JP2014047388A - HIGH-STRENGTH Ni BASE SUPERALLOY, AND TURBINE ROTOR BLADE OF GAS TURBINE USING THE SAME - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a Ni base superalloy having antioxidation and corrosion resistance in a level equal to or more than that of conventional alloys, and improved in creep strength at high temperature.SOLUTION: A Ni base alloy includes, in a mass ratio, Cr: 11.00-15.00%, Co: 4.00-9.00%, Al: 4.50-7.00%, Ti: 0.50-2.10%, W: 8.00-13.00%, Mo: 0.10-1.00%, Nb: 0.005-1.00%, C: 0.20% or less, B: 0.02% or less, with the balance being Ni and inevitable impurities.

Description

  The present invention relates to a Ni-base superalloy having high high-temperature strength and a turbine rotor blade of a gas turbine using the same.

  In recent years, due to increasing environmental awareness, such as saving fossil fuels, reducing carbon dioxide emissions, and preventing global warming, internal combustion engines have been improved in thermal efficiency. It is known that a heat engine such as a gas turbine or a jet engine can increase the thermal efficiency most effectively by operating the high temperature side of the Carnot cycle at a higher temperature. As the turbine inlet temperature rises, the importance of improving and developing materials used for high-temperature parts of gas turbines, that is, combustors, turbine rotor blades or stationary blades, is increasing.

In order to cope with this high temperature, a Ni-based heat-resistant alloy that is superior in high-temperature strength is applied in terms of material, and many Ni-based alloys are currently used. Ni-based alloys include normal cast alloys made of equiaxed crystals, unidirectionally solidified alloys made of columnar crystals, and single crystal alloys made of one crystal. In order to increase the strength of the Ni-based alloy, a large amount of solid solution strengthening elements such as W, Mo, Ta, and Co are added, and Al, Ti, and the like are added to form a strengthening phase γ′Ni ₃ (Al , Ti) phase is important to be precipitated.

Due to the increase in combustion temperature (turbine inlet temperature) accompanying efficiency improvement in recent years, the use of Ni-based alloys that are superior in high-temperature strength to Co-based alloys has been studied. In order to increase the strength of the Ni-based alloy, a large amount of solid solution strengthening elements W, Mo, Ta, Co, etc. are added, and Al, Ti, etc. are added to form a strengthening phase γ′Ni ₃ (Al , Ti) By using precipitation strengthening of the phase, it has excellent high temperature strength and thermal fatigue characteristics.

  On the other hand, due to soaring fuel prices, there is a movement to use low-quality fuel that contains many impurities that cause corrosion as fuel for industrial gas turbines, and the development of materials that combine high-temperature strength and corrosion resistance is also required. Yes. In such a material, it is desirable to add a large amount of Cr that forms a protective film. As an alloy that places importance on corrosion resistance, for example, a normal casting alloy disclosed in Japanese Patent Application Laid-Open No. 51-34819 (Patent Document 1) can be given.

However, in a conventional alloy, the more the solid solution strengthening element is included, the lower the stability of the material structure, and a hard and brittle harmful phase such as a σ phase is likely to precipitate when used for a long time. That is, it has been difficult to develop an alloy material having both excellent high temperature creep strength, corrosion resistance and oxidation resistance.

JP 51-34819 A

An object of the present invention is to provide a Ni-base superalloy having oxidation resistance and corrosion resistance equal to or higher than those of conventional alloys and improved creep strength at high temperatures.

The Ni-based alloy of the present invention has a mass ratio of Cr: 11.00 to 15.00%, Co: 4.00 to 9.00%, Al: 4.50 to 7.00%, Ti: 0.50 ˜2.10%, W: 8.00 to 13.00%, Mo: 0.10 to 1.00%, Nb: 0.005 to 1.00%, C: 0.20% or less, B: 0 It is characterized by containing 0.02% or less and the balance being Ni and inevitable impurities.

According to the present invention, it is possible to provide a Ni-base superalloy having oxidation resistance and corrosion resistance equivalent to or higher than those of conventional alloys and improved creep strength at high temperatures.

The graph which shows the creep rupture time with respect to an alloy test piece. The graph which shows the oxidation weight loss in the high temperature oxidation test with respect to an alloy test piece. The graph which shows the corrosion weight loss in the molten salt immersion corrosion test with respect to an alloy test piece. The figure which shows an example of the moving blade shape of a gas turbine. The figure which shows a gas turbine.

  Hereinafter, the present invention will be described in detail.

  FIG. 4 shows an example of a turbine rotor blade for an industrial gas turbine.

The turbine rotor blade 1 includes a blade part 110, a shank part 111, and a root part (dovetil part) 112, and has a size of 10 to 100 cm and a weight of about 1 to 10 kg. Moreover, the platform part 113 and the radial fin 114 are provided. The turbine rotor blade is a rotating part having a complicated cooling structure inside, and is exposed to a severe environment in which a centrifugal force during rotation and a load of thermal stress accompanying start and stop are repeatedly applied. As basic material properties, excellent high temperature creep strength, oxidation resistance to high temperature combustion gas atmosphere, and corrosion resistance are required.
As a result of investigating an alloy for obtaining a high temperature creep strength superior to that of a conventional material while maintaining corrosion resistance and oxidation resistance, the present inventors have found the present invention. .

  General means for producing the blades of a gas turbine include a technique using ordinary casting, unidirectional solidification casting, and single crystal casting. Unidirectionally solidified alloys and single crystal alloys are mainly used in the moving blades of small and lightweight jet engines (aviation gas turbines). However, a wing using a unidirectionally solidified alloy or a single crystal alloy has a complicated casting process, so that the casting yield when the wing is cast deteriorates. Particularly, the blades of industrial gas turbines are large in shape and complicated in shape, so that the casting yield is low, resulting in an expensive product.

Therefore, the present inventors have examined the alloys having higher high-temperature strength than the conventional materials while maintaining the corrosion resistance and the oxidation resistance as an alloy for ordinary casting by balancing each additive element of the alloy.
Hereinafter, the function and composition range of each component contained in the Ni-based alloy of the present invention will be described.

Cr: 11.00 to 15.00 mass%
Cr acts as a solid solution strengthening element and forms a dense oxide film, contributing to oxidation resistance and high temperature corrosion resistance in a corrosive atmosphere at high temperatures. In particular, in order to improve the corrosion resistance against molten salt corrosion, the effect increases as the Cr content increases. And the effect appears more prominently after exceeding 11.00% by mass. However, in the alloy of the present invention, since a large amount of the solid solution strengthening element W is added, if the amount of Cr is excessively large, a brittle TCP phase is precipitated and the high temperature strength is lowered. Therefore, it is desirable to balance with other alloy elements and to set the upper limit to 15.0% by mass. In this composition range, high strength and high corrosion resistance can be obtained. Preferably it is the range of 12.50-14.50 mass%.

Co: 4.00 to 9.00 mass%
Co has the effect of lowering the solid solution temperature of the γ 'phase and facilitating solution heat treatment. Especially when used in partial solution formation, as in the case of the alloy of the present invention, the solution rate can be reduced even at a low heat treatment temperature. It becomes possible to enlarge. In order to obtain the effect, the addition of at least 4% is necessary. However, excessive addition of Co destabilizes the γ ′ phase and rather leads to a decrease in strength. Therefore, Co needs to be 9.00% at the maximum. In this composition range, high high-temperature strength can be obtained. Preferably it is the range of 4.50-8.00 mass%.

Al: 4.50 to 7.00 mass%
Al is an essential element for forming the γ ′ phase (Ni ₃ Al), and in order to form the 60% fraction γ ′, it is necessary to add at least 4.50% or more. Al improves the oxidation resistance and corrosion resistance by forming an Al ₂ O ₃ protective film. However, if added excessively, the solid solution strengthening degree of the γ 'phase is lowered and the high-temperature strength is lowered. Therefore, the addition amount needs to be 7.00% at the maximum. In this composition range, when considering the balance between strength at high temperature, oxidation resistance, and corrosion resistance, the range is preferably 5.00 to 6.50 mass%.

Ti: 0.50 to 2.10% by mass
Ti has the effect of preventing the formation of a complex oxide of Cr and Al and improving the corrosion resistance of the alloy. However, the addition of Ti lowers the solidus temperature of the γ 'phase, so that the high-temperature strength is lowered. In order to tamper with the corrosion resistance against molten salt corrosion, a content of 0.50% by mass or more is necessary. . However, if added in excess of 2.10% by mass, the embrittled η phase is further precipitated, and the amount of γ ′ phase deposited as the γ ′ phase forming element depends on the amount of Ti added. In order to increase and control the amount of precipitation of the γ ′ phase, the upper limit must be 2.10% by mass. In an alloy containing 11.0 to 15.0% by mass of Cr as in the present invention alloy, preferably 0.50 to 1.50% by mass in consideration of the balance of strength, corrosion resistance and oxidation resistance at high temperatures. Range.

W: 8.00 to 13.00 mass%
W is an element having the most solid solution strengthening effect except for the expensive element Re, and mainly strengthens the γ phase by solid solution strengthening. Therefore, in order to obtain an excellent high temperature strength, a large amount of W is added to the present invention as long as the stability of the structure is maintained. W is a solid solution strengthening element similar to Mo and contributes to the improvement of the rigidity and the reduction of the diffusion coefficient. Contributes to strengthening. Reducing the lattice constant mismatch between the γ phase and the γ ′ phase improves the interfacial strength between the γ and γ ′ phases and improves the high temperature creep strength. W is an element that mainly enters the γ phase side, and on the contrary, Ta is an element that mainly enters the γ ′ phase side, which is a precipitated phase. An alloy with a large amount of W has a larger lattice constant on the γ phase side, and generally a smaller lattice constant mismatch defined by (lattice constant of γ ′ phase−lattice constant of γ phase) / (average of lattice constants of both phases). . Therefore, in order to reduce the lattice constant mismatch between the γ phase and the γ ′ phase, the amount of W is at least 8.00% or more. However, excessive addition of W deteriorates the phase stability of the alloy, leads to precipitation of the TCP phase and the like, and lowers the corrosion resistance. Therefore, it is necessary to regulate the maximum to 13.00%. Furthermore, the element Ta which makes it easy to precipitate the TCP phase by adding W and the compound is not added. Then, when importance is attached to phase stability, it is preferably in the range of 8.50 to 12.00% by mass.

Mo: 0.10 to 1.00% by mass
Since Mo has the same effect as W, it can be replaced with a part of W if necessary. In addition, since the solid solution temperature of the γ ′ phase is increased, the effect of improving the creep strength as in the case of W is obtained. And in order to acquire such an effect, content of 0.1 mass% or more is required, and creep strength improves as Mo content increases.

  On the other hand, Mo reduces the oxidation resistance and corrosion resistance of the alloy. In particular, as the content of Mo increases, the oxidation resistance is greatly deteriorated, so the upper limit must be 1.0% by mass. Further, Mo which is a factor of matrix deterioration due to μ phase precipitation is reduced, and instead, a large amount of W useful for matrix strengthening is added. Accordingly, the corrosion resistance and oxidation resistance characteristics at high temperatures are almost the same as those of conventional alloys, and when the creep strength is regarded as important, the composition range of the present invention is preferably in the range of 0.10 to 1.00% by mass. More preferably, it is in the range of 0.40 to 0.80% by mass.

Nb: 0.005 to 1.00% by mass
Nb has the effect of preventing the formation of a complex oxide of Cr and Al and improving the corrosion resistance of the alloy. On the other hand, the effect is smaller than that of Ta, but the effect of solid solution strengthening of the γ ′ phase is higher than that of Ti. Therefore, Nb is an effective element that can improve the corrosion resistance without reducing the high temperature strength, and it is necessary to add 0.60% or more. However, in order to maintain the phase stability of the γ ′ phase, the amount of Nb added needs to be 1.70% or less. When corrosion resistance is particularly important, addition of 1.0% or more is preferable. In consideration of the balance between strength, corrosion resistance and oxidation resistance at high temperatures, it is preferably in the range of 0.70 to 1.60% by mass, more preferably in the range of 0.80 to 1.50% by mass.

C: 0.20 mass% or less C forms MC type carbides with Ti, Nb, etc., M23C6 and M6C type carbides with Cr, W, Mo, etc., and prevents the grain boundaries from moving at high temperatures. The element has an effect of strengthening the grain boundary and plays an especially important role in the present invention. It is necessary to add 0.05% or more at least in order to exhibit this effect with ordinary cast material. Further, when it is desired to increase both strength and ductility, it is preferable to add 0.10% or more. However, if the amount of C is excessively increased, elements effective for solid solution strengthening of the γ phase and the γ ′ phase are taken into the carbide, so that the high temperature strength is lowered. Excess carbides also reduce fatigue strength. Therefore, the upper limit of C needs to be regulated to 0.20%. In the single crystal material, grain boundary strengthening is unnecessary, and the addition of C is 0 or 0.002 or less.

B: 0.02 mass% or less B has an effect of filling non-matching portions of crystal grain boundaries and increasing the bonding strength of crystal grain boundaries. In the alloy of the present invention, at least 0.005% of B needs to be added. When higher grain boundary strength is required as a normal cast material, it is desirable to add 0.010% or more. However, since B significantly lowers the melting point of the Ni-base superalloy, it is necessary to be 0.02% at the maximum.

Hf: 0 to 2.00% by mass, Zr: 0 to 0.05% by mass
Hf and Zr are segregated at the grain boundaries to slightly improve the strength of the grain boundaries. However, most of them form an intermetallic compound with Ni, that is, Ni ₃ Zr or the like, at the grain boundaries. Since this intermetallic compound lowers the ductility of the alloy and has a low melting point, it has little effective action such as lowering the melting temperature of the alloy and narrowing the solution treatment temperature range. Therefore, the upper limit was made 2.00 mass%, 0.50 mass%, and 0.05 mass%, respectively. Preferably, Hf is 0 to 0.10% by mass and Zr is 0 to 0.03% by mass.

O: 0 to 0.005 mass%, N: 0 to 0.005 mass%
Oxygen and nitrogen are impurities, both of which are often brought from alloy raw materials, O also enters from the crucible, and exists in the alloy as oxides (Al ₂ O ₃ ) and nitrides (TiN or AlN). To do. If these are present in the casting, it becomes the starting point of cracks during creep deformation, and the creep rupture life is reduced, or the fatigue life is reduced by starting fatigue crack generation. In particular, oxygen appears as an oxide on the surface of the casting, thereby causing a surface defect of the casting and reducing the yield of the casting. Therefore, the lower the content of these elements, the better. However, when manufacturing an actual ingot, it is impossible to make oxygen-free and nitrogen-free. % Or less is desirable.

  The Ni-based alloy composed of the above components, inevitable impurities, and the remaining Ni can obtain high-temperature strength and oxidation / corrosion resistance superior to conventional materials.

  The Ni-based alloy used for the test in this example is shown below. Table 1 shows the composition (% by mass) of the Ni-based alloy.

  Alloys No. A1 to A5 are alloy compositions showing the present invention, and Alloys No. B1 and B2 are alloy compositions showing conventional materials. For the test piece, a master ingot and a weighed alloy element were dissolved in an alumina crucible and cast into a flat plate having a thickness of 14 mm. After casting, the test piece was subjected to solution heat treatment and aging heat treatment.

Table 2 shows the conditions of the heat treatment performed on the alloy specimen.
In order to make the alloy composition uniform, solution heat treatment was performed at 1505 K for 2 h. After the solution heat treatment, air cooling was performed, and the subsequent aging heat treatment conditions were 1394 K / 4 hours / air cooling + 1352 K / 4 hours / air cooling + 1148 K / 16 hours / air cooling for all alloys. Thereafter, test piece processing was performed, and a creep rupture test, corrosion, and oxidation test were performed.

  From the heat-treated test piece, a creep test piece having a parallel part diameter of 6.0 mm and a parallel part length of 30 mm, a high-temperature oxidation test piece having a length of 25 mm, a width of 10 mm, and a thickness of 1.5 mm, and 15 mm × 15 mm × A 15 mm cube-shaped hot corrosion test piece was cut out, and the microstructure was examined with a scanning electron microscope to evaluate the structural stability of the alloy.

  Table 3 shows the conditions of the characteristic evaluation test performed on the alloy specimen.

The creep rupture test was performed under the condition of 1255K-137 MPa. In the high temperature oxidation test, the 1313 K-600 hour holding oxidation test was repeated three times, and the change in mass was measured. The high temperature corrosion test was conducted by immersing in a molten salt of 1123K (composition: Na ₂ SO ₄ : 75%, NaCl: 25%) for 25 hours 4 times (100 hours in total), and measuring the change in mass. did.

  These test results are summarized in Table 4.

  1 to 3 show the results of property evaluation tests of the respective alloys. FIG. 1 is a graph showing the results of creep rupture time at 1123K-314 MPa, FIG. 2 is a graph showing measurement results of oxidation weight loss in a high temperature oxidation test, and FIG. 3 is a measurement result of corrosion weight loss in a molten salt immersion corrosion test.

  As is apparent from the results shown in Table 4, the alloys A1 to A5 of this example have substantially the same oxidation resistance as compared to the existing alloy B1 (GTD111), and the high temperature creep rupture strength is greatly improved. It can be seen that the corrosion resistance is also improved. In particular, the creep rupture strength is significantly improved. Compared to the conventional material B1, the amount of W and AL is greatly increased, and in order to ensure the structural stability, the balance of other additive elements is emphasized and the high temperature strength is improved.

Compared to another existing alloy B2 (IN738LC), the oxidation resistance and corrosion resistance are slightly lowered, but the creep rupture time is about twice or more. In the alloy of this example, the creep strength at high temperature is improved by increasing the amount of W and Al added to B2.

  That is, according to the present invention, it has been recognized that a Ni-base superalloy having excellent high-temperature strength can be obtained by significantly improving the creep rupture life while maintaining high-temperature corrosion resistance and oxidation resistance characteristics.

  In the above embodiment, the effect as a normal casting material has been described. Further, it is very effective to use the alloy of the present invention as a unidirectionally solidified blade obtained by unidirectionally solidifying the alloy. It is a well-known fact that the creep rupture strength can be greatly improved by maintaining the corrosion resistance and the oxidation resistance by unidirectional solidification. In particular, the alloy of the present invention contains C and B effective for strengthening grain boundaries, and it is possible to add Hf effective for suppressing grain boundary cracking during casting as necessary. The alloy composition is also suitable as a unidirectional solidified material. Furthermore, a single crystal alloy is also applicable.

  As described above, according to the present invention, it is possible to obtain a Ni-based superalloy capable of ordinary casting having both excellent high temperature strength and oxidation / corrosion resistance.

  FIG. 5 is a diagram showing a gas turbine. In FIG. 5, 3 is a turbine blade (blade), 13 is a turbine stacking bolt, 18 is a turbine spacer, 19 is a distance piece, 20 is a first stage nozzle, 6 is a compressor disk, 7 is a compressor blade, 16 is a compressor nozzle, 8 is a compressor stacking bould, 9 is a compressor stub shaft, 4 is a turbine disk, 11 is a hole, and 15 is a combustor.

The gas turbine in the present embodiment applies a Ni-base superalloy having a high creep rupture strength and an excellent high-temperature strength to the gas turbine blade 3 and the gas turbine nozzle 20 so that the combustion gas temperature at the turbine inlet is the conventional one. It is possible to provide a gas turbine for ultra-high temperature power generation that can be made higher than the above and has high thermal efficiency.

Since the Ni-base superalloy according to the present invention has both high creep strength and oxidation / corrosion resistance, it is desirable to use it as a normal casting material for the 3- or 4-stage rotor blade 3 of a gas turbine. Further, since the alloy of the present invention contains C and B which are effective in strengthening grain boundaries, and Hf which is effective in suppressing grain boundary cracking at the time of casting, the gas turbine is used as a unidirectional cast material or a single crystal material. Can be used for the first and second stage turbine blades 3.

3 ... turbine blade, 4 ... turbine disk, 6 ... compressor disk, 7 ... compressor blade, 8 ... compressor stacking bould, 9 ... compressor stub shaft, 10 ... turbine stub shaft, 11 ... hole, 13 ... turbine stacking bolt, 15 ... Combustor, 16 ... Compressor nozzle, 18 ... Turbine spacer, 19 ... Distant piece, 20 ... First stage nozzle, 110 ... Wing part, 111 ... Shank part, 112 ... Root part (Dubtil part), 113 ... Platform part, 114 ... Radial fin.

Claims (4)

  By mass ratio, Cr: 11.00 to 15.00%, Co: 4.00 to 9.00%, Al: 4.50 to 7.00%, Ti: 0.50 to 2.10%, W: 8.00 to 13.00%, Mo: 0.10 to 1.00%, Nb: 0.005 to 1.00%, C: 0.20% or less, B: 0.02% or less, the balance Is a Ni-based alloy characterized by Ni and inevitable impurities.   The mass ratio of claim 1 is Cr: 12.50 to 14.50%, Co: 4.50 to 8.00%, Al: 5.00 to 6.50%, Ti: 0.50 to 1. 50%, W: 8.50-12.00%, Mo: 0.40-0.80%, Nb: 0.005-0.80%, C: 0.20% or less, B: 0.02% A Ni-based alloy comprising the following, the balance being Ni and inevitable impurities: 3. The Ni-base alloy according to claim 1, wherein the Ni-base alloy does not contain Ta and Re.   A turbine blade for a gas turbine comprising the Ni-based alloy according to any one of claims 1 to 3.