OF THE INVENTION The present invention has been made in view of such circumstances, and provides a nitriding method for steel and a heat treatment furnace using the same, which can efficiently perform a series of steps from pretreatment of steel to nitriding, and further, to form a uniform and deep nitride layer. The purpose.

In order to achieve the above object, the present invention maintains the steel in a first heat treatment furnace in a fluorine-based gas atmosphere under heating and fluorination treatment, and then in a second heat treatment furnace in a nitriding gas atmosphere in a heated state and nitriding treatment. In the method of nitriding steel, the second heat treatment furnace of the number corresponding to the amount of steel treatment per unit time of the first heat treatment furnace is provided, and the plurality of second steel materials in which the fluorination treatment is finished in the first heat treatment furnace in sequence. The nitriding method of the steel which is introduced into the heat treatment furnace and nitriding is the first point, and the lifting type inner cover which takes out the steel material from the inside and loads it freely and the pulling which covers the inner cover while maintaining a certain space. A bell-shaped outer cover is provided, and an inner cover is formed in the fluorination chamber, and a fluorine gas supply pipe is provided to the fluorination chamber. The exhaust pipe is installed, and the inner cover and the outer cover are formed as a heating chamber, and a fluorination treatment heat treatment furnace is provided in the heating chamber. A pull-up inner cover for freely loading and loading the fluorinated steel, and a pull-out bell-shaped outer cover for covering the inner cover while maintaining a constant space. The inner cover is placed in the nitriding chamber. A nitriding gas supply pipe and an exhaust pipe are formed in the nitriding treatment chamber, a heating chamber is formed between the inner cover and the outer cover, and a nitriding treatment chamber heating means is provided in the heating chamber. The heat treatment furnace of a dragon is made into the 3rd summary.

In the steel nitriding method of the present invention, as described above, in the same aspect as the above-described basic invention, a deep and uniform nitride layer can be formed by performing a special pretreatment using a fluorine-based gas on the steel prior to the nitriding treatment. The treatment cannot be performed in the same furnace, but in a separate furnace, and the establishment role of the furnace is reasonably established based on the steel throughput per unit time of the fluorination treatment furnace and the steel treatment per unit time of the nitrification treatment furnace. As a result, the amount of nitriding per unit time can be greatly improved.

In addition, according to the heat treatment furnace of the present invention, since the inner cover and the outer cover can be pulled up, even if the inner surface and the fan type of the inner cover are damaged by fluorine-based gas or nitridation gas, the inner cover is pulled up and the internal repair is simplified easily. I can do it quickly. In addition, the wear of the damage can be easily checked.

Next, the present invention will be described in detail.

In the present invention, the surface of the steel is pretreated using a fluorine-based gas. The fluorine-based gas is a gas containing a fluorine source component composed of NH ₃ , BF ₃ , CF ₄ , HF, SF ₆ , F ₂ alone or in a mixture in an inert gas such as N ₂ .

The NF ₃ , BF ₃ , CF ₄ , F ₂ is a gas at room temperature and SF ₆ is a liquid at room temperature. And these alone or a mixture of a mixture in an inert gas, such as in N ₂ constitute the fluorine-based gas according to the present invention with an inert gas.

Among the fluorine source components, NF ₃ is the most excellent and practical in terms of stability, reactivity, controllability, handleability and the like.

F ₂ is not very suitable because of its high reactivity and toxicity and difficulty in handling and furnace operation.

Gases such as BF ₃ , SF ₆ , and the like are also effective in forming the nitride layer, but are generally not suitable for the generation of reactants by harmful B and S. In addition, it is not preferable in terms of high sublimation FeCl _3, such as chloride generated as FCl _3.

In general, fluorine-based gas is used under a high temperature atmosphere, and in such a fluorine-based gas, a fluorine source component such as NF ₃ is set at a concentration of 0.05-20% (by weight).

Preferably it is 2-7%, More preferably, it is in the range of 3-5%. Among the steels of the present invention, various steels such as carbon steel and stainless steel are included. The shape of these steels is not particularly limited and may be processed even in the state of planks and coils, and may be in the form of screws or the like.

As described above, the steel of the present invention includes not only a monolithic material, but also an alloy in which alloys suitably blended with the above materials or alloys containing other alloy materials other than those mentioned above are used as the main component.

The present invention fluorinates the steel as follows, for example. That is, the steel is placed in a first heating furnace for fluorination treatment and heated to heat the steel to a temperature of 150-600 ° C, preferably 250-380 ° C. Next, a fluorine-based gas such as NF ₃ is introduced into the furnace in such a state, and the steel is placed at the above temperature, and 10-120 minutes, preferably 20-90 minutes, more preferably in a fluorine gas atmosphere such as NF ₃ . Keep 30-60 minutes.

As a result, the passivation film (mainly composed of an oxide film) on the steel surface is changed into a fluoride film. This reaction is performed based on the general formula mentioned later, for example.

FeO + 2F → FeF ₂ +1/20 ₂

Cr ₂ O ₃ + 4F → 2CrF ₂ +3/20 ₂

The process is carried out using a heat treatment furnace for fluorination treatment having a structure as shown in FIG. In the figure, "1" is a bell shaped outer cover, and "2" is a cylindrical inner cover covered with the outer cover.

The top portion of the outer cover 1 is integrally provided with a frame 10 having a locking portion 10a for pulling up to a hook such as a crane. Moreover, the upper part of the said inner cover 2 is integrally provided with the lid body 11 which has the latching part 11a in order to pull up to hooks, such as a crane. The inner cover 2 is formed of a fluorination chamber, and the space between the two covers 1, 2 is formed of a heating chamber.

&Quot; 3 " is a steel material which is freely loaded in the inner cover 2 and loaded therein. The steel material 3 is mounted on the frame 15 with the central hole 14 and extends upwardly from the central hole 14 and at the outer circumference of the frame 15. In the space between the second cylindrical mesh 17a extending to the left side, it is laminated in multiple stages via a porous cut-out plate 17b with a central hole.

"4" is a burner charging hole provided in the peripheral wall on the bottom side of the outer cover 1, and "4a" is an exhaust hole provided in the upper peripheral wall of the outer cover 1. "5" is the base, "6" is the air circulation fan of the furnace, facing the center hole 14 of the frame 15, the central hole 14 and the cylindrical mesh 16 extending upwards therefrom. The furnace air is circulated through.

&Quot; 7 " is installed in the pipe 7a extending downward from the base of the inner cover 2 by the heat exchanger. "8" is a forced cooling circulation blower installed in the pipe 7a connected to the heat exchanger 7, and "9" is a pipe for introducing fluorine-based gas such as NF ₃ into the inner cover 2.

&Quot; 12a " is an exhaust gas pipe which draws out the exhaust gas in the inner cover 2, and the middle part is divided into two, and one branch pipe 17 is provided with a valve 18 and the other branch pipe 19 The valve 20 and the vacuum pump 21 are provided in it.

When the exhaust gas pressure in the inner cover 2 is high, the path of the branch pipe 17 is used. When the exhaust gas pressure is low, the path of the branch pipe 19 for evacuating with the suction force of the vacuum pump 21 is used. &Quot; 12 " is a decontamination device and is connected to the end of the exhaust pipe 12a.

This decontamination apparatus 12 carries exhaust gas consisting of a pair of left and right activated carbon barrels 22, a heater coil 23 wound around the outer circumference thereof, and a fan heat exchanger 24 to a pair of left and right activated carbon cylinders 22. The residual, NF _3, etc. in the guided exhaust gas are thermally reacted with activated carbon to be converted into harmless CF ₄ , which is introduced into a fan heat exchanger 24 and cooled.

&Quot; 13 " is a scrubber installed in the pipe 25 extending from the heat exchanger 24, where water is contained therein and the exhaust gas introduced from the pipe 25 is bubbled to form HF in the exhaust gas. and an exhaust gas by dissolving in water a (dugoseo in the inner cover (2) NF ₃ is to generate by reaction with H ₂ O · H ₂₎ completely harmless and released into the atmosphere.

In this heat treatment furnace, a hook of a crane or the like (not shown) is hooked to the engaging portions 10a and 11a of the outer cover 1 and the inner cover 2 to thereby secure the outer cover 1 and the inner cover 2. Pull it up with a crane. In such a state, the steel material 3 is placed on the frame 15 as shown, and then the outer cover 1 and the inner cover 2 are lowered to the original state (state of FIG. 1).

Next, the thermal flame is radiated from a burner (not shown) charged in the burner charging hole 4 in a heating chamber formed between the outer cover 1 and the inner cover 2. As a result, the steel material 3 in the inner cover 2 is heated. Next, a fluorine treatment is carried out by introducing a fluorine-based gas such as NF ₃ into the inner cover 2 via the pipe 9 from the bottom thereof. In this way, the time for fluorination is usually about 30 to 60 minutes as described above.

Since the surface of the steel material 3 subjected to the fluorination treatment is covered with a fluoride film, the surface of the steel 3 is properly protected without oxidization even when shaken by outside air such as air. And stored in this state. Or it is put into the 2nd heating furnace for nitriding immediately, and nitriding is performed. The same structure as that of the first heating furnace is also used for the second heating furnace for nitriding treatment.

That is, the inner cover 2 and the outer cover are loaded by pulling up the inner cover 2 and the outer cover 1 using the heating furnace of the structure as described above to finish the fluorination treatment, and then the inner cover 2 and the outer cover. (1) is lowered and returned to its original state, and then a flame is blown from the burner in the space between the inner cover (2) and the outer cover (1) to heat the steel in the inner cover (2) to a nitriding temperature of 480-700 ° C. and by introducing a mixed gas with a single gas or NH ₃ and a carbon source consisting of NH ₃ through the pipe 9 at the lower side of the heating furnace in that state in the furnace and held 120 minutes.

In this process, the fluoride film is reduced or destroyed by H ₂ or a small amount of water (produced during the nitriding reaction), for example, as shown in the following formula, and an active steel surface is formed.

CrF ₄ + 2H ₂ → Cr + 4HF

2FeF ₃ + 3H ₂ → 2Fe + 6HF

In addition, the removal of the fluorinated film may be performed by blowing a mixed gas consisting of N ₂ and H ₂ or a gas of H ₂ alone before the introduction of the nitride gas to destroy the fluorinated film.

Rather, this is suitable in that troubles caused by the production of ammonium fluoride are eliminated.

An active nitrogen atom derived from nitriding gas acts on, and penetrates and diffuses into, the active steel surface thus formed. As a result, an ultrahard compound layer (nitride layer) containing nitrides such as CrN, Fe ₂ N, Fe ₃ N, Fe ₄ N, etc., is formed uniformly and deeply from the surface of the steel to the inside, and so on, the hard N atoms A diffusion layer is formed so that the compound layer + diffusion layer forms a prenitride layer.

As described above, according to the method of the present invention, the steel surface which appears simultaneously with the breakdown of the fluoride film is extremely activated, and nitrogen atoms act on the activated steel surface to form a superhard nitride layer uniformly to a deep region.

Example 1

As shown in FIG. 2, the fluorination treatment and the nitriding treatment were carried out with respect to the first heating furnace "A" for the fluorination treatment using two "A '" furnaces for the nitriding treatment.

[Fluorination]

After forming a plurality of austenitic SUS series screws (samples), each was steam-washed with trichloroethylene. Next, this was put into "A" with the first heating, heated to 300 ° C, and the fluorine-based gas containing N ₂ of 1% NF ₃ in the state of the inner cover per unit time in the inner cover. Twice the volume was added and held for 10 minutes.

After that, a part of the sample was taken out and the surface layers were compared, and it was confirmed that a fluoride film was formed on the entire surface.

[Nitriding]

Next, the fluorinated sample was moved to one of "A '" s by two second heatings, with NH ₃ of 25%, CO of 10%, H ₂ of 40%, and N of 25%. The mixed gas was introduced into the furnace and subjected to nitriding for 6 hours under the conditions described above.

After this treatment, the sample is air cooled and then taken out of the furnace. The nitride layer is formed uniformly on the surface of the obtained sample.

Next, during the above-mentioned nitriding treatment, a plurality of samples are subjected to fluorination treatment in the same manner as described above in the first heating furnace "A", and this time, two second heating furnaces "A '". The said sample was put into the "A '" furnace of the other side, and the nitriding process was performed in the same manner as the above.

In this way, two groups of "A '" are formed by nitridation treatment with one "A" by fluorination treatment and continuously operated so that the first heating furnace "A" has no time and can be efficiently nitrided. do.

Example 2

Two "A '" 2nd heating furnaces were knitted with respect to 1st "A" 1st heating furnace, and fluorination treatment and nitriding treatment were performed.

[Fluorination]

The pressure in the furnace was reduced to 100 torr, and treated at a temperature of 350 ° C. for 30 minutes by using a gas containing 0.1% NF ₃ as the fluorination treatment gas and the remaining portion consisting of N ₂ .

Other than this, the same process as in Example 1 was performed.

[Nitriding]

The nitriding treatment temperature was changed to 570 ° C., and the treatment time was changed to 5 hours using a nitrogen gas composed of 25% NH ₃ , 5% CO, 10% H ₂ , and the remaining portion N ₂ . Other than that, the same process as in Example 1 was carried out. In this example, the same effects as in Example 1 were obtained.

Example 3

Three sets of the second heating furnace "A '" were formed for the first heating furnace "A", and fluorination treatment and nitriding treatment were performed.

[Fluorination]

The reduced pressure in the furnace was treated 100torr, including 20% of NF ₃ and held for 40 minutes at a temperature of 330 ℃ using a fluoride gas, the rest consisting of N _2. Other than that was carried out in the same manner as in Example 1.

[Nitriding]

The nitriding treatment was carried out at 570 ° C. for 7 hours using a mixed gas composed of 25% NH ₃ , 10% CO ₂ , 25% H ₂ , and 40% N ₂ . Other than that was carried out in the same manner as in Example 1. Also in this embodiment, the same effects as in the first embodiment can be obtained.

Example 4

In the fluorination treatment of Example 1, the heating temperature of the first heating furnace “A” was changed to 200 ° C., and the fluorine gas was changed to N ₂ with the remaining part containing 1% of F ₂ . The amount of time was changed to keep 20 minutes by adding three times the volume in the inner commerce per unit time.

Aside from that, the same treatment as in Example 1 was carried out for the fluorination treatment and the nitriding treatment. The nitride layer of the obtained sample was the same as in Example 1 using NF ₃ for fluorination and was very good.

In the embodiment, a furnace having an outer cover and an inner cover is used in both the fluorination treatment first heating furnace and the nitriding treatment second heating furnace, but either furnace has a PIT type furnace. It is not impossible to use.

However, in such a furnace apparatus, it is difficult to take out and insert as a work, and it is difficult to replace and repair in case of wear loss of the furnace material by fluorination and nitriding.

As described above, the nitriding method of steel of the present invention maintains the steel in a heated state under a fluorine-based gas atmosphere to remove organic and inorganic foreign substances, and forms a passivation film such as an oxide film on the surface of the steel as a fluorinated film and then nitrides it.

In this way, the passivation film such as an oxide film on the steel surface is changed to a fluoride film, thereby protecting the steel surface in a good state. Therefore, the fluoride film formed on the steel surface protects the surface of the steel in a good state even if time passes during the nitriding treatment in the formation of the fluoride film.

As a result, an oxide film cannot be formed on the surface of the steel again. The fluorinated film is decomposed and removed during the subsequent nitriding treatment, and the surface of the steel is thereby exposed. This exposed metal surface is in an active state, and thus N atoms in the nitriding treatment easily enter the steel material. As a result, N atoms penetrate deeply and uniformly from the surface of the steel to form a good nitride layer.

In particular, in the nitriding method of the steel of the present invention, the fluorination treatment and the nitriding treatment are not carried out in the same furnace, but for the nitriding treatment which requires a considerably long time treatment for one fluorination treatment unit that can be treated in a relatively short time by installing each dedicated furnace. Since the operation of the plural condenser is carried out, there is no time spent in the fluorination treatment furnace, so that nitridation and efficiency can be realized.

In addition, since the inner cover and the outer cover can be pulled up, the heat treatment furnace of the present invention can easily and quickly repair the inner cover by lifting the inner cover even if the inner cover and the fan type are damaged by fluorine-based gas nitride gas. It does not take time for repair. It is also easy to check.

Claims (3)

A method of nitriding steels in which a steel material is placed in a first heat treatment furnace, maintained in a heated state under a fluorine-based gas atmosphere, and fluorinated, and then placed in a second heat treatment furnace in a heated state under a nitrogen gas atmosphere. In the heat treatment furnace of 1, the fluorinated steels are sequentially introduced into a plurality of second heat treatment furnaces and nitrided, and the total amount of steel processed per unit time in the second heat treatment furnace and the treatment per unit time in the first heat treatment furnace. And a total treatment time is shared by the first heat treatment furnace and the second heat treatment furnace so that the total amount of steels to be equal. It has a lifting type inner cover (2) for loading steel (3) inside and a lifting type bell shaped outer cover for covering the inner cover with a certain space, and the inner cover is formed into a fluorination chamber. A fluorine gas supply pipe 9 and an exhaust pipe 12a are provided in the fluorination treatment chamber, a heating chamber is formed between the inner cover and the outer cover, and a fluorination treatment chamber heating means is provided in the heating chamber. Fluorination treatment heat treatment furnace. It is equipped with a pull-out inner cover 2 for freely loading and loading the finished fluorinated steel 3 and a bell-shaped outer cover for covering the inner cover with a predetermined space therein. The inside of the cover is formed as a nitriding chamber, and the nitriding gas supply pipe 9 and the exhaust pipe 12a are provided in the nitriding chamber, and a heating chamber is formed between the inner cover and the outer cover, and the nitriding chamber in this heating chamber. A heat treatment furnace for nitriding treatment, characterized by providing a heating means.