Description
Tubular Threaded Joint Having Improved High Torque Performance
Technical Field
The present invention relates to a tubular threaded joint for use in makeup of
steel pipes and particularly oil country tubular goods and to surface treatment
thereof. A tubular threaded joint according to the present invention can exhibit
excellent galling resistance with certainty without application of a lubricating grease
such as compound grease which in the past has been applied to threaded joints when
connecting oil country tubular goods. Accordingly, a tubular threaded joint
according to the present invention can avoid the adverse effects on the global
environment and humans caused by compound grease. Furthermore, it does not
readily undergo yielding and can realize a metal-to-metal seal in a stable manner
even when the makeup operation is carried out with a high torque.
Background Art
Oil country tubular goods such as tubing and casing for use 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,
compound 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 goodshas a pin-box structure constituted by a member having male (external) threads and
referred to as a pin and a member having female (internal) threads and referred 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
referred to as 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 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 of the pin or box. 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, tothe 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 the 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 and
humans.
Compound grease contains a large amount of heavy metal powders such as
zinc, lead, and copper. 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
Patent Document 1 a tubular threaded joint having a viscous liquid or semisolid
lubricating coating formed thereon, and in Patent Document 2, they proposed a
tubular threaded joint which suppresses tackiness of the surface of a threaded joint
and reduces adhesion of foreign materials such as dirt, sand, and trash, which is a
drawback of a viscous liquid or a semisolid lubricating coating.
Patent Document 1: JP 2002-173692 A
Patent Document 2: JP 2004-53013 ASummary of the Invention
In a special threaded joint like the one shown in Figure 1 having a seal
portion and a shoulder portion, gas tightness is guaranteed by forming a
metal-to-metal seal between the seal portions of the pin and the box at the time of
makeup.
Figure 2 shows a torque chart (ordinate: torque, abscissa: number of turns) of
this type of threaded joint at the time of makeup. As shown in the figure, as the
number of turns increases, initially the torque gradually increases due to the threads
of the pin and box contacting each other. Subsequently, the seal portions of the
pin and the box contact and the rate of increase of torque increases. Finally, the
shoulder portions at the tip of the pin and the shoulder portion of the box contact
each other (a state referred to as shouldering), and when they begin to interfere with
each other (the torque at the start of this interference is referred to as the
shouldering torque and is indicated as Ts), the torque abruptly increases. Makeup
is completed when the torque reaches a prescribed makeup torque.
However, with a special threaded joint used in deep oil wells in which
compressive stresses and bending stresses are applied, makeup is sometimes carried
out at a torque which is higher than a usual torque so that loosening will not take
place. In this case, the shoulder portion at the end surface of the pin and the
shoulder portion of the box which contacts it sometimes undergoes yielding (the
torque at the time of yielding is referred to as the yield torque and is indicated as
Ty), and as shown in Figure 2, the shoulder portions sometimes plastically deform.
In a threaded joint which is made up with a high torque, it is advantageous
for the value of Ty - Ts (the torque-on-shoulder resistance, which is indicated as
∆Τ) to be large. However, with the tubular threaded joints described in Patent
Documents 1 and 2 which have a viscous liquid or semisolid lubricating coating, Ty
becomes low compared to when a conventional compound grease is applied. As a
result, it was found that ∆Τ becomes small and there is the problem that makeup
cannot be carried out with a high torque since the shoulder portions end up yielding
at a low 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 maintaining gas tightness, and a proper value therefor isprescribed depending on the internal diameter and the type of a joint.
The object of the present invention is to provide a tubular threaded joint
having a lubricating coating which does not contain harmful heavy metals such as
lead which impose a burden on the global environment, which imparts galling
resistance, gas tightness, and rust preventing properties, and which can provide a
large ∆Τ to the joint, thereby preventing yielding of the shoulder portions of the
joint even when makeup is carried out with a high torque.
It has been found that even if the composition of a lubricating coating is
simply varied so as to modify the coefficient of friction with the object of increasing
∆Τ, Ts and Ty generally behave in the same manner as each other. 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 shoulder portions of a pin and a box do not contact each other at a
prescribed makeup torque, and there are even situations in which makeup cannot be
completed (a phenomenon referred to as no shouldering).
The present inventors found that with a lubricating coating formed only of
substances imposing absolutely no burden or almost no burden on the global
environment, by employing a coating structure in which particular low friction
copolymer particles are dispersed in a highly viscous matrix, Ts can be suppressed
to a low value and Ty can be increased so that ∆Τ can be increased. The operating
mechanism of this lubricating coating is conjectured to be roughly as follows.
Figure 3(a) and 3(b) show the state of the contact surfaces of a pin and a box
at the start of makeup (shouldering) and just before the completion of makeup
(namely, under a low pressure and a high pressure), respectively, when the contact
surface of one of a pin and a box has a lubricating coating with the above-described
structure and the contact surface of the other member remains an uncoated metal
surface.
As shown in Figure 3(a), in the initial stage of shouldering when the pressure
is still low, the metal surface of the opposing member primarily contacts the low
friction copolymer particles protruding from the lubricating coating, so the
coefficient of friction is low, and accordingly Ts is low. On the other hand, as
shown in Figure 3(b), under a high pressure just before the completion of makeup,
the metal surface of the opposing member also contacts the coating made of ahighly viscous matrix, and the coefficient of friction increases. As a result, Ty
becomes high, and ∆Τ becomes large.
Under a high pressure, the copolymer particles which protruded from the
lubricating coating are embedded in the coating primarily due t o their plastic
deformation. When released from the pressure, the copolymer particles recover
the initial state in which they protrude from the lubricating coating, although they
wear to some extent. Therefore, the state shown in Figures 3(a) and 3(b) is also
maintained in the second and later cycle of makeup, and satisfactory galling
resistance is maintained.
The present invention is 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 lubricating coating comprising particles of a copolymer of a resin selected
from a silicone resin and a fluorocarbon resin with a thermoplastic resin dispersed
in a viscous matrix having a complex shear viscosity of at least 3000 Pa-sec at 25°
.
The matrix having a complex shear viscosity of at least 3000 Pa-sec at 25° C
can be said to be a highly viscous matrix. Both a silicone resin and a fluorocarbon
resin are known to have a low friction (hereunder these resins being collectively
referred to as low friction resins), and a copolymer of such resin with a
thermoplastic resin also has a low friction. Therefore, the lubricating coating used
in the present invention is characterized by having particles of a low friction
copolymer dispersed in a highly viscous matrix.
The highly viscous matrix exhibits a high viscous drag in a sliding interface
and therefore serves to increase the friction of the lubricating coating. On the
other hand, the copolymer particles serve to decrease the frictional drag in a sliding
interface.
During the formation of a lubricating coating, the low friction copolymer
particles are protruded from the coating surface such that the silicone or
fluorocarbon resin portion of the copolymer particles face 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, thereby forming the coating structure schematicallyshown in Figure 3(a). As a result, as described above, under a low pressure, 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, leading to a low Ts. On the other hand,
under a high 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, leading to a high Ty. In
this manner, a large ∆Τ can be realized.
Thus, in a threaded joint according to the present invention, the coefficient of
friction between the contact surfaces of a pin and a box at the time of sliding under
a low pressure differs from that at the time of sliding under a high pressure.
Specifically, the coefficient of friction measured under a pressure of 1GPa is
greater than that measured under a pressure of 0.3 GPa. The difference calculated
by subtracting the coefficient of friction measured at 0.3 GPa from that measured at
1GPa is preferably at least 0.02, more preferably at least 0.03, and most preferably
at least 0.05.
The coefficient of friction of a lubricating coating can be measured by the
FALEX Pin & Vee method (referred to hereinafter as the FALEX method) using a
FALEX Pin & Vee Block Machine in accordance with ASTM D 2625 (wear life
and load-carrying capacity of solid film lubricants) or ASTM D2670 (wear
properties of fluid lubricants).
In the present invention, the coefficient of friction of the contact surfaces of a
tubular threaded joint is measured using a test piece made of the same steel as the
tubular threaded joint and having the same solid lubricating coating formed after the
same preparatory surface treatment. The measurement is carried out under a high
pressure of 1GPa which corresponds to the maximum pressure applied to the seal
portions at the time of makeup of a tubular threaded joint and under a low pressure
of 0.3 GPa. In this manner, an average coefficient of friction in a steady friction
state prior to the occurrence of galling is determined under each of these two
pressures for comparison to each other. Of course, another conventional
laboratory friction tester may be used to measure the friction of coefficient.
Whichever method or apparatus is employed, the coefficient of friction of alubricating coating used in the present invention measured at a high pressure ( 1
GPa) is higher than that measured at a low pressure (0.3 GPa) when measurement is
carried out under the same conditions other than the applied pressure. 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 copolymer particles are preferably spherical particles, and more
preferably they are spherical particles of an acrylic-silicone copolymer with an
average particle diameter of 10 - 50 µη , and their content in the lubricating coating
is preferably 0.1 - 20 mass %.
The highly viscous matrix is preferably formed from at least one substance
selected from rosin-based materials including rosin and its derivatives, waxes, metal
soaps, and basic metal salts of an aromatic organic acid.
The lubricating coating may further contain graphite as a friction modifier.
The thickness of the lubricating coating is preferably 10 - 500 µιη.
The contact surface of at least one of the pin and the box having the
lubricating coating preferably undergoes, before the formation of the lubricating
coating, preparatory surface treatment by a method selected from at least one of
blasting, pickling, phosphate chemical conversion treatment, oxalate chemical
conversion treatment, borate chemical conversion treatment, electroplating, impact
plating, and a combination of these.
When the contact surface of only one member of the pin and the box has the
lubricating coating, the contact surface of the other member of the pin and the box
preferably undergoes surface treatment by a method selected from at least one of
blasting, pickling, phosphate chemical conversion treatment, oxalate chemical
conversion treatment, borate chemical conversion treatment, electroplating, impact
plating, and a combination of these.
A tubular threaded joint according to the present invention is preferably used
for connecting oil country tubular goods.
The present invention also relates to a method of connecting a plurality of oil
country tubular goods without applying a lubricating grease using the
above-described tubular threaded joint.
A tubular threaded joint according to the present invention impose little
burden on the global environment because the lubricating coating formed on thecontact surface does not contain harmful heavy metals such as lead unlike
compound grease. Nevertheless, the lubricating coating exhibits a large ∆Τ like
conventional compound grease, thereby making it possible to perform makeup
without the occurrence of yielding or galling of the shoulder portions even when
makeup is carried out with a high torque. Furthermore, galling can be suppressed
even under severe conditions such as encountered during unstable drilling in the sea.
Moreover, a tubular threaded joint according to the present invention suppresses
the formation of rust, its lubricating performance lasts even when makeup and
breakout are repeated, and it can guarantee gas tightness after makeup.
Brief Description of the Drawings
Figure 1 schematically shows the shoulder portions and the seal portions of a
pin and a box of a special threaded joint.
Figure 2 shows a typical torque chart at the time of makeup of a special
threaded joint.
Figures 3(a) and 3(b) schematically show the mechanism of operation of a
lubricating coating according to the present invention.
Figure 4 schematically shows the assembled structure of a steel pipe and a
coupling at the time of shipment of the steel pipe.
Figure 5 schematically shows the connecting portions of a threaded joint.
Figure 6(a) and 6(b) are explanatory views showing a contact surface of a
tubular threaded joint according to the present invention, Figure 6(a) showing an
example of surface roughening of a contact surface itself, and Figure 6(b) showing
an example of forming a preparatory surface treatment coating for surface
roughening of a contact surface.
Modes for Carrying Out the Invention
Below, a tubular threaded joint according to the present invention will be
explained in detail by way of example.
Figure 4 schematically shows the state of a steel pipe for an oil country
tubular good and a coupling at the time of shipment. A pin 1 having male threads
3a on its outer surface is formed on both ends of a steel pipe A, and a box 2 having
female threads on its inner surface is formed on both sides of a coupling B. A pinmeans 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. Although not shown in the drawing,
protectors for protecting the threads are mounted on the unconnected pin of the steel
pipe A and the unconnected box of the coupling B prior to shipment. These
protectors are removed prior to use of the threaded joint.
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 component. There are integral tubular
threaded joints which do not use a coupling and which have a pin on one end of a
steel pipe and a box on the other end thereof. A tubular threaded joint according to
the present invention can be either of these types of threaded joint.
Figure 5 schematically shows the structure of a typical tubular threaded joint
(referred to below simply as a threaded joint). The 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, a seal portion
4a positioned at the end of the steel pipe, and a shoulder portion 5 on the end
surface of the pipe. Correspondingly, the box 2 has female threads 3b, and a seal
portion 4b and a shoulder portion 5 to the rear of the threads. The seal portion and
the shoulder portion of each of the pin and the box constitute an unthreaded metal
contact portion.
The threads 3a and 3b, the seal portions 4a and 4b, and the shoulder portions
5 (namely, the threads and the unthreaded metal contact portions) of the pin 1 and
the box 2 are the contact surfaces of the threaded joint. These contact surfaces are
required to have galling resistance, gas tightness, and corrosion resistance. In the
past, for this purpose, a compound grease containing heavy metal powders was
applied, or a viscous liquid or semisolid lubricating coating was formed on the
contact surfaces. However, as stated above, the former had the problem that it had
an adverse effect on humans and the environment, and the latter had the problem
that when a high torque was applied during makeup, there was the possibility of the
shoulder portions yielding prior to the completion of makeup due to a small ∆Τ.
According to the present invention, as shown in Figures 6(a) and 6(b) with
respect to seal portions, the contact surface of at least one of the pin and the box iscoated with a lubricating coating 31a atop steel surface 30a or 30b. This
lubricating coating 31a exhibits the same excellent lubricating properties and effect
of maintaining gas tightness at the time of makeup of a threaded joint as a
conventional compound grease. Therefore, a threaded joint according to the
invention can prevent galling of a threaded joint and maintain gas tightness after
makeup without yielding of the shoulder portions even when makeup and breakout
are repeated with a high torque without using a compound grease.
The substrate for the lubricating coating 31a (namely, the contact surface of
the threaded joint) is preferably given a rough surface. As shown in Figure 6(a),
surface roughening may be achieved by direct surface roughening of the steel
surface 30a by blasting or pickling. Alternatively, as shown in Figure 6(b), it can
be achieved by forming a preparatory surface treatment coating 32 having a rough
surface on the steel surface 30b prior to forming the lubricating coating 31a.
The lubricating coating 31a can be formed by preparing a lubricating
coating-forming composition which if necessary is diluted with a suitable organic
solvent and applying it by a suitable method such as brush application, spraying, or
immersion, if necessary followed by drying to evaporate the solvent.
A 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 3, 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 easier to
perform preparatory surface treatment and application for forming a lubricating
coating on a short coupling than on a long steel pipe, so it is convenient to form a
lubricating coating on the contact surface of a coupling (usually on the contact
surface of a box). For a pin and a box which are not connected prior to shipment,
it is preferable to form a lubricating coating on the contact surfaces of both of a pin
and a box in order to simultaneously impart lubricating properties and rust
preventing properties to the surfaces. As a result, the lubricating properties and
gas tightness of the threaded joint can be prevented from worsening due to rust
formation.
The lubricating coating should cover the entirety of the contact surface of the
pin and/or the box, but the present invention also includes the case in which only a
portion of the contact surface (such as only the unthreaded metal contact portion) iscoated.
[Lubricating Coating]
In order to prevent galling at the time of makeup of steel pipes by a threaded
joint and to prevent rusting during storage, a lubricating coating is formed on the
contact surface of at least one of the pin and the box of a threaded joint. In the
present invention, in order to form a lubricating coating which has a large ∆Τ
sufficient to prevent yielding of the shoulder portions even when makeup is carried
out with a high torque and which can prevent the occurrence of galling and the
occurrence of rusting during storage, a lubricating coating is formed so as to have a
structure in which particular low friction copolymer particles are dispersed in a
highly viscous matrix.
A coating containing particles of a silicone resin or a fluorocarbon resin
(namely, a low friction resin) exhibits a low friction. However, with such a
lubricating coating which contains these particles made solely of a low friction resin,
since the bonding strength of the particles to the coating is not sufficient, the
particles tend to easily drop off when subjected to a frictional force. As a result,
even though the lubricating properties are initially good, the wear resistance and
durability of the coating decrease due to particles dropping off, and good lubricating
properties cannot be maintained.
Therefore, in the present invention, particles of a copolymer obtained by
copolymerizing the low friction resin with another resin (such as an acrylic resin or
a urethane resin) having a high affinity for the highly viscous matrix of the
lubricating coating are used. Use of such copolymer particles increases the affinity
of the particles for the highly viscous matrix and for the metal to be coated (contact
surface of a threaded joint) and hence the adhesion of the lubricating coating, and
good lubricating properties can be maintained.
Such low friction copolymer particles can be prepared by a copolymerization
reaction between a monomer of another resin and a low friction resin having into
which a functional group which is reactive with this monomer has been introduced.
The functional group which can be introduced into a low friction resin (such as a
silicone resin or a fluorocarbon resin) can be a (meth)acrylic group in the case of
copolymerization with an acrylic resin, a hydroxyl group in copolymerization with aurethane 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.
It is particularly advantageous to use acrylic-silicone copolymer particles as
copolymer particles in the present invention. These are particles of a copolymer of
a silicone resin with an acrylic monomer and can be prepared by copolymerizing a
polyorganosiloxane having a terminal free radically polymerizable 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 µηι .
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., the particles are
preferably spherical particles.
In the present invention, spherical acrylic-silicone copolymer particles
having an average particle diameter of 10 - 50 um are particularly preferred.
Spherical acrylic-silicone copolymer particles having an average particle diameter
of 30 µ are sold by Nissin Chemical Industry Co., Ltd. under the product name
Chaline R-170S. This product can be used as low friction copolymer particles in
the present invention.
The content of low friction copolymer particles and preferably
acrylic-silicone copolymer particles in a lubricating coating is preferably 0.2 - 20
mass %. If the content is less than 0.2 mass %, the above-described effects of the
present invention cannot be adequately obtained. On the other hand, if the content
exceeds 20 mass %, the uniformity of dispersion of the particles in the highlyviscous matrix and the adhesion and film-forming properties of the coating decrease,
and it may become difficult to form a quality coating. A more preferred content of
the copolymer particles is 1 - 15 mass %.
The lubricating coating has a highly viscous matrix having a complex shear
viscosity of at least 3000 Pa-sec at 25° C. Complex shear viscosity is measured in
accordance with JIS K 7244-10: 2005 (Plastics - Determination of dynamic
mechanical properties - Part 10 - Complex shear viscosity using a parallel plate
oscillatory rheometer). For example, viscosity can be measured using an ARES
viscoelastic rheometer manufactured by TA Instruments, with the measurement
mode being a twisting mode (parallel plate 25 mm) and the measurement frequency
being 1Hz.
Examples of substances capable of forming a matrix exhibiting a high
viscosity with a complex shear viscosity at 25° C of at least 3000 Pa-sec are one or
more substances selected from rosin-based substances including rosin and its
derivatives, waxes, metal soaps, and basic metal salts of an aromatic organic acid.
Of these substances, rosin-based substances are effective at increasing the
coefficient of friction of a lubricant layer, namely, at increasing ∆Τ, while waxes,
metal soaps, and basic metal salts of an aromatic organic acid are primarily
effective at preventing galling of a lubricating coating. Accordingly, a highly
viscous matrix is preferably a mixture of a rosin-based substance with one or more
of a wax, a metal soap, and a basic metal salt of an aromatic organic acid. This
highly viscous matrix is more preferably a mixture containing at least one of each of
a rosin-based substance, a wax, a metal soap, and a basic metal salt of an aromatic
organic acid. These components will be explained below.
A rosin-based substance selected from rosin and its derivatives in the
lubricating coating is effective at increasing the ∆Τ of the coating by increasing its
viscosity when it is subjected to a high pressure on a frictional surface. Rosin is a
natural resin secreted from trees of the pine family and is constituted by the three
elements carbon, hydrogen, and oxygen. Its main components are a resin acid
(rosin acid) having the formula C2oH3oO2 and colophonic acids having the formula
CnH + 0O4. Typical resin acids are abietic acid and d- or 1-pimaric acid.
Depending upon the method of collection, rosin can be classified as tall rosin,
gum rosin, and wood rosin. Any of these types may be used. Various rosinderivatives such as rosin esters, hydrogenated rosins, polymerized rosins, and
disproportionated rosins are commercially available, and these rosin derivatives
may be used as rosin-based substances.
The content of a rosin-based substance in the lubricating coating is
preferably at most 30 mass %. If the content exceeds 30 mass %, the composition
which is used for coating formation becomes highly viscous and the ease of forming
a coating may be worsened. In order to adequately obtain the above-described
effects of a rosin-based substance, the content of rosin in the lubricating coating is
preferably at least 5 mass %. A more preferred content is 5 - 20 mass %.
The lubricating coating in a tubular threaded joint according to the present
invention preferably does not contain heavy metals. The reason why compound
grease which has been used for lubrication of tubular threaded joints contains large
amounts of powder of soft heavy metals such as lead and zinc is for the purpose of
preventing galling by inhibiting direct contact between metals. In the present
invention, this function are provided by cooperation of a rosin-based substance and
other constituents of the highly viscous matrix together with the copolymer particles
contained in the lubricating coating. Therefore, the coating can exhibit a sufficient
lubricating performance without containing any heavy metals.
Wax not only has the effect of preventing galling by decreasing the friction
of a lubricating coating but it also decreases the fluidity of the lubricating 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 ceresin (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 in a lubricating coating is preferably at most 20 mass %.
If the content exceeds 20 mass %, the adhesion and strength of a lubricating coating
may decrease. The content is preferably at most 15 mass %. There is no
particular lower limit on the wax content, but it is preferably at least 2% in order to
obtain the above-described effects of wax with certainty.
A metal soap which is a salt of a fatty acid with a metal other than an alkalimetal can be contained in a lubricating coating in order to increase the antigalling
effect and rust preventing effect of the lubricating coating. The upper limit on its
content in a coating is 20 mass %. If its content exceeds 20 mass %, the adhesion
and strength of a lubricating coating may decrease. Preferably the content is at
most 15 mass %. There is no particular lower limit on the content of a metal soap,
but in order to be able to obtain the above-described effects with certainty, the
content of a metal soap is preferably at least 2 mass %.
The fatty acid portion 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 or fats such as beef tallow, lard, wool fat, palm oil, rapeseed oil, and coconut oil,
and a single carboxylic acid compound 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, or salicylic 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.
Like a metal soap, a basic metal salt of an aromatic organic acid may be
contained in a lubricating coating in order to increase the antigalling effect and the
rust preventing effect of a lubricating coating. Examples of basic metal salts of
aromatic organic acids are basic metal sulfonates, basic metal salicylates, basic
metal phenates, and basic metal carboxylates. These basic metal salts of aromatic
organic acids are each a salt constituted by an aromatic organic acid and a
stoichiometrically excessive amount of an alkali (an alkali metal or an alkaline earth
metal). At room temperature, they are a grease or a semisolid substance having
the excess alkali dispersed as a colloidal microparticles in oil. These basic metal
salts of aromatic organic acids exhibit a marked heavy-duty corrosion preventing
effect, and at the same time they exhibit a lubricating activity by physical
adsorption of the excess metal salt in the form of colloidal microparticles and by
chemical adsorption of organic acid groups.
The alkali which constitutes the cationic portion of the 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, ormagnesium. The same effect can be obtained whichever of these is employed.
The higher the basicity of the basic metal salt of an aromatic organic acid,
the greater the amount of the microparticles of the metal 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 basicity 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, the basic metal
salt of an aromatic organic acid preferably has a base number (JIS K 2501) (when
using two or more salts, the weighted average of the base numbers of the salts
taking their weights into consideration) is preferably at least 50 mg KOH/g.
However, if it has a base number exceeding 500 mg KOH/g, it increases the
hydrophilicity of a coating and decreases the rust preventing properties thereof. A
preferred base number is 100 - 500 mg KOH/g, and more preferably it is in the
range of 250 - 450 mg KOH/g.
As stated above, a basic metal salt of an aromatic organic acid is in the form
of a grease or a semisolid substance, and it can function as the matrix of a
lubricating coating. Therefore, its content in the lubricating coating may be a large
amount up to 65 mass %. A preferred content is 10 - 60 mass %, and a more
preferred content is 40 - 55 mass %.
In the present invention, in order to suppress the fluidity of the lubricating
coating at high temperatures and further improve galling resistance, graphite can be
contained in the lubricating coating as a friction modifier. It is preferable that the
graphite used be amorphous (earthy) graphite which does not produce a significant
decrease in the coefficient of friction of a coating.
The content of graphite in the lubricating coating is preferably 0.5 - 20
mass %. If it is less than 0.5 mass %, the effect of preventing galling is sometimes
inadequate. On the other hand, if it exceeds 20 mass %, the graphite may interfere
with the dispersibility of the copolymer particles and with the effect on friction at a
high makeup pressure (which produces an increased Ty).
In order to increase the uniformity of particulate components in the
lubricating coating or to improve the properties or appearance 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 variouslubricants and additives normally used in lubricating oils (such as an extreme
pressure agent). Lubricants refer to lubricating components which are liquid at
room temperature and which can be used in lubricating oils (including viscous
liquids such as grease). Examples of lubricants which can be used include
synthetic esters, natural oils and fats, and mineral oils.
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.
Examples of thermoplastic resins are polyethylene resins, polypropylene
resins, polystyrene resins, poly(methyl acrylate) resins, styrene-acrylate copolymer
resins, polyamide resins, and polybutene (polybutylene) resins. A copolymer or
blend of these resins or these resins and other thermoplastic resins can also be used.
It is preferable to use a thermoplastic resin having a density (JIS K 7 11 ) in the
range of 0.9 - 1.2. In addition, in view of the necessity for the resin to readily
deform at a frictional surface in order to exhibit lubricating properties, the thermal
deformation temperature (JIS K 7206) of the resin is preferably 50 - 150° C. A
preferable resin is a polybutene resin since it has a high viscosity under a high
pressure.
By incorporating a thermoplastic resin in the form of particles in a
lubricating coating, the resin exhibits a lubricating action similar to a solid lubricant
when it is introduced into a frictional interface, and it is particularly effective at
increasing galling resistance. Therefore, a thermoplastic resin is preferably present
in the lubricating coating in the form of a powder and particularly a powder having
spherical particles. In this case, if the composition used for forming the lubricating
coating contains an organic solvent, a thermoplastic resin which does not dissolve in
the solvent is selected. The powder of the thermoplastic resin can be dispersed or
suspended in the solvent. It does not matter if the resin swells in the solvent.
The powder of the thermoplastic resin preferably has a fine particle diameter
from the standpoints of increasing the coating thickness and increasing galling
resistance. However, if the particle diameter is smaller than 0.05 µη , gelling of
the lubricating coating-forming composition becomes marked, and it becomesdifficult to form a coating having a uniform thickness. If the particle diameter
exceeds 30 µηι, it becomes difficult to introduce the resin particles into the
frictional interface and the particles easily separate by sedimentation or floating in
the lubricating coating-forming composition, and it becomes difficult to form a
uniform coating. Accordingly, the particle diameter of the resin particles is
preferably in the range of 0.05 - 30 µπ and more preferably in the range of 0.07 -
20 µηι.
When the lubricating coating contains a thermoplastic resin, the content
thereof in the coating is preferably at most 10 mass % and more preferably in the
range of 0.1 - 5 mass %. The total amount of the above-described rosin-based
substance and the thermoplastic resin is preferably at most 30 mass %.
Examples of natural oils and fats which can be used as a lubricant 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 a lubricant.
A synthetic ester which can be used as a lubricant 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 trimethylopropane or
pentaerythritol.
Examples of fatty acid monoesters are monoesters of a carboxylic acid
having 12 - 24 carbon atoms with a higher alcohol having 8 - 20 carbon atoms.
Examples of dibasic acid diesters are diesters of a dibasic acid having 6 - 10 carbon
atoms with a higher alcohol having 8 - 20 carbon atoms. Examples of the fatty
acids in fatty acid esters of trimethylolpropane or pentaerythritol are those having 8
- 18 carbon atoms.
When a lubricating coating contains at least one of the above lubricants, the
content thereof is preferably at least 0.1 mass % in order to obtain an increase in
galling resistance. The content is preferably 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 resistanceof the lubricating coating when added in a small amount. Nonlimiting examples of
an extreme pressure agent are sulfurized oils, polysulfldes, 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 sulfurized oils are compounds having a sulfur content
of 5 - 30 mass % which is obtained by adding sulfur to an unsaturated animal or
vegetable oil such as olive oil, castor oil, rice bran oil, cottonseed oil, rapeseed oil,
soy bean oil, corn oil, beef tallow, or lard and heating the mixture.
Examples of preferred polysulfldes are polysulfldes of the formula:
R]-(S)c-R2 (wherein R and R2 may be the same or different and indicate an alkyl
group, an aryl group, an alkylaryl group, or an arylalkyl group each having 4 - 22
carbon atoms, and c is an integer from 2 to 5) and olefin sulfides containing 2 - 5
bonded sulfur atoms 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)(R6O)P(=S)-S]2-M
In the above formulas, R and R indicate an alkyl group, a cycloalkyl group,
an alkylcycloalkyl group, an aryl group, an alkylaryl group, or an arylalkyl group
each having up to 24 carbon atoms, R 4 and R5 indicate a hydrogen atom or an alkyl
group, a cycloalkyl group, an alkylcycloalkyl group, an aryl group, an alkylaryl
group, or an arylalkyl group each having up to 24 carbon atoms, and M indicates
molybdenum (Mo), zinc (Zn), or barium (Ba).
Particularly preferred examples include tricresyl phosphate and dioctyl
phosphate for phosphates; tristearyl phosphite, tridecyl phosphite, and dilauryl
hydrogen phosphite for phosphites; trialkyl thiophosphate in which each of R3, R4,
and R 5 is an alkyl group having 12 or 13 carbon atoms and alkyltriphenyl
thiophosphate for thiophosphates; and zinc dialkyl dithiophosphate in which each of
R3 and R is a primary or secondary alkyl group having 3 - 20 carbon atoms fordithiophosphoric acid metal salts.
A lubricating coating is formed by preparing a mixture of the constituents of
the coating in liquid form by the addition of a solvent and/or by heating and then
applying the resulting liquid to the contact surface of at least one of the pin and the
box of a threaded joint, and if necessary drying the coating.
Application after heating can be carried out by the so-called hot melt coating
method. A mixture of the constituents of a lubricating coating is heated to obtain a
viscosity suitable for application and is sprayed at the surface to be coated using a
spray gun having a temperature-maintaining mechanism. The surface being coated
is preferably preheated to roughly the same temperature of the coating composition.
When application is carried out at room temperature, a lubricating
coating-forming composition is prepared by adding a volatile solvent to a mixture
of the constituents of a lubricating coating. The volatile solvent evaporates during
the course of forming a coating, and substantially none of the solvent remains in the
lubricating coating. "Volatile" means that the solvent shows a tendency to
vaporize when present in the form of a coating at a temperature from room
temperature to 150° C. However, since a lubricating coating according to the
present invention may be a viscous liquid or a semisolid, it is permissible for a
slight amount of a solvent to remain in the coating.
There is no particular restriction on the type of solvent. Examples of
volatile solvents which are suitable for use in the present invention are
petroleum-derived solvents such as Solvent and mineral spirits which belong to
industrial gasoline prescribed by JIS K 2201, 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 150° C or higher,
and a final boiling point of 210° C or lower is preferred because it is relatively easy
to handle and it rapidly evaporates thereby leading to a short drying time.
In addition to the above-described components, the lubricating
coating-forming composition may contain additives such as an antioxidant, a
preservative, and a colorant.
The viscosity of the lubricating coating-forming composition (kinematic
viscosity in cSt measured by a Brookfield viscometer) may be appropriately
selected in accordance with the coating method. The viscosity is preferably atmost 4000 cSt at 40° C in the case of spray coating or immersion at room
temperature or at most 1000 cSt at 60° C in the case of brush coating.
The lubricating coating-forming composition can be prepared by first heating
some components such as wax of the highly viscous matrix in which the copolymer
particles are dispersed to a temperature of at least their melting point to melt them,
and then mixing the melt with the other components. Alternatively, the
composition can be prepared by dissolving or dispersing all of the components in an
organic solvent without melting wax.
The thickness of the lubricating coating is preferably in the range of 10 - 500
µ η for the following reasons. The lubricating coating is preferably sufficiently
thick to cover minute gaps in the contact surface area such as spaces between
threads. If the coating thickness is too small, the effects of supplying components
such as a rosin-based substance, a wax, a metal soap, a basic metal salt of an
aromatic organic acid, and graphite to the frictional surface from other gaps due to
hydrostatic pressure generated at the time of makeup can no longer be expected.
For this reason, the thickness of the lubricating coating is preferably at least 10 µι .
At the time of carrying out makeup which requires lubrication, the contact
surfaces of the box and the pin contact each other. Therefore, from the standpoint
of lubrication, it is sufficient to form a lubricating coating on the contact surface of
just one of the pin and the box. However, from the standpoint of preventing
rusting of the pin and the box when they are exposed to the atmosphere during
storage, it is preferable to form a lubricating coating on the contact surfaces of both
the pin and the box. The minimum coating thickness necessary for rust prevention
is also 10 µ . Accordingly, when a separate protecting means for preventing
rusting (such as previously connecting a pin and a box to each other or mounting a
protector) is not employed, a coating having a thickness of at least 10 µ is
preferably formed on the contact surfaces of both the pin and the box.
On the other hand, if the lubricating coating is too thick, not only is lubricant
wasted, but the prevention of environmental pollution, which is one of the objects of
the present invention, is impeded. From this standpoint, the thickness of the
lubricating coating preferably has an upper limit of around 500 µ . A preferred
thickness of the lubricating coating is 15 - 200 µη . However, as explained below,
when the surface roughness of the underlying contact surface (substrate) isincreased, the thickness of the lubricating coating is preferably larger than Rmax of
the substrate. When the substrate has a rough surface, the thickness of the
lubricating coating is the average value of the coating thickness of the entire coating,
which can be calculated from the surface area, the mass, and the density of the
coating.
As a general tendency, when the lubricating coating contains more than a
certain amount of a lubricant, it becomes a viscous liquid, while if the amount of a
lubricant is small or there is no lubricant, it becomes a semisolid.
[Preparatory Surface Treatment]
If a tubular threaded joint having a lubricating coating formed on the contact
surfaces of the pin and/or the box according to the present invention undergoes
preparatory surface treatment for surface roughening of the contact surfaces to
which the coating is applied such that the surface roughness is larger than the
surface roughness obtained by machining (3 - 5 µ ), the galling resistance often
increases. Accordingly, prior to forming a lubricating coating, preparatory surface
treatment for surface roughening is preferably performed on the contact surface to
be coated with the lubricating coating.
Examples of such preparatory surface treatment are blasting by projecting
blasting materials such as spherical shot or angular grit, pickling by immersion in a
strongly acidic solution such as sulfuric acid, hydrochloric acid, nitric acid, or
hydrofluoric acid to roughen the skin, chemical conversion treatment such as
phosphate treatment, oxalate treatment, or borate treatment (the roughness of the
crystal surface increases as the crystals which are formed develop), electroplating
with a metal such as Cu, Fe, Sn, or Zn or an alloy thereof (projections are
selectively plated, so the surface becomes slightly rougher), and impact plating
which can form a porous plated coating. Composite plating which forms an
electroplated coating having minute solid particles dispersed in metal may also be
employed as a method of imparting a roughened surface because the minute solid
particles protrude from the plated coating.
Whichever preparatory surface treatment method is used for the contact
surfaces, the surface roughness Rmax resulting from surface roughening by
preparatory surface treatment is preferably 5 - 40 µ . If Rmax is less than 5 µ ,the adhesion and retention of the lubricating coating may become inadequate. On
the other hand, if Rmax exceeds 40 µ η, friction increases, and the coating may be
unable to withstand shearing forces and compressive forces when a high pressure is
applied, thereby causing the coating to be easily destroyed or peeled off. Two or
more types of preparatory surface treatment for surface roughening may be
employed. The treatment can be carried out in a conventional manner.
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 µη , the coating thickness is preferably made
at least 5 µιη. There is no particular upper limit on the coating thickness, but
normally at most 50 µηι and preferably at most 40 µηι is sufficient. The formation
of a lubricating coating atop a porous coating which has been formed by preparatory
surface treatment can increase the adhesion of the lubricating coating by the
so-called "anchor effect". As a result, it becomes difficult for peeling of the solid
lubricating coating to take place under repeated makeup and breakout with contact
between metals being 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 phosphating treatment (treatment with manganese phosphate,
zinc phosphate, iron manganese phosphate, or zinc calcium phosphate) and impact
plating to form a zinc or zinc-iron alloy coating as a porous metal 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.
Phosphating treatment can be carried out by immersion or spraying in a
conventional manner. An acidic phosphating solution which is normally used for
zinc plated steel materials can be used. For example, a zinc phosphating solution
comprising 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 the6
2 5
duration of treatment may be up to 15 minutes depending upon the desired coating
thickness. In order to promote the formation of a coating, prior to phosphating
treatment, an aqueous surface conditioning solution containing colloidal titanium
can be supplied to the surface to be treated. After phosphating 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
apparatus. In the present invention, since it is sufficient to plate just the contact
surface, 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 is 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. 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 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.
From the standpoints of both corrosion resistance and adhesion, the thickness
of the zinc or zinc alloy layer formed by impact plating is preferably 5 - 40 um. If
it is less than 5 µπι, adequate corrosion resistance cannot be obtained, while if it
exceeds 40 µπ , the adhesion of the lubricating coating formed thereon may end up
decreasing. Similarly, the thickness of a phosphate coating is preferably in the
range of 5 - 40 µ η.
Two or more of the above-described types of preparatory surface treatment
may be combined.
As another type of preparatory surface treatment, although it produces almost
no surface roughening, certain types of electroplating in one or more 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 Cu, Sn, Ni, or alloys of these metals. Plating may besingle-layer plating or multiple-layer plating with two or more layers. Specific
examples of these types of electroplating are single plating with Cu, Sn, Ni, the
Cu-Sn alloy disclosed in JP 2003-74763 A, and Cu-Sn-Zn alloy, two-layer plating
by Cu plating and Sn plating, and three-layer plating by Ni plating, Cu plating, and
Sn plating. In the case of a tubular threaded joint made from a steel having a Cr
content exceeding 5% which is highly susceptible to galling, it is preferable to carry
out 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 having two or more layers with
metals selected from these alloys, Cu, Sn, and Ni (e.g., 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 three-layer plating by Ni
plating, Cu plating, and Sn plating).
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 with a
thickness of less than 1 m which is referred to as strike plating. The plating
thickness (the overall thickness in the case of multiple-layer plating) is preferably in
the range of 5 - 15 µηι .
It is also possible to form a solid corrosion protecting coating as another type
of preparatory surface treatment.
[Upper Dry Coating]
The above-described lubricating coating sometimes has some tackiness on its
surface. In this case, particularly when an oil country tubular good is placed
vertically, rust remaining on the inner surface or blasting particles which were
blasted in order to remove rust fall off, and they may adhere to the lubricating
coating and become embedded in the lubricating coating. Foreign matter that
becomes embedded in a coating cannot be completely removed by blowing with air,
and it worsens the lubricating properties of the coating. In order to eliminate this
problem, a thin dry solid coating may be formed atop the lubricating coating. This
dry solid coating can be a usual resin coating (such as 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 coatingmay also contain a small amount of wax.
[Surface Treatment of the Opposing Member]
When a lubricating coating according to the present invention is formed on
the contact surface of only one of the pin and the box (e.g., the box) of a tubular
threaded joint, the contact surface of the other member (e.g., the pin) which is not
coated with a lubricating coating may be left untreated, but preferably it is
roughened by the above-described preparatory surface treatment for surface
roughening. Thus, the surface roughening can be carried out by blasting
treatment; pickling; chemical conversion treatment such as with a phosphate,
oxalate, or borate; electroplating, composite plating to form a plated coating
containing minute solid particles; and a combination of two or more of these
methods. By doing so, when the other member is connected to a member having a
lubricating coating according to the present invention, the contact surface of the
other member which does not have a lubricating coating exhibits good retention of
the lubricating coating due to the; anchor effect provided by surface roughening, and
the galling resistance of the tubular threaded joint is increased.
If desired, in order to impart rust preventing properties, a known corrosion
protecting coating such as a coating of an ultraviolet curing resin or a thermosetting
resin can be formed atop the surface prepared by the preparatory surface treatment.
The corrosion protecting coating prevents the contact surface from contacting the
atmosphere. Therefore, even if water is formed due to condensation during
storage of a threaded joint and it contacts the joint, rust is prevented from
developing on the contact surface of the joint.
There are no particular limitations on surface treatment of the contact surface
of the opposing member of a threaded joint, and it is possible to carry out surface
treatment other than that described above. For example, a lubricating coating
other than one according to the present invention may be formed on the contact
surface of the opposing member.
Examples
The effects of the present invention will be illustrated by the following
examples and comparative examples. In the following explanation, the contactsurface of a pin including the male threads and the unthreaded metal contact portion
will be referred to as the pin surface, and the contact surface of a box including the
female threads and the unthreaded metal contact portion will be referred to as the
box surface. The surface roughness of a lubricating coating is expressed as Rmax.
The pin surface and the box surface (each having threads, a seal portion, and
a shoulder portion) of a special threaded joint (outer diameter: 17.78 cm (7 inches),
wall thickness: 1.036 cm (0.408 inches) made from the carbon steel A, the Cr-Mo
steel B, or the 13% Cr steel C shown in Table 1 (galling took place more readily in
the order from steel A to steel C) underwent the preparatory surface treatment
shown in Table 2. The surface roughness R of the surface prepared by the
preparatory surface treatment shown in Table 2 was the average value of ten-point
heights of irregularities.
A lubricating coating having the composition shown in Table 3 was formed
by spray coating of a lubricating coating- forming composition on the pin surface
and the box surface which had undergone preparatory surface treatment. For the
lubricating coatings of Example 4 and Comparative Example 3 which were solid at
room temperature, in accordance with the hot melt method, a lubricating
coating-forming composition not containing a solvent (having the same
composition as the lubricating coating) was heated to 130° C at the time of use, the
pin surface or the box surface to be coated was preheated to 130° C by induction
heating, and the lubricating coating-forming composition was applied using a spray
gun having a temperature maintaining mechanism. In this case, a lubricating
coating was formed by cooling. In the other examples, a lubricating
coating-forming composition diluted with a volatile organic solvent (mineral spirits)
was prepared and was applied by spray coating at room temperature. After
application, the volatile solvent was evaporated by air drying to form a lubricating
coating.
The content of the components in each lubricating coating shown in Table 3
is the mass % with respect to the total amount of nonvolatile components (exclusive
of a volatile solvent) in a lubricating coating-forming composition. A lubricating
coating having the same composition was formed on the pin surface and the box
surface.
Of the components shown in Table 3, the spherical copolymer particles wereacrylic-silicone copolymer particles (Chaline R-170S of Nissin Chemical Industry
Co., Ltd., average particle diameter of 30 µη ) which had a low friction. Of the
components used to form a highly viscous matrix, the rosin-based substance was a
hydrogenated rosin ester (Ester Gum H manufactured by Arakawa Chemical
Industries, Ltd.) which was a rosin ester, the wax was paraffin wax manufactured by
Nippon Seiro Co., Ltd., the metal soap was calcium stearate manufactured by DIC
Corporation, and the basic metal salt of an aromatic organic acid was Calcinate
C-400CLR manufactured by Crompton Corporation (a highly basic calcium
sulfonate having a base number of 400 mg KOH/g). The friction modifier was
amorphous graphite (Blue P manufactured by Nippon Graphite Industries, Ltd.).
The volatile organic solvent used for dilution in the case of spray coating at room
temperature was Cleansol which was mineral spirits manufactured by Nippon Oil
Corporation.
A test in which makeup and breakout were repeated for up to 10 times was
carried out on a tubular threaded joint having a lubricating coating formed in the
above-described manner on the pin surface and the box surface to evaluate galling
resistance.
In the repeated makeup and breakout test, makeup of a threaded joint was
carried out at a makeup speed of 10 rpm and a makeup torque of 20 kN-m, and after
breakout, the state of seizing of the pin surface and the box surface was visually
evaluated. When a damage caused by seizing was observed but it was not severe
and was repairable, repair was performed and makeup and breakout were continued.
The results of the makeup and breakout test are shown in Table 4.
Separately from the above test, a threaded joint sample was prepared under
the same conditions, and by carrying out makeup with a high makeup torque, a
torque chart like that shown in Figure 2 was prepared, and Ts (shouldering torque),
Ty (yield torque), and ∆Τ (= Ty - Ts, torque-on-shoulder resistance) were
determined 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 the torque
which appeared after interference of the shoulder portions occurred began to enter
the linear region (elastic deformation region). Ty was the torque at the start of
plastic deformation. Specifically, Ty was the torque when the change in torque
after reaching Ts began to lose linearity as the rotation proceeds.Table 4 shows the relative value of ∆Τ (= Ty - Ts) when the value of ∆Τ
obtained in the case of a conventional compound grease shown by Comparative
Example 1 of Table 3 is assigned a value of 100. A value of ∆Τ which is greater
than 100 means that ∆Τ was greater than when compound grease was applied, and
accordingly it was possible to perform makeup without yielding of the shoulder
portions and without the occurrence of galling even with a high makeup torque.
This means that galling can be suppressed even under severe conditions such as
occur during unstable excavation operations in the sea.
Separately, the coefficient of friction at an applied pressure of each of 0.3
GPa and 1 GPa was measured by the above-mentioned FALEX method using a pin
and two vee blocks prepared in accordance with ASTM D-2670, D-2625, D-3233,
and D-5620. The pin having a diameter of 6.35 mm (1/4 inches) was made of the
same steel as the threaded joint to be tested and underwent the same preparatory
surface treatment and formation of a lubricating coating as the pin surface of the
joint. The vee blocks each having a V-shaped groove with an angle of 96° and a
width of 6.35 mm (1/4 inches) were made of the same steel as the threaded joint to
be tested and underwent the same preparatory surface treatment and formation of a
lubricating coating as the box surface of the j oint. The value of ∆µ which is the
difference calculated by subtracting the coefficient of friction at a low pressure (0.3
GPa) from the coefficient of friction at a high pressure ( 1 GPa) is shown in Table 3.
A negaive value of ∆µ indicates that the coefficient of friction measured at a high
pressure is lower than that measured at a low pressure.Table 1
Steel composition of threaded joints (mass %, remainder is Fe and impurities)
Table 2
R: surface roughness (µηι), t : coating thickness (µη )Table 3
∆µ = (coefficient of friction at 1 GPa) minus (coefficient of friction at 0.3 GPa); a negaive value of ∆
the coefficient of friction at 1 GPa (at a high pressure) is lower than that at 0.3 GPa (at a low pressure)Table 4
'Number of cycles without problems
The ∆Τ ratio is the value relative to ∆Τ for Comparative Example 1,
which was assigned a value of 100
Example 1
The following surface treatment was carried out on a special threaded joint
made up of the carbon steel having composition A shown in Table 1.
The box surface which had been finished by machine grinding (surface
roughness of 3 m) was immersed for 10 minutes in a manganese phosphating
solution at 80 - 95° C to form a manganese phosphate coating with a thickness of 15
pm (surface roughness of 12 pm). The lubricating coating composition of
Example 1 shown in Table 3 (100 parts of the indicated composition was diluted
with 30 parts of an organic solvent) was applied by spraying to the box surface.
After evaporation of the solvent, a lubricating coating with a thickness of 50 um
was formed. In the examples, all the parts are parts by mass unless otherwise
indicated.
The pin surface which had been 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 having a thickness of 12 µιη (surface
roughness of 8 m) . A lubricating coating which was identical to that on the box
surface was formed on the pin surface.
The value of ∆µ was 0.05, which was greater than that of a coventional
compound grease (0.02 in Comparative Example 1), indicating that the coefficientof friction at a high pressure becomes much higher than that at a low pressure.
The value of ∆ Τ in a high torque test (indicated below as the ∆ Τ ratio) was
117% of the value of ∆ Τ in Comparative Example 1, which was assigned a value of
100. The ∆ Τ ratio was greatly increased compared to the ∆ Τ ratio of around 50%
for Comparative Examples 2 and 3, which did not contain low friction spherical
particles (acrylic-silicone copolymer particles). Moreover, it was found that the
∆ Τ for Example 1was greater than for a conventional compound grease
(Comparative Example 1). Accordingly, it was ascertained that this threaded joint
could be made up with a high torque without the occurrence of yielding of the
shoulder portions. In addition, in a makeup and breakout test, makeup and
breakout could be performed 10 times without the occurrence of galling.
Example 2
The following surface treatment was carried out on a special threaded joint
made of the 13% Cr steel having composition C shown in Table 1.
The box surface which had been finished by machine grinding (surface
roughness of 3 m) was subjected to electroplating by first Ni strike plating and
then by Cu plating to form a plated coating with a total thickness of 12 µ . The
lubricating coating-forming composition of Example 2 shown in Table 3 (100 parts
of the indicated composition were diluted with 30 parts of an organic solvent) was
applied by spraying to the box surface to form a lubricating coating with a thickness
of 52 µ after evaporation of the solvent.
The pin surface was given a surface roughness of 10 µ by sand blasting
with No. 80 sand, and the same composition as was used on the box surface was
applied to the pin surface to form a lubricating coating with a thickness of 50 µ .
The value of ∆ µ was 0.03. In a high torque test, the ∆ Τ ratio was 110%,
and it was confirmed that a higher torque ratio than in any of the comparative
examples was obtained in the same manner as in Example 1. Of course, makeup
and breakout could be carried out 10 times in the makeup and breakout test without
any problems.
Example 3
The following surface treatment was carried out on a special threaded jointW
3 5
made of the Cr-Mo steel having composition B shown in Table 1.
The box surface which had been finished by machine grinding (surface
roughness of 3 µ η) was subjected to electroplating first by Ni strike plating and
then by Cu-Sn-Zn alloy plating to form a plated coating having an overall thickness
of 7 µ . On the box surface, the lubricating coating-forming composition of
Example 3 shown in Table 3 (100 parts of the indicated composition were diluted
with 30 parts of an organic solvent) was applied by spraying. After evaporation of
the solvent, a lubricating coating with a thickness of 100 µη was formed.
The pin surface which had been finished by machine grinding (surface
roughness of 3 µ η) was immersed for 10 minutes in a zinc phosphating solution at
75 - 85° C to form a zinc phosphate coating with a thickness of 12 µ η (surface
roughness of 8 µηι) . A lubricating coating with a thickness of 100 µ η was formed
on the pin surface in exactly the same manner as for the box surface.
The value of ∆µ was a high value of 0.08. In the high torque test, it was
confirmed that the ∆Τ ratio was 130%, which was higher than in any of the
comparative examples. In the makeup and breakout test, makeup and breakout
could be performed 10 times without any problems.
Example 4
The following surface treatment was performed on a special threaded joint
made of the Cr-Mo steel having composition B shown in Table 1.
The box surface which had been 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 with an overall thickness of
7 µπι. The box surface which underwent this preparatory surface treatment was
then preheated to 130° C by induction heating, and the lubricating coating- forming
composition of Example 4 shown in Table 3 which was heated to 130° C was
applied to the preheated box surface with a spray gun having a temperature
maintaining mechanism. After cooling, a lubricating coating with a thickness of
50 µη was formed.
The pin surface which had been finished by machine grinding (surface
roughness of 3 µ η) was immersed for 10 minutes in a zinc phosphating solution at
75 - 85° C to form a zinc phosphate coating having a thickness of 12 µ (surfaceroughness of 8 µ ) . A lubricating coating having a thickness of 50 µηι was
formed in exactly the same manner as for the box surface on the pin surface.
The value of ∆ µ was a high value of 0.07. In a high torque test, the ∆ Τ
ratio was 128%, and it was confirmed that the ∆ Τ ratio was higher than in any of the
comparative examples. In the makeup and breakout test, makeup and breakout
could be performed 10 times without any problems.
As can been seen from the above results, there is a correlation between ∆ µ
and ∆ Τ . Thus, there is the tendency that the greater the ∆ µ, the greater the ∆ Τ .
In other words, by measurement of ∆ µ by the FALEX method, for example, using
small test pieces, it is possible to predict the value of ∆ Τ of a threaded joint and
hence makeup performance with a high torque thereof.
Comparative Example 1
The following surface treatment was performed on a special threaded joint
made of the carbon steel having composition A shown in Table 1.
The box surface which had been finished by machine grinding (surface
roughness of 3 µ η) was immersed for 10 minutes in a manganese phosphating
solution at 80 - 95° C to form a manganese phosphate coating with a thickness of 15
µηι (surface roughness of 12 um). A viscous liquid compound grease specified by
API standards was applied to the box surface to form a lubricating coating (the total
coating weight on the pin and the box was 50 grams, and the total coated area was
roughly 1400 cm2) .
The pin surface which had been finished by machine grinding (surface
roughness of 3 µ ι ) was immersed for 10 minutes in a zinc phosphating solution at
75 - 85° C to form a zinc phosphate coating with a thickness of 12 µ (surface
roughness of 8 µηι), and then the above-described compound grease was applied to
the pin surface in the above-described amount.
The value of ∆ µ was 0.02. As shown in Table 4, in a makeup and breakout
test, in 10 cycles of makeup and breakout, there was no apparent occurrence of
galling up to the 10th cycle. However, the compound grease in this example
contained heavy metal powder such as lead, so it could be considered harmful to
humans and the environment. In a high torque test, it had a high value of Ty
without the occurrence of yielding of the shoulder portions even when makeup was201 1/076016
37
carried out with a high torque, and it exhibited a high value of ∆Τ. ∆Τ at this time
was given a value of 100, and ∆Τ of the examples was compared to this value.
Comparative Example 2
The following surface treatment was performed on a special threaded joint
made of the Cr-Mo steel having composition B of Table 1.
The box surface which had been finished by machine grinding (surface
roughness of 3 µ ) 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 µη ) . The lubricating coating- forming composition of
Comparative Example 2 in Table 3 (prepared by diluting 100 parts of the indicated
composition with 30 parts of an organic solvent) which contained amorphous
graphite but did not contain acrylic-silicone copolymer particles was applied to the
box surface by spraying. After evaporation of the solvent, a lubricating coating
with a thickness of 80 µπ was formed.
The pin surface which had been finished by machine grinding (surface
roughness of 3 µπ ) was immersed for 10 minutes in a zinc phosphating solution at
75 - 85° C to form a zinc phosphate coating with a thickness of 12 µπ (surface
roughness of 8 µ η), and the same composition as for the box surface was sprayed
on the pin surface in the same manner as for the box surface to form a lubricating
coating having a thickness of 80 µη .
The value of ∆µ was -0.05, indicating that the coefficient of friction at a high
pressure is lower than that at a low pressure. As shown in Table 4, in the makeup
and breakout test, there was no occurrence of galling in 10 cycles of makeup and
breakout, and the results were extremely good. However, in the high torque test,
the ∆Τ ratio was an extremely low value of 50% compared to conventional
compound grease. Namely, it was confirmed that the ∆Τ ratio was not improved
at all with only a coating made from a highly viscous matrix and graphite.
Comparative Example 3
The following surface treatment was performed on a special threaded joint
made of the carbon steel having composition A shown in Table 1.
The box surface which had been finished by machine grinding (surfaceroughness of 3 µηι) was immersed for 10 minutes in a manganese phosphating
solution at 80 - 95° C to form a manganese phosphate coating with a thickness of 15
µηι (surface roughness of 12 µιη) . The box surface was preheated to 130° C by
induction heating, and the lubricating coating-forming composition of Comparative
Example 3 shown in Table 3 which was heated to 130° C was applied to the
preheated box surface with a spray gun having a temperature maintaining
mechanism. After cooling, a lubricating coating with a thickness of 50 µηι was
formed.
The pin surface which had been finished by machine grinding (surface
roughness of 3 µηι) was immersed for 10 minutes in a zinc phosphating solution at
75 - 85° C to form a zinc phosphate coating with a thickness of 12 µ (surface
roughness of 8 µηι) . The same composition as for the box surface was then
applied to the pin surface in the same manner as for the box surface to form a
lubricating coating having a thickness of 50 µηι.
The value of ∆µ was -0.06, indicating that the coefficient of friction at a high
pressure is lower than that at a low pressure. As shown in Table 4, in the makeup
and breakout test, there was no occurrence of galling in 10 cycles of makeup and
breakout, and the results were extremely good. However, in the high torque test,
the ∆Τ ratio was an extremely low value of 52% compared to a conventional
compound grease.
In order to evaluate rust preventing properties which are necessary for a
tubular threaded joint, the same preparatory surface treatment coating and
lubricating coating as for the box shown in Tables 2 and 3 were formed on a
separately prepared coupon test piece (70 mm x 150 mm x 1.0 mm thick). The test
piece was subjected to a salt spray test (according to JIS Z 2371 (corresponding to
ISO 9227), temperature of 35° C, duration of 1000 hours) and a humidity cabinet
test (according to JIS K 5600-7-2 (corresponding to ISO 6270), temperature of 50°
C, relative humidity of 98%, duration of 200 hours), and the occurrence of rusting
was investigated. It was confirmed that there was no occurrence of rust through
the end of either test with the threaded joints in the above examples.
In addition, when tubular threaded joints of each of the examples underwent
a gas tightness test and an actual use test in an actual drilling apparatus, it was
confirmed that an excellent lubricating coating having a higher ∆Τ than for aconventionally used compound grease was realized.
The present invention has been explained above with respect to embodiments
which are considered to be preferred at the present time, but the present invention is
not limited to the above-described embodiments. Variations which are not
contrary to the concept of the invention as construed from the claims and the overall
description are possible, and a threaded joint which incorporates such variations
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 lubricating
coating comprising particles of a copolymer of a resin selected from a silicone resin
and a fluorocarbon resin with a thermoplastic resin, said particles being dispersed in
a viscous matrix having a complex shear viscosity of at least 3000 Pa-sec at 25° C.
2. A tubular threaded joint as set forth in claim 1wherein the coefficient of
friction of the lubricating coating measured at a pressure of 1GPa is greater than
that measured at a pressure of 0.3 GPa.
3. A tubular threaded joint as set forth in claim 2 wherein the difference
calculated by subtracting the coefficient of friction at 0.3 GPa from that at 1GPa is
at least 0.02.
4. A tubular threaded joint as set forth in any one of claims 1to 3 wherein
the copolymer particles are spherical particles.
5. A tubular threaded joint as set forth in any one of claims 1 to 4 wherein
the copolymer particles are acrylic-silicone copolymer particles having an average
particle diameter of 10 - 50 µπ , and their content is 0.1 - 20 mass %.
6. A tubular threaded joint as set forth in any one of claims 1 - 5 wherein
the viscous matrix comprises at least one substance selected from rosin-based
substances including rosin and its derivatives, waxes, metal soaps, and basic metal
salts of aromatic organic acids.
7. A tubular threaded joint as set forth in any one of claims 1 to 5 wherein
the lubricating coating further contains graphite as a friction modifier.
8. A tubular threaded joint as set forth in any one of claims 1 to 7 whereinthe lubricating coating has a thickness of 10 - 500 µηι.
9. A tubular threaded joint as set forth in any one of claims 1 to 8 wherein
the contact surface of at least one of the pin and the box having the lubricating
coating 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
a combination of these before forming the lubricating coating.
10. A tubular threaded joint as set forth in any one of claims 1 to 9 wherein
the contact surface of one of the pin and the box has the lubricating coating, and the
contact surface of the other of the pin and the box has undergone surface treatment
by at least one method selected from at least one of blasting treatment, pickling,
phosphate chemical conversion treatment, oxalate chemical conversion treatment,
borate chemical conversion treatment, electroplating, impact plating, and a
combination of these.
11. A tubular threaded joint as set forth in any one of claims 1 to 10 for
connecting oil country tubular goods.
12. A method of connecting a plurality of oil country tubular goods without
applying a lubricating grease using a tubular threaded joint as set forth in any one of
claims 1 to 10.