Surface hardening treatment device and surface hardening treatment method

Abstract

Based on the nitriding potential in the processing furnace calculated by the in-furnace nitriding potential calculator and the target nitriding potential, the introduction amount of the ammonia gas is changed while the introduction amount of the ammonia decomposition gas is kept constant, such that the nitriding potential in the processing furnace is brought close to the target nitriding potential.

Claims

1. A surface hardening treatment method comprising: (i) arranging a work within a processing furnace of a surface hardening treatment device, where the surface hardening treatment device includes: a) an in-furnace atmospheric gas concentration detector configured to detect a hydrogen concentration or an ammonia concentration in the processing furnace, b) 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 c) a gas-introduction-amount controller configured to increase or decrease an introduction amount of an ammonia gas within a predetermined range of fluctuation while keeping an introduction amount of an 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, (ii) detecting, by using the in-furnace atmospheric gas concentration detector, a hydrogen concentration or an ammonium concentration in the processing furnace; (iii) calculating, by using the in-furnace nitriding potential calculator, a nitriding potential in the processing furnace based on the hydrogen concentration or the ammonia concentration detected in said step of detecting a hydrogen concentration or an ammonium concentration in the processing furnace; and (iv) treating the work by introducing an introduction amount of the ammonia gas and an introduction amount of the ammonia decomposition gas into the processing furnace, where the introduction amount of the ammonia gas and the introduction amount of the ammonia decomposition gas introduced into the processing furnace are controlled by said gas-introduction-amount controller, which thereby alters the introduction amount of the ammonia gas within a predetermined range of fluctuation while keeping the introduction amount of the ammonia decomposition gas constant based upon the nitriding potential calculated in said step of calculating a nitriding potential and the 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 method according to claim 1, wherein the gas-introduction-amount controller is configured for a plurality of surface hardening treatments for a work, and wherein the target nitriding potential is different for each of the respective surface hardening treatments within the plurality of surface hardening treatments, and wherein the target nitriding potential is constant within each of the respective surface hardening treatments of the plurality of surface hardening treatments.

3. A surface hardening treatment device for performing a gas nitriding treatment as a surface hardening treatment for a work arranged in a processing furnace by continuously introducing 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 increase or decrease an introduction amount of the ammonia gas within a predetermined range of fluctuation 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.

4. The surface hardening treatment device according to claim 3, wherein the gas-introduction-amount controller is configured for a plurality of surface hardening treatments for a work, and wherein the target nitriding potential is different for each of the respective surface hardening treatments within the plurality of surface hardening treatments, and wherein the target nitriding potential is constant within each of the respective surface hardening treatments of the plurality of surface hardening treatments.

5. The surface hardening treatment device according to claim 3, wherein the introduction amount of the ammonia gas is increased or decreased by means of a mass flow controller, and the introduction amount of the ammonia decomposition gas is increased or decreased by means of a manual flow meter.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a schematic view showing a surface hardening treatment device according to an embodiment of the present invention;

(2) FIG. 2 is a table showing results of nitriding potential controls as examples;

(3) FIG. 3 is a schematic view showing a surface hardening treatment device according to the invention disclosed in JP-B-6345320 (Patent Document 3); and

(4) FIG. 4 is a table showing results of nitriding potential controls as comparative examples.

DESCRIPTION OF EMBODIMENTS

(5) Hereinafter, a preferable embodiment of the present invention will be described. However, the present invention is not limited to the embodiment.

(6) (Structure)

(7) 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 nitriding treatment as a surface hardening treatment for a work S arranged in a processing furnace 2 by introducing only two kinds of furnace introduction gases, i.e., only an ammonia gas and an ammonia decomposition gas, into the processing furnace 2.

(8) 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.

(9) 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.

(10) 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.

(11) 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.

(12) 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. A main body of the sensor 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 sensor main body 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.

(13) In addition, after detecting the in-furnace atmospheric gas concentration, the atmospheric gas concentration detector 3 is configured to output an information signal including the detected concentration to the nitriding potential adjustor 4 and the recorder 6.

(14) 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 in the processing furnace 2, for example in correspondence with the date and time when the surface hardening treatment is performed.

(15) 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.

(16) 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 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 formula (5), 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.

(17) For example, the parameter setting device 15 is composed of a touch panel. Through the parameter setting device 15, the target nitriding potential can be set and inputted to be different values depending on time zones 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.

(18) The gas flow rate output adjustor 30 is configured to perform the PD control method in which respective gas introduction amounts of the two 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 an introduction amount of the ammonia gas while keeping an 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.

(19) 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.

(20) 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.

(21) 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 an initial introduction amount of the ammonia gas, which is 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 is 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. Then, the output values from the gas flow rate output adjustor 30 are transferred to the gas introduction amount controller 14.

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

(23) 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), the second supply valve 27 and a second flow meter 28.

(24) In the present embodiment, the ammonia gas and the ammonia decomposition gas are mixed in a furnace introduction gas pipe 29 before entering the processing furnace 2.

(25) 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).

(26) 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.

(27) 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.

(28) 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 operator's visual confirmation.

(29) 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).

(30) 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 furnace introduction gas supplier 25 and the second flow meter 28.

(31) The second flow meter 28 is formed by, for example, a mechanical manual flow meter such as a flow-type flow meter (which cannot finely change a flow rate within a short time period), and is interposed between the second supply valve 27 and the furnace introduction gas pipe 29. The second flow meter 28 can adjust a supply amount from the second supply valve 27 to the furnace introduction gas pipe 29 and can detect an actual supply amount thereof. The flow rate (opening degree) of the second flow meter 28 is manually adjusted so as to correspond to the control signal outputted from the gas introduction amount controller 14. The actual supply amount detected by the second flow meter 28 can be provided for an operator's visual confirmation.

(32) (Operation)

(33) Next, with reference to FIG. 2, 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 FIG. 2, 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.

(34) While the processing furnace 2 is heated, the ammonia gas and the ammonia decomposition 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 23 [l/min] and the initial introduction amount of the ammonia decomposition gas was set to 10 [l/min]. 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.

(35) 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 (sensor) 3, and thus it is effective to keep the on-off valve 17 closed.

(36) 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).

(37) 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 (0.7 in the example shown in FIG. 2) 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.

(38) 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 (0.8 in the example shown in FIG. 2) of the target nitriding potential and the standard margin (at a timing of about 35 minutes after starting the treatment in the example shown in FIG. 2), 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.

(39) 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. The detected hydrogen concentration signal or ammonia concentration signal is outputted to the nitriding potential adjustor 4 and the recorder 6.

(40) 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. Then, the gas flow rate output adjustor 30 performs the PID control method in which the respective gas introduction amounts of the two 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 by changing the introduction amount of the ammonia 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.

(41) Then, the gas flow rate output adjustor 30 controls the introduction amount of the ammonia gas as a result of the PID control method. Specifically, the gas flow rate output adjustor 30 determines the introduction amount of the ammonia gas, and the output value from the gas flow rate output adjustor 30 is transferred to the gas introduction amount controller 14.

(42) The gas introduction amount controller 14 transmits a control signal to the first supply amount controller 22 for the ammonia gas in order to realize the determined introduction amount of the ammonia gas.

(43) 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. As a specific example, in the example shown in FIG. 2, 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 2 ml (±1 ml), so that the nitriding potential can be controlled to the target nitriding potential (0.7) with extremely high precision since a timing of about 60 minutes after starting the treatment. (In the example shown in FIG. 2, recording of the respective gas introduction amounts and the nitriding potential was stopped at a timing of about 170 minutes after starting the treatment.)

(44) (Structure of Comparative Example)

(45) FIG. 3 is a schematic view showing a surface hardening treatment device according to the invention disclosed in JP-B-6345320 (Patent Document 3);

(46) In the surface hardening treatment device shown in FIG. 3, there is provided a second supply amount controller 126, which is another mass flow controller, between the second furnace introduction gas supplier 25 and the second supply valve 27. A gas flow rate output adjustor 130 is configured to perform a PID control method, in which the nitriding potential in the processing furnace 2 is brought close to the target nitriding potential by changing a flow rate ratio between the ammonia gas and the ammonia decomposition gas while keeping a total introduction amount of the ammonia gas and the ammonia decomposition gas constant.

(47) The gas flow rate output adjustor 130 is configured to control the introduction amount of each of the furnace introduction gases as a result of the PID control method. Specifically, the gas flow rate output adjustor 130 determines a flow rate ratio of the ammonia gas as a value within 0 to 100%, or a flow rate ratio of the ammonia decomposition gas as a value within 0 to 100%. In any case, since the sum of the two flow rate ratios is 100%, when one flow rate ratio is determined, the other flow rate ratio is also determined. Then, the output values from the gas flow rate output adjustor 130 are transferred to a gas introduction amount controller 114.

(48) The gas introduction amount controller 114 is configured to transmit control signals to the first supply amount controller 22 for the ammonia gas and a second supply amount controller 126 for the ammonia decomposition gas, respectively, in order to realize an introduction amount of each gas corresponding to the total introduction amount (total flow rate)×the flow rate ratio of each gas. In the present embodiment, the total introduction amount of the respective gases can also be set and inputted by a parameter setting device 115 for each different value of the target nitriding potential.

(49) The other structure of the treatment device shown in FIG. 3 is substantially the same as the treatment device according to the embodiment of the invention explained with reference to FIG. 1. In FIG. 3, the same portions as those of the treatment device shown in FIG. 1 are shown by the same reference numerals, and detailed explanation thereof is omitted.

(50) (Operation of Comparative Example)

(51) Next, with reference to FIG. 4, an operation of the surface hardening treatment device shown in FIG. 3 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 FIG. 4 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.

(52) While the processing furnace 2 is heated, the ammonia gas and the ammonia decomposition 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 30 [l/min] and the initial introduction amount of the ammonia decomposition gas was set to 10 [l/min]. These initial introduction amounts can be set and inputted by the parameter setting device 115. 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.

(53) In this comparative example as well, 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 (sensor) 3, and thus it is effective to keep the on-off valve 17 closed.

(54) 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).

(55) In addition, the in-furnace nitriding potential calculator 113 of the nitriding potential adjustor 4 calculates an in-furnace nitriding potential (which is initially a 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 (0.7 in the example shown in FIG. 4) and a standard margin. This standard margin can also be set and inputted by the parameter setting device 115, and is for example 0.1.

(56) 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 (0.8 in the example shown in FIG. 4) of the target nitriding potential and the standard margin (at a timing of about 25 minutes after starting the treatment in the example shown in FIG. 4), the nitriding potential adjustor 4 starts to control an introduction amount of each of the furnace introduction gases via the gas introduction amount controller 114. Herein, the on-off valve controller 16 opens the on-off valve 17.

(57) 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. The detected hydrogen concentration signal or ammonia concentration signal is outputted to the nitriding potential adjustor 4 and the recorder 6.

(58) The in-furnace nitriding potential calculator 113 of the nitriding potential adjustor 4 calculates the in-furnace nitriding potential based on the inputted hydrogen concentration signal or ammonia concentration signal. Then, the gas flow rate output adjustor 30 performs the PID control method in which the respective gas introduction amounts of the two kinds of furnace introduction gases are input values, the nitriding potential calculated by the in-furnace nitriding potential calculator 113 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 by changing the flow rate ratio between the ammonia gas and the ammonia decomposition gas while keeping the total introduction amount of the ammonia gas and the ammonia decomposition gas constant. by changing the introduction amount of the ammonia 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 115 are used. The setting parameter values may be different depending on values of the target nitriding potential.

(59) Then, the gas flow rate output adjustor 130 controls the introduction amount of each of the plurality of furnace introduction gases as a result of the PID control method. Specifically, the gas flow rate output adjustor 130 determines a flow rate ratio of each of the ammonia gas and the ammonia decomposition gas as a value within 0 to 100%, and the output values from the gas flow rate output adjustor 130 are transferred to the gas introduction amount controller 114.

(60) The gas introduction amount controller 114 transmits control signals to the first supply amount controller 22 for the ammonia gas and a second supply amount controller 126 for the ammonia decomposition gas, respectively, in order to realize an introduction amount of each gas corresponding to the total introduction amount×the flow rate ratio of each gas.

(61) 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. As a specific example, in the example shown in FIG. 4, a feedback control is performed with a sampling rate of about several hundred milliseconds, and each of the introduction amounts of the ammonia gas and ammonia decomposition gas is increased and decreased within a range of about 2 ml (±1 ml) (when the introduction amount of one of those gases is increased, the introduction amount of the other of those gases is decreased), so that the nitriding potential can be controlled to the target nitriding potential (0.7) with extremely high precision since a timing of about 50 minutes after starting the treatment. (In the example shown in FIG. 4, recording of the respective gas introduction amounts and the nitriding potential was stopped at a timing of about 145 minutes after starting the treatment.)

(62) (Comparison Against Comparative Example)

(63) As seen from the graphs shown in FIGS. 2 and 4, when 570° C. is adopted as the temperature condition and the target nitriding potential is set to 0.7, the treatment device shown in FIG. 1 (the embodiment of the invention) can achieve as high a control precision as the treatment device shown in FIG. 3 (JP-B-6345320: Patent Document 3) does.

(64) On the other hand, as seen from the structures shown in FIGS. 1 and 3, it is not necessary to provide a mass flow controller for controlling the introduction amount of the ammonia decomposition gas. Thus, the costs related to this element can be saved.

(65) Next, regarding the treatment device shown in FIG. 1 (the embodiment of the invention: Example), a range of achievable nitriding potential control was examined. As a result, as shown in the following table 1, it was confirmed that the treatment device shown in FIG. 1 can achieve a wide range of nitriding potential control on a lower nitriding potential side (for example, about 0.1 to 1.5 at 570° C.), which is similar to the treatment device shown in FIG. 3 (JP-B-6345320 (Patent Document 3): Comparative Example). That is to say, the usefulness of the treatment device shown in FIG. 1 was confirmed.

(66) TABLE-US-00001 TABLE 1 Set Values Measured Values Set Gas Flow Amount (l/min) Gas Flow Amount(l/min) Nitriding PID Temper- Total Nitriding Total Potential P I D ature NH3 Gas AX Gas Gas Potential Error NH3 Gas AX Gas Gas Treatment Example 1.5 6.2 133 34 570° C. Variable  2(Constant) — 1.5 0% Variable  2(Constant) about 58 1 Comparative 7.2 120 28 Variable Variable 60 1.5 0% Variable Variable 60 Example Treatment Example 1 6.2 133 34 570° C. Variable  5(Constant) — 1 0% Variable  5(Constant) about 48 2 Comparative 5.3 126 32 Variable Variable 50 1 0% Variable Variable 50 Example Treatment Example 0.7 6.2 133 34 570° C. Variable 10(Constant) — 0.7 0% Variable 10(Constant) about 38 3 Comparative 4.7 137 34 Variable Variable 40 0.7 0% Variable Variable 40 Example Treatment Example 0.4 6.2 133 34 570° C. Variable 15(Constant) — 0.4 0% Variable 15(Constant) about 33 4 Comparative 4.2 154 39 Variable Variable 40 0.4 0% Variable Variable 40 Example Treatment Example 0.1 6.2 133 34 570° C. Variable 19(Constant) — 0.1 0% Variable 19(Constant) about 27 5 Comparative 2.5 303 76 Variable Variable 30 0.1 0% Variable Variable 30 Example

(67) In the gas nitriding treatment around 570° C. (about 560 to 600° C.), the condition of K.sub.N=0.1 is a condition in order that no compound layer is generated. The condition of K.sub.N=0.2 to 1.0 is a condition in order that the γ′ phase is generated as a compound layer. The condition of K.sub.N=1.5 to 2.0 is a condition in order that the E phase is generated on a surface. In particular, it is known that the condition of K.sub.N=0.3 or the vicinity is a condition in order that the γ′ phase (which is important for practical use) can be generated as almost a single phase on a surface.

(68) In addition, as shown in the table 1, regarding the treatment device shown in FIG. 1 (the embodiment of the invention), it was confirmed that it is less necessary (even unnecessary for some cases) to finely change the setting parameter values (the set of “the proportional gain”, “the integral gain or the integration time” and “the derivative gain or the derivative time”) for the PID control method, depending on different values of the target nitriding potential.

DESCRIPTION OF REFERENCE SIGNS

(69) 1 Surface hardening treatment device 2 Processing furnace 3 Atmospheric gas concentration detector 4, 104 Nitriding potential adjustor 5 Temperature adjustor 6 Recorder 8 Stirring fan 9 Stirring-fan drive motor 10 In-furnace temperature measuring device 11 Furnace body heater 13 In-furnace nitriding potential calculator 14, 114 Gas introduction controller 15, 115 Parameter setting device (touch panel) 16 On-off valve controller 17 On-off valve 20 Furnace introduction gas supplier 21 First furnace introduction gas supplier 22 First supply amount controller 23 First supply valve 24 First flow meter 25 Second furnace introduction gas supplier 126 Second supply amount controller 27 Second supply valve 28 Second flow meter 29 Furnace introduction gas pipe 30, 130 Gas flow rate output adjustor 31, 131 Programmable logic controller 40 Exhaust gas pipe 41 Exhaust gas combustion decomposition apparatus