JPH0564705B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a method for forming a coating layer that improves the erosion resistance of a low-pressure final stage blade of a steam turbine blade. [Prior Art] 12Cr steel, which has excellent strength, is generally used for steam turbine blades. However, low-pressure final stage blades operated in humid steam for long periods of time are subject to erosion damage due to condensed water droplets. For this reason, conventional methods have been adopted in which a stellite plate is silver-brazed to the leading edge of a 12Cr steel wing to prevent damage. [Problems to be Solved by the Invention] However, the silver brazed Stellite board does not have a sufficient lifespan, and the Stellite board needs to be replaced after several years. Furthermore, silver brazing requires a high degree of skill, it is difficult to completely eliminate defects such as pinholes, and there are problems such as the fact that zinc and cadmium contained in silver brazing are harmful. The present invention provides a method for forming a coating layer on a turbine blade that has superior erosion resistance than conventional Stellite plates by using a low-pressure plasma spraying method, without performing the silver brazing work of Stellite plates, which has the above-mentioned problem. The purpose is to [Means for Solving the Problems] A method for forming an erosion-resistant coating layer for a turbine blade according to the present invention to achieve the above-mentioned object is a method for forming an erosion-resistant coating layer on a blade surface of a steam turbine by maintaining the temperature of the blade at 300 to 800°C. condition, with Co as the main component, Cr20~30%, W3
It is characterized by plasma spraying an alloy powder containing ~6% under reduced pressure to form a sprayed coating layer with a porosity of 5% or less. The erosion resistance properties of the sprayed coating layer depend on the adhesion between the blade and the sprayed coating layer and the bonding force between particles within the sprayed coating layer. In addition, the adhesion between the blade and the thermally sprayed coating layer and the interparticle bonding force within the thermally sprayed coating layer become stronger as the temperature of the blade increases during vacuum spraying. When the temperature of the blade is below 300°C, the adhesion between the blade and the sprayed coating layer decreases to below 10Kg/ ^mm2 , and further,
The interparticle bonding force within the thermally sprayed coating layer also decreases, and there is a risk that it will peel off in an erosion environment, making it impractical. If the blade is thermally sprayed without preheating, the blade will be deformed due to strain force caused by the difference in thermal expansion between the blade and the thermally sprayed coating layer. Therefore, by preheating the blade to 300 to 800°C, more preferably 400 to 600°C, the strain caused by the difference in thermal expansion can be minimized and deformation of the blade can be prevented. The temperature of the blade is
If the temperature exceeds 800°C, the blade will undergo thermal deformation and strength reduction, making it impossible to maintain its performance as a steam turbine blade. The above is the reason why the temperature of the blade was limited to 300 to 800°C in carrying out the present invention. Furthermore, the anti-erosion properties of the sprayed coating also depend on the porosity of the sprayed coating, defined below. Porosity (%) = (1 - (specific gravity of thermal spray coating layer / specific gravity of bulk material)) × 100 If there are many pores, erosion will develop around the pores, so the denser the coating layer with fewer pores, the better the erosion resistance. Excellent in sex. The thermal spray coating layer sprayed under reduced pressure by the method of the present invention is a laminate of fine, flat crystals, and has finely dispersed pores of 1 μm or less. The maximum value of the porosity of the sprayed coating layer, which exhibits better erosion resistance than conventional Stellite plates, is believed to be about 5%. In order to ensure high adhesion and interparticle bonding force of the sprayed coating layer, reduce porosity, and obtain a dense sprayed coating layer, the optimal pressure range of the vacuum chamber when performing the vacuum plasma spraying method is: It is 20 to 200 mmHg, more preferably 30 to 100 mmHg. Erosion damage is thought to occur when cracks occur at grain boundaries and the damage progresses as the crystal grains fall off, but since the crystal grains of the sprayed coating layer obtained by the method of the present invention are finely knitted, cracks occur at grain boundaries. Even if cracks occur, it is thought that erosion resistance is improved because the growth and progress of cracks is hindered. The composition of the powder used in carrying out the present invention is an alloy powder containing Co as the main component and 20 to 30% of Cr and 3 to 6% of W.
It may contain one or more other elements. The particle size of the powder is preferably 44 μm to 1 μm, more preferably
It is 25-5 μm. When the particle size of the alloy powder becomes coarse, the porosity increases and the interparticle bonding strength also decreases, resulting in deterioration of erosion resistance. In addition, if the particle size of the powder becomes too small, the feeding of the powder into the plasma becomes unstable.
It becomes difficult to form a dense and homogeneous coating layer. The plasma gas used in carrying out the present invention may be Ar+He, Ar+H ₂ or Ar+N ₂ mixed gas, but it is preferable to use Ar+H ₂ . Hereinafter, the present invention will be explained in more detail with reference to Examples. [Example] Example 1 Alloy powders a, b, and c having the composition (wt%) and particle size (μm) shown in Table 1 were sprayed under the thermal spraying conditions shown in Table 2.
Thermal spray coating layers A, B, and C were formed by vacuum spraying on the surface of 12Cr steel. The thickness of the sprayed coating layer is 0.7
~1.0mm. Steam turbine blades operated in wet steam are subject to drain erosion damage caused by condensed water droplets, but drain erosion caused by droplet collisions and cavitation erosion have exactly the same impact pressure generating mechanism that causes material damage. It is said that there is a good correlation between the two. The ultrasonic cavitation/erosion test method is a method commonly used to evaluate the erosion resistance of steam turbines, and the examples of the present invention were also conducted using this test method. A test piece shown in Figure 1 was prepared by processing 12Cr steel with a thermally sprayed coating layer on the surface, and the degree of erosion damage was evaluated using an ultrasonic cavitation erosion test method. The test conditions were a frequency of 19K.
The test was conducted by immersing the test piece shown in FIG. 1 in ion exchange water for 3 mm at Hz and amplitude of 40 μm. The surface of the test piece was polished in advance with a 3 μm diamond and used for the test. The temperature of the ion-exchanged water was kept constant at 24±1°C. As a comparison material, a stellite plate, which is conventionally soldered with silver to the leading edge of a turbine blade, was used, and the results shown in FIG. 2 were obtained. From the results shown in FIG. 2, the thermal spray coating layers A, B, and C of the present invention all exhibit better erosion resistance than the conventional Stellite plate. Example 2 Using alloy powder c shown in Table 1, vacuum spraying was carried out on a 12Cr steel surface under the spraying conditions shown in Table 3.
Thermal spray coating layers C, D, E, and F having different porosity were formed, and the degree of erosion damage was evaluated in the same manner as in Example 1. Figure 3 shows the relationship between the porosity of each thermal spray coating layer and the amount of weight loss after the 20 hour test. The thickness of each thermal spray coating layer is approximately 2 mm. In addition to the test piece shown in Figure 1, a 10 x 10 x 1.5 mm sample for specific gravity measurement is cut out from the thermal spray coating layer, and weighed in liquid as specified in JIS Z8807. The specific gravity was measured using the method. In addition, to determine the specific gravity of the bulk material, cut out a sample with the same dimensions as above from a sample obtained by plasma melting alloy powder C in a water-cooled steel crucible in an Ar atmosphere, measure the specific gravity in the same manner, and determine the porosity as the specific gravity of the bulk material. Ta. From the results shown in FIG. 3, it can be seen that when the porosity is approximately 5% or less, the thermally sprayed coating layer produced by the method of the present invention exhibits better erosion resistance than the conventional Stellite plate.

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[Table] [Effects of the Invention] As explained above, according to the present invention, problems associated with conventional silver brazing work can be avoided, and furthermore,
It is possible to form a surface coating layer for a turbine blade that has better erosion resistance than conventional silver-brazed Stellite plates.

[Brief explanation of the drawing]

Figure 1 shows the shape of the test piece used to evaluate the degree of erosion damage. FIG. 2 is a graph showing the results of an ultrasonic cavitation/erosion test. FIG. 3 is a graph showing the relationship between porosity and weight loss after a 20-hour test.

Claims (1)

[Claims] 1 The temperature of the blade is set to 300 on the blade surface of the steam turbine.
When maintained at ~800℃, Co is the main component,
Formation of an erosion-resistant coating layer for a turbine blade, characterized by plasma spraying an alloy powder containing 20-30% Cr and 3-6% W under reduced pressure to form a sprayed coating layer with a porosity of 5% or less. Method.