U.S. patent application number 09/984300 was filed with the patent office on 2002-05-02 for method of forging raw material for sintering and forging.
This patent application is currently assigned to UNISIA JECS CORPORATION. Invention is credited to Hatai, Yasuo, Iijima, Mitsumasa, Koizumi, Shin, Yoshimura, Takashi.
Application Number | 20020051725 09/984300 |
Document ID | / |
Family ID | 18806679 |
Filed Date | 2002-05-02 |
United States Patent
Application |
20020051725 |
Kind Code |
A1 |
Yoshimura, Takashi ; et
al. |
May 2, 2002 |
Method of forging raw material for sintering and forging
Abstract
A method of forging a raw material for sintering and forging.
The method comprises the steps of: (a) compacting metallic powder
containing iron as a main component and graphite to obtain a
compact having a predetermined density; (b) sintering the compact
at a temperature ranging from 700 to 1000.degree. C. to form a
sintered compact having a texture in which graphite is retained at
grain boundary of metal powder; (c) compressing the sintered
compact from two directions to obtain a compressed sintered
compact; and (d) extruding the compressed sintered compact upon
pressing the compressed sintered compact from the two directions in
a manner that a pressure in one of the two directions is reduced
relative to a pressure in the other of the two directions to
accomplish extrusion forging.
Inventors: |
Yoshimura, Takashi;
(Kanagawa, JP) ; Iijima, Mitsumasa; (Kanagawa,
JP) ; Koizumi, Shin; (Kanagawa, JP) ; Hatai,
Yasuo; (Kanagawa, JP) |
Correspondence
Address: |
Richard L. Schwaab
FOLEY & LARDNER
Washington Harbour
3000 K Street, N.W., Suite 500
Washington
DC
20007-5109
US
|
Assignee: |
UNISIA JECS CORPORATION
|
Family ID: |
18806679 |
Appl. No.: |
09/984300 |
Filed: |
October 29, 2001 |
Current U.S.
Class: |
419/28 |
Current CPC
Class: |
B22F 2998/10 20130101;
B22F 3/20 20130101; C22C 33/0228 20130101; B22F 2998/10 20130101;
B22F 3/02 20130101; B22F 3/10 20130101; B22F 3/20 20130101; B22F
3/02 20130101; C22C 33/02 20130101 |
Class at
Publication: |
419/28 |
International
Class: |
B22F 003/24 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 30, 2000 |
JP |
2000-330105 |
Claims
What is claimed is:
1. A method of forging a raw material for sintering and forging,
comprising the steps of: compacting metallic powder containing iron
as a main component and graphite to obtain a compact having a
predetermined density; sintering the compact at a temperature
ranging from 700 to 1000.degree. C. to form a sintered compact
having a texture in which graphite is retained at grain boundary of
metal powder; compressing the sintered compact from two directions
to obtain a compressed sintered compact; and extruding the
compressed sintered compact upon pressing the compressed sintered
compact from the two directions in a manner that a pressure in one
of the two directions is reduced relative to a pressure in the
other of the two directions to accomplish extrusion forging.
2. A method as claimed in claim 1, wherein the metallic powder
contains at least one selected from the group consisting of as
chromium, molybdenum, manganese, nickel, copper, tungsten, vanadium
and cobalt.
3. A method as claimed in claim 1, wherein the predetermined
density of the compact is not lower than 7.1 g/cm.sup.3.
4. A method as claimed in claim 1, wherein the compressing step and
the extruding step are successively carried out.
5. A method as claimed in claim 1, wherein the compressing step and
the extruding step are carried out without heating the sintered
compact.
6. A method as claimed in claim 1, wherein the sintered compact is
extruded under a forward extrusion in the extruding step.
7. A method as claimed in claim 1, further comprising the step of
preparing a die which has a compression section formed with a first
space in which the sintered compact is set to be compressed, and an
extrusion section continuous with the compression section and
formed with a second space continuous with the first space of the
compression section, the second space being smaller in sectional
area than the first space, wherein the compression step is carried
out by the compression section to increase a density of the
sintered compact to form a compressed sintered compact which is to
be extruded into the extrusion section, and the extruding step is
carried out by the extrusion section successively to the
compression step to form a forging.
8. A method as claimed in claim 7, wherein the first space of the
compression section of the die is shaped corresponding to a final
product.
9. A method as claimed in claim 1, wherein the two directions are
opposite directions.
10. A method of forging a raw material for sintering and forging,
comprising the steps of: compacting metallic powder containing iron
as a main component and graphite to obtain a compact; sintering the
compact at a temperature ranging from 700 to 1000.degree. C. to
form a sintered compact having a texture in which graphite is
retained at grain boundary of metal powder; filling the compact in
a forming space of a die; compressing the sintered compact in the
forming space of the die from opposite directions without heating
to obtain a compressed sintered compact; and extruding the
compressed sintered compact in the die without heating by
controlling pressures in the opposite directions in a manner that
the pressure in one of the opposite directions is decreased
relative to the pressure in the other of the opposite directions to
accomplish extrusion forging.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates to improvements in a method of
forging a raw material for sintering and forging in order to
produce a forging to be used as a mechanical part or the like, and
more particularly to the method of forging a sintered compact
containing iron as a main component and graphite.
[0002] Hitherto forging has been widely used for producing
mechanical parts. Additionally, in recent years, it has been
studied to produce a mechanical part first by sintering compacted
metallic powder to form a sintered compact and then by forging the
sintered compact. The metallic powder contains iron as a main
component and further contains a certain amount of graphite. It has
been known that crack tends to be readily produced in a product by
making extrusion forging on such a sintered compact.
[0003] This fact is described, for example, at pages 38 and 39 of a
technical text "Industrial Library 13--High Speed Forging
(published on Jun. 25, 1969 by Nikkan Kogyou Shinbunsha)".
According to this technical text, iron powder is subjected to
pre-compacting and sintering thereby to form a sintered compact
having a relative density of 78%, and then the sintered compact
undergoes extrusion forging under pressing upon loading a back
pressure of 4000 kg/cm.sup.2. This technical text recites that
production of crack cannot be prevented. Additionally, the
technical text recites that production of crack can be prevented in
case that the above sintered compact is subjected to extrusion
forging with a high speed hammer loading a back pressure of 3000
kg/cm.sup.2.
[0004] In the latter forging method, production of crack can be
prevented; however, a forming speed during the forging is high to
generate heat thereby inviting another disadvantage that such heat
causes the dimensional accuracy of a forging to lower.
[0005] Apart from the above, in recent years, a forging method as
disclosed in Japanese Patent Provisional Publication No. 2000-17307
has been devised and proposed. This forging method is summarized as
follows: Metallic powder is compacted to form a compact having a
certain density. Thereafter, the compact is sintered at
1300.degree. C. under vacuum thereby forming a sintered compact.
The sintered compact is located in a die and pressurized from
upward and downward directions under heating, in which a pressure
from the downward direction is reduced relative to that from the
upward direction thereby accomplishing extrusion forging. According
to this forging method, production of crack in a forging can be
prevented under the effects of heating during the extension forging
and application of the pressures from upward and downward
directions.
[0006] However, drawbacks have been encountered in such a
conventional forging method. Specifically, in case that metallic
powder as a raw material is prepared by mixing graphite with metal
powder containing iron as a main component, graphite is excessively
diffused in the metal powder to largely increase the hardness of
the sintered compact. Accordingly, if sufficient heat is not
applied to the sintered compact during the succeeding extrusion
forging, production of crack will occur in the resultant forging.
Thus, in the conventional forging method, carrying out such high
temperature heating is required during the extrusion forging,
thereby large-sizing and complicating a facility or forging machine
upon addition of a heating device while shortening the life of the
die and lowering the dimensional accuracy of the resultant
forging.
SUMMARY OF THE INVENTION
[0007] In view of the above, it is an object of the present
invention to provide an improved method of forging a raw material
for sintering and forging, which can effectively overcome drawbacks
encountered in conventional forging methods.
[0008] Another object of the present invention is to provide an
improved method of forging a raw material for sintering and
forging, which can securely prevent production of defects such as
crack and the like of a resultant forging without inviting
large-sizing and complication of a forging facility or machine,
shortening the life of a die and lowering the dimensional accuracy
of the resultant forging.
[0009] An aspect of the present invention resides in a method of
forging a raw material for sintering and forging. The method
comprises the steps of: (a) compacting metallic powder containing
iron as a main component and graphite to obtain a compact having a
predetermined density; (b) sintering the compact at a temperature
ranging from 700 to 1000.degree. C. to form a sintered compact
having a texture in which graphite is retained at grain boundary of
metal powder; (c) compressing the sintered compact from two
directions to obtain a compressed sintered compact; and (d)
extruding the compressed sintered compact upon pressing the
compressed sintered compact from the two directions in a manner
that a pressure in one of the two directions is reduced relative to
a pressure in the other of the two directions to accomplish
extrusion forging. Preferably, metallic powder contains at least
one selected from the group consisting of as chromium, molybdenum,
manganese, nickel, copper, tungsten, vanadium and cobalt.
[0010] Another aspect of the present invention resides in a method
of forging a raw material for sintering and forging. The method
comprises the steps of: (a) compacting metallic powder containing
iron as a main component and graphite to obtain a compact; (b)
sintering the compact at a temperature ranging from 700 to
1000.degree. C. to form a sintered compact having a texture in
which graphite is retained at grain boundary of metal powder; (c)
filling the compact in a forming space of a die; (d) compressing
the sintered compact in the forming space of the die from opposite
directions without heating to obtain a compressed sintered compact;
and (e) extruding the compressed sintered compact in the die
without heating by controlling pressures in the opposite directions
in a manner that the pressure in one of the opposite directions is
decreased relative to the pressure in the other of the opposite
directions to accomplish extrusion forging.
[0011] According to the present invention, in the sintered compact
obtained by sintering the compact at 700 to 1000.degree. C.,
binding among metals progresses in such a manner as to be able to
make a compression deformation while graphite is hardly diffused
and is dispersed at grain boundary. When this sintered compact is
compressed from two directions, it can be easily
compression-deformed under cold compression thereby forming the
high density compressed sintered compact. Then, this compressed
sintered compact is compressed from the two directions, in which
the pressure from one direction is reduced relative to that from
the other direction. As a result, the compressed sintered compact
is cold-extruded from the side of the other direction thereby
obtaining a forging having no defects such as crack and the
like.
[0012] Preferably, the predetermined density of the compact is not
lower than 7.1 g/cm.sup.3. With this feature, metal powder is in a
condition where contact among metal particles of the metal powder
is increased. Additionally, the composition of the sintered compact
is in a condition where graphite is retained at grain boundary of
the metal powder while precipitates such as carbide and the like
are hardly formed. As a result, the sintered compact is high in
hardness and high in elongation percentage while lubricating
characteristics at grain boundary of metal powder is increased
thereby to wholly raise the deformability of the sintered compact.
These effects are combined with the above effects of the particular
forging process thereby making it possible to prevent production of
defects such as crack and the like.
[0013] Preferably, the compressing step and the extruding step are
successively carried out. With this feature, the sintered compact
which has been subjected to a forming process at the compression
step can be transferred to the succeeding extruding step without
its work hardening. Accordingly, extrusion forging can be made
without trouble even a raw material which tends to readily make its
work hardening.
[0014] Preferably, the compressing step and the extruding step are
carried out without heating the sintered compact. With this
feature, the dimensional accuracy of the resultant forging can be
raised while thermal deterioration of a die can be prevented.
[0015] Preferably, the sintered compact is extruded under a forward
extrusion in the extruding step. With this feature, forging of a
long member can be realized without inviting crack or the like of
the long member.
[0016] Preferably, the step of preparing a die which has a
compression section formed with a first space in which the sintered
compact is set to be compressed, and an extrusion section
continuous with the compression section and formed with a second
space continuous with the first space of the compression section.
The second space is smaller in sectional area than the first space.
Here, the compression step is carried out by the compression
section to increase a density of the sintered compact to form a
compressed sintered compact which is to be extruded into the
extrusion section, and the extruding step is carried out by the
extrusion section successively to the compression step to form a
forging. With this feature, the compression section and the
extrusion section are formed continuous in the die, so that the
compression step and the extrusion step are successively carried
out.
[0017] Preferably, the first space of the compression section of
the die is shaped corresponding to a final product or resultant
forging. With this feature, a further processing is unnecessary
onto a part of the material remaining in a not-extruded state in
the compression section of the die, and therefore the material in
the compression section can be used as a product as it is.
[0018] The other objects and features of this invention will become
understood from the following description with reference to the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a vertical sectional view of an essential part of
an example of a forging machine carrying out a forging method
according to the present invention;
[0020] FIG. 2A is a fragmentary sectional view of a first step in
the forging method carried out by the forging machine of FIG.
1;
[0021] FIG. 2B is a fragmentary sectional view of a second step in
the forging method carried out by the forging machine of FIG. 1,
succeeding to the first step of FIG. 2A;
[0022] FIG. 2C is a fragmentary sectional view of a third step in
the forging method carried out by the forging machine of FIG. 1,
succeeding to the second step of FIG. 2B;
[0023] FIG. 3 is a schematic side view showing the shape of a
forging in experiment carried out to obtain experimental data of
FIGS. 4 and 5;
[0024] FIG. 4 is a graph representing the experimental data showing
the relationship between the not-extruded thickness and the density
of the forging of FIG. 3;
[0025] FIG. 5 is a graph representing the experimental data showing
the relationship between the density of the compact and the density
of the forging of FIG. 3;
[0026] FIG. 6A is a table containing experimental data representing
the relationship between the sintering temperature and the
elongation percentage of the sintered compact in terms of the
amount of graphite mixed with a metal powder (alloy steel powder)
same as that in Example 1;
[0027] FIG. 6B is a graph showing the experimental data of FIG.
6A;
[0028] FIG. 7A is a table containing experimental data representing
the relationship between the sintered temperature and the hardness
of the sintered compact in terms of the amount of graphite mixed
with the metal powder (alloy steel powder) same as that in Example
1;
[0029] FIG. 7B is a graph showing the experimental data of FIG.
7A;
[0030] FIG. 8A is a table containing experimental data representing
the relationship between the sintered temperature and the forming
load (flow stress) of the sintered compact in terms of the amount
of graphite mixed with the metal powder (allow steel powder) same
as that in Example 1;
[0031] FIG. 8B is a graph showing the experimental data of FIG.
7A;
[0032] FIG. 9 is a table containing experimental data representing
the experimental conditions and results of Examples 1 and 2 and
Comparative Example;
[0033] FIG. 10 is a table containing experimental data of the
dimensional accuracy of forgings which are produced respectively by
a conventional forging method and the forging method according to
the present invention;
[0034] FIG. 11 is a vertical sectional view showing the
conventional forging method used for obtaining the experimental
data of FIG. 10; and
[0035] FIG. 12 is fragmentary sectional view showing the forging
method according to the present invention used for obtaining the
experimental data of FIG. 10.
DETAILED DESCRIPTION OF THE INVENTION
[0036] According to the present invention, a method of forging a
raw material for sintering and forging comprises the steps of: (a)
compacting metallic powder (the raw material) containing iron as a
main component and graphite to obtain a compact having a
predetermined density; (b) sintering the compact at a temperature
ranging from 700 to 1000.degree. C. to form a sintered compact
having a texture in which graphite is retained at grain boundary of
metal powder; (c) compressing the sintered compact from two
directions to obtain a compressed sintered compact; and (d)
extruding the compressed sintered compact upon pressing the
compressed sintered compact from the two directions in a manner
that a pressure in one of the two directions is reduced relative to
a pressure in the other of the two directions to accomplish
extrusion forging. The above metallic powder preferably contains at
least one of hardening alloy elements such as chromium (Cr),
molybdenum (Mo), manganese (Mn), nickel (Ni), copper (Cu), tungsten
(W), vanadium (V), cobalt (Co) and the like.
[0037] An example of a forging machine for carrying out the forging
method according to the present invention will be discussed with
reference to FIGS. 1 and 2A to 2C.
[0038] The forging machine includes an upper ram 1 to which an
upper punch 2 is installed. A lower ram 3 is provided coaxially
with upper ram 1. A lower punch 4 having a diameter smaller than
that of upper punch 2 is installed to lower ram 3. A generally
cylindrical forging die 5 is fixedly installed to a stationary base
6. A sintered compact W.sub.0 is filled in a forming space 7 formed
inside die 5 so as to be subjected to a forming process. The
generally cylindrical inner surface (defining forming space 7) of
die 5 has a cylindrical large diameter section 8 and a cylindrical
small diameter section 9. A generally frustoconical or tapered
section 10 is formed between large and small diameter sections 8, 9
in such a manner as to smoothly connect the lower end of large
diameter section 8 and the upper end of small diameter section 9.
Upper punch 2 is inserted into large diameter section 8, whereas
lower punch 4 is inserted into small diameter section 9.
[0039] Upper ram 2 and lower ram 3 are operated to independently
move upward and downward. In lower ram 3, load to be applied
through lower punch 4 to sintered compact W.sub.0 or a compressed
sintered compact W.sub.1 is suitably controllable. In this example,
large diameter section 8 and tapered section 10 serve as a
compressing section for compressing the sintered compact or the
compressed sintered compact, while small diameter section 9 serves
as an extruding section for extruding the sintered compact or the
compressed sintered compact.
[0040] The forging machine of this example is configured to produce
a pinion shaft (final product) as a forging, used in an automotive
vehicle or the like. The pinion shaft includes a large diameter
section installed to a driving section of the vehicle, a small
diameter section to which a pinion is fixed, and a frustoconical or
tapered section connecting the large and small diameter section,
though not shown. The large diameter section, the small diameter
section and the tapered section of this pinion shaft correspond
respectively to large diameter section 8, small diameter section 9
and tapered section 10 of the inner surface of die 5. In other
words, during the extruding step, a material (or the sintered
compact) is extruded in a direction of from large diameter section
8 through tapered section 10 to small diameter section 9 of the
inner surface (defining forming space 7) of die 5, in which the
shape of the inner surface defining the forming space 7 is set such
that a part of the material extruded into small diameter section 9
becomes the small diameter section of the pinion shaft while a part
of the material remaining in a not-extruded state in large diameter
and tapered sections 8, 10 becomes the large diameter and tapered
sections of the pinion shaft as it is.
[0041] In the step of compacting the metallic powder, a pressure to
be impressed on the metallic powder is controlled to obtain the
compact having a density of not lower than 7.1 g/cm.sup.3,
preferably not lower than 7.3 g/cm.sup.3. This is because
compacting the metallic powder to form the compact having such a
high density as not lower than 7.1 g/cm.sup.3 increases the
contacting area among particles of the metal powder thereby raising
the toughness of a resultant product or forging. In case that the
density of the compact is not lower than 7.3 g/cm.sup.3, voids
among the metal particles become independent from each other so
that atmospheric gas in a furnace is difficult to enter the inside
of the compact, and therefore graphite tends to be readily retained
at the grain boundary without being diffused in the subsequent step
of sintering. This raises the hardness of sintered compact W.sub.0
and effectively suppresses the progress of carburizing causing a
reduction in elongation percentage of the resultant product, which
is a further effect to be expected. Additionally, since the compact
has been formed to have the high density as discussed above,
sintering due to a surface diffusion or melting at the contacting
surface among particles of the metal powder is made throughout a
wide range during the sintering step. Under the effect of such
sintering, sintered compact W.sub.0 can obtain a large elongation
percentage.
[0042] The temperature of sintering the compact is set in the range
of from 700 to 1000.degree. C. This is because joining of particles
of the metal powder by the sintering cannot progress at the
temperature lower than 700.degree. C. whereas graphite is
excessively diffused to obtain a too high hardness at the
temperature exceeding 1000.degree. C. Accordingly, by virtue of the
fact that the sintering temperature is set in the above range,
particles of the metal powder can be securely joined to each other
while graphite can be hardly diffused to remain at the grain
boundary. By this, the sintered compact becomes low in hardness and
high in elongation percentage while being raised in deformability
by large diameter section 8 of the inner surface of the die 5 as
shown in FIG. 2A. In this state, lower punch 4 is upwardly moved to
a certain level under operation of the lower ram 3, while the upper
punch 2 is downwardly moved under operation of the upper ram 1.
Thus, the sintered compact W.sub.0 is compressed by the upper punch
2 and the lower punch for a certain time and at a certain load
thereby densifying the texture of the sintered compact thereby
forming a compressed sintered compact W.sub.1 (this corresponds to
the compressing step). This compressed sintered compact W.sub.1
preferably has a density of 7.3 g/cm.sup.3 (corresponding to a
relative density of 93%), more preferably a density of 7.6
g/cm.sup.3 (corresponding to a relative density of 97%).
[0043] Subsequently, the load applied to the lower punch 4 is
reduced relative to that applied to upper punch 2, in which
compressed sintered compact W.sub.1 is gradually pushed or extruded
out into small diameter section 9 of the inner surface of die 5
while a certain compressive force is being applied to compressed
sintered compact w1. Upon such extrusion of compressed sintered
compact W.sub.1, forging is made on compressed sintered compact
W.sub.1 maintaining the minute texture of whole compressed sintered
compact W.sub.1. This forms a forging W.sub.2 having a high quality
without producing defects such as crack and the like. Forging
W.sub.2 is taken out from die 5 upon opening die 5 after the
forging.
[0044] During the step of forging, it is not carried out to extrude
whole compressed sintered compact W.sub.1 into small diameter
section 9 of the inner surface of die 5 so that a part
(corresponding to a certain thickness or height) of the forging
located at the large diameter section 8 remains not-extruded.
Accordingly, the thus obtained forging W.sub.2 is provided with the
tapered section and the large diameter section which are formed on
the upper end of the small diameter section of the forging.
[0045] Here, a variety of experiments were conducted in connection
with the forging method according to the present invention.
[0046] First, experiments for obtaining data shown in FIGS. 4 and 5
were conducted in accordance with the following forging method:
Compacting was made on four kinds of metallic powders whose main
component was iron containing 0.5% by weight of graphite so as to
obtain four kinds of compacts which had respectively densities of
6.5 g/cm.sup.3, 6.8 g/cm.sup.3, 7.1 g/cm.sup.3 and 7.4 g/cm.sup.3.
The four kinds of compacts were subjected to sintering at the above
sintering temperature range of 700 to 1000.degree. C. thereby
obtaining four kinds of sintered compacts. Each of the sintered
compacts was filled in the die of a forging machine similar to that
shown in FIG. 1, and then underwent a forward (downward) extrusion
under pressure from one direction, in which the reduction in area
of each sintered compact was 60%, thereby obtaining an extruded
sintered compact. The forward extrusion was an extrusion of each
sintered compact in a direction of an arrow F in FIG. 3 which
showed each sintered compact which had underwent the forward
extrusion. In the experiments, the densities of the extruded
sintered compacts were measured upon varying a not-extruded
thickness (See FIG. 3) which meant a thickness (axial dimension) of
a part remaining not-extruded thereby obtaining data shown in FIG.
4. In FIG. 4, a line F1 indicates the data of the compact which had
the density of 6.5 g/cm.sup.3 and was subjected to the forward
extrusion. A line F2 indicates the data of the compact which had
the density of 6.8 g/cm.sup.3 and was subjected to the forward
extrusion. A line F3 indicates the data of the compact which had
the density of 7.1 g/cm.sup.3 and was subjected to the forward
extrusion. A line F4 indicates the data of the compact which had
the density of 7.4 g/cm.sup.3 and was subjected to the forward
extrusion.
[0047] As apparent from FIG. 4, the density of the compact largely
affects extrusion of the sintered compact. When the density of the
compact was 6.5 g/cm.sup.3 or 6.8 g/cm.sup.3, it was not possible
to complete the extrusion to obtain a desired not-extruded
thickness so that the density of a resultant forging could not
exceed the value of 7.6 g/cm.sup.3 which was a standard value for
practical use. In contrast, when the density of the compact was 7.1
g/cm.sup.3 or 7.4 g/cm.sup.3, a resultant forging having the
density exceeding 7.6 g/cm.sup.3 was obtained.
[0048] Additionally, experiments were conducted in such a manner
that the forward extrusion was made on each of the sintered
compacts whose compacts had respectively the densities of 6.5
g/cm.sup.3, 6.8 g/cm.sup.3, 7.1 g/cm.sup.3 and 7.4 g/cm.sup.3. In
these experiments, the densities of a lower part a (at the side of
the small diameter section) shown in FIG. 3 and an upper part b (at
the side of the tapered section and the large diameter section)
shown in FIG. 3 were measured upon making the forward extrusion on
each of the sintered compacts. The data of this measurement were
shown in FIG. 5 in which a line a indicates the data of the lower
part a of the extruded sintered compact; and a line b indicates the
data of the upper part b of the extruded sintered compact. As
apparent from FIG. 5, in case that the densities of the compacts
were as high as 7.1 g/cm.sup.3 and 7.4 g/cm.sup.3, the densities of
both the lower part a and the upper part b take sufficient values
exceeding 7.6 g/cm.sup.3, and the difference between the densities
of the lower and upper part a, b was made small. Accordingly,
dispersion in densities of various parts in the resultant forging
can be suppressed lower.
[0049] FIGS. 6A and 6B respectively show experimental data and
graphs obtained under experiments in which forgings or products
were produced similarly to Example 1 which will be discussed after
and by varying the amount of graphite to be mixed with the alloy
steel powder (containing 1.0% by weight of chromium, 0.3% by weight
of molybdenum, 0.7% by weight of manganese and balance consisting
of iron and unavoidable impurities) in Example 1. The amount of the
graphite was varied as 0.1% by weight, 0.3% by weight, 0.5% by
weight and 1.0% by weight which were respectively indicated as
0.1%C, 0.3%C, 0.5%C, 1.0%C in FIG. 6A. The data and the graphs
represent the relationship between the sintering temperature and
the elongation percentage of the sintered compact. In FIG. 6B,
lines G1, G2, G3 and G4 indicate respectively the data of the
sintered compacts of the above graphite amounts of 0.1% by weight,
0.3% by weight, 0.5% by weight and 1.0% by weight.
[0050] FIGS. 7A and 7B respectively show experimental data and
graphs obtained under experiments in which forgings or products
were produced similarly to Example 1 and by varying the amount of
graphite to be mixed with the alloy steel powder in Example 1. The
amount of the graphite was varied as 0.1% by weight, 0.3% by
weight, 0.5% by weight and 1.0% by weight which were respectively
indicated as 0.1%C, 0.3%C, 0.5%C, 1.0%C in FIG. 7A. The data and
the graphs represent the relationship between the sintering
temperature and the Rockwell hardness of the sintered compact. In
FIG. 7B, lines G1, G2, G3 and G4 indicate respectively the data of
the sintered compacts of the above graphite amounts of 0.1% by
weight, 0.3% by weight, 0.5% by weight and 1.0% by weight.
[0051] As apparent from the data and graphs of FIGS. 6A to 7B, in
case that the sintering temperature is selected within the range of
700 to 1000.degree. C., binding among metals progresses thereby
providing a sintered compact elongation percentage for rendering
forging possible. Even if the sintering temperature is 1000.degree.
C. at which the hardness becomes the highest, the hardness can be
maintained at a value slightly higher than a Rockwell hardness
(B-scale) of 60 by adjusting the amount of graphite to be mixed
with the alloy steel powder. The value of Rockwell hardness
(B-scale) of 60 is generally the same as that obtained by making
annealing on a high strength cold forged steel; however, the
above-mentioned sintered compact in connection with FIGS. 7A and 7B
can obtain the value close to the Rockwell hardness (B-scale) of 60
without annealing.
[0052] The above-mentioned sintered compact which has been sintered
at the temperature ranging from 700 to 1000.degree. C. is filled in
the forging die and subjected to the compression and the extrusion
forging which are accomplished successively. During the compression
and the extrusion forging, voids in the metallic texture of the
sintered compact are squeezed thereby accomplishing densification
of the metallic texture and forming of the sintered compact. At
this time, sufficient graphite remains at the grain boundary of
metal powder in the sintered compact, and therefore a forming load
(flow stress or deformation resistance) MPa can be made very low as
depicted in FIGS. 8A and 8B. In other words, in the above-mentioned
sintered compact, diffusion of carbon is hardly made and therefore
the sintered compact is low in hardness and high in elongation
percentage. Additionally, graphite existing at metallic grain
boundary functions to promote slip among particles of the metal
powder, and therefore the forming load during the compression and
the extrusion becomes small thus making it possible to easily form
the forging into a desired shape. FIGS. 8A and 8B show experimental
data and graphs obtained under experiments in which forgings or
products were produced similarly to Example 1 and by varying the
amount of graphite to be mixed with the alloy steel powder in
Example 1. The amount of the graphite was varied as 0.1% by weight,
0.3% by weight, 0.5% by weight and 1.0% by weight which were
respectively indicated as 0.1%C, 0.3%C, 0.5%C, 1.0%C in FIG. 8A.
The data and the graphs represent the relationship between the
sintering temperature and the forming load (flow stress or
deformation resistance) MPa applied for the compression and the
extrusion of the sintered compact. In FIG. 8B, lines G1, G2, G3 and
G4 indicate respectively the data of the sintered compacts of the
above graphite amounts of 0.1% by weight, 0.3% by weight, 0.5% by
weight and 1.0% by weight.
[0053] In the forging method according to the present invention,
the compression and the extrusion forging of the sintered compact
are successively accomplished using the forging die. As a result,
the material or sintered compact cannot make its work hardening
after the compression step, and therefore there arises no problem
even in case of using a material which tends to readily make its
work hardening. Additionally, in this forging method, the
compression and the extrusion of the sintered compact are carried
out under a not-heated condition, thereby making it unnecessary
that the forging die is provided with an apparatus for heating the
die. This makes the forging machine small-sized and simplified
while preventing the dimensional accuracy of the resultant forging
from lowering due to heating. Further, not-heating the forging die
prevents the forging die from its thermal deterioration thereby
prolong the durability of the forging die.
[0054] FIG. 10 shows experimental data for the purpose of
comparison in dimensional accuracy of a resultant forging between a
conventional forging method and the forging method according to the
present invention. The resultant forging was generally
cup-shaped.
[0055] The conventional forging (hot forging) method was
accomplished as follows: As shown in FIG. 11, a sintered compact W
was filled in a forming hole 11 formed in a die 25. At this state,
a punch 22 is moved downward to press the central part of the
sintered compact W thereby to forge a generally cup-shaped
forging.
[0056] In contrast, in the forging method according to the present
invention accomplished using a forging machine similar to that
shown in FIG. 1 with the exception that the inner peripheral
surface of die 5 was cylindrical, as shown in FIG. 12, a core 11
was projected upward from a downward direction in a forming hole or
space 5a of the die 5. At this state, the sintered compact W.sub.0
is filled in the forming hole 12. Then, lower punch 4 was moved
upward while upper punch 2 is moved downward so as to press the
sintered compact W.sub.0. Thereafter, the pressing force of lower
punch 4 was reduced thereby to forge a generally cup-shaped
forging. This forging method was similar in forming and forging the
sintered compact to those in Example 1 (discussed after) with the
exception that the generally cup-shaped forging was formed in place
of the pinion shaft
[0057] As depicted in the experimental data shown in FIG. 10, in
case of the above conventional hot forging method, dispersion of
the outer diameter and the inner diameter of the resultant
cup-shaped forging are 1.0 mm. In contrast, in case of the forging
method according to the present invention, dispersions of the outer
diameter and the inner diameter of the resultant cup-shaped forging
are respectively 0.03 mm and 0.06 mm. These experimental data
reveal that a dimensional error due to thermal shrinkage is very
low in the forging method according to the present invention in
which no heat is applied. Additionally, in the forging method
according to the present invention, the forging can be easily taken
out from the die without forming a draft in the die. Furthermore,
according to the forging method of the present invention, the
sintered compact is formed under the forward extrusion while being
pressed from two directions, thereby making it possible to realize
the extrusion forging of a long member or sintered compact which
has conventionally been difficult to be forged.
EXAMPLES
[0058] The present invention will be more readily understood with
reference to the following Examples in comparison with Comparative
Example; however, these Examples are intended to illustrate the
invention and are not to be construed to limit the scope of the
invention.
Example 1
[0059] Graphite in an amount of 0.3% by weight was mixed with alloy
steel powder containing 1.0% by weight of chromium (Cr), 0.3% by
weight of molybdenum (Mo), 0.7% by weight of manganese (Mn) and
balance consisting of iron (Fe) and unavoidable impurities, thereby
forming metallic powder as raw material. This metallic powder was
compacted thereby forming a compact having a density of 7.4 g/cm3.
This compact was sintered in the atmosphere of nitrogen in a
furnace at 800.degree. C. (sintering temperature) for 60 minutes
thereby producing a sintered compact. The thus produced sintered
compact had an elongation percentage of 3.3% and a Rockwell
hardness (B-scale) of 48.6.
[0060] Subsequently, the sintered compact was filled in the die of
the forging machine shown in FIG. 1 and subjected to the
compression and the extrusion forging in the manner of
two-direction pressing under conditions in which the load of upper
punch 2 was 46 tonf; the forming or moving speed of upper ram 1 was
5 mm/sec.; the load of lower punch 4 was 15 tonf; the stopping time
of the both punches during the compression was 1 second; the
reduction in area of the sintered compact was 30%. As a result, a
forging or pinion shaft was produced; and the forming load (flow
stress) was 2333 MPa. The thus produced forging had no crack and
high in quality as shown in FIG. 9 in which the composition
"1.0Cr-0.3Mo-0.7Mn" indicates the composition of the alloy steel
powder containing 1.0% by weight of chromium (Cr), 0.3% by weight
of molybdenum (Mo), 0.7% by weight of manganese (Mn) and balance
consisting of iron (Fe) and unavoidable impurities.
[0061] For the purpose of comparison, the sintered compact filled
in the die was subjected to the forward extrusion in the direction
of the arrow F in FIG. 3, thereby forming a forging. Additionally,
the sintered compact filled in the die was subjected to a rearward
extrusion which was an extrusion of the sintered compact in the
opposite direction relative to the direction of the arrow F in FIG.
3, thereby forming a forging. As a result, in case of the forward
extrusion, apparent crack was produced in the extruded sintered
compact so that the forgeability is evaluated as no good (NG). In
case of the rearward extrusion, no apparent crack was produced in
the extruded sintered compact, and therefore the extruded sintered
compact seemed to be evaluated good (G) as shown in FIG. 9;
however, the forging obtained under the two-direction pressing was
largely high in quality as compared with that obtained under the
rearward extrusion.
COMPARATIVE EXAMPLE
[0062] The procedure of producing the sintered compact in Example 1
was repeated with the following exceptions: Graphite in an amount
of 0.5% by weight was mixed with the alloy steel powder thereby
forming metallic powder; the metallic powder was compacted thereby
forming a compact having a density of 7.1 g/cm.sup.3; and the
compact was sintered in the atmosphere of nitrogen gas in a furnace
at 1250.degree. C. for 60 minutes thereby producing a sintered
compact. The thus produced sintered compact had a relatively low
elongation percentage of 2.6% and a relatively high Rockwell
hardness (B-scale) of 75.0.
[0063] The sintered compact was subjected to the forging in the
manner of the two-direction pressing, the forward extrusion and the
rearward extrusion were made similarly to those in Example so as to
intend to form forgings. As a result of the above low elongation
percentage and high hardness of the sintered compact, it is
impossible to accomplish forging not only under the forward
extrusion and the rearward extrusion but also under the
two-direction pressing, and therefore the forgeability was
evaluated no good (NG) as shown in FIG. 9.
Example 2
[0064] The procedure of producing the sintered compact in Example 1
was repeated with the following exceptions: The metallic powder was
compacted at a compacting load of 2596 MPa thereby forming a
compact; the compact was sintered in the atmosphere of nitrogen gas
in a furnace at 900.degree. C. for 60 minutes thereby producing a
sintered compact. The thus produced sintered compact had an
elongation percentage of 5.7% and a Rockwell hardness (B-scale) of
55.1.
[0065] Subsequently, the sintered compact was filled in the die of
the forging machine shown in FIG. 1 and subjected to the
compression and the extrusion forging in the manner of
two-direction pressing under the same conditions as those in
Example 1 with the exception that the forming load (flow stress)
was 2596 MPa. As a result, a forging or pinion shaft was produced.
The thus produced forging had no crack and high in quality as shown
in FIG. 9.
[0066] Additionally, the sintered compact was subjected to the
forging in the manner of the forward extrusion and the rearward
extrusion similarly to those in Example 1, so as to intend to form
forgings. FIG. 9 depicts that the forgeability of the sintered
compact was evaluated good (G) in case of the two-direction
pressing, similarly to that in Example 1
[0067] As appreciated from the above, according to the forging
method of the present invention, the forging having no defects such
as crack and the like can be produced under a cold forging. This
makes it unnecessary to provide the forming machine or facility
with a heating device, thereby small-sizing and simplifying the
forging machine thus lowering a production cost of the forging.
Additionally, the dimensional accuracy of the forging can be
raised. Furthermore, deterioration of the die due to heat can be
prevented. In case that the compressing step and the extruding step
are successively carried out by using the forging die or the like
having the compression section continuous with the extrusion
section, forging can be easily accomplished even on a raw material
which tends to readily make its work hardening. Additionally, since
the sintered compact may be extruded under the forward extrusion in
the extruding step, forging can be easily made on a long member
which has been difficult to be forged.
[0068] The entire contents of Japanese Patent Application No.
2000-330105, filed Oct. 30, 2000, is incorporated herein by
reference.
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