U.S. patent application number 12/307662 was filed with the patent office on 2009-12-17 for powder forged member, powder mixture for powder forging, method for producing powder forged member, and fracture split type connecting rod using the same.
This patent application is currently assigned to HONDA MOTOR CO., LTD.. Invention is credited to Zenji Iida, Ryosuke Kogure, Masaaki Sato, Kentaro Takada, Minoru Takada.
Application Number | 20090311122 12/307662 |
Document ID | / |
Family ID | 38894552 |
Filed Date | 2009-12-17 |
United States Patent
Application |
20090311122 |
Kind Code |
A1 |
Sato; Masaaki ; et
al. |
December 17, 2009 |
POWDER FORGED MEMBER, POWDER MIXTURE FOR POWDER FORGING, METHOD FOR
PRODUCING POWDER FORGED MEMBER, AND FRACTURE SPLIT TYPE CONNECTING
ROD USING THE SAME
Abstract
A member produced by powder forging which retains machinability
and improved fatigue strength without having an increased hardness
and can retain self conformability after fracture splitting; a
powder mixture for powder forging; a process for producing a member
by powder forging; and a fracture splitting connecting rod obtained
from the member produced by powder forging. The member produced by
powder forging is one obtained by preforming a powder mixture,
subsequently sintering the preform, and forging the resultant
sintered preform at a high temperature. The free-copper proportion
in the sintered preform at the time when the forging is started is
10% or lower, and the member obtained through the forging has a
composition containing, in terms of mass %, 0.2-0.4% C, 3-5% Cu,
and up to 0.4% Mn (excluding 0), the remainder being iron and
incidental impurities, and has a ferrite content of 40-90%.
Inventors: |
Sato; Masaaki; (Hyogo,
JP) ; Takada; Minoru; (Tokyo, JP) ; Takada;
Kentaro; (Saitama, JP) ; Iida; Zenji;
(Saitama, JP) ; Kogure; Ryosuke; (Saitama,
JP) |
Correspondence
Address: |
WESTERMAN, HATTORI, DANIELS & ADRIAN, LLP
1250 CONNECTICUT AVENUE, NW, SUITE 700
WASHINGTON
DC
20036
US
|
Assignee: |
HONDA MOTOR CO., LTD.
Minato-ku, Tokyo
JP
|
Family ID: |
38894552 |
Appl. No.: |
12/307662 |
Filed: |
July 4, 2007 |
PCT Filed: |
July 4, 2007 |
PCT NO: |
PCT/JP2007/063377 |
371 Date: |
July 2, 2009 |
Current U.S.
Class: |
419/10 ; 419/28;
75/243; 75/245 |
Current CPC
Class: |
F16C 9/045 20130101;
Y10T 74/2142 20150115; F16C 7/023 20130101; B22F 2998/10 20130101;
C22C 33/0264 20130101; B21J 5/002 20130101; B21K 1/766 20130101;
B22F 2998/10 20130101; B22F 3/17 20130101; C22C 38/16 20130101;
C22C 38/04 20130101; F16C 7/02 20130101; B22F 3/10 20130101; B22F
3/17 20130101 |
Class at
Publication: |
419/10 ; 75/245;
75/243; 419/28 |
International
Class: |
B22F 3/12 20060101
B22F003/12; C22C 38/16 20060101 C22C038/16; B22F 3/24 20060101
B22F003/24 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 6, 2006 |
JP |
2006-186927 |
Claims
1. A powder forged member having excellent machinability and
fatigue strength, the powder forged member obtained by forging a
sintered preform at a high temperature, the sintered preform formed
by subjecting a powder mixture to preliminary compacting and
thereafter sintering the subjected compacted preform, the sintered
preform having a ratio of free Cu of 10% or less upon the start of
the forging, the component composition of the powder forged member
after the forging composed of, C: 0.2 to 0.4% by mass, Cu: 3 to 5%
by mass, Mn: 0.5% by mass or less (excluding 0), and the balance
iron with inevitable impurities, and the powder forged member
having a ferrite ratio of 40 to 90%.
2. The powder forged member having excellent machinability and
fatigue strength according to claim 1, wherein a relative density
to theoretical density is 97% or more.
3. The powder forged member having excellent machinability and
fatigue strength according to claim 2, wherein a hardness is HRC 33
or less, and a partial pulsating tensile fatigue limit is 325 MPa
or more.
4. The powder forged member having excellent machinability and
fatigue strength according to any one of claims 1 to 3, wherein the
powder forged member contains at least one machinability-improving
material in a total amount of 0.05 to 0.6% by mass, the
machinability-improving material selected from the group consisting
of MnS, MoS.sub.2, B.sub.2O.sub.3 and BN.
5. A fracture split type connecting rod produced by using the
powder forged member according to claim 1.
6. A powder mixture used as a raw material for a powder forged
member according to any one of claims 1 to 3, wherein a component
composition except a lubricant is composed of, C: 0.1 to 0.5% by
mass, Cu: 3 to 5% by mass, Mn: 0.4% by mass or less (excluding 0),
O: 0.3% by mass or less and the balance iron with inevitable
impurities.
7. The powder mixture for powder forging according to claim 6,
wherein the powder mixture is obtained by adding a graphite powder,
a copper powder and a lubricant into an iron-based powder composed
of, C: less than 0.05% by mass, O: 0.3% by mass or less and the
balance iron with inevitable impurities.
8. A powder mixture for powder forging used as a raw material for a
powder forged member according to claim 4, wherein a component
composition except a lubricant contains, C: 0.1 to 0.5% by mass,
Cu: 3 to 5% by mass, Mn: 0.4% by mass or less (excluding 0), O:
0.3% by mass or less, and at least one machinability-improving
material in a total amount of 0.05 to 0.6% by mass, and the balance
iron with inevitable impurities, the machinability-improving
material selected from the group consisting of MnS, MoS.sub.2 and
B.sub.2O.sub.3 and BN.
9. The powder mixture for powder forging according to claim 8,
wherein the powder mixture is obtained by adding a graphite powder,
a copper powder, at least one machinability-improving material
selected from the group consisting of MnS, MoS.sub.2,
B.sub.2O.sub.3 and BN, and a lubricant into an iron-based powder
composed of, C: less than 0.05%% by mass or less, O: 0.3%% by mass
or less or less and the balance iron with inevitable
impurities.
10. A method for producing a powder forged member having excellent
machinability and fatigue strength, the method comprising: a
compacting and sintering step of subjecting a powder mixture for
powder forging according to claim 6 to preliminary compacting and
thereafter sintering the subjected compacted preform to form a
sintered perform; and a forging step of forging the sintered
preform at a high temperature to form a powder forged member.
11. A method for producing a powder forged member having excellent
machinability and fatigue strength, the method comprising: a
compacting and sintering step of subjecting a powder mixture for
powder forging according to claim 7 to preliminary compacting and
thereafter sintering the subjected compacted preform to form a
sintered perform; and a forging step of forging the sintered
preform at a high temperature to form a powder forged member,
wherein the powder forged member contains at least one
machinability-improving material in a total amount of 0.05 to 0.6%
by mass, the machinability-improving material selected from the
group consisting of MnS, MoS.sub.2, B.sub.2O.sub.3 and BN.
12. A method for producing a powder forged member having excellent
machinability and fatigue strength, the method comprising: a
compacting and sintering step of subjecting a powder mixture for
powder forging according to claim 7 to preliminary compacting and
thereafter sintering the subjected compacted preform to form a
sintered perform; and a forging step of forging the sintered
preform at a high temperature to form a powder forged member.
13. A method for producing a powder forged member having excellent
machinability and fatigue strength, the method comprising: a
compacting and sintering step of subjecting a powder mixture for
powder forging according to claim 8 to preliminary compacting and
thereafter sintering the subjected compacted preform to form a
sintered perform; and a forging step of forging the sintered
preform at a high temperature to form a powder forged member.
Description
TECHNICAL FIELD
[0001] The present invention relates to a powder forged member
obtained by subjecting a powder mixture to preliminary compacting,
then sintering the subjected compacted preform, and thereafter
forging the obtained sintered preform, a powder mixture for powder
forging, a method for producing the powder forged member, and a
fracture split type connecting rod produced using the powder forged
member.
BACKGROUND ART
[0002] Conventionally, there has been widely carried out a powder
forging method for subjecting a powder mixture to preliminary
compacting, then sintering the subjected compacted preform, and
thereafter forging the obtained sintered preform to produce machine
parts. Examples of typical machine parts produced by the powder
forging method include a connecting rod and a bearing race.
Typically, the component composition of these machine parts using a
pure iron-based powder contains C: 0.45 to 0.65% by mass
(hereinafter, "% by mass" is merely represented as "%"), and Cu:
1.5 to 2% from the relationship of machinability and fatigue
strength of products on machining after forging, and the like. A
method for increasing the content of C or a method for increasing
both the contents of C and Cu is generally required for weight
saving or increase of fatigue strength of these machine parts.
Although the fatigue strength of the part is increased in the
methods for increasing the content of C, the hardness is also
increased. This causes a problem that the service life of a tool on
machining after forging is remarkably reduced to unfortunately
increase the product cost. In addition, there is a disadvantage
that the increased content of Cu causes the generation of cracks on
forging easily.
[0003] A method for adding a reheating process and a cooling
process after a forging process (see Patent Document 1), and a
method for adding other alloy elements such as Ni and Mo (see
Patent Document 2) are disclosed as another method for increasing
the fatigue strength of the machine part. However, the former
method causes the increase of processes and the latter method uses
expensive alloys, increasing the cost of the part and increasing
the hardness of the part as in the method for increasing the
content of C. This causes a disadvantage that the machinability is
reduced.
[0004] The above conventional methods decrease the toughness of the
part with the increase of the hardness, causing the fracture
surface to tend to become flat. When the part is produced using a
fracture dividing method adopted in the connecting rod or the like,
there is caused a particular problem of easily generating the
positional shift of the part on assembling the part (i.e., reducing
self-consistency).
[0005] Patent Document 1: Japanese Unexamined Patent Publication
No. 61-117203
[0006] Patent Document 2: Japanese Unexamined Patent Publication
No. 60-169501
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0007] It is an object of the present invention to provide a powder
forged member in which fatigue strength is improved while securing
its machinability without increasing its hardness, and
self-consistency after fracture split can be secured, a method for
producing the same, and a fracture split type connecting rod using
the powder forged member.
Means for Solving the Problems
[0008] In accordance with a first aspect of the present invention,
a powder forged member has excellent machinability and fatigue
strength, the powder forged member obtained by forging a sintered
preform at a high temperature, the sintered preform formed by
subjecting a powder mixture to preliminary compacting and
thereafter sintering the subjected compacted preform, the sintered
preform having a ratio of free Cu of 10% or less upon the start of
the forging, the component composition of the powder forged member
after the forging composed of, C: 0.2 to 0.4% by mass, Cu: 3 to 5%
by mass and Mn: 0.5% by mass or less (excluding 0), and the balance
iron with inevitable impurities, and the powder forged member
having a ferrite ratio of 40 to 90%.
[0009] In the powder forged member, a relative density to
theoretical density is preferably 97% or more.
[0010] In the powder forged member, it is preferable that a
hardness is HRC 33 or less, and a partial pulsating tensile fatigue
limit is 325 MPa or more.
[0011] It is preferable that the powder forged member contains at
least one machinability-improving material in a total amount of
0.05 to 0.6% by mass, the machinability-improving material selected
from the group consisting of MnS, MoS.sub.2, B.sub.2O.sub.3 and
BN.
[0012] In accordance with a second aspect of the present invention,
a fracture split type connecting rod is produced by using the
powder forged member of the first aspect.
[0013] In accordance with a third aspect of the present invention,
a powder mixture is used as a raw material for the powder forged
member of the first aspect, wherein a component composition except
a lubricant is composed of, C: 0.1 to 0.5% by mass, Cu: 3 to 5% by
mass, Mn: 0.4% by mass or less (excluding 0), O: 0.3% by mass or
less and the balance iron with inevitable impurities.
[0014] It is preferable that the powder mixture for powder forging
is obtained by adding a graphite powder, a copper powder and a
lubricant into an iron-based powder composed of, C: less than 0.05%
by mass, O: 0.3% by mass or less and the balance iron with
inevitable impurities.
[0015] In accordance with a fourth aspect of the present invention,
a powder mixture is used as a raw material for the powder forged
member of the first aspect, wherein a component composition except
a lubricant contains, C: 0.1 to 0.5% by mass, Cu: 3 to 5% by mass,
Mn: 0.4% by mass or less (excluding 0), O: 0.3% by mass or less,
and also at least one machinability-improving material in a total
amount of 0.05 to 0.6% by mass, and the balance iron with
inevitable impurities, the machinability-improving material
selected from the group consisting of MnS, MoS.sub.2,
B.sub.2O.sub.3 and BN.
[0016] It is preferable that the powder mixture for powder forging
is obtained by adding a graphite powder, a copper powder, at least
one machinability-improving material selected from the group
consisting of MnS, MoS.sub.2, B.sub.2O.sub.3 and BN, and a
lubricant into an iron-based powder composed of, C: less than 0.05%
by mass, O: 0.3% by mass or less and the balance iron with
inevitable impurities.
[0017] In accordance with a fifth aspect of the present invention,
a method for producing the powder forged member having excellent
machinability and fatigue strength of the first aspect, the method
includes: a compacting and sintering step of subjecting the powder
mixture for powder forging of the third aspect to preliminary
compacting and thereafter sintering the subjected compacted preform
to form a sintered perform; and a forging step of forging the
sintered preform at a high temperature to form a powder forged
member.
[0018] In accordance with a sixth aspect of the present invention,
a method for producing the powder forged member having excellent
machinability and fatigue strength of the first aspect includes: a
compacting and sintering step of subjecting the powder mixture for
powder forging of the fourth aspect to preliminary compacting and
thereafter sintering the subjected compacted preform to form a
sintered perform; and a forging step of forging the sintered
preform at a high temperature to form a powder forged member.
EFFECT OF THE INVENTION
[0019] The present invention increases the content of Cu as
compared with that of the conventional one instead of decreasing
the content of C of the powder forged member contrary to the
conventional one, and limits the ratio of free Cu in the sintered
preform upon the start of the forging. Thereby, since soft ferrite
is increased by the reduction of the content of C to suppress the
increase of hardness, the machinability can be secured and the
toughness can be maintained to ensure self-consistency after
fracture split. Furthermore, since the amount of diffusion of Cu
into ferrite is increased by the increase of the content of Cu and
the limit of the ratio of free Cu to promote solid solution
strengthening, the fatigue strength is also drastically improved.
The cracks of the powder forged member on forging can be prevented
by limiting the ratio of free Cu.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1(a) is a perspective view showing the shape and size
of a test piece of a powder forged member used for fatigue test of
Example, and FIG. 1(b) is a sectional view showing a section taken
along line A-A.
[0021] FIG. 2 is a sectional view showing an applied state of a
tensile load to a test piece of a powder forgedmember in fatigue
test.
[0022] FIG. 3 is a graph showing the relationship between ratio of
free Cu and fatigue limit.
[0023] FIG. 4 is a sectional view showing the microstructure of a
powder forged member.
BEST MODE FOR CARRYING OUT THE INVENTION
[0024] Hereinafter, the present invention will be described in
further details.
[Composition of Powder Forged Member]
[0025] First, the reason of limiting the composition of a powder
forged member according to the present invention, that is, a
component composition, structure, density and a ratio of free Cu in
a sintered preform will be described.
C: 0.2 to 0.4%
[0026] C is an indispensable element for ensuring the strength of a
base steel. Conventionally, the hardness and strength of the base
steel have been increased by increasing the content of C to
decrease ferrite and increase perlite in the structure of the base
steel. On the contrary, in the present invention, the content of C
is conversely decreased to 0.4% or less in order to suppress the
increase of the hardness of the base steel. However, since the
strength of the base steel cannot be sufficiently ensured even if
the content of Cu is increased when the content of C is excessively
decreased, the content of C is set to 0.2% or more. Therefore, the
content of C is set to 0.2 to 0.4%.
Cu: 3 to 5%
[0027] Cu is an element which is dissolved in a ferrite phase in
the structure of a base steel on heating for sintering and forging
to form a solid solution to exhibit solid solution strengthening
effect, and is partly precipitated on cooling to enhance the
strength of the base steel. In the conventional product, Cu is
almost used in an amount of about 2% of solid solution limit in the
ferrite phase near the eutectoid temperature of Fe--C system. On
the other hand, the solid solution limit of Cu in an austenite
phase is about 8%. Cu of 3% or more can be dissolved sufficiently
in the base steel to form a solid solution by increasing a heating
temperature as compared with that of the conventional product
and/or extending heating time. In the present invention, a larger
amount of Cu than that of the conventional product is dissolved in
the austenite phase to strengthen the solid solution of the ferrite
phase generated in a cooling process. The content of Cu of less
than 3.0% cannot exhibit the aimed solid solution strengthening
effect sufficiently. On the other hand, the content of Cu exceeding
5.0% causes the free Cu to remain easily. The extension of heating
time such as the extension of sintering time is required to limit
the ratio of free Cu to 10% or less, and consequently the
productivity is reduced. Therefore, the content of Cu is set to 3
to 5%, and preferably 3 to 4%.
Mn: 0.5% or Less (Excluding 0)
[0028] Mn is an element which has the deoxidizing effect of the
base steel and useful to increase hardenability and enhance the
strength of the base steel. However, Mn has a high affinity to
oxygen, and reacts with oxygen in atmosphere in a powder producing
process or in a sintering process of a product subjected to
preliminary compacting to easily produce an oxide. The content of
Mn exceeding 0.5% makes it difficult to reduce a Mn oxide and
remarkably reduce the quality characteristics of the powder forged
member such as the reduction of density and strength caused by the
Mn oxide. Therefore, the content of Mn is set to 0.5% or less
(excluding 0), and preferably 0.4% or less (excluding 0).
Balance: Iron and Inevitable Impurities
[0029] The powder forged member of the present invention may
contain P, S, Si, O, N and other elements as inevitable
impurities.
Ratio of Free Cu: 10% or Less
[0030] As described above, Cu nearly two times that of the
conventional product is used to strengthen the solid solution of
the ferrite phase, and non-dissolved Cu (i.e., free Cu) easily
remains in the base steel. Therefore, forging cracks may be
generated by hot brittleness on forging. In a severe case, the
possibility of the damage of the sintered preform is increased on
handling between a forming sintering process and a forging process.
Therefore, in the present invention, the ratio of free Cu in the
sintered preform upon the start of the forging is set to 10% or
less. Here, the ratio of free Cu, which means the ratio of
non-dissolved Cu in the base steel, of the total amount of Cu
added, can be quantitated by the following method. That is, the
section of the sintered preform as a member to be measured is
ground by paper and a buff, and is then etched by picric acid.
Three positions having a range of 0.2 mm.times.0.3 mm are
photographed by 400 magnifications using an optical microscope, and
the total area of portions of copper color is measured by image
processing. On the other hand, the total area of portions of copper
color of a reference material is measured by the same method. As
the reference material, there is used a product obtained by
sintering a compacted product compacted in the same component
compositions, shape and forming pressure as those of the member to
be measured under the condition of 1000.degree. C. for 20 minutes
where Cu is not dissolved substantially in the base steel. The
ratio of free Cu may be calculated using the following formula:
Ratio of free Cu (%)=[total area of portions of Cu color of member
to be measured]/[total area of portions of Cu color of reference
material].times.100.
Ferrite Ratio: 40 to 90%
[0031] When the powder forged member has a ferrite ratio of less
than 40%, the powder forged member has deficient toughness and
insufficient self-consistency after fracture split. On the other
hand, when the powder forged member has a ferrite ratio exceeding
90%, the powder forged member has excessively high toughness and
large elongation, causing deformation on fracture split to
deteriorate dimensional accuracy. Therefore, the ferrite ratio of
the powder forged member is set to 40 to 90%.
Relative Density to Theoretical Density: 97% or More
[0032] When the relative density to the theoretical density is less
than 97%, the degree of reduction in the fatigue strength of the
powder forged member becomes large. Therefore, the relative density
to the theoretical density of the powder forged member is
preferably 97% or more. When the relative density is set to 97% or
more, the hardness of the powder forged member becomes HRC 33 or
less and the partial pulsating tensile fatigue limit becomes 325
MPa or more. Therefore, there is provided a powder forged member
having secured machinability and excellent fatigue strength.
Machinability-Improving Material: Total Amount of 0.05 to 0.6%
[0033] A machinability-improving material may be added on
preliminary compacting (i.e., to a powder mixture for powder
forging) to improve the machinability of the powder forged member.
As the machinability-improving material, for example, a powder
composed of MnS, MoS.sub.2, B.sub.2O.sub.3 or BN may be used. They
may be used either singly or in the form of a combination of two or
more members. When the amount of the machinability-improving
material to be added is less than 0.05% in the total amount, the
machinability-improving effect is not sufficiently obtained. On the
other hand, when the amount of the machinability-improving material
to be added exceeds 0.6%, an area occupied by an iron material is
reduced, and nonmetal as the starting point of fatigue cracks is
increased, showing a tendency of reduction in the fatigue strength.
Therefore, the total amount of the machinability-improving material
to be added is preferably 0.05 to 0.6% in the total amount.
[Component Composition of Powder Mixture for Powder Forging]
[0034] Next, the reason of limiting the component composition of
the powder mixture for powder forging (hereinafter, merely referred
to as a "powder mixture") will be described.
C: 0.1 to 0.5%
[0035] It is necessary to adjust the content of C of the powder
mixture in consideration of the amount of oxygen in the powder
mixture and the kind of atmosphere gas on sintering so that the
content of C of the powder forged member finally obtained is set to
0.2 to 0.4%. That is, when inactive gas atmosphere such as N.sub.2
gas is used in the sintering process, C is oxidized and consumed by
oxygen in the powder mixture and impurities oxygen in atmosphere
gas. The content of C of the sintered preform (i.e., the powder
forged member) is lower than that of the powder mixture. Thereby,
the content of C of the powder mixture is adjusted to more than
0.2% and 0.5% or less which is higher than that of the powder
forged member. On the other hand, when atmosphere gas having high
carbon potential such as endothermic gas is used, carburization
caused by atmosphere gas usually advances to more than the amount
of oxidation consumption of C by oxygen in the powder mixture, and
the content of C of the sintered preform (i.e., the powder forged
member) becomes higher than that of the powder mixture. Thereby,
the content of C of the powder mixture is adjusted to 0.1% or more
and less than 0.4% which is lower than that of the powder forged
member. Therefore, the content of C of the powder mixture may be
set in the range of 0.1 to 0.5% while the change in the content of
C is predicted in accordance with the content of oxygen of the
powder mixture and the kind of sintering atmosphere gas.
O: 0.3% or Less
[0036] The variation of the consumed C amount is also larger when
the content of oxygen of the powder mixture is higher, and it
becomes difficult to set the content of C of the powder forged
member to the target of 0.2 to 0.4%. Thereby, the content of oxygen
of the powder mixture is set to 0.3% or less.
Other Components
[0037] Cu, Mn and the machinability-improving material are not
consumed and produced on sintering as in C. The content of each of
the components in the powder mixture is defined as the same as the
content of each of the components in the powder forged member
(although the value of the content of each of the components is
extremely slightly changed by the increase and decrease of the
amount of C on sintering in a precise sense, the value is within an
ignorable range).
[Method for Producing Powder Forged Member]
[0038] Next, a method for producing the powder forged member
satisfying the above composition will be described.
[0039] First, the change of the content of C on sintering is
predicted in accordance with the content of oxygen in an iron-based
powder and the kind of sintering atmosphere gas. A graphite powder
in which the content of C of the powder mixture is in the range of
0.1 to 0.5% so that the content of C after sintering is set to 0.2
to 0.4%, a copper powder in which the content of Cu is 3 to 5%, and
the machinability-improving material of the total amount of 0.05 to
0.6% if necessary are added into an iron-based powder. A proper
amount of a lubricant is further added thereto to produce a powder
mixture. This powder mixture is subjected to preliminary compacting
by a pressure compacting machine to produce a compacted
preform.
[0040] When the iron-based powder used in producing the powder
mixture is less compressibility, the density of the compacted
preform on preliminary compacting is hardly increased. The inside
of the sintered preform is oxidized during high temperature
conveyance to the forging process after sintering, and a phenomenon
in which the strength of the sintered preform is reduced by an
oxide film occurs even if the sintered preform is forged.
Therefore, in order to soften the iron-based powder and increase
the density of the compacted preform to prevent the internal
oxidation of the compacted preform, the content of C of the
iron-based powder is set to be less than 0.05%, preferably 0.04% or
less, and more preferably 0.02% or less.
[0041] Then, this compacted preform is sintered at a high
temperature to produce a sintered preform. Here, referring to
sintering condition, higher temperature and longer time are
preferable because the diffusion of Cu advances and the amount of
free Cu decreases as the temperature is higher or as time is
longer. However, when the content of Cu is, for example, 4%, the
ratio of free Cu can be set to 10% or less by sintering the preform
at 1190.degree. C. or more for 10 minutes.
[0042] This sintered preform is immediately forged with a
predetermined forging pressure at a high temperature without
cooling the sintered preform to obtain a powder forged member.
Higher forging pressure is preferable because the density of the
powder forged member becomes higher and the strength is increased
as the forging pressure is higher. However, when a connecting rod
having a shape and size as shown in, for example, FIG. 1 is formed,
the relative density to the theoretical density can be set to 97%
or more by forging the preform with a pressure of 6.0 ton/cm.sup.2
or more, resulting in the powder forged member having excellent
machinability and fatigue strength.
[0043] Although the example immediately forging the preform using
the temperature after sintering is described in the producing
method, the preform may be once cooled after being sintered, and
reheated to be forged. In this case, the preform is heated twice on
sintering and forging and the heating time becomes longer
inevitably. Thereby, even when the heating temperature is a
temperature (about 1050.degree. C. to about 1120.degree. C.)
further lower than the lower limit temperature (1190.degree. C.),
the ratio of free Cu can be set to 10% or less.
[0044] A fracture split type connecting rod produced using this
powder forged member has reduced tool abrasion on machining, and
suppress the increase in cost of parts, and has excellent fatigue
strength and self-consistency on assembling after fracture
split.
Example 1
Influence of Ratio of Free Cu
[0045] A graphite powder and a copper powder were added into a pure
iron-based powder having a component composition shown in Table 1
so that the contents of C and Cu after being sintered were
respectively 0.3% and 4%. Zinc stearate of 0.75% as a lubricant was
further added thereto, and they were mixed for 30 minutes to
produce a powder mixture. The powder mixture was subjected to
preliminary compacting with a compacting surface pressure of 6
ton/cm.sup.2 to produce a compacted preform.
TABLE-US-00001 TABLE 1 Components C Mn P S Si O N Content (mass %)
0.001 0.19 0.01 0.009 0.01 0.12 0.004
[0046] This compacted preform was dewaxed at 600.degree. C. for 10
minutes under N.sub.2 gas atmosphere, and was then sintered at
various temperatures of 1110 to 1260.degree. C. for 10 minutes to
produce a plurality of sintered preforms. The ratio of free Cu of
each of some sintered preforms was measured by using the method
described in the above [Composition of Powder Forged Member] The
remaining sintered preforms were immediately forged with a forging
pressure of 10 ton/cm.sup.2 to produce test pieces of powder forged
members imitating the shape of a connecting rod. Burr of each of
the test pieces was removed, and the surface scale was removed by
shot or the like to provide the test pieces to a pulsating tensile
fatigue test. FIG. 1 shows the shape and size of each of the test
pieces used for the fatigue test. FIG. 2 shows an applied state of
a tensile load to each of the test pieces in the fatigue test.
[0047] Table 2 and FIG. 3 show measurement and test results. As is
apparent from Table 2 and FIG. 3, as the sintering temperature is
higher, the ratio of free Cu decreases and the fatigue limit
increases. When the sintering time is 10 minutes, the ratio of free
Cu is 10% or less at the temperature of 1190.degree. C. or more,
and the fatigue limit of 325 MPa or more is obtained. FIG. 4 shows
comparatively the cross-sectional microstructures of a reference
material having a ratio of free Cu of 100%, a comparative material
having the ratio of 15% and an inventive material of 3%. In FIG. 4,
portions to which net hatching is applied have existing free
Cu.
TABLE-US-00002 TABLE 2 Test Sintering Ratio of Fatigue pieces
temperature free Cu limit No. (.degree. C.) (%) (MPa) Note 101 1110
82 245 Comparative 102 1140 56 275 example 103 1170 43 294 104 1180
19 324 105 1190 9.8 353 Inventive 106 1200 4.6 353 example 107 1230
2.1 363 108 1260 1.4 373
[0048] In Inventive Example, the ferrite ratio of the powder forged
member was about 70% at any sintering temperature.
Example 2
Influence of Contents of C and Cu
[0049] A graphite powder and a copper powder were added into a pure
iron-based powder having the same component composition as that of
Example 1 shown in Table 1 with the addition amounts of the
graphite powder and copper powder variously changed so that the
content of C and Cu after being forged were respectively 0.1 to
0.6% and 2 to 5% to produce a powder mixture. The powder mixture
was subjected to preliminary compacting in the same condition as
that of Example 1 described above to form a compacted preform. This
compacted preform was dewaxed at 600.degree. C. for 10 minutes
under N.sub.2 gas atmosphere, and was then sintered at 1120.degree.
C. for 30 minutes under N.sub.2 gas atmosphere to produce sintered
preforms. The sintered preforms were heated at 1050.degree. C. for
30 minutes under N.sub.2 gas atmosphere, and was then forged with a
forging pressure of 10 ton/cm.sup.2 to produce test pieces of
powder forged members imitating the shape of the same connecting
rod as that of Example 1 described above. These test pieces were
subjected to a tensile fatigue test in the same condition as that
of Example 1 described above, and the HRC hardness of each of the
surfaces of the test pieces after being machined was measured.
[0050] Furthermore, the following test was performed in order to
quantify self-consistency after fracture split. That is, a
disk-shaped test piece of a powder forged member having a diameter
of 90 mm.times.a thickness of 40 mm was produced in the same
condition as in the above description. This was machined to produce
a ring-shaped test piece having an outer diameter of 80 mm, an
inner diameter of 40 mm.times.a thickness of 20 mm and having a V
notch having a depth of 1 mm and an angle of 45 degrees on an inner
ring diagonal line. This test piece was subjected to tensile
fracture in the depth direction and right-angled direction of the
notch. A real area including micro unevenness of the fracture
surface was measured by using an optical three-dimensional
measurement device (produced by GFMesstechnik Company, type:
MicroCAD 3.times.4), and a ratio relative to a flat project area
ignoring the unevenness (referred to as a "fracture split area
ratio") was calculated. Furthermore, the presence or absence of the
shift of the engaged position of the fracture surface after
fracture split was visually investigated.
[0051] Table 3 shows test results. The ratio of free Cu of each of
the test pieces before being forged (upon the start of the forging)
exceeded 10% in test piece No. 222 having the content of Cu
exceeding 5%. However, the ratio was 10% or less in the other test
pieces.
TABLE-US-00003 TABLE 3 Chemical Fracture- Test composition Fatigue
Ferrite division pieces (mass %) Hardness limit ratio area ratio
No. C Cu (HRC) (MPa) (%) (-) Self-consistency Note 201 0.10 2.0
11.7 200 97 1.54 X: deformation caused Comparative 202 0.10 2.5
12.8 209 97 1.53 X: deformation caused Example 203 0.10 3.0 14.0
218 97 1.56 X: deformation caused 204 0.10 3.5 15.2 227 96 1.55 X:
deformation caused 205 0.10 4.0 16.4 236 96 1.54 X: deformation
caused 206 0.10 4.5 17.5 245 97 1.52 X: deformation caused 207 0.10
5.0 18.7 255 98 1.51 X: deformation caused 208 0.20 2.0 16.2 235
83.6 1.54 X: deformation caused 209 0.20 2.5 17.4 244 84.1 1.53 X:
deformation caused 210 0.20 3.0 18.5 307 84.6 1.51 .largecircle.
Inventive 211 0.20 3.5 19.7 316 85.1 1.50 .largecircle. Example 212
0.20 4.0 20.9 325 85.6 1.49 .largecircle. 213 0.20 4.5 22.1 334
86.1 1.48 .largecircle. 214 0.20 5.0 23.2 341 86.6 1.46
.largecircle. 215 0.30 2.0 20.7 270 66.9 1.46 .largecircle.
Comparative 216 0.30 2.5 21.9 280 67.4 1.45 .largecircle. Example
217 0.30 3.0 23.1 340 67.9 1.47 .largecircle. Inventive 218 0.30
3.5 24.3 346 68.4 1.45 .largecircle. Example 219 0.30 4.0 25.4 352
68.9 1.44 .largecircle. 220 0.30 4.5 26.6 357 69.4 1.43
.largecircle. 221 0.30 5.0 27.8 360 69.9 1.42 .largecircle. 222
0.30 6.0 28.0 306 70.1 Not Not measured Comparative measured
Example 223 0.40 2.0 25.3 315 50.2 1.44 .largecircle. 224 0.40 2.5
26.4 360 50.7 1.43 .largecircle. 225 0.40 3.0 27.6 363 51.2 1.42
.largecircle. Invention 226 0.40 3.5 28.8 365 51.7 1.41
.largecircle. Example 227 0.40 4.0 30.0 366 52.2 1.39 .largecircle.
228 0.40 4.5 31.1 367 52.7 1.38 .largecircle. 229 0.40 5.0 32.3 322
53.2 1.37 .largecircle. 230 0.50 2.0 29.8 343 33.5 1.40
.largecircle. Comparative 231 0.50 2.5 32.5 347 34 1.37
.largecircle. Example 232 0.50 3.0 33.1 349 34.5 1.36 X: shift
caused 233 0.50 3.5 33.3 358 35 1.36 X: shift caused 234 0.50 4.0
34.5 367 35.5 1.35 X: shift caused 235 0.50 4.5 35.7 376 36 1.34 X:
shift caused 236 0.50 5.0 36.8 357 36.5 1.32 X: shift caused 237
0.60 2.0 34.3 366 16.8 1.35 X: shift caused 238 0.60 2.5 35.5 375
17.3 1.34 X: shift caused 239 0.60 3.0 36.7 384 17.8 1.32 X: shift
caused 240 0.60 3.5 37.8 394 18.3 1.31 X: shift caused 241 0.60 4.0
39.0 403 18.8 1.30 X: shift caused 242 0.60 4.5 40.2 412 19.3 1.29
X: shift caused 243 0.60 5.0 41.4 200 19.8 1.28 X: shift caused
[0052] As shown in Table 3, the following is confirmed. Each of
Inventive Examples in which the contents of C and Cu, the ferrite
ratio and the ratio of free Cu were within the range defined in the
present invention, which had hardness of HRC 33 or less, had no
problem in machinability. Each of Inventive Examples had fatigue
limit of 300 MPa or more, specifically 325 MPa or more, except some
of Inventive Examples (test piece Nos. 210, 211). Inventive
Examples had no shift observed in the fracture surface after
fracture split and had no problem in self-consistency. Inventive
Examples satisfied machinability, fatigue strength and
self-consistency after fracture split simultaneously.
[0053] On the other hand, in Comparative Examples in which the
component composition and/or the ferrite ratio fall/falls out of
the range defined in the present invention, Comparative Examples,
which have hardness of HRC 33 or less, have fatigue limit up to 300
MPa except some Comparative Examples (test piece Nos. 230,231) and
cause deformation due to elongation in fracture split to reduce
dimensional accuracy (test piece Nos. 201 to 209). On the other
hand, in Comparative Examples having fatigue limit of 300 MPa or
more, the Comparative Examples have hardness exceeding HRC33 and
have deteriorated machinability, and cause engaged positional shift
of the fracture surface to cause a problem of self-consistency.
Therefore, it turns out that it is very difficult to obtain the
powder forged member simultaneously satisfying machinability,
fatigue strength and self-consistency after fracture split.
[0054] As shown in Table 3, the fracture split area ratio can be
used as the index representing self-consistency. When the fracture
split area ratio is less than 1.37, the engaged shift of the
fracture split surface occurs easily. On the other hand, when the
fracture split area ratio exceeds 1.51, it turns out that the
deformation due to elongation becomes remarkable and the
dimensional accuracy is deteriorated.
Example 3
Influence of Relative Density
[0055] Next, there were produced test pieces of powder forged
members having the same component composition (C: 0.3%, Cu: 3.5%)
as that of test piece No. 218 of Example 2 in the same condition as
that of Example 2 except that only a forging pressure was variously
changed in the range of 2.5 to 10 ton/cm.sup.2. The influence of
the relative density of the powder forged member exerted on the
fatigue limit was investigated. While the fatigue limit was
measured, the HRB hardness of each of the test pieces was also
measured. Table 4 shows test results.
TABLE-US-00004 TABLE 4 Test Forging Relative Fatigue pieces
pressure density Hardness limit No. (ton/cm.sup.2) (%) (HRB) (MPa)
218 10 99 105.0 346 301 7.5 98 100.0 338 302 9.5 99 101.5 340 303
6.0 97 97.0 329 304 4.0 95 91.5 316 305 3.5 94 86.5 299 306 2.5 93
80.0 286
[0056] As shown in above Table 4, it is confirmed that the fatigue
limit of 325 MPa or more could be ensured when the relative density
to the theoretical density was 97% or more.
Example 4
Influence of Machinability-Improving Material
[0057] Next, test pieces of powder forged members having the same
component composition (C: 0.3%, Cu: 3.5%) as that of the test piece
No. 218 of Example 2 as in Example 3 were produced in the same
manner as in Example 2 except that various machinability-improving
materials were added with the addition amount thereof changed. The
influence exerted on machinability was investigated. Referring to
machinability, a thrust force was measured when a hole was formed
from the surface of the test piece at the number of rotations of
200 rpm and the cutting speed of 0.12 mm/rev using an SKH drill
having a diameter of 5 mm. This was used as the index of
machinability. Table 5 shows the measurement results.
[0058] As is apparent from Table 5, the thrust force is reduced
with the increase of the addition amount of the
machinability-improving material to improve the machinability.
However, when the addition amount of the machinability-improving
material exceeds 0.6%, the large decrease trend of the fatigue
limit is observed even in any machinability-improving agent.
TABLE-US-00005 TABLE 5 Machinability - improving material Test
Amount to be Thrust Fatigue pieces added force Hardness limit No.
Kinds (mass %) (N) (HRC) (MPa) 218 -- 0.0 770 24.3 346 401 MnS 0.2
765 24.8 351 402 0.4 755 25.2 350 403 0.6 750 26.2 335 404 0.8 750
26.5 306 405 MoS.sub.2 0.8 750 25.5 308 406 0.6 750 25.8 338 407
B.sub.2O.sub.3 0.6 739 24.3 334 408 0.8 744 25.4 299 409 BN 0.6 746
24.9 336 410 0.8 749 26.3 316
Example 5
Influence of Oxygen Content of Powder Mixture
[0059] Next, the content of oxygen of a powder mixture was changed
using an iron-based powder having different content of oxygen, and
test pieces of powder forged members were produced in the same
condition as in that of Embodiment 1 described above. The contents
of C and Cu of the powder mixture after being forged were
respectively set to 0.3% and 4% as the target, and the addition
amount of graphite powder was set to 0.3%+(content % of oxygen of
iron-based powder-0.05%).times.3/4 to adjust the content of C.
Referring to this test piece, the content of C and the fatigue
limit were measured, and the influence of the content of oxygen of
the powder mixture exerted thereon was investigated.
[0060] Table 6 shows test results. As shown in Table 6, when the
content of oxygen of the iron-based powder (i.e., the powder
mixture) was 0.3% or less (test piece Nos. 501 to 503), the content
of C of the powder forged member was an approximate target content
of C. However, when the content of oxygen of the iron-based powder
(i.e., the powder mixture) exceeded 0.3% (test piece No. 504), it
turned out that the content of C of the powder forged member was
significantly shifted from the target content of C and fell out of
the appropriate range (0.2 to 0.4%) of the content of C defined in
the present invention to drastically reduce the fatigue
strength.
TABLE-US-00006 TABLE 6 Powder forged member Component Test Chemical
composition of composition Fatigue pieces iron-based powder (mass
%) (mass %) limit No. C Mn P S Si O C Cu (MPa) Note 501 0.001 0.19
0.01 0.009 0.01 0.012 0.31 4.00 352 Inventive 502 0.001 0.18 0.01
0.009 0.01 0.020 0.29 4.05 353 Example 503 0.001 0.18 0.01 0.009
0.01 0.030 0.30 4.00 351 504 0.001 0.19 0.01 0.009 0.01 0.040 0.15
3.95 267 Comparative Example
Example 6
Influence of Content of C of Iron-Based Powder
[0061] Next, an iron-based powder having different content of C was
used, and a powder mixture having the same component composition
was produced by adjusting the addition amount of a graphite powder.
Compacted preforms and test pieces of powder forged members were
produced in the same condition as in Embodiment 1 described above.
The contents of C and Cu after being forged were respectively set
to 0.3% and 4% as the target. The densities of the compacted
preform and powder forged member, and the fatigue limit of the
powder forged member were measured.
[0062] Table 7 shows test results. As is apparent from Table 7, the
decrease trend of the density of the compacted preform is shown
with the increase of the content of C of the iron-based powder.
When the content of C of the iron-based powder is 0.05% (test piece
No. 604), it turns out that the fatigue strength is drastically
reduced although the density of the powder forged member after
being forged is almost the same as that of a case where the content
of C is less than 0.05% (test piece No. 601 to 603).
TABLE-US-00007 TABLE 7 Powder forged Compacted member Test
Component composition of perform Fatigue pieces iron-based powder
(mass %) Density Density limit No. C Mn P S Si O (g/cm.sup.3)
(g/cm.sup.3) (MPa) Note 601 0.001 0.19 0.01 0.009 0.01 0.12 7.05
7.83 353 Inventive 602 0.005 0.18 0.01 0.008 0.01 0.12 6.90 7.83
352 Example 603 0.02 0.19 0.01 0.009 0.01 0.13 6.60 7.81 335 604
0.05 0.20 0.01 0.009 0.01 0.12 6.30 7.79 279 Comparative
Example
* * * * *