JPH02290904A - Method for sintering iron series powder - Google Patents

Abstract

PURPOSE:To obtain a high density and high strength material even if raw material powder is coarse by executing sintering under non-pressurizing, sintering under pressurized gas atmosphere of specific pressure and sintering under non- pressurizing at each specific temp. to a green compact of iron series powder after degreasing, in order. CONSTITUTION:After adding and mixing lubricant and binder to iron-series powder, this is compacted to obtain the green compact of iron-series powder. After degreasing this green compact, this is sintered at 1350-1450 deg.C under non- pressurizing and sintered at 1350-1450 deg.C under pressurizing gas atmosphere of >=20kg/cm<2> pressure and further, sintered at 1100-1350 deg.C under non- pressurizing. By this method, after making the void in the sintered body closed, as the void is eliminated with the pressurized sintering and coarsening of crystal grain of metal can be prevented, the high density and high strength iron-series powder metallurgical material is obtd.

Description

[Detailed Description of the Invention] Industrial Application Fields The present invention relates to a method for producing high-density (95% or more) iron-based powder metallurgy materials. This method has been developed mainly as a manufacturing method for parts with complex shapes because it can produce members with a final part shape or a shape close to it with high yield and high efficiency, and has excellent dimensional accuracy. 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%).
7g/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 notable are the use of fine metal powder for the former (the poor compressibility of fine powder was solved by the introduction of injection molding), and the use of HIP (hot isostatic pressing) for the latter. can give. Furthermore, regarding HIP technology, the pressure sintering method (US Patent No. 8 [i/00853, Japanese Patent Publication No. 83-50087
4) 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 price, the higher the price.

In addition, the above prior art (US patent US86/00853)
In the pressure sintering method that involves a short-term temperature rise (temperature spike) disclosed in accompanied by. 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.

Problems to be Solved by the Invention> As described above, methods for increasing the density of sintered bodies have been proposed, but there is still 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 conventional methods such as injection molding for producing sintered metal materials using fine powder as raw materials have improved the properties (density) and sintered metal materials. The purpose of this invention 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 Problem> 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 closed and then pressure sintered. 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 metallurgy material formed by molding and sintering iron-based powder, the present invention involves dewaxing the molded body of iron-based powder, sintering it at a temperature of 1350 to 1450°C without pressure, and reducing the temperature. Sintered in a pressurized gas atmosphere at 1350 to 1450°C and a pressure of 20 kg/cm2 or more, and further at a temperature of 1100°C.
This is a method for sintering iron-based powder, characterized by sintering at ~1350°C under no pressure.

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 unavoidable impurities and Si, Mn, Aj, which are added in small amounts as necessary in the production of iron powder.
! Examples include iron powder containing .

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.

The powder is compacted after adding and mixing the lubricant and 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 a desensitizer, a lubricant, a 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, and synthetic waxes such as low-molecular polyethylene, microcrystalline wax, paraffin wax, etc. There is wax, but 1f selected from these! ! Alternatively, use 2 fffl or more.

The desensitizing agent is selected depending on the combination with the main resin or wax, and specifically, di-2-ethylhexyl phthalate (DOP), diethyl phthalate (DEP), etc.
Examples include di-n-butyl phthalate (DHP).

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. Among batch-type kneaders, pressure kneaders, Banbury mixers, etc. Among the secondary machines, a twin-screw extruder or the like is advantageously suited. After kneading, granulation is performed using a beletizer or a crusher as necessary to obtain a composite for molding. Injection molding may be performed 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. Injection pressure is usually 500 to 20
It is about 00 atm.

After molding, heating is performed to remove the binder (dewaxing). The temperature increase rate at this time is 5 to 30° C./hr, and generally, it is heated to 600° C. 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 t without 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
If it exceeds t, the crystal grains will become coarser and the strength will deteriorate.

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, from the viewpoint of reducing C and 0, vacuum/ambience is preferable.

Note that gases that form the reducing atmosphere include hydrogen gas, ammonia decomposition (AX) gas, and hydrocarbon conversion (RX) gas.
Examples of the gas that forms the neutral atmosphere include nitrogen gas, argon gas, and the like.

Secondary sintering is performed at a temperature of 1350 to 1450°C and a pressure of 20°C.
It is carried out in a pressurized gas atmosphere of kg/cm' or more.

This step eliminates closed pores and achieves high density.

The temperature during secondary sintering is 1350-1450℃, but 1
At temperatures below 350°C, densification by pressure is insufficient; on the other hand, at temperatures above 1450°C, crystal grains become coarse and strength decreases, and this decrease in strength is also caused by subsequent pressureless sintering. I can't recover.

In addition, the pressure at this time is 20 kg/cm2 or more, but if the pressure is less than 20 kg/cm2, even if pressure sintering is performed at a high temperature, the pressurizing effect will be insufficient, and it will be difficult to achieve high density. On the other hand, even if the pressure exceeds 250 kg/cm2, the effect obtained by increasing the pressure will be small and it will be economically disadvantageous, so the upper limit of the pressure should be set at 250 kg/cm2.
/cm' is desirable.

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.

In the present invention, tertiary sintering is further performed at a temperature of 1100 to 1350°C without pressure. This step adjusts the grain size and increases the strength of the sintered body.

The temperature during tertiary sintering is 1100 to 1350°C, but 1
If the temperature is less than 100°C, there is no significant change in crystal grain size and no effect is obtained. On the other hand, if the temperature exceeds 1350°C, the crystal grains become coarse and the sintered body cannot be made to have high strength.

The atmosphere during pressureless sintering is as already described in the section of primary sintering, but tertiary sintering is preferably performed under normal pressure in a neutral gas atmosphere.

Although there is no restriction on the sintering time in any of the sintering steps, from the viewpoint of the technical effect and cost of the method of the present invention, the sintering time is preferably about 10 to 200 minutes in each step.

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

(Example 1) Average particle size is 34 μm, maximum particle size is 87 μm, and C:
0.04 t%, Si: 0.03 t%, Mn:
O, 10wt%, P: 0.011
wt%, S: 0.008 wt%, Ni: 0
．． Zinc stearate 1.23 wt%, MO:0.33 gastric t% to atomized iron powder. Owt% and 0.5wt% of graphite powder were added and mixed to prepare a molding compound. This is a 35L x 10W x 6.5H rectangular transverse rupture strength test piece with a compressed density of 6. Molding at 5 7 g/cn' (molding pressure 5 t/cm') L/ta. Dewaxing was performed at 600°C for 100 minutes in AX gas, and primary sintering was performed at 1300°C in a vacuum of 10-'Torr.
, 60 at 1360℃, 1450℃, or 1480℃
I did it for a minute. In addition, secondary sintering is performed using Ar gas, 40
kg/cm2 in a pressurized atmosphere for 30 minutes under the same temperature conditions as the primary sintering.
The test was carried out at 1250° C. for 60 minutes in an atmosphere of r gas and normal pressure.

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 sintering conditions and measurement results are shown in Table 1.

No.! Table * 100% = 7.84 g/cm' Inventive examples B and C had high density and transverse rupture strength. On the other hand, A (comparative example) with a low primary and secondary sintering temperature did not have closed pores, so both density and transverse rupture strength were low, whereas D (comparative example) with a high sintering temperature Although the density was high, the transverse rupture strength was low because the crystal grains were coarsened.

In addition, in E-H, which is a comparative example in which tertiary sintering was not performed, the transverse rupture strength was low because the crystal grains were not adjusted.

[Example 2] A molded article similar to that in Example 1 was dewaxed under the same conditions as in Example 1, and then heated at 136° C. in a vacuum of 10 −3 Torr.
Primary sintering was performed at 0° C. for 60 minutes, and then secondary sintering was performed at 1360° C. for 60 minutes in Ar gas and 40 kg/cm 2 ashes.

After that, Ar gas, normal pressure atmosphere, 1050°C, 115
Tertiary sintering was performed at 0°C, 1250°C, 1350°C, or 1400°C for 60 minutes to obtain transverse rupture strength test pieces.

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

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

Table 2 * 1 0 0% = 7.84 g/cm” J which is an invention example
-L all showed high transverse rupture strength. On the other hand,
Comparative example ■ with a low tertiary sintering temperature has a density that is almost at a satisfactory level, but the transverse rupture strength is insufficient due to the insufficient effect of grain adjustment. M, which is a high comparative example, had coarse grains and low transverse rupture strength. (Example 3) The same conditions as in Example 2 were used up to the primary sintering.

Then, in Ar gas, 10, 20, 40, 100, 23
0, or under a pressure of 350 k, g/am', 1
Secondary sintering was performed at 250° C. for 60 minutes, and tertiary sintering was performed at 1250° C. for 60 minutes in an atmosphere of Ar gas and normal pressure.

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.

Effects of the invention as shown in Table 3 The present invention provides a method for sintering iron-based powder that yields a high-density, high-strength iron-based powder metallurgical 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.

*100%=7.84g/am' Inventive examples 0 to S all showed high density.

On the other hand, N, which is a comparative example in which the atmospheric pressure during secondary sintering was low, was insufficient in density and transverse rupture strength. Note that S, which had a particularly high atmospheric pressure during secondary sintering, had a high density and transverse rupture strength, but the effect of increasing the density and transverse rupture strength with an increase in pressure was small and was not economical.

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 under no pressure at 0-1450℃, temperature 1350-1
A method for sintering iron-based powder, characterized by sintering in a pressurized gas atmosphere at 450°C and a pressure of 20 kg/cm^2 or more, and further sintering at a temperature of 1100 to 1350°C without pressure.