JP5367558B2 - How to improve residual stress in piping - Google Patents

Abstract

It is an object to provide a residual stress improving method for a pipe by imparting larger compressive residual stress on the pipe to sufficiently reduce tensile residual stress in order to prevent the stress corrosion cracking. With respect to a stress improving region where the residual stress of a pipe is to be improved, a load in the axial direction of the pipe is made such stress making axial strain of the outer surface of the pipe 0% or above and being yield stress of the pipe or below, and internal pressure of the pipe is raised. The pipe is plastically deformed and is expanded in the radial direction by the internal pressure. After the internal pressure is raised to the degree the pipe is plastically deformed, the internal pressure and the axial load are removed, and thereby compressive residual stress is imparted to a welding section and a heat affected zone which are the stress improving region of the inner surface of the pipe.

Description

  The present invention relates to a method for improving residual stress in piping, and more particularly to a method for improving residual stress in piping suitable for improving residual stress in the vicinity of a welded portion of piping.

  In piping used in a power plant, when welding is performed, residual stress is generated in the vicinity of the weld. Further, in consideration of corrosion resistance, stainless steel piping or nickel alloy steel piping is often used for the high temperature water piping. In piping using stainless steel, nickel alloy steel, or the like, stress corrosion cracking may occur when exposed to high temperature pure water for a long time while tensile residual stress is applied to the welded portion of the piping. Therefore, it is desirable to improve the residual stress by reducing the tensile residual stress generated by welding or changing to a compressive residual stress.

  As a method for improving the residual stress in the welded portion of the pipe, a method using an ice plug (ice plug) formed in the pipe has been proposed in the prior art documents.

  In the method for improving residual stress of piping described in Patent Document 1, refrigerant containers for forming ice plugs are respectively attached to the upstream side and the downstream side of the welded part of the pipe, and the refrigerant for expanding the pipe between each refrigerant container and the welded part. Install each container. First, ice plugs are formed at two locations upstream and downstream of the welded portion in the pipe by each refrigerant container for ice plug formation, and then water existing between the ice plugs is frozen by each refrigerant container for pipe expansion. Using the volume expansion of the generated ice, the vicinity of the welded portion of the pipe is pushed outward. As a result, the vicinity of the welded portion of the pipe is plastically deformed, and compressive residual stress is applied to the inner surface in the vicinity of the welded portion, thereby reducing the tensile residual stress or making the compressive residual stress.

  The method for improving the residual stress of a pipe described in Patent Document 2 cuts a surplus of a welded part or an outer surface of the pipe, and makes the thickness near the welded part of the pipe thinner than the thickness of the other part. When the thickness in the vicinity of the welded portion of the pipe is reduced, the amount of deformation in the vicinity of the welded portion at the time of pipe expansion due to the internal pressure of the pipe becomes larger than the amount of deformation in the vicinity of the welded portion. By expanding the pipe until it is plastically deformed, the tensile residual stress acting on the inner surface of the pipe in the vicinity of the weld is reduced or made into compressive residual stress.

  The method for improving the residual stress of a small-diameter pipe described in Patent Document 3 is that in the state where an axial tensile load is applied to the vicinity of the welded portion of the pipe, a temperature difference is applied to the inner and outer surfaces of the pipe to increase the internal pressure near the welded portion. Is expanded to give compressive residual stress to the inner surface of the pipe.

JP 2006-334596 A JP 2008-238190 A JP 2009-50906 A

  In the method for improving the residual stress of a pipe described in Patent Document 1, the residual stress is improved by the difference in stress between the inner surface and the outer surface during plastic deformation of the pipe. Therefore, depending on the diameter and thickness of the pipe, there is a possibility that the improvement of the residual stress of the pipe may not be sufficiently performed to prevent stress corrosion cracking. In the method for improving the residual stress of piping described in Patent Document 2, there is a possibility that the required strength of the piping may be lowered when the weld surplus or the outer surface of the piping is cut to prevent stress corrosion cracking. Moreover, in the residual stress improvement method for small-diameter pipes described in Patent Document 3, a plastic strain is generated on the inner surface by a tensile stress superimposed by an axial tensile load to give a compressive residual stress. In this case, depending on the magnitude of the tensile load to be applied, the improvement of the residual stress of the pipe may not be sufficiently performed to prevent stress corrosion cracking.

  The object of the present invention is to improve the residual stress by expanding the pipe by the internal pressure of the pipe, and by applying an axial load under a predetermined condition before or simultaneously with the increase of the internal pressure of the pipe, An object of the present invention is to provide a method for improving the residual stress of a pipe for applying a residual stress.

  According to the present invention, an axial load is applied to the pipe with respect to a stress improvement area having a tensile residual stress in a pipe filled with a fluid, and the pressure inside the pipe is increased to plasticize the stress improvement area. In the method for improving the residual stress of a pipe to be deformed, the load range in the pipe axial direction is a stress at which an axial strain of the pipe outer surface is 0% or more and a yield stress of the pipe or less. To do.

  The stress improvement region includes a welded part of the pipe and a heat affected part by the welded part, or a pipe expansion part of the pipe.

  Further, after the pipe is plastically deformed, the pressure in the pipe and the load in the axial direction of the pipe are unloaded. In addition, after the inner surface of the pipe starts plastic deformation, the pressure inside the pipe is unloaded when the outer surface of the pipe starts plastic deformation. Further, when the strain on the outer surface of the pipe reaches about 0.5%, the pressure inside the pipe is unloaded.

  Furthermore, the plastic deformation is confirmed by measuring a change in the circumferential strain with respect to the axial strain of the pipe outer surface.

  Furthermore, the detection position of the circumferential strain on the outer surface of the pipe is the vicinity of the welded portion, and the detection position of the axial strain is a region excluding the vicinity of the welded portion.

  According to the present invention, when an axial load is applied to the stress improvement region that increases the internal pressure of the pipe and improves the residual stress of the pipe, the axial strain on the outer surface of the pipe is 0% or more during plastic deformation of the pipe. By applying an axial load that is less than the yield stress of the pipe, it is possible to apply a large compressive residual stress to the inner surface of the stress improvement area of the pipe, improve the residual stress state, and prevent stress corrosion cracking. .

The schematic diagram which shows the residual stress improvement method of piping by this invention The schematic diagram which shows the residual stress improvement method of piping by this invention The schematic diagram which shows the residual stress improvement method of piping by this invention Explanatory drawing of the structure in the vicinity of the pipes and welds to be constructed according to the present invention The schematic diagram which shows the load apparatus of the axial load to piping in this invention The schematic diagram which shows the pipe expansion method in the residual stress improvement method of piping by this invention The graph which shows the axial direction strain conditions optimal for the residual stress improvement by this invention The graph which shows the axial direction strain conditions optimal for the residual stress improvement by this invention The graph which shows the axial direction strain conditions optimal for the residual stress improvement by this invention The graph which shows the optimal axial direction stress condition for the residual stress improvement by this invention The graph which shows the circumferential direction strain and stress at the time of the internal pressure rise of piping by this invention Graph showing axial and circumferential strain when the internal pressure of the pipe is increased according to the present invention Explanatory drawing which shows the example which evaluated the residual stress of the inner surface of piping made into the construction object of this invention The graph which shows the circumferential direction residual stress of the piping in which the welding residual stress exists by this invention The graph which shows the axial direction residual stress of the piping in which the welding residual stress exists by this invention Explanatory drawing which shows the example which evaluated the distortion | strain of the outer surface of piping made into the construction object of this invention

  The inventors examined a method that can apply a large compressive residual stress to the inner surface of the welded portion of the pipe and in the vicinity of the welded portion. As a result, the inventors increased the axial load and the internal pressure of the pipe to cause plastic deformation in the stress improvement region that improves the residual stress of the pipe, and the axial strain on the outer surface of the pipe is zero during the plastic deformation. It was newly found that it is desirable to apply a large compressive residual stress to the inner surface of the stress improvement region of the pipe by applying an axial load that is greater than or equal to% and less than the yield stress of the pipe.

This gives a greater compressive residual stress to the inner surface of the stress improvement area of the pipe, reduces the tensile residual stress state or changes to the compressive residual stress state, and further reduces the probability of stress corrosion cracking occurring in the pipe. Can do. Embodiments of the present invention reflecting the above examination results will be described below.
[Basic structure for residual stress improvement]
A method for improving the residual stress of a pipe according to a preferred embodiment of the present invention will be described with reference to FIGS. 1A to 1C, taking as an example the case of improving the residual stress in the vicinity of a welded part of a pipe in a power plant.

In FIG. 1A, a pipe 1 made of stainless steel (or nickel alloy) in a power plant is joined at its end by butt welding. Due to this butt welding, the inner surface of the welded portion 2 of the pipe 1 and the heat affected zone 3 in which the mechanical properties are changed by the heat of welding existing adjacent to both sides of the welded portion 2 are subjected to stress corrosion cracking. Since the residual tensile residual stress is generated, it is necessary to improve the stress. Therefore, an axial tensile load 4 is applied as shown in FIG. 1B. Next, as shown in FIG. 1C, an internal pressure 5 is applied to improve the residual stress in the welded portion 2 and the heat affected zone 3.
Here, the welded part is defined in the JIS as including a welded metal part (molten base metal + welded metal) and a surrounding heat-affected part. On the other hand, the welded portion in the present invention is used in a narrow sense consisting of a molten base material and a weld metal, and the heat affected zone is used separately from the welded portion.
[Measurement of residual stress by strain gauge]
The position for measuring the strain generated in the pipe 1 is determined. As shown in FIG. 2, in the vicinity of the welded portion, the inner surface 11 of the pipe is grooved to form a grooved surface 13 in order to align the inner surface during welding. In order to confirm that the tensile residual stress of the heat affected zone 3 and the grooved surface 13 of the pipe inner surface 11 has been improved, a measurement position of the strain gauge G1 in the circumferential direction of the pipe 1 is provided in the vicinity of the weld 2.

  On the other hand, the axial strain varies greatly depending on the measurement position because the groove processed surface 13 and the weld surplus 14 exist in the vicinity of the weld. Therefore, the measurement position of the strain gauge G2 in the axial direction of the pipe 1 is preferably provided at a position sufficiently away from the welded portion 2.

  When the strain gauge G1 is affixed to the outer surface 12 of the existing power plant internal pipe 1 for measuring the circumferential strain, it is difficult to measure the magnitude of the internal pressure 5. Since it is not possible to clearly determine whether the pipe 1 is plastically deformed only by a change in the circumferential strain, in this case, the circumferential strain and the axial strain are measured with respect to the axial strain by measuring the circumferential strain and the axial strain at the same measurement position. It is possible to confirm whether the pipe 1 is plastically deformed by measuring.

For measuring the circumferential strain, the outer diameter of the pipe 1 may be measured instead of using a strain gauge. When measuring the outer diameter of the pipe 1, an oil pen or the like is used on the outer surface 12 of the pipe 1 with respect to the determined measurement position of the outer diameter so as not to obscure the measurement position due to plastic deformation of the pipe 1 due to the increase in the internal pressure 5. Mark each. The measurement position of the outer diameter is measured using these markings, and the obtained measurement value is recorded as the initial value of the measurement position of the outer diameter. When measuring these, taking into account variations in the thickness of the pipe 1, two or more locations are measured in the respective cross sections perpendicular to the axis of the pipe 1 in the vicinity of the weld 2. [Axial load]
A configuration in which an axial load 4 is applied to the pipe 1 is shown in FIG. As a method for applying the axial load 4, a hydraulic chuck 21 and a hydraulic cylinder 22 are used. Hydraulic chucks 21 are attached to the upstream side and the downstream side of the welded portion 2 and the heat affected zone 3 where the residual stress of the pipe 1 should be improved. In order to apply the axial load 4 between the hydraulic chucks 21, a hydraulic cylinder 22 is attached between the hydraulic chucks 21. Two or more hydraulic cylinders 22 are attached at equal intervals in the circumferential direction of the pipe 1 so that bending deformation does not occur in the pipe 1 when the axial load 4 is applied by the hydraulic cylinder 22. When the internal pressure 5 of the pipe 1 rises, a load due to the internal pressure 5 is also generated in the axial direction of the pipe 1. For this reason, when the internal pressure 5 rises, the axial load 4 initially loaded by the hydraulic cylinder 22 also increases. The axial load 4 by the hydraulic cylinder 22 prevents the axial stress of the pipe 1 from exceeding the yield stress of the pipe 1 when the internal pressure 5 of the pipe 1 increases.
[Internal pressure increase method]
An axial load 4 is applied to the pipe 1 to increase the internal pressure 5 of the pipe 1. FIG. 4 illustrates a known internal pressure increasing method. As shown in FIG. 4A, an inner container 32 and an outer container 33 are attached to the outer surface 12 of the pipe 1 on the upstream side and the downstream side of the welded part 2 and the heat-affected part 3 to improve the residual stress of the pipe 1. It is done. The pipe 1 is filled with water 31.

  Each of the inner container 32 and the outer container 33 is hollow. As shown in (b), ethyl alcohol 34 is injected into the inner container 32 and the outer container 33 to such an extent that the pipe 1 is immersed. Dry ice 35 is put into the outer container 33 filled with ethyl alcohol 34 from the opening at the upper end. The water 31 in the pipe 1 is cooled at a position surrounded by the outer container 33 by the supplied dry ice 35. For this reason, the water 31 is frozen at each position in the pipe 1 to form the ice plug 36, and the water 31 is sealed by the pair of ice plugs 36.

  In order to form the ice plug 36 that exhibits a stronger sealing function in the pipe 1, the drying time in the outer container 33 is longer than the time required to form the ice plug 36 obtained in advance through experiments or the like. Continue cooling the pipe 1 with ice. As a result, a pair of ice plugs 36 that are firmly frozen on the inner surface of the pipe 1 are formed, and a sealed region filled with water 31 is formed between the ice plugs 36.

  Furthermore, as shown in (c), dry ice 35 is put into the inner container 32 filled with ethyl alcohol 34. The water 31 existing between the ice plugs 36 is cooled by the dry ice 35 in the inner container 32. At each position where the inner container 32 is attached, the pipe 1 is cooled below the freezing point, and the water 31 existing between the ice plugs 36 begins to freeze. When the water 31 becomes ice 37, the volume expands, so that the pressure of the water 31 existing between the ice plugs 36 begins to rise. The pressure in the pipe 1 rises between the ice plugs 36, and the pipe 1 forms a pipe expansion portion 38 radially outward between the inner containers 32 due to this pressure rise.

After it is confirmed that the pipe 1 is plastically deformed by the deformation of the pipe 1 or the measured circumferential strain reaches a set value, the dry ice 35 of the inner container 32 and the outer container 33 is discharged to the outside. The axial load 4 is unloaded after all of the ice plugs 35 and ice 36 in the pipe 1 are melted to form water 31 or simultaneously with the decrease in the internal pressure of the pipe 1. At this time, since the pipe 1 is plastically deformed as shown in (d), the pipe 1 is expanded even after the internal pressure 5 and the axial load 4 are unloaded.
[Optimum conditions for residual stress improvement]
A method for improving the residual stress of piping according to the present invention will be described with reference to FIGS. The inventors analyzed the residual stress generated in the pipe 1 by the finite element method for the method for improving the residual stress of the pipe of the present invention, and clarified the optimum conditions for the stress improvement.

  The pipe 1 to which the residual stress improvement method of the pipe targeted in this example is applied is made of stainless steel, the longitudinal elastic modulus is 195000 MPa, the Poisson's ratio is 0.3, and the yield stress is 270 MPa. The inner diameter of the pipe 1 was 20 mm, and the wall thickness was changed in three stages: 2 mm, 6 mm, and 10 mm.

  The internal pressure 5 of the pipe 1 having a thickness of 2 mm shown in FIG. 5A was increased from 0 MPa to 87 MPa. Further, the axial load generated in the pipe 1 due to the internal pressure 5 and the axial load 4 of the pipe 1 was applied to the end of the pipe 1 as a distributed stress in the axial direction. The axially distributed stress of the pipe 1 having a thickness of 2 mm is 129 MPa from the ratio of the inner diameter to the outer diameter of the pipe 1 when the axial load 4 is not applied, and from 0 MPa to 129 MPa simultaneously with the internal pressure 5 of the pipe 1. Raised. Here, the axial distribution stress means the axial distribution stress generated in the pipe at the time of loading.

  Next, in the pipe 1 having a thickness of 6 mm shown in FIG. 5B, when the axial load 4 was not applied, the internal pressure 5 was increased from 0 MPa to 147 MPa, and at the same time, the axial distributed stress was increased from 0 MPa to 94 MPa.

  Next, in the pipe 1 having a thickness of 10 mm shown in FIG. 5C, when the axial load 4 was not applied, the internal pressure 5 was increased from 0 MPa to 216 MPa, and at the same time, the axial distributed stress was increased from 0 MPa to 72 MPa.

  The axial stress calculated from the cross-sectional area of the pipe 1 when the axial load 4 was applied to each thick pipe 1 was applied as the axial distributed stress. For example, when an axial load 4 of 49 kN is applied to the pipe 1 having a thickness of 6 mm, it is calculated that an axial stress of 100 MPa is generated from the cross-sectional area of the pipe 1. Therefore, 100 MPa is added to the axial stress 147 MPa generated by the internal pressure 5 of the pipe 1 to load 247 MPa as the axial distributed stress.

  5A to 5C show the results of evaluating the optimum conditions for stress improvement by the finite element method when the axial load is changed. The horizontal axis in FIGS. 5A to 5C indicates the axial strain generated on the outer surface 12 of the pipe 1 when the axial load 4 and the internal pressure 5 of the pipe 1 reach the maximum value described above. The vertical axis indicates the residual stress generated on the inner surface 11 of the pipe 1 after unloading the internal pressure 5 and the axial distribution stress of the pipe 1, and indicates that the compressive residual stress is higher in the minus region. The vertical axis 0. The state of the unloaded and internal pressure bare tube is shown.

  From FIGS. 5A to 5C, it is clear that the improvement effect of the axial residual stress increases when the axial strain exceeds 0%. Therefore, the axial strain of the outer surface 12 of the pipe 1 when the internal pressure 5 and the axial load 4 are applied to the pipe 1 is preferably 0% or more. The effect of improving the circumferential residual stress is highest when the axial strain of the outer surface 12 of the pipe 1 is about 0%.

FIG. 6 shows the upper limit of the effect of improving the axial stress of the inner surface 11 of the pipe 1 with respect to the axial distributed stress applied to the end of the pipe 1. From FIG. 6, when the axial distribution stress exceeds the yield stress of 270 MPa, the improvement effect is reduced, or the improvement effect is lower than the state without the axial load 4. From these results, it is preferable that the axial load 4 is such that the axial strain of the outer surface 12 of the pipe 1 is 0% or more and the axial stress of the pipe 1 does not exceed the yield stress.
[Relationship between circumferential stress and strain]
7A and 7B show the results of evaluating the change when the internal pressure 5 of the pipe 1 having a thickness of 6 mm without the axial load 4 is increased by the finite element method.

  FIG. 7A shows changes in the circumferential stress of the inner surface 11 and the outer surface 12 with respect to the circumferential strain of the outer surface 12 when the internal pressure 5 of the pipe 1 is increased. When the internal pressure 5 is low and the circumferential strain is small, since the pipe 1 is elastically deformed, the circumferential stress generated on the inner surface 11 and the outer surface 12 of the pipe 1 is substantially the same.

  As the internal pressure 5 continues to rise, the pipe 1 begins to be plastically deformed. At this time, in the radial direction of the pipe 1, the inner surface 11 is loaded with the internal pressure 5, whereas the outer surface 12 is in an unloaded state. For this reason, the circumferential stress that starts plastic deformation differs between the inner surface 11 and the outer surface 12 of the pipe 1, and a stress difference occurs between the inner surface 11 and the outer surface 12 of the pipe 1.

  When the internal pressure 5 of the pipe 1 is unloaded, the pipe 1 is contracted in the radial direction by the amount of elastic deformation, and the circumferential stress is reduced. Since the amount of decrease in the circumferential stress between the inner surface 11 and the outer surface 12 of the pipe 1 is substantially the same, the greater the difference in stress between the inner surface 11 and the outer surface 12 generated after the plastic deformation of the pipe 1, the higher the effect of improving the residual stress. From FIG. 7A, the improvement effect becomes the highest when the outer surface 12 of the pipe 1 begins to be plastically deformed, that is, when the circumferential strain of the outer surface 12 is about 0.5%. Here, at the same location of the outer surface 12 of the pipe 1, it is determined whether the outer surface 12 of the pipe 1 is plastically deformed by measuring a change in the circumferential strain relative to the axial strain.

FIG. 7B shows a change in the circumferential strain with respect to the axial strain of the outer surface 12 when the internal pressure 5 of the pipe 1 is increased. The amount of change in the circumferential strain with respect to the axial strain is greatly changed at a circumferential strain of about 0.5% where the effect of improving the residual stress is the highest. When the pipe 1 is deformed in advance by welding or the like, the circumferential strain in which the pipe 1 starts plastic deformation is different from the present embodiment. For this reason, it is determined that the pipe 1 is plastically deformed by measuring a change in the circumferential strain with respect to the axial strain of the outer surface 12 of the pipe 1.
[Residual stress improvement results]
The results of improving the residual stress of the pipe of the present invention will be described with reference to FIG. The inventors evaluated the residual stress improvement effect in the present embodiment by the finite element method when the pipe 1 has a non-uniform shape in the axial direction by the welded portion 2 and there is a welding residual stress.

  The pipe 1 to which the residual stress improvement method for pipes of the present embodiment is applied is made of stainless steel, the longitudinal elastic modulus is 195000 MPa, the Poisson's ratio is 0.3, and the yield stress is 270 MPa. The inner diameter of the portion other than the welded portion 2 and the grooved surface 13 of the pipe 1 is 25 mm, and the wall thickness is 4.5 mm.

  FIG. 8 shows the residual stress on the inner surface 11 of the pipe 1 evaluated by the finite element method. It can be seen that a tensile residual stress of 100 MPa to 300 MPa is generated in the welded portion 2 and the groove processing surface 13.

  FIGS. 9A and 9B show an example in which the residual stress distribution on the inner surface 11 after unloading the internal pressure 5 and the axial load 4 of the pipe 1 improved in this embodiment is evaluated by a finite element method. When the axial load was no load, the internal pressure 5 of the pipe 1 was simultaneously increased from 0 MPa to 110 MPa, and the axial distributed stress was simultaneously increased from 0 MPa to 130 MPa. On the other hand, the axial distribution stress was changed to 100 MPa, 130 MPa (no load in the axial direction), 200 MPa, and 300 MPa. When the internal pressure 5 of the pipe 1 reaches 110 MPa, the axial strain at a position 20 mm from the weld center of the outer surface 12 of the pipe 1 is -0.04% when the axial distribution stress is 100 MPa, and 130 MPa. Was 0.02%, 0.18% at 200 MPa, and 1.23% at 300 MPa. From FIG. 9A, the residual stress in the pipe circumferential direction at the welded portion is largely located in the negative region, indicating a compressive residual stress, indicating an improvement in the stress state. Note that when the axial distribution stress exceeds the yield stress, 300 MPa, the circumferential stress improvement effect is low in the vicinity of the groove surface.

  Further, from FIG. 9B, the residual stress in the pipe axis direction at the welded portion is located in the minus region and indicates the compressive residual stress, indicating an improvement in the stress state. In addition, at an axial distribution stress of 100 MPa in which the axial strain becomes 0% or less when the internal pressure is 5 loads, a portion of the axial stress is 0 MPa or more in the vicinity of the groove processing surface, and a tensile compressive stress state is obtained.

  Therefore, even in a pipe in which deformation and residual stress exist in advance by welding, the axial load 4 is such that the axial strain of the outer surface 12 of the pipe 1 is 0% or more and the axial stress of the pipe 1 does not exceed the yield stress. It is preferable.

  FIG. 10 shows the analysis result by the finite element method of the circumferential and axial strains on the outer surface 12 of the pipe 1 when the internal pressure 5 and the axial load 4 of the pipe 1 improved in this embodiment are applied. FIG. 10 shows that the deformation amount of the pipe 1 other than the heat-affected zone 3 is larger than that of the heat-affected zone 3 depending on the shape of the welded portion 2 when the pipe 1 is expanded. Therefore, in order to confirm that the tensile residual stress of the heat affected zone 3 of the pipe inner surface 11 and the groove processing surface 13 has been improved, the measurement position of the strain gauge G1 in the circumferential direction of the pipe 1 is set near the welded portion 2. It is preferable to provide it. As for the axial strain, the vicinity of the welded portion greatly changes due to the influence of the groove processing surface 13 and the welding surplus 14. Therefore, the measurement position of the strain gauge G2 in the axial direction of the pipe 1 is preferably provided at a position sufficiently away from the welded portion 2.

  DESCRIPTION OF SYMBOLS 1 ... Pipe, 2 ... Welded part, 3 ... Heat-affected zone, 4 ... Axial load, 5 ... Internal pressure, 11 ... Pipe inner surface, 12 ... Pipe outer surface, 13 ... Groove processing surface, 14 ... Weld surplus, 21 ... Hydraulic chuck, 22 ... hydraulic jack, 31 ... water, 32 ... inner container, 33 ... outer container, 34 ... ethyl alcohol, 35 dry ice, 36 ... ice plug, 37 ... ice, 38 ... expanded part, G1 ... circumferential strain Gauge, G2 ... Axial strain gauge

Claims (9)

For a stress improvement region having tensile residual stress in a pipe filled with fluid, an axial load is applied to the pipe, and the pressure inside the pipe is increased to plastically deform the stress improvement area. In the residual stress improvement method,
The range of the load in the pipe axial direction is a stress at which the axial strain of the pipe outer surface is 0% or more and a stress equal to or lower than the yield stress of the pipe,
After the pipe is plastically deformed, the pressure in the pipe and the axial load of the pipe are unloaded,
A method for improving the residual stress of a pipe, comprising unloading the pressure inside the pipe when the strain on the outer surface of the pipe reaches about 0.5%.   The residual stress improvement method for piping according to claim 1, wherein the stress improvement region includes a welded portion of the pipe and a heat affected zone by the welded portion.   2. The residual stress improvement method for piping according to claim 1, wherein the stress improvement region includes an expanded portion of the piping. For a stress improvement region having tensile residual stress in a pipe filled with fluid, an axial load is applied to the pipe, and the pressure inside the pipe is increased to plastically deform the stress improvement area. In the residual stress improvement method,
The range of the load in the pipe axial direction is a stress at which the axial strain of the pipe outer surface is 0% or more and a stress equal to or lower than the yield stress of the pipe,
After the pipe is plastically deformed, the pressure in the pipe and the axial load of the pipe are unloaded,
After the pipe inner surface starts plastic deformation, when the pipe outer surface starts plastic deformation, the pressure inside the pipe is unloaded,
A method for improving residual stress in a pipe, wherein a plastic deformation is confirmed by measuring a change in a circumferential strain relative to an axial strain of the pipe outer surface.   5. The residual stress improvement method for piping according to claim 4, wherein the stress improvement region includes a welded portion of the pipe and a heat affected zone by the welded portion.   The residual stress improvement method for piping according to claim 4, wherein the stress improvement region includes a pipe expansion portion of the piping.   The residual stress improvement method for piping according to any one of claims 1 to 3, wherein a plastic deformation is confirmed by measuring a change in circumferential strain with respect to an axial strain of the outer surface of the piping. How to improve. The residual stress improvement method for piping according to any one of claims 4 to 6, wherein a detection position of a circumferential strain on the outer surface of the piping is a region where a heat-affected zone by the welded portion and a tensile residual stress of a groove surface is generated. A method for improving the residual stress of a pipe, characterized in that a detection position of an axial strain is set to a region which is not affected by a grooved surface and welding surplus . 8. The method for improving residual stress in piping according to claim 7, wherein the detection position of the circumferential strain on the outer surface of the piping is a region where the heat affected zone by the welded portion and the tensile residual stress of the groove surface is generated , and the axial strain is reduced. A method for improving residual stress of piping, characterized in that a detection position is an area that is not affected by a grooved surface and welding surplus .

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