JPH0112995B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an oil well joint with a chromium content, and more particularly to an oil well pipe joint whose seal portion has a specified hardness and melting point. The sealing performance of joints in oil country tubular goods is an important matter. Therefore, the joint structure shown in FIG. 1 is generally used to ensure the sealing performance. In other words, when OCTGs 1 are connected to each other (the other OCTG is not shown) using a coupling joint 2, a metal-metalyseal is used in which the threaded portion 3 ends and the tip thereof tapers when viewed from the pipe 1 side and contacts each other. Part 4
A circular arc seal part 5 having a center of curvature on the joint 2 side is formed at the inlet of this metal-to-metal seal part, and a press-fitting allowance is given to the seal parts 4 and 5 by rotating and tightening the joint 2. A seal is created by generating high surface pressure on the contact surfaces. Conventionally, the radius of curvature R of the arcuate seal portion 5 is as follows:
For high air tightness (1000 to 2000 atmospheres), 1 to 4 mm is used, and for medium air tightness (several hundred atmospheres), 200 to 300 mm is used. By the way, the sealing performance is related to the contact width as well as the surface pressure of the contact portion. To explain this with reference to FIG. 2, when the same load W acts on the arcuate seal portion 5 with different radii of curvature R ₁ and R ₂ , since the contact widths b ₁ and b ₂ are different, the surface pressure P ₁ and P ₂ are also different. And for high airtight use, the radius of curvature is 1~
4mm, and a surface pressure of about 300Kg/ ^mm2 is applied.
However, if the radius of curvature is increased, even though the maximum surface pressure is reduced as shown in FIG. 2, the contact width is increased, so this does not necessarily mean that the sealing performance is directly reduced. Furthermore, if the maximum surface pressure is set too high, seizure will occur when the joint is fastened, so it is important to appropriately set the value of the radius of curvature in order to prevent seizure. On the other hand, the conventional material for oil country tubular goods has been ordinary steel or Cr-Mo steel containing several weight percent of chromium (hereinafter referred to as Cr) or molybdenum (hereinafter referred to as Mo). As conditions become more and more severe, there is a demand for oil country tubular goods that exhibit sufficient performance such as corrosion resistance even in corrosive environments with H ₂ S and CO _2.In response to this demand, high chromium content containing 10% or more of Cr has been developed. Oil country tubular goods made of steel are used. It is necessary to perform some kind of surface treatment on the joint seal portion of oil country tubular goods in order to prevent the above-mentioned seizure and cracking. Therefore, conventional oil country tubular goods made of common materials have been chemically treated with zinc phosphate, manganese phosphate, or the like. However, even if this type of chemical conversion treatment is applied to the aforementioned high chromium-containing steel, the chemical reaction will not be sufficient and the adhesion will be poor, making it virtually difficult to prevent seizure and cracking. Ta. The present invention provides an oil well pipe joint that can reliably prevent seizure and cracking while achieving the desired sealing performance in order to cope with the difficulty of surface treatment peculiar to high chromium-containing steel as described above. The purpose is to To achieve this objective, the present inventor has conducted various studies and found that applying metal or alloy plating is an effective means in place of conventional chemical conversion treatment, and when selecting the metal or alloy plating, found that hardness and melting point are important factors for selection, leading to the completion of the present invention. That is, the present invention is made of a high chromium-containing steel containing 10% or more of chromium, and the threaded surface between the steel pipe and the joint ends and the tip thereof is tapered when viewed from the steel pipe side and contacts each other. A joint having a metal seal portion, at least the metal-to-metal seal portion has a Vickers hardness of 300 or less,
It is also characterized by being plated with a metal or alloy thereof having a melting point of 400°C or higher. Note that the high chromium-containing steel of the present invention contains 10% Cr.
In addition to the above, all steels containing Ni and other components, such as ferrite, martensitic, austenitic, two-phase ferrite and austenite, and other steels used as materials for oil country tubular goods are included. As mentioned above, high chromium-containing steel has poor adhesion of chemical conversion coatings. Therefore, the present inventor conducted various experiments on alternative surface treatment methods. One of the experiments is as follows. That is, a truncated cone-shaped test piece A with a tip of 2 mmφ as shown in Fig. 3 and a square plate test piece B as shown in Fig. 4 are prepared as test pieces, and the surface of test piece B is
Various surface treatments were applied to Bo. The test order is (1)
After applying a compound, which is a type of grease that is generally applied to the threaded part of oil country pipe joints, on test piece B, press the test piece A on top of it with a predetermined load. (2) Test piece A is actually (3) Continuously measure the torque during rotation, and determine that seizure has occurred when the torque increases rapidly as shown in Figure 6. (At the same time, the damage state on the surface of test piece B is also observed, and the results are used as a reference for determining the presence or absence of seizure.) (4) If no seizure has occurred,
Increase the pressing load and repeat steps (2) and (3) until seizure occurs. In this experiment, as shown in FIG. 5, the pressing load was increased in steps of 20 kgf/mm ² of contact surface pressure, and the pressing rotation time was 60 seconds for each step. Note that the continuous increase method may be used instead of the step load method. Here, as surface treatments for the surface Bo of test piece B, Cu plating, Zn plating, Ni-Zn alloy plating, Cr plating, Sn plating, and Ni plating were applied.
Seizure resistance of these various surface treatments (80Kg
The results are shown in the table ^below .

[Table] From the above table, it is clear that Cu plating, Zn plating, and Ni-Zn plating are suitable. This is thought to be due to the high melting point and low hardness. In other words, if the melting point is high, it will not melt due to the frictional heat when the joint is fastened, which will prevent seizure and cracking.If the hardness is low, the plating material will exert a lubricating effect on the contact surfaces when the joint is fastened, preventing seizure and It is thought that this is to contribute to preventing stuffiness. Therefore, we further conducted various experiments (not detailed) and investigated the critical values of melting point and Vickers hardness that would achieve the desired purpose, and found that even if the melting point is 400°C or higher and the Vickers hardness is 300 or lower, It became clear that something was wrong. Furthermore, when such a plating material is applied to a coupling joint or a metal-to-metal seal part of an oil country pipe, the radius of curvature of the arcuate seal part is approximately 1 to 4, which is conventionally used for high airtightness.
It was also found that 200 to 300 mm is sufficient for hollow-tight use. However, it is desirable to set the radius of curvature to 10 to 50 mm and to perform the above-mentioned plating treatment in order to satisfy both anti-seizure properties and sealing properties. On the other hand, the following tests were conducted on oil country tubular joints with each plating used in the above experiments. The steel types of oil country tubular goods are shown in the following table.

[Table] Each joint was tightened and untightened 20 times, and the seizure state in the metal-to-metal seal was observed. No burn-in was observed, but Sn plating, Cr plating and
Seizure was observed in Ni-metsuki. The pressure applied here is 80% of the pressure at which the inner surface of the pipe yields.
It also complies with the API (American Petroleum Institute) standard values. In addition, in the present invention, the plating treatment portion may be applied to the threaded portion in addition to the metal-to-metal seal portion. As described above, in the present invention, at least the metal-to-metal seal part is plated with a metal or its alloy having a Vickers hardness of 300 or less and a melting point of 400°C or more, so it is strong against high chromium-containing steel. It is possible to achieve close contact and also to reliably prevent burning and cracking.

[Brief explanation of drawings]

Figure 1 is a sectional view of the main parts of the joint, Figure 2 is a schematic diagram showing the relationship between radius of curvature and surface pressure, Figures 3 and 4 are front and perspective views of the test piece, and Figure 5. 6 is an explanatory diagram showing the load application method, and FIG. 6 is an explanatory diagram when determining the occurrence of seizure. 1... Oil country pipe, 2... Fitting, 3... Threaded part, 4
...Metal-metal seal part, 5...Circular seal part.

Claims (1)

[Claims] 1 Made of high chromium-containing steel containing 10% or more of chromium, it has a metal-to-metal seal part where the threaded surface between the steel pipe and the fitting ends and the tip tapers off when viewed from the steel pipe side and contacts each other. At least the metal-to-metal seal portion of the joint has a Vickers hardness of 300 or less and a melting point of 400°C.
An oil country pipe joint characterized by being plated with the metal or alloy thereof as described above.