Threaded Connection for Pipe and Method for Producing Threaded Connection for Pipe

Abstract

There is provided a threaded connection for pipe the tightening torque of which is easy to adjust. A threaded connection for pipe according to the present embodiment includes a pin and a box. The pin and the box have contact surfaces that include a thread part, a metal seal part, and a shoulder part, respectively. The threaded connection for pipe includes a ZnNi alloy plating layer on the contact surface of one of the pin and the box, and includes a CuSnZn alloy plating layer on the contact surface of the other one. The threaded connection for pipe further includes a lubricating coating in a liquid state or a semisolid state on at least one of the ZnNi alloy plating layer and the CuSnZn alloy plating layer.

Claims

1. A threaded connection for pipe comprising: a pin and a box each of which has a contact surface including a thread part, a metal seal part, and a shoulder part; a ZnNi alloy plating layer on the contact surface of one of the pin and the box, the ZnNi alloy plating layer being made of a ZnNi alloy; a CuSnZn alloy plating layer on the contact surface of another of the pin and the box, the CuSnZn alloy plating layer being made of a CuSnZn alloy; and a lubricating coating on at least one of the ZnNi alloy plating layer and the CuSnZn alloy plating layer, the lubricating coating being in a liquid state or a semisolid state.

2. The threaded connection for pipe according to claim 1, wherein the ZnNi alloy plating layer has a thickness of 1 to 20 m, the CuSnZn alloy plating layer has a thickness of 1 to 20 m, and the lubricating coating has a thickness of 30 to 300 m.

3. The threaded connection for pipe according to claim 1, wherein the ZnNi alloy consists of 85 to 91 mass % of Zn and 9 to 15 mass % of Ni, with the balance being impurities.

4. The threaded connection for pipe according to claim 1, wherein the CuSnZn alloy consists of 40 to 70 mass % of Cu, 20 to 50 mass % of Sn and 2 to 20 mass % of Zn, with the balance being impurities.

5. A method for producing a threaded connection for pipe comprising a pin and a box each of which has a contact surface including a thread part, a metal seal part, and a shoulder part, the method comprising: a step of forming a ZnNi alloy plating layer on the contact surface of one of the pin and the box, the ZnNi alloy plating layer being made of a ZnNi alloy; a step of forming a CuSnZn alloy plating layer on the contact surface of another of the pin and the box, the CuSnZn alloy plating layer being made of a CuSnZn alloy; and a step of forming a lubricating coating on at least one of the ZnNi alloy plating layer and the CuSnZn alloy plating layer, the lubricating coating being in a liquid state or a semisolid state.

6. The threaded connection for pipe according to claim 2, wherein the ZnNi alloy consists of 85 to 91 mass % of Zn and 9 to 15 mass % of Ni, with the balance being impurities.

7. The threaded connection for pipe according to claim 2, wherein the CuSnZn alloy consists of 40 to 70 mass % of Cu, 20 to 50 mass % of Sn and 2 to 20 mass % of Zn, with the balance being impurities.

8. The threaded connection for pipe according to claim 3, wherein the CuSnZn alloy consists of 40 to 70 mass % of Cu, 20 to 50 mass % of Sn and 2 to 20 mass % of Zn, with the balance being impurities.

9. The threaded connection for pipe according to claim 6, wherein the CuSnZn alloy consists of 40 to 70 mass % of Cu, 20 to 50 mass % of Sn and 2 to 20 mass % of Zn, with the balance being impurities.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0027] FIG. 1 is a graph illustrating the relation between the number of turns N and torque T in a threaded connection for pipe.

[0028] FIG. 2 is a diagram illustrating the configuration of a threaded connection for pipe according to the present embodiment.

[0029] FIG. 3 is a cross-sectional view of the threaded connection for pipe according to the present embodiment.

[0030] FIG. 4 is a cross-sectional view of an example of contact surfaces of the threaded connection for pipe (Pin) according to the present embodiment.

[0031] FIG. 5 is a cross-sectional view of an example of contact surfaces of the threaded connection for pipe (Box) according to the present embodiment.

[0032] FIG. 6 is a graph for illustrating a torque-on-shoulder resistance (T) in examples.

DESCRIPTION OF EMBODIMENTS

[0033] The present embodiment will be described below in detail with reference to the accompanying drawings. The same or equivalent elements will be denoted by the same reference numerals and the description thereof will not be repeated.

[0034] The present inventors conducted variety of studies about the relation between surface treatment on a contact surface of a threaded connection for pipe and torque-on-shoulder resistance T. Consequently, the following findings were obtained.

[0035] To increase the torque-on-shoulder resistance T, it is effective to decrease a shouldering torque Ts or to increase a yield torque Ty. A lubricating coating in a liquid state or a semisolid state changes its shape in accordance with a receiving pressure. Therefore, the lubricating coating in a liquid state or a semisolid state has a low frictional force in comparison with a lubricating coating in a solid state. A low frictional force makes both the shouldering torque Ts and the yield torque Ty decrease. When galling does not occur, yield torque Ty decreases remarkably. As a result, torque-on-shoulder resistance T decreases.

[0036] Meanwhile, as the receiving pressure increases, the lubricating coating in a liquid state or a semisolid state decreases in thickness. In a final phase of the fastening, a high pressure is applied to the lubricating coating. Therefore, the lubricating coating is torn locally, and base metals come into direct contact with each other. The chemical compositions of the base metals are normally the same or close to each other. In this case, galling is likely to occur. When galling occurs locally, a load imposed on a threaded connection for pipe rapidly rises with a small number of turns N of the fastening. For this reason, the adjustment of the tightening torque To becomes difficult.

[0037] In the case of the lubricating coating in a solid state as in Patent Literature 6, since the deformation amount of the lubricating coating corresponding to the pressure is small, the above-mentioned problem hardly occurs. On the other hand, as described above, in the case of the lubricating coating in a liquid state or a semisolid state, the base metals are easily in direct contact with each other during the fastening.

[0038] The present inventors found that by disposing specified alloy plating layers on contact surfaces of a pin and a box, torque-on-shoulder resistance T increases. Specifically, on a contact surface of one of the pin and the box, a ZnNi alloy plating layer made of a ZnNi alloy is disposed. On a contact surface of the other one of the pin and the box, a CuSnZn alloy plating layer made of a CuSnZn alloy is disposed. This disposition allows a yield torque Ty to be kept high.

[0039] The reason that the disposition of the aforementioned alloy plating layers allows the yield torque Ty to be kept high is conjectured as follows. ZnNi alloy plating and CuSnZn alloy plating have appropriate coefficients of friction. Therefore, when the ZnNi alloy plating layer and the CuSnZn alloy plating layer come into contact with each other, the frictional force therebetween increases, and the yield torque Ty increases. Furthermore, the ZnNi alloy plating and the CuSnZn alloy plating have high hardnesses and fusing points. Therefore, even in a final phase of the fastening, the ZnNi alloy plating layer and the CuSnZn alloy plating layer suffer minor damage, and a direct contact between base metals is inhibited. As a result, the yield torque Ty is kept high, which facilitates the adjustment of the tightening torque To.

[0040] The threaded connection for pipe in the present embodiment, which is completed based on the above findings, includes a pin and a box, a ZnNi alloy plating layer, and a CuSnZn alloy plating layer. The pin and the box each have a contact surface that includes a thread part, a metal seal part, and a shoulder part. The ZnNi alloy plating layer is disposed on a contact surface of one of the pin and the box. The CuSnZn alloy plating layer is disposed on a contact surface of the other one of the pin and the box. The threaded connection for pipe further includes a lubricating coating in a liquid state or a semisolid state on at least one of the ZnNi alloy plating layer and the CuSnZn alloy plating layer.

[0041] The threaded connection for pipe according to the present embodiment includes the ZnNi alloy plating layer on the contact surface of one of a pin and a box, and includes the CuSnZn alloy plating layer on the contact surface of the other one. Therefore, the threaded connection for pipe provides a high torque-on-shoulder resistance T, and the tightening torque To thereof is easy to adjust.

[0042] It is preferable that the ZnNi alloy plating layer mentioned above has a thickness of 1 to 20 m, the CuSnZn alloy plating layer has a thickness of 1 to 20 m, and the lubricating coating has a thickness of 30 to 300 m.

[0043] In this case, the torque-on-shoulder resistance T increases more stably.

[0044] Preferably, the ZnNi alloy mentioned above consists of 85 to 91 mass % of Zn and 9 to 15 mass % of Ni, with the balance being impurities.

[0045] Preferably, the CuSnZn alloy mentioned above consists of 40 to 70 mass % of Cu, 20 to 50 mass % of Sn and 2 to 20 mass % of Zn, with the balance being impurities.

[0046] A method for producing the threaded connection for pipe according to the present embodiment is a method for producing a threaded connection for pipe including a pin and a box. The pin and the box each have a contact surface that includes a thread part, a metal seal part, and a shoulder part. The producing method includes a ZnNi alloy plating layer forming step, a CuSnZn alloy plating layer forming step, and a coating forming step. In the ZnNi alloy plating layer forming step, a ZnNi alloy plating layer made of a ZnNi alloy is formed on a contact surface of one of the pin and the box. In the CuSnZn alloy plating layer forming step, a CuSnZn alloy plating layer made of a CuSnZn alloy is formed on a contact surface of the other one of the pin and the box. In the coating forming step, a lubricating coating in a liquid state or a semisolid state is formed on at least one of the ZnNi alloy plating layer and the CuSnZn alloy plating layer.

[0047] By the producing method according to the present embodiment, it is possible to produce a threaded connection for pipe that includes a ZnNi alloy plating layer on a contact surface of one of a pin and a box, and includes a CuSnZn alloy plating layer on a contact surface of the other one. The threaded connection for pipe provides a high torque-on-shoulder resistance T, and the tightening torque To thereof is easy to adjust.

[0048] Description will be made below about a threaded connection for pipe according to the present embodiment and a method for producing the threaded connection for pipe.

[Threaded Connection for Pipe]

[0049] A threaded connection for pipe includes a pin and a box. FIG. 2 is a diagram illustrating the configuration of a threaded connection for pipe according to the present embodiment. Referring to FIG. 2, the threaded connection for pipe includes a steel pipe 1 and a coupling 2. At the both ends of the steel pipe 1, a pin 3 is formed that includes an external thread part on its outer surface. At the both ends of the coupling 2, a box 4 is formed that includes an internal thread part on its inner surface. By fastening the pin 3 and the box 4, the coupling 2 is attached to an end of the steel pipe 1. Besides, there is an integral-type threaded connection for oil country tubular goods, which does not include a coupling 2 but includes a pin 3 provided at one end of the steel pipe 1 and a box 4 provided at the other end. The threaded connection for pipe according to the present embodiment is available for both of coupling-type threaded connections for pipe and integral-type threaded connections for pipe.

[0050] The pin 3 and the box 4 each have a contact surface that includes a thread part, a metal seal part, and a shoulder part. FIG. 3 is a cross-sectional view of the threaded connection for pipe according to the present embodiment. Referring to FIG. 3, the pin 3 includes an external thread part 31, a metal seal part 32, and a shoulder part 33. The box 4 includes an internal thread part 41, a metal seal part 42, and a shoulder part 43. The parts that come into contact with each other when the pin 3 and the box 4 are fastened are referred to as contact surfaces 34 and 44. Specifically, when the pin 3 and the box 4 are fastened, the thread parts (the external thread part 31 and the internal thread part 41) come into contact with each other, the metal seal parts (the metal seal parts 32 and 42) come into contact with each other, and the shoulder parts (the shoulder parts 33 and 43) come into contact with each other. In other words, the contact surfaces 34 and 44 include the thread parts 31 and 41, the metal seal parts 32 and 42, and the shoulder parts 33 and 43, respectively.

[0051] FIG. 4 and FIG. 5 is a cross-sectional view of an example of the contact surfaces 34 and 44 of the threaded connection for pipe according to the present embodiment. The alloy plating layers of the threaded connection for pipe according to the present embodiment differs on the contact surface of the pin 3 and on the contact surface of the box 4. In FIG. 4, the threaded connection for pipe includes a ZnNi alloy plating layer 100 on the contact surface 34 of the pin 3. In FIG. 5, the threaded connection for pipe includes a CuSnZn alloy plating layer 110 on the contact surface 44 of the box 4. The threaded connection for pipe further includes lubricating coatings 200 on both of the ZnNi alloy plating layer 100 and the CuSnZn alloy plating layer 110.

[0052] The disposition of the alloy plating layers according to the present embodiment is not limited to FIG. 4 and FIG. 5. The CuSnZn alloy plating layer 110 may be disposed on the contact surface 34 of the pin 3, and the ZnNi alloy plating layer 100 may be disposed on the contact surface 44 of the box 4.

[0053] Normally, metal plating is applied to threaded connection for pipe for the purpose of improving galling resistance or corrosion resistance. In this case, the threaded connection for pipe has a plating layer only on either the contact surface of the pin 3 or the contact surface of the box 4. Even with plating layers on both contact surfaces of the pin 3 and the box 4, the threaded connection for pipe has the plating layers of the same chemical composition on the contact surfaces of the pin 3 and the box 4. However, the threaded connection for pipe according to the present embodiment includes alloy plating layers of different chemical compositions on the contact surface of the pin 3 and on the contact surface of the box 4. Furthermore, each of the alloy plating layer of the threaded connection for pipe according to the present embodiment has specific chemical composition.

[ZnNi Alloy Plating Layer]

[0054] The ZnNi alloy plating layer 100 is disposed on a contact surface of one of the pin 3 and the box 4. The ZnNi alloy plating layer 100 is made of a ZnNi alloy. The ZnNi alloy consists of Zn and Ni, with the balance being impurities. Preferably, the ZnNi alloy consists of 85 to 91 mass % of Zn and 9 to 15 mass % of Ni, with the balance being impurities.

[0055] The chemical composition of the ZnNi alloy plating layer 100 is analyzed by the following method. Arbitrary 5 measurement points of the ZnNi alloy plating layer 100 are measured by using a handheld fluorescent X-ray analyzer (DELTA Premium DP-2000) manufactured by Olympus Innovex Co., Ltd. The measurement conditions are as follows. X-ray beam diameter: 3 mm, inspection time: 120 seconds, measurement mode: B 1, analysis software: Alloy Plus. The arithmetic average of the measurement results of the 5 measurement points is taken as the chemical composition of the ZnNi alloy plating layer 100.

[0056] The thickness of the ZnNi alloy plating layer 100 is not particularly limited. The thickness of the ZnNi alloy plating layer 100 is, for example, 1 to 20 m. When the thickness of the ZnNi alloy plating layer 100 is 1 m or more, the torque-on-shoulder resistance T can be increased stably. When the thickness of the ZnNi alloy plating layer 100 becomes more than 20 m, the above effect is however saturated.

[0057] The thickness of the ZnNi alloy plating layer 100 is measured by the following method. One of the contact surfaces 34 and 44, on which the ZnNi alloy plating layer 100 is formed, is brought into contact with a probe of a film thickness gauge of a phase-sensitive eddy-current type that is compliant with International Organization for Standardization (ISO) 21968 (2005). Measurement is made on a phase difference between a high-frequency magnetic field on an input side of the probe and an eddy current on the ZnNi alloy plating layer 100 that is excited by the high-frequency magnetic field. The phase difference is converted into the thickness of the ZnNi alloy plating layer 100.

[0058] The threaded connection for pipe may include the ZnNi alloy plating layer 100 partially on one of the contact surface 34 or 44 of the pin 3 and the box 4. The threaded connection for pipe may include the ZnNi alloy plating layer 100 entirely on one of the contact surface 34 or 44 of the pin 3 and the box 4. The metal seal parts 32 and 42 are subjected to a high interfacial pressure in particular in a final phase of the fastening. Therefore, in the case of disposing the ZnNi alloy plating layer 100 partially on one of the contact surfaces 34 and 44, it is preferable to dispose the ZnNi alloy plating layer 100 on one of the metal seal parts 32 and 42. That is, the threaded connection for pipe may include the ZnNi alloy plating layer 100 only on the metal seal part 32 or 42 on the contact surface 34 or 44 of one of the pin 3 and the box 4. Meanwhile, when the ZnNi alloy plating layer 100 is disposed entirely on one of the contact surfaces 34 and 44, the production efficiency of the threaded connection for pipe is increased.

[CuSnZn Alloy Plating Layer]

[0059] The CuSnZn alloy plating layer 110 is disposed on a contact surface of one of the pin 3 and the box 4 on which the ZnNi alloy plating layer 100 is not formed. The CuSnZn alloy plating layer 110 is made of a CuSnZn alloy. The CuSnZn alloy consists of Cu, Sn, and Zn, with the balance being impurities. Preferably, the CuSnZn alloy consists of 40 to 70 mass % of Cu, 20 to 50 mass % of Sn and 2 to 20 mass % of Zn, with the balance being impurities. The chemical composition of the CuSnZn alloy plating layer 110 is analyzed by the same method as the ZnNi alloy plating layer 100 mentioned above.

[0060] The thickness of the CuSnZn alloy plating layer 110 is not particularly limited. The thickness of the CuSnZn alloy plating layer 110 is, for example, 1 to 20 m. When the thickness of the CuSnZn alloy plating layer 110 is 1 m or more, the torque-on-shoulder resistance T can be increased stably. When the thickness of the CuSnZn alloy plating layer 110 becomes more than 20 m, the above effect is however saturated. The thickness of the CuSnZn alloy plating layer 110 is measured by the same method as that for the ZnNi alloy plating layer 100 mentioned above.

[0061] As with the ZnNi alloy plating layer 100, the CuSnZn alloy plating layer 110 may be disposed partially or entirely on one of the contact surfaces 34 and 44. That is, the threaded connection for pipe may include the CuSnZn alloy plating layer 110 partially on one of the contact surface 34 or 44 of the pin 3 and the box 4 on which the ZnNi alloy plating layer 100 is not formed. The threaded connection for pipe may include the CuSnZn alloy plating layer 110 entirely on one of the contact surface 34 or 44 of the pin 3 and the box 4 on which the ZnNi alloy plating layer 100 is not formed. In the case of disposing the CuSnZn alloy plating layer 110 partially on one of the contact surfaces 34 and 44, it is preferable to dispose the CuSnZn alloy plating layer 110 on one of the metal seal parts 32 and 42. That is, the threaded connection for pipe may include the CuSnZn alloy plating layer 110 only on one of the metal seal parts 32 or 42 on the contact surface 34 or 44 of the pin 3 and the box 4, on which the ZnNi alloy plating layer 100 is not formed. Meanwhile, when the CuSnZn alloy plating layer 110 is disposed entirely on one of the contact surfaces 34 and 44, the production efficiency of the threaded connection for pipe is increased.

[0062] The ZnNi alloy plating layer 100 and the CuSnZn alloy plating layer 110 have high hardnesses and fusing points. Therefore, even when fastening and loosening are repeated, damage to the ZnNi alloy plating layer 100 and the CuSnZn alloy plating layer 110 is inhibited. Consequently, the yield torque Ty is kept high even when fastening and loosening are repeated.

[0063] Furthermore, zinc (Zn) contained in the ZnNi alloy plating layer 100 and the CuSnZn alloy plating layer 110 is a base metal in comparison with iron (Fe), which is the principal component of the steel pipe. Therefore, the ZnNi alloy plating layer 100 and the CuSnZn alloy plating layer 110 have the effect of sacrificial protection, increasing the corrosion resistance of the threaded connection for pipe.

[Lubricating Coating]

[0064] The threaded connection for pipe includes a lubricating coating 200 on at least one of the ZnNi alloy plating layer 100 and the CuSnZn alloy plating layer 110. As shown in FIG. 4 and FIG. 5, the threaded connection for pipe may include the lubricating coating 200 in a liquid state or a semisolid state both on the ZnNi alloy plating layer 100 and on the CuSnZn alloy plating layer 110. The threaded connection for pipe may include the lubricating coating 200 in a liquid state or a semisolid state only on the ZnNi alloy plating layer 100. The threaded connection for pipe may include the lubricating coating 200 in a liquid state or a semisolid state only on the CuSnZn alloy plating layer 110.

[0065] The lubricating coating 200 is in a liquid state or a semisolid state. Here, the liquid state refers to a state of substance, which has a constant volume, but does not have a certain shape. When the lubricating coating 200 is in a liquid state, the lubricating coating 200 can change its shape freely and flow on the contact surface 34 and 44. The semisolid state refers to a state in which the lubricating coating 200 can flow on the contact surfaces 34 and 44 under an external load (pressure, heat, etc.) while changing its shape freely, as with the liquid state. Examples in the liquid state or the semisolid state include high-viscosity substances such as grease.

[0066] The lubricating coating 200 contains a well-known lubricant. Examples of the lubricant include SEAL-GUARD ECF from JET-LUBE Inc. Other examples of the lubricant include a lubricant that contains a rosin, a metallic soap, a wax, and a lubricant powder. The lubricant powder is, for example, an earthy graphite. The chemical compositions of the lubricating coating 200 disposed on the ZnNi alloy plating layer 100 and the lubricating coating 200 disposed on the CuSnZn alloy plating layer 110 may be the same or different from each other.

[0067] The thickness of the lubricating coating 200 is not particularly limited. The thickness of the lubricating coating 200 is, for example, 30 to 300 m. When the thickness of the lubricating coating 200 is 30 m or more, the effect of decreasing the shouldering torque Ts is further increased. When the thickness of the lubricating coating 200 becomes more than 300 m, the above effect is however saturated because a surplus of the lubricating coating 200 is removed from the contact surfaces 34 and 44 in the fastening.

[0068] The thickness of the lubricating coating 200 is measured by the following method. An arbitrary measurement point (area: 5 mm20 mm) of the metal seal portion 32 or 42 of the threaded connection for pipe is wiped off with absorbent cotton impregnated with ethanol. The amount of application of the lubricant is calculated from the difference between the weight of the absorbent cotton before wiping and the weight of the absorbent cotton after wiping. From the amount of application of the lubricant, the density of the lubricant, and the measurement point area, the average coating thickness of the lubricating coating 200 is calculated.

[0069] Unlike a lubricating coating in a solid state, the lubricating coating 200 in a liquid state or a semisolid state changes its shape under pressure. Therefore, the lubricating coating 200 generates a low frictional force. As a result, the shouldering torque Ts is decreased. Furthermore, unlike a lubricating coating in a solid state, the lubricating coating 200 in a liquid state or a semisolid state is not abraded even when fastening and loosening are repeated. In the fastening, the lubricating coating 200 in a liquid state or a semisolid state changes its shape and moves to a place where interfacial pressure is low. Then, in loosening, the lubricating coating 200 spreads out on the entire contact surface again. Therefore, the shouldering torque Ts of the threaded connection for pipe according to the present embodiment is kept low even when fastening and loosening are repeated.

[0070] Further, unlike a lubricating coating in a solid state, the lubricating coating 200 in a liquid state or a semisolid state partially breaks or moves to a place where interfacial pressure is low when the interfacial pressure is high (for example, in the fastening). Therefore, the ZnNi alloy plating layer 100 and the CuSnZn alloy plating layer 110 are contact each other from the beginning of the fastening. As a result, friction is increased without causing galling, and the yield torque Ty increased.

[Base Metal of Threaded Connection for Pipe]

[0071] The chemical composition of a base metal of the threaded connection for pipe is not particularly limited. Examples of the base metal include carbon steels, stainless steels, and alloy steels. Of the alloy steels, high alloy steels such as Ni alloys and duplex stainless steels containing alloying elements such as Cr, Ni, and Mo have high anticorrosion properties. Therefore, when these high alloy steels are used as the base metal, the corrosion resistance of the threaded connection for pipe is increased.

[Producing Method]

[0072] The method for producing a threaded connection for pipe according to the present embodiment is a method for producing the threaded connection for pipe mentioned before. The producing method includes a ZnNi alloy plating layer 100 forming step, a CuSnZn alloy plating layer 110 forming step, and a coating forming step.

[ZnNi Alloy Plating Layer Forming Step]

[0073] In the ZnNi alloy plating layer 100 forming step, the ZnNi alloy plating layer 100 made of a ZnNi alloy is formed on a contact surface of one of the pin 3 and the box 4. The ZnNi alloy plating layer 100 is formed by electrolytic plating. The electrolytic plating is performed in such a manner that the contact surface of one of the pin 3 and the box 4 is immersed in a plating bath containing zinc ions and nickel ions, and energized. The plating bath preferably contains zinc ion: 1 to 100 g/L and nickel ion: 1 to 50 g/L. The conditions for the electrolytic plating can be set as appropriate. The conditions for the electrolytic plating are, for example, a plating bath pH: 1 to 10, a plating bath temperature: 10 to 60 C., a current density: 1 to 100 A/dm.sup.2, and a time period of the treatment: 0.1 to 30 minutes.

[CuSnZn Alloy Plating Layer Forming Step]

[0074] In the CuSnZn alloy plating layer 110 forming step, the CuSnZn alloy plating layer 110 made of a CuSnZn alloy is formed on a contact surface of one of the pin 3 and the box 4 on which the ZnNi alloy plating layer 100 is not formed. The CuSnZn alloy plating layer 110 is formed by electrolytic plating. The electrolytic plating is performed in such a manner that the contact surface of one of the pin 3 and the box 4 on which the ZnNi alloy plating layer 100 is not formed is immersed in a plating bath containing copper ion, tin ion, and zinc ion, and energized. The plating bath preferably contains copper ion: 1 to 50 g/L, tin ion: 1 to 50 g/L, and zinc ion: 1 to 50 g/L. The conditions for the electrolytic plating can be set as appropriate. The conditions for the electrolytic plating are, for example, a plating bath pH: 1 to 14, a plating bath temperature: 10 to 60 C., a current density: 1 to 100 A/dm.sup.2, and a time period of the treatment: 0.1 to 40 minutes.

[Coating Forming Step]

[0075] After the alloy plating layers mentioned above are formed on the contact surfaces of the pin 3 and the box 4, the coating forming step is performed. In the coating forming step, the lubricating coating 200 in a liquid state or a semisolid state is formed on at least one of the ZnNi alloy plating layer 100 and the CuSnZn alloy plating layer 110.

[0076] The lubricating coating 200 is formed by applying the lubricant mentioned above on the alloy plating layers mentioned above. The method for the application is not particularly limited. Examples of the method for the application include spray application, brush application, and immersion. In the case of adopting the spray application, the lubricant may be sprayed while being heated and increased in fluidity. The lubricating coating 200 may be formed on part of the contact surfaces but is preferably formed on the entire contact surfaces uniformly. The coating forming step may be performed on one or both of the pin 3 or the box 4 including the ZnNi alloy plating layer 100, and the pin 3 or the box 4 including the CuSnZn alloy plating layer 110.

[Preconditioning Treatment Step]

[0077] The producing method may include a preconditioning treatment step before the ZnNi alloy plating layer 100 forming step and the CuSnZn alloy plating layer 110 forming step, as necessary. Examples of the preconditioning treatment step include pickling and alkaline degreasing. In the preconditioning treatment step, oil content and the like adhered to the contact surface is removed. The preconditioning treatment step may further include grinding work such as sandblast and mechanical grinding finishing. Only one of these preconditioning treatments may be performed, or more than one of these preconditioning treatments may be performed in combination.

Examples

[0078] Hereinafter, examples will be described. In the examples, the contact surface of a pin will be referred to as a pin surface, and the contact surface of a box will be referred to as a box surface. In addition, the symbol % in the examples means mass percent.

[0079] A steel pipe used in the present examples was VAM21HT from Nippon Steel & Sumitomo Metal Corporation. The steel pipe had an outer diameter: 127.00 mm (5 inches) and a wall thickness: 9.19 mm (0.362 inches). The type of the steel pipe was 13Cr steel. The chemical composition of the steel pipe was C: 0.19%, Si: 0.25%, Mn: 0.8%, P: 0.02%, S: 0.01%, Cu: 0.04%, Ni: 0.1%, Cr: 13%, Mo: 0.04%, and the balance: Fe and impurities.

[0080] A pin surface and a box surface of each test number were subjected to the mechanical grinding finishing (at a surface roughness Ra: 3 m). Subsequently, the preconditioning treatment shown in Table 1 was performed, the plating layers and the lubricating coating shown in Table 1 were formed, whereby the pin and the box of each test number were prepared.

TABLE-US-00001 TABLE 1 LUBRICATING TEST PRECONDITIONING PLATING LAYER COATING NUMBER TREATMENT (THICKNESS) (THICKNESS) 1 PIN ZnNi ALLOY PLATING LUBRICATING SURFACE LAYER COATING J (4.9 m) (120 m) BOX CuSnZn ALLOY LUBRICATING SURFACE PLATING LAYER COATING J (8.0 m) (120 m) 2 PIN ZnNi ALLOY PLATING LUBRICATING SURFACE LAYER COATING S (4.9 m) (80 m) BOX CuSnZn ALLOY LUBRICATING SURFACE PLATING LAYER COATING S (8.0 m) (80 m) 3 PIN CuSnZn ALLOY LUBRICATING SURFACE PLATING LAYER COATING S (8.0 m) (80 m) BOX ZnNi ALLOY PLATING LUBRICATING SURFACE LAYER COATING S (5.0 m) (80 m) 4 PIN SANDBLAST LUBRICATING SURFACE COATING D (120 m) BOX Cu PLATING LAYER LUBRICATING SURFACE (7.0 m) COATING D (120 m) 5 PIN SANDBLAST LUBRICATING SURFACE COATING J (120 m) BOX Cu PLATING LAYER LUBRICATING SURFACE (7.0 m) COATING J (120 m) 6 PIN SANDBLAST LUBRICATING SURFACE COATING J (120 m) BOX CuSnZn ALLOY LUBRICATING SURFACE PLATING LAYER COATING J (8.0 m) (120 m) 7 PIN SANDBLAST LUBRICATING SURFACE COATING S (80 m) BOX ZnNi ALLOY PLATING LUBRICATING SURFACE LAYER COATING S (5.0 m) (80 m) 8 PIN ZnNi ALLOY PLATING LUBRICATING SURFACE LAYER COATING S (4.9 m) (80 m) BOX ZnNi ALLOY PLATING LUBRICATING SURFACE LAYER COATING S (5.0 m) (80 m)

[Preconditioning Treatment Step]

[0081] The pin surface of test number 4 to test number 7 was subjected to sandblast as the preconditioning treatment. The surface roughness of each pin after the sandblast was an arithmetic average roughness Ra of 1.0 m and a maximum height roughness Rz of 5.2 m, the arithmetic average roughness Ra and the maximum height roughness Rz being specified in JIS B0601:2013.

[Plating Layer Forming Step]

[0082] On the pin surface and the box surface of each test number, the plating layers were formed. The formation of the plating layers was performed by electrolytic plating. The detailed producing conditions for the plating layers of each test number were those shown in Table 2. In the case of forming the ZnNi alloy plating layer as the plating layer (for the pin surfaces of test number 1, test number 2, and test number 8, and the box surfaces of test number 3, test number 7, and test number 8), use was made of trade name: DAIN Zinalloy N-PL from Daiwa Fine Chemicals Co., Ltd. as the plating bath, and an electrolytic bath was initially made-up. In the case of forming the CuSnZn alloy plating layer as plating layer (for the pin surface of test number 3, and the box surfaces of test number 1, test number 2, and test number 6), use was made of a plating bath from NIHON KAGAKU SANGYO CO., LTD. as the plating bath. In the case of forming the Cu plating layer as the plating layer (for the box surfaces of test number 4 and test number 5), use was made of a copper cyanide bath containing copper ion: 20 to 50 g/L and free cyanide: 7 to 20 g/L as the plating bath.

TABLE-US-00002 TABLE 2 PLATING PLATING BATH CURRENT PLATING TEST PLATING BATH TEMPERATURE DENSITY TREATMENT LAYER NUMBER LAYER pH ( C.) (A/dm.sup.2) TIME (min) COMPOSITION 1 PIN ZnNi 6.5 25 2 18 Zn: 85.3%, SURFACE ALLOY Ni: 14.7% PLATING LAYER BOX CuSnZn 14 45 2 40 Cu: about 63%, SURFACE ALLOY Sn: about 30%, PLATING Zn: about 7% LAYER 2 PIN ZnNi 6.5 25 2 18 Zn: 89.5%, SURFACE ALLOY Ni: 10.5% PLATING LAYER BOX CuSnZn 14 45 2 40 Cu: about 63%, SURFACE ALLOY Sn: about 30%, PLATING Zn: about 7% LAYER 3 PIN CuSnZn 14 45 2 40 Cu: about 63%, SURFACE ALLOY Sn: about 30%, PLATING Zn: about 7% LAYER BOX ZnNi 6.5 25 2 18 Zn: 85.3%, SURFACE ALLOY Ni: 14.7% PLATING LAYER 4 PIN SURFACE BOX Cu 14 60 3 20 Cu: 100% SURFACE PLATING LAYER 5 PIN SURFACE BOX Cu 14 60 3 20 Cu: 100% SURFACE PLATING LAYER 6 PIN SURFACE BOX CuSnZn 14 45 2 40 Cu: about 63%, SURFACE ALLOY Sn: about 30%, PLATING Zn: about 7% LAYER 7 PIN SURFACE BOX ZnNi 6.5 25 2 18 Zn: 85.3%, SURFACE ALLOY Ni: 14.7% PLATING LAYER 8 PIN ZnNi 6.5 25 2 18 Zn: 85.3%, SURFACE ALLOY Ni: 14.7% PLATING LAYER BOX ZnNi 6.5 25 2 18 Zn: 90.3%, SURFACE ALLOY Ni: 9.7% PLATING LAYER

[Coating Forming Step]

[0083] To the pin surface and the box surface of each test number, lubricant was applied at a normal temperature, whereby the lubricating coating was formed. For a lubricating coating J and a lubricating coating D, lubricant was applied by brush application. For a lubricating coating S, lubricant was diluted with mineral spirit, and spray application was performed, whereby the lubricating coating S was formed. For the formation of the lubricating coatings, the following lubricants were used.

TABLE-US-00003 Lubricating coating J Trade name: SEAL-GUARD (TM) ECF (TM) from JET-LUBE Corporation Lubricating coating S Rosin: 10% Ca stearate: 15% Wax: 10% Earthy graphite: 5% Basic Ca sulfonate: the balance Lubricating coating D Compound grease specified in American Petroleum

Institute (API) Bulletin 5A2

[0084] The lubricating coating J was what is called a yellow dope, which is classified as Yellow according to the guideline of Harmonised Offshore Chemical Notification Format (HOCNF). The lubricating coating S was a grease-like lubricating coating containing no heavy metals. For the formation of the lubricating coating S, the aforementioned lubricant was diluted with mineral spirit and used. The lubricating coating D was a compound grease containing heavy metals such as lead (Pb). Coating conditions were obtained in advance so that the thickness of each lubricating coating had the thickness shown in Table 1, and a lubricating coating was formed under the coating conditions.

[Torque-On-Shoulder Resistance Measurement Test]

[0085] Using the pin and the box of each test number, torque-on-shoulder resistance was measured. Specifically, the fastening was performed at a tightening speed of 10 rpm and with a tightening torque of 42.8 kN.Math.m. In the fastening, torque was measured, and a torque chart was created as shown in FIG. 6. N in FIG. 6 denotes number of turns. T in FIG. 6 denotes torque. Ts in FIG. 6 denotes shouldering torque. MTV in FIG. 6 denotes a torque value where a line segment L intersects with the torque chart. The line segment L is a straight line that has the same inclination as that of a linear region in the torque chart after shouldering and that is larger in number of turns than the linear region by 0.2%. Normally, to measure torque-on-shoulder resistance, yield torque (Ty) is used. However, in the present example, the yield torque (a boundary between the linear region and a nonlinear region in the torque chart after shouldering) was unclear. Thus, using the line segment L, MTV was defined. The difference between MTV and Ts was determined as a torque-on-shoulder resistance (T) The torque-on-shoulder resistance (T) was determined in the form of relative value using test number 4 as a reference (100%). The results of the measurement are shown in Table 3. Test number 4 is a typical combination of surface treatments for a threaded connection for pipe in the case of using a compound grease.

TABLE-US-00004 TABLE 3 T TEST NUMBER (%) 1 128 2 140 3 135 4 100 (REFERENCE) 5 74 6 91 7 88 8 90

[Result of Evaluation]

[0086] Referring Table 1 and Table 3, the threaded connections for pipe of test number 1 to test number 3 each included the ZnNi alloy plating layer on a contact surface of one of the pin and the box, and included a CuSnZn alloy plating layer on a contact surface of the other one. The threaded connections for pipe of test number 1 to test number 3 each included the lubricating coating on at least one of the ZnNi alloy plating layer and the CuSnZn alloy plating layer. Therefore, their torque-on-shoulder resistances were more than 100%.

[0087] Comparing test number 2 and test number 3, when the alloy plating layers of the pin surface and the box surface are exchanged with each other, resultant torque-on-shoulder resistances remained high. Therefore, it is conjectured that exchanging the kinds of the alloy plating layers has no great influence on their effect.

[0088] The threaded connections for pipe of test number 4 and test number 5 included neither ZnNi alloy plating layer nor CuSnZn alloy plating layer on both of their pin surfaces and box surfaces. Therefore, resultant torque-on-shoulder resistances were as low as 100% or less.

[0089] The threaded connection for pipe of test number 6 included the CuSnZn alloy plating layer on its box surface but included no plating layer on its pin surface. Therefore, a resultant torque-on-shoulder resistance was as low as 91%.

[0090] The threaded connection for pipe of test number 7 included the ZnNi alloy plating layer on its box surface but included no plating layer on its pin surface. Therefore, a resultant torque-on-shoulder resistance was as low as 88%.

[0091] The threaded connection for pipe of test number 8 included the ZnNi alloy plating layers on both of its pin surface and box surface but included no CuSnZn alloy plating layer. Therefore, a resultant torque-on-shoulder resistance was as low as 90%.

[0092] As seen from the above, the embodiment according to the present invention has been described. However, the aforementioned embodiment is merely an example for practicing the present invention. Therefore, the present invention is not limited to the previously-mentioned embodiment, and the previously-mentioned embodiment can be modified and practiced as appropriate without departing from the scope of the present invention.

REFERENCE SIGNS LIST

[0093] 3 pin [0094] 4 box [0095] 31, 41 thread part [0096] 32, 42 metal seal part [0097] 33, 43 shoulder part [0098] 100 ZnNi alloy plating layer [0099] 110 CuSnZn alloy plating layer [0100] 200 lubricating coating