JP2003119558A - Method for vacuum carburizing steel part - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce consumption of expensive unsaturated hydrocarbon gas, and miniaturize a vacuum exhaust system, along with forming a uniform carburized layer. SOLUTION: In a method for vacuum carburizing steel parts in a reduced pressure, this carburizing method is characterized by carburizing the steel parts in the unsaturated hydrocarbon gas with a first predetermined reduced pressure, and then carburizing them in the saturated hydrocarbon gas with a second predetermined reduced pressure, in the carburizing stage.

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 carburizing treatment has been carried out as one form of a surface treatment method for improving mechanical properties such as wear resistance of a steel material surface. In this vacuum carburizing process, steel parts are heated to a carburizing temperature of 870 to 1000 ° C. in a vacuum, and when a soaking state is reached, carburizing gas is supplied into the heating chamber and the heating chamber is maintained at the carburizing temperature. In this state, it is carried out by holding for a predetermined time under a predetermined reduced pressure (usually 10 to 70 KPa).

In this vacuum carburizing process, for example, as can be seen from FIG. 3 showing the carburizing time and the carbon concentration profile in the steel material in the vacuum carburizing process, 46% of the total carburizing amount is supplied within 5 minutes of the initial carburizing process. Therefore, in order to uniformly form the carburized layer on the entire surface of the steel part, it is important that the entire surface of the steel part is uniformly and sufficiently exposed to the carburizing gas at the initial carburization stage. Further, when a chain unsaturated hydrocarbon such as acetylene gas is used as a carburizing gas, the rate of thermal decomposition is high, so it is necessary to avoid soot generation in the heating chamber or the vacuum exhaust system. In order to prevent the generation of soot, Japanese Patent No. 2963869 proposes a method in which an acetylene-based gas is used as a carburizing gas and a steel part is vacuum carburized under a reduced pressure of 1 KPa or less. Specifically, the acetylene gas is continuously supplied during the entire carburizing period, while continuously exhausting the gas to carry out the carburizing treatment while maintaining the furnace pressure at 1 KPa or less.

[0004]

However, in this method, the pressure inside the furnace during the entire carburization period is maintained at 1 KPa or less and acetylene gas is introduced, so that the generation of soot can be avoided, but the treatment amount is small. If it is large, there is a problem that the uniformity of the carburized layer is not sufficient, the consumption of acetylene gas is large, and the vacuum exhaust system becomes large.

Therefore, it is an object of the present invention to form a uniform carburized layer, reduce consumption of expensive unsaturated hydrocarbon gas, and miniaturize a vacuum exhaust system.

[0006]

Means for Solving the Problems As a means for solving the above problems, the present invention provides a vacuum carburizing method for carburizing steel parts under reduced pressure, wherein an unsaturated hydrocarbon under a first predetermined reduced pressure during a carburizing period. The carburizing process is carried out under a second predetermined reduced pressure of a saturated hydrocarbon gas under a system gas.

Here, the term "first predetermined reduced pressure" means a degree of vacuum in which soot does not adhere to the surface of the treated material due to thermal decomposition of the unsaturated hydrocarbon type gas and the carbon supply amount can be reliably secured in the initial carburization stage. Specified by the type of unsaturated hydrocarbon gas used, and under the second predetermined reduced pressure, soot does not adhere to the surface of the treated material due to thermal decomposition of the saturated hydrocarbon gas, and the entire steel part is It refers to the degree of vacuum that can be exposed to carburizing gas, and is defined by the type of saturated hydrocarbon gas used.

In the carburizing process according to the present invention, the carburizing process is carried out by dividing the carburizing stage into two stages, an initial stage and a latter stage. As the unsaturated hydrocarbon type gas which is the carburizing gas used at the early stage of the carburizing period, Examples thereof include ethylene and acetylene, which can be used alone or in combination. The saturated hydrocarbon-based gas, which is a carburizing gas used in the latter part of the carburizing period, includes methane, ethane, propane, butane, etc. These can be used alone or in a mixture, but from the viewpoint of cost. Is 13A city gas (CH ₄ 88%, C
₂ H ₆ 6%, C ₃ H ₈ 3%, C ₄ H ₁₀ 3%) are preferably used.

In an embodiment of the present invention, 1.5-3.
Carburizing is carried out under acetylene gas of 5 KPa for 5 to 15 minutes, and subsequently, carburizing is carried out under 13A city gas of 5.0 to 8.0 KPa for 10 to 60 minutes.

The pressure in the early carburizing period is 1.5 KPa to 3.5 K
The reason for setting within the range of Pa is as follows. That is,
When the initial pressure setting value of the carburizing period is 3.5 KPa or less, it is possible to avoid the soot generated by the thermal decomposition of acetylene gas from adhering to the surface of the treated material, but if it exceeds 3.5 KPa, the acetylene gas in the heating chamber This is because it is difficult to avoid that soot is generated in large quantities due to thermal decomposition and soot adheres to the surface of the treated material. Further, if the pressure set value is less than 1.5 KPa, it becomes impossible to reliably secure the supply amount of acetylene gas at the initial carburization, which is an important factor for obtaining a uniform carburized layer.

The pressure in the latter half of the carburizing period is 5.0 to 8.
The reason for setting the range within 0 KPa is as follows. That is, in the carburizing treatment in the latter stage of the carburizing period, the decomposition rate of 13A city gas was significantly slower than that of acetylene gas, that is, 13C.
A The decomposition rate of methane, which is the main component of city gas, is about 1/1000 that of acetylene gas, which is difficult to decompose, and even if the pressure in the heating chamber is increased (that is, the residence time in the heating chamber is increased), Soot is not generated due to the decomposition of carburizing gas and soot can be prevented from adhering to the treated material, but at least 5.
This is because 0 KPa is required, and if it is less than 5.0 KPa, carburizing unevenness is likely to occur, and if it exceeds 8.0 KPa, soot accumulation is likely to occur.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The vacuum carburizing method for steel materials 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 batch type two-chamber type shown in FIG. A case where a vacuum carburizing furnace is used and acetylene gas is used as the carburizing gas in the early carburizing period and 13A city gas is used as the carburizing gas in the latter stage of the carburizing period will be described as an example.

The illustrated two-chamber type vacuum carburizing furnace comprises a charging / extracting chamber 1 and a heating chamber 9. The charging / extracting chamber 1 has a nitrogen supply source 4 through a gas supply line 2a and a valve 3a. While being connected to the first exhaust system line 5 and the first exhaust valve 6
Is connected to the vacuum exhaust system and the exhaust gas treatment system via. A charging / extracting door 7 is arranged at one end of the charging / extracting chamber 1, and a heating chamber 9 is connected to the other end thereof with an intermediate door 8 interposed. The heating chamber 9 includes a nitrogen supply source 4, an acetylene gas supply source 10 and a 13A city gas supply source 11 from a gas supply line 2b via valves 3b, 3c, 3d and gas supply lines 2, 2c, 2d.
Further, they are connected to the vacuum exhaust system and the exhaust gas treatment system via the second exhaust system line 12 and the second exhaust valve 13, respectively. It should be noted that the feeding / extracting chamber 1 is provided with an oil quenching tank therein, and the heating chamber 9 is provided with a heating means. A transfer means for transferring is provided, but these are omitted since they have the same structure as known ones.

When vacuum-carburizing a steel material to be treated using the two-chamber type vacuum carburizing furnace, first, a tray loaded with the material to be treated is loaded into the loading / extracting chamber 1, The charging / extracting chamber 1 and the heating chamber 9 are respectively evacuated through the exhaust system line 5 and the second exhaust system line 12 to reduce the pressure to a predetermined vacuum degree (about 0.02 KPa), and then the charging / combining process is also performed. The processing materials loaded on the trays in the extraction chamber 1 are heated under reduced pressure in the heating chamber 9
Located inside. Next, nitrogen gas is introduced into the heating chamber 9 from the nitrogen supply source 4 through the gas supply lines 2 and 2b,
The pressure inside the furnace is raised to a predetermined pressure (86.5 to 100 KPa), and the temperature inside the furnace is set to a carburizing temperature (870 to 870) under a nitrogen atmosphere.
The temperature is raised to 1000 ° C.

After the temperature rise is completed, the inside of the furnace is kept at the carburizing temperature for a predetermined time to carry out soaking. At this time, about 2
Evacuation of the heating chamber 9 at 0 minutes before (about 0.02 KP
a) Perform soaking under vacuum. After completion of soaking, acetylene gas is continuously supplied into the heating chamber 9 through the gas supply line 2c, and the initial carburizing treatment is performed for a predetermined time while maintaining the furnace pressure at 1.5 KPa to 3.5 KPa. Then, supply of acetylene gas is stopped, 13A city gas is continuously supplied into the heating chamber 9 through the gas supply line 2d, and carburization is performed for a predetermined time while maintaining the furnace pressure at 5.0 KPa to 8.0 KPa. Perform processing. After that, the supply of the 13A city gas was stopped, and the inside of the heating chamber 9 was evacuated (about 0.02 KPa).
Diffusion treatment is performed at the same temperature. When this carburizing / diffusion treatment is completed, nitrogen gas (N2) is supplied to the heating chamber 9 and the charging / extracting chamber 1 up to a predetermined pressure (86.5 to 100 KPa), and the quenching temperature (about 810 ~ 900 ° C)
Hold the temperature down. Then, the tray is moved to the charging / extracting chamber 1 and immersed in the oil quenching tank for quenching.

[0016]

[Example 1] Flat plate made of S15CK (50 mm x 500 mm x 3
mm) 80 sheets and SCM420 bag-made round bar (φ38 mm x
70 mm) as treatment material (total area 4.3 m ² ), these are loaded on the tray, the target effective carburizing depth is 0.6 mm, and the two-chamber vacuum carburizing furnace with the following specifications shown in Fig. 2 is used. Vacuum carburizing and quenching treatment was performed by the vacuum carburizing heat cycle shown in FIG. Furnace type: Two-chamber type (quenching: oil) Heating chamber volume: 7.8 m ³ (effective dimension: 1220 mm length x
760mm width x 710mm height) Vacuum pump: Oil rotary pump (450m ³ / hour) Mechanical booster (250m ³ / hour)

That is, after the tray on which the treatment material is mounted is placed in the heating chamber 9, the temperature inside the heating chamber 9 is raised to a carburizing temperature of 930 ° C. in a nitrogen atmosphere of 86.5 to 100 KPa, and 60 Heat soak for a minute. Approximately 20 minutes before the completion of soaking, vacuum exhaust is performed to 0.02 KPa (control value), and soaking is performed under vacuum. After completion of soaking, carburizing gas is supplied into the heating chamber 9 to perform carburizing treatment at a carburizing temperature of 930 ° C. for 25 minutes. During the initial carburizing period of 10 minutes, acetylene gas is continuously supplied at 12 L / min as carburizing gas, Carburizing treatment is performed while maintaining the pressure in the heating chamber 9 at 1.5 KPa (control value), and after 10 minutes, supply of carburizing gas is stopped, and then 13A city gas is supplied as carburizing gas at a rate of 55 L / min. And the pressure in the heating chamber 9 is maintained at 6.0 KPa (control value), and the carburizing treatment is continued for 15 minutes. Next, the inside of the heating chamber 9 is evacuated to 0.02 KPa (control value).
After the vacuum degree is set to 1, the diffusion process is performed at 930 ° C. for 42 minutes while maintaining the same vacuum degree. After the diffusion treatment, nitrogen gas is introduced into the heating chamber 9 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. After the diffusion treatment, the tray is moved to the inside of the charging / extracting chamber 1 in which nitrogen gas is introduced and the atmosphere is N2 atmosphere, and the treated material is put into oil at a temperature of 120 ° C. for oil quenching.

The effective carburized depth of the obtained treated material is 0.55.
A substantially uniform carburized layer was formed up to the inside of the bag hole at a depth of up to 0.60 mm, and soot was not attached to the surface of the treated material and soot was not accumulated in the trap portion, and good results were obtained.

[0019]

[Example 2] Flat plate made of S15CK (50 mm x 500 mm x 3
mm) 80 sheets and SCM420 bag-made round bar (φ38 mm x
70 mm) as treatment material (total area 4.3 m ² ), these are loaded on a tray, and the target effective carburizing depth is 0.9 mm, and vacuum carburizing and quenching treatment is performed under the following conditions in the same manner as in Example 1. went. In this example, the treatment in the latter stage of the carburizing period was divided into two stages and the carburizing gas supply amount was changed to two stages.

Carburizing treatment: Carburizing temperature: 930 ° C. Carburizing time: 60 minutes Carburizing gas: Early carburizing period: Acetylene gas late carburizing period: 13A City gas gas supply: Early carburizing period: 12 L / min x 10 minutes Carburizing period late A: 55 L / min x 15 min late carburizing period B: 40 L / min x 35 min Pressure inside heating chamber 9: early carburizing period: 1.5 KPa (control value) late carburizing period A: 6.5 KPa (control value) late carburizing period B : 6.5KPa (control value) Diffusion treatment: Diffusion temperature: 930 ° C Diffusion time: 150 minutes Heating chamber pressure: 0.02KPa (control value) Quenching treatment: Quenching temperature: 840 ° C Quenching liquid: Oil (temperature: 120 ° C) The obtained carburized material has an effective carburizing depth of 0.85 to 0.93 mm and a carburized layer is formed almost evenly up to the inside of the bag hole. Soot does not adhere to the surface of the treated material or soot is trapped in the trap part. No good results were obtained It was.

[0021]

[Embodiment 3] A hypothetical gear made of SCM420 was used as a treating material, and 30 pieces of this were loaded on the tray (total surface area: 4 m ² ).
Then, vacuum carburizing and quenching treatment was carried out in the same manner as in Example 1 under the following conditions with a target effective carburizing depth of 0.8 mm.

Carburizing treatment: Carburizing temperature: 950 ° C Carburizing time: 22 minutes Carburizing gas: Early carburizing period: Acetylene gas late carburizing period: 13A City gas supply: Carburizing period early: 12L / min × 10 minutes Carburizing period late: 50L / Min × 22 minutes Heating chamber pressure: Early carburizing period: 1.5 KPa (control value) Late carburizing period: 6.5 KPa (control value) Diffusion treatment: Diffusion temperature: 950 ° C Diffusion time: 92 minutes Heating chamber pressure: 0.1 02KPa (Control value) Quenching treatment: Quenching temperature: 850 ° C Quenching liquid: Oil (Temperature: 100 ° C) The obtained treated material has an effective carburizing depth of 0.76 to 0.84 mm and a uniform carburized layer is formed. Therefore, no soot was attached to the surface of the treated material and soot was not accumulated in the trap portion, and good results were obtained.

[0023]

As is apparent from the above description, the present invention is characterized in that during the initial carburizing period, unsaturated hydrocarbon gas (for example,
Acetylene gas), in the latter carburizing period, the carburizing process is performed with saturated hydrocarbon gas (for example, 13A city gas).
Soot can be prevented from adhering to the surface of the treated material and a uniform carburized layer can be formed. Furthermore, the consumption of expensive unsaturated hydrocarbon-based gas such as acetylene gas can be reduced and It can also 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 is a block diagram of a vacuum carburizing furnace used in the present invention.

FIG. 3 is a graph showing carburizing time and carbon concentration profile in steel

[Explanation of symbols]

1 ... Charge and extraction room 2 ... Gas supply line 3 ... Valve 4 ... Nitrogen supply source 5 ... First exhaust system line 6 ... First exhaust valve 7 ... Charging door 8 ... Middle door 9 ... Heating room 10 ... Acetylene gas supply source 11 ... City gas supply source 12 ... Second exhaust system line 13 ... Second exhaust valve

Claims (2)

[Claims] 1. A vacuum carburizing method for carburizing steel parts under reduced pressure, wherein during a carburizing period, a first predetermined reduced pressure of an unsaturated hydrocarbon-based gas is continuously followed by a second predetermined reduced pressure of a saturated hydrocarbon-based gas. A method for vacuum carburizing steel parts, characterized in that 2. The vacuum of a steel part according to claim 1, wherein the carburizing treatment is performed under an acetylene gas of 1.5 to 3.5 KPa and subsequently under a 13A city gas of 5.0 to 8.0 KPa. Carburizing method.