JP2020050946A - Ni-BASED SUPERALLOY - Google Patents

Abstract

To provide an Ni-based superalloy capable of being used as a high-temperature member for a coal-fired power-generating plant, the steam temperature of which is 700°C or more, and capable of producing a large-sized ingot while hardly causing defects of macrosegregation.SOLUTION: An Ni-based superalloy comprises, by mass, C:0.001-0.1%, Cr:15-20%, Co:15-25%, Al:3.0-6.0%, Mo:5.0-14.0%, Mg:0.01% or less, B:0.03 or less, Zr:0.03% or less, and the balance comprising Ni or inevitable impurities.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a Ni-based super heat-resistant alloy.

Increasing the steam temperature is effective for increasing the efficiency of a turbine used for coal-fired power generation. For this reason, an attempt has been made to increase the steam temperature in order to achieve higher efficiency. However, the heat resistance temperature of 12Cr ferritic heat-resistant steel currently used for turbine and rotor parts in many coal-fired power plants is set to 650 ° C at most, and the temperature is higher than 650 ° C. Therefore, it is considered that it cannot be used because of insufficient strength. Therefore, as a high-temperature member of a high-efficiency coal-fired power plant using steam of 700 ° C. or higher, application of a Ni-based super heat-resistant alloy having higher high-temperature strength than ferritic heat-resistant steel has been studied.
Meanwhile, an alloy disclosed in WO 2010/0386680 (Patent Document 1) is known as a Ni-based super heat-resistant alloy which is being studied for application as a high-temperature member of a 700 ° C.-class coal-fired power plant.

WO 2010/0386680 pamphlet

The Ni-base superalloy disclosed in Patent Document 1 described above has an alloy composition that has a high-temperature strength that can be used in an environment of about 700 ° C. and that hardly causes segregation during solidification. It is an excellent alloy that is compatible with ingot manufacturing. If excellent high-temperature strength can be realized in an environment of about 700 ° C. while maintaining the ease of manufacture of this large ingot, it can greatly contribute to improvement in power generation efficiency.
A Ni-base super heat-resistant alloy used in a high-temperature environment has excellent high-temperature strength and is used as various heat-resistant components. The Ni-base superalloy achieves high-temperature strength by adding a γ′-phase precipitation strengthening element such as Al, Ti, Nb, and Ta and a solid solution strengthening element such as Co, Mo, and Ta.
At the time of solidification, these precipitation strengthening elements and solid solution strengthening elements segregate in the γ phase and the liquid phase, which are primary crystals of the Ni-base superalloy, according to their respective distribution coefficients. Since the density is different between the concentrated liquid phase and the mother liquor phase generated by this segregation, the concentrated liquid phase flows using gravity as a driving force to form a string-like flow path. In the string-like flow path formed by this phenomenon, a phase different from the γ phase is densely crystallized from the concentrated liquid phase and becomes a macrosegregation defect which is one of solidification defects. In the present invention, the term “concentrated liquid phase” refers to a liquid portion in a solid-liquid coexisting region, and the term “mother liquid phase” refers to a liquid portion that is not a solid-liquid coexisting region. Shall be referred to as "liquid phase".

Since macrosegregation defects are much larger in size than the normal crystallization phase, they remain even after heat treatment and forging, and are likely to greatly reduce fatigue strength. Further, it is considered that the macro-segregation defect is more likely to occur when the cooling rate during solidification is lower, and is likely to occur when a larger ingot is manufactured.
Therefore, it is required that macro-segregation defects hardly occur in Ni-based super heat-resistant alloys applied to medium and large-sized products such as steam turbines and boilers used in coal-fired power plants, and large-sized ingots can be manufactured. Alloys that are unlikely to have macro-segregation defects are alloys that have a composition in which the density difference between the concentrated liquid phase and the mother liquor phase generated by segregation is small, but the composition of the concentrated liquid phase that is in the process of solidification is highly accurate. It is difficult to specify experimentally and computationally. For this reason, it is not easy to realize an alloy having high high-temperature strength and hardly causing macro-segregation defects.
An object of the present invention is to provide a Ni-based super heat-resistant alloy that can be used as a high-temperature member of a coal-fired power plant having a steam temperature of 700 ° C. or higher and that can produce a large ingot that is unlikely to generate macrosegregation defects. It is.

The present inventor has found that if the amount of Al is increased more than the Ni-base super heat-resistant alloy conventionally studied as a steam turbine member, the amount of the γ 'phase precipitated at a high temperature increases, so that higher high-temperature strength can be realized. I found that. Furthermore, the problem of reducing the density difference between the concentrated liquid phase and the mother liquor phase generated by segregation was examined, and the main metal elements added to the Ni-base superalloy were Al, Co, and Cr, which have relatively weak segregation tendency. In the case where only Mo having relatively strong segregation tendency is used, even if the amounts of Al, Co, and Cr change, the difference in density between the concentrated liquid phase and the mother liquor phase can be reduced by appropriately selecting the amount of Mo. It has been found that segregation defects are less likely to occur and a large ingot can be manufactured. As a result, the present inventors have found that an alloy that can be used as a high-temperature member of a coal-fired power plant having a steam temperature of 700 ° C. or more and that can produce a large ingot can be realized, and has reached the present invention.
That is, in the present invention, C: 0.001 to 0.1%, Cr: 15 to 20%, Co: 15 to 25%, Al: 3.0 to 6.0%, Mo: 5.0 by mass%. 〜14.0%, Mg: 0.02% or less, B: 0.03% or less, Zr: 0.03% or less, the balance being a Ni-based super heat-resistant alloy comprising Ni and unavoidable impurities.
Preferably, Mo is 9.0 to 12.0% by mass%.

  The Ni-base super heat-resistant alloy of the present invention has a high-temperature strength that can be used as a high-temperature member of a coal-fired power plant having a steam temperature of 700 ° C. or higher. Furthermore, since it is excellent in manufacturability of large ingots, it is possible to provide large ingots in which macro segregation defects are suppressed. Therefore, using the Ni-based super heat-resistant alloy of the present invention, it is possible to manufacture medium and large-sized parts that can be used in a coal-fired power plant having a steam temperature of 700 ° C. or higher.

Alloy No. 1 which is an example of the alloy of the present invention. 1 is a cross-sectional macrophotograph at a position of 65 mm in height of an ingot produced in an experiment performed to evaluate the productivity of large ingot No. 1.

An important feature of the present invention is that the alloy composition has high high-temperature strength and hardly generates macrosegregation defects.
The reasons for defining each chemical composition in the following range in the Ni-base superalloy of the present invention are as follows. In addition, it is described as mass% unless otherwise specified. The term “Ni-base superalloy” as used herein refers to a superalloy, a heat-resistant superalloy, or a Ni-based alloy used in a high-temperature region of 700 ° C. or higher, also called superalloy, and is strengthened by γ ′. Say alloy.
C: 0.001 to 0.1%
C forms a carbide by combining with an alloy element. The carbides precipitated at the grain boundaries increase the high-temperature strength by suppressing grain boundary sliding at high temperatures. However, if the C content is too large, carbides tend to precipitate in a stringer form in the structure after forging, and the strength properties are impaired, so the upper limit is 0.1%. A preferable lower limit for more surely obtaining the effect of C described above is 0.03%, and a preferable upper limit is 0.06%.

Cr: 15 to 20%
Cr combines with C to form carbides, thereby strengthening crystal grain boundaries and improving tensile strength at high temperatures. Further, by forming a dense oxide film containing Cr ₂ O ₃ on the surface, the oxidation resistance and the high-temperature corrosion resistance are improved. A high-temperature component of a steam turbine plant having a steam temperature of 700 ° C. or more needs to contain at least 15%. However, if a large amount of Cr is contained, the σ phase precipitates and the ductility and fracture toughness of the material deteriorate, so the upper limit is 20%. A preferred lower limit for more reliably obtaining the above-described effect of Cr is 16%, and a preferred upper limit is 18%.
Co: 15 to 25%
Co improves the high-temperature strength by replacing Ni with a solid solution in the matrix. Further, the inclusion of Co has the effect of lowering the solid solution temperature of the γ 'phase and facilitating hot working. When the amount of Al is increased to improve high-temperature strength and oxidation resistance, the γ 'phase is likely to precipitate and the hot workability at high temperatures is reduced. Workability can be maintained. However, if a large amount of Co is contained, precipitation of a harmful phase such as a σ phase or a μ phase is promoted, so the upper limit is 25%. A preferable lower limit for more reliably obtaining the effect of Co described above is 18%, and a preferable upper limit is 23%.

Al: 3.0 to 6.0%
Al forms a γ ′ (Ni ₃ Al) phase and improves the high-temperature strength of the alloy. When the amount of Al is insufficient, sufficient high-temperature strength cannot be obtained because the precipitation amount of the γ 'phase is small. Since the Ni-base alloy targeted by the present invention does not contain other γ′-phase precipitation strengthening elements, such as Ti, Ta and Nb, at least 3.0% of Al is required to obtain sufficient high-temperature strength. It is. If the amount of Al is too large, the solid solution temperature of the γ 'phase becomes high and hot working becomes difficult, so the upper limit was 6.0%. A preferable lower limit for more reliably obtaining the above-described effect of Al is 3.5%, and a preferable upper limit is 5.0%.
Mo: 5.0 to 14.0%
Mo strengthens the parent phase by solid solution. In addition, since Mo tends to be distributed to the liquid phase during solidification and increases the density of the molten metal, the generation of macrosegregation defects can be suppressed by adjusting the amount of Mo. In order to obtain a sufficient effect of suppressing macro-segregation defects in the Ni-base alloy targeted by the present invention, it is necessary to contain 5.0% or more Mo. However, Mo content exceeding 14.0% adversely affects high-temperature forgeability and high-temperature strength because a large amount of brittle harmful phase precipitates. Therefore, the upper limit of the content of Mo is set to 14.0%. A preferred lower limit for more reliably obtaining the Mo effect described above is 6.0%, further preferably 9.0%, and more preferably 9.5%. The preferable upper limit of Mo is 13.0%, and the more preferable upper limit is 12.0%.

Mg: 0.02% or less Mg is an optional element of the present invention, and can be added as needed. Mg can be added as a desulfurizing agent which forms a compound with S (sulfur) which is an embrittlement phase forming element when the alloy is melted. Addition of an appropriate amount of Mg has the effect of suppressing grain boundary segregation of S and improving hot workability. However, if an excessive amount of Mg is added, a low-melting-point phase of Mg is precipitated and the grain boundary strength is reduced. Therefore, the upper limit of the content of Mg is 0.02%. A preferable lower limit for more surely obtaining the effect of Mg described above is set to 0.001%. Note that Mg is preferably added within the range specified in the present invention in order to obtain the above-mentioned effects, but may be 0%.
B: 0.03% or less and Zr: 0.03% or less B (boron) and Zr are also selective elements of the present invention, and can be added as necessary. B and Zr can be added for grain boundary strengthening. B and Zr are segregated at the grain boundaries because of a large difference in atomic radius from Ni constituting the γ phase, which is the parent phase, and suppress grain boundary sliding. Therefore, it is considered to be beneficial for high-temperature strength. However, since a large amount degrades oxidation resistance, the upper limit is set to 0.03%. The preferable lower limits for more reliably obtaining the effects of B and Zr described above are preferably set to 0.001%. As for B and Zr, since both have the same effect, it is preferable to add at least one of them within the range specified in the present invention in order to obtain the above-mentioned effect, but each of them is 0%. It may be.
The balance: Ni and unavoidable impurities The balance is substantially Ni, but includes impurities unavoidably mixed in production. Although it is preferable that the impurity content is small, for example, Fe may inevitably be mixed in the range of 3% or less depending on the dissolved raw material, but Fe is acceptable if it is in the range of 3% or less. Elements other than Fe are permissible as long as each is in a range of up to about 0.1%.

The following examples illustrate the invention in more detail.
The productivity of large alloy ingots was evaluated using a small device that melts and solidifies in vacuum. In the present embodiment, an alloy composition that can obtain high strength even at a high temperature of at least 700 ° C. was selected.
An alloy material having a length of 220 mm, a height of 120 mm, a width of 40 mm, and 10 kg was produced by vacuum induction melting. The prepared material was set in an alumina crucible, and the material was melted in a vacuum atmosphere by raising the temperature of a carbon heater provided around the crucible to a set temperature of 1500 ° C. Subsequently, the temperature of the heater was lowered, and the melted material was solidified in one lateral direction by bringing the cooling plate into contact with the side of the crucible. Due to the contact of the cooling plate, solidification was achieved in which the cooling rate was high at a position close to the cooling plate and slowed as the distance from the cooling plate decreased. The cooling rate was confirmed by inserting a thermocouple into the crucible and measuring the temperature history of the alloy.

Table 1 shows the composition of the alloy material subjected to the experiment, and Table 2 shows the amounts of the γ 'phase at 700 ° C. and 800 ° C. based on thermodynamic calculations. Alloy No. 1, 2, 3, 4, 5, 6, and 7 are examples of the present invention. Alloy No. Reference numeral 11 denotes a Ni-based super heat-resistant alloy disclosed in Patent Document 1. Alloy No. Reference numeral 12 denotes an alloy equivalent to Waspaloy (trademark) used for various components as a Ni-based super heat-resistant alloy. The units of Mg and B in Table 1 are ppm.
As shown in Table 2, when comparing the amount of the γ 'phase at 700 ° C. of each alloy, the amount of the γ ′ phase of each of the alloys of the present invention was as high as 25% or more, and almost all of the γ ′ phases were at least 30%. It shows that high high-temperature strength can be obtained even at 700 ° C. or more. Further, when comparing the amount of the γ 'phase at 800 ° C., the amount of the γ ′ phase of the alloys of the present invention can be maintained at a high value of 18% or more in any case. This shows that sufficient high-temperature strength of 20.0% or more, which is equal to or more than that of Alloy No. 12, is obtained. In addition, No. Alloy 11 has a low γ 'phase content and low strength.

  The alloy No. FIG. 1 is a cross-sectional macrophotograph of the solidified ingot No. 1 at a height of 65 mm. In FIG. 1, the side where the shrinkage cavities occur (the right side in the figure) is the final solidification side, and the side without the shrinkage cavities (the left side in the figure) is the crucible side where the cooling plate is brought into contact. In FIG. 1, thermocouples are inserted at positions of about 10 mm, 30 mm, 50 mm, 70 mm, 110 mm, 150 mm, and 190 mm from the cooling plate side, and macrosegregation defects identified as black dots are approximately from the cooling plate side. It occurs from the position of 120 mm. Table 3 shows the evaluation results of each alloy. In this evaluation, the slowest cooling rate CR at a temperature 50 ° C. lower than the liquidus temperature among thermocouples at positions where macrosegregation defects did not occur in each evaluation was used as an evaluation index for the manufacture of large ingots of the alloy. Used. Alloy No. In the evaluation of 1, CR was the value of the thermocouple inserted at a position 110 mm from the end face on the cooling plate side, and was 2.93 ° C./min. Alloy No. In the evaluation of No. 4, CR is the value of the thermocouple inserted at a position 110 mm from the side face on the cooling plate side, and was 3.04 ° C./min. Alloy No. In the evaluation of No. 6, CR was a value of the thermocouple inserted at a position 110 mm from the side face on the cooling plate side, and was 3.23 ° C./min. Alloy No. In 2, 3, 5, 7, and 11, macro-segregation defects were all suppressed, so CR was set to the slowest cooling rate of all thermocouples. In this experiment, if a cooling rate equal to or higher than the CR shown in Table 3 is realized, macrosegregation defects of each alloy can be suppressed. Therefore, it can be said that as the CR is smaller, the alloy is less likely to generate macrosegregation defects.

  In order to confirm the manufacturability of the large ingot of the present invention, a comparison was made with a conventional example. The alloy No. of the present invention example. Each of 1, 2, and 3 has a smaller CR value than No. 12 (Waspalloy equivalent alloy), and it is understood that the large ingot manufacturability is better than that of Waspaloy equivalent alloy. Further, the alloy No. with the Mo amount in the range of 9.0 to 12.0%. In alloy Nos. 2, 3, 5, and 7, Macro-segregation defects are suppressed over the entire surface of the ingot as in the case of the Ni-based super heat-resistant alloy disclosed in Patent Document 1 of Japanese Patent No. It can be seen that 2, 3, 5, and 7 have the same level of large ingot manufacturability as the Ni-base superalloy disclosed in Patent Document 1. As shown in the above results, in the alloy system of the present invention, by appropriately selecting the amount of Mo, it is possible to realize very good large-sized ingot manufacturability.

INDUSTRIAL APPLICABILITY The present invention is excellent in large-sized ingot manufacturability and high-temperature strength, so that it can be applied to applications using large parts requiring high-temperature strength.

Claims (2)

  In mass%, C: 0.001 to 0.1%, Cr: 15 to 20%, Co: 15 to 25%, Al: 3.0 to 6.0%, Mo: 5.0 to 14.0% , Mg: 0.02% or less, B: 0.03% or less, Zr: 0.03% or less, the balance being Ni and unavoidable impurities. The Ni-based super heat-resistant alloy according to claim 1, wherein the Mo is 9.0 to 12.0% by mass%.