A recovery heat treatment method for repairing creep damage of K439B nickel-based superalloy

Abstract

The present application relates to the technical field of casting nickel-based superalloy, and particularly relates to a recovery heat treatment method for repairing creep damage of K439B nickel-based superalloy. 23 The recovery heat treatment method provided by the present application can repair the degraded structure of the nickel-based superalloy K439B used in a turbine casing, eliminate the C6 carbide and the eta phase, and make the structure of the alloy basically reach the level of the heat treatment state, so that the tensile property at room temperature and the high-temperature durability are close to or even exceed the level of the heat treatment state, thereby ensuring the service safety of an aero-engine, prolonging the service life of the engine casing, and reducing the maintenance cost of the aero-engine.

Description

Technical Field

[0001] This invention relates to the field of casting nickel-based superalloy technology, and in particular to a recovery heat treatment method for repairing creep damage in K439B nickel-based superalloy. Background Technology

[0002] Nickel-based superalloys possess excellent medium- and high-temperature strength, heat resistance, and corrosion resistance, making them a primary material for key hot-section components in advanced aero-engines and gas turbines. These components operate in complex environments under high temperature and pressure. The main strengthening mechanisms of these alloys include γ′ phase precipitation strengthening, grain boundary precipitation strengthening, second-phase strengthening, and solid solution strengthening. During service, the material undergoes a series of microstructural degradation processes, such as γ′ phase coarsening and rafting, carbide decomposition, and TCP phase precipitation, leading to a significant reduction in mechanical properties and severely impairing the service life of the castings, necessitating periodic component replacement. Recovery heat treatment can effectively adjust the size, quantity, and distribution of precipitates in serviceable components, restoring the alloy's microstructure and mechanical properties to a certain extent. This extends the component's service life, improves service safety, and enables it to be reused, while also reducing manufacturing costs, resulting in significant economic benefits.

[0003] K439B, a novel nickel-based superalloy, is intended for use in manufacturing complex thin-walled casing castings for fifth-generation turbofan engines operating at 800℃. These castings are prone to metallurgical defects such as porosity, chilling, and inclusions during manufacturing, necessitating welding. It has been reported that a high γ' phase content in the alloy is detrimental to weld repair. The γ' phase content and size are closely related to the Al and Ti contents. The K439B alloy has an Al+Ti content of 5.2 wt.% and a γ' phase content of 23%. The relatively low γ' phase content gives the alloy excellent weldability, but weakens its strengthening effect. Therefore, the grain boundary strengthening effect of the K439B alloy is also extremely important. When the M... 23 When C6 carbides are distributed in a granular manner, they can coordinate inter-grain deformation, giving the alloy higher strength. After long-term aging tests simulating service conditions, the K439B alloy exhibits coarsening of the γ′ phase size, degradation of MC carbides at grain boundaries to form a small amount of η phase, and simultaneous degradation of the M phase at grain boundaries. 23 The C6 carbide linkages are elongated and continue to grow, increasing the grain boundary width. This significantly reduces the alloy's room-temperature tensile properties and high-temperature creep rupture properties. Currently, no heat treatment methods for restoring the degraded microstructure of the K439B alloy have been reported.

[0004] Recovery heat treatment refers to the process of re-dissolving the strengthening phase of an alloy into the matrix and then re-precipitating it after heat treatment at different temperatures and times, so that the degraded structure is basically or completely restored to its initial morphology, thereby achieving the purpose of restoring mechanical properties. Chinese patent CN103643188B discloses a heat treatment method for K465 alloy, which has a very significant effect on restoring the performance of K465 turbine blades after a period of use. Chinese patent CN104878329B discloses a hot isostatic pressing method for repairing creep damage in DZ125 single-crystal alloy, which effectively restores the alloy's microstructure and mechanical properties after recovery treatment. Chinese patent CN110284087A focuses on the recovery treatment of creep damage in K403 alloy blades; after the recovery heat treatment process, both the microstructure and properties are restored to a new state. Chinese patent CN115584455A is a recovery heat treatment for single-crystal alloys, with the second solution treatment temperature being Tf,γ'±5℃, where Tf,γ' is the solution temperature at which the γ' phase is completely dissolved ±5℃. Unlike other alloys, the low content of the γ′ phase in K439B weakens its strengthening effect. To enhance grain boundary strengthening, the re-dissolution of the grain boundary precipitates η and M need to be considered. 23 The recovery of C6 carbides is particularly important at this stage, where the temperature and time settings during the recovery heat treatment process are crucial. Effective control of the temperature and time settings is necessary to manage the alloy's γ′ phase and grain boundary M. 23 C6 carbide precipitation and elimination of a small amount of η phase. Summary of the Invention

[0005] The purpose of this invention is to provide a practical and usable heat treatment method for restoring the degraded microstructure of nickel-based superalloy K439B used in turbine casings, thereby removing the coarsened γ' phase and grain boundary M 23 The elimination of C6 carbides and the η phase brings the alloy's microstructure to essentially the level of the heat-treated state. Its room-temperature tensile properties and high-temperature creep resistance approach or even exceed those of the heat-treated state, thereby ensuring the service safety of the aero-engine, extending the service life of the engine casing, and reducing aero-engine maintenance costs. The objective of this invention is achieved through the following process steps: A recovery heat treatment method for repairing creep damage in K439B nickel-based superalloy includes the following steps: S1. Solution treatment: Control the furnace temperature rise rate at 8-10°C per minute, raise the furnace temperature, stop heating and hold the temperature, evacuate the vacuum, and cool at the air cooling rate. S2. First-level aging treatment: Based on the S1 treatment, control the furnace temperature rise rate at 8-12℃ per minute, increase the furnace temperature, stop heating and hold at that temperature, evacuate, and cool at the air cooling rate. S3. Secondary aging treatment: Based on the S2 treatment, control the furnace temperature rise rate at 8-10℃ per minute, raise the furnace temperature, stop heating and hold the temperature, evacuate the vacuum, and control the cooling rate to air cooling rate. S4. Three-stage aging treatment: Based on the S3 treatment, the furnace temperature rise rate is controlled at 6~8℃ per minute. The furnace temperature is increased, heating is stopped and held at that temperature, vacuum is applied, and the cooling rate is the air cooling rate to obtain the repaired K439B nickel-based alloy.

[0006] The solution temperature defined in this invention is the complete solution temperature of the γ' phase in K439B alloy +80~110℃ (the purpose is to effectively dissolve the η phase and other harmful phases, while promoting the precipitation of more granular M phases at grain boundaries). 23 (C6 carbides), the complete solution temperature of the γ' phase varies among different alloys. After long-term aging for 5000 hours, the γ' phase in the damaged microstructure becomes severely coarsened. The selection of the solution time depends on the time required for the coarse γ' phase and the complete dissolution of the η phase within the alloy microstructure at that temperature. If the solution temperature is inappropriate, elongated γ' and η phases will appear at the grain boundaries.

[0007] The first-stage aging temperature and time also have a significant impact on the microstructure of the alloy. When the first-stage aging holding time is changed to 5 hours, the γ' secondary precipitation phase transformation in the alloy microstructure becomes coarser, and the grain boundaries become coarser and irregular. At this time, the room temperature tensile strength of the alloy is 1140 MPa, and the creep rupture performance is 61 h. When the first-stage aging temperature is changed to 1060℃, the room temperature tensile strength of the alloy is 1100 MPa, and the creep rupture performance is 69 h.

[0008] Existing technologies often employ solution treatment and multiple aging processes to repair alloys. However, due to the different alloys being repaired, their technical solutions differ from those of this invention. For example, in CN103643188B, step three involves heating the furnace to 1230℃ and holding for 4-4.5 hours, followed by furnace cooling. This not only fails to have a beneficial effect on the K439B alloy of this invention but also leads to coarser γ' secondary precipitation phase transformation, resulting in a decrease in the alloy's mechanical properties. Similarly, in CN110284087A, when the furnace temperature reaches 1220-1240℃, this solution treatment temperature indicates that the K439B alloy has already undergone initial melting, resulting in irreversible damage and the inability to recover its microstructure and mechanical properties. Furthermore, CN1048... The 78329B heat treatment is a recovery heat treatment for single-crystal alloys, and the solution treatment temperature used is 1235℃~1255℃. This process also causes initial melting for K439B alloys, resulting in irreversible damage to the alloy structure, and the microstructure and mechanical properties cannot be restored. For example, the CN115584455A heat treatment is a recovery heat treatment for single-crystal alloys, and the second solution treatment temperature is Tf,γ'±5℃, where Tf,γ' is the complete solution temperature of the γ' phase ±5℃. For K439B alloys, if the complete solution temperature ±5℃ is used, this temperature cannot completely eliminate the η phase inside the alloy structure, and granular carbides cannot precipitate at the grain boundaries. Therefore, the alloy structure and mechanical properties cannot be completely restored.

[0009] Therefore, existing heat treatment processes cannot effectively restore the microstructure and properties of damaged K439B.

[0010] In some embodiments, the furnace temperature in step S1 is raised to 1170°C to 1200°C, heating is stopped, and the temperature is maintained for 2 to 4 hours.

[0011] In some embodiments, the furnace temperature in step S2 is raised to 1060°C to 1090°C, heating is stopped, and the temperature is maintained for 3 to 5 hours.

[0012] In some embodiments, the furnace temperature in step S3 is raised to 830°C to 850°C, heating is stopped, and the temperature is maintained for 20 to 24 hours.

[0013] In some embodiments, the furnace temperature in step S4 is raised to 800°C, heating is stopped, and the temperature is maintained for 15 to 17 hours.

[0014] The applicant discovered in numerous experiments that when the furnace temperature in S1 is increased to 1180℃, elongated carbides precipitate at the grain boundaries, and η-phase precipitates at the grain boundaries, leading to a decrease in the alloy's mechanical properties. When the first-stage aging time in S2 is 5 hours, the secondary γ' phase in the alloy microstructure is large in size and irregular in shape, further resulting in a significant reduction in the alloy's room-temperature tensile properties and creep rupture life at 815℃ / 379MPa. When the second-stage aging temperature in S3 is 800℃, the size of the secondary γ' phase in the alloy microstructure is much smaller than the critical size, the volume fraction of the γ' phase is small, precipitation strengthening is weakened, and further... The crease life of the alloy at 815℃ / 379MPa was significantly reduced. When the secondary aging temperature was 870℃, after the recovery treatment, the secondary γ' phase in the alloy microstructure was large in size, extremely irregular in shape, and the grain boundaries were severely coarsened, resulting in a significant reduction in the room temperature tensile properties and the crease life at 815℃ / 379MPa. When the tertiary aging time in S4 was 4h, the morphology of the secondary γ' phase was not effectively optimized due to the short tertiary aging time. Furthermore, the secondary γ' phase was small in size and had a small volume fraction, which led to a reduction in the crease life of the alloy at 815℃ / 379MPa.

[0015] In some embodiments, the total creep rupture life of the repaired K439B nickel-based alloy is increased by 30-40% compared to the damaged test bar.

[0016] In some embodiments, the microstructure of the repaired K439B nickel-based alloy exhibits spherical dendrite trunks and interdendritic regions with γ′ phase morphology, an average γ′ phase size of no more than 60 nm, and granular M precipitates at grain boundaries. 23 In C6 carbides, the η phase at the grain boundaries completely disappears.

[0017] In some embodiments, the solution treatment, primary aging treatment, secondary aging treatment, and tertiary aging treatment are all carried out in a vacuum quenching furnace.

[0018] In some embodiments, the temperature uniformity of the vacuum quenching furnace is not lower than the requirements for Class III furnaces specified in HB5354.

[0019] In some embodiments, the initial furnace temperature of the vacuum quenching furnace is no greater than 150°C, and the furnace is evacuated to a vacuum level of 10°C. - 2 Below Pa.

[0020] Compared with the prior art, the present invention has the following beneficial effects: This invention provides a practical and usable recovery heat treatment method for repairing degraded microstructure of nickel-based superalloy K439B used in turbine casings, which removes the γ' phase and grain boundary M 23The elimination of C6 carbides and η phase allows the alloy's microstructure to essentially reach the level of the heat-treated state. The room temperature tensile properties and high temperature creep properties are close to or even exceed the level of the heat-treated state, thereby ensuring the service safety of the aero-engine, extending the service life of the engine casing, and reducing the maintenance cost of the aero-engine. Attached Figure Description

[0021] Figure 1 The images show the microstructure of the K439B alloy in the heat-treated state in Example 1. The left image shows the dendritic trunk γ' phase, and the right image shows the interdendritic γ' phase.

[0022] Figure 2 The image shows the microstructure of the K439B alloy after long-term aging at 800℃ for 5000 hours in Example 1. Figure 2 The left image (A) shows the dendritic trunk γ' phase. Figure 2 Figure A (right) shows the interdendritic γ' phase. Figure 2 B represents the grain boundary morphology.

[0023] Figure 3 The image shows the microstructure of the K439B alloy after solution treatment in Example 1. Figure 3 The left image (A) shows the dendritic trunk γ' phase. Figure 3 Figure A (right) shows the interdendritic γ' phase. Figure 3 B represents the grain boundary morphology.

[0024] Figure 4 The image shows the microstructure of the K439B alloy after repair in Example 1. Figure 4 The left image shows the dendritic trunk γ' phase. Figure 4 The right figure shows the γ' phase between dendrites.

[0025] Figure 5 The image shows the microstructure of the K439B alloy after solution treatment in Example 2. Figure 5 The left image (A) shows the dendritic trunk γ' phase. Figure 5 Figure A (right) shows the interdendritic γ' phase. Figure 5 B represents the grain boundary morphology.

[0026] Figure 6 The image shows the microstructure of the K439B alloy after repair in Example 2. Figure 6 The left image shows the dendritic trunk γ' phase. Figure 6 The right figure shows the γ' phase between dendrites.

[0027] Figure 7 The image shows a scanning electron microscope (SEM) image of the K439B alloy repaired in Comparative Example 1. Figure 7 The left image (A) shows the dendritic trunk γ' phase. Figure 7 Figure A (right) shows the interdendritic γ' phase. Figure 7B represents the grain boundary morphology after repair, with the left image showing the grain boundary morphology and the grain boundary precipitating elongated carbides, and the right image showing the precipitated strip-shaped η phase.

[0028] Figure 8 This is a light microscope image of the K439B alloy in Comparative Example 2 when the solution temperature is 1210℃.

[0029] Figure 9 The image shows a scanning electron microscope (SEM) image of the repaired K439B alloy in Comparative Example 3, where the left image (9) shows the γ' phase of the alloy dendrite trunk. Figure 9 The right figure shows the γ' phase between alloy dendrites.

[0030] Figure 10 The image shows a scanning electron microscope (SEM) image of the K439B alloy repaired in Comparative Example 4. Figure 10 The left image shows the dendritic trunk γ' phase, and the right image shows the interdendritic γ' phase. Figure 10 Figure B shows the grain boundary morphology. Detailed Implementation

[0031] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0032] Example 1

[0033] K439B alloy test bars were heat-treated using a DC-B30 / 16 box-type heat treatment furnace. The alloy was heated to above the solution temperature and then subjected to two-stage aging to obtain heat-treated test bars. Cylindrical specimens of Φ15 mm × 5 mm were cut from the heat-treated test bars and then ground and polished (using a UNIPOL-830 metallographic specimen pre-grinding machine and polishing machine). The specimens were then etched with an HNO3:HF:glycerol ratio of 1:2:1 for 15–120 s to prepare metallographic specimens for observing dendritic morphology and alloy phases. The microstructure after heat treatment was then observed, and the size and volume fraction of the γ′ phase and the grain boundary width were measured. Following this, the K439B alloy test bar underwent long-term aging at 800℃ for 5000h using a DC-B30 / 16 box-type heat treatment furnace. Thermocouples were used to monitor the sample temperature in real time during the experiment. Cylindrical samples of Φ15 mm × 5 mm were cut from the long-term aged test bar and subjected to grinding and polishing (using a UNIPOL-830 metallographic sample pre-grinding machine and polishing machine). The samples were then etched with an HNO3:HF:glycerol solution in a 1:2:1 ratio for 15–120 s to prepare metallographic samples for observing dendrite morphology and alloy phases. The microstructure after heat treatment was then observed, and the size and volume fraction of the γ′ phase and grain boundary width were measured. After obtaining the samples aged at 800℃ for 5000h, they were placed in a vacuum quenching furnace for recovery heat treatment.

[0034] The K439B alloy is composed of elements including Cr, Co, Al, Ti, Nb, W, Ta, B, Zr, C, and Ni. The main strengthening phases are the γ′ phase and carbides. The γ′ phase is primarily spherical, small in size and regular in shape. The average size of the γ′ phase in the dendrite trunk of the heat-treated alloy is 47 nm, and the average size of the γ′ phase between dendrites is 50 nm. Granular M atoms are uniformly precipitated at the grain boundaries. 23 The T6 alloy, after heat treatment, has a room temperature tensile strength of 1175.0 MPa and a creep rupture life of 116.83 h.

[0035] A recovery heat treatment method for repairing creep damage in K439B nickel-based superalloy includes the following steps: S1. Solution treatment: Control the furnace temperature rise rate at 10℃ per minute, raise the furnace temperature to 1205℃, stop heating and hold for 4 hours, then evacuate to 10℃. -2 Below Pa, the cooling rate is the air cooling rate; S2. First-level aging treatment: Based on the S1 treatment, control the furnace temperature rise rate at 10℃ per minute, raise the furnace temperature to 1086℃, stop heating and hold for 4 hours, then evacuate to 10℃. -2Below Pa, the cooling rate is the same as the air cooling rate: S3. Secondary Aging Treatment: Based on the S2 treatment, control the furnace temperature rise rate at 8℃ per minute, raise the furnace temperature to 850℃, stop heating and hold for 20 hours, then evacuate to 10℃. -2 For speeds below Pa, the cooling rate is controlled to be the air cooling rate. S4. Level 3 aging treatment: Based on the S3 treatment, control the furnace temperature rise rate at 8℃ per minute, raise the furnace temperature to 805℃, stop heating and hold for 17 hours, then evacuate to 10℃. -2 With a pressure below Pa and a cooling rate equal to that of air cooling, the repaired K439B nickel-based alloy was obtained.

[0036] The solution treatment, primary aging treatment, secondary aging treatment, and tertiary aging treatment are all carried out in a vacuum quenching furnace. The temperature uniformity of the vacuum quenching furnace is not lower than the relevant requirements for Class III furnaces specified in HB5354. The initial furnace temperature of the vacuum quenching furnace is not greater than 150°C, and the furnace is evacuated to 10°C. -2 Below Pa.

[0037] Example 2

[0038] K439B alloy test bars were heat-treated using a DC-B30 / 16 box-type heat treatment furnace. The alloy was heated to above the solution temperature and then subjected to two-stage aging to obtain heat-treated test bars. Cylindrical specimens of Φ15 mm × 5 mm were cut from the heat-treated test bars and then ground and polished (using a UNIPOL-830 metallographic specimen pre-grinding machine and polishing machine). The specimens were then etched with an HNO3:HF:glycerol ratio of 1:2:1 for 15–120 s to prepare metallographic specimens for observing dendritic morphology and alloy phases. The microstructure after heat treatment was then observed, and the size and volume fraction of the γ′ phase and the grain boundary width were measured. Following this, the K439B alloy test bar underwent long-term aging at 800℃ for 5000h using a DC-B30 / 16 box-type heat treatment furnace. Thermocouples were used to monitor the sample temperature in real time during the experiment. Cylindrical samples of Φ15 mm × 5 mm were cut from the long-term aged test bar and subjected to grinding and polishing (using a UNIPOL-830 metallographic sample pre-grinding machine and polishing machine). The samples were then etched with an HNO3:HF:glycerol solution in a 1:2:1 ratio for 15–120 s to prepare metallographic samples for observing dendrite morphology and alloy phases. The microstructure after heat treatment was then observed, and the size and volume fraction of the γ′ phase and grain boundary width were measured. After obtaining the samples aged at 800℃ for 5000h, they were placed in a vacuum quenching furnace for recovery heat treatment.

[0039] A recovery heat treatment method for repairing creep damage in K439B nickel-based superalloy includes the following steps: S1. Solution treatment: Control the furnace temperature rise rate at 10℃ per minute, raise the furnace temperature to 1190℃, stop heating and hold for 4 hours, then evacuate to 10℃. -2 Below Pa, the cooling rate is the air cooling rate; S2. First-level aging treatment: Based on the S1 treatment, control the furnace temperature rise rate at 10℃ per minute, raise the furnace temperature to 1075℃, stop heating and hold for 4 hours, then evacuate to 10℃. -2 Below Pa, the cooling rate is the same as the air cooling rate: S3. Secondary Aging Treatment: Based on the S2 treatment, control the furnace temperature rise rate at 8℃ per minute, raise the furnace temperature to 840℃, stop heating and hold for 20 hours, then evacuate to 10℃. -2 For speeds below Pa, the cooling rate is controlled to be the air cooling rate. S4. Level 3 aging treatment: Based on the S3 treatment, control the furnace temperature rise rate at 8℃ per minute, raise the furnace temperature to 795℃, stop heating and hold for 17 hours, then evacuate to 10℃. -2 With a pressure below Pa and a cooling rate equal to that of air cooling, the repaired K439B nickel-based alloy was obtained.

[0040] The solution treatment, primary aging treatment, secondary aging treatment, and tertiary aging treatment are all carried out in a vacuum quenching furnace. The temperature uniformity of the vacuum quenching furnace is not lower than the relevant requirements for Class III furnaces specified in HB5354. The initial furnace temperature of the vacuum quenching furnace is not greater than 150°C, and the furnace is evacuated to 10°C. - 2 Below Pa.

[0041] Comparative Example 1 The alloy was restored according to the method of Example 1, except that the solution temperature was set to 1180°C.

[0042] Comparative Example 2 The alloy was restored according to the method of Example 1, except that the solution temperature was set to 1210°C.

[0043] Comparative Example 3 The alloy was restored according to the method of Example 1, except that the holding time in S2 was 3 hours.

[0044] Comparative Example 4 The alloy was restored according to the method of Example 1, except that the holding time in S2 was 5 hours.

[0045] Performance testing Microscopic observation was performed on the K439B alloys repaired in the above examples and comparative examples, and the results are as follows: Figure 1 The images show the dendritic trunk and interdendritic structure of the K439B alloy after heat treatment in Example 1. It can be seen that the alloy structure precipitates spherical γ' phases, which are small in size and regular in shape. The average size of the γ' phase in the dendritic trunk of the heat-treated alloy is 47 nm, and the average size of the γ' phase in the interdendritic structure is 50 nm.

[0046] Figure 2 The microstructure of the K439B alloy after long-term aging at 800℃ for 5000h in Example 1 shows that after long-term aging, the γ′ phase of the K439B alloy coarsens, and the coarsening between dendrites is more obvious than that between dendrite trunks. The spherical γ′ phase evolves into a cubic phase. The average size of the γ′ phase in the dendrite trunk is 133.9nm, and the average size of the γ′ phase between dendrites is 137.9nm. Elongated carbides precipitate at the grain boundaries, and the MC carbides at the grain boundaries degenerate to form a small amount of η phase.

[0047] Figure 3 The image shows the microstructure of the K439B alloy in Example 1 after repair when the solution temperature was 1205℃. It can be seen that after the recovery heat treatment, the coarse, cubic γ′ phase completely disappeared, and smaller spherical γ′ phases re-precipitated. At the solution temperature of 1205℃, the sizes of the γ′ phases in the dendrite trunk and between the dendrites were 54.7 nm and 58.5 nm, respectively, with a volume fraction of 23%. The size, morphology, and distribution of the γ′ phase were basically consistent with the heat-treated state. Granular M-phase redeposited at the grain boundaries... 23 C6 carbides, the η phase at the grain boundaries completely disappears, and the alloy structure is basically restored.

[0048] Figure 4 In Example 1, when the first-stage aging temperature of S2 is 1086℃, it can be seen that after the recovery heat treatment, the dendrite trunk and the γ′ phase between dendrites have sizes of 54.6 nm and 58.7 nm, respectively, with a volume fraction of 23.3%. The grain boundary morphology is basically the same as that of the heat-treated state.

[0049] In step S3 of Example 1, when the secondary aging temperature is 850℃, it can be seen that after the recovery heat treatment, the dendrite trunk and the γ′ phase between dendrites have sizes of 54.9 nm and 58.5 nm, respectively, with a volume fraction of 23.3%. The grain boundary morphology is basically the same as that of the heat-treated state.

[0050] In step S4 of Example 1, when the third-stage aging temperature is 805℃, it can be seen that after the recovery heat treatment, the dendrite trunk and the γ′ phase between dendrites have sizes of 54.3 nm and 58.9 nm, respectively, with a volume fraction of 23%. The grain boundary morphology is basically the same as that of the heat-treated state.

[0051] Figure 5 The image shows the microstructure of the dendrite trunk and interdendritic space of K439B alloy after repair at a solution treatment temperature of 1190℃. At this temperature, the γ′ phase sizes in the dendrite trunk and interdendritic space are 54.0 nm and 58.0 nm, respectively, with a volume fraction of 22.8%. The grain boundary morphology is basically consistent with that in the heat-treated state.

[0052] Figure 6 The image shows the microstructure of the dendrite trunk and interdendritic space of K439B alloy after repair at a first-stage aging temperature of 1075℃. The γ′ phase sizes of the dendrite trunk and interdendritic space are 54.1 nm and 58.0 nm, respectively, with a volume fraction of 23%. The grain boundary morphology is basically consistent with that of the heat-treated state.

[0053] In Example 2, the microstructure of the K439B alloy after repair at a solution treatment temperature of 1190℃, including the dendrite trunk and interdendritic structures, showed that the γ′ phase sizes of the dendrite trunk and interdendritic structures were 54.0 nm and 58.0 nm, respectively, with a volume fraction of 22.8%. The grain boundary morphology was basically consistent with that of the heat-treated state.

[0054] In Example 2, the microstructure of the K439B alloy after repair at a first-stage aging temperature of 1075℃, including the dendritic trunk and interdendritic space, showed γ′ phase sizes of 54.1 nm and 58.0 nm, respectively, with a volume fraction of 23%. The grain boundary morphology was essentially the same as that of the heat-treated state.

[0055] In step S3 of Example 2, when the secondary aging temperature is 840℃, it can be seen that after the recovery heat treatment, the dendrite trunk and the γ′ phase between dendrites have sizes of 53.5 nm and 58.0 nm, respectively, with a volume fraction of 22.8%. The grain boundary morphology is basically the same as that of the heat-treated state.

[0056] In step S4 of Example 2, when the third-stage aging temperature is 795°C, it can be seen that after the recovery heat treatment, the dendrite trunk and the γ′ phase between dendrites have sizes of 53.3 nm and 58.9 nm, respectively, with a volume fraction of 22.9%. The grain boundary morphology is basically the same as that of the heat-treated state.

[0057] Figure 7 The image shows the microstructure of the alloy restored in Comparative Example 1. After restoration, the dendritic trunk and the γ′ phase between dendrites have sizes of 53.3 nm and 56.5 nm, respectively, with a volume fraction of 22.3%. It can be seen that the η phase in the alloy was not completely eliminated, and elongated carbides appeared at the alloy grain boundaries, resulting in a significant decrease in the alloy's creep life at 815℃ / 379MPa.

[0058] Figure 8The microstructure of the alloy after the recovery treatment in Comparative Example 2 is shown. After the recovery treatment, the alloy underwent initial melting, which will lead to a significant reduction in the room temperature mechanical properties and creep life at 815℃ / 379MPa.

[0059] Figure 9 To compare the microstructure of the alloy after restoration treatment in Example 4, the sizes of the γ′ phase in the dendrite trunk and between the dendrites after restoration treatment were 43.0 nm and 46.0 nm, respectively, with a volume fraction of 19.4%. It was found that the size of the secondary γ′ phase in the alloy microstructure was much smaller than the critical size, the volume fraction of the γ′ phase was small, the precipitation strengthening was weakened, and the creep life of the alloy at 815℃ / 379MPa was reduced.

[0060] Figure 10 The image shows the microstructure of the alloy restored in Comparative Example 3. After restoration, the dendrite trunk and interdendritic γ′ phase sizes are 120.0 nm and 125.1 nm, respectively, with a volume fraction of 24%. It was found that the secondary γ′ phase in the alloy microstructure is large in size, irregular in shape, and has a wide grain boundary, large grain boundary carbides, and increased grain boundary irregularity. These combined effects severely reduce the alloy's room temperature mechanical properties and creep rupture life at 815℃ / 379 MPa.

[0061] The K439B alloys repaired in the above embodiments and comparative examples were subjected to room temperature tensile tests and 815℃ / 379MPa creep rupture tests. Room temperature tensile properties were tested according to GB / T 228-2002 (Metallic Materials, Tensile Testing at Room Temperature), and creep rupture properties were tested according to GB / T 2039-1997 (Metallic Materials, Tensile Creep and Indestructibility Testing). The testing equipment used was a 3382 electronic universal testing machine and a SATEC M3 high-temperature creep-indestruction testing machine, respectively. The results are shown in Tables 1-3.

[0062] Table 1 shows the room temperature tensile properties and creep rupture properties at 815℃ / 379MPa of the alloy in its heat-treated state. Table 2 shows the room temperature tensile properties and high-temperature creep rupture properties tested using long-term aging (800℃ / 5000h) to simulate the alloy's service condition. Table 3 shows the room temperature tensile properties and creep rupture properties at 815℃ / 379MPa of the alloy after treatment with the recovery heat treatment method proposed in this invention. Table 2 shows that after long-term aging (800℃ / 5000h), the creep rupture life at 815℃ / 379MPa of the alloy decreased to 90.6h. After long-term aging, the creep rupture life at 815℃ / 379MPa of the alloy after treatment with the recovery heat treatment method proposed in this invention reached 122.6h, an improvement of 35% compared to the long-term aged state, and the room temperature tensile properties improved by 45%. Therefore, it can be seen that the microstructure, room temperature tensile properties, and creep rupture properties at 815℃ / 379MPa of the alloy after treatment with the recovery heat treatment method proposed in this invention are basically restored to the level of the heat-treated state.

[0063] Table 1. Room temperature tensile properties and creep rupture properties at 815℃ / 379MPa of K439B alloy in heat-treated state.

[0064] Table 2. Room temperature tensile properties and creep rupture properties at 815℃ / 379MPa of K439B alloy after long-term aging at 800℃ for 5000h.

[0065] Table 3. Room temperature tensile properties and creep rupture properties at 815℃ / 379MPa of K439B alloy after recovery heat treatment.

[0066] As can be seen from the above experimental data, the solution treatment and multi-stage aging treatment at specific temperatures and times used in this invention can significantly improve the creep life of the alloy, and the experimental results of Example 1 are the best.

[0067] The above description represents the preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A recovery heat treatment method for repairing creep damage in K439B nickel-based superalloy, characterized in that, Includes the following steps: S1. Solution treatment: Place the K439B nickel-based superalloy to be repaired into the furnace, control the furnace temperature rise rate at 8-10℃ per minute, raise the furnace temperature, stop heating and hold the temperature, evacuate the vacuum, and cool at the air cooling rate. S2. First-level aging treatment: Based on the S1 treatment, the furnace temperature is increased, heating is stopped and the temperature is maintained, vacuum is drawn, and the cooling rate is the air cooling rate. S3. Secondary aging treatment: Based on the S2 treatment, the furnace temperature is increased, heating is stopped and the temperature is maintained, vacuum is drawn, and the cooling rate is controlled to be the air cooling rate. S4. Third-level aging treatment: Based on the S3 treatment, the furnace temperature is increased, heating is stopped and held at that temperature, vacuum is applied, and the cooling rate is the same as air cooling rate, resulting in the repaired K439B nickel-based alloy. in: In step S1, the furnace temperature is raised to 1205°C, heating is stopped, and the temperature is maintained for 2-4 hours. In step S2, the furnace temperature is raised to 1060℃~1090℃, heating is stopped, and the temperature is maintained for 4 hours. In step S3, the furnace temperature is raised to 830℃~850℃, heating is stopped, and the temperature is maintained for 20~24 hours. In step S4, the furnace temperature is raised to 780-820℃, heating is stopped, and the temperature is maintained for 15-17 hours.

2. The method for restoring heat treatment according to claim 1, characterized in that, The furnace temperature rise rate for the first-stage aging treatment is 8–12°C per minute.

3. The method for restoring heat treatment according to claim 1, characterized in that, The furnace temperature rise rate during the secondary aging treatment is 8–10 °C per minute.

4. The method for restoring heat treatment according to claim 1, characterized in that, The furnace temperature rise rate for the three-stage aging treatment is 6-8°C per minute.

5. The method for restoring heat treatment according to claim 1, characterized in that, The solution treatment, first-level aging treatment, second-level aging treatment, and third-level aging treatment are all carried out in a vacuum quenching furnace.

6. The method for restoring heat treatment according to claim 5, characterized in that, The initial furnace temperature of the vacuum quenching furnace shall not exceed 150°C, and the furnace shall be evacuated to a vacuum level of 10°C. -2 Below Pa.

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