Designation of Document] Specification
[Title of the Invention] THREADED JOINT FOR STEEL PIPE
[Technical Field]
[OOOl]
The present invention relates to a threaded joint for a steel pipe having
superior lubricating performance and rust prevention performance. The threaded
joint for a steel pipe according to the present invention exhibits sufficient rust
prevention performance even in a tough storage environment, has sufficient lt~hricating
performance for connecting oil country tubular goods (OCTG), and can also be used
for simple repair. Therefore, this threaded joint is particularly suitable for use in
connecting OCTG.
Priority is claimed on Japanese Patent Application No. 2012-130134, filed on
June 7,2012, the content of which is incorporated herein by reference.
packground Art]
[0002]
Oil countty tubular goods (for example, a tubing through which a fluid such
as crude oil flows; and a casing which surrounds a tubing), which are used in oil
drilling for mining crude oil or gas oil, are generally several tens of meters long and
are connected using threaded joints until they have a length to reach an oil well. In
the past, the depth of an oil well was 2,000 m to 3,000 m. However, in recent deep
oil wells such as undersea oil wells, there are cases where the depth of an oil well
reaches a range of 8,000 m to 10,000 m or deeper.
[0003]
Depending on the usage enviromnent thereof, a threaded joint for a steel pipe
used for connecting oil countty tubular goods is affected by a load such as an axial
tension force caused by the weight of oil country tubular goods and a threaded joint; a
complex pressure such as an internal or external surface pressure; and heat in the
ground. Therefore, the threaded joint is required to maintain the airtightness of oil
country tubular goods without being damaged even in such a tough environment.
[0004]
FIG. 1 is a diagram schematically illustrating an assembly state of oil countly
tubular goods A and a coupling B which is a threaded joint component. In the oil
cout~trytu bular goods A, a pin 1 having male threads 11 (refer to FIG. 2) is formed on
external surfaces of both ends thereof. In the coupling B, a box 2 having female
threads 21 (refer to FIG. 2) is formed on internal surfaces of both ends thereof. An
end of the oil country tubular goods A is connected to an end of the coupling B in
advance.
FIG. 2 is a diagram schematically illustrating the details of a pin-box structure
of a typical threaded joint (also referred to as a special threaded joint) used for
connecting the oil country tubular goods A. The pin 1 is a joint component having
the male threads 11 and is typically formed on an end poi-tion of the oil country tubular
goods A. The box 2 is a joint component having the female threads 21 and is
typically formed on an internal surface of the threaded joint component (coupling B).
The pin 1 further includes, in addition to the male thleads 11, a seal portion 13 and a
shoulder portion 14 which are positioned closer to a tip end side of a steel pipe than the
threaded portion. Correspondingly, the box 2 further includes, in addition to the
female threads 21, a seal portion 23 which is positioned inside of the joint portion; and
a shoulder poi-tion 24 which comes into contact with the shoulder portion 14 of the pin
1. The seal portions 13 and 23 and the shoulder postions 14 and 24 of the pin 1 and
the box 2 are an unthreaded metal contact poi-tion of the threaded joint for a steel pipe.
Reference numeral 12 represents a lip portion at a tip end of the pin 1 and reference
numeral 31 represents a portion where the pin I is not in contact with the box 2 (a
portion in which bleeding oil remains during connecting).
[OOOS]
An end of the oil country tubular goods A is inserted into the coupling B, and
the male threads 11 and the female threads 21 are connected to each other. As a result,
the seal portion 13 of the pin 1 and the seal portion 23 of the box 2 come into contact
with each other at an appropriate interference amount to form a metal seal. This
metal seal secures the airtightness of the oil countly tubular goods A connected to the
coupling B. In this kind of threaded joint, in particular, in the seal portions 13 and 23,
non-repairable seizure called galling may occur.
[0006]
When the tubing and casing are lowered to an oil well, there are cases where a
threaded joint connected thereto is disconnected due to various kinds of problems; they
are pulled up from the oil well; and then they are connected and lowered again.
American Petroleum Institute (MI) requires galling resistance and airtightness in the
sense that galling does not occur and the airtightness of oil country tubular goods is
secured even when makeup (connecting) and breakout (disconnecting) are performed
10 times in the case of a tubing joint or 3 times in the case of a casing joint.
[0007]
In order to improve airtightness and galling resistance during connecting, in
tlie related art, a viscous liquid lubricant (referred to as "compound grease") containing
heavy metal powder is coated onto a contact surface (the threaded portion and
untlreaded metal contact portion) of the threaded joint. In API standard BULSA2,
such a compound grease is specified. The compound grease also exhibits rust
prevention performance of preventing rust on the coated contact surface.
[OOOS]
However, since the compound grease contains a large amount of powder of
heavy metals such as zinc, lead, and copper, the coated grease may be washed and
overflow on an external surface during the connecting of the threaded joint. As a
result, there is a possibility that, in particular, harmful heavy metals such as lead may
have an adverse effect on the environment (in particular, marine organisms). In
addition, since the work environment deteriorates due to the coating operation of the
compound grease, there is a concern of an adverse effect on a human body by the
coating operation. Therefore, a threaded joint for a steel pipe capable of exhibiting
superior galling resistance without using the compound grease is required.
[0009]
As a threaded joint which can be used for connecting oil country tubular
goods without coating the conrpound grease, many threaded joints for a steel pipe have
been proposed until now in which a contact surface thereof is coated with a solid
lubricant coating film composed of an organic or inorganic resin containing lubricating
powder such as molybdenum disulfide or graphite.
[OOlO]
As described above, in a threaded joint for a steel pipe having a seal portion,
since galling is likely to occur particularly in tlre seal portion, it is necessaly that
rusting of the seal portion be prevented. This is because galling is likely to occur
when the rust occurs. In addition, changes in the shape of the seal portion by
corrosion causes deterioration in ai~tiglitness. In a threaded joint for a steel pipe
which is not coated with tlre compound grease, since the rust preventiori performance
by the compound grease is not exhibited, the rust prevention of the sealportion is
further important. However, the above-described solid lubricating coating film does
not have sufficient rust prevention performance for preventing a cut portion of a joint
including a seal portion from being rusted when being stored in a highly corrosive
environment for a long period of time.
[OOII]
Patent Docunient 1 discloses a threaded joint for a steel pipe having superior
rust prevention performance that includes an ultraviolet ray-curable resin coating film
formed of a photocurable composition, which is curable by the irradiation of ultraviolet
rays, on a contact surface of the threaded joint.
[Prior Art Document]
[Patent Document]
[0012]
[Patent Document I] Japanese Unexamined Patent Application, First
Publication No. 2011-12251
[Disclosure of the Invention]
[Problem that the Invention is to solve]
[0013]
The threaded joint for a steel pipe disclosed in Japanese Unexamined Patent
Application, First PublicationNo. 2011-12251 is effective for preventing rust on a
contact surface (threaded portions and unthreaded metal contact portions of a pin and a
box) of the threaded joint for a steel pipe, but there are cases where rust prevention
performance is not always sufficient and rust cannot be completely prevented when the
storage state is not appropriate or when the storage environment is tough. For
example, oil country tubular goods may be stored in a tough environment such as a
coast or a desert. Therefore, even when the storage environment is poor, rust
prevention is required.
[0014]
I11 addition, when the cleanliness of a coating surface is insufficient during the
coating of an ultraviolet ray-curable resin composition, there are cases where coating
defects are generated on a formed ultraviolet ray-curable coating film and rust
prevention performance is locally insufficient. For example, when point-like rust is
generated in a seal portion, the galling resistance and airtightness of a threaded joint
significantly deteriorates. Therefore, it is necessaly that a seal poition, in particular,
be completely coated such that coating defects are not generated thereon.
[0015]
Furthermore, in a coating film formed of an ultraviolet ray-curable resin
composition, there are cases where necessary lubricating performance is not
sufficiently obtained. In addition, even when defects are observed in the coating film,
there is a problem in that defects are not easily repaired.
[OOI 61
The present invention has been made in consideration of the above-described
circumstances, and an object thereof is to provide a threaded joint for a steel pipe
including an ultraviolet ray-curable resin coating film in which;even in a tough storage
environment, sufficient last prevention performance for preventing the lusting of a
threaded joint for a steel pipe, used for connecting oil countty tubular goods, is
obtained; sufficient lubricating performance is obtained as a threaded joint for a steel
pipe used for connecting oil countly tubular goods; and even when coating defects are
generated during coating, the defects can be easily repaired.
[Means for Solving the Problems]
[0017]
The present invention adopts the following means in order to solve the abovedescribed
problems and achieve the object.
(1) According to an aspect of the present invention, there is provided a
threaded joint for a steel pipe including a pin and a box each of which is provided with
a contact surface that includes: an unthreaded metal contact portion having a seal
portion; and a threaded portion, the threaded joint including: an ultraviolet ray-curable
resin coating film that is formed on the contact surface of at least one of the pin and the
box; and an acrylic silicone resin coating film that is formed on at least a portion of a
surface of the ultraviolet ray-curable resin coating film.
[0018]
(2) In the threaded joint for a steel pipe according to (I), the acrylic silicone
resin coating film may be formed on a portion which covers at least the seal portion on
the surface of the ultraviolet ray-curable resin coating film.
(3) In the threaded joint for a steel pipe according to (1) or (2), the acrylic
silicone resin coating film may be formed on the entire surface of the ultraviolet raycurable
resin coating film.
(4) In the threaded joint for a steel pipe according to any one of (1) to (3), the
ultraviolet ray-curable resin coating film may contain a colorant.
[Advantageous Effects of the Invention]
[0019]
According to the above-described aspect, at least a portion of the contact
portions of the threaded joint for a steel pipe, preferably, a portion including the seal
poltion where galling is particularly likely to occur and which is important in terms of
the airtightness of the threaded joint is coated with the coating film having a two-layer
structure of the ultraviolet ray-curable resin coating film and tlie acrylic silicone resin
coating film as an upper layer formed thereon. As a result, it is possible to provide a
threaded joint for a steel pipe capable of preventing the rusting of the threaded joint for
a steel pipe even when being stored in a tough environment; and exhibiting sufficient
lubricating perfornlance for connecting oil country tubular goods.
[0020]
In addition, even if tlie cleanliness of the contact surface is insufficient and
coating defects are generated on the ultraviolet ray-curable resin coating film, the
defects can be repaired with the acrylic silicone resin coating film according to the
above-described aspect. Therefore, rust prevention performance can be reliably
obtained.
[Brief Description of the Drawings]
[0021]
FIG. 1 is a diagram schematically illustrating an assembly state of oil country
tubular pipes and a coupling which is a threaded joint component.
FIG. 2 is a diagram schematically illustrating a connecting portion of a
threaded joint for a steel pipe.
FIG. 3 shows images illustrating the results of a rust prevention test in
Example 1.
FIG. 4 is a torque chart illustrating the change of torque with respect to the
number of turns of a threaded joint for a steel pipe.
[Best Mode for Carrying Out the Invention]
[0022]
Hereinbelow, an embodiment of the present invention will be described.
Since the basic configuration of a threaded joint for a steel pipe according to the
embodiment are the same as that illustrated in FIGS. 1 and 2, the description will be
made referring to FIGS. 1 and 2 for convenience of description.
The threaded joint for a steel pipe according to the embodiment includes a pin
1 and a box 2 each of which is provided with a contact surface that has a threaded
portion; and an unthreaded metal contact portion having a seal portion and a shoulder
portion. That is, referring to FIGS. 1 and 2, the pin 1 includes male threads 11 as the
threaded portion and includes a seal portion 13 and a shoulder portion 14 as the
unthreaded metal contact portion. In addition, the box 2 includes female threads 21
as the tlueaded portion and includes a seal portion 23 and a shoulder portion 24 as the
unthreaded metal contact portion.
The threaded joint for a steel pipe according to the embodiment includes an
ultraviolet ray-curable resin coating film that is formed on the contact surface
(preferably on the entire contact surface) of at least one (that is, only the pin 1, only the
box 2, or both the pin 1 and the box 2) of the pin 1 and the box 2; and an acrylic
silicone resin coating film that is formed on at least a portion of a surface of the
ultraviolet ray-curable resin coating film, preferably on a portion which covers the seal
portions 13 and 23.
100231
Ultraviolet Ray-Curable Resin Coating Film
The ultraviolet ray-curable resin coating film can be formed using an
appropriate ultraviolet ray-curable resin coating material which is applicable to a steel
material. Generally, the ultraviolet ray-curable resin coating material contains at least
a monomer, an oligomer, and a photopolymerization initiator and may further contain
various kinds of additives such as a pigment, a rust preventing agent, a defoaming
agent, and a leveling agent. An example of such an ultraviolet ray-curable resin
coating material is disclosed in Japanese Unexamined Patent Application, First
Publicatiotl No. 2011-12251 as a photocurable composition. However, the ultraviolet
ray-curable resin coating material is not limited thereto. In addition, co~nmercially
available ultraviolet ray-curable resin coating materials may be used.
[0024]
As the ultraviolet ray-curable resin coating material, a material is preferable in
which the rust prevention property of a coating film is superior; the curing rate is high;
and a thick film can be obtained. In addition, in consideration of a coating
environment, it is preferable that a solventless ultraviolet ray-curable resin coating
material be used. In order to enhance the lubricating property of a formed ultraviolet
ray-curable resin coating film, at least one lubricant selected from waxes, metallic
soaps, PTFE resin, and fibrous fillers may be added to the coating material in an
amount of, for example, not more than 30% by mass in terms of solid content. As the
rust preventing agent for enhancing the rust prevention performance of the ultraviolet
ray-curable resin coating film, for example, aluminum tripolyphosphate or aluminum
phosphite can be used. In this case, the amount thereof added is preferably less than
or equal to 10% by mass in terms of solid content. As the pigment for coloring the
coating film, a material which does not significantly impair the transparency of the
coating film can be used. Examples of such a pigment include fluorescent pigments
and light color pigments.
[0025]
As the ultraviolet ray-curable coating material, it is particularly preferable that
an epoxy acrylic resin having superior rust prevention performance be used. This
kind of ultraviolet ray-curable coating material contains acrylic acid (or methacrylic
acid)-modified epoxy resin, obtained by adding acrylic acid (or methacrylic acid) to
- 10 -
epoxy resin as the oligomer. In this case, an ultraviolet ray-curable resin coating
material (for example, an urethane acrylic resin) other than the above-described
coating material can be used. Examples of commercially available ultraviolet raycurable
resin coating materials include an epoxy acrylic resin coating material which is
manufactured by Chugoku Marine Paints Ltd. and has a trade name of Aulrex.
[0026]
The ultraviolet ray-curable resin coating film can be formed by coating the
contact surface ('preferably, the entire contact surface) of at least one of the pin 1 and
the box 2 of the threaded joint for a steel pipe with the ultraviolet ray-curable resin
coating material using an appropriate method; and irradiating the coated contact
surface with ultraviolet rays. As a coating method of the ultraviolet ray-curable
coating material, it is preferable that spray coating be used. However, other coating
methods such as brush coating or dip coating may be used. The irradiation of
ultraviolet rays may be performed using an appropriate radiation soufce corresponding
to the photopolymerization initiator to be used in the coating material.
[0027]
The thickness of the ultraviolet ray-curable resin coating film can be adjusted
within a range of 1 pm to 100 pm, preferably, within a range of 5 pm to 30 pm.
Generally, since the ultraviolet ray-curable resin coating film has superior adhesion
with a substrate (the contact surface of the threaded joint for a steel pipe) and is an
impermeable coating film having high transparency, rust prevention performance is
superior.
[0028]
Acrylic Silicone Resin Coating Film
The acrylic silicone resin coating film is formed on at least a portion of a
surface of the ultraviolet ray-curable resin coating film. That is, in the threaded joint
for a steel pipe according to the embodiment, a coating film having the two-layer
structure of the ultraviolet ray-curable resin coating film as the lower layer and the
acrylic silicone resin coating film as the upper layer is formed on at least a portion of
the contact poltion of at least one of the pin 1 and the box 2. By providing the acrylic
silicone resin coating film on the ultraviolet ray-curable resin coating film, not only the
above-described repairing effect but also the lubricating performalice of the threaded
joint for a steel pipe can be improved. As a result, galling is difficult to occur.
[0029]
When the acrylic silicone resin coating film is formed on only aportion of the
surface of the ultraviolet ray-curable resin coating film, it is preferable that the acrylic
silicone resin coating film be formed on a portion which covers the seal portion, in
which galling is most likely to occur, on the surface of the ultraviolet ray-curable resin
coating film. For example, a configuration may be adopted in which the threaded
portions are coated with only the ultraviolet ray-curable resin coating film; and the seal
portions and the shoulder portions (that is, the unthreaded metal contact portions) are
coated with the coating film having the two-layer structure of the ultraviolet raycurable
resin coating film as the lower layer and the acrylic silicone resin coating film
as the upper layer. Since the ultraviolet ray-curable resin coating film and the acrylic
silicone resin coating film with which the threaded portions and the seal portions are
coated have superior transparency, the states of the threaded portions and the seal
portions can be observed above the coating film. In particular, the pill 1 is easily
damaged because the tbxeaded portion (male threads 11) and the seal poition 13 are
formed on the external surface. Therefore, it is desired that the damage state of the
threaded portion be checked in advance for each connecting. According to the
embodiment, even when both the ultraviolet ray-curable resin coating film and the
acrylic silicone resin coating film are coated, the damage state of the threaded joint can
be verified, for example, by visual illspection as in the case where no coating film is
provided.
[0030]
In this case, the acrylic silicone resin coating film as the upper layer can be
formed on a portion of a contact surface not having the seal portion, for example, for
the purpose of repair. Specific examples of such a case include a case where a
pinhole, a non-uniformly coated portion, or the like is generated on the ultraviolet raycurable
resin coating film as the lower layer; and a case where the ultraviolet raycurable
resin coating film is partially damaged for some reasons. In such cases, there
is a concern of deterioration of the rust prevention property such as the rusting of
damaged portions. However, by forming the ac~ylicsi licone resin coating film on the
portions, the damage is repaired and the deterioration of the lust prevention property
can be prevented. When the ultraviolet ray-curable resin coating film is damaged, it
is preferable that the acrylic silicone resin coating film be formed such that the vicinity
of damaged portions is also coated. When the ultraviolet ray-curable resin coating
film is colored with a colorant such as a fluorescent pigment, damaged portions and
lion-unifo~mlyc oated portions of the coating film can be checked by visual inspection
or the irradiation of ultraviolet rays.
[003 11
In consideration of the rust prevention propetty and the lubricating property, it
is preferable that the acrylic silicone resin coating film as the upper layer be formed on
the entire surface of the ultraviolet ray-curable resin coating film. When the
ultraviolet ray-curable resin coating film as the lower layer and the acrylic silicone
resin coating film as the upper layer are fortned on the entire contact surface, as
described below i11 Examples, the torque on shoulder resistatlce is increased and the
connecting torque specified by the tlueaded joint for a steel pipe can be adjusted to a
high value. As a result, the additional effect of increasing the sealing property
(airtightness) of the threaded joint for a steel pipe can be obtained.
[0032]
An acrylic silicone resin coating material includes a reaction product of
silicone resin and acrylic resin as the coating film-forming component. This coating
material can form a dense resin coating film having a siloxane bond as the skeleton and
has a strong adhesion force with an undercoat layer. The formed coating film can
have high performance in adhesion, gloss maintainability, color maintainability, water
resistance, chemical resistance (acid resistance and alkali resistance), oil resistance,
contamination resistance, and long-term weather resistance, in addition to water
repellency by the silicone component. The acrylic silicone resin coating material
includes an oil type (solvent type; strong solvent type and weak solvent type) and an
aqueous type (reaction curing type emulsion coating material). Both types can be
used, but an oil type material is preferable because a coating film can be obtained by
drying at a relatively low temperature within a short period of time. In particular, it is
preferable that a normal temperature curing-type material be used. Since the
temperature curing-type acrylic silicone resin material has a superior quick-drying
property, heating is not required, a coating film can be rapidly formed, and the
continuous operation can be performed. Therefore, there are no adverse effects on the
productivity of the threaded joint for a steel pipe.
[0033]
The present inventors have thoroughly investigated various kinds of materials
of the resin coating film which is formed as the upper layer formed on the ultraviolet
ray-curable resin coating film; and as a result, have found that, when the acrylic
silicone resin coating film is formed as the upper layer, the rust prevention property is
particularly improved and superior lust prevention performance is exhibited in a
corrosive environment under salt spray conditions, as compared to an acrylic resin
coating material or a fluororesin coating material which is generally known to have
superior corrosion resistance.
[0034]
As the ac~ylicsi licone resin coating material, commercially available products
can be used. Examples of commercially available acrylic silicone resin coating
materials include an acrylic silicone resin material which is adopted as an oil silicone
lacquer spray manufactured by Kanpe Ilapio Co., Ltd.
[0035]
The acrylic silicone resin coating material is coated onto the contact surface of
the threaded joint for a steel pipe, on which the ultraviolet ray-curable resin coating
film is formed, using the same appropriate method as that of the description of the
ultraviolet ray-curable resin coating material followed by volatilizing the solvent to
cure a coating film. As a result, a cured coating film can be formed.
It is preferable that the thickness of the acrylic silicone resin coating film be in
a range of 1 pm to 100 pm. When the thickness of the acrylic silicone resin coating
film is less than 1 pm, the rust prevention property significantly deteriorates. When
the thickness of the acrylic silicone resin coating film is greater than 100 pm, the
lubricating property significantly deteriorates. In the viewpoint of coexistence with
the lust prevention property and the lubricating property, it is more preferable that the
thickness of the acrylic silicone resin coating film be in a range of 5 pm to 10 pm.
When the thickness of the acrylic silicone resin coatitrg film is in a range of 5
pm to 10 pm, an imnprove~ne~oift a coilstant rust prevention perfo~lnancec all be
confirmed. However, when tlie thickness of the acrylic silicone resin coating film is
greater than 10 pm, an improvement of a further lust prevention performance cannot
be confirmed. On tlie other hand, when the thickness of the acrylic silicoile resin
coating film becomes greatel; the galling resistance is inhibited. As a result, the
allowance frequency of the repetition with connection and disconnection decreases.
The ultraviolet ray-curable resin coating film is obtained through a curing
process in which a coating material containing a monomer, an oligomer, a
photopolymerization initiator, and the like are issadiated with ultraviolet rays to
produce radicals from the photopolylnerization initiator; the monomer is converted into
a polymer by these radicals; and the coating material is cured. On the other hand, the
acrylic silicone resin coating film is obtained through a curing process in which in a
coating material (for example, oil silicone lacquer spray) containing acrylic silicone
resin, a volatile solvent, a curing agent, and the like, the solvent volatilizes; and the
coating material is cured.
That is, the curing processes of the ultraviolet ray-curable resin coating film
and the acrylic silicone resin coating film are basically totally different from each other,
[0036]
Undercoat Treatment
The threaded portions and the unthreaded metal contact portions, which are
the contact surface of the theaded joint for a steel pipe on which the ultraviolet raycurable
resin coating film is formed, are formed through a cutting process including
thread cutting, and the surface roughness thereof is generally about 3 pni to 5 pm.
When the surface roughness of the contact surface is greater than tlie above-described
range, the adhesion of a coating film formed thereon can be increased. As a result,
the performance of the threaded joint such as the connecting property and rust
prevention property can be improved. To that end, before the formation of the
ultraviolet ray-curable resin coating film, an undercoat treatment capable of increasing
the surface roughness may be performed on the contact surface of the threaded joint for
a steel pipe.
[0037]
Examples of such an undercoat treatment include a blast treatment of blasting
an abrasive such as a spherical shot material or a horn-shaped grid material; and an
acid washing treatment of dipping the contact surface in a strong acid solution such as
sulfuric acid, hydrochloric acid, nitric acid, or hydrofluoric acid to roughen the surface.
These treatments can increase the surface roughness of a base material itself.
[0038]
Other examples of the undercoat treatment include chemical conversion
coating such as a phosphating treatment, an oxalating treatment, or a borating
treatment; and metal plating.
In the chemical conversion treatment, a conversion coating film formed of
needle crystal and having a large surface roughmess is formed. As a result, the surface
roughness is increased and the adhesion of a solid rust prevention film and a solid
lubricating coating film formed thereon can be increased. In order to filrther improve
the rust prevention property, it is preferable that an undercoat layer (lower layer formed
below the ultraviolet ray-curable resin coating film) be formed of zinc phosphate,
manganese phosphate, or the like.
[0039]
Metal plating can improve the galling resistance of the threaded joint for a
steel pipe, and partial metal plating can increase the surface roughness. Examples of
metal plating for increasing the surface roughness include plating of coppel; iron, and
an alloy thereof using an electroplating method; impact plating of zinc or a zinc alloy
in which particles, obtained by coating iron cores with zinc, an zinc-iron alloy, or the
like, are blown by a centrifugal force or an air pressure, zinc particles or zinc-iron alloy
particles are deposited, and a porous metal coating film is formed; and composite metal
plating of dispersing solid fine particles in a metal to form a coating film. In
particular, the impact plating is effective for increasing the surface roughness.
[0040]
No matter which method is used in the undercoat treatment of the contact
surface, it is preferable that the surface roughness Rz be adjusted to 5 pm to 40 pm
after the surface roughening of the undercoat treatment. When the surface roughness
Rz is less than 5 pm, there are cases in which the adhesion with a coating film formed
on the contact surface is insufficient. On the other hand, when the surface roughness
Rz is greater than 40 pm, the friction of the surface increases and there are cases where,
when a high surface pressure is applied thereto, a coating film formed on the contact
surface cannot endure a shearing force and a compression force and is broken or peeled
off. As the undercoat treatment for surface roughening, two or more kinds of
treatments may be used in combination.
[0041]
Other Coating Films
When the ultraviolet ray-curable resin coating film as the lower layer and the
acrylic silicone resin coating film as the upper layer are formed on the contact surface
of at least one (for example, the pin 1) of the pin 1 and the box 2 of the threaded joint
for a steel pipe, the treatment on the contact surface of the other one (for example, the
box 2) is not particularly limited. For example, only the ultraviolet ray-curable resin
coating film as the lower layer may be formed. Alternatively, various kinds of
surface treatments of a threaded joint for a steel pipe, which have been disclosed and
will be disclosed in the related art, may be applied thereto.
[0042]
The coating film having a two layer structure according to the present
invention of the ultraviolet ray-curable resin coating film as the lower layer and the
acrylic silicone resin coating film as the upper layer has a superior rust prevention
property. However, when this coating film having the two-layer structure is formed
on the contact surfaces of both the pin 1 and the box 2, there are cases where galling
resistance is insufficient for the connecting of the threaded joint for a steel pipe. In
this case, it is preferable that the coating film having the two-layer structure according
to the present invention be formed on the contact surface of at least one of the pin 1
and the box 2; and a solid lubricating coating film described below be formed on the
contact surface of the other one.
[0043]
Examples of the solid lubricating coating film include a coating film which
contains lubricating powder and optionally other components in an organic or
inorganic resin. Examples of the lubricating powder include molybdenum disulfide,
tungsten disulfide, graphite, and polytetrafluoroethylene. Examples of the resin
which is a binder include organic resins such as epoxy resin, polyamide resin, and
polyamideimide resin; and silicon or titanium compounds (for example, alkyl silicate,
silica sol, and titania sol) which can form an silica or titania inorganic resin coating
film. The solid lubricating coating film may be a thin lubricating film having no
stickiness disclosed in WO 2007/04223 1 in which solid lubricant particles having
plastic or viscoplastic rlieological behavior (flow characteristic) are dispersed in a solid
matrix. This lubricating coating film call be formed by coating, using a hot melt
method, a composition which contains, for example, polyethylene as a thermoplastic
polymer; wax (for example, carnauba wax) and metallic soap (for example, zinc
stearate) as lubricating components; and calcium sulfonate as a corrosion inliibitor.
Wax, metallic soap, calcium sulfonate, and the like can be added to the abovedescribed
solid lubricating coating film containing lubricating powder in the resin.
[0044]
The thickness of the solid lubricating coating film is not particularly limited,
but is generally in a range of 5 ptn to 100 pm and preferably in a range of 5 pm to 50
pm. When the solid lubricating coating film is formed, similarly, in order to improve
the coating adhesion, it is preferable that the above-described undercoat treatment for
increasing the surface roughness be performed on the contact surface to be coated.
[0045]
Since the solid lubricating coating film is generally opaque, it is preferable
that the solid lubricating film be applied to the contact surface of the box 2 in which
the tllreaded portion is not easily damaged compared to the pin 1; and that the twolayer
coating film according to the present invention including the ultraviolet raycurable
resin coating film as the lower layer and the acrylic silicone resin coating film
as the upper layer be formed on the contact surface of the pin 1. By doing so, since
the two-layer coating film has superior transparency, the two-layer coating film is
typically formed on external surfaces of both ends of a long steel pipe and thus the
state of the threaded portion of the pin portion which is easily damaged can be
observed by visual inspection from the above of the two-layer coating film which
covers the pin 1.
In the above-described embodinlent, the threaded joint for a steel pipe
including the shoulder portions 14 and 24 has been described as an example.
However, the present invention can be applied to a threaded joint for a steel pipe not
including the shoulder portions 14 and 24. For example, the threaded joint for a steel
pipe is a threaded joint that a thread (for example, a wedge-shaped thread or the like)
itself has a locking function which stops the rotation of connection.
[0046]
The effects of the present invention will be described using the following
examples. Herei~lbelowt,h e contact surface including the threaded portion and the
unthreaded metal contact portion of the pin will be referred to as "pin surface"; and the
contact surface including the threaded portion and the unthreaded metal contact portion
of the box will be referred to as "box surface". In the examples, " % represents "%
by mass" unless specified otherwise.
[Examples]
[0047]
Example 1
In this example, various kinds of resin coating films were formed on the
ultraviolet ray-curable resin coating film which covers the contact surface of the
tl~eadedjo int for a steel pipe to investigate the rust prevention property. For
reference, the same test was performed for a case where only the ultraviolet raycurable
resin coating film was formed and a ease where a penetrating lubricant CRC 5-
56 (KURE 5-56, manufactured by Kure Engineering Ltd.) which is known to have a
high rust prevention effect was coated on the ultraviolet ray-curable resin coating film.
[0048]
A thread cutting treatment was performed on a portion of a carbon steel pipe
(outer diameter: 177.8 mm) approximately 120 mm distant fsom a pipe end. This
threaded portion was spray-coated with a commercially available ultraviolet raycurable
resin coating material of epoxy acrylic resin (Aulrex 550, manufactured by
Chugoku Marine Paints Ltd.) and was irradiated with ultraviolet rays at room
temperature for 60 seconds to cure the coating film. Spray-coating and irradiation of
ultraviolet rays were performed while rotating the steel pipe. A high-pressure
mercury lamp was used as an ultraviolet light source, aud the irradiation was
approximately 500 m~/crn'. In this way, the ultraviolet ray-curable resin coating film
having an average thickness of 30 pm was formed on the surface of the threaded
portion.
[0049]
The threaded portion on which the ultraviolet ray-curable resin coating film
was formed was cut into plural strip specimens. Various kinds of commercially
available resin coating materials or the above-described penetrating lubricant CRC 5-
56 was spray-coated on the ultraviolet ray-curable resin coating films of the strip
specimens. The three kinds of coating materials used are as follows.
(1) Thermosetting acrylic resin coating material: Acrylic lacquer spray a
(manufactured by Dai Nippon Co., Ltd.)
(2) Fluororesin coating material: Spray coating material of
polytetrafluoroethylene (nianufactured by Henkel Japan Ltd.)
(3) Acrylic silicone resin coating material: Silicone lacquer spray (acrylic
silicone resin coating material; manufactured by Kanpe Hapio Co., Ltd.)
After coating, the coating films were naturally dried according to the
instruction of the coating material. Various kinds of resin coating materials or the
penetrating lubricant CRC 5-56 was coated for about 5 seconds.
Using the strip specimens on which the ultraviolet ray-curable resin coating
film as the lower layer and the resin coating film or the penetrating lubricant coating
film as the upper layer were formed, a salt spray test (aqueous 5% NaCl solution,
40°C) was performed for 200 hours according to JIS 22371. The rust area ratios of
the specimens 200 hours after the spray salt test were evaluated by visual inspection,
and the results thereof are shown in Table 1 and FIG. 3.
I Table 1
I Film I
I
As illustrated in Table 1 and FIG. 3, under the condition 1-1 that only the
Upper-Layer
Coating
Film
Rust Area
Ratio
ultraviolet ray-curable resin coating film was formed and the resin coating film was not
Conditions
formed as the upper layer, lust was observed on 50% or higher of the entire surface of
1-3
the threaded portion of the specimen. Meanwhile, under the condition 1-2 that the
Lower-
Layer
Coating
1-1
None
50% or
higher
1-4
Ultraviolet Ray-Curable Resin Coating Film
1-2
Fluororesin
40% to 50%
1-5
Acrylic
Silicone
Resin
0% to 10%
CRC 5-56
40% to 50%
Acrylic
Resin
30% to 40%
penetrating lubricant CRC 5-56 which is known to have a high rust prevention effect
was coated for tlie upper layer, the rust area ratio was reduced to 40% to 50%. Under
the condition 1-4 that the fluororesin was coated for the upper layer, tlie same effect
was obtained. Under the condition 1-3 that the acrylic resin was coated for the upper
layer, the rust area ratio was reduced to 30% to 40% but was insufficient. On the
other hand, under the condition 1-5 that the acrylic silicone resin was coated for the
upper layer, the rust area ratio could be reduced to approximately 10% or lower. It
was clearly seen that the effect of the acrylic silicone resin was particularly higher as
the upper-layer resin coating film for enhancing the rust prevention properly of the
ultraviolet ray-curable resin coating film.
[0053]
Example 2
It was verified in Example 1 that the rust prevention effect of the coating film
having the two-layer structure of the ultraviolet ray-curable resin coating film as the
lower layer and the acrylic silicone resin coating film as the upper layer is significantly
high. However, in the case of a threaded joint for a steel pipe, the connecting
property of the tbreaded joint is the most important performance, and there is a concern
that the formation of the acrylic silicone resin coating film may inhibit this
performance. Therefore, the influence of the two-layer coating film on connecting
performance and airtightness was inspected.
[0054]
The pin surface of a tlxeaded joint for a steel pipe (VAMTOP, registered trade
name) having a seal portion, which was formed at a pipe end of an OCTG steel pipe
having an outer diameter of 177.8 mm, was spray-coated with the same ultraviolet raycurable
resin coating material of epoxy acrylic resin as that used in Example 1 and was
irradiated with ultraviolet rays at room temperature for 60 seconds to cure the coating
film. Spray-coating and irradiation of ultraviolet rays were performed while rotating
the steel pipe. The ultraviolet light source, the irradiation dose, and the thickness of
the coating film were the same as those of Example 1. In this way, the ultraviolet raycurable
resin coating film was formed on the entire pin surface containing a seal
portion and a shoulder portion.
[0055]
Regarding the pin surface which was coated with the ultraviolet ray-curable
resin coating film, only the seal portion and the shoulder portion of the pin tip end or
the entire pin surface containing the threaded portion was spray-coated with the same
acrylic silicone resin coating material as that used in Example 1 and was naturally
dried to folm a coating film. Coating was performed while rotating the steel pipe.
The thickness of the coating film was approximately 5 pm.
lo0561
As shown in Table 2, the condition 2-1 was that the acrylic silicone resin was
not coated (only the ultraviolet ray-curable resin coating film was coated); the
condition 2-2 was that the acrylic silicone resin coating film was formed on only the
pin tip end (the seal portion and the shoulder portion); and the condition 2-3 was that
the acrylic silicone resin coating film was formed on the entire pin surface containing
the threaded portion.
lo0571
On the box surface, under any conditions, a solid lubricating solid film
(thickness: 50 P I ) containing high basic calcium sulfonate as a major component in a
substrate having a wax component was formed.
The undercoat treatment performed before forming the ultraviolet ray-curable
resill coating film was a zinc phosphating treatment on the pin surface and was a
copper-tin-zinc ternary alloy plating treatment on the box treatment.
[0058]
Using the threaded joint subjected to the surface treatment as described above,
the connecting performance and the airtightness were inspected as follows.
Connecting Performance
The connecting performance was evaluated based on the influence on the
shouldering torque and the torque on shoulder resistance. The shouldering torque
refers to the torque value when the shoulder portions of the pin and the box are in
contact with each other (when interference starts). 111 addition, the torque on shoulder
resistance refers to the torque difference between the shouldering torque and the torque
at the time of the yield deformation of the shoulder portion. As the torque on
shoulder resistance is higher, the specified torque value during the connecting of the
threaded joint can be set to be higher. Therefore, when connecting is performed at the
specified torque value, the interference amount of the seal portion, that is, the
airtightness of the threaded joint can be increased. When the threaded joint was
finally connected at the specified torque value, the difference between the specified
torque value and the shouldering torque value affects the contact pressure of the seal
portions of the pin and the box. Therefore, the control of the shouldering torque
value and the torque on shoulder resistance value is important.
[0059]
Using the threaded joint in which the pin surface and the box surface were
treated as described above, connecting started at a predetermined rotating speed.
Connecting continued by applying the torque while monitoring the torque with respect
to the number of turns until it was confirmed that the joint shoulder yielded. From
the monitoring results (torque chart illustrating the torque change with respect to the
number of turns; refer to FIG. 4), the torque at the point at which the rapid increase of
toque increases was obtained as the shouldering torque Ts; and the difference between
the torque (yield torque Ty) at tlie point at which the rapid increase of toque is stopped
and the shouldering torque Ts was obtained as the torque on shoulder resistance AT.
The test results under the respective conditions are collectively shown in Table 2.
[0060]
Table 2
Coating Conditions ofAcrylic Silicone Shouldering Torque
Coating on Only I
(ft-lbs)
4851
Resin Coating Film
I Shoulder Portion) I
Condition 2-1
Condition 2-2
No Coating
Torque On Shoulder
Pin Tip End (Seal
Portion and
Condition 2-3
Resistance (ft-lbs)
4432
[0061]
As can be seen from the results of Table. 2, as compared to the condition 2-1,
the value of the shouldering torque Ts was reduced under the condition 2-2 and was
further reduced under the condition 2-3. Under the condition 2-3 that the acrylic
silicone resin coating film was coated on tlie entire pin surface, the value of the
shouldering torque Ts was reduced by approximately 11% as compared to the
condition 2-1. It was confirmed from these results that the formation of the acrylic
Coating On Entire
Pin Surface
4307
silicone resin coating film on the ultraviolet ray-curable resin coating film does not
inhibit the lubrication of the joint. Against the optimum torque To illustrated in FIG.4,
the lower limit of the shouldering torque Ts is substantially within a range of 5% to
15%, and the upper limit of the shouldering torque Ts is substantially within a range of
70% to 85%. Therefore, the variation of approximately 11% is a level which
becomes no problems.
[0062]
Meanwhile, the torque on shoulder resistance AT, which is the torque
difference between the shoulder torque Ts and the torque (yield torque Ty) at the time
of shoulder deformation, was increased by approximately 11% particularly under the
condition 2-3 that the acrylic silicone resin coating film was formed on the entire pin
surface, as compared to a case where the acrylic silicone resin coating film was not
formed. The increase in the torque on shoulder resistance AT represents that the
specified connecting torque is set to be higher correspondingly. Accordingly, the seal
performance of the thseaded joint for a steel pipe can be increased.
[0063]
Airtightness
The threaded joint for a steel pipe in which the pin surface and the box surface
were treated as described above was connected with the specified torque (7470 ft-lbs)
and was heated at 1 80°C according to a method of IS01 3679 to perform an
airtightness test under a complex load of internal pressure/external pressure and
tensioll/compression. It was confirmed fsom the results for the airtightness that the
existence of the acrylic silicone resin coating film does not affect the airtightness
performance.
[0064]
It was found from the above results that, when the acrylic silicone resin
coating film is formed on the ultraviolet ray-curable resin coating film, the rust
prevention property of the threaded joint for a steel pipe can be significantly improved;
there are no adverse effects on the airtightness of the threaded joint for a steel pipe; and
the specified torque can be set to be high because the torque on shoulder resistance AT
is increased, thereby obtaining the effect of improving the sealing property of the
threaded joint for a steel pipe.
[Explanation of References]
[0065]
A: oil country tubular goods
B: coupling
1: pin
2: box
11 : male thread
2 1 : female thread
13,23: seal portion
14,24: shoulder portion
[Designation of Document] CLAIMS
[Claim 1]
A theaded joint for a steel pipe including a pin and a box each of which is
provided with a contact surface that includes: an unthreaded metal contact portion
having a seal portion; and a threaded portion, the theaded joint comprising:
an ultraviolet ray-curable resin coating film that is formed on the contact
surface of at least one of the pin and the box; and
an acrylic silicone resin coating film that is formed on at least a portion of a
surface of the ultraviolet ray-curable resin coating film.
[Claim 2]
The threaded joint for a steel pipe accolding to claim 1,
I
wherein the acrylic silicone resin coating film is formed on a portion which
covers at least the seal portion on the surface of the ultraviolet ray-curable resin
coating film.
[Claim 3]
The threaded joint for a steel pipe according to claim I or 2,
wherein the acrylic silicone resin coating film is formed on an entire suface
of the ultraviolet ray-curable resin coating film.
[Claim 4]
The threaded joint for a steel pipe according to any one of claims 1 to 3,
wherein the ultraviolet ray-curable resin coating film contains a colorant.