JP2009012030A - Method for improving residual stress in inner surface of pipe, and pipe improved in residual stress in inner surface thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for improving residual stresses in an inner surface of a pipe, in which the residual tensile stresses in the axial direction of the inner surface of the pipe at a part to be improved, can be improved, and a pipe improved in residual stresses in inner surface thereof. <P>SOLUTION: Stress improvement heat input areas L5-L7 are set by deviating the position in the axial direction of a pipe 20 with respect to the part O to be improved where the residual tensile stress in the inner surface of the pipe 20 must be improved. The welding heat input higher than those in areas L1-L4 including the part O for improvement is given to an outer surface of the pipe at the set stress improvement heat input areas L5-L7. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a method for improving residual stress on an inner surface of a pipe and a pipe having improved residual stress on the inner surface.

  In the welded portion of austenitic stainless steel or high nickel base alloy, chromium carbide precipitates at the grain boundaries due to welding heat. As a result, a chromium-deficient layer is formed in the immediate vicinity of the crystal grain boundary, and this chromium-deficient layer becomes sensitized (sensitivity to corrosion). Further, generally high tensile residual stress exists on the surface in the vicinity of the weld. If a material with high tensile residual stress is used in a harsh corrosive environment in a sensitized state, stress corrosion cracking may occur. That is, when the three factors of material sensitization, high tensile residual stress, and severe corrosion environment overlap, the risk of stress corrosion cracking increases.

  Conventionally, stress corrosion cracking of austenitic stainless steel has been considered to occur in the weld heat affected zone of materials having a high carbon content such as JIS type 304 stainless steel. Therefore, the part exposed to the corrosive environment at the welded part of type 304 stainless steel was added with an element that is low in carbon content and difficult to be sensitized for the purpose of improving material factors among the factors that cause stress corrosion cracking. It has been replaced with JIS standard type 316L stainless steel.

  However, in recent years, the possibility that stress corrosion cracking occurs from the heat-affected zone of the welded part of type 316L stainless steel cannot be denied. Over time, it is becoming clear that the improvement in material factors using Type 316L stainless steel is not always sufficient. Furthermore, it cannot be said that the weld metal of stainless steel, which has been considered that cracks do not progress in the conventional knowledge, does not cause stress corrosion cracking. From such a possibility, in order to suppress stress corrosion cracking, not only improvement of material factors but also improvement of residual stress factors and corrosion environment factors has become an urgent task.

  On the other hand, one measure for suppressing the occurrence of stress corrosion cracking is to reduce the tensile residual stress in a region exposed to a corrosive environment. As a method of relieving the tensile residual stress of the welded joint part of the pipe, build-up welding is performed so as to cover the outside of the pipe including the existing welded joint part of the pipe, and then the pipe is cooled to shrink the build-up part. A method of generating compressive residual stress on the inner surface of an existing welded joint has been proposed (see Patent Document 1).

JP 2005-111513 A

  When overlay welding is performed on the outer surface of the pipe as in the technique described in Patent Document 1, shrinkage deformation occurs in the circumferential direction of the pipe due to shrinkage of the overlay welding portion, and the circumferential stress on the inner surface of the pipe is changed to compressive stress. Become. Overlay welding of the outer surface of the pipe is a very effective method when compressing the circumferential stress.

  However, Patent Document 1 does not particularly define the heat input conditions for overlay welding, the thickness / position of the overlay weld, and the like. Depending on the width and position of build-up welding, deformation that protrudes on the inner surface of the pipe occurs. In this case, the axial stress on the inner surface of the pipe becomes a tensile stress. If tensile residual stress remains in the vicinity of the butt weld on the inner surface of the pipe, it becomes a factor inducing the occurrence of cracks. Therefore, in order to suppress the occurrence and development of cracks, it is necessary to improve not only the circumferential direction of the improved portion but also the tensile stress in the axial direction.

  An object of the present invention is to provide a method for improving the residual stress on the inner surface of a pipe that can improve the residual tensile stress in the axial direction of the inner surface of the pipe at the location to be improved, and a pipe with improved residual stress on the inner surface.

  In order to achieve the above object, the present invention sets and sets the stress improvement heat input region by shifting the position in the axial direction of the pipe with respect to the improved portion that needs to improve the tensile residual stress on the inner surface of the pipe. Apply welding heat to the outer surface of the pipe in the stress improvement heat input area.

  ADVANTAGE OF THE INVENTION According to this invention, the residual tensile stress concerning the axial direction of the pipe inner surface of a to-be-improved location can be improved.

  Embodiments of the present invention will be described below with reference to the drawings.

  First, in the present specification, “improved location” means a location where it is necessary to improve the tensile residual stress on the inner surface of the pipe. Typically, for example, a welded portion or welded pipe such as a pipe joint is used. This is an area in the vicinity of the part (for example, within 10 mm). “Welding heat input” is a value obtained by approximately converting the amount of heat generated by arc generated when welding is performed, and is determined by the welding current, welding voltage, and welding speed. Therefore, unless otherwise specified, the concept of “giving welding heat input” in the present specification includes both a case where a weld metal such as a wire is supplied (build-up) and a case where no weld metal is supplied. Further, the “stress improving heat input region” means a region that gives welding heat input that contributes to the improvement of the residual stress of the improved portion, and is a region closer to the improved portion than the region (including the improved portion). High welding heat input is given compared to (region). In the present specification, the concept of “giving higher heat input to the stress-improving heat input region than the region including the improved portion” includes the stress improving heat input region relative to the region including the improved portion. The case where a lower welding heat input is applied and the case where no welding heat input is applied to the region are included. Based on these, the principle of improving the residual stress of the present invention will be described next.

  As shown in FIG. 1, a build-up portion (welded metal portion) 30 having a thickness tw and a length W on one side is laminated on the outer surface of the pipe 20 having an outer radius R and a tube wall thickness tp. The residual stress distribution on the inner surface of the pipe 20 when overlay welding is performed will be described.

  For example, as shown in Table 1 below, the results of measuring the stress distribution in the circumferential direction and the axial direction of the inner surface of the pipe for four cases AD having different pipe dimensions and welding conditions are shown in FIG.

  In the graph of FIG. 2, the residual stress (MPa) is on the vertical axis, and the distance (mm) in the axial direction from the center position O in the axial direction (the central axis direction of the pipe 20) of the built-up portion 30 is on the horizontal axis. . On the vertical axis, a positive value represents tensile residual stress and a negative value represents compressive residual stress. As can be seen from FIG. 2, both the circumferential residual stress and the axial residual stress have different distributions depending on the outer radius R and the thickness tp of the pipe 20.

On the other hand, FIG. 3 is a graph obtained by making the horizontal axis of the graph of FIG. 2 dimensionless by (Rtp) ^1/2 calculated from the outer radius R of the pipe 20 and the plate thickness tp. As shown in FIG. 3, the residual stress distribution on the inner surface almost coincides with the circumferential residual stress and the axial residual stress in each case AD. That is, if the stress distribution is obtained by making the axial distance from the center position O of the built-up portion 30 non-dimensional with (Rtp) ^1/2 , the inner surface of the pipe can be obtained regardless of the outer radius R or the thickness tp of the pipe 20. The residual stress distribution can be calculated.

  Here, as can be seen from FIG. 3, the circumferential stress on the inner surface of the pipe is slightly inclined to the compressive stress although there is a region slightly inclined to the tensile stress side, and becomes 0 (as the distance from the center position O of the built-up portion 30 increases. Asymptotically approaching zero). That is, by applying build-up welding to the outer surface of the pipe 20, the circumferential stress on the inner surface of the pipe is compressed and improved to a state in which stress corrosion cracking is unlikely to occur.

  On the other hand, the axial stress on the inner surface of the pipe is a tensile stress in the vicinity of the center position O of the built-up portion 30. In other words, when overlay welding is performed on the outer surface of the location where the residual stress is desired to be improved (the location to be improved), the circumferential stress on the inner surface of the pipe becomes compressive stress, but the axial stress on the inner surface of the pipe becomes tensile stress. Therefore, in order to suppress the stress corrosion cracking of the improved portion by performing build-up welding on the outer surface of the pipe, instead of simply forming the built-up portion 30 on the outer surface of the region including the improved portion, It is necessary to consider the positional relationship with the build-up part 30.

Accordingly, when viewing FIG. 3, the axial stress acting on the inner surface of the pipe is inclined toward the tensile stress side near the center position O of the built-up portion 30, but is rapidly inclined toward the compressive stress side as the distance from the center position O increases. Then, it becomes 0 at a position of about 2.7 (Rtp) ^1/2 . The region having a distance of 2.7 (Rtp) ^1/2 or more from the center position O, as the distance from the center position O, asymptotically approaches zero after inclined slightly tensile stress side. From this, when overlay welding is performed on the outer surface of the pipe, the heat generated by forming the overlay portion 30 in a region where the distance from the center position O of the overlay portion 30 is 2.7 (Rtp) ^1/2 or more. It can be seen that there is little influence on the effect, and even if the build-up portion 30 is formed at a distance of 2.7 (Rtp) ^1/2 or more from the improved portion, it does not affect the improvement of the residual stress at the improved portion. Therefore, in order to improve the axial stress of the improved portion, it is sufficient to form the built-up portion 30 in a region within a distance of 2.7 (Rtp) ^1/2 from the improved portion.

Next, the relationship between the build-up shape on the outer surface and the distribution of the axial residual stress on the inner surface of the pipe will be examined for the pipe illustrated in FIG. 4 is assumed to have an outer diameter of 609.6 mm (outer radius R is 304.8 mm) and a thickness tp of 38.9 mm, for example. On the outer surface of the pipe shown in FIG. 4, it is assumed that build-up welding in which a build-up portion 30 having a thickness of 0.5 tp and tp is formed on the outer face of the pipe as shown in FIG. The axial residual stress of the pipe inner surface in the regions L1 to L7 having a width 0.25 (Rtp) ^1/2 of the continuous built-up portion 30 was determined by a robust design method. The robust design method is a known method. The evaluation result by the robust design method of the relationship between the build-up thickness tw of each area | region L1-L7 of the build-up part 30 of a pipe outer surface and the axial direction stress of a pipe inner surface is shown in FIG.

In FIG. 6, the case where the cross-sectional shape of the built-up portion 30 is set to a welding thickness with relatively high sensitivity among tw = 0, 0.5 tp, and tp in each of the regions L1 to L7. It can be assumed that it is a suitable condition for generating a high axial compressive stress on the inner surface of the location to be improved (the center of the region L1, that is, the center position O). In the regions L1 to L3, tw = 0 is a favorable condition (that is, when the built-up portion 30 is formed, the axial stress on the inner surface of the improved portion O becomes tensile stress). In the region L4, the influence of the weld thickness tw on the axial stress on the inner surface of the improved portion O is relatively small. In the regions L5 to L7, tw = tp is a good condition (that is, the thickening of the built-up portion 30 indicates that the axial stress on the inner surface of the improved portion O can be inclined toward the compression side). FIG. 7 shows the shape of the built-up portion 30 for applying compressive stress to the improved portion O, which is set based on this result. As can be seen from FIG. 7, in order to compress the axial stress on the inner surface of the pipe of the improved portion O, it is necessary to build up a region away from the improved portion O. What needs to be built up is a region far from the region L4 when viewed from the improved portion O. Here, for example, the central portion of the region L4 is taken and the region in which the axial distance of the pipe from the improved portion O is 0.75 (Rtp) ^1/2 or more. The position closest to the portion to be improved O in the region L4 may be the starting point of the build-up portion 30, and the build-up portion 30 may be formed in a region of 0.625 (Rtp) ^1/2 or more.

As described above, in order to make the circumferential residual stress and the axial residual stress on the inner surface of the pipe of the improved portion O compressive and to suppress the stress corrosion cracking of the improved portion O, it is preferable to reduce the stress from 0 O to 0. 625 (Rtp) ^{1/2 to} 2.7 (Rtp) ^1/2 region, more preferably 0.75 (Rtp) ^{1/2 to} 2.7 (Rtp) ^1/2 region It is effective to set the heat input region and form the built-up portion 30 in the stress improving heat input region. In this way, by performing overlay welding on the pipe outer surface of the stress-improving heat input region that is displaced in the axial direction by a specified range from the location to be improved that needs to suppress the tensile residual stress, the piping at the location to be improved It is possible to suppress the circumferential tensile residual stress and the axial tensile residual stress on the inner surface, and to achieve compression stress.

  In general, if high tensile residual stress is applied to the inner surface of the butt-welded portion of the joint portion of the pipe or in the vicinity of the butt-welded portion (for example, a portion within 10 mm), there is a concern about the occurrence of stress corrosion cracking over time The By positioning such a butt weld or its vicinity in the improved area and applying welding heat to the stress improved heat input area deviated from the improved area as described above, the stress corrosion cracking of the improved area is suppressed. be able to. In the stress-improving heat input region, the build-up portion 30 may be formed as described in the previous drawings, but welding is performed without supplying weld metal (without forming the build-up portion 30). You may just give heat input.

  Next, the case where the crack part has already generate | occur | produced in the piping 20 is demonstrated.

  For example, when a crack location exists on the inner surface of the pipe 20, it is necessary to repair the crack location so that the strength can be secured with the weld metal even if the crack of the tube wall progresses. For this reason, when the cracked portion is the improved portion O, it is impossible to build-up welding avoiding the improved portion O. Therefore, when cracks are detected on the inner surface of the pipe, the axial tensile stress on the inner surface of the pipe is improved by applying the weld overlay method in which the build-up portion 30 is formed on the outer surface of the region to be improved (cracked portion) O. To consider.

  In the weld overlay method, the welded metal part (overlaying part) that has been welded and welded is positioned as a structural strength member, and overlaying is performed so that sufficient structural strength is compensated even if circumferential cracks that have occurred in the pipe penetrate the entire circumference. Ensure the thickness of the part. In overlay welding by the weld overlay method, generally, an overlay portion having a constant thickness in the axial direction is formed as shown in FIG. On the other hand, on the basis of the concept of the present invention, the pipe outer surface of the stress improving heat input region was given higher welding heat input than the region closer to the improved portion O (the region including the improved portion O). The shape of the built-up portion in this case is shown in FIG. In FIG. 9, the weld metal is laminated so as to cover the area to be improved (cracked area) O to the stress improvement heat input area, and the overlay 30 is formed by further superimposing the weld metal in the stress improvement heat input area. It is.

  FIG. 10 is a view showing the axial stress distribution on the inner surface of the pipe when the cross-sectional welding shown in FIGS. 7, 8, and 9 is applied to the outer surface of the pipe. In the overlay welding by the general weld overlay method shown in FIG. 8, the tensile stress on the inner surface of the pipe in the vicinity of the improved portion O increases, whereas the weld overlay method of FIG. 9 to which the concept of the present invention is applied. In overlay welding, the residual stress on the pipe inner surface in the vicinity of the portion to be improved O can be inclined to the compression side to improve the axial residual stress. Further, in the build-up welding shown in FIG. 7, the residual stress on the inner surface of the pipe in the vicinity of the improved portion O is more remarkably compressed.

  Next, an example suitable for regular inspection of the built-up location after construction will be described.

  For example, in the case where the welded portion or the stress improved heat input region is welded as described above and then periodically inspected by ultrasonic flaws for the presence of defects such as cracks in the portion covered with the cladding portion 30 of the pipe 20. The thickness of the built-up portion 30 is preferably uniform. However, in the overlay welding having the shape shown in FIG. 9, the weld thickness of the regions L5 to L7 is different from the weld thickness of the regions L1 to L4.

  In the example shown in FIG. 9, the build-up portion 30 in the stress-improving heat input region is thickly laminated, thereby strengthening the contraction force of the build-up portion 30 in the stress-improving heat input region as compared to the region including the improved portion. The residual stress on the inner surface of the improved area was improved. The same effect as this can be obtained by increasing the total heat input of the welding heat input to the stress improving heat input region as compared with the region including the improved portion while keeping the thickness of the built-up portion 30 constant. Can do. FIG. 11 shows an embodiment of the present invention in which the total heat input of welding heat input to the stress improving heat input region is made larger than that of the region including the improved portion O while keeping the thickness of the built-up portion 30 constant. It was shown to.

  In the example shown in FIG. 11, when the regions L5 to L7 (L1 to L7 are assumed to have the same scale as FIG. 5) are set as the stress improving heat input region 1 and overlay welding is performed on the regions L1 to L7, The welding pass is adjusted by adjusting the heat input so that the total heat input of the stress improving heat input region 1 at the raised portion 30 is larger than the total heat input of the region 2 (regions L1 to L4) including the improved portion O. They are stacked. In this case, since the welding heat input is determined by the welding current, welding voltage, and welding speed as described above, for example, in the stress improving heat input region 1, the feeding speed of the weld metal is adjusted to adjust the welding portion 30. While suppressing the fluctuation of the wall thickness, the welding speed is slowed or the welding current / welding plug pressure is increased with respect to the welding pass in the region 2. Thereby, the axial direction residual tensile stress of the inner surface of the pipe 20 can be suppressed while forming the built-up portion 30 having a shape suitable for ultrasonic flaw detection in which the variation in thickness is suppressed.

  The above can be applied when the material of the pipe 20 and the material of the built-up portion 30 (welded metal) are the same, but can also be applied when the pipe 20 and the built-up portion 30 are different materials. For example, austenitic stainless steel has a larger coefficient of linear expansion than carbon steel. Therefore, when the weld metal of austenitic stainless steel is welded on the outer surface of the pipe 20 when the pipe 20 is carbon steel, a larger shrinkage force is applied to the pipe 20 than when the weld metal of carbon steel is used. Can do. In other words, when the stress improvement method of the present invention is applied to carbon steel pipes and low alloy steel pipes, the use of austenitic stainless steel weld metal as a build-up welding material will improve the effect of compressing the axial stress on the pipe inner surface. high. Therefore, by selecting the weld metal or the pipe material according to the contraction force to be ensured, it is possible to obtain the effect of improving the stress more effectively.

  Based on the above knowledge, several examples of the stress improvement method according to the embodiment of the present invention will be described.

  FIG. 12 is a schematic diagram showing the construction of Example 1 of the stress improvement method according to the present invention.

  In FIG. 12, when reducing the axial tensile residual stress of the pipe inner surface of the butt weld portion 6 which is an improved portion of the existing pipe 5 in a plant or the like, it corresponds to, for example, the regions L4 to L7 previously shown in FIG. A build-up portion 3 having a thickness tw = 0.5 tp is laminated in the region, and a build-up weld having a thickness tw = 0.5 tp is superimposed on the region corresponding to the regions L5 to L7, so that a build-up with a thickness tw = tp is made. A raised portion 3 is formed. Since the pipe 5 is an existing pipe, the TIG torch 4 is rotated and moved relative to the pipe 5 so that the built-up portion 3 is stacked.

  When installing the welding apparatus on the pipe 5, first, the rail 7 for moving the TIG torch 4 is fitted on the outer peripheral surface of the pipe 5. The rail 7 has a half-crack structure, and it is not necessary to cut the existing pipe 5 at the time of installation. Next, the moving mechanism 8 that supports the TIG torch 4 is installed on the rail 7. The moving mechanism 8 has a wheel that clamps the rail 7 from both sides in the axial direction of the pipe 5, and travels in the circumferential direction along the rail 7 by driving the wheel with a motor (not shown). . The arm 11 that supports the TIG torch 4 is supported so as to be slidable in the circumferential direction of the pipe 10 with respect to the moving mechanism 8 and has a function of moving the TIG torch 4 in the axial direction. A cable 9 is connected to the TIG torch 4, and electric power from a welding power source (not shown) is applied to the TIG torch 4 via the cable 9, whereby an arc is generated from the TIG torch 4 toward the pipe 5. In this example, two such welding apparatuses are installed opposite to each other in the axial direction as shown in FIG. Although the piping 5 is arrange | positioned substantially horizontal, it is not limited to this.

  When overlay welding is performed using the welding apparatus of FIG. 12, welding conditions such as welding current, welding voltage, flow rate of inert gas from the TIG torch 4, and the supply speed of the weld metal are input to a control device (not shown). Set and controlled by its controller. The welding conditions set and input to the control device may be general TIG welding conditions. After the operation is started, after the arc is started and the supply of the wire (welded metal) is started, the moving mechanism 8 travels in the circumferential direction of the pipe 5 and the ring-shaped built-up portion 3 starts to be formed on the outer peripheral surface of the pipe 5. At this time, since the TIG torch 4 is rotated with respect to the existing pipe 5 in this example, when the TIG torch 4 is continuously rotated in the same direction, the cable 9 connected to the TIG torch 4 is wound around the pipe 5. End up. Therefore, when the moving mechanism 8 makes one round of the outer surface of the pipe 5 and returns to the start position, the arc is stopped, and the moving mechanism 8 travels in the direction opposite to that during welding and returns to the start position. Next, the arm 11 is driven to move the TIG torch 4 in the axial direction, and the subsequent pass is welded. The subsequent pass welding is the same as the first pass except that the axial position of the TIG torch 4 is different. In this way, the built-up portion 3 is formed in the stress improving heat input region.

  In this embodiment, two sets of welding devices are installed in the pipe 5 and overlay welding is simultaneously performed in two stress improving heat input regions sandwiching the butt weld 6. A method of sequentially forming the two built-up portions 3 with the weld 6 interposed therebetween may be used. However, the latter method has an advantage that only one welding apparatus is required, and the former method has an advantage that the time required for welding can be reduced to ½ compared to the latter method.

  FIG. 13 is a schematic diagram showing the construction of Example 2 of the stress improvement method according to the present invention.

  The pipe 5 for improving the stress in this embodiment is not a pipe already installed in a plant or the like, but a pipe obtained by butt welding and joining a plurality of pipes in a factory, and both ends thereof are free ends. The shape of the built-up portion 3 in this example may be the same as that of the first embodiment described with reference to FIG.

  In the case of the present embodiment, since the pipe 5 can be rotated, it is not necessary to provide a rail in the pipe 5 and circulate the TIG torch around the pipe 5 during overlay welding. Therefore, in this example, the pipe 5 is installed in the positioner (rotary chuck device) 10, the pipe 5 is rotated with respect to the TIG torch 4, and the built-up portion 3 is formed on the outer surface of the pipe 5. At this time, by arranging the TIG torch 4 downward at the top position of the pipe 5, that is, the position vertically above the center line of the pipe 5, the welding efficiency of the weld metal can be increased. When overlay welding is performed one place at a time, one TIG torch 4 is installed, the arc of the TIG torch 4 is started, and the supply of wire (welded metal) is started. Next, the positioner 10 is rotated, and the TIG torch 4 is continuously welded while being moved in the axial direction of the pipe 5. And welding is complete | finished when it becomes the predetermined build-up thickness. Since the pipe 5 rotates with respect to the TIG torch 4, the cable 9 of the TIG torch 4 does not wind around the pipe 5.

  When two TIG torches 4 can be installed as shown in FIG. 13, two TIG torches 4 are installed on both sides in the axial direction across the butt weld 6 which is an improved portion, and at the same time, two overlaid parts 3 is formed. When two TIG torches 4 are used, the time required for welding can be reduced to ½ compared to the case where one TIG torch 4 is used.

  Embodiment 3 of the stress improvement method according to the present invention will be described.

  In the present embodiment, for example, the overlaid portion 3 is not formed in FIG. 12 or FIG. 13, and welding heat is applied to the outer surface of the pipe 5 without supplying the weld metal to melt the outer surface of the pipe 5. . In this case, a contraction force is applied to the outer peripheral portion of the pipe 5 by welding heat input, and local deformation occurs such that the axial stress on the inner surface of the pipe 5 is compressed. Thereby, the axial tensile residual stress is suppressed, and further, compression stress is achieved. In this way, the pipe 5 can be contracted and deformed only by welding heat input without laminating the built-up portion 3. In this example, the welding apparatus shown in FIG. 12 or 13 can be used as it is.

  Embodiment 4 of the stress improvement method according to the present invention will be described.

  The present embodiment is an application example to an existing pipe in which a crack has occurred in the weld heat affected zone of butt welding. It is sufficient to use the welding apparatus shown in FIG.

  For example, in FIG. 12, when the pipe 5 is cracked (for convenience, it is assumed that the butt weld 6 is a cracked part), the build-up part 3 is formed at the pipe outer surface position covering the cracked part 6 of the pipe 5. In this case, as shown in FIG. 8, if the weld welding is uniformly performed only in the region corresponding to the regions L1 to L4 including the cracked portion 6, the tensile residual stress on the inner surface of the pipe at the cracked portion 6 increases. . On the other hand, as shown in FIG. 9, in addition to the regions corresponding to the regions L1 to L4, the stress-improving heat input region corresponding to the regions L5 to L7 has a thicker thickness than the regions L1 to L4. If the part 3 is formed, the tensile residual stress of the pipe inner surface of the crack location 6 can be suppressed. In this construction, first, a thick built-up portion 3 that can ensure the necessary structural strength even if cracks grow around the entire circumference of the pipe 5 is formed on the outer surface of the pipe in the regions L1 to L7. Repair 5 Next, the built-up portion 3 is superimposed on the portions of the regions L5 to L7 that become the stress improving heat input region, and the tensile residual stress on the inner surface of the cracked portion of the pipe is improved.

  When the present embodiment is constructed using the welding device of FIG. 12, first, the rail 7 and the moving mechanism 8 are sequentially installed on the pipe 5 in the same manner as in the embodiment of FIG. The torch 4 is circulated around the pipe 5 to form the built-up portion 3 by one pass. Welding is started from the zenith portion of the pipe 5, and in order to prevent the cable 9 from being wound, every time the built-up portion 3 is formed in one pass, the moving mechanism 8 is run in the reverse direction and returned to the welding start point. Later, the TIG torch 4 is moved in the axial direction to form the next welding pass. In this way, the build-up portion 3 of the regions L1 to L7 is formed, and when the thickness of the build-up portion 3 of the regions L1 to L7 reaches a predetermined thickness, the thickness of the stress improving heat input region of the regions L5 to L7 is next. Overlay the thickened portion 3 to make it thicker. If two welding apparatuses are used as shown in FIG. 12, the time required for construction can be shortened to ½ compared to the case where two welding apparatuses are used.

  Moreover, when constructing this embodiment or the previous three embodiments, welding heat input may be applied in a state where a cooling medium (cooling water or the like) is present in the pipe 5. In this case, it is conceivable to enclose a cooling medium in the pipe 5, or to circulate the cooling medium in the pipe 5. For example, when the target is the existing pipe 5 as in the first and fourth embodiments, the cooling water of the plant constituted by the pipe may be circulated through the pipe 5. When there is a cooling medium inside the pipe 5, the inner surface side of the pipe is cooled, and the temperature difference between the outer surface of the pipe (high temperature side) and the inner surface of the pipe (low temperature) increases. In this way, when the temperature difference between the inner surface and the outer surface of the pipe is large, the inner surface of the pipe yields on the tension side and tensile plastic strain occurs, and as a result, the residual stress on the inner surface of the pipe becomes compressive stress both in the circumferential direction and in the axial direction. . Note that the temperature difference between the inner surface and the outer surface of the pipe 5 is larger when the coolant is circulated than when the cooling medium is sealed inside the pipe 5.

  Embodiment 5 of the stress improvement method according to the present invention will be described.

  This embodiment is an example suitable for repairing a cracked portion of the pipe 5 and inspecting the presence or absence of subsequent cracking by the ultrasonic flaw detection method. The region including the improved portion of the build-up portion 3 and the stress improving heat input. This is an example in which the thicknesses of the regions are substantially the same. In this case, for example, as shown in FIG. 11, the axial tensile stress on the inner surface of the pipe is improved with respect to the total heat input to the regions L1 to L4 where the axial stress on the inner surface of the pipe 5 increases on the tension side. The total heat input to the areas L5 to L7 to be increased is increased. At that time, the supply speed of the weld metal is adjusted to suppress the variation in the build-up thickness in the regions L1 to L7. By increasing the total heat input to the regions L5 to L7 as compared with the other regions L1 to L4, it is possible to improve the axial tensile stress remaining on the inner surface of the pipe at the improved portion.

  Also in the present embodiment, as in the fourth embodiment, overlay welding is performed on the outer surface of the pipe covering the crack of the pipe 5. At this time, when overlay welding is performed only on the outer surface of the region 2 corresponding to the regions L1 to L4 in FIG. 11, the tensile stress on the inner surface of the improved portion increases, so that the stress improvement corresponding to the regions L5 to L7 is achieved. When high welding heat input is applied to the outer surface of the heat input region 1 and build-up welding is performed, the tensile residual stress on the inner surface of the pipe at the improved portion can be improved.

  For example, in the case of TIG welding, the number of passes required to weld the same amount of lamination varies with the same amount of heat input by adjusting the amount of wire (welded metal) supplied. When the heat input and the welding speed are the same, the number of welding passes required to weld the same stack height increases as the supply amount of the wire (welded metal) decreases. An increase in the number of welding passes means an increase in the total heat input for welding the same stack height. Therefore, even if the same build-up thickness is used, the tensile residual stress on the inner surface of the pipe to be improved can be improved by performing build-up welding in the stress improving heat input region 2 under the condition that the number of welding passes increases. it can.

  Further, this embodiment can also be constructed by a welding apparatus as shown in FIG. In that case, after forming the build-up portion 3 in the regions L1 to L4 in the same manner as in Example 4, the build-up portion of the regions L5 to L7 is formed under the condition that the number of welding passes per unit length is increased. To go. For example, in the stress improvement heat input region 2, the welding current, the welding voltage, and the welding speed are set to the same conditions as the overlay welding in the regions L1 to L4, and the supply amount of the wire (welded metal) per unit time is delayed ( For example, about 1/2). When the supply amount of the weld metal is halved, in the stress improving heat input region 2, the number of welding passes required for overlay welding to the same thickness as the regions L1 to L4 is doubled. In addition, the total heat input per unit weld width is doubled.

  In this example, as a method for confirming that the stress improving heat input region 2 is given higher welding heat input than the regions L1 to L4 and has a high shrinkage force, the regions L1 to L4 and the regions L5 to L7 ( There is a method of comparing the size of the dendrite structure in the stress improving heat input region 2). In the region where more heat is applied, the dendrite structure becomes wider accordingly. Therefore, it is possible to determine the magnitude of the contraction force from the interval between the dendrite structures.

  Embodiment 6 of the stress improvement method according to the present invention will be described.

  The present invention is an example in which the first and third embodiments, which can be applied to existing piping, are applied to the cooling water piping of a boiling water nuclear power plant. In this case, assuming that the pipe 5 in FIG. 12 or FIG. 13 is an existing cooling water pipe (recirculation pipe etc.) of the boiling water nuclear power plant, the outer surface of the pipe 5 is stopped after the plant is stopped by periodic inspection or the like. By performing overlay welding, it is possible to impart compressive residual stress to the inner surface of the pipe at the improved location and suppress stress corrosion cracking. Especially in this Embodiment, since the piping 5 is a cooling water piping, the inner surface of the piping 5 can be efficiently cooled with the circulating cooling water. In addition to the same effects as those of the other embodiments, there is a great merit that it is not necessary to set an additional device for cooling. Of course, since the recirculation pump installed in the plant can also be used for the circulation of the cooling water, there is a great merit in that the number of equipments required for construction is small and the setup time is short.

It is a model figure of piping pipe-welded on the outer surface. It is a figure which shows the result of having measured the stress distribution of the circumferential direction of a pipe inner surface, and an axial direction about four cases from which piping dimensions and welding conditions differ. It is the graph which made the horizontal axis of the graph of FIG. 2 dimensionless using the outer radius and thickness of piping. It is a model figure of piping. It is a model figure of the overlay welding of the piping outer surface. It is the factor effect figure which showed the relationship between overlay welding and a residual stress. It is a model figure of the optimum overlay welding for improving the axial tensile stress of the pipe inner surface. It is a model figure of general overlay welding used with a weld overlay method. It is a model figure of overlay welding by the weld overlay method constructed based on the concept of the present invention. It is a figure which shows the axial direction stress distribution of the pipe inner surface at the time of constructing the welding of the cross-sectional shape shown in FIG. 7, FIG. 8, FIG. 9 on the pipe outer surface. It is a model figure of the overlay welding by the other example of the weld overlay method constructed based on the concept of this invention. It is the schematic diagram showing the mode of construction of Example 1 of the stress improvement method which concerns on this invention. It is the schematic diagram showing the mode of construction of Example 2 of the stress improvement method which concerns on this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Stress improvement heat input area | region 2 Area | region including a to-be-improved part 4 TIG torch 5 Piping 6 To-be-improved part 7 Rail 8 Moving mechanism 9 Cable 20 Piping O To-be-improved part L1-4 To-be-improved part area L5-7 Thermal zone R Outer radius of pipe tp Thickness of pipe

Claims (21)

  A stress-improving heat input region is set by shifting the position in the axial direction of the pipe with respect to the portion to be improved that needs to improve the tensile residual stress on the inner surface of the pipe. A method for improving residual stress on an inner surface of a pipe, characterized by providing a higher welding heat input compared to a region including an improved portion. 2. The method for improving residual stress in an inner surface of a pipe according to claim 1, wherein when the outer radius of the pipe is R and the thickness of the pipe is tp, the stress improving heat input region is taken from the improved portion in the axial direction of the pipe. A method for improving residual stress on an inner surface of a pipe, wherein the distance is a region of 0.75 (Rtp) ^1/2 or more. 2. The method for improving residual stress in an inner surface of a pipe according to claim 1, wherein when the outer radius of the pipe is R and the thickness of the pipe is tp, the stress improving heat input region is taken from the improved portion in the axial direction of the pipe. A method for improving residual stress on an inner surface of a pipe, characterized in that the distance is an area of 2.7 (Rtp) ^1/2 or less.   2. The method for improving residual stress of an inner surface of a pipe according to claim 1, wherein welding heat input is applied to at least the outer surface of the pipe in the stress improving heat input region without supplying weld metal.   5. The method for improving residual stress on an inner surface of a pipe according to claim 4, wherein welding heat input is applied by TIG welding.   2. The method for improving residual stress in an inner surface of a pipe according to claim 1, wherein welding heat is applied while supplying the weld metal, and the weld metal is laminated at least on the outer surface of the pipe in the stress improving heat input region. Stress improvement method.   2. The method for improving residual stress on an inner surface of a pipe according to claim 1, wherein the part to be improved is a welded portion.   2. The method for improving residual stress in an inner surface of a pipe according to claim 1, wherein welding heat is applied only to the outer surface of the pipe in the stress-improving heat input region without applying welding heat to the region including the improved portion. A method for improving residual stress on the inner surface of pipes. 2. The method for improving residual stress in an inner surface of a pipe according to claim 1, wherein when the outer radius of the pipe is R, the thickness of the pipe is tp, and there is a cracked portion on the inner surface of the pipe, the outer surface of the pipe is covered so as to cover the cracked portion. A method for improving residual stress on an inner surface of a pipe, characterized by laminating a weld metal and further superimposing the weld metal on the stress improvement heat input region separated from the crack location by 0.75 (Rtp) ^1/2 or more in the axial direction. . 2. The method for improving residual stress in an inner surface of a pipe according to claim 1, wherein if the outer radius of the pipe is R, the thickness of the pipe is tp, and there is a cracked portion on the inner surface of the pipe, the outer surface of the pipe is covered so as to cover the cracked portion. When laminating the weld metal, the stress-improving heat input region that is more than 0.75 (Rtp) ^{1/2 in the} axial direction from the cracked portion of the weld metal is more than the region closer to the cracked portion than that. A method for improving residual stress on the inner surface of a pipe, characterized by increasing the amount of heat input.   2. A method for improving residual stress on an inner surface of a pipe according to claim 1, wherein welding heat is applied to the outer surface of the pipe in the stress improving heat input region in a state where there is a cooling medium in the pipe.   The outer surface of the stress-improving heat input region shifted in the axial direction with respect to the improved portion that needs to improve the tensile residual stress on the inner surface is given higher welding heat input than the region including the improved portion. Piping characterized by being. In the piping according to claim 12, when the outer radius is R and the thickness is tp, the stress improving heat input region has a distance taken in the axial direction from the improved portion of 0.75 (Rtp) ^1/2 or more. Piping characterized by being an area of In the pipe of claim 12, when the outer radius is R and the thickness is tp, the stress improving heat input region has a distance taken in the axial direction from the improved portion of 2.7 (Rtp) ^1/2 or less. Piping characterized by being an area of   The piping according to claim 12, wherein the improved portion is a welded portion.   The piping according to claim 12, wherein the improved portion is a cracked portion. In the pipe of claim 16, when the outer radius is R and the thickness is tp, weld metal is laminated on the outer surface so as to cover the cracked part, and further, 0.75 (Rtp) in the axial direction from the cracked part. A pipe characterized in that a weld metal is superimposed on the stress-improving heat input region separated by ^½ or more. The pipe according to claim 16, wherein when the outer radius is R and the thickness is tp, a weld metal is laminated on the outer surface so as to cover the cracked portion, and the weld metal has a thickness of 0. 0 in the axial direction from the cracked portion. 75 (Rtp) Piping characterized in that the total heat input is larger in the stress-improving heat input region separated by ^1/2 or more than in the region on the cracked part side.   19. The pipe according to claim 18, wherein a dendrite structure of the weld metal in the stress-improving heat input region is opened more than a region closer to the crack portion.   13. The piping according to claim 12, wherein the piping is a plant piping, and welding heat is applied to an outer surface of the piping in the stress-improving heat input region with a cooling medium inside.   21. The piping according to claim 20, which is a cooling water piping for a boiling water nuclear power plant.

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