JPS6111282B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for preventing chipping of a green compact when manufacturing metal parts using a powder metallurgy method.

When manufacturing metal parts using powder metallurgy,
First, raw material powder is filled into a mold of a predetermined shape, and after pressure forming, the green compact is transferred from the forming press to a mesh belt conveyor via a chute or belt conveyor, and then sent into a sintering furnace. It is then subjected to a prescribed heat treatment to become a sintered product. In some cases, the green compact is stored in a heat-resistant container and sintered.

Now, since green compacts are fragile, they often chip or crack due to mutual collision or overturning during handling during transfer from a forming press to a sintering furnace. In such a case, not only will these products become defective, but it will also be necessary to sort out all the products that have been contaminated with these products, resulting in a loss of cost and labor. As a countermeasure to this problem, the compacting pressure is usually increased to increase the mechanical strength of the green compact, but this method shortens the life of the mold, and in severe cases can even damage the mold. It only applied below.

A small amount of lubricant is usually added to the raw material powder used in powder metallurgy. The purpose is to improve the fluidity of the raw material powder during molding, to be effective in making the density uniform in the green compact, and to reduce the friction between the die and the green compact during knockout, which occurs in the green compact. Prevention of cracks,
and extending the life of molds. The lubricant should be used so that it does not remain during sintering and inhibit sintering.
At the sintering temperature, almost all of the soap must be dispersed from the green compact, and metal soaps such as zinc stearate are usually added in an amount of 0.1 to 1.5% by weight.

The inventor of the present invention discovered that by heating the green compact to a temperature above the melting point of metal soap, the mechanical strength of the compact increases significantly after cooling, and utilizing this phenomenon, the compact heating By performing the heat treatment described above by directly connecting the apparatus, it is possible to substantially eliminate chipping of the molded body that occurs during subsequent transportation, alignment, etc. The details of the experiment that discovered this phenomenon will be explained below.

Experiment The following two types of samples were prepared for Fe-C and Fe-Cu-C systems, which are widely used as sintered mechanical parts.

For the former, 2% graphite powder, atomized iron powder
97.5% and 0.5% zinc stearate as a lubricant were added and mixed, and then molded at a molding pressure of 5.5 t/cm ² to obtain Sample 1. The density ratio of this sample is 87%, and the length, width, and thickness are 12.7, 31.8, and 5 mm, respectively. For the latter, 1.0% of the graphite powder, 1.5% of the electrolytic copper powder, and the former
Sample 2 was prepared by blending 96.7% of iron powder with 0.8% of zinc stearate and using a compacting pressure of 6 t/cm ² to produce a compact with a density ratio of 91%. The particle size of all these raw material powders is 100 mesh or less.

Bending strength was used as a measure of the mechanical strength of these samples, and the measurement was in accordance with MPIF standard 13-51T.
Samples 1 and 2 were heated in the air at a temperature of 100 to 500℃ for 10
FIG. 1 shows the measurement results of the bending strength after holding for a minute and cooling to room temperature.

As can be seen from this figure, when the heat treatment temperature exceeds the melting point of zinc stearate (approximately 140℃), the bending strength shows a rapid increase, reaches a maximum value around 200℃, and then returns to the value before heat treatment. Coming closer. This maximum value reached approximately 2.5 times that of the unheated value.

When the lubricant is replaced with the above-mentioned zinc stearate, lead stearate (melting point: 114°C), or calcium stearate (melting point: approximately 180°C), which has a different melting point, the lubricant shows an increase in strength through heat treatment. As with the former, the temperature at which the rise begins and the temperature at which the intensity reaches its maximum value are at or above their melting points.
The temperature was around 50℃.

It is assumed that this phenomenon is caused by the metal soap in the pores of the powder compact being melted by heat treatment and solidified to form a continuous layer, and the density of this layer increases the strength of the compact. That is, in the example shown in the above experiment, Sample 1 contained 9% of pores by volume in the molded state, and zinc stearate occupied about 60% of the pores. In addition, in sample 2, this occupies about 40% of the vacancies, which account for 13%. In this state, the density ratio of the zinc stearate powder is about 40 to 60% as described above, so this strength is considered to be extremely small, and the strength of the molded body is almost only that of the metal portion.

If the temperature of the compact is set above the melting point of zinc stearate, it will melt and form a continuous layer that adheres to the inner wall of the pores, and after solidification, the strength of this layer will be added to the strength of the metal part. , increasing the strength of the green compact.

The increase in strength begins from this melting point, and above the melting point approximately
The fact that the maximum value is reached at 50° C. indicates the temperature at which the formation of the continuous layer begins and its completion. In addition, in the temperature range above the maximum strength,
This can be explained by the fact that molten zinc stearate flows out of the green compact and is lost through evaporation, reducing the amount. As mentioned above, this phenomenon is observed not only in zinc stearate but also in metal soaps in general, and the lower limit of the heat treatment temperature at which the effect appears is the melting point of the added metal soap, and the upper limit is experimentally determined. From this fact, it was confirmed that the melting point was 260℃. The present invention is not limited to the above-mentioned iron-based powder compact, but is fully applicable to metal-based compacts as well. Moreover, when the metal soaps to be added are used in combination, they can be applied as is if the above-mentioned melting point is taken as the melting start temperature of the mixture.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between the heat treatment temperature of a green compact and the bending strength of the green compact. 〓〓〓

Claims (1)

[Claims] 1. After compression-molding metal powder to which metal soap has been added as a molding lubricant into a desired shape, the compacted powder is heated to a temperature above the melting point of the metal soap and below the dewaxing temperature, and then returned to room temperature. A method for strengthening green compacts in powder metallurgy, characterized by: