JP2007031759A - Gas nitriding method for metal - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a gas nitriding method for metal capable of uniformly depositing only a nitrogen diffusion layer without depositing any nitrogen compound layer. <P>SOLUTION: The gas nitriding method of metal comprises a fluorination step of generating a fluoride film on a surface of a workpiece by heating and holding the workpiece in a gas atmosphere containing fluorine source gas, and a nitriding step of depositing a nitrided layer by heating and holding the workpiece in a gas atmosphere containing nitrogen source gas after the fluorination, and depositing a nitrided layer by allowing nitrogen to be invaded from its surface. The nitriding treatment for uniformly depositing only the nitrogen diffusion layer can be consistently performed by using a tool with at least its surface formed of non-nitriding material for a tool for arranging the workpiece in a treatment furnace. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention is a metal gas nitriding method for improving wear resistance, strength, and the like by forming a nitride layer as a nitrogen diffusion layer on the surface, and forming the nitride layer without forming a nitrogen compound layer on the surface The present invention relates to a gas nitriding method for metal.

  As a means for improving mechanical properties such as wear resistance, seizure resistance and fatigue strength of steel materials, surface hardening treatment such as carburizing treatment and nitriding treatment is used in a wide range of fields including the automobile industry. Of these, the nitriding treatment is performed at a temperature lower than the transformation point of steel, so that the generation of strain is relatively small. Therefore, for parts that require accuracy in various shapes, mainly sliding parts. But it has been applied.

  At this time, the nitrogen compound layer formed on the surface of the steel material has high hardness, and its reactivity with the sliding metal material is lower than that of the base material, so it is particularly excellent in wear resistance and seizure resistance, but it has ductility. Therefore, when a high load, a bending load, a local load, or the like is applied, cracking or peeling occurs and the crack is likely to start. For this reason, it may be necessary to form only the nitrogen diffusion layer without forming the nitrogen compound layer in applications that require particularly high fatigue strength. Nowadays, such demands are increasing as machine parts become smaller and more advanced.

  Moreover, when coating a hard film | membrane by PVD method etc., the method of applying a nitriding process as the pre-processing has also been used. This is because the base metal of the coating film is cured by nitriding to increase the strength, thereby suppressing the deformation of the base metal due to the load received from the surface and improving the peeling resistance of the coating film. . At this time, when the above brittle nitrogen compound layer is formed on the surface of the metal as a base by nitriding, it becomes clear that the adhesion strength of the coating film is remarkably lowered, and on the contrary, the wear resistance is lowered. Yes. Therefore, a nitriding method for forming only the nitrogen diffusion layer is also required from such a surface.

At this time, the normally gaseous nitriding method is excellent in productivity, an attempt to implement the nitriding treatment to obtain only diffusion layer, or reduce the supply amount to the NH ₃ gas concentration and the NH ₃ gas in the processing furnace, the process It is necessary to drastically reduce the amount of NH ₃ decomposed in the processing furnace by extremely lowering the temperature. However, if the decomposition of NH ₃ is suppressed so that a nitrogen compound layer cannot be formed, the oxide film formed on the steel material surface cannot be sufficiently reduced, so that it becomes very difficult to obtain a uniform nitride layer. In particular, the thickness of the nitrided layer is likely to vary due to the difference in material and surface processing state, and it is difficult to stably form a thick diffusion layer alone. On the other hand, there is a method of decomposing a nitrogen compound layer by heating and maintaining at a high temperature in N ₂ or N _{2 to which a} small amount of H ₂ is added, for example, after performing the nitriding treatment. Since nitrogen is released from the outermost surface, a tensile stress is generated on the surface portion and the strength is lowered. Therefore, there are many adverse effects particularly in use applications that require fatigue strength.

  In some mechanical parts such as gears and crankshafts where high surface pressure is applied and fatigue strength is required, a method of removing the nitrogen compound layer by mechanical polishing after nitriding treatment is also implemented. However, although compressive stress that contributes to an increase in fatigue strength occurs on the surface after the nitriding treatment, the outermost surface changes to tensile stress due to polishing, so this method is also concerned about a decrease in fatigue strength. The In addition, since polishing may not be possible depending on cost problems and part shapes, it is not always an effective method.

  In addition, a method of physically removing the compound layer formed on the surface by applying strong shot blasting after nitriding is also used, but the applicable shape of the object to be processed is extremely limited. It is very difficult to stably control shot blasting conditions that can completely and uniformly remove a hard compound layer. Moreover, this method is not an effective means because it causes a cost increase and blasting cannot be applied depending on the use of the part.

On the other hand, in order to solve such a problem, a vacuum processing apparatus is used, the oxide film on the surface is removed by ion bombardment treatment, and subsequently, the ionized low concentration N is diffused in the steel material, thereby uniformizing only the diffusion layer A resulting nitriding method is disclosed.
Japanese Patent No. 2998746 JP 2004-292934 A

However, since the nitriding method as described above needs to be vacuum-processed, there are restrictions on the size and shape of the object to be processed, and the cost cannot be increased because a large amount of goods cannot be processed at once. There is a problem. In this respect, gas nitriding that can process a large number of products at one time regardless of the shape of the object to be processed is excellent in practical use including cost, but only a diffusion layer is generated without forming a nitrogen compound layer. In order to perform nitriding, it is necessary to suppress the concentration and decomposition of NH ₃ which is a nitriding gas.

However, an attempt to process by suppressing the decomposition of NH _3, not only the amount of active N generated by the decomposition of NH ₃ is decreased, from reducing the amount of generation of active H with reducing action, The oxide film formed on the surface of the steel material cannot be sufficiently reduced, and it becomes extremely difficult to form a uniform nitride layer. Therefore, as a pretreatment for the nitriding treatment, nitriding is performed by removing the oxide film and activating the surface to perform nitriding.

Here, with respect to a jig for placing a processed product in a furnace when performing gas nitriding, conventionally, austenitic stainless steel represented by SUS304 that is difficult to be nitrided, nickel-based represented by Inconel, etc. Heat resistant alloys have been used. However, when a fluorination treatment is performed as a pretreatment for the nitriding treatment, a nitriding reaction occurs even with the above-mentioned non-nitriding material, and further deterioration of the surface proceeds due to repeated use. . At this time, in order to stably perform the nitriding treatment without forming the nitrogen compound layer, it is necessary not only to suppress the decomposition rate of NH ₃ but also to maintain an appropriate decomposition rate according to the object to be processed. When the jig surface deteriorates, the catalytic action decreases and it becomes extremely difficult to maintain an appropriate decomposition rate. Adsorption of moisture in the air adversely affects the fluorination atmosphere and nitridation atmosphere. There was a problem that fluoridation and nitridation could not be performed.

In addition, when the fluorination treatment was performed as a pretreatment for the nitriding treatment, the concentration of NH ₃ that is a nitriding gas was simply suppressed by performing nitriding without forming the nitrogen compound layer and generating only the diffusion layer. When the treatment is performed, the surface is activated by the fluorination treatment, so that the N potential variation in the furnace with respect to the activated surface appears remarkably, and a sufficient diffusion layer cannot be obtained. There is a problem that a stable nitrided layer cannot be obtained.

  The present invention has been made in view of such circumstances, and a metal that can stably form only a diffusion layer without forming a nitrogen compound layer only by gas nitriding treatment regardless of the shape of the object to be processed. An object of the present invention is to provide a gas nitriding method.

  In order to achieve the above object, a first metal gas nitriding method of the present invention is a fluorination treatment step in which an object to be treated is heated and held in a gas atmosphere containing a fluorine source gas to form a fluoride film on the surface. And a nitriding process for forming a nitrided layer by heating and holding the object to be processed in a gas atmosphere containing a nitrogen source gas and injecting nitrogen from the surface after the fluorination process Then, the gist is to use a jig whose surface is made of a non-nitriding material at least as a jig for placing the workpiece in the processing furnace.

In order to achieve the above object, the second metal gas nitriding method of the present invention is a fluorination method in which a workpiece is heated and held in a gas atmosphere containing a fluorine source gas to form a fluoride film on the surface thereof. And a nitriding treatment step of forming a nitride layer by heating and holding the object to be treated in a gas atmosphere containing a nitrogen source gas and allowing nitrogen to enter from the surface after the fluorination treatment. The gist is a nitriding method, wherein the nitrogen source gas contains NH ₃ and H ₂ , and the volume ratio of NH ₃ / H ₂ in the nitrogen source gas is set to 0.05 or more and 5 or less.

In other words, the first gas nitriding method of the present invention can maintain a stable decomposition rate of NH ₃ over a long period of time by using a jig whose surface is made of a non-nitriding material as a jig. It realizes production processing. Thus, by using a jig whose surface is made of a non-nitriding material at least as a jig, even when fluorination treatment is performed as a pretreatment for the nitriding treatment process, the jig undergoes a nitriding reaction. It does not occur, and surface deterioration does not progress even after repeated use. For this reason, there is almost no decrease in the catalytic action due to deterioration of the jig surface and adsorption of moisture to the deteriorated portion. Therefore, it is possible to suppress the decomposition rate of NH ₃ and maintain an appropriate decomposition rate according to the object to be processed, eliminate the adverse effect on the fluorination atmosphere and the nitriding atmosphere, and stably decompose NH ₃ over a long period of time. The rate can be maintained. Thus, stable nitriding treatment can be realized under conditions where the nitrogen compound layer is not formed, and only the diffusion layer can be stably formed without forming the nitrogen compound layer only by gas nitriding treatment.

Further, in the second gas nitriding method of the present invention, the nitrogen source gas contains NH ₃ and H ₂ , and the volume ratio of NH ₃ / H ₂ in the nitrogen source gas is 0.05 or more and 5 or less, After the fluorination treatment, nitriding treatment is performed in a gas atmosphere containing NH ₃ and H ₂ , and the composition ratio is controlled to form only a uniform diffusion layer without forming a nitrogen compound layer. . That is, when only the diffusion layer is formed by gas nitriding as described above, the oxide film formed on the surface of the object to be processed becomes a major obstacle. Since the film can be removed, for example, even when the supply amount of NH ₃ is very small, N generated by the decomposition of NH ₃ can smoothly enter the object to be processed, and a uniform nitride layer can be easily formed. It can be obtained.

In this case, a method of suppressing the decomposition amount of NH _{3 to} an extent that a nitrogen compound layer cannot be formed is important. As the method, in the present invention, a method of adding H ₂ to the atmosphere in addition to NH ₃ is used. The reason why the decomposition rate of NH ₃ can be suppressed by adding H ₂ is that active N is generated by the decomposition of NH ₃ and penetrates into the steel, as shown in the following equilibrium reaction formula. In this reaction, H ₂ is also generated simultaneously with the generation of active N. Therefore, when H ₂ is added to the atmosphere, the reaction hardly proceeds to the right side, and NH ₃ It is because decomposition | disassembly of is suppressed.
2NH ₃ = 2N + 3H ₂

The reason why the above method is used is that when the NH ₃ concentration is simply decreased, the variation in the processing furnace becomes large. This is because in the case where a sufficient amount of NH ₃ gas is not supplied, This is probably because the supplied NH ₃ decomposes before it reaches the entire furnace, resulting in variations in gas concentration in the furnace. Therefore, by supplying a sufficient amount of NH ₃ gas to reach the furnace and suppressing decomposition with H ₂ gas, variations in the processing furnace can be minimized. In addition, this method is sufficiently effective in a temperature range in which normal nitriding is performed at about 300 to 600 ° C. only by adding H ₂ gas at the same time as NH ₃ gas. Therefore, decomposition of NH ₃ gas in gas nitriding is suppressed. This is an extremely effective method.

In addition, as described above, if the decomposition rate is suppressed so that a nitrogen compound layer cannot be formed by a normal gas nitriding treatment, the oxide film formed on the surface of the steel material cannot be sufficiently reduced, thereby obtaining a uniform nitrided layer. However, the surface where the oxide is removed by the fluorination treatment to form the fluoride easily reduces the fluoride covering the surface even in a reducing atmosphere where the decomposition amount of NH ₃ is very small. Since it is activated by being removed, nitrogen can easily enter by the above method, and only a uniform diffusion layer can be formed.

Thus, after fluorination treatment, a nitriding gas containing NH ₃ as the main component and containing H ₂ is used, the nitrogen source gas contains NH ₃ and H ₂ , and NH ₃ / H in the nitrogen source gas is used. The volume ratio of ₂ is 0.05 or more and 5 or less, and the effect of the oxide film on the metal surface can be removed by heating and holding the workpiece in an atmosphere in which the decomposition of NH ₃ is controlled by the treatment temperature and gas composition. At the same time, it is possible to allow nitrogen to penetrate to the highest nitrogen concentration within a range in which the material to be treated does not form a nitrogen compound layer only by controlling the gas composition. For this reason, it is possible to stably form a uniform nitrogen diffusion layer without unevenness on the surface of the object to be processed. Furthermore, it is possible to carry out mass production processing even for complex shapes, large dies, etc., taking advantage of processing based on gas nitriding.

In the first gas nitriding method, the nitrogen source gas may contain NH ₃ and H ₂ , and the volume ratio of NH ₃ / H ₂ in the nitrogen source gas may be 0.05 or more and 5 or less.

  In the first gas nitriding method, the non-nitriding material may be a metal material containing nickel as a main component and not containing iron.

  In the first gas nitriding method, the non-nitriding material may be ceramic.

  Next, the best mode for carrying out the present invention will be described.

  The gas nitriding method of the present invention includes (1) a fluorination treatment step in which an object to be treated is heated and held in a gas atmosphere containing a fluorine source gas to form a fluoride film on the surface thereof, and (2) the fluorination treatment. Thereafter, a nitriding treatment step is performed in which the object to be treated is heated and held in a gas atmosphere containing a nitrogen source gas, and nitrogen is introduced from the surface to form a nitride layer.

A jig whose surface is made of a non-nitriding material is used as a jig for placing the workpiece in the processing furnace. As the nitrogen source gas, a gas containing NH ₃ and H _{2 and} having a volume ratio of NH ₃ / H ₂ in the nitrogen source gas of 0.05 to 5 is used.

In this way, the object to be processed is placed on a jig made of a non-nitriding material, and the object to be processed placed on the jig made of the non-nitriding material is heated in a gas atmosphere containing a fluorine source gas. A fluorination treatment step for holding and generating a fluoride film on the surface, and a nitriding gas mainly composed of NH ₃ and H ₂ in an atmosphere in which the gas ratio is optimized within a range in which the compound layer is not formed A nitriding process is performed by heating and holding an object nitrided layer placed on a jig made of a non-nitriding material at an optimum temperature in order to obtain a target nitrided layer.

  The metal targeted by the nitriding method of the present invention is not particularly limited as long as it is a material in which N can be dissolved, such as carbon steel and alloy steel, as well as stainless steel and nickel base alloy. Can be applied.

In addition, the jig material targeted by the present invention is not particularly limited as long as it is a non-nitriding material. For example, if it is a metal material, it may be a nickel-based alloy containing no iron, pure nickel, or non-metal. Examples thereof include ceramic materials such as alumina and zirconia. In particular, pure nickel is somewhat low in strength, but it is easy to produce jigs of various shapes suitable for the object to be processed. Furthermore, the surface has a large effect as a catalyst when NH ₃ decomposes, and nitride is formed. Since the change in the surface state including that which does not occur is extremely small, stable catalytic action can be maintained over a long period of time, so it is considered to be the most suitable material.

  The jig material only needs to be formed of a non-nitriding material at least on the surface. For example, a non-nitriding metal plating layer such as nickel plating, nickel phosphorus plating, gold plating, or platinum plating may be formed. Moreover, it can also comprise by forming the coating layer of a non-nitriding metal material and a ceramic material by techniques, such as thermal spraying.

(1) Fluorination treatment step In the fluorination treatment step, first, the object to be treated is heated and held in a gas atmosphere containing a fluorine source gas to remove oxides formed on the surface of the object to be treated, and to form a fluoride film. Let it form.

As the fluorine source gas in the fluorination treatment, a fluorine-based gas (fluorine compound gas or fluorine gas) that is a halogen-based substance having a stronger affinity than oxygen with respect to Fe or Cr that is a base material component forming an oxide. Containing gas). Examples of the fluorine-based gas include a fluorine compound, for example, a gas containing a main component gas composed of a fluorine compound gas such as NF ₃ , BF ₃ , CF ₄ , SF ₆ or F ₂ gas. Normally, this main component gas is diluted with a diluent gas such as N _{2 and} used as a fluorine-based gas.

Of the main component gases used for these fluorine-based gases, NF ₃ is the most excellent in terms of reactivity, handling properties, etc., and is practical. The object to be treated under the fluorine gas atmosphere, for example 200 to 580 ° C. in a _{N 2} atmosphere containing NF _3, preferably is NF ₃ is decomposed by holding 10 to 60 minutes in the temperature range of 200 to 500 ° C. As a result, active F is generated and substituted with the oxide on the surface of the object to be processed, so that a fluoride more stable than the oxide is formed. Since this fluoride is easily reduced when exposed to a reducing atmosphere, a surface without an oxide or the like that becomes a barrier when N enters and dissolves by this method appears.

  Therefore, it is possible to easily obtain a uniform nitride layer by performing nitriding after fluorination. At this time, the concentration of the fluorine compound or fluorine in the fluorine-based gas atmosphere is preferably 1000 to 100000 ppm.

  The treatment temperature for the fluorination treatment is preferably 200 to 580 ° C, more preferably 200 to 500 ° C.

  By the above method, in the fluorination treatment step, the activated fluorine atoms destroy and remove the inorganic and organic contaminants adhering to the surface of the object to be treated, and the surface is purified, and the fluorine atoms are oxidized. It reacts with the film to change to a fluoride film, and the surface of the object to be processed is covered and protected by the fluoride film.

  And since this fluoride film is decomposed and disappeared by the nitriding process in the subsequent step, the surface of the object to be processed becomes activated, and N quickly penetrates and diffuses into the activated surface, A uniform and deep diffusion layer is formed.

  With respect to this fluorination treatment step, nitriding treatment can be carried out using the same furnace as it is, and for example, a method in which a fluorination treatment chamber and a nitridation treatment chamber are separately carried out in a continuous furnace is also possible.

(2) Nitriding treatment step After the fluorination treatment step, the object to be treated is heated and held in a nitriding gas atmosphere mainly composed of NH ₃ and H ₂ , and nitrogen is introduced from the surface thereof to form a nitrogen compound layer. A nitriding treatment is performed to form only a nitrogen diffusion layer without forming.

As the atmosphere gas for nitriding, a mixed gas containing NH ₃ as a nitriding gas and H ₂ for suppressing decomposition thereof is used. The nitriding gas may be a non-nitriding gas such as N ₂ as another component.

Regarding the concentrations of NH ₃ and H ₂ , it is necessary to apply a concentration at which the nitrogen compound layer is not formed on the object to be processed. However, the concentration varies depending on the nitriding temperature, the material of the object to be processed, the surface processing state, and the like. The optimum density setting is appropriately performed.

The NH ₃ decomposition is controlled by the volume ratio of NH ₃ / H ₂ in the nitrogen source gas. The capacity ratio of NH ₃ / H ₂ is, for example, 10/20 = 0.5 if the atmosphere gas contains 10% NH _{3 and} 20% H ₂ . The appropriate ratio varies for the above reasons, but the ratio of H _{2 to} NH ₃ is preferably 0.05 or more and 5 or less. At this time, if the ratio is less than 0.05, the NH ₃ concentration is too low, and therefore, there is a high possibility that a variation in the thickness of the nitride layer particularly in various places in the furnace tends to occur and a stable mass production process cannot be realized. . On the other hand, when the ratio exceeds 5, the effect of suppressing the decomposition of NH ₃ by H ₂ cannot be sufficiently obtained.

When applying the above ratio, a non-nitriding gas such as N ₂ can also be mixed, but the gas is preferably a non-oxidizing gas. For example, an oxidizing gas such as O ₂ or CO ₂ promotes the decomposition of NH ₃ by reaction with NH ₃ or H ₂ , making it difficult to stably suppress the decomposition amount of NH ₃ . Accordingly NH ₃ and H ₂ gases other than the NH ₃ and H ₂ at a temperature range of applying either reactive with no N ₂ or the like of the gas is preferably a reducing gas such as CO.

At this time, since the ease of forming the nitrogen compound layer varies depending on the type of steel material and the processing state of the surface, etc., the above processing temperature and gas composition can be determined with sufficient consideration, but the above nitriding treatment The heating temperature in the process is preferably set to 350 to 600 ° C. At this time, if the temperature for nitriding is less than 350 ° C., the decomposition of NH ₃ is small and it is easy to suppress the decomposition itself, but in order to secure a sufficient thickness of the cured layer, the processing time becomes too long and the productivity is increased. This is because it gets worse. Conversely, when the nitriding temperature exceeds 600 ° C., the decomposition rate of NH ₃ is high and the suppression effect by H ₂ does not function sufficiently.

  The nitriding method of the present invention can be performed by, for example, a heat treatment furnace as shown in FIG.

  This heat treatment furnace includes a process gas supply port 3 for supplying a process gas such as a nitriding gas to a furnace body 1 having a heater 2, an agitation fan 6 for agitating the process gas introduced into the furnace, and an agitation A fan motor 5 and an exhaust gas exhaust port 4 for exhausting exhaust gas are provided.

The process gas supply port 3 is hydrogen gas cylinder 14, an ammonia gas cylinder 13, a nitrogen gas cylinder 12, NF ₃ gas cylinder 11 is connected, so as to supply hydrogen gas as the process gas, respectively, ammonia gas, nitrogen gas, NF ₃ gas ing. In the figure, 7 is a gas supply line main valve, 8 is a pneumatically operated valve, 9 is a pressure reducing valve, 10 is a gas flow meter, and 15 is an exhaust gas abatement device.

Using the above heat treatment furnace, for example, NF ₃ gas and nitrogen gas are supplied to perform fluorination treatment, and nitriding treatment is performed by supplying a nitriding gas containing NH ₃ and H ₂ as follows.

That is, first, the workpiece is placed in a furnace using a jig made of a non-nitriding material such as a pure nickel material, and the atmosphere in the furnace is sufficiently N ₂ or the like to prevent oxidation during temperature rise. After the substitution, the mixture is heated to 200 to 580 ° C, preferably 200 to 500 ° C. When the in-furnace object to be treated is soaked, a gas containing NF ₃ or the like is introduced into the furnace to perform a fluorination treatment for forming a fluoride film on the surface of the object to be treated.

Next, the supply of the NF ₃ gas is stopped, a nitriding gas containing NH ₃ and H ₂ is introduced into a furnace heated to 350 to 600 ° C., and the nitriding treatment is performed on the workpiece to which the fluorination treatment is performed. Do.

At this time, the nitriding gas is introduced into the furnace with the NH ₃ concentration adjusted so that the NH ₃ concentration is 2 volume% or more and 50 volume% or less, preferably 3 volume% or more and 30 volume% or less. Is preferred. This is because when the NH ₃ concentration is less than 2% by volume, the variation in the furnace for forming the nitride layer becomes large. Further, in the range where the NH ₃ concentration exceeds 50% by volume, for example, when the nitriding temperature is low, it is possible to form a nitride layer without forming a nitrogen compound layer, but in addition to the amount of NH ₃ used, NH ₃ This is because H ₂ for suppressing decomposition of H ₂ needs to have a high concentration, so that the amount of H ₂ used is increased, resulting in an increase in cost required for the nitriding treatment. Therefore, NH ₃ concentration of 2 volume% to 50 volume% or less, preferably 30 volume% or less than 3% by volume, the proper _NH 3 / _{H 2} ratio is added an appropriate amount of _{H 2} in order to suppress the decomposition of the NH ₃ In addition, by diluting it with a gas such as N _{2 and} using it as a nitriding gas, it is possible to perform a process at a reduced cost as much as possible.

At this time, it is preferable that the nitriding gas is always introduced into the furnace during the nitriding process, and about 50 to 90% by volume of the introduced NH ₃ amount is discharged from the furnace, and the remainder is adjusted to contribute to nitriding. . When the amount of NH ₃ discharged from the furnace is increased, the amount of N that contributes to nitriding is insufficient, and the variation in the furnace for forming the nitride layer increases, and conversely, when the amount of NH ₃ discharged from the furnace is decreased. This is because the amount of N contributing to nitriding becomes excessive, and a nitrogen compound layer may be formed.

Even when the amount of decomposition of NH ₃ is very small, the fluoride formed on the surface by the fluorination treatment is easily reduced and removed from the surface by active H generated by the decomposition. This exposes the active metal surface. Then, N generated along with H by decomposition of NH ₃ enters and diffuses into the steel material on the active metal surface to form a nitride layer. At this time, the active metal surface from which the fluoride film has been removed not only facilitates the entry of N, but also facilitates the formation of a nitrogen compound layer, thereby sufficiently reducing the decomposition rate of NH _3. It is necessary to keep.

At this time, in the case not implementing the above fluorination treatment, for example a low temperature the process temperature to 400 ° C. stand or subjected to nitriding treatment to suppress the decomposition of NH ₃ in such a way as adding of H ₂ to NH ₃ However, since the amount of decomposition of NH ₃ is small, the oxide film on the surface cannot be sufficiently reduced, and when the decomposition rate of NH ₃ is suppressed so as not to form the nitrogen compound layer, a uniform nitride layer should be formed. Is extremely difficult.

In addition, when a non-nitriding material is not used for the jig, even if a non-nitriding material such as Inconel material is used, the surface is nitrided when fluorination is performed. Progresses. The surface of the nitrided jig changes its NH ₃ decomposition rate, and the surface becomes rough and the amount of gas adsorbed increases. This ensures stable treatment when used repeatedly. It becomes impossible to do. For this reason, in this invention, the jig | tool comprised from the non-nitriding material is used. Among non-nitriding materials, in addition to the ease of processing when making jigs, not only nitrides but also pure nickel materials that are difficult to form fluoride in the temperature range where fluorination treatment is performed It is more preferable to use as.

As described above, the present invention uses a non-nitriding jig, performs fluorination, and further performs nitriding in a state where the decomposition rate of NH ₃ is controlled to be low by H ₂ . Only the nitrogen diffusion layer can be formed uniformly without forming a brittle nitrogen compound layer on the surface of the processed material.

  Next, examples will be described.

  Using a material tempered and tempered with high-speed tool steel SKH51 material and tempered to hardness HRC63, prepare a sample whose surface is polished with emery paper # 2000 in order to accurately check the presence or absence of the compound layer after nitriding treatment, The sample was placed on a net-like jig made of a pure nickel material, placed in the furnace 1 shown in FIG. 1, and subjected to fluorination treatment and nitridation treatment under the following conditions.

◆ Example (a)
Jig: Pure nickel fluoride: The temperature was raised to 320 ° C., NF ₃ gas was introduced into the furnace and held for 15 minutes.
Nitridation: After the above fluorination treatment, the temperature was raised to 480 ° C., and the NH ₃ gas, H ₂ gas, and N ₂ gas were held for 2 hours in a gas atmosphere in a ratio of 1: 2: 7.
◆ Comparative example (b)
Jig: Pure nickel fluorination: Same conditions as in Example (a) Nitrification: After the above fluorination treatment, the temperature is raised to 480 ° C., then NH ₃ gas and N ₂ gas are in a gas atmosphere with a ratio of 1: 9 Hold for 2 hr.
◆ Comparative example (c)
Jig: Pure nickel fluoride: None Nitrification: After the temperature was raised to 480 ° C., NH ₃ gas and N ₂ gas were held for 2 hours in a gas atmosphere in a ratio of 1: 9.

  A photograph of these samples observed by corroding the cross section with a 5% nitric acid alcohol solution is shown in FIG. Moreover, the result of having measured the compound layer thickness, the diffused layer thickness, and the surface hardness is shown in FIG.

  In Example (a), the nitrogen compound layer is not formed on the surface, only the diffusion layer is uniformly formed, and the surface hardness is as high as 1300 Hv or higher.

In the comparative example (b), although a uniform nitride layer is obtained due to the effect of the fluorination treatment, the NH ₃ decomposition rate by H ₂ is not controlled. _It can be seen that a nitrogen compound layer of about 3 μm, which is not corroded by the above-mentioned corrosive liquid, is formed on the surface even though the concentration of ₃ is lowered to 10%.

  In the comparative example (c), since the fluorination treatment is not performed, the diffusion layer depth varies widely and the surface hardness is considerably low even though a nitrogen compound layer of about 1 μm is formed on the surface. There is.

A sample was prepared by quenching and tempering a cold tool steel SKD11 material and tempering it to a hardness of HRC60, and polishing the surface with emery paper # 1000. The sample was placed in the furnace 1 shown in FIG. Fluorination treatment and nitridation treatment were performed under the following conditions.
◆ Example (d)
Jig: A net-like jig made of pure nickel material and a jig made of 5 mm diameter holes formed at intervals of 10 mm on a plate made of alumina.
Fluorination: The temperature was raised to 350 ° C., NF ₃ gas was introduced into the furnace and held for 15 minutes.
Nitridation: After the fluorination, the temperature was raised to 570 ° C., and NH ₃ gas, H ₂ gas and N ₂ gas were held for 3 hours in a gas atmosphere in a ratio of 1: 5: 4.
◆ Comparative example (e)
Jig: Ni-based heat-resistant alloy containing 70% or more of Ni and about 16% of Cr: nitridation same as Example (d): same as Example (d) ◆ Comparative example (f)
Jig: Austenitic stainless steel SUS304
Fluorination: Same as Example (d) Nitrification: Same as Example (d)

  In order to evaluate the stability of the treatment, the above fluorination treatment and nitriding treatment were repeatedly carried out, and the surface hardness of the sample after the nitriding treatment at the number of repetitions of 1, 5, and 10 was measured. The transition is shown in FIG.

In Example (d), a pure nickel material that does not cause a nitriding reaction and a ceramic material such as alumina are used as jig materials, so that stable hardness is obtained even when nitriding is repeated, and only the diffusion layer is uniform. It was also confirmed that it was formed.
In Comparative Example (e), the surface hardness gradually decreases when the number of treatments increases.
In the comparative example (f), a great decrease in hardness is observed.

  When the cross section of the jig after 10 times of use was examined, it was confirmed that a nitride layer of about 5 μm was formed with the nickel-based heat-resistant alloy and about 150 μm with SUS304. In particular, in SUS304, it was confirmed that an infinite number of cracks considered to be grain boundary cracks occurred from the surface to a depth of about 20 μm. This indicates that the nitriding reaction proceeds not only in stainless steel but also in nickel-based heat-resistant alloys containing 70% or more of Ni.

Thus, when the surface of the jig undergoes nitridation, the catalytic action of the surface decreases, the decomposition rate of NH ₃ decreases, and cracks occur in the nitrided layer due to repeated use, particularly in the atmosphere. When it is necessary to highly control the decomposition rate of NH ₃ so as not to form a nitrogen compound layer by adversely affecting the treatment by increasing the amount of adsorption of gas such as water vapor when exposed It is considered that a stable hardness cannot be obtained.

A rotating bend test piece using maraging steel (Fe-18Ni-9Co-5Mo-0.6Ti-0.1Al) was prepared, subjected to solution treatment at 850 ° C, and then subjected to aging treatment at 480 ° C for 2 hours. Samples were prepared and subjected to fluorination treatment and nitriding treatment under the following conditions.
◆ Example (g)
Jig: A jig made of pure nickel.
Fluorination: The temperature was raised to 480 ° C., and NF ₃ gas was introduced into the furnace and held for 10 minutes.
Nitridation: After fluorination, NH ₃ gas, H ₂ gas, and N ₂ gas were held for 1 hr in a gas atmosphere in a ratio of 3: 2: 5 while keeping the temperature at 480 ° C.
◆ Comparative example (h)
Jig: A jig made of pure nickel.
Fluorination: Same nitridation as in Example (g): Following fluorination, the temperature was soaked to 480 ° C., and NH ₃ gas and N ₂ gas were held for 1 hr in a gas atmosphere at a ratio of 3: 7.

  FIG. 5 shows the results of a bending fatigue test performed on the test pieces of the above Example (g) and Comparative Example (h) and only the aging test pieces.

  The fatigue strength of the example (g) is clearly improved compared to the comparative example (h) and the aging-only test piece. It can be seen that by forming only the diffusion layer uniformly without forming the nitrogen compound layer, it is very effective as a means for improving the fatigue strength when a particularly large bending stress is applied.

  In the comparative example (h), the fatigue strength on the high stress side is particularly low. From the cross-sectional observation result of the test piece, it is confirmed that the compound layer is formed on the surface about 2 μm. Especially when the stress to be applied is high, the fragile compound layer is cracked by bending stress and the crack It is considered that the fracture due to the progress of the inside has occurred more clearly.

  The high-speed tool steel SKH51 material was tempered to a quenching / tempering hardness HRC63. A sample of 20 × 20 × 7 mm was cut out from this material, a sample having a 20 × 20 mm surface polished to emery paper # 2000 was prepared, and fluorination treatment and nitridation treatment were performed under the following conditions.

◆ Example (i)
Jig: A reticulated jig made of pure nickel material Fluoride: The temperature was raised to 300 ° C. and NF ₃ gas was introduced into the furnace and held for 20 minutes.
Nitridation: The temperature was raised to 500 ° C., and NH ₃ gas, H ₂ gas, and N ₂ gas were held for 3 hours in a gas atmosphere in a ratio of 1: 3: 6.
◆ Comparative example (j)
Jig: A reticulated jig made of pure nickel material: Nitrogen same as in Example (i): After heating to 500 ° C., a gas atmosphere in which NH ₃ gas and N ₂ gas have a ratio of 1: 9 For 3 hours.

  The 20 × 20 mm surface of each test piece was mirror-polished by buffing, and then a TiN film was coated to a thickness of about 4 μm by the PVD method.

  For these test pieces, in order to evaluate the adhesion strength of the TiN film, the result of measuring the critical load until the coating film is broken using a scratch tester equipped with a diamond indenter is shown in FIG.

  In Example (i), the surface of the test piece that was not subjected to nitriding was mirror-polished and then the TiN coating was similarly improved over the load bearing value.

  Comparative Example (j) shows a very low load-bearing value, but this is because a nitrogen compound layer of about 5 μm is formed on the surface, so even if the surface is polished, the adhesion of the coating film It is thought that the nature is very low.

  Thus, since only a stable diffusion layer can be formed by a method that does not use a vacuum processing apparatus, even with a large mold that could not be processed by a method using a conventional ion nitriding furnace, By using the nitriding method of the present invention, it is possible to perform nitriding before coating.

As described above, the first gas nitriding method of the present invention maintains a stable NH ₃ decomposition rate over a long period of time by using a jig whose surface is made of a non-nitriding material as a jig. It is possible to achieve stable production processing. Thus, by using a jig whose surface is made of a non-nitriding material at least as a jig, even when fluorination treatment is performed as a pretreatment for the nitriding treatment process, the jig undergoes a nitriding reaction. It does not occur, and surface deterioration does not progress even after repeated use. For this reason, there is almost no decrease in the catalytic action due to deterioration of the jig surface and adsorption of moisture to the deteriorated portion. Therefore, it is possible to suppress the decomposition rate of NH ₃ and maintain an appropriate decomposition rate according to the object to be processed, eliminate the adverse effect on the fluorination atmosphere and the nitriding atmosphere, and stably decompose NH ₃ over a long period of time. The rate can be maintained. Thus, stable nitriding treatment can be realized under conditions where the nitrogen compound layer is not formed, and only the diffusion layer can be stably formed without forming the nitrogen compound layer only by gas nitriding treatment.

Further, in the second gas nitriding method of the present invention, the nitrogen source gas contains NH ₃ and H ₂ , and the volume ratio of NH ₃ / H ₂ in the nitrogen source gas is set to 0.05 or more and 5 or less. After the nitriding treatment, nitriding treatment is performed in a gas atmosphere containing NH ₃ and H ₂ and the composition ratio is controlled so that only the uniform diffusion layer is formed without forming the nitrogen compound layer. That is, when only the diffusion layer is formed by gas nitriding as described above, the oxide film formed on the surface of the object to be processed becomes a major obstacle. Since the film can be removed, for example, even when the supply amount of NH ₃ is very small, N generated by the decomposition of NH ₃ can smoothly enter the object to be processed, and a uniform nitride layer can be easily formed. It can be obtained.

  The present invention can be used for nitriding of steel materials, and particularly when the surface is subjected to a high tensile stress or bending stress, or when a coating treatment such as a PVD method is performed after the nitriding treatment, the surface is brittle. The present invention can be applied to application fields where it is necessary not to form a nitrogen compound layer.

It is sectional drawing which shows an example of a processing furnace. It is a cross-sectional observation result of the test piece made from SKH51. It is a compound layer thickness of a SKH51 test piece, a hardened layer thickness, and a surface hardness measurement result. It is a surface hardness measurement result of the test piece made from SKD11. It is a bending fatigue strength test result of a maraging steel test piece. It is a scratch test result of the SKH test piece coated with a TiN film.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Furnace body 2 Heating heater 3 Process gas supply port 4 Exhaust gas discharge port 5 Stirrer fan motor 6 Stirrer fan 7 Gas supply line main valve 8 Pneumatic pressure valve 9 Pressure reducing valve 10 Gas flow meter 11 NF ₃ Gas cylinder 12 Nitrogen gas cylinder 13 Ammonia gas cylinder 14 Hydrogen gas cylinder 15 Exhaust gas abatement system

Claims (5)

  A fluorination treatment step of heating and holding the object to be treated in a gas atmosphere containing a fluorine source gas to form a fluoride film on the surface thereof; and after the fluorination treatment, the object to be treated in a gas atmosphere containing a nitrogen source gas. A nitriding method of metal for performing a nitriding treatment step of forming a nitrided layer by intruding nitrogen from the surface of the object to be heated and forming a nitrided layer, as a jig for placing the object to be processed in a processing furnace A metal gas nitriding method characterized by using a jig having at least a surface thereof made of a non-nitriding material. The metal gas nitriding method according to claim 1, wherein the nitrogen source gas contains NH ₃ and H ₂ , and a volume ratio of NH ₃ / H ₂ in the nitrogen source gas is 0.05 or more and 5 or less.   3. The metal gas nitriding method according to claim 1, wherein the non-nitriding material is a metal material containing nickel as a main component and not containing iron.   The metal gas nitriding method according to claim 1, wherein the non-nitriding material is ceramic. A fluorination treatment step of heating and holding the object to be treated in a gas atmosphere containing a fluorine source gas to form a fluoride film on the surface thereof; and after the fluorination treatment, the object to be treated in a gas atmosphere containing a nitrogen source gas. A nitriding method of a metal for performing a nitriding treatment step of forming a nitrided layer by intruding nitrogen from the surface by heating and holding a treatment object, wherein the nitrogen source gas contains NH ₃ and H ₂ , and the nitrogen A metal gas nitriding method, wherein a volume ratio of NH ₃ / H ₂ in a source gas is set to 0.05 or more and 5 or less.