JPH08325701A - Vacuum carburization method and device and carburized product - Google Patents

Abstract

PURPOSE: To make it possible to uniformly carburize each part of a work including the inside wall surfaces of a deep recessed part and to save the amt. of gas and heat quantity to be used by suppressing the generation of soot in a vacuum carburization method. CONSTITUTION: The work M is vacuum heated in a heating chamber 2 of a vacuum carburization furnace 1 and gaseous acetylene is supplied from a carburizing gas source C into the heating chamber 1. The work is subjected to a carburization treatment by maintaining a vacuum state of <=1Pa in the heating chamber 2 by a vacuum evacuation source V.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a vacuum carburizing method, a carburizing apparatus used for carrying out this method, and a steel product carburized by the above method.

[0002]

2. Description of the Related Art In the most widely used carburizing method for surface modification of steel, gas carburizing is generally used in a gas atmosphere. However, gas carburizing involves generation of an abnormal surface layer and high temperature carburizing. There are problems such as incomplete furnace structure, generation of soot in high-concentration carburization, large number of management items of carburizing conditions and complexity, etc.Vacuum carburizing furnace was developed to overcome these problems. It is a carburizing method.

In the conventional vacuum carburizing method, a gaseous chain saturated hydrocarbon is used as a carburizing gas. That is, the gaseous chain saturated hydrocarbon is a methane-based gas, and includes methane gas (CH ₄ ) and propane gas (C
₃ H ₈ ), butane gas (C ₄ H ₁₀ ), etc. are used, and these carburizing gases are directly supplied to the heating chamber of a vacuum carburizing furnace in which a work made of steel is heated to about 900 to 1000 ° C. Then, thermal decomposition is carried out in the heating chamber, and activated carbon generated at that time is made to enter the surface of the steel material, and is carburized and diffused from the surface.

In this case, since the carburizing gas needs to be sufficiently spread over the entire surface of the work, the heating chamber containing the work is kept in a vacuum state, and the carburizing gas is supplied into the heating chamber while stirring or pulsing. Due to fluctuations in the furnace pressure caused by charging, the carburizing gas was sufficiently supplied to the surface of the work.

By the way, in the conventional vacuum carburizing method, it is generally recognized that the carburizing property is strong and hydrocarbon is used as the carburizing gas.
Gaseous chain saturated hydrocarbons such as the methane-based gases mentioned above have been used.

The reason is that among those skilled in the art, methane-based gas is stable in the temperature range up to about 1100 ° C. for carburizing steel, but the stability decreases as the molecular weight increases, and soot is generated. It is recognized that the carburizing power becomes stronger, while gaseous chain unsaturated hydrocarbons such as acetylene-based gas are more unstable than methane-based gas and thermal decomposition is more active than carburization reaction. However, it was recognized that even if used as a carburizing gas, it would only generate soot and was not suitable as a carburizing gas at all (Mamoru Kawakami "Metal Surface Hardening Heat Treatment Technology" Maki Shoten 1969 10 See page 139, issued 25th of May).

Therefore, in practice, as a carburizing gas in vacuum carburization, methane gas (CH ₄ ) and propane gas (C ₃ ) which are methane series gases which are gaseous chain saturated hydrocarbons are used.
H ₈ ), butane gas (C ₄ H ₁₀ ), etc. were only used, and no acetylene-based gas, which is a gaseous chain unsaturated hydrocarbon, was observed.

[0008]

However, although the conventional vacuum carburizing method has solved the problem of quality in gas carburizing, it still has the following problems.

That is, 1. Soot is often generated, and maintenance work is complicated and dirty. 2. It is difficult to perform uniform carburization unless the amount of gas inserted into the heating chamber of the furnace is reduced and the amount of gas is increased. 3. Insufficient carburization into small diameter deep holes or narrow gaps in the work. 4. Equipment costs are high and limited to special purposes. It has lower productivity and higher treatment cost than gas carburizing.

The following equation shows the mechanism of thermal decomposition of the conventionally used carburizing gas.

[0011]

[Equation 1]

In the above formula, [C] is activated carbon that contributes to carburization. However, the activated carbon decomposed in the furnace space other than the surface of the work becomes soot as it is, which causes soot generation in vacuum carburization.

Measures for reducing the amount of soot generated include: a. To dilute the amount of carburizing gas in the furnace as much as possible, use the feed gas diluted with an inert gas (gas pressure is conventional), b. An oxygen source (for example, alcohol) is mixed into the carburizing gas to such an extent that an abnormal layer is not generated, and a part of the activated carbon is used as CO for carburizing, and the residual CO gas is discharged to the outside of the furnace. C. Although it has effects other than soot countermeasures, it generates plasma in the vicinity of the work surface, ionizes the dilute carburizing gas and attracts it to the work surface, effectively uses it for carburization, and reduces decomposition and soot in other furnace spaces. (Plasma carburization), etc.

According to these measures, the amount of soot generated can be reduced in any case, but this causes a problem that the equipment cost and the processing cost increase and the original merit of vacuum carburization is impaired. is there.

Further, in the conventional vacuum carburization using the methane-based gas as the carburizing gas, when the intervals between the loaded works are insufficient, or when the works have deep holes with a small diameter or narrow gaps, the entire work is Even if it is attempted to uniformly carburize over a wide range, it is impossible to obtain a sufficient carburizing depth when the adjacent workpieces are too close to each other, as well as in a deep hole or a narrow gap c, and variations in carburizing depth cannot be avoided. For example, even if a gas circulating device, a gas stirring device, a gas high-speed injection device, etc. are installed in the heating chamber in the furnace for carburizing, if the workpiece has a hole with an inner diameter of 4 mm and a depth of 28 mm, While the effective carburizing depth on the outer peripheral surface was about 0.51 mm, the effective carburizing depth at the bottom of the hole was about 0.30 mm.

Such variations in carburizing depth are caused by decomposition gas produced when the carburizing gas used has a large number of hydrogen atoms compared to the number of carbon atoms and is decomposed to generate atomic carbon in the heating chamber. As the number of molecules of hydrogen gas, etc., increases, the mean free path (mean free pa
It is estimated that th) is made smaller.

In order to carry out the carburizing treatment so that the inner wall surface of the small-diameter hole can also secure a predetermined carburizing depth, carbon is supplied into the hole or a carburizing gas is supplied more than necessary. In addition, the gas is fluidized and agitated to carry out the carburizing treatment, resulting in an increase in the amount of soot generated.

In view of the above-mentioned problems, the present invention can suppress the generation of soot, uniformly carburize each portion including the inner wall surface of the deep recessed portion over the entire work, and the amount of gas used and An object of the present invention is to provide a vacuum carburizing method and apparatus that require a small amount of heat and a carburized steel product.

[0019]

In the vacuum carburizing method according to the present invention, a work made of steel is vacuum-heated in a heating chamber of a vacuum carburizing furnace, and a carburizing gas is supplied into the heating chamber for carburizing treatment. The method is characterized in that a gaseous chain unsaturated hydrocarbon is used as a carburizing gas, and a carburizing process is performed in a heating chamber in a vacuum state of 1 kPa or less.

As the above-mentioned gaseous chain unsaturated hydrocarbon, it is desirable to use acetylene gas, especially acetylene gas.

Further, the vacuum carburizing method according to the present invention can be applied not only to the carburizing treatment but also to the carbonitriding treatment in which nitrogen (N) is infiltrated into the surface of the steel at the same time as carbon (C). In that case, for example, ammonia gas (NH ₃ ) may be added as a gaseous nitrogen source in addition to acetylene gas as a carburizing gas.

Further, the vacuum carburizing apparatus according to the present invention comprises a vacuum carburizing furnace equipped with a heating chamber for heating a work made of steel, a carburizing gas source for supplying acetylene-based gas into the heating chamber, and a vacuum for the heating chamber. It is characterized in that it is provided with a vacuum exhaust source for exhausting, and performs vacuum carburization in a vacuum state of 1 kPa or less.

Further, the carburized steel product according to the present invention is provided with a closed end hole having an inner diameter D, and the region of the inner wall surface of the closed end hole having substantially the same carburizing depth is the opening of the closed end hole. It is formed over the range of the depth L from the end, and the value of the depth L is 12 to 50 in terms of L / D ratio.
It is characterized by being within the range of.

In order to realize soot-free vacuum carburization (pressure-reduced gas carburization), it is desirable that only carbon that directly contributes to carburization is decomposed in the furnace. It is desirable that the material decomposes or reacts only on the surface of the work as much as possible and does not decompose or react on other furnace materials or furnace spaces.

In view of this condition, a chemically unstable and active gas is more preferable as the carburizing gas than the stable methane-based gas used as the carburizing gas in the conventional vacuum carburizing method.

Therefore, in the vacuum carburizing method according to the present invention,
Chain unsaturated hydrocarbon gas, which is more chemically active than the chain saturated hydrocarbon gas such as methane gas and propane gas, and which is easy to react and decompose, is used as the carburizing gas in the temperature range up to about 1100 ° C where the steel material is carburized. use.

However, when the residence time in the furnace exceeds the limit, these unstable gases are more easily thermally decomposed than the conventionally used saturated hydrocarbon gas to generate soot. It is necessary to discharge the gas from the furnace within a time range that is sufficient for reactive decomposition on the surface of the work but insufficient for thermal decomposition, by strictly limiting the residence time.

Therefore, in the vacuum carburizing method according to the present invention, in order to shorten the residence time of the carburizing gas in the furnace, the furnace pressure is set to 1 kPa or less, which is extremely lower than that in the conventional vacuum carburizing method, and the surface of the work is reduced. Although a decomposition reaction occurs, a vacuum carburizing method has been realized which hardly generates soot in the furnace space.

Further, in the conventional vacuum carburizing method, the gas pressure is raised to a certain degree in order to quickly move the compound gas decomposed on the surface of the work and finished supplying carbon to uniformly distribute the new supply gas ( 15 ~ 70kPa)
The amount of carbon supplied to the surface of the work is secured by stirring the inside of the furnace with a fan or the like, or by using a pulse system for gas injection to reduce the amount of compound gas by decompressing and supplying a new high pressure gas by pulse injection. Of course, this would also provide a much larger amount of carburizing gas than is needed for carburizing, further promoting the generation of soot.

[0030]

On the other hand, in the vacuum carburizing method according to the present invention, a gaseous chain unsaturated hydrocarbon is used as a carburizing gas, and this gaseous chain unsaturated hydrocarbon is used. The ethylene gas (C ₂ H ₄ ) and the acetylene gas (C ₂ H ₂ ) have a smaller number of hydrogen atoms than the number of carbon atoms, unlike the conventionally used methane-based gas.

Therefore, even if the carburizing gas is decomposed so as to generate atomic carbon in the heating chamber, the number of molecules of hydrogen gas, which is a decomposition product gas, does not increase, so that it is attempted to come into contact with the work as carburizing gas molecules. It is possible to reduce the interference of hydrogen gas molecules and the like during the operation. As a result, the pressure during the carburizing process may be low, the mean free path of the carburizing gas molecules may be extended, and the carburizing gas molecules may easily enter the inner peripheral surface of the deep recess of the work. Since the carburizing gas molecule is an unsaturated hydrocarbon that is chemically active and easily decomposes even at high temperature and without taking time, it easily reacts and decomposes on the surface of the work in a short time to form atomic carbon. In addition to being able to supply to the work surface, each part of the work can be uniformly carburized.

The uniformity of carburization becomes remarkable as the furnace pressure is lowered. By the way, when a carburizing process is performed on a work having a closed end hole having an inner diameter D, a region where the total carburizing depth is almost equal on the inner wall surface of the closed end hole,
When it is assumed that the closed end hole is formed over the range of the depth L from the open end, when the carburizing process is performed with the furnace pressure of 0.02 kPa, the value of the depth L is 36 in L / D ratio. Reached Further, if the pressure in the furnace is lowered, the value of the depth L in the region where the total carburizing depth is substantially equal can be increased to about 50 in L / D ratio. Such a value cannot be achieved by vacuum carburization or plasma carburization as well as conventional gas carburization.

Further, in the present invention, since the carburizing process in the heating chamber is carried out at an extremely low pressure of 1 kPa or less as compared with the conventional vacuum carburization, the suction means for maintaining the low pressure after being supplied into the heating chamber. Time to be inhaled, ie
The residence time of the carburizing gas in the heating chamber becomes shorter. If the residence time is shortened, it becomes possible to remove the carburizing gas that has not been decomposed and carburized from the heating chamber before it is decomposed in the heating chamber to generate soot. Can be prevented.

Therefore, even if an unstable and easily decomposed gaseous unsaturated hydrocarbon is used as the carburizing gas, the necessary amount of carburizing gas is catalytically decomposed to the surface of the work in a short time to carburize. As a result, the undecomposed carburizing gas that easily generates soot can be immediately discharged to the outside of the heating chamber together with the generated gas after decomposition (hydrogen gas etc.), which prevents the generation of soot. It became possible to carburize the work. Further, since the decomposition product gas can be discharged to the outside of the heating chamber in a short time, it is possible to further extend the mean free path of the carburizing gas molecules, which contributes to uniform carburization of each part of the work.

Furthermore, by measuring the amount of carburizing gas discharged from the exhaust pump to an appropriate amount and controlling the amount of carburizing gas to be introduced into the heating chamber, the amount of carburizing gas used is minimized. Can be suppressed to

Further, in the vacuum carburizing method according to the present invention, since a chain-like unsaturated hydrocarbon which is chemically active and easily reacts and decomposes is used as the carburizing gas, the conventional methane-based gas is used. Even if the carburizing gas is not supplied in excess of the required amount, the work surface can be easily reacted, decomposed and carburized, and the carburizing gas is supplied in the total amount required for carburizing the work surface. The number of carbon atoms within about twice the amount of carbon is sufficient. By the way, in the conventional vacuum carburizing, about tens of times the total carbon amount required for carburizing was supplied into the furnace. Furthermore, in the vacuum carburizing method according to the present invention, vacuum carburizing is performed at a low pressure of 1 kPa or less, and since the heating chamber itself exhibits an adiabatic effect to the outside of the heating chamber, heat dissipation is small and the temperature inside the heating chamber is maintained. It is possible to reduce the amount of heat required for this.

Therefore, in the vacuum carburizing method according to the present invention, as the carburizing gas, a chain-like unsaturated hydrocarbon in the form of gas, which has not been considered in the past for only causing the generation of soot, is used, Compared with the conventional vacuum carburizing method, it is possible to suppress the generation of soot, uniformly carburize each part of the work including the inner wall surface of the deep recess, and further reduce the amount of gas and heat used. The effect can be obtained.

Further, in the vacuum carburizing method according to the present invention,
Since the heating chamber is set to a low pressure of 1 kPa or less and the heating chamber itself exerts a heat insulating effect against the outside of the heating chamber, it is possible to reduce the need for water cooling or heat insulating protection of the heating chamber itself. The outer wall of the vacuum container including the chamber does not have to have a special heat insulating structure by only maintaining the structure at a low pressure, which can contribute to reduction of manufacturing steps and manufacturing cost of the vacuum carburizing furnace.

Ion carburizing and plasma carburizing are known as methods for carburizing a work at a low pressure. However, even with these carburizing methods, if the work has a deep recess,
Due to the reason that the ionized gas cannot reach the bottom of the recess, it is unavoidable that carburizing unevenness occurs, and soot is less generated than the conventional vacuum carburizing method, but like the vacuum carburizing method of the present invention. There is a drawback that the generation of soot cannot be suppressed and the equipment cost is high.

When acetylene gas is used among ethylene gas and acetylene gas as the gaseous chain unsaturated hydrocarbon used in the present invention, the number of hydrogen atoms formed is smaller than that of ethylene gas. It is active and easy to carry out carburizing treatment, and it is possible to reduce the amount used and the treatment cost.

Further, in addition to acetylene gas as a carburizing gas, for example, ammonia gas (NH ₃ ) is added as a gaseous nitrogen source to carry out carbonitriding treatment, whereby quenching treatment from a lower temperature becomes possible. Therefore, the distortion can be reduced.

[0042]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a diagram showing an embodiment of a vacuum carburizing apparatus according to the present invention. A vacuum carburizing furnace 1 includes a heating chamber 2 covered with a vacuum container 4 and a cooling chamber 3 adjacent to the heating chamber 2. I have it.

The heating chamber 2 is composed of a heating element 2a and a heat insulating material 2b which are chemically and strength stable in vacuum and in the atmosphere of a high temperature environment. As the heating element 2a, it is possible to use, for example, a recrystallized silicon carbide-based heating element or one having an alumina sprayed coating layer formed on the surface thereof. A high-purity ceramic fiber can be used as the heat insulating material 2b. The cooling chamber 3 has an outer wall formed of a part of the vacuum container 4 and includes an oil tank 3a.

A vacuum exhaust source V is connected to both the heating chamber 2 and the cooling chamber 3, and the carburizing gas source C capable of supplying acetylene gas to the heating chamber 2 by dissolving acetylene gas in acetone. Is connected to the cooling chamber 3. An inert gas source G such as nitrogen gas that can pressurize the inside of the cooling chamber 3 to atmospheric pressure or more is connected to the cooling chamber 3.

A carry-in door 5 is provided at the upstream end of the heating chamber 2.
An intermediate door 6 is provided at the downstream end, and a carry-out door 7 is provided at the downstream end of the cooling chamber 3.
And an internal transfer device 8 that transfers the work M from the upstream to the downstream from the heating chamber 2 to the cooling chamber 3. An elevating table 9 for loading and unloading the work M into and from the oil tank 3a is installed in the cooling chamber 3. Further, the heating chamber 2 is provided with a heating section whose front and rear ends are closed by an internal carry-in door 5a and an internal intermediate door 6a.

Next, a vacuum carburizing method using the vacuum carburizing apparatus having such a structure will be described with reference to FIG. The heating chamber 2 is preheated to a predetermined temperature under atmospheric pressure.

The first process carry-in doors 5 and 5a are opened, and the first work M1 is heated in the heating chamber 2
It is carried in, and the carry-in doors 5 and 5a are immediately closed.

Second step While the heating chamber 2 is evacuated to 0.05 kPa by the vacuum exhaust source V, the first work M1 is heated to a predetermined temperature (900 ° C.).
It is vacuum-heated up to, and then acetylene gas is supplied from the carburizing gas source C into the heating chamber 2 (at this time, the inside of the heating chamber 2 becomes 0.1 kPa) to perform the carburizing treatment. Then, the supply of acetylene gas is stopped, and the inside of the heating chamber 2 is again set to 0.0
Diffusion treatment is performed under a vacuum of up to 5 kPa, and further soaking and heating are performed to lower the quenching temperature to 850 ° C. In addition,
Meanwhile, the cooling chamber 3 is evacuated.

Third step The intermediate doors 6, 6a are opened, the first work M1 is transferred onto the elevating table 9 of the cooling chamber 3 by the internal transfer device 8, and the intermediate doors 6, 6a are immediately closed.

Fourth step Cooling chamber 3 is supplied by supplying an inert gas from inert gas source G.
While raising the pressure above the atmospheric pressure, the elevator 9 is lowered to
The work M1 is quenched. Meanwhile, the high temperature heating chamber 2
Air is introduced into the inside to make it into an atmospheric state, and then the carry-in doors 5 and 5
a is opened, the second work M2 is loaded into the heating chamber 2,
Immediately, the carry-in doors 5 and 5a are closed. The reason why the cooling chamber 3 is pressurized above atmospheric pressure is to prevent the air from entering the cooling chamber 3 when the air is introduced into the heating chamber 2.

Fifth step The lift table 9 is raised, the carry-out door 7 is opened, and the first work M is opened.
1 is carried out of the furnace 1, the carrying-out door 7 is immediately closed, and the cooling chamber 3 is vacuum-cooled. Meanwhile, the second work M2 is handled in the same manner as the second step.

Hereinafter, in a steady state, the third to fifth steps are repeated and the carburizing process of the work is sequentially performed.

As an example of the work carburized in this manner, the outer diameter dimension is 20 as shown in the sectional view of FIG.
mm, length 30 mm, work sample 10 having a closed end hole 11 having an inner diameter of 6 mm and a depth of 28 mm and a closed end hole 12 having an inner diameter of 4 mm and a depth of 28 mm, a width of 400 mm, a length of 600 mm,
300 pieces are juxtaposed on a jig with a height of 50 mm, and the jigs are stacked in 6 stages and placed in the heating chamber 2. Carburizing temperature is 900 ° C., carburizing time is 40 minutes, diffusion time is 70 minutes, and quenching temperature is 850 ° C. In this case, the effective carburizing depth t ₀ of each work is 0.51
While it was around mm, the effective carburizing depth t ₂ at the bottom of the small-diameter closed end hole 12 was around 0.49 mm. That is,
According to the vacuum carburizing method of the present embodiment, this is 0.02
It has been proved that each part can be uniformly carburized with a variation of around mm.

Even when this test was repeated several hundred times, no soot was found in the heating chamber 2. Further, even if the carburized in the same way as the above-mentioned work sample 10, a closed end hole having an inner diameter of 4 mm and a depth of 50 mm is provided in a sample whose length is almost doubled, and the effective carburizing depth and hole on the outer peripheral surface are The difference from the effective carburizing depth of the bottom can be suppressed within a range of about 0.03 mm, which indicates that the vacuum carburizing method of the present embodiment can uniformly carburize each part.

By the way, when the work sample 10 is carburized by the conventional vacuum carburizing method using the conventional methane-based gas as the carburizing gas, the carburizing time is set to about twice, and the carburizing time is 10 times or more for the carburizing. Even if gas is supplied, the effective carburizing depth on the outer peripheral surface of the work sample 10 is 0.51 mm.
And the effective carburizing depth at the bottom of the 4 mmφ hole 12 is 0.30 mm.
Then, uneven carburization occurred. Further, in the conventional vacuum carburizing method, if the carburizing treatment is repeated 5 to 20 times, a large amount of soot is accumulated in the heating chamber 2 even after burnout, and cleaning is required. As a matter of course, in the case of gas carburizing generally performed, carburization of the bottom of the hole 12 cannot be expected at all.

In the vacuum carburizing method of the present invention, since the heating chamber is carburized in a vacuum state of 1 kPa or less, even if acetylene gas is used as the carburizing gas,
Carburizing treatment can be performed by eliminating uneven carburization of the work and suppressing soot generation. However, when carburizing treatment is performed with a pressure exceeding 1 kPa in the heating chamber, it becomes difficult to suppress soot generation and carburization also occurs. Unevenness is not desirable.

The lower the pressure in the heating chamber is, the more
It is possible to increase the effect of the method of the present invention, and further,
From the viewpoint that the heat insulating effect of the heating chamber itself can be effectively exerted, water cooling, heat retention, etc. are not required and the energy saving effect can be enhanced, it is desirable that the heating chamber has 0.3 kPa or less,
More desirably, it is preferable to carry out the carburizing treatment under a reduced pressure of 0.1 kPa or less.

FIG. 4 shows a sample (SCM41) having an inner diameter of 6 mm and a closed end hole having a depth of 27 mm and having an outer diameter of 20 mm and a length of 30 mm.
5), at a temperature of 930 ° C., holding time, carburizing time and diffusion time (see FIG. 2) were 30 minutes and 3 respectively.
It is a graph which shows the relationship of the carburizing depth with respect to the pressure in the furnace when the carburizing treatment is performed using acetylene gas as 0 minutes and 45 minutes, and the soot generation state. A polygonal line A is a graph showing a change in carburizing depth at the bottom of the closed end hole, and a polygonal line B is a graph showing a change in carburizing depth at the surface of the work sample.

As is clear from FIG. 4, with respect to the surface of the sample, when the pressure in the furnace is 1.0 kPa or less, a substantially constant carburizing depth is obtained. However, in order to uniformly carburize the inside and outside of the closed end hole, it is desirable that the furnace pressure be 0.3 kPa or less.

From the viewpoint of soot generation, there is no problem if the furnace pressure is 1.0 kPa or less.

FIG. 5 shows a sample (SCM415) having an inner diameter of 3.4 mm, a closed end hole having a depth of 175 mm and an outer diameter of 20 mm, and a length of 182 mm, which is subjected to the carburizing method of the present invention to form a carburized layer. It is a sectional view showing a formed state, and a graph showing uniformity of carburization. In this case, the furnace temperature is 930 ° C., the furnace pressure is 0.02 kPa, and the sum of carburizing time and diffusion time is 430.
The sample loading conditions are the same as described above.

As is clear from FIG. 5, a region where the total carburizing depth on the inner wall surface of the closed end hole is approximately equal (2.1 mm) reaches a depth of 122 mm from the inlet of the closed end hole, and a depth of 156 mm. The total carburizing depth became zero. That is, when it is assumed that a region of the inner wall surface of the closed end hole having a total carburizing depth substantially equal to that of D is formed from the open end of the closed end hole to the region of depth L, The value of L reaches 36 in the L / D ratio. Thus, as the pressure inside the furnace decreases, the uniformity of carburization also increases. Further, if the pressure in the furnace is lowered, the value of the depth L in the region where the total carburizing depth is substantially equal can be increased to about 50 in L / D ratio.

[Brief description of drawings]

FIG. 1 is a sectional view showing an embodiment of a vacuum carburizing apparatus according to the present invention.

FIG. 2 is a diagram showing an operation pattern of the vacuum carburizing furnace according to the present invention.

FIG. 3 is a sectional view of a sample carburized by a vacuum carburizing method according to the present invention.

FIG. 4 is a graph showing the relationship between carburizing depth and pressure in the furnace and the soot generation state when the vacuum carburizing method according to the present invention is carried out.

FIG. 5 is a cross-sectional view showing the entire carburized layer in a sample subjected to the vacuum carburizing method of the present invention and a graph showing the uniformity of carburizing depth.

[Explanation of symbols]

 1 Vacuum carburizing furnace 2 Heating chamber 3 Cooling chamber M Work V Vacuum exhaust source C Carburizing gas source G Inert gas source

Claims (7)

[Claims] 1. A vacuum carburizing method in which a workpiece made of steel material is vacuum-heated in a heating chamber of a vacuum carburizing furnace and a carburizing gas is supplied into the heating chamber to perform carburizing treatment. A vacuum carburizing method characterized in that a gaseous chain unsaturated hydrocarbon is used, and a carburizing process is performed in a vacuum state of 1 kPa or less in the heating chamber. 2. The vacuum carburizing method according to claim 1, wherein the gaseous chain unsaturated hydrocarbon comprises an acetylene-based gas. 3. The vacuum carburizing method according to claim 2, wherein the acetylene-based gas comprises acetylene gas. 4. The carburizing process is performed by adding a gaseous nitrogen source to the carburizing gas.
The vacuum carburizing method described. 5. A vacuum carburizing furnace having a heating chamber for heating a work made of steel, a carburizing gas source for supplying an acetylene-based gas into the heating chamber, and a vacuum exhaust source for evacuating the heating chamber. A vacuum carburizing device, which is equipped with a vacuum carburizing method of 1 kPa or less. 6. A closed end hole having an inner diameter of D is provided, and a region having substantially the same carburizing depth on the inner wall surface of the closed end hole is formed over a range of a depth L from an open end of the closed end hole. Carburized steel product, wherein the value of the depth L is within the range of 12 to 50 in terms of L / D ratio. 7. The carburized steel product according to claim 6, wherein the carburizing depth on the inner wall surface of the closed end hole is substantially equal over a region of 12 to 36 in terms of L / D ratio.