JP5127749B2 - Ni-base alloy for turbine rotor of steam turbine and turbine rotor of steam turbine using the same - Google Patents

Description

  The present invention relates to a Ni-based alloy for a turbine rotor of a steam turbine, and a turbine rotor of a steam turbine using the Ni-based alloy.

In thermal power plants, attention is focused on technologies for suppressing carbon dioxide emissions from the viewpoint of protecting the global environment, and there is an increasing need for higher efficiency in power generation.
In order to increase the power generation efficiency of the steam turbine, it is effective to increase the steam temperature of the steam turbine. In recent thermal power plants, the steam temperature of the turbine has risen to 600 ° C. or more, and in the future Is expected to rise to 650 ° C. and further to 700 ° C. or higher.

  The turbine rotor that supports the rotating blades that receive high-temperature steam rotates to generate high stress due to the high-temperature steam circulating around it. Therefore, the turbine rotor of the steam turbine needs to withstand high temperature and high stress, and an alloy having excellent strength, ductility, and toughness in a region from room temperature to high temperature is required as a material constituting the turbine rotor.

In particular, when the steam temperature exceeds 700 ° C., a conventional iron-based material lacks high-temperature strength, and therefore, use of a Ni-based alloy has been studied (for example, see Patent Document 1).
Ni-based alloys have been widely used mainly as jet engine and gas turbine materials because of their high-temperature strength and corrosion resistance. Typical examples thereof include Inconel 617 alloy (made by Special Metal Co.) and Inconel 706 alloy ( Special metal)).

As a mechanism for strengthening the high temperature strength of the Ni-based alloy, by adding Al or Ti, a gamma prime phase (Ni ₃ (Al, Ti)) or a gamma double prime phase (Ni ₃ Nb) is known as a precipitated phase, or both of these phases are precipitated to ensure high temperature strength. For example, Inconel 706 alloy is an example.

  Also known is an Ni-based alloy such as Inconel 617 alloy in which Co and Mo are added to strengthen the Ni-based matrix (solid solution strengthening) to ensure high temperature strength.

Japanese Unexamined Patent Publication No. 7-150277

  As described above, as a turbine rotor material for a steam turbine exceeding 700 ° C., the use of a Ni-based alloy having a higher high-temperature strength than an iron-based material has been studied, while maintaining the hot workability of the Ni-based alloy, There is a need for an improved composition that satisfies high temperature strength, forgeability, and the like.

  Therefore, the present invention has been made in response to the above-described conventional circumstances, and maintains a hot workability, and is excellent in both high-temperature strength and forgeability. It aims at providing the turbine rotor of the used steam turbine.

  One aspect of the Ni-based alloy for the turbine rotor of the steam turbine of the present invention is, by mass, C: 0.01 to 0.15%, Cr: 18 to 28%, Co: 10 to 15%, Mo: 8 to 12%, Al: 0.5 to less than 1.5%, Ti: 0.7 to 3.0%, B: 0.001 to 0.006%, the balance being made of Ni and inevitable impurities It is characterized by.

    One aspect of the turbine rotor of the steam turbine of the present invention is a turbine rotor that is provided in a steam turbine into which high-temperature steam is introduced, and at least a predetermined portion is made of the above-described Ni-based alloy for the turbine rotor of the steam turbine. It is characterized by that.

  According to the present invention, it is possible to provide a Ni-based alloy for a turbine rotor of a steam turbine excellent in both high-temperature strength and forgeability while maintaining hot workability, and a turbine rotor of a steam turbine using the same. .

The structure photograph of the Ni base alloy of this embodiment. The graph which shows the result of a greeble test. Structure photograph of Ni-based alloy. Another structural photograph of Ni-based alloy.

Hereinafter, embodiments of a Ni-based alloy for a turbine rotor of a steam turbine according to the present invention and a turbine rotor of a steam turbine using the same will be described.
Ni-based alloys such as Inconel 706 alloy and Inconel 617 alloy are extremely useful materials as turbine rotor materials. However, in order to further improve the efficiency of steam turbine power generation equipment, hot workability of Ni-based alloys (drawing, etc.) While maintaining the above, it is necessary to satisfy high temperature strength (mechanical strength at high temperature) and forgeability.

For example, Inconel 617 alloy is an alloy in which high temperature strength is improved by solid solution strengthening of a Ni-based matrix by adding cobalt (Co) and molybdenum (Mo). However, solid solution strengthening alone is not always sufficient.
Therefore, the present invention aims to further increase the strength by utilizing precipitation strengthening in addition to solid solution strengthening.

Hereinafter, the content of the high intensity | strength is demonstrated in detail.
The Ni-based alloy for turbines of the steam turbine of the present invention is based on the composition of the Inconel 617 alloy, which is a typical Ni-based alloy, and has been improved in strength characteristics and forgeability at high temperatures by adjusting the addition.
The Ti content of the conventional Inconel 617 alloy is about 0.6% by mass, and precipitation strengthening cannot be expected at this level of content. Therefore, by increasing the Ti content to 0.7 to 3.0 mass%, precipitated gamma 'phase (gamma prime phase _{(Ni 3 (Al, Ti)} )) to increase the amount of precipitation of. Furthermore, FIG. 3 shows a structural photograph of the Ni-based alloy when the Al concentration is set to 1.6% by mass or more and the Ti concentration is set to 0.7% by mass or more in order to improve the high-temperature strength. In FIG. 3, as indicated by arrows, precipitation of a harmful phase called σ phase was observed. The composition of the Ni-based alloy shown in FIG. 3 is Ni-1.8Al-1.3Ti-23Cr-12Co-9Mo-0.1Ta-0.3Nb (the numbers described before the constituent components are The content rate (mass%) of a structural component and the remainder are Ni). Therefore, in the present invention, in order to prevent the precipitation of harmful phases, the Al concentration is set to less than 0.5 to 1.5% by mass, tantalum (Ta) and niobium (Nb) are added as necessary, and the γ ′ phase is added. In addition to the stable precipitation, the stability of the γ ′ phase itself was improved, and as a result, the strength of the Ni-based alloy could be increased.

Embodiments of the Ni-based alloy for the turbine rotor of the steam turbine of the present embodiment are as follows.
(Alloy 1) By mass%, C: 0.01 to 0.15%, Cr: 18 to 28%, Co: 10 to 15%, Mo: 8 to 12%, Al: 0.5 to 1.5% Less than, Ti: 0.7-3.0%, B: 0.001-0.006%, The Ni-base alloy for turbine rotors of the steam turbine which the remainder consists of Ni and an unavoidable impurity.
(Alloy 2) By mass%, C: 0.01 to 0.15%, Cr: 18 to 28%, Co: 10 to 15%, Mo: 8 to 12%, Al: 0.5 to 1.5% Less than, Ti: 0.7-3.0%, B: 0.001-0.006%, Ta: 0.1-0.7%, the remainder of the steam turbine consisting of Ni and inevitable impurities Ni-base alloy for turbine rotor.
(Alloy 3) By mass%, C: 0.01 to 0.15%, Cr: 18 to 28%, Co: 10 to 15%, Mo: 8 to 12%, Al: 0.5 to 1.5% Less than, Ti: 0.7-3.0%, B: 0.001-0.006%, Nb: 0.1-0.4%, the remainder of the steam turbine consisting of Ni and inevitable impurities Ni-base alloy for turbine rotor.
(Alloy 4) By mass%, C: 0.01 to 0.15%, Cr: 18 to 28%, Co: 10 to 15%, Mo: 8 to 12%, Al: 0.5 to 1.5% Less than, Ti: 0.7-3.0%, B: 0.001-0.006%, Ta: 0.1-0.7%, Nb: 0.1-0.4%, the balance A Ni-based alloy for a turbine rotor of a steam turbine consisting of Ni and inevitable impurities.
(Alloy 5) By mass, C: 0.01 to 0.15%, Cr: 18 to 28%, Co: 10 to 15%, Mo: 8 to 12%, Al: 0.5 to 1.5% Less than, Ti: 0.7-3.0%, B: 0.001-0.006%, Ta + 2Nb (Molar ratio of Ta and Nb is 1: 2): 0.1-0.7% A Ni-based alloy for a turbine rotor of a steam turbine, the balance being Ni and inevitable impurities.
In the following description, “%” representing the composition component of the alloy is “% by mass” unless otherwise specified.

Here, in the inevitable impurities in the Ni-based alloys for the turbine rotor of the steam turbine of the above (Alloy 1) to (Alloy 5), among the inevitable impurities, at least the Si content is 0.1% or less and the Mn content Is desirably reduced to 0.1% or less.
Next, the reasons for limiting the component ranges of the respective compositions in the Ni-based alloy for turbine rotors of the steam turbine of the present embodiment described above will be described.

(1) C (carbon)
C is useful as a constituent element of M ₂₃ C ₆ type carbide, which is a strengthening phase. Particularly in a high temperature environment of 650 ° C. or higher, precipitation of M ₂₃ C ₆ type carbide during turbine operation causes creep of the alloy. This is one of the factors that maintain strength. It also has the effect of ensuring the fluidity of the molten metal during casting. When the C content is less than 0.01%, a sufficient amount of precipitation of carbide cannot be secured, so that the high-temperature strength is lowered and the fluidity of the molten metal during casting is significantly lowered. On the other hand, when the C content exceeds 0.15%, the segregation tendency of the components increases when producing a large ingot, and the generation of M ₆ C type carbide which is an embrittlement phase is promoted. Strength is improved, but forgeability is reduced. Therefore, the C content is determined to be 0.01 to 0.15%.

(2) Cr (chromium)
Cr is an essential element for increasing the oxidation resistance, corrosion resistance, and high temperature strength of the Ni-based alloy. Further, it is indispensable as a constituent element of M ₂₃ C ₆ type carbide, and particularly in a high temperature environment of 650 ° C. or higher, the creep strength of the alloy is maintained by precipitating M ₂₃ C ₆ type carbide during turbine operation. . Moreover, Cr can improve the oxidation resistance in the environment of high temperature steam. When the Cr content is less than 18%, the oxidation resistance decreases. On the other hand, when the Cr content exceeds 28%, precipitation of M ₂₃ C ₆ type carbides is remarkably promoted to increase the coarsening tendency. Therefore, the Cr content is set to 18 to 28%.

(3) Co (cobalt)
Co has a function of strengthening the parent phase by dissolving in the parent phase in the Ni-based alloy. When the Co content is less than 10%, the high-temperature strength decreases. On the other hand, if the Co content exceeds 15%, a harmful intermetallic compound phase is generated, and the forgeability is further reduced. Therefore, the Co content is set to 10 to 15%.

(4) Mo (molybdenum)
Mo has the effect of increasing the strength of the parent phase by dissolving in the Ni parent phase, and the stability of the carbide can be increased by partially replacing the M ₂₃ C ₆ type carbide. When the Mo content is less than 8%, the above-described effects are not exhibited. On the other hand, when the Mo content exceeds 12%, the segregation tendency of components increases when a large ingot is manufactured. Promotes the formation of M ₆ C type carbides, which are embrittled phases. Therefore, the Mo content is set to 8 to 12%.

(5) Al (aluminum)
Al forms a γ 'phase together with Ni, and can strengthen the Ni-based alloy by precipitation. When the Al content is less than 0.5%, the high temperature strength decreases. On the other hand, when the Al content is 1.5% or more, there is a possibility that precipitation of an embrittlement phase called a σ phase is promoted with a decrease in forgeability. Therefore, the Al content is set to 0.5 to less than 1.5% (0.5% or more and less than 1.5%).

(6) Ti (titanium)
Ti, like Al, can form a γ ′ phase together with Ni and strengthen the Ni-based alloy. When the Ti content is less than 0.7%, the high temperature strength is equivalent to that of the conventional material. On the other hand, if the Ti content exceeds 3%, the hot workability decreases, the forgeability decreases, and the notch sensitivity increases. Therefore, the Ti content is set to 0.7 to 3.0%.

(7) B (boron)
B segregates at the grain boundary and can improve high temperature characteristics. This effect can be exhibited when the B content is 0.001% or more. However, if the B content exceeds 0.006%, grain boundary embrittlement may occur. Therefore, the content rate of B is set to 0.001 to 0.006%.

(8) Ta (tantalum)
Ta has an effect of stabilizing the precipitation strengthening phase by dissolving in the γ ′ phase. When the Ta content is less than 0.1%, the stabilizing effect is not exhibited. On the other hand, when the Ta content exceeds 0.7%, the high temperature strength is improved, but the forgeability is lowered. Therefore, the Ta content is set to 0.1 to 0.7%.

(9) Nb (Niobium)
Nb, like Ta, has the effect of solid-dissolving in the γ ′ phase to increase the high-temperature strength and stabilize the precipitation strengthening phase. When the Nb content is less than 0.1%, the above effects are not exhibited. When the Nb content exceeds 0.4%, the high temperature strength is improved, but the forgeability is lowered. Therefore, the Nb content is set to 0.1 to 0.4%.
In addition, when Ta and Nb are contained in the range of (Ta + 2Nb) in the range of 0.1 to 0.7%, the γ ′ phase is dissolved to increase the high-temperature strength and stabilize the precipitation strength. . When the content of (Ta + 2Nb) is less than 0.1%, the above effect is not improved as compared with the conventional material. On the other hand, when the content of (Ta + 2Nb) exceeds 0.7%, Strength is improved, but forgeability is reduced. In this case, Ta and Nb are each contained at least 0.01% or more.

(10) Si (silicon), Mn (manganese), Fe (iron), Cu (copper) and S (sulfur)
Si, Mn, Fe, Cu, and S are classified as inevitable impurities in the Ni-based alloy for the turbine rotor of the steam turbine of this embodiment. It is desirable that the residual content of these inevitable impurities is as close to 0% as possible. Of these inevitable impurities, at least Si and Mn are preferably suppressed to 0.1% or less, respectively.
In the case of ordinary steel, Mn prevents brittleness by using S (sulfur) due to brittleness as MnS. However, the S content in the Ni-based alloy is extremely small, and it is not necessary to add Mn. Therefore, in the Ni-based alloy for the turbine rotor of the steam turbine of this embodiment, it is desirable that the Mn content is 0.1% or less and the residual content is as close to 0% as possible.
In the case of ordinary steel, Si is added to supplement the corrosion resistance. However, since the Ni-based alloy has a large Cr content and can sufficiently secure corrosion resistance, in the Ni-based alloy for the turbine rotor of the steam turbine of the present embodiment, the Si content is set to 0.1% or less as much as possible. It is desirable to make the residual content close to 0%.

  FIG. 4 is a structural photograph of a conventional Inconel 617 alloy. FIG. 1 [Composition: Ni-0.05C-1.15Al-1.8Ti-23Cr-12Co-9Mo-0.1Ta-0.3Nb-0.003B] is the Ni base for the turbine rotor of the steam turbine of this embodiment. It is a structure photograph of an alloy. As shown in FIG. 1, the Ni-based alloy for the turbine rotor of the steam turbine of the present embodiment is indicated by an arrow in the γ parent phase while suppressing the precipitation of the σ phase by setting the alloy composition range of the alloy. Thus, it became possible to precipitate fine γ ′ stably.

Next, the preferable manufacturing method of the Ni-base alloy for turbine rotors of the steam turbine of this embodiment is demonstrated.
The alloy whose components are adjusted as described above is melted and cast by a conventional method. Thereafter, the ingot is subjected to stabilization treatment, normal hot forging, and solution treatment. In the solution treatment after hot forging, it is desirable that the temperature be higher than the melting temperature of the γ ′ phase and lower than the local melting start temperature. The conditions for the stabilization treatment and the solution treatment vary depending on the alloy composition and the size of the processed material, but the stabilization treatment can be performed, for example, by heating at a temperature range of 1000 to 1250 ° C. for 3 to 72 hours, On the other hand, the solution treatment can be performed, for example, by heating in a temperature range of 1000 to 1200 ° C. for 3 to 24 hours and then rapid cooling. These processes may be performed in multiple stages. Furthermore, if necessary, early precipitation of γ ′ can be achieved by performing an aging treatment for 3 to 24 hours in a temperature range of 700 to 800 ° C.

(Evaluation of high temperature strength characteristics and manufacturability)
In the present embodiment, the alloy composition, high temperature strength characteristics and manufacturability of the Ni-based alloy will be described with reference to the table. 20 kg of Ni-based alloy having chemical components shown in Table 1 was melted in a vacuum induction melting furnace, and forgings were obtained from 26 types of ingots. In the 21 types of comparative examples, in order to evaluate the range of the addition amount of each element shown in the present embodiment, the addition amount was adjusted out of the range. The remaining five types are examples. In addition, the comparative example 1 has a chemical component equivalent to the Inconel 617 alloy which is a conventional material. In Table 1, Si, Mn, Fe, Cu, and S are inevitably mixed.

  Each of the 26 types of forgings was obtained by cutting and removing the black skin structure from a solid cylindrical ingot having a diameter of about 125 mm and a length of about 210 mm. The forged material after removal of the black skin was about 120 mm in diameter and about 200 mm in length. These forgings were subjected to stabilization treatment at 1180 ° C. for 6 hours, and immediately thereafter hot forging was performed. Hot forging was carried out until the forging ratio reached 3, but at that time, the temperature of the forging material was measured, and when the temperature of the forging material decreased to 1000 ° C, the forging operation was temporarily suspended and reheating at 1180 ° C was performed. Carried out. When the forging ratio reached 3, that is, when the total length of the forged product reached 600 mm, the forging was finished and allowed to cool. At this point, the diameter of the forged product was about 70 mm. After cooling, the surface of the forged product was observed to check for forging cracks.

Next, each of the forged products was heated at 1170 ° C. for 4 hours, and then subjected to a solution treatment that was cooled with water. The forged product after the solution treatment was subjected to an aging treatment at 750 ° C. for 10 hours. Test pieces were appropriately collected from the forged product after the aging treatment and used for various tests.
Results of tensile strength test (0.2% proof stress) from room temperature (23 ° C.) to high temperature (700 ° C. and 800 ° C.) and forging for Comparative Examples 1-21 and Examples 1-5 after solution treatment and aging treatment The situation is shown in Table 2. The tensile test was performed according to JIS Z 2241 (metal material tensile test method). The temperature conditions in the tensile test, 700 ° C. and 800 ° C., were set in consideration of the temperature conditions during the normal operation of the steam turbine and the temperature taking into account the safety factor. In Table 2, “forging ratio” is a value of L ₁ / L ₀ when the lengths before and after forging are L ₀ and L _1, and “number of reheats” is “forging ratio” in the forging process. This is the number of times the forged object is reheated during the period up to 3. “Forging crack” is a result of visual observation of the presence or absence of “forging crack” after forging. “No” indicates that there is no “forging crack”, and “Yes” indicates that it is present. “Forgeability” is a result of evaluation of forgeability, “◯” is determined to be good, and “x” is that forgeability is not good.

  As shown in Table 2, Examples 1 to 5 were found to have high 0.2% yield strength at each temperature and excellent forgeability. The examples were found to have both high temperature strength and forgeability improved by precipitation / solid solution strengthening as compared with the comparative examples.

(Gleeble test)
Table 3 shows the results of a greeble test in which hot workability was evaluated for Comparative Example 1 (conventional material: equivalent to Inconel 617) and Examples 1 to 5 shown in Table 1. The greeble test was conducted at 900 ° C., 1000 ° C., 1100 ° C., 1200 ° C. and 1300 ° C. with a tensile rate of 10% strain / second. FIG. 2 is a graph showing a greeble test result in each sample shown in Table 3. Here, the horizontal axis in FIG. 2 indicates the test temperature (° C.), and the cross-sectional area reduction ratio (reduction of area) shown on the vertical axis is the post-test relative to the cross-sectional area of the test piece before the test. It means the ratio of the cross-sectional area that is reduced from the cross-sectional area before the test in the test piece (after fracture). That is, when this value is large, it has excellent hot workability.

  As shown in Table 3, Examples 1 to 5 are equivalent to the conventional materials, and have a drawing value of 50% or more in the forging temperature range of 900 to 1300 ° C., and there is no problem in manufacturing. all right.

The Ni-based alloy for the turbine rotor of the steam turbine of the present embodiment improves the high-temperature strength while maintaining the workability of the conventional Ni-based alloy by finely adjusting the component range of the composition in the conventional Ni-based alloy. be able to. For this reason, the Ni-based alloy for the turbine rotor of the steam turbine of this embodiment can obtain high reliability even in a high-temperature environment as a turbine rotor material of a steam turbine into which high-temperature steam is introduced.
According to the Ni-based alloy for the turbine rotor of the steam turbine of the present embodiment, the high-temperature strength and forgeability are maintained while maintaining the hot workability of the conventional Ni-based alloy by being configured in the component range of the composition described above. Both can be improved.

  Moreover, the turbine rotor penetrating through the steam turbine into which the high-temperature steam is introduced can be made of any one of the above-described Ni-based alloys for the turbine rotor of the steam turbine of this embodiment. That is, all the parts of the turbine rotor of the steam turbine may be made of this Ni-based alloy, or a part of the turbine rotor of the turbine that is particularly hot may be made of this Ni-based alloy. Here, as a part of the turbine rotor of the steam turbine that becomes high temperature, specifically, the entire region of the high-pressure steam turbine unit or the region from the high-pressure steam turbine unit to a part of the intermediate-pressure steam turbine unit can be cited. According to this turbine rotor, the high-temperature strength can be improved, and it has high reliability even in a high-temperature environment.

  In addition, this invention is not limited only to the said embodiment, You may change a component in the range which does not deviate from the summary in an implementation stage. In addition, various inventions can be configured by appropriately combining a plurality of components disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, constituent elements over different embodiments may be appropriately combined.

Claims (4)

  In mass%, C: 0.01 to 0.15%, Cr: 18 to 28%, Co: 10 to 15%, Mo: 8 to 12%, Al: less than 0.5 to 1.5%, Ti: A steam turbine comprising 0.7 to 3.0%, Ta: 0.1 to 0.7%, B: 0.001 to 0.006%, the balance being made of Ni and inevitable impurities Ni-base alloy for turbine rotors. The Ni-base alloy for a turbine rotor, by mass%, Nb: 0.1 to 0.4% is further characterized by containing claim 1 Symbol mounting of the steam turbine of the turbine rotor for the Ni-based alloy. Among the unavoidable impurities, in mass%, Si: 0.1% or less, Mn: 0.1% or less according to claim 1, wherein the steam turbine of the turbine rotor for the Ni-based alloy, characterized in that. A turbine rotor penetrating a steam turbine into which high-temperature steam is introduced,
A turbine rotor of a steam turbine, wherein at least a predetermined portion is made of the Ni-based alloy for a turbine rotor of a steam turbine according to any one of claims 1 to 3 .