Ni-Based Alloy Member Manufacturing Method

The present disclosure relates to a Ni-based alloy member manufacturing method.

Priority is claimed on Japanese Patent Application No. 2022-98014 filed on Jun. 17, 2022, the content of which is incorporated herein by reference.

A Ni-based alloy is often used for a high-temperature member (such as a turbine blade) used in a thermal power plant and an aircraft turbine in order to satisfy mechanical properties required in a high-temperature environment.

In the Ni-based alloy used for a high-temperature member, high strength is obtained by precipitation of a γ′ (gamma prime) phase (L1structure) with a crystal lattice aligned with a γ (gamma) phase (FCC phase) in the γ phase, which is a matrix phase.

In a member (Ni-based alloy member) formed using a γ′ phase precipitation-strengthened Ni-based alloy that is strengthened by precipitation of a γ′ phase, a solid-solution heat treatment for dissolving the γ′ phase precipitated during casting in a base material is performed. When the Ni-based alloy member is cast, there is a difference in thermal expansion coefficient between the Ni-based alloy, and a mold and a core, resulting in an internal strain. When a solid-solution heat treatment is performed at a high temperature in a state where the internal strain is present, recrystallized grains that cause a decrease in strength characteristics are generated by using the internal strain as a driving force.

As a method for suppressing recrystallization, PTL 1 discloses a method of manufacturing a nickel-based single crystal superalloy article, the method including steps of: a stage of casting a nickel-based superalloy single crystal article in which a coarse γ′ phase is present in a γ-phase matrix; a stage of, when the single crystal article is heated to a solution heat treatment temperature to dissolve the γ′ phase in a γ phase in a stage in which stress concentration is formed in the single crystal article during solidification after the casting or during subsequent handling, forming the stress concentration with enough strength to cause recrystallization; a step of heating the single crystal article at a recovery temperature lower than a recrystallization temperature at a portion where the stress concentration is present to reduce the strength of the stress concentration; a solution heat treatment step of heating the article at a temperature lower than a solidus temperature of the article and higher than the recrystallization temperature and the recovery temperature after the reducing of the stress concentration to dissolve the γ′ phase in the γ phase; and a precipitation step of precipitating a subsequently refined γ′ phase in the γ-phase matrix, while maintaining a single crystal structure of the article.

In addition, PTL 2 discloses a method of manufacturing a Ni-based alloy regenerated member, the method including: a solution heat treatment/non-recrystallization heat treatment step S2 of performing a solid heat treatment/non-recrystallization heat treatment on a used member, which is a Ni-based alloy member used for a predetermined time in a turbine, at a temperature of higher than a solid-solution temperature of a γ′ phase by 10° C. or more and lower than a melting point of a γ phase by 10° C. or less for a holding time in a time range in which recrystallized grains of the γ phase are not generated; and an aging heat treatment step S3 of performing an aging heat treatment on the used member subjected to the solid heat treatment/non-recrystallization heat treatment to precipitate the γ′ phase in the γ phase, in which, in a case where a locking curve of a predetermined crystal plane of grains of the γ phase is measured by an XRD method for the used member after the solid heat treatment/non-recrystallization heat treatment S2, a half-width of the locking curve is 0.25° or more and 0.30° or less.

[PTL 1] Japanese Unexamined Patent Application Publication No. 59-64593.

[PTL 2] Japanese Unexamined Patent Application Publication No. 2019-112702.

However, in the manufacturing methods disclosed in PTL 1 and PTL 2, in a case where the γ′ phase is precipitated in an amount of 50 volume % or more, there may be cases where recrystallization cannot be sufficiently suppressed.

The present disclosure has been made to solve the above-described problems, and an object of the present disclosure is to provide a Ni-based alloy member manufacturing method capable of suppressing the generation of recrystallized grains even in a case where a γ′ phase is precipitated in an amount of 50 volume % or more.

A Ni-based alloy member manufacturing method of the present disclosure includes: a casting step; a first strain removal heat treatment step; a solution heat treatment step; and an aging heat treatment step, in which, in the casting step, a Ni-based alloy cast material having a chemical composition in which a γ′ phase is precipitable in an amount of 50 volume % or more in a γ phase in the aging heat treatment step, in the first strain removal heat treatment step, when a solid-solution temperature of the γ′ phase is denoted by Ts° C., the Ni-based alloy cast material after the casting step is heated in a first strain removal temperature range of Ts×0.90° C. or higher and Ts° C. or lower for one hour or longer, in the solution heat treatment step, when a melting point of the γ phase is denoted by Tm° C., the Ni-based alloy cast material after the first strain removal heat treatment step is heated from the first strain removal temperature range to a solution heat treatment temperature range of higher than Ts+t1° C. and Tm° C. or lower, and a temperature is held for two hours or longer in the solution heat treatment temperature range, and the t1 is 10° C. or lower.

According to the aspect of the present invention, it is possible to provide a Ni-based alloy member manufacturing method capable of suppressing the generation of recrystallized grains even in a case where a γ′ phase is precipitated in an amount of 50 volume % or more.

As a result of intensive studies conducted by the present inventors, it was found that in a Ni-based alloy having a chemical composition with which a γ′ phase (for example, a NiAl phase) can be precipitated in an amount of 50 volume % or more, recrystallization can be significantly reduced by removing a strain that causes the recrystallization in a predetermined temperature range and then dissolving the γ′ phase without cooling. The present invention has been made based on the above-described findings. The Ni-based alloy is, for example, an alloy containing 50 mass % or more of Ni and one or more kinds of alloys selected from the group consisting of Cr, W, Al, Ta, Co, Mo, Ti, C, and B.

Hereinafter, a Ni-based alloy member manufacturing method Saccording to a first embodiment will be described with reference to.is a flowchart of the Ni-based alloy member manufacturing method Saccording to the embodiment. The Ni-based alloy member manufacturing method Sincludes a casting step S, a first strain removal heat treatment step S, a solution heat treatment step S, and an aging heat treatment step S. Hereinafter, each step will be described. In the present specification, a numerical range represented using “to” refers to a range including numerical values before and after “to” as a lower limit and an upper limit. In the present specification, a temperature such as a heating temperature refers to a temperature of a surface of a Ni-based alloy cast material.

In the casting step S, the Ni-based alloy cast material in which a γ′ phase can be precipitated in an amount of 50 volume % or more in a γ phase in the aging heat treatment step Sis cast. A casting method is not particularly limited. The Ni-based alloy cast material can be manufactured, for example, by a lost-wax method. Since an active metal such as Al is contained, it is preferable to perform dissolution and casting in a vacuum. For example, the Ni-based alloy cast material is obtained by dissolving components constituting a Ni-based alloy, and injecting the obtained molten metal into a gap formed by a mold, a core, and the like.

A chemical composition of the Ni-based alloy cast material is not particularly limited as long as the γ′ phase can be precipitated in an amount of 50 volume % or more in the γ phase in the aging heat treatment step S. Such a Ni-based alloy cast material is, for example, a Ni-based alloy having a chemical composition including, by mass %, Cr: 5% to 15%, W: 3% to 10%, Al: 3.0% to 7.0%, Ta: 3% to 15%, Co: 0% to 15%, Mo: 0% to 5%, Ti: 0% to 5.0%, C: 0% to 0.10%, B: 0% to 0.05%, and a remainder including Ni and impurities.

Cr is an element that generates M23C6 precipitates that improves strength at a high temperature. In addition, inclusion of Cr also improves oxidation resistance in a high temperature environment. In order to obtain the above-described effect, a Cr content is preferably 5% or more. The Cr content is preferably 8% or more. In a case where the Cr content is more than 15% m, precipitation of a harmful phase is caused, which leads to a decrease in strength and a decrease in ductility of a Ni-based alloy member. Therefore, the Cr content is preferably 15% or less.

W is an element that dissolves in the γ phase, which is a matrix of the Ni-based alloy, and contributes to an improvement of the strength of the Ni-based alloy member by solid solution strengthening. In order to obtain the above-described effect, a W content is preferably 3% or more. The W content is more preferably 4% or more. In a case where the W content is more than 10%, the precipitation of a harmful phase is caused, which leads to a decrease in strength and a decrease in ductility of the Ni-based alloy member. Therefore, the W content is preferably 10% or less. The W content is more preferably 8% or less.

Al is an element that generates the γ′ phase that improves the strength of the Ni-based alloy member at a high temperature. Al is also an element having an effect of improving oxidation resistance and corrosion resistance at a high temperature. In order to obtain the above-described effect, an Al content is preferably 3.0% or more. The Al content is more preferably 3.5% or more. In a case where the Al content is more than 7.0%, weldability of the Ni-based alloy member decreases, and there is a concern that cracks occur during manufacturing or repair of the Ni-based alloy member. Therefore, the Al content is preferably 7.0% or less. The Al content is more preferably 5.5% or less.

Ta is an element that generates the γ′ phase which improves the strength of the Ni-based alloy member at a high temperature. In order to obtain the above-described effect, a Ta content is preferably 3% or more. The Ta content is more preferably 4% or more. In a case where the Ta content is more than 15%, MC carbides that are stable at a high temperature are generated within crystal grains, and M23C6 that contributes to the strength of the Ni-based alloy member at a high temperature is less likely to be generated. Therefore, the Ta content is preferably 15% or less. The Ta content is more preferably 11% or less.

Co is an element having an effect of improving a solid-solution temperature of the γ′ phase that improves the strength of the Ni-based alloy member at a high temperature. Co is also an element that contributes to stabilization of the γ′ phase at a high temperature. In a case where the Co content is more than 15%, precipitation of a harmful phase in the Ni-based alloy member is caused, which leads to a decrease in strength and a decrease in ductility of the Ni-based alloy member. Therefore, the Co content is preferably 15% or less. The Co content is more preferably 10% or less. Since Co may not be contained, a lower limit of the Co content is 0%.

Mo is an element that dissolves in the γ phase, which is the matrix of the Ni-based alloy, and contributes to the improvement of the strength of the Ni-based alloy member by solid solution strengthening. In a case where a Mo content is more than 5%, precipitation of a harmful phase is caused, which leads to a decrease in strength and a decrease in ductility of the Ni-based alloy member. Therefore, the Mo content is preferably 5% or less. The Mo content is more preferably 3% or less. Since Mo may not be contained, a lower limit of the Mo content is 0%.

Ti is an element that generates the γ′ phase that improves the strength of the Ni-based alloy member at a high temperature. Ti is also an element that contributes to an improvement of oxidation resistance and corrosion resistance of the Ni-based alloy member at a high temperature. When a Ti content is more than 5%, weldability of the Ni-based alloy member decreases, and there is a concern that cracks occur in the Ni-based alloy member during manufacturing or repair. Therefore, the Ti content is preferably 5.0% or less. The Ti content is more preferably 3.5% or less. Since Ti may not be contained, a lower limit of the Ti content is 0%.

C is an element constituting M23C6 precipitates that contribute to the improvement of the strength of the Ni-based alloy member at a high temperature. In a case where a C content is more than 0.10%, the number of MC carbides precipitated in the crystal grains increases, and there is a concern that intragranular strength increases and the ductility decreases. Therefore, the C content is preferably 0.10% or less. Since C may not be contained, a lower limit of the C content is 0%.

B is an element having an effect of improving high-temperature creep strength of the Ni-based alloy member by strengthening grain boundaries when present at the grain boundaries. In a case where a B content is more than 0.05%, borides are generated, and there is a concern that the ductility of the Ni-based alloy member decreases. Therefore, the B content is preferably 0.05% or less. Since B may not be contained, a lower limit of the B content is 0%.

The remainder of the Ni-based alloy cast material of the present disclosure includes Ni and impurities. Here, the impurities are components that are mixed in the raw materials or during manufacturing steps when the Ni-based alloy cast material is cast. The impurities are allowed within a range in which the effects of the Ni-based alloy member of the present disclosure can be obtained.

The chemical composition of the Ni-based alloy cast material can be analyzed by using a known method. For example, the chemical composition of the Ni-based alloy cast material can be analyzed by an inductively coupled plasma mass spectrometry.

In the first strain removal heat treatment step S, when the solid-solution temperature of the γ′ phase is denoted by Ts° C., the Ni-based alloy cast material after the casting step Sis heated in a first strain removal temperature range of Ts×0.90° C. or higher and Ts° C. or lower for one hour or longer. It is possible to remove internal strain (internal strain accumulated due to a difference in thermal expansion between the Ni-based alloy cast material, and the mold and the core during cooling after casting) formed inside the Ni-based alloy cast material formed in the casting step S. The solid-solution temperature of the γ′ phase refers to a temperature at which the γ′ phase is completely dissolved in a matrix phase. The solid-solution temperature of the γ′ phase can be obtained by performing calculation using thermodynamic calculation software (for example, JMatPro manufactured by Sente Software Ltd.) based on the chemical composition.

In a case where a first strain removal temperature is lower than Ts×0.9° C., the temperature is low and a volume percentage of the γ′ phase becomes excessively high, so that the internal strain of the Ni-based alloy member cannot be sufficiently removed. Therefore, the first strain removal temperature is Ts×0.9° C. or higher. In a case where the first strain removal temperature exceeds Ts° C., the temperature is high, and the γ′ phase is (disappeared) dissolved, so that recrystallized grains are likely to be generated. Therefore, the first strain removal temperature is Ts° C. or lower.

In the first strain removal heat treatment step S, a heating time in the first strain removal temperature range is one hour or longer. In a case where the heating time is shorter than one hour, the internal strain of the Ni-based alloy cast material cannot be sufficiently removed.

A temperature rising rate from room temperature (5° C. to 35° C.) to the first strain removal temperature range is preferably 50° C./min or slower. In a case where the Ni-based alloy cast material is rapidly heated, there is a possibility that the temperature rises to the first strain removal temperature range or higher. In a case where the temperature exceeds the first strain removal temperature range, there is a possibility that recrystallized grains are generated. Therefore, the temperature rising rate from room temperature to the first strain removal temperature range is 50° C./min or slower.

In the solution heat treatment step S, when a melting point of the γ phase is denoted by Tm° C., the Ni-based alloy cast material after the first strain removal heat treatment step Sis heated from the first strain removal temperature range to a solution heat treatment temperature range of higher than Ts+t1° C. and Tm° C. or lower, and the temperature is held for two hours or longer in the solution heat treatment temperature range. t1 is 10° C. or lower. t1 is preferably 1° C. or higher. t1 is more preferably 5° C. or higher. In the solution heat treatment step S, the Ni-based alloy cast material is heated from the first strain removal temperature range to the solution heat treatment temperature range without being cooled. As a result, the γ′ phase can be dissolved without generating strain caused by a difference in thermal expansion between the γ phase and the γ′ phase. Therefore, the recrystallized grains can be significantly reduced. The melting point Tm of the γ phase can be obtained by performing calculation using thermodynamic calculation software (for example, JMatPro manufactured by Sente Software Ltd.) based on the chemical composition.

In addition, in the Ni-based alloy cast material after casting, the γ′ phase is precipitated in a coarse state, and uneven distribution of chemical components occurs. In the solution heat treatment step S, the γ′ phase is dissolved, thereby achieving homogenization. In the solution heat treatment step S, the γ phase preferably occupies 100%, but may contain another phase as long as the strength at a high temperature is not reduced. In a case where the Ni-based alloy cast material after completing the first strain removal heat treatment step Sand the solution heat treatment step Sis observed with a microscope, a single γ phase exhibiting a dendritic pattern with no recrystallized grains can be confirmed.

In a case where the solution heat treatment temperature is Ts+t1° C. or lower, there may be cases where the γ′ phase cannot be sufficiently dissolved. Therefore, the solution heat treatment temperature is higher than Ts+t1° C. t1 is 10° C. or lower. t1 is preferably 1° C. or higher. t1 is more preferably 5° C. or higher. In a case where the solution heat treatment temperature exceeds the melting point Tm° C. of the γ phase, the γ phase is dissolved. Therefore, the solution heat treatment temperature is Tm° C. or lower.

A temperature rising rate from the first strain removal temperature range to the solution heat treatment temperature range is preferably 50° C./min or slower. In a case where the Ni-based alloy cast material is rapidly heated, there is a possibility that the temperature rises to the solution heat treatment temperature range or higher. In a case where the temperature exceeds the solution heat treatment temperature range, there is a possibility that the γ phase is dissolved. Therefore, the temperature rising rate from the first strain removal temperature range to the solution heat treatment temperature range is 50° C./min or slower.

After the temperature is raised to the solution heat treatment temperature range, the temperature is held for a certain period of time in the solution heat treatment temperature range. In a case where the time for holding the temperature is shorter than two hours, there is a possibility that the γ′ phase is not sufficiently dissolved. Therefore, in the solution heat treatment step S, the time for holding the temperature is two hours or longer.

After holding the temperature in the solution heat treatment temperature range, the Ni-based alloy cast material is cooled from the solution heat treatment temperature range to room temperature. A cooling method is, for example, gas cooling. The volume percentage of the γ′ phase, which is a strengthening phase, is adjusted to a target volume percentage in the subsequent aging heat treatment step S. Since there is a concern that an unexpected γ′ phase is precipitated during cooling after holding the temperature in the solution heat treatment temperature range, it is preferable to set a cooling rate to be as fast as possible. Therefore, the cooling rate is preferably 10° C./min or faster.

By performing the aging heat treatment step Son the Ni-based alloy cast material after the solution heat treatment step S, the γ′ phase can be precipitated, and the Ni-based alloy member of the present disclosure can be obtained. In the aging heat treatment step S, it is preferable to heat the Ni-based alloy cast material after the solution heat treatment step Sin an aging heat treatment temperature range of 850° C. or higher and 870° C. or lower for two hours to 20 hours. As a result, precipitation of the γ′ phase in an amount of 50 volume % or more in the γ phase is facilitated.

A temperature range (aging heat treatment temperature range) at which the Ni-based alloy cast material is aged after the solution heat treatment step Sis preferably 850° C. or higher and 870° C. or lower. In this aging heat treatment temperature range, the volume percentage of the γ′ phase precipitated in the γ phase can be easily adjusted to 50 volume % or more, which is preferable.

A time for heating the Ni-based alloy cast material after the solution heat treatment step Sin the aging heat treatment temperature range is preferably two hours to 20 hours. For this heating time, the volume percentage of the γ′ phase precipitated in the γ phase can be easily adjusted to 50 volume % or more, which is preferable.

The volume percentages of the γ phase and the γ′ phase after the aging heat treatment step Scan be evaluated by observing a cross section of the Ni-based alloy member with a scanning electron microscope (SEM).shows an electron micrograph obtained by the SEM observation of the Ni-based alloy member after the aging heat treatment step S. As shown in, in the Ni-based alloy member of the present disclosure, rectangular γ′ phases and γ phases in a lattice pattern in gaps between the rectangular γ′ phases are observed. For example, an area percentage of the γ′ phase can be evaluated from the obtained observation image by using image processing software. The volume percentage of the γ′ phase can be set as the area percentage of the γ′ phase obtained from the electron micrographs of the cross section.

As described above, according to the Ni-based alloy member manufacturing method according to the present embodiment, it is possible to suppress recrystallized grains.

Next, a Ni-based alloy member manufacturing method SB according to a second embodiment will be described with reference to.is a flowchart of the Ni-based alloy member manufacturing method SB according to the embodiment. The Ni-based alloy member manufacturing method SB includes a casting step S, a first strain removal heat treatment step S, a second strain removal heat treatment step S, a solution heat treatment step S, and an aging heat treatment step S. Hereinafter, each step will be described.

In the casting step S, the Ni-based alloy cast material in which a γ′ phase can be precipitated in an amount of 50 volume % or more in a γ phase in the aging heat treatment step Sis cast. A casting method is not particularly limited. The Ni-based alloy cast material can be manufactured, for example, by a lost-wax method. Since an active metal such as Al is contained, it is preferable to perform dissolution and casting in a vacuum. For example, the Ni-based alloy cast material is obtained by dissolving components constituting a Ni-based alloy, and injecting the obtained molten metal into a gap formed by a mold, a core, and the like.

A chemical composition of the Ni-based alloy cast material is not particularly limited as long as the γ′ phase can be precipitated in an amount of 50 volume % or more in the γ phase in the aging heat treatment step S. Such a Ni-based alloy cast material is, for example, a Ni-based alloy having a chemical composition including, by mass %, Cr: 5% to 15%, W: 3% to 10%, Al: 3.0% to 7.0%, Ta: 3% to 15%, Co: 0% to 15%, Mo: 0% to 5%, Ti: 0% to 5.0%, C: 0% to 0.10%, B: 0% to 0.05%, and a remainder including Ni and impurities.

In the first strain removal heat treatment step S, when the solid-solution temperature of the γ′ phase is denoted by Ts° C., the Ni-based alloy cast material after the casting step Sis heated in a first strain removal temperature range of Ts×0.90° C. or higher and Ts° C. or lower for one hour or longer. It is possible to remove internal strain (internal strain accumulated due to a difference in thermal expansion between the Ni-based alloy cast material, and the mold and the core during cooling after casting) formed inside the Ni-based alloy cast material formed in the casting step S. The solid-solution temperature of the γ′ phase refers to a temperature at which the γ′ phase is completely dissolved in a matrix phase. The solid-solution temperature of the γ′ phase can be obtained by performing calculation using thermodynamic calculation software (for example, JMatPro manufactured by Sente Software Ltd.) based on the chemical composition.

In a case where a first strain removal temperature is lower than Ts×0.9° C., the temperature is low and a volume percentage of the γ′ phase becomes excessively high, so that the internal strain of the Ni-based alloy member cannot be sufficiently removed. Therefore, the first strain removal temperature is Ts×0.9° C. or higher. In a case where the first strain removal temperature exceeds Ts° C., the temperature is high, and the γ′ phase is (disappeared) dissolved, so that recrystallized grains are likely to be generated. Therefore, the first strain removal temperature is Ts° C. or lower.

In the first strain removal heat treatment step S, a heating time in the first strain removal temperature range is one hour or longer. In a case where the heating time is shorter than one hour, the internal strain of the Ni-based alloy cast material cannot be sufficiently removed.

A temperature rising rate from room temperature (5° C. to 35° C.) to the first strain removal temperature range is preferably 50° C./min or slower. In a case where the Ni-based alloy cast material is rapidly heated, there is a possibility that the temperature rises to the first strain removal temperature range or higher. In a case where the temperature exceeds the first strain removal temperature range, there is a possibility that recrystallized grains are generated. Therefore, the temperature rising rate from room temperature to the first strain removal temperature range is 50° C./min or slower.