U.S. patent application number 15/942815 was filed with the patent office on 2018-10-04 for method of producing wear-resistant iron-based sintered alloy.
This patent application is currently assigned to TOYOTA JIDOSHA KABUSHIKI KAISHA. The applicant listed for this patent is FINE SINTER CO., LTD., TOYOTA JIDOSHA KABUSHIKI KAISHA. Invention is credited to Yuki KAMO, Takeshi NAKAMURA, Nobuyuki SHINOHARA, Yoshihisa UEDA, Takanori YONEDA.
Application Number | 20180282844 15/942815 |
Document ID | / |
Family ID | 63672204 |
Filed Date | 2018-10-04 |
United States Patent
Application |
20180282844 |
Kind Code |
A1 |
SHINOHARA; Nobuyuki ; et
al. |
October 4, 2018 |
METHOD OF PRODUCING WEAR-RESISTANT IRON-BASED SINTERED ALLOY
Abstract
A wear-resistant iron-based sintered alloy made of a mixed
powder including first hard particles, second hard particles,
graphite particles, and iron particles is produced. The first hard
particles are Fe--Mo--Ni--Co--Mn--Si--C alloy particles. The second
hard particles are Fe--Mo--Si alloy particles. The mixed powder
includes the first hard particles at 5 mass % to 50 mass %, the
second hard particles at 1 mass % to 8 mass %, and the graphite
particles at 0.5 mass % to 1.5 mass % when a total amount of the
above particles is set as 100 mass %. In a sintering process,
sintering is performed so that the hardness of the first hard
particles becomes 400 to 600 Hv and the hardness of the second hard
particles exceeds 600 Hv. Then, an oxidation treatment is performed
so that a density difference between before and after the oxidation
treatment in a sintered product becomes 0.05 g/cm.sup.3 or
more.
Inventors: |
SHINOHARA; Nobuyuki;
(Tajimi-shi, JP) ; KAMO; Yuki; (Okazaki-shi,
JP) ; UEDA; Yoshihisa; (Kasugai-shi, JP) ;
YONEDA; Takanori; (Kasugai-shi, JP) ; NAKAMURA;
Takeshi; (Kasugai-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
TOYOTA JIDOSHA KABUSHIKI KAISHA
FINE SINTER CO., LTD. |
Toyota-shi
Kasugai-shi |
|
JP
JP |
|
|
Assignee: |
TOYOTA JIDOSHA KABUSHIKI
KAISHA
Toyota-shi
JP
FINE SINTER CO., LTD.
Kasugai-shi
JP
|
Family ID: |
63672204 |
Appl. No.: |
15/942815 |
Filed: |
April 2, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B22F 5/106 20130101;
B22F 2301/35 20130101; B22F 2999/00 20130101; B22F 3/16 20130101;
B22F 3/10 20130101; C22C 33/0207 20130101; B22F 2201/05 20130101;
F01L 2301/00 20200501; B22F 2009/0824 20130101; C22C 38/12
20130101; B22F 2302/40 20130101; C22C 38/105 20130101; C22C 38/04
20130101; B22F 2201/10 20130101; B22F 2003/248 20130101; B22F
2201/02 20130101; F01L 3/02 20130101; F01L 2303/00 20200501; C22C
33/0285 20130101; B22F 3/02 20130101; C22C 38/02 20130101; B22F
2999/00 20130101; C22C 33/0207 20130101; C22C 33/0285 20130101;
C22C 1/055 20130101; B22F 2999/00 20130101; B22F 3/10 20130101;
B22F 2201/10 20130101; B22F 2999/00 20130101; B22F 2003/248
20130101; B22F 2201/05 20130101 |
International
Class: |
C22C 33/02 20060101
C22C033/02; C22C 38/12 20060101 C22C038/12; C22C 38/10 20060101
C22C038/10; C22C 38/04 20060101 C22C038/04; C22C 38/02 20060101
C22C038/02; B22F 3/16 20060101 B22F003/16; B22F 5/10 20060101
B22F005/10 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 4, 2017 |
JP |
2017-074255 |
Claims
1. A method of producing a wear-resistant iron-based sintered alloy
comprising: a molding process in which a mixed powder including
hard particles, graphite particles, and iron particles is
compact-molded into a molded product for a sintered alloy; and a
sintering process in which the molded product for the sintered
alloy is sintered while C of the graphite particles of the molded
product for the sintered alloy diffuses into the hard particles and
the iron particles, wherein the hard particles include first hard
particles and second hard particles, wherein the first hard
particles include Mo: 20 mass % to 70 mass %, Ni: 5 mass % to 40
mass %, Co: 5 mass % to 40 mass %, Mn: 1 mass % to 20 mass %, Si:
0.5 mass % to 4.0 mass %, and C: 0.5 mass % to 3.0 mass %, with the
balance including Fe and inevitable impurities when an amount of
the first hard particles is set as 100 mass %, wherein the second
hard particles include Mo: 60 mass % to 70 mass %, and Si: 2.0 mass
% or less, with the balance including Fe and inevitable impurities
when an amount of the second hard particles is set as 100 mass %,
wherein the mixed powder includes the first hard particles at 5
mass % to 50 mass %, the second hard particles at 1 mass % to 5
mass %, and the graphite particles at 0.5 mass % to 1.5 mass % when
a total amount of the first hard particles, the second hard
particles, the graphite particles, and the iron particles is set as
100 mass %, and wherein, in the sintering process, sintering is
performed so that the hardness of the first hard particles becomes
400 to 600 Hv and the hardness of the second hard particles exceeds
600 Hv, after the sintering process, an oxidation treatment is
performed on a sintered product sintered from the molded product
for the sintered alloy so that a part of iron contained in an iron
matrix derived from the iron particles becomes triiron tetraoxide,
and the oxidation treatment is performed so that a difference
between a density of the sintered product before the oxidation
treatment and a density of the sintered product after the oxidation
treatment becomes 0.05 g/cm.sup.3 or more.
2. The method according to claim 1, wherein 10 mass % or less of Cr
is additionally added to the first hard particles when the amount
of the first hard particles is set as 100 mass %.
3. The method according to claim 1, wherein a particle size of the
second hard particles is in a range of 100 .mu.m or less.
Description
INCORPORATION BY REFERENCE
[0001] The disclosure of Japanese Patent Application No.
2017-074255 filed on Apr. 4, 2017 including the specification,
drawings and abstract is incorporated herein by reference in its
entirety.
BACKGROUND
1. Technical Field
[0002] The present disclosure relates to a method of producing a
wear-resistant iron-based sintered alloy including hard particles
suitable for improving wear resistance of a sintered alloy.
2. Description of Related Art
[0003] A sintered alloy based on iron may be applied to a valve
seat and the like. Hard particles may be included in the sintered
alloy in order to further improve wear resistance. When hard
particles are included, graphite particles and iron particles are
mixed into hard particles to form a powder, and the mixed powder is
compact-molded into a molded product for a sintered alloy. Then,
generally, the molded product for a sintered alloy is heated and
thus it is sintered and becomes a sintered alloy.
[0004] As a method of producing such a sintered alloy, a method of
producing a wear-resistant iron-based sintered alloy in which a
mixed powder in which hard particles, graphite particles, and iron
particles are mixed is compact-molded into a molded product for a
sintered alloy, and the molded product for a sintered alloy is
sintered while C of the graphite particles of the molded product
for a sintered alloy diffuses into the hard particles and the iron
particles has been proposed (for example, refer to Japanese
Unexamined Patent Application Publication No. 2004-156101 (JP
2004-156101 A).
[0005] Here, the hard particles include Mo: 20 mass % to 70 mass %,
C: 0.2 mass % to 3 mass %, and Mn: 1 mass % to 15 mass %, with the
balance including inevitable impurities and Co. The mixed powder
includes the hard particles at 10 mass % to 60 mass % and the
graphite particles at 0.2 mass % to 2 mass % when the total amount
of the hard particles, the graphite particles, and the iron
particles is set as 100 mass %. Since hard particles are dispersed
into such a sintered alloy, it is possible to prevent abrasive
wear.
SUMMARY
[0006] However, a matrix material connecting hard particles of a
wear-resistant iron-based sintered alloy produced in the production
method described in JP 2004-156101 A is soft because it is an Fe--C
material in which C of the graphite particles has diffused into the
iron particles. Therefore, when the wear-resistant iron-based
sintered alloy and a metallic material of a sliding counterpart
component that comes in contact therewith are in metal-contact with
each other, a contact surface of the wear-resistant iron-based
sintered alloy is likely to be plastically deformed and adhesive
wear easily occurs on the contact surface. In order to prevent such
problems, it is desirable to increase the hardness of the
wear-resistant iron-based sintered alloy. However, there is a risk
of the machinability of the wear-resistant iron-based sintered
alloy deteriorating accordingly and it is difficult to achieve both
adhesive wear resistance and machinability.
[0007] The present disclosure provides a method of producing a
wear-resistant iron-based sintered alloy through which it is
possible to secure machinability while preventing adhesive
wear.
[0008] The inventors expected that the adhesive wear of a contact
surface would accelerate when the iron matrix of a wear-resistant
iron-based sintered alloy was plastically deformed as described
above. In this regard, the inventors studied addition of other hard
particles through which it is possible to prevent plastic
deformation of an iron matrix in addition to the hard particles
through which abrasive wear has so far been prevented. Therefore,
the inventors focused on molybdenum as a main component of the hard
particles, and found that, when an iron-molybdenum intermetallic
compound and a molybdenum carbide precipitated during sintering are
interspersed in an iron matrix, it is possible to control plastic
deformation of the iron matrix. In addition thereto, the inventors
acquired the new finding that, when oxidizing a part of the iron of
the iron matrix derived from the iron particles to triiron
tetraoxide, it is possible to improve the wear resistance thereof
without deteriorating the machinability of the sintered alloy.
[0009] An aspect of the present disclosure relates to a method of
producing a wear-resistant iron-based sintered alloy including a
molding process in which a mixed powder including hard particles,
graphite particles, and iron particles is compact-molded into a
molded product for a sintered alloy; and a sintering process in
which the molded product for the sintered alloy is sintered while C
of the graphite particles of the molded product for the sintered
alloy diffuses into the hard particles and the iron particles,
wherein the hard particles include first hard particles and second
hard particles, wherein the first hard particles include Mo: 20
mass % to 70 mass %, Ni: 5 mass % to 40 mass %, Co: 5 mass % to 40
mass %, Mn: 1 mass % to 20 mass %, Si: 0.5 mass % to 4.0 mass %,
and C: 0.5 mass % to 3.0 mass %, with the balance including Fe and
inevitable impurities when an amount of the first hard particles is
set as 100 mass %, wherein the second hard particles include Mo: 60
mass % to 70 mass %, and Si: 2.0 mass % or less, with the balance
including Fe and inevitable impurities when an amount of the second
hard particles is set as 100 mass %, wherein the mixed powder
includes the first hard particles at 5 mass % to 50 mass %, the
second hard particles at 1 mass % to 5 mass %, and the graphite
particles at 0.5 mass % to 1.5 mass % when a total amount of the
first hard particles, the second hard particles, the graphite
particles, and the iron particles is set as 100 mass %, and
wherein, in the sintering process, sintering is performed so that
the hardness of the first hard particles becomes 400 to 600 Hv and
the hardness of the second hard particles exceeds 600 Hv, after the
sintering process, an oxidation treatment is performed on a
sintered product sintered from the molded product for the sintered
alloy so that a part of iron contained in an iron matrix derived
from the iron particles becomes triiron tetraoxide, and the
oxidation treatment is performed so that a difference between a
density of the sintered product before the oxidation treatment and
a density of the sintered product after the oxidation treatment
becomes 0.05 g/cm.sup.3 or more.
[0010] According to the present disclosure, it is possible to
secure machinability while preventing adhesive wear.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] Features, advantages, and technical and industrial
significance of exemplary embodiments of the disclosure will be
described below with reference to the accompanying drawings, in
which like numerals denote like elements, and wherein:
[0012] FIG. 1 is a schematic conceptual view of a wear test used in
examples and comparative examples; FIG. 2 is a schematic conceptual
view of a machinability test used in examples and comparative
examples;
[0013] FIG. 3A is a graph showing results of wear test wear amount
proportions with respect to amounts of first hard particles added
in Examples 1 to 3 and Comparative Examples 1 and 9;
[0014] FIG. 3B is a graph showing results of tool wear amount
proportions with respect to amounts of the first hard particles
added in Examples 1 to 3 and Comparative Examples 1 and 9;
[0015] FIG. 4A is a graph showing results of wear test wear amount
proportions with respect to amounts of second hard particles added
in Examples 1, 4, and 5 and Comparative Examples 3, 4, and 9;
[0016] FIG. 4B is a graph showing results of tool wear amount
proportions with respect to amounts of the second hard particles
added in Examples 1, 4, and 5 and Comparative Examples 3, 4, and
9;
[0017] FIG. 5A is a graph showing results of wear test wear amount
proportions with respect to amounts of graphite particles added in
Examples 1, 6, and 7 and Comparative Examples 5, 6, and 9;
[0018] FIG. 5B is a graph showing results of tool wear amount
proportions with respect to amounts of the graphite particles added
in Examples 1, 6, and 7 and Comparative Examples 5, 6, and 9;
[0019] FIG. 6A is a graph showing results of wear test wear amount
proportions with respect to the hardness of the first hard
particles in Examples 1, 3, 5, and 8 and Comparative Examples 8 and
9;
[0020] FIG. 6B is a graph showing results of tool wear amount
proportions with respect to the hardness of the first hard
particles in Examples 1, 3, 5, and 8 and Comparative
[0021] Examples 8 and 9;
[0022] FIG. 7A is a graph showing results of wear test wear amount
proportions with respect to the density difference in the sintered
products in Examples 1 to 8 and Comparative Examples 7 and 9;
[0023] FIG. 7B is a graph showing results of tool wear amount
proportions with respect to the density difference in the sintered
products in Examples 1 to 8 and Comparative Examples 7 and 9;
[0024] FIG. 8A is a picture of a surface of a test piece according
to Example 1 after a wear test;
[0025] FIG. 8B is a picture of a surface of a test piece according
to Comparative Example 7 after a wear test;
[0026] FIG. 9A is a picture of a structure of the test piece
according to Example 1;
[0027] FIG. 9B is a picture of a structure of a test piece
according to Comparative Example 5;
[0028] FIG. 9C is a picture of a structure of a test piece
according to Comparative Example 6;
[0029] FIG. 10A is a graph showing results of wear test wear amount
proportions in Examples 1 and 9 and Comparative Example 10; and
[0030] FIG. 10B is a graph showing results of tool wear amount
proportions in Examples 1 and 9 and Comparative Example 10.
DETAILED DESCRIPTION OF EMBODIMENTS
[0031] An embodiment of the present disclosure will be described
below in detail. A molded product for a sintered alloy (hereinafter
referred to as a molded product) according to the present
embodiment is obtained by compact-molding a mixed powder including
first and second hard particles, graphite particles, and iron
particles to be described below. A wear-resistant iron-based
sintered alloy (hereinafter referred to as a sintered alloy) is
obtained by sintering the molded product while C of the graphite
particles diffuses into the hard particles and the iron particles.
The hard particles, the molded product obtained by compact-molding
a mixed powder in which the hard particles are mixed, and the
sintered alloy obtained by sintering the molded product will be
described below.
[0032] 1. First Hard Particles
[0033] The first hard particles are particles that are mixed as a
raw material into the sintered alloy and have high hardness with
respect to iron particles and an iron matrix of the sintered alloy,
and thus prevent abrasive wear of the sintered alloy.
[0034] The first hard particles are particles made of a
Co--Mo--Ni--Fe--Mn--Si--C alloy. Specifically, the first hard
particles include Mo: 20 mass % to 70 mass %, Ni: 5 mass % to 40
mass %, Co: 5 mass % to 40 mass %, Mn: 1 mass % to 20 mass %, Si:
0.5 mass % to 4.0 mass %, and C: 0.5 mass % to 3.0 mass %, with the
balance including Fe and inevitable impurities when the amount of
the first hard particles is set as 100 mass %. In addition, Cr may
be added to the first hard particles in a range of 10 mass % or
less as necessary. The hardness of the first hard particles before
sintering is preferably in a range of 400 to 600 Hv.
[0035] The first hard particles can be produced by preparing molten
metal in which the above composition is mixed together in the above
proportions and performing an atomization treatment in which the
molten metal is atomized. In addition, as another method, a
solidified body in which molten metal has solidified may be formed
into a powder by mechanical grinding. As the atomization treatment,
either a gas atomization treatment or a water atomization treatment
may be performed. However, in consideration of sinterability and
the like, a gas atomization treatment is more preferable because
round particles are obtained.
[0036] Here, lower limit values and upper limit values of the above
hard particle composition can be appropriately changed according to
the degree of importance of characteristics of application
components in consideration of reasons for limitation to be
described below and the hardness, solid lubricity, adhesiveness,
and cost in such ranges.
[0037] 1-1. Mo: 20 Mass % to 70 Mass %
[0038] In the composition of the first hard particles, Mo can
generate Mo carbide together with C of a carbon powder during
sintering and improve the hardness and wear resistance of the first
hard particles. In addition, regarding Mo, since under a high
temperature usage environment, Mo in a solid solution state and Mo
carbide are oxidized to form a Mo oxide film, it is possible to
obtain favorable solid lubricity for a sintered alloy.
[0039] Here, when a content of Mo is less than 20 mass %, not only
the amount of Mo carbide generated is reduced, but also an
oxidation initiation temperature of the first hard particles
increases, and generation of Mo oxide under a high temperature
usage environment is prevented. Thereby, the solid lubricity of the
obtained sintered alloy is insufficient, and the abrasive wear
resistance thereof decreases. On the other hand, when a content of
Mo exceeds 70 mass %, not only it is difficult to produce the first
hard particles using an atomizing method, but also adhesiveness
between the hard particles and the iron matrix decreases. More
preferably, a content of Mo is 30 mass % to 50 mass %.
[0040] 1-2. Ni: 5 Mass % to 40 Mass %
[0041] In the composition of the first hard particles, Ni can
enlarge an austenitic structure of the matrix of the first hard
particles and improve the toughness thereof. In addition, Ni can
increase an amount of Mo in a solid solution state of the first
hard particles, and improve the wear resistance of the first hard
particles.
[0042] Further, Ni diffuses into the iron matrix of the sintered
alloy during sintering, can enlarge the austenitic structure of the
iron matrix, increase the toughness of the sintered alloy, increase
an amount of Mo in a solid solution state in the iron matrix, and
improve wear resistance.
[0043] Here, when a content of Ni is less than 5 mass %, the above
effects of Ni are expected to be unlikely. On the other hand, when
a content of Ni exceeds 40 mass %, although the above effects of Ni
are maximized, the cost of the first hard particles increases. More
preferably, a content of Ni is 20 mass % to 40 mass %.
[0044] 1-3. Co: 5 Mass % to 40 Mass %
[0045] In the composition of the first hard particles, similarly to
Ni, Co can enlarge the austenitic structure in the matrix of the
first hard particles and the iron matrix of the sintered alloy, and
improve the hardness of the first hard particles.
[0046] Here, when a content of Co is less than 5 mass %, the above
effects of Ni are expected to be unlikely. On the other hand, when
a content of Co exceeds 40 mass %, although the above effects of Co
are maximized, the cost of the first hard particles increases. More
preferably, a content of Co is 10 mass % to 30 mass %.
[0047] 1-4. Mn: 1 Mass % to 20 Mass %
[0048] In the composition of the first hard particles, since Mn
efficiently diffuses from the first hard particles into the iron
matrix of the sintered alloy during sintering, it is possible to
improve adhesiveness between the first hard particles and the iron
matrix. In addition, Mn can enlarge the austenitic structure in the
matrix of the first hard particles and the iron matrix of the
sintered alloy.
[0049] Here, when a content of Mn is less than 1 mass %, since an
amount of Mn diffusing into the iron matrix is small, the
adhesiveness between the hard particles and the iron matrix
decreases. Thus, the mechanical strength of the obtained sintered
alloy decreases. On the other hand, when a content of Mn exceeds 20
mass %, the above effects of Mn are maximized. More preferably, a
content of Mn is 2 mass % to 8 mass %.
[0050] 1-5. Si: 0.5 mass % to 4.0 mass % In the composition of the
first hard particles, Si can improve adhesiveness between the first
hard particles and a Mo oxide film. Here, when a content of Si is
less than 0.5 mass %, the above effects of Si are expected to be
unlikely. On the other hand, when a content of Si exceeds 4.0 mass
%, moldability of the molded product deteriorates and the density
of the sintered alloy decreases. More preferably, a content of Si
is 0.5 mass % to 2 mass %.
[0051] 1-6. C: 0.5 Mass % to 3.0 Mass %
[0052] In the composition of the first hard particles, C combines
with Mo to form Mo carbide, and can improve the hardness and wear
resistance of the first hard particles. Here, when a content of C
is less than 0.5 mass %, the wear resistance effect is not
sufficient. On the other hand, when a content of C exceeds 3.0 mass
%, moldability of the molded product deteriorates and the density
of the sintered alloy decreases. More preferably, a content of C is
0.5 mass % to 2 mass %.
[0053] 1-7. Cr: 10 Mass %
[0054] Hereinafter, in the composition of the first hard particles,
Cr can prevent excessive oxidation of Mo during use. For example,
addition of Cr is effective when a usage environment temperature of
the sintered alloy is high, an amount of the Mo oxide film
generated in the first hard particles increases, and the Mo oxide
film peels off from the first hard particles,.
[0055] Here, when a content of Cr exceeds 10 mass %, formation of
the Mo oxide film in the first hard particles is prevented too
much. Here, under a corrosive environment such as an alcohol fuel
environment, in order to improve corrosion resistance, it is
desirable to add Cr. On the other hand, under an environment in
which adhesive wear is likely to occur, in order to accelerate
oxidation, it is desirable to reduce a content of Cr.
[0056] 1-8. Particle Size of First Hard Particles
[0057] The particle size of the first hard particles can be
appropriately selected according to an application, a type, and the
like of the sintered alloy. However, the particle size of the first
hard particles is preferably in a range of 44 .mu.m to 250 .mu.m,
and more preferably in a range of 44 .mu.m to 105 .mu.m.
[0058] Here, when hard particles with a particle size less than 44
.mu.m are included as the first hard particles, since the particle
size is too small, wear resistance of the wear-resistant iron-based
sintered alloy may decrease. On the other hand, when hard particles
with a particle size greater than 250 .mu.m are included as the
first hard particles, since the particle size is too large, the
machinability of the wear-resistant iron-based sintered alloy may
deteriorate.
[0059] 2. Second Hard Particles
[0060] Similarly to the first hard particles, the second hard
particles are particles that are mixed as a raw material into the
sintered alloy and have high hardness with respect to the iron
particles and an iron matrix of the sintered alloy. The second hard
particles are particles that significantly increase the hardness of
the sintered alloy when added in a small amount, prevent plastic
deformation of the iron matrix of the sintered alloy, and as a
result, decrease adhesive wear of the sintered alloy.
[0061] The second hard particles are particles made of an Fe--Mo
alloy, and include Mo: 60 mass % to 70 mass %, and Si: 2.0 mass %
or less, with the balance including Fe and inevitable impurities
when the amount of the second hard particles is set as 100 mass %.
The hardness of the second hard particles before sintering is
preferably in a range of 600 to 1600 Hv.
[0062] A solidified body in which molten metal has solidified is
formed into a powder by mechanical grinding to produce the second
hard particles. In addition, like the first hard particles, the
second hard particles may be produced through a gas atomization
treatment, a water atomization treatment, or the like.
[0063] 2-1. Mo: 60 Mass % to 70 Mass %
[0064] In the composition of the second hard particles, Mo can
generate Mo carbide together with C of a carbon powder during
sintering and improve the hardness and wear resistance of the
second hard particles. In addition, regarding Mo, since under a
high temperature usage environment, Mo in a solid solution state
and Mo carbide are oxidized to form a Mo oxide film, it is possible
to obtain favorable solid lubricity for a sintered alloy. In
addition, when molybdenum carbide precipitates at a grain boundary
of the iron matrix during sintering, it is possible to prevent
plastic deformation of the iron matrix and adhesive wear during
use.
[0065] Here, when a content of Mo is less than 60 mass %, it is
difficult to prevent plastic deformation of the iron matrix
according to molybdenum carbide described above and adhesive wear
resistance decreases. On the other hand, when a content of Mo
exceeds 70 mass %, it is difficult to produce second hard particles
using a grinding method, and a yield thereof decreases.
[0066] 2-2. Si: 2.0 Mass % or Less
[0067] When Si is contained in the composition of the second hard
particles, it is easy to produce the second hard particles using a
grinding method. Here, when a content of Si exceeds 2.0 mass %, the
hardness of the second hard particles increases, moldability of the
molded product deteriorates, the density of the sintered alloy
decreases, and also the machinability of the sintered alloy
deteriorates.
[0068] 2-3. Particle Size of Second Hard Particles
[0069] The particle size of the second hard particles can be
appropriately selected according to an application, a type, and the
like of the sintered alloy. However, the particle size (maximum
particle size) of the second hard particles is preferably in a
range of 100 .mu.m or less and more preferably, 75 .mu.m or less.
Thereby, the second hard particles can be uniformly dispersed into
the matrix and it is possible to increase the hardness of the
sintered alloy. Here, when hard particles with a particle size
greater than 100 .mu.m are included as the second hard particles,
since the particle size is too large, the machinability of the
sintered alloy may deteriorate. Here, the particle size of the
second hard particles is preferably 1.mu.m or more in consideration
of production.
[0070] 3. Graphite Particles
[0071] The graphite particles may be either natural graphite
particles or artificial graphite particles or a mixture thereof as
long as C of the graphite particles can diffuse into the iron
matrix and hard particles in a solid solution state during
sintering. The particle size of the graphite particles is
preferably in a range of 1.mu.m to 45 .mu.m. As a powder including
preferable graphite particles, a graphite powder (CPB-S
commercially available from Nippon Kokuen Group) can be
exemplified.
[0072] 4. Iron Particles
[0073] The iron particles serving as the matrix of the sintered
alloy are iron particles containing Fe as a main component. As a
powder including iron particles, a pure iron powder is preferable.
However, a low alloy steel powder may be used as long as
moldability during compact-molding does not deteriorate and
diffusion of elements such as Mn of the above first hard particles
does not decrease. As the low alloy steel powder, an Fe--C powder
can be used. For example, a powder having a composition including
C: 0.2 mass % to 5 mass %, the balance including inevitable
impurities and Fe when the amount of the low alloy steel powder is
set as 100 mass % can be used. In addition, such a powder may be a
gas atomized powder, a water atomized powder or a reduced powder.
The particle size of the iron particles is preferably in a range of
150 .mu.m or less.
[0074] 5. Mixing Ratio of Mixed Powder
[0075] A mixed powder including the first hard particles, the
second hard particles, the graphite particles, and the iron
particles is prepared. The mixed powder includes the first hard
particles at 5 mass % to 50 mass %, the second hard particles at 1
mass % to 5 mass %, and the graphite particles at 0.5 mass % to 1.5
mass % when the total amount of the first hard particles, the
second hard particles, the graphite particles, and the iron
particles is set as 100 mass %.
[0076] The mixed powder may include only the first hard particles,
the second hard particles, the graphite particles, and the iron
particles, but may include about several mass % of other particles
as long as the mechanical strength and wear resistance of the
obtained sintered alloy do not decrease. In this case, when the
total amount of the first and second hard particles, the graphite
particles, and the iron particles is 95 mass % or more with respect
to the mixed powder, sufficient effects can be expected. For
example, at least one type of particles for improving machinability
selected from the group consisting of sulfides (for example, MnS),
oxides (for example, CaCO.sub.3), fluorides (for example, CaF),
nitrides (for example, BN), and oxysulfides may be included in the
mixed powder.
[0077] Since the first hard particles are included at 5 mass % to
50 mass % with respect to the total amount of the first hard
particles, the second hard particles, the graphite particles, and
the iron particles, it is possible to improve both the mechanical
strength and the abrasive wear resistance of the sintered
alloy.
[0078] Here, when the first hard particles are included at less
than 5 mass % with respect to the total amount, as can be clearly
understood from experiments performed by the inventors to be
described below, a sufficient effect of the abrasive wear
resistance according to the first hard particles cannot be
exhibited.
[0079] On the other hand, when the amount of the first hard
particles exceeds 50 mass % with respect to the total amount, since
the amount of the first hard particles is too large, when the
molded product is molded from the mixed powder, it is difficult to
mold the molded product. In addition, since there is greater
contact between the first hard particles and a part in which iron
particles are sintered becomes smaller, the abrasive wear
resistance of the sintered alloy decreases.
[0080] Since the second hard particles are included at 1 mass % to
5 mass % with respect to the total amount of the first hard
particles, the second hard particles, the graphite particles, and
the iron particles, as described above, it is possible to prevent
plastic deformation of the iron matrix during use and reduce the
adhesive wear of the sintered alloy.
[0081] Here, when a content of the second hard particles is less
than 1 mass % with respect to the total amount, as can be clearly
understood from experiments performed by the inventors to be
described below, the adhesive wear resistance of the sintered alloy
decreases. On the other hand, when a content of the second hard
particles exceeds 5 mass % with respect to the total amount, the
machinability of the sintered alloy deteriorates.
[0082] Since the graphite particles are included at 0.5 mass % to
1.5 mass % with respect to the total amount of the first hard
particles, the second hard particles, the graphite particles, and
the iron particles, after sintering, without melting the first and
second hard particles, C of the graphite particles can diffuse in a
solid solution state into the first and second hard particles, and
additionally, a pearlite structure can be secured in the iron
matrix. Therefore, it is possible to improve both the mechanical
strength and the wear resistance of the sintered alloy.
[0083] Here, when a content of the graphite particles is less than
0.5 mass % with respect to the total amount, since the ferrite
structure of the iron matrix is likely to increase, the strength of
the iron matrix itself of the sintered alloy decreases. On the
other hand, when a content of the graphite particles exceeds 1.5
mass % with respect to the total amount, a cementite structure
precipitates and the machinability of the sintered alloy
deteriorates.
[0084] 6. Method of Producing Wear-Resistant Iron-Based Sintered
Alloy
[0085] In this manner, the obtained mixed powder is compact-molded
into a molded product for a sintered alloy (molding process). In
the molded product for a sintered alloy, the first hard particles,
the second hard particles, the graphite particles, and the iron
particles are included at the same proportion as in the mixed
powder.
[0086] While C of the graphite particles of the molded product for
a sintered alloy diffuses into the first and second hard particles
and the iron particles, the molded product for a sintered alloy
that is compact-molded is sintered to produce a sintered product
(sintering process). In this case, not only is there greater
diffusion of iron from the iron matrix (iron particles) into the
first and second hard particles but also the second hard particles
do not contain carbon. Therefore, carbon of the graphite particles
easily diffuses into the second hard particles, Mo carbide is
generated at a grain boundary between the second hard particles,
and the hardness of the sintered alloy can increase.
[0087] In the present embodiment, sintering is performed by
adjusting a sintering temperature and a sintering time so that the
hardness of the first hard particles becomes 400 to 600 Hv and the
hardness of the second hard particles exceeds 600 Hv. Regarding the
hardnesses of the first and second hard particles in the obtained
sintered alloy, these hardnesses are values measured using a micro
Vickers hardness testing machine at a measurement load of 0.1 kgf.
When the hardness of the first hard particles is set to be within
such a range, it is possible to secure wear resistance and
machinability for the sintered alloy. Here, when the hardness of
the first hard particles is less than 400 Hv, a difference in
hardness from the iron matrix in which carbon is in a solid
solution state is small, and the wear resistance of the sintered
alloy decreases. On the other hand, when the hardness of the
sintered alloy exceeds 600 Hv, the machinability of the sintered
alloy may deteriorate.
[0088] In addition, when the hardness of the second hard particles
is set to be within such a range, it is possible to improve the
wear resistance of the soft iron matrix. Here, when the hardness of
the second hard particles is less than 600 Hv, the wear resistance
of the sintered alloy may decrease.
[0089] The hardnesses of the first and second hard particles can be
adjusted by appropriately setting proportions of the components in
the above content range, the content of the graphite particles, the
sintering temperature and the sintering time. The sintering
temperature may be about 1050.degree. C. to 1250.degree. C., and
particularly, about 1100.degree. C. to 1150.degree. C. The
sintering time at the above sintering temperature may be 30 minutes
to 120 minutes, and more preferably 45 minutes to 90 minutes. The
sintering atmosphere may be a non-oxidizing atmosphere such as an
inert gas atmosphere. As the a non-oxidizing atmosphere, a nitrogen
gas atmosphere, an argon gas atmosphere, a vacuum atmosphere, and
the like may be used.
[0090] The matrix of the iron-based sintered alloy obtained by
sintering preferably includes a structure containing pearlite in
order to secure its hardness. The structure containing pearlite may
be a pearlite structure, a mixed pearlite-austenitic structure, or
a mixed pearlite-ferrite structure. In order to secure wear
resistance, ferrite with low hardness is preferably contained in a
small amount.
[0091] After the sintered product is prepared, an oxidation
treatment is performed on the sintered product so that a part of
iron contained in the iron matrix derived from the iron particles
become triiron tetraoxide (Fe.sub.3O.sub.4). The oxidation
treatment is performed so that a density difference between before
and after the oxidation treatment in the sintered product becomes
0.05 g/cm.sup.3 or more. In the oxidation treatment, oxides mainly
including triiron tetraoxide are generated. Therefore, the mass of
the sintered product after the oxidation treatment increases.
Therefore, a higher density difference indicates a larger amount of
triiron tetraoxide generated.
[0092] When a density difference between before and after the
oxidation treatment in the sintered product is set to 0.05
g/cm.sup.3 or more, it is possible to improve the wear resistance
of the sintered alloy. Here, when a density difference between
before and after the oxidation treatment in the sintered product is
less than 0.05 g/cm.sup.3, since a proportion of triiron tetraoxide
in the sintered alloy is small, adhesive wear is accelerated due to
metal contact with a counterpart member. As a result, the wear
resistance of the sintered alloy decreases.
[0093] In such an oxidation treatment, for example, under a water
vapor atmosphere, the sintered product is heated in temperature
conditions of 500.degree. C. to 600.degree. C. for 30 minutes to 90
minutes. Therefore, in the above range of the density difference,
iron (Fe) which is a matrix of the sintered product can be oxidized
to triiron tetraoxide (Fe.sub.3O.sub.4).
[0094] 7. Application of Wear-Resistant Iron-Based Sintered
Alloy
[0095] The sintered alloy obtained in the above production method
has a higher mechanical strength and wear resistance under a high
temperature usage environment than those in the related art. For
example, it can be suitably used for a valve system (for example, a
valve seat and a valve guide) and a wastegate valve of a
turbocharger of an internal combustion engine in which compressed
natural gas or liquefied petroleum gas is used as a fuel under a
high temperature usage environment.
[0096] For example, when a valve seat of an exhaust valve of an
internal combustion engine is made of a sintered alloy, even if a
wear pattern in which adhesive wear when the valve seat and the
valve come in contact with each other and abrasive wear when the
two slide over each other are combined develops, the wear
resistance of such a valve seat is improves compared with in the
related art. In particular, under a usage environment in which
compressed natural gas or liquefied petroleum gas is used as a
fuel, although a Mo oxide film is unlikely to be formed, in such an
environment, it is possible to reduce adhesive wear.
[0097] Examples in which the present disclosure is realized in
practice will be described below together with comparative
examples.
EXAMPLE 1
Optimal Amount of First Hard Particles Added
[0098] A sintered alloy according to Example 1 was produced
according to the following production method. As the first hard
particles, hard particles (commercially available from Daido Steel
Co., Ltd) produced from an alloy including Mo: 40 mass %, Ni: 30
mass %, Co: 20 mass %, Mn: 5 mass %, Si: 0.8 mass %, and C: 1.2
mass %, with the balance including Fe and inevitable impurities
(that is, Fe-40Mo-30Ni-20Co-5Mn-0.8Si-1.2C) using a gas atomizing
method were prepared. The first hard particles were classified into
a range of 44 .mu.m to 250 .mu.m using a sieve according to JIS
standard Z8801. Here, "granularity of particles" in this
specification is a value obtained by classification according to
this method.
[0099] As the second hard particles, second hard particles
(commercially available from Kinsay Matec Co., Ltd) produced from
an Fe-65 alloy including Mo: 65 mass %, with the balance including
Fe and inevitable impurities using a grinding method were prepared.
The second hard particles were classified into a range of 75 .mu.m
or less.
[0100] Next, a graphite powder (CPB-S commercially available from
Nippon Kokuen Group) including graphite particles and a reduced
iron powder (JIP255M-90 commercially available from JFE Steel
Corporation) including pure iron particles were prepared. The above
first hard particles, second hard particles, and graphite particles
in proportions of 40 mass %, 3 mass %, and 1.1 mass %,
respectively, with the remaining iron particles (specifically 55.9
mass %) were mixed together using a V type mixer for 30 minutes.
Thereby, a mixed powder was obtained.
[0101] Next, using a molding die, the obtained mixed powder was
compact-molded into a ring-shaped test piece at a pressurizing
force of 588 MPa to form a molded product for a sintered alloy
(compact-molded product). The compact-molded product was sintered
in an inert atmosphere (nitrogen gas atmosphere) at 1120.degree. C.
for 60 minutes to obtain a sintered product. The sintered product
was oxidized by heating under a water vapor atmosphere in heating
conditions of 550.degree. C. for 50 minutes. Thereby, a sintered
alloy (valve seat) test piece according to Example 1 was
formed.
EXAMPLES 2 and 3
Optimal Amount of First Hard Particles Added
[0102] In the same manner as in Example 1, sintered alloy test
pieces were prepared. Examples 2 and 3 were examples for evaluating
an optimal amount of first hard particles added. Examples 2 and 3
differed from Example 1 in that, as shown in Table 1, the first
hard particles were added at a proportion of 5 mass % and 50 mass
%, respectively, with respect to the entire mixed powder.
EXAMPLES 4 and 5
Optimal Amount of Second Hard Particles Added
[0103] In the same manner as in Example 1, sintered alloy test
pieces were prepared. Examples 4 and 5 were examples for evaluating
an optimal amount of second hard particles added. Examples 4 and 5
differed from Example 1 in that, as shown in Table 1, the second
hard particles were added at a proportion of 1 mass % and 5 mass %,
respectively, with respect to the entire mixed powder.
EXAMPLES 6 and 7
Optimal Amount of Graphite Particles Added
[0104] In the same manner as in Example 1, sintered alloy test
pieces were prepared. Examples 6 and 7 were examples for evaluating
an optimal amount of graphite particles added. Examples 6 and 7
differed from Example 2 in that, as shown in Table 1, the graphite
particles were added at a proportion of 0.5 mass % and 1.5 mass %,
respectively, with respect to the entire mixed powder.
EXAMPLE 8
Hardness of First Hard Particles
[0105] In the same manner as in Example 1, a sintered alloy test
piece was prepared. Example 8 differed from Example 1 in that the
sintering temperature was lower than in Example 1 and the
hardnesses of the first hard particles of the sintered product
after sintering was lowered (refer to Table 1, 545 Hv).
Comparative Examples 1 and 2
Comparative Examples for Optimal Amount of First Hard Particles
Added
[0106] In the same manner as in Example 1, sintered alloy test
pieces were prepared. Comparative Examples 1 and 2 were comparative
examples for evaluating an optimal amount of first hard particles
added. Comparative Examples 1 and 2 differed from Example 1 in
that, as shown in Table 1, the first hard particles were added at a
proportion of 0 mass % (that is, not added) and 60 mass %,
respectively, with respect to the entire mixed powder. Here, in
Comparative Example 2, it was not possible to mold a molded product
from the mixed powder.
Comparative Examples 3 and 4
Comparative Examples for Optimal Amount of Second Hard Particles
Added
[0107] In the same manner as in Example 1, sintered alloy test
pieces were prepared. Comparative Examples 3 and 4 were comparative
examples for evaluating an optimal amount of second hard particles
added. Comparative Examples 3 and 4 differed from Example 1 in
that, as shown in Table 1, the second hard particles were added at
a proportion of 0 mass % and 10 mass %, respectively, with respect
to the entire mixed powder. In addition, in Comparative Example 3,
the graphite particles were added at a proportion of 0.8 mass
%.
Comparative Examples 5 and 6
Comparative Examples for Optimal Amount of Graphite Particles
Added
[0108] In the same manner as in Example 1, sintered alloy test
pieces were prepared. Comparative Examples 5 and 6 were comparative
examples for evaluating an optimal amount of graphite particles
added. Comparative Examples 5 and 6 differed from
[0109] Example 1 in that, as shown in Table 1, the graphite
particles were added at a proportion of 0.4 mass % and 1.6 mass %,
respectively, with respect to the entire mixed powder.
Comparative Example 7
comparative Example for Density Difference in Sintered Product
[0110] In the same manner as in Example 1, a sintered alloy test
piece was prepared. In Comparative Example 7, a molding pressure
during compact-molding was higher than in Example 1, and the
density before the oxidation treatment was higher. Therefore, there
were less pores inside the sintered product, and thus generation of
oxides was prevented and an increase in density of the sintered
product after the oxidation treatment was reduced (that is, the
density difference was reduced).
Comparative Example 8
Comparative Example for Hardness of First Hard Particles
[0111] In the same manner as in Example 1, a sintered alloy test
pieces was prepared. Comparative Example 8 differed from Example 1
in that the sintering temperature was higher than in Example 1 and
the hardness of the first hard particles of the sintered product
after sintering was higher (refer to Table 1, 650 Hv).
Comparative Example 9
[0112] In the same manner as in Example 1, a sintered alloy test
piece was prepared. Comparative Example 9 differed from Example 1
in that particles including a Co-40Mo-5Cr-0.9C alloy corresponding
to the hard particles described in JP 2004-156101 A were used as
the first hard particles, no second hard particles were added, and
no oxidation treatment were performed on the sintered product after
sintering.
[0113] <Hardness Test>For the sintered alloy test pieces
according to Examples 1 to 8 and Comparative
[0114] Examples 1 to 9, the hardnesses of the first hard particles
and the second hard particles were measured using a micro Vickers
hardness testing machine at a measurement load of 0.1 kgf. The
results are shown in Table 1.
[0115] <Density Measurement Test>
[0116] Masses of the sintered alloy test pieces according to
Examples 1 to 8 and Comparative Examples 1, and 3 to 8 before and
after the oxidation treatment were measured. The measured mass was
divided by a volume calculated from the size of the test piece, and
densities of the test piece (sintered product) before and after the
oxidation treatment were calculated. In addition, a density
difference between before and after the oxidation treatment in the
test piece (sintered product) was calculated. The results are shown
in Table 1.
[0117] <Wear Test>
[0118] Using a testing machine in FIG. 1, a wear test was performed
on the sintered alloy test pieces according to Examples 1 to 8 and
Comparative Examples 1, and 3 to 9, and wear resistances thereof
were evaluated. In this test, as shown in FIG. 1, a propane gas
burner 10 was used as a heating source, and a sliding part between
a ring-shaped valve seat 12 made of the sintered alloy prepared as
above and a valve face 14 of a valve 13 was placed in a propane gas
combustion atmosphere. The valve face 14 was obtained by performing
nitrocarburizing according to EV12 (SEA standard). The temperature
of the valve seat 12 was controlled such that it was 250.degree.
C., a load of 25 kgf was applied by a spring 16 when the valve seat
12 was brought into contact with the valve face 14, the valve seat
12 was brought into contact with the valve face 14 at 3250
times/min, and the wear test was performed for 8 hours.
[0119] The total amount of a wear depth in the axial direction of
the valve seat 12 and the valve face 14 after the wear test was
measured as a wear test wear amount, and a value obtained by
dividing the wear test wear amount by the value in Comparative
Example 9 was calculated as a wear test wear amount proportion. The
results are shown in Table 1.
[0120] FIGS. 3A, 4A, 5A, 6A, and 7A show plotted results of
corresponding wear test wear amount proportions among Examples 1 to
8 and Comparative Examples 1, and 3 to 9 in which the horizontal
axis represents the amount of first hard particles added, the
amount of second hard particles added, the amount of graphite
particles added, the hardness of the first hard particles, and the
density difference in the sintered product in that order.
[0121] In addition, surfaces of the test pieces after the wear test
according to Example 1 and Comparative Example 7 after the wear
test were observed under a microscope. The results are shown in
FIG. 8A and FIG. 8B. FIG. 8A is a picture of the surface of the
test piece according to Example 1 after the wear test and FIG. 8B
is a picture of the surface of the test piece according to
Comparative Example 7 after the wear test.
[0122] The test pieces of Example 1, Comparative Example 5, and
Comparative Example 6 before the wear test were etched using nital,
and the structure of the sintered alloy was observed under a
microscope. The results are shown in FIG. 9A to FIG. 9C. FIG. 9A is
a picture of the structure of the test piece according to Example
1, FIG. 9B is a picture of the structure of the test piece
according to Comparative Example 5, and FIG. 9C is a picture of the
structure of the test piece according to Comparative Example 6.
[0123] <Machinability Test>
[0124] Using a testing machine shown in FIG. 2, a machinability
test was performed on the sintered alloy test pieces according to
Examples 1 to 8 and Comparative Examples 1, and 3 to 9, and the
machinability thereof was evaluated. In this test, six test pieces
20 with an outer diameter of 30 mm, an inner diameter of 22 mm, and
a total length of 9 mm were prepared for each of Examples 1 to 8
and Comparative Examples 1, and 3 to 9. Using an NC lathe, a test
piece 20 rotated at a rotational speed of 970 rpm was cut in a wet
type traverse using a titanium nitride aluminum-coated cemented
carbide tool (cutting tool) 30 with a cut depth of 0.3 mm, while
being fed at 0.08 mm/rev, and over a cutting distance of 320 m.
Then, a maximum wear depth of a flank of the tool 30 was measured
as a tool wear amount using an optical microscope and a value
obtained by dividing the tool wear amount by the value in
Comparative Example 9 was calculated as a tool wear amount
proportion. The results are shown in Table 1.
[0125] FIGS. 3B, 4B, 5B, 6B, and 7B show plotted results of
corresponding tool wear amount proportions among Examples 1, and 3
to 8 and Comparative Examples 1 to 9 in which the horizontal axis
represents the amount of first hard particles added, the amount of
second hard particles added, the amount of graphite particles
added, the hardness of the first hard particles, and the density
difference in the sintered product in that order.
TABLE-US-00001 TABLE 1 First hard Second hard Oxidation treatment
Graphite particles particles Before After particles Wear test
Amount Amount treatment treatment Density Amount wear Tool wear
Sample Hardness added Hardness added Density Density difference
added amount amount name (Hv) (mass %) (Hv) (mass %) (g/cm.sup.3)
(g/cm.sup.3) (g/cm.sup.3) (mass %) proportion proportion Example 1
560 40 1080 3 6.96 7.09 0.13 1.1 0.12 0.69 Example 2 568 5 940 3
7.07 7.13 0.06 1.1 0.33 0.59 Example 3 555 50 920 3 6.89 7.10 0.21
1.1 0.10 0.77 Example 4 570 40 1000 1 7.00 7.15 0.15 1.1 0.14 0.56
Example 5 585 40 1090 5 6.92 7.10 0.18 1.1 0.13 0.86 Example 6 566
40 1020 3 6.98 7.13 0.15 0.5 0.18 0.66 Example 7 562 40 960 3 7.00
7.08 0.08 1.5 0.13 0.84 Example 8 545 40 980 3 6.80 6.99 0.19 1.1
0.06 0.67 Comparative -- 0 1010 3 7.03 7.10 0.07 1.1 0.46 0.54
Example 1 Comparative 570 60 990 3 -- -- -- 1.1 Not Not Example 2
possible possible to mold to mold Comparative 560 40 -- 0 6.98 7.12
0.14 0.8 0.20 0.54 Example 3 Comparative 577 40 1080 10 6.76 7.00
0.24 1.1 0.10 0.97 Example 4 Comparative 560 40 1000 3 6.97 7.12
0.15 0.4 0.34 0.65 Example 5 Comparative 560 40 990 3 6.78 7.04
0.26 1.6 0.13 0.94 Example 6 Comparative 540 40 1100 3 7.16 7.20
0.04 1.1 0.42 0.70 Example 7 Comparative 650 40 990 3 6.96 7.12
0.16 1.1 0.13 0.90 Example 8 Comparative 880 40 None -- -- -- --
1.1 1.00 1.00 Example 9
[0126] (Result 1: Optimal Amount of First Hard Particles Added)
[0127] As shown in FIG. 3A, the wear test wear amount proportions
of Examples 1 to 3 were lower than in Comparative Examples 1 and 9.
The wear test wear amount proportions decreased in the order of
Example 2, Example 1, and Example 3. Therefore, when the first hard
particles were added, the abrasive wear resistance of the sintered
alloy was assumed to be improved. However, in Comparative Example
2, it can be said that moldability of the molded product
deteriorated because too much first hard particles were added.
Based on the above, the optimal amount of first hard particles
added was 5 mass % to 50 mass % with respect to the entire mixed
powder.
[0128] Here, as shown in FIG. 3B, the tool wear amount proportions
of Examples 1 to 3 were smaller than in Comparative Example 9. The
tool wear amount proportion increased in the order of Example 2,
Example 1, and Example 3. However, it is thought that, when more
first hard particles were added than in Example 3, the
machinability of the sintered alloy deteriorated and the tool wear
amount proportion increased.
[0129] (Result 2: Optimal Amount of Second Hard Particles
Added)
[0130] As shown in FIG. 4A, the wear test wear amount proportions
of Examples 1, 4, and 5 and Comparative Example 4 were lower than
in Comparative Examples 3 and 9. However, as shown in FIG. 4B, the
tool wear amount proportion of Comparative Example 4 was higher
than in Examples 1, 4, and 5. Here, when the surface of the test
piece after the wear test was observed, Comparative Example 3 had
more scratches due to adhesive wear than the other examples.
[0131] Therefore, it is thought that the second hard particles
improved the hardness of the sintered alloy after sintering,
prevented plastic deformation of the iron matrix of the sintered
alloy during use, and reduced adhesive wear of the sintered alloy.
Specifically, it is thought that, since the second hard particles
did not contain Ni, Co, and the like unlike the first hard
particles, the iron matrix around the second hard particles was
able to harden than in the first hard particles, molybdenum carbide
precipitated at the grain boundary of the iron matrix during
sintering, and thus the hardness of the iron matrix after sintering
was improved.
[0132] Based on the above, when an amount of second hard particles
added was too small, the surface of the sintered alloy after the
wear test was easily scraped off. On the other hand, it is thought
that, as in Comparative Example 4, when an amount of second hard
particles added was too large, the sintered alloy after sintering
was too hard, and machinability deteriorated. Based on the above
result, the optimal amount of second hard particles added was 1
mass % to 5 mass % with respect to the entire mixed powder.
[0133] (Result 3: Optimal Amount of Graphite Particles Added)
[0134] As shown in FIG. 5A, the wear test wear amount proportions
of Examples 1, 6, and 7 and Comparative Example 6 were lower than
in Comparative Examples 5 and 9. However, as shown in FIG. 5B, the
tool wear amount proportion of Comparative Example 6 was higher
than in Examples 1, 6, and 7.
[0135] As shown in FIG. 9A, in the structure of the sintered alloy
shown in Example 1, a pearlite structure was formed. However, as
shown in FIG. 9C, in the structure of the sintered alloy shown in
Comparative Example 6, a cementite structure was formed due to an
increased amount of graphite particles. It is thought that, as a
result, the tool wear amount proportion of Comparative Example 6
was higher than in Examples 1, 6, and 7.
[0136] On the other hand, it is thought that, as shown in FIG. 9B,
in the structure of the sintered alloy shown in Comparative Example
5, since the structure had ferrite as a major part thereof, the
wear test wear amount proportion of Comparative Example 5 was
higher than in Examples 1, 6, and 7 and Comparative Example 6.
Therefore, the optimal amount of graphite particles added at which
it was possible to secure a pearlite structure in the iron matrix
after sintering was 0.5 mass % to 1.5 mass % with respect to the
entire mixed powder.
[0137] (Result 4: Optimal Hardness of First Hard Particles)
[0138] As shown in FIG. 6A, the wear test wear amount proportions
of Examples 1, 3, 5, and 8 and Comparative Example 8 were lower
than in Comparative Example 9. However, as shown in FIG. 6B, the
tool wear amount proportion of Comparative Example 8 was higher
than in Examples 1, 3, 5, and 8.
[0139] It is thought that, since the hardness of the first hard
particles in Comparative Example 9 was higher than in Examples 1,
3, 5, and 8 and Comparative Example 8, a counterpart component wore
more, and the wear test wear amount proportion of Example 9 was
higher than in the other examples. On the other hand, it is thought
that, in Examples 1, 3, 5, and 8, since the hardness (Hv) of the
first hard particles was lower than in Comparative Example 8 and
was 600 or less, the tool wear amount proportions of Examples 1, 3,
5, and 8 were lower than in Comparative Example 8. Here, in
Examples 1, 3, 5, and 8, it can be said that, since first hard
particles with a hardness of 400 Hv or more were secured, wear
resistance was secured.
[0140] Therefore, the hardness of the first hard particles after
sintering is preferably in a range of 400 to 600 Hv. Here, in
consideration of the second hard particles that improved the wear
resistance of the iron matrix, in the condition of the above range
of the amount added, it was necessary for the hardness of the
second hard particles to be higher than the hardness of the first
hard particles and to exceed at least 600 Hv.
[0141] (Result 5: Optimal Density Difference in Sintered
Product)
[0142] As shown in FIG. 7A, the wear test wear amount proportions
of Examples 1 to 8 were lower than in Comparative Example 7 and 9.
As shown in FIG. 7B, the tool wear amount proportion of Comparative
Example 9 was higher than in Examples 1 to 8, and Comparative
Example 7.
[0143] In Comparative Example 7, since a density difference between
before and after the oxidation treatment in the sintered product
was less than 0.05 g/cm.sup.3, an amount of oxides mainly including
triiron tetraoxide in the sintered product was smaller than in the
sintered products of Examples 1 to 8. Therefore, metal contact with
a counterpart component was promoted, and as shown in FIG. 8B, in
the test piece (sintered product) of Comparative Example 7, and it
is thought that adhesive wear with a counterpart component
accelerated. On the other hand, in Examples 1 to 8, the wear
resistance of the sintered alloy is thought to have been higher
than in Comparative Example 7 since there was almost no such
adhesive wear, (for example, refer to Example 1, FIG. 8A).
Therefore, it was necessary to perform an oxidation treatment so
that a density difference between before and after the oxidation
treatment in the sintered product became 0.05 g/cm.sup.3 or
more.
EXAMPLE 9
Optimal Particle Size of Second Hard Particles
[0144] In the same manner as in Example 1, a sintered alloy test
piece was prepared. Example 9 was an example for evaluating an
optimal particle size of the second hard particles. Example 9
differed from Example 1 in that, as the second hard particles,
second hard particles classified to have a particle size
(granularity) in a range of greater than 75 .mu.m and 100 .mu.m or
less were used.
Comparative Example 10
Comparative Example for Optimal Particle Size of Second Hard
Particles
[0145] In the same manner as in Example 1, a sintered alloy test
piece was prepared. Comparative Example 10 is a comparative example
for evaluating an optimal particle size of the second hard
particles. Comparative Example 10 differed from Example 1 in that,
as the second hard particles, second hard particles classified to
have a particle size in a range of greater than 100 .mu.m and 150
.mu.m or less were used. Here, the test piece according to
Comparative Example 10 was a sintered alloy included in the scope
of the present disclosure and was set as Comparative Example 10 for
convenience to allow comparison with Examples 1 and 9.
[0146] In the same manner as in Example 1, the wear test and the
machinability test were performed on the test pieces of Example 9
and Comparative Example 10, and a wear test wear amount and a tool
wear amount were measured. The results are shown in FIG. 10A and
FIG. 10B together with the above results of Example 1.
[0147] FIG. 10A is a graph showing results of the wear test wear
amount proportions in Examples 1 and 9 and Comparative Example 10
and FIG. 10B is a graph showing results of the tool wear amount
proportions in Examples 1 and 9 and Comparative Example 10.
[0148] (Result 6: Optimal Particle Size of Second Hard
Particles)
[0149] As shown in FIG. 10A, the wear test wear amount proportions
of Examples 1 and 9 and Comparative Example 10 were similar.
However, as shown in FIG. 10B, the tool wear amount proportions of
Examples 1 and 9 were lower than in Comparative Example 10, and the
tool wear amount proportion of Example 1 was the lowest among the
examples. This is because, in Comparative Example 10, since the
particle size of the second hard particles was too large, the
machinability of the test piece (sintered product) deteriorated in
some cases. Based on this result, the particle size (maximum
particle size) of the second hard particles was preferably in a
range of 100 .mu.m or less, and more preferably, the particle size
(maximum particle size) of the second hard particles was in a range
of 75 .mu.m or less.
[0150] While embodiments of the present disclosure have been
described above in detail, the present disclosure is not limited to
these embodiments, and various design modifications can be
made.
* * * * *