JP2008260994A - Manufacturing method of carburized products - Google Patents

Abstract

The present invention provides a carburized product manufacturing method capable of sufficiently reducing the oxygen concentration in a furnace without evacuating and obtaining a carburized product having a uniform carburized layer.
First, a mixed gas of hydrogen and nitrogen is introduced into a chamber in which a work W is stored to perform atmosphere replacement. This reduces residual oxygen. After the dew point is lowered to −50 ° C. or lower (more preferably −55 ° C. or lower), carburization is started by supplying a mixed gas of acetylene and nitrogen. As a result, the residual oxygen in the atmosphere is extremely reduced, and carburization is performed with almost no oxide film on the surface of the workpiece W. For this reason, carburization is not hindered by the oxide film. Thus, a carburized product having a good carburized layer without unevenness is produced without vacuuming.
[Selection] Figure 1

Description

  The present invention relates to a method for manufacturing a carburized product in which a steel target product is carburized with a carburizing gas. More specifically, the present invention relates to a method of manufacturing a carburized product that enables uniform carburization without uneven carburization at normal pressure.

In carburizing using hydrocarbons as a carbon source, activated carbon generated by decomposition of hydrocarbons at high temperatures is adsorbed on the surface of the steel and penetrates into the interior. Such a carburizing method is called a direct carburizing method in contrast to a method of carburizing via a state such as carbon monoxide. In the direct carburizing method, oxygen remaining in the furnace atmosphere may cause defects such as uneven carburizing depth. This is because the presence of oxygen causes carburization in the presence of an oxide film on the steel surface. The unevenness occurs because the oxide film thickness is not uniform. Therefore, for example, in the technique of Patent Document 1, the inside of the furnace is evacuated prior to carburizing. A gas for carburizing treatment is introduced after reducing the amount of oxygen remaining in the furnace by evacuation. This is intended to prevent the above carburization failure.
JP 2005-120404 A

  However, the conventional techniques described above have the following problems. That is, since vacuuming is performed prior to carburizing, a large vacuum exhaust device is required. For this reason, it is inevitable to increase the scale of facilities. It is also conceivable to replace the furnace atmosphere by introducing an inert gas such as nitrogen at atmospheric pressure instead of evacuating. However, this does not sufficiently remove oxygen, and eventually carburization unevenness cannot be prevented.

  The present invention has been made in order to solve the problems of the conventional carburizing technique described above. That is, the object is to provide a method for manufacturing a carburized product that can sufficiently reduce the oxygen concentration in the furnace without evacuating and obtain a carburized product having a uniform carburized layer.

In order to solve this problem, in the present invention, a dew point of the atmosphere around the target product is introduced by introducing a mixed gas of non-carburizing reducing gas and inert gas into the room containing the target product made of steel. Is reduced to -50 ° C or lower, more preferably to -55 ° C or lower. Alternatively, by introducing a mixed gas, the oxygen concentration of the atmosphere is set to 7.2 × 10 ⁻²⁰ volume% or less, more preferably 2.6 × 10 ⁻²⁰ volume% or less. After that, carburization is performed with the atmosphere around the target product as an atmosphere containing hydrocarbons. Carburized products are manufactured by such a gas-phase carburizing method.

  For this reason, in the carburized product manufacturing method of the present invention, the air that was originally present is pushed out by introducing the mixed gas into the room. Also, oxygen in the air is removed by reacting with the non-carburizing reducing gas. This significantly reduces the residual oxygen concentration in the atmosphere without evacuation. At the same time, the oxide film on the surface of the object is clearly removed. Since carburizing is performed in this state, a carburized product having a uniform and uniform carburized layer can be obtained.

  ADVANTAGE OF THE INVENTION According to this invention, the manufacturing method of the carburized product in which the oxygen concentration in a furnace is fully reduced without evacuating and the carburized product which has a non-uniform | heterogenous carburized layer is obtained is provided.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the best mode for embodying the present invention will be described in detail with reference to the accompanying drawings. In this embodiment, carburized products are manufactured by carburizing formed steel products by the following procedure. 1. The target product is stored in the furnace.
2. A mixed gas of hydrogen and nitrogen is introduced into the furnace, and the atmosphere in the furnace is replaced by this mixed gas. This atmosphere replacement is performed until the dew point of the furnace atmosphere falls below a predetermined value set to -50 ° C or lower.
3. The product is heated to a predetermined carburizing temperature (about 950 ° C.).
4). Carburize by introducing a mixed gas of hydrocarbon and nitrogen into the furnace.

  The carburized product manufacturing method of this embodiment can be carried out in either the heating furnace with a preparation chamber in FIG. 1 or the heating furnace without a preparation chamber in FIG. The heating furnace with a preparation chamber in FIG. 1 has a preparation chamber (also called a purge chamber) 1 and a processing chamber 2. An outer door 3 is provided in front of the preparation chamber 1, and an intermediate door 4 is provided between the preparation chamber 1 and the processing chamber 2. The preparation chamber 1 is provided with a first gas introduction port 5, an exhaust port 6, and a dew point meter 9. The processing chamber 2 is also provided with a second gas introduction port 7 and an exhaust port 8. The second gas introduction port 7 is arranged at a position slightly inside the middle door 4 (right side in the figure). The processing chamber 2 has a heating function. Of course, the processing chamber 2 is provided with a processed product outlet, but this is not shown because it is not related to the features of the present invention.

  The heating furnace without a preparation chamber in FIG. 2 has a processing chamber 12 and a door 13. The processing chamber 12 is provided with a first gas introduction port 15, a second gas introduction port 17, an exhaust port 16, and a dew point meter 19. The processing chamber 12 has a heating function.

  1 and 2, first gas inlets 5 and 15 are inlets for supplying a mixed gas of hydrogen and nitrogen into the room. The second gas inlets 7 and 17 are inlets for supplying a mixed gas of acetylene and nitrogen, which are unsaturated hydrocarbons, into the room. These gas inlets are appropriately provided with an on-off valve, a flow rate adjusting valve, and the like. In addition, any gas introduction port can introduce only nitrogen gas. Any gas introduced is a dry gas that does not contain water vapor.

  The exhaust ports 6, 8, 16 are appropriately provided with on-off valves, check valves, and the like. The exhaust ports 6, 8, and 16 do not have a vacuum exhaust function, but allow gas to flow out in the room when gas is introduced from the gas inlet and the pressure in the room rises. This prevents the indoor pressure from fluctuating from atmospheric pressure.

  In the processing chamber 2 in the heating furnace with a preparation chamber shown in FIG. 1, a large number of trays on which the workpieces W are placed are usually arranged and an automatic transfer device is provided. That is, a heating furnace with a preparation room is often used for continuous processing. In that case, the processing chamber 2 is longer than that shown in FIG. 1, and a plurality of second gas introduction ports 7 are provided along the longitudinal direction thereof. On the other hand, the heating furnace without a preparation chamber in FIG. 2 is used for batch processing in which the workpieces W are replaced each time. In either case, oil quenching and other post-treatment facilities can be provided.

  The detailed procedure is described below. In this procedure, each door is closed unless otherwise specified. In addition, gas shall not be introduced from each gas inlet unless otherwise specified. First, the procedure in the case of using the heating furnace with the preparation chamber of FIG. 1 will be described. When this heating furnace with a preparation chamber is used, the dew point is maintained in a state of a predetermined value or less in the processing chamber 2 in advance.

[1. Product insertion]
The outer door 3 is opened, the workpiece W is inserted into the preparation chamber 1, and the outer door 3 is closed.

[2. Replacement of furnace atmosphere]
When the outer door 3 is closed, a mixed gas of hydrogen and nitrogen is introduced into the preparation chamber 1 from the first gas inlet 5. The hydrogen concentration of the gas mixture to be introduced must be low enough not to explode due to mixing with acetylene. This is because the indoor gas in the preparation chamber 1 and the indoor gas in the processing chamber 2 come into contact when the middle door 4 is opened later. By this introduction, the atmosphere in the preparation chamber 1 is gradually discharged through the exhaust port 6.

  As a result, oxygen and water vapor originally contained in the atmosphere in the preparation chamber 1 are exhausted from the preparation chamber 1. For this reason, the oxygen concentration and humidity decrease. Due to the decrease in humidity, water molecules adsorbed on the surface of the workpiece W, the wall surface of the furnace, etc. are liberated, but they are also discharged. Further, the oxide film on the surface of the work W is reduced by hydrogen to generate moisture, which is also discharged. As a result, both oxygen concentration and humidity decrease. A decrease in humidity means a decrease in dew point. For this reason, by observing the dew point measured by the dew point meter 9, the degree of decrease in residual oxygen in the preparation chamber 1 can be monitored. The decrease in residual oxygen corresponds to the decrease in the oxide film on the surface of the workpiece W.

  For this reason, in this embodiment, the atmosphere in the preparation chamber 1 is continuously replaced with the hydrogen-nitrogen mixed gas until the dew point becomes equal to or lower than a predetermined value set to -50 ° C or lower. If the dew point drops to this level, the residual oxygen concentration is 0.0062% by volume or less. In this state, the surface of the workpiece W no longer has an oxide film that inhibits carburization. If the dew point is lowered to −55 ° C. or lower, the residual oxygen concentration is 0.037% by volume or lower, which is even better.

[3. heating]
When the dew point falls below a predetermined value, the supply of the mixed gas from the first gas inlet 5 is stopped. Then, the inner door 4 is opened, and the workpiece W is moved from the preparation chamber 1 to the processing chamber 2. Then, the inner door 4 is closed. Further, the interior of the processing chamber 2 is heated to raise the temperature to about 950.degree. Thereby, the workpiece | work W is also heated to about 950 degreeC. Note that the preparatory chamber 1 may have some preheating function.

[4. Carburizing]
Then, a mixed gas of acetylene and nitrogen is introduced into the processing chamber 2 from the second gas introduction port 7. Then, the workpiece W is carburized. At this time, the oxide film is almost removed from the surface of the workpiece W due to the above-described dew point reduction. For this reason, adsorption of carbon atoms to the surface of the workpiece W and further penetration are hardly hindered. By maintaining this state (for example, about 10 minutes), sufficient carbon is allowed to enter the workpiece W, and then the temperature is kept as it is for diffusion of the intruded carbon (for example, about 5 minutes). In this holding, the supply of acetylene may be cut off to make the atmosphere only nitrogen. In this way, a carburized product having a carburized layer formed almost uniformly on the surface is manufactured.

  When continuous processing is performed using the automatic transfer function, the inside of the processing chamber 2 is heated to the carburizing temperature in advance. Then, a mixed gas of acetylene and nitrogen is allowed to flow from the second gas inlet 7 close to the inner door 4. Further, only nitrogen is allowed to flow from the second gas introduction port 7 far from the inner door 4. In this way, the workpiece W automatically conveyed in the processing chamber 2 is first placed in a mixed atmosphere of acetylene and nitrogen, and then placed in an atmosphere containing only nitrogen. As a result, diffusion continues after carburizing. In addition, when one workpiece W is sent into the processing chamber 2, the preparation chamber 1 starts a dew point lowering process for the next workpiece W. Thus, the carburizing process can be continuously performed on a large number of workpieces W.

  Next, the procedure in the case of using the heating furnace without the preparation chamber of FIG. 2 will be described. Even in this case, the basic idea is the same.

[1. Product insertion]
The door 13 is opened, the workpiece W is inserted into the processing chamber 12, and the door 13 is closed.

[2. Replacement of furnace atmosphere]
When the door 13 is closed, a mixed gas of hydrogen and nitrogen is introduced into the processing chamber 12 from the first gas inlet 15. As a result, the dew point in the processing chamber 12 decreases in the same manner as described in the description of the heating furnace with a preparation chamber. That is, residual oxygen decreases.

[3. heating]
When the oxygen concentration in the processing chamber 12 decreases, the temperature increase in the processing chamber 12 is started. As a result, the workpiece W is also heated.

[4. Carburizing]
When the dew point falls below a predetermined value and the workpiece W reaches the target temperature, the introduction of the mixed gas of hydrogen and nitrogen from the first gas inlet 15 is stopped, and instead of the acetylene and nitrogen from the second gas inlet 17 Introduce mixed gas. When the mixed gas of acetylene and nitrogen is sufficiently supplied, turn off acetylene for diffusion and supply only nitrogen. Thereby, the workpiece W is carburized. In this way, a carburized product having a carburized layer formed almost uniformly on the surface is manufactured.

  Examples will be described below. In this example, a cylindrical test piece having a diameter of 18 mm and an axial length of 50 mm was used as the workpiece W. As a material of the test piece, JIS-SCr420H which is a chromium-containing case-hardened steel was used. As the heating furnace, the one without the preparation room shown in FIG. 2 was used. Of course, the same test can be performed using the heating furnace with a preparation chamber shown in FIG.

Replacement of the furnace atmosphere of “2.” was performed under the following conditions.
Supply gas composition: 97% by volume of nitrogen + 3% by volume of hydrogen
Target dew point: 5 levels shown in Table 1 (of which 2 levels are comparative examples)
Here, the reason why hydrogen is 3% by volume is to prevent explosion due to mixing with acetylene as described above. In other words, the hydrogen explosion limit is 3.5% by volume under the mixed gas component during carburization described later. For this reason, a certain margin is provided on the safe side. The “oxygen concentration” in Table 1 is a value calculated by the following procedure for the concentration of residual oxygen when the dew point of the atmosphere decreases to the target value.

First, the free energy ΔG of the atmosphere is
ΔG = −57250 + 4.48TlogT−2.21T
It is represented by Here, T is the dew point. Also,
H ₂ + (1/2) O ₂ = H ₂ O
The equilibrium constant k of the system is obtained by using the partial pressure P (**) of each component,
k = P (H ₂ O) / P (H ₂ ) / P (O ₂ ) ^1/2
It is represented by In addition, between free energy ΔG, dew point T, and equilibrium constant k,
logk = -ΔG / RT
There is a relationship. Here, R is a gas constant. Thus, the partial pressure of oxygen can be obtained by obtaining the free energy ΔG from the dew point T and further obtaining the equilibrium constant k. The oxygen concentration is determined by taking the ratio to the total pressure.
Source: O-Kubaschewski and Ell Evans: Metallurgical Thermochemistry, Pergamon Press Ltd, London (1956), p331

Carburization (including diffusion) of “4.” was performed under the following conditions.
-Carburizing furnace temperature: 950 ° C
Composition of supply gas: 99% by volume of nitrogen + 1% by volume of acetylene
Holding time: 600 seconds, diffusion furnace temperature: 950 ° C
Supply gas composition: nitrogen only Retention time: 300 seconds

  With respect to the five-level specimens thus obtained, the carburization depth after the treatment was completed was measured by observing the cross-sectional structure with a microscope. The measurement location was a central position in the length direction on the side surface of the cylinder, and four locations were measured at equal intervals in the circumferential direction. The measurement results are shown in Table 2. “0 °”, “90 °”, “180 °”, and “270 °” in Table 2 indicate four measurement points in the circumferential direction. “Average”, “MAX”, and “MIN” indicate an average value, a maximum value, and a minimum value of four measured values for each level.

  The results of Table 2 are shown in the graph of FIG. In the graph of FIG. 3, “level” in Tables 1 and 2 is indicated by numbers with parentheses. This shows the following. In levels 1 and 2 which are examples, the carburization depth is about 0.18 mm on average, and a carburization depth of about 0.12 mm is secured at the minimum. Thus, very good carburization results were obtained for levels 1 and 2. Therefore, “judgment” in Table 2 is a double circle. In Example Level 3, compared with Levels 1 and 2, the lowest value is slightly inferior, but the average value is not much different. There is no occurrence of uncarburized areas. Thus, level 3 gave good results. Therefore, “judgment” in Table 2 is defined as a single circle. On the other hand, at levels 4 and 5 as comparative examples, the lowest value was “0”. That is, the occurrence of an uncarburized region was observed. In particular, at level 5, it was hardly carburized as a whole.

  Of course, the reason why such a remarkable difference appears between the example and the comparative example is the low dew point at the end of the atmosphere replacement, in other words, the difference in the residual oxygen concentration. At levels 4 and 5 which are comparative examples, the atmosphere replacement is completed before the dew point is lowered so much. For this reason, the carburizing process was started with a large amount of oxide film remaining on the surface of the test piece. This resulted in poor carburization.

On the other hand, in levels 1 to 3 as examples, atmosphere substitution was performed until the dew point was below -50 ° C. For this reason, it was used for the carburizing process with the residual oxygen concentration falling below 7.2 × 10 ⁻²⁰ vol%. For this reason, the carburizing process was started with the oxide film on the surface of the test piece almost removed. As a result, good carburization results were obtained.

In particular, at levels 1 and 2, atmosphere substitution was performed until the dew point was below -55 ° C. For this reason, it was used for the carburizing process in a state where the residual oxygen concentration was less than 2.6 × 10 ⁻²⁰ vol%. For this reason, the carburizing process was started in a state where the oxide film on the surface of the test piece was almost removed. This resulted in particularly good carburizing results.

  As described in detail above, in the present embodiment and this example, the carburizing process is performed after the residual oxygen is sufficiently reduced by supplying a mixed gas of hydrogen and nitrogen into the furnace. As a result, a good carburized product can be obtained by removing the oxide film on the surface of the workpiece while maintaining the atmospheric pressure without evacuation.

  Note that this embodiment is merely an example, and does not limit the present invention. Therefore, the present invention can naturally be improved and modified in various ways without departing from the gist thereof.

  For example, ammonia can be used in place of hydrogen as the non-carburizing reducing gas used during atmosphere replacement. Further, the hydrocarbon used in the carburizing process is not limited to acetylene, but may be ethylene or propane. As the inert gas, a rare gas such as argon can be used in addition to nitrogen. The steel material of the target material may be anything as long as it is generally subjected to carburizing treatment such as chrome steel and chromoly steel in addition to ordinary steel. The treatment temperature may be determined according to the steel type and other conditions. Moreover, it can also implement in the state which stopped the vacuum exhaust function using the processing furnace which has a vacuum exhaust function.

  The present invention can be used, for example, for carburizing treatment after forming various parts as follows. That is, piston pins and cam gears in internal combustion engines, various gears in vehicle drive transmission systems, constant velocity joints, and the like.

It is a schematic diagram which shows the heating furnace with a preparation chamber used with this form. It is a schematic diagram which shows the heating furnace without the preparation room used with this form. It is a graph which shows the carburizing depth of the test piece which concerns on an Example and a comparative example.

Explanation of symbols

1 Preparation chamber 2, 12 Processing chamber 5, 15 First gas inlet (hydrogen, nitrogen)
7, 17 Second gas inlet (acetylene, nitrogen)
044 Dew point meter

Claims (4)

In a method for manufacturing a carburized product in which a steel target product is carburized in the gas phase,
By introducing a mixture of non-carburizing reducing gas and inert gas into the room containing the target product, the dew point of the atmosphere around the target product is reduced to -50 ° C or lower,
Then, carburizing product manufacturing method characterized by performing carburization by making the circumference of an object article into the atmosphere containing hydrocarbons. The method of manufacturing a carburized product according to claim 1,
A decarburization temperature of −55 ° C. or lower when the dew point is lowered. In a method for manufacturing a carburized product in which a steel target product is carburized in the gas phase,
By introducing a mixture of non-carburizing reducing gas and inert gas into the room containing the target product, the oxygen concentration in the atmosphere around the target product is reduced to 7.2 x 10 ^-20 vol% or less. Make a decline,
Then, carburizing product manufacturing method characterized by performing carburization by making the circumference of an object article into the atmosphere containing hydrocarbons. In the manufacturing method of the carburized product of Claim 3,
A method for producing a carburized product, wherein the oxygen concentration is adjusted to 2.6 × 10 ⁻²⁰ vol% or less when the oxygen concentration is lowered.

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