Note: Descriptions are shown in the official language in which they were submitted.
CA 02988556 2017-12-06
NSSMC Ref. FP152761W00
Our Ref. 102W0222P1
AUSTENITIC HEAT-RESISTANT ALLOY AND WELDED STRUCTURE
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 Al 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 Aldiscussed above states that limiting the amounts
1
CA 02988556 2017-12-06
NSSMC Ref. FP152761W00
Our Ref. 102W0222P1
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 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 % 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 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 7Ø
[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] 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
2
CA 02988556 2017-12-06
NSSMC Ref. FP152761W00
Our Ref. 102W0222P1
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 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
3
CA 02988556 2017-12-06
NSSMC Ref. FP152761W00
Our Ref. 102W0222P1
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 % 0, 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 7Ø 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
4
CA 02988556 2017-12-06
NSSMC Ref. FP152761W00
Our Ref. 102W0222P1
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]
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.
[0025] C: 0.04 to 0.14 %
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 %.
[0026] Si: 0.05 to 1 %
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 %.
[0027] Mn: 0.5 to 2.5 %
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
CA 02988556 2017-12-06
NSSMC Ref. FP152761W00
Our Ref. 102W0222P1
more preferably 0.7 %. The upper limit of Mn content is preferably 2 %, and
more preferably 1.5 %.
[00281 13: up to 0.03 %
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 %.
[0029] S: less than 0.001 %
Similar to P, sulfur (5) 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 %.
[0030] Ni: 23 to 32 %
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 %.
[0031] Cr: 20 to 25 %
6
CA 02988556 2017-12-06
NSSMC Ref. FP152761W00
Our Ref. 102W0222P1
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 %.
[0032] W: 1 to 5 %
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
%. 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 %.
[0033] Nb: 0.1 to 0.6 %
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 %.
[0034] V: 0.1 to 0.6%
7
CA 02988556 2017-12-06
NSSMC Ref. FP152761W00
Our Ref. 102W0222P1
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 %.
[0035] N: 0.1 to 0.3 %
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 %.
[0036] B: 0.0005 to 0.01% =
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 %.
[0037] Sn: 0.001 to 0.02 %
Tin (Sn) has the effect of increasing the penetration depth during
8
CA 02988556 2017-12-06
NSSMC Ref. FP152761W00
Our Ref. 102W0222P1
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 0.0015 %, and more preferably 0.002 %. The upper
limit of Sn content is preferably 0.018 %, and more preferably 0.015 %.
[0038] Al; up to 0.03 %
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).
[0039] 0; up to 0.02 %
Oxygen (0) 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 0 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 0 content is preferably 0.018 %, and more
preferably 0.015 %. No lower limit needs to be set; still, it should be noted
that decreasing 0 excessively causes an increase in steel-manufacturing cost.
In view of this, the lower limit of 0 content is preferably 0.0005 %, and more
preferably 0.0008 %.
[0040] 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.
[0041] 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
9
CA 02988556 2017-12-06
NSSMC Ref. FP152761W00
Our Ref. 102W0222P1
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.
[0042] 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.
[0043] First Group¨Ti: 0 to 0.5 %
The element belonging to the first group is Ti. Ti improves the creep
strength of the alloy through precipitation strengthening.
[0044] Ti: 0 to 0.5 %
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 %.
[0045] Second Group¨Co: 0 to 2 %, Cu: 0 to 4 %, Mo: 0 to 4 %
The elements belonging to the second group are Co, Cu, and Mo.
These elements improve the creep strength of the alloy.
[0046] Co: 0 to 2 %
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 %.
[0047] Cu: 0 to 4 %
Similar to Ni and Co, copper (Cu) stabilizes the austenite
CA 02988556 2017-12-06
NSSMC Ref. FP152761W00
Our Ref. 102W0222P1
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 %.
[0048] Mo: 0 to 4 %
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 %.
[0049] Third Group¨Ca: 0 to 0.02 %, Mg: 0 to 0.02 %, REM: 0 to 0.2 %
The elements belonging to the third group are Ca, Mg and REM.
These elements improve hot workability of the alloy.
[0050] Ca: 0 to 0.02 %
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 %.
[0051] Mg: 0 to 0.02 %
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
11
CA 02988556 2017-12-06
NSSMC Ref. FP152761W00
Our Ref. 102W0222P1
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 %.
[0052] REM: 0 to 0.2 %
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 %.
[0053] "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.
[0054] 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.
[0055] [Microstructure]
Grain Size Number: 2.0 or more and less than 7.0
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 7Ø
[0056] 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
7Ø
[0057] The microstructure having the above-specified grain size can be
12
CA 02988556 2017-12-06
NSSMC Ref. FP152761W00
Our Ref. 102W0222P1
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 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 C for 3 to 45 minutes before water cooling, and yet more preferably
holding the alloy at temperatures of 1140 to 1210 C for 3 to 30 minutes
before water cooling.
[0058] 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
[0059] The present invention will be described in more detail below using
examples. The present invention is not limited to these examples.
[0060] 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 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 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.
[0061] [Table 1]
13
P"
.. 8 i TABLE 1
i
(r) tn
n Chemical composition (in mass %,
balance being Fe and impurities)
c-r- La, Mark -
-
5' c Si Mn P S Ni Cr W Nb V N B Al 0 Sn
Other
- , -
-
H 7 , 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
)-+z (t) 2_ 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
CD crq i:71,
Ti: 0.12, Ca: 0.002,
Sm 1-1 C4 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
C o 0.-
Cu: 0.41, Mo: 0.03
0 1-µ=
C1--' - -
'CS (c) 14:
Nd: 0.015, Co: 0.08,
t5-D . "< 0 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
Mg 0.001
0= 4,, ,._,
- -.
(1) o 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 0.0021 *_ 30.5 21.9
3.1 _ 0.38 0.31 , 0.198 0.0015 0.007 , 0.008 - * Ti:
0.18
8
;:l.. -= 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 , 0.033 * Nd:
0.010
= 0 H 0.08 0.25
0.95 _ 0.015 0.0008 _ 22.4 * 24.6 2.5 0.45 0.21 , 0.221 0.0024 0,004
0.010 , 0.010
1 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
C.)
0-
P
Po = J 0.07 0.26 0.85 0.015 0.0006 25.6 24.5 2.2 0.16 0.08 *0,165
0.0018 0.006 0.009 0.004 0
rn
I-, * indicates
that the value is outisde the range specified by the present invention. 1.,
Fi'
0
0
u,
.4=== n Cl)
'-ii= 'ZS
IV
0
Cr Ft
F
,
CD 0
,
IV
I
W P.
0
Cr
CD
c,
O pu
<
cD AD
18-
Z
rig
U)
Cf)
,-, = c-p-
c-F ====
or, n4
(-D
aci .
R pj
1-1 0
CD
0
0 go
C 1--,=
l-," 1-,1
CD c-t-
AD pp
0 CT))
(-1- ,-
Cl)
Lt3
1-, 0
CA 02988556 2017-12-06
NSSMC Ref. FP152761W00
Our Ref. 102W0222P1
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.
[0063] 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".
[0064] [Weld Crack Resistance]
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.
[0065] Aging was performed on one of the welded-joint parts for each mark
at 700 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".
[0066] [Creep-Rupture Strength]
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 C and under 167 MPa, conditions that result
CA 02988556 2017-12-06
NSSMC Ref. FP152761W00
Our Ref. 102W0222P1
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.
[0067] [Performance Evaluation Results]
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.
[0068] [Table 21
TABLE 2
M Grain Weldability Weld crack Creep¨rupture
ark
size 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 1.7 * passed acceptable unacceptable not tested
A-6 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.
[0069]
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 7Ø 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.
16
CA 02988556 2017-12-06
NSSMC Ref. FP152761W00
Our Ref. 102W0222P1
[00701 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.
[0071] 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.
[0072] 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.
[0073] 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.
[0074] 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.
[0075] 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.
[0076] 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
[0077] 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.
17
