JP3373638B2 - Laser peening method - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser peening method for changing a stress state of a metal surface in a transparent liquid to a predetermined value.

[0002]

2. Description of the Related Art A core structure of a light water cooling type nuclear reactor is made of a material such as austenitic stainless steel or nickel base alloy having sufficient corrosion resistance and high temperature strength under high temperature and high pressure water environment.

However, non-replaceable members are exposed to a high temperature and high pressure environment for a long period of time due to long-term operation of the plant, and the core materials such as shrouds are irradiated with neutrons, which causes them. There is concern about the problem of material deterioration that occurs. Particularly in the vicinity of the welded portion of the in-core structure, the material is sensitized and the residual tensile stress is formed by the heat input of the welding, so that there is a potential risk of stress corrosion cracking.

In recent years, various material surface modification techniques have been developed as countermeasures for preventive maintenance techniques in response to the extended operation period of plants. As part of this, a countermeasure construction method is being developed to prevent stress corrosion cracking by positively changing the surface residual stress from tensile to compression. For example, surface residual stress improvement techniques have been developed by methods such as shot peening or water jet peening.

In shot peening, a steel ball of about 0.3 to 1.2 mm is accelerated by using high-pressure air or centrifugal force, and the kinetic energy of the steel ball plastically deforms the surface of the working portion to form a compressive residual stress on the surface. It is a technology.

In addition, water jet peening requires 10
This is a technique to form a compressive residual stress on the surface by spraying ultra-high pressure water of about 00 atm from the nozzle tip and a shock wave when the water hammer action and cavitation break. Both of them have been proved to be effective against stress corrosion cracking by construction in water and have been partially put into practical use.

On the other hand, as a conventional surface treatment method for a metal material using a laser, quenching by heating, annealing, glazing (amorphization) by surface melting and solidification, alloying (alloy layer formation), and cladding (high Melting point material coating), and impact hardening due to instantaneous evaporation. The purpose of each of these is to control the surface hardness of the material, and to improve wear resistance, impact resistance, corrosion resistance and the like.

In order to check the integrity of the equipment installed between the shroud installed in the reactor upper chamber of the light water cooled reactor and the reactor pressure vessel and the surface of both, the mast or the tip of the rod. A device equipped with an underwater television camera is used for visual inspection, but work to change the surface stress state of the reactor internals has not been performed.

Further, an underwater swimming type inspection robot for visual inspection having a higher degree of freedom is used. However, this is also for visual inspection, and work to change the surface stress state of the structure has not been performed.

The basic principle of a conventional shot peening method using a laser will be described with reference to FIGS. 4 (a) and 4 (b).

FIG. 4A schematically shows the moment when the work piece 2 placed in the transparent liquid 1 is irradiated with the pulsed laser light 3. The transparent liquid 1 used here may be any as long as it is transparent to the wavelength of the pulsed laser light 3.

At the moment of irradiation, the pulsed laser beam 3 is absorbed on the surface of the workpiece 2, and only the pole surface is instantly heated and rapidly vaporized to generate high-temperature and high-pressure plasma 4. Due to the instantaneous ejection of the high temperature plasma 4, an impact force 5 is applied to the workpiece 2 as a reaction force thereof. It is a basic understanding of the phenomenon that the surface of the workpiece 2 is compressed and plastically deformed by the impact force 5.

FIG. 4B shows that the irradiation of the pulsed laser beam 3 is finished and the surface of the workpiece 2 is plastically deformed. As a result, compressive residual stress 6 is applied to the surface of the workpiece 2.

In the place where this phenomenon occurs, the transparent liquid 1 has an effect of confining the generated plasma 4 by the inertial force of the transparent liquid 1. In the transparent liquid 1, an impact force of several tens of times or more can be obtained as compared with the case of irradiation in air or vacuum. In addition, laser light 3 is generated by the cooling action of the transparent liquid 1.
It is possible to minimize the thermal effect of the irradiation of.
In this way, compressive stress can be left on the surface of the workpiece 2.

FIG. 5 shows the surface phenomenon depending on the power density of the laser light and the irradiation time. The power density is plotted on the vertical axis and the pulse width is plotted on the horizontal axis. From FIG. 5, it is understood that processing such as micro-removal, drilling, cutting, and heat treatment can be performed by selecting the pulse width and power density of the laser light.

FIG. 6 shows the energy range of various laser processings, in which the power density is plotted on the vertical axis and the irradiation time is plotted on the horizontal axis. Also from FIG. 6, impact hardening, drilling, glazing, welding cutting and quenching can be performed by selecting the relationship between the power density and the irradiation time.

[0017]

When using shot peening to improve the surface residual stress of a structure in a narrow part such as an annulus part between a shroud and a reactor pressure vessel, it is difficult to completely recover shots. In addition, when the shot peening operation is performed in the atmosphere, dust is generated, which makes the operation difficult.

When the work for changing the surface stress state of a structure is performed by using water jet peening, a jet reaction force is generated. Therefore, an automatic machine for changing the precise surface stress state by remote control in a narrow place is used. There is a problem that needs to be developed and that development is difficult.

When the amount of heat input to the work piece due to laser irradiation is large, the surface of the work piece may be melted or the metal structure may be changed, so that tensile residual stress may be formed on the surface instead of compression. is there. Further, in order to give a sufficient amount of compressive residual stress to the surface of the work piece, it is necessary to secure the density of plasma per unit time and unit area.

Therefore, in the case of directly irradiating the workpiece with the laser beam, there are problems that the irradiation conditions are limited depending on the material of the workpiece, and strict control of the processing conditions is required.

The present invention has been made in order to solve the above-mentioned problems, and it is not necessary to collect shots, there is no working environment for generating dust due to shots, and residual stress on the surface of a metal material that does not require reaction force compensation. An object of the present invention is to provide a highly reliable laser peening method capable of changing the state and improving the efficiency of processing.

[0022]

The invention according to claim 1 is
It is transparent to a work piece with a transparent liquid on the surface.
Pulse laser on the surface of the work piece by transmitting liquid
By irradiating the light, characterized in that to the residual compressive stress to the surface. In the invention according to claim 2, the pulse width of the pulsed laser light is 10 psec to 1 μsec or less, and the energy E per pulse of the laser light, the pulse time width t, and the laser spot area S on the workpiece are It is characterized in that the condition of 10 ⁶ <E / (t × S) <10 ¹² [w / cm ² ] is satisfied. The invention according to claim 3 is
It is characterized in that an optical device capable of condensing a laser beam is used to control the laser spot area on the workpiece within a limited range. The invention according to claim 4 is characterized in that a voltage is applied between the electrode and the workpiece, which are arranged to face the irradiation surface of the workpiece. The invention according to claim 5 is characterized in that an electrolytic solution is used as the transparent liquid . The invention according to claim 6 is characterized in that a coating layer that does not transmit laser light is formed on the surface of the workpiece, and the laser light is irradiated from above the coating layer. According to a seventh aspect of the present invention, stainless steel is used for the workpiece, and as the pretreatment for forming the coating layer, the oxide film is removed to expose the stainless steel base material layer, and then the coating layer is formed. It is characterized by irradiating laser light. According to an eighth aspect of the present invention, when a stainless steel is used for the work piece, the coating layer is a low melting point metal of Al, Sn, In, W,
It is made of at least one selected from the group consisting of refractory metals such as Mo, Ta, Nb, Ti, Zr, and Hf or noble metals such as platinum, gold, and palladium, and the coating layer has a thickness of 0.05 To 0.2 mm, 0.03 for refractory metals
To 0.15 mm, and in the case of precious metals, the range is 0.03 to 0.25 mm. In the invention according to claim 9, when the work layer is made of a material other than stainless steel and the coating layer is made of the same material as the work material, the thickness of the coating layer is selected in the range of 0.2 to 0.25 mm. It is characterized by being done.

[0023]

[Function] In the present invention, the pulse width of the pulsed laser light is 10 ps
When ec is 1 μsec or more, energy per pulse E of the pulsed laser light, pulse time width t, and laser spot area S on the workpiece, 10 ⁶ <E / (t ×
S) It is desirable to satisfy the condition of <10 ¹² [w / cm ² ].

The reason is the condition necessary for compressing the residual stress of the workpiece. If this condition cannot be satisfied,
In some cases, sufficient compression may not be obtained as surface residual stress, or only tensile stress due to thermal effects may remain.
Detailed irradiation conditions are determined by the target material, the required residual stress intensity, the depth that causes the stress change, the allowable thickness of the heat-affected zone, and the like.

In order to obtain the above-mentioned irradiation conditions, an optical device capable of converging laser light for the purpose of changing the laser spot area S on the workpiece is provided, and electrodes facing each other are provided near the laser light irradiation surface. A voltage of several tens to several hundreds of V synchronized with a direct current or a laser pulse is applied between the workpieces.

When the work of changing the surface stress state of the work piece is performed by using the laser beam, the purpose of increasing the generation efficiency of plasma on the surface of the work piece when operating the pulsed laser and It is effective in processing to form a coating layer that does not transmit laser light for the purpose of reducing the influence of heat.

By forming a coating layer on the processed surface of the work piece, the thermal effect on the material is suppressed as much as possible, plasma is generated more effectively, and the residual stress of the work piece is improved by the plasma confinement effect in water. Can be processed efficiently and with high reliability.

By irradiating a laser after forming the coating layer or directly on the oxide film, the processing margin can be secured and the processing reliability can be improved. Further, when the laser is directly irradiated on the oxide film, the processing time can be greatly shortened.

By doing so, a compressive residual stress can be applied to the surface of the workpiece by the impact force generated by the laser light irradiation. Further, since the laser beam is used, it is possible to limit the processed portion in detail. In addition, since no reaction force is generated during work, an unnecessary load is not applied to the operating device and the strength of the device does not need to be increased, and a device that is easy to handle can be obtained and work efficiency can be improved.

Further, by using the thermally and chemically stable water as the transparent liquid, it becomes possible to more reliably apply the compressive residual stress to the surface of the workpiece, and the radiation shielding effect of water is provided. Safe processing is possible even when radioactive material is used as the work piece.

By limiting the laser irradiation conditions, compressive stress can be reliably left on the workpiece.

Even if the laser output per pulse is not sufficient by reducing the irradiation area by the lens or the condenser mirror, the irradiation can be performed under the condition that the compressive stress can remain.

Depending on the laser irradiation conditions such as the pulse width not being sufficiently short, tensile stress due to heat may remain in the surface layer of the workpiece, but when the thickness of this surface layer is unacceptable. The heat-affected zone can be removed by the laser-controlled discharge between the laser light irradiation surface of the workpiece and the electrodes installed in the vicinity of the laser light irradiation surface, and the surface with residual compressive stress can be reliably obtained.

Similarly, a transparent liquid is used as an electrolytic solution, and electropolishing is performed between the laser-irradiated surface of the workpiece and the electrodes installed in the vicinity of the laser-irradiated surface, so that a surface on which compressive stress remains can be reliably obtained. be able to.

By forming a coating layer on the surface to be processed, the thermal influence on the object to be processed can be suppressed as much as possible to generate plasma more effectively, and the residual stress on the surface of the object to be processed can be improved by the plasma confinement effect in water. Can be performed efficiently and with high reliability.

When the laser beam is directly applied to the processed part, the irradiation conditions are limited depending on the material of the work piece, and strict control of the processing condition is required. On the other hand, laser light having an appropriate thickness is used. By forming a coating layer that does not permeate through, the coating layer material not only serves as a plasma supply source,
It also has the effect of reducing the heat input to the material of the processing part as much as possible, which is advantageous in actual machine processing because it widens the processing margin.

The material of the coating layer does not transmit laser light,
That is, any material may be used as long as it is an absorbing material, such as an insulator, a semiconductor, or a metal, but metal is advantageous because it is necessary to form a very thin coating layer on the surface of the processed portion. As a forming method, a metal thin film may be attached with an adhesive, physical vapor deposition, coating, or any other method.

In practice, when laser peening is applied as a preventive maintenance method, the surface of the reactor internal structure is covered with an oxide film due to long-term operation of the plant. In order to form the coating layer with a uniform film thickness, it is necessary to remove the oxide film by means of grinding or the like in consideration of the adhesion of the coating layer and the like.

When the metal thin film is a low melting point metal such as A1, Sn and In, irradiation is performed by applying a voltage of 0.05 to 0.2 mm and a voltage of tens to hundreds of volts between the workpiece and the counter electrode. As a result, the thermal effect on the material is suppressed to a minimum, and since the coating layer material is turned into plasma, the coating layer material does not remain on the surface of the processed portion after laser irradiation and the coating material removal step after processing can be omitted. Can be simplified.

Similarly, the coating layer materials are W, Mo, Ta, N.
It may be similar to a high melting point such as b, Ti, Zr, or Hf, or a furnace internal structure such as stainless steel or Inconel. In that case, the same effect can be obtained by applying a voltage of 0.03 to 0.15 mm and applying a voltage of several tens to several hundreds of volts between the workpiece and the counter electrode.

When platinum, gold, palladium, and zinc are the coating layer materials, the coating layer materials are positively left so that not only the residual stress improving effect by laser peening but also the corrosion resistance improving effect by the coating layer can be obtained. Since it can be expected, it is very effective as a countermeasure against stress corrosion cracking. In this case, the thickness of the coating layer is required to be 0.03 mm or more, but if it is 0.25 mm or more, sufficient compressive residual stress is not applied to the surface of the processed part, so the upper limit of the film thickness is 0.25 mm. .

The same effect can be obtained by using the oxide film formed on the surface of the reactor internal structure as the coating film. In that case, not only the coating layer need not be formed, but the process can be simplified, and the effect of removing the oxide film while improving the surface residual stress by direct laser irradiation can be expected.

Further, the processing is usually carried out in the atmosphere, but it is more advantageous to improve the residual stress by carrying out the processing in a medium such as water which transmits laser light. When processing in the atmosphere, high temperature and high pressure plasma generated by laser irradiation tends to diverge, but when processed in water, there is a radiation shielding effect, high stability, and a plasma confinement effect. The reaction can effectively form a residual stress in the material.

For the same purpose, instead of water, a work piece is provided in the electrolytic solution, a negative electrode is provided facing the laser light irradiation surface, and a negative electrode is synchronized between this work piece and the work piece. Electrolytic polishing is performed by applying a voltage of several V to several tens of V.

When applied to reactor internals, working in reactor water promotes the effect of improving residual stress and is effective in securing the reliability of working.

[0046]

EXAMPLE A first example of the laser peening method according to the present invention will be described with reference to FIG.

FIG. 1 is a conceptual diagram of a laser peening apparatus for leaving a compressive stress on the surface of a workpiece 2 installed in a water tank 7 filled with a transparent liquid 1. The pulsed laser light 3 emitted from the pulsed laser device 8 is
It is transmitted to the moving reflecting mirror 10 by the reflecting mirror 9. The laser light 3 reflected by the movable reflecting mirror 10 is applied to the workpiece 2.

At this time, the movable reflecting mirror 10 moves so as not to irradiate the same surface of the workpiece 2 for each pulse irradiation and to control the processing position. However, although this example shows movement in only one dimension, it is possible to perform two-dimensional processing or processing on the workpiece 2 having a curved surface by using a plurality of movable reflecting mirrors 10 as needed. Will also be possible.

Further, in this example, the case where the laser light 3 is transmitted by the structure of the reflecting mirrors 9 and 10 is shown, but a transmission optical system such as one or a bundle of optical fibers may be used. The same effect can be obtained.

A relationship shown in FIG. 5 is known between the pulse width and the power density of laser light capable of leaving a compressive stress on the surface of the workpiece 3. It is shown that impact hardening occurs when the pulse width is 1 μsec or less and the power density is 10 ⁸ (w / cm ² ) or more.

In this impact hardening, as shown in FIG. 4A, when the surface of the work piece 2 is irradiated with the pulsed laser beam 3, plasma 4 is shocked to generate a shock force 5 of the work piece 2.
This is because it acts on the surface of. Lasers that satisfy the conditions for generating this impact hardening include glass laser, Y
There are AG laser, copper vapor laser, excimer laser, etc.

The effect of laser shock hardening can be enhanced by providing glass, transparent liquid or the like on the surface of the workpiece 2. When water is used as the transparent liquid, the near-infrared to near- ultraviolet pulsed laser light 3 having good water permeability is desirable. The blue pulsed laser light 3 of a copper vapor laser is suitable for long-distance transmission in water. A harmonic wave of YAG laser light can be used.

Next, the operation and effect of the first embodiment will be described. A visible laser capable of obtaining a giant pulse is used as a laser, and a SUS304 steel plate material having a thickness of 10 mm (a thickness at which the material is not deformed by laser irradiation) is placed in a container filled with pure water as a transparent liquid.

The surface is irradiated with a laser having a laser beam diameter of 6 mmφ, a pulse width of about 5 nsec and a pulse energy of about 200 mJ (peak intensity E / (S × t) = 140 MW / cm).
² ) Then, when the irradiated surface is electrolytically polished to a depth of about 5 μm and the surface residual stress is measured using an X-ray residual stress measuring device,
A maximum residual compressive stress of about -50 MPa is obtained.

[0055] In the first embodiment, the laser beam focal length 50mm convex lens focused the beam diameter 0.5Fai,
Laser irradiation (Peak intensity E / (S × t) = 5GW / cm
² ) Then, when the irradiated surface is electrolytically polished to a depth of about 5 μm and the surface residual stress is measured using an X-ray residual stress measuring device,
A maximum residual compressive stress of about -450 MPa is obtained.

In the first embodiment, when the laser device 8 has a pulse width of about 8 psec and the test piece is irradiated with laser (peak intensity E / (S × t) = 1.3 TW / cm ² ), water Laser light scattering occurs due to breakdown, but when the surface to be irradiated is electrolytically polished to a depth of approximately 5 μm and the surface residual stress is measured using an X-ray residual stress measuring device, a maximum residual compressive stress of approximately −680 MPa is obtained. To be

Further, 10 psec <pulse width of laser light <
1 μsec, 10 ⁶ W / cm ² <E / (S × t) <10 ¹² W /
With a laser irradiation condition of cm ² , a compressive residual stress can be formed on the material surface in a laser transparent liquid.

When the heat-affected zone is formed on the surface layer with a thickness exceeding the allowable range, the compressive residual stress 6 can be formed on the surface of the workpiece 2 by removing it by electrolytic polishing.

Next, a second embodiment of the present invention will be described with reference to FIG. In the second embodiment, a discharge electrode is provided in the vicinity of the laser irradiation surface in the first embodiment, and the heat-affected zone that occurs at the same time as the stress improving process by laser irradiation is removed by electric discharge.

The basic structure of the second embodiment is the same as that of the first embodiment, but the electrode 11 is installed facing the workpiece 2 with a distance of several tens of μm. The electrode 11 has a ring shape so as not to obstruct the optical path of the laser light 3. A power supply 12 is connected between the electrode 11 and the workpiece 2 to provide a potential difference. A variable voltage DC power supply is used as the power supply 12.

Next, the operation of the second embodiment will be described. When the work piece 2 is irradiated with the pulsed laser light 3 as in the first embodiment, high-temperature high-pressure plasma 4 is generated on the irradiation surface as shown in FIG. A compressive stress can be left on the surface of the workpiece 2 by the impact force 5 on the high-pressure plasma 4,
Electric discharge induced by the plasma 4 is generated between the electrode 11 and the workpiece 2, and the heat-affected zone is removed. (See FIG. 4B) Next, the effect of the second embodiment will be described. A visible laser capable of obtaining a giant pulse is used as a laser, and a SUS304 steel plate material having a thickness of 10 mm (a thickness at which the material is not deformed by laser irradiation) is placed in a container filled with pure water as a transparent liquid.

The laser beam diameter is 0.5 mm on the surface.
A laser of φ (focused with a plano-convex lens having a focal length of 50 mm) is irradiated (peak intensity E / (S × t) = 5 GW / cm ² ).

At the same time, when an applied voltage of -200 V is applied to the electrode 8, the surface residual stress of the irradiation surface is measured using an X-ray residual stress measuring device, and a maximum compressive residual stress of about -600 MPa is obtained. .

In the second embodiment, an electrolytic solution was used instead of pure water, the electrode 11 was placed 5 mm away from the workpiece 2, an applied voltage of -5 V was applied to the electrode 11, and laser irradiation was performed. (Peak intensity E / (S × t) = 5 GW / cm ² ).
When the surface residual stress of the irradiation surface is measured using an X-ray residual stress measuring device, a maximum compressive residual stress of about -600 MPa can be obtained.

Next, a third embodiment of the present invention will be described with reference to FIG. In the third embodiment, as shown in FIG. 3, a coating layer 13 of, for example, an aluminum film of about 0.1 mm is formed by physical vapor deposition on the surface of the workpiece 2 of the first embodiment shown in FIG. It is to improve the surface stress by irradiation.

The operation and effect of the third embodiment will be described.

A YAG laser was used as a laser, and a transparent liquid was filled with pure water.
mm (thickness that does not cause material deformation by laser irradiation)
SUS304 steel plate material (aluminum film thickness of about 0.1 mm is formed by physical vapor deposition) is installed, and a laser with a pulse width of about 5 nsec and pulse energy of about 200 mJ is focused on the surface by a convex lens with a focal length of 50 mm. And irradiation (peak intensity E / (S × t) = 5 GW / cm ² ).

[0068] If the surface residual stress of the irradiated surface to measure with an X-ray residual stress measuring device, a maximum of about - residual compressive stress of 540MPa is obtained. In the part that received the laser irradiation,
The aluminum film formed by vapor deposition does not exist due to evaporation.

In the third embodiment, a titanium film of about 0.25 mm is formed by physical vapor deposition on the surface of the work piece 2 (Inconel 600) of the first embodiment, and laser irradiation is performed to improve the surface stress. You can

The operation and effect of the modification of the third embodiment will be described. Using a YAG laser as the laser, a container with a depth of 1 m filled with pure water as the transparent liquid, and a thickness of 10
mm (thickness that does not cause material deformation by laser irradiation)
Inconel 600 steel plate material (a titanium film with a thickness of about 0.25 mm is formed by physical vapor deposition) is installed.

The pulse width of the laser is about 5 nse on the surface.
c, laser with pulse energy of about 200 mJ, focal length 5
Converging and irradiating with a 0 mm convex lens (peak intensity E /
(S × t) = 5 GW / cm ² ).

[0072] If the surface residual stress of the irradiated surface to measure with an X-ray residual stress measuring device, a maximum of about - residual compressive stress of 430MPa is obtained. In the part that received the laser irradiation,
The titanium film formed by vapor deposition does not exist due to evaporation.

The operation and effect of the above modification of the third embodiment will be described. Using a YAG laser as the laser, in a container with a water depth of 1 m filled with pure water as the transparent liquid,
10 mm thick (thickness that does not cause deformation of the material due to laser irradiation), 50 mm square SUS304 steel plate material (thickness by physical vapor deposition is approximately
A 0.1 mm aluminum film is formed), and the one with bead-on-plate welding (formation of tensile residual stress of 350 MPa) is installed in the center with both ends restrained.

The pulse width of the laser is about 5 nse on the surface.
c, laser with pulse energy of about 200 mJ, focal length 5
Converging and irradiating with a 0 mm convex lens (peak intensity E /
(S × t) = 5 GW / cm ² ).

[0075] If the surface residual stress of the irradiated surface to measure with an X-ray residual stress measuring device, a maximum of about - residual compressive stress of 450MPa is obtained. In the part that received the laser irradiation,
The aluminum film formed by vapor deposition does not exist due to evaporation.

When such a test piece is immersed in boiling 42% magnesium chloride for 72 hours and subjected to a stress corrosion cracking test, stress corrosion cracking occurs in the welded portion not irradiated with laser. There is no cracking in the broken part.

In the third embodiment, a titanium film of about 0.25 mm is formed as a coating layer 13 by physical vapor deposition on the surface of the workpiece 2 (Inconel 600) in the vicinity of the welding bead where tensile residual stress is formed, and the laser is applied. To improve the surface stress.

A YAG laser was used as a laser, and a transparent liquid was filled with pure water.
mm (thickness that does not cause deformation of material due to laser irradiation), 50 mm square Inconel 600 steel plate material (tin film with a thickness of about 0.15 mm is formed by physical vapor deposition) with both ends restrained, with a bead at the center On-plate welding (formation of tensile residual stress of 300 MPa) is installed.

The pulse width of the laser is about 5 nse on the surface.
c, laser with pulse energy of about 200 mJ, focal length 5
Converging and irradiating with a 0 mm convex lens (peak intensity E /
(S × t) = 5 GW / cm ² ).

[0080] If the surface residual stress of the irradiated surface to measure with an X-ray residual stress measuring device, a maximum of about - residual compressive stress of 560MPa is obtained. In the part that received the laser irradiation,
The tin film formed by vapor deposition does not exist due to evaporation.

Next, a fourth embodiment of the present invention will be described with reference to FIG. The third embodiment is to affix a 0.1 mm SUS304 foil as a coating layer 13 on the surface of the workpiece 2 of the first embodiment with an adhesive and irradiate a laser to improve the surface stress. .

Next, the operation and effect of the fourth embodiment will be described. A YAG laser is used as the laser, and a container with a water depth of 1 m filled with pure water as the transparent liquid has a thickness of 10 mm (thickness that does not cause deformation of the material due to laser irradiation) SUS3.
04 Install a steel plate (attach SUS304 foil on the surface with an adhesive).

The pulse width of the laser is about 5 nse on the surface.
c, laser with pulse energy of about 200 mJ, focal length 5
Converging and irradiating with a 0 mm convex lens (peak intensity E /
(S × t) = 5 GW / cm ² ).

[0084] If the surface residual stress of the irradiated surface to measure with an X-ray residual stress measuring device, a maximum of about - residual compressive stress of 400MPa is obtained. In the part that received the laser irradiation,
The SUS304 foil attached with the adhesive does not exist after evaporation.

Next, a fifth embodiment of the present invention will be described.
The fifth embodiment is to form an oxide film as a coating layer 13 on the surface of the workpiece 2 (SUS304) of the first embodiment and irradiate a laser to improve the surface stress.

The operation and effect of the fifth embodiment will be described. A YAG laser is used as the laser, and a container with a depth of 1 m filled with pure water as the transparent liquid is a SUS304 steel plate with a thickness of 10 mm (a thickness that does not cause deformation of the material due to laser irradiation) (dissolved oxygen content 10 ppm, conductivity). Place an oxide film on the surface by dipping in 0.2 μS / cm, 288 ℃ high temperature pure water for 500 hours.

On its surface, the pulse width of the laser is about 5 nse.
c, laser with pulse energy of about 200 mJ, focal length 5
Converging and irradiating with a 0 mm convex lens (peak intensity E /
(S × t) = 5 GW / cm ² ).

[0088] If the surface residual stress of the irradiated surface to measure with an X-ray residual stress measuring device, a maximum of about - residual compressive stress of 320MPa is obtained. The oxide film in the portion irradiated with the laser is evaporated and removed.

[0089]

According to the present invention, it is possible to change the stress state of the metal surface without the need for reaction force compensation, and it is possible to improve the processing efficiency and reliability. Further, by limiting the thickness of the coating layer formed on the surface of the workpiece during laser peening, the step of removing the coating layer after laser irradiation can be omitted. Further, when applied to the in-furnace internal structure, the oxide film on the surface of the in-furnace internal structure can be removed by laser irradiation, and at the same time, the stress improving treatment can be performed with plasma generated by evaporation of the oxide coating layer.

[Brief description of drawings]

FIG. 1 is a conceptual diagram showing a laser peening apparatus for explaining a first embodiment of a laser peening method according to the present invention.

FIG. 2 is a conceptual diagram of an apparatus for explaining a second embodiment of the laser peening method according to the present invention.

FIG. 3 is an apparatus conceptual diagram for explaining a third embodiment of the laser peening method according to the present invention.

FIG. 4A is a schematic diagram for explaining the principle of a conventional laser peening method, and FIG. 4B shows a state in FIG. 4A in which laser irradiation is completed and a compressive residual stress is applied to the surface of the workpiece. Pattern diagram.

FIG. 5 is a characteristic diagram showing a surface phenomenon depending on the power density of laser light and irradiation time.

FIG. 6 is a characteristic diagram showing energy ranges of various laser processes.

[Explanation of symbols]

1 ... Transparent liquid, 2 ... Workpiece, 3 ... Pulse laser light,
4 ... Plasma, 5 ... Impact force, 6 ... Compressive residual stress, 7 ... Water tank, 8 ... Pulse laser device, 9 ... Reflecting mirror, 10 ... Moving reflecting mirror, 11 ... Electrode, 12 ... Power supply, 13 ... Coating layer.

─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Shigehiko Mukai 8 Shinsita-cho, Isogo-ku, Yokohama-shi, Kanagawa Toshiba Corporation Yokohama office (72) Minoru Obata Shin-Sugita-cho, Isogo-ku, Yokohama, Kanagawa Prefecture Toshiba Corporation Inside the Yokohama Works (72) Inventor Katsuhiko Sato 8 Shinsita-cho, Isogo-ku, Yokohama-shi, Kanagawa Inside the Yokohama Works, Toshiba Corporation (56) Reference JP-A-6-57335 (JP, A) JP-A-3-72092 (JP) , A) JP 55-54524 (JP, A) JP 63-38565 (JP, A) JP 5-80187 (JP, A) JP 5-65530 (JP, A) JP 1-162720 (JP, A) JP 63-243217 (JP, A) JP 6-81153 (JP, A) JP 6-884 (JP, A) JP 55-82780 (JP, A) JP-A-3-99790 (J , A) (58) investigated the field (Int.Cl. ^7, DB name) B23K 26/00 B23K 26/12 B23K 26/18 C21D 7/06

Claims (9)

(57) [Claims] 1. A work piece having a surface provided with a transparent liquid.
On the other hand, the transparent liquid is transmitted to the surface of the workpiece.
By applying a pulse laser beam, a laser peening method which comprises causing the residual compressive stress to the surface. 2. The pulse width of the pulsed laser light is 10 ps
1 μsec or less from ec, 10 ⁶ <E / (t × S) <10 ^{12 with} respect to energy E per pulse of laser light, pulse time width t, and laser spot area S on the workpiece.
The laser peening method according to claim 1, wherein the condition [w / cm ² ] is satisfied. 3. The laser peening method according to claim 1, wherein an optical device capable of condensing a laser beam is used to control the laser spot area on the workpiece within a limited range. 4. The laser peening method according to claim 1, wherein a voltage is applied between the electrode and the workpiece, which are arranged to face the irradiation surface of the workpiece. 5. The laser peening method according to claim 1, wherein an electrolytic solution is used as the transparent liquid . 6. The laser peening method according to claim 1, wherein a coating layer that does not transmit laser light is formed on the surface of the workpiece, and the laser light is irradiated from above the coating layer. 7. A stainless steel is used for the workpiece,
7. The laser peening method according to claim 6, wherein as a pretreatment for forming the coating layer, the oxide film is removed to expose the stainless steel base material layer, and then the coating layer is formed and laser light is irradiated. . 8. The coating layer when stainless steel is used for the workpiece is a low melting point metal of Al, Sn, In, W, M
o, Ta, Nb, Ti, Zr, Hf, a high melting point metal, or at least one selected from noble metals such as platinum, gold and palladium. The thickness of the coating layer is 0.05 when the low melting point metal is used. To 0.2 mm, 0.03 for refractory metals
7. The laser peening method according to claim 6, wherein the range is from 0.15 mm to 0.15 mm, and in the case of noble metal, 0.03 to 0.25 mm. 9. If the coating layer is made of the same material as the workpiece when the workpiece is a material other than stainless steel, the thickness of the coating layer is selected within the range of 0.2 to 0.25 mm. The laser peening method according to claim 6, wherein