U.S. patent application number 16/952178 was filed with the patent office on 2021-05-27 for nickel-based superalloy for diffusion bonding and method for diffusion bonding using the same.
The applicant listed for this patent is KOREA ATOMIC ENERGY RESEARCH INSTITUTE. Invention is credited to Jong Bae Hwang, Eung Seon Kim, Min Hwan Kim, In Jin SAH.
Application Number | 20210156004 16/952178 |
Document ID | / |
Family ID | 1000005276725 |
Filed Date | 2021-05-27 |
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United States Patent
Application |
20210156004 |
Kind Code |
A1 |
SAH; In Jin ; et
al. |
May 27, 2021 |
NICKEL-BASED SUPERALLOY FOR DIFFUSION BONDING AND METHOD FOR
DIFFUSION BONDING USING THE SAME
Abstract
The present invention relates to a nickel-based superalloy for
diffusion bonding, which includes a surface depletion layer in a
state in which an aluminum (Al) or titanium (Ti) content is
depleted, the surface depletion layer being formed to a depth of 50
.mu.m or less from a surface for diffusion bonding, and a method
for diffusion bonding using the same.
Inventors: |
SAH; In Jin; (Sejong-si,
KR) ; Hwang; Jong Bae; (Chungju-si, KR) ; Kim;
Eung Seon; (Seoul, KR) ; Kim; Min Hwan;
(Sejong-si, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KOREA ATOMIC ENERGY RESEARCH INSTITUTE |
Daejeon |
|
KR |
|
|
Family ID: |
1000005276725 |
Appl. No.: |
16/952178 |
Filed: |
November 19, 2020 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B23K 20/02 20130101;
C22C 19/055 20130101 |
International
Class: |
C22C 19/05 20060101
C22C019/05; B23K 20/02 20060101 B23K020/02 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 27, 2019 |
KR |
10-2019-0154595 |
Claims
1. A nickel-based superalloy for diffusion bonding, comprising: a
surface depletion layer in which an aluminum (Al) or titanium (Ti)
content is depleted, wherein the surface depletion layer is formed
to a depth of 50 .mu.m or less from a surface for diffusion
bonding.
2. The nickel-based superalloy of claim 1, wherein the nickel-based
superalloy comprises 20.0 to 24.0 wt % of chromium (Cr); 10.0 to
15.0 wt % of cobalt (Co); 8.0 to 10.0 wt % of molybdenum (Mo); 0.8
to 1.5 wt % of aluminum (Al); 0.6 wt % or less of titanium (Ti);
and the remainder of nickel (Ni).
3. The nickel-based superalloy of claim 1, wherein when the surface
depletion layer is divided into an upper surface depletion layer
and a lower surface depletion layer based on the halfway point of
the depth of the surface depletion layer, and the upper surface
depletion layer has a higher degree of depletion of Al or Ti
content compared to the lower surface depletion layer.
4. A method for diffusion bonding of a nickel-based superalloy,
comprising: (a) forming an outer oxide film and an inner oxide by
pre-oxidizing a nickel-based superalloy parent material; (b)
removing the formed outer oxide film and an inner oxide-containing
layer; and (c) preparing a diffusion-bonded nickel-based superalloy
material by performing diffusion bonding on the nickel-based
superalloy from which the outer oxide film and inner
oxide-containing layer are removed, wherein, in Step (c), the
nickel-based superalloy comprises a surface depletion layer in a
state in which an Al or Ti content is depleted, and the surface
depletion layer is formed to a depth of 50 .mu.m or less from a
surface for diffusion bonding.
5. The method of claim 4, wherein the pre-oxidation in Step (a) is
performed under atmospheric conditions at 600 to 1,200.degree. C.
for 0.1 to 500 hours.
6. The method of claim 4, wherein, in Step (a), the outer oxide
film comprises chromium oxide and has a thickness of 0.1 to 50
.mu.m.
7. The method of claim 4, wherein, in Step (a), the inner oxide
comprises aluminum oxide, titanium oxide or a combination thereof,
and has a depth of 1 to 100 .mu.m.
8. The method of claim 4, wherein, in Step (c), during the
diffusion bonding, the secondary phase formation by Al or Ti at the
interface between the nickel-based superalloys is inhibited or
reduced.
9. The method of claim 4, further comprising: (d) thermally
treating the formed diffusion-bonded nickel-based superalloy
material at 1,000 to 1,200.degree. C. for 1 to 100 hours.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to and the benefit of
Korean Patent Application No. 10-2019-0154595, filed on Nov. 27,
2019, the disclosure of which is incorporated herein by reference
in its entirety.
BACKGROUND
1. Field of the Invention
[0002] The present invention relates to a nickel-based superalloy
for diffusion bonding (particularly, an alloy derived from a parent
material of a solid solution strengthened nickel-based superalloy)
and a method for diffusion bonding using the same.
2. Discussion of Related Art
[0003] Bonding is classified into liquid-phase bonding and
solid-phase bonding according to whether two materials to be bonded
(a metal, a ceramic, a polymer, etc.) are phase-transformed from a
solid phase to a liquid phase during bonding. As one method of
solid-phase diffusion, diffusion bonding is a technique for bonding
materials using a diffusion phenomenon of atoms. Accordingly, it is
known that a diffusion-bonded material has mechanical performance
and microstructures at an interface and near the interface at the
same levels as those of a parent material.
[0004] The prior art focuses on presenting optimal conditions by
regulating variables (temperature, compressive load, environment,
surface condition, post heat treatment) used for diffusion bonding.
The diffusion bonding method according to the prior art is
extremely limited in grain boundary movement due to secondary phase
formation at an interface. This is a phenomenon in which an oxide
is formed at an interface due to aluminum (Al) and titanium (Ti)
intentionally included in a solid solution strengthened
nickel-based superalloy to improve oxidation/corrosion resistance
and ensure excellent high temperature strength, and has a problem
of weak integrity of the interface under a high temperature
operating condition.
[0005] In addition, to address the formation of planar grains at an
interface, the prior art showed that it is possible to form
spherical grains similar to a parent material at the interface by
promoting interdiffusion due to composition differences of elements
by inserting an intermediate insert (Ni, Ni--Cr foil, etc.) before
diffusion bonding or applying the main component (Ni, etc.) of a
parent material to a surface using various deposition methods.
However, this may increase costs due to an additional process, and
it is still difficult to apply it to a high temperature apparatus
with a complicated shape. In addition, the prior art may be a
method of addressing the formation of planar grains at the
interface, but basically, there is a problem of secondary phases
remaining at the interface.
PRIOR ART
Patent Document
[0006] Korean Patent Publication No. 10-1527112 (Jun. 2, 2015).
SUMMARY OF THE INVENTION
[0007] The present invention is directed to providing a
nickel-based superalloy for diffusion bonding of a nickel-based
superalloy, which includes a surface depletion layer in a state in
which an aluminum (Al) or titanium (Ti) content is depleted to
inhibit or reduce the secondary phase formation by Al or Ti at the
interface between the nickel-based superalloys, wherein the surface
depletion layer is formed to a depth of 50 .mu.m or less from a
surface for diffusion bonding.
[0008] However, technical problems to be solved in the present
invention are not limited to the above-described problems, and
other problems which are not described herein will be fully
understood by those of ordinary skill in the art from the following
descriptions.
[0009] The present invention provides a nickel-based superalloy for
diffusion bonding, which includes a surface depletion layer in a
state in which an Al or Ti content is depleted, wherein the surface
depletion layer is formed to a depth of 50 .mu.m or less from a
surface for diffusion bonding.
[0010] In addition, the present invention provides a method for
diffusion bonding of a nickel-based superalloy, which includes: (a)
pre-oxidizing a parent material of a nickel-based superalloy to
form an outer oxide film and an inner oxide; (b) removing the
formed outer oxide film and an inner oxide-containing layer; and
(c) preparing a diffusion-bonded nickel-based superalloy material
by performing diffusion bonding on the nickel-based superalloy from
which the formed outer oxide film and inner oxide-containing layer
are removed. In Step (c), the nickel-based superalloy includes a
surface depletion layer in a state in which an Al or Ti content is
depleted, and the surface depletion layer is formed to a depth of
50 .mu.m or less from a surface for diffusion bonding.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The above and other objects, features and advantages of the
present invention will become more apparent to those of ordinary
skill in the art by describing in detail exemplary embodiments
thereof with reference to the accompanying drawings, in which:
[0012] FIGS. 1A to 1C are a scanning electron microscope (SEM)
image of a nickel-based superalloy in which an outer oxide film, an
inner oxide and a depletion layer are formed after pre-oxidation at
850.degree. C. according to Example 1 and electronic probe
microanalyzer (EPMA) graphs thereof;
[0013] FIGS. 2A to 2C are scanning electron microscope (SEM) images
of a nickel-based superalloy in which an outer oxide film, an inner
oxide and a depletion layer are formed after pre-oxidation at
930.degree. C. according to Example 2 and electronic probe
microanalyzer (EPMA) graphs thereof; and
[0014] FIGS. 3A to 3E are stress-strain curves at various
temperatures (room temperature, 500.degree. C., 700.degree. C.,
800.degree. C. and 900.degree. C.) of a diffusion-bonded material
according to Example 2.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
[0015] While researching the inhibition or reduction of the
secondary phase formation by Al or Ti at the interface between
nickel-based superalloys during diffusion bonding of the
nickel-based superalloy, the inventors prepared a nickel-based
superalloy including a surface depletion layer in a state in which
an Al or Ti content is depleted through pre-oxidation and removal
and then performed diffusion bonding thereon, thereby confirming
excellent mechanical characteristics (stress and strain), and thus
the present invention was completed.
[0016] Hereinafter, the present invention will be described in
detail.
[0017] Nickel-Based Superalloy for Diffusion Bonding
[0018] The present invention provides a nickel-based superalloy for
diffusion bonding, which includes a surface depletion layer in a
state in which an aluminum (Al) or titanium (Ti) content is
depleted, and is formed to a depth of 50 .mu.m or less from a
surface for diffusion bonding.
[0019] The "nickel-based superalloy for diffusion bonding" refers
to a matrix in a state in which at least a part or all of an inner
oxide-containing layer is removed while an outer oxide film formed
through pre-oxidation from a nickel-based superalloy parent
material is completely removed, and is subjected to diffusion
bonding. In this regard, the "pre-oxidation," "outer oxide film"
and "inner oxide" will be described later
[0020] The nickel-based superalloy parent material preferably uses
a solid solution strengthened nickel-based superalloy parent
material having a low Al or Ti content to effectively form a
surface depletion layer in which an Al or Ti content is depleted in
the nickel-based superalloy, but the present invention is not
limited thereto. Meanwhile, the use of a precipitate-strengthened
nickel-based superalloy parent material having a relatively high Al
or Ti content has a limit in effectively forming a surface
depletion layer only through pre-oxidation. In the present
invention, as the nickel-based superalloy parent material, Alloy
617 was used.
[0021] Here, since Al or Ti is used to form an inner oxide in the
nickel-based superalloy parent material, the nickel-based
superalloy includes a surface depletion layer in which an Al or Ti
content is depleted, compared to the nickel-based superalloy parent
material. Meanwhile, since chromium (Cr) is used to form an outer
oxide film in the nickel-based superalloy parent material, the
nickel-based superalloy may include a surface depletion layer in
which a chromium (Cr) content is also depleted compared to the
nickel-based superalloy parent material.
[0022] Accordingly, although the entire composition of the
nickel-based superalloy has a low Al or Ti content compared to the
nickel-based superalloy parent material, and may also have a low Cr
content, the entire composition of the nickel-based superalloy is
as follows in detail.
[0023] Specifically, the nickel-based superalloy may include 20.0
to 24.0 wt % of Cr; 10.0 to 15.0 wt % of Co; 8.0 to 10.0 wt % of
Mo; 0.8 to 1.5 wt % of aluminum (Al); 0.6 wt % or less of Ti; and
the remainder of Ni.
[0024] Hereinafter, the role and effect of each element will be
described:
[0025] (1) Chromium (Cr)
[0026] Cr is an element for increasing high temperature oxidation
resistance, and a preferable Cr content is 20.0 to 24.0 wt %. Here,
when the Cr content is less than 20.0 wt %, there is a problem in
formation of a stable high temperature oxide film, and when the Cr
content is more than 24.0 wt %, there is a problem caused by
secondary phase formation according to high temperature aging.
[0027] (2) Cobalt (Co)
[0028] Co is an element for increasing high temperature strength
due to a solid solution strengthening effect, and a preferable
cobalt content is 10.0 to 15.0 wt %. Here, when the Co content is
less than 10.0 wt %, there is a problem caused by a decrease in
effective solid solution strengthening effect, and when the Co
content is more than 15.0 wt %, there is a problem caused by
secondary phase formation according to high temperature aging.
[0029] (3) Molybdenum (Mo)
[0030] Mo is an element for increasing high temperature strength
according to a solid solution strengthening effect, and a
preferable Mo content is 8.0 to 10.0 wt %. Here, when the Mo
content is less than 8.0 wt %, there is a problem caused by a
decrease in effective solid solution strengthening effect, and when
the Mo content is more than 10.0 wt %, there is a problem caused by
secondary phase formation according to high temperature aging.
[0031] (4) Aluminum (Al)
[0032] A is an element for increasing high temperature corrosion
resistance and high temperature strength according to a precipitate
strengthening effect, and a preferable Al content is 0.8 to 1.5 wt
%. Here, when the Al content is less than 0.8 wt %, there is a
problem caused by a decrease in high temperature corrosion
resistance and a decrease in effective precipitate strengthening
effect, and when the Al content is more than 1.5 wt %, there is a
problem caused by secondary phase formation according to high
temperature aging.
[0033] (5) Titanium (Ti)
[0034] Ti is an element for increasing high temperature strength
according to a precipitate strengthening effect, and a preferable
Ti content is 0.6 wt % or less, and preferably, more than 0 to 0.6
wt %. Here, when the Ti content is more than 0.6 wt %, there is a
problem caused by secondary phase formation according to high
temperature aging.
[0035] (6) Nickel (Ni)
[0036] Ni is an element serving as a base metal.
[0037] Other than these metals, the nickel-based superalloy may
further include a maximum 3.0 wt % of iron (Fe), and a maximum 1.0
wt % of one or more selected from the group consisting of Mn,
carbon (C), copper (Cu), silicon (Si), sulfur (S), boron (B) and
phosphorus (P).
[0038] The surface depletion layer is in a state in which an Al or
Ti content is depleted, compared to the nickel-based superalloy
parent material, and the surface depletion layer is formed to a
depth of 50 .mu.m or less from a surface for diffusion bonding
(that is, a surface directly subjected to diffusion bonding), and
preferably a depth of 30 .mu.m or less, but the present invention
is not limited thereto.
[0039] Due to the surface depletion layer, it is possible to move a
planar grain boundary formed at an interface through diffusion
according to composition differences of elements during diffusion
bonding, and the secondary phase formation by Al or Ti at an
interface may be effectively inhibited or reduced, thereby forming
intermetallic bonds.
[0040] As the surface depletion layer is closer to the surface of
diffusion bonding, it may have a gradient in which the degree of
depletion of an Al or Ti content increases. In other words, when
the surface depletion layer is divided into an upper surface
depletion layer and a lower surface depletion layer based on the
halfway point of the depth of the surface depletion layer, the
upper surface depletion layer has a higher degree of depletion of
Al or Ti content compared to the lower surface depletion layer, but
the present invention is not limited thereto. Therefore, as the
diffusion bonding progresses on the surface of the surface
depletion layer, planar grains formed at the interface may be
formed into spherical grains through the diffusion according to
composition differences of elements. Thus, the surface depletion
layer may have the same levels of mechanical properties as those of
a parent material at room temperature and high temperature.
[0041] Method for Diffusion Bonding of Nickel-Based Superalloy
[0042] The present invention provides a method for diffusion
bonding of a nickel-based superalloy, which includes: (a) forming
an outer oxide film and an inner oxide by pre-oxidizing a
nickel-based superalloy parent material; (b) removing the formed
outer oxide film and an inner oxide-containing layer; and (c)
preparing a diffusion-bonded nickel-based superalloy material by
performing diffusion bonding on the nickel-based superalloy from
which the outer oxide film and inner oxide-containing layer are
removed, and in Step (c), the nickel-based superalloy includes a
surface depletion layer in a state in which an Al or Ti content is
depleted, and the surface depletion layer is formed to a depth of
50 .mu.m or less from a surface for diffusion bonding.
[0043] First, the method for diffusion bonding for a nickel-based
superalloy according to the present invention includes forming an
outer oxide film and an inner oxide by pre-oxidizing a nickel-based
superalloy parent material (Step (a)) and removing the formed outer
oxide film and inner oxide-containing layer (Step (b)).
[0044] The pre-oxidation refers to oxidation under a high
temperature condition for forming an outer oxide film and an inner
oxide in the nickel-based superalloy parent material, and may be
performed under atmospheric conditions including oxygen at 600 to
1,200.degree. C. for 0.1 to 500 hours, and preferably at 700 to
1,050.degree. C. for 10 to 500 hours, but the present invention is
not limited thereto.
[0045] The "outer oxide film" used herein refers to an oxide film
formed by reaction with the external environment through
pre-oxidation on the surface of the nickel-based superalloy parent
material, may include chromium oxide, and specifically, a nickel
oxide (NiO) layer, a nickel-chromium composite oxide
(NiO--Cr.sub.2O.sub.3) layer and a chromium oxide (Cr.sub.2O.sub.3)
layer may be sequentially formed based on the external environment
through a reaction between oxygen provided from the external
environment and nickel (Ni) and chromium (Cr). Here, the thickness
of the outer oxide film may be 0.1 to 50 .mu.m, and preferably 1 to
10 .mu.m, but the present invention is not limited thereto.
[0046] Under the outer oxide film, a depletion layer in which a Cr
content is depleted and a depletion layer in which an Al or Ti
content is depleted may be formed.
[0047] The "inner oxide" used herein refers to an oxide formed in a
matrix direction through pre-oxidation based on a surface of the
nickel-based superalloy parent material, and may be formed by
partial aggregation in the form of an icicle in the matrix. Through
the reaction between oxygen provided from the external environment
and aluminum (Al) or titanium (Ti), an aluminum oxide, a titanium
oxide or a combination thereof may be formed. Here, the depth of
the inner oxide may be 1 to 100 .mu.m, and preferably, 5 to 30
.mu.m, but the present invention is not limited thereto.
[0048] The outer oxide film may be completely removed, and the
inner oxide-containing layer may be at least partially or
completely removed. Here, the inner oxide-containing layer may only
contain the inner oxide, but the inner oxide formed by partial
aggregation in the form of an icicle may be formed in a matrix in a
state in which an Al or Ti content is depleted. Here, the outer
oxide film and the inner oxide may be removed through various known
methods such as mechanical polishing.
[0049] The method for diffusion bonding of a nickel-based
superalloy according to the present invention includes preparing a
diffusion-bonded nickel-based superalloy material by performing
diffusion bonding on the nickel-based superalloy from which the
diffusion bonding on the outer oxide film and inner
oxide-containing layer are removed (Step (c)).
[0050] The nickel-based superalloy from which the outer oxide film
and inner oxide-containing layer are removed, which is subjected to
the diffusion bonding, includes a surface depletion layer in a
state in which an Al or Ti content is depleted, and the surface
depletion layer may be formed to a depth of 50 .mu.m or less from
the surface for diffusion bonding, which has been described above,
and thus duplicated descriptions will be omitted.
[0051] As the surface depletion layer is closer to the surface of
diffusion bonding, it may have a gradient in which the degree of
depletion of Al or Ti content increases, and as the diffusion
bonding progresses on the surface of the surface depletion layer,
planar grains formed at the interface may be effectively formed
into spherical grains through diffusion according to composition
differences of elements. In addition, during the diffusion bonding,
the secondary phase formation by Al or Ti at the interface between
the nickel-based superalloys is inhibited or reduced, and
intermetallic bonds may also be formed. Therefore, the same levels
of mechanical properties as those of the parent material may be
maintained at room temperature and high temperature.
[0052] Specifically, the diffusion bonding may be performed under a
vacuum condition at 1,000 to 1,200.degree. C. under a compressive
load of 10 to 20 MPa for 1 to 5 hours.
[0053] Selectively, the method for diffusion bonding of a
nickel-based superalloy according to the present invention may
further include thermally treating the formed diffusion-bonded
nickel-based superalloy material at 1,000 to 1,200.degree. C. for 1
to 100 hours (Step (d)).
[0054] Through the thermal treatment, planar grains remaining at
the interface may be additionally formed into spherical grains by
additional diffusion of atoms. In addition, after the thermal
treatment, cooling to 10 to 30.degree. C. may be performed through
furnace cooling, air cooling or quenching.
[0055] In addition, the present invention may provide a plate heat
exchanger manufactured using the method for diffusion bonding of a
nickel-based superalloy.
[0056] As described above, the present invention relates to a
nickel-based superalloy including a surface depletion layer in a
state in which an Al or Ti content is depleted (particularly, an
alloy derived from a solid solution strengthened nickel-based
superalloy parent material) and a method for diffusion bonding
using the same. Because of the surface depletion layer, it is
possible to move a planar grain boundary formed at the interface
through diffusion according to composition differences of elements
during diffusion bonding, and the secondary phase formation by Al
or Ti at the interface may be effectively inhibited or reduced,
thereby forming intermetallic bonds. Thus, since the
diffusion-bonded nickel-based superalloy material prepared
according to the present invention maintains the same level of
stress as that of the parent material at room temperature and high
temperature, and particularly, has excellent strain at the parent
material level at a high temperature of approximately 700.degree.
C. or more, it can be applied to the plate heat exchanger
industry.
[0057] To increase the efficiency of a high temperature apparatus
used in various industries (chemical, petrochemical, nuclear power
and power generation fields), higher temperature and pressure
conditions are required, and thus stainless steel currently used in
the plate heat exchanger industry is expected to be replaced with a
solid solution strengthened nickel-based superalloy in the near
future. In addition, in the global heat exchanger market in 2016,
the traditional shell & tube heat exchanger still accounted for
the largest share, that is, 3.35 billion dollars, but in the trend
of downsizing, lightening and modularization of a high temperature
apparatus, plate-frame heat exchangers (3 billion dollars in 2016)
are pursuing at a high annual growth rate of 7.11% on average.
Comprehensively considering these points, the present invention is
expected to activate the stagnant domestic manufacturing field
according to this trend of the times and contribute to the export
of products and technology through innovation.
[0058] Hereinafter, to help in understanding the present invention,
exemplary examples will be suggested. However, the following
examples are merely provided to more easily understand the present
invention, and not to limit the present invention.
EXAMPLES
Example 1
[0059] As nickel-based superalloy parent materials, two plates of
the same type of material such as Alloy 617 and UNS N06617 were
prepared, and the composition thereof is shown in Table 1
below.
TABLE-US-00001 TABLE 1 Ni Cr Co Mo Fe Mn Al Ti C Cu Si S B P
Composition 52.61 22.20 12.30 9.52 1.26 0.08 1.09 0.37 0.090 0.01
0.14 <0.002 0.002 <0.002 (wt %)
[0060] The plate-shaped Alloy 617 and UNS N06617 were pre-oxidized
under atmospheric conditions at 850.degree. C. for 100 to 200 hours
to form an outer oxide film (containing chromium oxide) and an
inner oxide (containing aluminum oxide and titanium oxide),
respectively (see FIGS. 1A to 1C).
[0061] As confirmed from the SEM image of FIG. 1A, it is confirmed
that both the thickness of the outer oxide film and the depth of
the inner oxide increase as the pre-oxidation time elapses.
[0062] In addition, as confirmed from the EPMA graph of FIG. 1B,
the thickness of the outer oxide film is determined by a point
where the final peak of the Cr composition is greatly reduced, and
it is confirmed that the point is spaced approximately 4 .mu.m
apart from a surface (the boundary between the environment and the
outer oxide film after the outer oxide film is formed by
pre-oxidation).
[0063] In addition, as confirmed from the EPMA graph of FIG. 1C,
the depth of the inner oxide is determined by the maximum point
where the Al- or Ti-based inner oxide is formed in the matrix, and
it is confirmed that the point is spaced approximately 20 .mu.m
(that is, the depth of the inner oxide is approximately 16 .mu.m)
apart from a surface (the boundary between the environment and the
outer oxide film after the outer oxide film is formed by
pre-oxidation). Meanwhile, it was confirmed that the Al or Ti
content is depleted to a depth of approximately 20 .mu.m under the
outer oxide film.
Example 2
[0064] A process was performed by the same method as described in
Example 1, except that plane-shaped Alloy 617 and UNS N06617 were
pre-oxidized under atmospheric conditions at 930.degree. C. for 100
to 200 hours to form an outer oxide film (containing chromium
oxide) and an inner oxide (containing aluminum oxide and titanium
oxide), respectively (see FIGS. 2A to 2C).
[0065] As confirmed from the SEM image of FIG. 2A, it is confirmed
that as the pre-oxidation temperature increases and the
pre-oxidation time elapses, both the thickness of the outer oxide
film and the depth of the inner oxide increase.
[0066] In addition, as confirmed from the EPMA graph of FIG. 2B,
the thickness of the outer oxide film is determined by a point
where the final peak of the Cr composition is greatly reduced, and
it is confirmed that the point is spaced approximately 8 .mu.m
apart from a surface (the boundary between the environment and the
outer oxide film after the outer oxide film is formed by
pre-oxidation).
[0067] In addition, as confirmed from the EPMA graph of FIG. 2C,
the depth of the inner oxide is determined by the maximum point
where the Al- or Ti-based inner oxide is formed in the matrix, and
it is confirmed that the point is spaced approximately 32 .mu.m
(that is, the depth of the inner oxide is approximately 24 .mu.m)
apart from a surface (the boundary between the environment and the
outer oxide film after the outer oxide film is formed by
pre-oxidation). Meanwhile, it was confirmed that the Al or Ti
content is depleted to a depth of approximately 40 .mu.m under the
outer oxide film.
[0068] Afterward, both the formed outer oxide film and inner
oxide-containing layer were removed by mechanical polishing,
followed by ethanol washing. Subsequently, it is confirmed that the
depth of the surface depletion layer is spaced approximately 16
.mu.m apart from the surface for diffusion bonding. Subsequently, a
diffusion-bonded material was prepared by performing diffusion
bonding on the surface for diffusion bonding at 1,150.degree. C.
under a compressive load of 14 MPa and a vacuum of
1.times.10.sup.-5 Torr for 2 hours.
[0069] The prepared diffusion-bonded material was then subjected to
final thermal treatment at 1,120.degree. C. for 20 hours.
[0070] Experimental Example: Measurement of Stress-Strain Curve at
Various Temperatures
[0071] Stress-strain curves at various temperatures (room
temperature, 500.degree. C., 700.degree. C., 800.degree. C. and
900.degree. C.) for the finally thermally-treated diffusion-bonded
material according to Example 2 and plate-shaped Alloy 617 and UNS
N06617 (parent materials) were plotted and compared according to
ASTM E8/E8M and E21, and the results are shown in FIGS. 3A to 3E,
respectively.
[0072] As confirmed from the stress-strain curves of FIGS. 3A to
3E, it is confirmed that the diffusion-bonded material according to
Example 2 maintained the same levels of stress as those of the
parent materials at both room temperature and high temperature.
Particularly, it is confirmed that the diffusion-bonded material
according to Example 2 has excellent strain at the parent material
level at a high temperature of approximately 700.degree. C. or
more.
[0073] The present invention relates to a nickel-based superalloy
(particularly, an alloy derived from a parent material of a solid
solution strengthened nickel-based superalloy) including a surface
depletion layer in a state in which an Al or Ti content is depleted
and a method for diffusion bonding using the same. Due to the
surface depletion layer, it is possible to move a planar grain
boundary formed at an interface through diffusion according to
composition differences of elements during diffusion bonding, and
the secondary phase formation by Al or Ti at an interface may be
effectively inhibited or reduced, thereby forming intermetallic
bonds. Therefore, the diffusion-bonded nickel-based superalloy
material according to the present invention maintains the same
level of stress as that of a parent material at room temperature or
high temperature, and has excellent strain at a parent material
level, particularly, at a high temperature of approximately
700.degree. C. or more so that it can be applied to the plate heat
exchanger industry.
[0074] It should be understood by those of ordinary skill in the
art that the above description of the present invention is
exemplary, and the exemplary embodiments disclosed herein can be
easily modified into other specific forms without departing from
the technical spirit or essential features of the present
invention. Therefore, the exemplary embodiments described above
should be interpreted as illustrative and not limited in any
aspect.
* * * * *