Description
Tubular Threaded Joint Having Improved High Torque Performance
Technical Field
This invention relates to a tubular threaded joint used for connecting steel
pipes and particularly oil country tubular goods and to a surface treatment method
therefor. A tubular threaded joint according to the present invention can reliably
exhibit excellent galling resistance without the application of a lubricating grease
such as compound grease which in the past has been applied to threaded joints each
time when makeup of oil country tubular goods is carried out. Therefore, a
tubular threaded joint according to the present invention can avoid the adverse
effects on the global environment and humans caused by compound grease. In
addition, the joint does not readily yield even if makeup is carried out with a high
torque, thereby making it possible to realize a stable metal-to-metal seal with an
adequate operating margin.
Background Art
Oil country tubular goods such as tubing and casing used for excavation of
oil wells for exploitation of crude oil or gas oil are usually connected with each
other (made up) using tubular threaded joints. In the past, the depth of oil wells
was 2,000 - 3,000 meters, but in deep wells such as recent oil fields in the sea, the
depth sometimes reaches 8,000 - 10,000 meters or larger. The length of oil
country tubular goods is typically 10 some meters, and tubing through which a
fluid such as crude oil flows is surrounded by a plurality of casings. Therefore,
the number of oil country tubular goods which are connected by threaded joints
reaches a huge number.
Since tubular threaded joints for oil country tubular goods are subjected in
their environment of use to loads in the form of tensile forces in the axial direction
caused by the mass of oil country tubular goods and the joints themselves,
compound pressures such as internal and external pressures, and geothermal heat,
they need to maintain gas tightness without being damaged even in such severe
environments.
Typical tubular threaded joints used for connecting oil country tubular
goods (also referred to as special threaded joints) have a pin-box structure. A pin,
which is a joint component having male (external) threads, is typically formed on
the outer surface of both ends of an oil country tubular good, and a box, which is a
counterpart joint component having female (internal) threads which engage with
the male threads, is typically formed on the inner surface of both sides of a
coupling, which is a separate member. As shown in Figure 1, the pin has a
shoulder portion (also referred to as a torque shoulder) formed on the end surface at
the tip of the pin and a seal portion formed between the end surface and the male
threads. Correspondingly, the box has a seal portion and a shoulder portion
located in the rear of the female threads and adapted to contact the seal portion and
the shoulder portion of the pin, respectively. The seal portions and the shoulder
portions of the pin and the box constitute unthreaded metal contact portions of a
tubular threaded joint, and the unthreaded metal contact portions and the threaded
portions of the pin and the box constitute contact surfaces of a tubular threaded
joint. Below-described Patent Document 1 discloses an example of such a special
threaded joint.
In order to perform makeup of this tubular threaded joint, one end (the pin)
of an oil country tubular good is inserted into a coupling (box), and the male
threads and the female threads are tightened until the shoulder portions of the pin
and the box contact each other and interfere with a suitable torque. As a result,
the seal portions of the pin and the box intimately contact each other to form a
metal-to-metal seal which guarantees the gas tightness of the threaded joint.
Due to various troubles occurring during the process of lowering tubing or
casing into an oil well, it is sometimes necessary to loosen a threaded joint which
has been made up, raise the joint from the oil well, retighten it, and again lower it
into the well. API (American Petroleum Institute) requires galling resistance such
that unrepairable seizing referred to as galling does not take place and gas tightness
is maintained even if tightening (makeup) and loosening (breakout) are carried out
0 times on a joint for tubing and 3 times on a joint for casing.
In order to increase galling resistance and gas tightness, a viscous liquid
lubricant (lubricating grease) referred to as compound grease or dope and
containing heavy metal powder has been previously applied to the contact surfaces
of a threaded joint each time makeup is carried out. Such compound grease is
prescribed by API BUL 5A2.
With the object of increasing the retention of compound grease and
improving sliding properties, it has been proposed to subject the contact surfaces of
a threaded joint to various types of surface treatment to form one or more layers
such as nitride treatment, various types of plating including galvanizing and
dispersion plating, and phosphate chemical conversion treatment. However, as
stated below, the use of compound grease may have adverse effects on the
environment and humans.
Compound grease contains large amounts of powder of heavy metals such
as zinc, lead, and copper. At the time of makeup of a threaded joint, the applied
grease is washed off or overflows to the outer surface, and it can have an adverse
effect on the environment and especially on sea life particularly due to harmful
heavy metals such as lead. In addition, the process of applying compound grease
worsens the work environment and the work efficiency, and there is also a concern
of harm to humans.
In recent years, as a result of the enactment in 1998 of the OSPAR
Convention (Oslo-Paris Convention) aimed at preventing marine pollution in the
Northeast Atlantic, strict environmental regulations are being enacted on a global
scale, and in some regions, the use of compound grease is already being regulated.
Accordingly, in order to avoid harmful effects on the environment and humans
during the excavation of gas wells and oil wells, a demand has developed for
threaded joints which can exhibit excellent galling resistance without using
compound grease.
As a threaded joint which can be used to connect oil country tubular goods
without application of compound grease, the present applicant proposed in Patent
Document 2 a threaded joint for steel pipes having a viscous liquid or semisolid
lubricating coating and in Patent Document 3 a threaded joint for steel pipes having
a solid lubricating coating.
Patent Document 1: JP 5-87275 A
Patent Document 2: JP 2002-173692 A
Patent Document 3:WO 2009/072486
Summary of the Invention
As stated above, with a special threaded joint like that shown in Figure 1
constituted by a pin and a box each with a seal portion, the seal portions of the pin
and the box form a metal-to-metal seal to guarantee gas tightness at the end of
makeup.
Figure 2 shows a torque chart at the time of makeup of this type of threaded
joint (ordinate: torque, abscissa: number of turns). As shown in this figure, as
rotation takes place, the threaded portions of the pin and the box initially contact
and torque gradually increases. Subsequently, the seal portions of the pin and the
box contact, and the rate of increase of torque increases. Eventually, the shoulder
portion at the tip of the pin and the shoulder portion of the box contact and begin to
interfere (the torque at the start of this interference is referred to as the shouldering
torque Ts), upon which the torque abruptly increases. Makeup is completed when
the torque reaches a predetermined makeup torque. The optimum torque in Figure
2 means the torque that is optimal for the completion of makeup with achieving an
amount of interference in the seal portions which is necessary for guaranteeing gas
tightness. A proper value for the optimum torque is predetermined depending on
the internal diameter and the type of a joint.
However, in a special threaded joint used in very deep wells in which
compressive stresses and bending stresses are applied, makeup is sometimes carried
out with a torque which is higher than usual to prevent the tightened thread from
loosening. In this case, the shoulder portion at the end of the pin and the shoulder
portion of the box which it contacts sometimes yield, leading to plastic deformation
of the shoulder portion of at least one member of the pin and the box. As a result,
as shown in Figure 2, the rate of increase of torque suddenly decreases. The
torque at the time when yielding and plastic deformation occur is referred to as the
yield torque Ty. Yielding of the shoulder portions leads to a failure of gas
tightness.
In a threaded joint which is made up with a high torque, it is advantageous
for the value of [Ty minus Ts] (Ty- Ts = DT ,or the torque-on-shoulder resistance)
to be large. However, in the tubular threaded joints described in Patent Document
2 having a viscous liquid or semisolid lubricating coating, Ty is low compared to
when a conventional compound grease is applied. As a result, DT becomes small,
and the shoulder portions yield at a low makeup torque, so it is sometimes
impossible to perform makeup with a high torque. In the tubular threaded joints
described in Patent Document 3 having a solid lubricating coating as well, DT
becomes smaller than that of a conventional compound grease.
The object of the present invention is to provide a tubular threaded joint
which does not readily undergo yielding of its shoulder portions even when it is
made up with a high torque and which has a lubricating coating which does not
contain harmful heavy metals, which has excellent galling resistance, gas tightness,
and rust-preventing properties, and which can afford a large DT to the joint.
It was found that even if the composition of a lubricating coating is varied
so as to vary its coefficient of friction, DT does not greatly change because Ts and
Ty typically vary in the same direction. For example, if the coefficient of friction
of a lubricating coating increases, Ty increases, but Ts also increases (a
phenomenon referred to as high shouldering). As a result, in the worst case, the
condition referred to as no-shouldering occurs in which the shoulder portions do
not contact at a predetermined makeup torque and makeup cannot be completed.
The present inventors found that with a tubular threaded joint having a
viscous liquid or solid lubricating coating which does not contain harmful heavy
metals which impose a burden on the global environment, by forming a highfriction
solid lubricating coating on a portion of the contact surface (the threaded
portion and the unthreaded metal contact portion) of at least one of a pin and a box
such as on the shoulder portion which is initially contacted and preferably on the
unthreaded metal contact portion including the seal portion and the shoulder
portion, and forming on at least the remaining portion of the contact surface a
lubricating coating selected from a viscous liquid lubricating coating and a solid
lubricating coating having a lower coefficient of friction than the high-friction solid
lubricating coating, a tubular threaded joint is obtained which has a large DT and
which does not undergo no-shouldering while having sufficient galling resistance,
gas tightness, and rust-preventing properties.
The mechanism by which a large DT is achieved is thought to be generally
as follows.
Makeup of a tubular threaded joint is carried out by inserting a pin into a
box and then rotating the pin or the box. Initially only the threaded portions of the
pin and the box contact and threadingly engage with each other. In the final stage
of makeup, the seal portions and the shoulder portions begin to contact, and
makeup is completed when a predetermined amount of interference is obtained
between the seal portions and the shoulder portions.
As shown in Figure 5(A), for example, with a tubular threaded joint having
a high-friction solid lubricating coating on the seal portions and the shoulder
portions of the contact surfaces of both a pin and a box and a lubricating coating
having a lower coefficient of friction on the remaining portion (primarily the
threaded portions), while only the threaded portions of the pin and the box initially
contact, a low friction state is achieved by the lubricating coating having a low
coefficient of friction which covers the threaded portions, so Ts becomes low. In
the final stage of makeup, when the seal portions and the shoulder portions start to
contact, the high-friction solid lubricating coatings which cover these portions
contact, causing a high-friction state to occur and causing Ty to increase. As a
result, DT is increased.
The present invention, which is based on this knowledge, is a tubular
threaded joint constituted by a pin and a box each having a contact surface
comprising an unthreaded metal contact portion including a seal portion and a
shoulder portion and a threaded portion, characterized in that the contact surface of
at least one of the pin and the box has a first lubricating coating and a second
lubricating coating, the first lubricating coating being a solid lubricating coating
formed on a portion of the contact surface including the shoulder portion, the
second lubricating coating being selected from a viscous liquid lubricating coating
and a solid lubricating coating and formed on at least the portion of the contact
surface where the first lubricating coating is not present, the first lubricating
coating having a coefficient of friction which is higher than that of the second
lubricating coating, the second lubricating coating being positioned on top in a
portion where both the first lubricating coating and the second lubricating coating
are present.
The portion of the contact surface having the first lubricating coating may
be just the shoulder portion, but preferably it is the entirety of the unthreaded metal
contact portion, namely, the seal portion and the shoulder portion.
The second lubricating coating may be provided just on the portion of the
contact surface which does not have the first lubricating coating, or it may be
provided on the entire contact surface having the first lubricating coating. In the
latter case, the second lubricating coating is positioned on top of the first
lubricating coating.
Preferred coating thicknesses of each coating are as follows.
The coating thickness of the first lubricating coating is 5 - 40 m i.
The coating thickness of the viscous liquid lubricating coating as a second
lubricating coating is 5 - 200 m . However, when this viscous liquid lubricating
coating is positioned on top of the first lubricating coating, the total of the coating
thickness of the first lubricating coating and that of the viscous liquid lubricating
coating is at most 200 m h.
The coating thickness of the solid lubricating coating as a second
lubricating coating is 5 - 150 mh . However, when this solid lubricating coating is
positioned on top of the first lubricating coating, the total of the coating thickness
of the first lubricating coating and that of the second solid lubricating coating is at
most 150 m .
When the contact surface of only one of the pin and the box has the first
lubricating coating and the second lubricating coating as described above, there are
no particular limitations concerning the contact surface of the other member of the
pin and the box, and it may be in an untreated state (for example, it may be in a
state after the below-described preparatory surface treatment). Preferably,
however, at least a portion of the contact surface of the other member and
preferably the entirety of the contact surface has any of the following surface
treatment coatings formed thereon:
1) a lubricating coating selected from a viscous liquid lubricating coating
and a solid lubricating coating,
2) a solid anticorrosive coating, or
3) a lower layer in the form of a lubricating coating selected from a viscous
liquid lubricating coating and a solid lubricating coating, and an upper layer in the
form of a solid anticorrosive coating.
The solid anticorrosive coating is preferably a coating based on an
ultraviolet curing resin. The lubricating coating may be either the abovedescribed
first lubricating coating or the second solid lubricating coating.
The contact surface of at least one and preferably both of the pin and the
box can be previously subjected to surface treatment by a method selected from one
or more of blasting treatment, pickling, phosphate chemical conversion treatment,
oxalate chemical conversion treatment, borate chemical conversion treatment,
electroplating, and impact plating in order to increase the adhesion and the
retention of a coating formed atop the contact surface and/or to increase the galling
resistance of the threaded joint.
In a tubular threaded joint according to the present invention, a lubricating
coating which is formed on its contact surfaces exhibits a large DT as observed
with a coating made of a conventional lubricating grease such as compound grease
which contains harmful heavy metals. Therefore, even at the time of makeup with
a high torque, it is possible to perform makeup without the occurrence of yielding
or galling of the shoulder portions. In addition, galling can be suppressed even
under severe conditions such as during unstable excavation operations in the sea.
Furthermore, since the lubricating coating contains substantially no harmful heavy
metals such as lead, it poses almost no burden on the global environment. A
tubular threaded joint according to the present invention suppress the occurrence of
rust, and it can continue to exhibit a lubricating function even when subjected to
repeated makeup and breakout, so it can guarantee gas tightness after makeup.
Brief Explanation of the Drawings
Figure 1 schematically shows the unthreaded metal contact portions (the
shoulder portions and seal portions) of a special threaded joint.
Figure 2 is a typical torque chart at the time of makeup of a special
threaded joint.
Figure 3 schematically shows the assembled structure of a steel pipe and a
coupling at the time of shipment of the steel pipe.
Figure 4 schematically shows a cross section of a special threaded joint.
Figures 5(A) - 5(C) show examples of the structure of coatings on a tubular
threaded joint according to the present invention.
Figures 6(A) - 6(C) show examples of the structure of different coatings on
a tubular threaded joint according to the present invention.
Modes for Carrying Out the Invention
Below, embodiments of a tubular threaded joint according to the present
invention will be explained in detail by way of example. The present invention is
not limited to the below-mentioned embodiments.
Figure 3 schematically shows the state of a typical tubular threaded joint at
the time of shipment. A pin 1 having a male threaded portion 3a is formed on the
outer surface of both ends of a steel pipe A, and a box 2 having a female threaded
portion 3b is formed on the inner surface of both sides of a coupling B. The
coupling B is previously connected to one end of the steel pipe A. Although not
shown in the drawing, a protector for protecting the threaded portions is previously
mounted on the unconnected pin of the steel pipe A and the unconnected box of the
coupling B before shipment. These protectors are removed from the threaded
joint before use.
As shown in the drawing, in a typical tubular threaded joint, a pin is formed
on the outer surface of both ends of a steel pipe and a box is formed on the inner
surface of a coupling, which is a separate member. There are also integral tubular
threaded joints which do not utilize a coupling and in which one end of a steel pipe
is made a pin and the other end is made a box. A tubular threaded joint according
to the present invention can be of either type.
Figure 4 schematically shows the structure of a special threaded joint
(referred to below simply as a threaded joint), which is a typical tubular threaded
joint used for connecting oil country tubular goods. This threaded joint is
constituted by a pin 1 formed on the outer surface of an end of a steel pipe A and a
box 2 formed on the inner surface of a coupling B. The pin 1 has a male threaded
portion 3a, a seal portion 4a located near the tip of the steel pipe, and a shoulder
portion 5a at its end surface. Correspondingly, the box 2 has a female threaded
portion 3b, and a seal portion 4b and a shoulder portion 5b on its inner side.
The seal portions and the shoulder portions of the pin 1 and the box 2 are
unthreaded metal contact portions, and the unthreaded metal contact portions
(namely, the seal portions and the shoulder portions) and the threaded portions are
the contact surfaces of the threaded joint. These contact surfaces need to have
galling resistance, gas tightness, and rust-preventing properties. In the past, to
provide these properties, (a) a compound grease containing heavy metal powder has
been applied to the contact surface of at least one of the pin and the box, or (b) a
viscous liquid, semisolid, or solid lubricating coating has been formed on the
contact surface. However, as stated above, (a) has the problem that it has an
adverse effect on humans and the environment, and (b) has the problem of a small
DT whereby when makeup is carried out with a high torque, there is the possibility
of yielding of the shoulder portions occurring before completion of makeup.
A threaded joint according to the present invention has a first lubricating
coating and a second lubricating coating on the contact surface of at least one
member of the pin and the box. The first lubricating coating is a solid lubricating
coating formed on a portion of the contact surface including at least the shoulder
portion. The second lubricating coating is selected from a viscous liquid
lubricating coating and a solid lubricating coating and formed on at least the
portion of the contact surface where the first lubricating coating is not present.
The first lubricating coating is a coating having relatively high friction with a
coefficient of friction which is higher than the coefficient of friction of the second
lubricating coating.
Below, the first lubricating coating will be referred to as a high-friction
solid lubricating coating, and when the second lubricating coating is a solid
lubricating coating, that solid lubricating coating will sometimes be referred to as a
second solid lubricating coating.
In the locations close to the threaded portions between the threaded
portions and the seal portions of the pin and the box of a threaded joint, a portion
where the pin and the box do not contact each other when the threaded joint is
made up is provided with the object of preventing lubricating components from
oozing out at the time of makeup of the threaded joint. In some threaded joints, a
non-contacting region where the pin and the box intentionally do not contact is
provided. Such portions where the pin and the box do not contact each other at
the time of makeup are not part of the contact surfaces, and it does not matter
whether a coating according to the present invention is applied to these portions.
A high-friction solid lubricating coating which is the first lubricating
coating is formed on just a portion of the contact surface of one or both of the pin
and the box which includes the shoulder portion. The portion of the contact
surface having the high-friction solid lubricating coating may be just the shoulder
portion, but preferably it is the entire unthreaded metal contact portion including
the seal portion and the shoulder portion. Namely, the high-friction solid
lubricating coating is preferably formed on the seal portion and the shoulder
portion of the contact surface of at least one of the pin and the box. At least the
remaining portion of the contact surface which does not have the high-friction solid
lubricating coating has a second lubricating coating selected from a viscous liquid
lubricating coating and a solid lubricating coating formed thereon. The second
lubricating coating may be formed on the entire contact surface, in which case the
second lubricating coating is positioned on top of the high-friction solid lubricating
coating (namely, it forms an upper layer). It is also possible for the second
lubricating coating to be formed just on the portion where the high-friction solid
lubricating coating is not present (e.g., just on the threaded portion).
When the contact surface of only one member of the pin and the box has
the high-friction solid lubricating coating and the second lubricating coating, there
is no particular limitation on surface treatment of the contact surface of the other
member of the pin and the box. For example, a high-friction solid lubricating
coating which may be the same as or different from the first lubricating coating, a
viscous liquid lubricating coating or a solid lubricating coating which may be the
same as or different from the second lubricating coating, a solid anticorrosive
coating, and a combination of a lower layer in the form of a lubricating coating and
particularly a viscous liquid lubricating coating and an upper layer in the form of a
solid anticorrosive coating can be formed on at least a portion of the contact surface
and preferably on the entire contact surface of the other member. Alternatively,
the contact surface of the other member can be left untreated, or it can be subjected
to just the below-described preparatory surface treatment for surface roughening
(such as phosphate chemical conversion treatment).
Figures 5(A) - (C) and Figures 6(A) - (B) show various possible
embodiments of combinations of the first and second lubricating coatings. In
these figures, of the male threads of the threaded portion of the pin 1, the thread 3a'
at the extreme end and closest to the seal portion 4a are formed with an incomplete
shape which is observed at the start of thread cutting. By making the thread at the
extreme end of the pin incomplete threads, stabbing of the pin becomes easier, and
the possibility of damage to the threaded portion of the box at the time of stabbing
of the pin is decreased.
Figure 5(A) shows an embodiment in which the unthreaded metal contact
portions (the seal portions and the shoulder portions) of the contact surfaces of both
the pin and the box have a high-friction solid lubricating coating 10, and the
remainder of each contact surface, which is primarily the threaded portion, has a
second lubricating coating 11.
Figure 5(B) shows an embodiment in which the unthreaded metal contact
portions of the contact surfaces of both the pin and the box have a high-friction
solid lubricating coating 10, and a second lubricating coating 11 which covers the
entirety each contact surface is formed atop each high-friction solid lubricating
coating 10.
Figure 5(C) shows an embodiment in which one of the pin and the box (the
pin in the figure) has a high-friction solid lubricating coating 10 which covers the
unthreaded metal contact portion and atop it a second lubricating coating 1 which
covers the entire contact surface in the same manner as in Figure 5(B), and the
entire contact surface of the other member (the box in the figure) is coated with a
second lubricating coating 11.
Figure 6(A) shows an embodiment in which one of the pin and the box (the
pin in the figure) has a high-friction solid lubricating coating which covers the
unthreaded metal contact portion and a second lubricating coating 11 which covers
the remainder of the contact surface in the same manner as in Figure 5(A), and the
entire contact surface of the other member (the box in the figure) is covered by a
second lubricating coating 11.
Figure 6(B) shows an embodiment in which one of the pin and the box (the
box in the figure) has a high-friction solid lubricating coating 10 which covers the
unthreaded metal contact portion and a second lubricating coating 11 which covers
the remainder of the contact surface in the same manner as in Figure 5(A), and the
entire contact surface of the other member (the pin in the figure) is covered by a
solid anticorrosive coating 12.
Figure 6(C) shows an embodiment in which one of the pin and the box (the
pin in the figure) has a high-friction solid lubricating coating 10 which covers the
unthreaded metal contact portions and atop it a second lubricating coating 11 which
covers the entire contact surface in the same manner as in Figure 5(B), and the
entire contact surface of the other member (the box in the figure) is covered by a
high-friction solid lubricating coating 10.
It is understood by those skilled in the art that a tubular threaded joint
according to the present invention can have a coating structure which is a
combination of coatings other than the combinations described above. For
example, the second lubricating coating 11 on one of the pin and the box in Figure
5(A) or on the pin in Figure 6(A) can be replaced by a solid anticorrosive coating.
In this case, the second lubricating coating 11 which is present on only one member
covers the portion on which the high-friction solid lubricating coating is not formed
including at least the threaded portion as shown in Figure 6(B).
Next, various coatings which cover the contact surfaces of a tubular
threaded joint according to the present invention will be explained. Unless
otherwise specified, % with respect to the content of components of the coatings
means mass %. This content is substantially the same as the content based on the
total solids content (the total content of nonvolatile components) of a coating
composition for forming a lubricating coating.
[High-Friction Solid Lubricating Coating]
The high-friction solid lubricating coating is a solid lubricating coating
having a relatively high coefficient of friction compared to the second lubricating
coating. It produces a high-friction state in the final stage of makeup of a threaded
joint (starting when the shoulder portions of the pin and the box contact until the
seal portions intimately contact with a predetermined amount of interference),
thereby increasing DT by increasing Ty and making it difficult for yielding of the
shoulder portions to take place even when makeup is carried out with a high torque.
In the present invention, a high-friction solid lubricating coating which has
such an effect is provided so as to cover a portion of the contact surface including
at least the shoulder portion of at least one of a pin and a box. Preferably, the
entirety of the unthreaded metal contact portion including the seal portion and the
shoulder portion is covered by the high-friction solid lubricating coating. When a
threaded joint has a plurality of seal portions and shoulder portions, it is preferable
to cover the entirety of the seal portions and the shoulder portions with a highfriction
solid lubricating coating. However, the objective of increasing DT can be
achieved even if only the shoulder portions where contact initially takes place in
the final stage of makeup of a threaded joint are covered with a high-friction solid
lubricating coating. The location where a high-friction solid lubricating coating is
formed can be suitably set in accordance with the shape of a joint and the required
performance.
Even when a second lubricating coating 11 is formed atop the high-friction
solid lubricating coating 10 such as on the pin 1 and the box 2 as shown in Figure
5(B) or on the pin 1 as shown in Figure 5(C), a high-friction state is achieved by
the high-friction solid lubricating coating 10 in the final stage of makeup, and a
desired effect of increasing DT can be achieved. The high-friction solid
lubricating coating needs to have a higher coefficient of friction than the second
lubricating coating 1 . A certain degree of adhesion to the substrate (the contact
surfaces of the pin and the box, which may be in an as-machined state or may have
a preparatory surface treatment coating such as one formed by phosphate chemical
conversion treatment or metal plating) is necessary.
An example of a high-friction solid lubricating coating which is suitable for
use in the present invention is a coating comprising an organic resin or inorganic
polymer which contains little or no solid lubricating particles (such as in an amount
of at most 5 mass %, preferably at most 3 mass %, and more preferably at most 1
mass % based on the total solids content).
A particularly preferred high-friction solid lubricating coating is a solid
lubricating coating which is formed from a film-forming composition which is used
for lubricating treatment before hydroforming of steel. Specific examples of such
a composition are Surflube C291 manufactured by Nippon Paint Co., Ltd. (based
on a water-soluble resin) and Gardolube L6334 and L6337 manufactured by
Chemetall GmbH. A solid lubricating coating formed from this type of
composition has a higher coefficient of friction than a lubricating coating used for
lubricating threaded joints (such as a lubricating coating selected from a viscous
liquid lubricating coating and a second solid lubricating coating used in the present
invention), and it forms a solid lubricating coating having good adhesion and
affinity to a lubricating coating. However, the solid lubricating coating which is
formed still has good lubricating properties and sliding properties, so as shown in
Figure 5(A) and Figure 6(B), for example, even if a second lubricating coating
having a low coefficient of friction is not present on the unthreaded metal contact
portion including the shoulder portion, galling resistance necessary for makeup and
sufficient gas tightness after makeup are obtained if a second lubricating coating is
present on the threaded portions of at least one of the pin and the box.
Another high-friction solid lubricating coating which can be used is a
coating comprising the same components as the below-described second solid
lubricating coating but which has a reduced content of a solid lubricant (lubricating
powder).
The coefficient of friction of a solid lubricating coating or a viscous liquid
lubricating coating can be measured in accordance with ASTM D2625 (load
carrying capacity and lifespan of solid film lubricants) or ASTM D2670 (wear
properties of fluid lubricants) by the Falex pin and Vee block method (referred to
below as the Falex method) using a Falex pin and Vee block machine. In the
Falex method, blocks (Vee blocks) having a tip with a V-shaped opening are
disposed facing opposite sides of a pin, and the pin is rotated while applying a
predetermined pressure loading to the blocks to measure the coefficient of friction.
Measurement of the coefficient of friction can be carried out using test
pieces constituted by blocks and a pin which are taken from a steel billet made of
the same material as used in a tubular threaded joint and which have undergone the
same preparatory surface treatment and surface coating treatment. Measurement
is carried out at around 1 GPa, which corresponds to the maximum pressure of the
seal portions at the time of makeup of a tubular threaded joint, and the average
coefficient of friction in a steady frictional state before the occurrence of galling
can be compared. Of course, a high-friction solid lubricating coating according to
the present invention can be selected based on the coefficient of friction measured
using another friction measuring apparatus normally used in a laboratory.
Whatever the measurement method, it is sufficient for the coefficient of friction of
the high-friction solid lubricating coating to be higher than the coefficient of
friction of the second lubricating coating when measurement is carried out under
the same conditions.
As long as the high-friction solid lubricating coating according to the
present invention has a higher coefficient of friction than the viscous liquid
lubricating coating or the second solid lubricating coating used as the second
lubricating coating, there is no particular lower limit on the coefficient of friction of
the high-friction solid lubricating coating. However, in order to adequately
achieve the objective of increasing Ty and increasing DT, the coefficient of friction
of the high-friction solid lubricating coating is preferably larger by a certain extent
than the coefficient of friction of the second lubricating coating. Preferably, the
coefficient of friction of the high-friction solid lubricating coating is at least 1.5
times, more preferably at least 2 times, and most preferably at least 2.5 times the
coefficient of friction of the second lubricating coating.
The coefficient of friction of the high-friction solid lubricating coating as
measured by the above-stated Falex method is preferably at least 0.06, more
preferably at least 0.08, and most preferably at least 0.1. Since an extremely high
coefficient of friction has an adverse effect on the galling resistance of a threaded
joint, the coefficient of friction of the high-friction solid lubricating coating is
preferably at most 0.25 and more preferably at most 0.20.
The thickness of the high-friction solid lubricating coating is preferably 5 -
40 mh . If it is less than 5 mhi, the effect of producing a high level of friction at the
time of contact and galling resistance may be inadequate. On the other hand, if it
exceeds 40 m i, not only does the friction-increasing effect reach a limit but an
adverse effect on the properties of the seal portion may develop.
The high-friction solid lubricating coating can be formed by coating
methods well known to those skilled in the art. In order to form a high-friction
solid lubricating coating on a portion of the contact surface of the pin and/or the
box, namely, on only the shoulder portion or on the unthreaded metal contact
portion including the seal portion and the shoulder portion, spray coating can be
carried out while shielding with a suitable means the portions where it is not
desired to form the high-friction solid lubricating coating. Upon drying to
evaporate solvents after application, a high-friction solid lubricating coating is
formed.
[Viscous Liquid Lubricating Coating]
A viscous liquid lubricating coating can be formed using a lubricating
grease which has been conventionally used to improve the galling resistance of the
contact surfaces of a threaded joint. It is preferable to use a lubricating grease
referred to as green dope which has little adverse effect on the environment and
contains no or little heavy metal powder.
A preferred example of such a viscous liquid lubricating coating is a
coating comprising a suitable amount of a base oil and at least one material selected
from a rosin-based material, wax, metal soap, and a basic metal salt of an aromatic
organic acid. Of these components, a rosin-based material is effective primarily at
increasing the coefficient of friction of a lubricating coating, namely, at increasing
DT, while wax, metal soap, and a basic metal salt of an aromatic organic acid are
effective primarily at preventing galling of a lubricating coating. Therefore, it is
possible for a coating to exhibit adequate lubricating performance even if it does
not contain a powder of a soft heavy metal such as lead or zinc. A particularly
preferable viscous liquid lubricating coating comprises all of a rosin-based
material, wax, metal soap, and a basic metal salt of an aromatic organic acid.
A rosin-based material is selected from rosin and its derivatives. When it
is contained in a lubricating coating, it becomes highly viscous when it undergoes a
high pressure in a frictional interface. As a result, it is effective at increasing DT
of the coating. The rosin which is used may be any of tall rosin, gum rosin, and
wood rosin, and various rosin derivatives such as rosin esters, hydrogenated rosins,
polymerized rosins, and disproportionated rosins can also be used. The content of
the rosin-based material in the lubricating coating is preferably 5 - 30% and more
preferably 5 - 20%.
Wax not only has the effect of preventing galling by decreasing the friction
of a lubricating coating, it also decreases the fluidity of the coating and increases
the coating strength. Any of animal, vegetable, mineral, and synthetic waxes can
be used. Examples of waxes which can be used are beeswax and whale tallow
(animal waxes); Japan wax, carnauba wax, candelilla wax, and rice wax (vegetable
waxes); paraffin wax, microcrystalline wax, petrolatum, montan wax, ozokerite,
and ceresine (mineral waxes); and oxide wax, polyethylene wax, Fischer-Tropsch
wax, amide wax, and hardened castor oil (castor wax) (synthetic waxes). Of
these, paraffin wax with a molecular weight of 150 - 500 is preferred. The wax
content of a lubricating coating is preferably 2 - 20 %.
A metal soap, which is a salt of a fatty acid with a metal other than an
alkali metal, is effective at increasing the galling-preventing effect and the rustpreventing
effect of a coating. Its content is preferably 2 - 20 %.
The fatty acid of a metal soap is preferably one having 12 - 30 carbon
atoms from the standpoint of lubricating properties and rust prevention. The fatty
acid can be either saturated or unsaturated. Mixed fatty acids derived from natural
oils and fats such as beef tallow, lard, wool fat, palm oil, rapeseed oil, and coconut
oil, and single compounds such as lauric acid, tridecyclic acid, myristic acid,
palmitic acid, lanopalmitic acid, stearic acid, isostearic acid, oleic acid, elaidic acid,
arachic acid, behenic acid, erucic acid, lignoceric acid, lanoceric acid, a sulfonic
acid, salicylic acid, and a carboxylic acid may be used. The metal salt is
preferably in the form of a calcium salt, but it may also be a salt of another alkaline
earth metal or a zinc salt. The salt may be either a neutral salt or a basic salt.
A viscous liquid lubricating coating may contain a basic metal salt of an
aromatic organic acid selected from basic sulfonates, basic salicylates, basic
phenates, and basic carboxylates as a rust-preventing agent. Each of these basic
metal salts of an aromatic organic acid is a salt of an aromatic organic acid with
excess alkali (an alkali metal or an alkaline earth metal) in which the excess alkali
is present as colliodal fine particles dispersed in oil. These basic metal salts are a
grease or a semisolid substance at room temperature, and exhibit a lubricating
action in addition to a rust-preventing action. The alkali which constitutes the
cation part of a basic metal salt of an aromatic organic acid may be an alkali metal
or an alkaline earth metal, but preferably it is an alkaline earth metal and
particularly calcium, barium, or magnesium, each providing the same effect. Its
content in the lubricating coating is preferably 10 to 70%.
The higher the base number of the basic metal salt of an aromatic organic
acid used as a rust-preventing agent, the greater the amount of the fine particles of
the salt which function as a solid lubricant, and the better the lubricating properties
(galling resistance) which can be imparted by the lubricating coating. When the
base number exceeds a certain level, the salt has the effect of neutralizing acid
components, and the rust-preventing effect of the lubricating coating is increased.
For these reasons, it is preferable to use one having a base number (JIS K 2501) of
50 - 500 mgKOH/g. A preferred base number is 100 - 500 g KOH/g, and more
preferably it is in the range of 250 - 450 mg KOH/g.
In order to suppress the fluidity of a viscous liquid lubricating coating at
high temperatures and further increase its galling resistance, the lubricating coating
may contain a lubricating powder. The lubricating powder can be any harmless
one which is not toxic and which does not overly decrease the coefficient of
friction. A preferred lubricating powder is graphite. Amorphous graphite which
produces little decrease in the coefficient of friction is more preferred. The
content of a lubricating powder is preferably 0.5 - 20%.
In order to increase the uniformity of dispersion of a solid lubricating
powder in the lubricating coating or to improve the properties of the lubricating
coating, the lubricating coating may include components other than those described
above, such as one or more components selected from organic resins and various
oils and additives normally used in lubricating oils (such as an extreme pressure
agent).
Oils refer to lubricating components which are liquid at room temperature
and which can be used in lubricating oils. Oils themselves have lubricating
properties. Examples of oils which can be used include synthetic esters, natural
oils, and mineral oils. The above-described rust-preventing agents (basic salts of
aromatic organic acids) also have lubricating properties, so they also function as
oils. The properties of a lubricating coating vary with the content of oil. If a
coating does not contain an oil or if the oil content is too low, the lubricating
coating does not become a viscous liquid lubricating coating and instead becomes a
solid lubricating coating. In the present invention, such a lubricating coating can
also be used as a solid lubricating coating.
An organic resin and particularly a thermoplastic resin suppresses tackiness
of the lubricating coating and increases the thickness of the coating, and when it is
introduced into a frictional interface, it increases galling resistance and decreases
friction between contacting metal portions even when a high makeup torque (a high
pressure) is applied. Therefore, it may be contained in a lubricating coating. In
such cases, it is preferable to use a resin in powder form having a particle diameter
in the range of 0.05 - 30 m and more preferably in the range of 0.07 - 20 m h .
Some examples of thermoplastic resins are polyethylene resins,
polypropylene resins, polystyrene resins, poly(methyl acrylate) resins, styreneacrylic
acid ester copolymer resins, polyamide resins, and polybutene
(polybutylene) resins. A copolymer or blend or of these resins or of these resins
and other thermoplastic resins can also be used. The density of the thermoplastic
resin (JIS K 7 112) is preferably in the range of 0.9 - 1.2. In addition, in view of
the necessity for the resin to readily deform on a frictional surface in order to
exhibit lubricating properties, the thermal deformation temperature (JIS K 7206) of
the resin is preferably 50 - 150° C.
When the lubricating coating contains a thermoplastic resin, the content
thereof in the coating is preferably at most 10 % and more preferably in the range
of 0.1 - 5 %. The total content of the above-described rosin-based material and
the thermoplastic resin is preferably at most 30 %.
Examples of natural oils and fats which can be used as an oil include beef
tallow, lard, wool fat, palm oil, rapeseed oil, and coconut oil. A mineral oil
(including a synthetic mineral oil) having a viscosity at 40° C of 10 - 300 cSt can
also be used as an oil.
A synthetic ester which can be used as an oil can increase the plasticity of
the thermoplastic resin and at the same time can increase the fluidity of the
lubricating coating when subjected to hydrostatic pressure. In addition, a
synthetic ester with a high melting point can be used to adjust the melting point and
hardness (softness) of the lubricating coating. Examples of a synthetic ester are
fatty acid monoesters, dibasic acid diesters, and fatty acid esters of
trimethylolpropane or pentaerythritol.
Examples of fatty acid monoesters are monoesters of carboxylic acids
having 12 - 24 carbon atoms with higher alcohols having 8 - 20 carbon atoms.
Examples of dibasic acid diesters are diesters of dibasic acids having 6 - 10 carbon
atoms with higher alcohols having 8 - 20 carbon atoms. Examples of fatty acids
which constitute a fatty acid ester of trimethylolpropane or pentaerythritol are ones
having 8 - 18 carbon atoms.
When a lubricating coating contains at least one of the above oils, the
content of the oil is preferably made at least 0.1 mass % in order to obtain an
increase in galling resistance. The content is preferably made at most 5 mass % in
order to prevent a decrease in the coating strength.
An extreme pressure agent has the effect of increasing the galling
resistance of the lubricating coating when added in a small amount. Nonlimiting
examples of an extreme pressure agent are vulcanized oils, polysulfides, and
phosphates, phosphites, thiophosphates, and dithiophosphoric acid metal salts.
When an extreme pressure agent is contained in a lubricating coating, its content is
preferably in the range of 0.05 - 5 mass %.
Examples of preferred vulcanized oils are compounds which contain 5 - 30
mass % of sulfur and are obtained by adding sulfur to unsaturated animal or
vegetable oils such as olive oil, castor oil, rice bran oil, cottonseed oil, rapeseed oil,
soy bean oil, corn oil, beef tallow, and lard and heating the mixture.
Examples of preferred polysulfides are polysulfides of the formula R S)
R2 (wherein R and R2 may be the same or different and are an alkyl group having
4 - 22 carbon atoms, an aryl group, an alkylaryl group, or an arylalkyl group, and c
is an integer from 2 to 5) and olefin sulfides containing 2 - 5 sulfur bonds per
molecule. Dibenzyl disulfide, di-tert-dodecyl polysulfide, and di-tert-nonyl
polysulfide are particularly preferred.
Phosphates, phosphites, thiophosphates, and dithiophosphoric acid metal
salts may be of the following general formulas.
phosphates: (R3O)(R4O)P(=O)(OR5)
phosphites: (R3O)(R4O)P(OR5)
thiophosphates: (R3O)(R4O)P(=S)(OR5)
dithiophosphoric acid metal salts: [(R3O)(R 0 )P(=S)-S ]2-M
In the above formulas, R3 and R are each an alkyl group having 1 to 24
carbon atoms, a cycloalkyl group, an alkylcycloalkyl group, an aryl group, an
alkylaryl group, or an arylalkyl group, R 4 and R5 are each a hydrogen atom, an
alkyl group having 1 to 24 carbon atoms, a cycloalkyl group, an alkylcycloalkyl
group, an aryl group, an alkylaryl group, or an arylalkyl group, and M is
molybdenum (Mo), zinc (Zn), or barium (Ba).
In addition to the above-described components, the viscous liquid
lubricating coating may contain an antioxidant, a preservative, a colorant, and the
like.
A viscous liquid lubricating coating can be formed by applying a coating
composition to the contact surfaces of at least one of the pin and the box of a
threaded joint, and drying the coating if necessary. Depending on the coating
method, the composition which is used may contain a volatile organic solvent in
addition to the above-described components.
When the coating composition is a solid or semisolid at room temperature,
it may be applied after being heated to lower its viscosity (for example, it may be
applied with a spray gun in the form of a hot melt).
When heating is not employed, a solvent is contained in the coating
composition to decrease the viscosity of the composition to a viscosity sufficient
for application. As a result, the coating thickness and the composition of the
lubricating coating which is formed are made uniform and coating formation can be
carried out efficiently. Examples of preferred solvents are petroleum solvents
such as solvents corresponding to industrial gasoline prescribed by JIS K 2201,
mineral spirits, aromatic petroleum naphtha, xylene, and Cellosolves. Two or
more of these may be used in combination. A solvent having a flash point of at
least 30° C, an initial boiling point of at least 150° C, and a final boiling point of at
most 210° C is preferred because it is relatively easy to handle and it rapidly
evaporates, so the drying time can be short.
A preferred coating thickness of the viscous liquid lubricating coating is 5 -
200 m and more preferably 15 - 200 m . The lubricating coating is preferably
sufficiently thick to fill minute interstices in the contact surfaces such as the spaces
between threads. If the coating thickness is too small, the effects of components
such as a rosin-based material, wax, metal soap, or lubricating powder being
supplied to the frictional surface from the interstices due to the action of hydrostatic
pressure which develops at the time of makeup can no longer be expected, and the
galling resistance of a threaded joint worsens. Furthermore, when the lubricating
coating contains a rust-preventing agent, the rust-preventing effect becomes
inadequate. On the other hand, making the coating thickness too large is not only
wasteful, but it runs counter to preventing environmental pollution, which is one of
the objects of the present invention. When a viscous liquid lubricating coating is
formed atop a high-friction solid lubricating coating 10 as a second lubricating
coating 11 as shown in Figures 5(B) and 5(C), the total coating thickness of the
high-friction solid lubricating coating and the viscous liquid lubricating coating is
preferably at most 200 m h .
[Second Solid Lubricating Coating]
A solid lubricating coating which is used to form the second lubricating
coating in the form of the second solid lubricating coating in the present invention
is basically constituted by a powder having a solid lubricating action (referred to as
a lubricating powder) and a binder. This coating can be formed by applying a
dispersion having a lubricating powder dispersed in a binder-containing solution.
The lubricating powder is strongly adhered to the surface of a threaded joint in a
state in which it is dispersed in the binder in the coating, and at the time of makeup,
it is stretched by the makeup pressure to a reduced thickness. As a result, it
increases the galling resistance of a threaded joint.
Examples of a lubricating powder include but not limited to molybdenum
disulfide, tungsten disulfide, graphite, fluorinated graphite, zinc oxide, tin sulfide,
bismuth sulfide, organomolybdenum compounds (e.g., a molybdenum
dialkylthiophosphate or a molybdenum dialkylthiocarbamate), PTFE
(polytetrafluoroethylene), and BN (boron nitride). One or more of these can be
used.
From the standpoints of the adhesion and rust-preventing properties of the
solid lubricating coating, graphite is a particularly preferred lubricating powder,
and from the standpoint of film-forming properties, amorphous graphite is more
preferred. A preferred content of lubricating powder in the solid lubricating
coating is 2 - 1 mass %. In the present invention, it is necessary for the
coefficient of friction of the second solid lubricating coating to be lower than the
coefficient of friction of the high-friction solid lubricating coating. The
coefficient of friction of the second solid lubricating coating can be adjusted by the
content of the lubricating powder. Accordingly, as stated above, if the content of
a lubricating powder is made small, this type of solid lubricating coating can also
be used as a high-friction solid lubricating coating.
The binder can be an organic resin or an inorganic polymer.
The organic resin is preferably one having heat resistance and suitable
hardness and wear resistance. Examples of such a resin are thermosetting resins
such as epoxy resins, polyimide resins, polycarbodiimide resins, phenolic resins,
furan resins, and silicone resins; and thermoplastic resins such as polyolefins,
polystyrenes, polyurethanes, polyimides, polyesters, polycarbonates, acrylic resins,
thermoplastic epoxy resins, polyamide-imide resins, polyether ether ketones, and
polyether sulfones. The resin which is used may be a copolymer or a blend of two
or more resins.
When the binder is a thermosetting resin, from the standpoints of adhesion
and wear resistance of a thermosetting solid lubricating coating, it is preferable to
perform heat setting treatment. The temperature of this heat setting treatment is
preferably at least 120° C and more preferably 150 - 380° C, and the treatment time
is preferably at least 30 minutes and more preferably 30 - 60 minutes.
When the binder is a thermoplastic resin, it is possible to employ a coating
composition using a solvent, but it is also possible to form a thermoplastic solid
lubricating coating without a solvent using the hot melt method. In the hot melt
method, a coating composition containing a thermoplastic resin and a lubricating
powder is heated to melt the thermoplastic resin, and the composition which has
become a low viscosity fluid is sprayed from a spray gun having a temperature
maintaining ability which maintains a constant temperature (normally a
temperature which is around the same as the temperature of the composition in a
molten state). The heating temperature of the composition is preferably 10 - 50°
C higher than the melting point of the thermoplastic resin (the melting temperature
or the softening point). In this method, it is suitable to use a thermoplastic resin
having a melting point of 80 - 320° C and preferably 90 - 200° C.
The substrate which is coated (namely, the contact surface of the pin and/or
the box) is preferably preheated to a temperature higher than the melting point of
the thermoplastic resin. As a result, it is possible to obtain good coating
properties. When the coating composition contains a small amount (such as at
most 2 mass %) of a surface active agent such as polydimethyl siloxane, it is
possible to form a good quality coating even if the substrate is not preheated or
even if the preheating temperature is lower than the melting point of the base resin.
After coating, the thermoplastic resin is solidified by cooling the substrate by air
cooling or natural cooling to form a solid lubricating coating atop the substrate.
The inorganic polymer is a compound having a three-dimensionally
crosslinked structure of metal-oxygen bonds such as Ti-O, Si-O, Zr-O, Mn-O, Ce-
O, or Ba-O. This compound can be formed by hydrolysis and condensation of a
hydrolyzable organometallic compound typified by a metal alkoxide, although
hydrolyzable inorganic compound such as titanium tetrachloride can also be used.
A preferred metal alkoxide which can be used is one having lower alkoxy groups
such as methoxy, ethoxy, isopropoxy, propoxy, isobutoxy, butoxy, or tert-butoxy
groups. A preferred metal alkoxide is an alkoxide of titanium or silicon, and a
titanium alkoxide is particularly preferable. Among these, titanium isopropoxide
is preferred due to its excellent film-forming properties.
The inorganic polymer may contain an alkyl group which may be
substituted with a functional group such as an amine or an epoxy group. For
example, it is possible to use an organometallic compound in which some of the
alkoxy groups are replaced by an alkyl group containing a functional group as is
the case with silane coupling agents and titanate coupling agents.
When the binder is an inorganic polymer, a lubricating powder is added to
a solution of a metal alkoxide or a partial hydrolysate thereof and dispersed therein,
and the resulting composition is applied to the contact surface of at least one of a
pin and a box. The resulting coating may be subjected to humidifying treatment
and then heated if necessary, thereby allowing hydrolysis and condensation of the
metal alkoxide to proceed and forming a solid lubricating coating in which a
lubricating powder is dispersed in a coating formed from an inorganic polymer
having metal-oxygen bonds.
Even when using any of the above-described binders, when the coating
composition contains a solvent, the solvent may be any of water, a water-miscible
organic solvent such as an alcohol, or a water-immiscible organic solvent such as a
hydrocarbon or an ester. Two or more types of solvents may be used in
combination.
In addition to a lubricating powder, various additives such as a rustpreventing
agent can be added to the solid lubricating coating within a range that
does not impair the galling resistance of the coating. For example, the rustpreventing
properties of the solid lubricating coating itself can be improved by
adding one or more of zinc powder, a chromium pigment, silica, and an alumina
pigment. A particularly preferred rust-preventing agent is calcium ion exchanged
silica. A solid lubricating coating may also contain an inorganic powder in order
to adjust the sliding properties of the coating. Examples of such an inorganic
powder are titanium dioxide and bismuth oxide. These rust-preventing agents,
inorganic powders, and the like (namely, powder components other than a
lubricating powder) can be present in a total amount of up to 20% of the solid
lubricating coating.
In addition to the above components, the solid lubricating coating may
contain auxiliary additives selected from a surface active agent, a colorant, an
antioxidant, and the like in an amount of at most 5%, for example. It is also
possible to contain an extreme pressure agent, a liquid lubricant, or the like in a
very small amount of at most 2 %.
For the same reasons as given for the viscous liquid lubricating coating, the
thickness of the solid lubricating coating is preferably 5 - 150 m and more
preferably 20 - 100 m h. When the solid lubricating coating is formed atop a highfriction
solid lubricating coating, the total thickness of the high-friction solid
lubricating coating and the solid lubricating coating is preferably at most 150 m h.
[Solid Anticorrosive Coating]
As stated above with respect to Figure 4, during the time until actual use of
a tubular threaded joint, a protector is often mounted on a pin or box which has not
been connected to another member. It is necessary for a solid anticorrosive
coating not to be destroyed under at least a force applied during mounting of a
protector, not to be dissolved even when exposed to water which condenses below
the dew point during transport or storage, and not to easily soften at high
temperatures exceeding 40° C. Any coating which can satisfy such properties can
be used as a solid anticorrosive coating. For example, a solid anticorrosive
coating may be a thermosetting resin coating optionally containing a rustpreventing
component.
A preferred solid anticorrosive coating is a coating based on an ultraviolet
curing resin. Known resin compositions comprising at least a monomer, an
oligomer, and a photopolymerization initiator can be used as an ultraviolet curing
resin.
Examples of monomers include but are not limited to polyvalent (di-, tri-,
or higher) esters of a polyvalent alcohol with a (meth)acrylic acid, various
(meth)acrylate compounds, N-vinylpyrrolidone, N-vinylcaprolactam, and styrene.
Examples of oligomers include but are not limited to epoxy (meth)acrylates,
urethane (meth)acrylates, polyester (meth)acrylates, polyester (meth)acrylates,
polyether (meth)acrylates, and silicone (meth)acrylates.
Useful photopolymerization initiators are compounds having absorption in
the wavelength range of 260 - 450 nm, including benzoin and its derivatives,
benzophenone and its derivatives, acetophenone and its derivatives, Michler's
ketone, benzil and its derivatives, tetralkylthiuram monosulfide, thioxanes, and the
like. It is particularly preferred to use a thioxane.
From the standpoints of coating strength and sliding properties, a solid
anticorrosive coating formed from an ultraviolet curing resin may contain an
additive selected from a lubricant and/or a fibrous filler and a rust-preventing agent.
Examples of a lubricant are metal soaps such as calcium stearate and zinc stearate,
and polytetrafluoroethylene (PTFE) resin. An example of a fibrous filler is
acicular calcium carbonate such as Whiskal sold by Maruo Calcium Co., Ltd..
One or more of these additives can be added in an amount of 0.05 - 0.35 parts by
mass with respect to 1 part by mass of the ultraviolet curing resin. Examples of a
rust-preventing agent are aluminum tripolyphosphate and aluminum phosphite. It
can be added in a maximum amount of around 0.10 parts by mass with respect to 1
part by mass of the ultraviolet curing resin.
A solid anticorrosive coating which is formed from an ultraviolet curing
resin is often transparent. From the standpoint of facilitating quality inspection of
the resulting solid anticorrosive coating either visually or by image processing
(investigating whether there is a coating and the uniformity or nonuniformity of the
coating thickness), the solid anticorrosive coating may contain a colorant. The
colorant which is used can be selected from pigments, dyes, and fluorescent
materials. The amount of a colorant is preferably a maximum of 0.05 parts by
mass with respect to one part by mass of the ultraviolet curing resin.
A preferred colorant is a fluorescent material. A fluorescent material may
be any of fluorescent pigments, fluorescent dyes, and fluophors used in fluorescent
paints, but preferably it is a fluorescent pigment. A solid anticorrosive coating
which contains a fluorescent material is colorless or transparent with a color under
visible light, but when it is irradiated with a black light or ultraviolet rays, it
fluoresces and becomes colored, making it possible to ascertain the presence of a
coating or whether there is unevenness of the coating. Furthermore, as it is
transparent under visible light, the material underneath the solid anticorrosive
coating, namely, the surface of the substrate can be observed. Accordingly,
inspection for damage to the threaded portions of the threaded joint is not impeded
by a solid anticorrosive coating.
After a composition based on an ultraviolet curing resin is applied to a
contact surface of a threaded joint, it is irradiated with ultraviolet light to cure the
coating, resulting in the formation of a solid anticorrosive coating based on an
ultraviolet curing resin. Irradiation with ultraviolet light can use a usual
commercially available ultraviolet light irradiation apparatus having an output
wavelength in the range of 200 - 450 nm. Examples of a source of ultraviolet
light are a high pressure mercury vapor lamp, an ultrahigh pressure mercury vapor
lamp, a xenon lamp, a carbon arc lamp, a metal halide lamp, and sunlight.
The coating thickness of a solid anticorrosive coating (the overall coating
thickness when there are two or more layers of an ultraviolet curing resin) is
preferably in the range of 5 - 50 m and more preferably in the range of 10 - 40
um. If the coating thickness of the solid anticorrosive coating is too small, it does
not adequately function as a anticorrosive coating. On the other hand, if the
coating thickness of the solid anticorrosive coating is too large, the solid
anticorrosive coating is sometimes destroyed under the force of mounting when
installing a protective member such as a protector, and corrosion prevention ends
up being inadequate.
A solid anticorrosive coating based on an ultraviolet curing resin is a
transparent coating, so the condition of the substrate can be observed through the
coating without removing it, and it is possible to inspect the threaded portions
before makeup from atop the coating. Accordingly, by forming the solid
anticorrosive coating on the contact surface of a pin, it is possible to easily inspect
for damage of the threaded portion of the pin which is typically formed on the outer
surface of an end of a steel pipe and which is easily damaged.
As stated above with respect to the high-friction solid lubricating coating,
each of the above-described viscous liquid lubricating coating, solid lubricating
coating, and solid anticorrosive coating is preferably applied by spray coating.
Spray coating includes hot melt coating.
As shown in Figure 5(A), when a high-friction solid lubricating coating is
formed on the unthreaded metal contact portion of a contact surface and a second
lubricating coating is formed on the threaded portion which is the remaining
portion of the contact surface, either the high-friction solid lubricating coating or
the second lubricating coating may be formed first. In this case, particularly when
the second lubricating coating is a solid lubricating coating, it is preferable to make
the thicknesses of the high-friction solid lubricating coating and the solid
lubricating coating approximately the same (for example, within ± 15 m h) so that a
large step does not develop at the border between the two types of coatings.
When the second lubricating coating is a viscous liquid lubricating coating, it has a
large ability to deform at the time of makeup, so the second lubricating coating and
the high-friction solid lubricating coating may have a large difference in their
thicknesses. Normally, the viscous liquid lubricating coating has a larger coating
thickness than the high-friction solid lubricating coating.
[Preparatory Surface Treatment]
In a tubular threaded joint according to the present invention in which a
high-friction solid lubricating coating and a second lubricating coating and in some
cases also a solid anticorrosive coating are formed on the contact surfaces of a pin
and/or a box, if preparatory surface treatment for surface roughening is carried out
on the contact surfaces which are the substrate for the coatings so that the surface
roughness is greater than 3 - 5 mp , which is the surface roughness after machining,
the coating adhesion increases, and there is a tendency for the desired effects of the
coatings to be enhanced. Accordingly, before forming a coating, it is preferable to
carry out preparatory surface treatment on the contact surfaces to roughen the
surfaces.
When a coating is formed atop a contact surface having a large surface
roughness, the thickness of the coating is preferably larger than Rmax of the
contact surface so as to completely cover the contact surface. When the contact
surface is rough, the thickness of a coating is the average value of the overall
coating thickness which is calculated from the area, the mass, and the density of the
coating.
Examples of preparatory surface treatment for surface roughening are
blasting treatment by projecting a blasting material such as spherical shot or
angular grit, pickling by immersion in a strong acid such as sulfuric acid,
hydrochloric acid, nitric acid, or hydrofluoric acid to roughen the surface, chemical
conversion treatment such as phosphate treatment, oxalate treatment, or borate
treatment (as the resulting crystals grow, the roughness of the crystal surface
increases), electroplating with a metal such as Cu, Fe, Sn, or Zn or an alloy of these
metals (projections are selectively plated, so the surface is slightly roughened), and
impact plating which can form a porous plated coating. As one type of
electroplating, composite plating which forms a plated coating in which minute
solid particles are dispersed in metal is possible as a method of imparting surface
roughness because the minute solid particles project from the plated coating.
Preparatory surface treatment may use two or more methods in combination.
Treatment can be carried out in accordance with known methods.
Whichever preparatory surface treatment method is used for the contact
surfaces, the surface roughness R ax produced by preparatory surface treatment
for surface roughening is preferably 5 - 40 m . If Rmax is less than 5 mih,
adhesion of a lubricating coating formed thereon and retention of the coating may
become inadequate. On the other hand, if Rmax exceeds 40 m , friction
increases, the coating may be unable to withstand shearing forces and compressive
forces at the time of a high pressure, and the coating may be easily destroyed or
peeled off.
From the standpoint of the adhesion of the lubricating coating, preparatory
surface treatment which can form a porous coating, namely, chemical conversion
treatment and impact plating are preferred. With these methods, in order to make
Rmax of the porous coating at least 5 mh , the coating thickness is preferably made
at least 5 mpi. There is no particular upper limit on the coating thickness, but
normally at most 50 mh and preferably at most 40 m h is sufficient. If a
lubricating coating is formed atop a porous coating which is formed by preparatory
surface treatment, the adhesion of the lubricating coating is increased by the socalled
"anchor effect". As a result, it becomes difficult for peeling of the solid
lubricating coating to take place under repeated makeup and breakout, contact
between metals is effectively prevented, and galling resistance, gas tightness, and
corrosion resistance are further increased.
Particularly preferred types of preparatory surface treatment for forming a
porous coating are phosphate chemical conversion treatment (treatment with
manganese phosphate, zinc phosphate, iron manganese phosphate, or zinc calcium
phosphate) and formation of a zinc or zinc-iron alloy coating by impact plating. A
manganese phosphate coating is preferable from the standpoint of adhesion, and a
zinc or zinc-iron alloy coating which can be expected to provide a sacrificial
corrosion-preventing effect by zinc is preferable from the standpoint of corrosion
resistance.
Phosphate chemical conversion treatment (phosphating) can be carried out
by immersion or spraying in a conventional manner. An acidic phosphating
soluiton which is normally used for zinc-plated materials can be used as a chemical
conversion treatment solution. For example, a zinc phosphating solution
containing 1 - 150 g/L of phosphate ions, 3 - 70 g/L of zinc ions, 1 - 100 g/L of
nitrate ions, and 0 - 30 g/L of nickel ions can be used. It is also possible to use a
manganese phosphating solution which is normally used for threaded joints. The
temperature of the solution can be from room temperature to 100° C, and the
duration of treatment can be up to 15 minutes depending upon the desired coating
thickness. In order to promote the formation of a coating, prior to phosphate
treatment, an aqueous surface conditioning solution containing colloidal titanium
can be supplied to the surface to be treated. After phosphate treatment, washing is
preferably performed with cold or warm water followed by drying.
Impact plating can be carried out by mechanical plating in which particles
are impacted with a material to be plated inside a rotating barrel, or by blast plating
in which particles are impacted against a material to be plated using a blasting
machine. In the present invention, it is sufficient to plate just a contact surface, so
it is preferable to employ blast plating which can perform localized plating. The
thickness of a zinc or zinc alloy layer which is formed by impact plating is
preferably 5 - 40 m h from the standpoints of both corrosion resistance and
adhesion.
For example, particles having an iron core coated with zinc or a zinc alloy
are blasted against the contact surface to be coated. The content of zinc or a zinc
alloy in the particles is preferably in the range of 20 - 60 mass %, and the diameter
of the particles is preferably in the range of 0.2 - 1.5 mm. As a result of blasting,
only the zinc or zinc alloy which is the coating layer of the particles adheres to the
contact surface which forms a substrate, and a porous coating made of zinc or a
zinc alloy is formed atop the contact surface. This impact plating can form a
porous metal plated coating having good adhesion to a steel surface regardless of
the composition of the steel.
As another type of preparatory surface treatment, although it produces
almost no surface roughening effect, electroplating in one or more specific layers
may improve the adhesion of the lubricating coating to the substrate and may
improve the galling resistance of a tubular threaded joint.
Examples of such preparatory surface treatment for a lubricating coating
are electroplating with a metal such as Cu, Sn, or Ni or alloys of these metals.
Plating may be single-layer plating or multiple-layer plating with two or more
layers. Specific examples of this type of electroplating are Cu plating, Sn plating,
Ni plating, Cu-Sn alloy plating, Cu-Sn-Zn alloy plating, two-layer plating by Cu
plating and Sn plating, and three-layer plating by Ni plating, Cu plating, and Sn
plating. Particularly a tubular threaded joint made from a steel having a Cr
content exceeding 5% is susceptible to galling, and therefore it is preferably
subjected to preparatory surface treatment in the form of single-layer plating with a
Cu-Sn alloy or a Cu-Sn-Zn alloy or multiple-layer plating with two or more layers
selected from these alloy platings and Cu plating, Sn plating, and Ni plating such as
two-layer plating by Cu plating and Sn plating, two-layer plating by Ni plating and
Sn plating, two-layer plating by Ni plating and Cu-Sn-Zn alloy plating, and threelayer
plating by Ni plating, Cu plating, and Sn plating are preferred.
These types of plating can be formed by the method described in JP 2003-
74763 A. In the case of multiple-layer plating, the lowermost layer of plating
(usually Ni plating) is preferably an extremely thin plating layer referred to as
strike plating and having a thickness of less than 1 mh . The plating thickness (the
overall thickness in the case of multiple-layer plating) is preferably in the range of
5 - 15 mp .
It is possible to form a solid anticorrosive coating as another preparatory
surface treatment method.
When the second lubricating coating is a viscous liquid lubricating coating,
in order to reduce the surface tackiness of this coating, a thin, dry solid coating
(e.g., having a thickness of 10 - 50 m ) may be formed as an upper layer of the
lubricating coating. This dry solid coating can be a usual resin coating (such as a
coating of an epoxy resin, a polyamide resin, a polyamide-imide resin, or a vinyl
resin) and it can be formed from either a water-based composition or an organic
solvent-based composition. The coating may also contain a small amount of wax
in order to afford lubricity.
Examples
The effects of the present invention will be illustrated by the following
examples and comparative examples. In the following explanation, the contact
surface of a pin including the threaded portion and the unthreaded metal contact
portion will be referred to as the pin surface, and the contact surface of a box
including the threaded portion and the unthreaded metal contact portion will be
referred to as the box surface. The surface roughness is expressed as Rmax.
Unless particularly specified, % means mass %.
The pin surface and the box surface of commercially available special
threaded joints (VAM TOP with an outer diameter of 17.78 cm (7 inches) and a
wall thickness of 1.036 cm (0.408 inches) manufactured by Sumitomo Metal
Industries, Ltd.) made from carbon steel A, Cr-Mo steel B, or 13% Cr steel C
having the composition shown in Table 1 were subjected to preparatory surface
treatment as shown in Table 2. Then, as shown in Table 3, a high-friction solid
lubricating coating and a second lubricating coating selected from a viscous liquid
lubricating coating and a solid lubricating coating and sometimes a solid
anticorrosive coating were formed on the pin surface and the box surface.
The details of treatment and the coating composition will be described
below. In Table 3, the unthreaded metal contact portion means the seal portion
and the shoulder portion, and the threaded portion means the portion of the contact
surface other than the seal portion and the shoulder portion. When forming
different coatings on the unthreaded metal contact portion and the threaded portion,
first the high-friction solid lubricating coating was formed on the unthreaded metal
contact portion, and then the indicated lubricating coating was formed on the
threaded portion. When forming a lubricating coating on the threaded portion, a
shielding plate was used so as not to form the lubricating coating atop the highfriction
solid lubricating coating which was formed on the unthreaded metal contact
portion. However, the border between these coatings need not be clear, and the
effects of the present invention can be obtained even when there is an overlapping
region of around 1 mm.
The coefficients of friction of the high-friction solid lubricating coating, the
viscous liquid lubricating coating, and the solid lubricating coating which were
formed were the maximum coefficients of friction under steady state conditions
when the coefficients of friction were measured by the above-mentioned Falex
testing method with a pressure of 1 GPa. Measurement was carried out in
accordance with ASTM D2670. The pin used for measurement had a diameter of
6.35 mm (1/4 inch), and 2 Vee blocks had a V-shaped groove with an included
angle of 96° and a groove width of 6.35 mm (1/4 inch). The pin and the blocks
were prepared by cutting them from a billet of the same steel as the threaded joint
to be tested, and they underwent the same preparatory surface treatment and
coating treatment as the surface of the pin and the box, respectively, of the threaded
joint to be tested.
A high torque test in which makeup was carried out with a high makeup
torque was performed on a tubular threaded joint which was prepared in the abovedescribed
manner to obtain a torque chart like that shown in Figure 2. The values
for Ts (the shouldering torque), Ty (the yield torque), and DT (the torque-onshoulder
resistance = Ty - Ts) were measured on the torque chart.
Ts was the torque at the start of interference of the shoulder portions.
Specifically, Ts was the torque when the change in torque which appeared when the
shoulder portions interfered began to enter a linear region (region of elastic
deformation). Ty was the torque at the start of plastic deformation. Specifically,
Ty was the torque when the torque began to leave the linear region after Ts was
reached in which the change in torque with the number of turns was linear. DT (=
Ty - Ts) was made 100 for Comparative Example 1 in Table 3 using a conventional
compound grease. Table 4 shows the results of comparison of other examples
with this value of DT.
A repeated makeup and breakout test was carried out on each tubular
threaded joint, and galling resistance was evaluated. In the repeated makeup and
breakout test, makeup of a threaded joint was carried out with a makeup speed of
10 rpm and a high makeup torque of 20 kN-m, and after breakout, the state of
galling of the pin surface and the box surface was investigated. In cases in which
seizing scratches which developed due to makeup were light and repeated makeup
was possible if repair was performed, repair was carried out and makeup and
breakout were continued. Makeup was carried out 0 times (for 0 cycles).
Table 4 also shows the results of this test.
Table 1
Table 2
R: surface roughness (mpi); t : coating thickness (mh )
Table 3
Table 4
1) A value of at least 95 is acceptable for practical use.
(Example 1)
The pin surface and the box surface of a special threaded joint made of
carbon steel having composition A shown in Table 1were subjected to preparatory
surface treatment and coating treatment as described below to form the coating
structure shown in Figure 5(A).
[Box Surface]
After finishing by machine grinding (surface roughness of 3 mih), the box
surface underwent preparatory surface treatment by immersion for 10 minutes in a
manganese phosphating solution at 80 - 95° C to form a manganese phosphate
coating having a thickness of 15 mh (surface roughness of 12 m h).
Surflube C291 manufactured by Nippon Paint Co., Ltd. which was diluted
with water to a strength of 10% was applied by spray coating to the unthreaded
metal contact portion (the seal portion and the shoulder portion) of the box surface
which had undergone the preparatory surface treatment to form a high-friction solid
lubricating coating having a coating thickness of approximately 10 m h after drying.
The coefficient of friction of this solid lubricating coating was 0.1. The threaded
portion (the portions other than the seal portion and the shoulder portion) of the box
surface which had undergone the preparatory surface treatment was treated so as to
form a viscous liquid lubricating coating thereon in the following manner.
The composition of the viscous liquid lubricating coating was 15% of a
hydrogenated rosin ester (Ester Gum H manufactured by Arakawa Chemical
Industries, Ltd.), 48% of a highly basic calcium sulfonate as a basic metal salt of an
aromatic organic acid (Calcinate C-400CLR manufactured by Crompton
Corporation, base number of 400 mg KOH/g), 17% of calcium stearate as a metal
soap (manufactured by DIC Corporation), 10% of amorphous graphite as a solid
lubricant (Blue P manufactured by Nippon Graphite Industries, Ltd.), and 10% of
paraffin wax.
After the above-described composition was diluted with 30 parts by mass
of an organic solvent (Exxsol D40 manufactured by Exxon Mobil Corporation) per
100 parts by mass of the composition to lower its viscosity, it was applied to the
threaded portion of the box surface by spray coating. After evaporation of the
solvent, a viscous liquid lubricating coating having a thickness of approximately 50
mih was formed. The coefficient of friction of this lubricating coating was 0.04.
[Pin Surface]
After finishing by machine grinding (surface roughness of 3 m i), the pin
surface was subjected to preparatory surface treatment by immersing for 10
minutes in a zinc phosphating solution at 75 - 85° C to form a zinc phosphate
coating (surface roughness of 8 mpi ) with a thickness of 12 mi .
The same treatment as for the box surface to form lubricating coatings was
carried out on the pin surface which had undergone the preparatory surface
treatment. Namely, the above-described high-friction solid lubricating coating
was formed on the unthreaded metal contact portion, and the above-described
viscous liquid lubricating coating was formed on the threaded portion. The
coating thickness and the coefficient of friction of each coating were the same as
for the box surface.
As can be seen from Table 4, the value of D T in a high torque test was such
that the ratio of D T when the value of D T for Comparative Example 1 was given a
value of 100 (referred to below as the D T ratio) was 125%. Compared to the D T
ratio of around 50% for Comparative Example 2 which did not have a high-friction
solid lubricating coating on the seal portion or the shoulder portion (the entirety of
the pin surface and the box surface was coated with a viscous liquid lubricating
coating), the D T ratio was greatly increased.
Moreover, D T in Example 1 was increased by 25% with respect to D T of
the reference example using compound grease (Comparative Example 1).
Accordingly, it was verified that the threaded joint of Example 1 could be made up
with a high torque without the occurrence of yielding of the shoulder portions. In
the repeated makeup and breakout test, makeup and breakout could be performed
10 times without the occurrence of galling.
(Example 2)
The pin surface and the box surface of a special threaded joint made of the
13% Cr steel having composition C shown in Table 1 were subjected to the belowdescribed
preparatory surface treatment and coating treatment to form the coating
structure shown in Figure 5(C).
[Box Surface]
After finishing by machine grinding (surface roughness of 3 m ), the box
surface underwent Ni strike plating and then Cu plating by electroplating to form a
plated coating with an overall thickness of 12 m i. The surface roughness after
this preparatory surface treatment was 3 mhi.
The same viscous liquid lubricating coating as described in Example 1 was
formed by spray coating on the entirety of the box surface which had undergone the
preparatory surface treatment. The coating thickness of the viscous liquid
lubricating coating after evaporation of the solvent was 80 mh , and its coefficient
of friction was 0.04.
[Pin Surface]
The pin surface was subjected to preparatory surface treatment by
sandblasting with No. 80 sand to give a surface roughness of 10 mh .
Undiluted Gardolube L6334 manufactured by Chemetall GmbH was
applied by spray coating to the unthreaded metal contact portion (the seal portion
and the shoulder portion) of the pin surface which had undergone the preliminary
surface treatment to form a high-friction solid lubricating coating having a
thickness of approximately 15 m . The coefficient of friction of this high-friction
solid lubricating coating was 0.15. The same viscous liquid lubricating coating as
was formed on the box surface was formed to the same coating thickness on the
entire pin surface including the unthreaded metal contact portion on which the
high-friction solid lubricating coating had been formed.
In the high torque test, the DT ratio was 112%, confirming that DT was
larger than for Comparative Example 1 which used compound grease. Of course,
makeup and breakout could be carried out 10 times without any problem in the
repeated makeup and breakout test.
(Example 3)
The pin surface and the box surface of a special threaded joint made of the
Cr-Mo steel having composition B shown in Table 1 were subjected to the belowdescribed
preparatory surface treatment and coating treatment to form the coating
structure shown in Figure 6(C).
[Box Surface]
After finishing by machine grinding (surface roughness of 3 mpi), the box
surface underwent Ni strike plating followed by Cu-Sn-Zn alloy plating by
electroplating to form a plated coating having an overall thickness of 7 mpi. The
surface roughness after the preparatory surface treatment was 2 m .
The unthreaded metal contact portion and the threaded portion of the box
surface which had undergone the preparatory surface treatment was coated by spray
coating with Surflube C291 manufactured by Nippon Paint Co., Ltd. which was
diluted with water to a strength of 10% to form a high-friction solid lubricating
coating (coefficient of friction of 0.1) having a coating thickness of approximately
10 m h after drying.
[Pin Surface]
After finishing by machine grinding (surface roughness of 3 mh ), the pin
surface was immersed for 10 minutes in a zinc phosphating solution at 75 - 85° C
for preparatory surface treatment to form a zinc phosphate coating (surface
roughness of 8 mhi) having a thickness of 12 mh .
The unthreaded metal contact portion of the pin surface which had
undergone the preparatory surface treatment was coated by spray coating with
Surflube C291 manufactured by Nippon Paint Co., Ltd. which was diluted with
water to a strength of 10% to form a high-friction solid lubricating coating with a
coating thickness of approximately 10 mpi (coefficient of friction of 0.1) after
drying. Then, the viscous liquid lubricating coating described in Example 1 was
formed on the solid lubricating coating and on the threaded portion (namely, on the
entire pin surface) by the same method as in Example 1 to a coating thickness of
approximately 50 m i .
In the high torque test, the DT ratio was 110%, confirming that DT was
larger than for the compound grease of Comparative Example 1. In the repeated
makeup and breakout test, makeup and breakout were performed 10 times without
any problems.
(Example 4)
The pin surface and the box surface of a special threaded joint made of the
the Cr-Mo steel having composition B shown in Table 1 were subjected to the
below-described preparatory surface treatment and coating treatment to form a
coating having the structure shown in Figure 6(B).
[Box Surface]
After finishing by machine grinding (surface roughness of 3 mpi), the box
surface underwent Ni strike plating followed by Cu-Sn-Zn alloy plating by
electroplating to form a plated coating having an overall thickness of 7 m . The
surface roughness after the preparatory surface treatment was 2 m .
The unthreaded metal contact portion of the box surface which had
undergone the preparatory surface treatment was coated by spray coating with
Surflube C291 manufactured by Nippon Paint Co., Ltd. which was diluted to a
strength of 10% to form a high-friction solid lubricating coating (coefficient of
friction of 0.1) having a coating thickness of approximately 50 m after drying.
On the threaded portion of the box surface which had undergone the preparatory
surface treatment, a solid lubricating coating was formed in the following manner.
A lubricating coating composition having the below-described composition
was heated at 120° C in a tank equipped with a stirrer to maintain a molten state
having a viscosity suitable for coating, while the box surface which had undergone
the preparatory surface treatment described above was preheated to 120° C by
induction heating. Using a spray gun having a spraying head with a heat retaining
mechanism, the above-described molten lubricating coating composition was
applied to the threaded portion of the preheated box surface. After cooling, a solid
lubricating coating having a thickness of 50 mhi (coefficient of friction of 0.03) was
formed.
The composition of the lubricating coating composition was as follows:
15% carnauba wax,
15% zinc stearate,
5% liquid polyalkyl methacrylate (Viscoplex™ 6-950 manufactured by
Rohmax Corporation),
49% corrosion inhibitor (NA-SUL™ Ca/W1935 manufactured by King
Industries, Inc.),
3.5% amorphous graphite
1% zinc oxide,
5% titanium dioxide,
5% bismuth trioxide,
1% silicone (polydimethyl siloxane), and
antioxidants (made by Ciba-Geigy Corporation):
0.3% Irganox™ L150 and
0.2% Irgafos™ 168.
[Pin Surface]
After finishing by machine grinding (surface roughness of 3 mi ), the pin
surface was immersed for 10 minutes in a zinc phosphating solution at 75 - 85° C
to form a zinc phosphate coating (surface roughness of 8 mih) having a thickness of
1 m . On the entire pin surface which had undergone this preparatory surface
treatment, a solid anticorrosive coating was formed from an ultraviolet curing resin
in the following manner.
A coating composition was prepared by adding aluminum zinc phosphate
as a rust-preventing agent and polyethylene wax as a lubricant to an epoxy acrylic
resin-based ultraviolet curing resin paint composition (solventless type)
manufactured by Chugoku Marine Paints, Ltd. The resulting coating composition
contained 94% resin, 5% rust-preventing agent, and 1% lubricant based on the total
solids content. This coating composition was applied by spraying to the entire pin
surface and was irradiated with ultraviolet rays (wavelength of 260 nm) from an
air-cooled mercury vapor lamp having an output of 4 kW to cure the coating. The
resulting coating had a thickness of 25 m and was colorless and transparent. The
male threaded portion of the pin could be inspected through the coating either with
the naked eye or with a magnifying glass.
In the high torque test, the DT ratio was 105%. The DT ratio was greatly
increased compared to Comparative Example 3 in which a high-friction solid
lubricating coating was not formed on the unthreaded metal contact portion (the
seal portion and the shoulder portion) of the box surface. In addition, the DT ratio
was increased compared to Comparative Example 1 which used a conventional
compound grease. In the repeated makeup and breakout test, makeup and
breakout could be carried out 10 times without any problems.
(Comparative Example 1)
The pin surface and the box surface of a special threaded joint made of the
carbon steel having composition A shown in Table 1 were subjected to the belowdescribed
preparatory surface treatment and coating treatment.
[Box Surface]
After finishing by machine grinding (surface roughness of 3 m ), the box
surface underwent preparatory surface treatment by immersion for 10 minutes in a
manganese phosphating solution at 80 - 95° C to form a manganese phosphate
coating having a thickness of 15 mh (surface roughness of 12 m ). A viscous
liquid compound grease in accordance with API BUL 5A2 was applied to the box
surface which had undergone this preparatory surface treatment to form a
lubricating coating. The coated amount of the compound grease was a total of 50
g on the pin and the box. The coated area was a total of roughly 1400 cm2.
[Pin Surface]
After finishing by machine grinding (surface roughness of 3 mp ), the pin
surface was immersed for 10 minutes in a zinc phosphating solution at 75 - 85° C
to form a zinc phosphate coating (surface roughness of 8 mh ) having a thickness of
1 m . The same compound grease as was used on the box surface was applied to
the pin surface which had undergone this preparatory surface treatment.
As shown in Table 3, during 10 cycles of makeup and breakout in the
repeated makeup and breakout test, there was no occurrence of galling through the
tenth cycle. However, compound grease contains heavy metal such as lead, so it
is harmful to humans and the environment.
In the high torque test, the joint exhibited a high value of Ty with a large
value of D T by which yielding of the shoulder portions did not occur even when
makeup was carried out with a high torque. The values for D T ratio in the other
examples was calculated with the value of D T at this time being made 100.
(Comparative Example 2)
The pin surface and the box surface of a special threaded joint made of the
Cr-Mo steel having composition B in Table 1 were subjected to the following
preparatory surface treatment and coating treatment.
[Box Surface]
After finishing by machine grinding (surface roughness of 3 m ), the box
surface was immersed for 10 minutes in a manganese phosphating solution at 80 -
95° C to form a manganese phosphate coating with a thickness of 12 m h (surface
roughness of 10 m ). The viscous liquid lubricating coating described in
Example 1 was formed by the same method on the entire box surface which had
undergone this preparatory surface treatment. After evaporation of the solvent, a
viscous liquid lubricating coating having a thickness of approximately 60 mi was
formed. The coefficient of friction of this lubricating coating was 0.04.
[Pin Surface]
After finishing by machine grinding (surface roughness of 3 mhi ), the pin
surface was immersed for 10 minutes in a zinc phosphating solution at 75 - 85° C
to form a zinc phosphate coating (surface roughness of 8 mh ) having a thickness of
12 m . The same viscous liquid lubricating coating as on the box surface was
formed to a thickness of 60 mpi on the entire pin surface which had undergone the
preparatory surface treatment.
In the repeated makeup and breakout test, the results were extremely good
with no occurrence of galling in 10 cycles of makeup and breakout. However, in
the high torque test, the D T ratio was an extremely small value of 52% compared to
the conventional compound grease (Comparative Example 1). Namely, it was
again confirmed that if the contact surfaces of a tubular threaded joint are entirely
coated only with a viscous liquid lubricating coating having a low coefficient of
friction, the D T ratio is greatly reduced.
(Comparative Example 3)
The pin surface and the box surface of a special threaded joint made of the
Cr-Mo steel having composition B in Table 1 were subjected to the following
preparatory surface treatment and coating treatment.
[Box Surface]
After finishing by machine grinding (surface roughness of 3 mhi ), the box
surface underwent preparatory surface treatment by immersion for 10 minutes in a
manganese phosphating solution at 80 - 95° C to form a manganese phosphate
coating having a thickness of 12 m (surface roughness of 10 um). The same
solid lubricating coating as described in Example 4 was formed by the same
method on the entire box surface which had undergone the preparatory surface
treatment. After cooling, a solid lubricating coating having a thickness of
approximately 50 mih (coefficient of friction of 0.03) was formed.
[Pin Surface]
After finishing by machine grinding (surface roughness of 3 mih ), the pin
surface was immersed for 10 minutes in a zinc phosphating solution at 75 - 85° C
to form a zinc phosphate coating (surface roughness of 8 mpi ) having a thickness of
12 mpi . The same ultraviolet curing resin coating (coating thickness of 25 m ) as
described in Example 4 was formed by the same method on the entire pin surface
which had undergone the preparatory surface treatment.
In the repeated makeup and breakout test, the results were extremely good
with no occurrence of galling in 10 cycles of makeup and breakout. However, in
the high torque test, the D T ratio was an extremely small value of 70% compared to
conventional compound grease.
(Comparative Example 4)
The pin surface and the box surface of a special threaded joint made of the
Cr-Mo steel having composition B in Table 1 were subjected to the following
preparatory surface treatment and coating treatment.
[Box Surface]
After finishing by machine grinding (surface roughness of 3 m h), the box
surface was immersed for 10 minutes in a manganese phosphating solution at 80 -
95° C to form a manganese phosphate coating with a thickness of 12 m (surface
roughness of 10 m h) . The same viscous liquid lubricating coating as described in
Example 1 was formed by the same method on the entire box surface which had
undergone this preparatory surface treatment. After evaporation of the solvent, a
viscous liquid lubricating coating having a thickness of approximately 60 mpi was
formed. The coefficient of friction of this lubricating coating was 0.04.
[Pin Surface]
After finishing by machine grinding (surface roughness of 3 mp ), the pin
surface was immersed for 10 minutes in a zinc phosphating solution at 75 - 85° C
to form a zinc phosphate coating (surface roughness of 8 mpi) having a thickness of
1 m h. The same high-friction solid lubricating coating as formed on the
unthreaded metal contact portion of the pin surface in Example 1was formed to a
thickness of 10 m h on the entire pin surface which had undergone the preparatory
surface treatment.
In the repeated makeup and breakout test, the makeup torque was
constantly high from the first cycle, and galling occurred in the fifth cycle making
it unable to continue the test. In the high torque test, the DT ratio was a small
value of 61% compared to the conventional compound grease (Comparative
Example 1). Namely, when the entire contact surface of one member of a
threaded joint was coated with a high-friction solid lubricating coating, the galling
resistance was greatly impaired, and due to a considerable increase in the
shouldering torque, the DT ratio was not improved.
(Comparative Example 5)
The pin surface and the box surface of a special threaded joint made of the
Cr-Mo steel having composition B in Table 1 were subjected to the following
preparatory surface treatment and coating treatment.
[Box Surface]
After finishing by machine grinding (surface roughness of 3 mih), the box
surface was immersed for 10 minutes in a manganese phosphating solution at 80 -
95° C to form a manganese phosphate coating having a thickness of 12 m (surface
roughness of 10 mih) . The same high-friction solid lubricating coating as formed
on the unthreaded metal contact portion of the box surface in Example 4 was
formed to a thickness of about 20 m on the entire box surface which had
undergone the preparatory surface treatment.
[Pin Surface]
After finishing by machine grinding (surface roughness of 3 mh ), the pin
surface was immersed for 10 minutes in a zinc phosphating solution at 75 - 85° C
to form a zinc phosphate coating (surface roughness of 8 m ) having a thickness of
1 mhi. The same ultraviolet curing resin coating (coating thickness of 25 m ) as
described in Example 4 was formed by the same method on the entire pin surface
which had undergone the preparatory surface treatment.
In the repeated makeup and breakout test, galling occurred in the first
cycle, and the test terminated. This premature galling made it unable to evaluate
by the high torque test. It was confirmed that the combination of coatings in this
example affords poor lubricity leading to a significant worsening in galling
resistance, which is the fundamental performance required for a tubular threaded
joint.
(Other Tests)
In order to investigate the rust-preventing properties of the tubular threaded
joints manufactured in Examples 1 - 4, the same preparatory surface treatment and
formation of lubricating coating or coatings as for the box in Table 2 were
performed on a separately prepared coupon test piece (70 mm x 150 mm x 1.0 mm
thick). Each test piece was subjected to a salt spray test (in accordance with JIS Z
2371 (corresponding to ISO 9227) at a temperature of 35° C for 1000 hours) or a
humidity resistance test (in accordance with JIS K 5600-7-2 (corresponding to ISO
6270) at a temperature of 50° C and a relative humidity of 98% for 200 hours), and
the occurrence of rust was investigated. As a result, it was ascertained that there
was no occurrence of rust on the tubular threaded joints of Examples 1 - 4 in either
of the tests.
When each of the examples of tubular threaded joints underwent a gas
tightness test and an actual use test in an actual excavating apparatus, each joint
exhibited satisfactory properties. It was confirmed that makeup could be stably
carried out with these joints even when the makeup torque was high due to the
values forAT which were larger than with conventionally used compound grease.

Claims
1. A tubular threaded joint constituted by a pin and a box each having a
contact surface comprising an unthreaded metal contact portion including a seal
portion and a shoulder portion and a threaded portion, characterized in that the
contact surface of at least one of the pin and the box has a first lubricating coating
and a second lubricating coating, the first lubricating coating being a solid
lubricating coating formed on a portion of the contact surface including the
shoulder portion, the second lubricating coating being selected from a viscous
liquid lubricating coating and a solid lubricating coating and formed on at least the
portion of the contact surface where the first lubricating coating is not present, the
first lubricating coating having a coefficient of friction which is higher than that of
the second lubricating coating, the second lubricating coating being positioned on
top if there is a portion of the contact surface in which both the first lubricating
coating and the second lubricating coating are present.
2. A tubular threaded joint as set forth in claim 1 wherein the portion of
the contact surface including the shoulder portion on which the first lubricating
coating is formed is an unthreaded metal contact portion of the contact surface.
3. A tubular threaded joint as set forth in claim 2 wherein the unthreaded
metal contact portion of the contact surface of at least one of the pin and the box
has the first lubricating coating, and the threaded portion of the contact surface has
the second lubricating coating.
4. A tubular threaded joint as set forth in claim 2 wherein the unthreaded
metal contact portion of the contact surface of at least one of the pin and the box
has the first lubricating coating, and the entirety of the contact surface has the
second lubricating coating formed atop the first lubricating coating.
5. A tubular threaded joint as set forth in claim 1 wherein the contact
surface of one member of the pin and the box has the first lubricating coating
formed on a portion of the contact surface including the shoulder portion and the
second lubricating coating formed on at least the portion of the contact surface
where the first lubricating coating is not present, and the contact surface of the
other member of the pin and the box has a coating selected from a lubricating
coating which is selected from a viscous liquid lubricating coating and a solid
lubricating coating; a solid anticorrosive coating; and a two-layer coating
comprising a lower layer in the form of a lubricating coating selected from a
viscous liquid lubricating coating and a solid lubricating coating and an upper layer
in the form of a solid anticorrosive coating.
6. A tubular threaded joint as set forth in claim 5 wherein the solid
anticorrosive coating is based on an ultraviolet curing resin.
7. A tubular threaded joint as set forth in any one of claims 1 - 6 wherein
the contact surface of at least one of the pin and the box is subjected to surface
treatment by a method selected from blasting, pickling, phosphate chemical
conversion treatment, oxalate chemical conversion treatment, borate chemical
conversion treatment, electroplating, impact plating, and two or more of these
methods prior to forming the lubricating coating or anticorrosive coating.
8. A tubular threaded joint as set forth in any one of claims 1 - 6 wherein
the first lubricating coating has a thickness of 5 - 40 mih.
9. A tubular threaded joint as set forth in claim 8 wherein the second
lubricating coating is a viscous liquid lubricating coating having a thickness of 5 -
200 m , and when this second lubricating coating is positioned atop the first
lubricating coating, the total thickness of the first lubricating coating and the
second lubricating coating is at most 200 mh .
10. A tubular threaded joint as set forth in claim 8 wherein the second
lubricating coating is a solid lubricating coating having a thickness of 5 - 150 mpi,
and when this second lubricating coating is positioned atop the first lubricating
coating, the total thickness of the first lubricating coating and the second
lubricating coating is at most 150 µπ.