JPS6245008B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The technology of manufacturing high-performance mechanical parts by powder hot forging is becoming more and more important year by year. This is because consumers of mechanical parts are seeking materials with better performance and more economically.

One of the important techniques in powder hot forging is the problem of dimensional accuracy of parts. This is because, in the powder hot forging method, which manufactures mechanical parts without almost any machining, unlike castings or cut products, finishing with high precision without extra machining is the most important point for economical efficiency.

In conventional hot forging methods, the article is forged in the temperature region of the austenitic phase of the steel and then transformed during a cooling process. Therefore, the lifespan of molds that are exposed to high temperatures and high pressures is short, and mold costs have become a major problem.

Furthermore, due to the transformation of the steel that occurs during the cooling process after forging, the change in volumetric dimensions is large, which causes a decrease in dimensional accuracy. In addition, dimensional accuracy is significantly affected by variations in temperature during forging, resulting thermal expansion, and variations in residual stress, making it difficult to produce high-precision parts.

As for the life of the mold, especially as the shape becomes finer, stress concentration occurs in the part concerned and wear becomes more severe.

On the other hand, in the cold forging method, although the problems caused by hot forging, which were mainly caused by heat, were solved, the life of the mold was even deteriorated. This is because the deformation resistance of the forged material increases significantly,
As a result, the pressure required is several times that of hot forging. Furthermore, the strength characteristics of the obtained cold forged parts are such that the residual rate of pores is high, and the micro cracks that occur due to poor plastic deformability are a source of fracture.
It wasn't enough.

As described above, in conventional powder forging methods, hot and cold forging methods each have their advantages and disadvantages, and there has been no method that comprehensively satisfies sufficient strength characteristics, dimensional accuracy, and economic efficiency.

The present invention utilizes powder metallurgy to realize a method that satisfies both of these requirements at the same time.

That is, in the first step, hot forging is performed to satisfy the strength characteristics of the material and to adjust the macroscopic shape. In order to achieve this, the iron-based alloy powder is made into a powder preform, which is then heated to the normal hot forging temperature of 1000 to 1200°C to create a mold temperature that leaves pores only in the surface layer. Hot forged. At this time, the powder preform may be previously sintered in a conventional manner before hot forging.

Normal hot forging is carried out in a mold heated to 250 to 400°C, but when a mold at a temperature like the one used in the first step of the present invention is used, the hot forged object is The moment it is placed in the mold, its surface cools rapidly. Therefore, even when forged, the plastic deformation of the surface portion is small, so pores remain in the surface layer that has come into contact with the mold.

On the other hand, the internal temperature is maintained at a normal temperature, so there are no pores and the forged body has high strength. However, as mentioned above, since the temperature of the surface of the forged object is low, it is difficult to undergo plastic deformation, and it is difficult to obtain a high-performance mechanical component with the desired dimensional accuracy by using only the first step. The hot forging mold absorbs the heat of the forged object from its surface and raises its temperature, so by lowering the temperature of the surface of the forging and strengthening the cooling of the mold,
Temperature rise can be suppressed. If the temperature rise was suppressed, the mold material would be less loaded at high temperatures, leading to an extension of mold life. According to the present invention, the hot forged body thus obtained is subjected to a second step of processing.

That is, in the second step, a fine shape in a limited area is created or corrected. 1st
In this step, the portions where the holes remain are deformed, thereby increasing the dimensional accuracy of the minute portions.

In the second step, the mold temperature is heated to 250 to 400°C, which is used in normal hot forging. The effect can be obtained.

However, in consideration of dimensional changes when the forged body is cooled, it is preferable to heat the forged body to 250 to 400°C.

For the above reasons, in the second step, if both the temperature of the mold and the hot forged body are in the range of 250 to 400°C, it is possible to manufacture high-performance mechanical parts with high dimensional accuracy.

The feature of the present application is that the first step of hot forging satisfies the strength characteristics of the material and also adjusts the macroscopic shape. At this time, fine shapes that adversely affect the life of the mold are not created. In the second step, a fine shape in a limited area is created or corrected. In this case, the creation and straightening of the second step requires extremely high pressures if a dense material with no pores is to be processed by hot forging, which ultimately leads to faster wear of the mold. For this reason, in the present invention, holes are left in a specific part of the surface layer in the first step, and the remaining parts are hot-forged so that there are no holes. We have discovered that the life of the mold can be significantly extended. The forged object obtained in this way has a high dimensional accuracy because the dimensions and shape are created and corrected at low temperatures, and there are very few defects in the material because it is hot processed.
It can be used as an excellent mechanical part. Moreover, as already mentioned, the life of the mold is extended by more than three times compared to the conventional single-step method, so that the cost of the mold can be avoided from impairing economic efficiency. Of course, the dimensions,
It goes without saying that the improvement in shape accuracy eliminates the need for expensive machining, such as turning, broaching, grinding, etc., thereby significantly increasing economic efficiency.

Example 1 A ring-shaped powder preform prepared to have a composition of Fe-2Ni-0.5Mo-0.3C was hot forged in a mold at a mold temperature of 60°C so as to have surface pores. Take over, 300
A forged object at 350°C was processed in a mold held at 350°C.

The obtained forged body had a total density of 7.82 g/cc, and very few pores were observed near the surface layer.

On the other hand, a forged body that was simply hot forged at a mold temperature of 250°C as a conventional method had an overall density of 7.83 g/cc. Comparing the dimensional accuracy of both, it was 52mm.
The diameter of the former was ±0.02mm, while the latter was ±0.05mm.

Example 2 Using a mold press, -100 mesh alloy powder (Fe-2Ni-0.5Mo-0.3C) was molded at a molding pressure of 7 ton/cm ² to form a ring-shaped green compact (outer diameter 60 mmφ, inner diameter
35mmφ, thickness 20mm) was obtained. The compact density is 7.1
g/cc.

This molded body was sintered at 1260°C for 1 hour in a nitrogen gas flow to obtain a sintered body with a specific gravity of 7.30g/cc, and then heated at 1100°C for 20 minutes in nitrogen gas. Hot forging was carried out in a mold at a pressure of 8 tons/cm ² .

Since the surface layer of the molded body was exposed to heat by the mold, the temperature decreased and workability deteriorated, and a preliminary forged body with little increase in density due to hot forging and a relatively large number of pores in the surface layer was obtained. When this was forged again at a mold temperature of 300° C. and a forged body temperature of 250° C., a dimensional accuracy of ±0.02 mm was obtained as in Example 1.

Claims (1)

[Claims of Claims] 1. After preforming iron-based alloy powder, hot forging is performed at a mold temperature that leaves pores only in the surface layer, and then the mold and the body to be forged are heated to 250 to 400°C. A method for manufacturing mechanical parts using powder hot forging, which is characterized by hot forging. 2 After preforming the iron-based alloy powder, it is sintered and hot forged at a mold temperature that leaves pores only in the surface layer, and then the mold and the forged body are heated to 250 to 400°C to heat it. A method for manufacturing mechanical parts using powder hot forging, which is characterized by inter-forging.