Nickel-tungsten intermediate alloy and multi-step heat treatment preparation method thereof

Abstract

The invention discloses a multi-step heat treatment preparation method of a nickel-tungsten intermediate alloy. The method comprises the following steps: 1, selecting nickel powder and tungsten powder with high purity, low oxygen content and specific particle size as raw materials; 2, uniformly mixing nickel powder, tungsten powder and absolute ethyl alcohol to obtain mixed powder; 3, performing cold isostatic pressing on the mixed powder to prepare a biscuit; 4, performing sintering and multi-step heat treatment on the biscuit to obtain a sintered body; and fifthly, the nickel-tungsten intermediate alloy is obtained after machining. By strictly controlling sintering and a multi-step heat treatment process, a solid solution phase of nickel is promoted to react with tungsten, a reaction product continues to react with tungsten, ablation of a high-melting-point and high-density elementary substance tungsten phase in the alloy is achieved, uniform distribution of nickel and tungsten elements is promoted, preparation of the low-cost, high-efficiency and high-purity nickel-tungsten intermediate alloy is achieved, and the method is suitable for industrial production. And the final product meets the application requirement in nickel-based high-temperature alloy smelting, and the method is simple and easy to implement.

Description

Technical Field

[0001] The invention belongs to the technical field of alloy preparation, and in particular relates to a nickel-tungsten master alloy and a multi-step heat treatment preparation method thereof. Background Art

[0002] Nickel-based superalloys have excellent structural stability, oxidation resistance, hot corrosion resistance and high-temperature mechanical properties, making them widely used in aerospace, shipbuilding and other fields. With the improvement of nickel-based superalloy generations, the composition of elements in the alloy has also changed significantly, and the temperature bearing capacity of the alloy has also gradually improved.

[0003] At present, refractory elements are indispensable in commercially available nickel-based high-temperature alloys, among which the content of tungsten is 3~7wt%. After the addition of tungsten, under the combined effect of solid solution strengthening and carbide grain boundary dispersion strengthening mechanism, the nickel-based high-temperature alloy exhibits excellent high-temperature mechanical properties. Compared with nickel, tungsten has a large atomic radius (135pm), a high melting point (3422℃), and a high density (19.26g / cm 3 ), the liquid surface tension is large (σ W =2300mN / m, T=3695K), resulting in serious tungsten segregation in the nickel-based high-temperature alloy prepared by the smelting method, which has an adverse effect on the performance of the final product.

[0004] In order to facilitate the addition of high melting point tungsten elements, prevent metal overheating, shorten the smelting time, reduce metal burning, and obtain a nickel-based high temperature alloy with uniform and accurate composition, the application of nickel-tungsten master alloy is of great significance. According to the nickel-tungsten binary alloy phase diagram, the nickel-tungsten master alloy with a tungsten content of less than 62wt% has a melting point of 1455℃~1655℃, which is close to the melting temperature of pure nickel and is suitable for the smelting of nickel-based high temperature alloys.

[0005] At present, there are four methods for preparing nickel-tungsten master alloys: vacuum induction melting method, electron beam melting method, aluminothermic method outside the furnace, and powder sintering method. The invention patents with publication numbers CN106756243A, CN109182843A, and CN110358947A respectively disclose the technology of preparing nickel-tungsten master alloys by vacuum induction melting method, electron beam melting method, and aluminothermic method outside the furnace. Among them, the nickel-tungsten master alloy prepared by the melting method is prone to tungsten element segregation, and has high energy consumption and cost; the nickel-tungsten master alloy prepared by the aluminothermic method outside the furnace has poor density, unstable composition, and is easy to introduce impurity elements such as aluminum and oxygen.

[0006] Compared with the above-mentioned smelting method and the aluminothermic method outside the furnace, the nickel-tungsten master alloy prepared by the powder sintering method has the advantages of uniform composition and low cost. The invention patents with publication numbers CN109825752A and CN102312132A have successively disclosed the technology of preparing nickel-tungsten master alloy by vacuum high-temperature sintering. The tungsten powder used in the former accounts for 38wt%~55wt% of the total weight of the alloy, and the sintering temperature is 800℃~1450℃, but due to the large particle size of the selected powder and the lack of carbon and oxygen isolation measures during the alloy sintering process, the composition distribution in the final product is uneven, and the content of impurity elements such as carbon, oxygen, and nitrogen exceeds the standard. By comparing the processes, it can be seen that the latter adds a ball milling process compared to the former, thereby reducing the particle size of the nickel-tungsten powder, promoting the formation of the nickel-tungsten master alloy, and ensuring the uniformity of the mixing of the nickel-tungsten powder; however, the production capacity of the ball milling process limits its large-scale application, and the mutual collision between the grinding balls during the ball milling process will also introduce additional impurities into the alloy. At the same time, it is difficult to eliminate the single tungsten phase at a sintering temperature of 1250℃~1350℃. In order to eliminate the single tungsten phase, the invention patent with publication number CN102329973A adds a high-temperature smelting process on the basis of patent CN102312132A. Although the single tungsten phase in the nickel-tungsten alloy is successfully eliminated by the high-temperature smelting method, the cost is also greatly increased. Summary of the invention

[0007] The technical problem to be solved by the present invention is to provide a multi-step heat treatment preparation method for a nickel-tungsten master alloy in view of the deficiencies of the above-mentioned prior art. The method comprises the following steps: mixing nickel powder and tungsten powder to form a compact, and then sequentially performing sintering and multi-step heat treatment. By strictly controlling the sintering and multi-step heat treatment processes, the ablation of the high melting point and high density single tungsten phase in the alloy is achieved, and the uniform distribution of the nickel and tungsten elements is promoted, thereby achieving the preparation of a low-cost and high-efficiency nickel-tungsten master alloy, and solving the problems of the high cost of the existing preparation method, the uneven composition of the nickel-tungsten master alloy, the easy segregation, and the high impurity elements.

[0008] In order to solve the above technical problems, the technical solution adopted by the present invention is: a multi-step heat treatment preparation method of nickel-tungsten master alloy, characterized in that the method comprises the following steps: Step 1. Raw material selection: Select nickel powder with a mass purity of more than 99.9%, a particle size of less than 100μm, and an oxygen content of no more than 1000ppm, and tungsten powder with a mass purity of more than 99.9%, a particle size of less than 25μm, and an oxygen content of no more than 1000ppm as raw materials; Step 2, powder mixing: according to the composition of the target product nickel-tungsten master alloy, weigh the nickel powder and tungsten powder in step 1 and mix them with anhydrous ethanol in a mixer to obtain a mixed powder; Step 3, green blank molding: the mixed powder obtained in step 2 is dried and loaded into a rubber mold, and after vacuum packaging, it is placed in a cold isostatic press and maintained at a pressure of 200MPa to 300MPa for 10 minutes to obtain a green blank; Step 4, sintering and multi-step heat treatment: placing the green blank obtained in step 3 in a vacuum sintering furnace, sintering at a vacuum degree of 0.001Pa~0.05Pa and a temperature of 1400℃~1600℃ for 2h, then performing multi-step heat treatment, first cooling to 1030℃~1060℃ for 1h~3h, then cooling to 1005℃~1025℃ for 1h~3h, cooling to room temperature with the furnace, and obtaining a sintered body; Step 5: Machining: The sintered body obtained in step 4 is machined to remove 1 mm to 2 mm of the surface to obtain a nickel-tungsten master alloy.

[0009] The above-mentioned multi-step heat treatment preparation method of a nickel-tungsten master alloy is characterized in that the particle size ratio of the nickel powder to the tungsten powder in step one is 1~2:1. During the powder mixing process, the particle size, density and content of different powders will affect the uniformity of powder mixing. According to the practical experience of powder mixing, when the product of the particle size, density and content of different powders is close, the mixing uniformity of the powder is better. At the same time, because the diffusion rate of tungsten (W) in nickel (Ni) at the same temperature is much higher than the diffusion rate of nickel in tungsten (the diffusion coefficient of tungsten in nickel at 1450°C is 1.2×10 -13 m 2 / s~1.6×10 - 13 m 2 / s, which is much higher than its diffusion coefficient in tungsten, 1.5×10 -13 m 2 / s~3.0×10 -22 m 2 / s), the present invention controls the particle size ratio of nickel powder to tungsten powder, on the one hand, to ensure that the nickel powder and tungsten powder are fully mixed, and on the other hand, to ensure that the tungsten powder particles are smaller than the nickel powder particles, so that in a short time during the subsequent sintering process, the tungsten element can be fully dissolved in the elemental nickel and melted, which significantly improves the diffusion capacity of the tungsten element, ensures the uniform distribution of the high melting point elemental tungsten phase in the product nickel-tungsten intermediate alloy, and promotes the full solid solution of the excess elemental tungsten phase in the nickel matrix.

[0010] The above-mentioned multi-step heat treatment preparation method of a nickel-tungsten master alloy is characterized in that, in the process of mixing powders in step 2, firstly, nickel powder and anhydrous ethanol accounting for 5% of the mass of nickel powder are placed in a mixer, and the barrel speed is 20 rpm to 60 rpm for 30 min, and tungsten powder is added after standing for 15 min, and the barrel speed is continued to be 20 rpm to 60 rpm for 3 h to 5 h. The present invention uses anhydrous ethanol as a binder, and fully spreads anhydrous ethanol on the surface of nickel powder through a first powder mixing, and then uses a second powder mixing to achieve the bonding of small-particle tungsten powder on the surface of nickel powder. At the same time, the addition of anhydrous ethanol reduces the fluidity of the powder, thereby achieving the purpose of maintaining the uniformity of the mixed powder, and then ensuring the uniform distribution of nickel-tungsten elements and high-melting-point single tungsten phases in the product nickel-tungsten master alloy.

[0011] The above-mentioned multi-step heat treatment preparation method of a nickel-tungsten master alloy is characterized in that after the mixed powder is prepared in step 2, the blank forming process in step 3 is completed within 8 hours. The present invention limits the time between the mixing of nickel powder and tungsten powder and the cold isostatic pressing, thereby avoiding the particle segregation phenomenon of the high-density tungsten powder due to long-term standing, thereby destroying the mixing uniformity of the nickel-tungsten alloy mixed powder, further affecting the forming of the blank and the uniformity of the alloy composition, and causing the segregation of the high-melting-point single tungsten phase in the alloy.

[0012] The above-mentioned multi-step heat treatment preparation method of a nickel-tungsten master alloy is characterized in that, during the sintering described in step 4, a tantalum sheet with a mass purity of more than 99% is first used to wrap the blank, and a sponge titanium with a mass purity of more than 99.5% is placed in the furnace chamber of the vacuum sintering furnace. The present invention is based on the oxide free energy diagram and the relevant binary phase diagram, and selects tantalum sheets and sponge titanium to isolate the contamination of nickel-tungsten alloys by impurities such as carbon and oxygen. According to the oxide free energy diagram, compared with nickel and tungsten, titanium and tantalum are more likely to react with elements such as carbon, oxygen, and sulfur to form carbides, oxides, sulfides, etc.; according to the binary phase diagram, nickel and titanium can generate nickel-titanium intermediate compounds on the titanium-rich side at a temperature of 942°C. Therefore, the present invention selects tantalum sheets that do not react with nickel and tungsten to wrap the blank, and selects sponge titanium that is easy to react with elements such as carbon, oxygen, and sulfur to absorb impurity gases in the furnace, thereby ensuring the purity of the nickel-tungsten master alloy after sintering.

[0013] The above-mentioned multi-step heat treatment preparation method of a nickel-tungsten master alloy is characterized in that, during the multi-step heat treatment process described in step 4, an infrared thermometer is used in combination with a K-type thermocouple equipped with a vacuum sintering furnace to monitor and control the heat treatment temperature of each step. Since the temperature range of the multi-step heat treatment is too narrow, the present invention uses an infrared thermometer combined with a K-type thermocouple method equipped with a sintering furnace to monitor and control the heat treatment temperature of each step, and uses manual auxiliary control of the heat treatment temperature to ensure the accuracy of the heat treatment temperature.

[0014] The present invention Figure 1Based on the Ni-W binary alloy phase diagram shown in the figure, the sintering and heat treatment process of nickel-tungsten master alloy is designed. Specifically, the green billet formed by cold isostatic pressing is first sintered at a vacuum of 0.001Pa~0.05Pa and a temperature of 1400℃~1500℃ for 2h. The large solubility (about 40wt%) and diffusion capacity of tungsten in nickel at high temperature are used to promote the formation of nickel-tungsten solid solution phase, thereby ensuring the ablation of most of the tungsten element; at the same time, according to Figure 1 From the Ni-W binary alloy phase diagram, it can be seen that although the content of high-melting-point elemental tungsten is extremely small (about 1wt%~5wt%) in the temperature range of 1068℃~1495℃, elemental tungsten is a thermodynamically stable phase and it is difficult to eliminate it simply by high-temperature heat treatment and sintering. In order to eliminate the small amount of elemental tungsten phase remaining after the nickel-tungsten master alloy is sintered at high temperature, the present invention needs to continue to perform multiple steps of heat treatment, and the heat treatment temperature needs to be controlled to 1002℃~1068℃ to ensure that elemental tungsten reacts with the solid solution phase of nickel (1068℃) and the NiW phase (1027℃), thereby realizing the consumption of high-temperature residual elemental tungsten. Specifically, the present invention first cools down to 1030℃~1060℃ and keeps warm for 1h~2h to ensure that the nickel solid solution phase reacts with the remaining tungsten components to generate a NiW phase with a lower melting point, and then cools down to 1005℃~1025℃ and keeps warm for 1h~2h to ensure that the NiW phase fully reacts with the remaining tungsten to generate a NiW2 phase with a lower melting point. Since the reactant NiW phase in the latter step reaction is the product in the previous step reaction, ensuring the formation of the NiW phase through multi-step heat treatment is the key to achieving the ablation of excess single tungsten phase in the alloy. Then, in the process of cooling to room temperature with the furnace, the NiW phase and NiW2 phase formed by heat treatment will react with the nickel solid solution phase and form the Ni4W phase. Finally, all the excess single tungsten phases in the alloy are completely ablated, a part of which is solid-dissolved in the single nickel, and the other part reacts with nickel to generate the NiW phase, NiW2 phase or Ni4W phase, avoiding the residual single W phase.

[0015] In addition, the present invention also discloses a nickel-tungsten master alloy prepared by the method as described above, characterized in that the element mass fraction of the nickel-tungsten master alloy is: Ni 40%~62%, and the balance is W and inevitable impurities, the inevitable impurities include non-metallic impurities O, C, S with a mass fraction of less than 0.01%, and metal impurities Al, Mo, Pb, Sn, Ta, Ti with a mass fraction of less than 0.01%, and the phase composition of the nickel-tungsten master alloy includes Ni4W phase and other nickel-tungsten phases, and other nickel-tungsten phases are derived from nickel solid solution phase, NiW phase and NiW2 phase, and the nickel-tungsten elements are evenly distributed in the nickel-tungsten master alloy without tungsten element segregation.

[0016] In addition, the present invention also discloses a nickel-tungsten master alloy prepared by the method as described above, characterized in that the element mass fraction of the nickel-tungsten master alloy is: Ni 40%~62%, and the balance is W and inevitable impurities, the inevitable impurities include non-metallic impurities O, C, S with a mass fraction of less than 0.01%, and metal impurities Al, Mo, Pb, Sn, Ta, Ti with a mass fraction of less than 0.01%, and the phase composition of the nickel-tungsten master alloy includes Ni4W phase and other nickel-tungsten phases, and other nickel-tungsten phases are derived from nickel solid solution phase, NiW phase and NiW2 phase, and the nickel-tungsten elements are evenly distributed in the nickel-tungsten master alloy without tungsten element segregation. The present invention comprehensively considers the melting point of the alloy and the subsequent multi-step heat treatment process characteristics, and controls the Ni mass fraction in the nickel-tungsten master alloy to be no more than 62% to ensure that its melting characteristics can meet the requirements of the smelting process; at the same time, during the research process of the present invention, it is found that when the Ni mass fraction in the nickel-tungsten master alloy is lower than 40%, W can be completely dissolved in the Ni matrix after high-temperature sintering, and there is no need to adopt the subsequent multi-step heat treatment steps. When the Ni mass fraction in the nickel-tungsten master alloy is more than 40%, W residue is inevitably generated after high-temperature sintering, and the subsequent multi-step heat treatment steps of the present invention can effectively eliminate the W residue; therefore, the present invention controls the element mass fraction of the nickel-tungsten master alloy to be: Ni 40%-62%, and the balance is W and inevitable impurities, which not only meets the restriction requirements of the alloy melting point on the components, but also adapts to the specific requirements of the subsequent multi-step heat treatment process on the Ni content.

[0017] Compared with the prior art, the present invention has the following advantages: 1. The present invention selects nickel powder and tungsten powder, mixes them and presses them into green compacts, and then sequentially performs sintering and multi-step heat treatment. By strictly controlling the sintering and multi-step heat treatment processes, the ablation of the high melting point and high density single substance tungsten phase in the alloy is achieved, and the uniform distribution of the nickel and tungsten elements is promoted, thereby achieving the preparation of a low-cost and high-efficiency nickel-tungsten master alloy, and the final product meets the application requirements in the smelting of nickel-based high-temperature alloys.

[0018] 2. Compared with the disadvantages of increased cost, reduced efficiency and decreased alloy purity of powder mixing using a ball mill in the prior art, the present invention ensures the uniformity of powder mixing and the density of the formed blank by controlling the particle size and purity of nickel powder and tungsten powder, adding a wetting agent, and controlling the powder mixing and molding process. The nickel-tungsten intermediate alloy is prepared by combining the powder sintering process to ensure the composition uniformity and controllability of the nickel-tungsten intermediate alloy. The method is simple, more economical and efficient, and at the same time avoids the serious tungsten element segregation caused by the large atomic radius, high melting point, high density and high liquid surface tension of the tungsten element during the smelting process.

[0019] 3. During the sintering process of the present invention, tantalum sheets are used to wrap and place titanium sponge, thereby isolating the alloy from possible pollution by elements such as carbon, sulfur, and oxygen in the sintering environment, thereby ensuring the purity of the nickel-tungsten master alloy.

[0020] 4. Compared with the prior art that directly cools to room temperature after high-temperature sintering, resulting in residual high-melting-point tungsten phase in the alloy, the present invention adds a multi-step heat treatment process after high-temperature sintering. Through the solid solution phase reaction of elemental tungsten and nickel and the continued reaction of the reaction product NiW phase with elemental tungsten, the excess high-melting-point tungsten phase in the alloy is melted, thereby obtaining a high-quality nickel-tungsten intermediate alloy.

[0021] 5. Compared with the disadvantage of the prior art that the aluminothermic method outside the furnace is easy to introduce impurity elements, the powder sintering process of the present invention does not use aluminum powder as a reducing agent, and the sintering process is in a high vacuum state, which effectively avoids the introduction of impurity elements. Therefore, the nickel-tungsten intermediate alloy prepared by the method of the present invention has lower content of aluminum, carbon, oxygen, sulfur and other elements and higher purity.

[0022] 6. The physical phase of the nickel-tungsten master alloy of the present invention is composed of Ni4W phase, and one or more of nickel solid solution phase, NiW phase and NiW2 phase. The nickel and tungsten elements in the nickel-tungsten master alloy are evenly distributed without tungsten element segregation, and is suitable for the preparation of nickel-based high-temperature alloys.

[0023] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is the phase diagram of Ni-W binary alloy.

[0025] Figure 2 This is the XRD spectrum of the green billet prepared in Example 1 of the present invention after sintering and then directly furnace-cooling to room temperature.

[0026] Figure 3 The XRD spectrum of the nickel-tungsten master alloy prepared in Example 1 of the present invention.

[0027] Figure 4a This is a secondary electron image of the nickel-tungsten master alloy prepared in Example 1 of the present invention.

[0028] Figure 4b This is a scanning diagram of the Ni element energy spectrum of the nickel-tungsten master alloy prepared in Example 1 of the present invention.

[0029] Figure 4c This is a scanning diagram of the W element energy spectrum of the nickel-tungsten master alloy prepared in Example 1 of the present invention. DETAILED DESCRIPTION

[0030] Example 1 The preparation method of this embodiment comprises the following steps: Step 1. Raw material selection: Select nickel powder with a mass purity of more than 99.9%, a particle size of 1 μm, and an oxygen content of no more than 1000 ppm, and tungsten powder with a mass purity of more than 99.9%, a particle size of 1 μm, and an oxygen content of no more than 1000 ppm as raw materials; Step 2, powder mixing: according to the composition of the target product nickel-tungsten master alloy Ni-38wt%W, nickel powder and anhydrous ethanol accounting for 5% of the mass of nickel powder are weighed and placed in a mixer, and the mixer is operated at a barrel speed of 20 rpm for 30 minutes. After standing for 15 minutes, tungsten powder is added, and the mixer is operated at a barrel speed of 20 rpm for 3 hours; Step 3, green blank molding: the mixed powder obtained in step 2 is placed in an oven at 120°C for 30 minutes for drying, then loaded into a rubber mold, and after vacuum packaging, placed in a cold isostatic press, and maintained at a pressure of 300 MPa for 10 minutes to obtain a green blank; After the mixed powder is prepared in step 2, the green blank forming process in step 3 is completed within 8 hours; Step 4, sintering and multi-step heat treatment: placing the green blank obtained in step 3 in a vacuum sintering furnace, wrapping the green blank with a tantalum sheet with a mass purity of more than 99%, placing a sponge titanium with a mass purity of more than 99.5% in the furnace chamber of the vacuum sintering furnace, and then sintering at a vacuum degree of 0.001Pa~0.05Pa and a temperature of 1400°C for 2h, followed by multi-step heat treatment, first cooling to 1030°C for 1h, then cooling to 1005°C for 1h, and cooling to room temperature with the furnace to obtain a sintered body; during the multi-step heat treatment process, an infrared thermometer combined with a K-type thermocouple equipped with the vacuum sintering furnace is used to monitor and control the heat treatment temperature of each step; Step 5: Machining: The sintered body obtained in step 4 is machined to remove a 1 mm thick surface to obtain a nickel-tungsten master alloy.

[0031] Figure 2 The XRD pattern of the green blank prepared in this embodiment after sintering and direct furnace cooling to room temperature is as follows: Figure 2 It can be seen that in addition to the main phases Ni4W, NiW and other intermediate compounds, there is still a minor phase W phase, indicating that the single substance W phase is in a thermodynamically stable state at high temperature.

[0032] Figure 3 The XRD pattern of the nickel-tungsten master alloy prepared in this embodiment is as follows: Figure 3 It can be seen that the peak of the W phase in the figure has disappeared, indicating that the W element in the nickel-tungsten master alloy has been completely dissolved in the Ni phase, or has undergone an alloying reaction with Ni, that is, the single W phase has been completely dissolved.

[0033] Figure 4aThis is the secondary electron image of the nickel-tungsten master alloy prepared in this example. Figure 4b~Figure 4c The energy spectrum scanning diagram of Ni and W elements of the nickel-tungsten master alloy prepared in this embodiment is shown in FIG. Figure 4a~Figure 4c It can be seen that the nickel and tungsten elements are evenly distributed in the figure, and there is no segregation or residue of the high-melting-point elemental tungsten, which indicates the high homogeneity of the nickel-tungsten master alloy, that is, the nickel-tungsten elements are evenly distributed without segregation.

[0034] The composition of the nickel-tungsten master alloy prepared in this embodiment was tested, and the results are shown in Table 1 below.

[0035] Table 1

[0036] As can be seen from Table 1, the impurity elements such as Al, Mo, Pb, Si, Sn, Ta, and Ti in the nickel-tungsten master alloy prepared in this embodiment are very low, indicating that the present invention effectively improves the purity of the nickel-tungsten master alloy by controlling the purity and particle size of the raw material tungsten powder and nickel powder; at the same time, the impurity elements such as C, O, and S in the nickel-tungsten master alloy are much lower than 0.01wt%, indicating that the present invention adopts tantalum sheet wrapping, sponge titanium placement and other means to effectively suppress the pollution of the sintering environment (graphite furnace chamber, air, etc.) to the nickel-tungsten master alloy product.

[0037] Example 2 The preparation method of this embodiment comprises the following steps: Step 1. Raw material selection: Select nickel powder with a mass purity of more than 99.9%, a particle size of 2 μm, and an oxygen content of no more than 1000 ppm, and tungsten powder with a mass purity of more than 99.9%, a particle size of 1 μm, and an oxygen content of no more than 1000 ppm as raw materials; Step 2, powder mixing: according to the composition of the target product nickel-tungsten master alloy Ni-50wt%W, nickel powder and anhydrous ethanol accounting for 5% of the mass of nickel powder are weighed and placed in a mixer, and the mixer is operated at a barrel speed of 20 rpm for 30 min. After standing for 15 min, tungsten powder is added, and the mixer is operated at a barrel speed of 40 rpm for 4 h. Step 3, green blank molding: the mixed powder obtained in step 2 is placed in an oven at 120°C for 30 minutes for drying, then loaded into a rubber mold, and after vacuum packaging, placed in a cold isostatic press, and maintained at a pressure of 250 MPa for 10 minutes to obtain a green blank; After the mixed powder is prepared in step 2, the green blank forming process in step 3 is completed within 8 hours; Step 4, sintering and multi-step heat treatment: placing the green blank obtained in step 3 in a vacuum sintering furnace, wrapping the green blank with a tantalum sheet with a mass purity of more than 99%, placing a titanium sponge with a mass purity of more than 99.5% in the furnace chamber of the vacuum sintering furnace, and then sintering at a vacuum degree of 0.001Pa~0.05Pa and a temperature of 1500°C for 2h, followed by multi-step heat treatment, first cooling to 1045°C for 2h, then cooling to 1015°C for 2h, and cooling to room temperature with the furnace to obtain a sintered body; during the multi-step heat treatment process, an infrared thermometer combined with a K-type thermocouple equipped with the vacuum sintering furnace is used to monitor and control the heat treatment temperature of each step; Step 5: Machining: The sintered body obtained in step 4 is machined to remove a surface with a thickness of 1.5 mm to obtain a nickel-tungsten master alloy.

[0038] The nickel-tungsten master alloy prepared in this embodiment was tested for composition, and the results are shown in Table 2 below.

[0039] Table 2

[0040] As can be seen from Table 2, the impurity elements such as Al, Mo, Pb, Si, Sn, Ta, and Ti in the nickel-tungsten master alloy prepared in this embodiment are very low, indicating that the present invention effectively improves the purity of the nickel-tungsten master alloy by controlling the purity and particle size of the raw material tungsten powder and nickel powder; at the same time, the impurity elements such as C, O, and S in the nickel-tungsten master alloy are much lower than 0.01wt%, indicating that the present invention adopts tantalum sheet wrapping, sponge titanium placement and other means to effectively suppress the pollution of the sintering environment (graphite furnace chamber, air, etc.) to the nickel-tungsten master alloy product.

[0041] After testing, the physical phase of the nickel-tungsten master alloy prepared in this embodiment consists of a nickel solid solution phase, a Ni4W phase and a NiW phase, and the nickel and tungsten elements are evenly distributed in the nickel-tungsten master alloy, and there is no segregation of tungsten elements. The reason is that the physical phase composition of the nickel-tungsten master alloy includes the Ni4W phase and the NiW phase in the equilibrium state, but due to the influence of kinetic factors, complete thermodynamic equilibrium cannot be achieved. Therefore, the final physical phase of the nickel-tungsten master alloy consists of three phases: a nickel solid solution phase, a Ni4W phase and a NiW phase.

[0042] Example 3 The preparation method of this embodiment comprises the following steps: Step 1. Raw material selection: Select nickel powder with a mass purity of more than 99.9%, a particle size of 1 μm, and an oxygen content of no more than 1000 ppm, and tungsten powder with a mass purity of more than 99.9%, a particle size of 1 μm, and an oxygen content of no more than 1000 ppm as raw materials; Step 2, powder mixing: according to the composition of the target product nickel-tungsten master alloy Ni-60wt%W, nickel powder and anhydrous ethanol accounting for 5% of the mass of nickel powder are weighed and placed in a mixer, and the mixer is operated at a barrel speed of 20 rpm for 30 min. After standing for 15 min, tungsten powder is added, and the mixer is operated at a barrel speed of 60 rpm for 5 h. Step 3, green blank molding: the mixed powder obtained in step 2 is placed in an oven at 120°C for 30 minutes for drying, then loaded into a rubber mold, and after vacuum packaging, placed in a cold isostatic press, and maintained at a pressure of 200 MPa for 10 minutes to obtain a green blank; After the mixed powder is prepared in step 2, the green blank forming process in step 3 is completed within 8 hours; Step 4, sintering and multi-step heat treatment: placing the green blank obtained in step 3 in a vacuum sintering furnace, wrapping the green blank with a tantalum sheet with a mass purity of more than 99%, placing a sponge titanium with a mass purity of more than 99.5% in the furnace chamber of the vacuum sintering furnace, and then sintering at a vacuum degree of 0.001Pa~0.05Pa and a temperature of 1600°C for 2h, followed by multi-step heat treatment, first cooling to 1060°C for 3h, then cooling to 1025°C for 3h, cooling with the furnace to room temperature, and obtaining a sintered body; during the multi-step heat treatment process, an infrared thermometer combined with a K-type thermocouple equipped with the vacuum sintering furnace is used to monitor and control the heat treatment temperature of each step; Step 5: Machining: The sintered body obtained in step 4 is machined to remove a 2 mm thick surface to obtain a nickel-tungsten master alloy.

[0043] The nickel-tungsten master alloy prepared in this embodiment was tested for composition, and the results are shown in Table 3 below.

[0044] Table 3

[0045] It can be seen from Table 3 that the impurity elements such as Al, Mo, Pb, Si, Sn, Ta, and Ti in the nickel-tungsten master alloy prepared in this embodiment are very low, indicating that the present invention effectively improves the purity of the nickel-tungsten master alloy by controlling the purity and particle size of the raw tungsten powder and nickel powder; at the same time, the impurity elements such as C, O, and S in the nickel-tungsten master alloy are much lower than 0.01wt%, indicating that the present invention adopts tantalum sheet wrapping, sponge titanium placement and other means to effectively suppress the pollution of the sintering environment (graphite furnace chamber, air, etc.) to the nickel-tungsten master alloy product.

[0046] After testing, it is found that the physical phase of the nickel-tungsten master alloy prepared in this embodiment consists of a Ni4W phase and a nickel solid solution phase, and the nickel and tungsten elements in the nickel-tungsten master alloy are evenly distributed without any segregation of tungsten elements.

[0047] The above is only a preferred embodiment of the present invention and does not limit the present invention in any way. Any simple modification, change and equivalent change made to the above embodiment according to the technical essence of the invention still falls within the protection scope of the technical solution of the present invention.

Claims

1. A multi-step heat treatment method for preparing a nickel-tungsten master alloy, characterized in that: The method comprises the following steps: Step 1. Raw material selection: Select nickel powder with a mass purity of more than 99.9%, a particle size of less than 100μm, and an oxygen content of no more than 1000ppm, and tungsten powder with a mass purity of more than 99.9%, a particle size of less than 25μm, and an oxygen content of no more than 1000ppm as raw materials; Step 2, powder mixing: according to the composition of the target product nickel-tungsten master alloy, weigh the nickel powder and tungsten powder in step 1 and mix them with anhydrous ethanol in a mixer to obtain a mixed powder; Step 3, green blank molding: the mixed powder obtained in step 2 is dried and loaded into a rubber mold, and after vacuum packaging, it is placed in a cold isostatic press and maintained at a pressure of 200MPa to 300MPa for 10 minutes to obtain a green blank; Step 4, sintering and multi-step heat treatment: placing the green blank obtained in step 3 in a vacuum sintering furnace, sintering at a vacuum degree of 0.001Pa~0.05Pa and a temperature of 1400℃~1600℃ for 2h, then performing multi-step heat treatment, first cooling to 1030℃~1060℃ for 1h~3h, then cooling to 1005℃~1025℃ for 1h~3h, cooling to room temperature with the furnace, and obtaining a sintered body; Step 5: Machining: The sintered body obtained in step 4 is machined to remove 1 mm to 2 mm of the surface to obtain a nickel-tungsten master alloy.

2. The multi-step heat treatment method for preparing a nickel-tungsten master alloy according to claim 1, characterized in that: The particle size ratio of the nickel powder to the tungsten powder in step 1 is 1-2:

1.

3. The multi-step heat treatment method for preparing a nickel-tungsten master alloy according to claim 1, characterized in that: In the powder mixing process described in step 2, first put the nickel powder and anhydrous ethanol accounting for 5% of the mass of the nickel powder in a mixer, run the mixer at a barrel speed of 20 rpm to 60 rpm for 30 minutes, let it stand for 15 minutes, then add tungsten powder, and continue to run the mixer at a barrel speed of 20 rpm to 60 rpm for 3 hours to 5 hours.

4. The multi-step heat treatment method for preparing a nickel-tungsten master alloy according to claim 1, characterized in that: After the mixed powder is prepared in step 2, the green blank forming process in step 3 is completed within 8 hours.

5. The multi-step heat treatment method for preparing a nickel-tungsten master alloy according to claim 1, characterized in that: During the sintering in step 4, firstly, a tantalum sheet with a mass purity of more than 99% is used to wrap the green blank, and a titanium sponge with a mass purity of more than 99.5% is placed in the furnace chamber of a vacuum sintering furnace.

6. The multi-step heat treatment method for preparing a nickel-tungsten master alloy according to claim 1, characterized in that: During the multi-step heat treatment process described in step 4, an infrared thermometer is used in combination with a K-type thermocouple equipped in the vacuum sintering furnace to monitor and control the temperature of each heat treatment step.

7. A nickel-tungsten master alloy prepared by the method according to any one of claims 1 to 6, characterized in that: The element mass fraction of the nickel-tungsten master alloy is: Ni 40%~62%, and the balance is W and inevitable impurities, the inevitable impurities include non-metallic impurities O, C, S with a mass fraction of less than 0.01%, and metal impurities Al, Mo, Pb, Sn, Ta, Ti with a mass fraction of less than 0.01%, and the phase composition of the nickel-tungsten master alloy includes Ni4W phase and other nickel-tungsten phases, and other nickel-tungsten phases are derived from nickel solid solution phase, NiW phase and NiW2 phase. The nickel and tungsten elements are evenly distributed in the nickel-tungsten master alloy, and there is no segregation of tungsten elements.

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