JPH08193256A - Method for nitriding steel - Google Patents

Abstract

PURPOSE: To prevent the occurrence of uneven nitriding at the time of nitriding treatment and also to remarkably reduce the cost of nitriding treatment. CONSTITUTION: This method is a method for nitriding a steel by allowing nitrogen to react with the surface of the steel and forming a hard nitrided layer. At this time, prior to nitriding, the steel is treated as follows: (A) The steel is heated and held in a gaseous atmosphere containing fluorine compound gas or fluorine gas and also containing air in the amount equivalent to 0.5-20vol.% of the whole or oxygen gas equivalent to 0.1-4vol.% of the whole; (B) The steel is heated and held in a gaseous atmosphere containing fluorine compound gas or fluorine gas and then heated and held in a gaseous atmosphere containing air in the amount equivalent to 0.5-2Ovol.% of the whole or oxygen gas equivalent to 0.1-4vol.% of the whole; (C) The steel is heated and held in a gaseous atmosphere containing air in the amount equivalent to 0.5-100vol.% of the whole or oxygen gas equivalent to 0.1-20vol.% of the whole and then heated and held in a gaseous atmosphere containing fluorine compound gas or fluorine gas.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel nitriding method for forming a nitride layer on the surface of steel to improve wear resistance and the like.

[0002]

2. Description of the Related Art For the purpose of improving mechanical properties such as wear resistance, corrosion resistance, and fatigue strength, the nitriding method for forming a nitride layer on the surface of steel or the carbonitriding method conventionally adopted is It looks like this: (A) Method using cyanated molten salt such as NaCN, KCNO (tuftride method) (b) Nitrogen (ion nitriding) by glow discharge (c) Ammonia or mixed gas of ammonia and gas having carbon source (for example, RX gas) Nitriding (gas nitriding, gas soft nitriding)

Of these, the method (a) is a harmful solution.
Since molten salt is used, it has a future potential in terms of work environment, waste treatment, etc.
Not good for In addition, the method of (b) is a low vacuum N₂
+ H ₂It is nitrided by glow discharge in an atmosphere.
The effect of the oxide film due to the cleaning action accompanying the putting
Although it will decrease, uneven nitriding will occur due to local temperature difference.
Cheap. In addition, this method has restrictions on the shape and size of the processed object.
There is a problem that the cost is large and the cost is high. further,
The method of (c) above is a process
There is a problem with stability, and to obtain a deep nitrided layer,
There is also a problem that it takes a long time.

In general, steel is nitrided at a temperature of 500 ° C. or higher. However, adsorption and diffusion of nitrogen on the surface layer of the steel requires high activity of the metal surface, and organic and inorganic fouling is prevented. Of course, it is desirable that there be no oxide film or O ₂ adsorption film. Further, the presence of the oxide film is not preferable because it promotes the dissociation degree of ammonia which is a nitriding gas. However, it is actually impossible to prevent the formation of an oxide film in the gas nitriding method. For example, even in the case of case-hardening steel or structural steel not containing a large amount of chromium, at a temperature of 400 ° C to 500 ° C, NH _A thin oxide material is formed even in an atmosphere of ₃ or NH ₃ + RX. This tendency is further strengthened in steel types containing a large amount of elements such as chromium having a high affinity for oxygen.

The formation of such an oxide varies depending on the surface condition and processing conditions of the same component, resulting in the formation of a non-uniform nitrided layer. As a typical example, in the case of cold-worked austenitic stainless steel, for example, even if the passivation film on the surface is completely removed by washing with hydrofluoric nitric acid before inserting it into the processing furnace, satisfactory nitriding is possible. It is almost impossible to form layers. Note that uneven nitriding occurs not only in gas soft nitriding but also in nitriding steel or stainless steel with only ammonia (gas nitriding). Further, even in the case of ordinary structural steel, there is a basic problem that uneven nitriding is likely to occur in the case of a complicated part such as a gear.

As a means for improving the above-mentioned essential problems of gas nitriding and gas soft nitriding, a method of inserting a vinyl chloride resin into a furnace together with a treated material (work), or CH ₃ C
In the past, a method of sprinkling 1 or the like and heating to 200 to 300 ° C. to generate HCl to prevent the generation of an oxide and remove it, or a method of plating the surface in advance to suppress the oxide has been used in the past. Although it has been proposed, it is the current situation that it has not been put to practical use. Chlorides such as FeCl ₂ and FeCl ₃ are precipitated on the surface of steel by HCl, but these are extremely brittle at temperatures below the nitriding temperature and easily sublimate and evaporate, so that no chloride film is formed.
Although it has some effect of suppressing the oxide film, it is not practically effective due to the complexity of handling itself and the significant damage to the furnace material.

[0007]

As described above, the conventional method has a problem that inorganic foreign matters due to the residual after cleaning before nitriding treatment and uneven nitriding due to the oxide film of the object to be treated are generated. For the purpose of effectively solving such a problem, the inventors of the present invention, prior to nitriding, have made steel an atmosphere of a fluorine compound or a gas containing fluorine (hereinafter referred to as “fluorine-based gas”). It was found that it is effective to form a fluoride film on the surface layer of steel by heating and holding it for a long time, and already applied for a patent (Japanese Patent Application No. 1-177660).
No.) In this way, when treated with a fluorine-based gas, the activated fluorine atoms destroy and remove the pollutants of inorganic and organic substances adhering to the steel surface to clean the surface, and the fluorine atoms react with the oxide film. Then, it changes into a fluoride film and the steel surface is covered and protected by the fluoride film. The fluoride film is decomposed and disappeared by the nitriding treatment in the next step, and at the same time, the steel surface is activated. Then, nitrogen atoms permeate and diffuse into the activated steel surface, and a rapid and uniform nitride layer is formed. However, in the process of actually continuing the operation, the above-mentioned fluorine-based gas is expensive and its consumption is considerably large, so that the cost of the nitriding treatment itself becomes high, and improvement thereof has been strongly demanded.

[0008]

To achieve the above object, the method for nitriding steel according to the present invention is a method for nitriding steel in which nitrogen is reacted on the surface of the steel to form a hard nitriding layer.
Prior to nitriding, the following fluorination treatment (A), (B) or (C) is performed. (A) In a gas atmosphere containing a fluorine compound gas or a fluorine gas, the gas atmosphere containing 0.5 to 20% by volume of air or 0.1 to 4% by volume of oxygen gas. , Keep the steel heated. (B) After heating and holding the steel in a gas atmosphere containing a fluorine compound gas or a fluorine gas, 0.5 to 2 of the whole
The steel is heated and held in a gas atmosphere containing air corresponding to 0% by volume or oxygen gas corresponding to 0.1 to 4% by volume. (C) After heating and holding the steel in a gas atmosphere containing 0.5 to 100% by volume of air or 0.1 to 20% by volume of oxygen gas, a fluorine compound gas or a fluorine gas is included. The steel is heated and held in a gas atmosphere.

[0009]

ACTIONS That is, the present inventors have conducted a series of studies for the purpose of improving the previously proposed method. As a result, prior to the nitriding of steel, while heating and holding the steel in the furnace first, when introducing the fluorine-based gas into the furnace and performing the fluorination treatment in that state, the fluorine-based gas alone is not used as the atmosphere gas. However, 0.5 to 20% by volume of air in a fluorine-based gas atmosphere, or 0.1 to 4% by volume (hereinafter abbreviated as "%") of oxygen gas is contained in the atmosphere. The fluorination treatment can reduce the consumption of the fluorine-based gas as compared with the previously proposed method, and has the same effect as the previously proposed method, or even more than that [(the fluorine atoms are attached to the steel surface (Effects of destructive removal of pollutants of inorganic and organic substances), (Effects of oxide film on the steel surface reacting with fluorine atoms to change to a fluoride film, and the steel surface is covered and protected by the fluoride film), ( Fluoride film, by the nitriding treatment of the next step, To the solution disappears, a state where the steel surface is activated, the effect of achieving rapid and uniform penetration diffusion of nitrogen atoms)] found that can be obtained. Further, as described above, the fluorination treatment does not necessarily have to be performed in the state where the fluorine-based gas and the air coexist. That is, in a gas atmosphere consisting of a fluorine-based gas, after heating and holding the steel to form a fluoride film on the steel surface, or at the same time as forming it, the air or oxygen is mixed with nitrogen or ammonia to mix. The gas may be introduced into the furnace to create a gas atmosphere, and the steel may be heat-treated in this gas atmosphere. Further, air or oxygen is mixed with nitrogen gas or the like, introduced in the form of a mixed gas, and the steel is heated and held in this gas atmosphere, and then the above-mentioned fluorine-based gas is introduced into the furnace, and in this gas atmosphere The steel may be heated and held at. It has been found that even in this case, the same effect as when the fluorine-based gas and air or oxygen are used simultaneously can be obtained.

Next, the present invention will be described in detail.

The fluorine-based gas (fluorine compound gas or gas containing fluorine gas) used in the present invention is a fluorine compound such as NF ₃ , BF ₃ , CF ₄ , S.
Examples thereof include fluorine compound gas such as F ₆ and gas containing F ₂ gas. Usually, a fluorine-based gas is composed of the fluorine compound gas, the F ₂ gas and a diluent gas (N ₂ gas or the like) for diluting the F ₂ gas. Among the fluorine compound gas and the F ₂ gas used for these fluorine-based gases, NF ₃ is the most excellent and practical in terms of reactivity, handleability and the like. Under the above fluorine-based gas atmosphere, a workpiece such as steel is treated with, for example, NF ₃
In this case, after heating and holding at a temperature of 250 to 600 ° C. to surface-treat the workpiece, nitriding treatment (or carbonitriding treatment) is performed using a known nitriding gas such as ammonia. Usually, the NF _{3 and the} like are used after being diluted with nitrogen gas, as described above. In such a fluorination treatment, the concentration of the fluorine compound or fluorine in the fluorine-based gas atmosphere is 1000 to 100000 pp on a volume basis.
m (same below).

The present invention is a combination of the action of the above-mentioned fluorine-based gas and the action of air or oxygen gas, which is the greatest feature. In this invention, there are the following three modes for the combination of the air or oxygen gas and the fluorine-based gas. The first mode is a mode in which air or oxygen is introduced into the fluorine-based gas and mixed. As described above, when the fluorine-based gas is mixed with air or oxygen, the total amount of the fluorine-based gas and the air or the like mixed with the fluorine-based gas is set to include 0.5 to 20% of air. It Further, in the case of oxygen, it is set so that 0.1 to 4% of oxygen is contained in the above whole. In the second aspect, the steel is heated and held in a gas atmosphere composed of a fluorine-based gas to form a fluoride film on the steel surface at the same time or after the formation of the fluoride film, and then the whole (entire atmosphere gas) is exposed to air. Is 0.5 to 20% or oxygen is 0.1
This is a mode in which air or oxygen is introduced so as to be present at ˜4%, for example, in the form of a mixed gas with nitrogen gas or NH ₃ gas. Furthermore, in the third aspect, prior to the introduction of the fluorine-based gas, air or oxygen is introduced into the furnace in the form of a mixed gas mixed with an inert gas such as nitrogen gas, and steel is introduced into this atmosphere gas. After heating and holding, there is a mode in which the fluorine-based gas is introduced into the furnace to form a fluoride film on the steel surface. In this case, prior to the introduction of the fluorine-based gas, the air introduced into the furnace or oxygen is set so that the entire atmosphere gas in the furnace contains 0.5 to 100% of air. In the case of oxygen, the total amount of oxygen is 0.1 to 2
It is set to contain 0%.

In the first and second aspects, no good effect can be obtained even if the air deviates from the above range or the oxygen deviates from the above range. In this case, the air to be used is usually purified air, which has a reduced content of hydrocarbons and impurities such as water and carbon dioxide. As the oxygen gas, pure oxygen gas may be used as it is, or pure oxygen gas diluted with another diluent gas such as N ₂ may be used. Also in this case, the amount of pure oxygen is set so as to correspond to 0.1 to 4% of the whole.

As the holding time in the atmosphere, an appropriate time may be selected depending on the type of steel, the shape and size of the work, the heating temperature, etc., and it is usually ten minutes to several tens of minutes.

The method of the present invention will be described more specifically.
For example, degreasing and cleaning a steel work, and heat it as shown in Fig. 1.
Insert into the processing furnace 1. This furnace 1 is equipped with a heater installed inside the outer shell 2.
Pit furnace with stainless steel inner container 4 inside
Then, the gas introduction pipe 5 and the exhaust pipe 6 are inserted. Gas guidance
Into the inlet pipe 5, a cylinder, a flow meter 17, a valve 18, etc.
Therefore, the gas is supplied. The atmosphere gas inside is the motor
It is agitated by a fan 8 rotating at 7. Work 10
Is placed in a wire mesh container 11 and inserted into the furnace.
In the figure, 13 is a vacuum pump, and 14 is an abatement device. this
Fluorine-based gas such as NF from the cylinder in the furnace₃And N₂
Introduce air from the cylinder while introducing the mixed gas of
And heat to a predetermined reaction temperature. NF₃Is 250-6
At a temperature of 00 ° C, active group F is generated, and this F remains on the surface.
Removes existing organic and inorganic foreign substances and
Fe, Cr substrate, or FeO, Fe₃O_Four, Cr₂
O ₃Reacts rapidly with oxides such as
Just like FeF on the surface₂, FeF₃, CrF₂, Cr
F_FourA very thin fluoride film containing a compound such as is formed.

[0016]

Embedded image FeO + 2F → FeF ₂ +1/20 ₂

[0017]

Embedded image Cr ₂ O ₃ + 4F → 2CrF ₂ +3/20 ₂

By this fluorination treatment reaction, the oxide film on the surface of the work is converted into a fluoride film, and a fluoride film is formed on the work surface. In this case, the atmosphere gas contains not only fluorine compound gas and F ₂ gas but also air, and oxygen (O ₂ ) in the air forms an O ₂ film on the surface of the formed fluoride film. However, it is considered that the fluoride film is reinforced. Since such an O ₂ film reinforces the fluorinated film, it is possible to prevent uneven nitriding from occurring in the next step of nitriding, and at the same time, to use an expensive fluorine compound, F, which has a high correlation with the prevention of uneven nitriding. _It will save the consumption of ₂ gas and eventually reduce the cost of nitriding.

Further, the fluorination reaction as described above is not limited to the case where the fluorine-based gas is mixed with air and oxygen at the same time, and after the steel is heated and held in the fluorine-based gas atmosphere in the furnace, the Alternatively, oxygen gas is introduced, and air equivalent to 0.5 to 20% by volume of the entire atmosphere gas or 0.
A gas atmosphere containing oxygen gas corresponding to 1 to 4% by volume is created, and the steel is heated and held in this gas atmosphere. Then, the same effect can be obtained as with the above simultaneous mixing. Further, in the fluorination treatment reaction as described above, air or oxygen is introduced into the furnace together with an inert gas or the like prior to the introduction of the fluorine-based gas, and 0.5 to 10
Air equivalent to 0% by volume, or 0.1 to 20% by volume
It can be obtained by creating a gas atmosphere containing oxygen gas corresponding to and heating and holding steel in this gas atmosphere.

The workpiece treated in this manner is continuously treated under a non-oxidizing atmosphere such as N ₂ atmosphere for 480-70.
When heated to a nitriding temperature of 0 ° C. and added with NH ₃ or a mixed gas of NH ₃ and a gas having a carbon source (for example, RX gas), the fluorinated film is heated by H ₂ or a trace amount of water, for example, the following formula: It is presumed that the active metal matrix is formed by reduction or destruction as described above.

[0021]

[Chemical Formula 3] CrF ₄ + 2H ₂ → Cr + 4HF

[0022]

Embedded image 2FeF ₃ + 3H ₂ → 2Fe + 6HF

As described above, at the same time when the active metal matrix is formed, N of the active group is adsorbed and penetrates into and diffuses into the metal. As a result, CrN, Fe ₂ N, Fe are formed on the surface.
A compound layer containing a nitride such as ₃ N or Fe ₄ N is formed.

The formation of such a compound layer is the same as in the conventional nitriding method, but in the conventional method, an oxide film formed while the temperature is raised from room temperature to the nitriding temperature, and at this time, it is adsorbed Since the surface activity is lowered by the O ₂ content, the degree of N adsorption on the surface is low and the N is non-uniform.
Further, such nonuniformity is magnified by the fact that it is practically difficult to keep the degree of decomposition of NH ₃ uniform in the furnace. As described above, in the present invention, since the fluoride film formed on the steel surface is reinforced by the O ₂ film, it is possible to prevent the occurrence of uneven nitriding and save the consumption amount of the expensive main component gas. . Then, as a result of performing the fluorination treatment in this manner, in the present invention, the adsorption of N on the surface of the work is performed uniformly and quickly.

One of the major characteristics of the operating process of the present invention is NF as a reaction gas for forming a fluoride film.
No reactivity at normal temperature, such as _3, by using a gaseous easy to handle material, simple process such as the process compared to methods such as using a chlorine source of PVC liquid plating and solid is continuous operation There is a point. The tufftride method can be said to be an excellent method in terms of the effect of improving the throwing power of the nitrided layer and the fatigue strength, but it cannot be said to be a method open to the future in that it requires a large cost for the work environment and pollution equipment. In the above process, only a simple device for detoxifying the treated waste gas is sufficient, the unevenness of nitriding can be eliminated with the throwing power equivalent to or higher than that of the tufftride method, and the tufftride method can be carburized at the same time as carburizing. However, pure nitriding is also possible.

Next, examples will be described together with comparative examples.

[0027]

After being heading formed by Example 1 and Comparative Examples 1 to 3] SUS305-based wire to "screw" washed Freon, placed in a treatment furnace 1 as shown in FIG. 1, the NF ₃ 40000p
It was kept at 320 ° C. for 15 minutes in an N ₂ gas atmosphere containing pm and 50,000 ppm (5% by volume) of air. After that, the mixture was heated to 580 ° C., a mixed gas of 50% NH ₃ + 50% N ₂ was introduced into the furnace to carry out nitriding treatment for 3 hours, and then air-cooled and taken out.

The nitrided layer of the obtained workpiece has a uniform thickness,
The hardness is such that the cross-sectional hardness of the thread portion is Hv = 350-
While 360, the overall surface hardness is Hv = 120
It was 0-1250.

On the other hand, as Comparative Example 1, the same work was washed with chlorofluorocarbon and put in the above-mentioned furnace, and 570 in 75% NH _3.
After heating at ℃ for 3 hours, almost no nitride layer was formed.

Further, as Comparative Example 2, in the above-mentioned Example 1, the air is added in an amount of 0.5 to 20%, which is within the range of the present invention.
Was treated in the same manner as in Example 1 except that the range was changed to 0.4%. The nitride layer of the workpiece thus treated is non-uniform, and the surface hardness is Hv = 480-1250.
It can be seen that there is a large variation, and the performance is significantly lower than in Example 1.

As Comparative Example 3, the same treatment as in Example 1 was carried out except that in Example 1 air was set to 21%, which is the upper limit of the present invention. It can be seen that the nitrided layer of the workpiece thus treated is also non-uniform and the surface hardness of the whole also varies, and the performance is significantly lower than that of Example 1.

[0032]

Example 2 and Comparative Examples 4 and 5 After tapping the SUS305 tapping screw with acetone, put it in the furnace shown in FIG.
F ₃ of 35000ppm and O ₂ of 7000pp
m (0.7%)-containing N ₂ atmosphere, the temperature was kept at 300 ° C. for 15 minutes, then the temperature was raised to 500 ° C., and the temperature was kept at N ₂ + 90% H ₂ for 30 minutes, and then 20% NH ₃ + 80% RX ( H ₂ O and CO ₂ are removed by incomplete combustion of methane, propane, etc. in air, and the composition is roughly N ₂ + CO.
(20%) + H ₂ (30%). ) For 3 hours and taken out. In this case, a nitride layer of 40 to 50 μm was uniformly formed on the entire screw surface.

Further, as Comparative Example 4, the same treatment as in Example 2 was carried out except that the concentration of oxygen was changed to 0.05%, which was out of the range of 0.1 to 4% in the present invention. However, in the obtained work, the nitride layer has a non-uniform thickness, and the hardness is such that the surface hardness of the screw head is Hv = 430 to 120.
The performance was significantly lower than that of Example 2, such as a large variation of 0.

As Comparative Example 5, the oxygen concentration deviates from the oxygen range of the present invention of 0.1 to 4%.
The treatment was performed in the same manner as in Example 2 except that the percentage was changed to
The obtained work has a non-uniform thickness of the nitrided layer, and the hardness is such that the surface hardness of the screw head is Hv = 430 to 1150.
The performance was significantly lower than that of Example 2, such as large variations.

[0035]

[Example 3 and Comparative Examples 6 and 7] SUS304, which has been subjected to strong cold stretching and then subjected to strong cutting polishing finish on the surface
Put the shaft furnace shown in FIG. 1, NF ₃ 25,000
N containing 5000 ppm (0.5%) of ppm and O _{2
2 In} atmosphere, heat at 320 ° C for 10 minutes to fluorinate, then heat to 580 ° C, 50% NH ₃ + 50%
The mixed gas of RX was held for 2 hours and taken out. As a result, the surface hardness Hv = 1150 to 1280 (base material hardness Hv
= 350-420) A uniform nitride layer having a thickness of 40 μm was obtained.

On the other hand, in Comparative Example 6, the same parts were cleaned with alcohol and then fluorinated under a mixed gas containing 50,000 ppm of NF _3, and then, Example 3 was used.
Was treated under the nitriding conditions of. In addition, as Comparative Example 5, NF ₃
The input concentration was the same as in Example 3, except that no O ₂ was added at all and the mixture was heated to 580 ° C. and subjected to the nitriding treatment under the same nitriding conditions as in Example 3. As a result, NF of Comparative Example 5
_{When 3} was doubled, the same uniform hardened layer as in Example 3 was formed, but in Comparative Example 7, it was partially 15 to 2
There was uneven nitriding such that only a 0 μm nitride layer was formed.

[0037]

Example 4 After washing the polished samples processed from steel quality material SKD61, a furnace shown in FIG. 1, NF ₃ 45000
It was obtained by holding in N ₂ gas containing 2000 ppm (0.2%) of O ₂ and 350 ° C. for 60 minutes, then raising the temperature to 550 ° C. and heating in 75% NH ₃ for 3 hours. The thickness of the nitride layer was 0.15 mm. No uneven nitriding was observed in this nitride layer.

[0038]

[Example 5] SU of the test material used in Example 3
After cleaning the shaft of S304 with acetone, the shaft was put into the furnace shown in FIG. 1 and held at 350 ° C. for 20 minutes in an N ₂ atmosphere containing 50,000 ppm of NF ₃ . Then heat, 45
N ₂ + 6% Air in the furnace for 30 minutes until reaching 0 ° C.
The atmosphere was a mixed gas of (air). Next, in the furnace 50
Switch to nitriding atmosphere gas of% NH ₃ + 50% RX, 5
The temperature was raised to 80 ° C., the state was maintained for 60 minutes, and the shaft was taken out. As a result, the surface hardness H on the shaft surface
A uniform nitriding hard layer with v = 1150 to 1250 and a thickness of 30 μm was formed.

[0039]

[Example 6] SU of the test material used in Example 3
After cleaning the shaft of S304 with acetone, the shaft was put into the furnace shown in FIG. 1 and the inside of the furnace was made a mixed gas atmosphere of N ₂ + 6% Air (air). Then, in the atmosphere, the shaft was held at 350 ° C. for 30 minutes. Next, in the furnace, NF ₃
N ₂ gas containing 50000 ppm was introduced, and this was held at 350 ° C. for 20 minutes in this gas atmosphere for fluorination treatment. Next, the inside of the furnace was switched to a nitriding atmosphere gas of 50% NH ₃ + 50% RX, the temperature was raised to 580 ° C., the state was maintained for 60 minutes, and the shaft was taken out. as a result,
Surface hardness Hv = 1150-1250 on the shaft surface,
A uniform nitriding / hardening layer having a thickness of 30 μm was formed.

[0040]

As described above, the method for nitriding steel according to the present invention is
Prior to nitriding treatment, steel is heat-treated in a mixed gas of fluorine-based gas and air or oxygen to fluorinate, or steel is heat-treated in a fluorine gas atmosphere and then air or oxygen is introduced. Fluorination treatment, or before introducing the fluorine-based gas, air or oxygen is introduced into the furnace together with an inert gas such as N ₂ to heat and hold the steel, and then the fluorine-based gas is introduced. The steel is heated and held in a gas atmosphere. As a result, activated fluorine atoms act on the steel surface to remove contaminants of inorganic and organic substances on the steel surface, replace the oxide film on the surface with a fluoride film, and form a fluoride film on the steel surface layer. It forms and protects, then the fluoride film is removed during the nitriding process and an activated steel matrix is produced, so that the surface of the nitrogen-activated steel matrix during nitriding is rapidly and uniformly applied. The effect of the fluorine compound gas or the like, which penetrates and diffuses to form a favorable nitride layer, is promoted by the air and oxygen gas. That is, in the present invention, a fluorine-based gas and air or oxygen are used in combination during the fluorination treatment. Therefore, the generated fluoride film is reinforced by the O ₂ film to prevent uneven nitriding from occurring during the nitriding treatment, and at the same time, to save consumption of expensive fluorine-based gas that is closely related to the prevention of uneven nitriding. In the end, the cost of the nitriding treatment can be greatly realized. Therefore, it becomes possible to form an inexpensive nitride layer on a wide range of steels. In addition, this invention is a steel type,
Regardless of the processing stage, the pretreatment state, etc., a good nitride layer can be obtained, and nitriding is possible even for parts having holes or slits. Furthermore, even for steel types that are difficult to nitride such as austenitic stainless steel and various heat resistant steels,
There are advantages such as easy nitriding.

[Brief description of drawings]

FIG. 1 is a sectional view showing an example of a processing furnace used in the present invention.

Claims (1)

[Claims] 1. A method for nitriding steel in which a hard nitriding layer is formed by reacting nitrogen on the surface of steel, which comprises the following fluorination treatment (A), (B) or (C) prior to nitriding. A method for nitriding steel, which comprises performing. (A) In a gas atmosphere containing a fluorine compound gas or a fluorine gas, the gas atmosphere containing 0.5 to 20% by volume of air or 0.1 to 4% by volume of oxygen gas. , Keep the steel heated. (B) After heating and holding the steel in a gas atmosphere containing a fluorine compound gas or a fluorine gas, 0.5 to 2 of the whole
The steel is heated and held in a gas atmosphere containing air corresponding to 0% by volume or oxygen gas corresponding to 0.1 to 4% by volume. (C) After heating and holding the steel in a gas atmosphere containing 0.5 to 100% by volume of air or 0.1 to 20% by volume of oxygen gas, a fluorine compound gas or a fluorine gas is included. The steel is heated and held in a gas atmosphere.