JP2017035712A - Peening processing method and turbine rotor wheel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a peening processing method in which a high residual stress can be certainly given that is of compression in such a direction that a crack is easily generated at the surface of a workpiece, and a turbine rotor wheel.SOLUTION: A peening processing method includes a preparation step S10 in which a test body having the same material as a workpiece is prepared, a first peening step S11 in which laser peening is implemented against the test body while changing the ratio of a laser irradiation density of a first scanning direction to that of a second scanning direction orthogonal to the first scanning direction, an evaluation step S12 in which a compression residual stress corresponding to each ratio of the laser irradiation density is evaluated in each of the first scanning direction and the second scanning direction, an extraction step S13 in which extracted is the ratio of the laser irradiation density in that the ratio of the residual stress in the first scanning direction to that of the second scanning direction is 0.3 to 0.7, and a second peening step S14 in which, setting the direction in that a crack is easily generated as the second scanning direction, laser peening is implemented against the surface of the workpiece with the extracted ratio of the laser irradiation density.SELECTED DRAWING: Figure 1

Description

  Embodiments described herein relate generally to a peening method for applying compressive residual stress to a surface of a workpiece and a turbine rotor wheel to which the processing method is applied.

  In general, as a method of improving the fatigue strength of structures and equipment used in industrial plants such as thermal power plants and nuclear power plants and preventing the occurrence of stress corrosion cracking, the surface of the workpiece is plastically deformed to give compressive residual stress. The peening process is widely known. Typical peening processes include shot peening and laser peening.

  Shot peening means that a hard ball (shot) such as metal or ceramic collides with the surface of the work piece at a high speed and forms a compressive stress portion where the compressive stress remains on the surface of the work piece. This is a technique for improving the surface hardening and wear resistance (for example, Patent Document 1).

  On the other hand, the laser peening process using a laser beam instead of a hard sphere generates a high-pressure plasma on the surface of the workpiece by irradiation with the laser beam, and compressive stress is applied to the surface of the workpiece by the reaction when the plasma expands. Forming part.

  In this laser peening process, it is possible to form the compressive stress part to a deeper part of the work piece, so that the surface hardening of the work piece and the improvement of the wear resistance can be expected more than the shot peening process. (For example, patent document 2).

JP-A-8-174422 JP 2011-115853 A

  By the way, in a thermal power plant, a nuclear power plant, and the like, there are many devices used in a high temperature environment, for example, there is a turbine rotor wheel 10 which is a device that affects the output of the turbine shown in FIG.

The turbine rotor wheel 10 is provided with a Christmas-like implanted portion 11 in the circumferential direction of the wheel, and a moving blade (not shown) of a gas turbine is connected to the implanted portion 11.
Gas turbines increase in temperature to about 1300 ° C. during operation, depending on their power. For this reason, the cooling hole 12 is provided in the lower part of the implantation part 11, and the inside of the gas turbine moving blade under operation is cooled by deriving the cooling gas from the cooling port 12 in the moving blade direction of the gas turbine. Is done.

  Although the corners of the cooling holes 12 are rounded, the surface of the corners is discontinuous in shape, so that a stress concentration phenomenon in which the stress partially increases occurs. For this reason, there is a possibility that a crack called hold time cracking (Hold Time Cracking) due to the stress corrosion oxidation phenomenon at the crystal grain boundary may occur in the depth direction. FIG. 11B shows an example when a crack occurs in the cooling hole 12.

  In order to prevent cracks in the cooling holes 12 or delay the generation time, it is necessary to apply a high compressive residual stress to the stress concentration portion of the cooling holes. The residual stress is caused by changes in mechanical properties due to temperature. Therefore, it is necessary to reliably apply a compressive residual stress to a device such as the cooling port 12 used in a high temperature environment in a direction in which cracks are likely to occur.

  The conventional peening treatment method does not consider a high-temperature environment or an environment in which residual stress is applied, and does not provide compressive residual stress with high accuracy in a direction in which cracks are likely to occur.

  The present invention has been made in view of such circumstances, and provides a peening treatment method and a turbine rotor wheel that can reliably apply high compressive residual stress in a direction in which cracks are likely to occur on the surface of a workpiece. The purpose is to provide.

  In the peening processing method according to the embodiment of the present invention, a peening processing method for applying compressive residual stress to the surface of the workpiece by laser peening, the preparation step for preparing a specimen of the same material as the workpiece And a first peening step of changing the ratio of the laser irradiation density in the first scanning direction and the second scanning direction perpendicular to the first scanning direction to the test body, and the laser irradiation density. The evaluation step for evaluating the compressive residual stress corresponding to each ratio in the first scanning direction and the second scanning direction, respectively, and the ratio of the residual stress in the first scanning direction to the second scanning direction is 0. An extraction step for extracting a ratio of the laser irradiation density from 3 to 0.7, and a direction in which cracks are likely to occur with respect to the surface of the workpiece, the second scanning direction. Set, characterized in that it comprises a second peening step of performing said laser peening at a ratio of extracted said laser irradiation density, the.

  In the peening treatment method according to the embodiment of the present invention, a peening treatment method for imparting compressive residual stress to the surface of the workpiece by shot peening, the preparation step for preparing a specimen of the same material as the workpiece And a first peening step in which the shot peening is performed by changing a ratio of a shot injection density between a first injection direction and a second injection direction perpendicular to the first injection direction with respect to the specimen, and the shot injection An evaluation step for evaluating the compressive residual stress corresponding to each density ratio in each of the first injection direction and the second injection direction, and a ratio of the residual stresses in the first injection direction and the second injection direction. An evaluation step for extracting a ratio of the shot spray density in which 0.3 is 0.7, and a direction in which cracks are likely to occur with respect to the surface of the workpiece. It is set to a second injection direction, and a second peening step of carrying out the shot peening at a ratio of extracted said shot injection density, characterized in that it comprises a.

  In the turbine rotor wheel according to the embodiment of the present invention, the cooling port formed in the lower part of the Christmas-like implantation portion that is arranged in the circumferential direction of the turbine rotor wheel and to which the moving blades of the gas turbine are connected is the above peening method. It is characterized by being applied to the surface of the corner portion of

  The embodiment of the present invention provides a peening treatment method and a turbine rotor wheel that can reliably apply a residual stress with high compression in a direction in which cracks are likely to occur on the surface of a workpiece.

The flowchart of the peening processing method which concerns on this embodiment. (A) is a figure which shows a block-shaped test body, (B) is explanatory drawing which shows the laser irradiation surface of a test body, (C) is explanatory drawing which shows an example of the scanning of a laser. The graph which shows the surface residual stress of each of the x direction and y direction provided by laser peening. The graph which shows the relationship between the ratio of the surface residual stress of the x direction and y direction provided by laser peening, and the ratio of the residual stress of the y direction with respect to an equivalent stress. The graph which shows the relationship between the ratio of the laser irradiation density of ax direction and ay direction, and the ratio of the surface residual stress of ax direction and ay direction. (A) is explanatory drawing which shows the case where the peening processing method concerning this embodiment is applied to the cooling hole of a turbine rotor wheel, (B) is an arrow line view from the A direction of FIG. 6 (A). (A) is explanatory drawing which shows the case where the peening processing method concerning this embodiment is applied to the welding part of piping of a reactor internal structure, (B) is the elements on larger scale of a welding part. The test result which shows the time transition of the surface residual stress before and behind heat processing when the test body performs the laser peening process concerning this embodiment. The graph which shows the relationship between the surface residual stress (equivalent stress) at the time of raising temperature, and an elastic limit. The graph which extrapolated the prediction result of the residual stress after the heat processing derived | led-out by the calculation formula to the test result shown in FIG. (A) is the schematic which shows a turbine rotor wheel, (B) is the elements on larger scale which show the case where the crack generate | occur | produced in the cooling hole of the turbine rotor wheel.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.
The peening treatment method according to the present embodiment uses laser peening, for example, thermal power such as turbine rotor wheels and piping of reactor internals, and residual compression on the surfaces of various devices used in industrial plants such as nuclear power plants. This is a surface modification method for a workpiece to which stress is applied.

  FIG. 1 shows a flowchart of a peening processing method according to the present embodiment. In the following description, the first scanning direction of the laser beam is defined as the x direction, and the second scanning direction orthogonal to the first direction is defined as the y direction. The x direction is arbitrarily set for a workpiece to be peened.

  In preparation step S10, a plurality of specimens made of the same material as the workpiece are prepared. Since the turbine rotor wheel and reactor internal structure that are the workpieces are mainly composed of all-stainless stainless steel or nickel-base alloy, the specimen is made of the same material as these materials. . FIG. 2A illustrates one of the produced block-shaped specimens.

  In the first peening step S11, laser peening is performed on the prepared specimen in two scanning directions, the x direction and the y direction. The x direction is arbitrarily set with respect to the specimen.

  Laser peening irradiates a pulse laser having a diameter of 1 mm or less with water on the surface of the specimen. By this laser irradiation, the evaporation of the evaporated metal is hindered by water, so that an impact force acts on the surface of the metal and a residual stress in the compression direction is applied to the surface.

As a laser to be used, a YAG laser fundamental wave having a wavelength of 1064 nm or a harmonic having a YAG laser having a wavelength of 532 nm made half the fundamental wavelength can be applied. Further, the peak power of the laser beam is set to be, for example, 1 to several tens GW / cm ^{2 on} the surface of the irradiation target.

  FIG. 2B is a diagram showing a laser irradiation surface when laser peening is performed on the surface of the specimen in two scanning directions of the x direction and the y direction, and FIG. 2C shows the laser to be scanned. An example of a beam is shown. As shown in FIG. 2C, the surface of the specimen is irradiated with the laser beam by alternately changing the two scanning directions of the x direction and the y direction.

  In the first peening step S11, laser peening is performed on each of the specimens by fixing the laser irradiation area and changing the ratio of the laser irradiation density in the x direction and the laser irradiation density in the y direction. As a result, a plurality of test bodies that are laser-peened by changing the ratio of the laser irradiation density in the two directions are produced. The laser irradiation density is obtained, for example, by the number of pulse laser irradiations per unit area.

  In the evaluation step S12, the compressive residual stress corresponding to each ratio of the laser irradiation density is evaluated for each of the x direction and the y direction using the manufactured specimen. Residual stress in two directions on the surface of the specimen is measured by X-ray diffraction.

  FIG. 3 shows an example when the residual stress on the surface is measured by the X-ray diffraction method after the laser peening treatment on the specimen. As shown in FIG. 3, when laser peening is performed in two scanning directions, residual stresses in the x direction and the y direction have different stress values.

  In FIG. 3, residual stress in shot peening is shown as a comparative example. Shot peening is a process in which a large number of steel balls are applied to the surface of a material to give compressive residual stress to the surface of the workpiece. Usually, the residual stress in the x direction and the y direction on the processing surface is almost the same value. .

Here, when the residual stress in the x direction and the y direction differs depending on the direction, the stress value can be evaluated as the equivalent stress σ _eq shown in the following equation (1). σ _x indicates the residual stress in the x direction, and σ _y indicates the residual stress in the y direction. If the residual stress in the x direction and the residual stress in the y direction are the same as in shot peening, the equivalent stress is equal to the residual stress in the x direction and the y direction.

FIG. 4 shows the ratio of the residual stress σ _{x applied} by laser peening to the residual stress σ _x in the y direction and the residual stress σ _{y in the} y direction, and the equivalent stress σ _eq , calculated based on the above equation (1). It is a graph which shows the ratio of residual stress (sigma) _y of the y direction with respect to.

As shown in FIG. 4, when the ratio sigma _{x /} sigma _y of residual stress sigma _y of residual stress sigma _x and y direction of the x-direction is 0.5, the residual stress in the y-direction with respect to the equivalent stress sigma _eq sigma _y Is minimal. That is, since the change in stress is determined by the equivalent stress σ _eq , when σ _x / σ _y is 0.5, the residual stress σ _y in the compression direction in the y direction is maximized. Therefore, when laser peening is performed on the workpiece at a ratio of the laser irradiation density at which σ _x / σ _y is 0.5, the compressive residual stress σ _y is maximized in the y direction.

In the extraction step S13, the ratio of the laser irradiation density at which the ratio in the residual stress in the x direction and the y direction becomes 0.5 is extracted from the evaluation result of the compressive residual stress corresponding to each ratio of the laser irradiation density in the evaluation step S12. The Although the maximum effect is the ratio of the laser irradiation density at which σ _x / σ _y is 0.5, it can be seen that a certain effect can be obtained if 0.3 to 0.7 as can be seen from FIG. It is done.

  FIG. 5 is a graph evaluating the relationship between the ratio of the laser irradiation density in the x direction and the y direction and the ratio of the surface residual stress in the x direction and the y direction. From this evaluation result, it is possible to extract the ratio A of the laser irradiation density at which the ratio in the residual stress in the x direction and the y direction becomes 0.5.

  In the second peening step S14, the direction in which cracks are likely to occur is set in the y direction with respect to the actual surface of the workpiece, and laser peening is performed at the ratio of the extracted laser irradiation density.

  FIG. 6A is a diagram illustrating a case where the peening process according to the present embodiment is performed on the cooling hole 12 of the turbine rotor wheel 10 (see FIG. 11). FIG. 6B shows an arrow view from the A direction.

  At the corner of the cooling hole 12 where the stress is concentrated, the direction in which cracks are assumed is set in the y direction, and laser peening is performed at the ratio of the laser irradiation density extracted in the extraction step S13. Thereby, the residual stress with high compression is reliably provided in the direction in which the crack is assumed.

  FIG. 7A is a diagram illustrating a case where the peening process according to the present embodiment is performed on the welded portion 14 of the pipe 13 of the furnace internal structure. FIG. 7B shows a partially enlarged view of the welded portion 14.

  In the pipe 13, a part affected by the heat of welding is generated along the welded part 14, and stress corrosion cracking may occur along the axial direction of the pipe 13.

  In a site where stress corrosion cracking is assumed, the direction in which cracking is assumed is set in the y direction, and laser peening is performed at the ratio of the laser irradiation density extracted in extraction step S13. Thereby, the residual stress with high compression is reliably provided in the direction in which stress corrosion cracking is assumed.

  FIG. 8 is a test result showing a time transition of the surface residual stress before and after the heat treatment when the laser-peening according to the present embodiment is performed on a block-shaped test body. In addition, in FIG. 8, when shot peening is performed, the time transition of the surface residual stress before and after the heat treatment is shown as a comparative example.

In the laser peening, residual stress sigma _y of residual stress sigma _x and y direction of the x-direction becomes a different value, the residual stress sigma _y in the y direction before heating (the position of the time 0) shows a good compression value, It can be seen that high residual stress remains even after heating. On the other hand, in the shot peening, the residual stress sigma _y is approximately the same next to the residual stress sigma _x and y direction of the x-direction, the residual stress of compression both directions after heating is reduced.

  In this way, peening is evaluated in two directions, the x direction and the y direction, and the ratio of the irradiation density of the laser that increases the compressive residual stress in the y direction is evaluated in advance. Then, the direction in which there is a possibility of cracking of the workpiece (the direction obtained from past data of the crack or the direction in which cracking may be obtained by analysis) is set in the y direction and evaluated. By carrying out the peening process at an irradiation density of, a highly compressive residual stress can be reliably applied.

  In shot peening, the ratio of the shot injection density that increases the compressive residual stress in the y direction, as in laser peening, by adjusting the injection density of the shot injected into the workpiece in each of the x and y directions. It is possible to obtain the same effect as the above-described laser peening by evaluating in advance.

  Specifically, shot peening is performed on the specimen by changing the ratio of the shot injection density in the x direction and the y direction. Then, a ratio of shot injection density at which the ratio of residual stress in the x direction and y direction becomes 0.5 is extracted. And then. Shot peening is performed at the ratio of the extracted jet density, with the direction in which cracks are likely to occur is set in the y direction with respect to the actual surface of the workpiece. As a shot, it is desirable to use a steel ball having a diameter of 0.1 mm or less.

  Next, a method for predicting changes in residual stress before and after heat treatment in a high-temperature environment will be studied.

FIG. 9 is a graph comparing the surface residual stress and the elastic limit with respect to the temperature (solid line in the figure) when the temperature of the heat treatment is increased. The surface residual stress in laser peening is indicated by equivalent stress σ _eq .

As shown in FIG. 9, the equivalent stress σ _eq in laser peening is a value close to the elastic limit. That is, the change in the residual stress before and after the heat treatment can be predicted based on the elastic limit with respect to the temperature.

Further, a method for calculating in advance the temporal change of the residual stress after the heat treatment will be described.
The equation representing the change in creep strain ε _c , which is a strain dependent on time, is the following equation (2) in which the amount of change in creep strain dε _c / dt per unit time depends on the residual stress σ (equivalent stress) and time t: It is represented by Note that the constants n, m, and A in the formula (2) are derived based on the actual change in creep strain over time using a specimen made of the same material as the workpiece.

dε _c / dt = Aσ ⁿ t ^m (2)

From this equation, the following equation (3) is derived on the assumption that the decrease in elastic strain ε _e that determines the magnitude of residual stress is equal to the increase in creep strain ε _c . Furthermore, the change in residual stress is calculated by integration over time. In laser peening, it is assumed that the ratio between the residual stress σ _{x in the x} direction and the residual stress σ _{y in} the y direction does not change before and after the heat treatment. E is Young's modulus.

dσ / dt = Edε _e / dt = −Edε _c / dt = −EAσ ⁿ t ^m (3)

  FIG. 10 is a graph obtained by extrapolating the test results shown in FIG. 8 with prediction results (dotted lines in the figure) indicating the temporal change in the residual stress after heating derived from the calculation formula. As shown in FIG. 10, it can be seen that the temporal change in the residual stress after the heat treatment can be roughly predicted. Thus, by predicting the time change of the compressive stress in a high temperature environment, it can be used for maintenance work such as replacement of equipment.

  According to the peening process of the present embodiment described above, peening evaluation is performed in two directions of the x direction and the y direction, and the ratio of the irradiation density of the laser that increases the compressive residual stress in the y direction is evaluated in advance. Then, by setting the direction in which the workpiece may be cracked to the y direction and performing the peening process at the evaluated laser irradiation density, it is possible to reliably apply a high compressive residual stress.

  Furthermore, the corner part of the cooling port formed in the lower part of the Christmas-like implantation part which is arrange | positioned in the circumferential direction of the turbine rotor wheel demonstrated in background art as an example, and to which the moving blade of a gas turbine is connected is demonstrated as an example. If it is adopted for the peening treatment on the surface in the depth direction, the effects described in the above peening treatment method can be obtained with certainty.

  Although several embodiments of the present invention have been described, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the scope of the invention. These embodiments and modifications thereof are included in the scope and gist of the invention, and are included in the invention described in the claims and the equivalents thereof.

  DESCRIPTION OF SYMBOLS 10 ... Turbine rotor wheel, 11 ... Implanted part, 12 ... Cooling hole, 13 ... Piping, 14 ... Welded part.

Claims (4)

A peening treatment method for applying compressive residual stress to the surface of a workpiece by laser peening,
A preparation step of preparing a specimen of the same material as the workpiece;
A first peening step in which the laser peening is performed by changing a ratio of a laser irradiation density in a first scanning direction and a second scanning direction orthogonal to the first scanning direction to the test body;
An evaluation step for evaluating the compressive residual stress corresponding to each ratio of the laser irradiation density in each of the first scanning direction and the second scanning direction;
An extraction step of extracting a ratio of the laser irradiation density at which a ratio of residual stress in the first scan and the second scan direction is 0.3 to 0.7;
A second peening step of setting the direction in which cracks are likely to occur with respect to the surface of the workpiece as the second scanning direction and performing the laser peening at a ratio of the extracted laser irradiation density; A peening processing method comprising: A peening method for imparting compressive residual stress to the surface of a workpiece by shot peening,
A preparation step of preparing a specimen of the same material as the workpiece;
A first peening step in which the shot peening is performed by changing a ratio of a shot injection density in a second injection direction perpendicular to the first injection direction to the test body;
An evaluation step of evaluating the compressive residual stress corresponding to each ratio of the shot injection density in each of the first injection direction and the second injection direction;
An evaluation step of extracting a ratio of the shot injection density at which a ratio of residual stress in the first injection direction and the second injection direction is 0.3 to 0.7;
A second peening step of setting the direction in which cracks are likely to occur to the surface of the workpiece as the second injection direction and performing the shot peening at a ratio of the extracted shot injection density; A peening processing method comprising:   The peening treatment method according to claim 2, wherein the shot peening is performed using a steel ball having a diameter of 0.1 mm or less.   The peening treatment method according to any one of claims 1 to 3, wherein a cooling port formed in a lower portion of a Christmas-like implantation portion disposed in a circumferential direction of a turbine rotor wheel and connected to a moving blade of a gas turbine. A turbine rotor wheel characterized by being applied to a corner surface.