JP4605718B2 - Pre-treatment method for vacuum carburizing furnace heating chamber - Google Patents

Description

  The present invention relates to a pretreatment method for a vacuum carburizing furnace heating chamber in which a hydrocarbon-based gas is decomposed at a high temperature under reduced pressure to carburize the surface of an iron-based work (member) such as a steel product.

  Conventionally, in the vacuum carburizing method, carbon that did not react with iron is hatched, and a polymer of hydrocarbon gas accumulates inside the heat insulating material, so that the heat insulating property deteriorates with time, excessive energy, time, etc. Therefore, so-called burnout or the like is performed in which air is periodically introduced into the soot and hydrocarbon gas polymer accumulated in the heat insulating material and burned (Patent Document 1 or the like). Also, the heat insulating material is replaced with a new one due to deterioration of the heat insulating material or the like. Also, repairs and suspensions due to vacations are performed regularly or irregularly. In such a case, the amount of oxygen in the heating chamber changes. Further, even in the operation state, the amount of residual / adsorbed oxygen varies depending on the amount of air brought in by the work and the base, the surface area of the work, the degree of oxidation of the work surface, and the material and state of the in-furnace structure.

  However, carburizing conditions change depending on the amount of oxygen in these furnaces, the amount of air (oxygen) leaks, the amount of oxygen volatilized from the furnace structure, and the amount of oxygen brought in with the workpiece. Cannot be secured.

Therefore, in Patent Document 2, the amount of oxygen is measured while measuring the amount of oxygen in the atmosphere gas and the thermal conductivity of the atmosphere gas in an atmosphere gas containing a carbon-containing compound under a reduced pressure of 13 to 4000 Pa, for example. Carburizing is carried out while adjusting the composition of the atmospheric gas so that the degree of decomposition of the carbon-containing compound is maintained at a predetermined value. The oxygen concentration is measured by a zirconia oxygen sensor, and the degree of decomposition of the carbon compound is measured by the thermal conductivity of the atmospheric gas. Further, it is disclosed that this zirconia oxygen sensor can accurately measure a slight difference in oxygen concentration in a very small concentration range of oxygen partial pressure of about 1 × 10 ⁻²⁰ atm.

If the output value of the zirconia oxygen sensor suddenly increases and reaches 1200 mV in 1 to 2 minutes when butane gas is introduced into the heating chamber using such a zirconia oxygen sensor, the amount of oxygen in the carburizing chamber However, the carburizing process is continued, assuming that the carburizing process is not hindered. Conversely, when the output value of the zirconia sensor does not increase rapidly and is about 1150 mV or less, the carburizing process is stopped because normal carburizing process cannot be performed even if butane gas is introduced. Moreover, without stopping the carburizing process, the introduction of butane gas is stopped and the introduction of hydrogen is switched to reduce the amount of oxygen in the carburizing chamber. It is stated that the carburizing process will continue. In this way, the status of each sensor is grasped, and the carburization status is monitored and controlled.
JP-A-2-115357 JP 2004-59959 A

  However, in such a conventional method, since the determination is performed while carburizing, if the oxygen amount does not decrease even if the amount of oxygen is switched to hydrogen gas, the work is returned to the outside of the heating chamber, the cause is removed, and then again. Since the workpiece is loaded and carburized while judging the amount of oxygen, the influence of the heat load on the workpiece is inevitable. Further, there is a problem that consumption of hydrocarbon gas, hydrogen gas, etc. increases, and raw material gas, heating energy, etc. are wasted. Furthermore, a large amount of oxygen is brought into the furnace at the time of heat insulation replacement or burnout. In this case, several times or a long period of time are required until a dummy work is inserted and the oxygen amount falls below a predetermined amount. There has been a problem that a wasteful operation such as entering a regular (normal operation) operation is required after the operation. Further, the cause of residual oxygen in the furnace includes leakage, oxygen volatilized from the structure in the furnace, and oxygen brought in with the work, but the individual effects thereof are not disclosed.

Furthermore, the zirconia oxygen sensor has a problem that it deteriorates early when exposed to a hydrocarbon gas for a long time, requires frequent replacement, and is expensive. Although the voltage output value of the zirconia oxygen sensor is used for the determination, it is described that the sensitivity of the zirconia oxygen sensor can measure an extremely small amount of oxygen concentration with an oxygen partial pressure of about 1 × 10 ⁻²⁰ atm. The actual oxygen partial pressure is not specifically mentioned.

  In view of the above-described problems, the problem of the present invention is that consumption of gas such as hydrocarbon gas and hydrogen gas is small, and energy is not wasted. Further, the present invention provides a vacuum carburizing method without waste of processes. Furthermore, it is to extend the life of the zirconia oxygen sensor and reduce the replacement frequency.

  In addition to mechanical causes such as leaks, the present inventors have made various studies on the amount of oxygen in the heating chamber. As a result, oxygen entering the heating chamber along with workpieces, jigs, and transport devices, and opening / closing of the intermediate door Oxygen in the inert gas that intrudes can be easily reduced by introducing ordinary vacuum exhaust and oxygen replacement gas such as hydrogen gas, hydrocarbon gas or nitrogen gas, and the amount of oxygen can be reduced in a short time. Less impact on carburizing quality. On the other hand, after the heating chamber was opened to the outside for a long time due to repairs and holidays, the carburizing quality was found to be very poor after replacing the heat insulating material or after burning out with oxygen. In addition, when small repairs or changes in carburizing conditions, etc., we found that quality may vary even if the heating chamber is stopped and the temperature of the heating chamber is once lowered to or near room temperature. This is because the oxygen adhering to and absorbed in the furnace structure, especially the heat insulating material, that is, the adsorbed oxygen, is not easily exhausted. I knew that.

Based on this knowledge, in the present invention, the cooling chamber having a furnace door that enables loading and unloading of the workpiece, and the cooling chamber has a heating chamber connected via an intermediate door, and the heating chamber is iron-based. In a vacuum carburizing furnace that carries a workpiece, decomposes hydrocarbon-based gas under reduced pressure and high temperature, reacts carbon on the iron surface of the workpiece, and carburizes the workpiece surface, without bringing the workpiece into the heating chamber, The intermediate door of the heating chamber is closed and sealed, the heating chamber is heated and evacuated, and the oxygen partial pressure in the heating chamber becomes 1 × 10 ^−19.6 atm or less which becomes a carburizing quality stable region in carburizing treatment. The above-mentioned problems have been solved by providing a vacuum carburizing pretreatment method for a vacuum carburizing furnace heating chamber that discharges adsorbed oxygen of the furnace internal structure in the heating chamber.

In other words, the workpiece is not carried in, and a carburizing gas or the like is not introduced, and the sealed heating chamber is evacuated so that the oxygen partial pressure in the heating chamber becomes a carburizing quality stable region in the carburizing process. The oxygen adsorbed in the furnace structure is discharged out of the furnace until 10 ^-19.6 atm or less . The discharge time takes several minutes or 30 minutes to several hours, sometimes 12 hours or more depending on the amount and state of adsorbed oxygen. However, pretreatment of vacuum carburizing in the heating chamber in this way makes it possible to reduce the amount of adsorbed oxygen that affects the carburizing process, so even if you enter the immediate operation (normal operation), it is stable without being affected by the amount of oxygen. Carburized quality can be obtained. Moreover, constant monitoring of the amount of oxygen during the carburizing process is not necessary.

When the operation interval of the vacuum carburizing furnace is a normal operation operation (continuous operation or constant operation for several hours every day), the amount of adsorbed oxygen stored in the furnace structure is small. On the other hand, it is effective to perform this pretreatment when the operation is stopped for a long time, burned out, the heat insulating material is replaced, the heating chamber is stopped, or the like. Therefore, in the invention according to claim 2, the pretreatment method for the heating chamber is performed when the oxygen partial pressure in the heating chamber exceeds 1 × 10 ^−19.6 atm, which is a carburizing quality stable region in the carburizing process , or When exceeding , for example, the pretreatment method of the vacuum carburizing furnace heating chamber performed when the adsorbed oxygen of the furnace internal structure in the heating chamber increases is provided. An example of the case where the amount of adsorbed oxygen in the furnace structure increases is after the completion of burnout or after replacement of the furnace structure. Moreover, it may occur even after the heating chamber has been cooled or released to the atmosphere during periodic inspections, changes in carburizing conditions, temporary suspension of operation, or the like. In this case, after restarting the heating chamber, the heating chamber is heated and evacuated, and if the oxygen partial pressure in the heating chamber does not become 1 × 10 ^−19.6 atm or less , which is the carburizing quality stable region in the carburizing process, the adsorbed oxygen Therefore, pretreatment of the vacuum carburizing furnace heating chamber of the present invention is performed.

  Thus, the amount of oxygen adsorbed by the in-furnace structure may be determined by measuring the amount of oxygen according to the present invention. Therefore, even in the case of normal operation, the amount of oxygen is measured by the pretreatment method of the present invention even when changing the type of workpiece or carburizing conditions, or during a short pause or inspection without stopping the heating chamber. It is preferable. In this case, since the oxygen partial pressure often reaches a predetermined value in a short time, there are few useless processes and it can be easily confirmed.

It is preferable that the measurement of the oxygen partial pressure and the evacuation process of adsorbed oxygen, etc. are matched with the carburizing conditions. Therefore, in the invention according to claim 3, the oxygen partial pressure is a pretreatment method for vacuum carburizing in a vacuum carburizing furnace heating chamber, which is an oxygen partial pressure at a vacuum carburizing temperature after heating in the heating chamber. did. By making the temperature of the heating chamber the same as the vacuum carburizing temperature, the variation can be reduced. The temperature is preferably 850 ° C. or higher and 1050 ° C. or lower, which is a general vacuum carburizing temperature. The reason why the predetermined value of the oxygen partial pressure is set to 1 × 10 ^−19.6 atm or less is that, as will be described later, as a result of investigating the relationship between the measured value of the oxygen partial pressure and the carburizing quality of the product by carburizing treatment, the oxygen partial pressure is 1 × 10 ^{− This} is because there is little variation in quality at ^19.6 atm and carburization quality is good. Furthermore, it is more certain if it is 1 × 10 ^−19.7 atm to 1 × 10 ⁻²⁰ atm or less. In Patent Document 1, although the value of 1 × 10 ⁻²⁰ atm is disclosed for the accuracy of the zirconia oxygen sensor, it is not a specific threshold value. The measurement of the oxygen partial pressure is preferably a zirconia oxygen sensor, but is not limited thereto.

According to the present invention, it is not necessary to measure the oxygen concentration during the carburizing process. Therefore, in the invention according to claim 4, the oxygen partial pressure is measured using a zirconia oxygen sensor that is easily affected by a hydrocarbon gas. pretreatment method for vacuum carburizing vacuum carburizing furnace heating chamber to be provided for. In addition, in the invention according to claim 5, vacuum carburizing pretreatment in a vacuum carburizing furnace heating chamber that prevents a contact reaction between the zirconia oxygen sensor and the hydrocarbon gas in the heating chamber at least during vacuum carburizing. It was a method.

In the present invention, the enclosed heating chamber is evacuated, and the adsorbed oxygen of the in-furnace structure until the oxygen partial pressure in the heating chamber becomes 1 × 10 ^−19.6 atm or less , which is a carburizing quality stable region in the carburizing process. Since the amount of adsorbed oxygen is reduced to a small amount and stable carburizing quality can be obtained immediately after the start of operation, the amount of oxygen during carburizing is excessive, so there is no need to re-carburize the hydrocarbon gas. There is little consumption of gas such as hydrogen gas, and there is no waste of energy. In addition, a vacuum carburizing method with no waste of processes has been provided. In addition, since constant monitoring of the amount of oxygen during the carburizing process is not required, exposure of the zirconia oxygen sensor to carbon-based hydrogen gas can be reduced, extending the life of the zirconia oxygen sensor and reducing the replacement frequency. it can.

In the second aspect of the present invention, the heating chamber is heated and evacuated after the heating chamber is restarted, and the oxygen partial pressure in the heating chamber becomes a carburizing quality stable region in the carburizing treatment, 1 × 10 ^−19.6 atm. When it is not below, the oxygen partial pressure is lowered to 1 × 10 ^-19.6 atm which becomes the carburizing quality stable region in the carburizing process when the adsorbed oxygen in the furnace structure increases after burnout or after heat insulation replacement. As a result, useless time can be reduced, the quality during normal operation is stabilized, and the burden on the operator is reduced. Further, in the invention described in claim 3, since the heating chamber is set at a vacuum carburizing temperature and the predetermined value of the oxygen partial pressure is 1 × 10 ^−19.6 atm or less, the reproducibility at the time of carburizing is good and the variation is small. A small and stable vacuum carburizing process is possible.

  Furthermore, in the invention described in claim 4, since a zirconia oxygen sensor is used, high-precision measurement is possible. In the invention described in claim 5, the zirconia oxygen sensor and the hydrocarbon-based gas are used during vacuum carburization. As a result, the zirconia oxygen sensor has a long life, and there is no need to replace it, making maintenance easier.

  Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a cross-sectional explanatory view of a vacuum carburizing furnace showing an embodiment of the present invention. The vacuum carburizing furnace 1 includes a cooling chamber 4 having a furnace door 3 in which a workpiece (not shown) can be carried in and out, a heating chamber 6 connected via an intermediate door 5, and a lower part of the cooling chamber 4. It is comprised from the oil tank 7 arrange | positioned. The cooling chamber 4 and the heating chamber 6 are each made to have a sealed structure by closing the intermediate door 5 and the furnace door 3, and the cooling chamber can be evacuated by a vacuum exhaust device (not shown). A vacuum pump 15 is connected to the heating chamber 6 through on-off valves 10 and 12, a pneumatic operation valve 11, and an in-furnace pressure control valve 13 operated by air pressure, so that the heating chamber can be evacuated. The on-off valves 10 and 12 are for maintenance, the on-off valves 10 and 11 are normally open, 12 is normally closed, and the heating chamber 6 and the vacuum pump 15 are opened and closed by a pneumatically operated valve 11 and a furnace pressure control valve 13. The In addition, a gas supply device (not shown) is provided so that carburizing gas, nitrogen gas, air, and the like can be supplied into the heating chamber, and nitrogen gas and air can be supplied into the cooling chamber.

  Quenching oil is stored in the oil tank 7, and oil quenching is enabled by immersing the work that has been carburized by heating. A heat insulating material 16 is disposed in the heating chamber to ensure the furnace temperature. As the heat insulating material 16, for example, an alumina / silica ceramic heat insulating material or the like is used. The same heat insulating material 16 is also used for the intermediate door 5. A furnace pressure gauge 8 and a furnace thermometer 9 are provided in the heating chamber 6. In addition to the heat insulating material 16, the furnace internal structure includes an oxygen adsorbing material such as a ceramic tube, a ceramic hearth, and a ceramic rail.

  In carburizing and quenching, the work is carried into the cooling chamber 4 from the furnace door 3. The furnace door 3 is closed and the cooling chamber is evacuated. After the predetermined exhaust of the cooling chamber 4 is performed, the intermediate door 5 is opened, the work is carried into the heating chamber 6, and the intermediate door is closed. In this state, the workpiece is heated, and after the soaking, carburizing, diffusion, and temperature lowering steps, the intermediate door 5 is opened, the workpiece is transferred to the cooling chamber 4, the intermediate door is closed, and the workpiece is immersed in the oil tank 7. Pull up the workpiece after quenching, open the furnace door and carry the workpiece out of the furnace. Since such a vacuum carburizing furnace is known, a detailed description thereof will be omitted.

In particular, in the present invention, the detection pipe 22 is inserted into the heating chamber 6 and connected to the vacuum pump 15 via the open / close valve 20, the sensor mounting pipe 21, and the open / close valves 23 and 24 in this order. An oxygen partial pressure detector 31 of the zirconia oxygen sensor 30 is inserted into the sensor attachment tube 21 so as to be able to contact and react with the gas in the attachment tube 21. The zirconia oxygen sensor 30 is provided with a measurement control unit (not shown) to display the oxygen partial pressure. A signal is output when the oxygen partial pressure reaches a predetermined value. Since the function, control, measurement method, and the like of the zirconia oxygen sensor are known, detailed description thereof will be omitted. The on-off valves 23 and 24 are arranged on both sides of a long pipe, and improve the efficiency of the vacuum pump.

In such a vacuum carburizing apparatus 1, the pretreatment of the vacuum carburizing furnace heating chamber of the present invention is performed as follows. The furnace door 3 and the intermediate door 5 of the heating chamber are closed and sealed while the work chamber is not carried in and the heating chamber 6 is left empty. The on-off valves 20, 23, 24 are opened so that the oxygen partial pressure of the heating chamber 6 acts on the zirconia oxygen sensor. While monitoring the furnace temperature with a furnace thermometer 9 using a radiant tube (not shown), the furnace is heated to a predetermined temperature of 850 ° C. to 1050 ° C. At the same time, the electromagnetic on-off valve 13 is opened, the vacuum pump 15 is operated, and the pressure inside the furnace is monitored by the furnace pressure gauge 8 until the oxygen partial pressure in the heating chamber reaches a predetermined value of 1 × 10 ^−19.6 atm. Evacuate. Since the zirconia oxygen sensor is attached to the sensor attachment tube 21 and disposed between the heating chamber 6 and the vacuum pump 15, the oxygen partial pressure in the heating chamber can be accurately measured.

When the oxygen partial pressure in the heating chamber reaches a predetermined value of 1 × 10 ^−19.6 atm or less, it is determined that the discharge of adsorbed oxygen in the furnace structure in the heating chamber is complete, and the on-off valve 20 is closed to prepare. finish. When the vacuum pump is stopped, the on-off valves 23 and 24 are closed to prevent the backflow of the vacuum pump oil. Thereafter, the work is carried into the cooling chamber from the furnace door as in the conventional case, and carburizing and quenching is performed by a predetermined operation. Note that when there is almost no decrease in oxygen partial pressure, a mechanical problem such as a leak may be considered. In this case, needless to say, it is necessary to stop the pretreatment and remove the cause.

  By performing such pretreatment, the heating chamber is heated and evacuated after the carburizing conditions are changed, repaired, or restarted after a long-term shutdown, and the oxygen partial pressure in the heating chamber is not less than a predetermined value, Even after burnout or after replacement of the furnace internal structure, the normal operation operation can be promptly started, and the carburization quality becomes stable. Further, at the time of vacuum carburizing, at least the on-off valves 20 and 23 are closed, so that the zirconia oxygen sensor and the hydrocarbon gas in the heating chamber do not react with each other. The open / close valves 20, 23, and 24 are manual valves. This is because the oxygen partial pressure is measured after burnout or the like, and thus is used less frequently. Instead of pneumatic valves, solenoid valves, and the like. Automatic measurement is also possible.

An embodiment of the pretreatment according to the present invention will be described. 2 (a) and 2 (b) are graphs showing changes in oxygen partial pressure and furnace temperature over time when pretreatment of the present invention is performed after burnout, and the horizontal axis is the time axis. The horizontal axis time width of (a) is 3 hours, and the horizontal axis time width of (b) is 1 hour. The size and capacity of the heating chamber are the same specifications. However, in the case of FIG. 2A, the temperature in the heating chamber becomes 924 ° C. in about 20 minutes and the oxygen partial pressure gradually decreases, but even after 3 hours, the oxygen partial pressure is about 1 × 10 ⁻¹⁹ atm. The oxygen partial pressure reached 1 × 10 ⁻²⁰ atm after 6 hours. In the case of FIG. 2B, the temperature in the heating chamber becomes 955 ° C. in about 40 minutes, the oxygen partial pressure quickly decreases, and the oxygen partial pressure reaches 1 × 10 ⁻²⁰ atm within 10 minutes. m, 1 × 10 ⁻²¹ atm to 1 × 10 ⁻²² atm. Thus, it can be seen that even after the same burnout, the degree of decrease in oxygen partial pressure varies greatly depending on the situation after burnout, the amount of adsorbed oxygen in the furnace structure, and the state. In addition, the initial change of FIG.2 (b) is because there was a leak from the inspection window of a heating chamber.

  As described above, since the time until the oxygen partial pressure in the heating chamber is stabilized varies widely, as in Patent Document 2, even if the vacuum carburization is performed while measuring the oxygen concentration in the furnace or the like, There are many variations in time and components. Therefore, if it takes time to enter a stable operation, a lot of useless work and energy and gas are consumed. On the other hand, in the pretreatment of the present invention, although there is a heating step, it is performed without supplying a carburizing gas such as a hydrocarbon-based gas, so that wasteful energy and gas consumption can be suppressed, Quality can be further stabilized.

  Next, the carburizing quality of the product for the pretreatment situation will be described. FIG. 3 is a graph showing the relationship between the oxygen partial pressure in the furnace and the product quality. The horizontal axis shows the elapsed time after burnout, and the figure showing the oxygen partial pressure at that time shows the upper and lower limits of the carburization depth when a workpiece (gear) is loaded and carburized at a predetermined oxygen partial pressure. Is plotted according to the oxygen partial pressure curve after burnout. The left vertical axis represents the oxygen partial pressure, and the right vertical axis represents the carburization depth (mm). Note that the same heating chamber is used as the heating chamber, the heating chamber is heated to 950 ° C., and evacuated to a predetermined oxygen partial pressure with a vacuum pump. As described above, this oxygen partial pressure is not oxygen released in the heating chamber but oxygen released from the adsorbed oxygen of the structure in the heating chamber, and is obtained by evacuation over a relatively long time. Value. Immediately after reaching the specified oxygen partial pressure, the tray on which the workpieces (gears) are arranged is brought into the heating chamber, all carburized under the same conditions, and the carburized depth of the workpiece in the tray is measured. The lower limit was measured. It should be noted that the carburization depth is required to be in the range of 0.8 mm as the upper limit and 0.5 mm as the lower limit as product quality. In FIG. 3, the upper limit value and the lower limit value are shown.

As shown in FIG. 3, when the oxygen partial pressure during the pretreatment is 1 × 10 ^−19.3 atm, the lower limit value of the carburization depth is 0.3 mm, the upper limit value is 0.65, and the lower limit value is 1 × 10 ^−19.5 atm. The value is 0.35 mm and the upper limit value is 0.6, and the carburization depth varies greatly. The lower limit value of the carburization depth is less than the target lower limit value 0.5, and the quality is inferior. Further, the oxygen partial pressure is increased, and when 1 × 10 ^−19.5 atm to 1 × 10 ^−19.6 atm, the lower limit value of the carburization depth is 0.6 mm, the upper limit value is 0.65, and the variation is very small. Furthermore, when the oxygen partial pressure is 1 × 10 ^−19.8 atm, a lower carburization depth lower limit value of 0.65 and an upper limit value of 0.7 can ensure a deeper carburization depth. Thus, it can be said that the oxygen partial pressure is around 1 × 10 ^−19.5 atm as the quality transition region, an oxygen partial pressure higher than this becomes an unstable region, and a lower oxygen partial pressure becomes a stable region. Thus, it was confirmed that the oxygen partial pressure in the pretreatment stage has a great influence on the heat treatment quality. In addition, from these results, it was confirmed that it can be said that the carburizing quality is not affected if at least the oxygen partial pressure during the pretreatment is 1 × 10 ^-19.6 atm or less. More preferably, it is more preferably less than 1 × 10 ^−19.6 atm. In addition, since there are influences, such as an oxygen partial pressure and a measurement error, it cannot be overemphasized that a suitable value is selected suitably in an actual machine.

It is a section explanatory view of a vacuum carburizing furnace showing an embodiment of the invention. It is a graph which shows the change of the oxygen partial pressure with respect to the time passage at the time of the time of the pretreatment of the present invention after the burnout is completed, and the temperature in the furnace, the horizontal axis is the time axis, the horizontal axis time width of (a) is 3 hours, the horizontal time width of (b) is 1 hour. It is a graph which shows the relationship between the oxygen partial pressure in a furnace, and the product quality by a carburizing process.

Explanation of symbols

1 Vacuum carburizing furnace 5 Intermediate door 6 Heating chamber 16 Furnace structure (heat insulation, ceramic heater, ceramic rail)
30 Zirconia oxygen sensor

Claims (5)

A cooling chamber having a furnace door that allows loading and unloading of workpieces, and a heating chamber connected to the cooling chamber via an intermediate door , carrying iron-based workpieces into the heating chamber, In a vacuum carburizing furnace that decomposes under reduced pressure and high temperature to cause carbon to react on the iron surface of the workpiece and carburizes the workpiece surface, the intermediate door of the heating chamber is closed and sealed without bringing the workpiece into the heating chamber. The heating chamber is heated and evacuated until the oxygen partial pressure in the heating chamber becomes 1 × 10 ^−19.6 atm or less which is a carburizing quality stable region in the carburizing process . A pre-treatment method for vacuum carburizing in a vacuum carburizing furnace heating chamber, wherein adsorbed oxygen is discharged. The pretreatment method for the heating chamber is performed when the oxygen partial pressure in the heating chamber exceeds or is assumed to exceed 1 × 10 ^−19.6 atm, which is a carburizing quality stable region in the carburizing process. pretreatment method of vacuum carburizing of vacuum carburizing furnace heating chamber according to claim 1,. 3. The pretreatment method for vacuum carburizing in a vacuum carburizing furnace heating chamber according to claim 1 , wherein the oxygen partial pressure is an oxygen partial pressure at a vacuum carburizing temperature after heating in the heating chamber. 4. The pretreatment method for vacuum carburizing in a vacuum carburizing furnace heating chamber according to claim 1, wherein the oxygen partial pressure is measured using a zirconia oxygen sensor. 5. The pretreatment method for vacuum carburizing in a vacuum carburizing furnace heating chamber according to claim 4, wherein the zirconia oxygen sensor and the hydrocarbon gas in the heating chamber are prevented from contacting and reacting at least during vacuum carburizing.

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