JPH0257605A - Production of sintered alloy having excellent dimensional precision - Google Patents

Abstract

PURPOSE:To produce the title sintered alloy with reduced variance in density and having excellent dimensional precision by forming the stainless steel powder having a specified particle size with a binder, removing the binder by heating, then sintering the formed product under specified conditions, and further sintering the product under pressure. CONSTITUTION:The particle size of the raw powder of ferritic and/or austenitic stainless steel is adjusted as follows. Namely, the content of the particles having <=5mum size is controlled to 3-20wt.%, that of the particles having 5-10mum size to 40wt.%, and that of the particles having 10-30mum size to 5-35wt.%. A binder is added to the metal powder and mixed, and the mixture is formed. The binder in the formed product is removed by heating in a nonoxidizing atmosphere. The heat-treated formed product is sintered at 1,050-1,350 deg.C and at a reduced pressure of <=0.1Torr, and then sintered at 1,250-1,350 deg.C in a nonoxidizing atmosphere. The sintered body is then further sintered at a pressure of <=100kgf/cm<2>. By this method, a sintered alloy with reduced variance in density and contraction by sintering and having excellent dimensional precision is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to parts manufactured by powder metallurgy, and particularly to a sintered alloy with excellent dimensional accuracy.

<Prior Art> In recent years, the production of sintered parts by powder metallurgy has shown remarkable growth, and the range of applications of sintered parts is expanding. In particular, as the shapes of automobile parts, electronic/electrical parts, and office parts that use stainless steel become more complex, the manufacturing method is also replacing the cutting method with the powder metallurgy method.

However, sintered alloys produced by powder metallurgy have density variations, and due to these variations, the dimensional accuracy of the sintered alloys has been poor.

<Problems to be Solved by the Invention> Therefore, as described above, an object of the present invention is to minimize the variation in shrinkage due to sintering, that is, the variation in density, in order to correct the dimensional accuracy of the sintered alloy.

Means for Solving the Problems> As a result of various studies, the present inventors found that the particle size structure of the metal powder is 3 to 20% by weight of 5 μm or less, 5 to 10 μm
40% by weight or more, 5-35% by weight of 10-30μm
Using ferritic and/or austenitic stainless steel powder as a raw material powder, a binder is added to the metal powder, mixed, molded, the binder in this molded body is removed by heating in a non-oxidizing atmosphere, and then After sintering at a temperature of 1050 to 1350°C under a reduced pressure of 0.1 Torr or less, sintering at a temperature of 1250 to 1350°C in a non-oxidizing atmosphere, and then pressure sintering at a temperature of 100 kgf/cm2 or less. Provided is a method for manufacturing a sintered alloy with excellent dimensional accuracy.

In addition, the particle size structure of the metal powder is 3 to 20% by weight of 5 μm or less, 40% by weight or more of 5 to 10 μm, and 10 to 30 μm.
using a raw material powder containing 0.5 to 50 weight% of Ni and the balance being Fe as the metal powder, adding a binder to this,
After mixing and shaping, the binder in this compact is removed by heating in a non-oxidizing atmosphere, followed by sintering at a temperature of 1100 to 1300°C in a reducing atmosphere, followed by pressure sintering at 100 kgf/c+n' or less. To provide a method for manufacturing a sintered alloy with excellent dimensional accuracy.

The present invention will be explained in detail below.

In the present invention, the particle size structure of the raw metal powder is most important for improving dimensional accuracy.

The dimensional change due to sintering is the largest, with a shrinkage rate of 15% or more from the molded body to the sintered body, and it is important to suppress dimensional variations during this process.

The particle size of the raw material powder influences sinterability, and is considered to be the cause of variation in shrinkage rate. In other words, the finer the powder, the easier it is to sinter and the larger the shrinkage, but the coarser the powder, the slower the sintering.
As the shrinkage becomes smaller, the shrinkage rate varies as a whole and dimensional accuracy deteriorates.

Therefore, by adjusting the particle size structure of the raw material powder, a sintered alloy with excellent dimensional accuracy can be obtained.

In the present invention, the particle size composition of the metal powder that is the raw material is: 5 μm or less; 3 to 20% by weight; 5 to 10 μm;
m: 40% by weight or more, 10-30 μm: 5-35% by weight
It is good to contain.

Containing 3 to 20% by weight of powder with a particle size of 5 μm or less is
Because fine powder with a particle size of 5 μm or less progresses easily in sintering,
This is because the rate of pore blockage is high.
Pore clogging is important in the subsequent pressure sintering process, and experiments have confirmed that a density of 92% or more is required to obtain the effect of pressure.

Further, the reason why the content is set to 3% by weight or more is that if it is less than 3% by weight, it is not effective as a sintering accelerator and no increase in closed porosity is observed.

Moreover, if it exceeds 20% by weight, the closed porosity increases, but the variation in shrinkage increases and dimensional accuracy decreases.

Next, it is preferable to contain 40% by weight or more of powder having a particle size of 5 to 10 μm.

If it is less than 40% by weight, fluidity decreases and moldability is impaired,
As a result, the molded product exhibits non-uniform filling, which reduces dimensional accuracy and is therefore unsuitable.

Further, it is preferable to contain particles having a particle size of 10 to 30 μm in an amount of 5 to 35% by weight.

Although raw material powder with a particle size of 10 to 30 μm has inferior sinterability compared to powder with a particle size of 10 to 30 μm, it is mixed for the purpose of maintaining the shape of the molded body, suppressing shrinkage tendency, and improving fluidity. Furthermore, when a molded article containing a binder is subjected to a deactivation mixture treatment, a particle size structure containing an appropriate amount of coarse powder makes it easier for the binder to escape because the gaps between the particles become larger.

It should be noted that if the content of the raw material powder with a particle size of 10 to 30 μm is less than 5% by weight, the above-mentioned effects will not be seen, so it is not preferable.

In addition, if the content exceeds 35% by weight, fluidity decreases and formability is impaired, and as a result, the uneven filling state within the molded object cannot be resolved even by sintering, resulting in a decrease in dimensional accuracy. So I don't like it.

It should be noted that if the particle size exceeds 30 μm, the sinterability will be extremely deteriorated, and therefore it is not suitable.

In the present invention, since a powder having a particle size of 30 μm or less is used, an appropriate amount of a binder is added to the copper powder, mixed, and then molded. This is because copper powder alone causes defects such as lamination and cracking during molding, resulting in a decrease in yield.

The binder may be a thermoplastic resin, wax, or a mixture of both for molding.

Additionally, a plasticizer, lubricant, degreasing accelerator, etc. are added as necessary.

Thermoplastic resins include acrylic, polyethylene,
One type or a mixture of two or more types such as polypropylene type and polystyrene type can be selected, and the waxes include:
1 ff! selected from natural waxes such as beeswax, wood wax, montan wax, etc., and synthetic waxes such as low molecular weight polyethylene, microcrystalline wax, paraffin wax, etc. Alternatively, two or more types can be selected and used.

The plasticizer is selected depending on the combination with the main resin or wax, and di-2-ethylhexyl phthalate (DOP), di-ethyl phthalate (DEP), di-n-butyl phthalate (DHP), etc. are used. can.

As the lubricant, higher fatty acids, fatty acid amides, fatty acid esters, etc. can be used, and waxes may also be used as the lubricant in some cases.

Further, as a degreasing accelerator, a sublimable substance such as camphor can also be added.

The binder varies depending on the molding method, but for example, in the injection molding method used to mold parts with complex shapes, it is 10% by weight.
Requires about 100% of binder.

After molding, the temperature is raised and maintained at a constant rate in a non-oxidizing atmosphere to remove the binder. If the heating rate is too fast at this time, the product will crack or swell, so
Raise the temperature at °C/h.

After removing the binder, it is sintered to achieve densification. The binder remains without being completely removed, but the method for removing this residual carbon varies depending on the raw material composition.

In the present invention, when the metal powder is ferritic and/or austenitic stainless steel powder, the final sintering process is performed by sintering the residual carbon and oxygen in the oxide film present on the surface of the stainless steel powder under reduced pressure. Consolidated impurities c.

Reduce as much as possible.

In the case of Fe--Ni, it is preferable to sinter in a non-oxidizing atmosphere. In particular, since the Fe-Ni type does not contain any elements that have a strong affinity for oxygen, residual carbon can be sufficiently removed by sintering in a reducing atmosphere.

In the case of stainless steel, it is preferable to heat it from room temperature to 1050 to 1350°C in a vacuum of 0.1 Torr or less. Subsequently, it is preferable to heat and maintain the temperature up to 1250 to 1350° C. in a non-oxidizing atmosphere such as nitrogen or Ar.
Through this process, a sintered alloy having a density ratio of 92% or more can be obtained. After that, at the same temperature, 10
By maintaining the pressure at 0 kgf/cm2 or less, a sintered alloy with excellent dimensional accuracy can be obtained.

In the case of stainless steel, corrosion resistance is required, the environment in which it is used is relatively harsh, and improvements in corrosion resistance as well as dimensional accuracy are desired. Among alloying elements, Cr is particularly effective in improving corrosion resistance, but the vapor pressure of Cr is 1O-3T at 1300℃.
orr, and depending on the degree of vacuum in vacuum sintering, C
Since r evaporates and the Cr concentration on the surface decreases, corrosion resistance deteriorates significantly. Therefore, by suppressing Cr evaporation,
It is important to avoid making the degree distribution non-uniform.

This is accomplished by the present invention's low temperature vacuum sintering followed by high temperature sintering.

The copper powder according to the present invention has a composition containing Cr, which is a hardly reducible element. In the present invention, compared to reduction in a hydrogen atmosphere used in a normal sintering process, vacuum sintering allows reduction to be easily promoted by the action of contained C, and as a result, a high-density sintered body can be obtained. be able to.
The sintering action begins at the point of contact between particles and proceeds by the solid-state diffusion of atoms. However, if the powder surface is covered with oxides, the diffusion of atoms is blocked and densification does not proceed, resulting in the formation of a sintered body. High density is not achieved. That is, in order to obtain high density, it is necessary to reduce the Cr-based oxide. For this purpose, it is sintered under reduced pressure. C when exceeding 0.1 Torr
The upper limit is set at 0.1T because the reduction reaction of r-based oxides is difficult to proceed.
It was set as orr.

The reason why the temperature range for vacuum sintering is set to 1050 to 1350°C is because the Cr-based oxide is not sufficiently reduced at temperatures lower than 1050°C, so the oxide remains and inhibits subsequent sintering. Therefore, the lower limit was set at 1050°C.

On the other hand, when sintering is performed at a temperature exceeding 1350° C., the amount of Cr evaporated from the surface increases, and not only does the concentration distribution become non-uniform, but defects such as the appearance of a liquid phase and loss of shape are observed. Therefore, the upper limit was set to 1350°C.

Subsequently, in order to achieve densification and homogenization of alloying elements by diffusion, non-oxidizing; in ambient air, 1250-1350
Sinter at °C. The previous stage of low-temperature vacuum sintering creates contact points between the particles and sintering begins, but by increasing the temperature even higher, diffusion is promoted and sintering progresses, making the remaining pores smaller and more spheroidal. 125 made the atmosphere non-oxidizing.
This is to suppress Cr evaporation at high temperatures of 0° C. or higher.

In the case of Fe--Ni alloys, residual carbon can be removed in a reducing atmosphere; for example, industrially used decomposed ammonia gas, hydrogen gas, etc. can be used. Moreover, if the atmosphere is maintained in a wet hydrogen atmosphere at a dew point of 0 to 30° C. and then changed to a dry atmosphere, the removal of residual carbon will be more effective.

At this time, atmospheric sintering is performed under atmospheric pressure, but it is necessary that the pores are sufficiently closed in order to obtain a pressurizing effect in the next step. Therefore, the sintering temperature range is
The temperature is preferably 100°C to 1300°C.

If the temperature is less than 1100°C, the diffusion rate is slow, a sintered density (92% or more) that provides a pressurizing effect cannot be obtained, and the dimensional accuracy of the final sintered body decreases.

On the other hand, a sintering temperature exceeding 1300° C. is economically disadvantageous in an industrial atmosphere sintering furnace because the heating element and refractory are rapidly consumed, resulting in high costs. Furthermore, since no significant effect on density increase was observed even if the temperature exceeded this limit, the upper limit was set at 1300°C. Subsequently, by pressurizing with an inert gas of 100 kgf/c [02 or less while maintaining the sintering temperature, a sintered alloy with small dimensional variations can be manufactured.

It is preferable that the pressurizing pressure is 100 kgf/cm2 or less. Due to the reduced pressure or atmosphere sintering in the previous process, the pores remaining inside the sintered body become spheroidal and occluded, and each pore is divided and exists near the grain boundaries, so it is easy to move, and the inside of the pore is The gas partial pressure inside the pores is very small due to vacuum or reducing gas, and can disappear at low pressure. Furthermore, even if the pressure exceeded 100 kgf/cm2, no further significant effect was observed, so taking economic efficiency into consideration, the upper limit was set at 100 kgf/cm2.

Note that even if the metal powder of the present invention inevitably contains 1% or less of nonmetallic inclusions, there is no particular influence.

<Example> The present invention will be specifically described below based on Examples.

(Examples 1 to 3, Comparative Examples 1 to 4) Average particle size of raw material powder is 2.1 μm: 0 to 50% by weight, 8
．． 5LIS316 adjusted by classification to be 2 μm; 30% by weight or more, 14°8 μmho ~ 50% by weight
Water atomized steel powder with the following composition was prepared.

Add thermoplastic resin and wax to this powder at 9% of the raw material powder.
:1 and kneaded using a pressure kneader. After kneading, the mixture was cooled, crushed into pellets, and molded into 30 rectangular parallelepipeds with a length of 40 mm, a width of 20 mm, and a diameter of 3 mm per liter using an injection molding machine. Next, in a nitrogen atmosphere, the temperature was increased at a rate of 10°C.
/h to 600°C to remove the binder so that the 070 molar ratio in the molded body was 1.0 to 2.0.

It was sintered at 1150°C in vacuum (<10-3 Torr) for over 1 hour, and then at 1300°C in an Ar gas atmosphere at normal pressure.
After sintering at ℃ for 2 hours, the furnace pressure was 50.100 kgf/a.
The pressure was increased to m2 and maintained for 1 hour.

After cooling, the dimensions in the width direction were measured using a micrometer, and the average value and standard deviation were determined for 20 samples.

The results are shown in Table 1.

As shown in Examples 1 to 3 of Table 1, when the particle size structure of the raw material powder was within the range of the present invention, a final sintered body with excellent dimensional accuracy and small dimensional variations was obtained.

On the other hand, in Comparative Example 1, since there was no fine powder of 2.1 μm, the closed porosity was low, so uniform shrinkage could not be obtained, and it is thought that the dimensional accuracy decreased.

In Comparative Example 2, there were many fine powders of 2.1 μm, and the shrinkage ratio of the fine powders was increased, which is thought to be the reason for the variation in dimensions.

Comparative Example 3 had the largest amount of coarse powder with a diameter of 14.8 μm, and it is thought that non-uniform density in the compact was the cause of the dimensional variation in the final sintered compact.

In Comparative Example 4, since there was no coarse powder at all, the deactivation of the composite material was not completed completely, and the amount of residual carbon was large, resulting in the appearance of a liquid phase, which caused shrinkage and non-uniform dimensional changes. .

(Example 4.5, Comparative Example 5.6) As a raw material powder, water atomized iron powder was adjusted to an average particle size of 2.0 to 18°0 μm by classification, and carbonyl Ni powder (average particle size: 8 μm) was added to this. was added in an amount of 2% by weight, formed in the same manner as in Examples 1 to 3, and treated with a deactivating agent. Thereafter, it was sintered at 1250° C. in a hydrogen atmosphere for 2 hours.
Next, it was held at 1250° C. for 1 hour under an argon atmosphere and a pressure of 100 kgf/cm 2 .

After cooling, the dimension in the width direction was measured using a micrometer, and the average standard deviation was determined for 20 samples. The results are shown in Table 2.

As can be seen from Table 2, when the particle size structure of the raw material powder was within the range of the present invention, results with small dimensional variations were obtained.

On the other hand, in Comparative Example 5, there was a large amount of fine powder, and as sintering progressed in a reducing atmosphere, the amount of shrinkage increased, which is thought to have caused variations in dimensions.

Comparative Example 6 had the largest amount of coarse powder, which is thought to have caused non-uniform filling of the metal powder during molding, causing variations in the dimensions of the final sintered body.

<Effects of the Invention> As described above, the present invention sets the particle size structure of the raw material powder to 5 μm or less: 3 to 20% by weight, 5 to 10 μm = 40% by weight or more,
10 to 30 μm: 5 to 35% by weight, a binder is added to the powder and mixed, and after molding, the binder of the molded body is heated and removed in a non-oxidizing atmosphere, and then sintered under reduced pressure or atmospheric pressure. By performing pressure sintering after sintering, it is possible to provide a manufacturing method that yields a sintered alloy with excellent dimensional accuracy.

Claims (2)

[Claims] (1) The particle size structure of the metal powder is 3 to 20% by weight of 5 μm or less, 40% by weight or more of 5 to 10 μm, and 10 to 30 μm.
Ferritic and/or austenitic stainless steel powder containing 5 to 35% by weight is used as a raw material powder, a binder is added to the metal powder, mixed, molded, and the binder in this molded body is placed in a non-oxidizing atmosphere. Then, after sintering at a temperature of 1050 to 1350°C under a reduced pressure of 0.1 Torr or less, sintering at a temperature of 1250 to 1350°C in a non-oxidizing atmosphere, and further heating at a temperature of 100 kgf/cm^2 or less. A method for producing a sintered alloy with excellent dimensional accuracy, which is characterized by pressure sintering. (2) The particle size structure of the metal powder is 3 to 20% by weight of 5 μm or less, 40% by weight or more of 5 to 10 μm, and 10 to 30 μm.
using a raw material powder containing 0.5 to 50 weight% of Ni and the balance being Fe as the metal powder, adding a binder to this,
After mixing and shaping, the binder in this compact is removed by heating in a non-oxidizing atmosphere, followed by sintering at a temperature of 1100-1300°C in a reducing atmosphere, followed by pressure sintering at 100 kgf/cm^2 or less. A method for producing a sintered alloy with excellent dimensional accuracy.