JP2017002372A - Manufacturing method and heat treating method of cast, and high frequency quenching device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method of a cast casting a work using flaky graphite cast iron as a raw material without cracking when the work surface of the work is performed high frequency quenching.SOLUTION: When high frequency induction heating is executed, a frequency of electric current flowing to a coil is monitored. A base time is a time when the frequency changed (time c), and electric power supplied to a coil is lowered from the base time, and the state is maintained during some period of time. Then, a cooling step of stopping the high frequency induction heating and quenching the work is executed.SELECTED DRAWING: Figure 2

Description

The present invention relates to a method for producing a cast product.
The present invention also relates to a method for heat-treating an iron-based material by induction heating. The present invention further relates to an induction hardening apparatus.

Casting has an advantage that a complicated shape can be easily formed. Known casting materials include flake graphite cast iron and spheroidal graphite cast iron.
Castings using flake graphite cast iron are lower in hardness and rigidity and inferior in wear resistance than casts using spheroidal graphite cast iron.
For this reason, flake graphite cast iron is often used as a material for casting a member used in a stationary state, such as a frame of a machine tool or a leg of a chair.
However, flake graphite cast iron has the advantages that the material is inexpensive and the flow of hot water is smooth.

For this reason, flake graphite cast iron is used as a material for casting members that have movement, such as valve bodies, blades, gears, shafts, etc., which are members of mechanical devices such as valves and blowers, and that receive sliding, friction, etc. There is a market demand for it. That is, there is a market demand for manufacturing machine parts using flake graphite cast iron.
When casting a member that receives friction or the like using flake graphite cast iron as a raw material, the mold is cooled and embedded with gold in order to increase the hardness of the portion that receives friction or the like.

The cast product cast by the mold in which the cooling metal is embedded has such a high hardness that the sliding surface can withstand practical use.
Further, since the other parts have a relatively low hardness, post-processing performed after casting is easy.

In other words, machine parts are often post-processed by machining such as drilling, milling, and lathe after casting, but members made of flake graphite cast iron are made of cold metal. The hardness of parts other than the chilled sliding surface is low, and the cutting work with a drill or the like is easy.
Further, induction hardening is known as a method for hardening a workpiece using an iron-based material.

JP 2004-133935 A

In the induction hardening, the workpiece can be partially heated, and only a part of the workpiece can be partially cured by partially quenching the workpiece. Therefore, post-processing of the workpiece is easy.
In addition, induction hardening has a number of features such as the fact that the quenching process can be completed in a short time and that less energy is required for quenching.

Therefore, the present inventors have considered that a machine part (work) is formed by casting, and then a part requiring hardness is induction-hardened.
The effects assumed by the present inventors were as follows.
(1) Since it is not necessary to embed a cooling metal during casting, it is easy to mold.
(2) Since only specific parts can be cured, post-processing after casting is easy.

However, when a flake graphite cast iron was used as a raw material and a workpiece in the shape of a machine part was cast, and that specific part was induction-hardened, an unexpected problem occurred.
That is, cracks occurred frequently when a workpiece made of flake graphite cast iron was subjected to high frequency induction heating and then rapidly cooled.
In particular, cracks tend to occur frequently when the cross-sectional shape of the part to be quenched is out of a perfect circle or the thickness is not uniform. In other words, if a part with a protrusion in the cross-sectional shape of the part to be quenched or a part with a thinner part than other parts is subjected to high-frequency induction heating and then rapidly cooled, cracks will occur in the protruding part and the thin part. There were many.
The present invention addresses this problem, and provides a heat treatment method that does not cause cracking when casting a work using flake graphite cast iron as a raw material and induction-quenching that specific part, and a method for producing a cast product To do.
Moreover, this invention makes it a subject to develop the heat processing method and induction hardening apparatus which do not produce a crack.

For the experiment, the inventors cut carbon steel, formed a workpiece having a projecting portion and a thin portion, and induction-hardened, but the occurrence of cracking was small. That is, a work having the shape of a machine part was formed using carbon steel as a raw material, and the work was induction-hardened, but the occurrence of cracks was small.
Similarly, a work having the same shape was cast using spheroidal graphite cast iron as a raw material, and induction hardening was performed, but the occurrence of cracks was small.
Thus, it has been concluded that cracks frequently occur when flake graphite cast iron is used as a raw material.
Since cracks frequently occur in the protruding part and the thin part, the temperature of the protruding part and the thin part is expected to be higher than other parts when induction heating is carried out, and attempts were made to improve this. Since the protrusion and the thin part have a small volume, it is difficult to make the temperature the same as that of other parts during high-frequency induction heating.

Then, after approving that the temperature variation occurs in the workpiece during high-frequency heating, it was considered to keep the entire temperature variation within a predetermined range.
In other words, quenching is possible if the highest temperature part of the workpiece is controlled so that cracks do not occur during rapid cooling and the lowest temperature part reaches a quenchable temperature. Moreover, it was thought that generation | occurrence | production of a crack could be prevented.

The principle and problems will be described below.
Iron-based materials have a ferrite-pearlite structure at room temperature, but when the temperature rises and exceeds the transformation point (AC1), an austenite structure begins to form, and when the AC3 transformation point is exceeded, the ferrite-pearlite structure completely changes to an austenite structure. To do.
Here, if the workpiece is not a cast iron but a carbon steel, the workpiece remains in a solid state even when the AC3 transformation point is exceeded. Therefore, if the workpiece is a carbon steel, the ferrite pearlite structure is completely changed to an austenite structure beyond the AC3 transformation point, and then the workpiece is rapidly cooled to form a dense martensite structure to increase the hardness.
In the case of hypoeutectoid steel, the actual quenching is performed by raising the temperature to a temperature exceeding the AC3 transformation point and quenching, and in the case of hypereutectoid steel, it is between the AC1 transformation point and the AC3 transformation point. Raise the temperature to quench.

On the other hand, flake graphite cast iron selected by the present inventors as a material for machine parts (workpieces) contains a large amount of carbon, and liquefaction starts when AC3 is exceeded. Therefore, when the temperature of a workpiece made of flake graphite cast iron is increased over AC3 and rapidly cooled, the frequency of occurrence of cracks increases. That is, flake graphite cast iron can be quenched and the temperature range in which cracks do not occur is narrower than that of carbon steel or the like.
For this reason, when quenching a workpiece made of flake graphite cast iron, there is little variation in the temperature that is allowed between the temperature of a portion that tends to be high, such as the protruding portion and the thin portion, and other portions. Therefore, when the work is subjected to high-frequency induction heating, if the surface temperature of the work is accurately detected and the temperature variation of each part can be within an allowable range, cracking does not occur.

  However, it is difficult to accurately detect the surface temperature of the workpiece at the time of high-frequency induction heating, and a method for performing high-frequency induction heating while monitoring the temperature has to be abandoned.

As a result of further studies by the present inventors, it has been found that the oscillation device used in the induction hardening apparatus exhibits an interesting behavior during induction heating.
In other words, when an oscillation device is connected to an induction heating coil via a current transformer and a work made of an iron-based material is induction-heated by the induction heating coil, if the output volume of the oscillation device is kept constant, The temperature is raised as shown in FIG.
That is, the iron-based material rises again after a certain time, although the temperature rise curve becomes dull near the magnetic transformation point (between the actual AC1 transformation point and the AC3 transformation point).
On the other hand, paying attention to the oscillation frequency of the oscillation device, as shown in FIG. 1B, the frequency is stable for a certain time, then increases slightly, and then stabilizes again. That is, the oscillation frequency of the oscillation device always changes little by little, but changes out of the change locus so far. More specifically, the frequency suddenly changes from a stable state to an upward trend.

The reason why the oscillation frequency of the oscillation device changes out of the change locus so far is considered to be because the permeability of the workpiece changes. That is, the iron-based material has a magnetic transformation point, and the magnetic permeability of the workpiece changes greatly before and after the magnetic transformation point. The magnetic transformation point is a temperature higher than the transformation point (AC1) at which the austenite structure starts to form.
Therefore, it can be said that the time when the oscillation frequency of the oscillation device changes is the time when the magnetic transformation point is exceeded, and the time when the ferrite pearlite in the workpiece starts to change its structure to austenite. Further, since the magnetic transformation point is a temperature lower than the temperature of AC3, the work is in a solid state.

Therefore, when the oscillation frequency of the oscillation device is monitored during high-frequency induction heating, it is possible to know the passage time of the magnetic transformation point. If the oscillation frequency of the oscillation device rises off the change trajectory up to that point, it may be considered that the magnetic transformation point is passed, and the temperature of the workpiece exceeds AC1 and the structure is austenitic.
Therefore, if the current or power supplied to the induction heating coil is lowered when the oscillation frequency of the oscillation device is increased, the temperature of the workpiece is maintained near the magnetic transformation point, the austenite structure is generated, and the solid state is maintained. Will be maintained.
Therefore, when the oscillation frequency of the oscillation device rises, if the current or power supplied to the induction heating coil is lowered and the workpiece is rapidly cooled after maintaining this state, the workpiece is not cracked and the workpiece is cured. be able to.

Note that, as a secondary effect due to the change in the oscillation frequency of the oscillation device, the current and power supplied to the induction heating coil also change. That is, when the workpiece temperature passes through the magnetic transformation point, the current and power supplied to the coil also change.
The current supplied to the coil gradually decreases as shown in FIG. 1C even though the output volume of the oscillation device is constant. Therefore, it is possible to know the passage time of the magnetic transformation point even if the current or power supplied to the induction heating coil is detected. That is, it may be considered that the transformation point has been passed if the current or power supplied to the induction heating coil tends to decrease.

  The invention according to claim 1 developed on the basis of the above-mentioned knowledge, in a casting manufacturing method including a casting step of molding a flake graphite cast iron workpiece and a quenching step of quenching the workpiece, The quenching step includes an induction heating step of induction heating the workpiece using an induction heating device including an oscillation device and an induction heating coil. In the induction heating step, the coil is subjected to induction heating when the workpiece is induction heated. Monitor at least one of the current, power, and / or frequency of the current flowing in the coil, and supply to the coil when any of these changes or changes from the previous change trajectory. The controlled heating process that reduces the current or power to be maintained and maintains the state for a certain period of time is performed, and then the cooling process that quenches the workpiece by stopping induction heating is performed. Which is a method for producing a casting, characterized.

  The invention according to claim 2 monitors at least one of the current and power flowing in the coil and the frequency of the current flowing in the coil, and when the current or power flowing in the coil tends to decrease, The method for producing a cast product according to claim 1, wherein when the frequency of the flowing current is changed, or when a change tendency appears, is set as a reference time, and the suppression heating process is performed based on the reference time. It is.

In the present invention, during the induction heating, at least one of the current flowing through the coil, the electric power, and the frequency of the current flowing through the coil is monitored. Then, when the current or power flowing through the coil tends to decrease or when the frequency of the alternating current flowing through the coil changes, the current or power supplied to the coil is decreased and the state is maintained for a certain period of time. “To decrease the current or power supplied to the coil and maintain the state for a certain period of time” means to maintain a state where the current or power is lower than the peak time. The current or power at this time is constant. There is no need.
As a specific measure for “decreasing the current or power supplied to the coil”, for example, there is a method of reducing the voltage applied to the coil by reducing the volume of the oscillation device.
As described above, the temperature of the workpiece at the time when the current or power flowing through the coil tends to decrease, or when the frequency of the alternating current flowing through the coil tends to rise, is near the magnetic transformation point (actual AC1 transformation point and Between AC3 transformation points). Therefore, at the reference time of the present invention, it can be said that the temperature of the workpiece is near the transformation point (between the actual AC1 transformation point and the AC3 transformation point).
In the present invention, the current or power supplied to the coil is reduced by monitoring the current flowing through the coil, etc., and indirectly knowing the passage of the transformation point and then reducing the output volume. As a result, after passing through the magnetic transformation point, the workpiece maintains a temperature in the vicinity of the magnetic transformation point for a certain time, and the ferrite / pearlite structure is transformed into an austenite structure. After that, when induction heating is stopped and rapidly cooled, the austenite structure is transformed into a martensite structure.
In the present invention, the temperature of the workpiece is not excessively increased. Further, by maintaining the temperature near the magnetic transformation point for a certain time, the temperature variation of each part of the work is reduced. For this reason, the work is unlikely to crack after rapid cooling.

  The invention according to claim 3 is the method of manufacturing a cast product according to claim 1 or 2, wherein the suppression heating step is performed after the frequency of the current flowing in the coil tends to increase.

  According to a fourth aspect of the present invention, in the induction heating process, only a specific part of the work is induction-heated, and the part to be induction-heated has a protrusion and / or a part whose thickness is thinner than other parts. It is a manufacturing method of the casting in any one of Claims 1 thru | or 3 characterized by the above-mentioned.

  According to a fifth aspect of the present invention, there is provided a heat treatment method in which a work made of an iron-based material is induction-heated using an induction heating coil connected to an AC generating source having an oscillation device. When heating, at least one of the current flowing through the coil, the electric power, or the frequency of the current flowing through the coil was monitored, and when any of these changed out of the previous change trajectory, or a tendency to change appeared In this case, the current or power supplied to the coil is reduced to maintain the state for a certain time, and then the induction heating is stopped.

  According to a sixth aspect of the present invention, there is provided a high-frequency quenching apparatus including a high-frequency oscillation device and an induction heating coil, comprising monitoring means for monitoring at least one of a current flowing through the coil, a power, or a frequency of a current flowing through the coil. And when any of these changes out of the change trajectory up to that point, or when a change tendency appears, suppression heating control is performed to reduce the current or power supplied to the coil. Induction hardening device.

  According to the method for manufacturing a cast product and the heat treatment method of the present invention, there are few cracks in the quenching process, and the yield is high. Moreover, when the induction hardening apparatus of this invention is used, there are few cracks of a workpiece | work and a yield is high.

It is the graph which arranged the temperature change at the time of carrying out high frequency induction heating of the workpiece | work made from flake graphite cast iron, the fluctuation | variation of a frequency, and the fluctuation | variation of an electric current on the same time axis. It is the graph which arranged the temperature change at the time of carrying out the high frequency induction heating of the workpiece | work made from flake graphite cast iron, the fluctuation | variation of a frequency, and the fluctuation | variation of an electric current on the same time axis according to the method of embodiment of this invention. It is a conceptual diagram of the high frequency induction heating apparatus used when implementing the heat processing method of embodiment of this invention. It is a flowchart which shows the process of the heat processing method of embodiment of this invention. FIG. 3 is a sketch of a cross-section after cutting a workpiece manufactured by the method for manufacturing a cast product according to the embodiment of the present invention and further etching the workpiece, and the workpiece reduces the current supplied to the coil from the reference time to 1 .5 seconds, after which induction heating is stopped and the workpiece is rapidly cooled. FIG. 4 is a sketch of a cross-section after cutting a workpiece manufactured by the method for manufacturing a cast product according to the embodiment of the present invention and further etching the workpiece; the workpiece reduces the power supplied to the coil from the reference time and changes its state to 6; .5 seconds, after which induction heating is stopped and the workpiece is rapidly cooled.

Embodiments of the present invention will be further described below. The present embodiment is a method of manufacturing a machine part (cast product), which includes a step of casting a workpiece 100 having a predetermined shape and hardening a part of the workpiece 100 by induction hardening.
The workpiece 100 is formed by casting as described above. The material used for casting is flake graphite cast iron. A specific part of the workpiece 100 is induction-hardened.
The work 100 is a shaft having an oval cross-sectional shape, and a screw portion 101 is provided in a part thereof. The workpiece 100 is induction-hardened only in a certain region (hereinafter, quenching region 103, FIG. 3) having an oval cross-sectional shape.
That is, the cross-sectional shape of the part (quenched region 103) where the workpiece 100 is quenched is not a perfect circle but an oval shape, and has a protrusion 102. In the method for manufacturing a cast product according to this embodiment, the entire periphery of the quenching region 103 is induction-quenched after casting.
A high-frequency induction heating device 2 for quenching the workpiece 100 includes an induction heating coil 3, a current transformer 5, a high-frequency oscillator 6 and a control device 7.
The induction heating coil 3 is a known one-turn coil.

  The high frequency oscillator 6 is a device that is connected to a commercial power source (not shown) and generates a high frequency current. The oscillation frequency of the high-frequency oscillator 6 can be set by a volume or the like (including a variable capacitor or the like), but the oscillation frequency varies depending on the state of the induction heating coil 3 or the like. In other words, the frequency can be increased or decreased by the volume 20 of the control device 7, but the high frequency oscillator 6 has an oscillation frequency determined by a tuning circuit including the induction heating coil 3. If the inductance of 3 changes, the oscillation frequency will change.

Moreover, the high frequency oscillator 6 can change the voltage applied to the induction heating coil 3 by the volume 21 of the control device 7, and can change the output current (current flowing through the induction heating coil 3) as a result. As a result, the power supplied to the induction heating coil 3 changes.
However, the output current of the high-frequency oscillator 6 varies not only with the resistance of the volume 21 but also with the inductance of the induction heating coil 3, the capacitance of each part, and the oscillation frequency.

In addition to having the volumes 20 and 21 described above, the control device 7 incorporates a program for executing a control process described later.
That is, the control device 7 reduces the voltage applied to the induction heating coil 3 for a certain period of time when the oscillation frequency of the high-frequency oscillator 6 is changed and the program for monitoring the oscillation frequency of the high-frequency oscillator 6 is changed. A program for reducing the output current and then stopping the oscillation of the high-frequency oscillator 6 is incorporated. More specifically, when the oscillation frequency of the high-frequency oscillator 6 rises off the locus of change so far, a program for reducing the output current for a certain time and then stopping the oscillation of the high-frequency oscillator 6 is built-in. Has been.
In the present embodiment, the control device 7 is a monitoring unit that monitors the frequency of the current flowing through the coil.
In the present embodiment, the suppression heating control is executed based on a signal from the control device 7.

Next, the quenching method of the present embodiment will be described in accordance with actual steps.
The high-frequency induction heating device 2 is such that the induction heating coil 3 is connected to a high-frequency oscillator 6 via a current transformer 5. Then, the quenching region 103 of the workpiece 100 is brought close to the induction heating coil 3 of the high frequency induction heating device 2 and heat treatment is started. The following processing is automatically executed in accordance with a program built in the control device 7.

That is, in step 1, the high-frequency oscillator 6 is energized, and the induction heating coil 3 is energized. In the present embodiment, the workpiece 100 is rotated during induction heating. The induction heating coil 3 is eccentrically moved following the rotation of the workpiece 100.
As a result of the high-frequency current being applied to the induction heating coil 3, an induction current is generated in the quenching region 103 of the workpiece 100, and the quenching region 103 is heated. The temperature of the quenching region 103, the supply current (or power) to the induction heating coil 3 and the oscillation frequency of the high frequency oscillator 6 immediately after the high frequency current is passed through the induction heating coil 3 are as shown by period A in FIG. is there.
That is, the oscillation frequency rises within a short time (from 0 to a) as shown in the graph F. The supply current to the induction heating coil 3 rises as shown in the graph D. The temperature of the quenching region 103 increases rapidly.

  In the subsequent period B, the oscillation frequency is in a stable state as shown in the graph F. The supply current continues to rise as shown in graph D. The temperature of the quenching region 103 continues to rise substantially uniformly, although the rising curve is somewhat dull.

Then, after the oscillation starts, the oscillation frequency suddenly increases. That is, according to the graph of FIG. 2, the change locus of the oscillation frequency from time a to time b is a locus that tends to increase in parallel or little by little, but deviates from the change locus until then after the time b. The oscillation frequency changes.
That is, the approximate straight line of the oscillation frequency from time b to time c is clearly different from the approximate straight line until that time (between time a and time b). In other words, the value of the oscillation frequency after the time b is not on the extension line of the approximate straight line of the oscillation frequency from the time b to the time c.

In this way, the oscillation frequency rises abnormally between time b and time c. That is, although the oscillation frequency is constantly changing, the change tendency and the change gradient are clearly changed, and the rising frequency of the start frequency becomes remarkable. For example, the frequency changes by a certain amount or a certain ratio within a certain time.
In this embodiment, the time point (time c) when the oscillation frequency changes by a certain frequency is used as a reference point.
The standard of the rate of change is about 2 to 10 percent, and it is desirable to use the case where the rate of change is about 3 to 7 percent per second as a reference point.

Here, in the program employed in this embodiment, the oscillation frequency is monitored in steps 2 and 3 as shown in FIG. 4, and when the oscillation frequency rises by a certain frequency, the routine proceeds from step 3 to step 4 to induce induction heating. The supply current to the coil 3 is forcibly reduced. More specifically, the voltage applied to the induction heating coil 3 is reduced, and the suppression heating control is executed.
The reduction rate of the voltage applied to the induction heating coil 3 is desirably 20% to 70%. Actually, it is an induction heating coil so that the temperature increase per unit time of the workpiece 100 is 50 degrees centigrade / second or less, more desirably 30 degrees centigrade / second or less, and more desirably about 25 degrees centigrade / second by preliminary tests. The voltage applied to 3 is reduced.
Further, at a time point c, which is a reference point, a predetermined timer is turned on in step 5 and the timer waits for the timer to finish counting in step 6.

At this time, the temperature of the quenching region 103, the supply current (or power) to the induction heating coil 3 and the oscillation frequency of the high-frequency oscillator 6 are as shown in FIG. 2, and the temperature of the quenching region 103 is the magnetic transformation point. The temperature is maintained within a certain temperature range immediately after passing through. That is, the temperature of the quenching region 103 is maintained at a temperature that has passed the magnetic transformation point, and the temperature change is small.
Since the supply current is forcibly reduced as described above, it is kept low as shown in the graph D and kept constant.
The oscillation frequency tends to increase, but tends to increase slightly.

When it is confirmed in step 6 that the timer has finished counting, the process proceeds to step 7 where the current supply to the induction heating coil 3 is stopped, and in step 8 the quenching region 103 is rapidly cooled.
It should be noted that it is desirable to provide a slight delay time between the time when the current supply to the induction heating coil 3 is stopped and the time when the quenching region 103 is rapidly cooled to cool the work 100 by air.

  The workpiece 100 manufactured by the above-described process is less likely to crack. Further, as shown in FIGS. 5 and 6, the work surface is evenly quenched.

In the present embodiment, the supply current (or power) is reduced at time c (reference time), but the amount of decrease in the current (or power) is preferably such that the workpiece 100 maintains the reference temperature. However, in practice, there may be a slight temperature increase.
Further, the quenching depth can be adjusted by the time for maintaining the state where the supply current (or power) is reduced.
When the time for maintaining the state where the supply current (or power) is reduced is short, the quenching depth becomes shallow as shown in FIG. On the contrary, the state in which the supply current is reduced is maintained, but if the supply current is long, the quenching depth becomes deep as shown in FIG.

  In the above-described embodiment, the time point (time c) when the oscillation frequency changes within a certain range is set as the reference time, but the time point when the change tendency appears (time b) may be set as the reference time. Further, the reference time may be determined based on the current value instead of the frequency.

The present invention does not limit the shape, structure, and application of the cast product, but is effective as a method for manufacturing a casing, gears, shafts, and other cast products.
The technical idea of the present invention can also be applied to heat treatment of iron materials of any shape.

2 High-frequency induction heating device 3 Induction heating coil 5 Current transformer 6 High-frequency oscillator 7 Control device 20 Volume 21 Volume 100 Work axis

Claims (6)

In a casting process for forming a workpiece made of flake graphite cast iron, and a method for producing a cast product having a quenching step of quenching the workpiece,
The quenching step includes an induction heating step of induction heating the workpiece using an induction heating device including an oscillation device and an induction heating coil. In the induction heating step, the coil is subjected to induction heating when the workpiece is induction heated. Monitor at least one of the current, power, and / or frequency of the current flowing in the coil, and supply to the coil when any of these changes or changes from the previous change trajectory. A method for producing a cast product, comprising: carrying out a suppression heating step of reducing a current or electric power to be maintained and maintaining the state for a certain period of time, and then carrying out a cooling step of stopping induction heating and rapidly cooling the workpiece.   Monitoring at least one of the current flowing through the coil, the power, or the frequency of the current flowing through the coil, and when the current or power flowing through the coil tends to decrease, or when the frequency of the current flowing through the coil changes, or The method for producing a cast product according to claim 1, wherein a time point at which a change tendency appears is set as a reference time, and the suppression heating step is performed based on the reference time.   The method for producing a cast product according to claim 1 or 2, wherein the suppression heating step is performed after the frequency of the current flowing through the coil tends to increase.   In the induction heating process, only a specific part of the work is induction-heated, and the part to be induction-heated has a protrusion and / or a part whose thickness is thinner than other parts. The manufacturing method of the casting in any one of thru | or 3.   In a heat treatment method in which an induction heating coil connected to an AC generating source equipped with an oscillation device is used to induction heat a work made of an iron-based material, the current flowing through the coil when the work is induction heated Monitoring at least one of power and / or frequency of current flowing in the coil, and supplying current to the coil when any of these changes or changes from the previous change trajectory, or A heat treatment method characterized by reducing power to maintain the state for a certain period of time and then stopping induction heating.   A high-frequency quenching apparatus including a high-frequency oscillation device and an induction heating coil has monitoring means for monitoring at least one of a current flowing through the coil, a power, or a frequency of a current flowing through the coil. A high-frequency quenching apparatus is characterized in that suppression heating control is performed to reduce the current or power supplied to the coil when a change occurs outside of the change locus or when a change tendency appears.

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