JP4092215B2 - Heat treatment furnace atmosphere control device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an atmosphere control method and apparatus for a heat treatment furnace, and more particularly to an atmosphere control method and apparatus for a heat treatment furnace that performs gas carburization, gas carbonitriding, bright atmosphere heat treatment, and the like.
[0002]
[Prior art]
Conventionally, as a heat treatment method such as gas carburizing, a gas (hereinafter referred to as an endothermic gas), which is a mixture of a hydrocarbon-based gas and air and is transformed using an endothermic modified gas generator, is supplied into the furnace, In order to obtain a carbon potential, a method of adding a hydrocarbon-based gas (hereinafter referred to as an enriched gas) has been widely employed. However, in recent years, from the viewpoint of energy saving, by introducing a hydrocarbon-based gas and an oxidizing gas directly into a furnace as shown in Japanese Patent Laid-Open Nos. 61-159567 and 4-63260, etc. There is a tendency that a direct carburizing method in which carburizing is performed without the need for a shift gas generating furnace is gradually adopted.
[0003]
[Problems to be solved by the invention]
However, in the method disclosed in Japanese Patent Laid-Open No. 61-159567, the oxidizing gas added to the furnace is oxygen, and the CO partial pressure is about 29% when CH ₄ is used as the hydrocarbon gas, C ₄ H ₁₀ In the case of Japanese Patent Laid-Open No. 4-63260, the CO partial pressure of the atmosphere is increased to around 40% when compared with the conventional method when CO ₂ is used and butane is used as the hydrocarbon gas. Although the carburizing time can be greatly shortened compared to the conventional method, the grain boundary oxidation of the treated product increases due to the high CO partial pressure compared to the conventional method.
[0004]
In general, the CO partial pressure in the furnace atmosphere fluctuates when a large amount of air is mixed in the furnace when charging and unloading the processed material. Nevertheless, in the method disclosed in Japanese Patent Laid-Open No. 4-63260, the supply amount of hydrocarbon gas is adjusted so that the value of the carbon potential of the atmosphere is constant. Due to the change in weight and surface area, the fluctuation of the atmosphere is large, the fluctuation of the carbon potential is also large, and the variation in the surface carbon concentration of the steel becomes large.
[0005]
In addition, the carburization rate in the direct carburizing method is strongly influenced by the carburizing period and the diffusion period. In the former, direct decomposition of hydrocarbon gas or the like (raw gas) is the main effect on carburization, and in the latter, the Boudouard reaction is the main. Therefore, in the case of direct introduction of the hydrocarbon gas or the like into the furnace, the degree of decomposition differs depending on the amount of addition and the temperature of the atmosphere (of course, depending on the loaded state of the charged product). As a result, there has been a problem that hydrocarbon-based gas or the like is added more than necessary for carburizing and becomes soot and accumulates in the furnace, or the processed material is sooted.
[0006]
In addition, when operating without knowing that it is within the sooting range described above, there has been a problem of shortening the life of the oxygen sensor.
[0007]
The object of the present invention is to eliminate the above-mentioned conventional drawbacks.
[0008]
[Means for Solving the Problems]
The atmosphere control device for a heat treatment furnace of the present invention includes a furnace shell, a heater for heating in the furnace, a CO partial pressure measuring means in the furnace, a carbon potential calculating means in the furnace, and butane gas and oxidizing gas in the furnace. And means for controlling the amount of butane gas and oxidizing gas introduced into the furnace. The means for controlling the oxidizing gas has a CO partial pressure in the furnace of 30%, or 28.5. It is a means for stopping the supply of the oxidizing gas when a set value in the range of 31.5% is reached.
[0009]
In the atmosphere control apparatus for a heat treatment furnace according to the present invention, the means for controlling the oxidizing gas may stop the butane so that the carbon potential in the furnace reaches a set value after the supply of the oxidizing gas is stopped. It is a means for controlling the amount of gas supply.
[0010]
Further, in the atmosphere control device for the heat treatment furnace of the present invention , the oxygen partial pressure in the furnace and CH ₄ The Rukoto to have a partial pressure measuring means, characterized.
[0013]
The oxidizing gas is air or CO ₂ gas.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings.
[0015]
FIG. 1 is an explanatory view of an atmosphere control apparatus for a heat treatment furnace according to the present invention.
[0016]
In FIG. 1, 1 is a furnace shell, 2 is a heat-resistant brick forming the furnace shell 1, 3 is a fan for stirring the atmosphere, 4 is a heater for heating, 5 is a thermocouple for controlling the temperature in the furnace, and 6 is a furnace, for example. Direct insertion type zirconia type solid electrolyte oxygen partial pressure measurement sensor, 7 CO ₂ partial pressure measurement tube, 8 CH ₄ partial pressure measurement tube, 9 CO partial pressure analyzer, 10 CH ₄ partial pressure analysis Apparatus, 11 is a supply pipe for hydrocarbon gas introduced into the furnace, 12 is a control valve thereof, 13 is a supply pipe for oxidizing gas introduced into the furnace, 14 is a control valve thereof, 15 is a carbon potential calculation device, Reference numeral 16 denotes a regulator that sends a regulation signal to the regulation valves 12 and 14.
[0017]
FIG. 2 shows the relationship between the carburizing time and the carburizing depth due to the difference in carbon potential. When the carbon potential during carburizing is high, it is already possible to finish carburizing in a shorter time than when it is low. It is known that in the Fe-C system equilibrium diagram, it is not suitable for actual operation when entering the sooting zone as shown by the oblique lines in FIG.
[0018]
In order to increase the carbon potential, a large amount of enriched gas (hydrocarbon-based gas) is preferably added. Looking at the passage of time after the addition of the enriched gas, as shown in FIG. 3, when the charging weight is constant at 150 kg and C ₄ H ₁₀ gas is used, A (flow rate 2.5 liter / min), B (1.4 In both cases (liter / min) and C (1.0 liter / min), as the carburizing time t elapses, the amount of residual CH ₄ starts to decrease and then increases, and the treated product generates sooting. On the other hand, in the case of D (0.5 liter / min), the amount of residual CH ₄ becomes almost constant and sooting does not occur. The difference is that in the case of A (2.5 liters / min), B (1.4 liters / min), and C (1.0 liters / min), the steel has absorbed carbon due to the large amount of addition. The reason is that undecomposed CH ₄ increases, whereas D (0.5 liter / min) is because the steel can absorb carbon. Therefore, analyzing the amount of residual CH ₄ and controlling its value will prevent sooting.
[0019]
In the Fe-C equilibrium diagram, the maximum carbon solid solution amount is constant when the temperature is determined, and sooting can be prevented by measuring the oxygen partial pressure corresponding to that value.
[0020]
As shown in FIG. 4, the carburization rate changes according to the carbon transfer coefficient β, and the carbon transfer coefficient β becomes maximum when the CO partial pressure in the carburizing furnace is 50%. On the other hand, the increase in the CO partial pressure also increases the O ₂ partial pressure in the furnace. The relationship between the grain boundary oxide layer depth from the surface and the CO partial pressure (CO partial pressure is proportional to the O ₂ partial pressure) is as shown in FIG.
[0021]
It is known that the influence of the depth of the grain boundary oxide layer on the material strength is generally limited to 13.5 μm. Therefore, the optimum CO partial pressure is determined from the intersection of the grain boundary oxide layer 13.5 μm and the CO partial pressure. The value is about 30% CO when the hydrocarbon gas is butane. In the present invention, when the CO partial pressure in the furnace reaches about 30%, this is judged from the analysis result of the CO analyzer 9 and the above oxidation. The sex gas control valve 14 is closed.
[0022]
As is clear from the experimental results in FIG. 6, CH ₄ and CO ₂ react stoichiometrically 1: 1, so that the opening of the valve 14 is about 30% CO when the hydrocarbon gas is butane. Although it is adjusted so that it can be changed to the center, there is actually an amount of O ₂ brought in by the processed material or an amount of leaked air due to the confidentiality of the furnace body, and the ratio of CO ₂ / CH ₄ is not necessarily 1: 1. Therefore, the valves 12 and 14 are controlled to open and close based on the CO partial pressure measurement result. The same effect can be obtained even when the flow rate of the oxidizing gas is kept constant and the flow rate of the hydrocarbon-based gas is controlled.
[0023]
Carbon potential equation case of controlling the CO to about 30% constant as described _{above, <C> + O 2/} 2 → Kp number equilibrium constant than CO, carbon potential (the activity) a _c, the oxygen partial pressure PO If it is ₂ , formula (1)
[0024]
a _c = CO / Kp · PO ₂ ^1/2 −−− (1)
[0025]
When the temperature is constant and CO is constant, Kp is also constant, and the carbon potential _ac is expressed as a function of PO ₂ ^{1/2 when} the oxygen partial pressure is PO ₂ . In order to obtain the target carbon potential, when the value of the oxygen electromotive force is less than the target value, the hydrocarbon gas valve 12 is opened. When the target value is exceeded, the valve 12 is closed.
[0026]
The carbon potential can be obtained by substituting the CO analysis result into the above formula (1) and calculating CO and O ₂ .
[0027]
When the temperature fluctuates, a change in Kp (for example, calculated by logKp = 58840.6 / T + 4.583), which is temperature adjustment, is automatically calculated, and is substituted into equation (1) for calculation. .
[0028]
Example 1
[0029]
Using a batch furnace, 150 kg of treated material was charged, and carburizing operation was performed at 930 ° C. for 4 hours using C ₄ H ₁₀ gas as a hydrocarbon gas and CO ₂ gas as an oxidizing gas. Regarding the CO partial pressure during operation, the surface carbon content of the treated material, and the carburization depth, the difference between the conventional method of Japanese Patent Laid-Open Nos. 61-159567 and 4-63260 and the present invention was investigated. The result is as shown in FIG. That is, in the conventional method, the CO fluctuation corresponding to CO% is 23 to 40% when the hydrocarbon gas is butane, whereas according to the present invention, the target is 30% when the hydrocarbon gas is butane. 28.5 to 31.5% (30% ± 1.5%). Further, in the conventional method, the variation of the surface carbon amount is 0.7 to 1.70% with respect to the target set surface carbon amount of 1.20%, whereas according to the present invention, it is 1.10 to 1.30%. Can be controlled within the range, and variations thereof are reduced.
[0030]
Similarly, the variation of the depth with respect to the target value of 0.7 mm for the carburization depth can be improved from 0.55 to 0.85 mm to 0.6 to 0.8 mm.
[0031]
FIG. 8 shows the change over time of the additive gas and the time course of the CO partial pressure that led to the above results. The maximum flow rates of C ₄ H ₁₀ gas and CO ₂ gas passing through the valves 12 and 14 were 2.5 liters / min, respectively. In the vicinity of the temperature increase, C ₄ H ₁₀ and CO ₂ flow in the maximum amount installed in this case, but the valves 12 and 14 are immediately operated according to the analysis result of CO, and as a result, It can also be understood that the control was performed with an accuracy of 30% ± 1.50%.
[0032]
Incidentally, as shown in FIG. 3, in the case of addition of butane 1.0 L / min or more hydrocarbon gas over time, CH ₄ amount is increased, this residual CH ₄ is not selected It is accumulated in the furnace as decomposition, and sooting increases.
[0033]
Therefore, as apparent from FIG. 8, when the temperature reaches 930 ° C., but is added in the case of butane as the hydrocarbon-based gas, the amount of sooting occurs when the addition amount is 2.5 liters / min. According to the method of the present invention, the addition amount is gradually reduced and sooting is prevented.
[0034]
In the present invention, the case where the charging weight was varied from (150 kg / 2) to (150 kg × 2), the weight was constant, the surface area was reduced to ½, and the test was increased by 6 times. However, in the case of butane as the hydrocarbon-based gas, the CO fluctuation in the atmosphere could be controlled to 30 ± 1.50% CO as shown in FIG.
[0035]
(Example 2)
[0036]
FIG. 9 is a cross-sectional view of carburization using an endothermic gas (CO: about 23%) that is conventionally performed in the case of butane as a hydrocarbon gas and carburization according to the present invention (CO: about 30%). The microstructure is shown, the left side of the photo is from the endothermic gas, and the right side is from the present invention. In any picture, the left side shows the surface, indicating that there is grain boundary oxidation. However, in the comparison between the two, both are about 10 μm of grain boundary oxidation, and there is no big difference between them. That is, the grain boundary oxidation does not increase so much because the CO is controlled to about 30%.
[0037]
(Example 3)
[0038]
FIG. 10 shows the difference between the conventional method and the method of the present invention on the carburization depth when 150 kg of treated material is carburized at 930 ° C. From this, it is understood that the present invention is carburized deeply by about 19% in the carburizing for a certain time as compared with the case of the endothermic gas. Therefore, in the case of carburizing at a certain depth, the carburizing time can be shortened as compared with the conventional method.
[0039]
Example 4
[0040]
Figure 11 is a C ₄ H ₁₀ gas and CO ₂ by the present invention using a gas and a conventional endothermic method using C ₄ H ₁₀ gas as a source gas and enriched gas of endothermic gas, treatment temperature 930 A comparison of the amount of gas used in the case of carburizing treatment with an effective hardened layer depth (corresponding to 0.4% C) of 1 mm with the carbon potential kept constant at 1.0 ° C. is shown. As a result, when the carburizing treatment is performed by the method according to the present invention, C ₄ H ₁₀ used for obtaining an effective hardened layer depth of 1 mm as compared with the case where the carburizing treatment is performed by the conventional endothermic gas method. The amount of gas was reduced by 69%.
[0041]
As the hydrocarbon-based gas, a liquid containing carbon atoms, for example, an alcohol or a gas, for example, a gas mainly containing a hydrocarbon such as acetylene, methane, propane or butane, preferably methane, propane or butane gas is used.
[0042]
As the oxidizing gas, air or CO ₂ gas is used.
[0043]
In the present invention, from the analysis result of the CH ₄ analyzer 10, the control valve 12 is closed when the value of CH ₄ changes from a decrease to an increase, and the inflow of the hydrocarbon gas C _X H _Y is stopped. ₄ Sooting can be prevented by controlling the amount not to increase.
[0044]
In the present invention, the oxygen partial pressure is measured by measuring the electromotive force of the oxygen partial pressure measuring sensor 6, and the sooting is also performed by closing the control valve 12 when the oxygen partial pressure reaches the set value. Can be prevented.
[0045]
【The invention's effect】
As described above, according to the present invention, in the atmospheric heat treatment such as gas carburizing, gas carbonitriding, and bright heat treatment, the addition amount of the hydrocarbon gas and the oxidizing gas for making the CO partial pressure of the atmosphere constant is increased. By controlling, it is possible to eliminate the effects of changes in the package (weight / surface area) of the processed material and changes in the holding time of the furnace, and to stabilize the quality of the processed material while keeping the carbon potential constant.
[0046]
In addition, sooting can be prevented beforehand by controlling the amount of addition of hydrocarbon gas or the like corresponding to the CH ₄ partial pressure and oxygen partial pressure of the atmosphere.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram of an atmosphere control apparatus for a heat treatment furnace according to the present invention.
FIG. 2 is a diagram showing the influence of carburizing time on the carburizing depth due to the difference in carbon potential.
FIG. 3 is a diagram showing the relationship between the amount of residual CH ₄ and the carburizing time due to the difference in the enriched gas addition amount.
FIG. 4 is a diagram showing the influence of a CO + H ₂ gas component on the carbon transfer coefficient.
FIG. 5 is a diagram showing the influence of CO% on the grain boundary oxide layer depth.
FIG. 6 is a diagram showing the relationship between CO% and CO ₂ / CH ₄ .
FIG. 7 is a diagram showing a comparison between a conventional method and the present invention with respect to CO% variation, surface carbon content variation, and carburization depth variation.
FIG. 8 is a diagram showing changes in CO%, residual CH ₄ content, added C ₄ H ₁₀ , and CO ₂ flow rates as a result of carburizing at 930 ° C.
FIG. 9 is a comparative view of micrographs showing grain boundary oxidation.
FIG. 10 is a diagram showing the difference between the conventional method and the present invention in the relationship between the effective carburizing depth and the carburizing time.
FIG. 11 is a comparative explanatory view of gas consumption between the conventional method and the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Furnace shell 2 Heat-resistant brick 3 Atmosphere stirring fan 4 Heating heater 5 Thermocouple 6 Oxygen partial pressure measuring sensor 7 CO ₂ partial pressure measuring tube 8 CH ₄ Partial pressure measuring tube 9 CO partial pressure analyzer 10 CH ₄ Partial pressure analyzer 11 Hydrocarbon gas supply pipe 12 Control valve 13 Oxidizing gas supply pipe 14 Control valve 15 Carbon potential calculation device 16 Controller

Claims (5)

  A furnace shell, a heater for heating in the furnace, a CO partial pressure measuring means in the furnace, a carbon potential calculating means in the furnace, a means for introducing butane gas and oxidizing gas into the furnace, and these butane gas and oxidizing gas And the means for controlling the oxidizing gas is a means for stopping the supply of the oxidizing gas when the CO partial pressure in the furnace reaches 30%. An atmosphere control device for a heat treatment furnace characterized by the above.   A furnace shell, a heater for heating in the furnace, a CO partial pressure measuring means in the furnace, a carbon potential calculating means in the furnace, a means for introducing butane gas and oxidizing gas into the furnace, and these butane gas and oxidizing gas The means for controlling the amount of oxygen introduced into the furnace is controlled by the oxidizing gas when the CO partial pressure in the furnace reaches a set value in the range of 28.5 to 31.5%. An atmosphere control device for a heat treatment furnace, characterized in that it is means for stopping the supply of a characteristic gas.   The means for controlling the oxidizing gas is a means for controlling the supply amount of the butane gas so that the carbon potential in the furnace reaches a set value after the supply of the oxidizing gas is stopped. The atmosphere control device for a heat treatment furnace according to claim 1 or 2. _4. The atmosphere control apparatus for a heat treatment furnace according to claim 1, further comprising means for measuring oxygen partial pressure in the furnace and CH ₄ partial pressure. Atmosphere control device of the heat treatment furnace of claim 1, 2, 3 or 4, wherein the said oxidizing gas is air or CO ₂ gas.

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