JPH06228732A - Method for nitriding austenitic stainless steel product - Google Patents

Abstract

PURPOSE:To obtain a product combining high corrosion resistance with high surface hardness by heating and holding an austenitic stainless steel product under a chlorine gas atmosphere and then nitriding the product by heating and holding it at specific temp. under a nitriding atmosphere. CONSTITUTION:An austenitic stainless steel product is heated and held, e.g. at about 300-550 deg.C under a chlorine gas atmosphere, by which the penetrability of N atoms is improved. As the austenitic stainless steel material, stainless steels containing >=22wt.% Cr and further >=1.5% Mo can also be used, other than the ordinary 18-8 stainless steel. Subsequently, this product is held in a heated state at <=450 deg.C, preferably about 380-420 deg.C, under a nitriding atmosphere containing NH3, etc. By the procedure, the surface layer of the product is formed into a dense and uniform nitrided layer of about 10-50mum thickness. It is preferable to further clean the nitrided layer with an HNO3-containing strong mixed acid solution. By this method, the high hardness product excellent in corrosion resistance and having about Hv900-1200 Vickers hardness can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of nitriding an austenitic stainless steel product having both high corrosion resistance and high surface hardness.

[0002]

2. Description of the Related Art Conventionally, stainless steel products, particularly 18-8 series stainless steel products having a chromium content of about 18% by weight (hereinafter abbreviated as "%") and a nickel content of about 8%, have high corrosion resistance and excellent corrosion resistance. It is most widely used because of its workability. However, these materials do not have quench-hardenability, and work-hardenability does not significantly improve hardness. Therefore, it is unsuitable for application as a material for parts requiring high frictional strength, and in general, for example, a martensitic stainless steel product having a quench hardening property is often used as a substitute. However, recently, the number of 18-8 type stainless steel products whose hardness is improved by nitriding and hardening is increasing. The nitriding temperature of this stainless steel product is usually set to about 550 to 570 ° C.
It is set to about ℃.

[0003]

However, both the martensitic stainless steel product and the 18-8 stainless steel nitride product have a very low corrosion resistance as compared with the untreated austenitic stainless steel product. have. In particular, the nitriding-hardened 18-
As a result of the research conducted by the present inventors, the deterioration in corrosion resistance of the 8 series stainless steel products is considered to be caused by the following causes. That is, crystalline Cr nitrides (CrN, Cr ₂ N, etc.) are generated in the formed nitride layer, so that solid solution C is formed.
Activated C, which is indispensable for the formation of a passive film that should have the original corrosion resistance retention function of stainless steel, with a significantly reduced r concentration.
This is because r is completely lost. Therefore, when nitriding a stainless steel product, the corrosion resistance must be sacrificed, which limits the application of the austenitic stainless steel product by improving the hardness in nitriding hardening.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a method for nitriding an austenitic stainless steel product having excellent corrosion resistance and high surface hardness.

[0005]

In order to achieve the above object, the present invention is to hold an austenitic stainless steel product in a heated state in a chlorine-based gas atmosphere, and then keep it in a nitriding atmosphere at 450 ° C. or lower. The first gist is the method of nitriding the austenitic stainless steel product, which is formed by holding the heated state and forming the surface layer of the austenitic stainless steel product in the nitrided layer. After forming,
A second gist is a method of nitriding an austenitic stainless steel product which is brought into contact with a strong mixed acid solution containing HNO ₃ to clean the surface thereof.

[0006]

That is, the present inventors have conducted a series of studies in order to obtain a stainless steel product having high hardness without impairing the corrosion resistance inherent in the stainless steel product. In the course of the research, as described above, the crystalline Cr nitride, which is the driving force for hardening the stainless steel surface in the conventional nitriding treatment, lowers the active Cr concentration on the one hand and loses the corrosion resistance of the stainless steel product. I will let you. In other words, the formation of crystalline Cr nitrides (CrN, Cr ₂ N, etc.) in the formed nitride layer significantly reduces the concentration of solid solution Cr, and a passive film that should have the original corrosion resistance maintaining function of stainless steel. It was found that the active Cr, which is indispensable for the formation of Cr, was completely lost. As a result of further research, such a phenomenon was remarkable when the nitriding hardening treatment was performed at a heating temperature higher than 450 ° C. In order to avoid this, the stainless steel product was treated with a chlorine-based gas. Vickers hardness Hv = when the surface treatment is performed to make it easy for N atoms to permeate and nitriding is performed at a heating temperature of 450 ° C. or lower.
The inventors have found that a nitride layer having a high surface hardness of about 900 to 1200 can be formed, and the deterioration of corrosion resistance can be relatively reduced as compared with the case of the conventional high temperature nitriding treatment, and the present invention has been completed. In addition, as a result of an X-ray diffraction analysis in the nitride layer formed by processing at a temperature of 420 ° C. or lower, no crystalline Cr nitride or iron nitride is confirmed, and good corrosion resistance is obtained. It was found that the formation factor of the nitride hardened layer was due to the formation of amorphous nitride. Further, as described above, after the surface layer is formed into the nitrided layer, a cleaning treatment (post-treatment) with a strong mixed acid solution containing HNO ₃ is more preferable. Thus, in the nitriding method of the present invention,
The purpose is to include the post-processing as described above.

Next, the present invention will be described in detail.

The austenitic stainless steel product according to the present invention holds the austenitic stainless steel product in a heated state under a chlorine-based gas atmosphere, and then holds the austenitic stainless steel product in a heated state below a specific temperature in a nitriding atmosphere. It is obtained by forming a surface layer of a stainless steel product as a nitride layer. Further, after the surface layer is formed on the nitride layer in this manner, it is more suitable to contact the surface with a strongly mixed acid solution containing HNO ₃ to clean the surface.

Material of the above austenitic stainless steel product
The most austenitic stainless steel material
18-8 type austenite, which is a typical stainless steel product
Stainless steel material is used, but higher corrosion resistance
Is required, so that the amount of active chromium can be increased.
Contains 22% or more of chromium and is austener at room temperature.
A stainless steel having an iron structure is used. Also, Mori
Austenitic stainless steel containing 1.5% or more of bude
Even if a material is used, the same high corrosion resistance as above can be obtained.
The addition of this molybdenum is based on the above-mentioned 18-8 type austenite.
Corrosion resistance is improved with respect to stainless steel materials
Like Further, the austenitic type used in the present invention
For stainless steel materials, molybdenum of 1.5% or more, black
Austenite-ferrite containing 22% or more of aluminum
Phase stainless steel (SUS329J ₁, SUS329J
₂) Is also included. Such austenite-ferrai
By applying the above treatment to the duplex stainless steel material
High corrosion resistance similar to the above can be obtained. This place
In the case of nitriding, HNO₃・ HF, H
NO₃・ Dip in a strong acid (mixed acid) such as HCl,
By removing the thickness of 3μm to 5μm from the surface, the corrosion resistance is further improved.
improves. The immersion temperature may be room temperature, but if necessary 4
Heat to 0 ° C to 50 ° C.

Examples of the chlorine-based gas used in the chlorine-based gas atmosphere in which the austenitic stainless steel product is placed and treated include chlorine compounds such as Cl ₂ , HCl and CHCl _3, which are used alone or in combination. To be done. These are used in a gas state when used.
In addition to these, other chlorine compounds containing Cl in the molecule can also be used as the above chlorine-based gas in the form of gas. Further, Cl ₂ gas generated by thermally decomposing such a chlorine compound gas in a thermal decomposition apparatus or C prepared in advance
L ₂ gas can also be used as the chlorine-based gas.
Such chlorine compound gas and Cl ₂ gas are mixed and used depending on the case. Then, the chlorine compound gas, C
The chlorine-based gas such as l ₂ gas can be used alone, but it is usually used after being diluted with an inert gas such as N ₂ gas. The concentration of the chlorine-based gas itself in the diluted gas is, for example, 10,000 to 100,000 p.
pm, preferably 20,000 to 70,000 pp
m, more preferably 30,000 to 50,000 ppm.

In the present invention, first, the above-mentioned non-nitrided austenitic stainless steel product is placed in a chlorine-based gas atmosphere of the above concentration, kept in a heated state, and treated with a chlorine-based compound gas. In this case, the heating and holding is performed by heating and holding the austenitic stainless steel product at a temperature of, for example, 300 to 550 ° C. The holding time of the austenitic stainless steel product in the chlorine-based gas atmosphere may be selected as an appropriate time according to the type of product, the shape and size of the product, the heating temperature, etc. Well set. By treating an austenitic stainless steel product under such a chlorine-based gas atmosphere, "N" atoms can permeate from the surface of the stainless steel product to the inside. It is considered that this is because the passivation film on the stainless steel surface is destroyed by chlorination and the [N] atom is easily penetrated.

[0012] As described above, the austenitic stainless steel product in which the "N" atom is easily permeated by the chlorine-based gas treatment is then held in a heated state in a nitriding atmosphere and subjected to the nitriding treatment. In this case, the nitriding gas to make nitriding atmosphere, a mixed gas of a single gas is used consisting of only NH _3, also gas with NH ₃ and a carbon source (e.g. RX gas), for example, NH ₃ and CO
A mixed gas of CO ₂ and CO ₂ is also used. It is also possible to use a mixture of both. Usually, an inert gas such as N ₂ is mixed with the above single gas or mixed gas for use. In some cases, H ₂ gas may be further mixed with these gases and used.

In such a nitriding atmosphere, the fluorinated austenitic stainless steel product is held in a heated state. In this case, maintaining the heating state
The temperature is set to 450 ° C. or lower at a lower temperature than before. Particularly preferably, it is 380 to 420 ° C. This is the greatest feature of the present invention. That is, when the temperature exceeds 450 ° C., crystalline CrN is generated in the nitride layer, the active Cr concentration is lowered, and the corrosion resistance of stainless steel is deteriorated. In particular, by performing the nitriding treatment in the temperature range of 420 ° C. to 380 ° C., it is possible to obtain the same corrosion resistance as the excellent corrosion resistance of the austenitic stainless steel that is the base material. At the nitriding treatment temperature of 370 ° C., even if the nitriding treatment is performed for 24 hours, the depth of the nitride hardened layer is 10 μm or less,
Poor industrial value. The nitriding treatment time is usually 10 to 2
It is set to 0 hours. By this nitriding treatment, the surface layer of the austenitic stainless steel product is formed into a dense and uniform nitride layer having a thickness of 10 to 50 μm, generally 20 to 40 μm (the entire layer is composed of one layer). As a result, the hardness of the base material of the austenitic stainless steel product is Hv = 25.
While the surface hardness is 0 to 450, the surface hardness reaches Vv = 900 to 1200 in Vickers hardness. The thickness of the nitride layer formed at this time basically depends on the nitriding temperature and the nitriding treatment time.

The above chlorination treatment and nitriding treatment are carried out, for example, in a metallic muffle furnace as shown in FIG. That is, in this muffle furnace, treatment with chlorine gas is first performed, and then nitriding treatment is performed. Figure 1
1, 1 is a muffle furnace, 2 is its outer shell, 3 is a heater, 4 is an inner container, 5 is a gas inlet pipe, 6 is an exhaust pipe, 7 is a motor, 8 is a fan, 11 is a wire mesh cage, and 13 is Vacuum pump, 14 is an exhaust gas treatment device, 15 and 16 are cylinders,
Reference numeral 17 is a flow meter, and 18 is a valve. An austenitic stainless steel product 10 is put in the furnace 1, a cylinder 16 is connected to a flow path, and a chlorine-based gas such as Cl ₂ is introduced into the furnace 1 to be heated and treated with the chlorine-based gas, and then an exhaust pipe. The gas is extracted from 6 by the action of the vacuum pump 13 to be detoxified in the exhaust gas treatment device 14 and discharged to the outside. Next, the cylinder 15 is connected to the flow path, a nitriding gas is introduced into the furnace 1 for nitriding treatment, and then the gas is discharged to the outside via the exhaust pipe 6 and the exhaust gas treatment device 14. Through this series of operations, the chlorine-based gas treatment and the nitriding treatment are performed.

Particularly, the above chlorine-based compound gas is treated.
At the time of chlorine-based gas, Cl ₂Is preferable to use
is there. That is, the above Cl₂Is gaseous at room temperature and atmospheric pressure
And N₂Dilute with and blow it into the furnace relatively easily.
You can In addition, as described above, the low temperature region of 450 ° C or lower
When nitriding in the region, after nitriding, the outermost layer of the nitride layer is not
A thin high temperature oxide film is formed. This high temperature oxide film
Absorbs moisture over time and causes red rust. This removal
(Cleaning) is formed into a complicated shape like a screw material
With products, it is difficult to perform physical removal such as polishing.
Therefore, the above removal becomes a problem. Cannot be physically removed
If not, HNO₃・ Effective to remove with strong acid such as HF
Is. Hardened layer formed at nitriding temperature of 480 ° C or higher
Has extremely poor corrosion resistance, so it can be easily
This cannot be adopted because it disappears, but it is not related to the present invention.
Corrosion resistance is the base material for austenitic stainless steel products.
Due to its closeness, even with such immersion, most of the hardened layer
The oxide scale can be removed by leaving. Note that scale removal
HNO is₃Alone, even if it rises to 60 ℃ ~ 70 ℃
Hard to achieve. HNO as above₃・ By HF processing
The high temperature oxide film that causes red rust cannot be removed.
So that a nitrided hard layer with good corrosion resistance can be obtained.
It In particular, this is a metastable form like the SUS304 system.
Effective for parts that have been forged from materials, for example screw materials
It Since these products have complicated surface layers,
Since it is difficult to remove the mechanical scale after aging, keep corrosion resistance
It is convenient. The above screw materials are not limited to screws in a narrow sense.
Various screws, bolts, nuts, pins, bushes,
Bets etc. are included. In addition, strong acid is HNO as described above.
₃-Not only HF but also HNO₃-Other mixed acids such as HCl
Can be given. And the above process is not limited to immersion.
It also includes spraying and processing depending on play etc.
It

When the high temperature oxide film is removed by the above acid solution containing HF, the outermost surface layer of the high temperature oxide film is removed by 3 to 5 parts.
By removing about μm, the oxide film can be completely removed.

[0017]

As described above, the present invention, under the chlorine-based gas atmosphere, by holding the austenitic stainless steel product in a heated state, destroys the passive film on the surface of the austenitic stainless steel product, and then The surface layer of the austenitic stainless steel product is formed into a nitriding hardened layer by nitriding by holding it in a heated state at 450 ° C. or lower in a nitriding atmosphere. That is, according to the study by the present inventors, austenitic stainless steel is
Contains an element that easily reacts with the “N” atom to form a hard intermetallic compound, and the chlorinated film formed earlier also allows the “N” atom to pass through during nitriding and hardening. During the nitriding treatment, “N” atoms penetrate into the surface layer of the austenitic stainless steel product at a predetermined depth and in a uniform state. As a result, it becomes possible to form a dense and uniform nitriding hardened layer with a predetermined depth only on the surface layer of the austenitic stainless steel product, and the surface hardness is significantly improved.
Moreover, in the present invention, as compared with the conventional high temperature treatment, the low temperature 4
Since the nitriding treatment is performed at a temperature of 50 ° C. or less, deterioration of high corrosion resistance, which is an inherent property of austenitic stainless steel products, is suppressed, and an austenitic stainless steel product having both high hardness and high corrosion resistance is provided. can get. In order to maintain such corrosion resistance, in particular, an austenitic stainless steel material generally used as a heat-resistant steel such as SUS310 containing chromium in 18-8 stainless steel or more, and containing 1.5% or more of molybdenum. Austenitic stainless steel material and molybdenum of 1.5%
The above is remarkable when an austenite-ferrite dual phase stainless material containing 22% or more of chromium is used as a base material. When molybdenum is included, the chromium concentration is 18%
Even before and after, no deterioration in corrosion resistance was observed.

Next, examples will be described together with comparative examples.

[0019]

[Example 1] SUS316 plate material as-solution-treated (chromium content 17.7%, nickel content 13%,
Molybdenum (2%) was prepared, and this was placed in the muffle furnace 1 shown in FIG.
The temperature was raised to 00 ° C. And in that state, chlorine-based gas (C
(1 ₂ 50 vol% + N ₂ 50 vol%) was put in the furnace 1 to the atmospheric pressure state, and in that state, the holding chlorination treatment was carried out for 40 minutes. Next, after the chlorine gas was discharged from the furnace 1, the nitriding gas (NH ₃ 50 vol% + N ₂ 25 vol%
+ H _{2 (} 25 vol%) was introduced, the temperature inside the furnace 1 was raised to 420 ° C., and the state was maintained for 12 hours to perform nitriding treatment and take out.

The above-mentioned SUS nitrided
When the surface hardness of the 316 plate material was examined, it was found that the Vickers hardness reached Hv = 800 to 880, and the thickness of the nitrided and hardened layer was 15 μm.

Further, in order to electrochemically investigate the corrosion resistance of the nitrided SUS316 plate material, it was subjected to an anode polarization bending test (according to JIS G 0579). The result is shown in FIG. From the above FIG. 2, comparing the levels of the passive state holding current in the vicinity of the passive state region (broken line X),
It can be seen that the nitriding agent plate material (curve A) is hardly deteriorated as compared with the base material (curve B) which is not nitrided.

[0022]

Comparative Example 1 The nitriding temperature was changed to 500 ° C. and the nitriding time was changed to 8 hours. Otherwise, the SUS316 plate was chlorinated and nitrided in the same manner as in Example 1.
When the surface hardness of the above-mentioned SUS316 plate material subjected to the nitriding treatment was examined, Hv = 11 in Vickers hardness
50 to 1250, and the thickness of the hardened nitride layer is μm.
Met.

Further, in order to electrochemically investigate the corrosion resistance of the nitrided SUS316 plate material, it was subjected to an anodic polarization test as described above. The result is shown in FIG.
From the above FIG. 3, comparing the order of the passivity holding current density level in the vicinity of the passivation region (broken line X), the nitrided plate material (curve C) is compared with the non-nitrided base material (curve D). It can be seen that there is a difference of three digits or more and the corrosion resistance is significantly deteriorated.

Further, the product of Example 1 and the product of Comparative Example 1 were subjected to a salt spray test (SST) according to JIS 2371.
As a result, the product of Comparative Example 1 generated rust in 1.5 hours. On the other hand, the product of Example 1 did not rust even after 320 hours. As described above, the rust did not occur in the product of Example 1 despite the fact that both the product of Example 1 and the product of Comparative Example 1 were subjected to the nitriding treatment. This is considered to be because the base material structure before nitriding has a close structure and a complete austenite structure, so that the Cr activity remains sufficient.

[0025]

Example 2 SUS316 plate material (chrome content 17.8)
%, Nickel content 12%, molybdenum 2%), a processed plate material (internal hardness Hv = 310 to 320) was prepared, and the surface was finished by emery paper No. 1000 and buffing. Then, this was subjected to chlorination treatment in the same manner as in Example 1, and then subjected to nitriding treatment at 390 ° C. for 36 hours in the same manner as in Example. The surface hardness of this sample is Hv = 10
At 50 to 1080, the thickness (depth) of the cured layer was 20 μm. Furthermore, when this sample was subjected to the SST test, it did not rust even after 600 hours.

[0026]

Example 3 Cold-rolled SUS310 plate material containing 24.9% Cr and 19.1% Ni (internal hardness Hv = 3
70-390) was prepared. This was chlorinated and nitrided by the same method as in Example 1. When the surface hardness of the SUS310 plate material thus nitrided was examined, Hv = 980 to 1020 in Vickers hardness.
And the thickness of the nitriding hardened layer was 15 μm.
Then, in order to electrochemically investigate the corrosion resistance of the nitrided SUS310 plate material, it was subjected to an anode polarization bending test (JIS G 0579) in the same manner as described above. The result is shown in FIG. From FIG. 4 above, comparing the high corrosion current density levels in the vicinity of the passive state region (broken line X), the nitrided plate material (curve E) is the base material that is not nitrided (curve F).
It is understood that it has a good corrosion resistance with a difference of about one digit as compared with.

Further, the product of Example 3 was subjected to the SST test. As a result, rust did not occur even after 680 hours. This is because the product of Example 3 has a large amount of defects on the surface caused by cold working, but has sufficient Cr activity remaining to stably retain the passive film even after the nitriding treatment. It is believed that there is.

[0028]

Example 4 A cold rolled product of SUS310 plate material (internal hardness Hv = 370 to 390) similar to the product of Example 2 containing 24.9% of Cr and 19.1% of Ni was used as Example 2. After the same polishing, it was placed in the heat treatment furnace shown in FIG. 1, the inside of the furnace was sufficiently vacuum-purged, and the temperature was raised to 400 ° C. In that state,
Chlorine gas (Cl ₂ 50vol% + N ₂ 50vol%)
Was blown for 10 minutes at a flow rate 10 times the furnace volume (11 liters) per unit time. Then, at the same temperature, nitriding gas (NH ₃ 50 vol% + N ₂ 25 vol% + H ₂ 25
%) Was introduced for 8 hours. Then, the nitriding gas was shut off, a chlorine-based gas was blown in for 10 minutes, and then the nitriding gas was again subjected to nitriding treatment for 8 hours. The surface hardness of the SUS310 plate material thus nitrided is the same as in Example 2 above.
It was almost the same as the product, but the thickness (depth) of the cured layer was 10μ
It was m. Furthermore, as a result of performing an SST test, 680
No rusting occurred over time.

[0029]

Example 5 An austenitic stainless rolled material (SUS309) containing 22.7% Cr and 13% Ni was prepared. Using this sample as a base material, chlorination treatment and nitriding treatment were performed in the same manner as in Example 1. When the surface hardness of the austenitic stainless steel material thus nitrided was examined, the Vickers hardness was Hv = 1.
070 to 1080, and the thickness of the nitrided hard layer is 18 μm.
It was m. Then, this was subjected to the SST test. As a result, rust did not occur even after 680 hours.

[0030]

Example 6 Using austenitic stainless steel material (XM7) having a Cr content of 19% and a Ni content of 9%, a tapping screw and a socket screw were formed by forging. This sample was chlorinated and nitrided by the same method as in Example 1. When the surface hardness of the austenitic stainless steel material subjected to the nitriding treatment in this way was examined, it was Hv = 1050-108 in Vickers hardness.
0, and the thickness of the nitrided hard layer was 16 μm. Then, the nitrided austenitic stainless steel screws and socket screws were subjected to an SST test. As a result, red rust occurred in 24 hours. After the red rust occurred, the SST test was further held for 48 hours, and the degree of rusting was remarkably slight as compared with the degree of rusting of the product of Comparative Example 1.

[0031]

Example 7 The tapping screw and socket screw prepared in Example 6 were chlorinated and nitrided in the same manner as in Example 1. However, the nitriding temperature is 380 ° C.
Above, the nitriding treatment time was changed to 20 hours. The surface hardness of the sample thus nitrided is Hv = 7.
At 50 to 780, the thickness (depth) of the nitrided hard layer was 8 μm. Further, in the SST test, red rust was generated at 40 hours, but even after 48 hours, the degree of rusting was much smaller than that of the 500 ° C. nitriding treated product of Comparative Example 1.

As is clear from the above examples, 450 ° C.
By performing the nitriding treatment at the temperature below, it can be seen that the corrosion resistance is relatively improved as compared with the case where the nitriding treatment is performed at a temperature higher than 450 ° C. It is affected by processing conditions, material composition, processing temperature, etc. Generally, an austenitic stainless steel product is subjected to some processing and added with material strength, so that many surface defects occur. And, for example,
In the case of 18-8 series stainless steel material such as SUS304,
Depending on the application conditions, it may be considered that the corrosion resistance is not sufficiently retained even if nitriding treatment at 400 ° C or lower is performed.
In such a case, an austenitic stainless steel material containing a larger amount of chromium than the above 18-8 stainless steel material or an austenitic stainless material containing 1.5% or more of molybdenum, which is currently used as heat resistant steel. It is possible to expect the corrosion resistance close to that of the base material by performing the nitriding treatment using the above as described above.

[0033]

[Embodiment 8] The nitriding-processed tapping screw and socket screw made of austenitic stainless steel material (XM7) obtained in Embodiments 6 and 7 were tested at 35 ° C.
The surface high temperature oxide layer was removed (cleaned) by immersing in a 15% HNO ₃ solution containing 15% HF for 1 hour. Then, the product that had been removed was subjected to an SST test. As a result, in Example 6 and Example 7, the red rust on the spot which occurred in 24 hours did not occur even after 480 hours had elapsed. The surface hardness of the tapping screw before acid cleaning is Hv = 1.
080 to 1080, the depth of the hardened layer was 16 μm, but after the treatment, the hardness Hv was 820 to 830 and the depth of the hardened layer was 10 μm. On the other hand, as shown in Comparative Example 1, 50
In the case of 316 material nitrided at 0 ° C, as a result of the same acid cleaning,
The hardened layer of 50 μm disappeared entirely, and the surface hardness showed the hardness of the base material.

[Embodiment 9] In place of the austenitic stainless steel material of Embodiment 6, an austenitic-ferrite dual phase stainless steel material (S containing chrome content 23% and molybdenum 2% (S
US329J ₁ ) was used to forge tapping screws and socket screws. This sample was chlorinated and nitrided by the same method as in Example 1.
When the surface hardness of the product thus treated was examined, it was found that the Vickers hardness reached Hv = 1100 to 1120, and the thickness of the nitride layer was 20 μm. Next, this nitriding-treated product was immersed in the same mixed acid containing HF as in Example 8 in the same manner to remove the surface oxide layer. As a result, the nitride hardened layer had a thickness of 12 μm and a surface hardness of Hv.
= 880-900. And this was used for the SST test. As a result, red rust did not occur after 480 hours.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a furnace used for nitriding treatment of the present invention.

FIG. 2 is a curve diagram of current density and voltage of an austenitic stainless steel material subjected to nitriding treatment according to the present invention.

FIG. 3 is a curve diagram of current density and voltage of an austenitic stainless steel material subjected to nitriding treatment according to the present invention.

FIG. 4 is a curve diagram of current density and voltage of an austenitic stainless steel material subjected to nitriding treatment according to the present invention.

Claims (6)

[Claims] 1. A chlorine-based gas atmosphere is used to hold an austenitic stainless steel product in a heated state.
A method for nitriding an austenitic stainless steel product, which comprises maintaining the surface of the austenitic stainless steel product as a nitriding layer by holding it in a nitriding atmosphere at a temperature of 450 ° C. or lower. 2. The austenitic stainless steel according to claim 1.
Made of austenitic stainless steel depending on product nitriding method
After forming the surface layer of the product on the nitride layer, HNO ₃Strength including
Characterized by contacting a mixed acid solution and cleaning its surface
Method for nitriding austenitic stainless steel products. 3. The austenitic stainless steel product is a processed product using an austenitic stainless steel material having a chromium content of 22% by weight or more.
A method for nitriding the described austenitic stainless steel product. 4. The austenitic stainless steel product is a processed product obtained by processing an austenitic stainless steel material containing 1.5% by weight or more of molybdenum.
A method for nitriding the described austenitic stainless steel product. 5. The austenitic stainless steel product according to claim 1, wherein the austenitic stainless steel product is a processed product obtained by processing an austenite-ferrite dual phase stainless steel material containing 1.5% by weight or more of molybdenum and 22% by weight or more of chromium. Nitriding method for stainless steel products. 6. The method for nitriding an austenitic stainless steel product according to claim 1, wherein the austenitic stainless steel product is a stainless screw.