JP2010094715A - Method for improving residual stress of piping - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for improving residual stress of a piping which is capable of imparting larger compressive residual stress to the piping. <P>SOLUTION: A pair of constraint members 4, outer containers 15A, 15B and inner containers 16A, 16B are mounted on an outer surface of a piping 1 with water 17 being present therein at upstream and downstream sides of a weld part 2. Dry ice 22 is placed in the outer containers 15A, 15B to cool the water 17 in the piping 1. An ice plug 23 is formed in the piping 1 at the position of each outer container. Thereafter, dry ice 22 is placed in the inner containers 16A, 16B to cool the water 17 present between the ice plugs 23. The water 17 starts to freeze, and its volume is inflated, and the piping 1 is expanded in the radial direction between the ice plugs 23. Plastic deformation occurs at the position where the weld part 2 is brought into contact with a constraint member 4 of the piping 1. Compressive residual stress is imparted in each inner surface at the position where the weld part 2 is brought into contact with the constraint member 4 of the piping 1. <P>COPYRIGHT: (C)2010,JPO&INPIT

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 generated by welding.

  As a method for improving the residual stress of the welded portion of the pipe, a method using an ice plug (ice plug) formed in the pipe is proposed in Japanese Patent Laid-Open Nos. 2005-95948 and 2006-334596. Yes.

  The method for improving the residual stress of piping described in Japanese Patent Application Laid-Open No. 2005-95948 is that the refrigerant containers are respectively attached to the upstream and downstream sides of the welded portion of the piping to form ice plugs in the piping. The pipe is plastically deformed by the volume expansion of the frozen ice by freezing the water between the ice plugs. By this plastic deformation, compressive residual stress is applied to the inner surface of the pipe. In order to limit the maximum amount of plastic deformation of the pipe at the time of plastic deformation of the pipe, a restraining jig is attached to the outer surface of a place (for example, a welded portion) where the pipe is plastically deformed.

  In the method for improving residual stress of piping described in JP-A-2006-334596, refrigerant containers for forming ice plugs are respectively attached at two locations on the upstream side and downstream side of the welded portion of the pipe, and these refrigerant containers are welded to each other. A refrigerant container for pipe expansion is attached at two locations between the sections. Ice generated by forming ice plugs at two locations upstream and downstream of the welded portion in the pipe by each refrigerant container for forming ice plugs, and freezing water existing between the ice plugs by each refrigerant container for expanding pipes The vicinity of the welded part of the pipe is pushed outward using the volume expansion of the pipe. 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 near the welded portion.

  Japanese Patent Application Laid-Open No. 5-154683 also describes a method for improving the stress of the welded portion of the pipe. In this stress improvement method, a restraint member is attached around the pipe on the upstream side and the downstream side of the welded portion, and the pipe is heated and thermally expanded. When the pipe is thermally expanded, a clamping force is applied to the pipe by the restraining member, and compressive stress is applied not only to the circumferential stress but also to the axial stress. Thereby, the residual stress of piping is improved.

JP-A-2005-95948 JP 2006-334596 A Japanese Patent Laid-Open No. 5-154683

  In the method for improving residual stress of piping described in Japanese Patent Application Laid-Open No. 2005-95948, a restraining jig is attached so as to surround the piping at a position where stress is improved including the outer surface of the welded portion. In the vicinity of the welded portion of the pipe, a surplus where the weld metal is raised to a size larger than necessary is formed. The restraining jig has a function of limiting the maximum amount of plastic deformation of the pipe, but it is necessary to create it in consideration of surplus. However, because the shape of the surplus differs for each welded part, it is necessary to create a restraining jig in consideration of the surplus for each welded part, but create a restraining jig according to the shape of the surplus. It is difficult. When the surplus of the welded part of the pipe is large, even if the pipe is expanded due to the volume expansion of ice in the pipe, the surplus will come into contact with the inner surface of the restraining jig, and the expansion of the welded part will be restricted by the restraining jig. May be insufficient. For this reason, there is a possibility that the residual stress is not improved as planned in the welded portion of the pipe.

  In the method for improving the stress of a pipe using a restraining member described in JP-A-5-154683, the pipe is tightened by the restraining member using thermal expansion of the pipe due to heating or shrinkage of overlay welding. For this reason, as described in FIG. 3 of Japanese Patent Application Laid-Open No. 5-154683, a large axial tensile stress is generated in the portion of the pipe in contact with the restraining member.

  An object of the present invention is to provide a method for improving the residual stress of a pipe, which can give a larger compressive residual stress to the pipe.

  A feature of the present invention that achieves the above-described object is to dispose a restraining member that surrounds the outer surface of the pipe at a position away from a stress improvement region that improves the residual stress of the pipe and suppresses deformation that extends in the radial direction of the pipe, Thereafter, the pressure in the pipe is increased in a state where the outer surface of the stress improvement region is not surrounded by the restraining member, and the stress contact region and the portion in contact with the restraining member of the pipe are plastically deformed.

  Place the restraint member around the outer surface of the pipe at a position away from the stress improvement area of the pipe, and raise the pressure in the pipe to plastically deform the part that is in contact with the pipe restraint member. A large compressive residual stress can be applied to the inner surface of the region.

  According to the present invention, a larger compressive residual stress can be applied to a stress improvement region (for example, a welded portion) of piping.

  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 increase the pressure in the pipe in which the restraint member is attached in the vicinity of the weld, and plastically deform the stress improvement region of the pipe (weld, etc.) and the place in contact with the pipe restraint member. Newly found that is desirable. By this plastic deformation, a large compressive residual stress can be applied to the inner surface of the stress improvement region of the pipe. For this reason, the probability that stress corrosion cracking occurs in the piping can be further reduced. As a method of increasing the pressure in the pipe with the restraint member attached to the pipe, a method of utilizing the volume expansion of ice by freezing the water in the pipe, and pressurizing the water in the pipe to reduce the pressure of this water There is a way to increase it.

  Examples of the present invention reflecting the above examination results will be described below.

  A method for improving the residual stress of a pipe according to the first embodiment which is a preferred embodiment of the present invention will be described with reference to FIG. This embodiment will be described by taking as an example the case of improving the residual stress in the vicinity of a welded portion of a pipe in a power plant.

  The end 1 of the stainless steel (or nickel alloy) pipe 1 of the power plant is joined by butt welding. Due to this butt welding, the inner surface of the welded portion 2 of the pipe 1 and the heat-affected portion 3 that is adjacent to the welded portion 2 on both sides of the welded portion 2 and whose mechanical properties are changed by the heat of welding are applied. Tensile residual stress that causes stress corrosion cracking has occurred. The welded part 2 and the heat-affected part 3 of the pipe 1 are in a range where the stress needs to be improved because tensile residual stress is generated by butt welding.

  As shown in FIG. 1 (A), a pump 5 and an on-off valve 8 are provided in the pipe 1 that should improve the residual stress in the welded part 2 and the heat affected zone 3. A pipe 7 is connected to the pipe 1 upstream and downstream of the pump 5, and a pressure control valve 6 is provided in the pipe 7.

  The position for measuring the outer diameter of the pipe 1 is determined. The measurement position of the outer diameter is preferably in the vicinity of the weld 2 and the position where the restraint member 4 is attached. The outer surface of the pipe 1 is marked on the outer surface of the pipe 1 with an oil-based pen or the like at the determined outer diameter measurement position so that the positions of the pipe 1 are not known due to plastic deformation of the pipe 1 accompanying the increase in internal pressure. 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 in each cross section perpendicular to the axis of the pipe 1 in the vicinity of the weld 2 and in the vicinity of the attached restraining member 4. Measure. Instead of measuring the outer diameter using the marking, a strain gauge is attached to the outer surface of the pipe 1 in the vicinity of the weld 2 and in the vicinity of the attached restraining member 4, and the circumference of the pipe 1 is measured using these strain gauges. You may measure the deformation | transformation of the piping 1 by measuring the distortion | strain of a direction. Thus, by determining the position where the outer diameter of the pipe 1 is measured and measuring the amount of change in the outer diameter of the pipe, it can be confirmed whether the entire pipe 1 is plastically deformed when the pipe 1 expands.

  An annular restraining member 4 made of stainless steel is attached to the pipe 1 upstream and downstream of the welded portion 2, specifically upstream and downstream of the heat affected zone 3, respectively (see FIG. 1B). The constraining member 4 includes two semicircular members. When the constraining member 4 is attached to the pipe 1, the semicircular members are arranged around the pipe 1 and coupled to each other using bolts or the like. The inner surface of the restraining member 4 is in contact with the outer surface of the pipe 1. The outer surface of the pipe 1 between the pair of restraining members 4 is not restrained by any restraining member. The outer surfaces of the welded portion 2 and the heat affected zone 3 are not restrained by the restraining members.

  The on-off valve 8 is fully closed, and the pressure control valve 6 is fully opened. Thereafter, the pump 5 is driven. The water in the pipe 1 pressurized by the pump 5 circulates through the pipe 7. The pressure control valve 6 is gradually closed to increase the pressure in the pipe 1 downstream from the pump 5. The amount of change in the outer diameter of the pipe 1 is measured at the outer diameter measurement position of the pipe 1 described above, and the opening degree of the pressure control valve 6 is decreased until the set deformation amount is obtained. The set deformation amount is a deformation amount that causes plastic deformation in the pipe 1 at the position of the pipe 1 in contact with the restraining member 4. By increasing the pressure in the pipe 1 as described above, the pipe 1 is deformed so as to spread in the radial direction. The amount of deformation in the radial direction at two locations where the pipe 1 is in contact with the restraining member 4 is smaller than the amount of deformation of the pipe 1 other than these positions because of the restraint by the restraining member 4. Thereby, the deformation | transformation which becomes convex toward the outer side in the radial direction arises in the piping 1 centering on the welding part 2 (refer FIG.1 (C)).

  As a result, plastic deformation occurs at the position of the pipe 1 in contact with the restraining member 4 as well as the welded portion 2 and the heat affected zone 3. This is because the restraining member 4 also expands somewhat outward due to the pressure increase in the pipe 1. The outer diameters of the pipes 1 are measured at the positions with the above-described markings (in the vicinity of the welded portion 2 and in the vicinity of the attached restraining member 4). When the internal pressure of the pipe 1 increases, plastic deformation occurs at the positions of the pipe 1 where the welded portion 2 and the restraining member 4 are in contact, and when the measured value of the outer diameter at those positions reaches the set value, the pressure control valve 6 And the pressure in the pipe 1 at the weld 2 and the heat affected zone 3 is reduced. The amount of deformation toward the outside in the radial direction in the welded portion 2 and the heat-affected zone 3 that is the stress improvement region is larger than the amount of deformation of the pipe 1 at the position in contact with the restraining member 4. The driving of the pump 5 is stopped and the restraining member 4 is removed from the pipe 1. The opening degree of the pressure control valve 6 may be increased when the strain measured with the strain gauge described above reaches a set value.

  FIG. 2 shows the shape of the pipe 1 after the pipe 1 is plastically deformed and the restraining member 4 is removed. Compressive residual stress is applied to the inner surfaces of the welded portion 2 and the heat affected zone 3 by plastic deformation due to the pressurization of water in the pipe 1. Since the pipe 1 is plastically deformed with the two restraining members 4 attached, the pipe 1 from which the restraining member 4 has been removed protrudes outward in the radial direction at the welded portion 2 and is inward at the position where the restraining member 4 is attached. It has a shape that is recessed toward the surface. As described above, since plastic deformation occurs at a position of the pipe 1 where the restraining member 4 is in contact, in this embodiment, the compressive residual stress on the inner surfaces of the welded portion 2 and the heat affected zone 3 is attached to the restraining member 4. However, it becomes larger than the compressive residual stress applied by the increase in the internal pressure of the pipe 1.

  A description will be given of the axial residual stress generated in the welded portion 2 and the portion in contact with the restraining member 4 of the pipe 1. At a location where the pipe 1 is in contact with the restraining member 4, a tensile residual stress is generated in the central portion in the radial direction of the pipe 1, and a compressive residual stress 12 is generated on the inner surface 10 side and the outer surface 11 side of the pipe 1. The portion of the pipe 1 that is in contact with the restraining member 4 is recessed toward the inside of the pipe 1. However, when the entire pipe 1 spreads outward in the radial direction due to internal pressure and is plastically deformed, the inner surface 10 side is described above. As described above, the compressive residual stress 12 is generated. Since the portion of the pipe 1 that has been in contact with the restraining member 4 is plastically deformed to the extent that compressive residual stress is generated, the distribution of the residual stress in the radial direction of the pipe 1 in the welded portion 2 is as shown in FIG. . That is, a large residual compressive stress 12 is generated on the inner surface 10 side of the pipe 1. A tensile residual stress 13 is generated on the outer surface 11 side of the pipe 1 of the weld 2.

  In the present embodiment, the restraint member 4 surrounding the pipe 1 is placed in contact with the pipe 1 upstream and downstream of the region (for example, the welded portion 1 and the heat-affected zone 3) where the residual stress of the pipe 1 is improved, By increasing the water pressure by the pump 5, the pressure in the pipe 1 is increased, and the pipe 1 is plastically deformed not only at the welded part 2 and the heat affected part 3 but also at the contact part of the restraining member 4. For this reason, the restraint member 4 described in Japanese Patent Application Laid-Open No. 2006-334596 is provided on the inner surface of the welded portion 2 and the heat-affected zone 3 of the pipe 1 in which the outer surface is not restrained by the restraint member 4. It is possible to apply a compressive residual stress larger than the respective residual stress improving methods. In this embodiment, in order to deform the welded part 2 and the heat-affected part 3 to which the restraint member is not attached and the position of the pipe 1 where the restraint member 4 is attached due to the pressure increase in the pipe 1, -Tensile residual stress on the inner surface of the pipe 1 at the position where the restraint member 4 is in contact with the pipe, as compared with the method for improving the residual stress of the pipe in which the restraint member is arranged at the welded portion and the heat affected zone, as described in Japanese Patent No. Can be reduced.

  Such an embodiment has an outer diameter in which compression residual stress may not be applied to the inner surface in the residual stress improvement methods described in JP-A-2005-95948 and JP-A-2006-334596, respectively. Even for a large pipe 1 (for example, a pipe having an outer diameter of 60 mm or more), compressive residual stress can be applied to the inner surfaces of the welded portion 2 and the heat affected zone 3. Therefore, this embodiment can prevent stress corrosion cracking in the pipe 1 having a large outer diameter. In the present embodiment, even in the pipe 1 having an outer diameter of less than 60 mm and a small outer diameter, it is possible to apply compressive residual stress to the inner surfaces of the welded portion 2 and the heat affected zone 3 as a matter of course.

  In this embodiment, since the on-off valve 8 is fully closed when the pressure in the pipe 1 is increased, the internal pressure of the pipe 1 also acts on the on-off valve 8. For this reason, as will be described in detail in the second embodiment, the pressure also acts on the stretching of the pipe 1 in the axial direction. The compressive residual stress generated by stretching the pipe 1 in the axial direction is added to the compressive residual stress generated by the outward deformation in the radial direction of the pipe 1. Therefore, the present embodiment can further improve the residual stress of the pipe 1.

  Only one restraining member 4 may be attached in contact with the outer surface of the pipe 1 upstream (or downstream) of the heat affected zone 3 to increase the pressure in the pipe 1 as described above. In this case, due to an increase in the internal pressure of the pipe 1, plastic deformation occurs at a position where the welded part 2, both the upstream and downstream heat affected parts 3 of the welded part 2, and the pipe 1 are in contact with the restraining member 4. Produce. For this reason, compressive residual stress is given to the inner surface of the welded part 2 and both heat affected parts 3. The effect of improving the residual stress due to plastic deformation at the position in contact with the restraint member 4 is increased at the welded portion 2 and the heat affected zone 3 close to the restraint member 4. It becomes small in the heat affected zone 3 located on the side.

  In this embodiment, the thickness of the restraining member 4 attached to the pipe 1 may be thicker than that of the restraining member 4 used when obtaining the radial residual stress distribution of the pipe 1 shown in FIG. By attaching a pair of thick restraining members 4 at the same position as in FIG. 1B and increasing the pressure in the pipe 1 as described above, the restraint of the welded portion 2, the heat affected zone 3, and the pipe 1 is restrained. Each of the positions where the members 4 are attached is spread outward in the radial direction and plastically deformed. In this case, the distribution of the residual stress in the radial direction at the portion of the pipe 1 in contact with the restraining member 4 is as shown in FIG. A tensile residual stress 13 that is smaller than the residual stress is formed on the inner surface 10 side of the pipe 1. Even in such a case, the plastic deformation of the pipe 1 between the restraining members 4 becomes larger than the residual stress improvement methods described in JP-A-2005-95948 and JP-A-2006-334596, respectively. And the residual compressive stress which arises in the inner surface of the heat affected zone 3 also becomes large. Compressive residual stress can be applied to the inner surfaces of the welded part 2 and the heat-affected part 3, for example, not only for pipes with a small outer diameter but also for pipes with a large outer diameter of 60 mm or more. If the thickness of the restraint member 4 becomes too thick, plastic deformation will not occur at the position of the pipe 1 where the restraint member 4 is attached. For this reason, the thickness of the restraining member 4 is set so that plastic deformation occurs at the position of the pipe 1 where the restraining member 4 is attached.

  A method for improving the residual stress of a pipe according to embodiment 2 which is another embodiment of the present invention will be described with reference to FIG. A pipe 1 to which the method for improving residual stress of a pipe according to this embodiment is applied is a pipe provided in a power plant, which is made of stainless steel and has an outer diameter of 60 mm and a thickness of 5 mm.

  As in the first embodiment, a pair of restraining members 4 are provided upstream and downstream of the welded portion 2 in the pipe 1 that should improve the residual stress in the welded portion 2 and the heat-affected zone, which are stress improvement regions. The pipe 1 is surrounded and attached to the pipe 1 upstream and downstream of the heat affected zone. Further, the outer containers 15A and 15B and the inner containers 16A and 16B are attached to the outer surface of the pipe 1 (see FIG. 6A). The pipe 1 is filled with water 17.

  Attachment of the restraining member 4 to the pipe 1 is performed in the same manner as in the first embodiment. The restraining member 4 is made of stainless steel, has a width of 10 mm, and a thickness of 1.5 mm. A distance 19 between the pair of restraining members 4 is 50 mm. The welded part 2 is located in the middle between the pair of restraining members 4. When the restraining member 4 is attached to the pipe 1, the inner diameter of the restraining member 4 is the same as the outer diameter of the pipe 1, and the inner surface of the restraining member 4 is in contact with the outer surface of the pipe 1.

  Inner containers 16 </ b> A and 16 </ b> B are arranged at positions away from the restraining member 4 in the direction from the welded part 2 to the restraining member 4, and attached to the pipe 1. The pair of restraining members 4 are disposed between the inner container 16A and the inner container 16B, and the distance between the inner container 16A and the inner container 16B is 300 mm. The inner containers 16A and 16B whose upper ends are opened are divided into two parts in the vertical direction before being installed in the pipe 1, and the structure divided into two after being attached with the pipe 1 in the vertical direction is bolted to each other. Are combined. Each length of the inner containers 16A and 16B in the axial direction of the pipe 1 is 200 mm.

  Outer containers 15A and 15B are arranged at positions away from inner containers 16A and 16B with welding portion 2 as a base point. The inner containers 16A and 16B are disposed between the outer container 15A and the outer container 15A. The width of each of the gap 21 formed between the outer container 15A and the inner container 16A and the gap 21 formed between the outer container 15B and the inner container 16B is 100 mm. The outer containers 15A and 15B whose upper ends are opened are divided into two parts in the vertical direction before being installed in the pipe 1, and the structure divided into two after being attached with the pipe 1 in the vertical direction is bolted to each other. Are combined. The length of the outer containers 15A, 15B in the axial direction of the pipe 1 is also 200 mm.

  As in the first embodiment, marking is performed at positions where the outer diameter of the pipe 1 set in the vicinity of the welded portion 2 and in the vicinity of the attached restraining member 4 is measured.

  The inner containers 16A and 16B and the outer containers 15A and 15B are hollow inside. Ethyl alcohol 18 is poured into the inner containers 16A and 16B and the outer containers 15A and 15B to such an extent that the pipe 1 is immersed. Dry ice 22 is put into the outer containers 15A and 15B filled with the ethyl alcohol 18 from the opening at the upper end. The supplied dry ice 22 cools the water 17 in the pipe 1 at a position surrounded by the outer container 15A and the water 17 in the pipe 1 at a position surrounded by the outer container 15B. For this reason, the water 17 freezes in each position in the piping 1, and the ice plug 23 is formed (refer FIG. 6 (B)). The water 17 between the ice plugs 23 is sealed by the ice plug 23. In order to form the ice plug 23 that exhibits a stronger sealing function in the pipe 1, the inside of the outer containers 15A and 15B is longer than the time required to form the ice plug 23 obtained in advance through experiments or the like. Continue cooling the pipe 1 with dry ice. As a result, a pair of ice plugs 23 that are firmly frozen on the inner surface of the pipe 1 are formed, and a sealed region filled with water 17 is formed between these ice plugs 23.

  As shown in FIG. 6C, dry ice 22 is put into the inner containers 16A and 16B filled with ethyl alcohol 18. The water 17 existing between the ice plugs 23 is cooled by the dry ice 22 in the inner containers 16A and 16B. At each position where the inner containers 16A and 16B are attached, the pipe 1 is cooled below the freezing point, and the water 17 existing between the ice plugs 23 begins to freeze. Since the volume expands when the water 17 becomes ice, the pressure of the water 17 existing between the ice plugs 23 starts to rise. The pressure in the pipe 1 rises between the ice plugs 23, and the pipe 1 is expanded outwardly in the radial direction between the inner container 16A and the inner container 16B by this pressure increase. The outer diameter of the pipe 1 is measured at each marked measurement position (in the vicinity of the weld 2 and in the vicinity of the attached restraining member 4). Check if the measured value of each outer diameter has reached the set value. When the measured value of the outer diameter does not reach the set value, the cooling of the pipe 1 by the dry ice 22 in the inner containers 16A and 16B is continued. When the measured value of the outer diameter reaches the set value, the dry ice 22 in the outer containers 15A and 15B and the inner containers 16A and 16B is discharged to the outside, and the cooling of the pipe 1 by the dry ice 22 is stopped. The amount of increase in the outer diameter of the pipe 1 is 1% of the outer diameter. If the outer diameter of the pipe 1 is 60 mm, the cooling is stopped after an increase of about 0.6 mm is confirmed at the position where the weld 2 and the pipe 1 are in contact with the restraining member 4.

  When the outer diameter of each position of the pipe 1 reaches the set value due to the deformation of the pipe 1, the pipe 1 is plastically deformed at the position where the pipe 1 is in contact with the restraining member 4 and the stress improvement region. Yes. The plastic deformation in the stress improvement region (the welded portion 2 and the heat affected zone) is larger than the plastic deformation at the position in contact with the restraining member 4 of the pipe 1.

  After the dry ice 22 in the outer containers 15A, 15B and the inner containers 16A, 16B is discharged to the outside and all the ice in the pipe 1 has melted to become water 17, the restraining member 4, the outer containers 15A, 15B and the inner The containers 16A and 16B are removed from the pipe 1. The outer containers 15A and 15B and the inner containers 16A and 16B can be removed before the ice in the pipe 1 is completely melted if the cooling is stopped. In a state in which these are removed, as shown in FIG. 6D, between the inner container 16A and the inner container 16B attached to the pipe 1, the pipe 1 faces outward in the radial direction in the welded portion 2. And has a shape that is recessed inward in the radial direction at the position where the restraining member 4 is attached. This shape is the same as the shape of the pipe 1 shown in FIG.

  In the present embodiment, each effect produced in the first embodiment can be obtained. In the present embodiment in which the water 17 existing between the ice plugs 23 is cooled, when the pressure in the pipe 1 rises, this pressure not only pushes the pipe 1 outward in the radial direction, but also the pipe 1 In the axial direction, the pipe 1 is stretched in the axial direction by acting on each ice plug 23 adhering to the inner surface of the pipe 1. For this reason, since the axial stress is applied to the axial residual stress of the pipe 1, the compressive residual stress generated by the outward deformation in the radial direction of the pipe 1 is generated by stretching the pipe 1 in the axial direction. Compressive residual stress is added. Also in this embodiment, the residual stress can be further improved.

  The inventors analyzed the residual stress generated in the pipe 1 using the finite element method for the residual stress improvement methods for the pipes of Examples 1 and 2, and clarified the mechanism of stress improvement in these Examples. did. FIG. 7A shows a model corresponding to Examples 1 and 2 used in the above analysis. This model is for a pipe 1 made of stainless steel and having an outer diameter of 60 mm and a wall thickness of 5 mm. A pair of restraining members 4 made of stainless steel surround each outer surface of the pipe 1 so that each of the restraining members 4 The inner surface is disposed in contact with the outer surface of the pipe 1. In the model, the width of the restraint member 4 was 10 mm, the thickness of the restraint member 4 was 1.5 mm, and the distance D between the pair of restraint members 4 was changed to 10 mm, 50 mm, and 100 mm. The pressure applied to the inner surface of the pipe 1 was increased from 0 MPa to 71.33 MPa, and after the pressure increased to 71.33 MPa, the pressure was decreased to 0 MPa. After the pressure applied to the inner surface of the pipe 1 decreased to 0 MPa, the restraining member 4 was removed from the pipe 1.

  First, changes in stress and strain generated in the pipe 1 when the internal pressure of the pipe 1 is raised and when the internal pressure is lowered will be described. The stress and strain at a position sufficiently away from the restraining member 4 of the pipe 1 when the internal pressure of the pipe 1 rises and falls vary as shown in FIG. FIG. 8A shows changes in axial stress and strain on the inner and outer surfaces of the pipe 1 when the internal pressure of the pipe 1 increases and decreases. Changes in the axial stress are shown in FIG. 8 (B), the changes in the stress and strain in the circumferential direction of the inner and outer surfaces of the pipe 1 when the internal pressure of the pipe 1 increases and decreases, and in FIG. 8 (C). This can be explained based on changes in stress and strain in the radial direction of the inner surface and outer surface of the pipe 1 when the internal pressure of the pipe 1 rises and falls. According to FIG. 8 (B) and FIG. 8 (C), since the strain in the circumferential direction of the pipe 1 has a larger absolute value than the strain in the radial direction, a strain having an opposite sign to the circumferential direction is present in the axial direction of the pipe 1. Arise.

  When the internal pressure increases, there is a difference in strain between the inner surface and the outer surface in the circumferential direction and the radial direction of the pipe 1. Here, when the strain in the circumferential direction of the pipe 1 exceeds about 0.2%, the pipe 1 starts plastic deformation. When plastic deformation is started, the difference in the radial strain is larger than the difference in the circumferential strain of the pipe 1, so that the axial stress on the inner surface of the pipe 1 becomes a compressive stress. Since the pipe 1 is plastically deformed, compressive stress remains on the inner surface. Note the change in the stress in the circumferential direction of the pipe 1 shown in FIG. Since pressure acts on the inner surface of the pipe 1, the inner surface of the pipe 1 starts plastic deformation with a lower stress than the outer surface of the pipe 1. Thereby, when a stress difference is generated between the inner surface and the outer surface of the pipe 1 and the pressure inside the pipe 1 is reduced, the inner surface of the pipe 1 can be made compressive stress by the stress difference. For these reasons, the internal pressure of the pipe 1 is increased and the pipe 1 is deformed outward in the radial direction until the pipe 1 is plastically deformed. The stress can be a compressive stress.

  FIG. 7B shows the analysis result of the residual stress generated in the pipe 1 by the finite element method using the model shown in FIG. This residual stress is an axial stress on the inner surface of the pipe 1. The horizontal axis of FIG. 7B is the middle between the pair of restraining members 4 attached to the pipe 1, that is, the distance 25 from one restraining member 4 and the distance 26 from the other restraining member 4 are equal. A position on the inner surface of the pipe 1 is a base point 27, and a distance 28 in the axial direction of the pipe 1 from the base point 27 is shown. The vertical axis represents the axial stress. The region from the base point 27 to 10 mm toward the restraining member 4 was used as the stress improvement region 29 where the residual stress should be improved, and the residual stress generated in the pipe 1 was analyzed by the finite element method. When the distance D between the restraint members 4 is 10 mm, which is smaller than 2√RT, the pair of restraint members 4 are too close to each other, so that the axial stress is the tensile residual stress in the stress improvement region 29. The circumferential stress in this case is compressive residual stress. R is the inner radius of the pipe 1 and T is the thickness of the pipe 1. When the distance D is 50 mm larger than 2√RT and smaller than 5√RT, the axial stress in the stress improvement region 29 is a compressive residual stress as compared with the case where the restraint member 4 is not attached. When the distance D is 50 mm, the circumferential stress in the stress improvement region 29 is also a large compressive residual stress. When the distance D is 100 mm, which is larger than 5√RT, the pair of restraining members 4 are too far away from each other, so that the stress improving effect due to the mounting of the restraining members 4 cannot be obtained in the stress improving region 29. Based on the above examination results, the pair of restraining members 4 are preferably attached to the pipe 1 so as to satisfy 2√RT <D <5√RT.

  Next, FIG. 9 shows the result of analyzing the axial residual stress generated in the pipe 1 when the distance D satisfying 2√RT <D <5√RT is 30 mm, 40 mm and 50 mm by the finite element method. By attaching the restraining member 4 so that the stress improvement region 29 is located within 9.5 mm from the base point 27 when the distance D is 30 mm, within 12.8 mm when the distance D is 40 mm, and within 15.5 mm when the distance D is 50 mm, The residual stress improvement effect can be enhanced. Based on the above examination results, it is desirable to attach the restraining member 4 so that the stress improvement region 29 is located within 0.3 D from the base point 27.

  In the methods for improving the residual stress of each pipe in the first and second embodiments, as described above, the increase in the internal pressure of the pipe 1 not only deforms the pipe 1 outward in the radial direction, but also the pipe. A force for pulling 1 in the axial direction is also generated. This tensile force causes a tensile stress in the axial direction of the pipe 1. FIG. 10 shows the distribution of residual stress by analysis using the finite element method when a load corresponding to the tensile stress generated in the axial direction of the pipe 1 is applied to the end of the pipe 1. When the internal pressure of the pipe 1 rises, axial stress is generated, so that both the axial stress and the circumferential stress of the pipe 1 have a higher residual stress improvement effect than when no load corresponding to the axial stress is applied. can get.

It is explanatory drawing which shows the process of the residual stress improvement method of piping of Example 1 which is one suitable Example of this invention, (A) is a block diagram of the piping system which applies the residual stress improvement method of Example 1, (B) is explanatory drawing which shows the state which attached a pair of restraint member to piping, and (C) raises the pressure in piping and shows the deformation | transformation state of piping after implementing the residual stress improvement method of Example 1. FIG. It is explanatory drawing shown. It is explanatory drawing which shows the state which removed a pair of restraining member from the state of FIG.1 (C). FIG. 3 is an enlarged view of a part III in FIG. 2. It is an enlarged view of the IV section of FIG. FIG. 3 is an enlarged view of a part III in FIG. 2 when the internal pressures of the pipes are different. It is explanatory drawing which shows the process of the residual stress improvement method of piping of Example 2 which is another Example of this invention, and attached the restraint member etc. to the pipe which applies the residual stress improvement method of Example 2 (A). Explanatory drawing which shows a state, (B) is explanatory drawing which shows the state which formed the pair of ice plugs in piping using the dry ice in a pair of outer container, (C) is the pressure between the ice plugs in piping. Explanatory drawing which shows the state which raised and deform | transformed piping toward the outer side, (D) is explanatory drawing which shows the deformation | transformation state of piping after implementing the residual stress improvement method of Example 2. FIG. It is explanatory drawing which shows the analysis result of the residual stress which arose in the inner surface of piping by the residual stress improvement method of piping in Example 1 and 2, (A) is explanatory drawing of the model used for the analysis, (B) is a pair. It is a characteristic view which shows the axial direction stress and circumferential direction stress of piping at the time of changing the distance between these restraint members. It is a characteristic view showing the relation between stress and strain when the internal pressure of piping is changed, (A) is a characteristic view showing changes in axial stress and axial strain of piping, and (B) is circumferential stress in piping. And (C) is a characteristic diagram showing changes in the radial stress and radial strain of the pipe. It is a characteristic view which shows the axial direction stress of the welding part of piping at the time of changing the distance between a pair of restraining members. It is a characteristic view which shows the axial direction stress and the circumferential direction stress of piping after applying the residual stress improvement method of Example 1 and 2.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Pipe, 2 ... Welded part, 3 ... Heat-affected part, 4 ... Restraint member, 5 ... Pump, 8 ... Open / close valve, 10 ... Pipe inner surface, 11 ... Pipe outer surface, 15A, 15B ... Outer container, 16A, 16B ... Inner container, 17 ... water, 18 ... ethyl alcohol, 22 ... dry ice, 23 ... ice plug.

Claims (8)

  A restraining member that surrounds the outer surface of the pipe at a position away from the stress improvement region that improves the residual stress of the pipe and suppresses deformation spreading in the radial direction of the pipe is disposed, and then the outer surface of the stress improvement region is Residual stress improvement of piping characterized in that the pressure in the piping is raised without being surrounded by the restraining member, and the stress improving region and the portion of the piping that is in contact with the restraining member are plastically deformed. Method.   The method for improving residual stress of piping according to claim 1, wherein the stress improvement region includes a welded portion of the pipe, and the arrangement of the restraining member is performed upstream and downstream of the welded portion.   The method for improving residual stress of a pipe according to claim 1 or 2, wherein when the restraining member is disposed, the restraining member is brought into contact with an outer surface of the pipe.   The method for improving residual stress of a pipe according to claim 1, wherein the pressure in the pipe is lowered after the pipe is plastically deformed.   The increase in pressure in the pipe causes the valve provided in the pipe to be fully closed at a position away from the stress improvement area, and then leaves the stress improvement area in a direction from the valve toward the stress improvement area. The method for improving residual stress in piping according to any one of claims 1 to 3, wherein the method is carried out by increasing the pressure of the liquid in the piping by a pump provided in the piping at a certain position.   The rise in pressure in the pipe forms ice plugs at two locations upstream and downstream of the stress improvement region in the pipe where water is present, and between the ice plugs where the arrangement position of the restraint member exists. The method for improving residual stress in piping according to any one of claims 1 to 3, which is performed by freezing water existing in the piping. When the thickness of the pipe is T, the inner radius of the pipe is R, and the distance between the restraining member disposed upstream and the restraining member disposed downstream is D, the distance D is
2√RT <D <5√RT
The method for improving residual stress in piping according to claim 2, wherein each of the restraining members is arranged so that   The stress improvement region is within a range of 0.3D on the upstream side and the downstream side, respectively, based on the middle position between the restraining member disposed upstream and the restraining member disposed downstream. The method for improving a residual stress of a pipe according to claim 2, wherein the method is located.

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