JP2005213618A - Method and device for modifying surface of structure - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method and a device for modifying a surface of a structure capable of efficiently and reliably preventing occurrence of damages by modifying the surface by material improvement of an execution part through a solution treatment and by compressive stress through stress improvement. <P>SOLUTION: The method for modifying the surface of the structure comprises a solution treatment step of heating the surface of an execution part of the structure at a temperature in the range of ≥ 900°C and ≤ 1,100°C by the high frequency induction heating, and a step of controlling the average cooling rate to be ≥ 30°C/sec when lowering the temperature of the surface of the execution part to 300°C from the solution treatment temperature. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a surface modification method and apparatus for preventing the occurrence and development of cracks such as stress corrosion cracking of a structure made of austenitic stainless steel.

  Conventionally, examples of damage to reactor internal structures made of austenitic stainless steel have been limited to heat-affected zones caused by welding, and have occurred at sites where stress corrosion cracking susceptibility has deteriorated due to thermal sensitization. However, in recent years, many damages that are considered to have occurred due to a mechanism different from conventional stress corrosion cracking, such as work hardening in the manufacturing stage, tensile residual stress, and material non-uniformity, have been reported.

These damages also occur in parts far away from the weld line compared to conventional stress corrosion cracking due to thermal sensitization, and as a preventive maintenance measure, it is more targeted for a larger area than before. An efficient construction method is required. The preventive maintenance techniques that have been put into practical use include stress improvement techniques such as laser peening and shot peening, and surface modification techniques using laser solution and melting (for example, Patent Documents 1 and 2).
JP 2001-287062 A (page 4, Fig. 1-3), Japanese Unexamined Patent Publication No. 7-75893 (5th page, FIG. 3)

  However, both of these stress improvement technologies and surface modification technologies were developed for the purpose of constructing a limited area near the weld line, and damage that occurs at sites sufficiently away from the weld line as described above. In order to be applied as preventive maintenance technology, dramatic improvement in construction efficiency is required.

  In addition, as mentioned above, it has been pointed out that the generation mechanism may be affected not only by material deterioration during the plant operation period but also by thermal history and processing history at the manufacturing stage. However, it may not be sufficient countermeasure technology.

  Therefore, the present invention has been made in view of the above-mentioned problems of the prior art, and the surface of the construction part is improved by a material improvement by solution treatment and a compressive stress by a stress improvement process, which is more efficient than the conventional technique. In addition, it is an object of the present invention to provide a surface modification method for a structure and an apparatus thereof that can reliably prevent the occurrence of damage.

  In order to achieve the above-mentioned object, the invention of the surface modification method for a structure according to claim 1 is directed to heating the surface of the construction part in the range of 900 ° C. or higher and 1100 ° C. or lower by the heating part of the high frequency induction heating device. And a cooling step in which the temperature of the surface of the construction part is cooled from the solution treatment temperature to 300 ° C. at an average cooling rate of 30 ° C./second or more by a cooling device.

  The invention of the surface modification method for a structure according to claim 8 includes a step of identifying a crack depth existing on the surface of the structure by a pre-construction inspection, and a control of construction parameters according to claim 5 or 6. It is characterized in that crack propagation is prevented from the step of forming a compressive stress layer having a depth greater than the crack depth identified.

  Further, the invention of the surface reforming apparatus for a structure according to claim 9 includes a heating unit provided with a high-frequency magnetic flux induction coil disposed facing the surface of the structure construction part, and a heating unit drive for moving the heating unit. A mechanism, a high-frequency power supply that supplies high-frequency power to the high-frequency magnetic flux induction coil of the heating unit, and a control signal and driving power to the heating unit drive mechanism to supply the heating unit to the surface of the construction unit. It is characterized by comprising a heating unit drive mechanism control device that moves at a speed.

  According to the present invention, by performing surface modification by material improvement by solution treatment and compressive stress by stress improvement treatment, production history of reactor internal equipment, stress corrosion cracking due to material deterioration during service, etc. It is possible to provide a highly efficient and effective surface modification method and reforming apparatus that prevent cracks from occurring and further suppress the growth of existing cracks.

Embodiments of the present invention will be described below with reference to the drawings.
(Example 1)
FIG. 1 is a conceptual diagram showing a use state of an embodiment according to a surface reforming apparatus for an in-furnace structure of the present invention.
In FIG. 1, reference numeral 1 denotes an austenitic stainless steel shroud installed in a reactor pressure vessel (not shown), which is composed of an upper shell, an intermediate shell, and a lower shell. The connection part between the intermediate | middle part trunk | drum 2 and the upper trunk | drum 3 which mounts a board is noted and shown.

  As shown in the figure, the upper cylinder 3 has an inner diameter and an outer diameter larger than the intermediate cylinder 2, and both the intermediate cylinder 2 and the upper cylinder 3 have intermediate widths corresponding to the dimensional difference in diameter. The upper lattice plate 6 is mounted on the intermediate ring 4 via the base 5 by welding via the ring 4. The shroud 1 and the upper lattice plate 6 are called a reactor internal structure. Reference numeral 7 denotes an upper ring that is welded to the upper end of the upper body 3. Reference numeral 8 denotes an operating floor located at the top of the reactor pressure vessel.

  Reference numeral 10 denotes a surface modification apparatus for performing surface modification of the shroud 1 which is the in-furnace structure configured as described above. The surface modification apparatus 10 is composed of the following elements.

  11 is a column placed on the center line C of the shroud 1 on the upper surface of the upper grid plate 6, and this column 11 accommodates a drive unit therein and is rotated at the upper end by the drive unit. Arm 12 is installed. Note that the rotation center of the rotating arm portion 12 coincides with the shroud 1 center line C. The rotating arm 12 has a tip that is sufficiently dimensioned to reach the upper ring 7 of the upper body 3. A wheel 13 having a tread on the upper end surface of the upper ring 7 is provided on the lower surface of the tip. It is attached so that the load of the rotating arm portion 12 is shared and supported by the support column 11 and the wheel 13.

  In addition to attaching the wheel 13 to the tip of the rotating arm 12, a heating unit support member 14 is further attached downward. A heating section (heating head) 15 is attached to the tip of the heating section support member 14. This heating unit 15 has a built-in high-frequency magnetic flux induction coil that is a component of the high-frequency induction heating device. The high-frequency magnetic flux generated by this high-frequency magnetic flux induction coil induces eddy currents in the construction part, and this eddy current causes construction. The part is heated to the solution treatment temperature. In the present invention, the above-described portion including the support column 11, the rotating arm portion 12, the wheel 13, and the heating portion support member 14 is referred to as a heating portion driving mechanism 16 for convenience.

  A heating unit positioning mechanism 17 is attached to the tip of the heating unit 15 to keep the distance between the heating unit 15 itself and the surface of the construction part of the shroud 1 constant. The reason why the distance between the heating unit 15 and the back surface of the construction part of the shroud is kept constant is to suppress the fluctuation of the heat input to the surface of the construction part as much as possible.

  In the example of FIG. 1, the heating part support member 14 is set to a length in which the heating part 15 is positioned in the vicinity of the weld line on the outer periphery of the intermediate part ring 4, but is not limited to this. For example, when the surface modification construction process is performed on the welded portion of the intermediate shell of the shroud 1 (not shown) or the vicinity of the lower ring, the length and shape of the heating portion support member 14 are appropriately selected so as to conform to it. That's fine.

  On the other hand, on the operating floor 8 located above the reactor pressure vessel, a power source and a control device for the heating unit 15 and the heating unit drive mechanism 16 are mounted. That is, during the surface modification construction process of the shroud 1, the high-frequency power supply device 18 configured by an inverter circuit for supplying high-frequency power of a desired frequency to the high-frequency magnetic flux induction coil of the heating unit 15, and the heating unit 15 Is mounted on the operating floor 8 with a heating unit driving mechanism control device 19 for controlling the heating unit driving mechanism 16 so as to move at a predetermined speed (for example, a constant speed) with respect to the construction unit.

In addition, in order to control the calorific value of a construction part efficiently and accurately, it is necessary to feed back the detected temperature of the construction part surface and to control the high frequency power supplied to a heating part. For this reason, although not shown in the present embodiment, a temperature sensor for detecting the surface temperature in the vicinity of the construction part is provided, and a detection signal of this temperature sensor is fed back to the high-frequency power supply device 18 based on a deviation signal from the set value. And controlling the firing angle of the inverter circuit to control the output frequency and the output power.
The high frequency power output from the high frequency power supply device 18 is supplied to the heating unit 15 through the high frequency transmission cable 20.

  The heating unit drive signal and the drive power from the control device 19 are supplied to a drive unit (not shown) housed in the column 11 through the cable 21. The rotating arm 12 is rotated at a constant speed by the drive unit, and the heating unit 15 installed at the tip of the heating unit support member 14 is moved at a constant speed with respect to the construction part of the shroud 1.

  In addition to the components from the support column 11 to the cable 21 described above, the surface modification device 10 is a cooling device (22 in FIG. 2) for rapidly cooling the construction part that has been solution-treated, although not shown. Is also provided.

  In general, it is considered that high tensile stress remains on the surface of the construction part that is subject to preventive maintenance due to the overlap of tensile residual stress by welding and tensile stress by machining. By cooling rapidly with a cooling device after solution treatment, a sufficiently high compressive stress can be formed by reducing the tensile stress or controlling the cooling process.

  FIG. 2 is a block diagram illustrating the heating unit 15 of the surface modification apparatus 10 described above. Reference numeral 22 denotes a cooling device not shown in FIG. 1, and has, for example, a cold air injection unit or a cold water injection unit in order to rapidly cool the construction unit.

Next, an embodiment of a surface modification method for a reactor internal structure or the like using the surface modification apparatus 10 described above will be described.
In this example, instead of the shroud 1 actually used in the nuclear power plant, a SUS316L steel mock-up specimen made of the same material as the shroud 1 was used, and the surface modification treatment was performed as follows. .

  First, in a state where the output of the high frequency power supply 18 is set to 250 kW and the frequency is set to 300 kHz, a heating unit 15 constituted by a planar heating coil is connected to a lower part (not shown) of an intermediate part cylinder 2 and a lower part cylinder (not shown) of the shroud 1. The ring end face is arranged at a site located about 100 mm high and about 50 mm wide.

  Next, by rotating the rotating arm 12 of the heating unit driving mechanism 16 at a constant speed, a solution treatment for about 3 minutes is performed by high-frequency induction heating while moving the heating unit 15 at a constant speed with respect to the construction part. went.

  As the solution treatment temperature in this case, since the shroud 1 is made of austenitic stainless steel, 900 ° C. or more and 1100 ° C. or less is optimal as a condition for stabilizing the austenite phase and further sufficiently diffusing and making the material uniform. . When the surface temperature of the center of the construction part was actually measured, it was confirmed that the surface temperature was maintained at about 1050 ° C.

  Next, after the solution treatment, water was sprayed from the cooling device 22 to the construction part, and the temperature of the construction part was rapidly cooled from about 1050 ° C. to near room temperature. FIG. 3 is a graph showing the relationship between the cooling rate (° C./second) of the construction part after the solution treatment and the residual stress formed on the surface. According to the graph of FIG. 3, in the case of a cooling rate of 30 (° C./second) or more that generates a large temperature difference between the surface and the inside, the effect of improving the residual stress after the solution treatment is remarkable. This is because in the process of cooling the construction part held at a high temperature, a large temperature difference occurs between the surface and the inside of the material, so stress due to thermal shrinkage occurs, and the surface residual stress is displaced to the compression side, This is because the stress is displaced toward the tension side in the interior where the temperature falls later. Furthermore, since the working part surface is rapidly heated by high frequency induction heating in this embodiment, it is easy to form a steep temperature gradient into the thickness of the plate. Therefore, in combination with the cooling method after solution treatment, a high compressive stress can be obtained. Not only can it be formed, it is also possible to control the thickness of the compressive stress layer.

  In addition, in order to confirm the stress improvement effect of the construction part, the residual stress distribution of the circumferential stress from the surface of the shroud 1 to a depth of about 1 mm was measured by the X-ray stress measurement method. FIG. 4 shows the measurement results after surface modification treatment by high-frequency heating, with the horizontal axis representing the depth (mm) from the surface and the vertical axis representing the residual stress (MPa). From the graph of FIG. 4, it can be seen that a compressive stress of about 650 MPa (minus sign is a compressive stress) is formed when the surface, that is, a depth of 0 mm, and the compressive stress layer reaches a depth of about 0.5 mm.

  Fig. 5 shows specimens taken from both the surface-modified and non-processed parts of the mock-up specimen, and observed the metal structure and measured the hardness distribution from the surface. The Vickers hardness (Hv) is taken on the vertical axis, and the hardness distribution measurement results of the construction part and the non-working part are compared and shown. As can be seen from FIG. 5, there is a work hardened layer that is considered to be due to machining with a depth of about 0.4 mm in the non-worked part indicated by the solid line, but in the part subjected to the surface modification work process, the broken line As shown by the above, the work-hardened layer disappears and has almost the same hardness as the inside. In this case, the structure was a healthy austenite single phase structure, and a ferrite phase and an extreme processed structure were not observed.

  FIG. 6 is a diagram showing the frequency dependence of the compressive stress layer formed by rapid cooling treatment by water jet from the cooling device 22 after the solution surface is subjected to solution treatment by high-frequency induction heating. The depth (mm) from the surface of the modified construction part is taken, and the residual stress (MPa) is taken on the vertical axis. In FIG. 6, the solid line is a characteristic diagram when the frequency is 50 kHz and the output is 300 kW, the alternate long and short dash line is the frequency when the frequency is 100 kHz and the output is 260 kW, and the broken line is the characteristic diagram when the frequency is 300 kHz and the output is 250 kW. In any construction part, the solution treatment condition is 1050 ° C. × 3 minutes. In the case of high-frequency induction heating, the skin depth of the eddy current is deeper as the frequency is lower due to the skin effect, so the solution region is considered to extend to the inside of the material. The compressive stress layer thickness formed by rapid cooling also changes, and the deeper the compressive stress layer is formed, the lower the frequency.

  In the above description, a high-frequency induction heating device has been described as the heating means of the construction part. However, as the heating means, radiant heating means such as a heater or a lamp can also be applied. However, the high-frequency induction heating means can induce only eddy currents in the construction member and heat only the surface layer, so it has good controllability, local heating is possible, the surroundings are not heated, energy efficiency is good, and rapid heating The device has excellent features such as being relatively compact. In addition, as described above, in the case of high-frequency induction heating, the skin depth of eddy current is deeper if the operating frequency is low, and shallow if it is high, so the skin depth of eddy current is controlled by changing the frequency according to the target member. Thus, the solution depth can be freely changed. For these reasons, the high frequency induction heating means is optimal as the heating means for the construction part.

As described above, in this example, the surface of the construction part of the structure is subjected to a solution treatment by heating in a range of 900 ° C. to 1100 ° C. by high frequency induction heating, and then the surface temperature of the construction part is set to 30. Cooling from the solution treatment temperature to 300 ° C at an average cooling rate of ℃ / sec or more, realizing material improvement and compressive stress by stress improvement treatment at the same time, higher efficiency and reliability than before High preventive maintenance methods can be provided.
The present invention is not limited to the apparatus and method described in the first embodiment, and may be described as follows.

(Example 2)
When high frequency induction heating is applied as a heating means for the construction part, as described below, as a construction control parameter that determines the solution treatment temperature, the modified layer depth, the surface compressive stress value after construction, and the compressive stress layer depth, Any one or two or more of the output frequency, output power, moving speed (or heating holding time by the heating unit) of the heating unit of the high-frequency induction heating device, or the construction unit cooling rate by the cooling device may be combined.

  That is, it is necessary to control the solution treatment temperature, the reformed layer thickness, the compressive stress value to be formed, and the compressive stress layer depth to a value more suitable for the material properties of the construction part. In the case of high frequency induction heating, the frequency that determines the skin depth of the eddy current is an important parameter that determines the depth of the solution layer and the depth of the compressive stress layer. On the other hand, the solution treatment temperature is mainly controlled by the output of the heating device that determines the eddy current density along with the frequency. The cooling rate is an important parameter for determining the residual compressive stress value and its depth. Further, in the case of a mobile device, the moving speed of the heating device that governs the thermal diffusion into the surroundings and inside the member due to heat conduction during construction is a major parameter.

  In any case, it is necessary to select the appropriate combination of construction parameters to control the solution treatment temperature, modified layer depth, compressive stress value, and compressive stress layer depth according to the expected material properties of the construction part. is there. On the other hand, in the case of the batch type apparatus of Example 3 described later, the heating and holding time is a construction parameter instead of the heating unit moving speed.

  In this way, by applying compressive stress value and depth control technology by controlling the construction parameters, if a crack exists, a crack is formed by forming a compressive stress layer that is greater than the crack depth. It is also possible to suppress progress.

(Example 3)
In the construction method using the surface modification apparatus 10 shown in FIG. 1, a moving method is used in which the heating apparatus is moved at a constant speed by the heating unit drive mechanism 16 to perform the surface modification construction process. The movement method is not limited.

  In the third embodiment, a surface modification treatment is performed on a predetermined partial region of the surface of the construction portion of the structure by holding the heating device for a predetermined time in a fixed state, and then the surface modification treatment range is partially The present invention provides a method (batch method) in which surface modification treatment is sequentially performed so as to overlap, and finally the surface modification treatment is performed over the entire construction target portion.

Example 4
You may make it apply the surface modification process mentioned above to the welding part at the time of construction of a reactor internal structure. In this case, it is possible to improve the reliability and prolong the service life of the equipment subjected to the surface modification treatment.

(Example 5)
If the presence of cracks on the surface of the reactor internal structure is known, the crack depth is quantified in advance by means of ultrasonic flaw detection or ECT (eddy current flaw detection), and then the surface modification described above. By applying a treatment method and forming a compressive stress layer having a depth equal to or greater than the crack depth on the surface of the construction part, a compressive stress field may be applied to the crack tip to prevent crack propagation.
This method can prevent crack propagation over a long period of time due to the compressive stress field formed at the crack tip in the case of a device having a small load stress in service such as a shroud.

(Example 6)
In the case where the surface modification treatment method of the present invention is applied in the air as in the case of application to a reactor internal structure at the time of construction, forced cooling by the cooling device 22 shown in FIG. It is necessary to cool rapidly by forced water cooling. However, when it is applied as a preventive maintenance method, it is assumed that it is underwater construction, so when cooling after solution treatment, water is forced to be sent from natural convection or around the heating part to the construction part. A special cooling device such as the cooling device 22 of FIG. 1 is not necessarily required because it will be cooled rapidly. Therefore, in the case of the preventive maintenance method, the construction apparatus can be miniaturized and access to the narrow part is also possible.

The conceptual diagram which shows the example of application of the surface modification apparatus of this invention. The block block diagram of the surface modification apparatus shown in FIG. The figure which shows the relationship between the cooling rate of a shroud mockup test body, and the residual stress formed in the surface. The stress distribution figure which shows the compressive-stress layer formed in the shroud mockup test body surface by the high frequency induction heating and the rapid cooling process. The hardness distribution figure of the construction part which shows the solution treatment effect by the high frequency induction heating of a shroud mockup test body surface layer part, and a non-application part. The stress distribution figure which shows the frequency dependence of the compressive-stress layer thickness formed in the shroud mockup test body surface by the high frequency induction heating and the rapid cooling process.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Shroud, 2 ... Middle part cylinder, 3 ... Upper part body, 4 ... Middle part ring, 5 ... Base, 6 ... Upper lattice board, 7 ... Upper ring 8 ... Operating floor, 10 ... Surface modification apparatus, 11 ... Post , 12 ... Rotating arm part, 13 ... Wheel, 14 ... Heating part support member, 15 ... Heating part (heating head), 16 ... Heating part drive mechanism, 17 ... Heating part positioning mechanism, 18 ... High frequency power supply device, 19 ... Heating Part drive mechanism control device, 20 ... high frequency transmission cable, 21 ... heating part drive mechanism signal and drive power cable, 22 ... cooling device.

Claims (11)

  A solution treatment step of heating the construction part surface of the structure within a range of 900 ° C. or more and 1100 ° C. or less by the heating part of the high frequency induction heating device, and an average of the temperature of the construction part surface of 30 ° C./second or more by the cooling device A method for surface modification of a structure, comprising a cooling step of cooling from a solution treatment temperature to 300 ° C. at a cooling rate.   2. The structure according to claim 1, wherein the solution treatment step heats the construction part surface while moving the heating part of the high-frequency induction heating device at a constant speed with respect to the construction part surface of the structure. Method for surface modification of objects.   In the solution treatment step, the heating unit of the high-frequency induction heating device is heated and held for a predetermined time in a part of the surface of the construction part in a state where the heating part is fixed to face a part of the part of the surface of the construction part of the structure. Then, a solution treatment is performed, and after that, the solution is heated and held for a predetermined time in a state where the heating unit is fixed so as to face another region, and thereafter, this is repeated to perform a solution treatment on the entire construction target region. The surface modification method for a structure according to claim 1, wherein:   4. The surface modification method for a structure according to claim 2, wherein the cooling step is performed by at least one of cooling means by air injection or cooling means by water injection.   The solution control temperature, the modification depth, the surface compressive stress value after construction, and the construction control parameters that determine the compressive stress depth are the output frequency and output power of the heating part in the solution treatment step. 3. The surface modification method for a structure according to claim 2, wherein any one of a moving speed and a cooling speed in a cooling step or a combination of two or more parameters is used.   The solution control temperature, the modification depth, the surface compressive stress value after construction, and the construction control parameters that determine the compressive stress depth are the output frequency and output power of the heating part in the solution treatment step. 4. The method for surface modification of a structure according to claim 3, wherein any one of a heating retention time and a cooling rate in the cooling step or a combination of two or more parameters is used.   The method for surface modification of a structure according to any one of claims 1 to 6, wherein a surface modification treatment is performed on a welded portion of the structure at the time of construction of the reactor internal structure.   A step of identifying the crack depth existing on the surface of the structure by pre-construction inspection, and forming a compressive stress layer greater than the identified crack depth by controlling the construction parameters described in claims 5 or 6 A method for modifying the surface of a structure, wherein crack propagation is prevented from occurring.   A heating unit provided with a high-frequency magnetic flux induction coil disposed opposite to the surface of the construction part of the structure, a heating unit driving mechanism for moving the heating unit, and supplying high-frequency power to the high-frequency magnetic flux induction coil of the heating unit A high-frequency power supply device and a heating unit driving mechanism control device that supplies a control signal and driving power to the heating unit driving mechanism to move the heating unit at a predetermined speed with respect to the surface of the construction unit, Surface modification equipment for structures.   The surface modification apparatus for a structure according to claim 9, further comprising a cooling unit that forcibly cools a surface of a construction portion of the structure. The surface modification apparatus according to claim 9, further comprising a mechanism for maintaining a constant distance between a surface of the construction portion of the structure and the heating portion.

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