JP5883727B2 - Gas nitriding and gas soft nitriding methods - Google Patents

Description

  The present invention provides a gas nitriding method, a gas soft nitriding method, and a gas nitriding apparatus that can be suitably used for these processing methods, capable of high-precision atmosphere control by a simple system and capable of reducing equipment costs and operating costs. The present invention relates to a gas soft nitriding apparatus.

  Gas nitriding treatment is a surface hardening treatment that improves strength, wear resistance, friction resistance, and galling resistance by generating a compound of nitrogen and an alloy component such as Al or Cr in the diffusion layer on the surface of the component. The processing is performed by supplying a mixed gas of ammonia gas and nitrogen gas or a mixed gas of ammonia gas and ammonia decomposition gas into the processing furnace.

On the other hand, the gas soft nitriding treatment is a treatment for improving wear resistance, friction resistance, and galling resistance by generating a hard compound layer of iron and nitrogen on the surface of carbon steel or the like, It is performed by introducing a mixed gas of ammonia gas and RX gas, a mixed gas of ammonia gas, nitrogen gas and carbon dioxide gas or the like into the processing furnace.
These gas nitriding treatments and gas soft nitriding treatments have a lower processing temperature than the carburizing and carbonitriding treatments and do not require a quenching (quenching) treatment. Has been applied.

In surface hardening treatment using such a mixed gas, it is generally difficult to detect the atmosphere in the furnace and control it in terms of the characteristics and performance of the sensor used, and the mixed gas can be used without referring to the furnace atmosphere. In addition to excessive nitriding, there were concerns about operational costs and environmental problems.
On the other hand, Patent Document 1 calculates the in-furnace gas composition by detecting the hydrogen concentration based on the thermal conductivity of the in-furnace gas, and determines the ratio of the introduced gas in the furnace based on the result. It has been proposed.

JP 2011-26627 A

  That is, in the surface hardening treatment method described in the cited reference 1, a plurality of types of in-furnace introduction gases including at least one in-furnace introduction gas that generates hydrogen in the treatment furnace are introduced into the treatment furnace, When performing the surface hardening treatment of the article to be processed disposed in the processing furnace, the hydrogen concentration of the furnace gas is detected based on the thermal conductivity of the furnace gas in the processing furnace, and the furnace concentration is determined based on the hydrogen concentration. The internal gas composition is calculated, and in accordance with the in-furnace gas composition and a preset in-furnace gas mixing ratio, a plurality of types of in-furnace introduced gases are set so that the in-furnace gas composition becomes the set in-furnace gas mixing ratio. While maintaining the ratio of the introduction amount into the processing furnace at a constant value, control the total introduction amount or individually introduce the introduction amounts of multiple types of furnace introduction gas so that the furnace introduction gas flow rate ratio changes. I try to control it.

  However, in the method described in the above publication, a mixed gas of ammonia gas and nitrogen gas (gas nitriding treatment) or a mixed gas of ammonia gas, nitrogen gas and carbon dioxide gas (gas soft nitriding) is used. The decomposition rate of ammonia gas changes due to the intervention of nitrogen gas and carbon dioxide gas. Therefore, it is impossible to accurately determine the nitriding potential only by detecting the hydrogen concentration, and even if the hydrogen concentration is fed back, it is impossible to control the nitriding potential at a constant level. There was a problem that processing was difficult.

  The present invention has been made to solve the above-described problems in conventional gas nitriding or gas soft nitriding, and its object is to control the atmosphere with high accuracy in spite of a simple system. An object of the present invention is to provide a gas nitriding method and a gas soft nitriding method that can reduce the amount of gas consumed, and an apparatus that can be suitably used for these processing methods.

  As a result of intensive investigations to achieve the above object, the present inventors have substantially reduced the supply gas in the furnace to the ammonia gas after the product to be processed in the furnace has been soaked to the processing temperature, that is, during the nitriding process. The ammonia gas was supplied into the furnace through two large and small channels. Then, a relatively small amount of ammonia gas is continuously supplied into the furnace from the first path, while the hydrogen partial pressure in the furnace is detected, and the hydrogen partial pressure is kept constant from the second path. It has been found that the above object can be achieved by controlling the supply of ammonia gas on and off, and the present invention has been completed.

  That is, the present invention is based on the above knowledge, and in the gas nitriding method of the present invention, the article to be processed stored in an airtight atmosphere furnace is predetermined in an inert gas or an inert gas and an ammonia gas. When the ammonia gas is supplied into the furnace and the nitride layer is formed on the surface of the article to be treated in ammonia gas and ammonia decomposition gas while maintaining the treatment temperature after the temperature is raised to the treatment temperature, the ammonia gas Are supplied from the first path and the second path, respectively, and continuously supplied from the first path, while the hydrogen partial pressure in the atmosphere furnace is detected, and the detected value is the nitriding potential of the furnace atmosphere. It is characterized in that the ammonia gas supply from the second path is on-off controlled so that the hydrogen partial pressure calculated in advance so as to be a desired value.

  Further, in the gas soft nitriding method of the present invention, the temperature of the article to be processed stored in an airtight atmosphere furnace is increased to a predetermined processing temperature in an inert gas or in an inert gas and an ammonia gas, and then the processing temperature. The ammonia gas and carbon dioxide gas are supplied into the furnace while forming a nitride layer on the surface of the article to be treated in ammonia gas, ammonia decomposition gas, and carbon dioxide gas. The first and second paths are respectively supplied and continuously supplied from the first path, while the hydrogen partial pressure in the atmosphere furnace is detected, and the detected value indicates the nitriding potential of the furnace atmosphere. The supply of ammonia gas and carbon dioxide gas from the second path is on-off controlled so as to obtain a hydrogen partial pressure calculated in advance to obtain a desired value.

  The gas nitriding apparatus of the present invention can be suitably used for the gas nitriding method, and includes an airtight atmosphere furnace that accommodates a product to be processed, a heating unit that heats the inside of the furnace, and a predetermined temperature inside the furnace. In-furnace temperature control means, in-furnace pressure control means for maintaining the pressure in the furnace at atmospheric pressure or higher, first ammonia supply means for continuously supplying ammonia gas into the furnace, and the furnace A second ammonia supply means for intermittently supplying ammonia gas therein, an inert gas supply means for supplying an inert gas into the furnace, a hydrogen sensor for detecting a hydrogen partial pressure in the furnace, and the hydrogen According to the present invention, there is provided atmosphere control means for performing on / off control of ammonia supply from the second ammonia supply means according to a detected value of the hydrogen partial pressure in the furnace by the sensor.

  Further, the gas soft nitriding apparatus of the present invention can be suitably used in the gas soft nitriding method, and includes an airtight atmosphere furnace that accommodates a product to be processed, a heating means that heats the inside of the furnace, and the inside of the furnace. In-furnace temperature control means for maintaining a predetermined temperature; in-furnace pressure control means for maintaining the atmospheric pressure in the furnace at atmospheric pressure or higher; first ammonia supply means for continuously supplying ammonia gas into the furnace; Second ammonia supply means for intermittently supplying ammonia gas into the furnace, first carbon dioxide supply means for continuously supplying carbon dioxide into the furnace, and intermittent supply of carbon dioxide into the furnace Second carbon dioxide gas supplying means, an inert gas supplying means for supplying an inert gas into the furnace, a hydrogen sensor for detecting a hydrogen partial pressure in the furnace, and a hydrogen partial pressure in the furnace by the hydrogen sensor. According to the detected value, the second ammonia The gas supply from the supply means and the second carbon dioxide supply means on - is characterized by having a controlled atmosphere means for turning off control.

Further, the first ammonia supply means and the first carbon dioxide gas supply means, and the second ammonia supply means and the second carbon dioxide gas supply means are respectively integrated into a first gas supply means and a second gas supply means, and these It is also possible to supply a mixed gas of ammonia gas and carbon dioxide from the supply means.
That is, an airtight atmosphere furnace for storing the product to be processed, a heating means for heating the inside of the furnace, an in-furnace temperature control means for maintaining the inside of the furnace at a predetermined temperature, and an atmospheric pressure in the furnace above atmospheric pressure Furnace pressure control means for holding, first gas supply means for continuously supplying ammonia gas and carbon dioxide gas into the furnace, and second gas supply for intermittently supplying ammonia gas and carbon dioxide gas into the furnace Means, an inert gas supply means for intermittently supplying an inert gas into the furnace, a hydrogen sensor for detecting a hydrogen partial pressure in the furnace, and a detected value of the hydrogen partial pressure in the furnace by the hydrogen sensor. Accordingly, an atmosphere control unit that performs on-off control of gas supply from the second gas supply unit can be provided.

  According to the present invention, since the gas supply during processing is substantially only ammonia gas, the nitriding potential of the furnace atmosphere can be maintained at a desired value by controlling the hydrogen partial pressure in the furnace to be constant. As a result, the processing accuracy is improved, the waste of the supply gas is eliminated, and the operation cost and the processing cost of the exhaust gas can be reduced. In addition, since the decomposition of ammonia gas is extremely slow at the nitriding temperature, in order to control this reaction, the supply of ammonia gas is controlled on and off based on the hydrogen partial pressure in the furnace, and from another route. By supplying continuously, hunting and negative pressure in the furnace hydrogen concentration are prevented, controllability is improved, and processing with excellent reproducibility becomes possible.

FIG. 4 is an example of a Railer state diagram showing the relationship between the nitriding potential Kn and the processing temperature affecting the nitriding product phase. It is the schematic which shows the structure of the gas nitriding apparatus by this invention. It is a time chart which shows the outline of the atmosphere control in the gas nitriding method of this invention. It is the schematic which shows the structure of the gas soft nitriding apparatus by this invention. It is a time chart which shows the outline of the atmosphere control in the gas nitriding method of this invention. It is a time chart which shows the time relationship between temperature and supply gas in the Example of the gas nitriding process by this invention. It is a time chart which shows the time relationship between the temperature and supply gas in the Example of the gas soft nitriding process by this invention.

  Hereinafter, the gas nitriding method and the gas soft nitriding method of the present invention will be described more specifically and in detail with reference to the drawings together with the structures of the apparatuses used.

  In the gas nitriding or gas soft nitriding method of the present invention, as described above, substantially only ammonia gas is supplied to the furnace as the nitriding gas, and as described below, the hydrogen concentration in the furnace By detecting (partial pressure), the nitriding potential of the furnace atmosphere can be obtained. Therefore, by maintaining the hydrogen partial pressure in the furnace at a value that provides the nitriding potential necessary for performing the desired nitriding treatment, it is possible to perform nitriding treatment as desired without excess or deficiency, and waste of supply gas Will be resolved.

  At this time, in the present invention, the supply of the ammonia gas is divided into two systems, and the ammonia gas is continuously supplied from the first path to prevent the atmosphere from becoming negative pressure so that the inside of the furnace does not become a negative pressure. On the other hand, the supply of ammonia gas from the second path is controlled on and off according to the detected value of the hydrogen partial pressure. This prevents the concentration of hydrogen in the furnace due to control delay, in other words, hunting of the nitriding potential, improves the accuracy of atmosphere control, and enables processing with excellent reproducibility without variation.

Here, the nitriding potential Kn is an index indicating the nitriding power of the furnace atmosphere, and is expressed by the following equation.
Kn = P _NH3 / P _H2 ^3/2
(In the formula, P _NH3 represents the partial pressure of ammonia (NH ₃ ), and P _H2 represents the partial pressure of hydrogen (H ₂ )).

Next, the relationship between the nitriding potential Kn and the hydrogen concentration will be described.
That is, the ammonia gas (NH ₃ ) supplied into the furnace is thermally decomposed into nitrogen (N ₂ ) and hydrogen (H ₂ ) based on the following formula.
NH ₃ → (1/2) N ₂ + (3/2) H ₂

Here, when the unit amount of ammonia gas is introduced into the furnace, if the decomposition rate of ammonia gas is α, the undecomposed ammonia amount is 1-α, the generated nitrogen amount is α / 2, and the generated hydrogen amount is 3α / 2 and the total amount of these gases is 1 + α.
Therefore, the ammonia partial pressure P _NH3 , the nitrogen partial pressure P _N2 , and the hydrogen partial pressure P _H2 are as follows.
P _NH3 = (1-α) / (1 + α) (1)
P _N2 = 1/2 · α / (1 + α) (2)
P _H2 = _3/2 · α / (1 + α) (3)

When α is eliminated from the above (1) to (3), and P _NH3 and P _N2 are each represented by P _H2 ,
P _NH3 = (3-4P _H2 ) / 3
P _N2 = P _H2 / 3
Therefore, the nitriding potential Kn = P _NH3 / P _H2 ^3/2 is
Kn = (3-4P _H2 ) / 3P _H2 ^3/2 = (1-4P _H2 / 3) / P _H2 ^3/2
Is calculated and, by detecting the hydrogen partial pressure P _H2 in the furnace, so that the nitriding potential Kn in the furnace atmosphere is determined.

Tables 1 and 2 summarize the relationship between the hydrogen partial pressure _PH2 and the nitriding potential Kn based on the above relational expressions. If the nitriding potential Kn necessary for forming a desired nitrided layer is determined, from these tables, Note this Request hydrogen partial pressure P _H2 for the nitriding potential Kn purpose, in the gas soft-nitriding of the present invention, although carbon dioxide is supplied with ammonia gas, the amount of ammonia It is confirmed that it is almost negligible because it is a very small amount of about 3 to 5% of the gas.

  FIG. 1 is a state diagram showing a relationship between a nitriding potential Kn affecting a nitridation formation phase and a processing temperature, which is known as a railer phase diagram, and is used for performing a gas nitriding (or soft nitriding) process on a steel part. It can be used to obtain a nitriding potential Kn necessary for forming a nitride layer having a phase composition of

However, the railer phase diagram described above is created based on an equilibrium state in which the nitriding potential of the workpiece surface made of pure iron matches the nitriding potential of the furnace atmosphere. Some errors may occur.
Therefore, in such a case, or in the case of soft nitriding with a relatively large amount of carbon dioxide added, correction may be made by a few preliminary tests as necessary to obtain the necessary nitriding potential in advance. desirable.

FIG. 2 shows an example of an apparatus used for the gas nitriding method of the present invention. The gas nitriding apparatus 1 shown in the figure includes a gas nitriding furnace (airtight atmosphere furnace) 2.
The inside of the gas nitriding furnace 2 is lined with a heat insulating material made of ceramic fiber or the like, and although not shown in the figure, heating means such as a heater is disposed along the inner wall of the heat insulating material, The product to be processed stored in the container is heated. In addition to a temperature sensor (thermocouple) for detecting the temperature in the furnace, the furnace temperature control means for adjusting the power supply to the heating means based on the temperature detected by the sensor and maintaining the furnace at a predetermined temperature It has. Furthermore, a fan for stirring the furnace atmosphere is also installed. The temperature information from the temperature sensor is also input to the furnace atmosphere control means 8 described later.

The gas nitriding furnace 2 includes a pressure gauge 3a for detecting the pressure in the furnace, a valve 3b for discharging the gas in the furnace, and an in-furnace pressure control means 3, and a valve corresponding to a value detected by the pressure gauge 3a. The opening degree of 3b is adjusted so that the pressure in the furnace does not become a negative pressure so as to prevent the inflow of air into the furnace.
The valve 3b is connected to a cracking furnace 4 containing a Ni-based catalyst, and surplus ammonia gas that has not been consumed in the processing in the gas nitriding furnace 2 is decomposed and combusted in the exhaust gas. After being combusted with the sex gas component, it is released to the atmosphere.

The gas nitriding furnace 2 is provided with a hydrogen sensor 5 for detecting the hydrogen partial pressure in the furnace, while first ammonia supplying means 6 for supplying ammonia gas (NH ₃ ) as nitriding gas. And the second ammonia supply means 7 are connected to each other.
Then, the detection signal from the hydrogen sensor 5 is output to the furnace atmosphere control means 8, and the furnace atmosphere control means 8 includes the first ammonia supply means 6, the second ammonia supply means 7, and an inertness described later. A valve opening / closing signal is output to the gas supply means 9.

  As the hydrogen sensor, for example, a measurement tube having hydrogen molecule permeability (for example, made by Ipson, Germany: HydroNit-sonde) can be used, but it can be directly attached to the furnace body, It is desirable to use a thermal conductivity sensor (for example, SE: H2 manufactured by Stange, Germany) from the viewpoint of continuously measuring the hydrogen concentration.

The first ammonia supply means 6 includes a manual valve 6a for adjusting the flow rate of ammonia gas and an electromagnetic valve 6b for opening and closing the supply path. During the gas nitriding process, the manual valve 6a is operated regardless of the furnace atmosphere. A relatively small amount of ammonia gas adjusted by 6a is always continuously supplied into the furnace.
On the other hand, the second ammonia supply means 7 is similarly provided with a manual valve 7a and an electromagnetic valve 7b, and the manual valve 7a is adjusted to a relatively large opening. In the gas nitriding process, the electromagnetic valve 7b is driven to open and close in response to a signal from the furnace atmosphere control means 8 output with reference to the hydrogen partial pressure in the furnace by the hydrogen sensor 5. That is, the hydrogen partial pressure in the furnace can be maintained at a predetermined value by controlling on / off the ammonia supply from the second supply means 7 into the furnace.

Further, the gas nitriding furnace 2 is supplied with an inert gas for removing the reactive gas in the furnace at the start and end of the process, and in this embodiment, nitrogen gas (N ₂ ) is supplied as an inert gas. Means 9 are connected.
This inert gas supply means 9 also includes a similar manual valve 9a and electromagnetic valve 9b. The electromagnetic valve 9b is opened in response to a signal from the furnace atmosphere control means 8, and the amount of nitrogen adjusted by the manual valve 9a is adjusted. Gas is supplied into the furnace.

  Next, a gas nitriding process using the above apparatus will be described with reference to FIG.

Prior to the processing, the opening of the manual valves 6a, 7a and 9a of the first ammonia supply means 6, the second ammonia supply means 7 and the inert gas supply means 9 is manually adjusted, and the respective gas supply amounts are adjusted. Keep it. Thereafter, the gas supply from these supply means is performed by opening and closing the electromagnetic valves 6b, 7b and 9b based on the signal from the furnace atmosphere control means 8, and the opening degree of these manual valves 6a, 7a and 9a is fixed. It will be.
The gas supply amounts from the first ammonia supply means 6, the second ammonia supply means 7 and the inert gas supply means 9 are 20 to 60% of the furnace volume of the nitriding furnace 2 as flow rates per hour, It is desirable to be about 50 to 150% and 40 to 100%.

First, when the processed workpiece is cooled or taken out from the furnace, and the subsequent workpiece (work) is charged into the furnace, the electromagnetic valve 9b of the inert gas supply means 9 is opened. Nitrogen gas is supplied into the furnace, and the nitriding gas in the furnace is discharged.
Then, after the installation of the product to be processed in the furnace is completed, the heater as the heating means is energized and the temperature rise is started, but the inert gas is used until the furnace temperature reaches a predetermined processing temperature. The supply of nitrogen from the supply means 9 is continued.

On the other hand, as the temperature rise is started, the electromagnetic valve 6b of the first ammonia supply unit 6 is driven to open, and supply of a relatively small amount of ammonia gas from the supply unit 6 is started. At this time, if the supply amount of ammonia gas is large, only the high temperature portion of the article to be processed in the temperature rising process with uneven temperature may be nitrided non-uniformly. It is desirable to suppress.
The ammonia supply from the first ammonia supply means 6 is continuously performed until the nitriding process is completed and the temperature of the article to be processed is started to be lowered.

  When the furnace temperature reaches a predetermined processing temperature, the electromagnetic valve 7b of the second ammonia supply means 7 is driven to open, and supply of a relatively large amount of ammonia gas from the supply means 7 is started. A large amount of furnace gas is discharged.

After the furnace temperature reaches the specified temperature, when some time passes and the temperature of the workpiece reaches the nitriding temperature, the atmosphere control, that is, the hydrogen partial pressure in the furnace is maintained at a predetermined value. Therefore, on / off control of ammonia supply by opening and closing the electromagnetic valve 7b is started according to the detection value by the hydrogen sensor 5.
As a result, the atmosphere in the furnace is maintained at a nitriding potential commensurate with the target nitriding treatment, and a nitride layer having a desired thickness and hardness can be formed on the surface of the article to be treated.

FIG. 4 shows an example of an apparatus used for the gas soft nitriding method of the present invention. The gas soft nitriding apparatus 10 shown in the figure includes a gas soft nitriding furnace (airtight atmosphere furnace) 11.
This gas soft nitriding apparatus 10 is basically the same as the above-described gas nitriding apparatus 1 except for the fact that it further includes carbon dioxide supply means 12 and 13 and includes the structure of the atmospheric furnace. Description of parts is omitted by giving the same reference numerals.

  That is, the gas soft nitriding apparatus 10 has, as a gas supply system, a first gas supply system for supplying carbon dioxide gas into the furnace in addition to the first and second ammonia supply means 6 and 2 and the inert gas supply means 9. First and second carbon dioxide gas supply means 12 and 13 are provided.

The first carbon dioxide supply means 12 includes a manual valve 12a that adjusts the flow rate of carbon dioxide supplied into the furnace and an electromagnetic valve 12b that opens and closes this supply path. Regardless, a relatively small amount of carbon dioxide gas adjusted by the manual valve 12a is continuously supplied into the furnace.
On the other hand, the second carbon dioxide supply means 13 is similarly provided with a manual valve 13a and an electromagnetic valve 13b, and the manual valve 13a is adjusted to a relatively large opening. In the gas soft nitriding process, the electromagnetic valve 13b is driven to open and close in response to a signal from the furnace atmosphere control means 8 output with reference to the hydrogen partial pressure in the furnace by the hydrogen sensor 5. . That is, on / off control of carbon dioxide supply from the second supply means 13 into the furnace is performed by opening and closing the electromagnetic valve 13b.

  Next, a gas soft nitriding process using the above apparatus will be described with reference to FIG.

Prior to processing, similarly, the opening of the manual valves 6a, 7a and 9a of the first and second ammonia supply means 6 and 7 and the inert gas supply means 9 are manually adjusted, and the respective gas supply amounts are adjusted. While adjusting, the opening degree of the manual valves 12a and 12a of the first and second carbon dioxide gas supply means 12 and 13 is adjusted manually.
The gas supply amounts from the first carbon dioxide gas supply means 12 and the second carbon dioxide gas supply means 13 are about 1 to 3% and 3 to 9% of the internal volume of the soft nitriding furnace 11 as flow rates per hour, respectively. It is desirable to adjust the opening degree of the manual valves 12a and 13a so that

  From the end of the process to the start of raising the temperature of the next article to be processed, the electromagnetic valve 9b of the inert gas supply means 9 is opened to supply nitrogen gas into the furnace. Then, after the installation of the product to be processed in the furnace is completed, energization to the heater as the heating means is started, but the nitrogen from the inert gas supply means 9 is kept until the furnace temperature reaches a predetermined processing temperature. Supply will continue.

  At the same time as the temperature rise is started, the electromagnetic valve 6b of the first ammonia supply means 6 is opened to start supplying a relatively small amount of ammonia gas. The ammonia supply from the first ammonia supply means 6 is subjected to nitriding treatment. The process is continuously performed until the temperature of the article to be processed is started.

  When the furnace temperature reaches a predetermined processing temperature, the electromagnetic valves 7b, 12a and 12b of the second ammonia supply means 7, the first carbon dioxide supply means 12 and the second carbon dioxide supply means 13 are opened. Thus, a relatively large amount of ammonia gas from the second ammonia supply means 7, a relatively small amount of carbon dioxide gas from the first carbon dioxide gas supply means 12, and a relatively large amount of carbon dioxide gas from the second carbon dioxide gas supply means 13. Is started, and the furnace gas containing a large amount of nitrogen gas is discharged.

After the furnace temperature reaches the specified temperature, when some time passes and the temperature of the workpiece reaches the nitriding temperature, the atmosphere control, that is, the hydrogen partial pressure in the furnace is maintained at a predetermined value. Therefore, on / off control of ammonia supply by opening and closing the electromagnetic valve 7b is started according to the detection value by the hydrogen sensor 5. At this time, the carbon dioxide supply from the second carbon dioxide supply means 13 is also controlled to be turned on and off in the same manner as the ammonia supply.
As a result, the atmosphere in the furnace is maintained at a nitriding potential commensurate with the target nitriding treatment, and a nitride layer having a desired thickness and hardness can be formed on the surface of the article to be treated.

  Before the soft nitriding process is finished, the electromagnetic valves 12a and 12b are closed to cut off the carbon dioxide supply from the first and second carbon dioxide supply means 12 and 12, thereby reducing the carbon dioxide concentration in the furnace. The precipitation of ammonium carbonate during cooling can be suppressed.

  In the above-described gas soft nitriding apparatus, the first and second ammonia supply units 6 and 7 and the first and second carbon dioxide gas supply units 12 and 13 are illustrated as examples. However, the first ammonia supply means 6 and the first carbon dioxide gas supply means 12 are integrated into a first gas supply means (for continuous supply), while the second ammonia supply means 7 and the second carbon dioxide gas supply means 13 are integrated. As a second gas supply means (for on-off control), a mixed gas of ammonia gas and carbon dioxide gas may be supplied from each supply means.

  EXAMPLES Hereinafter, although this invention is demonstrated concretely based on an Example, this invention is not limited by such an Example.

[Example 1 (gas nitriding)]
A test piece (diameter: 15 mm, height: 30 mm) made of hot tool steel SKD61 specified in JIS G4404 was prepared.
Then, for the purpose of forming a nitride layer having a cured layer thickness of 50 μm and a surface hardness of about 1000 with a micro Vickers hardness on the surface of such a test piece, it has a structure as shown in FIG. Then, in a pit type gas nitriding furnace having an in-furnace effective dimension of 1000 mm diameter × 1500 mm height, gas nitriding treatment was performed by the treatment cycle shown in FIG.

As shown in FIG. 6, the nitriding temperature is set to 490 ° C., the nitrogen gas supply amount from the inert gas supply means 9, the ammonia gas supply amount (small) from the first ammonia supply means 6, and the second ammonia The ammonia gas supply amount (large) from the supply means 7 was adjusted as follows using the manual valves 9a, 6a and 7a.
Nitrogen gas (N ₂ ): ₂ Nm ³ / hour Ammonia gas (NH ₃ : small): 1 Nm ³ / hour Ammonia gas (NH ₃ : large): 2.4 Nm ³ / hour

That is, as shown in FIG. 6, while supplying the nitrogen gas in a state where nitrogen gas is supplied from the inert gas supply means 9 and the atmosphere in the furnace is replaced with nitrogen after the test piece is stored in the furnace. Start heating up.
Simultaneously with the start of the temperature rise or when the temperature in the furnace rises to some extent, the electromagnetic valve 6b is driven to open to start supplying a small amount of ammonia gas from the first ammonia supply means 6. This small supply of ammonia gas is continued until the processing is completed.

When the furnace atmosphere temperature reaches a processing temperature of 490 ° C., the electromagnetic valve 7 b is driven to open, and a large amount of ammonia gas is supplied from the second ammonia supply means 7.
Then, after the furnace temperature has been raised to 490 ° C., about 30 minutes have passed, the test piece temperature has been made uniform to the processing temperature of 490 ° C., and when the decomposition of ammonia gas has started, the furnace atmosphere control has started. Then, on / off control of ammonia gas is performed by opening and closing the electromagnetic valve 7b.

In this atmosphere control, since the nitriding potential Kn for obtaining the nitride layer having the above thickness and hardness is previously determined to be “56”, the nitriding potential Kn in the furnace atmosphere is set to 56. , control for maintaining the furnace hydrogen partial pressure _{P H2} to a value of 0.065 (6.5%) obtained from Table 1 is performed.
Then, after such on / off control of the ammonia gas is continued for two and a half hours, the gas supply from the second ammonia supply means 7 is stopped, the gas nitriding process is completed, and the heater as the heating means is energized. The furnace temperature is lowered by the shutdown. At this time, the supply of nitrogen gas from the inert gas supply means 9 is resumed.

About the test piece which finished the said gas nitriding process, the hardness distribution was measured with the micro Vickers hardness meter, and the formation condition of the nitride layer was confirmed.
The results were as shown in Table 3, and it was confirmed that the cured layer thickness was about 50 μm, the surface hardness was 1046 Hv, and a nitride layer almost as intended was formed.

Note that the consumption of ammonia gas required for the gas nitriding treatment is about 7.5 m ³ , and the conventional nitriding treatment is performed without controlling the furnace hydrogen partial pressure PH ₂ by a conventional method. It has been found that a reduction of about 40% is possible compared to the required amount of ammonia gas of 10 to 15 m ³ .

[Example 2 (gas soft nitriding)]
A test piece (diameter 15 mm, height 30 mm) made of steel black core malleable cast iron FCD450 specified in JIS G5702 is prepared, and a compound layer (surface hardness of 500 to 800 Hv, about 5 to 20 μm) is prepared. 4 in a gas nitrocarburizing furnace having a structure as shown in FIG. 4 and having an effective dimension in the furnace of 760 mm width × 1200 mm length × 800 mm height to form a white layer). Gas nitrocarburizing treatment by cycle was performed.

As shown in FIG. 7, the processing temperature is set to 570 ° C., the nitrogen gas supply amount from the inert gas supply means 9, the ammonia gas supply amount (small) from the first ammonia supply means 6, and the second ammonia supply About the ammonia gas supply amount (large) from the means 7, it adjusted as follows with the manual valve 9a, 6a, 7a. The carbon dioxide supply amount (small) from the first carbon dioxide supply means 12 and the carbon dioxide supply amount (large) from the second carbon dioxide supply means 13 are as follows by the manual valves 12a and 13a. It was adjusted.
Nitrogen gas (N ₂ ): 1 Nm ³ / hour Ammonia gas (NH ₃ : small): 1 Nm ³ / hour Ammonia gas (NH ₃ : large): 4 Nm ³ / hour Carbon dioxide (CO ₂ : small): 1 NL / hour Carbonic acid gas (CO _2: large): 4NL / hour

That is, as shown in FIG. 7, after the test piece is stored in the furnace, nitrogen gas is supplied from the inert gas supply means 9 and the atmosphere in the furnace is replaced with nitrogen, while supplying the nitrogen gas. Start heating up.
Simultaneously with the start of the temperature rise or when the temperature in the furnace rises to some extent, the electromagnetic valve 6b is driven to open to start supplying a small amount of ammonia gas from the first ammonia supply means 6. This small supply of ammonia gas is continued until the processing is completed.

When the furnace atmosphere temperature reaches the processing temperature of 570 ° C., the electromagnetic valve 7 b is driven to open, and a large amount of ammonia gas is supplied from the second ammonia supply means 7. At the same time, the electromagnetic valves 12b and 13b are driven to open, and a small amount of carbon dioxide from the first carbon dioxide supply means 12 and a large amount of carbon dioxide from the second carbon dioxide supply means 13 are started.
Then, after the furnace temperature was raised to 570 ° C., about 1 hour passed, when the test piece temperature became uniform to the processing temperature of 570 ° C., nitrogen gas was almost exhausted, and decomposition of ammonia gas started. Then, the furnace atmosphere control is started, and on / off control of ammonia gas is performed by opening and closing the electromagnetic valve 7b. Then, in conjunction with the opening / closing drive of the electromagnetic valve 7b, the opening / closing of the electromagnetic valve 13b of the second carbon dioxide supply means 13 is also started.

In this atmosphere control, since it is previously determined that the nitriding potential Kn for obtaining the soft nitrided layer having the above compound thickness and surface hardness is “3.7”, the nitriding potential Kn of the furnace atmosphere is set to to 3.7, control for maintaining the furnace hydrogen partial pressure _{P H2} to a value of 0.30 (30%) obtained from Table 1 is performed.
Then, when such on-off control of ammonia gas and carbon dioxide supply is continued for 2 hours, the supply of carbon dioxide from the first and second carbon dioxide supply means 12 and 13 is first stopped. On the other hand, continuous supply of a small amount of ammonia gas from the first ammonia supply means 6 and on / off control of ammonia gas from the second ammonia supply means 7 are continued for another hour.

  When 1 hour has passed since the supply of carbon dioxide gas was stopped, the supply of ammonia gas from the first and second ammonia supply means 6 and 7 was stopped, and the gas soft nitriding process was completed. Then, the furnace temperature is lowered by stopping energization of the heater as the heating means. At this time, the supply of nitrogen gas from the inert gas supply means 9 is resumed.

About the test piece which finished the said process, the hardness distribution was measured with the micro Vickers hardness meter, and the formation condition of the soft nitriding layer was confirmed.
As a result, the thickness of the compound layer was 9.2 μm, the surface hardness was 786 Hv, and it was confirmed that a soft nitrided layer almost as intended was formed.

In addition, the consumption amount of ammonia gas required for the above gas soft nitriding treatment is about 15 m ³ , and in order to perform the same nitriding treatment without controlling the in-furnace hydrogen partial pressure PH ₂ by a conventional method. It has been found that a reduction of about 40% is possible compared to the required amount of ammonia gas of 20 to 30 m ³ .

1 Gas nitriding equipment 2 Gas nitriding furnace (airtight atmosphere furnace)
3 In-furnace pressure control means 5 Hydrogen sensor 6 First ammonia supply means 7 Second ammonia supply means 8 In-furnace atmosphere control means 9 Inert gas supply means 10 Gas soft nitriding apparatus 11 Gas soft nitriding furnace (airtight atmosphere furnace)
12 First carbon dioxide supply means 13 Second carbon dioxide supply means

Claims (9)

The temperature of the article to be processed stored in an airtight atmosphere furnace is increased to a predetermined processing temperature in an inert gas or in an inert gas and an ammonia gas, and then the ammonia gas is put into the furnace while maintaining the processing temperature. In a gas nitriding method for forming a nitride layer on the surface of the article to be treated in ammonia gas and ammonia decomposition gas by supplying
The ammonia gas is supplied from the first path and the second path, respectively, and continuously supplied from the first path,
The hydrogen partial pressure in the atmosphere furnace is detected, and the detected value from the second path is set so as to be a hydrogen partial pressure calculated by the following formula in advance in order to set the nitriding potential of the furnace atmosphere to a desired value. A gas nitriding method characterized in that ammonia gas supply is controlled on and off.
Kn = (1-4P _H2 / 3) / P _H2 ^3/2 (formula)
(Kn in the formula represents a nitriding potential, and P _H2 represents a hydrogen partial pressure) 2. The gas nitriding method according to claim 1, wherein the hydrogen partial pressure is detected by a heat conduction sensor mounted in an atmospheric furnace. The temperature of the article to be processed stored in an airtight atmosphere furnace is increased to a predetermined processing temperature in an inert gas or in an inert gas and an ammonia gas, and then the ammonia gas is put into the furnace while maintaining the processing temperature. And a gas soft nitriding method for forming a nitride layer on the surface of the article to be treated in ammonia gas, ammonia decomposition gas, and carbon dioxide gas by supplying carbon dioxide gas,
While supplying the ammonia gas and the carbon dioxide gas from the first path and the second path, respectively, and continuously supplying from the first path, respectively,
The hydrogen partial pressure in the atmosphere furnace is detected, and the detected value from the second path is set so as to be a hydrogen partial pressure calculated by the following formula in advance in order to set the nitriding potential of the furnace atmosphere to a desired value. A gas soft nitriding method, wherein supply of ammonia gas and carbon dioxide gas is controlled on and off.

_{Kn = (1-4P H2 / 3)} / P H2 3/2
(Kn in the formula represents a nitriding potential, and P _H2 represents a hydrogen partial pressure) (formula) 4. The gas soft nitriding method according to claim 3 , wherein the hydrogen partial pressure is detected by a heat conduction sensor mounted in an atmospheric furnace. An air-tight atmosphere furnace for storing the workpieces;
Heating means for heating the inside of the furnace;
Furnace temperature control means for maintaining the furnace at a predetermined temperature;
Furnace pressure control means for maintaining the pressure in the furnace at atmospheric pressure or higher;
First ammonia supply means for continuously supplying ammonia gas into the furnace;
A second ammonia supply means for intermittently supplying ammonia gas into the furnace;
An inert gas supply means for supplying an inert gas into the furnace;
A hydrogen sensor for detecting the hydrogen partial pressure in the furnace,
A gas nitriding apparatus comprising atmosphere control means for on-off control of ammonia supply from the second ammonia supply means in accordance with a detected value of the hydrogen partial pressure in the furnace by the hydrogen sensor. 6. The gas nitriding apparatus according to claim 5 , wherein the hydrogen sensor is a heat conduction type direct mounting type sensor. An air-tight atmosphere furnace for storing the workpieces;
Heating means for heating the inside of the furnace;
Furnace temperature control means for maintaining the furnace at a predetermined temperature;
Furnace pressure control means for maintaining the pressure in the furnace at atmospheric pressure or higher;
First ammonia supply means for continuously supplying ammonia gas into the furnace;
A second ammonia supply means for intermittently supplying ammonia gas into the furnace;
First carbon dioxide supply means for continuously supplying carbon dioxide into the furnace;
A second carbon dioxide gas supply means for intermittently supplying carbon dioxide into the furnace;
An inert gas supply means for supplying an inert gas into the furnace;
A hydrogen sensor for detecting the hydrogen partial pressure in the furnace,
According to the present invention, there is provided atmosphere control means for performing on / off control of gas supply from the second ammonia supply means and the second carbon dioxide gas supply means in accordance with a detected value of the hydrogen partial pressure in the furnace by the hydrogen sensor. Gas soft nitriding equipment. An air-tight atmosphere furnace for storing the workpieces;
Heating means for heating the inside of the furnace;
Furnace temperature control means for maintaining the furnace at a predetermined temperature;
Furnace pressure control means for maintaining the pressure in the furnace at atmospheric pressure or higher;
First gas supply means for continuously supplying ammonia gas and carbon dioxide gas into the furnace;
Second gas supply means for intermittently supplying ammonia gas and carbon dioxide gas into the furnace;
An inert gas supply means for supplying an inert gas into the furnace;
A hydrogen sensor for detecting the hydrogen partial pressure in the furnace,
A gas soft nitriding apparatus comprising atmosphere control means for performing on-off control of gas supply from the second gas supply means in accordance with a detected value of the hydrogen partial pressure in the furnace by the hydrogen sensor. The gas soft nitriding apparatus according to claim 7 or 8 , wherein the hydrogen sensor is a heat conduction type direct mounting type sensor.

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