JPS62211325A - Improvement of residual stress of double metallic pipe or the like - Google Patents

Abstract

PURPOSE:To suppress the generation and growth of a corrosion crack by subjecting the surface of a base pipe to quick heating under specific conditions while cooling water is made to exist between the base pipe and a thermal sleeve therein, then to slow cooling, thereby imparting a residual compressive stress to the surface of the base pipe. CONSTITUTION:The cooling water is fed into double pipes successively provided to a nozzle 2 of a pressure vessel 1 for a nuclear reactor and while the cooling water is made to exist in the annular hollow part between the base pipe 3 and the thermal sleeve therein, the base pipe 3 in the range of a part 9 to be treated is quickly heated from the outside surface by a heating means 7. The quantity of heat input is so set that the heating time for the surface of the base pipe 3 attains 20-50% of Fourier number, the penetration depth of heating 20-60% of the wall thickness of the base pipe 3 and the surface temp. >=450 deg.C. The part near the outside surface of the wall of the pipe 3 is thus led to the high temp. state. The thermal stress above the yield point is generated by the temp. difference from the part near the inside surface in this stage. The temp. difference of the wall of the base pipe 3 is removed by slow cooling, by which the state of imparting the residual compressive stress to the inside surface, i.e., the part 9 to be treated is attained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for improving residual stress in metal pipes, and is particularly applicable to double metal pipes that have a thermal sleeve and have a structure where fluid stagnation areas are likely to occur. The present invention relates to a method for improving residual stress that is suitable for this purpose.

"Prior Art" Generally, it is known that corrosion cracking progresses rapidly in metallic materials such as austenitic stainless steel, which is often used in nuclear power plants, chemical plants, etc., when tensile stress and corrosion factors coexist. It is being

Conventionally, in order to improve the stress in such metal tubes, cooling water is passed through the metal tube and induction heating is applied to the metal tube to create a temperature difference that causes thermal stress above the yield point on the inner and outer surfaces of the metal tube. A stress improvement method has been considered in which residual compressive stress is generated on the inner surface near a welded part such as a joint of a metal pipe.

"Problems to be Solved by the Invention" However, although this method is applicable to simple shapes such as straight pipes, when it comes to double pipes with thermal sleeves, the main pipe and thermal sleeve A stagnation area of the cooling water is generated between the two and the convection phenomenon of the cooling water is hindered, so there is a problem that it becomes difficult to provide the temperature difference necessary for improving the residual stress.

The present invention aims to effectively solve the problems of the prior art.

"Means to Solve the Problem" The method of improving residual stress in double metal pipes, etc. in the present invention is to remove the surface of the main pipe with cooling water present between the main pipe and the thermal sleeve inside the main pipe. Heating time is 2 of Fourier number
0% to 50%, heating penetration depth is 20% of main pipe wall thickness
After rapid heating to a surface temperature of 450°C or higher, the temperature is slowly cooled, and there is a difference between the time for temperature rise due to heating and the time for heat transfer to the inner wall of the main pipe. Taking advantage of this phenomenon, the area near the outer surface of the main pipe wall is brought to a high temperature state, and the temperature difference between the area near the inner surface and the inner surface at this time causes
By generating thermal stress above the yield point and then cooling to remove the temperature difference on the main pipe wall, a state is created in which compressive residual stress is applied to the inner surface, which is the part to be treated.

"Example" Hereinafter, an example of the method for improving residual stress in a double metal pipe or the like according to the present invention will be described with reference to FIGS. 1 to 3.

In this example, austenitic stainless steel M
The target of treatment is a double pipe made of C8US304) with a wall thickness of 19 mm.The double pipe shown in FIG. It has a structure in which a thermal sleeve 4 is disposed inside, and it is assumed that the thermal sleeve 4 is installed horizontally. The heating means, reference numeral 8 is a welded joint, and reference numeral 9 is a part to be treated.

[Cooling water insertion process] When cooling water is sent into the double pipe with this structure as shown by the arrow in Figure 1, the cooling water flows into the reactor pressure vessel l via the thermal sleeve 4. However, in the ring-shaped hollow portion 6 between the main pipe 3 and the thermal sleeve 4, a portion where cooling water is filled but no flow occurs, that is, a stagnation region remains.

However, due to the insertion of the cooling water and the heat transfer between the cooling water and the main pipe, each part of the double pipe has a fairly uniform temperature, for example, the temperature of the cooling water (normal temperature).

[Rapid heating step] Next, the heating means 7 is operated to perform high-frequency induction heating of the main pipe 3 in the area to be treated 9 from the outer surface. At this time, the heating time for the main pipe surface is 20% to 50% of the Fourier number, and the heating penetration depth is 20% to 6% of the main pipe wall thickness.
0%, and the amount of heat input is set so that the surface temperature of the main pipe 3 is in the range of 450° C. or higher.

In Fig. 2, the metal tube is a double tube made of austenitic stainless steel, the tube wall thickness of the main tube 3 is 19 mm, and the heating penetration depth is 9111ffi (approximately 50% of the tube wall thickness).
, shows the relationship between the time immediately after heating and the distance from the outer surface when heating is performed under the conditions that the heating time is 23 seconds (50% of the Fourier number) and the maximum temperature during heating is 450°C.

From the results shown in FIG. 2, it is clear that a large temperature difference occurs in the thickness direction of the main tube wall.

During this rapid heating process, heat transfer takes place from the high-temperature part to the low-temperature part, so the temperature of the main tube wall gradually tends to increase, but as specified by +, & 1lI-/
rXg ura-y+ IJ + b l f 1wt
kmnNMF; n411! I (20% seconds in the example)
For this reason, the extent to which the temperature of the inner surface increases due to heat transfer during rapid heating is within a practically acceptable range.

In addition, in the state of the main pipe wall immediately after heating, there is a large temperature difference between the high temperature part represented by the heating penetration depth and the low temperature part due to delay in heat transfer, so curve A in Fig. 3
As shown in , compressive stress occurs in the heated part near the outer surface due to hindering thermal expansion, and tensile stress occurs in the low temperature part due to thermal expansion.In particular, in the heated part, compressive stress is reduced due to the high temperature state In order to exceed the yield point, plastic deformation occurs.

[Slow cooling process] On the other hand, after the rapid heating process, if cooling water is continued to be fed as shown by the arrow in Figure 1, or if slow cooling is performed in a natural state, the high temperature part will be cooled during the cooling process. Due to the transfer of heat, the inner surface temperature of the main pipe increases, and depending on the case, nucleate boiling or film boiling occurs, and cooling water is replaced in the ring-shaped hollow part 6, thereby promoting cooling. , the temperature of the main pipe wall gradually becomes uniform and eventually reaches the temperature of the cooling water. When the temperature inside the main pipe wall is equalized, as shown by curve B in Figure 3, the inner surface spaced apart from the outer surface has a dimensional shrinkage caused by the plastic deformation. , a state in which compressive residual stress is generated, that is, a state in which compressive residual stress is applied to the portion to be processed can be achieved.

[Explanation of conditions for imparting compressive residual stress] To provide a supplementary explanation of the processing conditions when the rapid heating step and slow cooling step described above are combined, in austenitic stainless steel, according to research by the present inventors, the following Objectives can be achieved under certain conditions.

That is, in order to provide a large temperature difference between the inside and outside of the main pipe wall, it is necessary for the heating time (to) to satisfy the following equation.

0, 2 <a-to/L″< 0, 5---
---Ci) However, a: Temperature transfer rate (m/h) L: Thickness of main pipe wall When this is rearranged using the Fourier number (F), it is expressed by the following formula.

0.2F<t. <0.6F...(11) However, F = a-to/L' Furthermore, at the same time, the heating penetration depth S) is the thickness of the main pipe wall [
7), it is necessary to satisfy the following equation.

0.2L<S<0.6L (iii) Here, the heating penetration depth S) is a dimension that determines the heat generation distribution in the thickness direction of the main tube wall during high-frequency induction heating. . Generally, the heat generation density (W) during high-frequency induction heating is expressed by the following formula.

W = Wo e−j−−(iv) However, Wo=
Heating density at the surface j=2X/S X: distance from the outer surface The heating penetration depth (S (cm)) is S = (
1/ (2π)) Cp / (μrx to″″’)
)'・'−・・・−(V) However, ρ: Specific resistance (Ω・
cm) μ: relative magnetic permeability r: frequency (H2) By changing the frequency, the heating depth S) can be controlled.

Under the conditions (ii) and (iii) above, the temperature of the outer surface is 4
By heating in a range of 50° C. or higher, it is possible to obtain an effective temperature distribution for applying compressive residual stress to the target portion to be treated.

If the heating time is longer or the heating penetration depth S) is larger than this condition, the inner surface temperature will rise considerably and a large temperature difference will be obtained between the inside and outside of the main pipe wall. It disappears. On the other hand, if the heating time is short or the heating penetration depth is small, only the surface temperature will rise and only a very shallow part near the outer surface will undergo plastic deformation, making it difficult to obtain sufficient compressive residual stress on the inner surface. becomes difficult.

Note that the upper limit of the outer surface temperature needs to be set so as not to adversely affect the structure of the main tube wall. For example, in the case of austenitic stainless steel, the upper limit should be limited to 550° C. in order to avoid creating sensitized zones in the structure.

[About the Moe treatment process] When performing the treatment process of one example that includes the three conditions, the treatment range is limited, so the residual stress improvement effect may be reduced, but such In this case, prior to the rapid heating step, the residual stress can be improved by heating the part to be treated at a temperature below the above-mentioned upper limit temperature and applying the residual stress improvement method described in the conventional technology. By taking such measures in advance, the effect of improving residual stress can be improved by combining the effects of the subsequent implementation of the method of the present invention.

Up to this point, we have explained the double pipe in the example in Figure 1, but other similar pipes in which the cooling water stagnates, or pipes in which the cooling water does not stagnate, can also be used within the rapid heating time. Of course, similar residual stress improvement methods can be used as long as cooling is negligible.

1-Effects of the Invention As explained above, the residual stress reforming method for double metal pipes, etc. of the present invention is effective because of the difference between the temperature rise time due to rapid heating and the heat transfer time to the inner wall of the main pipe. is brought to a high temperature state, creating a temperature difference between it and other parts of the main pipe wall,
Thermal stress above the yield point occurs in the main pipe wall, causing plastic deformation, and then, when the temperature difference in the main pipe wall disappears due to slow cooling,
Because the compressive residual stress is applied to the inner surface, which is the part to be treated, due to the contraction of the plastically deformed part, a thermal sleeve exists inside the main pipe, and the ring-shaped hollow part Even when such a structure is formed, compressive residual stress can be applied to the inner surface of the main tube in the vicinity, thereby producing excellent effects such as suppressing the occurrence and growth of corrosion cracks.

[Brief explanation of drawings]

The drawings show an example in which the method of improving residual stress in a double metal pipe according to the present invention is applied to an austenitic stainless steel pipe. Figure 1 is a longitudinal cross-sectional view of the treated state of the double pipe, and Figure 2 shows the heating time. is 23 seconds and heating penetration depth is 9n+n+
Figure 3 shows the relationship between the distance from the outer surface and the generated stress when the heating time is 23 seconds and the heating penetration depth is 9 mm. It is a curve diagram. l... Reactor pressure vessel, 2... Nozzle, 3... Main tube, 4... Thermal sleeve,
5... Safe end, 6... Ring-shaped hollow part, 7... Heating means, 8... Welded joint, 9... Part to be treated. .

Claims (1)

[Claims] With cooling water present between the main pipe and the thermal sleeve inside it, the heating time for the main pipe surface is 20% to 50% of the Fourier number, and the heating penetration depth is 20% of the main pipe wall thickness.
% to 60% and a surface temperature of 450° C. or higher, followed by slow cooling.