JP7602718B2 - Method for manufacturing steam turbine blades - Google Patents

Description

The present invention relates to a manufacturing method for a steam turbine blade and a steam turbine blade. In particular, the present invention relates to a manufacturing method for a steam turbine blade using precipitation hardening stainless steel as a base material, which has both erosion resistance and stress corrosion cracking resistance, and a steam turbine blade.

In general, in steam turbines, condensed steam collides with the steam turbine blades rotating at high speed, causing erosion wear at the leading edge (inlet side) of the steam turbine blades, significantly shortening the lifespan of the steam turbine blades. In addition, steam turbine blades used in geothermal power generation contain a large amount of corrosive elements such as sulfur and chlorine in the steam, which causes stress corrosion cracking (hereinafter referred to as SCC), which also significantly shortens the lifespan.

As a measure to prevent such a decrease in life span, a method is known in which the leading edge of a steam turbine blade is subjected to a surface treatment (hardening treatment) by flame quenching to prevent the occurrence of erosive wear (see, for example, Patent Document 1). However, there is a problem in that it is difficult to control the temperature of the quenched part. This causes problems such as (1) if temperature control is insufficient, the hardness of the quenched part does not meet the target value, (2) the hardness and metal structure of the quenched part become non-uniform, making it difficult to achieve stable quality, and (3) the erosion life span is short because the quenching depth is shallow (≦2 mm), and therefore the steam turbine blade must be replaced at least every 10 years.

As a method to improve the problems of Patent Document 1, a method of laser hardening has been proposed (see Patent Document 2, for example). This method is said to achieve stable quality through precise heat input control, form a high-hardness layer, and achieve a deep hardening depth.

To achieve both deep penetration and high corrosion resistance, it is known that irradiating the leading edge surface of a steam turbine blade with a laser beam at 1050°C to 1100°C, followed by heat treatment at a temperature of 240°C to 260°C for a holding time of 10 to 100 hours, makes it possible to achieve both high strength and high corrosion resistance (see, for example, Patent Document 3).

Japanese Patent Application Publication No. 8-225828 JP 2013-209912 A JP 2017-222912 A

The method disclosed in Patent Document 3 is extremely useful from the viewpoint of imparting high strength and high corrosion resistance to a specified stainless steel base material. However, this method cannot be applied to precipitation hardening stainless steel, and there is a problem in that steam turbine blades using precipitation hardening stainless steel base material cannot achieve both high strength and high corrosion resistance.

As a result of intensive research, the inventors have come up with a heat treatment method capable of imparting high strength and high corrosion resistance to precipitation hardened stainless steel, and have completed the present invention. That is, according to one embodiment, the present invention relates to a method for manufacturing a steam turbine blade, comprising: a heat treatment step of irradiating a leading edge portion of a precipitation hardened stainless steel base material processed into a steam turbine blade shape with laser light at a surface temperature of 1150°C to 1300°C and with an irradiation area of ⁴⁰⁰ mm2 or more, and a post-heat treatment step of performing, after the heat treatment step, a post-heat treatment at a surface temperature of the leading edge portion of 400°C to 600°C for a holding time of 1 to 10 hours.

In the manufacturing method of the steam turbine blade, it is preferable that the heat treatment process includes a process of detecting the surface temperature of the leading edge of the laser irradiated part based on the thermal radiation from the laser irradiated part, and a process of feedback controlling the laser output so that the surface temperature of the leading edge of the laser irradiated part is 1150°C to 1300°C.

In the method for manufacturing the steam turbine blade, it is preferable that the post-heat treatment step is performed by heating the entire base material or by locally heating the laser irradiated area.

In the manufacturing method of the steam turbine blade, it is preferable that after the post-heat treatment process, a high-hardness layer having a hardness of 430 Hv or more and a hardening depth of 2 mm or more is formed on the leading edge surface.

In the manufacturing method of the steam turbine blade, it is preferable that the amount of δ-ferrite precipitated on the leading edge surface is 5% or less after the post-heat treatment process.

According to another embodiment, the present invention relates to a steam turbine blade manufactured by any of the manufacturing methods described above.

In another embodiment, the present invention relates to a steam turbine blade in which a high-hardness layer having a hardness of 430 Hv or more and a hardening depth of 2 mm or more is formed at the leading edge of a precipitation-hardened stainless steel base material, and the amount of precipitation of δ-ferrite on the surface of the high-hardness layer is 5% or less.

The manufacturing method of the present invention makes it possible to form an erosion-resistant hardened layer on the leading edge of a steam turbine blade made of precipitation-hardened stainless steel base material. This makes it possible to suppress the occurrence of erosion and SCC in the steam turbine blade.

FIG. 1 is a diagram showing a schematic diagram of a heat treatment process by laser irradiation of a steam turbine blade according to the present invention. FIG. 2 is a diagram showing locations of a steam turbine blade where corrosion is likely to occur. FIG. 3 is a graph showing the relationship between laser spot size to achieve a required hardening depth and leading edge thickness. FIG. 4 is a scanning electron microscope photograph of the surface of a steam turbine blade according to the embodiment. FIG. 5 is a scanning electron microscope photograph of the surface of a steam turbine blade of a comparative example.

Below, an embodiment of the present invention will be described with reference to the drawings. However, the present invention is not limited to the embodiment described below. Also, the drawings are illustrative schematic diagrams for explaining the present invention, and the dimensions and relative positional relationships of each component in the drawings do not limit the present invention.

According to one embodiment, the present invention provides a method for manufacturing a steam turbine blade, comprising the following steps.
(1) A heat treatment process in which a laser beam is irradiated to a leading edge of a precipitation hardened stainless steel base material processed into a steam turbine blade shape at a surface temperature of 1150°C to 1300°C with an irradiation area of 400 ^mm2 or more. (2) A post-heat treatment process in which, after the heat treatment process, a post-heat treatment is performed at a surface temperature of 400°C to 600°C on the leading edge with a holding time of 1 to 10 hours.

The first step is a heat treatment step by laser irradiation. Figure 1 is a conceptual diagram showing an example of a turbine blade hardening treatment device 2 used to carry out the first step. The laser irradiation step will be described below with reference to Figure 1. The hardening treatment device 2 includes a laser generator 21, a laser irradiation head 22, an irradiation head position detector 23, a laser light scanning mechanism 24, a control device 25, and a radiation light detector 26. Note that in Figure 1, a device that irradiates only one surface of the base material with a laser is used, and the other surface is not irradiated with a laser. However, the heat treatment step by laser irradiation is not limited to the device used, and can be carried out using any device that can carry out a similar process.

The heat treatment process using laser irradiation can be performed on the precipitation hardened stainless steel base material processed into the shape of a steam turbine blade. Any type of precipitation hardened stainless steel base material can be heat treated, for example, martensitic precipitation hardened stainless steel, semi-austenitic precipitation hardened stainless steel, austenitic precipitation hardened stainless steel, etc., and erosion resistance and SCC resistance can be imparted. The specified precipitation hardened stainless steel material can be formed into a specified blade shape to form the base material in this embodiment. Before the laser irradiation, the laser irradiated part of the base material may be subjected to physical surface treatment such as polishing with abrasive paper, or other surface treatment that does not affect the properties such as the strength of the base material, as necessary. This is to suppress the variation in the laser absorption rate.

The laser irradiated portion is a portion where corrosion is likely to occur when used as a steam turbine blade, and may be, for example, a portion near the blade tip where the peripheral speed is high, or a peripheral portion where the thickness is thin, and is typically the surface of the blade leading edge. Figure 2 is a schematic diagram of a typical steam turbine blade. In Figure 2, the blade leading edge 10 of the steam turbine blade 1, indicated by a dotted line, is a portion where corrosion is likely to occur. Typically, this portion is irradiated with a laser spot S, and heat treatment (hardening) by laser irradiation can be performed.

A laser generator 21 and a laser irradiation head 22 can be used for the laser irradiation. As the laser generator 21 (laser light source), a semiconductor laser is preferably used. The laser irradiation head 22 has a focusing lens (not shown) that focuses the laser light L generated by the laser generator 21, and the laser light L that passes through this focusing lens is irradiated onto the surface of the blade leading edge portion 10 of the steam turbine blade 1. The laser irradiation condition is that the irradiation area (laser spot area) is 400 ^mm2 or more. By increasing the irradiation area, the hardening volume increases, and it becomes possible to increase the hardening depth. If the irradiation area is less than 400 ^mm2 , the hardening depth is insufficient and falls below the required hardening depth of 2 mm. In this specification, the hardening depth refers to the depth of a portion having a hardness of 430 Hv or more when the Vickers hardness of the cross section of the laser irradiated portion is measured. The hardening depth is also sometimes called the hardening depth. In this specification, the Vickers hardness is exemplified, but the hardness can be specified by measuring using other hardness tests such as Rockwell, Brinell, and Knoop hardness, and converting the hardness into a Vickers hardness value. The required hardening depth can be achieved by changing the irradiation area according to the thickness of the leading edge of the steam turbine blade within a range of 400 ^mm2 or more. The shape of the laser spot is not particularly limited, but can be a square, rectangular, circular, or elliptical shape. In addition, when the laser spot shape is rectangular, the length of the side in the width direction can be longer than the length of the side in the scanning direction of the laser light, or vice versa. The same applies to the case of an ellipse.

The laser irradiation head 22 is positioned in such a manner that it is possible to prevent the laser light L reflected by the base material surface of the laser irradiation section from entering the laser device. Specifically, it is preferable to position the laser light L so that it is irradiated at an angle of, for example, 45° to the base material surface, but this is not limited to a specific irradiation angle.

In the heat treatment process by laser irradiation, the laser light is irradiated at a temperature in the range of 1150°C to 1300°C. Here, the temperature in the range of 1150°C to 1300°C refers to the surface temperature of the laser irradiated part. This temperature is also called the solution temperature. If the solution temperature is less than 1150°C, the hardening depth is significantly reduced and falls below the required hardening depth of 2 mm. Also, if the solution temperature is higher than 1300°C, the SCC resistance is significantly reduced due to the formation of δ-ferrite and the coarsening of the grain boundaries. In the hardening treatment device 2 shown in Figure 1, the surface temperature of the laser irradiated part can be controlled to a constant value by a temperature control system consisting of the control device 25 and the radiation detector 26. Therefore, in the heat treatment process by laser irradiation, the laser irradiated part can be precisely heat-treated at a temperature at which the surface of the base material does not melt.

The surface temperature of the irradiated portion can be controlled by feedback control, and includes, for example, a process of detecting the temperature of the laser irradiated portion based on the thermal radiation from the laser irradiated portion, and a process of feedback controlling the laser output using the laser irradiated portion temperature as an index so that the temperature of the leading edge surface is within the temperature range of 1150°C to 1300°C.

In the heat treatment process by laser irradiation, the laser light is scanned in the X and Y directions, so that the hardening treatment can be performed on the entire laser irradiation portion corresponding to the leading edge of the turbine blade under the above irradiation area and temperature conditions. The scanning speed of the laser light can be, for example, 1 to 5 mm/sec, but is not limited to a specific value. The laser light scanning can be performed by the laser light scanning mechanism 24. The laser light scanning mechanism 24 drives the laser irradiation head 22 to scan the laser light L in two-dimensional directions. The laser light scanning mechanism 24 can be composed of, for example, an X-direction driving mechanism that drives the laser irradiation head 22 in the X direction (left and right direction in FIG. 1) and a Y-direction driving mechanism that drives the laser irradiation head 22 in the Y direction (direction perpendicular to the paper surface in FIG. 1) (neither is shown). The irradiation head position detector 23 detects the position of the laser irradiation head 22 and may be attached to the laser light scanning mechanism 24. Since the base material is usually processed into a three-dimensional shape, the three-dimensional shape of the base material is stored in advance using a touch sensor or the like. The laser irradiation head 22 is then positioned using the irradiation head position detector 23, and scanning is performed using the laser light scanning mechanism 24.

An example of feedback control in a heat treatment process by laser irradiation using the device shown in Figure 1 will be described in more detail. Laser light L is irradiated from the laser irradiation head 22 to the front edge portion 10 of the base material. Next, when the laser light L is scanned in two dimensions by the laser light scanning mechanism 24, the control device 25 takes in the output of the irradiation head position detector 23 and determines whether the laser irradiation head 22 has reached the irradiation completion position of the laser light L. Here, if the laser irradiation head 22 has reached the laser irradiation completion position, the output of the laser generator 21 is reduced to zero, and the heat treatment process by laser irradiation is terminated.

If the laser irradiation head 22 has not reached the laser irradiation completion position, the control device 25 takes in the output of the thermal radiation detector 26, converts the temperature based on the intensity of the thermal radiation light M, which is the output, and determines whether it is 1300°C or less. If the converted temperature is above 1300°C, the control device 25 controls the laser output of the laser generator 21 so that the intensity of the thermal radiation light M corresponds to a temperature of 1300°C or less. Specifically, the control device 25 controls the laser output of the laser generator 21 according to a function whose variable is the deviation between the detected intensity of the thermal radiation light M and the intensity corresponding to the upper limit temperature. Then, the process returns to the step of taking in the output of the irradiation head position detector 23.

On the other hand, if the intensity of the thermal radiation light M detected by the thermal radiation light detector 26 is equal to or lower than the intensity corresponding to the upper limit temperature of 1300°C, the control device 25 judges whether the intensity of the thermal radiation light M detected by the thermal radiation light detector 26 is equal to or higher than the lower limit intensity corresponding to the lower limit temperature of 1150°C. If the intensity is equal to or higher than the lower limit intensity, the process returns to the step of acquiring the output of the irradiation head position detector 23. If the intensity of the thermal radiation light M detected by the thermal radiation light detector 26 is lower than the lower limit intensity, the control device 25 controls the output of the laser generator 21 so that the output is equal to or higher than the lower limit intensity corresponding to the lower limit temperature of 1150°C. Specifically, the control device 25 controls the output of the laser generator 21 so that the output of the laser generator 21 is equal to, for example, 40 W added to the current output. Then, the process returns to the step of acquiring the output of the irradiation head position detector 23.

In this way, feedback control is repeated from the start to the end of laser light irradiation. This allows a heat treatment process to be carried out using laser irradiation with precise temperature control.

When the heat treatment process using laser irradiation is completed, the processed part has a martensite structure with minute amounts of inclusions such as Cr carbides precipitated.

After the heat treatment process by laser irradiation, preferably after the temperature of the base material has dropped to about room temperature, a post-heat treatment process is carried out. At this time, a heat removal process such as gas spraying may be carried out to remove heat from the base material. It is preferable to carry out the post-heat treatment process, for example, one hour to one day after the temperature of the base material has dropped to about room temperature. This is to prevent quench cracks during the post-heat treatment process.

The post-heat treatment process is a process in which post-heat treatment is performed at a temperature at which the front edge surface is 400°C to 600°C, with a holding time of 1 to 10 hours. The treatment temperature in the post-heat treatment process can also be called the precipitation strengthening temperature. By performing post-heat treatment at the precipitation strengthening temperature, precipitates whose main component is Cu or the like are formed in the base material, and the hardness of the front edge can be improved. In addition, the post-heat treatment process can promote the diffusion of Cr and reduce the sensitization of the metal structure that may occur in the laser irradiation process. The post-heat treatment process is preferably performed by heating the entire base material. This is because the post-heat treatment process takes up to about 10 hours, and localized heating leads to poor productivity. However, if time is not an issue in terms of productivity, localized heating may be used.

For example, the base material is heated in an atmosphere by placing it in a heating furnace and holding it at a temperature in the range of 400°C to 600°C for 1 to 10 hours. If the precipitation strengthening temperature is lower than 400°C, the amount of precipitates is insufficient, and if it is higher than 600°C, the precipitates become coarse and sufficient hardness cannot be obtained. The precipitation strengthening temperature refers to the surface temperature of the base material, not the temperature inside the furnace, and is, for example, a value measured by a thermocouple or the like attached to the surface of the base material. If the holding time is shorter than 1 hour, the amount of precipitates formed is insufficient. The upper limit of the holding time is preferably within 10 hours. This is because if heated for a long period of time, the precipitates become coarse and the hardness decreases. The holding time is preferably determined depending on the temperature. For example, if the holding temperature is 400°C to 500°C, the holding time can be 7 to 10 hours, and if the holding temperature is 500°C to 600°C, the holding time can be 1 to 7 hours.

More preferably, the base material is placed in a furnace at room temperature, heated at a rate of, for example, 50 to 150°C/h, and held at a constant temperature within the range of 400°C to 500°C for the specified time mentioned above, then slowly cooled in the furnace and removed after returning to room temperature.

Localized heating can be achieved, for example, by irradiating the leading edge, which has previously been heat-treated with laser irradiation, with a laser again at the precipitation strengthening temperature. In this case, the base material surface temperature can be adjusted by feedback control, as in the previous heat treatment process.

The steam turbine blade obtained through the above process has a high-hardness layer with a hardness of 430 Hv or more and a hardening depth of 2 mm or more, preferably 6 mm or more. The state of the structure at this point is that fine precipitates (main component Cu, etc.) are formed in the martensite structure. The amount of δ-ferrite formed in the laser irradiated area is 5% or less. The amount of δ-ferrite formed here can be calculated by taking multiple images of the surface of the laser irradiated area and calculating the area of δ-ferrite structure per observation area.

The rupture life of the steam turbine blade obtained through the above process is 3000 hours or more. The rupture life is defined as the life until rupture occurs when a tensile test is performed by immersing a sample with a stress of 600 MPa in an aqueous solution containing 30,000 ^ppm Cl- and at a temperature of 80°C, in which _N2 gas is bubbled.

The manufacturing method of the steam turbine blade according to this embodiment can be applied to the manufacture of any steam turbine blade using a precipitation hardening stainless steel base material, and is particularly suitable for the manufacture of steam turbine blades for thermal power generation. The obtained steam turbine blade has both erosion resistance and SCC resistance.

The present invention will now be described in more detail with reference to examples. However, the present invention is not limited to the following examples.

In the following examples and comparative examples, austenitic precipitation hardened stainless steel was used as the base material, processed into the shape of a steam turbine blade. In order to suppress variation in laser absorption rate, the leading edge surface of the base material processed into the shape of a steam turbine blade was polished in advance with abrasive paper with a roughness of approximately #60 to #120. A semiconductor laser (manufactured by Laserline) was used for the heat treatment process using laser irradiation.

(1) Laser spot size and base material thickness A heat treatment process was performed with different laser spot sizes on steam turbine blade base materials with different thicknesses of the leading edge. The heat treatment temperature at this time was controlled so that the leading edge surface temperature was 1250°C, and the laser scanning speed was 1 mm/s. Figure 3 is a graph showing the relationship between the laser spot size that achieves the required hardening depth of 2 mm or more and the thickness of the leading edge. The hardening depth was measured by measuring the Vickers hardness of the cross section of the laser irradiated part, and the depth of the part with a hardness of 430 Hv or more was taken as the hardening depth. From Figure ³ , the thickness of the leading edge and the laser spot size that achieves the required hardening depth are in a proportional relationship, and although the numerical values are not shown in the graph, it was confirmed that the required hardening depth can be obtained for a general steam turbine blade by setting the laser spot size to 400 mm2 or more.

(2) Heat treatment temperature, δ-ferrite formation, and corrosion resistance Steam turbine blades of the examples and comparative examples were manufactured by changing the heat treatment temperature. The steam turbine blade of the examples was obtained by performing a heat treatment process by laser irradiation with a laser spot size of 1600 ^mm2 and a surface temperature of 1150°C, followed by a post-heat treatment process of heating in a furnace at 425°C for 8 hours. The steam turbine blade of the comparative example was obtained in the same manner, except that the surface temperature in the heat treatment process by laser irradiation was set to over 1300°C. Scanning electron microscope photographs of the steam turbine blade surfaces at the locations where laser irradiation was performed in the examples and comparative examples after the post-heat treatment process were measured.

Figure 4 shows a photograph of the surface of the steam turbine blade of the example, and Figure 5 shows a photograph of the surface of the steam turbine blade of the comparative example. No δ-ferrite formation was observed in the sample of Figure 4, and similarly in other fields of view, there was no δ-ferrite formation, or even if there was, the amount of formation was 5% or less. In Figure 5, the formation of δ-ferrite structure 3 was visible, and the amount of formation was more than 5%.

A corrosion resistance test was conducted on the steam turbines of the examples and the comparative example. The test conditions were as follows: a sample was immersed in an aqueous solution with ^Cl- : 30,000 ppm and temperature: 80°C, bubbled with _N2 gas, to which a stress of 600 MPa was applied, and a tensile test was conducted to measure the life until fracture. As a result, the fracture life of the steam turbine of the example was 3,000 hours or more, and the fracture life of the steam turbine of the comparative example was 900 hours.

(3) Solution temperature, precipitation hardening temperature and hardness A plate-shaped austenitic precipitation hardening stainless steel base material was subjected to a heat treatment process by laser irradiation and a post-heat treatment process at different temperatures, and the Vickers hardness of the surface of the laser irradiated area was measured. The laser spot size during laser irradiation was 1600 ^mm2 in both cases, and the post-heat treatment process was carried out for 8 hours. The results are shown in Table 1.

From Table 1, it was confirmed that the surface hardness was 430 Hv or more by performing a heat treatment process by laser irradiation in a specific solution temperature range and a post-heat treatment process in a precipitation strengthening temperature range. In addition, although no data is shown in Table 1, samples that were subjected to a heat treatment process by laser irradiation in the range of 1150 to 1300°C and then a post-heat treatment process in the range of 400 to 500°C achieved a hardening depth of 2 mm or more, particularly a hardening depth of about 7 to 9 mm. Note that, referring to Table 1, there were samples that were able to achieve a surface hardness of 430 Hv or more even when heated at 1100°C, but the hardening depth did not reach 2 mm.

The steam turbine blades manufactured by the method of the present invention are preferably used for power generation. For example, they are preferably used as steam turbine blades for thermal power generation, and are particularly preferably used as turbine blades that are subject to high stress.

Reference Signs List 1 Precipitation hardening stainless steel base material, 10 Front edge portion 2 Hardening treatment device, 21 Laser generator, 22 Laser irradiation head 23 Irradiation head position detector, 24 Laser beam scanning mechanism 25 Control device, 26 Radiation light detector

Claims (4)

a heat treatment step of irradiating a portion of the precipitation hardened stainless steel base material processed into a steam turbine blade shape , which corresponds to a leading edge portion of the steam turbine blade, with a laser beam at a surface temperature of 1150°C to 1300°C and over an irradiation area of 400 mm2 or ^more ;
and a post-heat treatment step of performing a post-heat treatment at a surface temperature of the leading edge portion of 400° C. to 600° C. for a holding time of 1 to 10 hours after the heat treatment step ,
The heat treatment step comprises:
detecting a surface temperature of the front edge portion of the laser irradiation portion based on thermal radiation light from the laser irradiation portion;
feedback-controlling the laser output so that the surface temperature of the front edge of the laser irradiation portion is 1150° C. to 1300° C.;
Including,
a high-hardness layer having a hardness of 430 Hv or more and a hardening depth of 2 mm or more is formed on the leading edge surface after the post-heat treatment step . 2. The method for manufacturing a steam turbine blade according to claim 1 , wherein the post-heat treatment step is performed by heating the entire base material or by locally heating a laser irradiated portion. 3. The method for manufacturing a steam turbine blade according to claim 1 , wherein after the post-heat treatment step, an amount of precipitation of δ-ferrite calculated from an area of δ-ferrite structure per observation area on the surface of the leading edge of the steam turbine blade is 5% or less. A steam turbine blade in which a high-hardness layer having a hardness of 430 Hv or more and a hardening depth of 2 mm or more is formed at the leading edge of a precipitation- hardened stainless steel base material, and the amount of precipitation of δ-ferrite calculated from the area of δ-ferrite structure per observation area on the surface of the high-hardness layer is 5% or less.