JP4861703B2 - Method for activating metal member surface - Google Patents

Description

  The present invention relates to a pretreatment method for a metal member that activates the surface of the metal member prior to performing diffusion permeation treatment such as nitriding or carburizing on the metal member.

  In order to improve mechanical properties such as wear resistance and fatigue strength, gas nitriding and gas carburizing methods that form a nitrided or carburized layer on the surface of metal members are mainly widely used for members made of iron-based materials. Has been.

  When these treatments are applied to the surface of a member made of alloy steel, especially high alloy steel, nitrogen and carbon penetrate and diffuse into the surface of the metal member due to the passivation film (oxide, etc.) present on the member surface. This hinders the processing of the above-mentioned members and causes problems. For this reason, the activation process of the surface of a metal member is performed prior to these diffusion penetration processes. The most widely adopted surface activation treatment is a method using a chloride compound typified by a marcomizing treatment. As the chloride, vinyl chloride resin, ammonium chloride, methylene chloride or the like is used.

  The chloride is heated together with a metal member in a processing furnace. These chlorides are decomposed by the heating to generate HCl, and the generated HCl destroys (denatures) the passivation film on the surface of the metal member to activate the surface, such as nitriding or carburizing in the next step. Ensures diffusion and infiltration treatment.

  However, the surface activation of the metal member by the chloride as described above not only causes the cracked and generated HCl to wear out the inner wall surface of the furnace made of brick or metal, but also in ammonia gas, which is an atmospheric gas, in gas nitriding and gas soft nitriding. To produce ammonium chloride, which accumulates in the furnace and exhaust system and causes troubles, and remains on the surface of the metal member (workpiece) to prevent corrosion resistance and fatigue strength of the member. This is causing a decline.

In recent years, a metal member surface activation method using a fluorine compound (NF ₃ ) belonging to the same halogen group has been put into practical use as an alternative to the method using chloride (for example, Patent Document 1). The NF ₃ is decomposed by heating to generate fluorine, and the generated fluorine changes the passivation film on the surface of the metal member to a fluoride film to activate the surface of the metal member. However, the method for activating a metal member surface with a fluorine compound (NF ₃ ) requires advanced treatment for detoxifying NF ₃ and HF contained in the exhaust gas, which hinders the spread of the method.

  The activation method of the metal member surface using the halide has problems such as a problem of deposits in the furnace, wear of the inner wall surface of the furnace, or a facility for detoxifying the exhaust gas. From such a background, the development of the activation method of the metal member surface which does not use a halide is advanced.

In the ammonia gas nitriding method described in Patent Document 2, a reducing radical generated by pyrolysis of acetone and CO are generated on the surface of a high chromium alloy steel member, which is a workpiece, thereby forming a passivated film on the surface of the alloy steel member. This is a method for reducing and activating. According to this method, acetone is thermally decomposed according to the following equation (1) on the surface of the heated high chromium alloy steel member, and reducing radicals and CO are generated on the surface of the high chromium alloy member.
2 (CH ₃ ) CO → 2CH ₃ · + CO (1)
The oxide film (MO) on the surface of the metal member is reduced by the following equation (2).
5MO + 2CH _3- > 5M + 2CO + 3H ₂ O (2)
Since the main component of the surface oxide film of the high chromium alloy steel member is Cr ₂ O ₃ , 5Cr ₂ O ₃ + 6CH ₃ · → 10Cr + 6CO + 9H ₂ O (3)
The CO produced according to the above formulas (1) to (3) reacts with ammonia as the atmospheric gas to produce HCN according to the following formula (4).
CO + NH ₃ → HCN + H ₂ O (4)
The HCN produced by the above formula (4) reduces the passivated film on the surface of the high chromium alloy member by the following reaction.
Cr ₂ O ₃ + 6HCN → 2Cr (CN) ₃ + 3H ₂ O (5)
C and N of the produced Cr (CN) ₃ diffuse into the surface of the high chromium alloy member, contribute to carburizing and nitriding, and no residue is generated on the member surface.

On the other hand, the activation reaction of the high chromium alloy steel member surface by the chloride is expressed by the following formula (6).
Cr ₂ O ₃ + 6HCl → 2CrCl ₃ + 3H ₂ O (6)
The chromium chloride remains on the surface of the member and becomes a causative substance of the member.
JP-A-3-44457 Japanese Patent Application No. 9-38341

  As described above, the method described in Patent Document 2 is excellent in that the problem of the method for activating a metal member surface with chloride described in Patent Document 1 is solved in principle. However, since the method described in Patent Document 2 uses liquid acetone at room temperature and normal pressure, it requires a device for introducing acetone vapor, and the flow rate of acetone is not easy, so a metal member having a uniform active surface. There is a drawback that it is difficult to obtain.

In order to solve the above problems, the present inventors have completed the present invention by developing a method that uses a compound that is a gas at normal temperature and pressure instead of acetone, which has a problem in handling.
That is, the configuration of the present invention is as follows.
1. A mixed gas containing a carbon supply compound that is a gas at normal temperature and pressure and ammonia as essential components is heated to 300 ° C. or higher in a heating furnace, and in the heated mixed gas, a metal member, a metal furnace inner wall, or a metal jig A method for activating a surface of a metal member, characterized in that HCN is produced by the catalytic action of and the produced HCN is allowed to act on the surface of the metal member.

2. 2. The method for activating a surface of a metal member according to 1, wherein the carbon supply compound is one or more compounds selected from acetylene, ethylene, propane, butane, and carbon monoxide.

3. The metal member surface according to 1 above, wherein the metal furnace inner wall or the metal jig contains one or more metals selected from Fe, Ni, Co, Cu, Cr, Mo, Nb, V, Ti, and Zr. Activation method.

4). 2. The method for activating a metal member surface as described in 1 above, wherein the HCN concentration generated in the furnace is 100 mg / m ³ or more and the dew point of the atmosphere gas in the furnace is 5 ° C. or less.

  According to the present invention, the surface passivating film of a high alloy steel member in which diffusion permeation treatment such as gas nitriding method, gas carburizing method or the like for forming a nitrided layer, carburized layer or carbonitrided layer on the surface of a metal member is difficult The HCN gas is generated in the furnace using the gas normally treated in the gas heat treatment, and the catalytic action of the surface of the metal to be treated and / or the metal furnace material is used to passivate the gas. By activating the surface of the alloy steel member, conventionally, there are no harmful effects such as deposits in the furnace, wear on the wall surface of the furnace, and exhaust gas detoxification treatment, which has been a problem in the activation treatment with halides. It is possible to provide an activation treatment method for the surface of a metal member that is useful as a pre-treatment of the diffusion permeation treatment.

Next, the present invention will be described in more detail with reference to the best mode for carrying out the invention.
According to Patent Document 2, CH ₃ .multidot. (Methyl radical) generated by the thermal decomposition of acetone in the formula (1) reduces the oxide film on the surface of the metal member. The CO produced by the equations (1) and (2) reacts with the atmosphere gas ammonia on the metal surface to produce HCN. HCN acts on the metal oxide film according to the equation (5).

The inventors of the present invention used CH ₃ · and HCN (reaction product of CO, which is another thermal decomposition product, and ammonia in the atmospheric gas) generated by the thermal decomposition of acetone, as expressed by the above formulas (2) and (5). From the comparison of formulas, the effect on the passivation film is similar, and the presence of both CH _3. And HCN is a sufficient condition for activation of the surface of the high chromium alloy steel member, but it is not necessarily a necessary condition. Therefore, focusing on HCN, we worked on the development of HCN generation method on the metal surface and confirmation of the activation effect on the metal member surface by HCN.

A gas selected from nitriding atmosphere gas (NH ₃ : N ₂ = molar ratio 1: 1) and various carbon-containing compounds that are gases at normal temperature and pressure is introduced into a muffle furnace made of SUS310S and heated to 550 ° C. Then, the production of HCN was examined. As a result, it was confirmed that carbon monoxide, carbon dioxide, acetylene, ethylene, propane and butane each clearly produced HCN in combination with ammonia.

  On the other hand, except that the inner wall of the muffle furnace was replaced with a brick furnace, the same experiment as described above was performed and the amount of HCN produced was analyzed. As a result, HCN was not detected in all cases. From this, it became clear that the catalytic action of the metal surface is an essential condition for the HCN generation reaction by ammonia and these gases.

The HCN generation reaction by ammonia and the carbon-containing compound can be expressed by the following formulas, respectively.
NH ₃ + CO → HCN + H ₂ O (7)
2NH ₃ + 2CO ₂ → 2HCN + H ₂ O + O ₂ (8)
2NH ₃ + C ₂ H ₂ → 2HCN + 3H ₂ (9)
2NH ₃ + C ₂ H ₄ → 2HCN + 4H ₂ (10)
3NH ₃ + C ₃ H ₈ → 3HCN + 7H ₂ (11)
4NH ₃ + C ₄ H ₁₀ → 4HCN + 9H ₂ (12)

Nitriding atmosphere gas (NH _3: N ₂ = 1: 1 mole ratio) and comparison of the amount of HCN by the reaction of gas selected from various carbon-containing compounds of the nitriding atmosphere gas (NH _3: N ₂ = mole ratio 1: 1) each carbon-containing compound is contained in an equivalent ratio of 1%, the inner wall is introduced into a muffle furnace made of SUS310S, heated to 550 ° C. for 30 minutes, and the formulas (7) to (12) The reaction was carried out. As a result, the amount of HCN produced by each carbon-containing compound was in the following order.
C ₂ H ₂ >CO> C ₂ H ₄ > C ₄ H ₁₀ > C ₃ H ₈ > CO ₂

Regarding these carbon-containing compounds that have been confirmed to generate HCN by reaction with a nitriding atmosphere gas, each of these compounds is introduced into a heating furnace at the initial stage of nitriding treatment to determine whether or not there is an activation action. Evaluation was performed using a plate material. As a result, C ₂ H ₂ , CO, C ₂ H ₄ , C ₄ H _10, and C ₃ H ₈ are more uniform in nitriding and nitrogen intrusion in the SUS304 plate material than in the control nitriding treatment in which no carbon-containing compound is introduced. A clear effect was observed in the weight increase due to. On the other hand, when CO ₂ is used, there is no difference from the control nitriding treatment in both uniform nitriding property and increase in the weight of the test piece, and CO ₂ has an activation effect on the surface of the SUS304 plate material. There wasn't.

It is a by-product of the HCN generation reaction of the above formula (8) that the activation action is not obtained on the surface of the SUS304 plate material even though HCN is generated by introducing CO _{2 in} the furnace. It is presumed to be due to re-oxidation of the surface of the SUS304 plate material due to the oxidizing action of O ₂ and H ₂ O. Regarding CO, HCN is generated as described above. This contradicts the phenomenon in which stainless steel is not uniformly nitrided in a gas soft nitriding atmosphere in which ammonia and RX gas containing CO exist. The reason is considered as follows. Here, the RX gas is a mixture of hydrocarbon gas (for example, propane gas, butane gas, natural gas) and air at a substantially chemical equivalent, and decomposes in a catalyst layer maintained at 1000 ° C. to decompose CO, H ₂ (N ₂ ). It is a gas that contains a small amount of CO ₂ and H ₂ O as a main component and is widely used as a carburizing gas.

CO contained in NH ₃ : RX gas = molar ratio 1: 1, which is a typical composition of gas soft nitriding, is about 10% by volume. Therefore, it is estimated that there is enough HCN necessary for activation of the metal member surface in the gas nitrocarburizing furnace, but a considerable amount of H ₂ O (around 2% by volume) is included in the RX gas whose dew point is not controlled. Since about 0.5% by volume of CO ₂ is present, it is judged that the surface of the SUS304 plate material activated by these oxidation actions is reoxidized, and the penetration of nitrogen into the plate material surface is prevented. Is done.

Therefore, when CO gas is selected as the carbon supply compound for activating the metal member surface, it is desirable to use a single CO gas instead of the RX gas. However, since the required injection amount of CO gas in the present invention is about 1/10 (capacity) of the gas soft nitriding atmosphere, the influence of H ₂ O and CO _{2 in} the RX gas is reduced. In some cases, it can be used as a source.

Judging from the formulas on the right side of the reaction formulas (7) to (12), the by-product oxidation action in the case of CO ₂ is the highest among these compounds having a cyanogenic action, followed by CO. All hydrocarbon compounds generate reducible hydrogen. Therefore, it is desirable to select a hydrocarbon compound as the carbon supply compound in order to prevent reoxidation.

The activation action of the alloy steel member surface according to the present invention is due to HCN. The activation effect depends on the HCN concentration in the furnace atmosphere. The appropriate concentration of HCN for obtaining a satisfactory activating effect is in the range of 100 to 30,000 mg / m ³ . If the concentration of HCN is less than 100 mg / m ³ , the above activation effect cannot be expected. On the other hand, if the concentration of HCN exceeds 30,000 mg / m ³ , the activation effect is saturated, which is not only economically disadvantageous, but also sooting (carbon generation in the furnace) due to thermal decomposition of the carbon supply compound. Since it happens, it is not preferable.
The dew point of the furnace atmosphere gas is preferably 5 ° C. or less. When the dew point is higher than 5 ° C., the metal surface activated by the HCN gas is reoxidized by H ₂ O in the atmosphere and passivated again.

  The environmental advantage of the method of the present invention is that, as explained in the reaction formula (5), the HCN that contributes to the activation of the metal member surface is taken into the member surface and is used for nitriding and carburizing the member. HCN that contributes and does not leave any residue on the surface of the member and is discharged as exhaust gas without contributing to the reaction can be easily detoxified with the ammonia combustion device attached to the nitriding device, and a new additional The equipment is unnecessary.

  A further advantage of the present invention is a reduction in nitriding time due to smooth progress of the nitriding process. Gas nitriding of a metal member is usually performed according to the following schedule.

The metal member was set in the furnace, and after the atmosphere in the furnace was vacuum purged or replaced with nitrogen gas, a nitriding atmosphere gas (NH ₃ + N ₂ ) was introduced 1 to 10 times the furnace volume per hour while After the temperature is raised to the nitriding temperature, it is maintained at a constant temperature. During the treatment, the pressure in the furnace is maintained at about atmospheric pressure + 0.5 kPa by a pressure valve, and the exhaust gas pushed out is burned and decomposed by an exhaust gas combustion device.

  In the method using the fluorine-based gas disclosed in Patent Document 1, as described in the example of Japanese Patent No. 2501925, after introducing the fluorinated gas and performing the activation treatment of the member, It is necessary to introduce the nitriding atmosphere gas into the furnace after exhausting the fluorinated gas.

  In contrast, in the present invention, in the step of raising the temperature of the metal member to the nitriding temperature, a carbon supply compound is introduced into the nitriding atmosphere gas, HCN is generated to activate the surface of the metal member, and then the carbon supply compound is introduced. By stopping, it is possible to shift to the nitriding process as it is. As a result, the processing time of the nitriding step is greatly shortened, and the reoxidation phenomenon on the surface of the metal member, which has been a problem in the conventional processing when shifting from the activation to the nitriding step, can be solved in principle. .

  The technical features and effects of the present invention are as described above. Hereinafter, preferred embodiments of the present invention will be described. In the treatment furnace used in the present invention, the inner wall is preferably made of metal, but even if the inner wall is not made of metal, the metal member to be treated becomes an HCN catalyst, and the metal member is held in the furnace. The jig may be made of metal. Examples of the metal constituting the metal inner wall, metal member, and jig include one or more metals selected from Fe, Ni, Co, Cu, Cr, Mo, Nb, V, Ti, and Zr. It is preferable.

  Examples of the metal member to be surface activated by the method of the present invention include cold mold steel, hot mold steel, plastic mold steel, high speed tool steel, powder high speed tool steel, and chromium molybdenum steel. , Maraging steel, austenitic stainless steel, ferritic stainless steel, martensitic stainless steel, martensitic heat resistant steel, austenitic heat resistant steel, nickel-base superalloy, and the like. In the inside, it is placed according to a conventional method by a suitable jig and subjected to surface activation treatment.

  The gas for surface treatment supplied into the furnace is a carbon supply compound and ammonia which are gases at normal temperature and pressure, and each is supplied into the furnace from a dedicated cylinder. These gases are set in a furnace with a metal member, and the atmosphere in the furnace is purged with vacuum or replaced with nitrogen gas, and then the nitriding atmosphere gas (ammonia alone or ammonia + nitrogen gas or ammonia + nitrogen gas + hydrogen gas) After establishing the reducing atmosphere, the temperature rise is started and the carbon supply compound of the present invention is introduced. When ammonia gas and the carbon supply compound are heated to 300 ° C. or higher in the furnace, HCN is generated by the catalytic action of the metal surface. The ratio of the flow rate of ammonia as the nitriding atmosphere gas and the flow rate of the carbon feed compound to be introduced should be in the range of 1: 0.0001 to 1: 0.1. When the flow ratio of the carbon supply compound is lower than 1: 0.0001, the activation effect cannot be obtained because the amount of HCN produced is low. When the flow rate ratio of the carbon supply compound exceeds 1: 0.1, the activation effect is saturated, which is economically disadvantageous.

  The carbon supply compound is one or more gaseous compounds selected from acetylene, ethylene, propane, butane and carbon monoxide as described above, and can be supplied into the processing furnace simultaneously with the ammonia-containing gas as described above. It is preferable for the efficient use of the carbon supply compound that the introduction of the carbon supply compound is preferably started when the temperature of the ammonia-containing gas in the furnace reaches about 300 ° C., but the concentration of the carbon supply compound in the furnace atmosphere is reduced. In order to shorten the treatment time by raising the temperature early, it is desirable to introduce a carbon supply compound at the same time as the temperature rise is started and to generate HCN from the initial stage.

  Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples. The following examples and comparative examples were carried out using a processing furnace having the structure shown in FIG. In FIG. 1, 1 is a muffle furnace, 2 is its outer shell, 3 is a heater, 4 is an inner container (retort), 5 is a gas introduction pipe, 6 is an exhaust pipe, 7 is a motor, 8 is a fan, and 9 is a metal. Jig, 10 is a gas guide cylinder, 11 is an umbrella, 12 is a vacuum pump, 13 is an exhaust gas combustion device, 14 is a carbon supply compound gas cylinder, 15 is an ammonia gas cylinder, 16 is a nitrogen gas cylinder, 17 is a hydrogen gas cylinder, and 18 is a flow rate. A total of 19 is a gas control valve.

[Example 1]
The SUS310S muffle furnace with an internal volume of 100 L shown in FIG. 1 is used, SUS304 plate material is set in the furnace, NH ₃ gas and N ₂ gas are fed at a flow rate of 200 L / H, and the temperature is increased from room temperature to 550 ° C. in 75 minutes. Warm up. The injection of acetylene gas 2 L / H was started when the ambient temperature reached 100 ° C. (18 minutes after the start of temperature increase). After the temperature was raised to 550 ° C., the atmospheric temperature was maintained for 2 hours. At this point, acetylene gas injection was stopped, while NH ₃ gas and N ₂ gas were allowed to flow at 550 ° C. for an additional 4 hours to advance nitriding. The heating was stopped and only the N ₂ gas was allowed to flow to cool the furnace. When the ambient temperature reached 100 ° C. or lower, the test piece in the furnace was taken out.

Further, the exhaust gas from the furnace was branched, and a part of the exhaust gas was absorbed in a 2% by mass aqueous caustic soda solution to perform HCN analysis. From the analysis result of the HCN absorption liquid, the average HCN concentration in the furnace atmosphere during the acetylene gas injection period was 8,000 mg / m ³ . The weight increase of the SUS304 test piece before and after nitriding was measured and found to be 20 g / m ² . A SUS304 test piece was cut and polished, etched with a marble solution, and the cut surface was observed with an optical microscope. As a result, a nitrided layer having a uniform thickness of 50 μm was formed (FIG. 2 shows a photomicrograph at a magnification of 500 times). ). When the surface hardness of the test piece was measured at 5 points using a Vickers hardness tester, all values were distributed between Hv = 1200 and 1250.

[Example 2]
SUS304 plate material was set in the muffle furnace used in Example 1, NH ₃ gas and N ₂ gas were fed at a flow rate of 200 L / H, respectively, and the temperature was raised from room temperature to 550 ° C. in 75 minutes. At the time when the ambient temperature reached 100 ° C. (18 minutes after the start of temperature increase), injection of propane gas 5 L / H was started. After the temperature was raised to 550 ° C., the ambient temperature was maintained for 2 hours. At this point, the injection of propane gas was stopped, while NH ₃ gas and N ₂ gas were allowed to flow at 550 ° C. for an additional 4 hours to advance nitriding, The heating was stopped and only N ₂ gas was allowed to flow and the furnace was cooled. When the ambient temperature reached 100 ° C. or lower, the test piece in the furnace was taken out.

Further, the exhaust gas from the furnace was branched, and a part of the exhaust gas was absorbed in a 2% by mass aqueous caustic soda solution to perform HCN analysis. From the analysis result of the HCN absorption liquid, the average HCN concentration in the furnace atmosphere during the propane gas injection period was 400 mg / m ³ . The weight increase of the SUS304 test piece before and after nitriding was measured and found to be 18 g / m ² . When the SUS304 test piece was cut and polished, etched with a marble liquid, and the cut surface was observed with an optical microscope, a nitride layer having a uniform thickness of 45 μm was formed. When the surface hardness of the test piece was measured at 5 points using a Vickers hardness tester, all values were distributed between Hv = 1200 and 1250.

Example 3
SUS304 plate material was set in the muffle furnace used in Example 1, NH ₃ gas and N ₂ gas were fed at a flow rate of 200 L / H, respectively, and the temperature was raised from room temperature to 550 ° C. in 75 minutes. The injection of 5 L / H of CO gas was started when the ambient temperature reached 100 ° C. (18 minutes after the start of temperature increase). After the temperature was raised to 550 ° C., the atmospheric temperature was maintained for 2 hours. At this point, CO gas injection was stopped, while NH ₃ gas and N ₂ gas were allowed to flow at 550 ° C. for an additional 4 hours to proceed nitriding, The heating was stopped and only N ₂ gas was allowed to flow and the furnace was cooled. When the ambient temperature reached 100 ° C. or lower, the test piece in the furnace was taken out.

Further, the exhaust gas from the furnace was branched, and a part of the exhaust gas was absorbed in a 2% by mass aqueous caustic soda solution to perform HCN analysis. From the analysis results of the HCN absorption liquid, the average HCN concentration in the furnace atmosphere during the CO gas injection period reached 1,000 mg / m ³ . It was 18 g / m < ² > when the weight increase before and behind the nitridation process of a SUS304 test piece was measured. When the SUS304 test piece was cut and polished, etched with a marble liquid, and the cut surface was observed with an optical microscope, a nitride layer having a uniform thickness of 45 μm was formed. When the surface hardness of the test piece was measured at 5 points using a Vickers hardness tester, all values were distributed between Hv = 1200 and 1250.

Example 4
SUS304 plate material was set in the muffle furnace used in Example 1, NH ₃ gas and N ₂ gas were fed at a flow rate of 200 L / H, respectively, and the temperature was raised from room temperature to 550 ° C. in 75 minutes. The injection of 5 L / H of C ₂ H ₄ gas was started when the ambient temperature reached 100 ° C. (18 minutes after the start of temperature increase). Maintaining the ambient temperature for 2 hours after raising the temperature to 550 ° C., at this point, the injection of C ₂ H ₄ gas is stopped, while NH ₃ gas and N ₂ gas are allowed to flow at 550 ° C. for an additional 4 hours to advance nitriding. After that, the heating was stopped and only the N ₂ gas was allowed to flow, and the furnace was cooled. When the ambient temperature became 100 ° C. or lower, the test piece in the furnace was taken out.

Further, the exhaust gas from the furnace was branched, and a part of the exhaust gas was absorbed in a 2% by mass aqueous caustic soda solution to perform HCN analysis. From the analysis result of the HCN absorption liquid, the average HCN concentration in the furnace atmosphere during the C ₂ H ₄ gas injection period reached 1200 mg / m ³ . It was 18 g / m < ² > when the weight increase before and behind the nitridation process of a SUS304 test piece was measured. When the SUS304 test piece was cut and polished, etched with a marble liquid, and the cut surface was observed with an optical microscope, a nitride layer having a uniform thickness of 45 μm was formed. When the surface hardness of the test piece was measured at 5 points using a Vickers hardness tester, all values were distributed between Hv = 1200 and 1250.

Example 5
SUS304 plate material was set in the muffle furnace used in Example 1, NH ₃ gas and N ₂ gas were fed at a flow rate of 200 L / H, respectively, and the temperature was raised from room temperature to 550 ° C. in 75 minutes. The injection of 5 L / H of C ₄ H ₁₀ gas was started when the ambient temperature reached 100 ° C. (18 minutes after the start of temperature increase). The ambient temperature is maintained for 2 hours after the temperature is raised to 550 ° C. At this point, the injection of C ₄ H ₁₀ gas is stopped, while NH ₃ gas and N ₂ gas are allowed to flow at 550 ° C. for an additional 4 hours to advance nitriding. After that, the heating was stopped and only the N ₂ gas was allowed to flow, and the furnace was cooled. When the ambient temperature became 100 ° C. or lower, the test piece in the furnace was taken out.

Further, the exhaust gas from the furnace was branched, and a part of the exhaust gas was absorbed in a 2% by mass aqueous caustic soda solution to perform HCN analysis. From the analysis result of the HCN absorption liquid, the average HCN concentration in the furnace atmosphere during the C ₄ H ₁₀ gas injection period reached 600 mg / m ³ . It was 18 g / m < ² > when the weight increase before and behind the nitridation process of a SUS304 test piece was measured. When the SUS304 test piece was cut and polished, etched with a marble liquid, and the cut surface was observed with an optical microscope, a nitride layer having a uniform thickness of 45 μm was formed. When the surface hardness of the test piece was measured at 5 points using a Vickers hardness tester, all values were distributed between Hv = 1200 and 1250.

[Comparative Example 1]
SUS304 plate material was set in the muffle furnace used in Example 1, NH ₃ gas and N ₂ gas were fed at a flow rate of 200 L / H, respectively, and the temperature was raised from room temperature to 550 ° C. in 75 minutes. After raising the temperature to 550 ° C. and maintaining the ambient temperature for 6 hours, the NH ₃ gas and the N ₂ gas are kept flowing and the nitriding is continued. Then, the heating is stopped and only the N ₂ gas is kept flowing and the furnace is cooled. When the temperature became 100 ° C. or lower, the test piece in the furnace was taken out.

The exhaust gas from the furnace was branched, and a part of the exhaust gas was absorbed in a 2% by mass aqueous caustic soda solution for HCN analysis. As a result of analysis of the HCN absorption liquid, it was confirmed that no HCN was detected and no HCN was present in the furnace atmosphere. The increase in weight of the SUS304 test piece before and after nitriding was measured and found to be 10 g / m ² . When the SUS304 test piece was cut and polished, etched with a marble solution, and the cut surface was observed with an optical microscope, a non-uniform thickness of 8 to 18 μm was formed (FIG. 3 shows a microscope with a magnification of 500 times). Show the photo). When the surface hardness of the test piece was measured at 5 points using a Vickers hardness tester, it varied greatly as Hv = 500 to 1100, and the absolute value was also lower than that of the example.

  According to the present invention, the surface passivating film of a high alloy steel member in which diffusion permeation treatment such as gas nitriding method, gas carburizing method or the like for forming a nitrided layer, carburized layer or carbonitrided layer on the surface of a metal member is difficult The HCN gas is generated in the furnace using the gas normally treated in the gas heat treatment, and the catalytic action of the surface of the metal to be treated and / or the metal furnace material is used to passivate the gas. By activating the surface of the alloy steel member, conventionally, there are no harmful effects such as deposits in the furnace, wear on the wall surface of the furnace, and exhaust gas detoxification treatment, which has been a problem in the activation treatment with halides. It is possible to provide an activation treatment method for the surface of a metal member that is useful as a pre-treatment of the diffusion permeation treatment.

The figure which shows the structure of the processing furnace used in this invention. Micrograph of cut surface of test piece of Example 1 Micrograph of cut surface of test piece of Comparative Example 1

Explanation of symbols

1: Muffle furnace 2: Outer shell 3: Heater 4: Inner vessel (retort)
5: Gas introduction pipe 6: Exhaust pipe 7: Motor 8: Fan 9: Metal jig 10: Gas guide cylinder 11: Umbrella 12: Vacuum pump 13: Exhaust gas combustion device 14: Carbon supply compound gas cylinder 15: Ammonia gas cylinder 16: Nitrogen gas cylinder 17: Hydrogen gas cylinder 18: Flow meter 19: Gas control valve

Claims (5)

A mixed gas containing a hydrocarbon compound that is a gas at normal temperature and pressure and ammonia as essential components is heated to 300 ° C. or higher in a heating furnace, and in the heated mixed gas, a metal member, a metal furnace inner wall, or a metal jig A method for activating a surface of a metal member, characterized in that HCN is generated by the catalytic action of and the generated HCN is allowed to act on a passivation film on the surface of the metal member. Hydrocarbon compounds, acetylene, ethylene, method for activating the surface of the metal member according to claim 1 which is one or more compounds selected or propane and pigs down al.   The metal member according to claim 1, wherein the metal furnace inner wall or the metal jig contains one or more metals selected from Fe, Ni, Co, Cu, Cr, Mo, Nb, V, Ti, and Zr. Surface activation method. 2. The method for activating a metal member surface according to claim 1, wherein the HCN concentration generated in the furnace is 100 mg / m ³ or more and the dew point of the atmosphere gas in the furnace is 5 ° C. or less. 2. The method for activating a surface of a metal member according to claim 1, wherein the temperature at which a mixed gas containing a hydrocarbon compound that is a gas at normal temperature and pressure and ammonia as essential components is heated in a heating furnace is 300 ° C. to 550 ° C. 3. .