JP2732403B2 - Ammonia gas nitriding method for non-nitridable metal materials - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a method for nitriding an ammonia-nitridable metal material with ammonia gas. More specifically, the present invention provides a first step of exposing a metal nitride-resistant material to hydrogen gas to reduce a surface oxide film of the metal nitride-resistant material, and applying a vacuum or nitrogen gas A second step of releasing hydrogen absorbed by the non-nitridable metal material from the non-nitridable metal material by exposing it to water, and a third step of nitriding the non-nitridable metal material with ammonia gas at a low temperature. The present invention relates to an ammonia gas nitriding method for a metal nitride material.

[Conventional technology and its problems]

Conventionally, an ammonia gas nitriding method is known as a method of hardening a metal material. However, a high chromium alloy containing stainless steel has a defect that it cannot be easily gas-nitrided because it has an oxide film on its surface to prevent ammonia gas nitriding. Therefore, in order to perform ammonia gas nitriding of these hardly nitrided metal materials, conventionally, as a pretreatment, a surface oxide film for preventing gas nitriding is removed by washing with an acidic solution, and then nitriding is performed at a relatively high temperature. I was However, when nitriding is performed at a high temperature, sufficient surface hardness cannot be obtained. As a method of cleaning with an acidic solution, for example, an austenitic stainless steel which is a typical example of a non-nitridable metal material having a surface oxide film that prevents nitriding of ammonia gas and having no surface hardening method other than nitriding. Japanese Industrial Standards: JIS (Japa
n Industrial Standard) SUS304 steel (hereinafter simply JIS SUS
In the case of ammonia gas nitriding of 304 steel, the surface oxide film was removed using a mixed solution of hydrofluoric acid and nitric acid or hydrochloric acid as a pretreatment. However, in the method of removing the surface oxide film with an acidic solution, the mixed acid used, the waste liquid after pickling, and the chromium oxide and other heavy metal oxides contained in the waste liquid are extremely harmful. Has a very serious drawback in that pollution must be carried out at a high cost. For this reason, a method that does not use an acidic solution has been proposed as a pretreatment for nitriding.

As a method of nitriding a non-nitridable metal material without using an acidic solution as a pretreatment for nitriding, for example, Japanese Patent Application Laid-Open No. 60-165370 discloses a method in which a stainless steel is heated to a high temperature in a vacuum as a pretreatment to activate the stainless steel surface. Then, a method of performing a nitriding treatment at a high temperature in a high-purity nitrogen gas atmosphere has been proposed.
However, in the surface activation by the pretreatment of this method, the subsequent nitriding must be performed at a high temperature of 950 to 1150 ° C. When nitriding is performed at such a high temperature, the hardness of the nitrided layer formed on the obtained nitrided stainless steel continuously decreases from the surface toward the inside, and even the surface having the highest hardness is sufficiently reduced. It has the disadvantage of not being cured.

Further, Japanese Patent Application Laid-Open No. 60-125365 proposes a method in which a stainless steel surface is activated by wet blasting as a pretreatment, and then nitriding is performed with a mixed gas of ammonia gas and nitrogen gas or ammonia gas. However, this method is not preferable because the surface of the stainless steel is physically shaved by the pretreatment so as to have a satin finish, so that the surface roughness of the obtained nitrided stainless steel becomes large. In addition, in the nitrided stainless steel obtained by this method, the hardness of the nitrided layer continuously decreases from the surface toward the inside,
There is a disadvantage that the hardening of the surface showing the highest hardness is not sufficient. In addition, in this method, since wet blasting is performed as a pretreatment, treatment such as encapsulating the stainless steel with hard particles is performed to prevent the stainless steel surface from being oxidized during the transition from the pretreatment to the nitriding treatment. It is necessary, and the operation is complicated, which is not preferable.

Further, Japanese Patent Application Laid-Open No. 58-54186 discloses that stainless steel is subjected to air annealing or glass bead peening at a high temperature as a pretreatment, and then nitriding of the stainless steel at a high temperature of 800 ° C. or more in a nitrogen-hydrogen atmosphere. A method of doing so has been proposed. However, air annealing is not preferable because an oxide is generated on the surface, and the method using glass bead peening is not preferable because the surface roughness becomes large similarly to the pretreatment by the wet blast method described above. Furthermore, since the nitrided stainless steel obtained by this method is nitrided at a high temperature of 800 ° C. or higher, the surface hardness is not so high,
There are drawbacks such as insufficient wear resistance.

[Means and actions for solving the problems]

In view of the above, the present inventors, as a pretreatment of the nitridation of the non-nitridable metal material, not to mention that there is no danger of pollution,
Without unnecessarily roughening the surface of the non-nitridable metal material,
After activating the surface by removing the surface oxide film, nitriding is performed at a low temperature, and as a result, diligent research is conducted to obtain a method of nitriding a non-nitridable metal material that can obtain a metal material having sufficient surface hardness. As a result, as a pretreatment for nitriding, the metal nitride-resistant material is exposed to hydrogen gas to reduce the surface oxide film of the metal nitride-resistant material, thereby nitriding the metal nitride-resistant material with ammonia gas at a low temperature of 300 to 570 ° C. Can be
It has been found that a non-nitridable metal material having excellent surface hardness can be obtained by this method. The present invention has been completed based on the above findings.

That is, an object of the present invention is to provide a method capable of performing a pretreatment without fear of pollution and nitriding a non-nitridable metal material at a low temperature.

According to the present invention, the non-nitridable metal material is used in an amount of 0.01
a first step of exposing to a hydrogen gas at a temperature of 600 to 1350 ° C. under a pressure of rr to reduce a surface oxide film of the non-nitridable metal material;
A second step of releasing the hydrogen absorbed by the non-nitridable metal material from the non-nitridable metal material by exposing the reduced-nitride metal material subjected to the reduction treatment to vacuum or nitrogen gas at a temperature of 90 to 570 ° C. , And 300 to 570 ° C.
A third step of exposing the mixture to an atmosphere of ammonia gas or a mixed gas containing ammonia gas at a temperature of 3 g.

When performing the method of the present invention, an atmosphere-controllable tank,
Although a furnace equipped with a temperature control device and a gas exhaust device is used,
Further, it is desirable from the viewpoint of safety to use a furnace equipped with an explosion-proof device.

The reduction of the surface oxide film of the non-nitridable metal material as a pretreatment for nitriding is performed by placing the non-nitridable metal material in a furnace and introducing hydrogen gas into the furnace. In the present invention, the non-nitridable metal material is
A metal material that cannot be easily gas-nitrided because it has an oxide film on its surface that prevents nitridation. Examples of such non-nitridable metal materials include high chromium alloys,
Chromium, etc., but for high chromium alloys, use stainless steel (for example, JIS SUS as described in JIS G4303).
Steel) and heat-resistant steel (for example, JIS SUH steel as described in JIS G4311) are typical examples, among which JIS SUS304 steel of austenitic stainless steel, JIS SUS430 steel of ferritic stainless steel, martensitic steel JIS SUS410 steel and JIS SUS440 steel of stainless steel and JIS SUS630 steel of precipitation hardening stainless steel are particularly typical. In the case of an alloy consisting of iron and chromium alone, the method of the present invention is extremely effective for those having a chromium content of 4% by weight or more, particularly 8% by weight or more. The pressure of the hydrogen gas is generally 0.01 to 1520 Torr. If the pressure is less than 0.01 Torr, the reduction is not sufficient, and the effect of activating the surface of the non-nitridable metal material is insufficient. On the other hand, when the pressure exceeds 1520 Torr, surface activation by reduction is sufficient,
For example, explosion-proof equipment and pressure-resistant equipment at high temperatures become expensive, which is not preferable. The temperature for the reduction is appropriately selected depending on the metal material to be nitrided and the like, but is in the range of 600 to 1350 ° C, and generally in the range of 800 to 1250 ° C. The time for reduction depends on the temperature, but is at least 15 minutes, generally 1 to 4 hours, but there is no particular problem in reducing for a longer time.
When the temperature is lower than 600 ° C., the reduction reaction does not easily occur, and the surface activation of the hardly nitrided metal material is insufficient. On the other hand, when the temperature exceeds 1350 ° C., the metal crystal grains become too coarse, so that the strength of the parent phase of the metal material itself at a low temperature of about room temperature cannot be obtained. Furthermore, 1350
If the temperature exceeds ℃, depending on the material used, the liquid metal phase is partially melted and a liquid phase is generated.

The nitriding treatment of the non-nitridable metal material in the method of the present invention includes:
This is performed as follows. First, after the surface activation of the non-nitridable metal material by reduction, which is a pretreatment, while maintaining the atmosphere in the furnace as it is, or after evacuating, or an oxygen-free gas atmosphere other than hydrogen such as nitrogen or argon After lowering the temperature in the furnace to 570 ° C or lower. Although the lower limit of the temperature to be lowered is not limited, it is usually not always necessary to lower the temperature to room temperature or lower. Next, the temperature is kept as it is, or after a predetermined nitriding temperature condition, an ammonia gas is introduced into the furnace, and the non-nitridable metal material is exposed in an ammonia gas atmosphere. The nitriding temperature is
The temperature is 300 to 570 ° C., but if particularly excellent surface hardness is to be obtained, the temperature is preferably 450 to 570 ° C., more preferably 50 to 570 ° C.
0 to 570 ° C. is more preferred. However, for special applications, such as wear resistance under small stresses such as gears of precision measurement equipment, such as when corrosion resistance is required, or when it is necessary that the amount of deformation due to nitriding is within several μm, The nitriding treatment is preferably performed at a temperature of 350 to 500 ° C. According to the method of the present invention, nitriding can be performed at a low temperature of 300 to 570 ° C., so that the compound layer 2 (external nitride layer) having excellent corrosion resistance as shown in FIG. 1 is formed. An excellent metal material can be obtained in terms of surface. Nitriding temperature is 300 ℃
If it is less than 3, nitriding hardly occurs, and the surface hardness of the obtained metal material is insufficient. On the other hand, when the temperature exceeds 570 ° C., the hardness of the obtained metal material is insufficient. Further 740
At a high temperature exceeding ℃, the compound layer (external nitride layer) as described above is not formed, and the corrosion resistance of the obtained metal material is not sufficiently improved. In the nitriding treatment, a mixed gas containing ammonia gas, for example, a mixed gas of ammonia gas and hydrogen gas or a mixed gas of ammonia gas and nitrogen gas may be used instead of ammonia gas. In the case of ammonia gas and hydrogen gas, the compound layer 2 (external nitride layer) having excellent corrosion resistance as shown in FIG. 1 is formed, so that a metal material excellent in corrosion resistance can be obtained. If the nitriding temperature is lower than 300 ° C., nitriding is unlikely to occur, and the thickness of the obtained metal material is insufficient. Meanwhile, 57
If the temperature exceeds 0 ° C., the hardness of the obtained nitrided layer is insufficient. At a high temperature exceeding 740 ° C., the compound layer (external nitride layer) as described above is not formed, and the corrosion resistance of the obtained metal material is not sufficiently improved. In the nitriding treatment, a mixed gas containing ammonia gas, for example, a mixed gas of ammonia gas and hydrogen gas or a mixed gas of ammonia gas and nitrogen gas may be used instead of ammonia gas. In the case of a mixed gas of ammonia gas and hydrogen gas,
It is necessary to contain at least 5% by volume of ammonia gas. In the case of a mixed gas of ammonia gas and nitrogen gas, it is necessary to contain at least 20% by volume of ammonia gas. The pressure of ammonia gas or a mixed gas containing ammonia gas is 500 to 1520T
orr is selected. The time required for nitriding is appropriately selected depending on the purpose, but is at least about 15 minutes, and is generally about several hours to 100 hours industrially.

The ammonia gas nitriding method of the metal nitride-resistant material of the present invention, specifically, the metal nitride material used during the reduction treatment of the surface oxide film of the metal nitride-resistant material using hydrogen gas as a pretreatment for nitriding, Hydrogen may be absorbed by the non-nitridable metal material depending on the reduction processing temperature, and in such a case, the metal material finally obtained by the nitriding may become brittle. Therefore, in such a case, when shifting from the reduction treatment to the nitridation treatment, a hydrogen-free atmosphere, that is, a vacuum in the furnace as described above, or an atmosphere in the furnace to an oxygen-free atmosphere other than hydrogen as described above. The substitution is preferable because the absorbed hydrogen can be released from the hardly nitrided metal material.

As the oxygen-free atmosphere other than hydrogen, a nitrogen gas atmosphere is preferable.

The release treatment of the hydrogen absorbed by the non-nitridable metal material is, specifically, after the surface activation of the non-nitridable metal material by reduction, which is a pretreatment, the inside of the furnace is evacuated or the atmosphere in the furnace is replaced with hydrogen. It is performed by replacing the atmosphere with another oxygen-free atmosphere and then cooling to a temperature of 570 ° C. or less. The temperature at which hydrogen is released under vacuum is not limited and can be lowered to room temperature.However, at a temperature lower than about 90 ° C., the rate of hydrogen release becomes slow, so that the temperature is usually about 90 ° C. or higher. It is preferably performed at a temperature. The temperature for releasing hydrogen does not necessarily need to be kept constant, and hydrogen may be released while continuously lowering the temperature.

When the process of releasing hydrogen absorbed by the non-nitridable metal material is performed, the nitriding process of the non-nitridable metal material is performed in a predetermined manner after the furnace is kept in a vacuum or after an inert gas is introduced into the furnace. After the temperature is increased, ammonia gas is introduced to expose the non-nitridable metal material in an ammonia gas atmosphere, or first, ammonia gas is introduced into a furnace and then set to a predetermined nitriding temperature to set the non-nitridable metal material in an ammonia gas atmosphere. Perform by exposing the material. N ₂ , He, Ne, Ar, Kr and Xe can be used as the inert gas, but N ₂ and Ar are preferred in terms of cost. The temperature and time of the nitriding treatment are each 300 to 570 ° C., preferably 450 to 570 ° C., as in the case where the hydrogen releasing treatment is not performed.
More preferably at 500 to 570 ° C, for at least about 15 minutes,
Industrially, it is generally several hours to about 100 hours. Further, similarly to the case where the hydrogen release treatment is not performed, the above-mentioned mixed gas containing ammonia gas may be used in the nitriding treatment instead of the ammonia gas. The pressure of ammonia gas or a mixed gas containing ammonia gas is 500 to 152.
0 Torr.

The metal nitride material obtained by the ammonia gas nitriding method of the present invention has a nitride layer having an excellent hardness consisting of a compound layer (external nitride layer) and a diffusion layer (internal nitride layer). It can be clearly distinguished from wood. The nitrided layer maintains a substantially constant hardness throughout regardless of the depth from the surface. Further, since the compound layer has excellent corrosion resistance, a metal material excellent not only in surface hardness but also in corrosion resistance can be obtained by the method of the present invention. The presence of this nitrided layer can be confirmed by micrographs, X-ray diffraction, etc., and the hardness of the nitrided layer of the metal nitride material can be measured by, for example, JIS Z2251 (1980) Vickers hardness test according to Japanese Industrial Standards. .

Hereinafter, the present invention will be described in detail with reference to examples, but the present invention is not limited thereto.

〔Example〕

Example 1 An austenitic stainless steel (JIS SUS304L steel) block (20 mm x 15 m) was placed in a furnace equipped with a tank capable of controlling the atmosphere, a temperature controller, a gas exhaust device, and an explosion-proof device for exhaust gas.
m × 30mm) and introduce 0.8Torr hydrogen gas into the furnace
The block was exposed to hydrogen gas at 1200 ° C. for 1 hour to reduce the surface oxide film of the block. While maintaining the atmosphere in the furnace, the temperature was lowered to 200 ° C.
After reducing the pressure to 0 ^-3 Torr or less, the furnace was cooled to room temperature to release hydrogen absorbed in the block. Nitrogen gas was introduced into the furnace to raise the pressure in the furnace to 760 Torr, and the temperature was raised again to 550 ° C. in the furnace. Thereafter, ammonia gas was introduced into the furnace to completely replace the nitrogen gas with ammonia gas, and the pressure in the furnace was set to 760 Torr, and the block was exposed to an ammonia gas stream for 19 hours to perform nitriding. After nitriding, the block was cooled with water.

The obtained nitrided stainless steel block was cut perpendicularly to its surface with a micro cutter, and the cut surface was treated with a corrosive liquid (5 g FeCl ₂ +2 ml HCl + 96 ml C ₂ H ₅ OH) and then 400 times with an optical microscope. Photographed on a larger scale. As a result of observing the obtained photograph, as shown in FIG.
Compound layer 2 (external nitride layer) and diffusion layer 3 (internal nitride layer)
A nitride layer having a thickness of about 95 μm is formed, and the nitride layer is visually distinguished from the parent phase 4 of stainless steel and observed. In this nitrided layer, a portion having a thickness of several μm to about 10 μm from the surface 1 was a compound layer 2 having excellent corrosion resistance which was not corroded by a corrosive liquid. The hardness of the nitrided layer is measured in the same cross section as above for the nitrided stainless steel block before the treatment with the corrosive liquid.
According to the method of 251 (1980), Shimadzu HMV-2000
When measured with a Vickers hardness tester at a measurement load of 100 g, the microhardness [HV (100 gf )] About 1100. Furthermore,
The surface of the nitrided block was converted to a Philips X-ray diffractometer NORELCO (target Co, acceleration voltage 35 kV, current 10 mA, C
o-Kα ray λKα, = 1.78890 (Å), λKα ₂ = 1.7927
8 (Å), using an iron filter)
As shown in Fig. 3, the base material of the block is JIS SUS304.
The nitrides Fe ₃ N, γ′-Fe ₄ N and ε-Fe _2.5 N with iron in the L steel and the CrN nitride with chromium were detected, indicating that a nitrided layer was formed.

Example 2 The same block as that used in Example 1 was placed in the furnace used in Example 1, and 1.0 Torr of hydrogen gas was introduced into the furnace.
The block was exposed to hydrogen gas at 4 ° C. for 4 hours to reduce the surface oxide film of the block. The temperature was lowered to 550 ° C. while maintaining the atmosphere in the furnace. Furnace temperature 5
When the temperature reached 50 ° C., nitrogen gas was introduced into the furnace, and hydrogen gas was completely replaced with nitrogen gas, so that the pressure in the furnace was 760 Torr. Thereafter, 760 Torr of ammonia gas was introduced into the furnace, the nitrogen gas was replaced with ammonia gas, and the block was exposed to ammonia gas for 20 hours to perform nitriding. After nitriding, the block was cooled with water.

The obtained nitrided stainless steel was cut in the same manner as in Example 1, and the hardness was measured by the same method as in the example.
As shown in the diagram plotted by the mark △ in FIG. 2, a nitrided layer having a thickness of about 125 μm was observed to be formed, and the hardness of the nitrided layer was almost equal to the distance from the surface of the block. Irrespective of the micro hardness (HV (100g
f)] about 1050, which is excellent.

Example 3 The same block as used in Example 1 was placed in the furnace used in Example 1, and 10 Torr of hydrogen gas was introduced into the furnace at 700 ° C.
The block was exposed to hydrogen gas for 1 hour to reduce the surface oxide film of the block. The temperature was lowered to 300 ° C. while maintaining the atmosphere in the furnace. When the temperature reached 300 ° C, the atmosphere in the furnace was replaced with ammonia gas within 1 minute, the pressure in the furnace was set to 760 Torr, and the block was heated to 300 ° C.
It was exposed to ammonia gas for 49 hours to perform nitriding. After nitriding, the block was cooled with water.

The surface of the obtained nitrided stainless steel was coated with an X-ray diffractometer XD-610 manufactured by Shimadzu Corporation (target Cu, acceleration voltage 30 kV, current 1
0 mA, Cu-Kα ray λKα, = 1.54050 (Å) λKα ₂ = 1.
As a result of analysis using 54433 (Å), using a nickel filter), as shown in FIG.
In the JIS SUS304L steel, nitrides Fe ₃ N and γ′-Fe ₄ N with iron and nitrides Cr ₂ N with chromium were detected, indicating that a nitrided layer was formed.

〔The invention's effect〕

The ammonia gas treatment method for a non-nitridable metal material according to the present invention uses a conventional method for reducing the surface oxide film of the non-nitridable metal material with hydrogen gas as a pre-treatment when nitriding the non-nitridable metal material to activate the surface. Not only there is no danger of pollution due to waste liquid which becomes a problem in the method using the acidic solution of
Since the nitridation of the hardly nitrided metal material can be performed at a lower temperature as compared with other conventional methods not using an acidic solution, a metal nitride material having sufficient hardness can be obtained. Further, since the nitrided metal material can be nitrided at a low temperature, a corrosion-resistant compound layer is formed on the surface of the nitrided metal material, so that a metal nitride material having further improved corrosion resistance can be obtained. Since the method of the present invention has the above-mentioned advantages, it is highly desirable to provide a metal nitride material which does not require pollution prevention measures, can save cost, has sufficient hardness, and has excellent corrosion resistance. It has an effect.

[Brief description of the drawings]

FIG. 1 is a photomicrograph of a metal structure of a cross section of a nitrided stainless steel obtained by the method of the present invention. FIG. 2 is a graph showing the relationship between the depth (μm) from the surface of the nitrided stainless steel obtained by the method of the present invention and Vickers hardness [HV (100 gf)]. FIG. 3 and FIG. 4 are graphs showing the results of X-ray diffraction analysis of the surface layer of the nitrided stainless steel obtained by the method of the present invention. 1 ... surface, 2 ... compound layer (external nitride layer), 3 ... diffusion layer (internal nitride layer), 4 ... stainless steel matrix

Continuation of the front page (72) Inventor Shiomi Kikuchi 5-48 Ichichoda, Seika-cho, Soraku-gun, Kyoto Prefecture (56) References JP-A-55-14851 (JP, A) "Metal material surface engineering" by Hiroshi Asada Coro Published by Nasha Co., Ltd. (Showa 43-12-10) 30P15-17 line

Claims (1)

(57) [Claims] A first step of exposing a metal nitride-resistant material to hydrogen gas under a pressure of 0.01 to 1520 Torr at a temperature of 600 to 1350 ° C. to reduce a surface oxide film of the metal nitride-resistant material, A second step of releasing the hydrogen absorbed by the non-nitridable metal material from the non-nitridable metal material by exposing the received non-nitridable metal material to vacuum or nitrogen gas at a temperature of 90 to 570 ° C .; A third step of exposing the non-nitridable metal material to an atmosphere of ammonia gas or a mixed gas containing ammonia gas at a temperature of 300 to 570 ° C.