JPH08302425A - Method for surface treatment of steel member - Google Patents

Abstract

PURPOSE: To provide a steel member whose surface property and fatigue strength are improved by achieving the surface treatment where the bearing pressure to the yield stress is specified after achieving the shot peening on the surface of the steel member. CONSTITUTION: The shot peening is achieved on the surface of a steel member. The surface treatment is further achieved on the surface of the steel member by the bearing pressure of >=1/20 to <=1/2 of the yield stress. The shot peening is preferably achieved under the condition where the arc height value is >=0.40mmA. The bearing pressure in the surface treatment of the surface hardened steel member is preferably achieved based on the yield stress to be obtained as the yield stress equivalent stress (Mpa)=-337+3.40×HV, where HV is the maximum value of the Vickers hardness on the surface layer. Only fine projected parts of the surface roughness are plastically deformed to improve the surface property, and the excellent surface compressive residual stress is generated to improve the fatigue strength.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of surface-treating steel members for the purpose of improving the fatigue strength of gears, bearings, shafts, etc., which often cause fatigue failure due to repeated stress.

[0002]

2. Description of the Related Art With respect to mechanical structural members such as gears, bearings, and shafts that are subjected to repeated stress, improvement of fatigue strength is an important issue, and many methods have been proposed. Among them, shot peening is to project hard particles such as steel balls onto the material surface at high speed, and mainly due to two factors, an increase in surface hardness and the application of compressive residual stress to the surface layer, Greatly improves fatigue strength. In recent years, in order to give higher fatigue strength, the projection strength of shot peening is increased and the influence depth is increased.

On the other hand, another method for improving the fatigue strength is surface rolling (rolling). This is to press a rotating body such as a roll against the surface of the target member under a certain rolling pressure, and in this case, not only the effect of imparting surface compressive residual stress can be obtained as in shot peening, but also the surface texture. Can also be improved.

[0004]

However, the increase in the shot peening projection intensity increases the depth of influence, but on the contrary, decreases the compressive residual stress value of the outermost surface, which promotes the initiation of cracking. There is a risk. further,
Since the surface roughness deteriorates as the shot peening strength increases, this point is also regarded as a problem.

As a method of compensating for the compressive residual stress on the outermost surface, it is known that the fatigue strength is improved by applying a two-step peening method in which ordinary shot peening is performed and thereafter particles having a smaller particle size are projected. Has been. However, in this case, the surface quality is not so improved,
For this reason, the fatigue strength improving effect is not always sufficiently satisfactory.

On the other hand, without performing shot peening,
When only the surface processing and rolling method is applied, a high hardness material whose surface Vickers hardness is about 250 or more by being subjected to surface hardening treatment by appropriate heat treatment such as carburizing, induction hardening and nitriding in advance. In the above, extremely high pressure is required to plastically deform from the surface to a certain depth. Therefore, not only a large-scale device is required, but also if a large surface pressure is applied to such a member having low toughness, cracks may be introduced or, in some cases, collapse may occur.

The present invention has been made in view of the above-mentioned conventional problems, and an object thereof is to provide a surface processing method for a steel member capable of imparting high fatigue strength while improving surface properties with a simpler construction. I am trying.

[0008]

In order to achieve the above-mentioned object, a method for surface-treating a steel member according to the present invention comprises a step of subjecting the surface of a steel member to shot peening and then yielding the surface of the member. The feature is that surface processing is performed with a surface pressure of 1/20 or more and 1/2 or less of the stress. The shot peening is preferably performed under the condition that the arc height value is 0.40 mmA or more.

On the other hand, the surface pressure of the surface-hardened steel member at the time of the above-mentioned surface processing is set by setting the maximum value of the Vickers hardness in the surface layer to HV, yield stress equivalent stress (MPa) =-337 + 3.40. It is performed based on the yield stress equivalent stress (MPa) obtained by × HV.

[0010]

[Operation] When a compressive residual stress is applied to the surface by shot peening, both the initiation and propagation of fatigue cracks are suppressed, but if the shot peening strength is increased to increase the effect, the compression of the outermost surface will occur. Residual stress is insufficient. Therefore, in the present invention, in order to compensate for this, surface rolling is continuously performed. This surface processing is mild with a surface pressure of 1/20 or more and 1/2 or less of the yield stress value of the surface layer. As a result, only the convex portions having a small surface roughness are plastically deformed to improve the surface quality, and good surface compressive residual stress is generated, so that the fatigue strength is improved.

In the range of 1/2 or less, the higher the surface pressure is, the larger the amount of improvement in fatigue strength tends to be. However, the effect is remarkably enhanced if the yield stress value is 1/10 or more. . On the other hand, when the surface pressure is increased to more than 1/2 of the yield stress, not only the residual compressive stress value near the surface becomes insufficient but also deformation causes microcracks, which causes damage and fatigue strength. Occurs when there is a decrease. Therefore, by subjecting the surface to a surface pressure in the range of ½ or less, the fatigue strength can be improved without giving extra damage to the material.

On the other hand, the application of compressive residual stress by shot peening for suppressing the initiation and propagation of fatigue cracks is
It is necessary to have a certain depth or more. From this perspective,
The arc height value is preferably 0.40 mmA or more, and 0.60
If it is mmA or more, a greater effect can be obtained. The upper limit of the arc height value is not particularly limited, but if the arc height value is unnecessarily increased, there is a concern that the surface roughness will remarkably increase or that the characteristics may be deteriorated due to the introduction of cracks. Since both the depth and the amount of the residual stress applied are arc height values of about 1.3 mmA or less, the level of the object of the present invention can be sufficiently achieved, and it is preferable to set this level as the upper limit.

If the surface hardening treatment is performed by an appropriate heat treatment (carburizing, induction hardening, nitriding, etc.) before the shot peening, it is difficult to measure the yield stress of the surface layer. The maximum value of Vickers hardness HV is
If the stress exceeds 400, the yield stress equivalent stress [MPa] = − 337 + 3.40 × HV is used to obtain the equivalent stress equivalent to the yield stress, and based on this, the surface pressure during surface processing may be set.

[0014]

EXAMPLES Next, specific examples of the present invention will be described. As shown in Table 1, JIS SCM 420 steel is used, which is carburized at 930 ° C for 3 hours, then held at 850 ° C for 10 minutes, oil-cooled and oil-quenched, and then at 180 ° C. And tempered for 2 hours.

Next, using a shot having a grain size of 0.8 mm,
Arc height value is 0.3mmA, 0.4mmA, 0.5mm
Shot peening was performed under various conditions of A, 0.6 mmA, and 0.8 mmA to prepare test pieces. Then, for various test pieces produced under the conditions of each shot peening,
Further, the surface rolling is performed by the roller rolling method under the condition that the ratio of the surface pressure to the yield stress is varied within the range of 1/20 to 1/2, whereby the shot peening condition and the surface rolling condition are obtained. Test pieces with various combinations with were prepared.

As the surface processing rolling, a method of pressing a roller-shaped rotating body having a small radius of curvature against the surface of the test piece at a set pressure was adopted. Incidentally, there are some other methods as such surface processing rolling, and the method is not limited to the roller rolling method described above, but from the viewpoint of the contact area (the smaller the area, the higher the surface with the lower load). Since the pressure can be obtained) and it can be easily applied to the case where the surface of the component is a curved surface, the above method is simple.

On the other hand, various surface pressures at the time of surface processing and rolling were measured by measuring the surface hardness of a test piece surface-reinforced by carburization.
The HV 780 obtained as the maximum value of the surface hardness was substituted into the following equation, and the yield stress equivalent stress was set based on 2315 MPa. Yield stress equivalent stress [MPa] =-337 + 3.40 × HV Then, for each of the above test pieces, stress concentration factor = 1.3
As a result, a fatigue test by Ono-type rotary bending test was performed.
The test results and the surface roughness measurement results are shown in Table 2 together with the shot peening conditions and the surface processing and rolling conditions.
Examples 1 to 14 are shown below.

In the same table, corresponding to each of Examples 1 to 14, only the carburizing treatment was performed and the shot peening and the surface processing rolling were not performed (Comparative Example 1), and the shot peening after the carburizing treatment. Only subjected to no surface processing and rolling (Comparative Examples 2 to 5, 8),
Surface rolling after shot peening with a ratio of surface pressure to yield stress exceeding 1/2 (Comparative Examples 6, 7 and 9), surface treatment without shot peening after carburizing treatment Rolled only (Comparative Examples 10 and 1)
1) is also shown.

Comparative Example 12 is one in which two-step peening was performed after the carburizing treatment, in which the shot particle size in the first step was
The value is 0.6 mm / arc height value 0.8 mmA, and the second shot grain size is 0.3 mm / arc height value 0.2 mmA.

[0020]

[Table 1]

[0021]

[Table 2]

As is clear from Table 2, fatigue strength is improved by rolling after shot peening. Although the effect of rolling is obtained even by light rolling with a pressure of 1/20 of the equivalent yield stress of the surface layer (Example 4),
Especially, the effect is large when it is set to 1/10 or more. In addition, the condition that the surface rolling exceeds 1/2 of the yield stress (Comparative Example 6.7.
On the contrary, in 9), the effect is decreased, and as in Comparative Example 7,
Depending on the conditions, the fatigue strength before rolling (Comparative Example 5) is lower than that. It is considered that this is because the residual stress value near the outermost surface was rather small and some damage such as the generation of microcracks was imparted.

On the other hand, regarding the influence of the shot peening strength, when the arc height value is 0.3 mmA as in Example 1, the fatigue strength is improved by the rolling process, but the effect is relatively small. This is considered to be due to insufficient suppression of crack growth due to the small depth of application of compressive residual stress. On the other hand, when the arc height value is 0.4 mmA and 0.5 mmA, the effect of improving the fatigue strength is increased, and when the rolling process is performed at 0.6 mmA or more, the fatigue strength is significantly improved. Has been achieved. As shown in Comparative Examples 10 and 11, the effect of rolling only was examined without performing shot peening, but it was found that the effect of improving fatigue strength was smaller than that of each example.

The compressive residual stress distribution in the two-step peened material (arc height value of the first step is 0.8 mmA) shown in Comparative Example 12 is 1/3 of the yield stress shown in Example 13 Although it was separately confirmed that the fatigue strength was lower than that, and considering the measurement results of the surface roughness, the improvement of the surface roughness had a great influence on the improvement of the fatigue strength. It is presumed to have.

As described above, by performing the light surface working rolling with the surface pressure of 1/2 or less of the yield stress of the surface layer after the shot peening, the result showing better fatigue strength than the conventional one was obtained.

[0026]

As described above, according to the present invention,
After shot peening, by performing a mild surface processing that plastically deforms only the minute projections of the surface roughness in the surface layer, it is difficult to obtain a large fatigue strength improvement effect especially with only surface processing rolling. Even for a high material, the compressive residual stress is deeply applied to the inside, and the residual stress on the outermost surface is increased, and the surface property is improved. As a result, with a simpler structure, it becomes possible to further improve the surface properties and the fatigue strength as compared with the conventional case.

Claims (3)

[Claims] 1. After shot peening is applied to the surface of a steel member, 1/20 of the yield stress is further applied to the surface of the member.
As described above, the surface processing method for a steel member is characterized by performing surface processing with a surface pressure of 1/2 or less. 2. The method for surface treating a steel member according to claim 1, wherein the shot peening is performed under an arc height value of 0.40 mmA or more. 3. The surface pressure of the steel member subjected to the surface hardening treatment at the time of the surface processing is set so that the maximum value of the Vickers hardness in the surface layer is HV, and the yield stress equivalent stress (MPa) = − 337 + 3.40. 3. The method for surface-treating a steel member according to claim 1 or 2, which is carried out based on a yield stress equivalent stress (MPa) obtained by xHV.