JPS6411709B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a gas carburizing method, and in particular,
A method using nitrogen as the main component of the atmospheric gas,
The pressure inside the closed processing furnace where the processed products are installed is reduced, then nitrogen gas is introduced to restore the pressure to a constant level, and then hydrocarbons and oxidizing gas are introduced to remove CO, CO ₂ and the furnace. The internal temperature is measured, the target amount of CO ₂ is calculated and set based on this, and the amount of enrichment gas introduced into the furnace is controlled based on the deviation between it and the analyzed value of the amount of CO ₂ , and the amount of CO 2 in the atmospheric gas is controlled. To maintain the equilibrium carbon concentration of the carburizing atmosphere at a desired value even when the carbon concentration varies, to significantly save the amount of hydrocarbons, and to obtain accurate carburizing results. Commonly used gas carburizing methods include:
There are two types: a conversion furnace method using an endothermic conversion gas, and a dripping method in which an organic liquid is directly injected into the furnace. In the shift furnace system, when hydrocarbon gas and air are mixed at a fixed ratio and passed through a catalyst in a shift furnace heated to 1050°C, the following reaction occurs. a.CxHy+b・(O ₂ +3.76N ₂ )→c・H ₂ +d・CO
+3.76×b・N ₂ This metamorphosed gas is sent to the carburizing furnace as a carrier gas, and a similar hydrocarbon CxHy is fed as an enrichment agent to increase the carburizing concentration, thereby creating a predetermined atmosphere inside the furnace. Adjust to achieve target carburizing. The amount of gas sent in at this time varies slightly depending on the surface area of the product being processed, but it requires approximately 6 to 10 times the volume of the furnace, and this used gas is a source of pollution and danger. From the standpoint of prevention, these are burned and disposed of near the furnace. Therefore, the actual efficiency of the gas used is extremely low and uneconomical. In addition, in the dripping carburizing method, when an organic liquid is directly dripped into a carburizing furnace heated to the carburizing temperature, it decomposes as CxHyOz→a・CO+b・H ₂ . The gas produced by this decomposition corresponds to the carrier gas of the shift furnace system, and it also contains hydrocarbons.
CxHy is added or an organic liquid that generates free carbon is premixed, and these are simultaneously added dropwise to provide an enriching effect and form a carburizing atmosphere.
In this case as well, the amount of organic liquid to be dripped into the furnace varies slightly depending on the surface area of the product to be treated, but it depends on the volume of the furnace.
It is necessary to generate 2.5 to 4 times the amount of gas, and this used gas is burned and disposed of near the furnace, from the same viewpoint as the shift furnace system. As described above, the currently used gas carburizing method requires a large amount of gas, and most of the gas is discharged outside the furnace without being used. This discharged gas contains large amounts of CO and H ₂ and is extremely dangerous, so it is all burned in front of the furnace. In contrast to such methods, from the viewpoint of energy saving and resource saving, which have recently been called for, in gas carburizing treatment, the gas necessary for carburizing is generated in the furnace without using a conversion furnace, and N ₂ gas is used as a purge gas. An N2 _- based carburizing method using However, like the above-mentioned method, this method uses open equipment and requires a large amount of gas to be fed. Additionally, a large amount of N ₂ gas is sent during startup, requiring early replacement of residual air. Furthermore, regarding the stability of the atmosphere, the gas used is an oxidizing gas and a hydrocarbon gas, which are mixed and transformed in the furnace, and the gas exchange rate is high.
That is, because the gas flow rate is high, sufficient gas transformation is not performed, and carburization due to undecomposed gas occurs, making it difficult to obtain a stable carburizing atmosphere. Generally, the carburizing mechanism when steel is heat treated in a carburizing atmosphere is given by the following reaction equation. 2CO[C]+CO ₂ ......(1) From the above equation, the carbon concentration in the carburizing atmosphere has traditionally been controlled by measuring the partial pressure of CO ₂ in the atmospheric gas components. There is. However, since this control method is based on the premise that the amount of CO is constant, there is a drawback that precise control cannot necessarily be expected. In other words, the gas composition of the atmosphere gas produced by the above-mentioned shift furnace method varies depending on the type of hydrocarbon gas used for shift conversion and the mixing ratio of hydrocarbon gas and air. In the carburizing method, in which atmospheric gas is packed together with the steel to be treated in a completely sealed container such as a furnace, the gas composition changes depending on the amount of enrichment gas supplied, so the partial pressure of CO changes due to this. Got tired,
With conventional control methods that measure only CO ₂ ,
It has been considered extremely difficult to control the carbon concentration on the surface of steel materials to a desired value. Therefore, as a technology to eliminate such difficulties,
Methods for measuring compositional gases other than CO ₂ have come to be proposed. For example, in JP-A-51-149135 and JP-A-51-149136, CO, CO ₂ , CH ₄ and H ₂ are 14539 No. 14539 and Japanese Patent Application No. 52-
In each publication No. 14540, CO, H ₂ and H ₂ O are each measured, and the carbon concentration in the atmosphere calculated from these analytical values is compared with the desired equilibrium carbon concentration set value, and the enrichment gas is adjusted according to the deviation. The amount of addition is controlled. Although these methods are effective in allowing more precise control than conventional methods, they
Since different types are required, the structure and arrangement of the analysis device and the arithmetic device become large and complicated, and their management is difficult. This invention solves the problems of conventional carburizing methods, and its purpose is to provide an economical gas carburizing method that does not waste the gas or its raw materials. _CO2
It is an object of the present invention to provide a method for easily and precisely controlling the carbon concentration in a carburizing atmosphere by measuring two types of composition gases: In particular, it is an object of the present invention to provide a method for controlling the equilibrium carbon concentration in a carburizing atmosphere to a constant level by modifying the amount of CO ₂ corresponding to a desired equilibrium carbon concentration in response to fluctuations in CO and changes in furnace temperature. Furthermore, an object of the present invention is to make the quality of steel materials uniform and highly accurate.Another object of the invention is to provide a less dangerous gas carburizing method. The object of the present invention is to provide a gas carburizing method that has a simple implementation device and is easy to operate. That is, the present invention, as in the illustrated embodiment, reduces the pressure in the heat treatment furnace 1 in which the steel material W is installed in a sealed state with a vacuum pump P, and then introduces nitrogen gas into the furnace to restore the pressure to a constant level. , after that,
Hydrocarbons and oxidizing gas are introduced, and the amount of CO ₂ and CO is analyzed from the atmospheric gas. Target corresponding to equilibrium carbon concentration
The amount of CO ₂ is calculated and set, and hydrocarbons or a mixture of hydrocarbons and oxidizing gas are controlled as enrichment gas based on the deviation between the target amount of CO ₂ and the analysis value of the amount of CO ₂ in the atmosphere gas. The present invention relates to a closed gas carburizing method characterized by: In detail, a sealed heat treatment furnace 1 is used to speed up the gas replacement in the furnace, the processed product W is installed inside the heat treatment furnace 1, and a vacuum pump P is used.
After that, nitrogen gas is introduced and the temperature is raised to take a stabilization time. After that, a highly decomposable hydrocarbon C ₃ H ₈ and an oxidizing gas (air) are added at the initial stage. Introduce CO to a concentration of 5 to 15%. The introduced hydrocarbons immediately decompose as follows. C ₃ T ₈ → 3 [C] + 4H ₂ ...... (2) The gas in the furnace is measured with an infrared analyzer, and the mixture of hydrocarbon gas CxHy or hydrocarbon and oxidizing gas is determined according to the preset carbon concentration. Add the mixture (which will become the enrichment gas) and control the atmosphere. In the reaction of formula (1) above, since the furnace is sealed, it is not easily affected by undecomposed gas, and CO and H ₂ can be stably obtained. Hydrocarbon gas C ₃ H ₈ as enriching agent
When used, it reacts with H ₂ O and CO ₂ in the furnace as follows. 3H ₂ O＋C ₃ H ₈ →3CO＋7H ₂ ………(3) 3CO ₂ +C ₃ H ₈ →6CO＋4H ₂ ………(4) This generated CO is expressed by the following formula as well as the CO gas in formula (1). Has a carburizing effect. [Fe] γ + 2CO → [Fe-C] γ + CO ₂
......(5) The exhaust gas sent into the furnace is discharged outside the furnace by a small pressure regulating valve that is adjusted to a constant pressure, but the amount of gas is extremely small. In addition, the dangerous gases BO and _H2 contained are so small that they are either burned or exhausted to the outside air. The reason why an oxidizing gas is used here is that since the furnace is a closed type, it is necessary to assist in the production of CO. Next, the control method of the present invention is carried out using the following arithmetic expression. That is, if K is the equilibrium constant in the reaction equation (1) of the carburizing mechanism described above, the following equilibrium equation holds true. K=Pco ₂ /ac・Pco ₂ ………(6) Where, ac: saturation level Pco, Pco ₂ : partial pressure of CO and CO ₂ , respectively ac=equilibrium carbon concentration CPeq/saturated carbon concentration CPsat
......(7) Now, by substituting equation (7) into equation (6) and transforming it, Pco ₂ = S x Pco ₂ /CPeq ......(8) Here, S = CPsat / K S is processing Constant determined by temperature (temperature coefficient)
It is. From equation (8), it can be seen that the target Pco ₂ should be corrected by the change in Pco. Embodiments illustrating the control method of the present invention will be described below. FIG. 1 is a block diagram showing an embodiment of the control device used in the present invention. A heat treatment furnace 1 maintained at a constant processing temperature is equipped with a motor M and a fan F facing into the furnace. A heater H is installed inside the furnace, and a vacuum pump P is connected to the furnace. A steel material W is charged into this heat treatment furnace 1 and carburized. Inside the heat treatment furnace 1, nitrogen, air, and hydrocarbons are supplied from a supply source (not shown) through a pipe line 2, and enrichment gas is similarly supplied from a supply source (not shown) through a pipe line 3 via a control valve 4. is supplied to generate an atmospheric gas having a predetermined equilibrium carbon concentration. This atmospheric gas is sent from the pipe 5 to a CO analyzer 6 connected to the pipe 5. The amount of CO analyzed by the CO analyzer 6 is input to the calculation device 10 as a CO analysis signal. Further, the temperature inside the furnace is measured by the thermocouple 7 and used as a temperature signal, or a signal corresponding to the temperature inside the furnace is set in advance and similarly inputted into the calculation device 10. On the other hand, a desired surface carbon concentration of the steel material is set in the carbon concentration setting device 8, and this is similarly inputted to the calculation device 10 as a target carbon concentration setting signal. The arithmetic unit 10 has a built-in arithmetic circuit of the above-mentioned formula 8, and the CO analysis signal input thereto,
A calculation is performed using the temperature signal and the target carbon concentration setting signal to calculate the target amount of CO ₂ corresponding to the equilibrium carbon concentration of the atmospheric gas. This target _CO2 amount is _CO2
Input as the setting value of the controller 9. In addition, the atmospheric gas in the furnace is connected to pipe 5.
The CO ₂ amount is analyzed by the CO ₂ analyzer 11, and the CO ₂
It is fed back to the CO ₂ controller 9 as an analysis signal. The CO ₂ controller 9 compares the target CO ₂ amount signal input as a set value from the arithmetic unit 10 with the CO ₂ analysis signal input as a measured value from the CO ₂ analyzer 11, and calculates the amount of CO 2 generated thereby. A control valve 4 connected to the pipe line 3 is actuated by the deviation signal, and the opening degree of the control valve 4 is adjusted depending on the magnitude of the deviation signal. In this way, the amount of enrichment gas supplied into the heat treatment furnace 1 is increased or decreased, and the equilibrium carbon concentration of the atmospheric gas within the furnace is maintained at a desired value. As a method for analyzing the amount of CO ₂ and the amount of CO in the atmospheric gas in the above embodiments, for example, an infrared absorption method, a gas chromatography method, or other appropriate method is employed. Also, as the CO ₂ controller, a servo setting type is suitable. As the arithmetic unit, either an analog type or a digital type can be employed. FIG. 2 is a block diagram showing the calculation process when an analog calculation device is used in the embodiment shown in FIG. CO analyzer 6 (Figure 1)
The CO analysis signal Pco is input to a square calculator 11, subjected to a square calculation, and sent as Pco ₂ to the next multiplier 13. On the other hand, the temperature signal from the thermocouple 7 is converted by the function generator 14 into a temperature coefficient S (=CPsat/K) corresponding to the temperature inside the furnace, and sent to the multiplier 13. The multiplier 13 multiplies the signals Pco ₂ and S, and inputs the signal as Pco ₂ ·S to the next divider 15. The divider 15 calculates this Pco ₂・
S is divided by the target carbon concentration setting signal CPeq from the carbon concentration setting device 8 and outputted to the CO ₂ controller 9 as Pco ₂ ·S/CPeq. In this way, the target CO ₂ amount corresponding to the equilibrium carbon concentration of the atmospheric gas is input to the CO ₂ controller 9 as a set value. Figure 3 shows a comparison of the analysis results of the surface carbon concentration of the treated steel material obtained by carburizing using the control method of this invention with the analysis results using the conventional infrared CO ₂ analysis method. That's right. The carburizing time in the figure is the elapsed time after the inside of the furnace reached the carburizing temperature (930°C) and atmospheric control was started. The carburizing treatment using the method of this invention in the same figure is as follows:
This was done according to the following steps. first,
After the steel material to be treated is charged into the furnace, the inside of the furnace is evacuated to 0.1 to 1.0 torr using a vacuum pump. After that, carrier gas is filled to near atmospheric pressure and the chamber is sealed. Then, when the temperature is raised and the furnace temperature has recovered, enrichment gas is supplied according to the control method of the present invention, and carburization is carried out for a predetermined time while maintaining the equilibrium carbon concentration of the atmospheric gas at a predetermined value. As mentioned above, this invention
A target CO ₂ amount corresponding to the desired equilibrium carbon concentration is set from the CO amount, furnace temperature, and target carbon concentration on the surface of the steel material, and the enrichment gas is adjusted based on the deviation between this set value and the CO ₂ amount in the atmospheric gas. It is configured to control the supply amount. Therefore, according to the present invention, it is possible to control the equilibrium carbon concentration of the atmospheric gas to be constant no matter how the amount of CO in the atmospheric gas changes. Moreover, the method analyzes two types of composition gases, the amount of CO and the amount of CO ₂ in the atmospheric gas, and corrects the target amount of CO ₂ according to the fluctuations in the amount of CO, so it is an extremely simple method. Not only is it possible to precisely control the atmospheric gas, but it is also possible to obtain uniform, highly accurate, and excellent quality steel products. Further, according to the present invention, not only enrichment gas but also carrier gas can be significantly reduced, and the demand for resource saving can be fully met. An example of gas composition analysis values obtained during this treatment is shown in the following table.

[Table] FIG. 4 shows the processing pattern. The amount of gas used in this method is 5 to 15% of the nitrogen gas used for purging after the initial vacuum
The amount of hydrocarbon C ₃ H ₈ used to achieve the CO concentration is the largest, and the amount of hydrocarbon (enrichment gas) used for subsequent atmosphere adjustment is extremely small. Comparing the case of carburizing using the conventional open method using a heat treatment furnace of the same volume and the case of this invention, the carburization is 1/18th as compared to the conversion furnace method, and 8 times less than that of the dripping method. Good results were obtained by using a carrier gas of 1. The depth of carburization, surface carbon concentration, etc. were no different from conventional results.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing an embodiment of the control device used in the present invention, Fig. 2 is a block diagram showing an embodiment of the calculation process, and Fig. 3 is the relationship between the surface carbon concentration of the steel to be treated and the carburizing time. FIG. 4 is a graph showing the processing pattern. In the figure, 1 is a heat treatment furnace, 4 is a control valve, 6 is a CO analyzer, 8 is a carbon concentration setting device, 9 is a CO ₂ controller, 1
0 is a calculation device, and 11 is a CO ₂ analyzer.

Claims (1)

[Claims] 1. The inside of the heat treatment furnace in which the steel products are installed is sealed and the pressure is reduced using a vacuum pump, and then nitrogen gas is introduced into the furnace to restore the pressure to a constant level. After that, hydrocarbons and oxidizing gas are introduced, from the atmospheric gas
The amount of _CO2 and the amount of CO are analyzed, and based on the analysis value of the amount of CO, the temperature inside the furnace, and the target carbon concentration on the surface of the steel product,
Target CO ₂ corresponding to the equilibrium carbon concentration of the atmospheric gas
Calculating and setting the amount of CO ₂ and controllingly feeding hydrocarbon or a mixture of hydrocarbon and oxidizing gas as enrichment gas based on the deviation between the target amount of CO ₂ and the analysis value of the amount of CO 2 in the atmosphere gas. A closed gas carburizing method characterized by: