WELDING MATERIAL FOR AUSTENITIC HEAT RESISTANT STEELS,
AND WELD METAL AND WELDED JOINT OBTAINED BY USING THE
SAME MATERIAL
TECHNICAL FIELD
[0001]
The present invention relates to a welding material for austenitic heat
resistant steels, and a weld metal and a welded joint obtained by using the
said welding material. More particularly, the present invention relates to a
welding material suitable for welding austenitic heat resistant steels employed
for equipment used at high temperatures, such as power generation boilers,
and a weld metal and a welded joint obtained by using the said welding
material.
BACKGROUND ART
[0002]
In recent years, from the viewpoint of environmental loading reduction,
the power generation boilers and the like have increasingly been operated at
higher temperature and pressure on a worldwide scale, and accordingly the
material used for such equipment has been required to have a greater high
temperature strength.
[0003]
As a material that meets such a requirement, for example, the Patent
Literature 1 proposes an austenitic heat resistant steel having an excellent
creep strength in a high temperature range of 700°C or higher due to a large
amount ofW contained.
[0004]
When the austenitic heat resistant steel is used for a structure, the
assembly is generally carried out by welding. At this time, the base metal
may be used as a welding material as it is. Further, the welding may be
•
performed by using a welding material for high Ni alloys, such as AWS A5.142005
ERNiCrCoMo·l.
[0005]
In general, however, the weld metal consisting of the austenitic heat
resistant steel is highly susceptible to hot cracking at the welding time, and
the prevention of the said cracking may be required. The "hot cracking at the
welding time" include the "solidification cracking" and the "ductility·dip
cracking" .
[0006]
Furthermore, for the base metal, high temperature strength is ensured
as the microstructure is regulated by rolling and heat treatment after melting,
whereas the weld metal is used in, an as·solidified microstructure in most
cases. Therefore, if the base metal is used as a welding material without
being subjected to rolling and heat treatment, sufficient resistance to hot
cracking is not attained at the welding time in some cases, or it is difficult, in
some cases, to attain mechanical properties such as creep strength equivalent
to those of the base metal.
[0007]
In addition, although the creep strength is excellent, the welding
material for high Ni alloys is not preferable in some cases from the viewpoint
of economy because of its high cost, and moreover in the case where the
components thereof are much different from those of the material to be welded,
sufficient resistance to hot cracking at the welding time is not attained in some
cases.
[0008]
On the other hand, the Patent Literature 2 proposes a welding material
for austenitic heat resistant steels excellent in high temperature strength, in
:~
3
which eutectic carbides of Nb and Ti are utilized, and thereby both of
prevention of hot cracking at the welding time and creep strength are attained.
[0009]
A welded structure consisting of an austenitic heat resistant steel, which
is used at high temperatures, has a problem of occurrence of cracking in weld
zones on account of a long period of use at high temperatures in addition to the
problem of hot cracking at the welding time.
[0010]
For example, in the Non-Patent Literatures 1 and 2, it is pointed out
that the weld zone of 18Cr-8Ni type austenitic heat resistant steels, undergo
intergranular cracking in a weld heat affected zone (hereinafter, referred to as
a "HAZ") on account of a long period of heating.
[0011]
In these Non-Patent Literatures, as a factor affecting the intergranular
cracking in the HAZ, carbides such as M23Ca and NbC are suggested.
[0012]
In the Non-Patent literature 3, in the HAZ of a Ni base heat resistant
alloy, a fact that the intergranular cracking occurs during post weld heat
treatment is pointed out, and in addition to the precipitation of y' phase, the
influence of grain boundary segregation of S is suggested.
[0013]
Furthermore, in the Non-Patent Literature 4, a study has been carried
out on the preventive measures against intergranular cracking in the HAZ of
the weld zone of an 18Cr-8Ni-Nb type austenitic heat resistant steel at the
time of a long period of heating. The measures are proposed from the
viewpoint of welding process such that the decrease in welding residual
stresses caused by a proper application of post weld heat treatment is effective
in preventing cracking.
-y
4
[0014]
Thus, from long ago, there has been known a phenomenon that cracking
occurs in the HAZ when an austenitic heat resistant steel is used for a long
period. In recent years, however, as many kinds of alloying elements are
contained in order to increase the strength of material, there has been a
tendency for cracking occurrence at the time of a long period of heating to
become remarkable even in weld metal.
[0015]
However, the mechanism of cracking occurred in the weld zone during a
long period of use has not yet been elucidated, and further, the measures
against the said cracking, especially, the measures against the cracking from
the viewpoint of the material of weld metal have not been established.
[0016]
The weld metal consisting of a high strength austenitic heat resistant
steel disclosed in the Patent Literature 2 is remarkably excellent in resistance
to hot cracking at the welding time. However, in terms of cracking which
occurs during a long period of use under recent severe service conditions
(hereinafter, referred to as "stress relaxation cracking"), the said weld metal
has room for some improvement.
LIST OF PRIOR ART DOCUMENTS
PATENT LITERATURES
[0017]
Patent Literature 1: JP 2004-3000 A
Patent Literature 2: JP 2008-207242 A
NON-PATENT LITERATURES
[0018]
Non-Patent Literature 1: R.N. Younger et al.: Journal of The Iron and
Steel Institute, October (1960), p.188
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S
Non"Patent Literature 2: R.N. Younger et aL: British Welding Journal,
December (1961), p.579
Non"Patent Literature 3: 19awa et aL: Journal of the Japan Welding
Society, Vol. 47 (1978) No. 10, p.679
Non"Patent Literature 4: Naiki et al.: Ishikawajima Harima
Engineering Review, Vol. 15 (1975) No.2, p.209
DISCLOSURE OF THE INVENTION
PROBLEMS TO BE SOLVED BY THE INVENTION
[0019]
The present invention has been made in view of the above"described
situation, and accordingly objectives thereof are to provide a welding material
for austenitic heat resistant steels having excellent resistance to hot cracking
at the welding time and to provide a weld metal obtained by using the said
welding material, which has resistance to hot cracking during welding,
resistance to stress relaxation cracking during a long period of use at high
temperatures, and an excellent creep strength. Furthermore, another
objective of the present invention is to provide a welded joint consisting of the
weld metal obtained by using the said welding material and a base metal
having an excellent creep strength at high temperatures.
MEANASFORSOL~NGTHEPROBLEMS
[0020]
In order to achieve the above objectives, the present inventors first
carried out detailed investigation of stress relaxation cracking which occur in a
weld metal during a long period of use at high temperatures. As a result, the
following items (a) to (c) were clarified.
[0021]
(a) The stress relaxation cracking occurs at the boundaries of columnar
crystals of the weld metal.
•
[0022]
(b) The cracking fracture surface is poor in ductility, and on the fracture
surface, the concentration of P and S is confirmed, especially, the
concentration of S being remarkable.
[0023]
(c) In the microstructure in the vicinity of a cracking portion, fine carbo"
nitrides and intermetallic compounds precipitate in large amounts within the
grains.
[0024]
From the above"clarified items (a) to (c), the present inventors reached
the following concfusions (d) to (f).
[0025]
(d) The stress relaxation cracking is the cracking that is opened by the
application of welding residual stresses and external stresses at the grain
boundary that is weakened by the segregation of P and S, especially of S,
during solidification at the welding time and during the subsequent heating at
high temperatures.
[0026]
(e) In the case where large amounts of carbo-nitrides and intermetallic
compounds precipitate finely within the grains, since the deformability within
the grains decreases, stress concentration on the grain boundary surface
occurs. Therefore, cracking is liable to occur on account of the superimposing
action of the stress concentration and weakened grain boundary.
(D The above-described mechanism is suggested concerning similar
cracking in HAZ in the Non-Patent Literature 3. The Non-Patent Literature
3 states that, to reduce S that weakens the grain boundary or to fix S, the
containing of Ca and Mg is effective in preventing the cracking. However, the
weld metal is generally used in a state of an as·solidified microstructure, and
it is presumed that the phenomenon is different from that of HAZ that is based
on the base metal being thermally refined by a heat treatment or the like.
Therefore, the possibility that the measures against cracking in HAZ proposed
in the Non"Patent Literature 3 can be also taken against stress relaxation
cracking is little. Specifically, Ca and Mg that are proposed in the Non·
Patent Literature 3 have very strong affinity for oxygen, and are liable to form
oxides during welding. Therefore, the amounts of Ca and Mg acting
effectively on the fixation of S in the weld metal are affected by the welding
conditions. For this reason, it is difficult to stably achieve the effect of fixing
S by using Ca and Mg. Furthermore, an extreme reduction in impurity
elements leads to a marked increase in steel making cost, so that it is difficult
to apply this mechanism to mass·produced industrial products.
[0027]
Accordingly, the present inventors carried out further detailed studies to
prevent stress relaxation cracking. As a result, it was revealed that the
susceptibility to stress relaxation cracking can be decreased by the following
measures of items (g) and (h).
[0028]
(g) The contents of Sand P in the weld metal, which segregate at grain
boundaries and weaken the grain boundaries, are regulated so as to be within
specific ranges.
[0029]
(h) The contents of elements that precipitate as fine carbides or
intermetallic compounds and cause an increase in resistance to deformation
within the grains, specifically Ti and Nb, are regulated so as to be within
specific ranges.
[0030]
However, although the measures of the items (g) and (h) were taken,
stress relaxation cracking could not be prevented completely. In addition, it
was revealed that since the precipitation strengthening effect cannot be
achieved sufficiently, the desired high creep strength cannot be attained.
[0031]
Accordingly, the present inventors advanced studies, and resultantly
found that by containing high'concentration W, both of prevention of stress
relaxation cracking and the securement of desired high creep strength can be
attained. The reasons for this are thought to be as described in the following
items (i) and G).
[0032]
(i) The increase of W content decreases the grain boundary segregation
energy of S during a long period of use, reduces the concentration of S into the
grain boundaries, and indirectly restrains the grain boundaries from
weakening.
[0033]
G) W contributes to the improvement in creep strength as a solid,
solution strengthening element. The decrease in deformability within the
grains in this case is less than that in the case where fine carbo'nitrides and
intermetallic compounds precipitate.
[0034]
However, it was clarified that in the case where high'concentration W is
contained, although the stress relaxation cracking that occurs in the weld
metal during a long period of use at high temperatures can be prevented, the
susceptibility to solidification cracking during welding increases inversely.
[0035]
Accordingly, the present inventors further carried out studies to prevent
solidification cracking during welding. As a result, a finding of the following
item (k) was obtained.
[0036]
(k) By controlling the contents of Cr and C so as to be within specific
ranges, specifically by making, by mass percent, the C content exceeding 0.05%
and not more than 0.18% in the case where 20 to 25% of Cr is contained,
solidification cracking during welding can be prevented.
[0037]
From the microstructure observation result of weld metal, the reason for
this is thought to be the following item (I).
[0038]
(I) In the case where the contents of C and Cr are controlled so as to be
within specific ranges, C mainly combines with Cr in the solidification process
of weld metal, and eutectic solidification of (Cr,MhaC6 and austenite occurs.
As a result, the disappearance of liquid phase at the solidification time
quickens, so that the solidification cracking during welding can be prevented.
[0039]
In addition, it could be confirmed that the control for making the
contents of C and Cr within proper ranges is also effective in preventing
ductility-dip cracking during welding.
[0040]
From the above-described findings, the present inventors obtained a
finding that by using a welding material for austenitic heat resistant steels in
which, by mass percent, C: exceeding 0.05% and not more than 0.18% and W:
exceeding 8.0% and not more than 13.0% are contained with an alloy
containing Cr: 20 to 25% and Ni: exceeding 40% and not more than 50% being
a base, the resistance to hot cracking during welding, the resistance to stress
%
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•
relaxation cracking during a long period of use at high temperatures, and the
desired high creep strength can be ensured.
[0041]
By using this welding material for austenitic heat resistant steels, there
can be obtained a weld metal having resistance to hot cracking during welding,
resistance to stress relaxation cracking during a long period of use at high
temperatures, and an excellent creep strength and a welded joint consisting of
a base metal of austenitic heat resistant steel excellent in high temperature
strength.
[0042]
When a welded joint is obtained by using this welding material, the use
of an austenitic heat resistant steel excellent in high temperature strength,
which contains, by mass percent, Ni: 40 to 50%, Cr: 20 to 25%, and W: 6.0 to
10.0%, as a base metal is preferable because, for the base metal as well, an
excellent creep strength can be ensured. The austenitic heat resistant steel
excellent in high temperature strength, which is used as the base metal, may
be an austenitic heat resistant steel having the same or different chemical
composition as or from that of the welding material in accordance with the
present invention.
[0043]
As the base metal, there is preferably used an austenitic heat resistant
steel excellent in high temperature strength, comprising, by mass percent, C:
0.04 to 0.12%, Si: 0.5% or less, Mn: 1.5% or less, p: 0.03% or less, S: 0.01% or
less, Ni: 40 to 50%, Cr: 20 to 25%, W: 6.0 to 10.0%, Mo: 0.2% or less, Nb: not
less than 0.05% and less than 0.60%, Ti: 0.02 to 0.20%, N: 0.02% or less, B:
0.005% or less, and AI: 0.04% or less, with the balance being Fe and impurities.
[0044]
·~
\ The term "impurities" so referred to in the phrase "the balance being Fe
and impurities" indicates those impurities which are mixed on account of
various factors in the production process including raw materials such as ore
or scrap when the welding material or the heat resistant steel is produced on
an industrial scale.
[0045]
The present invention has been accomplished on the basis of the abovedescribed
findings. The main points of the present invention are the welding
materials of the following items (1) and (2), the weld metal of the following
item (3), and the welded joints of the following items (4) to (6).
[0046]
(1) A welding material for austenitic heat resistant steels, having a
chemical composition comprising, by mass percent, C: exceeding 0.05% and not
more than 0.18%, Si: 0.5% or less, Mn: 1.5% or less, Ni: 40 to 50%, Cr: 20 to
25%, W: exceeding 8.0% and not more than 13.0%, Ti: 0.01 to 0.2%, N:
exceeding 0.03% and not more than 0.20%, and AI: 0.01.% or less, with the
balance being Fe and impurities, in which the contents of 0, P and S as the
impurities are, 0: 0.02% or less, p: 0.008% or less, and S: 0.005% or less.
[0047]
(2) The welding material for austenitic heat resistant steels according to
the above (1), which contains, by mass %, Nb: less than 0.60% in lieu of a part
of Fe.
[0048]
(3) A weld metal obtained by using the welding material for austenitic
heat resistant steels according to the above (1) or (2).
[0049]
•
(4) A welded joint consisting of the weld metal according to the above (3)
and a base metal of an austenitic heat resistant steel excellent in high
temperature strength.
[0050]
(5) The welded joint according to the above (4), wherein the base metal of
an austenitic heat resistant steel excellent in high temperature strength
contains, by mass percent, W: 6.0 to 10.0%, Ni: 40 to 50%, and Cr: 20 to 25%.
[0051]
(6) The welded joint according to the above (4), wherein the base metal of
an austenitic heat resistant steel excellent in high temperature strength
comprises, by mass percent, C: 0.04 to 0.12%, Si: 0.5% or less, Mn: 1.5% or less,
p: 0.03% or less, S: 0.01% or less, Ni: 40 to 50%, Cr: 20 to 25%, W: 6.0 to 10.0%,
Mo: 0.2% or less, Nb: not less than 0.05% and less than 0.60%, Ti: 0.02 to
0.20%, N: 0.02% or less, B: 0.005% or less, and AI: 0.04% or less, with the
balance being Fe and impurities.
ADVANTAGEOUS EFFECTS OF THE INVENTION
[0052]
According to the present invention, a welding material for austenitic
heat resistant steels having excellent resistance to hot cracking at the welding
time can be provided, and also a weld metal having resistance to hot cracking
during welding, resistance to stress relaxation cracking during a long period of
use at high temperatures, and an excellent creep strength can be provided by
using the said welding material. Further, a welded joint consisting of the
weld metal having resistance to hot cracking during welding, resistance to
stress relaxation cracking during a long period of use at high temperatures,
and an excellent creep strength and a base metal having a creep strength
excellent at high temperatures can be provided by using the said welding
material.
MODE FOR CARRYING OUT THE INVENTION
[0053]
The reason why the chemical composition of the welding material for
austenitic heat resistant steels is restricted in the present invention is as
described below. In the following description, the symbol "%" for the content
of each element means "% by mass".
[0054]
C: exceeding 0.05% and not more than 0.18%
Carbon (C) is an austenite·forming element, and is also an element
effective in enhancing the stability of austenitic structure at the time of use at
high temperatures. Moreover, in the present invention, C is an important
element for preventing hot cracking at the welding time. That is to say, C
combines mainly with Cr in the solidification process to produce eutectic
carbides, to quicken the disappearance of liquid phase, and to make the
microstructure of final solidification portion a lamellar structure of (Cr,MhaCa
and austenite. As a result, the remaining mode of liquid phase changes from
a plane shape to dot shape, and also the stress concentration on a specific
surface is restrained, so that solidification cracking can be prevented.
Furthermore, since C increases the final solidification boundary area, which
serves as a segregation site of impurities, C also contributes to the prevention
of ductility'dip cracking during welding and the decrease in susceptibility to
stress relaxation cracking during the use at high temperatures. In order to
sufficiently achieve the above'described effects within the Cr content range of
the present invention, to be described later, it is necessary that C exceeding
0.05% be contained. However, in the case where C is contained excessively,
excessive C, which does not turn to carbides during solidification, precipitates
finely as carbides during the use at high temperatures, so that the
susceptibility to stress relaxation cracking is rather increased. Therefore, the
.~
\4
content of C is set in the range of exceeding 0.05% and not more than 0.18%.
The preferable lower limit of the C content is 0.06%, and the preferable upper
limit thereof is 0.15%.
[0055]
Si: 0.5% or less
Silicon (si) is contained as a deoxidizer; however, it segregates at
columnar grain boundaries at the time of solidification of weld metal, lowers
the melting point of liquid phase, and increases the susceptibility to
solidification cracking. Therefore, the content of Si must be set to 0.5% or less.
The content of Si is preferably 0.3% or less. However, in the case where the
content of Si is decreased excessively, the deoxidizing effect is not achieved
sufficiently. Consequently, the index of cleanliness of the steel increases, in
other words, the cleanliness of the steel decreases, and moreover the
production cost increases. Therefore, the lower limit of the Si content is not
particularly restricted; however, the preferable lower limit thereof is 0.01%.
If at least 0.01% of Si is contained, the deoxidizing effect can be achieved.
The lower limit of the Si content is more preferably 0.02%.
[0056]
Mn: 1.5% or less
Like Si, manganese (Mn) is contained as a deoxidizer. Mn lowers the
activity of N in the weld metal, and thereby restrains the scattering of N
coming from the arc atmosphere, so that Mn also contributes to the
securement of strength. However, an excessive content of Mn causes
embrittlement. Therefore, the content of Mn must be set to 1.5% or less.
The content of Mn is preferably 1.2% or less. The lower limit of the Mn
content is not particularly restricted; however, the preferable lower limit
thereof is 0.01%. If at least 0.01% of Mn is contained, the above-described
· Jk'5
effect can be achieved. The lower limit of the Mn content is more preferably
0.02%.
[0057]
Ni: 40 to 50%
Nickel (Ni) is an element effective in obtaining the austenitic structure,
and is also an essential element for ensuring the structural stability during a
long period of use and thus attaining the sufficient creep strength. In order to
achieve the above-described effects, it is necessary that 40% or more of Ni be
contained. However, since Ni is an expensive element, a high content of Ni
exceeding 50% leads to an increase in cost. Therefore, the content of Ni is set
to 40 to 50%. The preferable lower limit of the Ni content is 40.5%, and the
preferable upper limit thereof is 48.5%. The more preferable lower limit of
the Ni content is 41%, and the more preferable upper limit thereof is 46%.
[0058]
Cr: 20 to 25%
Chromium (Cr) is an essential element for ensuring oxidation resistance
and corrosion resistance at high temperatures. Cr combines with C in the
solidification process to produce eutectic carbides and to prevent solidification
cracking and ductility-dip cracking during welding, and also has an effect for
reducing the susceptibility to stress relaxation cracking during the use at high
temperatures. In order to achieve the above-described effects, it is necessary
that 20% or more of Cr be contained. However, if the content of Cr becomes
excessive and exceeds 25%, the structural stability at high temperatures
deteriorates, and the creep strength decreases. Therefore, the content of Cr is
set to 20 to 25%. The preferable lower limit of the Cr content is 20.5%, and
the preferable upper limit thereof is 24.5%. The more preferable lower limit
of the Cr content is 21%, and the more preferable upper limit thereof is 24%.
[0059]
ow6
W: exceeding 8.0% and not more than 13.0%
Tungsten (W) is an element that dissolves in matrix and contributes
enormously to the improvement in creep strength at high temperatures
exceeding 700°C. Moreover, W decreases the grain boundary segregation
energy of S, reduces the concentration of S at the grain boundaries during the
post weld heat treatment and during the use at high temperatures. Therefore,
W restrains the grain boundaries from weakening, and indirectly contributes
to the prevention of stress relaxation cracking. In order to ensure these
effects sufficiently and to attain both of the resistance to stress relaxation
cracking and the creep strength during the use at high temperatures, it is
necessary in terms of the relationship with other elements constituting the
present invention that W exceeding 8.0% be contained. However, even ifWis
contained excessively, the said effects are saturated; hence the toughness and
creep strength are rather deteriorated. Further, since W is an expensive
element, a high content of W exceeding 13.0% leads to an increase in cost.
Therefore, the content ofW is set in the range of exceeding 8.0% and not more
than 13.0%. The preferable lower limit of the W content is 8.2%, and the
preferable upper limit thereof is 12.8%. The more preferable lower limit of
the Wcontent is 8.5%, and the more preferable upper limit thereof is 12.5%.
[0060]
Ti: 0.01 to 0.2%
Titanium (Ti) precipitates within the grains as fine carbo"nitrides, and
contributes to the improvement in creep strength at high temperatures. In
order to achieve this effect, it is necessary that 0.01% or more of Ti be
contained. However, if the content of Ti becomes excessive and exceeds 0.2%,
Ti precipitates in large amounts, so that the deformation resistance within the
grains is enhanced remarkably, and the susceptibility to stress relaxation
cracking during the use at high temperatures is increased. Therefore, the
content of Ti is set to 0.01 to 0.2%. The preferable lower limit of the Ti
content is 0.03%, and the preferable upper limit thereof is 0.15%.
[0061]
N: exceeding 0.03% and not more than 0.20%
Nitrogen (N) is an austenite-forming element, and is also an element
effective in enhancing the stability of austenitic structure at the time of use at
high temperatures. Further, N is an element that dissolves in matrix, and
contributes to the improvement in tensile strength. In order to achieve the
above-described effects, it is necessary that N exceeding 0.03% be contained.
However, if the content of N becomes excessive and exceeds 0.20%, N
precipitates in large amounts as nitrides during a long period of use, so that
the deformation resistance within the grains is enhanced remarkably, the
susceptibility to stress relaxation cracking during the use at high
temperatures is increased, and moreover the production of blowholes is caused
at the welding time. Therefore, the content of N is set in the range of
exceeding 0.03% and not more than 0.20%. The vreferable lower limit of the
N content is 0.05%, and the preferable upper limit thereof is 0.18%. The more
preferable lower limit of the N content is 0.07%, and the more preferable upper
limit thereof is 0.17%.
[0062]
Al: 0.01% or less
Aluminum (AI) is contained as a deoxidizer. However, if Al is contained
in large amounts, it impairs the cleanliness remarkably, and deteriorates the
workability of welding material and the ductility of weld metal. Therefore,
the content of AI must be set to 0.01%or less. The AI content is preferably set
to 0.008% or less. The lower limit of the Al content may be at a level of
contents of impurities.
[0063]
One welding material for austenitic heat resistant steels of the present
invention has a chemical composition comprising elements ranging from C to
AI described above with the balance being Fe and impurities, in which the
contents of 0, P and S as the impurities being restricted to the ranges
described below.
[0064]
0: 0.02% or less
Oxygen (0) exists as an impurity. In the case where 0 is contained in
large amounts, it deteriorates the workability of welding material and the
ductility of weld metal. Therefore, the content of 0 must be set to 0.02% or
less. The content of 0 is preferably set to 0.015% or less.
[0065]
p: 0.008% or less
Phosphorous (P) is an element contained as an impurity. P lowers the
melting point of final solidification portion at the time of solidification of weld
metal, increases the susceptibility to solidification cracking remarkably, and
brings about grain boundary embrittlement during the use at high
temperatures, thereby decreasing the resistance to stress relaxation cracking.
Therefore, the content of P must be set to 0.008% or less. The content of P is
preferably set to 0.006% or less.
[0066]
S: 0.005% or less
Like P, sulfur (S) is an element contained as an impurity. S lowers the
melting point of final solidification portion at the time of solidification of weld
metal, and increases the susceptibility to solidification cracking. Further, S is
an element that precipitates and concentrates at the grain boundaries during
the use at high temperatures, and remarkably enhances the susceptibility to
stress relaxation cracking. Therefore, the content of S must be set to 0.005%
or less. The content of S is preferably set to 0.003% or less.
[0067]
Another welding material for austenitic heat resistant steels of the
present invention has a chemical composition containing less than 0.60% of Nb
in lieu of a part of Fe in the "balance being Fe and impurities".
[0068]
Hereunder, the effects of containing Nb and the reasons for the
restriction of content of Nb, which is an optional element, are explained.
[0069]
Nb: less than 0.60%
Like Ti, niobium (Nb) is an element that precipitates within the grains
as fine carbo-nitrides, and it contributes to the improvement in creep strength
at high temperatures. Therefore, according to need, Nb can be contained.
However, if the content of Nb becomes excessive and 0.60% or more, large
amounts of carbo-nitrides precipitate and fine intermetallic compounds
precipitate, so that the deformation resistance within the grains is enhanced
remarkably, and the susceptibility to stress relaxation cracking during the use
at high temperatures is increased. Therefore, if Nb is contained, the content
of Nb is set to less than 0.60%. When Nb is contained, the content of Nb is
preferably set to 0.50% or less.
[0070]
On the other hand, in the case where Nb is contained, in order to stably
achieve the above"described effect of Nb, it is preferable that the content of Nb
be set to 0.01% or more, further preferably 0.05% or more. When Nb IS
contained, the content of Nb is still more preferably set to 0.10% or more.
[0071]
The above is a detailed description of the chemical composition of the
welding materials for austenitic heat resistant steels in accordance with the
present invention. These welding materials have excellent resistance to hot
cracking at the welding time, respectively. By using these welding materials,
a weld metal having resistance to hot cracking during welding, resistance to
stress relaxation cracking during a long period of use at high temperatures,
and an excellent creep strength can be obtained. Further, by using these
welding materials, a welded joint consisting of the weld metal having
resistance to hot cracking during welding, resistance to stress relaxation
cracking during a long period of use at high temperatures, and an excellent
creep strength and a base metal having an excellent creep strength at high
temperatures can be obtained.
[0072]
When a welded joint is obtained by using the welding materials for
austenitic heat resistant steels in accordance with the present invention, the
use of an austenitic heat resistant steel excellent in high temperature strength,
which contains W: 6.0 to 10.0%, Ni: 40 to 50%, and Cr: 20 to 25%, as a base
metal is preferable because the base metal as well has excellent ductility and
creep strength in the high temperature zone of 700°C or higher. The
austenitic heat resistant steel excellent in high temperature strength, which is
used as the base metal, may be an austenitic heat resistant steel having the
same or different chemical composition as or from that of the welding
materials for austenitic heat resistant steels in accordance with the present
invention.
[0073]
Hereunder, the reason why, in the case where the austenitic heat
resistant steel excellent in high temperature strength is used as the base
'.~'
2..-metal, the base metal preferably contains W: 6.0 to 10.0%, Ni: 40 to 50%, and
Cr: 20 to 25% is explained in detail.
[0074]
W: 6.0 to 10.0%
Tungsten (W) is an element that dissolves in matrix and contributes
enormously to the improvement in creep strength at high temperatures
exceeding 700°C as in the weld metal. Unlike the weld metal that is used as'
solidified condition, the base metal is homogenized by heat treatment, so that
the effect ofW is achieved more easily. Therefore, the base metal preferably
contains W, and the content of W may be 6.0% or more. However, W is an
expensive element and the containing thereof leads to an increase in cost.
Therefore, ifW is contained, the content ofWis preferably set to 10.0% or less.
The more preferable lower limit of the W content in the base metal is 7.0%,
and the more preferable upper limit thereof is 9.8%. The still more preferable
lower limit of the W content in the base metal is 7.5%, and the still more
preferable upper limit thereof is 9.5%.
[0075]
Ni: 40 to 50%
Nickel (Ni) is an element effective in obtaining the austenitic structure,
and is also an effective element for ensuring the structural stability during a
long period of use and thus attaining the sufficient creep strength as in the
weld metal. In order to achieve these effects, the base metal preferably
contains Ni, and the content of Ni is preferably set to 40% or more as in the
weld metal. On the other hand, Ni is an expensive element and the
containing thereof leads to an increase in cost. Therefore, if Ni is contained,
the content of Ni is preferably set to 50% or less. The more preferable lower
limit of the Ni content in the base metal is 40.5%, and the more preferable
upper limit thereof is 48.5%. The still more preferable lower limit of the Ni
.~.
2-2
content in the base metal is 42%, and the still more preferable upper limit
thereof is 47%.
[0076]
Cr: 20 to 25%
Chromium (Cr) is an element effective in ensuring oxidation resistance
and corrosion resistance of the base metal at high temperatures as in the wElld
metal. In order to achieve the effects equivalent to those of the weld metal,
the base metal preferably contains Cr, and the content of Cr is preferably set
to 20% or more. However, if the content of Cr becomes excessive, the
structural stability at high temperatures deteriorates, and the creep strength
decreases. Therefore, if Cr is contained, the content of Cr is preferably set to
25% or less. The more preferable lower limit of the Cr content in the base
metal is 20.5%, and the more preferable upper limit thereof is 24.5%. The
still more preferable lower limit of the Cr content in the base metal is 21%,
and the still more preferable upper limit thereof is 24%.
[0077]
The base metal of austenitic heat resistant steel excellent in high
temperature strength preferably contains elements each having the content
described below in addition to W, Ni and Cr of the above·described content
range, with the balance being Fe and impurities.
[0078]
C: 0.04 to 0.12%
Carbon (C) is an austenite-forming element, and is also an element
effective in enhancing the stability of austenitic structure at the time of use at
high temperatures as in the weld metal. Unlike the weld metal that is used
as·solidified condition, the base metal is homogenized by heat treatment, so
that the effect of C is achieved more easily, and also the measures against weld
cracking need not be taken. Therefore, the base metal preferably contains C,
",22" .
23
and the content of C may be 0.04% or more. However, if the content of C
becomes excessive, coarse carbides are produced during the use at high
temperatures, and the creep strength rather decreases. Therefore, if C is
contained, the content of C is preferably set to 0.12% or less. The more
preferable lower limit of the C content in the base metal is 0.05%, and the
more preferable upper limit thereof is 0.10%.
[0079]
Si: 0.5% or less
Silicon (SO has a deoxidizing effect. However, an excessive content of Si
leads to deterioration of toughness. Therefore, if the base metal contains Si,
the content of Si is preferably set to 0.5% or less, and more preferably 0.4% or
less. However, in the case where the content of Si is decreased excessively,
the deoxidizing effect is not achieved sufficiently. Consequently, the index of
cleanliness of the steel increases, in other words, the cleanliness of the steel
decreases, and moreover the production cost increases. Therefore, the lower
limit of the Si content in the base metal is not particularly restricted; however,
the preferable lower limit thereof is 0.01%. If at least 0.01% of Si is contained,
the deoxidizing effect can be achieved. The lower limit of the Si content is
more preferably 0.02%.
[0080]
Mn: 1.5% or less
Like Si, manganese (Mn) has a deoxidizing effect. However, an
excessive content of Mn leads to embrittlement. Therefore, in the case where
the base metal contains Mn, the content ofMn is preferably set to 1.5% or less,
and more preferably 1.2% or less. The lower limit of the Mn content in the
base metal is not particularly restricted; however, the lower limit thereof is
preferably 0.01%. If at least 0.01% of Mn is contained, the deoxidizing effect
can be achieved. The lower limit of the Mn content is more preferably 0.02%.
[0081]
p: 0.03% or less
Phosphorous (p) is an element contained as an impurity. If the content
of P becomes excessive, the creep ductility decreases. Unlike the weld metal,
the base metal does not require the measures against weld cracking, and an
extreme decrease in the P content leads to a marked increase in steel making
cost. Therefore, the P content in the base metal is preferably set to 0.03% or
less, and more preferably 0.02% or less.
[0082]
S: 0.01% or less
Like P, sulfur (S) is contained as an impurity. If the content of S
becomes excessive, the creep ductility decreases. Unlike the weld metal, the
base metal does not require the measures against weld cracking, and an
extreme decrease in the S content leads to a marked increase in steel making
cost. Therefore, the S content in the base metal is preferably set to 0.01% or
less, and more preferably 0.008% or less.
[0083]
Mo: 0.2% or less
Like W, molybdenum (Mo) is an element that dissolves in matrix and
contributes to the improvement in creep strength at high temperatures
exceeding 700°C. Therefore, the base metal preferably contains Mo.
However, Mo is an expensive element, and is also an element that decreases
the phase stability. Therefore, if Mo is contained, the content of Mo is
preferably set to 0.2% or less. The preferable lower limit of the Mo content in
the base metal is 0.02%.
[0084]
Nb: not less than 0.05% and less than 0.60%
-.2<25
Niobium (Nb) is an element that precipitates within the grains as fine
carbo"nitrides, and contributes to the improvement in creep strength at high
temperatures. In the base metal that is less susceptible to stress relaxation
cracking during the use at high temperatures than the weld metal, Nb can be
contained positively to increase the strength. For this reason, the base metal
preferably contains Nb, and the content of Nb is preferably set to 0.05% or
more. However, if the content of Nb becomes excessive, large amounts of
carbo"nitrides are produced, thereby decreasing the toughness. Therefore, if
Nb is contained, the content of Nb is preferably set to less than 0.60%. The
more preferable lower limit of the Nb content in the base metal is 0.10%, and
the more preferable upper limit thereof is 0.50%.
[0085]
Ti: 0.02 to 0.20%
Titanium (Tn is an element that precipitates within the grains as fine
carbo'nitrides, and contributes to the improvement in creep strength at high
temperatures. In the base metal that is less susceptible to stress relaxation
cracking during the use at high temperatures than the weld metal, Ti can be
contained positively to increase the strength. For this reason, the base metal
preferably contains Ti, and the content of Ti is preferably set to 0.02% or more.
However, if the content of Ti becomes excessive, large amounts of carbo"
nitrides are produced, thereby decreasing the toughness. Therefore, if Ti is
contained, the content of Ti is preferably set to 0.20% or less. The more
preferable lower limit of the Ti content in the base metal is 0.05%, and the
more preferable upper limit thereof is 0.15%.
[0086]
N: 0.02% or less
Nitrogen (N) is an element effective in stabilizing the austenitic phase,
and is also an element that dissolves in matrix, and contributes to the
.. ~
improvement in tensile strength. On the other hand, N decreases the· hot
workability remarkably. Therefore, in the base metal, the upper limit of the
N content should be controlled more stringently than in the weld metal, and
the content is preferably set to 0.02% or less. The more preferable upper limit
of the N content in the base metal is 0.01%.
[0087]
B: 0.005% or less
Boron (B) is an element that segregates at grain boundaries during the
use at high temperatures to strengthen the grain boundaries, and is also
effective in improving the creep strength by finely dispersing grain boundary
carbides. For this reason, the base metal preferably contains B. However, if
the content of B becomes excessive, the susceptibility to liquation cracking in
the HAZ enhances. Therefore, if B is contained, the content of B is preferably
set to 0.005% or less. The preferable lower limit of the B content in the base
metal is 0.0002%.
[0088]
Al: 0.04% or less
Aluminum (AI) has a deoxidizing effect. However, if the content of AI
becomes excessive, Al impairs the cleanliness remarkably, and deteriorates the
workability at the time of production of base metal. Despite this fact, in the
base metal, AI may not produce oxides during welding as in the weld metal
and further degrade the cleanliness. Therefore, if AI is contained, the content
of AI is preferably set to 0.04% or less. The more preferable upper limit of the
Al content in the base metal is 0.03%.
[0089]
The following examples illustrate the present invention more specifically.
These examples are, however, by no means limited to the scope of the present
invention.
EXAMPLES
[0090]
From an ingot obtained by melting and casting a material having the
chemical composition shown in Table 1 on an experimental basis, plates for
base metals to be welded with 12 mm in thickness, 50 mm in width and 100
mm in length were manufactured by hot forging, hot rolling, heat treatment
and machining.
[0091]
Furthermore, from ingots obtained by melting and casting materials of
marks A to F having the chemical compositions shown in Table 2 on an
experimental basis, welding materials (welding wires) having an outside
diameter of 1.2 mm and a length of 1000 mm were manufactured by hot
forging, hot rolling and machining.
[0092]
[Table 1]
Table 1
Chemical composition (% by mass) Balance: Fe and impurities
C Si Mn P S Ni Cr W Ti Nb AI B N
0.08 0.18 1.04 0.010 0.001 45.2 22.8 8.7 0.09 0.20 0.03 0.001 0.007
[0093]
[Table 2]
00 C'l 00 I:"- c.o C'l
l:'- c.o c.o u:l ~ u:l Z .... .... .... .... .... ....
0 0 0 0 0 0
.... CQ C'l u:l 0) CQ
0 00 00 00 00 00 00 0s:i 0 0 0 0 0 0 ~~
C1J
Q)
Q) .... ~ .... .... .... '.0 0 0 0 0 0 0CQ .S
'§ :;a 0 0 0 ~ 0 ~
0 0 0 0 0 0 ~
-8 v v Q)
.~ CIl
Q)
"d 0) ~ CQ -.:j4 ~
Z ~ C'l 00 u:l
~ 0 0 I 0 I 0 Q)
il -:5 Q)
li:.l 0 1; .... 0 .... 0) 0
~ .... .... .... .... 0 ....
'aJ 0 0 0 0 0 0
<.J "0
s:l Q)
~ c.o 0) c.o CQ ~ -.:j4 ~ ~ ~
C'l .... .... c.o C':! ....
~ ai ai ........ ~ ........ r:..: Q)
'Ul il il I-t
C1J .... l:'- .... ~ ~ l:'- C1J
~
I-t 0 t-: ~ ~ ~ .... ~ a 0 e-i .... .... .... .... e-i .0..
~ C'l C'l C'l C'l C'l C'1 .. ~
c.o 00 0) c.o u:l 00 ~
~ ... CQ C':! ~ ~ ~ .-4 0(J
'-' z c.) CQ CQ u:l CQ cO
s:l ~ -.:j4 ~ -.:j4 ~ -.:t' Q)
...0..... .... .... .... .... .... ~ .-4 ...... Q) C1J 0 0 0 0 0 0 "0
~ 00 0 0 0 0 ~ 0 ... 0 0 0 0 0 CIl a 0 1j
0
C'l C'l .... C'l C'l
0
<.J .-4
0 0 0 0 0 0 ~ Pot 0 0 0 0 C! 0 ~
...... 0 0 0 0 0 0
a -.:j4 .... Q)
~
u:l C'l CQ 0 C'1
,.Q 00 00 00 00 ~ 00 CIl
0 0 0 0 0 0 Q)
0 ..til
(J
c.o c.o t-- 0) 0 00 ... ... "0 0 0 0 0 ~ 0 en 0 0 0 0 0 0 .9
il
0 0 0) 0 C'1 0 0 ~ .... .... 0 .... ~ .-4
0 0 0 0 0 0
il a
...!4 Q)
~ < C:Q 0 Cl ~ li:.l tl ~
[0094]
The above plates for base metals to be welded were machined for
providing each of them with a shape of V"groove with an angle of 30° and a
root thickness of 1 mm in the longitudinal direction. Then each of them was
subjected to four side"restrained welding onto a commercial steel plate of
SM400B, specified in JIS G 3160(2008), with 25 mm in thickness, 200 mm in
·.';»..
width and 200 mm in length, using "DNiCrFe·3" specified in JIS Z 3224 (1999)
as a covered electrode.
[0095]
Thereafter, each plate for base metals to be welded was subjected to
multi·layer welding in the groove using the said welding materials of marks A
to F, by the TIG welding under a heat input condition of 9 to 15 kJ/cm; and
there~y two welded joints were manufactured for each of the marks of welding
materials.
[0096]
For each of the marks, the test described below was conducted in the
state in which one welded joint was as·welded condition, and the remaining
welded joint had been subjected to an aging heat treatment of 700°C x 500
hours.
[0097]
For each of specimens cut off from five locations of each of the welded
joints of each marks, that is to say, the as·welded welded joint and the aging
heat·treated welded joint of each of the marks, the transverse cross section
thereof was mirror-like polished and etched, and thereafter the presence of
cracking in the weld metal was examined by using an optical microscope. A
welded joint in which no cracking was recognized in all of the five specimens
was regarded as "acceptable".
[0098]
Furthermore, from the as·welded welded joint in which no cracking had
been recognized in the weld metal as the result of microscopic observation,
round bar creep rupture test specimens were cut off so that the weld metal is
positioned in the center of the parallel part. Thereafter the said specimens
were subjected to a creep rupture test under the conditions of 700°C and 147
MPa corresponding to the desired base metal plate rupture time of about 1000
.~
30
hours. A welded joint in which the base metal had ruptured was regarded as
"acceptable".
[0099]
The results of the above tests are shown in Table 3.
[0100]
The symbol "0" in the "Cracking observation result" column in Table 3
indicates an "acceptable" welded joint in which no cracking had been
recognized in all of the five specimens. On the other hand, the symbol "x"
indicates that cracking had been recognized in at least one of the five
specimens.
[0101]
In addition, the symbol "0" in the "Creep rupture test result" column
indicates an "acceptable" welded joint in which the base metal had ruptured.
On the other hand, the symbol "x" indicates that the base metal had not
ruptured. The symbol ,,_It in welding material mark E denotes that cracking
had been recognized in the weld metal of the specimen cut off from the as"
welded welded joint, and therefore the creep rupture test was not carried out.
[0102]
[Table 3]
•
•
Table 3
Mark of Cl'ackin~ observation result Creep rupture
welding as·welded aging heat"treated test result
matel'ial welded joint welded joint
A 0 0 0
B 0 0 0
C 0 0 0
* D 0 x 0
* E x x -
* F 0 x x
The mark * indicates falling outside the conditions regulated by
the present invention.
[0103]
From Table 3, it is apparent that in the weld metals of the welded joints
that were welded by using the welding materials of marks A to C, in which the
chemical composition was within the range regulated by the present invention,
neither stress relaxation cracking during aging heat treatment nor hot
cracking during welding occurred, and a high creep strength was attained.
[0104]
In contrast, in the weld metals of the welded joints that were welded by
using the welding materials of marks D to F, in which the chemical
composition was out of the range regulated by the present invention, the
occurrence of at least either ones of stress relaxation cracking during aging
heat treatment and hot cracking during welding was recognized.
[0105]
That is to say, in the welded joint that was welded by using the welding
material of mark D, in which the contents of Wand Nb were out of the range
regulated by the present invention, carbo"nitrides containing Nb and
intermetallic compounds were produced excessively within the grains, the
..~"
1'2
•
deformation resistance within the grains was high, and an effect of reducing
the grain boundary concentration of S could not be achieved; and thus stress
relaxation cracking during aging heat treatment occurred.
[0106]
In the welded joint that was welded by using the welding material of
mark E, in which the C content was as low as 0.02% and lower than the
content necessary for producing sufficient eutectic carbides in the solidification
process, hot cracking during welding occurred.
[0107]
In the welded joint that was welded by using the welding material of
mark F, in which the W content was out of the range regulated by the present
invention, stress relaxation cracking during aging heat treatment occurred,
and a satisfactory creep strength was not attained.
[0108]
As described above, it can be seen that in the case where a welding
material having a chemical composition that is within the range regulated by
the present invention is used, there can be obtained a weld metal having
resistance to hot cracking during welding, resistance to stress relaxation
cracking during a long period of use at high temperatures, and an excellent
creep strength.
INDUSTRIAL APPLICABILITY
[0109]
According to the present invention, a welding material for austenitic
heat resistant steels having excellent resistance to hot cracking at the welding
time can be provided, and also a weld metal having resistance to hot cracking
during welding, resistance to stress relaxation cracking during a long period of
use at high temperatures, and an excellent creep strength can be provided by
using the said welding material. Further, a welded joint consisting of the
·~.
33
•
weld metal having resistance to hot cracking during welding, resistance to
stress relaxation cracking during a long period of use at high temperatures,
and an excellent creep strength and a base metal having a creep strength
excellent at high temperatures can be provided by using the said welding
material.
,

•
•
..~ We claim:
1. A welding material for austenitic heat resistant steels, having a
chemical composition comprising, by mass percent, C: exceeding 0.05% and not
more than 0.18%, Si: 0.5% or less, Mn: 1.5% or less, Ni: 40 to 50%, Cr: 20 to
25%, W: exceeding 8.0% and not more than 13.0%, Ti: 0.01 to 0.2%, N:
exceeding 0.03% and not more than 0.20%, and AI: 0.01% or less, with the
balance being Fe and impurities, in which the contents of 0, P and S as the
impurities are, 0: 0.02% or less, p: 0.008% or less, and S: 0.005% or less.
2. The welding material for austenitic heat resistant steels according to
claim 1, which contains, by mass %, Nb: less than 0.60% in lieu of a part of Fe.
3. A weld metal obtained by using the welding material for austenitic
heat resistant steels according to claim 1 or 2.
4. A welded joint consisting of the weld metal according to claim 3 and
a base 'metal of an austenitic heat resistant steel excellent in high temperature
strength.
5. The welded joint according to claim 4, wherein the base metal of an
austenitic heat resistant steel excellent in high temperature strength contains,
by mass percent, W: 6.0 to 10.0%, Ni: 40 to 50%, and Cr: 20 to 25%.
6. The welded joint according to claim 4, wherein the base metal of an
austenitic heat resistant steel excellent in high temperature strength
comprises, by mass percent, C: 0.04 to 0.12%, Si: 0.5% or less, Mn: 1.5% or less,
p: 0.03% or less, S: 0.01% or less, Ni: 40 to 50%, Cr: 20 to 25%, W: 6.0 to 10.0%,
Mo: 0.2% or less, Nb: not less than 0.05% and less than 0.60%, Ti: 0.02 to
0.20%, N: 0.02% or less, B: 0.005% or less, and AI: 0.04% or less, with the
balance being Fe and impurities.