TECHNICAL FIELD
[0001] The present invention relates to a welded joint, and more
particularly to a welded joint having good metal-dusting resistance.
BACKGROUND ART
[0002] Demands for clean energy fuels such as hydrogen, methanol, liquid
fuels (or gas-to-liquids (GTL)), or dimethyl ether (DME) are expected to grow
significantly. Reforming equipment that are indispensable for
manufacturing such synthetic gases are preferably large-sized ones with
high thermal efficiency that are suitable for mass production. Further,
reforming equipment for conventional petroleum refining or in petrochemical
plants, for example, or ammonium production equipment or hydrogen
production equipment using petroleum as a raw material, for example, are
increasingly using heat exchange for waste heat collection to increase energy
efficiency.
[00031 To effectively utilize such heat in high-temperature gases, heat
exchange in the temperature range of 400 to 800 "C, which is lower than
conventional targets, is becoming important. Corrosion accompanying
carburization of high-Cr-high-Ni-Fe alloy-based metal materials used in
reaction tubes or heat exchangers in this temperature range is drawing
attention as a problem.
[00041 Synthetic gases produced by such reaction equipment, that is, gases
containing Hz, CO, COz, Hz0 and a hydrocarbon such as methane are usually
in contact with the metal material of the reaction tube or the like at a
temperature of 1000 "C or higher. In this temperature range, elements
such as Cr or Si which have stronger tendencies to oxidize than Fe or Ni, for
example, are selectively oxidized on the surface of the metal material such
that a fine film of chromium oxide, silicon oxide or the like is formed, thereby
preventing corrosion. However, in members at relatively low temperatures,
such as heat-exchanging members, the diffusion of elements from the
interior of the metal material toward the surface is insufficient such that the
formation of oxidized films with corrosion-preventing effect is delayed.
Further, since gases with a composition including hydrocarbon become
carburizing, carbon enters the metal material through its surface, causing
carburization.
[0005] When carburization progresses in an ethylene decomposition furnace
tube, for example, and a carburization phase made of carbides of Cr or Fe, for
example, is formed, this portion expands in volume. As a result, fine cracks
can easily develop and, in the worst case scenario, the tube breaks while
being used. Further, when a metal surface is exposed, carbon deposition (or
caulking) occurs at the surface, with metal working as a catalyst, decreasing
the flow passage area in the tube or decreasing heat transfer properties.
[0006] Also, heating furnace tubes in a catalytic cracking furnace which
increase the octane number of naphtha produced by distillation of petroleum
can be a severely carburizing environment made of hydrocarbon and
hydrogen, causing carburization or metal dusting.
[0007] If such cracking, wear or clogging of the tube spread, this may cause
a defect or the like in the equipment and interrupt its operation. In view of
this, sufficient consideration is needed for selecting materials for equipment
members.
[0008] To prevent corrosion caused by such carburization or metal dusting,
various measures have been developed.
[00091 Traditionally, for such equipment members, high-Cr-high-Ni-Fe
alloys have been used. For example, JP 2001-107196 A discloses a welded
joint where a chemical composition is defined and the relationship between
the Si, Cu or S content and the Nb, Ta, Ti and Zr contents and the
relationship between the Ni, Co and Cu contents are limited to certain
ranges. According to this document, this welded joint has good corrosion
resistance and weld-crack resistance in a sulfuric-acid environment.
However, this welded joint has a low Si content, making it difficult to use in
an environment where metal dusting may occur.
[0010] JP 2002-235136 A discloses a welded joint made of an Ni-based
heat-resistant alloy where it is proposed to actively include A1 and a
relationship between the amount of grain-boundary melting and the fixing
force at grain boundaries is defined. According to this document, this
welded joint has good carburization resistance and high-temperature
strength. However, in this welded joint, an increase in the Si content to
ensure metal-dusting resistance may cause solidification cracks to develop
during welding, making it difficult to provide both metal-dusting resistance
and solidification-cracking resistance during welding.
[0011] WO 20091107585 proposes a metal material with increased C in a
steel containing Si and Cu to reduce crack sensitivity in a heat-affected zone
(hereinafter referred to as HAZ) during welding. However, a high C content
increases solidification crack sensitivity during welding and also decreases
creep ductility.
[0012] JP 2007- 186727 A and JP 2007- 186728 A propose including one or
more of P, S, Sb and Bi in appropriate amounts to reduce gas dissociative
absorption (i.e. gaslmetal surface reaction). These elements segregate on a
metal surface, which makes it possible to reduce carburization and corrosion
due to metal dusting significantly even when they are not added in excessive
amounts. However, these elements segregate not only on a metal surface
but also along the boundaries of metal crystal grains, which leaves problems
in hot workability and weldability.
[0013] WO 20121524983 A proposes a metal material where the C content in
a steel containing Si and Cu is limited to reduce solidification crack
sensitivity and the Ti and A1 contents are limited to reduce HAZ crack
sensitivity. However, this document does not disclose welding materials
required to weld metal materials to construct a structure.
[00141 JP 2006-45597 A proposes a welding material and a welded joint
using it where an appropriate amount of Ti is added to reduce the adverse
effects of Si.
DISCLOSURE OF THE INVENTION
[0015] Research by the present inventors showed that, even if certain
metal-dusting resistances of a base material and a welding material are
provided, when a welded joint is fabricated, a certain metal-dusting
resistance may not be provided in the weld metal under some welding
conditions.
[0016] An object of the present invention is to provide a welded joint having
good metal-dusting resistance.
[00171 A welded joint according to the present invention is a welded joint
obtained by welding using a welding material having a chemical composition
including, in mass%, Cr: 15.0 to 35.0 %; and Ni: 40.0 to 70.0 %, including: a
base material having a chemical composition of, in mass%: C: 0.03 to
0.075 %; Si: 0.6 to 2.0 %; Mn: 0.05 to 2.5 %; P: up to 0.04 %; S: up to 0.015 %;
Cr: more than 16.0 % and less than 23.0 %; Ni: not less than 20.0 % and less
than 30.0 %; Cu: 0.5 to 10.0 %; Mo: less than 1 %; Al: up to 0.15 %; N: 0.005 to
0.20 %; 0 : up to 0.02 %; Ca: 0 to 0.1 %; REM: 0 to 0.15 %; V: not less than 0 %
and less than 0.5 %; and Nb: 0 to 2 %, a balance being Fe and impurities; and
a first-layer weld metal having a chemical composition including, in mass%,
an Fe content ranging from 10 to 40 %.
[0018] The present invention provides a welded joint having good
metal-dusting resistance.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] [FIG. I] FIG. 1 is a schematic cross-sectional view of a welded joint.
EMBODIMENTS FOR CARRYING OUT THE INVENTION
[0020] The present inventors attempted to find out how to improve the
metal-dusting resistance of a welded joint.
[0021] The metal-dusting resistance of a material is improved if the
material contains elements such as Si, Cu and P. However, these elements
significantly increase the solidification crack sensitivity of the material
during welding. Solidification cracks during welding are produced when, at
a stage close to the end of the solidification process in the welding and in
which a liquid phase in film form is present mainly along crystal grain
boundaries, the strain applied by solidification contraction or thermal
contraction exceeds the deformability of the weld metal. The solidification
crack sensitivity during welding may be reduced by improving the
deformability of the weld metal. However, this requires changes to
components of the base material, which is in contradiction to the objective of
ensuring metal-dusting resistance. In view of this, the present inventors
attempted to find out how to ensure metal-dusting resistance and
solidification-crack resistance during welding without changing components
of the base material.
LO0221 As discussed above, even when certain metal-dusting resistances of
the base material and welding material are provided, when the welded joint
is fabricated, a certain metal-dusting resistance of the weld metal may not be
provided under some welding conditions. Although details of its mechanism
are not clear, it was found out that, in a region where the Fe content and Ni
content in the weld metal are ill-balanced, the solidified microstructure may
facilitate metal dusting. The first layer of the weld metal can be easily
affected by the dilution ratio with respect to the base material such that the
Fe content and the Ni content can easily become ill-balanced. Thus, if the
first layer is a surface in contact with gases, metal dusting can easily occur.
Lo0231 To provide a welded joint having a metal-dusting resistance
substantially equal to that of the base material, the Fe content in the weld
metal must be limited. More specifically, if adjustments are done such that
the Fe content in the first layer of the weld metal is 40 mass% or lower, a
welded joint having a metal-dusting resistance substantially equal to that of
the base material can be provided. On the other hand, if the Fe content in
the first layer of the weld metal is too low, metal dusting can easily occur.
In view of this, adjustments must be done such that the Fe content in the
first layer of the weld metal is 10 mass% or higher.
LO0241 The welded joint of the present invention was made based on the
above findings. A welded joint according to an embodiment of the present
invention will be described in detail below. In the description below, "%" for
the content of an element means mass percentage.
LO0251 The welded joint according to the present embodiment is obtained by
welding a base material with a welding material having a chemical
composition including Cr: 15.0 to 35.0 % and Ni: 40.0 to 70.0 %. The welded
joint may be, for example, steel pipes or steel plates with their adjacent
endsledges welded together. The welded joint includes a base material and
a weld metal. The weld metal is formed by a part of the base material and
the welding material melting together and solidifying. The first-layer weld
metal has a chemical composition with an Fe content ranging from 10 to
40 %.
LO0261 [Chemical Composition of Base Material]
The base material has the chemical composition described below.
[0027] C: 0.03 to 0.075 %
Carbon (C) bonds with Cr, for example, to form a carbide, thereby
increasing the strength of the base material. On the other hand, if an
excessive amount of C is contained, creep ductility at high temperatures
decreases. In view of this, the C content in the base material should be in
the range from 0.03 to 0.075 %. The preferable lower limit of the C content
in the base material is 0.035 %, and the more preferable lower limit is 0.04 %.
The preferable upper limit of the C content in the base material is 0.07 %.
[0028] Si: 0.6 to 2.0 %
Silicon (Si) has a strong affinity for oxygen, and forms Si-based
oxidized scales under a protective oxidized-scale layer of Cr203, for example,
thereby blocking carburizing gases. On the other hand, if an excessive
amount of Si is contained, hot workability and weldability significantly
deteriorates. In view of this, the Si content in the base material should be
in the range from 0.6 to 2.0 %. The preferable lower limit of the Si content
in the base material is 0.8 %. The preferable upper limit of the Si content in
the base material is 1.5 %.
[0029] Mn: 0.05 to 2.5 %
Manganese (Mn) deoxidizes steel. Mn also stabilizes the austenitic
phase. Mn further improves the workability and weldability of the steel.
On the other hand, if an excessive amount of Mn is contained, the protective
oxidized-scale layer's ability to block carburizing gases is impaired. In view
of this, the Mn content in the base material should be in the range from 0.05
to 2.5 %. The preferable lower limit of the Mn content in the base material
is 0.1 %. The preferable upper limit of the Mn content in the base material
is 2.0 %.
[0030] P: up to 0.04 %
Phosphorus (PI is an impurity entering steel from a raw material or
the like during smelting. P deteriorates the hot workability and weldability
of the steel. In view of this, the P content in the base material should be up
to 0.04 %. It is preferable to reduce the P content in the base material to the
lowest level possible. The preferable upper limit of the P content in the base
material is 0.03 %, and the more preferable upper limit is 0.025 %.
[0031] S: up to 0.015 %
Sulfur (S) is an impurity entering steel from a raw material or the
like during smelting. S deteriorates the hot workability and weldability of
the steel. This effect is of particular relevance in a steel with a high Si
content or Cu content. In view of this, the S content in the base material
should be up to 0.015 %. It is preferable to reduce the S content in the base
material to the lowest level possible. The preferable upper limit of the S
content in the base material is 0.005 %, and the more preferable upper limit
is 0.002 %.
Lo0321 Cr: more than 16.0 % and less than 23.0 %
Chromium (Cr) forms a protective oxidized-scale layer of Cr203, for
example, and blocks carburizing gases. Thus, Cr gives carburization
resistance, metal-dusting resistance and caulking resistance to steel. On
the other hand, Cr bonds with C to form carbides, thereby decreasing creep
ductility. Further, Cr decreases the creep strength of the austenitic phase.
This effect is particularly significant in steels with high Si and Cu contents.
In view of this, the Cr content in the base material should be more than
16.0 % and less than 23.0 %. The preferable lower limit of the Cr content in
the base material is 18.0 %. To define an upper limit, the Cr content in the
base material is preferably less than 20.0 %.
[0033] Ni: not less than 20.0 % and less than 30.0 %
Nickel (Ni) stabilizes an austenitic phase. Ni also decreases the
enter rate of C entering the steel. Ni further increases the
high-temperature strength of the steel. On the other hand, if an excessive
amount of Ni is to be contained, this requires increased costs. Further, if an
excessive amount of Ni is contained, this may facilitate caulking or metal
dusting. In view of this, the Ni content in the base material should be not
less than 20.0 % and less than 30.0 %. The preferable lower limit of the Ni
content in the base material is 22.0 %. The preferable upper limit of the Ni
content in the base material is 28.0 %.
Lo0341 Cu: 0.5 to 10.0 %
Copper (Cu) prevents surface reaction between a carburizing gas and
a metal, thereby improving the metal-dusting resistance of the metal. Cu
also stabilizes the austenitic phase. On the other hand, if an excessive
amount of Cu is contained, the weldability of the steel deteriorates. In view
of this, the Cu content in the base material should be in the range from 0.5 to
10.0 %. The preferable lower limit of the Cu content in the base material is
1.5 %. The preferable upper limit of the Cu content in the base material is
6.0 %.
Lo0351 Mo: less than 1 %
Molybdenum (Mo) is an impurity. If Mo is contained in 1 % or more,
this causes production of an intermetallic compound such as o phase, which
deteriorates microstructure stability and hot workability. In view of this,
the Mo content in the base material should be less than 1 %. On the other
hand, an excessive reduction in Mo requires increased costs. Thus, the
preferable lower limit of the Mo content in the base material is 0.05 %.
E00361 Al: less than 0.15 %
Aluminum (Al) deoxidizes steel. On the other hand, if an excessive
amount of A1 is contained, a large amount of nitride is formed, which
decreases the toughness of the steel. In view of this, the A1 content in the
base material should be less than 0.15 %. The lower the A1 content, the
better it is. However, an excessively low amount of A1 results in insufficient
deoxidization. Further, an excessively low amount of A1 increases the index
of cleanliness of the steel. Further, an excessively low amount of A1 requires
increased costs. Thus, the preferable lower limit of the A1 content in the
base material is 0.03 %.
Lo0371 N: 0.005 to 0.20 %
Nitrogen (N) increases the high-temperature strength of steel. N
further increases the activity of C in the base material, thereby improving
the metal-dusting resistance of the base material. On the other hand, if an
excessive amount of N is contained, the hot workability of the steel
deteriorates. In view of this, the N content in the base material should be in
the range from 0.005 to 0.20 %. The preferable lower limit of the N content
in the base material is 0.010 %. The preferable upper limit of the N content
in the base material is 0.15 %.
[0038] 0 : up to 0.02 %
Oxygen (0) is an impurity. If an excessive amount of 0 is contained,
the hot workability during production of the base material deteriorates.
Further, if an excessive amount of 0 is contained, the toughness and
ductility of the weld metal decrease. In view of this, the 0 content in the
base material should be up to 0.02 %. The preferable upper limit of the 0
content in the base material is 0.01 %.
[00391 The balance in the chemical composition of the base material is Fe
and impurities. Impurity means an element originating from ore or scraps
used as the raw material of steel or an element that has entered for various
reasons during the manufacturing process.
[00401 In the chemical composition of the base material according to the
present embodiment, some of Fe above may be replaced by one or two
selected from at least one of the first and second groups described below.
First group: Ca: up to 0.1 %; REM: up to 0.15 %
Second group: V: less than 0.5 %; Nb: up to 2 %
[00411 First group (Ca: up to 0.1 %; REM: up to 0.15 %)
Calcium (Ca) and rare-earth metals (REMs) are optional elements.
That is, Ca and REMs need not be contained in the base material. Both Ca
and REMs improve the hot workability of steel. Thus, one or two of Ca and
REMs may be contained in the base material as necessary. If effects of Ca
and REMs in the base material are desired, it is preferable that at least one
of them is contained in 0.005 % or more.
100421 "REM" is a general term for 17 elements in total, i.e. Sc, Y and the
lanthanoids, and the content of REMs is the total content of one or more
REM elements. Further, REMs are typically contained in misch metal.
Thus, for example, misch metal may be added such that the REM content is
in the above-described range.
LO0431 On the other hand, if an excessive amount of Ca is contained, the
weldability of steel deteriorates. In view of this, the upper limit of the Ca
content in the base material should be 0.15 %. If an excessive amount of
REMs is contained, the weldability of the steel deteriorates. In view of this,
the upper limit of the REM content in the base material should be 0.15 %.
Only one of Ca and REMs may be contained, or two of them may be combined
and contained. If two of them are combined and contained, the preferable
upper limit of the total content is 0.2 %.
100441 Second group (V: less than 0.5 %; Nb: up to 2 %)
Vanadium (V) and Niobium (Nb) are optional elements. That is, V
and Nb need not be contained in the base material. Both V and Nb improve
the high-temperature strength of steel. Thus, one or both of V and Nb may
be contained in the base material as necessary. If effects of V are desired, it
is preferable that V is contained in 0.002 % or more; if effects of Nb are
desired, it is preferable that Nb is contained in 0.005 % or more.
LO0451 On the other hand, if an excessive amount of V is contained, the
weldability of the steel deteriorates. In view of this, to define an upper limit,
the V content in the base material should be less than 0.5 %. If an excessive
amount of Nb is contained, the weldability of the steel deteriorates. In view
of this, the upper limit of the Nb content in the base material should be 2 %.
Only one of V and Nb may be contained, or the two may be combined and
contained. If the two are combined and contained, the preferable upper
limit of the total content is 2.2 %.
[0046] [Chemical Composition of First Layer of Weld Metal]
The weld metal of the welded joint according to the present
embodiment includes a first layer having a chemical composition with an Fe
content ranging from 10 to 40 %. Solidification segregation occurs in weld
metal. Although details of its mechanism are not clear, in a region where
the Fe content and Ni content in the weld metal are ill-balanced, the
solidified microstructure may facilitate metal dusting. The first layer of the
weld metal can be easily affected by the dilution ratio with respect to the
base material such that the Fe content and the Ni content can easily become
ill-balanced. A first layer with an Fe content ranging from 10 to 40 % has a
metal-dusting resistance substantially equal to that of the base material.
[0047] First layer of the weld metal (first-layer weld metal) means the first
layer defined in JIS B 0190. More specifically, first-layer weld metal means
the weld metal fabricated by the first pass of welding. The weld metal is
formed by the base material and welding material melting together and
solidifying. The greater the pass number, the less the weld metal is affected
by the composition of the base material (i.e. diluted by the base material)
such that the components of the welding material become dominant. In
view of this, the component ranges of only the first-layer weld metal, which
can be easily affected by the dilution by the base material, must be managed
and its performance must be ensured.
[0048] FIG. 1 is a schematic cross-sectional view of a welded joint 1, which is
one example welded joint. The welded joint 1 includes a base material 10
and a weld metal 20. In this example, the weld metal 20 includes a
first-layer weld metal 21 and second and subsequent layers 22.
[0049] The first layer of the weld metal preferably has the chemical
composition described below.
[0050] C: 0.01 to 0.15 %
Carbon (C) increases the strength of the weld metal. On the other
hand, if an excessive amount of C is contained, the toughness of the weld
metal decreases. In view of this, the C content in the first layer of the weld
metal should be in the range from 0.01 to 0.15 %. The preferable lower limit
of the C content in first layer of the weld metal is 0.03 %, and the more
preferable lower limit is 0.05 %. The preferable upper limit of the C content
in the first layer of the weld metal is 0.12 %, and the more preferable upper
limit is 0.10 %.
[0051] Si: 0.01 to 4.0 %
Silicon (Si) has a strong affinity for oxygen, and forms Si-based
oxidized scales under a protective oxidized-scale layer of Cr203, for example,
thereby blocking carburizing gases. On the other hand, if an excessive
amount of Si is contained, hot workability and weldability significantly
deteriorate. In view of this, the Si content in the first layer of the weld
metal should be in the range from 0.01 to 4.0 %. The preferable lower limit
of the Si content in the first layer of the weld metal is 0.08 %, and the more
preferable lower limit is 0.3 %. The preferable upper limit of the Si content
in the first layer of the weld metal is 3.5 %, and the more preferable upper
limit is 2.0 %.
Lo0521 Mn: 0.05 to 3.0 %
Manganese (Mn) deoxidizes steel. Mn also stabilizes the austenitic
phase. Mn further improves the workability and weldability of the steel.
On the other hand, if an excessive amount of Mn is contained, the protective
oxidized-scale layer's ability to block carburizing gases is impaired. In view
of this, the Mn content in the first layer of the weld metal should be in the
range from 0.05 to 3.0 %. The preferable lower limit of the Mn content in
the first layer of the weld metal is 0.1 %. The preferable upper limit of the
Mn content in the first layer of the weld metal is 2.5 %.
lo0531 P: up to 0.03 %
Phosphorus (P) is an impurity entering steel from a raw material or
the like during smelting. P deteriorates the hot workability and weldability
of the steel. P further increases the solidification crack sensitivity during
welding in the weld metal. In view of this, the P content in the first layer of
the weld metal should be up to 0.03 %. It is preferable to reduce the P
content in the first layer of the weld metal to the lowest level possible. The
preferable upper limit of the P content in the first layer of the weld metal is
0.025 %.
Lo0541 S: up to 0.015 %
Sulfur (S) is an impurity entering steel from a raw material or the
like during smelting. S deteriorates the hot workability and weldability of
the steel. In view of this, the S content in the first layer of the weld metal
should be up to 0.015 %. It is preferable to reduce the S content in the first
layer of the weld metal to the lowest level possible. The preferable upper
limit of the S content in the first layer of the weld metal is 0.005 %, and the
more preferable upper limit is 0.002 %.
[0055] Cr: more than 16.0 % and less than 32.0 %
Chromium (Cr), in a high-temperature use environment, bonds with
C that has entered the welded joint and thus delays the growth of carburized
layers, thereby increasing metal-dusting resistance. On the other hand, if
an excessive amount of Cr is contained, the toughness of the weld metal
decreases. In view of this, the Cr content in the first layer of the weld metal
should be more than 16.0 % and less than 32.0 %. The preferable lower
limit of the Cr content in the first layer of the weld metal is 18.0 %. To
define an upper limit, the Cr content in the first layer of the weld metal is
preferably less than 23.0 %, and more preferably less than 20.0 %.
Lo0561 Ni: not less than 20.0 %
Nickel (Ni) increases high-temperature strength and microstructure
stability. Ni further increases corrosion resistance when present together
with Cr. Ni also prevents metal dusting. In view of this, the Ni content in
the first layer of the weld metal should be not less than 20.0 %. The
preferable upper limit of the Ni content in the first layer of the weld metal is
80 %.
[0057] Cu: 0.03 to 5.0 %
Copper (Cu) prevents surface reaction between a carburizing gas and
a metal, thereby improving the metal-dusting resistance of the metal. Cu
also stabilizes the austenitic phase. On the other hand, if an excessive
amount of Cu is contained, the solidification cracking sensitivity during
welding increases. In view of this, the Cu content in the first layer of the
weld metal should be in the range from 0.03 to 5.0 %. The preferable lower
limit of the Cu content in the first layer of the weld metal is 0.04 %. The
preferable upper limit of the Cu content in the first layer of the weld metal is
4.5 %.
[0058] Al: up to 1.0 %
Aluminum (Al) deoxidizes steel. On the other hand, if an excessive
amount of A1 is contained, a large amount of nitride is formed, which
decreases the toughness of the steel. Further, if an excessive amount ofAl
is contained, welding operability deteriorates. In view of this, the A1
content in the first layer of the weld metal should be up to 1.0 %. The lower
the A1 content, the better it is. However, an excessively low amount of A1
results in insufficient deoxidization. Further, an excessively low amount of
A1 increases the index of cleanliness of the steel. Further, an excessively
low amount of A1 requires increased costs. Thus, the preferable lower limit
of the A1 content in the first layer of the weld metal is 0.003 %.
COO591 N: 0.005 to 0.20 %
Nitrogen (N) increases the high-temperature strength of steel. N
further increases the activity of C in the weld metal, thereby improving the
metal-dusting resistance of the weld metal. On the other hand, if an
excessive amount of N is contained, the ductility of the weld metal decreases.
In view of this, the N content in the first layer of the weld metal should be in
the range from 0.005 to 0.20 %. The preferable lower limit of the N content
in the first layer of the weld metal is 0.010 %. The preferable upper limit of
the N content in the first layer of the weld metal is 0.15 %.
[0060] 0: up to 0.02 %
Oxygen (0) is an impurity. If an excessive amount of 0 is contained,
the toughness and ductility of the weld metal decrease. In view of this, the
0 content in the first layer of the weld metal should be up to 0.02 %. The
preferable upper limit of the 0 content in the first layer of the weld metal is
0.01 %.
[0061] Ti: 0 to 0.5 %
Titanium (Ti) is an optional element. That is, Ti need not be
contained in the first layer of the weld metal. Ti bonds with Ni and
precipitates within grains in the form of fine particles of an intermetallic
compound, and is effective in providing a certain creep strength at high
temperatures. Thus, Ti may be contained as necessary. When Ti is to be
contained, the preferable lower limit is 0.005 %. However, if the Ti content
is high, particularly above 0.5 %, then, during use at high temperatures,
particles of the intermetallic compound phase rapidly become more coarse,
which excessively decreases creep strength and toughness and, during
welding, decreases the cleanliness of the weld metal, deteriorating
weldability. In view of this, the upper limit of the Ti content in the first
layer of the weld metal should be 0.5 %.
Lo0621 Mo: 0 to 8 %
Molybdenum (MO) is an optional element. That is, Mo need not be
contained in the first layer of the weld metal. Mo dissolves in a matrix and
contributes to increasing creep strength at high temperatures. Thus, Mo
may be contained as necessary. If Mo is to be contained, the preferable
lower limit is 0.05 %. However, if the Mo content is high, particularly above
8 %, then, the stability of the austenite decreases, which decreases creep
strength. In view of this, the upper limit of the Mo content in the first layer
of the weld metal should be 8 %.
[0063] Nb: 0 to 3 %
Niobium (Nb) is an optional element. That is, Nb need not be
contained in the first layer of the weld metal. Nb dissolves in a matrix or
precipitates in the form of a carbonitride, thereby contributing to improving
creep strength at high temperatures. Thus, Nb may be contained as
necessary. If Nb is to be contained, the preferable lower limit is 0.05 %.
However, if the Nb content is high, particularly above 3 %, then, a large
amount of carbonitride precipitates, which decreases the ductility of the steel.
In view of this, the upper limit of the Nb content in the first layer of the weld
metal should be 3 %.
[0064] Co: 0 to 15 %
Cobalt (Co) is an optional element. That is, Co need not be
contained in the first layer of the weld metal. Co stabilizes the austenitic
phase and increases creep strength. Thus, Co may be contained as
necessary. If Co is to be contained in the first layer of the weld metal, the
preferable lower limit is 0.01 %. On the other hand, if an excessive amount
of Co is to be contained, this requires increased costs. In view of this, the
upper limit of the Co content in the first layer of the weld metal is 15.0 %.
The preferable upper limit of the Co content in the first layer of the weld
metal is 14.5 %.
Coo651 The balance in the chemical composition of the first layer of the weld
metal is Fe and impurities. However, as discussed above, the first layer of
the weld metal has an Fe content ranging from 10 to 40 %.
[0066] [Chemical Composition of Welding material]
As discussed above, the welded joint according to the present
embodiment is obtained by welding using a welding material having a
chemical composition including Cr: 15.0 to 35.0 % and Ni: 40.0 to 70.0 %.
The welding material, together with a part of the base material, forms the
weld metal.
Coo671 Cr: 15.0 to 35.0 %
Chromium (Cr) forms a protective oxidized-scale layer of Crz03, for
example, and blocks carburizing gases. Thus, Cr gives carburization
resistance, metal-dusting resistance and caulking resistance to steel. On
the other hand, Cr bonds with C to form carbides, thereby decreasing creep
strength. This effect is particularly significant in a steel with high Si and
Cu contents. The Cr content in the welding material should be in the range
from 15.0 % to 35.0 %. The preferable lower limit of the Cr content in the
welding material is 16.0 %. The preferable upper limit of the Cr content in
the welding material is 33.0 %, and the more preferable upper limit is 30.0 %,
and the yet more preferable upper limit is 28.0 %.
[00681 Ni: 40.0 to 70.0 %
Nickel (Ni) stabilizes an austenitic phase. Ni also decreases the
enter rate of C entering the steel. Ni further increases the
high-temperature strength of the steel. To cause Ni in the welding material
to exhibit these effects, Ni must be contained in 40 % or more. On the other
hand, when an excessive amount of Ni is contained in the steel, the steel is
saturated in terms of Ni's effects. In view of this, the Ni content in the
welding material should be in the range from 40.0 to 70.0 %. The preferable
lower limit of the Ni content in the welding material is 45.0 %.
[00691 In addition to Cr and Ni, the welding material preferably has the
chemical composition described below.
[0070] C: 0.01 to 0.15 %
Carbon (C) increases the strength of the weld metal. On the other
hand, if an excessive amount of C is contained, the solidification cracking
sensitivity during welding increases. In view of this, the C content in the
welding material should be in the range from 0.01 to 0. 15 %. The
preferable lower limit of the C content in the welding material is 0.03 %, and
the more preferable lower limit is 0.05 %. The preferable upper limit of the
C content in the welding material is 0.12 %, and the more preferable upper
limit is 0.10 %.
[0071] Si: up to 4.0 %
Silicon (Si) deteriorates hot workability and weldability. In view of
this, the Si content in the welding material should be up to 4.0 %. The
preferable upper limit of the Si content in the welding material is 3.5 %, and
the more preferable upper limit is 2.0 %, and the yet more preferable upper
limit is 1.5 %. If a high corrosion resistance is needed in the welded joint,
the lower limit of the Si content in the welding material is preferably 0.5 %.
[0072] Mn: 0.01 to 3.5 %
Manganese (Mn) deoxidizes steel. Mn also stabilizes the austenitic
phase. Mn further improves the workability and weldability of the steel.
On the other hand, if an excessive amount of Mn is contained, hot
workability during production of the welding material is impaired. In view
of this, the Mn content in the welding material should be in the range from
0.05 to 3.5 %. The preferable lower limit of the Mn content in the welding
material is 0.05 %, and the more preferable lower limit is 0.10 %. The
preferable upper limit of the Mn content in the welding material is 3.0 %.
[0073] P: up to 0.03 %
Phosphorus (P) is an impurity entering steel from a raw material or
the like during smelting. P deteriorates the hot workability and weldability
of the steel. P further increases the solidification crack sensitivity during
welding in the weld metal. In view of this, the P content in the welding
material should be up to 0.03 %. It is preferable to reduce the P content in
the welding material to the lowest level possible. The preferable upper
limit of the P content in the welding material is 0.025 %.
[0074] S: up to 0.015 %
Sulfur (S) is an impurity entering steel from a raw material or the
like during smelting. S deteriorates the hot workability and weldability of
the steel. In view of this, the S content in the welding material should be up
to 0.015 %. It is preferable to reduce the S content in the welding material
to the lowest level possible. The preferable upper limit of the S content in
the welding material is 0.005 %, and the more preferable upper limit is
0.002 %.
LO0751 Cu: 0.01 to 4.0 %
Copper (Cu) prevents surface reaction between a carburizing gas and
a metal, thereby improving the metal-dusting resistance of the metal. Cu
also stabilizes the austenitic phase. On the other hand, if an excessive
amount of Cu is contained, the solidification cracking sensitivity during
welding increases. In view of this, the Cu content in the welding material
should be in the range from 0.01 to 4.0 %. The preferable lower limit of the
Cu content in the welding material is 0.03 %. The preferable upper limit of
the Cu content in the welding material is 3.5 %.
[0076] Al: 0 to 1.5 %
Aluminum (Al) is an optional element. That is, A1 need not be
contained in the welding material. A1 deoxidizes steel. On the other hand,
if an excessive amount ofAl is contained, a large amount of nitride is formed,
which decreases the toughness of the steel. Further, if an excessive amount
of A1 is contained, welding operability deteriorates. In view of this, the A1
content in the welding material should be up to 1.5 %, and preferably up to
1.0 %. However, an excessively low amount ofAl results in insufficient
deoxidization. Further, an excessively low amount of A1 increases the index
of cleanliness of the steel. Further, an excessively low amount of A1 requires
increased costs. Thus, the preferable lower limit of the A1 content in the
welding material is 0.003 %. If a high strength at high-temperature is
needed in the welded joint, it is preferable that the lower limit of the A1
content in the welding material is 0.15 %.
Lo0771 N: 0.005 to 0.1 %
Nitrogen (N) increases the high-temperature strength of steel. N
further increases the activity of C in the welding material, thereby
improving the metal-dusting resistance of the welding material. On the
other hand, if an excessive amount of N is contained, the hot workability
during production of the welding material deteriorates. In view of this, the
N content in the welding material should be in the range from 0.005 to 0.1 %.
The preferable lower limit of the N content in the welding material is
0.010 %. The preferable upper limit of the N content in the welding
material is 0.05 %.
[0078] 0: up to 0.03 %
Oxygen (0) is an impurity. If an excessive amount of 0 is contained,
hot workability during production of the welding material deteriorates.
Further, the toughness and ductility of the weld metal decrease. In view of
this, the 0 content in the welding material should be up to 0.03 %. The
preferable upper limit of the 0 content in the welding material is 0.02 %.
[0079] Ti: 0 to 1.0 %
Titanium (Ti) is an optional element. That is, Ti need not be
contained in the welding material. Ti bonds with Ni and precipitates within
grains in the form of fine particles of an intermetallic compound, and is
effective in providing a certain creep strength at high temperatures. Thus,
Ti may be contained as necessary. When Ti is to be contained, the
preferable lower limit is 0.15 %. However, if the Ti content is high,
particularly above 1.0 %, then, during use at high temperatures, particles of
the intermetallic compound phase rapidly become more coarse, which
excessively decreases creep strength and toughness and, during production
of an alloy, decreases cleanliness, deteriorating manufacturability. In view
of this, the upper limit of the Ti content in the welding material should be
1.0 %.
[0080] Mo: 0 to 15 %
Molybdenum (Mo) is an optional element. That is, Mo need not be
contained in the welding material. Mo dissolves in a matrix and contributes
to increasing creep strength at high temperatures. Thus, Mo may be
contained as necessary. If Mo is to be contained, the preferable lower limit
is 0.01 %, and the more preferable lower limit is 7 %. However, if the Mo
content is high, particularly above 15 %, then, the stability of the austenite
decreases, which decreases creep strength. In view of this, the upper limit
of the Mo content in the welding material should be 15 %.
[0081] Nb: 0 to 5 %
Niobium (Nb) is an optional element. That is, Nb need not be
contained in the welding material. Nb dissolves in a matrix or precipitates
in the form of a carbonitride, thereby contributing to improving creep
strength at high temperatures. Thus, Nb may be contained as necessary.
If Nb is to be contained, the preferable lower limit is 0.005 %. However, if
the Nb content is high, particularly above 5 %, then, a large amount of
carbonitride precipitates, which decreases the ductility of the steel. In view
of this, the upper limit of the Nb content in the welding material should be
5 %.
[00821 Co: 0 to 15 %
Cobalt (Co) is an optional element. That is, Co need not be
contained in the welding material. Co stabilizes the austenitic phase and
increases creep strength. Thus, Co may be contained as necessary. If Co is
to be contained in the welding material, the preferable lower limit is 0.01 %.
On the other hand, if an excessive amount of Co is to be contained, this
requires increased costs. In view of this, the upper limit of the Co content in
the welding material is 15.0 %. The preferable upper limit of the Co content
in the welding material is 14.5 %.
[0083] If, particularly, high-temperature strength is to be ensured, it is
preferable that the chemical composition of the welding material includes Al:
0.15 to 1.5 %; Ti: 0.15 to 1.0 %; and Mo: 7 to 15 %, and further includes at
least one of Nb: 0.1 to 5 % and Co: 0.1 to 15 %.
LO0841 Further, if, particularly, corrosion resistance is to be ensured, it is
preferable that the chemical composition of the welding material includes Si:
0.5 to 4.0 %.
[0085] [Manufacturing Method]
First, an example method of manufacturing the base material will be
described. A steel having the chemical composition for the base material
described above is smelted. The smelting may be performed by an electric
furnace, an Ar-O:! mixture bottom-blown decarburizing furnace (AOD
furnace), or a vacuum decarburizing furnace (VOD furnace). The smelted
steel is used to produce an ingot by the ingot-making method. Alternatively,
the smelted steel may be used to produce a slab by continuous casting.
[0086] The ingot or slab is used to produce a base material. The base
material may be a steel plate or steel pipe, for example. The steel plate may
be produced by subjecting the ingot or slab to hot working such as hot forging
or hot rolling, for example. The steel pipe may be produced by, for example,
subjecting the ingot or slab to hot working to produce a round billet, and
subjecting the round billet to hot working such as piercing-rolling, hot
extrusion or hot forging. Alternatively, the steel pipe may be produced by
bending a steel plate to form an open pipe and welding those edges of the
open pipe that extend in the longitudinal direction.
[0087] Well-known heat treatment is performed on the base material, as
necessary.
[00881 Next, an example method of manufacturing the welding material will
be described. A steel having the chemical composition for the welding
material described above is smelted. The smelted steel is cast into an ingot.
The ingot is hot-worked to produce a welding material. The welding
material may be in the shape of a rod or block.
[00891 Again, a well-known heat treatment is performed on the welding
material, as necessary.
[0090] The welding material described above is used to weld the base
material described above. This provides a welded joint. The welding
method may be, for example, TIG welding, MIG welding, MAG welding, or
submerge welding. During welding, a part of the base material and the
welding material melt together and solidify to form a weld metal.
[00911 At this time, the dilution rate with respect to the base material is
adjusted such that the Fe content in the first layer of the weld metal is in the
range from 10 to 40 %. More specifically, the heat input during welding of
the first layer and the rate at which the welding material is fed are adjusted
depending on the chemical composition of the base material and the chemical
composition of the welding material. As the heat input during welding of
the first layer increases, the dilution ratio with respect to the base material
increases and the Fe content in the first layer of the weld metal increases.
As the rate at which the welding material is fed increases, the dilution ratio
with respect to the base material decreases and the Fe content in the first
layer of the weld metal decreases.
EXAMPLES
[0092] The present invention will be described in more detail using
Examples. The present invention is not limited to these Examples.
[00931 Steels labeled with characters A and B having the chemical
compositions shown in Table 1 were melted in a laboratory to produce ingots.
The ingots were subjected to hot forging, cold rolling, heat treatment and
machining to produce steel pipes (base materials) with an outer diameter of
25.4 mm, a thickness of 3.3 mm and a length of 60 mm.

We claim:
1. A welded joint obtained by welding using a welding material
having a chemical composition including, in mass%, Cr: 15.0 to 35.0 %; and
Ni: 40.0 to 70.0 %, comprising:
a base material having a chemical composition of, in mass%:
C: 0.03 to 0.075 %;
Si: 0.6 to 2.0 %;
Mn: 0.05 to 2.5 %;
P: up to 0.04 %;
S: up to 0.015 %;
Cr: more than 16.0 % and less than 23.0 %;
Ni: not less than 20.0 % and less than 30.0 %;
Cu: 0.5 to 10.0 %;
Mo: less than 1 %;
Al: up to 0.15 %;
N: 0.005 to 0.20 %;
0: up to 0.02 %;
Ca: 0 to 0.1 %;
REM: 0 to 0.15 %;
V: not less than 0 % and less than 0.5 %; and
Nb: 0 to 2 %,
a balance being Fe and impurities; and
a first-layer weld metal having a chemical composition including, in
mass%, an Fe content ranging from 10 to 40 %.
2. The welded joint according to claim 1, wherein the base material
has a chemical composition including, in mass%, one or two selected from the
group consisting of:
Ca: 0.005 to 0.1 %; and
REM: 0.005 to 0.15 %.
3. The welded joint according to claim 1 or 2, wherein the base
material has a chemical composition including, in mass%, one or two selected
from the group consisting of:
V: not less than 0.002 % and less than 0.5 %; and
Nb: 0.005 to 0.15 %.
4. The welded joint according to any one of claims 1 to 3, wherein
the first-layer weld metal has a chemical composition of, in mass%:
C: 0.01 to 0.15 %;
Si: 0.01 to 4.0 %;
Mn: 0.05 to 3.0 %;
P: up to 0.03 %;
S: up to 0.015 %;
Cr: more than 16.0 % and less than 32.0 %;
Ni: not less than 20.0 %;
Cu: 0.03 to 5.0 %;
Al: up to 1.0 %;
N: 0.005 to 0.2 %;
0 : up to 0.02 %;
Ti: 0 to 0.5 %;
Mo: 0 to 8 %;
Nb: 0 to 3 %;
Co: 0 to 15 %; and
Fe: 10 to 40 %,
a balance being impurities.
5. The welded joint according to any one of claims 1 to 4, wherein
the welding material has a chemical composition of, in mass%:
C: 0.01 to 0.15 %;
Si: up to 4.0 %;
Mn: 0.01 to 3.5 %;
P: up to 0.03 %;
S: up to 0.015 %;
Cr: 15.0 to 35.0 %;
Ni: 40.0 to 70.0 %;
Cu: 0.01 to 4.0 %;
Al: 0 to 1.5 %;
N: 0.005 to 0.1 %;
0: up to 0.03 %;
Ti: 0 to 1.0 %;
Mo: 0 to 15 %;
Nb: 0 to 5 %; and
Co: 0 to 15 %,
a balance being Fe and impurities.
6. The welded joint according to claim 5, wherein the welding
material has a chemical composition including, in mass%:
Al: 0.15 to 1.5 %;
Ti: 0.15 to 1.0 %; and
Mo: 7 to 15 %,
the chemical composition of the welding material further including at
least one of Nb: 0.1 to 5 % and Co: 0.1 to 15 %.
7. The welded joint according to claim 5, wherein the welding
material has a chemical composition including, in mass%:
Si: 0.5 to 4.0 %.