Vacuum furnace

Abstract

To provide a vacuum furnace capable of heat treatment in which an amount of dissolved gas is increased in cooling oil.SOLUTION: A vacuum furnace includes a heating chamber for heating a workpiece, a quenching chamber having an oil tank for storing cooling oil in which dissolved air is reduced in advance and cooling the workpiece heated in the heating chamber by immersing it in the cooling oil, and feeding means for feeding inert gas to be dissolved in the cooling oil to the cooling oil while the workpiece is housed in the heating chamber.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a vacuum furnace. [Background technology]

[0002] During heat treatment of workpieces such as metal parts, the formation of an oxide film on the workpiece surface can affect quality. Therefore, techniques for suppressing oxide film formation are known. Patent Document 1 points out the increased cost of vacuum furnaces and proposes a technique for reducing oxygen in cooling oil when performing heat treatment in an air atmosphere. However, the technique in Patent Document 1 is considered insufficient to suppress oxide film formation because heat treatment is performed in an air atmosphere. Generally, a vacuum furnace that performs heat treatment under reduced pressure is advantageous for suppressing oxide film formation. Patent Document 2 discloses an apparatus for performing heat treatment under reduced pressure, which injects gas onto the workpiece while immersing it in cooling oil. Patent Document 3 also discloses an apparatus for heat treatment under reduced pressure, which discloses a method (sometimes called the hyperpascal method) in which cooling oil is injected into a quenching chamber during the vapor film stage of quenching and the pressure in the quenching chamber is reduced during the boiling stage.

[0003] On the other hand, in a vacuum furnace, when new oil is used as the cooling oil, the quenching chamber containing the new oil is depressurized for a certain period of time (for example, overnight) to sufficiently reduce the amount of air dissolved in the new oil. One of the purposes of this is to prevent the formation of an oxide film on the workpiece surface. The new oil is then repeatedly used to cool the workpieces, but in a vacuum furnace, the quenching chamber is depressurized when it is connected to the atmosphere to load and unload the workpieces, so the amount of air dissolved in the cooling oil is maintained at a relatively low level.

[0004] However, if the amount of gas dissolved in the cooling oil is low, the cooling performance may decrease. Non-Patent Document 1 points out that cooling performance decreases when the amount of dissolved gas is low, and explains that an experiment using a beaker confirmed that cooling performance can be restored by stirring the cooling oil and increasing the amount of dissolved gas. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 3151681 [Patent Document 2] International Publication No. 2018 / 123246 Brochure [Patent Document 3] Patent No. 6533146 [Non-patent literature]

[0006] [Non-Patent Document 1] Katsumi Ichiya, Behavior of Heat Treatment Oil under Reduced Pressure, "Industrial Heating", Japan, Japan Industrial Furnace Association, November 15, 2006, Vol. 43, No. 6, pp. 20-26 Summary of the Invention [Problem to be solved by the invention]

[0007] Non-Patent Document 1 concludes that because it takes time for the amount of dissolved gas to increase due to stirring of the cooling oil, even if the cooling oil is repeatedly decompressed and stirred in a vacuum furnace, the effect on the cooling performance due to fluctuations in the amount of dissolved gas is small, and that this does not pose a problem in terms of leveling the performance of the cooling oil. However, from the perspective of vacuum furnace development, there is room for improvement in heat treatment performance by improving the performance of the cooling oil.

[0008] An object of the present invention is to provide a vacuum furnace capable of performing heat treatment by increasing the amount of gas dissolved in the cooling oil. [Means for solving the problem]

[0009] According to the present invention, a heating chamber for heating the workpiece; a quenching chamber having an oil tank for storing cooling oil in which dissolved air has been reduced in advance, in which the workpiece heated in the heating chamber is immersed in the cooling oil to be cooled; a supply means for supplying an inert gas to be dissolved in the cooling oil to the cooling oil while the workpiece is accommodated in the heating chamber. A vacuum furnace is provided. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide a vacuum furnace that is capable of performing heat treatment by increasing the amount of gas dissolved in the cooling oil. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is an explanatory diagram of a vacuum furnace according to an embodiment of the present invention. [Figure 2] An explanatory diagram of the quenching chamber of the vacuum furnace in Figure 1. [Figure 3] 2A and 2B are flowcharts showing an example of processing by the control unit, and 2C is an explanatory diagram of the operation of the vacuum furnace of FIG. 1. [Figure 4] 10 is a flowchart showing an example of processing by a control unit. [Figure 5] (A) to (F) are explanatory diagrams of the operation of the vacuum furnace in Figure 1. [Figure 6] (A) to (D) are explanatory diagrams of the operation of the vacuum furnace in Figure 1. [Figure 7] 4 is a timing chart showing the states of the heating chamber and the quenching chamber during heat treatment. [Figure 8] FIG. 4 is an explanatory diagram showing an example of the supply timing of an inert gas. [Figure 9] 10 is a flowchart showing another example of processing by the control unit. [Figure 10] 10 is a timing chart showing another state of the heating chamber and the quenching chamber during heat treatment. [Figure 11] 10(A) to 10(C) are explanatory views showing another example of the supply unit. [Figure 12] 10A and 10B are explanatory views showing another example of the supply unit. [Figure 13] 10 is a timing chart showing another state of the heating chamber and the quenching chamber during heat treatment. [Figure 14] FIG. 4 is an explanatory diagram of a vacuum furnace according to another embodiment of the present invention. [Figure 15] FIG. 15 is an explanatory diagram of the operation of the vacuum furnace of FIG. [Figure 16] FIG. 4 is an explanatory diagram of a vacuum furnace according to another embodiment of the present invention. [Figure 17] (A) is an explanatory diagram of the test specimen, and (B) is an explanatory diagram of the test contents. [Figure 18] (A) to (C) are diagrams showing the experimental results. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, the embodiments will be described in detail with reference to the accompanying drawings. Note that the following embodiments do not limit the scope of the invention as claimed, and not all combinations of features described in the embodiments are necessarily essential to the invention. Two or more of the features described in the embodiments may be arbitrarily combined. Furthermore, the same reference numerals are used for the same or similar components, and redundant explanations will be omitted.

[0013] First Embodiment <Vacuum furnace configuration> FIG. 1 is an explanatory diagram of a vacuum furnace 1 according to one embodiment of the present invention, and FIG. 2 is an explanatory diagram of a quenching chamber 3 of the vacuum furnace 1, where FIG. 1 shows the structure of the vacuum furnace 1 as viewed from the side, and FIG. 2 shows the structure of the vacuum furnace 1 as viewed from the front.

[0014] The vacuum furnace 1 is equipped with a heating chamber 2 and a quenching chamber 3 arranged in series, and is a facility for performing heat treatment on a workpiece W. The workpiece W is, for example, a plurality of machine parts stored in a basket. The heating chamber 2 is partitioned by a housing 20, and an insulated container 21 made of a heat insulating material is provided inside the heating chamber 2. The insulated container 21 is kept airtight, but has a door 21a on the side of the quenching chamber 3, and when the door 21a is open, the workpiece W can be carried into the insulated container 21 and removed from the insulated container 21 to the outside. The door 21a is moved by an actuator (not shown).

[0015] A transport unit 22 for transporting the workpiece W is provided at the bottom of the insulated container 21. The transport unit 22 is a unit that forms a hearth and moves the workpiece W inside and outside the insulated container 21, and is a roller conveyor in this embodiment. However, the transport unit 22 is not limited to a roller conveyor, and may be a chain conveyor or a fork-type transport robot.

[0016] A plurality of heaters 23 are also provided inside the insulated container 21. The inside of the insulated container 21 can be heated by the heaters 23, and the workpiece W stopped on the transport unit 22 can be subjected to heat treatment. A circulation fan 24 is provided in the heating chamber 2. The blades of the circulation fan 24 are arranged inside the insulated container 21, and can circulate the gas inside the insulated container 21.

[0017] A carburizing gas supply unit 7 is provided in the heating chamber 2. The carburizing gas supply unit 7 includes a gas reservoir 70 and a control valve V5. Compressed gas for carburizing (e.g., acetylene gas) is stored in the gas reservoir 70, and carburizing gas can be supplied into the insulated container 21 by opening the control valve V5.

[0018] The quenching chamber 3 is partitioned by a housing 30, and an opening 31a is formed in the wall on the front side thereof as an opening for carrying in and out the workpiece W, and an opening 31b is formed in the partition between the quenching chamber 3 and the heating chamber 2 for transporting the workpiece W between the quenching chamber 3 and the heating chamber 2. The opening 31a is opened and closed by a door 34, and the opening 31b is opened and closed by a door 35. The doors 34 and 35 are moved by the drive of an actuator (not shown).

[0019] The quenching chamber 3 includes an upper transfer chamber 31 and a lower oil tank 32. Cooling oil is stored in the oil tank 32. The transfer chamber 31 is provided with a transfer unit 33. In this embodiment, the transfer unit 33 is a roller conveyor. However, the transfer unit 33 is not limited to a roller conveyor and may be a chain conveyor.

[0020] The transport unit 33 is raised and lowered by a lifting unit 36. The lifting unit 36 ​​includes an actuator 36a and a hoisting device 36b that connects the actuator 36a to the transport unit 33. The lifting unit 36 ​​raises and lowers the transport unit 33 between the transport position shown by the dashed lines in FIG. 1 and FIG. 2 and the immersion position shown by the solid lines in FIG. 2. The hoisting device 36b is, for example, a chain, and the actuator 36a is a fluid cylinder that advances and retracts the hoisting device. The workpiece W on the transport unit 33 is immersed in the cooling oil when the transport unit 33 descends to the immersion position. The lifting unit 36 ​​can also continuously raise and lower the transport unit 33 (i.e., the workpiece W) over a short distance near the immersion position. The oil tank 32 is equipped with a heater 37, and the temperature of the cooling oil 32a can be adjusted by the heat generated by the heater 37.

[0021] The oil vat 32 is provided with a plurality of circulation units 39. Each circulation unit 39 is an agitation unit that includes a drive source 39a, such as a motor, provided outside the oil vat 32 and blades 39b provided inside the oil vat 32, and agitates the cooling oil 32a. Within the oil vat 32, a plurality of L- or J-shaped circulation passages 38 are formed by passage walls 38w. Each circulation passage 38 is a cylindrical passage having an inlet 38a and an outlet 38b. The inlet 38a is located at the top of the oil vat 32, and the outlet 38b is located at the bottom of the oil vat 32, particularly below the transfer unit 33 at the immersion position.

[0022] The blades 39b are disposed in the circulation passage 38, and their rotation generates a circulation flow from the inlet 38a to the outlet 38b as shown by the arrows in Figure 2. The cooling oil exiting from the outlet 38b causes the cooling oil around the workpiece W to flow, improving the cooling performance of the workpiece W.

[0023] The vacuum furnace 1 is equipped with a decompression unit 5 and an inert gas supply unit 6. The decompression unit 5 has a vacuum pump 50 and control valves V1 and V2. By operating the vacuum pump 50 and opening the control valve V1, the pressure in the quenching chamber 3 can be reduced, for example, the quenching chamber 3 can be evacuated. Furthermore, by opening the control valve V2, the pressure in the insulated container 21 can be reduced, for example, the insulated container 21 can be evacuated. The vacuum pump 50 is shared by the heating chamber 2 and the quenching chamber 3. However, a configuration in which each chamber is provided with its own vacuum pump may also be used.

[0024] The inert gas supply unit 6 is a unit that supplies an inert gas such as nitrogen and includes a gas supply source 60 and control valves V3, V4, and V6. The gas supply source 60 is, for example, a tank that stores compressed inert gas. When the control valve V3 is opened, the inert gas is supplied from the gas supply source 60 to the transfer chamber 31 of the quenching chamber 3, thereby restoring the pressure in the quenching chamber 3. When the control valve V4 is opened, the inert gas is supplied from the gas supply source 60 to the insulated container 21 of the heating chamber 2, thereby restoring the pressure in the insulated container 21.

[0025] In order to reduce the amount of oxygen dissolved in the cooling oil 32a, the dissolved air in the cooling oil 32a is reduced by decompression in advance, reducing the amount of dissolved gas. As a result, the cooling performance of the cooling oil 32a is reduced. When the control valve V6 is opened, an inert gas is supplied to the cooling oil 32a. By supplying the inert gas to the cooling oil 32a and dissolving it, the amount of dissolved gas in the cooling oil 32a is increased, improving the performance of the cooling oil 32a. In this embodiment, the gas supply source 60 is shared by the transfer chamber 31, the oil tank 32, and the insulated container 21. However, a configuration in which each chamber has its own individual gas supply source may also be used.

[0026] An injection section 61 is provided at the tip of the pipe leading from the control valve V6 into the oil tank 32. The inert gas is injected from the injection section 61 into the cooling oil 32a. By injecting the inert gas into the cooling oil 32a, the efficiency of dissolution can be improved. The injection section 61 is provided with a micronization section 62 that micronizes the bubbles of the inert gas injected from the injection section 61. The micronization section 62 is made of, for example, a porous material. The micronization section 62 may be a mechanism that generates microbubbles or nanobubbles. By micronizing the bubbles of the inert gas, the efficiency of dissolution in the cooling oil 32a can be improved.

[0027] The location where the inert gas is supplied to the cooling oil 32a can be selected at any location, but in this embodiment, it is within the circulation passage 38. Injecting the inert gas into the circulation passage 38 allows the inert gas to mix with the circulating flow of the cooling oil 32a, improving the dissolution efficiency of the inert gas. Furthermore, the circulating flow is a downward flow from the inlet 38a to the outlet 38b, flowing away from the liquid surface (oil surface) of the cooling oil 32a. Injecting the inert gas into this flow prevents the inert gas from prematurely escaping from the oil surface to the cooling oil 32a. Furthermore, in this embodiment, the inert gas is injected into the circulation passage 38 at a location upstream of the blades 39b in the flow direction of the circulating flow. After injection, the inert gas is mixed with the cooling oil 32a by the blades 39b, improving the dissolution efficiency of the inert gas in the cooling oil 32a. The circulation unit 39 may operate continuously, or may operate only during quenching of the workpiece W and during supply of the inert gas to the cooling oil 32a.

[0028] The control unit 9 automatically controls the vacuum furnace 1. The control unit 9 includes a processing unit, a memory unit, and an input / output interface. The processing unit is a processor such as a CPU, which executes programs stored in the memory unit. The memory unit is a storage device such as a ROM, RAM, or hard disk, which stores information necessary for control in addition to the programs executed by the processing unit. The input / output interface sends and receives signals between the processing unit and external devices.

[0029] The input / output interface receives detection results from a group 90 of sensors, such as sensors that measure the pressure and temperature of each chamber and sensors that detect the transport position of the workpiece W, and the processing unit outputs control signals to each actuator, such as the motor of the vacuum furnace 1 and fluid devices such as control valves, based on the input detection results, to drive them. An operation panel 91 is connected to the control unit 9. The operation panel 91 is, for example, a touch panel. The user can use the operation panel 91 to give instructions for various settings and operations of the vacuum furnace 1.

[0030] <Control example> The following describes an example of control processing executed by the processing unit of the control unit 9. Unless otherwise specified, each control valve and each door is closed.

[0031] <Settings> FIG. 3(A) is a flowchart showing an example of a process for accepting operation settings made by a user. In S1, a setting screen is displayed on the operation panel 91. The setting screen displays various setting items, for example, in a menu format. The user can call up the desired setting item and input the setting content. An example of an operation setting that can be set by the user is the condition for supplying inert gas from the injection unit 61 to the cooling oil 32a. Examples of this supply condition include the timing of supplying the inert gas, the supply time, or the supply mode (continuous, intermittent), etc.

[0032] In S2, the user's input regarding the setting items is accepted via the operation panel 91. In S3, the setting contents input in S2 are saved in the memory unit. This completes the process. The vacuum furnace 1 will then operate according to the saved setting contents.

[0033] <Vacuuming new oil> When the cooling oil 32a is new oil, it contains a large amount of dissolved oxygen. Therefore, as a preparatory process for heat treatment of the workpiece W, the air dissolved in the cooling oil is reduced in advance by depressurizing the pressure using the depressurizing unit 5. Figure 3(B) shows an example of processing executed by the processing section of the control unit 9, which is executed in response to a user's instruction. Figure 3(C) is an explanatory diagram of the operation of the vacuum furnace 1.

[0034] In S11, a start command from the user is accepted. The process begins with new oil contained in the oil tank 32. In S12, the vacuum pump 50 is operated and the control valve V1 is opened to evacuate the quenching chamber 3. As shown schematically in FIG. 3(C), the gas inside the quenching chamber 3 is exhausted. The pressure inside the quenching chamber 3 is reduced to, for example, less than 10 Pa. The oxygen dissolved in the new oil is desorbed and exhausted outside the furnace by the decompression unit 5.

[0035] In S13, it is determined whether a termination condition is met. The termination condition may be, for example, the passage of time or a user-specified termination time. In the case of the passage of time, for example, half a day to a day, this may be sufficient to remove dissolved air from the cooling oil 32a. If the termination condition is met, the process proceeds to S14, where the control valve V1 is closed and the vacuum pump 50 is stopped to terminate the evacuation.

[0036] <Example of heat treatment> An example of a series of heat treatments for the workpiece W will be described. Fig. 4 shows an example of processing executed by the processing section of the control unit 9, and Figs. 5(A) to 6(D) are explanatory diagrams of the operation of the vacuum furnace 1. In S21, the workpiece W is carried into the quenching chamber 3. Fig. 5(A) shows the operation of the vacuum furnace 1 at this time. After the inert gas supply unit 6 supplies inert gas to the quenching chamber 3 to bring the quenching chamber 3 to atmospheric pressure, the door 34 is opened and the workpiece W is carried into the quenching chamber 3 from outside the furnace. At this time, the quenching chamber 3 is open to the atmosphere.

[0037] In S22, the quenching chamber 3 is evacuated by the decompression unit 5. Figure 5(B) shows the operation of the vacuum furnace 1 at this time. The vacuum pump 50 is operated and the control valve V1 is opened to reduce the pressure inside the quenching chamber 3. The pressure inside the quenching chamber 3 is reduced to, for example, less than 10 Pa. The air that entered the quenching chamber 3 when the workpiece W was loaded is exhausted to the outside of the furnace. At this time, the air dissolved in the cooling oil 32a is also degassed and exhausted to the outside of the furnace, which prevents the increase of dissolved oxygen in the cooling oil 32a due to exposure to the atmosphere when the workpiece W was loaded.

[0038] In S23, the workpiece W is transported from the quenching chamber 3 to the heating chamber 2. Figure 5(C) shows the operation of the vacuum furnace 1 at this time. The doors 35 and 21a are opened, and the transport units 33 and 22 are driven to transport the workpiece W to the heating chamber 2.

[0039] Next, in step S24, the heating chamber 2 is evacuated by the decompression unit 5, and then heat treatment is performed in step S25. In step S26, the quenching chamber 3 is processed by supplying inert gas to the transfer chamber 31 by the inert gas supply unit 6 to increase the pressure inside the quenching chamber 3. Figure 5(D) shows the operation of the vacuum furnace 1 at this time.

[0040] The vacuum pump 50 is operated, and the control valve V2 is opened to reduce the pressure inside the heating chamber 2. Air may have entered the heating chamber 2 when the workpiece W was transported therein, so this air is exhausted to the outside of the furnace. The pressure inside the heating chamber 2 is reduced to, for example, less than 10 Pa, but heating by the heater 23 begins at approximately 50 Pa. In parallel, the control valve V3 is opened to supply inert gas to the transport chamber 31 using the inert gas supply unit 6, thereby increasing the pressure inside the quenching chamber 3. The increased pressure here is intended to suppress degassing of the inert gas and promote dissolution when the inert gas is subsequently supplied to the cooling oil 32a.

[0041] In parallel with the heat treatment in the heating chamber 2 (S25), in S27, the control valve V3 is closed to stop the supply of inert gas to the transfer chamber 31, while the control valve V6 is opened to supply inert gas to the cooling oil 32a. Figure 5(E) shows the operation of the vacuum furnace 1 at this time. The amount of dissolved gas can be increased while the amount of dissolved oxygen in the cooling oil 32a remains low. The heat treatment in the heating chamber 2 includes a step of opening the control valve V5 using the carburizing gas supply unit 7 to supply carburizing gas into the insulated container 21.

[0042] Upon completion of the heat treatment (S25) in the heating chamber 2, in S28 the control valve V6 is closed to terminate the supply of inert gas to the cooling oil 32a. In S29, to balance the air pressure between the heating chamber 2 and the quenching chamber 3 (for example, a pressure slightly lower than atmospheric pressure), the control valves V3 and V4 are opened and inert gas is supplied to the transfer chamber 31 and the insulated container 21 by the inert gas supply unit 6. Figure 5(F) shows the operation of the vacuum furnace 1 at this time, with inert gas being supplied to both the transfer chamber 31 and the insulated container 21. The supply of inert gas to the cooling oil 32a has now terminated.

[0043] In S30, the workpiece W is transported from the heating chamber 2 to the quenching chamber 3. Figure 6(A) shows the operation of the vacuum furnace 1 at this time. Doors 21a and 35 are opened, and the transport units 22 and 33 are driven to transport the workpiece W to the quenching chamber 2. Once the transport of the workpiece W is complete, doors 21a and 35 are closed, and the insulated container 21 is evacuated by the decompression unit 5.

[0044] In S31, the workpiece W is quenched. Figure 6(B) shows the operation of the vacuum furnace 1 at this time. The lifting unit 36 ​​lowers the transport device 33 together with the workpiece W to the immersion position. The workpiece W is immersed in the cooling oil 32a and rapidly cooled.

[0045] When quenching is complete, the lifting unit 36 ​​raises the transfer device 33 together with the workpiece W to the transfer position, and the workpiece W is carried out of the furnace in S32. Figures 6(C) and 6(D) show the operation of the vacuum furnace 1 at this time. As shown in Figure 6(C), the control valve V3 is opened, and the inert gas supply unit 6 supplies inert gas to the transfer chamber 31, restoring the pressure in the quenching chamber 3 to atmospheric pressure. Thereafter, as shown in Figure 6(D), the door 34 is opened, and the transfer device 33 carries the workpiece W out of the furnace. This completes the heat treatment of the workpiece W.

[0046] In S33, it is determined whether or not there is a next workpiece W to be processed, and if there is a next workpiece W, the process returns to S21 and the same process is repeated.

[0047] The state of the vacuum furnace 1 during the heat treatment in the process shown in Figure 4 will be described with reference to Figure 7. Figure 7 is a timing chart showing the timing of supplying inert gas to the cooling oil 32a ("inert gas supply"), pressure changes in the quenching chamber 3 ("quenching chamber pressure"), the timing of depressurization in the quenching chamber 3 ("quenching chamber depressurization"), pressure changes in the heating chamber 2 ("heating chamber pressure"), and the timing of depressurization in the heating chamber 2 ("heating chamber depressurization"). Figure 7 assumes that after the workpiece is loaded, the heat treatment in the heating chamber 2 proceeds through the steps of heating and soaking → carburizing → diffusion → temperature reduction → soaking, and then the workpiece is loaded into the quenching chamber 3, oil-cooled, and then removed from the furnace. Note that "loading the workpiece" here refers to the transport of the workpiece W from the quenching chamber 3 to the heating chamber 2.

[0048] In "Workpiece Loading," the air pressure in the quenching chamber 3 is reduced to pressure P1 (for example, less than 10 Pa) by vacuuming in S22. At time t1, the workpiece W is transported from the quenching chamber 3 to the heating chamber 2. Once the workpiece W has been transported to the heating chamber 2, doors 35 and 21a are closed, and then the heating chamber 2 is vacuumed in S24. The heating chamber 2 is basically always depressurized to a vacuum state (for example, less than 10 Pa), but since there is a possibility that a small amount of air may have entered the heating chamber 2 when the workpiece W is transported from the quenching chamber 3 to the heating chamber 2, the heating chamber 2 is vacuumed (degassed) just to be sure.

[0049] After the doors 35 and 21a are closed, the air pressure inside the quenching chamber 3, which has been reduced to pressure P1 (for example, less than 10 Pa) by evacuation in S22, is increased to pressure P2 in S26 to promote dissolution of the inert gas into the cooling oil 32a. The pressure P2 is, for example, in the range of 1 kPa to 9000 kPa, preferably 10 kPa to 2000 kPa, and more preferably 30 kPa to 90 kPa. In the example of FIG. 7, pressure P2 is assumed to be lower than atmospheric pressure (about 30 kPa).

[0050] When the air pressure inside the heating chamber 2 drops to 50 Pa or less, the heater 23 is driven to start heating the workpiece W. When the temperature inside the heating chamber 2 reaches a predetermined temperature, the heater 23 is driven to maintain that temperature and perform heat equalization. Meanwhile, in parallel with the start of heating the workpiece W, in the quenching chamber 3, the inert gas is started to be added to the cooling oil 32a. Because the air pressure inside the quenching chamber 3 has been increased to pressure P2, dissolution of the inert gas into the cooling oil 32a is promoted more rapidly than in the state of less than 10 Pa immediately after evacuation.

[0051] While inert gas is being supplied to the cooling oil 32a, the inert gas that is not dissolved in the cooling oil 32a increases the air pressure in the transfer chamber 31. Therefore, the pressure reduction unit 5 periodically reduces the pressure in the quenching chamber 3 to maintain the air pressure in the quenching chamber 3 at pressure P2. While the increase in air pressure in the quenching chamber 3 itself does not pose any particular problems, when the workpiece W is transferred from the heating chamber 2 to the quenching chamber 3, the pressure in the heating chamber 2 (particularly the insulated container 21) and the quenching chamber 3 must be restored to pressure P3 (e.g., 80 to 90 kPa) to balance the air pressure. To facilitate air pressure control in the quenching chamber 3, the air pressure in the quenching chamber 3 is maintained at pressure P2. In Figure 7, the "quenching chamber pressure" pulsates during the "inert gas supply" period. This represents the increase in air pressure in the transfer chamber 31 due to the inert gas that is not dissolved in the cooling oil 32a, and the reduction in the air pressure in the quenching chamber 3 due to the decompression unit 5.

[0052] During the heat treatment, the inside of the insulated container 21 is maintained at a vacuum state (less than 10 Pa). In this embodiment, a single vacuum pump 50 is used to reduce the pressure in each chamber. If the capacity of the vacuum pump 50 is small, it may be difficult to reduce the pressure in multiple chambers simultaneously. Therefore, the decompression timing of the "quenching chamber decompression" and the "heating chamber decompression" do not overlap, and a decompression unit 5 is used to alternately reduce the pressure in the two chambers.

[0053] During the carburizing process, carburizing gas is supplied to the insulated container 21 by the carburizing gas supply unit 7, causing the air pressure to increase. To control the air pressure inside the insulated container 21, the decompression unit 5 is used only to decompress the insulated container 21 during the carburizing process. For this reason, the supply of inert gas to the cooling oil 32a is temporarily stopped during the carburizing process. Of course, the supply of inert gas to the cooling oil 32a may also be continued during the carburizing process.

[0054] In the temperature-lowering process after the diffusion process, the operation of some of the heaters 23 is stopped and the temperature inside the insulated container 21 is gradually lowered to a predetermined temperature. After the subsequent soaking process, the heat treatment in the heating chamber 2 is completed. After that, in order to transport the workpiece W from the heating chamber 2 to the quenching chamber 3, the quenching chamber 3 and the heating chamber 2 are each restored to pressure P3. Once the workpiece W is transported from the heating chamber 2 to the quenching chamber 3, it is immersed in cooling oil 32a (oil cooling). The workpiece is then removed from the furnace. The heating chamber 2 is again evacuated by the decompression unit 5.

[0055] It takes time to dissolve the inert gas in the cooling oil 32a and increase the amount of dissolved gas. In this embodiment, while the workpiece is in the heating chamber 2, that time is used to supply the inert gas to the cooling oil 32a, thereby increasing the amount of dissolved gas. In this embodiment, the supply of the inert gas to the cooling oil 32a is carried out over the heating / soaking, diffusion, temperature reduction, and soaking steps. This ensures a supply time of, for example, about 60 to 120 minutes, which, combined with the direct injection of the inert gas from the injection unit 61 onto the cooling oil 32a, can sufficiently increase the amount of dissolved gas in the cooling oil 32a.

[0056] Second Embodiment Another example of the timing of supplying the inert gas to the cooling oil 32a will be described with reference to Fig. 8. As described above, it takes time to dissolve the inert gas in the cooling oil 32a and increase the amount of dissolved gas, so the user can set an appropriate supply timing depending on the time of the heat treatment, etc.

[0057] The example of EX1 in FIG. 8 shows an example in which an inert gas is supplied to the cooling oil 32a during the first half of the heat treatment (a period close to the timing of carrying the workpiece W into the heating chamber 2, for example, until heating and soaking). The example of EX2 in FIG. 8 shows an example in which an inert gas is supplied to the cooling oil 32a during the second half of the heat treatment (a period close to the timing of carrying the workpiece W out of the quenching chamber 3, for example, until diffusion, cooling, and soaking). Either of the timings of EX1 and EX2 can be used, but considering the possibility of degassing of dissolved gas in the cooling oil 32a, the example of EX2, which is a period close to quenching, is advantageous.

[0058] The example of EX3 in FIG. 8 shows an example in which the inert gas is supplied to the cooling oil 32a in two separate time periods, the first half of the heat treatment and the second half of the heat treatment. This is the same supply timing as in the first embodiment, and is suitable for cases where there is a problem with the supply of inert gas during the heat treatment. The example of EX4 in FIG. 8 shows an example in which the inert gas is supplied to the cooling oil 32a in the middle of the heat treatment. This is suitable for cases where there is a problem with the supply of inert gas during the first or second half of the heat treatment.

[0059] In the example of EX5, the supply of inert gas to the cooling oil 32a starts from the heat treatment stage, and the supply ends just before the workpiece W is transported to the quenching chamber 3 and immersed in the cooling oil 32a. This makes it easier to ensure a longer supply time.

[0060] In the examples described above, inert gas is not supplied to the cooling oil 32a while the workpiece W is being quenched (the time period when the workpiece W is not immersed in the cooling oil 32a). This has the advantage of stabilizing the properties of the cooling oil 32a during quenching. In contrast, the supply timing shown in Figure 8 can also be adopted. In the example shown, the supply of inert gas to the cooling oil 32a begins at the heat treatment stage, and continues while the workpiece W is being quenched (while the workpiece W is immersed in the cooling oil 32a). This makes it easier to ensure a longer supply time.

[0061] In any case, by using the time during which the workpiece W is present in the heating chamber 2 to supply an inert gas to the cooling oil 32a, it is possible to secure a time for increasing the amount of dissolved gas in the cooling oil 32a.

[0062] <Third Embodiment> In the example of FIG. 4 of the first embodiment, an inert gas was supplied to the cooling oil 32a at S27 every time the workpiece W (or every time the quenching chamber 3 was opened to the atmosphere at S21). This is advantageous in terms of sufficiently maintaining the amount of dissolved gas in the cooling oil 32a.

[0063] However, an inert gas may be supplied to the cooling oil 32a for each of a plurality of workpieces W. FIG. 9 is a flowchart showing an example thereof. Only the processing different from the example of FIG. 4 will be described. In this example, an inert gas is supplied to the cooling oil 32a every time three workpieces W are processed. The number of processed workpieces W may be set by the user.

[0064] After transporting the workpiece W to the heating chamber 2 at S23, the processes of S41 to S43 are performed as the processing on the quenching chamber 3 side. At S41, it is determined whether or not the variable N indicating the number of times the workpiece W has been processed is 3. If N = 3, the process proceeds to S42; if N ≠ 3, the process proceeds to S43. At S42, the variable N is reset to 0 and the process proceeds to S26. At S43, the variable N is incremented by one, and the processes of S26 to S28 are skipped and the process proceeds to S29.

[0065] In this embodiment, since an inert gas is supplied to the cooling oil 32a for each of a plurality of workpieces W, it is possible to prevent unnecessary supply of the inert gas.

[0066] <Fourth Embodiment> In the first embodiment, an example in which the pressure in the quenching chamber 3 is maintained at the pressure P2 (<P3) during the supply of the inert gas to the cooling oil 32a has been described, but the pressure P2 may be higher than the pressure P3 during the repressurization. When the pressure P2 is higher, the dissolution of the inert gas in the cooling oil 32a is promoted.

[0067] Fig. 10 is a timing chart showing the state of the vacuum furnace 1 during heat treatment in this embodiment, which replaces Fig. 7. Differences from the example in Fig. 7 will be described.

[0068] In this embodiment, the air pressure inside the quenching chamber 3, which has been reduced to pressure P1 (for example, less than 10 Pa) by evacuation in S22 of FIG. 4, is increased to pressure P2' in S26 to promote dissolution of the inert gas into the cooling oil 32a. Pressure P2' is, for example, approximately 130 kPa. While the inert gas is being supplied to the cooling oil 32a, the inert gas that has not dissolved in the cooling oil 32a increases the air pressure inside the transfer chamber 31. Therefore, the pressure inside the quenching chamber 3 is periodically reduced by the pressure reducing unit 5 to maintain the air pressure inside the quenching chamber 3 at pressure P2'.

[0069] When the pressure is restored (S29 in FIG. 4), the pressure reduction unit 5 reduces the air pressure in the quenching chamber 3 to a pressure P3, and equilibrates it with the air pressure in the heating chamber 2.

[0070] Fifth Embodiment In the first embodiment, the gas supply source 60 is shared by the transfer chamber 31, the insulated container 21, and the oil tank 32 as the inert gas supply unit 6, but a dedicated unit may be provided to supply inert gas to the cooling oil 32a of the oil tank 32.

[0071] 11(A) shows an example. The illustrated inert gas supply unit 6A includes a gas supply unit 63, a tank T for storing inert gas, and check valves 67a and 67b. The gas supply unit 63 includes a cylinder 65 and an actuator 64 that reciprocates a piston 65a in the cylinder 65. The actuator 64 includes, for example, a fluid circuit or an electric motor.

[0072] The cylinder 65 has a suction port 65b and a discharge port 65c, and the suction port 65b communicates with the tank T via a check valve 67a. The discharge port 65c communicates with the oil tank 32 via a check valve 67b.

[0073] When the piston 65a moves in the direction D1, negative pressure is created inside the cylinder 65, the check valve 67a opens, and inert gas is sucked into the cylinder 65 from the tank 66. At this time, the check valve 67b is closed. When the piston 65a moves in the direction D2, the check valve 67b opens, and the inert gas in the cylinder 65 is supplied to the cooling oil 32a in the oil tank 32. By continuously reciprocating the piston 65a in the directions D1 and D2, the inert gas can be supplied to the cooling oil 32a in the oil tank 32. This increases the amount of dissolved gas in the cooling oil 32a.

[0074] Figure 11(B) shows another example. The illustrated inert gas supply unit 6B has the same structure as the inert gas supply unit 6A in Figure 11(A), but the inert gas supply source is different. The suction port 65b of the inert gas supply unit 6B is connected to the inside of the transfer chamber 31 via a check valve 67a. Except when the workpiece W is being loaded or unloaded (S21, S32 in Figure 4), the transfer chamber 31 is filled with inert gas by the inert gas supply unit 6. The inert gas supply unit 6B uses the inside of the transfer chamber 31 as the inert gas supply source.

[0075] Fig. 11(C) shows another example. The inert gas supply unit 6C in Fig. 11(C) is a unit built into the quenching chamber 3, and is an air pump that sends the inert gas in the transfer chamber 31 into the cooling oil 32a in the oil tank 32.

[0076] Figure 12(A) shows yet another example. The inert gas supply unit 6D of Figure 12(A) includes an ejector 68 and a liquid feed pump 69. The ejector 68 is disposed inside the quenching chamber 3, and the liquid feed pump 69 is disposed outside the quenching chamber 3. The ejector 68 has a main body 681 that forms a linear oil passage, and a gas passage portion 682 that intersects with the oil passage. The oil passage of the main body 681 has an inlet 681a and an outlet 681b.

[0077] The main body 681 is immersed in the cooling oil 32 a, and the gas passage portion 682 extends from the main body 681 to above the oil surface of the cooling oil 32 a and communicates with the transfer chamber 31 .

[0078] The main body 681 has a cylindrical space 681c formed to surround the oil passage, and the space 681c communicates with the gas passage portion 682. A plurality of gas passages 681d communicate between the space 681c and the oil passage.

[0079] The liquid feed pump 69 sucks the cooling oil 32a from the oil tank 32 and discharges the sucked cooling oil 32a to the inlet 681a of the ejector 68. The discharged cooling oil 32a passes through the main body 681 and is sprayed from the outlet 681b into the oil tank 32. As the cooling oil 32a passes through the main body 681, negative pressure is generated in the space 681c and the gas passage 681d, and the inert gas in the transfer chamber 31 passes through the gas passage portion 682, the space 681c, and the gas passage 681d and mixes with the cooling oil 32a passing through the main body 681. The cooling oil 32a mixed with the inert gas is sprayed from the outlet 681b. In this way, the inert gas is supplied to the cooling oil 32a, and the amount of dissolved gas in the cooling oil 32a can be increased.

[0080] Fig. 12(B) shows another example. The inert gas supply unit 6E in Fig. 12(B) has the same configuration as the inert gas supply unit 6D in Fig. 12(A), but the ejector 68 is disposed outside the quenching chamber 3.

[0081] The ejector 68 and the liquid feed pump 69 constitute a circulation section that returns the cooling oil 32a discharged from the oil tank 32 to the oil tank 32, and an inert gas is supplied to the cooling oil 32a in the ejector 68 located midway along the circulation path. The cooling oil 32a mixed with the inert gas is sprayed into the oil tank 32 via the outlet 681b and the pipe 69a. The cooling oil 32a mixed with the inert gas may be sprayed into the transfer chamber 31 from the pipe 69b instead of the pipe 69a. The sprayed cooling oil 32a falls onto the cooling oil 32a in the oil tank 32 and mixes with it.

[0082] Sixth Embodiment In each of the inert gas supply units 6B to 6E in Figures 11(B) to 12(B), the inert gas supply source is the transfer chamber 31, so even if the inert gas is supplied to the cooling oil 32a, the air pressure inside the quenching chamber 3 does not increase. Therefore, there is no need to reduce the pressure inside the quenching chamber 3 to maintain a constant air pressure inside the quenching chamber 3. Rather, when the inert gas dissolves in the cooling oil 32a, the air pressure inside the quenching chamber 3 decreases, so the control valve V3 is opened to supply the inert gas from the gas supply source 60 to the transfer chamber 31. Figure 13 is a timing chart showing the state of the vacuum furnace 1 during heat treatment in this embodiment, which replaces the one in Figure 7. Differences from the example in Figure 7 will be described below.

[0083] In this embodiment, the decompression of the quenching chamber 3 by the decompression unit 5 is performed only by vacuuming in S22 in Fig. 4, and thereafter there is no need to decompress the quenching chamber 3. The decompression unit 5 is used exclusively for controlling the pressure in the heating chamber 2 (insulated container 21).

[0084] The supply of inert gas to the cooling oil 32a is also carried out during the carburizing process. Since the supply of inert gas to the cooling oil 32a reduces the air pressure inside the quenching chamber 3, at times t2 and t3, the control valve V3 is opened to supply inert gas from the gas supply source 60 to the transfer chamber 31. This maintains the air pressure inside the quenching chamber 3 at pressure P2.

[0085] Seventh Embodiment The above-described embodiments can also be used in conjunction with a hyperpascal method. FIG. 14 is an explanatory diagram of a vacuum furnace 1A showing one such example. The vacuum furnace 1A is a vacuum furnace 1 to which a hyperpascal unit 10 has been added. The hyperpascal unit 10 includes a sub-tank 11 containing cooling oil 32a, a pressurized tank 12 containing compressed inert gas, and a vacuum tank 13. The vacuum tank 13 is connected to a vacuum pump 50 via a control valve V14. The vacuum tank 13 is pre-evacuated by opening the control valve V14 and depressurizing the vacuum pump 50. The vacuum tank 13 is also connected to the upper part of the transfer chamber 31 via a control valve V15. By opening the control valve V15, the vacuum tank 13 sucks gas from the transfer chamber 31, thereby reducing the pressure inside the quenching chamber 3. Multiple pairs of the vacuum tank 13 and the control valves 14 and 15 may be provided.

[0086] The sub-tank 11 is connected to the pressurized tank 12 via a control valve V12, and is also connected to the oil tank 32 via a control valve V11. The pressurized tank 12 has an internal pressure equal to or higher than atmospheric pressure. By opening the control valves V11 and V12, the sub-tank 11 is pressurized by the compressed gas in the pressurized tank 12, and the cooling oil 32a in the sub-tank 11 can be supplied to the oil tank 32 in a short period of time. The sub-tank 11 is also connected to a vacuum pump 50 via a control valve V13. By opening the control valve V13 and depressurizing the sub-tank 11 with the vacuum pump 50, a portion of the cooling oil 32a in the oil tank 32 can be returned to the sub-tank 11.

[0087] An example of control of the hyperpascal unit 10 during quenching will now be described. As shown in FIG. 14, the lifting unit 36 ​​lowers the transport unit 33 together with the workpiece W to the immersion position, immersing the workpiece W in the cooling oil 32a. At approximately the same time, the control valves V12 and V11 are opened, and the cooling oil 32a in the sub-tank 11 is supplied to the oil tank 32. As a result, the oil level of the cooling oil 32a in the quenching chamber 3 rises to the top of the transfer chamber 31, as shown in FIG. 15. In other words, in this embodiment, the transfer chamber 31 also temporarily constitutes an oil tank. Furthermore, the pressure in the quenching chamber 3, which is connected to the pressurized tank 12 and sub-tank 11, rises due to Pascal's principle.

[0088] Thereafter, during the boiling stage in the critical region and the convection stage after the boiling stage, the control valve V15 is opened to connect the vacuum tank 13 to the transfer chamber 31, thereby reducing the pressure inside the quenching chamber 3. At this time, of the multiple sets of vacuum tanks 13 and control valves 14 and 15, each vacuum tank 13 is connected to the transfer chamber 31 in stages. This allows the pressure inside the quenching chamber 3 to be reduced in proportion to the cooling rate of the workpiece W. After all the vacuum tanks 13 are connected to the transfer chamber 31, when the process shifts from the boiling stage to the convection stage, each control valve 14 is opened to connect the vacuum pump 50 to the transfer chamber 31, and the vacuum pump 50 reduces the pressure inside the quenching chamber 3.

[0089] In this manner, quenching is performed. In quenching using the hyperpascal method, the amount of cooling oil 32a is increased during the vapor film stage to pressurize the quenching chamber 3, thereby preventing the generation of a vapor film on the surface of the workpiece W and increasing the cooling rate of the workpiece W. In addition, the pressure in the quenching chamber 3 can be reduced at a rate proportional to the cooling rate of the workpiece W. This delays the transition of the heat transfer mode from nucleate boiling to the convection stage. This ensures uniform heating of the workpiece W before it enters the danger zone.

[0090] After quenching, in preparation for the next quenching, control valve V15 is closed and each vacuum tank 13 is evacuated. Control valves V12 and V14 are closed and control valve V13 is opened. The vacuum pump 50 reduces the pressure inside the sub-tank 11, and the cooling oil 32a is returned from the oil tank 32 to the sub-tank 11. Thereafter, control valves V11 and V12 are closed.

[0091] Eighth Embodiment The vacuum furnaces 1 and 1A in the above embodiments are configured so that the workpiece W is carried into the quenching chamber 3 from outside the furnace, transported to the heating chamber 2 via the quenching chamber 3, and the heat-treated workpiece W is then carried out from the quenching chamber 3. However, it is also possible to employ a configuration in which the workpiece W is carried into the heating chamber 2 from outside the furnace, and the heat-treated workpiece W is then carried out from the quenching chamber 3. Figure 16 shows one such example.

[0092] In the vacuum furnace 1B, an opening 20a serving as an entrance for the workpiece W is formed in the wall of the housing 20 of the heating chamber 2 on the side opposite the quenching chamber 3, and the opening 20a is opened and closed by a door 25. The door 25 is moved by an actuator (not shown). In the insulating container 21A that replaces the insulating container 21, the wall facing the opening 20a is configured with an openable and closable door 21b.

[0093] In the vacuum furnace 1B, the workpiece W is first carried into the heating chamber 2, and after heat treatment, is transported to the quenching chamber 3 where it is quenched. After quenching, the workpiece W is carried out of the quenching chamber 3 to the outside of the furnace.

[0094] In this embodiment as well, while the workpiece W is present in the heating chamber 2, an inert gas can be supplied to the cooling oil 32a to increase the amount of gas dissolved in the cooling oil 32a.

[0095] Ninth Embodiment A hardening test was conducted using a test specimen. Fig. 17(A) shows a cross-sectional view and a right side view of test specimen 100. Test specimen 100 is a cylindrical steel material (diameter 50 mm, length 100 mm, SCM435). Test specimen 100 has holes drilled in three places on its end face, and thermocouples 101 to 103 are inserted into the holes. Thermocouple 101 is located near the outer periphery of the end face, thermocouple 103 is located near the center of the end face, and thermocouple 102 is located near a distance of one-quarter of the diameter from the center of the end face.

[0096] In the quenching test, the test piece 100 was heated to approximately 900°C by heat treatment together with a data logger, and then immersed in cooling oil. The data logger recorded the temperature detection results of each thermocouple 101-103 for a predetermined period of time. The test was conducted under four conditions. Figure 17(B) is an explanatory diagram of the test contents. "Pressure" refers to the air pressure inside the quenching chamber during quenching. "High Pascal Method" indicates whether or not the high pascal method described in the seventh embodiment was applied. "Inert Gas Supply" indicates whether or not an inert gas was injected into the cooling oil before quenching to increase the amount of dissolved gas. If "Yes," nitrogen gas was supplied to the cooling oil as an inert gas for 90 minutes.

[0097] Figures 18(A) to 18(C) show the test results. Figure 18(A) shows the test results for thermocouple 101, Figure 18(B) shows the test results for thermocouple 102, and Figure 18(C) shows the test results for thermocouple 103. R1 corresponds to number 1, R2 corresponds to number 2, R3 corresponds to number 3, and R4 corresponds to number 4.

[0098] Comparing R1 and R3, which are test results for No. 1 and No. 3, where the hyperpascal method was not used, it can be seen that R3 has a faster cooling rate than R1. The faster the cooling rate, the higher the quenched hardness. It can be seen that the cooling performance of the cooling oil is improved by injecting inert gas into the cooling oil to increase the amount of dissolved gas.

[0099] Comparing R2 and R4, which are the test results for No. 2 and No. 4, which were performed using the hyperpascal method, it can be seen that R4 has a faster cooling rate than R2. It can be seen that even when the hyperpascal method is performed, the cooling performance of the cooling oil is improved by injecting inert gas into the cooling oil to increase the amount of dissolved gas.

[0100] <Other embodiments> In each of the above embodiments, the case where carburizing oil quenching is performed as the heating treatment in the heating chamber 2 is exemplified, but the content of the heating treatment is not limited to carburizing oil quenching, and can be applied to various heating and cooling treatments using a liquid (e.g., quenching oil) (e.g., vacuum oil quenching, solution hardening, solid solution hardening, precipitation hardening, etc.). Although the embodiments of the invention have been described above, the invention is not limited to the above-described embodiments, and various modifications and variations are possible within the scope of the gist of the invention. [Explanation of symbols]

[0101] 1~1B vacuum furnace, 2 heating chamber, 3 quenching chamber

Claims

1. A heating chamber for heating the workpiece; a quenching chamber having an oil tank for storing cooling oil in which the amount of dissolved air has been reduced in advance, and in which the workpiece heated in the heating chamber is immersed in the cooling oil to be cooled; A supply means for supplying an inert gas to be dissolved in the cooling oil to the cooling oil while the workpiece is accommodated in the heating chamber. A vacuum furnace characterized by:

2. 2. The vacuum furnace of claim 1, The supplying means does not supply the inert gas to the cooling oil while the workpiece is immersed in the cooling oil in the quenching chamber. A vacuum furnace characterized by:

3. 2. The vacuum furnace of claim 1, A setting means is provided that allows a user to set the supply conditions of the inert gas by the supply means. A vacuum furnace characterized by:

4. 2. The vacuum furnace of claim 1, The quenching chamber is pressurized by supplying an inert gas before the inert gas is supplied to the cooling oil by the supply means. A vacuum furnace characterized by:

5. 5. The vacuum furnace according to claim 4, The air pressure in the quenching chamber after the pressure increase is lower than atmospheric pressure. A vacuum furnace characterized by:

6. 2. The vacuum furnace of claim 1, An inert gas is supplied to a space above the liquid level of the cooling oil in the quenching chamber, The supply means takes in the inert gas in the space and supplies the inert gas to the cooling oil. A vacuum furnace characterized by:

7. 7. The vacuum furnace according to claim 6, The supplying means is built into the quenching chamber. A vacuum furnace characterized by:

8. 2. The vacuum furnace of claim 1, The supply means includes an injection unit that injects the inert gas, The injection part is provided with a micronizing means for micronizing bubbles of the inert gas injected from the injection part. A vacuum furnace characterized by:

9. 2. The vacuum furnace of claim 1, The workpiece is carried from outside the furnace through the quenching chamber into the heating chamber. A vacuum furnace characterized by:

10. 2. The vacuum furnace of claim 1, a pressure reducing means for reducing the pressure in the quenching chamber; As a preparatory process for the heat treatment of the workpiece, the air dissolved in the cooling oil is reduced in advance by reducing the pressure of the pressure reducing means. A vacuum furnace characterized by:

11. 2. The vacuum furnace of claim 1, The supply means supplies an inert gas into the oil tank. A vacuum furnace characterized by:

12. 2. The vacuum furnace of claim 1, The supply means is A circulation unit that returns the cooling oil discharged from the oil tank to the oil tank; and An inert gas is supplied to the cooling oil circulating through the circulation section. A vacuum furnace characterized by:

13. The vacuum furnace of claim 1, The supplying means supplies an inert gas to be dissolved in the cooling oil to the cooling oil for each of the workpieces while the workpieces are accommodated in the heating chamber. A vacuum furnace characterized by:

14. The vacuum furnace of claim 1, A pressure reducing means for reducing the pressure in the quenching chamber; A control means for controlling the pressure reducing means and the supply means. A vacuum furnace characterized by:

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