CN111074332B - Heat treatment method for rapidly eliminating microsegregation in single crystal high-temperature alloy - Google Patents
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Abstract
The invention discloses a heat treatment method for rapidly eliminating microsegregation in a single crystal high-temperature alloy, which comprises the following steps: s1, preparing a single crystal test rod of a single crystal superalloy by adopting a directional solidification process, wherein the single crystal superalloy contains high-melting-point rare metal; s2, determining the primary melting temperature of the single crystal superalloy by adopting a metallographic method; s3 homogenization treatment: and (3) preserving the temperature of the single crystal test bar for 1-20h at the temperature 1-20 ℃ higher than the initial melting temperature, and cooling the single crystal test bar to room temperature in air. The homogenization heat treatment temperature adopted by the invention is higher than the initial melting temperature of the single crystal superalloy, and through reasonable selection and collocation of the heat treatment temperature and the heat preservation time, the dendritic crystal segregation in the alloy and the initial generated initial melting structure can be finally eliminated, the performance of the alloy is not influenced, and the long-term structure stability of the alloy can be improved.
Description
Technical Field
The invention relates to the technical field of heat treatment of single crystal high-temperature alloys, in particular to a heat treatment method for rapidly eliminating microsegregation in a single crystal high-temperature alloy.
Background
The high-temperature alloy (Superalloy) is a high-temperature structural material taking iron, nickel and cobalt as matrixes, can be used in a high-temperature environment with the temperature of more than 600 ℃, can bear harsh mechanical stress, has high-temperature lightness, excellent oxidation resistance and hot corrosion resistance, excellent creep deformation and fatigue resistance and good tissue stability, and is widely applied to hot-end parts of advanced power propulsion systems such as turbine engines and the like. The single crystal high temperature alloy eliminates crystal boundary, obviously reduces crystal boundary strengthening elements for lowering melting point, improves the initial melting temperature of the alloy, can carry out solution treatment in a higher temperature range, and has greatly improved strength compared with isometric crystal and oriented column crystal high temperature alloys, thereby being widely applied and being a key material for preparing advanced aeroengines.
In recent years, in order to meet the requirement of the increasing thrust-weight ratio of the engine, the proportion of refractory strengthening elements such as Re, W, Mo and the like in the nickel-based high-temperature alloy is increasing, the temperature bearing capacity of the alloy is improved, and the performance of the method is deteriorated, such as a large amount of casting defects, serious microsegregation and the like formed in the directional solidification process; the strengthening elements also change the solute distribution of the directional solidification, and the serious dendrite segregation exists in the alloy cast structure, namely, W, Re and other elements segregate in dendrite stems and Al and Ta elements segregate among dendrites, so that the structure stability and the mechanical property of the alloy are seriously influenced. For this reason, a subsequent homogenization heat treatment method is required to eliminate the element segregation and obtain a uniform texture structure. In order to avoid the initial melting of the alloy, the temperature of the traditional homogenization heat treatment cannot be higher than the initial melting temperature of the alloy, so that dendritic crystal segregation still exists after the alloy homogenization treatment. Although the homogenization treatment temperature of the alloy can be increased by a step-by-step or continuous heating method, the homogenization treatment system of the method is complicated, the homogenization time is long, for example, the CMSX-10 alloy has the homogenization time of 45 hours, and the segregation of the refractory Re element cannot be completely eliminated.
Disclosure of Invention
In order to overcome the defects of the prior art, the invention aims to provide a novel heat treatment method for rapidly eliminating the microsegregation in the single crystal superalloy, the adopted homogenization heat treatment temperature is higher than the initial melting temperature of the single crystal superalloy, and through reasonable selection and matching of the heat treatment temperature and the heat preservation time, the dendritic crystal segregation and the initially generated initial melting structure in the alloy can be finally eliminated, so that the long-term structure stability of the alloy is improved.
The purpose of the invention is realized by adopting the following technical scheme:
a heat treatment method for rapidly eliminating microsegregation in a single crystal superalloy comprises the following steps:
s1: preparing a single crystal test bar of a single crystal high temperature alloy by adopting a directional solidification process, wherein the single crystal high temperature alloy contains high-melting-point rare metals;
s2: determining the initial melting temperature of the single crystal high-temperature alloy by adopting a metallographic method;
s3: homogenizing: and (3) preserving the temperature of the single crystal test bar for 1-20h at the temperature 1-20 ℃ higher than the initial melting temperature, and cooling the single crystal test bar to room temperature in air.
Further, the high-melting-point rare metal is one or more of rhenium, tungsten, molybdenum, tantalum and niobium.
The directional solidification process is a high-speed solidification method or a liquid metal cooling method.
Further, the single crystal superalloy is a nickel-based single crystal superalloy.
Preferably, the nickel-based single crystal superalloy is a DD33 alloy.
The heat treatment method for rapidly eliminating the microsegregation in the single crystal superalloy comprises the following steps:
s1: preparing the DD33 single crystal test bar of the DD33 alloy by adopting a high-speed solidification method or a liquid metal cooling method;
s2: determining the DD33 initial melting temperature of the DD33 alloy by adopting a metallographic method;
s3: homogenization treatment: and (3) preserving the heat of the DD33 single crystal test rod for 10-20h at the temperature which is 10-20 ℃ higher than the initial melting temperature of the DD33, and cooling the DD 3578 single crystal test rod to room temperature in air.
Preferably, the nickel-based single crystal superalloy is a PWA1484 alloy.
The heat treatment method for rapidly eliminating the microsegregation in the single crystal superalloy is characterized by comprising the following steps of:
s1: preparing a PWA1484 single-crystal test bar of the PWA1484 alloy by adopting a high-speed solidification method or a liquid metal cooling method;
s2: determining the initial melting temperature of the PWA1484 alloy by adopting a metallographic method;
s3: homogenizing: and (3) preserving the temperature of the PWA1484 single crystal test bar for 4-7h at the temperature 5 ℃ higher than the primary melting temperature of the PWA1484, and cooling in air to room temperature.
Compared with the prior art, the invention has the beneficial effects that:
1. according to the Fick diffusion second law, the higher the homogenization heat treatment temperature is, the shorter the diffusion time is, and the higher the homogenization efficiency is; when the temperature is too high, although segregation can be reduced more quickly, isothermal solidification does not occur at high temperature, the initial melting phenomenon cannot be eliminated, and the performance of the alloy is seriously affected. The heat treatment method comprehensively considers the diffusion speed of alloy elements and the influence factors of primary melting elimination, and adopts a specific homogenization treatment temperature higher than the primary melting temperature of the alloy. In the range of 1-20 ℃ that the homogenization treatment temperature is higher than the initial melting temperature of the alloy, the initial melting phenomenon of the alloy begins to appear, in the proper heat preservation process, low-melting-point elements in the liquid phase diffuse to the solid phase, high-melting-point elements in the solid phase diffuse to the liquid phase, so that the solidification temperature of the liquid phase rises, isothermal solidification occurs in the heat preservation process, the initial melting phenomenon disappears, the effects of eliminating dendritic crystal segregation and initial melting tissues in the alloy are finally realized, and the long-term structure stability of the alloy is improved.
2. The invention adopts ultra-high temperature to carry out homogenization heat treatment, and matches with proper heat preservation time, thereby eliminating dendrite segregation to the maximum extent, not influencing the performance of the material, and improving the long-term structure stability of the alloy.
Drawings
FIG. 1 is a morphology graph of an initial melting structure of DD33 alloy after 1340 deg.C/1 h heat treatment in example 1;
FIG. 2 is a diagram of the morphology of the primary molten structure of the DD33 alloy of example 1 after 1350 ℃ homogenization treatment for different holding times; a. 1350 ℃/1 h; b. 1350 ℃/5 h; c. 1350 ℃/10 h; d. 1350 ℃/20 h;
FIG. 3 is a morphology graph of an initial melting structure of DD33 alloy after 1340 ℃ homogenization treatment in example 2 and with different holding times; a. 1340 ℃/1 h; b. 1340 ℃/2 h; c. 1340 ℃/10 h; d. 1340 ℃ per 20 h;
FIG. 4 is a primary molten microstructure of the PWA1484 alloy of example 3 after 1315 ℃/1h heat treatment;
FIG. 5 is a graph showing the morphology of an initial molten structure of the PWA1484 alloy of example 3 after homogenization treatment at 1320 ℃ for different holding times; a. 1320 ℃/0 h; b. 1320 ℃/1 h; c. 1320 ℃/2 h; d. 1320 ℃/4 h; e. 1320 ℃/7 h; f. 1320 ℃/10 h;
FIG. 6 is a graph of the microstructure of the alloy after 1100 deg.C/500 h long term aging after heat treatment of example 1 and comparative example 1; a. 1350 ℃/20 h; b. 1330 ℃/20 h.
Detailed Description
The present invention will be further described with reference to the accompanying drawings and the detailed description, and it should be noted that any combination of the embodiments or technical features described below can be used to form a new embodiment without conflict.
A heat treatment method for rapidly eliminating microsegregation in a single crystal superalloy comprises the following steps:
s1: preparing a single crystal test rod of a single crystal high temperature alloy by adopting a directional solidification process, wherein the single crystal high temperature alloy contains one or more of high-melting-point rare metals Re (rhenium), W (tungsten), Mo (molybdenum), Ta (tantalum) and Nb (niobium); the solidification process is a high-speed solidification method or a liquid metal cooling method;
s2: determining the primary melting temperature of the single crystal high-temperature alloy by adopting a metallographic method;
s3: homogenizing: and (3) preserving the temperature of the single crystal test bar for 1-20h at the temperature 1-20 ℃ higher than the initial melting temperature of the alloy, and cooling the single crystal test bar to room temperature in air.
The selection of the temperature difference that the homogenization treatment temperature is higher than the alloy initial melting temperature is related to the segregation degree of the alloy, the temperature in the temperature difference range of the embodiment can be screened according to the segregation degree of the alloy to carry out homogenization treatment, and the effects of eliminating dendritic crystal segregation and the initial melting structure of the alloy can be realized. The method is suitable for the high-temperature alloy with serious segregation, and as a further preferable scheme, the single-crystal high-temperature alloy is nickel-based single-crystal high-temperature alloy.
Different kinds and amounts of high-melting point rare metals are added, and different methods are adopted to prepare single crystal high-temperature alloy, the generated segregation degrees are different, and the initial melting temperatures of the alloy are different, so that the preparation methods of the high-melting point rare metals and the alloy need to be comprehensively considered, and appropriate parameters are selected.
The homogenization treatment of step S3 includes charging, holding, and air cooling to room temperature. The heat treatment method of the embodiment is less influenced by the heating rate, and the alloy can be charged in a furnace for heat preservation after the furnace temperature is heated to the homogenization treatment temperature; the alloy can be charged firstly, and then the temperature is gradually increased to the homogenization treatment temperature along with the furnace temperature, so that the operation is more convenient, and the parameters are easier to control.
Example 1
A heat treatment method for rapidly eliminating microsegregation in a single crystal superalloy comprises the following steps:
s1: preparation of Single Crystal test rods
Preparing a DD33 single crystal test rod by adopting a high-speed solidification method of a directional solidification process which is conventional in the field according to the conventional composition and formula of the DD33 alloy;
s2: determination of incipient melting temperature
The initial melting temperature of the single crystal superalloy DD33 is determined to be 1340 ℃ by adopting a metallographic method, and the specific operation is as follows:
the single crystal superalloy DD33 of the embodiment is subjected to heat preservation for 1 hour at 1330 ℃, 1335 ℃ and 1340 ℃ respectively and then is quenched with water; the surface oxide layer is removed by linear cutting, grinding, polishing and corrosion are carried out, the microstructure appearance is observed by a metallographic microscope, and the primary melting structure appearance is found by a 1340 ℃ heat preservation sample, namely the primary melting temperature of the single crystal superalloy DD33 is 1340 ℃.
S3: ultra-high temperature homogenization treatment
Cutting the DD33 single crystal test rod line in S1 into 20mm thick samples for experiment, homogenizing, placing the samples into a homogenizing furnace at 1350 ℃, preserving heat for 1-20h, and cooling to room temperature.
The sample in S3 is put into a homogenizing treatment furnace at 1340 ℃ for heat preservation for 1h, then air cooling is carried out to room temperature, and the morphology of the primary melting structure of the DD33 alloy after 1340 ℃ and 1h heat treatment is observed by a Scanning Electron Microscope (SEM), and the result is shown in figure 1, and the small amount of primary melting of the alloy can be seen from figure 1.
Meanwhile, the shapes of the primary molten structures of the alloy in the embodiment at the temperature of 1350 ℃ for 1h, 5h, 10h and 20h are respectively observed by a scanning electron microscope, and the results are shown in fig. 2, wherein a is the shape of the primary molten structure of the alloy DD33 in the embodiment after 1350 ℃/1h homogenization treatment, b is the shape of the primary molten structure of the alloy DD33 in the embodiment after 1350 ℃/5h homogenization treatment, c is the shape of the primary molten structure of the alloy DD33 in the embodiment after 1350 ℃/10h homogenization treatment, and d is the shape of the primary molten structure of the alloy DD33 in the embodiment after 1350 ℃/20h homogenization treatment.
As can be seen from FIG. 2, all the eutectic in the as-cast structure was melted after the homogenization treatment for 1 hour, but the area fraction of the liquid phase gradually decreased by isothermal solidification as the homogenization time was prolonged; after 20h of incubation, the liquid phase disappeared completely.
Example 2
A heat treatment method for rapidly eliminating microsegregation in a single crystal superalloy comprises the following steps:
s1: preparation of single crystal test bar
Preparing a DD33 single crystal test rod by adopting a liquid metal cooling method of a directional solidification process in the field according to the conventional composition and formula of the DD33 alloy;
s2: determination of incipient melting temperature
Determining the primary melting temperature of the single crystal superalloy DD33 to be 1320 ℃ by adopting a metallographic method; the specific operation is as follows:
the DD33 alloy of the embodiment is respectively subjected to heat preservation for 1h at 1310 ℃, 1315 ℃ and 1320 ℃ and then water quenching; the surface oxide layer is removed by linear cutting, grinding, polishing and corrosion are carried out, the microstructure appearance is observed by a metallographic microscope, and the appearance of the primary melting structure is found by a 1320 ℃ heat preservation sample, namely the primary melting temperature of the DD33 alloy in the embodiment is 1320 ℃.
S3: ultra-high temperature homogenization treatment
The DD33 single crystal test rod line in the embodiment S1 is cut to 20mm in thickness to be used as an experimental sample for homogenization treatment, the sample is placed into a homogenization treatment furnace at 1340 ℃, the temperature is kept for 1-20h, and then the sample is cooled to room temperature in air.
The morphology of the primary molten structure of the alloy in this embodiment after 1h, 2h, 10h and 20h of holding time was observed by a scanning electron microscope, and the result is shown in fig. 3, where a is the morphology of the primary molten structure of the DD33 alloy 1340 ℃/1h after homogenization treatment in this embodiment, b is the morphology of the primary molten structure of the DD33 alloy 1340 ℃/2h after homogenization treatment in this embodiment, c is the morphology of the primary molten structure of the DD33 alloy 1340 ℃/10h after homogenization treatment in this embodiment, and d is the morphology of the primary molten structure of the DD33 alloy 1340 ℃/20h after homogenization treatment in this embodiment.
As can be seen from FIG. 3, after homogenization for 2h, all the eutectic in the alloy structure is melted, the melting area fraction gradually decreases with the increase of homogenization time, and after heat preservation for 20h, the initial melting completely disappears.
Example 3
A heat treatment method for rapidly eliminating microsegregation in a single crystal superalloy comprises the following steps:
s1: preparation of single crystal test bar
Preparing a PWA1484 single-crystal test bar by adopting a high-speed solidification method of a conventional directional solidification process in the field according to the conventional composition and formula of the PWA1484 alloy;
s2: determination of incipient melting temperature
Determining the primary melting temperature of the single crystal superalloy PWA1484 to be 1315 ℃ by adopting a metallographic method; the PWA1484 alloy of the embodiment is subjected to water quenching after being respectively subjected to heat preservation for 1 hour at 1305 ℃, 1310 ℃ and 1315 ℃; the surface oxide layer is removed by linear cutting, grinding, polishing and corrosion are carried out, the microstructure appearance is observed by a metallographic microscope, and the primary melting structure appearance is found by a 1315 ℃ heat preservation sample, namely the primary melting temperature of the PWA1484 alloy of the embodiment is 1315 ℃.
S3: ultra-high temperature homogenization treatment
The PWA1484 single crystal test bar wire in this example S1 was cut to 20mm thick and homogenized, and the sample was placed in a homogenization furnace at 1320 ℃ for 0-10h, and then cooled to room temperature by air.
The sample in S3 was placed in a homogenizing furnace at 1315 ℃ and kept warm for 1 hour, then cooled to room temperature by air cooling, and the morphology of the primary melt structure of the PWA1484 alloy of this example after heat treatment at 1315 ℃/1 hour was observed by a scanning electron microscope, and as a result, as shown in FIG. 4, a small amount of primary melting of the alloy occurred as seen from FIG. 4.
The shapes of the primary melting structures of the alloys at the holding times of 0h, 1h, 2h, 4h/7h and 10h at the temperature of 1320 ℃ in the embodiment are respectively observed by a scanning electron microscope, and the results are shown in FIG. 5, in the figure, a is the morphology of the primary molten structure of the PWA1484 alloy after 1320 ℃ per 0h homogenization treatment, in the figure b is the morphology of the primary molten structure of the PWA1484 alloy of the present embodiment after 1320 ℃ per 1h homogenization treatment, in the figure, c is the morphology of the primary molten structure of the PWA1484 alloy after 1320 ℃ per 2h homogenization treatment, in the figure, d is the morphology of the primary molten structure of the PWA1484 alloy after 1320 ℃/4h homogenization treatment, in the figure, e is the morphology of the primary molten structure of the PWA1484 alloy after 1320 ℃/7h homogenization treatment, in the figure, f is the morphology of the primary molten structure of the PWA1484 alloy after 1320 ℃ C./10 h homogenization treatment.
FIG. 5 shows that a large amount of initial melting still exists in the alloy when the temperature is kept for 1 h; when the temperature is kept for 2 hours, the initial melting is reduced; when the temperature is kept for 4 hours, the initial melting basically disappears; when the temperature is kept for 7 hours, no primary melting exists, and loose small holes are formed; the effect of 10h of heat preservation is close to 7 h.
Comparative example 1
A heat treatment method for rapidly eliminating microsegregation in a single crystal superalloy is the same as that of example 1 except that the homogenization temperature is 1330 ℃, and details thereof are omitted.
Test example 1
Before homogenization treatment in example 1 and comparative example 1 and during homogenization treatment and heat preservation for 20 hours, the chemical components of main elements between dendrites and dendrites at different positions of an alloy sample are respectively measured by an energy spectrometer, the average value of the chemical components is obtained by multipoint measurement, and the segregation coefficient of each element is further calculated, and the result is shown in table 1.
TABLE 1 comparison of elemental microsegregation after homogenization at different temperatures
The results in Table 1 show that the segregation coefficients of the refractory elements such as Re, W and Ta of the as-cast alloy tend to 1 after 1350 ℃/20h, which indicates that the segregation of the positive segregation elements Re, W and Ta segregated in the dendrite trunk is completely eliminated. Compared with the conventional heat treatment comparative example 1(1330 ℃/20h), the segregation of the elements Re, W and Ta is greatly reduced, and the segregation elimination effect is better.
Test example 2
Alloy long term aging structure stability detection
After the alloy is respectively subjected to 1350 ℃/20h heat treatment of example 1 and 1330 ℃/20h heat treatment of comparative example 1, the alloy is aged for 500h at 1100 ℃, a scanning electron microscope is adopted to observe the microstructure of the alloy, and the structure appearance after the long-term aging of 1100 ℃/500h is shown in figure 6. In the figure, a is the tissue morphology after 1100 ℃/500h long-term aging after 1350 ℃/20h heat treatment, and b is the tissue morphology after 1100 ℃/500h long-term aging after 1330 ℃/20h heat treatment.
As can be seen from FIG. 6, the alloy after 1350 ℃/20h heat treatment has no precipitation of TCP phase, while the alloy after 1330 ℃/20h conventional heat treatment has obvious precipitation of TCP phase, which shows that the heat treatment method of the invention greatly improves the long-term structure stability of the alloy.
The above embodiments are only preferred embodiments of the present invention, and the scope of the present invention is not limited thereto, and any insubstantial changes and substitutions made by those skilled in the art based on the present invention are within the scope of the present invention claimed in the present invention.
Claims (3)
1. A heat treatment method for rapidly eliminating microsegregation in a single crystal superalloy is characterized by comprising the following steps:
s1: preparing a single crystal test bar of a single crystal high temperature alloy by adopting a directional solidification process, wherein the single crystal high temperature alloy contains high-melting-point rare metals; the single-crystal superalloy is a nickel-based single-crystal superalloy, and the nickel-based single-crystal superalloy is a DD33 alloy or a PWA1484 alloy;
s2: determining the initial melting temperature of the single crystal high-temperature alloy by adopting a metallographic method;
s3: homogenizing: when the nickel-based single crystal superalloy is a DD33 alloy, a single crystal test rod of the DD33 alloy is kept at the temperature 10-20 ℃ higher than the initial melting temperature of the DD33 alloy for 10-20 hours, and air-cooled to room temperature;
when the nickel-based single crystal superalloy is PWA1484 alloy, the single crystal test rod of the PWA1484 alloy is kept for 4-7 hours at the temperature 5 ℃ higher than the primary melting temperature of the PWA1484 alloy, and is cooled to room temperature by air.
2. The thermal processing method for rapidly eliminating microsegregation in a single crystal superalloy as in claim 1, wherein the refractory rare metal is one or more of rhenium, tungsten, molybdenum, tantalum, and niobium.
3. The thermal treatment method for rapidly eliminating microsegregation in a single crystal superalloy as in claim 1, wherein the directional solidification process is a high-speed solidification process or a liquid metal cooling process.
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| CN106756249A (en) * | 2016-12-09 | 2017-05-31 | 中国科学院金属研究所 | A kind of nickel-base high-temperature single crystal alloy of high intensity and tissue stabilization and preparation method thereof |
| CN106636759B (en) * | 2017-01-05 | 2018-09-21 | 中国科学院金属研究所 | A kind of high thermal stability high-strength nickel based single-crystal high-temperature alloy that platinum family element is strengthened |
| CN107460419B (en) * | 2017-06-30 | 2019-08-02 | 江苏大学 | A kind of tissue and performance optimization new process of low rhenium single crystal casting |
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