JPH05105908A - Method for shot-blasting powder sintered article - Google Patents

Abstract

PURPOSE:To increase the fatigue strength of a powder sintered article by shot- blasting the sintered article in multiple stages using shot grains with the sizes gradually decreased. CONSTITUTION:The gap formed in compacting between the adjacent raw powder grains tends to remain as open pores in the sintered article even after sintering, and the open pores are formed along the grain boundary of the raw material grain and extended to the depth from the surface. Since the open pores are not closed by a single shot blasting, shot grains having a larger diameter than the open pore are used to close the open pores, and the large open pores which have not been closed is smoothed. The remaining open pores are further smoothed by medium size shot grains, and the surface is finally finished, as required, by fine shot grains. The open pores on the surface of the sintered article are surely eliminated in this way.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for shot blasting a powder sintered product.

[0002]

2. Description of the Related Art In recent years, powder metallurgy technology has been drawing attention, in which raw material powders are mixed, shaped into a predetermined shape, and then fired to obtain a sintered body. In such powder metallurgy technology, segregation of component elements does not occur, it is possible to commercialize difficult-to-process materials, and extremely fine crystal structure can be obtained. There is also an advantage that secondary processing can be omitted.

However, the powder sintered product has a defect that the gap between the adjacent raw material powder particles is likely to remain as an open pore even after sintering, and this becomes a starting point of fatigue cracking, so that the fatigue strength is low.

In order to eliminate such a defect, a method of performing compressive residual stress on the surface layer by performing shot pinning or shot blasting is adopted (for example, Japanese Patent Laid-Open No. 2-232303). In addition, a method has also been adopted in which the open pores are surely closed by increasing the start temperature and the end temperature of shot pinning to increase the fatigue strength (for example, JP-A-61-56203). However, in the former method, as a result of damaging the surface, the fatigue strength may be rather lowered when residual stress is removed by surface treatment or the like. Further, the latter method is not sufficient in terms of blocking open pores, and further improvement in fatigue strength is desired.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a shot blasting method for a powder sintered product, which can effectively increase the fatigue strength of the powder sintered product.

[0006]

In order to solve the above-mentioned problems, the present invention is a shot blast method for a powder-sintered product, which comprises applying shot particles to the powder-sintered product to adjust the surface thereof. Provided is a shot blasting method for a powder sintered product, which comprises performing a multi-stage shot blasting process using a plurality of stages of shot grains sequentially from a shot grain to a fine grain side.

[0007]

As described above, in the sintered product, the gap between the adjacent raw material powder particles during molding tends to remain as open pores even after sintering, and the open pores are along the grain boundaries of the raw material particles. It penetrates deeply from the surface (see Fig. 1 (a)).
Therefore, it is not possible to sufficiently close the open pores with the one-step shot blasting.

In the present invention, as shown in FIG.
First, for example, the open pores are closed using shot particles having a particle diameter larger than the open pore diameter, and the large open pores that cannot be closed are smoothed. Then, FIG. 1 (c)
As shown in, the open pores remaining in the medium shot grains are further smoothed, and finally the surface is adjusted by fine grain shots if necessary. Therefore, the open pores on the surface of the sintered product can be surely eliminated, and the fatigue strength of the powder sintered product can be effectively increased.

[0009]

Embodiments of the present invention will be described in detail below.

In this embodiment, a titanium alloy is used as the powder sintered product. Powder of crushed titanium sponge as a starting material, which passed through a 100-mesh sieve (-
# 100, particle size is 149 μm or less), and a master alloy (prealloy) is mixed into this to obtain a Ti-6Al-4V alloy composition of 5.2 ton / cm ^2. Molded at 1260 ℃
It was baked in. The average particle size of the master alloy is 5μ.
It was m.

When the surface of the titanium alloy sintered body produced as described above was observed, many open pores were observed. The surface roughnesses R _max and R _a of this sintered body were measured and found to be 16.3 μm and 1.5 μm, respectively. The relationship between the number of repeated ruptures and the maximum load is shown in Fig. 2, and the fatigue strength is 28 kgf / mm ² Met.

Next, with respect to this sintered product, coarse grain shot # 40 (shot grain size to 400 μm) and medium grain shot #
150 (shot grain size ~ 100 μm), fine grain shot #
Shot blasting treatment was sequentially performed with 300 (shot particle size to 50 μm). When the surface of the sintered product was observed thereafter, there were almost no open pores. As a result of measuring the surface roughnesses R _max and R _a ) of this sintered body, they were 4.8 μm and 0.8 μm, respectively, and it was confirmed that they were improved as compared with the as-sintered state. The relationship between the number of repeated ruptures and the maximum stress is as shown in Fig. 2. The fatigue strength is 41kgf.
/ Mm ² Then, it was confirmed that it was significantly improved compared to the as-sintered state.

For comparison, as a result of shot blasting only with coarse-grained shots, open pores remained on the surface, and surface roughness was R _max = 7.2 μm, R _a = 1.3 μm.
It was confirmed that the surface condition was inferior to that of the multi-stage shot of the present invention. The relationship between the number of repeated ruptures and the maximum load is shown in Fig. 2, and the fatigue strength is 41kgf.
/ Mm ² It was confirmed that it was inferior to that of the multi-stage shot of the present invention. As described above, the effectiveness of the multi-stage shot was confirmed.

Although the titanium alloy is shown in this embodiment, it is needless to say that the present invention is not limited to this and any powder sintered product can be applied. Further, although the shot grains having a three-stage grain size are used, the shot grains are not limited to this.
Of course, it is effective if the number of stages is equal to or higher than that.

[0015]

According to the present invention, there is provided a method of shot blasting a powder sintered product which can effectively increase the fatigue strength of the powder sintered product.

[Brief description of drawings]

FIG. 1 is a diagram for explaining the principle of the present invention.

FIG. 2 is a diagram showing the relationship between the number of repeated fractures and the maximum stress.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Takahiro Fujita 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Steel Pipe Co., Ltd.

Claims (1)

[Claims] 1. A shot blasting method for a powder sintered product, which comprises applying shot particles to a powder sintered product to arrange the surface thereof, wherein shot particles of a plurality of stages are successively used from a coarse shot particle to a fine particle side. A method of shot blasting a powder-sintered product, which comprises performing a multi-stage shot blasting process.