JP2010084553A - Turbine blade and steam turbine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an inexpensive steam turbine stator blade without using alloy coating such as thermal spray and a sintered body, which is excellent in oxidation resistance, and of which efficiency is not decreased even after operation for a long period of time. <P>SOLUTION: A turbine blade uses stainless steel containing 8-15 mass% of chromium as base material, and has an oxide film on a surface of the base material. Members constituting the turbine blade are formed by processing a forging material. After surfaces of the members are polished to be 0.5a or less in surface roughness and the polished members are welded to be in a shape of a turbine stator blade, thermal processing is performed at an actual operation temperature or more. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a steam turbine stationary blade having a protective oxide film on its surface, and relates to a steam turbine stationary blade having a protective oxide film on its surface.

  In recent years, steam turbines are required to have high power generation efficiency, and the steam temperature tends to increase. When the steam temperature is 566 ° C. or higher, 12% Cr stainless steel is generally used as a material for the stationary blade. As an effect of the surface roughness of the stationary blade on the efficiency, it is known that an oxide scale is generated on the surface of the stationary blade during long-time operation, and the efficiency is lowered by roughening the surface. On the high-pressure stage side, in order to prevent oxidation due to high-temperature steam, a chromium carbide coating or nitriding treatment is applied to prevent a decrease in efficiency according to an increase in surface roughness. In particular, on the intermediate / low pressure stage side, after the surface-polished forged material is welded, it is used after the surface is smoothed again by surface grinding in order to remove the oxide scale generated during the stress relief annealing.

  Japanese Patent Application Laid-Open No. 2002-309303 (Patent Document 1) discloses a method of forming a metal particle composition containing an organic medium on a steel surface by applying and sintering fine metal particles for forming an alloy. ing.

  Japanese Patent Publication No. 2007-507604 (Patent Document 2) discloses a method for producing a nanostructured coating having improved wear resistance and erosion resistance using a corrosion-resistant binder matrix.

JP 2002-309303 A Special table 2007-507604

  When an alloy coating is formed by thermal spraying or sintering, although it is excellent in oxidation resistance and wear resistance, there is a problem that costs increase. In addition, the oxidation resistance may be lowered by improving the wear resistance as in the nitriding treatment.

  On the other hand, a stationary blade with only surface polishing that does not form a film on the surface is oxidized during long-time operation, and the surface becomes rough, thereby reducing efficiency.

  As described above, none of the conventional techniques satisfy the maintenance of turbine performance and the cost.

  The present invention has been made in view of the circumstances as described above, and its purpose is low cost without using an alloy coating such as thermal spraying or sintered body, excellent oxidation resistance, and after a long time operation. Provided is a steam turbine stationary blade whose efficiency is not lowered even if it exists.

  The present invention is based on stainless steel containing 0.1 to 1.0% Mn and 8 to 15% Cr by weight, and has a protective oxide film containing Cr, Mn and Fe on its surface, The steam turbine stationary blade is characterized in that the average particle size of the oxide in the oxide film is 20 to 70 nm.

  The present invention also provides the steam turbine stationary blade according to the above-described steam turbine stationary blade, wherein the surface roughness Ra is 1.6a or less.

  The inventors focused on the surface roughness of the steam turbine vane and the average particle size of the oxide, and studied the steam turbine efficiency and surface properties. As a result, based on Cr stainless steel containing 0.1 to 1.0% Mn and 8 to 15% Cr by weight, and having a protective oxide film containing Cr, Mn, Fe on the surface, It has been found that a steam turbine stationary blade having an average oxide particle size of 20 to 70 nm in the oxide film has excellent oxidation resistance and suppresses performance degradation even after long-time operation.

When the base 8-15% Cr stainless steel is oxidized in the normal atmosphere, Fe and Cr are oxidized to produce a scale of FeCr ₂ O ₄ . Since this scale is not protective, oxidation cannot be suppressed, and a scale of magnetite Fe ₃ O ₄ is formed in the outer layer of the FeCr ₂ O ₄ scale after long-time operation. In addition, when 9-13% Cr stainless steel is oxidized in a low oxygen partial pressure environment, Cr is preferentially oxidized because the standard free energy of formation of oxide is lower than that of Fe. The amount of Cr is insufficient to uniformly produce a protective chromia Cr ₂ O ₃ film. However, in 9-13% Cr stainless steel containing 0.1-1.0% Mn, the standard free energy of formation of Mn oxide is even lower than that of Fe and Cr, so that this is oxidized in a low oxygen partial pressure environment. In this case, it was found that Mn oxide was produced in a nodular form and Cr-rich oxide was produced in other parts, thereby suppressing oxidation during long-time operation.

  As for the surface roughness, it is desirable that the oxide is not generated because the surface becomes rough as the surface oxide grows. However, in 8-15% Cr steel, it is important to suppress oxide growth as a method other than coating with an alloy or ceramic. The present inventors have found that the growth of oxide scale is remarkably suppressed when the particle diameter of the oxide particles constituting the protective oxide film is 20 to 70 nm. On the other hand, no matter how much the particle size of the oxide particles is maintained on the nano order, the effect is not seen if the roughness of the surface on which the oxide is generated is large. As a result of various studies, in order to suppress an increase in surface roughness and prevent a decrease in efficiency even after long-time operation, the protective oxide film has a particle size of 20 to 70 nm, It is important that Ra is 1.6a or less, and the present invention has been achieved.

  According to the present invention, it is possible to suppress generation of oxide scale during operation in a steam turbine stationary blade, which is low in cost, excellent in oxidation resistance, and does not decrease in efficiency even after long-time operation. It became possible to provide.

  The present invention has the features as described above, and embodiments of the present invention will be described below, but the present invention is not limited to the following examples.

  First, a steam turbine using the present invention will be described.

  FIG. 1 is an example of a steam turbine plant in which the steam turbine blades of the present invention are applied as medium pressure stationary blades 14 and high pressure stationary blades 15. The steam at 566 ° C. supplied from the boiler is guided to the high-pressure casing 18 through the main steam pipe 28. The steam passes through the nozzle 38, the high pressure stationary blade 15 changes the direction of the flow of the steam and increases the speed of the steam due to the pressure difference, and the high pressure moving blade 16 converts the steam energy into rotational energy and rotates the rotor 33 Then, power is generated by a generator coupled to the rotor 33.

  FIG. 2 is a view showing a single blade type steam turbine stationary blade. After processing the forged material, the blade portion 50 and the inner shroud portion 51, and the blade portion 50 and the outer shroud portion 52, which are prepared by polishing the surface to a surface roughness Ra of 0.4a, are welded to form a turbine stationary blade shape. Then, it was manufactured by performing a heat treatment at 650 ° C. for 4 hours. In order to form the oxide film of the present invention on the surface of the turbine blade by heat treatment after welding, blasting or polishing after welding, which was conventionally required when the surface roughness Ra was polished to 1.6a, was created. The removal of the oxide scale by the above and the subsequent cleaning step are unnecessary.

  The oxide film of the present invention is formed on the surface of a turbine blade. The oxide film includes oxide particles, and the oxide particles have a particle size of 20 to 70 nanometers.

  The oxide film has a surface roughness Ra of 1.6a or less, preferably 1.0a or less, and more preferably 0.5a or less. For the surface roughness, the maximum height Ry, the ten-point average roughness Rz, the arithmetic average roughness Ra, and the like are used depending on how to obtain the surface roughness. The average roughness in this invention shows arithmetic average roughness Ra. A reference length is extracted from the roughness curve in the direction of the average line, the absolute values of deviations from the average line to the roughness curve in this extracted part are summed, and the average value is obtained by expressing in micrometers.

  The oxide film component mainly includes Cr, Fe, O, and Mn.

  The turbine blade having the oxide film of the present invention makes it possible to suppress the generation of oxide scale during operation, and is low in cost, excellent in oxidation resistance, and reduced in efficiency even after prolonged operation. It is possible not to.

Although the effect of the heat treatment condition can be seen even in the air, it is desirable to have a low oxygen partial pressure in an inert gas atmosphere such as Ar or in a vacuum. In particular, it is preferably 1 × 10 ⁻¹² atm or less. The heat treatment temperature is higher than the actual operating temperature. In the case of a blade having a welded structure, it is desirable to be a post-weld stress relief annealing temperature at the time of manufacture. In the case of a blade without a welded structure, the blade material is used. It is desirable that the temperature is not higher than the tempering temperature. 650-690 degreeC is especially preferable. The heat treatment is performed for a long time in a low-oxygen atmosphere, so that a Cr-rich oxide film with higher protection is formed. However, in practice, a short time is desirable in the process. In particular, 3 to 12 hours are preferable.

  Hereinafter, the reasons for limiting the components of the turbine blade used in the present invention will be described.

  Cr improves the corrosion resistance and oxidation resistance in steam. Moreover, hardenability is improved and there is also an effect of improving toughness and strength. If the content is less than 8.0%, these effects are not sufficient, and if it exceeds 15.0%, a δ ferrite phase is formed, so that the creep rupture strength and toughness are lowered. In particular, a range of 9.0 to 13.0 is preferable.

  Mn should be at least 0.1% in order to produce Mn oxide on the nodules. On the other hand, if added in a large amount, creep embrittlement tends to occur, so the content is made 1.0% or less. In particular, the range of 0.5 to 1.0% is preferable.

  Other elements that may be included include C, Si, Ni, Mo, V, W, Nb, N, Cu, Al, and unavoidable impurities S and P. It is preferable not to impair the strength.

  Table 1 shows the chemical composition of the stainless steel used for the steam turbine stationary blade in this example.

  The oxide film was evaluated with a test piece having the above composition.

  The steel ingot subjected to the high frequency melting furnace was hot forged at a temperature of 850 to 1150 ° C. to obtain a 30 mm square. Quenching was performed at 1024 to 1052 ° C. for 1 hour, followed by oil cooling, and tempering was performed at 620 ° C. or higher for 2 hours and then air cooling. A test piece having a size of 20 × 20 × 5 mm was cut from a 30 mm square test material, the surface was polished with # 600 emery paper, and then degreased and washed with acetone.

  Next, heat treatment was performed in the air at a temperature of 690 ° C. for 4 hours. The temperature increase and temperature decrease rates are 100 ° C. every hour.

  FIG. 3 shows a schematic diagram of surface properties after heat treatment in the atmosphere. When observed at a magnification of 2000 times, an oxide having a particle size of about 1 μm was uniformly formed on the surface. As a result of EDX analysis, the main components of the oxide were Cr, Fe, O, and Mn. Further, the surface roughness Ra slightly increased by heat treatment in the atmosphere.

  Using this test piece, an oxidation test for 1000 hours was performed in the atmosphere at a temperature of 650 ° C., and the properties of the oxide formed on the steel surface were observed using a scanning electron microscope.

  FIG. 4 is a schematic diagram of surface properties after an atmospheric oxidation test at 650 ° C. for 1000 hours after heat treatment in the atmosphere. When observed at a magnification of 5000, an oxide formed by combining a relatively large square oxide having a particle size of about 2 to 4 μm and an oxide having a particle size of about 1 μm was confirmed. It can be considered that the oxide having a particle size of about 1 μm in FIG. 3 has grown to 2 to 4 μm due to oxidation during operation.

  Moreover, about the said test piece, the surface roughness after grinding | polishing, after heat processing, and after an atmospheric oxidation test was performed.

  FIG. 5 is a diagram showing the relative values of the surface roughness when the surface roughness before heat treatment is 1 respectively. In the air heat treatment, the surface roughness increased after the heat treatment and during the operation time, but the increase was confirmed to be very small. When the surface roughness is increased, the efficiency of the turbine is decreased. Therefore, it was confirmed that the increase in the surface roughness was suppressed even when heat treatment in the atmosphere was applied to the stationary blade. Furthermore, the removal of the oxide scale by blasting or polishing after welding, which is necessary in the conventional process, and the subsequent cleaning process are unnecessary, and an effect of cost reduction can be obtained.

  A case where a test piece similar to that of Example 1 is prepared and heat treatment is performed in a low oxygen partial pressure will be described.

  The steel ingot subjected to the high frequency melting furnace was hot forged at a temperature of 850 to 1150 ° C. to obtain a 30 mm square. Quenching was performed at 1024 to 1052 ° C. for 1 hour, followed by oil cooling, and tempering was performed at 620 ° C. or higher for 2 hours and then air cooling. A test piece having a size of 20 × 20 × 5 mm was cut from a 30 mm square specimen, and the surface was polished with # 600 emery paper, and then degreased and washed with acetone.

Next, heat treatment was performed at a temperature of 690 ° C. for 4 hours in a low oxygen partial pressure with an oxygen partial pressure of 1 × 10 ⁻¹² atm or less. The temperature increase and temperature decrease rates are 100 ° C. every hour. Using these test pieces, an oxidation test was conducted for 1000 hours in the atmosphere at a temperature of 650 ° C., and the properties of the oxide formed on the steel surface were observed using a scanning electron microscope.

  FIG. 6 shows a schematic view of the surface properties after heat treatment in a low oxygen partial pressure, observed at the same magnification of 2000 as in FIG. The generation of oxide on the surface is very small compared to the atmospheric SR material of FIG. This corresponds to the initial surface of the steam turbine stationary blade according to the present invention.

  FIG. 7 is a diagram showing a schematic view of the surface properties after heat treatment in a low oxygen partial pressure, with the surface of FIG. 6 observed at a magnification of 80000 times. An oxide having a particle size of about 50 nm was uniformly formed on the surface. As a result of EDX analysis, the main components of the oxide were Cr, Fe, O, and Mn. Further, the surface roughness Ra was reduced by the heat treatment in the low oxygen partial pressure.

  FIG. 8 is a schematic diagram of the surface properties after an atmospheric oxidation test at 650 ° C. for 1000 hours after heat treatment in a low oxygen partial pressure. When observed at the same magnification of 5000 as in FIG. 4, it was confirmed that the particle size of the oxide was at most smaller than 2 μm and most smaller than 1 μm. Compared with FIG. 4, it is clear that the oxide particle size after long-time operation is smaller in the heat treatment at a low oxygen partial pressure than in the atmospheric heat treatment.

  Further, in FIG. 5, the relative values of the surface roughness before and after the heat treatment and after the oxidation test when the surface roughness before the heat treatment is set to 1 for the heat treatment in a low oxygen partial pressure are shown. In the heat treatment at a low oxygen partial pressure, it became clear that the surface roughness after the heat treatment and during the operation time decreased compared with that before the heat treatment. When heat treatment in low oxygen partial pressure is applied to a stationary blade, it is confirmed that by having the protective coating according to the present invention, generation of oxide scale after long-time operation is suppressed, and turbine performance can be maintained at low cost. It was done.

Moreover, the same test was performed by changing the temperature conditions of the heat treatment. The heat treatment temperature of Example 1 and Example 2 was changed to 650 ° C., and the heat treatment for 4 hours was performed in the atmosphere and at a low oxygen partial pressure of 1 × 10 ⁻¹³ atm or less. As a result, the ratio of the surface roughness before and after the heat treatment was 1.28 in the low oxygen partial pressure with respect to 1.38 in the atmosphere. It was confirmed that the protective film according to the present invention was effective even when performed at 650 ° C.

1 is a cross-sectional view of a high / medium pressure integrated steam turbine according to the present invention. It is a figure which shows a single blade type | mold steam turbine stationary blade. It is a schematic diagram of the surface property after heat processing in air | atmosphere. It is a schematic diagram of the surface property after carrying out an atmospheric oxidation test after the heat treatment in the atmosphere. It is a figure which shows the relative value of surface roughness when the surface roughness before heat processing is set to 1. It is a schematic diagram of the surface property after heat processing in a low oxygen partial pressure. It is a schematic diagram at the time of observing at higher magnification than FIG. 5 about the surface property after the heat processing in a low oxygen partial pressure. It is a schematic diagram of the surface property after carrying out an atmospheric oxidation test after the heat treatment in a low oxygen partial pressure.

Explanation of symbols

14 Medium-pressure stationary blade 15 High-pressure stationary blade 16 High-pressure moving blade 17 Medium-pressure moving blade 18 High-pressure internal compartment 19 High-pressure external compartment 20, 21 Medium-pressure internal compartment 22 Medium-pressure external compartment 25 Flange, elbow 28 Main steam inlet 33 High and medium pressure rotor shaft 38 Nozzle box 43 Bearing 50 Wing 51 Inner shroud 52 Outer shroud

Claims (11)

  A turbine blade having a base material of stainless steel containing 8 to 15% by mass of chromium, the surface of the base material having an oxide film. The turbine blade according to claim 1,
The turbine blade according to claim 1, wherein the oxide film includes oxide particles, and the oxide particles have a particle size of 20 to 70 nanometers. The turbine blade according to claim 1,
The turbine blade according to claim 1, wherein the oxide film has a surface roughness Ra of 1.6a or less. The turbine blade according to claim 1,
The turbine blade according to claim 1, wherein the base material contains iron as a main component and contains at least manganese and chromium. A turbine blade according to claim 4, wherein
The said base material contains 0.1-1.0% of Mn, The steam turbine blade characterized by the above-mentioned. A method for producing a vane for a turbine based on a stainless steel containing 8 to 15% by mass of chromium,
The members constituting the turbine blade are formed by forging processing, the surface of the member is polished to a surface roughness of 0.5a or less, and the polished members are welded to form a turbine vane shape. A method for producing a turbine vane, characterized in that the heat treatment is performed at a temperature higher than the inside temperature.   The method for producing a turbine vane according to claim 6, wherein the ground surface roughness Ra of the ground substrate is 1.6a or less.   It is a manufacturing method of the stationary blade for turbines described in Claim 6, Comprising: Time until an oxide particle with a particle size of 20-70 nanometer precipitates in the oxide film formed in the said member surface by the said heat processing The manufacturing method of the stationary blade for turbines characterized by performing.   The method for manufacturing a turbine vane according to claim 6, wherein the heat treatment is performed at 650 ° C to 690 ° C. 7. The method for manufacturing a turbine vane according to claim 6, wherein the heat treatment is performed at an oxygen partial pressure of 1 × 10 ⁻¹² atm or less.   7. The method for manufacturing a turbine vane according to claim 6, wherein a processing surface of the blade is in a range including a working fluid passage portion.

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