JP3973795B2 - Gas carburizing method - Google Patents

Description

[0001]
【Technical field】
The present invention relates to a carburizing method capable of obtaining high processing efficiency.
[0002]
[Prior art]
Carburizing is widely used as a means of improving mechanical properties such as pitting resistance and wear resistance of steel parts. As a carburizing method, there is a gas carburizing method using a hydrocarbon gas as a carburizing agent.
A conventional gas carburizing method will be briefly described with reference to FIG. This figure shows time in the horizontal axis, temperature in the vertical axis, furnace temperature T, enrich gas introduction timing C, and the like.
[0003]
As known from the figure, in the conventional gas carburizing method, first, the material to be processed is fed into the furnace of the carburizing furnace which has been heated to the processing temperature T _{0 in} advance (A). Since the furnace door is opened when the material to be treated is fed, the temperature in the furnace decreases. Next, the introduction of the enriched gas is started (C) when the furnace temperature is restored to the processing temperature T ₀ (B). This raises the carbon potential in the furnace and carburizes the material to be treated. Normally, an endothermic gas for preventing oxidation is always introduced into the furnace. Further, at the end of carburizing (D), a quenching process is usually performed.
[0004]
[Problems to be solved]
However, the conventional gas carburizing method has the following problems.
That is, the introduction of the enriched gas is performed after the furnace temperature returns to the processing temperature T ₀ . This is because if enriched gas is introduced at a stage where the temperature in the furnace is low, soot is easily generated, which affects the quality of the material to be processed.
[0005]
Since the enriched gas is not present in the furnace, the carburization does not proceed during the period from the delivery of the material to be treated to the recovery of the furnace temperature. Therefore, the period between points A and B in FIG. 4, that is, the period from when the material to be treated is fed to the recovery of the furnace temperature to the treatment temperature T ₀ is a time zone that does not contribute to carburization.
[0006]
On the other hand, carburizing itself may be allowed to proceed even if the carbon potential in the furnace is in an appropriate state even when the temperature does not reach the treatment temperature T ₀ . However, during the temperature rise in the furnace temperature T, as described above, the introduction of the enriched gas tends to lead to the generation of soot, so that an appropriate amount of enriched gas is introduced while suppressing the generation of soot to reduce the carbon potential in the furnace. It was very difficult to improve moderately.
[0007]
The present invention has been made in view of such conventional problems, and even when the furnace temperature is lower than the processing temperature, an appropriate amount of enriched gas can be introduced without generating soot, and the method can be efficiently performed. An object of the present invention is to provide a gas carburizing method capable of carburizing.
[0008]
[Means for solving problems]
In the invention of claim 1, the material to be treated is fed into the furnace of the carburizing furnace, and then the furnace temperature is raised to the carburizing temperature and an enriched gas is introduced into the furnace to raise the carbon potential. In the gas carburizing method of carburizing the workpiece,
The carburizing furnace includes a CO ₂ sensor that samples the atmospheric gas in the furnace and detects the CO ₂ concentration in the atmospheric gas outside the furnace, and detects the oxygen concentration in the atmospheric gas in the furnace. With an oxygen sensor,
The introduction of the enriched gas starts during the temperature increase of the furnace temperature, and the apparent amount calculated based on the CO ₂ concentration measured by the CO ₂ sensor is used to control the amount of the enriched gas introduced. carbon and potential CP _1, obtains a difference (CP ₂ -CP ₁₎ the carbon potential CP ₂ apparent that calculated based on the oxygen concentration measured by the oxygen sensor, the difference (CP ₂ -CP ₁₎ is a predetermined value per to adjust the flow rate of the enriched gas to approach,
As the predetermined value, a value smaller than the maximum value capable of suppressing the generation of soot in the furnace is adopted, and the difference (CP ₂ −) is controlled by on / off control or PID control using the predetermined value as a control target value. The gas carburizing method is characterized in that the flow rate of the enriched gas is kept at a maximum until CP ₁ ) approaches the predetermined value .
[0009]
The most notable point in the present invention is that the introduction of the enriched gas starts in the middle of the temperature rise when the furnace temperature does not reach the carburizing temperature, and two apparent carbons are detected from the detected values of the two different sensors. The potentials CP ₁ and CP ₂ are obtained, and the flow rate of the enriched gas is controlled using the difference (CP ₂ −CP ₁ ).
[0010]
As described above, the CO ₂ sensor is configured to sample the atmospheric gas in the furnace and measure the CO ₂ concentration outside the furnace. Thus, the CO ₂ sensor can measure the CO ₂ concentration with relatively high accuracy without being influenced by the tendency of soot generation in the furnace. Therefore, the value of the carbon potential CP ₁ calculated from the CO ₂ concentration is obtained as a value that is not significantly affected by the tendency of soot generation in the furnace.
[0011]
On the other hand, the oxygen sensor is installed in the furnace as described above, and measures the oxygen concentration in the state in which the oxygen sensor is in direct contact with the atmospheric gas in the furnace. Therefore, the oxygen sensor is relatively affected by the tendency of soot generation in the furnace. Therefore, the value of the carbon potential CP ₂ calculated from the oxygen concentration is a value affected by the tendency of soot generation in the furnace.
[0012]
As described above, the above two types of carbon potentials CP ₁ and CP ₂ are not only different in the object of the data that is the basis of the calculation, but also different in the degree of influence from the in-furnace state. Therefore, when the tendency of soot generation in the carburizing furnace changes, a large difference occurs between CP ₁ and CP ₂ .
[0013]
Specifically, when it becomes easy to generate soot in the furnace, the carbon potential CP ₂ obtained from the oxygen concentration tends to be larger than the carbon potential CP ₁ obtained from the CO ₂ concentration. That is, when the tendency to generate soot in the furnace increases, the difference between the two carbon potentials (CP ₂ −CP ₁ ) tends to increase.
[0014]
Using this characteristic, if the amount of enriched gas introduced is controlled so that the difference between the two carbon potentials (CP ₂ −CP ₁ ) approaches a predetermined value that can suppress the generation of soot, soot in the furnace Occurrence can be suppressed.
Here, the predetermined value is a value that can suppress the generation of soot in the furnace. For example, a value slightly smaller than the maximum value that can suppress the generation of soot in the furnace can be employed.
[0015]
There are various methods for controlling the amount of the enriched gas introduced. For example, there is a method in which the predetermined value of the difference (CP ₂ −CP ₁ ) is set as a control target value, and the flow rate of the enriched gas is adjusted by so-called on / off control, PID control, or the like.
[0016]
Next, the effects of the present invention will be described.
In the present invention, as described above, two apparent carbon potentials CP ₁ and CP ₂ are obtained from detection values of two different sensors, and the difference between them (CP ₂ −CP ₁ ) approaches the predetermined value. The flow rate of the enriched gas is controlled. Therefore, an appropriate amount of enriched gas can be introduced without generating soot in the furnace.
[0017]
This control method can be applied even during a temperature rise with a low furnace temperature. For this reason, even during the temperature rise in the furnace, an appropriate amount of enriched gas that does not generate soot can be introduced into the furnace, and the carbon potential can be increased moderately. Therefore, the carburization can be started during the temperature rise in the furnace, which has not been conventionally carburized, and the carburization completion time can be made earlier than before.
[0018]
Therefore, according to the present invention, even when the furnace temperature is lower than the processing temperature, an appropriate amount of enriched gas can be introduced without generating soot, and carburizing can be performed efficiently. Can be provided.
[0019]
Next, as in the invention of claim 2, the CO ₂ sensor can be configured to measure the CO ₂ concentration by an infrared absorption method. Since the infrared absorption method utilizes the fact that infrared rays having a specific wavelength are absorbed by the natural vibration of molecules, quantitative analysis and automation thereof are easy. Therefore, the measurement of the CO ₂ concentration can be performed accurately and easily.
[0020]
According to a third aspect of the present invention, the oxygen sensor is configured to measure using an electromotive force generated by a difference between an oxygen partial pressure in the furnace atmosphere gas and an oxygen partial pressure in the standard gas. be able to. The principle of using this electromotive force is shown by the well-known Nernst equation.
Examples of the above-described oxygen sensor include a zirconia sensor using zirconia as a solid electrolyte.
[0021]
As in the invention of claim 4, the introduction of the enriched gas is preferably started when the furnace temperature of the carburizing furnace exceeds a predetermined temperature. As said predetermined temperature, it can set to the temperature which can accelerate the progress of carburizing, for example. Specifically, the temperature is preferably 700 ° C. or higher. As a result, the efficiency of carburization can be improved.
[0022]
Further, as in the invention of claim 5, CP ₁ and CP ₂ can be expressed by the following relational expressions (I) and (II).
CP ₁ = (As · Pco ² ) / Pco ₂ · K ₁ . . . . . . . (I),
CP ₂ = (As · Pco) / Po ₂ ^1/2 · K ₂ . . . . . . . (II),
As: saturated carbon concentration (mass%) in austenite at the treatment temperature,
Pco : partial pressure of carbon monoxide,
Pco ₂ : the measured partial pressure of carbon dioxide,
Po ₂ : the measured partial pressure of oxygen,
K ₁ : Equilibrium constant of <C> + CO ₂ = 2CO,
K ₂ : <C> + (1/2) O ₂ = equilibrium constant of CO,
<C>: C dissolved in the material to be treated
[0023]
By using the relational expressions (I) and (II), the calculation accuracy of the carbon potential can be improved.
In the relational expressions (I) and (II), As, K ₁ and K ₂ can be expressed by the following relational expressions.
As = 0.23290−5.6957 × 10 ⁻⁴ (T−273) + 1.8830 × 10 ⁻⁶ (T−273) ² ,
K ₁ = exp {(170707−174.473T) / (− 8.3144T)},
K2 = exp {(111713 + 87.6548T) /8.3144T},
Here, T is a processing temperature.
[0024]
As in the invention of claim 6, the enriched gas is preferably natural gas. Natural gas has the property of producing less soot than butane gas, which is generally used as an enriched gas, and can further enhance the effect of suppressing soot generation.
[0025]
DETAILED DESCRIPTION OF THE INVENTION
Exemplary Embodiment A gas carburizing method according to an exemplary embodiment of the present invention will be described with reference to FIGS.
In the gas carburizing method of this example, the workpiece 8 is fed into the furnace of the carburizing furnace 1 and then the furnace temperature is raised to the carburizing temperature (about 930 ° C.) and the enriched gas 7 is introduced into the furnace. Then, the carbon potential is raised to carburize the workpiece 8.
[0026]
As shown in FIG. 1, the carburizing furnace 1 includes a CO ₂ sensor 2 that samples the atmosphere gas in the furnace and detects the CO ₂ concentration in the atmosphere gas outside the furnace, and the furnace atmosphere in the furnace. An oxygen sensor 3 for detecting the oxygen concentration in the gas is provided.
The introduction of enriched gas 7 is started during heating of the furnace temperature, and, in controlling the introduction amount of the enriched gas 7, were calculated based on the CO ₂ concentration measured by the CO ₂ sensor 2 determined the carbon potential CP ₁ of the apparent difference between the carbon potential CP ₂ apparent that calculated based on the oxygen concentration measured by the oxygen sensor 3 (CP ₂ -CP _1), the difference (CP ₂ -CP ₁ ) Is adjusted so as to approach the predetermined value M.
[0027]
This will be described in detail below.
In FIG. 1, the carburizing furnace 1 of this example, the equipment which this has, etc. are shown. The carburizing furnace 1 is provided with the CO ₂ sensor 2 and the oxygen sensor 3.
The CO ₂ sensor 2 is installed outside the furnace as described above, and is configured to periodically sample the atmospheric gas in the furnace via the pipe 21. The CO ₂ sensor 2 is a sensor using an infrared absorption method.
[0028]
As shown in FIG. 1, the oxygen sensor 3 uses zirconia 31 as a solid electrolyte to generate an electromotive force based on the difference between the oxygen partial pressure in the furnace atmosphere gas and the oxygen partial pressure in the atmosphere. The oxygen partial pressure in the furnace atmosphere gas is calculated from the electric power. This calculation is performed by the control unit 5 described later.
The carburizing furnace 1 is provided with a temperature sensor 4. The temperature sensor 4 periodically measures the temperature of the furnace atmosphere gas (furnace temperature).
[0029]
The carburizing furnace 1 is connected to a cylinder 70 containing natural gas (LNG) 7 as an enriched gas via a pipe 61 and a flow rate controller 6. The flow rate controller 6 is configured to adjust the flow rate of the natural gas 7 in accordance with instructions from the control unit 5 described later.
As shown in FIG. 1, the CO ₂ sensor 2, oxygen sensor 3, temperature sensor 4, and flow rate controller 6 are all electrically connected to the control unit 5.
[0030]
In this example, the amount of enriched gas introduced is controlled as follows.
As shown in the flowchart of FIG. 2, first, after the workpiece 8 is fed into the furnace in step S1, it is determined in step S2 whether the temperature in the furnace T has decreased to 800.degree. . Here, 800 ° C. is an example of a temperature at which the progress of carburizing can be accelerated, and it is of course possible to change to another temperature depending on the characteristics of the carburizing furnace.
[0031]
When the furnace temperature T becomes 800 ° C. or higher, introduction of the enriched gas is started in step S3. That is, the control unit 5 controls the flow rate controller 6 to maximize the amount of enriched gas (natural gas) 7 introduced.
[0032]
Next, in step S4, the control unit 5 determines the CO ₂ concentration (Pco ₂ ) from the CO ₂ sensor 2, the oxygen concentration (Po ₂ ) from the oxygen sensor, and the temperature (T) of the atmospheric gas in the furnace from the temperature sensor. Are periodically received, and the carbon potentials CP ₁ and CP ₂ are calculated periodically based on these values.
[0033]
The calculation at this time was performed by the following relational expressions (I) and (II).
CP ₁ = (As · Pco ² ) / Pco ₂ · K ₁ . . . . . . . (I),
CP ₂ = (As · Pco) / Po ₂ ^1/2 · K ₂ . . . . . . . (II),
As: saturated carbon concentration (mass%) in austenite at the treatment temperature,
(As = 0.23290−5.6957 × 10 ⁻⁴ (T−273) + 1.8830 × 10 ⁻⁶ (T−273) ² ),
Pco : partial pressure of carbon monoxide,
Pco ₂ : the measured partial pressure of carbon dioxide,
Po ₂ : the measured partial pressure of oxygen,
K ₁ : Equilibrium constant of <C> + CO ₂ = 2CO,
(K ₁ = exp {(170707−174.473T) / (− 8.3144T)}),
K ₂ : <C> + (1/2) O ₂ = equilibrium constant of CO,
(K ₂ = exp {(111713 + 87.6548T) /8.3144T}),
<C>: C dissolved in the material to be treated
T: Furnace temperature.
[0034]
Next, in step S5, the difference between CP ₁ and CP ₂ (CP ₂ −CP ₁ ) is obtained, and it is determined whether or not this is equal to or less than a predetermined value M. In this example, the predetermined value M is set to 0.2. Note that the value of the predetermined value M differs depending on the type of carburizing furnace, the type of material to be treated, and the like.
[0035]
If the difference (CP ₂ −CP ₁ ) exceeds the predetermined value M, the flow rate of the enriched gas is reduced in step S6. Specifically, the flow rate controller 6 is controlled by the control unit 5 to reduce the flow rate of the enriched gas.
There are various methods for restricting the flow rate. In this example, the flow rate is completely zero. This is so-called on / off control.
On the other hand, if the difference (CP ₂ −CP ₁ ) is less than or equal to the predetermined value M in step S5, the amount of enriched gas introduced is maintained at the maximum in step S7.
[0036]
In this example, by controlling the flow rate of the enriched gas, the difference between the two apparent carbon potentials (CP ₂ −CP ₁ ) approaches the predetermined value M, and soot generation in the furnace is ensured. Can be suppressed. For this reason, the enrichment gas can be introduced even during the temperature increase of the furnace temperature, which has been easy to generate soot.
[0037]
Therefore, it is possible to effectively use the time during which the temperature inside the furnace, which has not been carburized, is raised, as the carburizing time, and the carburizing completion time can be made faster than before.
This will be described with reference to FIG. As in FIG. 4, time is plotted on the horizontal axis and temperature is plotted on the vertical axis, showing the transition of the furnace temperature T and the processing material feed timing A and the timing at which the furnace temperature is restored to the carburizing temperature T _0. B, timing C when the introduction of the rich gas is started, and the like are shown.
[0038]
As can be seen from a comparison between FIG. 3 and FIG. 4, in this example (FIG. 3), the enrich gas introduction start timing C is earlier than in the conventional case (FIG. 4). For this reason, the total carburizing time (A to D) is shorter in this example than in the prior art. Therefore, in this example, the carburization efficiency can be improved as compared with the conventional case.
On the other hand, by using the above-mentioned control method, generation of soot in the furnace can be suppressed even when an enriched gas is introduced during the temperature rise. Therefore, the quality of the material to be processed can be maintained in a sufficiently good state.
[0039]
In this example, the flow control of the rich gas is performed by on / off control using the predetermined value M as a control target value. Instead of this, it is of course possible to use so-called PID control.
[0040]
【The invention's effect】
As described above, according to the present invention, even when the furnace temperature is lower than the processing temperature, an appropriate amount of enriched gas can be introduced without generating soot, and carburizing can be performed efficiently. A carburizing method can be provided.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram showing the configuration of a carburizing furnace in an embodiment.
FIG. 2 is a flowchart illustrating a flow control method for an enriched gas in the embodiment.
FIG. 3 is an explanatory diagram showing the start timing for introducing an enriched gas in the embodiment.
FIG. 4 is an explanatory diagram showing the start timing for introducing an enriched gas in a conventional example.
[Explanation of symbols]
1. . . Carburizing furnace,
2. . . CO ₂ sensor,
3. . . Oxygen sensor,
4). . . Temperature sensor,
5). . . Control unit,
6). . . Flow controller,
7). . . Enrich gas (natural gas),

Claims (6)

Carrying the material to be treated into the furnace of the carburizing furnace, then raising the furnace temperature to the carburizing temperature and introducing an enriched gas into the furnace to raise the carbon potential and carburizing the material to be treated. In the gas carburizing method,
The carburizing furnace includes a CO ₂ sensor that samples the atmosphere gas in the furnace and detects the CO ₂ concentration in the atmosphere gas outside the furnace, and detects the oxygen concentration in the atmosphere gas in the furnace. With an oxygen sensor,
The introduction of the enriched gas starts during the temperature rise of the furnace temperature, and the apparent amount calculated based on the CO ₂ concentration measured by the CO ₂ sensor is used to control the amount of the enriched gas introduced. carbon and potential CP _1, obtains a difference (CP ₂ -CP ₁₎ the carbon potential CP ₂ apparent that calculated based on the oxygen concentration measured by the oxygen sensor, the difference (CP ₂ -CP ₁₎ is a predetermined value per to adjust the flow rate of the enriched gas to approach,
As the predetermined value, a value smaller than the maximum value capable of suppressing the generation of soot in the furnace is adopted, and the difference (CP ₂ −) is controlled by on / off control or PID control using the predetermined value as a control target value. A gas carburizing method characterized in that the enriched gas introduction flow rate is maintained at a maximum until CP ₁ ) approaches the predetermined value . The gas carburizing method according to claim 1, wherein the CO ₂ sensor is configured to measure a CO ₂ concentration by an infrared absorption method.   3. The oxygen sensor according to claim 1, wherein the oxygen sensor is configured to measure using an electromotive force generated by a difference between an oxygen partial pressure in the furnace atmosphere gas and an oxygen partial pressure in the standard gas. Characterized gas carburizing method.   The gas carburizing method according to any one of claims 1 to 3, wherein the introduction of the enriched gas is started when the temperature inside the carburizing furnace exceeds a predetermined temperature. In any one of claims 1 to 4, the CP ₁ and CP ₂ are the following relationships,
CP ₁ = (As · Pco ² ) / Pco ₂ · K ₁ ,
CP ₂ = (As · Pco) / Po ₂ ^1/2 · K ₂ ,
As: saturated carbon concentration (mass%) in austenite at the treatment temperature,
Pco : partial pressure of carbon monoxide,
Pco ₂ : the measured partial pressure of carbon dioxide,
Po ₂ : the measured partial pressure of oxygen,
K ₁ : Equilibrium constant of <C> + CO ₂ = 2CO,
K ₂ : <C> + (1/2) O ₂ = equilibrium constant of CO,
<C>: C dissolved in the material to be treated
A gas carburizing method characterized by the following:   The gas carburizing method according to any one of claims 1 to 5, wherein the enriched gas is natural gas.

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