JPS597554B2 - Duplicate pipe weld joint processing method - Google Patents

Description

[Detailed Description of the Invention] This invention forms compressive residual stress in the inner pipe in advance to prevent stress corrosion cracking at the welded joint where unit pipes are joined together in a double pipe used for oil pipes, etc. It belongs to the technology of

Accordingly, the disclosed invention is a method of welding double pipes, each of which is made by stacking an austenitic stainless steel pipe as an inner pipe and a carbon steel pipe as an outer pipe, and welding them together at the abutting parts of their ends. The present invention relates to a method for treating residual stress, especially tensile residual stress in an inner pipe, so as to prevent stress corrosion cracking from occurring during fluid transport. On the other hand, internal pressure was applied from inside the tube to cause plastic deformation beyond the elastic limit, and then the tube contracted after applying a set stress. The present invention relates to a method for treating a welded joint of a double pipe in such a way that a compressive stress that does not reach the critical tensile stress that causes corrosion cracking is generated.

As is well known, the transport fluids used in various plant piping, oil transmission pipes, oil country tubular goods, etc. are often corrosive, containing corrosive substances such as chlorides, sulfides, acids, and alkalis. In order to deal with pressure loss during the process, the pressure is often increased.

Therefore, in consideration of corrosion resistance, pressure resistance, etc., for the above-mentioned transportation piping, for example, the inner tube is made of highly corrosion-resistant austenitic stainless steel tube, and the inner tube is made of polymerized carbon steel tube with strong pressure resistance. A double-pipe tube made of so-called stainless clad steel pipe is used to improve physical properties and mechanical properties.

By the way, as mentioned above, the austenitic stainless steel and carbon steel have good properties as a result of transportation pipes, but due to their physical property that they have a difference in coefficient of thermal expansion, they cannot be used in the cladding process. An extremely large tensile residual stress is generated due to the difference in mixture content between the time of high-temperature hot rolling and the time when the clad steel plate is cooled to room temperature.

By the way, under the severe conditions in which transport pipes are actually used, ie, when transporting corrosive fluids, the problem of so-called stress corrosion cracking occurs.

That is, according to theory and experiments, it has been found that stress corrosion cracking in boiling magnesium chloride rapidly occurs in SUS316 when the tensile residual stress exceeds 10 kg/i.

In particular, it has been found that the tensile residual stress is more severe as it approaches the yield stress, and in actual long-distance transport pipes, SUS3L6 is 15kg/m1? Since a certain tensile stress acts on the steel and exceeds the above-mentioned critical tensile stress of 10 kg/71t1t2, stress corrosion cracking appears at an early stage.

To address this issue, the applicant's earlier patent application filed in 1984-49
In the invention of No. 654 (filed on April 24, 1978), in a double pipe of stainless steel lined pipe, the pipe is expanded from the inside in a predetermined manner and the difference in yield stress between the inner and outer pipes is used to deform the pipe after plastic deformation. tube contraction,
A tensile residual stress is applied to the outer pipe and a compressive residual stress is applied to the inner pipe to prevent the generation of tensile stress exceeding the critical tensile stress for stress corrosion cracking. Developed practical possibilities.

By the way, in the piping of double pipes such as double pipes used in actual use, the pipe bodies are manufactured to prevent stress corrosion cracking as described above, but due to constraints such as factories and transportation, unit pipe bodies of a predetermined length are used. Therefore, in reality, it is necessary to connect and extend at least one pair of subsequent tube bodies, which is generally done through butt welding means.

Meanwhile, as shown in Fig. 1, a double pipe 3 with an austenitic stainless steel pipe 1 as the inner pipe and a carbon steel pipe 2 as the outer pipe,
3 is welded to the outer tube 2 and the inner tube 1, the outer tube 2 and the inner tube 1 are axially symmetrical about the joint 4 until the joint 4 is cooled from the high temperature state during welding to the normal temperature state. Compressive residual stress σP and tensile residual stress σ gradually decrease to the maximum value in the length direction.
, is distributed as residual formation.

It has also been found that the tensile residual stress of the inner tube is so large that it approaches the yield stress.

In addition, as mentioned above, oil pipes, etc. are transported under high pressure to cover pressure loss during transport, so SUS3L6 has a pressure of 15k.
A tensile stress of about 9/Mm2 is applied, which easily exceeds the critical tensile stress of 10k9/Mm2 for stress corrosion cracking. Therefore, even if stress corrosion cracking has been taken care of in the general part of the pipe, if tensile residual stress that is likely to cause stress corrosion cracking is formed in the area around the joint 4, the joint 4 will be damaged. The disadvantage is that cracks occur early.

To deal with this, it is difficult in terms of equipment to carry out the tube expansion method of the earlier application over the entire length after welding, and it is also possible to rapidly cool parts of the pipe, but it is also difficult in terms of equipment and due to the material quality. There are disadvantages of deterioration.

The purpose of this invention is to take measures against stress corrosion cracking in the general parts of double-walled pipes, etc. In view of the problem of stress corrosion cracking that occurs in the welded joints of pipes during operation, After butt welding the double pipes together, a tube expansion device is inserted through the tube end opening and set at the weld joint, expanding the joint and plastically deforming it beyond its elastic limit to eliminate the radial strain. After the tube shrinks, the difference in yield stress is used to create a compressive residual stress so that only an operating tensile stress lower than the critical stress corrosion cracking tensile stress finally acts on the inner tube where the tensile stress remains. In addition, an excellent double pipe welded joint is created in which tensile residual stress is formed in the outer layer pipe to prevent stress corrosion cracking from occurring over the entire length of the pipe, thereby maintaining the transport capacity of the pipe. This paper aims to provide a processing method. Next, an embodiment of the present invention which fulfills the above-mentioned object Vc will be described below with reference to FIG. 2 and the following drawings.

In the following, parts having the same features as those in FIG. 1 will be described using the same reference numerals. Fig. 2 shows a double-walled pipe.As an embodiment, when a specified tube expansion device is inserted into the inner pipe of the double-walled pipe, set at the welded joint, and expanded, the strain increases in the radial direction with respect to the pipe expansion stress σ. The amount of strain ξ is as gentle as the 0B curve in the outer tube because the yield stress is small, whereas the inner tube has a steep curve in proportion to the 0A curve and the small yield stress.

Then, plastic deformation is performed beyond the elastic limit, and the strain set value ξ is determined based on experiments and theory. When this is reached, the application of tube expansion stress is stopped and the tube shrinking process begins.

Therefore, the stress and strain in both tubes are reduced, and finally the set strain amount is reached, that is, the set compressive residual stress - σ
The amount of strain ξ such that 1-10k9/M7f7- is formed.

, and a corresponding tensile residual stress +ξ8 is formed in the outer tube. In this way, the difference in yield stress is used not only to remove the tensile residual stress in the inner tube during welding, but also to
It is possible to apply compressive residual stress prior to operation.

The pre-applied compressive residual stress, for example, the above-mentioned 10k
9/Ml2 is ξ. It can be obtained by determining , and can be selected in a predetermined manner.

In this way, the obtained double pipe has not only its general part, but also
The welded joint is also pre-formed with a compressive residual stress of, for example, -10k9/Mi, so even if it is transported under pressure so that a tensile stress of, for example, 15k9/Ml2 is applied in the form of an oil pipe, etc. Minus 5kg/77! Only tensile stress of i acts on the inner tube, and the stress corrosion. 1 of the critical tensile stress for cracking
Since it does not reach 0k9/Mi, stress corrosion cracking does not occur over the entire length.

Although means such as hydraulic pipe expansion and mechanical pipe expansion can be adopted as appropriate, the embodiment shown in Fig. 3 is a hydraulic pipe expansion mode, in which an austenitic stainless steel pipe is used as the inner pipe 1 and a carbon steel pipe is used as the outer pipe. When a predetermined number of unit length clad steel double pipes 3 having a predetermined shape as pipes 2 are connected, butt welding is performed to each other at a joint portion 4.

Measures have been taken to prevent stress corrosion and cracking of the circumference and each double pipe 3. Therefore, regarding the welded double pipes 3, 3, one of the
That is, a setting ring 6 having a U-shaped cross section and a set diameter slightly smaller than the inner pipe 1 is inserted through the opening 5 of the shorter pipe length, and the weld joint 4VC is appropriately set.

The inside of a hard ring-shaped rubber tube 8, which is installed in the inner cavity 7 of the settling ring 6 in advance, passes through the settling ring 6 and is connected to a hydraulic cylinder (not shown) via a pressure hose 9. The cylinder is operated to apply hydraulic pressure to the rubber tube 8f1C to directly apply tube expansion stress to the joint portion 4 of the inner tube 1, thereby exerting a tube expansion action on the inner tube 1 and the outer tube 2. As the joint portion 4 is expanded in this manner, the inner and outer tubes 1 and 2 undergo plastic deformation exceeding their elastic limit, following the stress-strain curve shown in FIG. 2, due to the difference in their yield points.
The strain gauge 10 measures the amount of strain in both over time.

Thus, as described above, the preset distortion amount ξ. When reaching this point, the pressure oil supply is stopped and both pipes 1 and 2 are contracted.

Then, in the tube shrinking process, the set strain amount ξ is
．． The compressive residual stress ξ is finally set in the inner tube 1 by selecting ξ.

However, a tensile residual stress ξ3 is also formed in the outer tube 2＼
Become. Further, the embodiment shown in FIG. 4 is an embodiment based on a mechanical tube expansion means set for the joint portion 4, and the hydraulic cylinder
A split ring 1 is provided around the ring 1 through a tongue 12.
3 is inserted into the joint part 4 from the end part 5 in the contracted state, and pressurized oil is supplied from a hydraulic pump (not shown) through the pressure hose 9 to operate the hydraulic cylinder to retract the tongue 12. The split ring 13 is used to evenly expand the joint part 4 from the inside, and the plastic deformation beyond its elastic limit and the set strain amount ξ. The operations such as stopping expansion and contracting the tube are the same as in the above embodiment, and the formation of compressive residual stress that does not cause stress corrosion cracking in the inner tube is also the same.

However, it goes without saying that the embodiments of the present invention are not limited to the above embodiments, and are not limited to double pipes, but also triple pipes, and various pipe expansion means can be employed. As described above, according to the present invention, when a plurality of laminated pipes are polymerized from pipes having different yield points, and the pipes of a set unit length are joined and extended via a welded joint, the welded joint is accessed from the inside of the pipe. A compressive residual stress is formed in the inner tube so that an operating tensile force below the design stress corrosion cracking limit tensile force acts on the inner tube. By applying expansion strain to the tube to stop the tube expansion, and then contracting the inner tube and outer tube based on the difference in yield stress to form the compressive residual stress in the inner tube, the double overlap as described above is achieved. When the pipe is operated as an oil pipe, etc., even if a pumping pressure is applied and the operating tensile stress is applied, the tensile force can always be kept below the critical tensile stress for stress corrosion cracking. There is an excellent effect in that there is no possibility of stress corrosion cracking occurring in the welded joint.

In particular, when the unit duplex pipe is subjected to stress corrosion cracking treatment, there is an effect that stress corrosion cracking does not occur over the entire length.

Therefore, double pipes made of stainless clad steel materials, especially,
In the case of double-walled pipes made of austenitic stainless clad steel materials, the latent tensile residual stress that is inevitably formed during the steel manufacturing process is also removed, and corrosive substances such as natural gas, petroleum chlorides, sulfides, acids, alkalis, etc. Stress corrosion cracking does not occur even when transporting fluid containing substances, and the advantages of corrosion resistance, good weldability, and good workability, which are the inherent good characteristics of stainless steel clad pipes, can be utilized.

[Brief explanation of the drawing]

Fig. 1 is an explanatory diagram of residual stress distribution in a butt-welded joint of a double pipe, Fig. 2 and the following are explanatory diagrams of embodiments of the present invention, and Fig. 2 is an explanatory diagram of compressive residual stress formation strain-stress relationship. Figures 3 and 4 are schematic illustrations of the tube expansion method. 1...Inner pipe, 2...Outer pipe, 3...
...Double pipe, 4...Joint part, -σA...
...Compressive residual stress.

Claims (1)

[Claims] 1. In a method for handling welding residual stress at a welded joint where double pipes made by stacking a plurality of pipes with different yield points are joined at their ends, the joint where the double pipes are welded together. After applying internal pressure to locally expand the pipe and expanding it beyond the elastic limit, the inner pipe welded joint had a welding tensile residual stress based on the difference in yield stress between the duplex pipes. A double pipe welding hand treatment method characterized in that compressive residual stress is generated in the double pipe welding part.