Salt bath heat treatment device

Abstract

A salt bath heat treatment device (10) is provided with: a salt bath tank (1) for accommodating a nitrate bath containing molten nitrate so as to cool a metal member during constant-temperature heat treatment; the water adding device (2) is used for adding water into the nitrate bath in the salt bath tank (1); a humidity detection device (5) that includes a humidity sensor (53a) that outputs a signal corresponding to the humidity caused by water vapor generated from the nitrate bath to which water has been added by the water addition device (2); and a control device (100) configured to control the water addition device (2) so as to adjust the amount of water added to the nitrate bath on the basis of a signal output from the humidity sensor (53a) of the humidity detection device (5).

Description

Technical Field

[0001] The present invention relates to the technical field of salt bath heat treatment of metal components using a salt bath. Background Art

[0002] Heretofore, salt bath heat treatment of metal components using a salt bath (salt bath), particularly isothermal heat treatment, has been carried out. This isothermal heat treatment is different from normal oil quenching or water quenching (i.e., quenching and tempering treatment), and is a heat treatment including the following steps: in order to cool the heated metal component, the metal component is placed in a salt bath at a certain temperature to maintain the temperature of the metal component. Specifically, the isothermal heat treatment includes austempering treatment and martensite quenching treatment. In these heat treatments, a nitrate bath containing molten nitrate is widely used as a cooling medium.

[0003] In the austempering treatment, first, the metal component is heated and held in a non-oxidizing atmosphere at 800°C to 900°C for 15 minutes to 60 minutes to transform into an austenite structure. Then, the metal component is immediately immersed in a nitrate bath maintained at about 300°C to 430°C for rapid cooling, and then held in the nitrate bath for 15 minutes to 60 minutes. Thereby, the austenite structure transforms into a uniform bainite structure.

[0004] A component subjected to such austempering treatment has advantages such as excellent mechanical properties such as toughness and impact resistance and small heat treatment distortion compared to a component of the same hardness subjected to general quenching and tempering treatment. However, in the austempering treatment, mainly carbon steels for mechanical structures and carbon tool steels are used, but since their alloy content is small, they lack hardenability. Therefore, in the austempering treatment, generally, the thickness of the component that can be processed at an industrial site is limited to 2 mm or less.

[0005] On the other hand, in the martensite quenching treatment, first, the metal component is heated and held in a non-oxidizing atmosphere at 800°C to 900°C for 15 minutes to 60 minutes to transform into an austenite structure. Then, the metal component is immediately immersed in a nitrate bath maintained at about 160°C to 250°C for rapid cooling, and then held in the nitrate bath for 5 minutes to 60 minutes. Thereby, the austenite structure transforms into a uniform martensite structure.

[0006] Compared with components of the same hardness that have undergone general quenching and tempering treatment, components that have undergone such martensitic quenching treatment have the advantage of less deformation caused by heat treatment. Both martensitic quenching treatment and quenching and tempering treatment are heat treatment methods for obtaining martensitic structure by cooling a metal component through the Ms point (the temperature at which the phase transformation from austenite structure to martensite structure begins). However, in quenching and tempering treatment, oil, water, water-soluble quenching agents, etc. are used as cooling media, and rapid cooling is performed until a temperature region lower than the Ms point. Therefore, in the cooling stage, a large temperature difference is generated between the surface part and the interior, the thin part and the thick part, etc. of the processed component, resulting in a deviation in the timing of the phase transformation to martensitic structure, and sometimes unexpectedly causing large heat treatment deformation.

[0007] In contrast, the nitrate bath used for martensitic quenching treatment has a higher temperature as a cooling medium compared to oil, water, etc. Therefore, after rapidly cooling the processed component to near the Ms point, it can be maintained in a temperature region close to the Ms point. Therefore, it is possible to cause the phase transformation to martensitic structure while minimizing the temperature difference generated between the surface part and the interior, the thin part and the thick part, etc. of the processed component, and heat treatment deformation can be suppressed. However, in martensitic quenching treatment, since the temperature of the cooling medium is high, the cooling rate during rapid cooling is slower than that of oil and water, and there is a tendency that the components that can be processed are limited to smaller-sized components.

[0008] Here, in order to expand the component size that can be processed by the isothermal heat treatment as described above, it is considered effective to increase the cooling ability of the nitrate bath as a cooling medium. It has been known that in order to increase the cooling ability of the nitrate bath, it is preferable to add water to the nitrate bath (for example, Non-Patent Documents 1 and 2). When water is added to the nitrate bath, intermolecular bonding occurs between the water molecules and the nitrate. Since the energy of this intermolecular bonding is small, it is broken at the moment of contact with the component heated to 800°C to 900°C. As a result, the latent heat of vaporization of the component is taken away, and the water molecules become vapor bubbles and stir the nitrate bath around the component, thereby improving the cooling performance.

[0009] Prior Art Documents

[0010] Non-Patent Documents

[0011] Non-Patent Document 1: "Heat Treatment", Vol. 17, No. 2, pp. 92 - 97 (Toshio Kubota, Kazuo Sato)

[0012] Non-Patent Document 2: "Industrial Heating", Vol. 23, No. 2, pp. 23 - 31 (Tsugio Yonemura) Summary of the Invention

[0013] Problems to be Solved by the Invention

[0014] According to the insights of the inventors of the present application, in order to reliably improve the cooling capacity of the nitrate bath, it is preferable to add an appropriate amount of water to the nitrate bath. Specifically, water can be added in such a way that the water content rate of the nitrate bath (the ratio of the water contained in the nitrate bath) is maintained at a set value.

[0015] The present invention has been completed based on the above insights. The object of the present invention is to improve the cooling capacity of a nitrate bath by controlling the addition of water to the nitrate bath in a salt bath heat treatment apparatus that uses a nitrate bath for heat treatment.

[0016] Means for Solving the Problem

[0017] To achieve the above object, the present invention is a salt bath heat treatment apparatus that heat-treats a metal component using a salt bath, characterized in that the salt bath heat treatment apparatus includes: a salt bath tank that houses a nitrate bath containing molten nitrate to cool the metal component during isothermal heat treatment; a water addition device that adds water to the nitrate bath in the salt bath tank; a humidity detection device that includes a humidity sensor that outputs a signal corresponding to the humidity brought by the water vapor generated from the nitrate bath to which water has been added by the water addition device; and a control device that is configured to control the water addition device based on the signal output from the humidity sensor of the humidity detection device to adjust the amount of water added to the nitrate bath.

[0018] According to the present invention configured as described above, the salt bath heat treatment apparatus controls the amount of water added to the nitrate bath based on a state value corresponding to the water content rate of the nitrate bath, that is, the humidity brought by the water vapor generated in the nitrate bath. Thereby, an amount of water corresponding to the water content rate of the nitrate bath can be appropriately added to the nitrate bath. Therefore, according to the present invention, the cooling capacity based on the nitrate bath can be reliably improved and the cooling capacity can be maintained constant. As a result, the size of the components that can be processed by isothermal heat treatment can be increased.

[0019] In the present invention, it is preferable that the control device is configured to control the water addition device in such a way that the water content rate of the nitrate bath is set to a value equal to or higher than a specified value.

[0020] According to the present invention configured as described above, the nitrate bath can be set to a sufficient water content rate, and the cooling capacity based on the nitrate bath can be effectively improved.

[0021] In the present invention, it is preferable that the control device is configured to control the water addition device in such a way that the amount of water added to the nitrate bath is equal to or greater than the evaporation amount of water from the nitrate bath.

[0022] According to the present invention configured as described above, since water equal to or greater than the evaporation amount of water from the nitrate bath is added, the water content rate of the nitrate bath can be reliably ensured.

[0023] In the present invention, preferably, the humidity detection device further includes: a water vapor sampling unit including a sampling container and a float, the sampling container sampling the atmosphere gas above the nitrate bath containing water vapor generated from the nitrate bath, the float floating the sampling container on the liquid surface of the nitrate bath; and a water vapor supply pipe communicating with the sampling container of the water vapor sampling unit and supplying the atmosphere gas sampled by the sampling container to the humidity sensor.

[0024] According to the present invention configured as such, since the sampling container is floated on the liquid surface of the nitrate bath by the float, the water vapor on the liquid surface of the nitrate bath can be reliably sampled by the sampling container.

[0025] In the present invention, preferably, the humidity detection device further includes: a water vapor sampling unit including a sampling container that samples the atmosphere gas above the nitrate bath containing water vapor generated from the nitrate bath; a water vapor supply pipe communicating with the sampling container of the water vapor sampling unit and supplying the water vapor sampled by the sampling container to the humidity sensor; and a gas supply pipe communicating with the sampling container of the water vapor sampling unit and supplying an inert gas to the sampling container, the sampling container supplying the sampled atmosphere gas to the humidity sensor via the water vapor supply pipe by using the inert gas supplied from the gas supply pipe.

[0026] According to the present invention configured as such, the water vapor sampled by the sampling container can be reliably supplied to the humidity sensor by the inert gas supplied from the gas supply pipe to the sampling container.

[0027] In the present invention, preferably, the humidity sensor of the humidity detection device is provided on the water vapor supply pipe, and the humidity detection device further includes a heater that heats at least the portion of the water vapor supply pipe measured by the humidity sensor.

[0028] According to the present invention configured as such, by heating the portion of the water vapor supply pipe measured by the humidity sensor by the heater, the detection value based on the humidity sensor can be stabilized. As a result, the control of the water addition device based on the signal from the humidity sensor can be stably performed.

[0029] In the present invention, preferably, the water addition device includes a water addition pipe immersed in the nitrate bath, water is added to the nitrate bath from the water addition pipe, and the salt bath heat treatment device further includes an injection device including a gas supply pipe that supplies a predetermined gas to the water addition pipe of the water addition device. When the addition of water from the water addition device stops, the injection device supplies a predetermined gas from the gas supply pipe to the water addition pipe, thereby filling the water addition pipe with the predetermined gas.

[0030] According to the present invention configured as described above, when the water addition is stopped, the water addition pipe is filled with a prescribed gas supplied from the gas supply pipe of the injection device, so that it is possible to suppress the negative pressure state in the water addition pipe. As a result, it is possible to prevent the molten nitrate in the nitrate bath from flowing back into the water addition pipe and solidifying to cause clogging.

[0031] In another aspect, in order to achieve the above object, the present invention is a salt bath heat treatment apparatus for heat-treating a metal component using a salt bath, characterized in that the salt bath heat treatment apparatus includes: a salt bath tank that stores a nitrate bath containing molten nitrate to cool the metal component during isothermal heat treatment; a water addition device that adds water to the nitrate bath in the salt bath tank; a water content estimation device that estimates the water content of the nitrate bath to which water has been added by the water addition device; and a control device that is configured to control the water addition device based on the water content estimated by the water content estimation device to adjust the amount of water added to the nitrate bath.

[0032] According to the present invention configured as described above, it is also possible to appropriately add an amount of water corresponding to the water content of the nitrate bath to the nitrate bath, reliably improve the cooling ability based on the nitrate bath, and maintain the cooling ability constant. As a result, it is possible to increase the size of the components that can be processed by isothermal heat treatment.

[0033] Advantages of the Invention

[0034] According to the present invention, in a salt bath heat treatment apparatus using a nitrate bath for heat treatment, by reliably controlling the addition of water to the nitrate bath, it is possible to reliably improve the cooling ability of the nitrate bath. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 It is a schematic structural diagram showing the overall structure of a salt bath heat treatment apparatus according to an embodiment of the present invention.

[0036] Figure 2 It is a schematic structural diagram showing a water vapor sampling unit of a salt bath heat treatment apparatus according to an embodiment of the present invention.

[0037] Figure 3 It is a schematic structural diagram showing a humidity detection unit of a salt bath heat treatment apparatus according to an embodiment of the present invention.

[0038] Figure 4 It is a block diagram showing the electrical structure of a salt bath heat treatment apparatus according to an embodiment of the present invention.

[0039] Figure 5 It shows an example of the relationship between the water content of the nitrate bath and the absolute humidity detected by the humidity sensor.

[0040] Figure 6 An example of the result of water addition control showing an embodiment of the present invention.

[0041] Figure 7 An example of the result of water addition control showing a comparative example. Detailed Embodiment

[0042] Hereinafter, a salt bath heat treatment apparatus according to an embodiment of the present invention will be described with reference to the drawings.

[0043] [Structure of Salt Bath Heat Treatment Apparatus]

[0044] First, with reference to Figures 1 to 4 the structure of the salt bath heat treatment apparatus according to an embodiment of the present invention will be specifically described. Figure 1 is a schematic structural diagram showing the overall structure of the salt bath heat treatment apparatus of the present embodiment. Figure 2 is a schematic structural diagram showing the water vapor sampling section of the salt bath heat treatment apparatus of the present embodiment. Figure 3 is a schematic structural diagram showing the humidity detection section of the salt bath heat treatment apparatus of the present embodiment. Figure 4 is a block diagram showing the electrical structure of the salt bath heat treatment apparatus of the present embodiment.

[0045] As Figure 1 shown, the salt bath heat treatment apparatus 10 mainly includes: a salt bath tank 1 that stores a nitrate bath containing molten nitrate; a water addition device 2 that adds water to the nitrate bath in the salt bath tank 1; an injection device 4 that fills the piping of the water addition device 2 with an inert gas; and a humidity detection device 5 that monitors the humidity brought by the water vapor generated from the nitrate bath. In addition, the salt bath heat treatment apparatus 10 also has a control device 100 (refer to Figure 4 ). The salt bath heat treatment apparatus 10 is a heat treatment system that uses a nitrate bath as a cooling medium and is used for cooling in isothermal quenching treatment, martensitic quenching treatment, and other constant temperature heat treatments of metal components.

[0046] In the salt bath heat treatment apparatus 10, a melting heater 1a is provided in a salt bath tank 1. The nitrate introduced into the salt bath tank 1 is melted by the melting heater 1a to form a nitrate bath. The temperature of the nitrate bath is determined according to the type of nitrate and the constant temperature heat treatment conditions, etc. The melting point of the nitrate varies depending on the mixing ratio of the nitrate raw materials, but the melting point of a commonly used low melting point nitrate bath is, for example, about 140°C. If the nitrate bath is used at a temperature near the melting point, problems such as uneven melting state and increased carry-out amount of the nitrate bath adhering to the components, resulting in a higher replenishment frequency, will occur. Therefore, it is preferable to set the temperature of the nitrate bath to 40°C or more (for example, 180°C or more) of the melting point of the used nitrate bath. In addition, from the viewpoint of suppressing the breakdown of the salt composition due to thermal decomposition and reducing salt fumes, it is preferable to make the temperature of the nitrate bath 430°C or less. In addition, the size of the salt bath tank 1 is, for example, a length of 3100 mm, a width of 1000 mm, and a depth of 1200 mm. In addition, when the inventor of the present case conducted experiments for the present invention, a cylinder with a diameter of 470 mm and a depth of 750 mm was used as the salt bath tank 1.

[0047] Next, the water addition device 2 of the salt bath heat treatment apparatus 10 first has: a tap water pipe 21 that introduces tap water (industrial water, drinking water); a tap water supply valve 22 that switches the supply and cut-off of tap water; a flow meter 23 that adjusts the flow rate of tap water; and a pure water manufacturing device 24 that manufactures pure water from tap water. Tap water contains components such as calcium and magnesium, but when they react with the nitrate bath, they accumulate as impurities in the nitrate bath, which may cause a decrease in the cooling capacity of the nitrate bath and equipment failures. Therefore, the pure water manufacturing device 24 preferably manufactures pure water having a conductivity of 10 μS / cm or less and a TOC (Total Organic Carbon) value of 1000 ppb or less at 25°C, for example.

[0048] In addition, in Figure 1 In the example of the salt bath heat treatment apparatus 10 shown, the water addition device 2 has the pure water manufacturing device 24, but the water addition device 2 may not have such a pure water manufacturing device 24. In the case where the water addition device 2 does not have the pure water manufacturing device 24, the water addition device 2 directly uses tap water or the like, that is, the water addition device 2 adds tap water or the like to the nitrate bath.

[0049] In addition, the water addition device 2 further has: a tank 25 that stores the pure water (hereinafter simply referred to as "water") manufactured by the pure water manufacturing device 24 as described above; a water addition pump 27 that draws the water stored in the tank 25; a water addition pipe 26 through which the water drawn by the water addition pump 27 passes; and water addition pipes 26a and 26b that branch the water addition pipe 26 into two at the downstream side of the water addition pump 27.

[0050] The lowermost downstream portions of the water supply pipes 26a and 26b are immersed in the nitrate bath within the salt bath tank 1, and the front ends (the lowermost downstream ends) are open to add the water within the tank 25 to the nitrate bath. For example, the pipe diameters of the water supply pipes 26a and 26b are 6 cm. Specifically, the water supply pipes 26a and 26b may be immersed in the nitrate bath for a length (immersion depth) of 10% to 90%, more preferably 30% to 70% of the depth of the nitrate bath within the salt bath tank 1 (the distance from the inner bottom surface of the salt bath tank 1 to the liquid level of the nitrate bath). This is because the temperature of the nitrate bath is much higher than the boiling point of water. Therefore, if the immersion depth is too shallow, splashing of the liquid level may occur due to a steam explosion (the water added to the nitrate bath immediately evaporates), which is dangerous. On the contrary, if the immersion depth is too deep (the front ends of the water supply pipes 26a and 26b are too close to the inner bottom surface of the salt bath tank 1), the water supply pipes 26a and 26b may be deformed or the bottom of the salt bath tank 1 may be impacted due to a steam explosion.

[0051] Here, in Figure 1 In the example of the salt bath heat treatment apparatus 10 shown, the water supply pipes 26a and 26b of the water addition device 2 are immersed in the nitrate bath to add water to the nitrate bath (hereinafter also simply referred to as "addition in the salt bath"), but instead of this addition in the salt bath, dripping of water onto the surface of the nitrate bath (hereinafter also simply referred to as "surface dripping in the salt bath") may be performed. However, from the following reasons, addition in the salt bath is preferred compared to surface dripping in the salt bath. First, the water content rate of addition in the salt bath is better than that of surface dripping in the salt bath. That is to say, by addition in the salt bath, the nitrate bath can be set to a saturated state of water with less water volume and in a shorter time compared to surface dripping in the salt bath. On the other hand, compared to addition in the salt bath, the saturated water content rate value of surface dripping in the salt bath becomes lower (in other words, the cooling capacity is lower). This is because, in surface dripping in the salt bath, most of the water dripped onto the nitrate bath immediately evaporates on the liquid surface.

[0052] In addition, the number of the immersed water supply pipes 26a and 26b is not limited to two, and one or more than three water supply pipes may also be used. In this case, the number of the water supply pipes to be applied may be determined according to the capacity, shape, etc. of the salt bath tank 1.

[0053] In addition, flow meters 28a and 28b for adjusting the water flow rate (i.e., the water addition amount), flow sensors 29a and 29b for separately monitoring the water addition amount from the flow meters 28a and 28b, and water addition valves 30a and 30b for switching the supply (addition) and cutoff of water from the water supply pipes 26a and 26b to the nitrate bath are respectively provided in the water supply pipes 26a and 26b.

[0054] Here, in the water addition device 2, the amount of water added from the water addition pipes 26a and 26b to the nitrate bath (representing the flow rate; the same applies hereinafter) is controlled by adjusting the output of the water addition pump 27. In this case, from the viewpoint of suppressing the steam explosion caused by adding water to the nitrate bath as described above (the degree of this phenomenon tends to increase as the amount of water added increases), the upper limit of the amount of water added can be suppressed to 30 ml / min or less, more preferably 20 ml / min or less. In addition, the temperature of the nitrate bath can be further considered corresponding to the setting of the upper limit of the amount of water added, and based on this, the number of the above-mentioned water addition pipes can be determined. In addition, the adjustment of the upper limit of the amount of water added is not limited to adjusting the output of the water addition pump 27, and the upper limit of the amount of water added can also be adjusted by changing the opening degrees of the water addition valves 30a and 30b.

[0055] In addition, the water addition device 2 further includes: a water level gauge 31 that detects the water storage amount of the tank 25; and a protection valve 32 that is provided in the water addition pipe 26 and returns the water in the water addition pipe 26 to the tank 25.

[0056] Next, the injection device 4 of the salt bath heat treatment device 10 includes: a gas supply pipe 41 that introduces an inert gas such as nitrogen; a gas supply valve 42 that switches the supply and cutoff of the inert gas; a pressure reducing valve 43 that reduces the pressure of the inert gas; gas supply pipes 41a and 41b (each provided with a check valve in the middle), which are branched into two from the gas supply pipe 41 on the downstream side of the pressure reducing valve 43; and flow meters 44a and 44b that adjust the flow rates of the inert gas passing through the gas supply pipes 41a and 41b, respectively. The gas supply pipes 41a and 41b of the injection device 4 are connected (communicated) to the downstream sides of the water addition valves 30a and 30b (and the check valves provided in the water addition pipes 26a and 26b) on the downstream side of the check valves, respectively.

[0057] When the addition of water performed by the water addition device 2 stops, such an injection device 4 operates to open the gas supply valve 42 so as to supply an inert gas from the gas supply pipes 41a and 41b to the water addition pipes 26a and 26b. As a result, the water addition pipes 26a and 26b (more precisely, the regions on the downstream side of the check valves provided in them) are filled with the inert gas. The reason for adopting such a structure is as follows.

[0058] When the addition of water by the water addition device 2 is stopped, the water addition pump 27 is stopped and the water addition valves 30a and 30b are closed. However, in this case, a state is formed in which water remains in the water addition pipes 26a and 26b on the downstream side of the water addition valves 30a and 30b. If the water addition pipes 26a and 26b are left in this state, the molten nitrate in the nitrate bath may flow back into the water addition pipes 26a and 26b and solidify, resulting in clogging. This is because the residual water remaining in the water addition pipes 26a and 26b in the part immersed in the nitrate bath flows out into the bath, causing a negative pressure state in the water addition pipes 26a and 26b.

[0059] Therefore, as described above, when the water addition is stopped, it is effective for the injection device 4 to operate in such a way as to supply an inert gas to the water addition pipes 26a and 26b. As a result, the water remaining in the water addition pipes 26a and 26b is discharged into the bath, and the water addition pipes 26a and 26b are filled with the inert gas, thus more reliably preventing a negative pressure state from occurring in the water addition pipes 26a and 26b. As a result, it is possible to prevent the molten nitrate in the nitrate bath from flowing back into the water addition pipes 26a and 26b and solidifying, resulting in clogging.

[0060] In addition, in order to prevent the residual water from being discharged suddenly into the nitrate bath, the inert gas can be supplied from the injection device 4 to the water addition pipes 26a and 26b at a flow rate of about 5 ml / min to 30 ml / min. In addition, in the injection device 4, it is not limited to using an inert gas, and air or the like can also be used.

[0061] Next, the humidity detection device 5 of the salt bath heat treatment device 10 includes: a water vapor sampling unit 51 that samples the water vapor generated from the nitrate bath; a water vapor supply pipe 52 that supplies the water vapor sampled by the water vapor sampling unit 51; and a humidity detection unit 53 that detects the humidity brought by the water vapor supplied from the water vapor supply pipe 52. In addition, as a component for supplying an inert gas such as nitrogen to the water vapor sampling unit 51, the humidity detection device 5 further includes: a gas supply pipe 54 that introduces the inert gas; a gas supply valve 55 that switches the supply and cut-off of the inert gas; a pressure reducing valve 56 that reduces the pressure of the inert gas; and a flow meter 57 that adjusts the flow rate of the inert gas.

[0062] As Figure 2As shown, the water vapor sampling section 51 of the humidity detection device 5 includes: a sampling container 51a that temporarily stores the water vapor sampled from the nitrate bath; and a float 51c that is installed at the lower part of the sampling container 51a to float the sampling container 51a on the liquid surface of the nitrate bath. Specifically, the sampling container 51a has an opening 51b at the bottom (i.e., the bottom is open), is formed to be hollow, and is inserted into a through hole that is provided at the central part of the float 51c and extends in the vertical direction. In such a sampling container 51a, the lower part is filled with nitrate, and the atmosphere gas containing water vapor enters the upper part from the liquid surface of the nitrate.

[0063] For example, the sampling container 51a of the present embodiment is made of iron and has the following shape: a cylindrical part with a diameter of 4.2 cm and a height of 16 cm, and a frustum-shaped cylindrical part with a lower end diameter of 4.9 cm, an upper end diameter of 3.3 cm, and a height of 5.1 cm provided on the cylindrical part.

[0064] In addition, the above-mentioned gas supply pipe 54 communicates with the side wall of the frustum-shaped cylindrical part of the sampling container 51a, and an inert gas is supplied from the gas supply pipe 54. Also, a water vapor supply pipe 52 communicates with it, and the sampled atmosphere gas (containing water vapor) and the inert gas from the gas supply pipe 54 are supplied to the water vapor supply pipe 52 together. In this case, the sampling container 51a supplies the sampled atmosphere gas to the water vapor supply pipe 52 using the inert gas supplied from the gas supply pipe 54 (that is, using the inert gas as a so-called carrier gas). The water vapor supply pipe 52 of the present embodiment extends upward from the upper end of the frustum-shaped cylindrical part of the sampling container 51a.

[0065] In addition, for example, the float 51c of the water vapor sampling section 51 is made of iron and is in the shape of a cylinder with a diameter of 20 cm and a height of 10 cm, so that the sampling container 51a floats on the liquid surface of the nitrate bath in such a way as to follow the rise and fall of the liquid surface of the nitrate bath and keep the length (immersion depth) immersed in the nitrate bath constant. In one example, the float 51c floats the sampling container 51a with an immersion depth of about 50 mm (preferably an immersion depth of about 20 mm to 100 mm).

[0066] Next, as Figure 3As shown, the humidity detection unit 53 of the humidity detection device 5 has a humidity sensor 53a, which is provided on the steam supply pipe 52 and outputs a signal corresponding to the humidity (specifically, the absolute humidity) of the atmosphere gas supplied from the steam supply pipe 52. In addition, the humidity detection unit 53 has a smoke recovery filter 53b on the steam supply pipe 52 upstream of the humidity sensor 53a. The smoke recovery filter 53b is used to recover the volatile components (salt smoke) generated in the nitrate bath and taken into the steam supply pipe 52. For example, the smoke recovery filter 53b is composed of absorbent cotton or the like.

[0067] In addition, the humidity detection unit 53 has a heater 53c, which heats the part of the steam supply pipe 52 where the humidity sensor 53a and the smoke recovery filter 53b are provided. For example, the heater 53c is composed of an electric heating wire or the like (the electric heating wire is schematically shown in Figure 3 . The heater 53c is provided to keep the part (hereinafter referred to as "humidity sensor measurement unit") 53a1 measured by the humidity sensor 53a in the steam supply pipe 52 at a specified temperature. In addition, it is also possible to provide a temperature sensor on the steam supply pipe 52 and adopt feedback control for the temperature control performed by the heater 53c based on the temperature detected by the temperature sensor.

[0068] The signal (corresponding to the absolute humidity) output from the humidity sensor 53a is affected by the temperature of the humidity sensor measurement unit 53a1 and the flow rate of the inert gas from the gas supply pipe 54. Therefore, in order to stably detect the signal, the heater 53c is kept at a certain temperature (for example, a temperature in the range of 40°C to 70°C), and in addition, the inert gas from the gas supply pipe 54 is supplied at a constant flow rate (for example, a constant flow rate in the range of 10 l / h to 1000 l / h, preferably 30 l / h to 500 l / h, more preferably 50 l / h to 250 l / h).

[0069] In addition, it is not limited to the method of detecting the absolute humidity by the humidity sensor 53a. Instead of detecting the absolute humidity, it is also possible to adopt methods of detecting the dew point, frost point, wet bulb temperature, water vapor partial pressure, mixing ratio, enthalpy, etc. (these parameters also correspond to "humidity" in the present invention). Specifically, as the humidity sensor 53a, a polymer-based humidity sensor, a metal oxide-based humidity sensor, an electrolyte-based humidity sensor, etc. can be used. In addition, the measurement method of humidity can be capacitive or resistive.

[0070] Next, with reference to Figure 4 , the electrical structure of the salt bath heat treatment device 10 of the present embodiment will be described. As Figure 4As shown, the salt bath heat treatment apparatus 10 is controlled by a control device 100. Detection signals are supplied to the control device 100 from the flow sensors 29a and 29b of the water addition device 2, the liquid level gauges 31, and the humidity sensor 53a of the humidity detection device 5. Further, based on these detection signals, the control device 100 supplies control signals to the tap water supply valve 22, the water addition pump 27, the water addition valves 30a and 30b, and the protective valve 32 of the water addition device 2, the gas supply valve 42 of the injection device 4, and the gas supply valve 55 of the humidity detection device 5 to control them.

[0071] For example, the control device 100 is constituted by a computer having: one or more processors (typically a CPU); and memories such as a ROM and a RAM that store various programs (including basic control programs such as an OS and application programs that are started on the OS and implement specific functions) and various data to be interpreted and executed on the processor. Further, as Figure 4 shown, it is not limited to using a single control device 100 that uniformly controls the salt bath heat treatment apparatus 10. In other examples, control devices (microcontrollers, etc.) respectively built into various valves, pumps, etc. may also be used.

[0072] Here, in the present embodiment, the main control performed by the control device 100 will be described. First, based on the detection signal detected by the humidity sensor 53a of the humidity detection device 5 and corresponding to the absolute humidity of the atmosphere gas containing water vapor generated in the nitrate bath, the control device 100 controls the output of the water addition pump 27 of the water addition device 2 to adjust the amount of water added to the nitrate bath. Typically, the control device 100 performs feedback control on the output of the water addition pump 27 based on the relationship between the absolute humidity corresponding to the detection signal of the humidity sensor 53a and a prescribed target value to adjust the amount of water added. Hereinafter, the output control of the water addition pump 27 executed by such a control device 100 will be referred to as "water addition control". Details of this water addition control will be described later.

[0073] In addition, regarding the water addition device 2, the control device 100 controls the tap water supply valve 22 based on the water storage amount of the tank 25 corresponding to the detection signal from the liquid level gauge 31. Specifically, when the water storage amount detected by the liquid level gauge 31 is less than a specified lower limit amount, the control device 100 controls to open the tap water supply valve 22. Thereby, tap water is supplied to the pure water manufacturing device 24, and pure water is manufactured by the pure water manufacturing device 24. On the contrary, when the water storage amount detected by the liquid level gauge 31 is equal to or more than a specified upper limit amount, the control device 100 controls to close the tap water supply valve 22. Thereby, by cutting off the supply of tap water to the pure water manufacturing device 24, the manufacturing of pure water is stopped. By controlling the tap water supply valve 22 based on the liquid level gauge 31 in this way, the water storage amount of the tank 25 is set to an amount between the specified lower limit amount and the specified upper limit amount.

[0074] In addition, regarding the water addition device 2, the control device 100 controls the water addition valves 30a, 30b and the protection valves 32 on the water addition pipes 26a, 26b based on the water addition amount corresponding to the detection signals from the flow sensors 29a, 29b on the water addition pipes 26a, 26b. Specifically, when the water addition amount detected by one or both of the flow sensors 29a, 29b is equal to or more than a specified value, the control device 100 controls to close one or both of the water addition valves 30a, 30b and open the protection valve 32. Thereby, when an abnormality occurs in the water addition performed by the water addition device 2, the water addition from the water addition pipes 26a, 26b to the nitrate bath is stopped, and the water in the water addition pipes 26a, 26b returns from the protection valve 32 to the tank 25. As a result, the water addition device 2, the salt bath tank 1, etc. are protected.

[0075] In addition, regarding the injection device 4, when the water addition by the water addition device 2 stops, specifically when the water addition valves 30a, 30b of the water addition device 2 are closed, the control device 100 controls to open the gas supply valves 42 to supply inert gas from the gas supply pipes 41a, 41b to the water addition pipes 26a, 26b respectively. Thereby, as described above, by filling the inert gas into the water addition pipes 26a, 26b when the water addition stops, it is possible to prevent the molten nitrate from flowing back in the water addition pipes 26a, 26b and solidifying to cause blockage.

[0076] [Water addition control]

[0077] Next, the water addition control of the present embodiment will be specifically described. As described above, in the present embodiment, the control device 100 controls the output of the water addition pump 27 of the water addition device 2 based on a signal corresponding to the absolute humidity of the atmosphere gas detected by the humidity sensor 53a of the humidity detection device 5 to adjust the amount of water added to the nitrate bath. Although the absolute humidity detected by the humidity sensor 53a is the humidity brought by the water vapor generated in the nitrate bath, according to the insight of the inventor of the present application, this absolute humidity is closely related to the ratio (water content rate) of the water contained in the nitrate bath.

[0078] Specifically, with reference to Figure 5 , the relationship between the humidity of the atmosphere gas containing the water vapor generated in the nitrate bath and the water content rate of the nitrate bath will be described. Figure 5 is a graph obtained through experiments showing an example of the relationship between the water content rate (horizontal axis) of the nitrate bath and the absolute humidity (vertical axis) of the atmosphere gas on the nitrate bath detected by the humidity sensor 53a. Specifically, Figure 5 shows the absolute humidity (g / m 3 ) detected by the humidity sensor 53a at each water content rate when the water content rates of the nitrate baths at 300 °C and 400 °C are set to various values. Thus, as shown by the solid line and the dotted line in the figure, it can be seen that there is a correlation (specifically, a linear relationship) between the water content rate and the absolute humidity. Therefore, it can be said that the water content rate of the nitrate bath can be corresponded to based on the absolute humidity of the atmosphere gas on the nitrate bath.

[0079] Therefore, in the present embodiment, the control device 100 performs water addition control based on the water content rate corresponding to the absolute humidity detected by the humidity sensor 53a. Specifically, the control device 100 controls the amount of water added based on the water addition pump 27 so that the water content rate of the nitrate bath becomes above a specified value (hereinafter referred to as "target water content rate"). This target water content rate is specified based on, for example, the water content rate when the nitrate bath is in a saturated state (a state where water is added to the limit in the nitrate bath). However, since the water content rate when the nitrate bath is in a saturated state varies depending on the temperature of the nitrate bath, the target water content rate can be set according to the temperature of the nitrate bath. In this case, the target water content rate to be set can be applied according to the temperature of the nitrate bath in consideration of the results of prior experiments or specified simulations, etc.

[0080] Specifically, in order to set the water content rate of the nitrate bath to the above-mentioned target water content rate, the control device 100 performs feedback control on the water addition pump 27 to adjust the water addition amount. That is, when the water content rate corresponding to the absolute humidity detected by the humidity sensor 53a is less than the target water content rate, the control device 100 controls the water addition pump 27 in a manner of increasing the water addition amount. On the other hand, when the water content rate corresponding to the absolute humidity detected by the humidity sensor 53a is equal to or higher than the target water content rate, the control device 100 controls the water addition pump 27 in a manner of decreasing the water addition amount. Thereby, the water content rate of the nitrate bath can be maintained at the target water content rate. Therefore, the nitrate bath can be maintained in a state with a relatively high cooling capacity. In addition, by reducing the water addition amount when the water content rate is equal to or higher than the target water content rate as described above, the amount of water added to the nitrate bath can be saved. As a result, the cartridge replacement cycle of the pure water manufacturing device 24 can be prolonged, or the humidity rise at the industrial site can be suppressed to improve the working environment.

[0081] In addition, in terms of control, regarding the electrical signal detected by the humidity sensor 53a, it is not necessary to specifically convert it into a value of a meaningful parameter such as absolute humidity or water content rate. As long as the value of the signal detected by the humidity sensor 53a is less than the value of the signal corresponding to the above-mentioned target water content rate, the control device 100 controls the output of the water addition pump 27 in a manner of increasing the water addition amount. On the other hand, when the value of the signal detected by the humidity sensor 53a is greater than the value of the signal corresponding to the target water content rate, the control device 100 controls the output of the water addition pump 27 in a manner of decreasing the water addition amount. The correspondence between the value of the signal detected by the humidity sensor 53a and the water content rate of the nitrate bath (refer to Figure 4 , for example, approximated by a linear function) is preferably calibrated in advance through prior experiments or specified simulations according to each condition (such as temperature) of the nitrate bath.

[0082] In addition, the control device 100 preferably controls the water addition pump 27 in such a way that the water addition amount (in other words, the water addition speed) is equal to or greater than the evaporation amount of water from the nitrate bath (in other words, the evaporation speed) (water addition amount ≥ evaporation amount of water). The evaporation amount of water from the nitrate bath is a quantity corresponding to the temperature of the nitrate bath, the liquid area of the nitrate bath, the number of water addition pipes 26a and 26b (in the Figure 1 example shown, it is 2), etc. Therefore, through prior experiments or specified simulations, etc., the evaporation amount of water from the nitrate bath can be obtained based on these parameters. The control device 100 preferably uses such an evaporation amount as the lower limit value of the water addition amount, for example.

[0083] [Function and Effect]

[0084] Next, the functions and effects of the salt bath heat treatment apparatus 10 of the present embodiment will be described. In the present embodiment, the salt bath heat treatment apparatus 10 includes: a salt bath tank 1 that stores a nitrate bath containing molten nitrate for cooling in isothermal heat treatment of a metal component; a water addition device 2 that adds water to the nitrate bath in the salt bath tank 1; a humidity detection device 5 that includes a humidity sensor 53a that outputs a signal corresponding to the humidity brought by water vapor generated from the nitrate bath to which water has been added by the water addition device 2; and a control device 100 that is configured to control the water addition device 2 based on the signal output from the humidity sensor 53a of the humidity detection device 5 to adjust the amount of water added to the nitrate bath.

[0085] In such a salt bath heat treatment apparatus 10, the amount of water added to the nitrate bath is controlled based on the humidity brought by water vapor from the nitrate bath corresponding to the water content rate of the nitrate bath. Therefore, according to the present embodiment, it is possible to appropriately add an amount of water corresponding to the water content rate of the nitrate bath to the nitrate bath. Therefore, according to the present embodiment, it is possible to reliably improve the cooling ability of the nitrate bath and maintain the cooling ability constant. As a result, it is possible to increase the size of the components that can be processed by isothermal heat treatment.

[0086] In addition, in the present embodiment, the control device 100 controls the water addition device 2 so that the water content rate of the nitrate bath is set to a specified value (for example, a target water content rate) or more. Therefore, it is possible to set the nitrate bath to a sufficient water content rate and effectively improve the cooling ability of the nitrate bath.

[0087] In addition, in the present embodiment, the control device 100 controls the water addition device 2 so that the amount of water added to the nitrate bath is equal to or more than the evaporation amount of water from the nitrate bath, thereby reliably ensuring the water content rate of the nitrate bath.

[0088] In addition, in the present embodiment, the water addition device 2 includes water addition pipes 26a and 26b immersed in the nitrate bath, and water is added to the nitrate bath from the water addition pipes 26a and 26b. The salt bath heat treatment apparatus 10 further includes an injection device 4 that includes gas supply pipes 41a and 41b that supply an inert gas to the water addition pipes 26a and 26b. When the addition of water from the water addition device 2 stops, the injection device 4 supplies an inert gas from the gas supply pipes 41a and 41b to the water addition pipes 26a and 26b, thereby filling the water addition pipes 26a and 26b with the inert gas. Thereby, when the water addition stops, it is possible to suppress the water addition pipes 26a and 26b from becoming a negative pressure state. As a result, it is possible to prevent the molten nitrate in the nitrate bath from flowing back in the water addition pipes 26a and 26b and solidifying to cause clogging.

[0089] In addition, in the present embodiment, the humidity detection device 5 further includes: a water vapor sampling unit 51 including a sampling container 51a and a float 51c. The sampling container 51a samples the atmosphere gas above the nitrate bath containing the water vapor generated from the nitrate bath, and the float 51c floats the sampling container 51a on the liquid surface of the nitrate bath; and a water vapor supply pipe 52 communicating with the sampling container 51a of the water vapor sampling unit 51 to supply the atmosphere gas sampled by the sampling container 51a to the humidity sensor 53a. Thus, by floating the sampling container 51a on the liquid surface of the nitrate bath using the float 51c, the sampling container 51a can reliably sample the atmosphere gas above the nitrate bath containing the water vapor generated from the nitrate bath under the same conditions all the time.

[0090] In addition, in the present embodiment, the humidity detection device 5 further includes a gas supply pipe 54 communicating with the sampling container 51a of the water vapor sampling unit 51 to supply an inert gas to the sampling container 51a. The sampling container 51a supplies the sampled atmosphere gas to the humidity sensor 53a via the water vapor supply pipe 52 using the inert gas supplied from the gas supply pipe 54. Thereby, the atmosphere gas sampled by the sampling container 51a can be reliably supplied to the humidity sensor 53a by the inert gas supplied from the gas supply pipe 54 to the sampling container 51a.

[0091] In addition, in the present embodiment, the humidity sensor 53a of the humidity detection device 5 is provided on the water vapor supply pipe 52, and the humidity detection device 5 further includes a heater 53c that heats at least the humidity sensor measurement unit 53a1 measured by the humidity sensor 53a in the water vapor supply pipe 52. Thus, by heating the humidity sensor measurement unit 53a1 using the heater 53c, the signal detected by the humidity sensor 53a can be stabilized. As a result, the water addition control based on the signal detected by the humidity sensor 53a can be stably performed.

[0092] Here, with reference to Figure 6 and Figure 7 , the effects in the case of performing water addition control in a state where the humidity sensor measurement unit 53a1 is heated by the heater 53c will be specifically described. Figure 6 An example of the result of the present embodiment in which the humidity sensor measurement unit 53a1 is heated by the heater 53c to perform water addition control is shown. Figure 7 An example of the result of a comparative example in which water addition control is performed without using such a heater 53c is shown. In Figure 6 and Figure 7In [the figure], the horizontal axis represents time, and the vertical axis represents the absolute humidity of water vapor detected by the humidity sensor 53a and the temperature of the water vapor supply pipe 52 provided with the humidity sensor measurement unit 53a1.

[0093] In the present embodiment, water addition control is performed in such a manner that the absolute humidity of water vapor from the nitrate bath heated to 380 °C is maintained at a target humidity (exemplified as the absolute humidity when the nitrate bath is in a saturated state). In particular, in the present embodiment, this water addition control is performed in a state where the humidity sensor measurement unit 53a1 is kept warm at approximately 65 °C by the heater 53c. On the other hand, in the comparative example, the heater 53c is stopped while continuing such water addition.

[0094] As Figure 6 shown, according to the present embodiment, it can be seen that the temperature of the humidity sensor measurement unit 53a1 is maintained substantially constant by the heater 53c, and on the other hand, it is maintained at a substantially constant value (target humidity) after the absolute humidity of the water vapor rises. Such a result is obtained because the temperature of the humidity sensor measurement unit 53a1 is maintained constant by the heater 53c, and thus the absolute humidity detected by the humidity sensor 53a is stabilized.

[0095] In contrast, as Figure 7 shown, in the comparative example, it can be seen that the temperature of the humidity sensor measurement unit 53a1 gradually decreases, and on the other hand, the absolute humidity of the water vapor is not maintained at a constant value. Specifically, the absolute humidity increases as the temperature decreases. Generally, the absolute humidity changes according to the temperature. Therefore, even when an atmosphere gas containing substantially the same amount of water vapor is supplied to the humidity sensor 53a, the absolute humidity increases when the temperature decreases.

[0096] Based on such results, by performing water addition control in a state where the humidity sensor measurement unit 53a1 is kept warm by the heater 53c as in the present embodiment, the absolute humidity (signal) detected by the humidity sensor 53a can be stabilized. As a result, water addition control based on this absolute humidity (signal) can be stably performed.

[0097] [Modification example]

[0098] In the above-described embodiment, the humidity brought by the water vapor generated from the nitrate bath is detected by the humidity sensor 53a, and water addition control is performed based on this humidity. However, in the modification example, water addition control may also be performed without using such humidity. Based on the history of the amount of water added to the nitrate bath, etc., the change in the humidity brought by the water vapor generated from the nitrate bath when water is added to the nitrate bath can be predicted to some extent (for example Figure 6)。Although such humidity changes vary depending on the temperature state of the nitrate bath and the like, if prior experiments or specified simulations are carried out, it is also possible to predict the humidity changes corresponding to the temperature state of the nitrate bath and the like.

[0099] Therefore, in the modified example, the humidity is inferred without detecting it, and water addition control is performed accordingly. In this case, since the humidity is not detected, it is also possible to directly infer the water content rate of the nitrate bath without specifically inferring the humidity. Therefore, in the modified example, it is also possible to take into account the results of prior experiments or specified simulations, etc., infer the water content rate of the nitrate bath based on the temperature state of the nitrate bath, the history of the amount of water added to the nitrate bath, etc., and perform water addition control based on the inferred water content rate. Additionally, in such a modified example, the above control device 100 corresponds to the "water content rate inference device" and the "control device" in the present invention.

[0100] The above embodiments are examples for explaining the present invention, and the present invention is not limited to these embodiments. The present invention can be implemented in various ways as long as it does not deviate from its gist.

[0101] Reference Numeral Explanation

[0102] 1: Salt bath tank; 2: Water addition device; 4: Injection device; 5: Humidity detection device; 10: Salt bath heat treatment device; 26, 26a, 26b: Water addition pipes; 27: Water addition pump; 41, 41a, 41b: Gas supply pipes; 51: Water vapor sampling section; 51a: Sampling container; 51c: Float; 52: Water vapor supply pipe; 53: Humidity detection section; 53a: Humidity sensor; 53b: Smoke recovery filter; 53c: Heater; 54: Gas supply pipe; 100: Control device.

Claims

1. A salt bath heat treatment device that uses a salt bath to heat treat metal components, characterized in that, This salt bath heat treatment apparatus includes: A salt bath tank that houses a nitrate bath containing molten nitrate for cooling in the isothermal heat treatment of the metal component; A water addition device that adds water to the nitrate bath in the salt bath tank; A humidity detection device that includes a humidity sensor which outputs a signal corresponding to the humidity brought by the water vapor generated from the nitrate bath to which water has been added by the water addition device; and A control device configured to control the water addition device based on the signal output from the humidity sensor of the humidity detection device to adjust the amount of water added to the nitrate bath.

2. The salt bath heat treatment device according to claim 1, characterized in that, The control device is configured to control the water addition device so that the water content rate of the nitrate bath is set to a specified value or more.

3. The salt bath heat treatment device according to claim 1 or 2, characterized in that, The control device is configured to control the water addition device so that the amount of water added to the nitrate bath is equal to or more than the evaporation amount of water from the nitrate bath.

4. The salt bath heat treatment device according to claim 1, characterized in that, The humidity detection device further includes: A water vapor sampling unit that includes a sampling container and a float. The sampling container samples the atmosphere gas above the nitrate bath containing the water vapor generated from the nitrate bath, and the float makes the sampling container float on the liquid surface of the nitrate bath; And A water vapor supply pipe that communicates with the sampling container of the water vapor sampling unit and supplies the atmosphere gas sampled by the sampling container to the humidity sensor.

5. The salt bath heat treatment device according to claim 1, characterized in that, The humidity detection device further includes: A water vapor sampling unit that includes a sampling container which samples the atmosphere gas above the nitrate bath containing the water vapor generated from the nitrate bath; A water vapor supply pipe that communicates with the sampling container of the water vapor sampling unit and supplies the water vapor sampled by the sampling container to the humidity sensor; and A gas supply pipe that communicates with the sampling container of the water vapor sampling unit and supplies an inert gas to the sampling container, The sampling container supplies the sampled atmosphere gas to the humidity sensor via the water vapor supply pipe using the inert gas supplied from the gas supply pipe.

6. The salt bath heat treatment device according to claim 4 or 5, characterized in that, The humidity sensor of the humidity detection device is provided on the water vapor supply pipe, The humidity detection device further includes a heater that heats at least the portion measured by the humidity sensor in the water vapor supply pipe.

7. The salt bath heat treatment device according to claim 1, characterized in that, The water addition device includes a water addition pipe immersed in the nitrate bath, and water is added to the nitrate bath from the water addition pipe. The salt bath heat treatment apparatus further includes an injection device that includes a gas supply pipe for supplying a specified gas to the water addition pipe of the water addition device. When the addition of water from the water addition device stops, the injection device supplies the specified gas from the gas supply pipe to the water addition pipe, thereby filling the water addition pipe with the specified gas.

8. A salt bath heat treatment device that uses a salt bath to heat treat metal components, characterized in that, This salt bath heat treatment apparatus includes: A salt bath tank that houses a nitrate bath containing molten nitrate for cooling in the isothermal heat treatment of the metal component; A water addition device that adds water to the nitrate bath in the salt bath tank; A water content estimation device that estimates the water content of the nitrate bath to which water has been added by the water addition device; And A control device configured to control the water addition device based on the water content estimated by the water content estimation device to adjust the amount of water added to the nitrate bath.

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