JP2020084219A - Atmosphere control method and atmosphere control device - Google Patents

Abstract

To provide an atmosphere control method capable of more accurately detecting a hydrogen concentration in an atmosphere in which a surface treatment of diffusing nonmetal on a surface of copper by impregnation is carried out, and more suitably performing atmosphere control based upon the detection.SOLUTION: Feedback control is performed in which a gas including at least a supply source gas for a nonmetal element is introduced into a treatment furnace 30, a proton-conductive solid electrolyte hydrogen sensor 1 detects a hydrogen concentration in the atmosphere in the treatment furnace, and the flow rate of at least the supply source gas of the gas which should be introduced into the treatment furnace is varied based upon the detected hydrogen concentration. An object surface treatment is one of nitriding steps for gas nitriding, gas soft nitriding, and vacuum carbonitriding using ammonia as the nitrogen supply source gas, or one of carburizing steps for vacuum carburizing and vacuum carbonitriding using acetylene as the carbon supply source gas.SELECTED DRAWING: Figure 1

Description

The present invention relates to an atmosphere control method for surface treatment in which a nonmetal is permeated and diffused on the surface of steel, and an atmosphere control device using the atmosphere control method.

In the nitriding steps of gas nitriding, gas soft nitriding, and vacuum carbonitriding, by introducing ammonia gas into a processing furnace as a nitrogen supply source and heating, the atomic nitrogen generated by the decomposition of the following formula (1) is converted into steel. Permeate and diffuse into. As a result, a diffusion phase (α phase) in which nitrogen is dissolved, and a compound phase of the iron-nitrogen compound (ε phase, γ′ phase) are generated on the surface of the steel. These phases have different properties and their formation depends on the processing conditions. Therefore, it is necessary to control the atmosphere in the processing furnace in addition to the processing temperature and the processing time in order to obtain the required characteristics depending on the intended use of the object.
2NH ₃ → 2N +3H ₂ (1)

On the other hand, in the carburizing process of vacuum carburizing and vacuum carbonitriding, by introducing and heating a hydrocarbon gas such as acetylene or ethylene as a carbon supply source, the carbon generated by the decomposition of hydrocarbon permeates the surface of the steel. Spread. As an example, the case where carbon is produced from acetylene is shown in the following formula (2). By such infiltration and diffusion of carbon, a layer having a high carbon concentration is formed on the surface of the steel. The carbon concentration and the penetration depth of the surface depend on the treatment conditions. Therefore, as in the case of the above-mentioned nitriding, it is necessary to control the atmosphere in the processing furnace in addition to the processing temperature and the processing time in order to obtain the characteristics required depending on the application of the processing object. Becomes
C ₂ H ₂ → 2C + H ₂ (2)

Therefore, a surface hardening treatment device has been proposed in which the concentration of hydrogen gas in the atmosphere in the treatment furnace that performs the surface treatment is detected by a thermal conductivity type hydrogen sensor (Patent Document 1). In this proposal, the in-furnace gas composition is calculated from the hydrogen gas concentration detected by the thermal conductivity type hydrogen sensor, and the gas in the furnace is adjusted so that the calculated in-reactor gas composition becomes a preset in-reactor gas mixing ratio. Control the amount introduced. As this control, the first control for controlling the total introduction amount into the furnace and the flow rate ratio of the plurality of introduction gases are maintained while the flow rate ratios of the plural kinds of gases introduced into the furnace are kept constant. It has been proposed to selectively execute any one of the second controls that individually control the introduction amount so as to change.

However, the thermal conductivity type hydrogen sensor used in the surface hardening treatment apparatus of Patent Document 1 has some drawbacks as follows. First, in the thermal conductivity type hydrogen sensor, the hydrogen concentration is measured based on the thermal conductivity of gas, but the thermal conductivity of gas differs depending on the type of gas. Therefore, if a gas other than the gas used for creating the calibration curve is present in the atmosphere, the hydrogen concentration cannot be accurately measured. For example, in the gas nitriding treatment, undecomposed ammonia gas remains in the treatment furnace in addition to nitrogen gas and hydrogen gas produced by the decomposition of ammonia. Therefore, the hydrogen concentration cannot be accurately measured by the thermal conductivity type hydrogen sensor calibrated on the basis of the calibration curve created by the combination of hydrogen and nitrogen. Further, in the atmosphere in the processing furnace for performing the gas soft nitriding treatment, in addition to nitrogen gas, hydrogen gas, residual ammonia gas, and carbon source gas, at least four kinds of gas exist. Therefore, the hydrogen concentration cannot be accurately measured by the thermal conductivity type hydrogen sensor calibrated based on the calibration curve created by the combination of the two gases of hydrogen and another gas.

In vacuum carburizing and vacuum carbonitriding, the pressure inside the furnace is changed as the process progresses. Particularly, in vacuum carbonitriding, the suitable pressure is largely different between the carburizing process and the nitriding process. When the pressure inside the furnace changes, the gas density naturally changes, so the heat conduction also changes. If the thermal conductivity type hydrogen sensor is calibrated in a certain pressure range, almost accurate measurement can be made within that pressure range, but if the pressure fluctuation exceeds that range, the thermal conductivity type hydrogen sensor is used. It is difficult to measure the hydrogen concentration accurately.

Unless the detected hydrogen concentration is a value that accurately reflects the actual hydrogen concentration, it is difficult to appropriately control the atmosphere of the processing furnace based on the detection result.

Further, in Patent Document 1, although the thermal conductivity type hydrogen sensor is described as “directly mounted on the furnace body”, the sensor element itself has a high temperature for surface treatment (500° C. to 600° C. in vacuum, vacuum). Since carburizing cannot withstand 900° C. to 950° C.), the sensor body including the sensor element is placed outside the furnace, and the pipe leading to the sensor element is connected to the processing furnace. Since the in-furnace gas flows into the pipe by natural convection and is guided to the sensor element, it takes time for the in-furnace gas to reach the sensor element. In fact, according to the site where the thermal conductivity type hydrogen sensor is used to detect the hydrogen concentration in the processing furnace, the delay of this detection reaches several tens of seconds. If the detection is delayed with respect to the change in the hydrogen concentration in the furnace gas, the control based on the detection result is also delayed.

Patent No. 5629436

Therefore, in view of the above situation, the present invention can more accurately detect the hydrogen concentration or the hydrogen partial pressure in the atmosphere in which the surface treatment for permeating and diffusing the nonmetal into the surface of the steel is performed, and the atmosphere control based on the detection can be performed. It is an object to provide an atmosphere control method that can be more appropriately performed and an atmosphere control device that uses the atmosphere control method.

In order to solve the above problems, the atmosphere control method according to the present invention,
"An atmosphere control method for controlling the atmosphere of a processing furnace for performing a surface treatment for permeating and diffusing a non-metallic element on the surface of steel,
Introducing a gas containing at least the non-metal element source gas into the processing furnace,
The hydrogen concentration or hydrogen partial pressure in the atmosphere of the processing furnace is detected by a hydrogen sensor having a proton conductive solid electrolyte as a sensor element,
Based on the detected hydrogen concentration or hydrogen partial pressure, to perform feedback control to change the flow rate of at least the source gas in the gas to be introduced into the processing furnace,
The surface treatment is
A specific nitriding treatment, which is one of the nitriding steps in gas nitriding, gas soft nitriding, and vacuum carbonitriding, in which the supply source gas for supplying nitrogen is ammonia, or
Vacuum carburizing using acetylene as the source gas for supplying carbon, and a specific carburizing process which is one of the carburizing steps in vacuum carbonitriding."

The "specific nitriding treatment" refers to a nitriding step in gas nitriding, gas soft nitriding, or vacuum carbonitriding, which is a target of the present invention among treatments for nitriding steel. Further, the "specific carburizing treatment" refers to a carburizing step in vacuum carburizing or vacuum carbonitriding which is a target of the present invention among the treatments for carburizing steel.

Hereinafter, the "supply source gas that supplies nitrogen" may be referred to as a "nitrogen supply source gas", and the "supply source gas that supplies carbon" may be referred to as a "carbon supply source gas".

A sensor that uses a "solid electrolyte" as a sensor element uses the principle of a concentration battery in which a potential difference occurs due to a difference in the concentration of the same ions, and the concentration of the gas to be detected (or gas) in two spaces sandwiching the solid electrolyte. When the partial pressure is different, the electromotive force generated in the solid electrolyte is measured. If the concentration (or gas partial pressure) of the gas to be detected is known in the first space of the two spaces, the measured electromotive force and the temperature of the sensor element are used in the second space according to the Nernst equation. The gas concentration (or gas partial pressure) can be known. Alternatively, while the gas concentration in the first space is kept constant, the gas concentration (or gas partial pressure) in the second space is changed to measure the electromotive force and create a calibration curve in advance, The gas concentration (or gas partial pressure) in the second space can be known from the measured value of the electromotive force when the gas concentration (or gas partial pressure) is unknown.

"Hydrogen sensor using a solid electrolyte having proton conductivity as a sensor element" (hereinafter sometimes referred to as "solid electrolyte hydrogen sensor") is an electromotive force caused purely by hydrogen concentration (or hydrogen partial pressure). Changes. Therefore, even if a gas containing hydrogen atoms other than hydrogen or another type of gas exists in the atmosphere to be measured, the hydrogen concentration (or hydrogen partial pressure) can be accurately detected without being affected by those gases. be able to. This makes it possible to accurately perform atmosphere control based on the detected hydrogen concentration (or hydrogen partial pressure).

It should be noted that, based on the total pressure of the gas atmosphere to be measured, the other can be known if one of the hydrogen partial pressure and the hydrogen concentration is known. Therefore, in the following, instead of the description of "hydrogen concentration or hydrogen partial pressure", simply "hydrogen Sometimes referred to as "concentration".

The temperature range in which the proton conductive solid electrolyte shows a correlation between the hydrogen concentration and the electromotive force is 400°C to 1000°C. This temperature range covers the processing temperature at which the specific nitriding treatment or the specific carburizing treatment is performed. Therefore, in the solid electrolyte hydrogen sensor, the sensor element can be inserted into the internal space of the processing furnace. Therefore, the hydrogen concentration can be detected promptly in response to the change of the hydrogen concentration in the atmosphere of the processing furnace, and the prompt feedback control can be performed based on the detection. In addition, as will be described later in detail, the solid electrolyte hydrogen sensor takes a shorter time than the thermal conductivity type hydrogen sensor until the detected value that changes reflecting the change in the hydrogen concentration in the atmosphere becomes stable. From this point as well, the hydrogen concentration can be detected promptly in response to the change in the hydrogen concentration in the atmosphere of the processing furnace, and the prompt feedback control can be performed based on the detection.

Further, as will be described later in detail, the solid electrolyte hydrogen sensor can accurately detect the hydrogen concentration even if the atmosphere to be measured is depressurized. Therefore, even when the surface treatment is vacuum carburization or vacuum carbonitriding, there is an advantage that the hydrogen concentration can be accurately detected without any problem, and appropriate feedback control can be performed based on the detection.

The atmosphere control method according to the present invention, in addition to the above configuration,
The atmosphere of the processing furnace may include three or more kinds of gases including the supply source gas and the hydrogen gas.

As described above, the thermal conductivity of gas differs depending on the type of gas. Therefore, when detecting the hydrogen concentration with a thermal conductivity type hydrogen sensor, if the atmosphere to be measured is a mixed gas containing a gas other than hydrogen gas, the gas concentration and thermal conductivity are It is necessary to create a calibration curve indicating the relationship in advance. However, when the mixed gas contains three or more kinds of gases, it is extremely difficult to create a calibration curve showing the relationship between the gas concentration and the thermal conductivity. Therefore, when the atmosphere of the processing furnace contains three or more kinds of gases, if the hydrogen sensor is of the thermal conductivity type, the hydrogen concentration in the atmosphere of the processing furnace cannot be accurately detected. On the other hand, in the solid electrolyte sensor, as described above, the electromotive force changes purely due to only the hydrogen concentration, so no matter how many kinds of gas exist in the atmosphere of the processing furnace, in addition to hydrogen gas. The hydrogen concentration can be accurately detected without any problem, and appropriate feedback control can be performed based on the detection.

The atmosphere control method according to the present invention, in addition to the above configuration,
"The feedback control is a target value of an index value that reflects the atmosphere of the processing furnace calculated based on the detection of hydrogen concentration or hydrogen partial pressure by the hydrogen sensor, and a past index that is the index value before a predetermined time. It is done by comparing with the value,
The past index value is the index value 0.5 seconds to 10 seconds ago.”

In the case of the specific nitriding treatment in which the nitrogen supply source gas is ammonia, the “index value reflecting the atmosphere of the processing furnace” is the hydrogen concentration in the atmosphere of the processing furnace and the ammonia concentration in the atmosphere of the processing furnace according to the above formula (1). Alternatively, the nitriding potential of the atmosphere of the processing furnace can be used. Here, the nitriding potential is defined as K _N by the following equation (3) and is an index representing the nitriding ability of the atmosphere.
K _N =P _NH3 /P _H2 ^3/2 (3)
K _N : Nitriding potential P _NH3 : _Partial pressure of ammonia gas P _H2 : _Partial pressure of hydrogen gas

On the other hand, in the case of the specific carburizing process in which the carbon source gas is acetylene, the “index value reflecting the atmosphere of the processing furnace” is the hydrogen concentration in the atmosphere of the processing furnace and the atmosphere of the processing furnace according to the above formula (2). It can be the acetylene concentration or the carburizing potential of the atmosphere of the processing furnace. Here, the carburizing potential is defined as Kc by the following equation (4) and is an index showing the carburizing ability of the atmosphere.
K _C =(P _C2H2 /P _H2 ) ^1/2 (4)
K _C : Carburizing potential P _C2H2 : _Partial pressure of acetylene gas P _H2 : _Partial pressure of hydrogen gas

As described above, when measuring the hydrogen concentration in the atmosphere of the processing furnace where the processing temperature is several hundreds of degrees or more with the thermal conductivity type hydrogen sensor, the sensor body including the sensor element is placed outside the furnace, and the gas inside the furnace is Since it is guided to the sensor element through the pipe, it generally takes several tens of seconds until the gas in the furnace reaches the sensor element. Therefore, when performing feedback control by comparing the past index value that is the index value before the predetermined time with the destination of the index value, the past index value that can be referred to is a value that is several tens of seconds or more before. Therefore, the atmosphere of the processing furnace changes during the several tens of seconds, and proper control cannot be performed.

On the other hand, the solid electrolyte hydrogen sensor responds promptly to changes in the hydrogen concentration and stabilizes the electromotive force value in a short time of about 0.5 seconds, as will be described later in detail. Therefore, it is possible to perform feedback control using the previous index value for a very short time of 0.5 seconds to 10 seconds as the past index value. Therefore, it is possible to perform the appropriate feedback control that promptly reflects the ever-changing atmosphere change of the processing furnace. Here, if the past index value is an index value 0.5 seconds to 2 seconds ago, it is more desirable for the same reason, and it is even more desirable if it is an index value 0.5 seconds to 1 second ago.

At the site where surface treatment is actually performed, the atmosphere inside the furnace changes suddenly every time the object to be processed (workpiece) is replaced, but the replacement of the work piece takes a short time interval of several minutes. It is sometimes done in. In addition, the new work has a high reaction progress rate due to the surface treatment. Therefore, although the atmosphere control for prompt feedback has been conventionally requested, the atmosphere control of the present configuration, which refers to the previous index value for a very short time of 0.5 to 10 seconds, is It was possible for the first time by using a solid electrolyte hydrogen sensor as a sensor.

The atmosphere control method according to the present invention, in addition to the above configuration,
"The feedback control is a target value of an index value that reflects the atmosphere of the processing furnace calculated based on the detection of hydrogen concentration or hydrogen partial pressure by the hydrogen sensor, and a past index that is the index value before a predetermined time. It is performed by contrast with the value, the magnitude of the rate of change when changing the flow rate of at least the supply source gas in the gas to be introduced into the processing furnace, between the past index value and the target value. It can be changed based on the magnitude of the difference."

In the feedback control that changes the flow rate of the gas introduced into the processing furnace, by changing the magnitude of the rate of change of the gas flow rate based on the magnitude of the difference between the past index value and the target value, details will be described later. In addition, the index value can be more accurately matched to the target value. Further, even if the past index value to be compared with the target value is the index value before a longer time, it is possible to perform the appropriate feedback control that converges the index value to the target value.

The atmosphere control method according to the present invention, in addition to the above configuration,
The detection of the hydrogen concentration or hydrogen partial pressure in the atmosphere of the processing furnace by the hydrogen sensor is performed in a state where the sensor element is inserted in the internal space of the processing furnace.

As described above, in the proton conductive solid electrolyte, the temperature range showing the correlation between the hydrogen concentration and the electromotive force is 400° C. to 1000° C., and includes the temperature range of the atmosphere in which the surface treatment is performed. Therefore, the present configuration in which the sensor element is inserted into the internal space of the processing furnace can be realized. Therefore, the hydrogen concentration can be detected promptly in response to the change of the hydrogen concentration in the atmosphere of the processing furnace, and the prompt feedback control can be performed based on the detection.

In addition, in the conventional technology that uses a thermal conductivity type hydrogen sensor as the hydrogen sensor and detects the hydrogen concentration in the atmosphere of the processing furnace that performs gas soft nitriding, the gas temperature decreases in the pipe that guides the furnace gas to the sensor element. By doing so, there was a problem that ammonium carbonate was deposited and a pipe was clogged. On the other hand, in this configuration, since the sensor element of the solid electrolyte hydrogen sensor is inserted into the internal space of the high temperature processing furnace, there is an advantage that ammonium carbonate does not precipitate from the gas in the furnace. If the sensor element is inserted into the internal space of the processing furnace, soot may adhere to the sensor element when the supply source gas is acetylene. However, even if a soot layer is formed on the surface of the solid electrolyte sensor element, the hydrogen gas passes through the soot layer and generates an electromotive force that accurately reflects the concentration, so that the hydrogen concentration can be detected accurately without any problem. Make sure you can.

Next, the atmosphere control device according to the present invention,
"Solid electrolyte sensor element, reference electrode provided on the surface of the sensor element, and measurement provided on the surface of the sensor element in a second space partitioned from the first space in contact with the reference electrode A sensor probe provided with an electrode, a hydrogen sensor provided with means for calculating hydrogen concentration or hydrogen partial pressure based on an electromotive force generated between the reference electrode and the measurement electrode,
A control means for controlling a gas to be introduced into a processing furnace for performing a surface treatment for permeating and diffusing a non-metallic element on the surface of the steel;
The control means is
Based on the detection of hydrogen concentration or hydrogen partial pressure in the atmosphere of the processing furnace by the hydrogen sensor, calculate an index value that reflects the atmosphere of the processing furnace, the target value of the index value a predetermined time before the index value By performing a feedback control for controlling the flow rate of at least the source gas of the non-metal element in the gas to be introduced into the processing furnace."

The atmosphere control device of this configuration uses the above atmosphere control method, and exhibits the above-described effects.

As described above, according to the present invention, it is possible to more accurately detect the hydrogen concentration or the hydrogen partial pressure in the atmosphere in which the surface treatment for permeating and diffusing the nonmetal into the surface of the steel is performed, and the atmosphere control based on the detection can be performed more accurately. An atmosphere control method that can be appropriately performed, and an atmosphere control device that uses the atmosphere control method can be provided.

It is a block diagram of the surface treatment apparatus provided with the atmosphere control apparatus which uses the atmosphere control method which is one Embodiment of this invention. (A) to (c) are simulation results for the atmosphere control with reference to the index value before a predetermined time when the rate of change in the flow rate of the gas introduced into the processing furnace is 1%. (A) to (c) are simulation results when the rate of change of the flow rate of the gas introduced into the processing furnace is set to 0.5% or 0.1% in the atmosphere control with reference to the index value before the predetermined time. .. (A) to (d) For the atmosphere control with reference to the index value before a predetermined time, the magnitude of the rate of change of the flow rate of the gas introduced into the processing furnace is based on the magnitude of the difference between the index value and the target value. It is a simulation result in the case of changing by. It is a graph which compared the result which detected the hydrogen concentration by the solid electrolyte hydrogen sensor about the mixed gas of several types of gas with the result which detected it by the thermal conductivity type hydrogen sensor. 6 is a graph in which a solid electrolyte hydrogen sensor and a thermal conductivity type hydrogen sensor are compared with each other for a response speed when a hydrogen concentration of an atmosphere to be measured changes and a time until a detected value becomes stable. (A) to (d) When the pressures of the atmospheres to be measured are different, the relationship between the hydrogen concentration in the introduced gas and the detected hydrogen concentration was compared between the solid electrolyte hydrogen sensor and the thermal conductivity type hydrogen sensor. It is a graph. It is a block diagram of the surface treatment apparatus of a form different from the surface treatment apparatus of FIG.

Hereinafter, an atmosphere control method, an atmosphere control apparatus using the atmosphere control method, and a surface treatment apparatus controlled by the atmosphere control apparatus, which are specific embodiments of the present invention, will be described. The atmosphere control method of the present embodiment is an atmosphere control method for controlling the atmosphere f of the processing furnace 30 that performs a surface treatment for permeating and diffusing nonmetallic elements on the surface of steel. A gas containing a supply source gas was introduced, and the hydrogen concentration or hydrogen partial pressure in the atmosphere f of the processing furnace 30 was detected by the solid electrolyte hydrogen sensor 1 having the solid electrolyte having proton conductivity as the sensor element 11 and detected. Based on the hydrogen concentration or hydrogen partial pressure, feedback control is performed to change at least the flow rate of the source gas in the gas to be introduced into the processing furnace 30.

The surface treatment to which this atmosphere control method is applied is any of gas nitriding using ammonia as a nitrogen source gas, gas soft nitriding, and a nitriding step in vacuum carbonitriding, and a vacuum using acetylene as a carbon source gas. It is either a carburizing process or a carburizing process in vacuum carbonitriding.

First, an atmosphere control device that uses the atmosphere control method of the present embodiment, a solid electrolyte hydrogen sensor 1 that is the configuration of the atmosphere control device, and a surface treatment device E1 that includes the atmosphere control device will be described with reference to FIG. ..

The surface treatment apparatus E1 includes a treatment furnace 30, a gas introduction pipe 31, a gas discharge pipe 32, a gas introduction device 25, and an atmosphere control device. In addition, the atmosphere control device includes the solid electrolyte hydrogen sensor 1 and the atmosphere control unit 21.

More specifically, the processing furnace 30 is a heating furnace, and when performing gas nitriding or gas soft nitriding as the surface treatment, the atmosphere f of the processing furnace 30 can be set to a temperature of at least 500°C to 600°C. When vacuum carburizing or vacuum carbonitriding is performed as the surface treatment, the one capable of setting the atmosphere f of the processing furnace 30 to a temperature of at least 900° C. to 950° C. is used.

The gas introduction pipe 31 is a pipe for introducing a gas for surface treatment into the inside of the treatment furnace 30, one end of which is open inside the treatment furnace 30, and the other end of which is connected to the gas introduction device 25. There is. The gas discharge pipe 32 is a pipe for discharging gas from the processing furnace 30 to the outside, one end of which is opened inside the processing furnace 30, and the other end of which is opened outside the processing furnace 30. Here, the surface treatment performed using the surface treatment apparatus E1 is vacuum carburization or vacuum carbonitriding, and the surface treatment apparatus E1 includes a suction pump as a decompression device 41 connected to the gas discharge pipe 32 and a pressure gauge 42. However, when the surface treatment is gas nitriding or gas nitrocarburizing, the decompression device 41 and the pressure gauge 42 may be omitted.

The solid electrolyte hydrogen sensor 1 includes a sensor probe 10 and a sensor control device 1D. The sensor probe 10 mainly includes a sensor element 11 of a proton conductive solid electrolyte, a reference electrode p1, a measurement electrode p2, and a cylindrical holder 18. The sensor element 11 has a cylindrical shape with a bottom, and an outer peripheral surface thereof and an inner peripheral surface of the holder 18 are hermetically sealed by a sealing portion 19 in a state of being opened inside the holder 18. Thereby, the internal space of the holder 18 is airtightly divided into the first space S1 and the second space S2. The reference electrode p1 is formed on the surface of the sensor element 11 in the first space S1, and the measurement electrode p2 is formed on the surface of the sensor element 11 in the second space S2. The reference electrode p1 and the measurement electrode p2 are electrically connected to a voltmeter (not shown) by lead wires L1 and L2, respectively, and the electromotive force generated between the reference electrode p1 and the measurement electrode p2 is measured. It

The sensor probe 10 is inserted into the processing furnace 30 so that the end of the sensor element 11 on the side of the measurement electrode p2 is located in the internal space of the processing furnace 30, and the rest is outside the processing furnace 30. Therefore, the atmosphere f of the processing furnace 30 is the second space S2 for the sensor probe 10.

Further, the sensor probe 10 further includes a thermocouple 13 for measuring the temperature of the sensor element 11, and a reference gas supply pipe 14 for supplying a reference gas to the first space S1, both of which are provided outside the processing furnace 30. It is inserted in the sensor probe 10. When the solid electrolyte is of a type that can use the atmosphere as the reference gas, the first space S1 may be opened to the atmosphere without the reference gas supply pipe 14.

The sensor control device 1D includes a microcomputer including a main storage device, an auxiliary storage device, and a microprocessor, and a sensor control program that causes the microcomputer to function as sensor control means is stored in the main storage device. The sensor control means converts the electromotive force of the thermocouple 13 into the temperature of the sensor element 11, the electromotive force generated between the reference electrode p1 and the measurement electrode p2, the hydrogen concentration in the reference gas, and the sensor element 11. Hydrogen gas concentration calculation means for calculating the hydrogen gas concentration in the atmosphere f of the processing furnace 30 based on the temperature, transmission means for transmitting the calculated hydrogen gas concentration to the atmosphere control unit 21, hydrogen gas concentration and the temperature of the sensor element 10. A storage means for storing the detection result of the above in the auxiliary storage device is mainly provided.

The gas introduction device 25 introduces a gas containing at least a source gas into the processing furnace 30, and opens and closes a gas-filled cylinder and a flow path that connects the cylinder and the atmosphere f of the processing furnace 30. A valve and an opening/closing valve adjusting device for adjusting the opening/closing and opening of the opening/closing valve. When introducing a mixed gas containing a plurality of types of gas into the processing furnace 30, a cylinder, an on-off valve that opens and closes a flow path that connects the cylinder and the atmosphere f, and an on-off valve adjusting device are provided for each gas. ..

The atmosphere control unit 21 includes a microcomputer including a main storage device, an auxiliary storage device, and a microprocessor, and an atmosphere control program that causes the microcomputer to function as an atmosphere control unit is stored in the main storage device. The atmosphere control means, based on the hydrogen concentration in the atmosphere f of the processing furnace 30 sent from the sensor control device 1D and the flow rate of the gas introduced into the processing furnace 30, sets an index value that reflects the atmosphere f of the processing furnace 30. The index value calculating means for calculating is compared with the target value before the predetermined time, and the flow rate of the gas to be introduced into the processing furnace 30 is determined based on the comparison. And a flow rate adjusting means for controlling.

When the gas introduced into the processing furnace 30 (hereinafter sometimes referred to as “introduced gas”) is a mixed gas containing a gas of another type in addition to the supply source gas, the flow rate adjusting means adjusts the flow rate of the gas. Is to change only the flow rate of the source gas in the mixed gas, to change the flow rate of the mixed gas as a whole while keeping the flow rate ratio of the plural kinds of gas constant, or to change the flow rate of each of the plural kinds of gas. It can be set to

As an example, the case where the surface treatment is gas nitriding and the gas introduced into the treatment furnace is a mixed gas containing nitrogen gas, hydrogen gas, and an inert gas (argon) in addition to ammonia gas as a nitrogen supply source gas , Shows the result of simulating the atmosphere control. Here, the index value reflecting the atmosphere of the processing furnace is nitrogen potential K _N , the target value of the index value is set to K _N =4.5, and the flow rate ratio of the mixed gas is NH ₃ :N ₂ :H ₂ :Ar=8:5:1:1 is maintained, and when K _N before the predetermined time is smaller than the target value, the flow rate of the entire mixed gas is reduced by 1%, and K _N before the predetermined time is larger than the target value. At that time, the flow rate of the entire mixed gas was increased by 1%. That is, this is an example in which the rate of change when changing the flow rate of the introduced gas based on detection of the hydrogen concentration in the atmosphere of the processing furnace is 1%.

Further, when the flow rate of the source gas in the introduced gas decreases, the residence time of the source gas in the processing furnace increases and decomposition proceeds, so when the flow rate of the entire mixed gas is reduced by 1%, the source gas The decomposition rate of (here, ammonia) is assumed to increase by 1%. On the other hand, when the flow rate of the source gas in the introduced gas increases, the residence time of the source gas in the processing furnace decreases and it becomes difficult for decomposition to proceed. Therefore, when the flow rate of the entire mixed gas is increased by 1%, It is assumed that the decomposition rate of the source gas is reduced by 1%. Then, the simulation was started from the time when the decomposition rate of ammonia at the start of the treatment was 20% and the K _N calculated based on the hydrogen concentration in the atmosphere was 4.15. Fig. 2(a) shows the change in the index value when the index value before 1 second is compared with the target value and the atmosphere is controlled. Fig. 2(a) shows the index value when the index value before 2 seconds is compared with the target value and the atmosphere is controlled. 2(b) shows the change in the index value when the atmosphere is controlled by comparing the index value 10 seconds before with the target value and FIG. 2(c).

As is clear from FIG. 2(a), when the index value one second before is compared with the target value and the atmosphere is controlled, the nitriding potential K _N in the atmosphere of the processing furnace is quickly reduced to about 6 seconds. It was adjusted to the target value of 4.5, and after that it fluctuated very slightly (K _N =4.5±0.03), but remained almost constant. Also, when the atmosphere is controlled by comparing the index value 2 seconds before with the target value, the nitriding potential K _N in the atmosphere is promptly changed to the target value in a short time of about 7 seconds as shown in FIG. 2B. It was adjusted to 4.5. After that, although the fluctuation range was slightly larger than when the index value one second ago was referred to (K _N =4.5±0.1), the fluctuation was only small and the nitriding potential K _N was kept almost constant. Was done.

On the other hand, when the atmosphere value is controlled by comparing the index value 10 seconds before with the target value, the nitriding potential K _N has a width larger than the difference between the initial value and the target value, as shown in FIG. It fluctuated and could not be converged to the target value K _N =4.5. The solid electrolyte hydrogen sensor responds quickly when the hydrogen concentration changes in the measurement target atmosphere, and the detected hydrogen concentration stabilizes within 0.5 seconds from the time when the hydrogen concentration changes in the measurement target atmosphere. As will be described later, as described above, the atmosphere value can be controlled by comparing the index value for a short time of 2 seconds and 1 second with the target value. That is, by using the solid electrolyte hydrogen sensor, it is possible to control the atmosphere of the processing furnace by comparing the index value 0.5 seconds to 2 seconds before with the target value.

Here, in the feedback control based on the detection of the hydrogen concentration, when changing the flow rate of the introduced gas, by setting the rate of change smaller than 1% in the above example, the time required to converge to the target value is long. However, the index value in comparison with the target value can be used as the index value for a longer time (the index value when going back to the past). Specifically, the composition of the mixed gas introduced into the processing furnace, the decomposition rate of ammonia at the start of the treatment, the target value of the nitrogen potential K _N as an index value, and the value of K _N at the start of the treatment are set as in the above example. The same setting is performed, and the flow rate of the entire mixed gas introduced into the processing furnace is changed at a rate of change of x% by comparing the index value before the predetermined time with the target value (the index value before the predetermined time is less than the target value. When it is small, the flow rate of the mixed gas is reduced by x%, and when the index value before the predetermined time is larger than the target value, the flow rate of the mixed gas is increased by x%). In the control, the change rate x% is set to 0.5%. A simulation and a simulation with a change rate x% of 0.1% were performed. The result is shown in FIG. The decomposition rate of ammonia was also set to the same change rate (0.5% or 0.1%).

As shown in FIG. 3A, when the rate of change of the flow rate of the introduced gas is 0.5%, as in the case where the rate of change is 1%, in the control in which the index value 10 seconds before is referred to, The index value fluctuated in a range larger than the difference between the initial value and the target value, and could not be converged to the target value. On the other hand, when the rate of change of the flow rate of the introduced gas is set to a small value of 0.1%, as shown in FIGS. 3B and 3C, the index value to be compared with the target value is 10 seconds before. It was possible to adjust the index value to almost the target value regardless of the index value or the index value 30 seconds before. More specifically, when the index value 10 seconds ago is referred to, K _N =4.5±0.07 is adjusted, and when the index value 30 seconds ago is referred to, K _N =4.5±0. It was 0.17.

In the above atmosphere control in which the rate of change when changing the flow rate of the mixed gas introduced into the processing furnace based on the detection of the hydrogen concentration is x%, the magnitude of the rate of change x% changes as the process progresses. If you change it based on the size of the difference between the target value and the index value, you can use the index value for a longer time (the index value when going back in the past) as the index value to compare with the target value. The index value can be accurately adjusted to a value that matches the target value. Specifically, the composition of the mixed gas introduced into the processing furnace, the decomposition rate of ammonia at the start of processing, the target value of the nitrogen potential K _N as an index value, and the value of K _N at the start of processing are the same as above. The rate of change x% of the flow rate of the introduced gas was set and changed as follows. That is, the control is started at the change rate x%=2%, and when the difference between the index value and the target value before the predetermined time is larger than 0.1, the change rate x% is maintained while (x%=x %), when the difference between the index value before the predetermined time and the target value is 0.1 or less, x%=(1/10)x% was changed. Therefore, as the detected index value gets closer to the target value, the rate of change changes to 2%, 0.2%, 0.02%, 0.002%. Also, the decomposition rate of ammonia is assumed to change at the same rate.

From the results of this simulation, the case where the past index value to be compared with the target value is the index value 1 second ago is shown in FIG. 4A, and the case where the past index value is 10 seconds ago is shown in FIG. 4B. FIG. 4C shows the case where the index value is 30 seconds before, and FIG. 4D shows the case where the index value is 60 seconds before. As is clear from these figures, the longer the past index value is, the longer it takes for the index value to begin to change so that it approaches the target value. Is long, but in any case, it coincides with the target value in an extremely short time of 3 to 6 seconds after the index value starts to change. Then, after the index value coincides with the target value, unlike the examples shown in FIGS. 2 and 3, there is no small change that increases or decreases across the target value.

In this way, by controlling the atmosphere in which the nitriding potential K _N , which is an index value that reflects the atmosphere of the processing furnace, is adjusted to the target value, the phase generated on the surface of the steel after the processing can be made the desired phase. Specifically, the phase generated by nitriding includes a diffusion phase (α phase) in which nitrogen is dissolved, an ε phase (Fe _2-3 —N) that is a compound phase of an iron-nitrogen compound, and a γ′ phase ( Fe ₄ -N), the ε phase is a brittle and porous phase, whereas the γ′ phase is a dense phase, and the α phase is a high hardness phase formed in a deeper portion than the compound phase. .. As described above, the properties of the generated phases differ greatly, so it is desirable to generate a desired phase according to the properties required by the application of the object to be treated and the like. Using a state diagram (Lehrer state diagram) showing the relationship between these generated phases (α phase, ε phase, γ′ phase), nitriding potential K _N , and processing temperature, the target value for generating the desired phase is obtained. The nitriding potential K _N and the processing temperature to be set can be set in advance.

Since the proton conductive fixed electrolyte produces an electromotive force that purely reflects the hydrogen gas concentration, by using the solid electrolyte hydrogen sensor as the hydrogen sensor, the source gas (NH ₃ ) And hydrogen gas, three or more types of gas (in the above, four types including N ₂ and Ar) are present, the hydrogen gas concentration can be accurately detected. More specifically, as shown in FIG. 5, each is a mixed gas containing hydrogen gas and another kind of gas and having a hydrogen concentration of 1%. However, four kinds of mixed gas having different gas combinations, that is, , 1% hydrogen/99% nitrogen gas A, 1% hydrogen/99% argon gas B, 1% hydrogen/5% propane/94% argon gas C, and 1% hydrogen/1% ammonia/98 argon % Gas D was used, the gas in the atmosphere to be measured was switched to gases A, B, C, D, and A in this order, and the hydrogen concentration was detected by the solid electrolyte hydrogen sensor. As a result, as is clear from FIG. 5, the detected hydrogen concentration was almost constant at 1% even when the gas was switched.

On the other hand, when a thermal conductivity type hydrogen sensor is used as the hydrogen sensor and the gases in the atmosphere to be measured are switched to gases A, B, C, D, and A in this order, the detected hydrogen concentration is shown in FIG. Shown together. The conversion of the detected value by this thermal conductivity type hydrogen sensor into hydrogen concentration was performed using a calibration curve prepared by a combination of hydrogen-nitrogen. As is clear from FIG. 5, for gas A, which is a combination of gases for which a calibration curve was created, the hydrogen concentration was detected almost accurately, but for gases B, C, and D containing other types of gas, The hydrogen concentration could not be detected accurately.

This is because, as shown in Table 1, the thermal conductivity differs depending on the type of gas. When the mixed gas contains three or more kinds of gases, it is impossible to create a calibration curve with the combination of the gases, or it is extremely difficult to make some assumptions to simplify the handling. .. Therefore, the atmosphere control method using the solid electrolyte hydrogen sensor as the hydrogen sensor is a case where a mixed gas of many kinds of gases is introduced into the processing furnace, and if the thermal conductivity type hydrogen sensor is used as the hydrogen sensor. Even if proper feedback control cannot be performed because accurate hydrogen concentration cannot be detected, there is no problem and it is possible to perform appropriate feedback control based on accurate hydrogen concentration detection. There is.

Although the case where the surface treatment is gas nitriding has been described above, the atmosphere control method of the present embodiment can be applied to gas soft nitriding in which the nitrogen supply source gas is ammonia. In the gas soft nitriding, a mixed gas of NH ₃ and an endothermic modification gas RX (CO, N ₂ , H ₂ ), a mixed gas of NH ₃ , N ₂ and CO ₂ or a mixed gas introduced into the processing furnace is used. , A mixed gas of NH ₃ , N ₂ , CO ₂ , and H ₂ can be used. In gas soft nitriding, the atmosphere in the processing furnace contains four or more types of gas, including ammonia and hydrogen gas, which are nitrogen supply source gases, so if the number of gases is three or more, the hydrogen concentration cannot be accurately detected. The superiority of the atmosphere control method of the present embodiment over the atmosphere control method using the hydrogen sensor is more apparent.

The atmosphere control method of the present embodiment can also be suitably applied to the case where vacuum carburization in which the atmosphere in the processing furnace is depressurized and vacuum carbonitriding are used as the surface treatment. The solid electrolyte hydrogen sensor can accurately detect the hydrogen concentration as shown in FIG. 7 by the same solid electrolyte hydrogen sensor regardless of whether the atmosphere to be measured is atmospheric pressure or reduced pressure. Here, FIG. 7 uses the surface treatment apparatus E1 having the above-described configuration, in which the sensor element includes the solid electrolyte hydrogen sensor 1 formed of SrZr _0.95 Yb _0.05 O _3-α which is a proton conductive solid electrolyte. Then, while changing the hydrogen concentration in the gas introduced into the processing furnace 30, when the total pressure of the atmosphere f of the processing furnace 30 is atmospheric pressure (101325 Pa), 5000 Pa, 1000 Pa, and 200 Pa, the hydrogen concentration in the introduced gas is changed. It is the result of calculating the hydrogen concentration on the basis of the electromotive force at that time while changing it to 100%, 80%, 50%, 30%. The temperature of the sensor element inserted in the internal space of the processing furnace 30 was 600°C.

For comparison, a solid-state hydrogen sensor 1 is replaced by a thermal conductivity type hydrogen sensor, and a surface treatment apparatus R of a comparative example having the same configuration as the surface treatment apparatus E1 is used, and the atmosphere f is similarly set. The hydrogen concentration was detected by changing the pressure of H and the hydrogen concentration in the introduced gas. The results are shown together with FIG. 7.

From FIG. 7, it is clear that in the surface treatment apparatus E1 using the solid electrolyte hydrogen sensor 1, the concentration of the introduced hydrogen gas is accurately detected at any pressure. On the other hand, in the surface treatment apparatus R of the comparative example using the thermal conductivity type hydrogen sensor, there is a difference between the hydrogen concentration in the introduced gas and the calculated hydrogen concentration, and the difference is when the pressure is as small as 200 Pa. It was very big. From these, it is considered that the atmosphere control method of the present embodiment using the solid electrolyte hydrogen sensor is suitable for vacuum carburization and vacuum carbonitriding performed under reduced pressure and in which the pressure changes with the treatment. Was given.

Next, a surface treatment device E2 including an atmosphere control device that is a solid electrolyte hydrogen sensor 2 of another form will be described with reference to FIG. The surface treatment device E2 is different from the surface treatment device E1 in that a tubular member 35 is connected in the middle of the gas discharge pipe 32, and the sensor probe 10b is disposed on the tubular member 35. In addition to the same configuration as the probe 10, a heater 15 for heating the sensor element 11 is further provided, and the sensor control unit of the sensor control device 2D detects the same temperature as the sensor control unit of the sensor control device 1D. In addition to the means, the hydrogen concentration calculating means, the transmitting means, and the storing means, a temperature adjusting means is further provided. The temperature adjusting means adjusts the current output to the heater 15 based on the temperature of the sensor element 11 detected by the thermocouple 13 to adjust the temperature of the sensor element 11.

More specifically, the tubular member 35 is in a room temperature space separated from the processing furnace 30. The sensor probe 10b is attached to the tubular member 35 so that the end of the sensor element 11 on the measurement electrode p2 side is located in the internal space of the tubular member 35. That is, the internal space of the tubular member 35 is the atmosphere of the solid electrolyte hydrogen sensor 2 to be measured and is the second space S2 for the sensor probe 10b. Note that FIG. 8 exemplifies a case where a suction pump as a pressure reducing device 41 is connected to the gas discharge pipe 32 downstream of the tubular member 35 and a pressure gauge 42 is attached to the tubular member 35. Although the configuration is one in which vacuum carburizing or vacuum carbonitriding is performed, when the surface treatment is gas nitriding or gas nitrocarburizing, the decompression device 41 and the pressure gauge 42 may be omitted.

The solid electrolyte hydrogen sensor 2 and the surface treatment apparatus E2 having the above-described configuration are used in the case where the processing furnace 30 does not have a hole for inserting the sensor probe 10b or the processing furnace 30 is a rotary furnace. It is adopted when 10b cannot be inserted into the internal space of the processing furnace 30. In such a configuration, the change in the hydrogen concentration in the furnace gas cannot be detected unless the furnace gas passes through the gas exhaust pipe 32 into the inner space of the tubular member 35 and reaches the sensor element 11. The situation is the same as that of the surface treatment apparatus of Patent Document 1 that uses the thermal conductivity type hydrogen sensor, that is, the surface treatment apparatus that takes time until the gas in the furnace reaches the sensor element through the pipe. However, as shown in FIG. 6, the solid electrolyte hydrogen sensor takes a shorter time to stabilize the output value in response to the change in the hydrogen concentration than the thermal conductivity type hydrogen sensor. Therefore, it is possible to perform more appropriate atmosphere control by performing feedback at shorter time intervals than feedback control based on detection of hydrogen concentration by the thermal conductivity hydrogen sensor.

Here, FIG. 6 shows the detection of the hydrogen concentration by the solid electrolyte hydrogen sensor 2 when the hydrogen concentration in the atmosphere f of the processing furnace 30 is switched from 50% to 80% in the surface treatment apparatus E2, and It is shown in comparison with detection of hydrogen concentration when a thermal conductivity type hydrogen sensor is used instead.

In the case where the hydrogen sensor is the solid electrolyte hydrogen sensor 2, the detected hydrogen concentration sharply rises from 50% at a certain point, and it is considered that the gas in the furnace reached the sensor element 11 at this point. With this point as the starting point (zero seconds), the hydrogen concentration detected by the solid electrolyte hydrogen sensor 2 rises to 80% in an extremely short time of less than 0.5 seconds, and stabilizes rapidly. On the other hand, when the hydrogen sensor is a thermal conductivity type hydrogen sensor, it is considered that the in-furnace gas in which the hydrogen concentration has changed to 80% has reached the sensor element via the pipeline from the above point (starting point). After a short delay, the hydrogen concentration started to rise, and after gradually increasing, the hydrogen concentration reached 80%. The time when the detected value of the hydrogen concentration by the thermal conductivity type hydrogen sensor becomes stable is 1 second or more later than the case of the solid electrolyte hydrogen sensor 2. Therefore, even in this configuration in which the sensor probe 10b is not inserted into the internal space of the processing furnace 30, the atmosphere control method using the solid electrolyte hydrogen sensor 2 is more advantageous than the atmosphere control method using the thermal conductivity type hydrogen sensor.

The time T to reach the starting point is the time until the gas in the furnace passes through the gas exhaust pipe 32, flows into the internal space of the tubular member 35, and reaches the sensor element. If it takes time for the in-furnace gas to reach the sensor element in this way, the feedback control is delayed by that amount. However, as described above with reference to FIG. By changing the size based on the size of the difference between the index value and the target value, it is possible to appropriately perform the atmosphere control in which the index value matches the target value.

Here, the temperature range in which the proton conductive solid electrolyte has a clean correlation between the hydrogen concentration and the electromotive force is 400°C to 1000°C. Therefore, in the solid electrolyte hydrogen sensor 2 included in the surface treatment apparatus E2, the temperature of the sensor element 11 is at least 400° C. by heating with the heater 15. Further, in the surface treatment device E2, the material of the gas discharge pipe 32 and the tubular member 35 is a metal having a high thermal conductivity. As a result, heat is transferred from the sensor probe 10b heated by the heater 15 and the temperature of the sensor element 11 reaches at least 400° C. to the metal gas exhaust pipe 32 through the metal tubular member 35. The tube becomes hot without being heated directly. As a result, the temperature of the gas in the processing furnace 30 is prevented from decreasing and ammonium carbonate or the like is prevented from depositing before the gas reaches the sensor element 11. It is not necessary to directly heat the gas exhaust pipe 32 and the tubular member 35, but it is more preferable to coat the outer peripheral surfaces of these with a heat insulating material.

Although the present invention has been described above with reference to the preferred embodiments, the present invention is not limited to the above-described embodiments, and various improvements are made within the scope not departing from the gist of the present invention as shown below. And the design can be changed.

For example, in the above, as the sensor probes 10 and 10b of the solid electrolyte hydrogen sensors 1 and 2, a mode in which the sensor element 11 having a cylindrical shape with a bottom closes the internal space of the cylindrical holder 18 has been exemplified. As long as the sensor element is held by the holder so that the first space in contact with the reference electrode and the second space in contact with the measurement electrode are partitioned, the shape of the sensor element and the manner of holding by the holder are not limited. For example, a bottomed tubular sensor element closes one end of the holder with its opening facing the inside or outside of the holder, and a columnar or flat sensor element closes the internal space of the holder. Alternatively, the columnar or flat sensor element may close one end of the holder.

1, 2 Solid electrolyte hydrogen sensor (hydrogen sensor)
1D, 2D sensor control device 10, 10b sensor probe 11 sensor element 21 atmosphere control unit 25 gas introduction device 30 processing furnace 31 gas introduction pipe 32 gas discharge pipe f atmosphere (atmosphere of processing furnace)
p1 reference electrode p2 measurement electrode S1 first space S2 second space

Claims (6)

An atmosphere control method for controlling the atmosphere of a processing furnace for performing a surface treatment for permeating and diffusing a nonmetallic element on the surface of steel,
Introducing a gas containing at least the non-metal element source gas into the processing furnace,
The hydrogen concentration or hydrogen partial pressure in the atmosphere of the processing furnace is detected by a hydrogen sensor having a proton conductive solid electrolyte as a sensor element,
Based on the detected hydrogen concentration or hydrogen partial pressure, to perform feedback control to change the flow rate of at least the source gas in the gas to be introduced into the processing furnace,
The surface treatment is
A specific nitriding treatment, which is one of the nitriding steps in gas nitriding, gas soft nitriding, and vacuum carbonitriding, in which the supply source gas for supplying nitrogen is ammonia, or
A method for controlling atmosphere, comprising vacuum carburizing using acetylene as the source gas for supplying carbon, and specific carburizing treatment which is one of carburizing steps in vacuum carbonitriding. The atmosphere control method according to claim 1, wherein the atmosphere of the processing furnace contains three or more kinds of gases including the supply source gas and the hydrogen gas. The feedback control is a target value of an index value reflecting the atmosphere of the processing furnace, which is calculated based on detection of hydrogen concentration or hydrogen partial pressure by the hydrogen sensor, and a past index value which is the index value before a predetermined time. It is done by contrasting with
The atmosphere control method according to claim 1 or 2, wherein the past index value is the index value 0.5 seconds to 10 seconds ago. The feedback control is a target value of an index value reflecting the atmosphere of the processing furnace, which is calculated based on detection of hydrogen concentration or hydrogen partial pressure by the hydrogen sensor, and a past index value which is the index value before a predetermined time. And the magnitude of the rate of change when changing the flow rate of at least the source gas in the gas to be introduced into the processing furnace, the difference between the past index value and the target value. The atmosphere control method according to claim 1 or 2, wherein the method is changed based on the size of the. 5. The detection of the hydrogen concentration or hydrogen partial pressure in the atmosphere of the processing furnace by the hydrogen sensor is performed in a state where the sensor element is inserted into the internal space of the processing furnace. Atmosphere control method described in one. A solid electrolyte sensor element, a reference electrode provided on the surface of the sensor element, and a measurement electrode provided on the surface of the sensor element in a second space partitioned from a first space in contact with the reference electrode. A sensor probe comprising: a hydrogen sensor comprising means for calculating hydrogen concentration or hydrogen partial pressure based on an electromotive force generated between the reference electrode and the measurement electrode;
A control means for controlling a gas to be introduced into a processing furnace for performing a surface treatment for permeating and diffusing a non-metallic element on the surface of the steel;
The control means is
Based on the detection of hydrogen concentration or hydrogen partial pressure in the atmosphere of the processing furnace by the hydrogen sensor, calculate an index value that reflects the atmosphere of the processing furnace, the target value of the index value a predetermined time before the index value By performing a feedback control for controlling the flow rate of at least the source gas of the non-metallic element in the gas to be introduced into the processing furnace, the atmosphere control device is compared with the past index value.

Images