Salt-bath heat treatment apparatus

Abstract

In order to subject a metal member to cooling under isothermal heat treatment, a salt-bath heat treatment apparatus 10 comprises: a salt bath tank 1 which accommodates a nitrate salt bath including a molten nitrate salt; a water addition device 2 to add water to the nitrate salt bath in the salt bath tank 1; a humidity detection device 5 comprising a humidity sensor 53a to output a signal corresponding to humidity caused by water vapor generated from the nitrate salt bath with water added by the water addition device 2; and a control device 100 configured to control the water addition device 2 to adjust an addition amount of water to the nitrate salt bath, based on the signal output from the humidity sensor 53a of the humidity detection device 5.

Description

TECHNICAL FIELD

[0001] The present invention relates to the field of techniques for performing a salt-bath heat treatment of a metal member using a salt bath.BACKGROUND ART

[0002] Heretofore, it has been practiced to perform a salt-bath heat treatment of a metal member, particularly to isothermal heat treatment, using a salt bath. This isothermal heat treatment is a heat treatment comprising the step of, for cooling of a heated metal member, putting the metal member into a salt bath kept at a constant temperature to hold the temperature of the metal member, and is different from normal oil-quenching or water-quenching (i.e., quenching and tempering). Specifically, the isothermal heat treatment includes austempering, and martempering. In these heat treatments, a nitrate salt bath including a molten nitrate salt is widely used as a cooling medium.

[0003] In the austempering, a metal member is first heated and held in a non-oxidizing atmosphere at 800 to 900° C. for 15 to 60 minutes, thereby being transformed into an austenite microstructure. Immediately thereafter, this metal member is immersed in a nitrate salt bath kept at about 300 to 430° C., thereby being rapidly cooled, and then held in the nitrate salt bath for 15 to 60 minutes. This allows the austenite microstructure to be transformed into a uniform bainite microstructure.

[0004] The member subjected to such austempering has an advantage of being excellent in mechanical properties such as toughness and shock resistance, and small in heat treatment distortion, as compared with a member subjected to commonly-quenched and tempered and having comparable hardness. However, although the austempering is mainly used for carbon steels use or high carbon steels, these steels are poor in hardenability because they contain alloy elements in a small amount. Therefore, in the austempering, the thickness of a member heat-treatable in industrial sites is generally limited to 2 mm or less.

[0005] On the other hand, in the martempering, a metal member is first heated and held in a non-oxidizing atmosphere at 800 to 900° C. for 15 to 60 minutes, thereby being transformed into an austenite microstructure. Immediately thereafter, this metal member is immersed in a nitrate salt bath kept at about 160 to 250° C., thereby being rapidly cooled, and then held in the nitrate salt bath for 5 to 60 minutes. This allows the austenite microstructure to be transformed into a uniform martensite microstructure.

[0006] The member subjected to such martempering has an advantage of being small in distortion due to heat treatment, as compared with a member subjected to commonly-quenched and tempered and having comparable hardness. Each of the martempering and the quenching and tempering is a heat-treating technique of cooling a metal member such that the temperature thereof passes through the Ms point (a temperature at which the transformation from the austenite microstructure to the martensite microstructure is initiated), thereby obtaining the martensite microstructure. However, in the quenched and tempered, oil, water, a water-soluble quenching agent, or the like, is used as a cooling medium, and the metal material is rapidly cooled to a temperature region lower than the Ms point. Thus, in the cooling stage, a large temperature difference occurs between a surface portion and an inner portion, a thin portion and a thick portion, or other portions, of a to-be-treated member, and a difference in the timing of transformation to the martensite microstructure occurs therebetween, thereby possibly causing undesirably large heat treatment distortion.

[0007] Compared with this, since the nitrate salt bath used for the martempering is higher than oil, water, or the like, in terms of temperature as a cooling medium, a to-be-treated member can be held in a temperature range close to the Ms point after being rapidly cooled to around the Ms point. Thus, the temperature difference occurring between the surface and inner portions, the thin and thick portions, or other portions, of the to-be-treated member, can be reduced as much as possible, and in this state, the to-be-treated member can be transformed into the martensite microstructure, so that it is possible to suppress the heat treatment distortion. However, due to the high temperature of the cooling medium, the cooling rate during the rapid cooling in the martempering is less than that of oil or water, and therefore there has been a tendency for a heat-treatable member to be limited to a small-sized member.

[0008] Here, in order to expand the range of size of a member heat-treatable by the above-mentioned isothermal heat treatment, it is considered effective to improve the cooling capacity of the nitrate salt bath as a cooling medium. Heretofore, it has been known that adding water to a nitrate salt bath is a good way to improve the cooling capacity of the nitrate salt bath (for example, non-patent documents 1 and 2). When water is added to a nitrate salt bath, molecules of water are intermolecularly bonded to a nitrate salt, wherein since energy of this intermolecular bond is small, the intermolecular bond will be cut at the moment of contact with a member heated up to 800 to 900° C. As a result, the latent heat of vaporization is taken away from the member, and the water molecules become vapor bubbles to agitate the nitrate salt bath around the member, thereby providing an improved cooling capacity.PRIOR ART DOCUMENTSNon-Patent Document

[0009] Non-Patent Document 1:“Heat Treatment” Vol. 17, No. 2, pp. 92-97 (Toshio KUBOTA, and Kazuo SATO)

[0010] Non-Patent Document 2:“Industrial Heating” Vol. 23, No. 2, pp. 23-31 (Tsugio YONEMURA)SUMMARY OF INVENTIONTechnical Problem

[0011] According to the knowledge of the present inventors, it is desirable to add an appropriate amount of water to a nitrate salt bath in order to adequately realize an improvement in cooling capacity by the nitrate salt bath. Specifically, it is advisable to add water to maintain a water content rate of the nitrate salt bath (the rate of water contained in the nitrate salt bath) at a set value.

[0012] The present invention has been made based on the above knowledge. It is an object of the present invention to, in a salt-bath heat treatment apparatus for performing heat treatment using a nitrate salt bath, improve the cooling capacity of the nitrate salt bath by controlling the addition of water to the nitrate salt bath.Solution to Technical Problem

[0013] In order to achieve the above object, the present invention provides a salt-bath heat treatment apparatus for performing a heat treatment of a metal member using a salt bath. The salt-bath heat treatment apparatus comprises: a salt bath tank which accommodates a nitrate salt bath including a molten nitrate salt to cool the metal member under an isothermal heat treatment; a water addition device for adding water to the nitrate salt bath in the salt bath tank; a humidity detection device comprising a humidity sensor to output a signal corresponding to humidity caused by water vapor generated from the nitrate salt bath with water added by the water addition device; and a control device configured to control the water addition device to adjust an addition amount of water to the nitrate salt bath, based on the signal output from the humidity sensor of the humidity detection device.

[0014] According to the present invention configured as just described, the salt-bath heat treatment apparatus controls the addition amount of water to the nitrate salt bath, based on humidity caused by water vapor generated in the nitrate salt bath, which is a state value corresponding to a water content rate of the nitrate salt bath. This makes it possible to appropriately add water to the nitrate salt bath in an amount corresponding to the water content rate of the nitrate salt bath. Therefore, according to the present invention, it is possible to adequately improve cooling capacity by the nitrate salt bath, and keep the cooling capacity constant, so that it becomes possible to expand the range of size of a member heat-treatable by the isothermal heat treatment.

[0015] Preferably, in the present invention, the control device is configured to control the water addition device to set a water content rate of the nitrate salt bath at a given value or more.

[0016] According to the present invention configured as just described, it is possible to set the nitrate salt bath to have a sufficient water content rate, thereby effectively improving the cooling capacity by the nitrate salt bath.

[0017] Preferably, in the present invention, the control device is configured to control the water addition device such that the addition amount of water to the nitrate salt bath becomes equal to or greater than a vaporization amount of water from the nitrate salt bath.

[0018] According to the present invention configured as just described, water is added in an amount which is equal to or greater than the vaporization amount of water from the nitrate salt bath, so that it is possible to adequately ensure the water content rate of the nitrate salt bath.

[0019] Preferably, in the present invention, the humidity detection device further comprises: a water vapor sampling unit comprising a sampling container to sample an atmosphere gas lying over the nitrate salt bath and including the water vapor generated from the nitrate salt bath, and a float to enable the sampling container to float on a liquid surface of the nitrate salt bath; and a water vapor delivery pipe which is communicated with the sampling container of the water vapor sampling unit to deliver the atmosphere gas sampled by the sampling container to the humidity sensor.

[0020] According to the present invention configured as just described, the sampling container is floated on the liquid surface of the nitrate salt bath by the float, so that it becomes possible for the sampling container to adequately sample water vapor over the liquid surface of the nitrate salt bath.

[0021] Preferably, in the present invention, the humidity detection device further comprises: a water vapor sampling unit comprising a sampling container to sample an atmosphere gas lying over the nitrate salt bath and including the water vapor generated from the nitrate salt bath; a water vapor delivery pipe which is communicated with the sampling container of the water vapor sampling unit to deliver the atmosphere gas sampled by the sampling container to the humidity sensor; and a gas supply pipe which is communicated with the sampling container of the water vapor sampling unit to supply an inert gas to the sampling container, wherein the sampling container delivers the sampled atmosphere gas to the humidity sensor through the water vapor delivery pipe through use of the inert gas supplied from the gas supply pipe.

[0022] According to the present invention configured as just described, it is possible to reliably deliver water vapor sampled by the sampling container, to the humidity sensor, by means of the inert gas supplied from the gas supply pipe to the gas sampling container.

[0023] Preferably, in the present invention, the humidity sensor of the humidity detection device is provided on the water vapor delivery pipe, and the humidity detection device further comprises a heater to keep the water vapor delivery pipe warm at least in a portion thereof where a measurement by the humidity sensor is performed.

[0024] According to the present invention configured as just described, a portion of the water vapor delivery pipe in which measurement by the humidity sensor is performed is kept warm by the heater, so that it is possible to stabilize a detection value of the humidity sensor. This makes it possible to stabilize the control of the water addition device to be performed based on the signal from the humidity sensor.

[0025] Preferably, in the present invention, the water addition device comprises a water addition pipe immersed in the nitrate salt bath to add water from the water addition pipe to an inside of the nitrate salt bath, and the salt-bath heat treatment apparatus further comprises a flushing device comprising a gas supply pipe to supply a given gas to the water addition pipe of the water addition device, the flushing device being configured to supply the given gas from the gas supply pipe to the water addition pipe to fill the water addition pipe with the given gas, when the addition of water from the water addition device is stopped.

[0026] According to the present invention configured as just described, during stopping of the water addition, the water addition pipe is filled with the given gas supplied from the gas supply pipe of the flushing device, so that it is possible to suppress a situation where the inside of the water addition pipe is in a negative pressure state. This makes it possible to prevent the occurrence of a situation where the molten nitrate salt in the nitrate salt bath flows back through the water addition pipe, and then solidifies, casing clogging.

[0027] According to another aspect, in order to achieve the above object, the present invention provides a salt-bath heat treatment apparatus for performing a heat treatment of a metal member using a salt bath. The salt-bath heat treatment apparatus comprises: a salt bath tank which accommodates a nitrate salt bath including a molten nitrate salt to cool the metal member under an isothermal heat treatment; a water addition device for adding water to the nitrate salt bath in the salt bath tank; a water content rate estimation device configured to estimate a water content rate of the nitrate salt bath with water added by the water addition device; and a control device configured to control the water addition device to adjust an addition amount of water to the nitrate salt bath, based on the water content rate estimated by the water content rate estimation device.

[0028] According to the present invention configured as just described, it is possible to properly add water to the nitrate salt bath in an amount according to the water content rate of the nitrate salt bath to adequately improve cooling capacity by the nitrate salt bath, and maintain the cooling capacity constant. This makes it possible to expand the range of size of a member heat-treatable by the isothermal heat treatment.Effect of Invention

[0029] According to the present invention, in a salt-bath heat treatment apparatus for performing heat treatment using a nitrate salt bath, it is possible to adequately control the addition of water to the nitrate salt bath, thereby reliably improving the cooling capacity of the nitrate salt bath.BRIEF DESCRIPTION OF DRAWINGS

[0030] FIG. 1 is a schematic configuration diagram showing an overall configuration of a salt-bath heat treatment apparatus according to an embodiment of the present invention.

[0031] FIG. 2 is a schematic configuration diagram showing a water vapor sampling unit of the salt-bath heat treatment apparatus according to the embodiment of the present invention.

[0032] FIG. 3 is a schematic configuration diagram showing a humidity detection unit of the salt-bath heat treatment apparatus according to the embodiment of the present invention.

[0033] FIG. 4 is a block diagram showing an electrical configuration of the salt-bath heat treatment apparatus according to the embodiment of the present invention.

[0034] FIG. 5 shows an example of the relationship between the water content rate of a nitrate salt bath and the absolute humidity detected by a humidity sensor.

[0035] FIG. 6 shows an example of a result of water addition control according to the embodiment of the present invention.

[0036] FIG. 7 shows an example of a result of water addition control according to a comparative example.DESCRIPTION OF EMBODIMENTS

[0037] With reference to the accompanying drawings, a salt-bath heat treatment apparatus according to an embodiment of the present invention will now be described.Configuration of Salt-Bath Heat Treatment Apparatus

[0038] With reference to FIGS. 1 to 4, the configuration of the salt-bath heat treatment apparatus according to the embodiment of the present invention will first be specifically described. FIG. 1 is a schematic configuration diagram showing an overall configuration of the salt-bath heat treatment apparatus according to this embodiment. FIG. 2 is a schematic configuration diagram showing a water vapor sampling unit of the salt-bath heat treatment apparatus according to this embodiment. FIG. 3 is a schematic configuration diagram showing a humidity detection unit of the salt-bath heat treatment apparatus according to this embodiment. FIG. 4 is a block diagram showing an electrical configuration of the salt-bath heat treatment apparatus according to this embodiment.

[0039] As shown in FIG. 1, the salt-bath heat treatment apparatus 10 mainly comprises: a salt bath tank 1 which accommodates a nitrate salt bath including a molten nitrate salt salt; a water addition device 2 to add water to the nitrate salt salt bath in the salt bath tank 1; a flushing device 4 to fill a pipe of the water addition device 2 with an inert gas; and a humidity detection device 5 to monitor humidity caused by water vapor generated from the nitrate salt bath. The salt-bath heat treatment apparatus 10 also comprises a control device 100 (see FIG. 4). The salt-bath heat treatment apparatus 10 is a heat treatment system for subjecting a metal member to cooling under isothermal heat treatment such as austempering or martempering, using a nitrate salt bath as a cooling medium.

[0040] The salt bath tank 1 of the salt-bath heat treatment apparatus 10 is provided with a melting heater 1a. When a nitrate salt is put in the salt bath tank 1, it is melted by the melting heater 1a, to create a nitrate salt bath. The temperature of the nitrate salt bath is determined based on the type of nitrate salt, conditions for isothermal heat treatment, and others. The melting point of a nitrate salt varies depending on the blend ratio of nitrate salt raw materials, but the melting point of a commonly-used low-melting-point nitrate salt bath is, e.g., about 140° C. If the nitrate salt bath is used at a temperature around its melting point, there arises problems such as unevenness of the molten state of the nitrate salt bath, and an increase in the amount of a portion of the nitrate salt bath to be taken out due to adhesion to members, leading to a high-frequency supplementation. Thus, it is preferable to set the temperature of a nitrate salt bath used to a value greater than the melting point of the nitrate salt bath, by 40° C. or more (e.g., 180° C. or more). Further, from a viewpoint of suppressing breakage of a salt composition due to thermal decomposition, and reducing salt fumes, it is preferable that the temperature of the nitrate salt bath is set to 430° C. or less. Further, the size of the salt bath tank 1 has, e.g., a length of 3100 mm, a width of 1000 mm, and a depth of 1200 mm. It should be noted that, when conducting experiments for the present invention, the present inventors used a pot having a diameter of 470 mm and a depth of 750 mm, as the salt bath tank 1.

[0041] Next, the water addition device 2 of the salt-bath heat treatment apparatus 10 comprises a tap water pipe 21 into which tap water (industrial water or clean water) is introduced, a tap water supply valve 22 to switch between supply and shutoff of tap water, a flowmeter 23 to adjust the flow rate of tap water, and a pure water production device 24 to produce pure water from tap water. The tap water contains calcium, magnesium, and other components, and when these components react with the nitrate salt bath, the resulting reaction products will be accumulated as impurities in the nitrate salt bath, possibly leading to deterioration in the cooling capacity of the nitrate salt bath, equipment failure, etc. Therefore, the pure water production device 24 is preferably configured to produce pure water having a conductivity of 10 μS / cm or less as measured, e.g., at 25° C., and a TOC (Total Organic Carbon) value of 1000 ppb or less.

[0042] In the salt-bath heat treatment apparatus 10 exemplified in FIG. 1, the water addition device 2 is equipped with the pure water production device 24. However, it should be noted that the water addition device 2 does not have to be equipped with such a pure water production device 24. When the water addition device 2 is not equipped with the pure water production device 24, the water addition device 2 will use tap water or the like, directly, i.e., the water addition device 2 will add tap water or the like to the nitrate salt bath.

[0043] The water addition device 2 further comprises: a water storage tank 25 to store pure water (pure water will hereinafter be simply written as “water”) produced by the pure water production device 24 in the above manner; a water addition pump 27 to pump up the water stored in the water storage tank 25; a water addition pipe 26 through which the water pumped up by the water addition pump 27 passes; and two water addition pipes 26a, 26b into which the water addition pipe 26 is branched on a downstream side of the water addition pump 27.

[0044] Each of the water addition pipes 26a, 26b has a downstream-side portion immersed in the nitrate salt bath in the salt bath tank 1, and a distal end (downstreammost end) of downstream-side portion is opened such that the water in the water storage tank 25 is added to the inside of the nitrate salt bath. For example, the pipe diameter of each of the water addition pipes 26a, 26b is 6 cm. Specifically, it is advisable to immerse each of the water addition pipes 26a, 26b in the nitrate salt bath by a length (immersion depth) equivalent to 10 to 80%, preferably 30 to 70%, of the depth of the nitrate salt bath in the salt bath tank 1 (the distance from an inner bottom surface of the salt bath tank 1 to the liquid level of the nitrate salt bath). This is because since the temperature of the nitrate salt bath is fairly higher than the boiling point of water, if the immersion depth is excessively small, steam explosion (phenomenon that water added to the nitrate salt bath is immediately vaporized) can cause jumping-up of the liquid surface, which is dangerous, and, conversely, if the immersion depth is excessively large (the distal end of each of the water addition pipes 26a, 26b is set at a position excessively close to the inner bottom surface of the salt bath tank 1), there is concern that the steam explosion can cause a situation where the water addition pipes 26a, 26b are deformed, or an impact is applied to the bottom of the salt bath tank 1.

[0045] Here, in the salt-bath heat treatment apparatus 10 exemplified in FIG. 1, the water addition pipes 26a, 26b of the water addition device 2 are immersed in the nitrate salt bath to perform addition of water to the inside of the nitrate salt bath (hereinafter, referred to simply as “salt-bath inside addition”). Instead of this salt-bath inside addition, dripping of water onto the surface of the nitrate salt bath (hereinafter, referred to simply as “salt-bath surface dripping” may be performed. However, for the following reasons, it is preferable to perform the salt-bath inside addition, rather than the salt-bath surface dripping. First, the salt-bath inside addition is more excellent than salt-bath surface dripping in terms of the efficiency of water impregnation. That is, according to the salt-bath inside addition, the nitrate salt bath can be set to a saturated state in water, using a smaller amount of water and within a shorter period of time, as compared with the salt-bath surface dripping. On the other hand, in the salt-bath surface dripping, the value of saturated water content rate becomes lower (i.e., the cooling ability becomes lower) than those in the salt-bath inside addition. This is because most of water dripped to the nitrate salt bath is immediately vaporized on the liquid surface.

[0046] The number of water addition pipes 26a, 26b to be immersed is not limited to two, but one water addition pipe or three or more water addition pipes may be used. In this case, the number of water addition pipes to be used may be determined according to the capacity, shape, and / or the like, of the salt bath tank 1.

[0047] The water addition pipes 26a, 26b are provided, respectively, with flowmeters 28a, 28b for adjusting the flow rate of water (i.e. the addition amount of water (water addition amount)), flow sensors 29a, 29b for monitoring the water addition amount separately from the flowmeters 28a, 28b, and water addition valves 30a, 30b for switching between supply (addition) and cutoff of water from the water addition pipes 26a, 26b to the nitrate salt bath.

[0048] Here, in the water addition device 2, the water addition amount (which shall indicate flow rate; the same shall apply hereinafter) from the water addition pipes 26a, 26b to the nitrate salt bath is controlled by adjusting the output of the water addition pump 27. In this case, from the viewpoint of suppressing the steam explosion due to water addition to the nitrate salt bath as described above (as the water addition amount increases, the degree of this phenomenon tends to become larger), it is advisable to limit the upper limit of the water addition amount to 30 ml / min or less, preferably 20 ml / min or less. Further, it is advisable to determine the number of the water addition pipes, in conformity to the above setting of the upper limit of the water addition amount, and in consideration of the temperature of the nitrate salt bath. It should be noted that the present invention is not limited to adjusting the upper limit of the water addition amount by adjusting the output of the water addition pump 27, but the upper limit of the water addition amount may be adjusted by changing the opening degree of the water addition valves 30a, 30b.

[0049] The water addition device 2 further comprises a liquid level meter 31 to detect a storage amount of water in the tank 25, and a protective valve 32 provided in the water addition pipe 26 to return water in the water addition pipe 26 to the tank 25.

[0050] Next, the flushing device 4 of the salt-bath heat treatment apparatus 10 comprises: a gas supply pipe 41 into which an inert gas such as nitrogen gas is introduced; a gas supply valve 42 to switch between supply and cutoff of the inert gas; a pressure reduction valve 43 to reduce the pressure of the inert gas; gas supply pipes 41a, 41b formed by branching the gas supply pipe 41into two on the downstream side of the pressure reduction valve 43 (a check valve is interposed in each of the gas supply pipes 41a, 41b); and flowmeters 44a, 44b to adjust the flow rate of the inert gas passing through the gas supply pipes 41a, 41b, respectively. Ends of the gas supply pipes 41a, 41b of the flushing device 4 on the downstream side of the check valves are connected to (communicated with) the water addition pipes 26a, 26b on the downstream side of the water adding valves 30a, 30b (and check valves provided in the water addition pipes 26a, 26b), respectively.

[0051] Such a flushing device 4 is operable, when the addition of water by the water addition device 2 is stopped, to open the gas supply valve 42 to supply the inert gas from the gas supply pipes 41a, 41b to the water addition pipes 26a, 26b. This allows the water addition pipes 26a, 26b (more accurately, regions on the downstream side of the check valves provided therein) to be filled with the inert gas. The reason for employing such a configuration is as follows.

[0052] 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, 30b are closed. However, in this case, water remains in portions of the water addition pipes 26a, 26b on the downstream side of the water addition valves 30a, 30b. If the water addition pipes 26a, 26b are left in this state, the molten nitrate salt in the nitrate salt bath can flow back through the water addition pipes 26a, 26b, so that the nitrate salt solidifies, possibly causing clogging. This is because the residual water in portions of the water addition pipes 26a, 26b immersed in the nitrate salt bath flows out into the bath, and thus the insides of the water addition pipes 26a, 26b are brought into a negative pressure state.

[0053] Therefore, when the addition of water is stopped, it is effective to operate the flushing device 4 to supply the inert gas to the water addition pipes 26a, 26b, as described above. By doing this, water remaining in the water addition pipes 26a, 26b is discharged into the bath, and simultaneously the water addition pipes 26a, 26b are filled with the inert gas, thereby more reliably avoiding a situation where the water addition pipes 26a, 26b are brought in a negative pressure state. This makes it possible to prevent the occurrence of a situation where the molten nitrate salt in the nitrate salt bath flows back through the water addition pipes 26a, 26b, and then solidifies, causing clogging.

[0054] In order to prevent the residual water from being discharged to the nitrate salt bath at once, it is advisable to supply the inert gas from the flushing device 4 to the water addition pipes 26a, 26b at a flow rate of about 5 to 30 ml / min. Further, the present invention is not limited to using an inert gas in the flushing device 4, but air or the like may be used.

[0055] Next, the humidity detection device 5 of the salt-bath heat treatment apparatus 10 comprises: a water vapor sampling unit 51 to sample water vapor generated from the nitrate salt bath: a water vapor delivery pipe 52 to deliver the water vapor sampled by the water vapor sampling unit 51; and a humidity detection unit 53 to detect humidity caused by the water vapor delivered from the water vapor delivery pipe 52. In addition, as a component for supplying an inert gas such as nitrogen gas to the water vapor sampling unit 51, the humidity detection device 5 further comprises: a gas supply pipe 54 into which an inert gas is introduced; a gas supply valve 55 to switch the supply and cutoff of the inert gas; a pressure reduction valve 56 to reducer the pressure of the inert gas; and a flowmeter 57 to adjust the flow rate of the inert gas.

[0056] As shown in FIG. 2, the water vapor sampling unit 51 of the humidity detection device 5 comprises: a sampling container 51a in which the water vapor sampled from the nitrate salt bath is temporarily stored; and a float 51c attached to a lower portion of the sampling container 51a to enable the sampling container 51a to float on the liquid surface of the nitrate salt bath. Specifically, the sampling container 51a is formed to have an opening 51b in the bottom thereof (i.e., have an open bottom), and in a hollow shape, and is fitted into a through-hole provided in a central region of the float 51c to extend in an up-down direction. In such a sampling container 51a, the nitrate salt is filled in a lower portion thereof, and an atmosphere gas containing water vapor is taken into an upper portion thereof from the liquid surface of the nitrate salt.

[0057] For example, the sampling container 51a in this embodiment is made of iron, and formed in a shape which comprises: a cylindrical portion having a diameter of 4.2 cm and a height of 16 cm; and a truncated-cylindrical portion provided on the top of the cylindrical portion to have a lower end diameter of 4.9 cm, an upper end diameter of 3.3 cm, and a height of 5.1 cm.

[0058] Further, the gas supply pipe 54 is communicated with a side wall of the truncated-cylindrical portion of the sampling container 51a to supply the inert gas from the gas supply pipe 54, and the water vapor delivery pipe 52 is communicated with the side wall, so that the sampled atmosphere gas (containing water vapor) is delivered to the water vapor delivery pipe 52 together with the inert gas from the gas supply pipe 54. In this case, the sampling container 51a delivers the sampled atmosphere gas to the water vapor delivery pipe 52 through the use of the inert gas supplied from the gas supply pipe 54, i.e., using the inert gas as a so-called carrier gas. The water vapor delivery pipe 52 in this embodiment extends upwardly from an upper end of the truncated-cylindrical portion of the sampling container 51a.

[0059] Further, for example, the float 51c of the water vapor sampling unit 51 is made of iron and formed in a cylindrical shape having a diameter of 20 cm and a height of 10 cm, wherein it is configured to enable the sampling container 51a to float on the liquid surface of the nitrate salt bath in such a manner as to follow rising and lowering of the liquid surface of the nitrate salt bath, and keep a length immersed in the nitrate salt bath (immersion depth) constant. In one example, the float 51c is configured to enable the sampling container 51a to float with an immersion depth of about 50 mm (the immersion depth is preferably set in the range of about 20 to 100 mm).

[0060] Then, as shown in FIG. 3, the humidity detection unit 53 of the humidity detection device 5 comprises a humidity sensor 53a provided on the water vapor delivery pipe 52 to output a signal corresponding to humidity, specifically, absolute humidity of the atmosphere gas delivered from the water vapor delivery pipe 52. Further, the humidity detection unit 53 comprises a fume collection filter 53b to collect volatile components (salt fumes) generated in the nitrate salt bath and taken into the water vapor delivery pipe 52, on the water vapor delivery pipe 52 on the upstream side of the humidity sensor 53a. For example, the fume collection filter 53b is composed of absorbent cotton or the like.

[0061] Further, the humidity detection unit 53 comprises a heater 53c to heat a portion of the water vapor delivery pipe 52 provided with the humidity sensor 53a and the fume collection filter 53b. For example, the heater 53c is composed of a heating wire or the like (FIG. 3 schematically illustrates a heating wire). The heater 53c is provided to keep a portion 53al where measurement by the humidity sensor 53a is performed in the water vapor delivery pipe 52 (hereinafter referred to as “humidity sensor measurement site”) warm at a given temperature. For this keep-warm control by the heater 53c, a temperature sensor may be further provided on the water vapor delivery pipe 52, and feedback control may be employed based on the temperature detected by this temperature sensor.

[0062] A signal (corresponding to absolute humidity) output from the humidity sensor 53a receives influences of the temperature of the humidity sensor measurement site 53a1 and the flow rate of the inert gas from the gas supply pipe 54. Thus, in order to stably detect the above signal, the heater 53c is kept warm at a constant temperature, e.g., within the range of 40 to 70° C., and the inert gas from the gas supply pipe 54 is supplied at a constant flow rate, e.g., within the range of 10 to 1000 l / h, preferably 30 to 500 l / h, more preferably 50 to 250 l / h.

[0063] It should be noted that the present invention is not limited to the aspect in which the absolute humidity is detected by the humidity sensor 53a, and an aspect may be employed in which dew point, frost point, wet-bulb temperature, water vapor partial pressure, mixing ratio, enthalpy, or the like may be detected instead of the absolute humidity (these parameters are also equivalent to the “humidity” in the present invention). Specifically, it is possible to use, as the humidity sensor 53a, a polymer-based humidity sensor, a metal oxide-based humidity sensor, an electrolyte-based humidity sensor, and the like. Further, a measurement method for humidity may be an electrostatic capacitance measurement method or may be an electric resistance measurement method.

[0064] Next, with reference to FIG. 4, an electrical configuration of the salt-bath heat treatment apparatus 10 according to this embodiment will be described. As shown in FIG. 4, the salt-bath heat treatment apparatus 10 is controlled by the control device 100. Detection signals are supplied to this control device 100 from the flow sensors 29a, 29b and the liquid level meter 31 of the water addition device 2, and the humidity sensor 53a of the humidity detection device 5. 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, 30b, and the protective valve 32 of the water addition device 2, the gas supply valve 42 of the flushing device 4, and the gas supply valve 55 of the humidity detection device 5, respectively, to control them.

[0065] For example, the control device 100 is composed of a computer comprising: one or more processors (typically CPU(s)); various programs (including a basic control program such as an OS, and an application program capable of being activated on the OS to realize a specific function) to be interpreted and executed on the one or more processor; and a memory, such as a ROM or a RAM, which stores various data. It should be noted that the present invention is not limited to using the single control device 100 to comprehensively control the salt-bath heat treatment apparatus 10, as shown in FIG. 4. In another embodiment, plural control devices (such as microcontrollers) built in various valves, pumps, and others, separately, may be used.

[0066] Here, major controls performed by the control device 100 in this embodiment will be described. First, the control device 100 controls the output of the water addition pump 27 of the water addition device 2 so as to adjust the addition amount of water to the nitrate salt bath, based on a detection signal detected by the humidity sensor 53a of the humidity detection device 5, corresponding to the absolute humidity of the atmosphere gas including water vapor generated in the nitrate salt bath. Typically, the control device 100 subjects the output of the water addition pump 27 to feedback control, so as to adjust the water addition amount, based on the relationship between the absolute humidity corresponding to the detection signal of the humidity sensor 53a and a given target value. This control of the output of the water addition pump 27 executed by the control device 100 will hereinafter be referred to as “water addition control”. The details of this water addition control will be described later.

[0067] Further, with respect to the water addition device 2, the control device 100 controls the tap water supply valve 22, based on the storage amount of water in the tank 25 corresponding to the detection signal from the liquid level meter 31. Specifically, when the storage amount of water detected by the liquid level meter 31 is less than a given lower limit amount, the control device 100 performs control to open the tap water supply valve 22. By doing this, tap water is supplied to the pure water production device 24, and thus pure water is produced by the pure water production device 24. On the other hand, when the storage amount of water detected by the liquid level meter 31 is equal to or greater than a given upper limit amount, the control device 100 performs control to close the tap water supply valve 22. By doing this, the supply of tap water to the pure water production device 24 is cut off, so that the production of pure water is stopped. By such control of the tap water supply valve 22 based on the liquid level meter 31, the storage amount of water in the tank 25 is set to an amount between the given lower limit amount and the given upper limit amount.

[0068] With respect to the water addition device 2, the control device 100 also controls the water addition valves 30a, 30b on the water addition pipes 26a, 26b, and the protective valve 32, based on the water addition amount corresponding to the detection signal 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 greater than a given value, the control device 100 closes one or both of the water addition valves 30a, 30b and performs control to open the protective valve 32. Thus, when abnormality occurs in the addition of water by the water addition device 2, the addition of water from the water addition pipes 26a, 26b to the nitrate salt bath is stopped, and water in the water addition pipes 26a, 26b is returned to the tank 25 through the protecting valve 32. This allows for protection of the water addition device 2, the salt bath tank 1, etc.

[0069] Further, with respect to the flushing device 4, when the addition of water by the water addition device 2 is stopped, specifically, when the water adding valves 30a, 30b of the water addition device 2 are closed, the control device 100 performs control to open the gas supply valve 42 so as to supply the inert gas from each of the gas supply pipes 41a, 41b to a corresponding one of the water addition pipes 26a, 26b. By doing this, at the time of stopping of the addition of the water, the insides of the water addition pipes 26a, 26b are filled with an inert gas to prevent the occurrence of the situation where the molten nitrate salt flows back through the water addition pipes 26a, 26b, and then solidifies, casing clogging, as described above.Water Addition Control

[0070] Next, the water addition control according to this embodiment will be specifically described. As described above, in this embodiment, the control device 100 controls the output of the water adding pump 27 of the water addition device 2 so as to adjust the addition amount of water to the nitrate salt bath, 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. Although the absolute humidity detected by the humidity sensor 53a is humidity caused by water vapor generated in the nitrate salt bath, according to knowledge of the present inventors, it is strongly correlated with the rate of water contained in the nitrate salt bath (water content rate).

[0071] Specifically, with reference to FIG. 5, the relationship between the humidity of the atmosphere gas containing water vapor generated in the nitrate salt bath and the water content rate of the nitrate salt bath will be described. FIG. 5 is an experimentally obtained graph showing an example of the relationship between the water content rate of the nitrate salt bath (horizontal axis) and the absolute humidity of the atmosphere gas lying over the nitrate salt bath detected by the humidity sensor 53a (vertical axis). Specifically, FIG. 5 shows absolute humidity (g / m3) detected by the humidity sensor 53a at each water content rate, as measured when the water content rate of the nitrate salt bath maintained at 300° C. or 400° C. is set to various values. As indicated by the solid line and the broken line in the figure, it can be seen that there is a correlation (specifically, linear relationship) between the water content rate and the absolute humidity. Therefore, it can be said that the water content rate of the nitrate salt bath can be derived from the absolute humidity of atmosphere gas lying over the nitrate salt bath, based on the correlation therebetween.

[0072] Therefore, in this embodiment, the control device 100 performs the 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 water addition amount by the water addition pump 27, such that the water content rate of the nitrate salt bath becomes equal to or greater than a given value (hereinafter referred to as “target water content rate”). For example, this target water content rate is set on the basis of a water content rate as measured when the nitrate salt bath is in a saturated state (state in which water is added to the nitrate salt bath to the utmost limit). However, since the water content rate as measured when the nitrate salt bath is in the saturated state varies according to the temperature of the nitrate salt bath, it is advisable to set the target water content rate according to the temperature of the nitrate salt bath. In this case, it is advisable to apply a target water content rate to be set according to the temperature of the nitrate salt bath, in consideration of results of pre-experiments, given simulations, and / or the like.

[0073] Specifically, the control device 100 subjects the water addition pump 27 to feedback control, so as to adjust the water addition amount to set the water content rate of the nitrate salt bath to the target water content rate. 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 to increase 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 greater than the target water content rate, the control device 100 controls the water addition pump 27 to reduce the water addition amount. This makes it possible to maintain the water content rate of the nitrate salt bath at the target water content rate. Thus, the nitrate salt bath can be maintained in a state in which its cooling capacity is high. In addition, by reducing the water addition amount when the water content rate is equal to or greater than the target water content rate, as described above, it is possible to save the amount of water to be added to the nitrate salt bath. This makes it possible to extend the length of a cartridge replacement cycle of the pure water production device 24, or to suppress an increase in humidity of an industrial site to improve work environment.

[0074] It should be noted that from the viewpoint of control, an electric signal detected by the humidity sensor 53a needs not be daringly converted to the value of a significant parameter such as absolute humidity or water content rate, and it it is only necessary for the the control device 100 to control the output of the water addition pump 27 so as to increase the water addition amount when the value of a signal detected by the humidity sensor 53a is less than the value of a signal corresponding to the target water content rate, and to control the output of the water addition pump 27 so as to reduce the water addition amount 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. It is preferable to preliminarily calibrate the correlation between the value of the signal detected by the humidity sensor 53a and the water content rate of the nitrate salt bath (see FIG. 4; e.g., linear function approximation), e.g., by preliminarily performing pre-experiments, given simulations, and / or the like, under each condition (e.g., temperature) of the nitrate salt bath.

[0075] Further, the control device 100 preferably controls the water addition pump 27 such that the water addition amount (i.e., water addition speed) becomes equal to or greater than the vaporization amount (i.e., vaporization speed) of water from the nitrate salt bath (water addition amount≥vaporization amount of water). The vaporization amount of water from the nitrate salt bath is an amount corresponding to the temperature of the nitrate salt bath, the area of the liquid surface of the nitrate salt bath, and the number of the water addition pipes 26a, 26b (two in the embodiment illustrated in FIG. 1). Therefore, by performing pre-experiments, given simulations, and / or the like, it is possible to determine the vaporization amount of water from the nitrate salt bath, based on the above parameters. For example, the control device 100 preferably utilizes such a vaporization amount as a lower limit of the water addition amount.Functions and Effects

[0076] Next, the functions and effects of the salt-bath heat treatment apparatus 10 according to this embodiment will be described. In this embodiment, in order to subject a metal member to cooling under isothermal heat treatment, the salt-bath heat treatment apparatus 10 comprises: a salt bath tank 1 which accommodates a nitrate salt bath including a molten nitrate salt; a water addition device 2 to add water to the nitrate salt bath in the salt bath tank 1; a humidity detection device 5 comprising a humidity sensor 53a to output a signal corresponding to humidity caused by water vapor generated from the nitrate salt bath with water added by the water addition device 2; and a control device 100 configured to control the water addition device 2 to adjust an addition amount of water to the nitrate salt bath, based on the signal output from the humidity sensor 53a of the humidity detection device 5.

[0077] In this salt-bath heat treatment apparatus 10, the addition amount of water to the nitrate salt bath is controlled, based on humidity caused by water vapor from the nitrate salt bath, corresponding to the water content rate of the nitrate salt bath. Thus, according to this embodiment, water can be appropriately added to the nitrate salt bath in an amount corresponding to the water content rate of the nitrate salt bath. Therefore, according to this embodiment, it is possible to adequately improve cooling capacity by the nitrate salt bath, and keep the cooling capacity constant, so that it becomes possible to expand the range of size of a member heat-treatable by the isothermal heat treatment.

[0078] In this embodiment, the control device 100 controls the water addition device 2 to set the water content rate of the nitrate salt bath at a given value (e.g., the target water content rate) or more, so that it is possible to set the nitrate salt bath to have a sufficient water content rate, thereby effectively improving the cooling capacity by the nitrate salt bath.

[0079] In this embodiment, the control device 100 controls the water addition device 2 such that the addition amount of water to the nitrate salt bath becomes equal to or greater than the vaporization amount of water from the nitrate salt bath, so that it is possible to adequately ensure the water content rate of the nitrate salt bath.

[0080] In this embodiment, the water addition device 2 comprises water addition pipes 26a, 26b each immersed in the nitrate salt bath, wherein the water addition device is configured to add water from the water addition pipes 26, 26b to the inside of the nitrate salt bath, wherein the salt-bath heat treatment apparatus 10 further comprises a flushing device 4 comprising gas supply pipes 41a, 41b to supply an inert gas to the water addition pipes 26a, 26b, wherein the flushing device 4 is configured to, when the addition of water from the water addition device 2 is stopped, supply the inert gas from the gas supply pipes 41a, 41b to the water addition pipes 26a, 26b to fill the water addition pipes 26a, 26b with the inert gas. Thus, it is possible to suppress a situation where the insides of the water addition pipes 26a, 26b are in a negative pressure state, so that it is possible to prevent the occurrence of a situation where the molten nitrate salt in the nitrate salt bath flows back through the water addition pipes 26a, 26b, and then solidifies, casing clogging.

[0081] In this embodiment, the humidity detection device 5 further comprises: a water vapor sampling unit 51a comprising a sampling container 51a to sample an atmosphere gas lying over the nitrate salt bath and including the water vapor generated from the nitrate salt bath, and a float 51c to enable the sampling container 51a to float on a liquid surface of the nitrate salt bath; and a water vapor delivery pipe 52 which is communicated with the sampling container 51a of the water vapor sampling unit 51 to deliver the atmosphere gas sampled by the sampling container 51a to the humidity sensor 53a. In this way, the sampling container 51a is floated on the liquid surface of the nitrate salt bath by the float 51c, so that it becomes possible for the sampling container 51a to adequately sample water vapor lying over the nitrate salt bath and including water vapor generated from the nitrate salt bath, constantly under the same conditions.

[0082] In this embodiment, the humidity detection device 5 further comprises: a gas supply pipe 54 which is communicated with the sampling container 51a of the water vapor sampling unit 51 to supply an inert gas to the sampling container 51a, wherein the sampling container 51a delivers the sampled atmosphere gas to the humidity sensor 53 through a water vapor delivery pipe 52 through use of the inert gas supplied from the gas supply pipe 54. Thus, it is possible to reliably deliver water vapor sampled by the sampling container 51a, to the humidity sensor 53a, by means of the inert gas supplied from the gas supply pipe 54 to the gas sampling container 51a.

[0083] In this embodiment, the humidity sensor 53a of the humidity detection device 5 is provided on the water vapor delivery pipe 52, wherein the humidity detection device 5 further comprises a heater 53c to keep the water vapor delivery pipe 52 warm at least in a humidity sensor measurement site 53a1 where measurement by the humidity sensor 53a is performed. In this way, the humidity sensor measurement site 53a1 is kept warm by the heater 53c, so that it is possible to stabilize a signal detected by the humidity sensor 53a. This makes it possible to stably perform the water addition control based on the signal detected by the humidity sensor 53a.

[0084] Here, with reference to FIGS. 6 and 7, the effect when the water addition control is performed under the condition that the humidity sensor measurement site 53a1 is kept warm by the heater 53c will be specifically described. FIG. 6 illustrates an example of a result obtained in this embodiment in which the water addition control is performed while the humidity sensor measurement site 53a1 is kept warm by the heater 53c, and FIG. 7 illustrates an example of a result obtained in a comparative example in which the water addition control is performed without using such a heater 53c. In FIGS. 6 and 7, 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 delivery pipe 52 provided with the humidity sensor measurement site 53a.

[0085] In this embodiment, the water addition control is performed such that the absolute humidity of water vapor from the nitrate salt bath heated to 380° C. is maintained at a target humidity (as shown by way of exemplification, an absolute humidity as measured when the nitrate salt bath is in the saturated state). Particularly in this embodiment, the water addition control is performed under the condition that the humidity sensor measurement site 53a1 is kept warm at about 65° C. by the heater 53c. On the other hand, in the comparative example, the heater 53c is stopped while continuing the addition of water.

[0086] As shown in FIG. 6, it can be seen that according to this embodiment, the temperature of the humidity sensor measurement site 53a1 is maintained approximately constant by the heater 53c, while the absolute humidity of the water vapor is maintained at an approximately constant value (target humidity) after the absolute humidity of the water vapor rises. This result is obtained because the temperature of the humidity sensor measurement site 53a1 is maintained constant by the heater 53c, and therefore the absolute humidity detected by the humidity sensor 53a is stabilized.

[0087] In contrast, as shown in FIG. 7, it can be seen that in the comparative example, the temperature of the humidity sensor measurement site 53a1 gradually decreases, while the absolute humidity of the water vapor is not maintained at a constant value, specifically, the absolute humidity increases along with the decrease in the temperature. Generally, since the absolute humidity changes according to the temperature, even if an atmosphere gas containing approximately the same amount of water vapor is delivered to the humidity sensor 53a, the absolute humidity increases when the temperature decreases.

[0088] In view of such a result, the water addition control is performed under the condition that the humidity sensor measurement site 53a1 is kept warm by the heater 53c, as in this embodiment, so that it is possible to stabilize the absolute humidity (signal) detected by the humidity sensor 53a, thereby stably performing the water addition control based on the absolute humidity (signal).Modifications

[0089] In the above embodiment, humidity caused by water vapor generated from the nitrate salt bath is detected by the humidity sensor 53a, and the water addition control is performed based on the detected humidity. However, in one example of modifications, the water addition control may be performed without using such humidity. A change in humidity caused by water vapor generated from the nitrate salt bath when water is added to the nitrate salt bath (e.g. FIG. 6) can be predicted, to some extent, from the history of the amount of water added to the nitrate salt bath. Although such a change in humidity varies according to a temperature state or the like of the nitrate salt bath, a change in humidity according to the temperature state of the nitrate salt bath or the like can also be predicted by preliminarily performing pre-experiments, given simulations, and / or the like.

[0090] Thus, in this modification, the water addition control is performed by estimating the humidity, without detecting the humidity. In this case, since the humidity is not detected, it is not necessary to daringly estimate the humidity, and instead, the water content rate of the nitrate salt bath may be directly estimated. Therefore, in another example of the modifications, the water content rate of the nitrate salt bath may be estimated, based on the temperature state of the nitrate salt bath, the history of the amount of water added to the nitrate salt bath, and / or the like, in consideration of ore-explements, given simulations, and / or the like, and then the water addition control may be performed based on the estimated water content rate. In this modification, the control device 100 is equivalent to the “water content rate estimation device” and the “control device” in the present invention.

[0091] The above embodiment is an exemplification for explaining the present invention, and the present invention is not limited to the embodiment. The present invention may be implemented in various forms without departing from the spirit and scope thereof as set forth in appended claims.LIST OF REFERENCE SIGNS1: salt bath tank

[0093] 2: water addition device

[0094] 4: flushing device

[0095] 5: humidity detection device

[0096] 10: salt-bath heat treatment apparatus

[0097] 26, 26a, 26b: water addition pipe

[0098] 27: water addition pump

[0099] 41, 41a, 41b: gas supply pipe

[0100] 51: water vapor sampling unit

[0101] 51a: sampling container

[0102] 51c: float

[0103] 52: water vapor delivery pipe

[0104] 53: humidity detection unit

[0105] 53a: humidity sensor

[0106] 53b: fume collection filter

[0107] 53c: heater

[0108] 54: gas supply pipe

[0109] 100: control device

Claims

1. A salt-bath heat treatment apparatus for performing a heat treatment of a metal member using a salt bath, the salt-bath heat treatment apparatus comprising:a salt bath tank which accommodates a nitrate salt bath including a molten nitrate salt to cool the metal member under an isothermal heat treatment;a water addition device for adding water to the nitrate salt bath in the salt bath tank;a humidity detection device comprising a humidity sensor to output a signal corresponding to humidity caused by water vapor generated from the nitrate salt bath with water added by the water addition device; anda control device configured to control the water addition device to adjust an addition amount of water to the nitrate salt bath, based on the signal output from the humidity sensor of the humidity detection device.

2. The salt-bath heat treatment apparatus according to claim 1, wherein the control device is configured to control the water addition device to set a water content rate of the nitrate salt bath at a given value or more.

3. The salt-bath heat treatment apparatus according to claim 1, wherein the control device is configured to control the water addition device such that the addition amount of water to the nitrate salt bath becomes equal to or greater than a vaporization amount of water from the nitrate salt bath.

4. The salt-bath heat treatment apparatus according to claim 1, wherein the humidity detection device further comprises:a water vapor sampling unit comprising a sampling container to sample an atmosphere gas lying over the nitrate salt bath and including the water vapor generated from the nitrate salt bath, and a float to enable the sampling container to float on a liquid surface of the nitrate salt bath; anda water vapor delivery pipe which is communicated with the sampling container of the water vapor sampling unit to deliver the atmosphere gas sampled by the sampling container to the humidity sensor.

5. The salt-bath heat treatment apparatus according to claim 1, wherein the humidity detection device further comprises:a water vapor sampling unit comprising a sampling container to sample an atmosphere gas lying over the nitrate salt bath and including the water vapor generated from the nitrate salt bath;a water vapor delivery pipe which is communicated with the sampling container of the water vapor sampling unit to deliver the atmosphere gas sampled by the sampling container to the humidity sensor; anda gas supply pipe which is communicated with the sampling container of the water vapor sampling unit to supply an inert gas to the sampling container,wherein the sampling container delivers the sampled atmosphere gas to the humidity sensor through the water vapor delivery pipe through use of the inert gas supplied from the gas supply pipe.

6. The salt-bath heat treatment apparatus according to claim 4, wherein the humidity sensor of the humidity detection device is provided on the water vapor delivery pipe, and wherein the humidity detection device further comprises a heater to keep the water vapor delivery pipe warm at least in a portion thereof where a measurement by the humidity sensor is performed.

7. The salt-bath heat treatment apparatus according to claim 1,wherein the water addition device comprises a water addition pipe immersed in the nitrate salt bath to add water from the water addition pipe to an inside of the nitrate salt bath, andwherein the salt-bath heat treatment apparatus further comprises a flushing device comprising a gas supply pipe to supply a given gas to the water addition pipe of the water addition device, the flushing device being configured to supply the given gas from the gas supply pipe to the water addition pipe to fill the water addition pipe with the given gas, when the addition of water from the water addition device is stopped.

8. A salt-bath heat treatment apparatus for performing a heat treatment of a metal member using a salt bath, the salt-bath heat treatment apparatus comprising:a salt bath tank which accommodates a nitrate salt bath including a molten nitrate salt to cool the metal member under an isothermal heat treatment;a water addition device for adding water to the nitrate salt bath in the salt bath tank;a water content rate estimation device configured to estimate a water content rate of the nitrate salt bath with water added by the water addition device; anda control device configured to control the water addition device to adjust an addition amount of water to the nitrate salt bath, based on the water content rate estimated by the water content rate estimation device.

9. The salt-bath heat treatment apparatus according to claim 5, wherein the humidity sensor of the humidity detection device is provided on the water vapor delivery pipe, and wherein the humidity detection device further comprises a heater to keep the water vapor delivery pipe warm at least in a portion thereof where a measurement by the humidity sensor is performed.

Images