U.S. patent application number 12/484597 was filed with the patent office on 2009-12-17 for heat treatment method of a ni-based superalloy for wave-type grain boundary and a ni-based superalloy produced accordingly.
This patent application is currently assigned to KOREA INSTITUTE OF MACHINERY & MATERIALS. Invention is credited to Baig Gyu Choi, Hyun Uk Hong, Hi Won Jeong, Chang Yong Jo, In Soo Kim, Seong Moon Seo, Young Soo Yoo.
Application Number | 20090308508 12/484597 |
Document ID | / |
Family ID | 41161336 |
Filed Date | 2009-12-17 |
United States Patent
Application |
20090308508 |
Kind Code |
A1 |
Hong; Hyun Uk ; et
al. |
December 17, 2009 |
Heat Treatment Method of a Ni-Based Superalloy for Wave-Type Grain
Boundary and a Ni-Based Superalloy Produced Accordingly
Abstract
The present invention suggests a method of heat treatment of a
Ni-based superalloy that improves resistance against creep, fatigue
and stress corrosion cracking while being economical and easy, and
a Ni-based superalloy produced by using the same. The method and
the superalloy of the present invention include solution treatment
at the high temperature region during a heat treatment process
after manufacturing or final cold working fabrication. Immediately
following the solution treatment, the material is slowly cooled at
1.about.15.degree. C./minute down to the intermediate temperature
region for aging treatment. After the slow cooling stage, aging
treatment is directly performed by holding it at the intermediate
temperature region for the prescribed time. Lastly, the aging
treatment is followed by air-cooling stage.
Inventors: |
Hong; Hyun Uk;
(Gyeongsangnam-do, KR) ; Kim; In Soo;
(Gyeongsangnam-do, KR) ; Choi; Baig Gyu;
(Gyeongsangnam-do, KR) ; Jo; Chang Yong;
(Gyeongsangnam-do, KR) ; Yoo; Young Soo;
(Gyeongsangnam-do, KR) ; Jeong; Hi Won;
(Gyeongsangnam-do, KR) ; Seo; Seong Moon;
(Gyeongsangnam-do, KR) |
Correspondence
Address: |
EDELL, SHAPIRO & FINNAN, LLC
1901 RESEARCH BOULEVARD, SUITE 400
ROCKVILLE
MD
20850
US
|
Assignee: |
KOREA INSTITUTE OF MACHINERY &
MATERIALS
Gyeongsangnam-do
KR
|
Family ID: |
41161336 |
Appl. No.: |
12/484597 |
Filed: |
June 15, 2009 |
Current U.S.
Class: |
148/675 ;
148/426 |
Current CPC
Class: |
C22F 1/10 20130101; C22C
19/03 20130101 |
Class at
Publication: |
148/675 ;
148/426 |
International
Class: |
C22F 1/10 20060101
C22F001/10; C22C 19/03 20060101 C22C019/03 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 16, 2008 |
KR |
1020080056386 |
Claims
1. A heat treatment method of a Ni-based superalloy for wave-type
grain boundary in heat treatment stage after producing or
processing a Ni-based superalloy, comprising: a solution treatment
process at the high temperature region; a slow cooling process at
1.about.15.degree. C./minute down to the intermediate temperature
region for direct aging treatment after the solution treatment; an
aging treatment process by holding it for the prescribed time at
the intermediate temperature region for the aging treatment
immediately after the slow cooling process; and an air-cooling
process after the aging treatment.
2. The heat treatment method of the Ni-based superalloy according
to claim 1, wherein the slow cooling process comprises, a stage in
which incomplete wave-type grain boundaries form at some of flat
grain boundaries formed during the solution treatment process; and
a stage in which the incomplete wave-type grain boundaries grow
into stable wave-type grain boundaries, incomplete wave-type grain
boundaries form at the flat grain boundaries, and planar carbides
begin to precipitate at the wave-type grain boundaries.
3. The heat treatment method of the Ni-based superalloy according
to claim 2, wherein aging treatment process is characterized by
that; most of the incomplete wave-type grain boundaries transform
into stable wave-type grain boundaries; and the precipitated
carbides form incoherent interfaces by growing into planar shapes
towards the opposite grain while being coherent with one grain
constituting the wave-type grain boundary.
4. The heat treatment method of a Ni-based superalloy for wave-type
grain boundary according to claim 1, wherein the solution treatment
is processed at 1000.about.1200.degree. C. during the solution
treatment time, and the aging treatment is processed at
700.about.900.degree. C. during the aging treatment time.
5. A Ni-based superalloy for wave-type grain boundary wherein
wave-type grain boundaries are included and the planar carbides are
placed at the grain boundaries away from one another.
6. The Ni-based superalloy according to claim 5, wherein the
carbides create a coherent interface with one grain with the above
grain boundary, and make an incoherent interface by growing toward
the opposite grain.
7. The Ni-based superalloy for wave-type grain boundary according
to claim 5, wherein the array of incoherent interfaces of the
planar carbides formed at the wave-type grain boundaries is zigzag
pattern.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority under 35 U.S.C. .sctn.119
to Application No. KR 10-2008-0056386 filed on Jun. 16, 2008,
entitled "A Heat Treatment Method of a Ni-Based Superalloy for
Wave-Type Grain Boundary and a Ni-Based Superalloy Produced
Accordingly," the entire contents of which are hereby incorporated
by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to a heat treatment method of
a Ni-based superalloy and a Ni-based superalloy produced by using
the same, and more particularly, to the heat treatment method of a
Ni-based superalloy improving resistance against intergranular
fracture caused by creep, fatigue, stress corrosion cracking, etc.,
and a Ni-based superalloy with wave-type or serrated grain
boundaries.
BACKGROUND
[0003] Ni-based superalloy is used as a material for high
temperature components such as power assembly for aero and
industrial gas turbines because it is excellent in formability,
weldability, corrosion resistance, high temperature mechanical
properties, etc. The material is exposed to harsh environment like
constant or complex strain cycle due to high temperature exposures
and mechanical loads during operation, and ends in failure caused
by damage from creep, fatigue, stress corrosion cracking, etc.
Therefore, improving resistance of the material against main damage
mechanisms such as creep, fatigue, stress corrosion cracking, etc.
has been an important issue to manufacturers, component
fabricators, operating companies, etc.
[0004] As illustrated in FIG. 1, the existing heat treatment
processes applied to manufacturing and processing of a wrought
nickel based superalloy, NIMONIC 263, which is widely used for
combustion lines for industrial gas turbines, transition ducts,
etc. will be examined. The method generally contains water-cooling
(over 50.degree. C./second) after solution treatment (over 5
minutes at 1000.about.1200.degree. C.) at the high temperature
region. Then, after the prescribed time, the 2nd heat treatment
process is applied by air-cooling after aging treatment (over 5
hours at 700.about.900.degree. C.) at the intermediate temperature
region. The above-stated heat treatment simply dissolves coarse
carbides and .gamma.' particles into the .gamma. matrix at the
solution treatment process after manufacturing or cold working,
precipitates the carbides at grain boundaries in advance at the
aging treatment process, and simultaneously distributes the
.gamma.' particles uniformly within the matrix. Accordingly, the
purposes are to enhance thermal stability of the material, decrease
grain boundary sensitization, and improve high temperature strength
of the material. However, this kind of heat treatment method cannot
improve resistance against creep, fatigue and stress corrosion
cracking satisfactorily at the present time. Therefore, a heat
treatment method that is more economical and simple while improving
the resistance remarkably is required.
[0005] Korean Patent Publication No. 1999-024668 discloses the heat
treatment method of a Ni-based superalloy for improving corrosion
resistance. The above-referenced patent suggested a heat treatment
method in which resistance of grain boundary fracture improves by
changing the shapes of grain boundaries within the material to wavy
shapes through slowing down the cooling speed to
0.1.about.5.degree. C./minute in all the temperature range to room
temperature or in a certain range after solution treatment at the
high temperature, and again treating with an agent. However, this
method is not economically efficient because the heat treatment
takes too long time since it cools the material in a relatively
slow speed, and the grain size becomes larger since the material is
exposed to a high temperature for a long time. In addition,
.gamma.' particles become coarsened and various harmful phases can
be precipitated, therefore, although resistance against stress
corrosion cracking might be improved, it can deteriorate tensile
properties and high temperature mechanical properties like creep,
fatigue, etc. Accordingly, it is deemed that the method can be
hardly applied to actual industrial spots.
SUMMARY
[0006] The present invention suggests a method of heat treatment of
a Ni-based superalloy that improves resistance against creep,
fatigue and stress corrosion cracking while being economical and
easy, and a Ni-based superalloy produced by using the same. The
heat treatment method of a Ni-based superalloy of the present
invention to accomplish the above-stated technical concerns
includes producing or processing a Ni-based superalloy and then,
performing solution treatment at the high temperature region during
a heat treatment process. Immediately following the solution
treatment, the material is slowly cooled at 1.about.15.degree.
C./minute down to the intermediate temperature region for aging
treatment. After the slow cooling stage, aging treatment is
immediately performed by holding it at the intermediate temperature
region for the prescribed time. Lastly, the aging treatment is
followed by air-cooling stage.
[0007] In the heat treatment method of the present invention, the
above-stated slow cooling stage consists of three processes; the
first process in which wave-type grain boundaries begin to form at
some of flat grain boundaries made during the solution treatment;
the second process in which some of the wave-type grain boundaries
formed grow with stable amplitude and frequency while more
wave-type grain boundaries form at the flat grain boundaries; and,
the third process in which planar carbides begin to precipitate at
the said some wave-type grain boundaries. In addition, in the aging
treatment process, most of the wave-type grain boundaries formed
grow into wave-type grain boundaries with stable amplitude and
frequency, and the carbides precipitated can stably grow in planar
shapes with low interfacial energy on the wave-type grain
boundaries.
[0008] More preferably in the present invention, the solution
treatment is processed for the prescribed time at
1000.about.1200.degree. C. and the aging treatment can be processed
for the prescribed time at 700.about.900.degree. C.
[0009] The superalloy of the present invention for accomplishing
the different technical tasks has wave-type grain boundaries in
which planar carbides are arrayed apart from each other at the
grain boundaries. At this time, the planar carbide shares the
coherent interface with one grain while sharing the incoherent
interfaces with the opposite grain. The array of incoherent
interfaces of planar carbide particles formed at wave-type grain
boundaries is zigzag pattern.
[0010] According to the heat treatment method of a Ni-based
superalloy and a Ni-based superalloy produced using the same by the
present invention, it is possible to improve resistance against
intergranular fracture caused by creep, fatigue, stress corrosion
cracking, etc., and at the same time, to conduct a time- and
cost-efficient heat treatment while maintaining basic properties of
a Ni-based superalloy by leading to precipitation of low-density
carbides with low interfacial energy and improving cohesive
strength between the grain boundaries and the matrix through
changing the shapes of grain boundaries to wave-type shapes.
[0011] The foregoing and other objects, features, aspects and
advantages of the present invention will become more apparent from
the following detailed description of the present invention when
considering the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is a drawing illustrating the existing heat treatment
process.
[0013] FIG. 2A is a drawing illustrating the heat treatment process
of the present invention, while FIG. 2B is a drawing illustrating
to conceptually explain changes of microstructures according to the
process of FIG. 2A.
[0014] FIG. 3 and FIG. 4 are photos showing microstructures of
NIMONIC263 alloys resulted from the existing heat treatment method
and the heat treatment method in the present invention,
respectively.
[0015] FIG. 5 and FIG. 6 are photos showing the fractured surfaces
after tensile test conducted at room temperature of NIMONIC263
alloys resulted from the existing heat treatment method and the
heat treatment method in the present invention, respectively.
[0016] FIG. 7A and FIG. 7B are graphs illustrating a creep test
result conducted under 760.degree. C./295 MPa and 815.degree.
C./180 MPa, respectively, using NIMONIC263 alloys resulted from the
existing heat treatment method and the heat treatment method in the
present invention.
DETAILED DESCRIPTION
[0017] Therefore, technical concerns the present invention intends
to accomplish are to improve resistance against creep, fatigue and
stress corrosion cracking, and to provide an economical and easy
heat treatment method of a Ni-based superalloy. In addition,
another technical concern of the invention is to provide a Ni-based
superalloy produced by using the method.
[0018] Exemplary embodiments of the present invention will now be
described based upon the accompanying drawings. The following
embodiments may be modified variably, and the scope of the present
invention is not limited to those embodiments. The embodiments of
the present invention are provided to more perfectly explain the
present invention to the skilled person in the art.
[0019] Firstly, embodiments of the present invention will suggest
the main damage mechanism of a Ni-based superalloy and how to
overcome the damage, and further explain a heat treatment method
embodying the method. Here, main damage mechanism of a Ni-based
superalloy like creep, fatigue, stress corrosion cracking, etc. is
defined as grain boundary damage to meet the convenience of
explanation.
[0020] In case of grain boundary damage, a main damage mechanism of
a Ni-based superalloy, cracks mainly initiate and propagate along
brittle grain boundaries. Accordingly, resistance against grain
boundary damage can be improved by reducing energy of the grain
boundaries themselves, increasing the crack propagation distance,
and changing morphology and characteristics of the carbides, that
is, particles precipitated at the grain boundaries. The embodiments
of the present invention suggest reducing energy of the grain
boundaries as mentioned above, increasing the crack propagation
distance, and changing morphology and characteristics of the grain
boundary carbides through the formation of wave-type or serrated
grain boundaries.
[0021] The wave-type grain boundaries improve resistance against
grain boundary damage for the following reasons. First of all, it
improves cohesiveness with the matrix by reducing misorientation
degree between two adjacent grains, and makes crack propagation
distance longer by changing grain boundary configuration. In
addition, the carbides precipitated at the grain boundaries become
planar while having low-density and stabilized low interfacial
energy. Although the planar carbides form at the same wave-type
grain boundary, the preference of each carbide particle to one
grain selection for sharing coherency is alternating so that the
array of incoherent interfaces of carbide particles formed at the
wave-type grain boundary is zigzag pattern.
[0022] As stated above, characteristics of the carbides are
modified so as to be favorable for resistance against grain
boundary damage by forming the wave-type grain boundaries. That is,
the density of incoherent interface between the carbides and the
matrix providing a preferential site for cavitation or crack
formation becomes lower and stabilized so that the resistance
against cavity or crack formation could be improved. Moreover, the
zigzag array of incoherent interface of carbide makes it more
difficult for cavities or cracks to interlink to form an
intergranular path for crack propagation; therefore, a lower rate
of crack propagation along the grain boundaries.
[0023] Hence, the embodiments of the present invention suggest how
to lead planar carbide particles by forming wave-type grain
boundaries.
[0024] Although there are various models regarding the formation of
serrated or wave-type grain boundaries, recently, the present
inventors have found that grain boundary serration occurs
spontaneously in the absence of carbides as a result of the total
free energy minimization of a material. That is, at the high
temperature region, straight-line flat grain boundaries develop in
order to reduce the surface area term as small as possible because
the influence of surface energy is bigger than misorientation
between two adjacent grains. The grain boundaries tend to serrate
to have several wavy segments in order to lower their interfacial
free energy at the intermediate temperature region where the
misorientation term becomes more important than surface area
term.
[0025] Considering this occurrence model of wave-type grain
boundaries, the following prerequisites are essential in order to
form wave-type grain boundaries in a Ni-based superalloy of the
present invention.
[0026] Firstly, carbide precipitation at the grain boundaries
should be retarded as much as possible because of the following
reasons; the carbide particles may inhibit the boundary movement as
pinning points, and it might be difficult to modify the carbide
characters like density and shape if the carbides form prior to the
grain boundary serration. Thus, the supersaturation of carbon atoms
should be suppressed. Secondly, sufficient temperature and time
should be provided for the grain boundaries to move largely; A
thermal equilibrium state should be continuously maintained during
cooling from a higher solution to a lower aging treatment
temperature, since grain boundary serration is known to occur
spontaneously.
[0027] In order to satisfy the above-mentioned prerequisites, the
embodiments of the present invention suggest holding a Ni-based
superalloy for the prescribed time at the high temperature region
in which the carbides are dissolved; slowly cooling down to under
the intermediate temperature region in which misorientation between
two adjacent grains is important; and then, immediately conducting
aging treatment at the same temperature. In addition, the method
maintained basic characteristics required by Ni-based superalloy
while creating wave-type grain boundaries. Accordingly, the present
invention suggests a new heat treatment method that is simpler than
the existing heat treatment methods and corresponds with the
purpose of the present invention.
[0028] The present invention suggests the optimum heat treatment
conditions that lead wave-type grain boundaries while maintaining
the grain size and the volume fraction of .gamma.' particles
through heat treatment tests with various conditions. Detailed
conditions include holding at the high temperature region for the
prescribed time for solution treatment; slowly cooling down to the
intermediate temperature region for aging treatment; immediately
conducting the aging treatment at the intermediate temperature
region; and then successively air-cooling. At this time, the slow
cooling down to the intermediate temperature region is performed at
1.about.15.degree. C./minute.
[0029] The heat treatment process of the present invention can be
compared with prior methods as follows. The prior inventions
applied a two-step heat treatment method in which solution
treatment is processed at the high temperature region
(1000.about.1200.degree. C.), water cooling is conducted (over
50.degree. C./second), and aging treatment is again performed at
the intermediate temperature region (700.about.900.degree. C.).
However, the current invention is a one-step heat treatment method
in which slow cooling is conducted down to the intermediate
temperature region immediately after solution treatment, and then
heat treatment is completed after holding it untouched at the
intermediate temperature region.
[0030] FIG. 2A is a drawing illustrating the heat treatment process
of the present invention, while FIG. 2B is a drawing illustrating
to conceptually explain changes of microstructures according to the
process of 2A. Here, the heat treatment temperature and the heat
treatment duration are examples of representative conditions for
heat treatment, but don't limit the range of the present invention.
For this, a Ni-based superalloy hot-rolled NIMONIC 263 was
used.
[0031] Referring to FIGS. 2A and 2B, the heat treatment method of
the present invention is divided into a solution treatment process
(Step a), a slow cooling process (Steps b.about.c), an aging
treatment process (Step d) and an air-cooling process. That is,
first of all, for the solution treatment, solution treatment
duration, e.g. for over five minutes, is maintained at
1000.about.1200.degree. C. of high temperature region. Then, the
material is cooled slowly with a speed of 1.about.15.degree.
C./minute to the intermediate temperature region or the aging
treatment temperature of 700.about.900.degree. C. Later, the aging
treatment temperature of 700.about.900.degree. C. is maintained for
over five hours, then, the heat treatment is completed after
air-cooling.
[0032] The solution treatment process is processed during the
solution duration that dissolves coarse carbides and .gamma.'
particles sufficiently resulting in enough solution treatment of
the superalloy in the present invention, but does not cause grain
growth. At this time, grain boundaries of the material after the
solution treatment or Step a are flat (20).
[0033] Wave-type grain boundaries begin to form during the slow
cooling process with a speed of 1.about.15.degree. C./minute to the
intermediate temperature region. The grain boundaries begin to have
wave-type partially at Step b called the early stage of slow
cooling process. At this time, the amplitude and frequency of the
wave-type grain boundaries (22) at the early stage of a slow
cooling process have not developed completely (this will be
referred to as incomplete wave-type grain boundaries for
convenience).
[0034] On the other hand, in Step c, the incomplete wave-type grain
boundaries (22) are forming continuously and some of them grow into
complete wave-type grain boundaries (24) with stable wave-type,
which is called the late stage of a slow cooling process. Namely,
incomplete wave-type grain boundaries (22), complete wave-type
grain boundaries (24) and some flat grain boundaries (20) with no
wave-type coexist at the late stage of slow cooling process. At
this time, planar carbides (30) begin to precipitate at the
incomplete wave-type grain boundaries (22) and the complete
wave-type grain boundaries (24) and precipitation hardened phase
.gamma.' begins to form at the matrix. The planar carbide (30)
shares the coherent interface with one grain constituting the
wave-type grain boundary (22 and 24) while sharing the incoherent
interface with opposite grain.
[0035] The slow cooling process is immediately followed by the
aging treatment process, and after a prescribed time passes, most
of wave-type grain boundaries grow to complete wave-type grain
boundaries (24) as stated in Step d and the precipitated carbides
(30) grow to form planar carbides (32). At this time, the carbides
(32) grow creating an incoherent interface to the direction of the
opposite grain of the coherent interface, while sharing the
coherency with one grain. At this time, the array of incoherent
interfaces of the planar carbides is zigzag pattern because of
crystallographic variants of the wave-type grain boundary
itself.
[0036] The aging treatment process is processed during the aging
treatment duration in which sufficient aging treatment takes place
securing no microstructural changes under exposure to the same
aging treatment temperature region (700.about.900.degree. C.), by
uniformly distributing .gamma.' particles of the superalloy within
the matrix and stabilizing the carbides at the grain boundaries,
coinciding with the purpose of the present invention. At this time,
the planar carbides (32) grow stably at the complete wave-type
grain boundaries (24).
[0037] The planar carbides (32) with completed aging treatment
process are arrayed away from each other by the wave-type
boundaries (24). Although the planar carbides (32) form at the same
serrated grain boundary, the preference of each carbide (32) to one
grain selection for sharing coherency is alternating so that the
array of incoherent interfaces of the carbide particles (32) formed
at wave-type grain boundaries is zigzag pattern.
[0038] In brief, the interfacial energy of the grain boundary
itself can be lowered significantly because of transformation from
the flat grain boundaries (20) to the complete wave-type grain
boundaries (24). In addition, the density of the carbides (32)
precipitated on the wave-type grain boundaries with low interfacial
energy becomes lower while incoherent interfacial energy of the
carbides (32) significantly becomes lower because they grow to
stable planar carbides. Further, the array of incoherent interfaces
of the planar carbides can be a zigzag pattern because of
crystallographic variants of the wave-type grain boundary
itself.
[0039] In the present invention, the reason why the slow cooling is
limited to 1.about.15.degree. C./minute at process to the aging
treatment temperature immediately after the solution treatment is
due to concern about that basic mechanical characteristics can be
deteriorated since the grains and precipitation hardened .gamma.'
phase coarsen as the exposure time becomes longer in case that the
cooling speed is under 1.degree. C./minute. In addition, if the
cooling speed exceeds 15.degree. C./minute, it is impossible to
obtain wave-type grain boundaries because carbides are precipitated
first since there is no enough time for the grain boundaries to
transform into wave-type grain boundaries.
[0040] On the other hand, in case that the material is slowly
cooled at 1.about.15.degree. C./minute in the entire temperature
region from the solution treatment temperature to room temperature
after the solution treatment, the material cannot be used as it is
and requires separate aging treatment causing extra time and cost
because .gamma.' precipitation and thermal stability are not
sufficient. If the material is slowly cooled at 1.about.15.degree.
C./minute of the different temperature region from the aging
treatment temperature of the present invention after the solution
treatment, not only wave-type grain boundaries do not form, but
also aging treatment should be conducted again.
[0041] In addition, microstructure would be shown as Step c if the
material is water-quenched quickly after the slow cooling process
suggested in the present invention. That is, incomplete wave-type
grain boundaries (22), complete wave-type grain boundaries (24) and
flat grain boundaries (20) coexist at this stage. The carbon atoms
are at the state of supersaturation due to the water-quenching, so
if aging treatment is conducted, granular shapes of carbides with
high density are precipitated at the incomplete wave-type grain
boundaries (22), the complete wave-type grain boundaries (24) and
even the flat grain boundaries (20). These cases have higher
interfacial energy than the present invention.
<Experiment Examples>
[0042] FIG. 3 is a photo showing microstructures of NIMONIC263
alloys resulted from the existing heat treatment method. The below
one is an enlarged photo of near a grain boundary. Solution
treatment was performed for about 30 minutes at the temperature of
1150.degree. C., and the material was water-quenched to room
temperature (over 50.degree. C./second), and then, aging treatment
was conducted again for about 8 hours at the temperature of
800.degree. C., and then the material is air-cooled. As illustrated
in the photo, with respect to the microstructures of the existing
alloy, it was found that small granular carbides are precipitated
with high density at straight-line flat grain boundaries. It was
identified that the size of grains is 60.about.70 .mu.m.
[0043] FIG. 4 is a photo showing microstructures of NIMONIC263
alloys resulted from the heat treatment method in the present
invention. The below one is an enlarged photo of near a grain
boundary. Solution treatment was performed for about 30 minutes at
the temperature of 1150.degree. C., and immediately the material
was slowly cooled down to the aging treatment temperature of
800.degree. C. at the speed of 10.degree. C./minute, and then the
material is air-cooled after holding for 8 hours at 800.degree.
C.
[0044] According to FIG. 4, it was found that, in the
microstructures of the embodiments of the present invention,
wave-type grain boundaries are well developed and planar carbides
with low interfacial energy are precipitated at the grain
boundaries with low density. At this time, the size of grains is
70.about.80 .mu.m which is similar to microstructures obtained from
ordinary heat treatment.
[0045] Characteristics of alloys obtained from the existing heat
treatment method as illustrated FIG. 3 and alloys obtained from the
heat treatment method in the present invention as illustrated FIG.
4 are to be examined in the following.
[0046] [Table 1] is results of tensile test of each alloy conducted
at room temperature.
TABLE-US-00001 TABLE 1 Yield Tensile Grain Size Strength Strength
Elongation Sample (.mu.m) (MPa) (MPa) (%) Alloy from the existing
62 640 1083 23.3 heat treatment Alloy from the present 75 622 1079
38.1 invention
[0047] As we know from the above table, an alloy from the present
invention presented similar yield strength and tensile strength to
an alloy from the existing heat treatment. However, it was found
that elongation indicating ductility significantly increased from
23.3% of the existing alloy to 38.1%.
[0048] FIG. 5 and FIG. 6 are photos showing the fractured surfaces
after tensile test conducted at room temperature of NIMONIC263
alloys obtained from the existing heat treatment method and the
heat treatment method in the present invention, respectively. At
this time, heat treatment is as explained above. As illustrated, it
was found that the grain boundary facets of the existing alloy were
separated easily and fractured without particular plastic
deformation.
[0049] However, as illustrated in FIG. 6 of the present invention,
considerable deformation such as dimples and shearing on the
wave-type grain boundary facets were found although the fracture
mode remains essentially intergranular. Hence, it was found that
the alloy of the present invention is fractured through sufficient
plastic deformation up to just before fracture. In another words,
the alloy of the present invention has relatively stronger cohesive
strength between the grain boundaries and the matrix than the
existing alloy. This result may be considered as one of elements
increasing ductility as stated in [Table 1].
[0050] In the concrete, in the present invention, the carbides (32)
with completed aging treatment are planar carbides that exist away
from one another on the stable wave-type grain boundaries (24). The
planar carbides (32) form by turns in a zigzag pattern (FIG. 4a and
FIG. 4b) towards two adjacent grains, not the array of incoherent
interfaces placed in one direction by growing toward the only grain
out of the two grains constituting wave-type grain boundaries (24).
Therefore, characteristics of the carbides (32) can be changed so
as to be favorable to resistance against grain boundary damage
because the wave-type grain boundaries (24) form as stated above.
That is, the density of incoherent interfaces between the carbides
(32) and the matrix providing preferential site for cavitation or
crack formation becomes lower and energy becomes more stable,
causing the lower rate of grain boundary cracking. Even though
grain boundary cracking is initiated, sufficient plastic
deformation is made up to just before fracture because crack
propagation along the grain boundary through interlinking is
delayed due to the incoherent interfaces in a zigzag pattern.
[0051] FIG. 7A and FIG. 7B are graphs illustrating creep test
results conducted under 760.degree. C./295 MPa and 815.degree.
C./180 MPa, respectively, using NIMONIC263 alloys obtained from the
existing heat treatment method and the heat treatment method in the
present invention.
[0052] From FIG. 7A and FIG. 7B, it was verified that the heat
treatment of the present invention leads to excellent creep
properties regardless of test conditions. More concretely, creep
rupture life increased from about 129 hours to about 178 hours, and
creep strain also increased from about 6% to about 11% in the test
under 760.degree. C./295 MPa. In addition, creep rupture life
increased from about 181 hours to about 252 hours, and creep strain
increased from about 17% to about 20% in the test under 815.degree.
C./180 MPa.
[0053] As the present invention may be embodied in several forms
without departing from the characteristics thereof, it should also
be understood that the above-described embodiments are not limited
by any of the details of the foregoing description, therefore,
various variations are possible by a person of ordinary skill in
the pertinent art within the range of technical features of the
present invention.
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