JP2983793B2 - Gas high carbon carburizing method for steel parts - Google Patents

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 carburizing time and improves furnace maintainability, and has an energy saving effect. And is economical.

[0002]

2. Description of the Related Art Recently, in order to improve mechanical properties such as pitting resistance and abrasion resistance, high carbon carburizing is often applied to mechanical parts. The high carbon carburization raises the carbon potential at the time of carburization, and precipitates carbide (carbide) harder than martensite together with the formation of martensite. In order to precipitate the carbide finely, It is necessary to generate enough carbide nuclei in the early stage of carburizing.

[0003] Conventionally, for this purpose, a method has been adopted in which carburization is performed at a reduced temperature to generate nuclei, or a method of once cooling to room temperature and reheating and quenching to refine the carbides. However, in the conventional method using a carrier gas, there is a limit to the increase in the carbon potential during carburization,
It takes a long time for the treatment, and there is a problem that soot is generated in large quantities due to sooting.

[0004]

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

[0005] Furthermore, the surface carbon content is usually reduced to 0.1% in carburizing.
To be 2-3%, which is more than 8 to 1.0%,
The carbon potential of the atmosphere in the furnace must be higher than the carbon solid solubility limit of austenite (Acm line) at the processing temperature. However, it is difficult to use a carrier gas (RX gas). In addition, large amounts of soot generated by sooting make it difficult to control the atmosphere in the furnace, and furthermore, the refractories and metals in the furnace are eroded, thereby impairing the durability of the carburizing furnace. Even the problem remained.

[0006] In view of the above circumstances, the present invention provides a processing method (hereinafter simply referred to as "use of carbon dioxide and supply of raw material gas directly into a furnace").
It is called direct carburizing method. ) To increase carbon potential, minimize carburizing time and improve furnace maintenance while minimizing sooting, and have energy-saving and economical effects. In the case of high Cr alloy steel, the growth of grain boundary oxidation can be suppressed.

[0007]

In order to solve the above-mentioned problems, the present invention employs a method of using carbon dioxide and a method of supplying a raw material gas directly into a furnace. That is, in the furnace
After charging the article to be treated, the gas for purging in the furnace and the subsequent gas
Supply carbon dioxide as an oxidizing gas in an amount necessary for carburizing
When the purge of the furnace gas is completed,
Stop supplying carbon dioxide gas as gas, and then
The temperature is increased while supplying only the amount of carbon dioxide necessary for the carburizing treatment, and the hydrocarbon gas is supplied upon completion of the temperature increase,
Carburizing treatment is performed by controlling the carbon potential in the furnace to be higher than the carbon solid solubility limit at that temperature.

[0008]

According to the present invention, after the article to be treated is charged in the furnace, carbon dioxide is introduced into the furnace, the nitrogen gas and the like in the furnace are purged, and at the same time a fixed amount of carbon dioxide is supplied while the article is treated. Raise the temperature of the product. In this step, if a hydrocarbon gas, which is a raw material gas, is supplied while the temperature of the article to be processed is low, sooting is caused.

When the temperature rise is completed, a hydrocarbon gas as a raw material gas is supplied into the furnace so that the carbon potential in the furnace is controlled to be higher than the carbon solid solubility limit (Acm) at that temperature. Carburizing is performed. It is desirable that the stirring fan be stopped until the temperature rise is completed, and the temperature be raised early.

As described above, when carbon dioxide gas is considered as the oxidizing gas, it is considered that the reaction in the furnace with the hydrocarbon gas as the raw material gas is represented by the following formula 1.

[0011]

Embedded image Now, assuming a typical CH ₄ as a hydrocarbon gas, the following equation 2 is obtained.

[0012]

Embedded image The free energy of this reaction is as follows. ΔG = 239580-51.26 TlogT-109.
07T [J / mol] In the low temperature region, the reaction of the formula 2 becomes difficult to proceed to the right side,
The higher the temperature, the easier it is to move to the right. Now, considering the reaction equilibrium constant Kp, at 700 ° C., Kp =
At 930 ° C., Kp = 5.9 × 10 ^{9 at} 930 ° C.
It can be seen that the temperature greatly depends on the temperature.

Therefore, when the article to be treated is charged into the furnace and the hydrocarbon gas is introduced into the furnace when the atmosphere temperature in the furnace is lowered, the CO concentration in the furnace is not increased. Conversely, it causes soaking due to cracking.

[0014]

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

Then, when the replacement of the furnace gas is completed, the supply of carbon dioxide as the furnace purge gas is stopped, and the temperature is increased while only the fixed amount of carbon dioxide is supplied. For example, a gas is supplied at a rate of 0.5 to 5 liters per minute, and simultaneously, a stirring fan is driven to perform a process for a predetermined time.

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

[0017]

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

FIG. 4 shows that the same article to be treated is carburized at 880 ° C. for 25 hours by using a conventional method, that is, using an RX gas, and then cooled to 660 ° C. in the same manner as in the above embodiment. 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 the cross-sectional structure.

When the two are compared, they are almost the same in terms of the structure and hardness distribution. However, the processing time requires 25 hours in the conventional method, whereas the processing time in the present invention is only 16 hours. According to the method, carburizing gas and electrical energy can be saved significantly.

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

That is, the article to be treated is charged into the furnace,
After a lapse of 0 minutes (before completion of heating), a fixed amount of carbon dioxide (0.5 to 5 liters per minute) and hydrocarbon gas (0.5 to 5 liters per minute) are supplied as oxidizing gas. And
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.

Comparison between the two shows that FIG. 6 obtained by the present invention.
The grain boundary oxidation of the processed product of FIG. 7 has stopped growing at about 10 to 15 μm, but the grain boundary oxidation of the processed product of FIG. 7 has grown to 40 μm or more, and has developed in a network form.
It has been found that the method of the present invention has an 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 early stage of carburizing,
In addition, the amount of carburizing gas used is smaller than that of the conventional method of using RX gas. In addition, since the gas is supplied after the furnace is heated, the occurrence of sooting is small, and the integrity of the furnace can be improved, and at the same time, the carburizing time can be reduced. Accordingly, an effect of saving energy and being economical can be obtained. Further, the effect of suppressing the growth of grain boundary oxidation during carburization can be obtained for high Cr alloy steel.

[Brief description of the drawings]

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

FIG. 2 is a hardness distribution diagram of a processed product according to the method of the present invention.

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

FIG. 4 is a hardness distribution diagram of a processed product according to a conventional method.

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

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

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

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) C23C 8/22

Claims (1)

(57) [Claims] 1. After the article to be treated is charged in the furnace, the furnace
Gas for gas and the amount of oxidizing gas required for subsequent carburizing
And supply of carbon dioxide as
Then, supply of carbon dioxide as the gas for purging in the furnace is performed.
And then remove the amount of carbon dioxide necessary for the carburizing process.
The temperature is raised while the temperature is being supplied , and when the temperature is raised, hydrocarbon gas is supplied, and the carbon potential in the furnace is controlled to be higher than the carbon solid solubility limit at that temperature to carburize. A gas high carbon carburizing method for steel parts, characterized by performing a treatment .