Method for improving hot working performance of nickel-based superalloy for molten salt reactor through combination of pre-deformation and homogenization

Abstract

The invention belongs to the technical field of high-temperature alloy heat treatment, and discloses a method for improving the hot working performance of a nickel-based high-temperature alloy for a molten salt reactor by combining pre-deformation and homogenization. The method comprises the steps that raw materials of the nickel-based superalloy for the molten salt reactor to be improved are sequentially subjected to vacuum induction melting, pouring and electroslag remelting refining treatment, and an electroslag remelting refined alloy cast ingot is obtained; the electroslag remelting refined alloy cast ingot is sequentially subjected to heating, heat preservation and pre-deformation treatment and is air-cooled to the room temperature, and a pre-deformed cast ingot is obtained; and the pre-deformed cast ingot is subjected to homogenization treatment and air cooling to the room temperature, and the method is completed. The method is a novel heat treatment process combining pre-deformation and high-temperature homogenization, the hot working performance of the alloy is optimized, the high-temperature alloy uniform in structure and excellent in mechanical property is obtained, and application in the field of high-temperature molten salt energy systems is met.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of high-temperature alloy heat treatment, and more particularly relates to a method for improving hot workability of a nickel-based high-temperature alloy for molten salt reactor by combining pre-deformation with homogenization. BACKGROUND

[0002] Ni-W-Cr alloy for high-temperature molten salt energy system, i.e. nickel-based high-temperature alloy for molten salt reactor, contains a large amount of refractory element tungsten (W≥19wt.%) which helps to ensure high-temperature strength of the alloy through solid solution strengthening, and at the same time makes the alloy have excellent corrosion resistance and radiation resistance. However, this also brings some problems, such as serious dendritic segregation, cracking during forging deformation and low recrystallization degree, which need to be solved urgently. For example, if high-temperature deformation is directly performed, it will often lead to uneven deformation caused by coarse eutectic phase or carbide, thereby forming cracks in the later deformation process. If the conventional homogenization heat treatment system is used before deformation, due to the addition of a large amount of W element, a long time is needed to eliminate dendritic segregation, which leads to serious oxidation of the alloy, and at the same time, the grain boundary carbide is severely coarsened, and the intragranular lamellar carbide is precipitated, which leads to strain concentration during subsequent forging process and causes the alloy to crack. Moreover, the coarse grain boundary carbide is easy to form crack source, leading to intergranular cracking and causing the mechanical properties of the alloy to decrease. Therefore, the Ni-W-Cr alloy for high-temperature molten salt energy system is not suitable for direct deformation, nor is it suitable for conventional homogenization heat treatment.

[0003] The service environment determines that the alloy needs to meet many conditions at the same time, such as excellent molten salt corrosion resistance, radiation resistance, high-temperature mechanical properties (≥800℃) and long-term service performance, etc. Therefore, the alloy mainly ensures the service life through solid solution strengthening, and realizes grain boundary strengthening through carbide pinning to ensure high-temperature grain boundary strength. Further, the element of the alloy has a narrow adjustable range, and the method of adjusting the alloy composition alone cannot be used to control the carbide and optimize the hot workability.

[0004] CN107841648A discloses a method for improving the hot workability of a platinum-tungsten alloy, which makes the ingot organization have no obvious coarse columnar crystal, has a uniform composition, and has a fine equiaxed crystal structure, thereby improving the success rate of ingot breakdown and improving the hot workability. However, this method is not applicable to high-W alloy used in high-temperature molten salt energy system. CN110373620A discloses a method for improving the hot workability of a high-γ' phase volume fraction nickel-based precipitation strengthened high-temperature alloy, which remelts the high-γ' phase volume fraction nickel-based precipitation strengthened high-temperature alloy, and performs five times of rod heat treatment, upsetting, elongation and annealing treatment. However, this method is also not applicable to the alloy (Ni-W-Cr alloy) used in high-temperature molten salt energy system.

[0005] In summary, the traditional homogenization and reheating deformation method is not suitable for the Ni-W-Cr alloy used in the high-temperature molten salt energy system, and therefore it is necessary to develop a new method to solve the serious segregation behavior of the Ni-W-Cr alloy used in the high-temperature molten salt energy system and optimize the hot working performance thereof. SUMMARY

[0006] The purpose of the present application is to overcome the deficiencies of the prior art and provide a method for improving the hot working performance of a nickel-based high-temperature alloy for a molten salt reactor by combining pre-deformation and homogenization. The present application is a new type of heat treatment process combining pre-deformation and high-temperature homogenization, which optimizes the hot working performance of the alloy and obtains a high-temperature alloy with uniform structure and excellent mechanical properties, meeting the application requirements in the field of high-temperature molten salt energy systems.

[0007] To achieve the above-mentioned purpose, the present application provides a method for improving the hot working performance of a nickel-based high-temperature alloy for a molten salt reactor by combining pre-deformation and homogenization, which comprises the following steps:

[0008] S1: subjecting the raw material of the nickel-based high-temperature alloy for a molten salt reactor to be improved to vacuum induction melting (VIM), pouring and electroslag refining (ESR) treatment in sequence to obtain an electroslag refining alloy ingot;

[0009] S2: subjecting the electroslag refining alloy ingot to heating, holding and pre-deformation treatment in sequence, and air cooling to room temperature to obtain a pre-deformation ingot;

[0010] S3: subjecting the pre-deformation ingot to homogenization treatment, and air cooling to room temperature to complete the method.

[0011] According to the present application, preferably, the nickel-based high-temperature alloy for a molten salt reactor comprises, based on the total weight of the alloy, C 0.02-0.08%, Al <0.3%, Cr 5.8-7.0%, W 19.0-26.0%, Zr <0.1%, Si <0.5%, Mn <0.8%, Ti 0-0.3%, Nb 2.0-4.0%, and the balance of Ni.

[0012] In the present application, the vacuum induction melting (VIM), pouring and electroslag refining (ESR) treatment are treatment processes known to those skilled in the art and capable of obtaining the alloy of the present application, and the process parameters are process parameters known to those skilled in the art and capable of obtaining the alloy of the present application.

[0013] According to the present application, preferably, the operating conditions of the vacuum induction melting (VIM) treatment include a melting temperature of 1490-1510°C, a vacuum degree of 0.4x10 -2 -0.6x10 -2 Pa, a refining temperature of 1540-1560°C, and a refining time of 8-10 min.

[0014] According to the application, preferably, the pouring is carried out under inert gas condition, and the temperature of the pouring is 1450-1490℃.

[0015] According to the application, preferably, in the step S2, the heating rate is 5-20℃ / min, and after heating to 1150-1220℃, the temperature is kept for 30min-2h, and then pre-deformation treatment is carried out at 1150-1220℃, the deformation amount of the pre-deformation treatment ranges from 5% to 15%, and the strain rate of the pre-deformation treatment is 0.5-1.5s -1 .

[0016] In the application, the air cooling is cooling in air, and the temperature of the room temperature is 15-25℃.

[0017] According to the application, preferably, in the step S3, the operation condition of the homogenization treatment includes: the heating rate is 5-20℃ / min, the temperature of the homogenization treatment is 1150-1220℃, and the holding time of the homogenization treatment is 30min-2h.

[0018] According to the application, preferably, after the method, the alloy obtained by the homogenization treatment is subjected to hot working high-temperature deformation treatment. The hot working high-temperature deformation treatment includes high-temperature forging deformation treatment and rolling deformation treatment, wherein the forging reduction and pass parameters are limited according to the actual requirements in the art.

[0019] The beneficial effects of the technical scheme of the application are as follows:

[0020] The application is a new type of heat treatment process combining pre-deformation and high-temperature homogenization, and the pre-deformation amount, homogenization treatment temperature and time are reasonably controlled, the alloy hot working performance is finally optimized, and then a high-temperature alloy with uniform structure and excellent mechanical properties is obtained, which is put into use in actual production, meets the application in the field of high-temperature molten salt energy system, and provides a new idea and theoretical basis for developing the next generation of energy system alloy.

[0021] The present application is directed to alloy refractory element segregation, deformation difficulties and other problems, according to the element multi-channel diffusion theory, a method for improving the hot working properties of the nickel-based high-temperature alloy for molten salt reactor by combining pre-deformation and homogenization is designed, which introduces dislocations, stacking faults and other defects in the pre-deformation of the alloy material, in terms of element diffusion, pre-deformation reduces the secondary interdendritic spacing, reduces the M2C phase transition activation energy, and promotes the remelting of the metastable M2C phase, both of which help to increase the concentration gradient of the elements. In addition, the introduced dislocations and substructures further promote the diffusion of segregated elements and the remelting of segregated structures, thereby shortening the homogenization treatment time, preventing the over-coarsening of carbides and grains, and avoiding the precipitation of intragranular lamellar carbides. Moreover, the pre-deformation breaks the as-cast carbides, promotes the uniform distribution of grain boundary M6C carbides, and reduces the formation of crack sources in the grain boundary weak zone.

[0022] During subsequent hot working deformation, the defects introduced by pre-deformation provide more carbide nucleation sites, making the carbide distribution more dispersed, thereby refining the grain boundary carbides and reducing the cracking sensitivity at the grain boundary. In addition, during subsequent hot working deformation, pre-deformation also increases the energy storage in the grains, introduces a continuous dynamic recrystallization mechanism (CDRX), and ultimately improves the degree of dynamic recrystallization in the alloy, thereby improving the microstructure, making the structure more uniform, significantly improving the hot working properties (hot ductility), and improving the mechanical properties of the alloy.

[0023] Other features and advantages of the present application will be described in detail in the following specific embodiments. BRIEF DESCRIPTION OF DRAWINGS

[0024] The above and other objects, features and advantages of the present application will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which like reference characters refer to like parts throughout the figures, and in which:

[0025] Figure 1 A flow chart of the preparation method of the molten salt reactor nickel-based high-temperature alloy of the present application and Comparative Example 1 is shown.

[0026] Figure 2 The as-cast microstructure map (light microscope) of the electroslag remelting refined alloy ingot obtained by the method of the present application is shown.

[0027] Figure 3 The homogenization microstructure map (light microscope) of the alloy obtained by the preparation method of the molten salt reactor nickel-based high-temperature alloy of Comparative Example 1 is shown.

[0028] Figure 4The microstructure (light microscope) of the pre-deformed ingot obtained by the method for improving hot workability of nickel-based superalloy for molten salt reactor provided by the embodiment 1 of the present application is shown.

[0029] Figure 5 The homogenized microstructure (light microscope) of the homogenized alloy obtained by the method for improving hot workability of nickel-based superalloy for molten salt reactor provided by the embodiment 1 of the present application is shown.

[0030] Figure 6 The microstructure (scanning electron microscope: ZEISS Gemini 300) of the homogenized alloy obtained by the method for improving hot workability of nickel-based superalloy for molten salt reactor provided by the embodiment 1 of the present application due to the pre-deformation induced partial twinning is shown.

[0031] Figure 7 The microstructure (scanning electron microscope: Inspect F50 field emission scanning electron microscope) of the homogenized alloy obtained by the method for improving hot workability of nickel-based superalloy for molten salt reactor provided by the embodiment 1 of the present application due to the pre-deformation providing more nucleation sites is shown.

[0032] Figure 8 The carbide cracking diagram (scanning electron microscope: ZEISS Gemini 300) of the homogenized alloy obtained by the preparation method of the nickel-based superalloy for molten salt reactor of the comparative example 1 of the present application due to high-temperature forging deformation treatment is shown.

[0033] Figure 9 The microstructure (scanning electron microscope: ZEISS Gemini 300) of the homogenized alloy obtained by the method for improving hot workability of nickel-based superalloy for molten salt reactor provided by the embodiment 1 of the present application after high-temperature forging deformation treatment is shown.

[0034] Figure 10 The IPF diagram (inverse pole figure) (scanning electron microscope: ZEISS Gemini 300) of the homogenized alloy obtained by the preparation method of the nickel-based superalloy for molten salt reactor of the comparative example 1 of the present application after high-temperature forging deformation treatment is shown.

[0035] Figure 11 The IPF diagram (scanning electron microscope: ZEISS Gemini 300) of the homogenized alloy obtained by the method for improving hot workability of nickel-based superalloy for molten salt reactor provided by the embodiment 1 of the present application after high-temperature forging deformation treatment is shown.

[0036] Figure 12Fig. 1 shows the microstructure of the rolled plate after high-temperature forging deformation treatment and rolling deformation treatment of the homogenized alloy obtained by the method for improving hot workability of nickel-based superalloy for molten salt reactor provided by Embodiment 2 of the present application (scanning electron microscope: Inspect F50 field emission scanning electron microscope). DETAILED DESCRIPTION

[0037] Preferred embodiments of the present application will be described in more detail below. Although the following describes preferred embodiments of the present application, it should be understood that the present application can be implemented in various forms and should not be limited by the embodiments set forth herein. Rather, these embodiments are provided so that the present application is more thoroughly and completely conveyed to those skilled in the art, and the scope of the present application is fully conveyed to those skilled in the art.

[0038] Embodiment 1

[0039] The present embodiment provides a method for improving hot workability of nickel-based superalloy for molten salt reactor by combining pre-deformation with homogenization. The alloy to be improved includes, based on the total weight of the nickel-based superalloy for molten salt reactor to be improved: C 0.04%, Al 0.1-0.3%, Cr 6%, W 24%, Zr 0.04%, Si 0.3%, Mn 0.6%, Ti 0.1-0.2%, Nb 2%, and the balance of Ni.

[0040] The method includes the following steps:

[0041] S1: raw materials of the nickel-based superalloy for molten salt reactor to be improved are added to a high-frequency vacuum induction furnace, the melting temperature is 1500°C, the vacuum degree is 0.5x10 -2 Pa, after the raw materials of the alloy to be improved are completely melted, the temperature is refined at 1550±10°C for 8-10 min, then inert gas is introduced and the alloy liquid is poured into an ingot, the pouring temperature is controlled in the range of 1450±20°C, to obtain a vacuum induction melting ingot. After the vacuum induction melting ingot is cooled, the sprue is cut off and the polished and polished ingot is placed in an electroslag remelting device as an electrode. After three steps of remelting operation of arc striking, total melting of the dissolving agent and top sealing and shrinkage filling, an electroslag remelted and refined alloy ingot is obtained, as shown in Fig. 2. Figure 2

[0042] S2: the electroslag remelted and refined alloy ingot is heated to 1200°C at a heating rate of 15°C / min, and then is kept at 1200°C for 30 min-2h, and then is subjected to pre-deformation treatment at 1200°C, the deformation amount is in the range of 10%, the strain rate of the pre-deformation treatment is 1s -1 , and is cooled to room temperature in air, to obtain a pre-deformed ingot, as shown in Fig. 3. Figure 4

[0043] ​​S3: heating at a heating rate of 5-20℃ / min to 1200℃, homogenizing the pre-deformed ingot at 1200℃ for 30-40min, and cooling to room temperature in air, to complete the method, and the microstructure of the alloy after homogenization is shown in Figures 5-7 .

[0044] After the method of this embodiment is completed, the homogenized alloy is sequentially subjected to heating, holding and high-temperature forging deformation treatment, to obtain a high-temperature forged alloy, as shown in Figure 9 , 11 , and as can be seen from Figure 9 and 11 , the high-temperature forged alloy after pre-deformation and homogenization treatment of this embodiment has no cracks, and the degree of recrystallization is improved.

[0045] High-temperature forging operation parameters of this embodiment:

[0046] 1. Upsetting forging.

[0047] First, the alloy ingot is heat treated, and then it is determined whether to perform one-time upsetting forging on the ingot according to the height-diameter ratio of the ingot, and the upsetting forging method is upsetting, and the forged blank is air-cooled;

[0048] The heating temperature for forging is 1100-1250℃, the holding time is 5-120min, a anvil with a diameter much larger than the material is used during upsetting, and the blank needs to be chamfered after upsetting.

[0049] 2. Intermediate forging

[0050] The blank obtained after upsetting forging is heated and held at a temperature of 1100-1250℃ for 5-30min, and then subjected to multiple times of open-die forging.

[0051] 3. Final forging

[0052] The blank after intermediate forging is finally forged at a temperature below the phase transition point to ensure the size requirements, and the alloy should also meet the corresponding uniformity and grain size. The forged blank is air-cooled to obtain an alloy plate blank meeting the specifications.

[0053] Example 2

[0054] This embodiment provides a method for improving the hot working properties of a nickel-based high-temperature alloy for a molten salt reactor by combining pre-deformation and homogenization. The nickel-based high-temperature alloy to be improved is the same as in Example 1.

[0055] The method comprises the following steps:

[0056] S1: The same as in Example 1, to obtain an electroslag remelted refined alloy ingot.

[0057] S2: heating the alloy ingot to 1200°C at a heating rate of 15°C / min, keeping the electroslag remelting refined alloy ingot at 1200°C for 30min-2h, and then performing a pre-deformation treatment at 1200°C with a deformation range of 15% and a strain rate of 1s -1 , cooled to room temperature in air to obtain a pre-deformed ingot;

[0058] S3: heating the pre-deformed ingot to 1200° C. at a heating rate of 5-20° C. / min, homogenizing the pre-deformed ingot at 1200° C. for 30-40 min, and cooling the pre-deformed ingot to room temperature in air to complete the method.

[0059] After the method of this embodiment is completed, the alloy obtained by homogenization treatment is subjected to heating, heat preservation, high temperature forging deformation treatment and rolling deformation treatment in sequence to obtain a rolled plate. The microstructure diagram of the rolled plate is as shown in FIG. Figure 12 As shown by Figure 12 It can be seen that the rolled plate obtained in this embodiment has a uniform structure, which is beneficial to the improvement of the mechanical properties of the alloy.

[0060] The high temperature forging operating parameters are the same as those in Example 1.

[0061] Specific parameters of rolling deformation:

[0062] Plates are formed by hot rolling. For 50mm thick slabs, three or more passes should be used. The starting rolling temperature should be ≥1100°C, the finishing rolling temperature should be ≥900°C, and the rolling speed should be 20mm / s-180mm / s.

[0063] First fire (3 passes, each pass reduces the thickness of the original plate by about 5-15%), then heat in the furnace for 25-30 minutes;

[0064] Second fire (4 passes, each pass reducing 5-8% of thickness), return to the furnace and heat for 15-20 minutes;

[0065] The third fire (6 passes, each pass reducing 2-5% of the thickness), the number of fires or the amount of reduction per pass can be increased according to the experimental requirements.

[0066] Repeat the above rolling steps until the set total deformation is achieved after several passes, and then slowly air cool the billet. The billet for rolling has sufficient head and tail cuts to remove shrinkage cavities and severely segregated parts.

[0067] Comparative Example 1

[0068] This comparative example provides a method for preparing a nickel-based high-temperature alloy for a molten salt reactor (conventional production process). The raw materials of the nickel-based high-temperature alloy for a molten salt reactor in this comparative example are the same as those of the nickel-based high-temperature alloy for a molten salt reactor to be improved in Example 1.

[0069] The method comprises the following steps:

[0070] S1: same as example 1, to obtain the electroslag remelted and refined alloy ingot.

[0071] S2: heating at a heating rate of 5-20℃ / min to 1200℃, homogenizing the electroslag remelted and refined alloy ingot at 1200℃ for 6-8h, and cooling to room temperature in air, to complete the method, to obtain the alloy of the present comparative example, as shown in Figure 3

[0072] After the end of the method of the present comparative example, the alloy obtained by homogenization is sequentially subjected to heating, holding and high temperature forging deformation treatment, to obtain a high temperature forged alloy, as shown in Figure 8 10 , and it can be known from Figure 8 and 10 that the high temperature forged alloy after homogenization of the present comparative example has cracking and low recrystallization degree.

[0073] The high temperature forging operation parameters are same as example 1.

[0074] The above has described the embodiments of the present application, the above description is exemplary, not exhaustive, and is not limited to the disclosed embodiments. Many modifications and changes are obvious to those skilled in the art without departing from the scope and spirit of the described embodiments.​​

Claims

1. A method for improving the hot working performance of nickel-based high-temperature alloys for molten salt reactors by combining pre-deformation and homogenization, characterized in that: The method comprises the following steps: S1: subjecting the raw material of the nickel-based high-temperature alloy for molten salt reactor to be improved to vacuum induction melting, pouring and electroslag remelting refining in sequence to obtain an electroslag remelting refined alloy ingot; S2: heating, holding, and pre-deforming the electroslag remelting refined alloy ingot in sequence, and air-cooling to room temperature to obtain a pre-deformed ingot; S3: homogenizing the pre-deformed ingot and air-cooling it to room temperature to complete the method.

2. The method for improving the hot working performance of nickel-based high-temperature alloy for molten salt reactor by combining pre-deformation and homogenization according to claim 1, wherein: Calculated by the total weight of the nickel-based high-temperature alloy for molten salt reactors, the alloy includes: C 0.02-0.08%, Al<0.3%, Cr 5.8-7.0%, W 19.0-26.0%, Zr<0.1%, Si<0.5%, Mn<0.8%, Ti 0-0.3%, Nb2.0-4.0%, and the balance is Ni.

3. The method for improving the hot working performance of nickel-based high-temperature alloy for molten salt reactor by combining pre-deformation and homogenization according to claim 1, wherein: The operating conditions of the vacuum induction melting process include: a melting temperature of 1490-1510°C, a vacuum degree of 0.4×10 -2 -0.6×10 -2 Pa, refining temperature is 1540-1560℃, and refining time is 8-10min.

4. The method for improving the hot working performance of nickel-based high-temperature alloy for molten salt reactor by combining pre-deformation and homogenization according to claim 1, wherein: The pouring is performed under inert gas conditions at a temperature of 1450-1490°C.

5. The method of improving the hot working performance of nickel-based high-temperature alloy for molten salt reactor by combining pre-deformation and homogenization according to claim 1, wherein: In step S2, the heating rate is 5-20°C / min, and the temperature is raised to 1150-1220°C and then kept warm for 30min-2h, and then pre-deformation treatment is performed at 1150-1220°C. The deformation range of the pre-deformation treatment is 5%-15%, and the strain rate of the pre-deformation treatment is 0.5-1.5s -1 .

6. The method of improving the hot working performance of nickel-based high-temperature alloy for molten salt reactor by combining pre-deformation and homogenization according to claim 1, wherein: In step S3, the operating conditions of the homogenization treatment include: a heating rate of 5-20° C. / min, a homogenization treatment temperature of 1150-1220° C., and a homogenization treatment holding time of 30 min-2 h.

7. The method for improving the hot working properties of nickel-based high-temperature alloy for molten salt reactor by combining pre-deformation and homogenization according to any one of claims 1 to 6, wherein: After the method is completed, the alloy obtained by the homogenization treatment is subjected to a hot working and high temperature deformation treatment.

Images