KR100661130B1 - Method for nitriding stainless steel by post-plasma - Google Patents

Abstract

A method for nitriding stainless steel by post-plasma is provided to secure high nitriding characteristics even at a low bias voltage by performing nitriding using a screen mesh in which post-plasma is formed, thereby increasing temperature uniformity and nitriding ion density within a furnace. A method for nitriding stainless steel by post-plasma comprises: a plasma generation process(S10) of generating post-plasma by applying an electric power of 4 to 8 kW to a screen mesh after maintaining an atmosphere within a furnace as a mixed gas of Ar and H2 and heating temperature of the furnace to 300 to 500 deg.C; a surface activation process(S20) of removing residual gas and impurities on a surface of stainless steel by oxidative and reductive gases after generating the post-plasma; a nitride layer forming process(S30) of nitriding the stainless steel at a temperature of 450 deg.C or less by injecting a nitriding reaction gas onto the stainless steel passing through the surface activation process; and a cooling process(S40) of cooling the stainless steel passing through the nitride layer forming process by lowering temperature within the furnace.

Description

Nitriding method for stainless steel using post plasma {Method for nitriding stainless steel by post-plasma}

1 is a schematic diagram showing the structure of a nitriding device according to the prior art,

2 is a block diagram listing the order of the stainless steel nitriding method according to the present invention;

3A and 3B are schematic views showing comparisons of furnace internal structures with and without a post plasma, respectively, according to the present invention;

Figure 4 is an experimental photograph showing the comparison of the nitrided S-phase nitriding and CrN treatment according to the present invention,

5 is an experimental photograph showing a comparison between the deviation of the post-plasma nitriding and ion-nitridation treated specimens according to the present invention,

6 is an enlarged experimental photograph showing a surface photograph of a post plasma nitriding and ion nitriding treated specimen according to the present invention;

7 is a table showing surface roughness values of specimens nitrided by a method different from that of post plasma nitriding according to the present invention;

8 is an experimental photograph comparing the surface of the specimen subjected to nitriding treatment with another method of post plasma nitriding according to the present invention;

9 is a graph showing the depth and surface hardness of the cured layer of the specimen by the post plasma nitridation of the present invention,

10 is a graph illustrating a comparison of wear characteristics of post-plasma nitriding and unnitriding specimens according to the present invention;

11 is a graph illustrating a comparison between post plasma nitriding and unnitriding corrosion characteristics in accordance with the present invention.

* Description of Major Symbols in Drawings *

10: no 20: screen net

25 plate 30 gas pipe

35 gas hole 40 gas injection unit

50: power supply device S10: plasma generation process

S20: Surface activation process S30: Nitride layer forming process

S40: Cooling process H: Heater

P: Specimen (Stainless Steel) PS: Plasma

Vp: Vacuum Pump

The present invention relates to a stainless steel nitriding method, and more particularly to providing a stainless steel nitriding method using a post plasma to increase the temperature uniformity and the nitride ion density inside the furnace to ensure high nitriding properties and surface roughness. have.

Recently, austenitic stainless steel has been used in various fields such as food, medicine, and tableware because of its high corrosion resistance. However, due to its low hardness, automotive austenitic stainless steels have severe restrictions on their use.

In order to improve such a problem, the surface hardness technology of austenitic stainless steel is being developed at a rapid speed.

In general, surface hardening treatment methods include high frequency heat treatment, carburization, nitriding, and the like, in particular, nitriding is abrasion and fatigue resistance by diffusing and infiltrating nitrogen from the surface of a metal material in the temperature range of 500 to 600 ° C. Hardening technology to improve the corrosion resistance and the like.

Unlike other surface hardening methods such as carburization and high frequency heat treatment, the nitriding method is used as a surface hardening treatment method of stainless steel because it hardens the heat treatment deformation of the metal and does not degrade mechanical properties because it hardens at a low temperature below the transformation point of steel.

Nitriding methods having the above characteristics are further divided into gas nitriding, salt bath nitriding, ion nitriding, plasma nitriding, and the like according to nitriding agents and nitriding properties.

First, gas nitriding is a method of forming a compound layer and a nitride layer in a metal material by putting ammonia (NH ₃ ) gas in a closed space for about 1 to 100 hours. have.

Then, the salt yokjil anger cyanide (CN ^-), cyanate (CNO ^-) in a way that the nitriding process is a metallic material in a liquid phase using a salt, the compound layer and the nitride layer of high hardness in a short period of time of about 4 hours to considerable depth It has been widely applied to the mass production process of automotive parts because of the advantages that can be obtained, the cooling rate is fast, there is no restriction of steel grade, and the installation cost is very low.

However, in the case of the above-mentioned gas nitriding and salt bath nitriding, the surface hardness can be improved by forming a high hardened layer on the stainless steel surface, but due to the nature of the nitriding process of nitriding at 500 ° C. or higher, CrN is precipitated. There was a problem of lowering the corrosion resistance and significantly reducing the Cr ₂ O ₃ layer that causes corrosion resistance.

In contrast, ion nitriding is capable of nitriding stainless steel at a low temperature of 450 ° C. or less by using a high bias, thereby preventing the precipitation of CrN.

However, the above-mentioned ion nitriding not only can not control the temperature precisely by the glow formed on the surface of the specimen, but also causes a problem that the surface of the specimen is damaged by the arc and the aging caused.

In addition, due to the characteristics of ion nitriding at different temperatures and low energy of the glow, there is a problem in that the variation of the specimen is large due to failure to uniformize the temperature in the working space, and the nitriding treatment is difficult in the hole or the small gap. .

On the other hand, modern nitriding technology has been developed aiming at the quality of ion nitriding and the production of gas nitriding, and a representative nitriding technique for achieving the above-mentioned goal is post plasma nitriding.

Referring to the post plasma nitriding, as illustrated in FIG. 1, the nitriding gas is passed through a plasma generated outside the specimen P to coat and nitride the specimen P, and the inside of the furnace 1. The board | substrate 2 which has a cathode power supply is installed in the center, the specimen P is provided on the board | substrate 2, and the heater H is installed in the furnace inside.

In addition, the gas injection unit 4 is installed at the upper part of the furnace 1, and a vacuum pump Vp is installed at the lower part so as to generate plasma PS around the specimen P. In addition, power is supplied to the substrate 2 and the heater H through the power supply device 5.

Here, if the plasma (PS) is described briefly, the atoms are separated from the nucleus and the electrons at a temperature of millions to tens of millions, and are vigorously moved as they are, so that they are electrically neutral gas, which is called plasma.

The post plasma nitriding described above allows nitriding treatment of the specimen by passing a nitriding gas through the plasma, thereby allowing nitriding without directly applying a glow to the treated specimen. Therefore, there is no surface damage of the specimen by the arc or the like, and there is no limitation of the shape of the specimen, so that the above-mentioned disadvantages of the ion nitriding method can be solved.

However, since the post plasma nitriding is not directly applied to the surface of the specimen, there is a problem that the performance of nitriding is deteriorated because there is no surface oxide film removal and activation process due to the sputtering effect of ions as in the ion nitriding method. At low temperatures, it was difficult to ensure a uniform temperature inside the furnace only by radiant heat from the heater, which resulted in a closed end where uniform nitriding was not possible on the specimen surface.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems. As a result, nitriding is carried out using a screen network on which a post plasma is formed. It is to provide a stainless steel nitriding method using a post plasma to ensure the characteristics.

Another object of the present invention is to provide a stainless steel nitriding method using a post plasma to improve the efficiency of nitriding and to secure a product having a high surface roughness by allowing radicals passing through the plasma to be directly used for nitriding.

According to the nitriding method using the post plasma of the present invention for achieving the above object, when the austenitic stainless steel is charged and heated in the furnace, the gas is passed through a plasma generated outside the stainless steel to nitride the stainless steel. In the method, the atmosphere inside the furnace is maintained by a mixed gas of Ar and H ₂ , the temperature is heated to 300 ~ 500 ℃, and the plasma is generated by applying a power of 4 ~ 8kW to the screen net to generate a post plasma and; A surface activation process of removing the residual gas and impurities on the surface of the stainless steel by oxidizing and reducing gas after generating the post plasma; A nitride layer forming step of nitriding at a temperature of 450 ° C. or less by injecting a nitriding reaction gas into the stainless steel that has undergone the surface activation step; It characterized in that it comprises a cooling step of cooling the stainless steel passed through the nitride layer forming step by dropping the temperature inside the furnace.

When described in detail with reference to the accompanying drawings a preferred embodiment of the present invention.

FIG. 2 is a block diagram sequentially listing a method of nitriding using a post plasma according to the present invention. The plasma generating step (S10), the surface activation step (S20), the nitride layer forming step (S30), and the cooling step (S40) are shown. Divided into.

First, the plasma generation step S10 is a step of generating the post plasma PS in the screen net 20 installed outside the stainless steel P. The atmosphere inside the furnace 10 is a mixture of Ar and H ₂ . After maintaining, using the heater (H) to heat the temperature in the furnace 10 to 300 ~ 500 ℃, to apply a power of 4 ~ 8kW to the screen net 20 to generate a post plasma (PS).

Here, looking at the reason for using 4 ~ 8kW as the power for forming the post plasma (PS), if too high power causes the internal temperature of the furnace caused by the post plasma (PS), applying too low power In this case, not only the influence of temperature uniformity is small but also the nitriding performance is lowered.

At this time, Figure 3a schematically shows the inside of the furnace 10 used in the above-described process, will be described below, by installing a heater (H) in the furnace 10 inside the stainless steel (P) The furnace 10 is heated, and a vacuum pump Vp is installed below the furnace 10 to deform the atmosphere inside the furnace 10 to a vacuum state.

In addition, an upper portion of the furnace 10 is provided with a gas injection unit 40 for introducing a working gas into the gas pipe 30 to be described later, the furnace 10 between the gas injection unit 40 and the furnace 10. Baratron gauge (G) for maintaining and measuring the degree of vacuum inside, and a throttle valve (Thv) for adjusting the amount of gas to be injected into the furnace (10), respectively. In addition, the furnace 10 is electrically connected to a PC or the like to measure the plasma (PS) and the power supply state in real time, thereby allowing overall control of the nitriding process.

In addition, a plate 25 is installed inside the center of the furnace 10, and a plurality of stainless steels P to be nitrided are provided on the plate 25 in multiple stages. In addition, the outside of the stainless steel (P) is provided with a screen net 20 to cover the stainless steel (P), the screen net 20 is the upper screen net 20a and a plurality (in the present invention two) It is possible to supply DC power from the power supply device 50 to each screen net 20 by installing separately by the side screen net 20b.

The screen net 20 is formed of a metal material so that the cathode power is applied to generate the plasma PS in the screen net 20. The outside of the screen network 20 is provided with a spiral gas pipe 30 into which oxidative and reducible gas and nitriding reaction gas are injected, and a plurality of gas holes 35 are formed in the gas pipe 30. In addition, the gas pipe 30 is connected to the gas inlet 40 outside the furnace 10 to receive gas.

Subsequently, the surface activation step S20 generates plasma PS in the screen net 20 installed outside the stainless steel P, and then oxidizes and reduces gas to the gas pipe 30 outside the screen net 20. Pass through the plasma (PS) to remove the residual gas and impurities on the surface of the stainless steel (P), the surface activation step (S20) is pre-treated in three steps.

In other words, the surface activation step (S20) is a pretreatment through a homogenization treatment step, an oxidation treatment step, a reduction treatment step.

Such a surface activation step (S20) will be described with reference to FIG. 3. First, in the homogenization treatment step, the temperature in the furnace 10 is raised to an oxidation temperature at a vacuum pressure of 1 × 10 ⁻² Torr, followed by stainless steel (P). Homogenize the surface of) for 5-10 minutes.

In the oxidation treatment step, the surface of the stainless steel P is homogenized and immediately oxidized using an oxidizing gas (eg, nitrous oxide (N ₂ O)) for 10 to 30 minutes. In addition, the reduction treatment step by raising the temperature in the furnace 10 to the nitriding temperature, using a reducing gas (for example, hydrogen (H ₂ )) for 10 to 30 minutes on the surface of the oxidation-treated stainless steel (P). Reduction treatment.

Here, the redox gas may be applied to air other than nitrous oxide (N ₂ O) or water (H ₂ O), and nitrogen trifluoride (NF ₃ ) or the like may be used as an activating gas having the same role.

On the other hand, the nitride layer forming step (S30) is a nitriding treatment by injecting a nitriding reaction gas into the stainless steel (P) through the surface activation step (S20), the nitriding reaction gas through the gas pipe 30 screen screen ( It is to pass the plasma (PS) generated in 20).

In this case, in the nitride layer forming process (S30), the stainless steel (P) is nitrided at a temperature of 450 ° C. or lower, because when the nitride is treated at 500 ° C. or higher, CrN is formed on the surface of the stainless steel (P). This is because the deposited and deposited CrN layer breaks the Cr ₂ O ₃ layer, thereby eliminating corrosion resistance, which is inherent in austenitic stainless steel (P), and is easily corroded or easily broken by CrN formation.

In addition, the above-mentioned nitriding reaction gas is nitrogen, hydrogen, and a nitriding accelerator, and the nitriding accelerator is a gas containing carbon (C), propane (C ₃ H ₈ ), acetylene (C ₂ H ₂ ), and methane. Any one of (CH ₄ ) is used. Among them, it is appropriate to use a propane (C ₃ H ₈₎ propane (C ₃ H _8), containing 1.2wt% of the weight of the case of using a.

Referring to the operation and effect of the present invention configured as described in detail as follows.

In order to nitrate austenitic stainless steel (P) (example: STS303, 304, 310, and 316) through a nitriding method using a post plasma of the present invention, first, stainless steel (P) is introduced into the furnace 10. After charging, the degree of vacuum inside the furnace 10 was maintained at 1 × 10 ⁻² Torr using a vacuum pump Vp, and the inside of the furnace 10 was supplied with power of 4 to 8 kW to the screen net 20. The temperature of is raised to 450-500 degreeC which is an oxidation temperature, and the post plasma PS is generated in the screen net 20.

At this time, after 30 minutes after the post plasma (PS) is generated in the screen net 20, the temperature change of the stainless steel (P) was measured, as shown in Figure 3a, the maximum temperature is 566 ℃, the minimum temperature is 556 ℃ It was shown that the temperature of the specimens (stainless steel) P disposed on the lower, left, and right sides was approximately 560 ± 5 ° C.

On the other hand, Figure 3b is a diagram showing the structure of the furnace 10 in a state in which the post-plasma PS is not generated in the screen net 20, by heating the inside of the furnace 10 using only the heater (H). As a result of measuring the temperature change of the stainless steel (P), there was a temperature difference of more than 30 ° C from the highest temperature of 547 ° C to the lowest temperature of 509 ° C. It appeared badly.

This is because the screen net 20 on which the post plasma PS is formed minimizes the heat dissipation to the outside and is ionized by the post plasma PS such that radical particles having high energy are actively active inside the screen net 20 so that the temperature is increased. It can be confirmed that it increases the uniformity.

In addition to the above phenomenon, since the ion plasma density of the inside of the screen network 20 is increased due to the generation of the post plasma PS, the plasma PS can be formed on the specimen by the low bias voltage.

That is, in the case of the specimen without the post plasma PS, a pressure of 4 torr and a high bias voltage of 10 kW or more are required to generate the plasma PS, but in the case of the specimen in which the post plasma PS is generated, 1 kW Uniform plasma PS can be formed on the specimen even with the following low bias voltage.

As such, the advantage of the nitriding treatment with a low bias voltage is to prevent the risk of arcing that may occur in the nitriding performed by the high bias voltage, and to minimize the nonuniformity of the nitriding quality due to the power variation due to the height. .

On the other hand, after generating the post plasma PS in the screen net 20 as mentioned above, the surface of the stainless steel P is homogenized for 5 to 10 minutes. Then, the oxidizing gas (for example, N ₂ O) flowing from the gas hole 35 is passed through the post plasma PS generated in the screen net 20 to oxidize the surface of the stainless steel P for 10 to 30 minutes. (S12).

After the oxidation treatment, the reducing gas (for example, H ₂ ) flowing from the gas hole 35 is passed through the plasma PS generated in the screen network 20 to reduce the surface of the specimen P for 10 to 30 minutes. By the treatment, the surface activation step (S20) of the specimen P is completed.

As such, the surface of the stainless steel (P) having undergone the surface activation process (S20) is formed very thick and uniform. Therefore, by removing the residual gas and impurities in the surface layer of the specimen (P) through a simple surface activation process (S20) using an oxidizing and reducing gas to increase the surface reactivity of the specimen (P), it is possible to improve the performance of the nitriding will be.

On the other hand, after the surface activation step (S20) described above, the pressure in the furnace 10 is set to 1-5torr while the temperature in the furnace 10 is maintained within 450 ° C, and the nitriding reaction gas is transferred to the screen net 20. By passing through the generated plasma (PS) to perform the nitride layer forming process (S30) of the specimen (P) for about 2 to 8 hours.

On the other hand, when the nitride layer forming step (S30) is completed, the cooling step (S40) for reducing the temperature inside the furnace 10 is to be completed, thereby completing the nitriding treatment of the specimen (P). Here, the carbon-containing gas used as the nitriding accelerator of the nitriding reaction gas serves to promote the rate of nitrogen diffusion in the steel during nitriding, thereby significantly increasing the nitriding treatment rate.

At this time, in the nitride layer forming process (S30) it is important to maintain the temperature of the nitriding within 450 ℃, this is to prevent the precipitation of CrN. As such, the supersaturated tissue made without the formation of CrN precipitates at low temperature is called S-phase (supersaturated or expanded austenite γ _N ). In FIG. 4, experimental photographs of S-phase nitrided specimens and CrN-nitride specimens are shown. Indicated.

In the above experimental pictures, the S-phase forming specimens did not cause corrosion even after long-term maintenance in the marble solution, whereas in the case of CrN-formed specimens, it was easily corroded by the marble solution and thus appeared black.

On the other hand, Figure 5 is a photograph comparing the uniformity of the nitrided stainless steel (P) by using the ion nitriding and post plasma, respectively, in the case of stainless steel (P) ion nitride treatment at 450 ℃ in the upper, lower, center The temperature deviations of the specimens were large, and the uniformity was 18 ~ 22㎛.

On the other hand, in the case of post-plasma (PS) -nitrided stainless steel (P), even if the position of the upper, lower, left, and right were measured, the deviation was small as 8 ~ 10㎛, showing a high uniformity. It can be confirmed that the uniformity according to.

6 is a photograph showing the uniformity of specimens of the nitrided specimen (SKD61) and ionized specimen (SKD61) using the post plasma (PS), in the case of the specimen using the post plasma (PS) In the case of ion nitrided specimens, differences in the shape of the nitride layer can be found by the high energy density of the edge portion, whereas the center and edge portions show uniform nitride layers.

On the other hand, in the case of the ion nitriding described above, sputtering occurs continuously during nitriding due to plasma glow formed on the specimen itself, resulting in very poor surface roughness.

FIG. 7 is a table listing surface roughness values according to the nitriding treatment methods of SKD 61 species. In the case of nitriding treatment using post plasma (PS) nitriding, the ion nitriding and gas nitriding have a Ra value of 0.1 or more. The surface roughness value (0.05) is also excellent compared with the surface roughness value (0.05) of radical nitridation which is known to have a good surface roughness value.

In addition, even when comparing the SEM image of the surface of the S-316 nitrided STS316 specimen of Figure 8 it can be seen that the surface roughness is superior to the specimen prepared by conventional ion nitriding method or European ASN nitriding technology.

As such, the advantage of excellent surface roughness value is that since the hardened layer of S-phase is about 10㎛ thin, if you want roughness less than 0.1㎛ due to the characteristics of the product, it is impossible by ion nitriding method. It is possible.

On the other hand, Figure 9 is a graph showing the hardness profile of the S-phase nitrided specimen, the surface hardness is very low, such as 800 ~ 900 Hv0.025, because the depth of the formed layer is thin as 15㎛ because it is processed at a low temperature.

On the other hand, when the CrN layer is formed, the hardness value of the specimen is high as about 1400 ~ 1600Hv0.025, and can be formed up to 100 μm or more depending on the treatment time since it is treated at 450 ° C. or more. However, when the CrN layer is formed, it has a very weak property, easily broken or broken even with a small impact, and wear resistance is also poor.

Accordingly, in FIG. 10, the wear resistance of the S-phase nitrided and unnitrided specimens was tested and compared with the loads of 2N, 5N, and 10N, respectively, and the S-phase nitrided specimens were not nitrided. It can be seen that the wear resistance is significantly improved compared to the specimen.

In addition, FIG. 11 is a graph diagram comparing the corrosion characteristics of the S-phase nitrided specimen and the non-nitrided specimen, and it can be seen that the corrosion resistance of the S-phase nitrided specimen is improved compared to the non-nitrided specimen. .

On the other hand, the present invention has been described in detail only with respect to the specific examples described above it will be apparent to those skilled in the art that various modifications and variations are possible within the technical scope of the present invention, it is natural that such variations and modifications belong to the appended claims. .

As described above, the present invention forms a high ion density by activating the reaction gas that has passed through the screen net by using a screen net having a post plasma formed thereon, and thus the temperature uniformity of the specimen (stainless steel). Increasing it will reduce the deviation of the specimen and at the same time improve the nitriding performance.

In addition, the uniform temperature treatment of the specimen is possible even at low temperature and low power due to the temperature uniformity increasing effect of the post plasma. Therefore, it is possible to prevent the precipitation of CrN generated on the surface of stainless steel during high temperature nitriding treatment, and to perform nitriding treatment while maintaining corrosion resistance, thereby further improving surface hardness and wear resistance of stainless steel. There is also.

In addition, instead of forming a plasma glow on the specimen itself and nitriding it as in a conventional ion nitriding process, a post plasma is formed on an external screen network and nitrided without directly applying a glow to the treated specimen, thereby resulting in ion nitriding. It also has the effect of solving the problem of temperature unevenness, arcing and edging that can occur when the glow is formed on the specimen, and the problem that nitriding is impossible in the hole or small gap.

In particular, by nitriding without applying glow directly to the specimen as described above, it prevents the continuous sputtering to occur, which is more excellent and excellent surface compared to other nitriding processes such as gas nitriding and radical nitriding as well as conventional ion nitriding There is also an effect of showing the roughness, and thus a post-treatment step such as a polishing step is unnecessary.

In addition, the metal screen net covers the specimen and uses a heater to heat the specimen, which not only increases energy density and temperature uniformity, but also enables active radicals that pass through the plasma to be used directly for nitriding, further enhancing the efficiency of the nitriding. There is also an effect.

Claims (1)

In the case of charging and heating austenitic stainless steel in the furnace, the method of nitriding the stainless steel by passing a gas through a plasma generated outside the stainless steel, The atmosphere inside the furnace 10 is maintained with a mixed gas of Ar and H ₂ , the temperature is heated to 300 to 500 ° C., and 4 to 8 kW of electric power is applied to the screen network 20 to generate a post plasma (PS). Plasma generating step (S10) and; A surface activation step (S20) of generating the post plasma (PS) and then removing residual gas and impurities on the surface of the stainless steel (P) by oxidizing and reducing gas; A nitride layer forming step (S30) of injecting a nitriding reaction gas into the stainless steel (P) having undergone the surface activation step (S20) and nitriding at a temperature of 450 ° C. or less; And a cooling step (S40) of cooling the stainless steel (P) through the nitride layer forming step (S30) by lowering the temperature inside the furnace (10).

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