JPH09324255A - Gas carburizing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve carburizability, to reduce the variation of carburizing concentration and carburized depth, and to improve quality by applying heating, at the gas carburizing, to a material to be carburized in an oxidizing atmosphere to modify its surface by oxidation and then performing carburizing. SOLUTION: Prior to carburizing treatment, a material to be treated is heated to 200-750 deg.C in an oxidizing atmosphere to undergo surface modification by oxidation. Further, by regulating the partial pressure of oxygen in the oxidizing atmosphere to 10<-20> to 10<-5> MPa at this time, oxidation is uniformized and modification is efficiently performed. The object in performing this pretreatment for carburizing is to efficiently modify surface state into a carburizable state, because, in general, machining is previously applied to the material to be carburized and, as a result, compositional changes are brought about in the surface to be carburized, and, in many cases, the surface is in a state unsuitable for carburizing treatment, e.g. a composition different from that of the base material is formed or a passivity layer is formed. Further, because of modification by oxidation, uniform carburizing can be done, independently, e.g. of adhesion of cutting oil, etc., and also carburizing velocity can be improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas carburizing method applied to modify the surface characteristics of an object to be processed such as a processed part.

[0002]

2. Description of the Related Art Conventionally, when a product to be processed is carburized by a gas carburizing method, if oil and fats such as cutting oil adhere to the surface of the product to be carburized, It is known that variations in surface carbon concentration and carburizing depth occur after the completion of treatment. Therefore, prior to carburizing,
A technique has been proposed in which an oxidizing gas such as air is supplied to the carburizing furnace to remove oils and fats, and the amount of deposited carbon is reduced so that a uniform carburizing treatment can be performed (for example, Japanese Patent Publication No. -51739 gazette).

However, in actual carburization, even in the case of dry cutting products that do not use cutting oil or the like, the carburized state such as the surface carbon concentration and carburized depth after the carburizing treatment may vary. The cause is not always clear at present, but it is presumed that the surface is altered for some reason, and as a countermeasure, it is necessary to perform shot blasting on the processed product to clean the surface. Has been done.

However, when the surface of the article to be treated is cleaned by shot blasting, the effect is only to prevent the carburization variation, and the other positive effect cannot be obtained, but the processing is not performed. There is a problem that the number of processes is increased and the cost is increased.

The present invention solves the above problems and improves the carburizing speed by improving the carburizing speed by modifying the surface of the article to be treated to a state suitable for carburizing and reducing the carburizing quality. An object is to provide a gas carburizing method that can be improved.

[0006]

In order to achieve the above object, the inventors of the present invention have conducted various experiments and examinations, and when the object to be treated is heat-treated under a predetermined oxidizing condition prior to the carburizing treatment. The present invention has been completed by further experiments and studies, knowing that the carburization rate can be improved and the carburization variation can be suppressed and good carburization can be performed.

That is, in the gas carburizing method of the present invention, the article to be treated is heated to 200 to 750 ° C. in an oxidizing atmosphere to oxidize and modify the surface of the article to be treated, followed by carburizing treatment. It is characterized by performing. According to the carburizing method of the present invention, it becomes possible to reduce variations in carburizing depth, surface carbon concentration, etc., regardless of the presence or absence of adhesion of cutting oil, etc., and to improve carburizing speed. . This is because by heating to a predetermined temperature in a predetermined oxidizing atmosphere, an oxide layer is formed on the surface of the object to be treated, and this oxide layer improves the carburizing property of the object to be treated, and the carburizing depth and This is considered to be due to the function of suppressing the variation in the surface carbon concentration. In the gas carburizing method of the present invention, the treatment temperature is 200 to 750.
The range is in the range of ° C, but when the treatment temperature is less than 200 ° C, a sufficient oxide layer is not formed on the surface of the article to be treated, and the carburizing property is improved, and the carburizing depth and the surface carbon concentration vary. The expected effect cannot be obtained in terms of reduction,
On the other hand, if the temperature exceeds 750 ° C., the thickness of the oxide layer (iron oxide etc.) formed on the surface of the article to be treated becomes large, and it becomes scale to peel off, which is not preferable.

Further, the gas carburizing method of the present invention is characterized in that the oxygen partial pressure in the oxidizing atmosphere is 10 ^-20 to 10 ^-5 MPa. Oxygen partial pressure in oxidizing atmosphere is 1
By setting it to 0 ^-20 to 10 ^-5 MPa, it becomes possible to uniformly oxidize and modify the surface of the article to be treated, and it is possible to efficiently perform good carburization with little variation in carburization. become. The oxygen partial pressure in the oxidizing atmosphere is set in the range of 10 ⁻²⁰ to 10 ⁻⁵ MPa because the oxygen partial pressure is lower than 10 ⁻²⁰ MPa, a sufficient oxide layer is formed on the surface of the article to be treated. However, it is difficult to obtain the above-mentioned desired effects, and when the oxygen partial pressure exceeds 10 ⁻⁵ MPa, a strong oxide film is formed on the surface of the object to be treated, which rather hinders the carburization reaction. It depends.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be shown below and the features thereof will be described in more detail. In addition,
FIG. 1 is a diagram showing a carburizing process according to an embodiment of the gas carburizing method of the present invention. In this embodiment, the JI
After heat-rolling a rolled rod of S SCM420 steel (diameter: 17 mm), cutting and cutting were performed to form a cylinder having a diameter of 15 mm and a length of 20 mm, which was used as a sample (processed article).

Then, the article to be treated was placed in a basket and carburized and quenched under the conditions shown in FIG.
In the carburizing step, the temperature of the atmosphere and the oxygen partial pressure in the atmosphere are shown in Table 1 between points A and B in the temperature raising step of FIG.
After performing the pretreatment by changing the conditions as shown in, the carburizing treatment was performed.

In this embodiment, after the temperature raising step, 9
CP = 0.97-1.02 at the maximum temperature of 30 ° C.
After carburizing for 1 hour at CP = 0.89 to 0.92 for 0.5 hour, the oil was cooled. Then, the effects of temperature and oxygen partial pressure on the carburizing quality were examined for the carburized and quenched samples.

[0012]

[Table 1]

In order to check the carburizing quality, as shown in FIG. 2, the sample was taken out from nine positions P marked with circles on the basket 1 containing the object to be treated, and the depth of 50 μm from the surface. Of carbon and the effective carburizing depth of 550 Hv or more were investigated. Table 1 shows the average value and standard deviation of the surface carbon concentration and the effective carburizing depth measured for the 9 points of samples collected from the above 9 positions P. In Table 1, the sample numbers marked with * are comparative examples outside the scope of the present invention, and others are examples within the scope of the present invention.

As shown in Table 1, temperatures of 200 to 750
In the carburized product of Comparative Example (Sample Nos. 7 to 10) which was subjected to carburizing treatment after being heated under the condition that the temperature was out of the range of 10 ° C. and an oxygen partial pressure of 10 ⁻²⁰ to 10 ⁻⁵ MPa, variations in surface carbon concentration. Results in that the carburizing rate is large and the carburizing rate is not sufficient (that is, the value of the effective carburizing depth is small), but the temperature is 200 to 750 ° C. and the oxygen partial pressure is 10 ⁻²⁰ to 10 −10.
^In the carburized products (Sample Nos. 1 to 6) of the examples of the present invention, which were carburized after being heated at ^-5 MPa,
It can be seen that the variation in the surface carbon concentration is reduced and the carburizing rate is improved.

In general, the carburized object is carburized as a pre-process, so that the machined surface has a compositional change and has a composition different from that of the base metal, or a passivation layer is formed. Although it is often in an unsuitable state for treatment, as described above, it is heated at a temperature of 200 to 750 ° C. and an oxygen partial pressure of 10 ⁻²⁰ to 10 ⁻⁵ MPa, and then carburized. According to the method of the present invention, the state of the surface is modified (formation of an oxide film) and the carburizing treatment can be efficiently performed.

In the above embodiment, at the maximum temperature of 930 ° C., CP = 0.97-1.02 for 1 hour, C
The case of performing the carburizing treatment at P = 0.89 to 0.92 for 0.5 hour and then performing the oil cooling has been described as an example, but the specific conditions of the carburizing step are limited to the above-described examples. Not a thing.

In the above embodiment, the product to be processed is JI.
The case where the rolled rod of S SCM420 steel is processed has been described, but the constituent material of the object to be processed is not limited to this, and the object to be processed made of various materials is carburized and quenched. In some cases, the present invention can be applied.

The present invention is not limited to the above embodiment in other respects, and various applications and design changes can be made within the scope of the invention.

[0019]

As described above, according to the gas carburizing method of the present invention, the object to be treated is heated to 200 to 750 ° in an oxidizing atmosphere.
By heating to C to oxidize and modify the surface of the article to be treated, the carburizing treatment is performed.
Regardless of the presence or absence of adhesion of cutting oil, it is possible to modify the surface of the object to be treated to a uniform and uniform state, improving the carburizing speed and reducing carburizing variations to improve carburizing quality. Can be made.

The oxygen partial pressure in the oxidizing atmosphere is set to 10
By setting the pressure to ^-20 to 10 ^-5 MPa, it becomes possible to uniformly oxidize and modify the surface of the object to be treated, and it is possible to perform carburizing treatment with less variation in carburizing quality more efficiently. Become.

[Brief description of drawings]

FIG. 1 is a diagram showing a carburizing step according to an embodiment of a gas carburizing method of the present invention.

FIG. 2 is a diagram showing a sampling method for a product to be processed in the embodiment of the present invention.

[Explanation of symbols]

 1 Basket P Sampling position

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yoshiro Takebayashi 229 Kahata Town, Tenri City, Nara Koyo Lindbergh Co., Ltd.

Claims (2)

[Claims] 1. An article to be treated is treated in an oxidizing atmosphere at 200-75.
A gas carburizing method, characterized in that by heating to 0 ° C., the surface of the article to be treated is oxidized and modified, and then carburizing is performed. 2. The oxygen partial pressure in the oxidizing atmosphere is 10 ^−20.
The gas carburizing method according to claim 1, wherein the gas carburizing method is 10 to ⁵ MPa.