JP2007016273A - Hierarchical surface reforming process of austenitic stainless steel component - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a complex plasma surface reforming process by which the surface hardness is enhanced without deteriorating corrosion resistance of austenitic stainless steel and the wear resistance is improved by making a hardened layer deeper; and a process for successively performing the penetration-diffusion of each of carbon and nitrogen by using plasma. <P>SOLUTION: A hierarchical surface reforming process comprises a low temperature carbon penetration-diffusion process for penetrating and diffusing carbon into an austenite phase by applying glow discharge to a material 22 to be treated of austenitic stainless steel in a gaseous hydrocarbon atmosphere in a vacuum vessel 11 and a low temperature nitrogen penetration-diffusion process for penetrating and diffusing nitrogen into the austenite phase by using glow discharge in a nitrogen atmosphere in the same vacuum vessel 11, which is performed successively after the carbon penetration-diffusion process. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a hierarchical surface modification method for increasing surface hardness and deepening a hardened layer without impairing the corrosion resistance of austenitic stainless steel parts.

Conventionally, as a method of improving the wear resistance of austenitic stainless steel parts, for example, as shown in Non-Patent Document 1, the surface hardness is increased by the plasma carburizing technique and the plasma nitriding technique described in Non-Patent Document 2. Surface modification methods are widely used.
However, it is a well-known fact that the processing temperature is high, chromium carbide precipitates in plasma carburization, chromium nitride precipitates in plasma nitriding, the passive film on the surface is removed, and the corrosion resistance of austenitic stainless steel is impaired.
Moreover, there are a carburizing method (Non-Patent Document 3) and a nitriding method (Non-Patent Document 4) performed using a temperature of about 400 ° C. as surface modification methods that do not impair the corrosion resistance.
However, the surface hardness is high in the low-temperature carburization, but the hardened layer is shallow. For example, in the example of Non-Patent Document 3, the surface hardness is high in low-temperature carburizing, but the hardened layer is shallow, for example, in the example of Non-Patent Document 3, the surface hardness is high. The thickness is about 1000 (HV), but the depth of the hardened layer is about 4 to 40 μm. Further, for example, in the example of Non-Patent Document 4, there is a drawback that the surface hardness is 290 to 460 (HV) and the depth of the hardened layer is about 2 to 5 μm at low temperature nitriding.

Summary of Surface Technology 105th Conference 2002,03.04 P120 ~ 121 Ion nitriding method Nikkan Kogyo Shimbun July 10, 1976 P51-57 Thermochemical Surface Engineering of Stainless Steel (Reprinted from Surface Engineering 1999, 15 (1), P49-54) Heat treatment Vol.25 No.4 August 1985 P191〜195

  The object of the present invention is to increase the surface hardness without impairing the corrosion resistance of the austenitic stainless steel, and to deepen the hardened layer to improve the wear resistance and to continuously improve the carbon and carbon by plasma. An object of the present invention is to provide an apparatus for osmotic diffusion of nitrogen.

The present invention includes a low-temperature carbon permeation diffusion step in which carbon is infiltrated and diffused into an austenitic phase using glow discharge in an austenitic stainless steel part in a hydrocarbon-based gas atmosphere in a vacuum vessel, and continuously in the same vacuum vessel. It consists of a low temperature nitrogen permeation diffusion process in which nitrogen is permeated and diffused into the austenite phase using glow discharge in a nitrogen atmosphere, increasing the surface hardness and deepening the hardened layer without impairing the corrosion resistance of the austenitic stainless steel parts. It is characterized by that.
The low temperature permeation diffusion step is performed in a vacuum vessel maintained at 1 Pa to 2.6 kPa, and carbon permeation diffusion uses a hydrocarbon gas such as methane gas, propane gas, and acetylene gas, and argon gas as a dilution gas, or In addition to using hydrogen gas, the low-temperature nitrogen permeation diffusion step uses nitrogen gas for nitrogen permeation diffusion and uses argon gas or hydrogen gas as a dilution gas.
Further, the hydrocarbon gas concentration used in the low temperature carbon permeation diffusion step is preferably about 1 to 3%, and the nitrogen gas concentration used in the low temperature nitrogen permeation diffusion step is preferably about 10 to 30%. Further, in the low temperature carbon permeation diffusion step, the carbon permeation diffusion temperature is preferably 400 to 450 ° C., and in the low temperature nitrogen permeation diffusion step, the nitrogen permeation diffusion temperature is preferably 380 to 420 ° C.

As described above, the present invention includes a low-temperature carbon permeation diffusion step in which carbon is infiltrated and diffused into an austenitic phase using glow discharge in an austenitic stainless steel part in a hydrocarbon-based gas atmosphere in a vacuum vessel, as described above. It consists of a low-temperature nitrogen permeation diffusion process in which nitrogen is continuously permeated and diffused into the austenite phase using a glow discharge in a nitrogen atmosphere. Since this is a low-temperature treatment, chromium carbide is precipitated in plasma carburizing and chromium nitride is precipitated in plasma nitriding. The surface hardness can be increased and the hardened layer can be deepened without impairing the corrosion resistance of the austenitic stainless steel part.
That is, in the plasma hierarchical surface processing method of the present invention, the austenitic stainless steel part is set in an atmosphere in which the hydrocarbon-based gas is diluted with at least hydrogen gas, and the temperature is maintained at 400 to 450 ° C. While carbon is permeated and diffused from the surface by glow discharge while maintaining the hydrocarbon gas at a concentration of 1 to 3%, the temperature is maintained at 380 to 420 ° C. and the nitrogen gas is maintained at a concentration of 10 to 30%. Nitrogen penetrates and diffuses from the surface by glow discharge, and carbon and nitrogen are dissolved in the austenite phase, so that chromium carbide and chromium nitride are not precipitated, corrosion resistance is not impaired, surface hardness is increased, and a hardened layer is formed. Since it is deepened, it is easy to save energy and has a low environmental impact, and has excellent effects such as being usable for automobile parts, machine parts, and electrical parts.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the best mode for carrying out the present invention will be described in detail with reference to the drawings. FIG. 1 schematically shows a plasma hierarchical surface modification apparatus 10 for performing the plasma hierarchical surface processing method of the present invention.
The plasma layer surface processing apparatus 10 includes a vacuum vessel 11, a vacuum exhaust device 12, a gas supply device 13, a DC plasma power supply 14, an operation panel 15, and the like.

<Vacuum container>
The vacuum vessel 11 has a cylindrical observation window 16 so that the temperature of the material 22 to be treated of austenitic stainless steel and the state of glow discharge can be observed through the heat-resistant glass. The vacuum vessel 11 has a double structure to prevent overheating, and cooling water is passed between the outer wall and the inner wall to cool the water. The vacuum vessel also has a port for supplying hydrocarbon gas, nitrogen gas, hydrogen gas and the like. Further, an insulated electrode 17 is assembled on the floor wall of the vacuum vessel 11 and protrudes into the vacuum vessel. A processing table 18 on which a processing material 22 made of austenitic stainless steel is placed is attached to the tip of the electrode 17.

<Vacuum exhaust device and gas supply device>
The vacuum exhaust device 12 includes an oil rotary vacuum pump 20 and a mechanical booster pump 21 and exhausts the inside of the vacuum vessel 11 to a predetermined pressure. Moreover, the gas supply apparatus 13 adjusts the gas flow rate so that it may become predetermined value with each gas flowmeter, and mixes hydrocarbon gas, nitrogen gas, and hydrogen gas in a gas mixing container. The gas mixed in the gas mixing container is supplied into the vacuum container 11 by the mass flow controller. Of course, you may supply directly to a vacuum container, without using a gas mixing container. The pressure inside the vacuum vessel 11 is controlled by the mass flow controller and the vacuum exhaust device 12 so as to become a predetermined value while being measured by a Pirani vacuum gauge.

<DC plasma power supply>
The DC plasma power source 14 applies a voltage of several hundred volts between the cathode 17 and the vacuum vessel 11 to generate plasma on the surface of the austenitic stainless steel material 22 by glow discharge.
Therefore, the plasma power source 14 uses an appropriate power source such as direct current, high frequency, or microwave.
In the embodiment for the present embodiment, no special device is provided for raising the temperature of the material 22 to be processed. This is because the plasma hierarchical surface reforming apparatus 10 generates glow discharge on the surface thereof, so that the carbon ions, nitrogen ions, hydrogen ions, etc. are applied to the surface of the material 22 to be treated of austenitic stainless steel connected to the cathode. This is because they are attracted electrically and collide with the surface, and the kinetic energy of carbon ions, nitrogen ions, hydrogen ions, etc. at that time is converted into thermal energy and the material 22 to be processed is heated. Of course, the material to be treated 22 may be heated to a predetermined temperature using an appropriate heater as the heating source.
The temperature of the material 22 to be treated of austenitic stainless steel is measured by an infrared radiation thermometer 19 through an observation window 16 made of heat resistant glass. Of course, the temperature may be measured using a thermocouple.

Next, the hierarchical surface modification method for an austenitic stainless steel part according to the present invention will be described using an example in which the plasma hierarchical surface processing apparatus 10 is used.
The processing material 18 of austenitic stainless steel, for example, SUS316L, is placed on the processing table 18 of the plasma level surface reforming apparatus 10, and then the vacuum vessel 11 is sealed. Next, the vacuum exhaust device 12 is operated, and the inside of the vacuum vessel 11 is exhausted to 1 Pa or less. Next, hydrogen gas is supplied from the gas supply device 13 into the vacuum vessel 11, the supply amount of the hydrogen gas is controlled by a mass flow controller so that the inside of the vacuum vessel 11 is maintained at 2.6 Pa, and the DC plasma power source 14. Thus, a voltage of −200 to −300 V is applied to the electrode 17 to generate glow discharge on the surface of the jig and the austenitic stainless steel workpiece 22. Then, in the hydrogen gas atmosphere, the discharge voltage and discharge current are increased, and the atmospheric pressure in the vacuum vessel 11 is increased stepwise. By such an operation, the austenitic stainless steel material 22 is continuously heated, and the discharge output is adjusted so that the atmospheric pressure is 532 Pa and the austenitic stainless steel material 22 is maintained at 450 ° C. Do.

  Further, when the atmospheric pressure was 532 Pa and the temperature of the material 22 to be treated of austenitic stainless steel reached 450 ° C., a hydrocarbon-based gas such as methane gas was used to supply the vacuum container 11. The hydrocarbon gas and the hydrogen gas are adjusted with the gas flowmeter so that the hydrocarbon gas concentration in the atmosphere becomes 1%, and carbon permeation diffusion is performed for 20 hours.

  Next, after performing the permeation diffusion of the carbon, the supply of the hydrocarbon gas from the gas supply device 13 and the output of the DC plasma power source 14 are stopped to stop the plasma discharge, and the austenitic stainless steel is processed. The temperature of the material 22 is lowered to 400 ° C. Then, when the temperature of the material 22 to be treated of austenitic stainless steel reaches 400 ° C., the nitrogen gas and hydrogen gas from the gas supply device 13 are supplied to the gas flow meter so that the nitrogen gas concentration in the atmosphere becomes 30%. Then, the vacuum vessel 11 is supplied until it reaches 532 Pa, and the DC plasma power supply 14 applies the voltage of the electrode 17 to generate glow discharge on the surface of the processing base 22 and the austenitic stainless steel workpiece 22. And nitrogen permeation diffusion was performed for 40 hours. After the nitrogen permeation and diffusion, the austenitic stainless steel treated material 22 was cooled in the vacuum vessel 11.

In the present invention, the hydrocarbon gas concentration used for carbon permeation diffusion is preferably about 1 to 3%.
If it is 1% or less, there is almost no permeation and diffusion of carbon from the surface. Further, if it is 3% or more, the amount of dissociated carbon increases, and austenitic stainless steel is sooted and carburized, and a part that is not carburized is generated.

Further, the concentration of nitrogen gas used for the permeation diffusion of nitrogen is preferably about 10 to 30%.
If it is 10% or less, the permeation and diffusion of nitrogen from the surface becomes slow. For example, when the nitrogen gas concentration is 10% compared to the case where the nitrogen gas concentration is 30%, the permeation and diffusion of nitrogen from the surface of the material to be processed becomes slow, and the processing time becomes about 1.5 times.
On the other hand, if it is 30% or more, chromium nitride is produced, chromium on the surface is reduced, the passive film is removed, and the corrosion resistance is lowered.

The carbon penetration diffusion temperature is preferably 400 to 450 ° C.
Below 400 ° C., carbon permeates and diffuses into the austenite phase, but takes a long time. For example, when treated at 450 ° C., it took 35 hours when treated at 390 ° C. in order to obtain the same surface hardness as obtained at 20 hours.
Further, when the temperature exceeds 470 ° C., chromium carbide precipitates and the corrosion resistance is lowered.
On the other hand, the penetration temperature of nitrogen is preferably 380 to 420 ° C. Below 380 ° C., nitrogen does not penetrate and diffuse, and the hardness does not increase. Above 420 ° C., chromium nitride precipitates and the corrosion resistance decreases.

FIG. 2 shows a metallographic microstructure near the surface of the material 22 to be treated of the austenitic stainless steel subjected to the above treatment.
As shown in FIG. 2, even if nitrogen permeation and diffusion are performed after carbon permeation and diffusion, carbon compound and nitrogen compound are not deposited on the surface of the 4% nitric acid alcohol solution, and corrosion resistance is not impaired. I was able to confirm.

FIG. 3 is a graph showing respective surface hardness and cross-sectional hardness distribution curves of the austenitic stainless steel subjected to the above-described processing method. The material 22 to be treated of the austenitic stainless steel has a micro Vickers hardness. The surface hardness and cross-sectional hardness are measured with a total load of 0.245N.
As shown in FIG. 3, the material 22 to be treated of austenitic stainless steel that has been subjected to carbon diffusion at 450 ° C. for 20 hours and then nitrogen diffusion at 400 ° C. for 40 hours has a surface hardness of Hv1090. Further, the depth of the hardened layer by the cross-sectional hardness distribution was confirmed to be about 70 μm.
As a comparative example, the surface hardness of an austenitic stainless steel subjected to carbon penetration diffusion at 450 ° C. for 20 hours is Hv620, the hardened layer depth is about 25 μm, and the austenite system subjected to nitrogen penetration diffusion at 400 ° C. for 80 hours. The surface hardness of stainless steel is Hv1200, the depth of the hardened layer is about 20 μm, the surface hardness is low with carbon penetration diffusion, the hardened layer is shallow, and the surface hardness is high with nitrogen penetration diffusion, the hardened layer Was confirmed to be shallow.

  As understood from the above, the plasma composite surface processing method of the present invention does not precipitate chromium carbide and chromium nitride, does not impair corrosion resistance, increases surface hardness, and deepens the hardened layer, thus saving energy. It is easy to use and has a low environmental impact, and can be used for automobile parts, machine parts, electrical parts, and the like.

It is the general | schematic figure which showed schematically the plasma hierarchical surface modification | reformation apparatus which enforces the method of surface-processing the austenitic stainless steel part of this invention. It is a metallographic microstructure near the surface of the austenitic stainless steel. It is the graph which showed each surface hardness and cross-sectional hardness distribution curve of the austenitic stainless steel.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Vacuum container 12 Vacuum exhaust apparatus 13 Gas supply apparatus 14 DC plasma power supply 15 Operation panel 16 Observation window 17 made of heat-resistant glass 17 Electrode 18 Treatment stand 19 Infrared radiation thermometer 20 Oil pressure vacuum pump 21 Mechanical booster pump 22 Material to be treated

Claims (5)

  A low temperature carbon infiltration and diffusion process in which carbon is infiltrated and diffused into the austenite phase using glow discharge in an austenitic stainless steel part in a hydrocarbon gas atmosphere in a vacuum vessel, and in a nitrogen atmosphere continuously in the same vacuum vessel It consists of a low temperature nitrogen permeation diffusion process in which nitrogen is permeated and diffused into the austenite phase using glow discharge, characterized by increasing the surface hardness and deepening the hardened layer without impairing the corrosion resistance of the austenitic stainless steel parts. Hierarchical surface modification method for austenitic stainless steel parts.   The low-temperature carbon permeation diffusion step is performed in a vacuum vessel maintained at 1 Pa to 2.6 kPa, and carbon permeation diffusion is performed using a hydrocarbon-based gas such as methane gas, propane gas, or acetylene gas, and argon gas or hydrogen gas as a dilution gas. The low-temperature nitrogen permeation and diffusion step uses nitrogen gas for nitrogen permeation and diffusion, and uses argon gas or hydrogen gas as a dilution gas. The hierarchical surface of an austenitic stainless steel part according to claim 1, Modification method.   The hierarchical surface modification method for an austenitic stainless steel part according to claim 2, wherein the hydrocarbon gas concentration used in the low temperature carbon permeation diffusion step is 1 to 3%.   The hierarchical surface modification method for austenitic stainless steel parts according to claim 2 or 3, wherein the nitrogen gas concentration used in the low temperature nitrogen permeation diffusion step is 10 to 30%. 5. The low temperature carbon permeation diffusion step, wherein the carbon permeation diffusion temperature is 400 to 450 ° C., and in the low temperature nitrogen permeation diffusion step, the nitrogen permeation diffusion temperature is 380 to 420 ° C. A hierarchical surface modification method for austenitic stainless steel parts.

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