JP2006152410A - Method and device for heat treatment of tube - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method and a device for heat treatment of a tube in which the residual stresses in a weld metal part and a weld heat-affected zone of an inner surface of an existing tube are converted into the compressive stresses, and the compressive stresses are generated in the inner surface of the existing pipe. <P>SOLUTION: In the method for heat treatment of the pipes to constitute a piping system, coolant is allowed to stay inside the pipe, and the outer surface of the pipe is heated at an arbitrary position. After generating the temperature distribution with less temperature difference in a tubular wall surface of the pipe at the heated portion, coolant is allowed to flow. By converting the residual stresses in the weld metal part and the weld heat-affected zone of the inner surface of the pipe into the compressive stresses, stress corrosion cracks generated from the weld metal part and the weld heat-affected zone can be suppressed. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for suppressing stress corrosion cracking by converting a tensile residual stress existing on the inner surface of a pipe constituting a piping system of a plant into a compressive residual stress.

  In the welds of stainless steel and nickel-base alloy steel, chrome carbide precipitates at the grain boundaries due to welding heat, resulting in the formation of a chrome-depleted layer in the immediate vicinity of the grain boundaries, making the chrome-depleted layer sensitized. (Phenomenon that becomes more sensitive to corrosion) occurs. On the other hand, generally high tensile residual stress is generated on the surface near the welded part of the pipes that make up the piping system of the plant. Therefore, stress corrosion cracking occurs when the material is sensitized and used in a severe corrosive environment. Wake up. That is, when the three factors of material sensitization, high tensile residual stress, and corrosive environment overlap, the risk of stress corrosion cracking increases.

  In order to suppress the occurrence of stress corrosion cracking, one of the countermeasures is to reduce the tensile residual stress in the area exposed to the corrosive environment. As a method of reducing the tensile residual stress on the inner surface of a welded part of an existing pipe in a plant, “Pipe system heat treatment method” described in Japanese Patent Publication No. 957324 (Patent Document 1) and Japanese Patent Application Laid-Open No. 55-110729 (Patent Publication). There is a “method for improving residual stress in steel pipes” described in literature 2). The former, after assembling the plant piping system, causes the temperature difference between the inner surface of the piping and the outer surface of the piping by heating the piping from the outer surface while flowing the coolant in the piping constituting the piping system. This is a method of reducing the tensile residual stress on the inner surface of the pipe by compressive yielding the outer surface and tensile yielding the inner surface. In the latter case, the pipe is heated in a state where there is no coolant in the pipe, the temperature distribution in the pipe wall of the pipe is made uniform, and then the coolant is supplied into the pipe so that the inner surface of the pipe and the pipe This is a method of reducing the tensile residual stress on the inner surface of the pipe by generating a temperature difference with the outer surface, tensile yielding the inner surface, and compressing yielding the outer surface.

Patent Publication No. 957324 Japanese Patent Laid-Open No. 55-110729

  In the prior art of Patent Document 1, the temperature difference formed in the pipe wall is gradual. The conventional technology of Patent Document 2 has a feature that it can be heated with a simple heating device and a steep temperature gradient can be obtained in the vicinity of the inner surface of the pipe. When applying to this piping system, it is necessary to put in and out the coolant in the entire piping system.

  An object of the present invention is to suppress the occurrence of stress corrosion cracks on the inner surface for a long period of time in a simple method in the pipe of an existing pipe welded joint.

  In order to achieve the above object, the present invention is characterized in that, in a heat treatment method for piping constituting a piping system of a plant, an arbitrary position on the outer surface of the piping is heated in a state where a coolant is stagnated in the piping. Thus, after forming a gentle temperature gradient in the tube wall, the coolant in the tube is flowed to form a steep temperature gradient in the vicinity of the inner surface of the pipe. Heat treatment method and apparatus. After the pipe is heated, a temperature difference is generated between the pipe inner surface and the outer surface of the pipe, and by rapidly cooling the inner surface of the pipe, the inner surface of the pipe is tensile yielded, the outer surface of the pipe is compressed and yielded, and the temperature difference between the inner and outer surfaces of the pipe is eliminated. When this occurs, the tensile residual stress on the inner surface of the pipe is reduced.

  Here, it is preferable that water is used as the coolant because it is simple. Further, when the piping system is a piping constituting a nuclear reactor, the coolant may be reactor water. Thus, it is possible to provide a heat treatment method that forms a steep temperature gradient in the vicinity of the inner surface of the pipe and converts the residual stress on the inner surface into a compressive stress in a state in which the reactor water serving as the coolant is stagnated in the pipe. Furthermore, any of induction heating and direct current heating may be used as the pipe heating means.

  The above-described heat treatment method for piping is to maintain a certain gap between the outer surface of the piping and the heating coil in the induction heating heating coil that is a means for heating the piping and the piping that generates compressive residual stress on the inner surface. The coolant is supplied to the spacer, the piping that is attached to the heating coil and allows the coolant to circulate inside, the transformer and power supply for supplying current to the heating coil, and the coolant circulation piping that is attached to the heating coil Use a heat treatment device composed of a coolant circulation mechanism and a circulation pump provided in the piping system, or supply current to the direct current heating terminal and the direct current heating terminal which are means for heating the piping. Heat treatment apparatus and piping system comprising: a transformer for power supply; and a coolant circulation mechanism for supplying coolant to a coolant circulation pipe attached to a terminal for direct current heating It can be achieved by using a circulation pump provided.

  According to the present invention, after the plant piping system is assembled, a compressive residual stress can be generated on the inner surface of the piping constituting the piping system, particularly in the vicinity of the weld metal part, thereby preventing stress corrosion cracking of the piping system. be able to.

  A first embodiment of the present invention will be described with reference to FIGS.

  FIG. 1 shows an embodiment when the present invention is applied to a boiling water nuclear power plant.

  The reactor pressure vessel 1 contains a core part 2 loaded with nuclear fuel, a steam separation dryer 3 and a core shroud 4 surrounding the core part 2. A plurality of control rods 5 are inserted into the core portion 2. The operation of the control rod 5 is performed by a control rod drive device 7 controlled by a control rod drive controller 6.

  The reactor pressure vessel 1 is filled with light water, which is cooling water, to some extent above the core 2. During operation of the reactor, the circulating pump 9 provided in the pipe 8 is driven, so that the cooling water in the reactor pressure vessel 1 is jetted through the recirculation system pipe 8 and the riser pipe (connected to the pipe 8) 10. It reaches into the pump 11. One end of the riser tube 10 is inserted into the reactor pressure vessel 1 and reaches the upper end of the jet pump 11. The valve 12 provided in the pipe 8 is open. The cooling water that has reached the inside of the jet pump 11 reaches the core 2 through the lower plenum 13. While rising up the core 2, the cooling water takes heat from the nuclear fuel and becomes steam.

  The steam passes through the steam separator / dryer 3, is then sent to the turbine 14, and is condensed in the condenser 15. The turbine 14 is driven, and the generator 16 connected thereto rotates. The water generated by being condensed in the condenser 15 is sent into the reactor pressure vessel 1 by the feed water pump 17.

  As described above, the pipe 8 and the riser pipe 10 constitute a recirculation pipe that is one pipe system of the boiling water nuclear power plant.

  Both end portions of the heating coil 18 wound around the pipe 8 are connected to a high frequency oscillator 19. A high-frequency heating device 20 that is a kind of induction heating device includes a heating coil 18 and a high-frequency oscillator 19.

  After the assembly of the recirculation system piping that connects the reactor pressure vessel 1 and the upper end of the jet pump 11 is completed, the circulation pump 9 is driven, and cooling water is allowed to flow into the recirculation system piping. Thereafter, the circulation pump 9 is stopped, and the cooling water is stagnated in the pipe 8. Next, the pipe 8 is heated from the outer surface of the pipe by the heating coil 18 of the high-frequency heating device 20. At this time, the temperature of the outer surface of the pipe 8 is adjusted so as to exceed the temperature at which the stress of the outer surface of the pipe 8 becomes equal to or higher than the compressive yield stress. On the other hand, the temperature of the inner surface of the pipe 8 rises to 100 ° C. or higher because the stagnant cooling water starts boiling. The temperature distribution in the pipe wall surface or the pipe inner surface temperature is estimated from the output value of the temperature measuring means (for example, thermocouple) (not shown) and the preset heating time.

  When it is determined that the wall surface temperature distribution of the pipe has a predetermined distribution or the pipe inner surface temperature has reached a predetermined temperature, the circulation pump 9 is driven, and the pipe 8 and the riser pipe 10 are placed in the reactor pressure vessel 1. Supply cooling water.

  In this way, heating is performed from the outer surface of the pipe 8 to create a state in which the temperature difference between the inner and outer surfaces of the pipe 8 is small, and then the inner surface of the pipe 8 is cooled to form a steep temperature gradient near the inner surface of the pipe 8. Then, the inner surface of the pipe 8 is tensile yielded.

  Thereafter, when the heating of the heating coil 18 is stopped and the temperature is lowered, compressive residual stress is generated on the inner surface of the pipe 8 and tensile residual stress is generated on the outer surface of the pipe.

  According to the present invention, after the recirculation system pipe is assembled, the pipe can be heat-treated, and a compressive residual stress can be generated on the inner surface of a necessary part of the recirculation system pipe.

  Moreover, since the cooling water in the reactor pressure vessel 1 can be supplied into the recirculation system pipe, it is not necessary to newly provide a device for supplying the cooling water into the recirculation system pipe.

  The embodiment shown in FIG. 2 generates compressive residual stress on the inner surface of the pipe in the vicinity of the welded portion 22 in the pipe welded to the pipe 21a and the pipe 21b.

  A heating coil 25 is spirally attached to the pipe 21 via a spacer 23 and a holder 24. An induced current generated by the power source 27 is supplied from the transformer 28 to the ends 26a and 26b of the heating coil 25 via the cables 29a and 29b.

  Further, the cooling water is supplied to the heating coil 25 from the end portion 26a and the end portion 26b of the heating coil 25 from the cooling water circulation pump 30 via the hoses 31a and 31b.

  A thermocouple 32 for measuring the temperature of the surface of the welded portion is attached to the weld metal portion on the outer surface of the pipe. The thermocouple 32 sends a voltage due to the thermoelectromotive force to the control device 34 via the cable 33.

  Note that cooling water is supplied into the pipe 21 from a supply system (not shown).

  Next, the heating coil 25 that is a component to be mounted on the pipe, the spacer 23 that supports the heating coil 25, and the holder 24 will be described in order.

  FIG. 3 shows a support state of the heating coil 25. The heating coil 25 uses a pipe-shaped copper tube. The heating coil 25 is supported by the holder 24.

FIG. 4 shows a cross section when virtually cut by a plane having the central axis of the pipe 21 as a normal line. The heating coil 25 is held by a holder 24 (not shown), and is installed in the pipe 1 via a spacer 23 that is an insulator.

  FIG. 5 shows a state where the heating coil 25 is separated for installation in the pipe. The heating coil 25 can be separated into a semicircular arc shape, and an electrical connection and a coolant flow path can be connected by upper and lower holders 24 (not shown) serving as connection portions. Therefore, at the time of installation, it is possible to appropriately take a gap between the pipe surface and the heating coil only by attaching the spacer to the pipe surface.

FIG. 6 shows a cross-sectional structure of the upper and lower holders 24 serving as connection portions. The end of the heating coil 25 is connected to the holder 24. At the upper part of the holder 24, an entrance / exit 35 serving as a coolant flow path is provided. By connecting the entrance / exit 35 with a heat-resistant hose or the like, a coolant circuit can be configured.

  Next, the procedure for heat treatment using this apparatus will be described. In FIG. 2, after preparing the apparatus in the state shown in the figure, the cooling water is filled in the pipes 21a and 21b joined by the welded portion 22 by a circulation pump (not shown). The control device 34 starts the cooling water circulation pump 30 and starts supplying cooling water to the heating coil 25. Next, a current is passed through the heating coil 25. Due to the current flowing through the heating coil 25, an induction current is induced in the pipe, and the pipe generates heat. The temperature of the outer surface of the pipe is measured by the thermocouple 32 and sent to the control device 34. The controller 34 estimates the temperature distribution on the pipe wall surface of the pipe or the pipe inner surface temperature from the output of the thermocouple 32 and a preset heating time.

  When it is determined that the temperature distribution on the pipe wall surface of the pipe or the pipe inner surface temperature has reached a predetermined value, the control device 34 drives a circulation pump (not shown) to flow the cooling water in the pipe. After the predetermined temperature gradient is obtained, the current supply to the heating coil 25 is stopped.

  In the embodiment shown in FIG. 7, the pipe heating means of the embodiment shown in FIG.

  Ring-shaped terminals 36 a and 36 b for direct current heating are attached to the pipe 21. Current generated by the power source 38 is supplied from the transformer 39 to the end portions 37a and 37b of the ring-shaped terminals via the cables 29a and 29b.

  Furthermore, the cooling water is supplied to the ring-shaped terminals 36a and 36b from the end portions 37a and 37b of the ring-shaped terminals via the hoses 31a and 31b from the cooling water circulation pump 30.

  A thermocouple 32 for measuring the temperature of the surface of the welded portion is attached to the weld metal portion on the outer surface of the pipe. The thermocouple 32 sends a voltage due to the thermoelectromotive force to the control device 34 via the cable 33.

  Note that cooling water is supplied into the pipe 21 from a supply system (not shown).

  Next, the procedure for heat treatment using this apparatus will be described. In FIG. 7, after preparing the apparatus in the state shown in the figure, the inside of the pipe 21 a and the pipe 21 b joined by the weld portion 22 is filled with cooling water. The control device 34 starts the cooling water circulation pump 30 and starts supplying the cooling water to the ring terminals 36a and 36b. Next, a current is passed through the ring terminals 36a and 36b. The piping 21 sandwiched between the ring-shaped terminals 36a and 36b is energized and heated, and the piping generates heat. The temperature of the pipe surface is measured by the thermocouple 32 and sent to the control device 34. The controller 34 estimates the temperature distribution on the pipe wall surface of the pipe or the pipe inner surface temperature from the output of the thermocouple 32 and a preset heating time.

  When it is determined that the temperature distribution on the pipe wall surface of the pipe or the pipe inner surface temperature has reached a predetermined value, the control device 34 drives a circulation pump (not shown) to flow the cooling water in the pipe. After a predetermined temperature gradient is obtained, current supply to the ring-shaped terminals 36a and 36b is stopped.

  The reason why the compressive residual stress is generated on the inner surface of the pipe in the embodiment described above will be described below.

  When the piping is heated without flowing the cooling water in the piping, the cooling water stagnating in the piping starts to boil, so the temperature of the inner surface of the piping rises to 100 ° C or more, which is the boiling temperature of the cooling water, The temperature distribution in the wall tends to be a curve A shown in FIG. To is the temperature of the outer surface of the pipe. When the cooling water in the pipe is flowed after forming the temperature distribution as shown by the curve A, the inner surface of the pipe is rapidly cooled and approaches the cooling water temperature, and the temperature distribution in the pipe wall is the curve B shown in FIG. become that way. Moreover, when the temperature change of the pipe inner and outer surface over time is schematically shown, it is as shown in FIG. In order to compare an example of the temperature distribution in the tube wall by the conventional method of heating the outer surface while cooling the inner surface, the curve C in FIG. 8 and the relationship between temperature and time are shown by a two-dot chain line in FIG. is there.

  Based on the temperature gradient according to the present invention shown by the curve B, as shown in FIG. 10, a large tensile stress σi is generated on the inner surface of the pipe and a compressive stress σo is generated on the outer surface of the pipe, so that the tensile stress σi exceeds the yield strength of the material. For example, the outer surface of the pipe will yield tensile, and when cooled to room temperature, compressive stress σri remains on the inner surface of the pipe and tensile stress σro remains on the outer surface of the pipe as shown in FIG.

  FIG. 12 shows the relationship between stress and strain. If the temperature difference between the inner surface of the pipe and the outer surface of the pipe is small, the yield stress does not exceed the inner surface of the pipe and the outer surface of the pipe. In FIG. If the heating is stopped, both the inner and outer surfaces of the pipe return to 0, and the residual stress does not change. On the other hand, if the temperature difference between the pipe inner surface and the pipe outer surface is sufficiently large, the stress distribution in the radial direction of the pipe is as shown in FIG. 10, and the tensile yield stress σy on the inner surface of the pipe and the compressive yield stress on the outer surface of the pipe − It exceeds σy. The stress in the pipe wall surface at that time is set to B1 and B2 in FIG. When heating is stopped at this point, the process of B1 and E1 is followed on the inner surface of the pipe, and if the entire pipe is lowered to the ambient temperature, the point C1 is reached. On the other hand, the outer surface of the pipe follows the process of B2 and E2 and reaches point C2. For this reason, compressive residual stress is generated by applying positive strain to the inner surface of the pipe, and tensile residual stress is generated by applying negative strain to the outer surface of the pipe.

  The above-described stress-strain relationship in FIG. 12 is when there is no residual stress, but the same concept holds true when there is tensile residual stress on the inner and outer surfaces, depending on the temperature difference between the inner surface of the pipe and the outer surface of the pipe. It may be considered that the generated stress is superimposed on the original tensile residual stress. Therefore, in order to yield the inner and outer surfaces of the pipe, the temperature difference may be small as compared with the case where there is no initial residual stress.

It is explanatory drawing at the time of applying the heat processing method of piping by this invention to a boiling water nuclear power plant. It is explanatory drawing which shows the 1st Example of the heat processing method of piping by this invention. It is explanatory drawing which shows the attachment structure of a heating coil. It is explanatory drawing which shows the example which attached the heating apparatus of the 1st Example by this invention to piping. It is explanatory drawing which shows the state removed from piping by dividing into 2 the heating apparatus of 1st Example by this invention. It is explanatory drawing which shows the cross-section of a holder. It is explanatory drawing which shows the 2nd Example of the heat processing method of piping by this invention. It is explanatory drawing which showed the temperature gradient which arises in a pipe wall graphically. It is explanatory drawing which showed the relationship between temperature and time schematically. It is explanatory drawing which shows the stress distribution in a pipe wall when the flow of a coolant is started and a temperature difference arises in the pipe inner and outer surfaces. It is explanatory drawing which shows the stress distribution in the pipe wall after performing the heat processing of this invention. It is explanatory drawing which shows stress-strain relationship.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Reactor pressure vessel, 2 ... Core part, 3 ... Steam separation dryer, 4 ... Core shroud, 5 ... Control rod, 6 ... Control rod drive controller, 7 ... Control rod drive device, 8, 21 ... Piping, DESCRIPTION OF SYMBOLS 9 ... Circulation pump, 10 ... Riser pipe, 11 ... Jet pump, 12 ... Valve, 13 ... Lower plenum, 14 ... Turbine, 15 ... Condenser, 16 ... Generator, 17 ... Feed water pump, 18 ... Heating coil, 19 DESCRIPTION OF SYMBOLS ... High frequency oscillator, 20 ... High frequency heating device, 22 ... Welding part, 23 ... Spacer, 24 ... Holder, 25 ... Heating coil, 26 ... Heating coil end, 27, 38 ... Power supply, 28, 39 ... Transformer, 29, 33 ... Cable, 30 ... Cooling water circulation pump, 31 ... Hose, 32 ... Thermocouple,
34 ... Control device, 35 ... Coolant inlet / outlet, 36 ... Ring-shaped terminal for direct current heating, 37 ... End of ring-shaped terminal, A ... Temperature distribution before coolant flow, B ... Temperature distribution after coolant flow C: Temperature distribution according to the conventional method.

Claims (7)

In the heat treatment method for the pipes constituting the plant,
A heat treatment method for a pipe, characterized in that a coolant is retained in the pipe, the pipe is heated in a state where the coolant is stopped, and the coolant is flowed after the heating.   The pipe heat treatment method according to claim 1, wherein water is used as the coolant. In the heat processing method of piping described in Claim 1,
The heat treatment method for piping, wherein an inner surface of the piping is 100 ° C. or higher.   2. The heat treatment method for piping according to claim 1, wherein the piping is a piping system constituting a nuclear reactor, and the coolant is reactor water.   2. The pipe heat treatment method according to claim 1, wherein the pipe heating means is induction heating or direct current heating. A welding repair method for piping constituting a plant,
A step of welding the pipe, a step of stagnating water in the pipe of at least the weld repaired portion, a step of heating the weld repaired portion, and a step of circulating the water in the pipe that has continued the heating; And a welding repair method for piping. A heat treatment device for piping having a circulation device and containing Cr,
It has a heating coil for induction heating, a spacer for maintaining a certain gap between the outer surface of the pipe and the heating coil, and a transformer and a power source for supplying current to the heating coil. Heat treatment equipment for piping.

Images