Stock material or miscellaneous articles – Hollow or container type article – Polymer or resin containing
Reexamination Certificate
2003-02-11
2004-12-07
Nolan, Sandra M. (Department: 1772)
Stock material or miscellaneous articles
Hollow or container type article
Polymer or resin containing
C428S036910, C428S457000, C428S469000, C428S418000, C285S094000, C285S333000, C285S390000, C285S334000
Reexamination Certificate
active
06827996
ABSTRACT:
TECHNICAL FIELD
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
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.
As schematically shown in
FIG. 1
, a male thread
3
A is normally formed on the outer surface at both ends of a steel pipe A to form a pin
1
, and a female thread
3
B 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.
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.
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.
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.
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.
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 &mgr;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° C.
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.
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.
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).
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° 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° 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° 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.
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.
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
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:
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
drying the applied coating by multistage heating which includes at least first stage heating in the temperature range of from 70° C. to 150° C. and second stage heating in the range of from higher than 150° C. to 380° C. to form a solid lubricant coating on the contact surface.
The process may further include, prior to the coating application step, a step of heating the contact surface t
Bruenjes Chris
Clark & Brody
Nolan Sandra M.
Sumitomo Metal Industries Ltd.
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