JP2012092424A - Method and device of gas carburizing - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for gas carburizing, capable of performing heat treatment in a stable atmosphere and also reducing equipment cost.SOLUTION: Converted gas is produced using hydrocarbon gas, air and a catalyst, and low-COconverted gas which has a carbon potential higher than that of the converted gas and is reduced in COconcentration is produced. One of the converted gas, the low-COconverted gas and a gaseous mixture of the converted gas and the low-COconverted gas is used as atmospheric gas in a series of heat treatment processes for carburizing.

Description

  The present invention relates to a gas carburizing method and a gas carburizing apparatus suitable for use in carrying out the method.

  As a conventional gas carburizing method, there is one using an endothermic shift gas generated in a shift furnace. In order to compensate for the weak carburizing power of the shift gas, an enriched gas is supplied into the furnace in addition to the shift gas. By performing carburizing, a predetermined surface carburizing concentration is secured. For example, as disclosed in Patent Document 1, the carbon potential of the furnace atmosphere (hereinafter referred to as CP) is controlled by adjusting the flow rate of the modified gas, the enriched gas, the air, or the like.

  On the other hand, a method for adjusting the atmosphere in the furnace by supplying only the modified gas generated in a predetermined CP for each process such as preheating, carburizing, cooling, quenching and the like in the furnace is also proposed. (Patent Document 2).

  Furthermore, a raw material mixed gas, which is a mixture of hydrocarbons and source gases such as carbon dioxide and oxygen, is introduced into a shift furnace having a catalyst layer to generate a shift gas, and after cooling the shift gas, moisture and carbon dioxide are adsorbed. There has also been proposed a method in which the separated gas is used alone or mixed with the modified gas as a carburizing atmosphere gas (Patent Document 3).

JP 2006-283116 A JP 2009-91632 A Japanese Patent Laid-Open No. 2004-10952

  However, when enriched gas, air, or the like is supplied into the furnace in addition to the metamorphic gas as described in Patent Document 1, the atmosphere tends to vary, so that there is a possibility that the quality of carburization may vary. In addition, when air is added, the object to be processed may be oxidized, so that the efficiency of the carburizing process and the quality of the object to be processed are reduced. Furthermore, various kinds of precision instruments such as various analyzers, calculators, sensors, and flow meters are necessary for CP control, leading to an increase in equipment cost.

  As in the case of Patent Document 2, in order to supply different CP conversion gas for each processing step, a plurality of gas conversion furnaces are necessary, and an increase in equipment cost is inevitable. In addition, if a high CP shift gas is generated in the shift furnace for the carburizing process, the shift furnace may be sooted.

  Since the thing of patent document 3 supplies hydrocarbon, a carbon dioxide, oxygen, nitrogen as needed, and single gas as a source gas, cost increase cannot be avoided and lacks practicality.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a gas carburizing method and a gas carburizing apparatus that can perform heat treatment in a stable atmosphere and can reduce equipment costs.

In order to solve the above problems, the gas carburizing method according to the present invention generates a modified gas using a hydrocarbon-based gas, air, and a catalyst, and has a higher carbon potential than the modified gas and a low CO ₂ concentration. generates a low CO ₂ converted gas, the reformed gas, the low CO ₂ converted gas, one of the mixed gas of the said converted gas low CO ₂ converted gas, as the atmosphere gas in a series of heat treatment processes for carburizing (Claim 1).

According to the present invention, in each of a series of heat treatment processes for carburizing, the reformed gas, the low CO ₂ converted gas, from a gas mixture of the converted gas and the low CO ₂ converted gas, calculated to the treated product Heat treatment can be performed by selecting an optimal gas according to the quality to be obtained. Since the low CO ₂ modified gas has a higher CP value than the modified gas, it exhibits a greater carburizing power. Therefore, it is not necessary to add the enriched gas as in the conventional case, and the hydrocarbon gas is not decomposed unlike the enriched gas, so that the heat treatment can always be performed in a stable atmosphere. There is no need for a plurality of transformation furnaces, and a large number of precision instruments are not necessary for CP control, which is advantageous in terms of equipment cost.

  In the gas carburizing method according to the present invention, the series of heat treatment steps for carburizing may be performed while moving an object to be processed between a plurality of heat treatment chambers, or in a single heat treatment chamber. A series of heat treatment steps may be sequentially performed.

As a preferred embodiment, the first modification process using the modification gas alone as the atmosphere gas, or using both the modification gas and the low CO ₂ modification gas as the atmosphere gas, and the modification gas, A mode in which both the low CO ₂ modified gas is used as an atmospheric gas or a second heat treatment step in which the low CO ₂ modified gas is used alone as an atmospheric gas is exemplified (Claim 2).

Further, whether to use the converted gas alone as the atmosphere gas, or may be a mode comprising a third and fourth heat treatment step of using both the said reformed gas low CO ₂ converted gas as the atmosphere gas ( Claim 3).

  As a specific example, the first heat treatment step is a temperature raising step, the second heat treatment step is a carburizing step, the third heat treatment step is a diffusion step, and the fourth heat treatment step is a temperature drop. It is a process (claim 4).

As a preferred embodiment, the low CO ₂ modified gas can be generated by separating CO ₂ contained therein from the modified gas (Claim 5).

  As a preferred embodiment, in the carburizing step included in the series of heat treatment steps, an aspect (Claim 6) in which the flow velocity in the furnace of the atmospheric gas is a flow velocity of 1.5 m / s or more, A mode (Claim 7) in which the flow rate in the furnace is a flow rate at which the number of atmosphere replacements in the furnace is 29 times / h or more can be adopted. According to these aspects, the effective hardened layer depth can surely reach the target value, and the carburizing time can be shortened.

On the other hand, a gas carburizing apparatus according to the present invention connects a shift furnace that generates a shift gas using a hydrocarbon gas, air, and a catalyst, and a heat treatment chamber that performs a series of processes for the carburization and the shift furnace. a reformed gas conduit to a low CO ₂ converted gas generator for generating a CO ₂ concentration is less low CO ₂ reformed gas has a higher carbon potential than the converted gas, low CO of the low CO ₂ converted gas generator A low CO ₂ metamorphic gas line connecting the _two metamorphic gas outlets and the heat treatment chamber, the metamorphic gas line and the low CO ₂ metamorphic gas line disposed on the heat treatment chamber, low CO ₂ converted gas, in which and a valve for switching between whether to introduce any of a gas mixture of the low CO ₂ converted gas and the reformed gas (claim 8).

According to the gas carburization apparatus, by switching of the valve, in the heat treatment chamber, the reformed gas, the low CO ₂ converted gas, from a gas mixture of the said converted gas low CO ₂ converted gas, the treated product The optimum gas can be selected and supplied according to the required quality. Since the low CO ₂ modified gas has a higher CP value than the modified gas, it exhibits a greater carburizing power. Therefore, it is not necessary to add an enriched gas as in the prior art, and therefore heat treatment can be performed in a stable atmosphere at all times. Since a plurality of transformation furnaces are not required as in the conventional example, and a large number of precision instruments are not required for CP control, this is advantageous in terms of cost.

  In the gas carburizing apparatus according to the present invention, the heat treatment chamber may be a plurality of heat treatment chambers that perform a series of processes for carburization, or a single heat treatment chamber that performs a series of processes for carburization. It may be.

As a preferred embodiment, a mode in which the low CO ₂ shift gas generating device is a CO ₂ adsorber that adsorbs and separates CO ₂ contained therein from the shift gas is exemplified (claim 9).

It is a distribution diagram showing the piping system of the gas carburizing device concerning one embodiment of the present invention. It is a processing-process figure in one Example of this invention. It is a graph which shows the relationship between the flow velocity which concerns on one Example of this invention, and the depth from the test piece surface whose carbon concentration is 0.3 wt%. It is a graph which shows the relationship between the flow volume which concerns on one Example of this invention, and the depth from the test piece surface whose carbon concentration is 0.3 wt%.

  Hereinafter, a gas carburizing method and a gas carburizing apparatus according to an embodiment of the present invention will be described with reference to the accompanying drawings.

Gas carburizing method according to the present invention is to produce a reformed gas by reforming furnace as the hydrocarbon gas and air feed gas, the CO ₂ concentration is less low CO ₂ reformed gas has a higher carbon potential than the reformed gas One of the generated modified gas, the low CO ₂ modified gas, and the mixed gas of the modified gas and the low CO ₂ modified gas is used as an atmospheric gas in a series of heat treatment steps for carburizing.

  In the present invention, the series of heat treatment steps for carburizing may be performed while moving an object to be processed between a plurality of heat treatment chambers, or a series of heat treatment steps in a single heat treatment chamber. It may be performed sequentially.

The shift gas can be generated using a conventionally known shift furnace. The shift gas generated in the shift furnace varies depending on the mixing ratio of hydrocarbon gas (for example, 13A gas, natural gas, propane, butane, etc.) and air, the target gas composition, and the CP value. generally, CO: 18 to 25 _vol%, H 2: 30-41 _vol%, N 2: 35 to 50 _vol%, CO 2: _{0.12~0.25 vol%,} H 2 O: 0.6% by volume _hereinafter, CH 4: is preferably 0.04 vol% or less. When 13A (city gas) is used as the hydrocarbon-based gas, an example of the modified gas composition is as follows (when 850 ° C. and CP value is 0.8).

N ₂ (nitrogen): about 39.85% by volume
H ₂ (hydrogen): about 36% by volume
CO (carbon monoxide): about 24% by volume
CO ₂ (carbon dioxide): about 0.15% by volume
The low CO ₂ converted gas from reformed gas generated in the reforming furnace can be generated by adsorptive separation of CO ₂ using a conventionally known CO ₂ adsorber. Therefore, the low CO ₂ modified gas has a smaller CO ₂ ratio than the modified gas. Specifically, CO ₂ is preferably less than 0.15% by volume, more preferably 0.1% by volume or less, and 0.05% by volume or less.

In the present invention, the method for producing the low CO ₂ modified gas is not limited to the above method, and a mixture of a plurality of gases such as N ₂ , H ₂ , CO, CO ₂ and the like has the same composition as the low CO ₂ modified gas. It may be a gas, which is also referred to as a low CO ₂ modified gas for convenience.

In the present invention, the low CO ₂ modified gas includes a case where CO ₂ is zero.

The CP (carbon potential) of the low CO ₂ shift gas is higher than the CP of the shift gas due to the reduction of CO ₂ compared to the shift gas, and the carburizing power is increased by using this as the carburizing atmosphere gas. . In addition, it is no longer necessary to use the enriched gas that has been added in the past to compensate for the weak carburizing power of the metamorphic gas, there is no decomposition of hydrocarbon gases, and carburizing is performed in a stable carburizing atmosphere gas. Carburizing unevenness and sooting are less likely to occur. This advantage is obtained wherein only the low CO ₂ converted gas not only when performing carburizing as the atmospheric gas, even when performing carburizing with a mixed gas of the low CO ₂ converted gas and the reformed gas as the atmosphere gas It is done.

The inventors of the present invention, a mixture of four gases CO and CO ₂ and _{H 2} and _{N 2,} and reformed gas, to produce more CO ₂ concentration of small low CO ₂ converted gas which, of SCr420 Gas carburizing treatment was performed on the workpiece (workpiece). The test furnace is gold furnace and the carburization time is 2 hours. Table 1 shows the carburizing conditions and carburizing atmosphere gas composition, and Tables 2 and 3 show the carburizing results.

In these tables, Conventional Example 1 is obtained by carburizing with a modified gas, and Examples 2 to 4 are those obtained by carburizing with a low CO ₂ modified gas. The CP of the atmospheric gas (low CO ₂ modified gas) of the example is higher than the CP of the atmospheric gas (modified gas) of the conventional example by a small amount of CO ₂ concentration.

In Table 1, the CP value was determined by a calculation formula of “(CO concentration) ² / (CO ₂ concentration) × Kp (equilibrium constant) × saturated carbon amount (saturated carbon amount of steel at carburizing temperature)”.

In Table 1, CH ₄ is considered to be produced in a trace amount by the reaction.

  As shown in Table 3, in both the conventional examples and the examples, there was no precipitation of carbides in the surface structure, and good carburization could be performed. Further, as shown in Table 2, the examples had higher carburizing ability and higher surface carbon concentration of the workpiece than the conventional examples, and good results were obtained. There was no sooting in any of the examples.

  When a shift gas having a CP of 1.2 at 950 ° C. was generated in the shift furnace, sooting occurred in the shift furnace.

By the test, the superiority of gas carburizing according to the low CO ₂ converted gas to the gas carburizing according converted gas was demonstrated.

  Next, a gas carburizing apparatus suitable for use in carrying out the method of the present invention will be described.

As shown in FIG. 1, a gas carburizing apparatus 100 according to an embodiment of the present invention includes a heat treatment chamber having a series of heat treatment chambers including a heating chamber (preheating chamber) 1, a carburizing chamber 2, a diffusion chamber 3, and a descending chamber 4. The unit 5, a shift furnace 6 that generates an atmospheric gas used in the heat treatment chambers 1, 2, 3, and 4, and low CO ₂ shift gas generators 7 and 7 are provided. The shift furnace 6 and the heat treatment chambers 1, 2, 3, 4 are connected by a shift gas pipe 8. Further, the low CO ₂ shift gas outlets 9 and 9 of the low CO ₂ shift gas generators 7 and 7 and the thermal chambers 1, 2, 3, and 4 are connected by a low CO ₂ shift gas pipe 10. .

  The shift furnace 6 is the same as that conventionally known and incorporates a catalyst (not shown) such as a nickel catalyst. Hydrocarbon gas such as methane, propane, butane, 13A (city gas) and air are introduced into the shift furnace 6 and pass through a catalyst layer heated to, for example, 1080 ° C. The modified gas is generated by the catalytic reaction (metamorphic reaction) at this time.

A cooling device 11 is attached to the shift furnace 6. The generated metamorphic gas at 1080 ° C. is cooled to room temperature by the cooling device 11 and then led out to the metamorphic gas pipe 8. In the case where the cooling device 11 is not attached to the shift furnace 6, a cooling device is disposed on the shift gas pipe 8 between the shift furnace 6 and the low CO ₂ shift gas generators 7, 7. You just have to.

The low CO ₂ converted gas generator device 7 generates a low CO ₂ converted gas CO ₂ concentration is less than reformed gas produced in the reforming furnace 6. As the low CO ₂ modified gas generating device 7, a conventionally known CO ₂ adsorber filled with a CO ₂ adsorbent can be used. In this case, said reformed gas generated in the reforming furnace 6 is introduced into the CO ₂ adsorber 7,7 in branch conduit 12, 12, the CO ₂ of the shift gas with CO ₂ adsorbent to adsorption separation. As the CO ₂ is adsorbed and separated, the modified gas becomes the low CO ₂ modified gas. The low CO ₂ metamorphic gas is led through the low CO ₂ metamorphic gas line 10 toward the heating chamber 1, the carburizing chamber 2, the diffusion chamber 3, and the descending greenhouse 4. By using the CO ₂ adsorber 7, moisture (H ₂ O) in the shift gas is also efficiently removed simultaneously with CO ₂ , and a low CO ₂ shift gas having a higher CP than the shift gas can be obtained. .

As another example of the low CO ₂ modified gas generating device 7, a low CO ₂ modified gas is generated by mixing a plurality of gases such as N ₂ , H ₂ , CO, and CO ₂ without using the modified gas. It is also possible to employ a gas mixing device (not shown).

In the example of FIG. 1, two CO ₂ adsorbers 7 and 7 are connected to the shift gas pipe 8 so as to be switchable. This is because CO ₂ adsorption and CO ₂ desorption are alternately performed between the two CO ₂ adsorbers 7 and 7 so that continuous operation can be performed without any problem. The separated CO ₂ is discharged from the CO ₂ adsorbers 7 and 7 to the outside through the discharge pipe 13.

A CO ₂ analyzer 14 is disposed in the exhaust line 13 and the low CO ₂ metamorphic gas line 10, and the CO ₂ concentration in the gas flowing through both lines is constantly monitored and measured.

A large number of valves 1 a to 4 a and 1 b to 4 b are disposed on the shift gas pipe 8 and the low CO ₂ shift gas pipe 10. These valves, in each of the series of heat treatment chamber 1, 2, 3, 4, the reformed gas, introducing one of a gas mixture of the low CO ₂ converted gas, the reformed gas and the low CO ₂ converted gas It is a valve for switching whether to do. A gas mixing unit 15 is disposed on the downstream side of each valve. In the gas mixing unit 15, and a reformed gas to be supplied through the reformed gas conduit 8, a low CO ₂ converted gas supplied is a predetermined composition are mixed through the low CO ₂ converted gas conduit 10.

By the operation of the valves 1a to 4a and 1b to 4b, the modified gas is introduced alone or the mixed gas is introduced into the temperature raising chamber 1 as a temperature raising step. In the carburizing chamber 2, as the carburizing step, the low-CO ₂ modified gas is introduced alone or the mixed gas is introduced. In the diffusion chamber 3, the modified gas is introduced alone or the mixed gas is introduced as a diffusion step. In the temperature drop room 4, the modified gas is introduced alone or the mixed gas is introduced as a temperature lowering step. The atmospheric gas in each of the heat treatment chambers 1, 2, 3 and 4 has a higher carburizing power as the proportion of the low CO ₂ metamorphic gas increases.

  Which gas is introduced into each heat treatment chamber in each process and how much gas is appropriately determined according to the quality required for the processed product. However, the metamorphic gas is not supplied to the carburizing chamber 2 alone. This is because the modified gas alone has a low CP value and a desired carburization cannot be obtained.

In addition, the low CO ₂ metamorphic gas is not supplied alone to the heating chamber 1, the diffusion chamber 3, and the descending greenhouse 4. This is because if a low CO ₂ metamorphic gas is supplied to the temperature raising chamber 1 and the temperature lowering chamber 4 alone, carbides may be deposited on the workpiece surface at low temperatures during temperature raising and temperature lowering. This is because if the low CO ₂ metamorphic gas is supplied to the diffusion chamber 3 alone, the surface carbon concentration of the workpiece is excessively increased, and the amount of precipitated austenite is expected to increase in the subsequent oil quenching process.

Preferably, the shift gas is generated in a shift furnace aiming at the CP of the shift gas used in the descending greenhouse 4, and this shift gas is supplied to the descending chamber 4 alone, and the carburizing chamber 2 is supplied with a low CO ₂ shift gas alone. Then, the mixed gas of the shift gas and the low CO ₂ shift gas is supplied to the temperature raising chamber 7 and the diffusion chamber 3.

A part of the atmospheric gas involved in the heat treatment in each of the heat treatment chambers 1, 2, 3, 4 is burned and discharged as exhaust gas through the exhaust pipe 16, and the other part is sent to the CO _{2 through the} CO ₂ demounting pipe 17. _{The two} adsorbers 7 and 7 are sent to the CO ₂ adsorber 7 and used as heating means for CO ₂ desorption.

  According to the gas carburizing apparatus 100, it is possible to always perform heat treatment on the workpiece with the optimum atmospheric gas in each of the heat treatment chambers 1, 2, 3, and 4 by operating the valves 1a to 4a and 1b to 4b. . There is no problem that the equipment cost is increased as in the conventional apparatus. Further, since it is not necessary to use an enriched gas, there is an advantage that the processing can be performed in a stable atmosphere and the quality of the processed product is improved.

  The illustrated example is an example in which the heat treatment section 5 includes a plurality of heat treatment chambers for performing a series of processes for carburizing, but the gas carburizing apparatus according to the present invention is not limited to this, and for carburizing. Also included is a mode in which the series of processes is performed in a single heat treatment chamber.

  Furthermore, the inventors of the present invention have sought for preferred flow rates and flow rates of carburizing atmosphere gases when carrying out the method of the present invention.

  First, as an experimental apparatus, an apparatus capable of replacing and inserting an inner pipe into an existing gold furnace furnace is prepared, and the inside of the inner pipe is used as a furnace. As the inner pipe, pipes having various inner diameters are prepared, and the inner pipe is changed to change the inner diameter of the pipe so that the flow rate and flow rate of the carburizing atmosphere gas flowing in the inner pipe are changed.

FIG. 2 is a process diagram in this experiment. As shown in FIG. 2, first, nitrogen gas was supplied into the furnace (inside the inner pipe), and the temperature was raised to 950 ° C. in 10 minutes. After raising the temperature and holding for 10 minutes, CO ₂ gas was supplied, and the mixture was further kept for 5 minutes in an atmosphere of N ₂ + CO ₂ . Next, a low CO ₂ gas (see Table 4 for the gas composition) was supplied into the furnace and held for 30 minutes to stabilize the atmosphere. Thereafter, a test piece TP (test piece: SCr420) was inserted into the furnace and held for 70 minutes, and carburizing was performed while continuously supplying the low CO ₂ metamorphic gas. After the carburizing process, a diffusion process was performed for 14 minutes and a temperature lowering process was performed for 14 minutes, followed by a quenching process. By using the inner pipe having various inner diameters (see the pipe inner diameter in Table 5), the flow rate and flow rate of the low CO ₂ metamorphic gas during the carburizing process can be changed to various values.

The results of the experiment are shown in Table 5.

  In Table 5, three examples from the top, that is, Comparative Example 1 and Examples 1 and 2, are the experimental results when the flow velocity is changed (the flow rate is constant). The following examples, that is, Example 3 and Comparative Examples 2 to 4, are experimental results when the flow rate is changed (the flow rate is constant).

  With reference to Table 5, the preferred flow rate and flow rate of the carburizing atmosphere gas will be considered.

<About flow velocity>
Even when carburizing with the low CO ₂ gas having a high CP value, if the flow rate is small as in Comparative Example 1, the depth (position) from the specimen surface where the carbon concentration targeted in this experiment is 0.3 wt% It can be seen that it does not reach 0.75 mm or more. On the other hand, from Examples 1 and 2, when the flow velocity is 3.04 m / s and 5.98 m / s, a target sufficient carburization depth can be obtained, and carburization can be performed faster than before. FIG. 3 is a graph showing the relationship between the flow velocity and the depth (position) from the surface of the test piece having a carbon concentration of 0.3 wt% using this test result. FIG. 3 shows that the flow rate of the carburizing atmosphere gas should be 1.5 m / s or more in order to obtain the target depth (0.75 mm).

  In order to make the flow rate of the carburizing atmosphere gas 1.5 m / s or more within the work installation range of the carburizing furnace or diffusion chamber of the carburizing furnace, fans for gas agitation are required vertically and horizontally in the carburizing chamber or diffusion chamber. Therefore, it is preferable to suppress the variation in the flow rate, and a stable carburizing process can be realized.

  The gas flow velocity (wind velocity) can be measured with an anemometer. In addition, increasing the flow rate increases the carbon transfer coefficient (β value), and the increase in flow rate is not related to the increase in flow rate (substance supply amount).

<About flow rate>
From Comparative Examples 2 to 4, when the flow rate of the carburizing atmosphere gas is small, that is, when the number of gas replacements in the furnace is 5.1 to 19 times per hour and the flow rate is about 0.1 m / s, the target carburizing is performed. It can be seen that the depth cannot be obtained. On the other hand, when the flow rate is large as in Example 3, the target carburizing depth is obtained, and carburizing can be performed earlier than in the past. FIG. 4 is a graph showing the relationship between the flow rate and the depth (position) from the surface of the test piece having a carbon concentration of 0.3 wt% using this test result. FIG. 4 shows that in order to obtain the target depth (0.75 mm), the flow rate of the carburizing atmosphere gas may be set to a flow rate that is 29 times / h or more in the furnace atmosphere replacement.

  The number of atmosphere replacements in the furnace is a value obtained by dividing the gas flow rate per hour supplied into the furnace by the furnace volume. In order to increase the number of times the atmosphere in the furnace is replaced, new gas may be continuously supplied into the furnace, but it is preferable from the viewpoint of cost that the furnace gas is circulated and supplied into the furnace. Moreover, it turns out that a carbon transfer coefficient ((beta value)) will fall when a flow rate (number of substitutions) is increased, and in order to improve carburizing efficiency, it is more preferable to increase a gas flow rate than a gas flow rate.

1 temperature raising chamber 2 the carburizing chamber 3 diffusion chamber 4 descending greenhouses 6 reforming furnace 7 low CO ₂ converted gas generator 8 reformed gas pipe 9 low CO ₂ converted gas outlet 10 low CO ₂ converted gas conduit 1a to 4a, 1B to 4B valve

Claims (9)

To generate a reformed gas by using the hydrocarbon gas and air and a catalyst, to produce a CO ₂ concentration is less low CO ₂ reformed gas has a higher carbon potential than the converted gas, the reformed gas, the low CO _A gas carburizing method in which any one of ₂ metamorphic gas, a mixed gas of the metamorphic gas and the low CO ₂ metamorphic gas is used as an atmosphere gas in a series of heat treatment steps for carburizing. A first heat treatment step in which the metamorphic gas is used alone as an atmospheric gas, or both the metamorphic gas and the low CO ₂ metamorphic gas are used as an atmospheric gas, and both the metamorphic gas and the low CO ₂ metamorphic gas The gas carburizing method according to claim 1, further comprising: a second heat treatment step using the low CO ₂ modified gas alone as the atmospheric gas. 3. The method according to claim 1, further comprising third and fourth heat treatment steps in which the shift gas is used alone as an atmospheric gas, or both the shift gas and the low CO ₂ shift gas are used as an atmospheric gas. Gas carburizing method.   The first heat treatment step is a temperature raising step, the second heat treatment step is a carburizing step, the third heat treatment step is a diffusion step, and the fourth heat treatment step is a temperature lowering step. Item 4. The gas carburizing method according to Item 3. The gas carburizing method according to any one of claims 1 to 4, wherein the low CO ₂ modified gas is generated by separating CO ₂ contained therein from the modified gas.   6. The gas carburizing method according to claim 1, wherein in the carburizing step included in the series of heat treatment steps, a flow rate in the furnace of the atmospheric gas is a flow rate of 1.5 m / s or more.   The gas according to any one of claims 1 to 6, wherein in the carburizing step included in the series of heat treatment steps, the flow rate of the atmospheric gas in the furnace is a flow rate at which the number of replacements in the furnace atmosphere is 29 times / h or more. Carburizing method. A shift furnace that generates a shift gas using a hydrocarbon gas, air, and a catalyst; a shift gas pipe that connects the shift furnace and a heat treatment chamber that performs a series of processes for carburizing; and the shift gas coupled with low CO ₂ converted gas generator, and a low CO ₂ converted gas outlet of the low CO ₂ converted gas generator the heat treatment chamber for generating a CO ₂ concentration is less low CO ₂ reformed gas has a high carbon potential A low CO ₂ metamorphic gas line, disposed on the metamorphic gas line and the low CO ₂ metamorphic gas line, and in the heat treatment chamber, the metamorphic gas, the low CO ₂ metamorphic gas, the metamorphic gas and the A gas carburizing apparatus comprising: a valve that switches which of a mixed gas with a low CO ₂ metamorphic gas is introduced. The gas carburizing apparatus according to claim 8, wherein the low CO ₂ shift gas generating apparatus is a CO ₂ adsorber that adsorbs and separates CO ₂ contained therein from the shift gas.

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