JP2000129418A - Vacuum carburizing method for steel parts and apparatus therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cost effective carburizing method for steel parts which is capable of rapidly executing vacuum carburizing enhanced in accuracy without the occurrence of unequal carburization and sooting and an apparatus therefor. SOLUTION: This vacuum carburizing method for the steel parts comprises supplying gaseous saturated chain hydrocarbon to a heating chamber 1 of a vacuum nonoxidizing atmosphere and subjecting the steel parts 2 to a carburizing treatment by heating these parts to 1000 to 1200 deg.C by a high-frequency heating means 3, then stopping the supply of the chain saturated hydrocarbon, discharging this gas and executing a diffusion treatment in the vacuum nonoxidizing atmosphere and further, repeating the carburizing treatment and the diffusion treatment at a suitable number of times.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method and an apparatus for carburizing steel parts, and more particularly to a method and an apparatus for carburizing steel parts under reduced pressure.

[0002]

2. Description of the Related Art Conventionally, as a method and an apparatus for carburizing steel parts under reduced pressure, an apparatus for heating an object to be processed in a heating chamber maintained under reduced pressure by a high-frequency heating means (Japanese Patent Laid-Open No. 59-259).
No. 74), a method of using a chain unsaturated hydrocarbon as a carburizing gas, and performing a carburizing treatment in a heating chamber under a vacuum of 1 kPa or less (Japanese Patent Laid-Open No. 8-325701). .

[0003]

As described above, in the method and apparatus for carburizing steel parts under reduced pressure, in view of the fact that conditions such as decompression and heating slightly affect the carburizing accuracy of steel parts, the present invention is based on carburizing. It is an object of the present invention to provide an economical carburizing method and apparatus capable of performing reduced-pressure carburization with high accuracy in a short time without causing unevenness and sooting.

[0004]

A first aspect of the present invention is to supply a chain saturated hydrocarbon gas to a heating chamber in a reduced-pressure, non-oxidizing atmosphere.
The steel part is heated to 1000 to 1200 ° C. by high-frequency heating means under a reduced pressure of 33 to 13.3 kPa to carry out carburizing treatment. Thereafter, the supply of the chain saturated hydrocarbon gas is stopped and discharged, and the reduced pressure non-oxidizing atmosphere is obtained. The diffusion process is performed below.

According to the first aspect, the inside of the heating chamber is from 1.33 to 1.33.
Putting under a reduced pressure of 13.3 kPa is 1.33 kPa
It is because it was confirmed that uneven carburization occurs below, and that carbon that does not directly participate in carburization becomes soot at 13.3 kPa or more, so-called sooting occurs.

The use of high-frequency heating means that only steel parts can be heated, so that a heat insulating material for the heating chamber is not required and the structure of the heating chamber can be simplified, which is economical. Furthermore, the heating temperature was raised above the normal carburizing temperature in order to shorten the carburizing time,
In addition, stopping and discharging the supply of the chain saturated hydrocarbon gas in the diffusion treatment after the carburizing treatment means that the remaining chain saturated hydrocarbon gas is immediately discharged after the carburizing treatment, thereby causing the generation of a soot. It is because it can suppress.

A second aspect of the present invention is characterized in that, after the carburizing and diffusing treatments of the first aspect, the carburizing and diffusing treatments are repeated an appropriate number of times. According to the second aspect, it is easy to adjust and secure the required carburizing depth,
Carburizing accuracy can be improved.

According to a third aspect of the present invention, the carburizing time during the carburizing process and the diffusing process that is repeated an appropriate number of times is set to 5 seconds to 3 minutes.

If the time is less than 5 seconds, the steel part is not sufficiently heated, and the supply of carbon to the steel part in one cycle becomes insufficient. If the carburizing cycle time is short,
Although the diffusion cycle time is also shortened, it is difficult to exhaust to the degree of vacuum required for the diffusion within the time.

[0010] In a fourth aspect, the output of the high-frequency heating according to the first, second or third aspect is set to 10 kHz or less. That is, it is for sufficiently heating the inside of the steel part in a short time, and if heating is performed with a high frequency output of 10 kHz or more, only the surface of the steel part is heated, and it takes time to heat the inside.

A fifth aspect of the present invention is directed to an apparatus for performing the reduced-pressure carburizing method, wherein a heating chamber having airtightness, a reduced-pressure exhaust device for converting the heating chamber to a non-oxidizing atmosphere, and a chain-like connection to the heating chamber are provided. A saturated hydrocarbon gas supply device, a high-frequency heating device for heating a steel component supplied into the heating chamber, a pressure control device for maintaining the pressure in the heating chamber at 1.33 to 13.3 kPa during carburizing, and a cooling chamber. And a configuration consisting of:

[0012]

An embodiment of the present invention will be described below. FIG. 1 is a schematic sectional view of a vacuum carburizing apparatus of the present invention. In the figure, 1 is a heating chamber having airtightness, 2
Is a steel part as an object to be processed, 3 is a high-frequency heating device, 4 is a vacuum evacuation device such as a pump, and 5 is a chain saturated hydrocarbon gas supply device to the heating chamber 1 (hereinafter simply referred to as a “gas supply device”). ), 6 is a control valve, 7 is a pressure control device in the heating chamber 1, 8 is a cooling chamber, 9 is an opening / closing door provided between the heating chamber 1 and the cooling chamber 8, 10 is a fan, and 11 is a fan. Fanmo
Reference numeral 12 denotes a transfer device.

Next, the vacuum carburizing method of the present invention is carried out as follows using the above-described vacuum carburizing apparatus of the present invention.

The steel part 2 is set in the heating chamber 1 and the evacuation device 4 is operated to move the inside of the heating chamber 1 to 1.33 × 10 ^−3.
The pressure is reduced to kPa, and the non-oxidizing atmosphere is maintained.

The high-frequency heating device 3 allows the steel part 2
Is rapidly heated to 1000-1200 ° C.

The pressure control device 7 is controlled to supply a chain saturated hydrocarbon gas (propane, methane, methane, etc.) from the gas supply device 5 so that the pressure in the heating chamber 1 becomes 1.33 to 13.3 kPa.
Butane gas-Hereinafter simply referred to as "gas". ) Is supplied.

After performing the carburizing process for a certain period of time, the supply of the gas is stopped and discharged, and the diffusion process is performed in a reduced-pressure, non-oxidizing atmosphere.

Further, if the required carbon concentration and carburizing depth are not obtained, the above and the above are alternately repeated based on the other initial treatment plan.

When it has been confirmed that the required carburizing depth of the steel part 2 has been obtained, the opening / closing door 9 is opened under reduced pressure, and the steel part 2 is moved to the cooling chamber 8 until the Ar ₃ transformation point is reached. Cooling is performed (referred to as primary cooling).

The steel part 2 is returned to the heating chamber 1 under reduced pressure, and is reheated to an austenitizing temperature or higher by high-frequency heating and is held for a predetermined time.

The steel part 2 is moved to the cooling chamber 1 under reduced pressure to perform quenching (see FIG. 2 described later).

[0022]

Example 1 JIS. The SCr420H was machined after forging and formed into a ring shape. The shape is outer diameter Φ190mm, inner diameter Φ140mm, thickness 25
mm, weighs 2.36 g, and has a surface area of 531 cm ² . Further, the test piece has a diameter of Φ5 mm and a diameter of Φ8 mm,
An unpenetrated hole with a depth of 15 mm was machined (hereinafter "steel part 2").
Say ).

The steel part 2 is set in the heating chamber 1 of the present invention, and the inside of the heating chamber 1 is depressurized to 0.67 × 10 ⁻³ kPa or less by operating the depressurizing and exhausting device 4. The steel part was rapidly heated to 1100 ° C. by high-frequency induction heating at 0.8 kHz and kept at a uniform temperature for 3 minutes.
At this time, the required time from room temperature to 1100 ° C. was 3 minutes.

Next, propane gas as a chain saturated hydrocarbon gas is instantaneously introduced until the pressure in the heating chamber 1 reaches 7.98 kHz, and then the pressure controller 7 controls the pressure so as to maintain the pressure. The decompression exhaust device 4 and the gas supply device 5 were controlled to perform carburizing treatment.

Since the carbon supply rate to the steel part by the reduced pressure carburizing at 1100 ° C. is very high, the carburizing treatment can be performed in a short time despite the large amount of solute carbon and the diffusion rate of austenite at 1100 ° C. The surface carbon content of the steel part 2 reaches the solid solubility limit, and then precipitates cementite, forms crystal grain boundaries, and forms alloy carbide in the grains.

The precipitation of the alloy carbide reduces the quenching property due to the reduction of the alloy component of the parent phase, especially Cr in case-hardened steel. Precipitation causes not only a decrease in surface hardness but also a decrease in material hardness.

In addition, cementite precipitated in the form of a network at the grain boundaries also causes a reduction in material strength. Further steel parts 2
When the surface carbon concentration reaches the solid solubility limit, soot tends to precipitate on the surface of the steel part 2 and inhibits the carburizing reaction.

In order to prevent the deterioration of the quality of the steel part 2 as described above, the carburizing treatment is performed by setting the carburizing time within a range in which the surface carbon content of the steel part 2 does not reach the austenite solid solubility limit. It is desirable to perform a diffusion process. For this reason, in the high-temperature reduced-pressure carburizing treatment, it is preferable to repeat the carburizing and diffusion field cycle an appropriate number of times in a short time. Table 1 below
2 shows a cycle of carburization and diffusion in the first embodiment.

[0029]

[Table 1] According to Table 1, the initial carburizing time is 120 seconds, but the subsequent carburizing time when the carburizing process and the diffusion process are repeated is 30 seconds, respectively. After the carburizing and diffusion treatments shown in Table 1 were completed, primary cooling, reheating, soaking, and quenching were performed in a non-oxidizing atmosphere in order to refine crystal grains coarsened during the high-temperature treatment. .

The primary cooling is performed from the heating chamber 1 which has been reduced to 0.67 × 10 ⁻³ kPa after the final cycle diffusion.
The steel part 2 is moved to the cooling chamber 8 evacuated and evacuated to 0.67 × 10 ⁻³ kPa, and the door 7 between the steel chamber 2 and the heating chamber 1 is closed.
1.01 × 10 ² kPa (atmospheric pressure) N ₂ gas is introduced, and the temperature of the steel part 2 is set to 500 to 600 using the fan 9.
It went until it reached ° C.

Next, the steel part 2 was reheated to 850 ° C.
After maintaining the soaking for minutes, quenching was performed. The quenching is performed in the same manner as in the primary cooling, except that the steel part 2 is transferred from the heating chamber 1 to the cooling chamber 8 in a non-oxidized state evacuated to 0.67 × 10 ⁻³ kPa.
After transferring, 7.98X10 the N ₂ gas in the cooling chamber 8 ² (6
ber) and gas quenching was performed. The results of the reduced pressure carburizing treatment are shown in Table 2 below.

[0032]

[Table 2] Table 2 shows the test pieces, that is, the carburized depths of the Φ5 mm hole and the Φ8 mm hole of the steel part 2 at the 15 mm bottom with respect to the carburized depth on the outer peripheral surface of the steel part 2. It is shown separately. That is, as the ratio is closer to 1, it means that there is no uneven carburizing even in the steel part 2 having a complicated shape.

As apparent from Table 2, 6.65 to 65
In the case of a carburizing treatment pressure of 13.3 kPa, it was confirmed that uniform carburizing was performed to the deep part of each hole, and no sooting was observed. Also, 1.33 to 6.65k
At the carburizing treatment pressure of Pa, slight carburizing unevenness was observed, but it was confirmed that the treatment was possible without generation of sooting.

When the carburizing treatment pressure is 13.3 kPa or more, the steel parts 2 and the inside of the heating chamber 1 are stinged, which causes not only inconvenience to the continuous treatment but also uneven carburizing. This is because the amount of soot deposited on the work surface becomes very large and inhibits the carburizing reaction.

[0035]

Embodiment 2 JIS. The processing was performed using a ring gear processed after forging the SCr420. The ring gear has an outer diameter of 218 mm, a module of 2.4, a tooth width of 33 mm, and a weight of 3.4 kg.

The ring gear was heated by high-frequency induction heating at 1 kHz. Other processing conditions are the same as in the first embodiment. FIG. 2 shows the entire process. In the figure, A is a primary heating step, B is a soaking step, C is a carburizing-diffusion treatment cycle, D is a primary cooling step, E is a reheating step, F is a soaking step, and G is N ₂ gas. It is quenching. Table 3 shows the carburizing-diffusion treatment cycle of C.

[Table 3] FIG. 3 shows the hardness measurement results of the tooth surface portion and the tooth bottom portion of the ring gear subjected to the processing, FIG. 4 shows the tooth surface portion, and FIG. 5 shows the photograph of the cross-sectional structure of the tooth bottom portion (X400).

As is apparent from FIG. 3, the hardness distribution of the tooth surface and the tooth bottom is almost the same, and the tissue and carburizing concentration of the tooth surface and the tooth bottom are also similar as is apparent from FIGS. It was almost the same, and no grain boundary oxide layer was observed at all.

[0038]

According to the method of the present invention, it is possible to carry out reduced-pressure carburization with high precision in a short time without causing uneven carburization and sooting, and it is economical. The effect that the method can be effectively implemented is obtained.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view showing one embodiment of a reduced-pressure carburizing apparatus of the present invention.

FIG. 2 is a processing time and temperature curve diagram of Example 2.

FIG. 3 is a hardness measurement curve diagram of a tooth surface portion and a tooth bottom portion of a ring gear according to a second embodiment.

FIG. 4 is a micrograph (X400) showing a metal structure of a tooth surface portion of a ring gear after the processing of Example 2.

FIG. 5 is a micrograph (X400) showing a metal structure of a tooth root portion of a ring gear after processing in Example 2.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Heating room 2 Steel part 3 High frequency heating device 4 Decompression exhaust device 5 Chain saturated hydrocarbon gas supply device 7 Pressure control device 8 Cooling device

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Hidehisa Torasawa 1-8-2 Marunouchi, Chiyoda-ku, Tokyo Dowa Mining Co., Ltd. (72) Inventor Jun Takahashi 1-2-2, Marunouchi, Chiyoda-ku, Tokyo Dowa Mining Co., Ltd. (72) Inventor Shinichi Takemoto 1-8-2 Marunouchi, Chiyoda-ku, Tokyo Dowa Mining Co., Ltd. (72) Inventor Itaka Nakahiro 1-8-2 Marunouchi, Chiyoda-ku, Tokyo No. Dowa Mining Co., Ltd. F-term (reference) 4K028 AA01 AC03

Claims (5)

[Claims] 1. A chain-like saturated hydrocarbon gas is supplied to a heating chamber in a reduced-pressure, non-oxidizing atmosphere, and a steel part is heated to 1000 to 1 by a high-frequency heating means under a reduced pressure of 1.33 to 13.3 kPa.
A reduced pressure carburizing method for a steel part, comprising heating to 200 ° C. to perform carburizing treatment, and then stopping and discharging the supply of the chain saturated hydrocarbon gas and performing diffusion treatment in a reduced-pressure, non-oxidizing atmosphere. 2. A chain-shaped saturated hydrocarbon gas is supplied to a heating chamber in a reduced-pressure, non-oxidizing atmosphere, and the steel part is heated to 1000 to 1 by a high-frequency heating means under a reduced pressure of 1.33 to 13.3 kPa.
Carburizing treatment is performed by heating to 200 ° C., and then the supply of the chain saturated hydrocarbon gas is stopped and discharged, and diffusion treatment is performed under a reduced-pressure, non-oxidizing atmosphere, and the carburizing treatment and the diffusion treatment are repeated an appropriate number of times. A reduced pressure carburizing method for a steel part. 3. The method according to claim 2, wherein the carburizing time at the time of repeating the carburizing treatment and the diffusion treatment is within 5 seconds to 3 minutes. 4. The reduced-pressure carburizing method according to claim 1, wherein the output of the high-frequency heating is 10 kHz or less. 5. A heating chamber having airtightness, a decompression exhaust device for converting the heating chamber into a non-oxidizing atmosphere, a device for supplying a chain saturated hydrocarbon gas to the heating chamber, and a steel part supplied into the heating chamber , A pressure control device for maintaining the pressure in the heating chamber at 1.33 to 13.3 kPa during carburizing, and a cooling chamber.