JP2016044335A - Surface treatment method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a surface treatment method that, when the surface of a metal member used in a high-temperature use environment is polished after being subjected to peening treatment, can maintain compressive residual stress imparting effect caused by the peening treatment.SOLUTION: A surface treatment method for treating the surface of a metal member includes: a first step of imparting a compressive residual stress to the surface of the metal member; and a second step of smoothly polishing the surface of the metal member, to which a compressive residual stress has been imparted in the first step, so that the surface roughness of the metal member becomes 1 μm or less.SELECTED DRAWING: Figure 1

Description

  Embodiments described herein relate generally to a surface treatment method.

  In general, there is a technique for imparting compressive residual stress to a metal member by performing a peening treatment on the surface of the metal member to prevent fatigue strength of the metal member and occurrence of stress corrosion cracking.

  Examples of peening processing techniques include shot peening, laser shock peening, laser peening, water jet peening, and ultrasonic peening.

JP 2011-115853 A Japanese Patent No. 4985644

  The conventional technology is a technology for applying compressive residual stress to a metal member by laser peening or ultrasonic peening. However, the surface of the member to which compressive residual stress is applied usually remains peened and the surface of the member becomes rough. There is.

  If the usage environment of the metal member is, for example, from room temperature to about 100 ° C., it is only necessary to prevent fatigue strength and stress corrosion cracking from occurring by peening, but this technology will exceed several hundred to 1000 ° C. It is considered to be applied to equipment used in a high temperature environment such as a gas turbine of a power plant. In the case of equipment used in such a high temperature environment, the surface of the metal member becomes rough. It is considered that cracks are likely to occur.

  Therefore, it is necessary to process the surface of the metal member once peened, but the effect of the compressive residual stress applied to the metal member may be lost by simply polishing the surface.

  The problem to be solved by the present invention is to maintain the effect of imparting compressive residual stress by peening when polishing the surface of a metal member after peening the metal member used in a high-temperature environment. An object of the present invention is to provide a surface treatment method capable of performing

  In the surface treatment method for a metal member, the surface treatment method of the embodiment includes: a first step of applying compressive residual stress to the surface of the metal member; and the metal member of which the compressive residual stress is applied in the first step. And a second step of smoothly polishing the surface of the metal member so that the surface roughness is 1 μm or less.

(A) is a perspective view which shows the turbine wheel as a member (process target member) of the object which performs surface treatment, (b) is the elements on larger scale of the turbine wheel of (a). It is a flowchart which shows the surface treatment method of 1st Embodiment. It is a figure which shows the internal residual stress distribution at the time of performing the surface treatment of 1st Embodiment. It is a figure which shows the change of the surface roughness of 1st Embodiment. It is a figure which shows the mode of the improvement of the creep rupture time by the 1st process of 1st Embodiment. It is a figure which shows the relationship between surface roughness and creep rupture time in 1st Embodiment. It is a figure which shows internal residual stress distribution at the time of performing the surface treatment of 2nd Embodiment. It is a figure which shows the change of the surface roughness of 2nd Embodiment.

Hereinafter, embodiments will be described in detail with reference to the drawings.
(First embodiment)
The first embodiment will be described with reference to FIGS.

  FIG. 1 is a view showing a turbine wheel as a member to be surface-treated, FIG. 2 is a flowchart showing a surface treatment procedure for the turbine wheel, and FIG. 3 is an internal residual stress distribution when the surface treatment is performed on the turbine wheel. FIG. 4 is a diagram showing a change in surface roughness when the turbine wheel is subjected to surface treatment, and FIG. 5 is a diagram showing a result of the creep test performed on the test piece subjected to the surface treatment in the first step. FIG. 6 and FIG. 6 are diagrams showing the results of a creep test performed on test pieces having different surface roughnesses.

  As shown in FIG. 1A, a turbine wheel 1 as a metal member to be processed includes an implanted portion 2 (groove portion) for fitting and fixing a gas turbine rotor blade (not shown), and the implanted portion 2 ( And a cooling port 3 provided on the inner surface of the groove portion). The implantation part 2 has a groove shape called a Christmas tree part, and a cooling port 3 is present at the base part.

  The inside of the gas turbine rotor blade in operation is cooled by deriving the cooling gas from the cooling port 3 provided on the inner surface of the implanted portion 2 of the turbine wheel to the cooling port inside the gas turbine rotor blade.

  In an actual plant (actual plant), although the gas turbine varies depending on its output, the temperature rises to about several hundred degrees Celsius during operation, and the temperature at the cooling port 3 also rises to nearly 500 degrees Celsius. In such a temperature environment, it has been confirmed that a crack called “Hold Time Cracking” due to a stress corrosion oxidation phenomenon at a grain boundary occurs in the cooling port 3. In order to prevent the occurrence of cracks, it is effective to form a compressive residual stress on the surface of the implanted portion 2 including the cooling port 3.

  Therefore, in this embodiment, as shown in FIG. 2, a compressive residual stress is applied to the portion of the cooling port 3 of the implanted portion 2 of the turbine wheel 1 by performing a laser peening process in the first step.

  In the second step, the surface roughening is performed by polishing (flap wheel finishing) with a flap wheel on the cooling port 3 portion of the implanted portion 2 to which compressive residual stress is applied by performing laser peening in the first step. The surface of the implantation part 2 is smoothed so that the thickness becomes 1 μm or less.

  More specifically, as shown in FIG. 1 (b), a laser peening process (laser peening) is performed on a construction range A of about 2 mm in the flange of the cooling port 3 of the implanted portion 2 of the turbine wheel 1 (FIG. 2). Step S101) After the construction, it is visually confirmed that the laser peening is performed on the surface (Step S102).

  The processing conditions (conditions such as compressive residual stress, depth, and surface roughness) for the surface of the implanted portion 2 of the turbine wheel 1 (metal member) in the first step are the creep test, creep fatigue test, and high temperature fatigue test. The decision shall be based on at least one of the results.

  Thereafter, the surface is polished with a flap wheel (step S103), and the surface of the cooling port 3 is finished in a smooth state substantially equal to the surface roughness value before the surface treatment while leaving a compressive residual stress.

  Thereafter, the polished state of the surface is again visually confirmed (step S104). The flap wheel is a rotary polishing device in which a polishing cloth made of an alumina abrasive is arranged radially from the center of the wheel with a shaft.

  In the finishing process of the flap wheel, the arithmetic average roughness Ra representing the surface roughness is finished to be, for example, 1.0 μm or less. In addition, since a 1st process and a 2nd process are different processes, about the process target member which the 1st process was complete | finished, the polishing by the flap wheel which is a 2nd process is implemented sequentially and rapidly.

Next, the effect of this surface treatment method will be described with reference to FIG.
The graph shown in FIG. 3 is obtained by performing the same processing as the processing target member on the surface using a test specimen made of the same material as the processing target member, and measuring the residual stress of the test specimen for each step by the X-ray diffraction method. It is a graph of values.

  Specifically, the residual stress was measured before the surface treatment, after the first step, and after the second step in the range of about 10 mm in diameter of the surface of the test specimen, and expressed as curves 4, 5 and 6 of the residual stress distribution. It is a thing. In the measurement, the surface of the surface-treated specimen is electropolished and the residual stress on the surface is measured.

  In the figure, the direction of (+) is tensile stress and the direction of (−) is compressive stress. Looking at the curve 4 of the residual stress distribution before the surface treatment, the residual stress before the surface treatment shows that the tensile residual stress is generated only in the shallow part of the surface by machining, and the residual stress in the deeper part is almost zero. It was.

  When laser peening, which is the first step, is performed on this surface, the residual stress on the surface becomes a compressive residual stress as indicated by curve 5 of the residual stress distribution, and the depth is also a value exceeding 500 μm.

  Then, a polishing process (finishing process) using a flap wheel, which is the second process, is performed on the surface subjected to the laser peening.

  Although the surface residual stress generated by the wheel is compression, its depth is shallower than laser peening, usually less than 100 μm. Therefore, looking at the curve 6 of the residual stress distribution, it can be seen that the influence of the flap wheel appears at a position shallower than 100 μm, and the distribution is substantially the same as that in the first step at a position deeper than that.

  The influence depth of the first step is deeper than the influence depth of the second step. However, regarding the residual stress near the surface, the compressive residual stress of the second step is less than the residual stress of the first step. It may be higher depending on the situation, and the distribution near the surface may be different from this result.

  FIG. 4 shows the measurement results of the test specimen (test piece), the results obtained by the same method as in FIG. 3, and shows the change in surface roughness in each step. FIG. 4 shows the arithmetic average roughness Ra as a value indicating the surface roughness.

  As shown in FIG. 4, the surface roughness before the surface treatment is 1 μm or less, and when the laser peening that is the first step is performed, the surface roughness rises slightly, and when the polishing treatment by the flap wheel that is the second step is performed. It is almost the same as the value before the surface treatment.

  FIGS. 5 and 6 are diagrams showing the results of a creep test using a specimen, FIG. 5 is a diagram showing the effect of laser peening, which is the first step, and FIG. 6 is a diagram showing the effect of surface roughness on the creep rupture time. .

  FIG. 5 is a result of a creep test at 500 ° C., which is a result of confirming a fracture time by applying a constant load to the two test pieces 51 and 52. In FIG. 5, the rupture time of the two test pieces 51 and 52 before the surface treatment is about 7000 hours, and both the test pieces 51 and 52 subjected to the first step do not break even if they exceed 11000 hours. The test was stopped at this point. When this result is seen, the test piece which implemented the 1st process extended the fracture | rupture time reliably rather than the test piece before surface treatment, and the improvement effect by a 1st process is recognized.

  FIG. 6 shows the result of a creep test performed at 600 ° C. for each of a plurality of test pieces (four test pieces in this example), and shows the relationship between the surface roughness and the rupture time. In this test, unlike the state after the first step, the test piece has almost no compressive residual stress.

  In FIG. 6, when the numerical value of the surface roughness Ra is reduced, the breaking time becomes longer. By making the surface of the test piece smooth, the breaking time can be lengthened, and the improvement effect by the flap wheel finishing (second process) is achieved. I understand.

  FIG. 5 and FIG. 6 are the results of testing at a temperature higher than the actual use temperature in order to confirm the effect. Based on these results, the surface treatment conditions of the first step and the second step were determined. .

  As described above, according to the first embodiment, after applying the compressive residual stress to the surface of the cooling port 3 of the implanted portion 2 of the turbine wheel 1 by the laser peening process in the first step, the second step Then, the surface of the cooling port 3 portion of the implanted portion 2 is smoothly polished to 1 μm or less, which is the surface roughness before the surface treatment, by the flap wheel finish, so that the compressive residual stress is maintained on the surface and inside of the implanted portion 2. The surface of the cooling port 3 portion of the planting part 2 can be smoothed to a predetermined roughness that can be used in a high temperature environment exceeding 500 ° C.

  In the said embodiment, although the 2nd process was made into flap wheel finishing, you may give the polishing process (process) by a belt sander, N strip, etc. other than this, for example. That is, in the second step, any one of the flap wheel, the belt sander, and the N strip may be polished.

(Second Embodiment)
A second embodiment will be described with reference to FIG.
FIG. 7 is a diagram showing changes in internal residual stress distribution due to the test specimen, and FIG. 8 is a diagram showing changes in surface roughness when surface treatment is performed.

  The surface of the test body is subjected to the same processing as the actual processing target member. In the second embodiment, the laser peening process is performed in both the first step and the second step. Laser peening was carried out under different construction conditions.

  The output of the laser peening in the first step is almost the same as that in the first embodiment, and the laser output of the laser peening in the second step is made smaller than the laser output of the laser peening in the first step. The residual stress of this specimen was measured by the X-ray diffraction method. In the measurement of the residual stress, electrolytic polishing was performed on a range of about 10 mm in diameter measured by the test piece (test body), and the residual stress of the surface in the range was measured.

  In FIG. 7, the (+) direction is tensile stress and the (−) direction is compressive stress. In the residual stress distribution curve 4 before the surface treatment, the tensile residual stress exists only in the shallow portion of the surface, and the residual stress in the deeper portion is almost zero.

  When laser peening, which is the first step, is performed on this surface, the residual stress on the surface becomes a compressive residual stress as shown in the residual stress distribution curve 7 after the first step, and the depth is also a value exceeding 500 μm. .

  Further, laser peening under different conditions as the second step was performed on this surface. The depth of the compressive residual stress of the laser peening in the second step is shallower than the depth of the residual stress in the first step. For this reason, in the residual stress distribution curve 8 after the second step, at a position shallower than 500 μm (position around 200 μm), the effect appears and the residual stress is lowered, which is slightly different from the residual stress distribution in the first step. However, at a deeper position, the distribution is almost the same as in the first step, and it can be seen that there is an effect of maintaining the residual stress.

  FIG. 8 shows the measurement result of the test specimen as in FIG. 7, and the arithmetic average roughness Ra. As shown in FIG. 8, when the surface roughness before the surface treatment is 1 μm or less and the laser peening as the first step is performed, the surface roughness rises slightly and the surface roughness becomes about 1.6 μm, which is the second step. When laser peening is performed under different conditions, the value is approximately 1 μm or less, which is almost the same as the value before the surface treatment. In such construction, the depth of the compressive residual stress was deep, and the surface of the test specimen could be smooth.

As described above, according to the above-described embodiment, the laser peening is performed in the first step on the implanted portion 2 of the turbine wheel 1 as a member of the equipment used in a high temperature use environment, and then in the second step. By maintaining the surface of the implanted portion 2 by flap wheel finishing (first embodiment) or laser peening (second embodiment) with weak laser output, the effect of applying compressive residual stress by the peening process in the first step is maintained. While the surface of the implantation part 2 can be smoothed.
According to at least one embodiment described above, the effect of applying compressive residual stress by peening is maintained when the surface of a metal member is polished after peening is performed on a metal member used in a high-temperature use environment. can do.

  Although the embodiment of the present invention has been described, this embodiment is presented as an example and is not intended to limit the scope of the invention. The novel embodiment 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.

  A ... Construction range, 1 ... Turbine wheel, 2 ... Implanted part, 3 ... Cooling port.

Claims (8)

In the surface treatment method for metal members,
A first step of applying compressive residual stress to the surface of the metal member;
And a second step of smoothly polishing the surface of the metal member so that the surface roughness of the metal member to which compressive residual stress is applied in the first step is 1 μm or less.   The surface roughness of the second step is smoother than the surface roughness after the first step, and the depth of the compressive residual stress in the second step is shallower than the depth of the compressive residual stress after the first step. The surface treatment method according to claim 1.   The surface treatment method according to claim 1, wherein the first step applies compressive residual stress to the surface of the metal member by peening.   The surface treatment method according to any one of claims 1 to 3, wherein the second step is a polishing treatment by any one of a flap wheel, a belt sander, an N strip, and laser peening.   The surface treatment method according to claim 3, wherein the peening in the first step is laser peening.   When both the process of the said 1st process and the said 2nd process are made into laser peening, the construction conditions of the said 2nd process are made into laser output smaller than the laser output of the laser peening of the said 1st process. The surface treatment method according to any one of claims 1 to 5.   The surface treatment method according to any one of claims 1 to 6, wherein a value, a depth, and a surface roughness of the compressive residual stress applied to the surface of the metal member are conditions for the surface treatment.   8. The processing condition for the surface of the metal member in the first step is determined based on a result of at least one of a creep test, a creep fatigue test, and a high temperature fatigue test. Surface treatment method.

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