JP2020196943A - Gas carburizing method and gas carburizing apparatus - Google Patents

Abstract

To provide a carburizing method for relaxing deviation between the measured value and actual value of cp and reducing cost required for enrichment.SOLUTION: The gas carburizing method for performing a carburizing step and a diffusion step in the state of having a material to be processed in an atmosphere comprises: changing a target carbon potential to be targetted in the carburizing step in the atmosphere from a relatively high first set value to a relatively low second set value during the carburizing step; promptly increasing the actual carbon potential in the atmosphere to raise the carburization ability of the atmosphere to achieve the reduction of carburization time and the uniformity of carburization depth; and reducing cost required for enrichment without wastefully consuming enrichment gas.SELECTED DRAWING: Figure 4

Description

The present invention relates to gas carburizing methods and devices.

In the gas carburizing treatment, generally, the treated product is charged into a treatment chamber having a carburizing atmosphere and held by heating. The atmosphere is generally formed by filling the treatment chamber with a metamorphic gas containing CO gas. When carburizing, the carbon potential (hereinafter referred to as "CP") is insufficient with the modified gas alone. That is, the carbon concentration of the obtained carburized layer does not rise sufficiently, and sufficient quenching hardness and the depth of the hardened layer cannot be obtained. Therefore, in order to adjust the CP of the metamorphic gas, an enriched gas is introduced into the furnace.

The applicant has grasped the following Patent Documents 1 and 2 as prior art documents relating to a gas carburizing method using an enriched gas.

Japanese Unexamined Patent Publication No. 62-26013 Japanese Unexamined Patent Publication No. 2015-129324

The above-mentioned Patent Document 1 has the following description. Although the "heat treatment path" is an error, it is quoted as it is.
[Gazette, page 3, lower left column, line 19 to lower right column, line 20]
In this example, a material made of nickel-chromium steel (SNC 415) was selected as the material to be treated, and the heat treatment operation was started. In this operation, as shown in the attached drawing, industrial methanol was fed into the heat treatment path at a flow rate of 2000 ml / hr as an organic liquid for generating a carrier gas. Then, the temperature inside the heat treatment furnace was maintained at 920 ° C., and 30 minutes after the holding, propane gas (100%) was supplied as an enriched gas into the heat treatment furnace at a flow rate of 3 l / min, and at the same time, as a super enriched gas. Similarly, propane gas (100%) was supplied into the heat treatment furnace at a flow rate of 5 l / min. Then, about 15 minutes after the feeding, the atmospheric gas component concentration reached a predetermined value (measured by the carbon sensor), so that the feeding of the super enriched gas was stopped and the feeding amount of the enriched gas was gradually reduced. By doing so, the atmospheric gas component concentration was maintained at a predetermined value. As a result, the atmosphere in the furnace can be set to a predetermined gas component concentration in a considerably shorter time than before, and the gas component concentration can be stably maintained.
[Gazette, page 3, upper right column, 2nd to 6th lines]
By the way, as the organic gas for the enriched gas described above, a paraffin-based hydrocarbon gas is desirable, and among these, methane gas, propane gas, butane gas, etc., which are easily thermally decomposed and are relatively inexpensive, are suitable.

The above-mentioned Patent Document 2 has the following description.
[0028]
The enriched gas is added to the carrier gas in order to increase the carbon concentration (carbon potential) of the atmospheric gas in the furnace 10a. That is, by supplying the enriched gas, carburizing with the atmospheric gas in the furnace 10a becomes possible. Conventionally, chain saturated hydrocarbons such as methane (CH ₄ ), propane (C ₃ H ₈ ) or butane (C ₄ H ₁₀ ) have been used as enriched gases in gas carburization under normal pressure. In the embodiment, acetylene (C ₂ H ₂ ), which is a chain unsaturated hydrocarbon, is used.

In the gas carburizing treatment, generally, in the carburizing step, carbon is permeated into the steel surface from the surface of the treated product and diffused inside in a relatively high CP atmosphere. In the subsequent diffusion step, the permeated carbon is diffused in the depth direction in a relatively low CP atmosphere, and at the same time, the carbon concentration on the steel surface is adjusted to a predetermined concentration. As the enriched gas introduced to adjust the CP of the above atmosphere, a natural gas or a saturated hydrocarbon gas such as propane or butane is generally used.

In order to measure the CP of the atmosphere, generally, the O ₂ concentration in the atmosphere is measured by the O ₂ sensor, and the CP is calculated from the O ₂ concentration and the temperature in the furnace. Two are correlated, if residual values decrease the O ₂ concentration CP is raised, CP is lowered if the O ₂ concentration goes up.

On the other hand, the CP in the gas carburizing treatment is controlled by setting a target set value and controlling the introduction amount of the enriched gas so that the measured value calculated from the O ₂ concentration approaches the above set value. When the CP is controlled at a high value, the amount of enriched gas introduced is increased, and when the CP is controlled at a low value, the amount of enriched gas introduced is decreased.

However, in the above O ₂ sensor, the measurement accuracy of the O ₂ concentration becomes unstable due to the influence of methane derived from the enriched gas. That is, the platinum electrode of the O ₂ sensor serves as a catalyst, and methane is decomposed in the vicinity of the sensor. H ₂ is generated by this decomposition, and the H ₂ concentration rises locally near the sensor. The H ₂ reacts with the surrounding O _2, and the O ₂ concentration decreases locally only in the vicinity of the sensor.

Then, the CP calculated based on the O ₂ concentration and the temperature inside the furnace will be higher than the actual CP of the atmosphere. Therefore, the CP controlled by using the O ₂ sensor has a local value only in the vicinity of the sensor, and the actual atmosphere does not reach the set value.

The above-mentioned methane is derived from the hydrocarbon gas used as the enriched gas and is mixed in the atmosphere. That is, if natural gas is used as the enriched gas, methane is contained as the main component. If propane or butane is used as the enriched gas, methane is produced by its decomposition.

Further, in general, in the gas carburizing treatment, CP is set to a different value and controlled in the carburizing step and the diffusion step. In the carburizing process, CP is controlled at a relatively high set value, and a large amount of carbon permeates the surface of the treated product and diffuses inside. In the diffusion step, CP is controlled with a relatively low set value to optimize the surface carbon concentration of the treated product, and at the same time, the carbon permeated into the treated product is diffused in the depth direction.

As described above, when methane is mixed in the atmosphere, even if the CP controlled based on the O ₂ concentration reaches the target set value, the CP in the actual atmosphere does not reach it. If the CP is controlled at a high set value in the carburizing step, the amount of enriched gas introduced increases, and the amount of methane mixed in increases accordingly. As the amount of methane mixed in increases, the dissociation between the CP controlled based on the O ₂ concentration and the CP in the actual atmosphere increases.

Therefore, the actual CP and CP to control deviates, even if the carburizing treatment in accordance with measurements of CP based on the O ₂ concentration, carburization ability of the atmosphere has is not sufficient. In particular, in the initial stage of the carburizing process, the CP by measurement rises when the CP in the actual atmosphere does not rise sufficiently, which limits the introduction of enrichment. For this reason, there arises a problem that the processing time becomes long and the carburizing depth varies.

On the other hand, as a CP measurement method, there is also a method of calculating from the CO ₂ concentration in the atmosphere. However, the O ₂ sensor is suitable as a sensor for controlling the CP to the target set value, and is often used. This is because the O ₂ sensor is easier to handle than the CO ₂ sensor, and its characteristics such as maintainability and reaction speed are suitable for the purpose.

For this reason, the problem that the measured value of CP and the actual value deviate from each other due to the mixing of methane as described above is left untouched.

Patent Document 1 discloses a technique in which the enrichment amount is temporarily increased for a certain period of time (super enrichment) at the initial stage of carburizing to raise CP in a short time.
However, as described in Patent Document 1, when the flow rate of propane gas is increased by super enrichment, a large amount of methane is also generated by the decomposition. Therefore, the problem that the measured value of CP and the actual value deviate from each other due to the above-mentioned mixing of methane becomes greater.

In Patent Document 2, acetylene is used as an enriched gas that suppresses the generation of methane due to decomposition.
However, acetylene gas is difficult to handle and is extremely costly. There is also difficulty in transporting in a gas container. Therefore, it is not suitable for general-purpose processing and is not widely used.

An object of the present invention is as shown below, and an object of the present invention is to solve the above problems.
Measurements of CP based on the O ₂ concentration as well as relax the divergence of the actual value, to provide a carburizing method and apparatus for saving the cost of enriched.

The gas carburizing method according to claim 1 employs the following configuration in order to achieve the above object.
It is a gas carburizing method in which a carburizing step and a diffusion step are performed in the presence of a processed product in the atmosphere.
During the carburizing step, the target carbon potential of the atmosphere, which is the target in the carburizing step, is changed from a relatively high first set value to a relatively low second set value.

The gas carburizing method according to claim 2 employs the following configuration in addition to the configuration according to claim 1.
The first set value and the second set value are set with reference to the first calculated carbon potential calculated from the O ₂ concentration in the atmosphere.

The gas carburizing method according to claim 3 employs the following configuration in addition to the configuration according to claim 1 or 2.
In the change from the first set value to the second set value, the second calculated carbon potential calculated from the CO ₂ concentration in the atmosphere becomes a predetermined value relatively lower than the second set value. Do that as a trigger.

The gas carburizing method according to claim 4 employs the following configuration in addition to the configuration according to any one of claims 1 to 3.
Ethylene gas is used as the enriched gas to be introduced during the carburizing step.

The gas carburizing method according to claim 5 employs the following configuration in addition to the configuration according to any one of claims 1 to 4.
A hydrocarbon gas is used as the enriched gas to be introduced during the diffusion step.

The gas carburizing method according to claim 6 adopts the following configuration in order to achieve the above object.
It is a gas carburizing method in which a carburizing step and a diffusion step are performed in the presence of a processed product in the atmosphere.
Ethylene gas is used as the enriched gas to be introduced during the carburizing process.
A hydrocarbon gas is used as the enriched gas to be introduced during the diffusion step.

The gas carburizing method according to claim 7 adopts the following configuration in addition to the configuration according to claim 6.
During the carburizing step, the target carbon potential of the atmosphere, which is the target in the carburizing step, is changed from a relatively high first set value to a relatively low second set value.

The gas carburizing method according to claim 8 adopts the following configuration in addition to the configuration according to claim 7.
The first set value and the second set value are set with reference to the first calculated carbon potential calculated from the O ₂ concentration in the atmosphere.

The gas carburizing method according to claim 9 adopts the following configuration in addition to the configuration according to claim 7 or 8.
In the change from the first set value to the second set value, the second calculated carbon potential calculated from the CO ₂ concentration in the atmosphere becomes a predetermined value relatively lower than the second set value. Do that as a trigger.

The gas carburizing apparatus according to claim 10 adopts the following configuration in order to achieve the above object.
One or more treatment chambers that perform carburizing and diffusion steps with the treated product present in the atmosphere.
A setting changing means for changing the target carbon potential of the atmosphere, which is the target in the carburizing step, from a relatively high first set value to a relatively low second set value during the carburizing step is provided.

The gas carburizing apparatus according to claim 11 adopts the following configuration in addition to the configuration according to claim 10.
Further provided with a first calculation means for calculating the first calculation carbon potential from the O ₂ concentration in the above atmosphere.
The first set value and the second set value are set with reference to the first calculated carbon potential.

The gas carburizing apparatus according to claim 12 adopts the following configuration in addition to the configuration according to claim 10 or 11.
Further provided with a second calculation means for calculating the second calculation carbon potential from the CO ₂ concentration in the above atmosphere.
The setting changing means changes the setting value by triggering that the second calculated carbon potential becomes a predetermined value relatively lower than the second setting value.

The gas carburizing apparatus according to claim 13 adopts the following configuration in addition to the configuration according to any one of claims 10 to 12.
Further provided is an ethylene introduction means for introducing ethylene gas into the treatment chamber as the enriched gas during the carburizing step.

The gas carburizing apparatus according to claim 14 employs the following configuration in addition to the configuration according to any one of claims 10 to 13.
Further provided is a hydrocarbon introduction means for introducing a hydrocarbon gas into the processing chamber as the enriched gas in the diffusion step.

The gas carburizing apparatus according to claim 15 adopts the following configuration in order to achieve the above object.
One or more treatment chambers that perform carburizing and diffusion steps with the treated product present in the atmosphere.
Ethylene introduction means for introducing ethylene gas into the treatment chamber as the enriched gas during the carburizing process, and
It is provided with a hydrocarbon introducing means for introducing a hydrocarbon gas into the processing chamber as the enriched gas in the diffusion step.

The gas carburizing apparatus according to claim 16 adopts the following configuration in addition to the configuration according to claim 15.
Further provided with a setting changing means for changing the target carbon potential of the atmosphere, which is the target in the carburizing step, from a relatively high first set value to a relatively low second set value during the carburizing step. There is.

The gas carburizing apparatus according to claim 17 adopts the following configuration in addition to the configuration according to claim 16.
Further provided with a first calculation means for calculating the first carbon potential from the O ₂ concentration in the above atmosphere,
The first set value and the second set value are set with reference to the first carbon potential.

The gas carburizing apparatus according to claim 18 adopts the following configuration in addition to the configuration according to claim 16 or 17.
Further provided with a second calculation means for calculating the second calculation carbon potential from the CO ₂ concentration in the above atmosphere.
The setting changing means changes the setting value by triggering that the second carbon potential becomes a predetermined value relatively lower than the second setting value.

In the gas carburizing method according to claim 1, the carburizing step and the diffusion step are performed in a state where the treated product is present in the atmosphere. During the carburizing step, the target carbon potential of the atmosphere, which is the target in the carburizing step, is changed from a relatively high first set value to a relatively low second set value.
As described above, in the initial stage of the carburizing process in which the measured value of the carbon potential rises due to the influence of methane derived from the enriched gas, the target carbon potential of the above atmosphere is set to a relatively high first set value. This prevents the introduction of enrichment from being restricted before the actual carbon potential of the above atmosphere rises. The actual carbon potential of the above atmosphere rises rapidly, increasing the carburizing capacity of the above atmosphere. Therefore, the carburizing treatment time can be shortened and the carburizing depth can be made uniform. Further, in the initial stage of the carburizing process in which the carbon potential needs to be rapidly increased, the amount of enriched gas introduced is increased by the control based on the first set value. After changing to the second set value, the amount of enriched gas introduced is reduced. Therefore, the cost required for enrichment can be reduced without wasting the enrichment gas.

In the gas carburizing method according to claim 2, the first set value and the second set value are set with reference to the first calculated carbon potential calculated from the O ₂ concentration in the atmosphere.
The first set value and the second set value are based on the O ₂ concentration detected by the O ₂ sensor. Therefore, the measured value of the first calculated carbon potential tends to increase due to the influence of methane derived from the enriched gas. Therefore, in the initial stage of the carburizing step, by setting the target carbon potential of the atmosphere to a relatively high first set value, the effect of rapidly increasing the actual carbon potential of the atmosphere can be remarkably obtained. it can.

In the gas carburizing method according to claim 3, the change from the first set value to the second set value is changed from the CO ₂ concentration in the atmosphere, and the second calculated carbon potential is the second set value. Triggered to be a relatively lower predetermined value.
Since the second calculated carbon potential is calculated from the CO ₂ concentration, the measured value does not fluctuate due to the influence of methane and shows the actual carbon potential. Therefore, by changing from the first set value to the second set value at the timing when the actual carbon potential approaches the second set value, the increased actual carbon potential is changed to the second set value thereafter. Can be controlled as a target. Therefore, the carbon potential of the atmosphere can be accurately controlled to the target value.

The gas carburizing method according to claim 4 uses ethylene gas as the enriched gas to be introduced during the carburizing step.
Ethylene gas produces less methane due to decomposition than hydrocarbon gas. Therefore, there is little tendency for the measured value of carbon potential to increase due to the influence of methane. Therefore, in the initial stage of the carburizing step, by setting the target carbon potential of the atmosphere to a relatively high first set value, the effect of rapidly increasing the actual carbon potential of the atmosphere can be remarkably obtained. it can.

The gas carburizing method according to claim 5 uses a hydrocarbon gas as the enriched gas to be introduced during the diffusion step.
Hydrocarbon gas is cheaper than the above ethylene gas. Further, in the diffusion step, since the amount of enriched gas introduced is small, there is little negative effect that the measured value and the actual value of the carbon potential deviate due to the influence of methane. In addition, ethylene gas functions as a carburizing accelerator, and the carbon concentration of the carburized layer may increase more than necessary. Therefore, in the diffusion step, an inexpensive enriched gas suitable for diffusion can be used, and the cost required for enrichment can be reduced.

In the gas carburizing method according to claim 6, the carburizing step and the diffusion step are performed in a state where the treated product is present in the atmosphere. Ethylene gas is used as the enriched gas introduced during the carburizing step, and hydrocarbon gas is used as the enriched gas introduced during the diffusion step.
The above ethylene gas produces less methane due to decomposition than the hydrocarbon gas. Therefore, there is little tendency for the measured value of carbon potential to increase due to the influence of methane. Therefore, the actual carbon potential of the above atmosphere is rapidly increased, and the carburizing capacity of the above atmosphere is increased. Therefore, the carburizing treatment time can be shortened and the carburizing depth can be made uniform.
Further, the hydrocarbon gas is cheaper than the above ethylene gas. Further, in the diffusion step, since the amount of enriched gas introduced is small, there is little negative effect that the measured value and the actual value of the carbon potential deviate due to the influence of methane. In addition, ethylene gas functions as a carburizing accelerator, and the carbon concentration of the carburized layer may increase more than necessary. Therefore, in the diffusion step, an inexpensive enriched gas suitable for diffusion can be used, and the cost required for enrichment can be reduced.

The gas carburizing method according to claim 7 sets the target carbon potential of the atmosphere, which is the target in the carburizing step, from a relatively high first set value to a relatively low second set value during the carburizing step. Change to.
As described above, in the initial stage of the carburizing process in which the measured value of the carbon potential rises due to the influence of methane derived from the enriched gas, the target carbon potential of the above atmosphere is set to a relatively high first set value. This prevents the introduction of enrichment from being restricted before the actual carbon potential of the above atmosphere rises. The actual carbon potential of the above atmosphere rises rapidly, increasing the carburizing capacity of the above atmosphere. Therefore, the carburizing treatment time can be shortened and the carburizing depth can be made uniform. Further, in the initial stage of the carburizing process in which the carbon potential needs to be rapidly increased, the amount of enriched gas introduced is increased by the control based on the first set value. After changing to the second set value, the amount of enriched gas introduced is reduced. Therefore, the cost required for enrichment can be reduced without wasting the enrichment gas.

In the gas carburizing method according to claim 8, the first set value and the second set value are set with reference to the first calculated carbon potential calculated from the O ₂ concentration in the atmosphere.
The first set value and the second set value are based on the O ₂ concentration detected by the O ₂ sensor. Therefore, the measured value of the first calculated carbon potential tends to increase due to the influence of methane derived from the enriched gas. Therefore, by using ethylene gas as the enrichment gas in the carburizing step, the effect of rapidly increasing the actual carbon potential of the above atmosphere can be remarkably obtained. Further, in the initial stage of the carburizing step, by setting the target carbon potential of the atmosphere to a relatively high first set value, the effect of rapidly increasing the actual carbon potential of the atmosphere can be remarkably obtained. it can.

In the gas carburizing method according to claim 9, when changing from the first set value to the second set value, the second calculated carbon potential calculated from the CO ₂ concentration in the atmosphere is the second set value. Triggered to be a relatively lower predetermined value.
Since the second calculated carbon potential is calculated from the CO ₂ concentration, the measured value does not fluctuate due to the influence of methane and shows the actual carbon potential. Therefore, by changing from the first set value to the second set value at the timing when the actual carbon potential approaches the second set value, the increased actual carbon potential is subsequently changed to the second set value. Can be controlled as a target. Therefore, the carbon potential of the atmosphere can be accurately controlled to the target value.

The gas carburizing apparatus according to claim 10 includes one or more treatment chambers for performing a carburizing step and a diffusion step in a state where the treated product is present in the atmosphere. Further, the setting changing means changes the target carbon potential of the atmosphere, which is the target in the carburizing step, from a relatively high first setting value to a relatively low second setting value during the carburizing step. ..
As described above, in the initial stage of the carburizing process in which the measured value of the carbon potential rises due to the influence of methane derived from the enriched gas, the target carbon potential of the above atmosphere is set to a relatively high first set value. This prevents the introduction of enrichment from being restricted before the actual carbon potential of the above atmosphere rises. The actual carbon potential of the above atmosphere rises rapidly, increasing the carburizing capacity of the above atmosphere. Therefore, the carburizing treatment time can be shortened and the carburizing depth can be made uniform. Further, in the initial stage of the carburizing process in which the carbon potential needs to be rapidly increased, the amount of enriched gas introduced is increased by the control based on the first set value. After changing to the second set value, the amount of enriched gas introduced is reduced. Therefore, the cost required for enrichment can be reduced without wasting the enrichment gas.

The gas carburizing apparatus according to claim 11 includes a first calculation means for calculating the first calculated carbon potential from the O ₂ concentration in the above atmosphere. The first set value and the second set value are set with reference to the first calculated carbon potential.
The first set value and the second set value are based on the O ₂ concentration detected by the O ₂ sensor. Therefore, the measured value of the first calculated carbon potential tends to increase due to the influence of methane derived from the enriched gas. Therefore, in the initial stage of the carburizing step, by setting the target carbon potential of the atmosphere to a relatively high first set value, the effect of rapidly increasing the actual carbon potential of the atmosphere can be remarkably obtained. it can.

The gas carburizing apparatus according to claim 12 includes a second calculation means for calculating the second calculated carbon potential from the CO ₂ concentration in the atmosphere. The setting changing means changes the setting value by triggering that the second calculated carbon potential becomes a predetermined value relatively lower than the second setting value.
Since the second calculated carbon potential is calculated from the CO ₂ concentration, the measured value does not fluctuate due to the influence of methane and shows the actual carbon potential. Therefore, by changing from the first set value to the second set value at the timing when the actual carbon potential approaches the second set value, the increased actual carbon potential is subsequently changed to the second set value. Can be controlled as a target. Therefore, the carbon potential of the atmosphere can be accurately controlled to the target value.

The gas carburizing apparatus according to claim 13 includes an ethylene introducing means for introducing ethylene gas into the processing chamber as an enriched gas during the carburizing step.
Ethylene gas produces less methane due to decomposition than hydrocarbon gas. Therefore, there is little tendency for the measured value of carbon potential to increase due to the influence of methane. Therefore, in the initial stage of the carburizing step, by setting the target carbon potential of the atmosphere to a relatively high first set value, the effect of rapidly increasing the actual carbon potential of the atmosphere can be remarkably obtained. it can.

The gas carburizing device according to claim 14 includes a hydrocarbon introducing means for introducing a hydrocarbon gas into the processing chamber as an enriched gas during the diffusion step.
Hydrocarbon gas is cheaper than the above ethylene gas. Further, in the diffusion step, since the amount of enriched gas introduced is small, there is little negative effect that the measured value and the actual value of the carbon potential deviate due to the influence of methane. In addition, ethylene gas functions as a carburizing accelerator, and the carbon concentration of the carburized layer may increase more than necessary. Therefore, in the diffusion step, an inexpensive enriched gas suitable for diffusion can be used, and the cost required for enrichment can be reduced.

The gas carburizing apparatus according to claim 15 includes one or more treatment chambers for performing a carburizing step and a diffusion step in a state where the treated product is present in the atmosphere. The ethylene introducing means introduces ethylene gas into the processing chamber as the enriched gas during the carburizing step. The hydrocarbon introducing means introduces the hydrocarbon gas into the processing chamber as the enriched gas in the diffusion step.
The above ethylene gas produces less methane due to decomposition than the hydrocarbon gas. Therefore, there is little tendency for the measured value of carbon potential to increase due to the influence of methane. Therefore, the actual carbon potential of the above atmosphere is rapidly increased, and the carburizing capacity of the above atmosphere is increased. Therefore, the carburizing treatment time can be shortened and the carburizing depth can be made uniform.
Further, the hydrocarbon gas is cheaper than the above ethylene gas. Further, in the diffusion step, since the amount of enriched gas introduced is small, there is little negative effect that the measured value and the actual value of the carbon potential deviate due to the influence of methane. In addition, ethylene gas functions as a carburizing accelerator, and the carbon concentration of the carburized layer may increase more than necessary. Therefore, in the diffusion step, an inexpensive enriched gas suitable for diffusion can be used, and the cost required for enrichment can be reduced.

The gas carburizing apparatus according to claim 16 sets a target carbon potential of the atmosphere, which is a target in the carburizing step, from a relatively high first set value to a relatively low second set value during the carburizing step. It is provided with a setting changing means for changing to.
As described above, in the initial stage of the carburizing process in which the measured value of the carbon potential rises due to the influence of methane derived from the enriched gas, the target carbon potential of the above atmosphere is set to a relatively high first set value. This prevents the introduction of enrichment from being restricted before the actual carbon potential of the above atmosphere rises. The actual carbon potential of the above atmosphere rises rapidly, increasing the carburizing capacity of the above atmosphere. Therefore, the carburizing treatment time can be shortened and the carburizing depth can be made uniform. Further, in the initial stage of the carburizing process in which the carbon potential needs to be rapidly increased, the amount of enriched gas introduced is increased by the control based on the first set value. After changing to the second set value, the amount of enriched gas introduced is reduced. Therefore, the cost required for enrichment can be reduced without wasting the enrichment gas.

The gas carburizing apparatus according to claim 17 includes a first calculation means for calculating the first carbon potential from the O ₂ concentration in the above atmosphere. The first set value and the second set value are set with reference to the first carbon potential.
The first set value and the second set value are based on the O ₂ concentration detected by the O ₂ sensor. Therefore, the measured value of the first calculated carbon potential tends to increase due to the influence of methane derived from the enriched gas. Therefore, by using ethylene gas as the enrichment gas in the carburizing step, the effect of rapidly increasing the actual carbon potential of the above atmosphere can be remarkably obtained. Further, in the initial stage of the carburizing step, by setting the target carbon potential of the atmosphere to a relatively high first set value, the effect of rapidly increasing the actual carbon potential of the atmosphere can be remarkably obtained. it can.

The gas carburizing apparatus according to claim 18 includes a second calculation means for calculating a second calculated carbon potential from the CO ₂ concentration in the above atmosphere. The setting changing means changes the setting value by triggering that the second carbon potential becomes a predetermined value relatively lower than the second setting value.
Since the second calculated carbon potential is calculated from the CO ₂ concentration, the measured value does not fluctuate due to the influence of methane and shows the actual carbon potential. Therefore, by changing from the first set value to the second set value at the timing when the actual carbon potential approaches the second set value, the increased actual carbon potential is changed to the second set value thereafter. Can be controlled as a target. Therefore, the carbon potential of the atmosphere can be accurately controlled to the target value.

It is a block diagram which shows the gas carburizing apparatus of one Embodiment to which this invention was applied. It is a diagram which shows the CP behavior of the comparative example 1. FIG. It is a diagram which shows the CP behavior of the comparative example 2. It is a diagram which shows the CP behavior of an Example.

Next, a mode for carrying out the present invention will be described.

◆ Gas carburizing method In the gas carburizing method of the present embodiment, the carburizing step and the diffusion step are performed in a state where the treated product is present in the atmosphere.

[Processed product]
The processed product targeted by the gas carburizing method of the present embodiment is steel. Generally, low carbon steel or medium carbon steel is targeted, and the carbon steel material for machine structure described in JIS G 4051, the alloy steel material for machine structure described in JIS G 4052, 4053, and the alloy steel material for machine structure described in JIS G 4805. High carbon chrome bearing steel materials, tool steels described in JIS G 4401 and the like can be used. Specifically, for example, S09C, S15C, S20C and the like can be mentioned. Further, as the alloy steel, alloy steels such as SMN, SCr and SCM materials can be used.

[Carburizing device]
The gas carburizing method of the present embodiment can be carried out by using a carburizing device capable of controlling the atmosphere and temperature. The carburizing device has a processing chamber having an inlet for a carrier gas, an enriched gas, and an oxidizing gas. The processing chamber is provided with loading / unloading means such as an opening / closing door and a conveyor device for loading and unloading processed products. A quenching tank can be attached to the carburizing device as needed.
Details of the carburizing device as an embodiment of the present invention will be described later.

[Process outline]
As described above, the carburizing step and the subsequent diffusion step are performed. Prior to the carburizing step, a gas replacement step and a temperature raising step can be performed. Before the heating step is completed and the carburizing step is performed, a pre-heating step can be performed if necessary. After the diffusion step, a post-heating step can be performed if necessary. Further, after the diffusion step or the post-equilibrium step, a quenching step can be performed if necessary.

The gas replacement step, the temperature raising step, the preheat soaking step, the carburizing step, the diffusion step, and the post soaking step are performed by charging the treated product into a treatment chamber filled with a predetermined atmosphere. The above atmosphere can be formed by introducing a necessary gas for forming an atmosphere into the treatment chamber and keeping the pressure above atmospheric pressure.

[Gas for atmosphere]
The gas for the atmosphere forming, metamorphic inclusive carburizing CO gas into the processing chamber gas, substituted or N ₂ gas for performing dilution, carbon potential (hereinafter referred to as "CP") enriched gas to control the Or oxidizing gas is used.

As the modified gas, for example, an endothermic modified gas commonly known as RX gas can be used. The endothermic modified gas is a gas produced by a reaction in which a hydrocarbon gas such as propane and air are passed through a nickel catalyst heated to 1050 ° C. The composition of the endothermic modified gas is, for example, CO: 20.9 to 23.9% + CO ₂ : 0.18% + H ₂ : 30.4 to 38.9% + balance N ₂ .

As the enriched gas, ethylene gas can be used in the present embodiment, particularly in the carburizing step. As the enrichment gas in the carburizing step, the ethylene gas and the hydrocarbon gas can be used in combination. Further, in the diffusion step, a hydrocarbon gas can be used as the enriched gas. As the hydrocarbon gas, for example, methane, ethane, propane, butane and the like can be used. By introducing the enriched gas, CO ₂ and H ₂ O, which are oxidizing gases in the furnace atmosphere, can be reduced and CP can be increased.

The oxidizing gas can increase the oxidizing gas in the atmosphere in the furnace and lower the CP. For example, if a high CP is maintained for a long time in the carburizing step, the CP in the furnace atmosphere may not drop to a desired value even if the introduction of the enriched gas is stopped in the next diffusion step. In such a case, a small amount of oxidizing gas can be supplied into the furnace to quickly reduce the CP to a desired value. As the oxidizing gas, for example, CO ₂ , air, nitrogen gas bubbling in water and containing water can be used.

[Explanation of each process]
In the gas replacement step, the opening / closing door is opened and the processed product is charged into the processing chamber, so that a large amount of air that has entered the processing chamber is discharged and replaced with atmospheric gas. Specifically, a carburizing metamorphic gas containing CO gas is introduced into the treatment chamber to maintain the pressure above atmospheric pressure and discharge the atmosphere. Further, instead of the above-mentioned metamorphic gas, it can be replaced with N ₂ gas.

The temperature raising step is a step of raising the temperature in a predetermined time until the temperature in the processing chamber cooled by charging the treated product reaches a predetermined carburizing temperature. In the atmosphere of the temperature raising step, CP can be controlled by introducing the modified gas as a carrier gas and, if necessary, introducing an enriched gas or an oxidizing gas.

In the preheating step, after the temperature of the treatment chamber is raised to a predetermined carburizing temperature, the treatment chamber is maintained at the carburizing temperature for a period of time until the entire charged processed product uniformly reaches the carburizing temperature. It is a process to do. In the atmosphere of the preheating step, the modified gas can be introduced as a carrier gas, and an enriched gas or an oxidizing gas can be introduced as needed to control the CP.

In the carburizing step, after the whole of the treated product reaches the carburizing temperature uniformly, the product is kept in a treatment chamber in a carburizing atmosphere for a predetermined time to permeate and diffuse carbon from the surface of the treated product. In the atmosphere of the carburizing step, the modified gas can be introduced as a carrier gas, and an enriched gas or an oxidizing gas can be introduced as needed to control the CP.
Enrichment and CP control during the carburizing process will be described in detail later.

In the diffusion step, after the completion of the carburizing step, the treated product is heated and held for a predetermined time to adjust the concentration of carbon that has penetrated into the surface layer of the treated product to a predetermined concentration, and is diffused inward. In the atmosphere of the diffusion step, the modified gas can be introduced as a carrier gas, and an enriched gas or an oxidizing gas can be introduced as needed to control the CP.
The enrichment and CP control of the diffusion step will be described in detail later.

In the post-heating step, after the completion of the diffusion step, for example, before quenching, the entire treated product is uniformly held for a time until a predetermined quenching temperature is reached. In the atmosphere of the post-heating step, the supply of the carrier gas, the enriched gas and the oxidizing gas is stopped, and the atmosphere in the treatment chamber is replaced with the inert gas.

In the above quenching step, a processed product maintained at a predetermined quenching temperature is put into quenching oil and rapidly cooled to cause martensitic transformation.

[Enrichment of carburizing process and CP control]
In the carburizing step, as described above, the modified gas is introduced as a carrier gas, and if necessary, an enriched gas or an oxidizing gas is introduced to form a carburizing atmosphere.

The CP of the above atmosphere can be controlled by increasing or decreasing the amount of the enriched gas introduced. That is, if the amount of enriched gas introduced is increased, the CP becomes high, and if the amount of enriched gas introduced is decreased, the CP becomes low. To further lower the CP, an oxidizing gas is introduced.

Here, CP refers to the equilibrium carbon concentration on the surface of the treated product when the carburizing treatment is performed in a predetermined carburizing atmosphere. An atmosphere with a CP of 1% means that the carbon concentration on the surface of the treated product when carburized in that atmosphere until equilibrium is reached is 1% by weight.

The CP is controlled by increasing or decreasing the amount of enriched gas introduced based on the difference between the set value of the target CP and the measured value obtained by measuring the CP of the atmosphere. The above-mentioned set value is a value at which the target carbon potential of the above-mentioned atmosphere is set. As the measured value, the measured value calculated from the O ₂ concentration in the atmosphere is used.

Therefore, if the measured value is lower than the set value, the amount of enriched gas introduced is increased to raise the CP. If the measured value is higher than the set value, the amount of enriched gas introduced is reduced to lower the CP. To further lower the CP, an oxidizing gas is introduced.

In the present embodiment, the target CP of the atmosphere, which is the target in the carburizing step, is changed from a relatively high first set value to a relatively low second set value during the carburizing step. In particular, the second set value can be the surface carbon concentration to be aimed at in the carburizing step. Therefore, the first set value can be a value higher than the surface carbon concentration aimed at in the carburizing step. For example, the first setting value can be set to 1.5% and the second setting can be set to 1.1%.

In the present embodiment, the first set value and the second set value are set based on the first calculated CP calculated from the O ₂ concentration in the atmosphere. A zirconia type O ₂ sensor is used for measuring the O ₂ concentration. These O ₂ sensors measure the O ₂ concentration in the above atmosphere by utilizing the electromotive force generated by the difference between the oxygen partial pressure in the furnace and the oxygen partial pressure in the atmosphere outside the furnace. The O ₂ concentration in the above atmosphere correlates with the CP, and when the O ₂ concentration decreases, the CP increases, and when the O ₂ concentration increases, the CP decreases.

As described above, the O ₂ sensor deteriorates the measurement accuracy of the O ₂ concentration due to the influence of methane derived from the enriched gas. As a result, the measured value of the first calculated CP takes a higher value than the actual CP. Therefore, if the CP set value is equivalent to the surface carbon concentration aimed at in the carburizing process, the amount of enriched gas introduced is considered to have risen to the set value even though the actual CP is lower than the set value. Miscontrol that reduces it.

In the present embodiment, by setting the initial stage of the carburizing step to the first set value higher than the second set value, the enriched gas introduction amount does not lead to the above-mentioned erroneous control of the enrich gas introduction amount. Can be controlled properly. The initial stage of the carburizing step is set to a first set value higher than the second set value, and the first set value is changed to the second set value at a predetermined timing thereafter.

The change from the first set value to the second set value is that the second calculated CP calculated from the CO ₂ concentration in the atmosphere becomes a predetermined value relatively lower than the second set value. Is used as a trigger. For example, when the first setting value is 1.5% and the second setting is 1.1%, the predetermined value as the trigger can be 1%.

The CO ₂ concentration can be measured by, for example, an infrared CO ₂ analyzer of the NDIR method. The CO ₂ concentration can also be measured with an infrared CO / CO ₂ analyzer. The CO ₂ concentration in the above atmosphere correlates with CP, and when the CO ₂ concentration decreases, the CP increases, and when the CO ₂ concentration increases, the CP decreases. Unlike the first calculated CP, the accuracy of the second calculated CP measured based on the CO ₂ concentration does not deteriorate due to the influence of methane.

In this embodiment, ethylene gas is used as the enriched gas to be introduced during the carburizing step. Ethylene gas produces less methane due to decomposition than hydrocarbon gas. Therefore, there is little tendency for the measured value of CP to increase due to the influence of methane. Therefore, in the initial stage of the carburizing step, the target CP of the atmosphere is set to the first set value, and the effect of rapidly increasing the actual CP can be remarkably obtained.

[Enrichment of diffusion process and CP control]
In this embodiment, a hydrocarbon gas is used as the enriched gas to be introduced during the diffusion step. That is, ethylene gas is used as the enriched gas introduced during the carburizing step, and hydrocarbon gas is used as the enriched gas introduced during the diffusion step.

The CP control of the diffusion step can be performed, for example, with a third set value lower than the second set value as the target CP. For example, when the first set value is 1.5% and the second setting is 1.1% in the carburizing step, the target CP of the diffusion step can be set to 0.8%. ..

The third set value in the CP control of the diffusion step is set with reference to the first calculated CP calculated from the O ₂ concentration in the atmosphere.

◆ Gas carburizing device FIG. 1 shows an embodiment of a gas carburizing device to which the present invention is applied. This gas carburizing device realizes the above-mentioned gas carburizing method.

The gas carburizing apparatus of the present embodiment includes a processing chamber 1, a first calculation means 10, a second calculation means 20, a setting change means 30, an ethylene introduction means 40, a hydrocarbon introduction means 50, a carrier gas introduction means 60, and an enrichment control means. 70, the oxidizing gas introduction means 80 is provided. In this example, air is introduced as the oxidizing gas.

The processing chamber 1 performs a carburizing step and a diffusion step in a state where the treated product is present in the atmosphere. The illustrated one illustrates a carburizing step and a diffusion step performed in one processing chamber 1. For example, two or more treatment chambers 1 may be provided, for example, the carburizing step and the diffusion step are performed in separate treatment chambers 1. The processing chamber 1 is provided with a stirring device 2 for stirring the atmosphere and a heating device (not shown) for heating the internal charging space.

The first calculation means 10 calculates CP (which is the first calculation CP) from the O ₂ concentration and temperature in the atmosphere. The O ₂ concentration is detected by the O ₂ sensor 11. As the O ₂ sensor 11, a zirconia type sensor can be used as described above. The first set value and the second set value used as the target value of CP control during the carburizing step are set based on the first calculated CP.

The second calculation means 20 calculates the second calculation CP from the CO ₂ concentration in the atmosphere. The CO ₂ concentration can be detected by the CO ₂ analyzer 21. The second calculation means 20 calculates the CP (the second calculation CP) in the processing chamber 1 based on the CO ₂ concentration detected by the CO ₂ analyzer 21 and the furnace temperature detected by the thermometer 22. calculate.

The carrier gas introducing means 60 introduces the carrier gas into the processing chamber 1 at a predetermined introduction flow rate by the carrier gas flow meter 61 and the carrier gas on-off valve 62.

The ethylene introduction means 40 introduces ethylene gas into the processing chamber 1 as an enriched gas during the carburizing step through the ethylene on-off valve 41 and the ethylene introduction path 42.

The hydrocarbon introducing means 50 introduces a hydrocarbon gas into the processing chamber 1 as an enriched gas during the diffusion step through the hydrocarbon on-off valve 51 and the hydrocarbon introduction path 52.

The enrichment control means 70 includes an enrichment gas flow meter 71, an enrichment gas on-off valve 72, and an enrichment gas control valve 73. By adjusting the opening and closing of the enrichment gas control valve 73, the amount of the enrichment gas introduced into the processing chamber 1 is controlled.

The oxidizing gas introducing means 80 includes an oxidizing gas flow meter 81, an oxidizing gas on-off valve 83, and an oxidizing gas regulating valve 82. By adjusting the opening and closing of the oxidizing gas on-off valve 83, the amount of the oxidizing gas introduced into the processing chamber 1 is controlled.

The setting changing means 30 changes the target CP of the atmosphere, which is the target in the carburizing step, from a relatively high first setting value to a relatively low second setting value during the carburizing step. As described above, the setting changing means 30 sets the first set value and the second set value with reference to the first calculated CP. Further, the setting changing means 30 changes the setting value by triggering that the second calculation CP becomes a predetermined value relatively lower than the second setting value. Specifically, the set value is changed by, for example, transmitting a valve opening signal to the enrich gas on-off valve 73 of the enrich control means 70 to control the amount of the enrich gas introduced into the processing chamber 1. Further, if necessary, a valve opening signal is transmitted to the oxidizing gas introducing means 80, and the set value is changed by controlling the amount of the oxidizing gas introduced into the processing chamber 1.

Further, the setting changing means 30 transmits an enrichment switching signal to the ethylene on-off valve 41 and the hydrocarbon on-off valve 51, whereby ethylene gas is introduced into the processing chamber 1 as an enriched gas during the carburizing step. Then, during the diffusion step, a hydrocarbon gas is introduced into the processing chamber 1 as an enriched gas.

The gas carburizing device of the present embodiment can realize the above-mentioned gas carburizing method by the above configuration.

◆ Effect of the embodiment The gas carburizing method of the present embodiment has the following effects. The gas carburizing device of the present embodiment also has the same effect.

In this embodiment, the carburizing step and the diffusion step are performed in a state where the treated product is present in the atmosphere. During the carburizing step, the target carbon potential of the atmosphere, which is the target in the carburizing step, is changed from a relatively high first set value to a relatively low second set value.
As described above, in the initial stage of the carburizing process in which the measured value of the carbon potential rises due to the influence of methane derived from the enriched gas, the target carbon potential of the above atmosphere is set to a relatively high first set value. This prevents the introduction of enrichment from being restricted before the actual carbon potential of the above atmosphere rises. The actual carbon potential of the above atmosphere rises rapidly, increasing the carburizing capacity of the above atmosphere. Therefore, the carburizing treatment time can be shortened and the carburizing depth can be made uniform. Further, in the initial stage of the carburizing process in which the carbon potential needs to be rapidly increased, the amount of enriched gas introduced is increased by the control based on the first set value. After changing to the second set value, the amount of enriched gas introduced is reduced. Therefore, the cost required for enrichment can be reduced without wasting the enrichment gas.

In this embodiment, the first set value and the second set value are set with reference to the first calculated carbon potential calculated from the O ₂ concentration in the atmosphere.
The first set value and the second set value are based on the O ₂ concentration detected by the O ₂ sensor. Therefore, the measured value of the first calculated carbon potential tends to increase due to the influence of methane derived from the enriched gas. Therefore, in the initial stage of the carburizing step, by setting the target carbon potential of the atmosphere to a relatively high first set value, the effect of rapidly increasing the actual carbon potential of the atmosphere can be remarkably obtained. it can.

In the present embodiment, the second calculated carbon potential calculated from the CO ₂ concentration in the atmosphere for changing from the first set value to the second set value is relatively lower than the second set value. Triggered to reach a predetermined value.
Since the second calculated carbon potential is calculated from the CO ₂ concentration, the measured value does not fluctuate due to the influence of methane and shows the actual carbon potential. Therefore, by changing from the first set value to the second set value at the timing when the actual carbon potential approaches the second set value, the increased actual carbon potential is subsequently changed to the second set value. Can be controlled as a target. Therefore, the carbon potential of the atmosphere can be accurately controlled to the target value.

In this embodiment, ethylene gas is used as the enriched gas to be introduced during the carburizing step.
Ethylene gas produces less methane due to decomposition than hydrocarbon gas. Therefore, there is little tendency for the measured value of carbon potential to increase due to the influence of methane. Therefore, in the initial stage of the carburizing step, by setting the target carbon potential of the atmosphere to a relatively high first set value, the effect of rapidly increasing the actual carbon potential of the atmosphere can be remarkably obtained. it can.

In this embodiment, a hydrocarbon gas is used as the enriched gas to be introduced during the diffusion step.
Hydrocarbon gas is cheaper than the above ethylene gas. Further, in the diffusion step, since the amount of enriched gas introduced is small, there is little negative effect that the measured value and the actual value of the carbon potential deviate due to the influence of methane. In addition, ethylene gas functions as a carburizing accelerator, and the carbon concentration of the carburized layer may increase more than necessary. Therefore, in the diffusion step, an inexpensive enriched gas suitable for diffusion can be used, and the cost required for enrichment can be reduced.

In this embodiment, the carburizing step and the diffusion step are performed in a state where the treated product is present in the atmosphere. Ethylene gas is used as the enriched gas introduced during the carburizing step, and hydrocarbon gas is used as the enriched gas introduced during the diffusion step.
The above ethylene gas produces less methane due to decomposition than the hydrocarbon gas. Therefore, there is little tendency for the measured value of carbon potential to increase due to the influence of methane. Therefore, the actual carbon potential of the above atmosphere is rapidly increased, and the carburizing capacity of the above atmosphere is increased. Therefore, the carburizing treatment time can be shortened and the carburizing depth can be made uniform.
Further, the hydrocarbon gas is cheaper than the above ethylene gas. Further, in the diffusion step, since the amount of enriched gas introduced is small, there is little negative effect that the measured value and the actual value of the carbon potential deviate due to the influence of methane. In addition, ethylene gas functions as a carburizing accelerator, and the carbon concentration of the carburized layer may increase more than necessary. Therefore, in the diffusion step, an inexpensive enriched gas suitable for diffusion can be used, and the cost required for enrichment can be reduced.

Next, an embodiment will be described together with a comparative example.

Using the gas carburizing device shown in FIG. 1, gas carburizing was performed under the following treatment conditions.
[Processing conditions]
Carrier gas: Carburizing gas containing CO as a carbon source (flow rate: 10 L / min)
Processed product: SCM415H Round bar Enriched gas: Ethylene gas, propane gas (maximum flow rate is 3%)
CP control: O ₂ probe sensor used,
Air, enriched ON-OFF control Atmosphere analyzer: Fuji Electric ZSV
Detects CO%, CO ₂ %, and methane%
Calculate CP (second calculation CP)
Treatment pattern: Pre-uniform heat 930 ° C x 20 minutes (CP set value is 0.3%)
Carburizing 930 ° C x 90 minutes
(Comparative example; CP set value 1.1%)
(Example; first set value 1.5% → second set value 1.1%)
Diffusion 930 ° C x 120 minutes (CP set value is 0.8%)
Quenching 830 ° C x 30 minutes (oil temperature 100 ° C)

[Comparative Example 1]
FIG. 2 shows the behavior of CP in Comparative Example 1. The horizontal axis is the elapsed time and the vertical axis is CP.
As the enriched gas, propane gas was used through the carburizing step and the diffusion step.
The CP set value of the carburizing step is 1.1%, and the CP set value of the diffusion step is 0.8%.

The solid line is the behavior of the first calculated CP based on the O ₂ concentration. It is changing according to the above-mentioned set value.

The dotted line is the behavior of the methane concentration in the atmosphere. The concentration increased significantly at the beginning of carburizing (about 1.6%), then gradually decreased during carburizing, and remained unchanged in the diffusion process.

The alternate long and short dash line is the behavior of the second calculated CP based on the CO ₂ concentration. At the initial stage of carburizing with a high methane concentration, the second calculated CP (the actual CP of the atmosphere) did not rise sufficiently, and the deviation from the first calculated CP was large. As the methane concentration decreases, the second calculated CP gradually increases, but the target value of 1.1% has not been reached during the carburizing process. In the diffusion step, the enrichment amount decreases because the set value of CP decreases, and the methane concentration also decreases accordingly, so that the difference between the second calculated CP and the first calculated CP is extremely small.

[Comparative Example 2]
FIG. 3 shows the behavior of CP in Comparative Example 2. The horizontal axis is the elapsed time and the vertical axis is CP.
As the enriched gas, ethylene gas was used through the carburizing process and the diffusion process.
The CP set value of the carburizing step is 1.1%, and the CP set value of the diffusion step is 0.8%.

The solid line is the behavior of the first calculated CP based on the O ₂ concentration. It is changing according to the above-mentioned set value.

The dotted line is the behavior of the methane concentration in the atmosphere. The concentration increased at the initial stage of carburizing (about 0.5%), then gradually decreased during carburizing, and remained unchanged in the diffusion step.

The alternate long and short dash line is the behavior of the second calculated CP based on the CO ₂ concentration. At the initial stage of carburizing with a high methane concentration, the second calculated CP (the actual CP of the atmosphere) did not rise sufficiently, and the deviation from the first calculated CP was large. As the methane concentration decreases, the second calculated CP gradually increases and finally reaches the target value of 1.1% just before the end of the carburizing process. In the diffusion step, the enrichment amount decreases because the set value of CP decreases, and the methane concentration also decreases accordingly, so that the difference between the second calculated CP and the first calculated CP is extremely small.

〔Example〕
FIG. 4 shows the behavior of CP in the examples. The horizontal axis is the elapsed time and the vertical axis is CP.
The carburizing process was enriched with ethylene gas. The first set value at the initial stage of carburizing was set to CP 1.5%, and after 3500 seconds, it was changed to the second set value of CP 1.1%.
The diffusion step was enriched with propane gas. The diffusion step CP set value is 0.8%.

The solid line is the behavior of the first calculated CP based on the O ₂ concentration. It shows that the first set value of 1.5% was changed to the second set value of CP 1.1% during the carburizing step and was set to 0.8% in the diffusion step.

The dotted line is the behavior of the methane concentration in the atmosphere. The concentration of methane generated at the initial stage of carburizing is suppressed to about 0.8%, and gradually decreases while being controlled at the first set value of 1.5%. While controlling with the second set value of 1.1%, the diffusion step is almost flat. As the amount of enriched gas introduced increases, methane rises to about 0.8%, but it is much less than propane.

The alternate long and short dash line is the behavior of the second calculated CP based on the CO ₂ concentration. Since ethylene gas is used for enrichment and the first set value is raised to 1.5%, the second calculated CP rises even at the beginning of carburizing with a high methane concentration, and the methane concentration begins to fall. Then, it rises to nearly 1.1%, which should be the target value. After that, after changing to the second set value of 1.1%, it remains around 1.1%. In the diffusion step, the enriched gas is switched to propane, and the discrepancy between the second calculated CP and the first calculated CP is extremely small.

The CP of the atmosphere at the initial stage of carburizing is higher than that of the comparative example, so that the carburizing efficiency is improved. Since the amount of methane produced from ethylene gas is small, even if a high CP is controlled as the target value at the initial stage of the carburizing process, sooting does not occur during that period.

[Carburizing depth]
The effective carburizing depth was measured from the cross-sectional hardness of the treated product. The hardness distribution in the depth direction from the surface was measured, and the depth at which Hv550 or more was defined as the effective carburizing depth. The results are as follows.
Example: 0.747 mm
Comparative Example 1: 0.661 mm
Comparative Example 2: 0.683 mm

In Examples, a deeper effective carburizing depth was obtained as compared with Comparative Examples 1 and 2. When the above effective carburizing depth is converted into the difference in carburizing time, the carburizing time can be expected to be shortened by about 20% in Examples as compared with Comparative Example 1.

◆ Summary In this example, ethylene gas was used as the enriched gas in the carburizing process, and propane gas was used as the enriched gas in the diffusion process. Further, in the carburizing step, the feedback control of the CO ₂ analyzer was used to control the setting value of CP based on the O ₂ concentration from 1.5% to 1.1%. As a result, the discrepancy between the CP based on the O ₂ concentration and the CP in the actual atmosphere is reduced, the CP can be controlled accurately, and the carburizing efficiency is improved.

In this embodiment, expensive acetylene gas is not used as the enrichment gas in the carburizing process. Moreover, in the diffusion step, an inexpensive hydrocarbon gas was used as the enriched gas. By improving the control of such carburizing atmosphere, stable carburizing quality of surface carbon concentration and low cost have been realized.

In the diffusion step, a hydrocarbon gas is used as the enriched gas instead of the ethylene gas. Ethylene gas suppresses the generation of methane, but functions as a carburizing accelerator (used as a carburizing gas in vacuum carburizing). Therefore, when it is used enriched in the diffusion step, the surface carbon concentration of the steel may increase more than necessary. Therefore, in this embodiment, a hydrocarbon gas is used as the enriched gas in the diffusion step. Since the CP set value is lower in the diffusion step than in the carburizing step, the amount of enriched gas introduced is small, and the difference between the CP set value and the actual value due to the production of methane is small.

Finally, the differences between Patent Documents 1 and 2 mentioned at the beginning and the present embodiment will be described.

[Differences between Patent Document 1 and this example]
In Patent Document 1, in order to raise the CP of the atmosphere in the furnace quickly, the control of enrichment is divided into super enrichment and normal enrichment, and super enrichment and normal propane enrichment are supplied at the initial stage of carburizing. A carbon sensor (O ₂ sensor) is used for CP measurement. Therefore, the CP value based on the O ₂ concentration and the CP value of the atmosphere inside the furnace should be significantly different from each other due to the influence of methane in the initial stage of carburizing.

On the other hand, in this embodiment, CP calculation based on CO / CO ₂ analysis, which is not easily affected by methane, is performed in parallel, and the amount of enriched gas introduced is increased to obtain the CP value of the furnace atmosphere at the initial stage of carburizing. The CP setting value based on the O ₂ concentration was set to about 1.5% in order to raise the value quickly. The CP setting value based on the O ₂ concentration was changed to 1.1% (CO /) triggered by the fact that the CP value of the atmosphere inside the furnace (CP calculation value in CO / CO ₂ analysis) reached 1.0%. Instead of CO ₂ analysis, a certain period of time may be used as a trigger). In addition, since the enriched gas in the carburizing process is ethylene gas, less methane is generated, so there is little discrepancy between the measured CP based on the O ₂ concentration and the CP in the actual atmosphere.

Further, although Patent Document 1 lists many types of enriched gas, it does not disclose that the enriched gas is changed in the carburizing step and the diffusion step.

[Differences between Patent Document 2 and this example]
Also in Patent Document 2, in order to raise the CP of the atmosphere in the furnace quickly, the control of enrichment is divided into super enrichment and normal enrichment, and acetylene enrichment and ethylene enrichment are normally supplied to super enrichment at the initial stage of carburizing. Patent Document 2 [0042] A carbon sensor (O ₂ sensor) is used for CP measurement.

In Patent Document 2, only one CP is set at the time of carburizing (CP = 0.9% in the example). That is, even if the enrichment flow rate is increased by super enrichment and normal enrichment, if the target is about CP = 0.9%, the CP value in the furnace atmosphere at the initial stage of carburizing should rise slowly.

On the other hand, in this embodiment, the calculation in the CO / CO ₂ analysis, which is not easily affected by methane, is performed in parallel, and the amount of enriched material introduced is increased in order to quickly raise the CP of the atmosphere in the furnace at the initial stage of carburizing. That is, the CP setting value based on the O ₂ concentration is set to 1.5%, and the CP calculation value in the CO / CO ₂ analysis reaches 1.0% as a trigger, and the CP setting value based on the O ₂ concentration is set. Is changed to 1.1%. Instead of CO / CO ₂ analysis, it is also possible to use the elapse of a certain period of time as a trigger. The enriched gas in the carburizing process is ethylene gas. Since the amount of methane generated is small, the difference between the CP value based on the O ₂ concentration and the actual CP of the atmosphere is small.

In Patent Document 2, the type of enriched gas is acetylene or ethylene as an example, but it is not disclosed that the enriched gas is changed in the carburizing step and the diffusion step.
In this embodiment, ethylene gas, which suppresses the generation of methane, is used as the enriched gas during carburizing, and is switched to the commonly used propane gas during diffusion (CP = 0.8% setting).

◆ Modifications Although the particularly preferred embodiments of the present invention have been described above, the present invention is not limited to the embodiments shown, and can be modified into various embodiments, and the present invention is various modifications. The purpose is to include an example.

For example, in the carburizing step, a mixed gas of ethylene gas and propane gas may be used as the enriched gas.

In the above embodiment, the change from the first set value to the second set value is performed with the trigger that the second calculated CP calculated from the CO ₂ concentration in the atmosphere reaches a predetermined value. , It is also possible to use the elapse of a predetermined time as a trigger.

1: Processing chamber 2: Stirrer 10: First calculation means 11: O ₂ sensor 20: Second calculation means 21: CO ₂ analyzer 22: Thermometer 30: Setting change means 40: Ethylene introduction means 41: Ethylene on-off valve 42: Ethylene introduction path 50: Hydrocarbon introduction means 51: Hydrocarbon on-off valve 52: Hydrocarbon introduction path 60: Carrier gas introduction means 61: Carrier gas flow meter 62: Carrier gas on-off valve 70: Enrich control means 71: Enrich gas Flow meter 72: Enrich gas on-off valve 73: Enrich gas control valve 80: Oxidizing gas introduction means 81: Oxidizing gas flow meter 82: Oxidizing gas control valve 83: Oxidizing gas on-off valve

Claims (18)

It is a gas carburizing method in which a carburizing step and a diffusion step are performed in the presence of a processed product in the atmosphere.
The target carbon potential of the atmosphere, which is the target in the carburizing step, is changed from a relatively high first set value to a relatively low second set value during the carburizing step. Method. The gas carburizing method according to claim 1, wherein the first set value and the second set value are set based on the first calculated carbon potential calculated from the O ₂ concentration in the atmosphere. In the change from the first set value to the second set value, the second calculated carbon potential calculated from the CO ₂ concentration in the atmosphere becomes a predetermined value relatively lower than the second set value. The gas carburizing method according to claim 1 or 2, wherein this is performed as a trigger. The gas carburizing method according to any one of claims 1 to 3, wherein ethylene gas is used as the enriched gas to be introduced during the carburizing step. The gas carburizing method according to any one of claims 1 to 4, wherein a hydrocarbon gas is used as the enriched gas to be introduced during the diffusion step. It is a gas carburizing method in which a carburizing step and a diffusion step are performed in the presence of a processed product in the atmosphere.
Ethylene gas is used as the enriched gas to be introduced during the carburizing process.
A gas carburizing method characterized in that a hydrocarbon gas is used as the enriched gas to be introduced during the diffusion step. The gas carburizing according to claim 6, wherein the target carbon potential of the atmosphere, which is the target in the carburizing step, is changed from a relatively high first set value to a relatively low second set value during the carburizing step. Method. The gas carburizing method according to claim 7, wherein the first set value and the second set value are set based on the first calculated carbon potential calculated from the O ₂ concentration in the atmosphere. In the change from the first set value to the second set value, the second calculated carbon potential calculated from the CO ₂ concentration in the atmosphere becomes a predetermined value relatively lower than the second set value. The gas carburizing method according to claim 7 or 8, wherein this is performed as a trigger. One or more treatment chambers that perform carburizing and diffusion steps with the treated product present in the atmosphere.
A setting changing means for changing the target carbon potential of the atmosphere, which is the target in the carburizing step, from a relatively high first set value to a relatively low second set value during the carburizing step is provided. A gas carburizing device characterized by. Further provided with a first calculation means for calculating the first calculation carbon potential from the O ₂ concentration in the above atmosphere.
The gas carburizing apparatus according to claim 10, wherein the first set value and the second set value are set with reference to the first calculated carbon potential. Further provided with a second calculation means for calculating the second calculation carbon potential from the CO ₂ concentration in the above atmosphere.
The gas carburizing apparatus according to claim 10 or 11, wherein the setting changing means changes the set value by triggering that the second calculated carbon potential becomes a predetermined value relatively lower than the second set value. .. The gas carburizing apparatus according to any one of claims 10 to 12, further comprising an ethylene introducing means for introducing ethylene gas into the processing chamber as the enriched gas in the carburizing step. The gas carburizing apparatus according to any one of claims 10 to 13, further comprising a hydrocarbon introducing means for introducing a hydrocarbon gas into the processing chamber as the enriched gas in the diffusion step. One or more treatment chambers that perform carburizing and diffusion steps with the treated product present in the atmosphere.
Ethylene introduction means for introducing ethylene gas into the treatment chamber as the enriched gas during the carburizing process, and
A gas carburizing apparatus including a hydrocarbon introducing means for introducing a hydrocarbon gas into the processing chamber as an enriched gas in the diffusion step. Further provided with a setting changing means for changing the target carbon potential of the atmosphere, which is the target in the carburizing step, from a relatively high first set value to a relatively low second set value during the carburizing step. The gas carburizing apparatus according to claim 15. Further provided with a first calculation means for calculating the first carbon potential from the O ₂ concentration in the above atmosphere,
The gas carburizing apparatus according to claim 16, wherein the first set value and the second set value are set with reference to the first carbon potential. Further provided with a second calculation means for calculating the second calculation carbon potential from the CO ₂ concentration in the above atmosphere.
The gas carburizing device according to claim 16 or 17, wherein the setting changing means changes the set value by triggering that the second carbon potential becomes a predetermined value relatively lower than the second set value.