A method of recovery heat treatment of a superalloy

Abstract

The application provides a recovery heat treatment method of a high-temperature alloy, and belongs to the field of alloy heat treatment. The recovery heat treatment method of the high-temperature alloy provided by the application comprises the following steps: segmentally heating a high-temperature alloy to be treated to a recovery temperature, and then segmentally cooling to obtain a recovered high-temperature alloy; the segmental heating comprises sequentially performed first heating, first holding and second heating; the heating rate of the first heating is 8-10 DEG C / min; the heating rate of the second heating is 6-8 DEG C / min; the segmental cooling comprises sequentially performed first cooling and second cooling; the cooling rate of the first cooling is 250-300 DEG C / min; and the cooling mode of the second cooling is air cooling. The high-temperature alloy treated by the method provided by the application has a structure close to the original structure of a non-service blade, and the cracking phenomenon of the workpiece caused by structure aging during the welding repair process can be effectively avoided.

Description

Technical Field

[0001] This invention relates to the field of alloy heat treatment technology, and in particular to a method for restoring the heat treatment of high-temperature alloys. Background Technology

[0002] K412 is a nickel-based precipitation-hardening equiaxed crystal cast superalloy developed based on ЖC3 alloy, commonly used to manufacture guide vanes for aero-engines. Superalloy blades typically operate under complex high-temperature and high-pressure conditions, enduring the erosion and corrosion of high-temperature exhaust gases and subjected to complex stress loads during service, making for harsh operating environments. With increasing service time, blades inevitably experience structural damage and performance degradation, even surface cracking, leading to blade failure. Due to the complex composition of blade materials and the cumbersome manufacturing process, replacing new blades is costly, often requiring refurbishment to restore performance. However, during refurbishment, it was found that after three or more argon arc welding (HGH3113) and brazing (HBNi82CrSiB) repairs to surface cracks, fluorescent staining revealed new surface crack defects.

[0003] Currently, for the problem of weld repair cracks caused by microstructure degradation in alloys during high-temperature service, hot isostatic pressing (HIP) or restorative heat treatment techniques are mostly used to control the microstructure of the alloy, restoring it to its initial morphology as much as possible or completely, thereby restoring its weldability. Existing HIP technology involves placing the workpiece in a high-temperature, high-pressure sealed container, using high-pressure argon gas as the medium, and applying isotropic static pressure to the workpiece to eliminate creep pores and reduce the dispersion of workpiece properties. However, if the pressure applied to the alloy during HIP treatment is too high, it may cause micro-cold plastic deformation under high pressure at a lower temperature, leading to recrystallization during subsequent heating, affecting the alloy's microstructure. Furthermore, HIP technology also suffers from the drawbacks of requiring expensive equipment and being more effective at repairing porosity and voids in alloys, with limited effectiveness in restoring degraded alloy microstructures.

[0004] Among existing heat treatment recovery techniques, Chinese patent "103643188A" (published on March 19, 2014) discloses a heat treatment method for repairing K465 alloy turbine blades. This invention effectively improves the service life of K465 alloy turbine blades by restoring the intragranular γ′ morphology after one service life. Chinese patent "110284087A" (published on September 27, 2019) discloses a heat treatment method for repairing creep damage in K403 nickel-based superalloy blades. This invention has a good repair effect on creep damage in K403 alloys with degraded microstructure. However, there are currently no reports, either domestically or internationally, on the restoration of the microstructure and weldability of K412 alloys or ЖC3 alloys that have suffered creep damage during service.

[0005] Therefore, providing a recovery heat treatment method to restore the microstructure and weldability of creep-damaged K412 or ЖC3 alloys has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0006] The purpose of this invention is to provide a method for restoring the heat treatment properties of high-temperature alloys. This method can restore the microstructure and weldability of creep-damaged K412 or ЖC3 alloys, thereby effectively extending the service life of alloy components.

[0007] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0008] This invention provides a method for the recovery heat treatment of a high-temperature alloy, comprising: heating the high-temperature alloy to be treated in stages to the recovery temperature, and then cooling it in stages to obtain the recovered high-temperature alloy;

[0009] The segmented heating includes a first heating, a first holding, and a second heating performed sequentially.

[0010] The heating rate of the first heating is 8-10℃ / min;

[0011] The second heating rate is 6–8 °C / min;

[0012] The segmented cooling includes a first cooling and a second cooling performed sequentially.

[0013] The cooling rate of the first cooling step is 250–300 °C / min;

[0014] The second cooling method is air cooling.

[0015] Preferably, the recovery temperature is 1130–1150°C, and the holding time at the recovery temperature is 420–450 min.

[0016] Preferably, the temperature of the first heat preservation is 800-820℃, and the time of the first heat preservation is 30-40 minutes.

[0017] Preferably, the endpoint temperature of the first cooling is 290–310°C.

[0018] Preferably, the high-temperature alloy to be treated is pretreated before the recovery heat treatment, and the pretreatment includes: sequentially subjecting the high-temperature alloy to be treated to corrosion treatment and ultrasonic cleaning.

[0019] Preferably, the temperature of the corrosion treatment is 50-60°C, and the time of the corrosion treatment is 40-50 minutes.

[0020] Preferably, the corrosive agent used in the corrosion treatment is a mixed solution of hydrochloric acid, citric acid and water.

[0021] Preferably, the recovery heat treatment is carried out under vacuum and inert gas protection conditions.

[0022] Preferably, the specific operation performed under the vacuum and inert gas protection conditions involves first evacuating the vacuum to 10... -2 Below Pa, then fill with inert gas to 2 × 10⁻⁶ Pa. 4 Pa.

[0023] Preferably, the high-temperature alloy to be treated is a creep-damaged K412 or ЖC3 alloy.

[0024] This invention provides a method for restoring the heat treatment of a high-temperature alloy, comprising: segmentally heating the high-temperature alloy to a recovery temperature, and then segmentally cooling it to obtain a restored high-temperature alloy; the segmented heating includes a first heating, a first holding, and a second heating in sequence; the heating rate of the first heating is 8-10℃ / min; the heating rate of the second heating is 6-8℃ / min; the segmented cooling includes a first cooling and a second cooling in sequence; the cooling rate of the first cooling is 250-300℃ / min; and the cooling method of the second cooling is air cooling. This invention employs a segmented heating method. By controlling the heating rate of each segment, stable heating conditions can be obtained, avoiding uneven element diffusion in the alloy caused by excessively rapid heating and reducing component segregation. Similarly, a segmented cooling method, by controlling the cooling rate and method, prevents the growth of the γ′ phase during cooling. Through the coordinated use of heating and cooling process parameters, the degraded γ′ phase and carbides in the high-temperature alloy to be treated dissolve, and fine-sized, uniform, and regularly shaped γ′ phases are re-precipitated, thereby restoring the alloy's microstructure and weldability and extending the service life of the alloy components. Example results show that the restoration heat treatment method provided by this invention, after restoring the K412 alloy blades after service, results in an alloy microstructure close to the original microstructure of the non-service blades. After tungsten inert gas welding (TIG) repair of the alloy blades, there are no obvious cracks at the weld, effectively preventing cracking caused by microstructure aging during the welding repair process. The blade's weldability is restored, effectively extending its service life and reducing aero-engine maintenance costs. Attached Figure Description

[0025] Figure 1 This is a flowchart of the recovery heat treatment in an embodiment of the present invention;

[0026] Figure 2 This is an optical microstructure diagram of the unused alloy blade in Comparative Example 1 of the present invention;

[0027] Figure 3 The image shows the electron microstructure of the unused alloy blade in Comparative Example 1 of this invention.

[0028] Figure 4 This is an optical microstructure image of the high-temperature alloy blade to be treated in an embodiment of the present invention;

[0029] Figure 5 This is an electron micrograph of the high-temperature alloy blade to be treated in an embodiment of the present invention;

[0030] Figure 6 This is an optical microstructure image of the high-temperature alloy blade after heat treatment in Example 1 of the present invention;

[0031] Figure 7 This is an electron micrograph of the high-temperature alloy blade after heat treatment in Example 1 of the present invention;

[0032] Figure 8 This is an optical microstructure image of the high-temperature alloy blade after heat treatment in Example 2 of the present invention;

[0033] Figure 9 This is an electron micrograph of the high-temperature alloy blade after heat treatment in Example 2 of the present invention;

[0034] Figure 10 This is an optical microstructure image of the high-temperature alloy blade after heat treatment in Example 3 of the present invention;

[0035] Figure 11 This is an electron micrograph of the high-temperature alloy blade after heat treatment in Example 3 of the present invention;

[0036] Figure 12 This is an optical microstructure diagram of the high-temperature alloy blade after heat treatment recovery in Comparative Example 2 of the present invention;

[0037] Figure 13 This is an electron micrograph of the high-temperature alloy blade after heat treatment recovery in Comparative Example 2 of the present invention;

[0038] Figure 14 This is a weld diagram of the high-temperature alloy blade to be treated in Embodiment 3 of the present invention;

[0039] Figure 15 This is a weld diagram of the high-temperature alloy blade after heat treatment in Embodiment 3 of the present invention. Detailed Implementation

[0040] This invention provides a method for the recovery heat treatment of a high-temperature alloy, comprising: heating the high-temperature alloy to be treated in stages to the recovery temperature, and then cooling it in stages to obtain the recovered high-temperature alloy.

[0041] In this invention, the high-temperature alloy to be treated is preferably a K412 or ЖC3 alloy suffering from creep damage. Limiting the high-temperature alloy to these types ensures better recovery of the alloy. In an embodiment of this invention, the high-temperature alloy to be treated is preferably a K412 alloy blade.

[0042] In this invention, the high-temperature alloy to be treated is preferably pretreated before undergoing the recovery heat treatment.

[0043] In this invention, the pretreatment preferably includes: subjecting the high-temperature alloy to be treated to corrosion treatment and ultrasonic cleaning in sequence.

[0044] In this invention, the preferred temperature for the corrosion treatment is 50–60°C, more preferably 55°C; the preferred time for the corrosion treatment is 40–50 min, more preferably 45 min. In this invention, the preferred etchant used for the corrosion treatment is a mixed solution of hydrochloric acid, citric acid, and water. This invention does not impose any particular limitation on the proportions of hydrochloric acid, citric acid, and water in the mixed solution; ratios commonly used by those skilled in the art can be adopted. This invention limits the temperature, time, and type of etchant for the corrosion treatment to the above-mentioned ranges to remove the surface coating of the high-temperature alloy to be treated, providing favorable conditions for subsequent recovery heat treatment.

[0045] In this invention, the solvent used for ultrasonic cleaning is preferably anhydrous ethanol. This invention does not impose any special limitations on the ultrasonic frequency, number of cleaning cycles, or time of the ultrasonic cleaning; conventional methods used by those skilled in the art to remove the corrosive agent from the surface of the high-temperature alloy to be treated are sufficient.

[0046] After ultrasonic cleaning, the present invention preferably further dries the ultrasonically cleaned alloy. The present invention does not have specific limitations on the drying temperature and time; common practices used by those skilled in the art to dry the alloy are sufficient.

[0047] In this invention, the segmented heating includes a first heating, a first holding, and a second heating performed sequentially; the heating rate of the first heating is 8–10 °C / min, preferably 8–9 °C / min; the heating rate of the second heating is 6–8 °C / min, preferably 6–7 °C / min. By limiting the heating rate of the segmented heating to the above range, this invention can obtain stable heating conditions and avoid uneven element diffusion in the alloy caused by excessively rapid heating rates, thus reducing component segregation.

[0048] In this invention, the temperature of the first heat preservation is preferably 800-820℃, more preferably 810℃; the time of the first heat preservation is preferably 30-40 min, more preferably 35 min. Limiting the temperature and time of the first heat preservation to the above range ensures the uniformity of alloying elements, which is beneficial for the subsequent recovery of alloy microstructure and welding performance.

[0049] In this invention, the recovery temperature is preferably 1130–1150°C, more preferably 1140–1150°C; the holding time at the recovery temperature is preferably 420–450 min, more preferably 420–430 min. Limiting the recovery temperature and the holding time within the above range ensures that the alloy can be restored to its original microstructure as much as possible and improves the weldability of the alloy.

[0050] In this invention, the recovery heat treatment is preferably carried out under vacuum or inert gas protection conditions.

[0051] In this invention, when the recovery heat treatment is performed under vacuum conditions, the vacuum level of the vacuum conditions is preferably less than 10. -2 Pa. By limiting the vacuum conditions to the above range, the oxidation of the alloy can be avoided.

[0052] In this invention, when the recovery heat treatment is carried out under inert gas protection conditions, it is preferable to first evacuate to 10 °C. -2 Below Pa, then fill with inert gas to 2 × 10⁻⁶ Pa. 4 Pa. In this invention, the inert gas is preferably argon; the purity of the argon is preferably ≥99.99%. This invention limits the conditions for restoring heat treatment under vacuum and inert gas protection to the range described above, which can prevent oxidation of the alloy.

[0053] In this invention, the recovery heat treatment is preferably carried out in a vacuum heating furnace.

[0054] In this invention, the segmented cooling includes a first cooling and a second cooling performed sequentially; the cooling rate of the first cooling is 250–300 °C / min, preferably 250–280 °C / min; the cooling method of the second cooling is air cooling. By limiting the cooling rate and method of the segmented cooling to the above range, this invention can avoid the growth of the γ′ phase during the cooling process.

[0055] In this invention, the endpoint temperature of the first cooling is preferably 290–310°C, more preferably 300°C. Limiting the endpoint temperature of the first cooling to this range ensures the recovery of the alloy's microstructure and improves its weldability.

[0056] In this invention, it is preferable to re-fill the vacuum furnace with an inert gas before the segmented cooling; the inert gas is preferably argon; the purity of the argon is preferably ≥99.99%; the amount of inert gas added is preferably such that the vacuum degree of the vacuum furnace is 0.2-0.3 MPa. This invention avoids oxidation of the alloy during the cooling process by re-filling the furnace with an inert gas before the segmented cooling.

[0057] This invention employs a segmented heating method. By controlling the heating rate of each segment, stable heating conditions can be obtained, avoiding uneven element diffusion in the alloy caused by excessively rapid heating and reducing component segregation. Similarly, a segmented cooling method, by controlling the cooling rate and method, can prevent the growth of the γ′ phase during cooling. Through the coordinated use of heating and cooling process parameters, the degraded γ′ phase and carbides in the alloy are dissolved, and small, uniform, and regularly shaped γ′ phases are re-precipitated, thereby restoring the alloy's microstructure and weldability and extending the blade's service life.

[0058] In an embodiment of the present invention, the recovery heat treatment process for the high-temperature alloy blade to be treated is as follows: Figure 1 As shown:

[0059] Remove the surface coating from the K412 alloy or ЖC3 blades, clean the sample and fixture, and place the sample into the furnace. Evacuate the furnace (pretreatment). Under vacuum or a protective atmosphere, raise the furnace temperature to 800–820°C at a heating rate of 8–10°C / min, hold at 800–820°C for 30–40 min (preheating), raise the furnace temperature to i°C (initial value of i is 1130) at a heating rate of 6–8°C / min, hold at i°C for 420–450 min (heating), introduce inert gas into the furnace, and lower the furnace temperature to 300°C (cooling). Open the furnace door and allow it to air-cool to room temperature. Prepare the metallographic structure and observe the blade microstructure.

[0060] The technical solutions of this invention will be clearly and completely described below with reference to the embodiments thereof. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0061] Example 1

[0062] A method for restoring the heat treatment of a high-temperature alloy, comprising the following steps:

[0063] The creep-damaged K412 alloy blades were placed in a corrosion solution made of hydrochloric acid, citric acid, and water, and kept at 55°C for 45 minutes. After corrosion treatment, the blades and blade holders were ultrasonically cleaned in anhydrous ethanol (analytical grade AR ≥ 99.7%), dried, and the initial temperature of the vacuum furnace was controlled at 25°C. The blades were then placed into the furnace, and the furnace was evacuated to a vacuum level of less than 10°C. -2 Pa, then fill with argon gas to a pressure of 2 × 10⁻⁶. 4 Pa, then the first heating is performed at a heating rate of 8℃ / min to 810℃, held for 35min, and then the second heating is performed at a heating rate of 6℃ / min to the recovery temperature of 1130℃, held for 420min. After the holding period, argon gas is introduced into the vacuum furnace for cooling, and the first cooling is performed at a cooling rate of 250℃ / min to 300℃. The furnace door is opened and the blades are taken out of the furnace for air cooling to the second cooling to room temperature, thus obtaining the recovered high-temperature alloy.

[0064] Comparative Example 1

[0065] Comparative Example 1 is an unused K412 alloy blade.

[0066] Optical microstructure of the unused K412 alloy blade in Comparative Example 1 is shown below. Figure 2 As shown; electron micrograph of the non-service K412 alloy blade in Comparative Example 1 is shown below. Figure 3 As shown; the optical microstructure of the high-temperature alloy blade to be treated in this embodiment of the invention is as follows. Figure 4 As shown; the electron microstructure of the high-temperature alloy blade to be treated in this embodiment of the invention is shown in the figure. Figure 5 As shown; the optical microstructure of the high-temperature alloy blade after heat treatment in Example 1 of this invention is shown in the figure. Figure 6 As shown; the electron microstructure of the high-temperature alloy blade after recovery heat treatment in Example 1 of this invention is shown in the figure. Figure 7 As shown; from Figures 2-7 As can be seen, the microstructure of the K412 alloy repaired in this embodiment is compared with... Figure 4 and 5 The microstructure of the high-temperature alloy to be treated shows that after a recovery heat treatment at 1130℃, the size and distribution of grain boundaries and γ′ precipitates in the K412 alloy begin to recover, but compared with... Figure 2 and 3 There is still a gap in the original microstructure of the medium alloy.

[0067] Example 2

[0068] The only difference between Example 2 and Example 1 is the recovery temperature at the second heating point of 1140°C; otherwise, they are the same as in Example 1.

[0069] Example 3

[0070] The only difference between Example 3 and Example 1 is the recovery temperature at the second heating point of 1150°C; otherwise, they are the same as in Example 1.

[0071] The optical microstructure of the high-temperature alloy blade after heat treatment in Example 2 of this invention is shown in the figure below. Figure 8 As shown; the electron microstructure of the high-temperature alloy blade after recovery heat treatment in Example 2 of this invention is shown in the figure. Figure 9 As shown; the optical microstructure of the high-temperature alloy blade after recovery heat treatment in Example 3 of this invention is shown in the figure. Figure 10 As shown; the electron microstructure of the high-temperature alloy blade after heat treatment in Example 3 of this invention is shown in the figure. Figure 11 As shown. By Figures 6-11 It can be observed that as the recovery temperature increases (1130–1150℃), the alloy microstructure of the blades after recovery heat treatment gradually approaches that of the original microstructure of the blades before service. Figures 2-5 and Figures 10-11 It can be seen that the strengthening phase γ′ in the K412 alloy microstructure after service is completely dissolved into the matrix when heated to 1150℃, and fine and uniform γ′ strengthening phase is redeprecipitated during the subsequent cooling process. Its size, distribution and morphology are close to the original microstructure of the unserviced blade.

[0072] Comparative Example 2

[0073] The only difference between Comparative Example 2 and Example 1 is that the temperature was lowered to 300°C by furnace cooling, and the furnace door was opened to allow the blades to be taken out of the furnace and air-cooled for a second cooling to room temperature, thus obtaining the restored high-temperature alloy.

[0074] The optical microstructure of the high-temperature alloy blade after recovery heat treatment in Comparative Example 2 of this invention is shown in the figure below. Figure 12 As shown; the electron microstructure of the high-temperature alloy blade after recovery heat treatment in Comparative Example 2 of this invention is shown in the figure. Figure 13 As shown, from Figures 6-7 and Figures 12-13 It can be seen that the cooling rate of the alloy affects the size of the precipitates in the alloy. Cooling at a higher rate can refine the size of the precipitates. If furnace cooling is used at a lower rate, the precipitates will grow sufficiently during the cooling process, which will damage the alloy's properties and reduce the effectiveness of the recovery heat treatment.

[0075] Test case

[0076] The high-temperature alloy blade to be treated in Example 3 and the restored high-temperature alloy blade obtained in Example 3 were repaired by tungsten inert gas welding (welding wire HGH3113).

[0077] The weld diagram of the high-temperature alloy blade to be treated in Example 3 is as follows: Figure 14 As shown; the weld diagram of the high-temperature alloy blade after restoration heat treatment in Example 3 is shown below. Figure 15 As shown. From Figure 14 and Figure 15 It can be seen that the macroscopic morphology of the weld seam of the high-temperature alloy blade without restoration heat treatment shows obvious cracks at the weld seam, indicating poor weld repairability of the blade; while the macroscopic morphology of the weld seam of the high-temperature alloy blade after restoration heat treatment shows no obvious cracks at the weld seam.

[0078] As can be seen from the test examples, after the alloy blades have been put into service, the 1150℃ recovery heat treatment process proposed in this invention can effectively prevent cracking caused by microstructure aging during the welding repair process, and the welding repair performance of the blades can be restored.

[0079] The recovery heat treatment method provided by this invention restores the alloy structure of the blade after it has been in service to a point where it is close to the original structure of the blade before it was put into service. After the alloy blade is repaired by tungsten inert gas welding, there are no obvious cracks at the weld, which can effectively avoid cracking caused by the aging of the structure during the welding repair process. The welding repair performance of the blade is restored, which can effectively extend the service life of the blade and reduce the maintenance cost of aero-engines.

[0080] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle 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 method for restoring the heat treatment of a high-temperature alloy, comprising the following steps: heating the high-temperature alloy to be treated in stages to the recovery temperature, and then cooling it in stages to obtain the restored high-temperature alloy; The segmented heating includes a first heating, a first holding, and a second heating performed sequentially. The heating rate of the first heating is 8~10℃ / min; The second heating rate is 6~8℃ / min; The segmented cooling includes a first cooling and a second cooling performed sequentially. The cooling rate of the first cooling step is 250~300℃ / min; The second cooling method is air cooling; The recovery temperature is 1130~1150℃, and the holding time at the recovery temperature is 420~450min; The temperature of the first heat preservation is 800~820℃, and the time of the first heat preservation is 30~40min; The endpoint temperature of the first cooling process is 290~310℃; The high-temperature alloy to be treated is a K412 or ЖC3 alloy with creep damage.

2. The method for restoring heat treatment according to claim 1, characterized in that, The high-temperature alloy to be treated is pretreated before the recovery heat treatment. The pretreatment includes: sequentially subjecting the high-temperature alloy to corrosion treatment and ultrasonic cleaning.

3. The method for restoring heat treatment according to claim 2, characterized in that, The temperature of the corrosion treatment is 50~60℃, and the time of the corrosion treatment is 40~50min.

4. The method for restoring heat treatment according to claim 2, characterized in that, The corrosion agent used in the corrosion treatment is a mixed solution of hydrochloric acid, citric acid and water.

5. The method for restoring heat treatment according to claim 1, characterized in that, The recovery heat treatment is carried out under vacuum and inert gas protection conditions.

6. The method for restoring heat treatment according to claim 5, characterized in that, The specific operation performed under the vacuum and inert gas protection conditions is to first evacuate to 10... -2 Below Pa, then fill with inert gas to 2 × 10⁻⁶ Pa. 4 Pa.

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