JPH07178668A - Shot peening processing method for steel part - Google Patents

Abstract

PURPOSE:To increase compression residual stress and maximum compression residual stress in a surface layer part and further to decrease surface roughness by performing a shot peening process by using a shot grain smaller than that in the preceding time. CONSTITUTION:After a peening process is applied by a condition of HRC50 or more shot grain hardness and 0.20 to 1.0mm shot grain size against a steel part of applying surface strengthening heat treatment of carburizing and carbonitriding or high frequency quenching, further a shot peening process in the second stage, strictly specifying the condition, is applied. Here based on an optimum arc height value (N) OPTIMUM obtained by a formula I, within a range of 0.03 to 0.45mm shot grain size (however, smaller than shot grain size in the first stage), a shot grain size and projecting time such as satisfying a formula II are calculated. Of this calculated value, an arbitrary condition is set to perform a shot peening process in the second stage. Where, K1 is 9.0X10<14>, (a) is -5.50, K2 is 2.8X10<-3>, (b) is 0.723, and (c) is 0.177.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention improves the fatigue strength of steel parts for machine structures such as gears and various shafts of automobiles, construction machines and industrial machines, which are repeatedly subjected to bending stress and shear stress. For details of the shot peening treatment method, the steel parts that have undergone surface hardening heat treatment such as carburizing, carbonitriding, or induction hardening are subjected to two-step shot peening treatment under appropriate conditions to achieve fatigue fracture of the steel parts. The present invention relates to a shot peening treatment method that has succeeded in easily increasing the resistance to.

[0002]

2. Description of the Prior Art Steel structural parts such as gears and various shafts of automobiles, construction machines and industrial machines, which are repeatedly subjected to bending stress and shear stress, are required to have high fatigue strength and wear resistance. Surface hardening heat treatment such as carburizing, carbonitriding, or induction hardening is effective for extending the life of such steel parts, and is widely applied. Also, for steel parts requiring higher fatigue strength, after subjecting to the surface hardening heat treatment as described above, further shot peening treatment is applied to give a compressive residual stress near the surface to improve the fatigue strength. Has been done.

A technique for subjecting a surface hardening heat treatment to a shot peening treatment further includes, for example, JP-A-60-
Methods such as 184627 and JP-A-61-170511 have been proposed. In the former method, in order to prevent a decrease in surface roughness after the shot peening treatment, first shot peening treatment is performed using shot particles having a particle diameter of 1.0 mm or more, and then the particle diameter is 0.3 mm. The second shot peening process is performed using the following shot particles. In the latter method, the second step shot peening treatment is performed in order to prevent the surface roughness from decreasing after the first step shot peening treatment and increase the compressive residual stress of the steel part.

Shot peening has the effect of imparting compressive residual stress in the vicinity of the surface and increasing the fatigue strength, but on the other hand it reduces the surface roughness of steel parts and is more likely to be the starting point of fatigue cracking. On the contrary, there was a problem that the fatigue strength was lowered, but it can be said that such a problem was overcome by performing the above-described two-step shot peening treatment.

[0005]

When higher fatigue strength is required for steel parts, higher compressive residual stress is required than before, but the two-stage shot peening treatment as shown in the above-mentioned prior art is required. However, such requirements cannot be met, and it is difficult to achieve the desired compressive residual stress.

The present invention has been made under such circumstances, and an object thereof is to achieve a higher compressive residual stress than ever, to remarkably improve the fatigue strength of steel parts, and further to improve the surface roughness. To provide a shot peening treatment method for obtaining a steel part having a small size.

[0007]

The shot peening treatment method of the present invention that has achieved the above object is a two-step shot peening process using shot particles having an HRC of 50 or more for a steel part subjected to a surface hardening heat treatment. The first step shot peening is performed by using shot particles having a shot particle size of 0.20 to 1.0 mm, and the two-step shot peening is performed by an optimum arc height obtained by the following formula (1). Based on the value σ _OPTIMUM , the shot particle diameter: within the range of 0.03 to 0.45 mm (however, smaller than the shot particle diameter of the first step), the shot particle diameter that satisfies the following formula (2) and The point is that the projection time is calculated, an arbitrary condition is set from the calculated values, and the second-stage shot peening process is performed. Optimum arc height value σ _OPTIMUM (mmA) = K ₁ · (hardness Hv of surface layer in as-heat-treated) ^a ± 0.05 (1) (K ₁ = 9.0 × 10 ¹⁴ , a = -5 .501) Optimum arc height value σ _OPTIMUM (mmA) = K ₂ · (shot particle size: μm) ^b · (projection time: seconds) ^c (2) (where K ₂ = 2.8 × 10 ⁻³ , b = 0.723, c = 0.177)

[0008]

The present inventors have studied from various angles in order to achieve the above object. As a result, for steel parts that have undergone surface strengthening heat treatment such as carburizing, carbonitriding or induction hardening,
Shot grain hardness: HRC 50 or more, shot grain size: 0.
After peening treatment under the condition of 20-1.0 mm,
Furthermore, they have found that the above object can be achieved satisfactorily by performing a second-stage shot peening treatment in which the conditions are strictly defined, and completed the present invention.

While explaining the processing conditions in the present invention, the operation in each processing will be described. First, the purpose of the shot peening treatment in the first step is to form high compressive residual stress deeply. From this point of view, it is preferable that the shot grain hardness and shot grain size are large. If the shot grain hardness is less than HRC50, it is difficult to cause large plastic deformation on the surface of the steel part, and it is necessary to set the shot grain hardness to HRC50 or more in order to obtain high compressive residual stress deeply. From the same viewpoint, the shot grain size needs to be 0.2 mm or more.
However, if the shot particle size exceeds 1.0 mm,
Since the deterioration of the surface roughness becomes remarkable, it is necessary to make the maximum 1.0 mm.

The purpose of the second-stage shot peening treatment is to increase the compressive residual stress and maximum compressive residual stress in the surface layer portion by using small shot grains and improve the surface roughness. From this point of view, in the present invention, the shot grain size at this time is defined as 0.03 to 0.45 mm. That is, if the shot grain size is less than 0.03 mm, it is difficult to cause plastic deformation in the surface layer portion, and high compressive residual stress cannot be obtained, and if it exceeds 0.45 mm, the surface roughness decreases. However, the shot grain size in the second-stage shot peening process needs to be smaller than the shot grain size in the first-stage shot peening process. The shot grain hardness needs to be HRC50 or higher because a high compressive residual stress cannot be obtained below HRC50, as in the case of the shot peening treatment in the first step. However, in the second-stage shot peening treatment, it is not sufficient to specify only the shot grain hardness and shot grain size, and the desired steel part may not be obtained.

Therefore, in the present invention, based on the arc height obtained by the equation (1), the shot particle diameter within the range of 0.03 to 0.45 mm satisfies the following equation (2). The projection time was calculated, and the requirement that the second stage shot peening process be performed by setting an arbitrary condition from the calculated values was defined. That is, the desired steel part was obtained by strictly defining the second-stage shot peening process conditions in this manner. The above equations (1) and (2) are derived by regression calculation, and the optimum arc height range of the second step shot peening treatment that optimizes the maximum compressive residual stress is set after the surface hardening heat treatment. Is calculated using the surface layer hardness Hv, and the shot grain size and the projection time during the second stage shot peening treatment are set by the arc height value. The term "surface layer hardness" as used herein means the hardness from the outermost surface after the surface hardening heat treatment to about 5 µmm.

The present invention will be described in more detail with reference to the following examples, but the following examples are not intended to limit the present invention, and any modification of the design of the present invention can be made without departing from the spirit of the preceding and the following. It is included in the technical scope.

[0013]

[Example] Six types of case-hardening steels (A to F) were used as test materials, and rotary bending fatigue test pieces of cutouts having a notch bottom of 8R and a shape factor of 2.0 were prepared. These test materials are
After carrying out carburizing and quenching by holding at 5 ° C. for 3 hours and oil cooling, carburizing and tempering was carried out by holding at 180 ° C. for 2 hours and air cooling. Subsequently, a two-stage shot peening treatment was performed under various conditions, and the fatigue strength was measured. At this time, the surface hardness after carburization (5 μm from the outermost surface) is substituted into the above formula (1) to find the range of the optimum arc height value σ _OPTIMUM ,
The shot grain size and the projection time which satisfy the above-mentioned optimum arc height value σ _OPTIMUM range within the shot grain size range of 0.03 to 0.25 mm are calculated from the formula (2), and based on this, the second stage Shot peening treatment was performed. The shot peening treatment conditions of each stage are shown in Table 1 together with the type of steel, the hardness of the surface layer after carburization and the optimum arc height value.

Fatigue test results, together with the characteristics (maximum compressive residual stress, surface roughness) after the two-step shot peening treatment,
It shows in Table 2. Tables 1 and 2 also show comparative examples that do not satisfy any of the requirements specified in the present invention.

[0015]

[Table 1]

[0016]

[Table 2]

From Tables 1 and 2, the following can be considered. First N
It is understood that o. 1 to 6 are all examples satisfying all the requirements specified in the present invention and exhibit excellent fatigue strength. On the other hand, Nos. 7 to 11 do not satisfy any of the requirements defined in the present invention, and the fatigue strength of each of them is inferior to those of the examples. These can be thought of as follows.

When the shot grain hardness was low in the first step shot peening treatment (No. 7) and when the shot grain hardness was low in the second shot peening treatment (No. 8), both of the examples ( The maximum compressive residual stress is lower than that of Nos. 1 and 2, and good fatigue strength is not obtained. Further, in the second step shot peening treatment, the one having a large shot grain size (No. 9, 11) had a poor surface roughness and had a fatigue strength higher than those of the examples (No. 3, No. 5). It's getting low. Further, the second stage shot peening treatment (No. 10) under the condition that the formula (2) is not satisfied is larger than that of the embodiment (No. 4) that satisfies the formula (2). Low compressive residual stress,
Fatigue strength is also low.

[0019]

The present invention is constructed as described above, and achieves a higher compressive residual stress than ever before, to remarkably improve the fatigue strength of steel parts, and to obtain a steel part having a small surface roughness. A shot peening treatment method for

Claims (1)

[Claims] 1. A steel part which has been subjected to a surface hardening heat treatment,
A two-stage shot peening treatment is performed using shot particles having an HRC of 50 or more, and the first stage shot peening treatment is performed with a shot grain size of 0.20 to 1.0 mm.
The two-step shot peening treatment using the shot particles of No. 1 is based on the optimum arc height value σ _OPTIMUM obtained by the following equation (1), and the shot particle diameter: 0.03 ~
Within the range of 0.45 mm (however, smaller than the shot grain size in the first step), the shot grain size and the projection time satisfying the following formula (2) are calculated, and any value is calculated. A shot peening treatment method for steel parts, characterized in that the second stage shot peening treatment is performed under the conditions set in step 1. Optimum arc height value σ _OPTIMUM (mmA) = K ₁ · (hardness Hv of surface layer in as-heat-treated) ^a ± 0.05 (1) (K ₁ = 9.0 × 10 ¹⁴ , a = -5 .501) Optimum arc height value σ _OPTIMUM (mmA) = K ₂ · (shot particle size: μm) ^b · (projection time: seconds) ^c (2) (where K ₂ = 2.8 × 10 ⁻³ , b = 0.723, c = 0.177)