DESCRIPTION
WELDING MATERIAL FOR Ni-BASED HEAT-RESISTANT ALLOY, AND WELD
METAL AND WELDED JOINT OBTAINED BY USING THE SAME MATERIAL
TECHNICAL FIELD
[OOO 11
The present invention relates to a welding material for Ni-based heat-resistant alloy,
and a weld metal and a welded joint obtained by using the welding material. More
particularly, the present invention relates to a welding material suitable for welding a Nibased
heat-resistant alloy employed for equipment used at high temperatures, such as
power generation boilers, and a weld metal and a welded joint obtained by using the
welding material.
BACKGROUND ART
[0002]
In recent years, from the viewpoint of reduction of environmental burdens, 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, a Ni-based heat-resistant
alloy defined by UNS NO6617 has been available. Also, Patent Documents 1 to 5 disclose
various Ni-based alloys. All of these alloys have a wide range of alloying elements
specified to meet the performance necessary as a base metal.
[0004]
When the Ni-based heat-resistant alloys are used for a structure, welding is generally
employed to assemble the structure.
[OOOS]
If the Ni-based alloy base metal is used as a welding material as it is when the
structure is assembled by welding, the susceptibility to high-temperature cracks during
welding at the weld metal can become high. Therefore, in some cases, the Ni-based alloy
for a base metal having a wide composition range is not diverted ai a welding material as it
is, but an alloy the component range of which has been controlled more strictly must be
used to prevent the hot cracking. The "high-temperature cracks during welding" include
"solidification cracks" and "ductility-dip cracks".
[0006]
On the other hand, as a welding material for Ni-based heat-resistant alloy used when
the structure is assembled by welding, AWS A5.14-2005 ER NiCrCoMo-1 has been known.
[0007]
Further, Patent Documents 6 to 8 propose various welding materials for Ni-based
alloy.
[0008]
Patent Document 6 proposes a welding material for oxide dispersion strengthening
alloy that is used for welding an oxide dispersion strengthening alloy having a high strength
to a heat-resistant alloy, and aimed at improving the strength by positively containing solidsolution
strengthening elements such as Mo and Nb.
[0009]
Patent Document 7 proposes welding materials for Ni-based alloy that are aimed at
increasing the strength by making the most of the solid-solution strengthening effect due to
Mo and W and the precipitation strengthening effect due to A1 and Ti.
[OO lo]
The Patent Document 8 proposes a welding material for Ni-based alloys that is
aimed at increasing both of the resistance to solidification cracking during welding and the
creep strength by containing Nb and W.
[OOl 11
A structure assembled by welding using the Ni-based heat resistant alloys and
welding materials for Ni-based heat resistant alloys is used at high temperatures, and
therefore has a concern for occurrence of cracking in weld zones when the structure is used
at high temperatures for a long period.
[OO 121
For example, Non Patent Document 1 points out that the intergranular cracks are
generated in a weld heat affected zone (hereinafter, referred to as a "HAZ") of a ~i-b&d
heat-resistant alloy during postweld heat treatment, and suggests that in addition to a
precipitation of y' phase, a grain boundary segregation of S affects the occurrence of the
intergranular cracks.
[00 131
Also, in Non Patent Document 2, a study has been carried out on the preventive
measures against intergranular cracks in the HAZ of the weld zone of an 18Cr-8Ni-Nb
based austenitic heat-resistant steel at the time of long-term heating. The measures are
proposed from the viewpoint of welding process such that the decrease in residual stresses
caused by a proper application of stress-relief heat treatment is effective in preventing
intergranular cracks in the HAZ.
[OO 141
Thus, from long ago, there has been known a phenomenon that cracks are generated
in the HAZ when a Ni-based heat-resistant alloy is used for a long time. In recent years,
however, as many kinds of alloying elements are contained to increase the strength of
material, there has been a tendency for crack generation at the time of long-term heating to
become remarkable even in weld metal.
[00 1 51
However, the mechanism of cracks generated in the weld zone during long-term use
has not yet been elucidated, and further, the countermeasures against cracks, especially, the
countermeasures against cracks from the viewpoint of the material of weld metal have not
been established.
[00 161
Therefore, the weld metal obtained by using AWS A5.14-2005 ER NiCrCoMo-1
known as the welding material for Ni-based heat-resistant alloy has an unsolved problem
of the generation of cracks during long-term use (hereinafter, referred to as "stress
relaxation cracks").
[00 1 71
Also, in Patent Documents 6 and 7 as well, the stress relaxation cracks are not
considered at all. Therefore, the weld metals obtained by using the welding materials
proposed in Patent Documents 6 and 7 also have an unsolved problem concerning the
stress relaxation cracks. Moreover, in the case where the welding materials proposed in
the Patent Documents 6 and 7 contain Ti and/or A1 in large amounts, there also arises a
problem of deteriorated weldability in fabrication.
[00 1 81
On the other hand, in the welding material proposed in the Patent Document 8 by
the present inventors, there is room for improvement in terms of securing higher creep
strength because this welding material contains little Ti.
LIST OF PRIOR ART DOCUMENTS
PATENT DOCUMENT
[00 191
Patent Document 1 : JP63-050440A
Patent Document 2: JP9-157779A
Patent Document 3 : JP2004-3000A
Patent Document 4: WO 200911 54161
Patent Document 5: JP2010-150593A
Patent Document 6: JP 10- 193 1 74A
Patent Document 7: WO 20 1010 13565
Patent Document 8: JP2008-20724244
NON PATENT DOCUMENT
[0020]
Non Patent Document 1: Igawa et al.: Journal of the Japan Welding Society, Vol.
47 (1978) No. 10, p.679
Non Patent Document 2: Uchiki et al.: MI Engineering Review, Vol. 15 (1975) No.
2, p.209
DISCLOSURE OF THE INVENTION
PROBLEMS TO BE SOLVED BY THE INVENTION
[002 11
The present invention has been made in view of the above-described situation, and
accordingly objectives thereof are to provide a welding material for Ni-based heat-resistant
alloy having resistance to high-temperature cracks excellent during welding and to provide
a weld metal obtained by using the welding material, which has resistance to hightemperature
cracks during welding, resistance to stress relaxation cracks during long-term
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 welding material and a base metal of Ni-based heat-resistant alloy excellent in
high-temperature strength.
MEANS FOR SOLVING THE PROBLEMS
[0022]
To achieve the above objectives, the present inventors first carried out detailed
investigation of stress relaxation cracks generated in a weld metal during long-term use at
high temperatures. As a result, the following items (a) to (c) were clarified.
[0023]
(a) The stress relaxation cracks are generated at the boundaries of columnar crystals
of the weld metal.
[0024]
(b) The crack fiacture surface is poor in ductility, and P and S are enriched on the
fiacture surface.
[0025]
(c) In the micro-structure in the vicinity of a crack portion, fine intermetallic
compounds precipitate in large amounts within the crystal grains.
[0026]
From the above-clarified items (a) to (c), the present inventors reached the
following conclusions (d) to (f).
[0027]
(d) The stress relaxation crack is a crack that is opened by the application of
residual stresses and external stresses at the grain boundary that is weakened by the
segregation of P and S during solidification during welding and the subsequent heating at
high temperatures.
[0028]
(e) In the case where large amounts of intermetallic compounds orland carbonitrides
precipitate finely within the grains, since the deformability within the grains
decreases, stress concentration on the grain boundary surface occurs. Therefore, cracks
are liable to be generated by the superimposing action of the stress concentration and
weakened grain boundary.
[0029]
(f) The above-described mechanism is suggested concerning similar cracks in HAZ
in Non Patent Document 1. Non Patent Document 1 states that, in order to reduce S that
weakens the grain boundary or to immobilize S, the containing of Ca and Mg is effective
in preventing the cracks. However, the weld metal is generally used in a state of an assolidified
micro-structure, and it is presumed that the phenomenon is different from that of
HAZ that is based on the base metal thermally refined by heat treatment or the like.
Therefore, it is unlikely that the countermeasures against cracks in HAZ proposed in Non
Patent Document 1 can be also taken against stress relaxation cracks. Specifically, Ca
and Mg that are proposed in the Non Patent Document 1 have a very strong affinity for
oxygen, and therefore are liable to form oxides during welding. Therefore, the yields of
Ca and Mg in weld metal after welding are affected by the welding conditions. For this
reason, it is difficult to stably achieve the effect of Ca and Mg. Furthermore, an extreme
reduction of impurity elements, especially a reduction of P content, leads to a significant
increase in production cost, so that it is difficult to apply this mechanism to mass-produced
industrial products.
[003 01
Accordingly, the present inventors carried out further detailed studies to prevent
stress relaxation cracks. As a result, it was revealed that the susceptibility to stress
relaxation cracks can be decreased by the following countermeasures of items (g) and (h).
[003 11
(g) The contents of S and P in the weld metal, which segregate at grain boundaries
and weaken the grain boundaries, are restricted so as to be within specific ranges.
[0032]
(h) The contents of elements that precipitate finely as intermetallic compounds
orland carbo-nitrides and cause an increase in resistance to deformation within the grains,
specifically Ti and Al, are restricted so as to be within specific ranges.
[0033]
However, although the countermeasures of the items (g) and (h) were taken, stress
relaxation cracks could not be prevented completely. In addition, it was revealed that
since the precipitation strengthening effect cannot be achieved sufficiently, a desired high
creep strength cannot be attained.
[0034]
Accordingly, the present inventors advanced studies, and resultantly found that by
containing high-concentration W and Cr, specifically, by containing, by mass percent, 7.0
to 13.0% of W and 25 to 35% of Cr, 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) to (k).
[0035]
(i) The increase in contents of W and Cr decreases the grain boundary segregation
energy of S and P, respectively, during a long period of use at high temperatures, reduces
the concentration of S and P, respectively, on the grain boundary, and restrains the grain
boundary fi-om weakening indirectly.
[003 61
Cj) 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 intermetallic compounds and carbo-nitrides precipitate.
[0037]
(k) Cr precipitates as an a-Cr phase, and contributes to the improvement in creep
strength. However, when this a-Cr phase precipitates, the decrease in deformability
within the grains is small as compared with the time when fine intermetallic compounds
and carbo-nitrides precipitate.
[003 81
However, it was clarified that in the case where the above-described highconcentration
W and Cr are contained, although the stress relaxation cracks that are
generated in the weld metal during long-term use at high temperatures can be prevented,
the susceptibility to solidification cracks during welding increases inversely.
[0039]
Accordingly, the present inventors further carried out studies to prevent
solidification cracks during welding. As a result, a finding of the following item (1) was
obtained.
[0040]
(1) By controlling the content of C so as to be within specific ranges, specifically by
making, by mass percent, the C content 0.06 to 0.18%, solidification cracks during welding
can be prevented.
[004 11
From the micro-structure observation result of weld metal, the reason for this is
thought to be the following item (m).
[0042]
(m) In the case where, by mass percent, 20 to 35% of Cr is contained and the
content of C are controlled so as to be within the above-described specific ranges, C
mainly combines with Cr in the solidification process of weld metal, and eutectic
solidification of (Cr, W23C6 and austenite occurs. As a result, the disappearance of liquid
phase during solidifying quickens, so that the solidification cracks during welding can be
prevented.
[0043]
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 cracks during welding.
[0044]
From the micro-structure observation results of weld metal, the reason for the above
confirmation is thought to be as described in the following item (n).
[0045]
(n) The carbide (Cr, M)23C6 formed by combining C with Cr in the solidification
process of weld metal exists at the columnar crystal boundary in a lamellar form. As a
result, the final solidification surface, which is the segregation site of impurity element, is
increased, and the above-described carbide (that is, (Cr, M)23C6) restrains the slippage of
grain boundary. Therefore, the ductility-dip crack during welding can be prevented.
[0046]
Also, the present inventors studied the weldability in fabrication. As a result, the
present inventors came to consider as described in the following item (0).
[0047]
(0) The deterioration of welding workability occurs on account of the fact that the
oxide scale with a high fusing point that contains much Al formed on the weld bead
surface of the preceding layer cannot be melted sufficiently by the following pass.
[0048]
Accordingly, the present inventors also studied in order to improve the welding
workability. As a result, the present inventors obtained the findings described in the
following item @).
[0049]
(p) By containing, by mass percent, more than 0.2% and not more than 1.5% of Ti
and less than 0.1% of Al in the welding material, the oxide scale formed on the weld bead
surface can be changed from oxide scale containing much A1 to oxide scale mainly
containing Ti, so that the welding workability is improved. Moreover, the fine
precipitation strengthening effect brought about by the intermetallic compounds of Ti and
A1 and the Ti carbo-nitrides can also be secured in the range in which the stress relaxation
cracking is not affected adversely.
[OOSO]
From the above-described facts, the present inventors obtained a finding that, for
the welding material for a Ni-based heat resistant alloy, excellent welding workability and
resistance to hot cracking during welding, and resistance to stress relaxation cracking and
desired high creep strength during a long period of use at high temperatures can be secured
by using an alloy containing, by mass percent, Cr: 25 to 35% and Ni: 45 to 55% as a base,
and by containing, by mass percent, C: 0.06 to 0.18%, W: 7.0 to 13.0%, Ti: more than
0.2% and not more than 1.5%, and Al: less than 0.1%.
[005 11
By using this welding material for Ni-based heat-resistant alloy, there can be
obtained a weld metal having resistance to high-temperature cracks during welding,
resistance to stress relaxation cracks during long-term use at high temperatures, and an
excellent creep strength and a welded joint consisting of a base metal of Ni-based heatresistant
alloy excellent in high-temperature strength.
[0052]
When a welded joint is obtained by using this welding material, the use of a Ni-based
heat-resistant alloy excellent in high-temperature strength, which contains, by mass percent,
Ni: 45 to 55%, Cr: 25 to 35%, and W: 3.5 to 8.5%, as a base metal is preferable because, for
the base metal as well, an excellent creep strength can be ensured. The Ni-based heatresistant
alloy excellent in high-temperature strength, which is used as the base metal, may
be a Ni-based heat-resistant alloy having the same or different chemical composition as or
from that of the welding material according to the present invention.
[0053]
As the base metal, there is preferably used a Ni-based heat-resistant alloy excellent in
high-temperature strength, consisting, by mass percent, of 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: 45 to 55%, Cr: 25 to 35%, W:
3.5% to 8.5%, Ti: 0.05 to 1.0%, Zr: 0.005 to 0.05%, N: 0.02% or less, B: 0.005% or less,
and Al: 0.05 to 0.3%, the balance being Fe and impurities.
[0054]
The "impurities" are elements that are mixed on account of various factors in the
production process including raw materials such as ore or scrap when the welding material
orland the heat-resistant alloy is produced on an industrial scale.
[0055]
The present invention was completed based on the above-described findings, and
involves the following welding materials, a weld metal, and welded joints.
[0056]
(1) A welding material for Ni-based heat-resistant alloy, having a chemical
composition comprising, by mass percent, C: 0.06 to 0.18%, Si: 0.5% or less, Mn: 1.5% or
less, Ni: 45 to 55%, Cr: 25 to 35%, W: 7.0 to 13.0%, Ti: more than 0.2% and not more than
1.5%, Al: less than 0.1 %, and N: 0.002 to 0.20%, with the balance of Fe and impurities, and
in the impurities, by mass percent, 0: 0.02% or less, P: 0.008% or less, and S: 0.005% or
less being contained.
[0057]
(2) The welding material foi~i-basedh eat-resistant alloy according to (I), wherein
the welding material has a chemical composition in which, in lieu of a part of Fe, Nb: 1.0
mass% or less is contained.
[0058]
(3) A weld metal obtained by using the welding material for Ni-based heat-resistant
alloy according to (1) or (2).
[0059]
(4) A welded joint consisting of the weld metal according to (3) and a base metal of a
Ni-based heat-resistant alloy excellent in high-temperature strength.
[0060]
(5) The welded joint according to (4), wherein the base metal of a Ni-based heatresistant
alloy excellent in high-temperature strength contains, by mass percent, W: 3.5 to
8.5%, Ni: 45 to 55%, and Cr: 25 to 35%.
[0061]
(6) The welded joint according to (4), wherein the base metal of a Ni-based heatresistant
alloy 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: 45 to
55%, 25 to 35%, W: 3.5 to 8.5%, Ti: 0.05 to 1.0%, Zr: 0.005 to 0.05%, N: 0.02% or less,
B: 0.005% or less, and Al: 0.05 to 0.3%, with the balance of Fe and impurities.
ADVANTAGEOUS EFFECTS OF THE INVENTION
[0062]
According to the present invention, a welding material for Ni-based heat-resistant
alloy having welding workability and resistance to high-temperature cracks excellent
during welding can be provided, and also a weld metal having resistance to hightemperature
cracks during welding, resistance to stress relaxation cracks during long-term
use at high temperatures, and an excellent creep strength can be provided by using the
welding material. Further, a welded joint consisting of the weld metal having resistance
to high-temperature cracks during welding, resistance to stress relaxation cracks during
long-term use at high temperatures, and an excellent creep strength and a base metal of Nibased
heat-resistant alloy excellent in high-temperature strength can be provided by using
the said welding material.
MODE FOR CARRYING OUT THE INVENTION
[0063]
The reason why the chemical composition of the welding material for Ni-based
heat-resistant alloy is restricted in the present invention is as described below. In the
explanation below, the symbol "%" of the content of each element means "mass%".
[0064]
C: 0.06 to 0.18%
C is an austenite producing element; and is also an element effective in enhancing
the stability of austenitic structure at the time of use at high temperatures. Further, in the
present invention, C is an important element for preventing high-temperature cracks at the
welding time. That is, 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, M)23C6 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
cracks can be prevented. Further, 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 cracks during welding and the decrease in susceptibility to stress relaxation
cracks during the use at high temperatures. In order to sufficiently achieve the abovedescribed
effects by the Cr content range of the present invention, described later, it is
necessary that 0.06% or more of C be contained. However, if 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
cracks is rather increased. Therefore, the C content is 0.06 to 0.18%. The preferable
lower limit of C content is 0.07%, and the preferable upper limit thereof is 0.15%.
[0065]
Si: 0.5% or less
Si is contained as a deoxidizer; however, it segregates at columnar crystal grain
boundaries during solidification of weld metal, lowers the melting point of liquid phase,
and increases the susceptibility to solidification cracks. Therefore, the Si content must be
0.5% or less. The Si content is preferably 0.3% or less. However, if Si content is
decreased excessively, the deoxidizing effect is not achieved sufficiently, the cleanliness of
alloy decreases, and the production cost increases. Therefore, although the lower limit of
Si content is not set specially, the preferable lower limit thereof is 0.01%. If at least
0.01% of Si is contained, the deoxidizing effect can be achieved. The further preferable
lower limit of Si content is 0.02%.
[0066]
Mn: 1.5% or less
Mn is contained as a deoxidizer like Si. However, if Mn is contained excessively,
embrittlement occurs. Therefore, the Mn content must be 1.5% or less. The Mn content
is preferably 1.2% or less. The lower limit of Mn content is not defined specifically;
however, the preferable lower limit thereof is 0.01%. If at least 0.01% of Mn is contained,
the deoxidizing effect can be achieved. The further preferable lower limit of Mn content
is 0.02%.
[0067]
Ni: 45 to 55%
Ni is an element effective in obtaining an austenitic structure, and is also an element
essential in ensuring structural stability at the time of long-term use and in attaining a
sufficient creep strength. In order to achieve the above-described effects, it is necessary
that 45% or more of Ni be contained. However, since Ni is an expensive element, a high
content of Ni exceeding 55% leads to an increase in cost. Therefore, the Ni content is 45
to 55%. The preferable lower limit of Ni content is 45.5%, and the preferable upper limit
thereof is 54.5%. The further preferable lower limit of Ni content is 46%, and the M e r
preferable upper limit thereof is 54%.
[0068]
Cr: 25 to 35%
Cr is an element essential in ensuring oxidation resistance and corrosion resistance
at high temperatures. Also, Cr precipitates as an a-Cr phase during a long period of use,
and also contributes to the improvement in creep strength. In addition, by being
contained in a proper range compositely with C, Cr is caused to combine with C in the
solidification process. Therefore, eutectic carbides are formed, and the solidification
cracking and ductility-dip crack during welding are prevented. Also, Cr combining with
C decreases the segregation energy of P during the use at high temperatures, and also
contributes indirectly to the decrease in susceptibility to stress relaxation cracking. In
order to achieve the above-described effects, it is necessary that 25% or more of Cr be
contained. However, if the Cr content becomes excessive and exceeds 35%, the structural
stability at high temperatures deteriorates, and the creep strength decreases. Therefore,
the Cr content is 25 to 35%. The preferable lower limit of Cr content is 25.5%, and the
preferable upper limit thereof is 34.5%. The further preferable lower limit of Cr content
is 26%, and the further preferable upper limit thereof is 34%.
[0069]
W: 7.0 to 13.0%
W is an element that dissolves in matrix and contributes enormously to the
improvement in creep strength at high temperatures exceeding 700°C. Also, W decreases
the grain boundary segregation energy of S, and reduces the concentration on the grain
boundary of S during the use at high temperatures. Therefore, W restrains the grain
boundary from weakening, and contributes indirectly to the prevention of stress relaxation
cracking. In order to achieve these effects sufficiently and to attain both of resistance to
stress relaxation cracks and creep strength during the use at high temperatures, it is
necessary in terms of the relationship with other elements constituting the present invention
that 7.0% or more of W be contained. However, even if W is contained excessively, the
effects are saturated, and the toughness and creep strength are rather decreased. Further,
since W is an expensive element, a high W content exceeding 13.0% leads to an increase in
cost. Therefore, the W content is in the range of 7.0 to 13.0%. The preferable lower limit
of W content is 7.2%, and the preferable upper limit thereof is 11.5%. The further
preferable lower limit of W content is 7.5%, and the further preferable upper limit thereof is
9.5%.
[0070]
Ti: more than 0.2% and not more than 1.5%
Ti precipitates within the grains as intermetallic compounds by combining with Ni,
or as carbo-nitrides by combining with carbon and nitrogen, and contributes to the
improvement in creep strength at high temperatures. Further, in the A1 content range of
the present invention, by making the oxide scale formed on the weld bead surface such as to
consist mainly of Ti, the welding workability is improved, and the formation of weld defects
is restrained. In order to achieve these effects, in relation to other elements constituting the
present invention, more than 0.2% of Ti must be contained. However, if the Ti content
becomes excessive and exceeds 1.5%, intermetallic compounds precipitate excessively, and
the deformation resistance within the grains is enhanced remarkably, so that the
susceptibility to stress relaxation cracks during the use at high temperatures is increased.
Therefore, the Ti content is exceeding 0.2% and not more than 1.5%. The preferable lower
limit of Ti content is 0.3%, and the preferable upper limit thereof is 1.4%.
[0071]
Al: less than 0.1 %
Al precipitates, like Ti, finely within the grains as intermetallic compounds by
combining with Ni, and contributes to the improvement in creep strength at high
temperatures. However, if the Al content becomes so excessive as to be 0.1 % or higher,
oxide scale consisting mainly of A1 having a high fusing point is formed on the weld bead
surface, and therefore the welding workability is deteriorated. Also, the increase in
susceptibility to stress relaxation cracking during the use at high temperatures caused by
the precipitation of intermetallic compound is not a little. Therefore, the content of A1 is
set to less than 0.1%. The preferable upper limit of the Al content is 0.05%. The lower
limit of the Al content is not placed especially; however, an extreme decrease in the A1
content leads to a rise in cost, so that the preferable lower limit thereof is 0.0002%. If at
least 0.0002% of A1 is contained, the advantageous effect of improving the creep strength
can be achieved. The further preferable lower limit of the A1 content is 0.0005%.
[0072]
N: 0.002 to 0.20%
N is an austenite-forming element, and is 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, precipitates as nitrides, and contributes to the
improvement in tensile strength and creep strength. In order to achieve the abovedescribed
effects, it is necessary that 0.002% or more of N be contained. However, if the
N content 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 N causes a formation of blowholes when welding is
performed. Therefore, the content of N is set to 0.002 to 0.20%. The lower limit of the
N content is preferably more than 0.03%, further preferably 0.035%. The preferable
upper limit of the N content is 0.18%.
LO0731
One of the welding materials for Ni-based heat-resistant alloy of the present
invention has a chemical composition consisting of elements ranging from C to N
described above, the balance being Fe and impurities, and the contents of 0 , P and S in
impurities being restricted to the ranges described below.
[0074]
0: 0.02% or less
0 exists as an impurity. If contained in large amounts, 0 deteriorates the
workability of welding material and the ductility of weld metal. Therefore, the 0 content
must be 0.02% or less. The 0 content is preferably 0.015% or less.
[0075]
P: 0.008% or less
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 cracks remarkably, and brings about grain boundary embrittlement during
the use at high temperatures, thereby decreasing the resistance to stress relaxation cracks.
Therefore, the P content must be 0.008% or less. The P content is preferably 0.006% or
less.
[0076]
S: 0.005% or less
S is an element contained as an impurity like P. S lowers the melting point of final
solidification portion at the time of solidification of weld metal, and increases the
susceptibility to solidification cracks. Further, S is an element that precipitates and
concentrates at the crystal grain boundaries during the use at high temperatures, and
remarkably enhances the susceptibility to stress relaxation cracks. Therefore, the S
content must be 0.005% or less. The S content is preferably 0.003% or less.
[0077]
Another of the welding materials for Ni-based heat-resistant alloy of the present
invention has a chemical composition containing 1 .O% or less of Nb in place of the abovedescribed
Fe.
[0078]
Hereunder, the operational advantages and the reasons for the restriction of content
of Nb, which is an optional element is explained.
[0079]
Nb: 1.0% or less
Like Ti, Nb is an element that precipitates within the grains as fine intermetallic
compounds andlor carbo-nitrides, and contributes to the improvement in creep strength at
high temperatures. Therefore, Nb may be contained as necessary. However, if the Nb
content becomes excessive and exceeds 1.0%, large amounts of fine intermetallic
compounds and/or carbo-nitrides 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, the content of Nb, if contained, is
1.0% or less. The content of Nb, if contained, is preferably 0.9% or less.
[0080]
On the other hand, in order to stably achieve the above-described effect of Nb, it is
preferable that the content of Nb, if contained, be 0.005% or more, further preferably
0.01% or more.
[008 11
The above is a detailed description of the chemical composition of the welding
materials for Ni-based heat-resistant alloy according to the present invention. These
welding materials each have excellent welding workability and resistance to hightemperature
cracks at the welding time. By using these welding materials, a weld metal
having resistance to high-temperature cracks during welding, resistance to stress relaxation
cracks during long-term 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 high-temperature cracks during weldihg, resistance to stress
relaxation cracks during long-term use at high temperatures, and an excellent creep
strength and a base metal of Ni-based heat-resistant alloy excellent in high-temperature
strength can be obtained.
[0082]
When a welded joint is obtained by using the welding materials for Ni-based heatresistant
alloy according to the present invention, the use of a Ni-based heat-resistant alloy
excellent in high-temperature strength, which contains W: 3.5 to 8.5%, Ni: 45 to 55%, and
Cr: 25 to 35%, 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 Nibased
heat-resistant alloy excellent in high-temperature strength, which is used as the base
metal, may be a Ni-based heat-resistant alloy having the same or different chemical
composition as or from that of the welding materials for Ni-based heat-resistant alloy
according to the present invention.
[0083]
Hereunder, the reason why, in the case where the Ni-based heat-resistant alloy
excellent in high-temperature strength is used as the base metal, the base metal preferably
contains W: 3.5 to 8.5%, Ni: 45 to 55%, and Cr: 25 to 35% is explained in detail.
[0084]
W: 3.5 to 8.5%
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, the base metal is homogenized by heat
treatment, so that the effect of W is achieved more easily. Therefore, the base metal
preferably contains W, and the content of W may be 3.5% or more. However, since W is
an expensive element and the containing thereof leads to an increase in cost, the content of
W, if contained, is preferably 8.5% or less. The W e r preferable lower limit of the W
content in the base metal is 3.8%, and the further preferable upper limit thereof is 8.2%.
[0085]
Ni: 45 to 55%
Ni is an element effective in obtaining an austenitic structure, and is also an element
effective in ensuring structural stability at the time of long-term use and in attaining a
sufficient creep strength as in the weld metal. To achieve these effects, the base metal
preferably contains Ni, and the content of Ni is preferably 45% or more as in the weld
metal. On the other hand, since Ni is an expensive element and the containing thereof
leads to an increase in cost, the content of Ni, if contained, is preferably 55% or less. The
further preferable lower limit of the Ni content in the base metal is 45.5%, and the M e r
preferable upper limit thereof is 54.5%. The still further preferable lower limit of the Ni
content in the base metal is 46%, and the still further preferable upper limit thereof is 54%.
[0086]
Cr: 25 to 35%
Cr is an element effective in ensuring oxidation resistance, corrosion resistance and
creep strength of the base metal at high temperatures as in the weld metal. To achieve
these effects, the base metal preferably contains Cr, and the content of Cr is preferably
25% or more as in the weld metal. However, if the Cr content becomes excessive, the
structural stability at high temperatures deteriorates, and the creep strength decreases.
Therefore, the content of Cr, if contained, is preferably 35% or less. The further
preferable lower limit of the Cr content in the base metal is 25.5%, and the further
preferable upper limit thereof is 34.5%. The still further preferable lower limit of the Cr
content in the base metal is 26%, and the still W e r preferable upper limit thereof is 34%.
[0087]
The base metal of Ni-based heat-resistant alloy 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, the balance being Fe and impurities.
[0088]
C is an austenite producing 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, the base metal is homogenized by
heat treatment, so that the effect of C is achieved more easily, and also the measures
against weld cracks need not be taken. Therefore, the base metal preferably contains C,
and the content of C may be 0.04% or more. However, if the C content becomes
excessive, coarse carbides are produced during the use at high temperatures, and the creep
strength rather decreases. Therefore, the content of C, if contained, is preferably 0.12%
or less. The further preferable lower limit of the C content in the base metal is 0.05%,
and the further preferable upper limit thereof is 0.1 0%.
[00 891
Si: 0.5% or less
Si has a deoxidizing action. Although the base metal does not require measures
against weld cracks as described above, if the Si content becomes excessive and exceeds
0.5%, the toughness decreases. Therefore, if the base metal contains Si, the content
thereof is preferably 0.5% or less. The Si content in the basemetal is further preferably
0.4% or less. However, if the Si content is decreased excessively, the deoxidizing effect
is not achieved sufficiently, the cleanliness of alloy decreases, and the production cost
increases. Therefore, although the lower limit of Si content in the base metal is not set
specially, the preferable lower limit thereof is 0.01%. If at least 0.01% of Si is contained,
the deoxidizing effect can be achieved. The further preferable lower limit of Si content is
0.02%.
[0090]
Mn: 1.5% or less
Mn has a deoxidizing action like Si. However, an excessive content of Mn leads
to embrittlement. Therefore, if the base metal contains Mn, the content of Mn is
preferably 1.5% or less, and further preferably 1.2% or less. The lower limit of the Mn
content in the base metal is not defined specifically; 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 further preferable lower limit of the Mn content is 0.02%.
[0091]
P: 0.03% or less
P is contained as an impurity. If the P content becomes excessive, the creep
ductility decreases. Unlike the weld metal, the base metal does not require the
countermeasures against weld cracks, and an extreme decrease in the P content leads to a
remarkable increase in production cost. Therefore, the P content in the base metal is
preferably 0.03% or less, and further preferably 0.02% or less.
[0092]
S: 0.01% or less
S is contained as an impurity like P. If the S content becomes excessive, the creep
ductility decreases. Unlike the weld metal, the base metal does not require the
countermeasures against weld cracks, and an extreme decrease in the S content leads to a
remarkable increase in production cost. Therefore, the S content in the base metal is
preferably 0.01 % or less, and further preferably 0.008% or less.
[0093]
Ti: 0.05 to 1.0%
Ti is an element that precipitates as fine intermetallic compounds and carbo-nitrides,
and contributes to the improvement in creep strength at high temperatures. For this
reason, the base metal preferably contains Ti, and the content of Ti is preferably 0.05% or
more. However, if the Ti content becomes excessive, large amounts of intermetallic
compounds and carbo-nitrides are produced, thereby decreasing the toughness. Therefore,
the content of Ti, if contained, is preferably 1.0% or less. The further preferable lower
limit of the Ti content in the base metal is 0.10%, and the M e r preferable upper limit
thereof is 0.95%.
[0094]
Zr: 0.005 to 0.05%
Zr dissolves in austenite, which is a matrix, and improves the strength at high
temperatures. Unlike the weld metal, the base metal does not melt. Therefore, in the
base metal, the above-described effect of active Zr such as to combine with 0 to form slag
in the weld metal can be put to practical use. Therefore, the base metal preferably
contains Zr, and the content thereof is preferably 0.005% or more. However, if Zr is
contained excessively, the creep ductility decreases. Therefore, if Zr is contained, the
content thereof is preferably 0.05% or less. The further preferable lower limit of the Zr
content in the base metal is 0.01%, and the further preferable upper limit thereof is 0.04%.
[0095]
N: 0.02% or less
N is an element effective in stabilizing the austenite phase, and is an element that
dissolves in matrix, and is effective in enhancing the tensile strength. On the other hand,
N significantly decreases the hot workability. Therefore, in the base metal, the upper
limit of the N content should be controlled strictly as compared with the weld metal, and is
preferably 0.02% or less. The further preferable upper limit of the N content in the base
metal is 0.01%. The lower limit of the N content in the base metal is not defined
specifically; however, the lower limit in terms of stabilization of austenite phase is
0.0005%.
[0096]
B: 0.005% or less
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 B content becomes excessive, the
susceptibility to liquation cracks of HAZ enhances. The lower limit of the B content in
the base metal is not defined specifically; however, the lower limit is preferably 0.0002%.
[0097]
Al: 0.05 to 0.3%
Al is an element that combines with Ni and finely precipitates within the grains as
fine intermetallic compounds, and contributes to the improvement in creep strength at high
temperatures. For this reason, in the base metal, which does not relate directly to the
welding workability, Al may be utilized positively to increase the strength. For this reason,
the base metal preferably contains Al, and the content of Al is preferably 0.05% or more.
However, if the Al content becomes excessive, intermetallic compounds precipitate
excessively, thereby decreasing the toughness. Therefore, the content of Al, if contained,
is preferably 0.3% or less. The further preferable lower limit of the A1 content in the base
metal is 0.06%, and the further preferable upper limit thereof is 0.25%.
[0098]
Hereunder, the present invention is explained more specifically by using Examples.
The present invention is not limited to these Examples.
EXAMPLES
[0099]
From an ingot obtained by melting and casting a material having the chemical
composition given in Table 1 on an experimental basis, a plate material measuring 12 mm
thick, 50 rnm wide, and 100 mrn long was prepared as a base metal to be welded by hot
forging, hot rolling, heat treatment, and machining.
[O 1 001
Further, from ingots obtained by melting and casting materials of symbols A to L
having the chemical compositions given in Table 2 on an experimental basis, welding
materials (welding wires) each having an outside diameter of 1.2 mm and a length of 1000
mrn were prepared by hot forging, hot rolling, and machining.
[OlOl]
[Table 11
Table 1
I Chemical composition (in maas%, balance: Fe and impurities) 1
[Table 21
Table 2
IS-ymLbol
* indicates tl w
Chemical co
Si I Mn I P
at conditions do no
mass%, balance: Fe and impurities)
CrI W I Ti 1 N b l A1 I N I 0
npoeition (in
30.6 ' 6.58 0.75 - 0.006 0.164 0.008
30.2 7,26 0.77 - 0.012 0.156 0.009
defined by the piwent invention.
S
0,001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
A V-type edge having an angle of 30" and a root thickness of 1 mm was formed in
Ni
60.1
60.0
60.3
60,2
60.1
'49.7
60.1
49.8
60.2
60.3
49.8
60.3
the longitudinal direction of the plate material for the base metal to be welded, and
satis$ thoel
thereafter the periphery of the plate material was restraint-welded onto a commercially sold
steel plate of SM4OOB specified in JIS G 3106 (2008), measuring 25 mm thick, 200 rnm
wide, and 200 mm long, by using a covered electrode of "E Ni 6625" specified in JIS Z
Thereafter, multi-layer welding was performed in the groove by TIG welding with a
heat input of 9 to 15 kJ/cm by using the welding materials of symbols A to L, and thereby
two welded joints were produced for each of the symbols of welding materials.
For each of the symbols, the test described below was conducted in the state in
which one welded joint was as welded, and the remaining welded joint had been subjected
to aging heat treatment of 700°C x 500 h.
[0 1061
That is to say, for each of the marks, specimens were cut off from five locations of
each of the as-welded welded joint and the aging heat-treated welded joint, and the
transverse cross section of each of the five specimens was mirror-like polished and etched,
and thereafter was observed under an optical microscope to examine the presence of lack
of fusion and the presence of cracking in the weld metal. A welded joint in which no
cracking was recognized in all of the five specimens observed under the optical
microscope was regarded as "acceptable". Also, for a welded joint in which no lack of
fusion was recognized, the welding workability was regarded as "excellent".
[0 1 071
Further, from the as-welded welded joint in which no lack of fusion and crack had
been recognized in the weld metal as the result of microscopic observation, a round-bar
creep rupture test specimen was sampled so that the weld metal is positioned in the center of
the parallel part, and a creep rupture test was conducted under the conditions of 700°C and
167 MPa so that the target rupture time of base metal plate material was 1000 h or longer.
A welded joint in which the creep rupture time was 1000 h or more, which was the target
rupture time of base metal plate material, was regarded as "acceptable".
[0 1 081
Table 3 gives the results of the above-described tests.
[0 1 091
The mark "0" in the "Crack observation result" column in Table 3 indicates an
"acceptable" welded joint in which no crack had been recognized in all of the five
specimens in the observation under the optical microscope. On the other hand, the mark
"x" indicates that a crack had been recognized in at least one of the five specimens in the
observation under the optical microscope.
[Ol lo]
Also, the symbol "0" in the "Welding workability test result" column in Table 3
indicates a welded joint in which no lack of fusion had been recognized in all of the five
specimens observed under the optical microscope, that is, the welding workability was
"excellent". On the other hand, the symbol "x" indicates that lack of fusion had been
recognized in at least one of the five specimens observed under the optical microscope.
[Olll]
Further, the symbol "0" in the "Creep rupture test resultt' column in Table 3
indicates an "acceptable" welded joint in which the creep rupture time was 1000 h or
longer, which is the target rupture time of the base metal plate. On the other hand, the
symbol "x" indicates that the creep rupture time did not reach 1000 h. The symbol "-" in
welding material mark J denotes that lack of fusion had been recognized in the weld metal
of the specimen cut off fkom the as-welded welded joint, and therefore the creep rupture
test was not carried out. Similarly, the mark "-" of welding material symbol L indicates
that a crack had been recognized in the weld metal of the specimen sampled from the aswelded
welded joint, and therefore the creep rupture test was not conducted.
[0112]
[Table 31
Table 3
* indicates that conditions do not satisfy
those defined by the present invention.
From Table 3, it is apparent that the welding materials of symbols A to G, in which
the chemical composition was within the range defined in the present invention are
excellent in welding workability, and in the weld metals of the welded joints that were
welded by using these welding materials, there was no lack of fusion, neither stress
relaxation cracks during aging heat treatment nor high-temperature cracks during welding
were generated, and also a high creep strength was attained.
[0114]
In contrast, in the welded joint that was welded by using the welding material of
mark H, in which the content of Ti exceeded the upper limit regulated by the present
invention, fine intermetallic compounds were formed excessively within the grains, and the
deformation resistance within the grains was high, so that stress relaxation cracking
occurred during the aging heat treatment.
In the welded joint that was welded by using the welding material of mark I, in
which the content of Ti was lower than the lower limit regulated by the present invention,
although no cracking occurred, intermetallic compounds necessary for securing the
strength were not formed, so that the rupture time did not reach 1000 h, and a satisfactory
creep strength was not attained.
[0116]
In the welded joint that was welded by using the welding material of mark J, in
which the content of A1 was out of the range defined by the present invention, in addition
to the fact that lack of fusion occurred and the welding workability was poor, fine
intermetallic compounds were formed excessively within the grains, and the deformation
resistance within the grains was high, so that stress relaxation cracking occurred during the
aging heat treatment.
[0117]
In the welded joint that was welded by using the welding material of mark K, in
which the content of W was out of the range defined by the present invention, although no
cracking occurred, the rupture time did not reach 1000 h, and a satisfactory creep strength
was not attained.
[0118]
In the welded joint that was welded by using the welding material of symbol L, in
which the C content was as low as 0.02% and was out of the range defined in the present
invention, solidification cracks were generated as the result that sufficient (Cr, M)23C6 could
not be produced in the final solidification portion.
[0119]
As described above, it can be seen that in the case where a welding material having
a chemical composition that is within the range defined in the present invention is used,
there can be obtained a weld metal having resistance to high-temperature cracks during
welding, resistance to stress relaxation cracks during long-term use at high temperatures,
and an excellent creep strength.
INDUSTRIAL APPLICABILITY
[O 1201
According to the present invention, a welding material for Ni-based heat-resistant
alloy having welding workability and resistance to high-temperature cracks excellent
during welding can be provided, and also a weld metal having resistance to hightemperature
cracks during welding, resistance to stress relaxation cracks during long-term
use at high temperatures, and an excellent creep strength can be provided by using the
welding material. Further, a welded joint consisting of the weld metal having resistance
to high-temperature cracks during welding, resistance to stress relaxation cracks during
long-term use at high temperatures, and an excellent creep strength and a base metal of Nibased
heat-resistant alloy excellent in high-temperature strength can be provided by using
the said welding material.
We claim:
1. A welding material- for Ni-based heat-resistant alloy, having a chemical
composition comprising, by mass percent, C: 0.06 to 0.18%, Si: 0.5% or less, Mn: 1.5% or
less, Ni: 45 to 55%, Cr: 25 to 35%, W: 7.0 to 13.0%, Ti: more than 0.2% and not more than
1.5%, Al: less than 0.1%, and N: 0.002 to 0.20%, with the balance of Fe and impurities, and
in the impurities, by mass percent, 0: 0.02% or less, P: 0.008% or less, and S: 0.005% or
less being contained.
2. The welding. material for Ni-based heat-resistant alloy according to claim 1,
wherein the welding material has a chemical composition in which, in lieu of a part of Fe,
Nb: 1.0 mass% or less is contained.
3. A weld metal obtained by using the welding material for Ni-based heat-resistant
alloy according to claim 1 or 2.
4. A welded joint consisting of the weld metal according to claim 3 and a base
metal of a Ni-based heat-resistant alloy excellent in high-temperature strength.
5. The welded joint according to claim 4, wherein the base metal of a Ni-based
heat-resistant alloy excellent in high-temperature strength contains, by mass percent, W: 3.5
to 8.5%, Ni: 45 to 55%, and Cr: 25 to 35%.
6. The welded joint according to claim 4, wherein the base metal of a Ni-based
heat-resistant alloy 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:
45 to 55%, 25 to 35%, W: 3.5 to 8.5%, Ti: 0.05 to 1.0%, Zr: 0.005 to 0.05%, N: 0.02% or
less, B: 0.005% or less, and Al: 0.05 to 0.3%, with the balance of Fe and impurities.