Nitriding furnace and nitriding method

Abstract

To provide a nitriding furnace and a nitriding method each of which enables highly accurate calculation of nitriding potential without using an ammonia gas analyzer in nitridization performed by supplying ammonia gas, nitrogen gas, hydrogen gas, and oxygen-based gas to a nitriding chamber.SOLUTION: A nitriding furnace 1 is provided, comprising: gas flow volume control equipment 3 detecting a supply flow volume value of each of ammonia gas, nitrogen gas, hydrogen gas, and oxygen-based gas; a hydrogen sensor 4 detecting a hydrogen partial pressure value PH2 in a nitriding chamber 2; and a nitriding potential calculation unit 5. The nitriding furnace 1 is configured in such a manner that: an ammonia partial pressure value PNH3 in the nitriding chamber 2 is calculated based on a signal of the hydrogen partial pressure value PH2, a signal of the supply flow volume of each gas, and a progress degree of thermal decomposition reaction of the ammonia gas obtained from an oxygen partial pressure value; and nitriding potential KN is calculated from the hydrogen partial pressure value PH2 and ammonia partial pressure value PNH3.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a nitriding furnace for nitriding steel members, and more particularly to a nitriding furnace which supplies ammonia gas, hydrogen gas, nitrogen gas or oxygen-based gas as a nitriding gas and performs nitriding. [Background technology]

[0002] Steel members such as gears used in automobile transmissions are required to have high pitting resistance and bending fatigue strength. To meet such requirements, a nitriding process is performed in which nitrogen penetrates into the steel member to form an iron nitride compound layer containing a γ' phase on the surface of the steel member. It is known that the amount of nitrogen that penetrates into the steel member and the morphology of the iron nitride compound layer change depending on the shape and material of the steel member, the amount of gas supplied, and the treatment temperature.

[0003] Therefore, in order to obtain high pitting resistance and bending fatigue strength, it is necessary to perform stable nitriding of the workpiece. The nitriding is performed by supplying a nitriding gas into a nitriding chamber in which the workpiece is housed. Therefore, in order to perform stable nitriding, it is necessary to control the atmosphere in the nitriding chamber. As a method for controlling the atmosphere in the nitriding chamber, the partial pressure P of ammonia gas in the nitriding chamber is set to 1000. NH3 and the partial pressure of hydrogen gas P H2 A method is known in which the atmosphere in the nitriding chamber is controlled based on the nitriding potential KN, which is obtained from the ratio of

[0004]

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[0005] Here, as a nitriding furnace for performing the nitriding treatment, Patent Document 1 discloses a nitriding furnace in which a nitriding gas consisting of ammonia gas, nitrogen gas, and air is supplied into a heating chamber in which the nitriding treatment is performed. In this nitriding furnace, the partial pressure P of the ammonia gas in the heating chamber is measured using a gas analyzer. NH3 and the partial pressure of hydrogen gas PH2 is being measured.

[0006] Patent Document 2 discloses a nitriding furnace to which ammonia gas and carbon dioxide gas are supplied. In the nitriding furnace described in Patent Document 2, the partial pressure P of hydrogen generated by the decomposition of ammonia gas is measured by a hydrogen sensor without using an ammonia analyzer. H2 In addition, in Patent Document 2, the presence of carbon dioxide gas is ignored because the amount of carbon dioxide gas is small compared to ammonia gas, and the relationship between the nitriding potential KN and the hydrogen concentration KN=(1-4P H2 / 3) / P H2 3 / 2 The derived relational expression and the hydrogen partial pressure P detected by the hydrogen sensor are H2 The nitriding potential KN is calculated from [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2016-194111 A [Patent Document 2] JP 2013-249524 A Summary of the Invention [Problem to be solved by the invention]

[0008] Since the ammonia gas analyzer described in Patent Document 1 is expensive, it is desirable not to use an ammonia gas analyzer from the viewpoint of cost reduction. In the nitriding furnace described in Patent Document 2, the hydrogen partial pressure P obtained by a hydrogen sensor without using an ammonia analyzer is H2 However, it is unclear whether the nitriding potential KN can be calculated in a similar manner when the nitriding gas supplied to the furnace contains gases other than ammonia gas.

[0009] Therefore, the present inventors have developed a method for supplying a mixed gas of ammonia (NH3) gas, nitrogen (N2) gas, hydrogen (H2) gas, and oxygen-based gas (e.g., air or oxygen gas) as a nitriding gas by using the formula KN=(1-4P H2 / 3) / P H2 3 / 2 As a result, it was found that the nitriding potential KN obtained by the above formula disclosed in Patent Document 2 differs from the nitriding potential KN calculated with high accuracy based on the ammonia partial pressure detected using an ammonia gas analyzer and the hydrogen partial pressure detected using a hydrogen sensor.

[0010] Therefore, in the past, when ammonia gas, nitrogen gas, hydrogen gas, or oxygen-based gas was supplied into the nitriding chamber as the nitriding gas, there was a problem that it was difficult to accurately calculate the nitriding potential KN without using an ammonia analyzer.

[0011] The present invention has been made in consideration of the above circumstances, and has an object to provide a nitriding furnace and a nitriding method that are capable of calculating the nitriding potential with high accuracy without using an ammonia gas analyzer in a nitriding process in which ammonia gas, nitrogen gas, hydrogen gas, or an oxygen-based gas is supplied to a nitriding chamber as a nitriding gas. [Means for solving the problem]

[0012] The inventors have focused on the fact that the reaction changes to the thermal decomposition reaction of ammonia gas depending on the amount of oxygen in the oxygen-based gas contained in the atmospheric gas in the nitriding chamber. Then, the ammonia partial pressure value P NH3 It was found that it is possible to calculate the ammonia partial pressure value P NH3 and the hydrogen partial pressure value P detected by the hydrogen sensor H2The present inventors have found that the above object can be achieved by calculating the nitriding potential KN using the above formula, and have completed the present invention.

[0013] One embodiment of the present invention for solving the above problem will be disclosed below. [1] A nitriding furnace that supplies ammonia gas, nitrogen gas, hydrogen gas, and oxygen-based gas to perform nitriding on a workpiece in a nitriding chamber, a gas flow rate control device for detecting a supply flow rate value of each of the ammonia gas, the nitrogen gas, the hydrogen gas, and the oxygen-based gas; The hydrogen partial pressure value P H2 A hydrogen sensor for detecting a nitriding potential calculation unit for calculating a nitriding potential KN in the nitriding treatment chamber, The oxygen-based gas is oxygen gas, air, or a mixture of oxygen gas and air, The nitriding potential calculation unit calculates the hydrogen partial pressure value P H2 The signal of the above, the signal of the supply flow rate value of each gas, and the oxygen partial pressure value in the oxygen-based gas before it flows into the nitriding chamber are input, and the ammonia partial pressure value P in the nitriding chamber is calculated based on the progress of the thermal decomposition reaction of the ammonia gas obtained from the input values. NH3 The hydrogen partial pressure value P H2 and the ammonia partial pressure value P NH3 and calculating a nitriding potential KN in the nitriding chamber from the following equation (1):

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[0014] According to the present invention, in a nitriding process in which ammonia gas, nitrogen gas, hydrogen gas, or oxygen-based gas is supplied to a nitriding chamber as a nitriding gas, the nitriding potential can be calculated with high accuracy without using an ammonia gas analyzer. [Brief description of the drawings]

[0015] [Figure 1]1 is a diagram showing a schematic configuration of a nitriding furnace according to an embodiment of the present invention; [Diagram 2] FIG. 13 is a diagram showing a schematic configuration of a nitriding furnace according to another embodiment. [Diagram 3] FIG. 2 is a diagram showing a schematic configuration of a nitriding furnace according to a reference example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. In this specification and the drawings, elements having substantially the same functional configurations are denoted by the same reference numerals, and duplicated description will be omitted.

[0017] <Nitriding furnace> As shown in FIG. 1, the nitriding furnace 1 according to this embodiment is a furnace for nitriding a metal workpiece W, such as a steel part, for example a gear for an automobile transmission, or a metal mold, by supplying multiple types of nitriding gases to the workpiece W.

[0018] The nitriding furnace 1 has a nitriding chamber 2 for performing the nitriding process on the workpiece W, a gas flow control device 3 for detecting the supply flow rate value of the gas supplied to the nitriding chamber 2, a hydrogen sensor 4 for detecting the hydrogen partial pressure value within the nitriding chamber 2, and a nitriding potential calculation unit 5 for calculating the nitriding potential KN.

[0019] Ammonia gas, nitrogen gas, hydrogen gas, and oxygen-based gas are supplied to the nitriding chamber 2 as nitriding gas. In this specification, the "oxygen-based gas" refers to oxygen gas, air, or a mixture of oxygen gas and air. The supply flow rate of each gas is not particularly limited, but for example, each gas is supplied at a ratio of 10 to 70% for ammonia gas, 5 to 40% for nitrogen gas, 10 to 70% for hydrogen gas, and 1 to 10% for oxygen-based gas, relative to the total supply flow rate of the nitriding gas. In this embodiment, the supply flow rate of each gas is a "volumetric flow rate", and the "volumetric flow rate" is a "flow rate converted to standard conditions" that does not depend on the actual gas temperature and pressure used, and the "standard conditions" are 101.3 kPa (1 atm) and 0°C.

[0020] The outside of the nitriding chamber 2 has ammonia gas supply unit 22a, nitrogen gas supply unit 22b, hydrogen gas supply unit 22c, and oxygen-based gas supply unit 22d as gas supply units 22. One end of pipes 23a to 23d is connected to each of the gas supply units 22a to 22d. The other end of pipes 23a to 23d is connected to a gas supply pipe 24 attached to the ceiling of the nitriding chamber 2. The ammonia gas, nitrogen gas, hydrogen gas, and oxygen-based gas supplied from each of the gas supply units 22a to 22d flow through the pipes 23a to 23d, join together at the gas supply pipe 24, and are supplied into the nitriding chamber 2.

[0021] The gas flow control device 3 is provided in each of the pipes 23a to 23d. The gas flow control device 3 has a mass flow sensor (not shown) which is a flow meter, and a solenoid valve (not shown). The gas flow control device 3 is configured to detect the flow rate value of the gas flowing in the pipe 23 by the mass flow sensor, and output a signal of the flow rate value to the nitriding potential calculation unit 5. The flow rate value detected here is the supply flow rate value of the gas supplied to the nitriding chamber 2. The gas flow control device 3 is also configured to perform control to adjust the opening degree of the solenoid valve so that the flow rate of the gas flowing in the pipe 23 becomes a predetermined flow rate. The configuration of the gas flow control device 3 is not limited to the configuration described in this embodiment, and may be configured to detect the supply flow rate values ​​of each of the ammonia gas, nitrogen gas, hydrogen gas, and oxygen-based gas supplied to the nitriding chamber 2, and output a signal of each supply flow rate value to the nitriding potential calculation unit 5.

[0022] The gas flow rate control device 3d for the oxygen-based gas in this embodiment has an oxygen sensor (not shown) capable of detecting the oxygen partial pressure in the oxygen-based gas. The gas flow rate control device 3d for the oxygen-based gas is configured to output a signal of the oxygen partial pressure value detected by the oxygen sensor to the nitriding potential calculation unit 5. If the oxygen partial pressure value can be specified without using an oxygen sensor, the oxygen partial pressure value may be input in advance to the nitriding potential calculation unit 5. In this way, the method of inputting the signal of the oxygen partial pressure value to the nitriding potential calculation unit 5 is not particularly limited, but the oxygen partial pressure value input to the nitriding potential calculation unit 5 is the oxygen partial pressure value in the oxygen-based gas before it flows into the nitriding treatment chamber 2.

[0023] The hydrogen sensor 4 is provided so as to be inserted inward from a side wall portion of the nitriding chamber 2. The hydrogen sensor 4 is also configured to output a signal of the hydrogen partial pressure value inside the nitriding chamber 2 to the nitriding potential calculation unit 5. There are no particular limitations on the type of hydrogen sensor 4, but it is preferable that the hydrogen sensor 4 be a sensor that can be placed inside the nitriding chamber 2 and uses a proton conductor with a recommended operating temperature of 400 to 600°C.

[0024] For example, when a hydrogen sensor that cannot be placed in the nitriding chamber 2 is used, in order to measure the hydrogen partial pressure in the nitriding chamber 2, it is necessary to install piping and a pump for sampling the atmospheric gas from the nitriding chamber 2. However, if a hydrogen sensor 4 using a proton conductor is used, the hydrogen sensor 4 can be placed in the nitriding chamber 2, so that piping and a pump for taking out the atmospheric gas from the nitriding chamber 2 are not required, and the number of parts of the nitriding furnace 1 can be reduced. In addition, since the hydrogen partial pressure can be measured in the nitriding chamber 2 without taking out the atmospheric gas outside the nitriding chamber 2, the detection accuracy of the hydrogen partial pressure value is improved. This also improves the accuracy of the nitriding potential KN that is finally calculated. Furthermore, since the time required to take out the atmospheric gas from the nitriding chamber 2 can be omitted, it becomes possible to calculate the nitriding potential KN in a shorter time.

[0025] The nitriding potential calculation unit 5 is configured to calculate the ammonia decomposition degree α of the ammonia gas supplied into the nitriding chamber 2 and the partial pressure value of the ammonia gas in the nitriding chamber 2 based on the hydrogen partial pressure value signal input from the hydrogen sensor 4, the supply flow rate value signals of each gas input from the gas flow control device 3, and the oxygen partial pressure value in the oxygen-based gas, and to calculate the nitriding potential KN in the nitriding chamber 2. The ammonia decomposition degree α is an index that indicates the degree of progress of the thermal decomposition reaction of the ammonia gas supplied into the nitriding chamber 2.

[0026] After the nitriding potential KN is calculated by the nitriding potential calculation unit 5, for example, an operator adjusts the supply flow rate of each gas based on the value of the nitriding potential KN so that a desired nitriding atmosphere is created inside the nitriding chamber 2. Alternatively, the supply flow rate of each gas may be automatically adjusted by providing a nitriding potential adjustment unit 6 as shown in Fig. 2.

[0027] The nitriding potential adjusting unit 6 is configured to compare the value of the nitriding potential KN input from the nitriding potential calculating unit 5 with the value of a target nitriding potential KN1 preset as a nitriding treatment condition. The nitriding potential adjusting unit 6 is also configured to calculate the supply flow rate of each gas required to set the nitriding potential KN to the target nitriding potential KN1, and to perform control to output a flow rate value signal to the gas flow rate control devices 3a-3d of each gas. Based on this signal, the supply flow rate of each gas is adjusted in each of the gas flow rate control devices 3a-3d.

[0028] The nitriding potential calculation unit 5 and the nitriding potential adjustment unit 6 are configured using a computer including, for example, a CPU (Central Processing Unit), a ROM (Read Only Memory), and a RAM (Random Access Memory). Note that the nitriding potential calculation unit 5 and the nitriding potential adjustment unit 6 may be configured using the same computer, or may be configured using different computers.

[0029] 2, the supply flow rates of the gases are automatically controlled in response to the ever-changing conditions in the nitriding chamber 2, making it easier to maintain the nitriding potential KN in the nitriding chamber 2 at the target nitriding potential KN1. This makes it easier to perform nitriding of the workpiece W under desired conditions, improving the quality of the nitriding of the workpiece W.

[0030] The nitriding chamber 2 is provided with a plurality of heaters 25 extending in the height direction. Each heater 25 is disposed so as to laterally surround the periphery of the workpiece W. The heaters 25 are provided to heat the nitriding gas to a predetermined temperature.

[0031] A fan 26 is provided on the ceiling of the nitriding chamber 2 to agitate the atmospheric gas in the nitriding chamber 2 and uniformize the temperature of the nitriding gas and the temperature of the workpiece W. A door (not shown) that can be opened and closed is attached to the side wall of the nitriding chamber 2, and the workpiece W is carried into the nitriding chamber 2 through the door.

[0032] One end of a gas exhaust pipe 27 that exhausts the atmospheric gas inside the nitriding chamber 2 is connected to the ceiling of the nitriding chamber 2, and the other end of the gas exhaust pipe 27 is connected to a vacuum pump (not shown) installed outside the nitriding chamber 2.

[0033] The nitriding furnace 1 in this embodiment is configured as described above. Next, a nitriding method using this nitriding furnace 1 will be described.

[0034] (Processing conditions) The nitriding gas is a mixed gas of ammonia gas, nitrogen gas, hydrogen gas, and oxygen-based gas. The ambient temperature in the nitriding chamber 2 is preferably 400 to 700°C. If the ambient temperature in the nitriding chamber 2 is 400°C or higher, the rate of generation of the iron nitride compound layer is increased, and the nitriding time can be shortened. If the ambient temperature in the nitriding chamber 2 is 700°C or lower, softening and distortion of the workpiece W can be suppressed. The ambient temperature in the nitriding chamber 2 is more preferably 400 to 600°C, and further preferably 500 to 600°C.

[0035] (Calculation of nitriding potential KN) When the nitriding furnace 1 in this embodiment performs nitriding under the above conditions, the nitriding potential calculation unit 5 performs calculation based on the following formula (2) to calculate the nitriding potential KN.

[0036]

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[0037] The degree of ammonia decomposition α in formula (2) is an index showing the degree of progress of the thermal decomposition reaction of the ammonia gas supplied into the nitriding chamber 2, and takes a value in the range of 0 to 1. For example, α=0 indicates a state in which the ammonia gas in the nitriding chamber 2 has not been decomposed at all, and α=1 indicates a state in which the ammonia gas in the nitriding chamber 2 has been completely decomposed. 1-α in formula (2) indicates the proportion of undecomposed ammonia gas in the ammonia gas supplied into the nitriding chamber 2.

[0038] Furthermore, the ammonia gas supplied into the nitriding chamber 2 is thermally decomposed into nitrogen gas and hydrogen gas according to the following formula. NH3 → (1 / 2)N2+ (3 / 2)H2 That is, 0.5α in formula (2) represents the ratio of the amount of nitrogen gas generated by the decomposition of ammonia gas to the amount of ammonia gas decomposed, and 1.5α in formula (2) represents the ratio of the amount of hydrogen gas generated by the decomposition of ammonia gas to the amount of ammonia gas decomposed.

[0039] In formula (2), p is the oxygen partial pressure value in the oxygen-based gas supplied into the nitriding chamber 2, and takes a value in the range of 0.20 to 1 (atm). For example, when the oxygen-based gas supplied is oxygen, p is 1. For example, when the oxygen-based gas supplied is air, p is usually 0.20. For example, when the oxygen-based gas supplied is a mixed gas of oxygen and air, p is greater than 0.20 and less than 1. 1-p in formula (2) indicates the nitrogen partial pressure in the oxygen-based gas supplied.

[0040] Furthermore, oxygen gas in the oxygen-based gas and hydrogen gas react in the nitriding chamber 2 based on the following formula: 2H2+O2→ 2H2O That is, -2p in formula (2) represents the ratio of the amount of hydrogen gas reduced by reaction with oxygen gas to the amount of oxygen gas in the oxygen-based gas supplied into the nitriding chamber 2. 2p in formula (2) represents the ratio of the amount of water vapor generated by the reaction between hydrogen gas and oxygen gas to the amount of oxygen gas in the oxygen-based gas supplied into the nitriding chamber 2.

[0041] f in Equation (2) NH3 in , f N2 in , f H2 in , f O2 in are the supply flow rates of ammonia gas, nitrogen gas, hydrogen gas, and oxygen-based gas supplied into the nitriding chamber 2, respectively. In this embodiment, the supply flow rate of each gas is a "volumetric flow rate." In this specification, the "volumetric flow rate" is a "flow rate converted to standard conditions" that is independent of the actual gas use temperature and pressure, and the "standard conditions" are 101.3 kPa (1 atm) and 0°C. The supply flow rate values ​​of each of the ammonia gas, hydrogen gas, nitrogen gas, and oxygen-based gas are detected by the gas flow control devices 3a to 3d.

[0042] f in Equation (2) NH3 out , f N2 out , f H2 out , f O2out , f H2O out are the volumetric flow rates of ammonia gas, nitrogen gas, hydrogen gas, oxygen-based gas, and water vapor in the atmospheric gas exhausted from the nitriding chamber 2, respectively.

[0043] f in Equation (2) H2 out When expressed as an equation, we obtain the following equation (3), and by transforming equation 3, we obtain the following equation (4).

[0044]

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[0045] Next, the hydrogen partial pressure P in the nitriding chamber 2 obtained by the hydrogen sensor 4 H2 is expressed as a volumetric flow rate ratio as shown in the following equation (5), and by transforming equation (5), the following equation (6) is obtained.

[0046]

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[0047] Here, by expanding the above-mentioned equation (2), we obtain the following equation (7).

[0048]

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[0049] By substituting this equation (7) into the term in brackets on the right-hand side of equation (6), we obtain the following equation (8).

[0050]

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[0051] Next, f in the above formula (4) H2 outBy substituting equation (8) into the above and transforming the equation into the following equations (8-1) to (8-4), the following equation (9) for calculating the ammonia decomposition rate α is obtained.

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[0053] P in Equation (9) H2 is the hydrogen partial pressure value, which is the value detected by the hydrogen sensor 4. NH3 in , f N2 in , f H2 in , f O2 in are the supply flow rate values ​​of ammonia gas, nitrogen gas, hydrogen gas, and oxygen-based gas, and are values ​​detected by the gas flow rate control device 3. p in formula (9) is the oxygen partial pressure value, and is a value detected by the gas flow rate control device 3 for oxygen gas or a value input in advance to the nitriding potential calculation unit 5. Therefore, there are no unknowns in formula (9) during the nitriding process, and by using formula (9), it is possible to calculate the ammonia decomposition degree α, which changes depending on the state of the atmospheric gas in the nitriding process chamber 2.

[0054] For example, when air is supplied as the oxygen-based gas into the nitriding chamber 2 and the oxygen partial pressure value p is set to 0.2, the ammonia decomposition degree α is calculated by the following formula.

[0055]

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[0056] Furthermore, for example, when oxygen gas is supplied as the oxygen-based gas into the nitriding chamber 2, if the oxygen partial pressure value p is set to 1, the ammonia decomposition degree α is calculated by the following formula.

[0057]

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[0058] Next, the ammonia partial pressure value P NH3 Calculate the ammonia partial pressure P NH3 is obtained from the volumetric flow rate as shown in equation (10) below.

[0059]

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[0060] Here, f in the above formula (2) NH3 out When expressed as an equation, we obtain the following equation (11).

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[0062] Then, by substituting the formula (11) into the numerator of the right-hand side of the formula (10) and the above formula (7) into the denominator of the right-hand side of the formula (10), the ammonia partial pressure value P NH3 The following equation (12) is obtained to calculate

[0063]

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[0064] α in formula (12) is the degree of ammonia decomposition, which is the value obtained from formula (9). NH3 in , f N2 in , f H2 in , f O2 inare the supply flow rate values ​​of ammonia gas, nitrogen gas, hydrogen gas, and oxygen-based gas, and are values ​​detected by the gas flow rate control device 3. p in formula (12) is the oxygen partial pressure value, and is a value detected by the gas flow rate control device 3 for oxygen gas, or a value input in advance to the nitriding potential calculation unit 5. Therefore, there are no unknowns in formula (12) during the nitriding process, and by using formula (12), the ammonia partial pressure value P NH3 can be calculated.

[0065] For example, when air is supplied as the oxygen-based gas into the nitriding chamber 2, if the oxygen partial pressure value p is set to 0.2, the ammonia partial pressure value P NH3 is calculated using the following formula:

[0066]

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[0067] In addition, for example, when oxygen is supplied as the oxygen-based gas into the nitriding chamber 2, if the oxygen partial pressure value p is set to 1, the ammonia partial pressure value P NH3 is calculated using the following formula:

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[0069] Finally, the hydrogen partial pressure value P detected by the hydrogen sensor 4 H2 and the ammonia partial pressure value P calculated by equation (12) NH3 Using this, the nitriding potential KN in the nitriding chamber 2 can be calculated from the following formula (1).

[0070]

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[0071] As will be shown in the examples described later, the nitriding potential KN calculated by the above method is a highly accurate value. That is, in the nitriding furnace 1 of this embodiment, the nitriding potential KN can be calculated with high accuracy without using an analyzer for ammonia gas. In particular, the calculation method of the nitriding potential KN according to this embodiment calculates the degree of progress of the thermal decomposition reaction of the ammonia gas in the nitriding chamber 2 from the oxygen partial pressure value in the supplied oxygen-based gas and the supply flow rate value of each gas, which was not considered in the past, and this point is different from the calculation method of the conventional nitriding potential KN. This difference makes it possible to calculate the nitriding potential KN with high accuracy.

[0072] Although an example of an embodiment of the present invention has been described above, the present invention is not limited to such an example. It is clear that a person skilled in the art can come up with various modified or revised examples within the scope of the technical ideas described in the claims, and it is understood that these also naturally belong to the technical scope of the present invention.

[0073] For example, in the above embodiment, the ammonia decomposition degree α is used as an index showing the progress of the thermal decomposition reaction of the ammonia gas supplied into the nitriding treatment chamber 2, and the ammonia partial pressure value P NH3 However, the ammonia partial pressure value P was calculated using another formula instead of using formula (12). NH3 Even in such a case, the ammonia partial pressure value P may be calculated using the ammonia decomposition degree α that takes into account the degree of progress of the thermal decomposition reaction of ammonia gas. NH3 By performing the calculation, the nitriding potential KN can be calculated more accurately than before. That is, the ammonia partial pressure value P NH3 If the calculation formula for the ammonia partial pressure P is a formula that takes into account the degree of progress of the thermal decomposition reaction of ammonia gas, NH3 The specific calculation formula is not particularly limited, but by using formula (12), the nitriding potential KN can be calculated more accurately.

[0074] In addition, the ammonia partial pressure value P NH3In the nitriding process in which ammonia gas, nitrogen gas, hydrogen gas, and oxygen-based gas are supplied as the nitriding gas, the degree of progress of the thermal decomposition reaction of ammonia gas, which changes based on the oxygen partial pressure value in the oxygen-based gas and the supply flow rate value of each gas, may be taken into consideration in order to accurately calculate the nitriding potential KN. That is, the ammonia partial pressure value P NH3 If the degree of progress of the thermal decomposition reaction of ammonia gas is taken into consideration in the calculation of (a), the nitriding potential KN can be calculated more accurately than before without using the ammonia decomposition degree α. However, by calculating the ammonia decomposition degree α using the above-mentioned formula (9), the nitriding potential KN can be calculated more accurately. EXAMPLES

[0075] Nitriding was carried out under the conditions shown in the following Reference Examples, Examples, and Comparative Examples, and the nitriding potential KN was calculated for each example.

[0076] <Reference example> In the reference example, nitriding was carried out using a nitriding furnace 11 having a structure shown in Fig. 3. The nitriding furnace 11 has a nitriding chamber 2 with a volume of about 330 L. A heater 25 is provided in the nitriding chamber 2 for heating the nitriding gas supplied into the nitriding chamber 2. The nitriding furnace 11 has an ammonia gas supply unit 22a, a nitrogen gas supply unit 22b, a hydrogen gas supply unit 22c, and an oxygen-based gas supply unit 22d, which are supply sources of the gases constituting the nitriding gas.

[0077] The nitriding gas is supplied from the gas supply pipe 24 into the nitriding chamber 2 through pipes 23a-23d connected to the gas supply units 22a-22d. The pipes 23a-23d are provided with gas flow control devices 3a-3d having mass flow sensors (not shown) as flow meters and solenoid valves (not shown). The gas flow control devices 33a-3d detect the flow rates of the gases flowing through the pipes 23a-23d with the mass flow sensors, and perform control to adjust the openings of the solenoid valves so that the flow rates of the gases supplied into the nitriding chamber 2 are predetermined flow rates.

[0078] In addition, the nitriding chamber 2 is provided with an ammonia partial pressure value P NH3 and a continuous gas analyzer 7 (manufactured by ABB, model AO2000-Uras26) that detects the hydrogen partial pressure value P H2 A hydrogen sensor 4 (Model Notorp-G, manufactured by TYK) using a proton conductor to detect hydrogen is provided. By using such a hydrogen sensor 4 using a proton conductor, the hydrogen sensor 4 can be disposed inside the nitriding chamber 2, eliminating the need for piping and pumps to extract atmospheric gas from the nitriding chamber 2 and allowing the number of parts in the nitriding furnace 1 to be reduced.

[0079] In addition, the nitriding furnace 11 detects an ammonia partial pressure P NH3 and the hydrogen partial pressure value P obtained by the hydrogen sensor 4. H2 The nitriding potential calculator 5 calculates the nitriding potential KN in the nitriding chamber 2 based on the signal from the nitriding potential calculator 5 according to the following equation (1).

[0080]

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[0081] The nitriding chamber 2 is provided with a fan 26 for agitating the nitriding gas in the nitriding chamber 2. In addition, the nitriding chamber 2 is provided with a gas exhaust pipe 27 for exhausting the atmospheric gas in the nitriding chamber 2.

[0082] The nitriding furnace 11 in the reference example was constructed as described above. When nitriding was performed using this nitriding furnace 11, ammonia gas, nitrogen gas, hydrogen gas, and air were supplied into the nitriding chamber 2 under the following condition A or condition B. (Condition A) Ammonia gas flow rate to the nitriding chamber: 16 L / min Nitrogen gas flow rate to the nitriding chamber: 2L / min Hydrogen gas flow rate to the nitriding chamber: 6L / min Air flow rate to nitriding chamber: 1.5L / min (Condition B) Ammonia gas flow rate to the nitriding chamber: 6L / min Nitrogen gas flow rate to the nitriding chamber: 7L / min Hydrogen gas flow rate to the nitriding chamber: 21L / min Air flow rate to nitriding chamber: 1.5L / min

[0083] The atmosphere in the nitriding chamber 2 was heated to 580° C. by the heater 25 while rotating the fan 26 at 1000 rpm. After maintaining this state for 60 minutes, the hydrogen partial pressure value P H2 and the ammonia partial pressure value P detected by the continuous gas analyzer 7 NH3 The nitriding potential KN was calculated from the above formula (1). In the nitriding furnace 11, the hydrogen partial pressure value P H2 and ammonia partial pressure value P NH3 Since both are actually measured values, the calculated nitriding potential KN is a value that is extremely close to the actual nitriding potential in the nitriding chamber 2.

[0084] As a result of calculating the nitriding potential KN by the above method, the nitriding potential KN under condition A was 1.42, and the nitriding potential KN under condition B was 0.25.

[0085] <Example> In the examples, nitriding was carried out using a nitriding furnace 1 having the structure shown in Fig. 1. The nitriding furnace 1 in the examples differs from the nitriding furnace 11 in the reference example shown in Fig. 3 in the following points. Ammonia partial pressure value P NH3 There is no continuous gas analyzer 7 for detecting Hydrogen partial pressure value P detected by hydrogen sensor 4 H2 The signal is input to the nitriding potential calculation unit 5. The signal of the supply flow rate value of each gas is input to the nitriding potential calculation unit 5. The value of the oxygen partial pressure p of the air (0.2) is input in advance to the nitriding potential calculation unit 5. The nitriding potential calculation unit 5 calculates the hydrogen partial pressure value P H2 Based on the supply flow rate values ​​of each gas and the above-mentioned oxygen partial pressure p, the ammonia decomposition degree α, which indicates the progress of the thermal decomposition reaction of ammonia gas, is calculated using the above-mentioned formula (9). The nitriding potential calculation unit 5 calculates the ammonia partial pressure value P by the above-mentioned formula (12) using the ammonia decomposition degree α obtained by formula (9). NH3 Calculate the nitriding potential KN using the above formula (1).

[0086] Nitriding was performed under the above conditions, and the nitriding potential KN was calculated. As a result, the nitriding potential KN under condition A was 1.41, and the nitriding potential KN under condition B was 0.25.

[0087] <Comparative Example> In the comparative example, the nitriding treatment was carried out using the same nitriding furnace 1 as in the example. In the comparative example, the hydrogen partial pressure value P H2 Using this, the nitriding potential KN was calculated according to the following formula disclosed in Patent Document 2. KN=(1-4P H2 / 3) / P H2 3 / 2

[0088] As a result, the nitriding potential KN under condition A was 1.25, and the nitriding potential KN under condition B was 0.16.

[0089] Table 1 shows the values ​​of the nitriding potential KN calculated in the above Reference Examples, Examples, and Comparative Examples.

[0090] [Table 1]

[0091] The nitriding potential KN calculated in the reference example is the hydrogen partial pressure P H2 The measured value of ammonia partial pressure P NH3Since the nitriding potential KN is calculated based on the measured values, it is a value that is extremely close to the actual nitriding potential in the nitriding chamber 2. However, the nitriding potential KN in the comparative example has a large error with respect to the nitriding potential of the reference example, and it is understood that the nitriding potential KN cannot be calculated with high accuracy by the calculation method of the comparative example. On the other hand, the nitriding potential KN in the example is within a range of ±0.05 of the nitriding potential KN calculated in the reference example, and it is understood that the nitriding potential KN can be calculated with high accuracy by the calculation method of the example. [Industrial Applicability]

[0092] The present invention can be applied to a nitriding furnace for performing nitriding treatment on metal workpieces, such as steel parts, such as gears for automobile transmissions, and metal molds. [Explanation of symbols]

[0093] 1, 11 Nitriding furnace 2. Nitriding chamber 3 Gas flow control equipment 4 Hydrogen sensor 5 Nitriding potential calculation section 6 Nitriding potential adjustment part 7 Continuous Gas Analyzer 22 Gas Supply Section 23 Piping 24 Gas supply pipe 25 Heater 26 Fans 27 Gas exhaust pipe

Claims

1. A nitriding furnace that supplies ammonia gas, nitrogen gas, hydrogen gas, and an oxygen-based gas to perform nitriding on a workpiece in a nitriding chamber, a gas flow rate control device for detecting a supply flow rate value of each of the ammonia gas, the nitrogen gas, the hydrogen gas, and the oxygen-based gas; The hydrogen partial pressure value P H2 A hydrogen sensor for detecting a nitriding potential calculation unit for calculating a nitriding potential KN in the nitriding treatment chamber, The oxygen-based gas is oxygen gas, air, or a mixture of oxygen gas and air, The nitriding potential calculation unit calculates the hydrogen partial pressure value P H2 a signal indicating the supply flow rate of each gas, and an oxygen partial pressure value in the oxygen-based gas before it flows into the nitriding chamber are inputted, The ammonia partial pressure value P in the nitriding chamber is calculated based on the degree of progress of the thermal decomposition reaction of the ammonia gas obtained from each input value. NH3 Calculate The hydrogen partial pressure value P H2 and the ammonia partial pressure value P NH3 and calculating a nitriding potential KN in the nitriding chamber from the following equation (1): [0010]

2. 2. The nitriding furnace according to claim 1, wherein the nitriding potential calculation unit is configured to calculate an ammonia decomposition degree α from the following formula (9) as an index showing a progress degree of the thermal decomposition reaction of the ammonia gas: [0025] However, the letters in the formula are as follows: P H2 : Hydrogen partial pressure (atm) p: Partial pressure of oxygen in oxygen-based gas (atm) f NH3 in :Ammonia gas supply flow rate (L / min) f N2 in :Nitrogen gas supply flow rate (L / min) f H2 in :Hydrogen gas supply flow rate (L / min) f O2 in : Oxygen gas supply flow rate (L / min)

3. The nitriding potential calculation unit calculates the ammonia partial pressure value P from the following formula (12) using the ammonia decomposition degree α. NH3 The nitriding furnace according to claim 2 , configured to calculate: [0030]

4. 4. The nitriding furnace according to claim 1, wherein the hydrogen sensor is a sensor using a proton conductor and is disposed within the nitriding chamber.

5. A nitriding method for supplying ammonia gas, nitrogen gas, hydrogen gas, and an oxygen-based gas to a workpiece in a nitriding chamber, comprising the steps of: The oxygen-based gas is oxygen gas, air, or a mixture of oxygen gas and air, The supply flow rate values ​​of the ammonia gas, the nitrogen gas, the hydrogen gas, and the oxygen-based gas, and the hydrogen partial pressure value P in the nitriding treatment chamber detected by a hydrogen sensor H2 and an ammonia partial pressure value P in the nitriding chamber based on the progress of the thermal decomposition reaction of the ammonia gas obtained from the oxygen partial pressure value in the oxygen-based gas before it flows into the nitriding chamber. NH3 Calculate The hydrogen partial pressure value P H2 and the ammonia partial pressure value P NH3 and calculating a nitriding potential KN in the nitriding chamber from the following equation (1): [0045]

6. 6. The nitriding method according to claim 5, wherein an ammonia decomposition degree α is calculated from the following formula (9) as an index showing a progress degree of the thermal decomposition reaction of the ammonia gas: [0050] However, the letters in the formula are as follows: P H2 : Hydrogen partial pressure (atm) p: Partial pressure of oxygen in oxygen-based gas (atm) f NH3 in :Ammonia gas supply flow rate (L / min) f N2 in :Nitrogen gas supply flow rate (L / min) f H2 in :Hydrogen gas supply flow rate (L / min) f O2 in : Oxygen gas supply flow rate (L / min)

7. Using the ammonia decomposition rate α, the ammonia partial pressure value P NH3 The nitriding method according to claim 6, further comprising the step of: [006]

8. the hydrogen sensor is a sensor using a proton conductor, The hydrogen partial pressure value P H2 The nitriding method according to any one of claims 5 to 7, wherein the hydrogen is detected by the hydrogen sensor disposed in the nitriding chamber.

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