JP3854851B2 - Carburizing method for steel parts - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a carburizing method for steel parts, and more particularly to a carburizing method capable of making the surface carbon concentration of steel parts uniform.
[0002]
[Prior art]
Conventionally, vacuum carburizing treatment has been performed 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 900 to 1050 ° C. in a vacuum, and when a soaking state is reached, a carburizing gas is supplied into the heating chamber, and the heating chamber is brought to the carburizing temperature. The carburization period in which carbon enters the steel material by holding it for a predetermined time under a predetermined reduced pressure (usually 10 to 70 kPa ) in the maintained state, and the supply of the carburizing gas is stopped and a vacuum (about 0.2 kPa ) and a diffusion period in which carbon is diffused into the steel material by holding for a predetermined time. In this vacuum carburizing process, the carburizing depth and surface carbon concentration are adjusted by controlling the carburizing time and the diffusion time. Note that propane, propylene, ethylene, acetylene, or the like is used as the carburizing gas.
[0003]
However, this vacuum carburizing process has the advantage that the carburizing time can be shortened as compared with the gas carburizing process performed in a carburizing atmosphere containing an endothermic metamorphic gas such as CO or H2, but it is processed at a high temperature and diffusion. Since the period is under vacuum, there is a problem that cementite is deposited on the sharp part of the steel part. In other words, carbon that has entered the surface of the steel part during the carburizing period diffuses into the steel part during the diffusion period, and this diffusion is caused by a portion having a small internal volume with respect to the unit surface (for example, a corner part of a prismatic part or a gear). Since it is slow in a tooth tip portion (hereinafter referred to as a pointed portion), when attempting to secure a desired surface carbon concentration in a portion other than the pointed portion (for example, a flat portion), cementite is deposited on the pointed portion, and conversely In addition, there is a problem that when a desired surface carbon concentration is secured at the sharpened portion, the portion other than the sharpened portion becomes insufficient in carburization (hardening hardness is insufficient).
[0004]
On the other hand, as a means for adjusting the surface carbon concentration in vacuum carburizing, a method of performing a diffusion period in a reduced pressure decarburizing atmosphere has been proposed in JP-A-61-117268. This method is designed to preferentially decarburize excessive carburization at sharp edges by holding CO2 gas alone or a mixed gas of CO2 gas and N2 gas intermittently or continuously for a predetermined time. It is.
[0005]
[Problems to be solved by the invention]
The carburizing method in which the diffusion period is performed in a decarburizing atmosphere gas has an advantage of preventing precipitation of cementite in the sharpened portion because the sharpened portion is preferentially decarburized over the smooth portion, but the diffusion treatment is performed at a diffusion time. Since it is performed by management, strict diffusion time management is required to make the surface carbon concentration uniform, and since the diffusion time varies depending on the shape of the steel part, it is not possible to mix and treat steel parts having different shapes There was a problem.
[0006]
Accordingly, an object of the present invention is to make the surface carbon concentration uniform and prevent precipitation of cementite.
[0007]
[Means for Solving the Problems]
As a means for solving the above-mentioned problems, the present invention provides a carburizing method for steel parts including a carburizing period and a diffusion period, wherein the carburizing period is performed in a carburizing gas under a predetermined reduced pressure, and the diffusion period is an atmosphere containing CO. In a gas (hereinafter referred to as CO-containing atmosphere gas), the carbon potential (CP) value in the atmosphere is managed.
[0008]
That is, according to the present invention, the diffusion period is performed in the CO-containing atmosphere gas, and at the same time, the carbon potential value in the atmosphere in the diffusion period is controlled according to the desired surface carbon concentration of the steel material component to be processed. This eliminates the drawbacks of the vacuum carburizing method.
[0009]
As the carburizing gas, unsaturated hydrocarbon gases such as ethylene and acetylene, and saturated hydrocarbons such as methane, ethane, propane, and butane are used. In a preferred embodiment, acetylene gas is used as the carburizing gas. Is done.
[0010]
Note that under the specified reduced pressure, the degree of vacuum is such that no soot adheres to the surface of the treated material due to thermal decomposition of the carburizing gas, and the carbon supply amount in the initial stage of carburizing can be ensured, and depending on the type of carburizing gas used. It is prescribed. When acetylene gas is used as the carburizing gas, the carburizing treatment is usually performed under 1.5 to 3.5 kPa of acetylene gas for 5 minutes to 2 hours.
[0011]
The reason why the pressure at the initial stage of the carburizing period is set within the range of 1.5 kPa to 3.5 kPa is as follows. That is, when the initial pressure setting value in the carburizing period is 3.5 kPa or less, it is possible to avoid the soot generated by the thermal decomposition of the acetylene gas from adhering to the surface of the treatment material, but when it exceeds 3.5 kPa , This is because it is difficult to avoid the generation of a large amount of soot due to the thermal decomposition of the acetylene gas and the soot from adhering to the surface of the treatment material. Further, when the pressure set value is less than 1.5 kPa , it is impossible to reliably secure the supply amount of acetylene gas at the initial stage of carburizing, which is an important factor for obtaining a uniform carburized layer.
[0012]
In the carburizing method according to the present invention, the diffusion period is performed under atmospheric pressure while managing the carbon potential value in the atmosphere in the CO-containing atmosphere gas. As the CO-containing atmosphere gas, an endothermic modified gas is used. used. The composition of the endothermic modified gas is CO: 20.9 to 23.9%, CO2: 0.15 to 0.2%, H2: 30.4 to 38.9%, and N2 remaining. When a steel part is heated in the CO-containing atmosphere gas, an equilibrium is established between the CO concentration in the atmosphere gas and the carbon in the steel, and the steel part is controlled by controlling the CO concentration in the atmosphere, that is, the carbon potential. It becomes possible to make the surface carbon concentration uniform.
[0013]
The control of the carbon potential value in the CO-containing atmosphere gas during the diffusion period is performed by directly or indirectly measuring the carbon potential in the CO-containing atmosphere in the furnace, and comparing the measured value with the set target value to obtain the difference. Depending on the difference, it can be carried out by increasing or decreasing the supply amount of the CO-containing atmosphere gas and its additive gas (hydrocarbon gas) into the furnace.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, in the carburizing method for steel parts according to the present invention, acetylene gas is used as the carburizing gas in the carburizing period, and a mixed gas of endothermic modified gas and nitrogen gas and the endothermic modified gas alone as the CO-containing atmosphere gas in the diffusion period Each case will be described with reference to FIG. 1 showing an example of a carburizing heat cycle of the method of the present invention.
[0015]
First, steel parts are arranged in the furnace heating chamber, and the heating chamber is replaced with nitrogen gas, and then the steel parts are heated to the carburizing temperature (900 to 1050 ° C.) by convection heating in a substantially atmospheric pressure N 2 atmosphere. . Next, after evacuating the heating chamber to a vacuum degree (about 0.02 kPa ) at which the oxides on the surface of the steel material such as chromium oxide and manganese oxide are reduced at the carburizing temperature, a predetermined amount of carbon supply source (for example, , Hydrocarbon-based gas) is introduced into the heating chamber, and the surface of the steel part is activated by heating and holding for 0.1 to 1 hour while maintaining the degree of vacuum and the temperature in the heating chamber.
[0016]
After the activation treatment, a predetermined amount of carburizing gas (for example, saturated hydrocarbon gas, unsaturated hydrocarbon gas, or a mixture thereof) is supplied into the heating chamber while maintaining the carburizing temperature, Carburizing treatment is performed while maintaining a vacuum level for a predetermined time (5 minutes to 2 hours). After the carburizing process, the supply of the carburizing gas is stopped, the CO-containing atmosphere gas is supplied into the heating chamber, the pressure is increased to atmospheric pressure, the heating chamber is maintained at the temperature, and the carbon potential of the furnace atmosphere is set to a predetermined value. While maintaining the value, diffusion treatment is performed by heating and holding for a predetermined time (5 minutes to 2 hours). After the diffusion treatment is completed, the temperature is lowered to the quenching temperature while maintaining the inside of the heating chamber in the above atmosphere, and the quenching treatment is performed after maintaining the same temperature for a predetermined time.
[0017]
In this way, by performing the diffusion period in a CO-containing atmosphere and controlling the carbon potential of the atmosphere to a predetermined constant value, decarburization is performed on the surface of the portion (pointed portion) that becomes excessively carburized. In addition, diffusion is performed on the surface of the flat portion, so that uniform surface carbon concentration can be obtained and cementite precipitation can be prevented. In addition, if a small volume intermediate chamber is provided between the charging chamber, the carburizing chamber, and the diffusion chamber with an openable / closable partition, and the processing material is sequentially transferred through the intermediate chamber, the atmosphere is disturbed. Carburizing and diffusion treatment can be performed without any.
[0018]
[Example 1]
Carburizing treatment was performed under the following conditions using a shaft made of SCR420 as a treatment material and a target effective carburization depth of 0.8 mm.
Carburizing treatment:
Carburizing temperature: 950 ° C
Carburizing time: 45 minutes Carburizing gas: Acetylene gas Processing pressure: 1.5 kPa (Control value)
Diffusion processing:
Diffusion temperature: 950 ° C
Diffusion time: 40 minutes Processing pressure: about 100 kPa
Atmosphere: Endothermic metamorphic gas + N2 gas (CO: 10% by volume)
CP value: 0.89
Quenching process:
Quenching temperature: 850 ° C
Quenching liquid: Oil (Temperature: 120 ° C)
[0019]
The effective carburized depth of the obtained treated material was 0.78 to 0.82 mm, and a substantially uniform carburized layer was formed. Thus, a good carburized material was obtained.
[0020]
[Example 2]
Carburizing treatment was performed under the following conditions with a gear made of SCM420 as a treatment material and a target effective carburization depth of 0.6 mm.
Carburizing treatment:
Carburizing temperature: 950 ° C
Carburizing time: 30 minutes Carburizing gas: Acetylene gas Processing pressure: 1.5 kPa (Control value)
Diffusion processing:
Diffusion temperature: 950 ° C
Diffusion time: 25 minutes Processing pressure: about 100 kPa
Atmosphere: Endothermic modified gas (CO: 23% by volume)
CP value: 0.85
Quenching process:
Quenching temperature: 850 ° C
Quenching liquid: Oil (Temperature: 120 ° C)
[0021]
The effective carburized depth of the obtained treated material was 0.60 to 0.65 mm, and a substantially uniform carburized layer was formed, and a good carburized material was obtained.
[0022]
[Example 3]
Carburizing treatment was performed under the following conditions with a large gear made of SCM420 as the treatment material and a target effective carburization depth of 1.2 mm.
Carburizing treatment:
Carburizing temperature: 950 ° C
Carburizing time: 120 minutes Carburizing gas: Acetylene gas Processing pressure: 1.5 kPa (Control value)
Diffusion processing:
Diffusion temperature: 950 ° C
Diffusion time: 85 minutes Processing pressure: about 100 kPa
Atmosphere: Endothermic metamorphic gas + N2 gas (CO: 15% by volume)
CP value: 0.85
Quenching process:
Quenching temperature: 850 ° C
Quenching liquid: Oil (Temperature: 120 ° C)
[0023]
The effective carburized depth of the obtained treated material was 1.20 to 1.24 mm, and a substantially uniform carburized layer was formed, and a good carburized material was obtained.
[0024]
[Example 4]
Carburizing treatment was performed under the following conditions with a linear block made of SCM415 as the treatment material and a target effective carburization depth of 1.6 mm.
Carburizing treatment:
Carburizing temperature: 950 ° C
Carburizing time: 200 minutes Carburizing gas: Acetylene gas Processing pressure: 1.5 kPa (Control value)
Diffusion processing:
Diffusion temperature: 950 ° C
Diffusion time: 160 minutes Processing pressure: about 100 kPa
Atmosphere: Endothermic metamorphic gas + N2 gas (CO: 8% by volume)
CP value: 0.8
Quenching process:
Quenching temperature: 850 ° C
Quenching liquid: Oil (Temperature: 120 ° C)
[0025]
The effective carburized depth of the obtained treated material was 1.56 to 1.62 mm, and a substantially uniform carburized layer was formed, and a good carburized material was obtained.
[0026]
【The invention's effect】
As is apparent from the above description, the present invention is a carburizing method including a carburizing period and a diffusion period, wherein the carburizing period is performed in a carburizing gas atmosphere under reduced pressure, and the diffusion period is performed in a CO-containing atmosphere. By controlling the carbon potential to a predetermined constant value, the surface of the part that will be excessively carburized will be decarburized and diffused throughout the surface, making the surface carbon concentration uniform and the precipitation of cementite. Can be prevented.
[Brief description of the drawings]
FIG. 1 shows an example of a carburizing heat cycle in the method of the present invention.

Claims (1)

In carburizing method of the steel parts, including carburizing period and the diffusion period, performs carburization phase, using acetylene gas as a carburizing gas at 1. 5 k P a ~3. Of 5 k P a carburizing gas, diffusion-life Is carried out at the carburizing temperature at the carburizing stage under atmospheric pressure while controlling the carbon potential value in the CO-containing atmosphere consisting of the endothermic shift gas alone or a mixed gas of the endothermic shift gas and nitrogen gas. Carburizing method for steel parts.

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