JP5548920B2 - Method for carburizing a workpiece having an edge - Google Patents

Description

  The present invention relates to a carburizing method for forming a carburized layer on a surface portion of a workpiece having an edge portion (for example, a gear), and more particularly to an improved plasma carburizing method.

  Plasma carburization of various steel materials (workpieces) is carried out by placing the work in a vacuum furnace and heating it to 850 ° C to 1100 ° C, introducing hydrocarbon gas such as methane as a reaction gas into the furnace, and causing glow discharge. This is a method of carburizing the surface of a work piece as a cathode, which can obtain a higher carburizing efficiency than the case of gas carburizing, and can obtain a uniform concentration distribution over the entire work surface even in general with a high carbon concentration. There is an advantage. After the plasma carburizing, the captured carbon is diffused into the workpiece and a carburized layer having a predetermined thickness is formed, so that a diffusion treatment is continuously performed as necessary.

  Carbon taken into the workpiece surface by plasma carburizing gradually diffuses into the workpiece during the carburizing treatment and the subsequent diffusion treatment, thereby forming a carburized layer having a predetermined thickness. However, when such plasma carburizing is applied to a workpiece having an edge like a gear, the carburized layer at the edge portion of the workpiece has a relatively high carbon concentration as compared with the flat portion of the workpiece. This is based on the difference in shape between the edge part and the flat part, and the edge part has a smaller surface area than the flat part, but the inner diffusible region is small, and the incorporated carbon is difficult to diffuse inside, This is because the surface carbon concentration is difficult to decrease.

  The unevenness of the surface carbon concentration at the edge portion and the flat portion is conspicuous in plasma carburizing and does not cause much problem in gas carburizing. In other words, since gas carburizing is carburizing under equilibrium conditions, the carbon concentration on the workpiece surface does not rise above the concentration that balances the carbon potential of the atmosphere, and the carburizing rate increases as the carbon concentration on the workpiece surface increases. Since it falls, the surface carbon concentration of the edge part and the flat part does not change greatly during the carburizing process. However, plasma carburizing is originally a carburizing under a non-equilibrium state, and even if the surface carbon concentration of the edge portion increases during the carburizing process, the rate of carbon incorporation is not different from that of the flat part. The difference in surface carbon concentration tends to be large. And the nonuniformity of the carbon concentration is not eliminated even if a normal diffusion treatment is performed.

  Therefore, at the stage where the diffusion treatment has been completed, even if there are no carbon concentration points exceeding the eutectoid point in the carburized layer in the flat part, the carbon concentration in the carburized layer at the edge part is less likely to decrease. In some cases, a concentration portion may remain, and in this case, a net-like carbide is formed along the grain boundary at the edge portion of the workpiece after cooling. The net-like carbide produced along the grain boundary is brittle and becomes the starting point of cracking, and once produced, it does not disappear easily even by the subsequent heat treatment. As a method for correcting such non-uniformity of carbon concentration, a method of continuously decarburizing after plasma carburizing has been proposed (Patent Document 1).

  In the diffusion treatment after the carburizing process by the method described in Patent Document 1 (described as decarburizing process in Patent Document 1), the carbon supplied by the carburizing process is thermally diffused inside the sample and a part thereof. Is released into the furnace atmosphere. This is a phenomenon that occurs also in the flat portion, and hinders the feature of plasma carburizing that can efficiently supply carbon.

JP 07-268601 A

  Therefore, the object of the present invention is to reduce the difference in carbon concentration between the edge part and the flat part when carburizing various workpieces with edge parts by the plasma carburizing method, and suppress the formation of net-like carbides generated at the edge part during cooling. Furthermore, the problem in the method described in Patent Document 1 is to be solved at the same time.

  The above object is achieved by the present invention described below. That is, according to the present invention, a workpiece having an edge portion is accommodated in a vacuum furnace, and after performing a plasma carburizing process (carburizing process) in which a carburizing gas is supplied and glow discharge is performed in the vacuum furnace, Carburizing a workpiece having an edge, characterized by having a step (diffusion step) of diffusing carbon in the workpiece surface layer into the workpiece by neutral or reducing gas plasma treatment containing an inert gas typified by Provide a method.

Moreover, after the said carburizing process or a diffusion process, performing the diffusion process in inert gas atmosphere can be added.
Moreover, after the said carburizing process or a diffusion process, performing the nitriding process in nitriding gas atmosphere can be added.

  In the method of the present invention, the carburizing treatment step and the diffusion step are preferably repeated twice or three times, and a step of oxidizing and removing carbides on the workpiece surface is added after the last diffusion step. You can also.

  In the method of the present invention, a step of coating a hard film having a film forming temperature of 100 to 300 ° C. may be added after the carburizing method.

  According to the present invention, when various workpieces having an edge portion are carburized by the plasma carburizing method, the difference in carbon concentration between the edge portion and the flat portion is small, and the generation of net-like carbides generated at the edge portion during cooling is suppressed. Furthermore, the problem in the method described in Patent Document 1 can be solved at the same time.

The figure explaining the Example of this invention The figure explaining the Example of this invention Electron micrograph of the surface of the test piece of Example 3 (magnification 2000 times) The figure explaining the Example of this invention The figure explaining the Example of this invention The figure explaining the Example of this invention

Next, the present invention will be described in more detail with reference to modes for carrying out the invention.
The workpiece to be carburized by the method of the present invention is case-hardened steel which is a steel material for machine structure, and has an edge portion. Examples of the workpiece having an edge portion include a differential gear, a planetary gear, and a final drive device that are power train components. In such a workpiece, the carbon concentration of the edge portion (corner portion) tends to be higher than that of the other portion (flat portion) in the conventional plasma carburizing process.

  The plasma carburizing apparatus used in the present invention is conventionally known. For example, the apparatus is capable of heating, applying plasma and carburizing under reduced pressure, and capable of cooling with pressurized gas. The equipment includes a vacuum chamber and a cooling chamber. have. In the vacuum chamber, a heater is installed on the outer peripheral wall, and an electrode connected to a plasma power source is installed inside the vacuum chamber also serving as a tray for workpieces (carburized products). The lower part of the apparatus is connected to a vacuum pump via an exhaust pipe to enable a reduced pressure atmosphere. And it is an apparatus which can supply argon, nitrogen, oxygen, hydrogen, and a hydrocarbon type gas to a vacuum chamber via an upper gas introduction nozzle. The cooling chamber can supply a maximum of 1 MPa of helium gas, and the helium gas can be circulated by a cooling fan and piping.

  The plasma carburizing process itself in the method of the present invention may be a conventionally known method, and the workpiece is housed in a vacuum furnace of the above apparatus, and a carburizing gas is supplied into the vacuum furnace to perform a plasma carburizing process for glow discharge. . A feature of the present invention is that the plasma carburization is followed by neutral or reducing gas plasma treatment containing an inert gas typified by argon. Further, a plasma carbonitriding process can be performed by supplying a nitriding gas after the carburizing process.

The method of the present invention will be described with reference to the embodiment shown in FIG. The workpiece is housed in the vacuum furnace of the above apparatus, and the temperature is raised to 850 to 1030 ° C. in about 1 to 2 hours while reducing the pressure in the furnace to about 10 ⁻³ to 10 ³ Pa. First, in order to first clean the surface of the workpiece, an argon gas containing 0 to 80% by volume of hydrogen is supplied into the furnace, the pressure is 50 to 200 Pa at the above temperature, the voltage is 500 to 700 V, and the current density is 0.00. Plasma treatment is performed for 15 to 60 minutes under conditions of ^{3 to} 1.2 mA / cm ² .

Plasma carburization is performed after the cleaning process. The carburizing condition may be a conventionally known condition. For example, a hydrocarbon gas such as methane gas is introduced into the furnace at a furnace temperature of 850 to 1030 ° C., the hydrocarbon gas partial pressure is 150 to 900 Pa, and the voltage is 280 to 350 V. Then, plasma carburizing treatment is performed for 5 to 60 minutes under the condition of current density of 0.1 to 0.4 mA / cm ² . In this carburizing process, the carbon concentration at the edge portion of the workpiece is about 0.1 to 0.8% higher in absolute value than the carbon concentration in the flat portion.

  After the carburizing process is completed, the supply of hydrocarbon gas is interrupted, and a neutral or reducing gas containing an inert gas typified by argon is supplied into the furnace. The carbon of the surface layer is diffused inside the workpiece. By this diffusion process, carbon on the surface of the carburized layer diffuses in the internal direction of the workpiece. At this time, after the carburizing treatment or the diffusion step, a step of diffusing carbon on the surface into the workpiece in a vacuum atmosphere or an inert gas atmosphere such as argon can be appropriately performed. Further, these diffusion treatments enable further carburization (secondary and tertiary carburization) on the surface of the workpiece. After the diffusion treatment is completed, helium gas for cooling is supplied from the upper part of the apparatus in the cooling chamber, and the sample surface temperature is set to 700 ° C. to room temperature, which is lower than the A1 transformation point. The above carburizing process and diffusion process are defined as the primary process (not including the cleaning process) (see FIG. 1).

The feature of the present invention resides in the diffusion treatment of the primary process described above. In the present invention, in order to achieve high-concentration carburization for precipitating spherical carbides, the secondary process and the primary process as shown in FIG. It is preferable to perform a tertiary process. The secondary process is the same as the primary process except that the carburization and diffusion temperatures are lower by about 0-100 ° C. than the primary process. The tertiary process is the same as the secondary process except for the last holding process. In the holding step, the supply of argon gas to the furnace is stopped and the inside of the furnace is maintained at about 10 ⁻³ to 10 ³ Pa, and the proper quenching temperature of the sample is about 750 to 930 ° C. in about 15 to 30 minutes. Then, the workpiece is cooled and taken out in the same manner as in the secondary process. Quenching is performed by this holding step.

The feature of the present invention resides in the diffusion process of the primary process described above. In the present invention, as shown in FIG. 5, a process for achieving an improvement in carburizing hardenability and an improvement in temper softening resistance can be performed. In the above process, the supply of a neutral or reducing gas containing an inert gas typified by argon or the like in the diffusion process is stopped, and a nitriding gas such as nitrogen is supplied at a temperature of 750 to 930 ° C. This is carried out by performing plasma nitriding for 10 to 30 minutes under the conditions of a temperature of 100 to 400 Pa, a voltage of 250 to 550 V, and a current density of 0.2 to 0.4 mA / cm ² (see Example 6).
In FIG. 1, neutral or reducing gas plasma treatment including an inert gas typified by argon is performed in all diffusion steps of the primary step to the tertiary step. However, the primary step and the secondary step are performed. The diffusion step of the step is performed by neutral or reducing gas plasma treatment containing an inert gas typified by argon, and the effect of the present invention can be obtained even when the diffusion treatment of the tertiary step is a vacuum diffusion treatment (Example 2). reference).

FIG. 2 shows another embodiment of the present invention. This embodiment is the same as the embodiment of FIG. 1 except that an oxidation step is provided between the diffusion treatment step and the holding step in the tertiary step of the embodiment shown in FIG. An infinite number of fine carbide fine particles remain on the surface of the carburized workpiece obtained in the embodiment shown in FIG. 1, and it may be required to remove the fine particles (such as peristaltic properties). . In the above oxidation step, the supply of neutral or reducing gas containing an inert gas typified by argon in the diffusion treatment is stopped, and an oxidizing gas such as oxygen or carbon dioxide is supplied at a temperature of 750 to 930 ° C. This is carried out by performing plasma oxidation treatment for 10 to 30 minutes under the conditions of the above-mentioned temperature, pressure of 100 to 400 Pa, voltage of 250 to 350 V, and current density of 0.2 to 0.4 mA / cm ² .

FIG. 6 shows another embodiment of the present invention. This embodiment is the same as the embodiment of FIG. 1 except that a nitriding step is provided between the diffusion treatment step and the holding step in the tertiary step of the embodiment shown in FIG. In some cases, it is required to improve the hardenability and temper softening resistance of the carburized workpiece obtained in the embodiment shown in FIG. In the nitriding step, the supply of a neutral or reducing gas containing an inert gas typified by argon in the diffusion treatment is stopped, and a nitriding gas such as nitrogen is supplied at a temperature of 750 to 930 ° C. This is performed by performing plasma nitriding for 10 to 30 minutes under the conditions of a pressure of 100 to 400 Pa, a voltage of 250 to 550 V, and a current density of 0.2 to 0.4 mA / cm ² .

After performing the quenching process of the above-described embodiment, the workpiece is accommodated in the vacuum furnace of the apparatus, and the pressure in the furnace is reduced to about 10 ⁻³ to 10 ³ Pa, and about 100 to 300 in about 1 to 2 hours. The temperature is raised to ° C. First, in order to first clean the surface of the workpiece, argon gas containing 0 to 80% by volume of hydrogen is supplied into the furnace, and the pressure is 5 to 50 Pa and the voltage is 400 to 700 V at the above temperature. Plasma treatment is performed for 60 minutes.
Coating is performed after the cleaning process. The carburizing condition may be a conventionally known condition. For example, a hydrocarbon gas such as methane gas is introduced into the furnace at a furnace temperature of 100 to 300 ° C., the hydrocarbon gas partial pressure is 5 to 50 Pa, and the voltage is 400 to 700 V. Plasma carburizing treatment is performed for 60 to 240 minutes.

Next, the present invention will be described more specifically with reference to examples and comparative examples.
[Sample material]
Using a case-hardened steel, which is a steel for machine structure shown in Table 1 below, a coin-shaped test piece of φ25 × 10 mm was created by machining.

[Plasma carburizing equipment]
The carburizing diffusion of the test piece was performed by carburizing treatment of the present invention using equipment capable of heating, applying plasma and carburizing under reduced pressure and capable of cooling with pressurized gas.
The facility has a vacuum chamber and a cooling chamber. In the vacuum chamber, a heater is installed on the outer peripheral wall, and an electrode connected to a plasma power source is installed inside the vacuum chamber also as a test piece tray. The lower part is connected to a vacuum pump via an exhaust pipe to enable a reduced pressure atmosphere. Argon, nitrogen, oxygen, hydrogen, and hydrocarbon gas can be supplied to the vacuum chamber via the upper gas introduction nozzle. The cooling chamber can supply a maximum of 1 MPa of helium gas, and the helium gas can be circulated by a cooling fan and piping.

[Evaluation method]
The carburized specimens obtained in the examples and comparative examples were evaluated by examining the surface carbon concentration, the structure observation, the cross-sectional hardness and the frictional wear characteristics. The evaluation method is as follows.
[Surface carbon concentration]
After carburizing treatment, the position of 5 μm from the surface was measured by glow discharge emission spectroscopic analysis.

[Tissue observation]
Coin-shaped specimens that have been carburized, diffused, quenched, and tempered are cut and polished, corroded with 3% nital, and then optical micrographs (observation magnification 1,000 times) are taken from the outermost surface to 50 μm for image analysis Then, the area ratio and the particle size were measured.
With respect to the network carbide, the presence or absence of the network carbide was observed under the same conditions as described above. Here, the net-like carbide is a precipitate in which coarse carbides form a net-like shape along the crystal grain boundary.

[Cross section hardness]
Based on JIS Z 2244, the measurement was performed from the surface toward the inside.
[Friction and wear characteristics]
Using a ball-on-disk friction and wear tester, the sliding mating member was SUJ2 with a diameter of 6 mm, and was tested under non-lubricating conditions under a load of 2 N, a peripheral speed of 300 rpm, and a sliding radius of 10 mm.

[Example 1]
The test piece was placed flat on a jig and carried into the processing chamber of the apparatus. A heating pattern is shown in FIG. As a primary process, the processing chamber atmosphere temperature was heated to 980 ° C. in a reduced pressure atmosphere of 1 × 10 ⁻³ Pa over 90 minutes. A gas mixture of 1 L / min of 20% Ar / 80% H ₂ was introduced at an in-furnace pressure of 50 Pa, and cleaning treatment with glow discharge plasma of 0.6 mA / cm ² · 500 V was performed for 30 minutes.

After completion of the cleaning process, 100% CH ₄ gas 10 L / min was subsequently introduced at a furnace pressure of 300 Pa, and carburizing process with glow discharge plasma of 0.4 mA / cm ² · 330 V was performed for 20 minutes. Subsequently, evacuation is performed, and the mixture is held in a vacuum atmosphere for 5 minutes. Subsequently, a gas mixture meter of 1% / min of 20% Ar / 80% H ₂ is introduced at a furnace pressure of 50 Pa, and 0.6 mA / cm ² is introduced. -Diffusion treatment with 500 V glow discharge plasma was performed for 40 minutes, and it was transported to a cooling chamber and cooled with pressurized He gas.

Subsequently, as a secondary process, reheating is performed in a reduced pressure atmosphere of 1 × 10 ⁻³ Pa for 90 minutes up to 900 ° C., and 100% CH ₄ gas 10 L / min is introduced at a furnace pressure of 300 Pa. Carburizing treatment with glow discharge plasma of 0.4 mA / cm ² · 330 V was performed for 20 minutes. Subsequently, evacuation is performed, and the mixture is kept in a vacuum atmosphere for 5 minutes. Subsequently, a mixed gas of 1% / min of 20% Ar / 80% H ₂ is introduced at a furnace pressure of 50 Pa, and 0.6 mA / cm ² · Diffusion treatment with glow discharge plasma of 500 V was performed for 40 minutes, and it was transported to a cooling chamber and cooled with pressurized He gas.

Further, as a third step, the processing chamber atmosphere temperature is reheated to 900 ° C. over 90 minutes in a reduced pressure atmosphere of 1 × 10 ⁻³ Pa, and 100 L CH ₄ gas 10 L / min is introduced at a furnace pressure of 300 Pa. Carburizing treatment with glow discharge plasma of 0.4 mA / cm ² · 330 V was performed for 20 minutes. Subsequently, evacuation is performed, and the mixture is kept in a vacuum atmosphere for 5 minutes. Subsequently, a mixed gas of 1% / min of 20% Ar / 80% H ₂ is introduced at a furnace pressure of 50 Pa, and 0.6 mA / cm ² · Diffusion treatment with 500 V glow discharge plasma was performed for 40 minutes. After evacuation, the temperature was lowered to a quenching temperature of 870 ° C. and held for 30 minutes, and then transferred to a cooling chamber and cooled with pressurized He gas.

  As a result of observing the structure of this test piece, the shape of the carbide precipitated at the end (edge) of the test piece became spherical, and the shape of the precipitated carbide at the end of the test piece, which was a concern for carburization, was improved. That is, ionized argon was sputtered and heated around the end of the test piece to promote carbon diffusion. Furthermore, the precipitation of reticulated carbide was suppressed by increasing the solid solubility limit of carbon by heating.

[Comparative Example 1]
In the primary process to the tertiary process, all diffusion processes were performed in a vacuum atmosphere, and the same processes as in Example 1 were performed except that. As a result of observing the structure of this test piece, the precipitation of spherical carbides occupied the whole in the flat part, but the carbon component was not completely dissolved in the end part of the test piece, and the reticulated carbide was found along the grain boundaries. Precipitated.

[Example 2]
In the third step only, all diffusion processes were performed in a vacuum atmosphere, and the other processes were performed in the same manner as in Example 1. As a result of observing the structure of the test piece, the shape of the carbide precipitated at the end of the test piece was spherical, and a structure in which the spherical carbide was finely precipitated in a large amount at the plane part was obtained. That is, in the primary process and the secondary process, the precipitation of mesh carbide was suppressed by promoting the diffusion of carbon and improving the carbon solid solution limit. Subsequently, by diffusing in a vacuum atmosphere in the third step, carbon that had been dissolved until the second step was precipitated as carbides in the grains, so that spherical carbide was precipitated at the end of the test piece, and the plane portion A structure with a large amount of carbide nuclei was obtained.

[Comparative Example 2]
In the third step only, diffusion treatment is performed in a glow discharge plasma atmosphere of 0.6 mA / cm ² · 500 V by introducing 1 L / min of a mixed gas of 20% Ar / 80% H _{2 at} a furnace pressure of 50 Pa. The same treatment as in Comparative Example 1 was performed.

  As a result of observing the structure of this test piece, the precipitation of spherical carbides occupied the whole in the flat part, but the carbon component was not completely dissolved in the end part of the test piece, and the reticulated carbide was found along the grain boundaries. Precipitated. In the primary process or the secondary process, the reticulated carbide had already precipitated, and the reticulated carbide did not re-dissolve in the plasma atmosphere of the tertiary process.

  Table 2 shows a summary of the surface carbon concentration, the carbide area ratio at the end, the average carbide particle size at the end, the presence or absence of mesh carbide at the end, and the cross-sectional hardness at a position of 50 μm from the surface of these test results described above. Show.

[Example 3]
In the third step, vacuum evacuation is performed to lower the quenching temperature to 870 ° C., and 1 L / min of a mixed gas of 80% O ₂ /20% H ₂ is introduced at a furnace pressure of 100 Pa, and 0.3 mA / cm ² · 260 V For 10 minutes in a glow discharge plasma atmosphere and conveyed to a cooling chamber for cooling with pressurized He gas. Otherwise, the same processing as in Example 1 was performed. A heat pattern is shown in FIG. As a result of observing the surface of this test piece with an electron microscope, the spherical carbide precipitated by carburization was lost. FIG. 3 shows an electron microscope observation photograph at a magnification of 2000 times.

[Example 4]
In the third step, vacuum evacuation is performed to lower the quenching temperature to 870 ° C., and 1 L / min of a mixed gas of 80% O ₂ /20% H ₂ is introduced at a furnace pressure of 100 Pa, and 0.3 mA / cm ² · 260 V For 30 minutes in a glow discharge plasma atmosphere and conveyed to a cooling chamber for cooling with pressurized He gas. Otherwise, the same processing as in Example 1 was performed. After the above treatment, a hard coating (DLC) was formed on the surface by plasma CVD. The ambient temperature at the start of film formation was 200 ° C. When the frictional wear characteristics of this test piece were evaluated using a ball-on-disk frictional wear tester, the sliding distance was 300 m.

[Example 5]
The test piece was placed flat on the jig and carried into the processing chamber. A heat pattern is shown in FIG. As a treatment step, the atmosphere in the treatment room was heated to 980 ° C. over 90 minutes in a reduced pressure atmosphere of 1 × 10 ⁻³ Pa. A gas mixture of 1 L / min of 20% Ar / 80% H ₂ was introduced at an in-furnace pressure of 50 Pa, and cleaning treatment with glow discharge plasma of 0.6 mA / cm ² · 500 V was performed for 30 minutes. After completion of the cleaning process, 100% CH ₄ gas 10 L / min was subsequently introduced at a furnace pressure of 300 Pa, and carburizing process with glow discharge plasma of 0.4 mA / cm ² · 330 V was performed for 20 minutes. Subsequently, evacuation is performed, and the mixture is held in a vacuum atmosphere for 5 minutes. Subsequently, a gas mixture meter of 1% / min of 20% Ar / 80% H ₂ is introduced at a furnace pressure of 50 Pa, and 0.6 mA / cm ² is introduced. -Diffusion treatment with 500 V glow discharge plasma was performed for 40 minutes, and it was transported to a cooling chamber and cooled with pressurized He gas.

  As a result of observing the structure of the test piece, no reticulated carbide was deposited at the end of the test piece, and the carbide precipitation at the end of the test piece, which was a concern for plasma carburization, was improved. That is, ionized argon was sputtered and heated around the end of the test piece to promote carbon diffusion. Furthermore, the precipitation of reticulated carbide was suppressed by increasing the solid solubility limit of carbon by heating.

[Comparative Example 3]
In the treatment process, all the diffusion treatment was performed in a vacuum atmosphere, and the other treatments were performed in the same manner as in Example 5. As a result of observing the structure of this test piece, the martensite structure and the retained austenite structure occupied the whole in the flat part, but the carbon component did not completely dissolve at the end part of the test piece, and the mesh along the grain boundary. A carbide was deposited.

[Example 6]
The test piece was placed flat on the jig and carried into the processing chamber. A heat pattern is shown in FIG. As a treatment step, the atmosphere in the treatment room was heated to 980 ° C. over 90 minutes in a reduced pressure atmosphere of 1 × 10 ⁻³ Pa. A gas mixture of 1 L / min of 20% Ar / 80% H ₂ was introduced at an in-furnace pressure of 50 Pa, and cleaning treatment with glow discharge plasma of 0.6 mA / cm ² · 500 V was performed for 30 minutes. After completion of the cleaning process, 100% CH ₄ gas 10 L / min was subsequently introduced at a furnace pressure of 300 Pa, and carburizing process with glow discharge plasma of 0.4 mA / cm ² · 330 V was performed for 20 minutes.

Subsequently, evacuation is performed, and the mixture is held in a vacuum atmosphere for 5 minutes. Subsequently, a gas mixture meter of 1% / min of 20% Ar / 80% H ₂ is introduced at a furnace pressure of 50 Pa, and 0.6 mA / cm ² is introduced. -Diffusion treatment with 500 V glow discharge plasma was performed for 40 minutes. Subsequently, the quenching temperature is lowered to 870 ° C., 1 L / min of a mixed gas of 33% N ₂ /67% H ₂ is introduced at a furnace pressure of 300 Pa, and the glow discharge plasma atmosphere is 0.3 mA / cm ² · 500 V. The sample was divided and conveyed to a cooling chamber, and cooled with pressurized He gas.
As a result of observing the structure of the test piece, no reticulated carbide was deposited at the end of the test piece, and the carbide precipitation at the end of the test piece, which was a concern for plasma carburization, was improved. That is, ionized argon was sputtered and heated around the end of the test piece to promote carbon diffusion.

[Comparative Example 4]
In the treatment process, the diffusion treatment was performed in a vacuum atmosphere, and the same treatment as in Example 6 was performed except that. As a result of observing the structure of this test piece, the martensite structure and the retained austenite structure occupied the whole in the flat part, but the carbon component did not completely dissolve at the end part of the test piece, and the mesh along the grain boundary. A carbide was deposited.

[Reference example]
In the third step, vacuum evacuation was performed and the temperature was lowered to a quenching temperature of 870 ° C., held in a vacuum atmosphere for 30 minutes, conveyed to a cooling chamber, and cooled with pressurized He gas. Otherwise, the same processing as in Example 1 was performed. After the above treatment, a hard coating (DLC) was formed on the surface by plasma CVD. The ambient temperature at the start of film formation was 200 ° C. When the frictional wear characteristics of this test piece were measured with a ball-on-disk tester, the sliding distance was 100 m.

  According to the present invention, when various workpieces having an edge portion are carburized by the plasma carburizing method, the difference in carbon concentration between the edge portion and the flat portion is small, and the generation of net-like carbides generated at the edge portion during cooling is suppressed. Furthermore, the problem in the method described in Patent Document 1 can be solved at the same time.

Claims (6)

A plasma in which a workpiece having an edge portion is housed in a vacuum furnace having an in- furnace temperature of 1030 ° C. or lower (except in the case of 900 ° C. or lower), and a carburizing gas is supplied into the vacuum furnace to perform glow discharge. after performing carburizing (carburized step), subsequently an inert gas typified by argon supply including - reducing gases in the furnace, the work by reducing gas plasma treatment containing an inert gas carbon in the surface layer have a step (diffusion process) to diffuse into the work internally, until the carburization step and the diffusion step and the primary process, these steps twice (second step), or 3 times (3 Next step) Carburizing method for a workpiece having an edge portion, characterized in that the secondary step and the tertiary step are repeated at a temperature lower by 0 to 100 ° C. than the primary step . The method for carburizing a workpiece according to claim 1 , wherein the furnace temperature is 980 to 1030 ° C. 3. The workpiece carburizing method according to claim 1, further comprising a nitriding step in a nitriding gas atmosphere after the last diffusion step. After performing the carburizing treatment step , the furnace is evacuated to a vacuum atmosphere or an inert atmosphere such as argon, followed by a diffusion step, and a reducing gas plasma treatment containing the inert gas is performed. The method for carburizing a workpiece according to any one of claims 1 to 3, wherein the gas plasma treatment is performed using hydrogen gas containing argon gas . After the last diffusion step, carburizing method of the work according to any one of claims 1 to 4 having an oxidation process for further oxidizing remove carbides of the workpiece surface.   The work carburizing method according to any one of claims 1 to 5, further comprising a step of coating a hard film having a film forming temperature of 100 to 300 ° C after the carburizing method.