JP2012117379A - CASTING Ni GROUP ALLOY FOR STEAM TURBINE AND CAST COMPONENT FOR THE STEAM TURBINE - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a casting Ni group alloy for a steam turbine excellent in high-temperature strength characteristics and casting property, and a casting component for the steam turbine manufactured using the casting Ni group alloy for the steam turbine.SOLUTION: The casting Ni group alloy for the steam turbine includes C: 0.01-0.15, Cr: 15-20, Co: 15-20, Co: 7-13, Mo: 4-7, Al: 0.3-2, Ti: 0.3-2, B: 0.001-0.006, and W+Mo (total of W and Mo): 9-12 by mass%, wherein the others include Ni and an inevitable impurity.

Description

  Embodiments of the present invention relate to a Ni-based alloy for steam turbine casting and a cast component of a steam turbine.

As a means for reducing CO ₂ emitted from coal-fired power generation equipment, it is effective to improve the efficiency of the coal-fired power generation system. For example, the power generation efficiency of the steam turbine provided in the coal-fired power generation equipment is improved. It can be realized by improving. To improve the power generation efficiency of the steam turbine, it is most effective to raise the turbine steam temperature. In recent thermal power plants, the steam temperature has risen to 600 ° C. or higher, and currently, thermal power generation systems having a steam temperature of 700 ° C. or higher are being developed worldwide.

  Cast parts such as a turbine casing, a valve casing, and a nozzle box included in a steam turbine that is exposed to high-temperature steam become hot and generate high stress. Therefore, materials having excellent strength, ductility, and toughness are demanded from the room temperature to a high temperature range as materials constituting them.

  The material currently used for the components constituting the steam turbine is an Fe-based alloy containing Fe as a main component and added with elements such as Cr and Mo in order to improve high-temperature strength. However, when the temperature of the steam is 700 ° C. or higher, the heat resistance temperature of the Fe-based alloy is exceeded, so application of a Ni-based alloy having a higher heat resistance temperature is being studied.

Typical examples of Ni-based alloys include Inconel 718 alloy (made by Special Metal) and Inconel 617 alloy (made by Special Metal). The strengthening mechanism of the Ni-based alloy is roughly divided into a precipitation strengthening type and a solid solution strengthening type. In a precipitation strengthened Ni-base alloy, by adding Al, Ti, Ta, Nb to Ni, a γ ′ (gamma prime: Ni ₃ (Al, Ti)) phase or γ ″ (gamma double prime: Ni ₃ Nb) The strength at high temperature is improved by precipitating a precipitation phase called “phase.” As a typical precipitation strengthened Ni-based alloy, the above-mentioned Inconel 718 alloy can be cited, while solid solution strengthened Ni-based alloy. Then, the matrix phase itself is strengthened by adding Co, Mo, W, etc. to Ni. As a typical solid solution strengthened Ni-based alloy, the above-mentioned Inconel 617 alloy can be mentioned.

Japanese Unexamined Patent Publication No. 7-150277

  As described above, application of a Ni-based alloy has been studied as a material for components of steam turbines used in a steam environment exceeding 700 ° C. The material of the components of the steam turbine is required to have a sufficiently high temperature strength in a steam environment exceeding 700 ° C. Further, since it is exposed to a high temperature for a long time, the soundness of the material for a long time is required.

  Therefore, the problem to be solved by the present invention is that a Ni-base alloy for casting of a steam turbine excellent in high-temperature strength characteristics and castability, and a cast component of a steam turbine manufactured using the Ni-base alloy for casting of this steam turbine Is to provide.

  In order to achieve the above object, the Ni-based alloy for cast components of a steam turbine according to an embodiment of the present invention is mass%, C: 0.01 to 0.15, Cr: 15 to 20, Co: 7 to 13, Mo: 4-7, Al: 0.3-2, Ti: 0.3-2, B: 0.001-0.006, W + Mo (total of W and Mo): 9-12, The balance consists of Ni and inevitable impurities.

  Hereinafter, an embodiment of the present invention will be described.

  For example, a Ni-based alloy such as Inconel 617 is a useful material that constitutes the components of steam turbines installed in high-temperature and high-pressure environments. When added, precipitation of a phase causing weakening of mechanical properties called a harmful phase is promoted.

  For example, when a strengthening element such as Al, Ti, or Mo is added to the conventional Inconel 617, the amount of precipitation of the γ ′ phase increases and the creep strength is improved. However, excessive addition of reinforcing elements promotes precipitation of plate-like or needle-like σ phases. This σ phase is an intermetallic compound mainly composed of Cr, Mo, Co, and Ni. It is believed that the hard and brittle sigma phase precipitates in the form of a plate or needle, thereby starting crack growth and degrading mechanical properties such as impact value, creep properties, and low cycle fatigue.

  In the Ni-based alloy for casting a steam turbine according to an embodiment of the present invention, the high temperature strength is improved by adding a strengthening element in an alloy range in which no harmful phase is precipitated.

  The Ni-based alloy for casting of a steam turbine in one embodiment according to the present invention is composed of the following composition component ranges. In the following description, “%” representing a composition component is “% by mass” unless otherwise specified.

  (M1) C: 0.01 to 0.15%, Cr: 15 to 20%, Co: 7 to 13%, Mo: 4 to 7%, Al: 0.3 to 2%, Ti: 0.3 to Ni-based alloy containing 2%, B: 0.001 to 0.006%, W + Mo (total of W and Mo): 9 to 12%, with the balance being Ni and inevitable impurities.

  (M2) C: 0.01 to 0.15%, Cr: 15 to 20%, Co: 7 to 13%, Mo: 4 to 7%, Al: 0.3 to 2%, Ti: 0.3 to 2%, B: 0.001 to 0.006%, Ta: 0.1 to 0.7%, W + Mo (total of W and Mo): 9 to 12%, the balance being from Ni and inevitable impurities Ni-base alloy.

  (M3) C: 0.01 to 0.15%, Cr: 15 to 20%, Co: 7 to 13%, Mo: 4 to 7%, Al: 0.3 to 2%, Ti: 0.3 to 2%, B: 0.001 to 0.006%, Nb: 0.1 to 0.4%, W + Mo (total of W and Mo): 9 to 12%, the balance being from Ni and inevitable impurities Ni-base alloy.

  (M4) C: 0.01 to 0.15%, Cr: 15 to 20%, Co: 7 to 13%, Mo: 4 to 7%, Al: 0.3 to 2%, Ti: 0.3 to 2%, B: 0.001 to 0.006%, Ta + 2Nb (sum of Ta content and twice Nb content): 0.1 to 0.7%, W + Mo (sum of W and Mo): A Ni-based alloy containing 9 to 12%, the balance being Ni and inevitable impurities.

  Of the inevitable impurities in the Ni-based alloys (M1) to (M4) described above, it is preferable that at least Si is suppressed to 0.1% or less and Mn to 0.1% or less. In addition, as an unavoidable impurity, Cu, Fe, S etc. are mentioned other than Si and Mn mentioned above, for example.

  The Ni-based alloy having the composition component range described above is suitable as a material constituting a cast component of a steam turbine having a temperature during operation of 680 to 750 ° C. Examples of cast components of the steam turbine include a turbine casing, a valve casing, and a nozzle box.

  Here, the turbine casing is a casing that constitutes a turbine casing in which a turbine rotor in which turbine blades are implanted penetrates, nozzles are disposed on an inner peripheral surface, and steam is introduced. The valve casing is a casing of a valve that functions as a steam valve that adjusts the flow rate of high-temperature and high-pressure steam supplied to the steam turbine or blocks the flow of steam. In particular, a valve casing in which steam having a temperature of 680 to 750 ° C. flows can be exemplified. The nozzle box is provided over the periphery of the turbine rotor for leading the high-temperature and high-pressure steam introduced into the steam turbine toward the first stage turbine blade through the first stage nozzle provided at the outlet. An annular steam flow path. These turbine casing, valve casing, and nozzle box are all installed in an environment exposed to high-temperature and high-pressure steam.

  Here, even if all the parts of the cast components of the steam turbine described above are configured by the Ni-based alloy described above, some parts of the cast components of the steam turbine that are particularly high in temperature are configured by the Ni-based alloy described above. May be. Here, specifically, the high temperature of the cast component of the steam turbine includes, for example, 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. . Further, the high temperature of the cast components of the steam turbine includes a main steam line section for introducing steam into the high-pressure steam turbine. In addition, the part from which the casting component of a steam turbine becomes high temperature is not restricted to these, For example, if it is a part from which temperature becomes about 680-750 degreeC, it will be contained in this.

  In addition, the Ni-based alloy having the composition component range described above is superior in high-temperature strength characteristics and castability than conventional Ni-based alloys. That is, by using this Ni-based alloy to form cast components for steam turbines such as turbine casings, valve casings, nozzle boxes, etc., turbine casings, valve casings, nozzle boxes, etc. that have high reliability even in high temperature environments Of cast parts can be provided.

  Next, the reason for limitation of each component range in the Ni-based alloy for casting of the steam turbine according to the embodiment of the present invention 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. In particular, in a high temperature environment of 650 ° C. or higher, it is possible to precipitate M ₂₃ C ₆ type carbide during operation of a steam turbine. This is one of the factors that maintain the creep 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 mechanical strength (high temperature strength characteristics, the same applies hereinafter) is reduced and the molten metal at the time of casting is reduced. The fluidity is significantly reduced. On the other hand, when the C content exceeds 0.15%, the tendency of component segregation during the production of a large ingot increases, and the impact value decreases due to grain boundary precipitation of coarse carbides. Therefore, the C content is determined to be 0.01 to 0.15%. Moreover, the more preferable range of the content rate of C is 0.05 to 0.1%.

(2) Cr (chromium)
Cr is an essential element for increasing the oxidation resistance, corrosion resistance and mechanical strength of the Ni-based alloy. In addition, it is indispensable as a constituent element of M ₂₃ C ₆ type carbide, and especially in a high temperature environment of 650 ° C. or higher, the M ₂₃ C ₆ type carbide is precipitated during operation of the steam turbine, so that the creep strength of the alloy is maintained. Is done. Moreover, Cr improves the oxidation resistance in a high temperature steam environment. When the Cr content is less than 15%, the oxidation resistance decreases. On the other hand, when the Cr content exceeds 20%, the precipitation of M ₂₃ C ₆ type carbides is remarkably promoted, thereby increasing the coarsening tendency and increasing the precipitation of the σ phase that is an embrittlement phase. Therefore, the Cr content is determined to be 15 to 20%. Moreover, the more preferable range of the content rate of Cr is 18 to 19%.

(3) Co (cobalt)
Co is a solid solution in the parent phase in the Ni-based alloy and improves the mechanical strength of the parent phase. However, if the Co content exceeds 13%, the hot water flowability decreases. Moreover, economical efficiency is impaired and manufacturing cost increases. On the other hand, when the Co content is less than 7%, the mechanical strength decreases. Therefore, the Co content is determined to be 7 to 13%. Moreover, the more preferable range of the content rate of Co is 11 to 13%.

(4) Mo (molybdenum)
Mo has the effect of improving the mechanical strength of the parent phase by solid solution in the Ni parent phase, and increases the stability of the carbide by partially replacing the M ₂₃ C ₆ type carbide. When the Mo content is less than 4%, precipitation of the σ phase is suppressed, but the above-described effects are not exhibited. On the other hand, when the Mo content exceeds 7%, the amount of precipitation of the σ phase increases, and the mechanical properties such as impact value, creep characteristics, and low cycle fatigue life are significantly deteriorated. Therefore, the Mo content is determined to be 4 to 7%. Moreover, the more preferable range of the content rate of Mo is 5 to 7%.

(5) Al (aluminum)
Al forms a γ ′ (Ni ₃ Al) phase together with Ni, and improves the mechanical strength of the Ni-based alloy by precipitation. When the Al content is less than 0.3%, the mechanical strength is not improved as compared with the conventional steel. On the other hand, when the Al content exceeds 2%, the mechanical strength is improved, but the castability is lowered. Therefore, the Al content is determined to be 0.3 to 2%. A more preferable range of the Al content is 1 to 1.5%.

(6) Ti (titanium)
Ti replaces Al in the γ ′ (Ni ₃ Al) phase to become (Ni ₃ (Al, Ti) and improves mechanical strength. When the Ti content is less than 0.3%, If the above effects are not exhibited and the Ti content exceeds 2%, the mechanical strength is improved, but the castability is reduced, so the Ti content is set to 0.3 to 2%. A more preferable range of the Ti content is 1 to 1.5%.

(7) B (boron)
B segregates at the grain boundaries and improves the mechanical properties. When the B content is less than 0.001%, the above-described effects are not exhibited. On the other hand, when the B content exceeds 0.006%, grain boundary embrittlement is caused and weldability is deteriorated. Therefore, the B content is determined to be 0.001 to 0.006%. Moreover, the more preferable range of the content rate of B is 0.003-0.004%.

(8) W (tungsten)
W, like Mo, has an effect of improving the mechanical strength of the parent phase by dissolving in the Ni parent phase. Further, W has a smaller influence on precipitation of the σ phase than Mo. When the content rate of W + Mo (total of W and Mo) exceeds 12%, precipitation of the σ phase becomes remarkable, and mechanical properties are deteriorated. On the other hand, when the W + Mo content is less than 9%, the effect of improving the mechanical strength is not exhibited. Therefore, the content rate of W + Mo is set to 9 to 12%. Moreover, the more preferable range of the content rate of W + Mo is 10 to 12%.

(9) Ta (tantalum)
Ta dissolves in the γ ′ (Ni ₃ (Al, Ti)) phase to improve the mechanical strength and stabilize the precipitation strength of the γ ′ phase. When the Ta content is less than 0.1%, the above effect is not improved as compared with the conventional steel, and when the Ta content exceeds 0.7%, the castability is lowered. Therefore, the Ta content is determined to be 0.1 to 0.7%. A more preferable range of the Ta content is 0.1 to 0.3%.

(10) Nb (Niobium)
Nb, like Ta, solidifies into the γ ′ (Ni ₃ (Al, Ti)) phase to improve the mechanical strength and stabilize the precipitation strength of the γ ′ phase. When the Nb content is less than 0.1%, the above effects are not improved as compared with the conventional steel. When the Nb content exceeds 0.4%, segregation occurs during melting and casting. Invite. Therefore, the Nb content is determined to be 0.1 to 0.4%. Moreover, the more preferable range of the content rate of Nb is 0.2 to 0.4%.

  Here, Ta and Nb described above may be added in combination. Since the atomic weight of Ta is approximately twice the atomic weight of Nb, the total of Ta content and twice the Nb content (Ta + 2Nb) is widely used as an equivalent amount of Ta.

  For example, when containing 0.2% Ta and 0.1% Nb, the equivalent amount of Ta (Ta + 2Nb) is 0.2 + 0.1 × 2 = 0.4%, and Ta alone contains 0.4%. It is considered that the same effect as that obtained can be obtained. The number of moles of Ta and Nb in 100 g of an alloy containing 0.2% Ta and 0.1% Nb is 0.0011 mol (0.2 / 180.9) in Ta and 0.0011 mol in Nb. (0.1 / 92.9 = 0.0011). That is, 0.0022 mol of the total atoms of Ta and Nb is present. On the other hand, the number of moles of Ta in 100 g of the alloy containing 0.4% of Ta alone is 0.0022 mol (0.4 / 180.9), which is almost the same as the total number of Ta and Nb. Ta atoms will be included.

When the content (content ratio) of the total content of the Ta content and twice the Nb content is in the range of 0.1 to 0.7%, the γ ′ phase (gamma prime phase : Ni ₃ (Al, Ti)) precipitation strength is improved. When this content is less than 0.1%, the above effect is not improved as compared with the conventional steel. When the content exceeds 0.7%, the mechanical strength is improved, but the castability is improved. Decreases. Moreover, the more preferable range of the content rate of Ta + 2Nb is 0.3 to 0.7%.

(11) Si (silicon), Mn (manganese), Cu (copper), Fe (iron) and S (sulfur)
Si, Mn, Cu, Fe and S are classified as inevitable impurities in the Ni-based alloy according to the present invention. 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.

  In the case of plain steel, Si is added to supplement the corrosion resistance. However, since the Ni-based alloy has a large Cr content and can sufficiently ensure corrosion resistance, the Ni-based alloy for casting of a steam turbine in one embodiment according to the present invention has a residual content of Si of 0.1% or less. It is desirable to make the residual content as close to 0% as possible.

  In the case of ordinary steel, Mn prevents brittleness by using S (sulfur) due to brittleness as MnS. However, the content of S in the Ni-based alloy is extremely small, and it is not necessary to add Mn. Therefore, in the Ni-based alloy for casting of a steam turbine in an embodiment according to the present invention, it is desirable that the residual content of Mn is 0.1% or less and that the residual content is as close to 0% as possible.

  Here, a Ni-based alloy for casting of a steam turbine according to an embodiment of the present invention and a method for producing a cast component of a steam turbine manufactured using the Ni-based alloy for casting parts will be described.

  In the Ni-based alloy for casting of a steam turbine according to an embodiment of the present invention, the composition components constituting the Ni-based alloy for cast parts are subjected to vacuum induction melting (VIM), and the molten metal is injected into a predetermined mold. The ingot is formed, and the ingot is subjected to solution treatment and aging treatment.

  In addition, the turbine casing, valve casing, and nozzle box, which are the casting parts of the embodiment according to the present invention, include, for example, the composition components that constitute the Ni-based alloy for casting of the steam turbine according to the embodiment of the present invention. It is manufactured by vacuum induction melting (VIM), injecting the molten metal into a mold for forming the shape of a turbine casing, a valve casing, and a nozzle box, casting in the atmosphere, and performing solution treatment and aging treatment.

  In addition, the turbine casing, valve casing, and nozzle box are obtained by melting (EF) the composition components constituting the Ni-based alloy for casting of the steam turbine according to an embodiment of the present invention into an argon-oxygen decarburization (AOD). ), The molten metal is poured into a mold for forming the shape of a turbine casing, a valve casing, and a nozzle box, cast in the atmosphere, and subjected to solution treatment and aging treatment.

  Here, in the solution treatment, it is preferable to maintain in the temperature range of 1050 to 1150 for 3 to 15 hours. Here, the solution treatment temperature is carried out in order to form a solid solution of the γ ′ phase precipitate. When the temperature is lower than 1050 ° C., the solution treatment temperature is not sufficiently solid. As a result, the strength decreases.

  Moreover, in an aging treatment, it is preferable to maintain in the temperature range of 700-800 degreeC for 10 to 48 hours. Thereby, the γ ′ phase can be precipitated at an early stage.

  In addition, the above-described method for producing a Ni-based alloy for a cast component of a steam turbine, a turbine casing, a valve casing, and a nozzle box is not limited to the above-described method.

  Hereinafter, it will be described that the Ni-based alloy for cast components of a steam turbine according to the present invention is excellent in high-temperature strength characteristics and castability.

(Evaluation of high-temperature strength characteristics and castability)
Here, it will be described that the Ni-based alloy in the chemical composition range of one embodiment of the present invention has excellent high-temperature strength characteristics and castability. Table 1 shows the chemical compositions of Sample 1 to Sample 32 used for the evaluation of the high temperature strength characteristics and castability. Samples 1 to 17 are Ni-based alloys in the chemical composition range of one embodiment of the present invention, and samples 18 to 32 are in the chemical composition range of one embodiment of the present invention. This is a comparative example. The Ni-based alloy in the chemical composition range of the embodiment of the present invention used here contains Fe, Cu, and S in addition to Si and Mn as unavoidable impurities.

  High temperature strength properties were evaluated by creep rupture test. In the creep rupture test, the Ni-based alloys of Sample 1 to Sample 32 having the chemical compositions shown in Table 1 were respectively melted in a vacuum induction melting furnace, and the hot metal portion was cut to prepare a 20 kg ingot. Subsequently, the ingot was subjected to a solution treatment at 1100 ° C. for 4 hours and an aging treatment at 780 ° C. for 30 hours to obtain a cast alloy. And the test piece of the predetermined size was produced from this casting alloy.

  Then, the creep rupture strength at a temperature of 700 ° C. and 100,000 hours was measured for the test piece of each sample. The creep rupture test was performed according to the standard of JIS Z 2271.

  Here, 700 ° C., which is a temperature condition in the creep rupture test, was set in consideration of a temperature condition during normal operation of the steam turbine and a temperature in consideration of the safety factor. Table 2 shows the measurement results of the creep rupture test.

  Further, castability of each sample was evaluated. In the evaluation of castability, the ingot described above was cut vertically into two parts, and the cut surface was subjected to JIS Z 2343-1 (Non-destructive test-penetration flaw detection test-Part 1: General rules: penetrant flaw detection test method and penetration instruction. Based on the pattern classification), a penetrant flaw detection test (PT) was performed, and the presence or absence of casting cracks was visually observed. Table 2 shows the results of evaluation of castability. Here, in Table 2, when there is no casting crack, it is indicated as “None”, and further, in order to show that the castability is excellent, the castability evaluation is indicated by “◯”. On the other hand, when there is a casting crack, it is indicated as “present”, and in order to indicate that the castability is inferior, the castability evaluation is indicated by “x”.

  As shown in Table 2, it was found that Sample 1 to Sample 17 had higher creep rupture strength and superior castability than Sample 18 to Sample 32.

  On the other hand, in the samples 18 to 32 according to the comparative example, results excellent in both high temperature strength characteristics and castability were not obtained.

  According to the embodiments described above, there are provided a steam turbine casting Ni-base alloy excellent in high-temperature strength characteristics and castability, and a steam turbine casting part produced using the steam turbine casting Ni-base alloy. It becomes possible.

  Although several embodiments of the present invention have been described, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the scope of the invention.

Claims (6)

  In mass%, C: 0.01 to 0.15, Cr: 15 to 20, Co: 7 to 13, Mo: 4 to 7, Al: 0.3 to 2, Ti: 0.3 to 2, B: 0.001 to 0.006, W + Mo (total of W and Mo): 9 to 12; balance is made of Ni and unavoidable impurities. Ni-based alloy for cast components of steam turbines.   In mass%, C: 0.01 to 0.15, Cr: 15 to 20, Co: 7 to 13, Mo: 4 to 7, Al: 0.3 to 2, Ti: 0.3 to 2, B: 0.001-0.006, Ta: 0.1-0.7, W + Mo (total of W and Mo): 9-12, The remainder consists of Ni and an unavoidable impurity, The steam turbine characterized by the above-mentioned Ni-base alloy for cast parts.   In mass%, C: 0.01 to 0.15, Cr: 15 to 20, Co: 7 to 13, Mo: 4 to 7, Al: 0.3 to 2, Ti: 0.3 to 2, B: A steam turbine comprising 0.001 to 0.006, Nb: 0.1 to 0.4, W + Mo (total of W and Mo): 9 to 12, and the balance being made of Ni and inevitable impurities Ni-base alloy for cast parts.   In mass%, C: 0.01 to 0.15, Cr: 15 to 20, Co: 7 to 13, Mo: 4 to 7, Al: 0.3 to 2, Ti: 0.3 to 2, B: 0.001-0.006, Ta + 2Nb (sum of Ta content and twice Nb content): 0.1-0.7, W + Mo (sum of W and Mo): 9-12 A Ni-based alloy for a casting component of a steam turbine, wherein the balance is made of Ni and inevitable impurities.   5. The steam turbine casting according to claim 1, wherein, among the unavoidable impurities, at least Si is suppressed to 0.1 mass% or less and Mn is suppressed to 0.1 mass% or less. Ni-based alloy for parts.   A cast component for a steam turbine, wherein at least a predetermined portion is produced by casting using the Ni-based alloy for a cast component of a steam turbine according to any one of claims 1 to 5.