JP2011038127A - Method for strengthening carburizing-treated component - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for strengthening a carburizing-treated component by which a high compressive residual stress can inexpensively and easily be imparted onto the outermost surface of a carburizing-treated component, and also, the distribution of the compressive residual stress can be changed according to the target component, when the method is performed by jointly using a carburizing treatment and a shot peenin. <P>SOLUTION: In the method for strengthening a carburizing-treated component, carburizing to a base material W is performed at a carburizing furnace. Regarding carburizing conditions, carburizing temperature is controlled to 950°C, next, temperature is reduced to 850°C, and finally, the object to be treated is subjected to oil quenching (oil cooling) into quenching oil of 130°C. Thereafter, the surface of the carburizing-treated component as the base material W subjected to the carburizing is subjected to shot peening with a mixture obtained in such a manner that the shot grains composed of grains made of steel and an abrasive composed of ceramics particles are mixed. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a technique for further increasing the fatigue strength of steel parts subjected to carburizing treatment for surface hardening by shot peening.

Conventionally, in order to improve the wear resistance and fatigue strength of steel parts, a technique of performing shot peening after carburizing the steel parts has been used.
The carburizing process is a technique for forming a surface layer having a high carbon content by diffusing carbon atoms from the surface of a steel part. The shot peening is a technique for imparting compressive residual stress to the surface of the component to be processed by striking hard particles such as steel particles called shot grains against the component to be processed with a compressed air flow or the like.

The shot peening can change the distribution of compressive residual stress applied to the surface of the component to be processed by changing the grain size and hardness of the shot grains.
The relationship between the grain size of shot grains and the distribution of compressive residual stress will be described with reference to FIG. Curves L11 to L13 in FIG. 4 indicate the relationship between the depth from the surface of the processed component and the compressive residual stress, and it is assumed that the grain size of the shot grains increases in this order. As shown in FIG. 4, the compressive residual stress can be applied to the portion where the depth from the surface of the processing component is large at the peak of each curve as the particle diameters of the curves L11, L12, L13 and the shot grains increase. is there.

  Furthermore, the relationship between the hardness of shot grains and the distribution of compressive residual stress will be described with reference to FIG. Curves L21 to L23 in FIG. 5 indicate the relationship between the depth from the surface of the processed component and the compressive residual stress, and it is assumed that the hardness of the shot grains increases in this order. As shown in FIG. 5, as the hardness of the curves L21, L22, and L23 and the shot grains increases, a large compressive residual stress can be applied at the peak of each curve.

  On the other hand, troostite or the like, which is a carburizing abnormal layer having a thickness of 20 μm to 30 μm, is formed on the surface of a steel part subjected to carburizing treatment (hereinafter referred to as carburized part). Since this carburizing abnormal layer is soft, even if shot peening is performed after carburizing as described above, it is difficult to impart compressive residual stress to the surface of the carburized part, and the cost for improving fatigue strength is reduced. There was a problem.

  Specifically, FIG. 6 shows a distribution of compressive residual stress in a carburized component that has been shot peened by the conventional technique. The prior art performs shot peening only for shot grains after carburizing treatment. In FIG. 6, the horizontal axis indicates the depth (μm) from the surface of the carburized component, and the vertical axis indicates the compressive residual stress (MPa) applied.

  As shown in FIG. 6, in shot peening in the prior art, although a large compressive residual stress can be applied as compared with before shot, carburization is performed as described above in a portion where the depth from the surface of the carburized component is shallower than about 30 μm. A sufficient compressive residual stress could not be applied because the abnormal layer remained. Specifically, the compressive residual stress is about -500 MPa to -1100 MPa in the depth portion where the carburized abnormal layer is generated.

As a first method for solving the above-described problem, a technique related to heat treatment that does not generate an abnormal carburization layer, such as vacuum carburization, has been proposed.
However, it is difficult to apply the technique to impart compressive residual stress to the surface of the carburized component because the lead time and work cost are enlarged compared to a general carburizing method. there were.

As a second method for solving the above problem, a technique for removing a carburized abnormal layer before performing shot peening has been proposed. Specifically, a method of chemically removing an abnormal carburization layer using electropolishing (see, for example, Patent Document 1) or a method of removing an abnormal carburization layer by performing shot peening separately (for example, Patent Document 2). See).
However, in the techniques described in Patent Document 1 and Patent Document 2, it is necessary to add a new process in order to remove the carburizing abnormal layer, which leads to an increase in lead time and work cost. was there.

As a third method for solving the above problem, there has been proposed a method of simultaneously removing a carburizing abnormal layer and applying compressive residual stress by one shot peening (see, for example, Patent Document 3).
However, in the technique described in Patent Document 3, the particle size and hardness of shot grains in shot peening are limited in order to effectively remove the carburizing abnormal layer. Specifically, the particle diameter of the shot grains is limited to between 0.15 mm and 0.3 mm in order to preferentially peel off the carburized abnormal layer. Further, since only the abnormal carburizing layer is removed without removing the normal carburized layer, the hardness of the shot grains is limited to between Hv 600 and 750.
For this reason, the technique described in Patent Document 3 causes a problem that the grain size and hardness of the shot grains are changed by shot peening and a desired compressive residual stress cannot be applied. In addition, since the distribution of compressive residual stress is determined as one pattern, the distribution of compressive residual stress cannot be changed depending on the target part.

Furthermore, a technique for performing shot peening by mixing shot grains and molybdenum disulfide has also been proposed (see, for example, Patent Document 4).
However, although the technique described in Patent Document 4 is intended to improve the strength of the carburized component, it is not intended to remove the carburizing abnormal layer. That is, the reduction in compressive residual stress due to the formation of a carburized abnormal layer could not be eliminated.

JP-A-62-203766 JP 61-265271 A JP-A-4-269166 JP 2005-187850 A

  Therefore, in view of the present situation, the present invention can impart desired compressive residual stress inexpensively and easily without adding a new process when carburizing treatment and shot peening are used in combination, and It is an object of the present invention to provide a method for strengthening a carburized component capable of changing the distribution of compressive residual stress depending on the target component.

  The problem to be solved by the present invention is as described above. Next, means for solving the problem will be described.

  That is, in Claim 1, it formed in the surface of the said carburizing process component by mixing and projecting the shot grain which consists of steel particles, and the abrasive | polishing agent which consists of ceramic particles on the surface of a carburization process component. Simultaneously with removing the abnormal carburizing layer, compressive residual stress is applied to the surface of the carburized component.

  According to a second aspect of the present invention, as the depth from the surface of the carburized component that imparts compressive residual stress increases, the particle size of the shot grains is greatly changed.

  According to a third aspect of the present invention, the hardness of the shot grain is greatly changed as the compressive residual stress applied to the carburized component increases.

  As effects of the present invention, the following effects can be obtained.

  According to the present invention, when performing carburizing treatment and shot peening in combination, high compressive residual stress can be imparted inexpensively and easily on the outermost surface of the carburized component without adding a new process, and The distribution of compressive residual stress can be changed depending on the target part.

(A) is the schematic of the base material to which the reinforcement | strengthening method of the carburizing process component which concerns on one Embodiment of this invention is applied, (b) is the figure which showed the conditions of the carburizing process similarly. (A) is an expanded sectional view of the carburized component that has been shot peened by the prior art, and (b) is an enlarged sectional view of the carburized component that has been shot peened by the method for strengthening a carburized component according to one embodiment of the present invention. The figure which showed distribution of the compression residual stress in the carburizing process component shot-peened by the reinforcement | strengthening method of the carburization process part which concerns on one Embodiment of this invention. The figure which showed the relationship between the particle size of shot grain, and distribution of compressive residual stress. The figure which showed the relationship between the hardness of shot grain, and distribution of compressive residual stress. The figure which showed the distribution of the compressive residual stress in the carburizing process component shot-peened by the prior art.

Next, embodiments of the invention will be described.
It should be noted that the technical scope of the present invention is not limited to the following examples, but broadly covers the entire scope of the technical idea that the present invention truly intends, as will be apparent from the matters described in the present specification and drawings. It extends.

An experiment conducted by the applicant of the present application regarding a method for strengthening a carburized component according to an embodiment of the present invention will be described with reference to FIG.
In the method for strengthening a carburized component according to the present embodiment, as shown in FIG. 1A, the base material W is a rectangular parallelepiped member whose material is SCr420, whose bottom is 20 mm, and whose length is 50 mm.
And carburizing with respect to the said base material W was performed in the carburizing furnace. As shown in FIG. 1B, the carburizing conditions are as follows: the carburizing temperature is set to 950 ° C., the carbon potential in the carburizing atmosphere is set to 0.8, then the temperature is lowered to 850 ° C., and finally the workpiece is put into the quenching oil at 130 ° C. Oil quenching (oil cooling).

Thereafter, shot grains and an abrasive were mixed and projected (shot peening) on the surface of the carburized component, which is the base material W subjected to carburization. As the shot grains, steel grains having a grain size of 0.8 mm and a hardness of Hv800 were used. Moreover, the said abrasive | polishing agent used the particle | grains made from aluminum oxide with a particle size of 0.5 mm. The abrasive is applicable as long as it is a general abrasive for ceramic particles.
Further, the shot grains and the abrasive used were mixed so that the weight of the shot grains was twice the weight of the abrasive. In addition, the shot coverage was set to 300%, the pressure was set to 0.3 MPa, and the shot grains and the abrasive were mixed to perform projection.

  Next, the experimental result regarding the strengthening method of the carburized component according to the present embodiment will be described with reference to FIGS. 2 and 3 in comparison with the experimental result in the prior art. In addition, the experiment in a prior art is what projected only a shot grain (shot peening), after performing a carburizing process similarly to this embodiment.

FIG. 2A is an enlarged cross-sectional view of a carburized component that has been shot peened by a conventional technique, and FIG. 2B is an enlarged cross-sectional view of the carburized component that has been shot peened by the method for strengthening a carburized component according to the present embodiment. is there.
With respect to the carburized component according to this embodiment, the carburized abnormal layer formed on the carburized component could be removed. Specifically, as for the experimental results in the prior art, as shown in FIG. 2 (a), the carburized abnormal layer remained with a thickness of 20 μm to 30 μm from the surface of the carburized component. As for the carburized parts, the shot grains and the abrasive were mixed and projected, so that the carburized abnormal layer could be removed by the abrasive as shown in FIG. 2 (b).

  FIG. 3 shows the distribution of compressive residual stress in the carburized part subjected to shot peening by the carburized part reinforcing method according to this embodiment. In FIG. 3, the horizontal axis indicates the depth (μm) from the surface of the carburized component, and the vertical axis indicates the compressive residual stress (MPa) applied.

  As shown in FIG. 3, in the experimental results using the method for strengthening a carburized component according to the present embodiment, a sufficient compressive residual stress could be applied by removing the carburizing abnormal layer as described above. . Specifically, a compressive residual stress of about -1200 MPa to -1400 MPa could be applied in a portion shallower than 30 μm from the surface of the carburized component. That is, compared with the carburized component according to the prior art shown in FIG. 6, the compressive residual stress can be improved about 2.3 times on the outermost surface.

  According to this embodiment, as described above, when carburizing treatment and shot peening are performed in combination, high compressive residual stress is imparted inexpensively and easily on the outermost surface of the carburized component without adding a new process. It can be done.

Moreover, the distribution of the compressive residual stress can be changed depending on the target part at the same time as the carburizing abnormal layer is removed by one shot peening. Specifically, since the carburizing abnormal layer is removed by the abrasive mixed with the shot grains, the shot grains can be used only for applying compressive residual stress.
Specifically, as the depth from the surface of the carburized component that imparts compressive residual stress increases, the particle size of the shot grains is greatly changed, and as the compressive residual stress imparted to the carburized component increases. The hardness of the shot grain is greatly changed.

  More specifically, when compressive residual stress is applied to a portion having a small depth from the surface of the carburized component, the particle size of the shot grains is in a range of 0.05 mm to 0.3 mm, and the carburizing treatment is performed. When compressive residual stress is applied to a portion having a medium depth from the surface of the part, the grain size of the shot grains is within a range of 0.3 mm to 0.8 mm, and the depth from the surface of the carburized part is When compressive residual stress is applied to a portion having a large thickness, the grain size of the shot grains is set within a range of 0.8 mm to 1.1 mm.

  Further, when applying a small compressive residual stress to the carburized component, the hardness of the shot grain is in the range of Hv450 to 600, and when applying a moderate compressive residual stress to the carburized component, When the hardness of the shot grains is in the range of Hv 600 to 700 and a large compressive residual stress is applied to the carburized component, the hardness of the shot grains is set in the range of Hv 700 to 900.

  W base material

Claims (3)

At the same time as removing the carburizing abnormal layer formed on the surface of the carburized part by mixing and projecting shot particles made of steel particles and abrasives made of ceramic particles on the surface of the carburized part, Applying compressive residual stress to the surface of the carburized component,
A method for strengthening a carburized component. As the depth from the surface of the carburized component imparting compressive residual stress increases, the particle size of the shot grains is greatly changed.
The method for strengthening a carburized component according to claim 1, wherein: As the compressive residual stress applied to the carburized component increases, the hardness of the shot grains is greatly changed.
The method for strengthening a carburized component according to claim 1 or 2, characterized in that

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