JPH05169255A - Method for welding tube with tube - Google Patents

Abstract

PURPOSE:To change it to the residual stress of compression that the residual stress distribution of inside surface of tube becomes the residual stress of tensile side being the source of the stress corrosion cracking at the conventional welding method by using the restricting ring and to prevent the stress corrosion cracking, at the time for welding the tube with the tube which constitute the welding construction. CONSTITUTION:A restricting rings 7 are attached at both sides of welding parts 2 of a cylindrical vessel 3. At the state that the restricting rings 7 are attached, the welding parts 2 are welded by using a welding torch 5. The welding residual stress of the neighborhood of weld zone of the inside surface of tube comes the solid line like by welding at the state that the restricting rings 7 are attached. The residual stress can be changed to the residual stress of compression side at the neighborhood of weld zone.

Description

Detailed Description of the Invention

[0001]

The present invention relates to a BWR nuclear power plant,
TECHNICAL FIELD The present invention relates to a welding method for welding parts of pipes and equipment such as chemical plants and cylindrical containers.

[0002]

2. Description of the Related Art For example, the method disclosed in Japanese Patent Laid-Open No. 55-110728 aims at improving residual stress by subjecting a structure after welding to a heat treatment method. FIG. 1 shows a conventional heat treatment method for a pipe butt joint. Heating body 1
Is heated from the outer circumference of the pipe, and at the same time, the inner surface is cooled by the cooling water 4. As a result, a temperature difference is created in the plate thickness direction, and during heating, the outer surface is compressed and the inner surface is yielded to the tensile side to improve stress. FIG. 2 shows the distribution of the residual stress in the axial direction generated during welding. The residual stress on the tensile side is 6, which causes stress corrosion cracking. FIG. 3 shows an example in which the heat treatment method shown in FIG. 1 is applied to improve the welding residual stress in FIG. The residual stress on the inner surface near the welded part exposed to the corrosive environment is compressive stress.

However, all of these examples of improvement are processing methods performed after welding pipes to each other, and residual stress cannot be improved simultaneously with welding. That is, post-treatment after welding was required.

[0004]

SUMMARY OF THE INVENTION According to the present invention, when the pipes constituting a welded structure are butt-welded to each other, in the conventional welding method, the residual stress distribution on the inner surface of the pipe causes stress corrosion cracking on the tensile side. By using a restraint ring, the residual stress of (1) is made to be a compressive residual stress to prevent stress corrosion cracking.

Further, the conventional heat treatment method is a treatment method performed after welding pipes to each other, and residual stress cannot be improved simultaneously with welding. In the present invention, the residual welding stress on the inner surface of the pipe is made into a residual compressive stress without post-processing after the completion of welding to prevent stress corrosion cracking.

[0006]

SUMMARY OF THE INVENTION The above-mentioned problems are solved when welding pipes constituting a welded structure after attaching a circumferential restraint device to the outer periphery of the pipe in the vicinity of the welded portion. It is achieved by

[0007]

According to the present invention, the residual stress of the welded portion on the inner surface of the pipe due to the welding of the pipes is reduced to the residual stress on the compression side, which does not cause a problem with stress corrosion cracking, by using the restraint device. Attempts to make compressive residual stresses without post-treatment. Similarly, when preheating is performed,
By using a restraint ring having a linear expansion coefficient smaller than that of the welded structure, the residual stress on the inner surface of the pipe is made to be the compressive residual stress.

[0008]

Embodiments of the present invention will be described below with reference to FIGS.

As shown in FIG. 4, restraint rings 7 are attached to both sides of the welded portion 2. The distance from the welded portion 2 to the restraint ring 7 varies depending on the welding conditions. For example, the material of the cylindrical container 3 is mild steel SS41, the material of the welding rod that is the source of the welded portion 2 is mild steel SS41, and the cylindrical container 3 has a plate thickness of 12 mm and an inner diameter of 100.
mm, axial length 1200 mm, heat input 1.2 J / mm
At this time, the center of the restraint ring 7 having a ring width of 20 mm may be attached at a position 20 mm away from the center of the welded portion 2.
Further, the stronger the binding force is, the better as long as it is within the range where the cylindrical container 3 is elastically deformed, that is, the range where it is not plastically deformed. Under the conditions of this time, the binding force was set to 150 MPa.

With the restraint ring 7 attached, the weld 2
Welding is performed by using the welding torch 5 in the same manner as the conventional method. The welding method this time is shield gas arc welding, and the groove of the cylindrical container 3 is 30 in the axial direction.
The one processed in the direction of ° was used.

By performing welding with the restraint ring 7 attached in this manner, the residual welding stress near the welded portion on the inner surface of the pipe becomes as indicated by the solid line 6. It can be seen that this is clearly improved to the residual stress of compression in the vicinity of the welded portion, as compared with the residual stress of ordinary welding shown by the solid line 6 in FIG.

FIG. 5 shows a time history of stress states on the inner surface of the pipe when the pipes are welded to each other. Here, time 0 is the welding start time, time t1 is the time when the temperature of the welded portion 2 reaches the maximum temperature during welding, and time t2 is the time after the end of welding.

The stress state when the restraint ring 7 is not attached is shown by a solid line 8. The welded portion 2 is heated by welding heat input and expands until time t1, but at this time, the inner surface of the pipe becomes a compressive stress due to the constraint of the periphery. Then, as it is cooled, the vicinity of the welded portion 2 contracts and is restrained from the surrounding structures, so that the tensile side residual stress finally exists on the inner surface of the pipe at time t2.

On the other hand, the stress state when welding is performed with the restraint ring 7 attached is as shown by the solid line 9. Before welding, a compressive stress exists on the inner surface of the pipe due to the restraining force of the restraining ring 7. Further, the thermal expansion due to welding is also suppressed by the restraint ring 7. After that, the same history as when there is no restraint ring 7 is taken, and at time t2 after the end of welding, a compressive residual stress state occurs.

FIG. 6 shows a method of welding the pipes after the preheating. The restraint ring 7 made of a material having a smaller linear expansion coefficient than that of the cylindrical container 3 is used, and the restraint ring 7 is attached and the whole is preheated. After that, by performing welding, the residual stress distribution on the inner surface of the pipe can be made a compressive residual stress even when preheating is performed.

FIG. 7 shows the time history of the stress state on the inner surface of the pipe when the pipe-to-pipe welding is performed after preheating.
Here, time 0 is the welding start time, time t1 is the time when the temperature of the welded portion 2 reaches the maximum temperature during welding, and time t2 is the time after the end of welding. The solid line 8 indicates the stress state when the restraint ring 7 is not attached without preheating, and the solid line 9 indicates the stress state when welding is performed with the restraint ring 7 attached. When the restraint ring 7 having a linear expansion coefficient smaller than that of the cylindrical container 3 is attached, a compressive stress exists at the time of preheating. Therefore, the time history of the stress state on the inner surface of the pipe when the pipe and the pipe are welded after preheating is indicated by the solid line 1
It becomes like 0. Finally, at the welding end time t2, compressive residual stress exists.

[0017]

According to the present invention, when the pipes constituting the welded structure and the pipes are butt-welded to each other, in the conventional welding method, the residual stress distribution on the inner surface of the pipes causes residual stress on the tensile side which causes stress corrosion cracking. By using a restraint ring at the place where the stress is generated, it is possible to make the residual stress of compression and prevent stress corrosion cracking.

[Brief description of drawings]

FIG. 1 is an explanatory view of a heat treatment method for a circumferential weld portion by a conventional heat treatment method.

FIG. 2 is an explanatory diagram of a welding residual stress distribution when normal welding is performed.

FIG. 3 is an explanatory diagram of improved residual stress after heat treatment by a conventional heat treatment method.

FIG. 4 is an explanatory view of an embodiment of a welding method using a restraint ring.

FIG. 5 is an explanatory diagram showing changes in residual stress over time when a welding method using a restraint ring is performed.

FIG. 6 is an explanatory view of an embodiment of a welding method performed after preheating the whole body by using a restraint ring having a linear expansion coefficient smaller than that of the welded structure.

FIG. 7 is an explanatory diagram showing a time history change of residual stress when a welding method is performed after preheating the whole body using a restraint ring.

[Explanation of symbols]

2 ... Welded portion, 3 ... Cylindrical container, 4 ... Cooling water, 5 ... Welding torch, 6 ... Inner surface residual stress distribution, 7 ... Restraint ring.

Claims (1)

[Claims] 1. A welding method for pipes and pipes, characterized in that, when welding pipes to each other, welding is performed after a circumferential restraint device is attached to the outer circumference of the pipe in the vicinity of the welded portion.