SURFACE HARDENING TREATMENT DEVICE AND SURFACE HARDENING TREATMENT METHOD

Abstract

The present, invention includes: an in-furnace atmospheric gas concentration detector configured to detect a hydrogen concentration or an ammonia concentration in a processing furnace; an in-furnace nitriding potential calculator configured to calculate a nitriding potential in the processing furnace based on the hydrogen concentration or the ammonia concentration detected by the in-furnace atmospheric gas concentration detector; and a gas-introduction-amount controller configured to change an introduction amount of each of the plurality of furnace introduction gases except for the ammonia decomposition gas while keeping an introduction amount of the ammonia decomposition gas constant. based on the calculated nitriding potential in the processing furnace and a target nitriding potential, such that the nitriding potential in the processing furnace is brought close to the target nitriding potential.

Claims

1. A surface hardening treatment device for performing a gas nitrocarburizing treatment as a surface hardening treatment for a work arranged in a processing furnace by introducing a plurality of furnace introduction gases including an ammonia gas and an ammonia decomposition gas, the surface hardening treatment device comprising an in-furnace atmospheric gas concentration detector configured to detect a hydrogen concentration or an ammonia concentration in the processing furnace, an in-furnace nitriding potential calculator configured to calculate a nitriding potential in the processing furnace based on the hydrogen concentration or the ammonia concentration detected by the in-furnace atmospheric gas concentration detector, and a gas-introduction-amount controller configured to change an introduction amount of each of the plurality of furnace introduction gases except for the ammonia decomposition gas while keeping an introduction amount of the ammonia decomposition gas constant, based on the nitriding potential in the processing furnace calculated by the in-furnace nitriding potential calculator and a target nitriding potential, such that the nitriding potential in the processing furnace is brought close to the target nitriding potential.

2. The surface hardening treatment device according to claim 1, further comprising an in-furnace oxygen concentration detector configured to detect an oxygen concentration in the processing furnace, wherein the in-furnace nitriding potential calculator is configured to calculate the nitriding potential in the processing furnace based on the hydrogen concentration or the ammonia concentration detected by the in-furnace atmospheric gas concentration detector and the oxygen concentration detected by the in-furnace oxygen concentration detector.

3. The surface hardening treatment device according to claim 1 wherein the gas-introduction-amount controller is configured to control the introduction amount C1, * * * , CN (N is an integer of one or more) of each of the plurality of furnace introduction gases except for the ammonia gas and the ammonia decomposition gas, using a factor of proportionality c1, * * * , cN assigned to each of the plurality of furnace introduction gases except for the ammonia gas and the ammonia decomposition gas, such that
C1=c1×(A+x×B), * * * ,cN=cN×(A+x×B) wherein the introduction amount of the ammonia gas is represented by A, the introduction amount of the ammonia decomposition gas is represented by B, and a predetermined constant is represented by x.

4. The surface hardening treatment device according to claim 3, wherein the predetermined constant x is within 0.4 to 0.6.

5. The surface hardening treatment device according to claim 4, wherein the predetermined constant x is 0.5.

6. The surface hardening treatment device according to claim 1, wherein the plurality of furnace introduction gases includes a carbon dioxide gas.

7. The surface hardening treatment device according to claim 1, wherein the plurality of furnace introduction gases includes a carbon monoxide gas.

8. The surface hardening treatment device according to claim 1, wherein the plurality of furnace introduction gases includes a carbon dioxide gas and a nitrogen gas.

9. The surface hardening treatment device according to claim 1, wherein the plurality of furnace introduction gases includes a carbon monoxide gas and a nitrogen gas.

10. A surface hardening treatment method of performing a gas nitrocarburizing treatment as a surface hardening treatment for a work arranged in a processing furnace by introducing a plurality of furnace introduction gases including an ammonia gas and an ammonia decomposition gas, the surface hardening treatment method comprising an in-furnace atmospheric gas concentration detecting step of detecting a hydrogen concentration or an ammonia concentration in the processing furnace, an in-furnace nitriding potential calculating step of calculating a nitriding potential in the processing furnace based on the hydrogen concentration or the ammonia concentration detected at the in-furnace atmospheric gas concentration detecting step, and a gas-introduction-amount controlling step of changing an introduction amount of each of the plurality of furnace introduction gases except for the ammonia decomposition gas while keeping an introduction amount of the ammonia decomposition gas constant, based on the nitriding potential in the processing furnace calculated at the in-furnace nitriding potential calculating step and a target nitriding potential, such that the nitriding potential in the processing furnace is brought close to the target nitriding potential.

11. A surface hardening treatment device for performing a gas nitrocarburizing treatment as a surface hardening treatment for a work arranged in a processing furnace by introducing a plurality of furnace introduction gases including an ammonia gas, an ammonia decomposition gas and a carburizing gas, the surface hardening treatment device comprising an in-furnace atmospheric gas concentration detector configured to detect a hydrogen concentration or an ammonia concentration in the processing furnace, an in-furnace nitriding potential calculator configured to calculate a nitriding potential in the processing furnace based on the hydrogen concentration or the ammonia concentration detected by the in-furnace atmospheric gas concentration detector, and a gas-introduction-amount controller configured to change an introduction amount of each of the ammonia gas and the carburizing gas while keeping an introduction amount of the ammonia decomposition gas constant, based on the nitriding potential in the processing furnace calculated by the in-furnace nitriding potential calculator and a target nitriding potential, such that the nitriding potential in the processing furnace is brought close to the target nitriding potential.

12. The surface hardening treatment device according to claim 11, wherein the gas-introduction-amount controller is configured to control the introduction amount C1 of the carburizing gas, using a factor of proportionality c1 assigned to the carburizing gas, such that
C1=c1×(A+x×B) wherein the introduction amount of the ammonia gas is represented by A, the introduction amount of the ammonia decomposition gas is represented by B, and a predetermined constant is represented by x.

13. A surface hardening treatment device for performing a gas nitrocarburizing treatment as a surface hardening treatment for a work arranged in a processing furnace by introducing a plurality of furnace introduction gases including an ammonia gas, an ammonia decomposition gas, a carburizing gas and a nitrogen gas, the surface hardening treatment device comprising an in-furnace atmospheric gas concentration detector configured to detect a hydrogen concentration or an ammonia concentration in the processing furnace, an in-furnace nitriding potential calculator configured to calculate a nitriding potential in the processing furnace based on the hydrogen concentration or the ammonia concentration detected by the in-furnace atmospheric gas concentration detector, and a gas-introduction-amount controller configured to change an introduction amount of each of the ammonia gas, the carburizing gas and the nitrogen gas while keeping an introduction amount of the ammonia decomposition gas constant, based on the nitriding potential in the processing furnace calculated by the in-furnace nitriding potential calculator and a target nitriding potential, such that the nitriding potential in the processing furnace is brought close to the target nitriding potential.

14. The surface hardening treatment device according to claim 13, wherein the gas-introduction-amount controller is configured to control the introduction amount C1 of the carburizing gas and the introduction amount C2 of the nitrogen gas, using a factor of proportionality c1 assigned to the carburizing gas and a factor of proportionality c2 assigned to the nitrogen gas, such that
C1=c1×(A+x×B) and C2=c2×(A+x×B) wherein the introduction amount of the ammonia gas is represented by A, the introduction amount of the ammonia decomposition gas is represented by B, and a predetermined constant is represented by x.

15. The surface hardening treatment device according to claim 2, wherein the gas-introduction-amount controller is configured to control the introduction amount C1, * * * , CN (N is an integer of one or more) of each of the plurality of furnace introduction gases except for the ammonia gas and the ammonia decomposition gas, using a factor of proportionality c1, * * * , cN assigned to each of the plurality of furnace introduction gases except for the ammonia gas and the ammonia decomposition gas, such that
C1=c1×(A+x×B), * * * ,cN=cN×(A+x×B) wherein the introduction amount of the ammonia gas is represented by A, the introduction amount of the ammonia decomposition gas is represented by B, and a predetermined constant is represented by x.

16. The surface hardening treatment device according to claim 15, wherein the predetermined constant x is within 0.4 to 0.6.

17. The surface hardening treatment device according to claim 16, wherein the predetermined constant x is 0.5.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0058] FIG. 1 is a schematic view showing a surface hardening treatment device according to a first embodiment of the present invention:

[0059] FIG. 2 is a graph showing a control of furnace introduction gases according to an example 1-1;

[0060] FIG. 3 is a graph showing a control of nitriding potential according to the example 1-1:

[0061] FIG. 4 is a graph showing a control of furnace introduction gases according to an example 1-3;

[0062] FIG. 5 is a graph showing a control of nitriding potential according to the example 1-3;

[0063] FIG. 6 is a table comparing the examples 1-1 to 1-3 with their respective comparative examples;

[0064] FIG. 7 is a schematic view showing a surface hardening treatment device according to a second embodiment of the present invention;

[0065] FIG. 8 is a graph showing a control of furnace introduction gases according to an example 2-2;

[0066] FIG. 9 is a graph showing a control of nitriding potential according to the example 2-2;

[0067] FIG. 10 is a table comparing the examples 2-1 to 2-3 with their respective comparative examples:

[0068] FIG. 11 is a schematic view showing a surface hardening treatment device according to a third embodiment of the present invention:

[0069] FIG. 12 is a graph showing a control of furnace introduction gases according to an example 3-2:

[0070] FIG. 13 is a graph showing a control of nitriding potential according to the example 3-2;

[0071] FIG. 14 is a table comparing the examples 3-1 to 3-3 with their respective comparative examples:

[0072] FIG. 15 is a schematic view showing a surface hardening treatment device according to a fourth embodiment of the present invention:

[0073] FIG. 16 is a table comparing the examples 4-1 to 4-3 with their respective comparative examples;

[0074] FIG. 17 is a schematic view showing a surface hardening treatment device according to a fifth embodiment of the present invention:and

[0075] FIG. 18 is a table comparing the examples 5-1 to 5-3 with their respective comparative examples.

DESCRIPTION OF EMBODIMENTS

[0076] Hereinafter, a preferable embodiment of the present invention will be described. However, the present invention is not limited to the embodiment.

[0077] (Structure)

[0078] FIG. 1 is a schematic view showing a surface hardening treatment device according to an embodiment of the present invention. As shown in FIG. 1, the surface hardening treatment device 1 of the present embodiment is a surface hardening treatment device for performing a gas nitrocarburizing treatment as a surface hardening treatment for a work S arranged in a processing furnace 2 by introducing an ammonia gas, an ammonia decomposition gas and a carbon dioxide gas into the processing furnace 2.

[0079] The ammonia decomposition gas is a gas called AX gas, and is a mixed gas composed of nitrogen and hydrogen in a ratio of 1:3. The work S is made of metal. For example, the work S is a steel part or a mold.

[0080] As shown in FIG. 1, the processing furnace 2 of the surface hardening treatment device 1 of the present embodiment includes: a stirring fan 8, a stirring-fan drive motor 9, a in-furnace temperature measuring device 10, a furnace body heater 11, an atmospheric gas concentration detector 3, a nitriding potential adjustor 4, a temperature adjustor 5, a programmable logic controller 31, a recorder 6, and a furnace introduction gas supplier 20.

[0081] The stirring fan 8 is disposed in the processing furnace 2 and configured to rotate in the processing furnace 2 in order to stir atmospheric gases in the processing furnace 2. The stirring-fan drive motor 9 is connected to the stirring fan 8 and configured to cause the stirring fan 8 to rotate at an arbitrary rotation speed.

[0082] The in-furnace temperature measuring device 10 includes a thermocouple and is configured to measure a temperature of the in-furnace gases existing in the processing furnace 2. In addition, after measuring the temperature of the in-furnace gases, the in-furnace temperature measuring device 10 is configured to output an information signal including the measured temperature (in-furnace temperature signal) to the temperature adjustor 5 and the recorder 6.

[0083] The atmospheric gas concentration detector 3 is composed of: a sensor capable of detecting a hydrogen concentration or an ammonia concentration in the processing furnace 2 as an in-furnace atmospheric gas concentration; and an oxygen sensor capable of detecting an oxygen concentration in the processing furnace 2 as an in-furnace oxygen concentration. A main body of each of the above sensors communicates with an inside of the processing furnace 2 via an atmospheric gas pipe 12. In the present embodiment, the atmospheric gas pipe 12 is formed as a single-line path that directly communicates the sensors' main bodies of the atmospheric gas concentration detector 3 and the processing furnace 2. An on-off valve 17 is provided in the middle of the atmospheric gas pipe 12. and configured to be controlled by an on-off valve controller 16.

[0084] In addition, after detecting the in-furnace atmospheric gas concentration and the in-furnace oxygen concentration, the atmospheric gas concentration detector 3 is configured to output an information signal including the detected concentrations to the nitriding potential adjustor 4 and the recorder 6.

[0085] The recorder 6 includes a CPU and a storage medium such as a memory. Based on the signals outputted from the in-furnace temperature measurement device 10 and the atmospheric gas concentration detector 3, the recorder 6 is configured to record the temperature and/or the atmospheric gas concentration and the oxygen concentration in the processing furnace 2, for example in correspondence with the date and time when the surface hardening treatment is performed.

[0086] The nitriding potential adjuster 4 includes an in-furnace nitriding potential calculator 13 and a gas flow rate output adjustor 30. The programmable logic controller 31 includes a gas introduction controller 14 and a parameter setting device 15.

[0087] The in-furnace nitriding potential calculator 13 is configured to calculate a nitriding potential in the processing furnace 2 based on the hydrogen concentration or the ammonia concentration and the oxygen concentration detected by the atmospheric gas concentration detector 3. Specifically, calculation formulas for the nitriding potential are programmed dependent on the actual furnace introduction gases in accordance with the same theory as the above formulas (5) to (9), and incorporated in the in-furnace nitriding potential calculator 13, so that the nitriding potential is calculated from the value of the in-furnace atmospheric gas concentration and the value of the oxygen concentration.

[0088] In the present embodiment, the introduction amount C1 of the carbon dioxide gas, which is a furnace introduction gas except for (other than) the ammonia gas and the ammonia decomposition gas, is controlled using a factor of proportionality c1 assigned to the carbon dioxide gas, such that C1=c1×(A+x=B), wherein the introduction amount of the ammonia gas is represented by A, the introduction amount of the ammonia decomposition gas is represented by B, and a predetermined constant is represented by x.

[0089] The parameter setting device 15 is composed of a touch panel, for example. Through the parameter setting device 15, the target nitriding potential, the processing temperature, the processing time, the introduction amount of the ammonia decomposition gas, the predetermined constant x, the factor of proportionality c1, and so on can be set and inputted for the same work. In addition, through the parameter setting device 15, setting parameter values for a PID control method can be set and inputted for each different value of the target nitriding potential. Specifically, “a proportional gain”, “an integral gain or an integration time”, and “a differential gain or a differentiation time” for the PID control method can be set and inputted for each different value of the target nitriding potential. The set and inputted setting parameter values are transferred to the gas flow rate output adjustor 30.

[0090] The gas flow rate output adjustor 30 is configured to perform the PID control method in which respective gas introduction amounts of the ammonia gas and the carbon dioxide gas among the three kinds of furnace introduction gases are input values, the nitriding potential calculated by the in-furnace nitriding potential calculator 13 is an output value, and the target nitriding potential (the set nitriding potential) is a target value. More specifically, in the present PID control method, the nitriding potential in the processing furnace 2 is brought close to the target nitriding potential by changing the introduction amount of the ammonia gas and the introduction amount of the carbon dioxide gas while keeping the introduction amount of the ammonia decomposition gas constant. In addition, in the present PID control method, the setting parameter values that have been transferred from the parameter setting device 15 are used.

[0091] Before the setting and inputting operation against the parameter setting device 15, it is preferable to perform pilot processes to obtain in advance candidate values for the setting parameter values of the PID control method. According to the present embodiment, even if (1) a state of the processing furnace (a state of a furnace wall and/or a jig), (2) a temperature condition of the processing furnace and (3) a state of the work (type and/or the number of parts) are the same, it is possible to obtain in advance candidate values for the setting parameter values (4) for each different value of the target nitriding potential, by an auto-tuning function that the nitriding potential adjustor 4 has in itself. In order to embody the nitriding potential adjustor 4 having such an auto-tuning function, a “UT75A” manufactured by Yokogawa Electric Co., Ltd. (a high-functional digital indicating controller, http://www.yokogawa.co.jp/ns/cis/utup/utadvanced/ns-ut75a-01-ja.htm) or the like can be used.

[0092] The setting parameter values (a set of “the proportional gain”, “the integral gain or the integration time” and “the derivative gain or the derivative time”) obtained as the candidate values can be recorded in some manner, and then can be manually inputted to the parameter setting device 15. Alternatively, the setting parameter values obtained as the candidate values can be stored in some storage device in a manner associated with the target nitriding potential, and then can be automatically read out by the parameter setting device 15 based on the set and inputted value of the target nitriding potential.

[0093] Before performing the PID control method, the gas flow rate output adjustor 30 is configured to determine an introduction amount of the ammonia decomposition gas, which is kept constant, and respective initial introduction amounts of the ammonia gas and of the carbon dioxide gas, which are subsequently changed. It is preferable to perform pilot processes to obtain in advance candidate values for these introduction amounts, so that the obtained values can be automatically read out by the parameter setting device 15 from some storage device or can be manually inputted to the parameter setting device 15. Thereafter, according to the PID control method, the introduction amount of the ammonia gas and the introduction amount of the carbon dioxide gas are changed (while the introduction amount of the ammonia decomposition gas is kept constant) such that the nitriding potential in the processing furnace 2 is brought close to the target nitriding potential while the above relationship of C1=c1×(A+x×B) is maintained. The output values from the gas flow rate output adjustor 30 are transferred to the gas introduction amount controller 14.

[0094] The gas introduction amount controller 14 is configured to transmit a control signal to a first supply amount controller 22 for the ammonia gas.

[0095] The furnace introduction gas supplier 20 of the present embodiment includes a first furnace introduction gas supplier 21 for the ammonia gas, the first supply amount controller 22, a first supply valve 23 and a first flow meter 24. In addition, the furnace introduction gas supplier 20 of the present embodiment includes a second furnace introduction gas supplier 25 for the ammonia decomposition gas (AX gas), a second supply amount controller 26, a second supply valve 27 and a second flow meter 28. Furthermore, the furnace introduction gas supplier 20 of the present embodiment includes a third furnace introduction gas supplier 61 for the carbon dioxide gas, a third supply amount controller 62, a third supply valve 63 and a third flow meter 64.

[0096] In the present embodiment, the ammonia gas, the ammonia decomposition gas and the carbon dioxide gas are mixed in a furnace introduction gas pipe 29 before entering the processing furnace 2.

[0097] The first furnace introduction gas supplier 21 is formed by, for example, a tank filled with a first furnace introduction gas (in this example, the ammonia gas).

[0098] The first supply amount controller 22 is formed by a mass flow controller (which can finely change a flow rate within a short time period), and is interposed between the first furnace introduction gas supplier 21 and the first supply valve 23. An opening degree of the first supply amount controller 22 changes according to the control signal outputted from the gas introduction amount controller 14, In addition, the first supply amount controller 22 is configured to detect a supply amount from the first furnace introduction gas supplier 21 to the first supply valve 23, and output an information signal including the detected supply amount to the gas introduction amount controller 14 and the recorder 6. This information signal can be used for correction or the like of the control performed by the gas introduction amount controller 14.

[0099] The first supply valve 23 is formed by an electromagnetic valve configured to switch between opened and closed states according to a control signal outputted from the gas introduction amount controller 14, and is interposed between the first supply amount controller 22 and the first flow meter 24.

[0100] The first flow meter 24 is formed by, for example, a mechanical flow meter such as a flow-type flow meter, and is interposed between the first supply valve 23 and the furnace introduction gas pipe 29. The first flow meter 24 detects a supply amount from the first supply valve 23 to the furnace introduction gas pipe 29. The supply amount detected by the first flow meter 24 can be provided for an operators visual confirmation.

[0101] The second furnace introduction gas supplier 25 is formed by, for example, a tank filled with a second furnace introduction gas (in this example, the ammonia decomposition gas).

[0102] The second supply amount controller 26 is formed by a mass flow controller (which can finely change a flow rate within a short time period), and is interposed between the second furnace introduction gas supplier 25 and the second supply valve 27. An opening degree of the second supply amount controller 26 changes according to the control signal outputted from the gas introduction amount controller 14. In addition, the second supply amount controller 26 is configured to detect a supply amount from the second furnace introduction gas supplier 25 to the second supply valve 27, and output an information signal including the detected supply amount to the gas introduction amount controller 14 and the recorder 6. This information signal can be used for correction or the like of the control performed by the gas introduction amount controller 14.

[0103] The second supply valve 27 is formed by an electromagnetic valve configured to switch between opened and closed states according to a control signal outputted from the gas introduction amount controller 14, and is interposed between the second supply amount controller 26 and the second flow meter 28.

[0104] The second flow meter 28 is formed by, for example, a mechanical flow meter such as a flow-type flow meter, and is interposed between the second supply valve 27 and the furnace introduction gas pipe 29. The second flow meter 28 detects a supply amount from the second supply valve 27 to the furnace introduction gas pipe 29. The supply amount detected by the second flow meter 28 can be provided for an operator's visual confirmation.

[0105] Herein, in the present invention, the introduction amount of the ammonia decomposition gas is not changed finely. Thus, the second supply amount controller 26 may be omitted, and a flow rate (an opening degree) of the second flow meter 28 may be manually adjusted correspondingly to the control signal outputted from the gas introduction amount controller 14.

[0106] The third furnace introduction gas supplier 61 is formed by, for example, a tank filled with a third furnace introduction gas (in this example, the carbon dioxide gas).

[0107] The third supply amount controller 62 is formed by a mass flow controller (which can finely change a flow rate within a short time period), and is interposed between the third furnace introduction gas supplier 61 and the third supply valve 63. An opening degree of the third supply amount controller 62 changes according to the control signal outputted from the gas introduction amount controller 14. In addition, the third supply amount controller 62 is configured to detect a supply amount from the third furnace introduction gas supplier 61 to the third supply valve 63, and output an information signal including the detected supply amount to the gas introduction amount controller 14 and the recorder 6. This information signal can be used for correction or the like of the control performed by the gas introduction amount controller 14.

[0108] The third supply valve 63 is formed by an electromagnetic valve configured to switch between opened and closed states according to a control signal outputted from the gas introduction amount controller 14, and is interposed between the third supply amount controller 62 and the third flow meter 64.

[0109] The third flow meter 64 is formed by, for example, a mechanical flow meter such as a flow-type flow meter, and is interposed between the third supply valve 63 and the furnace introduction gas pipe 29. The third flow meter 64 detects a supply amount from the third supply valve 63 to the furnace introduction gas pipe 29. The supply amount detected by the third flow meter 64 can be provided for an operator's visual confirmation.

Operation: Example 1-1

[0110] Next, with reference to FIGS. 2 and 3, an operation of the surface hardening treatment device 1 according to the present embodiment is explained. First, a work S to be processed is put into the processing furnace 2, and then the processing furnace 2 starts to be heated. In the example shown in FIGS. 2 and 3, a pit furnace having a size of φ 700×1000 was used as the processing furnace 2, 570° C. was adopted as the temperature to be heated, and a steel material having a surface area of 4 m.sup.2 was used as the work S.

[0111] While the processing furnace 2 is heated, the ammonia gas, the ammonia decomposition gas and the carbon dioxide gas are introduced into the processing furnace 2 from the furnace introduction gas supplier 20 according to their respective initial introduction amounts. In this example, as shown in FIG. 2, the initial introduction amount of the ammonia gas was set to 13 [l/min], the initial introduction amount of the ammonia decomposition gas was set to 19 [l/min], the initial introduction amount of the carbon dioxide gas was set to 1.03 [l/min], x=0.5 was set, and c1=0.053 was set. These initial introduction amounts can be set and inputted by the parameter setting device 15. Furthermore, the stirring fan drive motor 9 is driven and thus the stirring fan 8 rotates to stir the atmospheric gases in the processing furnace 2.

[0112] In the initial state, the on-off valve controller 16 closes the on-off valve 17. In general, as a pretreatment for the gas nitriding treatment, a treatment for activating a steel surface to make it easy for nitrogen to enter may be performed. In this case, a hydrogen chloride gas and/or a hydrogen cyanide gas or the like may be generated in the furnace. These gases may deteriorate the atmospheric gas concentration detector (sensors) 3, and thus it is effective to keep the on-off valve 17 closed.

[0113] In addition, the in-furnace temperature measurement device 10 measures a temperature of the in-furnace gases, and outputs art information in signal including the measured temperature to the nitriding potential adjustor 4 and the recorder 6. The nitriding potential adjustor 4 judges whether the state in the processing furnace 2 is still during the temperature rising step or already after the temperature rising step has been completed (a stable state).

[0114] In addition, the in-furnace nitriding potential calculator 13 of the nitriding potential adjustor 4 calculates an in-furnace nitriding potential (which is initially an extremely high value (since no hydrogen gas exists in the furnace), but decreases as decomposition of the ammonia gas (generation of the hydrogen gas) proceeds) and judges whether the calculated value has dropped lower than the sum of the target nitriding potential (0.6 in this example: see FIG. 3) and a standard margin. This standard margin can also be set and inputted by the parameter setting device 15, and is for example 0.1.

[0115] When it is determined that the temperature rising step has been completed and also it is determined that the calculated value of the in-furnace nitriding potential has dropped lower than the sum (0.7 in this example) of the target nitriding potential and the standard margin, the nitriding potential adjustor 4 starts to control an introduction amount of each of the furnace introduction gases via the gas introduction amount controller 14. Herein, the on-off valve controller 16 opens the on-off valve 17.

[0116] When the on-off valve 17 is opened, the processing furnace 2 and the so atmospheric gas concentration detector 3 communicate with each other, and then the atmospheric gas concentration detector 3 detects an in-furnace hydrogen concentration or an in-furnace ammonia concentration, and detects an in-furnace oxygen concentration. The detected hydrogen concentration signal or ammonia concentration signal and the detected oxygen concentration signal are outputted to the nitriding potential adjustor 4 and the recorder 6.

[0117] The in-furnace nitriding potential calculator 13 of the nitriding potential adjustor 4 calculates the in-furnace nitriding potential based on the inputted hydrogen concentration signal or ammonia concentration signal and the inputted oxygen concentration signal. Then, the gas flow rate output adjustor 30 performs the PID control method in which the respective gas introduction amounts of the ammonia gas and the carbon dioxide gas among the three kinds of furnace introduction gases are input values, the nitriding potential calculated by the in-furnace nitriding potential calculator 13 is an output value, and the target nitriding potential (the set nitriding potential) is a target value. Specifically, in the present PID control method, the nitriding potential in the processing furnace 2 is brought close to the target nitriding potential while the relationship of C1=c1×(A+x×B) is maintained, by changing the introduction amount of the ammonia gas and the introduction amount of the carbon dioxide gas while keeping the introduction amount of the ammonia decomposition gas constant. In the present PID control method, the setting parameter values that have been set and inputted by the parameter setting device 15 are used. The setting parameter values may be different depending on values of the target nitriding potential.

[0118] Then, the gas introduction amount controller 14 controls the introduction amount of the ammonia gas and the introduction amount of the carbon dioxide gas as a result of the PID control method. The gas introduction amount controller 14 transmits control signals to the first supply amount controller 22 for the ammonia gas, the second supply amount controller 26 for the ammonia decomposition gas (whose flow rate is constant) and the third supply amount controller 62 for the carbon dioxide gas, in order to realize the respective determined introduction amounts of the furnace introduction gases.

[0119] According to the control as described above, as shown in FIG. 3, the in-furnace nitriding potential can be stably controlled in the vicinity of the target nitriding potential. Thereby, the surface hardening treatment of the work S can be performed with extremely high quality. As a specific example. in the example shown in FIGS. 2 and 3, a feedback control is performed with a sampling rate of about several hundred milliseconds, and the introduction amount of the ammonia gas is increased and decreased within a range of about 3 ml (±1.5 ml), so that the nitriding potential can be controlled to the target nitriding potential (0.6) with extremely high precision since a timing of about 30 minutes after starting the treatment. (In the example shown in FIGS. 2 and 3, recording of the respective gas introduction amounts and the nitriding potential was stopped at a timing of about 190 minutes after starting the treatment.)

Operation: Example 1-2

[0120] Next, another case is explained as an example 1-2, in which the surface hardening treatment device 1 according to the present embodiment is used and the target nitriding potential is set to 0.4. In the example 1-2 as well, a pit furnace having a size of φ 700×1000 was used as the processing furnace 2, 570° C. was adopted as the temperature to be heated, and a steel material having a surface area of 4 m.sup.2 was used as the work S.

[0121] While the processing furnace 2 is heated, the ammonia gas, the ammonia decomposition gas and the carbon dioxide gas are introduced into the processing furnace 2 from the furnace introduction gas supplier 20 according to their respective initial introduction amounts. In this example, the initial introduction amount of the ammonia gas was set to 5.5 [l/min], the initial introduction amount of the ammonia decomposition gas was set to 25 [l/min], the initial introduction amount of the carbon dioxide gas was set to 0.95 [l/min], x=0.5 was set, and c1=0.053 was set. These initial introduction amounts can be set and inputted by the parameter setting device 15. Furthermore, the stirring fan drive motor 9 is driven and thus the stirring fan 8 rotates to stir the atmospheric gases in the processing furnace 2.

[0122] In the initial state, the on-off valve controller 16 closes the on-off valve 17.

[0123] In addition, the in-furnace temperature measurement device 10 measures a temperature of the in-furnace gases, and outputs an information signal including the measured temperature to the nitriding potential adjustor 4 and the recorder 6. The nitriding potential adjustor 4 judges whether the state in the processing furnace 2 is still during the temperature rising step or already after the temperature rising step has been completed (a stable state).

[0124] In addition, the in-furnace nitriding potential calculator 13 of the nitriding potential adjustor 4 calculates an in-furnace nitriding potential (which is initially an extremely high value (since no hydrogen gas exists in the furnace), but decreases as decomposition of the ammonia gas (generation of the hydrogen gas) proceeds) and judges whether the calculated value has dropped lower than the sum of the target nitriding potential (0.4 in this example) and a standard margin. This standard margin can also be set and inputted by the parameter setting device 15, and is for example 0.1.

[0125] When it is determined that the temperature rising step has been completed and also it is determined that the calculated value of the in-furnace nitriding potential has dropped lower than the sum (0.5 in this example) of the target nitriding potential and the standard margin, the nitriding potential adjustor 4 starts to control an introduction amount of each of the furnace introduction gases via the gas introduction amount controller 14, Herein, the on-off valve controller 16 opens the on-off valve 17.

[0126] When the on-off valve 17 is opened, the processing furnace 2 and the atmospheric gas concentration detector 3 communicate with each other, and then the atmospheric gas concentration detector 3 detects an in-furnace hydrogen concentration or an in-furnace ammonia concentration, and detects an in-furnace oxygen concentration. The detected hydrogen concentration signal or ammonia concentration signal and the detected oxygen concentration signal are outputted to the nitriding potential adjustor 4 and the recorder 6.

[0127] The in-furnace nitriding potential calculator 13 of the nitriding potential adjustor 4 calculates the in-furnace nitriding potential based on the inputted hydrogen concentration signal or ammonia concentration signal and the inputted oxygen concentration signal. Then, the gas flow rate output adjustor 30 performs the PID control method in which the respective gas introduction amounts of the ammonia gas and the carbon dioxide gas among the three kinds of furnace introduction gases are input values, the nitriding potential calculated in by the in-furnace nitriding potential calculator 13 is an output value, and the target nitriding potential (the set nitriding potential) is a target value. Specifically, in the present PID control method, the nitriding potential in the processing furnace 2 is brought close to the target nitriding potential while the relationship of C1=c1×(A+x×B) is maintained, by changing the introduction amount of the ammonia gas and the introduction amount of the carbon dioxide gas while keeping the introduction amount of the ammonia decomposition gas constant. In the present PID control method, the setting parameter values that have been set and inputted by the parameter setting device 15 are used. The setting parameter values may be different depending on values of the target nitriding potential.

[0128] Then, the gas introduction amount controller 14 controls the introduction amount of the ammonia gas and the introduction amount of the carbon dioxide gas as a result of the PID control method. The gas introduction amount controller 14 transmits control signals to the first supply amount controller 22 for the ammonia gas, the second supply amount controller 26 for the ammonia decomposition gas (whose flow rate is constant) and the third supply amount controller 62 for the carbon dioxide gas, in order to realize the respective determined introduction amounts of the furnace introduction gases.

[0129] According to the control as described above, the in-furnace nitriding potential can be stably controlled in the vicinity of the target nitriding potential. Thereby, the surface hardening treatment of the work S can be performed with extremely high quality. Specifically, a feedback control is performed with a sampling rate of about several hundred milliseconds, and the introduction amount of the ammonia gas is increased and decreased within a range of about 3 ml (±1.5 ml), so that the nitriding potential can be controlled to the target nitriding potential (0.4) with extremely high precision since a timing of about 30 minutes after starting the treatment.

Operation: Example 1-3

[0130] Next, further another case is explained as an example 1-3, in which the surface hardening treatment device 1 according to the present embodiment is used and the target nitriding potential is set to 0.2. In the example 1-3 as well, a pit furnace having a size of φ 700×1000 was used as the processing furnace 2, 570° C. was adopted as the temperature to be heated, and a steel material having a surface area of 4 m.sup.2 was used as the work S.

[0131] While the processing furnace 2 is heated, the ammonia gas, the ammonia decomposition gas and the carbon dioxide gas are introduced into the processing furnace 2 from the furnace introduction gas supplier 20 according to their respective initial introduction amounts. In this example, as shown in FIG. 4, the initial introduction amount of the ammonia gas was set to 2 [l/min], the initial introduction amount of the ammonia decomposition gas was set to 29 [l/min], the initial introduction amount of the carbon dioxide gas was set to 0.87 [l/min], x=0.5 was set, and c1=0.053 was set. These initial introduction amounts can be set and inputted by the parameter setting device 15. Furthermore, the stirring fan drive motor 9 is driven and thus the stirring fan 8 rotates to stir the atmospheric gases in the processing furnace 2.

[0132] In the initial state, the on-off valve controller 16 closes the on-off valve 17.

[0133] In addition, the in-furnace temperature measurement device 10 measures a temperature of the in-furnace gases, and outputs an information signal including the measured temperature to the nitriding potential adjustor 4 and the recorder 6. The nitriding potential adjustor 4 judges whether the state in the processing furnace 2 is still during the temperature rising step or already after the temperature rising step has been completed (a stable state).

[0134] In addition, the in-furnace nitriding potential calculator 13 of the nitriding potential adjustor 4 calculates an in-furnace nitriding potential (which is initially an extremely high value (since no hydrogen gas exists in the furnace), but decreases as decomposition of the ammonia gas (generation of the hydrogen gas) proceeds) and judges whether the calculated value has dropped lower than the sum of the target nitriding potential (0.2 in this example: see FIG. 5) and a standard margin. This standard margin can also be set and inputted by the parameter setting device 15, and is for example 0.1.

[0135] When it is determined that the temperature rising step has been completed and also it is determined that the calculated value of the in-furnace nitriding potential has dropped lower than the sum (0.3 in this example) of the target nitriding potential and the standard margin, the nitriding potential adjustor 4 starts to control an introduction amount of each of the furnace introduction gases via the gas introduction amount controller 14. Herein, the on-off valve controller 16 opens the on-off valve 17.

[0136] When the on-off valve 17 is opened, the processing furnace 2 and the atmospheric gas concentration detector 3 communicate with each other, and then the atmospheric gas concentration detector 3 detects an in-furnace hydrogen concentration or an in-furnace ammonia concentration, and detects an in-furnace oxygen concentration. The detected hydrogen concentration signal or ammonia concentration signal and the detected oxygen concentration signal are outputted to the nitriding potential adjustor 4 and the recorder 6.

[0137] The in-furnace nitriding potential calculator 13 of the nitriding potential adjustor 4 calculates the in-furnace nitriding potential based on the inputted hydrogen concentration signal or ammonia concentration signal and the inputted oxygen concentration signal. Then, the gas flow rate output adjustor 30 performs the PID control method in which the respective gas introduction amounts of the ammonia gas and the carbon dioxide gas among the three kinds of furnace introduction gases are input values, the nitriding potential calculated by the in-furnace nitriding potential calculator 13 is an output value, and the target nitriding potential (the set nitriding potential) is a target value. Specifically, in the present PID control method, the nitriding potential in the processing furnace 2 is brought close to the target nitriding potential while the relationship of C1=c1×(A+x×B) is maintained, by changing the introduction amount of the ammonia gas and the introduction amount of the carbon dioxide gas while keeping the introduction amount of the ammonia decomposition gas constant. In the present PID control method, the setting parameter values that have been set and inputted by the parameter setting device 15 are used. The setting parameter values may be different depending on values of the target nitriding potential.

[0138] Then, the gas introduction amount controller 14 controls the introduction amount of the ammonia gas and the introduction amount of the carbon dioxide gas as a result of the PID control method. The gas introduction amount controller 14 transmits control signals to the first supply amount controller 22 for the ammonia gas, the second supply amount controller 26 for the ammonia decomposition gas (whose flow rate is constant) and the third supply amount controller 62 for the carbon dioxide gas, in order to realize the respective determined introduction amounts of the furnace introduction gases.

[0139] According to the control as described above, as shown in FIG. 5, the in-furnace nitriding potential can be stably controlled in the vicinity of the target nitriding potential. Thereby, the surface hardening treatment of the work S can be performed with extremely high quality. As a specific example, in the example shown in FIGS. 4 and 5, a feedback control is performed with a sampling rate of about several hundred milliseconds, and the introduction amount of the ammonia gas is increased and decreased within a range of about 3 ml (±1.5 ml), so that the nitriding potential can be controlled to the target nitriding potential (0.2) with extremely high precision since a timing of about 30 minutes after starting the treatment. (In the example shown in FIGS. 4 and 5, recording of the respective gas introduction amounts and the nitriding potential was stopped at a timing of about 160 minutes after starting the treatment.)

[0140] (Explanation of Comparative Examples)

[0141] As comparative examples, controls of nitriding potential were performed. In each of them, the ammonia decomposition gas was not introduced, the ratio of the introduction amounts of the ammonia gas and the carbon dioxide gas was always maintained at 95:5, and the total introduction amount thereof was changed.

[0142] Specifically, the in-furnace nitriding potential calculator 13 of the nitriding potential adjustor 4 calculated the in-furnace nitriding potential based on the inputted hydrogen concentration signal or ammonia concentration signal and the inputted oxygen concentration signal. Then, the gas flow rate output adjustor 30 performed the PID control method in which the respective gas introduction amounts of the ammonia gas and the carbon dioxide gas were input values, the nitriding potential calculated by the in-furnace nitriding potential calculator 13 was an output value, and the target nitriding potential (the set nitriding potential) was a target value. More specifically, in the present PID control method, the nitriding potential in the processing furnace 2 was brought close to the target nitriding potential, by changing the total introduction amount of the ammonia gas and the carbon dioxide gas while keeping the ratio of the introduction amounts of the ammonia gas and the carbon dioxide gas constant.

[0143] However, in the above comparative examples, the nitriding potential could not be stably controlled.

Comparison Between Examples 1-1 to 1-3 and Comparative Examples

[0144] A table of the above results is shown as FIG. 6.

[0145] (Structure of Second Embodiment)

[0146] As shown in FIG. 7, in a second embodiment, a third furnace introduction gas supplier 61′ is formed by a tank filled with not a carbon dioxide gas but a carbon monoxide gas.

[0147] In the second embodiment, the introduction amount C1 of the carbon monoxide gas, which is a furnace introduction gas except for (other than) the ammonia gas and the ammonia decomposition gas, is controlled using a factor of proportionality c1 assigned to the carbon monoxide gas, such that C1=c1×(A+x×B), wherein the introduction amount of the ammonia gas is represented by A, the introduction amount of the ammonia decomposition gas is represented by B, and a predetermined constant is represented by x.

[0148] The other structure of the second embodiment is substantially the same as that of the first embodiment explained with reference to FIG. 1. In FIG. 7, the same parts as those of the first embodiment are shown by the same reference numerals, and detailed explanation thereof is omitted.

Operation: Example 2-1

[0149] Next, a case is explained as an example 2-1, in which the surface hardening treatment device according to the second embodiment is used and the target nitriding potential is set to 0.6. In the example 2-1 as well, a pit furnace having a size of φ 700×1000 was used as the processing furnace 2, 570° C. was adopted as the temperature to be heated, and a steel material having a surface area of 4 m.sup.2 was used as the work S.

[0150] While the processing furnace 2 is heated, the ammonia gas, the ammonia decomposition gas and the carbon monoxide gas are introduced into the processing furnace 2 from the furnace introduction gas supplier 20 according to their respective initial introduction amounts. In this example, the initial introduction amount of the ammonia gas was set to 5.5 [l/min], the initial introduction amount of the ammonia decomposition gas was set to 19 [l/min], the initial introduction amount of the carbon monoxide gas was set to 0.2 [l/min], x=0.5 was set, and c1=0.01 was set. These initial introduction amounts can be set and inputted by the parameter setting device 15. Furthermore, the stirring fan drive motor 9 is driven and thus the stirring fan 8 rotates to stir the atmospheric gases in the processing furnace 2.

[0151] In the initial state, the on-off valve controller 16 closes the on-off valve 17.

[0152] In addition, the in-furnace temperature measurement device 10 measures a temperature of the in-furnace gases, and outputs an information signal including the measured temperature to the nitriding potential adjustor 4 and the recorder 6. The nitriding potential adjustor 4 judges whether the state in the processing furnace 2 is still during the temperature rising step or already after the temperature rising step has been completed (a stable state).

[0153] In addition, the in-furnace nitriding potential calculator 13 of the nitriding potential adjustor 4 calculates an in-furnace nitriding potential (which is initially an extremely high value (since no hydrogen gas exists in the furnace), but decreases as decomposition of the ammonia gas (generation of the hydrogen gas) proceeds) and judges whether the calculated value has dropped lower than the sum of the target nitriding potential (0.6 in this example) and a standard margin. This standard margin can also be set and inputted by the parameter setting device 15, and is for example 0.1.

[0154] When it is determined that the temperature rising step has been completed and also it is determined that the calculated value of the in-furnace nitriding potential has dropped lower than the sum (0.7 in this example) of the target nitriding potential and the standard margin, the nitriding potential adjustor 4 starts to control an introduction amount of each of the furnace introduction gases via the gas introduction amount controller 14. Herein, the on-off valve controller 16 opens the on-off valve 17.

[0155] When the on-off valve 17 is opened, the processing furnace 2 and the atmospheric gas concentration detector 3 communicate with each other, and then the atmospheric gas concentration detector 3 detects an in-furnace hydrogen concentration or an in-furnace ammonia concentration, and detects an in-furnace oxygen concentration. The detected hydrogen concentration signal or ammonia concentration signal and the detected oxygen concentration signal are outputted to the nitriding potential adjustor 4 and the recorder 6.

[0156] The in-furnace nitriding potential calculator 13 of the nitriding potential adjustor 4 calculates the in-furnace nitriding potential based on the inputted hydrogen concentration signal or ammonia concentration signal and the inputted oxygen concentration signal. Then, the gas flow rate output adjustor 30 performs the PID control method in which the respective gas introduction amounts of the ammonia gas and the carbon monoxide gas among the three kinds of furnace introduction gases are input values, the nitriding potential calculated by the in-furnace nitriding potential calculator 13 is an output value, and the target nitriding potential (the set nitriding potential) is a target value. Specifically, in the present PID control method, the nitriding potential in the processing furnace 2 is brought close to the target nitriding potential while the relationship of C1=c1×(A+x×B) is maintained, by changing the introduction amount of the ammonia gas and the introduction amount of the carbon monoxide gas while keeping the introduction amount of the ammonia decomposition gas constant. In the present PID control method, the setting parameter values that have been set and inputted by the parameter setting device 15 are used. The setting parameter values may be different depending on values of the target nitriding potential.

[0157] Then, the gas introduction amount controller 14 controls the introduction amount of the ammonia gas and the introduction amount of the carbon monoxide gas as a result of the PID control method. The gas introduction amount controller 14 transmits control signals to the first supply amount controller 22 for the ammonia gas, the second supply amount controller 26 for the ammonia decomposition gas (whose flow rate is constant) and the third supply amount controller 62 for the carbon monoxide gas, in order to realize the respective determined introduction amounts of the furnace introduction gases.

[0158] According to the control as described above, the in-furnace nitriding potential can be stably controlled in the vicinity of the target nitriding potential. Thereby, the surface hardening treatment of the work S can be performed with extremely high quality. Specifically, a feedback control is performed with a sampling rate of about several hundred milliseconds, and the introduction amount of the ammonia gas is increased and decreased within a range of about 3 ml (±1.5 ml), so that the nitriding potential can be controlled to the target nitriding potential (0.6) with extremely high precision since a timing of about 20 minutes after starting the treatment.

Operation: Example 2-2

[0159] Next, another case is explained as an example 2-2, in which the surface hardening treatment device according to the second embodiment is used and the target nitriding potential is set to 0.4. In the example 2-2 as well, a pit furnace having a size of φ 700×1000 was used as the processing furnace 2, 570° C. was adopted as the temperature to be heated, and a steel material having a surface area of 4 m.sup.2 was used as the work S.

[0160] While the processing furnace 2 is heated, the ammonia gas, the ammonia decomposition gas and the carbon monoxide gas are introduced into the processing furnace 2 from the furnace introduction gas supplier 20 according to their respective initial introduction amounts. In this example, as shown in FIG. 8, the initial introduction amount of the ammonia gas was set to 3 [l/min], the initial introduction amount of the ammonia decomposition gas was set to 25 [l/min], the initial introduction amount of the carbon monoxide gas was set to 0.15 [l/min]. x=0.5 was set, and c1=0.01 was set. These initial introduction amounts can be set and inputted by the parameter setting device 15. Furthermore, the stirring fan drive motor 9 is driven and thus the stirring fan 8 rotates to stir the atmospheric gases in the processing furnace 2.

[0161] In the initial state, the on-off valve controller 16 closes the on-off valve 17.

[0162] In addition, the in-furnace temperature measurement device 10 measures a temperature of the in-furnace gases, and outputs an information signal including the measured temperature to the nitriding potential adjustor 4 so and the recorder 6. The nitriding potential adjustor 4 judges whether the state in the processing furnace 2 is still during the temperature rising step or already after the temperature rising step has been completed (a stable state).

[0163] In addition, the in-furnace nitriding potential calculator 13 of the nitriding potential adjustor 4 calculates an in-furnace nitriding potential (which is initially an extremely high value (since no hydrogen gas exists in the furnace), but decreases as decomposition of the ammonia gas (generation of the hydrogen gas) proceeds) and judges whether the calculated value has dropped lower than the sum of the target nitriding potential (0.4 in this example) and a standard margin. This standard margin can also be set and inputted by the parameter setting device 15, and is for example 0.1.

[0164] When it is determined that the temperature rising step has been completed and also it is determined that the calculated value of the in-furnace nitriding potential has dropped lower than the sum (0.5 in this example) of the target nitriding potential and the standard margin, the nitriding potential adjustor 4 starts to control an introduction amount of each of the furnace introduction gases via the gas introduction amount controller 14. Herein, the on-off valve controller 16 opens the on-off valve 17.

[0165] When the on-off valve 17 is opened, the processing furnace 2 and the atmospheric gas concentration detector 3 communicate with each other, and then the atmospheric gas concentration detector 3 detects an in-furnace hydrogen concentration or an in-furnace ammonia concentration, and detects an in-furnace oxygen concentration. The detected hydrogen concentration signal or ammonia concentration signal and the detected oxygen concentration signal are outputted to the nitriding potential adjustor 4 and the recorder 6.

[0166] The in-furnace nitriding potential calculator 13 of the nitriding potential adjustor 4 calculates the in-furnace nitriding potential based on the inputted hydrogen concentration signal or ammonia concentration signal and the inputted oxygen concentration signal. Then, the gas flow rate output adjustor 30 performs the PID control method in which the respective gas introduction amounts of the ammonia gas and the carbon monoxide gas among the three kinds of furnace introduction gases are input values, the nitriding potential calculated by the in-furnace nitriding potential calculator 13 is an output value, and the target nitriding potential (the set nitriding potential) is a target value. Specifically, in the present PID control method, the nitriding potential in the processing furnace 2 is brought close to the target nitriding potential while the relationship of C1=c1×(A+x×B) is maintained, by changing the introduction amount of the ammonia gas and the introduction amount of the carbon monoxide gas while keeping the introduction amount of the ammonia decomposition gas constant. In the present PID control method, the setting parameter values that have been set and inputted by the parameter setting device 15 are used. The setting parameter values may be different depending on values of the target nitriding potential.

[0167] Then, the gas introduction amount controller 14 controls the introduction amount of the ammonia gas and the introduction amount of the carbon monoxide gas as a result of the PID control method. The gas introduction amount controller 14 transmits control signals to the first supply amount controller 22 for the ammonia gas. the second supply amount controller 26 for the ammonia decomposition gas (whose flow rate is constant) and the third supply amount controller 62 for the carbon monoxide gas, in order to realize the respective determined introduction amounts of the furnace introduction gases.

[0168] According to the control as described above, as shown in FIG. 9, the in-furnace nitriding potential can be stably controlled in the vicinity of the target nitriding potential. Thereby, the surface hardening treatment of the work S can be performed with extremely high quality. Specifically, a feedback control is performed with a sampling rate of about several hundred milliseconds, and the introduction amount of the ammonia gas is increased and decreased within a range of about 3 ml (±1.5 ml), so that the nitriding potential can be controlled to the target nitriding potential (0.4) with extremely high precision since a timing of about 20 minutes after starting the treatment.

Operation: Example 2-3

[0169] Next, another case is explained as an example 2-3, in which the surface hardening treatment device according to the second embodiment is used and the target nitriding potential is set to 0.2. In the example 2-3 as well, a pit furnace having a size of φ 700×1000 was used as the processing furnace 2, 570° C. was adopted as the temperature to be heated, and a steel material having a surface area of 4 m.sup.2 was used as the work S.

[0170] While the processing furnace 2 is heated, the ammonia gas, the ammonia decomposition gas and the carbon monoxide gas are introduced into the processing furnace 2 from the furnace introduction gas supplier 20 according to their respective initial introduction amounts. In this example, the initial introduction amount of the ammonia gas was set to 1 [l/min], the initial introduction amount of the ammonia decomposition gas was set to 29 [l/min], the initial introduction amount of the carbon monoxide gas was set to 0.15 [l/min]. x=0.5 was set, and c1=0.01 was set. These initial introduction amounts can be set and inputted by the parameter setting device 15. Furthermore, the stirring fan drive motor 9 is driven and thus the stirring fan 8 rotates to stir the atmospheric gases in the processing furnace 2.

[0171] In the initial state, the on-off valve controller 16 closes the on-off valve 17.

[0172] In addition, the in-furnace temperature measurement device 10 measures a temperature of the in-furnace gases, and outputs an information signal including the measured temperature to the nitriding potential adjustor 4 and the recorder 6. The nitriding potential adjustor 4 judges whether the state in the processing furnace 2 is still during the temperature rising step or already after the temperature rising step has been completed (a stable state).

[0173] In addition, the in-furnace nitriding potential calculator 13 of the nitriding potential adjustor 4 calculates an in-furnace nitriding potential (which is initially an extremely high value (since no hydrogen gas exists in the furnace), but decreases as decomposition of the ammonia gas (generation of the hydrogen gas) proceeds) and judges whether the calculated value has dropped lower than the sum of the target nitriding potential (0.3 in this example) and a standard margin. This standard margin can also be set and inputted by the parameter setting device 15, and is for example 0.1.

[0174] When it is determined that the temperature rising step has been completed and also it is determined that the calculated value of the in-furnace nitriding potential has dropped lower than the sum (0.4 in this example) of the target nitriding potential and the standard margin, the nitriding potential adjustor 4 starts to control an introduction amount of each of the furnace introduction gases via the gas introduction amount controller 14. Herein, the on-off valve controller 16 opens the on-off valve 17.

[0175] When the on-off valve 17 is opened, the processing furnace 2 and the atmospheric gas concentration detector 3 communicate with each other, and then the atmospheric gas concentration detector 3 detects an in-furnace hydrogen concentration or an in-furnace ammonia concentration, and detects an in-furnace oxygen concentration. The detected hydrogen concentration signal or ammonia concentration signal and the detected oxygen concentration signal are outputted to the nitriding potential adjustor 4 and the recorder 6.

[0176] The in-furnace nitriding potential calculator 13 of the nitriding potential adjustor 4 calculates the in-furnace nitriding potential based on the inputted hydrogen concentration signal or ammonia concentration signal and the inputted oxygen concentration signal. Then, the gas flow rate output adjustor 30 performs the PID control method in which the respective gas introduction amounts of the ammonia gas and the carbon monoxide gas among the three kinds of furnace introduction gases are input values, the nitriding potential calculated by the in-furnace nitriding potential calculator 13 is an output value, and the target nitriding potential (the set nitriding potential) is a target value. Specifically, in the present PID control method, the nitriding potential in the processing furnace 2 is brought close to the target nitriding potential while the relationship of C1=c1×(A+x×B) is maintained, by changing the introduction amount of the ammonia gas and the introduction amount of the carbon monoxide gas while keeping the introduction amount of the ammonia decomposition gas constant. In the present PID control method, the setting parameter values that have been set and inputted by the parameter setting device 15 are used. The setting parameter values may be different depending on values of the target nitriding potential.

[0177] Then, the gas introduction amount controller 14 controls the introduction amount of the ammonia gas and the introduction amount of the carbon monoxide gas as a result of the PID control method. The gas in introduction amount controller 14 transmits control signals to the first supply amount controller 22 for the ammonia gas, the second supply amount controller 26 for the ammonia decomposition gas (whose flow rate is constant) and the third supply amount controller 62 for the carbon monoxide gas, in order to realize the respective determined introduction amounts of the furnace introduction gases.

[0178] According to the control as described above, the in-furnace nitriding potential can be stably controlled in the vicinity of the target nitriding potential. Thereby, the surface hardening treatment of the work S can be performed with extremely high quality. Specifically, a feedback control is performed with a sampling rate of about several hundred milliseconds, and the introduction amount of the ammonia gas is increased and decreased within a range of about 3 ml (±1.5 ml), so that the nitriding potential can be controlled to the target nitriding potential (0.2) with extremely high precision since a timing of about 30 minutes after starting the treatment.

Explanation of Comparative Examples

[0179] As comparative examples, controls of nitriding potential were performed. In each of them, the ammonia decomposition gas was not introduced, the ratio of the introduction amounts of the ammonia gas and the carbon monoxide gas was always maintained at 99:1, and the total introduction amount thereof was changed.

[0180] Specifically, the in-furnace nitriding potential calculator 13 of the nitriding potential adjustor 4 calculated the in-furnace nitriding potential based on the inputted hydrogen concentration signal or ammonia concentration signal and the inputted oxygen concentration signal. Then, the gas flow rate output adjustor 30 performed the PID control method in which the respective gas introduction amounts of the ammonia gas and the carbon monoxide gas were input values, the nitriding potential calculated by the in-furnace nitriding potential calculator 13 was an output value, and the target nitriding potential (the set nitriding potential) was a target value. More specifically, in the present MD control method, the nitriding potential in the processing furnace 2 was brought close to the target nitriding potential, by changing the total introduction amount of the ammonia gas and the carbon monoxide gas while keeping the ratio of the introduction amounts of the ammonia gas and the carbon monoxide gas constant.

[0181] However, in the above comparative examples, the nitriding potential could not be stably controlled.

Comparison Between Examples 2-1 to 2-3 and Comparative Examples

[0182] A table of the above results is shown as FIG. 10.

[0183] (Structure of Third Embodiment)

[0184] As shown in FIG. 11, a furnace introduction gas supplier 20′ of a third embodiment further includes a fourth furnace introduction gas supplier 71 for nitrogen gas, a fourth supply amount controller 72, a fourth supply valve 73 and a fourth flow meter 74.

[0185] The fourth furnace introduction gas supplier 71 is formed by, for example, a tank filled with a fourth furnace introduction gas (in this example, nitrogen gas).

[0186] The fourth supply amount controller 72 is formed by a mass flow controller (which can finely change a flow rate within a short time period), and is interposed between the fourth furnace introduction gas supplier 71 and the fourth supply valve 73. An opening degree of the fourth supply amount controller 72 changes according to the control signal outputted from the gas introduction amount controller 14. In addition, the fourth supply amount controller 72 is configured to detect a supply amount from the fourth furnace introduction gas supplier 71 to the fourth supply valve 73, and output an information signal including the detected supply amount to the gas introduction amount controller 14 and the recorder 6. This information signal can be used for correction or the like of the control performed by the gas introduction amount controller 14.

[0187] The fourth supply valve 73 is formed by an electromagnetic valve configured to switch between opened and closed states according to a control signal outputted from the gas introduction amount controller 14, and is interposed between the fourth supply amount controller 72 and the fourth flow meter 74.

[0188] The fourth flow meter 74 is formed by, for example, a mechanical flow meter such as a flow-type flow meter, and is interposed between the fourth supply valve 73 and the furnace introduction gas pipe 29. The fourth flow meter 74 detects a supply amount from the fourth supply valve 73 to the furnace introduction gas pipe 29. The supply amount detected by the fourth flow meter 74 can be provided for an operator's visual confirmation.

[0189] In the third embodiment, the introduction amount C1 of the carbon dioxide gas and the introduction amount C2 of the nitrogen gas, which are furnace introduction gases except for (other than) the ammonia gas and the ammonia decomposition gas, are controlled using a factor of proportionality c1 assigned to the carbon dioxide gas and a factor of proportionality c2 assigned to the nitrogen gas, such that C1=c1×(A+x×B) and C2=c2×(A+x×B), wherein the introduction amount of the ammonia gas is represented by A, the introduction amount of the ammonia decomposition gas is represented by B, and a predetermined constant is represented by x.

[0190] The other structure of the third embodiment is substantially the same as that of the first embodiment explained with reference to FIG. 1. In FIG. 11, the same parts as those of the first embodiment are shown by the same reference numerals, and detailed explanation thereof is omitted.)

Operation: Example 3-1

[0191] Next, a case is explained as an example 3-1, in which the surface hardening treatment device according to the third embodiment is used and the target nitriding potential is set to 1.0. In the example 3-1 as well, a pit furnace having a size of φ 700×1000 was used as the processing furnace 2, 570° C. was adopted as the temperature to be heated, and a steel material having a surface area of 4 m.sup.2 was used as the work S.

[0192] While the processing furnace 2 is heated, the ammonia gas, the ammonia decomposition gas, the carbon dioxide gas and the nitrogen gas are introduced into the processing furnace 2 from the furnace introduction gas supplier 20′ according to their respective initial introduction amounts. In this example, the initial introduction amount of the ammonia gas was set to 13 [l/min], the initial introduction amount of the ammonia decomposition gas was set to 19 [l/min], the initial introduction amount of the carbon dioxide gas was set to 2.2 [l/min], the initial introduction amount of the nitrogen gas was set to 20 [l/min], x=0.5 was set, c1=0.1 was set, and c2=0.9 was set. These initial introduction amounts can be set and inputted by the parameter setting device 15. Furthermore, the stirring fan drive motor 9 is driven and thus the stirring fan 8 rotates to stir the atmospheric gases in the processing furnace 2.

[0193] In the initial state, the on-off valve controller 16 closes the on-off valve 17.

[0194] In addition, the in-furnace temperature measurement device 10 measures a temperature of the in-furnace gases, and outputs an information signal including the measured temperature to the nitriding potential adjustor 4 and the recorder 6. The nitriding potential adjustor 4 judges whether the state in the processing furnace 2 is still during the temperature rising step or already after the temperature rising step has been completed (a stable state).

[0195] In addition, the in-furnace nitriding potential calculator 13 of the nitriding potential adjustor 4 calculates an in-furnace nitriding potential (which is initially an extremely high value (since no hydrogen gas exists in the furnace), but decreases as decomposition of the ammonia gas (generation of the hydrogen gas) proceeds) and judges whether the calculated value has dropped lower than the sum of the target nitriding potential (1.0 in this example) and a standard margin. This standard margin can also be set and inputted by the parameter setting device 15, and is for example 0.1.

[0196] When it is determined that the temperature rising step has been completed and also it is determined that the calculated value of the in-furnace nitriding potential has dropped lower than the sum (1.1 in this example) of the target nitriding potential and the standard margin, the nitriding potential adjustor 4 starts to control an introduction amount of each of the furnace introduction gases via the gas introduction amount controller 14. Herein, the on-off valve controller 16 opens the on-off valve 17.

[0197] When the on-off valve 17 is opened, the processing furnace 2 and the atmospheric gas concentration detector 3 communicate with each other, and then the atmospheric gas concentration detector 3 detects an in-furnace hydrogen concentration or an in-furnace ammonia concentration, and detects an in-furnace oxygen concentration. The detected hydrogen concentration signal or ammonia concentration signal and the detected oxygen concentration signal are outputted to the nitriding potential adjustor 4 and the recorder 6.

[0198] The in-furnace nitriding potential calculator 13 of the nitriding potential adjustor 4 calculates the in-furnace nitriding potential based on the inputted hydrogen concentration signal or ammonia concentration signal and the inputted oxygen concentration signal. Then, the gas flow rate output adjustor 30 performs the Pill control method in which the respective gas introduction amounts of the ammonia gas, the carbon dioxide gas and the nitrogen gas among the four kinds of furnace introduction gases are input values, the nitriding potential calculated by the in-furnace nitriding potential calculator 13 is an output value, and the target nitriding potential (the set nitriding potential) is a target value. Specifically, in the present PID control method, the nitriding potential in the processing furnace 2 is brought close to the target nitriding potential while the relationships of C1=c1×(A+x×B) and C2=c2×(A+x×B) are maintained, by changing the introduction amount of the ammonia gas, the introduction amount of the carbon dioxide gas and the introduction amount of the nitrogen gas while keeping the introduction amount of the ammonia decomposition gas constant. In the present PID control method, the setting parameter values that have been set and inputted by the parameter setting device 15 are used. The setting parameter values may be different depending on values of the target nitriding potential.

[0199] Then, the gas introduction amount controller 14 controls the introduction amount of the ammonia gas, the introduction amount of the carbon dioxide gas and the introduction amount of the nitrogen gas as a result of the PID control method. The gas introduction amount controller 14 transmits control signals to the first supply amount controller 22 for the ammonia gas, the second supply amount controller 26 for the ammonia decomposition gas (whose flow rate is constant), the third supply amount controller 62 for the carbon dioxide gas and the fourth supply amount controller 72 for the nitrogen gas, in order to realize the respective determined introduction amounts of the furnace introduction gases.

[0200] According to the control as described above, the in-furnace nitriding potential can be stably controlled in the vicinity of the target nitriding potential. Thereby, the surface hardening treatment of the work S can be performed with extremely high quality. Specifically, a feedback control is performed with a sampling rate of about several hundred milliseconds, and the introduction amount of the ammonia gas is increased and decreased within a range of about 3 ml (±1.5 ml), so that the nitriding potential can be controlled to the target nitriding potential (1.0) with extremely high precision since a timing of about 20 minutes after starting the treatment.

Operation: Example 3-2

[0201] Next, another case is explained as an example 3-2, in which the surface hardening treatment device according to the third embodiment is used and the target nitriding potential is set to 0.6. In the example 3-2 as well, a pit furnace having a size of φ 700×1000 was used as the processing furnace 2, 570° C. was adopted as the temperature to be heated, and a steel material having a surface area of 4 m.sup.2 was used as the work S.

[0202] While the processing furnace 2 is heated, the ammonia gas, the ammonia decomposition gas, the carbon dioxide gas and the nitrogen gas are introduced into the processing furnace 2 from the furnace introduction gas supplier 20′ according to their respective initial introduction amounts. In this example, as shown in FIG. 12, the initial introduction amount of the ammonia gas was set to 8 [l/min], the initial introduction amount of the ammonia decomposition gas was set to 25 [l/min], the initial introduction amount of the carbon dioxide gas was set to 2 [l/min], the initial introduction amount of the nitrogen gas was set to 18.5 [l/min], x=0.5 was set, c1=0.1 was set, and c2=0.9 was set. These initial introduction amounts can be set and inputted by the parameter setting device 15. Furthermore, the stirring fan drive motor 9 is driven and thus the stirring fan 8 rotates to stir the atmospheric gases in the processing furnace 2.

[0203] In the initial state, the on-off valve controller 16 closes the on-off valve 17.

[0204] In addition, the in-furnace temperature measurement device 10 measures a temperature of the in-furnace gases, and outputs an information signal including the measured temperature to the nitriding potential adjustor 4 and the recorder 6. The nitriding potential adjustor 4 judges whether the state in the processing furnace 2 is still during the temperature rising step or already after the temperature rising step has been completed (a stable state).

[0205] In addition, the in-furnace nitriding potential calculator 13 of the nitriding potential adjustor 4 calculates an in-furnace nitriding potential (which is initially an extremely high value (since no hydrogen gas exists in the furnace), but decreases as decomposition of the ammonia gas (generation of the hydrogen gas) proceeds) and judges whether the calculated value has dropped lower than the sum of the target nitriding potential (0.6 in this example) and a standard margin. This standard margin can also be set and inputted by the parameter setting device 15, and is for example 0.1.

[0206] When it is determined that the temperature rising step has been completed and also it is determined that the calculated value of the in-furnace nitriding potential has dropped lower than the sum (0.7 in this example) of the target nitriding potential and the standard margin, the nitriding potential adjustor 4 starts to control an introduction amount of each of the furnace introduction gases via the gas introduction amount controller 14. Herein, the on-off valve controller 16 opens the on-off valve 17.

[0207] When the on-off valve 17 is opened, the processing furnace 2 and the atmospheric gas concentration detector 3 communicate with each other, and then the atmospheric gas concentration detector 3 detects an in-furnace hydrogen concentration or an in-furnace ammonia concentration, and detects an in-furnace oxygen concentration. The detected hydrogen concentration signal or ammonia concentration signal and the detected oxygen concentration signal are outputted to the nitriding potential adjustor 4 and the recorder 6.

[0208] The in-furnace nitriding potential calculator 13 of the nitriding potential adjustor 4 calculates the in-furnace nitriding potential based on the inputted hydrogen concentration signal or ammonia concentration signal and the inputted oxygen concentration signal. Then, the gas flow rate output adjustor 30 performs the PO control method in which the respective gas introduction amounts of the ammonia gas, the carbon dioxide gas and the nitrogen gas among the four kinds of furnace introduction gases are input values, the nitriding potential calculated by the in-furnace nitriding potential calculator 13 is an output value, and the target nitriding potential (the set nitriding potential) is a target value. Specifically, in the present PID control method, the nitriding potential in the processing furnace 2 is brought close to the target nitriding potential while the relationships of C1=c1×(A+x×B) and C2=c2×(A+x×B) are maintained, by changing the introduction amount of the ammonia gas, the introduction amount of the carbon dioxide gas and the introduction amount of the nitrogen gas while keeping the introduction amount of the ammonia decomposition gas constant. In the present PID control method, the setting parameter values that have been set and inputted by the parameter setting device 15 are used. The setting parameter values may be different depending on values of the target nitriding potential.

[0209] Then, the gas introduction amount controller 14 controls the introduction amount of the ammonia gas, the introduction amount of the carbon dioxide gas and the introduction amount of the nitrogen gas as a result of the PID control method. The gas introduction amount controller 14 transmits control signals to the first supply amount controller 22 for the ammonia gas, the second supply amount controller 26 for the ammonia decomposition gas (whose flow rate is constant), the third supply amount controller 62 for the carbon dioxide gas and the fourth supply amount controller 72 for the nitrogen gas, in order to realize the respective determined introduction amounts of the furnace introduction gases.

[0210] According to the control as described above, as shown in FIG. 13, the in-furnace nitriding potential can be stably controlled in the vicinity of the target nitriding potential. Thereby, the surface hardening treatment of the work S can be performed with extremely high quality. Specifically, a feedback control is performed with a sampling rate of about several hundred milliseconds, and the introduction amount of the ammonia gas is increased and decreased within a range of about 3 ml (±1.5 ml), so that the nitriding potential can be controlled to the target nitriding potential (0.6) with extremely high precision since a timing of about 30 minutes after starting the treatment.

Operation: Example 3-3

[0211] Next, another case is explained as an example 3-3, in which the surface hardening treatment device according to the third embodiment is used and the target nitriding potential is set to 0.2. In the example 3-3 as well, a pit furnace having a size of 9 700×1000 was used as the processing furnace 2, 570° C. was adopted as the temperature to be heated, and a steel material having a surface area of 4 m.sup.2 was used as the work S.

[0212] While the processing furnace 2 is heated, the ammonia gas, the ammonia decomposition gas, the carbon dioxide gas and the nitrogen gas are introduced into the processing furnace 2 from the furnace introduction gas supplier 20′ according to their respective initial introduction amounts. In this example, the initial introduction amount of the ammonia gas was set to 3 [l/min], the initial introduction amount of the ammonia decomposition gas was set to 29 [l/min], the initial introduction amount of the carbon dioxide gas was set to 1.8 [l/min], the initial introduction amount of the nitrogen gas was set to 15.8 [l/min], x=0.5 was set, c1=0.1 was set, and c2=0.9 was set. These initial introduction amounts can be set and inputted by the parameter setting device 15. Furthermore, the stirring fan drive motor 9 is driven and thus the stirring fan 8 rotates to stir the atmospheric gases in the processing furnace 2.

[0213] In the initial state, the on-off valve controller 16 closes the on-off valve 17.

[0214] In addition, the in-furnace temperature measurement device 10 measures a temperature of the in-furnace gases, and outputs an information signal including the measured temperature to the nitriding potential adjustor 4 and the recorder 6. The nitriding potential adjustor 4 judges whether the state in the processing furnace 2 is still during the temperature rising step or already after the temperature rising step has been completed (a stable state).

[0215] In addition, the in-furnace nitriding potential calculator 13 of the nitriding potential adjustor 4 calculates an in-furnace nitriding potential (which is initially an extremely high value (since no hydrogen gas exists in the furnace), but decreases as decomposition of the ammonia gas (generation of the hydrogen gas) proceeds) and judges whether the calculated value has dropped lower than the sum of the target nitriding potential (0.2 in this example) and a standard margin. This standard margin can also be set and inputted by the parameter setting device 15, and is for example 0.1.

[0216] When it is determined that the temperature rising step has been completed and also it is determined that the calculated value of the in-furnace nitriding potential has dropped lower than the sum (0.3 in this example) of the target nitriding potential and the standard margin, the nitriding potential adjustor 4 starts to control an introduction amount of each of the furnace introduction gases via the gas introduction amount controller 14. Herein, the on-off valve controller 16 opens the on-off valve 17.

[0217] When the on-off valve 17 is opened, the processing furnace 2 and the atmospheric gas concentration detector 3 communicate with each other, and then the atmospheric gas concentration detector 3 detects an in-furnace hydrogen concentration or an in-furnace ammonia concentration, and detects an in-furnace oxygen concentration. The detected hydrogen concentration signal or ammonia concentration signal and the detected oxygen concentration signal are outputted to the nitriding potential adjustor 4 and the recorder 6.

[0218] The in-furnace nitriding potential calculator 13 of the nitriding potential adjustor 4 calculates the in-furnace nitriding potential based on the inputted hydrogen concentration signal or ammonia concentration signal and the inputted oxygen concentration signal. Then, the gas flow rate output adjustor 30 performs the PID control method in which the respective gas introduction amounts of the ammonia gas, the carbon dioxide gas and the nitrogen gas among the four kinds of furnace introduction gases are input values, the nitriding potential calculated by the in-furnace nitriding potential calculator 13 is an output value, and the target nitriding potential (the set nitriding potential) is a target value. Specifically, in the present PID control method, the nitriding potential in the processing furnace 2 is brought close to the target nitriding potential while the relationships of C1=c1×(A+x×B) and C2=c2×(A+x×B) are maintained, by changing the introduction amount of the ammonia gas, the introduction amount of the carbon dioxide gas and the introduction amount of the nitrogen gas while keeping the introduction amount of the ammonia decomposition gas constant. In the present PID control method, the setting parameter values that have been set and inputted by the parameter setting device 15 are used. The setting parameter values may be different depending on values of the target nitriding potential.

[0219] Then, the gas introduction amount controller 14 controls the introduction amount of the ammonia gas, the introduction amount of the carbon dioxide gas and the introduction amount of the nitrogen gas as a result of the PID control method. The gas introduction amount controller 14 transmits control signals to the first supply amount controller 22 for the ammonia gas, the second supply amount controller 26 for the ammonia decomposition gas (whose flow rate is constant), the third supply amount controller 62 for the carbon dioxide gas and the fourth supply amount controller 72 for the nitrogen gas, in order to realize the respective determined introduction amounts of the furnace introduction gases.

[0220] According to the control as described above, the in-furnace nitriding potential can be stably controlled in the vicinity of the target nitriding potential. Thereby, the surface hardening treatment of the work S can be performed with extremely high quality. Specifically, a feedback control is performed with a sampling rate of about several hundred milliseconds, and the introduction amount of the ammonia gas is increased and decreased within a range of about 3 ml (±1.5 ml), so that the nitriding potential can be controlled to the target nitriding potential (0.2) with extremely high precision since a timing of about 40 minutes after starting the treatment.)

Explanation of Comparative Examples

[0221] As comparative examples, controls of nitriding potential were performed. In each of them, the ammonia decomposition gas was not introduced, the ratio of the introduction amounts of the ammonia gas, the nitrogen gas and the carbon dioxide gas was always maintained at 50:45:5, and the total introduction amount thereof was changed.

[0222] Specifically, the in-furnace nitriding potential calculator 13 of the nitriding potential adjustor 4 calculated the in-furnace nitriding potential based on the inputted hydrogen concentration signal or ammonia concentration signal and the inputted oxygen concentration signal. Then, the gas flow rate output adjustor 30 performed the PID control method in which the respective gas introduction amounts of the ammonia gas, the nitrogen gas and the carbon dioxide gas were input values, the nitriding potential calculated by the in-furnace nitriding potential calculator 13 was an output value, and the target nitriding potential (the set nitriding potential) was a target value. More specifically, in the present PID control method, the nitriding potential in the processing furnace 2 was brought close to the target nitriding potential, by changing the total introduction amount of the ammonia gas, the nitrogen gas and the carbon dioxide gas while keeping the ratio of the introduction amounts of the ammonia gas, the nitrogen gas and the carbon dioxide gas constant.

[0223] However, in the above comparative examples, the nitriding potential could not be stably controlled.

Comparison Between Examples 3-1 to 3-3 and Comparative Examples

[0224] A table of the above results is shown as FIG. 14.

[0225] (Structure of Fourth Embodiment)

[0226] As shown in FIG. 15, in a fourth embodiment, a third furnace introduction gas supplier 61′ is formed by a tank filled with not a carbon dioxide gas but a carbon monoxide gas.

[0227] In the fourth embodiment, the introduction amount C1 of the carbon monoxide gas and the introduction amount C2 of the nitrogen gas, which are furnace introduction gases except for (other than) the ammonia gas and the ammonia decomposition gas, are controlled using a factor of proportionality c1 assigned to the carbon monoxide gas and a factor of proportionality c2 assigned to the nitrogen gas, such that C1=c1×(A+x×B) and C2=c2×(A+x×B), wherein the introduction amount of the ammonia gas is represented by A, the introduction amount of the ammonia decomposition gas is represented by B, and a predetermined constant is represented by x.

[0228] The other structure of the fourth embodiment is substantially the same as that of the third embodiment explained with reference to FIG. 11. In FIG. 15, the same parts as those of the third embodiment are shown by the same reference numerals, and detailed explanation thereof is omitted.

Operation: Example 4-1

[0229] Next, a case is explained as an example 4-1, in which the surface hardening treatment device according to the fourth embodiment is used and the target nitriding potential is set to 1.0. In the example 4-1 as well, a pit furnace having a size of φ 700×1000 was used as the processing furnace 2, 570° C. was adopted as the temperature to be heated, and a steel material having a surface area of 4 m.sup.2 was used as the work S.

[0230] While the processing furnace 2 is heated, the ammonia gas, the ammonia decomposition gas, the carbon monoxide gas and the nitrogen gas are introduced into the processing furnace 2 from the furnace introduction gas supplier 20′ according to their respective initial introduction amounts. In this example, the initial introduction amount of the ammonia gas was set to 13 [l/min], the initial introduction amount of the ammonia decomposition gas was set to 19 [l/min], the initial introduction amount of the carbon monoxide gas was set to 0.9 [l/min], the initial introduction amount of the nitrogen gas was set to 20 [l/min], x=0.5 was set, c1=0.04 was set, and c2=0.96 was set. These initial introduction amounts can be set and inputted by the parameter setting device 15. Furthermore, the stirring fan drive motor 9 is driven and thus the stirring fan 8 rotates to stir the atmospheric gases in the processing furnace 2.

[0231] In the initial state, the on-off valve controller 16 closes the on-off valve 17.

[0232] In addition, the in-furnace temperature measurement device 10 measures a temperature of the in-furnace gases, and outputs an information signal including the measured temperature to the nitriding potential adjustor 4 and the recorder 6. The nitriding potential adjustor 4 judges whether the state in the processing furnace 2 is still during the temperature rising step or already after the temperature rising step has been completed (a stable state).

[0233] In addition, the in-furnace nitriding potential calculator 13 of the nitriding potential adjustor 4 calculates an in-furnace nitriding potential (which is initially an extremely high value (since no hydrogen gas exists in the furnace), but decreases as decomposition of the ammonia gas (generation of the hydrogen gas) proceeds) and judges whether the calculated value has dropped lower than the sum of the target nitriding potential (1.0 in this example) and a standard margin. This standard margin can also be set and inputted by the parameter setting device 15, and is for example 0.1.

[0234] When it is determined that the temperature rising step has been completed and also it is determined that the calculated value of the in-furnace nitriding potential has dropped lower than the sum (1.1 in this example) of the target nitriding potential and the standard margin, the nitriding potential adjustor 4 starts to control an introduction amount of each of the furnace introduction gases via the gas introduction amount controller 14. Herein, the on-off valve controller 16 opens the on-off valve 17.

[0235] When the on-off valve 17 is opened, the processing furnace 2 and the atmospheric gas concentration detector 3 communicate with each other, and then the atmospheric gas concentration detector 3 detects an in-furnace hydrogen concentration or an in-furnace ammonia concentration, and detects an in-furnace oxygen concentration. The detected hydrogen concentration signal or ammonia concentration signal and the detected oxygen concentration signal are outputted to the nitriding potential adjustor 4 and the recorder 6.

[0236] The in-furnace nitriding potential calculator 13 of the nitriding potential adjustor 4 calculates the in-furnace nitriding potential based on the inputted hydrogen concentration signal or ammonia concentration signal and the inputted oxygen concentration signal. Then, the gas flow rate output adjustor 30 performs the PID control method in which the respective gas introduction amounts of the ammonia gas, the carbon monoxide gas and the nitrogen gas among the four kinds of furnace introduction gases are input values, the nitriding potential calculated by the in-furnace nitriding potential calculator 13 is an output value, and the target nitriding potential (the set nitriding potential) is a target value. Specifically, in the present PID control method, the nitriding potential in the processing furnace 2 is brought close to the target nitriding potential while the relationships of C1=c1×(A+x×B) and C2=c2×(A+x×B) are maintained, by changing the introduction amount of the ammonia gas, the introduction amount of the carbon monoxide gas and the introduction amount of the nitrogen gas while keeping the introduction amount of the ammonia decomposition gas constant. In the present PID control method, the setting parameter values that have been set and inputted by the parameter setting device 15 are used. The setting parameter values may be different depending on values of the target nitriding potential.

[0237] Then, the gas introduction amount controller 14 controls the introduction amount of the ammonia gas, the introduction amount of the carbon monoxide gas and the introduction amount of the nitrogen gas as a result of the PID control method. The gas introduction amount controller 14 transmits control signals to the first supply amount controller 22 for the ammonia gas, the second supply amount controller 26 for the ammonia decomposition gas (whose flow rate is constant), the third supply amount controller 62 for the carbon monoxide gas and the fourth supply amount controller 72 for the nitrogen gas, in order to realize the respective determined introduction amounts of the furnace introduction gases.

[0238] According to the control as described above, the in-furnace nitriding potential can be stably controlled in the vicinity of the target nitriding potential. Thereby, the surface hardening treatment of the work S can be performed with extremely high quality. Specifically, a feedback control is performed with a sampling rate of about several hundred milliseconds, and the introduction amount of the ammonia gas is increased and decreased within a range of about 3 ml (±1.5 ml), so that the nitriding potential can be controlled to the target nitriding potential (1.0) with extremely high precision since a timing of about 30 minutes after starting the treatment.

Operation: Example 4-2

[0239] Next, another case is explained as an example 4-2, in which the surface hardening treatment device according to the fourth embodiment is used and the target nitriding potential is set to 0.6. In the example 4-2 as well, a pit furnace having a size of φ 700×1000 was used as the processing furnace 2, 570° C. was adopted as the temperature to be heated, and a steel material having a surface area of 4 m.sup.2 was used as the work S.

[0240] While the processing furnace 2 is heated, the ammonia gas, the ammonia decomposition gas, the carbon monoxide gas and the nitrogen gas are introduced into the processing furnace 2 from the furnace introduction gas supplier 20′ according to their respective initial introduction amounts. In this example, the initial introduction amount of the ammonia gas was set to 8 [l/min], the initial introduction amount of the ammonia decomposition gas was set to 25 [l/min], the initial introduction amount of the carbon monoxide gas was set to 0.8 [l/min], the initial introduction amount of the nitrogen gas was set to 19.7 [l/min], x=0.5 was set, c1=0.04 was set, and c2=0.96 was set. These initial introduction amounts can be set and inputted by the parameter setting device 15. Furthermore, the stirring fan drive motor 9 is driven and thus the stirring fan 8 rotates to stir the atmospheric gases in the processing furnace 2.

[0241] In the initial state, the on-off valve controller 16 closes the on-off valve 17.

[0242] In addition, the in-furnace temperature measurement device 10 measures a temperature of the in-furnace gases, and outputs an information signal including the measured temperature to the nitriding potential adjustor 4 and the recorder 6. The nitriding potential adjustor 4 judges whether the state in the processing furnace 2 is still during the temperature rising step or already after the temperature rising step has been completed (a stable state).

[0243] In addition, the in-furnace nitriding potential calculator 13 of the nitriding potential adjustor 4 calculates an in-furnace nitriding potential (which is initially an extremely high value (since no hydrogen gas exists in the furnace), but decreases as decomposition of the ammonia gas (generation of the hydrogen gas) proceeds) and judges whether the calculated value has dropped lower than the sum of the target nitriding potential (0.6 in this example) and a standard margin. This standard margin can also be set and inputted by the parameter setting device 15, and is for example 0.1.

[0244] When it is determined that the temperature rising step has been completed and also it is determined that the calculated value of the in-furnace nitriding potential has dropped lower than the sum (0.7 in this example) of the target nitriding potential and the standard margin, the nitriding potential adjustor 4 starts to control an introduction amount of each of the furnace introduction gases via the gas introduction amount controller 14. Herein, the on-off valve controller 16 opens the on-off valve 17.

[0245] When the on-off valve 17 is opened, the processing furnace 2 and the atmospheric gas concentration detector 3 communicate with each other, and then the atmospheric gas concentration detector 3 detects an in-furnace hydrogen concentration or an in-furnace ammonia concentration, and detects an in-furnace oxygen concentration. The detected hydrogen concentration signal or ammonia concentration signal and the detected oxygen concentration signal are outputted to the nitriding potential adjustor 4 and the recorder 6.

[0246] The in-furnace nitriding potential calculator 13 of the nitriding potential adjustor 4 calculates the in-furnace nitriding potential based on the inputted hydrogen concentration signal or ammonia concentration signal and the inputted oxygen concentration signal. Then, the gas flow rate output adjustor 30 performs the PID control method in which the respective gas introduction amounts of the ammonia gas, the carbon monoxide gas and the nitrogen gas among the four kinds of furnace introduction gases are input values, the nitriding potential calculated by the in-furnace nitriding potential calculator 13 is an output value, and the target nitriding potential (the set nitriding potential) is a target value. Specifically, in the present PID control method, the nitriding potential in the processing furnace 2 is brought close to the target nitriding potential while the relationships of C1=c1×(A+x×B) and C2=c2×(A+x×B) are maintained, by changing the introduction amount of the ammonia gas, the introduction amount of the carbon monoxide gas and the introduction amount of the nitrogen gas while keeping the introduction amount of the ammonia decomposition gas constant. In the present PID control method, the setting parameter values that have been set and inputted by the parameter setting device 15 are used. The setting parameter values may be different depending on values of the target nitriding potential.

[0247] Then, the gas introduction amount controller 14 controls the introduction amount of the ammonia gas, the introduction amount of the carbon monoxide gas and the introduction amount of the nitrogen gas as a result of the PID control method. The gas introduction amount controller 14 transmits control signals to the first supply amount controller 22 for the ammonia gas, the second supply amount controller 26 for the ammonia decomposition gas (whose flow rate is constant), the third supply amount controller 62 for the carbon monoxide gas and the fourth supply amount controller 72 for the nitrogen gas, in order to realize the respective determined introduction amounts of the furnace introduction gases.

[0248] According to the control as described above, the in-furnace nitriding potential can be stably controlled in the vicinity of the target nitriding potential. Thereby, the surface hardening treatment of the work S can be performed with extremely high quality. Specifically, a feedback control is performed with a sampling rate of about several hundred milliseconds, and the introduction amount of the ammonia gas is increased and decreased within a range of about 3 ml (±1.5 ml), so that the nitriding potential can be controlled to the target nitriding potential (0.6) with extremely high precision since a timing of about 40 minutes after starting the treatment.

Operation: Example 4-3

[0249] Next, another case is explained as an example 4-3, in which the surface hardening treatment device according to the fourth embodiment is used and the target nitriding potential is set to 0.2. In the example 4-3 as well, a pit furnace having a size of φ 700×1000 was used as the processing furnace 2, 570° C. was adopted as the temperature to be heated, and a steel material having a surface area of 4 m.sup.2 was used as the work S.

[0250] While the processing furnace 2 is heated, the ammonia gas, the ammonia decomposition gas, the carbon monoxide gas and the nitrogen gas are introduced into the processing furnace 2 from the furnace introduction gas supplier 20′ according to their respective initial introduction amounts. In this example, the initial introduction amount of the ammonia gas was set to 3 [l/min], the initial introduction amount of the ammonia decomposition gas was set to 29 [l/min], the initial introduction amount of the carbon monoxide gas was set to 0.7 [l/min], the initial introduction amount of the nitrogen gas was set to 16 [l/min], x=0.5 was set, c1=0.04 was set, and c2=0.96 was set. These initial introduction amounts can be set and inputted by the parameter setting device 15. Furthermore, the stirring fan drive motor 9 is driven and thus the stirring fan 8 rotates to stir the atmospheric gases in the processing furnace 2.

[0251] In the initial state, the on-off valve controller 16 closes the on-off valve 17.)

[0252] In addition, the in-furnace temperature measurement device 10 measures a temperature of the in-furnace gases, and outputs an information signal including the measured temperature to the nitriding potential adjustor 4 and the recorder 6. The nitriding potential adjustor 4 judges whether the state in the processing furnace 2 is still during the temperature rising step or already after the temperature rising step has been completed (a stable state).

[0253] In addition, the in-furnace nitriding potential calculator 13 of the nitriding potential adjustor 4 calculates an in-furnace nitriding potential (which is initially an extremely high value (since no hydrogen gas exists in the furnace), but decreases as decomposition of the ammonia gas (generation of the hydrogen gas) proceeds) and judges whether the calculated value has dropped lower than the sum of the target nitriding potential (0.2 in this example) and a standard margin. This standard margin can also be set and inputted by the parameter setting device 15, and is for example 0.1.

[0254] When it is determined that the temperature rising step has been completed and also it is determined that the calculated value of the in-furnace nitriding potential has dropped lower than the sum (0.3 in this example) of the target nitriding potential and the standard margin, the nitriding potential adjustor 4 starts to control an introduction amount of each of the furnace introduction gases via the gas introduction amount controller 14. Herein, the on-off valve controller 16 opens the on-off valve 17.

[0255] When the on-off valve 17 is opened, the processing furnace 2 and the atmospheric gas concentration detector 3 communicate with each other, and then the atmospheric gas concentration detector 3 detects an in-furnace hydrogen concentration or an in-furnace ammonia concentration, and detects an in-furnace oxygen concentration. The detected hydrogen concentration signal or ammonia concentration signal and the detected oxygen concentration signal are outputted to the nitriding potential adjustor 4 and the recorder 6.

[0256] The in-furnace nitriding potential calculator 13 of the nitriding potential adjustor 4 calculates the in-furnace nitriding potential based on the inputted hydrogen concentration signal or ammonia concentration signal and the inputted oxygen concentration signal. Then, the gas flow rate output adjustor 30 performs the PID control method in which the respective gas introduction amounts of the ammonia gas, the carbon monoxide gas and the nitrogen gas among the four kinds of furnace introduction gases are input values, the nitriding potential calculated by the in-furnace nitriding potential calculator 13 is an output value, and the target nitriding potential (the set nitriding potential) is a target value. Specifically, in the present PID control method, the nitriding potential in the processing furnace 2 is brought close to the target nitriding potential while the relationships of C1=c1×(A+x×B) and C2=c2×(A+x×B) are maintained, by changing the introduction amount of the ammonia gas, the introduction amount of the carbon monoxide gas and the introduction amount of the nitrogen gas while keeping the introduction amount of the ammonia decomposition gas constant. In the present PID control method, the setting parameter values that have been set and inputted by the parameter setting device 15 are used. The setting parameter values may be different depending on values of the target nitriding potential.

[0257] Then, the gas introduction amount controller 14 controls the introduction amount of the ammonia gas, the introduction amount of the carbon monoxide gas and the introduction amount of the nitrogen gas as a result of the PID control method. The gas introduction amount controller 14 transmits control signals to the first supply amount controller 22 for the ammonia gas, the second supply amount controller 26 for the ammonia decomposition gas (whose flow rate is constant), the third supply amount controller 62 for the carbon monoxide gas and the fourth supply amount controller 72 for the nitrogen gas, in order to realize the respective determined introduction amounts of the furnace introduction gases.

[0258] According to the control as described above, the in-furnace nitriding potential can be stably controlled in the vicinity of the target nitriding potential. Thereby, the surface hardening treatment of the work S can be performed with extremely high quality. Specifically, a feedback control is performed with a sampling rate of about several hundred milliseconds, and the introduction amount of the ammonia gas is increased and decreased within a range of about 3 ml (±1.5 ml), so that the nitriding potential can be controlled to the target nitriding potential (0.2) with extremely high precision since a timing of about 40 minutes after starting the treatment.

Explanation of Comparative Examples

[0259] As comparative examples, controls of nitriding potential were performed. In each of them, the ammonia decomposition gas was not introduced, the ratio of the introduction amounts of the ammonia gas, the nitrogen gas and the carbon monoxide gas was always maintained at 50:48:2, and the total introduction amount thereof was changed.

[0260] Specifically, the in-furnace nitriding potential calculator 13 of the nitriding potential adjustor 4 calculated the in-furnace nitriding potential based on the inputted hydrogen concentration signal or ammonia concentration signal and the inputted oxygen concentration signal. Then, the gas flow rate output adjustor 30 performed the PID control method in which the respective gas introduction amounts of the ammonia gas, the nitrogen gas and the carbon monoxide gas were input values, the nitriding potential calculated by the in-furnace nitriding potential calculator 13 was an output value, and the target nitriding potential (the set nitriding potential) was a target value, More specifically, in the present PID control method, the nitriding potential in the processing furnace 2 was brought close to the target nitriding potential, by changing the total introduction amount of the ammonia gas, the nitrogen gas and the carbon monoxide gas while keeping the ratio of the introduction amounts of the ammonia gas, the nitrogen gas and the carbon monoxide gas constant. However, in the above comparative examples, the nitriding potential could not be stably controlled.

(Comparison Between Examples 4-1 to 4-3 and Comparative Examples)

[0261] A table of the above results is shown as FIG. 16.

[0262] (Structure of Fifth Embodiment)

[0263] As shown in FIG. 17, a furnace introduction gas supplier 20″ of a fifth embodiment further includes a fifth furnace introduction gas supplier 81 for carbon dioxide gas, a fifth supply amount controller 82, a fifth supply valve 83 and a fifth flow meter 84, in addition to the furnace introduction gas supplier 20′ of the fourth embodiment.

[0264] The fifth furnace introduction gas supplier 81 is formed by, for example, a tank filled with a fifth furnace introduction gas (in this example, carbon dioxide gas).

[0265] The fifth supply amount controller 82 is formed by a mass flow controller (which can finely change a flow rate within a short time period), and is interposed between the fifth furnace introduction gas supplier 81 and the fifth supply valve 83. An opening degree of the fifth supply amount controller 82 changes according to the control signal outputted from the gas introduction amount controller 14, In addition, the fifth supply amount controller 82 is configured to detect a supply amount from the fifth furnace introduction gas supplier 81 to the fifth supply valve 83, and output an information signal including the detected supply amount to the gas introduction amount controller 14 and the recorder 6. This information signal can be used for correction or the like of the control performed by the gas introduction amount controller 14.

[0266] The fifth supply valve 83 is formed by an electromagnetic valve configured to switch between opened and closed states according to a control signal outputted from the gas introduction amount controller 14, and is interposed between the fifth supply amount controller 82 and the fifth flow meter 84.

[0267] The fifth flow meter 84 is formed by, for example, a mechanical flow meter such as a flow-type flow meter, and is interposed between the fifth supply valve 83 and the furnace introduction gas pipe 29. The fifth flow meter 84 detects a supply amount from the fifth supply valve 83 to the furnace introduction gas pipe 29, The supply amount detected by the fifth flow meter 84 can be provided for an operator's visual confirmation.

[0268] In the fifth embodiment, the introduction amount C1 of the carbon monoxide gas, the introduction amount C2 of the nitrogen gas and the introduction amount C3 of the carbon dioxide gas, which are furnace introduction gases except for (other than) the ammonia gas and the ammonia decomposition gas, are controlled using a factor of proportionality c1 assigned to the carbon monoxide gas, a factor of proportionality c2 assigned to the nitrogen gas and a factor of proportionality c3 assigned to the carbon dioxide gas, such that C1=c1×(A+x×B). C2=c2×(A+x×B) and C3=c3×(A+x×B), wherein the introduction amount of the ammonia gas is represented by A, the introduction amount of the ammonia decomposition gas is represented by B, and a predetermined constant is represented by x.

[0269] The other structure of the fifth embodiment is substantially the same as that of the fourth embodiment explained with reference to FIG. 15. In FIG. 17, the same parts as those of the fourth embodiment are shown by the same reference numerals, and detailed explanation thereof is omitted.

Operation: Example 5-1

[0270] Next, a case is explained as an example 5-1, in which the surface hardening treatment device according to the fifth embodiment is used and the target nitriding potential is set to 1.0. In the example 5-1 as well, a pit furnace having a size of φ 700×1000 was used as the processing furnace 2, 570° C. was adopted as the temperature to be heated, and a steel material having a surface area of 4 m.sup.2 was used as the work S.

[0271] While the processing furnace 2 is heated, the ammonia gas, the ammonia decomposition gas, the carbon monoxide gas, the nitrogen gas and the carbon dioxide gas are introduced into the processing furnace 2 from the furnace introduction gas supplier 20″ according to their respective initial introduction amounts. In this example, the initial introduction amount of the ammonia gas was set to 13 [l/min], the initial introduction amount of the ammonia decomposition gas was set to 19 [l/min], the initial introduction amount of the carbon monoxide gas was set to 0.45 [l/min], the initial introduction amount of the nitrogen gas was set to 21 [l/min], the initial introduction amount of the carbon dioxide gas was set to 0.9 [l/min], x=0.5 was set, c1=0.02 was set, c2=0.94 was set, and c3=0.04. These initial introduction amounts can be set and inputted by the parameter setting device 15. Furthermore, the stirring fan drive motor 9 is driven and thus the stirring fan 8 rotates to stir the atmospheric gases in the processing furnace 2.

[0272] In the initial state, the on-off valve controller 16 closes the on-off valve 17.

[0273] In addition, the in-furnace temperature measurement device 10 measures a temperature of the in-furnace gases, and outputs an information signal including the measured temperature to the nitriding potential adjustor 4 and the recorder 6. The nitriding potential adjustor 4 judges whether the state in the processing furnace 2 is still during the temperature rising step or already after the temperature rising step has been completed (a stable state).

[0274] In addition, the in-furnace nitriding potential calculator 13 of the nitriding potential adjustor 4 calculates an in-furnace nitriding potential (which is initially an extremely high value (since no hydrogen gas exists in the furnace), but decreases as decomposition of the ammonia gas (generation of the hydrogen gas) proceeds) and judges whether the calculated value has dropped lower than the sum of the target nitriding potential (1.0 in this example) and a standard margin. This standard margin can also be set and inputted by the parameter setting device 15, and is for example 0.1.

[0275] When it is determined that the temperature rising step has been completed and also it is determined that the calculated value of the in-furnace nitriding potential has dropped lower than the sum (1.1 in this example) of the target nitriding potential and the standard margin, the nitriding potential adjustor 4 starts to control an introduction amount of each of the furnace introduction gases via the gas introduction amount controller 14. Herein, the on-off valve controller 16 opens the on-off valve 17.

[0276] When the on-off valve 17 is opened, the processing furnace 2 and the atmospheric gas concentration detector 3 communicate with each other, and then the atmospheric gas concentration detector 3 detects an in-furnace hydrogen concentration or an in-furnace ammonia concentration, and detects an in-furnace oxygen concentration. The detected hydrogen concentration signal or ammonia concentration signal and the detected oxygen concentration signal are outputted to the nitriding potential adjustor 4 and the recorder 6.

[0277] The in-furnace nitriding potential calculator 13 of the nitriding potential adjustor 4 calculates the in-furnace nitriding potential based on the inputted hydrogen concentration signal or ammonia concentration signal and the inputted oxygen concentration signal. Then, the gas flow rate output adjustor 30 performs the PID control method in which the respective gas introduction amounts of the ammonia gas, the carbon monoxide gas, the nitrogen gas and the carbon dioxide gas among the five kinds of furnace introduction gases are input values, the nitriding potential calculated by the in-furnace nitriding potential calculator 13 is an output value, and the target nitriding potential (the set nitriding potential) is a target value. Specifically, in the present PID control method, the nitriding potential in the processing furnace 2 is brought close to the target nitriding potential while the relationships of C1=c1×(A+x×B), C2=c2×(A+x×B) and C3=c3×(A+x×B) are maintained, by changing the introduction amount of the ammonia gas, the introduction amount of the carbon monoxide gas, the introduction amount of the nitrogen gas and the introduction amount of the carbon dioxide gas while keeping the introduction amount of the ammonia decomposition gas constant. In the present PID control method, the setting parameter values that have been set and inputted by the parameter setting device 15 are used. The setting parameter values may be different depending on values of the target nitriding potential.

[0278] Then, the gas introduction amount controller 14 controls the introduction amount of the ammonia gas, the introduction amount of the carbon monoxide gas, the introduction amount of the nitrogen gas and the introduction amount of the carbon dioxide gas as a result of the PID control method. The gas introduction amount controller 14 transmits control signals to the first supply amount controller 22 for the ammonia gas, the second supply amount controller 26 for the ammonia decomposition gas (whose flow rate is constant), the third supply amount controller 62 for the carbon monoxide gas, the fourth supply amount controller 72 for the nitrogen gas and the fifth supply amount controller 82 for the carbon dioxide gas, in order to realize the respective determined introduction amounts of the furnace introduction gases.

[0279] According to the control as described above, the in-furnace nitriding potential can be stably controlled in the vicinity of the target nitriding potential. Thereby, the surface hardening treatment of the work S can be performed with extremely high quality. Specifically, a feedback control is performed with a sampling rate of about several hundred milliseconds, and the introduction amount of the ammonia gas is increased and decreased within a range of about 3 ml (±1.5 ml), so that the nitriding potential can be controlled to the target nitriding potential (1.0) with extremely high precision since a timing of about 30 minutes after starting the treatment.

Operation: Example 5-2

[0280] Next, another case is explained as an example 5-2, in which the surface hardening treatment device according to the fifth embodiment is used and the target nitriding potential is set to 0.6. In the example 5-2 as well, a pit furnace having a size of φ 700×1000 was used as the processing furnace 2, 570° C. was adopted as the temperature to be heated, and a steel material having a surface area of 4 m.sup.2 was used as the work S.

[0281] While the processing furnace 2 is heated, the ammonia gas, the ammonia decomposition gas, the carbon monoxide gas, the nitrogen gas and the carbon dioxide gas are introduced into the processing furnace 2 from the furnace introduction gas supplier 20″ according to their respective initial introduction amounts. In this example, the initial introduction amount of the ammonia gas was set to 12 [l/min], the initial introduction amount of the ammonia decomposition gas was set to 25 [l/min], the initial introduction amount of the carbon monoxide gas was set to 0.5 [l/min], the initial introduction amount of the nitrogen gas was set to 23 [l/min], the initial introduction amount of the carbon dioxide gas was set to 1.0 [l/min]. x=0.5 was set, c1=0.02 was set, c2=0.94 was set, and c3=0.04. These initial introduction amounts can be set and inputted by the parameter setting device 15. Furthermore, the stirring fan drive motor 9 is driven and thus the stirring fan 8 rotates to stir the atmospheric gases in the processing furnace 2.

[0282] In the initial state, the on-off valve controller 16 closes the on-off valve 17.

[0283] In addition, the in-furnace temperature measurement device 10 measures a temperature of the in-furnace gases, and outputs an information signal including the measured temperature to the nitriding potential adjustor 4 and the recorder 6. The nitriding potential adjustor 4 judges whether the state in the processing furnace 2 is still during the temperature rising step or already after the temperature rising step has been completed (a stable state),

[0284] In addition, the in-furnace nitriding potential calculator 13 of the nitriding potential adjustor 4 calculates an in-furnace nitriding potential (which is initially an extremely high value (since no hydrogen gas exists in the furnace), but decreases as decomposition of the ammonia gas (generation of the hydrogen gas) proceeds) and judges whether the calculated value has dropped lower than the sum of the target nitriding potential (1.0 in this example) and a standard margin. This standard margin can also be set and inputted by the parameter setting device 15, and is for example 0.1.

[0285] When it is determined that the temperature rising step has been completed and also it is determined that the calculated value of the in-furnace nitriding potential has dropped lower than the sum (0.7 in this example) of the target nitriding potential and the standard margin, the nitriding potential adjustor 4 starts to control an introduction amount of each of the furnace introduction gases via the gas introduction amount controller 14. Herein, the on-off valve controller 16 opens the on-off valve 17.

[0286] When the on-off valve 17 is opened, the processing furnace 2 and the atmospheric gas concentration detector 3 communicate with each other, and then the atmospheric gas concentration detector 3 detects an in-furnace hydrogen concentration or an in-furnace ammonia concentration, and detects an in-furnace oxygen concentration. The detected hydrogen concentration signal or ammonia concentration signal and the detected oxygen concentration signal are outputted to the nitriding potential adjustor 4 and the recorder 6.

[0287] The in-furnace nitriding potential calculator 13 of the nitriding potential adjustor 4 calculates the in-furnace nitriding potential based on the inputted hydrogen concentration signal or ammonia concentration signal and the inputted oxygen concentration signal. Then, the gas flow rate output adjustor 30 performs the PIC control method in which the respective gas introduction amounts of the ammonia gas, the carbon monoxide gas, the nitrogen gas and the carbon dioxide gas among the five kinds of furnace introduction gases are input values, the nitriding potential calculated by the in-furnace nitriding potential calculator 13 is an output value, and the target nitriding potential (the set nitriding potential) is a target value. Specifically, in the present PID control method, the nitriding potential in the processing furnace 2 is brought close to the target nitriding potential while the relationships of C1=c1×(A+x×B), C2=c2×(A+x×B) and C3=c3×(A+x×B) are maintained, by changing the introduction amount of the ammonia gas, the introduction amount of the carbon monoxide gas, the introduction amount of the nitrogen gas and the introduction amount of the carbon dioxide gas while keeping the introduction amount of the ammonia decomposition gas constant. In the present PID control method, the setting parameter values that have been set and inputted by the parameter setting device 15 are used. The setting parameter values may be different depending on values of the target nitriding potential.

[0288] Then, the gas introduction amount controller 14 controls the introduction amount of the ammonia gas, the introduction amount of the carbon monoxide gas, the introduction amount of the nitrogen gas and the introduction amount of the carbon dioxide gas as a result of the PID control method. The gas introduction amount controller 14 transmits control signals to the first supply amount controller 22 for the ammonia gas, the second supply amount controller 26 for the ammonia decomposition gas (whose flow rate is constant), the third supply amount controller 62 for the carbon monoxide gas, the fourth supply amount controller 72 for the nitrogen gas and the fifth supply amount controller 82 for the carbon dioxide gas, in order to realize the respective determined introduction amounts of the furnace introduction gases.

[0289] According to the control as described above, the in-furnace nitriding potential can be stably controlled in the vicinity of the target nitriding potential. Thereby, the surface hardening treatment of the work S can be performed with extremely high quality. Specifically, a feedback control is performed with a sampling rate of about several hundred milliseconds, and the introduction amount of the ammonia gas is increased and decreased within a range of about 3 ml (±1.5 ml), so that the nitriding potential can be controlled to the target nitriding potential (0.6) with extremely high precision since a timing of about 40 minutes after starting the treatment.

Operation: Example 5-3

[0290] Next, another case is explained as an example 5-3, in which the surface hardening treatment device according to the fifth embodiment is used and the target nitriding potential is set to 0.2. In the example 5-3 as well, a pit furnace having a size of φ 700×1000 was used as the processing furnace 2, 570° C. was adopted as the temperature to be heated, and a steel material having a surface area of 4 m.sup.2 was used as the work S.

[0291] While the processing furnace 2 is heated, the ammonia gas, the ammonia decomposition gas, the carbon monoxide gas, the nitrogen gas and the carbon dioxide gas are introduced into the processing furnace 2 from the furnace introduction gas supplier 20″ according to their respective initial introduction amounts. In this example, the initial introduction amount of the ammonia gas was set to 3 [l/min], the initial introduction amount of the ammonia decomposition gas was set to 29 [l/min], the initial introduction amount of the carbon monoxide gas was set to 0.3 [l/min], the initial introduction amount of the nitrogen gas was set to 16 [l/min], the initial introduction amount of the carbon dioxide gas was set to 0.6 [l/min], x=0.5 was set, c1=0.02 was set, c2=0.94 was set, and c3=0.04. These initial introduction amounts can be set and inputted by the parameter setting device 15. Furthermore, the stirring fan drive motor 9 is driven and thus the stirring fan 8 rotates to stir the atmospheric gases in the processing furnace 2.

[0292] In the initial state, the on-off valve controller 16 closes the on-off valve 17.

[0293] In addition, the in-furnace temperature measurement device 10 measures a temperature of the in-furnace gases, and outputs an information signal including the measured temperature to the nitriding potential adjustor 4 and the recorder 6. The nitriding potential adjustor 4 judges whether the state in the processing furnace 2 is still during the temperature rising step or already after the temperature rising step has been completed (a stable state).

[0294] In addition, the in-furnace nitriding potential calculator 13 of the nitriding potential adjustor 4 calculates an in-furnace nitriding potential (which is initially an extremely high value (since no hydrogen gas exists in the furnace), but decreases as decomposition of the ammonia gas (generation of the hydrogen gas) proceeds) and judges whether the calculated value has dropped lower than the sum of the target nitriding potential (1.0 in this example) and a standard margin. This standard margin can also be set and inputted by the parameter setting device 15, and is for example 0.1.

[0295] When it is determined that the temperature rising step has been completed and also it is determined that the calculated value of the in-furnace nitriding potential has dropped lower than the sum (0.3 in this example) of the target nitriding potential and the standard margin, the nitriding potential adjustor 4 starts to control an introduction amount of each of the furnace introduction gases via the gas introduction amount controller 14. Herein, the on-off valve controller 16 opens the on-off valve 17.

[0296] When the on-off valve 17 is opened, the processing furnace 2 and the atmospheric gas concentration detector 3 communicate with each other, and then the atmospheric gas concentration detector 3 detects an in-furnace hydrogen concentration or an in-furnace ammonia concentration, and detects an in-furnace oxygen concentration. The detected hydrogen concentration signal or ammonia concentration signal and the detected oxygen concentration signal are outputted to the nitriding potential adjustor 4 and the recorder 6.

[0297] The in-furnace nitriding potential calculator 13 of the nitriding potential adjustor 4 calculates the in-furnace nitriding potential based on the inputted hydrogen concentration signal or ammonia concentration signal and the inputted oxygen concentration signal. Then, the gas flow rate output adjustor 30 performs the PID control method in which the respective gas introduction amounts of the ammonia gas, the carbon monoxide gas, the nitrogen gas and the carbon dioxide gas among the five kinds of furnace introduction gases are input values, the nitriding potential calculated by the in-furnace nitriding potential calculator 13 is an output value, and the target nitriding potential (the set nitriding potential) is a target value. Specifically, in the present PID control method, the nitriding potential in the processing furnace 2 is brought close to the target nitriding potential while the relationships of C1=c1×(A+x×B), C2=c2×(A+x×B) and C3=c3×(A+x×B) are maintained, by changing the introduction amount of the ammonia gas, the introduction amount of the carbon monoxide gas, the introduction amount of the nitrogen gas and the introduction amount of the carbon dioxide gas while keeping the introduction amount of the ammonia decomposition gas constant. In the present PID control method, the setting parameter values that have been set and inputted by the parameter setting device 15 are used. The setting parameter values may be different depending on values of the target nitriding potential.

[0298] Then, the gas introduction amount controller 14 controls the introduction amount of the ammonia gas, the introduction amount of the carbon monoxide gas, the introduction amount of the nitrogen gas and the introduction amount of the carbon dioxide gas as a result of the PID control method. The gas introduction amount controller 14 transmits control signals to the first supply amount controller 22 for the ammonia gas, the second supply amount controller 26 for the ammonia decomposition gas (whose flow rate is constant), the third supply amount controller 62 for the carbon monoxide gas, the fourth supply amount controller 72 for the nitrogen gas and the fifth supply amount controller 82 for the carbon dioxide gas, in order to realize the respective determined introduction amounts of the furnace introduction gases.

[0299] According to the control as described above, the in-furnace nitriding potential can be stably controlled in the vicinity of the target nitriding potential. Thereby, the surface hardening treatment of the work S can be performed with extremely high quality. Specifically, a feedback control is performed with a sampling rate of about several hundred milliseconds, and the introduction amount of the ammonia gas is increased and decreased within a range of about 3 ml (±1.5 ml), so that the nitriding potential can be controlled to the target nitriding potential (0.2) with extremely high precision since a timing of about 40 minutes after starting the treatment.

Explanation of Comparative Examples

[0300] As comparative examples, controls of nitriding potential were performed. In each of them, the ammonia decomposition gas was not introduced, the ratio of the introduction amounts of the ammonia gas, the nitrogen gas, the carbon monoxide gas and the carbon dioxide gas was always maintained at 50:47:1:2, and the total introduction amount thereof was changed.

[0301] Specifically, the in-furnace nitriding potential calculator 13 of the nitriding potential adjustor 4 calculated the in-furnace nitriding potential based on the inputted hydrogen concentration signal or ammonia concentration signal and the inputted oxygen concentration signal. Then, the gas flow rate output adjustor 30 performed the PID control method in which the respective gas introduction amounts of the ammonia gas, the nitrogen gas, the carbon monoxide gas and the carbon dioxide gas were input values, the nitriding potential calculated by the in-furnace nitriding potential calculator 13 was an output value, and the target nitriding potential (the set nitriding potential) was a target value. More specifically, in the present PID control method, the nitriding potential in the processing furnace 2 was brought close to the target nitriding potential, by changing the total introduction amount of the ammonia gas, the nitrogen gas, the carbon monoxide gas and the carbon dioxide gas while keeping the ratio of the introduction amounts of the ammonia gas, the nitrogen gas, the carbon monoxide gas and the carbon dioxide gas constant.

[0302] However, in the above comparative examples, the nitriding potential could not be stably controlled.

Comparison Between Examples 5-1 to 5-3 and Comparative Examples

[0303] A table of the above results is shown as FIG. 18.

DESCRIPTION OF REFERENCE SIGNS

[0304] 1 Surface hardening treatment device [0305] 2 Processing furnace [0306] 3 Atmospheric gas concentration detector [0307] 4 Nitriding potential adjustor [0308] 5 Temperature adjustor [0309] 6 Recorder [0310] 8 Stirring fan [0311] 9 Stirring-fan drive motor [0312] 10 In-furnace temperature measuring device [0313] 11 Furnace body heater [0314] 13 In-furnace nitriding potential calculator [0315] 14 Gas introduction controller [0316] 15 Parameter setting device (touch panel) [0317] 16 On-off valve controller [0318] 17 On-off valve [0319] 20, 20′. 20″ Furnace introduction gas supplier [0320] 21 First furnace introduction gas supplier [0321] 22 First supply amount controller [0322] 23 First supply valve [0323] 24 First flow meter [0324] 25 Second furnace introduction gas supplier [0325] 26 Second supply amount controller [0326] 27 Second supply valve [0327] 28 Second flow meter [0328] 29 Furnace introduction gas pipe [0329] 30 Gas flow rate output adjustor [0330] 31 Programmable logic controller [0331] 40 Exhaust gas pipe [0332] 41 Exhaust gas combustion decomposition apparatus [0333] 61, 61′ Third furnace introduction gas supplier [0334] 62 Third supply amount controller [0335] 63 Third supply valve [0336] 64 Third flow meter [0337] 71 Fourth furnace introduction gas supplier [0338] 72 Fourth supply amount controller [0339] 73 Fourth supply valve [0340] 74 Fourth flow meter [0341] 81 Fifth furnace introduction gas supplier [0342] 82 Fifth supply amount controller [0343] 83 Fifth supply valve [0344] 84 Fifth flow meter