Method for preparing ultra-high-purity invar alloy cast ingot based on forging diameter change

Abstract

The invention discloses a method for preparing an ultra-high-purity invar alloy cast ingot based on forging diameter changing. The method comprises the following steps that firstly, a cast ingot is formed through vacuum electron beam melting; secondly, forging and diameter changing are conducted; according to the method, vacuum electron beam cold bed smelting, forging diameter changing and vacuum consumable remelting form a short continuous process, the prepared and formed invar alloy ingot has the advantages of being large in size, high in purity and grain size, uniform and compact in structure, few in defect and the like by optimizing parameters, and the short continuous process has the advantages of reducing the cost, improving the production efficiency and the like at the same time. And the efficiency is improved.

Description

Technical Field

[0001] This invention relates to the field of alloy technology, and specifically to a method for preparing ultra-high purity Invar alloy ingots based on forging diameter modification. Background Technology

[0002] Invar alloy is a nickel-iron based precision alloy with an ultra-low coefficient of thermal expansion near room temperature. Its typical composition is 36% nickel (the balance being iron and trace amounts of other elements), hence it is also known as Fe-36Ni alloy. The core characteristic of this alloy is its extremely low average coefficient of thermal expansion, typically around 1.2 × 10⁻⁶, over a temperature range of -20°C to 100°C. -6 With a temperature of only about one-tenth that of ordinary steel, Invar alloys possess an anomalous property stemming from the counteracting effect of its unique ferromagnetism and lattice expansion. Due to its exceptional dimensional stability, Invar alloys are widely used in aerospace (such as satellite structural components and gyroscopes), precision instruments (such as optical platforms and laser cavities), electronic packaging, and as critical sealing materials for liquefied natural gas (LNG) carriers.

[0003] Current shortcomings: During the preparation of Invar alloy ingots, they are prone to repeated contact with refractory crucibles and slag, resulting in decreased ingot purity and poor microstructure uniformity, making it difficult to consistently obtain high-quality Invar alloys, which needs to be improved. Summary of the Invention

[0004] To address at least one of the aforementioned technical deficiencies, the present invention provides the following technical solution:

[0005] This application discloses a method for preparing ultra-high purity Invar alloy ingots based on forging and diameter modification, including the following steps: The first step is to melt and form an ingot using vacuum electron beam. The raw materials required for the Invar alloy ingot are added to the furnace for melting. Electron gun 1 melts the billet, and electron gun 2 refines the molten alloy in the crucible. The second step is forging and diameter adjustment. The ingot obtained in the first step is kept at 1100-1250℃ for 4-8 hours, and then upsetting and drawing is carried out to the required size. The initial forging temperature is ≥1050℃ and the final forging temperature is ≥950℃. The third step is vacuum self-consumable remelting. The surface of the forged ingot from the second step is polished, and then it is put into the furnace for melting. After melting, it is cooled in the furnace under high vacuum for at least 7 hours. The ingot is taken out of the furnace after the surface temperature drops below 200°C.

[0006] Furthermore, in the first step, the vacuum degree inside the furnace is 4.5-5.5×10⁻⁶. -3Pa, the furnace pressure rise rate is less than 1 Pa / h, electron gun one is used for melting at 300-320KW, electron gun two is used for melting at 250-270KW, melting temperature is 1500-1600℃, melting rate is 9-11kg / min, melting time is 3-4h, and the ingot is unloaded after melting and natural cooling for at least 4h.

[0007] Furthermore, in the first step, the Invar alloy raw material ratio is 70% Fe and 30% Ni, and it is made from electrolytic iron plates and electrolytic nickel plates with a purity greater than 99.99%.

[0008] Furthermore, in the second step, firstly, upsetting is performed axially at 1050-1150℃, with an upsetting deformation of 65-80%. Then, the furnace is heated to 980-1130℃ for radial elongation, with an elongation deformation of 20-40%. The furnace is then heated to 980-1130℃ a second time for axial upsetting, with an upsetting deformation of 50-70%. Finally, the furnace is heated to 980-1130℃ a third time for radial elongation, with an elongation deformation of 40-55%.

[0009] Furthermore, in the second step, the temperature is raised to 1200℃ at a rate of <100℃ / h under inert gas protection and held for 6 hours. After forging, the furnace is immediately transferred to a cooling furnace and cooled to 150℃ at a rate of <40℃ / h before being removed from the furnace.

[0010] Furthermore, in the third step, after the molten pool stabilizes during smelting, the smelting current is linearly increased to 9±0.3 kA within 5 minutes and steady-state smelting is carried out at a constant melting rate with a vacuum degree of <0.5 Pa. At the end of the smelting process, the smelting current is linearly reduced to 2 kA in 8-11 stages within 55±5 minutes. After remelting, the ingot is cooled in the furnace with a vacuum degree of ≤1 Pa for at least 7 hours and is taken out of the furnace after the surface temperature drops to 200°C.

[0011] Furthermore, in the third step, after the ingot is loaded into the crucible, a vacuum is first drawn to a vacuum level of <0.1 Pa. Then, an arc-starting current of 6 kA is applied between the electrode and the bottom homogeneous arc-starting material and maintained for 6 min.

[0012] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This invention uses a short continuous process consisting of vacuum electron beam cold bed melting, forging and diameter modification, and vacuum consumable remelting. By optimizing parameters, the Invar alloy ingots prepared have advantages such as large size, high purity, high grain size, uniform and dense structure, and few defects. The short continuous process also has the advantages of reducing costs and improving efficiency. Attached Figure Description

[0013] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0014] Figure 1 This is a structural diagram of the ingot prepared in the first step of Example 1; Figure 2 This is a structural diagram of the ingot prepared in the second step of Example 1; Figure 3 This is a structural diagram of the ingot prepared in the third step of Example 1. Detailed Implementation

[0015] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0016] Example 1 A method for preparing ultra-high purity Invar alloy ingots based on forging diameter modification includes the following steps: The first step involves vacuum electron beam melting and casting of ingots. The required proportions of raw materials (70% Fe, 30% Ni) for Invar alloy ingot casting are added to the furnace for melting. The Fe material is electrolytic iron plate with a purity greater than 99.99%, and the Ni material is electrolytic nickel plate with a purity greater than 99.99%. After sealing the furnace, a vacuum is drawn to a vacuum level of 5.0 × 10⁻⁶. -3 The furnace pressure rise rate was 0.6 Pa / h. Melting began at 35 kV and 7 A. The electron gun was started to heat the furnace, and the number of electron beam rings was adjusted to 25. The distance between the electron beam and the edge of the furnace was 10 mm. A dual-electron gun melting process was used. Electron gun one was used to melt the billet, and electron gun two was used to refine the Invar alloy melt in the water-cooled copper crucible. The ingot head was made of iron and had a diameter of 400 mm. During melting, the power of electron gun one was 310 kW. When the billet began to melt and drip into the water-cooled copper crucible, electron gun two was turned on and the power was adjusted to 260 kW for refining. The melting temperature reached 1560℃, the melting rate was controlled at 10 kg / min, and the total melting time was 3.5 h. After melting, the ingot was naturally cooled for 4 h before being vented and unloaded. The ingot shape was as follows. Figure 1 As shown.

[0017] The second step, forging and diameter adjustment, involves placing the ingot obtained in the first step into an annular heating furnace. Under argon protection, the temperature is raised to 1200℃ at a rate of 85℃ / h and held for 6 hours. Then, on a high-speed forging mill, upsetting and drawing are performed to the required dimensions. First, axial upsetting is performed at 1100℃ to a height of 500mm (upsetting deformation of 75%). Then, the ingot is reheated to 1100℃ for the first radial drawing (drawing deformation of 30%). Next, it is reheated to 1100℃ for a second axial upsetting (upsetting deformation of 60%). Finally, it is reheated to 1000℃ for a second radial drawing (drawing deformation of 50%). The final forging temperature is controlled at 960℃, resulting in a uniform long bar with a diameter of 250mm. The forged bar is immediately transferred to a cooling furnace and cooled to 120℃ at a slow cooling rate of 30℃ / h before being removed from the furnace. The ingot shape is as follows... Figure 2 As shown.

[0018] Step 3: Vacuum self-consuming remelting The surface of the forged Invar alloy ingot is ground to remove the oxide scale. Under argon protection, it is welded to the suspension rod and then coaxially assembled into a water-cooled copper crucible with an inner diameter of 320mm after cleaning before the furnace is opened. The furnace body is then closed and a vacuum of 0.05Pa is drawn to ensure the purity of the smelting environment.

[0019] When the melting process is started, an arc-starting current of 6kA is first applied between the electrode and the bottom homogeneous arc-starting material and maintained for 6 minutes. During this period, an axial alternating magnetic field of 50A is applied through the arc-stabilizing coil equipped in the furnace body to effectively constrain the arc, suppress deflection, and provide slight stirring for the molten pool.

[0020] After the molten pool stabilizes, the melting current is linearly increased and stabilized at 9kA within 5 minutes, entering the constant melting steady-state melting stage. During this stage, the melting rate is 3.5kg / min, and the corresponding dynamic vacuum degree is less than 0.5Pa.

[0021] When the remaining weight of the electrode melting is 85% of the initial weight (i.e., entering the final stage of melting), in order to reduce shrinkage cavities at the ingot head, the process switches to the programmed hot capping and feeding stage: within 55 minutes, the melting current is reduced from 9kA to 2kA in 10 uniform linear gradients, and the precisely controlled heat input provides continuous feeding to the top of the ingot.

[0022] After the remelting process, the ingot was cooled in a furnace under high vacuum (0.51 Pa) for 8 hours until its surface temperature dropped to 150°C before being removed from the furnace. Finally, by removing the bottom 50mm of the ingot and the top 200mm of the shrinkage cavity, a dense VAR finished ingot with a diameter of 320mm was obtained. The ingot morphology is as follows: Figure 3 As shown.

[0023] Comparative Example 1 Compared to Example 1, the difference lies in the following: In this example, the raw material ratio in the first step is Fe 67% and Ni 33%; the second step replaces the forging and diameter adjustment step with vacuum induction melting, as follows: The ingot formed in the first step is broken into small pieces, and these pieces are loaded into the crucible of a vacuum induction melting furnace with a magnesium oxide lining. After the furnace is closed, the vacuum system is activated to evacuate the furnace pressure to a vacuum degree of 0.06 Pa, and then power is supplied to start slow heating, so that the furnace charge is completely melted within 1 hour, and the temperature of the molten pool is raised to a refining temperature of 1550°C. At this temperature, refining is performed for 25 minutes. After refining, the crucible is tilted, and the molten liquid is poured into a cylindrical water-cooled copper mold with an inner diameter of 250 mm to obtain a cast VAR electrode with a diameter of 250 mm. The third step is the same as in Example 1.

[0024] Comparative Example 2 Compared with Example 1, the difference is that the forging diameter modification step in the second step is omitted.

[0025] Comparative Example 3 Compared to Example 1, the difference lies in the process flow: the first step (consistent with Example 1) is vacuum induction melting, and the second step is electroslag remelting and refining. The steps are as follows: First, the ingot prepared in the first step is vertically suspended as the cathode in the center of the ESR furnace crystallizer, which is a water-cooled copper crucible with an inner diameter of 400 mm. Pre-melted 70% CaF2-30% Al2O3 slag is placed at the bottom of the crucible, and remelting is carried out under argon protection. After startup, an AC current of 35V and 6000A is applied to generate strong resistance heat, which rapidly melts the solid slag into a high-temperature liquid slag pool; then the lower end of the electrode begins to melt, forming metal droplets that detach drop by drop, finally obtaining an Invar alloy ESR ingot with a diameter of 400 mm. The third step is forging and billet preparation, which involves heating a 400mm diameter ESR ingot to 1200℃ at a rate of 60℃ / h and holding it for 8 hours. Then, the billet is prepared on a large press, and the first axial upsetting is performed at 1100℃ with a height compression of 50%. After reheating in the furnace, the first radial drawing is performed at 1150℃ to forge a square billet with a cross-sectional side length of 380mm. The billet is then reheated in the furnace again, and the second axial upsetting is performed at 1100℃ with a height compression of 20%. Subsequently, the second radial drawing is performed at 1050℃ with a drawing deformation of 35%. Finally, the third finishing drawing is performed at 1000℃ with a drawing deformation of 20%. The final forging temperature is 900℃. After forging, the billet is directly placed in air to cool, and finally formed into an Invar alloy round billet with a diameter of 250mm.

[0026] To test the data, the head and tail of the prepared ingot were cut off by 50mm.

[0027] Cross sections were taken along the axial direction of the ingot at the head, middle, and tail. Samples were also taken at the radial edge, half radius, and center. The samples were tested using an oxygen, nitrogen, and hydrogen analyzer and a carbon and sulfur analyzer, as shown in Table 1.

[0028] Table 1

[0029] Conclusion: In all process routes involving vacuum induction melting (VIM), the oxygen and nitrogen content of the ingots increased by orders of magnitude. This demonstrates that the refractory crucible used in VIM is the core source of interstitial gas contamination. Comparative Example 2, lacking the "forging diameter adjustment" step, suffered from incomplete gas removal due to casting defects, resulting in the highest oxygen and nitrogen content. The carbon content trend was largely consistent with that of oxygen and nitrogen, further confirming the systematic impact of different process routes on material purity. Comparative Example 3, undergoing both VIM and ESR processes, had the highest carbon content. Hydrogen content was generally controlled at a low level, indicating that the risk of hydrogen embrittlement was effectively controlled.

[0030] Cross sections were taken along the ingot axis at the head, middle, and tail; samples were taken at the radial edge, half radius, and center, and their characteristic elements were detected by ICP-MS, as shown in Table 2.

[0031] Table 2

[0032] Conclusion: The magnesium and aluminum contents in Comparative Example 1 were abnormally high, while the contents of these elements were extremely low in Examples 1 and 2 of EBCHM alone. This constitutes direct evidence that these magnesium and aluminum elements did not originate from the raw materials, but were introduced by the VIM process. Magnesium, in particular, is a "fingerprint" characteristic of the eroded magnesium oxide refractory crucible used in VIM, definitively proving the inherent contamination that is unavoidable in this process. Meanwhile, Comparative Example 3 had the highest calcium and aluminum contents among all groups, clearly indicating that it suffered from both VIM crucible contamination and ESR slag contamination.

[0033] Cross sections were taken along the ingot axis at the head (T), middle (M), and tail (B); samples were taken at the radial edge (E), half radius (R / 2), and center position (C). Dendrite spacing, inclusions, and nickel content were detected by metallographic microscope and ICP-OES, as shown in Table 3.

[0034] Table 3

[0035] Conclusion: In Example 1, the inclusion rating remained stable at 0.5 at all locations. The inclusion ratings of the three comparative examples all showed a clear trend of "deterioration from the edge to the center", proving that the Invar alloy ingots obtained by this method have high purity and extremely uniform inclusion distribution.

[0036] In addition, the nickel content of the alloys prepared above was tested. It was found that the nickel content of Example 1 fluctuated the least in all positions of the ingot. Comparative Examples 1 and 3 showed typical characteristics of nickel-rich edges and nickel-poor cores. Comparative Example 2 showed severe nickel-poor cores and the most severe positive segregation. This proves that "forging and diameter modification" can achieve sufficient diffusion and homogenization of composition in the solid state.

[0037] Samples were taken from the transverse positions of the head (T), middle (M), and tail (B) of the ingot at half the radius (R / 2). The coefficient of thermal expansion, tensile strength, and elongation after fracture were tested using a thermal expansion tester and a universal testing machine. The results are shown in Tables 4, 5, and 6.

[0038] Table 4

[0039] Table 5

[0040] Table 6

[0041] Conclusion: Example 1 exhibits the best performance, with the smallest overall fluctuation range and highly similar CTE values ​​in the head, middle, and tail sections. In contrast, both the VIM-based process and the conventional duplex melting (EBCHM+VAR) show a significant axial performance gradient and a larger overall fluctuation range. Example 1 has the highest tensile strength and best plasticity, while Comparative Example 2 has the worst strength and plasticity, further demonstrating that "forging diameter modification" can effectively eliminate casting defects and improve mechanical properties. Example 1 has the smallest standard deviation in strength and plasticity data in the head, middle, and tail sections, indicating that its mechanical properties are highly consistent axially. The standard deviation of the ingot data prepared in the comparative examples is generally larger, especially the elongation fluctuation of Comparative Example 2, reflecting the performance instability caused by its uneven internal structure.

[0042] The above are merely preferred embodiments of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, any improvements and modifications made without departing from the principle of the present invention should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing ultra-high purity Invar alloy ingots based on forging diameter modification, characterized in that, Includes the following steps: The first step is to melt and form an ingot using vacuum electron beam. The raw materials required for the Invar alloy ingot are added to the furnace for melting. Electron gun 1 melts the billet, and electron gun 2 refines the molten alloy in the crucible. The second step is forging and diameter adjustment. The ingot obtained in the first step is kept at 1100-1250℃ for 4-8 hours, and then upsetting and drawing is carried out to the required size. The initial forging temperature is ≥1050℃ and the final forging temperature is ≥950℃. The third step is vacuum self-consumable remelting. The surface of the forged ingot from the second step is polished, and then it is put into the furnace for melting. After melting, it is cooled in the furnace under high vacuum for at least 7 hours. The ingot is taken out of the furnace after the surface temperature drops below 200°C.

2. The method for preparing ultra-high purity Invar alloy ingots based on forging diameter modification as described in claim 1, characterized in that: In the first step, the vacuum degree inside the furnace is 4.5-5.5×10⁻⁶. -3 Pa, the furnace pressure rise rate is less than 1 Pa / h, electron gun one is used for melting at 300-320KW, electron gun two is used for melting at 250-270KW, melting temperature is 1500-1600℃, melting rate is 9-11kg / min, melting time is 3-4h, and the ingot is unloaded after melting and natural cooling for at least 4h.

3. The method for preparing ultra-high purity Invar alloy ingots based on forging diameter modification as described in claim 2, characterized in that: In the first step, the raw material ratio of Invar alloy is 70% Fe and 30% Ni, and it is made from electrolytic iron plate and electrolytic nickel plate with a purity greater than 99.99%.

4. The method for preparing ultra-high purity Invar alloy ingots based on forging diameter modification as described in claim 1, characterized in that: In the second step, the material is first upset along the axial direction at 1050-1150℃, with an upsetting deformation of 65-80%. Then, it is heated back to 980-1130℃ for radial elongation, with an elongation deformation of 20-40%. It is then heated back to 980-1130℃ for a second axial upset, with an upsetting deformation of 50-70%. Finally, it is heated back to 980-1130℃ for a third radial elongation, with an elongation deformation of 40-55%.

5. The method for preparing ultra-high purity Invar alloy ingots based on forging diameter modification as described in claim 1, characterized in that: In the second step, the temperature is raised to 1200℃ at a rate of <100℃ / h under inert gas protection and held for 6 hours. After forging, the furnace is immediately transferred to a cooling furnace and cooled to 150℃ at a rate of <40℃ / h before being removed from the furnace.

6. The method for preparing ultra-high purity Invar alloy ingots based on forging diameter modification as described in claim 1, characterized in that: In the third step, after the molten pool stabilizes during melting, the melting current is linearly increased to 9±0.3 kA within 5 minutes and steady-state melting is carried out at a constant melting rate with a vacuum degree of <0.5 Pa. At the end of the melting process, the melting current is linearly reduced to 2 kA in 8-11 stages within 55±5 minutes. After remelting, the ingot is cooled in the furnace with a vacuum degree of ≤1 Pa for at least 7 hours and is taken out of the furnace after the surface temperature drops to 200℃.

7. The method for preparing ultra-high purity Invar alloy ingots based on forging diameter modification as described in claim 1, characterized in that: In the third step, after the ingot is loaded into the crucible, a vacuum is first drawn to a vacuum level of <0.1 Pa. Then, an arc-starting current of 6 kA is applied between the electrode and the bottom homogeneous arc-starting material and maintained for 6 min.

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