Nickel-based alloy for molten salt environment above 800°c and method for manufacturing the same

Abstract

The application relates to alloy technology and discloses a nickel-based alloy for a molten salt environment above 800 DEG C and a preparation method thereof. The inventor of the application finds that a Ni-(26-28)W-6Cr alloy added with 1.0-8.0% of Ta can obviously improve mechanical properties and high-temperature oxidation resistance. The chemical composition of the alloy is as follows: 26.0-28.0% of W, 6.0-8.0% of Cr, 0.1-0.5% of Si, 0-0.6% of Mn, 0-1.0% of Mo, 0-0.6% of Fe, 0.02-0.08% of C, 0-0.2% of Ti, 0-0.1% of Zr, 1.0-8.0% of Ta, and the balance of Ni.

Description

Technical Field

[0001] This application relates to the field of alloy materials technology, and in particular to nickel-based alloys for use in molten salt environments above 800°C. Background Technology

[0002] This section is intended to provide background or context for the embodiments of this application as set forth in the claims. The description herein is not intended to imply that it is prior art that has been disclosed, simply because it is included in this section.

[0003] Molten salts (fluorides and chlorides) have gained attention for their energy applications due to their extremely high thermal stability at high temperatures (>600℃) and low cost. The use of molten salts in energy systems such as molten salt reactors and concentrated solar power (CSP) can significantly increase outlet temperatures for high-efficiency energy conversions such as high-temperature hydrogen production. However, molten salts place extremely high demands on the performance of structural alloys. For example, the structural alloys for molten salt reactors are subjected to extreme operating environments such as high-temperature air, high-temperature molten salt corrosion, and neutron irradiation. Currently, the highest permissible temperature for high-temperature alloys used in molten salt environments (UNS N10003 alloy) is only 704℃, while high-temperature hydrogen production requires energy systems to operate at temperatures above 800℃. Therefore, developing structural alloys that can operate in molten salt environments above 800℃ is a pressing issue that needs to be addressed.

[0004] Chinese patent application number 201510612608.4 discloses a Ni-(26-28)W-6Cr(GH3539) alloy, which is a structural alloy specifically designed for molten salt environments. It has excellent mechanical properties, resistance to molten salt corrosion and radiation resistance, and is currently the most promising structural alloy for use in 800℃ molten salt environments.

[0005] However, this alloy currently has two drawbacks:

[0006] (1) Although the room temperature strength of Ni-(26-28)W-6Cr alloy can reach 930MPa, its strength at 850℃ is low, currently only reaching about 230MPa (see Mater. Sci. Eng. A, 668 (2016) 137.). For structural alloys to be used in molten salt environments, their high temperature strength still needs to be further enhanced.

[0007] (2) In order to obtain good resistance to molten salt corrosion, the Cr content in the alloy is designed to be only 6%, and the high-temperature oxidation resistance at 850℃ is poor (see Corros.Sci.,149(2019)87.). Summary of the Invention

[0008] The purpose of this application is to provide a nickel-based alloy for use in molten salt environments above 800°C and a method for preparing the same, which can improve the mechanical properties and high-temperature oxidation resistance of the alloy.

[0009] This application discloses a nickel-based alloy for use in molten salt environments above 800°C. The chemical composition of the alloy, by weight percentage, is: 26.0-28.0% W, 6.0-8.0% Cr, 0.1-0.5% Si, 0-0.6% Mn, 0-1.0% Mo, 0-0.6% Fe, 0.02-0.08% C, 0-0.2% Ti, 0-0.1% Zr, 1.0-8.0% Ta, with the balance being Ni.

[0010] In a preferred embodiment, the contents of Cr, Si, Mn, C, and Ta in the alloy are:

[0011] 6.0-8.0% Cr, 0.1-0.3% Si, 0.02-0.06% C, 1.0-6.0% Ta.

[0012] In a preferred embodiment, the content of B, N, S, and P in the alloy is less than 20 ppm.

[0013] In a preferred embodiment, the alloy does not contain Cu.

[0014] In a preferred embodiment, the alloy is free of Co.

[0015] In a preferred embodiment, the alloy is Al-free.

[0016] In a preferred embodiment, the method includes the following steps:

[0017] The master alloy is cast using a vacuum induction furnace;

[0018] Homogenization process;

[0019] Hot working.

[0020] In a preferred embodiment, the homogenization process is carried out at a temperature between 1150°C and 1300°C.

[0021] The homogenization process takes between 10 and 30 hours.

[0022] In a preferred embodiment, the processing temperature of the heat treatment is between 900°C and 1250°C.

[0023] In a preferred embodiment, the hot working is forging, hot rolling, or hot extrusion.

[0024] In the embodiments of this application, by adding 1.0-6.0% Ta and improving the proportions of other elements, the alloy achieves better high-temperature mechanical properties and high-temperature oxidation resistance. Compared with existing Ni-(26-28)W-6Cr alloys, the advantages of the nickel-based alloy for molten salt environments above 800°C in this application are: better high-temperature mechanical properties and significantly improved high-temperature oxidation resistance at 850°C. Attached Figure Description

[0025] Figure 1 The phase diagram of Ni-26W-6Cr-0.2Si-0.2Mn-0.03C alloys containing 0-12% Ta was calculated using JMatPro 7.0 (TTNi-8 database);

[0026] Figure 2 This is a microscopic morphology diagram of Example 3;

[0027] Figure 3 These are the static oxidation weight gain kinetic curves of Examples 1, 4, and 5 at 850℃ for 100 hours.

[0028] Figure 4 These are cross-sectional morphology and elemental distribution diagrams of statically oxidized samples from Examples 1, 4, and 5, taken at 850℃ for 100 hours. Detailed Implementation

[0029] Through long-term and in-depth research, the inventors of this application have discovered that adding 1.0-8.0% Ta to Ni-(26-28)W-6Cr alloy can significantly improve its mechanical properties and high-temperature oxidation resistance.

[0030] The service life of high-temperature alloys used in molten salt environments is expected to be more than ten years. They face extremely harsh service environments such as high temperature, strong neutron irradiation, and molten salt corrosion. Insufficient mechanical properties above 800℃ have become the biggest obstacle to improving the operating temperature of molten salt energy systems. Due to long-term service in high-temperature environments, the grains will coarsen during long-term service, resulting in a weakening of the strengthening effect. Therefore, fine-grain strengthening is not suitable for structural alloys used in molten salt environments. As for the second-phase strengthening effect, after systematic research, the optimal range of C content was determined to be 0.02-0.06%. Therefore, this application further improves the mechanical properties of the alloy from the perspective of solid solution strengthening. The main solid solution elements in Ni-(26-28)W-6Cr alloy are W and Cr. Among them, 6-8% Cr can better balance the alloy's resistance to high-temperature oxidation and resistance to molten salt corrosion. However, the solid solution limit of W+Cr in Ni is about 44%, so the W content cannot be further increased. The atomic radii of Ni, W, and Cr (see J Chem. Phys. 47 (1967) 1300–1307.) are 149 pm, 193 pm, and 166 pm, respectively, and the atomic radius of Ta is 200 pm. The inventors of this application have found that Ta has a stronger solid solution strengthening effect than W and Cr, and has a larger solid solubility in Ni, which can play a very good solid solution strengthening role.

[0031] Regarding high-temperature oxidation resistance, the inventors of this application discovered that elements commonly used in high-temperature alloys to improve high-temperature oxidation resistance, such as Cr, Al, and Si, are easily corroded in molten salt. Therefore, the content of these elements is strictly controlled, resulting in poor high-temperature oxidation resistance of the Ni-(26-28)W-6Cr alloy at 850℃. Research revealed that the main oxide component in the Ni-(26-28)W-6Cr alloy is NiWO4, where Ni has a valence state of +2. According to the Hauffe-Wagner theory, if alloying elements with valence states higher than +2 are doped into the alloy, the number of anion vacancies in NiWO4 will decrease, and the diffusion of cations will be reduced. Ta commonly has valence states of +4 to +6; adding Ta to the Ni-(26-28)W-6Cr alloy can improve its high-temperature oxidation resistance.

[0032] like Figure 1 As shown, according to the calculation results of JMatPro 7.0 (TTNi-8 database), Ta has a large solid solubility in the Ni-26W-6Cr-0.2Si-0.2Mn-0.03C alloy system. When its content does not exceed 9%, phases such as Laves and P that are harmful to high-temperature mechanical properties will not appear in the alloy.

[0033] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Experimental methods in the following embodiments that do not specify specific conditions are generally performed under conventional conditions or as recommended by the manufacturer. Unless otherwise specified, percentage contents mentioned in this application refer to weight percentage contents.

[0034] The nickel-based alloy for molten salt environments above 800°C according to embodiments of this application comprises the following chemical composition (by weight percentage): 26.0-28.0% W, 6.0-8.0% Cr, 0.1-0.5% Si, 0-0.6% Mn, 0-1.0% Mo, 0-0.6% Fe, 0.02-0.08% C, 0-0.2% Ti, 0-0.1% Zr, 1.0-8.0% Ta, with the balance being Ni.

[0035] Preferably, the alloy contains 6.0-8.0% Cr, 0.1-0.3% Si, 0-0.5% Mn, 0.02-0.06% C, and 1.0-6.0% Ta.

[0036] Preferably, the content of B, N, S, and P in the alloy is less than 20 ppm.

[0037] Preferably, the alloy is free of Cr, Co and Al.

[0038] Preferably, the alloy does not contain Co.

[0039] The preparation method of this alloy includes the following steps:

[0040] The master alloy is cast using a vacuum induction furnace;

[0041] Homogenization process;

[0042] Hot working.

[0043] Preferably, in one embodiment, the homogenization treatment temperature is between 1150°C and 1300°C; the homogenization treatment time is between 10 hours and 30 hours.

[0044] Preferably, in one embodiment, the processing temperature for hot working is between 900°C and 1250°C.

[0045] Optionally, in one embodiment, the hot working is forging, hot rolling, or hot extrusion.

[0046] The following seven examples within the above-mentioned composition range provide a detailed description of the performance of the nickel-based alloy for molten salt environments above 800°C provided by the present invention.

[0047] The alloys in these embodiments were obtained by mixing a Ni-26W-6Cr master alloy (composition as shown in Example 1) with high-purity Ta (99.99%) and other high-purity elements (all with purities higher than 99.95%) in different proportions, melting them in a vacuum induction furnace, and then performing subsequent heat treatment and processing. The chemical composition of the resulting alloys is shown in Table 1. Example 1 in Table 1 is the chemical composition of a prior art Ni-26W-6Cr master alloy without added Ta. Example 1 serves as a comparison object with the alloys of the present invention (Examples 2-7) to demonstrate the performance of the alloys of the present invention.

[0048] Table 1. Chemical composition (weight percentage) of the Ni-26W-6Cr alloy in the examples and comparisons.

[0049]

[0050] Figure 2 The microstructure of Example 3 shows that the average diameter of the alloy grains is about 60 μm, and the fine carbides are mainly distributed at the grain boundaries. There are no other precipitates besides the carbides.

[0051] Table 2 shows the room temperature tensile properties of Examples 1-7. When the amount of Ta added is 1.0-6.0%, the yield strength and tensile strength of the alloy are significantly improved, while the elongation does not change much. When the amount of Ta added is 8.0%, the strength of the alloy decreases slightly, but it is still higher than that of the Ni-26W-6Cr alloy without Ta (Example 1).

[0052] Table 2 Comparison of room temperature tensile mechanical properties of Examples 1-9

[0053] Sample Name Temperature (°C) Yield strength (MPa) Tensile strength (MPa) Elongation (%) Example 1 25 315 864 52.9 Example 2 25 351 878 53.1 Example 3 25 422 945 53.3 Example 4 25 562 1146 51.6 Example 5 25 587 1187 52.7 Example 6 25 465 971 49.1 Example 7 25 367 881 51.3

[0054] Table 3 shows the high-temperature (700-850℃) tensile properties of Examples 1-7. The mechanical properties of the Ta-added alloy are significantly stronger than those of the Ni-26W-6Cr alloy, and the trend is similar to that of room temperature tensile testing.

[0055] Table 3 Comparison of high-temperature (700-850℃) tensile mechanical properties of Examples 1-7

[0056]

[0057] Figure 3 The static oxidation kinetic curves for Examples 1, 4, and 5 at 850℃ for 100h show that the oxidation weight gain of the Ni-26W-6Cr alloy is significantly reduced after the addition of Ta. Figure 4The figures show the cross-sectional morphology and corresponding elemental distribution of the oxide samples from Examples 1, 4, and 5. It can be seen that the oxide film thickness of Example 1 (without Ta), a prior art example, is 41.2 μm, while the oxide film depths of Example 4 (containing 4% Ta) and Example 5 (containing 6% Ta) are 20.3 μm and 17.8 μm, respectively. The oxide depth of the alloy with added Ta is significantly reduced, and the addition of Ta mainly inhibits the formation of the inner oxide layer NiWO4, with little effect on the thickness of the outer oxide layer NiO. The elemental distribution diagram shows that Ta is enriched in the inner oxide NiWO4, effectively hindering the diffusion of oxygen into the matrix, thereby improving the alloy's high-temperature oxidation resistance.

[0058] All references to this application are incorporated herein by reference as if each reference were individually incorporated herein by reference. Furthermore, it should be understood that after reading the foregoing teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

Claims

1. A nickel-based alloy for use in molten salt environments above 800°C, characterized in that, The chemical composition of the alloy is, in percentage by weight: 26.2-26.4% of W, 6.2-6.3% of Cr, 0.12-0.13% of Si, 0.43-0.45% of Mn, 0.33-0.35% of Mo, 0.22-0.23% of Fe, 0.038-0.056% of C, 0.10-0.11% of Ti, 0.03-0.04% of Zr, 4.0-6.0% of Ta, the balance being Ni, wherein Ta is enriched in the internal oxide NiWO4, effectively hindering the diffusion of oxygen to the matrix.

2. The nickel-base alloy for above-800 °C molten salt environments of claim 1, wherein, The content of B, N, S, P in the alloy is less than 20 ppm.

3. The nickel-base alloy for above-800 °C molten salt environments of claim 1, wherein, The alloy does not contain Cu.

4. The nickel-base alloy for above-800 °C molten salt environments of claim 1, wherein, The alloy does not contain Co.

5. The nickel-base alloy for above-800 °C molten salt environments of claim 1, wherein, The alloy does not contain Al.

6. A method for producing a nickel-based alloy for use in a molten salt environment at 800°C or higher according to any one of claims 1 to 5, characterized by, The method comprises the following steps: casting the master alloy using a vacuum induction furnace; homogenization treatment; hot working.

7. The method of producing an alloy according to claim 6, wherein The treatment temperature of the homogenization treatment is between 1150°C and 1300°C; The treatment time of the homogenization treatment is between 10 hours and 30 hours.

8. The method of producing an alloy according to claim 6, wherein The working temperature of the hot working is between 900°C and 1250°C.

9. The method of producing an alloy according to claim 6, wherein The hot working is forging, hot rolling or hot extrusion.

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