Surface hardening device and surface hardening method

Abstract

To introduce ammonia gas, ammonia decomposition gas and nitrogen gas into a processing furnace having a temperature of 680°C or more to nitrify a product to be treated with gas as surface hardening.SOLUTION: The change of the introduction amounts of ammonia and nitrogen gases can approach nitriding potential in a processing furnace to a target nitriding potential while keeping the total introduction amount of three kinds of in-furnace introduction gases constant and keeping the introduction amount of ammonia decomposition gas constant corresponding to the nitriding potential in the processing furnace calculated by an in-furnace nitriding potential calculation device and the target nitriding potential. The introduction amount of the nitrogen gas is 40% or more to the total introduction amount of the three kinds of furnace introduction gases; and the introduction amount of the ammonia decomposition gas is 12% or more thereto.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to a surface hardening treatment apparatus and a surface hardening treatment method for performing surface hardening treatment, such as nitriding, soft nitriding, nitriding quenching, etc., on a metal workpiece. [Background technology]

[0002] Among the surface hardening treatments for steel and other metal workpieces, there is a high demand for nitriding, which is a low-strain treatment. Nitriding methods include the gas method, salt bath method, and plasma method.

[0003] Of these methods, the gas method is superior overall when considering quality, environmental friendliness, mass production, etc. Distortion caused by carburizing or carbonitriding with hardening or high-frequency hardening of machine parts can be improved by using nitriding treatment using the gas method (gas nitriding treatment). Soft nitriding treatment using the gas method with carburizing (gas soft nitriding treatment) is also known as a similar type of treatment to gas nitriding treatment.

[0004] Gas nitriding is a process in which only nitrogen is allowed to penetrate and diffuse into the workpiece to harden the surface. In gas nitriding, ammonia gas alone, a mixture of ammonia gas and nitrogen gas, ammonia gas and ammonia decomposition gas (consisting of 75% hydrogen and 25% nitrogen, also known as AX gas), or a mixture of ammonia gas, ammonia decomposition gas, and nitrogen gas is introduced into the treatment furnace to perform the surface hardening process.

[0005] On the other hand, gas soft nitriding is a process in which carbon is secondarily introduced and diffused into the workpiece together with nitrogen to harden the surface. For example, in gas soft nitriding, multiple types of furnace introduction gases, such as a mixed gas of ammonia gas, nitrogen gas, and carbon dioxide gas (CO2), or a mixed gas of ammonia gas, nitrogen gas, carbon dioxide gas, and carbon monoxide gas (CO), are introduced into the treatment furnace to perform surface hardening.

[0006] The basis of atmosphere control in gas nitriding and gas soft nitriding is the nitriding potential (KN The aim is to control the nitriding potential (K N ), the γ' phase (Fe4N) and ε phase (Fe 2-3 It is possible to obtain a wide range of nitriding qualities, such as by controlling the volume fraction of γ' phase and N, or by realizing a process in which the compound layer is not generated. For example, according to JP 2016-211069 (Patent Document 1), the selection of the γ' phase and thickening it improves bending fatigue strength and wear resistance, realizing further high performance of machine parts.

[0007] In the above-mentioned gas nitriding and gas soft nitriding, in order to control the atmosphere in the treatment furnace in which the workpiece is placed, an in-furnace atmosphere gas concentration measuring sensor is installed to measure the in-furnace hydrogen concentration or the in-furnace ammonia concentration. Then, the in-furnace nitriding potential is calculated from the measurement value of the in-furnace atmosphere gas concentration measuring sensor, and compared with the target (set) nitriding potential, and the flow rate of each introduced gas is controlled (Non-Patent Document 1). Regarding the method of controlling each introduced gas, a method of controlling the total introduction amount while keeping the flow rate ratio of the gases introduced into the furnace constant is well known (Non-Patent Document 2).

[0008] Patent Document 2 discloses an apparatus that can execute both a first control, which is a control mode that controls the total amount of gas introduced into the furnace while keeping the flow rate ratio of the gases introduced into the furnace constant, and a second control, which is a control mode that individually controls the amount of gas introduced into the furnace so that the flow rate ratio of the gases introduced into the furnace changes (only one of the two is selectively performed at the same time) (Patent Document 2). However, Patent Document 2 only discloses one specific example of a nitriding process in which the first control is effective (paragraphs 0096 and 0099 of Patent Document 2 state that "the total amount of ammonia gas and nitrogen gas introduced into the treatment furnace is controlled while maintaining NH3 (ammonia gas):N2 (nitrogen gas) = ​​80:20" to precisely control the nitriding potential 3,3), and does not disclose what kind of nitriding process or soft nitriding process it is effective to adopt the second control, nor does it disclose any specific example of an effective second control.

[0009] In addition, the method of controlling the total introduction amount while keeping the flow rate ratio of the gases introduced into the furnace constant has the advantage of being able to suppress the total amount of gas used, but it is also known that the control range of the nitriding potential is narrow. As a measure to address this problem, the present inventor developed a control method for realizing a wide nitriding potential control range (for example, about 0.05 to 1.3 at 580°C) on the low nitriding potential side, and obtained Patent No. 6345320 (Patent Document 3). In the control method of Patent No. 6345320 (Patent Document 3), the introduction amount of the multiple types of gases introduced into the furnace is individually controlled so that the nitriding potential in the processing furnace approaches the target nitriding potential by changing the flow rate ratio of the multiple types of gases introduced into the furnace while keeping the total introduction amount of the multiple types of gases introduced into the furnace constant. Furthermore, the present inventor developed a control method that can realize practical nitriding potential control by using only ammonia gas and ammonia decomposition gas as the gases introduced into the furnace, and obtained Patent No. 6503122 (Patent Document 4). In the control method of Patent No. 6503122 (Patent Document 4), the amount of ammonia gas introduced is changed while keeping the amount of ammonia decomposition gas introduced constant, thereby controlling the nitriding potential in the treatment furnace to approach a target nitriding potential.

[0010] (Basics of Gas Nitriding) To explain the basics of gas nitriding in chemical terms, in gas nitriding, a nitriding reaction represented by the following formula (1) occurs in a treatment furnace (gas nitriding furnace) in which the workpiece is placed. NH3 → [N] + 3 / 2H2 · · · (1)

[0011] At this time, the nitriding potential K N is defined by the following equation (2). K N =P NH3 / P H2 3 / 2 (2) Here, P NH3 is the partial pressure of ammonia in the furnace, and P H2 is the hydrogen partial pressure in the furnace. The nitriding potential K Nis well known as an index representing the nitriding ability of the atmosphere in a gas nitriding furnace.

[0012] On the other hand, in the furnace during the gas nitriding treatment, a part of the ammonia gas introduced into the furnace is thermally decomposed into hydrogen gas and nitrogen gas in accordance with the reaction of formula (3). NH3 → 1 / 2N2 + 3 / 2H2 (3)

[0013] In the furnace, the reaction of formula (3) mainly occurs, and the nitriding reaction of formula (1) can be almost negligible in terms of quantity. Therefore, if the ammonia concentration in the furnace consumed by the reaction of formula (3) or the hydrogen gas concentration generated by the reaction of formula (3) is known, the nitriding potential can be calculated. That is, since the amount of hydrogen and nitrogen generated is 1.5 moles and 0.5 moles, respectively, from 1 mole of ammonia, if the ammonia concentration in the furnace is measured, the amount of ammonia consumed by the reaction of formula (3) can be known, and the hydrogen concentration in the furnace can also be known, and the nitriding potential can be calculated. Alternatively, if the hydrogen concentration in the furnace is measured, the amount of hydrogen generated by the reaction of formula (3) can be known, and the amount of ammonia consumed by the reaction of formula (3) can be known, and the ammonia concentration in the furnace can also be known, and the nitriding potential can also be calculated.

[0014] The ammonia gas flowing into the gas nitriding furnace is discharged outside the furnace after circulating inside the furnace. That is, in the gas nitriding process, fresh (new) ammonia gas is constantly flowed into the furnace to replace the existing gas in the furnace, so that the existing gas is continuously discharged outside the furnace (pushed out by the supply pressure).

[0015] Here, if the flow rate of ammonia gas introduced into the furnace is low, the gas residence time in the furnace will be long, and the amount of ammonia gas that is decomposed will increase, and the amount of nitrogen gas + hydrogen gas generated by the decomposition reaction will increase. On the other hand, if the flow rate of ammonia gas introduced into the furnace is high, the amount of ammonia gas that is not decomposed and is discharged to the outside of the furnace will increase, and the amount of nitrogen gas + hydrogen gas generated in the furnace will decrease.

[0016] (Fundamentals of flow control) Next, regarding the basic matters of flow control, first, the case where the gas introduced into the furnace is only ammonia gas will be described. When the decomposition degree of the ammonia gas introduced into the furnace is s (0 < s < 1) (when 1 mole of ammonia gas is introduced, there are (1 + s) moles of gas in the furnace), the gas reaction in the furnace is represented by the following formula (4). NH3→(1 - s) / (1 + s)NH3+0.5s / (1 + s)N2+1.5s / (1 + s)H2 ···(4) Here, the left side is the gas introduced into the furnace (only ammonia gas), and the right side is the gas composition in the furnace. There are unreacted ammonia gas, nitrogen and hydrogen generated in a ratio of 1:3 by the decomposition of ammonia gas. Therefore, when measuring the hydrogen concentration in the furnace with a hydrogen sensor, 1.5s / (1 + s) on the right side corresponds to the measured value by the hydrogen sensor, and the decomposition degree s of the ammonia gas introduced into the furnace can be calculated from the measured value. Thereby, the ammonia concentration in the furnace corresponding to (1 - s) / (1 + s) on the right side can also be calculated. That is, the hydrogen concentration and the ammonia concentration in the furnace can be known only from the measured value of the hydrogen sensor. For this reason, the nitriding potential can be calculated.

[0017] Even when using a plurality of gases introduced into the furnace, the control of the nitriding potential KN is possible. For example, when two types of gases, ammonia and nitrogen, are used as the gases introduced into the furnace, and the introduction ratio is x:y (x and y are known and x + y = 1. For example, x = 0.5, y = 1 - 0.5 = 0.5 (NH3:N2 = 1:1)), the gas reaction in the furnace is represented by the following formula (5). xNH3+(1 - x)N2→x(1 - s) / (1 + sx)NH3+(0.5sx + 1 - x) / (1 + sx)N2+1.5sx / (1 + sx)H2 ···(5)

[0018] Here, the gas composition in the furnace on the right side is undecomposed ammonia gas, nitrogen and hydrogen generated by the decomposition of ammonia gas in a ratio of 1:3, and nitrogen gas (not decomposed in the furnace) on the left side that is introduced as it is. In this case, x can be recognized as a known value by quoting the value of the mass flow controller for each gas at each time (for example, x=0.5 at a certain time), and the only unknown in the hydrogen concentration in the furnace on the right side, that is, 1.5sx / (1+sx), is the decomposition degree s of ammonia. Therefore, as in the case of formula (4), the decomposition degree s of the ammonia gas introduced into the furnace can be calculated from the measurement value of the hydrogen sensor, and the ammonia concentration in the furnace can also be calculated. Therefore, the nitriding potential can be calculated.

[0019] When the flow rate ratio of the gas introduced into the furnace is not fixed, the hydrogen concentration in the furnace and the ammonia concentration in the furnace include two variables: the decomposition degree s of the ammonia gas introduced into the furnace and the introduction rate x of the ammonia gas. Generally, a mass flow controller (MFC) is used as a device to control the gas flow rate, so the introduction rate x of the ammonia gas can be continuously read as a digital signal based on the flow rate value. Therefore, the nitriding potential can be calculated by combining the introduction rate x and the measurement value of the hydrogen sensor based on formula (5).

[0020] (Special circumstances of high temperature gas nitriding) Both the above-mentioned Patent No. 6345320 (Patent Document 3) and the above-mentioned Patent No. 6503122 (Patent Document 4) are concerned with temperatures of about 600° C. or less.

[0021] On the other hand, in the high temperature range of 680°C or higher, the decomposition degree s of the ammonia gas introduced into the furnace becomes very high, so a control mode for mixing ammonia decomposition gas is not adopted, and nitrogen gas and ammonia gas are used instead.

[0022] Specifically, as described in JP 2007-46088 A (Patent Document 5), in the case of a furnace heated to 720° C., the amount of nitrogen gas introduced is 1 m 3Ammonia gas was introduced at a rate of 4 L / min per hour (Examples 1, 3, and 7 of Patent Document 5), and nitrogen gas was introduced at a rate of 1 m 3 Ammonia gas was introduced at a rate of 8 L / min per hour (Examples 2 and 4 to 6 of Patent Document 5), and nitrogen gas was introduced at a rate of 1 m 3 Ammonia gas was introduced at a rate of 4 L / min per hour (Example 1), and nitrogen gas was introduced at a rate of 1 m 3 In one example, ammonia gas was introduced at a rate of 6 L / min per hour (Example 8). [Prior art documents] [Patent documents]

[0023] [Patent Document 1] Patent Publication 2016-211069 [Patent Document 2] Patent No. 5629436 [Patent Document 3] Patent No. 6345320 [Patent Document 4] Patent No. 6503122 [Patent Document 5] Patent Publication No. 2007-46088 [Non-patent literature]

[0024] [Non-Patent Document 1] "Heat Treatment", Vol. 55, No. 1, pp. 7-11 (Yasushi Hiraoka, Yoichi Watanabe) [Non-Patent Document 2] "Nitriding and Nitrocarburizing of Iron", 2nd edition (2013), pp. 158-163 (Dieter Liedtke et al., Agne Technical Center) [Non-Patent Document 3] "Effect of Compound Layer Thickness Composed of γ'-Fe4N on Rotated-Bending Fatigue Strength in Gas-Nitrided JIS-SCM435 Steel", Materials Transactions, Vol.58, No.7(2017), pp.993-999 (Y.Hiraoka and A.Ishida) Summary of the Invention [Problem to be solved by the invention]

[0025] The present inventors have found that in the high temperature range of 680° C. or higher, particularly in the high temperature range of 800° C. or higher, the nitriding potential cannot be controlled with high precision in a control mode using only nitrogen gas and ammonia gas (Patent Document 5).

[0026] Furthermore, the present inventors have confirmed that in the high temperature range of 680°C or higher, particularly in the high temperature range of 800°C or higher, the nitriding potential cannot be controlled with high precision even in a control mode using only ammonia gas and ammonia decomposition gas (Patent Document 4).

[0027] The present invention has been devised based on the above findings. An object of the present invention is to provide a surface hardening apparatus and a surface hardening method that can realize practical nitriding potential control in a high temperature range of 680°C or higher, particularly in a high temperature range of 800°C or higher. [Means for solving the problem]

[0028] A first aspect of the present invention is a surface hardening treatment apparatus that introduces ammonia gas, ammonia decomposition gas, and nitrogen gas into a treatment furnace at a temperature of 680°C or higher and performs a gas nitriding treatment as a surface hardening treatment for a workpiece placed in the treatment furnace, characterized in that the surface hardening treatment apparatus comprises an in-furnace atmosphere gas concentration detection device that detects a hydrogen concentration or an ammonia concentration in the treatment furnace, an in-furnace atmosphere gas concentration detection device that calculates a nitriding potential in the treatment furnace based on the hydrogen concentration or the ammonia concentration detected by the in-furnace atmosphere gas concentration detection device, and a gas introduction amount control device that changes the introduction amount of the ammonia gas and the introduction amount of the nitrogen gas while keeping the total introduction amount of the three types of in-furnace introduction gas constant and keeping the introduction amount of the ammonia decomposition gas constant, in accordance with the nitriding potential in the treatment furnace calculated by the in-furnace atmosphere gas concentration detection device and a target nitriding potential, thereby bringing the nitriding potential in the treatment furnace closer to the target nitriding potential.

[0029] The present inventors have analyzed the principle of the first aspect of the present invention as follows. It has been known that practical nitriding potential control cannot be achieved by merely varying the amounts of ammonia gas and nitrogen gas introduced. That is, making the ratio of ammonia gas introduced larger than that of nitrogen gas does not increase the nitriding potential, and making the ratio of ammonia gas introduced smaller than that of nitrogen gas does not decrease the nitriding potential. The present inventors have discovered that it is effective to additionally introduce a certain amount of ammonia decomposition gas as a hydrogen source, and have arrived at the present invention.

[0030] However, since excessive introduction of ammonia decomposition gas into the furnace is disadvantageous in terms of the amount of ammonia decomposition gas used (cost), it is industrially required to keep the amount introduced to a minimum within a range that allows nitriding potential control. The present inventors have confirmed that in the first aspect of the present invention, the amount of the nitrogen gas introduced is 40% or more and the amount of the ammonia decomposition gas introduced is 12% to 24% of the total amount of the three types of gases introduced into the furnace, which is suitable in terms of both stabilization of nitriding potential control and processing costs.

[0031] A second aspect of the present invention is a surface hardening treatment apparatus that introduces ammonia gas, ammonia decomposition gas, and nitrogen gas into a treatment furnace at a temperature of 680°C or higher and performs a gas nitriding treatment as a surface hardening treatment for a workpiece placed in the treatment furnace, characterized in that the surface hardening treatment apparatus comprises an in-furnace atmosphere gas concentration detection device that detects a hydrogen concentration or an ammonia concentration in the treatment furnace, an in-furnace atmosphere gas concentration detection device that calculates a nitriding potential in the treatment furnace based on the hydrogen concentration or the ammonia concentration detected by the in-furnace atmosphere gas concentration detection device, and a gas introduction amount control device that changes the introduction amount of the ammonia gas and the total introduction amount of the three types of gases introduced into the furnace while keeping the introduction amount of the nitrogen gas constant and the introduction amount of the ammonia decomposition gas constant, in accordance with the nitriding potential in the treatment furnace calculated by the in-furnace atmosphere gas concentration detection device and a target nitriding potential, thereby bringing the nitriding potential in the treatment furnace closer to the target nitriding potential.

[0032] The present inventors have analyzed the principle of the second aspect of the present invention as follows. It has been known that practical nitriding potential control cannot be achieved by simply varying the amount of ammonia gas introduced while keeping the amount of nitrogen gas introduced constant. The present inventors have discovered that it is effective to additionally introduce a constant amount of ammonia decomposition gas as a hydrogen source, and have arrived at the present invention.

[0033] In the second embodiment, too, excessive introduction of ammonia decomposition gas into the furnace is disadvantageous in terms of the amount (cost) of ammonia decomposition gas used, and therefore, it is industrially required to keep the amount introduced to a minimum within a range that allows nitriding potential control. The present inventors have confirmed that, in the second embodiment of the present invention, the amount of the nitrogen gas introduced is 40% or more and the amount of the ammonia decomposition gas introduced is 12% to 24% of the total amount of the three types of gases introduced into the furnace, which is suitable in terms of both stabilization of nitriding potential control and processing costs.

[0034] In each of the above embodiments, 12% ammonia decomposition gas corresponds to 3% nitrogen gas + 9% hydrogen gas. Therefore, the first and second embodiments of the present invention are technically equivalent to the third and fourth embodiments of the present invention described below, respectively.

[0035] A third aspect of the present invention is a surface hardening treatment apparatus that introduces ammonia gas, hydrogen gas, and nitrogen gas into a treatment furnace at a temperature of 680°C or higher to perform a gas nitriding treatment as a surface hardening treatment for a workpiece placed in the treatment furnace, characterized in that the surface hardening treatment apparatus comprises an in-furnace atmosphere gas concentration detection device that detects a hydrogen concentration or an ammonia concentration in the treatment furnace, an in-furnace atmosphere gas concentration detection device that calculates a nitriding potential in the treatment furnace based on the hydrogen concentration or the ammonia concentration detected by the in-furnace atmosphere gas concentration detection device, and a gas introduction amount control device that changes the introduction amount of the ammonia gas and the introduction amount of the nitrogen gas while keeping the total introduction amount of the three types of in-furnace introduction gases constant and keeping the introduction amount of the hydrogen gas constant, in accordance with the nitriding potential in the treatment furnace calculated by the in-furnace atmosphere gas concentration detection device and a target nitriding potential, thereby bringing the nitriding potential in the treatment furnace closer to the target nitriding potential.

[0036] The present inventors have confirmed that in the third aspect of the present invention, the amount of the nitrogen gas introduced is 40% or more and the amount of the ammonia decomposition gas introduced is 9% to 18% relative to the total amount of the three types of gases introduced into the furnace, which is preferable in terms of both stabilizing the nitriding potential control and processing costs.

[0037] A fourth aspect of the present invention is a surface hardening treatment apparatus that introduces ammonia gas, hydrogen gas, and nitrogen gas into a treatment furnace at a temperature of 680°C or higher to perform a gas nitriding treatment as a surface hardening treatment for a workpiece placed in the treatment furnace, characterized in that the surface hardening treatment apparatus comprises an in-furnace atmosphere gas concentration detection device that detects a hydrogen concentration or an ammonia concentration in the treatment furnace, an in-furnace atmosphere gas concentration detection device that calculates a nitriding potential in the treatment furnace based on the hydrogen concentration or the ammonia concentration detected by the in-furnace atmosphere gas concentration detection device, and a gas introduction amount control device that changes the introduction amount of the ammonia gas and the total introduction amount of the three types of gases introduced into the furnace while keeping the introduction amount of the nitrogen gas constant and keeping the introduction amount of the hydrogen gas constant, in accordance with the nitriding potential in the treatment furnace calculated by the in-furnace atmosphere gas concentration detection device and a target nitriding potential, thereby bringing the nitriding potential in the treatment furnace closer to the target nitriding potential.

[0038] The present inventors have confirmed that in the fourth aspect of the present invention, the amount of the nitrogen gas introduced is 40% or more and the amount of the ammonia decomposition gas introduced is 9% to 18% relative to the total amount of the three types of gases introduced into the furnace, which is preferable in terms of both stabilizing the nitriding potential control and processing costs.

[0039] In each of the above aspects, the target nitriding potential is within a range of, for example, 0.01 to 0.20. It has been confirmed that the present invention makes it possible to realize such nitriding potential control over a relatively wide range (particularly, a relatively low range).

[0040] In each of the above aspects, the temperature inside the processing furnace is, for example, 800° C. or higher.

[0041] The present invention can also be recognized as a surface hardening method. That is, the first aspect of the method of the present invention is a surface hardening method for introducing ammonia gas, ammonia decomposition gas, and nitrogen gas into a treatment furnace at a temperature of 680°C or higher to perform gas nitriding as a surface hardening treatment of a workpiece placed in the treatment furnace, the surface hardening treatment method comprising: a furnace atmosphere gas concentration detection step for detecting a hydrogen concentration or an ammonia concentration in the treatment furnace; a furnace atmosphere gas concentration calculation step for calculating a nitriding potential in the treatment furnace based on the hydrogen concentration or the ammonia concentration detected by the furnace atmosphere gas concentration detection step; and a gas introduction amount control step for changing the introduction amount of the ammonia gas and the introduction amount of the nitrogen gas while keeping the total introduction amount of the three types of furnace introduction gases constant and keeping the introduction amount of the ammonia decomposition gas constant, according to the nitriding potential in the treatment furnace calculated by the furnace atmosphere gas concentration calculation step and a target nitriding potential, thereby bringing the nitriding potential in the treatment furnace closer to the target nitriding potential.

[0042] Alternatively, a second aspect of the method of the present invention is a surface hardening method for introducing ammonia gas, ammonia decomposition gas, and nitrogen gas into a treatment furnace at a temperature of 680°C or higher to perform a gas nitriding treatment as a surface hardening treatment of a workpiece placed in the treatment furnace, the surface hardening treatment method comprising: an in-furnace atmosphere gas concentration detection step for detecting a hydrogen concentration or an ammonia concentration in the treatment furnace; an in-furnace atmosphere gas concentration calculation step for calculating a nitriding potential in the treatment furnace based on the hydrogen concentration or the ammonia concentration detected by the in-furnace atmosphere gas concentration detection step; and a gas introduction amount control step for changing the introduction amount of the ammonia gas and the total introduction amount of the three types of gases introduced into the furnace while keeping the introduction amount of the nitrogen gas constant and keeping the introduction amount of the ammonia decomposition gas constant, in accordance with the nitriding potential in the treatment furnace calculated by the in-furnace atmosphere gas concentration calculation step and a target nitriding potential, thereby bringing the nitriding potential in the treatment furnace closer to the target nitriding potential.

[0043] Alternatively, a third aspect of the method of the present invention is a surface hardening method for introducing ammonia gas, hydrogen gas, and nitrogen gas into a treatment furnace at a temperature of 680°C or higher to perform a gas nitriding treatment as a surface hardening treatment of a workpiece placed in the treatment furnace, the surface hardening treatment method comprising: an in-furnace atmosphere gas concentration detection step for detecting a hydrogen concentration or an ammonia concentration in the treatment furnace; an in-furnace atmosphere gas concentration calculation step for calculating a nitriding potential in the treatment furnace based on the hydrogen concentration or the ammonia concentration detected by the in-furnace atmosphere gas concentration detection step; and a gas introduction amount control step for changing the introduction amount of the ammonia gas and the introduction amount of the nitrogen gas while keeping the total introduction amount of the three types of in-furnace introduction gases constant and keeping the introduction amount of the hydrogen gas constant, in accordance with the nitriding potential in the treatment furnace calculated by the in-furnace atmosphere gas concentration detection step and a target nitriding potential, thereby bringing the nitriding potential in the treatment furnace closer to the target nitriding potential.

[0044] Alternatively, a fourth aspect of the method of the present invention is a surface hardening method for introducing ammonia gas, hydrogen gas, and nitrogen gas into a treatment furnace having a temperature of 680°C or higher, and performing a gas nitriding treatment as a surface hardening treatment for a workpiece placed in the treatment furnace, the surface hardening treatment method comprising: an in-furnace atmosphere gas concentration detection step for detecting a hydrogen concentration or an ammonia concentration in the treatment furnace; an in-furnace atmosphere gas concentration calculation step for calculating a nitriding potential in the treatment furnace based on the hydrogen concentration or the ammonia concentration detected by the in-furnace atmosphere gas concentration detection step; and a gas introduction amount control step for changing the introduction amount of the ammonia gas and the total introduction amount of the three types of gases introduced into the furnace while keeping the introduction amount of the nitrogen gas constant and keeping the introduction amount of the hydrogen gas constant, in accordance with the nitriding potential in the treatment furnace calculated by the in-furnace atmosphere gas concentration calculation step and a target nitriding potential, thereby bringing the nitriding potential in the treatment furnace closer to the target nitriding potential. Effect of the Invention

[0045] According to the first aspect of the present invention, in the high temperature range of 680°C or higher, a feedback control system is adopted in which the amount of ammonia gas and the amount of nitrogen gas introduced are changed (the two are increased or decreased alternately) while the total amount of gases introduced into the furnace is kept constant, and at the same time, a constant amount (12% or more, more preferably 16% or more) of ammonia decomposition gas is additionally introduced, thereby making it possible to stabilize the nitriding potential control.

[0046] Alternatively, according to a second aspect of the present invention, a feedback control system is employed in which the amount of ammonia gas introduced and the total amount of gas introduced into the furnace are changed (both are increased or decreased) while the amount of nitrogen gas introduced is kept constant in a high temperature range of 680°C or higher, while a constant amount (12% or more, more preferably 16% or more) of ammonia decomposition gas is additionally introduced, thereby making it possible to stabilize the nitriding potential control.

[0047] Alternatively, according to a third aspect of the present invention, in a high temperature range of 680°C or higher, a feedback control system is employed in which the amount of ammonia gas and the amount of nitrogen gas introduced are changed (the two are increased or decreased alternately) while the total amount of gas introduced into the furnace is kept constant, and at the same time, a constant amount (9% or more, more preferably 12% or more) of hydrogen gas is additionally introduced, thereby making it possible to stabilize the nitriding potential control.

[0048] Alternatively, according to a fourth aspect of the present invention, a feedback control system is employed in which the amount of ammonia gas introduced and the total amount of gas introduced into the furnace are changed (both are increased or decreased) while the amount of nitrogen gas introduced is kept constant in a high temperature range of 680°C or higher, while a constant amount (9% or more, more preferably 12% or more) of hydrogen gas is additionally introduced, thereby stabilizing the nitriding potential control. [Brief description of the drawings]

[0049] [Figure 1] 1 is a schematic diagram showing a surface hardening treatment apparatus (batch-type heat treatment furnace) according to one embodiment of the present invention. [Diagram 2]FIG. 2 is a process diagram of one embodiment of the surface hardening treatment method of the present invention using the surface hardening treatment apparatus of FIG. [Diagram 3] FIG. 2 is a schematic diagram showing a main part of the surface hardening treatment device of FIG. [Figure 4] 1 is a table showing nitriding conditions for examples of the present invention. [Diagram 5] 1 is a table showing nitriding conditions for comparative examples of the present invention. [Figure 6] 1 is a table showing nitriding conditions for examples of the present invention. [Figure 7] FIG. 2 is a schematic view showing a modified example of the main part of the surface hardening treatment device of FIG. [Figure 8] 1 is a table showing nitriding conditions for examples of the present invention. [Figure 9] 1 is a table showing nitriding conditions for comparative examples of the present invention. [Figure 10] 1 is a table showing nitriding conditions for comparative examples of the present invention. [Figure 11] 1 is a table showing nitriding conditions for examples of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0050] Preferred embodiments of the present invention will be described below, but the present invention is not limited to the following embodiments.

[0051] [Example of processed item (workpiece)] The workpiece is a steel member. Specifically, it is a steel member made of carbon steel or alloy steel for mechanical construction, such as gears used in automatic transmissions. For example, a cylindrical ring gear or a cylindrical ring gear with a bottom is mounted on a multi-stage jig and subjected to nitriding treatment while being placed flat in a case (described later).

[0052] It is preferable that the steel members are pre-cleaned to remove dirt and oil before the nitriding treatment. Pre-cleaning is preferably performed by, for example, vacuum cleaning, which dissolves and replaces oil and the like in a hydrocarbon-based cleaning liquid and evaporates it to degrease and dry, or alkaline cleaning, which degreases the steel members with an alkaline cleaning liquid.

[0053] [Example of the configuration of a batch-type heat treatment furnace] FIG. 1 is a schematic diagram showing a surface hardening treatment apparatus (batch-type heat treatment furnace) 1 according to one embodiment of the present invention.

[0054] As shown in Fig. 1, the surface hardening treatment device (batch-type heat treatment furnace) 1 includes an entrance section 10, a heating chamber 11, a transport chamber 12, and an exit conveyor 13. A case 20 is placed in the entrance section 10, and a steel member as a workpiece is stored in the case 20. The maximum processing weight is 700 kg gross.

[0055] An inlet hood 22 having an openable and closable door 21 is attached to the inlet side (left side in FIG. 1) of the heating chamber 11. The heating chamber 11 has a retort structure, and the temperature inside the furnace is controlled to a predetermined temperature by heating the outer periphery of the retort with a heater (not shown). A plurality of types of gases for nitriding treatment are introduced into the heating chamber 11 while being controlled as described below (see FIGS. 3 and 7).

[0056] A fan 26 is attached to the ceiling of the heating chamber 11 to mix the gas introduced into the heating chamber 11 and make the heating temperature of the steel members uniform. A middle door 27 that can be opened and closed freely is attached to the outlet side of the heating chamber 11 (the right side in FIG. 1).

[0057] The transfer chamber 12 is provided with an elevator 30 that raises and lowers the case 20 in which the steel members are stored. A cooling chamber (oil tank) 32 that stores oil 31 for cooling is provided in the lower part of the transfer chamber 12. An exit hood 36 having an openable and closable door 35 is attached to the exit side of the transfer chamber 12 (the right side in FIG. 1).

[0058] Alternatively, the heating chamber 11 and the transfer chamber 12 may be formed as a processing chamber in the same space, and the steel members after heat treatment may be air-cooled by gas.

[0059] [Example of operation of a batch-type heat treatment furnace] In the surface hardening treatment apparatus (batch-type heat treatment furnace) 1 configured as described above, the case 20 containing the steel member is carried into the heating chamber 11 from the carrying-in section 10 by a pusher or the like. Then, after the steel member (the case 20 containing the steel member) is carried into the heating chamber 11, a treatment gas is introduced into the heating chamber 11, the treatment gas is heated to a predetermined temperature by a heater, and the steel member carried into the heating chamber 11 is subjected to a nitriding treatment while being stirred by a fan 26 (rotating at, for example, 1500 rpm).

[0060] FIG. 2 is a process diagram of one embodiment of the nitriding method of the present invention using the surface hardening treatment apparatus (batch-type heat treatment furnace) 1 of FIG.

[0061] In the example of FIG. 2, before the steel member (workpiece) is charged, the inside of the heating chamber 11 is preheated to, for example, 850° C. During this heating process, N2 gas is introduced at a flow rate of 7.5 (m 3 / h), and NH3 gas was introduced at a constant flow rate of 0.5 (m 3 / h), and AX gas is introduced at a constant flow rate of 2.0 (m 3 / h), the total flow rate is 7.5 + 0.5 + 2.0 = 10.0 (m 3 / h).

[0062] Next, a steel member (workpiece) is loaded into the heating chamber 11. At this time, the door 21 is opened, and the temperature inside the heating chamber 11 temporarily drops, as shown in Fig. 2. Thereafter, the door 21 is closed, and the temperature inside the heating chamber 11 is heated again to 850°C.

[0063] Even during the charging of steel components, in the example shown in Figure 2, N2 gas was 7.5 (m 3 / h), and NH3 gas was introduced at a constant flow rate of 0.5 (m 3 / h), and AX gas is introduced at a constant flow rate of 2.0 (m 3 / h), the total flow rate is 7.5 + 0.5 + 2.0 = 10.0 (m 3 / h).

[0064] Thereafter, a nitriding process is carried out. Specifically, for example, a value of 0.02 is adopted as the target nitriding potential, and the nitriding process is carried out at a temperature of 850° C. (corresponding to Example 1 described later).

[0065] Nitriding potential K N It is known that the partial pressure P(NH3) of NH3 gas and the partial pressure P(H2) of H2 gas are expressed by the following formula: K N = P(NH3) / P(H2) 3 / 2

[0066] In this nitriding process, the partial pressure P(NH3) of NH3 gas or the partial pressure P(H2) of H2 gas in the heating chamber 11 is measured, and the amount of processing gas introduced is feedback controlled so that the value of the nitriding potential calculated from the measured value falls within a range close to the target nitriding potential.

[0067] In the example of Figure 2, the partial pressure P(NH3) of NH3 gas in the heating chamber 11 is measured by an ammonia concentration analyzer (an example of an atmospheric gas concentration detection device 103: see Figures 3 and 7), and the amount of processing gas introduced is feedback controlled while the measured value is analyzed online (while the nitriding potential is calculated from the measured value).

[0068] Specifically, NH3 gas is set at an initial flow rate of 0.5 [m 3 / h], 0.47 to 0.53 [m 3 / h] range, and N2 gas was introduced at an initial flow rate of 7.5 [m 3 / h], 7.47 to 7.53 [m 3 / h], and the increase and decrease of both is done alternately (the total introduction amount of both is 8.0 [m 3 / h).

[0069] Furthermore, for example, AX gas (ammonia decomposition gas) is set at a constant flow rate of 2.0 [m 3 / h] is added.

[0070] The nitriding process is carried out for, for example, 90 minutes.

[0071] After the nitriding process is completed, a cooling process (quenching process) is performed. In the example shown in Fig. 2, the cooling process is performed for 20 minutes (an oil tank with an agitator is used, and the steel members are kept in oil (100°C) for 20 minutes). After the cooling process is completed, the cases 20 containing the steel members are discharged onto the discharge conveyor 13.

[0072] [Example of main components] FIG. 3 is a schematic diagram showing a main part of the surface hardening treatment device of FIG.

[0073] As shown in FIG. 3, the treatment furnace (heating chamber) 11 of the surface hardening treatment apparatus 1 of this embodiment is provided with an agitating fan 26, an agitating fan drive motor 109, a furnace temperature measuring device 110, a furnace body heating device 111, an atmospheric gas concentration detection device 103, a nitriding potential regulator 104, a temperature regulator 105, a programmable logic controller 131, a recorder 106, and a furnace introduction gas supply unit 120.

[0074] The stirring fan 26 is disposed in the processing furnace (heating chamber) 11 and rotates within the processing furnace (heating chamber) 11 to stir the atmosphere within the processing furnace (heating chamber) 11. An stirring fan drive motor 109 is connected to the stirring fan 26 and rotates the stirring fan 26 at an arbitrary rotation speed.

[0075] The furnace temperature measuring device 110 includes a thermocouple and is configured to measure the temperature of the furnace gas present in the processing furnace (heating chamber) 11. After measuring the temperature of the furnace gas, the furnace temperature measuring device 110 outputs an information signal (furnace temperature signal) including the measured temperature to the temperature controller 105 and the recorder 106.

[0076] The ambient gas concentration detection device 103 is composed of a sensor capable of detecting the hydrogen concentration or ammonia concentration in the processing furnace (heating chamber) 11 as the ambient gas concentration in the furnace. A detection body of the sensor communicates with the inside of the processing furnace (heating chamber) 11 via an ambient gas pipe 112. In this embodiment, the ambient gas pipe 112 is formed of a single-wire path that directly communicates the sensor body of the ambient gas concentration detection device 103 with the processing furnace (heating chamber) 11. An on-off valve 117 is provided midway through the ambient gas pipe 112, and the on-off valve is controlled by an on-off valve control device 116.

[0077] Moreover, the atmospheric gas concentration detector 103 detects the atmospheric gas concentration in the furnace, and then outputs an information signal including the detected concentration to the nitriding potential regulator 104 and the recorder 106 .

[0078] The recorder 106 includes a storage medium such as a CPU and a memory, and stores the temperature inside the treatment furnace (heating chamber) 11 and the atmospheric gas concentration inside the furnace based on output signals from the furnace temperature measuring device 110 and the atmospheric gas concentration detecting device 103, corresponding to, for example, the date and time when the surface hardening treatment was performed.

[0079] The nitriding potential regulator 104 has an in-furnace nitriding potential calculator 113 and a gas flow rate output regulator 130. The programmable logic controller 131 has a gas introduction amount controller 114 and a parameter setting device 115.

[0080] The in-furnace nitriding potential calculation device 113 is configured to calculate the nitriding potential in the treatment furnace (heating chamber) 11 based on the hydrogen concentration or ammonia concentration detected by the in-furnace atmosphere gas concentration detection device 103. Specifically, a calculation formula for the nitriding potential programmed based on a concept similar to that of equation (5) according to the actual gas introduced into the furnace is incorporated, and the nitriding potential is calculated from the value of the in-furnace atmosphere gas concentration.

[0081] The parameter setting device 115 is, for example, a touch panel, and is capable of inputting and setting the target nitriding potential, processing temperature, processing time, initial introduction amount of each introduced gas, and the like, for the same workpiece. It is also possible to input and set the setting parameter values ​​of PID control for each different value of the target nitriding potential. Specifically, it is possible to input and set the "proportional gain", "integral gain or integral time", and "differential gain or differential time" of the PID control for each different value of the target nitriding potential. The input and set setting parameter values ​​are transmitted to the gas flow output adjustment device 130.

[0082] In the first embodiment, the gas flow rate output adjusting device 130 performs PID control with the nitriding potential calculated by the furnace nitriding potential calculating device 113 as an output value, the target nitriding potential (set nitriding potential) as a target value, and the amounts of ammonia gas and nitrogen gas introduced among the three types of furnace introduction gases as input values. More specifically, in the PID control, the amounts of ammonia gas and nitrogen gas introduced are changed while the amount of ammonia decomposition gas introduced and the total amount of gas introduced into the furnace are kept constant, thereby making the nitriding potential in the treatment furnace (heating chamber) 11 approach the target nitriding potential. In addition, in the PID control, each set parameter value transmitted from the parameter setting device 15 is used.

[0083] Alternatively, in a second aspect, the gas flow rate output adjusting device 130 performs PID control with the nitriding potential calculated by the furnace nitriding potential calculating device 113 as an output value, the target nitriding potential (set nitriding potential) as a target value, and the amount of ammonia gas introduced among the three types of furnace introduction gases and the total amount of furnace introduction gases as input values. More specifically, in the PID control, the amount of ammonia gas introduced and the total amount of furnace introduction gases are changed while the amount of nitrogen gas introduced and the amount of ammonia decomposition gas introduced are kept constant, so that the nitriding potential in the treatment furnace (heating chamber) 11 approaches the target nitriding potential. Also, in the PID control, each set parameter value transmitted from the parameter setting device 15 is used.

[0084] It is preferable to obtain in advance candidate setting parameter values ​​for PID control for inputting settings to the parameter setting device 115 by performing pilot processing. In this embodiment, even if (1) the state of the processing furnace (state of the furnace walls and jigs), (2) the temperature conditions of the processing furnace, and (3) the state of the workpieces (type and number) are the same, (4) candidate setting parameter values ​​for each different value of the target nitriding potential can be obtained by the auto-tuning function of the nitriding potential regulator 104 itself. In order to configure the nitriding potential regulator 104 with the auto-tuning function, a UT75A (high-performance digital indicating regulator, http: / / www.yokogawa.co.jp / ns / cis / utup / utadvanced / ns-ut75a-01-ja.htm) manufactured by Yokogawa Electric Corporation or the like can be used.

[0085] The setting parameter values ​​acquired as candidates (a set of "proportional gain", "integral gain or integral time", and "differential gain or differential time") may be recorded in some form and manually input to the parameter setting device 115 according to the target processing content. However, the setting parameter values ​​acquired as candidates may be stored in some storage device in a form linked to the target nitriding potential, and may be automatically read out by the parameter setting device 15 based on the set and input value of the target nitriding potential.

[0086] Prior to PID control, the gas flow rate output adjusting device 130 determines the amount of gas introduced to be kept constant and the initial value of the amount of gas introduced to be changed based on the value of the target nitriding potential. Candidates for these values ​​are preferably obtained in advance by performing a pilot process, and are automatically read from a storage device or the like by the parameter setting device 115, or are manually input from the parameter setting device 115. Thereafter, the amount of gas introduced to be changed is determined (changed) according to the PID control so that the nitriding potential in the processing furnace (heating chamber) 11 approaches the target nitriding potential. The output value of the gas flow rate output adjusting device 130 is transmitted to the gas introduction amount control device 114.

[0087] The gas introduction amount controller 114 is adapted to send a control signal to a first supply amount controller 122 for ammonia gas.

[0088] The furnace introduction gas supply unit 120 of this embodiment has a first furnace introduction gas supply unit 121 for ammonia gas, a first supply amount control device 122, a first supply valve 123, and a first flow meter 124. The furnace introduction gas supply unit 120 of this embodiment also has a second furnace introduction gas supply unit 125 for ammonia decomposition gas (AX gas), a second supply amount control device 126, a second supply valve 127, and a second flow meter 128. Furthermore, the furnace introduction gas supply unit 120 of this embodiment has a third furnace introduction gas supply unit 171 for nitrogen gas, a third supply amount control device 172, a third supply valve 173, and a third flow meter 174.

[0089] In this embodiment, the ammonia gas, the ammonia decomposition gas, and the nitrogen gas are mixed in the furnace gas introduction pipe 129 before entering the processing furnace (heating chamber) 11.

[0090] The first furnace introduction gas supply unit 121 is formed of, for example, a tank filled with a first furnace introduction gas (ammonia gas in this example).

[0091] The first supply amount control device 122 is formed by a mass flow controller (capable of changing the flow rate in small increments within a short period of time) and is interposed between the first furnace introduction gas supply unit 121 and the first supply valve 123. The opening degree of the first supply amount control device 122 changes according to a control signal output from the gas introduction amount control device 114. The first supply amount control device 122 detects the supply amount from the first furnace introduction gas supply unit 121 to the first supply valve 123, and outputs an information signal including the detected supply amount to the gas introduction amount control device 114 and the recorder 106. The control signal can be used for correcting the control by the gas introduction amount control device 114, etc.

[0092] The first supply valve 123 is formed of an electromagnetic valve that switches between an open and closed state in response to a control signal output from the gas introduction amount control device 114, and is interposed between the first supply amount control device 122 and the first flow meter .

[0093] The first flow meter 124 is formed of, for example, a mechanical flow meter such as a flow type flow meter, and is interposed between the first supply valve 123 and the furnace introduction gas introduction piping 129. The first flow meter 124 detects the supply amount from the first supply valve 123 to the furnace introduction gas introduction piping 129. The supply amount detected by the first flow meter 124 can be used for visual confirmation by an operator.

[0094] The second furnace introduction gas supply unit 125 is formed of, for example, a tank filled with the second furnace introduction gas (ammonia decomposition gas in this example).

[0095] The second supply amount control device 126 is formed by a mass flow controller (capable of changing the flow rate in small increments within a short period of time) and is interposed between the second furnace introduction gas supply unit 125 and the second supply valve 127. The opening degree of the second supply amount control device 126 changes according to a control signal output from the gas introduction amount control device 114. The second supply amount control device 126 detects the supply amount from the second furnace introduction gas supply unit 125 to the second supply valve 127, and outputs an information signal including the detected supply amount to the gas introduction amount control device 114 and the recorder 106. The control signal can be used for correcting the control by the gas introduction amount control device 114, etc.

[0096] The second supply valve 127 is formed of an electromagnetic valve that switches between an open and closed state in response to a control signal output from the gas introduction amount control device 114, and is interposed between the second supply amount control device 126 and the second flow meter 128.

[0097] The second flow meter 128 is formed of a mechanical flow meter such as a flow type flow meter, and is interposed between the second supply valve 127 and the furnace introduction gas introduction piping 129. The second flow meter 128 detects the supply amount from the second supply valve 127 to the furnace introduction gas introduction piping 129. The supply amount detected by the second flow meter 128 can be used for visual confirmation by an operator.

[0098] However, in the present invention, since the amount of ammonia decomposition gas introduced is not fluctuated in small increments, the second supply amount control device 126 may be omitted, and the flow rate (opening) of the second flow meter 128 may be manually adjusted to correspond to the control signal output from the gas introduction amount control device 114.

[0099] The third furnace introduction gas supply unit 171 is formed of, for example, a tank filled with a third furnace introduction gas (nitrogen gas in this example).

[0100] The third supply amount control device 172 is formed by a mass flow controller (capable of changing the flow rate in small increments within a short period of time) and is interposed between the third furnace introduction gas supply unit 171 and the third supply valve 173. The opening degree of the third supply amount control device 172 changes according to a control signal output from the gas introduction amount control device 114. The third supply amount control device 172 detects the supply amount from the third furnace introduction gas supply unit 171 to the third supply valve 173 and outputs an information signal including the detected supply amount to the gas introduction amount control device 114 and the recorder 106. The control signal can be used for correcting the control by the gas introduction amount control device 114, etc.

[0101] The third supply valve 173 is formed of an electromagnetic valve that switches between an open and closed state in response to a control signal output from the gas introduction amount control device 114, and is interposed between the third supply amount control device 172 and the third flow meter 174.

[0102] The third flow meter 174 is formed of, for example, a mechanical flow meter such as a flow type flow meter, and is interposed between the third supply valve 173 and the furnace introduction gas introduction piping 129. The third flow meter 174 detects the supply amount from the third supply valve 173 to the furnace introduction gas introduction piping 129. The supply amount detected by the third flow meter 174 can be used for visual confirmation by an operator.

[0103] (Effects: Examples 1-1 to 1-4 (first embodiment at 850°C) and Comparative Example 1-2) Next, the operation of the first aspect of the surface hardening treatment device 1 of this embodiment will be described with reference to Figs.

[0104] During the nitriding process in the processing furnace (heating chamber) 11, ammonia gas, ammonia decomposition gas, and nitrogen gas are introduced into the processing furnace (heating chamber) 11 from the furnace introduction gas supply unit 120 at set initial flow rates.

[0105] (Example 1-1) In Example 1-1, as shown in Table 1 of FIG. 4, the nitriding temperature was 850° C., the nitriding time was 90 minutes, and the target nitriding potential was 0.02. As shown in FIG. 2, the set initial flow rate of ammonia gas was 0.5 [m 3 / h], and the set flow rate (constant) of ammonia decomposition gas is 2.0 [m 3 / h], and the initial flow rate of nitrogen gas was set at 7.5 [m 3 / h]. These set flow rates can be set and input in the parameter setting device 15. Furthermore, the stirring fan drive motor 109 is driven to rotate the stirring fan 26, and the atmosphere in the processing furnace (heating chamber) 11 is stirred.

[0106] In an initial state, the on-off valve control device 116 keeps the on-off valve 117 in a closed state. Generally, as a pre-treatment for the gas nitriding treatment, a treatment is performed to activate the steel surface to make it easier for nitrogen to enter. In this case, hydrogen chloride gas, hydrogen cyanide gas, etc. are generated in the furnace. Since these gases can deteriorate the furnace atmosphere gas concentration detection device (sensor) 103, it is effective to keep the on-off valve 117 in a closed state.

[0107] Moreover, an in-furnace temperature measuring device 110 measures the temperature of the gas in the furnace, and outputs an information signal including this measured temperature to the nitriding potential controller 104 and the recorder 106. The nitriding potential controller 104 judges the state inside the treatment furnace (heating chamber) 11 as to whether the temperature is being increased or whether the temperature increase has been completed (stable state).

[0108] Moreover, the furnace nitriding potential calculator 113 of the nitriding potential regulator 104 calculates the nitriding potential in the furnace (the atmosphere is below the set nitriding potential due to the influence of the atmosphere formed by the gas species introduced in advance) and judges whether or not it has fallen below the sum of the target nitriding potential (0.02 in this example) and a reference deviation value. This reference deviation value can also be set and input in the parameter setting device 115, and is, for example, 0.01.

[0109] When it is determined that the temperature rise is completed and that the calculated value of the nitriding potential in the furnace falls below the sum of the target nitriding potential and the reference deviation value (0.03 in this example), the nitriding potential regulator 104 starts controlling the amount of gas introduced into the furnace via the gas introduction amount controller 114. In response to this, the on-off valve controller 116 switches the on-off valve 117 to an open state.

[0110] When the on-off valve 117 is switched to an open state, the treatment furnace (heating chamber) 111 communicates with the atmospheric gas concentration detector 103, and the in-furnace atmospheric gas concentration detector 103 detects the in-furnace hydrogen concentration or the in-furnace ammonia concentration. The detected hydrogen concentration signal or ammonia concentration signal is output to the nitriding potential regulator 104 and the recorder 106.

[0111] The in-furnace nitriding potential calculation device 113 of the nitriding potential regulator 104 calculates the in-furnace nitriding potential based on the input hydrogen concentration signal or ammonia concentration signal. The gas flow rate output adjustment device 130 performs PID control with the nitriding potential calculated by the in-furnace nitriding potential calculation device 113 as an output value, the target nitriding potential (set nitriding potential) as a target value, and the respective amounts of ammonia gas and nitrogen gas introduced among the three types of gases introduced into the furnace as input values. Specifically, in the PID control, the amount of ammonia gas and nitrogen gas introduced are alternately increased and decreased while the amount of ammonia decomposition gas introduced is kept constant, thereby performing control such that the nitriding potential in the treatment furnace (heating chamber) 11 approaches the target nitriding potential while keeping the total amount of the three types of gases introduced into the furnace constant (within a range of about ±5%). In the PID control, each set parameter value set and input by the parameter setting device 115 is used. The set parameter value may be different depending on the value of the target nitriding potential.

[0112] Then, the gas introduction amount controller 114 controls the introduction amount of ammonia gas and the introduction amount of nitrogen gas as a result of PID control. The gas introduction amount controller 114 sends control signals to the first supply amount controller 122 for ammonia gas, the second supply amount controller 126 for ammonia decomposition gas (constant supply amount), and the third supply amount controller 172 for nitrogen gas, in order to realize the determined introduction amount of each gas.

[0113] By the above-mentioned control, the nitriding potential in the furnace can be stably controlled to be close to the target nitriding potential, and the surface hardening treatment of the steel member can be performed with extremely high quality. More specifically, by feedback control with a sampling time of about several hundred milliseconds, the amount of ammonia gas introduced was controlled to 0.47 to 0.53 [m 3 / h], and the amount of nitrogen gas introduced alternately (inversely) with this (increasing and decreasing) is 7.47 to 7.53 [m 3 / h], and it was confirmed that the nitriding potential could be controlled to be close to the target nitriding potential (0.02) with extremely high precision within a few seconds after the start of treatment.

[0114] (Example 1-2) In Example 1-2, as shown in Table 1 of FIG. 4, the nitriding temperature was 850° C., the nitriding time was 90 minutes, the target nitriding potential was 0.06, and the set initial flow rate of ammonia gas was 1.2 [m 3 / h], and the set flow rate (constant) of ammonia decomposition gas is 2.0 [m 3 / h], and the initial flow rate of nitrogen gas was set to 6.8 [m 3 / h].

[0115] Then, when it is determined that the temperature rise is completed and the calculated value of the nitriding potential in the furnace is determined to be lower than the sum (0.08 in this example) of the target nitriding potential (0.06 in this example) and the standard deviation value (0.02 in this example), the nitriding potential regulator 104 begins to control the amount of gas introduced into the furnace via the gas introduction amount control device 114.

[0116] With the rest of the control being substantially the same as in Example 1-1, the nitriding potential in the furnace could be stably controlled to be close to the target nitriding potential, and the surface hardening treatment of the steel member could be performed with extremely high quality. More specifically, the amount of ammonia gas introduced was controlled to 1.15 to 1.25 [m 3 / h], and the nitrogen gas introduction rate alternates between 6.75 and 6.85 [m 3 / h], and it was confirmed that the nitriding potential could be controlled to be close to the target nitriding potential (0.06) with extremely high precision from approximately 15 minutes after the start of treatment.

[0117] (Examples 1-3) In Example 1-3, as shown in Table 1 of FIG. 4, the nitriding temperature was 850° C., the nitriding time was 90 minutes, the target nitriding potential was 0.1, and the set initial flow rate of ammonia gas was 1.9 [m 3 / h], and the set flow rate (constant) of ammonia decomposition gas is 2.0 [m 3 / h], and the initial flow rate of nitrogen gas was set to 6.1 [m 3 / h].

[0118] Then, when it is determined that the temperature rise is completed and the calculated value of the nitriding potential in the furnace is determined to be below the sum (0.12 in this example) of the target nitriding potential (0.1 in this example) and the standard deviation value (0.02 in this example), the nitriding potential regulator 104 begins to control the amount of gas introduced into the furnace via the gas introduction amount control device 114.

[0119] With the rest of the control being substantially the same as in Example 1-1 and Example 1-2, the nitriding potential in the furnace could be stably controlled to be close to the target nitriding potential, and the surface hardening treatment of the steel member could be performed with extremely high quality. More specifically, the amount of ammonia gas introduced was controlled to 1.8 to 1.20 [m 3 / h], and the nitrogen gas introduction rate alternates between 6.0 and 6.2 [m 3 / h], and it was confirmed that the nitriding potential could be controlled to be close to the target nitriding potential (0.1) with extremely high precision from approximately 15 minutes after the start of treatment.

[0120] (Examples 1 to 4) In Example 1-4, as shown in Table 1 of FIG. 4, the nitriding temperature was 850° C., the nitriding time was 90 minutes, the target nitriding potential was 0.18, and the set initial flow rate of ammonia gas was 3.2 [m 3 / h], and the set flow rate (constant) of ammonia decomposition gas is 2.0 [m 3 / h], and the initial flow rate of nitrogen gas was set to 4.8 [m 3 / h].

[0121] Then, when it is determined that the temperature rise is completed and the calculated value of the nitriding potential in the furnace is determined to be lower than the sum (0.20 in this example) of the target nitriding potential (0.18 in this example) and the standard deviation value (0.02 in this example), the nitriding potential regulator 104 begins to control the amount of gas introduced into the furnace via the gas introduction amount control device 114.

[0122] With the rest of the control being substantially the same as in Examples 1-1 to 1-3, the nitriding potential in the furnace could be stably controlled to be close to the target nitriding potential, and the surface hardening treatment of the steel member could be performed with extremely high quality. More specifically, the amount of ammonia gas introduced was controlled to 3.1 to 3.3 [m 3 / h], and the nitrogen gas introduction rate alternates between 4.7 and 4.9 [m 3 / h], and it was confirmed that the nitriding potential could be controlled to be close to the target nitriding potential (0.18) with extremely high precision from approximately 15 minutes after the start of treatment.

[0123] (Comparative Example 1-2) In Comparative Example 1-2, as shown in Table 2 of FIG. 5, the nitriding temperature was 850° C., the nitriding time was 90 minutes, the target nitriding potential was 0.06 (similar to Example 1-2), and the set initial flow rate of ammonia gas was 1.2 [m 3 / h], no ammonia decomposition gas was introduced, and the initial flow rate of nitrogen gas was set to 6.8 [m 3 / h] (this is a control mode conforming to JP 2007-46088 A (Patent Document 5)).

[0124] Then, when it is determined that the temperature rise is completed and the calculated value of the nitriding potential in the furnace is determined to be lower than the sum (0.08 in this example) of the target nitriding potential (0.06 in this example) and the standard deviation value (0.02 in this example), the nitriding potential regulator 104 begins to control the amount of gas introduced into the furnace via the gas introduction amount control device 114.

[0125] Although the other aspects were controlled in a manner similar to that of Examples 1-1 to 1-4, it was not possible to stably control the nitriding potential in the furnace to the vicinity of the target nitriding potential. As a result, it was not possible to perform high-quality surface hardening of the steel members. More specifically, the amount of ammonia gas introduced was controlled to 0 to 1.5 [m 3 / h], and the nitrogen gas introduction rate alternates between 6.8 and 8.0 [m 3 / h], but the nitriding potential could not be converged to the vicinity of the target nitriding potential (0.06).

[0126] Example 2 In Example 2, as shown in Table 3 of FIG. 6, the nitriding temperature was 800° C., the nitriding time was 90 minutes, the target nitriding potential was 0.08, and the set initial flow rate of ammonia gas was 1.3 [m 3 / h], and the set flow rate (constant) of ammonia decomposition gas is 2.0 [m 3 / h], and the initial flow rate of nitrogen gas was set to 6.7 [m 3 / h].

[0127] Then, when it is determined that the temperature rise is completed and the calculated value of the nitriding potential in the furnace is determined to be lower than the sum (0.10 in this example) of the target nitriding potential (0.08 in this example) and the standard deviation value (0.02 in this example), the nitriding potential regulator 104 begins to control the amount of gas introduced into the furnace via the gas introduction amount control device 114.

[0128] With regard to other points, the nitriding potential in the furnace could be stably controlled to the vicinity of the target nitriding potential by controlling the furnace in a manner similar to that of Examples 1-1 to 1-4, and the surface hardening treatment of the steel member could be performed with extremely high quality. More specifically, the amount of ammonia gas introduced was controlled to 1.25 to 1.35 [m 3 / h], and the nitrogen gas introduction rate alternates between 6.65 and 6.75 [m 3 / h], and it was confirmed that the nitriding potential could be controlled to be close to the target nitriding potential (0.08) with extremely high precision from approximately 15 minutes after the start of treatment.

[0129] Example 3 In Example 3, as shown in Table 3 of FIG. 6, the nitriding temperature was 900° C., the nitriding time was 90 minutes, the target nitriding potential was 0.05, and the set initial flow rate of ammonia gas was 2.4 [m 3 / h], and the set flow rate (constant) of ammonia decomposition gas is 2.0 [m 3 / h], and the initial flow rate of nitrogen gas was set to 5.6 [m 3 / h].

[0130] Then, when it is determined that the temperature rise is completed and the calculated value of the nitriding potential in the furnace is determined to be lower than the sum (0.06 in this example) of the target nitriding potential (0.05 in this example) and the standard deviation value (0.01 in this example), the nitriding potential regulator 104 begins to control the amount of gas introduced into the furnace via the gas introduction amount control device 114.

[0131] With regard to other points, the nitriding potential in the furnace could be stably controlled to the vicinity of the target nitriding potential by substantially the same control as in Examples 1-1 to 1-4, and the surface hardening treatment of the steel member could be performed with extremely high quality. More specifically, the amount of ammonia gas introduced was controlled to 2.3 to 2.5 [m 3 / h], and the nitrogen gas introduction rate alternates between 5.5 and 5.7 [m 3 / h], and it was confirmed that the nitriding potential could be controlled to be close to the target nitriding potential (0.05) with extremely high precision from approximately 15 minutes after the start of treatment.

[0132] (Modifications of Examples 1-1 to 1-4, 2, and 3: Regarding the introduction ratio of ammonia decomposition gas) In all of Examples 1-1 to 1-4, 2, and 3, the total amount of the three types of gases introduced into the furnace (10 [m 3 / h]), the amount of ammonia decomposition gas introduced (2.0 [m 3 / h] was 20%.

[0133] The present inventors have confirmed that if this ratio is 12% or more, the nitriding potential capable of maintaining a practical surface nitrogen concentration can be controlled with extremely high precision to be close to the target nitriding potential in the high temperature range of 680°C or more, particularly in the high temperature range of 800°C or more.

[0134] On the other hand, the upper limit of this ratio depends on the target nitriding potential. Since most of the ammonia gas introduced into the furnace is decomposed into hydrogen gas and nitrogen gas, the introduction of ammonia decomposition gas is essential in terms of controlling the nitriding potential as described above, but if the amount introduced is too large, it may be an obstacle to achieving the target nitriding potential, and the introduction of unnecessary ammonia decomposition gas is uneconomical. It has been confirmed that when the target nitriding potential is at most about 0.2, the upper limit at which this can be achieved is 24% or less.

[0135] Specifically, the amount of ammonia decomposition gas introduced was 0.8 [m 3 / h], 1.2[m 3 / h], 1.6[m 3 / h], 2.4[m 3 / h], 2.8[m 3 / h], 3.2[m 3 / h], control was performed in substantially the same manner as in Examples 1-1 to 1-4, 2, and 3.

[0136] The amount of ammonia decomposition gas introduced is 1.2 [m 3 / h] (introduction ratio is 1.2 / 9.2 = 13%), 1.6 [m 3 / h] (introduction ratio is 1.6 / 9.6 = 17%), 2.4 [m 3 / h] (feedback ratio was 2.4 / 10.4=23%), as in Examples 1-1 to 1-4, 2, and 3, very highly accurate control of the nitriding potential was possible.

[0137] In addition, the amount of ammonia decomposition gas introduced was 2.8 [m 3 / h] (introduction ratio is 2.8 / 10.8 = 26%), 3.2 [m 3 / h] (introduction ratio was 3.2 / 11.2 = 28%), nitriding potential control with an acceptable degree of precision was possible.

[0138] On the other hand, the amount of ammonia decomposition gas introduced is 0.8 [m 3 / h] (introduction ratio was 0.8 / 8.8 = 9%), high-precision control of the nitriding potential was not possible.

[0139] (Replacement of ammonia decomposition gas with hydrogen gas and nitrogen gas) As will be apparent to those skilled in the art, 1.0 [m 3 / h] of ammonia decomposition gas is 0.75 [m 3 / h] hydrogen gas and 0.25 [m 3 / h] of nitrogen gas.

[0140] Therefore, as shown in FIG. 7, it is also possible to use an apparatus in which a tank 225 filled with hydrogen gas, instead of the tank 125 filled with ammonia decomposition gas, serves as the second furnace introduction gas supply unit.

[0141] In this case, in all of Examples 1-1 to 1-4, 2, and 3, the amount of hydrogen gas introduced was 1.5 [m 3 / h]), and the total amount of the three types of gas introduced into the furnace (10 [m 3 / h]) is 15%. On the other hand, the amount of nitrogen gas introduced is +0.5[m 3 / h] is added to the value.

[0142] Moreover, the above-mentioned suitable ammonia decomposition gas introduction ratio of 12% to 24% is equivalent to 9% to 18% when converted into a hydrogen gas introduction ratio.

[0143] (Effects: Examples 4-1 to 4-4 (Second embodiment at 850°C) and Comparative Example 4) Next, the operation of the second aspect of the surface hardening treatment device 1 of this embodiment will be described with reference to Figs.

[0144] During the nitriding process in the processing furnace (heating chamber) 11, ammonia gas, ammonia decomposition gas, and nitrogen gas are introduced into the processing furnace (heating chamber) 11 from the furnace introduction gas supply unit 120 at set initial flow rates.

[0145] (Example 4-1) In Example 4-1, as shown in Table 4 of FIG. 8, the nitriding temperature was 850° C., the nitriding time was 90 minutes, and the target nitriding potential was 0.02. As shown in FIG. 2, the set initial flow rate of ammonia gas was 0.8 [m 3 / h], and the set flow rate (constant) of ammonia decomposition gas is 2.0 [m 3 / h], and the set flow rate (constant) of nitrogen gas is 6 [m 3 / h]. These set flow rates can be set and input in the parameter setting device 15. Furthermore, the stirring fan drive motor 109 is driven to rotate the stirring fan 26, and the atmosphere in the processing furnace (heating chamber) 11 is stirred.

[0146] In an initial state, the on-off valve control device 116 keeps the on-off valve 117 in a closed state. Generally, as a pre-treatment for the gas nitriding treatment, a treatment is performed to activate the steel surface to make it easier for nitrogen to enter. In this case, hydrogen chloride gas, hydrogen cyanide gas, etc. are generated in the furnace. Since these gases can deteriorate the furnace atmosphere gas concentration detection device (sensor) 103, it is effective to keep the on-off valve 117 in a closed state.

[0147] Moreover, an in-furnace temperature measuring device 110 measures the temperature of the gas in the furnace, and outputs an information signal including this measured temperature to the nitriding potential controller 104 and the recorder 106. The nitriding potential controller 104 judges the state inside the treatment furnace (heating chamber) 11 as to whether the temperature is being increased or whether the temperature increase has been completed (stable state).

[0148] The nitriding potential calculator 113 in the nitriding potential regulator 104 calculates the nitriding potential in the furnace and, if the atmosphere is below the set nitriding potential due to the influence of the atmosphere formed by the gas species introduced in advance, judges whether the potential has fallen below the sum of the target nitriding potential (0.02 in this example) and a reference deviation value. This reference deviation value can also be set and input in the parameter setting device 115, and is, for example, 0.01.

[0149] When it is determined that the temperature rise is completed and that the calculated value of the nitriding potential in the furnace falls below the sum of the target nitriding potential and the reference deviation value (0.03 in this example), the nitriding potential regulator 104 starts controlling the amount of gas introduced into the furnace via the gas introduction amount controller 114. In response to this, the on-off valve controller 116 switches the on-off valve 117 to an open state.

[0150] When the on-off valve 117 is switched to an open state, the treatment furnace (heating chamber) 111 communicates with the atmospheric gas concentration detector 103, and the in-furnace atmospheric gas concentration detector 103 detects the in-furnace hydrogen concentration or the in-furnace ammonia concentration. The detected hydrogen concentration signal or ammonia concentration signal is output to the nitriding potential regulator 104 and the recorder 106.

[0151] The in-furnace nitriding potential calculation device 113 of the nitriding potential regulator 104 calculates the in-furnace nitriding potential based on the input hydrogen concentration signal or ammonia concentration signal. The gas flow rate output adjustment device 130 performs PID control with the nitriding potential calculated by the in-furnace nitriding potential calculation device 113 as an output value, the target nitriding potential (set nitriding potential) as a target value, and the amount of ammonia gas introduced among the three types of gases introduced into the furnace as an input value. Specifically, in the PID control, the amount of ammonia gas introduced and the total amount of gases introduced into the furnace are both increased and decreased while the amount of nitrogen gas introduced and the amount of ammonia decomposition gas introduced are kept constant, thereby performing control so that the nitriding potential in the treatment furnace (heating chamber) 11 approaches the target nitriding potential. In the PID control, each set parameter value set and input by the parameter setting device 115 is used. The set parameter value may be different depending on the value of the target nitriding potential.

[0152] Then, the gas introduction amount controller 114 controls the introduction amount of ammonia gas as a result of PID control. The gas introduction amount controller 114 sends control signals to the first supply amount controller 122 for ammonia gas, the second supply amount controller 126 for ammonia decomposition gas (constant supply amount), and the third supply amount controller 172 for nitrogen gas, in order to realize the determined introduction amount of each gas.

[0153] By the above-mentioned control, the nitriding potential in the furnace can be stably controlled to be close to the target nitriding potential, and the surface hardening treatment of the steel member can be performed with extremely high quality. More specifically, by feedback control with a sampling time of about several hundred milliseconds, the amount of ammonia gas introduced was controlled to 0.77 to 0.83 [m 3 / h], and it was confirmed that the nitriding potential could be controlled to be close to the target nitriding potential (0.02) with extremely high precision from approximately 15 minutes after the start of treatment.

[0154] (Example 4-2) In Example 4-2, as shown in the table of FIG. 8, the nitriding temperature was 850° C., the nitriding time was 90 minutes, the target nitriding potential was 0.06, and the set initial flow rate of ammonia gas was 1.8 [m 3 / h], and the set flow rate (constant) of ammonia decomposition gas is 2.0 [m 3 / h], and the set flow rate (constant) of nitrogen gas is 6 [m 3 / h].

[0155] Then, when it is determined that the temperature rise is completed and the calculated value of the nitriding potential in the furnace is determined to be lower than the sum (0.08 in this example) of the target nitriding potential (0.06 in this example) and the standard deviation value (0.02 in this example), the nitriding potential regulator 104 begins to control the amount of gas introduced into the furnace via the gas introduction amount control device 114.

[0156] With the rest of the control being substantially the same as in Example 4-1, the nitriding potential in the furnace could be stably controlled to be close to the target nitriding potential, and the surface hardening treatment of the steel member could be performed with extremely high quality. More specifically, the amount of ammonia gas introduced was controlled to 1.72 to 1.88 [m 3 / h], and it was confirmed that the nitriding potential could be controlled to be close to the target nitriding potential (0.06) with extremely high precision from approximately 15 minutes after the start of treatment.

[0157] (Example 4-3) In Example 4-3, as shown in Table 4 of FIG. 8, the nitriding temperature was 850° C., the nitriding time was 90 minutes, the target nitriding potential was 0.1, and the set initial flow rate of ammonia gas was 2.1 [m 3 / h], and the set flow rate (constant) of ammonia decomposition gas is 2.0 [m 3 / h], and the set flow rate (constant) of nitrogen gas is 6 [m 3 / h].

[0158] Then, when it is determined that the temperature rise is completed and the calculated value of the nitriding potential in the furnace is determined to be below the sum (0.12 in this example) of the target nitriding potential (0.1 in this example) and the standard deviation value (0.02 in this example), the nitriding potential regulator 104 begins to control the amount of gas introduced into the furnace via the gas introduction amount control device 114.

[0159] With the rest of the control being substantially the same as in Example 4-1 and Example 4-2, the nitriding potential in the furnace could be stably controlled to be close to the target nitriding potential, and the surface hardening treatment of the steel member could be performed with extremely high quality. More specifically, the amount of ammonia gas introduced was controlled to 2.0 to 2.2 [m 3 / h], and it was confirmed that the nitriding potential could be controlled to be close to the target nitriding potential (0.1) with extremely high precision from approximately 15 minutes after the start of treatment.

[0160] (Example 4-4) In Example 4-4, as shown in Table 4 of FIG. 8, the nitriding temperature was 850° C., the nitriding time was 90 minutes, the target nitriding potential was 0.18, and the set initial flow rate of ammonia gas was 2.1 [m 3 / h], and the set flow rate (constant) of ammonia decomposition gas is 2.0 [m 3 / h], and the set flow rate (constant) of nitrogen gas is 6 [m 3 / h].

[0161] Then, when it is determined that the temperature rise is completed and the calculated value of the nitriding potential in the furnace is determined to be lower than the sum (0.20 in this example) of the target nitriding potential (0.18 in this example) and the standard deviation value (0.02 in this example), the nitriding potential regulator 104 begins to control the amount of gas introduced into the furnace via the gas introduction amount control device 114.

[0162] With the rest of the control being substantially the same as in Examples 4-1 to 4-3, the nitriding potential in the furnace could be stably controlled to be close to the target nitriding potential, and the surface hardening treatment of the steel member could be performed with extremely high quality. More specifically, the amount of ammonia gas introduced was controlled to 2.5 to 2.65 [m 3 / h], and it was confirmed that the nitriding potential could be controlled to be close to the target nitriding potential (0.18) with extremely high precision from approximately 15 minutes after the start of treatment.

[0163] (Comparative Example 4-2-1) In Comparative Example 4-2-1, as shown in Table 5 of FIG. 9, the nitriding temperature was 850° C., the nitriding time was 90 minutes, the target nitriding potential was 0.06 (similar to Example 4-2), and the set initial flow rate of ammonia gas was 1.0 [m 3 / h], no ammonia decomposition gas was introduced, and the set flow rate of nitrogen gas was 10 [m 3 / h] (this is a control mode conforming to JP 2007-46088 A (Patent Document 5)).

[0164] Then, when it is determined that the temperature rise is completed and the calculated value of the nitriding potential in the furnace is determined to be lower than the sum (0.08 in this example) of the target nitriding potential (0.06 in this example) and the standard deviation value (0.02 in this example), the nitriding potential regulator 104 begins to control the amount of gas introduced into the furnace via the gas introduction amount control device 114.

[0165] Although the other controls were attempted in the same manner as in Examples 4-1 to 4-4, it was not possible to stably control the in-furnace nitriding potential to the vicinity of the target nitriding potential. As a result, it was not possible to perform high-quality surface hardening of the steel members. More specifically, the amount of ammonia gas introduced was controlled to 0 to 1.0 [m 3 / h], but the nitriding potential could not be converged to near the target nitriding potential (0.06).

[0166] (Comparative Example 4-2-2) In Comparative Example 4-2-2, as shown in Table 6 of FIG. 10, the nitriding temperature was 850° C., the nitriding time was 90 minutes, the target nitriding potential was 0.06 (similar to Example 4-2), and the set initial flow rate of ammonia gas was 6.0 [m 3 / h], no nitrogen gas was introduced, and the set flow rate of the ammonia decomposition gas was 2.0 [m 3 / h] (this is a control mode conforming to Japanese Patent No. 6503122 (Patent Document 4)).

[0167] Then, when it is determined that the temperature rise is completed and the calculated value of the nitriding potential in the furnace is determined to be lower than the sum (0.08 in this example) of the target nitriding potential (0.06 in this example) and the standard deviation value (0.02 in this example), the nitriding potential regulator 104 begins to control the amount of gas introduced into the furnace via the gas introduction amount control device 114.

[0168] Although the other controls were attempted in the same manner as in Examples 4-1 to 4-4, it was not possible to stably control the nitriding potential in the furnace to the vicinity of the target nitriding potential. As a result, it was not possible to perform high-quality surface hardening of the steel members. More specifically, the amount of ammonia gas introduced was controlled to 0 to 6.0 [m 3 / h], but the nitriding potential could not be converged to near the target nitriding potential (0.06).

[0169] (Comparative Example 4-2-3) In Comparative Example 4-2-3, as shown in Table 6 of FIG. 10, the nitriding temperature was 850° C., the nitriding time was 90 minutes, the target nitriding potential was 0.06 (similar to Example 4-2), and the set initial flow rate of ammonia gas was 6.0 [m 3 / h], no nitrogen gas was introduced, and the set flow rate of the ammonia decomposition gas was 4.0 [m 3 / h] (this is a control mode conforming to Japanese Patent No. 6503122 (Patent Document 4)).

[0170] Then, when it is determined that the temperature rise is completed and the calculated value of the nitriding potential in the furnace is determined to be lower than the sum (0.08 in this example) of the target nitriding potential (0.06 in this example) and the standard deviation value (0.02 in this example), the nitriding potential regulator 104 begins to control the amount of gas introduced into the furnace via the gas introduction amount control device 114.

[0171] Although the other controls were attempted in the same manner as in Examples 4-1 to 4-4, it was not possible to stably control the nitriding potential in the furnace to the vicinity of the target nitriding potential. As a result, it was not possible to perform high-quality surface hardening of the steel members. More specifically, the amount of ammonia gas introduced was controlled to 6.0 to 8.0 [m 3 / h], but the nitriding potential could not be converged to near the target nitriding potential (0.06).

[0172] Example 5 In Example 5, as shown in Table 7 of FIG. 11, the nitriding temperature was 800° C., the nitriding time was 90 minutes, the target nitriding potential was 0.08, and the set initial flow rate of ammonia gas was 2.1 [m 3 / h], and the set flow rate (constant) of ammonia decomposition gas is 2.0 [m 3 / h], and the set flow rate (constant) of nitrogen gas is 6 [m 3 / h].

[0173] Then, when it is determined that the temperature rise is completed and the calculated value of the nitriding potential in the furnace is determined to be lower than the sum (0.10 in this example) of the target nitriding potential (0.08 in this example) and the standard deviation value (0.02 in this example), the nitriding potential regulator 104 begins to control the amount of gas introduced into the furnace via the gas introduction amount control device 114.

[0174] With regard to other points, the nitriding potential in the furnace could be stably controlled to the vicinity of the target nitriding potential by controlling the nitriding potential in substantially the same manner as in Examples 4-1 to 4-4, and the surface hardening treatment of the steel member could be performed with extremely high quality. More specifically, the amount of ammonia gas introduced was controlled to 1.9 to 2.05 [m 3 / h], and it was confirmed that the nitriding potential could be controlled to be close to the target nitriding potential (0.08) with extremely high precision from approximately 15 minutes after the start of treatment.

[0175] Example 6 In Example 6, as shown in Table 7 of FIG. 11, the nitriding temperature was 900° C., the nitriding time was 90 minutes, the target nitriding potential was 0.05, and the set initial flow rate of ammonia gas was 1.9 [m 3 / h], and the set flow rate (constant) of ammonia decomposition gas is 2.0 [m 3 / h], and the set flow rate (constant) of nitrogen gas is 6 [m 3 / h].

[0176] Then, when it is determined that the temperature rise is completed and the calculated value of the nitriding potential in the furnace is determined to be lower than the sum (0.08 in this example) of the target nitriding potential (0.05 in this example) and the standard deviation value (0.02 in this example), the nitriding potential regulator 104 begins to control the amount of gas introduced into the furnace via the gas introduction amount control device 114.

[0177] With the rest of the control being substantially the same as in Examples 4-1 to 4-4, the nitriding potential in the furnace could be stably controlled to be close to the target nitriding potential, and the surface hardening treatment of the steel member could be performed with extremely high quality. More specifically, the amount of ammonia gas introduced was controlled to 1.85 to 1.98 [m 3 / h], and it was confirmed that the nitriding potential could be controlled to be close to the target nitriding potential (0.05) with extremely high precision from approximately 15 minutes after the start of treatment.

[0178] (Modifications of Examples 4-1 to 4-4, 5, and 6: Regarding the introduction ratio of ammonia decomposition gas) In Examples 4-1 to 4-4, 5, and 6, the total amount of the three types of gases introduced into the furnace (10 [m 3 / h]), the amount of ammonia decomposition gas introduced (2.0 [m 3 / h) was 19% to 23%.

[0179] The present inventors have confirmed that if this ratio is 12% or more, the nitriding potential can be controlled to be close to the target nitriding potential with extremely high precision in the high temperature range of 680°C or more, particularly in the high temperature range of 800°C or more.

[0180] On the other hand, regarding the upper limit of this ratio, although it is not as critical as the lower limit, it has been confirmed that if it is 24% or less, the nitriding potential can be controlled to be close to the target nitriding potential with extremely high precision in the high temperature range of 680°C or higher, particularly in the high temperature range of 800°C or higher.

[0181] Specifically, the amount of ammonia decomposition gas introduced was 0.8 [m 3 / h], 1.2[m 3 / h], 1.6[m 3 / h], 2.4[m 3 / h], 2.8[m 3 / h], 3.2[m 3 / h], control was performed in substantially the same manner as in Examples 4-1 to 4-4, 5, and 6.

[0182] The amount of ammonia decomposition gas introduced is 1.2 [m 3 / h] (introduction ratio is 1.2 / 8~1.2 / 9.8=12%~15%), 1.6[m 3 / h] (introduction ratio is 1.6 / 8.4~1.6 / 10.2=16%~19%), 2.4[m 3 / h] (feedback ratio was 2.4 / 9.2 to 2.4 / 11.0=22% to 26%), similar to Examples 1-1 to 1-4, 2 and 3, highly accurate nitriding potential control was possible.

[0183] In addition, the amount of ammonia decomposition gas introduced was 2.8 [m 3 / h] (introduction ratio is 2.8 / 9.6~2.8 / 11.4=25%~29%), 3.2 [m 3 / h] (introduction ratio was 3.2 / 10.0~3.2 / 11.8=27%~32%), nitriding potential control with an acceptable degree of precision was possible.

[0184] On the other hand, the amount of ammonia decomposition gas introduced is 0.8 [m 3 / h] (the introduction ratio was 0.8 / 7.6 to 0.8 / 9.4 = 9% to 11%), high-precision control of the nitriding potential was not possible.

[0185] (Replacement of ammonia decomposition gas with hydrogen gas and nitrogen gas) As will be apparent to those skilled in the art, 1.0 [m 3 / h] of ammonia decomposition gas is 0.75 [m 3 / h] hydrogen gas and 0.25 [m 3 / h] of nitrogen gas.

[0186] Therefore, as shown in FIG. 7, it is also possible to use an apparatus in which a tank 225 filled with hydrogen gas, instead of the tank 125 filled with ammonia decomposition gas, serves as the second furnace introduction gas supply unit.

[0187] In this case, in all of Examples 4-1 to 4-4, 5, and 6, the amount of hydrogen gas introduced was 1.5 [m 3 / h]), and the introduction ratio of the three types of gases to the total introduction amount of the furnace is 14% to 17%. On the other hand, the introduction amount of nitrogen gas is +0.5 [m 3 / h] is added to the value.

[0188] Moreover, the above-mentioned suitable introduction ratio of ammonia decomposition gas of 12% to 24% is equivalent to an introduction ratio of hydrogen gas of 9% to 18%. [Explanation of symbols]

[0189] 1 Surface hardening treatment equipment 10 Loading section 11 Heating chamber 12 Transport Room 13. Discharge conveyor 15 Parameter setting device 20 cases 21 Door 22 Entrance hood 26 Stirring fan 27 Middle Door 30 Elevator 31 Cooling Oil 21 Cooling room (oil tank) 35 Door 36 Exit Hood 103 Furnace atmosphere gas concentration detection device 104 Nitriding potential controller 105 Temperature controller 106 Recorder 109 Stirring fan drive motor 110 Furnace temperature measuring device 111 Furnace heating device 112 Atmospheric gas piping 113 Nitriding potential calculation device in furnace 114 Gas introduction amount control device 115 Parameter setting device 116 Opening and closing valve control device 117 On-off valve 120 Furnace gas supply section 121 First furnace gas supply section 122 First supply amount control device 123 First supply valve 124 1st flow meter 125 Second furnace gas supply section (ammonia decomposition gas tank) 126 Second supply amount control device 127 Second supply valve 128 2nd flow meter 129 Furnace gas introduction pipe 130 Gas flow output regulator 131 Programmable Logic Controller 171 Third furnace gas supply section 172 Third supply amount control device 173 Third supply valve 174 3rd flow meter 225 Second furnace gas supply section (hydrogen gas tank)

Claims

1. 1. A surface hardening treatment apparatus for introducing ammonia gas, ammonia decomposition gas, and nitrogen gas into a treatment furnace having a temperature of 680° C. or higher, and performing a gas nitriding treatment as a surface hardening treatment of a workpiece placed in the treatment furnace, a furnace atmosphere gas concentration detection device for detecting a hydrogen concentration or an ammonia concentration in the processing furnace; an in-furnace nitriding potential calculation device that calculates a nitriding potential in the processing furnace based on the hydrogen concentration or the ammonia concentration detected by the in-furnace atmosphere gas concentration detection device; a gas introduction amount control device which changes the amount of the ammonia gas and the amount of the nitrogen gas introduced into the processing furnace while keeping the total amount of the three types of gases introduced into the processing furnace constant and keeping the amount of the ammonia decomposition gas introduced constant in accordance with the nitriding potential in the processing furnace calculated by the nitriding potential calculation device and a target nitriding potential, thereby bringing the nitriding potential in the processing furnace closer to the target nitriding potential; A surface hardening treatment device comprising:

2. The amount of the nitrogen gas introduced is 40% or more, and the amount of the ammonia decomposition gas introduced is 12% to 24% of the total amount of the three types of gases introduced into the furnace. The surface hardening treatment device according to claim 1 .

3. 1. A surface hardening treatment apparatus for introducing ammonia gas, ammonia decomposition gas, and nitrogen gas into a treatment furnace having a temperature of 680° C. or higher, and performing a gas nitriding treatment as a surface hardening treatment of a workpiece placed in the treatment furnace, a furnace atmosphere gas concentration detection device for detecting a hydrogen concentration or an ammonia concentration in the processing furnace; an in-furnace nitriding potential calculation device that calculates a nitriding potential in the processing furnace based on the hydrogen concentration or the ammonia concentration detected by the in-furnace atmosphere gas concentration detection device; a gas introduction amount control device which changes the amount of the ammonia gas introduced and the total amount of the three types of gases introduced into the furnace, while keeping the amount of the nitrogen gas introduced constant and the amount of the ammonia decomposition gas introduced constant, in accordance with the nitriding potential in the processing furnace calculated by the furnace nitriding potential calculation device and a target nitriding potential, thereby bringing the nitriding potential in the processing furnace closer to the target nitriding potential; A surface hardening treatment device comprising:

4. The amount of the nitrogen gas introduced is 40% or more, and the amount of the ammonia decomposition gas introduced is 12% to 24% of the total amount of the three types of gases introduced into the furnace. The surface hardening treatment device according to claim 3 .

5. The target nitriding potential is in the range of 0.01 to 0.

20.

5. The surface hardening treatment device according to claim 1, wherein the surface hardening treatment device is a surface hardening treatment device.

6. The temperature in the processing furnace is 800° C. or higher. The surface hardening treatment device according to claim 5 .

7. A surface hardening method comprising introducing ammonia gas, ammonia decomposition gas, and nitrogen gas into a treatment furnace having a temperature of 680° C. or higher, and carrying out a gas nitriding treatment as a surface hardening treatment of a workpiece placed in the treatment furnace, the method comprising the steps of: a step of detecting an atmospheric gas concentration in the processing furnace, the step of detecting a hydrogen concentration or an ammonia concentration in the processing furnace; an in-furnace nitriding potential calculation step of calculating a nitriding potential in the processing furnace based on the hydrogen concentration or the ammonia concentration detected by the in-furnace atmosphere gas concentration detection step; a gas introduction amount control step of changing the amount of the ammonia gas and the amount of the nitrogen gas introduced while keeping the total amount of the three types of gases introduced into the furnace constant and keeping the amount of the ammonia decomposition gas introduced constant, in accordance with the nitriding potential in the processing furnace calculated by the nitriding potential calculation step and a target nitriding potential, thereby bringing the nitriding potential in the processing furnace closer to the target nitriding potential; A surface hardening treatment method comprising:

8. The amount of the nitrogen gas introduced is 40% or more, and the amount of the ammonia decomposition gas introduced is 12% to 24% of the total amount of the three types of gases introduced into the furnace.

8. The surface hardening method according to claim 7.

9. A surface hardening method comprising introducing ammonia gas, ammonia decomposition gas, and nitrogen gas into a treatment furnace having a temperature of 680° C. or higher, and carrying out a gas nitriding treatment as a surface hardening treatment of a workpiece placed in the treatment furnace, the method comprising the steps of: a step of detecting an atmospheric gas concentration in the processing furnace, the step of detecting a hydrogen concentration or an ammonia concentration in the processing furnace; an in-furnace nitriding potential calculation step of calculating a nitriding potential in the processing furnace based on the hydrogen concentration or the ammonia concentration detected by the in-furnace atmosphere gas concentration detection step; a gas introduction amount control step of changing the amount of the ammonia gas introduced and the total amount of the three types of gases introduced into the furnace, while keeping the amount of the nitrogen gas introduced constant and the amount of the ammonia decomposition gas introduced constant, in accordance with the nitriding potential in the processing furnace calculated by the nitriding potential calculation step and a target nitriding potential, thereby bringing the nitriding potential in the processing furnace closer to the target nitriding potential; A surface hardening treatment method comprising:

10. The amount of the nitrogen gas introduced is 40% or more, and the amount of the ammonia decomposition gas introduced is 12% to 24% of the total amount of the three types of gases introduced into the furnace.

10. The surface hardening method according to claim 9.

11. The target nitriding potential is in the range of 0.01 to 0.

20.

11. The surface hardening method according to claim 7, wherein the surface hardening treatment is carried out in a manner similar to that described above.

12. The temperature in the processing furnace is 800° C. or higher. The surface hardening method according to claim 11.

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