A heat treatment method for restoring the properties of a nickel-based single crystal superalloy

Abstract

The present disclosure discloses a method for heat treatment to restore the properties of a nickel-based single crystal superalloy, including: S100: Obtain the creep strain of the nickel-based single crystal superalloy sample after creep. If the strain is less than the strain threshold, perform step S200; if the strain is greater than the strain threshold, perform step S300; S200: Use the solution treatment temperature as the termination heating temperature, and perform heat treatment to restore the creeped nickel-based single crystal superalloy sample; S300: Perform heat treatment to restore the creeped nickel-based single crystal superalloy sample according to the set heat treatment parameters for restoration.

Description

Technical Field

[0001] The present disclosure belongs to the field of metal heat treatment, and particularly relates to a heat treatment method for restoring the properties of a nickel-based single crystal superalloy. Background Art

[0002] Aeroengines are the crown of industry and an important symbol to measure a country's comprehensive scientific and technological level, the strength of its scientific and technological industrial foundation, and its overall national strength. At present, the global aeroengine market scale is about 75 billion US dollars, and the output value of China's aeroengine market is only 3.076 billion US dollars. Among them, there are more than 4,200 turbofan engines, accounting for 44.7% of the total number. The high manufacturing price makes the turbofan engine account for more than 90% of the aeroengine market value. As a key hot-end component of aeroengines, turbine blades serve in harsh environments such as high speed, high temperature, and heavy load for a long time, and are listed as the first key component of aeroengines. Under the simultaneous action of high centrifugal load and high temperature load, turbine blades are extremely prone to fatigue cracks and high-temperature creep phenomena. With the increase of flight time, crack accumulation is likely to occur, resulting in fatigue fracture failure and creep fracture failure. Statistics on the failure data of various components of domestic and foreign aeroengines show that blade failures account for about 39% of engine failures.

[0003] The main form of high-temperature creep degradation is microstructural degradation (the rafting of the strengthening phase γ′ and the nucleation and growth of high-temperature creep holes). Creep will greatly affect the high-temperature performance and service life of superalloy parts. For components damaged by creep, there are generally two treatment methods, namely replacing them with new components or repairing the damaged components through technological means to extend their service life. However, the cost of replacing a brand-new turbine blade is about 400,000 yuan. The main reason for its high price is the complexity of the preparation process and the use of expensive rare earth elements. Therefore, choosing technological means to repair damaged components can greatly reduce economic costs and has great practical application value.

[0004] In the process of repairing damaged parts by technological means, the main applications are the methods of restoration heat treatment or hot isostatic pressing (HIP) + restoration heat treatment. If HIP + restoration heat treatment is used to repair damaged tissues, higher temperatures are required to dissolve the rafted γ' phase. When the HIP temperature approaches the solidus temperature of the alloy, there is a risk of recrystallization and primary melting inside the material. For example, under the conditions of 1250 °C / 150 Mpa, the element diffusion rate is greater, the pores are completely healed, but primary melting occurs in the tissue, resulting in a significant decrease in the alloy properties. When the temperature and pressure are simultaneously increased to the specified values during hot isostatic pressing of the alloy, small cold plastic deformations may occur under high pressure at a lower temperature, and then recrystallization may occur under the subsequent high temperature. Once recrystallization occurs in a single crystal, its properties will decrease significantly. This is because recrystallization will form transverse grain boundaries, which become weak regions in the single crystal structure. During application, the parts are prone to brittle fracture along the grain boundaries. Compared with the non-recrystallized single crystal creep parts, the life of the recrystallized single crystal creep parts is reduced from 260 hours to 40 hours.

[0005] In summary, it is necessary to develop a more efficient restoration heat treatment process to reduce the recrystallization risk of parts and improve the repair efficiency. Summary of the Invention

[0006] Aiming at the deficiencies in the prior art, the purpose of the present disclosure is to provide a method for restoring the properties of a nickel-based single crystal superalloy by heat treatment. This method uses a specific heating rate and heating range to heat-treat the creeped sample, and can restore the γ' phase of the sample after restoration heat treatment to the original state.

[0007] To achieve the above purpose, the present disclosure provides the following technical solutions:

[0008] A method for restoring the creep properties of a nickel-based single crystal superalloy, comprising the following steps:

[0009] S100: Obtain the creep strain of the nickel-based single crystal superalloy sample after creep. If the strain is less than the strain threshold, execute step S200; if the strain is greater than the strain threshold, execute step S300;

[0010] S200: Use the solution temperature as the termination heating temperature to perform restoration heat treatment on the creeped nickel-based single crystal superalloy sample;

[0011] S300: Perform restoration heat treatment on the creeped nickel-based single crystal superalloy sample according to the set restoration heat treatment parameters.

[0012] Preferably, in step S300, the restoration heat treatment parameters include: the starting heating temperature, the termination heating temperature, and the temperature range divided based on the starting heating temperature and the termination heating temperature.

[0013] Preferably, step S300 includes the following steps:

[0014] S301: Set the starting heating temperature and the ending heating temperature;

[0015] S302: Set different temperature ranges based on the starting heating temperature and the ending heating temperature;

[0016] S303: Set different heating rate increases within different temperature ranges to heat up the creeped nickel-based single crystal superalloy sample, so as to complete the recovery heat treatment.

[0017] Preferably, the starting heating temperature, the ending heating temperature, and the temperature ranges are set based on the recovery degree of the sample.

[0018] Preferably, the recovery degree of the sample is expressed as:

[0019]

[0020] Wherein, where S t is the total γ′ precipitation phase area, S r is the area of rafted γ′, and s is the sum of the area of the γ′ precipitation phase and the area of the γ phase.

[0021] Preferably, the temperature ranges include a low temperature region, a medium temperature region, and a high temperature region.

[0022] Preferably, the heating rate increase is set based on the release time of the residual stress in each temperature range.

[0023] Preferably, the method further includes the following steps:

[0024] S400: Perform standard heat treatment on the sample that has undergone recovery heat treatment in step S200 or step S300.

[0025] Preferably, in step S100, the strain threshold is 5%.

[0026] Compared with the prior art, the beneficial effects brought by the present disclosure are:

[0027] 1. The present disclosure uses specific heating rate increases and heating ranges to perform heat treatment on the creeped sample, which can not only restore the γ′ phase of the component after recovery heat treatment to its original state, effectively repair the high temperature creep degradation while avoiding recrystallization, and the properties of the repaired microstructure are close to the original level.

[0028] 2. The method of the present disclosure is simple, reliable, and low-cost, suitable for industrial production, and has broad application prospects in the refurbishment of third-generation and fourth-generation single crystal superalloy turbine guide vanes. Description of the Drawings

[0029] Figure 1 It is a flowchart of a heat treatment method after creep of a single crystal nickel-based superalloy provided by an embodiment of the present disclosure;

[0030] Figure 2 It is a scanning electron microscope image of an original creep strain sample provided by an embodiment of the present disclosure;

[0031] Figure 3 It is a scanning electron microscope image of a creep strain sample after recovery heat treatment provided by an embodiment of the present disclosure;

[0032] Figure 4 It is an electron backscatter diffraction pattern of a creep strain sample after recovery heat treatment provided by an embodiment of the present disclosure;

[0033] Figure 5 It is a scanning electron microscope image of an original creep strain sample provided by another embodiment of the present disclosure;

[0034] Figure 6 It is a scanning electron microscope image of a creep strain sample after recovery heat treatment provided by another embodiment of the present disclosure;

[0035] Figure 7 It is an electron backscatter diffraction pattern of a creep strain sample after recovery heat treatment provided by another embodiment of the present disclosure;

[0036] Figure 8 It is a scanning electron microscope image of the original microstructure state of a creep strain sample before creep provided by another embodiment of the present disclosure. Detailed implementation manners

[0037] The following will refer to the attached Figures 1 to 8 The specific embodiments of the present disclosure will be described in detail. Although the specific embodiments of the present disclosure are shown in the drawings, it should be understood that the present disclosure can be implemented in various forms and should not be limited by the embodiments set forth herein. On the contrary, these embodiments are provided so that the present disclosure can be more thoroughly understood and the scope of the present disclosure can be fully conveyed to those skilled in the art.

[0038] It should be noted that in the specification and claims, certain terms are used to refer to specific components. Those skilled in the art should understand that technicians may use different nouns to refer to the same component. The specification and claims do not distinguish components by the difference in nouns, but by the difference in the functions of the components. For example, the terms "comprising" or "including" mentioned throughout the specification and claims are open-ended terms and should be interpreted as "including but not limited to". The following description in the specification is for the purpose of describing the preferred embodiments of the present disclosure, but it is not intended to limit the scope of the present disclosure. The protection scope of the present disclosure shall be determined by the scope defined in the appended claims.

[0039] To facilitate the understanding of the embodiments of the present disclosure, the following will further explain with specific examples in conjunction with the drawings, and each drawing does not constitute a limitation to the embodiments of the present disclosure.

[0040] In one embodiment, as Figure 1 shown, the present disclosure provides a method for heat treatment to restore the creep performance of a nickel-based single crystal superalloy, including the following steps:

[0041] 1. Obtain a nickel-based single crystal superalloy sample that has undergone creep strain. Its microscopic morphology is as Figure 2 shown. After measurement, the porosity of the sample is obtained as 0.435%, which is greater than 0.3%. Then its creep strain is greater than the threshold strain of 5% (this threshold is obtained through experiments). At this time, it is necessary to perform heat treatment to restore the creeped sample by setting the heat treatment parameters for restoration.

[0042] 2. Set the heat treatment parameters for restoration according to the restoration degree ΔS of the sample. The heat treatment parameters for restoration specifically include the starting heating temperature, the ending heating temperature, and the temperature range;

[0043] In this step, the restoration degree ΔS is defined as: where S t is the total area of the γ′ precipitate phase (the subscript t represents the γ′ structure restored to a cubic shape, total), S rThe area of rafted γ′ is γ′r (the subscript r represents the γ′ structure that has undergone rafting), and s is the total area (i.e., the sum of the areas of the γ′ precipitates and the γ phase area). The sample is heat-treated at 1180 °C, and the residence time t1 (i.e., the release time required for the residual stress at this heat treatment temperature) is 110 min, and the recovery degree is ΔS1≈33%; the sample is heat-treated at 1240 °C, the residence time t2 is 75 min, and the recovery degree is ΔS2≈46%; the sample is heat-treated at 1260 °C, the residence time t3 is 50 min, and the recovery degree is ΔS3≈62%. According to the recovery degree ΔS at each stage, the starting heating temperature can be determined to be 1050 °C, and the ending heating temperature is 1260 °C.

[0044] Furthermore, the temperature range between the starting heating temperature and the ending heating temperature is divided into a low-temperature zone, a medium-temperature zone, and a high-temperature zone. Among them, the temperature range of the low-temperature zone is 1050 °C - 1180 °C, the temperature range of the medium-temperature zone is 1180 °C - 1240 °C, and the temperature range of the high-temperature zone is 1240 °C - 1260 °C.

[0045] It should be noted that the temperature ranges of the low-temperature zone, the medium-temperature zone, and the high-temperature zone are also set according to the recovery degree of the sample, that is, according to the area degree of γ′ restored to a cubic shape to determine the temperature range. Within this temperature range, the rafted structure of the sample will undergo derafting and recover to a certain cubic shape.

[0046] 3. After setting the temperature range, it is necessary to further set the heating rate within each temperature range. In this embodiment, the heating rate is set by the release time of the residual stress (residual stress refers to the self-equilibrated internal stress remaining in the object after the action of external forces or non-uniform temperature fields, etc. Non-uniform plastic deformation or phase transformation may cause residual stress. The existence of residual stress sometimes does not immediately manifest as a defect, but when the total stress exceeds the strength limit due to the superposition of the working stress and the residual stress during the operation of the part, cracks and fractures will occur. Most of the residual stress of the part can be eliminated by appropriate heat treatment.) in each temperature range. The heating rate is defined as: Specifically, within the low-temperature range, the heating rate is where ΔT1 is the temperature difference in the low-temperature zone, and t1 is the release time of the residual stress in the low-temperature range; within the medium-temperature range, the heating rate is where ΔT2 is the temperature difference in the medium-temperature zone, and t2 is the release time of the residual stress in the medium-temperature range; within the high-temperature range, the heating rate is Among them, ΔT3 is the temperature difference in the high-temperature zone, and t3 is the release time of the residual stress in the high-temperature range. According to the above formula, it can be calculated that when the temperature range in the low-temperature zone is 1050°C - 1180°C, t1 is 110 min, and the heating rate R1 is 1.2°C / min; when the temperature range in the medium-temperature zone is 1180°C - 1240°C, t2 is 75 min, and the heating rate R2 is 0.8°C / min; when the temperature range in the high-temperature zone is 1240°C - 1260°C, t3 is 50 min, and the heating rate R3 is 0.4°C / min.

[0047] Heat the creeped samples at the above different heating rates in the low-temperature range, medium-temperature range, and high-temperature range respectively, and the performance recovery of the samples can be completed.

[0048] Furthermore, it should be noted that the reason for setting different heating rates in each temperature range is as follows: Strain energy is released during the heat treatment process, and the positive and negative signs of dislocations annihilate. If too fast a heating rate is used, the residual stress cannot be fully released, and the dislocation reaction is incomplete, resulting in the appearance of recrystallization during the heat treatment process. Although too slow a heating rate can release the strain energy, it takes too long and has low efficiency. Therefore, according to experimental experience, heat treatment is carried out at different heating rates in different temperature ranges, which can not only fully release the strain energy, avoid recrystallization and restore the microstructure, but also take a short time and have high efficiency.

[0049] 4. After completing the above recovery heat treatment steps, the samples need to be subjected to standard heat treatment.

[0050] In this step, the specific process of the standard heat treatment is: keep the temperature at 1300°C for 30 min and then air-cool; after that, perform a primary aging treatment on the samples, and the specific process is: keep the temperature at 1120°C for 4 h and then air-cool; finally, perform a secondary aging treatment on the samples, and the specific process is: keep the temperature at 870°C for 20 h and then air-cool.

[0051] Use a scanning electron microscope to perform morphological characterization on the samples after the above recovery heat treatment steps, and the characterization results are as Figure 3 shown. The γ' phase transforms into γ' phase particles arranged in a more regular cubic shape, and the size of the γ' phase particles is between 300 and 500 nm, and the volume fraction of γ' is about 73%.

[0052] Furthermore, use electron backscatter diffraction to perform morphological characterization on the samples after the recovery heat treatment, and the characterization results are as Figure 4 shown. It can be found that the crystal orientation has not changed and no recrystallization phenomenon has occurred.

[0053] In another embodiment, the present disclosure also provides a method for heat treatment to restore the creep properties of a nickel-based single crystal superalloy, comprising the following steps:

[0054] 1. Obtain a nickel-based single crystal superalloy sample that has undergone creep strain. Its microscopic morphology is as Figure 5 shown. After measurement, the porosity of the sample is obtained as 0.031%, which is less than 0.3%. Then its creep strain is less than the threshold strain of 5%, and the solution heat treatment temperature can be directly selected for the restoration heat treatment;

[0055] 2. After completing the restoration heat treatment, the sample needs to be subjected to standard heat treatment. The specific process of the standard heat treatment is as follows: Insulate at a temperature of 1300 °C for 30 min and then air cool; thereafter, perform a primary aging treatment on the sample. The specific process is as follows: Insulate at a temperature of 1120 °C for 4 h and then air cool; finally, perform a secondary aging treatment on the sample. The specific process is as follows: Insulate at a temperature of 870 °C for 20 h and then air cool.

[0056] Using a scanning electron microscope, perform microscopic morphology characterization on the sample after the above heat treatment. The characterization results are as Figure 6 shown. The rafted and coarsened γ' phase transforms into cubic γ' phase particles with a relatively regular arrangement. The size of the square particles of the γ' phase is between 300 and 500 nm, and the volume fraction of γ' is about 69%.

[0057] Furthermore, use electron backscatter diffraction to perform microscopic structure morphology characterization on the sample after heat treatment. The characterization results are as Figure 7 shown. It can be found that the crystal orientation has not changed and no recrystallization occurs.

[0058] It should be noted that the reason for adopting different heat restoration treatment methods according to the relationship between the creep strain of the sample and the threshold strain in the above embodiments is as follows: Different strain amounts will introduce different amounts of deformation energy storage, and the deformation energy storage mainly exists in the creep-damaged components in the form of dislocations, etc. For components with a smaller strain amount, due to the smaller deformation energy storage, the restoration regime can directly select to raise the temperature to the solution temperature to dissolve γ', and through dislocation reactions to annihilate, thereby releasing the deformation energy storage and restoring the tissue. For components with a larger strain amount, if the temperature is directly raised to the solution temperature point to dissolve γ', the deformation energy storage will be released too quickly, and recrystallization will directly form at the stress concentration sites, which is extremely likely to cause recrystallization. Therefore, the restoration regime needs to lower the temperature and slowly release the residual stress energy to achieve the purpose of suppressing recrystallization. During the process of heating at a certain rate, the γ' phase slowly dissolves, the range where dislocations in the γ channels can move becomes larger, the dislocations can continuously react and annihilate, fully releasing the deformation energy storage and further restoring the damaged tissue.

[0059] The above has elaborated on the present disclosure by using specific embodiments, which is only for helping to understand the present disclosure and not for limiting the present disclosure. Any partial modification or replacement by those skilled in the art within the technical scope disclosed by the present disclosure shall be covered within the scope of the present disclosure.

Claims

1. A method for heat treatment to recover the creep properties of a nickel-based single crystal superalloy, comprising the following steps: S100: Obtain the creep strain of the nickel-based single crystal superalloy sample after creep. If the strain is less than the strain threshold, perform step S200; if the strain is greater than the strain threshold, perform step S300. The strain threshold is 5%; S200: Use the solution treatment temperature as the termination heating temperature to perform heat treatment for recovery on the creeped nickel-based single crystal superalloy sample; S3 S300: Perform heat treatment for recovery on the creeped nickel-based single crystal superalloy sample according to the set heat treatment parameters for recovery; The heat treatment parameters for recovery include: The starting heating temperature, the termination heating temperature, and the temperature ranges divided based on the starting heating temperature and the termination heating temperature; Step S300 includes the following steps: S301: Set the starting heating temperature and the termination heating temperature; S302: Set different temperature ranges based on the starting heating temperature and the termination heating temperature; S303: Set different heating rates within different temperature ranges to heat up the creeped nickel-based single crystal superalloy sample to complete the heat treatment for recovery; Among them, the heating rate is defined as: , specifically, in the low-temperature range, the heating rate is , where is the temperature difference in the low-temperature zone, is the release time of the residual stress in the low-temperature range; in the medium-temperature range, the heating rate is , where is the temperature difference in the medium-temperature zone, t2 is the release time of the residual stress in the medium-temperature range; in the high-temperature range, the heating rate is , where is the temperature difference in the high-temperature zone, t3 is the release time of the residual stress in the high-temperature range; The starting heating temperature, the termination heating temperature, and the temperature ranges are set based on the recovery degree of the sample; The recovery degree of the sample is expressed as: ， Among them, is the total area of γ' precipitation phase, is the area of rafted γ', is the sum of the area of γ' precipitation phase and the area of γ phase; The method further includes the following steps: S400: Perform standard heat treatment on the sample after heat treatment for recovery in step S200 or step S300, specifically including: Insulate at a temperature of 1300 °C for 30 min and air cool; Perform a primary aging treatment on the sample. The specific process is: Insulate at a temperature of 1120 °C for 4 h and air cool; Perform a secondary aging treatment on the sample. The specific process is: Insulate at a temperature of 870 °C for 20 h and air cool.

2. The method according to claim 1, wherein The temperature ranges include a low-temperature zone, a medium-temperature zone, and a high-temperature zone.

3. The method according to claim 1, wherein The heating rate is set based on the release time of the residual stress in each temperature range.

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