JPH10204612A - Dehydrogenation for machine parts - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a dehydrogenating method for machine parts capable of reducing hydrogen infiltrated at the time of carburizing treatment or carbonitriding treatment. SOLUTION: Machine parts are subjected to carburizing treatment or carbonitriding treatment under prescribed conditions and are thereafter held under heating in a vacuum to release diffusible hydrogen and nondiffusible hydrogen from the steel. In this way, the formation of a surface abnormal layer and the generation of soot can be prevented, and furthermore, hydrogen in the steel can be removed in a short time only by the increase of vacuum equipment without requiring the remarkable change of the equipment.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for dehydrogenating mechanical parts, and more particularly to a method for dehydrogenating mechanical parts such as bearings and gears used in automobiles, agricultural machines, construction machines and the like.

[0002]

2. Description of the Related Art Conventionally, mechanical parts which are required to have high wear resistance, fatigue strength or toughness, such as bearings and gears, are required to have improved functionality. To achieve this, the alloy steel is subjected to a surface hardening treatment such as a gas carburizing treatment or a carbonitriding treatment.

[0003] In such gas carburizing treatment and the like, a continuous furnace can be easily constructed. On the other hand, since furnaces and processed products are deposited due to the generation of soot, the furnace is contaminated or carburizing gas easily comes into contact with air. As a result, there is a disadvantage that the stability of product quality is lacking.

For this reason, in recent years, a method has been adopted in which a vacuum exhaust device is provided, and vacuum replacement is performed at the time of quenching such as after carrying in a furnace of a machined part to be processed or after carburizing.
Japanese Utility Model Laid-Open Publication No. 3-45946 proposes a technique for removing oxides on the surface of the treated material and preventing the adhesion of soot by reducing the oxygen partial pressure by performing a diffusion treatment under vacuum after the carburizing treatment. I have.

[0005]

However, in the above-mentioned conventional technology, the mechanical parts are exposed to hydrogen contained in the carburizing atmosphere during the gas carburizing treatment, so that the hydrogen gas is diffused into the steel of the mechanical parts. Get inside. In particular, when carburizing ultra-large mechanical parts requiring a long carburizing time, the amount of hydrogen remaining in the steel of the mechanical parts after the heat treatment increases, and there is a possibility that material defects due to hydrogen may occur.

[0006] As material defects caused by hydrogen, there are those called white spot defects and those called delayed fracture.

[0007] The white spot defect is a phenomenon in which hair-like internal defects occur during cooling after heat treatment or when left at room temperature. The white spot defect is caused when a high concentration of hydrogen is present in steel. It is considered that hydrogen is gasified in the vicinity of a material defect such as an inclusion and a crack is generated by applying a stress such as a transformation stress.

[0008] On the other hand, delayed fracture is a phenomenon in a high-strength steel in which mechanical parts are embrittled by hydrogen remaining in the steel and brittle fracture occurs at a stress much lower than the yield point. Over a wide range of loads through an incubation period of.

[0009] Such material defects caused by hydrogen have the following problems particularly in mechanical parts such as bearings having high strength and subjected to relatively large repetitive stress. That is, in a mechanical part made of such a high-strength material, when high-concentration hydrogen remains in steel and becomes brittle, microcracks are generated, leading to early peeling (flaking), and the life of the mechanical part is shortened. There was a problem that it was greatly reduced.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and an object of the present invention is to provide a method for dehydrogenating mechanical parts capable of reducing hydrogen entering during carburizing or carbonitriding. With the goal.

[0011]

Means for Solving the Problems The applicant of the present invention has conducted intensive studies to reduce the amount of hydrogen entering into mechanical parts during carburizing, and as a result, it has been found that, by heating and holding under vacuum, the diffusivity existing in steel is reduced. It has been found that impurities such as hydrogen can be removed.

The present invention is based on such findings and applies vacuum substitution to carburizing or carbonitriding. The method for dehydrogenating mechanical parts according to the present invention comprises the steps of carburizing or carbonitriding under predetermined conditions. After the treatment, a dehydrogenation treatment is performed by heating and holding under vacuum.

According to the present invention, the formation of an abnormal surface layer and the generation of soot can be prevented, and the hydrogen in steel is removed in a short time only by adding a vacuum equipment without requiring a major equipment change. It is possible to do.

[0014]

Next, embodiments of the present invention will be described in detail.

[0015] First, the applicant of the present application examined the effect of hydrogen entering steel on the life characteristics of mechanical parts.

A specimen of φ65 × 6 mm was prepared from SCr440 steel and carburized at 950 ° C. for 20 hours.
It was allowed to cool to room temperature. Next, the test piece after standing to cool is heated and held at 860 ° C. for 30 minutes, and then subjected to a quenching treatment (hardening heat treatment), and then to a tempering treatment at 160 ° C. for 2 hours. The test piece was charged with hydrogen by changing the electrolysis time according to the method, and then a life test was performed.

The hydrogen charging conditions are as follows.

[Hydrogen charging conditions] Electrolyte: 5% sulfuric acid solution Anode: Platinum Cathode: Test piece Current density: 0.02 A / cm ² Electrolysis time: 1, 3, 5 hours 1st edition, 10th to 21st
Page (Electric Steel Research Institute, Science and Engineering, May 25, 1969)
The test was performed using the thrust bearing steel tester described in (1).

The life of the test piece, each test piece was determined microscope or visible to the naked eye cracks or the time at which peeling occurs about 10% life (L ₁₀ life), the cumulative number of revolutions up to such time The life was thus evaluated.

The test conditions for the life test are as follows.

[Test conditions for life test] Surface pressure: 4900 MPa Rotational speed: 3000 cpm Lubricating oil: # 68 turbine oil (manufactured by Nippon Oil Co., Ltd.) Contaminant foreign matter: Composition: Fe ₃ C (cementite) powder Rockwell hardness : HRC52 Particle size: 74 to 147 mμ Incorporation amount: 300 ppm in lubricating oil In the hydrogen analysis, a test piece after carburizing and cooling (hereinafter referred to as “carburized and cooled product”) and a test piece with peeling after the life test were used. (Hereinafter referred to as "life test specimen"), and the hydrogen concentration in steel of the test piece was measured. Specifically, in the hydrogen analysis, hydrogen was released from the test piece by a vacuum heating method and the weight of hydrogen was measured by a mass spectrometer for both the carburized and cooled product and the life test product. The concentration was calculated.

Hydrogen concentration (ppm) = hydrogen weight / weight of test piece (1) For the carburized and cooled products, these carburized and cooled products were immediately stored in a dewar containing dry ice to reduce the hydrogen component. The test piece was removed from the dewar bottle while avoiding detachment from the test piece, and the surface was polished, degreased, and dried with cold air within one hour, and measurement of hydrogen analysis was started.

The measurement conditions for hydrogen analysis are as follows.

[Measurement conditions for hydrogen analysis] Heating furnace: infrared image furnace Heating temperature: room temperature to 700 ° C. Heating rate: 10 ° C./min FIG. 1 shows the hydrogen concentration (ppm) of the test piece and the life (L) of the test piece. ₁₀ life).

[0025] FIG from 1 apparent from, along with a decrease in the hydrogen concentration of the test piece, improves the L ₁₀ life, L ₁₀ life is improved dramatically when the hydrogen concentration is 1ppm or less.
Moreover, compared with the case of the above hydrogen concentration 1ppm of hydrogen concentration is below 0.8 ppm, L ₁₀ life seen that more than double. This is because when the hydrogen concentration becomes high, microcracks tend to occur due to hydrogen embrittlement, and as a result, premature peeling occurs. According to the life characteristic diagram of FIG. 1, the hydrogen concentration becomes 1 ppm, preferably 0.8 ppm or less. It has been found that durability can be drastically improved.

Next, as described above, a carburized cooled product subjected to carburizing treatment at 950 ° C. for 20 hours, and after carburizing treatment, at 950 ° C.
Heating for 30 minutes at a degree of vacuum of 1 Torr, followed by dehydrogenation treatment, followed by cooling of a test piece (hereinafter referred to as "dehydrogenated product") when the amount of hydrogen was measured by the vacuum heating method described above. The relationship between temperature and hydrogen release rate was measured.

FIG. 2 is a characteristic diagram showing the relationship between the heating temperature and the hydrogen release rate. In FIG. 2, characteristic A is the hydrogen release characteristic of the carburized and cooled product, and characteristic B is the dehydrogenation treatment. It shows the hydrogen release characteristics of the product.

As the temperature of both the carburized cooled product and the dehydrogenated product increases from room temperature, the peak of hydrogen release reaches 200 to 250.
° C and 450-500 ° C. In FIG. 2, hydrogen released on the low temperature side is diffusible hydrogen, and hydrogen released on the high temperature side is non-diffusible hydrogen.
It is said that diffusible hydrogen released on the low temperature side causes material defects such as white spot defects and delayed fracture.

The applicant of the present application has proposed a high-temperature vacuum melting method (JIS
The total hydrogen content of the test piece was measured according to Z2614), and the hydrogen concentration of the test piece was measured from the total hydrogen content. As a result, the hydrogen concentration of the carburized and cooled product was as high as 3.3 ppm. The hydrogen concentration of the product was found to be significantly reduced to 0.73 ppm. Moreover, the dehydrogenated product in the present embodiment has a total hydrogen concentration of 1 ppm in the test piece.
By doing the following, not only diffusible hydrogen but also non-diffusible hydrogen can be simultaneously removed in a short time treatment, and white spot defects caused by hydrogen penetrating into the test piece during carburizing and carbonitriding, It has been found that material defects such as delayed fracture can be prevented.

Next, critical values of the heating holding temperature and the degree of vacuum will be examined.

[Heat holding temperature] Since the diffusion rate of hydrogen is lower than the temperature of the so-called A ₁ transformation point, that is, the case where the structure is (ferrite + pearlite) is the fastest, the heating holding temperature is constant below the A ₁ transformation point. It is desirable to carry out the dehydrogenation treatment by holding and transforming to (ferrite + pearlite). In consideration of the hydrogen release characteristics shown in FIG. 2, when the dehydrogenation temperature is set to 450 ° C. or higher, both diffusible hydrogen and non-diffusible hydrogen can be removed. On the other hand, when the heating and holding temperature is 400 ° C. or lower, only the diffusible hydrogen can be removed only, and the total hydrogen concentration cannot be sufficiently reduced. That is, 450 ° C. is desirable as the lower limit temperature of the heating and holding temperature. The upper limit temperature of the heating and holding temperature is desirably a temperature immediately below the A ₁ transformation point for the above-described reason. However, after the carburizing treatment or the carbonitriding treatment, the same temperature (for example, 950 ° C. in the case of 950 ° C. and carburizing treatment) is used.
The same effect can be obtained even if vacuum dehydrogenation treatment is performed in the above.

It is to be noted that a desired dehydrogenation effect can be obtained by performing a carburizing treatment or a carbonitriding treatment, allowing the mixture to cool, and then reheating and maintaining the above heating and holding temperature. However, if the cooling rate of the cooling performed after the carburizing treatment or the carbonitriding treatment is too high, hydrogen remains, so that hydrogen gas precipitates as the temperature decreases, and white spot defects may occur. Therefore, it is necessary to perform slow cooling control for the cooling.

[Degree of vacuum] The following experiment was conducted on the effect of the degree of vacuum during the dehydrogenation treatment on the hydrogen concentration in steel.

One hundred thrust-type test pieces of φ65 × 6 mm of SCr440 steel were prepared, carburized at 950 ° C. for 20 hours, and allowed to cool to room temperature. Then
Dehydrogenation was performed by heating and holding at 950 ° C. for 30 minutes at various degrees of vacuum, and hydrogen analysis was performed. The hydrogen concentration was measured by the above-mentioned vacuum heating method.

FIG. 3 is a characteristic diagram showing the relationship between the degree of vacuum and the hydrogen concentration.

As is apparent from FIG. 3, the hydrogen concentration is reduced by performing the vacuum heat treatment, and the degree of vacuum is reduced to 1 Torr.
When the pressure is reduced below, the hydrogen concentration of the test piece becomes 1 ppm in a short time.
It was found to decrease below. On the other hand, the higher the degree of vacuum, the lower the hydrogen concentration and the higher the safety against material defects such as white spot defects and delayed fracture.
It is desirable to reduce the pressure to rr or less. As described above, in this embodiment, the degree of vacuum is set to 0.1 to 1 Torr.

As described above, in this embodiment, the degree of vacuum is set to 0.1.
Although it is set to 1 to 1 Torr, such a vacuum degree is a vacuum degree that can be reached by a general-purpose oil rotary pump, and it is economically inexpensive. In addition, the degree of vacuum of about 1 Torr is not an extremely high vacuum, and decarburization, denitrification, etc. do not occur because of the short-time treatment.

The method for dehydrogenating mechanical parts according to the present invention can be applied regardless of the type of steel. Although the diffusion rate of hydrogen is affected by the addition of alloying elements, the degree of the effect is small, but the diffusion rate of hydrogen is much more affected by the temperature, degree of vacuum, and the structure. This is because there is almost no difference in effect depending on the steel type at the heating holding temperature and the degree of vacuum.

[0039]

Next, embodiments of the present invention will be described specifically.

First, test specimens of steel type 1, steel type 2 and steel type 3 having different component compositions (all dimensions are φ65 × 6 mm)
Was subjected to a carburizing treatment or a carbonitriding treatment at 950 ° C. for 20 hours to perform a predetermined dehydrogenation treatment.

Table 1 shows the component compositions of steel type 1, steel type 2 and steel type 3. Table 2 shows the carburizing conditions or carbonitriding conditions, dehydrogenating conditions, and measurement results (L ₁₀ Life (cycle), hydrogen concentration (ppm)).
In Table 2, treatments A to D are examples of the present invention, and treatments E to H are comparative examples.

[0042]

[Table 1]

[0043]

[Table 2] After the treatments A to H were performed for the life measurement, 840 ° C.
In performing quench-hardening treatment, then subjected to 90 minutes tempering treatment at 160 ° C., it was measured L ₁₀ life by performing life tests.

Regarding the hydrogen concentration, the total hydrogen content in the steel was measured by the high-temperature vacuum melting method (JIS Z 2614), and the hydrogen concentration in the steel was calculated from the total hydrogen content.

The processing conditions of this embodiment (A to D) are as follows.

[Treatment A] After the carburizing treatment, the mixture was subjected to a dehydrogenation treatment at a temperature of 950 ° C. for 0.5 hour and a degree of vacuum of 0.1 Torr, and then allowed to cool to the atmosphere.

[Treatment B] After the carburizing treatment, the mixture was cooled with nitrogen gas to a temperature of 650 ° C., subjected to a dehydrogenation treatment at the temperature of 650 ° C. for 0.5 hour at a degree of vacuum of 0.1 Torr, and then allowed to cool to the atmosphere.

[Treatment C] After the carburizing treatment, the mixture was allowed to cool to room temperature, reheated to a temperature of 650 ° C.
After performing dehydrogenation treatment at a vacuum degree of 0.1 Torr for a time, the mixture was allowed to cool to the atmosphere.

[Treatment D] After carbonitriding, the temperature was 650 ° C.
The mixture was subjected to dehydrogenation treatment at a temperature of 650 ° C. for 0.5 hour and a degree of vacuum of 0.1 Torr, and then allowed to cool to the atmosphere.

On the other hand, the processing conditions of the comparative examples (E to H) are as follows.

[Treatment E] After the carburizing treatment, the mixture was cooled to a temperature of 250 ° C. with nitrogen gas, subjected to a dehydrogenation treatment at the temperature of 250 ° C. for 0.5 hour at a degree of vacuum of 0.1 Torr, and then allowed to cool to the atmosphere.

[Treatment F] After the carburizing treatment, nitrogen gas was cooled to a temperature of 650 ° C., and the temperature was 650 ° C. for 0.5 hour and the degree of vacuum was 2
After performing dehydrogenation treatment with Torr, it was allowed to cool to the atmosphere.

[Treatment G] After the carburizing treatment, the mixture was cooled to a temperature of 650 ° C. with nitrogen gas, and heated to a temperature of 650 ° C. in a nitrogen atmosphere (atmospheric pressure).
After dehydrogenation for 0.5 hour, the mixture was allowed to cool to the atmosphere.

[Treatment H] After carburizing, the mixture was allowed to cool to room temperature in the air, and then subjected to a dehydrogenation treatment at room temperature for 20 hours at a vacuum of 0.1 Torr.

As is clear from Table 2, in the treatments A to D, the hydrogen concentration was reduced to 1 ppm or less irrespective of the types of steel 1, steel 2, and steel 3, and their life was good. Is obtained.

On the other hand, in the treatment E, the diffusible hydrogen was released because the heating and holding temperature was as low as 250 ° C., but the non-diffusible hydrogen was not released, so that the hydrogen concentration was 2.23 ppm.
And becomes more than 1 ppm, L ₁₀ life embodiment process A~D
It is extremely low compared to. It is generally said that if there is no diffusible hydrogen, delayed fracture can be prevented.However, since non-diffusible hydrogen remains in the test piece, diffusible hydrogen is not It is considered that the behavior similar to the above is performed. Therefore, it is understood from these facts that it is necessary to remove not only diffusible hydrogen but also non-diffusible hydrogen.

In both processes F and G, the degree of vacuum was 1
Since the vacuum is lower than Torr, the amount of discharged hydrogen is reduced. Therefore, the hydrogen concentration of the specimen becomes more 1 ppm, L ₁₀ life is reduced.

In the treatment H, although the degree of vacuum is kept at 1 Torr or less for 20 hours, since the specimen is kept at room temperature, almost no hydrogen is discharged from the test piece and the hydrogen concentration in the test piece is high. maintaining, resulted in a result L ₁₀ life is extremely lowered.

[0059]

As described above, according to the method for dehydrogenating mechanical parts according to the present invention, after performing carburizing or carbonitriding under predetermined conditions, heating and holding under vacuum to perform dehydrogenating, It does not require a significant change of the conventional equipment, or adding vacuum equipment to the existing equipment, or if the vacuum replacement equipment is already equipped, only by diverting the vacuum replacement equipment, during carburizing treatment or carbonitriding processing It can greatly reduce the hydrogen concentration in the machine parts that have penetrated and increase the safety against material defects such as white spot defects and delayed fracture,
Machine parts with improved durability can be obtained.

[Brief description of the drawings]

FIG. 1 is a characteristic diagram showing a relationship between a hydrogen concentration and a service life of a machine component.

FIG. 2 is a characteristic diagram showing a relationship between a heating temperature and a hydrogen release rate from a mechanical part.

FIG. 3 is a characteristic diagram showing a relationship between a degree of vacuum and a hydrogen concentration.

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[Procedure amendment]

[Submission date] February 20, 1997

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0004

[Correction method] Change

[Correction contents]

For this reason, in recent years, a method has been adopted in which a vacuum exhaust device is provided, and vacuum replacement is performed at the time of quenching such as after carrying in a furnace of a machined part to be processed or after carburizing.
Further, by the real equitable 3-45946 discloses to reduce the oxygen partial pressure is subjected to diffusion treatment in vacuum after carburization, to remove oxides of treatment material surface, it is proposed a technique of preventing the soot of ing.

Claims (1)

[Claims] 1. A method for dehydrogenating mechanical parts, comprising subjecting a mechanical part to a carburizing treatment or a carbonitriding treatment under a predetermined condition, and then performing a dehydrogenation treatment by heating and holding under vacuum.