JPS5993586A - Method of relaxing stress of addition welding piping - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to welding residual stress in piping in which an appendage, particularly a cylindrical appendage, is fitted and attached by welding. Concerning stress relaxation methods for welded piping.

[Prior art]

Various types of welded piping are used in power plants, including nuclear power plants, and chemical plants. Generally, the tensile stress of the material's yield stress type is /A in these welds.
trading. This tensile residual stress causes stress corrosion in piping used in corrosive environments.

It is often the main cause of corrosion fatigue cracking, water, and A-embrittlement sidewalling. In order to prevent such cracks, the wires used under harsh conditions are subjected to residual stress relaxation heat treatment after welding.

Conventionally, this process has been commonly carried out by (1) stress relief annealing, which involves holding the temperature at 600 C/1 degree for several tens of minutes to several hours in a heating furnace, and then slowly cooling it, (2) locally damaging the welded area by fire or other means. There are local heat treatments that heat the area. The former method can be applied to piping manufactured in a factory during assembly, and has been shown to work well in practice. However, in many cases, method (1) cannot be applied to the installation U after assembly is completed or to the on-site Id connection U. If a crack is discovered after the start of operation and it becomes necessary to send the pipe to the US, method (1) cannot be applied unless the pipe is dismantled. On the other hand, method (2) has a wide range of applications, but lacks reliability and its effectiveness is unclear. If the method is incorrect, it may have the opposite effect.

After the assembly is completed and the plant begins operation, the following is a specific example of a pipe in a BW plant that cracked and required countermeasures.

Material deterioration and tensile residual stress occurred in the welded part of the 8US304 steel piping at the BW Hanseong Plant due to the welding heat cycle, and stress corrosion cracking occurred due to the combination of the corrosive environment of high-temperature water. However, it is difficult to dismantle this pipe, and even if it is temporarily dismantled, the stress-relieving annealing at about 600C as mentioned earlier will generate a sensitized structure in 5O830.4 steel, so the residual stress will be removed. Even if it is possible, it is impossible to apply conventional methods because it will cause material deterioration. Therefore, as shown in Fig. 1, weld metal 1
A concession-shaped groove 2 is machined on the outer circumference of the U3.3', and the previously formed groove is re-done while cooling the inner surface of U3.3' with a coolant 4 to prevent the inward direction from rising to the sensitization temperature. A method has been developed to reduce the tensile residual stress on the viewing surface by filling it with a melting solution. This method is effective for straight U welds as shown in this figure, and is already used in actual plants. However, this method does not have a stress relaxation effect on the one-step weld of branch d as shown in Figs. 2 and 3, and
Limited.

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for relieving residual stress at branching and U-welding portions of plant piping after the start of operation of an assembly T-distributor without disassembling the piping.

[Summary of the invention]

The biggest reason why the stress in the weld area can be alleviated by the method shown in Figure 1 is that by cooling the inner weld and performing the welding process 5,
m t in the thickness direction that occurs on the inner surface (low temperature) and outer surface (high temperature)
By i mΔ1゛, as shown in Fig. 4, the inner surface of u is 1
Tensile stress occurs during welding, and compressive stress occurs after rewelding. This means that the existing tensile residual stress will be relaxed before rewelding. In addition, due to the above-mentioned effect due to ΔT, rewelding causes local damage as shown in Figure 5.
The deformation in which a swells is the average temperature of the inner and outer surfaces of the re-welded part +11.
It depends on 4 and becomes fruit. This means that compressive stress is generated on the inner surface of a during bathing and contact, and tensile stress is generated after rewelding. However, in a circumferential joint such as this example in which pipes are welded, the effect of Tm is smaller than the effect of ΔIl+, and the relaxation of stress can be obtained.

However, when Kenpo is applied to the branch 'nm junctions as shown in Figs. 2 and 3, stress relaxation cannot be obtained because the effect of 1゛Yo is stronger than that of ∆T. However, compressive stress remains due to T at a location a little axially distant from the inner weld in FIG. In order to take advantage of the effect of the mounting rod of the present invention, the re-welding area and the initial bath contact area should not be aligned but separated from each other.

Embodiment FIG. 2 shows a branch U of a distribution/d branch structure to which the present invention is applied, in which a small diameter branch = #7 (7
')' Branch Sacell fc me (D'tt 8 (8'
) is attached by inset welding 9 (9').

7 (7') and 8 (8/) are joined by butt welding 10 (10'). FIG. 2 shows a cross section of the branch pipe portion of FIG. 1. When implementing the present invention in this arrangement a, first, as shown in FIG. 6, a bevel groove 11 with a depth h on the outer peripheral surface KI1mB of 6, which is 8 apart from 9, is machined along 9 with a force 11. After that, while cooling the inner surface of the tube with coolant 4,
1. Perform overlay welding. FIG. 7 shows the state after overlay welding 12 is performed while cooling the inner surface in this manner. FIG. 8 schematically shows a deformed state during build-up welding.

Maximum temperature on the outside surface during overlay welding! ('I', , , ) rises to the melting temperature, so it expands locally as shown by the solid line in Figure 48. However, the inner surface temperature (T-) is only about 300C because it is cooled.

This thermal deformation effectively relieves the welding residual stress on the inner surface of 9. Hereinafter, only relevant portions of the effects of the present invention will be selected and explained.

Figure 9 shows that during overlay welding, part 12 attempts to expand and deform in the radial direction corresponding to the average temperature of the inner and outer surfaces ['l', = (1',, 10 Tm)/2, but 12 Local bending indicates a situation where deformation occurs due to constraints in the vicinity of . As can be seen from the figure, during overlay rewelding, 12 is tensile stress (+), the inner surface of that part is compressive stress (-L12, the outer surface of 9 is compressive stress, and the inner surface is tensile stress). If all of these stresses are greater than the yield stress, the meat Ji1
．． After the bath welding is completed and the temperatures on the inner and outer surfaces become uniform, stress of the opposite sign to that during build-up remains, resulting in a situation as shown in the lower diagram of FIG. 9. That is, due to expansion deformation bending stress, 9
Compressive stress will remain on the inner surface.

Next, due to the average temperature T1, the inside and outside of 12 tries to expand uniformly, but due to the constraint of 6, compressive stress due to membrane stress occurs during welding, and the tensile stress due to membrane stress occurs on the inside and outside 1flj of 9. Membrane stress is stopped. This situation is shown in Figure 1O.
In the figure, when we fix that the closed and original overlay welds of 12 are uniformly hot, and look at this from the side of branch a7, 12 tries to extend in the 70-half-inch direction. From this, the generation of membrane stress can be understood (it becomes even more understandable if we assume that the main pipe 6 is a relatively large branch a compared to the branch I77)! I11! can).

When this film stress exceeds the yield point, the first
As shown in Fig. O and F, there is compression on the inner and outer surfaces of 9, and tensile stress remains on the inner and outer surfaces of 12. , inner surface and two low temperatures), tensile stress is generated on the outer surface during the build-up melt 4m, and after completion of the build-up, tensile film stress remains on the outer surface and compressive stress remains on the inner surface. This can be said to be bending stress dominated by the difference in internal and external surface temperatures.

That is, by performing overlay welding 12 on the outer surface of 1lls apart from 9, the inner surface of 9 has the following effects: expansion deformation dependent bending stress (Fig. 9), membrane stress (Fig. 1O), inner and outer surface temperature. Compressive residual stress is generated by three types of differentially dependent bending stress (iTii diagram). On the other hand, the inner surface of 12 is the 11th
A compressive residual stress occurs only when the effect shown in FIG. 9 is greater than the sum of the effects shown in FIGS. 9 and 10.

In the above example, the tensile residual stress on the inner surface of 9 was relaxed due to overlay welding, but in Figure 12, an elevated heating coil 14 is placed along 9 at a distance S from 9. established,
A high frequency magnetic current is applied to this from a high frequency power supply connected via a couple 15 and a torsion 16. In this case, the coolant 4 is flowing inside U to cool the inner surface and the current is υ11. vinegar. An induced current is induced on the outer surface of the mother wire (a) of the coil 14, resistance heat is generated, and the temperature rises. On the other hand, since the inner surface is cooled, it is kept at a low temperature. That is, in the previous example, heat was applied by overlay welding, but in this row, heat is applied by high frequency induction heating, and the effect is basically the same as in Figs. 9 to 41411. The difference is that in the case of overlaying, strictly speaking, the film stress effect is small because the molten jχ- moves, but in this example, the film stress effect is smaller than in the previous example because t, coil IM, and F generate heat at the same time. larger than

In order to make the film stress effect even more effective than the chestnut shown in FIG. 12, FIG.

The above example describes the branch U that connects a branch pipe to the main pipe, but as another example, fa connects a pipe or nozzle to a cylindrical volume g'4 or a rectangular container.
It can also be applied to the contact portion in exactly the same manner as described above.

[Effects of the Explanation] According to the present invention, the tensile welding IAwl stress on the inner surface of the nozzle fitting welding portion can be effectively alleviated.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram of the prior art, FIG. FIG. 6 is an explanatory diagram showing the preparatory stage for implementing the present invention, FIG. 7 is an explanatory diagram showing the implementation results, FIG. 8 is an explanatory diagram showing the deformation of the pipe during implementation of the present invention, and FIGS. FIG. 11 is an explanatory diagram showing the effect of the present invention, FIG. 12 is an explanatory diagram showing an apparatus+1Q configuration and implementation method showing an application example of the present invention, and FIG. 13 is a modification of FIG. 12. It is an explanatory diagram. 6...Main pipe, 8...8 units, 9...Built-in welding, 1
1... Bevel groove, 4... Coolant, 12... Overlay welding, 14... Heating coil, 17... Jtd wave C"
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Claims (1)

[Claims] 1. In the welding residual stress relief system for a pipe having an appendage attached by fitting and welding to the outer peripheral surface, the inner surface of the pipe is cooled with a coolant, and the inner surface of the pipe is cooled in the vicinity of the fitting weld and not including the weld. A stress relaxation method for appendage welded piping, which is characterized by heating a localized region on the outer peripheral surface of the piping along the welded part.