Steam turbine rotor blade and method for manufacturing steam turbine rotor blade

Abstract

A steam turbine rotor blade achieving both abrasion resistance and reliability, and a method for manufacturing a steam turbine rotor blade capable of obtaining such a steam turbine rotor blade are provided. A steam turbine rotor blade according to the invention is characterized by including a blade base material and an erosion shield formed on a surface of the blade base material, wherein the blade base material is composed of a titanium alloy, and the erosion shield is composed of a weld overlay layer including a parent phase composed of pure titanium in which a metal element is solid-dissolved or a titanium alloy in which a metal element is solid-dissolved, and a hard phase dispersed in the parent phase.

Claims

1. A steam turbine rotor blade, comprising: a blade base material that is composed of a titanium alloy; and an erosion shield formed on a surface of the blade base material, wherein the erosion shield is composed of: a weld overlay layer that includes a parent phase composed of pure titanium in which a metal element is solid-dissolved or a titanium alloy in which the metal element is solid-dissolved, and a hard phase that is dispersed in the parent phase, wherein the hard phase is composed of titanium silicide.

2. The steam turbine rotor blade according to claim 1, wherein the weld overlay layer has a melted and solidified structure.

3. The steam turbine rotor blade according to claim 1, wherein the metal element includes chromium.

4. The steam turbine rotor blade according to claim 1, wherein the erosion shield has a thickness of 20 mm or more.

5. The steam turbine rotor blade according to claim 1, wherein the titanium alloy is 6Al-4VTi or 15Mo-5Zr-3AlTi.

6. The steam turbine rotor blade according to claim 1, wherein the erosion shield is provided in a tip portion of the steam turbine rotor blade.

7. The steam turbine rotor blade according to claim 1, wherein the steam turbine rotor blade is a long blade to be used for a last stage of a steam turbine.

8. The steam turbine rotor blade according to claim 1, wherein the metal element includes iron.

9. A method for forming an erosion shield on a surface of a blade base material, the method comprising: forming a weld overlay layer by melting a welding raw material powder containing a raw material powder of parent phase particles composed of pure titanium or a titanium alloy and a raw material powder of inorganic compound particles by a heat source on the blade base material, and forming a hard phase that is dispersed in the parent phase, wherein the hard phase is composed of titanium silicide.

10. A method for manufacturing a steam turbine rotor blade comprising: the method for forming the erosion shield according to claim 9; forming a welding shield groove portion in a portion of the blade base material where the erosion shield is formed; annealing the weld overlay layer; and machining the weld overlay layer.

11. The method for manufacturing the steam turbine rotor blade according to claim 10, wherein the erosion shield is provided in a tip portion of the steam turbine rotor blade.

12. The method for manufacturing the steam turbine rotor blade according to claim 10, wherein the steam turbine rotor blade is a long blade to be used for a last stage of a steam turbine.

13. The method for manufacturing forming the erosion shield according to claim 9, wherein the heat source is a laser, a plasma transfer arc, or an electron beam.

14. The method for forming the erosion shield according to claim 9, wherein the weld overlay layer has a melted and solidified structure.

15. The method for forming the erosion shield according to claim 9, wherein the inorganic compound particles contain at least one of chromium carbide, chromium silicide, chromium boride, and iron boride.

16. The method for forming the erosion shield according to claim 9, wherein the erosion shield has a thickness of 20 mm or more.

17. The method for manufacturing forming the erosion shield according to claim 9, wherein a content of the inorganic compound particles in the welding raw material powder is from 5 to 15 vol %.

18. The method for manufacturing forming the erosion shield according to claim 9, wherein the titanium alloy is 6Al-4VTi or 15Mo-5Zr-3AlTi.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a schematic view showing one example of a steam turbine rotor blade according to the invention.

(2) FIG. 2A is a view schematically showing one example of a cross section of a blade base material and an erosion shield in FIG. 1.

(3) FIG. 2B is a view schematically showing another example of a cross section of a blade base material and an erosion shield in FIG. 1.

(4) FIG. 3 is a light micrograph of a weld overlay layer of Example (No. 2).

DESCRIPTION OF EMBODIMENTS

(5) Hereinafter, a steam turbine rotor blade and a method for manufacturing a steam turbine rotor blade according to the invention will be described in detail. However, the invention is not limited to the embodiments described herein, and the embodiments can be combined or modified without changing the gist of the invention.

(6) [Steam Turbine Rotor Blade]

(7) FIG. 1 is a schematic view showing one example of a steam turbine rotor blade according to the invention. As shown in FIG. 1, a steam turbine rotor blade 10 according to the invention includes a blade base material 11 composed of a titanium alloy, and an erosion shield 15 formed on a surface of the blade base material 11. In FIG. 1, the erosion shield 15 is formed at a tip of the blade base material 11 where the peripheral speed is high and abrasion further increases. In addition, the blade base material 11 includes a stub 14 for integrating the adjacent blade by torsion, and a continuous cover 13. This steam turbine rotor blade 10 is an axial entry type having a blade root portion 12 with an inverted Christmas tree shape.

(8) FIG. 2A is a view schematically showing one example of a cross section taken along the line A-A of the blade base material and the erosion shield in FIG. 1, and FIG. 2B is a view schematically showing another example of a cross section taken along the line A-A of the blade base material and the erosion shield in FIG. 1. As shown in FIGS. 2A and 2B, the erosion shield 15 according to the invention is formed of a weld overlay layer (hereinafter also referred to as overlay layer) (hereinafter the reference numeral 15 also refers to weld overlay layer). The weld overlay layer 15 may be formed from a side edge to a wide surface of the blade base material 11 as shown in FIG. 2A, or may be formed only at a side edge of the blade base material 11 as shown in FIG. 2B. This weld overlay layer 15 is characterized by including a parent phase composed of pure titanium in which a metal element is solid-dissolved or a titanium alloy in which a metal element is solid-dissolved, and a hard phase dispersed in the parent phase. Hereinafter, the respective phases will be described in detail.

(9) (1) Parent Phase

(10) The parent phase (matrix) constituting the weld overlay layer 15 is formed from the same material as that of the blade base material 11, that is, a material in which a metal element is solid-dissolved in pure titanium or a titanium alloy. As the titanium alloy, for example, 6Al-4VTi or 15Mo-5Zr-3AlTi can be used. By forming the parent phase of the weld overlay layer 15 from the same material as that of the blade base material 11, the difference in the thermal expansion coefficient from the blade base material 11 is reduced, and the close adhesion between the blade base material 11 and the weld overlay layer 15 can be enhanced.

(11) In the weld overlay layer 15 according to the invention, a metal element is solid-dissolved in the above-mentioned parent phase. By solid-dissolving the metal element in the above-mentioned parent phase in this manner, the parent phase is hardened so as to improve the hardness of the weld overlay layer 15, and therefore, the abrasion resistance can be improved. This metal element is derived from inorganic compound particles as described in the below-mentioned manufacturing method. It is necessary that the metal element should be a material which is solid-dissolved in the parent phase to harden the parent phase, and specifically, chromium (Cr), iron (Fe), and the like are preferred. Incidentally, whether the above-mentioned metal element is in a state of being solid-dissolved in the parent phase can be evaluated by determining the crystalline structure and lattice constant of the parent phase of the weld overlay layer 15 by X-ray diffraction (XRD) measurement.

(12) (2) Hard Phase

(13) The weld overlay layer 15 according to the invention includes a hard phase dispersed in the parent phase. By including such a hard phase, also the hardness of the weld overlay layer 15 can be improved in addition to the parent phase in which the above-mentioned metal element is solid-dissolved. This hard phase is a phase produced by reacting part of the parent phase with a material derived from inorganic compound particles as described in the below-mentioned manufacturing method. As the hard phase, a phase containing at least one of titanium carbide, titanium silicide, and titanium boride can be exemplified. Incidentally, this hard phase can be analyzed by XRD measurement of the weld overlay layer 15.

(14) The erosion shield 15 according to the invention is a weld overlay layer including the above-mentioned parent phase and hard phase, and has a melted and solidified structure, and therefore is clearly different from the structure of the hard film formed by vapor deposition in the above-mentioned PTL 1. Further, the thickness of the weld overlay layer of the invention is not particularly limited, however, a weld overlay layer having a thickness of 20 mm or more can be formed, and is thicker than the hard film having a film thickness of several tens of micrometers described in PTL 1, and therefore, the reliability of the steam turbine rotor blade can be enhanced.

(15) In PTL 2, the surface of the blade base material is melted, and a ceramic powder is fed to the melted region, whereby the weld overlay layer is formed. However, when the feeding amount of the ceramic powder is increased, the ceramic powder is not diffused in the base material, and therefore, the composition of the weld overlay layer to be obtained is the same as the composition of the ceramic powder, and thus, a weld overlay layer having a structure in which a hard phase derived from inorganic compound particles is dispersed in a parent phase composed of pure titanium in which a metal element is solid-dissolved or a titanium alloy in which a metal element is solid-dissolved as in the invention is not formed.

(16) Further, also in the case where the ceramic powder (SiC, BN, or the like) described in PTL 3 is used, a structure in which a hard phase derived from inorganic compound particles is dispersed in a parent phase composed of pure titanium in which a metal element is solid-dissolved or a titanium alloy in which a metal element is solid-dissolved as in the invention is not formed.

(17) The steam turbine rotor blade according to the invention includes the erosion shield having both excellent abrasion resistance and excellent reliability. The application place of the steam turbine rotor blade is of course not limited, but is preferably a last stage of a steam turbine where the humidity is highest, and in such a case, the effect of the invention can be exhibited most strongly.

(18) [Method for Manufacturing Steam Turbine Rotor Blade]

(19) Next, a method for manufacturing a steam turbine rotor blade according to the invention will be described. The method for manufacturing a steam turbine rotor blade according to the invention includes a step of forming an erosion shield on a blade base material 11, and this step of forming the erosion shield includes a step of forming a weld overlay layer by melting a welding raw material powder containing a raw material powder of parent phase particles and a raw material powder of inorganic compound particles by a heat source on the blade base material 11. Specifically, a mixed powder (welding raw material powder) obtained by mixing a raw material powder of parent phase particles (parent powder) constituting the above-mentioned parent phase with inorganic compound particles is prepared. Subsequently, the above-mentioned mixed powder is melted by a heat source on the blade base material 11 to form an overlay layer. The type of the heat source is not particularly limited, but examples thereof include a laser, a plasma transfer arc, and an electron beam.

(20) As the inorganic compound particles, inorganic compound particles in which some are solid-dissolved in the parent phase, and some react with the parent phase to forma hard phase are used. Specifically, the inorganic compound particles preferably contain at least one of chromium carbide, chromium silicide, chromium boride, and iron boride, and among these, from the viewpoint of improving the hardness of the weld overlay layer 15, chromium boride is particularly preferred. In the case of using such a compound, a metal element (chromium or iron) constituting the inorganic compound particles is solid-dissolved in the parent phase in the process of melting to harden the parent phase. Further, an element (carbon, silicon, or boron) other than the metal element to be solid-dissolved in the parent phase of the inorganic compound particles reacts with part of the parent phase to form a hard phase.

(21) The content of the inorganic compound particles in the raw material powder is preferably from 5 to 15 vol %. When the content is less than 5 vol %, an effect of hardening the parent phase is not sufficiently obtained. Further, when the content is more than 15 vol %, the hardening of the parent phase excessively proceeds to generate a crack when welding. Incidentally, a preferred range of the content does not depend on the type of the inorganic compound particles.

(22) Incidentally, as manufacturing steps, a step of forming a welding shield groove portion in a portion of the blade base material 11 where the erosion shield 15 is formed, a step of annealing the weld overlay layer 15, and a step of machining the weld overlay layer 15 into a final shape may be included. The annealing is preferably performed in vacuum by setting the retention temperature to 450 C. to 550 C. (450 C. or higher and 550 C. or lower) and the retention time to 8 to 10 hours. By selecting a heat source whose heat input is small, annealing to remove residual stress can be omitted.

(23) The method for manufacturing a steam turbine rotor blade according to the invention is characterized by using a mixed powder obtained by mixing a parent powder with an inorganic compound powder so as to obtain the structure of the weld overlay layer described above. According to such a manufacturing method, the metal element in the inorganic compound particles is solid-dissolved in the parent phase to harden the parent phase, and further, due to a hard phase produced by reacting the parent phase with the inorganic compound particles, the hardness of the weld overlay layer can be further improved.

EXAMPLES

(24) Hereinafter Examples will be described.

Example 1

(25) (I) Element Test

(26) (i) Fabrication of Test Piece

(27) A method for fabricating test pieces (Inventive Materials Nos. 1 to 7 and Comparative Materials Nos. 8 to 11) subjected to an element test will be described. As the base material of Nos. 1 to 10, a 6Al-4VTi alloy was prepared, and as the base material of No. 11, a 15Mo-5Zr-3AlTi alloy forged plate was prepared. With respect to No. 1 to 10, a parent powder (titanium-based powder) which is the raw material powder of the parent phase particles and an inorganic compound powder which is the raw material powder of the inorganic compound particles are fed from different powder pods to a welding portion of the base material to give a predetermined mixing ratio, and melted on the surface of the base material by a laser, whereby a weld overlay layer was formed. The composition of the welding material and the composition of the hard phase are shown in the following Table 1. Incidentally, the descriptions in the parentheses in the column of Inorganic compound powder each denote the content of the inorganic compound powder in the raw material powder.

(28) (ii) Evaluation

(29) As the evaluation of the microstructure of the overlay layer, light microscopic observation and XRD measurement were performed. Further, the hardness of the overlay layer was evaluated by the Vickers hardness test. The test was performed in accordance with JIS (Japanese Industrial Standards) Z 2244. The evaluation of the abrasion resistance was performed by the water jet test. The evaluation results are also shown in the following Table 1.

(30) TABLE-US-00001 TABLE 1 Evaluation results Raw material powder Abrasion Parent powder Inorganic resistance (titanium-based compound Structure of weld overlay layer Hardness (thinning No. powder) powder Parent phase Hard phase (HV) depth ratio) Remarks Inventive 1 6Al4VTi Cr carbide Cr solid-dissolved in Titanium 402 0.83 good material (5 vol %) 6Al4VTi carbide 2 6Al4VTi Cr carbide Cr solid-dissolved in Titanium 444 0.51 good (10 vol %) 6Al4VTi carbide 3 6Al4VTi Cr carbide Cr solid-dissolved in Titanium 523 0.23 good (15 vol %) 6Al4VTi carbide 4 6Al4VTi Cr silicide Cr solid-dissolved in Titanium 420 0.62 good (10 vol %) 6Al4VTi silicide 5 6Al4VTi Cr boride Cr solid-dissolved in Titanium 475 0.38 good (10 vol %) 6Al4VTi boride 6 6Al4VTi Fe boride Fe solid-dissolved in Titanium 488 0.39 good (10 vol %) 6Al4VTi boride 7 Pure titanium Cr carbide Cr solid-dissolved in Titanium 432 0.58 good (10 vol %) pure titanium carbide Comparative 8 6Al4VTi Cr carbide Cr solid-dissolved in Titanium 369 1.11 Abrasion material (2.5 vol %) 6Al4VTi carbide resistance is not improved. 9 6Al4VTi Cr carbide Cr solid-dissolved in Titanium 559 Crack is (20 vol %) 6Al4VTi carbide generated. 10 6Al4VTi Ti carbide 6Al4VTi Titanium 434 1.44 Abrasion (10 vol %) carbide resistance is not improved 11 15Mo5Zr3AlTi 15Mo5Zr3AlTi 375 1 currently (forged plate) (forged plate) used shield material

(31) In Table 1, Nos. 1 to 7 are Examples according to the invention, and Nos. 10 and 11 are Comparative Examples. Nos. 8 and 9 are Reference Examples, and are not known materials because the preferred parent powder and the preferred inorganic compound powder of the invention are used as the weld overlay layer, however, the content of the inorganic compound powder is outside the preferred range of the invention. From the results shown in Table 1, Nos. 1 to 7 according to the present Examples all have an excellent hardness and excellent abrasion resistance. As a result of evaluation of the crystalline structure and lattice constant of pure titanium or the titanium alloy to serve as the parent phase by XRD measurement with respect to the test pieces of Nos. 1 to 10, it was confirmed that the metal element is solid-dissolved in the parent phase, and the hard phase is formed and dispersed in the parent phase. Hereinafter, the evaluation results of the respective test pieces will be described in detail.

(32) In the light microscopic observation of No. 1, a spherical deposit with a size of several tens of micrometers was confirmed in the overlay layer. When the spherical deposit was analyzed by XRD measurement, it was confirmed that the deposit is titanium carbide which is not contained in the raw material, and therefore, a hard phase is formed.

(33) FIG. 3 is a light micrograph of the weld overlay layer of Example (No. 2). As shown in FIG. 3, it is clearly found that a hard phase (black portion) 31 is deposited and dispersed in a parent phase 30 (gray portion).

(34) When comparing No. 1 with No. 3, as the amount of Cr carbide increased, the deposition amount and the size of titanium carbide which is the hard phase increased, and the hardness increased. In Nos. 4 and 5, as the inorganic compound powder, Cr silicide and Cr boride were used, respectively, however, it was confirmed that titanium boride and titanium silicide are formed, respectively, as the hard phase in the overlay layer. In No. 6, as the inorganic compound powder, Fe boride was used, however, it was confirmed that titanium boride is formed in the overlay layer in the same manner as in No. 5. In No. 7, pure titanium was used as the titanium-based powder, however, titanium carbide was confirmed as the hard phase in the overlay layer in the same manner as in Nos. 1 to 3. From the comparison of Nos. 2, 4, and 5, when the hard phase is titanium boride, higher hardness and more excellent abrasion resistance than in the case where the hard phase is titanium carbide or titanium silicide were obtained.

(35) In No. 8 which is a comparative material, the content of the inorganic compound powder was not sufficient, and the parent phase was not sufficiently hardened, and therefore, improvement of the abrasion resistance was not observed. In No. 9 which is a comparative material, the content of the inorganic compound powder was excessive, and therefore, a crack was generated in the overlay layer. Further, in No. 10, titanium carbide was confirmed, however, melting by a laser was not sufficient, and the particle diameter was equivalent to that of the raw material powder. Therefore, the metal element was not solid-dissolved in the parent phase.

(36) The depth of thinning by water jet of the 15Mo-5Zr-3AlTi alloy of No. 11, which is a currently used erosion shield material, was set to 1.0, and relative comparison of the depth of thinning with the respective test pieces was performed. As a result, the amount of thinning of the inventive material was smaller than that of the comparative material, and therefore, improvement of the abrasion resistance was confirmed. Further, there was a tendency that as the amount of the inorganic compound is larger and the hardness is higher, the thinning amount decreases. In No. 9 which is a comparative material, a crack was generated in the overlay layer, and therefore, the abrasion resistance could not be evaluated. Further, in No. 10, the inorganic compound powder did not contain an element which reacts with the parent phase to form a hard phase, and the abrasion resistance was not improved.

Example 2

(37) (II) Test Production of Actual Machine

(38) An example in which the steam turbine rotor blade according to the invention was used in an actual machine will be described. A 40-inch steam turbine titanium long blade composed of a 6Al-4VTi alloy was fabricated. As a method for fabricating the steam turbine titanium long blade, closed-die forging was performed for a 6Al-4VTi alloy billet using a 40-inch blade die. Thereafter, a heat treatment for adjusting the microstructure was performed. Subsequently, the entire blade was machined into a predetermined shape, and a shield welding groove portion was provided in a tip portion. In the shield welding groove portion, the raw material shown in No. 2 in Table 1 was melted by a laser, and overlay welding was performed. As final processing, processing of the overlay layer, or surface polishing or bend straightening of the entire blade was performed, whereby a 40-inch long blade was formed.

(39) When a defect inspection of the erosion shield portion of the steam turbine rotor blade obtained by the above-mentioned step was performed, no problem was observed. Further, when the erosion resistance of the steam turbine titanium long blade was evaluated in a verification test facility, thinning due to erosion by long-term use was almost not observed and was at a level free from problems, and it was confirmed that the reliability is excellent.

(40) As described above, it was demonstrated that according to the invention, a steam turbine rotor blade achieving both abrasion resistance and reliability, and a method for manufacturing a steam turbine rotor blade capable of obtaining such a steam turbine rotor blade can be provided.

(41) Incidentally, the above-mentioned embodiments specifically describe the invention for facilitating the understanding of the invention, and the invention is not limited to those including all the configurations described above. For example, part of the configuration of a certain embodiment can be replaced with the configuration of another embodiment, or the configuration of another embodiment can be added to the configuration of a certain embodiment. In addition, part of the configuration of each embodiment can be deleted or replaced with another configuration, or another configuration can be added thereto.

REFERENCE SIGNS LIST

(42) 10: steam turbine rotor blade, 11: blade base material, 12: root portion, 13: continuous cover, 14: stub, 15: erosion shield, 30: parent phase, 31: hard phase