JPH06279981A - Method for carburizing steel parts to high carbon state with gas - Google Patents

Abstract

PURPOSE:To provide the economical method for carburizing steel parts to a high carbon state with gas with suppresses the generation of sooting, shorten the time for carburization, improves the maintenance characteristic of a furnace and has an energy lessening effect. CONSTITUTION:The gas in the furnace is replaced with carbon dioxide and the parts to be treated are heated up in a carbon dioxide atmosphere. Gaseous hydrocarbon is supplied into the furnace simultaneously with completion of the heating up. The parts to be treated are then carburized by controlling the carbon potential in the furnace to a potential higher than the potential at the solubility limit of the carbon at this temp. As a result, the growth of grain boundary oxidation at the time of the carburization is suppressed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high carbon carburizing method for steel parts, which suppresses the occurrence of sooting, shortens the carburizing time and improves the maintainability of the furnace, and further has an energy saving effect. And is economical.

[0002]

2. Description of the Related Art In order to improve mechanical properties such as pitting resistance and wear resistance, high carbon carburization has recently been often applied to mechanical parts. The high-carbon carburizing is to raise the carbon potential during carburization and to precipitate carbides (carbides) harder than martensite with the formation of martensite. In order to finely precipitate the carbides, It is necessary to generate a sufficient amount of carbide nuclei in the initial stage of carburization.

Conventionally, therefore, there has been adopted a method of lowering the temperature to carburize to generate nuclei, or to once cool to normal temperature and reheat and quench to finely refine the carbide. However, the conventional method using a carrier gas has a limit in the amount of increase in carbon potential during carburization,
It took a long time for the treatment, and there was a problem that a large amount of soot was generated due to the sooting.

[0004]

In the above-mentioned conventional method, the problems of complicated processing pattern and long processing time remain, and in the case of high Cr alloy steel (Cr≈2.5%). Since the Cr content is high, Cr oxides are generated on the surface, a very thick grain boundary oxide layer is generated, and the grain boundary oxide layer forms an abnormal surface layer during quenching and causes fatigue. The problem of adversely affecting strength remained.

Furthermore, the amount of surface carbon is usually 0.
Since it is 2 to 3%, which is more than 8 to 1.0%,
The carbon potential of the furnace atmosphere must be raised above the carbon solid solution limit (Acm line) of austenite at the processing temperature, but it is difficult when using a carrier gas (RX gas). Along with this, the soot generation due to sooting makes it difficult to control the atmosphere in the furnace, and the refractory materials and metals in the furnace are eroded to deteriorate the durability of the carburizing furnace. There was still a problem.

In view of the above circumstances, the present invention uses a carbon dioxide and a treatment method of directly supplying a raw material gas into a furnace (hereinafter, simply referred to as
It is called the direct carburizing method. ) Is adopted, the carbon potential is increased, the carburizing time is shortened and the maintainability of the furnace is improved while minimizing the sooting. Furthermore, it has an energy saving effect and is economical. In addition, regarding the high Cr alloy steel, the growth of the grain boundary oxidation can be suppressed.

[0007]

In order to solve the above problems, the present invention employs a method of using carbon dioxide and a treatment method of directly supplying a raw material gas into a furnace. That is, the present invention replaces the in-furnace gas with carbon dioxide, and raises the temperature of the article to be treated in a carbon dioxide atmosphere, and when the temperature rise is completed, the hydrocarbon gas is fed into the furnace to increase the carbon potential in the furnace. The carburizing treatment is performed by controlling to a temperature higher than the solid solubility limit of carbon at that temperature.

[0008]

In the present invention, after the object to be treated is charged into the furnace, carbon dioxide is introduced into the furnace, the nitrogen gas in the furnace is purged, and at the same time a fixed amount of carbon dioxide is supplied to the object to be treated. The temperature of the product is raised. In this step, if the hydrocarbon gas as the raw material gas is supplied while the temperature of the article to be treated is low, it causes the sooting, so that it is avoided.

When the temperature rise is completed, a hydrocarbon gas as a raw material gas is supplied into the furnace to control the carbon potential in the furnace to a state higher than the carbon solid solution limit (Acm) at that temperature. Then, carburization is performed. It is desirable that the stirring fan be stopped until the temperature rise is completed to achieve early temperature rise.

As described above, when carbon dioxide gas is considered as the oxidizing gas, the reaction of the raw material gas with the hydrocarbon gas in the furnace is considered to be the following formula 1.

[0011]

[Chemical 1] Now, when CH ₄ which is a typical hydrocarbon gas is considered, the following formula 2 is obtained.

[0012]

[Chemical 2] The free energy of this reaction is as follows. ΔG = 239580-51.26TlogT-109.
07T [J / mol] In the low temperature region, the reaction of the above formula 2 hardly proceeds to the right,
As the temperature shifts to the high temperature region, it becomes easier to proceed to the right. Now, considering the equilibrium constant Kp of the reaction, at 700 ° C., Kp =
In contrast to 6.9, at 930 ° C., Kp = 5.9 × 10 ⁹
It can be seen that it is extremely dependent on temperature.

Therefore, introducing the hydrocarbon gas into the furnace when the article to be treated is charged into the furnace and the atmospheric temperature in the furnace is lowered, not to increase the CO concentration in the furnace. On the contrary, it is a factor that causes suiting due to cracking.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 shows a typical processing pattern of the present invention. That is, the product to be treated is charged into the furnace (point a)
After that, carbon dioxide as a gas for purging in the furnace, for example,
Supply 10 to 100 liters for 1 minute to purge nitrogen gas and the like in the furnace. Furthermore, at the same time, quantification of the amount of carbon dioxide as an oxidizing gas required for the subsequent carburizing treatment, for example,
Continue to deliver 0.5-5 liters per minute.

Then, when the replacement of the gas in the furnace is completed, the supply of carbon dioxide as the gas for purging in the furnace is stopped, and the temperature is raised while supplying only the fixed amount of carbon dioxide. For example, 0.5 to 5 liters of gas is supplied per minute, and at the same time, the stirring fan is driven to perform processing for a predetermined time.

The following products were processed according to the above processing pattern. The chemical composition of the article to be treated is shown in Table 1 below.

[0017]

[Table 1] The concrete carburizing conditions are as follows: treatment temperature: 930 ° C., treatment time: 16 hours, after that the temperature is lowered to 660 ° C., the temperature is raised to 850 ° C., the temperature is held for 1 hour, and the steel is quenched in oil. is there. FIG. 2 shows the hardness distribution of the obtained treated product, and FIG. 3 shows a micrograph (× 400) showing its cross-sectional structure.

In FIG. 4, an article to be treated similar to the above is carburized by a conventional method, that is, using RX gas at 880 ° C. for 25 hours, and then cooled to 660 ° C. as in the above example, and Raise the temperature to 850 ° C and hold for 1 hour,
The hardness distribution of the treated product quenched in oil is shown, and FIG. 5 shows a micrograph (× 400) showing its cross-sectional structure.

Comparing the two, although the structures and hardness distributions are almost the same, the processing time required for the conventional method is 25 hours, whereas the processing time for the present invention is 16 hours. According to the method, the carburizing gas and the electric energy can be largely saved.

Further, FIG. 6 shows a cross-sectional composition image photograph (× 1500) of the treated product obtained in the present invention, and FIG. 7 shows a direct carburizing method at 930 ° C. at 16 ° C. without adopting the present invention. Cross-sectional composition image photograph (× 15)
00) is shown.

That is, the article to be treated is charged into the furnace and
After a lapse of 0 minutes (before completion of temperature rise), a fixed amount of carbon dioxide (0.5 to 5 liters per minute) and a hydrocarbon gas (0.5 to 5 liters per minute) as an oxidizing gas are supplied. Then
At the same time, the stirring fan is driven to raise the temperature to a predetermined processing temperature and the processing is performed for a predetermined time.

Comparing both, FIG. 6 obtained by the present invention.
Although the grain boundary oxidation of the treated product of No. 10 stops growing at about 10 to 15 μm, the grain boundary oxidation of the treated product of FIG. 7 grows to 40 μm or more, and further develops like a network.
The method of the present invention has been found to have the effect of suppressing the growth of grain boundary oxidation.

[0023]

According to the method of the present invention, a relatively high carbon potential can be obtained in the initial carburization.
In addition, the amount of carburizing gas used is smaller than that of the conventional method of using RX gas, and since it is supplied after the temperature inside the furnace is raised, sooting is less likely to occur and the maintainability of the furnace can be improved, and at the same time the carburizing time is shortened. As a result, it is possible to obtain the effect of being energy-saving and economical. Furthermore, it is possible to obtain the effect of suppressing the growth of grain boundary oxidation during carburization for high Cr alloy steel.

[Brief description of drawings]

FIG. 1 is a typical processing pattern diagram of the present invention.

FIG. 2 is a hardness distribution chart of a product treated by the method of the present invention.

FIG. 3 is a photomicrograph (× 400) showing a cross-sectional structure of a product treated by the method of the present invention.

FIG. 4 is a hardness distribution chart of a processed product by a conventional method.

FIG. 5 is a micrograph (× 400) showing a cross-sectional structure of a processed product according to a conventional method.

FIG. 6 is a composition image photograph (× 1) of a product treated by the method of the present invention.
500).

FIG. 7 is a composition image photograph (× 1500) of a product treated by a conventional direct carburizing method.

Claims (1)

[Claims] 1. The furnace gas is replaced with carbon dioxide, and the temperature of the article to be treated is raised in a carbon dioxide atmosphere. When the temperature rise is completed, hydrocarbon gas is supplied into the furnace to increase the carbon potential in the furnace. A gas high carbon carburizing method for steel parts, characterized by controlling to a temperature higher than the solid solubility limit of carbon at that temperature and carburizing.