JP3445968B2 - Vacuum carburizing method for steel parts - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for vacuum carburizing steel parts.

[0002]

2. Description of the Related Art Conventionally, vacuum carburization has been performed as a surface treatment method for improving the wear resistance and other mechanical properties of the surface of steel materials. In this vacuum carburizing treatment, steel parts are heated in vacuum to a carburizing temperature of 870 to 1000 ° C., and when a soaking state is reached, a carburizing gas is supplied into the heating chamber and the ambient temperature is brought to the carburizing temperature. 10 to 7 while maintaining
By holding under a reduced pressure of 0 KPa for a predetermined time, carbon generated by thermal decomposition of carburizing gas penetrates and diffuses on the surface of the steel material part.

In the above-mentioned vacuum carburizing treatment, in order to form a carburized layer uniformly on the entire surface of the steel material part, it is necessary that the entire surface of the steel material part is uniformly and sufficiently exposed to the carburizing gas at the initial stage of carburization. However, propane, propylene, ethylene, etc., which are usually used as carburizing gases, have relatively high stability, so in order to thermally decompose them, the heating chamber must be maintained at a high temperature and a large amount must be supplied. Inevitably, not only the amount of carburizing gas consumed will increase, but also a large amount of soot will be generated, so that there will be a problem that maintenance and management of the facility will require a great deal of cost.

To solve these problems, Japanese Patent No. 29
Japanese Patent No. 63869 proposes a method of using an acetylene-based gas as a carburizing gas and vacuum carburizing steel parts under a reduced pressure of 1 KPa or less. Specifically, while the acetylene gas is continuously supplied during the carburizing period, the pressure in the furnace is 1 KPa or less, preferably 0.3 by continuously exhausting the gas.
The carburizing treatment is performed by maintaining a constant value of KPa or less, more preferably 0.1 KPa or less.

[0005]

However, as described above, in order to form a carburized layer uniformly on the entire surface of the steel material part in a short time, the entire surface of the steel material part should be carburized gas (acetylene) at the initial carburization. It is necessary to be uniformly and sufficiently exposed to the gas), but in the above method, the soot generation during the carburizing period is maintained because the furnace pressure during the carburizing period is maintained at a high degree of vacuum, which is a constant value of 1 KPa or less. Although it can be avoided, the uniformity of the carburized layer is not sufficient, and carburizing gas (acetylene)
There is a problem that the consumption amount of the In particular, this becomes remarkable when the amount of treatment is large and it is necessary to supply a large amount of carburizing gas.

Therefore, it is an object of the present invention to prevent the soot from adhering to the treated material and to uniformly form a carburized layer and reduce the consumption of acetylene gas.

[0007]

As a means for solving the above problems, the present invention provides a vacuum carburizing method for carburizing steel parts under reduced pressure, in which a predetermined amount of acetylene gas is continuously supplied during a carburizing period. On the other hand, the exhaust is started by starting the exhaust at the upper limit pressure setting value and stopping the exhaust at the lower limit pressure setting value, and gradually decreasing the continuous supply of acetylene gas in the carburizing period after a predetermined time after the carburizing start. The upper limit pressure setting value and the lower limit pressure setting value are set to 1.1 KPa to 3.5 KPa, and the upper limit pressure setting value is set to be 1.5 times or more the lower limit pressure setting value.

In an embodiment of the present invention, the upper limit pressure set value is 1.7 to 3.5 KPa, preferably 2.0.
Within the range of 3.0 KPa, and the lower limit pressure set value is 1.1 to 1.65 KPa, preferably 1.1 to 1.3.
It is set to a value within the range of KPa.

The upper limit pressure setting value and the lower limit pressure setting value in the carburizing period are set within the range of 1.1 KPa to 3.5 KPa for the following reason. That is, if the maximum value of the upper limit pressure set value in the carburizing period is 3.5 KPa or less, even if the acetylene gas is thermally decomposed in the heating chamber, it is possible to avoid soot from adhering to the surface of the treated material, but it exceeds 3.5 KPa. And the soot generated by the thermal decomposition of acetylene gas in the heating chamber surface
This is because it is difficult to avoid the adhesion to the. Moreover, the minimum value of the lower limit pressure set value is set to 1.1 KPa, and if it is less than 1.1 KPa, it is an important factor for obtaining a uniform carburized layer. Because you cannot do it.

Further, the upper limit pressure set value in the heating chamber during the carburizing period is 1.5 times or more the lower limit pressure set value, that is,
Set the ratio of the upper limit pressure setting value to the lower limit pressure setting value (upper limit pressure setting value / lower limit pressure setting value) during intermittent exhaust to 1.5.
The reason for making it more than twice is that the acetylene gas (fresh carburizing gas) continuously fed can be surely brought into contact with the surface of the treated material, and if it is less than 1.5 times , carburizing unevenness is likely to occur. Is.

In another embodiment of the present invention,
As a carburizing gas, hydrogen gas is supplied in addition to acetylene gas. That is, the thermal decomposition reaction of acetylene is represented by C2H2 = 2C ↓ + H2, but since acetylene is easily thermally decomposed at a lower temperature than propane,
With acetylene gas alone, it may be unavoidable that a large amount of soot is generated due to the control delay of the exhaust system. However, when hydrogen gas (H2) is supplied into the heating chamber together with acetylene gas, the partial pressure of hydrogen gas in the heating chamber increases, so that the reaction in the above reaction formula is difficult to proceed to the right. Therefore, the decomposition rate of acetylene becomes slow, and even if the pressure in the heating chamber is increased (that is, the residence time in the heating chamber is lengthened), the generation of soot due to the decomposition of acetylene is reduced and the adhesion to the treated material can be prevented. .

Therefore, from another point of view, the present invention is
In a vacuum carburizing method for carburizing steel parts under reduced pressure, during the carburizing period, a specified amount of acetylene gas and hydrogen gas are continuously supplied, while starting exhaust at the upper limit pressure setting value and lower limit pressure setting value. While performing the exhaust consisting of stopping the exhaust at, gradually reduce the continuous supply of acetylene gas and hydrogen gas in the carburizing period after a predetermined time after the carburization start, and set the upper limit pressure setting value and the lower limit pressure setting value. The present invention provides a vacuum carburizing method for steel parts in which the upper limit pressure set value is set to 1.1 KPa to 8.0 KPa and the upper limit pressure set value is set to 1.5 times or more the pressure lower limit set value.

In a preferred embodiment, the upper limit pressure set value is 2.0 to 8.0 KPa, preferably 2.5 to
The value within the range of 7.0 KPa, and the lower limit pressure set value is 1.1 to 4.0 KPa, preferably 1.1 to 2.0 KP.
It is set to a value within the range of a.

In a preferred embodiment, the amount of hydrogen gas is set to 1 to 3 times the amount of acetylene gas, usually about 2 times.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION The vacuum carburizing method for steel according to the present invention can be carried out in any form such as a batch type vacuum carburizing furnace or a continuous type vacuum carburizing furnace. Here, the two-chamber type vacuum carburizing shown in FIG. A case of carrying out using a furnace will be described as an example.

The illustrated two-chamber type vacuum carburizing furnace comprises a charging chamber 1 and a heating chamber 11. The charging chamber 1 has an oil quenching tank and nitrogen through a gas supply line 2 and a valve 3a. While being connected to the supply source 4, it is connected to a vacuum exhaust system and an exhaust gas treatment system via a first exhaust system line 5a and a first exhaust valve 6a, and a charging door 7 is arranged at one end side of the charging chamber 1. It is set up. On the other hand, the heating chamber 11 is connected to the other end side of the charging chamber 1 with an intermediate door 8 interposed, and the acetylene gas supply source 9, through the gas supply lines 2b, 2c and the valves 3b, 3c. In the hydrogen gas supply source 10,
It is connected to the vacuum exhaust system and the exhaust gas treatment system via the second exhaust system line 5b and the second exhaust valve 6b, respectively. A heating means is provided in the heating chamber 11, and
Similar to the charging chamber 1, a transfer means for transferring the material to be processed is provided, but these are omitted because they have the same structure as a known one.

When vacuum-carburizing a steel part to be treated using the two-chamber vacuum carburizing furnace, first, a tray loaded with the substance to be treated is loaded into the loading chamber 1, and the first and second trays are loaded. 2 charging chamber 1 and heating chamber 11 via exhaust line 5a, 5b
The inside of each is evacuated to a predetermined degree of vacuum (approximately 0.02K
After the pressure is reduced to Pa), the processing material loaded on the tray in the charging chamber 1 is placed in the heating chamber 11 under reduced pressure. Then, nitrogen gas is introduced into the heating chamber 11 from the nitrogen supply source 4 through the gas supply line 2b, and the furnace pressure is set to a predetermined pressure (8
The temperature is increased to 6.5 to 100 KPa) and the temperature is increased to a carburizing temperature (870 to 1000 ° C.) under a nitrogen atmosphere.

When the temperature rise is completed, the carburizing temperature is maintained for a predetermined time for soaking. At this time, about 2
Evacuated the heating chamber 11 at 0 minutes before (about 0.02K
Pa), and soaking is performed under vacuum. When soaking is completed,
Acetylene gas (carburizing gas) is continuously fed into the heating chamber 11 through the gas supply line 2b, and after carburizing for a predetermined time, the heating chamber 11 is vacuum-exhausted (about 0.02 KPa) for diffusion treatment. .

During this carburizing period, the acetylene gas is continuously fed in such a large amount that the amount of acetylene gas introduced at the beginning of carburization is larger than the amount of introduction at the latter stage of carburization. On the other hand, the exhaust of the heating chamber 11 is intermittent exhaust (pulse exhaust). That is, when the pressure in the heating chamber 10 reaches the upper limit pressure setting value (maximum setting value: 3.5 KPa), the second
While the exhaust valve 6b is opened, when the lower limit pressure set value (minimum set value: 1.1 KPa) is reached, the second exhaust valve 6b is closed (pulse operation). When this carburizing / diffusion process is completed, nitrogen gas (N2) is supplied at a predetermined pressure (86.5 to 100 KP).
a) and quenching temperature (about 810
˜900 ° C.). Then, quenching is performed in the charging chamber 1.

In the above-mentioned embodiment, only acetylene gas is used as the carburizing gas, but when a mixed gas of acetylene gas and hydrogen gas is used as the carburizing gas, the following operation is performed. That is, in the same manner as in the above-described embodiment, soaking treatment is performed under vacuum, and when soaking is completed, acetylene gas and hydrogen continuously feed gas (carburizing gas) into the heating chamber 11, After carburizing for a period of time, the inside of the heating chamber 11 is evacuated (about 0.02 KPa) for diffusion treatment. The hydrogen gas is about twice as much as the acetylene gas. During this carburizing period, when the pressure in the heating chamber 11 reaches the upper limit pressure setting value (maximum setting value: 8 KPa), the second exhaust valve 6b is opened, while the lower limit pressure setting value (minimum setting value: 1. 1KPa)
When it reaches, the pulse operation for closing the second exhaust valve 6b is performed.

[0021]

[Embodiment 1] A transmission gear (φ180 mm) made of SCr420H was used as a treatment material, and this was put on a tray for 120 times.
Sheets are loaded (total surface area 8.5 m ² ) and the target effective carburizing depth is 0.
Using a two-chamber type vacuum carburizing furnace having the following specifications shown in FIG. 4 as 6 mm, vacuum carburizing and quenching treatment was performed in the vacuum carburizing heat cycle shown in FIG. Furnace type: 2 chamber type (quenching: oil) Heating chamber volume: 7.8 m ³ (effective dimension: 1220 mm length x 760 mm width x 710 mm height) Vacuum pump: Oil rotary pump (450 m ³ / hour) Mechanical booster (250 m ³ / h) that, after placing a tray equipped with a large number of treatment material into the heating chamber 11, in a nitrogen atmosphere at 86.5～100KPa, the heating chamber temperature is raised to carburization temperature 950 ° C.,
Heat soaks. Approximately 20 minutes before the completion of soaking, the chamber is evacuated, and soaking is further performed under a vacuum of 0.02 KPa for about 20 minutes.
Do for a minute. After completion of the soaking treatment, acetylene gas is continuously supplied as a carburizing gas into the heating chamber 11 to perform carburizing treatment. During the carburization period, 40 L / min from the start of carburization of acetylene gas to 5 L, 15 L / min for 5 to 10 min, 10 to 10 L / min.
During 25 minutes, it is continuously supplied while gradually decreasing to 9 L / min. During that time, when the pressure in the heating chamber 11 reaches the upper limit pressure set value of 2.0 KPa, the second exhaust valve 6b is opened to perform vacuum exhaust. However, the pressure in the heating chamber 11 is 1.1 which is the lower limit pressure setting value.
When the pressure reaches KPa, the pressure in the heating chamber 11 is controlled by performing intermittent exhausting, which consists of closing the second exhaust valve 6b. Next, the inside of the heating chamber 11 is evacuated to a vacuum degree of 0.02 KPa, and heating is performed for about 1.3 hours while maintaining the same vacuum degree to perform a diffusion process. After the diffusion treatment, nitrogen gas is introduced into the heating chamber 11 to create a N2 atmosphere at approximately atmospheric pressure, the temperature is lowered to the quenching temperature of 850 ° C. in the same atmosphere, and the temperature is maintained for 1 hour. Then, nitrogen gas was introduced into the charging chamber to create a N2 atmosphere at approximately atmospheric pressure, the tray was moved to the charging chamber 1, and
Oil quench in oil at a temperature of 0 ° C.

When the treated material was observed after vacuum carburizing and quenching, the effective carburizing depth was 0.59 to 0.61 mm, and no soot adhesion or carburizing unevenness was observed on the surface, and good results were obtained. The carburizing time and the carbon concentration profile in the steel material in the vacuum carburizing treatment are as shown in FIG. 2, and the carbon infiltration ratio into the steel material per unit time (that is, to the total invading carbon amount after carburization) The ratio of the amount of invading carbon after carburizing for a certain period of time) is as shown in FIG. From the results shown in FIG. 3, the carburizing temperature was 9
Under the conditions of 50 ° C and an effective carburizing depth of 0.6 mm, 46% of the total invading carbon amount penetrates 5 minutes after the start of carburizing and 18% of the total invading carbon amount 5 minutes after 5 minutes. You can see that Therefore, as described above, the supply amount of the carburizing gas is increased in the early stage of carburization, and the supply amount of the carburizing gas is reduced corresponding to the decrease in the invading carbon amount.

[0023]

[Example 2] Pinion gear made of SCr420H (φ30m
m) is treated material, 5000 pieces are loaded on the tray (total surface area: 26 m ² ) and the target effective carburizing depth is 0.6 mm.
As in Example 1, vacuum carburizing and quenching treatment was performed under the following conditions. Vacuum carburizing and quenching conditions Carburizing temperature: 950 ° C Carburizing gas: Acetylene gas Gas supply rate: 0-5 minutes ・ ・ ・ 120 L / min 5-10 minutes ・ ・ ・ 35 L / min 10-25 minutes ・ ・ ・ 20 L / min Heating chamber pressure: Upper limit set value ・ ・ ・3.0KPa Lower limit pressure setting value… 1.5KPa Quenching temperature: 840 ℃ Quenching: Oil quenching (Temperature: 120 ℃) After vacuum carburizing and quenching, the treated material was observed and the effective carburizing depth was 0.56 to 0.61mm. No soot adhesion or carburizing unevenness was observed on the surface, and good results were obtained.

[0024]

[Third Embodiment] A pinion gear made of SCr420H (φ30 m
m) as treated material, 5000 pieces are loaded on the tray (total surface area: 26m2), and the target effective carburizing depth is 0.6mm.
As in Example 1, vacuum carburizing and quenching treatment was performed under the following conditions. Vacuum carburizing and quenching conditions Carburizing temperature: 950 ° C Carburizing gas: acetylene gas + hydrogen gas Gas supply rate: 0-5 minutes acetylene (100 L / min) + hydrogen (200 L / min) 5-10 minutes acetylene (35 L / min) + Hydrogen (70 L / min) 10-25 minutes acetylene (20 L / min) + Hydrogen (40 L / min) Heating chamber pressure: Upper limit pressure setting value 7.0 KPa Lower limit pressure setting value 1.5 KPa Quenching temperature: 840 ° C Quenching: Oil quenching (Temperature: 120 ° C) After vacuum carburizing and quenching, the treated material was observed and no soot adhesion or uneven carburizing was observed on the surface, and the effective carburizing depth was 0.5.
Good results of 9 to 0.62 mm were obtained.

[0025]

[Comparative Example 1] SCr420H pinion gear (φ30 mm)
Was used as a treated material, and 5000 pieces were loaded on the tray (total surface area: 26 m ² ), and the target effective carburizing depth was 0.6 mm, and vacuum carburizing and quenching treatment was performed under the following conditions in the same manner as in Example 1. Vacuum carburizing and quenching conditions Carburizing temperature: 950 ° C Carburizing gas: Acetylene gas Gas supply rate: 0-5 minutes ・ ・ ・ 120 L / min 5-10 minutes ・ ・ ・ 35 L / min 10-25 minutes ・ ・ ・ 20 L / min Heating chamber pressure: Upper limit pressure set value ・ ・ ・4.0KPa Lower limit pressure setting value 1.5KPa Quenching temperature: 840 ℃ Quenching: Oil quenching (Temperature: 120 ℃) After vacuum carburizing and quenching, when the treated material was observed, soot adhered to the surface of the treated material and the effective carburized depth The unevenness was 0.48 to 0.61 mm. This is probably because the upper limit pressure setting value in the heating chamber during the carburizing period is not appropriate.

[0026]

[Comparative Example 2] Pinion gear made of SCr420H (φ30m
m) is treated material, 5000 pieces are loaded on the tray (total surface area: 26 m ² ) and the target effective carburizing depth is 0.6 mm.
As in Example 1, vacuum carburizing and quenching treatment was performed under the following conditions. Vacuum carburizing and quenching conditions Carburizing temperature: 950 ° C Carburizing gas: acetylene gas + hydrogen gas Gas supply rate: 0-5 minutes acetylene (120 L / min) + hydrogen (80 L / min) 5-10 minutes acetylene (35 L / min) + Hydrogen (50 L / min) 10-25 minutes acetylene (20 L / min) + Hydrogen (30 L / min) Heating chamber pressure: Upper limit pressure setting value 10.0 KPa Lower limit pressure setting value 1.5 KPa Quenching temperature: 840 ° C Quenching: Oil quenching (Temperature: 120 ° C) After vacuum carburizing and quenching, the treated material was observed, soot was adhered to the surface of the treated material, and the effective carburized depth was 0.40 to 0.62 mm, which was not practical for practical use. Met. This is probably because the upper limit pressure setting value of the pressure in the heating chamber and the amount of hydrogen added during the carburizing period are not appropriate.

[0027]

As is apparent from the above description, the present invention continuously supplies a predetermined amount of acetylene gas during the carburizing period while starting exhaust at the upper limit pressure setting value and lower limit pressure setting value. Since exhausting (pulse exhausting) consisting of stopping the exhausting is performed, the acetylene gas (fresh carburizing gas) continuously fed can be surely brought into contact with the surface of the processing material. Also, the pressure in the heating chamber at the beginning of carburization is 1.1 KPa.
Therefore, the supply amount of acetylene gas at the initial stage of carburization, which is an important factor for obtaining a uniform carburized layer, can be reliably secured. Furthermore, since the continuous supply of acetylene gas in the carburizing period is gradually reduced after a predetermined time after the start of carburizing, the consumption of acetylene gas can be reduced without sending in excessive acetylene gas. Moreover, according to the present invention, by the synergistic effect of the above effects, it is possible to avoid soot from adhering to the surface of the treated material,
A uniform carburized layer can be formed. In addition, since acetylene gas and hydrogen gas are supplied during the carburizing period, soot generation is suppressed, soot adhesion to the surface of the treated material can be avoided, and a uniform carburized layer pressure can be formed. Can be increased, and the vacuum exhaust device can be downsized.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of a vacuum carburizing heat cycle in the method of the present invention.

[Fig. 2] Graph showing carburizing time and carbon concentration profile in steel

FIG. 3 is a graph showing a carbon infiltration ratio into a steel material per unit time.

FIG. 4 is a block diagram of a vacuum carburizing furnace used in the present invention.

[Explanation of symbols]

1 ... Charge chamber 2 ... Gas supply line 3 ... Valve 4 ... Nitrogen supply source 5 ... First exhaust system line 6a, 6b ... Exhaust valve 7 ... Charge door 8 ... Intermediate door 9 ... Acetylene gas supply source 10 ... Hydrogen gas Source 11 ... Heating chamber

Claims (2)

(57) [Claims] 1. A vacuum carburizing method for carburizing steel parts under reduced pressure, wherein a predetermined amount of acetylene gas is continuously supplied during the carburizing period, while exhausting is started at an upper limit pressure set value and a lower limit pressure set value is set. In addition to performing exhaustion, the continuous supply of acetylene gas during the carburizing period is gradually reduced after a predetermined time after the start of carburizing, and the upper limit pressure setting value and the lower limit pressure setting value are set to 1.1.
A vacuum carburizing method for steel parts, characterized in that the upper limit pressure setting value is set to KPa to 3.5 KPa and the upper limit pressure setting value is set to 1.5 times or more of the lower limit pressure setting value. 2. A vacuum carburizing method for carburizing steel parts under reduced pressure, wherein a predetermined amount of acetylene gas and hydrogen gas are continuously supplied during the carburizing period, while exhausting is started at an upper limit pressure setting value. While performing the exhaust consisting of stopping the exhaust at the lower limit pressure setting value, the continuous supply of acetylene gas and hydrogen gas in the carburizing period is gradually reduced after a predetermined time after the carburization start, and the upper limit pressure setting value and A vacuum carburizing method for steel parts, characterized in that the lower limit pressure set value is set to 1.1 KPa to 8.0 KPa, and the upper limit pressure set value is set to 1.5 times or more of the lower limit pressure set value.