DESCRIPTION
AUSTENITIC STEEL WELDED JOINT
TECHNICAL FIELD
[oooll
The present invention relates to an austenitic steel welded joint.
Specifically, the present invention relates to a welded joint having not only
high strength required in high-pressure gas piping, but also high strength and
excellent hydrogen embrittlement resistance that are properties required in
high-pressure hydrogen gas piping at the same time.
BACKGROUND ART
[00021
Recently, various studies have enthusiastically been conducted on the
practical use of transportation equipment that utilizes hydrogen, natural gas
and the like as energy. To facilitate the practical use, it is required at the
same time to fulfill infrastructure of a usage environment for storing and
transporting such gases at high pressure, so that high-strength materials for
use in this usage environment, having tensile strength of more than 800 MPa,
have simultaneously been developed as well as planning of applications of
these materials.
[OOO~]
In such a background, as material for use in the above environment,
Patent Documents 1 to 3 suggest, for example, austenitic stainless steel having
high content of Mn for the purpose of enhancing solubility of N, and also
containing V or a combination of V and Nb so as to utilize solid-solution
strengthening of N and precipitation strengthening of Nitride, thereby
attaining high strengthening.
[0004]
In the case of using such high-strength austenitic steel containing a
large amount of N as a structure, assembly by welding is necessary, and
welded portions are required to have strength equal to that of base metal in
the light of a use performance thereof. For this purpose, Patent Documents 3
to 5 suggest welding materials (weld metal) having tensile strength of more
than 800 MPa by positively utilizing Al, Ti, and Nb, for example.
[00051
Unfortunately, both the welding materials and the weld metal produced
by using these welding materials require postweld heat treatment for
attaining high strength. Meanwhile, taking account of a real size of a large
structure, postweld heat treatment for such long hours greatly limits its
feasibility, and may also cause extreme increase in production cost.
LIST OF PRIOR ART DOCUMENTS
PATENT DOCUMENT
[0006]
Patent Document 1: WO 20041083476
Patent Document 2: WO 20041083477
Patent Document 3: WO 20041110695
Patent Document 4: JP5- 192785A
Patent Document 5: JP2010-227949A
DISCLOSURE OF THE INVENTION
PROBLEMS TO BE SOLVED BY THE INVENTION
[00071
An object of the present invention, which has been made in view of the
above situation, is to provide a welded joint having both high strength and
excellent hydrogen embrittlement resistance which are properties required in
high-pressure gas piping, particularly in high-pressure hydrogen gas piping
without the postweld heat treatment.
MEANS FOR SOLVING THE PROBLEMS
[0008l
In order to solve the aforementioned problems, the present inventors
have conducted careful studies. As a result, first, in order to attain high
strength in the weld metal without the postweld heat treatment, it has been
found that it is effective to make the most of solid-solution strengthening by
using N.
[00091
Further careful studies have been conducted on how to secure strength,
without the postweld heat treatment, in the weld metal equal to strength in
the base metal consisting of, by mass%, C: 0.1% or less, Si: 0.8% or less, Mn:
2.5 to 5.5%, Ni: 8 to 15%, Cr: 19 to 25%, Al: less than 0.05%, and N: 0.15 to
0.45%.
[oo 101
In addition, the hydrogen embrittlement susceptibility of the obtained
welded portions was evaluated, and careful studies were conducted so as to at
least attain the hydrogen embrittlement resistance equal to that of the base
metal.
[00111
As a result, the following findings (a) to (c) were uncovered.
loo 121
(a) It is possible to secure strength equal to that in the base metal by
containing N of 0.15% or more by mass% in the weld metal within a chemical
composition range of Cr: 18 to 24%, and Ni: 8 to 15% by mass%.
COO 131
(b) If a large amount of ferrite is contained in the weld metal, the ferrite
initiates hydrogen embrittlement cracks, and the cracks combine together to
propagate, thereby deteriorating hydrogen embrittlement resistance of the
weld metal. By adjusting the chemical composition, however, such that the
amount of the ferrite in the weld metal becomes 20% or less in area ratio, it is
possible to attain excellent hydrogen embrittlement resistance.
[00141
(c) If an austenite phase in the weld metal is unstable, the austenite
phase transforms to martensite caused by welding residual strain and
machining after the welding, thereby the hydrogen embrittlement
susceptibility becomes high. By adjusting the chemical composition,
specifically, if the chemical composition of the weld metal is adjusted so as to
satisfy
413 - 462(C + N) - 9.2Si - 8.1Mn - 13.7Cr - 9.5Ni - 18.5Mo I -70,
where a symbol of each element in the above Formula denotes a content
(mass%) of the element, it is possible to enhance the stability of the austenite,
and also to attain excellent hydrogen embrittlement resistance.
[00 151
It is also found that in order to contain N of 0.15% or more by mass% in
the weld metal within the composition range of Cr: 18 to 24%, and Ni: 8 to 15%
by mass% according to the above (a), it is preferable to carry out the following
methods to <3>.
[OO1 61
N scatters from the surface of a molten pool during the welding, so
that the amount of N remaining in the weld metal after the welding is
decreased. Hence, welding heat input is controlled and/or a groove shape is
selected so as to reduce the surface area of the molten pool during the welding.
[00171
<2> A gas made by mixing Np of 0 to 50% in volume% in Ar is used for a
shield gas so as to reduce scatters of N from the surface of the molten pool
during the welding. Note that the mixed N2 of 0% in volume% represents
that the Ar gas is used alone.
[0018]
<3> A gas made by mixing N2 of 0 to 100% in volume% in Ar is used as a
back shield gas so as to prevent scatters of N from the surface of the molten
pool on the backside during the fwst layer welding. Note that mixed Na of 0%
in volume% represents that the Ar gas is used alone. The mixed N2 of 100%
in volume% represents that the N2 gas is used alone.
[00191
The present invention has been accomplished based on the above
findings, and its gist is described by the austenitic steel welded joint set forth
in the following constitutions (1) and (2).
[00201
(1) An austenitic steel welded joint produced by welding base metal with
a gas tungsten arc welding process which uses welding material, wherein
the base metal has a chemical composition comprising: by mass%, C:
0.1% or less; Si: 0.8% or less; Mn: 2.5 to 5.5%; Ni: 8 to 15%; Cr: 19 to 25%; Al:
less than 0.05%; and N: 0.15 to 0.45%, the balance being Fe and impurities
that contain 0: 0.02% or less, P: 0.05% or less, and S: 0.03% or less, and
the welding material has a chemical composition comprising: by mass%,
C: 0.1% or less; Si: 0.8% or less; Mn: 1.5 to 4.5%; Ni: 8 to 15%; Cr: 18 to 24%;
Al: less than 0.05%; and N: 0.15 to 0.35%, the balance being Fe and impurities
that contain 0: 0.02% or less, P: 0.05% or less, and S: 0.03% or less,
wherein
weld metal has a chemical composition comprising: by mass%, C: 0.1% or
less; Si: 0.8% or less; Mn: 1.5 to 5.5%; Ni: 8 to 15%; Cr: 18 to 24%; Al: less than
0.05%; and N: 0.15 to 0.35%, the balance being Fe and impurities that contain
0: 0.02% or less, P: 0.05% or less, and S: 0.03% or less,
the chemical composition of the weld metal satisfies Formula (I),
and
an amount of ferrite in the weld metal is 20% or less in area ratio.
413 - 462(C + N) - 9.2% - 8.1Mn - 13.7Cr - 9.5Ni - 18.5Mo I -70...(1)
where a symbol of each element in Formula (1) denotes a content
(mass%) of the element.
[002 11
(2) The austenitic steel welded joint set forth in the above (I), wherein
the chemical composition of any one or more of the base metal, the welding
material, and the weld metal comprise one or more of, by mass%, V: 0.5% or
less, Nb: 0.5% or less, and Mo: 4.5% or less, in lieu of part of Fe.
[00221
"Impurities" of "Fe and impurities" as the balance denote minerals or
scraps as row materials, or substances mixed through an environment of a
manufacturing process and the like during industrial production of steel
materials.
ADVANTAGEOUS EFFECTS OF THE INVENTION
[00231
According to the present invention, it is possible to provide a welded
joint having both high strength and excellent hydrogen embrittlement
@ resistance which are properties required in high-pressure gas piping,
particularly in high-pressure hydrogen gas piping without the postweld heat
treatment.
BRIEF DESCRIPTION OF THE DRAWING
[00241
[Figure 11 Figure 1 is a drawing explaining a grooving shape.
MODE FOR CARRYING OUT THE INVENTION
[00251
The reason for limiting chemical compositions of base metal(s), welding
material, and weld metal, as well as a micro-structure of the weld metal in the
austenitic steel welded joint of the present invention is as follows.
[00261
In the following description, a symbol "%" of content of each element
denotes "mass %", hereinafter.
[00271
(A) Chemical compositions of base metal, welding material, and weld
metal
C: 0.1% or less (base metal, welding material, and weld metal)
C is an element effective for stabilizing austenite. C generates carbide,
however, at grain boundaries with heating during the welding, and
deteriorates corrosion resistance as well as causes deterioration of ductility.
Accordingly, the upper limit of the C content is defined to be 0.1%. The more
preferable upper limit of the C content is 0.08%. It is unnecessary to
specifically define the lower limit of the C content, but extreme decrease in the
C content causes significant increase in production cost. Accordingly, the
preferable lower limit of the C content is 0.002%.
[0028]
Si: 0.8% or less (base metal, welding material, and weld metal)
Si is an element effective as a deoxidizer, and effective for enhancement
of corrosion resistance. The excessive Si content, however, causes
deterioration of stability of the austenite, and deterioration of ductility, and
the excessive Si content in the weld metal causes segregation at columnar
crystal boundaries during the solidification, which decreases the fusing point
of the liquid phase, and increases solidification cracking susceptibility.
Accordingly, the Si content is defined to be 0.8% or less. The Si content is
preferably defined to be 0.6% or less. It is unnecessary to specifically define
the lower limit of the Si content, but extreme decrease in the Si content
hinders the sufficient deoxidation effect, and deteriorates cleanliness of steel,
as well as increases the production cost. Accordingly, the preferable lower
limit of the Si content is 0.01%.
[00291
Mn: 2.5 to 5.5% (base metal), 1.5 to 4.5% (welding material), 1.5 to 5.5%
(weld metal)
Mn is an element effective as a deoxidizer, and effective for stabilizing
the austenite. Mn increases solubility of N in molten metal during the
production of the base metal and during the welding, thereby indirectly
contributing to enhancement of strength. In order to sufficiently attain this
strength enhancing effect, the Mn content should be 2.5% or more in the base
metal. Meanwhile, the solidification speed of the weld metal is faster
compared to that of the base metal during its production, and the weld metal
has smaller decrease in N during the solidification, so that Mn is contained
1.5% or more in the weld metal. The Mn content in the welding material
which is to be melted during welding, and entire of which becomes the weld
metal is the same as that in the weld metal. The preferable lower limit of the
Mn content is 2.7% in the base metal, and 1.7% in the welding material and in
the weld metal. On the other hand, the excessive Mn content causes
deterioration of ductility. Hence, the upper limit of the Mn content in the
base metal and in the weld metal should be 5.5%. In order to secure the
workability of welding material, the upper limit of the Mn content in the
welding material should be more strictly restricted compared to the base metal
and the weld metal, so that the upper limit of the Mn content is 4,5%. The
preferable upper limit of the Mn content is 5.0% in the base metal and in the
weld metal, and 4.0% in the welding material.
[00301
Ni: 8 to 15% (base metal, welding material, and weld metal)
Ni is an essential element to obtain stable austenite, and the Ni content
of 8% or more is required for sufficiently attaining this effect. Ni is, however,
an expensive element, so that large content of Ni causes increase in cost, and
decreases solubility of N in the molten metal during the production of the base
metal, and during the welding. Accordingly, the upper limit of the Ni content
is defined to be 15%. The preferable lower limit of the Ni content is 8.2%, and
the preferable upper limit thereof is 14.5%.
[003 11
Cr: 19 to 25% (base metal), 18 to 24% (welding material and weld metal)
Cr is an essential element for securing corrosion resistance in the usage
environment. Cr is effective to increase solubility of N in the molten metal
during the production of the base metal and during the welding, and the Cr
content of 19% or more in the base metal is necessary for sufficiently attaining
this effect. Meanwhile, the solidification speed of the weld metal is faster
compared to that of the base metal during its production, and the weld metal
has smaller decrease in N during the solidification process, so that the Cr
content is defined to be 18% or more. The Cr content in the welding material
;e which is to be melted during the welding, and entire of which becomes the
weld metal is the same as that in the weld metal. The preferable lower limit
of the Cr content is 19.2 % in the base metal, and 18.2% in the welding
material and the weld metal. On the other hand, the excessive Cr content
causes unstable austenite, and may cause embrittlement depending on the
type of the gas environment. Accordingly, it is necessary to define the upper
limit of the Cr content in the base metal to be 25%. Since the micro-structure
of the weld metal becomes unstable due to solidification segregation, the Cr
content in the weld metal should be more strictly controlled, and thus the
upper limit of the Cr content in the weld metal is defined to be 24%. The
upper limit of the Cr content in the welding material which is to be melted
during the welding, and entire of which becomes the weld metal is the same as
that in the weld metal. The preferable upper limit of the Cr content is 24.5%
in the base metal, and 23.5% in the welding material and the weld metal.
[00321
Al: less than 0.05% (base metal, welding material, and weld metal)
A1 is contained as a deoxidizer as similar to Si and Mn. The excessive
Al content, however, generates a large amount of nitride, and causes
deterioration of ductility. Hence, the A1 content is defined to be less than
0.05%. The A1 content is preferably 0.04% or less. It is unnecessary to
specifically define the lower limit of the Al content, but extreme decrease in
the A1 content hinders sufficient attainment of the deoxidation effect, so that
the index of cleanliness of steel becomes increased, resulting in deterioration of
cleanliness and increase in production cost. Accordingly, the preferable lower
limit of the A1 content is 0.003%.
[00331
N: 0.15 to 0.45% (base metal), N: 0.15 to 0.35% (welding material and
weld metal)
N is an element soluble in the matrix and forms fine nitride, and thus N
is an essential element for ensuring the high strength. The N content should
be 0.15% or more so as to sufficiently attain this effect. In the base metal,
however, the N content of more than 0.45% causes deterioration of hot
workability during the production of the base metal. Hence, the upper limit
of the N content in the base metal should be 0.45%. In the weld metal, N
which is contained more than 0.35% cannot dissolve in a molten pool during
the welding, and causes blowholes and/or pits. Hence, the upper limit of the
N content in the weld metal should be 0.35%. In the welding material which
is to be melted during the welding, and entire of which becomes the weld metal,
the upper limit of the N content should be 0.35%, for the same reason as that
of the weld metal. The preferable lower limit of the N content in the base
metal is 0.16%, and the preferable upper limit thereof is 0.42%. In the
welding material and the weld metal, the preferable lower limit of the N
content is 0.16%, and the preferable upper limit thereof is 0.32%.
[00341
The base metal, the welding material, and the weld metal of the
austenitic steel welded joint of the present invention have a chemical
composition in which the aforementioned elements from C to N are contained,
the balance being Fe and impurities, and the contents of 0, P, and S as the
impurities are limited in the following ranges.
[00351
0: 0.02% or less (base metal, welding material, and weld metal)
0 exists as an impurity, and a large amount of 0 causes the
deterioration of hot workability of the base metal and the welding material
during the production of them, and also causes the deterioration of toughness
and ductility of the weld metal. Accordingly, the 0 content should be 0.02%
or less. The preferable upper limit of the 0 content is 0.01%.
LO0361
P: 0.05% or less (base metal, welding material, and weld metal)
P is contained as an impurity, and a large amount of P in the base metal
and in the welding material hinders the hot workability during the production
of them. In the weld metal, a large amount of P decreases the fusing point of
the liquid phase at the time of solidification and causes increase in
solidification cracking susceptibility. Accordingly, the P content is preferably
reduced as small as possible, but excessive reduction of the P content causes
increase in production cost of the steel; therefore the P content is defined to be
0.05% or less. The preferable P content is 0.03% or less.
[00371
S: 0.03% or less (base metal, welding material, and weld metal)
As similar to P, S is contained as an impurity, and a large amount of S
in the base metal and in the welding material hinders the hot workability
during the production of them. In the weld metal, a large amount of S
decreases the fusing point of the liquid phase at the time of solidification and
causes increase in solidification cracking susceptibility in. Accordingly, as
similar to P, the S content is preferably reduced as small as possible, but
excessive reduction of the S content causes increase in production cost of the
steel; therefore the upper limit of the S content is defined to be 0.03%. The
preferable S content is 0.01% or less.
[00381
The base metal, the welding material, and the weld metal of the
austenitic steel welded joint of the present invention may contain one or more
elements of V, Nb, and Mo in lieu of part of Fe in "Fe and the impurities" as
the balance.
[00391
Hereinafter, operational advantages of V, Nb, and Mo that are optional
elements will be described along with the reason for limiting the content of
each element.
[00401
V: 0.5% or less
V dissolves in the matrix or precipitates as carbide, and is an effective
element for enhancing the strength, and therefore may be contained. The
excessive V content of more than 0.5%, however, allows a large amount of
carbide to precipitate, which causes the deterioration of ductility. Accordingly,
the V content is defined to be 0.5% or less if contained. The preferable upper
limit of the V content is 0.4% if contained.
[00411
In order to stably attain the above effect of V, the V content is preferably
0.01% or more.
[00421
Nb: 0.5% or less
As similar to V, Nb may be contained because Nb dissolves in the matrix
or precipitates carbo-nitride, and is an effective element for enhancing the
strength. The excessive Nb content of more than 0.5%, however, allows a
large amount of carbo-nitride to precipitate, which causes the deterioration of
ductility. Accordingly, the Nb content is defined to be 0.5% or less if
contained. The preferable upper limit of the Nb content is 0.4% if contained.
[00431
In order to stably attain the above effect of Nb, the Nb content is
preferably 0.01% or more.
[00441
Mo: 4.5% or less
Mo is an effective element for enhancing the strength. Mo is also an
effective element for enhancing .the corrosion resistance in the usage
environment. Accordingly, Mo may be contained. Mo is, however, a very
expensive element, and the excessive Mo content of more than 4.5% causes
unstable austenite. Accordingly, the content of Mo is defined to be 4.5% or
less if contained. The preferable upper limit of the Mo content is 4% if
contained.
[00451
In order to stably attain the above effect of Mo, the Mo content is
preferably 0.1% or more.
Lo0461
Only any one of V, Nb, and Mo, or more than one of these elements in
combination may be contained. In the case of containing these elements in
combination, the total amount of these elements may be 5.5% in the case of
using the respective upper limits of the contents of V, Nb, and Mo, but this is
preferable 5.0% or less.
[00471
In addition, the chemical composition of the weld metal of the austenitic
steel welded joint of the present invention should satisfy the following Formula
(1).
[00481
413 - 462(C + N) - 9.2Si - 8.1Mn - 13.7Cr - 9.5Ni - 18.5Mo I -70...(1),
where a symbol of each element in Formula (1) denotes a content (mass%) of
the element.
[00491
The austenite in the weld metal is unstable because the weld metal has
a rapidly solidified micro-structure, so that the austenite phase transforms to
martensite caused by welding residual strain and machining after the welding,
and the hydrogen embrittlement resistance is deteriorated. However, the
austenite is stabilized when Formula (1) is satisfied, so that it is possible to
prevent the martensitizing caused by the welding residual strain and the
machining after the welding. The left term of Formula (1) is preferably -80 or
less, and it is more preferable if this is as small as possible.
[OO~O]
(B) Micro-structure of weld metal
The micro-structure of the weld metal is a rapidly solidified microstructure.
The ferrite generated at a high temperature in the solidifying
process remains even at a room temperature in the weld metal, which is
different from the base metal produced through solid-solution heat treatment.
The ferrite becomes embrittled in the hydrogen environment and initiates
fracture. Then, if it exists continuously, it combines together and propagate,
thereby deteriorating the hydrogen embrittlement resistance of the weld metal.
The amount of the ferrite of 20% or less in area ratio, however, hardly causes
the above problems. Accordingly, the amount of the ferrite in the weld metal
is defined to be 20% or less in area ratio. The amount of the ferrite in the
weld metal is preferably 15% or less in area ratio. The lower limit of the
amount of the ferrite in the weld metal may be 0% in area ratio, that is, the
weld metal may be fully austenite.
[005 11
The amount of the ferrite in the weld metal in area ratio becomes
increased if ferrite generating elements represented by Cr and Si become
increased relative to austenite generating elements represented by Ni, Mn,
and C in the weld metal. Accordingly, the above amount of the ferrite in the
weld metal in area ratio can be attained by adjusting such that the chemical
composition of the weld metal satisfies the aforementioned range, and the
(r excessive amount of the ferrite generating elements is not contained relative to
the austenite generating elements.
[00521
In order to produce the weld metal including the above chemical
composition and the amount of the ferrite by using the base metal and the
welding material having the aforementioned chemical composition, it is
preferable to make the surface area of the molten pool during the welding as
small as possible with the gas tungsten arc welding process.
[00531
In the welding with the gas tungsten arc welding process, it is preferable
to control the welding condition such that a crater surface area after the
completion of welding becomes 120 mm2 or less.
[00541
In the welding with the gas tungsten arc welding process, it is more
preferable to use a gas made by mixing an N2 gas in an Ar gas for a shield gas
and a back shield gas. The reason for this is as follows.
[00551
Compared to a gas metal arc welding, the gas tungsten arc welding
readily attains a defect-free, high-quality welded joint. In spite of this, this
welding process causes scatters of N from the surface of the molten pool during
the welding, so that the amount of N remaining in the weld metal after the
welding is decreased, resulting in deterioration of the strength. In order to
prevent this, it is effective to decrease the surface area of the molten pool
during the welding so as to reduce a region where N scatters. Specifically, the
surface area of the molten pool during the welding corresponds to the crater
surface area of each layer after the completion of welding; therefore, it is
preferable to adjust the welding condition such that the crater surface area
becomes 120 mm2 or less.
[00561
It is effective to mix the N2 gas in the shield gas or the backshield gas so
as to increase a partial pressure of N for the sake of reducing scatters of N
from the surface of the molten pool, and it nitrides the surface of the solidified
weld metal, thereby slightly contributing to strengthen the weld metal. If N2
in the shield gas or in the backshield gas becomes more than 50% in the
volume%, N which is dissolved in the weld metal at a high temperature cannot
be dissolved any more in the weld metal during solidification, and becomes
turned into N2; thus blowholes andlor pits may be generated in some cases.
Accordingly, it is preferable to define the upper limit of the N2 gas in the shield
gas or in the backshield gas to be 50% in volume%.
E00571
Hereinafter, the present invention will be more specifically described by
using examples, but the present invention is not limited to the examples.
EXAMPLES
[0058l
Ingots produced by laboratory dissolving and casting materials indicated
by reference symbols A to C having chemical compositions shown in Table 1
were subjected to hot forging, hot rolling, heat treatment, and machining so as
to produce steel plates of 3 mm in thickness, 50 mm in width, and 100 mm in
length for the welding base metal.
[00591
Ingots produced by laboratory dissolving and casting materials indicated
by reference symbols V to Z having chemical compositions shown in Table 2
were subjected to hot forging, hot rolling, and machining so as to produce
welding wires (welding material) of 1.2 mm in outer diameter.
[0060]
Each of the above produced steel plates for the welding base metal was
subjected to groove machining in its longitudinal direction as shown in Fig. 1,
and thereafter, a butt welding was carried out in combination of the base
metal and the welding material under the welding conditions as shown in
Table 3 with the gas tungsten arc welding process. The welding was
completed with two-layer welding, and weaving was also carried out on certain
joints during the welding.
[006 11
[Table 11
Table 1
[00621
[Table 21
Symbol
A
B
C
Table 2
Chemical composition (mass%) Balance: Fe and impurities
[00631
[Table 31
Symbol
V
W
X
Y
Z
C
0.031
0.030
0.068
An asterisk "*" represents deviation from the condition specified by the present invention.
Chemical composition (mass%) Balance: Fe and impurities
Si
0.41
0.42
0.39
V
0.22 -
C
0.035
0.012
0.078
0.013
0.018
A1
0.012
0.009
0.010
Nb
0.23
0.31
-
Mn
4.52
3.57
5.13
Si
0.38
0.11
0.20
0.41
0.35
N
0.32
0.40
0.42
0
0.007
0.005
0.005
P
0.017
0.017
0.015
Mn
1.80
1.50
2.20
1.90
1.60
S
0.001
0.001
0.001
S
0.004
0.006
0.005
0.008
0.005
P
0.020
0.020
0.040
0.015
0.022
Ni
12.2
9.8
8.8
Ni
8.4
8.3
9.7
13.8
8.1
Cr
21.9
20.5
24.1
Cr
*24.5
19.0
21.4
19.4
*24.9
Mo
2.1
2.5
-
Mo
3.2 -
-
3.8
3.5
V
-
-
0.01 -
Nb
-
-
0.05
-
-
A1
0.006
0.005
0.008
0.006
0.007
N
*0.37
0.15
0.16
0.18
0.15
0
0.006
0.006
0.005
0.003
0.006
Table 3
present invention.
[00641
The crater surface area of each layer at the time of completing the
welding was measured. The crater at the time of completing the welding
corresponds to the surface of the molten pool during the welding, and thus this
crater can be considered as the surface area of the molten pool during the
welding.
[00651
A cross sectional microscopic test specimen was collected from each
welded joint where the layer welding had been completed, and observation was
conducted with an optical microscope on each specimen for the sake of
investigating existence of weld defects.
[0066]
As a result of the observation with the optical microscope, the specimens
having no weld defects such as blowholes were determined as "acceptable".
For each welded joint that has been determined to be "acceptable", chemical
analysis of the weld metal was conducted, the amount of the ferrite in area
ratio in a central portion of the weld metal was measured, and a plate-like
tensile test specimen having the weld metal in its central parallel portion was
collected from each welded joint so as to conduct a tensile test at a normal
temperature.
[00671
In this tensile test, the specimens ruptured in their base metals were
determined to be "acceptable", and from each welded joint determined to be
"acceptable", a plate-like low strain rate tensile test specimen having the weld
metal in its parallel portion was collected, and each test specimen was
subjected to a low strain rate tensile test in the atmosphere, and in the highpressure
hydrogen environment of 85 MPa. The strain rate was 3 x 10-5/s,
and such a welded joint was determined to be "acceptable" in the low strain
rate tensile test that had a ratio of 90% or more between the reduction of area
after the rupture in the high-pressure hydrogen environment and the
reduction of area after the rupture in the atmosphere.
[OO~SI
Table 4 shows the examination result regarding the crater surface area
corresponding to the surface area of the molten pool during the welding, and
weld defects therein. Table 5 shows the chemical analysis result of the weld
OI metal, and the measurement result of the amount (area ratio) of the ferrite in
the weld metal. In addition, Table 6 shows the respective results of the
tensile test and the low strain rate tensile test.
[00691
In Table 4, the symbol "-" in the columns of "Crater Surface Area of First
Layer" and "Crater Surface Area of Second Layer" indicates that no
measurement was conducted because of occurrence of countless blowholes.
[00701
In Table 5, the symbol "- -" for the test reference numerals 517 and 526
indicates that no chemical analysis of the weld metal and no measurement of
the amount of the ferrite in the weld metal were conducted because of
confirmation of the weld defects.
[007 11
In the column "Tensile test" of Table 6, the symbol "0" indicates that the
specimen of interest was determined to be "acceptable" because rupture
occurred in the base metal, and the symbol "x" indicates that rupture occurred
in the weld metal. The symbol "-" indicates that no tensile test was conducted
because of confirmation of the weld defect.
[00721
Similarly, in Table 6, in the column "Low Strain Rate Tensile Test", the
symbol "0" indicates that the specimen of interest was determined to be
"acceptable" because the ratio of the reduction of area after the rupture in the
high-pressure hydrogen environment relative to the reduction of area after the
rupture in the atmosphere was 90% or more, and the symbol "x" indicates that
the above ratio was less than 90%. The symbol "-" indicates that no low
strain rate tensile test was conducted because of confirmation of the weld
defect (test reference numerals 517 and 526), or because of occurrence of the
.\ rupture in the weld metal in the tensile test (test reference numerals J19 and
J20).
[00731
[Table 41
Table 4
Test
reference
J 1
J 2
J3
J 4
J 5
J 6
J7
J 8
J 9
J10
J11
J12
J13
J14
J15
J16
J17
J 18
J 19
J20
J21
J 22
J23
J 24
J25
J26
J27
The symbol
layer" and "Crater surface area of second layer" indicates that
no measurement was conducted because of occurrence of
countless blowholes.
Crater surface
area of first
layer
(mm2)
8 2
84
81
82
9 1
89
92
9 0
7 7
7 8
8 0
76
80
82
82
84
-
8 3
126
134
7 5
84
8 1
82
83 -
87
"-" in the
Crater surface
area of second
layer
(mm2)
82
8 8
8 2
8 6
95
9 1
95
9 1
88
82
84
7 9
8 2
8 3
84
8 6
-
88
132
145
7 9
9 2
84
8 7
9 0 -
94
columns of "Crater
Weld defect
No defect
No defect
No defect
No defect
No defect
No defect
No defect
No defect
No defect
No defect
No defect
No defect
No defect
No defect
No defect
No defect
Countless blowholes
No defect
No defect
No defect
No defect
No defect
No defect
No defect
No defect
Countless blowholes
No defect
surface area of first
Table 5
Test
reference
J 1
J2
J3
J4
J5
J 6
J 7
J 8
J9
J10
J11
J12
J13
J14
J15
J16
J17
J18
J19
J20
J21
J22
J23
J24
J25
J26
J27
Left term
A symbol "- -" for the test reference numerals J17 and J26 indicates that no chemical analysis of the weld metal and no measurement of
the amount of the ferrite in the weld metal were conducted because of confirmation of the weld defects.
An asterisk "*" represents deviation from the condition specified by the present invention.
C
0.059
0.058
0.060
0.059
0.060
0.063
0.061
0.059
0.021
0.019
0.020
0.022
0.020
0.019
0.020
0.019 - -
0.019
0.055
0.051
0.023
0.019
0.034
0.074
0,033
- -
0.038
of
Si
0.28
0.29
0.28
0.29
0.28
0.27
0.28
0.29
0.41
0.40
0.39
0.41
0.40
0.43
0.40
0.41
- -
0.23
0.31
0.30
0.38
0.22
0.22
0.27
0.40
- -
0.37
Formula (1 )
Mn
3.13
3.17
3.08
3.15
2.71
2.64
2.68
2.75
3.03
2.92
2.95
2.97
2.55
2.53
2.63
2.50
- -
2.33
2.86
2.84
2.35
2.65
2.95
3.31
3.06
- -
2.98
: 41
P
0.031
0.030
0.030
0.031
0.029
0.033
0.032
0.031
0.015
0.016
0.016
0.014
0.016
0.017
0.016
0.015
- -
0.019
0.029
0.025
0.020
0.019
0.018
0.031
0.015
- -
0.019
Chemical
S
0.003
0.002
0.003
0.003
0.004
0.002
0.004
0.003
0.005
0.004
0.005
0.002
0.005
0.004
0,005
0.004
- -
0.004
0.003
0.002
0.003
0.004
0.004
0.003
0.005
- -
0.003
3-462(C+N)-9.2Si-8.1
composition
Ni
10.7
10.8
10.6
10.7
9.7
9.6
9.5
9.7
13.0
13.2
13.2
13.1
12.2
12.3
12.0
12.4
- -
8.9
9.7
9.5
8.7
9.8
8.5
9.4
12.0
- -
8.4
Cr
21.6
21.5
21.5
21.4
21.1
20.9
21.2
21.0
20.5
20.4
20.3
20.6
19.8
19.7
19.7
19.8
-
19.6
21.0
21.2
23.2
20.1
21.0
22.4
21.1
- -
*24.6
Mn-13.70-9.5Ni-18.5Mo
(mass%)
Mo
0.8
0.9
0.8
0.9
0.9
0.8
0.9
1.0
3.1
3.0
3.1
3.2
3.3
3.1
3.2
3.3
- -
1.0
1.2
1.1
3.1
0.8
-
-
2.4
- -
2.0
Balance:
V
0.09
0.09
0.08
0.09 -
-
-
-
0.10
0.09
0.09
0.10
0.01
0.01
0.01
0.01
- -
-
-
-
-
0.08
-
0.01
- -
Fe
Nb
0.12
0.13
0.12
0.12
0.15
0.13
0.14
0.15
0.10
0.09
0.08
0.09
0.12
0.13
0.14
0.11
- -
0.12
0.17
0.18
0.12
0.09
-
0.03
-
- -
-
and
Al
0.010
0.011
0.010
0.010
0.008
0.007
0.008
0.006
0.009
0.008
0.008
0.008
0.007
0.009
0.008
0.007
- -
0.007
0.008
0.009
0.008
0.008
0.007
0.009
0.007
- -
0.008
impurities
N
0.20
0.19
0.18
0.18
0.19
0.18
0.17
0.17
0.23
0.22
0.22
0.21
0.23
0.22
0.21
0.20
- -
0.18
"0.14
*0.13
0.20
0.19
0.20
0.21
0.23
- -
0.20
O
0.006
0.004
0.005
0.003
0.005
0.003
0.003
0.005
0.006
0.005
0.002
0.004
0.004
0.003
0.003
0.004 - -
0.006
0.005
0.005
0.006
0.007
0.006
0.005
0.004
- -
0.005
Area ratio
Value on left term of ferrite
of Formula (1)
-147
-144
-1 35
-1 37
-125
-115
-115
-116
-193
-185
-1 86
-1 88
-1 75
-166
-161
-162 - -
*-69
-107
-100
-171
-90
-89
-144
-184
- -
-178
(%)
9.2
8.0
8.6
8.2
6.5
7.2
6.8
7.5
7.4
7.2
7.6
7.8
11.2
11.5
12.0
10.8 - -
17.2
9.8
9.8
*38.6
13.1
11.3
9.8
7.8
- -
*41.1
[00751
[Table 61
Table 6
Inventive
example
Comparatlve
example
Inventwe
example
Comparatrve
example
The symbol
the rupture occurred In the base metal, and the symbol "x"
indicates that the rupture occurred In the weld metal The
symbol "-" lndlcates that no tensile test was conducted
The symbol "0" In the column "Low strain rate tensile test"
~ndicates that the ratio between the reduction of area after
the rupture In the high-pressure hydrogen environment and
the reduct~on of area after the rupture In the atmosphere was
90% or more, the symbol "x" Indicates that the above ratlo
was less than 90% The symbol "-" Indicates that no low
strain rate tenslle test was conducted
An asterisk "*" lndrcates a welded joint deviating from the
condition specified by the present lnvent~on
Test
reference
J 1
J 2
J 3
J 4
J 5
J 6
J 7
J 8
J 9
J I 0
J11
J12
J13
J14
J15
J16
*J17
*J18
*J19
*J20
*J21
J22
J23
J24
J25
*J26
*J27
"0" in the
Tensile
test
3
3
.>
3
3
3
(3
3
<>
3
3
,
3
3
-
z
x
x
,
c
3
-
i
column "Tensile
Low strain rate
tensile test
3
d
3
3
3
2
3
1
(I
>
1
1
~j
-
x -
-
x
)
J
>
-
x
test" lnd~cates that
Lo0761
In Tables 4 to 6, in the case of the test reference numerals 51 to J16 and
522 to 525 of Inventive examples that satisfied all the conditions specified by
the present invention, it is obvious that each welded joint had no weld defect
occurred therein, and had high strength and excellent hydrogen embrittlement
resistance at the same time.
[00771
To the contrary, in the case of the test reference numerals 517 to 521,
526, and 527 of Comparative examples, there was such a problem in each
welded joint that at least the occurrence of the weld defect was confirmed, the
rupture occurred in the weld metal, or the hydrogen embrittlement resistance
was poor.
[00781
Specifically, in the test reference numerals 517 and 526, the welding
material indicated by the reference character V whose amount of N was 0.37%,
which was more than the upper limit of the range specified by the present
invention, was used so that N could not be dissolved in the weld metal during
the welding, which generated countless blowholes (see Table 4).
[00791
In the test reference numeral J18, the value on the left term of Formula
(1) was -69 in the weld metal, which did not satisfy the condition of Formula
(11, so that martensite was generated through the deformation processing at
the time of the low strain rate tensile test, resulting in deterioration of the
hydrogen embrittlement resistance of the weld metal (see Table 5 and Table 6).
[oosol
In both the test reference numerals J19 and J 20, each crater surface
area was more than 120 mm2, and the scatters of N from the surface of the
molten pool became increased, and the amount of N in the weld metal was less
than the range specified by the present invention. Consequently, the rupture
occurred in the weld metal (see Tables 4 to 6).
[008 11
In the test reference numerals 521 and 527, the welding material
indicated by the reference character Z having the Cr content of 24.9% that was
more than the upper limit of the range specified by the present invention was
used. Consequently, in the test reference numeral 521, although the chemical
composition of the weld metal satisfied the range specified by the present
invention, a large amount of Cr relative to the amount of Ni was contained, so
that the amount of the ferrite became more than 20%, and the hydrogen
embrittlement susceptibility of the weld metal became increased. In addition,
in the test reference numeral 527, the amount of Cr in the weld metal was
more than the upper limit of the range specified by the present invention, and
a large amount of Cr was contained in the weld metal, so that the amount of
the ferrite became more than 20%, and the hydrogen embrittlement
susceptibility of the weld metal became increased (see Table 5 and Table 6).
INDUSTRIAL APPLICABILITY
[00821
According to the present invention, it is possible to provide a welded
joint having both high strength and excellent hydrogen embrittlement
resistance which are properties required in high-pressure gas piping,
particularly in high-pressure hydrogen gas piping without the postweld heat
treatment.

We claim:
1. An austenitic steel welded joint produced by welhng base metal with a gas a tungsten arc welding process which uses welchng material, wherein
the base metal has a chemical composition comprising: by mass%, C: 0.1% or less;
Si: 0.8% or less; Mn: 2.5 to 5.5%; Ni: 8 to 15%; Cr: 19 to 25%; Al: less than 0.05%; and N:
0.15 to 0.45%, the balance being Fe and impurities that contain 0 : 0.02% or less, P:
0.05% or less, and S: 0.03% or less, and
the welding material has a chemical composition comprising: by mass%, C: 0.1%
or less; Si: 0.8% or less; Mn: 1.5 to 4.5%; Ni: 8 to 15%; Cr: 18 to 24%; Al: less than 0.05%;
and N: 0.15 to 0.35%, the balance being Fe and impurities that contain 0: 0.02% or less,
P: 0.05% or less, and S: 0.03% or less,
wherein
weld metal has a chemical composition comprising: by mass%, C: 0.1% or less; Si:
0.8% or less; Mn: 1.5 to 5.5%; Ni: 8 to 15%; Cr: 18 to 24%; Al: less than 0.05%; and N:
0.15 to 0.35%, the balance being Fe and impurities that contain 0: 0.02% or less, P:
0.05% or less, and S: 0.03% or less,
the chemical composition of the weld metal satisfies Formula (I),
and
an amount of ferrite in the weld metal is 20% or less in area ratio.
413 - 462(C + N) - 9.2Si - 8.1Mn - 13.7Cr - 9.5Ni - 18.5Mo I -70...(1)
where a symbol of each element in Formula (1) denotes a content (mass%) of the
element.
2. The austenitic steel welded joint according to claim 1, wherein
the chemical composition of any one or more of the base metal, the welding
material, and the weld metal comprise one or more of, by mass%, V: 0.5% or less, Nb:
0.5% or less, and Mo: 4.5% or less in lieu of part of Fe.
Dated this 30th day of December, 2013.
Nippon Steel & fimitomo Metal Corporation ma~&m& (Dev Robinson)
of Amarchand & Mangaldas &
Suresh A. Shroff & Co.
Attorneys for the Applicant