Hardening method

Abstract

To provide a technique that reduces the difference of cooling rates in the vertical direction, without necessitating a deep cooling tank.SOLUTION: A hardening method for cooling a workpiece placed on a support table, comprises: a lowering control step of lowering the support table into a coolant in which the maximum value of the surface heat transfer coefficient at the boiling stage, accumulated while the coolant is in a non-flowing state in a cooling tank, is 6000 W / m2 K or more, such that the immersion of the workpiece is completed and the support table stops while a vapor film of the coolant is formed around the workpiece due to the heat of the workpiece; and a state-maintaining step of maintaining a state in which the coolant does not flow and the support table does not move, after stopping the lowering of the support table, at least until the surface of the workpiece undergoes martensitic transformation, so that no relative flow velocity is generated between the workpiece and the coolant.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a quenching method. [Background technology]

[0002] Conventionally, when quenching is performed using water as a coolant with high cooling capacity, there is known a technique for controlling the relative velocity between the workpiece and the coolant surrounding the workpiece. For example, Patent Document 1 discloses a configuration in which the workpiece is immersed in a coolant with an average flow velocity of 1.0 m / s or more, so that the entire surface is cooled from the boiling stage without forming a vapor film stage.

[0003] Patent Document 2 discloses a configuration in which, when a workpiece is moved into a water-based coolant and quenched, the relative speed between the workpiece and the coolant is made slower than the moving speed of the workpiece at least until the workpiece undergoes martensitic transformation. Patent Document 3 discloses a quenching method in which a metal member is immersed in heat treatment oil and quenched, in which the metal member is lowered into the heat treatment oil, and when the oil immersion start portion, which is the portion that is first submerged in the heat treatment oil, is in the vapor film stage where it is covered with a vapor film, the metal member is stopped and then raised.

[0004] Furthermore, Patent Document 4 discloses a configuration in which, in order to suppress the influence of the vapor film on the quenching results, the object to be quenched is immersed while suspended from a rod, and the vapor film is actively peeled off from the object to be quenched. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-97520 [Patent Document 2] International Publication No. 2020-203226 [Patent Document 3] Patent Publication No. 2021-147626 [Patent Document 4] Japanese Patent Application Publication No. 2023-153496 Summary of the Invention [Problem to be solved by the invention]

[0006] When performing quenching, which requires the internal hardness or depth of the hardened layer of a workpiece, it is necessary to rapidly cool the workpiece to increase the cooling rate inside the workpiece, so various coolants with high cooling capacity are used. However, when using a coolant with high cooling capacity, it is necessary to ensure that the cooling rate at each location on the workpiece is not different, otherwise distortion will remain after quenching.

[0007] For example, in the technology disclosed in Patent Document 1, a high flow rate is applied to the coolant, allowing the entire surface to cool from the boiling stage without forming a vapor film. However, in Patent Document 1, a suction mechanism is operated to apply a flow rate to the water until the cooling of the workpiece is complete. Therefore, even after the transition from the vapor film stage to the boiling stage, the coolant continues to flow at a rate until the cooling is complete. Furthermore, in the technology disclosed in Patent Document 1, a flow rate is applied from the top to the bottom of the workpiece. In this case, the coolant stagnates at the bottom of the workpiece, resulting in a low relative flow rate, while the coolant is agitated at the top of the workpiece, resulting in a high relative flow rate. Therefore, the top of the workpiece is cooled faster than the bottom. This results in a difference in the cooling rate until martensitic transformation occurs between the top and bottom of the workpiece, resulting in distortion of the workpiece after quenching is complete.

[0008] On the other hand, according to the technology disclosed in Patent Document 2, the relative speed between the workpiece and the coolant is slower than the moving speed of the workpiece, so the cooling speed of the workpiece is uniform in the vertical direction. However, in Patent Document 2, the workpiece is lowered until martensitic transformation occurs, and the coolant is caused to flow downward, so that the relative speed between the workpiece and the coolant is slower than the moving speed of the workpiece. Therefore, in order to maintain a slow relative speed until martensitic transformation occurs, the workpiece needs to be moved deeply downward, which requires a deep cooling tank.

[0009] In addition, in Patent Document 3, a metal member is lowered in a cooling tank, then raised, and quenching is completed during this raising process. This creates a relative flow velocity between the workpiece and the coolant until martensitic transformation occurs, resulting in distortion of the workpiece after quenching is complete. Furthermore, a very deep cooling tank is required, with a stroke length (descent depth) of 100 to 700 m.

[0010] In Patent Document 4, the object to be quenched is held in an unstable state to cause the vapor film to peel off as quickly as possible, and the idea of ​​actively utilizing the vapor film is not disclosed. The present invention has been made in consideration of the above-mentioned problems, and has as its object to reduce the difference in cooling rate between the vertical direction and thereby reduce heat treatment deformation without requiring a deep cooling tank. [Means for solving the problem]

[0011] The quenching method according to one embodiment is a quenching method for cooling a workpiece placed on a support table, in which the maximum surface heat transfer coefficient at the boiling stage is 6000 W / m 2 The method includes a lowering control step of lowering the support table into a coolant at a temperature of 0.5 K or higher, and controlling the support table so that the immersion of the workpiece is completed and the support table stops while a vapor film of the coolant is formed around the workpiece due to the heat of the workpiece; and a state maintaining step of maintaining the coolant in a non-flowing state and the support table in a non-moving state after the descent of the support table is stopped so that no relative flow velocity is imparted between the workpiece and the coolant, at least until the surface of the workpiece undergoes martensitic transformation.

[0012] The maximum surface heat transfer coefficient during boiling is 6000 W / m 2A coolant with a temperature of 0.1 K or higher has an extremely high cooling capacity. Therefore, when cooling begins in the boiling stage, cooling proceeds very rapidly, increasing the cooling rate inside the workpiece and ensuring the workpiece's internal hardness and hardened layer depth. On the other hand, when the coolant around the workpiece is flowing in one direction, the relative flow velocity with respect to the coolant is high upstream of the workpiece flow, while the relative flow velocity is low downstream, where the coolant tends to stagnate. As a result, the upstream portion of the flow experiences a faster cooling rate than the downstream portion. In particular, with a coolant with an extremely high cooling capacity, the difference in cooling rate between different portions of the workpiece becomes significant. This results in distortion of the workpiece after quenching.

[0013] To prevent the replacement of coolant by the flow of coolant, the workpiece is immersed in the coolant accumulated in a cooling tank without flowing, the support table is stopped, and a state in which no relative flow velocity is imparted between the workpiece and the coolant is maintained at least until the surface of the workpiece undergoes martensitic transformation. In this configuration, after the workpiece's descent is stopped, no relative velocity is generated between the workpiece and the surrounding coolant. This reduces the possibility of differences in cooling rate at different parts of the workpiece, reduces the possibility of uneven quenching progress, and reduces heat treatment deformation.

[0014] Furthermore, by maintaining a state in which there is no relative flow velocity between the workpiece and the coolant at least until the surface of the workpiece undergoes martensitic transformation, the difference in expansion associated with martensitic transformation between the upstream and downstream sides of the workpiece that would occur if a relative flow velocity were applied can be reduced, thereby reducing distortion in the workpiece after quenching. To prevent differences in the relative flow velocity of the surrounding coolant between different parts of the workpiece, it is important to proceed with quenching the workpiece in a non-flowing coolant. However, if the coolant is not flowing, a relative flow velocity will inevitably occur between the workpiece and the coolant during the process of immersing the workpiece in liquid coolant. Therefore, immersion of the workpiece is completed and stopped while the workpiece is still covered by a vapor film. This configuration allows the immersion process in a non-flowing coolant, which would otherwise generate a relative flow velocity, to be completed during the vapor film stage, where the cooling rate is slow, minimizing the impact of the relative flow velocity during the immersion process. Furthermore, a deep cooling bath is not required. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1A is a diagram showing a schematic diagram of an apparatus for carrying out the quenching method, and FIG. 1B is a diagram showing the state during quenching. [Figure 2] 1 is a flowchart showing a heat treatment process. [Figure 3] FIG. 10 is a diagram illustrating a stroke. [Figure 4] FIG. 10 is a diagram showing the surface heat transfer coefficient of a coolant. [Figure 5] FIG. 10 is a diagram for explaining the effect. [Figure 6] FIG. 3 is a diagram showing the temperature change and martensite fraction of the workpiece according to Example 1. [Figure 7] FIG. 10 is a diagram showing the amount of distortion of the workpiece according to Example 1. [Figure 8] FIG. 10 is a diagram showing the temperature change and martensite fraction of the workpiece according to Comparative Example 1. [Figure 9] FIG. 10 is a diagram showing the amount of distortion of the workpiece according to Comparative Example 1. [Figure 10]FIG. 1 is a diagram showing the strain, elongation, and internal hardness of the workpieces according to Example 1 and Comparative Example 1. [Figure 11] FIG. 1 is a diagram showing the hardness of the workpieces according to Example 1 and Comparative Example 2. [Figure 12] FIG. 10 is a diagram showing the temperature change and martensite fraction of the workpiece according to Example 2. [Figure 13] FIG. 10 is a diagram showing the amount of distortion of the workpiece according to Example 2. [Figure 14] FIG. 10 is a diagram showing the temperature change and martensite fraction of the workpiece according to Example 3. [Figure 15] FIG. 10 is a diagram showing the amount of distortion of the workpiece according to Example 3. [Figure 16] 10 is a diagram showing the amount of strain, the amount of elongation, and the internal hardness of the workpieces according to Examples 2 and 3. FIG. [Figure 17] FIG. 10 is a diagram showing an embodiment in which the workpiece S is placed at a plurality of different positions in the vertical direction. [Figure 18] 18A and 18B are diagrams showing the configuration of a ring gear as an object to be processed, and FIGS. 18C and 18D are diagrams showing the configuration of a drive shaft as an object to be processed. [Figure 19] 10A and 10B are diagrams showing examples of support stands when immersing a ring gear. [Figure 20] FIG. 10 is a schematic diagram illustrating the period during which a vapor film is maintained for each portion of a drive shaft when the drive shaft is immersed. [Figure 21] FIG. 10 is a diagram showing the state in which immersion is completed when the object to be treated is a drive shaft. [Figure 22] 10A and 10B are diagrams showing an embodiment in which drive shafts as workpieces are placed at a plurality of different positions in the vertical direction of the support table. [Figure 23] 10A and 10B are diagrams showing examples of support stands when immersing a drive shaft. [Figure 24] FIG. 2 is a diagram showing an insertion portion for inserting one drive shaft and a connecting portion 211c, as viewed from the direction of the center axis Ax of the drive shaft. DETAILED DESCRIPTION OF THE INVENTION

[0016] Here, the embodiments of the present invention will be described in the following order. (1) Configuration of an apparatus for carrying out the quenching method: (2) Heat treatment process: (3) Working Example: (4) Other embodiments:

[0017] (1) Configuration of an apparatus for carrying out the quenching method: FIG. 1A is a schematic diagram illustrating an apparatus for carrying out a quenching method according to one embodiment of the present invention. FIG. 1A shows the main components of the apparatus, and various configurations, such as the mechanisms for driving the components and the shapes of the components, can be employed. The apparatus shown in FIG. 1A is provided with a cooling tank 10 having a hollow rectangular parallelepiped portion. In this embodiment, a coolant W is stored in the cooling tank 10 in advance. In this embodiment, the coolant W is a liquid that generates a vapor film around the workpiece S to be quenched after the workpiece S is immersed, and maintains the vapor film at least until the immersion of the workpiece S is complete. Specific examples of the coolant W will be described later.

[0018] In this embodiment, the apparatus for carrying out the quenching method is equipped with a moving device 20 that raises and lowers the workpiece S. The moving device 20 has a support table 21 and a support part 22, and the workpiece S is placed on the support table 21. The support part 22 has a part that extends in the vertical direction, and the lower end of this part is connected to the support table 21. A motor M is connected to the support part 22, and the rotational force of the motor M is converted into raising and lowering motion of the support part 22 by a mechanism not shown, thereby moving the support table 21 up and down.

[0019] Various configurations may be used for moving the support part 22 in the up and down direction, and the drive source of the motor M may be various types of energy. Furthermore, the support part 22 may be moved up and down by various mechanisms such as electric, hydraulic, or pneumatic drive. The type of the motor M is not limited, and it may be a linear motor or the like.

[0020] In this embodiment, the motor M can change the lifting and lowering speed of the support table 21 on which the workpiece S is placed, and the lifting speed and lowering speed of the support table 21 can be specified by a control signal output from the control device to the motor M.

[0021] The number and placement (orientation) of the workpieces S to be placed on the support table 21 may be various. For example, a pallet may be attached to the support table 21, and multiple workpieces S may be placed on the pallet. In this specification, an example in which there is one workpiece S will be described. Of course, the support table 21, the support part 22, etc. may have various features, and for example, the support table 21 may be formed in a mesh or lattice pattern so that the support table 21 can be easily lowered.

[0022] In this embodiment, the workpiece S is a part that has been carburized. The carburization process may be performed by a carburizing treatment device (not shown in FIG. 1A ), as long as the workpiece S is heated in a furnace of any type and the carbon present around the workpiece S is carburized into the workpiece S. Of course, the configuration of the furnace is not limited, and the workpiece S may be transported while being carburized in the furnace, or the workpiece S may be located in a fixed position in the furnace and then removed after carburization. The mode of carburization is also not limited, and various modes of carburization may be used, such as gas carburization, liquid carburization, solid carburization, vacuum carburization (vacuum gas carburization), and plasma carburization. In any case, the workpiece S after carburization is set on the support table 21 and quenched.

[0023] (2) Heat treatment process: Next, the heat treatment process (carburizing and quenching) for the workpiece S will be described. FIG. 2 is a flowchart showing the heat treatment process according to this embodiment. In the heat treatment process, the workpiece S to be heat treated is set in a carburizing treatment device (step S100). Next, carburizing treatment is performed (step S105). The conditions for the carburizing treatment are determined based on the intended use of the workpiece S, etc. For example, a predetermined carbon-containing substance (gas, etc.) is introduced into the carburizing treatment device in which the workpiece S is set, and the workpiece S is heated to a target temperature at a predetermined heating rate. Then, once the workpiece S reaches the target temperature, it is maintained at the target temperature for a predetermined period of time.

[0024] Next, the carburized workpiece S is set on the moving device 20 (step S110). That is, the carburized workpiece S is placed on the support table 21. In this embodiment, the cooling tank 10 does not include a device for circulating the coolant W therein, and therefore the coolant W is not flowing within the cooling tank 10.

[0025] Next, the lowering speed Ve of the workpiece S is set to a predetermined speed (step S115). That is, a control signal is output to the motor M, and as a result, the lowering speed Ve of the support table 21 becomes the predetermined speed, and the lowering of the support table 21 begins. The predetermined speed is set so that the immersion of the workpiece S is completed while a vapor film of the coolant W is being formed around the workpiece S due to the heat of the workpiece S.

[0026] The immersion is complete when the upper end of the workpiece S is below the liquid level of the coolant W. In this embodiment, as shown in FIG. 3, the stroke ST is determined so that the upper end Es of the workpiece S is a predetermined distance Lg below the liquid level Sw of the coolant W. That is, the stroke ST is set so that the distance Lg between the upper end Es of the workpiece S and the liquid level Sw of the coolant W is greater than zero. Note that the distance Lg does not need to be excessively large and may be the minimum necessary length. That is, the distance Lg is set so that the workpiece S is not exposed to the outside of the coolant W, and when the descent of the workpiece S stops and the top of the workpiece S is cooled at a position the distance Lg from the liquid level Sw, a stable vapor film can be generated, and the coolant W continues to cover the workpiece S even during the boiling stage after the vapor film disappears. Specifically, the distance Lg can be set to the maximum waviness length (height) + 30 mm, for example. The distance Lg may be set to less than one or half the total height of the workpiece S.

[0027] The descent speed Ve may be set so that the immersion of the workpiece S is completed while a vapor film of the coolant W is being formed around the workpiece S, and may be set in accordance with the height H of the workpiece S, the characteristics of the coolant W, etc. Specifically, in this embodiment, the workpiece S is lowered until the distance (Lg+H) from the liquid level Sw of the coolant W to the bottom end of the workpiece S at the position where the descent of the support base 21 stops matches the stroke ST. The descent speed Ve is set to be faster than (stroke ST / maintenance period T of the vapor film around the workpiece S). Setting this speed allows the immersion of the workpiece S to be completed before the vapor film disappears.

[0028] If the vapor film maintenance period T is the same for the first and last immersed portions of the workpiece S, the difference in timing between when the vapor film disappears at the first immersed portion and when it disappears at the last immersed portion is, at most, equal to the difference in timing when the two portions were immersed. Therefore, in order to reduce the difference in timing when the vapor film disappears between the first and last immersed portions, it is preferable to move the support base 21 as quickly as possible within the vapor film maintenance period T after the first portion is immersed.

[0029] Therefore, (stroke ST / steam film maintenance period T of workpiece S) is the lower limit of the descent speed Ve, and the descent speed Ve is set to be greater than ST / T. For example, when a cylindrical workpiece S made of carburized SCM420 and 36 mm in height is used and immersed in a 20% water-soluble coolant with a stroke ST of 520 mm, a descent speed Ve of 400 mm / sec can be used.

[0030] The vapor film maintenance period T is the period during which the vapor film is formed. In this embodiment, the vapor film maintenance period T is the period from the time when the first immersed portion of the workpiece S begins to be immersed until the vapor film disappears around that portion and boiling begins. The vapor film is formed when the heat of the workpiece S is transferred to the coolant W, causing the coolant W to evaporate, and the vaporized coolant W exists between the surface of the workpiece S and the liquid coolant W.

[0031] The state in which a vapor film exists around the workpiece S is a state in which a vapor film exists over the entire outer surface of the workpiece S. In this embodiment, the state in which a vapor film exists around the workpiece S is assumed to be a state in which a vapor film exists over the entire outer surface of the workpiece S, but the case in which the vapor film disappears locally and temporarily on the outer surface of the workpiece S is not excluded. In other words, even if a state in which steam is temporarily absent occurs in a small part of the outer surface of the workpiece S, as long as the state in which steam disappears locally does not continue, it may be considered a state in which a vapor film exists.

[0032] When a vapor film of coolant W is in contact with the outer surface of the workpiece S, the surface heat transfer coefficient (amount of heat transferred per unit area and unit temperature difference) is smaller than when the liquid coolant W is in contact with the outer surface. Therefore, the vapor film maintenance period T is the period during which the change per unit temperature of the surface heat transfer coefficient between the coolant W and the workpiece S is below a predetermined value when the workpiece S is immersed in the coolant W. The surface heat transfer coefficient is defined in JIS Z8000-5:2014 "Quantities and units - Part 5: Thermodynamics" as the surface heat transfer coefficient α = Q / (A(Tw-Ta)). Here, Q is the amount of heat transferred (W) and A is the heat transfer area (m 2 ), Tw is the surface temperature (K) of the workpiece S, and Ta is the coolant temperature (K).

[0033] FIG. 4 shows the surface heat transfer coefficient of the coolant W. In FIG. 4, the horizontal axis represents temperature (surface temperature of the workpiece S minus the coolant temperature: °C), and the vertical axis represents the surface heat transfer coefficient. The horizontal axis represents the surface heat transfer coefficient for each of the coolants W. The temperature of the coolant W can be set to various temperatures, and the surface heat transfer coefficient varies depending on the temperature of the coolant W. Therefore, the difference between the surface temperature of the workpiece S and the temperature of the coolant W is represented as the temperature on the horizontal axis. The solid line represents a water-soluble coolant in which 10% of a polymer compound is dissolved in water. The dashed line represented by a series of dots represents a water-soluble coolant in which 20% of a polymer compound is dissolved in water. The dashed line represents hot oil. The dashed line represented by a series of lines represents cold oil. The dashed line represents water. In FIG. 4, the water-soluble coolant and water are at 25°C, the cold oil is at 80°C, and the hot oil is at 100°C. In FIG. 4, the temperature and surface heat transfer coefficient are shown for multiple points on the curve. In addition, the surface heat transfer coefficient of the coolant W was calculated from a cooling curve obtained in accordance with the cooling performance test method of JIS K 2 2 4 2:2 0 1 2 "Heat Treatment Oils."

[0034] The water-soluble coolant is a coolant W in which a water-soluble substance is dissolved in water, such as a polymer-based water-soluble coolant in which a polymer compound (polymer) such as polyalkylene glycol, polyvinyl alcohol, polyethylene glycol, sodium polyacrylate, or polyvinylpyrrolidone is dissolved. A polymer-based water-soluble coolant in which at least one of these is dissolved may also be used. The coolant W according to this embodiment is a water-soluble coolant whose main component is polyalkylene glycol. The concentration of the polymer compound is, for example, 5 to 30% by volume, 10 to 30% by volume, or 10 to 20% by volume. An example of such a water-soluble coolant is Daphne Plastics Quench DQ (registered trademark) manufactured by Idemitsu Kosan.

[0035] Examples of hot oils and cold oils include quenching oils made by adding various additives to mineral oils. Examples of hot oils include Bright Martemper Oil S (registered trademark) manufactured by Nippon Grease Co., Ltd., which uses low-sulfur refined mineral oil as a base oil and additives. Examples of cold oils include Daphne Master Quench A (registered trademark) manufactured by Idemitsu Kosan Co., Ltd., which uses paraffinic mineral oil as a base oil and additives.

[0036] FIG. 4 shows that a vapor film is formed when a workpiece S in an austenite state is immersed at a temperature higher than the A1 transformation line in the steel phase diagram. For example, in a 10% water-soluble coolant, the surface heat transfer coefficient is approximately constant over the temperature range from t1 to t2. This constant surface heat transfer coefficient is smaller than the surface heat transfer coefficient over most of the temperature range below t1. In this way, the state in which the surface heat transfer coefficient is approximately constant is a state in which a vapor film is formed around the workpiece S, making it more difficult for heat to be transferred from the workpiece S to the coolant W than when the workpiece S is surrounded by a liquid. In other words, in the temperature range in which the surface heat transfer coefficient is approximately constant, heat exchange occurs between the workpiece S and the coolant W via a gaseous vapor film, resulting in a smaller surface heat transfer coefficient than when heat exchange occurs with the liquid coolant W.

[0037] In the temperature range where the surface heat transfer coefficient is almost constant, as shown in Figure 4, the surface heat transfer coefficient is 2000 W / m 2 ·K or less. More preferably, the surface heat transfer coefficient is 1300 W / m 2 ·K or less. Furthermore, in the temperature range where the surface heat transfer coefficient is almost constant, the range where the surface heat transfer coefficient changes is 1080 to 1164 W / m for a water-soluble coolant with 10% polymer compound dissolved therein. 2 K, 1245~1285W / m in water-soluble coolant containing 20% ​​polymer 2 ·K. Also, in hot oil, it is 640W / m 2 Nearly constant at K, 500-680W / m in cold oil 2 K, 800-1000 W / m in water 2 ·K. Thus, the range of change in the temperature range where the surface heat transfer coefficient is almost constant is 200 W / m 2 ·K, preferably 100W / m 2 ·K or less, more preferably 70W / m 2 ·K or less, 40W / m 2 A coolant W with a temperature of 0.5K or less can be used.

[0038] By observing the surface heat transfer coefficient of the coolant W shown in Figure 4, it is possible to determine whether a vapor film will form around the workpiece S when the workpiece S is immersed. For example, if the workpiece S that has been carburized to a target temperature of approximately 850°C is immersed in a 10% water-soluble coolant, cooling begins and a vapor film will form, but this vapor film will remain until the temperature of the workpiece S reaches t1. When the workpiece S reaches temperature t1, the vapor film disappears and the workpiece S is covered with liquid coolant W.

[0039] As described above, during the period when the vapor film is formed, the surface heat transfer coefficient is relatively small and does not change significantly. In other words, while the workpiece S is immersed in the coolant W and the temperature is decreasing, the change in the surface heat transfer coefficient relative to the temperature decrease is equal to or less than a predetermined value. For example, if the change in the surface heat transfer coefficient per 10°C is 100 W / m 2.K or less. Of course, this default value is just an example, and the change in the surface heat transfer coefficient per 10°C is 80W / m 2 ·K or less, 50W / m 2 ·K or less, 20W / m 2 A value of K or less may be adopted.

[0040] The state in which the change in the surface heat transfer coefficient with respect to the amount of temperature drop is equal to or less than a predetermined value is maintained between temperature t2, at which the vapor film was formed immediately after the start of immersion, and temperature t1. When the temperature drops below t1, the change in the surface heat transfer coefficient with respect to the amount of temperature drop increases rapidly. In other words, it exceeds the predetermined value. Therefore, the vapor film maintenance period T is the period from when the change in the surface heat transfer coefficient with respect to the amount of temperature drop first falls below the predetermined value until it exceeds the predetermined value. The vapor film maintenance period T is determined, for example, by experiment, simulation, etc.

[0041] As described above, in the vapor film stage, the surface heat transfer coefficient is almost constant, but when the vapor film maintenance period T ends and the stage shifts to the boiling stage, it is preferable that the surface heat transfer coefficient becomes a large value. In the present invention, the maximum value of the surface heat transfer coefficient in the boiling stage is 6000 W / m 2 By using a coolant W with a temperature of 0.1 K or higher, the workpiece S can be cooled at high speed, the cooling rate of the inside of the workpiece S can be increased, and the internal hardness and depth of the hardened layer of the workpiece S can be ensured. Specifically, in the vapor film stage, the surface heat transfer coefficient is 2000 W / m 2 ·K or less, the amount of heat transferred from the workpiece S to the coolant W is suppressed compared to the boiling stage. On the other hand, in the boiling stage, the maximum surface heat transfer coefficient is 6000 W / m 2 ·K or higher, the surface heat transfer coefficient between the vapor film and boiling stages is at least 4000 W / m 2 ·K changes. Therefore, when the process moves to the boiling stage, a large amount of heat can be transferred from the workpiece S to the coolant W. Note that the hot oil shown in FIG. 4 is excluded from the present invention.

[0042] Furthermore, in this embodiment, it is necessary to complete the immersion of the workpiece S within the vapor film maintenance period T and stop the support table 21. For this reason, it is preferable that the vapor film maintenance period T is not excessively short, and that the vapor film is formed for a certain period of time. Specifically, it is preferable that the boiling initiation temperature, at which the vapor film formed around the workpiece S disappears and boiling begins, is 600°C or lower. Note that this boiling initiation temperature is also evaluated by subtracting the coolant temperature from the surface temperature of the workpiece S, similar to the temperature on the horizontal axis in Figure 4. For example, when the coolant temperature is 25°C, a boiling initiation temperature of 600°C means that the workpiece S transitions to the boiling stage when the surface temperature of the workpiece S reaches 625°C.

[0043] When using a coolant W with a boiling start temperature of 600°C or less, if immersion begins when the surface temperature of the workpiece S is about 800°C, a vapor film is maintained until the workpiece S is cooled to about 175°C. Therefore, immersion of the workpiece S can be completed within the vapor film maintenance period T without having to move the workpiece S at an excessively high speed. Furthermore, even if the workpiece S has a long vertical length or if the workpiece S is placed at multiple different positions in the vertical direction of the support table, immersion of the workpiece S can be completed within the vapor film maintenance period T.

[0044] If the boiling start temperature is 600°C or less, the immersion of the workpiece S can be completed within the vapor film maintenance period T. Examples of such coolants W include a 10% water-soluble coolant (boiling start temperature 492°C), a 20% water-soluble coolant (boiling start temperature 545°C), cold oil (boiling start temperature 540°C), and hot oil (boiling start temperature 560°C) shown in Figure 4. Note that when a coolant W with a boiling start temperature of 600°C or less is used, water shown in Figure 4 is excluded.

[0045] Furthermore, in this embodiment, if the vapor film is maintained for an excessively long period, rapid cooling cannot be performed, so it is preferable that the boiling initiation temperature is not excessively low. Therefore, for example, considering that the martensitic transformation temperature of the base material is about 420°C, it is preferable that the boiling initiation temperature be 450°C or higher.

[0046] Furthermore, in order to rapidly cool the workpiece S with the coolant W in the boiling stage, it is preferable to rapidly cool the workpiece S immediately after the transition from the vapor film stage to the boiling stage. For example, after the vapor film formed around the workpiece disappears and boiling begins, the rate of increase in the surface heat transfer coefficient per unit temperature decrease is 100 W / m 2 ·K 2 It is preferable that this is equal to or greater than this.

[0047] More specifically, the transition from the vapor film stage to the boiling stage results in a rapid increase in the surface heat transfer coefficient immediately after the transition to the boiling stage, since heat is transferred from the surface of the workpiece S to the coolant W more efficiently in the boiling stage than in the vapor film stage. This can be evaluated by the rate of increase in the surface heat transfer coefficient per unit temperature decrease during the temperature decrease process. If this rate of increase is 100 W / m 2 ·K 2 It has been found that if the temperature is above this, rapid cooling can be performed at a sufficient rate during the boiling stage.

[0048] The rate of increase in the surface heat transfer coefficient per unit temperature decrease is 100 W / m 2 ·K 2 If the temperature is more than this, the workpiece S can be rapidly cooled at the boiling stage. As such a coolant W, for example, a 10% water-soluble coolant (rate of increase: 121 W / m 2 ·K 2 ), 20% water-soluble coolant (rate of rise 170W / m 2 ·K 2 ), cold oil (rate of rise 144W / m 2 ·K 2 ), and the rate of increase is 100W / m 2 ·K 2 When the above coolant W is used, the water and hot oil shown in FIG. 4 are excluded.

[0049] In addition, in the case of 10% water-soluble coolant, 20% water-soluble coolant, and cold oil shown in Figure 4, the rate of increase in the surface heat transfer coefficient per unit temperature decrease is 100 W / m immediately after the transition from the vapor film stage to the boiling stage. 2 ·K2 The surface heat transfer coefficient is 6000W / m 2 Therefore, these coolants W rapidly cool the workpiece S at the boiling stage and have a surface heat transfer coefficient of 6000 W / m 2 This state of 0.5K or higher continues until the martensitic transformation has progressed sufficiently (for example, until the temperature drops to 300°C or lower). Therefore, the workpiece S can be cooled efficiently.

[0050] Returning to the explanation of the flowchart shown in Fig. 2, in step S115, when the lowering speed Ve is set to a predetermined speed, it is determined whether the depth of the bottom end of the workpiece S has reached the stroke ST (step S120). That is, it is determined whether the distance from the liquid level Sw of the coolant W to the position of the bottom end of the workpiece S coincides with the stroke ST. This determination can be realized by various configurations, and may be based on, for example, the detection results from various sensors, the driving time of the motor M, etc.

[0051] As described above, in this embodiment, the lowering speed Ve is set to a predetermined speed in step S115, and the workpiece S is lowered until the depth of its lower end reaches the stroke ST in step S120. Therefore, steps S115 and S120 allow the workpiece S to be lowered into the coolant W accumulated in the cooling tank 10, and the immersion of the workpiece S can be completed while a vapor film of the coolant W is being formed around the workpiece S due to the heat of the workpiece S. When the immersion process in this embodiment, in which the workpiece S is immersed in the coolant W that is not flowing in the cooling tank 10, inevitably generates a relative flow velocity between the workpiece S and the coolant. For this reason, in this embodiment, the immersion process, in which a relative flow velocity occurs, is completed during the vapor film stage, in which the cooling rate is slow. As a result, the influence of the relative flow velocity during the immersion process is minimized, i.e., the difference in the cooling rate of the workpiece S between the upstream and downstream sides of the relative flow velocity can be minimized.

[0052] In step S120, when it is determined that the depth of the lower end of the workpiece S has reached the stroke ST, the motor M stops the descent of the support table 21 (step S125). That is, the control device outputs a predetermined control signal to the motor M, stopping the operation of the motor M and stopping the descent of the support table 21. In this embodiment, the coolant W is not flowing within the cooling tank 10. In addition, in this embodiment, the cooling tank 10 is not provided with a mechanism for applying an external force to the coolant W using a propeller, pump, or the like to move it. Therefore, when the descent of the support table 21 is stopped, a relative flow velocity is not imparted between the workpiece S and the coolant W around the workpiece S.

[0053] Therefore, when the descent of the support table 21 stops, there is no factor causing the coolant W around the workpiece S to flow except for natural convection, and the area around the workpiece S becomes extremely stable. Therefore, according to this embodiment, there is no factor that promotes the cooling of the workpiece S, and the cooling rate of the workpiece S does not vary from part to part. As a result, the workpiece S is not cooled unevenly, and the possibility of uneven progress of quenching can be reduced.

[0054] In step S125, when the vapor film maintenance period T has elapsed while the lowering of the support table 21 is stopped, the vapor film disappears. When the vapor film disappears from around the workpiece S and the workpiece S comes into contact with the liquid coolant W, rapid cooling begins. When rapid cooling begins, the temperature of the workpiece S reaches the martensitic transformation start temperature Ms, and martensitic transformation progresses from the surface of the workpiece S.

[0055] In this embodiment, a predetermined period is specified in advance, which is a period during which the martensite fraction on the surface of the workpiece S reaches a predetermined value. The predetermined period can be defined, for example, by the length of time that has elapsed since the start of immersion. Here, the martensite fraction P is P(t)=1-exp(-b(Ms-t)) t:Temperature Ms: Martensitic transformation start temperature b: A constant determined by the material and carbon concentration (for example, in the case of SCM420, b = 0.143 for the base material, and b = 0.01 when carburized to a surface carbon concentration of 0.8 (mass%)) It is expressed as:

[0056] In this embodiment, the surface of the workpiece S is deemed to have undergone martensitic transformation when the martensite fraction of the surface of the workpiece S reaches a predetermined value. Since the workpiece S becomes harder and less susceptible to distortion as the martensite fraction increases, the condition that the martensite fraction reaches a predetermined value can be considered a condition for suppressing distortion to an allowable level or less. For example, various values ​​such as 21%, 28%, 50%, and 61% can be defined as the predetermined value depending on the allowable distortion level. The predetermined period, which is the time required for the martensite fraction to reach the predetermined value, can be determined in advance through experiments or simulations. In this embodiment, in step S130, it is determined whether the predetermined period has elapsed. If the predetermined period has elapsed, the surface of the workpiece S is deemed to have undergone martensite transformation and been sufficiently quenched. Whether the predetermined period has elapsed can be determined by, for example, measuring the elapsed time from the start of immersion using a timing device (not shown).

[0057] The cooling tank 10 according to this embodiment is not provided with a mechanism for moving the coolant W by applying an external force such as a propeller or a pump. Therefore, even after the support base 21 stops, the coolant W is not moved by an external force during the period until it is determined in step S130 that the predetermined period has elapsed. That is, the coolant W remains in a state of not being moved by an external force during the period until the surface of the workpiece S undergoes martensitic transformation. Therefore, there is little possibility that the flow of the coolant W will cause the cooling rate of the surface of the workpiece S to become uneven. Note that if the cooling tank 10 is provided with a mechanism for moving the coolant W, the coolant W may be moved to promote cooling after it is determined in step S130 that the predetermined period has elapsed.

[0058] In step S135, it is determined whether the workpiece S has reached a predetermined temperature. That is, the temperature at which the workpiece S is removed from the coolant W is determined in advance as the predetermined temperature. The determination of whether the predetermined temperature has been reached can be realized by various configurations, and may be performed, for example, based on the detection results by various sensors, measurement of the length of time, etc.

[0059] If it is determined in step S135 that the workpiece S has reached the predetermined temperature, the workpiece is raised (step S140). That is, the moving device 20 is controlled to raise the support table 21. This ends the state in which the workpiece S is immersed in the coolant W, and the heat treatment process is completed. Of course, this cooling process is just one example, and subsequent heat treatments such as tempering, annealing, high-frequency heating, etc. may also be performed.

[0060] Here, we will explain the effect of stopping the support base 21 by completing immersion within the vapor film maintenance period T while the coolant W is not flowing as shown in Figure 4. Figure 5 is a diagram showing the effect of each of the technical concepts adopted in this embodiment. In Figure 5, the technical concepts are shown element by element on the left side.

[0061] Specifically, the surface heat transfer coefficient at the boiling stage is 6000 W / m 2 When a coolant W having a temperature of 6000 W / m or higher is used, a large amount of heat can be transferred from the workpiece S to the coolant W after the transition to the boiling stage, the cooling rate of the inside of the workpiece S increases, and the hardenability of the inside of the workpiece S improves. In Figure 5, 2 The circle in the hardenability column indicates that the hardenability improves with the use of coolant W at temperatures above K.

[0062] On the other hand, the surface heat transfer coefficient at the boiling stage is 6000W / m 2A coolant W with a temperature of 0.1 K or higher has a very high cooling capacity, which makes it easy for distortion to occur in the workpiece S. That is, when the coolant W around the workpiece S is flowing in one direction, the relative flow velocity between the workpiece S and the coolant W is high upstream of the flow of the coolant W, making the coolant W more likely to be agitated. Furthermore, the coolant W is more likely to stagnate downstream of the flow of the coolant W, making the relative flow velocity low. Therefore, the cooling rate is faster in the upstream portion of the flow than in the downstream portion, resulting in a significant difference in cooling rate between different portions of the workpiece S. As a result, distortion occurs in the workpiece S after quenching. In Figure 5, the x in the heat treatment deformation column indicates that distortion is more likely to occur when a coolant W with a high cooling capacity is used.

[0063] Therefore, to prevent agitation and stagnation of the coolant W due to the flow of the coolant, after the workpiece S is immersed in the coolant W accumulated in the cooling tank 10 without flowing and the support base 21 is stopped, a state in which no relative flow velocity is applied between the workpiece S and the coolant W is maintained until the surface of the workpiece S undergoes martensitic transformation. Therefore, in this embodiment, after the transition to the boiling stage, a situation in which cooling is accelerated in some parts of the workpiece S and inhibited in other parts does not occur. Therefore, differences in the cooling rate between different parts of the workpiece S are unlikely to occur, and as a result, differences in the amount of deformation between different parts of the workpiece S are unlikely to occur. Furthermore, by maintaining a state in which no relative flow velocity is applied between the workpiece S and the coolant W at least until the surface of the workpiece S undergoes martensitic transformation, the difference in expansion associated with martensitic transformation between the upstream and downstream sides of the workpiece S that occurs when a relative flow velocity is applied is reduced, thereby reducing distortion of the workpiece S after quenching. Furthermore, in this embodiment, the immersion operation is performed so that immersion is completed during the vapor film stage of the coolant W and the support table is stopped. If the workpiece S is immersed in a non-flowing coolant W, a relative flow velocity occurs between the workpiece S and the coolant W. However, in this embodiment, the immersion process is completed during the vapor film stage, where the cooling rate is slow. Therefore, the influence of the relative flow velocity occurring around the workpiece S during the immersion process is small, i.e., the difference in the cooling rate of the workpiece S between the upstream and downstream sides of the relative flow velocity can be minimized. As a result, differences in the amount of deformation at each location of the workpiece S are less likely to occur. In Figure 5, the circles in the heat treatment deformation column indicate that differences in the amount of deformation at each location are less likely to occur.

[0064] However, if the coolant W is not allowed to flow until the surface of the workpiece S has transformed into martensitic when the workpiece has transitioned to the boiling stage, the cooling rate inside the workpiece S will be slower than when the coolant W is flowing around the workpiece S, which may result in the workpiece S not achieving the desired internal hardness or hardened layer depth, and the hardenability inside the workpiece S will be reduced. In Figure 5, the x in the hardenability column indicates that the hardenability is reduced when the coolant W is not allowed to flow until the surface of the workpiece has transformed into martensitic.

[0065] The decrease in hardenability can be prevented by increasing the amount of alloys (Cr, Mo, Mn, Ni, B, Si, V) that improve hardenability as the material of the workpiece S. However, since Mo, Ni, V, etc. are expensive, this leads to an increase in costs. Therefore, the decrease in hardenability can be rephrased as an increase in material costs. However, in this embodiment, the surface heat transfer coefficient at the boiling stage is 6000 W / m 2 ·Since coolant W, which has a high hardenability of K or higher, is used, there is no hardenability associated with not allowing coolant W to flow.

[0066] In addition, the rate of increase in the surface heat transfer coefficient during the boiling stage is 100W / m 2 ·K 2 If the above coolant W is used, rapid cooling can be achieved at the boiling stage, and the surface heat transfer coefficient at the boiling stage is 6000W / m 2 As with the use of a coolant W of 100 K or higher, the cooling rate inside the workpiece S increases, improving the hardenability inside the workpiece S. In Figure 5, the rate of increase in the surface heat transfer coefficient during the boiling stage is 100 W / m 2 ·K 2 The improved hardenability associated with the use of the above coolant W is indicated by the circles in the hardenability column.

[0067] In addition, the rate of increase in the surface heat transfer coefficient during the boiling stage is 100W / m 2 ·K 2 The cooling capacity of the above coolant W is so high that if the degree of cooling differs for each part of the workpiece S, the surface heat transfer coefficient at the boiling stage will be 6000 W / m 2Similarly to coolant W with a temperature of 0.5 K or higher, distortion is likely to occur. In FIG. 5, the fact that distortion is likely to occur as a result of using a coolant W with a high cooling capacity is indicated by an x ​​in the heat treatment deformation column. However, in this embodiment, a state in which no relative flow velocity is applied between the workpiece S and the coolant W is maintained until the surface of the workpiece S undergoes martensitic transformation, and further, the immersion operation is performed so that immersion is completed during the vapor film stage of the coolant W and the support table is stopped, so that heat treatment deformation is unlikely to occur as described above.

[0068] Furthermore, to complete immersion during the vapor film stage, the upper surface of the workpiece S must reach or fall below the liquid level within the vapor film maintenance period T, requiring the support table 21 to be lowered at a high speed. Furthermore, the faster the movement speed of the support table 21, the larger the equipment required to drive the support table 21 and the more durable it must be to withstand repeated quenching in the production process. For example, the device for lowering the support table 21 inevitably faces limitations such as an upper load capacity and an upper limit on the lowering speed, and increasing these limits would require excessive costs. Furthermore, if there is an upper limit on the lowering speed of the support table 21, the height of the workpiece S must be limited so that immersion can be completed at or below that lowering speed. This imposes limitations on the number of workpieces S that can be arranged vertically on the support table 21. In Figure 5, the x in the productivity column indicates that the high-speed lowering of the support table 21 makes equipment design difficult and also imposes limitations on the height and number of workpieces S that can be arranged vertically.

[0069] As described above, completing immersion during the vapor film stage makes it difficult to improve productivity. However, in this embodiment, productivity is prevented from decreasing by using a coolant W having a boiling start temperature of 600°C or less. Specifically, when a coolant W having a boiling start temperature of 600°C or less is used, for example, if immersion is initiated when the surface temperature of the workpiece S is approximately 800°C, the vapor film is maintained until the workpiece S is cooled to approximately 175°C. Thus, if the boiling start temperature is 600°C or less, sufficient time is secured to complete immersion during the vapor film stage. Therefore, immersion of the workpiece S can be completed within the vapor film maintenance period T without moving the workpiece S excessively quickly. In Figure 5, the circle in the productivity column indicates that sufficient time is secured to complete immersion during the vapor film stage by using a coolant W having a boiling start temperature of 600°C or less.

[0070] Thus, using a coolant W whose boiling start temperature is 600°C or lower allows for a sufficiently long vapor film maintenance period T, but if the vapor film maintenance period T is long, it takes time for the workpiece S to begin quenching after immersion begins. In other words, if the transition temperature to the boiling stage is low, the cooling rate inside the workpiece S becomes insufficient, leading to poor hardening (deterioration of internal hardness and hardened layer depth). In Figure 5, the x in the hardenability column indicates that the use of a coolant W whose boiling start temperature is 600°C or lower results in a decrease in hardenability.

[0071] As described above, when using a coolant W whose boiling start temperature is 600°C or less, it is difficult to increase the cooling rate. However, in this embodiment, the surface heat transfer coefficient at the boiling stage is 6000W / m 2 Use a coolant W of 100 K or higher, or the rate of increase in the surface heat transfer coefficient during the boiling stage is 100 W / m 2 ·K 2 By using the above-described coolant W, the cooling rate inside the workpiece S is increased, so that the hardenability does not decrease as described above.

[0072] As described above, the disadvantages of using each of the technical concepts of this embodiment are compensated for by the advantages of the other technical concepts. Therefore, this embodiment can provide a quenching method that does not require a deep cooling bath, reduces the difference in cooling rate between the top and bottom, and is less likely to cause distortion.

[0073] (3) Working Example: Next, the quenching process in the above-mentioned heat treatment step will be described. Here, an example is shown in which the workpiece S is a cylindrical shaft (diameter 36 mm, axial length 148 mm) made of SCM420, and the shaft is supported on a support stand 21 so as not to roll with its axial direction oriented horizontally, and quenched. The sample obtained by quenching the workpiece S in the heat treatment step shown in Figure 3 is called Example 1, and the sample quenched in the heat treatment step shown in Figure 3 with coolant W flowing in the cooling tank 10 is called Comparative Example 1.

[0074] In the heat treatment process of Comparative Example 1, the coolant W in the cooling tank 10 is caused to flow in a fixed direction around the workpiece S. This configuration can be realized, for example, by providing a flow unit, such as a propeller, inside the cooling tank 10 that uses an external force to move the coolant W, and by using the flow unit to cause the coolant W circulating inside the cooling tank 10 to flow in a fixed direction around the workpiece S. That is, in Comparative Example 1, quenching is performed in a state in which the coolant W flows around the workpiece S in the same direction as and parallel to the descending direction of the support table 21. Also, here, the magnitude of the flow velocity Vq of the coolant W is set to be equal to the magnitude of the descending velocity Ve. Therefore, while the support table 21 is descending, the relative velocity between the workpiece S and the coolant W is 0, but when the support table 21 stops, the coolant W has a downward relative velocity Ve (= Vq) with respect to the workpiece S.

[0075] In both Example 1 and Comparative Example 1, the stroke ST is 520 mm. In addition, in both Example 1 and Comparative Example 1, the coolant W is a 20% water-soluble coolant. The internal temperature of the workpiece S when it begins to be immersed in the coolant W is 850°C. The workpiece S has been carburized to a surface layer of 1 mm with a surface carbon concentration of 0.8 (mass%). The descent speed Ve is 400 mm / s in both Example 1 and Comparative Example 1, but the flow speed Vq of the coolant W is 0 mm / s in Example 1 and 400 mm / s downward in Comparative Example 1. In both Example 1 and Comparative Example 1, the temperature of the coolant W is room temperature (e.g., 25°C).

[0076] FIG. 6 is a graph showing the temperature change and martensite fraction of the workpiece S in Example 1. The horizontal axis represents time in logarithm, the left vertical axis represents temperature, and the right vertical axis represents martensite fraction. In FIG. 6, the graph shows the changes in the temperature and martensite fraction of the workpiece S over time after the workpiece S comes into contact with the coolant W. In FIG. 6, the solid line indicates the surface temperature and martensite fraction of the axially central portion at the bottom of the workpiece S. The dashed-dotted line also indicates the surface temperature and martensite fraction of the axially central portion at the top of the workpiece S. In the example, the martensite fraction at the top surface of the workpiece S overlaps with the martensite fraction at the bottom surface, so only the solid line is visible. Furthermore, the dashed-dotted line indicates the temperature and martensite fraction inside (the center) of the workpiece S.

[0077] Fig. 7 shows the change in the amount of distortion over time in Example 1. In Fig. 7, the horizontal axis represents time in logarithm, and the vertical axis represents the amount of distortion. The amount of distortion (amount of bending) indicates the degree to which the cylindrical axis of the workpiece S is bent in the vertical direction, and represents the distance between the top and bottom of the cylindrical axis of the workpiece S in the vertical direction. In addition, whether the center or the end of the cylindrical axis is upper is indicated by positive or negative. A positive value indicates a bending such that the center of the cylindrical axis is higher than the end (convex upward), and a negative value indicates a bending such that the center of the cylindrical axis is lower than the end (convex downward).

[0078] When the immersion of the workpiece S begins, a temperature difference occurs between the bottom and top surfaces of the workpiece S, as shown in Figure 6, reflecting the difference in the immersion start timing between the top and bottom of the workpiece S. In this state, as time passes, the workpiece S gradually cools down, with a temperature difference occurring between the bottom and top surfaces.

[0079] However, in Example 1, immersion is completed while the workpiece S is still covered with a vapor film, and the support table 21 is stopped, so the temperature difference between the bottom and top surfaces of the workpiece S does not become large. Specifically, when the workpiece S is immersed in the coolant W according to this embodiment, a vapor film forms around the immersed portion. Therefore, from the start of immersion until time te, when the vapor film maintenance period T has elapsed, heat exchange between the workpiece S and the coolant W occurs via the vapor film. Therefore, the cooling rate after the start of immersion is relatively slow, approximately 100°C / second. Figure 6 shows that in Example 1, the temperature difference between the bottom and top surfaces of the workpiece S remains almost constant from the start of immersion until time ts.

[0080] Distortion reflecting the temperature difference occurs in the workpiece S. When the lower part of the workpiece S is cooler than the upper part, as in Example 1, the upper part of the workpiece S undergoes a greater degree of thermal expansion than the lower part, causing the upper part to become longer and the lower part to become shorter in the axial direction. As a result, the amount of distortion of the workpiece S becomes positive (convex upward). In Example 1, as shown in FIG. 7, positive distortion occurs immediately after the start of immersion, and the amount of distortion gradually increases.

[0081] After the start of immersion, the support table 21 stops during the vapor film maintenance period T during which the vapor film is maintained. In FIG. 6, the support table 21 stops at time ts (1.3 seconds after the start of immersion). In this embodiment, the cooling tank 10 does not have a mechanism for circulating the coolant W, and the coolant W is not flowing. Therefore, after the support table 21 stops at time ts, the position of the coolant W around the workpiece S hardly changes.

[0082] Therefore, no relative flow velocity occurs between the workpiece S and the coolant W around the workpiece S, and there is little possibility that a difference in cooling rate will occur above and below the workpiece S due to a difference in relative flow velocity between the workpiece S and the coolant W. As a result, as shown in Figure 7, from immediately after the start of immersion through time ts to time te.

[0083] In this embodiment, the vapor film disappears at time te, which is later than time ts. In the shaft according to this embodiment, the axial end at the bottom of the workpiece S cools the fastest, and when this end reaches a temperature of 545°C in FIG. 4 (the surface temperature of the workpiece S is 570°C), the shaft transitions to the boiling stage. In the shaft according to this embodiment, when the periphery of a portion of the workpiece S transitions to the boiling stage, this transition triggers the vapor film around the shaft to peel off, causing the shaft to transition to the boiling stage. Therefore, in the example shown in FIG. 6, the vapor film peels off when the surface temperature of the top of the workpiece S is 710°C and the surface temperature of the bottom is 680°C.

[0084] When the vapor film disappears, the liquid coolant W comes into contact with the surface of the workpiece S. Because the surface heat transfer coefficient of the liquid coolant W is greater than that of the vapor film coolant W, cooling progresses rapidly after time te. For a while after time te, a temperature difference between the bottom and top of the workpiece S is observable, as shown in Figure 6. However, after the workpiece S is rapidly cooled, the cooling rate slows around time tb, as shown in Figure 6, and the temperature difference between the bottom and top gradually disappears. Therefore, from around time tb, the degree of thermal expansion at the top of the workpiece S approaches the degree of thermal expansion at the bottom, and the amount of distortion gradually decreases after time tb, as shown in Figure 7.

[0085] Then, before time tm when the surface temperature of the workpiece S reaches the martensitic transformation start temperature (represented as the carburized layer Ms point in FIG. 6 ), the temperature difference between the bottom and top of the workpiece S is almost eliminated, and the strain converges to its minimum value (approximately 9 μm) after time tb. As described above, in Example 1 shown in FIGS. 6 and 7 , the descent of the workpiece S is stopped while a vapor film is formed around the workpiece S, and the coolant W is maintained in a non-flowing state. Therefore, even during the period when the vapor film is formed and after the vapor film disappears and rapid cooling begins, the coolant W around the workpiece S does not flow, and no relative flow velocity occurs between the workpiece S and the coolant W. Therefore, differences in cooling rate due to differences in relative flow velocity are unlikely to occur depending on the part of the workpiece S. Therefore, partial cooling does not progress in the workpiece S, and temperature differences in the workpiece S are unlikely to occur. Therefore, distortion of the workpiece S is unlikely to occur.

[0086] When the surface of the workpiece S reaches a temperature below the martensitic transformation start temperature, martensite begins to form on the surface of the workpiece S. As shown in Figure 6, there is no difference in the change over time in the martensite fraction on the surface between the bottom and top of the workpiece S. Therefore, the amount of strain remaining in the workpiece S after quenching is very small.

[0087] FIG. 8 is a diagram showing the temperature change and martensite fraction of the workpiece S as Comparative Example 1. The horizontal axis represents time in logarithm, the left vertical axis represents temperature, and the right vertical axis represents martensite fraction. The graph in FIG. 8 shows the changes in the temperature and martensite fraction of the workpiece S over time after the workpiece S comes into contact with the coolant W. In FIG. 8, the surface temperature and martensite fraction at the bottom of the workpiece S are indicated by solid lines. The surface temperature and martensite fraction at the top of the workpiece S are indicated by dashed lines, and the temperature and martensite fraction at the interior (center) of the workpiece S are indicated by dashed lines.

[0088] Fig. 9 shows the change in the amount of strain over time in Comparative Example 1. In Fig. 9, the horizontal axis represents time in logarithm, and the vertical axis represents the amount of strain. The definition of the amount of strain is the same as that in Example 1 shown in Fig. 7.

[0089] When the immersion of the workpiece S begins, a slight temperature difference occurs between the lowermost and uppermost surfaces of the workpiece S, as shown in FIG. 8, reflecting the difference in the immersion start timing between the upper and lower parts of the workpiece S. However, in Comparative Example 1, the coolant W flows downward at the same speed as the descending speed of the workpiece S, so the temperature difference is smaller than in Example 1. Specifically, the descending speed Ve when the support base 21 moves the workpiece S downward and the flow speed Vq of the coolant W are the same in magnitude and direction. Therefore, the coolant W moves downward together with the workpiece S, making it difficult for a relative flow velocity to occur between the workpiece S and the coolant W.

[0090] 8, in Comparative Example 1, the workpiece is cooled while the temperatures of the bottom and top are close to each other during the vapor film maintenance period T. Note that, also in Comparative Example 1, heat exchange between the workpiece S and the coolant W during the vapor film maintenance period T occurs via the vapor film. Therefore, the cooling rate after the start of immersion is relatively slow, at about 100°C / second.

[0091] In the workpiece S, distortion reflecting the temperature difference occurs, and therefore, in Comparative Example 1, the amount of distortion is positive (convex upward) immediately after the start of immersion, reflecting the fact that the lower part of the workpiece S is cooler than the upper part. In Comparative Example 1, the temperature difference between the top and bottom of the workpiece S immediately after the start of immersion is smaller than in Example 1, and therefore the amount of distortion immediately after the start of immersion is smaller than in Example 1. In Comparative Example 1, the temperature difference between the top and bottom of the workpiece S is almost constant, and therefore the amount of distortion does not change much until time ts.

[0092] In Comparative Example 1, the support table 21 also stops during the vapor film maintenance period T during which the vapor film is maintained. In FIG. 8, the support table 21 stops at time ts (the time 1.3 seconds have elapsed since the start of immersion). However, in Comparative Example 1, the coolant W descends around the workpiece S at a descending velocity Vq. Therefore, when the support table 21 is stopped at time ts, the coolant W around the workpiece S flows downward. Therefore, in the axial center portion of the workpiece S, the coolant W is stirred on the upstream side of the flow of the coolant W (above the workpiece S), and the relative flow velocity between the workpiece S and the coolant W is high, while the coolant W stagnates on the downstream side (below the workpiece S), resulting in a low relative flow velocity.

[0093] Under such circumstances, cooling is promoted around the upper part of the workpiece S, and cooling is suppressed around the lower part of the workpiece S.

[0094] As described above, in Comparative Example 1, after the support table 21 stops at time ts, the upper part of the workpiece S cools more quickly than the lower part. Referring to Fig. 8, it can be seen that the surface temperature of the upper part of the workpiece S decreases earlier than the surface temperature of the lower part. In this case, as time passes, the degree of thermal expansion of the lower part of the workpiece S becomes greater than that of the upper part, and as shown in Fig. 9, after time ts, the amount of distortion changes from positive to negative.

[0095] In Comparative Example 1, the vapor film also disappears at time te, but in the rapid cooling stage immediately after the vapor film disappears, the surface temperature of the upper part is lower than that of the lower part, and cooling of the upper part is more rapid than that of the lower part. As a result, the amount of negative strain increases rapidly after time te.

[0096] After the workpiece S is rapidly cooled, the cooling rate slows down around time tb, as shown in FIG. 8, and therefore the temperature difference between the upper and lower parts is eliminated in Comparative Example 1 as well. Therefore, as shown in FIG. 9, the absolute value of the strain decreases after time tb. However, in Comparative Example 1, the flow of coolant W promotes cooling of the upper part, so even if the cooling rate slows down around time tb, the temperature difference between the upper and lower parts is difficult to eliminate. In this Comparative Example, as shown in FIG. 8, a temperature difference exists between the bottom and top of the workpiece S even at time tm, when the martensitic transformation start temperature (carburized layer Ms point) is reached on the surface of the workpiece S.

[0097] Thus, in this comparative example, there is a temperature difference between the bottom and top of the workpiece S when the martensitic transformation start temperature is reached, and the top of the workpiece S is consistently cooler than the bottom from time te to time tm. Therefore, considering the surface temperature of the workpiece S, it would seem that the bottom of the workpiece S has a greater degree of thermal expansion than the top, resulting in a downward convex shape. However, in the comparative example, as shown in Figure 9, the amount of strain of the workpiece S becomes a positive value at time tr, resulting in an upward convex shape.

[0098] This is thought to be because the martensitic transformation start temperature is reached earlier in the extremely superficial portion of the workpiece S than in a portion slightly deeper below the surface, causing volume expansion due to martensitic transformation. That is, in Figure 8, the portion slightly deeper below the surface is defined as the surface of the workpiece S, and the portion slightly deeper below the surface is labeled "surface martensite" to indicate the martensite fraction, but in the actual surface layer of the workpiece S, martensitic transformation may start earlier.

[0099] As described above, when a temperature difference occurs on the surface or surface layer of the workpiece S, the timing at which martensitic transformation begins varies depending on the part of the workpiece S. As a result, as shown in Fig. 9, the amount of strain in the workpiece S differs from the value predicted from only the surface of the workpiece S, and changes in a very complex manner over time.

[0100] Furthermore, the surface of the upper part of the workpiece S, which is slightly deeper than the surface layer, reaches the martensitic transformation start temperature at time tm, and the martensite fraction increases after time tm. At time tm, there is a temperature difference between the upper and lower surfaces of the workpiece S, and the lower part reaches the martensitic transformation start temperature later, so the martensite fraction increases in the lower part later than in the upper part.

[0101] Therefore, the change in volumetric expansion over time due to martensitic transformation differs between the upper and lower parts of the workpiece S, and such differences in the degree of volumetric expansion between parts of the workpiece S cause distortion. In Comparative Example 1 shown in FIG. 9, the amount of distortion changes due to various factors, resulting in a distortion of 22 μm after quenching is completed. As described above, in Comparative Example 1, the amount of distortion changes in a complex manner, resulting in a larger amount of distortion than in the example in which the coolant W does not flow after the support base 21 is stopped.

[0102] As explained above with reference to Fig. 6, in Example 1, the lowering of the support table 21 is stopped within the vapor film maintenance period T, and a state in which no relative flow velocity is applied between the workpiece S and the coolant W is maintained at least until the surface of the workpiece S transforms into martensitic. Therefore, even if there is a slight temperature difference between different parts of the workpiece S when the vapor film disappears, the temperature difference is eliminated or almost eliminated when the martensitic transformation start temperature is reached. Therefore, differences in the martensite formation rate between different parts of the workpiece S are unlikely to occur, and distortion is unlikely to occur.

[0103] 10 shows the distortion (bending) (μm), elongation (μm), and internal hardness (Hv) of the workpieces S of Example 1 and Comparative Example 1. The distortion (bending) is defined in the same way as in FIGS. 7 and 9. The elongation is the difference between the axial length of the workpiece S after quenching and the axial length of the workpiece S at room temperature that has not been subjected to heat treatment. The internal hardness is the Vickers hardness at the center of the workpiece S.

[0104] In the above example, comparing Example 1 and Comparative Example 1, the internal hardness is equivalent, but the distortion amounts are 9 μm and 22 μm, respectively, and the elongation amounts are 131 μm and 150 μm, respectively. Therefore, Example 1 has smaller distortion amounts and elongation amounts. As described above, during quenching in Example 1, the coolant W is not fluidized by an external force. Therefore, compared to Comparative Example 1, in which the coolant W flows and the cooling rate is uneven at each location of the workpiece S, Example 1 has smaller distortion amounts and elongation amounts.

[0105] As shown in Figure 4, if the liquid has a temperature range in which the change in the surface heat transfer coefficient relative to the temperature drop after immersion is less than a predetermined value, there is a vapor film maintenance period T, and by completing the immersion and stopping the descent within this vapor film maintenance period T, quenching can be performed with little heat treatment deformation and high surface hardness. However, if the maximum surface heat transfer coefficient in the boiling stage is 6000 W / m 2When a coolant W with a viscosity of 0.1 K or higher is used, the internal hardness and hardened layer depth of a workpiece S of the same material can be improved, resulting in a deeper effective hardening depth, compared to when hot oil is used. Figure 11 shows the hardness of Example 1, in which quenching was performed using a 20% water-soluble coolant at a descending speed Ve of 400 mm / s, as described above, and Comparative Example 2, in which quenching was performed using hot oil at a descending speed Ve of 400 mm / s, after which the relative speed between the workpiece S and the surrounding coolant W was set to 400 mm / s. In the graph shown in Figure 11, the horizontal axis represents the distance from the center, and the vertical axis represents Vickers hardness. Note that the center is the cylindrical axis of the workpiece S, and the distance from the cylindrical axis toward the bottom is shown as a negative value, while the distance from the cylindrical axis toward the top is shown as a positive value. In Figure 11, the Vickers hardness of Example 1 is shown as a solid line, and the Vickers hardness of Comparative Example 2 is shown as a dashed line. Also shown is a dashed line indicating a Vickers hardness of 513 HV, which serves as an index for evaluating the effective hardening depth.

[0106] As shown in this figure, the Vickers hardness of Comparative Example 2 is lower than that of Example 1 throughout the entire area from the surface to the interior of the workpiece S. For this reason, it can be said that Comparative Example 2 has a shallower effective hardening depth than Example 1. In this way, when a water-soluble coolant is used as the coolant W when quenching a workpiece S made of the same material, a workpiece S with a deeper effective hardening depth can be obtained than when hot oil is used.

[0107] (4) Other embodiments: The above embodiment is merely an example of how the present invention can be implemented, and various other embodiments are possible. For example, the workpiece S is not limited to a shaft; various objects, such as gears and building components, may be used as the workpiece. Furthermore, various positions may be adopted for the workpiece S during quenching. Any configuration may be adopted as long as the workpiece S is fixed to a portion, such as the support base 21, through which the workpiece S is lowered, and the workpiece S remains stationary in the coolant W, which is not flowing, as the support base 21 lowers. Furthermore, the atmospheric pressure of the cooling tank 10 may be adjusted to stabilize the generation of the vapor film. Specifically, the higher the atmospheric pressure in the cooling tank 10, the more stable the vapor film, and the lower the atmospheric pressure, the more unstable the vapor film. Therefore, the atmospheric pressure may be increased to a predetermined value or higher until the workpiece's descent is complete, and then reduced to a value lower than the predetermined value after the workpiece's descent is stopped.

[0108] Quenching may be any heat treatment in which a metal is heated to a predetermined temperature and then rapidly cooled, and various materials may be used as the workpiece. For example, the quenching is not limited to the quenching in which carburized steel is rapidly cooled as in the above-described embodiment, but may be a quenching in which carbon-containing steel is prepared in advance and then rapidly cooled after heating. Furthermore, the workpiece may be a material that has been subjected to carbonitriding or nitriding.

[0109] Furthermore, the material of the workpiece is not limited as long as it is subject to quenching. Examples of suitable workpieces include various types of steel, general rolled steel, carbon steel, alloy steel, carburizing steel, tool steel, spring steel, bearing steel, hot-rolled steel plate, cold-rolled steel plate, and carbon steel castings. Furthermore, suitable workpieces include steel materials defined by material standards such as JIS, SAE, and DIN, such as JIS S35C, JIS S45C, JIS SCM440, JIS SCM420, JIS SCM415, JIS SCR440, JIS SCR420, MSB20, DEG, and AG20. These materials may also be subjected to carburizing, carbonitriding, or nitriding treatments.

[0110] In the lowering control step, the support table is lowered into the coolant accumulated in the cooling tank in a non-flowing state, and the support table can be controlled so that the immersion of the workpiece is completed and the support table is stopped while a vapor film of the coolant is formed around the workpiece due to the heat of the workpiece. In other words, the lowering control step is performed so that the immersion is completed and the support table is stopped before the vapor film disappears. The cooling tank can be of any shape, capacity, etc., as long as it can accumulate the coolant W in a non-flowing state. Note that a device for applying an external force to the coolant W to cause it to flow may or may not be installed. If installed, the coolant W can be flowed after the surface of the workpiece has undergone martensitic transformation, allowing for early cooling and removal of the workpiece.

[0111] The coolant W may be any material capable of forming a vapor film around the workpiece S. In other words, the coolant W may be any liquid in which the formation of a vapor film reduces the heat transfer between the workpiece S and the coolant W compared to when the workpiece S is boiling. Typical examples of such coolants W include liquids in which, in a graph of the surface heat transfer coefficient vs. temperature, there is a temperature range in which the surface heat transfer coefficient between the workpiece S and the coolant W after immersion is flat (the change per unit temperature is below a predetermined value). In addition to the water-soluble coolants described above, various other coolants W may be used, such as various aqueous solutions in which various materials are dissolved in water, or quenching oil.

[0112] Furthermore, when the vapor film stage is changed to the boiling stage, heat is transferred from the workpiece to the coolant, and in order to perform quenching properly, it is preferable that the heat transfer be fast in the boiling stage. The degree of heat transfer can be evaluated by the surface heat transfer coefficient, and in order to perform quenching properly in the boiling stage, the maximum surface heat transfer coefficient in the boiling stage should be 6000 W / m 2·K or more. The surface heat transfer coefficient is the amount of heat per unit area and unit temperature when heat is transferred from the workpiece to the coolant, so the larger the surface heat transfer coefficient, the more rapid the cooling. The maximum surface heat transfer coefficient at the boiling stage is 6000 W / m 2 It is known that if the surface heat transfer coefficient is 7000 W / m or more, the cooling rate inside the workpiece S can be increased without causing the coolant around the workpiece to flow, martensitic transformation can occur, and quenching can be performed to improve the internal hardness and the depth of the hardened layer. Of course, the higher the surface heat transfer coefficient, the better, and the maximum value is 7000 W / m 2 ·K or more, 8000W / m 2 It is more preferable that the temperature is .K or higher.

[0113] The support base for lowering the workpiece may have various configurations. For example, as in the above-described embodiment, it may be a support base on which the workpiece is placed, a support base equipped with a support portion for supporting the workpiece and an insertion portion for inserting the workpiece, or a cage-shaped support base through which the coolant W passes. Alternatively, it may be a support base on which the workpiece can be fixed using various fixing members, and various other configurations may be adopted.

[0114] The vapor film is a film of vapor formed by the transfer of heat from the workpiece to the coolant W. In other words, the state in which the vapor is in direct contact with the workpiece and a layer of vapor is formed covering the workpiece is the state in which the vapor film is formed. The vapor film only needs to prevent direct contact between the workpiece and the liquid coolant W. The state in which the vapor film is formed is assumed to be a state in which steam is present over the entire outer surface of the workpiece and the outer surface of the workpiece is not in contact with the liquid coolant W, but the outer surface of the workpiece may also be in localized contact with the liquid coolant W to the extent that it does not affect distortion.

[0115] Immersion may be any process in which the workpiece is immersed in coolant W. In other words, immersion is considered complete when the workpiece changes from a state in which no coolant W is present around it to a state in which the entire workpiece is immersed in coolant W. Immersion is complete when the top of the workpiece is below the liquid surface of coolant W, but to complete immersion while a vapor film is formed, it is not necessary to lower the support table to an excessively deep position. For example, the support table may be lowered to a depth that will prevent the top of the workpiece from being exposed above the liquid surface of coolant W due to fluctuations in the liquid surface.

[0116] The support stand may be stopped as long as it can realize a state in which, when the support stand is stopped, no relative flow velocity occurs between the workpiece and the coolant W surrounding the workpiece. In other words, it is sufficient to realize a state in which, inside the coolant W that is not flowing, the workpiece also becomes stationary when the support stand is stopped, and no relative flow velocity occurs except for natural convection, etc.

[0117] The state maintaining step is a step of, after stopping the lowering of the support table, maintaining a state in which the coolant is not flowing and the support table is not moving so that no relative flow velocity is imparted between the workpiece and the coolant W, at least until the surface of the workpiece is transformed into martensitic. In other words, after the immersion is completed, it is sufficient that the workpiece is held motionless within the coolant W until the surface of the workpiece is transformed into martensitic.

[0118] Furthermore, it is preferable that the workpiece and the support stand are supported on the support stand by point contact or line contact. Even if the workpiece is in point contact or line contact, if the workpiece is fixed to the support stand by being supported or gripped at multiple positions (for example, three or more points), it is possible to realize a state in which no relative flow velocity is applied between the workpiece and the coolant W.

[0119] It should be noted that a state in which no relative flow velocity is imparted between the workpiece and the coolant W is maintained from the start of immersion until at least the surface of the workpiece is transformed into martensitic. The state in which a relative flow velocity is imparted can be achieved by a mechanism other than the device for lowering the support stand for the workpiece, for example, a mechanism for circulating the coolant W. If the mechanism does not cause the coolant W to flow, a state in which no relative flow velocity is imparted between the workpiece and the coolant W is maintained until the surface of the workpiece is transformed into martensitic.

[0120] In other words, during the lowering control process and the state maintaining process, the coolant W is not fluidized by a mechanism for fluidizing the coolant W, and is in a non-fluidized state unless slight movement such as natural convection is considered to be a flow. Thus, until the surface of the workpiece undergoes martensitic transformation, no external force is applied to the coolant W for the purpose of moving the coolant W, and in this sense, the coolant W does not circulate within the cooling tank. However, the coolant W may move due to an external force that is not intended to move it. For example, the coolant W may acquire a flow velocity of 0 or more as the workpiece descends, or the coolant W may move due to natural convection. Such movement is not included in the movement of the coolant W due to an external force.

[0121] After the surface of the workpiece has been transformed into martensitic, the coolant W may be moved by an external force. If the coolant W moves, a difference in cooling rate may occur between the upper and lower parts of the workpiece.

[0122] Therefore, if the coolant W flows around the workpiece while the support table is stopped, differences in the cooling rate of the workpiece may occur, which may cause distortion. However, after the entire surface of the workpiece has undergone martensitic transformation, the workpiece is hardened and distortion is unlikely to occur. Therefore, even if the coolant W is moved by an external force after the entire surface of the workpiece has undergone martensitic transformation, distortion will not occur or will be small. Therefore, if the coolant W is moved after martensitic transformation, cooling will be promoted, and the workpiece will be cooled quickly, allowing it to be removed from the coolant W.

[0123] Whether the surface of the workpiece has been transformed into martensitic can be determined by various methods, such as by the time elapsed after the workpiece was immersed. The time elapsed can be determined by the martensite fraction or the surface temperature of the workpiece. For example, the martensite fraction can be set to a standard value such as 21%, 28%, 50%, or 61%, and the time required for the martensite fraction to reach the standard value can be statistically determined.

[0124] 12 and 13 show Example 2, in which a shaft similar to that shown in FIGS. 6 and 7 is immersed in coolant W, and the coolant W is allowed to flow after the martensite fraction reaches 21%. The graph notation in FIGS. 12 and 13 is the same as that in FIGS. 6 and 7. In Example 2 shown in FIGS. 12 and 13, the coolant W is a 20% water-soluble coolant and is at room temperature (e.g., 25°C). The descending speed of the support base 21 is 400 mm / s. Furthermore, the flow velocity Vq of the coolant W is 0 mm / s until the martensite fraction reaches 21%. After the martensite fraction reaches 21%, the flow velocity Vq of the coolant W is 200 mm / s downward. Furthermore, the stroke ST is 520 mm, and the internal temperature of the workpiece S at the start of immersion in the coolant W is 850°C. The workpiece S is subjected to carburization treatment to a surface layer of 3 mm with a surface carbon concentration of 0.8 (mass%).

[0125] In Example 2 shown in Figures 12 and 13, the operation until the martensite fraction reaches 21% is generally the same as in Example 1 shown in Figures 6 and 7. Therefore, the changes in the surface temperature and martensite fraction until the martensite fraction reaches 21% are the same as in the examples shown in Figures 6 and 7. At time tm, when the surface temperature of the workpiece S reaches the martensitic transformation start temperature (represented as the carburized layer Ms point in Figure 12), martensitic transformation progresses. After time tm, the surface martensite fraction increases, as shown by the surface martensite (top) and (bottom) in Figure 12.

[0126] When the martensite fraction reaches 21% at time tw, the coolant W begins to flow. In this example, the flow velocity is 200 mm / s downward. When this flow velocity occurs around the workpiece S, the temperature of the upper part of the workpiece S drops more than that of the lower part after time tw. Furthermore, after time tw, the martensite fraction increases earlier in the upper part of the workpiece S than in the lower part. After time tw, the amount of strain fluctuates after time tw, as shown in Figure 13, reflecting the temperature difference (difference in the degree of thermal expansion) between the upper and lower parts of the workpiece S and the difference in the magnitude of the martensite fraction (difference in the degree of expansion associated with martensitic transformation). However, it eventually converges to a constant value. In this example, the flow of the coolant W begins when the martensite fraction is relatively low, so deformation occurs due to the difference in expansion associated with martensitic transformation. This deformation continues as the martensitic transformation progresses, resulting in a larger deformation than when there is no coolant flow, as shown in Figures 6 and 7. The final strain value is approximately 23 μm. When deformation is acceptable, allowing the coolant W to flow in this manner allows the temperature of the workpiece S to be lowered more quickly than when the coolant W is not allowed to flow, and allows the workpiece S to be removed from the cooling tank 10 more quickly.

[0127] Figures 14 and 15 show Example 3 in which a shaft similar to that shown in Figures 6 and 7 was immersed in coolant W, and the coolant W was allowed to flow after the martensite fraction reached 61%. The notation used for the graphs in Figures 14 and 15 is the same as that used in Figures 6 and 7. Example 3 shown in Figures 14 and 15 is the same as Example 2 shown in Figures 12 and 13, except for the timing at which the flow of coolant W by the flow section begins.

[0128] When the martensite fraction reaches 61% at time tw, the coolant W begins to flow. As a result, after time tw, the temperature of the upper part of the workpiece S drops more than that of the lower part. Also, after time tw, the martensite fraction increases earlier in the upper part of the workpiece S than in the lower part. After time tw, the amount of strain fluctuates after time tw as shown in Figure 15, reflecting the temperature difference between the upper and lower parts of the workpiece S (difference in the degree of thermal expansion) and the difference in the magnitude of the martensite fraction (difference in the degree of expansion associated with martensitic transformation), but eventually converges to a constant value. In this example, the final strain value is approximately 18 μm. Furthermore, after time tw when the martensite fraction on the surface reaches 61%, the results showed that even if the coolant W is allowed to flow, the degree of distortion is the same as when it is not allowed to flow. Therefore, in this embodiment, the coolant W can be allowed to flow without worsening the distortion once the martensite fraction reaches 61%, so the temperature of the workpiece S can be lowered earlier than when it is not allowed to flow, and the workpiece S can be removed from the cooling tank 10 earlier.

[0129] FIG. 16 shows the distortion (bending amount) (μm), elongation (μm), and internal hardness (Hv) of the workpiece S as Example 2 and Example 3 described above. The distortion (bending amount) is defined in the same way as in FIGS. 7 and 9 described above. The elongation and internal hardness are defined in the same way as in FIG. 10. Comparing Example 2 and Example 3, the internal hardness and elongation are equivalent, but the distortion is 23 μm and 18 μm, respectively. As described above, Example 2 has a lower martensite fraction and the coolant W was allowed to flow before the martensitic transformation had progressed sufficiently, resulting in a larger distortion than Example 3.

[0130] Furthermore, a plurality of workpieces S may be placed on the support table 21. The workpieces S may also be placed at a plurality of different positions in the vertical direction of the support table 21. Fig. 17 shows an example in which the workpieces S are placed at a plurality of different positions in the vertical direction of the support table 21. In this example, the configuration other than the support table 21 and the workpieces S placed on the support table 21 can be realized by the same configuration as the above-described embodiment.

[0131] 17, a plurality of workpieces S can be placed on the support table 21 at two different positions in the vertical direction. Here, three workpieces S are placed on the lower level of the support table 21 and three on the upper level, but it goes without saying that the workpieces S can also be placed in the depth direction of the drawing. Here, the workpiece S placed on the lower level is called the lowest workpiece S, and the workpiece S placed on the upper level is called the highest workpiece S. It goes without saying that the workpieces S can also be placed at three or more levels in the vertical direction.

[0132] In this embodiment, the immersion completion state is when the top end Eu of the top workpiece S is below the liquid level Sw of the coolant W. In this embodiment, as shown in FIG. 17 , the stroke ST is also determined so that the top end Eu of the top workpiece S is a predetermined distance Lg below the liquid level Sw of the coolant W. That is, the stroke ST is set so that the distance Lg between the top end Eu of the workpiece S and the liquid level Sw of the coolant W is greater than zero. Note that the distance Lg does not need to be excessively large and may be the minimum necessary length. That is, the distance Lg is set so that the workpiece S is not exposed to the outside of the coolant W, and when the descent of the workpiece S stops and the top of the workpiece S is cooled at a position distance Lg from the liquid level Sw, a stable vapor film can be generated, and the coolant W continues to cover the workpiece S even during the boiling stage after the vapor film disappears. Specifically, the distance Lg can be set to the maximum waviness length (height) + 30 mm, for example. The distance Lg may be set to less than one time the total height of the workpiece S, or one-half the total height, etc.

[0133] The descent speed Ve may be set so that the immersion of the top of the top workpiece S is completed while a vapor film of coolant W is forming around the bottom workpiece S, and may be set in accordance with the height H from the bottom end El of the bottom workpiece S to the top end Eu of the top workpiece S, the characteristics of the coolant W, etc. Specifically, in this embodiment, the descent continues until the distance (Lg+H) from the liquid level Sw of the coolant W to the bottom end El of the bottom workpiece S at the descent stop position of the support base 21 matches the stroke ST. The descent speed Ve is set to be faster than (stroke ST / time T for maintaining the vapor film around the bottom workpiece S).

[0134] When this speed is set, it is possible to complete the immersion of all of the workpieces S before the vapor film disappears. The support table 21 is lowered at the lowering speed Ve, and when the distance from the liquid level Sw matches the stroke ST, the support table 21 is stopped. With the above configuration, it is possible to complete the immersion of the topmost part (upper end Eu) of the topmost workpiece S and stop the support table 21 within the period in which the vapor film is formed on the bottommost workpiece S. Then, a state in which no relative flow velocity is applied between the workpieces S and the coolant W is maintained, at least until the surfaces of all of the workpieces S are transformed into martensitic.

[0135] According to this configuration, the movement of the workpieces S stacked vertically in multiple tiers is stopped while the lowest workpiece S is covered with a vapor film. Furthermore, the coolant W around the workpieces S does not flow until at least the surfaces of all of the workpieces S have been transformed into martensitic. Therefore, there is no factor that imparts a relative flow velocity between the workpieces S and the coolant W around all of the workpieces S to be treated, and there is no factor that partially accelerates the cooling of the workpieces S. This reduces the possibility of differences in cooling rate occurring in different parts of all of the workpieces S to be treated, thereby reducing the possibility of uneven progress of quenching. Furthermore, multiple workpieces S can be quenched at once.

[0136] In the above-described embodiment, the vapor film maintenance period T is the period from when the first immersed portion of the workpiece S begins to be immersed until the vapor film disappears around that portion and boiling begins. The vapor film maintenance period T is defined on the assumption that the area around the first immersed portion of the workpiece S will boil first, and this assumption holds true for many parts.

[0137] Furthermore, if the ease with which the vapor film peels off varies depending on the part of the workpiece S, this assumption can be established by positioning the part that peels off easily as the bottom end of the workpiece S when immersed. For example, the workpiece S includes parts of various shapes, and if there are sharp parts, the vapor film peels off more easily from the sharp parts than from non-sharp parts. In this case, the workpiece S may be placed on the support table 21 so that the sharp parts are at the bottom. Furthermore, if the workpiece S has many sharp parts, the sharp parts may be positioned at the bottom by orienting the workpiece S in a way that makes it easier to place it on the support table 21.

[0138] 18A and 18B show an example of a workpiece S having many sharp portions. In this example, the workpiece S is a ring gear Gr. The ring gear Gr is an annular part, and FIG. 18A shows the ring gear Gr as viewed along the central axis Ax of the ring, while FIG. 18B is a cross-sectional view showing the state cut along a plane including the central axis Ax of the ring. Teeth that mesh with other gears are formed on the outer periphery of the ring gear Gr. In FIG. 18A, the portions where the teeth are formed are indicated by dashed lines. The teeth of the ring gear Gr have sharper tips than other portions.

[0139] 18A and 18B, if the ring gear Gr is placed on the support base 21 so that the central axis Ax faces horizontally, the teeth will be at the bottom. Therefore, with this arrangement, the vapor film maintenance period T can be considered to be the period from when the first immersed portion of the workpiece S begins to be immersed until the vapor film disappears around that portion and boiling begins.

[0140] Figure 19 is a diagram showing an example of the support stand 210 when the ring gear Gr shown in Figures 18A and 18B is immersed. The support stand 210 is a stand on which the ring gear Gr can be placed in three vertical stages. Figure 19 shows the ring gear Gr in a cross section similar to that of Figure 18B, and also shows the support stand 210 cut away at that cross section.

[0141] The support base 210 has a mounting portion 210a on which the ring gear Gr is placed. The mounting portion 210a can be attached to the support base 210 at three different positions in the vertical direction. Recesses are formed in multiple positions in the mounting portion 210a, and the inner peripheral surface of the ring gear Gr can be positioned by engaging each of the recesses. The mounting portion 210a is set on the support base 210 with the inner peripheral surface of the ring gear Gr engaging each of the recesses. Of course, the workpiece S may also be placed in the depth direction of the drawing.

[0142] 19, in this example, the stroke ST is determined so that the upper end Eu of the topmost workpiece S is a predetermined distance Lg below the liquid level Sw of the coolant W. The workpiece S is lowered until the distance (Lg+H) from the liquid level Sw of the coolant W to the lower end El of the bottommost workpiece S at the position where the support base 210 stops lowering matches the stroke ST. The descent speed Ve is set to be faster than (stroke ST / steam film maintenance period T around the bottommost workpiece S).

[0143] When this speed is set, it is possible to complete the immersion of all of the workpieces S before the vapor film disappears. The support table 210 is lowered at the lowering speed Ve, and when the distance from the liquid level Sw matches the stroke ST, the support table 210 is stopped. With the above configuration, it is possible to complete the immersion of the top (upper end Eu) of the topmost workpiece S and stop the support table 210 within the period in which the vapor film is formed on the bottommost workpiece S. Then, a state in which no relative flow velocity is applied between the workpieces S and the coolant W is maintained, at least until the surfaces of all of the workpieces S are transformed into martensitic.

[0144] According to this configuration, the movement of the workpieces S stacked vertically in multiple tiers is stopped while the lowest workpiece S is covered with a vapor film. Furthermore, the coolant W around the workpieces S does not flow until at least the surfaces of all of the workpieces S have been transformed into martensitic. Therefore, there is no factor that imparts a relative flow velocity between the workpieces S and the coolant W around all of the workpieces S to be treated, and there is no factor that partially accelerates the cooling of the workpieces S. This reduces the possibility of differences in cooling rate occurring in different parts of all of the workpieces S to be treated, thereby reducing the possibility of uneven progress of quenching. Furthermore, multiple workpieces S can be quenched at once.

[0145] Furthermore, if the periphery of a portion different from the portion initially immersed transitions to the boiling stage first, the vapor film maintenance period T may be shorter. Figures 18C and 18D show an example of a treatment object S having locally sharp portions. In this example, the treatment object S is a drive shaft Sd. The drive shaft Sd is a substantially cylindrical component. Figure 18C shows the drive shaft Sd as viewed from a direction perpendicular to the central axis Ax, and Figure 18D is a cross-sectional view showing the drive shaft Sd cut along a plane including the central axis Ax of the ring. The drive shaft Sd is formed with a pinion gear portion Gp having teeth formed thereon that mesh with other gears in the circumferential direction centered on the central axis Ax, and a spline portion Sp having teeth formed thereon that mesh with other components.

[0146] In Figure 18C, details of the pinion gear portion Gp and the teeth formed on the spline portion Sp are omitted. In this embodiment, the teeth formed on the spline portion Sp have a shorter tooth height than the teeth formed on the pinion gear portion Gp, and the tooth tips are not sharp. Therefore, in the drive shaft Sd of this embodiment, the steam film is most likely to peel off around the tooth tips of the spline portion Sp. When the drive shaft Sd is placed on the support base 21 in the position shown in Figures 18C and 18D, the pinion gear portion Gp will transition to the boiling stage before the lower end, which begins to be immersed first. In this case, the steam film maintenance period T is considered to be the period from when the workpiece S begins to be immersed until the surrounding steam film disappears and a portion begins to boil.

[0147] Figure 20 is a schematic diagram illustrating the period during which a vapor film is maintained for each portion of the drive shaft Sd when the drive shaft Sd is immersed. Figure 20 shows the period during which a vapor film is maintained for each portion when the drive shaft Sd is immersed in the same orientation as Figures 18C and 18D, i.e., with the spline portion Sp below the pinion gear portion Gp and with the central axis Ax facing up or down. Note that Figure 20 shows periods Tl, Ts, and Tg during which a vapor film is maintained along the time axis t.

[0148] In Figure 20, it is assumed that the support base 21 on which the drive shaft Sd is placed is lowered, and that the immersion of the lower end El begins at time tls. Subsequently, it is assumed that the immersion of the portion Ps begins at time tss, and the immersion of the portion Pg begins at time tgs. When the immersion begins, a vapor film forms around each portion. In this example, the vapor film formed around the lower end El disappears at time tle, and the area around the lower end El transitions to the boiling stage after time tle. Furthermore, the vapor film formed around the portion Ps disappears at time tse, and the area around the portion Ps transitions to the boiling stage after time tse. Furthermore, the vapor film formed around the portion Pg disappears at time tge, and the area around the portion Pg transitions to the boiling stage after time tge.

[0149] In this example, the vapor film around the portion Pg disappears the fastest. In this case, the vapor film maintenance period T is the period from when the workpiece S begins to be immersed until the surrounding vapor film disappears and the portion Pg begins to boil, i.e., the period Tsd from time tls to time tge. In this case, in the lowering control process, the support table 21 is controlled so that the immersion of the workpiece S is completed and the support table 21 stops within the period Tsd. Figure 21 is a diagram showing the state where immersion is completed when the drive shaft Sd is the workpiece S. Figure 21 shows the state in which the workpiece S is placed on the support table 21 with the spline portion Sp facing downward relative to the pinion gear portion Gp.

[0150] 21, in this example, the stroke ST is determined so that the upper end Eu of the workpiece S is a predetermined distance Lg below the liquid level Sw of the coolant W. The distance Lg may be set in the same manner as in the above example. The descent speed Ve may be set so that the immersion of the workpiece S is completed while a vapor film of the coolant W is being formed around the pinion gear portion Gp, and may be set in accordance with the height H from the lower end El of the workpiece S to the upper end Eu of the workpiece S, the characteristics of the coolant W, etc.

[0151] Specifically, in this embodiment, the coolant W is lowered until the distance (Lg+H) from the liquid level Sw to the lower end El of the workpiece S at the lowering stop position of the support stand 21 matches the stroke ST. If the stroke from the liquid level Sw to the lower end Ep of the pinion gear portion Gp, which is the portion where the vapor film first peels off, is defined as the stroke STp, and the period from when the lower end Ep of the pinion gear portion Gp is ​​immersed until the periphery of the lower end Ep transitions to the boiling stage (see FIG. 20), the descent speed Ve is set to be faster than STp / Tg.

[0152] When this speed is set, the lower end Ep of the pinion gear portion Gp reaches the depth of the stroke STp faster than the time period Tg has elapsed since the start of immersion of the lower end Ep of the pinion gear portion Gp. Therefore, as shown in FIG. 20 , when the condition that the portion where the vapor film disappears the fastest is the portion Pg is satisfied, immersion can be completed and the support table 21 can be stopped while the vapor film is maintained all around the workpiece S. Furthermore, a state in which no relative flow velocity is applied between the workpiece S and the coolant W is maintained at least until the surface of the workpiece S transforms into martensitic material. This configuration reduces the possibility of differences in cooling rate between different portions of the workpiece S, thereby reducing the possibility of uneven progress of hardening.

[0153] 18C and 18D, the drive shaft Sd serving as the workpiece S is asymmetric in the vertical direction. Therefore, the distance of the stroke STp differs between when the spline portion Sp faces downward relative to the pinion gear portion Gp as shown in FIG. 21 and when the vertical direction is reversed, i.e., when the spline portion Sp faces upward relative to the pinion gear portion Gp. Specifically, when the spline portion Sp faces upward relative to the pinion gear portion Gp, the lower end of the pinion gear portion Gp is ​​located deeper than in the state shown in FIG. 21. Therefore, the stroke STp to the lower end of the pinion gear portion Gp becomes deeper. In this state, the lower limit value STp / Tg of the descent speed Ve becomes larger, and faster control is required.

[0154] For this reason, it is preferable that the workpiece S, which is asymmetric in the vertical direction, is placed on the support table 21 so that the part of the workpiece S where the surrounding vapor film disappears the earliest and boiling begins faces upward in the vertical direction. With this configuration, compared to when the workpiece S is placed on the support table 21 with the workpiece S facing in the opposite vertical direction, it is not necessary to make the descent speed Ve excessively fast, and the degree of freedom in designing the device that lowers the support table 21 is increased.

[0155] Furthermore, the workpieces S may be placed at a plurality of different positions in the vertical direction of the support table 21. Fig. 22 shows an example in which drive shafts Sd as the workpieces S are placed at a plurality of different positions in the vertical direction of the support table 21. In Fig. 22, a plurality of workpieces S can be placed at two different positions in the vertical direction on the support table 21. The number of workpieces S that can be arranged in the vertical and horizontal directions may be arbitrary, and the workpieces S may be placed in the depth direction of the drawing.

[0156] In this embodiment, the immersion is complete when the upper end Eu of the top workpiece S is below the liquid level Sw of the coolant W. In this embodiment, as shown in FIG. 22 , the stroke ST is determined so that the upper end Eu of the top workpiece S is a predetermined distance Lg below the liquid level Sw of the coolant W. This distance Lg may be set in the same manner as in the above-described example. The descent speed Ve may be set so that the immersion of the top workpiece S is complete while a vapor film of the coolant W is forming around the pinion gear portion Gp of the bottom drive shaft Sd. The descent speed Ve may be set in accordance with the height from the lower end Ep of the pinion gear portion Gp of the bottom drive shaft Sd to the upper end Eu of the top workpiece S, the characteristics of the coolant W, and the like.

[0157] Specifically, in this embodiment, the coolant W is lowered until the distance (Lg+H) from the liquid level Sw to the lower end El of the lowest workpiece S at the lowering stop position of the support stand 21 matches the stroke ST. If the stroke from the liquid level Sw to the lower end Ep of the pinion gear portion Gp, which is the portion of the lowest drive shaft Sd where the vapor film first peels off, is defined as the stroke STp, and the period from when the lower end Ep of the pinion gear portion Gp is ​​immersed until the periphery of the lower end Ep transitions to the boiling stage (see FIG. 20), the descent speed Ve is set to be faster than STp / Tg.

[0158] When this speed is set, it is possible to complete the immersion of all of the workpieces S before the vapor film on the lowest workpiece S disappears. The support table 21 is lowered at the lowering speed Ve, and when the distance from the liquid level Sw matches the stroke ST, the support table 21 is stopped. With the above configuration, it is possible to complete the immersion of the topmost part (upper end Eu) of the workpiece S on the topmost level and stop the support table 21 within the period in which the vapor film on the lowest workpiece S is formed. Then, a state in which no relative flow velocity is applied between the workpieces S and the coolant W is maintained, at least until the surfaces of all of the workpieces S are transformed into martensitic.

[0159] According to this configuration, the movement of the workpieces S stacked vertically in multiple tiers is stopped while the lowest workpiece S is covered with a vapor film. Furthermore, the coolant W around the workpieces S does not flow until at least the surfaces of all the workpieces have been transformed into martensitic. Therefore, there is no factor that imparts a relative flow velocity between the workpieces S and the coolant W around all of the workpieces S to be treated, and there is no factor that partially accelerates the cooling of the workpieces S. This reduces the possibility of differences in cooling rate occurring in different parts of all of the workpieces S to be treated, thereby reducing the possibility of uneven progress of quenching. Furthermore, multiple workpieces S can be quenched at once.

[0160] Figure 23 is a diagram showing an example of the support stand 211 when immersing the drive shaft Sd shown in Figures 18C and 18D. The support stand 211 is a stand on which multiple drive shafts Sd can be placed in two vertical stages. Figure 23 shows the drive shaft Sd in a cross section similar to that of Figure 18D, and also shows the support stand 211 cut at that cross section.

[0161] The support base 211 has insertion portions 211a and 211b into which the drive shaft Sd is inserted. The insertion portions 211a and 211b are members having inner circumferential surfaces that are slightly larger than the outer circumferential surface of the drive shaft Sd at multiple locations (three locations in this embodiment). These insertion portions 211a and 211b hold the drive shaft Sd in an upright position with the spline portion Sp of the drive shaft Sd inserted into them.

[0162] In this embodiment, insertion portions 211a and 211b are formed at two different positions in the up-down direction on one drive shaft Sd. In this embodiment, insertion portion 211a is connected to coupling portion 211c, and coupling portion 211c extends in a predetermined direction and is connected to another coupling portion 211c or a portion extending in the up-down direction from support base 211. Insertion portion 211b is connected to coupling portion 211d, and coupling portion 211d extends in a predetermined direction and is connected to another coupling portion 211d or a portion extending in the up-down direction from support base 211.

[0163] The insertion portions 211a and 211b are capable of inserting and holding the drive shaft Sd, and the connecting portions 211c and 211d are only required to be able to support the insertion portions 211a and 211b. Figure 24 shows the insertion portion 211a and connecting portion 211c for inserting one drive shaft Sd as viewed from the direction of the central axis Ax of the drive shaft Sd. As shown in Figure 24, the connecting portion 211c extends in three directions from a portion where the center of the drive shaft Sd is located, and an insertion portion 211a is formed on each of the connecting portions. The connecting portions 211c are connected to other connecting portions 211c at portions not shown.

[0164] 23, the coolant W is lowered until the distance (Lg+H) from the liquid level Sw to the lower end El of the lowest workpiece S at the lowering stop position of the support stand 21 matches the stroke ST. When the stroke from the liquid level Sw to the lower end Ep of the pinion gear portion Gp, which is the portion of the lowest drive shaft Sd where the vapor film first peels off, is defined as the stroke STp, and the period from when the lower end Ep of the pinion gear portion Gp is ​​immersed until the periphery of the lower end Ep transitions to the boiling stage (see FIG. 20), the descent speed Ve is set to be faster than STp / Tg.

[0165] By setting this speed, it is possible to complete immersion of all workpieces S before the vapor film on the lowest workpiece S disappears. The support table 211 is lowered at the lowering speed Ve, and when the distance from the liquid level Sw matches the stroke ST, the support table 211 is stopped. With the above configuration, it is possible to complete immersion of the top (upper end Eu) of the top workpiece S and stop the support table 211 within the period during which the vapor film is formed on the lowest workpiece S. Furthermore, a state in which no relative flow velocity is applied between the workpieces S and the coolant W is maintained at least until the surfaces of all workpieces S to be treated undergo martensitic transformation. With this configuration, the possibility of differences in cooling rate between different parts of the workpieces S is reduced, reducing the possibility of uneven progress of quenching. Furthermore, it is possible to quench multiple workpieces S at once. [Explanation of symbols]

[0166] 10...Cooling tank, 20...Transfer device, 21...Support stand, 22...Support part

Claims

1. A quenching method for cooling a workpiece placed on a support table, comprising: The maximum surface heat transfer coefficient during boiling stored in a non-fluidized state in the cooling bath is 6000 W / m ２ a lowering control step of lowering the support table into the coolant having a temperature of K or higher, and controlling the support table so that the immersion of the workpiece is completed and the support table stops while a vapor film of the coolant is being formed around the workpiece by the heat of the workpiece; a state maintaining step of, after stopping the lowering of the support table, maintaining a state in which the coolant is not flowing and the support table is not moving so that no relative flow velocity is imparted between the workpiece and the coolant at least until the surface of the workpiece is transformed into martensitic material; A quenching method comprising:

2. The coolant is the boiling initiation temperature at which the vapor film formed around the workpiece disappears and boiling begins is 600°C or lower; The method of hardening according to claim 1.

3. The coolant is The boiling initiation temperature at which the vapor film formed around the workpiece disappears and boiling begins is 450°C or higher. The method of hardening according to claim 2.

4. The coolant is After the vapor film formed around the object to be treated disappears and boiling begins, the rate of increase in the surface heat transfer coefficient per unit temperature decrease is 100 W / m ２ ・K ２ That's all. The method of hardening according to claim 1.

5. The coolant is a water-soluble coolant in which a polymer compound is dissolved in water. The method of hardening according to claim 1.

6. the polymer compound contains at least one of polyalkylene glycol, polyvinyl alcohol, polyethylene glycol, sodium polyacrylate, and polyvinylpyrrolidone; The coolant is a water-soluble coolant in which 5% by volume to 30% by volume of the polymer compound is dissolved in water. The method of hardening according to claim 5.

7. The period during which the vapor film is formed is: The period from when the first immersed portion of the workpiece begins to be immersed until the vapor film disappears around the portion and boiling begins. The method of hardening according to claim 1.

8. The period during which the vapor film is formed is: The period from when the workpiece starts to be immersed to when the surrounding vapor film disappears and a portion where boiling begins appears. The method of hardening according to claim 1.

9. The period during which the vapor film is formed is: The period during which the amount of change in the surface heat transfer coefficient between the coolant and the workpiece relative to the amount of temperature decrease during the process in which the workpiece is immersed in the coolant and the temperature decreases, from the time when the amount of change first becomes equal to or less than a predetermined value until the amount of change exceeds the predetermined value. The method of hardening according to claim 1.

10. The lowering speed of the workpiece in the lowering control step is the distance from the liquid surface of the coolant to the lower end of the workpiece at the position where the descent stops / the period during which the vapor film is formed is shorter; The method of hardening according to claim 1.

11. When the martensite fraction on the surface of the workpiece reaches a predetermined value, the surface of the workpiece is considered to have been transformed into martensite. The method of hardening according to claim 1.

12. When a predetermined period of time has elapsed after the start of immersion of the workpiece, the surface of the workpiece is considered to have been transformed into martensitic. The method of hardening according to claim 1.

13. The state in which no relative flow velocity is given between the workpiece and the coolant is as follows: a state in which the coolant is not moved by an external force and the support base is not moved; The method of hardening according to claim 1.

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