JP2018119194A - Method for modifying surface of structure - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve resistance to irradiation-induced stress corrosion cracking of structures composed of a metal material.SOLUTION: In this method, the following are carried out sequentially. A surface processing step of inducing plastic deformation to the surface of a structure that is an object 35 made of a metal material; a heating step of heating the surface processed in the surface processing step at a heating rate that avoids sensitization of the metal material; and a cooling step of cooling the surface heated in the heating step at a cooling rate that avoids sensitization of the metal material. A grain refinement layer 34 is thereby formed in a region of at least 100 μm in depth from the surface of the structure.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a surface modification method for a structure existing in an environment irradiated with neutrons.

  Since a nuclear power plant needs to be operated while maintaining the soundness of the metal material of the plant equipment for a long period of time, austenitic stainless steel having excellent strength and corrosion resistance is used as the metal material of the plant equipment. In particular, reactor internal structures as plant equipment are exposed to extremely harsh environments because they are exposed to long-term neutron irradiation and high-temperature high-pressure water. For this reason, in the reactor internal structure, irradiation-induced stress corrosion is caused by a decrease in Cr concentration at the grain boundary due to irradiation-induced segregation and irradiation embrittlement due to concentration of irradiation defects at the grain boundary. There is a concern of cracking. As a method of increasing the grain boundaries that are sinks of irradiation defects and suppressing Cr deficiency at the grain boundaries due to irradiation-induced segregation, this improves the resistance to irradiation-induced stress corrosion cracking. Technology.

  Various techniques have been publicly disclosed as a method for refining crystal grains in a nuclear reactor structure in a nuclear reactor plant (Patent Documents 1 to 4), and the process mainly constitutes the nuclear reactor structure. There are a strong processing step for imparting strong processing to a metal material, a heating step for heating the metal material subjected to strong processing, and a cooling step for cooling after heating.

  Further, there is a method of introducing a grain refinement layer only on the surface of a conventional reactor internal structure (Patent Documents 5 and 6). This method can be applied to a structure after assembly, and it is possible to avoid the coarsening of crystal grains by welding.

JP 2006-233292 A Japanese Patent No. 4843230 JP 2005-171278 A JP 2010-174308 A JP 2005-265449 A JP 2006-283043 A

  However, in the conventional grain refinement techniques for nuclear reactor nuclear reactor structures in Patent Documents 1 to 6 described above, the heating rate in the heating process, the cooling rate in the cooling process and sensitization (that is, the grain size of the crystal grains). The relationship with the formation of a Cr-depleted layer in the vicinity of the boundary is not described, and therefore a sufficient improvement in resistance to irradiation-induced stress corrosion cracking cannot be expected.

  Moreover, the crystal grain refinement techniques in Patent Documents 1 to 6 describe only the crystal grain refinement method, and do not disclose the construction to the reactor internal structure assuming a specific actual construction.

  The object of the present invention has been made in consideration of the above-mentioned circumstances, and provides a surface modification method for a structure that can improve the resistance to irradiation-induced stress corrosion cracking of a structure composed of a metal material. It is in.

  The surface modification method for a structure according to the present invention includes a surface processing step for performing processing accompanied by plastic deformation on a surface of a structure composed of a metal material, and a work surface processed by the surface processing step. A heating step of heating at a temperature rising rate that avoids sensitization of the metal material, and a cooling step of cooling the surface to be heated heated by the heating step at a temperature lowering rate that avoids sensitization of the metal material Then, a refined layer of crystal grains is formed in a depth region of at least 100 μm from the surface of the structure.

  According to the present invention, the crystal grain refinement layer is formed in the depth region of at least 100 μm from the surface of the structure, so that the grain boundaries of the crystal grains are increased. Thereby, the sink of an irradiation defect can increase and the irradiation embrittlement of a grain boundary can be suppressed, and also Cr deficiency at the grain boundary due to irradiation-induced segregation can be suppressed. From these things, the radiation-induced stress corrosion cracking resistance of the structure can be improved. In addition, heating is performed at a rate of temperature rise that avoids sensitization of the metal material in the heating process, and cooling is performed at a rate of temperature decrease that avoids sensitization of the metal material in the cooling process. Formation of a Cr-deficient layer in the vicinity of the grain boundary of the crystal grain on the surface of the structure can be suppressed. This also improves the radiation-induced stress corrosion cracking resistance of the structure.

The process drawing which shows one Embodiment of the surface modification method of the structure based on this invention. The perspective view which shows the enforcement condition of the surface processing process (shot peening process) of FIG. The schematic longitudinal cross-sectional view which shows a boiling water reactor provided with the shroud which is the enforcement object of FIG.1 and FIG.2. FIG. 4 is a perspective view showing the shroud, the upper lattice plate, and the core support plate of FIG. The schematic block diagram which shows the example by which the surface processing process of FIG.1 and FIG.2 is implemented by laser peening process. The schematic block diagram which shows the example by which the surface processing process of FIG.1 and FIG.2 is implemented by water jet peening process. The schematic block diagram which shows the example by which the surface processing process of FIG.1 and FIG.2 is implemented by hammering. The schematic block diagram which shows the example by which the surface processing process of FIG.1 and FIG.2 is implemented by cutting. The schematic block diagram which shows the example by which the surface processing process of FIG.1 and FIG.2 is implemented by grinder processing. The schematic block diagram which shows the example in which the heating process of FIG. 1 is implemented by laser irradiation heating. The schematic block diagram which shows the example by which the heating process of FIG. 1 is implemented by infrared heating. The schematic block diagram which shows the example by which the heating process of FIG. 1 is implemented by high frequency induction heating. The schematic block diagram which shows the example by which the heating process of FIG. 1 is implemented by combustion gas radiation heating. The graph which shows the TTS curve which prescribes | regulates sensitization of the austenitic stainless steel used in the heating process of FIG. The shape of the heating area | region by the heating head (laser head) of FIG. 10 and the heating distribution by this heating head are shown, and a heating area | region is each circular shape (A), rectangular shape (B), and V shape (C). Explanatory drawing which shows a case. (A) is a photomicrograph after ion irradiation is performed on austenitic stainless steel whose surface crystal grains are refined through the surface processing step, heating step and cooling step of FIG. 1, and (B) is a diagram. 16 is a micrograph showing the composition distribution of Cr atoms in the photograph of (A).

DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings.
FIG. 1 is a process diagram showing an embodiment of a surface modification method for a structure according to the present invention. Moreover, FIG. 2 is a perspective view which shows the enforcement condition of the surface processing process (shot peening process) of FIG. The surface modification method 10 of the structure of the present embodiment is a structure formed through, for example, a welding process, in this embodiment, a reactor structure (reactor pressure vessel 12) of the boiling water reactor 11 shown in FIG. , And the reactor internal structure accommodated in the reactor pressure vessel 12), the surface crystal of the structure is refined to thereby improve the radiation-induced stress corrosion cracking of the structure. It improves the performance. In this embodiment, a case will be described in which the crystal grains on the surface of the shroud 14 are refined among the above-described reactor internal structures.

  Here, as shown in FIG. 3, the boiling water reactor 11 is configured such that a shroud 14 that houses a core 13 is disposed in a reactor pressure vessel 12, and the core 13 includes a fuel assembly 15. . In order to support the fuel assembly 15, an upper lattice plate 16 is disposed on the upper portion of the shroud 14, and a core support plate 17 is disposed on the lower portion. In order to control the output of the fuel assembly 15, the control rod 18 is inserted into and removed from the core 13 by the control rod drive mechanism 19.

  Further, the reactor pressure vessel 12 includes a steam outlet 21 that guides steam evaporated by the heat of the fuel assembly 15 to a turbine (not shown), a feed water inlet 22 that guides feed water into the reactor pressure vessel 12, a reactor The recirculation water outlet 23 for introducing the recirculation water to be the driving water in the pressure vessel 12 to the recirculation system (not shown), and the recirculation water to be the driving water in the reactor pressure vessel 12 are introduced from the recirculation system. A recirculation water inlet 24 is arranged.

  The aforementioned reactor internal structure includes the shroud 14, the upper lattice plate 16, the core support plate 17, the steam separator 25, the steam dryer 26, and the like that are sequentially disposed above the core 13. Similar to the reactor pressure vessel 12, this in-reactor structure is made of austenitic stainless steel from the viewpoint of strength and corrosion resistance.

  As shown in FIG. 4, the shroud 14 shown in FIG. 3 includes an upper ring 27, an upper trunk 28, an intermediate ring 29, an intermediate trunk 30, a lower ring 31, and a lower trunk 32 from the top. Joined by welding. As a representative example of the weld line, a circumferential weld line 33 of the intermediate body cylinder 30 is shown by a broken line in FIG. The upper lattice plate 16 is positioned on the middle ring 29 and the core support plate 17 is positioned on the lower ring 31. In this case, the shroud 14, the upper lattice plate 16, and the core support plate 17 have a role of supporting the fuel assembly 15. If cracks or the like enter and damage these, it becomes difficult to maintain the long-term soundness of the nuclear power plant. Further, since the shroud 14, the upper lattice plate 16, and the core support plate 17 are located relatively close to the core 13, they are easily affected by neutrons over a long period of time.

  Now, the surface modification method 10 for a structure shown in FIG. 1 is a structure as an object 35 made of a metal material (for example, a shroud 14 made of austenitic stainless steel in the boiling water reactor 11). By carrying out a surface processing step, a heating step, and a cooling step sequentially on the surface of the substrate, a grain refinement layer is formed at a depth region of at least 100 μm from the surface of the structure (for example, the shroud 14) as the enforcement target 35. 34 is formed.

  In the surface processing step, strong processing with plastic deformation is performed on the surface of the structure (for example, the shroud 14) as the enforcement target 35 using the processing head 36. Moreover, a heating process heats the to-be-processed surface processed by the surface processing process with the temperature increase rate which avoids the sensitization (after-mentioned) of austenitic stainless steel using the heating head 37. FIG. Further, in the cooling step, the surface to be heated heated in the heating step is cooled by using the cooling head 38 at a temperature lowering rate that avoids sensitization (described later) of the austenitic stainless steel.

  The surface processing step described above is a step of performing at least one selected from the group consisting of peening and machining. Among these, the peening is at least one selected from the group consisting of shot peening, laser peening, and water jet peening. In the shot peening process, as shown in FIG. 1, a projection material (shot) 40 is projected from the shot peening head 39 as the processing head 36 onto the surface of the structure (for example, the shroud 14), and the structure (for example, the shroud) is impacted by the impact. 14) A strong working strain (plastic strain) is imparted to the surface. As the projection material 40, a steel ball or a small sphere having the same degree of hardness as this steel ball is used.

  This shot peening process is performed a plurality of times, for example, twice, and it is preferable that the projection material 40 having a different sphere diameter is used each time. In other words, in the surface modification method 10 for a structure, the crystal grain becomes finer as the amount of plastic strain applied to the surface of the structure (for example, the shroud 14) in the surface processing step is larger (higher). Generally, in the shot beaning process, the plastic strain is applied deeper from the execution surface of the construction object 35 to the inside as the sphere diameter of the projection material 40 is larger, and the enforcement of the enforcement object 35 is performed as the sphere diameter of the projection material 40 is smaller. It is given near the surface. Therefore, after projecting the projection material 40 having a large sphere diameter in the first round, the projection material 40 having a small sphere diameter is projected in the next round.

  Thereby, by combining the distribution characteristics of the plastic strain imparted by the projection material 40 having both spherical diameters, the plastic strain is imparted deeply from the surface to the inside of the structure (for example, the shroud 14) that is the construction target 35, and the structure. A large (high) plastic strain is imparted to the surface portion of the object (for example, the shroud 14).

  In the peening process including the above-described shot peening process, the peening head is configured to move (scan) at a constant speed while maintaining a constant interval with respect to the surface of the structure (for example, the shroud 14). Yes. FIG. 2 shows a case where the surface of the vicinity of the circumferential weld line 33 in the intermediate shell 30 of the shroud 14 is processed using a shot peening head 39. In this case, a rail 41 is installed along the circumferential direction of the shroud 14 so as to maintain a certain distance from the surface of the shroud 14, and the shot peening head 39 moves in the direction of arrow A at a constant speed on this rail 41 (scanning). ), The amount of plastic strain applied to the surface of the shroud 14 is made uniform in the circumferential direction of the shroud 14.

  The moving (scanning) speed of the shot peening head 39 that moves (scans) on the rail 41 becomes 100% coverage or more at which the projection material 40 is projected on the entire surface of the construction surface, and further, the arc height value (plastic strain) on the construction surface. The speed is set so that the projection material 40 is projected for a time sufficient to saturate the amount. Thereby, uniform and sufficient plastic strain can be applied in the circumferential direction of the shroud 14.

  After the processing head 36 (peening head including the shot peening head 39) moves (scans) on the rail 41, the position of the rail 41 is adjusted in a direction perpendicular to the moving direction, and then along the rail 41, The processing head 36 (peening head including the shot peening head 39) may be moved (scanned) again. Thereby, uniform plastic strain can be applied to the entire surface of the structure (for example, the shroud 14) by the processing head 36 (peening head including the shot peening head 39). Further, instead of the rail 41, the processing head 36 (peening head including the shot pini head 39) may be moved in the circumferential direction of the structure (for example, the shroud 14) by other means such as a robot arm or manually.

  Here, peening processing (laser peening processing, water jet peening processing and machining) other than shot peening will be described.

  As shown in FIG. 5, the laser peening process includes a laser peening head 44 as a processing head 36 (peening head), a lens 45, a laser oscillator 46, and a water feeder 47. The pulse laser 49 oscillated by the laser oscillator 46 is focused on the structure (the shroud 14) covered with the water film 48 by the lens 45 and irradiated, and the plasma 50 generated on the surface of the structure (the shroud 14). The shock wave caused by the surface processing of the structure (shroud 14).

  As shown in FIG. 6, the water jet peening process includes a water jet peening head 51 as a processing head 36 (peening head) and a pump 52. The water jet peening head 51 injects high-pressure water pressurized by the pump 52 onto the structure in the water 53 (the shroud 14), and uses the collapse pressure of the generated cavitation 54 to construct the structure (the shroud 14). Surface treatment is performed.

  The machining is at least one selected from the group consisting of hammering, cutting, and grinder processing. Among these, the hammering process includes a hammer 55 as the processing head 36, an actuator 56, and a control power source 57 as shown in FIG. 7. The control power source 57 generates a high-frequency electromagnetic force in the actuator 56, and the hammer 55 vibrates at a high frequency by this electromagnetic force, so that the tip of the hammer 55 strikes the surface of the structure (the shroud 14) to perform surface processing. I do.

  As shown in FIG. 8, the cutting process includes a cutter 58 as a processing head 36, a main shaft 59, and a rotating device 60. The rotating device 60 rotates the main shaft 59, and the cutter 58 is pressed against the structure (shroud 14) while rotating, whereby surface processing is performed on the structure (shroud 14) while discharging chips 61. Further, as shown in FIG. 9, the grinder processing includes a grindstone 62 as a processing head 36 and a rotating device 63. The rotating device 63 presses the structure (shroud 14) against the structure (shroud 14) while rotating the grindstone 62, thereby performing surface processing on the structure (shroud 14).

  In the heating step shown in FIG. 1, by generating plastic strain on the surface of a structure (for example, shroud 14) by a surface processing step, heating is performed to grow crystal grains with a high plastic strain as a nucleus, and fine grains are formed. This is a step of generating crystal grains. And this heating process is complete | finished before the produced | generated fine crystal grain couple | bonds and recrystallizes and becomes a coarse crystal grain, It transfers to a cooling process. The particle diameter of the fine crystal grains generated in this heating step is preferably 5 μm or less from the viewpoint of increasing the grain boundary of the crystal grains and improving the radiation-induced stress corrosion cracking resistance of the structure.

  This heating step is a step of performing at least one selected from the group consisting of laser irradiation heating (FIG. 10), infrared heating (FIG. 11), high frequency induction heating (FIG. 12), and combustion gas radiation heating (FIG. 13). is there.

  As shown in FIG. 10, the laser irradiation heating includes a laser heat source 65, a light guide unit 66, a laser head 67 as a heating head 37, and a temperature monitor 68. The laser head 67 is moved with respect to the structure (the shroud 14), and heat treatment is performed while monitoring the surface temperature of the structure (the shroud 14) with the temperature monitor 68. As shown in FIG. 11, the infrared heating includes an infrared heating lamp 70 as a heating head 37, a temperature controller 71, a temperature monitor 72, and a power source 73. The infrared heating lamp 70 is moved with respect to the structure (the shroud 14), and heat treatment is performed while the surface temperature of the structure (the shroud 14) is monitored by the temperature monitor 72.

  As shown in FIG. 12, the high-frequency induction heating includes a high-frequency power source 74, a cooler 75, a high-frequency current transformer 76, a heating coil 77 as a heating head 37, and a temperature monitor 78. A heating coil 77 is installed and moved around the structure (the shroud 14), and heat treatment is performed while monitoring the surface temperature of the structure (the shroud 14) with the temperature monitor 78. As shown in FIG. 13, the combustion gas radiant heating includes a combustion gas radiant head 79, an oxygen cylinder 80, and a fuel cylinder 81 as the heating head 37. Heat treatment is performed by moving the combustion gas radiation head 79 while radiating the combustion gas 82 from the combustion gas radiation head 79 to the structure (the shroud 14).

  In the heating process described above, the rate of temperature increase is performed at a speed that avoids sensitization of the metal material (for example, austenitic stainless steel) constituting the structure. For example, when austenitic stainless steel is held for a certain time in the vicinity of 600 ° C. to 800 ° C., Cr carbide in which Cr atoms and C atoms in the austenitic stainless steel are bonded is precipitated in the vicinity of the crystal grain boundary. A Cr-deficient layer is generated in the vicinity of the grain boundary. This phenomenon is called sensitization of a metal material (austenitic stainless steel). This Cr-deficient layer lowers the corrosion resistance of the grain boundary of the metal material (austenitic stainless steel), so that irradiation-induced stress corrosion cracking is likely to occur.

  FIG. 14 shows a constant temperature sensitization curve (TTS (time-temperature-sensitization) curve) created to estimate the possibility of sensitization based on the amount of carbon contained in austenitic stainless steel, the heating time and the heating temperature. 3 is a graph showing P. In the case of X2, the temperature increase rate in the heating process reaches a sensitization region surrounded by the TTS curve P, so that a Cr-depleted layer is generated at the grain boundary of the austenitic stainless steel. On the other hand, when the temperature rising rate in the heating process is X1, since it deviates from the sensitization region surrounded by the TTS curve P, the formation of a Cr-depleted layer at the grain boundary of the austenitic stainless steel is prevented. The temperature increase rate X1 is preferably 1 ° C./second or more, for example. In addition, there exists an advantage which can avoid the coarsening of the crystal grain of a metal material (for example, austenitic stainless steel) by making the temperature increase rate in a heating process high with 1 degree-C / sec or more as mentioned above.

  In the heating step shown in FIG. 1, the processing head 36 (shot peening head 39) shown in FIG. 2 is replaced with a heating head 37, and the heating head 37 is placed on the rail 41 on the surface of the structure (for example, the shroud 14). In contrast, the surface of the structure (for example, the shroud 14) is heated by moving (scanning) at a constant speed while maintaining a constant interval. Thereby, the heating amount in the scanning region scanned by the heating head 37 on the surface of the structure (for example, the shroud 14) is made uniform, and an equivalent thermal history is obtained. Further, the moving (scanning) speed of the heating head 37 is not increased because fine crystal grains are bonded and coarsened when the surface of the structure (for example, the shroud 14) is held at a high temperature for a long time. Set to speed.

  The rail 41 may be configured such that the position of the rail 41 can be adjusted in a direction perpendicular to the moving (scanning) direction of the heating head 37, and the entire surface of the structure can be heated by the heating head 37. Further, instead of the rail 41, the heating head 37 may be moved (scanned) at a constant speed while maintaining a constant interval with respect to the surface of the structure by a robot arm or manual operation.

  In the heating process shown in FIG. 1, the heating area by the heating head 37 is set such that the heating amount in the width direction of the heating area perpendicular to the movement (scanning) direction of the heating head 37 is constant. FIG. 15 shows a case where the heating head 37 is a laser head 67 of laser irradiation heating (FIG. 10), and this laser head 67 maintains a constant distance with respect to the surface of the structure (for example, the shroud 14) while maintaining a constant speed. The heating regions 83A, 83B, and 83C by the laser head 67 and the heating distributions 84A, 84B, and 84C to the structure (for example, the shroud 14) by the heating regions 83A, 83B, and 83C are shown. .

  In the circular heating region 83A, in the width direction B of the heating region 83A perpendicular to the moving (scanning) direction A of the laser head 67, a heating distribution 84A in which the heating amount at both ends is small and the heating amount is biased is exhibited. . On the other hand, in the rectangular heating region 83B and the V-shaped heating region 83C, heating with a constant heating amount in the width direction B of the heating regions 83B and 83C perpendicular to the moving (scanning) direction A of the laser head 67 is performed. Distribution 84B, 84C is exhibited. Accordingly, the structure (for example, the shroud 14) is adopted by adopting the rectangular heating region 83B and the V-shaped heating region 83C that provide the heating distributions 84B and 84C with a constant heating amount to the surface of the structure (for example, the shroud 14). It becomes possible to form a refined layer with uniform crystal grains on the surface 14).

  The laser head 67 for laser irradiation heating (FIG. 10) can set a rectangular heating region 83B and a V-shaped heating region 83C that give uniform heating distributions 84B and 84C by adjusting semiconductor elements. In the case of infrared heating (FIG. 11) or combustion gas radiation heating (FIG. 13), the surface to be heated of the structure (for example, the shroud 14) or the infrared heating lamp 70 or the combustion gas radiation head 79 is used. By performing masking, it becomes possible to set a rectangular heating region 83B and a V-shaped heating region 83C on the surface of the structure (for example, the shroud 14).

  The cooling process shown in FIG. 1 is performed in order to prevent the fine crystal grains generated in the heating process from being combined and coarsened. This cooling step is a step of performing at least one selected from the group consisting of water cooling, oil cooling, gas cooling, and air cooling. Here, water cooling uses water as a refrigerant. Oil cooling uses oil as a refrigerant. Gas cooling uses a gas injected as a refrigerant (for example, an inert gas or air). In air cooling, static air is used as a refrigerant.

  This cooling process is appropriately selected depending on the heat capacity of the structure (for example, the shroud 14), the construction environment, the ease of recovery of the refrigerant, and the like. Among these, water cooling is most desirable because water as a refrigerant can provide a high temperature drop rate, and the refrigerant itself is easily available and can be easily recovered. However, when the heating process is performed at a high temperature and in a short time, air cooling may be sufficient because heat is not easily conducted from the surface of the structure (for example, the shroud 14) to the inside.

  In addition, the cooling rate in the cooling process is performed at a speed that avoids sensitization of the metal material (for example, austenitic stainless steel) constituting the structure. For example, in the TTS curve P of FIG. 14 created to estimate the formation (ie, sensitization) of the Cr-depleted layer in the vicinity of the grain boundary of the austenitic stainless steel crystal grain, Since the cooling rate Y2 reaches the sensitized region surrounded by the TTS curve P, a Cr-depleted layer is generated at the grain boundary of the austenitic stainless steel.

  On the other hand, when the temperature decrease rate is Y1, since this temperature decrease rate Y1 deviates from the sensitization region surrounded by the TTS curve P, the formation of a Cr-depleted layer at the grain boundary of the austenitic stainless steel is prevented. The temperature lowering rate Y1 is preferably 1 ° C./second or more like the temperature rising rate X1 in the heating step. In addition, since the temperature-fall rate in a cooling process is 1 degree-C / second or more as mentioned above, there also exists an advantage which can avoid the coarsening of the crystal grain of the metal material (for example, austenitic stainless steel) of a structure.

With the configuration as described above, the following effects (1) to (4) are achieved according to the present embodiment.
(1) By performing the surface processing step, the heating step, and the cooling step shown in FIG. 1 in sequence, a crystal grain refinement layer 34 is formed in a depth region of at least 100 μm from the surface of the structure (for example, the shroud 14). As a result, the grain boundaries of the crystal grains increase. Thereby, the sink of irradiation defects increases, and irradiation embrittlement of the grain boundary can be suppressed even by neutron irradiation, and further, Cr deficiency at the grain boundary due to irradiation-induced segregation caused by neutron irradiation can be suppressed. From these things, the radiation-induced stress corrosion cracking resistance of the structure (for example, the shroud 14) can be improved.

  In FIG. 16A, ion irradiation simulating neutron irradiation was performed on austenitic stainless steel with refined crystal grains by sequentially performing the surface processing step, heating step, and cooling step of this embodiment. FIG. 16 (B) shows a micrograph showing the Cr composition distribution in the austenitic stainless steel crystal shown in FIG. 16 (A). In general, it is known that in an austenitic stainless steel, Cr deficiency occurs in the grain boundary 86 of the crystal grain 85 due to irradiation-induced segregation caused by neutron irradiation or ion irradiation.

  In this embodiment, as shown in FIGS. 16A and 16B, Cr atoms are uniformly distributed over the entire crystal grain 85 including the grain boundary 86, and no Cr deficiency occurs in the grain boundary 86. I understand that. For this reason, the austenitic stainless steel in which the surface processing, heating process, and cooling process of the present embodiment are sequentially performed to refine the surface crystals can be expected to improve the resistance to irradiation-induced stress corrosion cracking.

  (2) As shown in FIGS. 1 and 14, heating is performed at a heating rate (for example, heating rate X1) that avoids sensitization of a metal material (for example, austenitic stainless steel) in the heating process, and metallic material is used in the cooling process. Cooling is performed at a temperature decrease rate (for example, temperature decrease rate Y1) that avoids sensitization (for example, austenitic stainless steel). For this reason, it is possible to suppress the formation of a Cr-deficient layer in the vicinity of the grain boundary of the crystal grains on the surface of the structure (for example, the shroud 14) that has been heated and cooled. Also from this, the radiation-induced stress corrosion cracking resistance of the structure (for example, the shroud 14) can be improved.

  (3) As shown in FIG. 2, a peening head (shot peening head 39, laser peening head 44, water jet peening head 51) used in the peening process in the surface processing process, and a heating head 37 used in the heating process. The (laser head 67, infrared heating lamp 70, heating coil 77, combustion gas radiation head 79) uses a rail 41 or the like, for example, while maintaining a certain distance from the surface of the structure (for example, the shroud 14). , Move (scan) at a constant speed. As a result, the amount of plastic strain in the surface processing step and the amount of heating in the heating step can be made uniform on the surface of the structure (for example, the shroud 14). it can.

  (4) As shown in FIGS. 1 and 15, the heating area by the heating head 37 is a moving (scanning) direction A of the heating head 37, as shown by a rectangular heating area 83B and a V-shaped heating area 83C. The heating amount is set so as to be constant in the width direction B of the heating region perpendicular to. For this reason, the refinement | miniaturization layer with a uniform crystal grain can be formed in the surface of a structure (for example, shroud 14).

  As mentioned above, although embodiment of this invention was described, this embodiment is shown as an example and is not intending limiting the range of invention. This embodiment can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the spirit of the invention. It is included in the scope and gist of the invention, and is included in the invention described in the claims and the equivalent scope thereof.

  For example, in the present embodiment, the structure is described as being composed of austenitic stainless steel, but the present invention can also be applied to a structure composed of nickel-base alloy, martensitic stainless steel, or the like. In this embodiment, the case where the structure is a nuclear reactor nuclear reactor structure has been described. However, the structure may be a structure such as a spacecraft that exists in space or flies in space.

  DESCRIPTION OF SYMBOLS 10 ... Surface modification method, 11 ... Boiling water reactor, 14 ... Shroud (structure) 34 ... Refined layer, 36 ... Processing head, 37 ... Heating head, 39 ... Shot peening head, 40 ... Projection material, 41 ... Rail, 44 ... Laser peening head, 51 ... Water jet peening head, 67 ... Laser head, 70 ... Infrared heating lamp, 77 ... Heating coil, 79 ... Combustion gas radiation head, 83B, 83C ... Heating region, 84B, 84C ... Heat distribution, A ... movement (scanning) direction, B ... width direction, P ... TTS curve, X1 ... temperature increase rate, Y1 ... temperature decrease rate.

Claims (11)

A surface processing step of performing processing with plastic deformation on the surface of the structure composed of a metal material;
A heating step of heating the work surface processed by the surface processing step at a temperature rising rate that avoids sensitization of the metal material;
By sequentially performing a cooling step for cooling the surface to be heated heated by the heating step at a temperature lowering rate that avoids sensitization of the metal material,
A method for modifying a surface of a structure, wherein a refined layer of crystal grains is formed in a depth region of at least 100 μm from the surface of the structure. 2. The surface modification method for a structure according to claim 1, wherein the metal material is austenitic stainless steel, martensitic stainless steel, or a nickel-based alloy. The method for surface modification of a structure according to claim 1 or 2, wherein the surface processing step is at least one selected from the group consisting of peening and machining. 4. The surface modification method for a structure according to claim 3, wherein the peening is a step of performing at least one selected from the group consisting of shot peening, laser peening, and water jet peening. . 5. The surface modification of a structure according to claim 3, wherein the peening head used in the peening process moves at a constant speed while maintaining a constant distance with respect to the surface of the structure. Method. 5. The surface modification method for a structure according to claim 4, wherein the shot peening is performed a plurality of times, and a projection material having a different particle size is used each time. The surface modification method for a structure according to claim 3, wherein the machining is at least one selected from the group consisting of hammering, cutting, and grinder processing. The structure according to claim 1, wherein the heating step is a step of performing at least one selected from the group consisting of laser irradiation heating, infrared heating, high frequency induction heating, and combustion gas irradiation heating. Surface modification method. 9. The surface modification method for a structure according to claim 8, wherein the heating head used in the heating step moves at a constant speed while maintaining a constant interval with respect to the surface of the structure. 10. The surface modification of a structure according to claim 9, wherein, in the heating step, the heating area by the heating head has a constant amount of heating in the width direction of the heating area perpendicular to the moving direction of the heating head. Quality method. The method for surface modification of a structure according to claim 1 or 2, wherein the cooling step is a step of performing at least one selected from the group consisting of water cooling, oil cooling, gas cooling, and air cooling.