JP2000104743A - Rolling bearing and manufacture of same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a rolling bearing that ensures an extended life even under elevated temperatures and bearing pressures, and a manufacturing method of such rolling bearings. SOLUTION: This rolling bearing comprises an inner ring, an outer ring and rolling elements, at least any one of which consists of a secondary hardened steel and includes eutectic carbide and a surficial nitrogen diffusion layer. The surface has Vickers hardness of 1,000 or more and a residual compressive stress of 200 MPa or more.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rolling bearing and a method of manufacturing the same, and more particularly, to a rolling bearing used in a high-temperature atmosphere and a method of manufacturing the same.

[0002]

2. Description of the Related Art As a material for a high-temperature bearing, a high-speed tool steel (SKH material) whose hardness does not decrease even in a high-temperature atmosphere, T1, or a semi-high-speed tool steel M50 is used. These steels are Cr,
A large amount of carbide-forming elements such as Mo or W is contained, and carbides of these elements are precipitated during the tempering process, whereby a secondary hardening phenomenon is developed and a decrease in hardness during tempering is prevented.

[0003]

However, these steels already contain large carbides (eutectic carbides) during the melting process. For this reason, when the bearing is used under a high load condition, the eutectic carbide acts as a stress concentration source to shorten the life of the bearing, or the eutectic carbide falls off the surface to cause surface-originated separation. Conventionally, there has been no good method to prevent this, and it has not been possible to provide a long-life bearing using these steels under high surface pressure conditions.

Accordingly, the present invention has been made to solve the above-described problems, and provides a rolling bearing having a long life even when used under conditions of high temperature and high surface pressure, and a method of manufacturing the same. The purpose is to do so.

[0005]

Means for Solving the Problems The present inventors have conducted various experiments to extend the life of a rolling bearing under high temperature and high surface pressure conditions, and have obtained the following findings.

That is, the present inventors performed nitriding treatment in the ferrite temperature range during tempering treatment for secondary hardening, so that the surface hardness was reduced to HV1000 to 1200 even after the removal of the fragile compound layer on the surface. And found that the residual compressive stress of the surface layer can be increased to 200 MPa or more.

The rolling bearing of the present invention based on these findings includes an inner ring, an outer ring and a rolling element, wherein at least one of the inner ring, the outer ring and the rolling element is made of secondary hardened steel and has a eutectic carbide. And a nitrogen diffusion layer on the surface. Here, secondary hardened steel refers to steel that exhibits a secondary hardening phenomenon. And Vickers hardness HV of the surface layer is 100
0 or more, and the residual compressive stress of the surface layer is 200 MPa.
That is all.

[0008] In the rolling bearing configured as described above, since the secondary hardened steel is used, a decrease in hardness is prevented even when used in a high temperature environment due to the secondary hardening phenomenon. In addition, since the hardness of the base layer can be increased to a Vickers hardness of 1000 or more in the surface layer, the difference in hardness between the eutectic carbide and the surrounding steel (base layer) is reduced, and the eutectic carbide is prevented from becoming a stress concentration source. be able to. In addition, since a residual compressive stress of 200 MPa or more is applied to the surface layer, it is possible to prevent the eutectic carbide from falling off the surface, to greatly suppress the occurrence of surface-originated damage and normal internal-originated damage, The rolling life can be improved as compared with the conventional example. As described above, a rolling bearing having a long service life even under high temperature and high surface pressure can be obtained.

Further, a method of manufacturing a rolling bearing according to the present invention includes a step of forming at least one of an inner ring, an outer ring and a rolling element of a secondary hardened steel, a step of performing a tempering treatment on the secondary hardened steel, Performing a nitriding treatment in a ferrite temperature range during or after the tempering treatment.

According to such a process, the surface layer can be increased in Vickers hardness and the residual compressive stress in the surface layer can be increased by performing the nitriding treatment in the ferrite temperature range. As a result, it is possible to prevent the eutectic carbide from becoming a stress concentration source and prevent the eutectic carbide from falling off the surface. In addition, since the secondary hardened steel is used, secondary hardening phenomenon that precipitates eutectic carbide by tempering treatment appears,
A decrease in hardness can be prevented. As described above, it is possible to manufacture a rolling bearing having a long life even under high temperature and high surface pressure.

When the nitriding treatment in the ferrite temperature range is performed in the same step as the tempering treatment, the number of manufacturing steps can be reduced as compared with the case where these treatments are performed separately.

In order to maximize the effect of the nitriding treatment, it is preferable to perform the nitriding treatment at a temperature of 500 ° C. or more and 600 ° C. or less and 0.5 hours or more in the ferrite temperature range. It is more preferable to set the period between 0.5 hours and 3 hours.

[0013] The step of performing the nitriding treatment in the ferrite temperature range includes forming a diffusion layer by diffusing nitrogen on the surface of the secondary hardened steel, and forming a compound layer containing nitrogen. Preferably, the method further comprises a step of removing the compound layer and leaving a diffusion layer on the surface of the secondary hardened steel. Thereby, the fragile chemical layer formed on the surface layer can be removed.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In one embodiment of the present invention, a tempering process of a high-speed tool steel represented by SKH or M50 by gas nitrocarburizing or tuftride treatment for hardening the surface at a temperature of 550 ° C. to 580 ° C. A surface hardening treatment is performed using Thereafter, only the fragile compound layer formed on the surface layer is removed by polishing, leaving only a hardened layer in which nitrogen is diffused in the steel. Thereby, a hardened layer having high hardness and no brittleness remains on the surface layer portion of the bearing transfer surface.

[0015] The hardened layer is more likely to be hardened when the amount of alloying elements such as Cr, W or Mo is large, and the Vickers hardness HV can be easily increased to 1000 by applying these nitriding treatments to high-speed steel.
The above diffusion layer can be formed. 200M in this diffusion layer
Residual compressive stress of Pa or more, preferably more than 400 MPa is applied. Thereby, the difference in hardness between the eutectic carbide and the surroundings can be reduced to prevent stress concentration, and at the same time, prevent the eutectic carbide from falling off and form a compressive stress field effective for the rolling life. As a result, the life of occurrence of surface-originated damage and normal internal-originated damage can be significantly improved.

[0016]

An embodiment of the present invention will be described below.

M50 which is a semi-high speed steel (C: 0.83% by weight, Si: 0.19% by weight, Ni: 0.08% by weight,
Cr: 4.17% by weight, Mo: 4.25% by weight, V:
A roller made of 1.02% by weight and the balance Fe) was prepared.

After performing a gas soft nitriding treatment at a temperature of 580 ° C. for 100 minutes during the tempering process of this time, a test piece from which a white layer (thickness of 10 μm or more and 20 μm or less), which is a compound layer formed on the surface, is removed. Prepared. Further, a test piece not subjected to the gas soft nitriding treatment at the above time was also prepared. The surface hardness of the test piece subjected to the gas soft nitriding treatment was HV1200 after polishing (state of the diffusion layer).

Due to diffusion, the depth from the surface is about 0.2 m
A hardened layer (diffusion layer) remained clearly up to the portion of m.
The residual compressive stress is 0.05 mm deep from the surface.
Was 450 MPa in the portion (surface layer), 200 MPa in the portion having a depth of 0.1 mm from the surface, and almost 0 in the portion having a depth of 0.2 mm from the surface.

On the other hand, in the test piece not subjected to the gas nitrocarburizing treatment, the surface hardness was HV800, and the residual stress at a portion having a depth of 0.05 mm from the surface was almost 0.

A rolling test was performed on these test pieces under the conditions shown in Table 1. 4155M for contact stress value
It was set to Pa, and the conditions for using the bearing in the market were considerably severe.

[0022]

[Table 1]

As a result, the results shown in Table 2 were obtained.

[0024]

[Table 2]

As shown in Table 2, in the test pieces which were not subjected to the gas nitrocarburizing treatment, all of the seven test pieces subjected to the test peeled off from the carbide in a short time. On the other hand, a test piece which was subjected to gas nitrocarburizing treatment and polishing was tested.
Only three of the specimens showed a long life and then peeled off, and four specimens did not peel off within a predetermined time.
That is, in the test piece subjected to the gas nitrocarburizing treatment, the life was extended four times or more as compared with the test piece not subjected to the gas nitrocarburizing treatment.

Note that "L10" in Table 2 means a life corresponding to a survival probability of 90%, that is, a failure probability of 10%. “L50” means the life corresponding to a survival probability of 50%. Further, the “L10 ratio” indicates a value obtained by dividing “L10” of the sample subjected to gas nitrocarburizing by “L10” of the sample not subjected to gas nitrocarburizing.

The data shown in Table 2 are for M50 steel. Among steels whose hardness does not decrease significantly by the treatment, eutectic carbides are a source of stress concentration and have a short life. Steel (high-speed tool steel, heat-resistant steel,
The present invention is applicable to high alloy steels, martensitic stainless steels, and the like. The nitriding process can be widely applied not only to the gas soft nitriding process but also to the nitriding process in a ferrite temperature region where diffusion from a liquid phase or a solid phase such as a tuftride process is performed.

Further, although the gas nitrocarburizing treatment was performed during the tempering treatment, it was confirmed that the same effect can be obtained by performing the gas nitrocarburizing treatment after the tempering treatment.

Although the present invention has been described above, the embodiment shown here can be variously modified.

The embodiments and examples disclosed this time are to be considered in all respects as illustrative and not restrictive. The scope of the present invention is not limited to the above-described embodiments and examples, but is defined by the appended claims, and is intended to include any modifications within the scope and meaning equivalent to the appended claims. Is done.

[0031]

According to the present invention, it is possible to prevent the eutectic carbide from being a source of stress concentration and to prevent the eutectic carbide from falling off the surface, and to prevent a decrease in hardness even at a high temperature due to a secondary hardening phenomenon. Therefore, a rolling bearing having a long life even under high temperature and high surface pressure can be provided.

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Claims (4)

[Claims] 1. A rolling bearing comprising an inner ring, an outer ring and a rolling element, wherein at least one of the inner ring, the outer ring and the rolling element is made of a secondary hardened steel, has a eutectic carbide, and has a surface layer. It has a nitrogen diffusion layer, and the Vickers hardness HV of the surface layer is 1000 or more,
A rolling bearing, wherein the residual compressive stress of the surface layer is 200 MPa or more. 2. At least one of an inner ring, an outer ring and a rolling element.
Forming a second hardened steel from a secondary hardened steel, performing a tempering process on the secondary hardened steel, and performing a nitriding process in a ferrite temperature range during or after the tempering process. Production method. 3. The method for producing a rolling bearing according to claim 2, wherein the step of performing the nitriding treatment in the ferrite temperature range is performed at a temperature of 500 ° C. or more and 600 ° C. or less for 0.5 hour or more. 4. The step of performing a nitriding treatment in the ferrite temperature range includes forming a diffusion layer by diffusing nitrogen on the surface of the secondary hardened steel, and forming a compound layer containing nitrogen. The method of manufacturing a rolling bearing according to claim 2 or 3, further comprising a step of removing a compound layer to leave the diffusion layer on the surface of the secondary hardened steel.