JP2001081543A - Vacuum carburizing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent a generation of soot even in the case where hydrocarbon gas of >=2 carbon atoms is adopted and to carry out uniform carburizing treatment. SOLUTION: Hydrocarbon gas is introduced into a heating chamber, and carburizing treatment is performed under reduced pressure. In this vacuum carburizing method, hydrocarbon gas of prescribed amount is introduced for a prescribed time at the beginning of carburizing. The amount of subsequent introduction of the hydrocarbon gas is gradually decreased according to the predetermined pattern to carry out carburizing or is continuously controlled on the basis of the set amount of transmission of laser in the heating chamber.

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, and more specifically, to a method of directly supplying a hydrocarbon-based gas into a vacuum furnace to carburize a surface of a material to be treated, and subsequently performing a heat treatment under vacuum. Things.

[0002]

2. Description of the Related Art Vacuum carburizing involves heating a material to be treated to a carburizing temperature in a vacuum and, when a uniform temperature is reached, methane (C).
H _4), propane (C ₃ H _8), propylene (C ₃ H _6), ethylene (C ₂ H ₄₎ or acetylene (C ₂ H ₂₎ carburizing gas such as
(Carbon-based gas) is directly supplied into the furnace, carburized by holding for a predetermined time, and then diffusion is performed by holding for a predetermined time under vacuum. For the material, after diffusion treatment for the purpose of reducing the crystal grain size, A ₁
After cooling below the transformation, reheating is performed and quenching is performed.

As described above, vacuum carburization is a saturated carburization performed on an Acm line using a raw hydrocarbon gas.
A method of controlling the carbon potential of the carburizing atmosphere to adjust the surface carbon concentration as in other carburizing methods cannot be adopted. Therefore, in vacuum carburization, a desired amount of surface carbon and a desired carburized hardening depth are obtained by controlling the carburizing time and the diffusion time. In this case, the amount of carbon required for carburizing per unit surface area of the material to be treated can be theoretically calculated, but the required amount of carbon differs between the initial stage and the latter half of carburizing.

Incidentally, in order to examine the progress of carbon absorption in vacuum carburization, the amount of carbon absorption in vacuum carburization was measured assuming, for example, a case of 1000 ° C. and an effective carburization depth of 1.0 mm. The result shown in FIG. Note that FIG. 3 plots values measured every 1 minute from the start of carburization to 5 minutes and thereafter every 5 minutes. From the results shown in FIG. 3, the carbon content of about 20% of the total carburization required in 1 minute after the start of carburization and the carbon content of about 40% of the total carburization required in 5 minutes after the start of carburization were absorbed. It turns out that it is done.

In actual operation, if the amount of hydrocarbon gas calculated based on the theoretically required amount of carbon is used as the supply amount, variations in carburizing cannot be avoided. The carburizing process is performed by supplying three times the carburizing gas (hydrocarbon gas), and the carbon potential of the atmosphere during the process cannot be controlled as described above, so that the sooting in the furnace is performed. However, there is a problem that the soot adheres to the surface of the material to be treated and the glittering state cannot be maintained.

As a method for solving this problem, Japanese Patent Publication No. Sho 5
According to Japanese Patent Application Laid-Open No. 8-48626, an opening is provided in a part of a processing (carburizing) chamber, and interruption of an optical line due to generation of soot in the atmosphere discharged from the opening is detected by a light emitting device and a light receiving device to generate soot. At the point of detection, the supply of hydrocarbons is stopped, and after releasing the cutoff of the optical line due to the disappearance of soot, a certain amount of hydrocarbon gas is supplied again, and intermittent carburization is performed using the generation and disappearance of soot. A method for controlling the supply of a reactive gas has been proposed.

[0007]

However, in the above method, since the presence or absence of soot generation is detected by the sampling gas extracted from the carburizing chamber, a control delay is inevitable, and the supply of carburizing gas is stopped when the soot is stopped. Since the point of occurrence is detected, carburization occurs in a state in which soot is generated intermittently, and there is a problem that soot cannot be completely prevented from adhering to the surface of the material to be treated.

In the above method, when methane gas (CH ₄ ) is used as the carburizing gas, a methyl radical generation reaction, which is a rate-determining step of a chain reaction during thermal decomposition, is slow.
Even if the supply of hydrocarbons is stopped by detecting the interruption of the optical line due to soot, the adverse effect due to the control delay is relatively small, but as a carburizing gas, for example, a carbon number of 2 or more such as propane gas (C ₃ H ₈ ) When a hydrocarbon-based gas of
At a high temperature of about ゜ C, the decomposition rate is several thousand times faster than the methane group generation reaction, and a large amount of soot is generated. There is a problem that prevention of soot adhesion and uniform carburizing cannot be performed.

Accordingly, an object of the present invention is to prevent the generation of soot and perform a uniform carburizing treatment even when a hydrocarbon-based gas having 2 or more carbon atoms is employed. Yes, based on the following findings.

That is, from the results shown in FIG. 3, in the case of vacuum carburization, it is necessary to increase the amount of carburizing gas to be introduced at the beginning of carburizing, but to reduce the amount of carburizing gas to be introduced at the latter stage of carburizing. It turns out that you may do it.

Further, as described above, since the amount of carbon absorbed by the material to be treated such as steel is different between the initial stage of carburizing and the latter stage of carburizing, the carbon atom amount in the atmosphere in vacuum carburizing varies. When the laser transmission amount of the atmosphere was measured while supplying a constant amount of carburizing gas during the carburizing period, the result shown in FIG. 4 was obtained. From the figure, the laser transmission amount in the initial stage of carburizing was almost constant. It was found that the amount of laser transmission attenuated over time. This means that except for the initial stage of carburizing, the presence of a carburizing gas equal to or greater than the amount of carbon absorbed by the steel material, that is, the presence of carbon atoms that are not absorbed by the steel material,
Further, it means that the amount of carbon atoms in the atmosphere and the laser transmission amount in the atmosphere have a correlation.

[0012]

According to the present invention, there is provided a vacuum carburizing method for feeding a hydrocarbon-based gas into a heating chamber and performing carburizing treatment under reduced pressure as means for solving the above-mentioned problems.
At the start of carburization, a predetermined amount of hydrocarbon-based gas is supplied for a certain period of time, and then the amount of hydrocarbon-based gas supplied during the carburization period is gradually reduced to perform carburization.

From another viewpoint, the present invention relates to a vacuum carburizing method in which a hydrocarbon-based gas is fed into a heating chamber and carburized under reduced pressure. The amount of the hydrocarbon-based gas to be supplied during the subsequent carburizing period is controlled based on the set transmission amount of the laser in the heating chamber.

In a preferred embodiment, the set transmission amount of the laser is set to a laser transmission amount at the time when the laser transmission amount is attenuated by 5 to 10% from the laser transmission amount at the start of carburizing. This is because, when the set permeation amount is less than 5%, the concentration of free carbon atoms is too low and carburization takes time, and
If the amount of attenuation exceeds 10%, the concentration of free carbon atoms becomes too high, soot is gradually generated as the concentration increases.

As the hydrocarbon-based gas, methane (C
H ₄ ), propane (C ₃ H ₈ ), propylene (C ₃ H ₆ ), ethylene (C ₂ H ₄ ), or a known carburizing gas such as acetylene (C ₂ H ₂ ) can be used. For shortening, it is preferable to use a hydrocarbon gas having 2 or more carbon atoms alone or in combination of two or more.

In general, the process of vacuum carburization is performed during a heating period,
The temperature is divided into a carburization period and a diffusion period, and the temperature during the carburization period and the diffusion period is usually set appropriately at a temperature in the range of 800 to 1200 ° C. depending on the material to be treated. The pressure during the carburization period is usually 6.7 KPa or less, preferably 0.013 to 1.0.
It is performed under a pressure of 3 KPa.

In order to detect the concentration of free carbon atoms in the carburizing atmosphere, a laser which is monochromatic, has excellent directivity, and is easily affected by free carbon atoms emitted into the furnace atmosphere is employed. In order to radiate this laser into a carburizing atmosphere and detect the amount of laser transmitted therethrough, a laser light source and a laser light receiver are employed, which are disposed on the furnace wall so as to face each other.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The method of the present invention can be carried out, for example, using a vacuum carburizing furnace having a structure shown in FIG. In the drawing, a furnace body 1 constituting a vacuum carburizing furnace is provided with a circulation fan 2 for stirring gas inside the furnace at an upper part thereof, and a carburizing gas supply port 4 and a gas exhaust port 7 are formed in a furnace wall 3 respectively. I have. The heating chamber 5 in the furnace main body 1 is connected to a carburizing gas supply source (not shown) through a carburizing gas supply line 6 connected to the carburizing gas supply port 4. Evacuation system line 8 connected to exhaust port 7
Is connected to a vacuum evacuation device (not shown).

A laser projector 9 and a laser receiver 10 are disposed on the furnace wall 3 of the furnace body 1 with the heating chamber 5 interposed therebetween, and they detect the amount of laser transmission in the furnace atmosphere together with the converter 11. The laser beam emitted from the laser projector 9 is transmitted through the heating chamber 5 in the furnace.
And the transmission amount is detected as an electrical signal corresponding to the laser transmission amount by the laser receiver 10 and the converter 11, and the detected value is input to the control device 12.

The controller 12 controls the laser transmission amount in the furnace atmosphere based on the detection value detected by the transmission amount detection means so that the laser transmission amount is maintained at the set transmission amount as shown in FIG. The amount of carburizing gas supplied into the heating chamber 5 is gradually reduced by controlling a flow rate controller 13 provided in the carburizing gas supply line 6, for example, a mass flow meter or a control motor valve.

In practice, the inside of the furnace main body 1 is first evacuated to a vacuum of about 0.013 KPa by a vacuum evacuation device, and then the material is carburized at a temperature of, for example, 10% in a vacuum.
Heat to 00 ° C. After reaching soaking, the carburizing gas supply port 4 feeds a carburizing gas, for example, 30% by volume of propane gas.
And a mixed gas of 70% by volume of propylene gas are fed into the furnace to perform carburization.

Simultaneously with the supply of the carburizing gas, a laser is emitted from the laser projector 9 into the heating chamber 5, passes through the heating chamber 5 and reaches the laser receiver 10, where the amount of transmission is detected. This transmission amount is converted into an electric signal by the converter 11 and input to the control device 12 as the laser transmission amount at the start of carburization, and the set transmission amount in the carburizing period is set based on the laser transmission amount. The set transmission amount is set to a laser transmission amount of 5 to 10%, for example, 10% attenuated from the transmission amount, with the laser transmission amount at the start of carburization as 100%.

When the mixed gas (carburizing gas), which is three times the theoretically required amount of carbon, is fed into the furnace, carbon atoms are generated by thermal decomposition and released into the furnace, as shown in FIG. When the entire surface of the steel part in the heating chamber 5 reaches the saturation value of the Acm line at the carburizing temperature, the transmission amount of the laser transmitted through the heating chamber 5 decreases. The laser transmission amount of the atmosphere in the furnace is constantly detected by the laser light receiver 10, and the detected value is compared with the set transmission amount by the control device 12, and when the detected value decreases to the set transmission amount, the control device A signal is output from the heating chamber 12 and the flow controller 13 is controlled by the signal to reduce the opening.
The amount of carburizing gas supplied to the inside is reduced.

It is to be noted that the carburizing unevenness can be reduced by shortening the start time of the decrease in the amount of laser transmission, that is, by feeding in as much carburizing gas as possible within a range where soot is not generated. By managing time,
The carburization depth can be made more accurate.

Immediately after the amount of carburizing gas has been reduced,
Since the amount of carbon atoms absorbed by the material to be treated becomes larger than the amount of carbon atoms generated by the thermal decomposition of the carburizing gas, the amount of laser transmission tends to increase temporarily, but the amount of laser transmission tends to increase over time. Since the amount of carbon absorbed by the treatment material decreases, the amount of carbon atoms in the atmosphere increases, and the laser transmission amount in the furnace atmosphere starts to decrease again. When the laser transmission amount in the furnace atmosphere again decreases to the set transmission amount, the opening of the flow controller 13 is further reduced by the control device 12, so that the carburizing gas supplied into the heating chamber 5 is supplied. Thereafter, the amount of carburizing gas supplied into the heating chamber 5 is continuously decreased in the same manner, and the laser transmission amount is maintained at 90% of the laser transmission amount at the start of carburizing until the end of carburizing. You. When carburizing is over, the supply of carburizing gas is stopped,
The inside of the heating chamber 5 is again evacuated to a vacuum of about 0.13 KPa, and diffusion is performed while maintaining the temperature.

In the above embodiment, the amount of laser transmission through the furnace atmosphere in the heating chamber is detected, and the amount of carburizing gas supplied is gradually reduced so that the detected value is maintained at the set transmission amount. However, instead of this, the hydrocarbon system in the carburizing period according to a control pattern set in advance based on the scale of the vacuum carburizing furnace, the size and amount of the material to be treated, the desired surface carbon amount, the carburizing depth, etc. The gas supply amount may be gradually reduced.

[0027]

Therefore, according to the present invention, a predetermined amount of hydrocarbon-based gas is supplied for a certain period of time at the start of carburization, and then carburizing is performed while gradually reducing the amount of hydrocarbon-based gas supplied during the carburization period. In addition, it is possible to prevent the presence of excessive carburizing gas in the furnace, and to prevent the generation of soot.

Further, since the amount of laser transmission in the furnace atmosphere is directly detected in the furnace and the supply amount of the carburizing gas is controlled based on the detected value, there is no delay in the control of the supply of the carburizing gas. Indirectly grasps the carbon atomic weight in the furnace atmosphere at the time of the above, soot generation can be reliably avoided, and the carburizing temperature, the total surface area and the carburizing Irrespective of the progress of the carburization, it is possible to prevent the generation of soot and maintain the supply amount of the carburizing gas for avoiding the carburizing unevenness.

Further, the reference value for controlling the supply amount of the carburizing gas is 5 to 5
Since the laser transmission amount at the time of attenuation by 10% is employed, it is possible to prevent carburization unevenness and to avoid soot generation.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a carburizing apparatus used in the method of the present invention,

FIG. 2 is a diagram showing a relationship between a control pattern of a laser transmission amount and a carburizing gas supply amount in the method of the present invention.

FIG. 3 is a graph showing the relationship between carburization depth and carbon content.

FIG. 4 is a graph showing a relationship between a laser transmission amount and a carburizing time.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Furnace main body 2 ... Circulation fan 3 ... Furnace wall part 4 ... Carburizing gas supply port 5 ... Heating chamber 6 ... Carburizing gas supply line 7 ... Gas exhaust port 8 ... Vacuum exhaust system line 9 ... Laser projector 10 ... Laser reception Unit 11 ... Converter 12 ... Control device 13 ... Flow controller

Claims (3)

[Claims] In a vacuum carburizing method in which a hydrocarbon-based gas is fed into a heating chamber and carburized under reduced pressure, a predetermined amount of a hydrocarbon-based gas is fed for a certain time at the start of carburization, A vacuum carburizing method characterized by gradually reducing the amount of hydrocarbon-based gas to be fed and carburizing. 2. A vacuum carburizing method in which a hydrocarbon-based gas is fed into a heating chamber and carburized under reduced pressure, wherein a predetermined amount of a hydrocarbon-based gas is fed at the start of carburization, and the hydrocarbon in a subsequent carburizing period is introduced. A vacuum carburizing method characterized by controlling an amount of system gas to be supplied based on a set transmission amount of a laser in the heating chamber. 3. The vacuum carburizing method according to claim 2, wherein the set transmission amount of the laser is a laser transmission amount when the laser transmission amount is attenuated by 5 to 10% from the laser transmission amount at the start of carburizing.