JPH08277417A - Method for treating surface of steel worked slab having corner part - Google Patents

Abstract

PURPOSE: To impart high residual stress even to the one having a complicated shape such as a gear to a further depth and to moreover improve its fatigue strength. CONSTITUTION: A steel worked piece having a corner part is subjected to carburizing and quenching treatment and tempering treatment and is thereafter subjected to primary shot peening, and the treated face is subjected to grinding or cutting treatment and is thereafter subjected to secondary shot peening as well. As the cutting treatment, hard shaving is preferably executed. Moreover, in the case a gear is subjected to surface treatment, the primary shot peening is executed preferably in such a manner that shot grains having >=60HRc hardness and 0.6 to 0.8mm grain diameter are projected at 70 to 90m/sec projecting rate, and the secondary shot peening is executed preferably in such a manner that shot grains having 800 to 2000HV hardness and 0.05 to 0.2mm grain diameter projected at 70 to 90m/sec projecting rate.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a work piece having a complicated shape with corners, and more particularly to a method for surface treatment of a gear forming a drive system unit of a vehicle. More specifically, the present invention provides
The present invention relates to a surface treatment method capable of imparting a fatigue strength superior to that of a conventional steel work piece having a corner portion.

[0002]

2. Description of the Related Art Steel parts for mechanical structures, which are required to have high fatigue strength, have a complicated shape with corners like a gear forming a drive train unit of a vehicle. Some of them are required to have complicated properties such as a surface having abrasion resistance, strong resistance to repeated dynamic impact, and minimal distortion. In recent years, in particular, gears have been required to have higher fatigue strength so as to withstand the severe conditions of higher speed and higher load as the output of the engine increases. It cannot be increased in size due to a request, and since the shape is complicated, thermomechanical processing such as ausforming cannot be performed due to processing such as rolling. Therefore, as a general method of improving fatigue strength, it has been proposed to subject a gear to a carburizing treatment to harden the surface while maintaining the internal toughness, and then subject the gear to shot peening. According to this method, theoretically, the austenite remaining by the carburizing, quenching and tempering treatment is induced to be transformed into martensite by the stress applied by shot peening, so that the surface layer portion,
It is believed that compressive residual stress can be introduced with surface hardening.

Regarding the specific conditions of shot peening for a carburized steel work piece, in shot peening, the larger the particle size of the shot grains and the higher the projection speed, the greater the energy of the shot. To improve the shot peening effect,
It is necessary to project shot particles having a large particle size at a high speed, but if the shot particles are too large, a complicated shape (for example,
If the projection speed is too high, the surface roughness will increase or minute cracks will occur on the surface, and if the projection is carried out for a certain period, the fatigue strength will be saturated. From this, in JP-A-60-218422, the optimum conditions for shot peening are to select a shot grain diameter of 0.3 to 1.0 mm, a projection time of 5 to 40 minutes, and a projection speed of 35 to 50 m / s. Is proposed.

Further, in Japanese Patent Laid-Open No. 61-79719, shot peening is performed in two steps, and relatively large shot grains are used in primary shot peening.
It has been reported that in secondary shot peening, the optimum residual stress distribution morphology can be obtained by using relatively small shot grains. And, JP-A-6-14578
No. 5, in the case of applying the above-described two-step shot peening method to a carburized steel processed piece equipped with a gear, a primary shot peening is applied to the carburized steel processed piece by hardness Hv700.
As described above, shot particles having a grain size of 0.6 to 1.2 mm are used, and after tempering treatment at 100 to 200 ° C., secondary shot peening is further performed with a hardness of Hv 700 or more and a grain size of 0.1 to 0. It has been reported that the use of 5 mm shot grains can significantly improve the fatigue strength without missing corners such as the cutting edge.

However, in recent years, some mechanical parts, particularly gears, are required to have fatigue strength that can withstand even more severe conditions.

[0006]

Therefore, the present invention has a complicated shape like a gear constituting a drive train unit of a vehicle, and the surface of the gear has a wear resistance as a requirement for its mechanical properties. However, in order to obtain a steel work piece that is strong against repeated dynamic impact and is suitable as a steel part for machine structural use that requires complex characteristics such as minimal distortion, and has higher fatigue strength than before, An object is to provide a method for treating the processed piece.

[0007]

[Means for Solving the Problems] The effect of the two-step shot peening method, which has been conventionally recognized as having a mechanical strength suitable for use in a gear or the like, has been earnestly studied.
In carburizing and tempering steels with abnormal surface structure, fatigue is not always from the outermost surface because a soft surface abnormal structure due to internal oxidation of alloy elements is formed along the grain boundaries at a certain depth from the surface. Since it does not always start, and it may start from the tip, there must be a certain amount of compressive residual stress at the tip, but even with the two-stage shot peening method, after all, primary shot peening Since only the residual stress at the tip is introduced, in order to impart even higher fatigue strength to the carburized and tempered steel, it is necessary to perform the primary shot peening more strongly, but on the other hand, too much. If you perform strong primary shot peening,
Surface roughness is increased, and microcracks are generated on the surface, and secondary shot peening worsens it, which in turn reduces the fatigue life. If the steel workpiece has a shape with a corner like a gear, the corner of the gear may be missing before or immediately after it is actually used.

In Japanese Patent Laid-Open No. 6-145759, a treated surface is subjected to a tempering treatment at 130 to 200 ° C. between the primary shot peening and the secondary shot peening to withstand the impact during the secondary shot peening. Although it has been proposed to increase the toughness so as to obtain, the tempering process necessarily involves thermal deformation. Further, it is difficult to control the depth of structural transformation due to tempering with high accuracy, and the toughness will be increased more than necessary. More importantly, in the above method, the surface roughened by the primary shot peening is directly subjected to the secondary shot peening. Therefore, if the primary shot peening is strengthened, the tooth surface is scraped off during the secondary shot peening. On the other hand, if grinding is performed as a post-treatment after the secondary shot peening, the original advantage of the two-step shot peening cannot be utilized.

On the other hand, the inventor of the present invention surprisingly found that after carburizing and tempering the steel work piece having the corners, the shot peening was performed. , If the treated surface is subjected to cutting or grinding treatment, even if the primary shot peening is performed stronger than before, if the treated surface is prepared by the cutting or grinding treatment, and then the secondary shot peening is further applied to the treated surface, Needless to say, the corners are missing, no microcracks are observed on the treated surface, and
It was found that heat was not generated and therefore thermal deformation was not generated, and the present invention was completed based on the above findings.

According to the method for treating a steel work piece having a corner of the present invention, after carburizing and tempering treatment, primary shot peening is performed, the treated surface is ground or cut, and then, In addition, secondary shot peening is further applied to the treated surface.

The primary shot peening is more powerful than that used in the conventional two-stage shot peening method. Specifically, hardness Hc: 60 or more and particle size 0.6 to
0.8 mm shot particles are projected at a projection speed of 70 to 90 m / sec. When shot peening is performed in this range,
1400M when the depth from the surface is 50-60μm
Residual stress of Pa or more is applied. The projection time is
The peening effect was insufficient when it was less than 6 seconds, but after 19 seconds, it was almost saturated, so that it is preferably 6 to 19 seconds. The depth from the surface of carburizing is shallower than that of solid carburizing when the carburizing method used is liquid carburizing, but it cannot be used with precision gears if the tooth surface wears out by about 0.05 mm. In general, the tip of the abnormal surface tissue is 50-60.
Since the treatment is performed so as to exist at or near μm, the method of the present invention can consequently increase the magnitude of the compressive residual stress at the point corresponding to the tip.

Regarding the hardness of shot grains, Aihara et al., "Improvement of Fatigue of Carburized and Quenched Material by Shot Peening" (Proceedings of the Japan Society of Mechanical Engineers, Material Mechanics Conference 319)
As reported in (1990) 269-271), the higher the hardness is, the larger the impact force is, and the work-induced transformation of retained austenite existing in the carburizing and tempering state to martensite is promoted. , Hc:
It was set to 60 or more. It will be understood by those skilled in the art that the hardness of shot particles has an upper limit from the viewpoint of using shot particles that can be stably mass-produced. The material is, but is not limited to, steel or cast steel. Regarding the particle size, if it is smaller than 0.6 mm, the maximum value of the compressive residual stress cannot be given at the depth of 50 to 60 μm from the surface, and
If it is larger than 0.8 mm, peening to the groove portion of the gear becomes impossible with a commonly used gear shape, so the thickness is set to 0.6 to 0.8 mm. The projection speed is
It is 70 to 90 m / sec.

The cutting or grinding treatment is performed to roughen the surface by primary shot peening or to smooth the portion that has become the protrusion. The cutting or grinding allowance should be as small as possible,
At most, it is preferably 30 μm or less. This is because if it is too much, the compressive residual stress layer formed by carburizing and tempering treatment and further primary shot peening will decrease. As a processing method, in addition to a cutting method such as shaving processing or honing processing, a grinding method corresponding to the shape of the gear can be used. Of these, the preferred processing method is the hard shaving method. The hard shaving method is basically the same as the shaving method used as a finishing process after cutting a tooth to form a tooth mold, and the cutter used has a large number of grooves on the tooth surface. The line of intersection with the surface serves as a cutting edge to cut off the tooth flank of the gear. Since this cutter does not have No. 2, the cutting force is small, but the accuracy can be increased. The hard shaving method is characterized by using a tool with a CBN coating on the shaving cutter and rotating the shaving cutter and the gear together to perform synchronous processing. It has the advantage of being small. The cutting agent used at this time is preferably an oil having excellent lubricity and detergency.

The material of the cutter used for hard shaving is preferably CBN because it can be cut at high speed and can be cut efficiently.

When the gear is hard shaving, a shaving step of a larger or smaller size is inevitably formed before and after the dressing of the cutting tool and the degree of wear thereof. However, in the present invention, during the secondary shot peening. Since the shaving step is removed, the smoothness is maintained to a satisfactory level.

When used as a gear treatment, the secondary shot peening is performed so that a compressive residual stress of about -1000 MPa is preferably applied to the outermost surface in order to achieve an appropriate surface hardening effect. The hardness and projection speed of the grains are adjusted. Specifically, the particle size is 0.05
Is 0.2 mm and the hardness is Hv 800-2000. The projection speed is 70 to 90 m / s, and the projection time is 4 to 6 seconds. Usually, glass beads are used as shot grains.

[0017]

According to the treatment method of the present invention, a large residual compressive stress is applied to the tip of the abnormal surface tissue and a large surface hardness is imparted to the outermost surface, as compared with the conventional treatment methods. Moreover, even when the above-mentioned treatment is applied to the surface provided with the corners, at least the same treated surface as that obtained when the conventional treatment method is used can be obtained. That is, even if the processed product has a corner, no chipping occurs, the surface is smooth, there is no deformation, and it is not necessary to correct it before using it for practical use.

[0018]

EXAMPLES The following examples of the present invention are provided solely for the purpose of illustration and are not intended to limit the scope of the invention.

Example 1 A standard test piece of a fatigue test method was prepared from a low alloy steel for machine structure (for example, SCM420), and carburized and tempered for an effective hardening depth of 55 μm. After that, primary shot peening was performed under the following conditions.

Hardness of shot particles: HRc60 Particle size: Shot particles of 0.6 mm Material: Steel Air pressure: 5.5 kg / cm ² Projection amount: 18 kg / min Projection distance: 120 mm Projection time: 6 seconds Under these conditions, The projection speed was within the range of 70 to 90 m / sec.

The depth from the surface of the obtained test piece (μ
The relationship between m) and compressive residual stress (MPa) is shown in FIG.
As is clear from FIG. 1, the depth from the surface is 50-60.
The maximum compressive residual stress of about -1400 MPa was applied at the position of μm.

Subsequently, secondary shot peening was performed under the following conditions using shot particles having a smaller diameter than that used in the primary shot peening.

Shot particle hardness: Hv2000 Particle size: 0.2 mm Material: Glass beads Air pressure: 5.0 kg / cm ² Projection distance: 120 mm Projection amount: 1.6 kg / min Projection time: 6 seconds The projection speed is It was 80 m / sec. Depth (μm) from the surface of the obtained test piece and compressive residual stress (MPa)
The relationship with was as shown in FIG. As is clear from FIG. 2, a compressive residual stress of about −1000 MPa was applied to the outermost surface layer. However, the processing surface was rough.

On the other hand, in order to evaluate the method of the present invention, the treated surface was ground and finished under the following conditions between the primary shot peening and the secondary shot peening to obtain a depth of 30 μm from the surface. Grinded to a degree.

In this case, the surface was smoothed and the protrusions disappeared. The obtained relationship between the depth (μm) from the surface of the test piece and the compressive residual stress (MPa) was as shown in FIG. As is clear from FIG. 3, the outermost layer-
A compressive residual stress of about 1000 MPa was applied.
Further, it can be seen that the residual compressive stress of about -1400 MPa is applied to a portion having a depth of 50 to 60 μm from the surface which is considered to be the tip portion of the abnormal tissue.

Example 2 A helical gear having the following shape was manufactured from the same material as that used in the above example.

Module: 3.5, helix angle: 23 ° Number of teeth: 31 Tooth width: 30 And carburizing and tempering treatment, primary shot peening, and secondary shot peening of the gear under the same conditions. Was applied.

At this time, hard shaving was performed between the primary shot peening and the secondary shot peening under the following conditions. The shaving allowance is the same as the cutting allowance in the first embodiment.

Feed rate: 0.1 mm / rotation Circumferential speed: 2000 m / min No missing part was observed at the tooth tip. Also, the surface
Smoothness that does not require further grinding (specifically, R
_max : about 3 to 4 μm).

[0030]

According to the surface treatment method of the present invention, even in a complicated shape such as a gear, a large compressive residual stress can be applied deeper and fatigue strength can be further improved.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the compressive residual stress and the depth from the surface after primary shot peening of a standard test piece.

FIG. 2 is a diagram showing the relationship between the compressive residual stress and the depth from the surface of a standard test piece after the primary shot peening and the secondary shot peening.

FIG. 3 is a diagram showing the relationship between the compressive residual stress and the depth from the surface of the standard test piece after the primary shot peening and the secondary shot peening with the hard shaving interposed therebetween.

Claims (4)

[Claims] 1. A steel work piece having a corner portion is subjected to carburizing quenching treatment and tempering treatment, followed by primary shot peening, and the treated surface is ground or cut, and then further treated. A surface treatment method for a steel work piece, characterized by subjecting a surface to secondary shot peening. 2. The surface treatment method for a steel workpiece having a corner according to claim 1, wherein hard shaving is performed as the cutting treatment. 3. A gear is used as a processed piece, and primary shot peening is performed, and the hardness is HRc: 60 or more and the grain size is 0.
The method for surface treatment of a steel work piece according to claim 1 or 2, which is carried out by projecting shot particles of 6 to 0.8 mm at a projecting speed of 70 to 90 m / sec. 4. Secondary shot peening, hardness H
V: 800 to 2000, particle size 0.05 to 0.2 mm
The method for surface treatment of a steel workpiece according to claim 3, which is carried out by projecting the shot particles of No. 2 at a projection speed of 70 to 90 m / sec.