JPH02298204A - Method for sintering ferrous powder - Google Patents

Abstract

PURPOSE:To easily manufacture ferrous powder metallurgical material having high density and high strength in industrial scale by sintering under pressurized gas atmosphere specifying temp. and pressure after degreasing a green compact of the ferrous powder and sintering at the specified temp. under non-pressurizing. CONSTITUTION:After compacting by adding binder, plasticizer, etc., to the ferrous powder, this is heated to about 600 deg.C at about 5-30 deg.C/hr temp. raising velocity and immediately cooled, and degreasing is executed. Successively, this green compact is primarily sintered at 1350-1450 deg.C under non-pressurizing (the atmosphere-vacuum) to make closed void of void in the sintered body. Successively, the secondary sintering is executed at 1100-1350 deg.C under pressurized gas atmosphere (inert gas, reduced gas) at >=70atm pressure and the closed void is caused to disappear to make the high density of this. Then, both of the primary and secondary sinterings are executed for about 10-200min. By this method, even if any particle size of the raw material powder is used, the ferrous powder metallurgical material having high density and high strength is obtd. and the coat can be reduced.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to a method for producing a high-density (95% or more) iron-based powder metallurgy material, and more particularly, to a method for sintering iron-based powder. .

<Conventional technology> Iron-based powder metallurgy can produce parts with a final part shape or a shape close to it with high yield and high efficiency, and has excellent dimensional accuracy, so it is mainly used as a manufacturing method for parts with complex shapes. It has developed. However, the sintered density, which has a very important influence on various properties of the sintered material, is
Sintering at a temperature of 00 to 1350°C results in only 90% of the true density (that is, a density ratio of 90%), and problems often remain in functionality such as mechanical properties, corrosion resistance, and magnetic properties.

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. Materials with excellent properties have a density ratio of 90 to 95% (in the case of Fe, the density is 7.07 to 7.4%).
7 g/cm'') or higher, and improving 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 the raw material powder, and the other is due to improvements in the manufacturing process.

Particularly noteworthy are the use of fine metal powder for the former (the poor compressibility of powder was solved by the introduction of injection molding), and the use of HIP (hot isostatic pressing) for the latter. can be given. Furthermore, regarding HIP technology, the pressure sintering method (U.S. Patent υ586100853, Japanese Patent Application Publication No. 63-500874) has greatly improved the equipment cost.
) has also been proposed.

However, this pressure sintering method also requires improvement in terms of cost and implementation on an industrial scale. That is,
In this pressure sintering method, no consideration was given to the particle size of the raw material powder. Therefore, when manufacturing sintered metal materials, especially those using fine powder (particle size of about 44 μm or less, preferably 10 μm or less) as a raw material, such as injection molding, its economic efficiency (the raw material powder price is based on the average particle size The problem was that the smaller the size, the higher the price.

Mata, the above prior art (U.S. Pat. No. 11s86/0085)
In the pressure sintering method that involves a short-term temperature rise (temperature spike) disclosed in 3), it is necessary to limit the time for holding the sintered body at a high temperature to a short time, which is difficult for industrial purposes. accompanied by difficulty. Furthermore, it is difficult in industrial-scale furnaces to produce uniform temperature spikes throughout the furnace, resulting in large variations in density depending on the position of the sintered body within the furnace. arise.

<Problems to be Solved by the Invention> As described above, methods for increasing the density of sintered bodies have been proposed, but there is room for improvement in terms of cost and scale-up to an industrial scale.

The present invention has been made in view of these facts, and is a method for manufacturing sintered metal materials using conventional methods such as injection molding using fine powder as a raw material, which improves the properties (density) of sintered metal materials and its properties. The purpose is to provide a method for sintering iron-based powder that is highly economical (raw material cost). That is, the object of the present invention is to provide a method for industrially easily producing a high-density and high-strength iron-based powder metallurgical material even if the raw material powder is coarse particles.

Means for Solving the Problems> The present inventors have discovered that in order to obtain a high-density, high-strength iron-based powder metallurgy material, the pores of the sintered body are made closed, and then the pores are closed by pressure sintering. We thought that sintering should be performed under conditions that satisfy the requirements of eliminating pores and preventing coarsening of metal crystal grains, and after intensive study of these conditions, we arrived at the present invention.

That is, in the production of a powder metallurgical material formed by molding and sintering iron-based powder, the present invention is directed to dewaxing the iron-based powder compact, sintering it at a temperature of 1350 to 1450°C without pressure, and then This is a method for sintering iron-based powder, characterized by sintering in a pressurized gas atmosphere at a temperature of 1100 to 1350°C and a pressure of 70 atmospheres or more.

The present invention will be explained in detail below.

The iron-based powder to which the present invention is applied includes pure iron-based iron powder,
That is, the main component is Fe, and St%Mn%AJ is added in small amounts as necessary for unavoidable impurities and iron powder production.
2 and the like, and powders known as so-called low-alloy steel powders that undergo α#γ transformation within the range from room temperature to 1400°C.

The powder used may be produced by any of the high-pressure water atomization method, the reduction method, the carbonyl method, etc., or may be a fine metal powder obtained by pulverization or classification after these methods. Alternatively, a mixed powder thereof may be used.

The powder molded body may be molded using any known molding method, such as a known mold molding method in which an organic lubricant is added to the powder, or a compound formed by kneading it with an organic binder. Those molded by a known injection molding method can be used. In the case of complex-shaped parts, those molded by injection molding are preferred.

Here, powder compaction will be described.

Molding of the powder is performed after adding and mixing the binder.

Examples of binders for molding include higher fatty acids, fatty acid amides, fatty acid esters, etc., which are lubricants.

In the case of injection molding, a binder mainly consisting of a thermoplastic resin and/or wax is used, and if necessary, a plasticizer, lubricant, degreasing accelerator, etc. are added.

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, and synthetic waxes such as low-molecular polyethylene, microcrystalline wax, paraffin wax, etc. There are waxes, but one or more fffi selected from these 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 binder varies depending on the molding method in the post-process.
In normal mold press molding, the amount is 0.5 to 3.0% by weight, and in injection molding, it is about 10% by weight.

Batch-type or continuous-type kneaders can be used to mix and knead iron-based powder and binder in the case of injection molding.Batch-type kneaders include pressure kneaders, Banbury mixers, etc.; Among these, a twin-screw extruder and the like are advantageously suited. After kneading, granulation is performed using a pelletizer or pulverizer as necessary.
Obtain a molding compound.

Injection molding may be carried out using an injection molding machine used for normal injection molding, such as an injection molding machine for plastics or an injection molding machine for metal powder.The injection pressure is usually 500 to 20
It is about 00 a.m.

After molding, heating is performed to remove the binder (dewaxing). The temperature increase rate at this time is 5 to b.Heating is performed and immediately cooled. Note that if the temperature increase rate at this time is too high, cracks or blisters will occur in the obtained molded product, which is not preferable.

In the present invention, a molded body of iron-based powder made using a molding compound containing iron-based powder is dewaxed, for example, under the above conditions, and then sintered under the following conditions.

The primary sintering is performed at a temperature of 1350 to 1450°C and under no pressure. That is, by performing pressureless sintering at high temperature, the pores of the sintered body are closed.

The temperature of pressureless sintering is 1350 to 1450°C, but 1
If the temperature is lower than 350°C, the pores will not become closed, and high density cannot be achieved by subsequent pressurization. On the other hand, 1450
When the temperature exceeds ℃, the crystal grains become coarse and the strength deteriorates.

No pressure refers to normal pressure to vacuum. In addition, the atmosphere for pressureless sintering is vacuum, reducing or neutral (inert).
Although any atmosphere may be used, a vacuum atmosphere is preferable from the viewpoint of reducing C and O.

Note that gases that form a reducing atmosphere include hydrogen gas, ammonia decomposition (AX) gas, and hydrocarbon modification (RX) gas.
), gases, etc., and examples of gases forming a neutral atmosphere include nitrogen gas, argon gas, etc.

The secondary sintering is performed in a pressurized gas atmosphere at a temperature of 1100 to 1350°C and a pressure of 70 atmospheres or more.

This step eliminates closed pores and achieves high density.

The temperature during secondary sintering is 1100 to 1350°C, but 1
At temperatures below 100°C, the sintered metal is insufficiently softened;
Densification by pressurization was not achieved sufficiently, while 1350
If the temperature exceeds .degree. C., the crystal grains become coarse and the strength of the sintered body decreases.

Further, the pressure at this time is 70 atmospheres or more, but if it is less than 70 atmospheres, the atmospheric temperature is relatively low, so that the effect of pressurization, that is, densification cannot be sufficiently achieved. on the other hand,
Even if the pressure exceeds 400 atm, the effect obtained by increasing the pressure will be small and it will be economically disadvantageous, so it is desirable to set the upper limit of the pressure to 400 atm.

Gases to be used include those applicable to normal pressure sintering, such as neutral (inert) gases such as nitrogen gas and argon gas, and reducing gases such as hydrogen gas, AX gas, and RX gas. It's gas.

There is no particular restriction on the sintering time in either the secondary or secondary sintering process, but from the viewpoint of the technical effects and cost of the method of the present invention, it is recommended that each process take about 10 to 200 minutes. preferable.

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

[Example 1] The average particle size was 38 μm, the maximum particle size was 98 μm, and C: 0.
012wt%, Si: 0.02wt%, Mn: 0.12
wt%, P: 0.008wt%, S+0.010wt%, 0:0.1
Atomized iron powder containing 25 wt%, Mono, 96 wt%, zinc stearate 1.0 wt% and graphite powder 0.
3 wt% was added and mixed to form a molding comparand.
This was compressed into a 35' x lO' x 6.5' (7) rectangular transverse rupture strength test piece at a compression density of 6.53 g/cm'.
Mold molding (molding pressure 5 t/c@2) was performed.

Dewaxing was carried out in AX gas at 600°C for 100 minutes, and subsequent sintering was carried out at 130°C in a vacuum of 10-'T'orr.
It was carried out for 60 minutes at 0°C, 1360°C, 1450°C, or 1480°C. In addition, secondary sintering is performed using Ar gas, 1
The test was carried out at 1250° C. for 60 minutes in a pressurized atmosphere of 00 atm.

The density of the thus obtained transverse rupture strength test piece was measured by Archimedes' method.

Further, the transverse rupture strength was measured by a three-point bending test method.

The primary sintering conditions and measurement results are shown in Table 1.

Table 1 *100%=7.85 g/cm3 Inventive examples B and C had high density and transverse rupture strength. On the other hand, A (comparative example) with a low secondary sintering temperature had low density and transverse rupture strength because not all the pores were closed, while D with a high sintering temperature
(Comparative Example) had a high density, but the transverse rupture strength was low because the crystal grains were coarsened.

[Example 2] A molded article similar to Example 1 was dewaxed under the same conditions as Example 1, and then heated at 1360°C in a vacuum of 10-3 Torr.
Next, sintering was carried out for 60 minutes at Ar gas, 1
00atm atmosphere, 1150℃, 1250℃, 1350
℃ or 1400℃ for 60 minutes,
A transverse rupture strength test piece was obtained.

Regarding these, the density and transverse rupture strength were measured in the same manner as in Example 1.

The secondary sintering temperature and measurement results are shown in Table 2.

Table 2 *100%w7.85g/cm3 All of the invention examples F-H had high density and transverse rupture strength.

On the other hand, in E, a comparative example with a low secondary sintering temperature, high density was not achieved due to insufficient softening of the raw material metal, and as a result, the strength decreased due to pores. Transverse rupture strength was also insufficient.

On the other hand, Comparative Example I, which had a high secondary sintering temperature, had a high density, but the crystal grains were coarse and the transverse rupture strength was low.

[Example 3] The conditions up to the primary sintering were the same as in Example 2.

Then, in Ar gas, 50.70.100.200.4
00 or 60 at 1250℃ under pressure of 600 atm.
Secondary sintering was performed for 1 minute.

Regarding these, the density and transverse rupture strength were measured in the same manner as in Example 1.

The secondary sintering pressure and measurement results are shown in Table 3.

Table 3 *100%w7.85g/cm3 Inventive examples ni-0 all showed high density.

On the other hand, Comparative Example J, which had a low atmospheric pressure during secondary sintering, was insufficient in density and transverse rupture strength. Note that 0, which is an example in which the atmospheric pressure during secondary sintering is particularly high, has a high density and transverse rupture strength, but the effect of increasing density and transverse rupture strength due to the increase in pressure is small, and it is not economically viable. was considered disadvantageous.

<Effects of the Invention> The present invention provides a method for sintering iron-based powder that yields a high-density, high-strength iron-based powder metallurgy material.

Since the method of the present invention can be carried out using raw material powder of any particle size, costs can be reduced.

Furthermore, the method of the present invention is very useful because it can be carried out on an industrial scale.

Claims (1)

[Claims] (1) In the production of powder metallurgy materials made by molding and sintering iron-based powder, after dewaxing the iron-based powder compact, the temperature is 135
Sintered at 0-1450°C without pressure, then at a temperature of 11
A method for sintering iron-based powder, characterized by sintering in a pressurized gas atmosphere at 00 to 1350°C and a pressure of 70 atmospheres or more.