JP3005953B2 - Steel nitriding method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for nitriding steel in which a nitride layer is formed 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, fatigue strength and the like, a method conventionally used as a nitriding method for forming a nitride layer on the surface of steel or a carbonitriding method is as follows. It looks like this: (A) Method using cyanide molten salt such as NaCN, KCNO, etc. (Tufftride method) (b) Glow discharge nitriding (ion nitriding) (c) Ammonia or a mixed gas of ammonia and a gas having a carbon source (eg, RX gas) By gas (gas nitriding, gas nitrocarburizing)

[0003] Among them, the method (a) uses a harmful molten salt, and therefore is not preferable in the future in terms of working environment, waste disposal, and the like. The method (b) is based on low vacuum N ₂
Nitrogen is nitrided by glow discharge in a + H ₂ atmosphere. The effect of the oxide film is reduced due to the cleaning effect accompanying sputtering, but nitriding unevenness due to a local temperature difference is likely to occur. In addition, this method has a problem that the shape and size of the processed object are greatly restricted, and the cost is increased. further,
The method (c) has a problem in the stability of the treatment, such as uneven nitridation, and also requires a long time to obtain a deep nitride layer.

[0004] Generally, steel is nitrided at a temperature of 500 ° C or higher, but the adsorption and diffusion of nitrogen to the steel surface layer requires a high activity on the metal surface. of course, it is desirable that the adsorption film of the oxide film and O ₂ is not present. Further, the presence of the oxide film is not preferable in that 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 by the gas nitriding method. For example, even in the case of case hardening steel or structural steel not containing a large amount of chromium, even at a temperature of 400 ° C. to 500 ° C., NH _{Even in} an atmosphere of ₃ or NH ₃ + RX, a thin oxide substance is formed. This tendency is further enhanced in steel types containing a large amount of elements having a high affinity for oxygen such as chromium.

The formation of such an oxide varies depending on the surface condition, processing conditions, and the like even in the same part, and as a result, a non-uniform nitride layer is formed. As a typical example, for example, in the case of a cold-worked austenitic stainless steel, even if it is washed with hydrofluoric nitric acid before being inserted into the processing furnace to completely remove the passivation film on the surface, satisfactory nitriding can be achieved. It is almost impossible to form a layer. In addition, the non-uniform nitriding occurs not only in gas nitrocarburizing but also in nitriding (gas nitriding) of nitrided steel or stainless steel with only ammonia. Further, even in the case of ordinary structural steel, there is a basic problem that in the case of a component having a complicated shape such as a gear, nitriding unevenness easily occurs.

As means for improving the essential problems of gas nitriding and gas nitrocarburizing as described above, a method of inserting a vinyl chloride resin into a furnace together with a processed material (work), a method of 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 oxides and remove them, or a method of plating oxides on the surface in advance to suppress oxides, etc., have been used in the past. Although it has been proposed, it has hardly been put to practical use. HCl precipitates chlorides such as FeCl ₂ and FeCl ₃ on the steel surface, but these are extremely brittle at temperatures lower than the nitriding temperature, and are easily sublimated and evaporated, so that chloride films are not formed.
Although it has a slight oxide film suppression effect, it is not practically effective due to the complicated handling itself and the remarkable damage to the furnace material.

[0007]

SUMMARY OF THE INVENTION In view of the above circumstances, it is an object of the present invention to effectively eliminate the occurrence of organic and inorganic foreign matters remaining after cleaning before nitriding treatment and the occurrence of uneven nitriding due to an oxide film on an object to be treated. In order to achieve this object, it is an object to provide a simple system in the processing process.

[0008]

Means for Solving the Problems In order to solve the above problems, a method for nitriding steel according to the present invention is directed to a method for nitriding steel in which a hard nitrided layer is formed by reacting nitrogen on the surface of steel.
The steel is heated and held beforehand in a fluorine-containing reaction gas atmosphere to form a fluoride film on the surface layer, and then heated in a nitriding atmosphere to form a nitride layer and to form the above-mentioned fluoride film.
The above reaction gas after use is detoxified . Here, the nitriding method means a carbonitriding method, an oxynitriding method,
Includes various nitriding methods such as the sulphiditriding method.

As the fluorine-containing reaction gas used in the present invention, for example, NF ₃ , BF ₃ , CF ₄ , SF ₆ , NH
₄ F, there is a halogen gas containing a fluorine compound or fluorine, such as F _2. Among these fluorine compounds, reactivity,
NF ₃ is the most excellent in terms of handleability and is practical. Under a reaction gas atmosphere containing the fluorine compound, the steel workpiece is heated and maintained at a temperature of 150 to 350 ° C. in the case of, for example, NF ₃ , and after the workpiece is surface-treated, a known nitriding gas such as ammonia Is used to perform a nitriding treatment (or a carbonitriding treatment). The concentration of the fluorine compound in the reaction gas atmosphere containing fluorine is, for example, 1000 to 1000.
The retention time in the atmosphere is 0 ppm, and an appropriate time may be selected according to the type of steel, the shape and size of the work, the heating temperature, and the like.

[0010]

Next, an embodiment of the present invention will be described.

The method of the present invention will be described more specifically.
A steel work is, for example, degreased and washed, and inserted into a heat treatment furnace 1 as shown in FIG. The furnace 1 is a pit furnace in which a stainless steel inner container 4 is placed inside a heater 3 provided in an outer shell 2, and a gas introduction pipe 5 and an exhaust pipe 6 are inserted therein. The gas introduction pipe 5 has a flow meter 17 and a valve 1 from cylinders 15 and 16.
Gas is supplied via 8 or the like. The internal atmosphere is agitated by a fan 8 rotated by a motor 7. Work 1
No. 0 is put in a container 11 made of steel and inserted into the furnace.
In the figure, 13 is a vacuum pump and 14 is a known abatement device. A reaction gas containing fluorine, for example, a mixed gas of NF ₃ and N ₂ is introduced into the furnace, and heated to a predetermined reaction temperature.
NF ₃ generates an F component of an active group at a temperature of 250 to 400 ° C., and this F removes organic and inorganic foreign substances remaining on the surface and also removes Fe, Cr substrate or FeO,
It reacts quickly with oxides such as Fe ₃ O ₄ and Cr ₂ O ₃ ,
For example, as shown in the following formula, FeF ₂ , FeF
₃ , a very thin fluoride film containing a compound such as CrF ₂ or CrF ₄ in the metal structure is formed.

[0012]

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By this reaction, the oxide film on the work surface is converted into a fluoride film, and O ₂ adsorbed on the surface is also removed. And such a fluoride film is made of O ₂ , H ₂ ,
When H ₂ O is not present, it is considered to be stable at a temperature of 600 ° C. or less and prevent the formation of an oxide film on the metal substrate and the adsorption of O ₂ until the subsequent nitriding temperature.

Thus, the workpiece treated with the fluorine-containing reaction gas is heated to a nitriding temperature of 480 to 700 ° C. in a non-oxidizing atmosphere such as an N ₂ atmosphere.
NH ₃ or a gas containing NH ₃ and a carbon source (for example, R 3
When a mixed gas with X gas is added, the fluoride film is reduced or destroyed by H ₂ or a small amount of water, for example, as in the following formula, and it is presumed that an active metal base is formed.

[0015]

Embedded image

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

The formation of such a compound layer is the same as in the conventional nitriding method. However, in the conventional method, the oxide film formed during the temperature rise from the room temperature to the nitriding temperature, Since the surface activity is reduced by the amount of O _2, the degree of N adsorption on the surface is low and non-uniform.
Such non-uniformity is also magnified by the fact that it is practically difficult to keep the degree of decomposition of NH ₃ uniform in the furnace. In the present invention, since the adsorption of N on the work surface is performed uniformly and quickly, the above-mentioned problem does not occur. In this process, the fluoride film is 600 ° C
Since a stable passivation film is formed below, damage to the metal furnace material is extremely small.

One of the major features of the operation process of the present invention is that NF as a reaction gas for forming a fluoride film is used.
The use of a non-reactive, gaseous, easy-to-handle substance such as ₃ makes the process simpler, such as continuous processing compared to methods such as plating and using a solid PVC liquid chlorine source. It is in a point. The tuftride method can be said to be an excellent method in terms of the effect of improving the throwing power of the nitride layer and the improvement of the fatigue strength, but cannot be said to be a future method in that it requires a large cost for the working environment, pollution equipment and the like. In the above process, a simple device for detoxifying the treatment waste gas is sufficient, and it is possible to eliminate uneven nitriding with the same or higher throwing power as that of the tuftride system. However, only pure nitriding is possible.

[0019]

Example 1 and Comparative Example 1 A SUS305-based work-hardened product (screw) was washed with Freon, placed in a processing furnace 1 as shown in FIG. 1, and placed in a N ₂ gas atmosphere containing 5000 ppm of NF ₃ at 300 ° C. for 15 hours. Hold for minutes. Then 530 ° C
Then, a mixed gas of 50% NH ₃ + 50% N ₂ was introduced into the furnace, and a nitriding treatment was performed for 3 hours.

The thickness of the nitrided layer of the obtained work is uniform,
The hardness of the substrate was 260 to 280 Hv, while the surface hardness was 1100 to 1300 Hv.

On the other hand, as Comparative Example 1, the same work was washed with chlorofluorocarbon and then treated with hydrofluoric-nitric acid.
After heating at 530 ° C. and 570 ° C. for 3 hours in% NH ₃ , the thickness of the nitrided layer formed by any of the treatments varied greatly, and there were many portions where no nitrided layer was formed.

FIG. 2 shows the results of elemental analysis of the surface layer by EPMA for Example 1 and Comparative Example 1 (570 ° C.).
Shown in 3 and 4 show the microscopic structures near the surfaces of both.

[0023]

Example 2 After washing the tapping screw of SUS305 with acetone, the tapping screw was put in the furnace shown in FIG. 1 and kept at 280 ° C. for 15 minutes in an N ₂ atmosphere containing 5,000 ppm of NF _3, and then 470 mm.
Temperature and kept under N ₂ + 90% H ₂ for 30 minutes.
It was taken out by nitriding with 0% NH ₃ + 80% RX for 8 hours.

Although a nitride layer of 40 to 50 μm was formed on the entire surface of the screw, the surface hardness was Hv = 650 to 750.
This nitrided layer exhibited corrosion resistance to 5% sulfuric acid that was not inferior to that of the substrate.

[0025]

Example 3 and Comparative Example 2 A hot die part (SKD61) polished by emery was put into a furnace shown in FIG. 1 as a work, and was placed in an N ₂ atmosphere containing 3000 ppm of NF ₃ .
After heating at 300 ° C. for 15 to 20 minutes, the mixture was heated to 570 ° C. and treated with a mixed gas of 50% NH ₃ + 50% N ₂ for 3 hours. As a result, the surface hardness is 1000 to 1100 Hv
(Base material 450 to 500 Hv), a uniform nitrided layer having a thickness of 120 μm was obtained.

On the other hand, in Comparative Example 2, the same part was subjected to nitric acid cleaning at 570 ° C. for 3 hours after cleaning with hydrofluoric nitric acid, but the nitrided film was 90 to 100 μm at the thickest point, and had a large variation and a rough surface. It was terrible.

[0027]

Example 4 and Comparative Example 3 After cleaning the nitrided steel (SACM1), it was placed in the furnace shown in FIG. 1 and kept at 280 ° C. for 20 minutes in N ₂ gas containing 5000 ppm of NF ₃ , and then
After heating to 50 ° C. and heating in 75% NH ₃ for 12 hours, the thickness of the obtained nitrided layer was 0.42 mm.
As Comparative Example 3, when the same component was nitrided by the conventional method, the thickness of the nitrided layer was 0.28 mm.

[0028]

Embodiment 5 After cleaning the mold parts of the structural carbon steel (S45C), the NF ₃ gas was held at 300 ° C. for 20 minutes in an atmosphere containing 5000 ppm, and then the temperature was raised to 530 ° C. and NH ₃ + 50% After treatment with RX for 4 hours, the mixture was taken out with oil cooling. The hardness of the obtained nitrided layer is 450 to 480 Hv
Met. The result of the rotational bending fatigue strength test of this work was 44 kg / mm ² , which was equal to or higher than that of a conventional gas nitrocarburized product.

[0029]

As is apparent from the above description, the nitriding method of the present invention is an improvement over conventional gas nitriding and gas nitrocarburizing, and has made it possible to quickly obtain a uniform nitrided layer. Also, a good nitrided layer can be obtained regardless of the type of steel, processing stage, pretreatment state, etc., and nitriding can be performed even on a component having holes or slits. Furthermore, there is an advantage that nitriding can be easily performed even on a steel type that is difficult to nitride such as austenitic stainless steel.

[Brief description of the drawings]

FIG. 1 is a sectional view showing an example of a processing furnace.

FIG. 2 is a chart showing the results of elemental analysis.

FIG. 3 is a metallographic photograph of the surface layer of the product of Example 1 treated according to the present invention.

FIG. 4 is a photograph of the metal structure of the surface layer of Comparative Example 1;

──────────────────────────────────────────────────続 き Continuing on the front page (72) Kenzo Kitano, Inventor 1498-1 Koyamadacho, Kawachinagano, Osaka (72) Teruo Minato, 3-38-2, Shiroyamadai, Hashimoto, Wakayama, Japan (56) References 3-44457 (JP, A) JP-A-6-299317 (JP, A) JP-A-6-145951 (JP, A) JP-A-5-195193 (JP, A) JP-A-5-59530 (JP, A) A) JP-A-4-358057 (JP, A) JP-A-4-187755 (JP, A) JP-A-4-187754 (JP, A) JP-A-4-44457 (JP, A) (58) Survey Field (Int.Cl. ⁷ , DB name) C23C 8/00-8/26

Claims (1)

(57) [Claims] In a method for nitriding steel, in which a hard nitrided layer is formed by reacting nitrogen on the surface of steel, a steel film is heated and held in a reaction gas atmosphere containing fluorine in advance to form a fluoride film on the surface layer. thereby forming a nitride layer by heating in a nitriding atmosphere after, on after being used for generating the fluoride film
A method for nitriding steel, comprising detoxifying the reaction gas .