Patent Grant
10487384
The present invention relates to an Ni-based alloy product, an Ni-based alloy member produced of the Ni-based alloy product, a method for producing the Ni-based alloy product, and a method for producing the Ni-based alloy member.
How to improve thermal efficiency of high temperature devices, such as gas turbines and jet engines, is an important problem for many reasons including the need to reduce environmental impacts. An effective way of increasing thermal efficiency is to increase service temperatures.
Currently, a turbine inlet temperature of about 1300° C. is standard in a gas turbine. On the other hand, turbine components applicable to temperatures around 1700° C. are becoming commercially practical. Also, for gas turbine components such as turbine blades, Ni-based alloys of high heat-resistant superalloys are often used.
Meanwhile, high-strength Ni-based alloys applied to these gas turbines, jet engines, etc. derive their high mechanical strength from precipitating a γ′ phase (gamma prime phase, Ni3Al) therein. A γ′ phase is coherent with a γ phase in crystalline lattice, and the γ′ phase coherently precipitated in the γ phase (hereinafter referred to as a “coherent γ′ phase”) contributes greatly to the improvement in mechanical strength. In other words, the mechanical strength of Ni-based alloy members used in gas turbines, etc. can be improved by increasing the amount of the precipitated γ′ phase. However, such high-strength Ni-based alloy members with a high content of the precipitated γ′ phase have extremely poor cold workability due to their high hardness, and therefore high-strength Ni-based alloy members are not usually cold-worked.
For example, turbine blades mentioned above are produced of Ni-based alloys by precision forging, in which a γ′ phase precipitate is present at a ratio of 36 to 60 volume %, and cold working is not carried out in the production process due to their high hardness.
On the other hand, as for combustor components produced by cold working, hardness can be reduced by using Ni-based alloys in which a γ′ phase precipitate is present at a controlled ratio of 30 volume % or lower, thereby making cold working possible. However, such combustor components and other articles that can be cold-worked have lower mechanical strength than turbine blades or the like produced of Ni-based alloys including a γ′ phase precipitate at a ratio of 36 to 60 volume %. And, such Ni-based alloys including a γ′ phase precipitate of 30 volume % or lower are not adequate to fully satisfy requirements for the capability to tolerate increasingly high temperatures, as mentioned above.
As seen from the above, what is strongly needed in the art is to develop an Ni-based alloy member that is produced of an Ni-based alloy including a γ′ phase precipitate of 36 to 60 volume % and having a high durable temperature and that further has good cold workability. Also, a method for producing such a member is required.
Patent Literature 1 discloses a method for making an Ni-based superalloy article having a controlled grain size from a forging preform. In Patent Literature 1, there is described a controlling method of a grain size of an Ni-based superalloy, comprising the steps of hot die forging as the initial forging operations and isothermal forging as the subsequent forging operations. With this controlling method, a uniform grain size of approximately ASTM 6 to 8 can be achieved by carrying out hot die forging for the initial upset followed by isothermal forging and, if necessary, subsolvus annealing to provide a microstructure suitable for supersolvus heat treatment. It also describes that the hot die forging causes partial or complete recrystallization of the microstructure, which facilitates superplastic deformation in the subsequent isothermal forging operations. Moreover, Examples disclosed in Patent Literature 1 include a description about grain sizes when heat treatment is applied at 1850° F., 1900° F., and 1925° F.
Patent Literature 1: Japanese Unexamined Patent Application Publication No. Hei 9(1997)-302450.
With the method for controlling the grain size of an Ni-based superalloy described in Patent Literature 1, a uniform grain size can be achieved, and in addition, superplastic deformation can be facilitated. However, this does not solve the above-mentioned problem, that is, does not make it possible to provide an Ni-based alloy member including a γ′ phase precipitate at a ratio of 36 to 60 volume % and which has a high durable temperature and also has good cold workability. Furthermore, Patent Literature 1 does not provide a method for producing the Ni-based alloy member.
The present invention has been made in view of the above problems, and it is an objective to provide: an Ni-based alloy member in which a γ′ phase precipitate is present at a ratio of 36 to 60 volume % and which has a high durable temperature and also has good cold workability; a method for producing the member; an Ni-based alloy product to be used as a precursor of the Ni-based alloy member; and a method for producing the product.
According to one aspect of the present invention, there is provided an Ni-based alloy product having a two-phase structure composed of a γ phase and a γ′ phase that is incoherent with the γ phase in crystalline lattice parameters (hereinafter referred to as an “incoherent γ′ phase”), in which the incoherent γ′ phase is present at a ratio of 20 volume % or higher in the two-phase structure.
A hardness of the Ni-based alloy product can be decreased with increasing contents of the incoherent γ′ phase, thereby facilitating cold working. More preferable precipitation ratio of the incoherent γ′ phase is 25 volume % or higher. Also, the hardness is preferably 400 Hv or lower, more preferably 370 Hv or lower.
Moreover, in order to enhance ductility in cold working and improve cold workability, average grain size of the γ phase and the incoherent γ′ phase is preferably 100 μm or smaller, more preferably 50 μm or smaller.
The same advantages of the invention can be obtained even when carbides and different phases such as an η (eta) phase are present besides the incoherent γ′ phase. However, the total of such different phases is preferably 15 volume % or less.
Furthermore, the advantages of the present invention can be obtained even when some precipitates of a fine-grained coherent γ′ phase are present in the γ phase. However, it is preferable that the amount of the coherent γ′ phase be limited to a minimum.
The Ni-based alloy product according to the present invention is excellent in cutting machinability as well as in cold workability.
In order to produce the Ni-based alloy product according to the present invention, hot forging needs to be performed in a temperature range where the two phases of the γ phase and the incoherent γ′ phase can coexist. The reason is not only to precipitate the incoherent γ′ phase but also to obtain a fine microstructure by inhibiting the coarsening of the γ phase by the incoherent γ′ phase.
The hot forging needs to be performed at temperatures equal to or higher than 1000° C., at which the mechanical strength of the incoherent γ′ phase becomes lower. Furthermore, it is desirable that the incoherent γ′ phase be present at a ratio of 10 volume % or higher during the hot forging.
After the forging, the hardness of the Ni-based alloy can be decreased by increasing the incoherent γ′ phase, resulting in further enhanced hot workability.
In order to increase the incoherent γ′ phase, it is effective to conduct homogenization heat treatment at a temperature equal to or higher than 1000° C. and within a temperature range where the two phases of the γ phase and the γ′ phase coexist, preferably at a heating temperature of the final forging. And, after the homogenization heat treatment, it is effective to carry out slow cooling to a temperature 100° C. or more below the homogenization heat treatment temperature.
This slow cooling inhibits the precipitation of the coherent γ′ phase into the γ phase, which makes it possible to increase the incoherent γ′ phase.
A cooling rate of 100° C./h or slower is effective; a cooling rate of 50° C./h or slower is significantly effective; and a cooling rate of 20° C./h or slower is the most preferable.
Besides, an Ni-based alloy member according to the present invention is a Ni-based alloy member produced through cold working (including cutting machining), annealing, and solution and aging heat treatment of the Ni-based alloy product described above. And, the Ni-based alloy member comprises a γ phase and a coherent γ′ phase, in which the coherent γ′ phase is present at a ratio of 36 to 60 volume %, and has a predetermined shape.
When conducting solution heat treatment to redissolve the incoherent γ′ phase into a matrix, it is effective to apply a heat treatment at temperatures above a temperature at which the incoherent γ′ phase dissolves and becomes a solid solution completely. However, in the case where a grain size of the matrix becomes too coarse and the properties are degraded by the heat treatment, the coarsening of the crystalline grains can be inhibited by applying the solution heat treatment at temperatures at which the incoherent γ′ phase remains to some extent. In this case, the amount of the residual incoherent γ′ phase is preferably 10 volume % or less.
In addition, a method of an Ni-based alloy member according to the present invention includes the step of producing a precursor of an Ni-based alloy member that has a predetermined shape by cold-working the Ni-based alloy product produced by the method described above. The precursor of an Ni-based alloy member is subjected to solution and aging heat treatment so as to produce an Ni-based alloy member comprises a γ phase and a coherent γ′ phase, wherein the coherent γ′ phase is present at a ratio of 36 to 60 volume %.
According to an Ni-based alloy product and a method for producing the product of the present invention, the Ni-based alloy product produced by hot forging has a two-phase structure composed of a γ phase and a γ′ phase that is incoherent with the γ phase, wherein the γ′ phase is present at a ratio of 20 volume % or higher, which leads to excellent cold workability in the Ni-based alloy product. Also, according to an Ni-based alloy member and a method for producing the member of the present invention, by subjecting the above-mentioned Ni-based alloy product to cold working, forming it into a predetermined shape, and then subjecting it to solution and aging heat treatment, there can be obtained an Ni-based alloy member having a high durable temperature, in which the Ni-based alloy member comprises a γ phase and a coherent γ′ phase, the coherent γ′ phase being present at a ratio of 36 to 60 volume %.
Preferred embodiments of an Ni-based alloy product, a method for producing the product, an Ni-based alloy member, and a method for producing the member according to the present invention will be described below with reference to the accompanying drawings.
(First Embodiment of Method for Producing Ni-Based Alloy Member)
In the method for producing an Ni-based alloy member shown in the flowchart of
An Ni-based alloy member produced by the production method according to the present invention is made up of a γ phase and a γ′ phase that is coherent with the γ phase, wherein the γ′ phase is present at a ratio of 36 to 60 volume %, and has a high durable temperature. More specifically, the object to be produced by the production method of the present invention is an Ni-based alloy member wherein a γ′ phase that is thermodynamically stable in a temperature range of 700 to 900° C., in which the Ni-based alloy member is to be used, is present at a ratio of 36 to 60 volume %.
In producing an Ni-based alloy member of such high mechanical strength, first, an Ni-based alloy product (a product as a production base material for the Ni-based alloy member) that has a two-phase structure composed of a γ phase and an incoherent γ′ phase, wherein the incoherent γ′ phase is present at a ratio of 20 volume % or higher, is produced by hot-forging an Ni-based alloy material at a temperature equal to or higher than 1000° C. and at which the γ′ phase is precipitated at a ratio of 10 volume % or higher (step S10 in
An example of the ingredient composition of the Ni-based alloy product would be 12% of Co, 14% of Cr, 3.7% of Al, 2.6% of Ti, 1% of Nb, 1% of W, 2% of Mo, 0.01% of C, and the balance of Ni (all in volume %), wherein an incoherent γ′ phase is present at a ratio of 20 volume % or higher.
An Ni-based alloy product as an inventive example produced by hot forging has a microstructure shown in FIG. 3(b).
In
Incidentally, in the γ phase M′, Ni and Al atoms are randomly arranged, but in the γ′ phase P′, Ni and Al atoms are regularly arranged. While both are based on a face-centered cubic lattice, they are different as precipitates.
For comparison with the microstructure of the Ni-based alloy product of the inventive example shown in
As shown in
Meanwhile, a γ′ phase generally has good lattice coherence with a γ phase of a matrix. Therefore, a γ′ phase P precipitated within a crystalline grain of a γ phase M like
The inventors came up with a technical idea in which this γ′ phase P is not significantly higher in mechanical strength than the γ phase M, and that the coherent interface between the γ phase M and the γ′ phase P would enhance the mechanical strength of an Ni-based alloy member.
In other words, the inventors considered that the presence of a coherent interface between a γ phase M and a γ′ phase P, as shown in
So, by carrying out hot forging or applying heat treatment after the hot forging at a temperature equal to or higher than 1000° C. and at which two phases of a γ phase and a γ′ phase can coexist, there can be produced an Ni-based alloy product having a two-phase structure in which a γ phase M′ and a γ′ phase P′ that is incoherent with the γ phase M′ are aligned via incoherent grain boundaries B as shown in
Referring back to
Herein, “cold working” means working the Ni-based alloy product 1 into the shape of a desired final Ni-based alloy member by, for example, forging, rolling, or molding at a room temperature.
Because the Ni-based alloy product 1 used has the microstructure shown in
Enhancing ductility is effective in further improving this cold workability, and it is preferable that the crystalline grains of both the γ phase M′ and the incoherent γ′ phase P′ that form the Ni-based alloy product 1 be adjusted to 100 μm or smaller in grain size. It is more preferable that they be adjusted to 50 μm or smaller in grain size.
Regarding this grain size, the inventors have proved that by performing step S10, namely, the step of hot-forging an Ni-based alloy base material at a temperature equal to or higher than 1000° C. and at which a γ′ phase and a γ phase can coexist, a γ′ phase that is incoherent with the γ phase is precipitated, and this precipitated γ′ phase inhibits the grain growth of the γ phase. As a result, the grain size of both the γ phase and the γ′ phase can be adjusted to 100 μm or smaller.
By this cold working, there is produced a precursor of an Ni-based alloy member that is a precursor of Ni-based alloy members such as plates, rod-shaped wires, and even turbine blades to be used as gas turbine components.
However, the precursor of an Ni-based alloy member produced in step S20 has a microstructure in which no coherent interface is present between the γ phase and the γ′ phase to contribute to the enhancement of mechanical strength. Therefore, the precursor itself is not suitable for application as high-strength members.
Then, the precursor of an Ni-based alloy member is subjected to solution heat treatment so as to redissolve the incoherent γ′ phase into a matrix. Subsequently, the precursor is subjected to aging heat treatment so as to precipitate a coherent γ′ phase as an inclusion in the crystalline grains of the γ phase, which causes the formation of a coherent interface between the γ phase and the γ′ phase. Thus there is produced an Ni-based alloy member that has the microstructure shown in
Here, the microstructure shown in
Examples of the Ni-based alloy member produced in step S30 are shown in
Each of these Ni-based alloy members 10, 10A and 10B contains a γ′ phase at a ratio of 36 to 60 volume % or higher and has a high durable temperature due to a coherent interface formed between a γ phase and a γ′ phase that is coherent with this γ phase.
As described above, according to the production flow shown in
(Second Embodiment of Method for Producing Ni-Based Alloy Member)
The production method for an Ni-based alloy member shown in
For example, in the case where hot forging is performed at temperatures around 1200° C. in the initial stage and at around 1150° C. in the final stage, the subsequent heat treatment is applied for a predetermined time at a temperature around 1100° C., which is below the final stage temperature of the hot forging of about 1150° C., and then heat treatment is applied while controlling the temperature by slow-cooling the Ni-based alloy product to temperatures around 1000° C. or 900° C.
The inventors have revealed that by applying heat treatment after hot forging for a predetermined time at a temperature below the hot forging temperatures in the way described above, the incoherent γ′ phase can be increased to further lower the hardness of the Ni-based alloy product, which results in further improved cold workability.
[Cold Workability Verification Tests and Results Thereof]
The inventors produced test pieces of different ingredient compositions under different production conditions and conducted tests to verify the cold workability of each test piece. Table 1 below shows the ingredient compositions of the test pieces, and Table 2 shows the production conditions of the test pieces and cold working test results. Also, as for the test pieces for which heat treatment was applied after hot forging during their production, the details of the heat treatments A, B and C in Table 2 are shown in Table 3.
In producing each test piece, the base material of 20 kg was melted by vacuum induction melting, subjected to homogenization heat treatment, and subsequently hot-forged under the conditions shown in Table 2 into a round bar with a diameter of 15 mm.
In Comparative Example 1, hot forging was not performed, whereas in Comparative Examples 2 to 6, hot forging was performed. Hot forging was performed also in Inventive Examples 1 to 10, and as for Inventive Examples 5 to 10, one of the heat treatments A to C shown in Table 3 was applied after the hot forging.
The microstructure of each test piece was observed after the hot forging or after the subsequent heat treatment, and the content ratios of the γ phase and the incoherent γ′ phase were measured.
Furthermore, the cold working tests were conducted in the following procedure. First, each obtained round bar with a diameter of 15 mm was reduced in diameter 1 mm by 1 mm, by cold drawing. The cold drawing was performed three times until the diameter was reduced to 12 mm.
The cold working test results for the test pieces that could not be drawn successfully are denoted as “NG” in Table 2.
In contrast, the cold working test results for the test pieces that could be drawn successfully into a test piece with a diameter of 13 mm without cracking are denoted as “OK” in Table 2. Some test pieces were subsequently subjected to annealing at temperatures between 1000° C. and 1100° C. and cold working repeatedly to be successfully worked into a wire rod with a diameter of 3 mm.
As shown in Table 2, the cold working test results for test pieces of Comparative Examples 1 to 6 were all “NG”, whereas the cold working test results for test pieces of Inventive Examples 1 to 10 were all “OK”. In particular, it was easy to perform cold working of test pieces containing an incoherent γ′ phase precipitate in an amount of 25% or larger and having a hardness of 370 Hv or lower.
As for test pieces of Comparative Examples 1 to 6, despite the hot forging performed, the amount of the incoherent γ′ phase remained 0 volume %, resulting in a Vickers hardness (Hv) of over 400 before cold working, with which cold working was impossible. This is because, except for Comparative Example 4, the hot forging temperatures were higher than the solvus temperature of the γ′ phase and therefore no γ′ phase precipitation occurred during the hot forging. In Comparative Example 4, the hot forging temperatures were slightly lower than the solvus temperature of the γ′ phase, and therefore an incoherent γ′ phase was precipitated in a small amount, which, however, was not enough to improve cold workability. The solvus temperatures of the γ′ phase of Comparative Examples 1 to 6 were 1134° C., 1157° C., 1183° C., 1173° C., 1115° C., and 1154° C., respectively.
In contrast, the Vickers hardness (Hv) of each test piece of Inventive Examples 1 to 10 was lower than 400, which permits cold working.
In particular, Inventive Examples 5 to 10, for which any one of the heat treatments A to C was applied after the hot forging, each exhibited a Vickers hardness (Hv) that was relatively low as compared with Inventive Examples 1 to 3, for which no heat treatment was applied after the hot forging.
As can be seen from the above, it has been demonstrated that the hardness of an Ni-based alloy product can be further lowered to further improve its cold workability by applying homogenization heat treatment at a temperature equal to or higher than 1000° C. and within a temperature range in which the γ phase and the γ′ phase coexist after performing hot forging in the way described above and subsequently performing slow cooling to a temperature 100° C. or more below the homogenization heat treatment temperature.
Incidentally, test pieces of Inventive Examples 1 to 8 were successfully worked into a wire with a diameter of 2 mm by being subjected to annealing and cold drawing repeatedly after the first cold working test.
A relationship between the amount of the precipitated incoherent γ′ phase and the Vickers hardness before the cold working in Table 2 is shown in a graph form in
Here, tensile testing was conducted in two cases, at a room temperature and at 700° C. Also, creep testing was conducted at 700° C. and a load stress of 350 MPa.
While the preferred embodiments of the present invention have been described above with reference to the accompanying drawings, it should be noted that the specific constitution is not to be construed as limited to the embodiments and that any design modifications, etc. made without departing from the spirit and scope of the present invention are to be included in the present invention.
1 . . . Ni-based alloy product; 10, 10A, 10B . . . Ni-based alloy member; B . . . grain boundary; M . . . γ phase (matrix); P . . . γ′ phase (γ′ phase coherent with γ phase); and P′ . . . γ′ phase (γ′ phase incoherent with γ phase).
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2013/069367 | 7/17/2013 | WO | 00 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2015/008343 | 1/22/2015 | WO | A |
| Number | Name | Date | Kind |
|---|---|---|---|
| 4574015 | Genereux et al. | Mar 1986 | A |
| 4820353 | Chang | Apr 1989 | A |
| 5120373 | Miller et al. | Jun 1992 | A |
| 5131961 | Sato et al. | Jul 1992 | A |
| 5605584 | DeLuca et al. | Feb 1997 | A |
| 5649280 | Blankenship et al. | Jul 1997 | A |
| 5759305 | Benz et al. | Jun 1998 | A |
| 6059904 | Benz et al. | May 2000 | A |
| 8083124 | Darolia | Dec 2011 | B1 |
| 20050265887 | Toji et al. | Dec 2005 | A1 |
| 20100243111 | Imano | Sep 2010 | A1 |
| 20110223055 | Semba et al. | Sep 2011 | A1 |
| 20150368774 | Ota et al. | Dec 2015 | A1 |
| Number | Date | Country |
|---|---|---|
| 1053094 | Jul 1991 | CN |
| 1276022 | Dec 2000 | CN |
| 0 260 513 | Mar 1988 | EP |
| 0 361 524 | Apr 1990 | EP |
| 0 433 072 | Jun 1991 | EP |
| 0 787 815 | Aug 1997 | EP |
| 2 330 225 | Jun 2011 | EP |
| 63-145737 | Jun 1988 | JP |
| 2-97634 | Apr 1990 | JP |
| 9-302450 | Nov 1997 | JP |
| 2006-9143 | Jan 2006 | JP |
| WO 9923265 | May 1999 | WO |
| WO 2010038826 | Apr 2010 | WO |
| Entry |
|---|
| Chinese-language Office Action issued in counterpart Chinese Application No. 201380074789.9 dated May 31, 2016 with English translation (Seventeen (17) pages). |
| International Search Report (PCT/ISA/210) issued in PCT Application No. PCT/JP2013/069367 dated Sep. 3, 2013 with English translation (Four (4) pages). |
| Extended European Search Report issued in counterpart European Application No. 13889448.0 dated Jan. 2, 2017 (eight (8) pages). |
| Number | Date | Country | |
|---|---|---|---|
| 20160160334 A1 | Jun 2016 | US |
