U.S. patent application number 14/349122 was filed with the patent office on 2014-08-14 for iron-based alloy for powder injection molding.
This patent application is currently assigned to POSTECH ACADEMY-INDUSTRY FOUNDATION. The applicant listed for this patent is Jeonghyeon Do, Byeong-Joo Lee, Sung Hak Lee, Yang Su Shin. Invention is credited to Jeonghyeon Do, Byeong-Joo Lee, Sung Hak Lee, Yang Su Shin.
Application Number | 20140227124 14/349122 |
Document ID | / |
Family ID | 48141076 |
Filed Date | 2014-08-14 |
United States Patent
Application |
20140227124 |
Kind Code |
A1 |
Lee; Sung Hak ; et
al. |
August 14, 2014 |
IRON-BASED ALLOY FOR POWDER INJECTION MOLDING
Abstract
A ferrous alloy for powder injection molding is provided. The
ferrous alloy for powder injection molding includes iron (Fe) at
52.59-78.15 wt %, chromium (Cr) at 16.45-37.34 wt %, boron (B) at
3.42-7.76 wt %, silicon (Si) at 1.64-1.92 wt %, sulfur (S) at
0-0.21 wt %, carbon (C) at 0.16-0.18 wt %, and other inevitable
impurities.
Inventors: |
Lee; Sung Hak; (Pohang-si,
KR) ; Lee; Byeong-Joo; (Pohang-si, KR) ; Do;
Jeonghyeon; (Jinju-si, KR) ; Shin; Yang Su;
(Seoul, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Lee; Sung Hak
Lee; Byeong-Joo
Do; Jeonghyeon
Shin; Yang Su |
Pohang-si
Pohang-si
Jinju-si
Seoul |
|
KR
KR
KR
KR |
|
|
Assignee: |
POSTECH ACADEMY-INDUSTRY
FOUNDATION
Pohang-city
KR
|
Family ID: |
48141076 |
Appl. No.: |
14/349122 |
Filed: |
April 20, 2012 |
PCT Filed: |
April 20, 2012 |
PCT NO: |
PCT/KR2012/003071 |
371 Date: |
April 2, 2014 |
Current U.S.
Class: |
419/36 ; 420/42;
420/64 |
Current CPC
Class: |
B22F 3/225 20130101;
C22C 38/34 20130101; C22C 38/42 20130101; C22C 38/48 20130101; C22C
30/00 20130101; C22C 38/32 20130101; C22C 33/0285 20130101; B22F
3/12 20130101; C22C 38/60 20130101 |
Class at
Publication: |
419/36 ; 420/42;
420/64 |
International
Class: |
C22C 38/60 20060101
C22C038/60; C22C 38/34 20060101 C22C038/34; C22C 38/32 20060101
C22C038/32; B22F 3/12 20060101 B22F003/12 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 21, 2011 |
KR |
10-2011-0108028 |
Claims
1. A ferrous alloy for powder injection molding, comprising iron
(Fe) at 52.59-78.15 wt %, chromium (Cr) at 16.45-37.34 wt %, boron
(B) at 3.42-7.76 wt %, silicon (Si) at 1.64-1.92 wt %, sulfur (S)
at 0-0.21 wt %, carbon (C) 0.16-0.18 wt %, and other inevitable
impurities.
2. The ferrous alloy of claim 1, wherein a ratio X.sub.Cr/X.sub.B
of the chromium (Cr) and boron (B) is 1.0.
3. The ferrous alloy of claim 2, wherein a sum (X.sub.Cr+X.sub.B)
of a mole fraction of the chromium (Cr) and the boron (B) is 0.30
to 0.60.
4. The ferrous alloy of claim 1, wherein a sum
(X.sub.Fe+X.sub.Cr+X.sub.B) of a mole fraction of the iron (Fe),
the chromium (Cr), and the boron (B) is 0.9635.
5. The ferrous alloy of claim 4, wherein in a microstructure of the
ferrous alloy for powder injection molding, chromium boride
(Cr.sub.2B) is distributed in a network form within a ferrite
base.
6. The ferrous alloy of claim 5, wherein a volume fraction of the
chromium boride (Cr.sub.2B) is 51-91%.
7. The ferrous alloy of claim 6, wherein hardness of the ferrous
alloy for powder injection molding is 600-1600 VHN.
8. A method of injection molding a ferrous powder, the method
comprising: providing powder of a ferrous alloy for powder
injection molding according to claim 1; forming a powder mixture by
mixing the powder of the ferrous alloy for powder injection molding
and a binder; compression molding the powder mixture; removing the
binder by heating the powder mixture; and sintering the powder
mixture from which the binder is removed.
9. The method of claim 8, wherein the powder mixture is formed by
mixing the powder of the ferrous alloy for powder injection molding
according to claim 1, paraffin wax, tungsten carbide balls, and
heptanes in a container and rotating the container.
10. The ferrous alloy of claim 2, wherein a sum
(X.sub.Fe+X.sub.Cr+X.sub.B) of a mole fraction of the iron (Fe),
the chromium (Cr), and the boron (B) is 0.9635.
11. The ferrous alloy of claim 3, wherein a sum
(X.sub.Fe+X.sub.Cr+X.sub.B) of a mole fraction of the iron (Fe),
the chromium (Cr), and the boron (B) is 0.9635.
12. The ferrous alloy of claim 5, wherein in a microstructure of
the ferrous alloy for powder injection molding, chromium boride
(Cr.sub.2B) is distributed in a network form within a ferrite
base.
13. The ferrous alloy of claim 11, wherein in a microstructure of
the ferrous alloy for powder injection molding, chromium boride
(Cr.sub.2B) is distributed in a network form within a ferrite
base.
14. The ferrous alloy of claim 12, wherein a volume fraction of the
chromium boride (Cr.sub.2B) is 51-91%.
15. The ferrous alloy of claim 13, wherein a volume fraction of the
chromium boride (Cr.sub.2B) is 51-91%.
16. The ferrous alloy of claim 14, wherein hardness of the ferrous
alloy for powder injection molding is 600-1600 VHN.
17. The ferrous alloy of claim 15, wherein hardness of the ferrous
alloy for powder injection molding is 600-1600 VHN.
18. A method of injection molding a ferrous powder, the method
comprising: providing powder of a ferrous alloy for powder
injection molding according to claim 2; forming a powder mixture by
mixing the powder of the ferrous alloy for powder injection molding
and a binder; compression molding the powder mixture; removing the
binder by heating the powder mixture; and sintering the powder
mixture from which the binder is removed.
19. A method of injection molding a ferrous powder, the method
comprising: providing powder of a ferrous alloy for powder
injection molding according to claim 3; forming a powder mixture by
mixing the powder of the ferrous alloy for powder injection molding
and a binder; compression molding the powder mixture; removing the
binder by heating the powder mixture; and sintering the powder
mixture from which the binder is removed.
20. A method of injection molding a ferrous powder, the method
comprising: providing powder of a ferrous alloy for powder
injection molding according to claim 4; forming a powder mixture by
mixing the powder of the ferrous alloy for powder injection molding
and a binder; compression molding the powder mixture; removing the
binder by heating the powder mixture; and sintering the powder
mixture from which the binder is removed.
Description
TECHNICAL FIELD
[0001] The present invention relates to a ferrous alloy, and more
particularly, to a ferrous alloy for powder injection molding.
BACKGROUND ART
[0002] Powder injection molding (PIM) is new powder metallurgy
molding technology in which powder metallurgy technology and an
injection molding method, which is mass production technology of a
precise plastic component, are coupled.
[0003] A PIM process is a process of producing a component by
mixing a micro-powder and a polymer binder to be a subject of flow,
injecting the powder and the binder into a mold, removing the
binder from the mix, and finally sintering only the powder at a
high temperature.
[0004] Continued research and development and expanded application
of powder injection molding processes are ongoing because existing
component designers and users recognize the merits in that the
degree of freedom of component design by a combination of a
material used for powder injection molding technology and a shape
thereof is high, and diversified application materials and
processes having economic efficiency are being developed by
research institutions and powder injection molding technology
companies. That is, three-dimensional precision components may be
made entirely of a powder material such as a metal, a ceramic, a
carbide, and an intermetallic compound, and even of a processing
resistant material or a material that cannot be cast, because mass
production is available with few or no post-processes, so powder
injection molding technology is appropriate for economically
producing high value components.
[0005] Stainless steel that occupies most of a metal powder
injection molding market has a property of strength, hardness,
abrasion resistance, and corrosion resistance that a final powder
injection molding component requires with a change of chemical
composition.
[0006] Therefore, a stainless steel powder that is used for powder
injection molding variously exists as SUS304L, SUS316L, SUS430, and
SUS630, and for martensite-based stainless steel powder injection
molding, an additional heat treatment process is necessary.
[0007] Therefore, development of an alloy that can be applied to
various powder injection molding components without heat treatment
and with economical and various microstructures and properties
while fully satisfying a property of a common powder injection
molding component is requested.
[0008] The above information disclosed in this Background section
is only for enhancement of understanding of the background of the
invention and therefore it may contain information that does not
form the prior art that is already known in this country to a
person of ordinary skill in the art.
DISCLOSURE
Technical Problem
[0009] The present invention has been made in an effort to provide
a ferrous alloy having advantages of representing superior
hardness, abrasion resistance, and corrosion resistance than those
of stainless steel that is used for existing powder injection
molding and having a low production cost.
Technical Solution
[0010] An exemplary embodiment of the present invention provides a
ferrous alloy for powder injection molding including: iron (Fe) at
52.59-78.15 wt %, chromium (Cr) at 16.45-37.34 wt %, boron (B) at
3.42-7.76 wt %, silicon (Si) at 1.64-1.92 wt %, sulfur (S) at
0-0.21 wt %, carbon (C) at 0.16-0.18 wt %, and other inevitable
impurities.
[0011] A ratio X.sub.Cr/X.sub.B of the chromium (Cr) and boron (B)
may be 1.0.
[0012] A sum (X.sub.Cr+X.sub.B) of a composition of the chromium
(Cr) and the boron (B) may be 0.30 to 0.60.
[0013] A sum (X.sub.Fe+X.sub.Cr+X.sub.B) of a composition of the
iron (Fe), the chromium (Cr), and the boron (B) may be 0.9635.
[0014] In a microstructure of the ferrous alloy for powder
injection molding, chromium boride (Cr.sub.2B) may be distributed
in a network form within a ferrite base.
[0015] A volume fraction of the chromium boride (Cr.sub.2B) may be
51-91%. Hardness of the ferrous alloy for powder injection molding
may be 600-1600 VHN.
[0016] Another embodiment of the present invention provides a
method of injection molding a ferrous powder, the method including:
providing powder of the ferrous alloy for powder injection molding;
forming a powder mixture by mixing the powder of the ferrous alloy
for powder injection molding and a binder; compression molding the
powder mixture; removing the binder by heating the powder mixture;
and sintering the powder mixture from which the binder is
removed.
[0017] The powder mixture may be formed by mixing the powder of the
ferrous alloy for powder injection molding, paraffin wax, tungsten
carbide balls, and heptanes in a container and rotating the
container.
Advantageous Effects
[0018] Ferrous alloys for powder injection molding according to the
present invention form hard Cr.sub.2B boride with different volume
fractions, thereby greatly improving hardness and abrasion
resistance, compared with a conventionally used stainless
steel.
[0019] Further, ferrous alloys for powder injection molding
according to the present invention are cheaper in alloy price than
a commercial stainless steel by lowering a fraction of an alloy
element, and by reducing production cost by lowering a sintering
temperature and reducing sintering time, the ferrous alloys are
superior in price competitiveness.
DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 shows scanning electron microscope (SEM) photographs
illustrating a microstructure of a ferrous alloy for powder
injection molding according to an exemplary embodiment of the
present invention.
[0021] FIG. 2 shows Fe--Cr--B ternary system isothermal phase
diagrams of a ferrous alloy that is designed according to an
exemplary embodiment of the present invention.
[0022] FIG. 3 shows graphs illustrating calculation results of a
precipitation driving force change of Cr.sub.2B according to each
component composition on a constituent element (Fe, Cr, B, Si, S,
and C) basis of a basic alloy when designing a ferrous alloy
according to an exemplary embodiment of the present invention.
[0023] FIG. 4 shows graphs illustrating a fraction of equilibrium
phases existing at 1000.degree. C. according to a ratio of chromium
and boron in a ferrous alloy according to an exemplary embodiment
of the present invention.
[0024] FIGS. 5 to 7 show graphs illustrating an equilibrium phase
fraction according to a temperature of an alloy composition of 9
ferrous alloys that are designed according to an exemplary
embodiment of the present invention.
[0025] FIGS. 8 to 10 show SEM photographs of microstructures after
ferrous alloys are cast and heated according to an exemplary
embodiment of the present invention.
[0026] FIG. 11 illustrates X-ray diffraction analysis results of
ferrous alloys according to an exemplary embodiment of the present
invention.
[0027] FIG. 12 is a graph illustrating a price of an alloy element
according to a property (hardness reference) that is requested for
components.
MODE FOR INVENTION
[0028] These and other objects of the present application and a
method of obtaining them will become more readily apparent from the
detailed description given hereinafter together with the
accompanying drawings. While the present invention may be embodied
in many different forms, specific embodiments of the present
invention are shown in drawings and are described herein in detail,
with the understanding that the present invention is to be
considered as an exemplification of the principles of the invention
and is not intended to limit the invention to the specific
embodiments illustrated.
[0029] Hereinafter, a ferrous alloy for powder injection molding
according to an exemplary embodiment of the present invention will
be described with reference to the accompanying drawings. For
reference, detailed descriptions of well- known functions and
structures incorporated herein may be omitted to avoid obscuring
the subject matter of the present invention.
[0030] It is characterized that a ferrous alloy for powder
injection molding according to an exemplary embodiment of the
present invention includes iron (Fe) at 52.59-78.15 wt %, chromium
(Cr) at 16.45-37.34 wt %, boron (B) at 3.42-7.76 wt %, silicon (Si)
at 1.64-1.92wt %, sulfur (S) at 0-0.21 wt %, carbon (C) at
0.16-0.18 wt %, and other inevitable impurities.
[0031] A microstructure of the ferrous alloy is characterized in
that chromium boride (Cr.sub.2B) is distributed in a network form
within a ferrite base. Because a chromium boride (Cr.sub.2B) phase
according to the present invention is very hard, a shape and a
fraction distribution state of the precipitated chromium boride
phase may have a direct influence on entire hardness and abrasion
resistance of a specimen.
[0032] Because a chromium boride phase of the ferrous alloy forms a
network structure, an entire hardness distribution of the specimen
is uniform according to a position, and an applied load in an
abrasion environment is distributed to be superior in an abrasion
resistance property.
[0033] Further, it is characterized in that a volume fraction of
the chromium boride (Cr.sub.2B) is 51-91%.
[0034] It is characterized that hardness of the alloy is 600-1600
VHN.
[0035] In order to satisfy a condition of high hardness and a low
cost of the ferrous alloy that is requested in a powder injection
molding process, a high hardness property can be obtained using
precipitation of chromium boride, which is a compound of Cr and B
of a relatively cheap price.
[0036] A composition ratio of chromium (Cr) and boron (B) is fixed
to 1:1, and the sum (the sum of a mole fraction) of a composition
ratio of chrome (Cr) and boron (B) may be adjusted to 0.30-0.60. In
this case, the sum of a composition ratio (the sum of a mole
fraction) of iron (Fe), chromium (Cr), and boron (B), which are
major components, may be fixed to 0.9634.
[0037] The chromium (Cr) is generally an alloy element that is
added to increase hardenability of the alloy and to improve
corrosion resistance when quickly cooling, and in the present
invention, by bonding to boron (B), the chromium (Cr) is an element
that adjusts the chromium boride precipitation fraction.
[0038] When a content of chromium is 16.45 wt % or more, a
precipitation fraction of chromium boride is formed to be 50 vol %
or more, and thus hardness of about 600 VHN or more, which is a
high hardness request value required in a powder injection molding
process, can be obtained.
[0039] When a content of chromium exceeds 37.34 wt %, excessive
chromium boride is precipitated and thus toughness is deteriorated,
so content thereof is limited to 16.45-37.34 wt %.
[0040] A content of boron (B) is determined to be 3.42-7.76 wt %
according to such chromium (Cr) content.
[0041] Boron (B) is an element that is added for improving
hardenability, and in the present invention, boron (B) is an
element that adjusts a chromium boride precipitation fraction by
bonding to chromium. When a content of boron is 3.42 wt % or more,
a precipitation fraction of chromium boride becomes 50 vol % or
more, and thus hardness of about 600 VHN or more, which is a high
hardness request value required in a powder injection molding
process, can be obtained.
[0042] When a content of boron exceeds 7.76 wt %, excessive
chromium boride is precipitated and thus toughness is deteriorated,
so content thereof is limited to 3.42-7.76 wt %. A content of
chromium is determined to be 16.45-37.34 wt % according to such
content of boron.
[0043] In the present invention, silicone (Si) is an element that
performs a function of stabilizing a base with ferrite in a
sintering press of a powder injection molding process and improving
hardenability through solid solution reinforcement. When a content
of silicone is 1.64 wt % or less, a solid solution reinforcement
and ferrite stabilization effect is slight, and when a content of
silicone is 1.92 wt % or more, a solid solution reinforcement
effect does not increase in proportion thereto, and so content of
silicone is limited to 1.64-1.92 wt %.
[0044] Because sulfur generally forms a non-metallic inclusion and
deteriorates properties of an alloy, it is preferable to control
sulfur content to be as low as possible, but by adding sulfur up to
a limit at which a sulfide (FeS) is not formed, the present
invention attempts to obtain a base and chromium boride
stabilization effect. Therefore, it is preferable that an upper
limit thereof is set to 0.21 wt %.
[0045] Carbon (C) is an element that can efficiently improve
hardenability of an alloy, and in order to satisfy hardness that
the present invention desires, carbon (C) at 0.16 wt % or more
should be contained, and when carbon (C) at 0.18 wt % or more is
added, toughness is deteriorated and thus the content of carbon (C)
is limited to 0.16-0.18 wt %.
[0046] A method of injection molding a ferrous powder according to
another exemplary embodiment of the present invention includes
steps of: providing a powder of a ferrous alloy for powder
injection molding; forming a powder mixture by mixing the powder of
a ferrous alloy for powder injection molding and a binder;
compression molding the powder mixture; removing the binder by
heating the powder mixture; and sintering the powder mixture from
which the binder is removed.
[0047] The powder mixture is formed by mixing a powder of the
ferrous alloy for powder injection molding, paraffin wax, tungsten
carbide balls, and heptanes in a container and rotating the
container.
[0048] Compression molding of the powder mixture may be performed
by using a press at a pressure of 100 kgf/cm.sup.2 or more after
charging the powder mixture into a mold.
[0049] The binder may be removed by raising the temperature of the
compression molded powder mixture up to 500.degree. C. at a heating
rate of 2.degree. C./min or more and maintaining the temperature
for 1 hour.
[0050] The sintering may be performed by charging the powder
mixture from which the binder is removed into a heat treating
furnace, heating it to 1175.degree. C. at a heating rate of
3.degree. C./min or more in a hydrogen atmosphere, and maintaining
the temperature for 1 hour.
[0051] The present invention provides a ferrous alloy with high
hardness and at a low cost for powder injection molding that may
represent various properties by forming chromium boride (Cr.sub.2B)
of different volume fractions in a ferrite base by reducing a ratio
of an alloy element and changing a ratio of chromium (Cr) and boron
(B) based on a Fe-43Cr-5.6B-1.8Si-0.2S-0.17C (wt %) alloy.
[0052] Further, a microstructure of alloys according to the present
invention may have a structure in which other crystalline particles
are distributed in addition to chromium boride (Cr.sub.2B) and a
ferrite base. Hereinafter, the present invention will be described
in detail through a description of exemplary embodiments of the
present invention and comparative examples. However, the following
exemplary embodiments are merely examples for aiding comprehension
of the present invention, and the scope of the present invention is
not reduced or limited by the following exemplary embodiments.
Exemplary Embodiment 1
[0053] Table 1 illustrates a composition of a basic alloy as a
reference when designing a ferrous alloy for powder injection
molding according to an exemplary embodiment of the present
invention.
TABLE-US-00001 TABLE 1 Fe Cr B Si S C atomic % Bal. 38.15 35.79
22.40 2.77 0.27 0.61 wt % Bal. 49.23 43 5.6 1.8 0.2 0.17
[0054] Table 2 illustrates a composition of an alloy in which a
ferrous alloy for powder injection molding according to an
exemplary embodiment of the present invention is designed by
thermodynamic calculation.
TABLE-US-00002 TABLE 2 Alloy Ratio Fe Cr B Si S C Exemplary atom-
66.34 15.00 15.00 2.77 0.27 0.61 Embod- ic % iment 1 wt % 78.15
16.45 3.42 1.64 0.18 0.16 Exemplary atom- 51.34 22.50 22.50 2.77
0.27 0.61 Embod- ic % iment 2 wt % 65.54 26.75 5.56 1.78 0.20 0.17
Exemplary atom- 38.15 29.10 29.10 2.77 0.27 0.61 Embod- ic % iment
3 wt % 52.59 37.34 7.76 1.92 0.21 0.18 Exemplary atom- 66.34 18.46
11.54 2.77 0.27 0.61 Embod- ic % iment 4 wt % 75.86 19.66 2.55 1.59
0.18 0.15 Exemplary atom- 51.34 27.70 17.31 2.77 0.27 0.61 Embod-
ic % iment 5 wt % 62.49 31.39 4.08 1.70 0.19 0.16 Exemplary atom-
38.15 35.81 22.38 2.77 0.27 0.61 Embod- ic % iment 6 wt % 49.22
43.02 5.59 1.80 0.20 0.17 Exemplary atom- 66.34 20.63 9.38 2.77
0.27 0.61 Embod- ic % iment 7 wt % 74.50 21.57 2.04 1.57 0.17 0.15
Exemplary atom- 51.34 30.94 14.06 2.77 0.27 0.61 Embod- ic % iment
8 wt % 60.72 34.07 3.22 1.65 0.18 0.16 Exemplary atom- 38.15 40.01
18.19 2.77 0.27 0.61 Embod- ic % iment 9 wt % 47.33 46.22 4.37 1.73
0.19 0.16
[0055] A ferrous alloy for powder injection molding was arc melted
under an argon (Ar) atmosphere according to a composition of Table
2. An alloy that was used for arc melting included high purity Fe
(99.9 wt %), Si (99.99 wt %), and C (99 wt %), as well as
previously alloyed FeB (99.2 wt %), FeS (98.5 wt %), and FeCr (98.6
wt %) as a pre-alloy, and was turned over 4-5 times and repeatedly
melted so as to uniformize a mother alloy component.
[0056] By suction casting a mother alloy that is produced by arc
melting, a bar specimen having a thickness of 5 mm was cast.
Because the cast alloy is not in a homogeneous state because of
sufficient diffusion, unlike a powder injection molding process
product, the cast alloy was heat treated and cooled in a furnace in
a similar condition (maintained for 30 min at 1200.degree. C.) to a
sintering condition of a powder injection molding process.
[0057] Cast and heat treated alloys were etched with a Viella
solution (45 ml glycerol, 15 ml HNO.sub.3, and 30 ml HCl) and were
observed with a scanning electron microscope (SEM).
[0058] Phases existing within the alloy were analyzed with an X-ray
diffraction test method, and a volume fraction of chromium boride
(Cr.sub.2B) was measured by an image analyzer.
[0059] Entire hardness of the alloy was measured under a load of
300 g by a Vickers hardness device.
Exemplary Embodiment 2
[0060] By melting element powders of a ferrous alloy for powder
injection molding with a vacuum induction furnace with a 30 kg
capacity according to alloy compositions of Table 2, a mother alloy
ingot was produced. The used alloy included high purity Fe (99.9 wt
%), Si (99.99 wt %), and C (99 wt %), and previously alloyed FeB
(99.2 wt %), FeS (98.5 wt %), and FeCr (98.6 wt %) as a
pre-alloy.
[0061] The ingot was again melted at 1550.degree. C. in an argon
(Ar) atmosphere, and was produced as a spherical powder through
N.sup.2 gas atomization at an injection pressure of 20 bar.
[0062] By classifying this powder, a powder having a size of 25
.mu.m or less was used for powder injection molding.
[0063] In the present invention, in order to easily mix and to
reduce volume expansion, paraffin wax was used as a binder. A
ferrous powder with a weight of 97 g and 3 g of a binder were mixed
and charged into a high-density polyethylene (HDPE) container
having a capacity of 300 ml together with 20 ml of tungsten carbide
balls, and heptanes were filled to a volume of 250 ml.
[0064] By setting a rotation speed of the container to 45 rpm and
rotating for 24 hours, the ferrous powder and the binder were
mixed. After a powder mixture was dried at a hot plate at
55.degree. C., the powder mixture was charged into a circular mold
with a diameter of 13 mm and compressed and molded using a press at
room temperature with a pressure of 100 kgf/cm.sup.2.
[0065] In order to remove the binder from the resultant compact,
the temperature of the compact was raised to 500.degree. C. at a
heating rate of 2.degree. C./min, and the compact was maintained
and degreased at this temperature for 1 hour. In order to densify
the degreased compact, the compact was charged into a heat treating
furnace and a temperature thereof was raised up to 1175.degree. C.
at a heating rate of 3.degree. C./min in a hydrogen atmosphere, and
the compact was sintered for 1 hour.
Exemplary Embodiment 3
High Temperature Phase Equilibrium Analysis by Thermodynamic
Calculation
[0066] In order to design a changed alloy composition from a basic
composition of an alloy, a property determining element of the
alloy is determined, and a property (particularly, hardness) of the
alloy is closely related to precipitation of chromium boride
(Cr.sub.2B).
[0067] Accordingly, in order to estimate a chromium boride
(Cr.sub.2B) forming amount, a thermodynamic calculation was
performed. Software that is used for calculation is ThermoCalc,
which is a commercial thermodynamic calculation program, and as a
thermodynamic database, an upgraded version of TCFE2000 was
used.
[0068] FIG. 2 is a Fe--Cr--B ternary system isothermal phase
diagram that is formed using the database.
[0069] FIG. 3 illustrates a calculation result of a precipitation
driving force change of Cr.sub.2B according to each component
composition on a constituent element (Fe, Cr, B, Si, S, and C)
basis of a basic alloy at 1250.degree. C. (pink) and 1000.degree.
C. (yellowish green).
[0070] A precipitation driving force is obtained by calculating an
amount of a component to adjust in a range from 0 to two times an
original amount in a state in which a ratio between other
components is fixed.
[0071] It can be seen from FIG. 3 that when changing an amount of
iron (Fe), chromium (Cr), and boron (B), which are major
constituent elements of a basic alloy, a precipitation driving
force of chromium boride (Cr.sub.2B) is greatly changed. Therefore,
when selecting a basic alloy for designing an alloy, iron,
chromium, and boron were used as major variables.
[0072] When a basic alloy is used for PIM, the basic alloy is
sintered for a long time at 1200.degree. C. in a powder injection
process, and thus it may be considered that a sintered
microstructure has arrived at equilibrium.
[0073] At 1000.degree. C., which is a temperature that may be
thought of as a cooling temperature after arriving at equilibrium,
by adjusting a composition of iron, chromium, and boron, an
equilibrium phase fraction was calculated.
[0074] A composition was obtained when the sum
(X.sub.Fe+X.sub.Cr+X.sub.B) of a mole fraction of a major component
is maintained at 0.9634 based on a basic alloy of Table 1 as a
reference, but values of X.sub.Fe and (X.sub.Cr+X.sub.B) were
adjusted by setting a ratio X.sub.Cr/X.sub.B of Cr and B to three
cases of 1.0 (B ratio increase), 1.6 (basis ratio), and 2.2 (Cr
ratio increase).
[0075] FIG. 4 illustrates a fraction of equilibrium phases existing
at 1000.degree. C. in the three cases. As the boron ratio increases
and as the (X.sub.Cr+X.sub.B) value increases, an equilibrium phase
fraction of chromium boride (Cr.sub.2B) increases.
[0076] In a composition ratio of the three cases, by again
selecting three cases of (X.sub.Cr+X.sub.B)=0.30, 0.45, and 0.5819,
a total of 9 alloy compositions were obtained, and this is
represented in Table 2.
[0077] First, at each of compositions in which an X.sub.Cr/X.sub.B
ratio is 1.0, 1.6, and 2.2, cases in which the (X.sub.Cr+X.sub.B)
sums are 0.30, 0.45, and 0.5819 were divided into Exemplary
Embodiments 1 to 9.
[0078] Here, Exemplary Embodiment 6 of X.sub.Cr/X.sub.B=1.6 and
(X.sub.Cr+X.sub.B)=0.5819 is the same as an alloy of Table 1, which
is a reference composition.
[0079] FIGS. 5 to 7 are graphs illustrating an equilibrium phase
fraction according to a temperature of the 9 alloy compositions.
Thereby, thermodynamic calculation values of an equilibrium phase
fraction of chromium boride (Cr.sub.2B) at several temperatures may
be seen.
[0080] For example, in an alloy composition of Exemplary Embodiment
1 (X.sub.Cr/X.sub.B=1.0, X.sub.Cr+X.sub.B=0.30), at 1000.degree.
C., a Cr.sub.2B fraction is about 43 vol % and the remaining
elements are BCC .alpha.-Fe (ferrite) and FCC .gamma.-Fe
(austenite). When the temperature drops to room temperature, a
fraction of Cr.sub.2B is maintained, .gamma.-Fe is transformed into
a-Fe, and thus it is expected that a room temperature
microstructure is formed with Cr.sub.2B of 43 vol % and .alpha.-Fe
of 57 vol %.
Exemplary Embodiment 4
Microstructure of Ferrous Alloy
[0081] As long time sintering is performed at a high temperature,
PIM components arrive at a sufficient equilibrium state, but after
melting, because solidification by cooling occurs relatively
quickly, a cast alloy may not arrive in an equilibrium state.
[0082] Therefore, in order to obtain a microstructure having
arrived in a sufficient equilibrium state, heat treatment
corresponding to a sintering process of PIM was applied to a cast
alloy.
[0083] FIGS. 8 to 10 illustrate SEM microstructures of heat-treated
alloys after casting. After heat treatment, because diffusion
occurs, a microstructure thereof is considerably different from
that of the cast alloy.
[0084] When martensite is maintained for a long time at a high
temperature, the martensite is changed to tempered martensite,
i.e., ferrite in which micro-carbides are precipitated, and a form
of Cr.sub.2B is changed from a needle shape or a bar shape to a
spherical or oval shape.
[0085] Like a case of a cast alloy, as (X.sub.Cr+X.sub.B)
increases, a tendency that a Cr2B amount increases is represented.
A microstructure (FIG. 9(f)) of an alloy of Exemplary Embodiment 6
having the same composition as an alloy of a reference composition
is similar to that of a component that is powder injection molded
with the basic alloy powder of FIG. 1. It may be expected that a
microstructure of a heat-treated alloy after casting will be
similar to a microstructure of a component having passed through a
PIM process after making the alloy into powder.
[0086] FIG. 11 illustrates X-ray diffraction analysis results of
heat-treated alloys. Because a peak of .alpha.-Fe (ferrite) and
Cr.sub.2B is represented in all alloys, it can be seen that
Cr.sub.2B is distributed within a ferrite base. A precipitated
Cr.sub.2B fraction was measured and is represented in Table 3, and
was compared with a Cr.sub.2B fraction that is obtained from FIGS.
5 to 7 representing an equilibrium phase fraction.
Exemplary Embodiment 5
Hardness of Ferrous Alloy
[0087] Hardness of a cast alloy and a heat-treated alloy was
measured and is represented in Table 3. Hardness of a cast alloy
and a heat-treated alloy is different in the same chemical
composition because a microstructure of a cast alloy that is formed
with Cr.sub.2B and martensite is changed after heat treatment.
TABLE-US-00003 TABLE 3 Volume fraction of chromium boride (vol %)
Vickers hardness (VHN) Calculation After heat After heat Alloy
value treatment After casting treatment Exemplary 48 51 1107.5
(.+-.60.8) 595.4 (.+-.37.8) Embodiment 1 Exemplary 71 76 1200.5
(.+-.7.6) 1144.6 (.+-.123.7) Embodiment 2 Exemplary 89 91 1649.6
(.+-.105.1) 1624.7 (.+-.125.2) Embodiment 3 Exemplary 40 46 837.4
(.+-.52.8) 404.4 (.+-.14.6) Embodiment 4 Exemplary 57 59 735.3
(.+-.14.0) 648.9 (.+-.170.7) Embodiment 5 Exemplary 71 80 759.5
(.+-.33.4) 1260.5 (.+-.55.5) embodiment 6 Exemplary 34 42 658.0
(.+-.89.2) 325.7 (.+-.17.7) Embodiment 7 Exemplary 47 47 482.8
(.+-.18.3) 537.0 (.+-.58.2) Embodiment 8 Exemplary 60 66 617.6
(.+-.6.2) 962.5 (.+-.149.2) Embodiment 9
[0088] In the cast alloy, martensite is changed to ferrite after
heat treatment, and Cr.sub.2B arrives in an equilibrium state by a
diffusion effect by high temperature heat treatment and thus
hardness and a fraction of Cr.sub.2B increases. After heat
treatment, as martensite is changed to ferrite, hardness decreases,
and as hardness and a fraction of Cr.sub.2B increases, hardness
increases, while hardness of a cast alloy increases or decreases
after heat treatment according to a competition relationship
between the two.
[0089] That is, in an alloy composition having a high Cr.sub.2B
fraction, an increase effect of hardness and a fraction of
Cr.sub.2B is larger than a hardness decrease by martensite-ferrite
transformation, and thus after heat treatment, hardness increases.
However, in an alloy composition having a high ferrite fraction,
because a hardness decrease effect by martensite-ferrite
transformation is larger than an increase effect of hardness and a
fraction of Cr.sub.2B, a tendency that hardness decreases after
heat treatment is represented.
[0090] Ferrous alloys that are produced in this research may be
variously used for producing a PIM component according to a
fraction and hardness of Cr.sub.2B in consideration of similarity
of a microstructure (FIG. 9(f)) of an alloy of Exemplary Embodiment
6 and a microstructure (FIG. 1) of a component in which PIM is
performed with a basic alloy powder.
[0091] As a Cr.sub.2B fraction generally increases, hardness
increases, but a brittle fracture possibility increases. Therefore,
when applying to a PIM component, a high Cr.sub.2B fraction and
hardness are not always necessary and thus it is preferable to
effectively select an alloy having a property that is appropriate
for a property that is requested for the component.
[0092] For example, when a hardness condition that is requested for
a PIM component is 500-600 VHN, an alloy corresponding thereto, for
example, alloys of Exemplary Embodiments 1, 5, and 8, may be
applied. In this case, the alloy may have a merit that brittleness
decreases and toughness increases, compared with an alloy having
high hardness.
[0093] Further, when a Cr.sub.2B fraction increases, an amount of
an alloy element increases according to the increased Cr.sub.2B
fraction, and thus it is preferable to together consider economic
efficiency including a price of an alloy element as well as a
property that is requested for the PIM component.
[0094] FIG. 12 is a graph illustrating a price of an alloy element
according to a property (hardness reference) that is requested for
components. A price reference of an alloy element was formed based
on an LME daily price (based on Jul. 27, 2010).
[0095] A price of an alloy element that is contained in an alloy
that is made in this research is lower than that of a basic alloy,
and when hardness decreases, an alloy element price also
decreases.
[0096] Further, because a price of an alloy element is lower than
that of ferrite-based or martensite-based SUS630 stainless steel
(composition: Fe-17Cr-4Ni-4Cu-0.35Nb-0.07C (wt %), hardness: 360
VHN) that is much used for PIM, the alloy element may be fully used
as a replacement.
[0097] Therefore, when simultaneously considering a property and
economic efficiency that is requested for a PIM component, an alloy
of this research can be fully applied to a PIM component of various
conditions and has an excellent property and advantageous economic
efficiency.
[0098] As described above, as hard Cr.sub.2B adjusts a Cr.sub.2B
fraction from the present invention in which a PIM alloy that is
distributed in a ferrite base is designed, an alloy representing
various properties may be successfully produced.
[0099] Particularly, a Cr.sub.2B fraction and hardness was
estimated from a calculation result of a high temperature
equilibrium phase fraction by a thermodynamic calculation, and a
ferrous alloy having various Cr.sub.2B fractions and hardness was
produced using this.
[0100] As these alloys contain much Cr.sub.2B that is stable at a
high temperature and has excellent corrosion resistance, it is
expected that these alloys are much superior to an existing
stainless steel PIM component in terms of high temperature
property, abrasion resistance, and corrosion resistance, as well as
room temperature hardness, and thus there is a new possibility that
these alloys can be applied to a structural components that require
excellent properties.
[0101] Because an alloy of Exemplary Embodiment 3 represents
hardness of 1600 VHN or more, the alloy may be fully applied even
to a PIM component of a tungsten carbide alloy as well as stainless
steel. While this invention has been described in connection with
what is presently considered to be practical exemplary embodiments,
it is to be understood that the invention is not limited to the
disclosed embodiments, but, on the contrary, is intended to cover
various modifications and equivalent arrangements included within
the spirit and scope of the appended claims.
* * * * *