JPH0257615A - Method for high-density sintering iron-based powder - Google Patents

Abstract

PURPOSE:To economically produce a high-density dense iron-based sintering material by forming iron-based powder having specified particle diameter, sinterint the formed product at a specified temp., transiently cooling the product to a specified temp., and again sintering the product at a specified temp. CONSTITUTION:The iron-based powder obtained by the atomization process, etc., and having 3-25mum mean particle diameter is formed by injection molding, etc. The formed product is degreased, if necessary, and then sintered at 1,100-1,400 deg.C under no-pressure or reduced-pressure conditions. the sintered body is transiently cooled to 900 deg.C to cause gamma-to-alpha transformation, and the shift of the grain boundary is promoted. The sintered body is then again sintered in the gamma region at 1,100-1,400 deg.C, and further sintered, if necessary, in a compressed gas at 30-250atm. As a result, the body is densified, and an iron-based sintered material having >=about 95% density is obtained.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to the production of iron-based sintered materials using fine powder.
The present invention relates to a method for manufacturing a high-density sintered body.

<Prior art and its problems> Iron-based powder metallurgy can produce members with a final part shape or a shape close to it in small quantities and with high efficiency, and also has excellent dimensional accuracy, so it is mainly used for complex shapes. It has been developed as a manufacturing method for parts. However, the sintered density, which is extremely important for various properties of sintered materials, is usually only 90% of the true density (that is, 90% of the density ratio), which causes problems in functionality such as mechanical properties, corrosion resistance, and magnetic properties. was often left behind.

To solve this problem, it is effective to cause sufficient dimensional shrinkage and densification during sintering to achieve high density. Therefore, instead of the usual iron-based powder having an average particle size of several tens of μm, it is necessary to use fine powder with an average particle size of 25 μm or less to improve the sinterability during sintering.

However, with normal sintering patterns, there is a problem that sufficient densification does not occur and the density does not increase as expected. The density ratio is 90 to 95% (Fe
In this case, a density higher than 7.07 to 7.47 g/cm3) is desired, and improvement of not only the raw material powder but also the sintering method is an extremely important key.

These approaches to densification can be roughly divided into two types.
There are two types of methods: one is due to improved sinterability of raw material powder, and the other is due to improved manufacturing process. Particularly notable are the use of fine metal powder for the former (the poor compressibility of fine powder was solved by introducing the injection molding method mentioned above), and the use of HIP (hot isostatic pressing) for the latter. Uses etc. can be mentioned. Furthermore, regarding the HIP technology, the pressure sintering method (US Pat. No. 1,158,610,853, Japanese Patent Publication No. 1983-500,874), which has greatly improved the equipment cost, has high industrial value.

<Problems to be Solved by the Invention> The present invention uses a special sintering pattern in a method of sintering iron-based powder to significantly improve densification compared to conventional methods9.
It is an object of the present invention to provide a sintering method that allows obtaining a sintered material that achieves a density of 5% or more.

Means for Solving the Problems The present inventors have completed the present invention as a result of extensive research into the influence of firing conditions on densification of iron-based sintered materials.

That is, the first aspect of the present invention is a method for sintering iron-based powder, in which iron-based powder with an average particle size of 3 to 25 μm is molded, then sintered at 1100 to 1400°C, and once heated to 900°C.
Provided is a method for high-density sintering of iron-based powder, which is characterized in that the material is cooled to below .degree. C. and then sintered again at 1,100 to 1,400.degree.

A second aspect of the present invention is a method for sintering iron-based powder, in which iron-based powder with an average particle size of 3 to 25 μm is molded, then sintered at 1100 to 1400°C, and once cooled to 900°C or less. Provided is a method for high-density sintering of iron-based powder, which is characterized in that the powder is sintered again at 1100 to 1400°C, and then sintered in a pressurized gas at a pressure of 30 to 250 atmospheres.

The present invention will be explained in detail below.

The iron-based powder used as a raw material in the present invention is manufactured using a Microtrack particle size analyzer (LEEDS) that uses a laser scattering method.
& N0RTHRtlP COMPANY) is required to have an average particle size of 3 to 25 μm. If the thickness exceeds 25 μm, it is difficult to obtain a high density with a density ratio of 95% or more even using the method of the present invention. 3
If it is less than 3 μm, it is possible to increase the density by 95% or more without using this method, so it is intentionally excluded from the scope of this application, but of course, the sintering method of this application can be applied to powder less than 3 μm. , it is also possible to further increase the density.

The iron-based powder is pure iron-based iron powder, that is, the main component is Fe, with unavoidable impurities and a small amount of Si added as necessary in the production of iron powder.

Iron powder containing Mn, A42, etc. and α
Targets powders with #γ transformation.

The powder used may be fine metal powder produced by high-pressure water atomization method, reduction method, carbonyl method, crushing and classification, or mixed powder thereof.

Any known molding method may be used to mold the powder.
For example, a known molding method in which an organic lubricant is added to powder, or a known injection molding method in which the powder is kneaded with an organic binder and molded into a compound can be used. For complex-shaped parts, injection molding is preferred.

The powder is molded after the binder is added and mixed.

Examples of lubricants for mold molding include higher fatty acids, fatty acid amides, fatty acid esters, and the like. In the case of injection molding, a binder mainly consisting of a thermoplastic resin and/or wax is used, and a plasticizer, lubricant, degreasing accelerator, etc. are added as necessary.

Thermoplastic resins include acrylic, polyethylene,
There are polypropylene-based and polystyrene-based waxes, and waxes include natural waxes such as beeswax, Japanese wax, and montan wax, as well as low-molecular polyethylene, microcrystalline wax, paraffin wax, etc. There are synthetic waxes like this, but 1f is selected from these! I! Alternatively, two or more types are used.

The plasticizer is selected depending on the combination with the main resin or wax, but specifically, di-2-ethylhexyl phthalate (DOP), diethyl phthalate (DEP)
, di-n-butyl phthalate (DHP), and the like.

The amount of the binder varies depending on the molding method used in the post-process, and is approximately 0.5 to 3.0% by weight in normal mold press molding, and approximately 10% by weight in injection molding.

Batch type or continuous type kneaders can be used for mixing and kneading iron powder and binder in the case of injection molding. Among batch type kneaders, pressure kneaders, Banbury mixers, etc. Among the kneaders, twin-screw extruders and the like are advantageously suited. After kneading, granulation is performed using a pelletizer or pulverizer as necessary.
Obtain a molding compound.

Injection molding can be carried out using an injection molding machine commonly used for injection molding, such as an injection molding machine for plastics or an injection molding machine for metal powder. Injection pressure is usually 500 to 20
It is about 00 atm.

After molding, heating is performed to remove the binder. The temperature increase rate at this time is 5 to b, and numerically, it is heated to 600°C and immediately cooled. It should be noted that if the temperature increase rate at this time is too high, cracks or swelling will occur in the obtained molded product, which is not preferable.

The sintering temperature in the first stage is 1100 to 1400°C.
The first stage sintering is carried out under no pressure and is also very preferably carried out in a vacuum. If the sintering temperature is lower than 1100° C., diffusion of Fe atoms in the iron-based powder in the γ region is slow, and densification cannot be expected. On the other hand, even if the temperature exceeds 1400°C, the effects of the present sintering method cannot be exhibited any further and the sintering cost increases, so this is set as the upper limit temperature.

The sintering time at 1100 to 1400°C is preferably 0.5 to 2 hours.

Next, the sintered body is once cooled to below 900°C, and γ→α
cause perversion. As a result, in response to the grain size range of the iron-based powder, the grain boundaries of the particle bonding area move away from the bonding area, and when the next heating and sintering is performed, densification tends to be further promoted. The main point of the present invention is to increase the sintered density of the sintered material by utilizing the above.

Cooling to 900° C. or lower is preferably 700° C. or lower, more preferably to room temperature.

For cooling, the sintered body may be taken out of the furnace after the first stage of sintering, or the temperature may be lowered to 900°C or less in the same furnace, and the temperature may be raised again to continue the second stage of sintering. Good too.

In the second stage of sintering, the grain boundaries move fj due to the γ-α transformation, and the grain boundary diffusion path becomes dense and arranged to work advantageously. Then, sintering is carried out again at 1100 to 1400 °C to achieve the γ region. It is then sintered. The α-γ transformation occurs in the second stage of sintering, and the grain boundaries move again, but the stiffness is different from the position of the grain boundaries in the γ region during the first stage of sintering, and is advantageous for densification. This is a perfect arrangement.

In addition, the pores become closed pores due to grain boundary movement, and the pores tend to disappear in the next pressure sintering step. The reason for limiting the sintering temperature in the second stage is 1100℃ to make the sintered material denser.
A temperature higher than 1,400° C. is required, and even if the temperature exceeds 1400° C., no significant effect is observed, so this range is set.

In the normal one-time sintering method, the density increases with the sintering temperature, but the density does not reach 90% or more until the temperature exceeds 1300 tons. On the other hand, according to the two-step sintering method, which takes a cooling step between the first stage sintering and the second stage sintering,
'100 t: or more, the density ratio becomes 90% or more, and the effect of improving densification is extremely large. Tadasi 1
Even if the temperature exceeds 400°C, no further effect is observed.

The sintering time of the second stage is preferably 0.5 to 2 hours.

If the purpose is to achieve higher density, as shown in the second aspect of the present invention, this is further preferably followed by heating at the same temperature (
Pressure sintering is carried out in a pressurized gas of 30 to 250 atmospheres at a temperature in the range of 1100 to 1400°C. Since the pores become closed pores in the two steps of sintering before this pressurization step, the gas pressure applied in this step has an advantageous effect on pore elimination, and further densification occurs. Pressures below 30 atmospheres are insufficient for densification. On the other hand, even if the temperature exceeds 250 atm, no further effect is seen.

This results in a much higher density than normal sintering or a combination of sintering and pressure sintering.

<Example> Hereinafter, the present invention will be specifically explained with reference to Examples.

Ru.

(Example 1) 4% by weight of thermoplastic resin ( Polyethylene) and 8% by weight wax (paraffin wax) were added, kneaded with a pressure kneader, and granulated with a granulator.
Created a molding compound. Using the compound, a 40X10X5 molding machine was used for metal powder injection molding.
A sample of mm in size was molded.

The molded body was heated in N2 gas up to 600t at 5°C/m i
After degreasing by heating at a temperature increase rate of n, the mixture was immediately cooled.

After that, 1050.1150.1250.
Under the conditions of 1350.1450℃×100m1n, the first
A stepwise sintering was performed.

After the first stage sintering, it was cooled to room temperature, and then sintered in a 100 mL 'fS2 stage in H2 gas at the same temperature as the first stage sintering.

Table 2 shows the sintered density and sintered density ratio of the final sintered body obtained by this sintering method.

Table 2 also shows the results of a normal one-time sintering method conducted at the same temperature and held for 200 ml as a comparative example.

As is clear from Table 2, an iron-based sintered body having an excellent sintered density ratio can be obtained by the sintering method of the present invention.

Also, from this result, in order to obtain a sintered density ratio of 90% or more, the first
It can be seen that after the stage sintering, the first stage sintering temperature needs to be 1100°C or higher, and that even if the temperature exceeds 1400°C, the density ratio cannot be improved by increasing the temperature.

Table 1 Chemical composition (wt%) table of raw material powder (Example 2) Test powders with different average particle sizes were obtained by cutting coarse particles from water atomized iron powder with an average particle size of 31 μm by air classification (Table 3) I got it.

The wood sample powder was molded under the same conditions as in Example 1 and degreased.

A first stage sintering was performed at 1350° C. x 100 ml in H2 gas. After the first stage sintering, it was cooled to room temperature, and then a second stage sintering was performed under the same conditions as the first stage sintering.

Table 4 shows the sintered density and sintered density ratio of the final sintered body obtained by this sintering method.

From this result, in order to obtain a sintered density ratio of 90% or more, the average particle size of the raw material powder needs to be 25 μm or less, and 3.
It is shown that even if the particle size is less than μm, the improvement in density ratio due to particle size reduction is small.

table average Particle size (μs) t n (%) 0.08 0.07 0.08 0.06 0.07 0.10 0.09 0.09 0.09 0.09 0.35 0.33 0.33 0.32 0.31 0.009 0.009 0.009 0.009 0.009 table 0.010 o,ot.

O,010 0,010 0,010 0,41 O136 0,26 0,20 0,17 (Example 3) Average particle size 11μ having the same component composition in Table 1 as Example 1
water atomized iron powder for powder metallurgy with an average particle size of 13 μm
0.3% of graphite powder was added, 4% by weight of thermoplastic resin (polyethylene) and 8% by weight of wax (paraffin wax) were added, kneaded with a pressure kneader, and granulated with a granulator. , created a molding compound. Using the compound, 4
A specimen of 0x10x5 mm was molded. The molded body is N
The sample was heated to 600°C in two gases at a heating rate of 5°C/min, degreased, and immediately cooled.

After that, 1050.115o, 1250.
1350, 1450″Cx1o.

The first stage sintering was performed under conditions of min.

After the first stage sintering, after cooling to room temperature, 30m1n in a vacuum of 10-” Torr at the same temperature as the first stage sintering.
S2 stage sintering was performed.

After the second stage sintering, Ar gas was introduced over 20 m1n, and 9
The pressure was set to 7 atm and maintained at 30 mfn.

Table 5 shows the sintered density and sintered density ratio of the final sintered body obtained by this sintering method.

As a comparative example, Table 5 also shows the results of performing the pressure sintering for 80 minutes without performing the second stage sintering after the first stage sintering and cooling to room temperature.

As is clear from Table 5, an iron-based sintered body having an excellent sintered density ratio can be obtained by the sintering method of the present invention.

Table (Comparative Example 16.17) Samples after degreasing obtained under the same conditions as Example 1 were 1O-3
Sintering was performed in a vacuum of Torr under the conditions of 1250°C x +30m1n.

After the above sintering, Ar gas was introduced at 97 atmospheres over 20 ml and maintained at 15 ml or 60 ml. (The first stage of sintering and cooling to room temperature were not performed.

Table 6 shows the sintered density and sintered density ratio of the final sintered body obtained by this sintering method.

A comparison of the final sintered density ratios of Invention 8 to Comparative Examples 16 and 17 shows that the sintering method of the present invention significantly improves the density compared to the combination of conventional sintering and pressure sintering.

Table 6 (Example 4) The sample after degreasing obtained under the same conditions as Example 1 was subjected to first stage sintering under conditions of 1300° C. x 200 ml in an H2τ atmosphere of 1 atm.

After the first stage sintering, it was cooled to room temperature and then heated for 60 m in a vacuum of 10-'Torr at the same temperature as the first stage sintering.
A second stage sintering of 1n was performed.

After the second stage sintering, Ar gas was introduced over 2.0 m1n.
Pressure sintering at 300° C. was performed at various pressure levels for 60 m1n.

Table 7 shows the sintered density and sintered density ratio of the final sintered body obtained by this sintering method.

This result shows that to obtain a sintered density ratio of 95% or more, a pressure of 30 atm or more is required, and that if the pressure exceeds 250 atm, the density ratio will not improve much due to an increase in pressure.

(Example 5) The test powder shown in Table 3 of Example 2 was molded and degreased under the same conditions as Example 1 to obtain a sample.

This sample was then sintered under the same conditions as in Example 4, except that the sintering pressure was 95 atm.

Table 8 shows the sintered density and sintered density ratio of the final sintered body obtained by this sintering method.

From this result, in order to obtain a sintered density ratio of 95% or more, the average particle size of the raw material powder needs to be 25 μm or less, and 3.
It is shown that even if the particle size is less than μm, the improvement in density ratio due to particle size reduction is small.

<Effects of the Invention> The manufacturing method of the present invention performs a sintering process including a cooling process that causes transformation to γ-α in the manufacturing process of iron-based sintered material using fine powder, so it is different from conventional sintering. High-density ferrous sintered materials can be produced more economically than sintering methods or combination sintering methods of conventional sintering methods and pressure sintering methods.

same

Claims (2)

[Claims] (1) In the sintering method of iron-based powder, the average particle size is 3 to 25
After molding μm iron-based powder, 1100-1400℃
Sintered at 1100-1400℃, once cooled to below 900℃,
A high-density sintering method for iron-based powder characterized by sintering at ℃. (2) In the sintering method of iron-based powder, the average particle size is 3 to 25
After molding μm iron-based powder, 1100-1400℃
sintered at 1100~1400℃, cooled down to below 900℃, and then heated again at 1100~1400℃.
A method for high-density sintering of iron-based powder, characterized by sintering at a temperature of 0.degree. C. and then sintering in a pressurized gas at a pressure of 30 to 250 atmospheres.