U.S. patent application number 15/736395 was filed with the patent office on 2018-08-02 for austenitic heat-resistant alloy and welded structure.
The applicant listed for this patent is NIPPON STEEL & SUMITOMO METAL CORPORATION. Invention is credited to Hiroyuki HIRATA, Atsuro ISEDA, Kana JOTOKU, Toshihide ONO, Hiroyuki SEMBA, Katsuki TANAKA.
Application Number | 20180216215 15/736395 |
Document ID | / |
Family ID | 57609569 |
Filed Date | 2018-08-02 |
United States Patent
Application |
20180216215 |
Kind Code |
A1 |
HIRATA; Hiroyuki ; et
al. |
August 2, 2018 |
AUSTENITIC HEAT-RESISTANT ALLOY AND WELDED STRUCTURE
Abstract
An austenitic heat-resistant alloy has a chemical composition
of, in mass %: 0.04 to 0.14% C; 0.05 to 1% Si; 0.5 to 2.5% Mn; up
to 0.03% P; less than 0.001% S; 23 to 32% Ni; 20 to 25% Cr 1 to 5%
W; 0.1 to 0.6% Nb; 0.1 to 0.6% V; 0.1 to 0.3% N; 0.0005 to 0.01% B;
0.001 to 0.02% Sn; up to 0.03% AI; up to 0.02% 0; 0 to 0.5% Ti; 0
to 2% Co; 0 to 4% Cu; 0 to 4% Mo; 0 to 0.02% Ca; 0 to 0.02% Mg; 0
to 0.2% REM; and the balance being Fe and impurities. The alloy
microstructure has a grain size number in accordance with ASTM E112
of 2.0 or more and less than 7.0.
Inventors: |
HIRATA; Hiroyuki;
(Neyagawa-shi, Osaka, JP) ; SEMBA; Hiroyuki;
(Sanda-shi, Hyogo, JP) ; JOTOKU; Kana;
(Amagasaki-shi, Hyogo, JP) ; ISEDA; Atsuro;
(Kobe-shi, Hyogo, JP) ; ONO; Toshihide; (Houston,
TX) ; TANAKA; Katsuki; (Nishinomiya-shi, Hyogo,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NIPPON STEEL & SUMITOMO METAL CORPORATION |
Tokyo |
|
JP |
|
|
Family ID: |
57609569 |
Appl. No.: |
15/736395 |
Filed: |
June 2, 2016 |
PCT Filed: |
June 2, 2016 |
PCT NO: |
PCT/JP2016/066458 |
371 Date: |
December 14, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/42 20130101;
C22C 38/002 20130101; C22C 38/005 20130101; C22C 38/44 20130101;
C22C 38/50 20130101; C22C 38/52 20130101; C22C 38/06 20130101; C22C
38/02 20130101; C22C 38/58 20130101; C22C 38/001 20130101; C22C
38/00 20130101; C22C 38/48 20130101; C22C 38/54 20130101; C22C
38/46 20130101; C22C 38/008 20130101; C21D 2211/001 20130101; C22C
38/04 20130101 |
International
Class: |
C22C 38/58 20060101
C22C038/58; C22C 38/54 20060101 C22C038/54; C22C 38/52 20060101
C22C038/52; C22C 38/50 20060101 C22C038/50; C22C 38/48 20060101
C22C038/48; C22C 38/46 20060101 C22C038/46; C22C 38/44 20060101
C22C038/44; C22C 38/42 20060101 C22C038/42; C22C 38/06 20060101
C22C038/06; C22C 38/02 20060101 C22C038/02; C22C 38/00 20060101
C22C038/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 1, 2015 |
JP |
2015-132552 |
Claims
1. An austenitic heat-resistant alloy having a chemical composition
of, in mass %: 0.04 to 0.14% C; 0.05 to 1% Si; 0.5 to 2.5% Mn; up
to 0.03% P; less than 0.001% S; 23 to 32% Ni; 20 to 25% Cr; 1 to 5%
W; 0.1 to 0.6% Nb; 0.1 to 0.6% V; 0.1 to 0.3% N; 0.0005 to 0.01% B;
0.001 to 0.02% Sn; up to 0.03% Al; up to 0.02% O; 0 to 0.5% T; 0 to
2% Co; 0 to 4% Cu; 0 to 4% Mo; 0 to 0.02% Ca; 0 to 0.02% Mg; 0 to
0.2% REM; and the balance being Fe and impurities, the alloy having
a microstructure with a grain size represented by a grain size
number in accordance with ASTM E112 of 2.0 or more and less than
7.0.
2. The austenitic heat-resistant alloy according to claim 1,
wherein the chemical composition contains one or more elements
selected from one of the first to third groups provided below, in
mass %: first group: 0.01 to 0.5% Ti; second group: 0.01 to 2% Co,
0.01 to 4% Cu, and 0.01 to 4% Mo; and third group: 0.0005 to 0.02%
Ca; 0.0005 to 0.02% Mg; and 0.0005 to 0.2% REM.
3. A welded structure including the austenitic heat-resistant alloy
according to claim 1.
4. A welded structure including the austenitic heat-resistant alloy
according to claim 2.
Description
TECHNICAL FIELD
[0001] The present invention relates to an austenitic
heat-resistant alloy and a welded structure including this
alloy.
BACKGROUND ART
[0002] In recent years, worldwide efforts have been made to
increase steam temperatures and pressures during the operation of
thermal power boilers or the like to reduce loads to the
environment. Materials used in superheater tubes or reheater tubes
are required to have improved high-temperature strength and
corrosion resistance.
[0003] To meet these requirements, various austenitic
heat-resistant alloys containing large amounts of nitrogen have
been disclosed.
[0004] For example, JP 2004-250783 A proposes an austenitic
stainless steel with improved high-temperature strength and
corrosion resistance, where the N content is 0.1 to 0.35% and the
Cr content is higher than 22% and lower than 30%, and a metallic
microstructure is specified.
[0005] JP 2009-084606 A proposes an austenitic stainless steel with
improved high-temperature strength and corrosion resistance, where
the N content is 0.1 to 0.35% and the Cr content is higher than 22%
and lower than 30%, and impurity elements are specified.
[0006] JP 2012-1749 A discloses an austenitic heat-resistant steel
with improved high-temperature strength and hot workability
containing 0.09 to 0.30% N and having large amounts of Mo and W in
composite addition.
[0007] WO 2009/044796 A1 discloses a high-strength austenitic
stainless steel containing 0.03 to 0.35% N and one or more of Nb, V
and Ti.
DISCLOSURE OF THE INVENTION
[0008] These austenitic heat-resistant alloys are usually welded
for assembly and then used at high temperatures. However, when
welded structures using austenitic heat-resistant alloys having
high N contents are used at high temperatures for a prolonged
period of time, cracks called strain-induced precipitation
hardening (SIPH) cracks may occur in weld-heat-affected zones.
[0009] WO 2009/044796 A1 discussed above states that limiting the
amounts of the elements that cause embrittlement of the grain
boundaries and the elements that strengthen the grain interiors to
certain ranges prevents cracking that would occur during use for a
prolonged period of time. Indeed, these materials prevent cracking
under certain conditions. However, in recent years, the use of
austenitic heat-resistant alloys with large amounts of W, Mo etc.
added thereto to further improve properties such as
high-temperature strength has become widespread. For some weld
conditions, structure shapes and sizes, for example, these
austenitic heat-resistant alloys may not prevent cracking in a
stable manner. More specifically, they may not prevent cracking in
a stable manner for high welding heat inputs, heavy plate
thicknesses or high use temperatures such as above 650.degree.
C.
[0010] An object of the present invention is to provide an
austenitic heat-resistant alloy that provides good crack resistance
and high-temperature strength in a stable manner.
[0011] An austenitic heat-resistant alloy according to an
embodiment of the present invention has a chemical composition of,
in mass %: 0.04 to 0.14% C; 0.05 to 1% Si; 0.5 to 2.5% Mn; up to
0.03% P; less than 0.001% S; 23 to 32% Ni; 20 to 25% Cr; 1 to 5% W;
0.1 to 0.6% Nb; 0.1 to 0.6% V; 0.1 to 0.3% N; 0.0005 to 0.01% B;
0.001 to 0.02% Sn; up to 0.03% Al; up to 0.02% O; 0 to 0.5% Ti; 0
to 2% Co; 0 to 4% Cu; 0 to 4% Mo; 0 to 0.02% Ca; 0 to 0.02% Mg; 0
to 0.2% REM; and the balance being Fe and impurities, the alloy
having a microstructure with a grain size represented by a grain
size number in accordance with ASTM E112 of 2.0 or more and less
than 7.0.
[0012] The present invention provides an austenitic heat-resistant
alloy that provides good crack resistance and high-temperature
strength in a stable manner.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a cross-sectional view of a bevel produced for the
Examples, showing the shape of the groove thereof.
EMBODIMENTS FOR CARRYING OUT THE INVENTION
[0014] The present inventors conducted a detailed investigation to
solve the above-discussed problems, and revealed the following
findings.
[0015] The inventors thoroughly investigated SIPH cracks occurring,
during use, in welded joints using austenitic heat-resistant alloys
with high N contents. They found that (1) cracks developed along
grain boundaries in weld-heat-affected zones with coarse grains
near the fusion lines, and (2) clear concentrating of S was
detected on the fractured surfaces of cracks. They further found
that (3) large amounts of nitrides and carbonitrides had
precipitated within grains near the cracks. This was particularly
significant for high Nb contents. In addition, they found that (4)
the larger the initial grain size of the used austenitic
heat-resistant alloy, the larger the grain size in
weld-heat-affected zones became and the more likely cracking
occurred.
[0016] From these finding, they assumed that SIPH cracks occurred
because large amounts of nitrides and carbonitrides precipitate
within grains during use at high temperatures and thus the grain
interiors become less likely to be deformed, which leads to
concentration of creep deformations on grain boundaries and finally
to openings. S segregates on grain boundaries during welding or
during use and thereby decreases the bonding force of the grain
boundaries. Further, the larger the grain size, the smaller the
area of grain boundaries per unit volume. Grain boundaries work as
sites for producing nuclei for nitride and carbonitride particles.
Thus, the smaller the grain boundaries, the larger the amounts of
nitrides and carbonitrides that precipitate within grains. In
addition, creep deformations that are caused by external forces
applied during use, for example welding residual stress, are the
more likely to be concentrated on certain grain boundaries. Thus,
the inventors concluded that the larger the initial grain size of
the base material, the more likely cracking occurs. Particularly,
they concluded that, at high temperatures above 650.degree. C.,
precipitates precipitate in short periods of time and, in addition,
grain-boundary segregation occurs at early stages, making the
problems more apparent.
[0017] To prevent such cracking, it is effective to reduce elements
that increase the deformation resistance within the grains by using
precipitation strengthening or solute strengthening. However, these
elements are indispensable to provide sufficient creep strength at
high temperatures. Thus, the prevention of cracks and the provision
of sufficient creep strength at high temperatures are tradeoffs and
are difficult to achieve at the same time.
[0018] After extended research, the inventors revealed that, in
order to prevent SIPH cracking in an austenitic heat-resistant
alloy containing 0.04 to 0.14% C, 0.05 to 1% Si, 0.5 to 2.5% Mn, up
to 0.03% P, 23 to 32% Ni, 20 to 25% Cr, 1 to 5% W, 0.1 to 0.3% N,
0.0005 to 0.01% B, up to 0.03% Al, and up to 0.02% O, it is
effective to exactly control the Nb and S contents to be in the
range of 0.1 to 0.6% and below 0.001%, respectively, and to have an
initial grain size of the base material represented by a grain size
number as defined by the American Society for Testing and Material
(ASTM) of 2.0 or more. However, if the grain size is finer than
necessary and the Nb content is limited, the creep strength of the
base material does not reach a specified value. Thus, the inventors
found that the grain size as represented by grain size number needs
to be less than 7.0. In addition, they revealed that V, which has a
lower precipitation strengthening property than Nb, in a content of
0.1 to 0.6% is necessary to achieve a predetermined creep strength
without impairing SIPH crack resistance.
[0019] While the inventors determined that these steps indeed
prevent SIPH cracking, they found out during the research that
another problem may arise.
[0020] As discussed above, austenitic heat-resistant alloys are
generally welded for assembly. When they are welded, a filler
material is usually used. However, for small parts with thin wall
thicknesses, or even for components with heavy wall thickness for
root running or tack welding, gas shield-arc welding may be
performed without using a filler material. If the penetration depth
is insufficient at this time, unwelded abutting surfaces remain as
weld defects, and the strength required of a welded joint cannot be
obtained. While S reduces SIPH crack resistance, S has the effect
of increasing the penetration depth. Thus, the inventors found that
the problem of insufficient penetration depth tends to be apparent
if the S content is exactly controlled to be below 0.001% to
address the issue of SIPH crack resistance.
[0021] To prevent insufficient penetration depth, welding heat
input may be simply increased. However, increasing welding heat
input brings about grains coarsening in weld-heat-affected zones,
and the inventors failed to prevent SIPH cracking even when the
initial grain size of the base material had a grain size number of
2.0 or more.
[0022] After further research, the inventors found that, in order
to prevent insufficient penetration depth in a stable manner, it is
effective to have an Sn content in the range of 0.001 to 0.02%.
They concluded that this is because Sn can easily evaporate from
the surface of the molten pool during welding and ionize in the arc
to contribute to the formation of an electrifying path, thereby
increasing the current density of the arc.
[0023] The present invention was made based on the above-discussed
findings. An austenitic heat-resistant alloy according to an
embodiment of the present invention will now be described in
detail.
[0024] [Chemical Composition]
[0025] The austenitic heat-resistant alloy according to the present
embodiment has the chemical composition described below. In the
following description, "%" in the content of an element means mass
percent.
[0026] C: 0.04 to 0.14%
[0027] Carbon (C) stabilizes the austenite microstructure and forms
fine carbide particles to improve creep strength during use at high
temperatures. 0.04% or more C needs to be contained in order that
these effects are sufficiently present. However, if an excess
amount of C is contained, large amounts of carbides precipitate,
which reduces SIPH crack resistance. In view of this, the upper
limit should be 0.14%. The lower limit of C content is preferably
0.05%, and more preferably 0.06%. The upper limit of C content is
preferably 0.13%, and more preferably 0.12%.
[0028] Si: 0.05 to 1%
[0029] Silicon (Si) has a deoxidizing effect, and is effective in
improving the corrosion resistance and oxidation resistance at high
temperatures. 0.05% or more Si needs to be contained in order that
these effects are sufficiently present. However, if an excess
amount of Si is contained, the stability of the microstructure
decreases, which decreases toughness and creep strength. In view of
this, the upper limit should be 1%. The lower limit of Si content
is preferably 0.08%, and more preferably 0.1%. The upper limit of
Si content is preferably 0.6%, and more preferably 0.5%.
[0030] Mn: 0.5 to 2.5%
[0031] Similar to Si, manganese (Mn) has a deoxidizing effect. Mn
also contributes to the stabilization of austenite microstructure.
0.5% or more Mn needs to be contained in order that these effects
are sufficiently present. However, if an excess amount of Mn is
contained, this causes embrittlement of the alloy, and creep
ductility decreases. In view of this, the upper limit should be
2.5%. The lower limit of Mn content is preferably 0.6%, and more
preferably 0.7%. The upper limit of Mn content is preferably 2%,
and more preferably 1.5%.
[0032] P: Up to 0.03%
[0033] Phosphorus (P) is contained in the alloy in the form of an
impurity, and, during welding, segregates on grain boundaries in
weld-heat-affected zones, thereby increasing liquation cracking
susceptibility. P also decreases creep ductility after use for a
prolonged period of time. In view of this, an upper limit should be
set for P content, which should be 0.03% or lower. The upper limit
of P content is preferably 0.028%, and more preferably 0.025%. It
is preferable to minimize P content; however, reducing it
excessively causes increased steel-manufacturing cost. In view of
this, the lower limit of P content is preferably 0.0005%, and more
preferably 0.0008%.
[0034] S: Less than 0.001%
[0035] Similar to P, sulfur (S) is contained in the alloy in the
form of an impurity, and, during welding, segregates on grain
boundaries in weld-heat-affected zones, thereby increasing
liquation cracking susceptibility. S also segregates on grain
boundaries during use for a prolonged period of time and causes
embrittlement, which significantly reduces SIPH crack resistance.
To prevent these effects within the limits of the chemical
composition of the present embodiment, the S content needs to be
less than 0.001%. The upper limit of S content is preferably
0.0008%, and more preferably 0.0005%. It is preferable to minimize
S content; however, reducing it excessively causes increased
steel-manufacturing cost. In view of this, the lower limit of S
content is preferably 0.0001%, and more preferably 0.0002%.
[0036] Ni: 23 to 32%
[0037] Nickel (Ni) is an element indispensable for providing
sufficient stability of the austenite phase during use for a
prolonged period of time. 23% or more Ni needs to be contained in
order that this effect is sufficiently present within the limits of
Cr and W contents of the present embodiment. However, Ni is an
expensive element, and large amounts of Ni contained mean increased
costs. In view of this, the upper limit should be 32%. The lower
limit of Ni content is preferably 25%, and more preferably 25.5%.
The upper limit of Ni content is preferably 31.5%, and more
preferably 31%.
[0038] Cr: 20 to 25%
[0039] Chromium (Cr) is an element indispensable for providing
sufficient oxidation resistance and corrosion resistance at high
temperatures. Cr also forms fine carbide particles to contribute to
the provision of sufficient creep strength, too. 20% or more Cr
needs to be contained in order that these effects are sufficiently
present within the limits of Ni content of the present embodiment.
However, if an excessive amount of Cr is contained, the
microstructure stability of the austenite phase at high
temperatures deteriorates, which decreases creep strength. In view
of this, the upper limit should be 25%. The lower limit of Cr
content is preferably 20.5%, and more preferably 21%. The upper
limit of Cr content is preferably 24.5%, and more preferably
24%.
[0040] W: 1 to 5%
[0041] Tungsten (W) dissolves in the matrix, or forms fine
intermetallic compounds to significantly contribute to the
improvement of creep strength and tensile strength at high
temperatures. 1% or more W needs to be contained in order that
these effects are sufficiently present. However, if an excess
amount of W is contained, the deformation resistance with grains
becomes high and SIPH crack resistance reduces, and creep strength
may decrease. Further, W is an expensive element, and large amounts
of W contained mean increased costs. In view of this, the upper
limit should be 5%. The lower limit of W content is preferably
1.2%, and more preferably 1.5%. The upper limit of W content is
preferably 4.5%, and more preferably 4%.
[0042] Nb: 0.1 to 0.6%
[0043] Niobium (Nb) precipitates in the form of fine MX
carbonitride particles, and, in addition, precipitates in the form
of Z phase (CrNbN) within grains to significantly contribute to the
improvement of creep strength and tensile strength at high
temperatures. 0.1% or more Nb needs to be contained in order that
these effects are sufficiently present. However, if an excess
amount of Nb is contained, the strengthening property of these
precipitates is too high, which reduces SIPH crack resistance and
causes a decrease in creep ductility and toughness. In view of
this, the upper limit should be 0.6%. The lower limit of Nb content
is preferably 0.12%, and more preferably 0.15%. The upper limit of
Nb content is preferably 0.55%, and more preferably 0.5%.
[0044] V: 0.1 to 0.6%
[0045] Vanadium (V) precipitates in the form of fine MX
carbonitride particles within the grains to contribute to the
improvement of creep strength and tensile strength at high
temperatures. 0.1% or more V needs to be contained in order that
these effects are sufficiently present. However, if an excess
amount of V is contained, large amounts of carbonitrides
precipitate, which reduces SIPH crack resistance and causes a
decrease in creep ductility and toughness. In view of this, the
upper limit should be 0.6%. The lower limit of V content is
preferably 0.12%, and more preferably 0.15%. The upper limit of V
content is preferably 0.55%, and more preferably 0.5%.
[0046] N: 0.1 to 0.3%
[0047] Nitrogen (N) stabilizes the austenite microstructure, and
dissolves in the matrix or precipitates in the form of nitrides to
contribute to the improvement of high-temperature strength. 0.1% or
more N needs to be contained in order that these effects are
sufficiently present. However, if an excessive amount of N is
contained, it dissolves during use for a short period of time, or
large amounts of fine nitride particles precipitate within grains
during use for a prolonged period of time, thereby increasing the
deformation resistance within grains, which reduces SIPH crack
resistance. Further, creep ductility and toughness decrease. In
view of this, the upper limit should be 0.3%. The lower limit of N
content is preferably 0.12%, and more preferably 0.14%. The upper
limit of N content is preferably 0.28%, and more preferably
0.26%.
[0048] B: 0.0005 to 0.01%
[0049] Boron (B) provides fine dispersed grain-boundary carbide
particles to improve creep strength, and segregates on grain
boundaries to strengthen grain boundaries. 0.0005% or more B needs
to be contained in order that these effects are sufficiently
present. However, if an excess amount of B is contained, the weld
thermal cycle during welding causes a large amount of B to
segregate in weld heat affected zones near melt boundaries to
decrease the melting point of grain boundaries, thereby increasing
liquation cracking susceptibility. In view of this, the upper limit
should be 0.01%. The lower limit of B content is preferably
0.0008%, and more preferably 0.001%. The upper limit of B content
is preferably 0.008%, and more preferably 0.006%.
[0050] Sn: 0.001 to 0.02%
[0051] Tin (Sn) has the effect of increasing the penetration depth
during welding by evaporating from the molten pool to increase the
current density of the arc. 0.001% or more Sn needs to be contained
in order that these effects are sufficiently present. However, if
an excess amount of Sn is contained, the liquation cracking
susceptibility in weld-heat-affected zones during welding and the
SIPH crack susceptibility during use become high. In view of this,
the upper limit should be 0.02%. The lower limit of Sn content is
preferably 0.0016%, and more preferably 0.002%. The upper limit of
Sn content is preferably 0.018%, and more preferably 0.015%.
[0052] Al: Up to 0.03%
[0053] Aluminum (Al) has a deoxidizing effect. However, if an
excess amount of Al is contained, the cleanliness of the alloy
deteriorates, which decreases hot workability. In view of this, the
upper limit should be 0.03%. The upper limit of Al content is
preferably 0.025%, and more preferably 0.02%. No lower limit needs
to be set; still, it should be noted that decreasing Al excessively
causes an increase in steel-manufacturing cost. In view of this,
the lower limit of Al content is preferably 0.0005%, and more
preferably 0.001%. Al as used herein means acid-soluble Al (sol.
Al).
[0054] O: Up to 0.02%
[0055] Oxygen (O) is contained in the alloy in the form of an
impurity, and has the effect of increasing the penetration depth
during welding. However, if an excess amount of O is contained, hot
workability decreases and toughness and ductility deteriorate. In
view of this, the upper limit should be 0.02%. The upper limit of O
content is preferably 0.018%, and more preferably 0.015%. No lower
limit needs to be set; still, it should be noted that decreasing O
excessively causes an increase in steel-manufacturing cost. In view
of this, the lower limit of O content is preferably 0.0005%, and
more preferably 0.0008%.
[0056] The balance of the chemical composition of the austenitic
heat-resistant alloy in the present embodiment is Fe and
impurities. Impurity as used herein means an element originating
from ore or scrap used as raw material for the heat-resistant alloy
being manufactured on an industrial basis or an element that has
entered from the environment or the like during the manufacturing
process.
[0057] In the chemical composition of the austenitic heat-resistant
alloy in the present embodiment, some of the Fe may be replaced by
one or more elements selected from one of the first to third groups
provided below. All of the elements listed below are optional
elements. That is, none of the elements listed below may be
contained in the austenitic heat-resistant alloy of the present
embodiment. Or, only one or some of them may be contained.
[0058] More specifically, for example, only one group may be
selected from among the first to third groups and one or more
elements may be selected from this group. In this case, it is not
necessary to select all the elements belonging to the selected
group. Further, a plurality of groups may be selected from among
the first to third groups and one or more elements may be selected
from each of these groups. Again, it is not necessary to select all
the elements belonging to the selected groups.
[0059] First Group--Ti: 0 to 0.5%
[0060] The element belonging to the first group is Ti. Ti improves
the creep strength of the alloy through precipitation
strengthening.
[0061] Ti: 0 to 0.5%
[0062] Similar to Nb and V, Titanium (Ti) combines with carbon or
nitrogen to form fine carbide or carbonitride particles, thereby
contributing to the improvement of creep strength. These effects
are present if a small amount of Ti is contained. On the other
hand, if an excess amount of Ti is contained, large amounts of
precipitates are produced, which reduces SIPH resistance and creep
ductility. In view of this, the upper limit should be 0.5%. The
lower limit of Ti content is preferably 0.01%, and more preferably
0.03%. The upper limit of Ti content is preferably 0.45%, and more
preferably 0.4%.
[0063] Second Group--Co: 0 to 2%, Cu: 0 to 4%, Mo: 0 to 4%
[0064] The elements belonging to the second group are Co, Cu, and
Mo. These elements improve the creep strength of the alloy.
[0065] Co: 0 to 2%
[0066] Similar to Ni, cobalt (Co) is an austenite-forming element,
and increases the stability of the austenite microstructure to
contribute to the improvement of creep strength. These effects are
present if a small amount of Co is contained. However, Co is a very
expensive element, and large amounts of Co contained mean increased
costs. In view of this, the upper limit should be 2%. The lower
limit of Co content is preferably 0.01%, and more preferably 0.03%.
The upper limit of Co content is preferably 1.8%, and more
preferably 1.5%.
[0067] Cu: 0 to 4%
[0068] Similar to Ni and Co, copper (Cu) stabilizes the austenite
microstructure, and precipitates in the form of fine particles
during use to contribute to the improvement of creep strength.
These effects are present if a small amount of Cu is contained. On
the other hand, if an excessive amount of Cu is contained, this
causes a decrease in hot workability. In view of this, the upper
limit should be 4%. The lower limit of Cu content is preferably
0.01%, and more preferably 0.03%. The upper limit of Cu content is
preferably 3.8%, and more preferably 3.5%.
[0069] Mo: 0 to 4%
[0070] Similar to W, molybdenum (Mo) dissolves in the matrix and
contributes to the improvement of creep strength and tensile
strength at high temperatures. These effects are present if a small
amount of Mo is contained. On the other hand, if an excessive
amount of Mo is contained, the deformation resistance within grains
becomes high and SIPH crack resistance reduces, and creep strength
may decrease. Further, Mo is an expensive element, and large
amounts of Mo contained mean increased costs. In view of this, the
upper limit should be 4%. The lower limit of Mo content is
preferably 0.01%, and more preferably 0.03%. The upper limit of Mo
content is preferably 3.8%, and more preferably 3.5%.
[0071] Third Group--Ca: 0 to 0.02%, Mg: 0 to 0.02%, REM: 0 to
0.2%
[0072] The elements belonging to the third group are Ca, Mg and
REM. These elements improve hot workability of the alloy.
[0073] Ca: 0 to 0.02%
[0074] Calcium (Ca) improves hot workability during manufacture.
This effect is present if a small amount of Ca is contained. On the
other hand, if an excessive amount of Ca is contained, it combines
with oxygen to significantly decrease the cleanliness of the alloy,
which decreases hot workability. In view of this, the upper limit
should be 0.02%. The lower limit of Ca content is preferably
0.0005%, and more preferably 0.001%. The upper limit of Ca content
is preferably 0.01%, and more preferably 0.005%.
[0075] Mg: 0 to 0.02%
[0076] Similar to Ca, magnesium (Mg) improves hot workability
during manufacture. This effect is present if a small amount of Mg
is contained. On the other hand, if an excess amount of Mg is
contained, it combines with oxygen to significantly decrease the
cleanliness of the alloy, which decreases hot workability. In view
of this, the upper limit is 0.02%. The lower limit of Mg content is
preferably 0.0005%, and more preferably 0.001%. The upper limit of
Mg content is preferably 0.01%, and more preferably 0.005%.
[0077] REM: 0 to 0.2%
[0078] Similar to Ca and Mg, rare-earth metals (REMs) improve hot
workability during manufacture. This effect is present if a small
amount of REM is contained. On the other hand, if an excessive
amount of REM is contained, it combines with oxygen to
significantly decrease the cleanliness of the alloy, which
decreases hot workability. In view of this, the upper limit should
be 0.2%. The lower limit of REM content is preferably 0.0005%, and
more preferably 0.001%. The upper limit of REM content is
preferably 0.15%, and more preferably 0.1%.
[0079] "REM" is a collective term for a total of 17 elements, i.e.
Sc, Y and the lanthanoids, and "REM content" means the total
content of one or more REM elements. REMs are usually contained in
mischmetal. Thus, for example, mischmetal may be added to the alloy
such that the REM content is in the above-indicated range.
[0080] Particularly, Nd has a strong affinity for S and P, and has
the effect of reducing weld liquation cracking susceptibility by
forming sulfides or phosphides, and thus it is more preferable to
utilize Nd.
[0081] [Microstructure]
[0082] Grain Size Number: 2.0 or More and Less than 7.0
[0083] The austenitic heat-resistant alloy according to the present
embodiment has a microstructure having a grain size represented by
a grain size number in accordance with ASTM E112 of 2.0 or more and
less than 7.0.
[0084] In order to give sufficient SIPH crack resistance to the
weld-heat-affected zones of a welded structure using the austenitic
heat-resistant alloy of the present embodiment, the grains of the
microstructure before welding need to be fine grains, i.e. their
size as represented by grain size number in accordance with ASTM
E112 needs to be 2.0 or more, in order to prevent the grains in the
weld-heat-affected zones from becoming excessively coarse even
after being affected by the heat cycle from the welding. On the
other hand, if the grains are so fine as to have a grain size
number of 7.0 or more, the required creep strength is not obtained.
In view of this, the grain size number should be 2.0 or more and
less than 7.0.
[0085] The microstructure having the above-specified grain size can
be provided by performing a heat treatment on the alloy with the
above-specified chemical composition under appropriate conditions.
This microstructure may be achieved by, for example, shaping the
alloy of the above-specified chemical composition into a
predetermined shape by hot working or cold working before
performing a solution heat treatment in which it is held at
temperatures of 900 to 1250.degree. C. for 3 to 60 minutes before
water cooling. The higher the holding temperature of the solution
heat treatment and the longer the holding time, the larger the
grain size becomes (i.e. the smaller the grain size number
becomes). More preferably, the solution heat treatment involves
holding the alloy at temperatures of 1120 to 1220.degree. C. for 3
to 45 minutes before water cooling, and yet more preferably holding
the alloy at temperatures of 1140 to 1210.degree. C. for 3 to 30
minutes before water cooling.
[0086] The austenitic heat-resistant alloy according to an
embodiment of the present invention has been described. The present
embodiment provides an austenitic heat-resistant alloy providing
good crack resistance and high-temperature strength in a stable
manner.
EXAMPLES
[0087] The present invention will be described in more detail below
using examples. The present invention is not limited to these
examples.
[0088] The materials labeled A to J having the chemical
compositions shown in Table 1 were melted in a laboratory and
ingots were cast, which were subjected to hot forging and hot
rolling in the temperature range of 1000 to 1150.degree. C. to
provide plates with a thickness of 20 mm. These plates were further
subjected to cold rolling to the thickness of 16 mm. The plates
were subjected to a solution heat treatment in which they were held
at 1200.degree. C. for a predetermined period of time before water
cooling. After the solution heat treatment, they were machined to
plates with a thickness of 14 mm, a width of 50 mm and a length of
100 mm. From other plates subjected to the solution heat treatment,
samples to be used for microstructure observation were taken and
the grain size of the microstructure of each sample was measured in
accordance with ASTM E 112. From material A, materials with
different grain sizes were produced by changing the holding time of
the solution heat treatment in the range of 3 to 30 minutes.
TABLE-US-00001 TABLE 1 Chemical composition (in mass %. balance
being Fe and impurities) Mark C Si Mn P S Ni Cr W Nb V N B Al O Sn
Other A 0.09 0.28 0.98 0.017 0.0008 30.2 21.8 3.3 0.25 0.21 0.197
0.0023 0.005 0.009 0.004 B 0.08 0.32 1.02 0.008 0.0006 28.5 22.0
3.0 0.23 0.22 0.206 0.0017 0.006 0.008 0.012 Nd: 0.023 C 0.10 0.25
1.10 0.016 0.0005 27.1 21.7 2.7 0.18 0.19 0.174 0.0018 0.005 0.009
0.001 Ti: 0.12, Ca: 0.002, Cu: 0.41, Mo: 0.03 D 0.07 0.34 1.18
0.014 0.0004 30.6 22.3 2.9 0.21 0.19 0.185 0.0026 0.004 0.010 0.016
Nd: 0.015, Co: 0.08, Mg: 0.001 E 0.07 0.29 0.82 0.017 0.0002 29.8
22.4 2.8 0.22 0.21 0.211 0.0024 0.012 0.004 -- * F 0.11 0.29 0.96
0.021 .sup. 0.0021 * 30.5 21.9 3.1 0.38 0.31 0.198 0.0015 0.007
0.008 -- * Ti: 0.18 G 0.09 0.30 0.98 0.023 0.0003 30.3 22.0 2.7
0.42 0.29 0.215 0.0044 0.003 0.009 .sup. 0.033 * Nd: 0.010 H 0.08
0.25 0.95 0.015 0.0008 .sup. 22.4 * 24.6 2.5 0.45 0.21 0.221 0.0024
0.004 0.010 0.010 I 0.08 0.25 1.04 0.015 0.0007 30.9 22.0 3.1 0.24
0.20 0.188 0.0019 0.004 0.008 0.003 REM: 0.018 J 0.07 0.26 0.85
0.015 0.0006 25.6 24.5 2.2 0.16 .sup. 0.08 * 0.165 0.0018 0.006
0.009 0.004 * indicates that the value is outisde the range
specified by the present invention.
[0089] [Weldability]
[0090] The groove shown in FIG. 1 was provided along the
longitudinal direction of each plate produced as described above.
With grooved plates abutting each other, two joints for each mark
were subjected to butt welding using gas-tungsten arc welding to
produce welded joints. The welding did not use filler material, and
the amount of heat input was 5 kJ/cm.
[0091] Those of the obtained welded joints that had back beads with
a width of 2 mm or more across the entire length of the weld line
for both joint parts were determined to have good weldability in
fabrication and thus to have "passed" the test. Those that had a
portion for either joint part in which no back bead was present
were determined to have poor weldability in fabrication and thus to
be "unacceptable".
[0092] [Weld Crack Resistance]
[0093] Each of the above-described welded joints, with only a first
welded layer (i.e. root running), was placed on a commercial steel
plate equivalent to the SM400B plate specified by JIS G 3106 (2008)
(with a thickness of 30 mm, a width of 200 mm and a length of 200
mm), and restraint welding was performed on the four sides using a
covered arc welding rod ENi 6625 specified by JIS Z 3224 (2010).
Thereafter, a tig wire equivalent to the SNi 6625 wire specified by
JIS Z 3334 (2011) was used to perform a multi-layer welding in the
groove by TIG welding with a heat input of 10 to 15 kJ/cm, thereby
producing welded joints, two for each mark.
[0094] Aging was performed on one of the welded-joint parts for
each mark at 700.degree. C. for 500 hours. Samples were taken from
five points on each of the as-welded joints and welded joints after
aging, with the observation surface represented by a transverse
cross section of the joint (i.e. cross section perpendicular to the
weld bead). Mirror polishing and etching were performed on these
samples before inspection by optical microscopy to determine
whether cracks were present in the weld-heat-affected zones. Welded
joints where no cracks were found in any of the five samples were
determined to be "good" and those where cracks were found in one
sample were determined to be "acceptable", thus to have passed the
test. Those welded joints where cracks were found in two or more
samples were determined to be "unacceptable".
[0095] [Creep-Rupture Strength]
[0096] From those as-welded joints that have passed the weld crack
resistance test, round-bar creep-rupture test specimens were taken
such that the center of the parallel portion was made of welded
metal. Creep-rupture testing was conducted at 700.degree. C. and
under 167 MPa, conditions that result in a target fracture time for
the base material of about 1000 hours. The base material was
fractured, and those joints where the fracture time was 90% or more
of the fracture time of the base material (i.e. 900 hours or
longer) were determined to have "passed" the test.
[0097] [Performance Evaluation Results]
[0098] The performance evaluation results are shown in Table 2.
Table 2 also shows the grain size number of the austenitic
heat-resistant alloy for each mark.
TABLE-US-00002 TABLE 2 Grain Creep- size Weldability Weld crack
rupture Mark number in Fabrication as-welded aged test result A-1
2.3 passed good good passed A-2 3.7 passed good good passed A-3 5.4
passed good good passed A-4 6.8 passed good good passed A-5 .sup.
1.7 * passed acceptable unacceptable not tested A-6 .sup. 7.5 *
passed good good not passed B 3.5 passed good good passed C 3.4
passed good good passed D 3.6 passed acceptable acceptable passed E
3.1 unacceptable good good passed F 3.6 passed acceptable
unacceptable not tested G 3.4 passed unacceptable unacceptable not
tested H 3.8 passed good good not passed I 3.5 passed good good
passed J 3.2 passed good good not passed * indicates that the value
is outisde the range specified by the present invention.
[0099] Each of the welded joints using the austenitic
heat-resistant alloys with Marks A-1 to A-4, B to D and I as the
base material had an appropriate chemical composition, where the
initial grain size of the base material had a grain size of 2.0 or
more and less than 7.0. Each of these welded joints had a back bead
across the entire length after root running, and had good
weldability in fabrication. Further, though the thickness of the
base material was 14 mm, which is relatively large, no cracks were
produced in weld-heat-affected zones even after aging, meaning good
crack resistance. Further, the creep-rupture strength at high
temperatures was sufficient.
[0100] In the welded joint using the austenite heat-resistant alloy
with Mark A-5 as the base material, cracks that are believed to be
SIPH cracks were produced after aging. This is presumably because
the grain size of the austenitic heat-resistant alloy with Mark A-5
was too large.
[0101] The welded joint using the austenitic heat-resistant alloy
with Mark A-6 as the base material had good crack resistance, but
the creep-rupture time was below the target. This is presumably
because the grain size of the austenitic heat-resistant alloy with
Mark A-6 was too small.
[0102] In the welded joint using the austenitic heat-resistant
alloy with Mark E as the base material, no back bead was present in
some portions after root running. This is presumably because the Sn
content of the austenitic heat-resistant alloy with Mark E was too
low.
[0103] The welded joint using the austenitic heat-resistant alloy
with Mark F as the base material contained no Sn but a large amount
of S such that a sufficient back bead was produced. However, cracks
that are believed to be SIPH cracks were produced after aging.
[0104] In the welded joint using the austenitic heat-resistant
alloy with Mark G as the base material, directly after welding and
after aging, cracks that are believed to be liquation cracks and
SIPH cracks, respectively, were produced. This is presumably
because the Sn content of the austenitic heat-resistant alloy with
Mark G was too high.
[0105] In the welded joint using the austenitic heat-resistant
alloy with Mark H as the base material, the weldability in
fabrication and weld crack resistance were good but the required
creep strength was not satisfied. This is presumably because the Ni
content of the austenitic heat-resistant alloy with Mark H was too
low, impairing phase stability.
[0106] In the welded joint using the austenitic heat-resistant
alloy with Mark J as the base material, too, the required creep
strength was not satisfied. This is presumably because the amount
of V contained in the austenitic heat-resistant alloy with Mark J
was lower than the lower limit.
INDUSTRIAL APPLICABILITY
[0107] The present invention can be suitably used as an austenitic
heat-resistant alloy used as a high-temperature part such as a main
steam tube or high-temperature reheating steam tube in a thermal
power boiler.
* * * * *