JPH07217777A - Screw joint with excellent seizure resistance - Google Patents

Abstract

PURPOSE:To enhance the heat-resistance by forming a surface film having a hardness which is less than that of the mother material, on at least a metal sealing part of either one of a pin and a box, and by setting the thickness of the film to a value greater than the surface roughness of a metal sealing surface. CONSTITUTION:A box 1 and a pin 2 are engaged with each other by threads 4. In a metal sealing part 3 having a substantial sealability, either one of boundary contact surfaces of the pin and the box is formed thereover with a covering film layer 5. During screwing of the joint, the outer surface of the mother material opposed to such a surface treated layer slides. Setting the hardness of the surface treating material to a value less than that of the mother material, is an indispensable term for enhancing the heat resistance, in addition to the combination of the film thickness and the surface roughness. The film thickness of the surface film 5 is set to a value greater than the surface roughness of the slide surface opposing thereto, and further, the hardness of the film is set to a value less than that of the mother material, thereby it is possible to exhibit a sufficient heat-resistance.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to repeated make-up (tightening) and break-out (loosening) in oil well pipe threaded joints used for crude oil mining and line pipe threaded joints for transporting mined crude oil. Even if the joint does not seize, it relates to a screw joint that can be used repeatedly.

[0002]

2. Description of the Related Art Generally, a threaded joint is used for a tubing or a casing used for drilling an oil well. These threaded joints must be able to be reused without burning even after repeated make-ups and breakouts. API
(American Petroleum Institute) 10 for tubing fittings
It is stipulated that the joint can be used without seizure even after repeated make-up and breakout.

In order to prevent seizure of joints, surface treatments such as zinc plating, copper plating and phosphate treatment have hitherto been performed. In recent years, in premium joints having a metal sealing part that has been widely used, due to the high surface pressure due to metal-to-metal contact, the metal sealing part is in a situation where it is extremely easy to seize. A surface treatment was applied to the contact interface, and compound grease was applied to improve the seizure resistance.

Generally, the seizure resistance of the metal sealed portion is more preferable as the contact surface pressure is lower, but if it is too low, the gas in the pipe may leak, so that the surface pressure will not be seized and will not cause leakage. The most important point in the conventional joint design was to control the temperature. On the other hand, since the metal seal is turned by a tool called a bite, improving the cutting efficiency of the threaded joint made the metal seal rough, but the relationship between the surface texture and seizure resistance was not clear, and tribology was not found. From the point of view, the optimum seizure resistant design has not been achieved.

[0005]

Conventionally, in order to improve the seizure resistance of the metal seal portion of a premium joint, the joint is surface-treated, compound grease is applied, or contact surface pressure or sliding length is used. I was controlling it properly. An object of the present invention is to improve seizure resistance by providing a specific relationship between the surface roughness of the sliding surface and the film thickness of the surface treatment of the metal sealing portion, in addition to the conventional device.

[0006]

SUMMARY OF THE INVENTION The present invention has a pin having an external thread on the outer surface of the tube and an unthreaded tip following the external thread, and an internal thread on the inner surface of the tube and a threadless section corresponding to the unthreaded tip. In a threaded joint that consists of a box, and this non-threaded part slides during joint screwing to form a metal sealed part, at least the metal sealed part of either the pin or the box has a hardness equal to or lower than the base material. A threaded joint excellent in seizure resistance, characterized in that a surface coating is formed and the film thickness is made equal to or greater than the surface roughness of the opposing metal sealing surface.

It has been said that the cutting roughness of the metal sealed portion of the oil well pipe threaded joint affects seizure resistance of the sealed portion.
However, the smaller the surface roughness, the better the seizure resistance,
It was said that the larger the size, the better, and it was not clear that it was qualitative. As a mechanism of seizure, the present inventors have found that the probability that seizure occurs at the time when the base material is exposed to wear due to sliding of the coating on the opposing metal surface, and a make-up and break The residual coating thickness for each out was used as an index of seizure resistance.

FIG. 1 shows a schematic diagram of a typical premium joint. Here, 1 is a box, 2 is a pin, and they are respectively screwed with screws 4. The metal sealing portion 3 having a virtual sealing property is 3, and the coating layer 5 is applied to one of the contact interfaces of the pin and the box in this portion.
During the screwing of the joint, the surface of the base material facing the surface treatment layer slides.

FIG. 2 shows a schematic view of the sliding surface. Here, 6 is the box-side base material, and 7 is the pin-side base material.
The film 5 is attached to the box surface. In order to improve the seizure resistance of the sliding interface in such a structure, the coating film 5 is softer than the pin side base metal 7 and the coating layer thickness δ.
Is less than the roughness R _max of the opposing contact surfaces.

3 and 4, changes in the plating film thickness after repeating 10 make-ups and breakouts due to the relationship between the plating film thickness when galvanizing is used as the surface treatment layer and the contact surface roughness corresponding thereto. Indicates. In Fig. 3, the initial film thickness is 25
FIG. 4 shows the result when the initial film thickness is around 12 μm. As a result, the film thickness remains, and in order to maintain the seizure resistance, the thickness of the surface treatment layer is designed so as to be at least equal to the surface roughness, and the minimum plating film thickness is preferably 12 μm.
It is desirable to form it to m or more.

Usually, in turning, the surface roughness depends on the feed of the cutting tool and draws a regular pitch, but the influence of these factors other than the roughness on the seizure resistance was studied. FIG. 5 is a result of comparing the relationship between the roughness pitch and the galvanizing thickness after 10 times of make-up and break-out, and the deviation of the surface roughness is 7 μm. As a result,
It was found that the residual film thickness did not significantly depend on the roughness pitch, and the surface roughness alone may be designed as described above. The seizure resistance can be ensured not only by turning with a cutting tool but also when the surface is roughened like a sandblast as long as the conditions of the present invention are satisfied.

Since the surface roughness affects not only seizure property but also sealing property and joint production efficiency, it is 1 to 5 μm.
The range is practical, and the thickness of the coating is preferably 12 to 30 μm in consideration of the decrease in the substantial surface pressure due to wear.

It can be seen from the experimental results in Table 1 that the present invention can be applied regardless of the type of surface treatment. The decrease in film thickness is almost in agreement with the surface roughness. With these types of surface treatment, as long as the relationship between the film thickness and the surface roughness of the present invention is satisfied, 1
No seizure occurs in 0 times of make-up and break-out, but seizure occurs even in 1 or 2 times of make-up when the hardness of the surface-treated material is equal to or higher than the base metal strength like Cr plating and Ti coating. Therefore, setting the hardness of the surface-treated material to the base material or less is an essential condition for improving the seizure resistance in addition to the combination of the film thickness and the surface roughness.

[0014]

[Table 1]

Japanese Patent Laid-Open No. 61-124792 discloses an invention in which the seal surface roughness is specified for the purpose of improving the seizure resistance. However, such an invention specifies only the surface roughness on the side on which resin coating is performed. However, the present invention is different from the present invention in which the roughness of the contact surface opposite to the surface-treated film thickness is defined, and the seizure resistance is not sufficiently improved only by such a prior art.

Similarly, Japanese Patent Laid-Open No. 5-117870 discloses a technique relating to the roughness of the sliding surface for improving the seizure resistance, which is also the same as the above-mentioned known example.
The feature is that the thickness is 0 to 60 μm, which is different from the present invention.

An invention which refers to the roughness pitch of the sealing surface for the purpose of improving the seizure resistance is disclosed in Japanese Examined Patent Publication No. 41-6421. In this invention, the surface roughness of the sliding surface is made rough and the aim is to enclose compound grease in a shell-like gap, but in this study, the wear of the surface coating is reduced more than the effect of such grease. This is more suitable for improving the seizure resistance, and the shape and aim are different from those of the present invention.

[0018]

According to the present invention, the seizure resistance sufficient for practical use can be obtained by forming the film thickness of the surface coating to be equal to or more than the roughness of the opposite sliding surface and the coating hardness to be less than the base material hardness. Can be demonstrated.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a configuration of a threaded joint to which the present invention can be applied.

FIG. 2 is a diagram schematically showing a configuration of a sliding interface intended by the present invention.

FIG. 3 is a diagram showing the relationship between the surface roughness after repeated make-up and break-out 10 times at an initial film thickness of 25 μm and the film thickness reduction.

FIG. 4 is a graph showing the relationship between the surface roughness after repeated make-up and break-out 10 times at an initial film thickness of 12 μm and the film thickness reduction.

FIG. 5 is a diagram showing a relationship between a coating thickness reduction amount and a surface roughness pitch when the surface roughness is 7 μm.

[Explanation of symbols]

 1 Box 2 Pin 3 Metal Sealing Part 4 Screw 5 Coating 6 Box Base Material 7 Pin Base Material Screw

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Haruyuki Nagayoshi 1-1 No. 1 Tobita-cho, Tobata-ku, Kitakyushu-shi, Fukuoka New Nippon Steel Corporation Yawata Works

Claims (1)

[Claims] 1. A pin comprising a male screw on the outer surface of the pipe and a screwless tip portion following the male screw, and a box having a female screw on the inner surface of the pipe and a screwless portion corresponding to the screwless tip portion. In a threaded joint in which the no-body part slides during joint screwing to form a metal sealed part, at least the metal sealed part of either the pin or the box forms a surface coating having a hardness equal to or lower than that of the base material. A threaded joint with excellent seizure resistance, characterized in that the film thickness of the coating is made equal to or greater than the surface roughness of the opposing metal sealing surface.