JP7174943B2 - Nitriding equipment - Google Patents

Description

The present disclosure relates to a nitriding apparatus and a nitriding method.

Nitriding, which hardens steel materials and the like by dissolving nitrogen atoms in the material to be treated, improves wear resistance, fatigue strength, corrosion resistance, and the like of the material to be treated. Therefore, it has become an important technology in the fields of molds, sliding parts of engines, cutting tools, and the like.

Several methods are known for nitriding treatment, but the plasma nitriding method using plasma has the advantage of being able to perform nitriding treatment in a short period of time. However, since general plasma nitridation uses low-pressure plasma, it requires a vacuum vessel. For this reason, in order to nitride a large object, a large-scale apparatus having a large vacuum chamber is required. Moreover, it is unsuitable for applications such as local processing of a part of the object to be processed. As a method of performing plasma nitridation without using a vacuum vessel, a method using atmospheric pressure plasma has been studied (see, for example, Patent Document 1).

JP 2014-111821 A

However, in the method described in Patent Document 1, the influence of oxygen and the like in the atmosphere is very large, and stable nitriding cannot be performed. A method of setting a positive pressure state by providing a cover around the plasma jet has also been studied, but even if such a method is used, it is difficult to stabilize the nitriding treatment.

An object of the present disclosure is to enable stable nitriding by atmospheric pressure plasma.

One aspect of the nitriding apparatus of the present disclosure includes a plasma jet generating section that generates atmospheric pressure plasma, and a local sealing cover attached to the tip of the plasma jet generating section, wherein the local sealing cover is an object to be processed or Together with the plate on which the object to be processed is placed, it forms a closed space whose interior can be decompressed.

In one aspect of the nitriding apparatus, the local sealing cover may have a sealing member provided at a portion that contacts the surface of the object to be processed or the plate.

In one aspect of the nitriding apparatus, the opening of the localized sealing cover may be closed by the surface of the workpiece.

In the nitriding treatment method of the present disclosure, the nitriding apparatus of the present disclosure is used, and after the step of reducing the pressure in the closed space and the step of reducing the pressure, the plasma jet generation unit generates atmospheric pressure plasma of nitrogen, thereby introducing nitrogen atoms into the portion covered by the localized sealing cover.

In one aspect of the nitriding method, an inert gas is supplied to the plasma jet generating part between the step of reducing the pressure and the step of introducing nitrogen atoms, plasma is generated in the closed space while the pressure is reduced, and the A step of cleaning the surface of the object may be further included.

According to the nitriding apparatus of the present disclosure, stable nitriding can be performed by atmospheric pressure plasma.

1 is a cross-sectional view showing a nitriding apparatus according to one embodiment; FIG. FIG. 10 is a cross-sectional view showing a modification of the localized sealing cover; FIG. 10 is a cross-sectional view showing a modification of the localized sealing cover; It is a sectional view showing an example of a cooling mechanism. It is a sectional view showing an example of a heat shield. It is a graph which shows the distribution of oxygen in the depth direction. It is a graph which shows the change of the depth direction of hardness.

As shown in FIG. 1, the nitriding apparatus of the present embodiment includes a plasma jet generation unit 101 that generates atmospheric pressure plasma, and a tip portion of the plasma jet generation unit 101 that is attached to at least part of an object 150 to be processed. and a localized sealing cover 102 that can be covered. The local sealing cover 102 has an opening on the side opposite to the plasma jet generating part 101, the opening is closed by the surface of the object 150 to be processed, and the space sealed by the local sealing cover 102 and the object to be processed. 170 are formed. A vacuum evacuation unit 131 having a valve 133 and a vacuum pump 132 is connected to the space 170 to reduce the pressure inside the space 170 .

The plasma jet generating section 101 is a pulse arc type plasma jet device, and has a cylindrical outer electrode 111 and a substantially conical inner electrode 112 provided inside the outer electrode 111 . A pulse power supply 113 that supplies high-frequency power is connected between the external electrode 111 and the internal electrode 112 . A discharge region 115 is formed between the external electrode 111 and the internal electrode 112 .

A source gas is supplied from the gas supply unit 135 to the discharge region 115, and a high-frequency power is supplied from the pulse power supply 113, thereby generating pulsed arc plasma. The generated pulse arc plasma is injected as a plasma jet 160 from a nozzle 117 provided at the tip of the external electrode 111 . The gas supply unit 135 has, for example, a gas cylinder 136 and a valve 137 .

By using a gas containing nitrogen as the raw material gas, it is possible to generate a jet of nitrogen plasma containing high-density nitrogen atoms. By placing the workpiece 150 in front of the nozzle 117, the nitrogen atoms in the plasma jet 160 reach the surface of the workpiece 150 before recombining with nitrogen molecules. The self-heating of the atmospheric pressure plasma allows the surface temperature of the object 150 to be processed to be suitable for nitriding treatment, so that nitrogen atoms can be solid-dissolved and diffused into the object 150 to be processed. The temperature suitable for nitriding varies depending on the material of the object to be treated. For example, it is about 400 to 600.degree.

In the nitriding apparatus of this embodiment, the portion of the object 150 to be reached by the plasma jet 160 is covered with the local sealing cover 102, and the object 150 and the local sealing cover 102 form a sealed space 170. ing. By once decompressing the inside of the space 170, oxygen, water vapor, etc. in the space 170 can be exhausted. If the plasma jet 160 is applied to the object 150 to be processed in an atmosphere containing oxygen or the like, an oxide film may be formed on the surface of the object 150 to be processed. When an oxide film is formed on the surface of the object 150 to be processed, diffusion of nitrogen atoms supplied by the plasma jet 160 is hindered, and sufficient nitridation cannot be performed. However, since the nitriding apparatus of the present embodiment can greatly reduce the amount of oxygen in the atmosphere, the formation of an oxide film can be suppressed, and stable nitriding can be performed using atmospheric pressure plasma. On the other hand, since the object 150 to be processed is not housed in the vacuum chamber, even a large object can be nitrided.

In the nitriding apparatus of this embodiment, the local sealing cover 102 is configured to be in close contact with the surface of the object 150 to be treated and form a closed space 170 together with the object 150 to be treated. FIG. 1 shows an example in which the local sealing cover 102 has a cylindrical shape with an opening on the side opposite to the plasma jet generating section 101 . A flange 125 is provided on the periphery of the opening, and a seal member 126 made of an O-ring is attached to the flange 125 . Therefore, a closed space 170 is formed by the surface of the plate-shaped workpiece 150 and the local sealing cover 102 , and the pressure in the space 170 can be reduced by the vacuum exhaust section 131 .

The local airtight cover 102 is not limited to a cylindrical shape, and may have a square tube shape or a dome shape. Moreover, the flange 125 may be provided as necessary, and a configuration in which the flange 125 is not provided is also possible. FIG. 1 shows an example in which an object 150 to be processed is a flat plate and a local sealing cover 102 having a planar opening is used. However, as shown in FIG. 2, in the case of a workpiece 150 having an uneven surface, a local sealing cover 102 having an opening that conforms to the uneven surface of the workpiece 150 can be used.

As shown in FIG. 3, the workpiece 150 may be placed on a plate 155 and the plate 155 and the local sealing cover 102 may form a closed space 170 . In this way, even a small object 150 to be processed can be easily nitrided.

The size of the local sealing cover 102 may be appropriately set according to the size and shape of the object 150 to be processed. Depending on the material of the local sealing cover 102, it is preferable to set the size so that the distance from the plasma jet 160 can be several centimeters to several tens of centimeters. In this way, it is possible to avoid thermal deformation or thermal decomposition of the local sealing cover 102 by the plasma jet 160 .

The local sealing cover 102 is preferably made of a material having a heat resistance of about 100 degrees or higher. It can be formed from heat-resistant resin such as resin and polyimide resin. Further, by providing a cooling mechanism such as a cooling jacket in the local sealing cover 102, it becomes possible to use a material with even lower heat resistance.

In FIG. 1, in order to improve the adhesion between the local sealing cover and the object 150 to be processed, a seal member 126 made of an O-ring is provided at a portion of the local sealing cover 102 that contacts the object 150 to be processed. The material of the O-ring preferably has a heat resistance of about 100 degrees or more, and fluororubber, fluororubber coated with fluororesin, silicon rubber, metal O-ring, or the like can be used. By increasing the size of the localized sealing cover 102, it is also possible to use materials with low heat resistance. Moreover, the sealing member 126 is not limited to the O-ring, and may have other configurations as long as it can be brought into close contact with the local sealing cover 102 so as to depressurize it.

By pressing the local sealing cover 102 against the object 150 to be processed, the sealing member 126 is brought into close contact with the object 150 to be processed. Any mechanism may be used to press the local sealing cover 102 by applying weight. For example, a clamp or the like may be used.

As shown in FIG. 4, a cooling mechanism may be provided to limit the temperature rise of the localized sealing cover 102 and the seal member 126 . FIG. 4 shows an example in which the cooling mechanism is a cooling jacket 127 covering the entire localized sealing cover 102 . By flowing circulating water through the cooling jacket 127, temperature rise of the local sealing cover 102 and the sealing member 126 can be suppressed. Cooling jackets 127 can also be provided locally in particularly heat-sensitive areas or areas where temperature increases occur. For example, it is possible to adopt a configuration in which only the vicinity of the O-ring is cooled. The cooling mechanism is not limited to the water-cooling type, and may be an air-cooling type. When such a cooling mechanism is provided, a material with a low heat resistance temperature can be used for the local sealing cover 102 and the sealing member 126 .

Also, as shown in FIG. 5, a heat shield 128 can be provided around the plasma jet 160 . The heat shield plate 128 can be made of stainless steel, aluminum, or the like, for example. By providing the heat shield plate 128 , it is possible to suppress the temperature rise of the local sealing cover 102 due to the radiant heat from the plasma jet 160 . Both a cooling mechanism such as a jacket and a heat shield can be provided.

Nitriding of the workpiece 150 using the nitriding apparatus of the present embodiment can be performed as follows. First, the local sealing cover 102 corresponding to the shape of the object 150 to be processed is prepared. Next, the nitriding apparatus is arranged so that the plasma jet 160 acts on the processed position of the processed object 150 . After that, the evacuation unit 121 is operated to reduce the pressure in the space 170 covered by the local sealing cover 102 . From the viewpoint of sufficiently removing oxygen and the like in the space 170 and allowing solid solution and diffusion of nitrogen atoms, the pressure in the space 170 should be low, preferably 100 Pa or less, more preferably 10 Pa or less, and even more preferably. is 1 Pa or less. From the viewpoint of sufficient evacuation, the pressure reduction time is preferably 1 minute or longer, more preferably 10 minutes or longer, and even more preferably 30 minutes or longer. From the viewpoint of productivity, the time is preferably 5 hours or less, more preferably 3 hours or less, and even more preferably 1 hour or less.

After the space 170 is sufficiently decompressed, the source gas is supplied from the gas supply unit 104 to replace the discharge region 115 and the space 170 with the source gas, and the pressure is returned to the atmospheric pressure. The raw material gas can be, for example, a mixed gas of nitrogen gas and hydrogen gas. By adding hydrogen gas, it is possible to reduce a small amount of oxygen remaining in the raw material gas. In addition, the amount of NH radicals generated varies depending on the mixing ratio of hydrogen gas, and the diffusion range and concentration of nitrogen atoms can be controlled. In this case, the ratio of hydrogen gas to nitrogen gas is preferably 0.1% by volume or more, preferably 200% by volume or less, and more preferably 10% by volume.

After the inside of the space 170 is replaced with the raw material gas, high-frequency power is supplied from the pulse power source 113 to the outer electrode 111 and the inner electrode 112 to generate atmospheric pressure nitrogen plasma. The feed rate of the raw material gas is preferably 1 L/min to 100 L/min from the viewpoint of generating stable atmospheric pressure plasma. By irradiating the position to be processed of the object 150 to be processed with the plasma jet 160, nitrogen atoms can be dissolved and diffused into the position to be processed for nitriding. The irradiation time of the plasma jet 160 may be determined according to the required degree of nitriding treatment, and may be 0.1 hours or more and 24 hours or less.

When irradiating the plasma jet 160, the temperature of the object 150 at the position to be processed is set to a temperature higher than the temperature at which nitrogen atoms diffuse and lower than a temperature at which the object 150 is not deformed by heat. When the object 150 to be processed is a steel material, the temperature is preferably 400° C. or higher, more preferably 500° C. or higher, and preferably 600° C. or lower, more preferably 550° C. or lower. The temperature of the position to be treated can be controlled by changing settings such as the peak value of the pulse voltage supplied to the electrodes, the repetition frequency, and the duty ratio. The temperature of the position to be processed can be set to a predetermined temperature range by the plasma jet 160, and a heater that heats the entire or local area of the object to be processed 150 can also be used.

The pressure reduction of the space 170 within the localized sealing cover 102 and the purge with the source gas may be repeated multiple times. By repeating purging a plurality of times, it is possible to sufficiently remove oxygen and the like even if the pressure during decompression is high.

After decompressing the space 170 in the local sealing cover 102, when the pressure is returned to atmospheric pressure, the vacuum pump 122 is stopped and the raw material gas is supplied. Further, by adjusting the exhaustion by the vacuum pump 122 and the supply of the raw material gas, the space 170 can be made to have a slightly negative pressure. By setting the space 170 to a slightly negative pressure, it is possible to further reduce the intrusion of outside air into the space 170 . In this case, plasma can be stably generated if the pressure in the space 170 is higher than about 900 hPa.

After decompressing the space 170 in the localized sealing cover 102 and before generating the atmospheric pressure plasma, an inert gas such as argon (Ar) gas or helium (He) gas is supplied to the discharge region 115 to form an argon gas plasma. is generated, the surface of the object 150 to be processed can be bombarded. By performing the bombardment cleaning, the passivation layer and the like existing on the surface of the object 150 to be processed can be removed, facilitating the diffusion of nitrogen atoms.

When bombardment cleaning is performed, the supply rate of Ar gas or the like can be about several CC/min to several hundred CC/min, and the pressure in the space 170 can be about 80 Pa. Although the bombardment cleaning time is not particularly limited, it can be about 60 minutes.

The object to be treated by the nitriding apparatus of the present embodiment can be pig iron castings such as spheroidal graphite cast iron or gray cast iron, or steel materials such as carbon tool steel, alloy tool steel or high speed tool steel. It can also be used for nitriding aluminum, titanium, or alloys thereof. Since the nitriding apparatus of the present embodiment does not use a vacuum vessel for accommodating the object to be treated, it can be used for nitriding and hardening repaired portions of large molds for molding automobile bodies. In addition, since it can be nitrided locally, it can be used for applications such as selectively hardening only a part of a part. When hardening a predetermined region of the surface of the object to be treated, the treatment can be performed a plurality of times while shifting the mounting position of the nitriding apparatus of the present embodiment.

<Nitriding equipment>
Nitriding was performed by the nitriding apparatus shown in FIG. The localized sealing cover was cylindrical with a diameter of 15 cm and a height of 8 cm. The material of the local sealing device 102 was stainless steel. The width of the flange was 5 cm, and the sealing member was a commercially available fluororubber O-ring (wire diameter 5 mm x inner diameter 22.5 cm). The distance between the internal electrode of the plasma generating section and the external electrode was 20 mm, and the inner diameter of the nozzle was 4 mm.

<Measurement of oxygen distribution>
The oxygen distribution in the depth direction from the surface of the obtained sample was measured with an X-ray photoelectron spectrometer (K-Alpha, manufactured by Thermo Fisher Scientific). The X-ray spot size was 200 μm. For etching with argon ions, the beam energy was 1000 eV, the current setting was high level, and the raster size was 1 mm.

<Measurement of hardness change>
A cross section obtained by cutting the obtained sample was measured for change in hardness in the depth direction by a micro Vickers hardness tester (FM-300, manufactured by Futuretech). The load was 10 g and the holding time was 10 seconds.

(Example 1)
Alloy tool steel (JIS SKD61) and gray cast iron (JIS FC250) with a thickness of 5 mm and 2 cm square were nitrided as follows.

A sample was placed on a 28 cm square stainless steel plate with a thickness of 2 cm so that a space was generated around the sample, and a local sealing cover of the nitriding apparatus was placed on the stainless plate. The pressure in the space was reduced to 1 Pa for 1 hour by a vacuum pump. After that, the vacuum pump was stopped, nitrogen was supplied at a flow rate of 2 L/min and hydrogen was supplied at a flow rate of 20 L/min, and the pressure in the space was returned to atmospheric pressure. Subsequently, while supplying nitrogen at a flow rate of 19.8 L / min and hydrogen at a flow rate of 200 mL / min, a high voltage pulse with a peak value of ± 5 kV and a frequency of 21 kHz was applied to the internal electrode to generate atmospheric pressure plasma for 120 minutes. Nitriding was performed.

The distribution of oxygen in the depth direction was measured for a sample obtained by nitriding SKD61. A change in hardness in the depth direction was measured for a sample obtained by nitriding FC250.

(Comparative example 1)
Nitriding treatment was performed on SKD61 in the same manner as in Example 1, except that the pressure in the space was not reduced. About the obtained sample, the distribution of oxygen in the depth direction was measured.

(Comparative example 2)
The distribution of oxygen in the depth direction was measured for SKD61 which was not subjected to nitriding treatment.

(Comparative Example 3)
For FC250, which was entrusted with commercial low-pressure plasma nitriding treatment, changes in hardness in the depth direction were measured.

(Comparative Example 4)
For FC250, which was entrusted with commercial radical nitriding treatment, changes in hardness in the depth direction were measured.

As shown in FIG. 6, in the case of Comparative Example 2 in which nitriding treatment was not performed, oxygen was present on the outermost surface, but the amount of oxygen decreased rapidly. On the other hand, in the case of Comparative Example 1 in which the nitriding treatment was performed without the decompression treatment, oxygen existed deep inside, and it is clear that a relatively thick oxide film was formed in the plasma treatment. In the case of Example 1, in which the nitriding treatment was performed after the decompression treatment, the depth of oxygen existing was much shallower than that of Comparative Example 1, and it is clear that the formation of an oxide film was suppressed.

As shown in FIG. 7, in Example 1 in which the atmospheric pressure plasma nitriding treatment was performed, a stable nitriding treatment was performed with a small change in hardness in the depth direction. In addition, it achieves almost the same degree of hardness as Comparative Example 4 in which commercial radical nitriding treatment is performed. On the other hand, in Comparative Example 3, in which commercial low-pressure plasma nitriding treatment was performed, the hardness near the surface increased significantly, but the inside was not sufficiently nitrided.

INDUSTRIAL APPLICABILITY The nitriding apparatus of the present disclosure can perform stable nitriding using atmospheric pressure plasma, and is particularly useful as an apparatus for locally nitriding a large member that cannot be placed in a vacuum vessel or the like.

101 Plasma jet generation unit 102 Locally sealed cover 104 Gas supply unit 111 External electrode 112 Internal electrode 113 Pulse power source 115 Discharge area 117 Nozzle 121 Vacuum exhaust unit 122 Vacuum pump 125 Flange 126 Sealing member 127 Cooling jacket 128 Heat shield plate 150 Object to be processed 155 plate 160 plasma jet 170 space

Claims (7)

a plasma jet generation unit that generates atmospheric pressure plasma and ejects it from the tip nozzle;
a localized sealing cover attached to the tip of the plasma jet generating section;
a vacuum exhaust unit for decompressing the inside of the local sealing cover,
The local sealing cover has a cylindrical shape and has a top plate through which the tip nozzle penetrates, a side wall surrounding the tip nozzle, and an open end on the opposite side of the top plate. Forming a sealed space capable of decompressing the inside to 10 Pa or less by bringing the open end into contact with the placed plate,
A nitriding apparatus for nitriding a steel material, wherein the temperature at the position of the object to be treated is controlled to be 400° C. or higher and 550° C. or lower. 2. The nitriding apparatus according to claim 1, wherein said local sealing cover has a seal member provided at a portion in contact with the surface of said object to be processed or said plate. 3. The nitriding apparatus according to claim 1, wherein the opening of said localized sealing cover is closed by the surface of said workpiece. The nitriding apparatus according to any one of claims 1 to 3, further comprising a cooling mechanism for cooling said localized sealing cover. 5. The nitriding apparatus according to any one of claims 1 to 4, wherein the temperature at the position to be processed of said object to be processed is controlled within a predetermined range by irradiation with a plasma jet. Using the nitriding apparatus according to any one of claims 1 to 5,
a step of decompressing the closed space to 10 Pa or less ;
and a step of generating atmospheric pressure plasma of nitrogen by the plasma jet generating section after the step of decompressing to introduce nitrogen atoms into the portion of the object to be processed covered by the local sealing cover. , Nitriding method for steel. Between the step of reducing the pressure and the step of introducing the nitrogen atoms, an inert gas is supplied to the plasma jet generating section to generate plasma in a state where the pressure in the sealed space is reduced, and the surface of the object to be processed 7. The nitriding method according to claim 6, further comprising the step of cleaning.

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