U.S. patent application number 11/728598 was filed with the patent office on 2007-10-04 for method of manufacturing sintered body.
This patent application is currently assigned to Seiko Epson Corporation. Invention is credited to Hidefumi Nakamura.
Application Number | 20070231181 11/728598 |
Document ID | / |
Family ID | 38559212 |
Filed Date | 2007-10-04 |
United States Patent
Application |
20070231181 |
Kind Code |
A1 |
Nakamura; Hidefumi |
October 4, 2007 |
Method of manufacturing sintered body
Abstract
A method of manufacturing a sintered body includes a first
process of forming a metal powder containing carbon into a compact
having a predetermined shape and a second process of baking the
compact in a hermetically sealed space so as to produce a sintered
body. The hermetically sealed space has an atmosphere having a
pressure of 60 kPa to 140 kPa and contains a hydrogen gas and an
oxygen gas. The sum of partial pressures of the hydrogen gas and
the oxygen gas is not more than 3 Pa.
Inventors: |
Nakamura; Hidefumi; (Aomori,
JP) |
Correspondence
Address: |
HARNESS, DICKEY & PIERCE, P.L.C.
P.O. BOX 828
BLOOMFIELD HILLS
MI
48303
US
|
Assignee: |
Seiko Epson Corporation
|
Family ID: |
38559212 |
Appl. No.: |
11/728598 |
Filed: |
March 26, 2007 |
Current U.S.
Class: |
419/14 ;
419/58 |
Current CPC
Class: |
B22F 2999/00 20130101;
B22F 3/1007 20130101; B22F 2998/10 20130101; B22F 2998/10 20130101;
B22F 2999/00 20130101; B22F 3/1007 20130101; B22F 2999/00 20130101;
B22F 2201/11 20130101; B22F 3/225 20130101; B22F 1/0085 20130101;
B22F 2201/02 20130101; B22F 1/0085 20130101; B22F 3/02 20130101;
B22F 3/1007 20130101 |
Class at
Publication: |
419/14 ;
419/58 |
International
Class: |
B22F 3/10 20060101
B22F003/10 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 3, 2006 |
JP |
2006-102560 |
Claims
1. A method of manufacturing a sintered body, the method
comprising: forming a metal powder containing carbon into a compact
having a predetermined shape; and baking the compact in a
hermetically sealed space so as to produce a sintered body, the
hermetically sealed space having an atmosphere having a pressure of
60 kPa to 140 kPa and containing a hydrogen gas and an oxygen gas,
a sum of partial pressures of the hydrogen gas and the oxygen gas
being not more than 3 Pa.
2. The method as recited in claim 1, wherein the atmosphere of the
hermetically sealed space primarily contains an inert gas.
3. The method as recited in claim 2, wherein the inert gas
comprises an argon gas.
4. The method as recited in claim 1, wherein the metal powder has
an average particle diameter of 3 .mu.m to 30 .mu.m.
5. The method as recited in claim 1, wherein the metal powder has a
carbon content of 0.05 atm % to 2 atm %.
6. The method as recited in claim 1, wherein the metal powder is
made of an Fe-based alloy material.
7. The method as recited in claim 1, wherein the metal powder
forming process comprises: forming a composition containing the
metal powder and a binder into a predetermined shape so as to
produce a primary compact; and removing the binder from the primary
compact so as to produce a secondary compact as the compact.
8. The method as recited in claim 7, wherein the forming process of
the composition comprises metal injection molding of the
composition to produce the primary compact.
9. A sintered body manufactured by the method as recited in claim
1.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims priorities to Japanese Patent
Application No. 2006-102560 filed on Apr. 3, 2006 which is hereby
expressly incorporated by reference herein in its entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a sintered body and a
method of manufacturing a sintered body.
[0004] 2. Description of the Related Art
[0005] For example, a metal product is manufactured by sintering a
compact containing a metal powder in the following manner. A metal
powder and an organic binder are mixed and kneaded. The kneaded
compound is formed into a predetermined shape, so that a primary
compact is produced. Then the primary compact is degreased
(debound) so as to remove the organic binder from the primary
compact. Thus, a secondary compact (degreased compact) is produced.
Thereafter, the secondary compact is baked so as to produce a
sintered body.
[0006] The secondary compact is baked in a baking furnace under a
reduced-pressure (vacuum) atmosphere having a high vacuum of 13 Pa
(0.1 Torr) or less, a non-oxidizing atmosphere (see Japanese
laid-open patent publication No. 7-224348), or an air
atmosphere.
[0007] Meanwhile, in the field of powder metallurgy, mechanical
characteristics of a resultant sintered body have been improved by
properly adjusting the composition of a metal powder. Specifically,
it can be seen that mechanical characteristics such as tensile
strength and hardness are improved in a sintered body formed of
stainless steel powder having a given carbon (C) content (e.g.,
about 0.8 atm % to about 1.2 atm %).
[0008] However, when a secondary compact of a metal powder
containing carbon is to be baked by the aforementioned method
disclosed in Japanese laid-open patent publication No. 7-224348,
the following problems arise.
[0009] First, when a secondary compact is baked in a non-oxidizing
atmosphere, a hydrogen gas contained in the atmosphere reacts with
carbon in the secondary compact. Accordingly, carbon is
problematically desorbed from the secondary compact. Furthermore,
when a secondary compact is baked in an air atmosphere, an oxygen
gas contained in the atmosphere reacts with carbon in the secondary
compact. Accordingly, the same problem arises. If such a problem
arises, then the carbon content of a sintered body is lowered so as
to cause deterioration of mechanical characteristics.
[0010] Additionally, because a decrease of the carbon content
proceeds when the secondary compact is brought into contact with a
hydrogen gas and an oxygen gas in the atmosphere, it is likely to
depend upon a shape of the secondary compact. That is, the carbon
content tends to be lowered in a secondary compact having a
complicated shape with a large surface area. Accordingly, the
degree of the decrease of the carbon content varies depending upon
the shape of a secondary compact.
[0011] Second, the aforementioned high vacuum of 13 Pa (0.1 Torr)
or less may cause pressure drop in the baking furnace and
production of an oxygen gas from a material of components forming a
baking furnace depending upon the kind of the material. Therefore,
the same problem as described above may also arise. Furthermore, in
order to maintain a high vacuum, it is necessary to provide a
special pressure proof mechanism or an expensive pump for a high
vacuum in the baking furnace. Accordingly, cost of the baking
process is problematically increased.
SUMMARY OF THE INVENTION
[0012] The present invention has been made in view of the above
drawbacks. It is, therefore, a first object of the present
invention to provide a method of manufacturing a sintered body
which can efficiently manufacture a sintered body having a desired
carbon content at a low cost irrespective of a shape of the
sintered body.
[0013] A second object of the present invention is to provide a
sintered body manufactured by such a method.
[0014] According to a first aspect of the present invention, there
is provided a method of manufacturing a sintered body. This method
includes forming a metal powder containing carbon into a compact
having a predetermined shape and baking the compact in a
hermetically sealed space so as to produce a sintered body. The
hermetically sealed space has an atmosphere having a pressure of 60
kPa to 140 kPa and contains a hydrogen gas and an oxygen gas. The
sum of partial pressures of the hydrogen gas and the oxygen gas is
not more than 3 Pa.
[0015] With the above method, it is possible to efficiently produce
a sintered body having a desired carbon content at a low cost.
[0016] It is desirable that the atmosphere of the hermetically
sealed space primarily should contain an inert gas. It is also
desirable to use an argon gas as the inert gas.
[0017] Furthermore, it is desirable that the metal powder should
have an average particle diameter of 3 .mu.m to 30 .mu.m. It is
also desirable that the metal powder should have a carbon content
of 0.05 atm % to 2 atm %. Additionally, it is desirable to make the
metal powder of an Fe-based alloy material.
[0018] The metal powder forming process may include forming a
composition containing the metal powder and a binder into a
predetermined shape so as to produce a primary compact and removing
the binder from the primary compact so as to produce a secondary
compact as the compact. It is desirable that the forming process of
the composition should comprise metal injection molding of the
composition to produce the primary compact.
[0019] According to a second aspect of the present invention, there
is provided a sintered body manufactured by the above method. This
sintered body can have excellent mechanical characteristics.
[0020] The above and other objects, features, and advantages of the
present invention will be apparent from the following description
when taken in conjunction with the accompanying drawings which
illustrate preferred embodiments of the present invention by way of
example.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 is a flow chart showing a method of manufacturing a
sintered body according to an embodiment of the present invention;
and
[0022] FIG. 2 is a vertical cross-sectional view schematically
showing a baking furnace (hermetically sealed vessel) used in a
method of manufacturing a sintered body according to the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0023] A method of manufacturing a sintered body according to a
preferred embodiment of the present invention will be described
below with reference to FIGS. 1 and 2.
[0024] In a method of manufacturing a sintered body according to
the present invention, a compact of a metal powder containing
carbon is baked (sintered) under a predetermined atmosphere so as
to produce a sintered body having a desired carbon content.
[0025] FIG. 1 is a flow chart showing a method of manufacturing a
sintered body according to an embodiment of the present
invention.
[0026] The method shown in FIG. 1 uses a metal powder to produce a
sintered body. Specifically, as shown in FIG. 1, the manufacturing
method of a sintered body includes a composition preparation
process of preparing a composition containing a metal powder and an
organic binder, a formation process of forming the composition into
a primary compact, a removal process (degrease process/debinding
process) of removing the organic binder from the primary compact so
as to produce a secondary compact, and a baking process of baking
(sintering) the secondary compact so as to produce a sintered body.
These processes will be described in the order named.
[0027] 1) Composition Preparation Process
[0028] First, a metal powder and an organic binder are prepared and
kneaded by a kneader, so that a kneaded compound (composition) is
produced. The metal powder is dispersed uniformly in the kneaded
compound. It is desirable that the metal powder and the organic
binder mixed in the kneaded compound should not react with each
other or should have little reactivity with each other.
[0029] According to the present invention, a metal powder
containing carbon (C) is used as the metal powder. The material of
the metal powder is not limited to a specific one as long as it is
a metal material containing carbon. Examples of the material of the
metal powder includes stainless steel such as SUS-420 or SUS-440,
carbon steel, die steel, high speed tool steel, various kinds of
Fe-based alloy materials such as Fe--Ni--Co alloys or Fe--Ni alloys
including Fe.sub.2NiC or Fe.sub.8NiC, low-carbon steel, various
kinds of Ni-based alloy materials, and various kinds of Cu-based
alloy materials.
[0030] Meanwhile, in the field of powder metallurgy, mechanical
characteristics of a resultant sintered body have been improved by
properly adjusting the composition of a metal powder. Specifically,
it can be seen that mechanical characteristics such as tensile
strength and hardness are improved in a sintered body having a
given carbon (C) content (e.g., about 0.8 atm % to about 1.2 atm
%).
[0031] In the conventional technology, however, even if a metal
powder adjusted in carbon content in order that a sintered body has
a desired carbon content is used to produce a sintered body, it is
difficult to manufacture a sintered body having a desired carbon
content for the reasons descried later.
[0032] In contrast to the conventional technology, the present
invention demonstrates its effects and advantages effectively in a
case where a metal powder including an Fe-based alloy material is
used to produce a sintered body. Specifically, an Fe-based alloy
sintered body having a desired carbon content can be produced
efficiently by baking an Fe-based alloy powder containing carbon
under a predetermined atmosphere. The resultant sintered body
exhibits excellent mechanical characteristics.
[0033] It is desirable to use a metal powder having a carbon
content in a range of about 0.05 wt % to about 2 wt %, preferably
about 0.1 wt % to about 1.5 wt %. When a metal powder containing a
small amount of carbon is used to produce a sintered body as
described above, mechanical characteristics of the sintered body
often vary according to a slight change of the carbon content. The
present invention demonstrates its effects and advantages
effectively in producing a sintered body in which the carbon
content is required to be controlled with reliability.
[0034] The metal powder may contain two or more kinds of materials
having different compositions. In this case, it is possible to
produce a sintered body having an alloy composition that has not
heretofore been produced by casting. Furthermore, it is easy to
produce a sintered body having novel functions or many functions.
Accordingly, it is possible to widen functions and applications of
a sintered body.
[0035] The average particle diameter of the metal powder is not
limited to a specific value. Nevertheless, it is desirable that the
metal powder should have an average particle diameter of about 3
.mu.m to about 30 .mu.m, more preferably about 5 .mu.m to about 20
.mu.m. With use of the metal powder having an average particle
diameter in the above range, it is possible to increase the
flowability of a kneaded compound and thus obtain a kneaded
compound having excellent formability (ease of formation). As a
result, it is possible to enhance the density of a primary compact
in the subsequent formation process and hence produce a sintered
body having excellent mechanical characteristics as a final
product.
[0036] Meanwhile, if the metal powder has a particle diameter as
small as described above, the metal powder in a secondary compact
described later is brought into contact with the atmosphere with an
increased surface area. Accordingly, carbon tends to be desorbed in
the subsequent baking process. The present invention demonstrates
its effects and advantages effectively in a case where such a
secondary compact is baked so as to produce a sintered body.
[0037] For example, such a metal powder can be manufactured by an
atomization process (e.g., a water atomization method, a gas
atomization method, or a high-speed spinning water atomization
method), a reduction process, a carbonyl process, or a grinding
process. It is desirable to use a metal powder manufactured by an
atomization process. By an atomization process, it is possible to
efficiently produce an extremely fine metal powder. When such a
metal powder is employed as a material powder, a sintered body
having a fine crystalline structure and excellent mechanical
strength can be produced with reliability.
[0038] Furthermore, because each particle of a metal powder
manufactured by an atomization process has a spherical shape close
to a perfect sphere, a metal powder manufactured by an atomization
process has excellent dispersibility and flowability. Accordingly,
in the formation process, the kneaded compound can be filled into a
forming die at a high filling fraction. Thus, a primary compact and
a secondary compact having a fine complicated shape can readily be
produced in the formation process and the removal process, which
will be described later. However, when a secondary compact has a
fine complicated shape, it tends to have a large surface area
contacting the atmosphere. Therefore, carbon in the secondary
compact is likely to be desorbed in the subsequent baking process
because of reaction with an atmosphere gas. The present invention,
however, demonstrates its effects and advantages effectively even
in a case where a secondary compact having a complicated shape is
baked so as to produce a sintered body.
[0039] Examples of the organic binder include various kinds of
resins including polyolefine such as polyethylene, polypropylene,
and ethylene-vinyl acetate copolymer, acrylic resin such as
polymethyl methacrylate and polybutyl methacrylate, styrene resin
such as polystyrene, polyester such as polyvinyl chloride,
polyvinylidene chloride, polyamide, polyethylene terephthalate, and
polybutylene terephthalate, polyester, polyvinyl alcohol, and
copolymer thereof, a variety of wax, paraffin, higher fatty acid
(e.g., stearic acid), higher alcohol, higher fatty acid ester, and
higher fatty acid amide. One or more materials of the above
materials may be mixed in the organic binder.
[0040] Furthermore, it is desirable that the kneaded compound
should have an organic binder content of about 2 wt % to about 40
wt %, more preferably about 5 wt % to about 30 wt %. With the
kneaded compound having an organic binder content in the above
range, it is possible to form a primary compact with high
formability and enhance the density of the primary compact, so that
the primary compact can have excellent stability in shape.
Additionally, it is possible to reduce a difference in size between
the primary compact and the secondary compact, i.e., a shrinkage
percentage. As a result, the dimensional accuracy of the secondary
compact and the sintered body can be improved.
[0041] A plasticizer may be added to the kneaded compound. Examples
of the plasticizer include phthalate ester (e.g., DOP, DEP, and
DBP), adipic acid ester, trimellitic acid ester, and sebacic acid
ester. One or more materials of the above materials may be mixed in
the plasticizer.
[0042] For example, in addition to the metal powder, the organic
binder, and the plasticizer, various additives such as an
antioxidant, a degrease accelerator, and a surface-active agent may
be added to the kneaded compound as needed.
[0043] Kneading conditions vary depending upon various conditions
including the metal composition or the particle diameter of a metal
powder to be used, the composition of an organic binder, and the
amounts of the metal powder and the organic binder to be mixed. For
example, the kneading temperature is in a range of about 50.degree.
C. to about 200.degree. C., and the kneading time is in a range of
about 15 minutes to about 210 minutes.
[0044] Furthermore, the kneaded compound may be formed into pellets
(small lumps). For example, each pellet has a particle diameter of
about 1 mm to about 15 mm.
[0045] 2) Formation Process
[0046] Next, the kneaded compound is formed into a primary compact
having the same shape as a sintered body to be produced. A method
of producing (forming) a primary compact is not limited to a
specific one. For example, a primary compact may be formed by a
metal injection molding (MIM) method or a compression molding
method (compression powder molding method). Particularly, a metal
injection molding method is suitable for formation of a primary
compact.
[0047] The MIM method allows a relatively small primary compact or
a primary compact having a fine complicated shape to be produced
with a near net shape (a shape close to a final product). Thus, the
MIM method can advantageously make full use of characteristics of
the metal powder to be used. Although the MIM method can form a
fine complicated shape, it is likely to increase a surface area of
the secondary compact contacting the atmosphere. Accordingly, the
aforementioned tendency becomes more significant. The present
invention, however, demonstrates its effects and advantages
effectively even in a case where a secondary compact produced by an
MIM method is baked so as to produce a sintered body.
[0048] A method of producing a primary compact will be described
below with use of a typical example of an MIM method.
[0049] First, injection molding is performed on the kneaded
compound produced in the process 1) or pellets granulated from the
kneaded compound by an injection molding machine. As a result, a
primary compact having a desired shape and size is produced. In
this case, by properly selecting a forming die, it is possible to
produce a primary compact having a complicated shape with ease.
[0050] In the primary compact thus produced, the metal powder is
substantially dispersed uniformly in the organic binder.
[0051] The shape and size of the primary compact to be produced are
determined in expectation of a shrinkage of the primary compact due
to subsequent degrease and sintering.
[0052] Forming conditions in injection molding vary depending upon
various conditions including the composition or the particle
diameter of a metal powder to be used, the composition of an
organic binder, and the amounts of the metal powder and the organic
binder to be mixed. For example, the material temperature is
preferably in a range of about 80.degree. C. to about 200.degree.
C., and the injection pressure is preferably in a range of about 2
MPa to about 30 MPa (about 20 kgf/cm.sup.2 to about 300
kgf/cm.sup.2).
[0053] 3) Removal Process (Degrease Process)
[0054] A degrease process (binder removal process) is performed on
the primary compact produced in the process 2), so that a secondary
compact (degreased compact) is produced. For this degrease process,
heat treatment is performed under an atmosphere including an
oxidizing gas such as air or oxygen, a reducing gas such as
hydrogen or carbon monoxide, an inert gas such as nitrogen, helium,
or argon, or a mixed gas containing one or more of these gases, or
under an reduced-pressure atmosphere. In this case, conditions for
heat treatment vary to some extent, for example, depending upon the
temperature at which the organic binder begins to be decomposed.
The heat treatment is preferably performed at a temperature of
about 100.degree. C. to about 750.degree. C. for about 0.5 hour to
about 40 hours, more preferably at a temperature of about
150.degree. C. to about 600.degree. C. for about 1 hour to about 24
hours.
[0055] The degrease process with heat treatment may be performed by
a plurality of steps (stages) for various purposes (e.g., for the
purpose of shortening the degrease time). For example, a first half
of the degrease process may be performed at a low temperature while
a second half of the degrease process may be performed at a high
temperature. Alternatively, a low-temperature degrease step and a
high-temperature degrease step may be repeated.
[0056] Furthermore, the degrease process may be performed by
eluting a specific component in the organic binder or the additives
with use of a predetermined solvent (a fluid such as a liquid or a
gas).
[0057] Thus, the organic binder is removed. As a result, a
secondary compact is produced.
[0058] The organic binder may not be removed completely by the
degrease process. For example, a portion of the organic binder may
be left in the compact at the time of completion of the degrease
process.
[0059] In this manner, it is possible to produce a secondary
compact having excellent capability of maintaining its shape
(form).
[0060] In the present embodiment, the processes 1) to 3) constitute
a first process of forming a metal powder containing carbon into a
compact having a predetermined shape.
[0061] For example, the aforementioned metal powder may be pressed
into a predetermined shape so as to provide a compressed compact
having a desired shape. Such a compressed compact may be used
instead of the secondary compact produced in the process 3). In
this case, the pressing process constitutes a first process of
forming a metal powder containing carbon into a compact having a
predetermined shape.
[0062] 4) Baking Process
[0063] The secondary compact produced in the process 3) is baked in
a baking furnace or the like. Thus, the secondary compact is
sintered, so that a sintered body is produced (second process). By
the sintering, the metal powder is dispersed at interfaces between
particles so as to cause grain growth, thereby forming a
crystalline structure. As a result, it is possible to produce a
closely packed sintered body having a high density.
[0064] The baking temperatures vary to some extent depending upon
the composition of the metal powder and the like. For example, the
baking temperature is preferably in a range of about 1,000.degree.
C. to about 1,400.degree. C., more preferably about 1,100.degree.
C. to about 1,300.degree. C. Under the sintering temperature in the
above range, the dispersion and grain growth of the metal powder
are optimized to thereby provide a sintered body having excellent
characteristics such as mechanical strength, dimensional accuracy,
and external appearance. The sintering temperature in the sintering
process may vary (increase or decrease) with time in or out of the
above range.
[0065] The sintering period is preferably in a range of about 0.5
hour to about 7 hours, more preferably about 1 hour to about 4
hours.
[0066] Meanwhile, in the present invention, the metal powder
contains carbon at a given percentage. With the conventional
technology to produce a sintered body from such a metal powder, a
secondary compact is baked in a baking furnace under an a
non-oxidizing atmosphere such as an inert gas atmosphere or a
reducing gas atmosphere, an air atmosphere, or a reduced-pressure
atmosphere having a high vacuum (e.g., a pressure of 13 Pa (0.1
Torr) or less), thereby producing a sintered body.
[0067] If a secondary compact is baked under various gas
atmospheres such as a non-oxidizing atmosphere and an air
atmosphere, then carbon in the secondary compact reacts with
H.sub.2 (hydrogen gas) or O.sub.2 (oxygen gas) contained in the
atmosphere. As a result, the carbon in the secondary compact is
problematically desorbed from the secondary compact. Specifically,
when carbon in the secondary compact reacts with H.sub.2, it is
converted into a hydrocarbon gas such as CH.sub.4 and desorbed from
the secondary compact. Furthermore, when carbon in the secondary
compact reacts with O.sub.2, it is converted into CO or CO.sub.2
and desorbed from the secondary compact.
[0068] These reactions decrease the carbon content of the secondary
compact so that the carbon content of a sintered body as a final
product becomes lower than a desired carbon content. Thus, the
mechanical characteristics of the sintered body are deteriorated.
As described above, such a problem becomes more significant when
the metal powder has a low carbon content.
[0069] Additionally, because a decrease of the carbon content
proceeds when the secondary compact is brought into contact with
H.sub.2 or O.sub.2 contained in the atmosphere, it is likely to
depend upon a shape of the secondary compact. That is, the carbon
content tends to be lowered in a secondary compact having a
complicated shape with a large surface area. Accordingly, the
degree of the decrease of the carbon content varies depending upon
the shape of a secondary compact. This problem causes variations in
mechanical characteristics between a plurality of sintered bodies
having different shapes or between portions having different shapes
in a sintered body.
[0070] On the other hand, if a secondary compact is baked under a
reduced-pressure atmosphere having a high vacuum, then a pressure
in a baking furnace is lowered. Furthermore, O.sub.2 is produced by
decomposition (dissociation) of components of the baking furnace or
support members for supporting the secondary compact or by
dissociation of moisture adsorbed in these components or members.
Accordingly, the same problem as described above arises. Moreover,
large and expensive facilities are required for a vacuum pump or a
pressure vessel in order to maintain a high vacuum. Therefore,
manufacturing cost is unavoidably increased. Additionally, a
pressure vessel capable of maintaining a high vacuum has limitation
in volume. Accordingly, such a pressure vessel can accommodate only
a limited number of secondary compacts. As a result, the production
efficiency is problematically low.
[0071] According to the present invention, a secondary compact is
placed in a hermetically sealed space having an atmosphere in which
a pressure is adjusted to 60 kPa to 140 kPa (450 Torr to 1,050
Torr). Further, the sum of partial pressures of H.sub.2 (hydrogen
gas) and O.sub.2 (oxygen gas) in the atmosphere is adjusted to a
value not more than 3 Pa. Then the secondary compact is baked in
this hermetically sealed space. With this method, it is possible to
efficiently produce a sintered body containing carbon at a desired
percentage.
[0072] Furthermore, since the carbon content is prevented or
inhibited from being lowered, a variation in decrease of the carbon
content can be reduced. Therefore, it is possible to produce a
sintered body having desired mechanical characteristics.
[0073] According to the present invention, the sum of a partial
pressure of H.sub.2 and a partial pressure of O.sub.2 in the
atmosphere is set to be at most 3 Pa. Therefore, the amounts of
H.sub.2 and O.sub.2 to react with carbon in a secondary compact can
be reduced. Thus, carbon is prevented from being consumed to a
large extent.
[0074] In a case where an atmosphere gas is continuously supplied
into a conventional continuous baking furnace, since the atmosphere
gas unavoidably contains H.sub.2 gas and O.sub.2 gas as impurities,
the H.sub.2 gas and O.sub.2 gas are also supplied (replenished)
constantly to the baking furnace. Accordingly, even if a gas having
a low partial pressure of H.sub.2 and a low partial pressure of
O.sub.2 is used as the atmosphere gas, a decrease of the carbon
content of the secondary compact is unavoidably caused.
[0075] In contrast to the conventional method, according to the
present invention, a secondary compact is placed in a hermetically
sealed space. Therefore, once H.sub.2 and O.sub.2 contained in the
atmosphere are consumed by reactions with carbon in the secondary
compact, further consumption of carbon in the secondary compact can
be prevented. Thus, the secondary compact can be baked while carbon
in the secondary compact can be prevented or inhibited from being
reduced. Hence, it is possible to produce a sintered body having a
desired carbon content with ease.
[0076] According to the present invention, the sum of a partial
pressure of H.sub.2 and a partial pressure of O.sub.2 in the
atmosphere should not be more than 3 Pa. It is desirable that the
sum of the partial pressures should be at most 2.5 Pa, more
preferably at most 1.5 Pa. With this configuration, it is possible
to further reduce consumption of carbon in the secondary compact
and make the carbon content of a sintered body closer to a desired
value. As a result, it is possible to produce a sintered body
having better mechanical characteristics.
[0077] The secondary compact is placed in a hermetically sealed
space within a closed vessel or the like. The pressure of the
hermetically sealed space is adjusted to 60 kPa to 140 kPa (450
Torr to 1,050 Torr). This pressure can be maintained sufficiently
with a simple closed vessel because of a small difference from an
atmospheric pressure.
[0078] Furthermore, it is possible to shorten a period of time
required to increase or decrease the pressure in the closed vessel.
As a result, a production efficiency of a sintered body can be
enhanced.
[0079] The pressure of the hermetically sealed space should be in a
range of 60 kPa to 140 kPa (450 Torr to 1,050 Torr). It is
desirable that the pressure of the hermetically sealed space should
be in a range of about 80 kPa to about 120 kPa (about 600 Torr to
about 900 Torr). A simpler closed vessel can be used within this
range. Furthermore, when the pressure of the hermetically sealed
space is in the above range, a pressure difference between the
interior and exterior of the hermetically sealed space can be made
extremely small. Accordingly, the closed vessel does not need to be
equipped with a special pressure proof mechanism, and cost of the
baking process can be reduced.
[0080] The atmosphere in the closed vessel may contain any gas
components. Nevertheless, it is desirable that the atmosphere
primarily should contain an inert gas such as nitrogen, helium, or
argon. An inert gas is most unlikely to react with carbon or other
elements in the secondary compact. Therefore, it is possible to
prevent the composition in the secondary compact from unexpectedly
varying in the baking process.
[0081] Particularly, it is desirable to use an argon gas as the
inert gas. Since an argon gas is a noble gas, it has a low
reactivity with most of elements. Therefore, it is possible to more
reliably prevent the composition in the secondary compact from
unexpectedly varying in the baking process. Furthermore, since an
argon gas can be obtained at a relatively low cost among noble
gases, it can suitably be used as the atmosphere gas.
[0082] For example, the aforementioned baking process can be
performed by a baking furnace (closed vessel) as shown in FIG.
2.
[0083] FIG. 2 is a vertical cross-sectional view schematically
showing a baking furnace (closed vessel) 10 used in a method of
manufacturing a sintered body according to the present invention.
In the following description, the upper and lower sides in FIG. 2
will be referred to as "upper" and "lower," respectively.
[0084] As shown in FIG. 2, the baking furnace 10 includes a furnace
body 11 having an opening portion 12 formed in a side surface
thereof, a cover 20 capable of closing the opening portion 12 and
hermetically sealing the furnace body 11, and a stage 14 provided
in an internal furnace space 13 within the furnace body 11 for
supporting secondary compacts 30 thereon.
[0085] Furthermore, the baking furnace 10 also includes an gas
supply valve 15 provided at an upper portion of the furnace body
11, a gas supply device 40, and a pipe 16 connecting the gas supply
valve 15 and the gas supply device 40 to each other. Thus, the
baking furnace 10 is configured to supply an atmosphere gas from
the gas supply device 40 into the internal furnace space 13 or stop
the supply of the atmosphere gas via the pipe 16 and the gas supply
valve 15.
[0086] The baking furnace 10 has a gas discharge valve 17 provided
at a lower portion of the furnace body 11. Thus, the baking furnace
10 is configured to discharge the atmosphere gas in the internal
furnace space 13 to the exterior of the furnace body 11 or stop the
discharge of the atmosphere gas via the gas discharge valve 17.
[0087] In the internal furnace space 13, a heater 18 is provided
along a wall surface of the furnace body 11. The heater 18 is
connected to a power source device (not shown) via wiring. When the
heater 18 is supplied with electric power, it generates heat so as
to heat the atmosphere gas and the secondary compacts 30 in the
internal furnace space 13.
[0088] Baking operation of the secondary compacts 30 in the baking
furnace 10 will be described below.
[0089] First, the secondary compacts 30 are placed on the stage 14.
Then the opening portion 12 is closed by the cover 20.
Subsequently, the gas discharge valve 17 and the gas supply valve
15 are opened. Thus, the internal furnace space 13 is filled with
an atmosphere gas that meets the aforementioned conditions. Before
the supply of the atmosphere gas, the pressure in the internal
furnace space 13 may be reduced by a vacuum pump or the like as
needed. In such a case, it is possible to enhance the purity of the
atmosphere gas in the internal furnace space 13.
[0090] Thereafter, the gas discharge valve 17 and the gas supply
valve 15 are closed so that the internal furnace space 13 is
hermetically sealed.
[0091] Next, electric power is supplied to the heater 18 so that
the secondary compacts 30 are baked at the aforementioned baking
temperature for the aforementioned baking time. In this manner, a
sintered body is produced.
[0092] The sintered body thus produced has a desired carbon content
and demonstrates excellent mechanical characteristics.
[0093] Although a preferred embodiment of a method of manufacturing
a sintered body according to the present invention has been shown
and described in detail, it should be understood that the present
invention is not limited to the illustrated embodiment. For
example, any additional process may be added to a method of
manufacturing a sintered body as desired. Furthermore, the
atmosphere in the closed vessel may be changed during the baking
process as needed. The baking process may be performed subsequently
to the removal process (degrease process).
EXAMPLES
[0094] 1. Manufacturing Sintered Bodies
[0095] i) First, a powder of stainless steel SUS-440C produced by a
water atomization method (PF-20F made by EPSON ATMIX Corporation)
was prepared. This powder had an average particle diameter of 10
.mu.m. A mixture of polypropylene and wax (organic binder) was also
prepared. The powder of stainless steel and the mixture of
polypropylene and wax were weighed with a weight ratio of 9:1 and
mixed with each other. Thus, a mixed material was obtained.
[0096] ii) Then the mixed material was kneaded by a kneader, so
that a kneaded compound was produced.
[0097] iii) Next, injection molding was performed on the kneaded
compound under the following conditions by an injection molding
machine so as to produce primary compacts. At that time, primary
compacts having three types of shapes (Shape A, Shape B, and Shape
C) with different surface areas were produced. Ten primary compacts
were produced for each shape type. Accordingly, 30 primary compacts
were produced in total.
[0098] (Forming Conditions) [0099] Material temperature:
150.degree. C. [0100] Injection pressure: 11 MPa (110
kgf/cm.sup.2)
[0101] iv) Subsequently, heat treatment (degrease process) was
performed on the produced primary compacts under the following
conditions so as to produce secondary compacts (degreased
compacts).
[0102] (Degrease Conditions) [0103] Heating temperature:
500.degree. C. [0104] Heating time: 2 hours [0105] Heating
atmosphere: nitrogen gas
[0106] v) Then the produced secondary compacts were baked under the
following baking conditions. Thus, sintered bodies were produced
(Example I). The baked process was performed in a hermetically
sealed state in which the secondary compacts were housed in a
baking furnace capable of hermetically sealing the interior of the
furnace.
[0107] (Baking Conditions) [0108] Baking temperature: 1,235.degree.
C. [0109] Baking time: 6 hours [0110] Heating atmosphere: argon gas
(The sum of H.sub.2 partial pressure and O.sub.2 partial pressure
was 2.0 Pa.) [0111] Pressure of atmosphere: atmospheric pressure
(100 kPa)
[0112] Other sintered bodies were produced in the same manner as in
Example I except that the pressure of the heating atmosphere was
changed into 133 kPa (1,000 Torr) in Step v) (Example II). The sum
of H.sub.2 partial pressure and O.sub.2 partial pressure was 2.7
Pa.
[0113] Still other sintered bodies were produced in the same manner
as in Example I except that the pressure of the heating atmosphere
was changed into 67 kPa (500 Torr) in Step v) (Example III). The
sum of H.sub.2 partial pressure and O.sub.2 partial pressure was
1.3 Pa.
[0114] Other sintered bodies were produced in the same manner as in
Example I except that the sum of H.sub.2 partial pressure and
O.sub.2 partial pressure in the heating atmosphere was set to be
0.5 Pa in Step v) (Example IV).
[0115] Still other sintered bodies were produced in the same manner
as in Example I except that the baking process was performed with a
baking furnace capable of continuously supplying an atmosphere gas
in Step v) (Comparative Example V).
[0116] Other sintered bodies were produced in the same manner as in
Example I except that the sum of H.sub.2 partial pressure and
O.sub.2 partial pressure in the heating atmosphere was set to be 20
Pa in Step v) (Comparative Example VI).
[0117] II. Evaluating the Sintered Bodies
[0118] Composition analysis was performed on the sintered bodies
obtained in Examples I to IV and Comparative Examples V and VI with
an electron probe microanalyzer (EPMA).
[0119] First, composition analysis was performed on the powder of
SUS-440C used in Examples I to IV and Comparative Examples V and
VI. Then composition analysis was performed on the sintered bodies
obtained in Examples I to IV and Comparative Examples V and VI. The
carbon content of the powder of SUS-440C and the carbon content of
each sintered body were measured. The carbon contents of the
sintered bodies of Examples I to IV and Comparative Examples V and
VI were represented by a relative value on the assumption that the
carbon content in the powder of SUS-440C was defined by 1.
[0120] Next, the carbon contents of the thirty sintered bodies
obtained in each of Examples I to IV and Comparative Examples V and
VI were compared. Specifically, a difference between a maximum
value and a minimum value of the carbon contents of the thirty
sintered bodies was calculated as a range of the carbon contents.
The ranges of the carbon contents of the sintered bodies were
compared and evaluated. Table 1 shows results of the
evaluation.
TABLE-US-00001 TABLE 1 Baking Evaluation conditions results Partial
Range of pressure Carbon carbon of content contents H.sub.2 +
O.sub.2 Atmosphere (Relative (relative [Pa] gas value) values)
Reference example -- -- 1 -- (Metal powder) Example I 2.0 Sealed
0.37 0.04 (closed) Example II 2.7 Sealed 0.35 0.05 (closed) Example
III 1.3 Sealed 0.41 0.02 (closed) Example IV 0.5 Sealed 0.40 0.02
(closed) Comparative 2.0 Continuous 0.19 0.13 Example V supply
Comparative 20 Sealed 0.22 0.15 Example VI (closed)
[0121] As shown in Table 1, average values of the carbon contents
of the sintered bodies in Examples I to IV ranged from 0.35 to 0.41
as relative values in the assumption that the carbon content of the
powder of SUS-440C was defined by 1. In contrast to Examples I to
IV, average values of the carbon contents of the sintered bodies in
Comparative Examples V and VI ranged from 0.19 to 0.22. Thus, it
can be seen that desorption of carbon could be reduced in the
baking process of Examples I to IV as compared to Comparative
Examples V and VI.
[0122] Furthermore, when comparing ranges (or variations) of the
carbon contents of the sintered bodies obtained in Examples I to IV
and Comparative Examples V and VI, the ranges of the carbon
contents of the sintered bodies of Examples I to IV were much
narrower (smaller) than those of Comparative Examples V and VI.
This result shows that the present invention can efficiently
produce sintered bodies having a desired carbon content
irrespective of their shapes even if those sintered bodies have
different shapes (surface areas). Additionally, as can be seen from
the results, since the range of the carbon contents of sintered
bodies can be made small, the carbon contents of the sintered
bodies can be made close to a desired value.
[0123] Although certain preferred embodiments of the present
invention have been shown and described in detail, it should be
understood that various changes and modifications may be made
therein without departing from the scope of the appended
claims.
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