U.S. patent application number 15/738223 was filed with the patent office on 2018-06-28 for iron-based sintered body and method of manufacturing the same.
This patent application is currently assigned to JFE STEEL CORPORATION. The applicant listed for this patent is JFE STEEL CORPORATION. Invention is credited to Akio KOBAYASHI, Naomichi NAKAMURA, Itsuya SATO, Takuya TAKASHITA.
Application Number | 20180178291 15/738223 |
Document ID | / |
Family ID | 58288592 |
Filed Date | 2018-06-28 |
United States Patent
Application |
20180178291 |
Kind Code |
A1 |
TAKASHITA; Takuya ; et
al. |
June 28, 2018 |
IRON-BASED SINTERED BODY AND METHOD OF MANUFACTURING THE SAME
Abstract
Provided is an iron-based sintered body having excellent
mechanical properties. In the sintered body, the area fraction of
pores is 15% or less and the area-based median size D50 of the
pores is 20 82 m or less.
Inventors: |
TAKASHITA; Takuya;
(Chiyoda-ku, JP) ; KOBAYASHI; Akio; (Chiyoda-ku,
JP) ; NAKAMURA; Naomichi; (Chiyoda-ku, JP) ;
SATO; Itsuya; (Chiyoda-ku, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
JFE STEEL CORPORATION |
Chiyoda-ku, Tokyo |
|
JP |
|
|
Assignee: |
JFE STEEL CORPORATION
Chiyoda-ku, Tokyo
JP
|
Family ID: |
58288592 |
Appl. No.: |
15/738223 |
Filed: |
September 16, 2016 |
PCT Filed: |
September 16, 2016 |
PCT NO: |
PCT/JP2016/004259 |
371 Date: |
December 20, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/12 20130101;
C22C 33/0257 20130101; B22F 2301/35 20130101; B22F 3/24 20130101;
C22C 33/02 20130101; C21D 1/18 20130101; C22C 38/16 20130101; B22F
1/0059 20130101; B22F 3/16 20130101; C22C 38/00 20130101; B22F
2998/10 20130101; B22F 2003/248 20130101; B22F 2998/10 20130101;
B22F 1/0059 20130101; B22F 3/02 20130101; B22F 3/10 20130101; B22F
2201/30 20130101; B22F 2003/248 20130101 |
International
Class: |
B22F 3/24 20060101
B22F003/24; C22C 38/16 20060101 C22C038/16; C22C 38/12 20060101
C22C038/12; C22C 33/02 20060101 C22C033/02; B22F 3/16 20060101
B22F003/16 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 18, 2015 |
JP |
2015-185656 |
Claims
1.-9. (canceled)
10. An iron-based sintered body, comprising an area fraction of
pores in the iron-based sintered body of 15% or less, and an
area-based median size D50 of the pores of 20 .mu.m or less.
11. The iron-based sintered body according to claim 10, comprising
Mo, Cu, and C.
12. The iron-based sintered body according to claim 11, comprising
Mo in an amount of 0.2 mass % to 1.5 mass %, Cu in an amount of 0.5
mass % to 4.0 mass %, and C in an amount of 0.1 mass % to 1.0 mass
%.
13. The iron-based sintered body according to claim 10, wherein the
iron-based sintered body has been carburized, quenched, and
tempered.
14. The iron-based sintered body according to claim 11, wherein the
iron-based sintered body has been carburized, quenched, and
tempered.
15. The iron-based sintered body according to claim 12, wherein the
iron-based sintered body has been carburized, quenched, and
tempered.
16. A method of manufacturing an iron-based sintered body, the
method comprising: compacting (i) partially diffusion alloyed steel
powder in which Mo is adhered to the surface of particles of
iron-based powder by diffusion bonding with (ii) mixed powder for
powder metallurgy obtained by mixing at least Cu powder and
graphite powder at a pressure of 400 MPa or more to obtain a
compact; and then sintering the obtained compact at 1000.degree. C.
or higher for 10 min or more.
17. The method of manufacturing a high strength according to claim
16, the method further comprising carburizing, quenching, and
tempering after sintering the obtained compact.
18. The method of manufacturing an iron-based sintered body,
according to claim 16, wherein the mixed powder for powder
metallurgy contains Mo in an amount of 0.2 mass % to 1.5 mass % and
the balance consisting of Fe and incidental impurities.
19. The method of manufacturing an iron-based sintered body,
according to claim 17, wherein the mixed powder for powder
metallurgy contains Mo in an amount of 0.2 mass % to 1.5 mass % and
the balance consisting of Fe and incidental impurities.
20. The method of manufacturing an iron-based sintered body,
according to claim 16, wherein the partially diffusion alloyed
steel powder has a mean particle diameter of 30 .mu.m to 120 .mu.m
and a specific surface area of less than 0.10 m.sup.2/g, and a
circularity of particles of the partially diffusion alloyed steel
powder that have a diameter in a range of 50 .mu.m to 100 .mu.m is
0.65 or less.
21. The method of manufacturing an iron-based sintered body,
according to claim 17, wherein the partially diffusion alloyed
steel powder has a mean particle diameter of 30 .mu.m to 120 .mu.m
and a specific surface area of less than 0.10 m.sup.2/g, and a
circularity of particles of the partially diffusion alloyed steel
powder that have a diameter in a range of 50 .mu.m to 100 .mu.m is
0.65 or less.
22. The method of manufacturing an iron-based sintered body,
according to claim 18, wherein the partially diffusion alloyed
steel powder has a mean particle diameter of 30 .mu.m to 120 .mu.m
and a specific surface area of less than 0.10 m.sup.2/g, and a
circularity of particles of the partially diffusion alloyed steel
powder that have a diameter in a range of 50 .mu.m to 100 .mu.m is
0.65 or less.
23. The method of manufacturing an iron-based sintered body,
according to claim 19, wherein the partially diffusion alloyed
steel powder has a mean particle diameter of 30 .mu.m to 120 .mu.m
and a specific surface area of less than 0.10 m.sup.2/g, and a
circularity of particles of the partially diffusion alloyed steel
powder that have a diameter in a range of 50 .mu.m to 100 .mu.m is
0.65 or less.
24. The method of manufacturing an iron-based sintered body,
according to claim 16, wherein the amount of the Cu powder mixed is
0.5 mass % to 4.0 mass % of the mixed powder for powder
metallurgy.
25. The method of manufacturing an iron-based sintered body,
according to claim 17, wherein the amount of the Cu powder mixed is
0.5 mass % to 4.0 mass % of the mixed powder for powder
metallurgy.
26. The method of manufacturing an iron-based sintered body,
according to claim 18, wherein the amount of the Cu powder mixed is
0.5 mass % to 4.0 mass % of the mixed powder for powder
metallurgy.
27. The method of manufacturing an iron-based sintered body,
according to claim 19, wherein the amount of the Cu powder mixed is
0.5 mass % to 4.0 mass % of the mixed powder for powder
metallurgy.
28. The method of manufacturing an iron-based sintered body,
according to claim 20, wherein the amount of the Cu powder mixed is
0.5 mass % to 4.0 mass % of the mixed powder for powder
metallurgy.
29. The method of manufacturing an iron-based sintered body,
according to claim 21, wherein the amount of the Cu powder mixed is
0.5 mass % to 4.0 mass % of the mixed powder for powder metallurgy.
Description
TECHNICAL FIELD
[0001] This disclosure relates to an iron-based sintered body, and
relates in particular to an iron-based sintered body suitable for
manufacturing high strength sintered parts for automobiles, the
sintered body having high sintered density and having reliably
improved tensile strength and toughness (impact energy value) after
performing the processes of carburizing, quenching, and tempering
on the sintered body. Further, this disclosure relates to a method
of manufacturing the iron-based sintered body.
[0002] Powder metallurgical techniques enable producing parts with
complicated shapes in shapes that are extremely close to product
shapes (so-called near net shapes) with high dimensional accuracy,
and consequently significantly reducing machining costs. For this
reason, powder metallurgical products are used for various machines
and parts in many fields.
[0003] In recent years, there is a strong demand for powder
metallurgical products to have improved toughness in terms of
improving the strength for miniaturizing parts and reducing the
weight thereof and safety. In particular, for powder metallurgical
products (iron-based sintered bodies) which are very often used for
gears and the like, in addition to higher strength and higher
toughness, there is also a strong demand for higher hardness in
terms of wear resistance. In order to meet the above-mentioned
demands, iron-based sintered bodies of which components,
structures, density and the like are controlled suitably are
required to be developed, since the strength and toughness of an
iron-based sintered body varies widely depending on those
properties.
[0004] Typically, a green compact before being subjected to
sintering is produced by mixing iron-based powder with alloying
powders such as copper powder and graphite powder and a lubricant
such as stearic acid or lithium stearate to obtain mixed powder;
filling a mold with the mixed powder; and compacting the
powder.
[0005] The density of a green compact obtained through a typical
powder metallurgical process is usually around 6.6 Mg/m.sup.3 to
7.1 Mg/m.sup.3. The green compact is then sintered to form a
sintered body which in turn is further subjected to optional sizing
or cutting work, thereby obtaining a powder metallurgical product.
Further, when even higher strength is required, carburizing heat
treatment or bright heat treatment may be performed after
sintering.
[0006] Based on the components, iron-based powders used here are
categorized into iron powder (e.g. iron-based powder and the like)
and alloy steel powder. Further, when categorized by production
method, iron-based powders are categorized into atomized iron
powder and reduced iron powder. Within these categories specified
by production methods, the term "iron powder" is used with a broad
meaning encompassing alloy steel powder as well as iron-based
powder.
[0007] In terms of obtaining a sintered body with high strength and
high toughness, it is advantageous that iron-based powder being a
main component in particular allows alloying of the powder to be
promoted and high compressibility of the powder to be
maintained.
[0008] First, known iron-based powders obtained by alloying
include: [0009] (1) mixed powder obtained by adding alloying
element powders to iron-based powder, [0010] (2) pre-alloyed steel
powder obtained by completely alloying alloying elements, [0011]
(3) partially diffusion alloyed steel powder (also referred to as
composite alloy steel powder) obtained by partially adding alloying
element powders in a diffused manner to the surface of particles of
iron-based powder or pre-alloyed steel powder.
[0012] The mixed powder (1) mentioned above advantageously has high
compressibility equivalent to that of pure iron powder. However, in
sintering, the alloying elements are not sufficiently diffused in
Fe and form a non-uniform microstructure, which would result in
poor strength of the resulting sintered body. Further, since Mn,
Cr, V, Si, and the like are more easily oxidized than Fe, when
these elements are used as the alloying elements, they get oxidized
in sintering, which would reduce the strength of the resulting
sintered body. In order to suppress the oxidation and reduce the
amount of oxygen in the sintered body, it is necessary that the
atmosphere for sintering, and the CO.sub.2 concentration and the
dew point in the carburizing atmosphere are strictly controlled in
the case of performing carburizing after sintering. Accordingly,
the mixed powder (1) mentioned above cannot meet the demands for
higher strength in recent years and has become unused.
[0013] On the other hand, when the pre-alloyed steel powder
obtained by completely alloying the elements of (2) mentioned above
is used, the alloying elements can be completely prevented from
being segregated, so that the microstructure of the sintered body
is made uniform, leading to stable mechanical properties. In
addition, also in the case where Mn, Cr, V, Si, and the like are
used as the alloying elements, the amount of oxygen in the sintered
body can be advantageously reduced by limiting the kind and the
amount of the alloying elements. However, when the pre-alloyed
steel powder is produced by atomization from molten steel,
oxidation in the atomization of the molten steel and solid solution
hardening of steel powder due to complete alloying would be caused,
which makes it difficult to increase the density of the green
compact after compaction (forming by pressing). When the density of
the green compact is low, the toughness of the sintered body
obtained by sintering the green compact is low. Therefore, also
when the pre-alloyed steel powder is used, demands for higher
strength and higher toughness cannot be met.
[0014] The partially diffusion alloyed steel powder (3) mentioned
above is produced by adding alloying elements to iron-based powder
or pre-alloyed steel powder, followed by heating under a
non-oxidizing or reducing atmosphere, thereby partially diffusion
bonding the alloying element powders to the surface of particles of
iron-based powder or pre-alloyed steel powder. Accordingly,
advantages of the iron-based mixed powder of (1) above and the
pre-alloyed steel powder of (2) above can be obtained.
[0015] Thus, when the partially diffusion pre-alloyed steel powder
is used, oxygen in the sintered body can be reduced and the green
compact can have a high compressibility equivalent to the case of
using pure iron powder. Therefore, the sintered body has a
multi-phase structure consisting of a completely alloyed phase and
a partially concentrated phase, increasing the strength of the
sintered body.
[0016] As basic alloy components used in the partially diffusion
alloyed steel powder, Ni and Mo are used heavily.
[0017] Ni has the effect of improving the toughness of a sintered
body. Adding Ni stabilizes austenite, which allows more austenite
to remain as retained austenite without transforming to martensite
after quenching. Further, Ni serves to strengthen the matrix of a
sintered body by solid solution strengthening.
[0018] Meanwhile, Mo has the effect of improving hardenability.
Accordingly, Mo suppresses the formation of ferrite during
quenching, allowing bainite or martensite to be easily formed,
thereby strengthening the matrix of the sintered body. Further, Mo
is contained as a solid solution in a matrix to solid solution
strengthen the matrix, and forms fine carbides to strengthen the
matrix by precipitation.
[0019] As an example of the mixed powder for high strength sintered
parts using the above-described partially diffusion alloyed steel
powder, JP 3663929 B2 (PTL 1) discloses mixed powder for high
strength sintered parts obtained by mixing Ni: 1 mass % to 5 mass
%, Cu: 0.5 mass % to 4 mass %, and graphite powder: 0.2 mass % to
0.9 mass % to alloy steel powder in which Ni: 0.5 mass % to 4 mass
% and Mo: 0.5 mass % to 5 mass % are partially alloyed. The
sintered material described in PTL 1 contains 1.5 mass % of Ni at
minimum, and substantially contains 3 mass % or more of Ni
according to Examples of PTL 1. This means that a large amount of
Ni as much as 3 mass % or more is required to obtain a sintered
body having a high strength of 800 MPa or more. Further, obtaining
a material having a high strength of 1000 MPa or more by subjecting
a sintered body to carburizing, quenching, and tempering also
requires a large amount of Ni as much as for example 3 mass % or 4
mass %.
[0020] However, Ni is an element which is disadvantageous in terms
of addressing recent environmental problems and recycling, so its
use is desirably avoided as possible. Also in respect of cost,
adding several mass % of Ni is significantly disadvantageous.
Further, when Ni is used as an alloying element, sintering is
required to be performed for a long time in order to sufficiently
diffuse Ni in iron powder or steel powder. Moreover, when Ni being
an austenite phase stabilizing element is not sufficiently
diffused, a high Ni concentration area is stabilized as the
austenite phase (hereinafter also referred to as y phase) and the
other area where Ni is hardly contained is stabilized as other
phases, resulting in a non-uniform metal structure in the sintered
body.
[0021] As a Ni-free technique, JP 3651420 B2 (PTL 2) discloses a
technique associated with partially diffusion alloyed steel powder
of Mo free of Ni. That is, PTL 2 states that optimization of the Mo
content results in a sintered body having high ductility and high
toughness that can resist repressing after sintering.
[0022] Further, regarding a high density sintered body free of Ni,
JP H04-285141 A (PTL 3) discloses mixing iron-based powder having a
mean particle diameter of 1 .mu.m to 18 .mu.m with copper powder
having a mean particle diameter of 1 .mu.m to 18 .mu.m at a weight
ratio of 100:(0.2 to 5), and compacting the mixed powder and
sintering the green compact. In the technique disclosed in PTL 3,
iron-based powder having a mean particle diameter that is extremely
smaller than that of typical one is used, so that a sintered body
having a density as extremely high as 7.42 g/cm.sup.3 or more can
be obtained.
[0023] WO 2015/045273 A1 (PTL 4) discloses that a sintered body
having high strength and high toughness is obtained using powder
free of Ni, in which Mo is adhered to the surface of iron-based
powder particles by diffusion bonding to achieve a specific surface
area of 0.1 m.sup.2/g or more.
[0024] Further, JP 2015-014048 A (PTL 5) discloses that a sintered
body having high strength and high toughness is obtained using
powder in which Mo is adhered to iron-based powder particles
containing reduced iron powder by diffusion bonding.
[0025] JP 2015-004098 A (PTL 6) describes that Fe-Mn-Si powder is
added to iron powder particles of a small particle size and the
mixed powder is warm compacted in a lubricated mold, thereby
reducing the maximum pore length of the sintered body to obtain a
sintered body having high strength and high toughness.
CITATION LIST
Patent Literature
[0026] PTL 1: JP 3663929 B2
[0027] PTL 2: JP 3651420 B2
[0028] PTL 3: JP H04-285141 A
[0029] PTL 4: WO 2015/045273 A1
[0030] PTL 5: JP 2015-014048 A
[0031] PTL 6: JP 2015-004098 A
SUMMARY
Technical Problem
[0032] However, the sintered materials obtained in accordance with
the description of PTL 2, PTL 3, PTL 4, PTL 5, and PTL 6 above have
been found to have the following respective problems.
[0033] The technique disclosed in PTL 2 is designed to achieve high
strength by recompression after sintering. Accordingly, when a
sintered material is manufactured by a typical metallurgical
process, both sufficient strength and toughness are hardly achieved
at the same time.
[0034] Further, the iron-based powder used for the sintered
material described in PTL 3 has a mean particle diameter of 1 .mu.m
to 18 .mu.m which is smaller than normal. Such a small particle
diameter results in poor flowability of the mixed powder inducing
cracking and chipping of the green compact due to unevenness of the
powder in filling the mold. Therefore, it is difficult to obtain a
sintered body having sufficient strength and toughness.
[0035] Further, since the powder described in PTL 4 has extremely
large specific surface area, use of such powder results in low
flowability of the powder and induces cracking and chipping of the
green compact due to unevenness of the powder in filling the mold.
Therefore, it is difficult to obtain a sintered body having
sufficient strength and toughness.
[0036] Also for the sintered body described in PTL 5, as with the
technique described in PTL 4, reduced iron powder having extremely
large specific surface area is used, which results in low
flowability of the powder and induces cracking and chipping of the
green compact due to unevenness of the powder in filling the mold.
Therefore, it is difficult to obtain a sintered body having
sufficient strength and toughness.
[0037] The toughness of the sintered body disclosed in PTL 6 is
increased mainly by limiting the maximum pore length; however, high
strength and toughness are hardly achieved by only limiting the
maximum pore length, and further improvement is required.
[0038] It could be helpful to provide an iron-based sintered body
having excellent mechanical properties as well as a method of
manufacturing the same.
Solution to Problem
[0039] With a view to achieve the above objective, we made various
studies to obtain a sintered body having both high strength and
high toughness. As a result, we discovered the following: [0040]
for an iron-based sintered body obtained by pressing mixed powder
made of iron-based powder and additives and then sintering,
adjusting the mean diameter of pores in the sintered body
contributes to the improvement in the impact energy value due to
the dispersion of stress concentrations in the structure.
[0041] This disclosure is based on the aforementioned discoveries
and further studies. Specifically, the primary features of this
disclosure are described below.
[0042] 1. An iron-based sintered body, comprising an area fraction
of pores in the iron-based sintered body of 15% or less, and an
area-based median size D50 of the pores of 20 .mu.m or less.
[0043] 2. The iron-based sintered body according to 1. above,
comprising Mo, Cu, and C.
[0044] 3. The iron-based sintered body according to 2. above,
comprising Mo in an amount of 0.2 mass % to 1.5 mass %, Cu in an
amount of 0.5 mass % to 4.0 mass %, and C in an amount of 0.1 mass
% to 1.0 mass %.
[0045] 4. The iron-based sintered body according to any one of 1.
to 3. above, wherein the iron-based sintered body has been
carburized, quenched, and tempered.
[0046] 5. A method of manufacturing an iron-based sintered body,
the method comprising: compacting (i) partially diffusion alloyed
steel powder in which Mo is adhered to the surface of particles of
iron-based powder by diffusion bonding with (ii) mixed powder for
powder metallurgy obtained by mixing at least Cu powder and
graphite powder at a pressure of 400 MPa or more to obtain a
compact; and then sintering the obtained compact at 1000.degree. C.
or higher for 10 min or more.
[0047] 6. The method of manufacturing a high strength according to
5. above, the method further comprising carburizing, quenching, and
tempering after sintering the obtained compact.
[0048] 7. The method of manufacturing an iron-based sintered body,
according to 5. or 6. above, wherein the mixed powder for powder
metallurgy contains Mo in an amount of 0.2 mass % to 1.5 mass % and
the balance consisting of Fe and incidental impurities.
[0049] 8. The method of manufacturing an iron-based sintered body,
according to any one of 5. to 7. above, wherein the partially
diffusion alloyed steel powder has a mean particle diameter of 30
.mu.m to 120 .mu.m and a specific surface area of less than 0.10
m.sup.2/g, and a circularity of particles of the partially
diffusion alloyed steel powder that have a diameter in a range of
50 .mu.m to 100 .mu.m is 0.65 or less.
[0050] 9. The method of manufacturing an iron-based sintered body,
according to any one of 5. to 8. above, wherein the amount of the
Cu powder mixed is 0.5 mass % to 4.0 mass % of the mixed powder for
powder metallurgy.
Advantageous Effect
[0051] This disclosure can provide an iron-based sintered body
having both high strength and high toughness.
DETAILED DESCRIPTION
[0052] Our methods and products will be described in detail
below.
[0053] The area fraction of pores in the disclosed sintered body is
15% or less and the area-based median size D50 of the pores is 20
.mu.m or less.
[0054] Pores are unavoidably formed in the iron-sintered body
obtained by sintering a green compact obtained by compacting alloy
steel powder for powder metallurgy, and it is important to control
the pores for improving the strength and toughness of the sintered
body. That is, since smaller pores hardly act as starting points of
cracks, it is important that the area-based median size D50 of the
pores is 20 .mu.m or less. More preferably, the area-based median
size D50 is 15 .mu.m or less. When the median size D50 exceeds 20
.mu.m, the toughness is significantly reduced.
[0055] Here, the median size D50 of the pores can be measured in
the following manner.
[0056] First, a sintered body is embedded in a thermosetting resin.
A cross section is then mirror-polished and the cross section is
imaged using an optical microscope at 100.times. magnification over
a field of view of 843 .mu.m .times.629 .mu.m. The cross-sectional
area A of all the pores in 20 fields randomly selected from the
resulting micrograph of the cross section is measured. The
equivalent circle diameter d.sub.c that is the diameter of a circle
having an area equal to the measured cross-sectional area is
determined in accordance with the following equation (I). Next, the
areas of the pores are integrated in ascending order of the circle
equivalent diameter and a circle equivalent diameter at which the
integrated value is 50% of the total area of the pores is defined
as an area-based median size D50.
d.sub.c=2 {square root over (A/.pi.)} (I)
[0057] As described above, the median size D50 of the pores of the
sintered body is controlled to 20 .mu.m or less, since a median
size D50 exceeding 20 .mu.m increases pores having an indefinite
shape and such pores become stress concentrations when deformation
occurs, which reduces strength and toughness.
[0058] Here, in order to control the area fraction of the pores in
the sintered body to 15% or less and the median size D50 of the
pores to 20 .mu.m or less, partially diffusion alloyed steel powder
of mixed powder for powder metallurgy which is a material of the
sintered body is used. The partially diffusion alloyed steel powder
is obtained by adhering Mo powder particles to the surface of
iron-based powder particles, the steel powder particles having a
mean particle diameter of 30 .mu.m to 120 .mu.m a and specific
surface area of less than 0.10 m.sup.2/g, and particles of the
steel powder that have a diameter in a range of 50 .mu.m to 100
.mu.m has a circularity of 0.65. Thus, sintering is promoted in
manufacturing a sintered body to be described, so that a desired
sintered body can be obtained.
[0059] Since the number of pores is preferably smaller, the area
fraction of the pores in the sintered body is controlled to 15% or
less. This is because since an area fraction of the pores exceeding
15% reduces the content of metal in the sintered body, even if the
pore diameter is reduced, sufficient strength and toughness cannot
be obtained. Note that making the pores in the sintered body be 0%
requires significant effort and is not realistic. The pores in the
sintered body obtained by the following method is at least
approximately 5%.
[0060] Here, the area fraction of the pores in the sintered body
can be calculated by the following method.
[0061] In a manner similar to the above, the cross-sectional area A
of all the pores in 20 fields is measured and summed to find the
total pore area A.sub.t of all the observed fields. Dividing
A.sub.t by the total of the areas of all the observed fields gives
the area fraction of the pores.
[0062] Further, the length of the pores in the sintered body is
preferably smaller. The "mean maximum pore length" that is an
indicator of the length of the pores is calculated as follows.
First, the maximum value of the distance between two points on the
circumferential edge of each pore in the field of the above
micrograph of the cross section is found by image analysis and is
defined as the "pore length" of the pore. The "maximum pore length"
is longest among the "pore lengths" of all the pores included in a
field of view of the micrograph of the cross section. Further, the
"mean maximum pore length" is the arithmetic mean value of the
maximum pore lengths found for 20 fields selected randomly. Note
that in order to achieve sufficient mechanical properties, the mean
maximum pore length is preferably less than 100 .mu.m.
[0063] Further, the above sintered body preferably contains Mo, Cu,
and C. Mo has the effect of improving hardenability. Cu has the
effect of improving solid solution strengthening and hardenability
of iron-based powder. C has the effect of enhancing the strength of
iron-based sintered body by being precipitated as a solid solution
or fine carbide in iron. Preferred content range of the respective
elements contained in the disclosed iron-based sintered body is Mo:
0.2 mass % to 1.5 mass %, Cu: 0.5 mass % to 4.0 mass %, and C: 0.1
mass % to 1.0 mass %. When the elements are less than the above
range, the strength cannot sufficiently be increased, whereas when
they are added to be more than the above range, the structure is
extremely hardened and the toughness is reduced.
[0064] Next, a method of obtaining the above sintered body will be
described. The following method is a mere example, and the
disclosed iron-based sintered body may be obtained by a method
other than the following method.
[0065] In manufacturing a sintered body by sintering a green
compact obtained by compacting mixed powder for powder metallurgy,
the mixed powder is made into the green compact by compaction using
a punch by a technique in which the compaction is performed while
rotating the punch about an imaginary axis in the pressing
direction. This method can produce more shear strains in the mixed
powder than in typical compaction, facilitating plastic deformation
of the mixed powder, and the pores in the sintered body can have a
finer diameter.
[0066] Next, a method of manufacturing a sintered body,
particularly suitable for manufacturing a sintered body containing
Mo, Cu, and C will be described.
[0067] In this method, mixed powder for powder metallurgy
containing iron-based powder and additives is compacted by a
conventional method to form a green compact, and the green compact
is then sintered by a conventional method, thereby obtaining an
iron-based sintered body. On this occasion, with a view to
increasing the density of the sintered body, it is preferable that
Mo-concentrated portions are formed in sintered neck parts between
particles of the iron-based powder in the green compact; iron-based
powder having particles with low circularity is used to achieve
stronger entanglement between particles of the powder during
compaction thereby promoting sintering; and the sintering is also
promoted with suppressed Cu growth. When the density of a sintered
body is high, both strength and toughness are improved; however,
since a sintered body obtained by this manufacturing method has a
uniform metal structure, the mechanical properties of the sintered
body are stable with little variation, unlike conventional sintered
bodies, for example, those using Ni.
[0068] In order to obtain such a sintered body, the sintered body
is preferably manufactured using partially diffusion alloyed steel
powder described below as the iron-based powder of the above mixed
powder for powder metallurgy.
[0069] Mixed powder for powder metallurgy preferably used in this
disclosure is obtained by mixing partially diffusion alloyed steel
powder in which Mo is adhered by diffusion bonding to the surface
of particles of iron-based powder of which mean particle diameter,
circularity, and specific surface area are appropriate (hereinafter
also referred to as partially alloyed steel powder) with an
appropriate amount of Cu powder having a mean particle diameter in
a range described below as well as graphite powder.
[0070] Mixed powder for powder metallurgy according to this
disclosure will now be described in detail. Note that "%" herein
means "mass %" unless otherwise specified. Accordingly, the Mo
content, the Cu content, and the graphite powder content each
represents the proportion of the element in the entire mixed powder
for powder metallurgy (100 mass %).
(Iron-Based Powder)
[0071] As described above, the partially diffusion alloyed steel
powder is obtained by adhering Mo to the surface of particles of
the iron-based powder, and it is preferred that the mean particle
diameter is 30 .mu.m to 120 .mu.m, the specific surface area is
less than 0.10 m.sup.2/g, and particles having a diameter in a
range of 50 .mu.m to 100 .mu.m have a circularity (cross-sectional
circularity) of 0.65 or less. Here, when the iron-based powder is
partially alloyed, the particle diameter and the circularity hardly
change. Accordingly, iron-based powder having a mean particle
diameter and a circularity in the same range as that of the
partially diffusion alloyed steel powder is used.
[0072] First, the iron-based powder preferably has a mean particle
diameter of 30 .mu.m to 120 .mu.m and particles having a diameter
in a range of 50 .mu.m to 100 .mu.m preferably have a circularity
(roundness of the cross section) of 0.65 or less. For the reasons
described below, the partially alloyed steel powder is required to
have a mean particle diameter of 30 .mu.m to 120 .mu.m and
particles having a diameter in a range of 50 .mu.m to 100 .mu.m are
required to have a circularity of 0.65 or less. Accordingly, the
iron-based powder is also required to meet those conditions.
[0073] Here, the mean particle diameter of the iron-based powder
and the partially alloyed steel powder refers to the median size
D50 determined from the cumulative weight distribution, and is a
particle diameter found by determining the particle size
distribution using a sieve according to JIS Z 8801-1, producing the
integrated particle size distribution from the resulting particle
size distribution, and finding the particle diameter obtained when
the oversized particles and the undersized particles constitute 50%
by weight each.
[0074] Further, the circularity of the particles of iron-based
powder and partially alloyed steel powder can be determined as
follows. Although a case of iron-based powder is explained by way
of example, the circularity of partially alloyed steel powder
particles is also determined through the same process.
[0075] First, iron-based powder is embedded in a thermosetting
resin. On this occasion, the iron-based powder is embedded to be
uniformly distributed in an area with a thickness of 0.5 mm or more
in the thermosetting resin so that a sufficient number of cross
sections of the iron-based powder particles can be observed in an
observation surface exposed by polishing the powder-embedded resin.
After that, the resin is polished to expose a cross section of the
iron-based powder particles; the cross section of the resin is
mirror polished; and the cross section is magnified and imaged by
an optical microscope. The cross sectional area A and the
peripheral length Lp of the iron-based powder particles in the
resulting micrograph of the cross section are determined by image
analysis. Examples of software capable of such image analysis
include ImageJ (open source, National Institutes of Health). The
circle equivalent diameter dc is calculated from the determined
cross-sectional area A. Here, dc is calculated by the same equation
(I) as in the case of the pores.
d.sub.c=2 {square root over (A/.pi.)} (I)
[0076] Next, the peripheral length of a circular approximation of
each powder particle Lc is calculated by multiplying the particle
diameter dc by the number .pi.. The circularity C is calculated
from the determined Lc and the peripheral length Lp of the cross
section of each iron-based powder particle. Here, the circularity C
is a value defined by the following equation (II).
[0077] When the circularity C is 1, the cross-sectional shape of
the particle is a perfect circle, and a smaller C value results in
a more indefinite shape.
C=L.sub.c/L.sub.p (II)
[0078] Note that iron-based powder means powder having an Fe
content of 50% or more. Examples of iron-based powder include
as-atomized powder (atomized iron powder as atomized), atomized
iron powder (obtained by reducing as-atomized powder in a reducing
atmosphere), and reduced iron powder. In particular, iron-based
powder used in this disclosure is preferably as-atomized powder or
atomized iron powder. This is because since reduced iron powder
contains many pores in the particles, sufficient density would not
be obtained during compaction. Further, reduced iron powder
contains more inclusions acting as starting points of fracture in
the particles than atomized iron powder, which would reduce the
fatigue strength which is one of the important mechanical
properties of a sintered body.
[0079] Specifically, iron-based powder preferably used in this
disclosure is any one of as-atomized powder obtained by atomizing
molten steel, drying the atomized molten steel, and classifying the
resulting powder without performing heat treatment for e.g.,
deoxidation (reduction) and decarbonization; and atomized iron
powder obtained by reducing as-atomized powder in a reducing
atmosphere.
[0080] Iron-based powder satisfying the above-described circularity
can be obtained by appropriately adjusting the spraying conditions
for atomization and conditions for additional processes performed
after the spraying. Further, iron-based powder having particles of
different circularities may be mixed and the circularity of the
particles of the iron-based powder that have a particle diameter in
a range of 50 .mu.m to 100 .mu.m may be controlled to fall within
the above-described range.
(Partially Diffusion Alloyed Steel Powder)
[0081] Partially diffusion alloyed steel powder is obtained by
adhering Mo to the surface of particles of the above iron-based
powder, and it is required that the mean particle diameter is 30
.mu.m to 120 .mu.m, the specific surface area is less than 0.10
m.sup.2/g, and particles having a diameter in a range of 50 .mu.m
to 100 .mu.m have a circularity of 0.65 or less.
[0082] Thus, the partially diffusion alloyed steel powder is
produced by adhering Mo to the above iron-based powder by diffusion
bonding. The Mo content is set to be 0.2% to 1.5% of the entire
mixed powder for powder metallurgy (100%). When the Mo content is
less than 0.2%, the hardenability and strength of a sintered body
manufactured using the mixed powder for powder metallurgy are
poorly improved. On the other hand, when the Mo content exceeds
1.5%, the effect of improving hardenability reaches a plateau, and
the structure of the sintered body becomes rather non-uniform.
Accordingly, high strength and toughness cannot be obtained.
Therefore, the content of Mo adhered by diffusion bonding is set to
be 0.2% to 1.5%. The Mo content is preferably 0.3% to 1.0%, more
preferably 0.4% to 0.8 %.
[0083] Here, Mo-containing powder can be given as an example of a
Mo source. Examples of the Mo-containing powder include pure metal
powder of Mo, oxidized Mo powder, and Mo alloy powders such as
Fe-Mo (ferromolybdenum) powder. Further, Mo compounds such as Mo
carbides, Mo sulfides, and Mo nitrides can be used as preferred
Mo-containing powders. Theses material powders can be used alone;
alternatively, some of these material powders can be used in a
mixed form.
[0084] Specifically, the above-described iron-based powder and the
Mo-containing powder are mixed in the proportions described above
(the Mo content is 0.2% to 1.5% of the entire mixed powder for
powder metallurgy (100%)). The mixing method is not particularly
limited, and the powders can be mixed by a conventional method
using a Henschel mixer, a cone blender, or the like.
[0085] Next, mixed powder of the above-described iron-based powder
and the Mo-containing powder is heated so that Mo is diffused in
the iron-based powder through the contact surface between the
iron-based powder and the Mo-containing powder, thereby joining Mo
to the iron-based powder. Partially alloyed steel powder containing
Mo can be obtained by this heat treatment.
[0086] As the atmosphere for diffusion-bonding heat treatment, a
reducing atmosphere or a hydrogen-containing atmosphere is
preferable, and a hydrogen-containing atmosphere is particularly
suitable. Alternatively, the heat treatment may be performed under
vacuum.
[0087] Further, for example when a Mo compound such as oxidized Mo
powder is used as the Mo-containing powder, the temperature of the
heat treatment is preferably set to be in a range of 800.degree. C.
to 1100.degree. C. When the temperature of the heat treatment is
lower than 800.degree. C., the Mo compound is insufficiently
decomposed and Mo is not diffused into the iron-based powder, so
that Mo hardly adheres to the iron-based powder. When the heat
treatment temperature exceeds 1100.degree. C., sintering between
iron-based powder particles is promoted during the heat treatment,
and the circularity of the iron-based powder particles exceeds the
predetermined range. On the other hand, when a metal and an alloy,
for example, Mo pure metal and an alloy such as Fe-Mo are used for
the Mo-containing powder, a preferred heat treatment temperature is
in a range of 600.degree. C. to 1100.degree. C. When the
temperature of the heat treatment is lower than 600.degree. C., Mo
is not sufficiently diffused into the iron-based powder, so that Mo
hardly adheres to the iron-based powder. On the other hand, when
the heat treatment temperature exceeds 1100.degree. C., sintering
between iron-based powder particles is promoted during the heat
treatment, and the circularity of the partially alloyed steel
powder exceeds the predetermined range.
[0088] When heat treatment, that is, diffusion bonding is performed
as described above, since partially alloyed steel powder particles
are usually sintered together and solidified, grinding and
classification are performed to obtain particles having a
predetermined particle diameter described below. Specifically, in
order to achieve the predetermined particle diameter, the grinding
conditions are tightened or coarse powder is removed by
classification using a sieve with openings of a predetermined size,
as necessary. In addition, annealing may optionally be
performed.
[0089] Specifically, it is important that the mean particle
diameter of the partially alloyed steel powder is in a range of 30
.mu.m to 120 .mu.m. The lower limit of the mean particle diameter
is preferably 40 .mu.m, more preferably 50 .mu.m. Meanwhile, the
upper limit of the mean particle diameter is preferably 100 .mu.m,
more preferably 80 .mu.m.
[0090] As described above, the mean particle diameter of the
partially alloyed steel powder refers to the median size D50
determined from the cumulative weight distribution, and is a
particle diameter found by determining the particle size
distribution using a sieve according to JIS Z 8801-1, producing the
integrated particle size distribution from the resulting particle
size distribution, and finding the particle diameter obtained when
the oversized particles and the undersized particles constitute 50%
by weight each.
[0091] Here when the mean particle diameter of the partially
alloyed steel powder particles is smaller than 30 .mu.m, the
flowability of the partially alloyed steel powder is reduced, and
for example the productivity in compaction using a mold is
affected. On the other hand, when the mean particle diameter of the
partially alloyed steel powder particles exceeds 120 .mu.m, the
driving force is weakened during sintering and coarse pores are
formed around the coarse iron-based powder particles. This reduces
the sintered density and leads to reduction in the strength and
toughness of a sintered body and the sintered body having been
carburized, quenched, and tempered. The maximum particle diameter
of the partially alloyed steel powder particles is preferably 180
.mu.m or less.
[0092] Further, in terms of compressibility, the specific surface
area of the partially alloyed steel powder particles is set to be
less than 0.10 m.sup.2/g. Here, the specific surface area of the
partially alloyed steel powder refers to the specific surface area
of particles of the partially alloyed steel powder except for
additives (Cu powder, graphite powder, lubricant).
[0093] When the specific surface area of the partially alloyed
steel powder exceeds 0.10 m.sup.2/g, the flowability of the mixed
powder for powder metallurgy is reduced. Note that the lower limit
of the specific surface area is not specified; however, the lower
limit of the specific surface area achieved industrially is
approximately 0.010 m.sup.2/g. The specific surface area can be
controlled as desired by adjusting the particle size of coarse
particles of more than 100 .mu.m and fine particles of less than 50
.mu.m after diffusion bonding by sieving. Specifically, the
specific surface area is reduced by reducing the proportion of fine
particles or increasing the proportion of coarse particles.
[0094] Further, particles of the partially alloyed steel powder
that have a diameter of 50 .mu.m to 100 .mu.m are required to have
a circularity of 0.65. The circularity is preferably 0.60 or less,
more preferably 0.58 or less. Reducing the circularity increases
the entanglement between particles during compaction and improves
the compressibility of the mixed powder for powder metallurgy, so
that coarse pores in the green compact and the sintered body are
reduced. On the other hand, an excessively low circularity reduces
the compressibility of the mixed powder for powder metallurgy.
Accordingly, the circularity is preferably 0.40 or more.
[0095] The circularity of the partially alloyed steel powder
particles having a diameter of 50 .mu.m to 100 .mu.m can be
measured as follows. First, the particle diameter of the partially
alloyed steel powder particles is calculated in the same manner as
that of the above-described iron-based powder particles and is
expressed as dc, and the partially alloyed steel powder particles
having dc in a range of 50 .mu.m to 100 .mu.m are extracted. Here,
optical microscopy imaging performed is such that at least 150
particles of the partially alloyed steel powder that have a
diameter in a range of 50 .mu.m to 100 .mu.m can be extracted. The
circularity of the extracted partially alloyed steel powder
particles was calculated in the same manner as in the case of the
above-described iron-based powder.
[0096] Note that the particle diameter of the partially alloyed
steel powder particles is limited to 50 .mu.m to 100 .mu.m because
reducing the circularity of the particles of this range can most
effectively promote sintering. Specifically, since particles of
less than 50 .mu.m are fine particles which originally facilitate
sintering, reducing the circularity of such particles of less than
50 .mu.m does not significantly promote sintering. Further, since
particles having a particle diameter exceeding 100 .mu.m are
extremely coarse, reducing the circularity of those particles does
not significantly promote sintering.
[0097] In this disclosure, the remainder components in the
partially alloyed steel powder are iron and inevitable impurities.
Here, impurities contained in the partially alloyed steel powder
may be C (except for graphite content), O, N, S, and others, the
contents of which may be set to C: 0.02% or less, 0: 0.3% or less,
N: 0.004% or less, S: 0.03% or less, Si: 0.2% or less, Mn: 0.5% or
less, and P: 0.1% or less in the partially alloyed steel powder
without any particular problem. The content of O, however, is
preferably 0.25% or less. It should be noted that when the amount
of incidental impurities exceeds the above range, the
compressibility in compaction using the partially alloyed steel
powder decreases, which makes it difficult to obtain a green
compact having sufficient density by the compaction.
[0098] In this disclosure, a sintered body manufactured using mixed
powder for powder metallurgy is further subjected to carburizing,
quenching, and tempering, and Cu powder and graphite powder are
then added to the partially alloyed steel powder obtained as
described above for the purpose of achieving a tensile strength of
1000 MPa.
(Cu Powder)
[0099] Cu is an element useful in improving the solid solution
strengthening and the hardenability of iron-based powder thereby
increasing the strength of sintered parts. The amount of Cu added
is preferably 0.5% or more and 4.0 or less. When the amount of Cu
powder added is less than 0.5%, the advantageous effects of adding
Cu are hardly obtained. On the other hand, when the Cu content
exceeds 4.0%, not only does the effects improving the strength of
the sintered parts reach a plateau but also the density of the
sintered body is reduced. Therefore, the amount of Cu powder added
is limited to a range of 0.5% to 4.0%.The amount added is
preferably in a range of 1.0% to 3.0%.
[0100] Further, when Cu powder of large particle size is used, in
sintering a green compact of mixed powder for powder metallurgy,
molten Cu penetrates between particles of the partially alloyed
steel powder to expand the volume of the sintered body after
sintering, which would reduce the density of the sintered body. In
order to prevent the density of the sintered body from decreasing
in such a way, the mean particle diameter of the Cu powder is
preferably set to be 50 .mu.m or less. More preferably, the mean
particle diameter of the Cu powder is 40 .mu.m or less, still more
preferably 30 .mu.m or less. Although the lower limit of the mean
particle diameter of the Cu powder is not specified, the lower
limit is preferably set to be approximately 0.5 .mu.m in order not
to increase the production cost of the Cu powder unnecessarily.
[0101] The mean particle diameter of the Cu powder can be
calculated by the following method.
[0102] Since the mean particle diameter of particles having a mean
particle diameter of 45 .mu.m or less is difficult to be measured
by means of sieving, the particle diameter is measured using a
laser diffraction/scattering particle size distribution measurement
system. Examples of the laser diffraction/scattering particle size
distribution measurement system include LA-950V2 manufactured by
HORIBA, Ltd. Of course, other laser diffraction/scattering particle
size distribution measurement systems may be used; however, for
performing accurate measurement, the lower limit and the upper
limit of the measurable particle diameter range of the system used
are preferably 0.1 .mu.m or less and 45 .mu.m or more,
respectively. Using the system mentioned above, a solvent in which
Cu powder is dispersed is exposed to a laser beam, and the particle
size distribution and the mean particle diameter of the Cu powder
are measured from the diffraction and scattering intensity of the
laser beam. For the solvent in which the Cu powder is dispersed,
ethanol is preferably used, since particles are easily dispersed in
ethanol, and ethanol is easy to handle. When a solvent in which the
Van der Waals force is strong and particles are hardly dispersed,
such as water is used, particles agglomerate during the
measurement, and the measurement result includes a mean particle
diameter larger than the real mean particle diameter. Therefore,
such a solvent is not preferred. Accordingly, it is preferable that
Cu powder introduced into an ethanol solution is preferably
dispersed using ultrasound before the measurement.
[0103] Since the appropriate dispersion time varies depending on
the target powder, the dispersion is performed in 7 stages at 10
min intervals between 0 min and 60 min, and the mean particle
diameter of the Cu powder is measured after each dispersion time
stage. In order to prevent particle agglomeration, during each
measurement, the measurement is performed with the solvent being
stirred. Of the particle diameters obtained through the seven
measurements performed by changing the dispersion time by 10 min,
the smallest value is used as the mean particle diameter of the Cu
powder.
(Graphite Powder)
[0104] Graphite powder is useful in increasing strength and fatigue
strength, and graphite powder is added to the partially alloyed
steel powder in an amount in a range of 0.1% to 1.0%, and mixing is
performed. When the amount of graphite powder added is less than
0.1%, the above advantageous effects cannot be obtained. On the
other hand, when the amount of graphite powder added exceeds 1.0%,
the sintered body becomes hypereutectoid, and cementite is
precipitated, resulting in reduced strength. Therefore, the amount
of graphite powder added is limited to a range of 0.1% to 1.0%. The
amount of graphite powder added is preferably in a range of 0.2% to
0.8%. Note that the particle diameter of graphite powder to be
added is preferably in a range of approximately from 1 .mu.m to 50
.mu.m.
[0105] In this disclosure, the Cu powder and graphite powder
described above are mixed with partially diffusion alloyed steel
powder to which Mo is diffusionally adhered to obtain
Fe-Mo-Cu-C-based mixed powder for powder metallurgy, and the mixing
may be performed in accordance with conventional powder mixing
methods.
[0106] Further, in a stage where a sintered body is obtained, if
the sintered body needs to be further formed into the shape of
parts by cutting work or the like, powder for improving
machinability, such as MnS is added to the mixed powder for powder
metallurgy in accordance with conventional methods.
[0107] Next, the compacting conditions and sintering conditions
preferable for manufacturing a sintered body using the-above
described mixed powder for powder metallurgy will be described.
[0108] In compaction using the above mixed powder for powder
metallurgy, a lubricant powder may also be mixed in. Further,
compaction may be performed with a lubricant being applied or
adhered to a mold. In either case, as the lubricant, any of metal
soap such as zinc stearate and lithium stearate, amide-based wax
such as ethylenebisstearamide, and other well known lubricants may
suitably be used. When mixing the lubricant, the amount thereof is
preferably around from 0.1 parts by mass to 1.2 parts by mass with
respect to 100 parts by mass of the mixed powder for powder
metallurgy.
[0109] In manufacturing a green compact by compacting the disclosed
mixed powder for powder metallurgy, the compaction is preferably
performed at a pressure of 400 MPa to 1000 MPa. When the compacting
pressure is less than 400 MPa, the density of the resulting green
compact is reduced, and the properties of the sintered body are
degraded. On the other hand, a compacting pressure exceeding 1000
MPa extremely shortens the life of the mold, which is economically
disadvantageous. The compacting temperature is preferably in a
range of room temperature (approximately 20.degree. C.) to
approximately 160.degree. C.
[0110] Further, the green compact is sintered preferably at a
temperature in a range of 1100.degree. C. to 1300.degree. C. When
the sintering temperature is lower than 1100.degree. C., sintering
stops; accordingly, it is difficult to achieve the desired tensile
strength: 1000 MPa or more. On the other hand, a sintering
temperature higher than 1300.degree. C. extremely shortens the life
of a sintering furnace, which is economically disadvantageous. The
sintering time is preferably in a range of 10 min to 180 min.
[0111] A sintered body obtained using mixed powder for powder
metallurgy according to this disclosure under the above sintering
conditions through such a procedure can have higher density after
sintering than the case of using alloy steel powder which does not
fall within the above range even if the green density is the
same.
[0112] Further, the resulting sintered body may be subjected to
strengthening processes such as carburized quenching, bright
quenching, induction hardening, and a carbonitriding process as
necessary; however, even when such strengthening processes are not
performed, the sintered body using the mixed powder for powder
metallurgy according to this disclosure have improved strength and
toughness compared with conventional sintered bodies which are not
subjected to strengthening processes. The strengthening processes
may be performed in accordance with conventional methods.
[0113] The disclosed iron-based sintered body obtained as described
above preferably contains Mo: 0.2 mass % to 1.5 mass %, Cu: 0.5
mass % to 4.0 mass %, and C: 0.1 mass % to 1.0 mass %.
Specifically, the C content is preferably in a range of 0.1% to
1.0% with which the highest strengthening effect and the highest
fatigue strength improving effect can be achieved. When the C
content is less than 0.1%, the above advantageous effects cannot be
achieved. On the other hand, a C content exceeding 1.0% results in
a hypereutectoid sintered body, so that cementite is precipitated,
resulting in reduced strength. Therefore, the amount of C contained
in the sintered body is limited to a range of 0.1% to 1.0%.
Preferably, the C content is 0.2% to 0.8%. The preferred content of
Mo and Cu is determined as described above for the same reasons as
in the case of the above-described mixed powder for powder
metallurgy.
[0114] Note that when a lubricant and the like are mixed into the
above mixed powder for powder metallurgy in manufacturing a
sintered body, the amount of Mo, Cu, and C in the mixed powder for
powder metallurgy is controlled so that the amount of Mo, Cu, and C
contained the sintered body fall within the above range.
[0115] Further, the C content of the sintered body may change from
the amount of graphite added depending on the sintering conditions
(temperature, time, atmosphere, and others). Accordingly, when the
amount of the graphite powder added is controlled within the above
range depending on the sintering conditions, an iron-based sintered
body having a C content preferred in this disclosure (0.1% to 1.0%,
more preferably 0.2% to 0.8%) can be manufactured.
EXAMPLES
[0116] A more detailed description of this disclosure will be given
below with reference to examples; however, the disclosure is not
limited solely to the following examples.
Example 1
[0117] As-atomized powders having particles with different
circularities were used as iron-based powders. The as-atomized
powders were subjected to grinding using a high speed mixer
(LFS-GS-2J manufactured by Fukae Powtec Corp.) so that the
circularities of the particles varied.
[0118] Oxidized Mo powder (mean particle diameter: 10 .mu.m) was
added to the iron-based powders at a predetermined ratio, and the
resultant powders were mixed for 15 minutes in a V blender, then
subjected to heat treatment in a hydrogen atmosphere with a dew
point of 30.degree. C. (holding temperature: 880.degree. C.,
holding time: 1 h). Mo of a predetermined amount presented in Table
1 was then adhered to the surface of the particles of the
iron-based powders by diffusion bonding to produce partially
alloyed steel powders for powder metallurgy. Note that the Mo
content was varied as in Samples Nos. 1 to 8 presented in Table
1.
[0119] The produced partially alloyed steel powders were each
embedded into a resin and polishing was performed to expose a cross
section of the partially alloyed steel powder particles.
Specifically, the partially alloyed steel powders were each
embedded to be uniformly distributed in an area with a thickness of
0.5 mm or more in a thermosetting resin so that a cross section of
a sufficient number of partially alloyed steel powder particles can
be observed in the polished surface, that is, the observation
surface. After the polishing, the polished surface was magnified
and imaged by an optical microscope, and the circularity of the
particles was calculated by image analysis as described above.
[0120] Further, the specific surface area of the partially alloyed
steel powder particles was measured through BET theory. The
particles of each partially alloyed steel powder were confirmed to
have a specific surface area of less than 0.10 m.sup.2/g.
[0121] Subsequently, Cu powder of the mean particle diameter and
the amount presented in Table 1 was added to these partially
alloyed steel powders, and graphite powder (mean particle diameter:
5 .mu.m) of the amount also presented in Table 1 was added thereto.
Ethylenebisstearamide was then added in an amount of 0.6 parts by
mass to the resulting mixed powder for powder metallurgy: 100 parts
by mass, and the powder was then mixed in a V blender for 15
minutes.
[0122] Samples Nos. 9 to 25 used partially alloyed steel powder
equivalent to those used in Sample No. 5, yet the amounts of Cu
powders and graphite powders varied. Samples Nos. 26 to 31 used
basically the same partially alloyed steel powder as that of Sample
No. 5, of which mean particle diameter was adjusted by sieving.
Further, Samples Nos. 32 to 38 used partially alloyed steel powders
having circularities that varied.
[0123] After that, each mixed powder was compacted at a density of
7.0 g/cm.sup.3, thereby manufacturing bar-shaped green compacts
having length: 55 mm, width: 10 mm, and thickness: 10 mm and
ring-shaped green compacts having outer diameter: 38 mm, inner
diameter: 25 mm, and thickness: 10 mm (ten pieces each). The
compacting pressure was 400 MPa in each case.
[0124] The bar-shaped green compacts and the ring-shaped green
compacts were sintered thereby obtaining sintered bodies. The
sintering was performed under a set of conditions including
sintered temperature: 1130.degree. C. and sintering time: 20 min in
a propane converted gas atmosphere.
[0125] The measurement of outer diameter, inner diameter, and
thickness and mass measurement were performed on the ring-shaped
sintered bodies, thereby calculating the sintered body density
(Mg/m.sup.3). Further, the median size, area fraction, and mean
maximum pore length of pores in the sintered bodies were measured
in accordance with the above-described methods.
[0126] For the bar-shaped sintered bodies, five of them were worked
into round bar tensile test pieces (JIS No. 2), each having a
parallel portion with a diameter of 5 mm, to be subjected to the
tensile test according to JIS Z2241, and the other five were bar
shaped (unnotched) as sintered and had a size according to JIS
Z2242 to be subjected to the Charpy impact test according to JIS
Z2242. Each of these test pieces was subjected to gas carburizing
at carbon potential: 0.8 mass % (holding temperature: 870.degree.
C., holding time: 60 min) followed by quenching (60.degree. C., oil
quenching) and tempering (holding temperature: 180.degree. C.,
holding time: 60 min).
[0127] The round bar tensile test pieces and bar-shaped test pieces
for the Charpy impact test subjected to carburizing, quenching, and
tempering were subjected to the tensile test according to JIS Z2241
and the Charpy impact test according to JIS Z2242; thus, the
tensile strength (MPa) and the impact energy value (J/cm.sup.2)
were measured and the mean values were calculated with the number
of samples n=5.
[0128] The measurement results are also presented in Table 1. The
evaluation criteria are as follows. [0129] (1) Flowability of Mixed
Powder
[0130] Mixed powders for powder metallurgy: 100 g were introduced
into a nozzle having diameter: 2.5 mm.phi.. When the total amount
of powder was completely flown within 80 s without stopping, the
powder was judged to have passed (passed). When the powder required
a longer time to be flown or the total amount or part of the amount
of powder stopped and failed to be flown, the powder was judged to
have failed (failed). [0131] (2) Sintered Body Density
[0132] A sintered body density of 6.95 Mg/m.sup.3 or more, that is
equal to or higher than that of a conventional 4Ni material (4
Ni-1.5 Cu-0.5 Mo, maximum particle diameter of material powder: 180
.mu.m) was judged to have passed. [0133] (3) Tensile Strength
[0134] When the round bar tensile test pieces having been subjected
to carburizing, quenching, and tempering had a tensile strength of
1000 MPa or more, the test pieces were judged to have passed.
[0135] (4) Impact Energy Value
[0136] When the bar-shaped test pieces for the Charpy impact test
having been subjected to carburizing, quenching, and tempering had
an impact energy value of 14.5 J/cm.sup.2 or more, the test pieces
were judged to have passed.
[0137] Note that the test of the impact energy value was also
performed on the sintered body before carburizing, quenching, and
tempering.
TABLE-US-00001 TABLE 1 After carburizing, Partially alloyed
quenching, steel powder Sintered body tempering Mean Mo Cu Graphite
Cu Mo Cu C Pore Mean Impact Impact particle content content content
particle content content content area Median maximum energy Tensile
energy Sample diameter (mass (mass (mass diameter (mass (mass (mass
fraction pore size pore size value Density strength value No.
(.mu.m) Circularity %) %) %) (.mu.m) Flowability %) %) %) (%)
(.mu.m) (.mu.m) (J/cm.sup.2) (Mg/m.sup.3) (MPa) (J/cm.sup.2)
Evaluation Note 1 89 0.58 0.1 2.0 0.30 35 passed 0.1 2.0 0.3 14 22
110 24 7.02 1080 13.8 failed Comparative Example 2 91 0.60 0.2 2.0
0.30 35 passed 0.2 2.0 0.3 14 16 90 33 7.00 1125 14.7 passed
Example 3 92 0.61 0.4 2.0 0.30 35 passed 0.4 2.0 0.3 14 14 80 41
7.01 1150 15.6 passed Example 4 95 0.62 0.6 2.0 0.30 35 passed 0.6
2.0 0.3 14 15 82 40 7.01 1175 15.4 passed Example 5 91 0.58 0.8 2.0
0.30 35 passed 0.8 2.0 0.3 15 16 80 39 6.97 1185 15.1 passed
Example 6 88 0.63 1.0 2.0 0.30 35 passed 1.0 2.0 0.3 14 17 87 36
6.98 1195 14.8 passed Example 7 92 0.63 1.5 2.0 0.30 35 passed 1.5
2.0 0.3 15 17 90 34 6.95 1200 14.6 passed Example 8 93 0.62 2.0 2.0
0.30 35 passed 2.0 2.0 0.3 15 22 110 22 6.93 1230 13.6 failed
Comparative Example 9 91 0.58 0.8 0.2 0.30 35 passed 0.8 0.2 0.3 14
21 92 23 7.01 980 13.6 failed Comparative Example 10 91 0.58 0.8
0.5 0.30 35 passed 0.8 0.5 0.3 14 19 86 32 7.00 1015 14.6 passed
Example 11 91 0.58 0.8 1.5 0.30 35 passed 0.8 1.5 0.3 14 15 84 37
6.98 1135 15.1 passed Example 12 91 0.58 0.8 3.0 0.30 35 passed 0.8
3.0 0.3 15 14 78 39 6.97 1210 15.4 passed Example 13 91 0.58 0.8
4.0 0.30 35 passed 0.8 4.0 0.3 15 14 75 42 6.95 1180 15.9 passed
Example 14 91 0.58 0.8 5.0 0.30 35 passed 0.8 5.0 0.3 16 22 70 23
6.92 990 13.0 failed Comparative Example 15 91 0.58 0.8 2.0 0.05 35
passed 0.8 2.0 0.05 14 13 73 41 7.02 980 16.0 failed Example 16 91
0.58 0.8 2.0 0.15 35 passed 0.8 2.0 0.2 14 17 85 38 7.00 1090 15.2
passed Example 17 91 0.58 0.8 2.0 0.50 35 passed 0.8 2.0 0.5 14 16
90 32 6.98 1150 14.8 passed Example 18 91 0.58 0.8 2.0 1.00 35
passed 0.8 2.0 1.0 15 20 105 28 6.97 1180 14.5 passed Example 19 91
0.58 0.8 2.0 1.50 35 passed 0.8 2.0 1.5 15 26 125 20 6.97 1115 12.0
failed Comparative Example 20 91 0.58 0.8 2.0 0.30 55 passed 0.8
2.0 0.3 15 19 84 30 6.95 1110 14.5 passed Example 21 91 0.58 0.8
2.0 0.30 48 passed 0.8 2.0 0.3 15 18 87 29 6.96 1140 14.6 passed
Example 22 91 0.58 0.8 2.0 0.30 30 passed 0.8 2.0 0.3 14 15 83 35
6.98 1151 15.1 passed Example 23 91 0.58 0.8 2.0 0.30 24 passed 0.8
2.0 15 82 36 6.99 1160 15.1 passed Example 24 91 0.58 0.8 2.0 0.30
15 passed 0.8 2.0 0.3 14 16 81 38 7.00 1180 15.2 passed Example 25
91 0.58 0.8 2.0 0.30 1.5 passed 0.8 2.0 0.3 13 14 76 39 7.03 1210
15.6 passed Example 26 128 0.48 0.8 2.0 0.30 35 passed 0.8 2.0 0.3
16 22 100 29 6.93 995 14.0 failed Comparative Example 27 118 0.55
0.8 2.0 0.30 35 passed 0.8 2.0 0.3 14 18 82 31 6.98 1150 14.7
passed Example 28 98 0.57 0.8 2.0 0.30 35 passed 0.8 2.0 0.3 14 16
78 37 7.00 1135 15.4 passed Example 29 75 0.58 0.8 2.0 0.30 35
passed 0.8 2.0 0.3 14 15 74 42 7.01 1194 15.7 passed Example 30 60
0.59 0.8 2.0 0.30 35 passed 0.8 2.0 0.3 14 14 73 44 7.01 1230 16.0
passed Example 31 35 0.62 0.8 2.0 0.30 35 passed 0.8 2.0 0.3 14 13
72 46 6.99 1260 16.3 passed Example 32 28 0.64 0.8 2.0 0.30 35
failed -- -- -- -- -- -- -- -- -- -- failed Comparative Example 33
70 0.45 0.8 2.0 0.30 35 passed 0.8 2.0 0.3 14 12 71 47 7.01 1240
16.4 passed Example 34 69 0.54 0.8 2.0 0.30 35 passed 0.8 2.0 0.3
14 14 70 45 7.00 1213 16.1 passed Example 35 72 0.56 0.8 2.0 0.30
35 passed 0.8 2.0 0.3 14 14 74 43 6.99 1180 15.9 passed Example 36
69 0.60 0.8 2.0 0.30 35 passed 0.8 2.0 0.3 14 17 84 38 7.00 1140
15.0 passed Example 37 70 0.62 0.8 2.0 0.30 35 passed 0.8 2.0 0.3
15 19 85 35 6.97 1120 14.7 passed Example 38 71 0.67 0.8 2.0 0.30
35 passed 0.8 2.0 0.3 14 28 100 19 6.98 1001 12.0 failed
Comparative Example 39* 65 0.67 0.5 -- 0.30 35 passed 0.5 1.5 0.3
15 29 130 25 6.97 998 13.3 failed Comparative Example Sample No. 39
is a 4Ni material (Fe--4Ni--1.5Cu--0.5Mo) For each of Samples Nos.
1, 8, 9, 14, 19, 26, 38, and 39*, the median size D50 of the pores
in the sintered body exceeded 20 .mu.m, resulting in a low impact
energy value, lack of toughness, and reduced tensile strength.
[0138] Further, for comparing the effects of the components in the
sintered bodies, the Mo content in Sample Nos. 1 to 8, the Cu
content in Nos. 9 to 14, and the graphite content in Nos. 15 to 19
were contrasted. Similarly, Samples Nos. 20 to 25 were designed for
evaluating the effect of the Cu particle diameter, Nos. 26 to 31
for evaluating the effect of the alloyed particle diameter, and
Nos. 32 to 38 for evaluating the effect of the circularity and the
mean particle diameter of the partially alloyed steel powders.
Table 1 also presents the results of a 4 Ni material (4 Ni-1.5
Cu-0.5 Mo, maximum particle diameter of material powder: 180 .mu.m)
as the conventional material. The table demonstrates that our
examples exhibited better properties over the conventional 4 Ni
material.
[0139] As presented in Table 1, all of Examples of this disclosure
were sintered bodies having high tensile strength and
toughness.
Example 2
[0140] Three atomized iron powders having particles of different
specific surface areas and circularities were prepared. The
specific surface area and the circularity were adjusted by grinding
each atomized iron powder using a high speed mixer (LFS-GS-2J
manufactured by Fukae Powtec Corp.) and adjusting the mixing ratio
of coarse powder having a particle size of 100 .mu.m or more and
fine powder having a particle size of 45 .mu.m or less.
[0141] Oxidized Mo powder (mean particle diameter: 10 .mu.m) was
added to the iron-based powders at a predetermined ratio, and the
resultant powders were mixed for 15 minutes in a V blender, then
subjected to heat treatment in a hydrogen atmosphere with a dew
point of 30.degree. C. (holding temperature: 880.degree. C.,
holding time: 1 h). Mo of a predetermined amount presented in Table
2 was then adhered to the surface of the particles of the
iron-based powders by diffusion bonding to produce partially
alloyed steel powders for powder metallurgy. These partially
alloyed steel powders were each embedded into a resin and polishing
was performed to expose a cross section of the partially alloyed
steel powder particles. Subsequently, the cross section was
magnified and imaged by an optical microscope, and the circularity
of the particles was calculated by image analysis. Further, the
specific surface area of the partially alloyed steel powder
particles was measured through BET theory.
[0142] Next, 2 mass % of Cu powder having a mean particle diameter
of 35 .mu.m was added to these partially alloyed steel powders, and
0.3 mass % of graphite powder (mean particle diameter: 5 .mu.m) was
added thereto. Ethylenebisstearamide was then added in an amount of
0.6 parts by mass to the resulting mixed powder for powder
metallurgy: 100 parts by mass, and the powder was then mixed in a V
blender for 15 minutes. Each of the mixed powders was compacted at
a compacting pressure of 686 MPa, thereby manufacturing bar-shaped
green compacts having length: 55 mm, width: 10 mm, and thickness:
10 mm and ring-shaped green compacts having outer diameter: 38 mm,
inner diameter: 25 mm, and thickness: 10 mm (ten pieces each).
[0143] The bar-shaped green compacts and ring-shape green compacts
were sintered to obtain sintered bodies. The sintering was
performed under a set of conditions including sintered temperature:
1130.degree. C. and sintering time: 20 min in a propane converted
gas atmosphere.
[0144] The measurement of outer diameter, inner diameter, and
thickness and mass measurement were performed on the ring-shaped
sintered bodies, thereby calculating the sintered body density
(Mg/m.sup.3). Further, the median size, area fraction, and mean
maximum pore length of pores in the sintered bodies were measured
in accordance with the above-described methods.
[0145] For the bar-shaped sintered bodies, five of them were worked
into round bar tensile test pieces (JIS No. 2) having diameter: 5
mm to be subjected to the tensile test according to JIS Z2241, and
the other five were bar shaped (unnotched) as sintered to be
subjected to the Charpy impact test according to JIS Z2242. Each of
these test pieces was subjected to gas carburizing at carbon
potential: 0.8 mass % (holding temperature: 870.degree. C., holding
time: 60 min) followed by quenching (60.degree. C., oil quenching)
and tempering (holding temperature: 180.degree. C., holding time:
60 min).
[0146] The round bar tensile test pieces and bar-shaped test pieces
for the Charpy impact test subjected to carburizing, quenching, and
tempering were subjected to the tensile test according to JIS Z2241
and the Charpy impact test according to JIS Z2242; thus, the
tensile strength (MPa) and the impact energy value (J/cm.sup.2)
were measured and the mean values were calculated with the number
of samples n=5.
[0147] The measurement results are also presented in Table 2. The
acceptance criteria for the values of the properties were the same
as those in Example 1.
TABLE-US-00002 TABLE 2 Partially alloyed steel powder Mean Specific
Cu Sintered body particle surface Mo Cu Graphite particle Mo Cu
Sample diameter area content content content diameter content
content No. (.mu.m) Circularity (m.sup.2/g) (mass %) (mass %) (mass
%) (.mu.m) Flowability (mass %) (mass %) 40 78 0.55 0.07 0.4 2.0
0.3 35 passed 0.4 2.0 41 76 0.52 0.08 0.8 2.0 0.3 35 passed 0.8 2.0
42 76 0.59 0.13 0.4 2.0 0.3 35 -- 0.4 2.0 43 77 0.52 0.15 0.8 2.0
0.3 35 -- 0.8 2.0 44 76 0.67 0.12 0.4 2.0 0.3 35 -- 0.4 2.0 45 77
0.66 0.14 0.8 2.0 0.3 35 -- -- -- 46 75 0.68 0.06 0.4 2.0 0.3 35
passed 0.4 2.0 47 77 0.69 0.08 0.8 2.0 0.3 35 passed 0.8 2.0 After
carburizing, quenching, Sintered body tempering Mean Impact Impact
C Pore area Median maximum energy Tensile energy Sample content
fraction pore size pore size value Density strength value No. (mass
%) (%) (.mu.m) (.mu.m) (J/cm.sup.2) (Mg/m.sup.3) (MPa) (J/cm.sup.2)
Evaluation Note 40 0.3 14 16 85.0 35.0 7.01 1175 15.1 passed
Example 41 0.3 15 14 73.0 42.0 6.97 1194 15.7 passed Example 42 0.3
-- -- -- -- -- -- -- failed Comparative Example 43 0.3 -- -- -- --
-- -- -- failed Comparative Example 44 0.3 -- -- -- -- -- -- --
failed Comparative Example 45 -- -- -- -- -- -- -- -- failed
Comparative Example 46 0.3 12 25 110.0 21.0 7.10 1060 12.1 failed
Comparative Example 47 0.3 13 25 100.0 20.0 7.06 1075 12.3 failed
Comparative Example
[0148] As can be seen from Table 2, all the sintered bodies having
a median pore size D50 of 20 .mu.m or less had a high impact energy
value, excellent toughness, and high tensile strength. Further,
when partially alloyed steel powders having particles of a
circularity and a specific surface area within the disclosed range
were used, the target values of the sintered body density, the
tensile strength, and the impact energy value were achieved.
* * * * *