JP7340154B2 - Method for manufacturing Ni-based hot forged material - Google Patents

Description

The present invention relates to a method for manufacturing a Ni-based hot forged material made of a Ni-Cr-Fe alloy that provides a strengthening mechanism using the γ'' phase, which is a metastable phase of Ni ₃ Nb, and in particular, it relates to a method for producing a Ni-based hot forged material made of a Ni-Cr-Fe alloy that provides a strengthening mechanism using the γ'' phase, which is a metastable phase of Ni 3 Nb, and in particular, it relates to a method for manufacturing a Ni-based hot forged material made of a Ni-Cr-Fe alloy that provides a strengthening mechanism using the γ'' phase, which is a metastable phase of Ni 3 Nb, and in particular, it relates to a method for producing a Ni-based hot forged material made of a Ni-Cr-Fe alloy that provides a strengthening mechanism by the γ'' phase, which is a metastable phase of Ni 3 Nb. The present invention relates to a method for producing a Ni-based hot forged material having high mechanical strength.

As a Ni-based alloy with excellent mechanical strength at high temperatures, the γ'' (gamma double prime) phase, which is a metastable phase of tetragonal Ni ₃ Nb, is precipitated in the γ phase, which is the parent phase, and due to the interfacial coherent strain. Ni-based alloys that utilize a strengthening mechanism, such as Inconel 718 (trade name), are known. In such Ni-based alloys that have an austenite phase as their parent phase, grain size cannot be refined through phase transformation. Therefore, hot forging above the recrystallization temperature and heating and holding promotes recrystallization while suppressing the growth of recrystallized grains to maintain fine grains and obtain high mechanical strength. A method for producing a Ni-based hot forged material has been proposed.

For example, Patent Document 1 discloses a method of manufacturing a fine-grained Ni-based hot forged material using recrystallization. First, a hot forged material made of a Ni-based alloy containing Nb is heat-treated at a predetermined temperature of 900° C. or higher to precipitate a δ phase made of Ni ₃ Nb in the parent phase. Thereafter, finish forging is performed at a temperature lower than 900° C. and at a predetermined forging ratio or higher to disperse the cut pieces of the δ phase in the parent phase. Then, recrystallized grains are obtained by recrystallization while removing the machining strain of finish forging through solid solution treatment, and their growth is pinned by the δ phase. According to this, it is possible to produce a Ni-based alloy material having an average grain size of #7 or more and a maximum value of #4 or more as defined by ASTM E112.

Moreover, Patent Document 2 also discloses a method for manufacturing a fine-grained Ni-based alloy material using the pinning effect of the growth of recrystallized grains due to the δ phase. After performing a δ phase precipitation treatment in advance to precipitate the δ phase in the form of needles, the δ phase is heated at a temperature below 920 to 1025°C for 1 to 36 hours to partially dissolve the precipitated acicular δ phase into a solid solution with the fragmentation of the δ phase. Adjust the shape and amount of precipitation. After that, free forging, in which a predetermined amount of blow is applied above the recrystallization temperature, and reheating are repeated. Here, by adjusting the shape and precipitation amount of the δ phase and making the δ phase spheroidal and fine, the pinning effect on the growth of recrystallized grains can be further enhanced, and the average grain size is #8. They say they can do more than that.

Japanese Patent Application Publication No. 2003-226950 Japanese Patent Application Publication No. 2014-161861

In order to maintain fine grains, the step of conditioning heat treatment to spheroidize and refine the acicular δ phase is time consuming and, as a result, increases manufacturing costs. Furthermore, depending on the shape of the δ phase resulting from the conditioning heat treatment, especially if it becomes plate-like, the fracture toughness will decrease. Furthermore, since the γ'' phase as a reinforcing phase is also a metastable phase of tetragonal Ni ₃ Nb, if the δ phase remains in excess, the γ'' phase cannot be sufficiently precipitated, making it difficult to increase the mechanical strength. I can't.

The present invention has been made in view of the _above -mentioned circumstances, and its purpose is to maintain fine recrystallized grains and achieve high An object of the present invention is to provide a method for manufacturing a Ni-based hot forged material having mechanical strength.

The method for producing a Ni-based hot forged material according to the present invention is a method for producing a Ni-based hot forged material made of a Ni-Cr-Fe alloy that provides a strengthening mechanism using the γ'' phase, which is a metastable phase of Ni ₃ Nb. Therefore, the δ phase particles, which are the stable phase of Ni ₃ Nb, are precipitated at a temperature higher than the γ'' phase solvus temperature, and the δ phase particles are precipitated at a temperature higher than the γ'' phase solvus temperature. It includes a forging/recrystallization step in which the grain size growth of recrystallized grains is suppressed by phase particles, and prior to the forging/recrystallization step, the γ” phase particles are precipitated at a temperature below the γ” phase solvus temperature. The method is characterized by including a precipitation control step of controlling precipitation of the δ phase particles.

According to this invention, the precipitation of δ phase particles can be controlled by precipitating γ'' phase particles in the precipitation control process prior to the forging/recrystallization process, and the growth of recrystallized grains can be effectively suppressed to form fine particles. These fine recrystallized grains can provide high mechanical strength by utilizing the subsequent strengthening mechanism by the γ'' phase.

In the above invention, the δ phase particles may be characterized in that they are precipitated with the γ'' phase particles as nuclei and are mainly provided within the crystal grains.According to this invention, the δ phase particles are finely dispersed and precipitated. and can easily maintain fine recrystallization.

The above-described invention may be characterized in that hot forging is performed after the cross-sectional area ratio of the δ phase particles is set to 5% or more. According to this invention, maintenance of fine recrystallized grains is facilitated.

The above invention may be characterized in that the γ'' phase particles have an average particle diameter of 100 nm or more. According to this invention, the precipitation of the δ phase particles can be easily controlled.

The above-described invention may be characterized by including a high-temperature heat treatment step for solution treatment prior to the precipitation control step. According to this invention, the development of γ'' phase particles in the control process can be easily controlled, and as a result, fine recrystallized grains can be maintained.

FIG. 2 is a flow diagram showing an example of a method for manufacturing a Ni-based hot forged material according to the present invention. FIG. 3 is a diagram showing how δ phase particles and γ'' phase particles are precipitated in a crystal. (a) A heat treatment diagram according to a conventional method, and (b) a heat treatment diagram according to a manufacturing method in this example. These are SEM photographs of (a) a comparative example and (b) an example of cross-sectional structures in which δ-phase particles were precipitated by holding for 3 hours in a manufacturing test. These are SEM photographs of (a) a comparative example and (b) an example of cross-sectional structures in which δ-phase particles were precipitated by holding for 10 hours in a manufacturing test. These are SEM photographs of (a) a comparative example and (b) an example of cross-sectional structures in which δ-phase particles were precipitated by holding for 36 hours in a manufacturing test. It is a graph which shows the measurement result of the cross-sectional area ratio of delta phase particles.

A method for manufacturing a Ni-based hot forged material made of a Ni-Cr-Fe alloy as one embodiment of the present invention will be described along with FIG. 1 with reference to FIGS. 2 and 3.

The Ni-Cr-Fe alloy targeted here is an alloy that contains at least Nb and has a composition that provides a strengthening mechanism with a γ'' phase mainly composed of Ni ₃ Nb as a strengthening phase. For example, the final By precipitating γ'' phase particles through aging heat treatment after processing into a typical product shape, the mechanical strength required for the component is ensured. Examples of such Ni-Cr-Fe alloys include Alloy718, Alloy718plus, Alloy706, Alloy625, and FX550.

An example of the component composition of Alloy 718 is as follows. That is, in mass %, Ni: 50-55%, Cr: 17-21%, Al: 0.2-0.8%, Ti: 0.6-1.2%, Nb: 4.7-5. 6%, Mo: 2.8 to 3.3%, Co: 1.0% or less, the balance is Fe, and the content of element M is [M]% by mass, [C] + [Si] + [Mn ]+[P]+[S]+[Cu]+[B]+[Mg] is a component composition of 1.1% or less.

As shown in FIG. 1, first, an alloy ingot using the above-mentioned Ni-Cr-Fe alloy is prepared (S1). For example, an alloy ingot having predetermined dimensions is obtained by melting an alloy in a vacuum arc melting furnace and then subjecting the ingot to blooming.

Next, high-temperature heat treatment is performed as necessary (S2). In the high-temperature heat treatment, the alloy ingot is made into a solution to dissolve intermetallic compounds into solid solution, and at the same time, distortions caused by blooming forging are removed and crystal grains are refined. In addition, in the case of high-temperature heat treatment, there is no restriction on the subsequent cooling rate, and there is no restriction on the temperature reached by cooling, which can be set as appropriate. In other words, it may be cooled to room temperature or cooled to a holding temperature in a subsequent step.

Next, the alloy ingot is roughly forged as required (S3). In order to refine the crystal grains in the subsequent step, it is preferable that the grain size number is -2 or more after this step. For example, if the crystal grain size number is 5 or less at this stage, the crystal grains can be made finer by this embodiment.

By the way, in the manufacturing method using the above-mentioned strengthening mechanism using the γ'' phase as the strengthening phase, there is a known method in which δ phase particles are precipitated and then hot forged to suppress the grain size growth of recrystallized grains. In the case of such a method, the next step is generally to precipitate δ phase particles.

In contrast, in this example, a precipitation control step (S4) in which γ'' phase particles are precipitated was provided prior to the δ phase particle precipitation (S5a) step. γ'' phase particles, which are the metastable phase of Ni ₃ Nb, are precipitated (S4a). For example, the temperature can be set appropriately in the range of 800 to 900°C. As will be described later, the subsequent δ phase particle precipitation step ( The precipitation of δ phase particles in step S5a) is controlled by the γ” phase particles precipitated in this step.

Next, a forging/recrystallization (S5) process including a δ phase particle precipitation (S5a) process is performed. First, in the δ phase particle precipitation (S5a) step, heating is performed to a temperature above the γ'' phase solvus temperature and below the δ phase solvus temperature to precipitate δ phase particles, which are the stable phase of Ni ₃ Nb. For example, 900 to 1100 It can be set as appropriate in the range of ℃.In the hot forging (S5b) process, finish forging is performed at a temperature higher than the recrystallization temperature to split and refine the precipitated δ phase particles and to apply processing strain to the alloy ingot. In the recrystallization (S5c) step, the steel is heated and held at the recrystallization temperature.Then, the working strain accumulated in the hot forging (S5b) promotes recrystallization and can refine the crystal grains. At this time, the grain size growth of the recrystallized grains is suppressed by the δ phase particles, and the crystal grains of the obtained Ni-based hot forging material are kept fine.

Here, referring also to FIG. 2(a), in the case of the conventional method without the above-mentioned precipitation control (S4) step, first, by heating and holding at a predetermined temperature, the crystal grains a1 having the grain boundary 1 have a δ phase. When the particles are precipitated, δ phase particles 3 are precipitated along grain boundaries 1 like crystal grains a2, and extend from grain boundaries 1 toward the interior of the grains. A certain amount of holding time is required to extend the δ phase particles 3 into the grains like crystal grains a3.

On the other hand, in the case of the method of the present example that includes the precipitation control (S4) step, although the mechanism is not certain, the inventors speculated as follows. That is, as shown in FIG. 2(b), first, when a crystal grain b1 having a grain boundary 1 is heated and held at a temperature below the γ" phase solvus temperature to precipitate γ" phase particles, it becomes like a crystal grain b2. γ” phase particles 2 are dispersed and precipitated within the grains.Furthermore, when heated and held at a temperature higher than the γ” phase solvus temperature to precipitate δ phase particles, the δ phase particles 3 are separated from the grains as shown in crystal grain b3. Precipitate. This is considered to be because the δ phase particles 3 are precipitated using the γ'' phase particles 2 as precipitation nuclei.

In this way, in the crystal grains b3 according to the present example, since the δ phase particles are precipitated from within the crystal grains b2, even the small δ phase particles 3 are dispersed throughout the interior of the crystal grains b3. Therefore, according to this example, it is sufficient to obtain smaller δ-phase particles 3 than the crystal grains a3 obtained by the conventional method, and the δ-phase particle precipitation (S5a) step for obtaining small δ-phase particles 3 requires only a short time. It can be treated as a treatment.

For example, as shown in FIG. 3(a), in the conventional method, in the process of precipitating δ phase particles, the δ phase particles are precipitated by holding the temperature T2 higher than the γ'' phase solvus temperature for a time H2, The δ phase particles were grown to the inside of the grains. For example, when using Alloy 718, when the temperature T2 is 915°C, the time H2 needs to be 36 hours in order to grow the δ phase particles sufficiently to the inside of the grains. was there.

On the other hand, as shown in FIG. 3(b), in the method of this embodiment, in the γ'' phase particle precipitation (S4a) step, the γ'' Precipitating phase particles. In the δ phase particle precipitation (S5a) step, δ phase particles are precipitated by maintaining the temperature T2 equal to or higher than the γ'' phase solvus temperature for a time H2'. Similarly, for example, when Alloy 718 is used, the temperature T1 can be 870°C, the temperature T2 can be 915°C, the time H1 can be 10 hours, and the time H2' can be 10 hours. In other words, at least the total retention time can be shorter than that of the conventional method.

Further, by making the δ phase particles 3 relatively small, the δ phase particles can be divided into smaller particles in the hot forging (S5b) step, and finer and more uniform dispersion can be achieved. As a result, in the recrystallization (S5c) step, the pinning effect of the δ phase particles can be efficiently obtained, and the growth of the recrystallized particles can be suppressed more strongly.

In the manner described above, the Ni-based hot forged material of this example can be obtained.

Note that the Ni-based hot forged material is then machined as necessary and strengthened by precipitating the γ'' phase through aging treatment, but since this is well known, it will not be described in detail.

Further, it is preferable that the δγ'' phase particles precipitated in the precipitation control (S4) step have an average particle size of 100 nm or more, so that the δ phase particles are precipitated within the crystal grains in the δ phase particle precipitation (S5a) step. It can be made easier.

Further, before the hot forging (S5b) step of the forging/recrystallization (S5) step, the cross-sectional area ratio of the δ phase particles is preferably 5% or more, more preferably 8% or more, and 15% or more. More preferred. This makes it possible to further enhance the pinning effect of suppressing the growth of recrystallized grains in the recrystallization (S5c) step, thereby maintaining fine crystal grains.

[Manufacturing test]
Next, the results of a manufacturing test of a Ni-based hot forged material using the above-described manufacturing method will be explained using FIGS. 1 and 4 to 6.

In this test, Alloy 718 was used as the Ni-Cr-Fe alloy. The composition of the alloy used is, in mass%, Ni: 53.6%, Cr: 18.18%, Nb: 5.48%, Mo: 2.92%, Ti: 0.98%, Al: 0 .41%, C: 0.02%, B: 0.0007%, Mg: 0.0006% (remaining Fe).

Referring to FIG. 1, dimensions of 10 mm x 10 mm x 5 mm were obtained from an alloy ingot obtained through the steps of alloy ingot preparation (S1), high temperature heat treatment (S2), and rough forging (S3) using such an alloy. A plurality of test pieces were cut out. Then, among the multiple test pieces, some of them were used as examples and γ'' phase particles were precipitated in the precipitation control (S4) process, and the remaining parts were used as comparative examples and proceeded to the next process as they were. In the heat treatment (S2) step, the temperature was maintained at 1050° C. for 4 hours, and in the precipitation control (S4) step, the temperature was maintained at 870° C. for 10 hours.

Next, δ phase particles were precipitated for each of the test pieces of the example and the comparative example by a δ phase particle precipitation (S5a) step. In the δ phase particle precipitation (S5a) step, the holding temperature was 915°C, and six holding times were applied for both the example and the comparative example: 1 hour, 3 hours, 10 hours, 29 hours, 36 hours, and 100 hours. And so. For each test piece, the cross section was polished, electrolytically etched, observed with a SEM (scanning electron microscope), and a photograph of the structure was taken. Further, the cross-sectional area ratio of the δ phase particles was measured by image analysis of the SEM observation photograph taken after polishing and chemically etching the cross section. In addition, Vickers hardness was also measured. In addition, for each test piece that had the same component composition and the same treatment as those used in the Examples and Comparative Examples, the cross-sectional area ratio of the δ phase particles before heating and holding was also measured and recorded as the holding time of 0 hours. did.

As shown in Figure 4, when the holding time was 3 hours, in Comparative Example (a), the δ phase particles precipitated only at the grain boundaries and began to grow in an acicular shape from the grain boundaries toward the interior of the grains. observed. The growth of the δ-phase particles was partial, and many parts within the grains that were not reached by the growing δ-phase particles were observed. On the other hand, in Example (b), precipitation of δ phase particles was observed at the grain boundaries, while precipitation of δ phase particles was also observed within the grains. The cross-sectional area ratio of the δ phase particles was 4% in the comparative example and 9% in the example. Regarding the hardness, the comparative example had a hardness of 178 HV, and the working example had a hardness of 187 HV. In other words, it can be said that the crystallization and growth of the δ phase particles is faster in the example. In addition, in the example, although it becomes hard once due to the precipitation of γ'' phase particles in the precipitation control (S4) step, it is maintained at a temperature equal to or higher than the γ'' phase solvus temperature for 1 hour in the δ phase particle precipitation (S5a) step. If this is done, all the γ'' phase particles are dissolved in solid solution. Therefore, it is unlikely that the hardness of the examples is due to the remaining γ'' phase particles.

As shown in Figure 5, when the holding time is 10 hours, in Comparative Example (a), the growth of the δ-phase particles still reaches the inside of the grains only partially, and the portions that the δ-phase particles have not reached are Many were observed within the grains. On the other hand, in Example (b), it was observed that the δ phase particles grew and reached almost the entire area inside the particles. The cross-sectional area ratio of the δ phase particles was 5% in the comparative example and 19% in the example. The hardness was 183HV in the comparative example and 225HV in the example, which was even harder.

As shown in FIG. 6, when the holding time was 36 hours, in Comparative Example (b), it was observed that the δ phase particles grew and reached almost the entire area inside the particles. In Example (b), excessive growth of δ phase particles was observed. The cross-sectional area ratio of the δ phase particles was 19% in the comparative example, which is the same as in the example where the holding time was 10 hours, and was 22% in the example. The hardness was 229HV in the comparative example and 234HV in the example, which was even harder.

As shown in Figure 7, when the holding time was changed from 0 to 100 hours, the cross-sectional area ratio of the δ phase particles increased rapidly on the short time side in both Examples and Comparative Examples, and then gradually increased. did. In the comparative example, the cross-sectional area ratio of the δ phase particles was less than 10% until 20 hours, whereas in the example it exceeded 15% after 10 hours.

As described above, according to the example in which the γ'' phase particles were precipitated in the precipitation control (S4) step and then the δ phase particles were precipitated, the δ phase particles were precipitated in a shorter time than in the comparative example using the conventional method. It was found that it was possible to sufficiently grow the δ-phase particles.In addition, in the conventional method, 36 hours of holding time was required for heating and holding to precipitate the δ-phase particles.On the other hand, according to the above-mentioned example, the δ-phase particles The holding time in the particle precipitation (S5a) step is sufficient for 10 hours, which is sufficient to maintain fine crystal grains in the Ni-based hot forged material obtained through the hot forging (S5b) and recrystallization (S5c) steps. It was determined that

Note that if the Ni-Cr-Fe alloy provides a strengthening mechanism by the γ'' phase, which is the metastable phase of Ni _{3 Nb, it is possible to precipitate δ phase particles, which are the stable phase of Ni 3 Nb} . Similarly, γ'' phase particles are precipitated in the precipitation control (S4) process prior to the δ phase particle precipitation (S5a) process in the forging/recrystallization (S5) process, and a Ni-based hot forged material is similarly obtained. be able to. In other words, the present embodiment is applicable to Ni-Cr-Fe based alloys other than Alloy718 described above that provide a strengthening mechanism by the γ'' phase, such as Alloy718plus, Alloy706, Alloy625, and FX550.

Although typical embodiments of the present invention have been described above, the present invention is not necessarily limited to these, and those skilled in the art will understand without departing from the spirit of the present invention or the scope of the appended claims. , various alternative embodiments and modifications may be found.

1 Crystal grain 2 γ” phase particle 3 δ phase particle S4 Precipitation control (process)
S5 Forging/recrystallization (process)

Claims (5)

A method for producing a Ni-based hot forged material made of a Ni-Cr-Fe alloy that provides a strengthening mechanism by the γ'' phase, which is a metastable phase of Ni ₃ Nb, comprising:
The δ phase particles, which are the stable phase of Ni ₃ Nb, are precipitated at a temperature higher than the γ'' phase solvus temperature, and the δ phase particles are hot-forged and heated and held at a temperature higher than the recrystallization temperature for recrystallization. Includes a forging and recrystallization process that suppresses the grain size growth of recrystallized grains,
Prior to the forging/recrystallization step, the Ni method includes a precipitation control step of precipitating γ” phase particles at a temperature below the γ” phase solvus temperature to control precipitation of the δ phase particles. A method for producing base hot forged material. 2. The method for producing a Ni-based hot forged material according to claim 1, wherein the δ phase particles are precipitated within crystal grains using the γ'' phase particles as nuclei. 3. The method for producing a Ni-based hot forged material according to claim 1, wherein hot forging is performed after the δ phase particles have a cross-sectional area ratio of 5% or more. 4. The method for producing a Ni-based hot forged material according to claim 3, wherein the γ'' phase particles have an average particle size of 100 nm or more. 5. The method for producing a Ni-based hot forged material according to claim 1, further comprising a high-temperature heat treatment step for solution treatment prior to the precipitation control step.

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