Heat treatment process for regulating and controlling nickel-based superalloy casting structure after welding repair

Abstract

The invention discloses a heat treatment process for regulating and controlling a nickel-based superalloy casting structure after welding repair, and relates to the technical field of nickel-based superalloy heat treatment. A heat treatment process for regulating and controlling the structure of a nickel-based superalloy casting after welding repair comprises the following steps that hot isostatic pressing treatment is conducted on the nickel-based superalloy casting, the nickel-based superalloy casting is placed in a closed container to be heated and pressurized to 1150-1200 DEG C, and the pressure in the closed container is not smaller than 140 Mpa; then preserving heat and pressure for more than 180 minutes, and air-cooling to room temperature; and the casting subjected to hot isostatic pressing is subjected to solution treatment at the temperature of 1190 DEG C or below, heat preservation is conducted for 2 h, and air cooling is conducted to the room temperature. According to the method, pores and microcracks of the part, difficult to weld and repair, of the nickel-based superalloy casting can be eliminated, the structure can be homogenized, and the mechanical property of the material is improved.

Description

Technical Field

[0001] This invention proposes a heat treatment process for controlling the microstructure of nickel-based superalloy castings after weld repair. It belongs to the field of weld repair technology and is applicable to the heat treatment of repaired nickel-based superalloy components. Background Technology

[0002] K4002 is a precipitation-strengthened cast nickel-based superalloy with excellent oxidation resistance and thermal fatigue strength. Its various high-temperature properties meet the operational requirements of key hot-end components in next-generation aero-engines and gas turbines, and it has been successfully used in turbine blades and integrally cast turbines in various aero-engine models. However, these hot-end components have complex structures and manufacturing processes, and are prone to defects such as porosity, cracks, and looseness during manufacturing. Furthermore, they are susceptible to failure when used in harsh environments such as high temperature, high pressure, oxidation, and corrosion. To ensure that defective hot-end components meet service requirements, they must be replaced or repaired. However, complete replacement is very costly, while repair costs only 20% of the cost of replacement. Therefore, welding cladding is currently the primary method for repair.

[0003] Welding repair involves high heat source energy density and a large thermal gradient during the repair process, which easily leads to elemental segregation and high residual stress. K4002 nickel-based superalloy, being a difficult-to-weld alloy, is highly susceptible to cracking and defects under high stress. Although micro-beam plasma arc welding reduces heat input and almost eliminates cracks in the repaired area, the partial melting zone in the heat-affected zone caused by the increased heat input further affects the strength, hardness, and fatigue resistance of the welded joint. Furthermore, sporadic shrinkage cavities and other defects may appear in areas not treated by welding. To eliminate residual porosity and improve the uneven microstructure of the repaired joint, further microstructural control is necessary.

[0004] Hot isostatic pressing (HIP) is a powder metallurgy and material densification technology that integrates thermodynamics and mechanics. Its core principle is to apply extremely high temperatures and isotropic ultra-high pressures to the material simultaneously, subjecting the workpiece to uniform pressure from all directions at high temperatures. This compresses any discontinuous defects within the material, such as pores and cracks, achieving densification. Solution treatment is a process that reheats the γ' reinforcing phase and other strengthening phases until they dissolve into the γ matrix to form a supersaturated solid solution. This effectively homogenizes the microstructure and improves mechanical properties. Therefore, developing a post-processing technique that can help eliminate pore defects and homogenize the microstructure is of great significance. Summary of the Invention

[0005] In view of the limitations of the prior art, the present invention proposes a heat treatment process for regulating the microstructure of nickel-based superalloy castings after welding repair, which can eliminate casting porosity defects and improve microstructure without producing cracks or other defects.

[0006] The technical solution of this invention is as follows:

[0007] A heat treatment process for regulating the microstructure of nickel-based superalloy castings after weld repair includes the following steps:

[0008] First, a micro-beam plasma arc was used as a heat source to weld and repair the K4002 substrate, using HGH3113 welding wire.

[0009] Furthermore, two sets of metallographic and tensile samples were cut from the casting using an electrical discharge wire cutting machine, and labeled A and B respectively.

[0010] Furthermore, the B group samples were subjected to hot isostatic pressing to obtain hot isostatically pressed metallographic and tensile samples.

[0011] Preferably, the hot isostatic pressing temperature is 1150°~1200°C and the pressure is 140MPa.

[0012] Preferably, the heat preservation time is 180 minutes.

[0013] Preferably, the cooling method is air cooling.

[0014] Furthermore, the samples from group B were subjected to solution treatment to obtain metallographic and tensile specimens after solution treatment.

[0015] Preferably, the solution treatment temperature is 1160℃.

[0016] Preferably, the heating rate is 8–10 °C / min.

[0017] Preferably, the heat preservation time is 120 minutes.

[0018] Preferably, air cooling is used.

[0019] Furthermore, metallographic and tensile samples were obtained after two processes: hot isostatic pressing and solution treatment.

[0020] The surfaces of samples A and B were further cleaned using ultrasonic cleaning, and the oxide layer on the samples was removed using sandpaper.

[0021] Furthermore, after the samples were polished, the microstructures of A and B were observed using a scanning electron microscope.

[0022] Furthermore, the cross-sectional metallographic samples of groups A and B were subjected to hardness tests from top to bottom.

[0023] Furthermore, the tensile properties of the tensile specimens from groups A and B were tested using a testing machine.

[0024] Compared with the prior art, the present invention has at least the following beneficial effects:

[0025] This invention discloses a heat treatment process for regulating the microstructure of nickel-based superalloy castings after weld repair. In this heat treatment process, compared with the original sample A, the original porosity defects of sample B after hot isostatic pressing and solution treatment are eliminated. No continuously distributed liquid film or cracks are observed at the grain boundaries, and square primary γ' phases are redeprecipitated in the partially melted zone near the fusion line. Regarding microhardness, this post-treatment process significantly improves the microhardness of the cladding layer, while the microhardness of the heat-affected zone and the base material remains relatively unchanged, resulting in a more uniform microhardness distribution in the sample. In terms of tensile properties, the average tensile strength of sample B is slightly lower than that of sample A due to the dissolution of carbides in the heat-affected zone, but the elongation after fracture is increased, and the overall mechanical properties meet aerospace standards. Therefore, the heat treatment process of this invention can eliminate the original porosity defects while homogenizing the microstructure, balancing the microhardness of the cladding layer and the base material, and improving the elongation after fracture of the repaired casting. Attached Figure Description

[0026] Figure 1 Scattered holes and defects inside the casting

[0027] Figure 2 Tissue near the fusion line after welding repair

[0028] Figure 3 Structure near the fusion line after heat treatment

[0029] Figure 4 Microstructure of the heat-affected zone after heat treatment

[0030] Figure 5 Hardness distribution map

[0031] Figure 6 Stress-strain curve Detailed Implementation

[0032] To enable those skilled in the art to better understand the present invention, the present invention will be further described below with reference to specific embodiments.

[0033] The specific implementation steps of this invention are as follows:

[0034] (1) Using micro-beam plasma arc welding, HGH3113 welding wire with a diameter of 1.2 mm was used. The current was set to 50 A, the scanning speed was 2 mm / s, and the wire feeding speed was 0.4 m / min to perform welding repair on K4002 cast nickel-based high-temperature alloy plate.

[0035] (2) Two sets of cross-sectional metallographic samples and tensile test specimens were separated from the repaired plate using an electric wire EDM machine. They were labeled A and B respectively. The cutting speed was 7-10 mm / min.

[0036] (3) Use ultrasonic cleaning to clean the separated metallographic and tensile samples.

[0037] (4) The metallographic samples of Group A were subjected to processes such as mounting, grinding, polishing, and etching, and then observed under a Zeiss GeminiSem500 scanning electron microscope.

[0038] (5) The samples of group B were subjected to hot isostatic pressing (HIP). The specific steps of HIP were as follows: the metallographic and tensile samples after solution treatment were ultrasonically cleaned, and the oxide layer was removed with sandpaper. They were then placed in a hot isostatic pressing furnace under heat and pressure for densification. Argon gas of 99% purity was introduced to apply pressure, which was set to 140 MPa and 1180 °C. The heating rate was set to 10 °C / min. The temperature was held at 1180 °C for 3 hours. After the test, the samples were cooled to room temperature by air cooling.

[0039] (6) The B group samples after hot isostatic pressing were subjected to solution treatment. The specific implementation steps of solution treatment are as follows: ultrasonic cleaning was used to remove the stains on the sample surface after hot isostatic pressing. The B sample was sent into the material frame of the high temperature electric furnace. The heating rate was set to 8℃ / min, the solution treatment temperature was controlled to 1160℃, the holding time was 2h, and the sample was cooled to room temperature by air cooling after the treatment was completed.

[0040] (7) The metallographic samples of group B were subjected to processes such as mounting, grinding, polishing, and etching, and then sent to a Zeiss GeminiSem500 scanning electron microscope for observation, with a focus on checking for defects such as voids.

[0041] (8) The metallographic samples A and B were tested for hardness using an HVS-50Z Vickers hardness tester with a load of 500gf and a holding time of 15s.

[0042] (9) The two groups of tensile samples, A and B, were subjected to room temperature tensile tests using a KY-100KNOW electronic universal testing machine at a tensile speed of 0.5 mm / min.

[0043] In Example 1, comparing sample B with sample A, it can be seen that after hot isostatic pressing and solution treatment under these parameters, the pores and defects that were originally scattered in the heat-affected zone and the base material were healed under high temperature and pressure. The density of the sample was significantly improved, no pores were found, and square γ' phases were re-precipitated in some of the molten areas of the original heat-affected zone. The closer to the base material, the more elliptic the γ' phases became. No continuous liquid film was observed around the grain boundaries of the heat-affected zone. Both carbides and eutectic phases dissolved. Microhardness testing showed that the difference between the microhardness of the repair layer of sample B and the base material after hot isostatic pressing and solution treatment was significantly reduced compared to sample A. Tensile testing showed that because the high-strength carbides dissolved, the ultimate tensile strength of sample B was slightly reduced compared to sample A, from 735 MPa to 712 MPa, a decrease of only about 3%. However, the elongation after fracture increased from 18.4% to 28.2%, an increase of 53.2%. The strength decreased slightly while the plasticity increased significantly.

[0044] In summary, this invention effectively eliminates some of the non-removable diffuse porosity defects during the welding repair of nickel-based superalloys, homogenizes the microstructure, and significantly improves the plasticity of the repaired joint.

[0045] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A heat treatment process for eliminating defects in nickel-based superalloy castings, characterized in that, Specifically, the following steps are included: (1) Hot isostatic pressing: The nickel-based high-temperature alloy castings repaired by micro-beam plasma welding are subjected to hot isostatic pressing to improve the density of the microstructure and eliminate the pore defects that are difficult to repair by welding. (2) Solution treatment: The nickel-based superalloy castings after hot isostatic pressing are subjected to solution treatment to dissolve the pre-generated coarse γ' strengthening phase and eutectic phase, homogenize the microstructure, and improve mechanical properties.

2. The hot isostatic pressing method according to claim 1, characterized in that: In a sealed container, the nickel-based superalloy casting is heated to 1150–1200°C, the pressure inside the container is not less than 140 MPa, and the temperature and pressure are maintained for more than 180 minutes before being air-cooled to below 200°C.

3. The solution treatment method according to claim 1, characterized in that: In a high-temperature electric furnace, the nickel-based superalloy casting is heated to 1160°C, held for 120 minutes, and then air-cooled to room temperature.

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