JPS5944155B2 - Yousetsutsugitekouzou - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a welded joint structure, and particularly to a welded joint structure suitable for use in austenitic stainless steel piping for a nuclear reactor primary cooling system.

Examples of conventional pipe weld joint structures are shown in FIGS. 1 and 2.

FIG. 1 is a sectional view of the groove before welding, and when joining the welding pipe 1 and the pipe to be welded 2, the joint is processed as shown in FIG.

Since there is a dimensional difference between the welded pipe 1 and the welded pipe 2 within the specified tolerance, an inner diameter adjustment process 3 is performed to prevent a step from forming on the inner surfaces of the pipes 1 and 2 after welding. Further, the outer surfaces of the pipes 1 and 2 are provided with a U-shaped groove 4 to facilitate welding work. The pipes 1 and 2, which have been subjected to the inner diameter adjustment process 3 and the U-shaped bevel process 4, are butted together as shown in FIG. 1, and a welding rod is welded to the U-shaped portion 5 to join the pipes 1 and 2.

FIG. 2 is a cross-sectional view after welding, and shows the welded metal 6 of the U-shaped portion 5.
shrinks when cooled, so the welded part outer diameter Dw is the pipe outer diameter D
becomes smaller compared to The conventional pipe welding method described above has the following drawbacks.

Due to the contraction of the welded metal 6, tensile residual stress in the circumferential direction and the axial direction is generated in the inner portion of the pipe near the heat affected zone l.

The maximum value of the residual stress reaches approximately the yield stress of the material. Furthermore, the material near the heat affected zone T becomes sensitive due to heat input during welding. Furthermore, the shape of the welded part changes due to the inner diameter adjustment process, so it is susceptible to concentration of load stress. In particular, when a sudden change in shape is caused due to a defect in the inner diameter adjustment process, a large stress concentration occurs. Recently, stress corrosion cracking of austenitic stainless steel piping in the primary cooling system of light water reactors has become a problem.

Stress corrosion cracking is a complex phenomenon, and its causes are not completely understood, but it is generally accepted that stress corrosion cracking is more likely to occur when tensile stress is applied to areas where the inner surface of pipes is sensitive. It is a fact. As mentioned above, the inner surface of the pipe is sensitive near the welded part, and is subject to large tensile stress due to residual stress, etc., so stress corrosion cracking is extremely likely to occur. In fact, 9Oq of piping trouble due to stress corrosion cracking
B or more occurs near the weld. Therefore, to prevent stress corrosion cracking, it is effective to reduce sensitization and lower tensile stress.

For this reason, heat input management temperature management during welding, heat treatment after welding,
Efforts are being made to increase sensitivity, reduce residual stress, and load stress by examining piping routes, piping support methods, and developing new materials. In particular, the method of applying compressive stress to the inner surface of piping through peening is considered to be one of the effective methods for preventing stress corrosion cracking, but if the peening is uneven, large tensile stress is generated locally, which can cause It has drawbacks such as increased susceptibility to stress corrosion cracking, so it has not left the realm of research yet.

As mentioned above, various stress corrosion cracking prevention methods have been studied, but problems remain and stress corrosion cracking has not yet been completely prevented.

An object of the present invention is to provide a welded joint structure that can prevent stress corrosion cracking from occurring near the welded portion of piping. The present invention provides a pipe joint structure in which two pipes having a V- or U-shaped groove structure are connected by butt welding, in which a shrinkage force is exerted on at least the outer peripheral portion of the butt side ends of both pipes. It is characterized by a ring-shaped member attached.

A preferred embodiment of the pipe welding joint of the present invention is shown in FIGS. 3 and 4.

FIG. 3 is a cross-sectional view of the groove before welding, in which the welding pipe 1 and the pipe to be welded 2 are fitted with a shrink fit ring 8 on the outer periphery of the abutting side ends of both pipes, and then subjected to inner diameter adjustment processing 3 and U-shaped processing 4 is performed.

FIG. 4 is a sectional view after welding, and since the welded metal 6 of the U-shaped portion contracts when cooled, the welded portion pipe outer diameter Dw is smaller than the pipe outer diameter D.

Therefore, although tensile residual stress occurs in both the circumferential and axial directions in the inner part of the piping near the heat-affected zone 7, the shrinkage force of the shrink fit ring 8 cancels out at least the circumferential tensile residual stress. Then you can. Outer diameter 89.1Tf
rm, wall thickness 7.6m austenitic stainless steel piping, inner diameter 88.9m, wall thickness 76? When the austenitic stainless steel ring is shrink-fitted, approximately 24
kg/l! Since the compressive stress of AI12 is generated, it can be seen that if a similar ring-shaped member is provided at the pipe welding part, the circumferential tensile residual stress generated on the inner surface of the pipe due to welding can be canceled out.

Stress corrosion cracking in pipe welds occurs due to the combined effects of sensitization, tensile stress, and a corrosive environment; therefore, stress corrosion cracking in welds can be suppressed by reducing tensile stress. Further, the shrink fit ring 8 can also be used as a strength member to reduce stress applied to the welded portion.

The welded parts of piping have become sensitized and are susceptible to large stresses due to welding residual stress and stress concentration due to internal diameter adjustment processing, but reducing the stress level at the sensitized parts with the shrink fit ring 8 is effective in preventing stress corrosion cracking. It is. It is desirable that the material of the shrink fit ring 8 is the same as that of the pipes 1 and 2 from the viewpoint of corrosion prevention, but if there is a special requirement, a ring 8 made of a different material may be used.

For example, if ring 8, which has a smaller thermal expansion coefficient than piping 1 and 2, is used, the shrink fit stress can be increased as the temperature increases, and stress is not applied at low temperatures, but only at high temperatures. It is also possible to construct an additional ring 8. In the examples shown in FIGS. 3 and 4, a shrink fit ring was used to reduce the tensile stress near the heat affected zone, but the
In the embodiment shown in FIG. 6 and FIG. 6, a weld build-up 9 is provided in a ring shape.

FIG. 5 is a cross-sectional view of the groove before welding, in which the weld pipe 1 and the pipe to be welded 2 are subjected to inner diameter adjustment processing 3 and U-shaped processing 4 after providing a weld build-up 9 on the outer peripheral surface of the welded part. ing. 6th
The figure is a cross-sectional view after welding, and due to the shrinkage of the weld metal 6 in the U-shaped part, the inner part near the heat-affected zone 1 of the pipe is
Although tensile residual stress occurs in both the circumferential and axial directions,
Since compressive stress is applied by the weld build-up 9, tensile residual stress in at least the circumferential direction can be reduced.

In the embodiments shown in FIGS. 5 and 6, weld build-up is performed before pipe welding, but weld build-up may be performed after pipe welding. Further, in the embodiments shown in FIGS. 3 and 5, shrink fitting is performed before pipe welding, but the tightening ring may be attached after pipe welding. FIG. 7 shows an embodiment in which a removable tightening ring 8 is attached after pipe welding.

Above, we have described examples of welded joints that prevent stress corrosion cracking by adding compressive stress by providing a shrink fit ring, ring-shaped weld build-up, etc. to the welded part, but the effect of adding compressive stress by the shrink fit ring Since this is much larger than that produced by weld build-up, etc., it is considered effective to use a shrink-fit ring unless there are any special problems.

According to the present invention, it is possible to reduce tensile stress in a pipe weld and prevent stress corrosion cracking.

[BRIEF DESCRIPTION OF THE DRAWINGS] FIG. 1 is a vertical cross-sectional view of the conventional example before welding, FIG. 2 is a vertical cross-sectional view of the conventional example shown in FIG. 1 after welding, and FIG. 3 is a vertical cross-sectional view of the conventional example shown in FIG. FIG. 4 is a cross-sectional view of the embodiment of the present invention shown in FIG. 3 before welding; FIG. 5 is a vertical cross-sectional view of another embodiment of the present invention before welding; 6 is a longitudinal sectional view of the embodiment shown in FIG. 5 after welding, and FIG. 7 is a longitudinal sectional view of another embodiment of the present invention. DESCRIPTION OF SYMBOLS 1... Welded pipe, 2... Piping to be welded, 4... U-shaped grooved part, 6... Welded metal, 7... Heat affected zone,
8... Shrink fit ring, 9... Weld overlay part.

Claims (1)

[Claims] 1. In a pipe joint structure in which two pipes having a V- or U-shaped groove structure are connected by butt welding, a ring-shaped member having a contractile force is attached to at least the outer peripheral portion of the butt side ends of both pipes. A welded joint structure characterized by the installation of .