Method for producing stainless steel and method for manufacturing turbine blade

Abstract

To provide a stainless steel having low hydrogen embrittlement sensitivity and high strength.SOLUTION: A method for manufacturing a turbine blade includes the steps of: forming a blade body; performing first aging treatment on the blade body; and performing second aging treatment on the blade body on which the first aging treatment has been performed. The first aging treatment is performed at a temperature and for a time that fall within a γ phase formation region and outside a precipitation formation region, and the second aging treatment is performed at a temperature and for a time that fall outside the γ phase formation region and within the precipitation formation region.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to stainless steel, a turbine blade using the same, and a method for manufacturing the stainless steel.

Background Art

[0002] Stainless steel is used in various applications. For example, Patent Document 1 describes stainless steel used for turbine blades.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] The stainless steel described in Patent Document 1 is a precipitation hardening type martensitic stainless steel in which mechanical strength, toughness, and corrosion resistance are balanced at a higher level than before and which has excellent corrosion resistance. Here, since stainless steel with high strength generally has high susceptibility to hydrogen embrittlement, it cannot be used in a corrosive environment such as a steam turbine. By replacing it with a lightweight titanium alloy, the susceptibility to hydrogen embrittlement can be reduced, but the material cost increases and the materials are limited.

[0005] The present disclosure solves the above-described problems, and an object thereof is to provide a stainless steel having low susceptibility to hydrogen embrittlement and high strength, a turbine blade using the same, and a method for manufacturing the stainless steel.

Means for Solving the Problems

[0006] The present disclosure for achieving the above object is stainless steel in which the ratio of the austenite phase (γ phase) to the whole is 10% or more and the size of the precipitation phase is 0.5 μm or less.

[0007] The present disclosure for achieving the above object is stainless steel, in which the proportion of the austenite phase (γ phase) with respect to the whole is 10% or more, and the 0.2% proof stress is 1000 MPa or more.

[0008] Further, the present disclosure is a turbine blade including a blade body formed of the stainless steel according to any one of the above.

[0009] Further, the present disclosure is a method for manufacturing stainless steel, including a step of forming a blade body, a step of performing a first aging treatment on the blade body, and a step of performing a second aging treatment on the blade body on which the first aging treatment has been performed.

Advantages of the Invention

[0010] According to the present disclosure, it is possible to increase the strength while reducing the hydrogen embrittlement susceptibility.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Modes for Carrying Out the Invention

[0012] Hereinafter, with reference to the drawings, preferred embodiments of the present disclosure will be described in detail. Note that the present disclosure is not limited by this embodiment, and when there are a plurality of embodiments, those configured by combining each embodiment are also included. In addition, the components in the embodiments include those that can be easily assumed by those skilled in the art, those that are substantially the same, and those within the so-called equivalent range.

[0013] FIG. 1 is a flowchart showing an example of the manufacturing method of the present embodiment. FIG. 2 is an explanatory diagram showing an example of the manufacturing method of the present embodiment. The present disclosure is a manufacturing method of stainless steel. Stainless steel is steel containing 12% by mass or more of Cr (chromium) and 2% by mass or less of C (carbon) in Fe (iron). In the present embodiment, the manufacturing of a turbine blade using stainless steel will be described. Note that the use and shape of the stainless steel are not limited thereto. As the turbine blade, for example, it can be used as the moving blade of the final stage of a steam turbine. The turbine blade includes, for example, a blade body and a connecting portion that connects to the rotating body shaft. It is preferable that both the blade body and the connecting portion of the turbine blade are formed of stainless steel, but only the blade body may be formed of stainless steel.

[0014] As shown in FIG. 1, the manufacturing method of stainless steel according to the present embodiment includes, for example, a step of forming a blade body of a turbine blade such as a moving blade or a stationary blade of a steam turbine (step S12), a step of performing a first aging treatment (single-stage aging treatment) on the blade body (step S14), and a step of performing a second aging treatment (two-stage aging treatment) on the blade body that has undergone the first aging treatment (step S16). The manufacturing method of stainless steel is not limited to the above steps, and after the treatment in step S12, for example, processing of the blade body may be performed.

[0015] In step S12, the wing body is formed. The wing body is formed of stainless steel. The stainless steel of the present embodiment is preferably steel in which, in Fe, C is 0.1 mass% or less, Ni is 2.0 mass% or more and 12.0 mass% or less, Cr is 10.0 mass% or more and 18.0 mass% or less, Mo is 3.0 mass% or less, and Cu is 6.0 mass% or less. Further, the stainless steel preferably has Si of 0.15 mass% or less, Mn of 0.15 mass% or less, P of 0.05 mass% or less, S of 0.05 mass% or less, Al of 1.5 mass% or less, N of 0.05 mass% or less, Ti of 2.0 mass% or less, and Nb of 1.0 mass% or less. Note that the stainless steel contains inevitable impurities in addition to the above materials.

[0016] The wing body is formed by forging or the like using the above materials.

[0017] In step S14, a first aging treatment is performed on the wing body. In the first aging treatment, the wing body is placed in a heating furnace and heated to precipitate a γ phase in the matrix phase. The first aging treatment is performed by heat treatment at, for example, a first temperature for a predetermined time.

[0018] In step S16, a second aging treatment is performed on the wing body. The second aging treatment is continuously performed after the completion of the first aging treatment. Specifically, the second aging treatment is performed by changing the temperature conditions of the heating furnace in which the first aging treatment was performed. Alternatively, after performing the first aging treatment, the wing body is cooled to room temperature once, and then reheated to perform the second aging treatment. In the second aging treatment, intragranular precipitates are precipitated in the matrix phase by heating the wing body. The second aging treatment is performed by heat treatment at, for example, a second temperature for a predetermined time. The second temperature is lower than the first temperature.

[0019] The stainless steel, which is the wing body (base material), as shown in Fig. 2, has a range 12 where γ-phase is precipitated and a range 14 where intragranular precipitates are precipitated, depending on the time and temperature of the aging treatment. The ranges 12 and 14 vary according to the type of stainless steel, but range 12 is on the higher temperature side than range 14. Also, ranges 12 and 14 partially overlap. The stainless steel generates γ-phase under the condition of being included in range 12 and not being included in range 14. Both γ-phase and intragranular precipitates are generated under the condition of being included in both ranges 12 and 14. The stainless steel generates intragranular precipitates under the condition of not being included in range 12 and being included in range 14.

[0020] The manufacturing method of this embodiment performs two aging treatments at the temperature and time shown by the condition line 16. Specifically, the first aging treatment is carried out at temperature A 1 for a time from t 0 to t 1 , and the second aging treatment is carried out at temperature A 2 for a time from t 1 to t 2 . In Fig. 2, although the temperature changes from temperature A 1 to temperature A 1 to temperature A 2 at time t 2 , it is changed by a predetermined change amount based on the performance of the heating furnace. Or, it may be heated to temperature A 0 after being cooled to room temperature once. The temperature A 1 from time t 1 is a condition that is included in range 12 and not included in range 14. The temperature A 1 from time t 2 is a condition that is not included in range 12 and is included in range 14. In Fig. 2, a comparison condition line 18 is shown for comparison. The comparison condition line 18 is the case where the aging treatment is a single-stage aging treatment, and shows the case of heating at temperature A 2 from time t 3 to time t o to time t a .

[0021] As shown in Fig. 1, the stainless steel of this embodiment is subjected to two-step aging treatment. Also, as shown in Fig. 2, the stainless steel of this embodiment executes the first aging treatment (one-step aging treatment) under conditions that generate the γ phase, and executes the second aging treatment (two-step aging treatment) under conditions that generate intragranular precipitates.

[0022] Fig. 3 is an explanatory diagram showing an example of the stainless steel of this embodiment. Fig. 3 shows the observation results of the stainless steel manufactured by the manufacturing method of this embodiment and having a γ-phase amount of 15 vol%. The γ-phase amount (volume fraction amount of the γ phase) is calculated by measuring the diffraction peak intensities of the matrix phase and the γ phase using X-rays and multiplying them by a correction coefficient. Fig. 3 shows the state where the cross-section is magnified 2000 times, 7000 times, and 30,000 times. As shown in Fig. 3, in the stainless steel with a γ-phase amount of 15 vol%, the γ phase (austenite phase) 24 and the precipitate phase 26 precipitate in the α phase (matrix phase, ferrite phase) 20.

[0023] As shown in Fig. 3, in the stainless steel of this embodiment, the γ phase and the precipitate phase are dispersed and formed in the α phase. Also, in the stainless steel of this embodiment, it is preferable that the γ phase with respect to the whole is 10% or more. Further, the precipitate phase is an intermetallic compound phase such as Ni 3 Al, Ni 3 Ti or a Cu-rich phase, and the particle size is 0.5 μm or less.

[0024] Fig. 4 is a graph showing an example of the relationship between the strength of the stainless steel and the amount of the γ phase. Fig. 5 is a graph showing the relationship between the strength of the stainless steel and the aging conditions. Fig. 6 is a graph showing an example of the relationship between the strength of the stainless steel and the amount of the γ phase. Fig. 7 is a schematic diagram showing an example of the relationship between the amount of the γ phase and the fracture time.

[0025] The stainless steel is manufactured by single-stage aging treatment as shown by the comparative condition line 18 in FIG. 2. For example, CUSTOM455 (registered trademark) of maraging steel is defined to be manufactured by single-stage aging treatment according to standards such as ASM5617. In this case, the amount of γ-phase (γ amount) can be controlled by controlling the aging treatment conditions during manufacturing. As shown in FIG. 4, for the stainless steel, when the amount of γ-phase increases, the strength, for example, the 0.2% proof stress, tends to decrease. Here, the 0.2% proof stress is the stress at which the permanent strain during unloading becomes 0.2%. Also, for the stainless steel, as the amount of γ-phase increases, the hydrogen embrittlement sensitivity decreases. That is, it becomes less likely to be embrittled by hydrogen.

[0026] FIGS. 5 to 7 are examples of the case of stainless steel having components corresponding to CUSTOM455 of maraging steel. FIG. 5 shows a comparison of the strength between single-stage aging stainless steel and two-stage aging stainless steel. As shown in FIG. 5, for the stainless steel of the present embodiment, as the second aging treatment, by performing 470°C for 24 hours, the hardness can be made higher than that in the case of manufacturing by single-stage aging. Also, the precipitates of CUSTOM455 are carbides, Cu-rich phases, and intermetallic compounds (Ni 3 Ti).

[0027] FIG. 6 shows the relationship between the hardness and the γ amount between single-stage aging stainless steel and two-stage aging stainless steel of the present embodiment. As shown in FIG. 6, by performing two-stage aging, the strength can be increased even with the same γ amount.

[0028] FIG. 7 is a graph showing the relationship between the amount of γ-phase and the strength reduction due to hydrogen embrittlement sensitivity. In FIG. 7, the vertical axis is a logarithmic axis. FIG. 7 shows the relationship between the amount of γ-phase and the fracture time when CUSTOM455 is manufactured by single-stage aging. The fracture time was taken as the time when the test piece was visually confirmed. As shown in FIG. 7, for the stainless steel, as the amount of γ-phase increases, the fracture time becomes longer. That is, it can be seen that by increasing the amount of γ-phase, specifically, by setting the amount of γ-phase to 10% or more, the hydrogen embrittlement sensitivity can be lowered.

[0029] Here, FIG. 8 is a graph showing the relationship between the aging temperature and the amount of the γ phase. FIG. 8 is an example in the case of stainless steel having a composition corresponding to CUSTOM455 of maraging steel. In FIG. 8, the horizontal axis represents the aging temperature, and the vertical axis represents the amount of γ (the amount of the γ phase). The aging temperature on the horizontal axis is the aging temperature of single-stage aging. FIG. 8 shows the case where two-stage aging is performed at 470°C for 4 hours after performing single-stage aging for 1 hour. Further, FIG. 8 also shows the case where single-stage aging is performed for 1 hour for comparison and then manufactured.

[0030] As shown in FIG. 8, by performing two-stage aging, more γ phase can be formed in single-stage aging. Further, in the case of CUSTOM455, the single-stage aging is preferably performed at 625°C or higher and 700°C or lower, preferably 670°C or higher and 700°C or lower. By setting it to 700°C or lower, the γ phase can be suppressed, and by setting it to 625°C or higher, the precipitation of precipitates can be suppressed. Thereby, while suppressing the generation of the precipitation phase, the γ phase can be formed. Further, the single-stage aging is preferably 4 hours or less. Thereby, it is possible to suppress the coarsening of the γ phase. Further, by performing single-stage aging under the above conditions, fine precipitates suitable as precipitation phases can be precipitated during two-stage aging, and the strength can be improved.

[0031] From the above, the stainless steel of the present embodiment can increase the amount of the γ phase and improve the strength by manufacturing with two-stage aging that satisfies the above conditions. Thereby, while reducing the hydrogen embrittlement susceptibility, the strength can be increased.

[0032] As described above, the more the γ phase and the precipitation phase that trap hydrogen precipitate, the lower the hydrogen embrittlement susceptibility. Further, the strength tends to increase as the amount of intragranular precipitates increases and decrease as the amount of the γ phase increases. When manufacturing by single-stage aging, by increasing the aging temperature, the amount of γ considered to be effective for hydrogen trapping can be increased, but the strength decreases because the precipitation phase coarsens. That is, when the γ phase is increased by single-stage aging, fine precipitates are not formed, so the strength decreases.

[0033] The method for manufacturing a stainless steel according to the present disclosure includes a step of forming a blade body, a step of performing a first aging treatment on the blade body, and a step of performing a second aging treatment on the blade body that has undergone the first aging treatment. Thereby, it is possible to manufacture a high-strength stainless steel while reducing the hydrogen embrittlement susceptibility.

[0034] Also, the first aging treatment is preferably performed at a temperature and for a time that are included in the γ-phase generation region and not included in the precipitation generation region, and the second aging treatment is preferably performed at a temperature and for a time that are not included in the γ-phase generation region but are included in the precipitation generation region. Thereby, the γ-phase can be precipitated by the first aging treatment, and fine precipitates can be precipitated by the second aging treatment, and the performance of hydrogen embrittlement susceptibility and strength can be adjusted by each aging treatment.

[0035] Also, the second aging treatment is preferably at a temperature 100°C or more lower than the first aging treatment. Thereby, the first aging treatment can be performed in the γ-phase generation region, the second aging treatment can be performed in the precipitation generation region, and the execution of the aging treatment in the region where the γ-phase generation region and the precipitation generation region overlap can be suppressed.

[0036] Also, the first aging treatment is preferably 0.5 hours or more and 4 hours or less, and the second aging treatment is preferably 5 hours or more and 50 hours or less. Thereby, the γ-phase and the fine precipitates can be appropriately precipitated respectively.

[0037] Also, the first aging treatment is 550°C or more and 850°C or less, and the second aging treatment is 400°C or more and 600°C or less. Thereby, the γ-phase and the fine precipitates can be appropriately precipitated respectively.

[0038] The stainless steel of the present disclosure is manufactured by two-stage aging, with the γ-phase precipitating during the first aging and fine precipitates precipitating during the second aging. As a result, the stainless steel of the present disclosure has a γ-phase ratio of 10% or more with respect to the whole, and the size of the precipitated phase is 0.5 μm or less, that is, fine precipitates of 0.5 μm or less. Also, the stainless steel of the present disclosure has a γ-phase ratio of 10% or more with respect to the whole, and a 0.2% proof stress of 1000 MPa or more. Thereby, the γ-phase and fine precipitates can be appropriately formed, and while increasing the amount of the γ-phase, the strength can be improved. Thereby, while reducing the hydrogen embrittlement susceptibility, the strength can be increased.

[0039] The stainless steel of this embodiment is preferably a steel in which, in Fe, C is 0.1 mass% or less, Ni is 2.0 mass% or more and 12.0 mass% or less, Cr is 10.0 mass% or more and 18.0 mass% or less, Mo is 3.0 mass% or less, and Cu is 6.0 mass% or less. Also, it is more preferable that the stainless steel has Si of 0.15 mass% or less, Mn of 0.15 mass% or less, P of 0.05 mass% or less, S of 0.05 mass% or less, Al of 1.5 mass% or less, N of 0.05 mass% or less, Ti of 2.0 mass% or less, and Nb of 1.0 mass% or less. Note that the stainless steel contains inevitable impurities in addition to the above materials. By using the stainless steel of the above composition, the strength and durability can be made within an appropriate range. Also, even a stainless steel with a small amount of Ti can have a low hydrogen embrittlement susceptibility.

[0040] Also, as the stainless steel, it is preferably used for maraging steel. Externally, it is preferable to have a composition corresponding to COSTOM450 (registered trademark), COSTOM455 (registered trademark), COSTOM465 (registered trademark).

[0041] In the case of COSTOM450 (registered trademark), the stainless steel is a steel containing Fe, with C being 0.05 mass% or less, Ni being 6.00 mass% or more and 7.00 mass% or less, Cr being 14.00 mass% or more and 16.00 mass% or less, Mo being 0.50 mass% or more and 1.00 mass% or less, and Cu being 1.25 mass% or more and 1.75 mass% or less. Also, the stainless steel has Si being 1.00 mass% or less, Mn being 1.00 mass% or less, P being 0.030 mass% or less, S being 0.030 mass% or less, Al being 0.055 mass% or less, and Nb being 0.01 mass% or more and 0.75 mass% or less.

[0042] In the case of COSTOM455 (registered trademark), the stainless steel is a steel containing Fe, with C being 0.03 mass% or less, Ni being 7.50 mass% or more and 9.50 mass% or less, Cr being 11.00 mass% or more and 12.50 mass% or less, Mo being 0.5 mass% or less, and Cu being 1.50 mass% or more and 2.50 mass% or less. Also, the stainless steel has Si being 0.50 mass% or less, Mn being 0.50 mass% or less, P being 0.015 mass% or less, S being 0.015 mass% or less, Ti being 0.90 mass% or more and 1.40 mass% or less, N being 0.015 mass% or less, and Nb being 0.50 mass% or less.

[0043] In the case of COSTOM465 (registered trademark), the stainless steel is a steel containing Fe, with C being 0.02 mass% or less, Ni being 10.75 mass% or more and 11.25 mass% or less, Cr being 11.00 mass% or more and 12.50 mass% or less, and Mo being 0.75 mass% or more and 1.25 mass% or less. Also, the stainless steel has Si being 0.25 mass% or less, Mn being 0.25 mass% or less, P being 0.015 mass% or less, S being 0.010 mass% or less, and Ti being 1.50 mass% or more and 1.80 mass% or less.

[0044] Also, as the stainless steel, it is preferably used for precipitation hardening type stainless steel. Externally, it is preferably made of components corresponding to 13-8Mo steel and 17-4PH stainless steel.

[0045] In the case of 13-8Mo steel, the stainless steel is preferably a steel in which, in Fe, C is 0.05 mass% or less, Ni is 7.50 mass% or more and 8.50 mass% or less, Cr is 12.25 mass% or more and 13.25 mass% or less, and Mo is 2.00 mass% or more and 2.50 mass% or less. Further, in the stainless steel, Si is 0.10 mass% or less, Mn is 0.10 mass% or less, P is 0.010 mass% or less, S is 0.008 mass% or less, Al is 0.9 mass% or more and 1.35 mass% or less, and N is 0.010 mass% or less.

[0046] In the case of 17-4PH stainless steel, the stainless steel is a steel in which, in Fe, C is 0.07 mass% or less, Ni is 3.00 mass% or more and 5.00 mass% or less, Cr is 15.00 mass% or more and 17.50 mass% or less, Mo is 0.50 mass% or less, and Cu is 3.00 mass% or more and 5.00 mass% or less. Further, in the stainless steel, Si is 1.00 mass% or less, Mn is 1.00 mass% or less, P is 0.040 mass% or less, S is 0.030 mass% or less, N is 0.05 mass% or less, and Nb is 0.15 mass% or more and 0.45 mass% or less.

[0047] Any of the stainless steels is manufactured by the manufacturing method of the present embodiment and has a structure that satisfies the phase composition or conditions of the present embodiment, whereby the hydrogen embrittlement susceptibility can be reduced and the strength can be increased.

[0048] Further, the stainless steel of the present embodiment is preferably used for a turbine blade, more specifically, the final stage of a turbine rotor blade, and more preferably used for the turbine rotor blade of the final stage of a steam turbine. By using it as a turbine blade, it is less likely to be hydrogen embrittled and can satisfy the strength as a turbine blade, and can be used for long-term operation of a turbine.

Explanation of symbols

[0049] Regions 12 and 14 Condition line 16 Comparative condition line 18

Claims

1. a first aging step in which austenite phase is formed (by reversing transformation) in a portion of the martensite phase; a second aging step for forming a precipitation phase in a part of the martensite phase; Including, The first aging step is carried out at a temperature and for a time sufficient to form an austenite phase without forming precipitates in the martensite phase; The method for producing stainless steel, wherein the second aging step is carried out at a temperature and for a time such that precipitates form in the martensite phase and that the austenite phase is not formed.

2. The first aging step is carried out so that the volume ratio of the austenite phase to the whole is 10% or more, 2. The method for producing stainless steel according to claim 1, wherein the second aging treatment step is carried out so that the size of the precipitated phase is 0.5 μm or less.

3. 3. The method for producing stainless steel according to claim 1, wherein the second aging treatment step is performed at a temperature that is at least 100° C. lower than that of the first aging treatment step.

4. The first aging step is from 0.5 hours to 4 hours, The method for producing a stainless steel according to any one of claims 1 to 3, wherein the second aging treatment step is performed for a period of 5 hours or more and 50 hours or less.

5. The first aging step is performed at a temperature of 550° C. or higher and 850° C. or lower, The method for producing stainless steel according to any one of claims 1 to 4, wherein the second aging treatment step is performed at a temperature of 400°C or higher and 600°C or lower.

6. forming a blade from a stainless steel having a martensite matrix phase formed therein; and treating the stainless steel in the shape of the blade by the method for producing stainless steel according to any one of claims 1 to 5.

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