Abstract:
In a method for heat treatment of an existing pipe constituting a piping system, for converting the residual stress at a welded metallic portion and a welding heat influenced portion of the inside surface of the pipe, into a compressive stress and thereby generate the compressive stress in the inside surface of the existing pipe, a coolant is retained in the pipe; an arbitrary portion of the outside surface of the pipe is heated; thereby a temperature distribution little in temperature difference is produced in the wall surface of the pipe at the heated portion; and then the coolant is allowed to flow. By converting the residual stress at the welded metallic portion and the welding heat influenced portion of the inside surface of the pipe, into the compressive stress, stress corrosion cracking generated from the welded metallic portion and the welding heat influenced portion can be suppressed.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a method for converting a tensile residual stress existing in an inside surface of a pipe constituting a piping system of a plant, into a compressive residual stress to suppress stress corrosion cracking.  
         [0003]     2. Description of the Related Art  
         [0004]     In a welded portion of stainless steel or nickel-base alloy steel, chromium carbide is separated out at a crystal grain boundary due to heat of welding. As a result, a chromium-lack layer is produced near the pole of the grain boundary, and sensitization (a phenomenon that the sensitivity to corrosion increases) occurs in the chromium-lack layer. On the other hand, a high tensile residual stress is generally generated in the surface near a welded portion of a pipe constituting a piping system of a plant. Therefore, if the pipe is used in a severely corrosive environment in a state wherein the material has been sensitized, stress corrosion cracking occurs. That is, when three factors of sensitization of the material, a high tensile residual stress, and a corrosive environment, are superposed, the risk of stress corrosion cracking increases.  
         [0005]     To suppress the generation of stress corrosion cracking, reduction of the tensile residual stress in the region exposed to the corrosive environment is cited as one of measures. As methods for reducing the tensile residual stress of the inside surface of a welded portion of an existing pipe of a plant, there are “Method for Heat Treatment of Piping System” described in JP-B-957324 and “Method for Improving Residual Stress in Steel Pipe” described in JP-A-55-110729. The former is a method in which a piping system of a plant is assembled; a coolant is then allowed to flow in a pipe constituting the piping system; in this state, the pipe is heated through its outside surface to make a difference in temperature between the inside surface of the pipe and the outside surface of the pipe; the outside surface is compressive-yielded and the inside surface is tensile-yielded; and thereby the tensile residual stress of the inside surface of the pipe is reduced. The latter is a method in which a pipe is heated in a state wherein no coolant exists in the pipe, to uniformalize the temperature distribution in the wall of the pipe; a coolant is then supplied into the pipe to make a difference in temperature between the inside surface of the pipe and the outside surface of the pipe; the inside surface is tensile-yielded and the outside surface is compressive-yielded; and thereby the tensile residual stress of the inside surface of the pipe is reduced.  
         [0006]     In JP-B-957324 as described above, the difference in temperature generated in the wall of the pipe is gentle. JP-A-55-110729 has a characteristic feature that heating can be performed with a simple heater and a steep gradient of temperature can be obtained near the inside surface of the pipe. However, because the pipe is heated without cooling the inside surface of the pipe, it is required to take in/out a coolant on the whole of a piping system when applied to the piping system of an existing plant.  
       SUMMARY OF THE INVENTION  
       [0007]     An object of the present invention is to suppress the generation of stress corrosion cracking of an inside surface of a pipe of an existing pipe welded joint for a long time by a simple method.  
         [0008]     According to the present invention to attain the above object, a method for heat treatment of a pipe constituting a piping system of a plant, have a characteristic feature that an arbitrary portion of the outside surface of the pipe is heated in a state wherein a coolant is retained in the pipe, so as to make a gentle temperature gradient in the pipe wall; and then, by allowing the coolant to flow in the pipe, the temperature gradient in the wall of the pipe at the heated portion is changed into a steep temperature gradient near the inside surface of the pipe. After the pipe is heated, a difference in temperature is made in particular between the inside and outside surfaces of the pipe. By rapidly cooling the inside surface of the pipe, the inside surface of the pipe is tensile-yielded and the outside surface of the pipe is compressive-yielded. When the temperatures of the inside and outside surfaces of the pipe are equaled to each other, the tensile residual stress in the inside surface of the pipe is reduced.  
         [0009]     Here, the coolant is preferably water because it is easy to use. In the case that the pipe constitutes a piping system of a reactor, the coolant may be reactor water. Thereby, a method for heat treatment can be provided in which a steep temperature gradient is made near the inside surface of the pipe in a state wherein the reactor water as the coolant is retained in the pipe, and thereby the residual stress in the inside surface is converted into a compressive stress. Further, the pipe may be heated by either induction heating or direct electrical heating.  
         [0010]     The above-described method for heat treatment of piping can be realized by using an apparatus for heat treatment comprising a heating coil for induction heating as means for heating a pipe, a spacer for keeping the heating coil at a constant distance from the outside surface of the pipe in which a compressive residual stress is to be generated in the inside surface, the pipe attached to the heating coil and allowing the coolant to be circulated in the pipe, a transformer and a power supply for applying an electric current to the heating coil, and a coolant circulating mechanism for supplying the coolant in the pipe for coolant circulation attached to the heating coil; and a circulating pump provided in the piping system. Alternatively, the method can be realized by using an apparatus for heat treatment comprising a direct electrical heating terminal as means for heating the pipe, a transformer and a power supply for applying an electric current to the direct electrical heating terminal, and a coolant circulating mechanism for supplying the coolant in the pipe for coolant circulation attached to the direct electrical heating terminal; and a circulating pump provided in the piping system.  
         [0011]     According to the present invention, after a piping system of a plant is assembled, a compressive residual stress can be generated in the inside surface of a pipe constituting the piping system, in particular, near a welded metallic portion. As a result, stress corrosion cracking in the piping system can be prevented.  
         [0012]     Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  is an explanatory diagram of a case wherein a method for heat treatment of piping according to the present invention is applied to a nuclear power plant with a boiling-water reactor;  
         [0014]      FIG. 2  is an explanatory view showing a first embodiment of a method for heat treatment of piping according to the present invention;  
         [0015]      FIG. 3  is an explanatory view showing an attachment structure of a heating coil;  
         [0016]      FIG. 4  is an explanatory view showing an example in which a heater of the first embodiment according to the present invention is attached to a pipe;  
         [0017]      FIG. 5  is an explanatory view showing a state wherein the heater of the first embodiment according to the present invention is detached from the pipe by being divided into two parts;  
         [0018]      FIG. 6  is an explanatory view showing a sectional structure of a holder;  
         [0019]      FIG. 7  is an explanatory view showing a second embodiment of a method for heat treatment of piping according to the present invention;  
         [0020]      FIG. 8  is an explanatory graph graphically showing a temperature gradient produced in a pipe wall;  
         [0021]      FIG. 9  is an explanatory graph graphically showing a relation between temperature and time;  
         [0022]      FIG. 10  is an explanatory representation showing a stress distribution in a pipe wall when a coolant begins to flow and a difference in temperature is produced between inside and outside surfaces of the pipe;  
         [0023]      FIG. 11  is an explanatory representation showing a stress distribution in the pipe wall after heat treatment of the present invention is applied; and  
         [0024]      FIG. 12  is an explanatory graph showing a relation of stress-strain. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]     A first embodiment of the present invention will be described with reference to FIGS.  1  to  7 .  
         [0026]      FIG. 1  shows the first embodiment in which the present invention is applied to a nuclear power plant with a boiling-water reactor.  
         [0027]     A reactor pressure vessel  1  has therein an in core portion  2  to be charged with nuclear fuel; a steam water separator  3 ; and a core shroud  4  surrounding the in core portion  2 . A plurality of control rods  5  are inserted in the in core portion  2 . The control rods  5  are operated by a control rod driving system  7  under the control of a control rod drive controller  6 .  
         [0028]     The interior of the reactor pressure vessel  1  is filled up with light water as cooling water to a somewhat upper portion of the in core portion  2 . When the reactor is in operation, by driving a circulating pump  9  provided for a pipe  8 , the cooling water in the reactor pressure vessel  1  passes through the pipe  8  and a riser tube  10  (connected to the pipe  8 ) of a primary loop recirculation piping, and reaches the interior of a jet pump  11 . One end of the riser tube  10  is inserted in the reactor pressure vessel  1  to the upper end of the jet pump  11 . A valve  12  provided in the pipe  8  is opened. The cooling water reaching the interior of the jet pump  11  passes through a lower plenum  13  to the in core portion  2 . While moving upward in the in core portion  2 , the cooling water draws heat from nuclear fuel and changes into steam.  
         [0029]     The steam passes the steam water separator  3 , and then it is sent to a turbine  14  and condensed in a condenser  15 . The turbine  14  is driven, and a generator  16  connected to the turbine  14  is rotated. Water generated by condensation in the condenser  15  is sent to the interior of the reactor pressure vessel  1  by a feed water pump  17 .  
         [0030]     As described above, the pipe  8  and the riser tube  10  constitute a primary loop recirculation piping as one piping system of the nuclear power plant with a boiling-water reactor.  
         [0031]     Both ends of a heating coil  18  wound on the pipe  8  are connected to a high-frequency oscillator  19 . A high-frequency heating apparatus  20  as one kind of induction heating apparatus is made up of the heating coil  18 , the high-frequency oscillator  19 , and so on.  
         [0032]     After the assembly of the primary loop recirculation piping connecting the reactor pressure vessel  1  and the upper end of the jet pump  11  is completed, the circulating pump  9  is driven, so that the cooling water flows in the recirculation system pipe. Afterward, the circulating pump  9  is stopped, so that the cooling water is allowed to stagnate in the pipe  8 . Next, the pipe  8  is heated through the outside surface of the pipe by the heating coil  18  of the high-frequency heating apparatus  20 . At this time, the temperature of the outside surface of the pipe  8  is controlled so as to exceed a temperature at which the stress of the outside surface of the pipe  8  is not less than the compressive yield stress. On the other hand, the temperature of the inside surface of the pipe  8  rises to 100° C. or more because the stagnating cooling water begins to boil. The temperature distribution in the wall surface of the pipe or the temperature of the inside surface of the pipe is estimated from an output value of not-shown temperature measuring means (e.g., a thermocouple or the like) for the outside surface of the pipe, and a heating time set in advance.  
         [0033]     When the temperature distribution in the wall surface of the pipe is judged to become a predetermined distribution or the temperature of the inside surface of the pipe is judged to become a predetermined temperature, the circulating pump  9  is driven, and the cooling water in the reactor pressure vessel  1  is supplied into the pipe  8  and the riser tube  10 .  
         [0034]     As described above, the pipe  8  is heated through its outside surface so as to make a state wherein the difference in temperature between the inside and outside surfaces of the pipe  8  is little. Next, by cooling the inside surface of the pipe  8 , a steep temperature gradient is formed near the inside surface of the pipe, and thereby the inside surface of the pipe  8  is tensile-yielded.  
         [0035]     Afterward, heating by the heating coil  18  is stopped. When the temperature lowers, a compressive residual stress is generated in the inside surface of the pipe  8  and a tensile residual stress is generated in the outside surface of the pipe.  
         [0036]     According to the present invention, after the primary lop recirculation piping is assembled, heat treatment for the pipe can be performed and a compressive residual stress can be generated in the inside surface of a necessary portion of the primary loop recirculation piping.  
         [0037]     In addition, because the cooling water in the reactor pressure vessel  1  can be supplied into the primary loop recirculation piping, an apparatus for supplying the cooling water into the primary loop recirculation piping need not be newly provided.  
         [0038]     The embodiment shown in  FIG. 2  is to generate a compressive residual stress in the inside surface near a welded portion  22  of a pipe in which a pipe  21   a  and a pipe  21   b  are welded to each other.  
         [0039]     A heating coil  25  is spirally attached to the pipe  21  with spacers  23  and holders  24 . An induction current generated in a power source  27  is supplied from a transformer  28  through cables  29   a  and  29   b  to end portions  26   a  and  26   b  of the heating coil  25 .  
         [0040]     Further, the heating coil  25  has a structure to be supplied with the cooling water from the end portions  26   a  and  26   b  of the heating coil  25  through hoses  31   a  and  31   b  from a cooling water circulating pump  30 .  
         [0041]     A thermocouple  32  for measuring the temperature of the surface of the welded portion is attached to the welded metallic portion of the outside surface of the pipe. The thermocouple  32  sends a voltage caused by thermoelectromotive force to a controller  34  through a cable  33 .  
         [0042]     The cooling water is supplied into the pipe  21  by a not-shown feed system.  
         [0043]     Next, the heating coil  25  as a component to be attached to the pipe, and a spacer  23  and a holder  24  for supporting the heating coil  25 , will be described in order.  
         [0044]      FIG. 3  shows a supporting condition of the heating coil  25 . As the heating coil  25  used is a copper pipe. The heating coil  25  is supported by the holder  24 .  
         [0045]      FIG. 4  shows a sectional view when hypothetically cut along a plane a normal line of which is coincide with the central axis of the pipe  21 . The heating coil  25  is held by a not-shown holder  24  and set around the pipe  1  with a spacer  23  made of an insulator.  
         [0046]      FIG. 5  shows a state wherein the heating coil  25  is divided to be set around the pipe. The heating coil  25  can be divided into semicircular parts, and electrical connection and connection of flow passages for a coolant can be made by not-shown upper and lower holders  24  as connecting portions. Therefore, when setting, a proper space can be made between the pipe surface and the heating coil only by attaching the spacers in accordance with the pipe surface.  
         [0047]      FIG. 6  shows a sectional structure of an upper or lower holder  24  as a connecting portion. An end of the heating coil  25  is connected to the holder  24 . A port  35  as a flow passage for the coolant is provided in an upper portion of the holder  24 . By connecting a heat-resistant hose or the like to the port  35 , a circuit for the coolant can be constructed.  
         [0048]     Next, a procedure of a heat treatment operation using this apparatus will be described. In  FIG. 2 , the apparatus is provided in the shown state, and then the interiors of the pipes  21   a  and  21   b  connected at the welded portion  22  are filled up with the cooling water by a not-shown circulating pump. The controller  34  starts the cooling water circulating pump  30  to start supplying the cooling water to the heating coil  25 . Next, the current is applied to the heating coil  25 . An induction current is induced in the pipe by the current which flows in the heating coil  25 , and thereby heat generation of the pipe occurs. The temperature of the outside surface of the pipe is measured by the thermocouple  32 , and the output of the thermocouple  32  is sent to the controller  34 . The controller  34  estimates the temperature distribution in the wall surface of the pipe or the temperature of the inside surface of the pipe from the output of the thermocouple  32  and a heating time set in advance.  
         [0049]     When the temperature distribution in the wall surface of the pipe or the temperature of the inside surface of the pipe is judged to have become a predetermined value, the controller  34  drives a not-shown circulating pump, so that the cooling water is allowed to flow in the pipe. After a predetermined temperature gradient is obtained, the current supply to the heating coil  25  is stopped.  
         [0050]     In the embodiment shown in  FIG. 7 , the pipe heating means of the embodiment shown in  FIG. 2  is changed to direct electrical heating.  
         [0051]     Ring terminals  36   a  and  36   b  for direct electrical heating are attached to the pipe  21 . The current generated in a power source  38  is supplied from a transformer  39  through cables  29   a  and  29   b  to end portions  37   a  and  37   b  of the ring terminals.  
         [0052]     Further, the ring terminals  36   a  and  36   b  have structures to be supplied with the cooling water from the end portions  37   a  and  37   b  of the ring terminals through hoses  31   a  and  31   b  from a cooling water circulating pump  30 .  
         [0053]     A thermocouple  32  for measuring the temperature of the surface of the welded portion is attached to the welded metallic portion of the outside surface of the pipe. The thermocouple  32  sends a voltage caused by thermoelectromotive force to a controller  34  through a cable  33 .  
         [0054]     The cooling water is supplied into the pipe  21  by a not-shown feed system.  
         [0055]     Next, a procedure of a heat treatment operation using this apparatus will be described. In  FIG. 7 , the apparatus is provided in the shown state, and then the interiors of the pipes  21   a  and  21   b  connected at the welded portion  22  are filled up with the cooling water. The controller  34  starts the cooling water circulating pump  30  to start supplying the cooling water to the ring terminals  36   a  and  36   b . Next, the current is applied to the ring terminals  36   a  and  36   b . The pipe  21  sandwiched by the ring terminals  36   a  and  36   b  is electrically heated, and thereby heat generation of the pipe occurs. The temperature of the surface of the pipe is measured by the thermocouple  32 , and the output of the thermocouple  32  is sent to the controller  34 . The controller  34  estimates the temperature distribution in the wall surface of the pipe or the temperature of the inside surface of the pipe from the output of the thermocouple  32  and a heating time set in advance.  
         [0056]     When the temperature distribution in the wall surface of the pipe or the temperature of the inside surface of the pipe is judged to become a predetermined value, the controller  34  drives a not-shown circulating pump, so that the cooling water flows in the pipe. After a predetermined temperature gradient is obtained, the current supply to the ring terminals  36   a  and  36   b  is stopped.  
         [0057]     Here, the reason why a compressive residual stress is generated in the inside surface of the pipe in the above-described embodiments will be described below.  
         [0058]     When the pipe is heated in a state wherein the cooling water in the pipe does not flow, the cooling water which stagnates in the pipe begins to boil. Thus, the temperature of the inside surface of the pipe rises to 100° C., which is the boiling temperature of the cooling water, or more. As a result, the temperature distribution in the wall surface of the pipe has a tendency as shown by a curve A in  FIG. 8 . To represents the temperature of the outside surface of the pipe. When the cooling water in the pipe flows after the temperature distribution as shown by the curve A is formed, the inside surface of the pipe is rapidly cooled and the temperature of the inside surface gets near to the temperature of the cooling water. As a result, the temperature distribution in the wall surface of the pipe becomes a state as shown by a curve B in  FIG. 8 . When changes in temperature of the inside and outside surfaces of the pipe as the time elapses are graphically shown, it is as shown in  FIG. 9 . For comparison, an example of the temperature distribution in the wall surface of the pipe by a conventional method in which the outside surface is heated while the inside surface is cooled, is shown by a curve C in  FIG. 8 , and the relation between temperature and time is shown by a chain double-dashed line in  FIG. 9 .  
         [0059]     On the basis of the temperature gradient shown by the curve B according to the present invention, as shown in  FIG. 10 , a large tensile stress σi is generated in the inside surface of the pipe and a compressive stress σo is generated in the outside surface of the pipe. When the tensile stress σi exceeds the yield strength of the material, the outside surface of the pipe is tensile-yielded. When cooling to the normal temperature, as shown in  FIG. 11 , a compressive stress σri remains in the inside surface of the pipe and a tensile stress σro remains in the outside surface of the pipe.  
         [0060]      FIG. 12  shows a relation between stress and strain. When the difference in temperature between the inside and outside surfaces of the pipe is little, either of the inside and outside surfaces of the pipe does not exceed its yield stress. Thus, in  FIG. 12 , the stress-strain relation exists on the straight line of 0D2 in the outside surface of the pipe and on the straight line of 0D1 in the inside surface of the pipe. When heating is stopped, either of stress in the inside and outside surfaces of the pipe returns to zero, and the residual stress does not change. On the other hand, the difference in temperature between the inside and outside surfaces of the pipe is sufficiently large, the radial stress distribution of the pipe is as shown in  FIG. 10 , and the stress in the inside surface of the pipe exceeds the tensile-side yield stress σy and that in the outside surface of the pipe exceeds the compressive-side yield stress −σy. The stresses in the wall surface of the pipe at that time are shown by B 1  and B 2  in  FIG. 12 , respectively. When heating stops at this time, the stress in the inside surface of the pipe is changed in the course of B 1  to E 1 . When the temperature of the whole of the pipe lowers to the temperature of the atmosphere, the stress in the inside surface reaches the point C 1 . On the other hand, the stress in the outside surface of the pipe changed in the course of B 2  to E 2  and reaches the point C 2 . Therefore, a compressive residual stress is generated in the inside surface of the pipe because a positive strain is given, while a tensile residual stress is generated in the outside surface of the pipe because a negative strain is given.  
         [0061]     The above-described stress-strain relation of  FIG. 12  is in the case that no residual stress exists. However, even in the case that a tensile residual stress exists in the inside or outside surface, a similar method of thinking can be used and it may be thought that a stress generated due to the difference in temperature between the inside and outside surfaces of the pipe is superposed on the original tensile residual stress. Therefore, to yield the inside and outside surfaces of the pipe, a less difference in temperature suffices in comparison with a case wherein no initial residual stress exists.  
         [0062]     It should be further understood by those skilled in the art that although the foregoing description has been made on embodiments of the invention, the invention is not limited thereto and various changes and modifications may be made without departing from the spirit of the invention and the scope of the appended claims.