TECHNICAL FIELD
[0001]
The present disclosure relates to a threaded connection for pipes and a method
for producing a threaded connection for pipes.
BACKGROUND ART
[0002]
For drilling an oil field or a natural gas field, oil country tubular goods
(OCTG) are used. The oil country tubular goods (OCTG) are formed by connecting
a plurality of pipes in proportion to the depth of a well. The pipes are connected by
fastening threaded connections for pipes formed at end portions of the pipes. The
oil country tubular goods are drawn up and loosened for inspection or the like, and
after the inspection, fastened again and reused.
[0003]
A threaded connection for pipes includes a pin and a box. The pin has a pin
contact surface which includes an external thread part formed on an outer peripheral
surface of an end portion of a pipe. The pin contact surface may include a pin metal
seal part and a pin shoulder part. The box has a box contact surface which includes
an internal thread part formed on an inner peripheral surface of an end portion of a
pipe. The box contact surface may include a box metal seal part and a box shoulder
part. Hereunder, the pin contact surface and the box contact surface are simply
referred to as "contact surface".
[0004]
In fastening the pipes, the pin contact surface and the box contact surface are
brought into contact with each other. Accordingly, the pin contact surface and the
box contact surface are repeatedly subjected to strong friction in fastening and
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loosening the pipes. When the fastening and loosening are repeated and if these
regions have no sufficient durability against friction, galling (unrepairable seizure)
occurs. Therefore, threaded connections for pipes are required to have a sufficient
durability against friction, namely, an excellent galling resistance.
[0005]
To enhance the galling resistance, a compound grease that contains heavy
metal has been used in conventional practice. By applying the compound grease on
a surface of a threaded connection for pipes, the galling resistance of the threaded
connection for pipes can be increased. However, heavy metals such as Pb
contained in the compound grease may have an influence on an environment. For
this reason, there is a demand for developing a threaded connection for pipes that
does not need the compound grease.
[0006]
A technique relating to a threaded connection for pipes that has an excellent
galling resistance is proposed in Japanese Patent Application Publication No. 05-
149485 (Patent Literature 1). The threaded connection for pipes disclosed in Patent
Literature 1 forms a dispersed plating layer on a surface of a pin or a box, and in the
dispersed plating layer, one or more kinds of non-metallic phases and an additional
metallic phase are dispersed and caused to be eutectoid in a metal matrix. Patent
Literature 1 discloses that this provides a threaded connection for pipes that has an
excellent galling resistance even in fastening and loosening using a compound grease
containing no heavy metal component.
[0007]
There are studies including Patent Literature 1 for enhancing slidability by
dispersing a non-metallic material in a plating layer so as to enhance a galling
resistance or decrease a coefficient of friction. Specifically, Japanese Patent
Application Publication No. 2008-214666 (Patent Literature 2) proposes a threaded
connection for pipes enhanced a galling resistance to disperse a non-metallic material
in a plating layer.
[0008]
A threaded connection for pipes disclosed in Patent Literature 2 forms a lowfrictional
composite coating layer on a surface of a base material, and the low-
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frictional composite coating layer contains a carbon-nano material and a zinc
component. Patent Literature 2 discloses that this provides a threaded connection
for pipes that is excellent in low-frictional properties under heavy loads and so
excellent in fastening properties as to enable the thread member to withstand harsh
environments.
CITATION LIST
PATENT LITERATURE
[0009]
Patent Literature 1: Japanese Patent Application Publication No. 05-149485
Patent Literature 2: Japanese Patent Application Publication No. 2008-214666
SUMMARY OF INVENTION
TECHNICAL PROBLEM
[0010]
Meanwhile, for the purpose of increasing a mining recovery of oil or natural
gas, the deepening of oil wells has proceeded. In addition to increase in depths of
oil wells, a future increase in horizontal wells is expected. The horizontal well
refers to a well drilled vertically down to a stratum in which oil or natural gas is
buried (oil stratum), and then drilled in a horizontal direction or a direction inclining
from the horizontal direction along the oil stratum. Recent years have seen a
tendency of horizontal wells to increase in lengths of horizontal portions thereof. In
a horizontal well, a route of the well changes from a vertical direction to the
horizontal direction in the middle of the well. Therefore, in a bent portion where
the route of the well changes from the vertical direction to the horizontal direction, a
high stress is applied to entire oil country tubular goods. Therefore, a high stress is
also applied to a threaded connection for pipes in the bent portion. Moreover, in
drilling operation of a horizontal well, it is necessary to push the oil country tubular
goods into the well while rotating the oil country tubular goods in a circumferential
direction. When rotated under a high stress, the threaded connection for pipes is
likely to loosen. A loosened threaded connection for pipes leads to a reduction in
airtightness of the oil country tubular goods. For that reason, the threaded
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connection for pipes is required to be likely to hard to loosen even in a horizontal
well.
[0011]
By the techniques disclosed in Patent Literature 1 or 2, a galling resistance of
a threaded connection for pipes can be increased. However, in Patent Literature 1
or 2, there is no mention about the making less of looseness of a threaded connection
for pipes even if that is used for a horizontal well.
[0012]
An objective of the present disclosure is to provide a threaded connection for
pipes having an excellent galling resistance and being capable of preventing
looseness even when used for a horizontal well, and to provide a method for
producing the threaded connection for pipes.
SOLUTION TO PROBLEM
[0013]
A threaded connection for pipes according to the present disclosure includes a
pin, a box, and a Zn-Ni alloy plating layer. The pin has a pin contact surface that
includes an external thread part. The box has a box contact surface that includes an
internal thread part. The Zn-Ni alloy plating layer is disposed on or above at least
one of the pin contact surface and the box contact surface. The Zn-Ni alloy plating
layer contains graphite.
[0014]
A method for producing the threaded connection for pipes according to the
present disclosure includes a preparing step and a plating layer forming step. In the
preparing step, a pin, a box, and a plating solution are prepared. The pin has a pin
contact surface that includes an external thread part. The box has a box contact
surface that includes an internal thread part. The plating solution contains a zinc
ion, a nickel ion, and graphite. In the plating layer forming step, at least one of the
pin contact surface and the box contact surface is brought into contact with the
plating solution and subjected to electroplating, by which a Zn-Ni alloy plating layer
is formed on at least one of the pin contact surface and the box contact surface.
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ADVANTAGEOUS EFFECTS OF INVENTION
[0015]
The threaded connection for pipes according to the present disclosure has an
excellent galling resistance and is capable of suppressing looseness when used in a
horizontal well. The method for producing the threaded connection for pipes
according to the present disclosure can produce the threaded connection for pipes
described above.
BRIEF DESCRIPTION OF DRAWINGS
[0016]
[FIG. 1] FIG. 1 is a diagram illustrating results of Bowden sliding test
performed on Test Nos. 1, 3, and 7 in Example.
[FIG. 2] FIG. 2 is a diagram illustrating a configuration of an example of a
threaded connection for pipes of a coupling type.
[FIG. 3] FIG. 3 is a diagram illustrating a configuration of an example of a
threaded connection for pipes of an integral type.
[FIG. 4] FIG. 4 is a sectional view of an example of a threaded connection for
pipes.
[FIG. 5] FIG. 5 is a sectional view of an example of a threaded connection for
pipes according to the present embodiment.
[FIG. 6] FIG. 6 is a sectional view of an example of a threaded connection for
pipes according to another embodiment, the threaded connection for pipes being
different from the threaded connection for pipes illustrated in FIG. 5.
[FIG. 7] FIG. 7 is a sectional view of an example of a threaded connection for
pipes according to still another embodiment, the threaded connection for pipes being
different from the threaded connections for pipes illustrated in FIG. 5 and FIG. 6.
[FIG. 8] FIG. 8 is a sectional view of an example of a threaded connection for
pipes according to the present embodiment in a case where its pin and box each
include a lubricating coating.
[FIG. 9] FIG. 9 is a sectional view of an example of a threaded connection for
pipes according to another embodiment, the threaded connection for pipes being
different from the threaded connection for pipes illustrated in FIG. 8.
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[FIG. 10] FIG. 10 is a sectional view of an example of a threaded connection
for pipes according to still another embodiment, the threaded connection for pipes
being different from the threaded connections for pipes illustrated in FIG. 8 and FIG.
9.
[FIG. 11] FIG. 11 is a diagram illustrating results of Bowden sliding test
performed on Test Nos. 2, 4, and 5 in Example.
[FIG. 12] FIG. 12 is a diagram illustrating results of Bowden sliding test
performed on Test Nos. 6 and 8 in Example.
DESCRIPTION OF EMBODIMENTS
[0017]
The present embodiment will be described below in detail with reference to
the accompanying drawings. The same or equivalent portions will be denoted by
the same reference numerals, and description of the portions will not be repeated.
[0018]
The present inventors first focused their attention on a coefficient of friction,
and conducted studies about a galling resistance and looseness of a threaded
connection for pipes, the looseness occurring when the threaded connection for pipes
is used in a horizontal well. As a result, the following findings were obtained.
[0019]
A threaded connection for pipes is fastened and loosened with a given torque.
In the fastening and loosening, as described above, a pin contact surfaces and a box
contact surface slide on each other with a high interfacial pressure of, for example,
1.0 GPa or more applied to the contact surfaces. In other words, the contact
surfaces receive a strong friction in the fastening and the loosening. Accordingly,
when a coefficient of friction between the contact surfaces is high, frictional heat
generated in the fastening and the loosening increases, which easily causes adhesion
or galling. In contact between metals, the galling is remarkably likely to occur
when a coefficient of friction between the metals is, for example, more than 0.4.
For that reason, in conventional practice, reduction of a coefficient of friction
between contact surfaces has been attempted by applying a lubricating agent or by
forming a plating layer having a low coefficient of friction on the contact surfaces.
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For example, when a conventional API dope is used, the coefficient of friction of the
contact surfaces becomes about 0.05 to 0.2. In this case, the galling of the contact
surfaces becomes suppresses.
[0020]
Meanwhile, the present inventors have found that decreasing a coefficient of
friction makes the threaded connection for pipes easy to loosen. A torque required
in fastening and loosening is in a proportional relationship with a coefficient of
friction between contact surfaces. Specifically, the higher the coefficient of friction
between contact surfaces is, the higher the torque in fastening and loosening is. A
threaded connection for pipes that is insusceptible to looseness can be obtained if the
threaded connection for pipes maintains a high torque in loosening. Hereafter, a
capability to maintain a high torque in loosening will be referred to as a high-torque
maintenance property.
[0021]
Accordingly, when the coefficient of friction is decreased, although the
galling resistance of the threaded connection for pipes is increased, the threaded
connection for pipes is likely to easy to loosen. Meanwhile, when the coefficient of
friction is increased, although the looseness of the threaded connection for pipes is
suppressed, the galling resistance of the threaded connection for pipes is decreased.
As described, for the threaded connection for pipes, it has been considered to be
difficult to obtain both the excellent galling resistance and the high-torque
maintenance property.
[0022]
Meanwhile, as a result of detailed studies of the present inventors, it became
clear that in a case where a specified plating formed on or above the contact surfaces
contains graphite, the galling resistance of the threaded connection for pipes is
increased and further the high-torque maintenance property is increased. In this
regard, it will be described in detail with reference to the drawing.
[0023]
FIG. 1 is a diagram illustrating a part of results of Bowden sliding test
performed in Example. Referring to FIG. 1, Test No. 1 indicates a result of which a
Zn-Ni alloy plating layer containing no graphite is subjected to Bowden sliding test.
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Test No. 3 indicates a result of which a Zn-Ni alloy plating layer containing graphite
is subjected to Bowden sliding test. Test No. 7 indicates a result of which a Cu
plating layer containing graphite to subjected to Bowden sliding test.
[0024]
Referring to Test No. 1 and Test No. 3 in FIG. 1, the number of sliding until a
coefficient of friction became more than 0.4 of a Zn-Ni alloy plating layer containing
graphite (Test No. 3) is greater than that of a Zn-Ni alloy plating layer containing no
graphite (Test No. 1). As described above, for the threaded connection for pipes,
when the coefficient of friction became more than 0.4, the galling is remarkably
likely to occur. Accordingly, FIG. 1 indicates that the galling resistance of Zn-Ni
alloy plating layer increased by containing graphite.
[0025]
Further, referring to Test No. 3 in FIG. 1, in the Zn-Ni alloy plating layer
containing graphite, a high coefficient of friction became more than 0.2 after the
number of sliding was over 50 times. As a result of continued sliding, the
coefficient of friction became more than 0.4 after the number of sliding was over 200
times. Accordingly, in the Zn-Ni alloy plating layer containing graphite, the
coefficient of friction about 0.2 to 0.4 was maintained during 150 times at the
number of sliding. In short, in the Zn-Ni alloy plating layer containing graphite, the
number of sliding until the galling was occurred was large, and further, the number
of sliding during the high coefficient of friction was large. Therefore, it became
clear that the Zn-Ni alloy plating layer containing graphite is capable to achieve both
the excellent galling resistance and the high-torque maintenance property.
[0026]
Meanwhile, referring to Test No. 7 in FIG. 1, in the Cu plating layer
containing graphite, a coefficient of friction indicated about 0.1 until the number of
sliding was over 250 times. As a result of continued sliding, the coefficient of
friction became more than 0.4 at a stroke after the number of sliding was over 300
times. Accordingly, in the Cu plating layer containing graphite, the number of
sliding during the coefficient of friction maintained about 0.2 to 0.4 was small.
Therefore, the Cu plating layer containing graphite had the excellent galling
resistance and did not indicate the high-torque maintenance property.
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[0027]
As described above, as a result of detailed studies of the present inventors, it
became clear that both the galling resistance and the high-torque maintenance
property of the threaded connection for pipes are increased as a result from
containing graphite to the specified plating, that is, the Zn-Ni alloy plating layer
formed on or above the contact surfaces. Meanwhile, the detailed reason both the
galling resistance and the high-torque maintenance property of the threaded
connection for pipes are increased when the Zn-Ni alloy plating layer contains
graphite has not been clarified. The present inventors are considering that the
reason for this as follows.
[0028]
Zn-Ni alloy plating has a high hardness as compared with conventional Cu
plating. The higher the hardness, the higher the resistance is generated against a
stress that is applied in sliding in fastening at a remarkably high interfacial pressure
(e.g., 1.0 GPa or more). In addition, in Example described later, for the Test Nos.
in which the coefficient of friction has become more than 0.4 at the Bowden sliding
test, it was verified that the plating layers were peeled off from the steel sheet for
each Test No. after the Bowden sliding test. Meanwhile, in the Zn-Ni alloy plating
layers containing graphite (Test Nos. 2 to 5), the remained amount of the plating
layer was larger than that of the Zn-Ni alloy plating layers containing no graphite
(Test No. 1). Accordingly, in the Zn-Ni alloy plating layer, graphite may possibly
prevent the Zn-Ni alloy plating layer from being peeled off.
[0029]
Therefore, in the Zn-Ni alloy plating layer, the present inventors presume that
the coefficient of friction is increased by being peeled off the part of the plating layer
according to the number of sliding. In the Zn-Ni alloy plating layer partially peeled,
it is considered that the coefficient of friction increases to a range of 0.2 to 0.4.
Further, by the Zn-Ni alloy plating layer contains graphite, peeling of the Zn-Ni alloy
plating layer is suppressed, so as to the coefficient of friction may possibly maintain
within a range of 0.2 to 0.4. Furthermore, this tendency is not confirmed in the Cu
plating layer containing graphite. Therefore, the effect of which graphite prevents
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the plating layer from being peeled off so as to the coefficient of friction maintains
within a range of 0.2 to 0.4 is typical of the Zn-Ni alloy plating layer.
[0030]
By the aforementioned mechanism, the present inventors is considering that
the threaded connection for pipes having the Zn-Ni alloy plating layer containing
graphite achieve both the excellent galling resistance and the increased high-torque
maintenance property. Note that, by a mechanism different from the
aforementioned mechanism, there is a possibility that the Zn-Ni alloy plating layer
containing graphite enhances the galling resistance and the high-torque maintenance
property of the threaded connection for pipes. However, it has been proved by
Examples described later that the threaded connection for pipes can achieve both the
excellent galling resistance and the excellent high-torque maintenance property by
containing graphite in the Zn-Ni alloy plating layer.
[0031]
The threaded connection for pipes according to the present embodiment,
which has been completed based on the above findings, includes a pin, a box, and a
Zn-Ni alloy plating layer. The pin has a pin contact surface that includes an
external thread part. The box has a box contact surface that includes an internal
thread part. The Zn-Ni alloy plating layer is disposed on or above at least one of
the pin contact surface and the box contact surface. The Zn-Ni alloy plating layer
contains graphite.
[0032]
The threaded connection for pipes according to the present embodiment
includes the Zn-Ni alloy plating layer containing graphite. Therefore, the threaded
connection for pipes according to the present embodiment has an excellent galling
resistance and an excellent high-torque maintenance property. Therefore, the
threaded connection for pipes has an excellent galling resistance and is likely to hard
to loosen even when used for a horizontal well. A plating layer made of a Zn-Ni
alloy, graphite, and impurities will be herein referred to as a Zn-Ni alloy plating layer.
[0033]
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Preferably, when a content of a whole chemical composition of the Zn-Ni
alloy plating layer is 100.0 at%, a Ni content in the Zn-Ni alloy plating layer is 5.0 to
35.0 at%.
[0034]
In this case, a hardness of the Zn-Ni alloy plating layer is increased.
[0035]
Preferably, when the content of the whole chemical composition of the Zn-Ni
alloy plating layer is 100.0 at%, a graphite content in the Zn-Ni alloy plating layer is
30.0 to 60.0 at%.
[0036]
In this case, the high-torque maintenance property is further increased,
making the threaded connection for pipes further insusceptible to looseness even
when used for a horizontal well.
[0037]
The Zn-Ni alloy plating layer described above may have a thickness of 1.0 to
50.0 m.
[0038]
Preferably, the threaded connection for pipes may further include a lubricating
coating on or above one or more surface selected from the group consisting of the pin
contact surface, the box contact surface, and the Zn-Ni alloy plating layer.
[0039]
In this case, a lubricity of the threaded connection for pipes is increased.
[0040]
A method for producing the threaded connection for pipes according to the
present embodiment includes a preparing step and a plating layer forming step. In
the preparing step, a pin, a box, and a plating solution are prepared. The pin has a
pin contact surface that includes an external thread part. The box has a box contact
surface that includes an internal thread part. The plating solution contains a zinc
ion, a nickel ion, and graphite. In the plating layer forming step, at least one of the
pin contact surface and the box contact surface is brought into contact with the
plating solution, so as to form a Zn-Ni alloy plating layer on at least one of the pin
contact surface and the box contact surface by electrolytic plating.
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[0041]
The threaded connection for pipes according to the present embodiment and a
method for producing the threaded connection for pipes will be described below in
detail. In the following description, at% refers to the atomic percent.
[0042]
[Threaded Connection for Pipes]
A threaded connection for pipes includes a threaded connection for pipes of a
coupling type and a threaded connection for pipes of an integral type. FIG. 2 is a
diagram illustrating a configuration of a threaded connection for pipes of a coupling
type. Referring to FIG. 2, the threaded connection for pipes includes a pipe 1 and a
coupling 2. At both ends of the pipe 1, pins 3 each including an external thread part
on its outer surface are formed. At both ends of the coupling 2, boxes 4 having
internal thread parts on their inner surfaces are formed. By fastening the pin 3 and
the box 4, the coupling 2 is attached to an end of the pipe 1.
[0043]
FIG. 3 is a diagram illustrating a configuration of a threaded connection for
pipes of an integral type. Referring to FIG. 3, the threaded connection for pipes
includes a pipe 1. At one end of the pipe 1, a pin 3 including an external thread part
on its outer surface is formed. At the other end of the pipe 1, a box 4 including an
internal thread part on its inner surface is formed. By fastening the pin 3 and the
box 4, pipes 1 can be connected together. The threaded connection for pipes
according to the present embodiment is available for both of a threaded connection
for pipe or tube of a coupling type and a threaded connection for pipes of an integral
type.
[0044]
Parts that come into contact with each other when the pin 3 and the box 4 are
fastened are referred to as a pin contact surface 34 and a box contact surface 44.
FIG. 4 is a sectional view of an example of the threaded connection for pipes. In
the example of the threaded connection for pipes illustrated in FIG. 4, the pin 3
includes an external thread part 31, a pin metal seal part 32, and a pin shoulder part
33. The box 4 includes an internal thread part 41, a box metal seal part 42, and a
box shoulder part 43.
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[0045]
When the pin 3 and the box 4 are fastened, the thread parts (the external
thread part 31 and the internal thread part 41) come into contact with each other, the
metal seal parts (the pin metal seal part 32 and the box metal seal part 42) come into
contact with each other, and the shoulder parts (the pin shoulder part 33 and the box
shoulder part 43) come into contact with each other. In other words, when the pin
contact surface 34 includes the external thread part 31, the pin metal seal part 32, and
the pin shoulder part 33, the box contact surface 44 includes the internal thread part
41, the box metal seal part 42, and the box shoulder part 43. Note that, the threaded
connection for pipes according to the present embodiment does not have to include
the metal seal part and the shoulder part as long as that includes the thread part.
[0046]
In short, the pin 3 has the pin contact surface 34 that includes the external
thread part 31. As described above, the pin contact surface 34 may further include
the pin metal seal part 32 and the pin shoulder part 33. Therefore, the pin contact
surface 34 may not include the pin metal seal part 32 and the pin shoulder part 33 as
long as that includes the external thread part 31. The pin contact surface 34 may
include the external thread part 31 and the pin metal seal part 32, may include the
external thread part 31 and pin shoulder part 33, or may include the external thread
part 31, the pin metal seal part 32 and pin shoulder part 33.
[0047]
Also, the box 4 has the box contact surface 44 that includes the internal thread
part 41. As described above, the box contact surface 44 may further include the box
metal seal part 42 and the box shoulder part 43. Therefore, the box contact surface
44 may not include the box metal seal part 42 and the box shoulder part 43 as long as
that includes the internal thread part 41. The box contact surface 44 may include
the internal thread part 41 and the box metal seal part 42, may include the internal
thread part 41 and box shoulder part 43, or may include the internal thread part 41,
the box metal seal part 42 and box shoulder part 43.
[0048]
Hereinafter, the case where the pin contact surface 34 includes the external
thread part 31, the pin metal seal part 32 and the pin shoulder part 33, and the box
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contact surface 44 includes the internal thread part 41, the box metal seal part 42 and
the box shoulder part 43 as one example of the threaded connection for pipes
according to the present embodiment will be described.
[0049]
In the threaded connection for pipes including the metal seal parts and the
shoulder parts, the disposition of the thread parts, the metal seal parts and the
shoulder parts is not particularly limited. Specifically, in FIG. 4, on the pin 3, the
pin shoulder part 33, the pin metal seal part 32, and the external thread part 31 are
disposed in this order from an end of the pipe 1. Further, in FIG. 4, on the box 4,
the internal thread part 41, the box metal seal part 42, and the box shoulder part 43
are disposed in this order from an end of the pipe 1 or the coupling 2. Meanwhile,
the disposition of these is not limited to the disposition illustrated in FIG. 4 and can
be changed as appropriate.
[0050]
For example, as illustrated in FIG. 3, the external thread part, the pin metal
seal part, the pin shoulder part, the pin metal seal part, and the external thread part
may be disposed on the pin 3 in this order from an end of the pipe 1. In a similar
manner, on the box 4, the internal thread part, the box metal seal part, the box
shoulder part, the box metal seal part, and the internal thread part may be disposed in
this order from an end of the pipe 1 or the coupling 2.
[0051]
Therefore, the pin contact surface 34 of the pin 3 may include a plurality of
the external thread parts 31, may include a plurality of the pin metal seal parts 32 and
may include a plurality of the pin shoulder parts 33. In a similar manner, the box
contact surface 44 of the box 4 may include a plurality of the internal thread parts 41,
may include a plurality of the box metal seal parts 42 and the box shoulder parts 43.
[0052]
[Zn-Ni Alloy Plating Layer]
The threaded connection for pipes according to the present embodiment
includes the pin 3, box 4, and a Zn-Ni alloy plating layer. The Zn-Ni alloy plating
layer is disposed on or above at least one of the pin contact surface 34 and the box
contact surface 44.
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[0053]
FIG. 5 is a sectional view of an example of the threaded connection for pipes
according to the present embodiment. Referring to FIG. 5, the Zn-Ni alloy plating
layers 100 is disposed on both the pin contact surface 34 and the box contact surface
44. FIG. 6 is a sectional view of an example of the threaded connection for pipes
according to another embodiment, the threaded connection for pipes being different
from the threaded connection for pipes illustrated in FIG. 5. Referring to FIG. 6,
the Zn-Ni alloy plating layers 100 may be disposed only on the box contact surface
44. FIG. 7 is a sectional view of an example of the threaded connection for pipes
according to still another embodiment, the threaded connection for pipes being
different from the threaded connections for pipes illustrated in FIG. 5 and FIG. 6.
Referring to FIG. 7, the Zn-Ni alloy plating layers 100 may be disposed only on the
pin contact surface 34.
[0054]
The Zn-Ni alloy plating layer 100 has to be disposed on or above at least one
of the pin contact surface 34 and the box contact surface 44. Further, the Zn-Ni
alloy plating layer 100 may be disposed on or above either the whole or a part of at
least one of the pin contact surfaces 34 and the box contact surface 44. Specifically,
the pin metal seal part 32 and the box metal seal part 42, and the pin shoulder part 33
and the box shoulder part 43 are under a high interfacial pressure particularly in a
final phase of fastening. Therefore, in a case where the threaded connection for
pipes includes the pin metal seal part 32, the box metal seal part 42, the pin shoulder
part 33 and the box shoulder part 43, the Zn-Ni alloy plating layer 100 may be
disposed on or above at least one of the pin metal seal part 32, the box metal seal part
42, the pin shoulder part 33, and the box shoulder part 43. In contrast, in a case
where Zn-Ni alloy plating layer 100 is disposed entirely on or above at least one of
the contact surfaces 34 and 44, a production efficiency of the threaded connection for
pipes is increased.
[0055]
As described above, the Zn-Ni alloy plating layer 100 is made of a Zn-Ni
alloy, graphite, and impurities. Here, the Zn-Ni alloy contains zinc (Zn) and nickel
(Ni). Also, the Zn-Ni alloy may contain impurities. Here, the impurities of the
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Zn-Ni alloy plating layer 100 and the impurities of the Zn-Ni alloy mean substances
that are other than Zn, Ni and graphite, contained in the Zn-Ni alloy plating layer 100
or the Zn-Ni alloy during production of the threaded connection for pipes, and
contained at contents within ranges within which the impurities have no influence on
effects of the present disclosure.
[0056]
The Ni content in the Zn-Ni alloy plating layer 100 is not limited to a specific
content. However, when the content of the whole chemical composition of the Zn-
Ni alloy plating layer 100 is assumed to be 100.0 at%, the Ni content in the Zn-Ni
alloy plating layer 100 of 5.0 to 35.0 at% causes a hardness of the Zn-Ni alloy
plating layer 100 to increase.
[0057]
Therefore, when the content of the whole chemical composition of the Zn-Ni
alloy plating layer 100 is assumed to be 100.0 at%, the Zn-Ni alloy plating layer 100
preferably contains 5.0 to 35.0 at% of Ni. A lower limit of the Ni content in the Zn-
Ni alloy plating layer 100 is more preferably 10.0 at%. An upper limit of the Ni
content in the Zn-Ni alloy plating layer 100 is more preferably 30.0 at%.
[0058]
Note that, Zinc (Zn) contained in the Zn-Ni alloy plating layer 100 is a base
metal in comparison with iron (Fe), which is a principal component of the pipe. Zn
therefore has an effect of sacrificial protection, increasing a corrosion resistance of
the threaded connection for pipes.
[0059]
[Graphite]
Graphite is a substance formed of layered sheets (graphenes), each of which is
a hexagonal lattice into which carbon atoms are coupled. Graphite contains carbon
(C), and its balance consists of impurities. Since a bonding strength between layers
in graphene is weak, graphenes are prone to be peeled off from their layers. For
that reason, graphite is generally used as a lubricating agent. However, according
to the present embodiment, the Zn-Ni alloy plating layer 100 containing graphite
increases the galling resistance and increases the high-torque maintenance property
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of the threaded connection for pipes. As a result, the threaded connection for pipes
is capable of preventing looseness even when used for a horizontal well.
[0060]
[Graphite Content]
The graphite content in the Zn-Ni alloy plating layer 100 is not limited to a
specific content. However, when the content of the whole chemical composition of
the Zn-Ni alloy plating layer 100 is assumed to be 100.0 at%, 30.0 at% or more of
graphite content further increases the high-torque maintenance property of the Zn-Ni
alloy plating layer 100. In this case, looseness of a threaded connection for pipes is
further suppressed even when the threaded connection for pipes is used in a
horizontal well. In contrast, when the content of the whole chemical composition of
the Zn-Ni alloy plating layer 100 is assumed to be 100.0 at%, a normal Zn-Ni alloy
plating layer 100 can be formed in a stable manner when the graphite content is 60.0
at% or less. Therefore, when the content of the whole chemical composition of the
Zn-Ni alloy plating layer 100 is assumed to be 100.0 at%, the graphite content in the
Zn-Ni alloy plating layer 100 is preferably 30.0 to 60.0 at%. A lower limit of the
graphite content in the Zn-Ni alloy plating layer 100 is more preferably 40.0 at%.
An upper limit of the graphite content in the Zn-Ni alloy plating layer 100 is more
preferably 55.0 at%.

We claim:
1. A threaded connection for pipes, comprising:
a pin having a pin contact surface that includes an external thread part;
a box having a box contact surface that includes an internal thread part; and
a Zn-Ni alloy plating layer disposed on or above at least one of the pin contact
surface and the box contact surface, wherein
the Zn-Ni alloy plating layer contains graphite.
2. The threaded connection for pipes according to claim 1, wherein
when a content of a whole chemical composition of the Zn-Ni alloy plating
layer is 100.0 at%,
a Ni content in the Zn-Ni alloy plating layer is 5.0 to 35.0 at%.
3. The threaded connection for pipes according to claim 1 or claim 2, wherein
when the content of the whole chemical composition of the Zn-Ni alloy
plating layer is 100.0 at%,
a graphite content in the Zn-Ni alloy plating layer is 30.0 to 60.0 at%.
4. The threaded connection for pipes according to any one of claims 1 to 3,
wherein a thickness of the Zn-Ni alloy plating layer is 1.0 to 50.0 m.
5. The threaded connection for pipes according to any one of claims 1 to 4,
further comprising a lubricating coating on or above at least one selected from the
group consisting of the pin contact surface, the box contact surface, and the Zn-Ni
alloy plating layer.
6. A method for producing a threaded connection for pipes, the method
comprising the steps of:
preparing a pin having a pin contact surface that includes an external thread
part; a box having a box contact surface that includes an internal thread part; and a
plating solution containing a zinc ion, a nickel ion, and graphite; and
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forming a Zn-Ni alloy plating layer, by electrolytic plating, on at least one of
the pin contact surface and the box contact surface by bringing at least one of the pin
contact surface and the box contact surface into contact with the plating solution.