JPS6137015B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The disclosed technology belongs to the technical field of producing corrosion-resistant double pipes such as oil country tubular goods by forming axial compressive residual stress.

Therefore, in this invention, the diameter of the outer tube is increased by heating in advance, and a corrosion-resistant inner tube in a relatively cooled state is fitted in a relatively overlapping manner to the outer tube, and in this state, a tube expansion liquid pressure is applied to the inside of the inner tube. The inner tube is expanded and yielded, plastically deformed and integrated with the outer tube, and the diameter of both tubes is increased. After reaching the set diameter increase state, the expansion liquid is removed and both tubes are reduced in diameter to create a larger diameter. This relates to a manufacturing method that obtains a self-sealing double pipe by obtaining a tightness margin, and in particular, when both pipes reach the set diameter increase state, the pipe is expanded until the temperature difference between the two pipes becomes small. After maintaining the condition and generating sufficient axial fitting force that exceeds the friction limit surface pressure, the tube expansion liquid is removed and both tubes are contracted.The inner tube is heated to increase its diameter, and the outer tube is cooled and contracted to create a larger diameter. The present invention relates to a method of manufacturing a self-tightening double pipe in which a fitting force is obtained and a compressive residual stress is applied to the inner pipe by shrinking the outer pipe in the axial direction.

As is well known, in oil country tubular goods, nuclear power plant piping, etc., in order to provide pressure resistance, heat resistance, and corrosion resistance, double-layered pipes with an outer tube made of carbon steel and an inner tube made of stainless steel, for example, are used. It is starting to be adopted.

Therefore, it is necessary to provide strong bonding force between the inner and outer tubes in order to prevent boundary surface displacement and erosion during operation of the seed double tube, but it is necessary to provide a strong bonding force between the inner and outer tubes, and to ensure uniformity of fit and ease of manufacturing. Due to its various merits, the so-called thermal tube expansion method has been developed and has come to be adopted, as shown in many of the inventions of the applicant's earlier applications.

That is, as schematically shown in FIGS. 1 and 2, a carbon steel outer tube 1
The stainless steel inner tube 2, which has been previously heated to increase its diameter from A to B, is immersed in water up to the temperature of tap water and cooled and reduced in diameter from C to D, is fitted in a relative layer.
Predetermined seal plugs 3, 3' are attached to both ends of the inner pipe 2, and tap water 4 is fed as a pipe expansion liquid from one seal plug 3 to pressurize it, and the strain ε and stress F curves shown in FIG. The inner tube 2 is expanded along the line, and the inner tube 2 yields and comes into contact with the inner surface of the outer tube 1, and further plastically deforms and increases in diameter, and the inner and outer tubes 2 and 1 reach the set diameters E and F, respectively. After that, the expanded pipe water is removed, both pipes 1 and 2 are reduced in diameter, and the temperature is returned to the ambient temperature. A large interference margin ΔD was formed in 2 and a strong fitting force was applied to obtain a self-tightening double pipe.

However, in this type of manufacturing method, the expansion hydraulic pressure is immediately removed after increasing the diameter of both tubes 1 and 2 to the above-mentioned set diameter, so the outer tube 1 has an outer tube temperature T on the horizontal axis and an inner and outer tube 1 on the vertical axis. If we take the surface pressure f of 2, as shown by the straight line C ₁ , since the inner pipe 2 is still at a high temperature when the pipe expansion starts to remove the hydraulic pressure, the temperature continues to drop and approaches the atmospheric temperature, while the inner pipe 2 temperature increases.

Therefore, when viewed from the axial direction, the outer tube 1 is shortened and the inner tube 2 is expanded, as shown in _FIG . Only when the temperature of the tube 1 decreases will the axial displacement be prevented by the friction between the inner and outer tubes, but at this time there is a risk that sufficient compressive residual stress in the inner tube in the axial direction will not be formed.

Therefore, there is a potential risk of stress corrosion cracking in the axial direction, and in particular, in oil country tubular goods, large axial tensile stress is applied due to their own weight during operation, and in addition, they are exposed to harsh corrosive environments. However, there was a problem in that the stress that could be used under the critical stress for stress corrosion cracking had to be limited.

To deal with this, it is possible to apply an axial compressive force to the inner tube 2 during and after the tube expansion, but the disadvantage is that the equipment is extremely complicated, and management and control are complicated and costly. It was hot.

The purpose of this invention is to solve the problem of axial compressive residual stress in self-stressing double pipes based on the thermal expansion method based on the prior art, and to stop and maintain the set expansion state in the hydraulic expansion process. It is an object of the present invention to provide an excellent self-tensioning double pipe manufacturing method that can obtain sufficient axial fitting force without causing any misalignment, remove hydraulic pressure afterward, and apply a large axial compressive residual stress to the inner pipe. be.

The structure of the present invention in accordance with the above-mentioned object is to fit an inner tube which has been cooled and contracted into an outer tube whose diameter has been increased by heating in advance in a relatively overlapping manner,
The inner tube is expanded with a low-temperature liquid, plastically deformed to increase its diameter, and brought into contact with the outer tube.The diameter of both the inner and outer tubes is further increased, and when the set diameter is reached, the diameter increase is stopped and the state is maintained, thereby reducing the temperature of the inner and outer tubes. Reduce the difference and reduce the diameter of the outer pipe to increase the friction force by sufficiently increasing the surface pressure not only in the radial direction but also in the axial direction. After making sure that no axial deviation occurs, remove the tube expansion liquid and connect both tubes. As a result, the temperature increase and expansion of the inner tube is prevented by the above-mentioned axial frictional force, and the inner tube is concomitantly shortened due to the shortening of the outer tube, and a high axial compressive residual stress is applied to the inner tube, causing the inner tube to increase in temperature. The gist is to obtain a high interference margin through diameter increase and diameter reduction by cooling the outer tube, and to apply sufficient fitting force.

Next, an embodiment of the present invention will be described below with reference to FIGS. 1, 2, and 3, and according to FIG. 5. Note that the same parts as in FIGS. 1 and 4 will be described using the same reference numerals.

As described above, the carbon steel outer tube 1 of a set length is heated to a set temperature in advance to increase its diameter, and the stainless steel inner tube 2 is cooled and reduced in diameter by immersion in tap water, as shown in Fig. 2. , B, C, and D are set to the initial diameter and are relatively overlappingly fitted, and the tap water 4 is fed into the inner pipe 2 through the seal plugs 3, 3' and pressurized.
The tube expands along the stress and strain curve shown in Figure ε-F.

As a result, the inner tube 2 increases in diameter, yields, integrates with the outer tube 1, and undergoes plastic deformation, and both tubes 1 and 2 further increase in diameter,
When the set diameter reaches the set diameter, stop applying pressure to expand the tube, maintain the pressure, and wait until the temperature difference between the two tubes becomes small.

At this time, the hydraulic pipe expansion state is maintained, so
As shown in the _C2 line graph in Figure 3, when the temperature of the outer tube 1 starts to decrease, the critical friction surface pressure f ₀ against the inner tube 2 is already sufficiently high.
Since the surface pressure f ₁ of the inner and outer tubes 1 and 2 exceeds 1, axial displacement between the tubes 1 and 2 will not occur.

If the tube expansion pressure is removed when the outer tube temperature drops to T ₀ , by this time sufficient contact pressure between the inner and outer tubes has already been obtained that exceeds the limit frictional surface pressure due to the temperature drop up to that point. Even during the temperature drop of the outer tube, no deviation occurs between the inner and outer tubes in the axial direction, and axial compressive stress is introduced into the inner tube due to the axial temperature contraction of the outer tube.

The temperature T ₀ at which this pipe expansion pressure should be removed is determined experimentally and theoretically, and after the designed time has elapsed, the pipe expansion tap water 4 is removed and discharged through the seal plug 3.

Then, both pipes 1 and 2 contract to T and C as shown in Fig. 2, and a certain amount of interference is obtained in the radial direction, but
By leaving it as it is, the outer tube 1 cools down to the ambient temperature and its diameter decreases from T to E, while the inner tube 2 increases in diameter from T to N until it reaches the ambient temperature, and therefore a large interference of ΔD is obtained. A self-tightening double pipe 5 to which a strong fitting force is applied is obtained.

In the above process, the outer tube 1 is shortened as the temperature decreases in the axial direction as shown in FIG. A large compressive residual stress in the axial direction is imparted due to the stress relative to the warm stretching.

Therefore, the critical stress for axial stress corrosion cracking is set large.

Therefore, even if it is used in the oil country tubular goods mentioned above and is subjected to large tensile stress due to its own weight, stress corrosion cracking will not occur even in a severe corrosive environment.

It should be noted that the embodiments of the present invention are of course not limited to the above-mentioned embodiments; for example, the outer tube may also be plastically deformed during the tube expansion process.
Further, various embodiments can be adopted, such as the need not to pre-cool the inner pipe to shrink the inner pipe.

As described above, according to the present invention, the inner and outer tubes are hydraulically expanded by a thermal tube expansion method, the tube expansion is stopped at a set diameter, and the expansion hydraulic pressure is maintained to reduce the temperature difference between the inner and outer tubes. By removing the liquid after tube expansion, the process in which the outer tube, which has been previously heated to a high temperature, cools down relative to the inner tube, creates enough contact pressure between the two tubes to reach the limit of friction that does not cause axial deviation. Since it is applied in excess of the surface pressure, even if the tube expansion liquid is removed, there will be no axial displacement of the two tubes, and as the outer tube is shortened in the axial direction,
A sufficiently large compressive residual stress in the axial direction is imparted to the inner tube, and therefore, an excellent effect is achieved in that a self-tightening double tube with high radial fitting force and strong resistance to stress corrosion cracking can be obtained.

In addition, since the above-mentioned axial compressive residual stress can be obtained by simply stopping the pipe expansion hydraulic pressure for a set time, there is no need for any complicated equipment, the operation is simple, and there is no need for power, which has the advantage of reducing costs. There is no advantage.

[Brief explanation of the drawing]

FIG. 1 is a schematic explanatory diagram of tube expansion, FIG. 2 is an explanatory diagram of stress strain curves of tube expansion, and FIG. 3 is a comparative explanatory diagram of the relationship between the outer tube temperature and the inner and outer tube surface pressures between the conventional and the present invention.
FIG. 4 is an explanatory diagram of the axial deviation of the inner tube and the outer tube based on the prior art, and FIG. 5 is an explanatory diagram of applying compressive residual stress to the inner tube according to an embodiment of the present invention. 1...Outer pipe, 2...Inner pipe, 4...Tube expansion fluid, F...Tube expansion fluid pressure, ΔD...Clamping margin, 5...Double pipe, _f0 ...Friction limit surface pressure.

Claims (1)

[Claims] 1 Layer the cooling inner tube over the heated diameter-increasing outer tube, then apply expansion liquid pressure to the inner tube to expand the inner tube, expand both tubes together, and after increasing the diameter to a specified diameter, remove the expansion liquid and tighten. In a method for manufacturing a self-contained double-pipe tube through a process, the state in which the inner and outer tubes are integrally expanded to a predetermined diameter is maintained until the temperature difference between the inner and outer tubes becomes small, and the axial displacement friction limit surface pressure is exceeded. A method for manufacturing a self-contained double tube, characterized in that the tube expansion liquid is removed after surface pressure is obtained between the inner and outer tubes.