JP2018123831A - Screw joint for pipe and manufacturing method of screw joint for pipe - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a screw joint for pipe excellent in seizure resistance and corrosion resistance, and a manufacturing method of the screw joint for pipe.SOLUTION: A screw joint 1 for pipe includes a pin 4, a box 5 and an alloy plating layer 20. Each of the pin 4 and the box 5 includes a contact surface 40, 50 having a screw part 41, 51 and a no-thread metal contact part 42, 52. The alloy plating layer 20 comprises Zn-Co alloy. The alloy plating layer 20 is arranged on the contact surface 40, 50 of at least one of the pin 4 and box 5.SELECTED DRAWING: Figure 3

Description

  The present invention relates to a threaded joint for pipes and a method for producing a threaded joint for pipes, and more particularly relates to a threaded joint for oil well pipes and a method for producing a threaded joint for oil well pipes.

  Oil well pipes are used to mine oil and natural gas fields. The oil well pipe is formed by connecting a plurality of steel pipes according to the depth of the well. The steel pipes are connected by screwing pipe threaded joints formed at the ends of the steel pipes. The oil well pipe is pulled up for inspection or the like, unscrewed, inspected, screwed again, and used again.

  The threaded joint for pipes includes a pin and a box. The pin includes a male screw portion and an unthreaded metal contact portion formed on the outer peripheral surface of the tip portion of the steel pipe. The box includes an internal thread portion and an unthreaded metal contact portion formed on the inner peripheral surface of the distal end portion of the steel pipe. The threaded part of the pin and the box and the unthreaded metal contact part are repeatedly subjected to strong friction when the steel pipe is screwed and unscrewed. If these parts do not have sufficient durability against friction, goling (seizure that cannot be repaired) occurs when screwing and unscrewing are repeated. Therefore, the threaded joint for pipes is required to have sufficient durability against friction, that is, excellent seizure resistance.

  Conventionally, in order to improve seizure resistance, a compound grease containing a heavy metal called a dope has been used. By applying compound grease to the surface of the threaded joint for pipes, the seizure resistance of the threaded joint for pipes can be improved. However, heavy metals such as Pb, Zn and Cu contained in the compound grease may affect the environment. For this reason, development of the threaded joint for pipes which does not use compound grease is desired.

  International Publication No. 2009/072486 (Patent Document 1) proposes a threaded joint for pipes that is excellent in seizure resistance even without compound grease.

  The threaded joint for pipes described in Patent Document 1 includes a pin and a box each having a contact surface having a threaded portion and an unthreaded metal contact portion. The contact surface of the box has as a top layer a solid lubricating coating having a plastic or viscoplastic rheological behavior. The contact surface of the pin has a solid anticorrosive coating composed mainly of an ultraviolet curable resin as the uppermost layer. This eliminates the use of compound grease, suppresses the generation of rust, exhibits excellent seizure resistance and airtightness, and provides a threaded joint for pipes that has no stickiness on the surface and excellent appearance and inspection. And Patent Document 1.

  In order to suppress seizure of the threaded joint for pipes, it is effective to form a plating layer containing a metal having high hardness and melting point. Therefore, conventionally, copper (Cu) plating or Cu alloy plating has been used. The hardness and melting point of Cu are high. Therefore, when Cu is contained in the plating layer, the hardness and melting point of the entire plating layer are increased. Therefore, the seizure resistance of the pipe threaded joint is increased.

  Techniques for improving the seizure resistance of a threaded joint for pipes by Cu alloy plating are described in Japanese Patent Application Laid-Open No. 2003-074763 (Patent Document 2) and Japanese Patent Application Laid-Open No. 2008-215473 (Patent Document 3).

  The threaded joint for pipes described in Patent Document 2 includes a pin portion and a coupling. The coupling is a steel pipe containing 9% by mass or more of Cr. At both ends of the coupling, a box portion having a female screw and a metal-metal seal portion is provided. A Cu—Sn alloy layer is disposed on the surface of the female screw and the metal-metal seal portion of the coupling. Patent Document 2 describes that when this threaded joint for pipes is used, even if green dope (lubricant containing no Pb) is used, sealing performance is better than before and goling can be remarkably suppressed. Yes.

  The threaded joint for pipes described in Patent Document 3 includes a pin and a box. At least one contact surface of the pin and the box has a first plating layer made of a Cu—Zn alloy. As a result, even when green dope is applied, and even when it is not doped, it exhibits sufficient leakage resistance and seizure resistance, and also has excellent corrosion resistance, preventing the occurrence of crevice corrosion. Document 3 describes this.

  On the other hand, after the oil well pipe is manufactured, it is transported by a ship or the like and stored for a certain period until it is used. The transportation and storage of oil well pipes may take a long time. In addition, oil wells may be stored outdoors. When stored outdoors for a long period of time, rust is generated in the threaded joint for oil well pipes, and the airtightness and seizure resistance of the threaded joints for oil well pipes may be reduced. Therefore, an oil well pipe threaded joint is required to have excellent corrosion resistance in addition to the above-mentioned seizure resistance.

International Publication No. 2009/072486 JP 2003-074763 A JP 2008-215473 A

  Even if the techniques disclosed in Patent Documents 1 to 3 described above are used, the seizure resistance and corrosion resistance of the threaded joint for pipes may be low.

  An object of the present invention is to provide a threaded joint for pipes excellent in seizure resistance and corrosion resistance and a method for producing the same.

  The threaded joint for pipes of this embodiment includes a pin and a box, and an alloy plating layer. Each pin and box includes a contact surface having a threaded portion and an unthreaded metal contact portion. The alloy plating layer is made of a Zn-Co alloy. The alloy plating layer is disposed on at least one contact surface of the pin and the box.

  The manufacturing method of the threaded joint for pipes of this embodiment is a manufacturing method of the threaded joint for pipes provided with a pin and a box. Each pin and box includes a contact surface having a threaded portion and an unthreaded metal contact portion. The manufacturing method of this embodiment includes a preparation step and an alloy plating layer forming step. In the preparation step, a plating bath containing zinc ions and cobalt ions is prepared. In the alloy plating layer forming step, an alloy plating layer made of a Zn—Co alloy is formed on the contact surface of at least one of the pin and the box using a plating bath.

  The threaded joint for pipes of this embodiment is excellent in seizure resistance and corrosion resistance.

FIG. 1 is a diagram showing a configuration of a threaded joint for pipes according to the present embodiment. FIG. 2 is a cross-sectional view of the threaded joint for pipes according to the present embodiment. FIG. 3 is a cross-sectional view of the contact surface of the threaded joint for pipes according to the present embodiment. FIG. 4 is a cross-sectional view of a contact surface of a threaded joint for pipes according to another embodiment different from FIG.

  Hereinafter, this embodiment will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals and description thereof will not be repeated.

  The present inventors have made various studies on the seizure resistance and corrosion resistance of the threaded joint for pipes. As a result, the following knowledge was obtained.

  In order to improve the seizure resistance of the threaded joint for pipes, it is effective to form a plating layer having a high hardness and a high melting point on a threaded portion and a non-threaded metal contact portion (hereinafter referred to as a contact surface). If the hardness of the plating layer is high, the plating layer is less likely to be damaged when the screw joint for pipes is tightened and unscrewed. Furthermore, if the melting point of the plating layer is high, the plating layer is unlikely to melt even when the temperature is locally high during screw tightening and unscrewing of the pipe threaded joint.

  Therefore, in this embodiment, an alloy plating layer made of a Zn—Co alloy is formed on the contact surface. The hardness and melting point of the Zn—Co alloy are high. Therefore, the seizure resistance of the pipe threaded joint can be improved. In the case of a pure metal, zinc (Zn) has lower hardness and melting point than copper (Cu) conventionally used for plating. However, if it is a Zn-Co alloy, the plating layer has a sufficiently high hardness and a high melting point, and can enhance the seizure resistance.

  If a Zn-Co alloy is used, the corrosion resistance of the threaded joint for pipes can be further increased. Zinc (Zn) is a base metal compared to iron (Fe), nickel (Ni) and chromium (Cr). Therefore, if a plating layer containing zinc (Zn) is formed on the contact surface, the plating layer is preferentially corroded over the steel material (sacrificial corrosion protection). Thereby, the corrosion resistance of the threaded joint for pipes is increased.

  The threaded joint for pipes of this embodiment completed based on the above knowledge is provided with a pin and a box, and an alloy plating layer. Each pin and box includes a contact surface having a threaded portion and an unthreaded metal contact portion. The alloy plating layer is made of a Zn-Co alloy. The alloy plating layer is disposed on at least one contact surface of the pin and the box.

  The threaded joint for pipes of this embodiment includes an alloy plating layer made of a Zn—Co alloy. Therefore, it is excellent in seizure resistance and corrosion resistance.

  Preferably, the alloy plating layer contains more than 5 to 25% Co by mass%.

  In this case, the seizure resistance and corrosion resistance of the pipe threaded joint are further increased.

  Preferably, the thickness of the alloy plating layer is 1 to 20 μm.

  Preferably, the threaded joint for pipes further includes a solid lubricating coating on the alloy plating layer.

  In this case, the lubricity of the threaded joint for pipes is increased.

  The manufacturing method of the threaded joint for pipes of this embodiment is a manufacturing method of the threaded joint for pipes provided with a pin and a box. Each pin and box includes a contact surface having a threaded portion and an unthreaded metal contact portion. The manufacturing method of this embodiment includes a preparation step and an alloy plating layer forming step. In the preparation step, a plating bath containing zinc ions and cobalt ions is prepared. In the alloy plating layer forming step, an alloy plating layer made of a Zn—Co alloy is formed on the contact surface of at least one of the pin and the box using a plating bath.

  Preferably, the manufacturing method further includes a step of forming a solid lubricating film on the alloy plating layer.

  Hereinafter, the threaded joint for pipes and the manufacturing method thereof according to the present embodiment will be described in detail.

[Threaded joint 1 for pipes]
The threaded joint for pipes includes a pin and a box. FIG. 1 is a diagram showing a configuration of a threaded joint 1 for a pipe according to the present embodiment. With reference to FIG. 1, the threaded joint 1 for a pipe includes a steel pipe 2 and a coupling 3. At both ends of the steel pipe 2, pins 4 having male screw portions on the outer surface are formed. Boxes 5 having internal thread portions on the inner surface are formed at both ends of the coupling 3. The coupling 3 is attached to the end of the steel pipe 2 by screwing the pin 4 and the box 5 together. On the other hand, there is also an integral type threaded joint for pipes in which one end of the steel pipe 2 is a pin 4 and the other end is a box 5 without using the coupling 3. The threaded joint for pipes 1 of this embodiment can be used for both threaded couplings for pipes of the coupling system and the integral system.

  Pin 4 and box 5 have contact surfaces with threaded and unthreaded metal contacts. FIG. 2 is a cross-sectional view of the threaded joint 1 for a pipe according to the present embodiment. Referring to FIG. 2, pin 4 and box 5 include contact surfaces 40 and 50, respectively. The contact surface 40 of the pin 4 comprises a male thread part 41 and an unthreaded metal contact part 42. The unthreaded metal contact portion 42 includes a metal seal portion 43 and a shoulder portion 44. The contact surface 50 of the box 5 comprises an internal thread 51 and an unthreaded metal contact 52. The unthreaded metal contact portion 52 includes a metal seal portion 53 and a shoulder portion 54.

  In FIG. 2, in the pin 4, the shoulder portion 44, the metal seal portion 43, and the male screw portion 41 are arranged in order of the shoulder portion 44, the metal seal portion 43, and the male screw portion 41 from the tip of the pin 4. In the box 5, the shoulder portion 54, the metal seal portion 53, and the female screw portion 51 are arranged from the tip of the box 5 in the order of the female screw portion 51, the metal seal portion 53, and the shoulder portion 54. However, the shape of the contact surfaces 40 and 50 is not limited to the shape described in FIG. For example, contact surfaces 40 and 50 may not include shoulder portions 44 and 54. In this case, the metal seal portion 43 and the female screw portion 41 of the pin 4 are arranged in the order of the metal seal portion 43 and the male screw portion 41 from the tip of the pin 4. The metal seal portion 53 and the female screw portion 51 of the box 5 are arranged in the order of the female screw portion 51 and the metal seal portion 53 from the tip of the box 5.

  FIG. 3 is a cross-sectional view of the contact surfaces 40 and 50 of the pipe threaded joint 1 according to the present embodiment. Referring to FIG. 3, the threaded joint for pipes 1 includes an alloy plating layer 20 on at least one contact surface 40 and 50 of the pin 4 and the box 5.

[Alloy plating layer 20]
The pipe threaded joint 1 includes an alloy plating layer 20 made of a Zn—Co alloy on at least one of the contact surfaces 40 and 50 of the pin 4 and the box 5. The alloy plating layer 20 has a high hardness and melting point. Therefore, the seizure resistance of the pipe threaded joint 1 is increased. Zn contained in the alloy plating layer 20 is a base metal. Therefore, the corrosion resistance of the pipe threaded joint is increased.

  The alloy plating layer 20 is made of a Zn—Co alloy. The Zn—Co alloy contains Zn and Co, and the balance consists of impurities. Preferably, the Co content in the alloy plating layer is greater than 5 to 25% by mass and the Zn content is less than 75 to 95% by mass. The lower limit of the Co content in the alloy plating layer 20 is more preferably 10% by mass. The upper limit of the Co content in the alloy plating layer 20 is more preferably 20% by mass. The alloy plating layer 20 has a high Zn content. Therefore, the effect of sacrificial corrosion protection is great.

  The Co content in the alloy plating layer 20 is measured by the following method. The measurement is performed using a scanning electron microscope with an energy dispersive X-ray analyzer (EDS) (EDS: Pegasus manufactured by EDAX, SEM: ERA-8900FE manufactured by Elionix Co., Ltd.). Arbitrary three places on the surface of the alloy plating layer 20 are observed with a scanning electron microscope, and the composition is analyzed. Measurement magnification: 1500 to 5000 times, acceleration voltage: 15 to 30 kV, irradiation current: irradiation with an electron beam with a maximum of 1 nA, the intensity of Co-kα rays is measured, and the Co content (mass%) is calculated. The average value of 3 places is adopted and it is set as Co content.

  The thickness of the alloy plating layer 20 is preferably 1 to 20 μm. If the thickness of the alloy plating layer 20 is 1 μm or more, the seizure resistance and the corrosion resistance of the threaded joint 1 for pipes can be stably increased. If the thickness of the alloy plating layer 20 is 20 micrometers or less, the adhesiveness of plating will be stabilized. Therefore, the thickness of the alloy plating layer is preferably 1 to 20 μm.

  The thickness of the alloy plating layer 20 is measured by the following method. The pipe threaded joint 1 on which the alloy plating layer 20 is formed is cut at right angles to the longitudinal direction of the pipe threaded joint 1 to obtain a cut piece. The cut piece is embedded in the resin so that the cut surface becomes the observation surface. The observation surface is polished to prepare an observation sample. The cross section of the alloy plating layer 20 is observed with a scanning electron microscope (SEM), and the thickness of the alloy plating layer 20 is measured.

[Solid lubricating coating 21]
FIG. 4 is a cross-sectional view of the contact surfaces 40 and 50 of the threaded joint 1 for pipe according to another embodiment different from FIG. Referring to FIG. 4, preferably, the threaded joint for pipes 1 further includes a solid lubricating coating 21 on the alloy plating layer 20. In this case, the lubricity of the threaded joint for pipes 1 is increased. A well-known solid lubricant film 21 can be used. The solid lubricating coating 21 includes, for example, lubricating particles and a binder. The solid lubricating coating 21 may contain a solvent and other components as necessary.

The lubricating particles reduce the friction coefficient of the surface of the solid lubricating coating 21. The lubricating particles are not particularly limited as long as they have lubricating properties. Lubricating particles include, for example, graphite, MoS ₂ (molybdenum disulfide), WS ₂ (tungsten disulfide), BN (boron nitride), PTFE (polytetrafluoroethylene), CF _x (graphite fluoride) and CaCO ₃ (carbonic acid). One or more selected from the group consisting of calcium). Preferably, graphite, graphite fluoride, MoS ₂ and PTFE are used.

  The binder binds the lubricating particles into the solid lubricating coating 21. The binder is, for example, one or two selected from the group consisting of organic resins and inorganic resins. When using organic resin, 1 type selected from the group which consists of a thermosetting resin and a thermoplastic resin can be used. The thermosetting resin is, for example, one or two selected from the group consisting of epoxy resin, polyimide resin, polycarbodiimide resin, polyether sulfone, polyether ether ketone resin, phenol resin, furan resin, urea resin, and acrylic resin. More than a seed. The thermoplastic resin is, for example, one or more selected from the group consisting of polyamideimide resin, polyethylene resin, polypropylene resin, polystyrene resin, and ethylene vinyl acetate resin.

  When an inorganic resin is used, polymetalloxane can be used. The polymetalloxane refers to a polymer compound in which a repeating metal-oxygen bond is a main chain skeleton. Preferably, polytitanoxane (Ti—O) and polysiloxane (Si—O) are used. These inorganic resins are obtained by hydrolyzing and condensing metal alkoxide. The alkoxy group of the metal alkoxide is, for example, a lower alkoxy group such as a methoxy group, an ethoxy group, a propoxy group, an isopropoxy group, an isobutoxy group, a butoxy group and a tert-butoxy group.

  When it is necessary to dissolve or disperse the lubricating particles and the binder, a solvent is used. The solvent is not particularly limited as long as it can disperse or dissolve the components contained in the solid lubricant film 21. As the solvent, one or two selected from the group consisting of an organic solvent and water can be used. The organic solvent is, for example, one or two selected from the group consisting of toluene and isopropyl alcohol.

  The solid lubricating coating 21 can contain other components as necessary. Other components are, for example, rust inhibitors, corrosion inhibitors, surfactants, waxes, friction modifiers, pigments and the like. The respective contents of the lubricating particles, the binder, the solvent and other components are appropriately set.

[Base material of threaded joint 1 for pipes]
The composition of the base material of the pipe threaded joint 1 is not particularly limited. Examples of the base material include carbon steel, stainless steel, and alloy steel. Among alloy steels, duplex stainless steels containing alloy elements such as Cr, Ni and Mo and high alloy steels such as Ni alloys have high corrosion resistance. Therefore, if these high alloy steels are used as a base material, excellent corrosion resistance can be obtained in a corrosive environment containing hydrogen sulfide, carbon dioxide and the like.

[Production method]
The manufacturing method of the threaded joint 1 for pipes by this embodiment comprises a preparatory process and an alloy plating layer formation process.

[Preparation process]
In the preparation step, a plating bath containing zinc ions and cobalt ions is prepared. The plating bath further contains a solvent. The solvent is preferably water. Zinc ions are contained in the plating bath by dissolving a salt of zinc ions and anions (for example, zinc sulfate) in a solvent. Similarly, cobalt ions are contained in the plating bath by dissolving a salt of cobalt ions and anions (for example, cobalt sulfate) in a solvent. The anion is, for example, one or more selected from the group consisting of sulfate ion, chloride ion and pyrophosphate ion.

  The lower limit of the zinc ion content in the plating bath is preferably 10 g / L, more preferably 20 g / L in terms of zinc. The upper limit of the zinc ion content in the plating bath is preferably 55 g / L, more preferably 30 g / L in terms of zinc. The lower limit of the content of cobalt ions in the plating bath is preferably 10 g / L, more preferably 30 g / L, in terms of cobalt. The upper limit of the content of cobalt ions in the plating bath is preferably 50 g / L, more preferably 45 g / L, in terms of cobalt.

  The plating bath is optionally selected from one or two selected from the group consisting of a conductivity salt, an anodic dissolution accelerator, a complexing agent, a pH buffer, a surfactant, a reducing agent, a stabilizer, and other additives. It may contain seeds or more.

[Alloy plating layer forming process]
In the alloy plating layer forming step, the alloy plating layer 20 made of a Zn—Co alloy is formed on at least one of the contact surfaces 40 and 50 of the pin 4 and the box 5. The alloy plating layer 20 is formed by electroplating. Electroplating is performed by immersing at least one contact surface 40 and 50 of the pin 4 and the box 5 in the plating bath and energizing. The conditions for electroplating can be set as appropriate. The electroplating conditions are, for example, plating bath pH: 1 to 10, plating bath temperature: 10 to 60 ° C., current density: 1 to 100 A / dm ² , and processing time: 0.1 to 30 minutes.

[Solid lubricating film forming process]
Preferably, the manufacturing method includes a step of forming a solid lubricating coating 21 on the alloy plating layer 20 (a solid lubricating coating forming step). In the solid lubricant film forming step, first, a solid lubricant film composition (hereinafter also referred to as a composition) is prepared. The composition is formed by mixing the lubricating particles and binder described above. The composition may further contain the above-described solvent and other components.

  The obtained composition is applied onto the alloy plating layer 20. The method of application is not particularly limited. For example, the composition is sprayed onto the alloy plating layer 20 using a spray gun. The pin 4 or the box 5 to which the composition is applied is heated and dried. Thereby, the composition is cured, and a solid lubricating film 21 is formed on the alloy plating layer 20. The conditions for the heat drying can be appropriately set in consideration of the boiling point and melting point of each component contained in the composition. For compositions that do not use a solvent, the hot melt method can be used. In the hot melt method, the composition is heated to a fluid state. The composition in a fluidized state is sprayed using, for example, a spray gun having a temperature holding function. The pin 4 or the box 5 coated with the composition is cooled by air cooling or the like. Thereby, the composition is cured, and a solid lubricating film 21 is formed on the alloy plating layer 20.

[Other processes]
The manufacturing process may further include a pretreatment process before the alloy plating layer forming process, if necessary. The pretreatment process is, for example, pickling and alkali degreasing. In the pretreatment process, oil components and the like adhering to the contact surfaces 40 and 50 are washed. The pretreatment process may further include a grinding process such as a mechanical grinding finish. The pretreatment step may be performed before the preparation step or after the preparation step as long as it is before the alloy plating layer formation step.

  The manufacturing process may further include a phosphate film forming process as necessary. In the phosphate film forming step, a phosphate film is formed between the alloy plating layer 20 and the solid lubricating film 21. The adhesion of the solid lubricating coating can be enhanced by the phosphate coating. The phosphate is, for example, one or two selected from the group consisting of zinc phosphate and manganese phosphate. The phosphate film forming step can be performed by a known method.

  Examples will be described below. In the embodiment, the contact surface of the pin is referred to as the pin surface, and the contact surface of the box is referred to as the box surface. Moreover,% in an Example means the mass%.

  For the base material, Cr 13% steel (C: 0.18%, Si: 0.23%, Mn: 0.8%, P: 0.02%, S: 0.01), which is a kind of high alloy steel. %, Cu: 0.04%, Ni: 0.1%, Cr: 13%, Mo: 0.04%, balance: Fe and impurities). Using this Cr 13% steel, seamless steel pipes and couplings were produced. The seamless steel pipe had an outer diameter of 244.5 mm, a wall thickness of 13.84 mm, and a length of 1200 mm. On the outer surface of both ends of the seamless steel pipe, a pin having a male thread part and an unthreaded metal contact part was formed by cutting. Boxes having internal thread portions and unthreaded metal contact portions were formed on the inner surfaces of both ends of the coupling by cutting.

[Preparation process]
A plating bath containing zinc ions and cobalt ions was prepared. The plating bath was prepared by dissolving commercially available zinc sulfate heptahydrate, cobalt sulfate heptahydrate, ammonium chloride: 0.5 mol / L and boric acid: 0.5 mol / L in pure water. The zinc ion concentration and the cobalt ion concentration were adjusted so as to be 1.0 mol / L in total, and various types of plating baths were constructed by changing each ion concentration.

[Alloy plating layer forming process]
An alloy plating layer was formed on the surface of the boxes of Test No. 1 to Test No. 6. Specifically, an alloy plating layer made of a Zn—Co alloy was formed by immersing the coupling in a plating bath of each test number and energizing it. The plating conditions were plating bath pH: 3.5, plating bath temperature: 35 ° C., and current density: ^{2 to} 20 A / dm ² (constant current electrolysis method).

  A Cu—Sn—Zn alloy plating layer was formed on the surface of the test No. 7 box. Specifically, a Cu—Sn—Zn alloy plating layer was formed on the box surface by electroplating using a cyan bath containing copper ions, tin ions and zinc ions. The Cu—Sn—Zn alloy plating layer contained Zn: about 7%, Sn: about 40%, Cu: about 53%.

[Solid lubricating film forming process]
A solid lubricant film was further formed on the surface of each test number box. The composition for forming a solid lubricant film contained polyamideimide resin: 12% by mass, dimethyl sulfoxide: 45% by mass, PTFE particles: 5% by mass, and pure water: the balance. After spraying this composition on the alloy plating layer, preliminary drying (85 ° C., 10 minutes) and main heating (280 ° C., 30 minutes) were performed to form a solid lubricating film having an average film thickness of 30 μm.

  The pin surface was subjected to mechanical grinding finish (surface roughness 3 μm), and then a solid anticorrosive film was formed. The composition for forming a solid anticorrosive film contained an acrylic resin-based ultraviolet curable resin paint, aluminum phosphite, and polyethylene wax. The contents of aluminum phosphite and polyethylene wax were 0.05 and 0.01, respectively, with respect to the acrylic resin ultraviolet curable resin 1. After this composition was applied to the surface of the pin, the composition was cured by irradiating it with ultraviolet rays using a UV lamp (air-cooled mercury lamp, output 4 kW, ultraviolet wavelength: 260 nm). The thickness of the solid anticorrosion coating was 25 μm.

[Co content measurement test in alloy plating layer]
The Co content in the alloy plating layers of Test No. 1 to Test No. 6 was measured by the method described above. The results are shown in Table 1.

[Thickness measurement test of alloy plating layer]
The film thickness of the alloy plating layer of each test number was measured by the method described above. The results are shown in Table 1.

[Evaluation test for seizure resistance]
Seizure resistance was evaluated for the pins and boxes of each test number. Specifically, using a seamless steel pipe having a pin and a box of each test number and a coupling, screw tightening and unscrewing were repeated at room temperature. Screwing and unscrewing was repeated up to 10 times. The tightening torque for screw tightening was 49351.8 N · m (36400 ft · lbs). Each time the screw tightening and screw unwinding were performed once, the pin surface was visually observed. The number of occurrences of seizure was measured by visual observation. The results are shown in Table 1. In Table 1, “> 10” indicates that seizure did not occur even when screw tightening and screw unscrewing were repeated 10 times.

[Salt spray test]
The salt spray test was implemented using the test piece which has the same composition as the said coupling. The test piece was provided with the same alloy plating layer and solid lubricant film as the box surface of each test number. The size of the test piece was 70 mm in width, 50 mm in length, and 1 mm in thickness. The salt spray test was performed based on the method described in JIS Z2371 (2000). The test time was a maximum of 2000 hours. The time when rust was generated on the surface of the test piece of each test number was measured by visual observation. The results are shown in Table 1. In Table 1, “> 2000” indicates that rusting was not confirmed even after 2000 hours.

[Evaluation results]
Referring to Table 1, the threaded joints for pipes of Test No. 1 to Test No. 6 were provided with an alloy plating layer made of a Zn—Co alloy on at least one contact surface of the pin and the box. Therefore, compared with the conventional Cu-Sn-Zn alloy plating layer (test number 7), the outstanding seizure resistance and corrosion resistance were shown. Specifically, seizure did not occur in the threaded joints for pipes of Test No. 1 to Test No. 6 even when screw tightening and screw unscrewing were repeated six times. Furthermore, rust was not confirmed until 960 hours in the salt spray test of the threaded joint for pipes of test numbers 1 to 6.

  In the threaded joints for pipes of Test No. 1 to Test No. 4, the Co content in the alloy plating layer was in mass% and was in the range of more than 5 to 25%. Therefore, the seizure resistance and corrosion resistance were further improved as compared with the threaded joints for pipes of Test No. 5 and Test No. 6. Specifically, seizure did not occur in the threaded joints for pipes of Test No. 1 to Test No. 4 even when the screw tightening and unscrewing were repeated 10 times in the seizure resistance evaluation test. Furthermore, the pipe thread joints of Test No. 1 to Test No. 4 were not found to rust even after 2000 hours in the salt spray test.

  The embodiment of the present invention has been described above. However, the above-described embodiment is merely an example for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately changing the above-described embodiment without departing from the spirit thereof.

DESCRIPTION OF SYMBOLS 1 Pipe thread joint 4 Pin 5 Box 40, 50 Contact surface 41, 51 Thread part 42, 52 Unthreaded metal contact part 20 Alloy plating layer 21 Solid lubricating film

Claims (6)

A pin and a box each having a contact surface with a threaded portion and an unthreaded metal contact;
A threaded joint for pipes comprising an alloy plating layer made of a Zn-Co alloy on the contact surface of at least one of the pin and the box. The threaded joint for pipes according to claim 1,
The alloy plating layer is a threaded joint for pipes containing, by mass%, more than 5 to 25% Co. The threaded joint for pipes according to claim 1 or 2,
The threaded joint for pipes, wherein the alloy plating layer has a thickness of 1 to 20 μm. The pipe threaded joint according to any one of claims 1 to 3, further comprising:
A threaded joint for pipes comprising a solid lubricating coating on the alloy plating layer. A method for producing a threaded joint for pipes comprising a pin and a box each having a contact surface with a threaded portion and an unthreaded metal contact,
Preparing a plating bath containing zinc ions and cobalt ions;
Forming a plated alloy layer made of a Zn-Co alloy on the contact surface of at least one of the pin and the box using the plating bath. The method for manufacturing a threaded joint for pipes according to claim 5, further comprising:
A method for manufacturing a threaded joint for pipes, comprising a step of forming a solid lubricant film on the alloy plating layer.

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