U.S. patent application number 10/829292 was filed with the patent office on 2004-10-07 for threaded joint for steel pipes and process for the surface treatment thereof.
Invention is credited to Goto, Kunio.
Application Number | 20040195826 10/829292 |
Document ID | / |
Family ID | 26613442 |
Filed Date | 2004-10-07 |
United States Patent
Application |
20040195826 |
Kind Code |
A1 |
Goto, Kunio |
October 7, 2004 |
Threaded joint for steel pipes and process for the surface
treatment thereof
Abstract
This invention relates to a threaded joint for steel pipes which
comprises a pin and a box each having a contact surface including a
threaded portion and an unthreaded metal contact portion and which
can be prevented from galling during repeated fastening and
loosening without application of a compound grease. A solid
lubricant coating which comprises a lubricating powder such as
molybdenum disulfide and a resin binder is formed on the contact
surface of at least one of the pin and the box. The coating is
formed by applying a coating fluid and drying the applied coating
by first stage heating in the temperature range of from 70.degree.
C. to 150.degree. C. and second stage heating in the range of from
higher than 150.degree. C. to 380.degree. C. The resulting solid
lubricant coating has a hardness of 70-140 on the Rockwell M scale
and an adhesive strength of at least 500 N/m as determined by the
SAICAS (Surface And Interfacial Cutting Analysis System) method,
and it exhibits excellent galling resistance even in the
environment of high-temperature oil wells. Inclusion of ultraviolet
screening fine particles such as titanium oxide fine particles in
the solid lubricant coating increases the rust preventing
properties of the threaded joint.
Inventors: |
Goto, Kunio; (Kobe-shi,
JP) |
Correspondence
Address: |
CLARK & BRODY
1750 K STREET NW
SUITE 600
WASHINGTON
DC
20006
US
|
Family ID: |
26613442 |
Appl. No.: |
10/829292 |
Filed: |
April 22, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10829292 |
Apr 22, 2004 |
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10361556 |
Feb 11, 2003 |
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10361556 |
Feb 11, 2003 |
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PCT/JP02/03588 |
Apr 11, 2002 |
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Current U.S.
Class: |
285/94 ; 285/333;
285/55 |
Current CPC
Class: |
C23C 24/08 20130101;
C10M 2201/0413 20130101; F16L 15/00 20130101; Y10T 428/31529
20150401; C10M 2201/0663 20130101; C10M 111/04 20130101; Y10T
428/31678 20150401; C10N 2050/02 20130101; C10M 2201/0613 20130101;
C10M 2209/1013 20130101; E21B 17/042 20130101; F16L 15/001
20130101; C10M 2201/0623 20130101; C10N 2030/06 20130101; F16L
15/004 20130101; C10M 2209/1003 20130101; C10N 2050/08 20130101;
C10M 2201/0653 20130101; F16L 58/182 20130101; C10M 2217/0443
20130101; Y10T 428/1393 20150115; C10M 2201/0803 20130101; C10M
177/00 20130101; C10M 2201/084 20130101; Y10T 428/139 20150115;
C10N 2070/00 20130101; C10N 2080/00 20130101; C10N 2040/34
20130101 |
Class at
Publication: |
285/094 ;
285/055; 285/333 |
International
Class: |
F16L 055/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 11, 2001 |
JP |
2001-112884 |
May 1, 2001 |
JP |
2001-134576 |
Claims
1. A process for the surface treatment of a threaded joint for
steel pipes comprising a pin and a box each having a contact
surface including a threaded portion and an unthreaded metal
contact portion, characterized in that the process comprises the
steps: applying a coating fluid containing a resin and a
lubricating powder in a solvent to the contact surface of at least
one of the pin and the box, and drying the applied coating by
multistage heating which includes at least first stage heating in
the temperature range of from 70.degree. C. to 150.degree. C. and
second stage heating in the range of from higher than 150.degree.
C. to 380.degree. C. to form a solid lubricant coating on the
contact surface.
2. A process as set forth in claim 1, wherein the process further
includes, prior to the coating application step, a step of heating
the contact surface to be coated to a temperature of from
50.degree. C. to 200.degree. C.
3. A process as set forth in claim 1, wherein the solid lubricant
coating which is formed has a hardness of 70-140 on the Rockwell M
scale.
4. A process as set forth in claim 1, wherein the solid lubricant
coating formed has an adhesive strength of at least 500 N/m as
determined by the SAICAS (Surface And Interfacial Cutting Analysis
System) method.
5. A process as set forth in claim 1, wherein the lubricating
powder is a powder of one or more substances selected from
molybdenum disulfide, tungsten disulfide, graphite, boron nitride,
and ploytetrafluoroethylene.
6. A process as set forth in claim 1, wherein the contact surface
to which the coating fluid is applied has a surface roughness of
5-40 .mu.m Rmax.
7. A process as set forth in claim 1, wherein the contact surface
to which the coating fluid is applied has a porous coating layer
formed by pretreatment.
8-16. canceled
17. A process as set forth in claim 2, wherein the process further
includes, prior to the coating application step, a step of heating
the contact surface to be coated to a temperature of from
50.degree. C. to 200.degree. C.
18. A process as set forth in claim 2, wherein the solid lubricant
coating which is formed has a hardness of 70-140 on the Rockwell M
scale.
19. A process as set forth in claim 2, wherein the solid lubricant
coating formed has an adhesive strength of at least 500 N/m as
determined by the SAICAS (Surface And Interfacial Cutting Analysis
System) method.
20. A process as set forth in claim 2, wherein the lubricating
powder is a powder of one or more substances selected from
molybdenum disulfide, tungsten disulfide, graphite, boron nitride,
and ploytetrafluoroethylene.
21. A process as set forth in claim 2, wherein the contact surface
to which the coating fluid is applied has a surface roughness of
5-40 .mu.m Rmax.
22. A process as set forth in claim 2, wherein the contact surface
to which the coating fluid is applied has a porous coating layer
formed by pretreatment.
Description
TECHNICAL FIELD
[0001] This invention generally relates to a threaded joint for
steel pipes for use in connecting oil well pipes to each other.
More particularly, this invention relates to a threaded joint for
steel pipes which has a solid lubricant coating having excellent
galling resistance, gas tightness, and rust preventing properties
and which does not require the application of a compound grease
containing a heavy metal powder, which application was
conventionally carried out before each time fastening was performed
in order to prevent the joint from galling, and to a process for
surface treatment capable of forming such a solid lubricant
coating.
BACKGROUND ART
[0002] Oil well pipes which are steel pipes used in the drilling of
oil wells are connected with each other by a threaded joint for
steel pipes. The threaded joint is comprised of a pin having a male
thread and a box having a female thread.
[0003] As schematically shown in FIG. 1, a male thread 3A is
normally formed on the outer surface at both ends of a steel pipe A
to form a pin 1, and a female thread 3B is formed on both sides of
the inner surface of a separate joint member in the form of a
sleeve-shaped coupling B to form a box 2. As shown in FIG. 1, the
steel pipe A is normally shipped in a state in which a coupling B
is previously connected to one end.
[0004] A threaded joint for steel pipes is subjected to compound
pressures due to axial tensile forces caused by the weight of the
steel pipe and the coupling and internal and external pressures
underground, and it is also subjected to heat underground.
Therefore a threaded joint is required to maintain gas tightness
(sealability) without being damaged even under such conditions. In
addition, during the process of lowering oil well pipes, it is
often the case that a joint which has once been fastened is
loosened (unfastened) and then refastened. Therefore, according to
API (American Petroleum Institute), it is desired that there be no
occurrence of severe seizing called galling and that gas tightness
be maintained even if fastening (make-up) and loosening (break-out)
are carried out ten times for joints for tubing and three times for
joints for casing.
[0005] In recent years, in order to improve gas tightness, special
threaded joints which are capable of forming a metal-to-metal seal
have come to be generally used. In this type of threaded joint,
each of a pin and a box has an unthreaded metal contact portion in
addition to a threaded portion having a male or female thread, and
both the threaded portion and the unthreaded metal contact portion
form a contact surface between the pin and box. The unthreaded
metal contact portions of the pin and the box come into intimate
contact with each other to form a metal-to-metal seal portion and
contribute to an increase in gas tightness.
[0006] In such a threaded joint capable of forming a metal-to-metal
seal, a lubricating grease with high lubricity called a compound
grease has been used. This grease, which is a kind of liquid
lubricant, is applied to the contact surface of at least one of the
pin and the box prior to fastening. However, this grease contains a
large amount of harmful heavy metal powders. When the grease which
is squeezed out to the periphery during fastening is cleaned with a
cleaning agent, the compound grease and the used cleaning agent
flow out into the ocean or the soil and cause environmental
pollution, and this has come to be considered a problem. In
addition, there was the problem that the application of grease and
cleaning which were repeated before each fastening lowered the
working efficiency in the field.
[0007] As threaded joints for steel pipes which do not need the
application of a compound grease, JP 08-103724A, JP 08-233163A, JP
08-233164A, and JP 09-72467A disclose threaded joints in which a
solid lubricant coating comprising a resin as a binder and
molybdenum disulfide or tungsten disulfide as a solid lubricant is
applied to a threaded portion and an unthreaded metal contact
portion (namely, to the contact surface) of at least one of a pin
and a box.
[0008] In these Japanese patent publications, in order to increase
the adhesion between the solid lubricant coating and the substrate
steel, it is disclosed to form, as an undercoating layer for the
solid lubricant coating, a manganese phosphate chemical conversion
coating layer or a combination of a nitride layer and a manganese
phosphate chemical conversion coating layer, or to provide the
contact surface with surface roughness having an Rmax of 5-40
.mu.m. JP 08-103724A discloses that a solid lubricant coating is
formed by performing baking of an applied coating with heating for
20-30 minutes in the temperature range of 150-300.degree. C.
[0009] It might be expected that the use of a threaded joint in
which the contact surface of a pin and a box has a solid lubricant
coating formed by surface treatment to provide lubricity thereto
would make it possible to dispense with the application of a
compound grease and thus avoid the aforementioned problems
regarding the environment and working efficiency.
[0010] However, with a conventional solid lubricant coating, it is
not possible to attain a high anti-galling effect such as can be
obtained by application of a compound grease, and a seizing flaw
called galling occurs after fastening and loosening are repeated
several times. Thus, the effect of a conventional solid lubricant
coating for preventing galling was insufficient.
[0011] The decrease in galling resistance and gas tightness of a
threaded joint was significant, particularly when the storage
period of the threaded joint from its shipping out of the factory
(i.e., from the formation of a solid lubricant coating) to its
actual use on a rig site to fasten it was long (it is sometimes as
long as one or two years).
[0012] Furthermore, recently, a heat-resistant threaded joint for
steel pipes has been desired for use in high-temperature oil wells
in which the temperature reaches 250-300.degree. C., which is
higher than the temperature in conventional oil wells, or in
steam-injection oil wells into which steam at a high temperature
close to the critical temperature (e.g., around 350.degree. C.) is
injected in order to improve oil recovery. Therefore, it has been
required for a threaded joint to guarantee galling resistance and
gas tightness when a joint which has been fastened is subjected to
a heating test at a temperature of around 350.degree. C. and then
subjected to loosening and re-fastening. With the above-described
conventional solid lubricant coating, it was difficult to assure
these properties required for a heat-resistant threaded joint.
[0013] It is an object of this invention to provide a process for
the heat treatment of a threaded joint for steel pipes, which can
form a solid lubricant coating having excellent galling resistance
which can effectively suppress the occurrence of galling upon
repeated fastening and loosening even with a heat-resistant
threaded joint for steel pipes.
[0014] It is another object of this invention to provide a threaded
joint for steel pipes which can alleviate a decrease in galling
resistance and gas tightness without using a compound grease when
it is stored for a prolonged period from the formation of a solid
lubricant coating to its use on site.
DISCLOSURE OF THE INVENTION
[0015] According to one aspect, the present invention is a process
for the surface treatment of a threaded joint for steel pipes
comprising a pin and a box each having a contact surface including
a threaded portion and an unthreaded metal contact portion,
characterized in that the process comprises the steps:
[0016] applying a coating fluid containing a resin binder and a
lubricating powder in a solvent to the contact surface of at least
one of the pin and the box, and
[0017] drying the applied coating by multistage heating which
includes at least first stage heating in the temperature range of
from 70.degree. C. to 150.degree. C. and second stage heating in
the range of from higher than 150.degree. C. to 380.degree. C. to
form a solid lubricant coating on the contact surface.
[0018] The process may further include, prior to the coating
application step, a step of heating the contact surface to be
coated to a temperature of from 50.degree. C. to 200.degree. C.
[0019] The solid lubricant coating formed according to the process
of the present invention can possess a hardness of 70-140 on the
Rockwell M scale and an adhesive strength of at least 500 N/m as
determined by the SAICAS (Surface And Interfacial Cutting Analysis
System) method.
[0020] It has been found that a cause of insufficient galling
resistance of a conventional solid lubricant coating formed on the
contact surface of a threaded joint for steel pipes is insufficient
hardness of the coating, which is caused by insufficient drying of
the coating.
[0021] A solid lubricant coating for a threaded joint is generally
formed by applying a coating fluid containing a resin and a
lubricating powder (e.g., molybdenum disulfide powder) in a
volatile solvent to the contact surface of the threaded joint,
followed by heating to dry (or bake) the applied coating. In the
case where the applied coating is dried by heating at a temperature
of 150-300.degree. C. as employed in the prior art, even if the
beating is carried out for a prolonged period, it is not possible
to completely evaporate the solvent, and a minute amount of the
solvent and moisture is confined in the dried coating and leads to
the formation of internal defects, which prevent the coating from
having sufficient hardness and galling resistance. Such a solid
lubricant coating wears off when fastening and loosening are
repeated, and it ends up completely wearing out, thereby producing
metal-to-metal contact and causing galling.
[0022] In accordance with the above-described process of the
present invention, drying becomes complete by performing the drying
by at least two stages comprising a first stage heating at a lower
temperature and a second stage heating at a higher temperature,
resulting in the formation of a solid lubricant coating which has
higher hardness than that obtained in the case where drying is
performed by heating at a fixed temperature as employed in the
prior art and which is improved with respect to galling resistance,
wear resistance, adhesion, and rust preventing properties and
adapted even to the environment of high-temperature oil wells.
[0023] The present invention also relates to a threaded joint for
steel pipes comprising a pin and a box each having a contact
surface including a threaded portion and an unthreaded metal
contact portion, characterized in that the contact surface of at
least one of the pin and the box has a solid lubricant coating
formed thereon which comprises a lubricating powder selected from
molybdenum disulfide and/or tungsten disulfide and a resin, the
coating having a hardness of 70-140 on the Rockwell M scale and/or
an adhesive strength of at least 500 N/m as determined by the
SAICAS method.
[0024] According to another aspect, the present invention is a
threaded joint for steel pipes comprising a pin and a box each
having a contact surface including a threaded portion and an
unthreaded metal contact portion, characterized in that the contact
surface of at least one of the pin and the box has a solid
lubricant coating formed thereon which comprises a lubricating
powder, ultraviolet screening fine particles, and a resin
binder.
[0025] A cause for a decrease in galling resistance and gas
tightness encountered in a conventional threaded joint having a
solid lubricant coating comprising a resin and a lubricating powder
on the contact surface thereof when the joint is stored for a long
period is that the rust preventing properties of the solid
lubricant coating are markedly inferior to those of a compound
grease so that it is not capable of completely protecting the
contact surface of the threaded joint from rusting during storage.
If rust is caused on the contact surface of the pin or the box
during storage of such a threaded joint, the adhesion of the solid
lubricant coating of the joint decreases markedly, and blistering
and peeling of the coating occur. In addition, the contact surface
roughness increases due to the rust. As a result, when steel pipes
are connected by fastening a threaded joint, fastening becomes
unstable, leading to the occurrence of galling during fastening or
loosening and a decrease in gas tightness.
[0026] It was found that rusting during storage of a threaded joint
having a solid lubricant coating is mainly caused by aging or
deterioration with time of the resin used as a binder in the solid
lubricant coating, particularly by forming cracks in the coating
due to deterioration of the resin by ultraviolet light and allowing
moisture to penetrate through the cracks. In order to prevent a
solid lubricant coating from deteriorating by ultraviolet light, it
has been found that addition of inorganic ultraviolet screening
fine particles, not an organic ultraviolet absorbing agent, is
effective and that rusting of a threaded joint during long-term
storage is significantly suppressed by a solid lubricant coating
containing ultraviolet screening fine particles.
[0027] Preferably, the ultraviolet screening fine particles are
fine particles of one or more substances selected from titanium
oxide, zinc oxide, and iron oxide, and they have a mean particle
diameter of 0.01-0.1 .mu.m and are present in the solid lubricant
coating with a mass ratio of 0.1-50 parts to 100 parts of the resin
binder.
[0028] In the present invention, a lubricating powder is preferably
a powder of one or more substances selected from molybdenum
disulfide, tungsten disulfide, graphite, boron nitride, and
polytetrafluoroethylene.
[0029] It is also preferred that the contact surface on which a
solid lubricant coating is formed have a porous coating layer as a
primary coat underlying the solid lubricant coating.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] FIG. 1 is a diagram which schematically shows a typical
assembly of a steel pipe and a threaded coupling at the time of
shipment of the steel pipe.
[0031] FIG. 2 is a diagram which schematically shows a connecting
portion of a threaded joint for steel pipes according to the
present invention.
[0032] FIGS. 3a and 3b are diagrams showing examples of the heating
pattern (temperature profile) of first stage heating and second
stage heating in a process for the surface treatment of a threaded
joint for steel pipes according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0033] FIG. 2 is a diagram which schematically shows the structure
of a typical threaded joint for steel pipes. In the figure, 1 is a
pin, 2 is a box, 3 is a threaded portion, 4 is an unthreaded metal
contact portion, and 5 is a shoulder portion. In the following
description, an unthreaded metal contact portion will also be
referred to as simply a metal contact portion.
[0034] As shown in FIG. 2, a typical threaded joint is comprised of
a pin 1 having a threaded portion 3 (more precisely, a male thread
portion) and an unthreaded metal contact portion 4 formed on the
outer surface at an end of a steel pipe, and a box 2 having a
threaded portion 3 (more precisely, a female thread portion) and an
unthreaded metal contact portion 4 formed on the inner surface of a
threaded joint member (a coupling). However, the location of a pin
and a box is not limited to the depicted one. For example, a
coupling may be omitted by forming a pin at one end of a steel pipe
and a box at the other end of the pipe, or a pin (a male thread)
may be formed on a coupling with a box being formed at both ends of
a steel pipe.
[0035] The threaded portion 3 and the (unthreaded) metal contact
portion 4 on each of the pin and the box constitute a contact
surface of the threaded joint. The contact surface and particularly
the unthreaded metal contact portion which is more susceptible to
galling is required to have galling resistance. For this purpose,
in the prior art, a compound grease containing a heavy metal powder
was applied to the contact surface, but the use of a compound
grease involves many problems from the standpoints of the
environment and working efficiency.
[0036] In accordance with the present invention, a coating fluid
containing a binder resin and a lubricating powder in a solvent is
applied to the contact surface of at least one of the pin and the
box, and the applied coating is dried by heating to form a solid
lubricant coating. The solid lubricant coating formed on the
contact surface of a threaded joint is subjected to high pressure
of sliding while the threaded joint is fastened and loosened
repeatedly, thereby producing wear particles comprising the
lubricating powder. It is presumed that these wear particles
containing a lubricating powder are distributed over the entire
contact surface to contribute to prevention of metal-to-metal
contact at the contacting interface and alleviation of friction,
thereby is exhibiting an anti-galling effect.
[0037] It is desirable that the contact surface of at least one of
the pin and the box to which a coating fluid is applied be
previously roughened such that the surface has a roughness (Rmax)
of 5-40 .mu.m, which is greater than the surface roughness as
machined (3-5 .mu.m), in order to further improve the effect
achieved by the present invention. If the value of Rmax of the
surface to be coated is less than 5 .mu.m, the resulting solid
lubricant coating may have decreased adhesion. On the other hand,
if it is greater than 40 .mu.m, the coated surface may produce
increased friction and promote abrasive wear of the solid lubricant
coating, and the coating may not withstand repeated fastening and
loosening of the joint. However, the effect of the present
invention is of course obtainable even if the surface roughness is
not in the above-described range.
[0038] The method of surface roughening may be a method of
roughening the steel surface itself such as blasting with sand or
grit and dipping in a strong acid solution such as sulfuric acid,
hydrochloric acid, nitric acid, or hydrofluoric acid to roughen the
surface. Another possible method is to form a primary (underlying)
coat layer having a rougher surface than the steel surface to
roughen the surface to be coated.
[0039] Examples of a method of forming such a primary coat layer
include a method of forming a chemical conversion coating such as a
phosphate, oxalate, or borate treatment (in which the surface
roughness of the crystal layer increases as the crystals which are
formed grow), a method of electroplating with a metal such as
copper or iron (in which peaks or raised points are preferentially
plated so that the surface is slightly roughened), an impact
plating method in which particles having an iron core coated with
zinc or a zinc-iron alloy are blasted using centrifugal force or
pneumatic pressure to form a coating of zinc or a zinc-iron alloy,
a soft nitriding method forming a nitride layer (e.g.,
tufftriding), a composite metallic coating method in which a porous
coating comprising solid fine particles in a metal is formed, and
the like.
[0040] From the viewpoint of adhesion of a solid lubricant coating,
a porous coating, particularly a chemical conversion coating formed
by phosphating (with manganese phosphate, zinc phosphate,
iron-manganese phosphate, or zinc-calcium phosphate) or a coating
of zinc or a zinc-iron alloy formed by impact plating is preferred.
A more preferred coating is a manganese phosphate coating from the
standpoint of adhesion or a zinc or a zinc-iron alloy coating from
the standpoint of rust prevention. Both a phosphate coating formed
by chemical conversion treatment and a zinc or zinc-iron alloy
coating formed by impact plating are porous, so they can provide a
solid lubricant coating formed thereon with an increased
adhesion.
[0041] When a primary coat layer is formed, the thickness of the
layer is not restricted, but it is preferably in the range of 5-40
.mu.m from the standpoints of rust prevention and adhesion. With a
thickness of less than 5 .mu.m, sufficient rust prevention may not
be achieved. A thickness of greater than 40 .mu.m may cause a
decrease in adhesion of a solid lubricant coating formed
thereon.
[0042] The resin present in a solid lubricant coating may be any
resin capable of functioning as a binder. A resin having thermal
resistance and a reasonable level of hardness and wear resistance
is suitable. Examples of such a resin include thermosetting resins
such as epoxy resins polyimide resins, polycarbodiimide resins,
polyethersulfones, polyetheretherketone resins, phenolic resins,
furan resins, urea resins, and acrylic resins, as well as
thermoplastic resins such as polyamideimide resins, polyethylene
resins, silicone resins, and polystyrene resins.
[0043] Although the lubricating powder may be any powder exhibiting
lubricity, in view of the high load which is applied, it is
desirable to use a powder of one or more substances selected from
molybdenum disulfide, tungsten disulfide, graphite, boron nitride,
and PTFE (polytetrafluoroethylene). Particularly preferred is a
powder of molybdenum disulfide and/or tungsten disulfide, both
providing a high relaxation of wear and friction, or a mixture
thereof with other lubricating powder or powders.
[0044] Preferably, the lubricating powder has a mean particle
diameter in the range of 0.5-60 .mu.m. If it is less than 0.5
.mu.m, the powder tends to aggregate, and uniform dispersion of the
powder in a coating fluid may become difficult. As a result, there
are cases in which a desired solid lubricant coating having a
lubricating powder uniformly dispersed therein is not formed,
resulting in insufficient galling resistance. On the other hand, if
the mean particle diameter of the lubricating powder is larger than
60 .mu.m, the strength of the solid lubricant coating may be
decreased to such an extent that the occurrence of galling cannot
be prevented.
[0045] The ratio of the lubricating powder to the resin binder is
preferably such that the mass ratio of lubricating powder to binder
is in the range of 0.3-9.0 from the standpoint of galling
resistance. If the mass ratio of lubricating powder to binder is
less than 0.3, the amount of the lubricating powder in the
above-described wear particles may be insufficient, and galling
resistance may become poor. On the other hand, if the mass ratio is
greater than 9.0, the solid lubricant coating may have insufficient
strength, so it cannot withstand a high pressure and has a
decreased adhesion to the substrate surface, thereby causing the
galling resistance and gas tightness to deteriorate. The mass ratio
of the lubricating powder to the binder is preferably in the range
of 0.5-9.0 in view of galling resistance and more preferably in the
range of 1.0-8.5 further taking adhesion into consideration.
[0046] A solvent which is used to form a coating fluid may be a
single solvent or a mixed solvent selected from various low-boiling
solvents including hydrocarbons (e.g., toluene) and alcohols (e.g.,
isopropyl alcohol). Preferably, the solvent has a boiling
temperature of 150.degree. C. or below.
[0047] The coating fluid used to form a solid lubricant coating may
contain an additional constituent or constituents, in addition to a
solvent, a resin, and a lubricating powder. For example, one or
more powders selected from zinc powder, a chrome pigment, and
alumina may be added. In addition, a colorant may be present such
that the resulting solid lubricant coating is colored. If
appropriate, the coating fluid may contain one or more additives
such as a dispersant, an antifoaming agent, and a thickening
agent.
[0048] In an embodiment of the present invention, ultraviolet
screening fine particles are added to the coating fluid to form a
solid lubricant coating comprising a lubricating powder, a resin
and ultraviolet screening fine particles. Thus, it is possible to
significantly improve the rust preventing properties of a solid
lubricant coating while maintaining its galling resistance and gas
tightness, whereby the contact surface of a threaded joint is
inhibited from rusting caused by aging of the solid lubricant
coating formed thereon, and hence the occurrence of galling and a
decrease in gas tightness due to rusting are also suppressed. As a
result, even if a threaded joint having a solid lubricant coating
formed thereon is stored outdoors for a prolonged period, it is
prevented from suffering a significant deterioration in properties,
and its reliability as a product is significantly improved.
[0049] An organic ultraviolet absorbing agent (e.g., benzotriazole
or its derivative) is sometimes added to coating compositions in
order to improve their weatherability. In the present invention,
such an organic ultraviolet absorbing agent is not effective.
[0050] The ultraviolet screening fine particles which are used in
the present invention are not limited as long as they are fine
particles having a high absorbance and refractive index in the
ultraviolet region (300-400 nm in wavelength). Examples of
materials of such fine particles include titanium oxide, zinc
oxide, iron oxide, barium sulfate, silica, composite particles of
zirconia and a polyamide, and synthetic mica in which iron is
included.
[0051] By reason of a less adverse effect on galling resistance,
titanium oxide, zinc oxide, iron oxide, barium sulfate, and silica
are preferred. More preferred are titanium oxide, zinc oxide, and
iron oxide in view of uniform dispersibility of fine particles in a
coating.
[0052] As the ultraviolet screening fine particles, it is
preferable to use so-called ultra-fine particles having a mean
particle diameter in the range of 0.01-0.1 .mu.m from the
standpoint of the balance between ultraviolet screening properties
or aging with time of a solid lubricant coating and galling
resistance thereof, although larger particles up to those having a
mean particle diameter on the order of 2 .mu.m may be used. If the
ultraviolet screening fine particles have a mean particle diameter
of less than 0.01 .mu.m, they have a strong tendency toward
aggregation and may be distributed unevenly in a solid lubricant
coating, and the resistance to aging of the coating may be
insufficient. Ultraviolet screening fine particles having a mean
particle diameter of greater than 0.1 .mu.m may inhibit the
anti-galling properties of a lubricating powder, thereby
deteriorating the galling resistance of a solid lubricant
coating.
[0053] The content of the ultraviolet screening fine particles in a
solid lubricant coating is preferably such that the mass ratio of
the particles to 100 parts of the binder is in the range of 0.1-50
and more preferably 1-30. If the amount of the ultraviolet
screening fine particles is less than 0.1 parts based on 100 parts
of the resin, the ultraviolet screening effect may become
insufficient, and the solid lubricant coating may not be inhibited
from aging, thereby making it impossible to maintain rust
preventing properties, gas tightness, and galling resistance during
repeated fastening and loosening. Addition of ultraviolet screening
fine particles in an amount of more than 50 parts based on 100
parts of the resin may have a substantial adverse effect on the
strength, adhesion, and galling resistance of a solid lubricant
coating.
[0054] The above-described coating fluid which comprises a binder
resin, a lubricating powder, and optionally ultraviolet screening
fine particles in a solvent is applied to the contact surface
(threaded portion and unthreaded metal contact portion) of at least
one of the pin and the box. The application may be performed in any
suitable method known in the art including brush coating, dipping,
and air spraying.
[0055] It is desirable that the application be performed so as to
form a solid lubricant coating having a thickness of at least 5
.mu.m and not greater than 50 .mu.m. With a solid lubricant coating
having a thickness of less than 5 .mu.m, the amount of the
lubricating powder present therein may be small, and the
effectiveness of the coating in improving lubricity may be
decreased. When the thickness of the solid lubricant coating is
greater than 50 .mu.m, there are cases in which the gas tightness
is decreased due to insufficient tightening during fastening, or if
the pressure is increased in order to guarantee the gas tightness,
galling may occur easily, or the solid lubricant coating may peel
off easily.
[0056] After application, the applied coating is preferably dried
by heating to form a coating having an increased hardness. The
heating temperature is preferably 120.degree. C. or higher and more
preferably from 150.degree. C. to 380.degree. C. The duration of
heating may be determined based on the size of a threaded joint for
steel pipes, and it is preferably at least 20 minutes and more
preferably 30-60 minutes.
[0057] According to another embodiment of the present invention,
this heating for drying an applied coating is performed by at least
two stages. Thus, initially, first stage heating is carried out at
a lower temperature to sufficiently evaporate the solvent and
moisture from the inside of the coating while the coating remains
fluid. Thereafter, second stage heating is conducted in a
temperature range which is higher than that for the first stage
heating to further evaporate the solvent and moisture, thereby
making it possible to form a solid lubricant coating having a high
hardness and a high wear resistance. The solid lubricant coating
exhibits excellent galling resistance even in the environment of
high-temperature oil wells. It also possesses excellent rust
preventing properties.
[0058] Specifically, an applied coating is dried by multistage
heating which includes at least first stage heating in the
temperature range of from 70.degree. C. to 150.degree. C. and
second stage heating in the range of from higher than 150.degree.
C. to 380.degree. C. The heating period (duration of temperature
retention) for each heating stage may be determined depending on
the size of a threaded joint for steel pipes, and it is preferably
at least 20 minutes and more preferably 30-60 minutes.
[0059] The first stage heating which is performed at a temperature
of less than 70.degree. C. is not sufficiently effective for
evaporating the solvent and moisture from the inside of the applied
coating. If it is performed at a temperature of higher than
150.degree. C., the applied coating is solidified while the solvent
and moisture still remain inside, resulting in insufficient
hardening of the coating. Regarding the temperature for the second
stage heating, if it is 150.degree. C. or lower, it is difficult to
completely remove the solvent and moisture from the coating, and if
it is higher than 380.degree. C., an adequate hardness cannot be
obtained in view of the heat resistance of the solid lubricant
coating itself. The temperature range for the first stage heating
is preferably 80.degree. C.-140.degree. C. from the standpoint of
ease of evaporation of the solvent and moisture, and that for the
second stage heating is preferably 180.degree. C. and 350.degree.
C. in view of coating hardness.
[0060] FIGS. 3a and 3b show examples of temperature profiles
(heating patterns) of two stage heating consisting of first and
second stage heating. As shown in FIG. 3a, the first stage heating
may be followed by cooling before the second stage heating is
started, or as shown in FIG. 3b, the first and the second stage
heating may be carried out consecutively.
[0061] Furthermore, the first stage heating and/or the second stage
heating itself may be conducted by multistage heating so that the
entire heating is performed at temperatures of three or more
stages. However, from the viewpoint of economy, two stage heating
consisting of first and second stage heating is preferred.
[0062] In addition, both of the first and the second stage heating,
and particularly the first stage heating need not be performed by
maintaining a constant temperature as shown in the figures, but the
heating may be performed while slowly raising the temperature. In
the latter case, for the first stage heating, if the length of time
required to raise the temperature from 70.degree. C. to 150.degree.
C. is 20 minutes or longer, such heating is regarded as the first
stage heating according to the present invention. In the prior art,
when an applied coating is heated at a temperature of from
150.degree. C. to 300.degree. C., for example, the length of time
required to raise the temperature from 70.degree. C. to 150.degree.
C. is generally at most 5 minutes, and this is clearly different
from the present invention.
[0063] Prior to the application of a coating fluid, it is desirable
to heat (i.e., preheat) the contact surface to be coated (coating
surface) at a temperature of from 50.degree. C. to 200.degree. C.
for the purpose of increasing the adhesion of the resulting solid
lubricant coating. Preheating at a temperature of lower than
50.degree. C. provides little effect on improvement of adhesion. If
the preheating temperature is higher than 200.degree. C., the
applied coating fluid (applied coating) has a decreased viscosity,
thereby making it difficult to form a solid lubricant coating with
a sufficient thickness, and in fact the adhesion of the coating is
decreased. The duration of preheating may be determined in
accordance with the size of the threaded joint for steel pipes, and
it is preferable that the temperature of the coating surface be
maintained in the above-mentioned range throughout the coating
application. However, some effect on improvement of adhesion is
attainable even if the temperature immediately before the onset of
the coating application is in the above-described range without
subsequent temperature retention during the coating
application.
[0064] Both preheating and heating after coating application can be
carried out by a known ordinary method such as furnace heating or
hot air heating. In order to heat a box, it is efficient and
economical to heat it in a heating furnace to maintain the surface
at a predetermined temperature. A pin may be heated by inserting
only the threaded end portion into a heating furnace or by heating
with hot air to maintain the surface at a predetermined
temperature. For the aforementioned multistage heating, since it is
necessary to control the temperature within a certain range,
heating is preferably by furnace heating. The atmosphere in the
furnace is not limited and atmospheric air is sufficient.
[0065] When an applied coating is dried by the aforementioned
multistage heating, a well-hardened solid lubricant coating can be
formed. Preferably, the resulting solid lubricant coating has a
value of hardness in the range of 70-140 expressed as a Rockwell M
scale hardness prescribed by JIS-K7202 (hereunder simply referred
to as a Rockwell M hardness). A coating having a Rockwell M
hardness of less than 70 may cause a rapid increase in the amount
of wear when subjected to sliding friction occurring during
repeated fastening and loosening of the threaded joint, resulting
in insufficient galling resistance. If this hardness of the coating
is greater than 140, wear is too light to provide wear particles to
the contact surface in an amount sufficient to prevent the surface
from galling. In view of galling resistance, the Rockwell M
hardness of the coating is more preferably in the range of
90-140.
[0066] A solid lubricant coating which contains molybdenum
disulfide and/or tungsten disulfide as a lubricating powder and
which has been dried by a conventional drying method of one-stage
heating has a Rockwell M hardness on the order of 50. In accordance
with the present invention, it is possible for a threaded joint for
steel pipes having a solid lubricant coating containing molybdenum
disulfide and/or tungsten disulfide as a lubricating powder to have
a higher coating hardness which is in the range of 70-140 in
Rockwell M hardness.
[0067] It is desired for a solid lubricant coating formed on a
threaded joint for steel pipes to have excellent adhesion. This is
because the coating is subjected to shear stress under a high load
during fastening and loosening of the joint, and if the adhesion is
low, the coating ends up peeling off and failing to exhibit a
sufficient anti-galling effect.
[0068] There are various methods for evaluating the adhesion of a
coating. A simple and well-known method is the so-called grid cut
(adhesive tape peeling) test. However, this method cannot be
employed to test a solid lubricant coating of a threaded joint,
since the adhesion desired therefor is much higher than the level
measurable by the grid-cut test.
[0069] The present inventors found that the adhesion (peeling
resistance) of a solid lubricant coating formed on a threaded joint
can be quantitatively assessed by the adhesive strength measured by
the SAICAS (Surface And Interfacial Cutting Analysis System) method
detailed in the Japanese-language journal, "Toso Gijutsu (Coating
Technique)", April 1995, pp. 123-135 and that when this adhesive
strength of a solid lubricant coating is at least a certain value,
the coating is prevented from peeling off during fastening and
loosening even if it has a high hardness.
[0070] According to the SAICAS method, a sharp cutting edge is
forced against the surface of a coating under a load while the
substrate to which the coating adheres is moved in a horizontal
direction, thereby cutting the coating obliquely from the surface
to the interface with the substrate. After the edge reaches the
interface, the applied load is adjusted so that the cutting edge is
allowed to move horizontally along the interface. The adhesive
strength of the coating can be determined as the peel force per
peel width (width of the cutting edge) (N/m) required to peel the
coating while the edge is moved along the interface. A measuring
device for the SAICAS method is sold on the market by Daipla-Wintes
under the trade name SAICAS.
[0071] In a preferred embodiment of the present invention, a solid
lubricant coating formed on the contact surface of a threaded joint
as a substrate has an adhesive strength of at least 500 N/m as
measured by the SAICAS method. If the adhesive strength of the
coating to the substrate is less than 500 N/m, the coating may not
exhibit a sufficient anti-galling effect.
[0072] A solid lubricant coating which has been dried by multistage
heating according to the aforementioned embodiment of the present
invention tends to exhibit improved adhesive strength compared with
a similar coating which has been dried in a conventional manner.
The adhesive force can be further improved by performing the
above-described surface coarsening and/or preheating of the
substrate, if necessary.
[0073] Although a solid lubricant coating may be applied to the
contact surface of both the pin and the box, the objects of the
present invention can be achieved by applying the coating to only
one of these elements, and this is advantageous in terms of cost.
In such cases, the solid lubricant coating is formed by a
relatively easy operation if it is formed on the contact surface of
a box, which is shorter. The other joint element (preferably a
pin), to which the solid lubricant coating is not applied, may be
uncoated. In particular, when the pin and the box are temporarily
fastened to each other before shipment as shown in FIG. 1, the
other joint element, e.g., the pin, can be prevented from rusting
even if its contact surface is uncoated (e.g., even if it is
as-machined), since the contact surface of the pin is brought into
intimate contact with the coating formed on the contact surface of
the box by the temporary fastening. The solid lubricant coating may
be applied to only a part of the contact surface, particularly only
to the metal contact portion.
[0074] However, when a box is connected to a pin of a steel pipe at
one end of the pipe as shown in FIG. 1, the other pin of the steel
pipe which is located at the opposite end of the pipe and the
unconnected half of the box remain exposed to the atmosphere. These
exposed contact surfaces of the pin and the box may be subjected to
a suitable surface treatment to provide rust prevention with or
without lubricity, and/or may be protected by attachment of a
suitable protector. Such surface treatment may be applied to the
contact surface of the aforementioned other joint element.
[0075] A threaded joint for steel pipes according to the present
invention can be fastened without application of a compound grease,
but an oil may be applied to the solid lubricant coating or the
contact surface of the mating element to be connected, if desired.
In the latter case, the oil which is applied is not restricted, and
any of a mineral oil, a synthetic ester oil, and an animal or
vegetable oil may be used. Various additives such as a
rust-preventing agent and an extreme pressure agent which have
conventionally been used for lubricating oils may be added to the
oil. If such an additive is a liquid, it may be used alone as an
oil to be applied.
[0076] Useful rust-preventing agents include basic metal
sulfonates, basic metal phenates, basic metal carboxylates, and the
like. As an extreme pressure agent, known agents such as sulfur-,
phosphorus-, or chlorine-containing ones and organometal salts may
be used. In addition, other additives such as an anti-oxidant, a
pour point depressant, and a viscosity index improver may be added
to the oil.
[0077] The present invention provides a threaded joint for steel
pipes having a solid lubricant coating on the contact surface
thereof, the coating exhibiting improved galling resistance, gas
tightness, wear resistance, and rust preventing properties. As a
result, the threaded joint can be inhibited from galling during
repeated fastening and loosening without application of a compound
grease. This effect is maintained when the joint is used to dig a
crude oil well in a high-temperature environment such as a deep,
high-temperature oil well or a steam-injection oil well, or it
lasts when the threaded joint is left outdoors for a prolonged
period before the use of the joint at a rig site.
EXAMPLES
[0078] The present invention will be described more fully by the
following examples. These examples are purely for illustrative
purposes and are not intended to restrict the present invention. In
the following description, the contact surface of a pin is referred
to as a pin surface, and the contact surface of a box is referred
to as a box surface.
Examples 1-7 and Comparative Examples 1-4
[0079] The pin surface and the box surface of a threaded joint for
steel pipes [outer diameter: 7 inches (178 mm), wall thickness:
0.408 inches (10.4 mm)] made of a material selected from a carbon
steel A, a Cr-Mo steel B, a 13%-Cr steel C, and a high alloy steel
D each having a composition shown in Table 1 (galling occurring
most easily with D, and galling becoming successively more
difficult with C, B, and A) were subjected to one of the
combinations of surface treatment (surface pretreatment and
optionally formation of a solid lubricant coating) shown in Table 2
as No. 1 to 5, as described below for each example. Table 2 shows
the surface roughness in Rmax (R) of the pretreated surface and the
thickness (t) of a primary coat layer (pretreatment coating), as
well as the thickness of a lubricating coating (t) and the mass
ratio of a lubricating powder to a resin (binder) (M). In these
examples, pretreatment was applied to the contact surface of each
of the pin and the box, but a solid lubricant coating was formed on
one of the pin surface or the box surface. To the pin surface or
the box surface on which a solid lubricant coating was not formed,
a commercially available, ordinary rust-preventing oil which did
not contain heavy metal powder was applied in order to prevent the
surface from rusting. The fastening and loosening test was carried
out without removing the rust-preventing oil.
[0080] The coating fluid which was used to form the solid lubricant
coating was a dispersion in which a lubricating powder was
dispersed in a solution of a resin dissolved in a solvent. The
solvent which was used was a mixed solvent of ethanol/toluene
(50/50) for a polyamideimide resin, N-methyl-2-pyrrolidone/xylene
(65/35) for a phenolic resin, and tetrahydrofuran/cyclohexane
(50/50) for an epoxy resin. Preheating of the substrate prior to
application of the coating fluid and heating for drying after the
application were both carried out in atmospheric air using a
heating furnace. Table 3 shows a number for the type of surface
treatment (in Table 2), preheating temperature of the substrate
(temperature of the substrate before the coating fluid was
applied), and the heating conditions for drying the applied coating
after the fluid was applied (temperature.times.duration of heating
for first stage heating and second stage heating).
[0081] Separately, the same combinations of pretreatment and
formation of a solid lubricant coating as shown in Table 2 were
performed on a steel plate (10 mm.times.50 mm.times.2 mm thick)
having the same composition as the steel pipe used as a substrate.
Thus, the pretreatment which was performed was the same as
performed on the contact surface of the element on which a solid
lubricant coating was formed (i.e., box for No. 1 to 4 and pin for
No. 5 in Table 2). The resulting solid lubricant coating was
measured for adhesive strength and hardness. The adhesive strength
of the coating was measured using a measuring device SAICAS BN-1
manufactured by Daipla-Wintes. The coating hardness was measured in
terms of Rockwell M scale according to JIS-K7202. The results of
these measurements are also shown in Table 3.
[0082] Using a threaded joint which had been subjected to surface
treatment as described above, a test was carried out by repeating
fastening and loosening operations up to 20 times in the manner
shown in Table 4 while examining the occurrence of seizing or
galling. Thus, as shown in Table 4, fastening and loosening were
carried out at ambient temperature for the first to fourth, sixth
to fourteenth, and sixteenth to twentieth runs of operation, and
for the fifth and fifteenth runs, after fastening was carried out,
the threaded joint was subjected to heating for 24 hours at
350.degree. C. and then cooled before loosening was carried out at
ambient temperature. The fastening and loosening conditions
corresponded to the use conditions for a heat-resistant threaded
joint. The fastening speed was 10 rpm and the fastening torque was
10340 ft.multidot.lbs. The results of the occurrence of seizing or
galling are shown in Table 5.
1TABLE 1 Steel (mass %) Type C Si Mn P S Cu Ni Cr Mo A 0.24 0.30
1.30 0.02 0.01 0.04 0.07 0.17 0.04 B 0.25 0.25 0.80 0.02 0.01 0.04
0.05 0.95 0.18 C 0.19 0.25 0.80 0.02 0.01 0.04 0.10 13.0 0.04 D
0.02 0.30 0.50 0.02 0.01 0.50 7.00 25.0 3.20
[0083]
2TABLE 2 No Substrate Pretreatmemt Solid Lubriacating Coating 1 Pin
Grinding (R = 3) None Box Sand blasting (R = 31) PAI.sup.1) resin +
MoS.sub.2 (M = 4.0, t = 30) 2 Pin Grinding (R = 3) None Box 1.
Grinding (R = 4) Epoxy resin + MoS.sub.2 + Graphite 2. Mn
phosphating (t = 15, R = 20) (M = 4.0, t = 28) 3 Pin Grinding (R =
3) None Box 1. Grinding (R = 4) Phenolic resin + WS.sub.2 2. Cu
plating (t = 10, R = 11) (M = 4.0, t = 32) 4 Pin Grinding (R = 4)
None Box 1. Grinding (R = 4) PAI.sup.1) resin + MoS.sub.2 (M = 4.0,
t = 20) 2. Zn--Fe alloy coat (t = 7, R = 18) 5 Pin 1. Grinding (R =
4) PAI.sup.1) resin + MoS.sub.2 (M = 4.0, t = 28) 2. Zn phosphating
(t = 15, R = 20) Box 1. Grinding (R = 4) None 2. Mn phosphating (t
= 10, R = 10) (Notes) .sup.1)PAI resin = Polyamideimide resin; "R"
indicates a surface roughness, Rmax (.mu.m); "t" indicates the
thickness of a coating (.mu.m); and "M" indicates the mass ratio of
lubricating powder to binder.
[0084]
3TABLE 3 Surface Preheated Heating Coating SAICAS Treatment Surface
Conditions Rockwell Adhesive Steel Number in Temp. Temp. .times.
Duration M Scale Strength No. Type Table 2 (.degree. C.) (.degree.
C. .times. minutes) Hardness (N/m).sup.1 EXAMPLE 1 A No. 1 60 1st:
100 .times. 30 125 9820 2nd: 260 .times. 30 2 A No. 1 100 1st: 100
.times. 30 110 12010 2nd: 260 .times. 30 3 B No. 2 130 1st: 100
.times. 30 125 10520 2nd: 230 .times. 30 4 B No. 2 130 1st: 70
.times. 30 100 6840 2nd: 230 .times. 30 5 C No. 3 180 1st: 80
.times. 20 95 2570 2nd: 170 .times. 60 6 D No. 4 100 1st: 80
.times. 30 80 11470 2nd: 170 .times. 40 7 A No. 5 100 1st: 140
.times. 20 130 12470 2nd: 280 .times. 30 COMPARATIVE 1 A No. 2 175
150 .times. 50 60 460 2 A No. 1 180 240 .times. 50 50.about.125
240.about.10500 3 A No. 2 130 1st: 50 .times. 30 65.about.100
320.about.6900 2nd: 230 .times. 30 4 A No. 1 None 1st: 100 .times.
20 60 350 2nd: 410 .times. 30 .sup.1The adhesive strength in
Comparative Examples 2 and 3 fluctuated greatly.
[0085]
4TABLE 4 1st to 4th times fastening and loosening at ambient
temperature 5th time after fastening at ambient temperature,
heating for 24 hours at 350.degree. C., then cooling to ambient
temperature, and loosening 6th to 14th times fastening and
loosening at ambient temperature 15th time after fastening at
ambient temperature, heating for 24 hours at 350.degree. C., then
cooling to ambient temperature, and loosening 16th to 20th
fastening and loosening at ambient temperature times
[0086]
5 TABLE 5 Occurrence of seizing or galling.sup.1) in Example the
fastening run numbered below No..sup.3) 1 2 3 4 5 6 7 8 9 10 11 12
13 14 15 16 17 18 19 20 Ex. 1 .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. Ex. 2 .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. Ex. 3 .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. Ex. 4 .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .DELTA. .DELTA. .DELTA. .DELTA. Ex. 5
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .DELTA. .DELTA. .DELTA. .DELTA. .DELTA.
.DELTA. Ex. 6 .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .DELTA. .DELTA. .DELTA.
.DELTA. .DELTA. .DELTA. Ex. 7 .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. Com Ex. 1 .DELTA. X -- -- -- -- -- --
-- -- -- -- -- -- -- -- -- -- -- -- Com Ex. 2 .largecircle.
.largecircle. .largecircle. .largecircle. .DELTA. X -- -- -- -- --
-- -- -- -- -- -- -- -- -- Com Ex. 3 .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle. .DELTA.
.DELTA. X -- -- -- -- -- -- -- -- -- -- -- Com Ex. 4 .DELTA. X --
-- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- (Notes)
.sup.1).largecircle.: No seizure; .DELTA.: Slight seizure
(repairable); X: Galling (unrepairable); --: Not performed.
Example 1
[0087] A threaded joint made of a carbon steel of type A shown in
Table 1 was subjected to the following surface treatment.
[0088] The box surface was pretreated by blasting with #60 sand so
as to have a surface roughness of 31 .mu.m. Thereafter, the box was
preheated to 60.degree. C., and a solid lubricant coating of a
polyamideimide resin containing a lubricating powder of molybdenum
disulfide and having a thickness of 30 .mu.m was formed on the
contact surface. The solid lubricant coating contained molybdenum
disulfide with a mass ratio of molybdenum disulfide to
polyamideimide resin of 4:1. Drying of the applied coating was
performed by first stage heating for 30 minutes at 100.degree. C.
and, after cooling to ambient temperature, by second stage heating
for 30 minutes at 260.degree. C.
[0089] The pin surface was treated only by machine grinding
(surface roughness: 3 .mu.m).
[0090] In the following examples, the data shown in Table 2 are not
indicated.
Example 2
[0091] The procedure of Example 1 was repeated except that the
temperature at which the box was preheated prior to coating was
changed from 60.degree. C. to 100.degree. C. and the conditions for
heating after the application were changed in such a manner that
the first stage heating for 30 minutes at 100.degree. C. was
directly followed by the second stage heating for 30 minutes at
260.degree. C. without cooling.
Example 3
[0092] A threaded joint made of a Cr--Mo steel of type B shown in
Table 1 was subjected to the following surface treatment.
[0093] The box surface was pretreated, after machine grinding, by
forming a manganese phosphate chemical conversion coating thereon.
Thereafter, the box was preheated to 130.degree. C., and a solid
lubricant coating of an epoxy resin containing a lubricating powder
of a mixture of molybdenum disulfide and graphite (mass ratio=9:1)
was formed on the surface. Drying of the applied coating was
performed by first stage heating for 30 minutes at 100.degree. C.
and, after cooling to ambient temperature, by second stage heating
for 30 minutes at 230.degree. C.
[0094] The pin surface was treated only by machine grinding.
Example 4
[0095] The procedure of Example 3 was repeated except that the
temperature for first stage heating after the coating application
was changed from 100.degree. C. in Example 3 to 70.degree. C.
Example 5
[0096] A threaded joint made of a 13%-Cr steel of type C shown in
Table 1 was subjected to the following surface treatment.
[0097] The box surface was pretreated, after machine grinding, by
electroplating to form a copper coating. Thereafter, the box was
preheated to 180.degree. C., and a solid lubricant coating of a
phenolic resin containing a lubricating powder of tungsten
disulfide was formed on the box surface. Drying of the applied
coating was performed by first stage heating for 20 minutes at
80.degree. C. and, after cooling to ambient temperature, by second
stage heating for 60 minutes at 170.degree. C.
[0098] The pin surface was treated only by machine grinding.
Example 6
[0099] A threaded joint made of a high alloy steel of type D shown
in Table 1 was subjected to the following surface treatment.
[0100] The box surface was pretreated, after machine grinding, by
blast plating to form a zinc-iron alloy coating. Thereafter, the
box was preheated to 100.degree. C., and a solid lubricant coating
of a polyamideimide resin containing a lubricating powder of
molybdenum disulfide was formed on the box surface. Drying of the
applied coating was performed by first stage heating for 30 minutes
at 80.degree. C. and, after cooling to ambient-temperature, by
second stage heating for 40 minutes at 170.degree. C.
[0101] The pin surface was treated only by machine grinding.
Example 7
[0102] A threaded joint made of a carbon steel of type A shown in
Table 1 was subjected to the following surface treatment.
[0103] The box surface was subjected to only pretreatment which was
performed by machine grinding and then by forming a manganese
phosphate chemical conversion coating thereon.
[0104] The pin surface was pretreated, after machine grinding, by
forming a zinc phosphate chemical conversion coating thereon.
Thereafter, only the pin portion was placed in a heating furnace to
preheat it to 100.degree. C., and a solid lubricant coating of a
polyamideimide resin containing a lubricating powder of molybdenum
disulfide was formed on the pin surface. Drying of the applied
coating was performed by first stage heating for 20 minutes at
140.degree. C. and, after cooling to ambient temperature, by second
stage heating for 30 minutes at 280.degree. C. while only the pin
portion was placed in a heating furnace during heating.
[0105] As can be seen from Table 3, the solid lubricant coating
formed in each of Examples 1 to 7 was hardened and had a Rockwell M
hardness of at least 80. It also had a satisfactory adhesive
strength of at least 2500 N/m as measured by the SAICAS method.
Comparison between Examples 1 and 2 shows that a higher preheating
temperature in Example 2 resulted in a slightly decreased coating
hardness but an improved adhesive strength. Comparison between
Examples 3 and 4 shows that a higher temperature for the first
stage heating in Example 3 resulted in a higher value in both
coating hardness and adhesive strength due to more complete drying
of the coating.
[0106] Table 5 shows that in some of the threaded joints of
Examples 1-7, slight seizing occurred in the 15th and later runs of
a repeated fastening and loosening test which simulated a
high-temperature oil well, but even in such cases, fastening and
loosening could be repeated 20 times by surface dressing in all the
examples with no problems with respect to gas tightness. The
occurrence of slight seizing in Examples 5 and 6 was due to the
steel material of the threaded joint, which is susceptible to
seizing and galling. If the same solid lubricant coating as in
Example 5 or 6 were formed on a threaded joint of steel type A or
B, it is presumed that no seizing would occur. In Example 4, since
the temperature for first stage heating was lower as set forth
above, the resulting coating had a slightly low hardness, and
therefore slight seizing occurred in the 17th and later runs.
Comparative Example 1
[0107] A threaded joint made of a carbon steel of type A shown in
Table 1 was subjected to the following surface treatment.
[0108] The box surface was pretreated, after machine grinding, by
forming a manganese phosphate chemical conversion coating thereon.
Thereafter, the box was preheated to 175.degree. C., and a solid
lubricant coating of an epoxy resin containing a lubricating powder
of a mixture of molybdenum disulfide and graphite (mass ratio=9:1)
was formed on the box surface. Drying of the applied coating was
performed by one stage heating for 50 minutes at 150.degree. C.
[0109] The pin surface was treated only by machine grinding.
[0110] As shown in Table 5, in the fastening and loosening test,
slight seizing occurred in the first run. Fastening and loosening
for the second run proceeded after surface dressing, but galling
(severe seizing) occurred to such an extent that loosening became
impossible, so the test was terminated.
[0111] This example corresponds to a case in which only the first
stage heating in the present invention was performed for drying. In
this case, although the solvent and moisture evaporated to some
extent from the inside of the coating, evaporation was not complete
since second stage heating was not performed, and the resulting
coating had a low hardness. In addition, although preheating was
performed, the adhesive strength was also insufficient. Thus, the
insufficient hardness and adhesive strength of the solid lubricant
coating seemed to be responsible for the premature occurrence of
galling.
Comparative Example 2
[0112] The procedure of Example 1 was repeated except that the
preheating temperature was raised to 180.degree. C. and the coating
application was followed by heating which was performed by one
stage heating for 50 minutes at 240.degree. C.
[0113] As shown in Table 5, in the fastening and loosening test,
slight seizing occurred in the fifth run. Fastening and loosening
for the sixth run proceeded after surface dressing, but galling
occurred in the sixth run, so the test was terminated.
[0114] This example illustrates a conventional heating method and
corresponds to a case in which only the second stage heating in the
present invention was performed. In this case, since first stage
heating at a lower temperature was not performed, the wet coating
solidified rapidly, and the solvent and moisture were confined
within the coating, thereby causing a great fluctuation in the
hardness and adhesive strength of the resulting solid lubricant
coating. As a result, it is thought that galling occurred
readily.
Comparative Example 3
[0115] The procedure of Comparative Example 1 was repeated except
that the preheating temperature was lowered to 130.degree. C. and
the coating application was followed by heating which was performed
by first stage heating for 30 minutes at 50.degree. C. and, after
cooling to ambient temperature, by second stage heating for 30
minutes at 230.degree. C.
[0116] As shown in Table 5, in the fastening and loosening test,
slight seizing occurred in the seventh run. Fastening and loosening
for the sixth run proceeded after surface dressing, but galling
occurred in the ninth run, so the test was terminated. Since the
temperature for the first stage heating was too low, it is presumed
that evaporation of the solvent and moisture from the inside of the
coating which was solidifying became insufficient, and as in the
case of Comparative Example 2 which corresponds to a conventional
heating method, the hardness and adhesive strength of the resulting
solid lubricant coating fluctuated locally, thereby causing
galling.
Comparative Example 4
[0117] The procedure of Example 1 was repeated except that the
pretreated box was not preheated and the coating application was
followed by heating which was performed by first stage heating for
20 minutes at 100.degree. C. and, after cooling to ambient
temperature, by second stage heating for 30 minutes at 410.degree.
C.
[0118] As shown in Table 5, in the fastening and loosening test,
slight seizing occurred in the first run. Fastening and loosening
for the second run proceeded after surface dressing, but galling
occurred to such an extent that loosening became impossible, so the
test was terminated.
[0119] This result seemed to be due to the temperature for the
second stage heating, which was too high, whereby evaporation of
the solvent and moisture from the inside of the solid lubricant
coating became insufficient and the solid lubricant coating itself
became soft and peeled off quickly during fastening in the first
run. cl Examples 8-15 and Comparative Examples 5-6
[0120] The pin surface and the box surface of a threaded joint for
steel pipes [outer diameter: 7 inches (178 mm), wall thickness:
0.408 inches (10.4 mm)] made of a material selected from a carbon
steel A, a Cr--Mo steel B, a 13%-Cr steel C, and a high alloy steel
D each having a composition shown in Table 1 were subjected to one
of the combinations of surface treatment (surface pretreatment and
formation of a solid lubricant coating) shown in Table 6. The
details of the surface treatment will be described below for each
example.
[0121] As described later, in these examples and comparative
examples, a solid lubricant coating was formed only on the box
surface, while the pin surface was either in an as-machined state
or thereafter coated only with a primary coat layer. A commercially
available, ordinary rust-preventing oil which did not contain heavy
metal powder was applied to the pin surface in order to prevent the
surface from rusting. It should be understood by those skilled in
the art that if the solid lubricant coating were formed only on the
pin surface, the same results would be obtained.
[0122] Table 6 shows the data on the pretreatment, i.e., the
surface roughness in Rmax (R) of the substrate steel and the
thickness of a primary coat layer (t), for each of the pin and the
box, as well as the constitution of a solid lubricant coating,
i.e., the particular resin (binder), lubricating powder, and
ultraviolet screening fine particles which were used, the mass
ratio of lubricating powder to resin (M) and the mass ratio (mass
parts) of ultraviolet screening fine particles to 100 parts of
resin (U) in the solid lubricant coating, the mean particle
diameter of the ultraviolet screening fine particles (P), and the
thickness of the solid lubricant coating (t).
[0123] The lubricating powder which was used had the following mean
particle diameter:
6 Molybdenum disulfide powder (MoS.sub.2): 15 .mu.m Tungsten
disulfide powder (WS.sub.2): 4 .mu.m Graphite powder: 1 .mu.m Boron
nitride powder (BN): 2 .mu.m PTFE powder: 0.8 .mu.m.
[0124] Using a threaded joint which was treated as described above
in which a solid lubricant coating was formed on the pretreated box
surface and an oil was applied to the pretreated pin surface, the
pin and the box were connected without applying a tightening force
(torque), and the joint was subjected to an outdoor exposure test
(average temperature: 28-33.degree. C. and average relative
humidity: 60-70%) for 3 months while the pin and the box were
connected as above. After 3 months, the pin and the box were
unfastened, and the box was examined for cracking of the solid
lubricant coating formed on the box surface and for rusting of the
box surface.
[0125] Furthermore, using the threaded joint which had been
subjected to the outdoor exposure test, fastening and loosening
were repeated up to 20 times at ambient temperature, without
removing the oil applied to the pin surface, to examine for the
occurrence of seizing or galling. This test was carried out with a
fastening speed of 10 rpm and a fastening torque of 10340 ft-lbs.
Table 7 show the results of occurrence of seizing or galling (in
the sixth and later runs) and of cracking of the coating and
rusting of the contact surface.
7 TABLE 6 Pin Box TYPE No. ST.sup.1 Pretreatment Coating
Pretreatment Solid Lubricating Coating EXAMPLES 8 A Grinding, R = 2
Oil.sup.3 Sand blasting, PAI resin.sup.3, t = 28 R = 15 MoS.sub.2
(M = 3.8) TiO.sub.2 (U = 10.2, P = 0.03) 9 A 1. Grinding, R = 3
Oil.sup.3 1. Grinding, R = 3 PAI resin, t = 27 2. Zn phosphating,
2. Mn phosphating, MoS.sub.2 (M = 3.8) t = 15 t = 22 ZnO (U = 10.2,
P = 0.02) 10 B Grinding, R = 2 Oil.sup.3 1. Grinding, R = 3 Epoxy
resin, t = 22 2. Mn phosphating, WS.sub.2 (M = 1.0) t = 24
Fe.sub.3O.sub.4 (U = 5.1, P = 0.05) 11 C Grinding, R = 2 Oil.sup.3
1. Grinding, R = 3 Phenolic resin, t = 28 2. Cu plating, MoS.sub.2
+ graphite (M = 4.0) t = 6 TiO.sub.2 (U = 25.4, P = 0.01) 12 D 1.
Grinding, R = 3 Oil.sup.3 1. Grinding, R = 3 PAI resin, t = 28 2.
Zn-Fe alloy 2. Zn-Fe alloy BN (M = 4.5) coating, t = 6 coating, t =
7 ZnO (U = 47.5, P = 0.005) 13 A 1. Grinding, R = 3 Oil.sup.3
Grinding, R = 3 PAI resin, t = 25 2. Zn phosphating, MoS.sub.2 +
PTFE (M = 1.0) t = 15 TiO.sub.2(U = 60.9, P = 0.03) 14 A 1.
Grinding, R = 3 Oil.sup.3 1. Grinding, R = 3 PAI resin, t = 28 2.
Zn phosphating, 2. Mn phosphating, MoS.sub.2 (M = 3.8) t = 15 t =
21 TiO.sub.2 (P = 0.03) + ZnO (P = 0.015) (U = 0.08) 15 A 1.
Grinding, R = 3 Oil.sup.3 1. Grinding, R = 3 PAI resin, t = 28 2.
Zn phosphating, 2. Mn phosphating, MoS.sub.2 (M = 3.8) t = 15 t =
19 BaSO.sub.4 (U = 10.2, P = 1.00) .sup.2COMP 5 A Grinding, R = 3
Oil.sup.3 1. Grinding, R = 3 PAI resin, t = 30 2. Mn phosphating,
MoS.sub.2 (M = 4.0) t = 18 6 A Grinding, R = 3 Oil.sup.3 1.
Grinding, R = 3 PAI resin, t = 28 2. Mn phosphating, TiO.sub.2 (U =
1.0, P = 0.03) t = 18 (Notes) .sup.1ST: Steel Type; .sup.2COMP =
Comparative Examples; .sup.3Oil: Commercially available rust
preventing oil; .sup.4PAI resin = Polyamideimide resin; "R"
indicates a surface roughness, Rmax (.mu.m); "t" indicates the
thickness of a coating (.mu.m); "M" indicates the mass ratio of
lubricating powder to binder; "U" indicates the mass ratio of UV
screening fine particles relative to 100 parts of resin; and "P"
indicates the mean average particle diameter of UV screening fine
particles.
[0126]
8 TABLE 7 Occurrence of seizing/galling.sup.1) in Example the run
numbered below Occurence.sup.2) No..sup.3) 6 7 8 9 10 11 12 13 14
15 16 17 18 19 20 Cracks Rust Ex. 8 .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. Ex. 9 .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle. Ex. 10
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. Ex. 11 .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. Ex. 12 .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. Ex. 13 .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .DELTA. .DELTA. .DELTA.
.largecircle. .largecircle. Ex. 14 .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .DELTA. .DELTA. .DELTA. .DELTA.
.largecircle. Ex. 15 .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .DELTA. .DELTA. .DELTA. X
-- .DELTA. .largecircle. Com Ex. 5 .largecircle. .DELTA. .DELTA. X
-- -- -- -- -- -- -- -- -- -- -- X X Com Ex. 6 X in the 1st run. --
-- -- -- -- -- -- -- -- -- .largecircle. .largecircle.
.sup.1).largecircle.: No seizing or galling; .DELTA.: Slight
seizure (repairable); X: Galling (unrepairable); --: Not performed.
.sup.2)Occurrence of cracks: .largecircle.: No cracks; .DELTA.:
Slight cracks; X: Remarkable cracks Occurrence of rust:
.largecircle.: No rust; .DELTA.: Slight rust but unproblematic; X:
Remarkable rust (problematic). .sup.3)Ex: Example; Com Ex.:
Comparative Example
Example 8
[0127] A threaded joint made of a carbon steel having composition A
was subjected to the following surface treatment.
[0128] The box surface was pretreated by sand blasting with #80
sand to have a surface roughness of 15 .mu.m. A solid lubricant
coating of a polyamideimide resin containing a lubricating powder
of molybdenum disulfide and ultraviolet screening fine particles of
titanium oxide having a mean particle diameter of 0.03 .mu.m was
formed on the box surface. The solid lubricant coating had a
thickness of 28 .mu.m, and it contained the lubricating powder with
a mass ratio of 3.8 relative to the resin and the ultraviolet
screening fine particles with a mass ratio of 10.2 relative to 100
parts of the resin. The applied coating was dried by heating for 30
minutes at 260.degree. C. to make the resulting coating hard.
[0129] The pin surface was in an as-machined state produced by
grinding (surface roughness: 2 .mu.m).
[0130] In the following examples, the data shown in Table 6 are not
indicated.
Example 9
[0131] A threaded joint made of a carbon steel having composition A
was subjected to the following surface treatment.
[0132] The box surface was pretreated, after machine grinding, by
forming a manganese phosphate chemical conversion coating on that
surface. A solid lubricant coating of a polyamideimide resin
containing a lubricating powder of molybdenum disulfide and
ultraviolet screening fine particles of zinc oxide was formed on
the primary coat layer in the same manner as in Example 8.
[0133] The pin surface was pretreated, after machine grinding, by
forming a zinc phosphate chemical conversion coating on that
surface.
Example 10
[0134] A threaded joint made of a Cr--Mo steel having composition B
was subjected to the following surface treatment.
[0135] The box surface was pretreated, after machine grinding, by
forming a manganese phosphate chemical conversion coating on that
surface. A solid lubricant coating of an epoxy resin containing a
lubricating powder of tungsten disulfide and ultraviolet screening
fine particles of iron oxide was formed on the primary coat layer
in the same manner as in Example 8 except that the heating
temperature was changed to 230.degree. C.
[0136] The pin surface was in an as-machined state produced by
grinding.
Example 11
[0137] A threaded joint made of a 13%-Cr steel having composition C
was subjected to the following surface treatment.
[0138] The box surface was pretreated, after machine grinding, by
electroplating to form a copper coating. A solid lubricant coating
of a phenolic resin containing a lubricating powder of a mixture of
molybdenum disulfide and graphite and ultraviolet screening fine
particles of titanium oxide was formed on the primary coat layer in
the same manner as in Example 8 except that the heating temperature
was changed to 170.degree. C.
[0139] The pin surface was in an as-machined state produced by
grinding.
Example 12
[0140] A threaded joint made of a high alloy steel having
composition D was subjected to the following surface treatment.
[0141] The box surface was pretreated, after machine grinding, by
blast plating to form a zinc-iron alloy coating. A solid lubricant
coating of a polyamideimide resin containing a lubricating powder
of boron nitride and ultraviolet screening fine particles of zinc
oxide was formed on the primary coat layer in the same manner as in
Example 8.
[0142] The pin surface was pretreated, after machine grinding, by
blast plating to form a zinc-iron alloy coating on that
surface.
Example 13
[0143] A threaded joint made of a carbon steel having composition A
was subjected to the following surface treatment.
[0144] After the box surface was pretreated by machine grinding, a
solid lubricant coating of a polyamideimide resin containing a
lubricating powder of a mixture of molybdenum disulfide and PTFE
and ultraviolet screening fine particles of titanium oxide was
formed on the surface in the same manner as in Example 8.
[0145] The pin surface was pretreated, after machine grinding, by
forming a zinc phosphate chemical conversion coating on that
surface.
Example 14
[0146] A threaded joint made of a carbon steel having composition A
was subjected to the following surface treatment.
[0147] The box surface was pretreated, after machine grinding, by
forming a manganese phosphate chemical conversion coating on that
surface. A solid lubricant coating of a polyamideimide resin
containing a lubricating powder of molybdenum disulfide and
ultraviolet screening fine particles of a mixture of titanium oxide
and zinc oxide was formed on the primary coat layer in the same
manner as in Example 8.
[0148] The pin surface was pretreated, after machine grinding, by
forming a zinc phosphate chemical conversion coating on that
surface.
Example 15
[0149] A threaded joint made of a carbon steel having composition A
was subjected to the following surface treatment.
[0150] The box surface was pretreated, after machine grinding, by
forming a manganese phosphate chemical conversion coating on that
surface. A solid lubricant coating of a polyamideimide resin
containing a lubricating powder of molybdenum disulfide and
ultraviolet screening fine particles of barium sulfate was formed
on the primary coat layer in the same manner as in Example 8.
[0151] The pin surface was pretreated, after machine grinding, by
forming a zinc phosphate chemical conversion coating on that
surface.
[0152] As shown in Table 7, in the outdoor exposure test, slight
cracks were observed on the solid lubricant coating formed on the
box surface in Examples 14 and 15. However, in all of Examples 8 to
15 including those examples, no rust was found. Accordingly, it is
concluded that the rust preventing properties of a solid lubricant
coating can be guaranteed by addition of ultraviolet screening fine
particles.
[0153] In the fastening and loosening test, there was no occurrence
of galling in Examples 8 to 12 while fastening and loosening were
repeated 20 times, and gas tightness was maintained throughout. In
Examples 13 and 14 in which the amount of ultraviolet screening
fine particles was larger or smaller, slight seizing occurred in
the 18th and later runs, but fastening and loosening could be
continued up to the 20th run by performing surface dressing. In
Example 15, slight seizing occurred in the 16th run, and fastening
and loosening could be continued up to the 18th run. However, in
the 19th run, galling occurred, and the test was terminated. This
result seems to be because barium sulfate, which has a relatively
low ultraviolet screening effect, was used for the ultraviolet
screening fine particles, and its mean particle diameter was
relatively coarse (1 .mu.m). However, the galling resistance in
that example is still considered to be improved, when compared to
the result in Comparative Example 5 described below.
Comparative Example 5
[0154] A threaded joint made of a carbon steel having composition A
was subjected to the following surface treatment.
[0155] The box surface was pretreated, after machine grinding, by
forming a manganese phosphate chemical conversion coating on that
surface. A solid lubricant coating of a polyamideimide resin
containing a lubricating powder of molybdenum disulfide with no
ultraviolet screening fine particles was formed on the primary coat
layer in the same manner as in Example 8.
[0156] The pin surface was in an as-machined state produced by
grinding.
[0157] As shown in Table 7, in the outdoor exposure test, severe
cracks were observed on the solid lubricant coating formed on the
box surface. Since the cracks reached the substrate, the surface
was rusted noticeably. In the fastening and loosening test, slight
seizing occurred in the 7th and later runs. Fastening and loosening
could be continued up to the 8th run by performing surface
dressing, but galling occurred in the 9th run, so the test was
terminated.
Comparative Example 6
[0158] A threaded joint made of a carbon steel having composition A
was subjected to the following surface treatment.
[0159] The box surface was pretreated, after machine grinding, by
forming a manganese phosphate chemical conversion coating on that
surface. A solid lubricant coating of a polyamideimide resin
containing ultraviolet screening fine particles of titanium oxide
was formed on the primary coat layer in the same manner as in
Example 8.
[0160] The pin surface was in an as-machined state produced by
grinding.
[0161] As shown in Table 7, in the outdoor exposure test, no cracks
were observed on the solid lubricant coating formed on the box
surface. No rusting was noted, either. However, in the fastening
and loosening test, galling occurred in the first run, and the test
was terminated. The absence of a lubricating powder seems to be
responsible for the insufficient galling resistance.
* * * * *