Tubular Threaded Joint Having Improved Low Temperature Performance
Technical Field
This invention relates to a composition for forming a thermoplastic solid
lubricating coating used for surface treatment of tubular threaded joints for
connecting steel pipes and particularly oil country tubular goods, and to a tubular
threaded joint having a solid lubricating coating formed from this composition. A
tubular threaded joint according to the present invention can be used without
application of compound grease thereto, and it can exhibit improved galling
resistance and gas tightness even in an extremely low temperature environment, so
it can be used for excavation of oil wells particularly in extremely cold regions.
Background Art
Oil country tubular goods such as tubing and casing used in the excavation of
oil wells for recovery of crude oil or gas oil are normally connected with each other
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 offshore oil fields, the depth can reach
8,000 - 10,000 meters. The length of an oil country tubular good is typically 10
some meters, and tubing having a fluid such as crude oil flowing in its interior is
surrounded by a plurality of casings, and hence the number of oil country tubular
goods which are connected together can reach a huge number of a thousand or
more.
In their environment of use, tubular threaded joints for oil country tubular
goods are subjected to loads in the form of tensile forces in the axial direction
caused by the weight of oil country tubular goods and the joints themselves,
complex pressures such as inner and outer pressures, and geothermal heat.
Therefore, they must be able to guarantee gas tightness without being damaged even
under such severe environments.
A typical tubular threaded joint used for connecting oil country tubular goods
has a pin-box structure constituted by a member which has male (external) threads
and is referred to as a pin and a member which has female (internal) threads and isreferred to as a box. Typically, a pin is formed on both ends of an oil country
tubular good, and a box is formed on the inner surface of both sides of a threaded
joint component (a coupling).
As shown in Figure 1, a threaded joint which has excellent gas tightness and
is referred to as a special threaded joint has a seal portion and a shoulder portion
(also referred to as a torque shoulder) on each of the pin and the box. The seal
portion is formed on the outer periphery near the end surface closer to the end of the
pin than the male threads and on the inner periphery on the base of the female
threads of the box, and the shoulder portion is formed on the end surface at the end
of the pin and on the corresponding rearmost portion of the box. The seal portion
and the shoulder portion constitute an unthreaded metal contact portion of the pin or
box of the tubular threaded joint, and the unthreaded metal contact portion and the
threaded portion (male or female threads) constitute a contact surface the pin or box
thereof. By inserting one end (a pin) of an oil country tubular good into a coupling
(a box) and tightening the male threads of the pin and the female threads of the
coupling until the shoulder portions of the pin and the box are made to abut and
then interfere with a suitable torque, the seal portions of the pin and the box
intimately contact each other and form a metal-to-metal seal, thereby maintaining
the gas tightness of the threaded joint.
When tubing or casing is being lowered into an oil well, due to various
problems, a threaded joint which was once tightened is sometimes loosened, the
threaded joints are lifted out of the oil well, then they are retightened and lowered
into the well. API (American Petroleum Institute) requires galling resistance so
that gas tightness is maintained without the occurrence of unrepairable seizing
referred to as galling even when a joint undergoes tightening (makeup) and
loosening (breakout) 10 times for a joint for tubing and 3 times for a joint for
casing.
In order to increase galling resistance and gas tightness when performing
makeup of a threaded joint for oil country tubular goods, a viscous liquid lubricant
(a lubricating grease) which is referred to as compound grease and which contains
heavy metal powders is applied to a contact surface of a threaded joint (namely, to
the threads and the unthreaded metal contact portion of the pin or box).
Compound grease is prescribed by API Bulletin 5A2.In the past, it has been proposed to subject a contact surface of a threaded
joint to various types of surface treatment such as nitriding, various types of plating
including zinc plating and composite plating, and phosphate chemical conversion
treatment to form one or more layers in order to increase the retention of compound
grease or improve sliding properties. However, as described below, the use of
compound grease poses the threat of adverse effects on the environment a d
humans.
Compound grease contains a large amount of heavy metal powders such as
zinc, lead, and copper powders. At the time of makeup of a threaded joint, grease
which has been applied is washed off or overflows to the exterior surface, and there
is a possibility of its producing adverse effects on the environment and particularly
on marine life due to harmful heavy metals such as lead. In addition, the process
of applying compound grease worsens the work environment and working
efficiency, and there is a concern of its toxicity towards 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 restrictions 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 for connecting oil country tubular
goods without application of compound grease, the present applicants proposed in
WO 2006/1 0425 1 a tubular threaded joint in which the contact surface of at least
one of a pin and a box is coated with a two-layer coating having a viscous liquid or
semisolid lubricating coating and a dry solid coating formed atop it. The dry solid
coating can be formed from a thermosetting resin such as an acrylic resin or from an
ultraviolet curing resin. The viscous liquid or semisolid lubricating coating has
tackiness so that foreign matter easily adheres thereto, but by forming a dry solid
coating atop it, the tackiness is eliminated. Since the dry solid coating is destroyed
at the time of makeup of a threaded joint, it does not interfere with the lubricating
properties of the lubricating coating disposed beneath it.In WO 2007/42231, the present applicants disclosed a threaded joint having a
thin lubricating coating without tackiness which contains solid lubricant particles
dispersed in a solid matrix exhibiting plastic or viscoplastic rheological behavior
(flow properties) on the threads (of a pin and a box). The matrix preferably has a
melting point in the range of 80 - 320° C, and it is formed by spray coating in a
molten state (hot melt spraying), by flame coating of powder, or by spray coating of
an aqueous emulsion. A composition used in the hot melt method contains, for
example, polyethylene as a thermoplastic polymer, wax (such as carnauba wax) and
a metal soap (such as zinc stearate) as a lubricating component, and calcium
sulfonate as a corrosion inhibitor. .
In WO 2006/75774, the present applicants described a tubular threaded joint
in which the contact surface of at least one of a pin and a box is coated with a
two-layer coating comprising a solid lubricating coating comprising a lubricating
powder and a binder, and a solid corrosion-preventing coating which does not
contain solid particles formed atop the solid lubricating coating.
Patent Document 1: WO 2006/104251
Patent Document 2: WO 2007/4223 1
Patent Document 3: WO 2006/75774
Summary of the Invention
The tubular threaded joints described in above-mentioned Patent Documents
1 - 3 exhibit excellent adhesion and sliding properties of the solid lubricating
coatmg and sufficient galling resistance in cold to warm environments from around
-10° C to around +50° C. However, when it is exposed to an extremely cold
environment from - 60° C to -20° C, peeling of the solid lubricating coating due to a
decrease in adhesion and the occurrence of cracking due to embrittlement of the
coating easily occur. Furthermore, if makeup and breakout of a threaded joint are
carried out at such low temperatures, the torque becomes extremely high and the
number of times that connection can be performed as an index of galling resistance
becomes inadequate.
The object of the present invention is to provide a tubular threaded joint
which suppresses the formation of rust and exhibits excellent galling resistance and
gas tightness without using compound grease even in an extremely coldenvironment and which does not have a tacky surface.
As a result of studies aiming at realizing sufficient galling resistance, rust
resistance, and gas tightness without an extreme increase in the makeup and
breakout torques of a threaded joint even when it is used not only in cold, warm and
tropical regions where the air temperature is around -20° C to around +50 °C but
also in extremely cold regions at -60° C to -20° C, the present inventors made the
following findings.
1) A thermoplastic lubricating coating containing particular copolymer
particles such as acrylic-silicone copolymer particles in a thermoplastic polymer
matrix is effective.
2) Galling resistance is further improved when a coating contains a solid
lubricant in addition to the copolymer particles.
3) A polyolefin resin or an ethylene-vinyl acetate copolymer resin is
preferable as a thermoplastic polymer which serves as a matrix (base material) of
the coating, and graphite is preferable as a solid lubricant.
The present invention, which was completed based on the above findings, is
a composition for forming a thermoplastic solid lubricating coating on a tubular
threaded joint, characterized by comprising (1) a thermoplastic polymer as a coating
matrix and (2) particles of a copolymer of a resin selected from a silicone resin and
a fluorocarbon resin with a thermoplastic resin.
From another aspect, the present invention is a tubular threaded joint having
improved performance in a low temperature environment which is constituted by a
pin and a box, each having contact surface including threads and an unthreaded
metal contact portion, characterized in that a thermoplastic solid lubricating coating
which contains particles of a copolymer of a resin selected from a silicone resin and
a fluorocarbon resin with a thermoplastic resin in a thermoplastic polymer as a
coating matrix is formed as an uppermost surface treatment coating layer on the
contact surface of one or both of the pin and the box. This tubular threaded joint is
suitable for use in connecting oil country tubular goods. In one embodiment, the
thermoplastic solid lubricating coating is formed on the contact surface of one of the
pin and the box, and the contact surface of the other of the pin and the box has a
solid corrosion-protecting coating based on an ultraviolet curing resin as an
uppermost surface treatment coating layer.In the copolymer of a resin selected from a silicone resin and a fluorocarbon
resin with another thermoplastic resin which is used in the present invention, both
the silicone resin and the fluorocarbon resin have a low friction (hereunder these
resins being collectively referred to as low friction resins), and the copolymer itself
maintains a low friction. Therefore, particles of such copolymer function as
lubricating particles capable of conferring lubricity to a coating. Particles of this
copolymer may hereinafter be referred to as low friction copolymer particles.
Particles of a silicone resin or fluorocarbon resin alone have insufficient bonding
strength to a thermoplastic polymer which constitutes the matrix of a lubricating
coating. By copolymerizing the particles with a thermoplastic resin, the particles
have an increased bonding strength in the thermoplastic polymer matrix.
During the formation of a lubricating coating, the low friction copolymer
particles are protruded from the coating surface with the silicone or fluorocarbon
resin portion of the copolymer particles facing outwards due to the action of surface
tension and the affinity of the thermoplastic polymer matrix which is higher for the
thermoplastic resin of the copolymer than for the silicone or fluorocarbon resin
thereof. As a result, as shown in Figures 5(a) and 5(b), in the initial stage of
makeup of a threaded joint when the surface pressure is still low (in the state of a
low shouldering), the surface of the opposing member primarily contacts the low
friction copolymer particles protruding from the surface of the lubricating coating,
thereby decreasing the coefficient of friction of the coating. As the makeup
proceeds to produce a high tightening pressure, the protruding low friction
copolymer particles are embedded in the coating primarily due to their plastic
deformation, and the surface of the opposing member also contacts the
thermoplastic polymer matrix, thereby increasing the coefficient of friction of the
entire coating compared to that at a low tightening pressure. When the makeup
operation is repeated, the lubricating coating still retains the state shown in Figures
5(a) and 5(b) in the second and later cycle of makeup although the low friction
copolymer particles wear to some extent, and satisfactory galling resistance is still
maintained.
Roughly speaking, the coefficient of friction of a coating made solely of a
thermoplastic polymer matrix is on the order of 0.1 to 0.2, while that of a coating
containing low friction copolymer particles in the matrix is on the order of 0.01 to0.1. In particular, the coefficient of friction of the lubricating coating in the state
shown in Figure 5(a) is on the order of 0.05. In general, a coefficient of friction of
0.1 or greater is considered to be high friction, and a coefficient of friction of 0.05
or lower is considered to be low friction.
The low friction copolymer particles are preferably acrylic-silicone
copolymer particles and more preferably acrylic-silicone copolymer particles
having an average particle diameter of 10 - 40 micrometers. Their content in the
coating is preferably 0.5 - 30 mass %.
The thermoplastic polymer matrix is preferably one or more polymers
selected from a polyolefin resin and an ethylene-vinyl acetate copolymer resin.
The thermoplastic solid lubricating coating preferably further contains a solid
lubricant, and the solid lubricant is preferably graphite.
From another standpoint, the present invention is a method of manufacturing
a tubular threaded joint having a surface treatment coating layer, the tubular
threaded joint being constituted by a pin and a box each having a contact surface
including threads and an unthreaded metal contact portion, characterized by forming
a solid lubricating coating as an uppermost surface treatment coating layer on the
contact surface of at least one of the pin and box by application of a composition
containing low friction copolymer particles in a molten thermoplastic polymer
matrix followed by cooling to solidify the matrix material.
In one embodiment of this method, the solid lubricating coating is formed on
the contact surface of one member of the pin and the box, and a solid-
corrosion-protecting coating is formed on the contact surface of the other member
of the pin and the box as an uppermost surface treatment coating layer by
application of a composition based on an ultraviolet curing resin followed by
irradiation with ultraviolet light.
The present invention can form a thermoplastic solid lubricating coating
having excellent galling resistance on the contact surface of a tubular threaded joint
without using a compound grease. The solid lubricating coating has excellent
performance in a low temperature environment, and even in an extremely low
temperature environment of -60° C to -20° C, the makeup torque and breakout
torque of a threaded joint are not greatly increased and almost no deterioration is
observed in the solid lubricating coating. Furthermore, this coating exhibits thesame excellent galling resistance, gas tightness, and rust preventing properties as
achieved with compound grease.
Brief Explanation of the Drawings
Figure 1 schematically shows the unthreaded metal contact portions
(shoulder portions and the seal portions) of a pin and a box of a special threaded
joint.
Figure 2 schematically shows the assembled structure of a steel pipe and a
coupling at the time of shipment of the steel pipe.
Figure 3 schematically shows the connecting portion of a threaded joint.
Figure 4 is an explanatory view showing the contact surfaces of a tubular
threaded joint according to the present invention, Figure 4(a) shows an example of
surface roughening of a contact surface itself, and Figure 4(b) shows an example of
forming a preparatory surface treatment coating for surface roughening of a contact
surface.
Figure 5 schematically shows the mechanism of operation of a lubricating
coating 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 more specifically described by way of example.
Figure 2 schematically shows the state of a steel pipe for oil country tubular
goods and a coupling at the time of shipment. A pin 1 having male threads 3a is
formed on the outer surface of both ends of a steel pipe A, and a box 2 having
female threads 3b is formed on the inner surface of both sides of a coupling B. A
pin means a member of a threaded joint having male threads, and a box means a
member of a threaded joint having female threads. The coupling B is previously
connected to one end of the steel pipe A. Prior to shipment, a protector (not
shown) for protecting the threads is mounted on the pin of the steel pipe and the box
of the coupling B which are not connected to other members, and these protectors
are removed prior to use of a threaded joint.
In a typical tubular threaded joint, as shown in the figure, a pin is formed on
the outer surface of both ends of a steel pipe, and a box is formed on the innersurface of a coupling which is a separate component. There are also integral
tubular threaded joints which do not use a coupling and in which one end of a steel
pipe is made a pin having male threads on its exterior and the other end is made a
box having female threads on its interior. A tubular threaded joint according to the
present invention can be applied to either of these types.
Figure 3 schematically shows the structure of a typical tubular threaded joint.
A tubular threaded joint is constituted by a pin 1 formed on the outer surface of the
end of a steel pipe A, and a box 2 formed on the inner surface of a coupling B.
The pin 1 has male threads 3a and a seal portion 4a and a shoulder portion 5
positioned at the end of the steel pipe. Correspondingly, the box 2 has the female
threads 3b and a seal portion 4b and a shoulder portion on the side of the threads
remote from the end of the box.
In each of the pin 1 and the box 2, the seal portion and the shoulder portion
constitute an unthreaded metal contact portion, and the threads and the unthreaded
metal contact portion (namely, the seal portion and the shoulder portion) constitute
a contact surface of the threaded joint. Galling resistance, gas tightness, and
corrosion resistance are required for the contact surfaces of the pin and the box. In
the past, for this purpose, compound grease containing heavy metal powder has
been applied thereto. However, due to concerns of the adverse effects of heavy
metals on humans and the environment, threaded joints having a solid lubricating
coating which can be used for connection of oil country tubular goods without
application of compound grease are being studied. A solid lubricating coating is
typically a resin coating containing a solid lubricant.
However, if a conventional solid lubricating coating is used in an extremely
cold environment of -60° C to -20° C, there are the problems that the initial makeup
torque becomes high, the unthreaded metal contact portions for guaranteeing gas
tightness do not contact with a prescribed makeup pressure, and the threads are not
completely engaged (a condition referred to as no shouldering), galling easily takes
place during makeup, and even if makeup is achieved, the initial breakout torque at
the time of breakout becomes extremely high. Furthermore, when tongs used for
makeup of pipes have a low capacity, the problem that makeup cannot take place
due to insufficient torque may occur.
According to the present invention, as shown in Figures 4(a) and 4(b) withrespect to a seal portion, the contact surface of at least one of a pin and a box is
coated with a particular thermoplastic solid lubricating coating 31a formed as an
uppermost surface treatment coating atop a steel surface 30a or 30b. This solid
lubricating coating can exhibit a lubrication-imparting function even when it is
exposed to an extremely cold environment of -60° C to -20° C, it can prevent
galling of a threaded joint while preventing an increase in torque at the time of
makeup or breakout, and it can guarantee gas tightness after makeup.
The substrate for the solid lubricating coating 31a (namely, the contact
surface of the threaded joint) preferably has undergone surface roughening. As
shown in Figure 4(a), the surface roughening can be achieved by direct surface
roughening by blasting treatment or pickling of the surface of the steel 30a, or it can
be achieved by forming a preparatory surface treatment coating 32 having a rough
surface (such as a phosphate coating or porous zinc (alloy) plating) on the surface of
the steel 30b before forming the lubricating coating 31a.
The solid lubricating coating 31a can be formed by applying a thermoplastic
solid lubricating coating-forming composition heated at an temperature sufficient to
melt the thermoplastic polymer matrix by a suitable method such as spraying, brush
application, or immersion and then solidifying the coating by a known cooling
methods such as air cooling or natural cooling. Alternatively, a liquid composition
containing a solvent can be applied in a conventional manner.
A solid lubricating coating may be formed on the contact surfaces of both a
pin and a box, but for a pin and a box which are connected to each other prior to
shipment as shown in Figure 2, it is sufficient to form a lubricating coating on the
contact surface of just one of the pin and the box. In this case, it is convenient to
form a lubricating coating on the contact surface of a coupling (normally the contact
surface of a box) because coating application is easier on the coupling (a short
member) than on a long steel pipe.
For a pin and box which are not connected prior to shipment, a solid
lubricating coating can be formed on the contact surfaces of both a pin and a box to
simultaneously impart lubricating properties and rust preventing properties.
Alternatively, a solid lubricating coating may be formed on the contact surface of
just one of a pin and a box (such as the box), and a solid corrosion-protecting
coating may be formed on the contact surface of the other member (such as the pin).In either case, galling resistance, gas tightness, and rust resistance can be imparted
to a threaded joint. The solid corrosion-protecting coating is preferably an
ultraviolet cured coating, and it is preferably formed after preparatory surface
treatment for surface roughening.
The entirety of the contact surface of a pin and/or a box should be coated
with a lubricating coating, but the present invention also includes the case in which
only a portion of the contact surface (such as only the unthreaded metal contact
portions) is coated.
[Thermoplastic solid lubricating coating]
In the present invention, a thermoplastic solid lubricating coating is formed
on the contact surface of at least one of a pin and a box constituting a tubular
threaded joint. This solid lubricating coating is required to avoid the occurrence of
no shouldering in which the makeup torque at the start of makeup becomes high and
to prevent the initial breakout torque from becoming high in order to adequately
prevent galling when steel pipes are connected by the threaded joint not only in cold,
temperate, and tropical regions (at -20° C to +50° C), but also in extremely cold
regions (at -60° C to -20° C) as well as to prevent rusting during storage.
A composition for forming a thermoplastic solid lubricating coating
comprises a thermoplastic polymer matrix and low friction copolymer particles.
Accordingly, the thermoplastic solid lubricating coating which is formed has a
structure containing low friction copolymer particles dispersed in a thermoplastic
polymer matrix. Because the coating contains low friction copolymer particles,
the coating exhibits an effect of reducing friction, and it can markedly improve the
galling resistance of a threaded joint. Moreover, the low friction copolymer
particles can adequately exhibit the friction-reducing effect even at extremely low
temperatures.
It is preferable to use a thermoplastic polymer having a melting temperature
(or softening temperature; the same applies below) of 80° C - 320° C to form a
thermoplastic polymer matrix of a thermoplastic solid lubricating coating. The
melting temperature is more preferably in the range of 90° C - 200° C. If the
melting temperature of a thermoplastic polymer which forms a matrix of a coating,
it becomes difficult to perform application in a molten state such as is the case with
hot melt coating. On the other hand, if the melting temperature is too low, thesolid lubricating coating softens when it is exposed to a high temperature in tropical
regions or in summer even in temperate regions, leading to a deterioration in
performance.
Examples of thermoplastic polymers which can be used as a matrix material
in the present invention include, although not to limited thereto, polyolefins,
polystyrenes, polyurethanes, polyamides, polyesters, polycarbonates, acrylic resins,
and thermoplastic epoxy resins. The thermoplastic polymer may be a
homopolymer or a copolymer.
As described later, a contact surface of a tubular threaded joint which is a
substrate on which a lubricating coating is formed may be previously subjected to
preparatory surface treatment such as chemical conversion treatment or plating.
From the standpoints of the adhesion to the substrate, film-forming properties,
coatability, viscosity at the time of melting, and dispersibility of low friction
copolymer particles, it is preferable that the thermoplastic polymer which is used be
a mixture of a plurality of types of thermoplastic polymers having different
properties such as their melting point, softening point, and glass transition
temperature.
Particularly preferred thermoplastic polymers for use as a matrix material are
polyolefin resins and ethylene-vinyl acetate copolymer resins, and it is particularly
preferred to use a mixture of at least two polyolefin resins having different melting
points or softening points and an ethylene-vinyl acetate copolymer resin.
Low friction copolymer particles dispersed in a thermoplastic polymer
matrix exhibit the effect of decreasing friction and lowering the torque even at
extremely low temperatures. Therefore, a thermoplastic solid lubricating coating
containing these particles can exhibit a greatly decreased friction while maintaining
the adhesion of the coating even at extremely low temperatures of -60° C to -20° C.
This fact was first elucidated by the present inventors.
Low friction copolymer particles which are used in the present invention are
in the form of a powder of a copolymer obtained by copolymerization of a low
friction resin selected from a fluorocarbon resin such as polytetrafluoroethylene and
a silicone resin with a monomer of another thermoplastic resin. This copolymer
may be a block copolymer. Even when using particles of a copolymer of a low
friction resin such as a silicone resin or a fluorocarbon resin with a thermoplasticresin, the low friction resin portion which has good sliding properties at low
temperatures always faces the sliding surfaces, thereby making it possible to
maintain substantially the same level of good lubricating properties as when using
particles made solely of a low friction resin. In addition, the thermoplastic resin
portion of the copolymer particles is compatible with the thermoplastic polymer
forming a coating matrix, so the particles are strongly bonded to the matrix.
Therefore, even when a high tightening pressure is applied, the particles do not
easily drop off as is the case when particles made solely of a low friction resin are
used. Even though the lubricating properties are initially good when using
particles made solely of a low friction resin such as a silicone resin or a
fluorocarbon resin, the wear resistance and durability of the coating decrease due to
particles dropping off, and good lubricating properties cannot be maintained.
As the thermoplastic resin which forms a copolymer with a low friction resin,
it is preferable to select a resin having affinity for the thermoplastic polymer used as
a matrix of the thermoplastic resin coating. For example, it is possible to use a
thermoplastic resin which is of the same type as the thermoplastic polymer used as
the matrix of the coating. Some examples of a suitable thermoplastic resin are
acrylic resins, urethane resins, polyester resins, polycarbonate resins, polyimide
resins, and thermoplastic epoxy resins.
A copolymer of a low friction resin and a thermoplastic resin monomer can
be prepared by copolymerizing the thermoplastic resin monomer with a reactive
silicone or fluorocarbon resin having a functional group capable of reacting the
thermoplastic resin monomer which has previously been introduced into the resin.
The reactive functional group which can be introduced into a silicone or
fluorocarbon resin is a (meth)acrylic group in the case of copolymerization with an
acrylic resin, a hydroxyl group in copolymerization with a urethane resin, an epoxy
group, a carboxyl group, or a hydroxyl group in copolymerization with a polyester
resin, a phenolic group in copolymerization with a polycarbonate resin, an amino
group in copolymerization with a polyimide resin, and a hydroxyl group in
copolymerization with a thermoplastic epoxy resin.
An example of a low friction copolymer particle which can be
advantageously used in the present invention is an acrylic-silicone copolymer
particle. This is a particulate (powdery) copolymer obtainable bycopolymerization of a silicone resin with an acrylic monomer, and which can be
prepared by copolymerizing a polyorganosiloxane having a free radically
polymerizable terminal group (such as a (meth)acrylic group) with a (meth)acrylate
ester. The proportion of the polyorganosiloxane and the (meth)acrylate ester in
this copolymer is preferably 60 - 80 : 20 - 40 as a mass ratio. The size of the
copolymer particles is preferably such that the average particle diameter is in the
range of 10 - 400 micrometers.
Copolymerization can be carried out by emulsion polymerization or the like
using a suitable liquid medium and a free radical polymerization initiator. The
resulting copolymer in the form of an emulsion is subjected to solid-liquid
separation so as to recover the solids, and the desired copolymer particles are
obtained in the form of secondary particles which are aggregates of the minute
particles in the emulsion (primary particles). In the present invention, the particles
and particle diameter mean the secondary particles and the particle diameter of the
secondary particles, respectively. The shape of the copolymer particles may be
either amorphous or spherical, but preferably it is spherical, i.e., they are preferably
spherical particles.
Spherical acrylic-silicone copolymer particles having an average particle
diameter of 10 - 40 micrometers are particularly suitable for the present invention.
Spherical acrylic-silicone copolymer particles having an average particle diameter
of 30 micrometers are sold by Nissin Chemical Industry Co., Ltd. under the product
name Chaline R-l 70S. This product can be used as low friction copolymer
particles in the present invention.
The thermoplastic solid lubricating coating contains low friction copolymer
particles, preferably acrylic-silicone copolymer particles, dispersed in a
thermoplastic polymer matrix. In the case of a tubular threaded joint for
connecting oil country tubular goods, the content of the acrylic-silicone copolymer
particles in the thermoplastic solid lubricating coating is preferably in the range of
0.5 - 30 mass % and more preferably in the range of 1 - 20 mass %. If this content
is less than 0.5 mass %, the friction reducing effect and the adhesion of the coating
at extremely low temperatures become insufficient, while if the content exceeds 30
mass %, the ability to form a coating decreases, and it may become difficult to form
a quality coating.In order to further improve lubricating properties, the thermoplastic solid
lubricating coating may additionally contain various solid lubricants. A solid
lubricant means a powder having lubricating properties. Solid lubricants can be
roughly classified as follows:
(1) ones which exhibit lubricating properties due to having a crystal structure
which easily slides such as a hexagonal layer crystal structure (e.g., graphite, zinc
oxide, and boron nitride);
(2) ones exhibiting lubricating properties due to having a reactive element in
addition to a crystal structure (e.g., molybdenum disulfide, tungsten disulfide,
graphite fluoride, tin sulfide, and bismuth sulfide);
(3) ones exhibiting lubricating properties due to having chemical reactivity
(e.g., certain thiosulfate compounds), and
(4) ones exhibiting lubricating properties due to plastic or viscoplastic
behavior under a frictional stress (e.g., polytetrafluoroethylene (PTFE) and
polyamides).
Any of these types of solid lubricants can be used, but type (1) is preferred.
Solid lubricants of type (1) can be used by themselves, or they can be used in
combination with solid lubricants of type (2) and/or type (4).
Graphite is a particularly preferred solid lubricant from the standpoints of not
interfering with the effects of acrylic-silicone copolymer particles as well as
adhesion and rust prevention, and amorphous (earthy) graphite is still more
preferred from the standpoint of the ability to form a coating. The content of the
solid lubricant in the thermoplastic solid lubricating coating is preferably in the
range of 2 - 15 mass %.
In addition to a solid lubricant, the thermoplastic solid lubricating coating
may contain an inorganic powder for adjusting sliding properties. Examples of
such an inorganic powder are titanium dioxide and bismuth oxide. In order to
strengthen the rust preventing properties of a coating, the thermoplastic solid -
lubricating coating may contain an anticorrosive agent. An example of a preferred
anticorrosive agent is calcium ion exchanged silica. Commercially available
reactive water repellents may also be used. These inorganic powders,
anticorrosive agents, and other additives may be present in the thermoplastic
lubricating coating in a total amount of up to 20 mass %.In addition to the above-described components, the thermoplastic solid
lubricating coating may contain small amounts of other additives selected from
surface active agents, colorants, antioxidants, and the like in an amount of at most 5
mass %, for example. It may also contain an extreme pressure agent, a liquid
lubricant, or the like in an extremely small amount of at most 2 mass %.
According to the present invention, a solid lubricating coating-forming
composition for forming the above-described thermoplastic solid lubricating coating
(referred to below as a coating composition) is provided. This coating
composition may be a solventless (or non-solvent) composition consisting
essentially of the above-described components, or it may be a solvent-based
composition in which a thermoplastic polymer matrix is dissolved in a solvent.
A solventless coating composition can be prepared by, for example, adding
acrylic-silicone copolymer particles, a solid lubricant and other additives to a
molten thermoplastic polymer matrix followed by blending or kneading.
Alternatively, a powder mixture in which all the components in a powder state are
mixed can be used as a coating composition. A solventless coating material has
the advantages that it can form a lubricating coating in a short period of time and
that there is no evaporation of organic solvents which are harmful to the
environment.
Such a solventless coating composition can form a thermoplastic solid
lubricating coating by the hot melt method, for example. In this method, a coating
composition (containing the above-described thermoplastic polymer matrix and
various powders) which has been heated to cause the thermoplastic polymer matrix
to melt and form a fluid composition having a viscosity low enough for coating is
sprayed by a spray gun having the ability to maintain a fixed temperature (normally
around the same temperature as the composition in a molten state). The
temperature to which the composition is heated is preferably made 10° C - 50° C
higher than the melting point (the melting temperature or the softening temperature)
of the thermoplastic polymer matrix. It is acceptable for the low friction
copolymer particles in the coating composition (such as acrylic-silicone copolymer
particles) to partially melt during heating.
The substrate being coated (namely, the contact surface of a pin and/or a
box) is preferably preheated to a temperature higher than the melting point of thethermoplastic polymer matrix. By performing preheating, a good coating ability
can be obtained. When the coating composition contains a small amount (such as
at most 2%) of a surface active agent such as polydimethylsiloxane, a good coating
can be formed even if the substrate is not preheated or if the preheating temperature
is lower than the melting point of the polymer matrix.
The coating composition is heated and melted inside a tank equipped with a
suitable stirring apparatus, and it is supplied to the spraying head (which is
maintained at a prescribed temperature) of a spray gun through a metering pump by
a compressor and sprayed at the substrate. The temperature at which the inside of
the tank and the spraying head are maintained is adjusted in accordance with the
melting point of the polymer matrix in the composition.
The substrate is then cooled by air cooling or natural cooling to solidify the
thermoplastic polymer matrix and form a thermoplastic solid lubricating coating
according to the present invention atop the substrate. The thickness of a
thermoplastic solid lubricating coating formed in this manner is preferably in the
range of 10 - 200 µιη and more preferably in the range of 25 - 100 µ η. If the
thickness of the thermoplastic solid lubricating coating is too small, the lubricating
properties of a tubular threaded joint are insufficient and it becomes easy for galling
to occur at the time of makeup or breakout. This thermoplastic solid lubricating
coating has also corrosion resistance to some extent, but if the coating thickness is
too small, the corrosion resistance becomes inadequate and the corrosion resistance
of contact surface of a tubular threaded joint decreases.
On the other hand, making the thickness of the thermoplastic solid
lubricating coating too large not only wastes lubricant but also is contrary to
preventing environmental pollution, which is one object of the present invention.
When the thermoplastic solid lubricating coating and the below-described
solid corrosion-protecting coating which is formed as necessary are formed atop a
contact surface having its surface roughness increased by preparatory surface
treatment, they both preferably have a coating thickness larger than the roughness
Rmax of the substrate having an increased surface roughness. If the thickness is
not larger than this roughness, it is sometimes not possible to completely cover the
surface of the substrate. The coating thickness when the substrate has a rough
surface is the average value of the coating thickness of the entire coating, which canbe calculated from the surface area, the mass, and the density of the coating.
[Solid Corrosion-Protecting Coating]
When the above-described thermoplastic solid lubricating coating is formed
on the contact surface of only one of the pin and the box (such as the box) of a
tubular threaded joint, the contact surface of the other member (such as the pin) may
undergo just the below-described preparatory surface treatment. However, in
order to impart rust preventing properties, a solid corrosion-protecting coating is
preferably formed as an uppermost surface treatment coating layer on the contact
surface of the other member.
As described above with respect to Figure 1, up to the time when a tubular
threaded joint is actually used, a protector is often mounted on the pin and box
which have not been connected to another member. The solid corrosion-protecting
coating must not be destroyed under at least the force applied when mounting a
protector thereon, it must not dissolve when exposed to water which is formed by
condensation due to the dew point during transport or storage, and it must not easily
soften at a high temperature exceeding 40° C.
In a preferred embodiment of the present invention, a solid
corrosion-protecting coating which can satisfy these properties is formed from a
composition based on an ultraviolet curing resin, which is known to be able to form
a high strength coating. Known resin compositions comprising at least a monomer,
an oligomer, and a photopolymerization initiator can be used as an ultraviolet curing
resin. There are no particular limitations on the components or composition of an
ultraviolet curing resin as long as a photopolymerization reaction is produced by
irradiation with ultraviolet light to form a cured coating.
Some non-limiting examples of monomers are polyvalent (di, tri, or higher)
esters of polyhydric alcohols with (meth)acrylic acid, various (meth)acrylates,
N-vinylpyrrolidone, N-vinylcaprolactam, and styrenes. Some non-limiting
examples of oligomers are epoxy (meth)acrylates, urethane (meth)acrylates,
polyester (meth)acrylates, polyether (meth)acrylates, and silicone (meth)acrylates.
Useful photopolymerization initiators are compounds having absorption in a
wavelength region of 260 - 450 nm, examples of which are benzoin and its
derivatives, benzophenone and its derivatives, acetophenone and its derivatives,
Michler's ketone, benzil and its derivatives, tetralkylthiuram monosulfides, andthioxanes. It is particularly preferable to use thioxanes.
From the standpoints of coating strength and sliding properties, a solid
corrosion-protecting coating formed from an ultraviolet curing resin may contain
additives selected from lubricants, fibrous fillers, and anticorrosive agents in the
coating.
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 one part by mass of the ultraviolet curing resin. If the amount
is less than 0.05 parts, the strength of the coating is sometimes inadequate. On the
other hand, if the amount exceeds 0.35 parts, the viscosity of a coating composition
becomes high and the ease of coating decreases, and this sometimes leads to a
decrease in coating strength.
Examples of an anticorrosive agent are aluminum tripolyphosphate and
aluminum phosphite . The anticorrosive agent can be added i an amount of up to
0.10 parts by mass with respect to one part by mass of the ultraviolet curing resin.
A solid corrosion-protecting coating formed from an ultraviolet curing resin
is often transparent. From the standpoint of facilitating quality inspection (such as
inspection for the presence or absence of a coating or for uniformity or unevenness
of the coating thickness) by visual examination or by image processing of the solid
corrosion-protecting coating which is formed, the solid corrosion-protecting coating
may contain a colorant. Colorants which are used can be selected from pigments,
dyes, and fluorescent materials. Fluorescent materials sometimes do not give
coloration to a coating under visible light, but they give coloration to the coating at
least under ultraviolet light. Therefore, they are included as colorants in the
present invention. These colorants may be commercially available ones, and there
are no particular restrictions thereon as long as quality inspection of a solid
corrosion-protecting coating is possible visually or by image processing. Either
organic or inorganic colorants may be used.
The transparency of a solid corrosion-protecting coating decreases or
disappears when a pigment is added. If a solid corrosion-protecting coating
becomes non-transparent, it becomes difficult to inspect for damage of the threadsof the pin which forms a substrate. Accordingly, when a pigment is used, one
having a high degree of brightness such as a yellow or white pigment is preferred.
From the standpoint of corrosion prevention, the particle diameter of a pigment is
preferably as small as possible, and it is preferable to use a pigment with an average
particle diameter of at most 5 µι . Dyes do not greatly decrease the transparency
of a solid corrosion-protecting coating, so there are no problems with using a dye
having a strong color such as red or blue. The added amount of the pigment or a
dye is preferably a maximum of 0.05 parts by mass with respect to one part by mass
of the ultraviolet curing resin. If the amount exceeds 0.05 parts by mass, corrosion
resistance may decrease. A more preferred added amount is at most 0.02 parts by
mass.
A fluorescent material can be any of a fluorescent pigment, a fluorescent dye,
and a phosphor used in a fluorescent paint. Fluorescent pigments are roughly
categorized as inorganic fluorescent pigments and daylight fluorescent pigments.
Examples of inorganic fluorescentpigments are ones based on zinc sulfide or
zinc cadmium sulfide (containing a metal activator), halogenated calcium
phosphates, rare earth activated strontium chloroapatites, and the like. Two or
more of these can often be used in combination. Inorganic fluorescent pigments
have excellent resistance to weather and heat.
There are several types of daylight fluorescent pigments, but the main types
are synthetic resin solid solution types in which a fluorescent dye is incorporated
into a colorless synthetic resin to form a pigment. Fluorescent dyes can also be
used by themselves. Various types of inorganic or organic fluorescent pigments
and particularly synthetic resin solid solution types are used in fluorescent paints
and fluorescent printing inks, and these phosphors (fluorescent materials) can be
used as fluorescent pigments or fluorescent dyes.
A solid corrosion-protecting coating containing a fluorescent pigment or dye
is colorless or has a transparent color under visible light. However, when it is
irradiated with black light or ultraviolet light, it fluoresces and becomes colored,
and it becomes possible to ascertain whether or not a coating is present or to
ascertain unevenness in the coating thickness. As the coating is transparent under
visible light, the substrate underneath the solid corrosion-protecting coating, namely,
the surface of the substrate can be visually observed. Accordingly, visualinspection for damage of the threads of a threaded joint is not obstructed by the
solid corrosion-protecting coating.
The added amount of these fluorescent materials is preferably up to
approximately 0.05 parts by mass with respect to one part by mass of the ultraviolet
curing resin. If the added amount exceeds 0.05 parts by mass, corrosion resistance
may decrease. A more preferred added amount is at most 0.02 parts by mass.
In order to make it possible to carry out quality control not only of the solid
corrosion-protecting coating but also of the underlying threads, it is preferable to
use a fluorescent material and particularly a fluorescent pigment as a colorant.
After a composition based on an ultraviolet curing resin (including a
composition consisting essentially of components of an ultraviolet curing resin) is
applied to the contact surface of a threaded joint, the coating is cured by irradiation
with ultraviolet light to form a solid corrosion-protecting coating made from an
ultraviolet cured resin layer.
By repeating coating and irradiation with ultraviolet light, it is possible to
form a solid corrosion-protecting coating having two or more layers of an
ultraviolet curing resin. By using multiple layers of a corrosion-protecting coating,
the coating strength is further increased, the solid corrosion-protecting coating is not
destroyed even when a large force is applied at the time of makeup of a threaded
joint and the corrosion resistance of the threaded joint is further increased. In the
present invention, because a lubricating coating is not present beneath the solid
corrosion-protecting coating, it is not necessary for the solid corrosion-protecting
coating to be destroyed during makeup of a threaded joint. Not destroying the
solid corrosion-protecting coating increases the corrosion resistance of a threaded
joint.
Irradiation with ultraviolet light can be carried out using a commercially
available ultraviolet light irradiation apparatus having an output wavelength in the
region of 200 - 450 nm. Examples of a source of ultraviolet light are high pressure
mercury vapor lamps, ultrahigh pressure mercury vapor lamps, xenon lamps, carbon
arc lamps, metal halide lamps, and sunlight. The length of time for which
irradiation is performed and the strength of the irradiated ultraviolet light can be
suitably set by one skilled in the art.
The thickness of the solid corrosion-protecting coating (the total coatingthickness when there are two or more layers of an ultraviolet curing resin) is
preferably in the range of 5 - 50 µ and more preferably in the range of 10 - 40 µ .
It is preferably smaller than the thickness of the solid lubricating coating formed
on the opposing member. If the thickness of the solid corrosion-protecting coating
is too small, it does not adequately function as a corrosion-protecting coating, and
the corrosion resistance of a tubular threaded joint may be inadequate. On the
other hand, if the thickness of the solid corrosion-protecting coating exceeds 50 µ η,
when a protective member such as a protector having high gas tightness is mounted
on the end of an oil country tubular good, the solid corrosion-protecting coating
may be destroyed by the force at the time of mounting the protector, and the
corrosion resistance of a tubular threaded joint becomes inadequate. Furthermore,
at this time, powder produced by wear is discharged into the environment and the
work environment is worsened. In addition, a solid corrosion-protecting coating
having a thickness larger than the thickness of the solid lubricating coating on the
opposing member may interfere with the lubricating performance of the lubricating
coating.
Since a solid corrosion-protecting coating based on an ultraviolet curing resin
is transparent, the condition of a substrate can be observed without removing the
coating, and threads can be inspected from above the coating prior to makeup.
Accordingly, by forming this solid corrosion-protecting coating on the contact
surface of a pin in which threads are formed on its outer surface and hence are more
susceptible to damage than the threads of a box, it is possible to easily inspect for
damage to the threads of a pin while leaving the coating in place.
[Preparatory Surface Treatment]
The threads and seal portions of a pin and a box which constitute the contact
surfaces of a tubular threaded joint are formed by cutting operations including
thread cutting. Their surface roughness is typically around 3 - 5 µ η . If the
surface roughness of the contact surfaces is greater than this amount, the adhesion
of a coating formed atop them can be increased, and as a result, performance such
as galling resistance and corrosion resistance can be improved. Therefore, prior to
forming a coating, preparatory surface treatment which can increase the surface
roughness is preferably carried out on the contact surface of at least one and
preferably both of the pin and the box.Examples of such preparatory treatment are blasting by projecting blasting
material such as spherical shot or angular grit, and pickling by immersion in a
strongly acidic solution such as sulfuric acid, hydrochloric acid, nitric acid, or
hydrofluoric acid to roughen the skin. These treatments can increase the surface
roughness of the substrate itself.
Examples of another type of preparatory surface treatment are chemical
conversion treatment such as phosphate treatment, oxalate treatment, or borate
treatment, and metal plating. These methods form an undercoating layer having a
large surface roughness and a high adhesion on the surface of the substrate. A
chemical conversion coating formed by a chemical conversion treatment is made of
acicular crystals with a large surface roughness. Examples of metal plating are
electroplating with copper, iron, or alloys thereof (protrusions are preferentially
plated, so the surface becomes slightly rougher); impact plating with zinc or a zinc
alloy in which particles having an iron core coated with zinc or a zinc-iron alloy are
projected using centrifugal force or air pressure, thereby forming a porous metal
coating by deposition of zinc or zinc-iron alloy particles; and composite metal
plating in which a coating having minute solid particles dispersed in metal is
formed.
Whichever method is used for preliminary surface treatment of the contact
surface, the surface roughness R ax resulting from surface roughening by
preparatory surface treatment is preferably 5 - 40 µπ . If Rmax is less than 5 µ η,
the adhesion of the lubricating or corrosion-protecting coating which is formed atop
the roughened surface may be inadequate. On the other hand, if Rmax exceeds 40
µπ , friction increases, and the coating may not able resist shear forces and
compressive forces and may be easily destroyed or peel off when it is subjected to a
high tightening pressure. Two or more types of preparatory surface treatment for
surface roughening may be used in combination. In addition, different types of
preparatory surface treatment can be carried out on the pin and the box.
From the standpoint of adhesion of the solid corrosion-protecting coating or
the solid lubricating coating, preparatory surface treatment which can form a porous
coating is preferred. In particular, phosphate treatment using manganese
phosphate, zinc phosphate, iron manganese phosphate, or zinc calcium phosphate,
or impact plating to form a zinc or zinc-iron alloy coating is preferred aspreparatory surface treatment. From the standpoint of the adhesion of a coating
formed atop it, a manganese phosphate coating is preferred, and from the standpoint
of corrosion resistance, a zinc or zinc-iron alloy coating with which a sacrificial
corrosion effect due to zinc can be expected is preferred.
Manganese phosphate chemical conversion treatment is particularly
preferred as preparatory surface treatment for a solid lubricating coating, and zinc
phosphate chemical conversion treatment and zinc or zinc-iron alloy plating by
impact plating are particularly preferred as preparatory surface treatment for a solid
corrosion-protecting coating.
A coating formed by phosphate treatment and a zinc or zinc-iron alloy
coating formed by impact plating are both porous coatings. By forming a solid
corrosion-protecting coating or a solid lubricating coating atop such a porous
coating, the adhesion of the upper coating is increased by the so-called anchor effect
of the lower porous coating. As a result, it becomes difficult for peeling of the
solid lubricating coating or the solid corrosion-protecting coating to take place even
if makeup and breakout are repeated, and galling resistance, gas tightness, and
corrosion resistance are further increased.
Phosphate treatment can be carried out by immersion or spraying in a
conventional manner. An acidic phosphating solution which is commonly used for
zinc-plated steel materials can be used in this treatment. 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 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 in accordance with the desired
coating thickness. In order to accelerate coating formation, an aqueous surface
conditioning solution containing colloidal titanium may be supplied to the surface to
be treated prior to phosphate treatment. After phosphate treatment, washing is
preferably carried out 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 the material to be plated using a blasting
apparatus. In the present invention, it is sufficient to plate just the contact surfaceof a threaded joint, so it is preferable to employ blast plating which can perform
localized plating.
For example, a blasting material in the form of particles having an iron core
coated with zinc or a zinc alloy (such as zinc-iron alloy) is projected against a
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 particle diameter is preferably in
the range of 0.2 - .5 mm. Blasting of the particles causes only the zinc or zinc
alloy which is the coating layer of the particles to adhere to the contact surface, and
a porous coating made of zinc or a zinc alloy particles is formed atop the contact
surface. This impact plating can form a plated coating having good adhesion to a
steel surface regardless of the composition of the steel.
From the standpoints of corrosion resistance and adhesion, the thickness of a
zinc or a zinc alloy layer formed by impact plating is preferably 5 - 40 µ . If it is
less than 5 r n , sufficient corrosion resistance cannot be guaranteed. On the other
hand, if it exceeds 40 µ η, adhesion of a coating formed thereon sometimes
decreases. Similarly, the thickness of a phosphate coating is preferably in the
range of 5 - 40 µπ .
Another possible preparatory surface treatment is a particular type of single
or multiple layer electroplating, which is effective for increasing galling resistance
when used to form a substrate for a solid lubricating coating although it does not
provide a surface roughening effect. Examples of such plating are single-layer
plating with Cu, Sn, or Ni, single-layer plating with a Cu-Sn alloy as disclosed in JP
2003-74763 A, two-layer plating with a Cu layer and an Sn layer, and three-layer
plating with an Ni layer, a Cu layer, and an Sn layer. Cu-Sn alloy plating,
two-layer plating by Cu plating and Sn plating, and three-layer plating by Ni plating,
Cu plating, and Sn plating are preferred for a steel pipe made from a steel having a
Cr content of at least 5%. More preferred are two-layer plating by Cu plating and
Sn plating, three-layer plating by Ni strike plating, Cu plating, and Sn plating, and
Cu-Sn-Zn alloy plating. Such metal or alloy plating can be carried out by the
methods described in JP 2003-74763 A. In the case of multiple layer plating, the
lowermost plating layer (usually Ni plating) is preferably an extremely thin plating
layer having a thickness of less than 1 µπ formed by the so-called strike plating.
The thickness of plating (the overall thickness in the case of multiple layer plating)is preferably in the range of 5 - 15 µηι.
Examples
Below, examples of the present invention will be described. However, the
present invention is not limited by the examples. In the examples, the contact
surface of a pin will be referred to as the pin surface and the contact surface of a box
will be referred to as the box surface. Unless otherwise specified, percent and part
in the examples mean mass percent and part by mass, respectively.
Example 1
The pin surface and the box surface of a tubular threaded joint (outer
diameter of 17.78 cm (7 inches), wall thickness of 1.036 cm (0.408 inches)) made
of carbon steel (C: 0.21%, Si: 0.25%, Mn: 1.1%, P: 0.02%, S: 0.01%, Cu: 0.04%,
Ni: 0.06%, Cr: 0.17%, Mo: 0.04%, remainder: iron and impurities) were subjected
to the following preparatory surface treatment.
The pin surface which was finished by machine grinding (surface roughness
of 3 um) was immersed for 10 minutes in a zinc phosphating solution at 75 - 85° C
to form a zinc phosphate coating with a thickness of 8 um (surface roughness of 8
µη ) .
The box surface which was finished by machine grinding (surface roughness
of 3 um) 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 µ η (surface
roughness of 10 ^m).
A composition for forming a solid lubricating Coating having the
below-described composition was heated to 160° C in a tank equipped with a
stirring mechanism to form it into a molten state having a viscosity suitable for
coating, and the pin surface and the box surface which had undergone the
above-described preparatory surface treatment were preheated to 130° C by
induction heating. The solid lubricating coating-forming composition in which the
matrix polymer is in molten state was sprayed at both the pin surface and the box
surface using a spray gun having a spraying head with a temperature maintaining
function. After cooling, a solid lubricating coating having a thickness of 50 µπ
was formed.Composition of the Lubricating Coating-Forming Composition:
(Thermoplastic polymer matrix)
21.5% of a polyolefm resin (HM321 of Cemedine Co. Ltd., softening
point of 130° C),
1.5% of an ethylene-vinyl acetate copolymer resin (HM22 1 of
Cemedine Co., Ltd., softening point of 105° C), and
42% of a low molecular weight polyolefm (21OP of Mitsui Chemicals,
Inc., softening point of 123° C).
(Acrylic-silicone copolymer particles)
10% of Chaline R-170S (Nissin Chemical Industry Co., Ltd., average
particle diameter of 30 µι ) .
(Solid lubricant)
5% of amorphous graphite (Blue P of Nippon Graphite Industries,
Ltd., average particle diameter of 7 µ ι) .
A repeated makeup and breakout test was performed up to 10 times on a
tubular threaded joint treated as above (makeup speed of 10 rpm, makeup torque of
20 kN-m) at room temperature (approximately 20° C) and at approximately -40° C
by cooling the periphery of the threaded joint with dry ice. The shouldering torque
ratio and the breakout torque ratio on the first cycle (both were relative values with
the shouldering torque and the breakout torque at the time of makeup with a
compound grease being given a value of 100), the adhesion of the solid lubricating
coating (which was determined by whether there was peeling or cracking of the
coating when exposed to each temperature, and by the condition of the coating after
the first cycle of makeup and breakout), and the state of galling of the contact
surfaces of the pin and the box after repeated makeup (the number of times that
makeup could be performed without the occurrence of galling, up to a maximum of
10 times; when light galling which could be repaired occurred, repair was
performed and makeup was continued) were investigated. The results are shown
in Table 1.
As shown in Table 1, in Comparative Example 1 in which a solid lubricating
coating did not include the above-described acrylic-silicone copolymer particles, the
torque ratio at -40° C was extremely high, whereas in Example 1 in which a solid
lubricating coating contained the above-described copolymer particles, the torquelevel was around the same as when using a compound grease both at room
temperature and at -40° C. The adhesion of the coating was also good. There
was no occurrence of galling, and makeup and breakout could be performed 10
times.
Example 2
The pin surface and the box surface of the same tubular threaded joint made
of carbon steel as used in Example 1were subjected to the following surface
treatment.
The pin surface which was finished by machine grinding (surface roughness
of 3 µ ) was immersed for 10 minutes in a zinc phosphating solution at 75 - 85° C
to form a zinc phosphate coating with a thickness of 8 µ η (surface roughness of 8
µ ) . An ultraviolet curing resin coating compositions prepared by adding 0.05
parts of aluminum phosphite as an anticorrosive agent and 0.01 parts of
polyethylene wax as a lubricant to one part of the resin content of an acrylic
resin-based ultraviolet curing resin paint made by Chugoku Marine Paints Ltd. was
applied atop the zinc phosphate coating of the pin surface and was irradiated with
ultraviolet light under the following conditions to cure the coating and form an
ultraviolet cured resin coating having a thickness of 25 µπι on the pin surface. The
resulting solid corrosion-protecting coating was colorless and transparent, and the
male threads of the pin could be inspected with the naked eye or with a magnifying
glass from atop the coating.
UV lamp: Water-cooled mercury vapor lamp,
UV lamp output: 4 kW,
Wavelength of UV light: 260 nm.
The box surface which was finished by machine grinding (surface roughness
of 3 µ ) was subjected to electroplating first by Ni strike plating and then by
Cu-Sn-Zn alloy plating to form a plated coating having a total thickness of 8 µ .
A lubricating coating-forming composition having the following composition was
heated to 160° C in a tank having a stirring mechanism to obtain a molten state with
a viscosity suitable for coating. After the box surface which underwent the
above-described preparatory surface treatment was preheated to 130° C by
induction heating, the molten composition for forming a solid lubricating coatingwas applied to the preheated box surface using a spray gun having a spraying head
with a temperature maintaining mechanism. After cooling, a solid lubricating
coating having a thickness of 50 µ was formed on the box surface.
Composition of the Lubricating Coating-Forming Composition:
(Thermoplastic polymer matrix)
22.5% of a polyolefin resin (HM321 of Cemedine Co., Ltd., softening point
of 130° C),
22.5 % of an ethylene-vinyl acetate copolymer resin (HM221 of Cemedine
Co., Ltd., softening point of 105° C),
45% of a low molecular weight polyolefin (21OP of Mitsui Chemicals, Inc.,
melting point of 123° C);
(Acrylic-silicone copolymer particles)
5% of Chaline R-170S (Nissin Chemical Industry Co., Ltd., average
particle diameter of 30 µη );
(Solid lubricant)
5% of amorphous graphite (Blue P of Nippon Graphite Industries,
Ltd., average particle diameter of 7 µηι) .
A repeated makeup and breakout test of a tubular threaded joint was carried
out at room temperature and at approximately -40° C in the same manner as in
Example 1. As shown in Table 1, in Comparative Example 1 in which a solid
lubricating coating did not contain acrylic-silicone copolymer particles, the torque
ratio at -40° C was extremely high, whereas in Example 2 in which a solid
lubricating coating contained acrylic-silicone copolymer particles, the torque level
was approximately the same as when using compound grease both at room
temperature and at a low temperature of -40° C. The adhesion of the coating was
good. Furthermore, there was no occurrence of galling, and makeup and breakout
could be performed 10 times.
Example 3
The pin surface and the box surface of a tubular threaded joint (outer
diameter: 24.448 cm (9-5/8 inches), wall thickness: 1.105 cm (0.435 inches)) made
of a 13Cr steel (C: 0.19%, Si: 0.25%, Mn: 0.9%, P: 0.02%, S: 0.01%, Cu: 0.04%,
Ni: 0.1 1%, Cr: 13%, Mo: 0.04%, remainder: iron and impurities) which is moresusceptible to galling than carbon steel were subjected to the following surface
treatment.
To the pin surface which was finished by machine grinding (surface
roughness of 3 µιη), an ultraviolet curing resin coating composition prepared by
adding 0.05 parts of aluminum tripolyphosphate as an anticorrosive agent, 0.01 part
of polyethylene wax as a lubricant, and 0.003 parts of a fluorescent pigment to one
part of the resin content of an acrylic resin-based ultraviolet curing resin paint made
by Chugoku Marine Paints Ltd. was applied and irradiated with ultraviolet light
under the following conditions for curing to form an ultraviolet cured resin coating
having a thickness of approximately 25 µπι . The resulting solid
corrosion-protecting coating was colorless and clear, and the male threads of the pin
could be inspected with the naked eye or with a magnifying glass from above the
coating.
UV lamp: Water-cooled mercury vapor lamp,
UV lamp output: 4 kW,
Wavelength of UV light: 260 nm.
The box surface which was finished by machine grinding (surface roughness
of 3 um) was subjected to electroplating first by Ni strike plating and then by
Cu-Sn-Zn alloy plating to form a plated coating with a total thickness of 8 µη . A
lubricating coating-forming composition having the below-described composition
was then heated to 160° C in a tank equipped with a stirring mechanism to form a
composition haying a molten matrix material with a viscosity suitable for coating.
The box surface which underwent preparatory surface treatment in the
above-described manner was preheated to 150° C by induction heating, and then the
molten composition for forming a solid lubricating coating was applied to the
preheated box surface using a spray gun having a spraying head with a temperature
maintaining mechanism. After cooling, a solid lubricating coating with a thickness
of 100 µπι was formed.
Composition of the Lubricating Coating-forming Composition
(Thermoplastic polymer matrix)
20% of a polyolefin resin (HM321 of Cemedine Co., Ltd., softening
point of 130° C),
20% of an ethylene-vinyl acetate copolymer resin (HM221 ofCemedine Co., Ltd., softening point of 105° C),
40% of a low molecular weight polyolefin (210 P of Mitsui Chemicals,
Inc., melting point of 123° C);
(Acrylic-silicone copolymer particles)
10% of Chaline R-170S (Nissin Chemical Industry Co., Ltd., average
particle diameter of 30 µιη) ;
(Solid lubricant)
5% of amorphous graphite (Blue P of Nippon Graphite Industries,
Ltd., average particle diameter of 7 µ );
(Anticorrosive agent)
5% of Ca ion exchanged silica (Sylysia 52Mo of Fuji Silysia
Chemical, Ltd.).
A repeated makeup and breakout test of a tubular threaded joint was carried
out at room temperature and at approximately -40° C in the same manner as in
Example 1. As shown in Table 1, in Comparative Example 1 in which a solid
lubricating coating did not contain acrylic-silicone copolymer particles, the torque
ratio at -40° C was extremely high, whereas in Example 3 in which a solid
lubricating coating contained acrylic-silicone copolymer particles, the torque level
was approximately the same as when using compound grease both at room
temperature and at -40° C. The adhesion of the coating was good. In addition,
there was no occurrence of galling, and makeup and breakout could be performed
ten times.
Rust preventing properties which are necessary for a tubular threaded joint
were evaluated by forming the same solid lubricating coating as formed in
Examples 1 - 3 oh a box surface on a separately prepared coupon test piece of the
same steel (70 mm x 150 mm x 2 mm thick) and subjecting each test piece to a
humidity cabinet test (temperature of 50° C, relative humidity of 98%, duration of
200 hours). As a result, it was confirmed that there was no occurrence of rust in
any of Examples 1 - 3.
Comparative Example 1
The pin surface and the box surface of the same tubular threaded joint made
of carbon steel as used in Example 1were subjected to the following surfacetreatment.
The pin surface which was finished by machine grinding (surface roughness
of 3 µιη) was immersed for 10 minutes in a zinc phosphating solution at 75 - 85° C
to form a zinc phosphate coating with a thickness of 8 µ η (surface roughness of 8
µ ) . An ultraviolet curing resin coating composition prepared by adding 0.05
parts of aluminum phosphite as an anticorrosive agent and 0.01 parts of
polyethylene wax as a lubricant to one part of the resin content of an acrylic
resin-based ultraviolet curing resin paint made by Chugoku Marine Paint, Ltd was
applied atop the zinc phosphate coating and was irradiated with ultraviolet light
under the following conditions to cure the coating and form an ultraviolet cured
resin coating having a thickness of 25 µ η on the pin surface. The resulting solid
corrosion-protecting coating was colorless and transparent, and the male threads of
the pin could be inspected with the naked eye or with a magnifying glass from atop
the coating.
UV lamp: Air-cooled mercury vapor lamp,
UV lamp output: 4 k ,
Wavelength of UV light: 260 nm.
The box surface which was finished by machine grinding (surface roughness
of 3 µπι) was subjected to electroplating first by Ni strike plating and then by
Cu-Sn-Zn alloy plating to form a plated coating with a total thickness of 8 µ . A
lubricating coating-forming composition having the following composition (not
containing acrylic-silicone copolymer particles) was heated to 1 0° C inside a tank
having a stirring mechanism to obtain a molten state having a viscosity suitable for
coating, and after the box surface which had undergone the above-described
preparatory surface treatment was preheated to 120° C by induction heating, the
molten composition for forming a solid lubricating coating was applied to the
preheated box surface using a spray gun having a spraying head with a temperature
maintaining mechanism. After cooling, a solid lubricating coating with a thickness
of 50 µ was formed.
Composition of the Lubricating Coating-Forming Composition
(Thermoplastic polymer matrix)
22.5% of a polyolefin resin (HM321 of Cemedine Co., Ltd., softening
point of 130° C),22.5% of an ethylene-vinyl acetate copolymer resin (HM221 of
Cemedine Co., Ltd., softening point of 105° C),
45% of a low molecular weight polyolefin (210 P of Mitsui Chemicals,
Inc., melting point of 123° C);
(Solid lubricant)
5% of amorphous graphite (Blue P of Nippon Graphite Industries,
Ltd., average particle diameter of 7 µ ),
(Anticorrosive agent)
5% of Ca ion exchanged silica (Sylysia 52Mo of Fuji Silysia
Chemical, Ltd.).
A repeated makeup and breakout test of a tubular threaded joint was carried
out at room temperature and at approximately -40° C in the same manner as in
Example 1. As shown in Table 1, in Comparative Example 1which did not
contain acrylic-silicone copolymer particles, the torque ratio was high compared to
Examples 1 - 3 even at 20° C, and the torque ratio was extremely high at -40° C.
There were no problems with respect to the adhesion of the coating even at low
temperatures, but galling occurred on the fifth makeup, and the test was terminated.
Comparative Example 2 -
The pin surface and the box surface of the same tubular threaded joint made
of carbon steel as used in Example 1were subjected to the following surface
treatment.
The pin surface which was finished by machine grinding (surface roughness
of 3 µπι ) was immersed for 10 minutes in a zinc phosphating solution at 75 - 85° C
to form a zinc phosphate coating with a thickness of 8 µι (surface roughness of 8
µη ) . An ultraviolet curing resin coating composition prepared by adding 0.05
parts of aluminum phosphite as an anticorrosive agent and 0.01 parts of
polyethylene wax as a lubricant to one part of the resin content of an acrylic
resin-based ultraviolet curing resin paint made by Chugoku Marine Paint, Ltd was
applied atop the zinc phosphate coating and was irradiated with ultraviolet light
under the following conditions to cure the coating and form an ultraviolet cured
resin coating having a thickness of 25 µιη on the pin surface. The resulting solid
corrosion-protecting coating was colorless and transparent, and the male threads ofT/JP2011/076018
34
the pin could be inspected with the naked eye or with a magnifying glass from atop
the coating.
UV lamp: Air-cooled mercury vapor lamp,
UV lamp output: 4 kW,
Wavelength of UV light: 260 nm.
The box surface which was finished by machine grinding (surface roughness
of 3 µη ) was subjected to electroplating first by Ni strike plating and then by
Cu-Sn-Zn alloy plating to form a plated coating with a total thickness of 8 µπι. A
lubricating coating-forming composition having the following composition was
heated to 120° C inside a tank having a stirring mechanism to obtain a molten state
having a viscosity suitable for coating, and after the box surface which had
undergone the above-described preparatory surface treatment was preheated to 120°
C by induction heating, the molten composition for forming a solid lubricating
coating was applied to the preheated box surface using a spray gun having a
spraying head with a temperature maintaining mechanism. After cooling, a solid
lubricating coating with a thickness of 50 µηι was formed.
Composition of the Lubricating Coating-Forming Composition
9% of a polyethylene homopolymer (Licowax™ PE 520 of Clariant
Corporation),
15% of carnauba wax,
15% of zinc stearate,
5% of liquid polyalkyl methacrylate (Viscoplex™ 6-950 of Rohmax
Corporation),
40% of a corrosion inhibitor (NA-SUL™ Ca/Wl 935 of King
Industries Inc.), _
3.5% of fluorinated graphite,
1% of zinc oxide,
5% of titanium dioxide,
5% of bismuth trioxide,
1% of silicone resin particles (KMP-590 of Nissin Chemical Industry
Co., Ltd., average particle diameter of 2 µ η),
Antioxidant (manufactured by Ciba-Geigy Corporation)
0.3% ofIrganox™ L150,0.2% ofIrgafos ,
168.
A repeated makeup and breakout test of a tubular threaded joint was carried
out at room temperature and at approximately -40° C in the same manner as in
Example 1. As shown in Table 1, the torque ratio at -40° C in Comparative
Example 2, in which a conventional hot melt type solid lubricating coating was
formed on the box surface, was approximately 3 times as high as in Examples 1 - 3.
Furthermore, peeling of the coating was observed at -40° C. Galling developed
on the sixth makeup in the test, so the test was terminated.
Table 1
The present invention was explained above with respect to embodiments
which are currently thought to be preferred, but the present invention is not limited
to the above disclosed embodiments. It is possible to make variations within a
scope which is not contrary to the technical concept of the invention construed from
the claims and the overall description, and a threaded joint which incorporates such
changes should be understood as being encompassed by the technical scope of the
present invention.Claims
1. A tubular threaded joint comprising a pin and a box each having contact
surface including threads and an unthreaded metal contact portion, characterized in
that the contact surface of at least one of the pin and the box has a thermoplastic
solid lubricating coating formed as an uppermost surface treatment coating layer,
said thermoplastic solid lubricating coating containing particles of a copolymer of a
resin selected from a silicone resin and a fluorocarbon resin with a different
thermoplastic resin in a thermoplastic polymer matrix.
2. A tubular threaded joint as set forth in claim 1wherein the thermoplastic
solid lubricating coating is formed on the contact surface of one of the pin and the
box, and the contact surface of the other of the pin and the box has a solid
corrosion-protecting coating based on an ultraviolet curing resin as an uppermost
surface treatment coating layer.
3. A tubular threaded joint as set forth in claim 1 or claim 2 wherein the
particles of a copolymer are acrylic-silicone copolymer particles.
4. A tubular threaded joint as set forth in claim 3 wherein the
acrylic-silicone copolymer particles are spherical particles having an average
particle diameter of 10 - 40 um, and their content in the coating is 0.5 - 30 mass %.
5. A tubular threaded joint as set forth in claim 1 or 2 wherein the
thermoplastic polymer matrix comprises one or more resins selected from a
polyolefin resin and an ethylene-vinyl acetate copolymer resin.
6. A tubular threaded joint as set forth in claim 1 or 2 wherein the
thermoplastic solid lubricating coating further contains a solid lubricant.
7. A threaded joint as set forth in claim 6 wherein the solid lubricant is
graphite.8. A tubular threaded joint as set forth in claim 1 or 2 wherein the thickness
of the thermoplastic solid lubricating coating is 10 - 200 µπι .
9. A tubular threaded joint as set forth in claim 2 wherein the thickness of
the solid corrosion-protecting coating is 5 - 50 µ η.
10. A tubular threaded joint as set forth in claim 1 or 2 which is used for
connection of oil country tubular goods.
11. A composition for forming a thermoplastic solid lubricating coating on
a tubular threaded j oint characterized by comprising (1) a thermoplastic polymer
matrix material, and (2) particles of a copolymer of a resin selected from a silicone
resin and a fluorocarbon resin with a different thermoplastic resin.
12. A composition as set forth in claim 11wherein the particles of a
copolymer are acrylic-silicone copolymer particles.
13. A composition as set forth in claim 12 wherein the acrylic-silicone
copolymer particles are spherical particles having an average particle diameter of 10
- 40 µ and their content is 0.5 - 30 mass % of the total solids content of the
composition.
14. A composition as set forth in any of claims 11 - 13 wherein the
thermoplastic polymer matrix material is one or more resins selected from a
polyolefin resin and an ethylene-vinyl acetate copolymer resin.
15. A composition as set forth in any of claims 11 - 14 further containing a
solid lubricant.
16. A composition as set forth in claim 15 wherein the solid lubricant is
graphite.
17. A method of manufacturing a tubular threaded joint having a surfacetreatment coating layer, said tubular threaded joint comprising a pin and a box each
having a contact surface including threads and an unthreaded metal contact portion,
characterized by forming a solid lubricating coating as an uppermost surface
treatment coating layer on the contact surface of at least one of the pin and box by
application of a composition as set forth in any of claims 11 to 16 in which the
thermoplastic polymer matrix material is in molten state followed by cooling.
18. A method as set forth in claim 17 wherein the solid lubricating coating
is formed on the contact surface of one member of the pin and the box, and a solid
corrosion-protecting coating is formed on the contact surface of the other member
of the pin and the box as an uppermost surface treatment coating layer by
application of a composition based on an ultraviolet curing resin followed by
irradiation with ultraviolet light.