JPS60135526A - Heat treatment of weld zone of double pipe - Google Patents

Abstract

PURPOSE:To remove the residual tensile stress on the outside surface of an outside pipe of a double pipe and to prevent generation of stress corrosion cracking by heating the part near the weld zone of the outside surface of the inside pipe and the outside pipe and cooling the outside surface of the outside pipe, the outside surface in the exposed part of the inside pipe and the inside surface of the inside pipe where said part is superposed on the outside pipe. CONSTITUTION:Stress corrosion cracking 4 is generated by the residual tensile stress by the thermal influence in the stage of welding in a double pipe formed by welding an inside pipe 1 and an outside pipe 2 in a weld zone 3. An induction heating coil 5 is disposed in the pipe 1 and the walls of the inside and outside pipes 1, 2 near the weld zone 3 are heated in order to prevent such cracking. The outside surface of the outside pipe 2, the outside surface in the part where the inside pipe 1 projects from the outside pipe 2 and the inside surface of the inside pipe 1 in the part where both pipes 1, 2 are superposed are cooled by cooling water (a), (b). As a result, the residual compressive stress by the bending moment occurring in a temp. difference is generated in the outside surface of the outside pipe 2 near the weld zone 3 and the surface of the part where the inside pipe 1 projects from the outside pipe 2. The generation of the stress corrosion cracking owing to the residual tensile stress is thus prevented.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a heat treatment method for pipes, and in particular to a heat treatment method suitable for use in improving residual stress on the outer surface of a double pipe near the bath contact area. be.

"Prior Art" Some of the piping used in nuclear couplants, thermal power plants, chemical plants, etc. has a double-pipe structure, as shown in FIG. There is a structure in which the end of the outer core W2 is welded to the outer surface of the inner and outer surfaces of GL. In the case of such a double-tube structure, the inner and outer tubes 1 and
Residual tensile stress is generated on the outer surfaces of the inner and outer tubes 1 and 2 in the vicinity of the welded portion 3 due to the heat input when welding the tubes 2 .

By the way, in steel materials, it is known that when tensile stress and corrosion factors coexist, stress corrosion cracking rapidly progresses in a direction perpendicular to the tensile direction.

Therefore, in the case of a double-walled pipe in which tensile stress remains as shown in Figure 1, if a condition in which corrosion factors exist outside is applied, a coexistence state of tensile stress and corrosion factors will be formed, resulting in stress corrosion. This causes cracks 4 to occur.

As a means to solve these problems, it is conceivable to actively generate residual compressive stress on the surface where the corrosive factors of snow exist. For example, if a corrosion factor exists on the outside of the tube, the outer surface of d is cooled with cooling water (b), and the wire tube is heated from the inside by the induction heating coil 5, thereby increasing the temperature between the inner and outer surfaces of the tube. This is an application of a method that generates residual compressive stress on the outer surface of the tube by applying a difference and then cooling it.

However, when this method is applied to a double pipe as shown in Fig. 1, it is difficult to provide a sufficient temperature difference dT (T1 - T,) for the inner pipe 1, as shown in Fig. 2. However, the temperature difference d T” that can be given to the outer tube 2 is
('I's "4) tends to be insufficient.Therefore, there are problems that need to be improved, such as the difficulty of generating residual compressive stress near the outward joint of the double pipe. There is.

``Object of the Invention'' The present invention has been proposed in consideration of the circumstances of the prior art described above, and is a method for preventing stress corrosion cracking from occurring on the outer surface near the welded part of a double pipe. The object of the present invention is to provide a method for heat treatment of pipe welds.

"Structure of the Invention" The fourth aspect of the present invention is to heat the bath contact area between the outer surface of the inner tube and the end of the outer tube that covers the inner tube and the vicinity thereof, and to After cooling the outer surface of the protruding portion and the inner surface of the portion of the inner tube where the outer tube overlaps, and creating a moment in the opposite direction to the moment generated in the outer tube near the welded portion of the inner tube, A feature is that residual tensile stress on the outer surface of the outer tube is removed by cooling the inner and outer tubes.

``Example'' Hereinafter, an example in which the present invention is applied to an austenitic stainless steel pipe often used in atomic couplants and fire couplants will be described with reference to FIGS. 3 to 5.

In this embodiment, the inner pipe 1, which has a double M penetrating the wall of the reactor pressure vessel, and the outer 4ff, such as a nozzle that covers the inner pipe 1, are
2, and is intended to improve residual stress on the outer surfaces of both the inner and outer tubes 1 and 2 in the vicinity of the welded portion 3 between the outer surface of the inner tube 1 and the end of the outer tube 2.

That is, as shown in FIG.
While heating the tube wall of outer tube 2, the outer surface and inner surface of outer tube 2 are heated.
The outer surface of the portion of the l that protrudes from the outer tube 2 and the outer tube 2 of the inner tube 1
The arrows (a) and (0) in Figure 3 are on the inner surface of the part where the
Coolant (cooling water) is sprayed onto each surface as shown in . By performing the heating and cooling at the same time, the temperature difference of the tube wall I of the portion of the inner tube 1 protruding from the outer tube 2, which is shown as TI 1.6 in FIG.
In the same figure, there is a temperature difference shown by T, ·T on the tube wall where the outer tube 2 overlaps, and a temperature difference not shown by T, ·T1° is generated on the tube wall of the outer tube 2. let These temperatures are
T,>'1', ,T,>'r,,T,>T, respectively.
. and Tll>T,>T, and T, and T,
The temperature difference between the inner tube 1 and the inner tube 1 is sufficient to generate thermal stress in different directions between the inner and outer surfaces of the inner tube 1, which is equal to or higher than the yield point. Set above.

By creating this temperature difference, it is possible to create a thermal stress distribution as shown in FIG. 4 on the wall of the inner tube 1 protruding from the outer tube 2. That is, tensile stress +σ1 on the outer surface side and compressive stress -σ1 on the inner surface side.
cause In addition, as shown in FIG. 4, the portion where the inner tube 1 and the outer tube 2 overlap has a compressive stress of −σ3 on the outer surface side.
, a thermal stress distribution of tensile stress +σ is generated on the inner surface side. As a result, a thermal stress distribution in the opposite direction is generated in the inner tube 1 across the welded portion 3, and accordingly, a bending moment M1 is generated near the welded portion of the inner tube 1 as shown in FIG. .

On the other hand, To and T are in the outside 872. A thermal stress with tensile stress on the outer side and compressive stress on the inner side is generated based on the temperature difference between The free deformation of the outer tube 2 is more completely restrained by this, and the tensile stress +σ3 generated on the outer surface side of the outer tube ′w2 and the compressive stress −σ generated on the inner side are suppressed to cause sufficient plastic deformation in the outer tube 2. It can be made as large as possible.

Next, when the heating of both the inner and outer tubes 1 and 2 is stopped and both are cooled, residual compression occurs on the outer surface of the portion of the inner tube 1 protruding from the outer tube 2 and on the outer surface of the outer tube 2, as shown in FIG. It is possible to generate stress -σ4・-σ蓼.

"Effects of the Invention" As explained above, according to the heat treatment method for a welded part of a double pipe according to the present invention, a moment is generated in the vicinity of the welded part of the inner pipe in the opposite direction to the moment produced in the outer pipe. By restraining the thermal deformation of the tube, it is possible to generate a large thermal stress that causes plastic deformation on the outer surface of the outer tube. Therefore, even if the temperature difference that can be applied to the outer tube is insufficient, the tensile stress remaining on the outer surface of the outer tube during welding can be reliably removed or changed to residual compressive stress, improving the stress of the outer tube. can be implemented effectively. Furthermore, it is possible to apply a temperature difference sufficient to cause plastic deformation to the inner tube to the portion of the inner tube that protrudes from the outer tube, thereby reliably generating residual compressive stress on the outer surface of the inner tube. In combination with the above-mentioned effects, it is possible to prevent upper sliding stress corrosion cracking from occurring on the outer surface of the double pipe welded portion.

[Brief explanation of drawings]

In the drawings, Fig. 1 is a partially sectional side view showing an example of the structure of a double pipe used in atomic couplants or thermal power plants, etc., and Fig. 2 is a side view showing an example of the heat treatment method for the pipe. Figures 3 to 5 show an embodiment of the present invention, Figure 3 is a schematic diagram showing heating and cooling operations, and Figure M4 shows the inner and outer tubes during heating and cooling. FIG. 5 is a diagram showing the internal stress moment generated in the inner pipe and the residual stress in the outer pipe after cooling. 1...Inner pipe, 2...Outer pipe, 3...
...Welding part, 5...Induction heating coil. Applicant Ishikawajima Harima Heavy Industries Co., Ltd. Figure 3 Figure 4

Claims (1)

[Claims] A method for heat treating a welded portion between an outer surface of an inner tube and an end of an outer tube provided to cover an inner core tube, the method comprising: heating the vicinity of the welded portion of the cylinder/outer tube to a temperature lower than the transformation temperature; The outer surface of the portion of the inner tube that protrudes from the outer tube, the inner surface of the portion where the outer tube overlaps, and the outer surface of the outer tube are respectively cooled, and a moment is generated in the vicinity of the welded portion of the inner tube in a direction opposite to the moment generated in the outer tube. The second method is characterized in that the residual tensile stress on the outer surface of the outer tube is removed by causing plastic deformation due to tension on the outer surface side of the outer tube by generating a moment of , and then cooling the inner and outer tubes. Heat treatment method for heavy pipe welds.