JPS6237319A - Method for repairing metallic vessel or the like - Google Patents

Abstract

PURPOSE:To generate residual compressive stress near the front end of a defective part by heating a metallic wall near the defective part and cooling the exposed surface near the defective part to provide a temp. difference in the sheet thickness direction where the thermal stress above the yield point is generated to a metallic wall then removing the temp. difference. CONSTITUTION:A heating means 3 is operated to heat the metallic wall 1 near the defective part 2 of a metallic pipe which has a defect such as crack in the stage of repairing the above-mentioned metallic pipe. At the same time, cooling water is continuously forced into the metallic pipe to generate the temp. difference between both surfaces of the metallic wall 1. A temp. gradient is generated in the metallic wall 1 near the defective part 2 by such temp. difference. The residual compressive stress is generated near the defective part 2 when the heating means 3 is stopped and the pipe is allowed to cool of itself. The growth of the crack, etc. is thus prevented.

Description

Detailed Description of the Invention "Industrial Application Field J The present invention relates to a method for repairing metal containers, etc., and in particular applies compressive residual stress to areas where defects such as cracks have occurred to prevent the growth of cracks etc. This relates to a repair method that prevents this.

"Conventional technology and its problems" In general, containers, metal pipes, etc. made of metal materials such as austenitic stainless steel, which are often used in nuclear power plants, chemical plants, etc., have corrosion cracks, hydrogen cracks, etc. on the inner surface. When defects such as cracks occur due to cracks, fatigue, etc., the defects grow further due to tensile stress caused by internal pressure and stress fluctuations due to start-up and stoppage of plant operation, causing damage to the structure of metal containers, pipes, etc. Phenomena may occur that impair the mechanical strength of objects.

Therefore, when a defect occurs, methods such as repairing the defective part by means such as welding or replacing the part are adopted.

However, such methods involve high costs.

In addition, in the case of nuclear couplants, there is a problem of worker exposure during repair or parts replacement work.

The present invention has been made in order to effectively solve these problems, and aims to: 1) reduce costs, 2) shorten the construction period, 2) reduce the exposure of workers to radiation, and 2) eliminate defective parts. This patent is characterized in that compressive residual stress is concentratedly applied to the tip, etc., to prevent the subsequent growth of defective parts and improve reliability.

"Means for Solving the Problems" To achieve the above object, the present invention provides a method for repairing metal containers, etc. that have defects such as cracks, in which the metal wall near the defect is heated and the defect is removed. After cooling the exposed surface near the defective part and applying a temperature difference in the thickness direction that generates thermal stress exceeding the yield point in the metal wall, by removing this temperature difference, the exposed surface near the tip of the defective part is cooled. This causes residual compressive stress.

"Example" First, an outline of the method according to the present invention will be explained based on FIGS. 1 to 3, using an example in which the metal component is a metal tube. In the figure, reference numeral 1 indicates a wall of a metal tube (metal wall), reference numeral 2 indicates a defective portion such as a crack occurring on the inner surface of the metal wall l, and reference numeral 3 indicates a heating means such as an induction heating coil. Further, in order to carry out the method, a cooling means such as passing cooling water as shown by the arrow in FIG. 1 is required.

By operating the heating means 3, the metal wall 1 near the defective part 2 is heated, and by continuously passing cooling water into the metal tube, etc., the temperature is increased on both sides of the metal wall I. Make a difference. Due to this temperature difference, a temperature gradient as shown by the chain line θn in FIG. 2 is generated in the metal wall 1 near the defective portion.

Next, by stopping the operation of the heating means and allowing natural cooling, etc., the temperature difference in the metal wall 1 is removed, and the metal wall 1 is returned to a normal temperature state as indicated by the chain line θZ in FIG. 2.

By going through these processes of heating, applying a temperature difference, and removing a temperature difference, phenomena such as stress generation occur in the metal wall 1. That is, when the metal wall 1 is heated uniformly at room temperature or when the metal tube can freely expand thermally, no stress is generated in each part of the metal wall I, but due to temperature differences, Inside the metal wall l, mark the chain line in Figure 2.
If a temperature gradient as shown in Fig. 3 occurs, a point with a certain average temperature will try to expand or contract by the difference from the (old), so as shown by the curve σn in Fig. 3, a high fjL(
Compressive stress is generated on the outer surface side, which is at a temperature close to θh), and tensile stress is generated on the inner surface side, which is at a low temperature (temperature close to cooling water).

As shown in FIG. 3, if the crack in the defective part 2 has entered the metal wall 1, the defective part (crack)
In the part 2, no stress is generated because it can expand and contract in the width direction (horizontal direction in Figure 3), but in the vicinity of the crack tip, large plasticity due to high tensile stress occurs due to the stress concentration effect, etc. Deformation will occur.

Therefore, when the temperature difference is removed, a particularly large compressive residual stress is applied near the tip of the defect that has penetrated into the interior of the metal wall, as shown by the curve σZ in Figure 3. It is what it is.

In addition, the temperature difference (ΔT
c) is determined by the following formula within a range where the defective portion 2 is small and can be ignored.

ΔTc=4(1-ν)σy/Eα・---(i) Only 1
-5 v ・Bore No. 710σy: Stress E in the circumferential direction ・Young's coefficient α: Coefficient of linear expansion Note that when a defect occurs, residual compressive stress is concentrated at the tip and occurs. For this reason, depending on the depth (depending on the degree of depth), it may be possible to require a smaller temperature difference than in equation (1).

The metal composition is then replaced with a stainless steel tube (S) as shown in FIG.
US304), its dimensions are 72
0 mm, tube wall thickness is 30 mm, and 30% of tube wall thickness
Figures 4 to 7 show analysis examples in which a sample is formed as a defective part by adding cracks all around the circumference, and the sample is repaired as shown in Figures 1 to 3, that is, heat treated.
This will be explained based on the diagram.

In this case, the temperature difference required for the heat treatment for said repair is:
In (1) mentioned above, (7Y = 25 kg/mm',
30% crack, E= 19500kg/+nm', a=
When calculated by adding the 12 conditions, the result is approximately 200'C, so when this 200°C temperature difference is applied, the defective part shown by the chain line in Figure 4 and the normal part shown by the chain line ■ in Figure 4 A comparison was made with a part that is large and unaffected by the defect.

As shown in Figure 5, no stress is observed in the defect, but the tip, where stress growth is a concern, is approximately 88.
A residual compressive stress of about kgf/cm is applied, and subsequent crack growth can be effectively prevented.

As shown in Fig. 6, a residual compressive stress of approximately 27 kgf/cm2 can be applied to the inner surface of the metal wall in the normal part, but the residual compressive stress at the tip of the defective part in Fig. 5 is 30 kgf/cm2. It is about %.

Next, the case where a combination of external loads is applied will be explained with reference to FIG. When internal pressure or the like acts on the stainless steel pipe subjected to the above treatment and tensile stress in the axial direction overlaps during use, the residual stress curve changes like Δ-Mu〇-. depending on the degree of the overlapping stress. However, △
is the axial tensile stress caused by external load is 0,
62 kgf/mm2, ○ is 9, 2 kgf/mm2
mm2, · indicates a case of 12.3 kgf/mm'.

As a result, if the stress applied during the operation of general piping in a nuclear power plant, for example, is within the range of about 6 to 7 Jf'/'mm', compressive residual stress is maintained at the tip of the defective part. Therefore, it is possible to prevent the phenomenon of defective portions from growing.

In addition, although the case of stainless steel pipes has been explained in FIGS. 4 to 7, in the case of carbon steel pipes, a temperature difference of about 300°C is determined by the above equation (1), and other similar pipes, It can also be applied to containers etc. In addition to the example shown in Figure 1, methods for applying a temperature difference include surface quenching, in which cooling water is blown into the metal wall after heating it with a burner, and heating the metal wall while cooling water is present on the inner surface. It is also possible to use other methods such as Furthermore, although the residual stress improvement in the tube axis direction has been described, it can also be applied in the circumferential direction.

"Effects of the Invention" As explained above, the present invention heats the metal wall near the defective part and cools the exposed surface near the defective part, thereby generating thermal stress in the metal wall in the thickness direction that is higher than the yield point. After applying a temperature difference of , the temperature difference is removed to generate residual compressive stress in the vicinity of the defective part, so that the following effects can be achieved.

(a) Since the metal component is heated and cooled to create a temperature difference, it is easier to carry out than the method of removing defective parts and repairing by welding. It has few restrictions on shape and size, has a wide range of applications, and is highly practical.

(b) Compressive residual stress is applied intensively to the tip of the defect, which prevents the defect from growing, and even when tensile stress due to internal pressure acts on the metal wall, compressive residual stress is applied intensively to the tip of the defect. By maintaining residual stress,
It is possible to ensure safety and improve reliability.

(c) In addition to repairing the defective part, it is possible to bring the surface in the vicinity to a stable state with compressive residual stress, and to suppress the subsequent occurrence and growth of corrosion cracks, fatigue cracks, etc.

[Brief explanation of the drawing]

Fig. 1 is a partially cutaway perspective view of an example in which the metal component is a metal tube, illustrating the outline of the method according to the present invention, and Fig. 2 is an explanation of the temperature of the metal wall near the defective part in Fig. 1. Figure, 3rd
The figure is the stress curve diagram of the metal wall at each temperature in Figure 2, and Figure 4.
The figure is a partially cutaway perspective view of the stainless steel pipe in the analysis example, Figure 5 is a residual stress curve diagram in the thickness direction of the metal wall at the chain line v in Figure 4, and Figure 6 is the chain line ■ in Figure 4. Fig. 7 is a residual stress curve diagram in the thickness direction of the metal wall at a portion, and Fig. 7 is a residual core horn curve diagram of an analysis example of residual stress in a state where external loads are combined and applied. 1...Metal wall, 2...Defect part, 3...
・・・Heating means. Applicant Ishikawajima Harima Heavy Industries Co., Ltd. Figure 1 ooooo○○

Claims (1)

[Claims] In a method for repairing metal containers, etc. that have defects such as cracks, a plate that heats the metal wall near the defect and cools the exposed surface near the defect to generate thermal stress in the metal wall that exceeds the yield point. A method for repairing metal containers, etc., characterized by applying a temperature difference in the thickness direction and then removing the temperature difference.